Experimental performance of a compliant scroll expander for an organic Rankine cycle

Abstract

A small-scale scroll expander was developed and tested. The expander was based on a compliant scroll compressor having a displacement of ∼6.8 cm3. A test loop was also developed for determining expander performance parameters. The loop employed a compressor to pressurize the working fluid up to 2758 kPa and a heat exchanger to create the expander inlet conditions that would characterize an operating power cycle. The expander was tested with R134a as the working fluid. For power output measurements, a dynamometer was used where torque and rotational rate were measured. An important feature in adapting a scroll compressor to expander duty was the lubrication of the scroll wraps during operation and the lubrication required by the journal bearings within the expander housing. The approach used in this study was to dissolve a compatible oil into the circulating working fluid to lubricate the wraps, while a pump was employed to deliver pressurized oil to the bearings. The pressurized oil supply was also used to generate a controlled force to the upper stationary scroll wrap through a sealed piston assembly. This force maintained axial contact between the upper and lower scroll wraps during operation. After expansion, the R134a vapour passed through an orifice flow meter and then to a liquid-cooled heat exchanger. It then enters a compressor and heat exchanger to complete the flow loop. A series of performance tests were conducted on the expander at various expander rotational speeds, inlet and outlet pressures, and scroll sealing pressures. Test results show consistent isentropic efficiencies of over 70 per cent for the expander at its maximum efficiency operating points. The expander performance, however, is a function of speed, pressure ratio, and scroll sealing pressure.

Geometric Design of Scroll Expanders Optimized for Small Organic Rankine Cycles

“…The resulting avoidance of nonproductive parameter combinations conserves computation effort and accelerates the selection of optimal scroll geometries. From this data the desired volume expansion ratio R VP of the expander can be inferred from the working fluid properties, assuming 95% volumetric efficiency (reasonable for scrolls) [2,4,19] and using an imputed isentropic efficiency from, e.g., Refs. [3,4], which may be inclusive of downstream losses from a generator.…”

“…While several investigations of scroll expander models and validation experiments are described in the literature [2][3][4][5][6][7], discussion of the choice of the scroll geometry, the single feature upon which all other properties depend, is generally limited to the case of circle involutes in low volume ratio compressor applications [8,9]. In contrast, the present work explores the effect of varying the basic scroll geometry as a method for developing novel scroll machinery at the higher volume ratios needed for many ORC applications.…”

Slide Valves for Single-Screw Expanders Working Under Varied Operating Conditions

“…E "`B`r p,max´rp,0˘´t an pπ{2 ξq˘{`B`r p,max´rp,0˘´a rctan`B`r p,max´rp,0˘˘˘ ( 16) rp " r p´4 r p ; Nr ot " N rot´3 000 3000 ; p˚" p´10 10 (17) r p,0 " r p,0,n`a0 Nr ot (18) ϑ " ϑ n`a1 p˚`a 2 Nr ot (19) r p,max " r p,max,n`a3 p˚`a 4 Nr ot (20) y max " y max,n`a5 p˚`a 6 pNr ot´Nrot,n q…”

“…Expanders come in two types: dynamic (turbine) [8,9] and positive displacement (volumetric) [10] such as rotary vane [11][12][13][14], scroll [15][16][17][18][19], twin screw [20], single-screw [21][22][23][24][25][26][27] and reciprocating expanders [28]. In the medium-low power range, the displacement expander is a better choice than the turbo generator, because the former has a low flow rate, low rotational speed and a high pressure ratio [29].…”

Effect of resistive load on the performance of an organic Rankine cycle with a scroll expander

“…Wang et al [26] found a maximum expander isentropic efficiency of 77% and power output of 1kW from a scroll expander modified from a compliant scroll compressor using R134 as working fluid. More precisely, it is important to determine some operating parameters for achieving the system maximum energy efficiency; these parameters include pressure ratio, inlet condition and electrical load applied to ORC system.…”