This is a specific system that is used within Solar Hot Water systems. It is also referred to as an Active System. Whether the solar water heating system is deemed as a zero carbon or a low carbon system depends on how the water heating system is pumped and controlled. The use of electricity can reduce the carbon savings of a system by 10 to 20 % and is mainly used in low carbon system to circulate fluid through the collector.
Conventional low carbon designs, i.e. most of the systems in northern Europe, predominantly use a mains powered circulation pump for when the hot water tank is below the solar panels. When the storage tank is kept in the building a controller is necessary to distinguish when the water is hotter in the panels than in the tank, and a pump is necessary to transfer the fluid between the different parts. Some systems use electronic controllers these are beneficial as they allow for many different functions such as measuring the energy production, better safety functions, thermostatic and time-clock control of auxiliary heat, hot water circulation loops, display of error messages and alarms, remote display panels and remote or local data logging.
More modern carbon solar water heating systems are powered by solar electric (photovoltaic) pumps. These use a low flow diaphragm pump to move the water and a 5-20W PV panel which is turned in the same direction as the main solar panel. A diaphragm pump works through positive displacement, using a combination of the reciprocating action of a rubber, thermoplastic or teflon diaphragm and suitable non-return check valves to pump a fluid. When the volume of a chamber of the pump is increased (the diaphragm moving up), the pressure decreases, and fluid is drawn into the chamber. When the chamber pressure later increases in volume (the diaphragm moving down), the fluid previously drawn in is forced out. Finally, the diaphragm moving up once again draws fluid into the chamber, completing the cycle. The most common solar collector is the insulated glazed flat panel however other high efficiency panels are available, such as the evacuated tube collectors, as well as less expensive panels for other uses, such as polypropylene panels – for swimming pools. Evacuated collectors have several evacuated glass tubes which heat up solar absorbers and in turn the solar working fluid (water or an antifreeze mix). The vacuum within the evacuated tubes reduces heat losses, allowing them to reach considerably higher temperatures than most flat-plate collectors, because of this they can perform well in colder conditions.
This is not particularly advantageous in warmer climates unless really hot water is required. The high temperatures that can occur may require special system design to avoid or mitigate overheating conditions. A further advantage this design has over the flat-plate type is that the constant profile of the round tube means that the collector is always perpendicular to the sun's rays and therefore the energy absorbed is constant over the course of a day.
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Posted under Articles, House & Home, Renewable Energy
This post was written by Hannah Walker on July 16, 2008