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The Effect of Working Fluid Properties on the Performance of a Miniature View metadata, citation and similar papers at core.ac.uk brought to you by CORE provided by Archive Ouverte en Sciences de l'Information et de la Communication The effect of working fluid properties onthe performance of a miniature free piston expander for waste heat harvesting Sindhu Preetham Burugupally, Leland Weiss, Christopher Depcik To cite this version: Sindhu Preetham Burugupally, Leland Weiss, Christopher Depcik. The effect of working fluid proper- ties on the performance of a miniature free piston expander for waste heat harvesting. Applied Thermal Engineering, Elsevier, 2019, 151, pp.431-438. 10.1016/j.applthermaleng.2019.02.035. hal-02269568 HAL Id: hal-02269568 https://hal.archives-ouvertes.fr/hal-02269568 Submitted on 23 Aug 2019 HAL is a multi-disciplinary open access L’archive ouverte pluridisciplinaire HAL, est archive for the deposit and dissemination of sci- destinée au dépôt et à la diffusion de documents entific research documents, whether they are pub- scientifiques de niveau recherche, publiés ou non, lished or not. The documents may come from émanant des établissements d’enseignement et de teaching and research institutions in France or recherche français ou étrangers, des laboratoires abroad, or from public or private research centers. publics ou privés. The Effect of Working Fluid Properties on the Performance of a Miniature Free Piston Expander for Waste Heat Harvesting Sindhu Preetham Burugupallya,∗, Leland Weissb, Christopher Depcikc aDepartment of Mechanical Engineering, Wichita State University, Wichita, KS 67260 USA bDepartment of Mechanical Engineering, Louisiana Tech University, Ruston, LA 71272 USA cDepartment of Mechanical Engineering, University of Kansas, Lawrence, KS 66045 USA Abstract Power generation from waste heat sources at the miniature length scales may be generated by using phase change working fluids (liquid-to-vapor) in a traditional boiler-expander-condenser-pump system. This pa- per builds on our prior work of boiler and expander design by investigating the effects of working fluid properties on an expander unit based on a Free Piston (FPE) architecture. Here, using first principles, a lumped-parameter model of the FPE is derived by idealizing the FPE as a linear spring-mass-damper system. Moreover, a linear-generator model is incorporated to study the effects on useful power output from the FPE directly. As a result, insight into the thermodynamic processes within the FPE are detailed and general recommendations for working fluid selection are established. They include: (1) to achieve a higher FPE efficiency, it is desirable for a working fluid to have high specific heat ratio, and (2) a peak output voltage of about 20 V AC and peak output power of around 2 W can be generated by coupling a centimeter-scale electromagnetic energy converter to the FPE. Overall, this effort shows the promise of reliable miniature thermal power generation from low temperature waste heat sources. Keywords: Free piston, working fluids, waste heat harvesting, mathematical modeling, open cycle, linear generator. 1. Introduction change working fluids (liquid-to-vapor) in a tradi- tional boiler-expander-condenser-pump setup; here, Miniature thermal power generation systems (cen- the heat converts the phase of the working fluid timeter size and below) have the flexibility to oper- |liquid into vapor|subsequently, increasing pres- ate from a variety of available heat sources ranging sure that results in expander boundary work. For from the low energy density waste heat from elec- power generation at miniature length scales, linear- tronics to the high energy density of the heat of type expanders, such as reciprocating-type expanders combustion generated from gaseous or liquid fuels (e.g. free pistons), may be a preferred choice as com- [1, 2]. This flexibility can be achieved using phase pared to rotary-type expanders (e.g. radial-flow tur- bines) due to their simple geometry, lower fabrication cost, and absence of high mechanical stresses induced ∗Corresponding author by the rotary motion [3, 4]. Moreover, the use of Email addresses: these reciprocating-type architectures addresses some [email protected] (Sindhu of the existing technological challenges at the minia- Preetham Burugupally), [email protected] (Leland Weiss), ture length scale such as the issues arising from high- [email protected] (Christopher Depcik) speed operation and sealing of rotational systems [5]. (a) P, V, T g In macroscale reciprocating systems with a . k Mi,e crankshaft assembly, the friction losses from the as- CV m sembly can amount to more than 50% of the total Valve friction loss [6]. Hence, upon scaling down the sys- bem +b f tem size to a miniature length scale, friction losses Cylinder . ∆V(t) can become dominant due to increased surface area Leak, M x(t) = to volume ratios and higher operating speeds [7]. As l S a result, to reduce friction