The aim of this thesis was to investigate and challenge current thinking regarding the sensitivity of the thermal performance of active solar water heating systems to the control strategy employed, with particular attention to the effect of the control temperature differential settings between the solar absorber plate and the solar store at which the circulating pump is switched on and off (dTon and dToff). A mathematical analysis suggested that the performance should be more sensitive to the above parameters than is generally believed. The theoretical appraisal also suggested a relationship for the ratio dTon/dToff required for stable pump control. Measured data from a large-scale solar heating field trial were carefully correlated with the predictions of a computer model. The calibrated model was then used to carry out a sensitivity analysis into the effect of altering the control criteria. The results have shown that, contrary to current thinking, the long-term thermal performance of the system is significantly impaired by the use of a pump switch-on criterion (dTon) higher than 6°K, and that the sensitivity to this parameter increases with increasing dTon. The results have also revealed that the heat losses throughout the system are five or six times higher than theoretical calculations based on the insulation manufacturers' specifications predict. This is not an isolated result, but has been experienced on other monitored installations, and the implication is that the quoted figures for insulation performance (which are derived from tests under tightly controlled laboratory conditions) are extremely difficult to achieve in practice. It is argued that the above two observations are linked, and that a high switch-on criterion leads to significant amounts of collectable solar radiation being wasted as the collector absorber plate loses heat to the surrounding air without reaching a temperature sufficient to turn the circulating pump on. The effect of the control settings on pump switching stability was also investigated, and, whilst the point at which instability occurred did not agree precisely with the theoretical value, the general relationship between the ratio dTon/dToff and the number of pump switching cycles per year supported the mathematical hypothesis. The discrepancy was attributed to the difficulty of measuring the collector overall heat loss coefficient, U1, precisely. For the installation under study it was found that a value of dTon/dToff above 8 would ensure stable pump control. Interpreting optimum performance in terms of both annual solar energy output and pump switching stability, the combination of all the above results led to the general recommendation that a pump switch-on setting of 4-6°K with a switch-off setting of 0.1-0.5°K should be employed to achieve optimum performance of a solar water heating system utilising flat plate solar collectors in the UK. The switch-on criterion of 4-6°K can be achieved reasonably easily by the use of standard, inexpensive controllers and nickel-based temperature sensors. However, such controllers are not capable of consistently resolving temperature differentials to the degree required to meet the switch-off criterion of 0.1-0.5°K over the full operating temperature range. For large installations, therefore, the use of high quality controllers with calibrated platinum resistance thermometers as temperature sensors will prove cost-effective and is strongly recommended.
|Date of Award||1985|