Piston Expander Research Articles

Simulation, construction and evaluation of cheap piston expander for low-pressure power generation by compressed air as working fluid

Abstract This research presents piston expander for low-pressure (1–10 bars) power generation to use with organic Rankine cycle and compressed air storage system. The mathematical model is analyzed to show behavior of main parameters such as pressure ratio and charge/discharge volumes where these parameters influence to the work and an isentropic efficiency base on dimensionless analysis. The main contributions of this work are method to find the main parameters at the best performance, modification of diesel engine to use as expander and performance evaluation. From the simulation, we found that the high discharge volumes lead to high producing work and isentropic efficiency. The expander was tested to find the power output and isentropic efficiency of the proposed method including torque, speed, temperature and flow rate compared with the simulation results. From the experimental results, the modified expander by the proposed method can be operated by using low pressure working fluid. The engine produces maximum power output are 160 W, 257 W and 431 W at averaged speed of 683 rpm where pressure ratios are 3, 4 and 5. The averaged isentropic efficiency is 57%.

Performance study of a small-single piston expander using compressed air as working fluid

This research presents the small-single piston expander for small-scale power generation to use with compressed air storage system and ORC. The main contributions of this work are method to easily modify small-single piston engine into the expander and evaluation. The expander was tested to find the performance of method including torque, speed, temperature and flow rate are measured. The small-single-piston engine was used in the evaluation. From the experimental results, the modified engine by the proposed method can work by using compressed air working fluid. At the optimal point, the engine produces maximum powers are 160 W, 257 W, 431 W and 581 W. These powers are at speeds of 660 rpm, 685 rpm, 705 rpm and 710 bar respectively, where pressure charge are 3 bar, 4 bar, 5 bar and 6 bar. The averaged isentropic efficiency is 57%.

Experimental investigation of four volumetric expanders

The aim of this paper is to facilitate the selection of the expander for a small-scale organic Rankine cycle based on an experimental comparison of a piston, a screw and two scroll expanders. First, based on a literature review, a comparison between these three technologies of volumetric expansion machines is performed. Afterward, four displacement expanders (~2kW) are tested in a small-scale ORC unit with fluid R245fa: a variable speed swashplate piston prototype, an oil-free twin-screw, a modified lubricated hermetic scroll compressor operating at a constant speed and a variable speed modified scroll compressor. The maximum effective isentropic efficiencies measured are 53% for the piston expander and the screw expander, 76% for the variable-speed scroll and 81 % for the constant-speed scroll machine. However, these performance do not reflect the highest efficiencies achievable by each expander: the test-rig has experimental limitations in terms of mass flow rate and pressure drop (among others) which restrict the achievable operating conditions. The calibration of semi-empirical models based on the measurements allows to overcome this issue and to predict the isentropic efficiency in optimal conditions despite the limitations of the test-rig. When selecting an expander, other considerations than the efficiency have to be taken into account such as the flexibility, the operating conditions, the costs and the components compactness. Based on experimental results and practical considerations, some guidelines are drawn to help the reader to properly select a volumetric expander.

Experimental study on free piston linear generator (FPLG) used for waste heat recovery of vehicle engine

In this study, a novel free piston expander coupling with a linear generator prototype is presented to recover exhaust waste heat efficiently from vehicle engine. The free piston linear generator (FPLG) can be used in a small-scale organic Rankine cycle (ORC) system and can directly convert the thermodynamic energy of working fluid into electricity. The effects of intake pressure, operation frequency as well as external load resistance on operation characteristics and output performance are discussed. The results show that the piston displacement profile is similar to a sinusoidal wave. The piston amplitude increases significantly with the increase of the intake pressure. It is observed that the increase in the operation frequency brings about the decrease in both piston amplitude and motion symmetry. In addition, the peak piston velocity and power output show nearly linear relation with the intake pressure, which indicates that the power output is sensitive to the piston velocity. When the operation frequency is 1.0Hz and the external load resistance is 20Ω, the maximum peak velocity is about 0.69m/s and the highest power output 96W is obtained. The conversion efficiency of the FPLG increases with the increase of the intake pressure. But the conversion efficiency increases slightly when the intake pressure is higher than 2.2bar. When the operation frequency is 2.0Hz, energy conversion efficiency of the FPLG can reach up to 45.82% with an intake pressure of 2.6bar.

