Multi-Vane Expanders as Prime Movers for Low-Grade Energy ...

Applied Energy 16 (1984) 129-146

Multi-Vane Expanders as Prime Movers for Low-Grade Energy Organic Rankine-Cycle Engines

O. Badr, P. W. O'Callaghan, M. Hussein and S. D. Probert

School of Mechanical Engineering, Cranfield Institute of Technology, Cranfield, Bedford, MK43 0AL (Great Britain)

SUMMARY

The behaviour of the prime mover in a low-grade energy Rankine-cycle engine is one of the most important Jiwtors affecting the overall system perJormance.

A survey ojpublished data concerning various types oJexpander in use in Rankine engines and an analysis based on the concept oj similarity shows that, for the low power outputs, positive-displacement expanders hare potential advantages when compared with turbines. However, the multi-vane expander (i.e. the MVE) offers considerable promise as the most appropriate prime mover Jor organic Rankine-cycle engines utilizing solar energy or waste heat as energy inputs, especially in the developing countries. The jeatures and characteristics of the appropriate MVE's are discussed, together with the operational problems remaining to be soh'ed.

GLOSSARY

The unfortunately designated standard practice terms 'specific speed' and 'specific diameter' of an expander are defined as its rotational speed and characteristic diameter, respectively, when the outlet volume flow rate and the isentropic enthalpy drop across the expander both equal unity in their respective SI units.

129 Applied Energy 0306-2619/84/$03-00 ~ Elsevier Applied Science Publishers Ltd, England, 1984. Printed in Great Britain

130 O. Badr, P. W. O'Callaghan, M. Hussein, S. D. Probert

D h L N S 12 (,Xh)~s t/

N OMENCLATURE

Radial clearance between the rotor and stator of the expander (or between the piston and cylinder in the case of the piston expander) (m). Characteristic diameter of the expander (m). Axial-turbine blade height (m). Rotary-piston expander rotor length (m). Expander rotational speed (rpm). Stroke of the piston expander (m). Expander outlet volume flow rate (m3/s). Isentropic enthalpy drop across the expander (J/kg). Efficiency of the expander: it is dictated by the inlet total pressure and exhaust static pressure (0 < q < 1).

INTRODUCTION

Expanders in Rankine-cycle engines convert thermal energy into mechanical work. There are two basic types of expander: one uses the energy of expansion directly in a positive-displacement process, whereas the other converts enthalpy to kinetic energy and then to mechanical work. In the positive-displacement expander, a fixed amount of working fluid is contained by a piston or sealed rotor during its expansion. Turbines are expanders which utilize the resulting kinetic energy: the fluid flows continuously through the machine, work being able to be extracted as a result of the dynamic head of the working fluid.

PRIME MOVERS IN LOW-GRADE ENERGY RANKINE-CYCLE ENGINES

Curran 1 collected the available data up to September, 1979, for over 2000 operational Rankine engines which used various organic working fluids providing output powers from 0.1 to 1120 kW. The summary, presented in Table 1 and Fig. 1, attempts to systemise the vast disparity of available information for systems operating at their design points. (Unfortunately, in many cases this is incomplete, e.g. with respect to system cost and weight.) It must be emphasised that the correlations so emerging in Fig. 1

Multi-lane expanders as prime morers jot" Rankine-c vole engines 131

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represent the best lines through the available data and, as such, can be considered only as approximate design guidelines. Nevertheless, it is interesting to note that the low speed (~<5000rpm) devices are predominantly positive-displacement machines for producing power outputs up to about 10 kW, whereas the high speed ( > 5000 rpm) turbines have been adopted for meeting greater demands (> 10kW). The data

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population also indicates, for the ranges of rotational speed considered, that the power outputs for positive-displacement expanders rise with increasing speed, whereas the reverse behaviour applies for the turbines.

