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Single-stage steam absorption chiller

Figure 3 shows the pounds of CO₂ resulting from the production of 1 ton-hr of cooling by a single-stage steam absorption chiller. Natural gas fuels a boiler producing low-pressure steam at an assumed efficiency of 80 percent. This likely casts the system in the best-possible light, as Energy Star would assume an efficiency, including distribution losses, for a central plant more on the order of 69 percent.² Input energy includes natural gas as well as electricity for auxiliary equipment. CO₂ emissions are 132-percent higher than those of the high-efficiency electric centrifugal chiller.

Two-stage steam absorption chiller

Figure 4 shows the pounds of CO₂ resulting from the production of 1 ton-hr of cooling by a two-stage steam absorption chiller. The basic parameters are the same as those of the single-stage steam absorption chiller, except high-pressure, 100-psig steam is utilized, increasing the chiller coefficient of performance from 0.71 to 1.35. Input energy includes natural gas as well as electricity for auxiliary equipment. CO₂ emissions are 29-percent higher than those of the high-efficiency electric centrifugal chiller.

Gas-fired two-stage absorption chiller

Figure 5 shows the pounds of CO₂ resulting from the production of 1 ton-hr of cooling by a gas-fired two-stage absorption chiller. Notice the absence of a boiler because natural gas is utilized directly in the chiller. Input energy includes natural gas as well as electricity for auxiliary equipment. This proves to be the best of the absorption-chiller options, but CO₂ emissions still are 24-percent higher than those of the high-efficiency electric centrifugal chiller.

Natural-gas-engine/generator-powered electric chiller without heat recovery

Figure 6 shows the pounds of CO₂ resulting from the production of 1 ton-hr of cooling by a natural-gas-engine/generator-powered electric chiller without engine heat recovery. In this case, a nominal 400-kw natural-gas-fired engine/generator set is used to provide electricity to power the high-efficiency electric centrifugal chiller, along with electric auxiliary equipment, of Option 1. CO₂ emissions are 26-percent below those of Option 1, but at the cost of a significant increase in system complexity.