In this issue

This column features helpful information, innovative equipment, systems and applications utilities around the nation can use to save energy and improve service.

Consider absorption technology for waste heat recovery

Absorption technology operates on the "heat pump" principle and has been used in various forms for well over 100 years. The technology uses thermal energy rather than mechanical shaft energy for its operation. In the past, it has been used predominately for creating chilled water (with steam or fossil fuel combustion as the primary energy source) and for cooling in specialized industrial processes.

Complex technology

In the United States, absorption technology for chilling has been substantially replaced by electric motor-driven mechanical vapor compression technology. Many factors are responsible for the decline, including complexity of absorption systems and lack of understanding of absorption principles and applications, even by technically-trained individuals. But the primary factor is economic, due to the technology's comparatively poor energy conversion efficiency and today's high fuel costs.

For sake of simplicity, the operating principles of absorption technology will not be explained in this brief article. However, the technology has some very interesting characteristics that make it an option if the absorption system can be operated with recovered low-level waste heat energy. Many energy policymakers and conservationists have stated the largest opportunity for reducing industrial energy consumption in the U.S. is thermal energy recovery.

Cost-effective heat recovery

The most promising and economically viable potential applications of absorption technology are situations where a source of inexpensive thermal energy (or "free" energy in the case of heated waste streams) is available at temperatures ranging from approximately 200 degrees F to 400 degrees F, and the need for cooling or heating exists. Examples of these types of streams are hot exhaust gas discharges, hot water discharges and waste low-pressure steam.

The following example illustrates waste heat energy recovery resulting in reduced electrical energy consumption. A large food processing operation often has large heating loads handled by steam boilers, and cooling loads handled by mechanical cooling/freezing systems. One energy reduction scenario is to recover a significant amount of the heat energy in the boiler flue gases to provide the operating thermal energy for an absorption technology chilled water system. This could probably not be done directly, but would require an intermediate system installed between the boiler flue gas stream and the absorption chiller.

Substantial energy recovery is possible with this strategy, especially if the water vapor component of the boiler flue gas is condensed. Significant cooling loads could be displaced from existing mechanical-vapor compression systems, resulting in reduced overall electricity consumption.

Adopting absorption technology for energy recovery has been difficult to implement due to a lack of pre-packaged, pre-engineered systems. Typically, specialists in heat recovery with knowledge of the specific situation must design a customized system, adding cost, time and complexity. Even with these factors, the economics of potential energy recovery in many situations make exploring absorption technology a good idea.

For more information about absorption technology or technical assistance on this topic, please call the Power Line at 1-800-769-3756.