Comparison of the Organic Flash Cycle (OFC) to Other Advanced Vapor Cycles for Intermediate and High Temperature Waste Heat Reclamation and Solar Thermal Energy
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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 article info abstract Article history: The Organic Flash Cycle (OFC) is proposed as a vapor power cycle that could potentially improve the Received 27 August 2011 efficiency with which high and intermediate temperature finite thermal sources are utilized. The OFC’s Received in revised form aim is to improve temperature matching and reduce exergy losses during heat addition. A theoretical 22 February 2012 investigation is conducted using high accuracy equations of state such as BACKONE, SpaneWagner, and Accepted 24 March 2012 REFPROP in a detailed thermodynamic and exergetic analysis. The study examines 10 different aromatic Available online 22 April 2012 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 Keywords: Organic Rankine Cycle (ORC) mixture, and a transcritical CO2 cycle. Results showed aromatic hydrocarbons to be the better suited fl Solar thermal working uid for the ORC and OFC due to higher power output and less complex turbine designs. Results Waste heat also showed that the single flash OFC achieves comparable utilization efficiencies to the optimized basic Vapor cycle ORC. Although the OFC improved heat addition exergetic efficiency, this advantage was negated by Exergy analysis 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. Ó 2012 Elsevier Ltd. All rights reserved. 1. Introduction 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 As energy demands continue to rise, researchers continue to effective power cycle is crucial. search for alternative energy sources to generate electricity, as well One of the major sources of irreversibilities for vapor power as improve existing methods to maximize efficiency. In order to cycles stems from the heat addition process. The thermal source meet global energy demands, a greater reliance on electricity and working fluid must be separated by some temperature differ- generated from renewable energy sources such as solar thermal ence in order for heat transfer to occur; however, heat transfer and geothermal energy will become necessary. For solar thermal, across a finite temperature difference inherently causes irrevers- energy is obtained from a heated fluid circulating in a solar field, ibilities. Therefore, it is important to maintain good temperature whereas for geothermal, energy is obtained from hot brine that has matching between the heat exchanger streams to minimize these been extracted from a geothermal well. In addition to renewable types of irreversibilities [1,2]. A large degree of temperature mis- sources, thermal energy that in the past would have been released matching often does occur when the thermal source is single-phase and lost to the ambient such as hot exhaust exiting a gas turbine and possesses a near linear temperature profile along the heat and industrial waste heat, are now being reexamined as potential exchanger. For a vapor power cycle using a pure working fluid power sources. These aforementioned energy sources are often though, the working fluid is first heated as a liquid, undergoes termed as finite thermal energy reservoirs because the reservoir liquidevapor phase change, and if necessary, is further superheated temperature and its thermal energy decreases dramatically as heat 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 * Corresponding author. form, destroys potential work or exergy, and reduces the effec- E-mail address: [email protected] (T. Ho). tiveness of the heat exchangers [2]. To minimize temperature 0360-5442/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.energy.2012.03.067 214 T. Ho et al. / Energy 42 (2012) 213e223 Fig. 1. Variation in stream temperatures during heat addition process for single component, zeotropic, transcritical and flash cycles [13]. mismatching, researchers have proposed a number of possible avoiding isothermal phase change as shown in Fig. 1d. Once the solutions that include utilizing unconventional working fluids and working fluid is heated to a saturated liquid state, work is then innovative cycle configurations and designs. extracted using a two-phase expander. Similar to the transcritical The use of zeotropic mixtures as working fluids in vapor cycles cycle, the design of a reliable and efficient two-phase expander is has been proposed by a number of researchers as a possible method still ongoing, though significant strides have been made recently to improve temperature matching [3e9]. Zeotropic mixtures for screw-type and scroll-type expanders and reciprocating engines exhibit a unique characteristic known as a “temperature glide,” [20]. To avoid the requirement of a two-phase expander though, the which results in a variation in temperature during isobaric phase single-phase liquid could be throttled to a two-phase mixture after change. Temperature gliding can produce a better temperature heat addition. The liquid and vapor components of the mixture can match to the finite thermal reservoir by avoiding isothermal phase then be separated and work can be extracted from the saturated change. As shown in Fig. 1b, the zeotropic working fluid’s temper- vapor using a conventional and readily available Organic Rankine ature glide follows the temperature profile of the thermal reservoir Cycle (ORC) turbine. By using this type of configuration, a two- more closely. This reduces the irreversibilities in the heat addition phase turbine is no longer necessary, while the advantageous process and can potentially improve the net power output and temperature matching between streams shown in Fig. 1d is still utilization efficiency [10]. achieved. This resulting cycle is somewhat similar to the flash Another method that has been suggested is to avoid the phase steam cycle that utilizes high temperature and pressure geofluid change region completely by transferring heat at pressures above that has been extracted in the liquid state from a geothermal well. the critical pressure [11e16]. This would essentially avoid the Once extracted, the liquid geofluid is then throttled to a lower temperature mismatching encountered in the constant tempera- pressure or flashed to produce a liquidevapor mixture [10]. One ture phase change process. An illustration of the improved major disadvantage of the steam flash cycle though, is that the temperature matching is shown in Fig. 1c. A vapor cycle that steam after expansion contains a significant amount of moisture operates partly under supercritical conditions is known as a tran- because water is a “wet” fluid. Wet fluids exhibit a saturated vapor scritical cycle; working fluids that are often suggested for such curve on a temperature-entropy (T-S) diagram that is negatively cycles include carbon dioxide [13e15] and helium. Although sloped [4]. Isentropic expansion of a “wet” fluid from its saturated theoretically the transcritical cycle has merit, the design of a suit- vapor state will always produce a two-phase mixture with liquid able supercritical turbine for fluids other than water is still in the droplets forming. Although in reality large steam turbines often developmental phase [15].Afluid under supercritical conditions have isentropic efficiencies of 80%e90%, saturated steam cycles in can possess both liquid and vapor behaviors; thus tailoring both geothermal and nuclear power industries still require special a turbine specifically to one specific condition; i.e. liquid or vapor wet steam turbines. Wet steam turbines are constructed with behavior, becomes problematic [17] and the development of an expensive reinforcing materials to protect the blades from erosion appropriate supercritical turbine design remains ongoing. and damage caused by the liquid droplets [10]. The trilateral flash cycle [17e20], another proposed method to Expansion of a saturated vapor into the vapor dome can be improve temperature matching, faces similar challenges to that of avoided if a fluid with an infinite or positively sloped saturated the transcritical cycle. In the trilateral flash cycle, heat addition vapor curve on a T-S diagram was used instead of steam [3]. These occurs while the working fluid is a single-phase liquid, thereby fluids are known as “isentropic” or “dry” fluids, respectively. T. Ho et al. / Energy 42 (2012) 213e223 215 Turbine cost would be significantly reduced for the Organic Flash 3. Methods of analysis Cycle (OFC) since blade reinforcing materials are no longer neces- sary. Although uncontrolled expansion of the working fluid 3.1. Equations of state explicit in Helmholtz energy generates considerable irreversibilities during the throttling or flashing process [21], the reduction in exergy