Transient performance of a stepped solar still with built-in latent heat thermal energy storage

The transient performance of a stepped solar still with built-in latent heat thermal energy storage was studied. The still was designed for heating and humidification of agriculture greenhouses (GH) in remote areas. The solar still consists of five stepped basins with an inclined glass cover and is insulated on the bottom. The basin was placed on a slab filled with a layer of paraffin wax (phase change material, PCM) that acts as a latent heat thermal energy storage system (LHTESS). Air from GH enters the still from the bottom, flows between the basins and glass cover where it is heated and humidified, and then flows back into the GH. The still performance parameters investigated were analyzed, and the results compared with the case of a still without the LHTESS. The results showed that the still with LHTESS has an efficiency of 57%, and the total daily yield is about 4.6 L/m2. The still temperature as well as outlet air temperature and GH heat load are more uniform compared to the sinusoidal trends for the still without LHTESS. It was found that the relative humidity of circulating air increased along the still and always leaves at saturation conditions. The results indicated that decreasing the air flow rate has an insignificant influence on the still yield, while the GH heat load experiences a decrease. For a selected design and operational parameters, the still was able to provide heat for the GH for 24 h/d. This finding is important since heat could be provided to the GH at night and when it is most needed.

1. The transient thermal performance of a stepped solar still with a built-in LHTESS (for the heating and humidification of agricultural greenhouses) was investigated. The still has a simple design where the absorber consists of five of stepped (black coated and insulated) small basins carrying the stagnant saline water. The basins are placed on slab containers filled with paraffin wax for thermal energy storage. Air (from the greenhouse) was circulated between the hot basin water and the tilted glass cover (where it is heated and humidified), then flows back to the greenhouse. 2. The performance parameters of the still including component temperatures, greenhouse heat rates, outlet air temperatures and humidity, and yield and efficiency were studied. The results show that: C The still’s efficiency is 57% compared to 61% for the still without LHTESS. C The total daily yield is about 4.6 L/m2 compared to 4.9 L/m2 for the still without LHTESS. C The still’s temperatures are more uniform compared to the sinusoidal trends for the still without LHTESS. C The relative humidity of the circulating air increased along the still and always leaves at saturation conditions. C A decreasing air flow rate has insignificant influence on the system productivity; however, it influences the greenhouse heat load. C When using a selected combination of parameters (the absorber is placed on a 5-cm thickness of paraffin wax with a 50°C melting temperature, covering the glass after sunset), A C G h Hfg H — — — — — — I k L l M m p Pr Q s t T Th v w — — — — — — — — — — — — — — — Area, m2 Specific heat, J/kg °C Grashoff number Specific enthalpy, J/kg Latent heat of evaporation, J/kg Film coefficient of heat transfer, W/m2 °C Solar intensity, W/m2 Thermal conductivity, W/m.°C Length, m Liquid Mass, kg Mass rate, kg/s Pressure, Pa Prandtl number Heat transfer rate, W Solid Time, s Temperature, °C Thickness, m Velocity, m/s Humidity ratio, kgwater /kgair — — — — — — — — — — Absorbtivity Incremental rise Difference Emissivity Relative humidity Efficiency Reflectivity Radiation constant Summation Transmisivity Greek " * ) g N D F E J Subscripts a ab am av b C COND E eq Eff g gh h i inst K m o p pw R s sid s/l su sw w — — — — — — — — — — — — — — — — — — — — — — — — — — — — — Circulating air Absorber Ambient Average Base Convection Condensation Evaporation Equivalent Effective Glass Greenhouse Humidification Insulation Instantaneous Conduction Melting Outlet Projected Product water Radiation Sky Side (glass cover) Solid/liquid Sun Saline water Wind Inlet Outlet