Mechanical Drives - Gas Turbine Designs

Gas turbines are seldom selected to drive chiller compressors because the efficiency of the cogeneration system using a gas turbine relies heavily on recovering the engine's waste heat. Most sites simply don't have a use for all the waste heat. In cases where the heat can be used, the gas turbine is typically used to drive a generator to take maximum advantage of its power generating capabilities. The main problems associated with using gas turbines as chiller drives include:

1. Gas turbine power levels (and the resulting chilled water production) are significantly reduced (~ 25-35%) at high ambient temperature levels. This means that at the very time the site needs maximum power to drive a chiller compressor, the gas turbine is least capable of delivering it. One solution might be to use some of the chilled water production to cool gas turbine inlet air, but this also reduces net chilled water production.
   

2. Operating and maintenance procedures are relatively sophisticated. The engines must be protected against inlet dust, contaminants, frosting, or damage from foreign objects. When placed in the hands of qualified, experienced personnel, and run continuously, gas turbines have recorded extremely high annual availability and low maintenance costs. Unfortunately, chillers seldom run continuously.

3. If the gas turbine is fueled with natural gas, gas pressures have to be higher than with any other mechanical driver -- typically 300 - 400 psig for the gas turbine. These pressures aren't always available from suppliers, and therefore require a supplemental gas compressor. Since this gas compressor is relatively unreliable, a "spare" is usually added in the system design, making it an expensive design attribute. Coupled with the power used to compress the natural gas fuel input, this compressor becomes a significant element in the cost-effectiveness equation.

4. Careful matching of the turbine and compressor, both available in limited size increments is essential. Starting and stopping torques are specially important. These requirements typically increase the chiller cost not economically supportable.

This doesn't mean that the gas turbine is a necessarily bad choice for a mechanical drive application, it just highlights the primary concerns the designer and owner should consider in evaluating the alternatives. Therefore, it would be prudent to rely on qualified design professionals and reputable equipment manufacturers for gas turbine installed cost, operating characteristics, and site-specific performance estimates.