Evaluation of CHP plants

The approach of “Primary Energy Savings” (PES) is an internationally accepted method to assess and compare the “quality” of CHP plants. According to EU Directive 2004/8/EC, PES is calculated on bases of the following formula:

CHP E_eta = P/F (electrical efficiency of cogeneration plant)
Ref H_eta = reference efficiency of separate heat production
CHP H_eta =Q/F (thermal efficiency of cogeneration plant)
Ref E_eta = reference efficiency of separate electricity production
P = net electrical output
Q = net thermal output
F = fuel energy consumption of total CHP plant (GCV!)
P, Q, F – annual mean values!!!

Where a cogeneration plant delivers mechanical energy instead of electricity, this energy should be considered.All stated efficiencies are based on the following definition:

Efficiency reference values for separate production of heat and electricity:

The principles for defining these efficiency reference values referred to the separate heat and electricity production that cogeneration is intended to substitute having the following constraints:

- Comparison with separate electricity production shall be based on the principle that the same fuel types are compared
- Each cogeneration unit shall be compared with the best available and economically justifiable technology for separate production of heat and electricity on the market in the year of construction of the cogeneration unit incorporating ambient conditions of site
- The efficiency reference values for cogeneration units older than 10 years of age shall be fixed on the reference values of units of 10 years of age

Example:

Reference efficiency of a steam boiler may be 85%. For reference electrical efficiency, 30% is assumed. With these reference values, the above graph results. The x-axis displays the thermal CHP-efficiency, the blue lines the electrical CHP-efficiencies and the red lines the overall CHP efficiency.

Instead of separate production of process heat and electricity, with a CHP plant consisting of a backpressure turbine with an electrical efficiency of 20% and a thermal efficiency of 65% can be considered as realistic (overall CHP efficiency 85%). With this arrangement, up to 30% of primary energy can be saved compared to separate generation of electricity and process heat, as shown in the graph above. The cover page of this brochure shows the same example.