Silicon / Gallium Arsenide Phosphide dual junction PV step cells to achieve 35% efficiency

Multi-junction solar cells based on gallium and germanium have achieved solar to electricity conversion efficiencies of 46% in highly concentrated sunlight and an efficiency of 34.5% in ambient sunlight. However, these types of solar cells remain extremely expensive, and even the using highly concentrated sunlight which reduces the amount solar cell area required by a large factor, these cells have been unable to complete with the falling costs of conventional silicon solar cells.

A collaboration between researchers at The MASDAR institute of the Unite Arab Emirates and MIT has developed a dual junction solar cell which consists of a gallium arsenide phosphide cell grown on a silicon germanium substrate which is then bonded to a silicon substrate which acts as the bottom cell in the tandem cell design. The silicon germanium underlayer gives good performance properties to the gallium based top cell but blocks light from the bottom silicon cell. In the step cell design a patterning process is used to etch away part of the top cell exposing the silicon beneath directly to incoming solar radiation.

A news articlehas been published by the MASDAR institute describing some aspect of this dual junction cell design. An abstract of a Journal of Applied Physics paper about the theoretical performance limits of this cell design (38.7% max efficiency) is also available. The article and the abstract leave many puzzles in my mind about the physics of this cell and about the sources of cost reduction relative to the more usual designs of multi-junction gallium based cells.

Nevertheless the researchers are sufficiently enthused about the economic potential of this cell design that they are planning to create a startup company to try to commercialize it. Presumably the market will be in the currently moribund area of concentrated photovoltaics (CPV), which uses concentrating optics and dual axis trackers to produce electricity from high efficiency PV designs.

Green House Concentrated Solar Thermal

Do solar thermal troughs enclosed in green house have superior economics than thermal troughs sitting in the open air? The California company Glasspoint thinks so. Greenhouse Enclosed Concentrated Solar

Greentech Media recently posted an article about a 1GW concentrated solar power (CSP) plant to be installed in Oman by the California company Glass point Technologies. Greentech crows about this plant being the largest CSP installation in history, the previous record being 377MW for the IvanpahIvanpah CSP plant. However, the Omani plant is intended to produce steam rather than electricity. If one allows for the efficiency of conversion of steam thermal energy to electricity the solar fields of the two plants are probably similar in size.

Glasspoint’s marketing strategy of providing solar steam to the oil industry to improve the economics of heavy oil extraction is nothing to crow about from an environmental point of view. The natural gas displaced by the solar field will certainly not be left in the ground, and improving the economics of heavy oil extraction contribute less than nothing to leaving fossil carbon in the ground or to weaning humanity off of fossil fuels.

Nevertheless Glasspoint’ s technology of greenhouse enclosed CSP troughs is interesting. They claim that this technology offers capital cost and operating cost advantages compared to conventional open air CSP. The improved capital cost claim may seem surprising at first sight since the enclosing greenhouse is obviously an extra cost. However greenhouse panels are flat glass while concentrating solar troughs are parabolic mirrors which are much more expensive to manufacture. Inside of wind sheltering green houses the parabolic troughs can be made comparatively cheap lightweight materials.

Operating cost improvements come from utilizing automated cleaning equipment from the commercial green house industry to keep the flat green house panels free from dust.

If these innovations lower the cost of producing steam oil field injection then presumably they could also lower the cost of producing steam for running electrical generators in conventional CSP.