A heat-driven chiller can free electrical capacity—but it must reject more heat than the cooling it produces.
Contents
| Flow | Thermal power |
|---|---|
| Useful heat supplied to chiller | 1.0 MW |
| Cooling at assumed thermal COP 0.7 | 0.7 MW |
| Heat rejected, before auxiliary contribution | 1.7 MW |
| Electric chiller alternative at assumed COP 5 | 0.14 MW electrical input for 0.7 MW cooling |
“Heat-powered cooling” still needs a heat sink
An absorption chiller uses heat to drive a refrigeration cycle. That can turn engine heat into chilled water while reducing compressor-electric demand. The cooling tower or other rejection system must then dispose of both the input heat and the cooling load.
The example’s 1.7 MW rejection requirement is why a waste-heat chiller can disappoint when the tower is undersized. It is not a 0.7 MW tower simply because the cooling output is 0.7 MW.
Match temperature before counting the heat
Single-effect and double-effect absorption cycles require different driving temperatures and deliver different thermal efficiencies. Engine jacket water may suit some hot-water-driven equipment, while exhaust or steam can support higher-temperature arrangements. Require the chiller curve at the available supply/return temperatures and the design cooling-water condition.
The 0.7 COP in the example is an assumption consistent with a single-effect screening case, not a guarantee for a particular chiller. Pump and tower-fan electricity remain. Water treatment, makeup water and maintenance also belong in operating cost.
Value the electrical capacity it releases
At an assumed electric-chiller COP of 5, 0.7 MW of cooling would require 140 kW of compressor input. If that load coincides with a constrained site peak, freeing electrical capacity can be valuable as well as saving energy.
For example, 140 kW avoided over 5,000 hours at $0.10/kWh is $70,000/year before absorption-system auxiliaries. Deduct the value of heat that could have served another customer or process. “Waste” heat is not free when it has a better use.
Run both systems through the summer case
High cooling-water temperature can reduce absorption capacity at the same time site cooling demand rises. Evaluate generator output, usable heat, chiller performance and tower capacity together. Critical loads also need a plan for generator outages and low-load operation; a backup electric chiller may be part of the sensible design.
The best applications have steady simultaneous heat and cooling demand, a suitable heat grade, room for rejection equipment and a real value for released electrical capacity.