Closed-loop solar heating systems use solar energy to preheat water. The systems consist primarily of a water tank, a solar panel array (to collect heat from the Sun), and a heat transfer system (to transfer heat to the water tank). A heat-transfer fluid circulates through the system loop.
How it works:
 | The solar collector panels are heated by the Sun's rays; |
| The heat transfers to the transfer-fluid; |
| The fluid is pumped to the heat-exchanger; |
| The heat-exchanger transfers the collected heat to the potable water from the transfer-fluid. |
| The heated water is stored in a tank until it is used --such as when hot water pours from a faucet etc. |
| The transfer-fluid returns to the solar collectors for reheating. |
The heat transfer loop is completely filled with the transfer fluid. In fact, excessive bubbles can cause airlocks that stop the pumping action.
The fluid only circulates through the system when the panels are collecting a usable amount of heat from the sun's rays.
Closed loop systems usually have one or two tanks.
For two tank systems, one tank is used as a preheat solar storage tank, which initially holds the solar heated water. The other tank is a conventional water heater, which provides auxiliary water heating when there is not enough solar energy to completely heat the water. The two tanks are connected in series; as hot water is drawn from the conventional water heater for use, it is replenished by the preheated water from the solar storage tank.
A one tank system uses the same technique as the two tank system, only the tanks are combined into a single tank. The single tank functions primarily because of a thermaldynamic property, where heat rises. Just as hot air rises, hot water rises too. Cold water enters the bottom of the tank. The solar heat energy is transfered to the water via coils near the bottom of the tank. An electric heating element boosts water temperature in the upper half of the tank when solar energy is not sufficient to heat it alone. Hot water is extracted from the top of the tank to the hot water faucets.
Drainback systems are a kind of closed-loop system, where fluid is only in the transfer loop during pump operation.
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