3 Green Vehicles

Green Alternatives and National Energy Strategy: The Facts behind the Headlines

CHAPTER 3 Green Vehicles

The previous chapter dealt with reducing gasoline consumption without major technology changes. We can drive less; we can drive more efficiently; we can drive smaller, lighter automobiles. We can also get some improvement in fuel economy by making mechanical improvements to the internal combustion gasoline automobile. Doing these things will decrease gasoline consumption somewhat but not as much as desired. They will not achieve the goal of eliminating oil as a fuel source and replacing it with cleaner domestic and possibly renewable sources. To accomplish this, we need some serious engineering changes.

This chapter focuses on technology solutions for increasing gasoline fuel economy and for replacing gasoline with alternative fuels, either completely or partially. Each alternative has potential benefits and certain drawbacks and limitations. Each alternative breaks new engineering ground and has a large price tag. Understanding the engineering possibilities and trade-offs is important if we are to make the tough decisions we must face in the future. I discuss engineering alternatives that are available now or are under development: diesel, flex-fuel, natural gas, hybrid electric, series electric, plug-in electric, and hydrogen fuel-cell vehicles.

The first three types of alternative vehicles I discuss, diesel vehicles, flex-fuel vehicles, and natural gas vehicles (NGVs), are similar in that they have internal combustion engines for propulsion. The differences among them are the fuel and the fact that the “fuel tank” on the NGV is one or more compressed gas cylinders. There are several electric vehicles (EVs), which get some or all of their propulsion from an electric motor. The hybrid electric vehicle (HEV) is unique in that it has two parallel propulsion systems, one an internal combustion engine and the other an electric motor. The series electric vehicle (SEV) has all-electric drive but burns fossil fuel in an onboard generator; the generator is in series with the electric motor. The hybrid plug-in electric vehicle (H-PEV) is similar to the SEV, but its batteries can be charged from the power grid. These three are transitional vehicles between fossil-fuel internal combustion engine vehicles (ICEVs) and pure electric drive. The final two vehicles, the pure plug-in electric vehicle (P-PEV) and hydrogen fuel-cell vehicle (HFCV), are pure electric-motor propulsion systems that get all of their energy from the power grid.

Diesel Vehicles

Diesel engines are similar to gasoline engines in that they are powered by internal combustion and run on petroleum-based fuel. They are different in that compression rather than an electrical spark ignites the fuel and the fuel comes from a different fraction of crude oil. Diesel vehicles have been on the road for almost as long as gasoline engines, but they have generally been restricted to trucking in the United States because of a perception that they are noisy and polluting. However, diesels tend to be dependable, long lasting, powerful, and fuel-efficient, and they have been much more popular for personal automobiles in Europe for some time. Diesels have become reliable, efficient, quiet, and low-pollution, and they have been steadily gaining acceptance for automobiles in the United States. Diesel vehicles are roughly 30% to 35% more fuel-efficient than gasoline engines, and diesel fuel contains 10% more energy per gallon than gasoline. Modern diesels are attractive because fuel economy is better than with gasoline. Even though diesel fuel costs more than gasoline, the better fuel economy results in lower fuel cost per mile.

The functional layout of the diesel vehicle is shown in figure 3.1a. Similar to the conventional car discussed in chapter 2, the diesel vehicle has a fuel tank filled from an external source, an internal combustion engine, and a drivetrain that incorporates a transmission. Several models of diesel vehicles are currently available in the United States. The Volkswagen Jetta SportWagen is a useful example because it is available in otherwise identical diesel and gasoline versions. The diesel SportWagen has the leading fuel economy in the small-station-wagon category: 30 mpg in city driving, 41 mpg in highway driving, and 35 mpg overall (assuming 55% city driving and 45% highway driving); written succinctly as 30/41/35. This compares favorably with the gasoline version at 21/30/25. The diesel gets 40% better overall fuel economy.

The diesel Jetta produces lower greenhouse gas emissions than the gasoline version. I calculate 38% less CO₂ from the diesel than from the gasoline Jetta (3.0 tons per year versus 4.8 tons per year). Clearly, the Jetta diesel reduces CO₂ emissions per mile significantly compared with the gasoline version.

The saving in annual fuel cost is attractive. While diesel fuel costs more than gasoline, the Environmental Protection Agency (EPA) estimates the diesel Jetta saves $210 a year in fuel costs. However, the purchase price of the car offsets these benefits. The sticker price of the Jetta diesel is $4,600 higher than the gasoline model. Higher sticker price is common for alternative vehicles. In the case of the Jetta, it would take twenty-two years of fuel-cost saving to recoup the increased vehicle cost. On a simple financial-investment basis, the diesel does not make sense at current prices. It might be desirable for reducing pollution and fuel consumption, but there is little financial incentive for the average person to invest in the Jetta diesel.

When we look beyond fuel economy, the overriding issue is reducing demand for crude oil and dependence on imported oil. This leads to an interesting situation. A barrel of crude oil contains less diesel fuel than gasoline, about half as much. When we calculate the fuel economy as miles per barrel of crude, rather than miles per gallon of refined diesel fuel, the result is 338 miles per barrel of crude for the diesel versus 500 miles per barrel of crude for the gasoline version. That is, the diesel consumes more crude oil per mile than the gasoline version, even though it consumes less refined diesel fuel. As long as the number of diesels on the road remains small, modifying the mix of crude oils and adjusting the refining process can offset at least some of the difference. Modifying international trade in refinery products would also help. However, as the number of diesels on the road increases, the demand for crude oil will increase, just the opposite of what we are trying to accomplish.

Biodiesel

Biodiesel provides a possible way out of this unacceptable situation. Biodiesel fuel can run a diesel engine, but it comes from biomass rather than petroleum. From here on, I distinguish between “biodiesel,” which comes from plants and animal fat, and “petro-diesel,” which is the common petroleum-based diesel fuel.

Biodiesel is one of two biological alternatives to fossil fuel. I discuss ethanol, the leading contender for a possible alternative to gasoline, in the section on flex-fuel vehicles. In both cases, diluting petrofuel with biofuel reduces oil consumption. Whether this is successful depends on how the alternative fuel performs in the vehicle and the inevitable drawbacks introduced by the biofuel.

Ironically, when Rudolf Diesel was first working on his diesel engine invention in the late nineteenth century, he envisioned using vegetable oil as a fuel so that fuel for his engine would be available throughout the world. One demonstration engine at the 1900 Paris Exposition ran on peanut oil. The burgeoning petroleum economy and easy availability of kerosene and, later, diesel fuel pushed vegetable oil fuels into the background.¹ Even as late as 1912, shortly before his death, Diesel was extolling the virtues of vegetable oil as a fuel for his engines. Diesel’s vision not withstanding, kerosene from refining petroleum was plentiful and inexpensive in the late nineteenth century, and Diesel concentrated on kerosene as a fuel. By the early twentieth century, diesel engines had become more widely used, and an inexpensive, low-grade petroleum product called “diesel fuel” had become available. Diesel engines were optimized to take advantage of this new fuel, making it more difficult for them to burn vegetable oil. Even then, people were concerned because petrodiesel fuel was dirtier than vegetable oil fuels, but petroleum was inexpensive and plentiful, and petrodiesel became the standard fuel.

FIGURE 3.1 Fossil-Fuel Vehicle Layouts: a, conventional internal combustion engine vehicle (ICEV); b, hybrid electric vehicle (HEV); c, series electric vehicle (SEV); d, hybrid plug-in electric vehicle (H-PEV). The ICEV layout illustrates the common automobile with a fuel tank, internal combustion (IC) engine, and transmission. Liquid fossil fuel, gasoline or diesel, is stored in a simple fuel tank in conventional ICEVs. NGVs are ICEVs that use compressed natural gas stored in high-pressure (3,600 psi) cylinders for fuel. The HEV has two complete propulsion systems. One is the common internal combustion engine; the other is an electric drive system. The small battery is charged by regenerative braking or a small on-board genset. The SEV has a single propulsion motor, which is electric. Its small battery is charged by regenerative braking and an onboard genset. It does not have the fossil fuel IC engine of the HEV, and the battery cannot be charged by plugging into an external charging source. The H-PEV is similar to the P-PEV (see fig. 3.3), which gets all of its electricity from an external charging source, but unlike the P-PEV, the H-PEV has an onboard genset to provide electricity after the battery has depleted the external charge.

Source: Mehrdad Ehsani, Yimin Gao, and Ali Emadi, Modern Electric, Hybrid Electric, and Fuel Cell Vehicles: Fundamentals, Theory, and Design, 2nd ed. (Boca Raton, FL: CRC Press 2010).

Vegetable-oil fuel was not totally forgotten. It is clean, and vegetable oil sources are widely available in tropical climates. Research continued in the early twentieth century. The advent of World War II and the resulting disruption of transoceanic transportation and petroleum availability stimulated a resurgence of interest in vegetable oil as a biodiesel fuel, but interest again flagged after the war when petroleum supplies recovered. Biodiesel experienced another temporary resurgence because of the 1973 OPEC oil embargo. Another period of resurgence is now underway because of the pending crises of petroleum supply and global warming.