In order to compare the performances of the different types of expander for a specific application it is convenient to utilize the concept of similarity and thereby reduce the number of parameters needed to describe their characteristics. Such considerations show that four parameters are sufficient to describe completely the performance of geometrically similar expanders: these are, the Mach number, the Reynolds number at entry to the flow passage, the specific speed, Ns, and the specific diameter, Os .2

Barber and Prigmore 3 discovered that the Reynolds number and the Mach number of the expanding working fluid have only secondary effects on the performance of the expander. In addition, if the Reynolds number exceeds ]06 , changes in it have no apparent effect upon performance. If the Mach number is significantly less than unity, compressibility effects are small and the expander performance can be represented as a function of only two parameters--the specific speed and the specific diameter. These similarity parameters are defined 2'3 as:

Nil1~ 2

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D( Ah )i~ 1/4 Ds - ~ I / 2

For a specified volume flow rate and enthalpy change through the expander, the specific speed is a measure of the rotational speed of the expander's rotor. The specific diameter can be regarded as a measure of the size of the machine: it corresponds to the rotor diameter for a rotary machine and to the piston diameter for a reciprocating expander. Balje 2 used this similarity concept and the available information on expander performance data to compute the optimal geometries and maximum obtainable efficiencies at the design point operation for different types of expander, as functions of N s and Ds. He presented this information in the form of N s Ds diagrams. Figure 2 shows a compilation of Balje's results derived from Barber and Prigmore. 3 This Figure indicates that, for various ranges of specific speed, certain types of expander offer better performances than others. Throughout the low specific-speed regime, positive-displacement expanders are, efficiency-wise, superior to single- stage turbines. Balje 2 found that the rotary positive-displacement

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expanders exhibited about the same efficiencies as the pressure-staged single-disc turbine at specific diameters and, consequently, tip speeds which are only one-third to one-quarter of the values required for single- disc turbines. Also, only comparatively low pressure ratios were needed for the positive-displacement expanders.

Because high-performance turbines frequently operate at high rotational speeds, gearboxes are required to match the speed of the turbine to the requirements of the Rankine-cycle liquid feed pump and the imposed load (e.g. generator, irrigation pump or compressor). In high power applications, the gearbox efficiency is between 95 per cent and 98 per cent. However, for low power ranges, fixed parasitic losses in the gearbox can cause significant percentage reductions in the turbine's power output (i.e.

3 0 ~ 4 0 per cent for a 10kW turbine). Moreover, speed-reducing gearboxes are relatively expensive and heavy. Barber, 4 for example, reported that a 187 kW turbine/gearbox unit typically had a 10 kg turbine and a 23 kg gearbox.

Mobarak e t a l . 5 carried out a study to select a turbine for small Rankine-cycle power units in the range from 1 to 100kW (e.g. as used with solar power-generation units for small communities in rural areas of developing countries). They concluded that none of the conventional- type turbines would be suitable because of their low efficiencies. They recommended a specially designed ten-stage radial turbine, with a maximum estimated overall efficiency of 71 per cent at 100kW and 8300rpm, and a wheel diameter of about 0.52m, as the most suitable choice for that application. For the same power output, they estimated the efficiencies of the conventional reaction, impulse, radial and multi- stage turbines to be 62 per cent, 66 per cent, 68 per cent and 69 per cent, respectively when running at 6660, 13 320, 20 600 and 8500 rpm.

It is known that the efficiency of a turbine will diminish at rotational speeds other than the design-point speed, and that mechanical stresses and sealing problems may occur if the turbine is allowed to run at very high speeds. However, to maintain the speed of rotation of a turbine at the design poinL a complicated and expensive automatic speed control system is required. Casci e t a l . 6 reported that they fitted their axial flow turbine (which has an output of 40 kW at 42 120 rpm, and a rotor mean radius of 0.243m) with a servo-actuated throttle-valve at the turbine inlet and an overspeed tachometer for the protection of the rotating parts. Cipolla and M a r g a r f reported that the bellows-type, spring-loaded, high-speed rotary face-seals were ineffective in their

Multi-vane expanders as prime morers Jor Rankine-cyele engines 139

experimental centrifugal turbine (giving 95 kW at 34600rpm) at high speeds and that significant leakages of the working fluid (R-11) occurred.

