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Are absorption chillers energy efficient?

No way! However, they can be an option under certain circumstances, says Dan Mizesko…

| | Jun 18, 2017 | 11:07 am
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I have been reading with interest that District Cooling businesses in the GCC region might be studying and contemplating absorption chillers for District Cooling plants. District Cooling promotes itself as “energy efficient”, so to be clear, absorption chillers are not energy efficient; in fact, they are very energy intensive.

Now, before anyone in the absorption industry gets outraged, absorption chillers do make sense in certain circumstances, but I want to be clear with the readers, absorption chillers are not energy efficient.

SO, WHAT IS AN ABSORPTION CHILLER?

Absorption chillers are not as easy and straightforward to understand as a mechanical vapour-compression system, and in this region, Absorption Chiller Certified Engineers are very rare, so I will do my best to explain.

The refrigeration cycle for a conventional vapour-compression chiller and an absorption chiller is similar in that both produce chilled water through the evaporation and condensation of a refrigerant at different pressures within the machine. However, a conventional chiller uses a mechanical means to compress and transport the refrigerant vapour to the condenser, whereas an absorption chiller depends on a thermo-chemical process involving lithium bromide and water to establish the pressure differential in lieu of mechanical compression. While most vapour-compression chillers utilise electricity as their energy source to operate the machine, absorption chillers use heat as their energy. Typically, the heat is in the form of steam, hot water or through the direct combustion of Natural Gas.

The simplest absorption machines are residential refrigerators, with a gas flame at the bottom, ice cubes at the top and no electricity involved. An absorption chiller is larger and more complicated, but the basic principle is the same. The evaporator allows the refrigerant to evaporate and to be absorbed by the absorbent, a process that extracts heat from the chilled water loop. The combined fluids then go to the generator, which is heated by the hot water gas or steam, driving the refrigerant back out of the absorbent. The refrigerant then goes to the condenser to be cooled back down to a liquid, while the absorbent is pumped back to the absorber. The cooled refrigerant is released through an expansion valve into the evaporator, and the cycle repeats.

Absorption chillers are either lithium bromide-water (LiBr/H2O) or ammonia-water equipment. The LiBr/H2O system uses lithium bromide as the absorber and water as the refrigerant. The ammonia-water system uses water as the absorber and ammonia as the refrigerant. I will concentrate on the LiBr/H20 chiller for this article.

SINGLE-EFFECT ABSORPTION CHILLER

The single-effect “cycle” refers to the transfer of fluids through the four major components of the refrigeration machine – evaporator, absorber, generator and condenser. Single-effect LiBr/H2O absorption chillers use low-pressure steam, or hot water, as the heat source. The water is able to evaporate and extract heat in the evaporator, because the system is under a vacuum. The thermal efficiency of single-effect absorption systems is low. Although the technology is sound, the low efficiency has inhibited the cost competitiveness of single-effect systems. Most new single-effect machines are installed in applications, where waste heat is readily available. Single-effect chillers can be used to produce chilled water for air conditioning and for cooling process water and are available in capacities from 100 to 1,500 tonnes of refrigeration (TR).

DOUBLE-EFFECT ABSORPTION CHILLER

The desire for higher efficiencies in absorption chillers led to the development of double-effect LiBr/H2O systems. The double-effect chiller differs from the single-effect in that there are two condensers and two generators to allow for more refrigerant boil-off from the absorbent solution. The higher temperature generator uses the externally supplied steam to boil the refrigerant from the weak absorbent. The refrigerant vapour from the high- temperature generator is condensed and the heat produced is used to provide heat to the low-temperature generator. These systems use gas-fired combustors or high-pressure steam as the heat source. Double-effect absorption chillers are used for air conditioning and process cooling in regions where the cost of electricity is high relative to Natural Gas. Double-effect absorption chillers are also used in applications, where high-pressure steam, such as District Heating, is readily available. Although the double-effect machines are more efficient than single-effect machines, they have a higher initial manufacturing cost. There are special material considerations owing to increased corrosion rates (higher operating temperatures than single- effect machines), larger heat exchanger surface areas and more complicated control systems.

What’s important to remember is that again, water is the refrigerant in an absorption chiller, and lithium bromide as a salt is used to absorb the water. These are difficult and not easily understood concepts. However, water has a very high specific heat and latent heat of vapourisation, which makes it a great refrigerant.

How is water boiling at 212 degrees Fahrenheit going to create chilled water at 44 degrees Fahrenheit? The boiling temperature of water is a direct function of pressure, and at a pressure of 1 atmosphere (29.92 Hg), water boils at 212 degrees Fahrenheit. When the pressure on the water is decreased, the boiling temperature of water is lowered (absorption chillers must operate at a very low vacuum). Absorption chillers must be very tight and leak-free to generate chilled water. This, in my opinion, is a big disadvantage of absorption chillers in the GCC region, as leak-check and leak-arrest methods must be of a very high standard, and dehydration and vacuum standards must also be of the highest level. Are technicians and engineers in the region trained and experienced to meet the high standards? In the case of industrial absorption chillers, helium is used for leak-checking the chillers, so that even tiny weep-hole leaks in the shells are identified. The helium-based approach is necessary, because even the smallest of leaks can render an absorption chiller incapable of generating chilled water.

