An investigation of a system for improving the use of solar energy in greenhouses
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
A prototype system of panels 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 mainly 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
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