Solar Thermal Power Plants - Renewable Energy: Technology, Economics and Environment - page 189

changed. The plant is now provided with a tube saturated steam receiver which
supplies steam of 40 bar and 250 °C.
The northern heliostat field was erected from 2005 to 2006. It consists of 624
faceted glass/metal heliostats (T type) "Sanlúcar 120" of a mirror surface of
121 m 2 each. The cavity receiver is mounted on a tower of an approximate height
of 100 m, consisting of four 5.36 x 12.0 m tube panels. The thermal storage incor-
porated into the plant has a useful heat energy of 20 MWh permitting 30 min of
operation at 70 % of load /5-15/. Plant commissioning is scheduled for 2007.
Solar Tres. This plant is based on the know-how gathered during the construction
and operation of the Solar Two plant (using salt as heat transfer and heat storage
medium). This is why the project is called "Solar Tres" (being the Spanish transla-
tion for "Solar Three"). This solar tower power plant provided with a molten salt
tube receiver and an electric capacity of 15 MW has been exclusively designed for
solar operation. The northern heliostat field has 2,494 heliostats of a surface of
96 m 2 each. The heliostats to be used are of the faceted glass/metal heliostat type
(T type) equipped with highly reflecting mirrors in simplified design (solar multi-
ple of 3). It is planned that the receiver will have a heat capacity of 120 MW and
be of cylindrical molten salt tube design. The storage (600 MWh) incorporated
into the concept is to enable operation using heat from the storage for 16 h /5-15/.
Solgate. Solgate is a pilot solar tower power plant equipped with a closed volu-
metric receiver, a secondary concentrator and a ceramic absorber of an electric
nominal capacity of 250 kW designed for hybrid operation (i.e. combined opera-
tion using natural gas and solar radiation). The heliostats arranged in the PSA's
CESA-1 field have a mirror surface of 40 m 2 each (solar multiple of 1). To date,
operation conditions have permitted air outlet temperatures of up to 1,050 °C and
the direct drive of the gas turbine /5-9/
5.2.2 Economic and environmental analysis
The following sections are aimed at assessing solar tower power plants according
to economic and environmental parameters.
Economic analysis. Within the scope of the economic analysis, power generation
costs will be calculated for the discussed types of solar thermal power plants. In
line with the preceding assessment method, applied throughout this topic, the
costs for construction and operation are determined and distributed in the form of
annuities over the technical lifetime of the power plant. On the basis of these an-
nual amortisation costs and the provided electric energy, the electricity generation
costs per kilowatt hour are calculated. If not otherwise indicated, a technical life-
time of 25 years for all machine equipment and an interest rate of 4.5 % have been
assumed to allow for a comparison with other power generation options.