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Publication Title | WASTE HEAT ENERGY SUPERCRITICAL CARBON DIOXIDE RECOVERY CYCLE ANALYSIS AND DESIGN

Organic Rankine Cycle

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WASTE HEAT ENERGY SUPERCRITICAL CARBON DIOXIDE RECOVERY CYCLE ANALYSIS AND DESIGN

Trent Wells

Daniel Forgette

Ryan Okerson

Matthew DeVost

Kevin R. Anderson California State Polytechnic University Mechanical Engineering Department 3801 West Temple Ave

Pomona, CA 91768 tswells@csupomona.edu dtforgette@csupomona.edu raokerson@csupomona.edu medevost@csupomona.edu kranderson1@csupomona.edu

ABSTRACT

The US Department of Energy has estimated that 280,000 MW of recyclable waste heat is expelled annually by U.S. industries. Further estimates suggest that harvesting it could result in a savings of $70 billion to $150 billion per year (1). Thus, any efficiency increase will result in savings to energy producers. Supercritical carbon dioxide (SCO2) provides unique advantages over alternative waste heat recovery systems however, it also produces unique design challenges. We propose a novel energy recovery device based on a SCO2 regenerative Rankine cycle for small-scale (1kW to 5kW) heat recovery. This study presents a thermodynamic SCO2 cycle analysis for waste heat recovery from low temperature (200°C - 500°C) sources using small mass flow rates (20 – 60 grams/sec). This paper will present a prototype SCO2 cycle architecture including details of key system components. Preliminary modeling suggests that SCO2 systems are viable for low temperature waste heat recovery.

1. INTRODUCTION

SCO2 has been considered a viable alternative working fluid for power cycles since the 1960s because it provides several advantages over steam and helium (2-6). The density of SCO2 allows energy extraction devices to have a much smaller footprint than comparable steam and helium based turbo machinery (6). Additionally, the critical point of CO2 is very low (31.1 °C and 7.4 MPa) compared to other fluids,

Steve Cunningham Martin Stuart

Butte Industries

2549 N. Palm Drive Signal Hill, CA 90755 stevec@butteindustries.com martins@butteindustries.com

allowing for heat transfer from low temperature (200 °C – 500 °C) sources to the supercritical state (7). Operating in the single supercritical phase throughout the proposed cycle reduces the need for two-phase hardware (6). However, due to the operating pressure and highly variable, non-linear fluid properties, suitable hardware for industrial use did not exist until recently (4, 5). Advancements in compact heat exchangers and turbo machinery coupled with the drive for business to become “green” has revived interest in SCO2 power cycles leading to new solutions for energy addition and extraction (4, 5). The objective of this paper is to investigate the waste heat regenerative SCO2 Rankine cycle performance and feasibility with low flow rate through mathematical modeling.

2. REGENERATIVE RANKINE CYCLE LAYOUT

The waste heat regenerative Rankine cycle is made up of six components as shown on the next page in Fig. 1. The SCO2 cycle starts at a low side pressure above 7.5 MPa and a low side temperature of 35 °C, slightly above the critical point. After compression the SCO2 is brought to the high-side pressure of 20 MPa and a temperature of 36 °C, approximately 1 °C higher than the pre-compressed state. An internal heat exchanger then heats the pressurized SCO2 by exchange with low-pressure, post-expansion SCO2.

After exiting the internal heat exchanger the SCO2 is heated in a second heat exchanger where addition is done via waste heat, raising the temperature to its ultimate value of approximately 200 °C. The heated and pressurized SCO2 is

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