CHILLER PERFORMANCE STANDARD

The primary absorption chiller performance standard is AHRI Standard 560 (2000 Standard for Absorption Water Chilling and Water Heating Packages). AHRI Standard 560 applies to water-cooled single-effect steam chillers, water-cooled single-effect hot water chillers, water-cooled double- effect steam chillers, water-cooled double-effect hot water chillers and water-cooled double-effect direct-fired chillers. The standard provides testing standard conditions, rating requirements, minimum data requirements for published ratings and integrated part load value (IPLV) or non-standard part load value (NPLV). For performing the IPLV testing, AHRI Standard 560 has established standard conditions for absorption chillers, including:

• Entering condenser water temperature: 85 degrees Fahrenheit
• Condenser water flow rate: 3.6 gpm/tonne (single-effect indirect fired)
• 4.0 gpm/tonne (double-effect indirect fired, double-effect direct-fired)
• Condenser water-side fouling factor: 0.00025
• Evaporator leaving water temperature: 44 degrees Fahrenheit
• Evaporator water flow rate: 2.4 gpm/tonne Evaporator waterside fouling factor: 0.0001
• Tube-side fouling factor (steam): 0.000 (indirect fired)
• Tube-side fouling factor (hot water): 0.0001 (indirect fired)

It is very important to understand that chillers rarely operate at their maximum capacity. AHRI used typical building types and operations in 29 different cities to develop a chiller loading profile during a typical year. The resulting chiller loading profile is at 100% capacity about one per cent of the time, 75% capacity about 42% of the time, 50% capacity about 45% of the time, and 25% capacity about 12% of the time. These values are incorporated into the IPLV equations.

When evaluating different chiller energy usages, the IPLV provides the most accurate average chiller energy usage. Ultimately, the chiller’s energy usage is primarily based upon the “lift” or temperature difference between the chilled water leaving temperature and condenser water leaving temperature. Lowering the condenser water leaving temperature or raising the chilled water leaving temperature will reduce lift and the energy usage of the chiller. Raising the condenser water leaving temperature or lowering the chilled water leaving temperature will increase lift and the energy usage of the chiller (the above lift conditions make absorption chillers another challenge for utilisation in this region, especially for District Cooling systems).

ENERGY EFFICIENCY

Absorption chiller energy efficiency is based upon fuel consumption per tonne of cooling, whereas motor-driven vapour-compression chiller energy efficiency is based upon kW/tonne cooling. The co-efficient of performance (COP) is a method for determining overall chiller energy performance. As per OEM supplied information, the COP range for the different absorption chiller types are as follows (the higher the COP number, the more efficient the chiller):

Looking at the COP ranges, the single-effect chiller is the least energy-efficient absorption chiller type with the hot water, steam and direct-fired, double-effect absorption chillers being almost twice as energy efficient. The hot water and steam double-effect absorption chillers are the most energy efficient absorption chillers, but how do they compare with motor-driven vapor-compression chillers?

The two motor-driven vapor-compression chillers being utilised for energy efficiency comparison are the water-cooled rotary screw chiller and the water-cooled centrifugal chiller. As per OEM supplied information, the water-cooled rotary screw chiller has a COP range of 3.90-5.40, whereas the water-cooled centrifugal chiller has a COP range of 7.00-8.79. The result is that motor-driven vapor-compression chillers are 4-7 times more energy efficient than absorption chillers.

Absorption chillers have other major disadvantages compared to centrifugal chillers.

  • Larger capacity cooling towers are required in the case of absorption chillers, which would add USD25-30 per tonne to the cost of the plant. For a 25,000 TR plant, that is USD625,000/AED 2,300,000 plus of additional capital expense (capex) to the plant construction.
  • Absorption chillers are more expensive than centrifugal chillers, which means additional capex.
  • Absorption chillers have a much higher maintenance cost, which means additional operational expense (opex).
  • Absorption chillers require larger pumps, because more water flow is required with absorption chillers, which translates to additional capex.
  • Absorption chillers utilise substantially more condenser water consumption in evaporation than centrifugal chillers (50% more condenser water isconsumed in evaporation with an absorption chiller versus an electric-driven centrifugal chiller). Since water is a critical issue not only in the GCCregion but worldwide, this is a major disadvantage.
  • Absorption chillers have a large footprint versus centrifugal chillers

In a head-to-head energy-efficiency competition, motor-driven vapour-compression chillers will beat absorption chillers every time. However, there are specific applications where absorption chillers may have an advantage over motor-driven vapor-compression chillers.Typically…

• For a facility that has a cogeneration power plant or some other thermal energy-generating process with excess (waste) thermal energy, absorption chillers can utilise the excess thermal energy to produce chilled water instead of all the excess thermal energy being wasted.

• For a facility that has inadequate electrical infrastructure, or if bringing electrical infrastructure to the facility is cost-prohibitive, absorption chillers have a substantially lower electrical power requirement than motor-driven vapor-compression chillers.

• For a facility with high electrical power cost and low fuel cost, absorption chillers may have a lower operating cost than motor-driven vapor- compression chillers. However, the fuel cost must be substantially low to make it work.

• For a facility that requires substantial system reliability, the lower electrical requirements for absorption chillers will reduce emergency generator load requirements.

 

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Dan Mizesko is Managing Partner, U.S. Chiller Services Int, HVAC & Energy Services International. He can be contacted at dan@uschillerservices.com.


CPI Industry accepts no liability for the views or opinions expressed in this column, or for the consequences of any actions taken on the basis of the information provided here.


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