On hot days, the gas turbine output can reduce 10-20%. Typically, every 1 oF rise of ambient air temperature reduces the gas turbine output by approximately 0.3 to 0.5% . Stand-by units cost $300-450/kW plus maintenance fees. There is a clear business case for inlet cooling technologies as power augmentation options. In a presentation at Power Gen International, Ting Wang, Director Energy Conversion & Conservation Center at the University of New Orleans, laid out a few pinch points.
Fogging is of various types. Natural fog with droplet sizes of 1 to 10 mm. In saturated fog cooling, the air is saturated, which means 100% relative humidity. In overspray or high-fogging, more water is supplied than needed for saturation. In spray cooling, droplet inertia dominates, leading to a relatively large slip velocity. Droplets are greater than 20mm. In mist cooling, slip velocity is zero and droplet size is between 5~20mm.
In inlet cooling, an active refrigeration system achieves temperatures below wet bulb temperatures. Compressor-based refrigeration chillers or heat-driven absorption refrigeration based chillers are used. Systems can combine electric, absorption chilling with partial thermal storage.
In indirect chilling systems, the evaporative cooling is limited by wet bulb temperature. In direct wet systems, spray water is introduced into the air stream or onto a medium in the main inlet air stream.
Indirect “dry” systems are similar to the indirect chilling system except that the separated primary source is provided by evaporative cooling.
Excess water vapor causes increased specific heat, which causes less cooling during expansion, so exhaust temperature goes up. Higher exhaust temperature causes the GT controller to back off on fuel flow, which reduces the power boost and increases CO emissions. Can be corrected by modifying GT controls.
Gas Turbine inlet cooling can significantly augment the output power during hot or dry days, but not necessarily the thermal efficiency. Among various inlet cooling schemes, fogging is the most economic approach, costing $35-60 per kW. Fogging droplet size should be smaller than 20 microns to minimize erosion and maximize evaporation in the compressor. Wet compression increases axial velocity, blade inlet velocity, incidence angle and tangential component of velocity. Most erosion occurs at leading edge and one spot of trailing 10-9´edge of the rotor suction side.
