Solar Water Heaters

st150panelIn Southern Africa we are blessed with more warm sunny days than many other places on earth. Unfortunately we tend to take the sun for granted and have not made much use of the abundant energy it offers. The average amount of solar radiation per annum we receive as a country/region is among  the highest in the world.

In most households, 30-50% of the monthly electricity bill is used to heat water. Solar powered water heaters are thus an ideal source of alternative energy, because this energy is essentially free. Solar radiation can be extremely effective for heating water.

In essence, solar heaters work on a very a simple principle. Solar radiation is collected and then that energy is used to heat water. Simplistically, radiation is absorbed by black surfaces and reflected by shiny surfaces. Solar water heaters thus typically have some form of black collection surface which absorbs solar radiation and then transfers that heat to water. The problem that all solar water heaters need to solve is that heating energy is only available during the day between about 10 am and 4 pm.

Most people are used to electric geysers which can heat the water at any time of the day, so it doesn’t really matter what your daily hot water usage pattern is in those cases. However to get the best benefits from a solar water heating system your daily hot water usage pattern becomes very relevant.

The basic problem is this. Solar water heating systems are very good at providing a tank full of hot water at the end of the day because the sun has been heating the water in the storage tank for about five hours by then. But once the sun goes down, there is no more heat available until about 10am the next day. So if you need to heat water for the morning shower, after using all the heated water during the night, the only option is to revert back to using an electrical element for heating because the suns’ energy is not available at night.

On the other hand, if your storage tank is big enough, then it may be possible to still have enough hot water left in the morning, that was heated the day before by the sun.

Electric geysers tend to be sized by various conventions which result in say a 200 litre tank being considered suitable for a family of four. However one must remember that electric elements can heat the water from cold within about two hours. So an electric geyser doesn’t need to store as much water as the equivalent solar water system. To be self sufficient, a solar system has to store enough water to last right through the night until the sun is available for heating the next day. Typically one needs at least twice the storage capacity for a solar water heating system, if one wants to gain maximum benefit from the suns energy.

Many people use an existing geyser as the storage tank for a retrofitted solar water heating system. In those cases, only half the water required every 24 hours can usually be heated by the sun (simply because the tank is probably too small to store all the hot water needed each day), and so the rest has to be heated using conventional electric elements after the tank is emptied in the evening. So on average one can expect about a 50% saving in those cases.

If one installs a larger solar system using a much larger tank, one has to increase the size of the solar heating panels accordingly – all of which has a consequent effect on the cost of the system. One can then potentially use the sun to heat all the water needed each day.

However there are still a few issues to consider with a “full sized” solar system.

Hot water has the very beneficial property of separating from cold water when both are static in the same tank. This simple property of hot water allows you to take a long hot shower without it suddenly becoming cold after just a few minutes. As you use hot water during the shower, cold water is flowing into the bottom of the geyser and taking the place of the hot water used. The hot and cold water remain separate without mixing if the geyser is designed properly. If the hot and cold water mixed, you would experience a sudden drop in water temperature after using just a small amount of the hot water stored in the geyser.

Thus after the evening’s ablutions, the top half of the storage tank of a large solar water system will probably be full of hot water, and the bottom half full of cold water. However, as time goes by, heat transfer takes place between the hot water and the cold water. So not only is the hot water losing heat through the walls of the geyser, but it is also losing heat to the cold water co-existing in the tank. This heat transfer between hot and cold water doesn’t occur as rapidly as one might think, but by morning, the hot water in the top of the tank will have cooled down by at least 8-10 degrees. The consequence of this is that in solar water heating systems that are large enough to provide all the hot water required each day, one still has to accept the morning shower will potentially be colder than that of the evening shower – unless electricity was used to “boost” the temperature during the night, or two or more tanks are used in series which adds to the cost.

So even in large solar systems one will probably be using some electricity to heat water every day.

