POWER May 2010 - 112

Hybrid plants
9. Cost comparison between CSP and the RO and MED desalination options
for major plant systems. Source: Bechtel Power
Concentrating solar power/reverse osmosis
Concentrating solar power/multiple effect distillation
100
95
90
85
80
75
70
65
60
55
50
45
40
35
30
25
20
15
10
5
11. Typical O&M cost for an RO desalination
plant. Source: Bechtel Power
Miscellaneous, 7%
Chemicals,
11%
Membrane
11%
Power, 71%
Solar collector field
Power block
10. Cost of desalination equipment. Source: Bechtel Power
1,400
1,200
1,000
800
600
400
200
Multiple effect distillation
Seawater reverse osmosis
Desalination plant
All the desalination technologies are striving
to reduce cost, increase capacity and efficiency,
and reduce environmental impact.
The large number of CSP projects under
development for all types of technologies-
tower, trough, linear Fresnel-will lead to
improved equipment, better field experience,
and lower capital costs.
2,500
nature. Wind, PV, and CSP generate power
only when the wind blows and the sun shines.
Obviously, the benefits of energy storage
are invaluable in improving the grid stability,
power quality, and continuity of supply.
When compared with heat or electricity, the
desalination product-water-can be stored
very economically. This provides an additional
advantage for combining solar thermal
plants with desalination.
Due to variations in seasonal and diurnal
solar heat input, many solar fields for CSP
are designed to match an average insolation
flux. In the summer and during peak daylight
hours, an excess of heat is produce by
the solar field. In the absence of a storage
system capable of absorbing this heat, some
of the solar collectors or mirrors have to be
defocused. This process is called " energy
112
7,200
Water production (m3/day)
dumping. " By directing this excess heat
to the desalination plant, one can produce
water and store it for future use. This way
the capacity factor of the CSP plant could
be dramatically increased and the economic
picture improved.
Although water consumption remains relative
flat over the entire year, electric power
demand increases by more than 50% during
the summer due to air conditioning loads. The
combination of a hybrid solar power generation
and desalination plant would allow some
degree of discretionary allocation of the heat
input to either of the applications-power
or water-depending on the sizing of the
electrical generation equipment. Some of
the electrical power generated at off-peak
demand conditions could be used to produce
water and appropriately store it.
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45,500
It's All About the Money
In evaluating capital costs as well as operating
costs, one important criterion is
the size of the solar field for each of the
desalination technologies. Figure 9 provides
a comparison of the relative costs
of CSP combined with RO versus CSP
combined with MED. It can be seen that
the solar field as well as the conventional
power plant costs are higher for the RO option,
but the actual desalination plant cost
is lower (see Trieb, referenced above). A
comparison of the equipment cost for the
desalination equipment alone is shown in
Figure 10.
It is evident that the RO systems are
less expensive for all sizes of plants. When
compared with conventional systems, the
viability of such solar desalination systems
is dependent on the escalation of fuel
costs, operation and maintenance costs
of combined cycles, and market price for
water. The operating costs of an RO plant,
presented in Figure 11, indicate that the
power is the most significant contributor
(72%). For a desalination plant, a key factor
in cost reduction is an increase in process
efficiency by improving the recovery
power ratio. ■
-Dr. Justin Zachary is technology
manager and a Bechtel Fellow for Bechtel
Power Corp. Zachary is also a POWER
contributing editor. Colleen M. Layman is
manager of water treatment for Bechtel
Power Corp.
POWER | May 2010
Estimated cost ($/m3)
Percent of total cost
http://www.powermag.com
POWER May 2010

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