The current situation is slightly different from when Diesel was developing his engine. His original engine could burn vegetable oil directly. Since then, however, petrodiesel has become available, and engineers have optimized the diesel engine to take full advantage of the new fuel. One result was that vegetable oil, or any other biofuel, does not work as well in diesels now as it did then. The solution is either to modify the engine to burn biodiesel more efficiently or modify the biodiesel fuel. Industry’s choice has been to modify the fuel. The reason for this decision is telling. Essentially the industry concluded that modifying machine tooling and engine design and manufacturing would not be economically beneficial because biodiesel would never capture a significant fraction of the diesel market. The cost-effective approach is to modify the fuel to match the engine rather than the engine to match the fuel, with the result that producing biodiesel fuel is more complicated now than it was in Diesel’s time. The diesel industry itself does not think that biodiesel can be a major contributor to the diesel engine industry.

The industry is currently extracting biodiesel from plants, used vegetable cooking oil, and animal fats. I will not elaborate on the process because there are several books that cover the topic thoroughly,² and I am much more interested in production yields. With regard to plant sources, the plant has to be grown and harvested, and the important statistic is yield of biodiesel per acre of agricultural land (table 3.1).

The most common biodiesel crops in the United States are soybeans and corn, which provide very little biodiesel. Other crops grown in other countries with different climates and agricultural conditions might be more productive, but relying on them would raise the same concerns about dependence on foreign sources of fuel as we now have about foreign oil. Rising demand for biofuel has led to an increased amount of arable land being devoted to producing fuel rather than food. This means less food and higher food prices worldwide and has led to widespread deforestation throughout the world, with a detrimental effect on global warming.

TABLE 3.1 Biodiesel Crop Yields

The potential beneficial effect of biodiesel production from US agriculture is inevitably limited. As stated earlier, the United States consumes 64.3 billion gallons of diesel and 142 billion gallons of gasoline (208.3 billion gallons of fuel total) annually. If 10% of the 922.1 million acres of US farmland were devoted to soybeans, the yield of biodiesel would be 4.4 billion gallons, only 7% of the annual diesel consumption, or 2% of total vehicle fuel. Actual savings would be slightly different because biodiesel would be blended with petrodiesel in vehicle fuel, but the numbers are telling. The point is that taking a large percentage of US farmland out of growing food would make only a minor contribution to oil independence.

Biodiesel can also be produced from used cooking oil. The quality is not as good as biodiesel derived from plant crops because of contaminants and the successive rounds of heating to high temperature. However, at 50% yield, the 3 billion gallons of used cooking oil generated in the United States annually could theoretically provide 1.5 billion gallons of biodiesel. That is only 2% of diesel fuel demand. We could also get 750 million gallons of biodiesel (1% of US consumption of diesel) from 11 billion pounds of animal fat available from the meat-packing industry. All told, converting all the used cooking oil, all the animal fat, and 10% of US farmland to biodiesel production would barely offset 10% of current petrodiesel consumption. Moreover, increased demand for diesel would simply mean that biodiesel would provide a smaller percentage of total consumption of diesel.

I am uneasy depending on crops for fuel. What happens if we depend on the US soybean crop for fuel and the country enters a period like the 1930s dust bowl? Crop yields already vary from year to year, and now, with increasing concern about global warming and climate change, we should expect crops to be less dependable. To be somewhat whimsical, what happens if Americans stop eating french fries and the supply of cooking oil dries up? What happens if the United States turns vegetarian and the supply of animal fat dries up?

I cannot leave biodiesel without mentioning algae. Algae potentially surpass all other biodiesel feedstocks with regard to yield per acre. Early experiments by the National Renewable Energy Laboratory (NREL) indicate that maintaining high levels of production is difficult, but yields of 6,500 gallons per acre are possible.³ This is several hundred times the yield from growing soybeans. Experimentation ended because of high cost and difficulty getting consistent dependable yields. However, interest is increasing again, and algal biodiesel warrants careful monitoring. Inexpensive dependable algal biodiesel with yields anywhere close to the promise could be the breakthrough that weans us from gasoline.

Bottom Line

Diesel vehicles reduce greenhouse gases and pollution and increase fuel economy. However, they also increase demand for oil. Should the number of diesels on the road grow significantly, we will have to increase imports of crude oil.

In the short term, increased sticker price more than offsets the saving in fuel expense. This will probably change in the future, but now buying a diesel car is not a wise economic decision.

Domestic agricultural biodiesel cannot provide enough fuel to be a significant player in the long-term national energy crisis. Biodiesel has many advantages, and developing a dependable supply of biodiesel would certainly help to stretch fuel supplies, but the overall effect would be minor unless algal biodiesel becomes a reality. Algal biodiesel holds promise, and we should watch its development closely. The game changes significantly if a dependable, economical, source of algal biodiesel with yields anywhere near what seems to be feasible becomes available. In that case, diesel engines running on biodiesel would be a major benefit. Even then, however, depending on a crop could be risky. We should scrutinize dependability of crop yield very carefully.

Flex-Fuel Vehicles

During World War II, the fuel industry added tetraethyl lead to gasoline to boost octane. Lead turned out to be toxic and was banned. Several other additives for boosting performance were tried at various times. One of these, methyl tertiary butyl ether (MTBE), was added to gasoline to boost octane and oxygenate the fuel to make it burn more efficiently. This was successful, and the Clean Air Act of 1990 established MTBE as the standard additive. Unfortunately, MTBE turned out to be a carcinogen, and it was replaced by ethanol, an alcohol derived from corn. Since 2007, federal law has required all “gasoline” to be 10% ethanol and 90% gasoline, the mixture referred to as E10. While originally a performance-boosting additive, ethanol is now being considered for diluting gasoline. The idea is to replace 85% of the gasoline in each gallon of fuel with ethanol by mandating E85 fuel. However, ethanol is highly corrosive and, although E10 is not a problem for engines, E85 would damage conventional gasoline engines. Engines have to be modified to burn E85. A flex-fuel vehicle is capable of burning any mixture of gasoline and ethanol, at least up to E85. The concept is alluring: replace most of the gasoline consumed in vehicles, much of which comes from foreign oil, with domestic, renewable, corn alcohol. Unfortunately, the flex-fuel vehicle is promising in principle but problematic in execution.

One problem has to do with compression ratio. Ethanol has higher octane than gasoline. Indeed, it is used as an additive to gasoline to raise the octane. As long as the flex-fuel engine has to accommodate gasoline, the compression ratio has to be low, in keeping with gasoline. If engines burned only E85, the compression ratio could be higher, taking advantage of the higher octane of ethanol, and efficiency would be better. That is, keeping the engine flexible enough to burn either gasoline or E85 reduces the efficiency of burning E85. That said, there are several other reasons ethanol fuel is not a good idea.

Consider the effect on food supply. If we assume a corn-ethanol crop yield of 700 gallons per acre, devoting 10% of the 922.1 million acres of US cropland to ethanol production would provide 64.9 billion gallons of ethanol. Because driving range with E85 is 66% of the driving range with gasoline, this amount of ethanol would offset 30% of the gasoline consumed in the United States annually. That is, devoting a tenth of existing cropland to ethanol production would offset 30% of our gasoline consumption, whereas the same acreage devoted to biodiesel would offset only 7% of diesel consumption. Clearly, ethanol reduces gasoline demand more than biodiesel does, and devoting cropland to ethanol production would have a greater effect on reducing oil consumption.

Ethanol production that does not affect the food supply may be possible. Cellulosic ethanol is produced from wood, grasses, or the nonedible parts of plants. Ethanol may also be obtained from algae. Algenol Biofuels, Inc., is working closely with Mexican industry and the Mexican government to develop the technology.⁴ Construction of the pilot plant began in January 2010.⁵ The potential yield of ethanol from algae is similar to the potential yield of biodiesel from algae, up to 6,000 gallons per acre, so these sources might eventually produce ethanol economically and without negatively affecting the food supply

However, in addition to having the same negative effects on world food supplies and deforestation as biodiesel production, reliance on ethanol has several unique and severe drawbacks. First, ethanol contains less energy per gallon than gasoline (table 3.2). The range of a flex-fuel vehicle operating on E85 is three-quarters the range when operating on gasoline. This is an inconvenience to drivers on long trips, and experience in other countries where ethanol fuel is more widely used shows that many drivers switch back to gasoline on long trips to minimize the frequency of stops for refueling. Availability is another issue. As of late 2010, there were 2,347 filling stations in the United States that sold E85.⁶ There are no significant technological hurdles to overcome in producing E85, but it will be many years before E85 is widely available. Cost is another issue. E85 costs more per gallon than gasoline.⁷ Yielding fewer miles per gallon at a higher price per gallon, E85 costs almost twice as much per mile traveled than does gasoline. Operating a flex-fuel vehicle on E85, one would have to fill up more often and pay more per mile—not an attractive proposition. While cost will probably decrease with time and research, it is doubtful that range, which is limited by tank size and the lower energy density of ethanol, will ever be as great as we are used to with gasoline.

Another problem with ethanol is that it is highly corrosive. The current automobile engine and fuel system cannot stand up to high concentrations of ethanol. E10 is not harmful to current automobiles (boats and small airplane are another matter, discussed below), but E85 definitely is. Any metal, rubber, or fiberglass component that is subject to ethanol’s corrosiveness must be replaced to run on E85.⁸ Flex-fuel vehicles are engineered to burn E85 or any combination of E85 and gasoline without damage. This means that if E85 is mandated, not only all automobiles but all gasoline engines, including those for motorcycles, small boats, small airplanes, all-terrain vehicles, standby home generators, snowblowers, lawnmowers, power tools, and so on will have to be replaced with E85-tolerant models to avoid damage. The cost to the public would be enormous. If we were to mandate the use of E85 in automobiles and keep E10 available for other uses, ensuring that E85 is always used in automobiles would be difficult. Drivers would tend to use E10 because of its lower cost and greater driving range.

