Environmental Engineering Reference
In-Depth Information
Technical advances made in the meantime have helped to increase effi ciency and
reduce costs. One option was the use of direct solar steam. With this process water
is vaporized at a high pressure by the collectors and heated up to 500 °C. This steam
is led directly into a turbine, thereby rendering thermo oil and heat exchangers
superfl uous.
Temperature and Effi ciency
The Achilles heel of solar thermal power plants is not the solar collector that
concentrates the sunlight. Collectors easily achieve effi ciency of over 70%.
However, most of the valuable solar heat is lost during its conversion into electricity.
The steam turbines used in this process in solar power plants barely manage to achieve
35% effi ciency. In other words, 65% of the heat gained from the sun returns unused as
waste heat into the environment. The effi ciency of steam turbines results directly from
the temperature difference of the steam between entry into and exit from the turbines.
The exit temperature depends on the cooling and even at best is only minimally below
the ambient temperature. With parabolic trough power plants, the entry temperature,
contingent on the thermo oil, is currently just below 400 °C. With a temperature increase
to 500 degrees or higher, the effi ciency of turbines could easily reach 40%. But this is
the maximum even for steam turbines. Combined gas and steam turbines, which operate
at temperatures of over 1000 °C, achieve effi ciency of up to 60%. Gas and steam turbines
with high effi ciency can be used, for example, in solar tower power plants.
One question that is often asked is whether simple pipe collectors can be used to generate
electricity from general purpose water. In principle, this would be possible. However,
due to the extremely low temperatures, effi ciency would be too minimal to make this
approach economically viable. Even the waste heat in the environment would have
limited use. Solar power plants are normally situated in hot, sunny regions. However, a
demand for gigantic quantities of low-temperature heat does not really exist in these
areas.
7.2.2 Solar Tower Power Plants
With solar tower power plants, several hundred or even thousand rotating mirrors
are arranged around a tower. Called heliostats, these mirrors are individually con-
trolled by computer to track the movement of the sun and are orientated towards
the top of the tower. They must be orientated precisely within a fraction of a degree
so that the refl ected sunlight actually reaches the focal point. A receiver is located
here with an absorber, which, due to the highly concentrated sunlight, heats up to
temperatures of over 1000 degrees. Air or molten salt transports the heat. A gas or
steam turbine that drives a generator ultimately converts the heat into electric
energy.
With the tower concept with open volumetric receivers (Figure 7.7), air from the
environment is sucked by a blower through a receiver towards which the heliostats
are orientated. The materials used for the receiver are wire mesh, ceramic foam or
metallic or ceramic honeycomb structures. The receiver is heated up by the solar
radiation and transfers the heat to the sucked-through ambient air. The sucked-in
air cools the front side of the receiver. Very high temperatures only develop on the
