With an old radiator, a 220 liter water bin and a few random hoses and connectors I made a solar collector to heat my greenhouse during the night. The radiator heats 220 liter of water which will keep the greenhouse temperature up during the cold nights: the heat will slowly radiate out.
The radiator is packed in isolating foam material within a wooden box closed off with a 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. Simple, reliable and cheap. Obviously this does NOT work in cloudy conditions, but with clouds the temperature won’t drop as dramatically as with clear conditions. 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 with antifreeze.