How to power heating and A/C in an electric car

The move to battery powered electric cars poses several challenges when it comes to climate control within the vehicle. Gone is the hugely inefficient internal combustion engine who's coolant system can easily be tapped for cabin heating while powering a 3 - 5 kw air conditioning compressor takes on a whole new meaning when your typical EV consumes only 200 wh/mile to drive down the road at highway speeds.

In an effort to improve the energy efficiency of ICEs the US DoE has been funding development projects for Thermoelectric Vehicular Heating, Ventilation and Air Conditioning (HVAC) and for Waste Heat Recovery. There are several approaches to the problem.

The first is localized or zonal heating and cooling. Current vehicular HVAC technology heats and cools the thermal loads of the surrounding structures such as the headliner, windows, flooring, and seat backs in addition to the occupants. These systems consume between 3,500-5,000 W. To reduce this load heating or cooling can be plumbed directly into the seats. These seats are called Climate Controlled Seats (CCS) and usually feature internal ducting, their own blower and a perforated leather surface.

As the seat has direct contact with the occupant it has much higher thermal conductivity compared to air which is a poor conductor. With direct contact cooling or heating load per person could be reduced to less than 700 Watts compared to 5,000 W to heat/cool the entire cabin. Imagine the difference in power requirements between an electric blanket compared to an electric space heater that uses 10x the energy.

Reverse cycle air conditioning

In an EV with no ICE to drive the A/C compressor it has to be driven electrically much the same way as a residential air conditioner. Where some cars today have started to improve fuel efficiency with electrically driven power steering pumps and racks, the 12v electrical system in most cars isn't powerful enough to drive a 5 kw A/C compressor. However, most battery EVs have a 350 volt main battery with more than enough power to drive a compressor. The challenge is to reduce the load so that running the A/C doesn't overly drain the battery and dramatically reduce range.

As with residential air conditioners, variable speed compressors driven by inverters operate more efficiently, mostly because they avoid the high current drain require by repeatedly starting the electric motor driving the compressor. The combination of Climate Controlled Seats and variable speed compressor will reduce loads significantly compared to systems used on ICEs today. Also like residential air conditioners, with the addition of a simple valve, these A/C systems can be run in reverse cycle to provide heat as well as cooling.

Occupant cooling requirement are much reduced to start with in an Ev as the largest heat source in any vehicle, the ICE, is removed. In a normal fossil fueled car the engine, transmission, coolant and exhaust systems are all radiating enormous amounts of heat any time the engine is running. To get some idea, an ICE is only 15% energy efficient at the wheels, meaning up to 70% of the energy in the fuel is converted into waste heat. For every 100 kw of engine power, 300 kw of waste heat energy is produced. A significant percentage of that can radiate into the cabin increasing the need for air conditioning.

In super cold climates where cabin heating is essential, there are several sources in an EV. An electric vehicle is approx 80% energy efficient meaning there is some waste heat available from the power electronics, the motor and battery pack. In production Evs most of these parts have a liquid coolant system so cabin heating can be plumbed in exactly like a conventional car heater system and/or into climate controlled seats.

Effect Thermoelectrics

Another approach to zonal heating / cooling that is in the very early stages of development is the use of Peltier Effect Thermoelectric (TE) devices located in the dashboard, headliner, A&B pillars and seats / seat backs. TE devices convert electrical energy directly into either heat or cold. The DOE Vehicle Technologies (VT) Program has funded selected project teams headed by Ford and GM to develop automotive thermoelectric heating, ventilation and air-conditioning (TE HVAC) systems, using the zonal concept of cooling or heating only the occupants and not the whole cabin.

Current TE devices are very inefficient at around 5-10% compared with 40–60% achieved by conventional compression cycle systems so significant development is still required before TE HVAC could be used in EVs. Most effort is focused on multi-layer Quantum Well Thermoelectric devices originally developed for CPU cooling. Most of the efficiency gains are theoretical at this stage with 15% having been achieved in lab tests and up to 25% being 'possible'. These types of TE HVAC systems under development are being considered where the power to run them is generated from TE systems that generate energy from the waste heat in an ICEs exhaust and cooling system and could make it into production vehicles by 2012 to 2015.