1) Solar Collector Types
Solar water heating systems harness the heat in solar radiation to produce hot water. Domestic solar water heating system usually comprises of solar collectors and a water tank. The solar collectors absorb the heat in solar radiation to heat up water, either directly or indirectly. The hot water so produced is stored in the storage tank for subsequent use. An alternative arrangement is to use a packaged type solar water heater, with solar collectors and an insulated storage tank packaged together in one integrated unit.
Usually a solar water heating system is designed to fulfill a major part of the hot water requirements of the premise, with the rest being made up by electric heating (or gas heating).
The solar collector is by far the most widely used solar energy conversion device, and there are millions in use around the world. Solar collectors can be classified into two major types based on design, i.e. flat-plate collectors and evacuated-tube collectors, with the latter further divided into glass-glass type and glass-metal type.
(a) Flat-plate solar collectors
A flat-plate solar collector contains a metal absorber plate (made of copper or aluminum) enclosed in an insulated rectangular box with a glass or plastic cover. The absorber is usually painted black to maximize heat absorption. The tubes for the heat transfer medium (i.e. water), which are usually made of copper, are connected conductively to the absorber. When the solar radiation hits the absorber, the major part of it is absorbed and a small part reflected. The absorbed heat is conducted to the tubes or channels for the heat transfer medium.
(b) Evacuated-tube solar collectors
i. Glass-glass type
The collector contains parallel rows of transparent tubes. Each tube is made up of an outer glass tube and an inner glass tube. The inner tube is coated with an absorber coating that absorbs solar energy well but minimizes radiant heat loss. A thermal conducting plate with a U-tube is inserted into the inner glass tube. The water to be heated flows in the U-tube. Air is removed from the space between the outer glass tube and the inner glass tube to form a vacuum so as to reduce conductive heat loss.
ii. Glass-metal type
Glass-metal tubes are further divided into direct flow-through type and heat-pipe type.
For direct flow-through evacuated-tube collectors, the absorber in the form of metallic fins or metallic cylinder is installed inside the glass tube. Air is removed from the glass tube to create a vacuum. Water flows in U-pipe which is attached to the absorber inside the glass tube.
For heat-pipe evacuated-tube collectors, a heat pipe is attached to the absorber inside the vacuum glass tube. The heat pipe is filled with a working fluid with low boiling point (such as alcohol). At the upper end of the heat pipe is a condenser bulb where heat exchange takes place. The tubes are mounted, with the condenser bulbs up, into a manifold (or storage tank in the case of packaged solar water heater). Heat energy collected by the absorber fins vaporizes the working fluid, which rises into the condenser bulb in the form of vapour. Water from the recirculation loop flows through manifold and picks up the heat from the condenser bulbs. The condensate of the working fluid then returns to the collector heating zone by gravity.
The characteristic curve equation of a solar collector can be a first-order equation or a second-order equation.
First-order equation: η = ηo - k * ΔT / G
Above: Graph of efficiency and average temperature difference at constant irradiance level of 1000 W/m2, for typical solar collectors
Evacuated-tube collectors are usually more expensive than flat-plate collectors. However, as the k-value is lower, evacuated-tube collector can achieve a high efficiency even with large temperature difference between absorber and surrounding.
A solar water heating system generally requires a well-insulated storage tank to hold the heated water. The storage tank is often equipped with an auxiliary electric heater (or gas heater) to boost the temperature of the heated water when the thermal output of the solar collectors is not sufficient to meet the heating requirements. This may happen when solar energy is not available or insufficient, for example, at night, on a cloudy day, or in the cooler months of the year.
Other ancillary equipment includes controller and circulating pump, safety devices like temperature/pressure (T/P) relief valve and tempering (water mixing) valve. The controller monitors the collector and storage temperatures with temperature sensors, and operate a circulating pump at the appropriate temperature differential. A T/P relief valve releases water (through a discharge pipe to drain) if either the temperature or the pressure inside the storage tank gets too high. A tempering valve is used to control the temperature of the hot water being delivered to the user so as to prevent burns. The valve mixes the hot water with tap water to ensure that the water delivered to the user is kept below a safe temperature.
Solar water heating systems can be divided into 2 categories, the direct type (open-loop type) and the indirect type (closed-loop type), depending on whether the incoming is heated up directly or indirectly via a heat exchanger.
(a) Direct type (open-loop system)
In an open-loop system, the incoming water (i.e. water to be used) circulates directly through the solar collectors.
(b) Indirect type (closed-loop system)
In a closed-loop system, the incoming water is not heated up directly at the solar collectors. Instead, a working fluid (e.g. freon, propylene glycol, distilled water) is employed in the collectors and circulates in a closed loop. The working fluid circulates through a heat exchanger inside a water tank, where heat is transferred to the incoming water.
Packaged type solar water heaters, with solar collectors and an insulated storage tank integrated into one single unit, are also available in the market. Since the installation of this type of system is less complicated, it is suitable for smaller facilities and for household use. The packaged type solar water heaters are available in different sizes. A smaller one is sufficient for a family of two and a larger one can provide hot water for eight persons. These types of systems are suitable for low-density housing development such as villas, low-rise houses and village houses, etc. as these housing developments tend to have more roof space for installation.
Solar water heating system can be used for a wide range of applications. It may be suitable for establishments with regular hot water demand over the year such as hotels, hostels, households and hospitals. It is also suitable for sport centres and holiday resort villages. It can also be used for pre-heating of swimming pool water. Low-rise offices buildings with canteens and washrooms and or showers can also be considered for installation of solar water heating systems.