losses, free-piston based (b) architectures are being developed [8, 9, 10] where the P crankshaft assembly is completely eliminated. Sub- 2 3 P2=P i sequently, this removes crankshaft and bearing fric- tion losses; thereby, improving the overall system ef- 4 ficiency. Furthermore, the low friction losses and pure linear motion of the piston nearly eliminates P1=P o 1 5 the need for lubrication. These advantages moti- V vated researchers to investigate the performance of V1 V5 Free Piston Expanders (FPEs) coupled with linear TDC BDC alternators using mathematical models, simulations, and experiments [8]. Overall, they show that it is Figure 1: (a) A representation of FPE as a linear spring- possible to generate an output power O(101) W at mass-damper system. (b) Indicated pressure-volume diagram of the FPE. relatively low operating frequencies (< 10 Hz). Like others [8, 11], the unique features of the free- piston architecture encouraged prior efforts at devel- gated as potential candidates [8, 15] due to a negative oping a FPE for waste heat recovery at the minia- slope on the saturated vapor curve |hence called dry ture length scales [3, 12]. This previous work estab- fluids—that enables superheating and expansion at a lished many of the working principles [3, 12], as well lower pressure while remaining 100% vapor. This en- as the technology demonstration of various mechan- ables a greater output work from the FPE without ical components of the FPE [13, 14]. Generally, the the deleterious effects of condensed working fluids. centimeter-scale FPE under consideration (Fig. 1) As a result, these make an ideal choice for low tem- has a unique potential to harvest waste heat. Specif- perature waste heat harvesting applications. ically, the linear motion of the FPE allows coupling Therefore, this paper builds upon the prior foun- with a linear alternator for electricity generation sim- dation of boiler and FPE design through models and ilar to the work by Refs. [8, 15, 16]. Moreover, mul- experiments [3, 13, 12], while concentrating on the tiple boiler-expander units can be placed on waste effect of working fluid properties (Table 1) on the heat sources where larger power output is necessary; FPE performance. Unlike other approaches, where whereas, single units can produce milli-Watt scale the nonlinear spring stiffness of the working fluid is power for low power applications. linearized [18], the present study makes no such ap- Typically, waste heat sources are of low \availabil- proximation. In this study, thermodynamic processes ity" or exergy such that the temperature gradient is within the FPE are detailed and general recommen- relatively low [13]. Therefore, it is desirable to em- dations for working fluid selection are established. ploy working fluids with low boiling points (e.g., or- Further, the effect of leakage losses within the FPE is ganic working fluids) [17]. However, a proper choice studied, along with the determination of acceptable of the working fluid is necessary for the optimal per- leakage losses. In addition, an electromagnetic en- formance of the FPE. Different fluids (pure and mix- ergy conversion model is employed to simulate power tures), such as R-123 and R245fa, are being investi- output from the FPE. This provides an insight into 2 useful power generation resulting from the working haust (4 ! 5 ! 1) processes as depicted in Fig. 1b. fluid study. The cycle begins with the piston at state 1 (TDC) where superheated vapor at high pressure is injected 2. Free Piston Expander into the cylinder CV by the opening of the valve. This results in pressurization of the CV and motion 2.1. Design of the piston towards BDC. The valve remains open The FPE is a reciprocating-type expander sim- until the piston reaches state 3. Note that the in- ilar to an internal combustion engine with a pis- jection process 1 ! 2 ! 3 occurs for a predeter- ton in-cylinder arrangement, however, devoid of the mined time duration t13 obtained through paramet- crankshaft assembly. Superheated vapor at a high ric studies. Here, the process 1!2 occurs at constant pressure is injected into the cylinder control volume volume, while the process 2!3 occurs at constant (CV) through a valve that is meant to displace the pressure ∼ Pi. After the injection process, the valve piston mass (m) by a distance as a function of time closes and the expansion process 3 ! 4 begins where x(t) to produce boundary work (Fig. 1a). Here, the the CV expands until state 4. Next, the valve opens CV denotes a control volume with state parameters: and the exhaust process starts with the expended pressure (P ), volume (V ), and temperature (T ) at working fluid discharged out of the CV to ambient time t. The valve on the left is used for the injec- pressure Po (4 ! 5 ! 1). This process comprises of tion (M_ i) or exhaust (M_ e) of the working fluid. Fur- the blowdown phase of exhaust (4!5), and exhaust thermore, this valve is connected to the high pressure displacement (5!1).
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