Building Construction, Renovation & Maintenance

Solar Hot Water Systems

Solar Water Heater

Solar Water Heaters Concept, Design and Types

In a time when electricity is becoming more and more expensive and we need to reduce our dependence on fossil fuels the use of the sun to heat our water makes a lot of sense. Solar water heaters or not expensive, they can reduce our electricity bills, relieve the pressure on our strained electricity supplies, they are more reliable and less trouble than gas water heaters and are good for the planet. Solar water heaters have, of course, been around for many years, they are a simple concept so let us look at how they work, different designs and their advantages and disadavantages.


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The Design Of A Solar Water heater

How does a solar water heater work? A conventional solar water heater consist of a solar collector panel and a water tank. Water is supplied to the heater from the house cold water system and so is under pressure, this carries the water through the heater and on into the household hot water system.

The solar collector, a shallow box usually 2 metres high and 1 metre wide and about 8cms deep, is mounted on a roof facing the sun. The collector is usually connected to a horizontal water tank which lies across the top of the collector. Water from the tank runs down a pipe to the bottom of the collector panel, from here it flows up through a copper coil of pipe which snakes its way up inside the collector panel collecting heat from the sun as it goes. At the top of the collector a return pipe carries the now heated water back into the water tank.

It is a very simply system which depends on one fundamental principle - hot water rises while cold water sinks. The pipe taking the water from the tank to the collector must come from the bottom of the tank where the water is coolest. From here the cold water sinks, flowing down the pipe to the bottom of the collector. Here it enters the collector and starts to heat up rising up the pipe in the collector panel and back to the tank. The pipe feeds the heated water into the top of the tank where the water is hottest.

This may seem obvious but I have seen several solar water heaters that were either installed or repaired incorrectly so they did not work. In one case the tank was mounted below the collector!

A system that depends solely on water temperature differences for circulation is called a "passive" system. Sometimes the water tank is mounted away from, even below, the collector (you may want to hide it in the roof) in which case a small pump is used to circulate the water and this is called an "active" system.

Are Solar Water Heaters Effective?

In a word Yes, very. Many people think that they won't work on cloudy days but this is not true. The heating comes from the infra red rays in sunlight an infra red rays are able to penetrate clouds so while clouds can reduce their effectiveness you will still get a tank of hot water even on cloudy days.

Insulation

Insulation is very important to maximise heat collection and retention. The collector panel has a glass cover allowing the suns rays to enter but preventing heat loss to the surrounding air. The water tank is insulated with a thick covering of cellular polyurethane foam to prevent heat loss particularly during the night.

Improvements Over The Years

This simple concept has had a number of modifications over the years:

  • A thin copper collector plate is soldered to the snake of pipe inside the collector to increase the amount of heat being collected.
  • Insulation is added beneath the collector plate to reduce heat loss from the underside of the plate.
  • Special glass formulations have been developed to improve the transmission of the sun's heat into the collector panel.
  • In colder climates the design is modified to separate the heat absorption system from the water to be heated so that antifreeze can be used to prevent freezing of the absorption fluid.
  • A pressure relief valve is fitted to release pressure should the heater overheat.
  • Electric heater elements are usually installed so that if the water is not hot enough it can be "topped up" using electrical heating.
  • These days the copper plate and pipe have been replaced by much lower cost aluminium.

Water Temperature - How Hot?

Solar hot water heaters usually heat to 60 degrees centigrade, any hotter than this is considered dangerous with a risk of scalding an unsuspecting bather. In some countries there is a legal maximum termperature that heaters are allowed to reach.

Size of Solar Water Heaters

The standard storage capacity of solar water heaters tends to be 160 litres to 180 litres, 300 litres and larger are also available when ordered. Being larger than the standard 100 litres for standard electric heaters, solar heaters allow for heat loss and night time useage, 180 litres is ample for an average household.

The Water Tank

These days solar water heater tanks are usually made from ordinary mild steel coated with vitreous enamel coating (the manufacturers call it glass lining). The tank has a sacrificial anode to prevent rusting and the anodes are designed to last about 8 years. Once the anode is gone the tank will probably rust out. A company from Perth used to manufacture heaters with stainless steel tanks, sadly they were bought out by one of the larger Australian manufacturers and the stainless steel was replaced by enamelled steel.

Different Designs

Over the years a number of variations of this conventional design have been tried. One of interest used high intensity parabolic reflectors which tracked the sun focussing its rays on a single straight pipe. These were very effective (in fact too effective and had to be designed to turn away from the sun if they got too hot) but were too complicated and could not compete with the simplicity of the standard design.

Evacuated Glass Tube Solar Collectors

In more recent years a new development has been in the use of an array of evacuated tubes to replace the solar collector panel. The glass tubes are manufactured as two tubes one inside the other with an evacuated gap in between. A row of these tubes is pushed into the underside of the horizontal water tank. The water in the tank is free to run down inside the inner glass tube where it is heated by the sun and the hot water passes back up the tube and into the tank. The vacuum in the cavity between the glass tubes allows the sun's rays to pass into the tube while insulating the water from heat loss.

These heaters are popular and are usually markedly cheaper than the more conventional design. They have an advantage in that the large diameter glass tubes (5 cms) do not scale up with hard water in the way that the pipes in standard collector panels do. A downside is that, because of the way they are designed, these heaters cannot take pressurised water so a small pump is needed to pull the water from the tank to your hot water system.

Solar water heaters, of course, have a basic drawback in that once the sun sets no more heat (funny that) and most people want a hot shower in the morning. The larger tank capacity addresses this issue and users report that solar water heaters are highly effective.

Advantages and Disadvantages of Solar Water Heaters

So to summarise:

Advantages

  • They give you free hot water (once installed).
  • You still have hot water when there are power blackouts.
  • Usually simple systems that need very little maintenance.
  • Usually have an electric element backup.

Disadvantages

  • It is vitally important to select a good design.
  • Some are very expensive so may not manage to pay for themselves.
  • They only work during the day.

Low take up rates of solar water heaters

In spite of this takeup of solar heater has been disappointing but is probably due to the fact that the better quality imported units (Rheem and Solarhart) are very expensive (Rp40 to Rp60 million), a substantial investment. Cheaper units can cost anything from Rp11 to Rp20 million. A solar water heater will save between 2 and 3 million rupiah a year in water heating costs.


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Copyright © Phil Wilson February 2010
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