Factors Affecting the Performance of Rockstorages as Solar Energy Collection/Storage Systems for Greenhouses

Abstract

ABSTRACT DATA from 6 years operation were analyzed in order to identify important factors affecting the performance of rockstorages as devices for saving energy and enhancing CO2 enrichment in greenhouses. The data were taken from two similar rockstorages of slightly different design attached to two different size greenhouses. The data included seasons conducted with four different crops (lettuce, peas, tomatoes, and cucumbers), two different covers (single fiberglass and double polyethylene), with and without CO2 enrichment, systems of two sizes with similar wind exposures, and systems of the same size with different wind exposures. Fuel savings using the rockstorages ranged from a low of 54.8 MJ/m2 in the winter of 1981 to a high of 143.3 MJ/m2 in the spring of 1979. Percentage enrichment time ranged from 64.3% for the spring of 1984 to 100% in the winter of 1981, up to 45% more enrichment time than conventionally enriched treatments. Electrical energy debit, the additional electricity required to operate the rockstorages, ranged from 9.5 MJ/m^ to 34.6 MJ/m2 whereas collection efficiency varied from 8.0% to 19.3%. Equations for fuel savings, percentage enrichment time, electrical consumption increase, and collection efficiency were determined from the data and evaluated to provide a basis for comparing the effects of important seasonal differences. Fuel savings was found to be a non-linear function of average outside temperature and solar energy and to decrease with CO2 enrichment and increasing severity of wind exposure. Fuel savings under fiberglass was not significantly different from those under double polyethylene. No differences attributable to system size were found. Percentage enrichment time decreased linearly with increasing outside temperature and solar radiation and was found to be negatively affected by decreasing vent set point temperature but not by system size or severity of exposure. Electrical energy debit was found to increase linearly with increasing temperature and solar radiation as well as with CO2 enrichment, decreasing vent set point temperature, and system size. Collection efficiency was found to be a non-linear function of average outside temperature and to increase with CO2 enrichment; however, the increase was not beneficial. Peak efficiency was less under double polyethylene than under fiberglass and decreased with decreasing venting and heating set point temperature; however, the utilization of the energy was better under low temperature conditions so that total fuel savings increased.