Solar Energy Perspectives - IEA

Solar Energy Perspectives: The solar resource and its possible uses
Figure 2.10 Increase in collected energy on optimally titled collectors
versus horizontal ones
180 o
W
90 o
W
0 o
90 o
E
180 o
E
60 o
N
60 o
S
30 o
N
30 o
S
0 o
600 kWh/m 2 /y
550 kWh/m 2 /y
500 kWh/m 2 /y
450 kWh/m 2 /y
400 kWh/m 2 /y
350 kWh/m 2 /y
300 kWh/m 2 /y
250 kWh/m 2 /y
200 kWh/m 2 /y
150 kWh/m 2 /y
100 kWh/m 2 /y
50 kWh/m 2 /y
Note: The map shows the increase in collected energy gained by tilting the receiving surfaces at its optimal angle.
Source: Breyer and Schmidt, 2010a.
Key point
Collector tilt angle reduces geographical disparities in available solar energy resource.
But tracking the sun comes at a cost. Higher concentration factors require more precise tracking,
which entail higher costs. Furthermore, the diffuse radiation incoming onto the reflector does
not hit the receiver and is lost. The importance of this loss is shown in juxtaposing global normal
and direct normal irradiance levels worldwide (except the Poles) (Figure 2.11).
In any case, concentrating technologies can be deployed only where DNI largely dominates
the solar radiation mix, i.e. in sunny countries where the skies are clear most of the time,
over hot and arid or semi-arid regions of the globe. These are the ideal places for
concentrating solar power (CSP), concentrating photovoltaics (CPV), but also manufacturing
of solar fuels and, of course, other industrial uses of high-temperature solar heat. These are
also regions where solar desalination is likely to take place, given the usual scarcity of water.
In humid equatorial regions, sunshine is abundant but the diffuse component is relatively
more important so concentrating technology is less suitable. PV would work fine, but so do
solar water heaters and some other uses of solar heat, from crop drying to process heat, and
some forms of solar cooking.
It is often beyond 40° of latitude (north or south) or at high altitudes that solar space heating
is most profitable. Indeed, the same phenomenon that makes the air cooler at altitude (i.e.
the lower density of the atmosphere) also makes the sunshine stronger, when the weather is
fine. Here, land relief is important, as it heavily influences the availability of sunshine.
Concentrating PV has only a small share in the current PV market, and a very large majority
of the market for solar heat today is based on non-concentrating collectors. Concentrating
solar power takes all the current market for solar thermal electricity, and is the only available
technology option for manufacturing solar fuels.
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© OECD/IEA, 2011