Solar water heating systemsThere are two broad types of solar water heating systems:
1.Open circuit or direct systems circulate water directly from the collectors to the tank/cylinder. These types of systems are not normally freeze-resistant, nor are they resistant to hard water. Our climate is not 100%predictable, but if you do not live on the coastal band or if you live in an area where you get frost, then you should purchase an indirect system or a freeze-resistant direct system. The SABS performs a freeze test on systems. Due to technological advances there are direct systems that can work in areas that get frost and still will not freeze. These are listed as direct frost resistant systems. You will need to check these details both on the list of participating suppliers and with the installers themselves. PLEASE do not install a non-freeze resistant system if you know you get frost. The system may only need one frost occurrence to make the system fail completely. Note: the rebate will not be given to direct systems that are installed on the escarpment. Remember – a wind chill factor can significantly drop the temperature of an already frost-covered system – so if you usually only get only a small amount of frost, this does not mean the system will not freeze. Areas where the temperature drops below 4ºC should be considered frost areas. The water quality in your area will have an affect your systems performance and longevity. Open circuit/direct systems should not be used in areas with ‘hard’ water (water with high calcium/lime content). Look inside your kettle to check if your area has ‘hard’ water and if you find traces of scale either on the heating element or on the inside of the kettle this indicates lime in the water supply. If your soap does not lather well this could also be a sign of a high chemical content in your water. If your water is ‘soft’ – does not have a high calcium/lime content (e.g. treated water or water drawn from a borehole or river supply), then it should be suitable. The agent in your area should be able to advise you further or contact your local municipal water provider for further information. 2.Closed-circuit or indirect systems Contain a fluid (normally food-grade glycol) that is held inside a piping system and solar collector panel where it is heated by the sun. This fluid is then circulated through the piping system to the hot water cylinder/tank where the heat from the fluid is transferred to the water. The cooled fluid is then returned to the collector for reheating. Closed circuit / indirect systems can be used in all areas – including those that get frost (temperatures below 4ºC ) or have water with a high mineral content (‘hard’ water), as they are not affected by these circumstances. Closed-circuit systems could need to have the heat transfer fluid checked and replenished every few years. Your agent will discuss this maintenance with you. Water can be moved around the solar heating system in one of two ways 3.Forced circulation/pumped system: This system works with a small circulation pump, controlled by a temperature differential controller, which circulates the heat transfer fluid from the collectors to the heat exchanger in the solar geyser. Collectors are mounted outside on the roof, with the solar geyser installed in a space provided. Pumped systems can be more expensive than natural convection systems due to the added technology, particularly if anti-freeze measures are required. However, they can be the right solution if your roof structure and support beams (trusses) cannot support the weight of the solar water tank. If the pumps performance is maximised, then your system’s operation and savings will both be maximised. A temperature differential controller is used to control the pump: a.)This differential controller switches on the pump when the correct temperature is detected at the hot water outlet (the controller will only allow water to be released once the water in the collector is higher than that stored in the storage vessel/water tank at any given time). b.)The differential controller therefore also switches the pump off when the fluid in the collectors is within 5oC of the return temperature’s cold probe from the solar geyser. The pumps in forced circulation systems have low power requirements and direct current (DC) pumps powered by small photovoltaic (PV) panels which convert sunlight into DC electricity are sometimes used. Care needs to be taken here as photovoltaic technology works on light received and not heat. Should a photovoltaic pump be used, the pump should not be controlled using panel voltage. A differential temperature controller must be included. This will ensure that the system is not cooled at certain times in the late afternoon or when it is cloudy – and when there is sufficient light to run the pump but insufficient solar radiation to heat the water. The advantage of this system means that if you are concerned about aesthetics, you will not need to place the system on your roof. However, the disadvantage to this type of system is that unless you use photovoltaic technology to power the pump, if there is no electricity, you will not have hot water. 4.Natural convection/thermosyphon system Thermosyphon systems work with the natural laws of circulation – hot water rises and cold water sinks. As the water in the collector is heated, it rises naturally into the geyser, while the cooler, heavier water in the geyser flows down to the bottom of the collector, causing circulation through the system. To achieve circulation during the day and to limit reverse circulation at night, the water tank/cylinder must be above the collector. Thermosyphon systems can be freeze-resistant. Anti-freeze valves and built-in freeze resistance can be used by placing a closed circuit between the collector and the geyser; this means a heat transfer fluid will be used. Natural convection systems are mainly used in houses where the water tank can be installed at a higher level than the collector, either inside or on top of the roof – as long as the tank remains above the collector. This type of system works best if installed in houses with roofs pitched higher than 15º. Roofs with a lower pitch will still work, but overall effectiveness will be decreased – although additional inclined support frames can be supplied. Typical thermosyphon installations use a close-couple configuration. This means that the solar water tank and the collector will be in close proximity to each other, with the solar tank higher than the collector. Thermosyphon systems can be either close-couple or split systems. a.)In a close-couple system, the solar geyser and collector are mounted externally and close together. In thermosyphon circulation a close-couple system is very reliable if installed properly. b.)In a split thermosyphon system the solar geyser and collector are separated, with the geyser usually installed in the roof. The geyser must be above the collector with connecting pipes rising smoothly. The pipes should not level out or dip at any point, as this could cause poor circulation. Advantages: You do not need any power source to get hot water Systems are reliable and simple. You will maximise your energy savings. If the system bursts and the tank is on the roof your insurance, consequential damage and costs are likely to be less. 5.Pre-heat/pre-feed system configuration A pre-heat system (sometimes referred to as pre-feed) is when the solar-heated water from your solar water heating system is fed into your existing geyser. It is therefore connected in conjunction with your electrical geyser, as opposed to maximising only the solar water heater system. Most configurations of solar water heating systems can be used as pre-heaters. Standard domestic SABS electric geysers do not have the same insulating capabilities as external solar water tanks, so a lot of the heat energy can be lost when these geysers store the hot water. (The use of a geyser blanket can improve a conventional electric geyser’s efficiency). This configuration therefore works best where the energy needs are mostly during the day or early evening, as the hot water in the geyser is not circulated. Pre-heat systems can be good for increasing the amount of hot water available at the time of use. The major benefit of a pre-heat/feed system is that you increase the amount of stored hot water available. These systems still need to be sized and used adequately or else you could find yourself without hot water. Furthermore, the element in a pre-feed must be removed from the solar tank and only installed in the electric geyser. When the electric geyser bursts you need to drain the solar tank in order to place an element in the solar tank. In addition to this, if the solar tank is not adequately sized you could find that the system does not cater for your hot water requirements and that you need to heat the system electrically |
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