With an old radiator given to me by a friend, a 220 liter water bin and a few random hoses and connectors I made a solar collector to heat my greenhouse during the night.
Step 1: How I Did It.
The radiator heats 220 liter of water which will keep the greenhouse temperature up during the cold nights since the stored heat will radiate out slowly.
The radiator was sprayed pitch black and is packed around and on the back in isolating foam material enclosed in a wooden box which is closed off with a single sheet of plexiglass. It works like a charm: no pump is needed since the hot water will flow UP and thus create a flow which sucks the cold water in at the bottom of the radiator. Within 15 minutes the water went from 16 to 32 degrees celcius! The temperature did not go up much after that.
Step 2: Afterthoughts
This solution is simple, reliable and cheap. At first the setup did not work; it turned out that an airbubble was blocking the flow. I solved this by hosing water through the bottom outlet of the bin towards the radiator...
Obviously it will NOT work in cloudy conditions, but with clouds the temperature won't drop as dramatically as with clear conditions here in the Netherlands. Hard frosts may also damage the setup, so I tinker with the idea of making a version with a copper coil (connected to the radiator) as heat exchanger in the vat and fill the coil and radiator with antifreeze. The radiator is of the double type. I wonder if a single plated radiator would work better.
Since the bin is not isolated the heat storage capacity is limited. Isolating it will dramatically improve the heat storage capacity I guess, but then the bin will not work as a radiator at night. A solution could be to install a pump which can circulate the warm water under the beds of the greenhouse (through a hose or something) to keep the plants warm. This would involve a setup with solar panels for electricity, a battery to store the electricity, an arduino to make things work and a few things more. Doable, but more complicated. Your input much appreciated!
Update mid April: we had two nights of frost. The first was -2 Celcius which was perfect. The minimum temperature in the greenhouse went down to +2 very early morning, no damage was done to my young plants. The second night we went down to -5 and the greenhouse sunk to just around 0 Celcius. I had covered up all my plants to be sure but forgot a cucumber which froze. The rest of the young plants (annuals but also tomatoes and peppers) came through without damage. Lesson learned: be very aware!