Other cooling options
Dry cooling eliminates a thermal power plant’s dependence on cooling water. The plant’s steam is condensed inside finned tubes by blowing air across their exterior surfaces. The challenges of dry cooling include much higher capital and installation costs, a high efficiency penalty, increased exhaust gas emissions, and load limitations on hot days.
Currently, dry cooling is used, or viewed as an option of last resort, where water is very costly or limited in availability. There are now several plants in operation or under construction that use dry cooling; most are gas-fired, combined-cycle units. As a result, in the U.S. there is only limited experience with dry cooling of baseload-scale plants. Advanced technologies for dry cooling larger plants would be of great interest to power project developers if the technologies would reduce the efficiency and capital-cost penalties.
One developer framed the problem succinctly as follows: “We wouldn’t go to dry cooling unless we really had to, because of enormous capital and operating costs, and lower plant efficiency.” Efficient air cooling options must be expanded and made less costly for future plants.
Hybrid cooling represents a middle ground that may be more appealing and feasible for baseload plants. Hybrid cooling systems use a combination of both wet and dry cooling technologies to conserve water. Although they decrease the hot weather penalty, they reduce but don’t completely eliminate the need for cooling water. Hybrid systems can limit annual water use to 2% to 5% of what wet recirculating cooling systems use, although 20% to 80% is a more typical range. Generation efficiency and capacity generally increase with greater water use.
It is only where the costs of water are highest that air cooling is cost-competitive with water cooling (Figure 5). For example, if our 350-MW reference plant were in El Paso, Texas, dry cooling would be cost-competitive only when the cost of water exceeds about $3/kgal. Above that level, dry cooling would be preferred because its cost is unaffected by the cost of water.
5. Breakeven points. Comparing the costs of wet and dry cooling for two hypothetical 350-MW plants—one in Portland, Ore., and the other in El Paso, Texas. Source: EPRI, 2004
The magnitude of potential savings for generators in warmer climates approaches 20% of cooling costs. Look at the cost curves of Figure 5 for plants in El Paso, Texas, and Portland, Ore. (see POWER, September 2007, “Port Westward Generating Plant”). The difference is due entirely to El Paso’s hot weather penalty, which is on the order of $1.5 million/year in cooling costs, according to a 2004 EPRI report. The goal of ongoing research into improved air-cooled and/or hybrid technologies is to reduce costs for a plant of this capacity by a significant share. A reduction of 33% to 66% in the hot weather penalty would produce annual savings of $500,000 to $1,000,000.
Email
Comments
Print
Reuse
