Solar energy collection by antennas
Section snippets
INTRODUCTION
Solar cells, with the exception of their anti-reflection coatings, are quantum devices, only able to be understood and designed by application of quantum physics. However, the wave nature of light is routinely exploited at longer wavelengths in radio and microwave frequency bands. Photon energies are low at radio frequencies and a large number is required to give some particular power density. We tend to use wave models in that regime. At short wavelengths fewer photons are required for the
HISTORY OF SOLAR RECTENNAS
Bailey proposed the idea of collecting solar energy with devices based on the wave nature of light in 1972 (Bailey, 1972, Bailey et al., 1975). He suggested artificial pyramid or cone structures analogous to those found in nature and similar to dielectric rod antennas. His paper describes pairs of the pyramids as modified dipole antennas, each pair electrically connected to a diode (half wave rectifier), low-pass filter and load. The antenna elements needed to be several wavelengths long to
RELATED TECHNOLOGIES
A solar rectenna is similar to a simple radio telescope or radiometer (Burke and Graham-Smith, 1997) except that the radio telescope needs to measure the radiative power received through the antenna, often with a square-law detector which produces an output voltage proportional to the input power, while the rectenna needs to convert that power to useful work. There are also other technologies of relevance.
Filtering
Two filters are usually employed in rectennas (McSpadden et al., 1998, Nahas, 1975). An input filter between the antenna and the rectifier must (a) form an impedance match between the antenna and the subsequent circuitry, (b) ensure a continuous current path for power flow from the antenna despite the intermittent nature of the rectifier current, and (c) prevent the reradiation by the antenna of harmonic energy produced by the non-linear load presented by the rectifier (Brown, 1970). It was
EFFICIENCY LIMITS
Bailey (1972) suggested that 100% solar energy conversion efficiency may be possible but he also claimed the same upper limit for photovoltaic converters in general. There was at that time considerable confusion in the literature about whether the Carnot efficiency limit should apply to solar energy conversion (Bailey, 1980). Today, it is not questioned that it does and, indeed, more restrictive limits apply for any conceivable converter structure. Kraus (1988) also claimed the possibility of
Beam steering
It is common at microwave and millimeter wave frequencies to spatially steer the beam of a phased array antenna without any physical movement by controlling the phase of the signal from each element in the array. This suggests the future possibility of a concentrating solar energy collector with electronic pointing without moving parts.
Frequency up-conversion
The benefits available from up-conversion of sub-bandgap light passing though a bifacial solar cell have been outlined by Trupke et al. (Trupke et al., 2002).
CONClUSIONS
We have reviewed the history and principles of solar antennas and described their technological context. The thermodynamic limit on its performance was explained and quantified as 85.4% for concentrated solar input without frequency selectivity and 86.8% with selectivity. It remains an open question whether an ultimate limit exists closer to 93.3% but such considerations must account for all sources and consequences of electrical noise.
Acknowledgements
The Special Research Centre for Third Generation Photovoltaics is supported by the Australian Research Council’s Special Research Centres scheme. MAG acknowledges the support of a Federation Fellowship and RC is grateful to R. Gough and G. James (CSIRO) and N. Harder and G. Conibeer (UNSW) and H. Linke (U. Oregon) for helpful discussions and to J. Hansen for preparing the figures.
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