In Part 1 we talked about generating incremental steam using duct burners at extremely high thermal efficiencies for improved combined heat and power (CHP) systems. Now, let’s talk about trying to maximize power out of the gas turbine generator (GTG) with minimal parasitic spend. This is very important during peak demand periods and, coincidentally, also happens during the hottest time of the day (or year).
The GTG’s ISO-rated design is based on standard ambient air at 59°F, 60% relative humidity and 14.7 psia at sea level with no inlet and exhaust pressure drops. The GTG has an inlet air compressor that compresses ambient air before introducing it to the combustor. As the ambient temperature increases, the air density reduces (specific volume increases). The compressor is a “volume” machine and this reduction in air density implies a lessening in the overall mass flow rate of air through the suction of the compressor. This decreases power output and also fuel efficiency (or increases the heat rate). For example, an old frame-style and a new aeroderivative GTG may experience a 4% and 8% drop in power produced (compared to design-rated conditions), respectively, when ambient temperature rises to 80°F. Similarly, fuel efficiency can drop up to 2% when the ambient temperature increases to 80°F and fall 3.5% at 95°F ambient conditions. The manufacturer of your CHP system can provide a correlation between the ambient temperature and the net power produced for your specific GTG.