MODELING THE THERMAL-CHARACTERISTICS OF GREENHOUSE POND SYSTEMS

Greenhouse pond systems (GPS) can provide a good alternative for maint aining water temperature in aquacultural facilities. However, their th ermal characteristics are not well understood. The GPS model advanced in this paper describes the evolution of various heat and water vapor transfer fluxes, temperature and humidity at a given site under variou s climatic conditions. Simulation results show that, in a 1-m pond, a passive polyethylene GPS can yield a 5.2 degrees C increase in water t emperature compared with outside air temperature. The night temperatur e of the internal air in a passive GPS can be maintained a few degrees higher than that in a horticultural greenhouse. The main heat losses of the water in the GPS are thermal radiation to the cover, convection from the cover to the external air, and thermal radiation from the co ver to the sky. Reducing these three heat flux densities is the princi pal measure for maintaining water temperature or saving energy in a GP S. Water condensation frequently occurs on the inner surface of the co ver, which makes highly thermal-radiation-transparent covering materia ls like polyethylene become opaque to thermal radiation and behave Lik e low emissivity glass. Polyethylene is thus a more sound material for the GPS cover than glass. Mean water temperature in a passive polyeth ylene GPS is 0.6 degrees C higher than that in a glass GPS, while, in an active polyethylene GPS, the total heat demand is 9.2% lower than t hat in a glass GPS. From a temperature maintenance point of view, poly vinyl chloride is almost as effective as polyethylene. This model can provide a useful tool for optimum control of water temperature and eva luation of the economic potential for the active GPS. (C) 1998 Publish ed by Elsevier Science B.V. All rights reserved.