Boyd, David (1983) An investigation of a system for improving the use of solar energy in greenhouses. (PhD thesis), Kingston Polytechnic, .

Abstract

An original low cost modular system which collects excess solar energy entering a conventional cold greenhouse during the day and uses the stored energy at night for heating has been devised and analysed. During the day, fluid absorbs heat as it is pumped through semi-transparent panels mounted immediately inside the greenhouse roof and then returned to an insulated tank forming the thermal store. At night the warm fluid is recirculated through the panels and/or emitters near the plants to provide greenhouse heating. Semi-transparent panels were designed and constructed and their thermal operation analysed in a test rig. Semi-transparent panels not only collect energy from the sun but also, unlike conventional solar collectors, gain up to 70% of heat from adjacent warm air. A semi-empirical mathematical model of the thermal operation of the panels is presented which predicts panel energy gain with better than 20% accuracy. A prototype system of panel and store was constructed in a small greenhouse which had been partitioned into test and control compartments. Experiments were conducted into the operation of the system and its effect on the vertical temperature profile and illumination inside the greenhouse both with and without plants. At night, the panels heated plants mainiy by restricting radiative losses to the sky, so that leaf temperatures were elevated by up to 2.5 deg.C. Panels reduced light levels to about 65% of those in the control compartment. A detailed computer model was written to predict the illumination and temperatures inside a greenhouse fitted with a panel/store system. This model was validated and can therefore be used for the general analysis of semi-transparent solar panel systems and their effect in greenhouses. Panel/store systems were calculated to supply, for heating at night, about 6% of the annual solar energy incident on the greenhouse.

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