solar hot water heaters

July 3, 2007 at 9:39 pm (essays, mechanical engineering)

in my first semester back at tafe, doing mechanical engineering, i have been required to complete exactly one assignment. true to form, it was late. but that was ok. now that i’ve lost all my files i’m realising that there actually is a value to the work i’ve done, the research and compilation i’ve sweated over for countless semesters. value akin to that of a blog, in fact. so here is the assignment, in the blog. read it if you’re interested… but them’s the rules round here!

A solar hot water heater uses energy from the sun to heat water, thereby saving on non-renewable energy, greenhouse gas emissions and cost. There are many ways to build a solar hot water heater. The focus of this paper is on a domestic system that a non-specialist could adapt form their existing home hot water.

The main elements of a solar hot water heater are the collector and the tank, which are connected by insulated pipes. Other components, such as valves, pumps and booster heaters are optional, depending on choices and circumstances.

The solar collector (also called the absorber) is what differentiates solar systems from other hot water heaters. It consists of a metal sheet with channels, which in an open system either house water pipes or conduct water themselves. In a closed system, the channels or pipes conduct another liquid, generally with a lower freezing point, and the heat is transferred to the water protected below. The simplest method employs pipes, generally made of copper, and set into a sheet of aluminium, or copper to avoid the corrosion which occurs when different metals are in contact with each other.

To maximise the absorbed energy, all the metal is painted black, or ‘solar chrome’, a silvery selective surface, which is even better, as it absorbs more energy than it emits. This is all set in insulation to avoid wasting energy by heating the tray and roof. The transparent cover is usually a high impact glass. Low-iron ‘solar’ glass is even better, as it transmits near infrared energy better than less pure glasses.

The edges of the tray and cover must be well sealed to avoid water and other damage, with holes on the underside to release condensation, and strong brackets to attach it to the beams of the roof, beneath the tiles. The whole construction must also be able to withstand impact, abrasion, ultraviolet radiation and temperatures of at least a hundred degrees centigrade.

The collector needs to be approximately one square metre for every seventy-five litre of tank volume, So for a normal three hundred litre tank you should have four square metres of collector, more if there is shading, the glass is not kept clean, or it is not mounted at a suitable angle. It must face between 45 degrees either side of true North in the southern hemisphere, and be mounted at a specific angle related to the latitude of the location. In Sydney, at latitude 34 degrees, an angle of 34-39 degrees is the most efficient averaged over the entire year. However, increasing the angle to 44-49 degrees maximises efficiency in winter and reduces the problem of overheating in summer.

There are three main kinds of tanks. The most common commercially available solar hot water heater has a close-coupled tank, which is attached horizontally at the top of the collector, on the roof. This avoids the need for pumps with the thermosiphon effect, where the cold water is drawn down from the bottom of the storage tank, to be heated in the collector, after which it rises naturally back into the top of the tank. Installation, however, is be much more difficult than in models with separate tanks. A 300L tank exceeds 500kg, which would require a crane to lift it, and often reinforcement of the roof as well.

There are several advantages of using the house’s existing mains pressure tank, if one is available. It can be left at ground level, which makes installation, maintenance and replacement much easier, and it is already equipped with a mains pressure pump. Also, it will be an upright tank, which has better stratification than a close-coupled model, by allowing more distance between the heated and unheated water, which sit in layers in the same tank. Disadvantages include the increased distance of pipe and insulation required to connect the storage tank to the collectors, and a small cost in the running of the pump.

A third option is the low-pressure tank. This is the only type that is safe for a non-professional to create or adapt significantly. Low-pressure tanks can be made of a simple sheet of copper, but will also benefit from insulation. They are mounted in the ceiling to take advantage of the pressure of a head of water. If it can sit higher than the collector it can also use the thermosiphon effect. This system could still require roof reinforcement, and the lack of mains pressure is often undesirable.

All pressure tanks require temperature, pressure and vacuum relief valves. Low pressure systems can get away with vents. Temperature control, or tempering valves are also good at outlets such as basins and showers, but can be bypassed for washing machines and dishwashers, where scalding isn’t a risk. These can prevent boosters from kicking in, or mix in cold water when the temperature in the tank rises past set levels. Non-return valves are required on thermosiphon systems to prevent the water flowing backwards at night, and frost protection will also be required in frost prone areas.

Another component that is useful where winter sun isn’t always adequate is a booster heater. If a commercial mains pressure tank is used, a gas or electric heater will probably be already installed inside. This can be set to come on at night, only if a minimum temperature level has not been reached.

If attention is paid to all aspects, it is possible in summer to supply a house its total hot water requirement without reliance on non-solar energy. This can reduce the average power bill by a third. In a Sydney winter boosting will be required, but energy consumption will be drastically reduced.

References:

Fact Sheet 4.3 Solar Hot Water, from the Home Technical Manual: Design for Lifestyle and the Future http://www.greenhouse.gov.au/yourhome/technical/fs43.htm

Introduction to Renewable Energy Technologies Resource Book, January 2003 Edition. Published by the Renewable Energy Centre, Brisbane and North Point Institute of TAFE

Australian Standards AS 3500.4.2:1997 and AS 2712: 1993

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