Solar Hot Water Heater System

A lot of people believe that the heat pump – isn't a proper 'solar' powered system because it does not use collectors. These people are wrong! Heat pump systems don't need any solar collectors because they use a different type of solar energy. Where 'conventional' solar hot water systems need sun light to heat the water, heat pumps pull solar heat out of the air around us and use this heat to increase your water temperature. Although it sounds unlikely there is actually enough heat in the surrounding air – even on a winter's day – to heat your water right up to boiling!
Heat pump systems do use electricity to remove the heat from the air and transfer it to your water (they are basically air conditioners in "heating-mode"), but they use around 75% less electricity heating your water in this way – compared to a conventional electric water heater.
Here is a typical heat pump system – you can see the similarities to an air conditioner!

This type of system is the most common in Japan, Israel and Australia, which have enjoyed an installed base of millions of residential and commercial systems for the last 30 years. Like the ICS system, the thermosyphon system eliminates the circulation and control system.
However, circulation is provided by the thermosyphoning principle: The hot water storage tank is located higher than the solar collector and circulation flow is induced when the coldest water in the bottom of the storage tank falls by gravity through a circulation line into the bottom of the solar collector panel, where it is heated and rises. Water heated in the solar collector panel rises into a circulation line to a high point in the water storage tank.

Drainback systems have a few more components, but are specifically designed to provide fail-safe operation in climates with frequent freezing during the coldest winter weather. The solar collector and control system are the same as the direct circulation system. However, an antifreeze solution is circulated through the solar collector and back into a heat exchanger in the hot water storage tank. The addition of a heat exchanger adds to the cost of the system and creates some degree of heat transfer energy loss, and this combined with the fact that these systems are typically installed at higher latitudes, where incoming solar radiation is reduced, makes indirect solar water heating less viable than its direct circulation cousin.
Like the direct system, an electronic control system compares the temperature of a sensor located at the solar collector with the temperature of a sensor located in the bottom of the hot water storage tank (where the coldest water is located). When the solar collector temperature is warmer than the water in the bottom of the tank by some predetermined difference (four degrees, for example), the electronic control turns on a small pump However, in the drainback system, the fluid circulating throught the solar collector is separated from the potable water in the hot water storage tank.
Either water or a glycol solution is circulated through the solar collector and a drainback tank. When the pump stops, fluid in the solar collector "drains back" into the drainback tank, leaving the solar collector empty whenever it has no fluid circulating through it. A second circulating pump circulates potable water from the hot water storage tank through a heat exchanger in the drainback tank.

Integral collector storage systems, also called "batch" solar heaters, combine the hot water storage tank and the solar collector surface into a single component, eliminating the need for circulating pumps or automatic control systems. In its most simple implementation, a water storage tank painted black and sitting out in the sunlight is a rudimentary ICS system.
This type of system works best as a preheater for a conventional or tankless water heater. The cold water line that feeds the conventional water heater is diverted and sent first through the ICS solar module. Circulation is provided by utility mains pressure. In other words, when hot water is drawn out for use from the conventional water heater, the storage tank is replenished with solar heated water instead of cold water. This allows the electric or gas heater to work substantially less.

Also called an "active" or "open loop" system, this is the type of system most often installed in the central and southern areas of Florida, and other non-freezing sunbelt climates within the United States.
In the direct system, an electronic control system compares the temperature of a sensor located at the solar collector with the temperature of a sensor located in the bottom of the hot water storage tank (where the coldest water is located). When the solar collector temperature is warmer than the water in the bottom of the tank by some predetermined difference (four degrees, for example), the electronic control turns on a small pump, that draws cold water from the bottom of the hot water storage tank and circulates it through the solar collector. Solar heated water is returned to the top of the tank.
The circulating pump is very small and typically uses about the same amount of electricity as a 100-watt lightbulb. Another version of this system uses a small photovoltaic (solar electric) panel to operate a direct current (DC) circulating pump.