Like any internal combustion engine, the power output and efficiency of a combustion turbine decrease as ambient temperatures rise. This loss of power and efficiency is caused by a reduction in ambient air density at higher temperatures. Since turbines are mass flow machines with a volumetrically limited intake, less-dense intake air results in degradation of power output and increase in engine fuel consumption. This is a particular problem for small turbines, which are often used for peaking duty when power demands spike during hot weather.
For these reasons, many such turbines can benefit from some means of inlet air cooling. Inlet cooling increases inlet air density and increases the total heat content of the turbine’s exhaust, further adding to its economic effectiveness.
A number of methods for turbine inlet cooling have been developed, each with its pluses and minuses. Not all methods are appropriate for all applications:
- Fogging: A direct evaporative approach in which deionized (DI) water is sprayed into the air stream in a fine mist. DI water is used to prevent foreign materials (such as dissolved minerals) from entering the turbine. However, this method does not actually change the thermodynamic energy, or enthalpy, of the air. Obviously, a reliable source of DI water must be available, and several turbine manufacturers frown on this method (if they do not bar it entirely) because of potential damage to the turbine.
- Direct evaporative: Inlet air is passed through a wetted media. This approach offers a large surface area and a low pressure loss, and can reduce the temperature from approximately 70% to 90% of the difference between the entering dry bulb and wet bulb temperatures. Like inlet fogging, this method does not change inlet air enthalpy. Water treatment should follow gas turbine OEM requirements.
- Indirect evaporative. Inlet air is cooled in a heat exchanger, with evaporation of water taking place in a secondary air stream. Unlike direct evaporation, this method changes the enthalpy of the air stream by extracting heat through the heat exchanger into the secondary air stream, while not adding moisture to the turbine inlet air.
- Mechanical chilling: Inlet air is cooled using a mechanical chilling/refrigeration process with a cooling coil placed in the air stream. This method also changes air enthalpy and can offer substantial inlet cooling without adding water to the inlet air. However, the presence of the cooling coil can increase pressure loss in the inlet air stream compared to evaporative methods. This method also involves the highest parasitic loads and typically has the highest capital and operational costs.
The appropriate method is dependent on operational circumstances. For example, inlet fogging and conventional evaporative methods may be less effective in humid climates, and the greater expense of mechanical chilling may not be justified for smaller turbines.
Best of Both
Conventional methods can also be combined.
In multi-stage indirect/direct evaporative cooling, a method developed by Everest Sciences called ECOCool, air is first cooled by an indirect evaporative process that reduces the primary air dry bulb and wet bulb temperatures prior to entering an air washer, which is a direct evaporative process. This hybrid combination can cool the air several degrees below the ambient wet bulb temperature, which always becomes the limitation for conventional evaporative techniques, and thus ECOCool provides cooler and denser air than conventional evaporative methods. The parasitic power to operate the hybrid cooling system is essentially negligible. This process produces the greatest density and lowest inlet air temperature of any of the evaporative techniques, but it is more expensive.
Everest Sciences has also developed an inlet chilling method, ECOChill, that combines the benefits of conventional methods (Figure 1). In this hybrid process, indirect evaporative cooling is used in conjunction with supplemental mechanical chilling to achieve target temperatures comparable to conventional mechanical chilling systems. The advantage of this method is that lower inlet temperatures can be achieved using a fraction of the parasitic load of mechanical chilling, and without introducing additional moisture into the inlet air stream.
1. The ECOChill system combines indirect evaporative cooling with mechanical refrigeration. Source: Everest Sciences
While a hybrid system combining direct evaporative and mechanical chilling might seem attractive as well, such an approach would not be effective because of how mechanical refrigeration works. The power required for refrigeration is a function of the total enthalpy change between a starting temperature and the target temperature. Because direct evaporative techniques do not change the enthalpy of the airstream, there is no benefit in combining the two approaches, as the mechanical refrigeration system will need to work just as hard as it would standing alone.
Because indirect evaporation reduces the enthalpy of the airstream, however, its chilling effect is additive with mechanical refrigeration. This means that adding indirect evaporation will dramatically reduce the refrigeration capacity required to achieve a given target temperature and the amount of work the system must perform to reach the target temperatures. Thus, the parasitic load required to chill the air is also reduced.
Another benefit is inherent in indirect evaporation, which requires a secondary air stream. This high volume source of cooler-than-ambient air can be used for refrigerant condensing, making a separate cooling tower or radiators unnecessary, and further increasing the hybrid system efficiency.
In addition to the listed advantages, the ECOChill system has other benefits. The indirect evaporation system can be operated independently, allowing the mechanical refrigeration system to be shut off when its additional chilling capacity is not needed. This reduces overall parasitic loads as well as wear and tear.
The ECOChill system also has integrated filtration so installation can be a direct drop-in replacement for the filter house. Once set up, the system is fully automatic, maintaining turbine intake air at the target temperatures without the need for operator oversight.
Everest Sciences installed an ECOChill cooling system for a client in the U.S. Southwest. The client employs two small turbines to drive refrigeration compressors for cryogenic distillation of natural gas liquids. The existing direct evaporative system needed replacement and did not provide sufficient cooling and air density increase during many periods of the year (Figure 2). During some conditions, power output could drop more than 20% from rated capacity.
2. ECOChill’s combination of cooling technologies offers substantial improvements over traditional evaporative methods. Source: Everest Sciences
After installation of an ECOChill system, the turbines were able to maintain better-than-rated full power throughout most of the year because the system was able to reduce inlet temperatures to 50F, below ISO/ISA conditions (59F). The client’s estimated yearly return on investment was more than $700,000 from increased power output and reduced operating costs. Most ECOChill installations will see payback periods of less than two years, sometimes considerably less. Everest Sciences has recently commissioned a third unit in this same facility.
Another client operates a natural gas pipeline in the Midwest, where high summer temperatures caused the pipeline compressor driver, a gas turbine, to lose significant power, leading to throughput reductions in gas transportation. Everest Sciences supplied an ECOCool multi-staged indirect evaporative followed by air wash system. Not only does this system provide the client 45F to 50F inlet air temperatures, but its operation also is staged (something not possible with traditional direct evaporative cooling) to provide near–constantly cooled inlet air temperatures throughout the day. The installation provides cool weather–like conditions to the gas turbine all summer long and allows for consistent and steady power delivery to the pipeline compressor.
—Marcus Bastianen, PE is director of sales and marketing with Everest Sciences.