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Even among chiller experts, there are outdated misconceptions about the efficiency, flexibility and maintainability of absorption chillers. However, in recent years, absorption cooling technology has undergone significant changes that defy these assumptions. Today’s absorption chillers have proven to be efficient, reliable performers in many industrial cooling applications around the world, usually as part of a natural-gas or combined heat and power (CHP) system.


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Some absorption chillers are designed with a two-step evaporator and absorber cycle. Image provided by Johnson Controls Inc. (Click on the image to enlarge.)


They use water as a refrigerant and make efficient use of waste heat or low cost natural gas, making absorption chillers cost-effective solutions in circumstances where natural refrigerants are required, excess heat is available and grid congestion occurs. This is especially important as the industry transitions from ozone-depleting refrigerants to those with low global warming potential (GWP). Natural refrigerants, including those that use water, have low to no GWP.

A closer look at the real-world performance of lithium bromide/water absorption chillers in the 30- to 2,000-ton range can help clarify common misunderstandings about these units and illustrate their benefits.


Cost-Effective over the Life of the Chiller

It is a common belief that electric centrifugal chillers are far more cost-efficient than absorption chillers in typical applications. Because of this, some think absorption chillers are more expensive to operate and do not make sense financially to install at their plant.

The coefficient of performance (COP), a common way to measure efficiency, is the reason behind this misconception. COP is output, or cooling capacity, divided by input. Table 1 shows the COP of different 1,000-ton systems at testing conditions defined by the Air-Conditioning, Heating and Refrigeration Institute (AHRI).

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TABLE 1. The coefficient of performance (COP) is compared among different 1,000-ton chillers. Image provided by Johnson Controls Inc. (Click on the image to enlarge.)

The COP of the electric centrifugal chiller is higher than that of the absorption chillers, but it is important to consider two things. The first is that the COP of centrifugal chillers is calculated in a different way than absorption chillers. For electric chillers, the input is electric energy; for absorption chillers, the input is non-electric (thermal) energy. Second, the ultimate cost of operation, not the perceived efficiency based on an academic comparison of COP values, is what cost justifies one system over another.

To truly evaluate the cost-effectiveness of an absorption chiller, it is important to take into account how COPs correspond with actual energy costs (table 2).

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TABLE 2. The corresponding COP, design point and energy costs for different 1,000-ton chillers are shown. Image provided by Johnson Controls Inc. (Click on the image to enlarge.)

This comparison makes two assumptions. First, it assumes that the centrifugal and absorption chillers are working at design conditions with their respective COPs. Second, it assumes that the cost of natural gas is $4/MMBTU, and electricity is assumed at $0.1 kWh and $0.2 kWh with no demand charge (table 3).

The table reveals that, at these energy rates, the absorption chiller costs less to operate than the electric centrifugal chiller even though its COP is not nearly as high. This basic analysis demonstrates that one cannot conclude that an electric centrifugal chiller costs less to operate than absorption chillers solely based on COP values. In situations where the cost of natural gas is low, waste heat is available and electric costs are high, absorption chillers may offer the more cost-effective choice.

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TABLE 3. The breakdown of ton-hour expenses are compared between an absorption chiller fired by natural gas and an electric centrifugal chiller. Image provided by Johnson Controls Inc. (Click on the image to enlarge.)

A look at annual operational costs further supports that absorption chillers can be economical (table 4). The analysis, performed using computer simulation, considers an average U.S. city with average chiller performance. The considered chillers are single-stage steam, two-stage steam and direct-fired absorption chillers, all with a minimum entering condenser water temperature of 68°F (20°C), and an industry-average, 1,000-ton electric centrifugal chiller with variable-speed drive.

The model shows that, although it may have a higher-rated efficiency than an absorption chiller, an electric chiller does not necessarily cost less annually to operate. In this case, a plant may save more over the course of the year using absorption chillers. This is because absorption chillers are driven by low cost heat, which can lower the total cost of operation.

Lower operational costs can pay back capital investment more quickly. Although absorption chillers require a greater first cost than electric centrifugal chillers, an absorption chiller can often pay for itself faster through bigger operational cost savings. It is important to choose a chiller based on life-cycle cost rather than initial cost. While initial cost may be higher for one chiller over another, the initial cost, plus maintenance and annual energy costs over the life of the same chiller, may be lower in total.

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TABLE 4. A look at annual operational costs further supports how economical absorption chillers can be. Shown here are the operating costs of centrifugal and absorption chillers at two separate electric utility rates, assuming the rate of natural gas is $4/MMBTU and steam is waste ($2/1,000 lb). Image provided by Johnson Controls Inc. (Click on the image to enlarge.)

