Seasonal performance of a brine pond solar heat collector in New England

Abstract

The modified 20 metric ton, 20 m2 area Bloch-Tabor brine pond built on the premises of the New Alchemy Institute, Hatchville, Massachusntts, under Woods Hole Oceanographic Institution auspices has been in operation since early October 1976 through one of the coldest winters in recent New England history. Despite a "cold" start in October 1976 the core temperature declined only to 24°C in January-February 1977, reached a summer peak of 58 C in August 1977 and is now showing seasonal decline . Generally speaking, the pond has maintained a temperature about 20°C above ambient; showing only very sluggish responses to weather changes and a smooth response to seasonal changes of insolation and average wind speed. The pond, though small, has been operated in an unsheltered mode to see whether very large ponds (too large to be covered) could perform usefully in New England. As a result the open pond not only suffered evaporative losses and wind-mixing of the fresh water cap overlying the brine, but, when iced over, collected snow. Exposure to the elements also permitted accumulation of leaves, pollen, and dust on the surface which cast shadows. Almost daily cleaning was necessary but easily done with a drop of dishwashing detergent on the surface and a scoop net. The pond operated at an average efficiency of 24% (total daily output/daylight input x 100%) . The pond differs from the Bloch-Tabor design in utilizing coal as a black body absorber and brine concentrations of Calcium Chloride Hexahydrate near 45% to raise the refractive index to 1.42 and thus enhance the "fish-eye" or whole-sky radiation-trap effect. As a result the pond could acquire and trap radiation on cloudy as well as clear days to such good effect that it is difficult to distinguish between clear and overcast days in the recording thermograph records. Rainfall has a cooling effect from which recovery is rapid, and also "tops up" and freshens the sweet water cap over the brine. Precipitation has just about balanced evaporative losses from the surface since excesses overflow and P - E is generally positive in New Enqland. The experiment has been generally successful from the physical point of view (radiant heating of homes, for example), but its biological performance is marginal in mid-winter . Tropical species of plankton feeders, such as Talapia, grow best in water temperatures near 27°C which the pond at 24°C cannot supply in the coldest months. Anaerobic digesters for bioconversion of organic wastes into high grade fuels and fertilizers operate best at 35°C which, again, are not within the capabilities of the pond in mid-winter. Water plants, algae and the nitrogen-fixing bacteria associated with Azolla (water ferns) survive and reproduce at 24°C and can thus be held over in winter, but 30°C or so would improve their vitality. Obviously, some form of increased heat storage capacity, with very long time constants, is required.As a result of these findings and experiences an altogether different approach to solar heat collection and storage has been developed in which the heat excesses of summer are collected and stored below ground for use in winter: the annual-cycle, groundwater heat storage system. Preparations are being made to drive wells to and into the phreatic zone so as to pump several hundred-thousand gallons of cold groundwater to the surface for solar heating to about 45°C in summer, and return the heated water to the groundwater table for storage as a warm, buoyant lens for heat recovery in winter. The practicality of such a plan has been given careful study with much help from the geologists, hydrologists, and environmentalists of the U. S. Geological Survey, Environmental Protection Agency , and the National Water Well Association. Computer models of the "thermal onion" developed around heat storage well and heat recovery expectations have been made at the ETH, Zurich, Switzerland which suggest that a pilot experiment would provide valuable proof of the principle and indicate the scope of its applications. A much more serious constraint on brine pond usage is environmental . The concrete tank holding the pond survived the pressure of foot-thick ice without damage, but subsequent bulldozer operations near the tank undermined its footings. This provoked the thought that a serious leak in a large hypersaline pond could discharge brine into the vadose zone and eventually contaminate groundwater to such an extent that the water quality of wells in the vicinity would suffer for years. For this reason the present experiment has been terminated. Clearly, brine ponds should be built only in places where they do not impose an environmental threat, as in connection with marine aquaculture tank or polder heating where inadvertent salt leakage would be of no consequence. Theoretical and experimental study of the physics of brine pond efficiency indicates that the 20m2, 20-ton experiment just concluded, represents the lower limit of practical size. Theory suggests that a more nearly optimum pond would be about 10,000 m2 (one hectare) in area and 3 m deep with a much more gentle pycnocline developed in the upper 1 m. The pond should be quite fresh in the upper 20 cm and reach higher salt concentrations below the 1 m level. Since the coal layer tends to be neutrally buoyant at 40% brine concentrations it thus becomes involved in convection and shades the bottom layers. A lower salt concentration would prove advantageous in terms of heat collection and heat storage capabilities. Wave and wind-stirring action on a large pond would have to be suppressed, possibly by floating a grid of wooden booms on the surface. Large solar ponds are being studied at the Dead Sea Works in Israel. It would be very instructive to conduct similar experiments in the less favorable climate of New England.