Photo-thermal effect of carbon dispersions in water: augmenting community scale desalination with solar stills
Solar stills are simple desalination systems which are capable of producing low-cost freshwater for small community requirements. But the commercialization is limited from their low daily productivity. It is expected that by adding low-costblack carbon particles, the desalination rate can be enhanced due to their photothermal effect. These black carbon particles have strong solar absorption coefficient in the solar spectrum, and when used as dispersions in water, they increase the photothermal conversion effect which results in an increase in local fluid temperature. A higher surface temperature increases the evaporation rate due to localized heating. Experiments with biochar dispersions show that, while, the evaporation rate of water is mostly limited below 0.6 kg/m2-h, these low-cost dispersions can increase the evaporation to more than 1 kg/m2-h in open environments. The influence of these particles with regards to solar still productivity and its feasibility is then assessed for community scale desalination. Solar stills are compared against photovoltaic seawater reverse osmosis (PV-SWRO) and solar at plate collector integrated humidification dehumidification (FPC-HDH), and found to be cost competitive for small volumes of freshwater production. The feasibility assessment of a solar still system was performed for a 10000 m2 solar still with California as a reference location. It is estimated that freshwater can be produced at a rate of 27.7 m3=day to 30.8 m3/day for at least 100 households. The corresponding cost of water produced using levelized cost of water approach was found to be between $ 6.47/m3 and 7.14/m3 for nominal investment. With feasible investment options, the cost of water produced can be of only $ 1.58/m3 to 1.74/m3. In a supply system, the desalination cost has the major share, and if other costs such as feed intake costs, water treatment and brine disposal are to be added, the cost of water increases, but still expected to be close to marginal cost of freshwater supplied. Also, from the application standpoint, solar stills are much simpler to install, operate and maintain than other competing technologies which are more complex, thereby requiring constant supervision. The analysis performed indicates the feasibility of installing low-cost solar stills successfully in rural communities to produce low-cost freshwater.