Efficient capture and effective use of thermal energy is critical to successful CHP, from both the technical and financial perspective. In most CHP and district energy applications, the exhaust gas from the electric generation equipment is ducted to a heat exchanger to recover the thermal energy in the gas. Generally, these heat exchangers are air-to-water heat exchangers, where the exhaust gas flows over some form of tube and fin heat exchange surface and the heat from the exhaust gas is transferred to make hot water or steam.
In the majority of installations, a flapper damper or "diverter" is employed to vary flow across the heat transfer surfaces of the heat exchanger to maintain a specific design temperature of the hot water or steam generation rate. The hot water or steam is then used to provide hot water or steam heating and/or to operate thermally activated equipment, such as an absorption chiller for cooling or a desiccant dehumidifier for dehumidification.
Heat Recovery Steam Generators
Heat Recovery Steam Generators (or "HRSG," often pronounced “herzig”) are essentially boilers that capture or recover the exhaust of a prime mover such as a combustion turbine, natural gas or diesel engine to create steam.
The system consists of a bank of tubes that is mounted between the unit and the exhaust stack. Exhaust gases at temperatures of 800°F to 1200°F heat these tubes. Water is then pumped and circulated through the tubes and can be held under high pressure to temperatures of 370°F or higher resulting in the production of high pressure steam. Since the flue gas never comes in direct contact with the water, the steam can be safely used in thermally activated cooling equipment.
HRSGs, which range from 10-250 MW and have an efficiency of 60-85%, are typically found in many combined cycle power plants.
Heat recovery from a reciprocating engine is much more complicated than with a gas turbine due to the number of different heat streams that need to be tapped, as shown in the figure below.
Heat Streams from a Reciprocating Engine
Heat from the jacket water, lube oil and exhaust can all be captured, but not directly. The figure below shows a very typical heat recovery configuration for a reciprocating engine CHP application. The recovered heat for a reciprocating engine takes the form of low-pressure steam or hot water in all but the largest units. It should also be noted that the heat from the jacket water and cooling water must be ejected even if it is not being used, so often reciprocating engine sets will be equipped with a cooling tower to eject excess heat.
Typical Reciprocating Engine CHP Configuration
Absorption Chillers
Absorption chillers use heat instead of mechanical energy to provide cooling. This highly efficient technology uses less energy than conventional chilling equipment, and also cools buildings without the use of ozone-depleting chlorofluorocarbons (CFCs). Unlike conventional electric chillers, which use mechanical energy in a vapor compression process to provide refrigeration, absorption chillers primarily use heat energy with limited mechanical energy for pumping. These chillers can be powered by natural gas, steam, or waste heat.
Absorption chiller works by transferring its thermal energy from the heat source to the heat sink through an absorbent fluid and a refrigerant. The absorption chiller creates a refrigerative effect by absorbing and then releasing water vapor into and out of a lithium bromide solution (see figure below).
Simplified Absorption Cycle
Absorption chiller works by transferring its thermal energy from the heat source to the heat sink through an absorbent fluid and a refrigerant. The absorption chiller creates a refrigerative effect by absorbing and then releasing water vapor into and out of a lithium bromide solution (figure below).
Desiccant Dehumidifier
Desiccants can be used to remove humidity from air. This reduces the cost of air conditioning and improves indoor air quality. Space conditioning is comprised of two separate components:
• Sensible cooling - lowering the air temperature
• Latent cooling - reducing humidity in the air
By reducing moisture content of the air, desiccants reduce the latent cooling load on conventional AC equipment. Thus, the chiller load is primarily limited to only the sensible cooling (i.e., reducing the temperature). By dehumidifying the air, desiccants improve the efficiency of standard air conditioning equipment and thereby lower the cost for air conditioning. Controlling humidity (to less than 60%) helps prevent the growth of mold, bacteria, and microorganisms that cause allergies or are otherwise harmful to human health.
Desiccants use chemical (or physical) absorption of water vapor to hydroscopic materials. Heat is required to remove water from the desiccant, thereby regenerating it to be reused for further dehumidification. Thus, desiccants provide excellent heat loads for CHP systems throughout the cooling season.