Clean fossil fuel heat and power plants - General information

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Thermal efficiency

Thermal efficiency has a direct impact on the cost of electricity for fossil fuel power production. It is defined as heat supplied over total work output.

The characteristic of thermal efficiency of a power cycle can be illustrated by the Carnot principle. In a Carnot process, the power cycle undergoes a series of four reversible part processes.

1-2: Heat is supplied at a temperature level, T_h.
2-3: Work is generated by isothermal expansion.
3-4: Heat is rejected at a temperature level, T_c.
4-1: Work is supplied by isothermal compression.

Figure 1 illustrates the Carnot process. The total power output W equals the green area shown while the heat supply Q equals the complete hatched area.

1.
2.

From equation (1) and (2), the thermal efficiency of a Carnot process can be expressed by:

3.

Equation (3) shows that to achieve a high thermal efficiency, it is desirable to have a high temperature level for heat supply and a low temperature level for heat rejection.

Figure 1: The Carnot process

The temperature level for heat supply (T_h) is nomally much higher for a gas turbine power cycle (Brayton Cycle) compared to a steam turbine power cycle (Rankine Cycle), as illustrated in Figure 2. In a combined cycle power plant, the heat from the gas turbine exhaust gas is utilized in a steam cycle. Then the temperature level of heat rejection (T_c) may be the same for the steam power cycle and for the combined cycle configuration. The Carnot efficiency is given by:

3.

As T_h is higher for the combined cycle, and Tc is the same, the efficiency is higher for a combined cycle gas turbine based power production compared to a steam turbine based power production.

Figure 2: Temperature-entropy diagram

To separate steam turbine based power production from gas turbine based power production we use the thermology 1^st generation- and 2^nd generation oxyfuel. 1^st generation oxyfuel is steam turbine based power production with a relative low efficiency potential and 2^nd generation oxyfuel is gas turbine based power production with a relative high efficiency potential.

An oxygen fired gas turbine is not mature technology and 2^nd generation oxyfuel requires a gas turbine development to become commercial technology. Nebb focus mainly on 2^nd generation oxyfuel and we believe this is a promising technology for future zero emission power generation.

	1st generation oxyfuel	2nd generation oxyfuel
Main energy production unit	Steam turbine	Gas turbine
Temperature level of heat supply	low	high
Thermal efficiency potential	low	high
Technical maturity	Requires boiler redesign	Requires gas turbine development

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