Solar Energy Perspectives - IEA

Chapter 2: The solar resource and its possible uses
those where much of the increase in energy demand is expected to take place in the
coming decades.
By separating the direct and diffuse radiations, it is clear that direct sunlight differs more than
the global resource. There is more diffuse irradiance around the year in the humid tropics,
but not more total energy than in southern Europe or Sahara deserts (Figure 2.8).
Arid areas also exhibit less variability in solar radiation than temperate areas. Day-to-day
weather patterns change, in a way that meteorologists are now able to predict with some
accuracy. The solar resource also varies, beyond the predictable seasonal changes, from year
to year, for global irradiance and even more for direct normal irradiance. Figure 2.9 illustrates
this large variability in showing the global horizontal irradiance (GHI) in Potsdam, Germany
(top), and deviations of moving averages across 1 to 22 years (bottom). One clear message is
that a solar device can experience large deviations in input from one year to another. Another
is that a ten-year measurement is needed to get a precise idea of the average resource. This
is not measurement error – only natural variability.
Tilting collectors, tracking and concentration
Global irradiance on horizontal surfaces (or global horizontal irradiance [GHI]) is the
measure of the density of the available solar resource per surface area, but various other
measures of the resource also need to be taken into account. Global irradiance could also be
defined on “optimum” tilt angle for collectors, i.e. for a receiving surface oriented towards
the Equator tilted to maximise the received energy over the year.
Tilting collectors increases the irradiance (per receiver surface area) up to 35% or about
500 kWh/m 2 /y, especially for latitudes lower than 30°S and higher than 30°N (Figure 2.10).
The optimal tilt angle is usually considered to be equal to the latitude of the location, so the
receiving surface is perpendicular to the sun’s rays on average within a year. However, when
diffuse radiation is important, notably in northern Europe and extreme southern Latin
America, the actual tilt angle maximising irradiance can be up to 15° lower than latitude. The
economically optimal tilt angle can differ from the irradiance optimised tilt angle, depending
on the type of application and the impact of tilt angles on the overall investment cost.
Other potentially useful measures include the global irradiance for one-axis tracking surface
and the global irradiance for two-axis tracking surface. This defines the global normal
irradiance (GNI), which is the maximum solar resource that can be used. Direct irradiance is
more often looked at under the form of direct normal irradiance (DNI) – the direct beam
irradiance received on a surface perpendicular to the sun’s rays.
The respective proportions of direct and diffuse irradiance are of primary importance for
collecting the energy from the sun and have many practical implications. Nonconcentrating
technologies take advantage of the global radiation, direct and diffuse
(including the reflections from the ground or other surfaces) and do not require tracking.
If sun-tracking is used with non-concentrating solar devices, it need not be very precise
and therefore costly, while it increases the amount of collected solar energy. It allows
taking advantage of the best possible resource. This is worthwhile in some cases, but not
that many, as expanding a fixed receiver area is often a less costly solution for collecting
as much energy.
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© OECD/IEA, 2011