Sun and water

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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.

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READERS' COMMENTS

whats so great about this article
Posted by Markus on Monday 5 October 2009 at 11:20 UAE time


I don’t get the point of this article, this is elementary physics, something you can Google or wiki..Where is the business element in the piece?
Basically how can you apply this to real life and with a sound business case.......

Great Job
Posted by Khaldoon, Doha, Qatar on Sunday 4 October 2009 at 20:31 UAE time


Very interesting article, it reflects the deep knowledge of the author. well presented and organized, direct to the point, focus and rich.

good Job Eng. Muath, wishing you success and prosperities.

The everlasting energy source
Posted by Youssef Nasser on Tuesday 13 January 2009 at 11:41 UAE time


I never knew that the system could "over-heat", and thus making it a challenge to have in the gulf region.
All buildings should definately consider using solar power as the best way to heat water as this article suggests. However, it shouldn't just stop at "Sun & Water", which might solve the "over-heat" issue by utilising the extras to other means.
Afterall, the Sun is the life bearing planet!

excellent article
Posted by Mohamed Ibrahim, sharjah, U.A.E on Tuesday 13 January 2009 at 10:26 UAE time


Very rich and will presented article which show a good knowledge in the subject, well done Engineer Muath Bni Hani

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