TABLE 3.2 Energy Densities of Fuels

The boating industry provides indications of what might happen if there were a widespread switch to E85. Powerboats have had problems ever since 2007 when E10 was mandated. The first problem is that the corrosive ethanol fuel dislodges gunk from the walls of the fuel tank and fuel lines and dissolves fiberglass fuel tanks. The resulting contaminated fuel clogs fuel filters and causes engine stoppage and sometimes engine damage. The second problem is caused by ethanol’s affinity for water. Water in the fuel can cause a phase separation, with gasoline on top and a mixture of ethanol and water at the bottom. The ethanol/water blend may cause the engine to stop and may cause severe damage to the engine.⁹ Boat fuel tanks are usually made of fiberglass and are vented to the atmosphere, and they go long periods without use, during which the environment provides many opportunities for water to enter the fuel tank. The problems caused by E10 in gasoline engines other than those in modern automobiles are so severe that Oregon passed a law that went into effect January 1, 2009, exempting gasoline sold for use in boats, aircraft, all-terrain vehicles, classic cars, and gasoline-powered tools from the requirement to contain 10% ethanol.¹⁰ E10 is a problem for boating and small aircraft now. E85 will be a much more serious problem for more engines in the future.

While it is true that flex-fuel vehicles burning E85 would reduce gasoline consumption and demand for foreign oil, the negative effects are numerous and must be weighed very carefully. First, the driving range of E85 is significantly less than that of gasoline, and cost per mile is much higher. Getting people to switch from gasoline to E85 will be difficult. Second, extracting ethanol from corn, the current plan, would have a significant effect on world food supplies. In the broad scheme of things, people will accept ethanol only if we find a source that does not affect food supplies or lead to deforestation. Yields from cellulosic sources are probably too low to be useful, but this might change. Ethanol from algae might be the solution, but that is a long way off. Even if we find a source of abundant ethanol that does not affect the world food supply, there is the third problem: what to do with nonautomobile engines, such as small boats, small airplanes, snowblowers, and power tools. Turning them into flex-fuel engines would be expensive, and maintaining distinct supplies of E85 and gasoline (or E10) would be a logistical nightmare.

Natural Gas Vehicles

Although only one natural gas car is on the market as of this writing, other NGVs, such as buses and delivery trucks, are moderately commonplace. An internal combustion engine can burn natural gas as a fuel with relatively minor modifications. Fuel economy and cost per mile are similar to conventional vehicles. Moreover, natural gas burns cleaner than gasoline. NGVs definitely reduce emissions of greenhouse gases and pollution and certainly reduce demand for gasoline. In-vehicle fuel storage and refueling are the primary drawbacks.

The NGV layout is shown in figure 3.1a. The NGV is an ICEV similar to the common gasoline car, but the fuel is natural gas compressed to 3,600 psi, and the fuel is stored in a high-pressure cylinder.

A little arithmetic clearly illustrates the storage issue. The energy content of natural gas is 0.04 kWh per gallon at atmospheric pressure (table 3.2). Compared with gasoline’s 36.0 kWh per gallon, this is minute. One would need 940 gallons of natural gas at atmospheric pressure to provide the energy in one gallon of gasoline. To be practical in a vehicle, natural gas has to be stored in a much smaller volume.

There are two ways natural gas can be stored efficiently. The first method is cooling and liquefying it as liquefied natural gas. This provides good energy density by volume but requires special tanks to maintain very low temperature. This is expensive and therefore practical only for long-distance transportation, such as shipping, and for heavy-duty vehicles, such as large trucks, but it is impractical for private passenger vehicles. The second method of storing natural gas efficiently is to compress it. This is straightforward and requires nothing more complicated for storage than a strong storage tank and a compressor for filling the tank. When natural gas is compressed and stored at 3,600 psi, the standard for natural gas, the energy content is still only about a quarter of the energy in an equivalent volume of gasoline. In other words, to store the same amount of energy in natural gas as in gasoline, one needs about four times as much storage volume to get the same driving range as with gasoline. Moreover, the high pressure requires a strong, heavy tank. Natural gas storage takes up much more space and weighs much more than gasoline storage for equivalent quantities of energy.

How do natural gas cars stack up to gasoline cars? The Honda Civic is a good illustration because it comes in a both a gasoline model (the DX) and a natural gas model (the GX). Figure 3.2 compares the two. Predictably, the natural gas version weighs 10% more than the gasoline model, its range is 40% less, and its cargo space is 50% less. There also is a price penalty. While fuel economy is slightly better, 31 mpg of gasoline equivalent (mpgge) versus 29 mpg, and fuel savings are about $100 per year, the sticker price is $14,000 higher. It would take 140 years to recover the purchase price penalty. As we have found with all of the alternative technology vehicles, there is no financial incentive for selecting an NGV over the gasoline car. The main benefits are that natural gas is much less polluting than gasoline, emitting roughly 20% less pollution overall and 15% less CO₂, and switching to NGV reduces consumption of gasoline and oil.

FIGURE 3.2 Honda Civic Gasoline DX versus Honda Natural Gas GX. Although the gasoline and natural gas Honda Civic cars are otherwise similar, the natural gas model costs much more, weighs more, has half the cargo space, and has about half the range.

Source: Honda, “Civic,” http://automobiles.honda.com/civic/.

Sizing the fuel tank is a compromise between driving range and cargo space. To provide the same range as the gasoline model, the natural gas model would have to have a gas tank four times the size of the gasoline tank, undoubtedly taking away useful cargo space. Honda has decided to provide about half the range of the gasoline model, a barely acceptable 225 miles, with a gas tank about twice the size of the gasoline tank. Range is lost and some cargo space is lost.

Fueling an NGV requires filling the high-pressure natural gas tank, and how this is done depends on what is available and the complexity of the fueling station. Home refueling is possible with a “Phill” system.¹¹ The Phill system allows one to refuel at home from the household gas line. One must have a natural gas line and a garage to house the system, so availability of home refueling is very limited. Most townhouse and apartment dwellers would be out of luck. Another problem is that residential gas lines provide gas at low pressure, and this means the system has to compress the gas. Compressing heats gas, so the gas must not only be compressed but cooled. This is not difficult, but it takes time. Anyone familiar with filling SCUBA tanks or barbecue grill gas tanks knows the drill. Specifications for the Phill system indicate a fueling rate of about half a gallon of gasoline equivalent per hour, which is one hour for 12 miles of driving and nineteen hours to fill the Honda GX tanks. The cost of purchase, installation, and electric power to run the system are prohibitive. Home fueling is available now for the few people who have natural gas service and a garage, do not mind paying dearly for fuel, and do not object to overnight fueling. Widespread home refueling of private NGVs is prohibitively impractical and expensive, although it does not require infrastructure expansion.

At fueling stations equipped with high-pressure storage tanks, filling the car’s tank is fast and simple. The gas is precompressed and stored at high pressure. Filling the tank is simply a matter of connecting the high-pressure storage tank to the vehicle tank and letting the gas flow. Unfortunately, there are very few such “fast-fill” fueling stations. This will change in the future if demand for NGVs grows. However, given the lower range of NGVs, the network of fast-fill stations will have to be more extensive than the current network of gasoline stations. Building the necessary infrastructure will be slow and expensive.

The need for heavy high-pressure fuel tanks on the vehicle is a limiting factor for small cars. On smaller vehicles, the percentage of weight and volume devoted to the fuel tank increases. Consequently, natural gas is most appropriate for large vehicles. Application to fleet vehicles like buses and delivery vans is ideal. Because they are large vehicles, the fuel tank weight and size penalty is proportionally less. Because they could operate out of a central depot, they would need only a single fueling station, minimizing the required infrastructure development.

If we are to introduce NGV nationally, we need a national infrastructure for fueling. Since the range of passenger NGVs is about half that of gasoline vehicles (based on the Honda GX NGV as a model for future NGV cars), the number of natural gas refueling stations probably needs to be at least twice the number of gasoline stations. Considering that there are few if any public natural gas refueling stations outside of California (as of 2010), this is a tall order.

Should NGVs become commonplace throughout the country, demand for natural gas will increase, and we will have to work toward doubling the supply of natural gas and distribution pipelines. The effect on natural gas imports will be significant. We consume about 23.2 Tcf of natural gas annually. Most but not all is from domestic sources. As we saw in chapter 1, we import 16% of the natural gas we consume in the United States. The natural gas equivalent to all the gasoline consumed in the United States is 15.8 Tcf. This means that demand for natural gas would increase almost 70% if NGVs became the standard mode of transportation. If domestic production remains constant, the additional 15.8 Tcf would have to be imported, raising the amount of natural gas we import to 50% of consumption. This is a long way from the goal of minimizing dependence on foreign sources of energy.

Refueling an NGV at home is difficult, time consuming, expensive, and available only to people who have parking immediately adjacent to their homes and a natural gas line. Home refueling of NGVs would be impractical for most people and too expensive for all. Widespread NGV deployment will have to wait for the infrastructure of fast fueling stations to be developed, and the cost will depend on the size and complexity of the new infrastructure. Since the range of current NGVs is significantly shorter than we are used to with gasoline cars, there would have to be significantly more fast fueling natural gas stations than the current number of gasoline stations if we convert passenger cars from gasoline to NGVs. Infrastructure development would be complex and costly. If, on the other hand, we use natural gas only for fleets of vehicles that return periodically to a central depot for fueling, far fewer natural gas fueling stations would be needed. The complexity and cost of infrastructure development would be less and the development time shorter.

