Criteria for working fluids selection in low-temperature solar organic Rankine cycles

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, tfb@aua. gr

Abstract

The working fluid is one of the most important components of a Rankine cycle power system. It influences the performance and thereby the economics of the system. Therefore, special care should be taken when selecting the working fluid. In this paper a list of criteria that should fulfill a fluid to be considered as good for a Rankine cycle is given. These criteria include good heat transfer properties, non-toxicity, non-flammability, high efficiency, availability and low cost. Finally, a general methodology for working fluid selection made of three steps: (1) data collection, (2) data analysis and (3) decision is proposed.

Keywords: criteria, working fluids, methodology

1. Introduction

Steam has shown its ability to serve as working fluid in high temperature power plants. In low temperature (< 200 °С) or low-output power plants (< 10 kW), the use of water is not economically feasible [1-2]. Therefore, other fluids should be sought. Interest was found for refrigerants and some other fluids. These fluids are: Halons, hydrocarbons (HCs), hydrofluorocarbons (HFCs), chlorofluorocarbons (CFCs), hydrochlorofluorocarbons (HCFCs) and natural fluids like carbon dioxide, ammonia, air, etc. Unfortunately, some of the above categories of fluids were phased-out or are to be banned soon by the international regulations [3]; pushing manufacturers of substances and researchers to look for new environmentally friendly fluids. So, new categories of fluids like zeotropes, azeotropes and other multicomponent fluids were born [4]. Of various working fluids existing that could be used in Rankine power cycles, the problem that comes out when designing a cycle is the choice or the selection of the best suited fluid for a particular application. The choice of the working fluid is very important as it determines the efficiency and the economics of the power plant. If in literature many criteria are proposed for the working medium selection, very few works point how the best fluid can be picked up from a group of several potential candidates. In the present paper, we list these criteria and propose a general methodology for working fluids selection in organic Rankine cycles.

2. The criteria

Before going through a selection process of an item in a precise group of similar items, the criteria should be well established. This applies also to the selection of working fluids for Rankine cycles. For this particular case of organic fluids, many works have been reported in literature. Badr et al. [1], Stine and Geyer [2], Lee et al. [5], and Maizza and Maizza [6] are some authors who
investigated the criteria that should fulfill an ideal working fluid. In this section, these criteria are reported.