[Question] advice / solution needed to heatup a greenhouse

[SilverNodashi]

I have a 5m x 5m x 2.5m greenhouse, with plastic sheeting (not glass) and I need to keep the ambient temperature to 10c or 15c during the night.
There's currently a 2000w fan heater in, which raised the temp by about 2c - 3c during the night, but it was still 4c this morning and some plants have some "frostbyte". So I need to go and buy at least two more, but 6kw for 10 - 12hours will cost a small fortune to run on Eskom. So I need something more cost effective.

I have considered putting in some black water pipes, full of water, but don't yet have a workable solution for a heat source, during the night. Will a solar collector and circulation pump, during the day, heatup the water in the pipes warm enough to sustain some heat during the nights?
I prefer not to burn any wood or gas inside.

[desA]

Air-to-air heat-pump will save your budget.

Sacks for night-time lagging of the roof & walls. Lag anything that could see a clear sky at night.

[SilverNodashi]

Air-to-air heat-pump will save your budget.

Can you make a recommendation, to something affordable?
the only price I could fine online is R24,000! My whole winter electricity bill won't be that much, with my current heaters.

Sacks for night-time lagging of the roof & walls. Lag anything that could see a clear sky at night.

uhm... Please re-phrase, I missed half of what you said. I can't see any clear sky at night, from within the greenhouse.

[desA]

Ok - leave the heat-pump for now.

Lag walls & roof. The clear walls tend to emit a lot of radiant energy during the evening towards deep space. For standard walls (not clear), you're looking to basically insulate against conduction of heat from warm inside to cold outside. You could use any cheap insulating material - polystyrene panels from old fridge protectors, hessian sacking, old cheap blankets etc.

Can you get charcoal-type heaters, or even perhaps gas heaters? Turn heat down reasonably low & leave to run through the night. Only problem is there may be no temperature control - ie. doesn't cut off if green-house gets too hot. But, practically, in Winter, I don't see this really being an issue.