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Publication Title | Off-design operation of ORC and CO2 power production cycles for low temperature surplus heat recovery

Organic Rankine Cycle

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International Journal of Low-Carbon Technologies Advance Access published March 1, 2011

Off-design operation of ORC and CO2 power

production cycles for low temperature

surplus heat recovery

..............................................................................................................................................................

Harald Taxt Walnum*, Yves Ladam, Petter Neksa ̊ and Trond Andresen

SINTEF Energy Research, 7465 Trondheim, Norway

.............................................................................................................................................

Abstract

In process industry, large amounts of surplus heat are available. Electricity production is an interesting method to recover this energy. This paper focuses on the off-design operation of the Rankine cycles and compares the behaviour of transcritical CO2 cycles and an organic Rankine cycle (ORC) with R-123 as the working fluid. The simulations show that the ORC is more sensitive than the CO2 cycle to reduction in available heat, and will with only small changes get droplets in the inlet of the expander. With small increments in the available heat source, the CO2 cycle also seem to have a marginally better response without control of the process.

Keywords: Rankine cycle; power production; surplus heat

no Received 15 October 2010; revised 25 January 2011; accepted 26 October 2011

................................................................................................................................................................................

*Corresponding author: harald.taxt.walnum@sintef.

1 INTRODUCTION

In process industry, large amounts of energy are rejected to the ambient. Recovery of this surplus energy is a wide topic. Among the strategies for energy recovery, production of electri- city is very interesting, due to the versatility of this form of energy.

Power production from surplus heat sources is largely domi- nated by the steam process. It can be found in nuclear, oil- or gas-fired power plants, biomass-fired plants and even solar power plants. However, the steam process suffers from high capital cost and poor efficiency for medium-to-low tempera- ture heat sources (the borderline being 4008C) [1].

The organic Rankine cycle (ORC) is now a well-established technology for power production from low-temperature heat sources. It combines improved efficiency, and lower capital and operating costs. The working fluids used are organic compounds in the halocarbon or hydrocarbon families, fluids commonly used in the refrigeration industry.

Common applications for the technology are electricity production from geothermal fields [2–3], biomass plants [4] or as bottoming cycle for gas turbines [5–6]. More scarce applications are to be found in solar application [7–8] or energy recovery from waste heat in industry ([1] and D. Nadav, unpublished work). A commonly accepted limit for a profit- able energy recovery plant is 2008C for a gas heat source and 908C for a liquid heat source (S. Koren, private communication).

Research in ORC technology is very active, focusing both on component development [9] and working fluid selection [10 – 13].

Despite substantial improvements, power production from low-to-medium temperature heat sources is still handicapped by large investment cost and relatively poor efficiency. In addition, working fluids used are either toxic (ammonia), flammable (hydrocarbons) or very potent greenhouse gases, contributing to global warming (HFC refrigerants).

The transcritical Rankine power cycle recently received special attention [14–17] due to its performances for energy recovery from low-temperature sources. The transcritical process differs from the others, in that it absorbs heat at a supercritical pressure. Due to the temperature glide during heating of a single-phase fluid, compared with the constant temperature of an evaporating single-component fluid, it is possible to achieve a much better temperature approach with the heat source in the main heat exchanger. To achieve as low temperature differences as possible in a heat exchanger is important, as the exergy losses are directly coupled with the temperature difference between the fluids.

CO2 is a natural candidate as the working fluid for this technology. It combines high performance, low cost, low tox- icity, is non-flammable and has no environmental impact. A transcritical CO2 power cycle operates at relatively high press- ures, typically 100 bars at heat absorption. This gives a poten- tial for component size reduction and then, investment cost reduction. In addition, heat absorption without phase change

International Journal of Low-Carbon Technologies 2011, 0, 1–7

# The Author 2011. Published by Oxford University Press. All rights reserved. For Permissions, please email: journals.permissions@oup.com doi:10.1093/ijlct/ctr003

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