Rankine-cycle engines, utilizing low-grade energy, especially solar energy and waste heat, usually operate between small and varying temperature differences caused by the fluctuating input powers. However, turbines usually are highly inefficient at off-design conditions; that is, on part-loads. This indicates one of the basic disadvantages of using turbines in such applications, especially for low power outputs. Bahadri s summarized some of the major problems of using turbines as expanders in Rankine engines of less than 50kW output for solar-energy water pumping, particularly for low capacities, as: (i) very low efficiencies; (ii) high relative costs, especially for multiple-wheel expansion systems and (iii) the possibility of too high a moisture content in the expanded vapour, which can result in the erosion of the turbine blades.

Thus it can be concluded that positive-displacement expanders have potential advantages as prime movers in low-grade energy engines, especially for low-power levels. These attributes are: (i) high efficiency; (ii) the expanders operate at moderate speeds: they do not usually require speed-reducing gearboxes as turbines do: (iii) good "off-desigff performances: (iv) they can operate even with high moisture-content vapours and suffer little or no erosion ; (v) they require only simplified control systems: (vi) low cost; (vii) compactness and (viii) only simple manufacturing technologies need be employed.

Three types of positive-displacement expander are now in use as prime-movers for low-grade organic Rankine-cycle engines, namely, the reciprocating piston, the rotary screw (i.e. the helical system) and the rotary MVE (see Table 1). Although reciprocating piston expanders have reached an advanced state of development, as have turbines, including small size ones, the present states of design of the screw and the multi-vane rotary expanders for the Rankine-cycle engine application leave much to be desired. Hence reciprocating expanders still represent the most usual design in the majority of low-power Rankine-cycle proposals.

Rotary positive-displacement expanders are potentially serious com- petitors with reciprocating expanders at low-power levels. Such devices possess two main advantages:

(i) They exhibit better breathing characteristics due to the small fluid friction losses in the inlet and exhaust ports, compared with those across the valves of a reciprocating engine, and practically


(ii)

negligible clearance volumes. Also, the necessity for the existence of complicated valve-timing gears in reciprocating engines make them more bulky and costly to manufacture. Rotary positive-displacement expanders in operation create much less noise and vibration and so experience less balancing problems. Because they are rotary machines, connecting rods and crank- shafts are unnecessary. This, moreover, gives rotary expanders the advantages of compactness, less weight and lower cost.

Suri et al. 9 selected the helical expander as the energy converter in their experimental organic Rankine-cycle solar-powered plant (designed to provide 10kW of electricity) because of its better performance compared with piston engines and turbines in the 10 to 100kW range. The expander was capable of handling small volume flows of R- 114 with low or moderate shaft speeds at high efficiencies both under rated and part- load conditions. However, the expander, producing 16.5 kW (mechanical power) and running at 75 000 rpm at full load, was coupled to a three- phase alternator (running at 1500rpm) through a reduction-gear drive whilst the generator speed was controlled by regulating the vapour flow rate into the expander by means of a control valve. Merigoux and Pocard 1 o surveyed pertinent activities of Alsthom-Atlantique, France, in the solar energy field. Although a radial inflow turbine was suggested for the R-11 Rankine-cycle in the 50-500kW power-generation range, a screw expander with oil injection (running at --~ 4200 rpm and having ~ 70 per cent isentropic efficiency) was chosen as the prime mover in the R-113 Rankine-cycle in the 5-50 kW power range. This was regarded as a high reliability solution because of:

(i) The small variations in efficiency for large ranges of expansion ratios and rotational speeds.

(ii) The engine's ability to be used with low-concentration solar- energy collectors due to its high expansion-ratio capability.

(iii) Its aptitude to operate with most commercially available organic working fluids.

Lorenz et al. 11 described the solar farm project of the M.A.N. Corporation, West Germany, for the generation of mechanical and electrical energy in the range 15-500kW. The high-pressure steam generated with the concentrating sun-tracking collectors could be used to drive multi-stage turbines, screw expanders or piston engines. Regarding

Multi-vane expanders as prime movers.[or Rankine-cycle engines 141

the development of the energy-conversion circuit, Lorenz et al. concluded that, whereas multi-stage turbines of relatively high efficiency ( >_ 60 per cent) in the upper performance range were already available on the market, screw expanders of simple design would appear to offer advantages, especially in the lower and medium performance ranges for the solar farm. M .A.N.'s programme for developing screw machines of l 5 to 200 kW appeared promising because, with an efficiency of 50-70 per cent, much lower speeds (about 25 per cent of those of corresponding power output turbines), and so simpler construction and safer operation, could be expected.

