CHP prime mover technologies produce excess heat that is recycled for another thermal energy need, such as space heating, domestic hot water, air conditioning, humidity control, process steam for industrial steam loads, product frying, greenhouses, or nearly any other thermal energy need. The end result is significantly more efficient than generating power, heating, and cooling separately. Below are descriptions of some of the technologies that run on the recycled thermal energy. Which one employed depends on what output is needed and on the temperature and quantity of the excess heat available. It is often possible to employ more than one of these, either at the same time (i.e. air conditioning and humidity control) or seasonally (i.e. cooling in the summer and heating in the winter).
Efficient capture and effective use of thermal energy is essential for maximizing the energy savings and economic return of CHP. Cooling is often an especially-useful add-on, as it allows customers to reduce seasonal peak electric demand and allows future electric and gas grids to operate with more level loads.
In most CHP 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.
The next frontier in thermally-activated technologies for CHP – especially absorption chillers and desiccant dehumidifiers – is to factory design pre-engineered, integrated, packaged systems using standard, modular equipment, as opposed to using custom-designed and custom-engineered systems for each particular site. Some companies are making strides in smaller-scale integrated systems (small, medium and large commercial sites or small industrial sites); larger sites will still require custom work.
- Absorption Chillers – All chillers use energy to remove heat from a building. In the case of absorption chillers, the energy comes from heat. (Yes, although this sounds counterintuitive, they do use heat to produce cooling.) This heat can be supplied by natural gas, steam, or waste heat. This highly efficient technology uses less energy than conventional chilling equipment, and also cools buildings without the use of ozone-depleting chlorofluorocarbons (CFCs). Absorption chillers works by transferring the thermal energy from a heat source to a 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.
- Ammonia Absorption Chillers – Ammonia-based absorption chillers use a mixture of ammonia and water as a working pair, with ammonia as the refrigerant, rather than the lithium-bromide and water used in regular absorption chillers. Ammonia absorption chillers are used in a range of applications, from small refrigerators of less than 25 refrigeration tons (RT) of cooling capacity to mammoth heat-recovery machines installed with power plants. Ammonia is an excellent refrigerant with a high latent heat and excellent heat transfer characteristics. Compared to regular lithium bromide absorption chillers, ammonia absorption chillers are suited for certain industrial applications that require extremely cold temperatures (below 32 degrees F or 0 degrees C, where the water in lithium bromide systems freezes). Because of its toxicity, ammonia absorption chillers are often restricted to applications in which the equipment is outdoors to allow natural dilution of any leaks. Ammonia absorption chillers are most commonly found at large-scale food processing, petrochemical, and fertilizer plants. However, they are also newly available for residential and light commercial applications, generally in the range of 2 to 10 RT, which could be modularized into larger systems.
- Desiccant Dehumidifiers – Desiccants can be used to remove humidity from air. In humid climates, this reduces the cost of air conditioning and improves indoor air quality. Space conditioning is comprised of two separate components: 1) Sensible cooling – lowering the air temperature and 2) 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 either chemically or physically bond water vapor to hydroscopic materials. Once the dessicant material is saturated, it can be dried out using hot exhaust form a CHP system, and used again and again. Heat is required to remove water from the desiccant, thereby regenerating it to be reused for further dehumidification. Thus, desiccants are an excellent use of the heat from CHP systems throughout the cooling season.
- Heat Recovery Steam Generators (HRSGs) – 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 prime mover 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 megawatts 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. Heat from the jacket water, lube oil and exhaust can all be captured, but not directly. 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.