Modeling the thermal characteristics of greenhouse pond systems

Greenhouse pond systems (GPS) can provide a good alternative for maintaining water temperature in aquacultural facilities. However, their thermal 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 various climatic conditions. Simulation results show that, in a 1-m pond, a passive polyethylene GPS can yield a 5.2°C increase in water temperature compared with outside air temperature. The night temperature 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 cover to the sky. Reducing these three heat flux densities is the principal measure for maintaining water temperature or saving energy in a GPS. Water condensation frequently occurs on the inner surface of the cover, which makes highly thermal-radiationtransparent covering materials like polyethylene become opaque to thermal radiation and behave like low emissivity glass. Polyethylene is thus a more sound material for the GPS cover than glass. Mean water temperature in a passive polyethylene GPS is 0.6°C higher than that in a glass GPS, while, in an active polyethylene GPS, the total heat demand is 9.2% lower than that in a glass GPS. From a temperature maintenance point of view, polyvinyl chloride is almost as effective as polyethylene. This model can provide a useful tool for optimum control of water temperature and evaluation of the economic potential for the active GPS. © 1998 Published by Elsevier Science B.V. All rights reserved.