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Publication Title | A Numerical Procedure for the Preliminary Design of a ORC Power Plants with Positive Displacement Expanders

Organic Rankine Cycle

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WSEAS TRANSACTIONS on ENVIRONMENT and DEVELOPMENT T. Donateo, A. Fazio

A Numerical Procedure for the Preliminary Design of a ORC Power Plants with Positive Displacement Expanders

T. DONATEO, A. FAZIO Department of Engineering for Innovation University of Salento

Via per Arnesano, 73100

ITALY teresa.donateo@unisalento.it

Abstract: - Organic Rankine Cycle power plants use organic, high molecular mass fluids to allow heat recovery from lower temperature sources such as industrial waste heat, geothermal heat, solar etc. The expansion process is usually carried out by axial or radial turbines. However, when the available thermal power is less than 1000 kW, the usage of positive-displacement expander becomes more suitable. In the present investigation, only Scroll expanders have been taken into account. A model has been developed to evaluate the efficiency and the performance of both the overall ORC power plant and the positive displacement expander. A multi-objective genetic algorithm is used for the optimization of the power plant to select the main design parameter of the plant like evaporator pressure, overheating, thermal recovering, mass flow rate etc. The power plant has been optimized based on the local radiating solar energy distribution obtained by statistical data on the local site (Lecce - Italy) during the year 2007. Several alternative refrigerants, such as R123, R245fa and R134a have been taken into account.

Key-Words: - ORC, positive displacement expander, multi-objective optimization , solar energy.

1 Introduction

Power plants using a high molecular fluid to perform a Rankine cycle are named ORC (Organic Rankine Cycle) power plants ([1]-[4]). The most common ORC configuration consists essentially of a heat recovery vapor generator, a power turbine, a condenser and a circulating pump. The corresponding cycle is show in Fig. 1. Note that at the exit of the expander, the organic fluid can be in the state of over-heated vapor. Thus, a heat exchanger can be added to the standard configuration of an elementary Rankine power plant to transfer heat from the hotter working fluid to the colder liquid that exit the pump (regeneration). Working fluids are chosen so that they can be subjected to the evaporation phase and (if any) over- heating at a low temperature, thus enabling the recovery of heat from sources at low temperature, and with limited and discontinuous available power (solar, biomass, geothermal, waste heat in general). In addition to this requirement, the working fluids must present an acceptable pressure in the processes of heat exchange, low freezing point, high stability to temperature changes, high heat of vaporization, high density and low environmental impact. Moreover, they must be available at low cost, non-

flammable, non-corrosive and non-toxic in order to ensure safety. ASHRAE [5] offers a list of potential working fluid for ORC plant. The thermodynamic properties of such fluids can be evaluated by using the REFPROP (Reference Fluid Thermodynamic and Transport Properties Database) a software developed by the National Institute of Standards and Technology [6].

The use of energy sources with different characteristics in terms of available power and maximum temperature reached in the cycle requires, in fact, to tie the design parameters of the system (vaporization pressure, amount of overheating, condensing pressure, etc.) to the thermodynamic characteristics of the working fluid in order to achieve the best efficiency.

Several studies can be found in literature about the choice of the working fluid once the design parameters of the power plant have been defined. Among them, Wang and Zhao [7] analyze the use of binary zeotropic mixtures of R152a and R245fa as working fluids of the solar ORC in a direct vapor generation configuration.

E-ISSN: 2224-3496 186

Volume 10, 2014

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