Page 139 - A Comprehensive Guide to Solar Energy Systems

138 A COmPREHEnSIVE GuIdE TO SOlAR EnERGy SySTEmS

7.3.1 Main Components

The main four elements in a central receiver plants are (Fig. 7.6):
1. the heliostat field
2. the receiver
3. the power conversion system (PCS), and
4. the thermal storage system (TES)

The heliostat field of a plant with a central receiver is usually composed of thousands of
heliostats, which are reflecting surfaces supported on a vertical pylon. The reflecting sur-
face has two degrees of freedom (up/down, and right/left movements) to reflect the collect-
ed direct solar radiation onto the receiver. Each heliostat is provided with a local control in
continuous communication with the central control room to receive the commands related
to the position to be adopted by the reflecting surface to assure that the collected radiation
is reflected in the right direction toward the receiver. The local control manages the drive
unit of the heliostat according to the commands received from the central computer locat-
ed in the control room. The reflecting surface of each heliostat is composed of mirror seg-
ments, called “facets,” which are attached to a metallic structure and placed altogether in a
predefined way to have the small curvature required to project onto the receiver a reflected
image with a total area smaller than the area of the reflecting surface itself. This positioning
of each facet during the heliostat assembly process is performed with a very high accuracy.
As the distance between the central receiver and the outer heliostats of the solar field may
be of 1 km and even more in large plants, a very high degree of accuracy in the positioning
of the facets within the total reflecting surface of the heliostats is required (Fig. 7.7).
When the nominal power of the CSTP plant is small (>20 mW e ) the heliostats are placed
at one side of the tower only (at the north side in plants located in the north Hemisphere
and at the South side in plants located in the South Hemisphere) because a circular re-
ceiver would be less efficient due to higher optical losses.
There are different types of receivers for these solar systems. The first commercial plant
with a central receiver was the PS-10 plant built in 2007 by the company ABEnGOA in the
village of Sanlucar la mayor (Seville, Spain) [14]. This plant has a saturated-steam receiver
placed on a tower of 110 m, where the liquid water entering the receiver is converted into
saturated steam at 250°C and 40 × 10 Pa (40 bar) by the solar radiation reflected by 627
5
2
heliostats with a total reflecting surface of 75 216 m . The saturated steam feeds a 10 mW e
(net power) turbo-generator.
Because of the lower efficiency of steam turbines using saturated steam, other options
have been commercially implemented. For example, the company BrightSource Energy
(http://www.brightsourceenergy.com/) built the superheated steam plant, IVAnPAH, in
Ivanpah dry lake (California, uSA) , composed of three units of central receiver systems
with a total net power of 377 mW e . Since its start-up this is the largest central receiver plant
in operation to date. It is composed of 173 500 heliostats with a total reflecting surface of
2
2.6 × 10 m . The receiver installed in each of the three 140 m high towers, convert the feed
6