TECHNOLOGIES
Many technologies are commercially available for TIC. These technologies can be divided into the following major categories/groups:
All of the technologies listed above have pros and cons. Many published articles are available on these technologies. A number of these publications are listed in the Library section.
Wetted media is one of the evaporative cooling technologies in which cooling is achieved by evaporation of the water added to the CT inlet air. Historically, it is the first technology to be used for TIC. In this technology, the inlet air is exposed to a film of water in one of the many types of wetted media. A honey-comb-like medium is one of the most commonly employed media. The water used for wetting the medium may require treatment, depending upon the quality of water and the medium manufacturer's specifications. Wetted media can cool the inlet to within 85% to 95% of the difference between the ambient dry-bulb and wet-bulb temperature. It is one of the lowest capital and operating cost options. Its primary disadvantage is that the extent of cooling is limited by the wet-bulb temperature and is therefore, weather dependent. In addition, it also requires proper control of the chemistry of the re-circulating water that absorbs contaminants and monitoring of the media degradation. It works most efficiently during hot and dry weather and is less effective when ambient humidity is high. On an overall basis, this is the most widely used technology.
Fogging is another evaporative cooling technology. The basic idea in this technology is to add water to the inlet air by spraying very fine droplets of water. Fogging systems can be designed to produce droplets of variable sizes, depending on the desired evaporation time and ambient conditions. The water droplet size is generally less than 40 microns and on an average it is about 20 microns. The water used for fogging typically requires demineralization. Fogging systems can cool the inlet air by 95% to 98% of the difference between ambient dry-bulb and wet-bulb temperature and is therefore, slightly more effective than the wetted media. Its capital cost is very comparable to that for the wetted media and has similar limitations and disadvantages. It is the second most frequently applied technology for TIC. Some CT OEMs do not allow fogging due to compressor degradation and failures associated with fogging.
Overspraying/Wet Compression is yet another evaporative cooling technology in which more fog is added to the inlet air than can be evaporated under the conditions of the ambient air. The air stream carries the excess fog into the compressor section of the CT where it further evaporates, cools the compressed air and creates extra mass for boosting the CT output beyond that possible with the evaporative cooling technologies but its maintenance costs may potentially be higher due to the impingement characteristics and quality of the water ingested.
Mechanical Chiller systems can cool the inlet air to much lower temperatures than those possible with evaporative cooling and can maintain any desired inlet air temperature down to as low as 42oF, independent of ambient wet-bulb temperature. The mechanical chillers used in these systems could be driven by electric motors or steam turbines. Drawing the inlet air across cooling coils, in which either chilled water or refrigerant is circulated, cools it to the desired temperature. The chilled water can be supplied directly from a chiller or from a TES (Thermal Energy Storage) tank that stores ice or chilled water. A TES is typically used when there are only a limited number of hours required for inlet air cooling. TES can reduce overall capital costs because it reduces the chiller capacity requirements as compared to the capacity required to match the instantaneous on peak demand for cooling. Net power plant on peak capacity is greater as less or no electric energy is required to operate the chillers as they charge the TES system the night before using lower cost off-peak electricity. Somewhat offsetting these benefits a system with TES requires a larger site footprint for the TES tank. In summary, the advantages of a mechanical refrigeration system are that it can maintain the inlet air at much lower temperatures than those possible by other technologies and achieves the desired temperatures independent of weather or climate conditions. The primary disadvantage of this system is that it is capital cost intensive and also has higher parasitic loads (typically 0.70-0.81kW/RT) that lead to higher overall heat rates than those for the evaporative cooling technologies.
Absorption Cooling systems are similar to the mechanical refrigeration systems except that instead of using mechanical chillers, these systems use absorption chillers that require thermal energy (steam or hot water) as the primary source of energy and require much less electric energy than the mechanical chillers. Absorption cooling systems can be used to cool the inlet air to about 50oF. These systems can be employed with or without chilled water TES systems. Absorption chillers can be single-effect or double-effect chillers. The single-effect absorption chillers use hot water or 15-psig steam (18 lb./hr.RT) while the double-effect chillers require less steam (10 lb./hr.RT) but need the steam at higher pressure (115 psig). The advantage of this system is that it has much less parasitic load (typically 0.25 to 0.28 kW/RT) and its major disadvantage is that its capital cost is much higher than even mechanical refrigeration systems. The primary successful applications of Absorption chillers in power plants are where excess thermal energy is available and the conversion of this energy to high-value electric energy creates a winning situation for the user.
LNG Vaporization systems are useful for power plants plants located near a liquefied natural gas (LNG) facility. In supplying natural gas for power plant or other applications, LNG must be vaporized by some heat source. For applications in TIC, the inlet air is used as such a heat source.
Hybrid Systems incorporate some combination of technologies, for example mechanical and absorption chillers. Such a system is optimized for a specific plant based on the power demand and electric prices and availability of thermal energy. Other hybrids may include combinations of evaporative cooling and chiller-based systems.
