Articles
THE SOLAR POND AS AN ALTERNATIVE ENERGY SOURCE FOR GREENHOUSES
The average intensity in winter, when heat is needed, is much lower than in summer.
For instance, typical daily solar radiation for each month at Wooster, Ohio (41°N latitude) is shown in Figure 1 to be 3–4 times greater in June than in December.
Any system that can economically and efficiently store large quantities of heat might utilize the annual variability.
Solar ponds appear to be a type of solar system that can both collect and store year-around radiation for use primarily in the winter.
Solar ponds are heated by absorption of short wave radiation throughout the vertical profile and on the pond liner.
The pond is intended to be transparent and the lower levels absorb primarily visible and ultraviolet light.
Most of the infra-red light is absorbed at or near the surface of the pond and tends to be lost to the air by evaporation and conduction-convection.
The solar pond, as established for this study, has a high density convective zone of approximately 20% sodium chloride in the bottom half.
The top half of the pond is composed of a concentration gradient that goes from 20% at the center to nearly 0% at the surface.
Such gradients are non-convective and no circulation occurs.
Heat can only be lost to the surface by conduction through the gradient, which is relatively slow.
For example, the brine gradient has similar heat conductivity characteristics as water with a conductivity of 2240 j-m/hr-m2-°C at 38°C.
Natural solar ponds were first discovered in the early 1900’s in Hungary as noted by Kalecsinsky (1902). Tabor (1963) studied pond feasibility for power generation and was able to achieve small pond temperatures up to 90°C, but had numerous technical problems with large ponds.
Rabl and Nielsen (1975) have studied the solar pond as a solution to space heating of residences in Ohio and similar areas.
Rabl calculated that a pond equal in volume to a well insulated three-bedroom home could meet all of the winter space heat requirements of that home.
Nielsen (1975) further developed a unique salt gradient establishment procedure using a small pool and laboratory models.
Based on Rabl and Nielsen’s work, a full-scale experimental solar pond shown in Figure 2 was constructed adjacent to the Department of Agricultural Engineering Greenhouse at the Ohio Agricultural Research and Development Center (OARDC). The pond was designed to meet all of the winter heat requirements of a 186m2 three-bedroom home or a 98m2 greenhouse in Wooster, Ohio.