Collector Types

The function of the solar collector is to “collect” the radiant heat from the sun and transfer that to water in some way. All types of solar water collectors have the following main attributes:

  • A treated surface that absorbs incoming solar radiation – the surface treatment can be simple matt black paint or more sophisticated films applied to the surface that are specifically designed to enhance the ability of that surface to absorb solar radiation,
  • Water that is in thermal contact with the heated absorbing surface,
  • A layer of glass positioned in front of the absorbing surface. Glass has the useful property of “trapping” solar energy because it is relatively transparent to incoming solar radiation and thus allows it to pass through freely. But once that radiation is absorbed by the heated surface, the resultant thermal radiation re-released by the heated surface cannot pass through the glass as freely so the energy is effectively trapped behind the glass layer.
  • The heated surface and circulating water are insulated from the environment to ensure that heat gained is not just lost back into the environment.

There are two main types of solar collectors currently available on the market with one or two sub-variations thereof.

Flsolarflatpanel_smat plate collectors are the simplest types of collector. As their name implies, a simple flat plate is attached to an array of water tubes, arranged in some sort of insulated box with a glass front. Flat plate collectors are relatively easy to manufacture, and do not require very expensive materials. Thus flat plate collectors are relatively cheap. Given Southern Africa’s high average solar radiation levels, they are more than adequate for heating water in our climate.

vactube_smVacuum tube collectors are more efficient, sophisticated collectors. They usually consist of an array of glass tubes containing a thin sheet of copper, aluminium or treated glass with a special surface coating that maximises its solar radiation absorption ability. All the air is withdrawn from the tube to form a vacuum. The vacuum ensures that the absorption surface is very well insulated, so very little heat is lost.  Usually water is circulated through a small pipe running down the centre of the tube. The water pipe is thermally coupled to the absorption surface so that heat is transferred from the absorption surface into water circulating in the tube. In very sophisticated variations of the vacuum tube collector, a heat pipe is used to transfer the heat gained to water. Vacuum tubes were developed for climates in which the average annual solar radiation isn’t as high as we enjoy here in Southern Africa. Consequently the additional sophistication and expense of vacuum tube systems are not usually necessary here. In fact, they can lead to over-heating problems in which the water boils if one is not careful!

The size of the collector is matched to the storage tank size so that it can heat all that water within the 5 hours or so of effective heat available from the sun each day.

Solar collectors need to conform to SABS standards that ensure, among other things, that the glass used can withstand South African hail (many imported collectors have glass that’s too thin/weak). Please ensure that the collector you may use has passed the relevant SABS tests.

“Low Pressure” Systems

In most urban homes the local council supplies water at quite a high pressure (usually about 4-6 bar). This enables water to flow up pipes to the second floor as well as flow out of showers and hose pipes satisfactorily. Consequently your geyser, and all household pipe work needs to be able to withstand that pressure safely without bursting. All South African made geysers need to pass strict SABS tests in this regard.

However some of the cheaper solar tanks/systems imported from countries that don’t have as strict standards with regard to hot water systems cannot safely withstand our household water pressure. These systems have come to be known as “low pressure solar systems”. lowpress_smUsually they take the form of a water tank connected directly to a set of vacuum tubes with a much smaller feeder tank on the top of that. The feeder tank usually is open to the atmosphere and contains a float valve (similar to that in your toilet cistern) that enables the high pressure water from the household mains to flow into the main tank without pressurising it.

Low pressure solar systems are very cheap, but have some significant draw backs:

  • The hot water they supply cannot be fed straight back into the existing plumbing system of the house because that is all pressurised to 4-6bar, whilst the low pressure system is effectively at atmospheric pressure.
  • Low pressure hot water will not flow uphill, only downhill, so the low pressure tank needs to be at the highest point in the house.
  • Trying to mix low pressure hot water with high pressure cold water in a mixer tap can be problematic.
  • Obtaining a satisfying shower from a low pressure hot water system is going to be difficult.

For this reason, many low pressure systems are supplied with a powerful pump that is intended to try and increase the pressure of the hot water supplied back up to normal mains pressure. These obviously use electricity so the overall energy efficiency of the system is reduced. In addition pumps can be noisy and need periodic maintenance.

Low pressure solar water heating systems are ideally suited to rural applications such as bush lodges or rural homes where there is no high pressure water supplied by the local council.