MULTI-VAN E EXPANDERS

While rotary screw expanders appear to be more promising compared with turbines and piston engines for low power outputs they are not the most suitable prime movers for low-grade energy Rankine-cycle engines because:

(i) Their rotational speeds are higher than the recommended operational speeds of some of the driven equipment and, therefore, reduction gearboxes and speed control systems are required.

(ii) They need a relatively high level of technology in their production. This is an important consideration because of the available capabilities in the developing countries, even though great potential exists there for the installation of solar-energy systems for power generation.

The multi-vane vapour expander is basically a rotary, low-speed, positive-displacement machine. It exhibits performance characteristics which are not possessed by either the reciprocating expander or the turbine, yet it remains simple in design and incurs low production costs.

The concept of using a sliding-vane machine as an expander is not new. It has occupied an important r61e in industry as a qow-temperature', low- expansion air motor with an efficiency of 25 to 35 per cent. This low efficiency has so far been tolerated because, in most applications, the air motors operate for only short periods and therefore the total energy consumption is low. Also, the poor efficiency disadvantage is balanced by the low cost, high reliability and compactness. In the late 1960's and early 1970's, several studies were completed under the direction of Shouman to improve the understanding of the operation of sliding-vane expanders.


This resulted in an increase in the multi-vane air motor efficiency to over 50 per cent.12 The studies also revealed that the major source of power loss is fluid leakage and that the performance is greatly influenced by the location of the inlet and exhaust ports (i.e. port timing). Gill and Shouman 12 concluded that, by carefully controlling the tolerances (so leading to low leakage rates) and using appropriate port timings, the efficiency could be increased to well above 80 per cent. In a study carried out by Jacazio et al.13 to define where the main power losses occurred within the vane-type air motors, they discovered that, for their considered motor, 65 per cent of the power losses were due to the pressure drop at the inlet, whereas 20 per cent of the losses resulted from leakages, and the remaining losses ensued due to friction and outlet pressure drops. Although the working conditions for the air motor differ from those of the MVE in a Rankine-cycle engine, the modes of the power losses, excluding the heat transfer losses, are approximately the same. Moreover, some of the early MVE experimental investigations were carried out using commercially available vane-type air motors.

Engdahl and Tillman14 described the details of an automobile steam engine designed to operate in the way of a super-critical Rankine cycle. A design operating pressure ratio of 70 to 1 might possibly be realized in a multi-stage turbine. However, small steam-turbines, that run efficiently over a large range of operating conditions, are not available. A piston- type expander could be used, but the valving problems and lubrication difficulties at the high inlet temperature would put this device at a severe disadvantage.

Engdahl and Tillman believed that the most logical choice of expander is the compact, simple, high efficiency multi-vane device. The high efficiency was achieved through the proper choice of the port timing and reducing: (i) the pressure drop across the admission port (achieved by maintaining a low steam velocity); (ii) the friction losses (by selecting an appropriate combination of materials) and (iii) the leakage rate (by using appropriate seals).

Wolgemuth and Olson 15 carried out a study of the behaviour of vane- type expanders that included considering the transient charging and exhaust processes. They concluded that, in the Rankine-cycle power systems, where the power output is relatively low (i.e. in the vicinity of 75 kW), good breathing should be easy to achieve with vane expanders whereas, at such power levels, turbine efficiencies suffer significantly from partial admission losses.

Multi-vane expanders as prime movers.[or Rankine-cycle engines 143

Marsters and Ogbuefi 16 stated that both turbine and reciprocating expanders have disadvantages at low-power levels, the turbine being characterized by high rotational speeds and small blade-heights with correspondingly large losses, whereas the reciprocating engine requires complex valving and incurs balancing problems. Thus the MVE offers considerable promise as an appropriate prime mover for organic Rankine- cycles at these low-power levels. Marsters and Ogbuefi concluded that a significant increase in MVE efficiency may be achieved if particular attention is paid to: (i) the design characteristics, particularly the inlet and outlet porting and (ii) the friction and leakage losses.