A massive shift from gasoline to natural gas would reduce greenhouse gases and pollution and would decrease demand for gasoline and oil, but it would increase our dependence on foreign natural gas and hasten the time when we start experiencing shortages. Limiting natural gas to fleet vehicles would prolong our domestic reserves and minimize demand for imported natural gas and might achieve a better balance of gasoline and natural gas usage.

Finally, the size and weight penalty imposed by natural gas storage tanks limits our ability to improve fuel efficiency by making vehicles small and light. There is some vehicle size where the penalty associated with natural gas offsets the potential benefits of size and weight. The relative weight/size penalty is less prohibitive for fleet vehicles than for small cars.

For these reasons, the future of NGVs appears to lie in large vehicles, such as large taxis, buses, delivery vans, and other large fleet vehicles, fueled at central depots. The tank size/weight penalty is not severe for these larger vehicles, and refueling at a central fleet depot minimizes how much the natural gas infrastructure has to be expanded.

Hybrid Electric Vehicles

One has only to mention the Toyota Prius to get an earful about how good gasoline HEVs are in terms of fuel economy. Indeed, in every category of passenger car or light truck tabulated by the EPA in its Fuel Economy Guide 2009 that includes an HEV, the HEV is the fuel-economy leader. The 2009 Prius, which has a 48/45/46 mpg rating, has led the hybrids several years running.

Why is the HEV so successful, and what improvements may we expect from HEV technology in the future? What does HEV technology tell us about the goals of driving green in general?

“Hybrid” means that the vehicle uses more than one source of energy. The primary source for HEVs is gasoline, and the secondary source is electricity. That is, a standard gasoline engine is the primary source of propulsion, and an electric motor powered by a battery augments the gasoline engine in certain situations. Figure 3.1b shows the layout of an HEV, emphasizing the two parallel propulsion systems that make the vehicle a hybrid. The battery for the electric drive is charged either by regenerative braking or by the motor-generator set, or genset, which burns whatever fuel is used for the internal combustion engine propulsion system.

The HEV provides benefit in four ways. First, hybrid vehicles utilize varying numbers of elements of the improved standard technology already incorporated in conventional cars, such as a continuously variable transmission, cylinder control to improve efficiency of the engine and drivetrain, low-rolling-resistance tires, and streamlining to reduce losses. Though not strictly hybrid technologies, as they are common in standard gasoline engines as well, these elements improve fuel economy in both city and highway driving. Second, the electric motor comes on at high speed under certain circumstances to assist the gasoline engine when high power is required. This improves fuel economy slightly during highway driving because it allows the gasoline engine to be smaller and more efficient than otherwise would be possible (the 2010 Toyota Prius gasoline engine is a mere 98 horsepower). Third, the HEV almost eliminates idling loss by turning off the gasoline engine when the vehicle is not moving and turning it back on when the vehicle starts moving again. The integrated starter/generator technology makes this possible with no hesitation because the system uses the electric motor and battery for getaway while the gasoline engine starts and comes on line. Fourth, HEV technology reduces inertia losses by using regenerative braking to capture the inertia that otherwise would dissipate to the atmosphere as heat. The reclaimed inertia energy charges the battery and makes the whole electric motor scheme possible in the absence of external charging. Regenerative braking and the integrated starter/generator are the major unique contributors to HEV fuel economy, and both are possible only because of the electric propulsion system. Advanced internal combustion engine engineering is applicable to conventional automobiles and is not unique to HEV; streamlining and weight reduction are also applicable to conventional automobiles, and they benefit small cars operating at slower speeds more than they benefit large, boxy trucks and sport-utility vehicles (SUVs).

In 2007, I contemplated trading my Toyota Highlander in for a new model. Since an HEV model was available at the time, I seriously considered getting one. I didn’t. My experience provides a good example because the 2007 Toyota Highlander was available in otherwise similar HEV and conventional models. EPA fuel economy figures are 32/27/29 for the Highlander hybrid and 19/25/21 for the conventional version. Note that hybrid technology provides a lot of benefit in city driving (32 mpg versus 19 mpg) but not very much on the highway (27 mpg versus 25 mpg). Hybrid technology benefits highway driving mainly by using the electric motor to assist the gasoline engine at high speed and during acceleration. However, the atmospheric drag induced by the large, boxy SUV body overpowers the benefit of the hybrid assist in highway driving. The noticeable benefit in city driving is most certainly due to regenerative braking and eliminating idle losses. HEV technology would have saved me about $524 a year in fuel cost.¹² However, the hybrid version cost $10,000 more than the gasoline version. It would have taken twenty years to recover the purchase price penalty. I eventually decided to keep what I had. The improvement in fuel economy achieved by hybrid technology on a large, boxy vehicle was not worth the added cost.

Results from similar comparisons of smaller cars are different. The EPA Fuel Economy Guide 2009 shows, in particular, the top conventional and top hybrid cars in the compact category.¹³ Here the conventional Chevrolet Aveo gets 25/34/29 mpg, and the Honda Civic HEV gets 40/45/42 mpg. The benefit of the HEV over a conventional competitor is 45%, which is double the 21% improvement of the 2007 Highlander HEV over the conventional model. However, comparing the overall fuel economy of the conventional Chevrolet Aveo with the conventional Jeep Compass, one sees a 19% improvement, about the same as the improvement from the conventional 2007 Highlander to the hybrid 2007 Highlander. If I want to improve fuel economy, then, I can do as well or better by switching to a smaller conventional car than by switching to a hybrid in the same category, and it will cost me a lot less. Once again we see that the losses incurred by large, heavy, boxy vehicles are simply too severe to be overcome by technology.

Many of the engineering modifications incorporated in HEVs address engine and transmission inefficiency, air resistance, and tire rolling resistance common to all ICEVs. What distinguishes the HEV is a second, electric propulsion system in addition to the gasoline engine. The electric motor provides propulsion assistance at high speed, enables regenerative braking, and eliminates losses when idling. Most of the advanced engineering is just as beneficial on conventional vehicles, so it is not entirely clear how much the HEV-specific elements improve fuel economy relative to the added complexity and expense.

At the end of chapter 2, I estimated the likely maximum fuel economy for a standard gasoline engine automobile by looking at the energy flow (fig. 2.1) and estimating potential improvement in each area of loss. To do the same with an HEV, I assume that in addition to all of the improvements of the conventional vehicle, the integrated starter/generator saves 90% of the idling loss, regenerative braking recovers 85% of the inertial loss, and streamlining recovers 50% of the drag losses. The resulting estimate is 48 mpg in city driving and 43 mpg in highway driving. This is almost exactly the fuel economy for the 2009 Prius (48 mpg city and 45 mpg highway). My conclusion is that hybrid technology, like the latest standard engine technology, has already achieved about as much improvement as one can expect.

Comparing HEVs with standard cars shows the true cost. I compared vehicles in three categories. In the compact category, I compared the 2009 Honda Civic standard car with the Civic HEV. In the midsize category, I compared the latest Toyota Prius with the Nissan Versa. In the SUV category, I compared standard and HEV versions of the Ford Escape. In all categories, the HEV got better fuel economy and saved between $430 and $707 per year in fuel cost. Countering this saving was a price difference ranging from $8,000 to almost $16,000. It would take eighteen to twenty-two years before the increased purchase price is balanced by the fuel saving.

The 2010 Toyota Prius represents about as much as we can expect from engineering development of HEVs. Further improvement in fuel economy will consist of incremental improvements, such as using solar panels to help power accessories (as on the 2010 Prius) and making the vehicle lighter. However, it seems likely that the two-propulsion-system design will be superseded by mechanically simpler systems.

Electric Vehicles

The HEV is a very complex machine, having two parallel propulsion systems. Sometimes the gasoline engine propels the vehicle; sometimes the electric motor propels it; sometimes both propel it simultaneously. Most of the propulsion comes from the gasoline engine, with assistance from the electric motor. The HEV has allowed us to develop electric drive technology and demonstrate its advantages. The next conceptual development stage is the EV. These have been around for years for low-speed, short-distance applications like golf carts and factory runabouts. The design of current EVs suitable for highway use draw from experience with golf carts (and similar vehicles) and the HEV. The advantages of EVs are numerous. Electric motors are more efficient than internal combustion engines, they emit no greenhouse gases or pollution (though generating the electricity that drives them is another matter), they eliminate losses at idle, they minimize braking loss with regenerative braking, and they generally do not burn fossil fuel. Though some EVs do burn fossil fuel, P-PEVs do not.

Compare the diagram of the P-PEV in figure 3.3a with the HEV in figure 3.1b. Electric propulsion, battery charging from the power grid, and extreme simplicity are the key features. A P-PEV is plugged into an electrical outlet to charge the battery and runs on battery power until the battery is completely discharged. Table 3.3 summarizes the battery issues with two leading plug-in electric vehicles (PEVs), the Tesla Roadster and the Chevrolet Volt.¹⁴ (The Chevrolet Volt is not a P-PEV, as I will discuss later.) The battery on the Chevrolet Volt weighs 375 pounds and has a 16 kWh storage capacity. It is capable of storing enough energy for 40 miles of driving before it has to be recharged. The Tesla Roadster gets greater range, 240 miles versus 40, with a larger battery. The larger battery on the Tesla Roadster does not completely account for the much greater range. To prolong the life of the battery, the Chevrolet Volt does not allow its battery to discharge completely and does not charge it fully. Only 8.8 kWh, roughly half of the total capacity, is used. The Tesla Roadster, on the other hand, uses the full capacity. The Tesla Roadster gets much greater range than the Chevrolet Volt but uses a much larger and heavier battery and costs much more.

