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How Does It Work?

The information below about the solar hardware is now largely obsolete. Hydronic solar heating systems for small and medium sized applications have been superceeded by solar PV based heating systems which have none of the overheating, heat transfer fluid and maintenance issues, yet cost less to install and work better.

The fact that the solar energy can heat water is not very difficult to believe - most of us have felt how hot the water can get in a garden hose left out in the sun:) - so we just need the hardware to deliver that hot water into our homes.

The vast majority of solar heating systems sold in North America are two-tank systems. A two tank system is defined as a solar system that heats water in one storage tank that is then delivered to a conventional water heater tank (where water is heated by natural gas, electricity, propane, oil, etc). A variant of this design is where there is a solar storage tank that delivers water to an instantaneous or "tankless" water heater. The intent is that that solar energy is used to preheat the (cold) water before it is supplied to the conventional water heater. This will happen automatically when hot water is drawn from the tank - for use in showers and washing machines, etc. Because the water being supplied to the water heater is warm/hot the conventional heater does not have to use as much fuel/electricity to bring the water up to the desired delivery temperature.

The percentage of heat supplied by the solar system compared to the total amount of energy need to heat the water from cold to hot, is called the "solar fraction".

On a good, sunny, Summer day, a properly sized solar system may provide 80% to 100% of the heat required for the hot water. After a period of several cloudy, cold days the solar system may deliver no heat (which is why you need to have a conventional heater). Normally, solar systems in Ontario are designed to supply up to 50% of the annual hot water load. Attempting to size a solar system to deliver a larger percentage of the annual load can create problems. See: Solar water Heating - In Depth Information for details

There are solar water heating products coming onto the market that incorporate solar storage as part of a conventional hot water tank. The intent is that the solar system will heat the bottom half of the tank and a conventional heater - electric or natural gas - will heat the top half of the tank. While the systems work, the solar systems performance will often be compromised because the solar storage volume is too small. Each square metre of solar collector area should have at least 50  litres of solar storage. (Each square foot of solar collector should have a gallon of solar storage).These new single-tank systems typically have much less storage available for the solar system, and as a result: the solar sytem will operate at a significantly higher temperature causing it to, shut off prematurely, age more rapidly, and lose more energy through collector, pipe and tank heat loses. Single tank systems compromise performance in order to save the cost of an additional tank.

Happily PV solar water heating systems do not suffer from this problem in single tank configurations. As there is only a wire between the PV panels and the hot water storage tank - there is basically no energy-transfer-loss between the PV panels and the tank. The panels will continue to operate at their maximum efficiency independent of the temperature the tank reaches. This gives PV hot water systems a significant advantage in high-temperature applications - especially against evacuated tube systems.

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