1Tchanche Fankam Bertrand*, 1George Papadakis, 1Gregory Lambrinos and 2Antonios

Frangoudakis

Department of Natural Resources and Agricultural Engineering,

1 Laboratory of Agricultural Engineering,

2 Laboratory of Agricultural Constructions,

Agricultural University of Athens, 75 Iera Odos Street 11855 Athens, Greece Tel.: +30 (210) 529 4046; Fax: +30 (210) 529 4036 * Corresponding author, e-mail address: tfb@aua. gr

Abstract

This paper deals with the study of the effects of regeneration on low-temperature solar organic Rankine cycles utilizing R134a as working fluid. The configurations investigated are: basic ORC (organic Rankine cycle), regenerative ORC with internal heat exchanger, regenerative ORC with open feedwater heater and regenerative ORC with closed feedwater heater. The effects of the regeneration is assessed using a method of thermodynamic exergetic analysis called exergy — topological method based on the combination of exergy flow graphs, exergy loss graphs and thermoeconomic graphs. For a designed output power of 2 kW, the incorporation of an open feedwater is the most efficient way to increase the system performance. The thermal and exergetic efficiencies obtained were 3.95% and 9.16% respectively; this represents an increase of 6.83% and 6.93% in thermal and exergetic efficiencies respectively compared to the simple Rankine engine. The configuration with closed feedwater heater produces similar results but is less efficient. The incorporation of an internal heat exchanger produces negative effects. Keywords: organic Rankine cycles, exergy-topological method, regeneration, solar energy

1. Introduction

The lack of electricity or means of power generation seriously limits the development of many regions around the world. The increasing fossil fuels prices and the pollution derived from their combustion in power plants are two important reasons for which the attention should be shifted towards renewable energy sources. As can be seen today, the price of oil will still increase and the access to grid electricity will become more and more difficult for populations leaving in remote areas. In this context, solar energy can play a major role in sunny areas. Recent works [1-3] on low-temperature organic Rankine cycles show that this technology could be a solution for providing fresh water and electricity, and perhaps thermal comfort. Although the system’s components are well known technologies, the integration to a fully and efficiently working system is a challenge. A serious drawback of the low-temperature organic Rankine cycle is its low efficiency. An examination of the basic Rankine cycle reveals that the heat is transferred to the working fluid at relatively low temperature; this reduces the system thermal efficiency. A solution to remedy this shortcoming is to use a regenerator or a recuperator, which gives birth to several organic Rankine cycle configurations.

In this paper, the effects of the regeneration on the modified cycle with internal heat exchanger and regenerative cycles with open and closed feedwater heaters are analyzed using a new exergy approach called exergy-topological method [4-7].