Rejected Heat Storage or Rejection

The demands for prime power and thermal energy are not always perfectly matched. When configuring a system, it is usually most economical to satisfy non-varying baseloads. In theory, this means that available power and recovered thermal energy can be used all of the time. This perfect relationship, however, is not always attainable.

If available heat cannot be used when it is recovered, it must either be stored for later use or rejected to the environment. The most common types of storage associated with recovered heat from prime movers are hot water or chilled water generated by a heat recovery absorption chiller. A need for storage may indicate an imbalance in supply and demand that results in increased costs. Sometimes, however, storage is an economic enhancement technique that balances a system and improves efficiency, as when systems are purposely sized to use stored hot or chilled water during an electric utility's on-peak billing period. Figure 8-2 shows a large buffer tank used for hot water storage with a 17 MW gas turbine cogeneration system that provides electricity and heat for a large city district system.

Figure 8-3 is a schematic layout of a reciprocating engine-generator application featuring domestic hot water heating with series recovery of exhaust and engine coolant heat. In this application, two 10,000 gal (37,850 l) hot water storage tanks are used to balance the thermal load. The total heat recovered from the 300 kW system can

Fig. 8-2 Large Buffer Hot Water Storage Tank Featured in Cogeneration Application. Source: ABB
Fig. 8-3 300 kW Engine-Generator Set with Hot Water Heat Recovery System and 20,000 Gallon Storage Capacity. Source: Waukesha Engine Division

produce about 1,500 gal (5,678 l) per hour of 180°F (82°C) water, charging storage tanks in about 13 hours of continuous operation. The system is appropriate for a facility with a continuous electric load, but only an intermittent (e.g., daytime) hot water load. For example, during an 11 hour period of hot water demand (at up to 3,300 gph or 12,491 lph), the system could provide 1,500 gph (5,678 lph) heat recovery output, plus 1,800 gph (6,813 lph) from storage.

Quantifying Storage

The quantity of thermal energy that can be stored is dependent on the temperature difference that can be achieved and on the thermal qualities of the storage materials. The quantity of heat stored is expressed as:

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