APPLICATIONINVERTERS
Figure 5: The radiative efficiencies of two MQW top cells grown on a research reactor compared with a typical MQW middle cell.
integrated, combined heat-and-power applications that further reduce electrical generation costs. High temperature operation of the cells mean that the cooling water can be used in the building, because a temperature of 90 °C is high enough to run an absorption chiller providing air conditioning.
Meanwhile, the cells can generate electricity. With three times the efficiency of a second generation cell, they can be used on buildings in less than ideal locations, such as vertical walls in northern latitudes, and still generate more electricity per square meter over a year than second-generation cells.
efficiency of a triple-junction device with QWs in two of the cells will vary less with changes in spectral conditions than one incorporating wells into just one cell.
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Power generating window formed from transparent blinds feature Fresnel lenses that focus direct sunlight onto
luminescent light bars which further concentrate the
sunlight on cells in the frame. Diffuse light illuminates the room. (Courtesy
Solarstructure Ltd.)
Realising such high efficiencies in real devices requires the top cell to be radiatively efficient. Initial results, which we presented in Valencia, are promising. The radiative efficiency, which was extracted from measurements of the dark-current of MQW top cells grown on a research reactor at the EPSRC National Centre at the University of Sheffield, increases with concentration (see Figure 5). The initial values for radiative efficiency for the top cells are not as high as for the more mature middle cell, but it does increase with the number of quantum wells. If around 50 of these are incorporated into the top well, efficiencies in Figure 4 can be realised.
Focusing sunlight by a factor of 500 or so onto a cell causes it to heat up. Cooling can reduce this rise in temperature, which is beneficial for conventional triple-junction cells, because as they get hotter, the cell efficiency falls. In our novel cells this is far less of an issue, because they can be designed with deep wells in the top and middle cells, a modification that allows efficiencies of more than 40 percent to be achieved at operating temperatures of 90 °C. This cell heating can be put to good use, allowing this device to feature in
Our MQW cells are also ideal for use in smart, power generating windows. The blinds, transparent Fresnel lenses tracking the sun, focus the direct sunlight on a luminescent light bar while allowing the indirect sunlight into the room for internal illumination, thus reducing electricity demand. The QW cells are mounted in the window frame. The cooling water can be used for internal purposes including running air-conditioning, the demand for which is highest when the sun is shining.
Many other applications will open up for small- sized, high-concentration units offering more than double the electrical efficiency of first- and second- generation photovoltaics. If first-generation cells, which supplied the household’s electricity, were replaced by such a system covering the same roof area, the extra electricity produced could power the family electric car for the year, even in rainy England! In addition, this system could supply the household’s hot water requirements.
At Quantasol, we are well placed to capture a significant proportion of the increasing orders for third-generation cells, and are destined to become a major player in this market. The versatility of our cells enables them to maximise electrical energy harvesting in varying spectral conditions as well as to cope with the challenges provided by the ingenuity of concentrator manufacturers.
© 2011 Angel Business Communications. Permission required.
Further reading International Energy Agency, Photovoltaic Power Systems Programme. IEA-PVPS T1-19: 2010, http:/ /
www.iea-pvps.org/ S. Kurtz, National Renewable Laboratory Technical Report, NREL/TP-520-43208, (2009), http:/
www.osti.gov/bridge K.Arthur and K. Barnham Compound Semiconductor, Jan&Feb 2008, 22 A. Dobbin et al. submitted to Solar Energy Materials and Solar Cells, February 2011 K. Barnham et al., 5th World Conference on Photovoltaic Energy Conversion (WCPEC5) , Valencia, 234 (September 2010). K. Barnham et al. Nature Materials 5 161, (2006).
www.solar-pv-management.com Issue III 2011
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