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Publication Title | ORGANIC RANKINE CYCLE ASSOCIATED WITH AN ABSORPTION CHILLER FOR BIOMASS APPLICATIONS

Organic Rankine Cycle

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Tecnologia/Technology

ORGANIC RANKINE CYCLE ASSOCIATED WITH AN ABSORPTION CHILLER FOR BIOMASS APPLICATIONS

C. A. R. Sotomonte, S. Ribeiro, E. Oliveira, E. E. S. Lora, and O. J. Venturini

Universidade Federal de Itajubá Departamento de Engenharia Mecânica Bairro Pinheirinho CP. 37500-903, Itajubá, Mina Gerais, Brasil c.rodriguez32@unfei.edu.br

NOMENCLATURE

C cooling power, kW

P Pressure, kPa

T average fluid temperature, K

Wn net electric power, kW

m working fluid mass flow, kmol/s mB biomass consumption, kg/s

H enthalpy, kJ/kmol

S entropy, kJ/kmol K

Q heat flow, kJ/s

E exergy. kW

I irreversibility, kW

Greek symbols

η energy efficiency ε exergy efficiency

Subscripts

b boiler

t turbine

p pump

con condenser

is isentropic

evp evaporator

g electric generator F fuel

P product

INTRODUCTION

During the last years, Brazil has been studying the utilization of biomass as fuels for distributed generation systems aiming the diversification the

ABSTRACT

The main goal of this work is to develop a calculation process, based on the second law of thermodynamic, for evaluating the potential of a small Organic Rankine Cycle (ORC) cogeneration (electrical and cooling power) plant using biomass as fuel. Two different configurations and several organic working fluids are presented and assessed. A mathematical model has been developed to find thermodynamically suitable fluids for ORC in biomass cogeneration plants. The main results show that, the family of alkylbenzenes has higher exergetic efficiencies found to be around 16% almost 5 percent higher efficiency than the cycles that use siloxanes as working fluids; in spite of their lower cooling power capacity. The results demonstrate that the cycle efficiency is more dependent on the thermodynamic properties of the working fluids than on the system configuration

Keywords: Cogeneration, Exergetic Analysis, Biomass, Organic Rankine Cycle

country’s energetic matrix and also to supply electricity to small communities situated in isolated regions. Currently, several prime movers such as: Micro-turbines, Internal Combustion Engines and Stirling Engines are being proposed to generate electric energy for small scale applications. However, the utilization of biomass with these technologies requires a previous stage of the biomass conversion into a liquid or gaseous fuel via a chemical, thermochemical or biochemical process. Although feasible, most of these conversion technologies still do not have competitive cost, for this reason, the most used technology for energy conversion from biomass is the direct combustion.

As a result, electricity generation technologies, such as steam turbines, although having low efficiencies, would have a better performance due to their high level of development. For the improvement of these thermal systems, especially when they are related to efficiency gain, the reduction of losses in conversion process is mandatory. In this way, power and cooling cogeneration systems can be interesting; particularly for tropical regions such as the Brazilian Amazonia.

The application of combustion based biomass to electricity conversion technologies for small capacities is complicated by the fact that small conventional axial steam turbines have very low efficiencies, due to the small blades height and the necessity of partial flow implementation in the flow section. There are two solutions for this problem: the utilization of another fluid as the working one, an organic fluid (Organic Rankine Cycle- ORC), the utilization of another prime mover instead of the conventional axial turbines (radial turbines, screw or scroll expanders, or steam piston engines), or both

Engenharia Térmica (Thermal Engineering), Vol. 10 • No. 01 - 02 • June and December 2011 • p. 15-22 15

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