The main advantage of an integrated storage solar thermal system is simplicity (which is reflected in the cost) – no heat exchanger, no pump, no controller and no glycol. The disadvantage is the lack of freeze protection and requirement for the tank on the roof. This system was plumbed with innovative valving allowing the owner to run in 3 modes depending on solar gain and outdoor temperatures –
3 Modes of Operation in One System
Solar preheat (approx 6 months/year) – The standard mode in which the solar tank is used to preheat the main electric tank. If there is sufficient solar gain, the electric tank is fed with water above the thermostat setting, and the tank elements won’t turn on. If the solar gain is not sufficient, the electric tank tops up the tank to provide domestic hot water at a constant temperature.
Solar only (approx 4 months/year) – The electric hot water tank is off and the only heat in the system is provided by the sun, a thing of beauty. This requires reliable solar gain and conservative use.
Electric only (approx 2 months/year) – For those freezing periods on the coast, the integrated storage tank is drained and electric tank provides the domestic hot water.
This system is a pressurized “heat pipe” type meaning that the cylindrical tank is pressurized to the domestic water system pressure and the method of heating is through sealed heat pipes. The heat pipes (copper tubes shown in the photo) have a small amount of water that is heated by the sun, moves up into the cylindrical tank by a natural convective thermosyphon, heating the water in the tank. The evacuated tubes are a glass tube with a plenum of air between them that both insulates and increases the effective exposed tube area. An important part of this system is the reflector that allows the solar energy between the tubes to be reflected and collected on one of the tubes.
Siting the Collector –
A perfect solar roof, due south with almost no shading year round. This system was sited over the roof ridge to maximize solar gain and reduce distance to the hot water loads. The angle to the horizon was set to 41 degrees to optimize the sun in the shoulder season. For a couple of months in the winter (to prevent freezing), the system will be drained, so optimizing for those months or total annual energy (45 degrees), doesn’t make sense. Optimizing for the summer months (35 degrees) would provide a little more hot water when we have a surplus, but pay the penalty in the shoulder season.