According to the Energy Information Administration, in the United States, transportation uses 26.5% of the total energy, or 67.6% of the petroleum. To reduce the use of fossil energy, the transition to electric cars using rechargeable batteries, especially using Li ion batteries as storage medium and solar energy as the source, is the best approach. Electric cars have many desirable features:
• The intrinsic efficiency of electric motors is very high, typically 90%.
• The round-trip efficiency of energy storage in rechargeable batteries, especially Li ion batteries, is very high, typically around 90%.
• The mechanical structure of electric cars is much simpler than either the Otto engine cars or the diesel engine cars.
• The regenerative brake can be implemented naturally. Actually, the efficiency enhancement of hybrid cars, such as the Toyota Prius, is mainly due to the regenerative brake.
• As battery technology is progressing rapidly, the manufacturing cost of electric cars will decrease rapidly.
• They can make a natural connection to solar electricity.
Table 12.5: Efficiency of Several Automobiles.
Source: Sustainable Energy – without hot air, David J. C. MacKay .
• They are virtually noise free.
Let us first examine some technical data. In the United States, the efficiency of cars is measured by miles per gallon of gasoline, or mpg. Ine SI units, the most convenient measure is either kilometers per kilowatt-hours of energy, or the energy in kilowatt – hours required to drive 1 km. Because the energy content of gasoline is approximately 1.3×108 J/gallon and one mile is 1.609 km, a simple calculation gives 1 km/kWh = 22.37 mpg. Table 12.5 gives the measured data for several popular cars.
For several decades, hydrogen and fuel cell has been considered as an alternative to the Otto engine and the diesel engine for automobiles. Grandiose expressions such as the hydrogen age, hydrogen economy, and hydrogen era have been used. However, according to an analysis by Joseph J. Romm , a deputy energy secretary during the Clinton administration in charge of hydrogen projects, based on his hands-on experience, in the foreseeable future the use of hydrogen will not become a commercially viable method of energy storage. It is prohibitively expensive and notoriously dangerous. It is especially unsuitable for automobiles, because the density of storage of the highly compressed hydrogen is only one tenth that of gasoline. Besides, fuel cells have low efficiency (compared with rechargeable batteries) and low lifetime and use expensive precious metals.
In the future, solar photovoltaics will become the main source of electricity, a technology that works well with electric cars. In particular, solar photovoltaics can charge the batteries in electric cars without going through the grid. This approach has in-
Figure 12.10 Solar-powered electric car charging station in Kyoto. The charging station, supplied by Nissin Electric Co., has a battery pack, an inverter, and a rapid charging setup. Courtesy of Kyoto University.
Figure 12.11 Solar-powered electric car charging station in Tennessee. In the United States, taking another step in building its electric vehicle charging infrastructure, Tennessee is now home to the first of several solar-powered EV-charging stations .
herent advantages. It avoids the cost and energy loss due to the DC-AC inverter and the AC-DC inverter. Furthermore, the intermittency of solar energy is no longer a disadvantage because to charge batteries a very stable power source is not required. The advantage of solar-powered battery charger can be further improved by using the battery-swap procedure: Spare batteries are charged when there is sunlight. A car with a depleted battery can come to the charging station to swap for a fully charged one in a few minutes, probably even faster than filling the gas tank. Figure 12.10 shows a solar-powered electric car charging station in the city of Kyoto supplied by Nissin Electric Co. It has a battery pack, an inverter, and a rapid charging setup.
In the United States, both General Motor and Nissan will mass-produce electric cars, Chevy Volt and Nissan Leaf, in the state of Tennessee. Therefore, it makes sense for people in Tennessee to start laying a plug-in groundwork. That has already happened in Pulaski, Tennessee. In August 2010, the first solar parking lot opened — that is, parking spaces with an electric vehicle charger that is powered by solar energy; see Fig. 12.11. At the opening ceremony, after remarks highlighting American energy independence and job creation, Congressman Lincoln Davis switched on the array, sending the sun’s energy into the grid. It was stated that “All the components are American made. It is an example of how American small business and manufacturing are growing in the new green economy .”
12.1. A typical domestic hot water tank has a dimension of diameter D = 50 cm and height H = 135 cm, with a т = 5 cm thick insulation made of rigid foam polyimide, see Figure 1. If the difference of the external and internal temperatures is 45°C, what is the energy loss of this tank in watts? (The thermal conductivity of rigid foam polyimide is k = 0.026 W/(m°C).)
12.2. By storing hot water in that tank at 65°C and the environment temperature is 20°C, how long it takes to cool the water temperature down by 1°C?
12.3. By storing ice at 0°C in that insulated tank, with external temperature of 25°C, how long it takes for all the ice to melt? (The latent heat of ice is 335 kJ/liter.)