Solupro Renewable Energy » Solar Water Heaters

During the past few years solar water heating has been improved significantly through research to provide a low maintenance solution mainly aimed at the residential market.

This concept has several benefits, but also disadvantages:

• The main benefit of SWH is a reduction in electricity cost that is normally incurred by electrical resistance heating of sanitary water as found in conventional geysers. The cost of water heating typically contributes between 30% and 50% of a household’s electricity cost. Solar water heaters have been shown to save 55%-70% of water heating costs [2]. It can therefore save a significant fraction of the typical household’s electrical bill of between 17% and 35%.
• Unfortunately the disadvantage of the concept is the high capital cost of acquiring and installing such a unit. Even with recently increased electricity costs, payback periods for typical solar water heater installations still range between 3 to 4.5 years.
• The SWH industries response to the general negativity regarding pricing has been to offer smaller and therefore less costly systems to clients. Even though these systems still save an appreciable amount of energy, the fraction of savings is impacted negatively for large homes where hot water consumption far exceeds the heating capacity of the smaller SWH. This implies that the rest of the hot water requirements will still be heated by the geyser electrical resistance element.

When considering the application of a solar water heater, or any renewable source for the purpose of heating water, it is of paramount importance to take into account the fact that any such system is reliant primarily on the limited availability of the natural source. Naturally, in the case of a solar water heater, the ideal is to heat an ample amount of hot water for the entire demand during solar hours. It is imperative to appreciate the fact that the available solar heated volume needs to be managed; for even though a solar geyser is fitted with an electrical element to serve as a “back-up” thermal source, the function of the element is limited to non-peak supply periods – i.e. 02:00am to 06:00am, and 14:00pm to 18:00pm – and therefore excess demand beyond these periods will not rely on electrical backup or “boost heating”. The primary purpose of a solar water heater is to heat the total required capacity by harnessing solar energy and storing such heated water for consumption which would typically be demanded at night or early morning. Should the capacity prove too little, necessitating the electrical element to heat water (during the set times) for excess demand, the saving is compromised and ultimately results in a prolonged amortization period.

The type, complexity, and size of a solar water heating system is mostly determined by:

• The primary need:
  • Volume required;
  • Demand pattern / consumption habit;
  • Purpose.
• The demographical location of the intended site of application, with relation to solar radiation and annual kW/h.a yield;
• Prevalent summer and winter climate conditions specific to the demographic;
• Variation in ambient temperature during the day-night cycle;
• Likelihood of frost occurrence or the ambient temperature reaching temperatures below 5ºC;
• The nature of the roof structure designated to house the solar water heater, in terms of:
  • Structural strength and assembly;
  • Inclination (pitch);
  • Azimuth orientation; and
  • Practical setting.
• The source, quality and/or purity of household water supply.

  The minimum requirements of the system are typically determined by the amount or temperature of hot water required during winter, when a system’s output and incoming water temperature are typically at their lowest. The maximum output of the system is determined by the need to prevent the water in the system from becoming too hot.

Solar Water Heater Types

There are predominantly two types of solar water heaters:

1. Direct

   The term ‘direct’ literally refers to the fact that water is heated directly, by means of water circulating from the storage tank into the solar collector panel, where solar energy directly raises the water temperature, heated water transfers back into the storage tank.In conclusion, potable water is contained in the loop joining the collector panel and storage tank.In theory, direct systems are not suited to frost prone areas owing to the evident fact that water is not frost resistant. In an area where frost is prone to occur or where the ambient temperature is likely to decrease below 5ºC, the risk exist that water contained inside the collector panel will freeze, ultimately damaging the collector panel. Though there are various means of anti-freezing controls and measures that are added to direct systems as auxiliary system components, few have proven to be completely fail proof.

2. Indirect

   The term ‘indirect’ literally refers to the fact that water is heated indirectly through the addition of –

   • A frost resistant heat transfer fluid (HFT), most commonly being a dilution of propylene glycol; and
   • A heat exchange manifold.

Potable water remains contained inside the storage tank, whilst heat transfer fluid circulates from the heat exchange manifold (which may present itself as a central coil or as a chamber encircling the storage tank) into the solar collector panel, where solar energy raises the temperature of HTF, and heated HTF transfers back into the heat exchange manifold.

In conclusion, potable water cannot be contained inside the loop joining the collector panels and the heat exchange manifold.