Radial piston expander as a throttling valve in a heat pump: Focus on the 2-phase expansion

Two-phase expansion devices are a practicable solution for improving the energy efficiency of heat pumps. The study of the fluid behavior in such machines has not yet been investigated in detail. In the present study, a radial piston expander was used in a heat pump operating with R134a in place of the throttling valve. Special attention was paid to investigate both numerically and experimentally the behavior of the fluid inside a cylinder within a full cycle of the engine, at different inlet thermodynamic conditions and rotational speeds. The measured indicated cycles were compared with those predicted by a quasi-steady model and a rather large difference between them was noticed. Therefore, a non-isentropic model to predict the in-cylinder pressure trend during the expansion phase at closed valves, developed on the basis of a model proposed for water at atmospheric conditions, was applied. The results achieved with the improved non isentropic model showed a significantly higher accuracy in predicting the experimental results along the expansion phase at closed valves, thus filling the gap registered with the full isentropic models, which lack in this part of the cycle.

Use of Rolling Piston Expanders for Energy Regeneration in Natural Gas Pressure Reduction Stations—Selected Thermodynamic Issues

Gas pressure reduction stations are commonly applied to decrease the pressure of natural gas in the transmission pipelines. In such stations, natural gas is expanded in throttling valves without producing any energy. Through the use of expander in natural gas pressure reduction stations, it is possible to recover the pressure energy of the natural gas during expansion, and drive the electrical generator. Possible solutions include turbines and volumetric expanders. However, turbines are complicated and expensive, while volumetric expanders are simple and cheap. This paper presents an analytical modeling of rolling piston expander work conditions when adopted to natural gas expansion. The main objective of this research was therefore a comprehensive analysis of influence of varied sizes of the expander components and natural gas thermal properties at the inlet and at the outlet of the expander, on the expander output power. The analysis presented in this paper indicates that the rolling piston expander is a good alternative to the turbines proposed for energy recovery in natural gas pressure reduction stations.

Numerical characterization under uncertainties of a piston expander for exhaust heat recovery on heavy commercial vehicles

While nearly 30 percent of the fuel energy is lost as waste heat in the form of hot exhaust gases, exhaust heat recovery promises one of the biggest fuel economy potential regarding the technologies available in the next decade. Applied to heavy commercial vehicles (HCVs), buses or off road vehicles, a bottoming Rankine Cycle (RC) on exhaust heat shows a great potential in recovering the exhaust gases energy, even for part loads. The objective of this paper is to illustrates the interest in assessing the uncertainty of this kind of systems for getting a robust prediction of the associated numerical model. In particular, the focus here is on the simulation of a piston expander for exhaust heat recovery.Uncertainties associated to the experimental measurements are propagated through the numerical code by means of uncertainty quantification techniques. Several sources of uncertainties are taken into account at the same time, thus yielding various indications concerning the most predominant parameters, and their influence on several quantities of interest, such as the mechanical power, the mass flow and the exhaust temperature.

Performance measurement of a new concept reciprocating piston expander (RPE) using a newly developed small-scale dynamometer unit

This paper presents the progress of a small-scale dynamometer prototype development for performance measurement of a reciprocating piston expander (RPE). Since the available dynamometer systems in the market are limited to specific applications that require for the customization, their price normally very expensive. Since the current study on the RPE required a dynamometer unit, therefore, a new and cheaper dynamometer prototype that was suitable for RPE application has been developed. Using air as RPE working fluid, a case study has been carried out to measure its performance at different inlet fluid conditions, i.e., within 20°C–140°C and 3–5 bars. The results observed that the performance of RPE was proportionally increased to the increased of inlet fluid pressure and temperature. The maximum brake power produced was 27 Watt when the RPE operated at 140°C, 5 bars and the speed of 820 rpm. It also revealed that the changes in the pressure of inlet fluid can give significant change on the performance of the RPE due to its direct relation to the RPE actual rotating force. Although the RPE and dynamometer seem good being adapted to each other, both of them require some improvements to ensure both systems well operated and reliable.

Organic Rankine cycle – review and research directions in engine applications

Waste heat to power conversion using Organic Rankine Cycles (ORC) is expected to play an important role in CO 2 reductions from diesel engines. Firstly, a review of automotive ORCs is presented focusing on the pure working fluids, thermal architectures and expanders. The discussion includes, but is not limited to: R245fa, ethanol and water as fluids; series, parallel and cascade as architectures; dry saturated, superheated and supercritical as expansion conditions; and scroll, radial turbine and piston as expansion machines. Secondly, research direction in versatile expander and holistic architecture (NO x + CO 2 ) are proposed. Benefits of using the proposed unconventional approaches are quantified using Ricardo Wave and Aspen HYSYS for diesel engine and ORC modelling. Results indicate that, the implementation of versatile piston expander tolerant to two-phase and using cyclopentane can potentially increase the highway drive cycle power by 8%. Furthermore, holistic architecture offering complete utilisation of charge air and exhaust recirculation heat increased the performance noticeably to 5% of engine power at the design point condition.

