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Natural-gas-engine/generator-powered electric chiller with heat recovery

Figure 7 shows the pounds of CO₂ resulting from the production of 1 ton-hr of cooling by a natural-gas-engine/generator-powered electric chiller with heat recovery. In this case, the engine/generator-powered electric centrifugal chiller of the previous option has been equipped with a full engine heat-recovery system, one that includes engine jacket water and engine exhaust. For a typical internal-combustion, natural-gas-fired engine of this size, the shaft energy produced would be about 32 percent of the input energy, leaving about 68 percent to be dissipated as heat. For typical heat-recovery applications, the dissipated heat can be recovered as a percentage of input energy largely as follows:

• Jacket heat recovery (180°F water), 30 percent.

• Exhaust heat recovery (steam or hot water), 18 percent.

• Lube-oil/aftercooler heat recovery (130°F water), 8 percent (not utilized in this case).

• Unrecoverable losses, 12 percent.

For this application, then, 48 percent of the heat was considered recovered from the input-energy stream through jacket water and exhaust heat. Additional auxiliary electricity was considered for the jacket-water and exhaust heat-recovery pumping systems.

The natural gas not burned because of useful heat recovery becomes a credit to the process, reducing emissions to 0.35 lb of CO₂ per ton-hour of cooling provided, or 70-percent below the emissions of the high-efficiency electric centrifugal chiller (Figure 8). If heat is not recovered, emissions will be similar to those of a natural-gas-engine/generator-powered electric chiller without engine heat recovery (0.86 lb of CO₂ per ton-hour). Every real heat-recovery system operates somewhere between those two extremes.

Unfortunately, a natural-gas-engine/generator-powered electric chiller with heat recovery adds a significant amount of first cost and operating complexity to a system.