Ciclo Rankine

Renewable Energy 35 (2010) 989–996

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Renewable Energy
journal homepage: www.elsevier.com/locate/renene

Parametric theoretical study of a two-stage solar organic Rankine cycle for RO desalination
G. Kosmadakis*, D. Manolakos, G. Papadakis
Department of Natural Resources and Agricultural Engineering, Agricultural University of Athens, 75 Iera OdosStreet, 11855 Athens, Greece

a r t i c l e i n f o
Article history: Received 19 January 2009 Accepted 25 October 2009 Available online 12 November 2009 Keywords: Organic Rankine cycle Solar energy Desalination Specific fresh water cost

a b s t r a c t
The present work concerns the parametric study of an autonomous, two-stage solar organic Rankine cycle for RO desalination. The main goal ofthe current simulation is to estimate the efficiency, as well as to calculate the annual mechanical energy available for desalination in the considered cases, in order to evaluate the influence of various parameters on the performance of the system. The parametric study concerns the variation of different parameters, without changing actually the baseline case. The effect of the collectors’ slopeand the total number of evacuated tube collectors used, have been extensively examined. The total cost is also taken into consideration and is calculated for the different cases examined, along with the specific fresh water cost (V/m3). Ó 2009 Elsevier Ltd. All rights reserved.

1. Introduction In a solar Organic Rankine Cycle (ORC) for Reverse Osmosis (RO) desalination operating in thetemperature order of 130  C, the addition of a high temperature stage increases significantly both the overall efficiency and the desalinated water produced [1,2]. This can be explained by the fact that the maximum temperature of the thermodynamic cycle increases and therefore the efficiency increases accordingly, following the Carnot efficiency principle. The single-stage Solar Organic Rankine Cycle (SORC)has been experimentally evaluated [3–6] along with the detailed simulation of its performance [4,7]. With minor rearrangements and addition of some components to build the high temperature stage (e.g. heat exchangers, organic fluid feed pump etc.) the autonomous prototype single-stage, low-temperature SORC for RO desalination [3] can be transformed to a two-stage system with a significantly increasedperformance [2]. The research so far has been focused on both single and two-stage systems. On one hand attention has been given to the single-stage system in terms of optimizing its behaviour through experimental demonstration and on the other simulation codes have been developed and improved for the two-stage system, so that finally a transfer of know-how from one system to the other can beachieved. For the latter purpose it is crucial to distinguish, which parameters and in what extent influence the performance of the system. This can help to identify the different trends prevailing

during the design process and finally optimizing the system in greater extent. The parameters under investigation in the current study are the collectors’ slope and the total number of evacuated tubecollectors installed. The final scope of the ongoing research work is the improvement of the developed system and in the future this to be able to constitute an alternative desalination method competitive to the PhotoVoltaic cells for RO desalination (PV-RO) in the basis of techno-economic feasibility.

2. Materials and methods 2.1. Description of the system The simplified block diagram of Fig. 1,illustrates the basic concept of the system. The process can be briefly described as follows: The heat produced by the solar collectors is transformed to mechanical energy through the High Temperature Solar Organic Rankine Cycle (HTSORC), so called the ‘‘upper stage’’, operating at a temperature range from 130 to 140  C. The refrigerant R245fa has been selected for this cycle due to its appropriate...