Thermally-Activated Machines
Refrigeration Cycle Descriptions

Absorption Chiller Refrigeration Cycle

The basic cooling cycle is the same for the absorption and electric chillers. Both systems use a low-temperature liquid refrigerant that absorbs heat from the water to be cooled and converts to a vapor phase (in the evaporator section). The refrigerant vapors are then compressed to a higher pressure (by a compressor or a generator), converted back into a liquid by rejecting heat to the external surroundings (in the condenser section), and then expanded to a low- pressure mixture of liquid and vapor (in the expander section) that goes back to the evaporator section and the cycle is repeated.

The basic difference between the electric chillers and absorption chillers is that an electric chiller uses an electric motor for operating a compressor used for raising the pressure of refrigerant vapors and an absorption chiller uses heat for compressing refrigerant vapors to a high-pressure. The rejected heat from the power-generation equipment (e.g. turbines, microturbines, and engines) may be used with an absorption chiller to provide the cooling in a CHP system.

The basic absorption cycle employs two fluids, the absorbate or refrigerant, and the absorbent. The most commonly fluids are water as the refrigerant and lithium bromide as the absorbent. These fluids are separated and recombined in the absorption cycle. In the absorption cycle the low-pressure refrigerant vapor is absorbed into the absorbent releasing a large amount of heat. The liquid refrigerant/absorbent solution is pumped to a high-operating pressure generator using significantly less electricity than that for compressing the refrigerant for an electric chiller. Heat is added at the high-pressure generator from a gas burner, steam, hot water or hot gases. The added heat causes the refrigerant to desorb from the absorbent and vaporize. The vapors flow to a condenser, where heat is rejected and condense to a high-pressure liquid. The liquid is then throttled though an expansion valve to the lower pressure in the evaporator where it evaporates by absorbing heat and provides useful cooling. The remaining liquid absorbent, in the generator passes through a valve, where its pressure is reduced, and then is recombined with the low-pressure refrigerant vapors returning from the evaporator so the cycle can be repeated.

Absorption chillers are used to generate cold water (44°F) that is circulated to air handlers in the distribution system for air conditioning.

"Indirect-fired" absorption chillers use steam, hot water or hot gases steam from a boiler, turbine or engine generator, or fuel cell as their primary power input. Theses chillers can be well suited for integration into a CHP system for buildings by utilizing the rejected heat from the electric generation process, thereby providing high operating efficiencies through use of otherwise wasted energy.

"Direct-fired" systems contain natural gas burners; rejected heat from these chillers can be used to regenerate desiccant dehumidifiers or provide hot water.

Commercially absorption chillers can be single-effect or multiple-effect. The above schematic refers to a single-effect absorption chiller. Multiple-effect absorption chillers are more efficient and discussed below.

Back To Top

Multiple-Effect Absorption Chillers

In a single-effect absorption chiller, the heat released during the chemical process of absorbing refrigerant vapor into the liquid stream, rich in absorbent, is rejected to the environment. In a multiple-effect absorption chiller, some of this energy is used as the driving force to generate more refrigerant vapor. The more vapor generated per unit of heat or fuel input, the greater the cooling capacity and the higher the overall operating efficiency.

A double-effect chiller uses two generators paired with a single condenser, absorber, and evaporator. It requires a higher temperature heat input to operate and therefore they are limited in the type of electrical generation equipment they can be paired with when used in a CHP System.

Triple-effect chillers can achieve even higher efficiencies than the double-effect chillers. These chillers require still higher elevated operating temperatures that can limit choices in materials and refrigerant/absorbent pairs. Triple-effect chillers are under development by manufacturers working in cooperation with the U.S. Department of Energy.

Animation of a Direct-Fired Double-Effect Absorpton Chiller
(Courtesy of InterEnergy Software)

Back To Top

Desiccant Dehumidification Cycle for Solid Desiccants

A typical approach to using solid desiccants for dehumidifying air streams is by impregnating them into a light-weight honeycomb or corrugated matrix that is formed into a wheel. The desiccant-coated wheel is rotated through a "supply" or "process" air stream. The "active" section of the wheel removes moisture from the air and the dried air is routed to the building. By drying the air provided to a chiller, air-conditioning efficiencies are increased because a desiccant removes the moisture from the air more efficiently than a chiller or a direct-expansion (DX) evaporator does.

The other section of the wheel rotates through a "reactivation" or "regeneration" air stream that dries the desiccant out and carries the moisture out of the building. The desiccant can be reactivated with air that is either hotter or drier than the process air.

"Passive" desiccant wheels that are used in total energy recovery ventilators (ERVs) and enthalpy exchangers use dry building exhaust air for regeneration. These simple enthalpy wheels are generally less expensive but also less effective than active desiccant units.

The "active" desiccant wheel can dry the supply air continuously, to any desired humidity level, in all weather, regardless of the moisture content of building exhaust air. They are regenerated with hot air from a burner or other heat source (such as rejected heat from a power generation equipment in a CHP system). This allows them to be used independently of or in combination with building exhaust air and thus, allows more operational/control flexibility. Enthalpy wheels or heat pipes can be added to transfer energy from the supply side to the exhaust side, reducing energy requirements and boosting efficiency.

The ability of a desiccant dehumidifier to use the heat rejected from a turbine, microturbine, or engine-generator makes "active" desiccant systems well suited for integration into a CHP system for buildings providing dependable, low maintenance dehumidification performance at high operating efficiencies.

Animation of a typical Solid Desiccant Dehumidification Cycle.
(Courtesy of InterEnergy Software)

Back To Top

Desiccant Dehumidification Cycle for Liquid Desiccants

In a typical liquid desiccant system, shown below, the desiccant is distributed in one chamber (conditioner), using spray nozzles, where it contacts the passing process air stream to be dehumidified. Lithium chloride solution is the most common liquid desiccant used commercially. As the desiccant absorbs the moisture from the process air, heat is released. A cooling coil in the chamber (or chilled liquid desiccant itself) removes the heat of sorption, creating simultaneous desiccant dehumidification and aftercooling, providing latent and sensible cooling.

(Courtesy of Munters Corporation)

The moisture laden desiccant from the conditioning chamber is then pumped to the other chamber (regenerator), where heat is applied, using a heating coil. In the regenerator, heat drives off the water from the desiccant into an exhaust air stream. Heat to drive off the water could come from many sources, including exhaust gas streams from power generation and absorption cooling systems. The desiccant is now ready to be re-used in the conditioning chamber. It is pumped from the regeneration chamber, to be redistributed in the conditioning/dehumidification chamber.

An interchanger is often used to cool the warmer desiccant leaving the regenerator by exchanging heat with the cooler desiccant from the conditioner. Additional process air sensible cooling may be required to provide process control or comfortable space dry bulb temperatures.

One regenerator can handle desiccant from several conditioning chambers. Varying the concentration of desiccant in the solution controls humidity in the processed air.

Liquid desiccant systems not only control humidity in process air, but also scrub the air of particulates, killing bacteria and viruses.

Back To Top