Environmental Engineering Reference
In-Depth Information
limiting reflective losses. Along with the antireflective coating, significant develop-
ments have been made in improving absorber performance.
Most of heat collection elements are applied to stand-alone concentrated solar
power plants with synthetic oil as the heat transfer fluid. The synthetic oil decomposes
at high temperature, producing hydrogen which permeates across the tubes and ends up
in the vacuum annulus. The presence of hydrogen in the vacuum annulus is detrimental
for the thermal performance of the collector. For this reason the vacuum is usually filled
with getters which absorb hydrogen and other gases that eventually permeate during
operation. The getters are barium-based and, as additional components required for
the HCE, they affect the overall costs.
The adoption of vacuum glass is beneficial for HCE efficiency, but this requires a
dedicated glass-to-metal sealing. In addition, the adoption of bellows is necessary to
equalize the thermal expansion of metal and glass in order to guarantee the vacuum
conditions in the annulus. A drawback of the bellows is the shadow thrown onto
the absorber tubes, with consequent efficiency penalties. However, recently SCHOTT
PTR 70® have reduced the impact of bellows, leading to an active area above 96.7%
(SCHOTT PTR® 70 Brochure). Active length is defined as the active aperture area of
the receiver on the total receiver area.
Commercial HCE efficiency as a function of heat transfer fluid (HTF) temperature
is shown in Figure 14.3.6. Finally, in order to limit bending of the tube, HCE length
is usually equal to 4 m. For this reason, several HCEs are placed in series to make
a trough.
14.3.5 Structure
The structure, usually of metal, serves to hold the heat collection element and the
reflectors at the correct position. The parabolic shape of the reflector is usually formed
by the structure. The structure has to fulfil the following requirements: efficient use of
material, ease of transportation onto the site, easy to assemble, and able to withstand
atmospheric conditions for at least 30 years. Moreover, it must have a high torsional
and bending stiffness under wind loads (the structure must be designed to work under
wind conditions, typical of desert locations). For example, Andasol plant can con-
tinue operating in winds up to 13.6 m/s (about 50 km/h); above this wind speed, the
collectors are put in a wind-protected position. It must be remembered that the aper-
ture length of the parabola is in the range of 6 m and wind drag and lift can be really
significant. For these reasons, research activity on structure development is carried
out using computational fluid dynamics (CFD) analysis and wind tunnel testing to
determine the required torsional and bending stiffness in order to achieve a desired
interception factor at defined conditions. An example of CFD analysis on parabolic
troughs performed at Politecnico di Milano is shown in Figure 14.3.7.
Research activity has led to different types of structure becoming available on
the market, each based on a different concept. Flagsol's Eurotrough and Ultimate
Trough are based on a torque box design and made from galvanized steel (Graf and
Nava, 2011). The torque box guarantees savings on materials, uses thick and reliable
mirrors, and reduces wind loads compared to other configurations. SENER proposes
a torque tube plus stamped steel cantilever, with the mirrors supported by arms. The
design of Acciona solar power is based on recycled aluminium or steel struts and geo
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