Since in EVs it's much more important to think about energy efficiency, and there is no large source of engine heat avaliable, an overhaul of the cabin heating/AC is needed. Fortunately, the main traction batteries are an easy source of high voltage DC power, which can be used to supply power-hungry heating/cooling.
Heating:
There are a few viable approaches. There is nowhere near enough energy wasted as heat by the drivetrain components to warm the cabin even a little bit, so additional heating/AC is normally required. Remember that any high voltage wiring in the car needs care taken to ensure that it's safe, fused and with DC rated contactors to disconnect it. Standard mains-voltage rated AC relays are completely inadequate and will weld closed on their first operation if you're unlucky. Here are a few options;
- Reuse the stock heater core and plumb in a HVDC electric fluid-heater (powered from the main traction battery pack, so high voltage)
- Replace the stock heater core with a HVDC electric heating element (
i.e. cut a hole in the heater core and install a ceramic heating element
)
- Use an air-source heat pump (very expensive, but more energy efficient. New EVs such as the Tesla Model 3 now use heat-pumps)
It is a good idea to supplement the main heater (required for windscreen clearing) with heated seats & a heated steering wheel retrofit, as these are a much smaller drain on the battery. (Heated seats on full power could be 60W, an air heater might be anything up to 3000-4000W.)
Cooling:
It is not sensible to attach any mechanical loads onto the drivetrain (alternator, A/C compressor etc) as these are generally far less efficient than directly drawing power from the car's electrical system, especially since the main battery pack has high voltage to drive heaters/compressors etc.
Therefore if you want to maintain or install an air conditioning system, you need to replace the A/C compressor with an electric model. The most cost effective source of these is finding parts from scrapped EVs. These components are expensive however, and usually need some degree of signal spoofing, or replacement control systems to control them, as they come from modern cars and have CAN signalling everywhere.
Fortunately for most of us driving an MGF/TF, any time the weather's warm enough to need AC, we'll pop the roof down... Avoid the complexity of designing a replacement AC system and get the wind in your hair at the same time!