Experimental and Potential Analysis of a Single-Valve Expander for Waste Heat Recovery of a Gasoline Engine

In this paper, a Rankine cycle test system is established to recover exhaust energy from a 2.0 L gasoline engine. Experiments on the system’s performance are carried out under various working conditions. The experimental results indicate that the recovery power of the expander is strongly related to the load and speed of the gasoline engine. It is found that when the output power of the gasoline engine is 39.8–76.6 kW, the net power of the expander is 1.8–2.97 kW, which is equivalent to 3.9%–4.9% of the engine power. The performance simulation shows that the mass flow rate, power output, and isentropic efficiency of the piston expander are directly determined by the intake valve timing. Selecting a suitable intake valve timing can optimize the performance of the expander. The simulation results show that a 1 kW increment in power can be obtained only by selecting an optimum intake open timing. The experimental results further verify that the single-valve piston expander, because of its small dimensions, simple structure, and high speed, is appropriate, and has great potential for energy recovery of gasoline engine exhaust and has good prospects for engineering applications.

Energy recovery by means of a radial piston expander in a CO2 refrigeration system

Natural refrigeration fluids as CO2 represent a very promising alternative to hydrofluorocarbons (HFCs) but they generally entail lower COP values. The possibility of recovering the large throttling losses by using an expander could give a substantial contribution to the performance improvement. In this work, a reciprocating expander developed from a hydraulic motor was experimentally analyzed. Despite of the mechanical losses due to the different original application of the machine, the thermodynamic cycles showed very promising results with the adoption of this solution. The machine isentropic efficiency was 19%, while the indicated efficiency was 66%, showing a very interesting potential of the expander. Thus, by considering a reasonable value of mechanical efficiency about 65%, the isentropic efficiency would be around 40%. Consequently, the entailed COP potential improvement, compared to a refrigeration system with simple throttling valve is currently at 7.4%, while it might overcome 20%.

Performance Analysis of a Reciprocating Piston Expander and a Plate Type Exhaust Gas Recirculation Boiler in a Water-Based Rankine Cycle for Heat Recovery from a Heavy Duty Diesel Engine

The exhaust gas in an internal combustion engine provides favorable conditions for a waste-heat recovery (WHR) system. The highest potential is achieved by the Rankine cycle as a heat recovery technology. There are only few experimental studies that investigate full-scale systems using water-based working fluids and their effects on the performance and operation of a Rankine cycle heat recovery system. This paper discusses experimental results and practical challenges with a WHR system when utilizing heat from the exhaust gas recirculation system of a truck engine. The results showed that the boiler’s pinch point necessitated trade-offs between maintaining adequate boiling pressure while achieving acceptable cooling of the EGR and superheating of the water. The expander used in the system had a geometric compression ratio of 21 together with a steam outlet timing that caused high re-compression. Inlet pressures of up to 30 bar were therefore required for a stable expander power output. Such high pressures increased the pump power, and reduced the EGR cooling in the boiler because of pinch-point effects. Simulations indicated that reducing the expander’s compression ratio from 21 to 13 would allow 30% lower steam supply pressures without adversely affecting the expander’s power output.

Feasibility analysis and performance characteristics investigation of spatial recuperative expander based on organic Rankine cycle for waste heat recovery

This paper proposes a new concept of spatial recuperative expander which injects cold refrigerant during exhaust stroke as a measure of direct contact heat transfer. The commercial simulation tool GT-SUIT 7.4 is employed to model and verify the feasibility of spatial recuperative expander. The research contents are comprised of the following aspects: Firstly, the principles and performance characteristics between traditional reciprocating piston expander and spatial recuperative expander have been investigated. Secondly, the potential of spatial recuperation by adjusting cold refrigerant injection timing has been studied. Thirdly, the relation between expander performance and variable expansion ratio under constant operating condition has been discussed. Fourthly, the thermodynamic performance of spatial recuperative expander under various operating conditions has been examined. The simulation results indicate that: Firstly, the torque per unit mass, thermal efficiency, exergetic efficiency, isentropic efficiency and recuperative efficiency of optimum spatial recuperative expander are 51.00%, 6.74%, 20.79%, 5.68% and 11.36% higher than traditional reciprocating piston expander respectively. Secondly, the cold refrigerant injection timing has little influence on recuperative efficiency because the recuperation process can complete within 16.67ms. Thirdly, different operating conditions correspond to particular optimal expansion ratio. Fourthly, increasing the pump pressure and maintaining appropriate superheated degree can reconcile both power output and thermal economy. While increasing the refrigerant temperature and preserving suitable pump pressure could benefit both recuperative performance and power output.

