POWER May 2010 - 110

Hybrid plants
is an alternative to thermal desalination
processes. In this case as well as in MED,
the steam generated by the solar plant can
be used through a steam turbine to produce
the electric power needed to drive the RO
pumps. As an alternative for large, multiunit
RO systems, the high-pressure seawater
can be provided by a single pump driven by
a steam turbine. This arrangement is similar
to the steam-turbine-driven boiler feed
pumps in a fossil fuel power plant. Obviously,
a feasibility study must be conducted
to determine the most appropriate solution
for each situation.
Often, MED and RO are compared in
terms of overall performance and specifically
for energy consumption. Based on
several Bechtel internal studies as well as
open literature, one can conclude that in
specific cases, the CSP and RO (Figure 7)
require less energy than a similar CSP and
MED combination.
However, an analysis presented by
Franz Trieb ( " Concentrating Solar Power
for Seawater Desalination, " DLR report,
Stuttgart, 2007) suggests that, for several
locations, CSP/MED requires 4% to 11%
less input energy than CSP/RO. Therefore,
before any decision can be made on the
type of desalination technology to be used,
it is recommended that a detailed analysis
be conducted for each specific location,
evaluating the amount of water, salinity of
the input seawater, and site conditions. It
appears that CSP/MED provides slightly
better performance at sites with high salinity,
such as the Arabian Gulf, whereas
CSP and RO appear to be more suitable for
low-salinity waters in the Atlantic Ocean
or Mediterranean Sea.
One additional advantage of the RO system
is that the solar field might be located
away from the shoreline. The only connection
between the two is the production of
electricity to drive the RO pumps and other
necessary auxiliary loads.
Linear Fresnel technology development
could have a significant impact on desalination
if its design, thermal performance,
and cost can meet industry expectations.
Operating within the necessary temperature
ranges for MED as well as RO, despite
the lower optical efficiency, linear
Fresnel may prove to be the best solution
for standalone desalination plants. Figure
8 depicts a linear Fresnel system arrangement
that includes a steam turbine with a
geared system for direct drive of the seawater
pumps.
CSP Desalination and Storage
A key issue with deploying renewable energy
conversion systems is their intermittent
110
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6. Solar parabolic trough power plant with oil steam generator and
MED desalination. Source: Bechtel Power
Solar field
Solar
superheater
Steam
generator
Solar preheater
Brine
MED system
Seawater
Steam turbine
Daerator
Expansion vessel
7. Solar parabolic trough power plant with oil steam generator and
SWRO desalination. Source: Bechtel Power
Solar field
Solar
superheater
Steam
generator
Solar preheater
Desalinated
water
Reverse
osmosis
desalination
system
Daerator
Expansion vessel
8. Linear Fresnel power plant with RO system. Source: Bechtel Power
Brine
Steam
turbine
Seawater
Electric generator
Steam
Gear
Solar field
Desalinated
water
Reverse
osmosis
desalination
system
Brine
POWER | May 2010
Seawater
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POWER May 2010

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