• Introduction

Generator costs are typically represented by up to four different curves
1. Input/output Curve
2. Production-cost Curve
3. Heat-rate Curve
4. Incremental Cost Curve

Note:
1 Btu (British thermal unit) = 1054 J
1 MBtu = 1x106 Btu
1 MBtu = 0.293 MWh
3.41 Mbtu = 1 MWh

Input output Curve

It is the fundamental curve for a thermal plant / generating unit which represent the fuel required (in form of thermal energy per hour) to maintain various level of power generation (in MW).

Production Cost Curve

It is the curve for a thermal power plant / generating unit which represent the fuel cost required (in Rs/hr) to maintain various level of power generation (in MW). Fuel cost is expresses as Rs/MBtu.

Heat Rate Curve

If we divide, the I/O Curve with the corresponding output MW, we get the unit’s Heat Rate.

The heat rate curve plots the thermal energy required per MWh for the generator as a function of the generator’s MW output. Thus, the heat rate curve indicates the efficiency of the unit over its operating range.

Generally, units are least efficient at the minimum and maximum portions of their MW output capability and most efficient somewhere in the middle of their operating range.

Incremental Cost Curve

If we take the slope (derivative) of the IO curve at every point, we will come up with the unit’s incremental heat rate. The generator IO curve is usually approximated by a parabolic curve – therefore, the derivative is a straight line.

If we multiply the fuel cost and the IHR Curve, we will have the Incremental Cost Curve. Incremental cost is simply first derivative of the production cost (Rs/hr). This is the curve we use for Economic Dispatch.