Optimisation of fluid-flow in a flat plate solar water heater

At higher latitudes, where there is a lower annual solar availability, solar water heating can still make a valuable contribution towards water heating energy demand. Whereas systems at lower latitudes may use natural circulation to promote flow through the collector, at higher latitudes the collector flow rate is generally regulated by a temperature differential controller (TDC) operating a mains supplied circulation pump. With the pump switched on, the flow rate is constant. In addition, the requirement of freeze protection at higher latitudes increases system complexity, and hence cost, and can also reduce efficiency. A new freeze-tolerant solar domestic hot water system (SDHWS) is introduced. This system, termed freeze tolerant-photovoltaic (FT-PV), through virtue of using a collector that tolerates freezing, requires neither anti-freeze nor a drain back facility to afford frost protection. In contrast to the above mentioned TDC switched circulation pump, the FT-PV system uses a small solar electric, photovoltaic (PV), module to drive a pump. As PV output varies with irradiance, so the system will operate with a low and variable flow rate. In addition, as water can be circulated directly through the collector, the collector return can be fed directly into the storage tank, with the low flow rate promoting thermal stratification therein. For the FT-PV system storage tank, a model for defining the internal axial temperature profile in relation to that of the external wall during a cooling period is presented and validated. Furthermore, a model for simulating tank heat loss and destratification has been developed and, again, validated. A new method for determining the collector efficiency characteristic is presented. This method is validated through a comparison with empirically defined values, and also through application in software models that have been developed for simulating the collector outlet temperature. The optimum collector flow rate profile is defined. Presently, there are no "off the shelf' components available that will provide the above desired flow rate profile. Through considering the optimum operating condition, the present work details the selection of the most appropriate PV module/pump pairing for the present application. Through an experimental and analytical study of the FT-PV systems component characteristics and interactions, the effect of two simple methods of control on flow rate, and hence collector outlet temperature, is presented. While the flow rate profile is not ideal, the method of physical shading is seen to provide a collector outlet temperature (COT) profile that is closest to optimum. With this control strategy, the performance of the optimised FT-PV system is compared with that of a standard anti-freeze based system. For an identical load demand, irrespective of the method of control applied to the auxiliary heat source, the FT-PV system is shown to require less auxiliary input, representing a greater saving. The FT-PV system collector outlet temperature profile, and hence the system performance is, however, highly sensitive to both the position of, and method of control applied to, the auxiliary heater. The performance of both the FT-PV and reference systems may be improved through the intelligent control of the auxiliary heater. The above-mentioned method of analysis may be used to define the optimum degree of control for the larger commercial FT-PV system.