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Publication Title | Comparison of the Organic Flash Cycle (OFC) to other advanced vapor cycles for intermediate and high temperature waste heat reclamation and solar thermal energy

Organic Rankine Cycle

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Energy 42 (2012) 213e223

Contents lists available at SciVerse ScienceDirect Energy

journal homepage: www.elsevier.com/locate/energy

Comparison of the Organic Flash Cycle (OFC) to other advanced vapor cycles

for intermediate and high temperature waste heat reclamation and solar thermal energy

Tony Ho*, Samuel S. Mao, Ralph Greif

Department of Mechanical Engineering, University of California-Berkeley, Etcheverry Hall, Berkeley, CA 94720, USA

articleinfo

Article history:

Received 27 August 2011 Received in revised form

22 February 2012

Accepted 24 March 2012 Available online 22 April 2012

Keywords:

Organic Rankine Cycle (ORC) Solar thermal

Waste heat

Vapor cycle

Exergy analysis

1. Introduction

As energy demands continue to rise, researchers continue to search for alternative energy sources to generate electricity, as well as improve existing methods to maximize efficiency. In order to meet global energy demands, a greater reliance on electricity generated from renewable energy sources such as solar thermal and geothermal energy will become necessary. For solar thermal, energy is obtained from a heated fluid circulating in a solar field, whereas for geothermal, energy is obtained from hot brine that has been extracted from a geothermal well. In addition to renewable sources, thermal energy that in the past would have been released and lost to the ambient such as hot exhaust exiting a gas turbine and industrial waste heat, are now being reexamined as potential power sources. These aforementioned energy sources are often termed as finite thermal energy reservoirs because the reservoir temperature and its thermal energy decreases dramatically as heat

* Corresponding author.

E-mail address: tony.ho@berkeley.edu (T. Ho).

0360-5442/$ e see front matter ! 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.energy.2012.03.067

abstract

The Organic Flash Cycle (OFC) is proposed as a vapor power cycle that could potentially improve the efficiency with which high and intermediate temperature finite thermal sources are utilized. The OFC’s aim is to improve temperature matching and reduce exergy losses during heat addition. A theoretical investigation is conducted using high accuracy equations of state such as BACKONE, SpaneWagner, and REFPROP in a detailed thermodynamic and exergetic analysis. The study examines 10 different aromatic hydrocarbons and siloxanes as potential working fluids. Comparisons are drawn between the OFC and an optimized basic Organic Rankine Cycle (ORC), a zeotropic Rankine cycle using a binary ammonia-water mixture, and a transcritical CO2 cycle. Results showed aromatic hydrocarbons to be the better suited working fluid for the ORC and OFC due to higher power output and less complex turbine designs. Results also showed that the single flash OFC achieves comparable utilization efficiencies to the optimized basic ORC. Although the OFC improved heat addition exergetic efficiency, this advantage was negated by irreversibilities introduced during flash evaporation. A number of potentially significant improvements to the OFC are possible though which includes using a secondary flash stage or replacing the throttling valve with a two-phase expander.

! 2012 Elsevier Ltd. All rights reserved.

is transferred from the source to the power cycle. In order to make the most of these finite sources, the design of an efficient and effective power cycle is crucial.

One of the major sources of irreversibilities for vapor power cycles stems from the heat addition process. The thermal source and working fluid must be separated by some temperature differ- ence in order for heat transfer to occur; however, heat transfer across a finite temperature difference inherently causes irrevers- ibilities. Therefore, it is important to maintain good temperature matching between the heat exchanger streams to minimize these types of irreversibilities [1,2]. A large degree of temperature mis- matching often does occur when the thermal source is single-phase and possesses a near linear temperature profile along the heat exchanger. For a vapor power cycle using a pure working fluid though, the working fluid is first heated as a liquid, undergoes liquidevapor phase change, and if necessary, is further superheated as a vapor thereafter. Its temperature profile will first be near linear, then constant during phase change, and then near linear again, as shown in Fig. 1a. Temperature mismatching causes a pinch point to form, destroys potential work or exergy, and reduces the effec- tiveness of the heat exchangers [2]. To minimize temperature

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