Cogeneration - Designs

Cogeneration is generally defined as the coincident production of electricity and usable thermal energy from a single fuel or thermal input. For example, a water-cooled engine-generator can produce power and hot water. Further heat recovery, if economically worthwhile, could also recover exhaust gas energy.

Cooling system cogeneration designs are most often:

• Engine-driven chillers with heat recovery,
  
• Steam turbine-driven chillers in large cogeneration systems, or
  
• Steam absorption chillers used to condense "waste steam."

The easiest way to evaluate cogeneration system alternatives is to start with the site's heating loads. The following rules of thumb are useful in selecting the "prime mover:"

• Reciprocating engines work best for small heating loads (less than 2,000 Btu of heat per kW of power used), or whenever hot water heat recovery is desired.
  
• Gas turbines fit situations where 5,000 - 10,000 Btu of heat per kW of power is needed. This tends to be large hospitals, universities, and industrial plants.
  
• Steam turbines: are appropriate where 20,000+ Btu of heat per kW of power is needed. The best applications are usually large industrial plants.

These prime movers can drive electric generators, air compressors, process equipment, or chillers. The choice is based on annual operating hours and the integration of heat and power making the most sense. Consequently, most cogeneration designs use the prime mover to generate power rather than drive a chiller. Waste heat from each prime mover can be recovered to displace steam that would have otherwise been generated in a boiler, or can be used to produce cooling in an absorption chiller.

Select from these areas of interest . . .