Absorption Refrigeration

Absorption Refrigeration

Absorption refrigeration is the least intuitive of the solar refrigeration alternatives. Unlike the PV and solar mechanical refrigeration options, the absorption refrigeration system is considered a “heatdriven” system that requires minimal mechanical power for the compression process. It replaces the energy-intensive compression in a vapor compression system with a heatactivated “thermal compression system.” A schematic of a single-stage absorption system using ammonia as the refrigerant and ammonia-water as the absorbent is shown in Figure

6. Absorption cooling systems that use lithium bromide-water absorption-refrigerant working fluids can not be used at temperatures below 0°C (32°F).

The condenser, throttle and evaporator operate in the exactly the same manner as for the vapor compression system. In place of the compressor, however, the absorption system uses a series of three heat exchangers (absorber, regenerating intermediate

heat exchanger and a generator) and a small solution pump.

Ammonia vapor exiting the evaporator (State 6) is absorbed in a liquid solution of water-ammonia in the absorber. The absorption of ammonia vapor into the water-ammonia solution is analogous to a condensation process. The process is exothermic and so cooling water is required to carry away the heat of absorption. The principle governing this phase of the operation is that a vapor is more readily absorbed into a liquid solution as the temperature of the liquid solution is reduced.

The ammonia-rich liquid solution leaving the absorber (State 7) is pumped to a higher pressure, passed through a heat exchanger and delivered to the generator (State 1). The minimum mechanical power needed to operate the pump is given by Equation 1, the same equation that applies to the minimum power needed by a compressor. However, the power requirement for the pump is much smaller than that for the compressor since v, the specific volume of the liquid solution, is much smaller than the specific volume of a refrigerant vapor. It is, in fact, possible to design an absorption system that does not require any mechanical power input relying instead on gravity. However, grid-connected systems usually rely on the use of a small pump.

In the generator, the liquid solution is heated, which promotes desorption of the refrigerant (ammonia) from the solution. Unfortunately, some water also is desorbed with the ammonia, and it must be separated from the ammonia using the rectifier. Without the use of a rectifier, water exits at State 2 with the ammonia and travels to the evaporator, where it increases the temperature at which refrigeration can be provided.

This solution temperature needed to drive the desorption process with ammonia-water is in the range between 120°C to 130°C (248°F to 266°F). Temperatures in this range can be obtained using low cost non-tracking solar collectors. At these temperatures, evacuated tubular collectors may be more suitable than fl at-plate collectors as their effi ciency is less sensitive to operating temperature.

The overall effi ciency of a solar refrigeration system is the product of the solar collection effi ciency and the coeffi cient of performance of the absorption system. The effi ciency of an evacuated tubular collector for different levels of solar radiation and energy delivery temperatures is given in Figure 5. The COP for a single-stage ammonia-water system depends on the evaporator and condenser temperatures. The COP for providing refrigeration at –10°C (14°F) with a 35°C (95°F) condensing temperature is approximately 0.50. Advanced absorption cycle confi gurations have been developed that could achieve higher COP values. The absorption cycle will operate with lower temperatures of thermal energy supplied from the solar collectors with little penalty to the COP, although the capacity will be significantly reduced.

A number of barriers have prevented more widespread use of solar refrigeration systems. First, solar refrigeration systems necessarily are more complicated, costly, and bulky than conventional vapor compression systems because of the necessity to locally generate the power needed to operate the refrigeration cycle.

Second, the ability of a solar refrigeration system to function is driven by the availability of solar radiation. Because this energy resource is variable, some form of redundancy or energy storage (electrical or thermal) is required for most applications, which further adds to the system size and cost. The advantage of solar refrigeration systems is that they displace some or all of the conventional fuel use.

The operating costs of a solar refrigeration system should be lower than that of conventional systems, but at current and projected fuel costs, this operating cost savings would not likely compensate for their additional capital costs, even in a longterm life-cycle analysis. The major advantage of solar refrigeration is that it can be designed to operate independent of a utility grid.

Applications exist in which this capability is essential, such as storing medicines in remote areas. Of the three solar refrigeration concepts presented here, the photovoltaic system is most appropriate for small capacity portable systems located in areas not near conventional energy sources (electricity or gas). Absorption and solar mechanical systems are necessarily larger and bulkier and require extensive plumbing as well as electrical connections. In situations where the cost of thermal energy is high, absorption systems may be viable for larger stationary refrigeration systems.

The solar mechanical refrigeration systems would require tracking solar collectors to produce high temperatures at which the heat power cycle effi ciency becomes competitive. If the capital cost and effi ciency of tracking solar collectors can be signifi cantly reduced, this refrigeration system option could be effective in larger scale refrigeration applications.

Solar Mechanical Refrigeration

Solar Mechanical Refrigeration

Solar mechanical refrigeration uses a conventional vapour compression system driven by mechanical power that is produced with a solar-driven heat power cycle. The heat power cycle usually considered for this application is a Rankine cycle in which a fluid is vaporised at an elevated pressure by heat exchange with a fluid heated by solar

collectors. A storage tank can be included to provide some high temperature thermal storage.

The vapour flows through a turbine or piston expander to produce mechanical power, as shown in Figure 4. The fluid exiting the expander is condensed and pumped back to the boiler pressure where it is again vaporised.

