Solar Heating

Solar Hot Water

Solar energy can be used to heat domestic water. Systems are economical in the long term because they are long-lasting and the energy – sunlight – is free.

Solar water heating is best for:

• Homes or businesses with high water usage, e.g. large families, laundries, restaurants and health clubs
• Homes or businesses that currently heat their water with electricity
• Buildings with good solar exposure between 9 a.m. and 3 p.m

The main advantages of solar water heating are:

• Pollution resulting from conventional water heating is avoided. Unlike conventional water heaters, solar water heaters do not burn fuel or depend on electricity generation. They therefore avoid the creation of carbon dioxide, nitrogen oxides, sulfur dioxide and other air pollutants.
• Reduced vulnerability to fluctuations in fuel prices.
• Minimization of a country’s dependency on oil from abroad
• After the system has paid for itself, the hot water is free.
• The resale value of the property may increase

Systems

Solar water heating systems can be either active (more common) or passive.Active Systems rely on pumps to move the liquid between the collector and the storage tank. Passive Systems rely on gravity and the tendency for water to circulate naturally when it is heated.
The system can be either direct (open loop) or indirect (closed loop). In direct systems, the water that flows through the collectors is used in the household.

In indirect systems, a non-freezing liquid is used as a heat transfer fluid that is heated while flowing through the collector. The heat is transferred to the household water by means of a heat exchanger.

Active Systems

Active systems use electrically driven pumps, valves and controllers to circulate water or other heat transfer fluids through the collectors. They are usually more expensive than passive systems, but also more efficient. Active systems are usually easier to retrofit than passive systems, because their storage tanks need not be installed above or close to the collectors. However, they will not function in the event of a power failure because they need electricity.

Direct Active Systems

These are most appropriate for regions with a mild climate or only occasional freezing conditions. These systems convey the water from the collectors to the household taps. The systems consist of one or more collectors, a pump and a storage tank. A pump circulates the water from the tank up to the collector and back again. No anti-freeze solution or heat exchanger is involved.

Because the system has no heat exchanger, it is slightly more efficient than an indirect system. However, the system must incorporate special mechanisms to provide some degree of protection against freezing.

There are two main types of active direct systems, each of which deals with freeze conditions differently. When the temperature drops below zero, "recirculation" systems pump heated water through the collectors. "Draindown" systems drain the water from the collectors into a holding tank to avoid damage from freezing. This type of system is not recommended.

The recirculation system, has a differential controller and twotemperature sensors (one at the outlet of the collectors and the other at the bottom of the tank) that sense the temperature difference between water leaving the solar collector and the coldest water in the storage tank. When the water in the collector is about 10°C warmer than the water in the tank, the pump is turned on by the controller. When the temperature difference drops to about 1-3°C, the pump is turned off. In this way, heat is always transferred from the collector to the water when the pump operates.

In climates where freezing occurs infrequently, a recirculation-type differential control can be used to turn the circulation pump on when the collector inlet temperature falls to 5°C. The philosophy behind this design is that heating the collectors with hot water from the tank is an inexpensive way to protect against freezing, provided it is only required occasionally.

Recirculation systems are no longer very commonly used due to their vulnerability to freezing as a result of power failure, malfunction of the sensor or controller or damaged sensor wires.

A draindown system is a direct system in which the collectors are filled with domestic water under house pressure when there is no danger of freezing. Once the system is filled, a differential controller operates a pump to move water from the tank through the collectors.

A draindown valve, invented in the 1970s exclusively for these systems, provides the freeze protection function. When the collector inlet temperature falls to 5°C, the draindown valve, activated by the controller, isolates the collector inlet and outlet from the tank. It simultaneously opens a valve that allows water in the collector to drain away. A vacuum breaker is always installed at the top of the collectors to allow air to enter as water flows out at the bottom. Associated with the vacuum breaker, there is an automatic air vent to allow air to escape when the system fills again.

Draindown systems have proven to be the most problematic of all freeze protection systems. They are vulnerable to frozen vacuum breakers and air vents, damaged sensors or wiring, lack of proper pipe drainage and malfunction of the draindown valve. This type of system is now rarely installed, and is not recommended. Many existing systems of this type have been converted to drainback or indirect antifreeze systems.