There are a few engineering hurdles to overcome. The first is range, which is a critical issue. Chevrolet has decided that 40 miles per charge will satisfy most people. The argument seems to be that the average person drives 15,000 miles per year, which is just slightly greater than 40 miles per day. However, studies have shown that most Americans want 300 to 400 miles between fueling stops. The Tesla is much closer to this goal than the Volt. Greater range means more battery capacity, which implies more batteries with greater volume and greater weight. There is a trade-off. Greater weight in batteries means less efficient driving and greater range. There is a break-even point somewhere. Research and development (R&D) may improve matters, but batteries are not new technology, and achieving the eightfold improvement required to raise range to 320 miles with little or no price increase is unlikely.

FIGURE 3.3 All-Electric Vehicle Layouts: a, pure plug-in electric vehicle (P-PEV); b, hydrogen fuel-cell vehicle (HFCV). The P-PEV gets all of its propulsion from an electric motor and all of its electricity from an externally charged battery. The HFCV also gets all of its propulsion from an electric motor but almost all of its electricity from the fuel cell, supplemented by regenerative braking. Consequently, the battery can be smaller than on a P-PEV. HFCVs would be fueled most practically at fast fueling stations that would generate hydrogen on site by electrolysis, drawing on power from the electric power grid and storing high-pressure hydrogen to facilitate fast fueling.

Source: Ehsani, Yimin, and Emadi, Modern Electric, Hybrid Electric, and Fuel Cell Vehicles.

TABLE 3.3 Electric Vehicle Range Factors

Charging the battery is the second engineering issue. Charging time is limited by the current capacity of the charging circuit and by the maximum charging current the battery can accept without damage. Much has been said about the simplicity of charging the battery by plugging it into a household electric circuit, but the physics of electricity dictate that a common household circuit can only provide the Volt 40 miles of driving from seven hours of charging; a kitchen small-appliance circuit improves this to five hours (table 3.4). A high-power electric stove or central air-conditioner circuit improves charging time to slightly more than one hour, but very few people have a spare high-power circuit, and installing one is expensive. Home recharging will be limited to drivers with off-street parking who can live with recharging times measured in hours. Most apartment dwellers and townhouse residents will be out of luck. Recharging P-PEVs will become practical only when stations capable of high-power fast charging are at least as widely available as the 159,006 gasoline filling stations in the United States.¹⁵ The ideal fast charging station would provide electrical energy at the same rate that filling stations pump gasoline so that the time spent refueling an EV is the same as people are accustomed to. However, to charge the battery in the same amount of time that it takes to pump gasoline, one needs a 3,000 kW (3 MW) supply. Charging five EVs at the same time (most gasoline stations can fuel several cars at once) requires around 15 MW. This is a lot of power. To put it in perspective, the average capacity of US power plants is 62 MW;¹⁶ the entire output from an average power plant would be needed to power four fast-charge EV charging stations. While the foregoing analysis is indicative of the huge power levels needed for fast charging, such extremely fast charging is impractical because the battery cannot accept such huge currents. Optimum battery charging applies the maximum charging current to the battery for about an hour and then a decreasing current for roughly two additional hours.¹⁷ The limitation on charging current means that charging a fully discharged battery in less than one hour is not practical.

TABLE 3.4 Electric Vehicle Battery Charging Time

The third engineering hurdle is the effect on the national power grid. If we assume that fast charging stations are to be as widely available as gas stations, meeting the demand for electricity would be a big strain. First, consider the effect on the local distribution system. If I were to drive a Volt, I would consume 19 kWh of electricity per day. Reviewing my household bills, I currently consume 30 kWh per day on average. The electric car would increase my household consumption of electricity by 63%. A large number of people in my neighborhood suddenly doubling their consumption would severely strain the local distribution system. At the very least, the local distribution system would have to be upgraded as electric cars are deployed. Then consider the nationwide demand for electricity. Annual consumption of gasoline in the United States is equivalent to approximately 4.3 trillion kWh. Because of the efficiency of P-PEVs, they would require about half this, 2.42 trillion kWh of electricity. Now the annual US consumption of electricity is 3.5 trillion kWh. In order to replace all gasoline usage with electricity, we would have to almost double generating and distribution capacity. Doubling electricity consumption would also require a massive increase in the national power grid, especially transmission lines. There is already strong public resistance to new transmission lines; doubling the capacity would face strong opposition. More important is the need to double generating capacity. This would require massive construction of power-generating stations. Most important, most power plants now run on coal. Doubling the generating capacity with coal plants to support EVs might reduce dependence on oil, but it would also lead to unacceptable increases in pollution and damage to the environment. The P-PEV itself might be efficient and pollution-free, but generating the electricity to charge PEV batteries is less efficient and more polluting than gasoline.

The limited range of a P-PEV raises the question, what do you do if you run the battery dry? There is now no way to recharge the battery away from home and no means of getting road assistance. Chevrolet’s solution to this problem is to provide a small internal combustion engine motor and generator and a small fuel tank, variously referred to as a genset (motor-generator set)¹⁸ or auxiliary power unit. Figure 3.1d shows the layout of the H-PEV, which is powered by the power grid or an onboard generator. The sizing is such that the generator keeps the battery charged after the initial 40-mile charge has been depleted, resulting in an overall 300-mile range. The internal combustion engine is small and can run at constant optimal speed, so the overall efficiency is quite good. Chevrolet advertises 50 mpg for the Volt in gasoline-generator mode.

There is a lot of talk about how much battery R&D will improve the practicality of electric cars. EVs probably will become more practical, but we need to go slow here. R&D may improve cost, weight, size, and longevity, but charging time depends on the power available at the plug. Available charging power is the limitation, not the battery.

Fuel Economy

Comparing fuel economies of EVs and gasoline vehicles is complicated because one uses electricity and the other burns gasoline. The common method for comparing fuel efficiencies using the gallon-of-gasoline-equivalent concept works well for fossil fuels but gets a bit more complicated when we compare fossil fuels and electricity. Energy content values are easily compared. A gallon of gasoline contains 36.0 kWh of energy, so we can treat every 36.0 kWh of electric energy consumed as the equivalent of 1 gallon of gasoline. That is, 36.0 kWh is one “gallon of gasoline equivalent,” or 1 gge. An EV that drives 1 mile consuming 36.0 kWh of electric energy is getting 1 mpg of gasoline equivalent, or 1 mpgge. This works quite well if we think of electricity as a primary energy resource like wind or solar power. It gets complicated when we consider that electricity is actually a secondary energy resource generated from a fossil fuel in a power plant. One then has to take into consideration the conversion efficiency of the generator, be it the power plants feeding the national power grid or onboard gensets.

As I write this, I am watching a GM executive on television announce that the Chevrolet Volt will get 230 mpg fuel economy.¹⁹ This is an incredible claim, and it makes the Volt the first car to claim triple-digit fuel economy. The claim is correct but misleading. Here is a good example of the need to understand the facts in order to understand claims like this about fuel efficiency.

To get this fuel economy rating, technicians subjected the car to the standard EPA test schedule of 11 miles and found that it consumed 0.22 gallons of gasoline. They then calculated fuel economy by adding the 11 miles of the test to the 40 miles the Volt would get from a fully charged battery, for a total of 51 miles, and dividing that by the 0.22 gallons of gasoline consumed by the genset. This is 230 mpg.

What is the real fuel economy of the Volt? It depends on how you determine fuel economy. One way is to consider only operation powered by the genset. Here, the small gasoline tank and genset power the car for about 260 miles. Chevrolet claims fuel economy of 50 mpg when operating on the genset. Another way is to consider pure battery operation. The fully charged battery carries the car 40 miles. The battery capacity is 16 kWh, but to extend the life of the battery, control circuitry never allows the battery to discharge or charge fully. The useful energy in a single charge is 8.8 kWh. Since 36.0 kWh is the equivalent of 1 gallon of gasoline, this is 0.24 gge, so fuel economy is 163 mpgge. If we want to account for the fuel required to generate the electricity needed to charge the battery, the PEV does not fare so well. Because of losses in the battery charger, it must draw 11 kWh from the grid to get 8.8 kWh into the battery. Since an average power plant is one-third efficient, we need to put 33 kWh of energy into the power plant to get 11 kWh into the grid. Overall, this is 0.92 gge, so battery operation gives 44 mpgge. This tells me that the PEV gets 44 to 50 mpgge regardless of fuel.

Additional improvements are possible with EVs. Figure 3.4 shows the energy flow in an EV and is similar to figure 2.1, showing energy flow in a gasoline vehicle. Losses are allocated to drag, rolling resistance, and inertia/braking according to the percentages from figure 2.1.

Compare the energy flow and losses in cars with electric drive shown here with that of conventional internal combustion engine cars shown in chapter 2 (fig. 2.1). Electric motors are more efficient than internal combustion engines, and the electric car does not require a complicated transmission. This results in drag and weight being more important relative to the other losses. What stands out is that much more of the energy in the battery gets to road contact (46% city, 67% highway) with EVs than with internal combustion engines. This means that reducing drag, tire rolling resistance, and inertia have much more effect on fuel economy in electric cars than in gasoline cars. Indeed, if one assumes that drag and inertia are reduced 75% by extreme streamlining and weight reduction and tire rolling resistance is reduced 50%, one would eliminate 30% of the losses in city driving and 45% of the losses in highway driving. Fuel efficiency would almost double on the highway and improve by 50% in city driving. Such measures will have a major effect on extending the range of PEVs.

