1023 Gas Turbines

Gas turbines are a cost-effective CHP alternative for commercial and industrial end users with a base-load electric demand greater than about 5 MW. Although gas turbines can operate satisfactorily at part-load, they perform best at full power in base-load operation. Gas turbines are frequently used in U.S. district steam heating systems since their high quality thermal output can be used for most medium pressure steam systems.

Gas turbines for CHP can be in either a simple-cycle or a combined-cycle configuration. Simple-cycle applications are most prevalent in smaller installations of typically less than 25 MW. Waste heat is recovered in an HRSG to generate high- or low-pressure steam or hot water. The thermal product can be used directly or converted to chilled water with single- or double-effect absorption chillers.

The simple-cycle gas turbine has the lowest electric efficiency and power to heat ratio since there is no recovery of heat in the exhaust gas. Hot exhaust gas can be used directly in a process, or, by adding an HRSG, exhaust heat can generate steam or hot water. Combined cycles involving a steam turbine become economical for larger installations. The most advanced utility-class gas turbines achieve up to 60% electric generation efficiency — but achieving this high efficiency means that only very low-grade waste heat is available for CHP.

More energy can be extracted from the turbine by burning the oxygen-rich exhaust gas (supplemental firing). A duct burner is usually fitted within the HRSG to increase the exhaust gas temperature at efficiencies of 90% and greater.

10.2.3.1 Absorption Chilling

Absorption chilling systems can be provided to produce chilled water directly from the gas turbine exhaust (Figure 10.3). The most common application of absorption chilling, however, is to use low, 2 to 4 bar (~30 to 60 psig), or medium, 10 bar (~150 psig), pressure-saturated steam. Absorption chillers generate chilled water using a working fluid operating between high-temperature gas turbine exhaust and a lower-temperature sink (Figure 10.4). Most common absorption chillers in industrial applications use a lithium bromide (LiBr) and water solution to provide chilled water at 7°C (44°F). Lower water temperatures can be achieved with ammonia and water systems.

In a lithium bromide absorption chiller, chilled water is produced through the evaporation of water in the evaporator section. Water evaporation is induced by a concentrated solution of lithium bromide that has a high affinity for water vapor. As water vapor is absorbed in the absorber section, additional water evaporation chills the refrigerant by boiling at low pressure and temperature. The absorbent solution becomes diluted and is pumped to the regenerator, where steam is used to boil out excess water. The water vapor is condensed in the condenser section by a cooling water system and returned to the evaporator section. In a separate flow path, the strong lithium bromide returns to the absorber section.

Low pressure steam, 2 to 4 bar (~30 to 60 psig), is used in single-stage lithium bromide absorption chillers at a rate per refrigeration ton (RT) of 7.7 kg/RT (17 lbm/RT). Medium pressure steam, 10 bar (~150 psig), is used in two-stage absorption chillers at a rate of 4.5 kg/RT (10 lbm/RT).

FIGURE 10.3

Absorption cooling cycle.

FIGURE 10.3

Absorption cooling cycle.

Gas Turbine

Gas Turbine

Steam Turbine (Combined Cycle)

FIGURE 10.4

Gas turbine heat recovery.

Solar Stirling Engine Basics Explained

Solar Stirling Engine Basics Explained

The solar Stirling engine is progressively becoming a viable alternative to solar panels for its higher efficiency. Stirling engines might be the best way to harvest the power provided by the sun. This is an easy-to-understand explanation of how Stirling engines work, the different types, and why they are more efficient than steam engines.

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