Electric Cars and Air Conditioning

Written by Bryan Johannsen

We recently posted and article on operating an electric car in extremely cold environments. What about operating an electric car in the other extreme? Driving in the summer in south Florida almost requires air conditioning. How will it affect your EV?

Air conditioning in a car involves taking in outside air and cooling it using a compressor and a refrigerant. In fossil fuel powered cars the compressor is powered by a belt that is driven by the engine and uses air brought into the engine compartment from the grills used to cool the engine's radiator. It negatively affects fuel economy since turning the compressor creates an extra load on the engine.

Air conditioning in an electric car is much the same as it is in a gasoline powered car but the design of an electric vehicle presents a few problems for air conditioning. The added weight of course affects the range of the vehicle as does powering the compressor but powering the compressor must be done differently.

When a regular car is sitting at a stoplight of stuck in a traffic jam the engine is idling, turning over at a low RPM. This still allows the belt driven compressor to continue functioning when the car is stopped. One of the reasons an electric motor is so efficient is because when the car is stopped the motor is effectively "off". It stops turning completely as it doesn't need to stay ticking over to start going again when the accelerator is depressed. For an air conditioner this means that the typical belt driven configuration is either going to shut off when the vehicle is stopped or require the engine to "idle" enough to turn the compressor which would have a very detrimental effect on the car's range.

A better way to power the compressor is through a second electric motor attached directly to (or built in to) the compressor. This allows the compressor to continue to work when the vehicle is stopped and doesn't require the more powerful drive motor to run when the car isn't moving. This still creates extra drain on the batteries reducing the vehicle's overall range but it is a better solution than belt driven compressors in electric vehicles.

Another problem stems from outside air intake. A gas or diesel car has a grill opening in the front so air can get to the radiator to cool the car's engine. This same opening takes in air for the climate control system in the vehicle and is used for heating and cooling. Electric motors don't require a radiator to cool the engine so they don't require a grill opening at the front of car. This creates an opportunity for enhanced aerodynamic efficiency. That cooling system in a gas powered car creates a lot of drag as it takes in air and interrupts airflow over the body. An electric car can be designed to maximize airflow over the vehicle and create extremely low drag. This enhances the range of the vehicle by making it's power use more efficient. An opening added to the front of the vehicle to take in air for the compressor negates this and adds drag and reduces efficiency, though the opening doesn't need to be as large as it is to cool a radiator.

Solar power would be a good way to drive the compressor motor since in most cases the sun is shining when you want to use the air conditioner but solar panels are expensive and fragile and require quite a bit of surface area to produce the needed power.

Pre-cooling is a way to increase the air conditioner's efficiency. This involves cooling the vehicles interior while it is still plugged into the charger, this way the air conditioner doesn't have to work as hard to cool the vehicle when it is initially turned on.

Air conditioning in a electric vehicle is a touchy subject due to it's negative effects on efficiency but what is required is some new thinking on how to cool the vehicle instead of shoe-horning what currently works into a system that wasn't designed for it.

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