Percentage losses are quite different once the internal combustion engine is replaced by the electric motor. Indeed, the losses attributable to drag, tires, and inertia/braking are much larger factors. This means that making vehicles smaller, lighter, and more streamlined should have proportionally greater effect on fuel efficiency in EVs than in gasoline vehicles.

Batteries

A key component of EVs (especially P-PEVs and H-PEVs) is the battery. This raises several concerns. The first is size and weight. Batteries are large and heavy relative to the energy they can store. This is not a major problem for HEVs, since large storage capacity is not required. It is a major problem for PEVs, since the range of the car depends on the size of the battery. Lithium-ion batteries are the most common EV battery technology today. The Chevrolet Volt battery pack weighs 375 pounds and takes up 100 liters (L), or 22 gallons, volume. The battery has a 16 kWh capacity, so the effective energy density numbers are 94 watt-hours (Wh) per kilogram and 160 Wh/L. Both are less than nominal values for the lithium-ion battery alone,²⁰ indicating that the battery system involves a lot of machinery in addition to the battery itself. The Volt has a range of 40 miles from a power source weighing 375 pounds and taking up a volume of 22 gallons. A standard gasoline car getting 30 mpg would need 1.3 gallons of fuel weighing 8 pounds to cover the same distance. The Volt suffers a 50:1 weight penalty and a 20:1 volume penalty compared to gasoline. Herein is one of the drawbacks of PEVs. One can only hope that continued R&D will get the size and weight down in order to provide adequate driving range in an acceptable package.

FIGURE 3.4 Energy Flow in an Electric Vehicle: a, city driving; b, highway driving. These models, illustrate energy flow in P-PEVs (i.e., vehicles with all-electric drive powered by a large battery charged from the power grid). When we compare these diagrams with similar diagrams for gasoline cars (fig. 2.1), we see that the electric vehicle has zero idle loss, extremely low engine losses, and no thermodynamic losses. Electric motors are more efficient than internal combustion engines, and the electric car does not require a complicated transmission. As a result, drag and weight are more important relative to other losses for electric vehicles. Much more of the energy in the battery gets to road contact with electric vehicles (46% city, 67% highway) than is the case with internal combustion engines. This means that reducing drag, tire rolling resistance, and inertia have much more effect on fuel economy in electric cars than in gasoline cars.

Source: Based on data from Kurt M. Johnson, “A Plug-In Hybrid Electric Vehicle Loss Model to Compare Well-to-Wheel Energy Use from Multiple Sources” (master’s thesis, Virginia Polytechnic Institute and State University, 2008), 43, table 16.

The second concern is air temperature. Prius owners in the Washington, DC, area have told me that there is a noticeable decrease in fuel economy in the winter. This is understandable because battery capacity depends on temperature and decreases if the temperature is lower than 75°F. Capacity decreases to 80% of maximum at 32°F and to 50% of maximum at 5°F. That simply means that the range of the Volt drops from 40 miles to 20 miles at temperatures approaching zero. To protect the battery, the battery pack in the Chevrolet Volt disconnects if the temperature gets down to around freezing. This could be a serious issue for people living in cold climates. They would need to keep the car plugged into the electric grid to keep battery temperature ready for instant departure. If a connection to the grid is not available, one would have to consume battery power continuously to keep the battery warm and ready for instant departure or wait while the battery warms up. In any case, maintaining battery temperature consumes energy, thereby reducing overall fuel economy. Operation in areas of the country subject to near-freezing temperatures reduces performance and subjects the driver to inconvenience.

The third concern is the supply of lithium, the fundamental raw material for EV batteries. There are other battery technologies, but the lithium-ion battery is the current leading contender. Unfortunately, the United States does not have a good supply of lithium ²¹ (table 3.5). Most of the lithium in the world is in South America and China. A report by William Tahil of Meridian International Research estimates each kilowatt-hour of battery storage capacity requires 0.3 kg of lithium.²² The Chevrolet Volt’s 16 kWh battery requires 4.8 kg of lithium. Replacing 5% of the 251 million vehicles on the road each year with PEVs would require 60,000 metric tons of lithium. This is about 3 times current world production of lithium and 1.5 times total US reserves. Replacing all of our cars with PEVs would require 1.2 MMt of lithium, roughly 10% of world reserves. That is just for electric cars in the United States; supplying lithium for electric cars worldwide would be difficult.

TABLE 3.5 Sources of Lithium by Country (metric tons)

Comparison

When operating on the battery, the PEV achieves the goal of not burning fossil fuel and reaps the benefits of electric propulsion, no losses at idle, regenerative braking, and efficient electric drive. However, the battery imposes a significant penalty in weight and space. Comparing the Chevrolet Volt and the otherwise similar internal combustion engine Chevrolet Cobalt, figure 3.5 illustrates the extent of the penalty.

Note that the Volt, with its electric motor and battery weighs 654 pounds more than the Cobalt; the lithium-ion battery itself adds 375 pounds. The Volt has seating for one less passenger than the Cobalt and 3.3 cubic feet less cargo space. The high additional weight of the Volt, almost twice the weight of the battery, indicates that the Volt requires a lot of peripheral equipment in addition to the large battery. Overall, the penalty is 650 pounds and a loss of a quarter of the payload volume. That is a high price to pay for an all-electric range of 40 miles.

FIGURE 3.5 Chevrolet Gasoline Cobalt versus Chevrolet Volt Electric Vehicle. Though the gasoline-powered Cobalt and the electric-powered Volt when operating solely on its battery are otherwise similar, the Volt costs much more, weighs more, has less passenger capacity, less cargo space, and one-tenth the range.

Sources: Cobalt: Chevrolet, “Cobalt,” www.chevrolet.com/cobalt/. Volt: Chevrolet, “2011 Volt,” www.chevrolet.com/volt/.

Bottom Line

On the bright side, electric cars promise pollution-free operation and a path to weaning the automobile industry away from gasoline and oil. Unless one is satisfied with about 40 miles of driving per day and lives in a home with access to a charging circuit, the PEV will not be practical until the fast charging station infrastructure becomes as widely available as gasoline filling stations are today. This will take time. It will also be limited to availability of high-power charging sources. This will limit practicality of PEVs in remote locations.

Currently, if you want to drive your vehicle away from developed roads and filling stations, you simply take along extra cans of gasoline or have a drum of fuel delivered periodically. There is no practical way to carry extra electricity or deliver it in batch quantities.

Range on battery power is limited. Battery operation will be limited until batteries become much smaller and lighter. Meanwhile, some form of auxiliary power unit or genset will be required to provide electrical power, quite possibly for decades.

Limited availability of home charging outlets and long charging time for the outlets that are available will limit widespread deployment of PEVs until the power grid infrastructure expands. Extensive deployment of PEVs will require doubled generator capacity, an extensively expanded transmission line network, and widespread fast charging stations. Charging stations will have to be more widespread than existing gasoline stations to compensate for the shorter range and hence more frequent refueling of PEVs.

Doubling electricity demand will produce a huge increase in coal usage and corresponding pollution, and the public will not accept PEVs on a wide scale until a clean source of electricity is developed.

Low temperature will markedly reduce PEV performance in cold climates because keeping the battery warm will consume energy from the battery and adversely affect driving range.

Finally, there is a risk that the United States will become dependent on foreign sources of lithium for PEV batteries.

Series Electric Vehicles

H-PEVs, such as the Chevrolet Volt, are neither fish nor fowl. Similar to the HEV, which has two distinct propulsion systems, the H-PEV has two distinct power sources, an externally charged battery and a fossil-fuel-burning genset, although it has only one propulsion motor. Indeed, the Volt is more of a fossil-fuel-burning vehicle than an externally charged EV. When battery and fuel tank are both full, the Volt has a range of 40 miles on battery power and 260 miles on fuel. Anyone commuting more than 20 miles one way to work—as do 23% of commuters in the United States—would be driving on fuel at least part of the time.²³ Roughly 8% of commuters would be driving on fuel half of the time. And when driving to visit my in-laws, 450 miles away, I would be driving on fuel for over 90% of the trip. Why not simplify the vehicle by running it on fuel all the time? That is, consider the SEV, depicted in figure 3.1c. The SEV is similar to the H-PEV, but it requires no external charging. All electricity is supplied by an onboard fuel-burning genset. The battery can be smaller because the vehicle seldom operates without the genset producing electricity.

The SEV would provide most of the benefits of electric propulsion (i.e., efficiency, regenerative braking, no wasted power while idling) and does not have the range limitations of P-PEVs. Since the Chevrolet Volt is advertised as getting 50 mpg on fuel, an SEV should do at least as well, probably better because the battery would be smaller and lighter. Moreover, range would not be an issue for SEVs. On the downside, the vehicle would still be burning fuel. However, different gensets could be available for different fuels. For example, if algal biodiesel becomes practical, it could be used in a diesel genset on an SEV. Perhaps most important, the SEV could be a 50 mpg or better vehicle that does not require a power grid.

A functional SEV could be deployed today and would provide greatly improved fuel economy without needing battery development or increased charging infrastructure. The SEV could then be smoothly phased out in favor of PEVs as batteries and charging infrastructure are developed.

