Published on: 16/12/2008
Publication: Arabian Business
Country: United Arab Emirates

Muath Bani Hani looks at the benefits of a centralised domestic solar water heating systems in UAE applications.

A solar hot water system works by collecting solar radiation through solar panels fitted on a roof. The heat is then transferred to water storage for daytime use.

Solar collecting systems are usually classified as passive or active. Passive systems collect and distribute solar energy without the use of an auxiliary energy source, they are dependent upon building design and the thermal characteristic of the material used.

Active systems consist of components that are, to a large extent, independent of the building design and often require an auxiliary energy source (pumps) to circulate the water from the storage tank, through the collectors and back into storage.

The circulating pumps are turned on automatically by an electronic differential thermostat. This device reads the different temperature in the collectors and the storage tank. Pumps are turned on when the water temperature in the solar collectors is higher than the storage tank water.

It is recommended that a timer is used to automatically turn off the recirculation pumps between the hours of 12:00 am and 5:30 am, or that a variable speed pump is used for further pumping power optimisation.

When demand for hot water use is minimal the Aquastat only activates the recirculation or controls the VFD drive pumps when the water in the line falls below 150OF (65.6OC). Active systems are more easily applied to existing buildings.

Generally solar heating systems for domestic hot water include: a solar heating collector panel to collect the solar radiation that will be converted and stored as thermal energy; a fluid system (water) to transport the stored energy from the collector panel to its point of use; and a hot water storage tank to store the heated water by the solar collectors' panels for day and night use.

Collectors may also be classified as fat-plate or vacuum tube. High-performance flat-plate or vacuum tubes are generally required for cooling applications, since higher temperatures are needed to drive absorption-type cooling units.

Evacuated tubes are composed of two glass tubes made from tough borosilicate glass. The outer tube is transparent allowing the suns rays to pass through with minimum reflection.

The inner tube is covered with a special coating, which gives excellent solar radiation absorption and minimized reflection properties. The tops of both tubes are fused together and air in the space between the two layers of glass
is evacuated.

This "evacuation" of the air forms a vacuum, and is an excellent insulator. The insulation properties are very efficient that while the inside of the tube is so hot, the outer tube is cold to touch. The main problem with this type of collector is that the efficiency decreases significantly when they get covered by dirt and dust, so frequent cleaning is needed.

The flat-plate consists of an absorber plate, cover glass, insulation, and housing. The absorber plate is usually made of copper and coated to increase the absorption of solar radiation. The cover glass (or glasses) is used to reduce the convection and radiation losses from the absorber. The housing holds the absorber (insulated on the back and edges) and cover plates.

The working fluid (water, ethylene, glycol, air, etc.) is circulated in a serpentine fashion through the absorber plate to carry the solar energy to its point of use.

The temperature of the working fluid in a flat-plate collector may range from 30OC to 90OC, depending on the type of collector and the application. The collection efficiency of flat-plate collectors varies with design, orientation, time of day, and the temperature of the working fluid.

The amount of solar irradiation reaching the top of the outside glazing will depend on the location, orientation, and tilt of the collector. The amount of useful energy collected will also depend on the optical properties, the properties of the absorber plate and losses by conduction, convection, and radiation.

Solar collectors are classified as fixed or tracking. The tracking collectors are controlled to follow the sun throughout the day. Such systems are rather complicated and generally only used for special high-temperature applications. Fixed collectors are much simpler.

Although their position or orientation may be adjusted on a seasonal basis, they remain fixed over a day's time. Fixed collectors are less efficient than tracking collectors; nevertheless they are generally preferred as they less costly to buy and maintain. Solar panels shall be oriented toward the sun, with minimal or no shading during the day. The absorbed heat will be stored as hot water.

The United Arab Emirates has the highest dry and wet bulb temperatures in the area with a 108.3 OF (42.4 OC) dry bulb and 86.6 OF (30.34 OC) wet bulb, if solar collectors are exposed to these high temperatures the heat water may rapidly degrade and excessive pressures may be produced in the solar collector heat transfer loop, caused by the evaporation of the water inside the solar collectors.

This can lead to scalding in the hot water storage due to the high steam temperature and possibly a breakdown in the system.

This excess heat and pressure will lead to vibration in the pipes and the valves in the system and overheat the circulating pumps, which may destroy the seals, and water tends to leak from the pumps.

Many solutions are proposed to eliminate excess heat from solar systems; one is to cover the solar collector surface with a reflective opaque cover or precut sheets of plywood.

Another way is to use a heat dump package, which automatically activates a small circulating pump and a closed loop radiator when the hot water storage tank reaches a designated high-limit temperature. Once the water temperature drops below the limit and the hot water storage tank is ready to accept more heat, then the heat dump shuts off and the solar panels return to operation.

Muath Bani Hani is a mechanical engineer at DC PRO Engineering, green building MEP division, is a LEAD accredited professional and member of ASHRAE.