Solar Thermal Power Plants - Renewable Energy: Technology, Economics and Environment - page 186

Power plant cycles. The power plant cycles applied for solar tower power plants
are mainly based on conventional power plant components commercially avail-
able today; the currently achievable pressures and temperatures of the working
media applied for solar tower power plants are in line with the current power plant
technology. Solar tower power plants within the capacity range from 5 to
200 MW can thus be designed using commercially available turbines and genera-
tors including all required auxiliaries.

5.2.1.2 System concepts

According to the applied heat transfer fluid or working medium, different system
concepts are applied. Since open or cavity tube receivers reach working tempera-
tures of approximately 500 to 550 °C, they are predominantly applied for Rankine
cycles run by steam. Steam is either generated directly inside the receiver or by
the secondary circuit (e.g. molten salt).
Hot air of approximately 700 °C generated by open volumetric receivers can be
used within existing steam generators, similar to e.g. heat recovery boilers. The
inlet temperature can, for instance, be maintained constant by an incorporated
natural gas-fired duct burner, so that this concept is particularly suitable for hy-
bridisation (i.e. application of solar energy in combination with fossil fuels like
e.g. natural gas). Outlet gas/air is re-transferred to the receiver by means of a
blower so that at least up to approximately 60 % are re-circulated.
Another possibility is the so called inverse gas turbine process. Within such a
cycle an open volumetric air receiver is used and the hot air is directly fed into the
gas turbine where the air is expanded /5-10/. One advantage compared to a steam
cycle is a much simpler design. But so far such cycles have only been analysed
theoretically.
In the past, several solar tower power plants have been realised within R&D
projects sponsored by public money and industry. Within the following explana-
tions some of these research plants are presented.

Solar One. Solar One is a solar power plant of an electric capacity of 10 MW,
which was operated from 1982 to 1988 in the Californian Mojave Desert. This
plant proved the general feasibility of solar thermal power generation by tower
plants at the megawatt scale. Water was used as heat transfer fluid for the re-
ceiver. Among other difficulties, the plant showed the problem of maintaining
operation when there are cloud passages.
Fig. 5.10 shows the performance characteristics of the Solar One plant as a
typical example of performance characteristics of solar tower power plants. Ac-
cording to this figure, electricity is generated from a daily overall direct radiation
of 4 to 5 kWh/(m 2 d) onwards. With increasing direct radiation the electricity
output increases approximately linearly. The threshold from which electricity is
generated, is mainly determined by the water steam tube receiver technology. This