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Publication Title | PERFORMANCE EVALUATION OF A COMBINED ORGANIC RANKINE CYCLE AND AN ABSORPTION REFRIGERATION SYSTEM

Organic Rankine Cycle

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PERFORMANCE EVALUATION OF A COMBINED ORGANIC RANKINE CYCLE AND AN ABSORPTION REFRIGERATION SYSTEM

Andreea MARIN1 , Adrian UNTEA1 , Lavinia GROSU2, Alexandru DOBROVICESCU1, Diogo QUEIROS-CONDE2

1UNIVERSITY „POLITEHNICA“ OF BUCHAREST, 2UNIVERSITY OF PARIS WEST, LEME, FRANCE

Abstract. In this paper, exergy analysis is used to assess the exergetic performance of a solar powered Organic Rankine Cycle (ORC) and a LiBr Absorption Refrigeration System. The exergy efficiency and the exergy destruction rates are calculated for the whole combined system. The goal of this research is to highlight that this kind of systems is able to satisfy air-cooling and electrical power need for an existing building. Thus, cogeneration system using solar energy for combined cooling and electrical power in buildings is studied. The simulations for the ORC are performed for different working fluids and show that using R245fa as working fluid in the low-temperature solar Rankine cycle is suitable. For the LiBr absorption refrigeration system the coefficient of performance is calculated, as well as the influence of the inlet temperature in the Generator. Keywords: Organic Rankine Cycle, Absoption Refrigeration System, solar system, combined cycles, exergy analysis.

1. INTRODUCTION

In the current economical and energetic context, implementing technologies using renewable energy as heating source is offering a double advantage: the reduction of pollution and of the fuel cost. One of the main concerns of the modern human is to provide comfort in buildings. The main utilities that make a building „alive” are: electricity, domestic hot water, heating/cooling according to external ambiance. In this study the attention is focused on providing electricity and cooling during summer for a public establishment.

The usual way of providing this two utilities to a building are to connect to local electrical network and to install an electric driven chiller. In this paper the performance of a combined cycles (production of electricity using an ORC and cooling load assured by a single-effect lithium bromide/water absorption system) is studied. The two systems work together and the energy that drives the combined system is represented by the solar energy transferred to steam of 140oC. Using heat as an energy source offers the possibility to consider the sun as the fuel of the combined power - cooling system [1].

The whole system and each sub-system is analyzed according to first and second law of the thermodynamics. A computational model is deve- loped in Thermoptim software in order to study the

performance of the systems with different operating temperatures.

The ORC system is very similar to a steam Rankine cycle which uses an organic fluid instead of water in order to for generate electricity [2]. Organic Rankine Cycle is a promising process for conversion of low and medium temperature heat to electricity, which makes it suitable for solar applications. Organic fluids can operate at a much lower evaporation temperature and pressure than a conventional turbine using water as working fluid and still perform at a high efficiency [3-6]. The usual working fluids for an ORC are: HFC’s Freon, ammonia, butane, iso-pentane, toluene and they have in general a high molecular mass. Several fluids were compared in previous work and it was found that R245fa presents interesting performances for an ORC solar system, although for a high pressure [4].

The solar fluid at the exit of the heat exchanger of the ORC is conducted into a second heat exchanger which is the generator of the absorption cooling system (ACS). In this way potential of the lower temperature heat is used in the ACS in order to produce cooling effect in the evaporator (Figure 1).

The reason for choosing LiBr/H2O as working fluid instead of ammonia/water pair is:

– it does not requires a rectification column and dephlegmator;

TERMOTEHNICA 1/2013

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