Building Construction, Renovation, Maintenance & Advice

Solar Hot Water System Design

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"The Prunes Of Mother Earth"

To the East of Bali lies the Lombok Strait which, apart from being rather wet and very deep, is where the famous Wallace Line lies. Quite a while ago now Alfred Russel Wallace suggested a line of biological separation existed when he noticed that species to the North and West of the line were very different from species to the East and South. To the West animals tended to walk around on four legs of equal size while to the East and South they tended to hop with little legs at the front and great big legs in the rear. After considerable thought this biological oddity was down to the intense heat of the sun which makes the ground so hot that the wildlife has a tendency to hop around on their back legs saying things like "oh dear that's hot" or "eek my feet are burning."

He noticed that the people were also different. They had golden brown, very wrinkly skins and had obviously followed a different evolutionary path. He postulated that they had evolved from a rare species of dried plum and referred to them as the Prune people of the Great South Land.

This belief survived for years but things become unravelled when a young Prune, Kylie, refused to go out one day and wanted to stay at home watching Neighbours. After several years on the sofa she lost her browness, the wrinkles smoothed and she started to look rather like an ordinary person. The mystery of the Prune people was finally revealed.

It was discovered that they were, in fact, one of the great sun worshipping peoples of the world along with the Egyptians and the Incas. Research revealed that they would spend many hours each day down on the beaches worshipping the sun and this made their skins brown and wrinkly. They had developed a whole class structure based on skin texture and colour. The high priests and priestesses were the brownest and wrinkliest of all, the result of many hours of sun worship. With a religion based on a form of Paganism these people became known as the Prunes of Mother Earth, more commonly known as POMEs.

They have been the subject of extensive research, people have been studying old prunes, and while the archeologists were digging up Ming vases in China they were turning up suntan lotion bottles in The Great South Land. If ever you get the opportunity visit the National Museum where there is a particularly impressive collection of zinc cream jars from the Bjelke Peterson period.

Perhaps it was the wrinkly nature of their skins which tended to trap dirt but the POMEs became obsessed with washing and would become quite derogatory about people with ordinary washing habits. At one stage an international incident was created when a visiting dignitary, the Queen of England no less, was greeted by the head Prune with the words "what's wrong, ya scared of a bit of soap and water missus."

The decline of the Prunes started when it was discovered that so much washing gave them skin cancers but the final death knell came when the United Nations declared that it was wrong to discriminate against people on the basis of the wrinkliness or cleanliness of their skin. Wrinkles were no longer valued, the Prunes died out but their legacy lives on. Through their worship of the sun and their desire for cleanliness the Prunes used their technological genius to weld up a few bits of old pipe and make solar water heaters many of which survive to this day.

Solar Water Heaters Make A Lot Of Sense

In a time when we need to reduce our dependence on fossil fuels solar energy does make a lot of sense. It is good for the planet, it can reduce our electricity bills and relieve the pressure on our straining electricity supplies. Solar water heaters have, of course, been around for many years, they are a simple concept so let us look at how they work.

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.


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 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.

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. Solar water heaters tend to rust out and in tests I was involved in many years ago this was usually after about 8 years only just after they had paid for themselves.

If manufacturers of standard solar water heaters were to take a more ethical approach in their manufacturing design and methods solar water heaters could last much longer and probably more people would by them.

Anyone for boiled prunes?

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Copyright © Phil Wilson 2010
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9 December 2017 Copyright © Mr Fixit,
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