23-05-2012, 10:58 AM
Harnessing High-Altitude Solar Power
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Abstract
As an intermediate solution between Glaser’s satellite
solar power (SSP) and ground-based photovoltaic (PV) panels,
this paper examines the collection of solar energy using a highaltitude
aerostatic platform. A procedure to calculate the irradiance
in themedium/high troposphere, based on experimental data,
is described. The results show that here a PV system could collect
about four to six times the energy collected by a typical U.K.-based
ground installation, and between one-third and half of the total energy
the same system would collect if supported by a geostationary
satellite (SSP).
INTRODUCTION
THE DEVELOPMENT of new and cost-effective methods
to harness renewable energy has become crucial to
maintain the energy supply that underpins our society, and solar
power is one of the main candidates to make a substantial
contribution to fulfil our future energy requirements.
One of the major issues in the use of ground-based photovoltaic
(PV) panels to harvest solar power is the relatively low
energy density that is compounded by the fact that the power
output of the devices is strongly dependent on the latitude and
weather conditions. These factors have particularly hindered the
diffusion of PV in several countries with cloudy climates (e.g.,
north European countries). On the other hand, areas with high
solar irradiations (e.g., African deserts; see [1]) are remote from
most users and the losses over thousands of miles of cables
and the political issues entailed in such a large project, severely
reduce the economic advantages.
SOLAR IRRADIANCE VARIATION WITH ALTITUDE
Solar radiation traveling through the atmosphere is attenuated
by two main kinds of processes. The first process is defined as
scattering and it involves the air molecules (Rayleigh scattering)
and the larger particles that can change the direction of the
photons after an interaction (Mie scattering). The other main
process is the molecular absorption, in which the energy of
the photons is converted into some other forms. In both these
processes, energy is removed from the beam of light. The total
attenuation of the light, traveling through a mean is known as
extinction [8]. In the case of the Earth’s atmosphere, most of the
attenuation is due to scattering. As a result of these processes,
the global solar radiation falling on a surface can be divided
into two main components: direct (or beam) and diffuse. For
low angles of incidence of the sun beam and/or a cloudy sky,
the beam component can be very low so that most of the energy
captured (global) is actually the diffused component.
HIGH-ALTITUDE WINDS
The knowledge of the mean wind speed at a certain altitude
(and its statistical properties) is essential to calculate the aerodynamic
forces acting on the aerostat and in particular to determine
the forces along the mooring cable.
The wind speed data described in this section were provided
by the Natural Environment Research Council (NERC), from
the Mesosphere–Stratosphere–Troposphere (MST) radar station
located at Capel Dewi (52.42◦N, 4.01◦W), near Aberystwyth in
west Wales, U.K. This facility can provide vertical and horizontal
wind speed data, covering an altitude range from 2 to
20 km, with 300 m resolution. However, for this paper, only
the data up to 10 km were processed. The particular set of data
described here covers the period January–December 2007, and
measurements were acquired everyday continuously.
CONCLUSION
This paper has examined the possibility to harvest solar energy
in the high atmosphere, as an intermediate solution between
ground-based PV devices and SSP. Based on the real data concerning
the extinction parameters in the Earth’s atmosphere, it
has been demonstrated that at altitudes above 6 km, it is possible
to produce over four times the energy that is usually produced by
ground-based PV in the U.K. Compared with SSP the method
advocated in this paper allows to collect between one-third and
half of what could be collected by a geostationary satellite collector
(for the same size of PV system). However, the cost of
SSP is orders of magnitude higher than the solution advocated
here. Based on the realistic values of the relevant engineering
parameters, a concept design has been presented, and its preliminary
costing has shown that ASPG could be a viable facility
to harvest renewable energy.