Until the battery problem is solved, electric hybrids will continue to evolve. The next step is the plug-in hybrid, in which the battery is charged overnight from the grid. Since most people in cities usually drive no more than 30 miles (50 km) a day, a slightly larger battery will store enough energy for that, so that the gasoline engine need not be started except on weekends. Air quality in cities would be greatly improved. There are actually two types of plug-in hybrids. The usual one works like the Prius: the battery is charged from the grid as well as by the gaso­line motor. Two motors drive the car. In a series hybrid, a small motor runs only to charge the battery. The propulsion is entirely electric. The savings in fossil-fuel consumption and GHG emission have been estimated in a report by Electric Power Research Institute and the Natural Resources Defense Council (EPRI — NRDC).54 It turns out that it matters whether the battery is sized to give 10, 20, or 40 miles of electric driving.

The EPRI-NRDC report considers scenarios, nine in all, depending on whether PHEVs (plug-in hybrid electric vehicles) have a low, medium, or high penetration into the market, and whether the power industry makes a low, medium, or high effort to reduce their emissions. Although the nine results vary by a factor of 4, they are all good. The GHG reduction in 2050 is predicted to be between 163 and 612 million metric tons (in the USA). An idea of how they expect PHEVs to take over the market is shown in Fig. 3.53. If no progress is made in battery technology (which is unlikely), PHEVs will take over more than half the car market!

Table 3.1 compares various kinds of hybrids with normal cars.55 The data are for 12,000 miles of driving in year 2010. The normal hybrid generates its own electricity and therefore uses more gasoline than PHEVs, though less than gaso­line cars. PHEV10 is a PHEV that can go 10 miles on one charge. PHEV20 and 40 have bigger batteries to go 20 and 40 miles. All the hybrids are assumed to have

a gas motor averaging 38 miles/gallon. The PHEVs use more electricity from the grid and less gasoline, so their carbon footprints are smaller. Remember that electricity generated at a power plant uses less oil than electricity generated in the car. If the power plant uses hydroelectricity or nuclear power, the carbon footprint is more than halved.

How much money will a plug-in hybrid save? This depends, of course, on the battery size in the PHEV and on local prices; but here is an example. The break­down between electricity usage and gas usage in Table 3.1 is based on some data on driving habits. In electric drive, a Prius-type hybrid uses 150 W-hrs of electricity per kilometer.56 This works out to be 0.24 kWh/mile. In 2009, the average cost of residential electricity in the USA was 11.7 0/kWh. The cost of 2,477 kWh in the PHEV40 case is then 2477 x $0.117 = $290. In the PHEV40 column, we see that 107 gallons of gasoline are consumed. If we assume a price of $3.00/gallon, the gas cost is $321 and the total fuel cost is $611. These are the figures in the last column of Table 3.1. The other columns are calculated the same way. As for the “normal” cars, all the energy comes from gasoline, so there is no electricity cost. We see that hybrids save on the cost of fuel, but these savings may not offset the premium one pays for hybrids at present. For the plug-in hybrids, there is a “sweet spot” around the PHEV20, whose fuel costs are much lower than for the PHEV10 but not much higher than the PHEV40. Since most people do not drive 40 miles every day, the extra cost of a large battery is not worth it. However, individuals are not “most people”; they can buy a plug-in suited for their own driving habits.

There has been some concern about the effect of numerous plug-in hybrids on the grid. Since charging a PHEV on household current can take upwards of eight hours, most people would want 240-V service installed. Then charging can be done in 2-3 h. At this rate, however, as much as 6.6 kW of electricity is drawn. Each car that is plugged into that service is like adding three houses to the grid, each house with their lights on and air conditioner working.57 If every household has a plug-in, the local grid would have to be boosted. However, the EPRI-NRDC study shows that the industry experts are not worried. They show a profile in which 74% of the charging is done between 10 p. m. and 6 a. m., with a small daytime peak between 10 a. m. and 4 p. m. There are minima around 8:30 a. m. and 5:30 p. m. when people are commuting. The grid can handle that load, at least for the present.

Batteries

Electric cars can go a long way toward relieving our dependence on oil, but the bottleneck is the battery. We are spoiled by having cars that can go 300-400 miles (500-600 km) without refueling and can be filled up in 10 min. There has been no path-breaking invention in batteries in the last few decades. Figure 3.54 shows where we are. Each rectangle is the range occupied by one type of battery according to how much it weighs and how big it is compared to the energy it can store. Lighter batteries are to the right, and smaller batteries are near the top. At the bottom left is the old stand-by: the lead-acid battery used in conventional cars. It is heavy and big for the amount of energy it carries. The only improvement over the last 50 years is that they are now sealed, so that we don’t have to check the fluid level and add water every week or so. The first experimental electric cars carried a load of lead- acid batteries. One battery is only good for starting a car and keeping its headlights on for a few hours; it cannot move a car very far. The small carbon-zinc and alka­line batteries we use for small appliances and toys are off the chart because they are not rechargeable. Nickel-metal-hydride (NiMH) batteries, however, are success­fully used in cars, notably the Prius. These were chosen because they are safer than lithium and have proven reliability. The best we have at present is the lithium-ion battery. As Fig. 3.54 clearly shows, “lithiums” are lighter and smaller for the same amount of energy. They are used to power laptop computers, cell phones, cameras, and other small appliances. Their safety and reliability are, however, worrisome for use in cars. There is hope, however, because electric cars like the Tesla Roadster have shown that, if cost is not a consideration, sport-car performance can be

achieved with a 6800-cell Li-I battery good for 244 miles. With a 288 HP (215 kW) motor, the car goes 125 mph (200 kph) and accelerates 0-60 mph in 3.7 s. Charging the battery on 240 V takes 17 kW in 3.5 h.

Aside from cost, lithium batteries have two main problems. Safety is the main concern, since lithium batteries have been known to explode, as they did in some laptops a few years ago. When a short circuit occurs in such a battery, the chemicals can burn and cause short circuits in neighboring cells, which release more heat, starting a runaway reaction. Unlike hydrogen, which cannot burn without oxygen from the air, lithium batteries have the oxygen inside. The solution is to divide the lithium battery pack into small isolated units which are then connected together with wires. The second problem is life span, which depends on how often the battery is recharged. Even if it is not used, a lithium battery can lose as much as 20% of its capacity per year [33], as many laptop owners have found to their dismay. The number of charge-recharge cycles is limited to several thousand. For cars, 5,000 cycles would be good for 10 years for most drivers, and this is close to present technology. However, it would be hard to build enough extra capacity for the car to maintain its driving range for 10 years. Charging a lithium battery too fast or overcharging it could cause plating of the electrodes, which shortens it life. These problems are slowly being solved as companies move into this rapidly expanding market. The target price set by the US Advanced Battery Consortium for electric car batteries is $300/kWh. Lead-acid batteries cost about $45/kWh, compared with NiMH batteries, which cost $350/kWh for small ones to $700/kWh for ones used in cars. Right now Li-ion batteries cost $450/kWh [33]. Perhaps economy of scale will bring the prices down as electric cars overtake the market.