92Heating and Cooling Systems

There is little point in producing the ultimate energy-efficient electric vehicle, light, aerodynamic and with high motor and transmission efficiencies, and then waste precious energy by passing current directly through a resistance to heat the vehicle. With IC engine vehicles, copious waste heat will quickly warm the vehicle, although starting off on a cold morning may be unpleasant. For fuel cell vehicles or hybrids with internal combustion engines waste heat is also available, but with battery powered electric vehicles there is little waste heat and where heating is required this must be supplied from a suitable source. Of course, heating does not need to be supplied for electric vehicles such as bikes and golf buggies. Vehicle cooling is often needed in hot climates and this can also absorb considerable energy.

Batteries have a low specific energy and are expensive. It is better to store heat energy by using the specific heat, or latent heat of materials. As an example, one kilogram of

Electric Vehicle Technology Explained James Larminie and John Lowry © 2003 John Wiley & Sons, Ltd ISBN: 0-470-85163-5

water housed in a suitable insulated container and raised through 70°C above ambient contains 293 kJ or 81 Wh of heat. At 81 Wh.kg-1 this is a considerably better specific energy than both lead acid and NiCAD batteries.1 Early night storage heaters used the same principle for storing heat, but they used brick rather than water. More modern night storage heaters use the latent heat of fusion of materials such as wax, which gives an even higher specific energy than that obtained by heating water. Basically the wax is melted and kept in an insulated container. The heat can be drawn from the wax when required. A variation on this theme could be successfully used for storing heat in a vehicle. The heater could be recharged at the same time that the batteries were topped up and heat could be taken off as required. This is the basis of the RHP2 climate control system of Groupe Enerstat Inc. of Canada. For commuter vehicles this method of heating using thermal stores does have an advantage. A consequence would be that on cold days the vehicle would be warm as soon as the driver gets in, which would be a boon for short journeys.

A similar technique could be used for storing 'cool'. For example ice could be created at night and the latent heat of fusion released when required. The latent heat of fusion of ice/water is 92.7Wh.kg-1 and a further 17.3Wh.kg-1 can be obtained from heating the water to 15°C, giving a total specific energy of 110Wh.kg-1.

Both of these systems are relatively simple and are worth remembering as methods of heating and cooling electric vehicles. Schematics of both systems are shown in Figures 9.1 and 9.2.

Fuel burning heaters can be used to provide warmth. Such heaters have been used in battery vehicles used by the US postal service. It was said that such heaters could only

Electric current to supply heat

Warmer air out

Container with heat retentive material, such as molten wax, where the latent heat of fusion is released as the material cools and solidifies.

Cold air in

Figure 9.1 Schematic of storage heater system

1 Note that this means the ultra-low technology hot water bottle has a higher specific energy than most types of modern battery.

Figure 9.2 Schematic of cooling system

be controlled by opening the doors and letting the heat out, and the result was that the vehicles ended up using almost as much fuel as the diesel powered vehicles they replaced! For all sorts of reasons, this option must be considered a last resort. For one thing, the vehicle can no longer be classified as 'zero emission'.

Another way of heating and cooling an electric vehicle is to use a heat pump, as was done on the GM EV1. A heat pump is a device that actually provides more heat energy than the shaft or electrical energy put in. A schematic of a heat pump is shown in Figure 9.3. As its name implies, a heat pump pumps heat from one location to another.

When heating an electric vehicle, the heat pump would take heat from the outside air and pump the heat to the car interior. A reversible heat pump takes heat from the car and pumps it to the outside air, cooling the car. Heat pumps have a coefficient of performance which is typically 3 or more, in other words for every kilowatt of electrical input, 3 kW or more of heat will be pumped into or taken from the car. Refrigerators and

Shaft power

Shaft power

The heat pump removes air from cold source to a hot sink or vice-versa, and can be used to heat or cool the vehicle t

Outside air

Air inside vehicle

Figure 9.3 Schematic of a heat pump air conditioning units are examples of heat pumps. When air conditioning both heats and cools it is a reversible heat pump.

The design of heat pumps is complicated, however there are many air conditioning systems available which can be used as a basis. Firms who specialise in heat pumps and air conditioning should be consulted if you need a heat pump for an electric car. Heat pumps are also a feature of the RHP2 system alluded to earlier.

Car heating is clearly necessary, particularly in cold climates, but there is always some debate as to whether cooling is needed. Normally on a hot day in the UK the windows are opened. However, this will affect the aerodynamics considerably, increasing the drag and shortening the range. It would make better sense to install air conditioning in all electric vehicles where long range is required.

Most building designers know that the best way to heat a building is to keep the heat in and the best way to cool the building is to keep heat out. They also will tell you that the heating needs to come on half an hour before you get up. However, with cars they are either freezing cold, or when in strong sunlight get so hot that pets and small children who cannot free themselves can easily die from the extreme temperatures. Before a heating/cooling system is installed it is worth thinking about the best method of keeping the heat in and out hence minimising both the size of system and also minimising the amount of power used.

Certainly insulating material can be placed around the vehicle. Some modern insulating materials are very thin and would add little to the vehicle mass. Structural foam sandwich materials discussed in Chapter 8 would have good insulating properties and these materials would therefore serve two purposes.

The big problem with overheating stems from the fact that most cars are highly efficient passive solar heating systems; most would make good greenhouses. There is a large glazed area, allowing sun into the vehicle where it is absorbed by the interior as heat. This heat cannot be radiated, convected and conducted away at the same rate and so the vehicle temperature rises, often quite considerably. The sun will also hit the vehicle roof and be conducted through the roof, but this can be cut down by insulating the roof as discussed earlier. This problem can be considerably reduced by using glass covered with a selective coating, which considerably cuts down the amount of solar radiation that enters the vehicle.

There are some other ways in which vehicle heating and cooling can be aided. When at rest in sunshine in hot climates, air can be drawn through the vehicle bringing the internal temperature nearer to the outside air temperature. If the vehicle is being charged from external sources the fan can be powered from this. One neat design is to incorporate a solar panel in the roof to power the fan. It is also worth cooling or heating the vehicle half an hour or so before it is used, either by using a remote control or a time switch. Where a heat pump system is used the electricity would come from the charge point.

Hybrid Cars The Whole Truth Revealed

Hybrid Cars The Whole Truth Revealed

Hybrid Cars! Man! Is that a HOT topic right now! There are some good reasons why hybrids are so hot. If you’ve pulled your present car or SUV or truck up next to a gas pumpand inserted the nozzle, you know exactly what I mean! I written this book to give you some basic information on some things<br />you may have been wondering about.

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