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Domestic Water Pre-Heating (and home heating)
Deciding what type of collectors to use and the question of economics requires a little education on the subject. Go there to learn about large scale solar water heating potential with Powerstrip as well.
Anyone with a hot water tank can use solar energy. Keep in mind these things don't pay for themselves quickly so you want to get more out of it than a reduced energy bill. Add valves so you can take hot water directly off the coil (#10 below) in the solar tank. If its been sunny you can shower, wash the floors , car etc with free heated water from the sun directly. The temperature won't be as hot as the hot water tank usually and as its used up you'll notice the temp. dropping. This tells you you've used up the free heat and you can switch the valves back and take hot water from the hot water tank again. This system is the heart of any residential solar heating system. If you want to heat space its best done with separate coils in the floor (in floor heating). Just pump the low pressure solar water through the floor when you want to add heat to the space. You can add yet another pump to circulate the hot water tank through the coil in the solar tank. This allows you to heat the space with the hot water tank! This is a great solution for a house with some back rooms that don't currently have heat. I really don't recommend these systems to people who just want a quick return on their investment. You have to find joy in heating at no cost and using the earth's resourses in a sustainable responsible manner.
I wrote the above in 1994. Here in 2010 let me add that unfortunately what we have today is government rebates for solar water heaters using the approved technology which is the wrong technology. Boxed and glazed and evacuated tube systems are the staus quo. They have never gained market acceptance without subsidies and now with subsidies again, like in the early 70's when there was a big tax credit program, we're funding them again with tax dollars. We at Hot Sun won't even try to sell you our system now because the lure of easy money has a very strong appeal. Ironically and tellingly our friends at Fafco have been awarded a hige givernment grant to develop this idea and they have. Its a cheap version of this and its available in some areas after subsidies for zero dollars. Its nice to get free money from the government but are we really moving anything forward when our tax dollars pay for our neighbor's low grade overpriced contraptions?
As shown in the diagram cold water picks up heat from the solar storage tank(7) before entering your hot water tank(9). A differential temperature thermostat (5) turns the solar pump (4) on automatically whenever the roof temperature is higher than the tank temperature. At night or once the maximum tank temperature is reached the pump shuts off and the collectors(1) fully drain back to the 65 gallon tank (7). As a result this system can be applied in freezing conditions as long as all the piping slopes back to the tank and there is no exposed piping below tank level. Insulating outdoor piping doesn't prevent freeze damage. It just increases the time it takes to freeze.
The vacuum breaker (2) is standard with any solar system. It allows air to enter so the collectors can drain when the pump(4) is off.
The vacuum breaker (3) is existing in some plumbing codes It protects the hot water tank (9) from negative pressure (vacuum).
The float valve (6) kills power to the system if the tank level drops too low. When the lights on the differential thermostat go off its time to top up the level of water in the tank. This feature protects the pump in the event of a leak in the system.
An overflow tube (8) vents the tank and prevents overfilling.
A thermometer (11) is a simple way to monitor the temperature of the water leaving the solar tank.
A coil of copper pipe(10) in the tank allows the pressured cold water to exchange heat from the water in the solar storage tank.
The solar storage tank (7) is insulated to R-12. Even at 60C (140F) the tank will lose only a few degrees over night. Morning showers can take advantage of yesterday's sun.
Performance.
Our standard PMI-65 solar domestic water pre-heating system includes 2) 4'x10' Powermats (7.6 square meters) and 65 gallons (290 l) storage. Unfortunately our honest sales approach has killed the market for these and we're not making them anymore. However they are simple enough to fudge together yourself. I'm giving you the best design possible right here at no charge. many variations are possible.
In almost any household you will get more heat from a boxed and glazed or evacuated tube system of the same area. If you check you will find that usually competing systems are using far less collector area. If your hot water consumption is low enough a smaller, more expensive collector type system may be a better choice. As the load increases and economics start playing a role a swimming pool collector based system will deliver the most bang for the buck. As the load becomes huge we have the technology to achieve the good economics that become possible.