Gas Turbines

Gas turbine-driven vapor compression systems are typically applied only in large capacities. While available in capacities of 20 tons (70 kW^) or even less, they are

Fig. 37-47 Reciprocating Engine Matched with 1,500 kW Electric Generator and 1,000 Ton Air-Cooled Chiller. Source: Carrier Corp. and Caterpillar Engine Division

uncommon below the 1,200 ton (4,200 kW^) capacity range. However, with the advent of micro turbines, very small capacity packaged systems may achieve market entry. Gas turbines are attractive vapor compression drivers because they provide precise control, large quantities of high-quality recoverable exhaust energy, high reliability, and the capability for effective air emissions control.

Gas turbines are usually matched with centrifugal compressors, but, in some cases, lower capacity units are matched with custom-designed screw compressors. While centrifugal compressors typically operate at high operational speeds, gas turbine speeds are usually even higher, necessitating a speed reducer (gear) to match driver and compressor shaft speeds. With some vary large capacity systems, centrifugal compressor speed may actually be matched for direct drive by the gas turbine without use of gearing.

Single-shaft gas turbines may be effectively applied to full-capacity, base-loaded applications where their poor part-load efficiency and inability to effect speed control are not detriments. For applications where part-load operating time is significant, multi-shaft turbines are well suited. As with reciprocating engines, their variable speed operating capability provides for complimentary part-load performance of both the turbine and the compressor. Multi-shaft units can also quickly develop high starting torque to accelerate compressors to design operating speeds.

Key characteristics that support, but are not necessarily essential for, economical application of gas turbine drivers are extensive annual cooling requirement with concurrent baseload thermal energy requirements and a focus on reliability and electricity independence.

A potential application drawback is the concurrence of high inlet temperature conditions with peak cooling capacity requirements. Due, in part, to decreased air density and the impact of higher fluid temperatures on adiabatic head, the turbine's compressor absorbs more power when inlet air is hotter, thereby reducing total power available to the shaft. This can result in the system output being at its lowest when load requirements are at their highest. Inlet cooling, which can be provided for by available chilled water, can cost-effectively cool the turbine air inlet to improve performance. While this can be included in the application design, there will be a capital and operating cost penalty.

Heat Recovery

High mass flow and exhaust temperature characteristics provide large quantities of potentially recoverable heat, which can produce strong economic performance when matched with a sufficient existing concurrent thermal load requirement. Augmentation by supplementary firing in the oxygen-rich exhaust can significantly increase exhaust gas temperature and, therefore, steam (or hot water) production.

Alternatively, recuperation can be used to transfer heat from turbine exhaust to turbine compressor discharge air prior to combustion, thereby displacing a portion of the fuel requirements. This results in a thermal efficiency increase and corresponding decrease in recoverable energy due to the reduction in exhaust gas temperature — typically to about 600°F (315°C), and also a slight decrease in capacity.

In addition to using recovered heat for non-cooling process applications, recovered heat can be used to provide additional cooling output in a piggyback arrangement. Options include:

• A double-effect steam absorption chiller (or customized directly coupled heat recovery powered unit).

• A hot water recovery boiler to power a single-effect absorption chiller.

• A steam turbine-driven chiller (either condensing or non-condensing turbines may be used to increase overall system cooling output).

Steam injection may also be used to enhance capacity and cycle efficiency. Capacity enhancement with steam injection- (STIG-) cycle operation will be equal to or greater than the combined-cycle configuration, while overall thermal efficiency will be slightly lower.

Fig. 37-48 6,100 Ton Gas Turbine-Driven Chiller System Applied at a University Campus. Source: Stewart and Stevenson

Performance

Typical simple-cycle HHV energy input requirements for gas turbine vapor compression drive systems, under ARI standard conditions, range from 11,500 Btuh/ton (0.96 kWh/kWr), or 1.04 COP for smaller capacity systems, to 6,300 Btuh/ton (0.53 kWh/kWr), or 1.9 COP for very large systems. Taking into account recoverable heat and displaced boiler efficiency losses, net energy input requirements range from 4,000 Btuh/ton (0.33 kWh/kWr), or 3.0 COPnet, to 2,600 Btuh/ton (0.22 kWh/kWr), or 4.6 COPnet.

When steam injection or recuperation is used, energy input requirements may reach slightly under 5,000 Btuh/ton (0.42 kWh/kWr), or 2.4 COP. When matched with steam turbine-driven systems or absorption chillers powered by recovered heat, combined COPs in excess of 2.0 are achievable.

Maintenance

As with other types of combustion gas turbine applications, gas turbine-driven vapor compression systems, when applied, maintained, and operated properly, offer high reliability and relatively low life-cycle maintenance costs. However, a high-quality control and monitoring system and a hands-on understanding of the specific system in operation are important elements in successful operation.

Typically, the life-cycle OM&R cost on the turbine drive will range from $0.004 to $0.012 per ton-h ($0.001 to $0.03 per kWhr). Complete service contracts, including turbine engine replacement or overhaul insurance, are common. Third-party operation is somewhat common for larger systems.

Application Examples

Figure 37-48 shows a 6,100 ton (21,500 kWr) gas turbine-driven chiller system that provides chilled water and steam for a campus-wide distribution system at a large university. The system features a 5,400 hp (4,000 kW^) Allison gas turbine driving a York multi-stage, high-pressure centrifugal compressor operating on HCFC-22 refrigerant.

The turbine operates at a full load rated speed of 14,770 rpm to drive the centrifugal compressor at 13,600 rpm through a 1.086:1 gear reducer. Variable speed operation allows the system to operate efficiently under part-load conditions and minimizes the use of hot gas bypass under very low loads. The turbine exhaust is passed to an HRSG that generates 100,000 lbm/h (45,000 kg/h) of steam at a pressure of 150 psig (11.4 bar). The turbine exhaust heat is capable of generating about 25,000 lbm/h (11,250 kg/h) of steam, with the remainder of the heat to the HRSG provided by fired duct burners.

Figure 37-49 illustrates two gas turbine drive configurations. Both application options are used to achieve additional capacity through the use of recovered heat. The configuration on the top matches a gas turbine-driven centrifugal compressor with an absorption chiller. The configuration on the bottom is a traditional combined-cycle system matching a gas turbine-driven chiller with a steam turbine-driven chiller.

Figure 37-50 illustrates a system in which the gas turbine driver is paired in line with a smaller electric motor/generator. With this configuration, chiller capacity is matched with the combined shaft power output of both the turbine and the motor. This application configuration is used to extend operating hours and enhance system economics.

Under design cooling conditions, full turbine and motor power are used to drive the chiller. Under partial load, the turbine at full-load power is used alone to drive the chiller. As loads continue to fall, the turbine remains

Typical Mechanical/Absorption System

Renewable Energy 101

Renewable Energy 101

Renewable energy is energy that is generated from sunlight, rain, tides, geothermal heat and wind. These sources are naturally and constantly replenished, which is why they are deemed as renewable. The usage of renewable energy sources is very important when considering the sustainability of the existing energy usage of the world. While there is currently an abundance of non-renewable energy sources, such as nuclear fuels, these energy sources are depleting. In addition to being a non-renewable supply, the non-renewable energy sources release emissions into the air, which has an adverse effect on the environment.

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