Hydrogen Fuel-Cell Vehicles

The SEV generates electric power using an onboard fuel-burning genset. An alternative source of electricity would be a hydrogen fuel cell. An HFCV is an SEV that generates its electricity using an onboard hydrogen-consuming fuel cell (fig. 3.3b). The fuel cell generates electricity onboard simply by passing hydrogen through the fuel cell; hydrogen gas goes in, and electricity and water come out. An onboard storage tank supplies the hydrogen. One advantage of the HFCV over the P-PEV is that the battery can be much smaller, lighter, and less expensive because it is not the primary supplier of electricity for the motor, but this advantage is offset by the weight of the fuel cell and hydrogen storage tanks. Like the P-PEV, the HFCV produces no pollution or greenhouse gases. Except for the fact that the HFCV is fueled with hydrogen and not charged with electricity from the power grid, it can be thought of as a PEV with a hydrogen tank, fuel cell, and small battery replacing the large storage battery. Three advanced prototype passenger cars powered by hydrogen fuel cells were operating in 2010 (the Honda FCX Clarity, Chevrolet Equinox, and Toyota FCHV). Commercial availability of fuel cells is a decade away, but the technology has great promise.

Hydrogen Gas Supply

Onboard hydrogen storage is the first engineering issue with the HFCV. Hydrogen, like natural gas, has very little energy per cubic foot at normal temperature and pressure. The energy content of hydrogen gas at atmospheric pressure is only a quarter that of natural gas by volume (table 3.2). Onboard storage of hydrogen is more of a problem than with natural gas, though the sources of the problem are the same. As with natural gas, one could store hydrogen as a liquid, but storing it as a compressed gas would be preferred. At 5,000 psi, the pressurization of some of HFCVs, 1 gge of hydrogen takes up 9 gallons of volume. At 10,000 psi, 1 gge takes up 5 gallons. That is, even at 10,000 psi, hydrogen takes up five times the volume that gasoline would require for an equivalent amount of energy, and that is just for the gas itself. The storage tank adds weight and volume. The space penalty from hydrogen is more severe than with natural gas. Hydrogen is less practical for small passenger cars than natural gas simply because of the onboard storage constraint. One prototype, the Honda FCX Clarity, gets a range of 280 miles from a fuel tank almost the size of a 42-gallon oil barrel. By comparison, a gasoline car getting 30 mpg would have a range of over 1,100 miles with a gasoline tank this size.

The second issue is filling the hydrogen tank. More precisely, the issue is getting the hydrogen gas to the fueling station. Curiously, how hydrogen would be produced for the automotive market would depend on how the hydrogen is transported. As I said earlier, the energy content of hydrogen gas at atmospheric pressure is only a quarter that of natural gas by volume. Trucking hydrogen as compressed gas is feasible but prohibitively expensive for distances greater than 200 miles. Long-distance transport of liquid hydrogen via cryogenic pipeline would be preferable but also expensive. Moreover, the current infrastructure consists of only 700 miles of hydrogen pipeline, compared with 1 million miles of existing gasoline pipelines. The complexity of hydrogen transportation compared with natural gas and the need for massive hydrogen pipeline development probably mean that generating hydrogen at centralized production plants and transporting it to refueling stations is impractical. A better approach would be to produce hydrogen at each refueling station.

The next issue is how to generate the hydrogen. There are essentially two methods for manufacturing hydrogen: natural gas reforming and electrolysis.²⁴ Natural gas reforming currently produces almost all of the hydrogen used in the United States, but it seems unlikely that this method would be used for vehicle fuel. An argument in favor of reforming natural gas is that, if natural gas infrastructure were expanded to support NGVs, additional pipelines to support hydrogen production at fueling stations would not be needed. The increased natural gas infrastructure could serve both NGVs and HFCVs. While this sounds good at first, my concern is the efficient use of resources. Because of losses in the conversion sequence—natural gas to hydrogen to electricity—much more natural gas would be consumed per mile of driving than if the natural gas were used directly in NGVs. Using natural gas in NGVs would be a more efficient usage of natural resources and would generate less pollution and greenhouse gases than reforming it into hydrogen.

The most practical procedure for manufacturing hydrogen for HFCV would be to make hydrogen at the fueling station by electrolysis, passing electricity through water to make hydrogen and oxygen. If we expand the national electric infrastructure to support increasing demand and PEVs, no additional infrastructure development would be needed to support HFCV fueled by local electrolysis. Still, the scope of electric power grid expansion would increase, and the need for clean sources of electricity would increase significantly.

Comparison

While it is true that hydrogen weighs less than gasoline for equivalent energy content, the situation changes dramatically when the storage tank is considered. A gallon of gasoline weighs 2.8 kg; 1 gge of hydrogen weighs 1 kg. Compressed to 10,000 psi, 1 gge of hydrogen takes up 5 gallons. Current storage tank technology provides 6% of total weight as hydrogen. That is, 2.8 kg of compressed hydrogen in a tank weighs 17 kg. Hydrogen at 10,000 psi and the tank together weigh about six times the equivalent amount of gasoline and take up about six times the volume. The six-to-one weight and volume penalties are obvious when we compare similar vehicles, such as the Chevrolet Equinox gasoline and HFCV models (fig. 3.6).

The fuel system in the HFCV stores 4.2 kg of hydrogen at 10,000 psi weighs 300 pounds and takes up 42 gallons of space. The HFCV weighs quite a bit more than the conventional-engine car, even though the weight is minimized by aluminum doors and a carbon-fiber hood.

Hydrogen fuel-cell technology exacts a significant penalty in weight and payload space, both cargo space and passenger capacity. Indeed, the weight penalty is more severe than one might expect just from the weight and volume of the fuel-cell technology. The penalty is more than 800 pounds of added weight, loss of space for one passenger and almost half the cargo volume, and half the driving range. While the penalty is severe and driving range is less than desired, the driving range of the HFCV is greater than that of P-PEVs. Moreover, the temperature operating range of current hydrogen fuel cells is –13°F to 113°F, which makes the use of HFCVs in extremely hot or cold climates impractical.

Figure 3.7 compares energy efficiencies of P-PEVs and HFCVs by illustrating how effectively electric power from the power grid is used. Since consuming natural resources ultimately generates the power in the power grid, this figure compares how efficiently the two technologies use natural resources. The P-PEV battery is charged directly from the power grid. The battery charger converts alternating current (AC) from the power grid to direct current (DC) and stores the energy in the battery. Battery chargers are typically 85% efficient. Fueling the HFCV is more complicated. To generate hydrogen, the power supply converts AC from the power grid to DC, which powers the electrolyzer that manufactures hydrogen. The hydrogen is compressed and fed into the storage tank. Hydrogen flows from the storage tank into the fuel cell to generate electricity to power the car. In some cases, the power from the fuel cell goes directly to the propulsion motor; in other cases it goes to charge the battery. The corresponding end-to-end efficiencies differ, but both are nominally 30%. The HFCV’s efficiency is about one-third that of the battery-powered P-PEV, a comparatively inefficient use of natural resources. The benefit of the HFCV relative to P-PEV is unclear. On the one hand, the HFCV offers greater range and the possibility of roadside refueling from trucks carrying high-pressure hydrogen tanks. On the other hand, it uses natural resources less efficiently.

FIGURE 3.6 Chevrolet Equinox Gasoline Vehicle versus Equinox Hydrogen Fuel-Cell Vehicle. Although the Chevrolet Equinox gasoline and hydrogen fuel-cell models are otherwise similar, the fuel-cell car weighs more and has less passenger capacity, half the cargo space, and less than half the range. Cost is not compared, as the fuel-cell Equinox is not available for sale.

Sources: Gasoline model: Chevrolet, “2011 Equinox,” www.chevrolet.com/equinox/. Fuel-cell model: Consumer Guide Automotive, “2009 Chevrolet Equinox Fuel Cell: Overview,” http://consumerguideauto.howstuffworks.com/2009-chevrolet-equinox-fuel-cell.htm.

Bottom Line

Fuel cells are too expensive now to be practical, although prices are declining steadily.²⁵ The easiest and most economical method for providing hydrogen at fueling stations is to generate hydrogen by electrolysis at the fueling station. New hydrogen transportation infrastructure would not be needed because hydrogen fueling would take advantage of the improved electricity infrastructure that would be required by PEVs, and natural gas would not be diverted from direct use as a fuel. However, deployment of HFCVs would increase the need for a clean and abundant source of electricity.

FIGURE 3.7 Efficiency of Plug-In Vehicles versus Fuel-Cell Vehicles. This chart compares energy efficiencies of P-PEVs and HFCVs by examining how effectively electric power from the power grid is used. The P-PEV battery charger stores the energy in the battery, typically at 85% efficiency. The HFCV power supply converts AC from the power grid to DC to power the electrolyzer, which produces hydrogen, which in turn is compressed and fed into the storage tank and then flows into the fuel cell to generate electricity. In some cases, the power from the fuel cell may go directly to the propulsion motor (for 27% overall efficiency) or may be stored in a battery (32% overall efficiency).

Source: Data from Ulf Bossel, “Does a Hydrogen Economy Make Sense?” Proceedings of the IEEE 94, no. 10 (October 2006).

The fuel cell and 10,000 psi hydrogen tanks and peripheral equipment impose a significant weight and payload volume penalty on the HFCV. The range of HFCVs currently falls midway between the 40-mile range of the H-PEV Chevrolet Volt when operating on battery alone and the 244-mile range of the Tesla Roadster.

The fundamental trade-off is the more efficient use of resources by the P-PEV for the greater range of the HFCV. Moreover, even though transporting large quantities of hydrogen by truck is impractical, roadside refueling of HFCV is possible, whereas such roadside assistance to P-PEVs is not.

