Page 146 - A Comprehensive Guide to Solar Energy Systems

Chapter 7 • Concentrating Solar Thermal Power 145

The feasibility of supercritical CO 2 , (s-CO 2 ), is also being investigated to increase the
efficiency of CSTP plants with central receiver technology because it would allow higher
working temperatures in the receiver and the Rankine cycle of the PCS could be replaced
by a Brayton cycle resulting in higher efficiencies [18]. Because s-CO 2 has good thermo-
physical properties as a heat carrier medium, a significant effort is being devoted to using
it by R + d centres, located in countries such as the united States of America and Australia.
The density of s-CO 2 is similar to that of liquid water and allows for the pumping pow-
er needed in a compressor to be significantly reduced, thus increasing the thermal-to-
electric energy conversion efficiency of the Brayton cycle. This is the main reason why
the use of s-CO 2 is one of the key R + d topics included in the American SunShot initiative
(https://energy.gov/eere/sunshot/sunshot-initiative), which is a national effort support-
ed by the uS department of Energy to drive down the cost of solar electricity and support
the use of solar energy to replace fossil fuels. The target of SunShot for CSTP plants is to
−1
lower the cost of STE to $0.06 (kW h) . At present, the main short-term challenge in this
technology program is the implementation of a small (≤10 mW e ) experimental plant using
a Brayton cycle with s-CO 2 . Such a plant requires the design and manufacture of special
equipment that is not yet available in the market (this includes s-CO 2 heat exchangers and
turbine). A complete set of technical documents concerning power cycles and equipment
for s-CO 2 , as well as the main problems associated with this working fluid (i.e., corrosion
and erosion in the associated equipment) are available at: http://energy.sandia.gov/en-
ergy/renewable-energy/supercritical-co2.
Another interesting research topic related to central receiver technology is the use
of air, either at atmospheric pressure or under pressure. As air is freely available, the
development of a central receiver technology using air as working fluid is a very compel-
ling option. Central receivers using air at atmospheric pressure are made of ceramic or
metallic porous elements that heat the air flow circulating through them. The ceramic or
metallic porous elements composing the receivers are heated by the concentrated solar
radiation reflected by the heliostats, and the heat is then transferred to the air. The hot
air leaving the receiver is then sent to a thermal storage system and/or to a steam gen-
erator (air/water heat exchanger) where the superheated steam needed for the Rankine
cycle of the PCS is produced. The thermal storage vessel and medium suitable for this
type of central receiver system must withstand high-temperatures (>800°C) and heavy
thermal cycling without degradation. So far, pellets of alumina (Al 2 O 3 ) have been often
used as high-temperature storage medium in experimental facilities. Fig. 7.11 shows the
scheme of the pilot plant installed at the Plataforma Solar de Almería research facility
(PSA, www.psa.es).
Central receiver plants using atmospheric air have a lower efficiency than those with
molten salt receivers (Fig. 7.8). However, the simplicity and ease of operation of volumetric
receivers with atmospheric air could compensate for their lower efficiency. It must be kept
in mind that overall plant efficiency is not the only key aspect of a CSTP plant, because
the investment and operation and maintenance costs also play a significant role in the