Page 141 - A Comprehensive Guide to Solar Energy Systems

140 A COmPREHEnSIVE GuIdE TO SOlAR EnERGy SySTEmS

Both, the saturated-steam and superheated-steam central receiver plants have a
common problem: the difficulty to store thermal energy during sunlight hours to pro-
duce electricity when solar radiation is not available. Only small-capacity thermal stor-
age systems are cost-effective for this type of plant with the result that the stored energy
can only keep the turbine running at nominal power for minutes rather than hours. The
reason is that the saturated water and steam must be stored in very large and expensive
thick-wall steel vessels to provide a stable operation of the turbine during solar radia-
tion transients. Higher storage capacity would require many high-pressure vessels con-
nected in parallel, which are very expensive and would jeopardize the profitability of
the plant.
As the possibility of producing electricity at night is one of the main benefits of CSTP
plants, other central receiver technologies more suitable for thermal storage during
sunlight hours have been developed for commercial plants using central receivers. The
most popular one involves molten-salt receivers. A molten-salt receiver looks similar
to a saturated or superheated-steam receiver, because it is made of many parallel steel
tubes coated with a black thermal paint. The main difference is the raw material used for
the steel tubes and the wall thickness of the tubes, because the working pressure with
molten-salt is much lower (just a few bars at the receiver outlet). The molten-salt used
in these central receiver plants is the same as is used for thermal storage in plants with
PTC: a binary mixture of potassium nitrate (40%) and sodium nitrate (60%). The afford-
able price, low viscosity and good thermal stability (up to 600°C) of this binary salt make
it a very good option for use as both the thermal storage medium and the working fluid
in the central receiver. The first commercial CSTP plant with a central receiver using
molten salt in both the receiver and the thermal storage system was the GEmASOlAR
plant, which was developed by the company Torresol Energy in 2011 in the province of
Seville (Spain) (http://www.power-technology.com/projects/gemasolar-concentrated-
solar-power/).
This plant has a nominal net output of 19.9 MW e and is fitted with a molten salt ther-
mal storage system able to keep the plant running at full load for 15 h after sunset. Fig. 7.8
shows the scheme of this plant. Molten salt at 290°C is pumped to the receiver from the
cold storage tank to increase its temperature to 565°C as it circulates through the receiver.
Once heated, the molten salt is sent to the hot storage tank where it remains until used
in the steam generator to produce the superheated steam required to drive the turbo-
generator and thus produce the electricity exported to the grid. After the steam generator,
the molten salt is sent to the cold storage tank. As the amount of hot salt delivered by the
receiver during sunlight hours is more than that required to run the turbo-generator at full
load, the level of the molten salt inside the hot storage tank increases, thus charging the
storage system to produce electricity after sunset.
The good dispatchability and overall efficiency of this type of CSTP plant are boosting
the number of commercial projects being developed with this technology in many coun-
tries such as united States of America, morocco, South Africa, and China.
The PCS, also called Power Block, so far used in commercial CSTP plants with central
receivers is composed of a water/steam Rankine cycle with superheated steam, similar