“Direct” vs “Indirect Systems”

The simplest way of heating the water in your geyser using a solar water heating panel is to circulate that water through a solar collector panel during the day. This method is called the “direct” method because the actual hot water you eventually use was heated directly by the solar collector.

However, in areas where the temperature can drop below or close to freezing at night the direct method has a drawback. The water in the collector can freeze in those circumstances. Usually the pipes used in the collector are copper. Copper contracts when it freezes whilst water expands as it freezes – the consequence can be a burst pipe, either within the collector or the external pipe work leading to/from it.

One of the ways to avoid this problem is to adopt the “indirect” heating method. In this design the water circulating through the collector panel is contained in a closed loop, it does not mix with the water you will eventually use. The normal way this is achieved is via a specialised “solar geyser” which has separate compartments or internal heat exchangers that keep the two types of water separate while allowing heat to transfer. In this way the water in the tank is heated “indirectly” by the circulating water in the closed loop.

This arrangement allows one to add antifreeze (similar, but not exactly the same, as what you put in your car’s radiator) to the water circulating in the closed loop so that freezing of that water is prevented.

There are other ways of avoiding frost damage in direct systems, but the indirect method is obviously a good solution to the problem. The additional heat exchangers and specialised geysers required add to the cost of the system though.

One of the ways frost/freezing damage can be avoided on direct systems is via an intelligent controller that activates the circulation pump periodically when the temperature of the collector panel approaches, or drops below, freezing. This circulates warm water from the geyser through the panel for a few minutes every so often to stop any ice from forming.

An interesting point in indirect systems is that the water circulating in the closed loop is no longer pressurised, as it is separated from the pressurised water in the main compartment, so a low pressure collector can be used in that case, the tank however still has to be able to withstand mains pressure.

Thermosyphon vs Active Systems

Hot water naturally rises above cold water. If one places the storage tank above the solar collector panel and arranges the pipe work carefully, as the water is heated by the collector, it will flow into the storage tank on its own. Cold water will simultaneously flow out of the bottom of the tank into the collector. This process is called thermosyphon. In order to work, it needs careful arrangement of the tank in relation to the collector. But once working, no further intervention is required for the process to continue on its own whenever the collector heats up. Thermosyphon systems are usually more reliable because they don’t need additional pumps and electronic controllers to circulate the water. Some thermosyphon systems are “close-coupled” where the tank is joined directly to the collector.

Active Systems

In situations where the storage tank is inside the roof, such as when a conventional geyser is used as the storage tank, then it becomes more difficult to position the tank/geyser high enough above the collector to enable thermosyphon circulation. In those cases, where the collector is installed above the tank, a pump and electronic controller (that controls the pump) are used. This is called an “active” system. The electronic controller is the “brains” of an active system. Typically the controller uses water temperature sensors in the collector panel as well as the storage tank. When the collector panel temperature rises above the tank temperature, the controller switches on the circulation pump. When the panel temperature falls below the tank temperature, the pump is switched off. Clearly active systems require a small amount of electricity to power the small circulation pump as well as power for the electronic controller. Sometimes this electricity is supplied by a small photo voltaic solar panel and battery, and sometimes normal mains electricity is used to power active systems. The amount of electricity required to run an active system is very little, no more than a normal light bulb in most cases.

More sophisticated electronic controllers also perform other functions such as:

  • Allow you to set time periods in which the electric element is not active. This stops the electric element switching on during the day when the collector panel is heating the water for example. During periods when the electric element is active, the controller may also control what temperature the elements heat the water to.
  • They detect when the panel temperature is approaching zero and then circulate water from the tank through the panel to avoid ice formation.

Retrofitting to Existing Geysers

Solar systems can either come complete with their own specialised storage tanks, or alternatively some systems are designed to use the existing conventional geyser as the storage tank, these are often termed “Retrofit” systems.

Retrofit designs are usually “direct” systems in which the water from the geyser is heated directly by the collector panel. However, there are some retrofit systems that incorporate a special heat exchanger that is fitted into the conventional geyser, in which case water is heated indirectly by a mixture of water/antifreeze circulating through the collector panel and the heat exchanger in a closed loop.

Retrofit systems are usually cheaper because they don’t include the cost of an additional storage tank.

