Some cars (like the Toyota Prius) have a system where hot engine coolant is pumped into an insulated storage bottle. This can be used to heat the interior when the ICE is not running.
I seem to remember Saab experimenting with this for overnight storage, where the hot coolant would help heat up the engine when you start the car the next morning. Has this ever resulted in a usable product? And could this be retrofitted to other cars?
I'm looking into ways to improve my winter commute. A coolant water heater (electric or petrol) is the obvious solution (electric is difficult since I can't park my car on private property), but I'll consider alternatives.