Active Solar Water Heating Systems

Active Solar Systems use one or more electric pumps, valves and controllers to circulate water or other heat transfer fluids through the collectors. Active systems are also referred to as forced circulation systems and can be further categorised as either direct or indirect heating systems.

Open Loop

In the direct or open loop system water, one or more pumps are used to circulate water or working fluid through the collectors (Fig)

One of the major advantages of an active open loop system is its efficiency and low operating costs. However they are sensitive to freezing conditions and able to heat water to moderate temperature levels (~ 50 – 60⁰C), making it popular in regions with ample sunshine and moderate ambient conditions. They are also susceptible to damage due to corrosion and scale as a result of coming into contact with hard or acidic water. Various design modifications have been adopted to help overcome its sensitivity to freezing conditions. One such modification is the use of a drain back mode to circulate water from the storage tank to the solar collectors.

Drainback systems use distilled water as the heat transfer fluid in the collector loop. A pump is used to circulate water through the collectors. If the pump is switched off, the water drains back into the reservoir and collector via gravity as a result of the system being non-pressurized. As the system is reliant on water`s ability to drain back as influenced by gravity, it is essential that all piping above the drainback tank including the collectors are pointed downwards towards the direction of the drainback tank as seen in the figure below (Fig).

One of the major advantages of the drainbank tank is that it requires no maintenance except of the replacement of failed system components.

Closed Loop

In contrast to the open loop system, indirect or closed loop system involves the circulation of Heat Transfer Fluid (HTF) with the aid of a pump. Heat transfer fluid such as ethylene glycol and other refrigerants is circulated between the collector and the storage tank through the heat exchangeras seen in the figure below (Fig). Heat exchangers are used to transfer the heat from the fluid to the household water stored in the tanks.

Closed loop glycol systems offer good freeze protection and are reliable, making them popular in regions prone to consistent freezing temperatures. Compared to an open loop system, closed loop systems are generally more complicated as it requires either a tank with a heat exchanger coil or an external heat exchanger. The use of a heat exchanger would meanthat the collector loops would operate at slightly higher temperature conditions than the open loop system. This could have an impact on its performance and to minimise this, the collector loops are pressurized (Ruchi Shukla). The pump operates more like a heat pump and due to its reliance on solar energy;and its performance is compromised in low ambient temperature conditions (Ruchi Shukla).

Various studies have been undertaken to improve performance of the heat pumps, with two alternatives being proposed being the use of a variable capacity direct expansion solar assisted HP system (DX-SAHPS) and use of heat pipes (Ruchi Shukla).

DX-SAHPS uses a bare solar collector acting as an evaporator to be  used as a heat pump system. Observational results obtained by Chaturvedi et al. indicate that the coefficient of performance of the system can be improved by lowering the speed of the compressor. However it has to be noted that such a system shows optimal performance in summer compared to winter. A different design to the DX-SAHPS model by Kuan et al. used a bare flat plate collector as a source in addition to an evaporator for the refrigerant as seen in the figure below (Fig) . Based on a simulation model, it was concluded that collector efficiency was between 40 – 60% .

Use of heat pipes has also been introduced to enhance the performance of closed loop systems.The use of a closed loop system containing a heat pipe in addition to the conventional heat pump was tested/modelled by Huang et al. The system operated in heat pump mode under low solar radiation conditions, whilst under clear sunny conditions the heat pipe mode operated independently of electrical energy input to enhance thermal efficiency. The performance of the hybrid system was higher by about 28.7% in comparison to the normal heat pump mode of operation.

However it has to be noted that, solar energy is variable, and systems will not be as effective on cloudy days, so a booster system is required to provide water heating in periods of high demand or low solar gain. Some of the key design features that would have to be taken into consideration would include the size, location and type of collector panels, whether to use an open or closed loop heating system; and use of either pump of thermo-siphon for heat transfer. One of the important factors in determining efficiency of a SWH system is based on how well systems are configured and installed.