To heat with electric power you can use a heat pump ( also known as an air conditioner used for heating ) wich is up to 400% percent efficient.That means with 1 kw of electric power you get 4 kw of heat.with a ceramic heater for 1 kw of electric power you get a bit less than 1 kw of heat.
Yes, of course. I was considering mentioning that. But a heat pump has a much higher initial cost.
Actually, heat pumps combined with geothermal work really well together. It's easier to pump heat out of the ground that is 40 degrees than to pump heat out of air that is 10 degrees, let's say. This has been used to heat some homes, and do so very efficiently.
What a lot of people may not realize is just because something is cold doesn't mean it doesn't contain a lot of useful heat that could be used to warm things using a heat pump.
However, if the heat pump needs to be operating the entire night, every night during the Winter, in colder places (and especially northerly climates with shorter day lengths), I still suspect covering everything with thick insulation and using artificial grow lighting during the winter might be more energy efficient, even than a heat pump.
It depends on the specific situation and exact climate conditions. Initial price is also a big factor to consider too, since there might be a theoretically most energy efficient way to do things, but it might involve switching between 3 different strategies at different parts of the year. Obviously you would not really be getting your money's worth in that case. It doesn't make sense to buy a heat pump if it's only going to be used 1 month out of the year, for example. That money might have been better spent on going geothermal. So a lot of trade-offs.
Some greenhouses attempt to put the thermal mass inside the greenhouse, but it can be difficult or impractical to do that since a very large volume would be required. (I've looked into it and done some calculations)
Phase change eutectic salts are another possibility, but is generally impractically expensive considering the amount you would need. (I think that would only make sense if you were a large well-funded museum institution)
One last idea I thought of, if for some reason it was too difficult to dig into the ground, is have a smaller separate part of the greenhouse with a very large tank of water that was well insulated. Water would be circulated around in pipes. During the middle of the day, when the greenhouse is at its hottest, the water would be circulated to cool the greenhouse and absorb heat back into the insulated tank. Then during the middle of the night and early morning, the water would be circulated again to release the heat stored in the tank. This strategy would prevent the water in the tank from immediately releasing all its heat early in the night, and would also allow the greenhouse to heat up faster in the morning, without the cooler water keeping the temperature down. This could possibly help solve the problem of the greenhouse not having enough thermal energy to adequately heat the water for the night.