POWER May 2010 - 108

Hybrid plants
5. Solar tower technology. Courtesy:
Bright Source
Table 2. Summary of MED options evaluated. Source: Bechtel Power
Case 1
Seawater TDS (mg/l)
Seawater chlorinity (mg/l)
Distillate TDS (mg/l)
Recovery
Number of effects
Gain output radio
However, several disadvantages also exist,
including these:
■ Technology not as mature as trough, with
only recent, relatively small-scale commercial
development taking place.
■ Lower power cycle efficiency due to lower
steam temperature.
■ Lower optical efficiency and increased
heat losses due to absence of insulation
around the receiver tubes.
Expect the steam cycle efficiency to improve
as technology suppliers are able to increase
operating temperatures up to 450C.
Solar Tower. In this concept, a boiler on
top of a tall tower receives concentrated solar
radiation from a field of heliostats, which are
two-axis tracking mirrors. The heat transfer
media could be water or steam, molten salt,
liquid sodium, or compressed air.
In the more conventional arrangement, the
working fluid is water (Figure 5). The water
temperature is higher than in line-focus
systems-close to 545C. The power tower
can be connected to molten salt storage,
thus allowing the system to extend operating
hours or increase capacity during periods
when power is most valuable. The main
advantage of this technology is its ability to
provide high-temperature superheated steam.
The design requires accurate aiming and control
capabilities for the solar field heliostats
to maximize efficiency and avoid potential
damage to the receiver on top of the tower.
Integrate the Disparate Systems
Several scenarios were considered in analyzing
potential integrated desalination plant/
CSP power plant options. As part of the case
studies, the first parameter varied was the location
of the CSP-desalination/power plant
facility. Three locations, each possessing plentiful
insolation (exposure to sun) and lacking
freshwater supplies, were considered:
■ Egypt on the coast of the Mediterranean Sea
■ Saudi Arabia on the Red Sea
■ California, U.S., on the Pacific Ocean
108
Steam pressure (bar abs)
Steam temperature (C)
Distillate flow rates (m3/day)
Mediterranean Sea
35,000
21,000
10
0.2
10
8.5
3
135
2,500
7,200
45,500
Case 2 Pacific
Ocean
30,000
18,000
10
0.2
8
6.8
2
125
2,500
7,200
45,500
Case 3 Red Sea
45,000
27,000
10
0.15
12
10.2
3
135
2,500
7,200
45,500
Table 3. Summary of SWRO options evaluated. Source: Bechtel Power
Case 1
Seawater TDS (mg/l)
Seawater chlorinity (mg/l)
Permeate TDS (mg/l)
Seawater RO recovery (%)
Energy recovery device
Permeate flow rate (m3/day)
Mediterranean Sea
35,000
21,000
105
50
Pelton Wheel
40% recovery
2,500
7,200
45,500
For each location, installation of three
different-size desalination facilities, each designed
to provide freshwater for a different
power plant configuration that would supply
water for a new plant and a nearby town, was
evaluated:
■ 2,500 m3
■ 7,200 m3
■ 45,500 m3
/day (0.7 mgd)
/day (2 mgd)
/day (12 mgd)
For all three physical locations and all
three power plant/desalination system configurations,
installation of both MED and
SWRO as the desalination technology was
considered. Tables 2 and 3 provide a summary
of the desalination/power plant configuration
scenarios investigated.
The primary aim of CSP plants is to generate
electricity, yet a number of configurations
enable CSP to be combined with various
desalination methods. When compared with
photovoltaic (PV) or wind, CSP could provide
a much more consistent power output
when combined with either energy storage or
fossil fuel backup. The most suitable options
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Case 2 Pacific
Ocean
30,000
18,000
100
55
Pelton Wheel
40% recovery
2,500
7,200
45,500
Case 3 Red Sea
45,000
27,000
155
35
Pelton Wheel
40% recovery
2,500
7,200
45,500
will be described here.
A typical solar trough configuration combined
with a MED system where steam generated
is first expended in a noncondensing
turbine and then used in a conventional manner
for desalination (Figure 6). The steam
generated by a trough plant is superheated to
around 380C. As described earlier, the steam
temperature for the MED plant is around
135C. Therefore, there is sufficient energy
in the steam to produce electricity before it
is used in the MED plant. It is important to
emphasize that water production is the main
purpose of the plant-electricity is a byproduct.
While conventional combined-cycle
(CC) power plants can be configured in a
similar manner for desalination, a fundamental
difference exists in the design approach
for solar and for fossil fuel-fired plants. The
fuel for the solar plant is free; therefore, the
design is not focused primarily on efficiency
but on capital cost and capacity of the desalination
process. In contrast, for the CC power
plant, electricity production at the highest
possible efficiency is the ultimate goal.
As described previously, the RO system
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POWER May 2010

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