The system is made up of a large clay reservoir filled with water (you could add mosquito retardant, or something to prevent algae growth) with a pipe connection at the bottom, a 12v solar water pump, and garden tubing (example 1/2" garden drip irrigation feed pipe).
The operation is simple. During daylight hours the solar pump will power up and pump the cold water from the garden pipes into the clay reservoir. When the sun goes down and the water is heated as much as it will, the solar pump will stop working and the water will flow back by gravity to the water lines in the garden bed. Note, one of the garden lines has a vertical element (sealed at the top and filled with water) that allows air to compress at the top to ease water travel back and forth.
A difficulty of this design will be connecting the water pipe to the clay reservoir. You can simplify this by using a bladder inside the clay pot that has the hose connection and merely routing the piping through the pot. Alternately, you can drill a hole in the pot, and install a bulkhead fitting like from here (http://www.jehmco.com/html/bulkhead_fittings.html).
