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Publication Title | Efficient Organic Rankine Cycle (ORC) Power Generation with low grade energy as heat source: preliminary design of Effi low res ORC unit

Organic Rankine Cycle

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Recent Advances in Fluid Mechanics and Thermal Engineering

Efficient Organic Rankine Cycle (ORC) Power Generation with low grade energy as heat source: preliminary design of Effi low res ORC unit

M.GR. VRACHOPOULOS, N.S. TACHOS, M.K. KOUKOU, G.K. KARANASIOU Energy and Environmental Research Laboratory, Mechanical Engineering Department Central Greece University of Applied Sciences (TEI STEREAS ELLADAS)

344 00 Psachna, Evia Greece

e-mail: mvrachop@gmail.com https://renewablelab.wordpress.com/english

and

C. KARYTSAS

Center for Renewable Energy Sources and Saving (CRES)

19ο Km Marathonos Ave, 19009, Pikermi Attiki Greece

e-mail: kkari@cres.gr https://www.cres.gr

Abstract: - The Effi low res project intends to develop an ORC power generation unit working at low temperatures with improved overall efficiency and to improve cost-effectiveness, competitiveness and penetration of the ORC systems in the Greek market. The major goal is the performance optimization of an ORC unit by exploiting low temperature sources so as to make it cost effective. For that purpose a combined cycle has been designed which exploits the properties of the selected working fluid (refrigerant) that vaporize at low temperatures. Computational simulation techniques were used to investigate the performance of refrigerants selected for the specific application so as to substantiate the choice of the most suitable refrigerant for process optimization. Results show that R134a as a working fluid is an optimal solution to balancing three factors studied: required amount of heat, mass flow rate and cost.

Key-Words: ORC, geothermal energy, solar energy, refrigerant

1 Introduction

Renewable energy sources, such as solar thermal and geothermal energy are potentially energy sources that are capable to meet electricity demand. However, the moderate temperature heat from these sources cannot be converted efficiently to electrical power when using conventional power generation methods. For that reason and for the conversion of low-grade heat sources into electricity various cycles have been proposed [1-3] as the organic Rankine Cycle (ORC), supercritical Rankine cycle, Kalina cycle, etc. The literature shows extensive analyses and comparisons among different thermodynamic cycles and working fluids [1-12]. However, most of the comparisons were conducted under certain predefined temperature conditions and used only a few working fluids [4–12]. As ORC is much less complex and need less maintenance it is a promising process for conversion of low and medium temperature heat to electricity.

In Greece, there are areas with water sources with temperature higher than 80oC [13]. Also, classic solar thermal systems with flat solar collectors may produce temperature around or more than 80oC. Both cases have been exploited for various uses

mainly for space heating, greenhouse heating etc or other thermal uses [14]. The exploitation period of such systems is limited in periods with thermal needs, namely periods where these systems (especially the solar thermal systems) show decreased performance and present increased losses. However, the period that the above heats are available is during the whole year while they are maximized during summer where thermal needs are limited. For the improvement of the performance and exploitability of those applications during the whole year various technologies have been developed, the main one among them is cooling production technology or/and power generation with a relatively low efficiency especially at temperatures around or up to 80oC.

The Effi Low Res project intends to develop an ORC power generation unit working at low temperatures with improved overall efficiency by exploiting low temperature sources so as to make it cost effective. The ORC applies the principle of Rankine cycle steam but uses an organic fluid with a low boiling point to recover the heat from low temperature heat sources. The choice of refrigerant for a given application is a key to its successful

ISBN: 978-1-61804-311-5

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