Design, fabrication, and measurement of high efficiency concentrating photovoltaic modules leading to photovoltaic systems with low energy cost .

摘要

Adopting photovoltaics (PV) as one of the major electricity sources for terrestrial applications requires that it have a reduced energy cost. PV energy cost reduction can be achieved by combining market, tax and regulatory incentives, and research and development (R&D) support. This work studies the contribution of module efficiency to the cost of PV energy. It covers a range of research from quantifying the value of module efficiency in lowering PV energy cost, especially concentrating PV (CPV), to technically realizing high efficiency CPV modules, to predicting the low energy cost that result from this high CPV module efficiency.;This work starts with quantitative analysis of the value of module efficiency using levelized cost of energy (LCOE) as a metric. It concludes that, with the same baseline conditions, higher module efficiency leads to lower LCOE. Specifically, to make CPV competitive with flat plate PV, CPV module efficiency must be higher than 30% even if the tracker price is cut by 50%. After proving the significance of high CPV module efficiency, this work focuses on the design, fabrication, measurement and analysis of one high efficiency CPV module structure, and it predicts the low LCOE that would result from adopting such a CPV module. The design of this module structure is based on understanding the component efficiency losses that exist in current dominant high concentration (high-X) CPV adopting triple-junction (3-J) solar cells with two electrical contacts. These losses can be attributed to two main causes: (1) low tolerance to pointing error, essentially due to high-X; and (2) low resistance to spectrum change, essentially due to the two-contact structure of 3-J solar cells. To eliminate these two factors, a new CPV module structure was designed, fabricated and measured. This structure adopts middle concentration (mid-X) and multiple junctions with separate electrical contacts by using lateral spectrum splitting. To demonstrate the high efficiency of this new design, a prototype submodule is fabricated. The submodule is built as a test bed whose adjustability allows the sample to be switched and the configuration to be changed. Based on this test bed concept, a self-consistent measurement methodology is proposed that allows the identification of individual component efficiency. Thus, any problem existing in the design can be located. This submodule has been independently measured by two organizations. The measurements demonstrate an outdoor efficiency as high as 39.5%. An official, calibrated efficiency of 36.7% was measured by the National Renewable Energy Laboratory (NREL) and this result was reported by NREL as a record submodule efficiency. The official citation in the journal Progress on Photovoltaics stated "This is probably the highest efficiency yet measured for the experimental conversion of sunlight to electricity by any means". Detailed analyses of the measured component efficiencies and the comparison between the two independent measurements identify improvements that could lead to further increases in submodule efficiency. Specifically, the prediction of higher submodule efficiency by adopting immersed optics has been demonstrated by a variation design based on the same key concept of lateral spectra splitting. This submodule was developed by a team lead by DuPont and a new record submodule efficiency of 38.5% is reported. After these experiments that prove the high module efficiency, the role of high efficiency mid-X CPV that adopts lateral spectrum splitting and independent electrical contacts is compared to that of other PV technologies and this CPV is found to achieve the lowest PV energy cost.