Heat-absorbing Materials Useful for Increased Solar Still Efficiency

The solar still is the simplest form of solar-powered desalination. It uses the mechanisms of evaporation and condensation—the same processes used other forms of distillation—to purify the water. In a solar still, water is kept in an airtight container. As the water heats up, it evaporates and becomes water vapor. The lid of the still serves as the condenser to transform the purified water vapor back into water and the water slides down the slope of the lid to a collection point. Murugavel et al. (2010) built and tested a solar still with a roof-like glass lid, shallow basin, and insulation. They investigated the effects of various insulating and heat-absorbing materials on the efficiency of the still, since operation of the still depends on the amount of water evaporated, which depends on the amount of heat added to the water, among other things. The materials tested here included rocks, brick, metal, and cloth. The results of their testing showed that a ¾ inch layer of quartzite rock on the bed of the still performed the best. Murugavel et al. also performed theoretical calculations using energy balances and heat transfer equations to determine the theoretical efficiency possible for the chosen parameters. While the quartzite rock performed the best in tests, the actual efficiency was still nowhere near the theoretically possible one.—Erin Partlan
 
Murugavel, K., Sivakumar, S., Ahamed, J., Chockalingam, K., Srithar, K., 2010. Single basin double slope solar still with minimum basin depth and energy storing materials. Applied Energy, 87, 2, 514–523.
 
Murugavel et al. built and tested a solar still in Kovilpatti, India. They crafted a basin from mild steel plate, created a glass cover with a north and south slope, and insulated it with glass wool. In testing, a minimal water depth of 0.5 cm was used. Measurements were taken of the influx and outflux of water and of the temperature of the body of water and the water vapor. Also, atmospheric conditions were monitored to ensure that factors were controlled between test days. From incident solar radiation and ambient temperature data, the authors conclude that this is a valid assumption. The materials used to collect extra heat on the basin were ¼ inch quartzite rock, ¾ inch quartzite rock, ¼ inch washed stones, 1½ inch cement concrete pieces, 1¼ inch brick pieces, mild steel trimmings, and a light black cotton cloth. Multiple trials were run with each material, and while the overall productions hovered around 3.5 L/day of water, the ¾ inch quartzite material performed slightly better than the rest at 3.66 L/day of water.
 

In the theoretical testing, the authors use thermodynamic equations to model the heat influxes and outfluxes of the system. They note that they are novel in their approach as they use a variable term for the transmittance of solar energy through the glass cover, a term usually assumed to be constant. The resulting equations in their modeling are expressions for the instantaneous and overall water production of the solar still. However, when the parameters from the test of the ¾ inch quartzite rock are used, it was found that four-fold increase in the production rates was theoretically possible. While the authors note several areas of discrepancy—the change in water volume and depth over time, a higher proportion of water vapor inside the still, and differences in the absorptivity of the different testing materials—these results imply that the heat-absorbing material used has a minimal impact on improved efficiency, and that instead, focuses should be made on improving the design of the solar still itself to better utilize the incident energy. 

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