The efficiency of the Rankine cycle increases with increasing temperature of the vaporised fluid entering the expander, as shown in Figure 5 (bold line). The Rankine cycle efficiency in Figure 5 was estimated for a high-temperature organic fluid assuming that saturated vapour is provided to a 70% efficient expander and condensation occurs at 35°C (95°F). The efficiency of a solar collector, however, decreases with increasing temperature of the delivered energy. High temperatures can be obtained from concentrating solar collectors that track the sun’s position in one or two dimensions. Tracking systems add cost, weight and complexity to the system. If tracking is to be avoided, evacuated tubular, compound parabolic or advanced multi-cover flat plate collectors can be used to produce fl uid temperatures ranging between 100°C – 200°C (212°F – 392°F).

Vapour Compression Cycle

The Vapour Compression Refrigeration Cycle is a process that cools an enclosed space to a temperature lower than the surroundings. To accomplish this, heat must be removed from the enclosed space and dissipated into the surroundings. However, heat tends to flow from an area of high temperature to that of a lower temperature. How can we make the reverse happen? This is where the Vapour Compression Refrigeration Cycle steps in. During the cycle, a substance called the refrigerant circulates continuously through four stages. The first stage is called Evaporation and it is here that the refrigerant cools the enclosed space by absorbing heat. Next during the Compression stage, the pressure of the refrigerant is increased, which raises the temperature above that of the surroundings. As this hot refrigerant moves through the next stage, Condensation, the natural direction of heat flow allows the release of energy into the surrounding air. Finally, during the Expansion phase, the refrigerant temperature is lowered by what is called the auto-refrigeration effect. This cold refrigerant then begins the Evaporation stage again, removing more heat from the enclosed space.

Each of the four stages will now be revisited in detail, explaining the physical changes that occur in the refrigerant and the devices used to accomplish these changes. A visual representation of the cycle is displayed below. With the explanation of each stage, a picture of the part responsible is included to aid in identification.

Evaporation 

During this stage, the refrigerant travels through a device called an evaporator that has a large surface area and typically consists of a coiled tube surrounded by aluminium fins. The cold fluid is a mixture of liquid and vapour refrigerant as it begins this stage. While flowing through the evaporator, all the liquid evaporates and absorbs heat from the enclosed space. The energy absorbed is used to change the state of the refrigerant from liquid to vapour. This lowers the temperature of the space, along with whatever food or beverages are stored in it. The refrigerant exits this stage as a saturated vapour.

Compression

The heat that was absorbed in the Evaporation stage must be released into the surroundings, but this will not happen unless the temperature of the refrigerant is higher than the outside air. This is the purpose of the Compression stage. A device, predictably called a compressor, raises the pressure of the refrigerant vapour. Due to basic thermodynamic principles, this causes the temperature of the refrigerant to rise, leaving the stage as a superheated vapour. Energy is needed to power the compressor, which is why electricity is required to operate a refrigerator  

Condensation

Now that we have increased the temperature of the refrigerant above that of the surroundings, we can dissipate the heat necessary to continue the process. This is accomplished with a evince very similar to the evaporator. It also uses a coiled tube with aluminium fins, but may have different dimensions than the evaporator to accommodate the different state of the refrigerant. As the hot vapour flows through the condenser, the outside air removes energy and the refrigerant becomes a saturated liquid. At this point the slightest drop in pressure will initiate evaporation, which is the basis for the final stage of the process

Expansion

To begin a new cycle, all that must happen is a lowering of the refrigeration temperature to below that of the enclosure. This is the key to the entire cycle, because this was the problem that we started with. However, in this situation we can utilise what is called the auto-refrigeration effect. When a saturated liquid experiences a sudden drop in pressure, a small amount of liquid is instantly vaporised and the temperature of the mixture is drastically reduced. This cold liquid-vapour mixture can now begin a new cycle. The pressure drop is accomplished by the simplest, yet most important, part of the system – a simple flow restriction. This part is commonly called a throttle or expansion valve.

Summary

The Vapour Compression Refrigeration cycle is a simple four stage process that cools a small enclosed area to a temperature lower than the surroundings. During the cycle, a refrigerant removes heat from the enclosed space and dissipates it to the surrounding air. The Evaporator draws heat from the space as the cold liquid-vapour refrigerant evaporates. The Compressor increases the pressure of the vapour, which increases the temperature above that of the surrounding air. The condenser then releases heat to the surroundings as the hot vapour refrigerant condenses. This saturated liquid passes through the Expansion Valve, abruptly lowering the pressure and initiating the auto-refrigeration effect. This drops the temperature of the refrigerant and begins a new cycle.

Absorption Refrigeration Cycle

     Absorption - 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 vapour phase (in the evaporator section). The refrigerant vapours 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 vapour (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 vapours and an absorption chiller uses heat for compressing refrigerant vapours to a high-pressure. The rejected heat from the power-generation equipment (e.g. turbines, micro turbines, and engines) may be used with an absorption chiller to provide the cooling in a CHIP 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 vapour 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 vaporise. The vapours 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 vapours 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 CHIP system for buildings by utilising 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.

Multiple-Effect Absorption Chillers

                  

   In a single-effect absorption chiller, the heat released during the chemical process of absorbing refrigerant vapour 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 vapour. The more vapour 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 Absorption Chiller

(Courtesy of Inter Energy Software)

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, micro turbine, 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 Inter Energy Software)

   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 after cooling, 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 inter changer 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.