Co-generation plant operations should be decided only after comparing the running cost to cost of electricity.

With a growing demand for power security, co-generation presents as an attractive yet challenging opportunity. It yields heat and power from a single fuel source. Co-generation plant operations should be decided only after comparing the running cost to cost of grid electricity.

This comparison is always made during the feasibility stage of the plant. However during the years in operation, the cost of grid electricity changes or a mismatch occurs between the process heat and electrical requirements. This mismatch leads to:

Operations at off design conditions lowering operational efficiencies – due to mismatch between steam generator and turbine.
Steam venting – if the process steam demand is less that electricity demand.
Poor steam quality or pressure drops in process plant – Incorrect design of the utility system.
Low cogeneration efficiency.
Thus in case of co-generation systems it is important to ensure the right system configuration to ensure satisfactory performance. If the cost of steam is higher than the grid electricity cost, venting steam after the turbine not only wastes energy but may also not be financially viable.

Illustration:

The cost of electricity generation in cogeneration system is given by:

Electricity cost = Incremental fuel consumption x cost of fuel

Net electricity generated

Incremental fuel consumption = (Actual fuel consumed @ generation pressure – Theoretical fuel consumption for process steam @ 10bar)