Complete Report - University of New South Wales

4.5.7.3 Antenna Collection of Solar Energy
Researchers:
Richard Corkish, Samantha Wong, Gavin Conibeer
Research on the potential use of antennae for conversion of solar energy to direct current
electricity was revived during 2005, with a focus on the ultimate effi ciencies that could be
achieved.
Coherent, polarised, narrow-band radio waves may already be received and converted to
direct current by rectifying antennae, or “rectennas” but the conversion of broad spectrum,
incoherent, unpolarised solar radiation is more challenging. This work has continued to derive
insights from the fi eld of radio astronomy, where the accurate detection of such radiation is
well established. In particular, we have used the model of the “Dicke radiometer” to show that
the electrical noise output of an antenna receiving solar radiation can be represented as that
of a resistor at the sun’s black body temperature.
The effi ciency of conversion of the noise power from a resistor has been studied already
by others but our attempts to reproduce published results in a key paper repeatedly failed
and discussions with the author led to a correction (http://summa.physik.hu-berlin.de/
~ sokolov/).
That work investigated the rectifi cation by a diode, at one temperature, of the electrical noise
from a resistor at a higher temperature. Using the detailed balance method, the calculation
showed that the effi ciency of a system with a diode rectifi er as a heat engine must always be
less than the Carnot effi ciency. The reason for this thermodynamic irreversibility is that even
when no work is done, heat transport takes place between the heat baths.
The Carnot effi ciency increases with temperature. For an assumed black body temperatures
of 6000K for the sun and 300K for the receiver, the Carnot effi cieny equals 0.95. Figure
4.5.31 is a plot of the effi ciency of an ideal rectenna against the current drawn from the
device, with a peak effi ciency at 48.81%. (Dimensionless current is shown as this does not
depend on geometrical factors.)
Camot Efficiency = 0.95
Figure 4.5.31: Rectenna effi ciency vs.
dimensionless current for a Carnot effi ciency
of 95%.
0.5
0.4
While signifi cant practical challenges
remain for the possible implementation of
solar rectennas, the Centre’s work has now
developed a model to predict their ultimate
effi ciency.
Efficiency
0.3
0.2
-0.1
0
-4.5 -4 -3.5 -3 -2.5 -2 -1.5 -1 -0.5 -0
Normalised Current (dimensionless)
93