Experimental Investigation of a Small-Scale ORC Power Plant Using a Positive Displacement Expander with and without a Regenerator

Collings Collings,Wang Wang,Mckeown Mckeown,Yu Yu

doi:10.3390/en12081452

Abstract

While large-scale ORC power plants are a relatively mature technology, their application to small-scale power plants (i.e., below 10 kW) still encounters some technical challenges. Positive displacement expanders are mostly used for such small-scale applications. However, their built-in expansion ratios are often smaller than the expansion ratio required for the maximum utilisation of heat sources, leading to under expansion and consequently higher enthalpy at the outlet of the expander, and ultimately resulting in a lower thermal efficiency. In order to overcome this issue, one possible solution is to introduce an internal heat exchanger (i.e., the so-called regenerator) to recover the enthalpy exiting the expander and use it to pre-heat the liquid working fluid before it enters the evaporator. In this paper, a small-scale experimental rig (with 1-kW rated power) was designed and built that is capable of switching between regenerative and non-regenerative modes, using R245fa as the working fluid. It has been tested under various operating conditions, and the results reveal that the regenerative heat exchanger can recover a considerable amount of heat when under expansion occurs, increasing the cycle efficiency.

Highlights

There is an ever-growing demand for power generation worldwide, while there are serious concerns about the impact of the emissions of carbon dioxide and other pollutants associated with the generation of this power
For a waste heat source temperature of 90 ◦ C, the greatest efficiency and lowest levelised cost of electricity were given for cyclopropane, whereas the highest second-law efficiency was given by R143a
The results from the experimental rig show a clear increase in the first-law efficiency by the introduction of a regenerator into the cycle, and this increase in efficiency becomes greater at increased heat source temperatures

Summary

Introduction

There is an ever-growing demand for power generation worldwide, while there are serious concerns about the impact of the emissions of carbon dioxide and other pollutants associated with the generation of this power. Wei et al [28] compared regenerative and non-regenerative ORCs at heat source temperatures below 100 ◦ C for waste heat recovery applications using three dry working fluids, and found that the improvement in thermal performance associated with the regenerative cycle did not offset an increased capital cost under these conditions, resulting in a higher levelised cost of energy and a longer payback period for the regenerative cycles. These reported analyses show that the efficacy of a regenerator in economic terms is highly dependent on the working fluid, heat source, and application of the cycle. Temperature and pressure were measured at various points in the cycle, as well as the power output, enabling the efficiency and pressure losses in the cycle to be examined and compared between the two cycle configurations across a range of heat source temperatures

Experimental Set-Up

Definition of Parameters

The Test Case with Highest Thermal Efficiency

Comparison between Regenerative and Non-Regenerative Cycles

Conclusions