Coal, Gas

The Hot and Cold of Dry Cooling

Issue 10 and Volume 105.

By Brian K. Schimmoller,
Managing Editor

Demand for water is increasing in almost all sectors of the economy – residential, agricultural, industrial – and the power generation sector is no exception. The current avalanche of gas-fired power plants is associated with a tremendous thirst for water: water for cooling, for HRSG supply and makeup, for inlet air conditioning, etc.

Where demand for water is greatest, and supply is the lowest, particularly out West, air-cooled condensers are being evaluated and installed in larger numbers because of their ability to drastically cut water consumption levels. Water consumption at power plants equipped with air-cooled condensers is about one-tenth that of power plants equipped with conventional cooling towers. Reliant Energy Wholesale Group, for example, has committed to air-cooled condensers at multiple facilities, including the recently announced 580 MW Signal Peak plant in Pinal County, Ariz. and the 480 MW El Dorado plant operational near Las Vegas, Nev. Justification for the use of air-cooled condensers stems from their proven technical viability and from the relative ease with which dry cooling technology can gain regulatory approval and community acceptance.

The life-cycle costs of air-cooled condensers (capital and operating costs over the life of the plant) are generally higher than those of wet cooling towers. “The cost differences, however, are decreasing,” says Dr. Detlev Kroger, University of Stellenbosch, “due to the rapid increase in the cost of water in many places and recent developments in air-cooled condenser technology, such as newer single-row finned tubes, which make them less costly.

Despite the higher costs, the use of dry cooling technology can be expected to increase, strictly for the regulatory and public acceptance reasons mentioned above. A more subtle issue arises with respect to operations, however. At high dry bulb temperatures, the ability of the air-cooled condensers to condense the steam is hampered by the smaller temperature difference between the air and steam. The result is increased backpressure and reduced plant output. This issue is significant because the time when high dry bulb temperatures predominate – late afternoon in the summer – corresponds to the time when demand is highest. The ability of the plant to meet demand, therefore, is in question.

Recognizing the advantage of lower air temperatures in maintaining or restoring condenser performance, some facilities have considered the use of inlet air conditioning technology. Evaporative cooling and direct water spray technologies are potential options, although the huge volumes of air that have to be moved across air-cooled condenser fans make fogging-type systems uneconomical, says Kent Zammit, Manager of Cooling Water Technologies with EPRI. In association with the California Energy Commission, EPRI is developing an inlet conditioning system that would offer a cost-effective alternative for air cooling without exposing the fan blades or heat transfer coils to damaging water droplets. EPRI has completed pilot-scale tests and some commercial tests at the Crockett Cogenerating Station in Crockett, California, and is developing plans to conduct a full-scale test next summer at a larger plant that typically experiences high summer temperatures. Admittedly, inlet conditioning approaches mitigate one of the air-cooled condenser’s distinguishing features since they require some water consumption, but the consumption would be sporadic and much less than that of wet cooling towers. The EPRI system also may allow use of non-potable water sources.

Inlet air conditioning ahead of air-cooled condensers is not a new technology. Richard Leitz, vice president with Balcke-Durr, estimates that one-fourth of the industrial heat exchangers at chemical plants and refineries are equipped with humidification systems. These systems, which are installed in the circumference of the inlet to the fan, are typically designed for about an 80 F dry bulb temperature and achieve about 98 percent humidity. Leitz is not aware of any large power plants equipped with humidification systems for air-cooled condensers. Water quality is a major concern. Poor quality water can result in the precipitation of soft salts, which then get “baked” on the air-cooled condenser’s heat transfer tubes, collecting dust and inhibiting heat transfer. Because of the potential for these problems, spray systems should strictly be used for peak-shaving applications, not extended use, says Leitz.

Another technique for increasing an air-cooled condenser’s ability to provide the needed cooling during hot, peak load conditions is to increase the air flow rate through the condenser fan, says Leitz. By installing oversized two-speed fan motors or variable frequency drives that can span a larger load range, the fan has some reserve capacity to tap during hot weather conditions. The additional cost of the larger fan or VFD is not unsubstantial, but may be worth it in some instances.

The best alternative for enhancing the performance of the air-cooled condenser may have nothing to do with the air-cooled condenser operation itself. “Rather than spending money on the air-cooled condenser to regain a small amount of capacity, the money may be better spent in the design phase by optimizing the gas turbine, steam turbine and/or HRSG to get extra capacity,” says Leitz. For example, steam turbines are now available that can accommodate higher backpressures than older turbine models, according to EPRI’s Zammit.

The bottom line with respect to air-cooled condensers is that developers must carefully consider the interactive impacts among technology selection, expected weather patterns, unit dispatch, and risk tolerance before committing one way or another. If the plant is expected to operate significantly during periods of very hot weather, says Leitz, the best option may be to cater the entire plant design to greater power output. If the plant will operate in high temperatures a modest number of hours, the owner may want to consider installing variable frequency drives that can safely run at 110 percent load to maintain power production. If the plant will operate infrequently at high temperatures, the best bet might be to use a low-tech humidification system that can provide the boost when necessary without substantially exposing the heat exchange surfaces to damaging contaminants.

Air-cooled condensers provide effective cooling/condensing for power plant applications, but developers must be aware of their limitations and design accordingly, based on various potential operating scenarios.