The Advantages of a Solar Pre-heat Configuration  Apricus AP 22 on flat roof. © Sustainable Living Projects. Preheating means solar gain is used effectively in the pattern of domestic hot water consumption - in the early morning and evening. As these two points of use are in non sunshine hours it is best to increase the total amount of hot water available ("the battery bank") so that the water with solar gain replaces the water in the booster tank in the evening. For example, 4 people shower in the evening and consume 200L of hot water. While this water may have been heated by the sun to a high temperature, it is used and thus finished. As there is no solar radiation at the time of the depletion the electric element has to then heat the water up by the full delta T of say 40 degrees. This requires large current draw as the element has to work long and hard to reheat the total volume of new water.
However, if 200L of solar hot water is placed in a preheat configuration then the water containing the solar gain flows into the booster tank as it is depleted. The booster tank thus receives 200L of hot water which it merely has to maintain at this temperature until the morning. This water is then emptied in the morning wash hour and after that the sun will assist the reheating of the fresh cold water in the system. It is more efficient for the feeder/booster tank to reheat to a greater delta T during the day owing to higher ambient temperature and heat transfer via conduction from the solar gain in the preheat tank Also given that the total capacity of available hot water has been doubled it is unlikely that all the hot water in the system will ever be depleted – a further advantage. For the same reasons (usage patterns) North West orientations can perform well - more heat gain is achieved in the afternoon which is closest to the main usage period and the period of protracted non solar gain - night. Sustainable Living Projects recommends a variation of this principle in the circulating ring configuration which we recommend for new build. A circulating ring allows hot water to be at the tap instantly as hot water is slowly pumped around a ring main loop. Having hot water on demand saves water and thus heat as cold water does not have to be run off in order to reach hot water. We recommend this configuration for new build applications. Large vertical solar tanks can achieve the same advantages via good water stratification. |
