Solar hot water systems (for heating household water) usually have a small solar-powered pump, which runs faster the brighter the sun. So on a sunny day, when the water is getting hot quickly, the pump is running at max speed.

I have seen plans for a non-pumped system... I think it relied on the 'hot water rises' principle to move cool water into the bottom of the solar array and hot water out of the top.

If it's not a big greenhouse you would probably be ok with a solar battery, rather than a central heating system. Just fill a large, dark-coloured container with water and put it somewhere in the sun. It'll soak up heat in the day and release it at night.

Alternatively, build a chicken house along the north wall of the greenhouse. The chooks will warm the greenhouse with their body heat. Give them access to the greenhouse in winter (as a warm indoor run) and they will eat all the pests and manure the soil for you. They even breathe out carbon dioxide, which is good for your young plants!

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Resistance is fertile