Summary

The goals of a new energy strategy are to reduce gasoline consumption, reduce pollution caused by burning fossil fuels, and reduce dependence on foreign oil. How well do the alternative vehicles stack up against these goals? What should the strategy be for transitioning to alternative vehicles? And what are the implications for the national energy strategy? I believe that the data speak loudly and that the conclusions I present here are justified. However, unexpected developments are possible; some promising projects will fail, while some seemingly dubious projects will succeed. In the end, I am presenting my personal opinion of what the data mean.

The first thing we should do is stretch our gasoline supplies as much as possible. Improving the standard internal combustion engine is the simplest step. The newer gasoline cars are getting better and better fuel economy, and meeting the new CAFE standard will be a good first step. However, there is not much room for additional improvements in fuel economy through engineering. We have gone about as far as possible. The same is true of diesel cars. Moreover, while diesel fuel economy, in miles per gallon of diesel fuel, is superior to fuel economy in gasoline cars, measured in barrels of oil per mile, diesel cars are less efficient than gasoline cars. Allowing the percentage of cars using diesel engines to grow too much could increase oil consumption. The strategy should be to aim for the optimal balance of the two engines.

NGVs do not pose any technological uncertainty, and they would simultaneously reduce pollution and greenhouse gas emissions and reduce consumption of gasoline and diesel. However, a natural gas pipeline network is needed to support fueling stations. In addition, the need for compressed gas storage cylinders adds a weight and cargo space penalty, which becomes more and more severe as cars become smaller and lighter (and more fuel-efficient). A corollary of the cargo volume penalty is that the fuel tank capacity is reduced so that the driving range of an NGV is less than the range of a comparable gasoline car. The final consideration is that domestic natural gas supplies are limited, and we are already importing a significant amount of natural gas. Though extensive deployment of NGVs would reduce consumption of gasoline, it would simply exchange dependence on foreign oil for dependence on foreign natural gas. While NGVs can play an important role in reducing noxious emissions and reducing demand for oil, the optimum strategy would be to use natural gas on depot-fueled fleets of larger vehicles like delivery vans and buses. Refueling at depots would minimize the number of fueling stations and hence minimize the need for infrastructure development, and placing natural gas engines on larger vehicles would minimize the size and cargo volume penalty.

Flex-fuel vehicles, those that burn E85, ethanol mixed with gasoline, definitely extend our supply of gasoline. E85 is much more expensive per mile than gasoline, making it unattractive now. This could change in the future. Even so, driving range is much shorter with E85 than with gasoline. As long as gasoline is available, many drivers would simply continue to use gasoline. We will need to keep gasoline on the market or convert all of the small planes, boats, lawn tools, all-terrain vehicles, home generators, and other small engines to E85 as well. Moreover, growing corn for ethanol takes cropland out of food cultivation. With limited cropland, we would soon become dependent on foreign sources of ethanol, and the effect on world starvation would be catastrophic. Ethanol supplies will probably be quite limited. Moreover, the negative aspects of ethanol’s corrosiveness will limit widespread use of E85. Nonetheless, ethanol has some benefits in limited situations. Flex-fuel vehicles could extend our gasoline supply but only to a relatively small extent.

Biodiesel does not require engine modification, so one of my concerns with flex-fuel vehicles is not an issue for biodiesel and diesel engines. However, yields of biodiesel from crops or cooking oil are so low that they can never provide a significant percentage of US fuel demand, and crop-based biodiesel competes with food crops for arable land. We might as well take advantage of biodiesel fuel, but we must recognize that it will have only a minor effect on national demand for oil.

Biofuel, either ethanol or biodiesel, from algae might be a game changer. Demonstration projects have produced both ethanol and biodiesel from algae. Yields per acre are much higher than yields from crops, and land does not have to be taken out of food production. Successful development of algae as a source of energy would resolve many of the issues associated with biofuel.

Whatever fossil fuel is used—gasoline, diesel, or natural gas—HEV technology, as exemplified by the Toyota Prius, definitely improves fuel efficiency. Even so, while it reduces demand for gasoline, it does not eliminate it. Moreover, it involves complex and expensive machinery, having two propulsion systems, one gasoline and one electric. The HEV is probably the best thing we have today, but in the long run it has to be replaced by something simpler and less expensive.

The SEV is such an alternative. The SEV has a single, electric propulsion system powered by an onboard generator. It provides all the advantages of electric propulsion, it is simpler than the HEV, it does not require any infrastructure development, and it should provide at least the 50 mpg advertised for the Chevrolet Volt. The battery is smaller and lighter than required by a P-PEV, since the SEV almost never operates without the generator, and the motor driving the generator can be small and optimized for constant-speed operation.

There is another benefit from electric-propulsion vehicles. The distribution of energy losses to the various drivetrain components is quite different from that of an ICEV. This means that there is added opportunity for improving the fuel efficiency of conventional cars by addressing size, weight, drag, and tires. Fuel economy better than the Volt’s 50 mpg should be possible for conventional automobiles, but they will have to become smaller and lighter, and the safety of mixing small vehicles with behemoths on the highway will be a major issue.

The next step would be deploying PEVs. P-PEVs, like the Tesla Roadster, are powered solely by an externally charged battery. H-PEVs, like the Chevrolet Volt, are powered by an externally charged battery but also have a fuel-burning genset to provide electric power when the battery runs low. The pure electric car offers several advantages, not the least of which is complete elimination of demand for gasoline. Problems include the cost and size of the battery, what to do when the battery runs dry, and battery charging. Battery research should bring the size, weight, and price of batteries down. A network of fast charging stations would offset the concerns about running the battery dry, but charging the battery would remain problematic. It takes a long time to charge the battery unless a fast charging station with very large power capacity is available. Home charging is severely limited by the need for off-street parking and the hours required to charge the battery. Widespread deployment of P-PEVs will have to wait for a national network of fast charging stations.

The H-PEV, exemplified by the Chevrolet Volt, solves the problem of limited charging capability by putting a genset on the car. The Volt is similar to an SEV, but the Volt relies primarily on battery operation, and its genset is primarily for backup. Making the genset the sole source of electricity, with no provision for external charging capability, as in the SEV, changes the emphasis and would allow EV technology to be optimized. In particular, the battery could be much smaller and lighter.

Range and charging time will continue to be problems. It is doubtful that batteries will improve to the point that range comparable to what we now expect from our cars will be possible with practical batteries. We will have to get used to more frequent fueling stops, and more frequent fueling will require more fast charging stations than the current number of gasoline stations, which in turn would entail a massive infrastructure expansion. Moreover, the demand for electricity will increase significantly; increased demand from EVs, population growth, and the growth of electronic gadgets will double or triple demand in a few decades. Meeting that demand will require a massive increase in electricity-generating capacity. Even with fast charging stations, the charging time would be constrained by the maximum current the battery can accept without damage. It is doubtful that PEVs will ever be charged as fast as gasoline cars are fueled.

The HFCV is an SEV, but instead of a fossil-fuel-burning genset, it uses a hydrogen-gas-consuming fuel cell to generate electricity. Although they are too expensive now to be practical, fuel-cell prices are on a track that should make HFCVs economically feasible in the future. The problem is that the large, heavy storage tanks limit range and cargo space. It is doubtful that HFCVs will be able to provide the range we are accustomed to. The main issue is supplying hydrogen at fueling stations. Manufacturing hydrogen at large plants and distributing it throughout the country is one option. However, this is expensive and requires developing a hydrogen delivery infrastructure. Although this may be feasible, two other options are more efficient. One is to distribute natural gas to fueling stations and manufacture hydrogen from the natural gas at each fueling station. Pipeline expansion would be minimal as long as we are developing wider natural gas distribution to support NGVs. However, energy losses in the hydrogen manufacturing process would mean that we would get fewer miles per cubic foot of hydrogen in an HFCV than if we used the natural gas directly in an NGV. The final option is to generate hydrogen at the fueling stations from electricity through hydrolysis. This, too, would not require additional expansion of the infrastructure, assuming that the power grid is expanded to accommodate PEVs. However, losses in hydrogen manufacture mean that the HFCV would need much more electricity from the power grid than a PEV for the same range. Consequently, the trade-off between HFCVs and P-PEVs would seem to be possibly greater range for HFCVs against greater utilization of power grid energy for P-PEVs. Additionally, emergency roadside refueling of HFCVs might be possible with service vehicles carrying high-pressure tanks of hydrogen, whereas roadside refueling of P-PEVs is problematic.

SEVs could provide excellent fuel economy. If algal biofuel becomes a possibility, SEVs might provide transportation with all the features and benefits of current gasoline automobiles but with no consumption of oil and almost no pollution. If plentiful biofuel does not become reality, electric cars, either P-PEVs or HFCVs, would provide resource-efficient, low-pollution transportation with somewhat reduced convenience, in particular in the areas of range and off-highway operation.

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4 Green Energy Sources

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Green Alternatives and National Energy Strategy: The Facts behind the Headlines

CHAPTER 3

Green Vehicles

Diesel Vehicles

Biodiesel

Bottom Line

Flex-Fuel Vehicles

Natural Gas Vehicles

Hybrid Electric Vehicles

Electric Vehicles

Fuel Economy

Batteries

Comparison

Bottom Line

Series Electric Vehicles

Hydrogen Fuel-Cell Vehicles

Hydrogen Gas Supply

Comparison

Bottom Line

Summary

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