However, absorption chillers are suited for some applications better than others. The appropriate chillers must be carefully selected for each situation. If low cost, thermal-driving heat sources are not readily available, it is usually not economically sound to produce the driving heat source required to fire an absorption chiller.


Flexible Use, Operating Range and Energy Sourcing

The evolution of absorption cooling technology has resulted in much greater application versatility for industrial process cooling installations. Absorption chillers have been successfully applied for gas turbine inlet air-cooling applications, hazardous-area outdoor applications and other uses.

Although absorption chillers are typically longer and heavier than electric centrifugal chillers, it is possible to adjust the dimensions and achieve the same cooling capacity. This can help solve installation challenges such as tight spaces or other physical constraints.

Absorption chillers have greater real-world flexibility than might be expected from their standard operation ranges. For example, if the unit is working at 50 percent cooling load, the chilled water and the cooling (condenser) water flow rate may also be at 50 percent of the design value. The acceptable rate of change of flow is approximately 5 percent per minute (table 5).

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TABLE 5. Absorption chillers have greater real-world flexibility than might be expected from their standard operation ranges. Shown here are standard operating ranges of absorption chillers. Image provided by Johnson Controls Inc. (Click on the image to enlarge.)

The tube material used can affect absorption chiller cooling capacity. Materials typically include copper, 90/10 cupronickel (CuNi 90:10), stainless steel (SS316) and titanium, but if tube material is not copper, there will likely be a deration (7 ~ 18 percent) in cooling capacity.

This deration also happens when the cooling (condenser) water inlet temperature to the absorber section rises above the design value. Consider an industry-average, two-stage, 1,000-ton, steam-driven absorption chiller. If the cooling (condenser) water temperature increases from 85 to 93°F (29 to 34°C), the cooling capacity may drop to almost 750 tons (a 25 percent deration) while the COP stays relatively flat.

Absorption chillers typically require a larger cooling tower compared to a mechanical chiller with the same capacity. The values in table 6 provide an understanding of the heat rejection to the cooling tower.

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TABLE 6. The values provide an understanding of the heat rejection to the cooling tower. Image provided by Johnson Controls Inc. (Click on the image to enlarge.)

Typically, the heat rejection to the cooling tower for a single-stage and two-stage absorption chiller is 29 MBh/ton and 21 MBh/ton, respectively.

For additional flexibility, absorption chillers can be driven in several ways. They can be driven by a combination of direct-fired, plus hot water or steam plus hot water, multi-energy units. Direct exhaust gas or a combination of exhaust gas plus hot water plus direct gas-fired configurations are possible, too. Single-effect, double-lift designs can be driven by much lower entering hot water temperatures. Other special designs can provide leaving evaporator fluid temperature below 32°F (0°C), which is ideal for food and beverage applications.


Ease of Operation and Maintenance

One of the most common misconceptions about absorption chillers is that they are not easy to operate or maintain because the lithium bromide salt solution crystalizes. The truth is that modern absorption chillers are equipped with PLC controls that ensure the concentration percentage of the operating lithium bromide solution does not enter the crystallization zone.

The design method used can offer further protection against crystallization. For instance, one method uses a combination of two-step, evaporator-absorber and parallel (lithium bromide) flow design instead of a series-flow or reverse-flow design. In this two-step design, the entering absorber spray concentration is usually only 58 to 61.5 percent, depending on the type of unit and design condition. Besides low solution (salt) concentration, the unit operates with the lowest temperature and pressure in the first-stage (high temperature) generator section.

This type of low solution (salt) concentration cycle provides protection against crystallization. It also can improve the reliability of the system and extend its life by reducing corrosion and the amount of non-condensable gases.

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Absorption chillers have proven to be efficient, reliable performers in many industrial cooling applications around the world, usually as part of a natural-gas or combined heat and power (CHP) system. Shown here is a combined heat and power absorption chiller. Image provided by Johnson Controls Inc. (Click on the image to enlarge.)

With modern controls and well-considered designs, absorption chillers are straightforward to operate and maintain. Just like any other water-cooled chiller, cooling (condenser) water quality must be maintained in accordance with the manufacturer’s guidelines. The vacuum also requires maintenance; however, modern absorption chillers have an automatic purge system that performs this service. To determine the topping quantity of the corrosion inhibitor, the lithium bromide sample should be analyzed once or twice annually based on the frequency of operation. A thorough understanding of the cycle is vital for successful operation and maintenance, in accordance with a manufacturer’s guidance.

In conclusion, the time has come to see what absorption chillers really offer. Facilities searching for a chiller that makes use of existing thermal energy sources and low-GWP natural refrigerants are best positioned to reap their benefits. With improved controls, a two-step evaporator/absorber design, zero-GWP refrigerant and waste-heat recovery capabilities, modern absorption cooling technology is a key solution for a decarbonized future.