Eckard 17 described the activities carried out by the General Electric Company, USA, in designing its circular stator MVE and its measured performance characteristics with R-11 as the working fluid. A measured brake-efficiency of more than 80 per cent, at about 800 rpm, was obtained for the prototype (and 4 .1kW and 70 per cent brake efficiency at 1800 rpm): design improvements were expected to give 85 per cent brake efficiency for R-11. The brake efficiency remained high over the speed range 400 to 2200 rpm. The relatively flat efficiency/speed characteristic makes the MVE a potential candidate as a prime mover for Rankine-cycle engines utilizing fluctuating solar insolations or waste heat as energy inputs. Many vane and stator material combinations were evaluated to obtain wear and friction data. Extrapolated test results from expanders on tests for over 1000 h each indicated that well over 20 000 h lifetimes could be expected for some applications. Still there remained the problems of the inherently high leakage rates and friction, which had to be overcome.

Hussein, 18 Hussein et al. 19 and O'Callaghan e t a [ . 2° selected the MVE for an experimental low-grade energy engine, utilizing R-11 as the working fluid, as an attractive prime mover for solar power generators in the developing countries. Being extremely simple mechanically, needing no advanced technology and also incurring only a relatively low financial expenditure, it can be produced by local craftsmen. To establish the necessary practical data on the performance of the MVE for the analytical study and the future modifications, the experimental study was carried out using a modified reversed vane-type refrigeration compressor. Based on the information obtained, a 5 kW shaft power organic Rankine- cycle engine with an MVE as the prime mover was designed and built to drive an irrigation pump stimulated by solar energy. The project became known as the ~King Tut Project' and developed as a collaborative venture between Cranfield Institute of Technology, the Egyptian Government


and two British Companies--Denco Air Ltd. and GEC. The prototype is currently undergoing field trials at a new desert city near Cairo, Egypt.

FEATURES AND CHARACTERISTICS OF THE MULTI-VANE EXPANDERS

From the previous survey, the following features and characteristics emerged as desirable for an MVE which is to be used as the prime mover in a Rankine-cycle engine at low-power levels:

(i) Simple Construction Easily machined to tight tolerances. Conventional shaft seals and bearings. Easily lubricated. Self compensating for wear. Compact, lightweight and rugged. Requires little maintenance.

(ii) Low Noise and Vibration Low speed. No dynamic valves or gear train required. Intrinsically balanced.

(iii) High Brake Efficiency over Wide ranges of Shaft powers. Speed. Working fluid mass flow rates. Available energy inputs.

(iv) High Torque at Low or Zero Speed Self starting under load. Speed compatible with driven equipment, such as alternators, compressors, fans, pumps, etc. Smooth torque production without the need for flywheel storage.

(v) Relatively High Volumetric Expansion Ratios Range up to 10 via a single-stage expansion. Easily adjustable expansion ratios. Adaptable for many working fluids.

(vi) High Tolerances to a Wide Range of Vapour Qualities Can operate with wet vapours with little or no erosion. Avoids liquid compression damage. Requires only a simplified control system.

Multi-vane expanders as prime movers Jor Rankine-cycle engines 145

R E M A I N I N G P R O B L E M S C O N C E R N E D W I T H M U L T I - V A N E E X P A N D E R S

(i) The selection of the optimal working fluid for each particular application.

(ii) Reduction of breathing losses and the corresponding choice of optimal port timing.

(iii) Inherently high internal leakages and frictional dissipations of the expanders.

(iv) Reduction of heat transfer losses, especially with injected liquid lubricants.

These problems define the framework for future studies needed in order to achieve more reliable and higher efficiency MVE's.

R E F E R E N C E S

1. H. M. Curran, Use of organic working fluids in Rankine engines, J. Energy, 5(4) (July August, 1981), pp. 218- 23.

2. O. E. Balje, A study on design criteria and matching of turbomachines: Part A Similarity relations and design criteria of turbines, Trans. ASME, Journal oJEngineeringJor Power, 84 (January, 1962), pp. 83- 102.

3. R. E. Barber and D. E. Prigmore, Solar-powered heat engines, Solar Energy Handbook--Chap. 22, McGraw-Hill, New York, 1981.

