Direct Heat Engines

For the steam and gas-turbine power plants, it is likely to state three performance parameters established from experimental measurements:

• Thermal or cycle efficiency, η_cy = W/Q_H here Q_H is the heat transferred to the functioning fluid at higher temperature.
• Heating device efficiency, η_H = Q_H/? H_c here Δ H_c is the calorific value of the fuel.
• Overall efficiency, η₀ = W/?H_c = η_cy η_H.

When ηcy were equivalent to 100%, W would be equivalent to Q_H and there would be no heat refusal to sink. Except the sink were at the absolute zero, this would disobey the second law. In the heating device the products forever leave at a temperature higher than that of the incoming reactants, as it would be highly uneconomical to give the requisite heat transfer region to effect total cooling.

It is to be observed that though the word ‘efficiency’ is employed to symbolize ηcy and η_H, the two are completely different in type; η_H can theoretically achieve a value of 100%, whereas the maximum possible value of η_cy is much less than 100%. Since the relationship among W and QH and that among Q_H and Δ H_c . Since η_cy is forever very much less than 100%, therefore also is η₀. It is significant to realize that limited attention to the improvement of ηcy by the power station designer should be avoided, as steps taken to enhance η_cy might really cause η_H to fall if remedial measures are not accepted. An improvement in ηcy would not then be reflected in a analogous improvement in η₀, and it is this figure that is ultimately significant.  

The above performance parameters just give a means of stating the ‘measured’ performance of the plant. Additionally, or even more considerably, the engineer will be curious to know how much improved the performance could have been. Therefore, in addition to a performance parameter, that is solely a measure of real performance, a ‘performance criterion’ against that the measured values of η_cy, η_H and η₀ can be compared, becomes essential. In order to give a criterion against that to judge the measured cycle efficiency, η_cy, it is essential to perform a detailed study of ideal cycles operating under comparable circumstances. For the steam power plant, the suitable performance criterion will be the cycle effectiveness of an ‘ideal’ steam plant supplied with steam at the similar temperature and pressure, and exhausting to the similar condenser pressure, the Second Law proclaims that all irreversible procedures outcome in lost opportunities for generating work, and hence all procedures in the ideal plant should be reversible. The resultant reversible cycle is the Rankine cycle.

As the Rankine cycle efficiency is the rational principle of excellence against which to compare the measured cycle effectiveness of a real steam plant, the ratio of the latter to the previous will give a measure of the excellence of performance of the real plant. This ratio is termed as the ‘efficiency ratio’. It is a more informative measure of the plant performance than the cycle efficiency, as a statement of the latter expresses no information as to how much better, hypothetically, the performance could have been. As the total work output is nearly equivalent to the turbine work output, since the feed pump work input is little, the efficiency ratio is around equivalent to the isentropic efficiency of the turbine. For the simple gas turbine cycle, the ideal reversible cycle operating beneath comparable circumstances is the Joule or Brayton cycle.