Thermally-Activated Machines

Thermally activated machines are based on technologies that primarily use thermal energy, preferably heat from the exhaust gases of power generation equipment, instead of electric energy for providing heating, cooling, or humidity control for buildings. There are two primary candidates for thermally-activated machines for application in CHP systems:

• Absorption chillers
• Desiccant dehumidifiers

Follow the provided links for a Real Video streaming media presentation, in 56K or DSL/T1 formats, of an overview of cooling technologies that use thermal energy.

Absorption chillers

Absorption chillers are cooling machines that use heat as the primary source of energy for driving an absorption refrigeration cycle. These chillers require very little electric power (0.02 kW/ton) compared to electric chillers that need 0.47 to 0.88 kW/ton, depending upon the type of electric chiller.

Absorption chillers have fewer and smaller moving parts and are thus, quieter during operation than electric chillers. These chillers are also environmentally friendly in that they use water as a naturally benign refrigerant.

Commercially available absorption chillers can utilize one of the four sources of heat:

• Steam
• Hot water
• Exhaust gases
• Direct combustion

All absorption chillers, except those that use direct combustion, are excellent candidates for providing some, or all, cooling of the load in a CHP system for a building. Modern absorption chillers can also work as boilers for providing heating during winter and feature new electronic controls that provide quick start-up, automatic purge and greater turndown capability than many electric chillers. Maintenance contracts and extended warranties are also available on absorption chillers at costs similar to those for electric chillers. Many facilities across the U.S. are already benefiting from the use of absorption chillers.

Two types of absorption chillers are commercially available:

• Single effect
• Multiple effect

Compared to single-effect chillers, multiple-effect absorption chillers cost more to own (higher capital cost) but are more energy efficient and thus less expensive to operate (lower energy cost). The overall economic attractiveness of each chiller depends on many factors, including the cost of capital and cost of energy.

A Real Video streaming media presentation of absorption chillers is provided here in (telephone 56K) or (broadband DSL/T1) formats, courtesy of the American Gas Cooling Center.

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Desiccant dehumidifiers

There are two separate aspects of space conditioning for comfort cooling;

• Lowering the temperature of the air (sensible cooling), and
• Reducing humidity in the air (latent cooling)

It is important to control humidity to below 60% Relative Humidity (RH) to prevent growth of mold, bacteria and other harmful microorganisms in buildings and prevent adverse health effects.

Traditionally, lowering of temperature and humidity have been accomplished using a single piece of equipment (either by an electric chiller or an absorption chiller) that lowers the air temperature below its dew point temperature. Moisture in the incoming air condenses on the outside of a cooling coil over which the air passes and cooler air, containing less moisture, is sent to the space being conditioned. Reducing humidity in the air by cooling, often requires lowering the air temperature below a comfortable level and might necessitate some reheating of the dehumidified air.

Desiccant dehumidifiers reduce humidity in the air by using solid desiccant materials or liquid desiccant materials to attract and hold moisture. Desiccant dehumidifiers can operate independently of the chiller operation and can be operated in series with chillers. Recoverable heat from the exhaust gases of turbines and engines for power generation or engine-driven chillers can be used for regenerating desiccant material in these dehumidifiers.

In a CHP system, it might be desirable to first reduce moisture content of the air using a desiccant dehumidifier and then cool the dehumidified air by using conventional cooling equipment. By reducing moisture content of the air, desiccant dehumidifiers take care of the latent cooling load and thus reduce the total cooling load of the chillers to only the sensible cooling (reducing the temperature). Alternatively, a desiccant dehumidifier can be used to further dehumidify and partially reheat cool, saturated air leaving a conventional cooling coil. By positioning the desiccant dehumidifier after the cooling coil, dehumidification performance of the desiccant is enhanced. This allows the use of moderate or lower temperatures, typical of CHP systems, for regenerating the desiccant.

More information on desiccants is also available at the National Renewable Energy Laboratory's Advanced Desiccant Cooling and Dehumidification Program website and the Oak Ridge National Laboratory's Building Technology Center website. A Real Video streaming media presentation, on desiccant dehumidifiers, is provided here in (telephone 56K) or (broadband DSL/T1) formats, courtesy of the American Gas Cooling Center.

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