Performance Study On The Photovoltaic And Thermal(PV/T) Stepped Solar Desalination System With The Improved Bottom Space Structure

The photovoltaic thermal(PV/T)stepped solar still consists of a PV/T collector and a stepped solar still,which not only raises the evaporation temperature but also provides additional electricity.The stepped solar still serves as one of the main components of the desalination system,and its structural characteristics have crucial impacts on the performance of the desalination system,so it has become one of the research hotspots in the field of freshwater production using solar energy.In this article,three novel PV/T stepped solar stills with ameliorated bottom space structure have been proposed.Those desalination systems are featured respectively by bottom channel with working fluid circulation(WFC),bottom channel with saline water flowing around circulation(FAC),and bottom cavity air convection(CAC).The physical and mathematical models have been established,and the variations of components temperature,fresh water productivity,thermal efficiency,and exergy efficiency under the conditions of actual solar radiation intensity,ambient temperature,and ambient wind speed have been studied by numerical calculation methods.Additionally,the influence of the bottom channel depth on the performance of the WFC and FAC desalination systems have also been researched.In the end,the three desalination systems have been analyzed individually,the differences in performance and working principles have been revealed and demonstrated.On that basis,the selection of the engineering choice has been suggested.The results indicated that the performance of the three solar distillation systems proposed in this paper is higher than that of the traditional PV/T stepped solar still.In the WFC desalination system,more thermal energy has been stored and delivered because of the working fluid stayed in the bottom channel.Therefore,compared the WFC desalination system(the bottom channel depth H= 0.01m)with the traditional PV/T stepped solar still,the average temperature of the glass cover,saline water,and absorber plate of the stepped still have been increased by 11.8%,7.2%,and 9.7%respectively;the total daily freshwater production is 3.48 kg/d,which increased by about 32.3%;the daily total thermal efficiency up to 40.6%,which increased by about9.6%;the average exergy efficiency was 7.4%,which increased by 1%;the components with the most exergy losses are the PV/T collector and the absorber plate of the stepped still which accounted for 51.9% and 40.5% of the total exergy losses,respectively.The research on the impact of channel depth H showed that the optimal depth of the WFC system is about 0.02 m.Under this circumstance,the system performance is the best.That is,the average evaporation water temperature,daily total fresh water productivity and daily total thermal efficiency are 59.8 ?,3.50kg/d and 40.8%,respectively.The system structure and working principle of the FAC desalination system determine that the temperature rise and evaporation of saline water are more efficient.Therefore,compared with the WFC desalination system with the best performance(H=0.02m),when the channel depth H of the FAC system is also 0.02 m,the average temperature of the glass cover,saline water,and absorber plate of the stepped still have been increased by 2.0%,2.3% and 2.8% respectively;the total daily freshwater production reached 3.62 kg/d,which increased by 3.4%;the total daily thermal efficiency reached 42.3%,which increased by 1.5%.However the research on the channel depth H and the exergy analysis show that the optimal depth for the FAC system is about 0.01 m.Under this condition,compared with WFC desalination system,the average evaporation temperature,the daily total freshwater production and the daily total thermal efficiency are 63.2?,3.95kg/d and 45.9%,which have been improved by5.7%,12.9% and 5.1%,respectively.And in this case,the exergy performance of the FAC system is the best,with the average exergy efficiency of 10%,which is increased by 1.78%.In the CAC desalination system,the existence of air in the cavity of the stepped still makes the heat dissipation of the absorber plate more difficult,and at the same time,the structure of the stepped still is simpler.Therefore,when the air flow rate is 0.01m/s,the average temperature of the glass cover,saline water,and absorber plate of the stepped still increased by 2.7%,2.0%,and 2.0% respectively;the total daily freshwater production increased to 4.21 kg/d,which increased by 5.5%;the total thermal efficiency is 49.1%,which increased by 3.2%.However,the average exergy efficiency is lower than of the FAC system,which is 9.6% respectively.Therefore,if only considering the high-efficiency of fresh water production and the convenience of the manufacturing process,the CAC desalination system is worth recommending.