Solar Energy Perspectives - IEA

Solar Energy Perspectives: Solar heat
Parabolic troughs
Parabolic trough systems consist of parallel rows of mirrors (reflectors) curved in one
dimension (i.e. semi-cylindrical) to focus the sun’s rays. The mirror arrays can be more than
100 m long with the curved surface 5 m to 6 m across. The heat collectors are stainless steel
pipes (absorber tubes) with a selective coating (designed to allow pipes to absorb high levels
of solar radiation while emitting very little infra-red radiation). The pipes are insulated in an
evacuated glass envelope.
Simple small installations can have the trough rotate around a fixed pipe. With increased sizes
however, the mechanical forces soon become intractable and the rotation axis must be set at
the gravity centre of the device. This creates the need to move absorber tubes and therefore
ball joints or flexible hoses subject to potential leakages. The use of a low-pressure HTF
(synthetic oil) is the standard with troughs; direct steam generation is currently under study.
Parabolic troughs are the most widely used concentrators for solar thermal electricity (STE)
today (see Chapter 8) and represent 90% of the current market. Electricity generation
currently represents an even greater share of the market for parabolic troughs. Other
applications, such as rooftop devices to cogenerate heat, electricity and cold, and industrial
process heat applications, are commercialised, notably in the United States.
Parabolic troughs are usually oriented along a north-south axis and track the sun from east to
west. Orientation along an east-west axis would collect less energy over the year, but more
during the winter by reducing the cosine losses 2 resulting from the sun being low in the sky.
Combining both orientations in a single facility has been suggested but never put into practice.
A new step could possibly be reached with mirror-film reflectors of much larger size than
current glass-made troughs (Photo 7.6). Increased concentration factors would allow
increasing working temperatures while keeping good collector efficiencies, but would
require new HTF or working fluids.
At the linear focus of the semi-cylindrical parabola, the heat collector element is designed to
capture as much of the solar flux as possible, while minimising radiation and convection
thermal losses. The capture is facilitated by selective coating of the receiver tube, and
minimising losses by an evacuated transparent glass envelope. Different heat transfer or
working fluids can be piped through the collectors, depending on the use of the installation.
Parabolic troughs have a relatively good optical efficiency but need to be distanced from each
other to minimise shading, which happens when a mirror on the trough intercepts part of the
solar flux incident on another.
Fresnel reflectors
Linear Fresnel reflectors (LFRs) approximate the parabolic shape of trough systems, but use
long rows of flat or slightly curved mirrors to reflect the sun’s rays onto a downward-facing
linear, fixed receiver. Compact linear Fresnel reflectors (CLFRs) use two parallel receivers for
each row of mirrors (Figure 7.7).
2. Cosine loss is energy lost by not facing the sun's rays directly.
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© OECD/IEA, 2011