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Kristiansand, Norway

Strandberg R.,Norwegian University of Science and Technology | Strandberg R.,Teknova AS | Reenaas T.W.,Norwegian University of Science and Technology
Applied Physics Letters

So far, theoretical efficiency limits for the intermediate band solar cell have been calculated under the assumption that the absorptivity of the solar cell is 1 for all photon energies larger than the smallest subband gap. In the present work, efficiency limits have been calculated under the assumption that the cell is covered by spectrally selective reflectors. The efficiency limit for the 1 sun 6000 K black body spectrum is found to increase from 46.8% to 48.5% and the limit for the AM1.5G spectrum (as defined by ASTM G173-03) is found to increase from 49.4% to 52.0%. © 2010 American Institute of Physics. Source

Gjerstad K.,Teknova AS | Time R.W.,University of Stavanger | Bjorkevoll K.S.,Sintef
Journal of Non-Newtonian Fluid Mechanics

Flow of Bingham fluids in Couette-Poiseuille flow is studied. A set of analytically flow equations for flow between parallel plates/slots is presented, and used as a basis for developing simpler flow equations.A single flow equation explicit in the pressure gradient that approximates the full analytical solution for annulus is developed. For the special case when the diameter ratio approaches unity, we also present a simpler flow equation designed for slot flow.The new explicit flow equations are designed for application in dynamic ODE-based flow models, and may readily be combined with advanced pressure control and parameter estimation techniques.The performance of the new equation is shown to be good, and the concept presented here may be extended to the more general Herschel-Bulkley model. © 2012 Elsevier B.V. Source

Strandberg R.,Teknova AS | Reenaas T.,Norwegian University of Science and Technology
IEEE Transactions on Electron Devices

The optimal filling of the intermediate band (IB) of an IB solar cell is investigated. Using models based on detailed balance principles, it is shown that the optimal filling varies with the size of the subband gaps, the absorptivity of the cell, and the degree of the overlap between the absorption coefficients as well as the mutual sizes of the absorption cross sections for transitions over the subband gaps. The results of calculations that show how nonoptimal filling affects the cell efficiency are also presented. In several cases, a deviation from the optimal filling will only result in small changes in the efficiency. However, cases where the efficiency is reduced dramatically due to nonoptimal filling are also identified. For some cases, the negative impact of nonoptimal filling can be reduced by increasing the absorptivity of the cell or, when the effect of photofilling is significant, by increasing the light concentration. © 2010 IEEE. Source

Strandberg R.,Teknova AS
Conference Record of the IEEE Photovoltaic Specialists Conference

The most commonly used criteria for evaluation of the suitability of different intermediate band materials for use in intermediate band solar cells are their band gaps. One often sees that such an evaluation is made based on theoretical efficiency limits with non-overlapping absorption coefficients. In this work the theoretical efficiency limits for various degrees of overlap are calculated for relevant combinations of band gaps. It is found that the optimal position of the intermediate band moves towards the middle of the band gap when the overlap increases. It is also shown that overlap between the absorption coefficients can increase the theoretical efficiency for some band gap combinations. This work aims to serve as a rough guide for determining whether a combination of band gaps is promising for use in intermediate band solar cells or not. © 2012 IEEE. Source

The intermediate band materials BSSi214, Cu4CrGa 3S8, Cu4TiGa3S8, Mg 2In3VS8, S32Zn31Cr, and Te32Zn31Cr, as well as a certain configuration of InAs quantum dots in GaAs, are evaluated as candidates to implement highly efficient intermediate band solar cells. The evaluation implies calculating theoretical efficiencies by combining an existing mathematical model and the absorption coefficients for the investigated materials. The model takes into account the energy dependence and spectral overlaps of the absorption coefficients related to transitions between various pairs of electronic bands. The presented results represent theoretical efficiencies for flat-plate solar cells, without light-trapping schemes, based on absorption coefficients publicly available in scientific journals. Only BSSi214 and InAs quantum dots in GaAs turn out to have theoretical efficiencies close to or above the detailed balance efficiency limit for single-bandgap cells. It appears unlikely that cells made of the other materials will be able to show efficiencies higher than single-bandgap cells either due to unfortunate absorption coefficients or due to bandgap combinations that are too far from the optimal. The results highlight the fact that materials have to be selected with great care when attempting to make IBSC prototypes with higher efficiency than conventional solar cells. © 2013 IEEE. Source

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