4. R. E. Barber, Rankine-cycle systems for waste-heat recovery, Chemical Engineering (Nov. 25, 1974), pp. 101 6.

5. A. Mobarak, N. Rafat and M. Saad, Turbine selection for a small capacity solar-powered generator, Solar Energy International Progress Proceedings o/the International Symposium - Workshop on Solar Energ)', 16 22 June 1978, Cairo, Egypt, Vol. 3, pp. 1351 67.

6. C. Casci, G. Angilino, P. Ferrari, M. Gaia, G. Giglioli and E. Macchi, Experimental results and economics of a small (40 kW) organic Rankine- cycle engine, Proceedings oj 1980 IECEC. Aug. 18-22, No. 809199, pp. 1008 14.

7. G. Cipolla and R. Margary, Experimental Rankine-('vcle Engine Designed ./or Utilization oJ Low Temperature, Low Pressure Heat- Final R~Tort, Contract No. 196-76-7EEI, Final Report. Energy Commission of the European Communities, Brussels, 1981.

8. M.N. Bahadari, Solar water pumping, Solar Energy, 22 (1978), pp. 307 16. 9. R. K. Suri, S. Chandra, M. V. Krishnamurthy, S. Srinivasamurthy,

K. Berndorfer, H. Hopmann and D. Wolf, Development of small solar- power plants for rural areas in India, Proceedings ~t the I97,~ 1SES Congress, New Delhi, India, pp. 1722 7.


10. J. M. Merigoux and P. Pocard, Solar power units with screw expanders, Solar Energy International Progress--Proceedings oJ the International Symposium Workshop on Solar Energy, 16 22 June 1978, Cairo, Egypt, Vol. 3, pp. 1293 317.

11. J. Lorenz, J. Fuestel and M. Kraft, New developments for future solar- power plants, Solar Energy International Progress--Proceedings oJ the International Symposium--Workshop on Solar Energy, 16- 22 June 1978, Cairo, Egypt, Vol. 3, pp. 1318 28.

12. W. Gill and A. R. Shouman, State of the art review of sliding-vane machinery, ASME Paper No. 76-DET-90, presented at the Design Engineer- ing Technical Conference, Montreal, Quebec, Canada, Sept. 26 29, 1976.

13. G. Jacazio, B. Piombo, A. Romiti and A. Sola, The optimisation of the performance of vane-type air motors, Proceedings oJ the F(fih World Congress on Theory o/ Machines and Mechanisms, Montreal, Quebec, Canada, July 8- 13, 1979, pp. 607- 10.

14. R. E. Engdahl and E. S. Tillman, A compact supercritical-steam engine for automotive power, Proeeedings o/the 1971 1ECEC, Aug. 1971, No. 719110, pp. 802 12.

15. C. H. Wolgemuth and D. R. Olson, A study of breathing-in vane-type expanders, Proceedings ~[ the 1971 IECEC, Aug. 1971, No. 719163, pp. 1249 258.

16. G. F. Marsters and E. Ogbuefi, Rotary-vane expander development: Some design considerations, Proceedings o/ the 1972 1ECEC, Sept. 1972, No. 729048, pp. 249 254.

17. S. E. Eckard, Multi-vane expander as prime mover in low-temperature solar or waste-heat applications, Proceedings o/ the 1975 IECEC, Aug. 1975, No. 759204, pp. 1399 405.

18. M. Hussein, Low-grade energy engines, PhD Thesis, School of Mechanical Engineering, Cranfield Institute of Technology, 1981.

19. M. Hussein, P. W. O'Callaghan and S. D. Probert, Solar activated power generator utilising a multi-vane expander as the prime mover in an organic Rankine-cycle. Paper presented at the Solar World Forum, International Solar Energy Society, Brighton, 23 28 Aug., 1981.

20. P. W. O'Callaghan, M. Hussein and S. D. Probert, Solar thermodynamic water pump (Rotary expander/evacuated tube collectors)--The King Tut Project, Proceedings o/ the UK- ISES (Solar Energy jor Det'eloping Countries: R~/rigeration and Water Pumping), London, Jan. 27, 1982. pp. 77 91.

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