Subcritical Thermodynamic Cycles with Organic Medium and Isothermal Expansion

Krzysztof Kosowski,Marian Piwowarski

doi:10.3390/en13174340

Abstract

The efficiencies of the Organic Rankine Cycle (ORC) are not very high and only very seldom do they exceed 20%. The increase and optimization of initial parameters and certain modifications of the thermodynamic cycle make it possible to overcome these drawbacks. A new modified cycle has been described and analyzed in detail in the paper. Similarly to the Ericsson cycle for gas turbines, isothermal expansion in the turbine is suggested for the power plant with organic media. The new cycle and the typical ORC power plants have the same block diagram. The only difference is that expansion in the proposed cycle occurs not adiabatically but as an isothermal process. The thermodynamic calculations have been carried out for 11 various fluids and 4 different cycles. The obtained results have clearly shown that cycles with isothermal expansion (isothermal turbines) are characterized by remarkably higher efficiency than typical power plants with adiabatic turbines. The increase in efficiency varies from 6 to 12 percent points for cycles with saturated live vapor and from 4 to 7 percent points for cycles with superheated live vapor. The performed analyses have shown that it is possible to achieve a very high efficiency (over 45%) of organic cycle, which is a very competitive value. In such cases the proposed power plants can achieve an efficiency which is higher than that of modern steam turbine plants with supercritical parameters.

Highlights

The European power system will face a number of changes due to climate restrictions
For example, we consider the most advanced ultra-supercritical steam power plants in design or operation, we usually find two interstage superheaters, a number of feedwater heaters, live steam temperature as high as 760 ◦ C, and live steam pressure of about 35 MPa, which can be treated as limit values for those parameters today
The authors have proposed a new cycle for vapor power plants which increases the efficiency in the same temperature range by several percent points

Summary

Introduction

The European power system will face a number of changes due to climate restrictions. This is imposed by emission standards mainly for carbon dioxide [1,2]. The efficiency of power plants can be higher if we increase upper temperature and modify the cycles [30], making them more complex and applying additional elements [31]. We cannot apply very low values of the lower cycle temperature due to the surrounding conditions, nor can we increase the upper temperature because of material limitations. It appears that the efficiencies of particular cycle elements are extremely high and it is very difficult to expect remarkable progress in that field. The authors have proposed a new cycle for vapor power plants which increases the efficiency in the same temperature range by several percent points (from 4 to 12 percent points in the considered variants). These types of power plants have not been described in the literature so far

Modelling

Results and Discussion

Conclusions