Last month, 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).
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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 .
Different methodologies minimize this power reduction during peak operating periods. Most of these methodologies rely on the concept of cooling the intake air prior to the GTG inlet — generally called “gas turbine inlet cooling” (GTIC). Several technologies such as mechanical vapor compression (chilled water packages), fogging, wetted-media, etc., provide this cooling effect for hot ambient air. However, they come at a price and may have limitations due to geographic location, design of the GTG and the CHP system, etc.
Chemical plants and refineries may have areas where excess low-pressure steam (<20 psig) is vented or waste heat from process streams and heaters is available. Depending on the steam generated in the heat-recovery steam generator, the exhaust from the CHP system also may have enough waste heat. Some novel applications have recovered this waste heat to operate an absorption chiller system to provide the cooling effect for the inlet air. I often find potential applications of this technology but, due to a lack of application-specific understanding or financial hurdles, this option usually gets overlooked. Hence, I am bringing it up to get some of your grey cells thinking about it.
Waste heat serves as an excellent source of thermal energy to power an absorption chiller system. Absorption chiller systems use one of the two working fluid pairs: ammonia/water or lithium bromide/water. Generally, I find ammonia/water absorption systems in chemical plants and refineries, whereas lithium bromide/water applications are more prominent in central utility plants for large commercial buildings, manufacturing assembly plants, etc. Ammonia/water systems have the distinct advantage of producing temperatures below 32°F and so lend themselves to the process heat integration needed for process plants. Lithium bromide/water systems only can produce temperatures above 40°F. Once chilled water is produced by these systems, it is pumped to a heat exchanger coil (finned tube) located at the air inlet of the GTG. Some systems eliminate the chilled water loop and use a direct exchange coil.
Chilled water produced at 44°F generally suffices to get the design power capacity from the GTG — even on a very hot day. Because a very minimal amount of parasitic energy is needed to operate a waste-heat-fired absorption chiller system, the loss in power capacity and fuel efficiency due to hot ambient conditions can be virtually eliminated. This is a win-win situation and should be actively investigated whenever limitations of power production are observed in GTG operations.
I leave you with a task: compare the actual power production from your GTG versus design-rated conditions and optimize your CHP system’s operation to generate the most power at the highest fuel efficiency.
