Pre-heat Storage of Solar Water Heating Systems

In pre-heat storage, the energy is transmitted from the primary system. This is integrated with any other heat sources situated further ‘downstream’. These additional heat sources top-up any temperature shortfall from the solar-generated pre-heat water store before final distribution. This refers to heat sources intended to contribute to average daily demand, it does not mean manually-controlled sources used for occasional boost beyond mean daily demand or automatic sterilization routines. A suitable pre-heat solar the store will maximize the safe displacement of fossil fuel and the most significant variable in a store’s performance is the volume, which should be sized for household use or the absorber area

If an absorber is of the highest efficiency then, in order to achieve the maximum collector performance, higher rates of solar storage may be required – up to 100 liters per square meter of the absorber for some collector types. Such very large stores can be further improved by using advanced stratification or temperature-layering techniques.

It is important that the ‘hot-top’ of a combined store is not purposely depleted during the day as comfort levels or bacterial-cleansing ability may be reduced. Due to the predominately low-temperature range of transfer, such heat exchangers need much larger surface areas than conventional exchangers. Their performance is considerably affected by choice of materials and shape. As a general rule, copper-finned exchangers can be designed with a lower surface area than a smooth bore steel type, but in all cases, the net heat transfer should be at least 40W/K per m2 of exchanger per net unit area of collector absorber.

The heat exchanger between a solar primary and a secondary system should not reduce the maximum collector efficiency by more than 10 percent under full circulation. A similar problem will occur where an electric immersion element is not sufficiently high enough or a secondary pump acts to circulate hot water downwards through the store. Ideally, the collector return loop connection is made at the coolest part of the system, possibly adjacent to the store’s cold feed. In a combined store, a physical division between different heat sources should be used in preference to a ‘time control’ strategy relying purely on electronic devices.

Physical separation offers a more robust methSod of securing the essential solar pre-heat volume necessary for efficient solar collector performance. This standard allows the CO2 mitigation of a proposed system to be calculated and from this information, the most suitable store for a given solar primary system can be selected. A store for secondary water pre-heated by a solar water heater system should be tested and independently certified to or equivalent. Adequate labeling of the solar store is essential if correct commissioning, prediction of performance and future maintenance are to be ensured. This should be added to any other labeling required due to the presence of other heat sources. Solar store labeling should include Rating of each internal heat exchanger and description of the type. The maximum operating pressure of all primary heat exchangers, Stand-by heat loss rate, with allowance for variable solar temperatures.

Thermal stratification is the natural layering of buoyant warmer water over cold. Its effects are desirable in pre-heat storage in order to maintain effective heat transfer from the primary loop. For the same volume, the effect is most pronounced in tall, thin stores with an elevated height-width ratio. Stores in horizontal format have poor stratification and therefore are not recommended for solar storage. With the careful arrangement of exchange surfaces and choice of materials, the mixing sometimes caused by thermal eddies, cold feed inrush, and cylinder wall conduction can be reduced in stores of certain formats. These may use extra vertically stacked coils, tubes or internal separators to encourage the ‘chimney’ effect of buoyant warm water.

                                                                                                                           By Supreme Products