Performance Study Of Liquid And Solar Cells In HCPV System With Liquid Immersion Cooling

High concentration photovoltaics(HCPV)have been considered as an effective way to reduce the photovoltaic(PV)electricity cost.However,the solar cells under high illuminations will have increases in temperature.And the cells will experience both short-term(efficiency loss)and long-term(irreversible damage)degradation due to excess temperature.Therefore,cooling of photovoltaic cells is one of the main concerns when desiigning HCPV systems.Otherwise,HCPV will not have measurable contribution to the PV market.Aiming at eliminating the shortages of conventional CPV cooling approaches,a direct liquid-immersion cooling method is proposed for the densely packed III-V cells module under high concentrations.The thesis consists of four parts.The performance of liquid and solar cells in HCPV system with liquid immersion cooling were investigated.(1)Ten liquids were identified as candidates,which were selected in the four categories of liquids,namely synthetic oils,glycols,silicone oils and mineral oils.The spectral transmittance of the ten immersion liquid candidates was determined using transmittance measurements of two path-length cuvette via spectrophotometer.The calculated normalized photocurrent density(Jnp)of CPV solar cells indicated that the other candidate liquids can be used for immersing triple-junction CPV solar cells except for glycerol.Further,Therminol VP-1 immersion would cause the smallest power loss of three-junction CPV solar cells,followed by dimethyl silicon oil.(2)The reliability of immersion liquid candidates was assessed through designed UV light,damp heat and high temperature accelerated lifetime tests.The tests results showed that dimethyl silicon oil after tests would cause the smallest power loss of three-junction CPV solar cells and the other immersion liquids except for synthetic oils Therminol VP-1 and naphthenic oils Nytex 801 exhibited little degradation.By analysis the infrared spectrum of liquid which optical transmittance decreased significantly after accelerated lifetime tests,results showed that the oxygen in the air binds to the carbon atom in the liquid molecule to produce the keto-carbonyl under the effect of UV light,moisture and high heat environment.Which is the main reason for the opticaltransmittance of liquid decreased significantly.(3)The electrical characteristics of InGaP/InGaAs/Ge III–V triple junction CPV cells were examined through indoor solar simulator,which were in different kinds of liquids and also within different liquid thickness on top.The results showed that the electrical performance of the III–V triple junction CPV solar cells increased with increasing liquid thickness firstly,and then decreased with increasing liquid thickness.The efficiency and the maximum output power of the cell reached maximum when the liquid thickness is 4 mm.And the biggest percentage change is 8.99%.This is because the reduction in reflection and reduced cell surface recombination losses from surface adsorption of polar molecules.(4)Electrical characteristics of triple-junction CPV solar cells immersed liquids were investigated through designed UV light,damp heat and high temperature accelerated lifetime tests.The results showed that the most promising liquids to immerse triple-junction CPV solar cells selected for accelerated tests include dimethyl silicon oil and C14 n-alkane,Further,dimethyl silicon oil was the optimum liquid.The dark characteristic curves of solar cells which conversion efficiency decreased significantly after three accelerated lifetime tests were analyzed,results showed that the root causes of degradation can be attributed to liquids and solar cells interact with each other under the influence of environmental factors,as well as resulted in the number of defects around the chip perimeter increasing,which reduced the carrier lifetime and caused an increase inrecombination current.