Optical Transparency of Inexpensive Salt Solutions for Construction of Density-Gradient Solar Ponds

A laboratory testing protocol has been developed that will permit a comparative evaluation of the optical performance of candidate salts for use in stabilizing solar ponds. Extinction coefficients measured for the salts and for water in dual-beam spectrophotometer fitted with 10-cm quartz sample cells were input to a digital model that convolutes a solar spectrum with an assumed salt/water concentration gradient similar to that to be established in the solar pond. The model determined the fraction of available solar energy transmitted to the pond storage layer. This information was used to estimate the size of the pond needed to meet a given set of thermal demands if the candidate salt were to be used. One candidate salt, a by-product from a flue-gas desulfurization process, was tested according to this protocol. Results indicated that this salt would perform poorly in comparison to commercially available sodium chloride; however, a sensitivity analysis revealed sources of error in the spectrophotometric measurement procedure, which may have led to an unduly pessimistic prediction of performance. These errors are being corrected, and this and other candidate salts will be subject to further evaluation for use in ponds to be constructed on-site.