CSP Desalination and Storage
A key issue with deploying renewable energy conversion systems is their intermittent 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 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.
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.
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.
9. Cost comparison between CSP and the RO and MED desalination options for major plant systems. Source: Bechtel Power
10. Cost of desalination equipment. Source: Bechtel Power
11. Typical O&M cost for an RO desalination plant. Source: Bechtel Power
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.
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