Retrofit systems are often more discreet, because the geyser remains where it is, inside the roof, and all that is visible on the roof is the collector panel.

Operation

Operating a solar water heating system is relatively straightforward. Once installed and working properly, it should produce satisfyingly hot water once a day. There a number of characteristics of solar water heating systems to be aware of though.

Firstly, the sun is only available as a heating source from about 10am to 4pm, so that is when water will be heated. It will start getting hot at about ten and should reach full temperature at some time before about 4pm. If your water usage patterns suite that routine then solar water heating will be perfect for you. If not, you will probably still have to use some electricity to heat water at other times.

You can’t switch off the sun in the same way that you can switch off an electric element when the water is hot enough. The sun will keep heating the water in the collector regardless of its current temperature. Collector panels are designed to match the storage tank size so that they are not only big enough to heat the water in the tank within the time that the sun is effective, but they also must not be too big so that the water gets too hot. Boiling water creates steam. Steam creates additional pressure in the system, the hotter the steam gets, the more the pressure increases. Steam thus stores energy in the same way compressed air does. Water is incompressible so pressurised water does not store energy. If the pressure in the system increases to the point that it exceeds the strength of the geyser and the safety valve fails to open, an explosion will occur that will literally flatten your house! Clearly safety valves are EXTREMELY important in solar water heating systems.

Your water usage pattern also becomes relevant in this regard because if you don’t use hot water for an extended period then the maximum temperature of the water in the system can steadily climb each day as the sun adds more energy to the system than is being lost via heat loss through the pipe work etc. It is thus extremely important that the collector panel is correctly matched to the size of the storage tank for South African conditions, and that an approved T/P valve (temperature and pressure safety valve) is installed. It is also desirable that the control systems can deal with extended periods of zero hot water withdrawal – for example, some will circulate the hot water through the collector panel at night to cool it down.

For this reason it is advisable to cover the collector panel if you are not going to be using hot water for an extended period, to stop the sun heating the water when it isn’t needed.

Going Off Grid

If you intend to rely on solar energy exclusively for hot water (ie dispense with any form of electrical heating system), then you will need a fairly large, very well insulated tank to store the heated water. This would be far larger than the appropriate traditional electrical geyser size for your family. The larger tank provides sufficient buffer to compensate for the fact that heating only occurs during the day, and also may reduce the negative affect of overcast days, so that hot water is constantly available when required (although it may not always be piping hot). Studies conducted in the US by F.A Brookes in 1936 suggest a tank size of 130 liters per person in the family (this includes all daily requirements such as ablutions, dishwashing etc.). Using Brooke’s figures, a 4 person family would require a tank size of 520 liters. Clearly this is represents a substantial system, with consequent cost implications! Consult your supplier/installer for the most practical, cost effective options.

Installation

A solar water heating collector should be aligned so that it is tilted upwards from horizontal at about the same angle as the angle of latitude of its position on earth. For example, when installing a collector in Johannesburg, one should tilt the collector at 26 degrees from the horizontal towards north.

Equally important is the collector’s micro location with respect to local horizons and shadows during the day/year. Obviously if it’s stuck behind the chimney or under a tree for example, and thus only gets direct sun during part of the day, the collector will never perform the way it should. The north face of the roof is a good option in most cases.

Some people may be concerned with aesthetics. Different models have different levels of aesthetic impact. You will need to discuss this with your supplier/installer. Bare in mind that the collector could be installed at the rear of the house by mounting it on a suitably raised frame on the south facing roof or on an outbuilding’s roof.

Insulation of all hot water pipes is also important, particularly if the distance between the collector and the tank is long. Insulation of external pipe work is also important with regard to avoiding frost damage in regions where temperatures get close to or below freezing at night.

Water is a very heavy substance. One litre has a mass of roughly 1 kg, so the entire system can quickly reach quite large weights (200kg or more). Clearly the tank and collector need to be properly mounted with respect to the roof structure and load bearing walls, and one must ensure the structure can bear that additional load.

Furthermore, ensure that the system is installed in compliance with all the applicable standards. So make sure your unit is installed by competent people who are aware of, and conform to, all the relevant regulations.

Alternatively, if you feel up to it, you can install the system your self.

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