Preliminary Development of a Free Piston Expander–Linear Generator for Small-Scale Organic Rankine Cycle (ORC) Waste Heat Recovery System

A novel free piston expander-linear generator (FPE-LG) integrated unit was proposed to recover waste heat efficiently from vehicle engine. This integrated unit can be used in a small-scale Organic Rankine Cycle (ORC) system and can directly convert the thermodynamic energy of working fluid into electric energy. The conceptual design of the free piston expander (FPE) was introduced and discussed. A cam plate and the corresponding valve train were used to control the inlet and outlet valve timing of the FPE. The working principle of the FPE-LG was proven to be feasible using an air test rig. The indicated efficiency of the FPE was obtained from the p–V indicator diagram. The dynamic characteristics of the in-cylinder flow field during the intake and exhaust processes of the FPE were analyzed based on Fluent software and 3D numerical simulation models using a computation fluid dynamics method. Results show that the indicated efficiency of the FPE can reach 66.2% and the maximal electric power output of the FPE-LG can reach 22.7 W when the working frequency is 3 Hz and intake pressure is 0.2 MPa. Two large-scale vortices are formed during the intake process because of the non-uniform distribution of velocity and pressure. The vortex flow will convert pressure energy and kinetic energy into thermodynamic energy for the working fluid, which weakens the power capacity of the working fluid.

Investigations of a new free piston expander engine cycle

This work examines the design and operation of a new, small-scale FPE (Free Piston Expander) engine that operates using low temperature waste heat sources to produce useful power output. The FPE is based on a sliding-piston architecture that eliminates challenges associated with MEMS-based rotating systems. A nonlinear lumped-parameter model is derived to study the factors that control the performance of the FPE engine and its unique operating cycle. This basic analysis considers a closed cycle operation of the FPE with low thermal or heat inputs and dimensions on the order of several millimeters. Key system design and operating parameters such as piston mass, external load, and heat input are varied to identify conditions and trends for optimal performance. The model indicated the pressure-volume diagram resembles a constant pressure cycle for a certain set of operating conditions but is also condition dependent. Increased heat inputs to the FPE reduced the engine natural or operating frequency while increasing the power output. Thermal efficiencies of the FPE are shown to be predictably low, on the order of 0.2% due to the small heat input and operating temperature gradients associated with waste heat. Key design features are identified that reveal FPE efficiency, operating frequency, and output power are dependent on piston mass, external load, input heat-rate, and duration of heat input.

Experimental study of an oil-free steam piston expander for micro-combined heat and power systems

This paper presents an experimental study conducted on an oil-free steam piston expander for micro-combined heat and power systems. This expander can produce electrical power (between 740 and 2400W) with a significant range of supply temperature (between 260 and 340°C) and pressure (between 20 and 34bar). The expander electrical power output exhibits a fast dynamic response to a change of working or supply fluid conditions. The reached expander overall isentropic efficiency (including electrical generator efficiency) was between 19% and 40%.An empirical model has been developed in order to conduct a sensitivity analysis of the system by varying working variables such as supply and exhaust pressures, rotational speed and supply temperature. The parameters of this model have been identified using experimental results. The sensitivity analysis showed a limited increase of the electrical power output with a rotational speed until 900rpm and a reduction of the power output for values beyond 900rpm. It also highlighted the significant positive impact of the supply pressure on the electrical power output and the negative impact of the superheating on the expander overall isentropic efficiency.The obtained results are useful for the future control of the expander integrated into a Rankine cycle.Since the fluid at the expander exhaust is steam at a pressure close to 1 bar, it is possible to produce heat at a temperature close to 80°C, which is sufficient for most domestic applications (heating or direct hot water production).

Experimental investigation on the CO2 transcritical power cycle

CO2 has perfect environmental properties and has great potential to become a very ideal working fluid for power cycle. In the laboratory, a CO2 transcritical power cycle system was established, using a rolling piston expander. Experimental study was carried out on the operating parameters, the electric power generated and the thermal efficiency. The pump operating speed and the load resistance were used to regulate the operating parameters. The results showed that there was a sudden decrease for the electric power generated in the start-up process. The electric power rose with increasing the converter frequency. When the converter frequency kept constant, the electric current declined with increasing the load resistance. In the experimental study, the steady electric power generated could reach about 1100 W and the thermal efficiency 5.0% when the high pressure was about 11 MPa and the low pressure was about 4.6 MPa. Though the isentropic efficiency, about 21.4%, was unsatisfactory, it still has important significance for the study on CO2 expander.

Improvement and experimental research of CO2 two-rolling piston expander

Owing to the poor performance of the previous CO2 two-rolling piston expander prototype, improvements were made to increase its efficiency. The improvement measures included increasing the expansion ratio by minimizing the size of the intermediate channel and adopting some sealing methods to block the leakages, such as integrating the intermediate plate and filling O-rings in the seal grooves of the end covers. To reduce the leakage and irreversible losses, lubrication of the expander was investigated as a measure. The improved expander was tested in the CO2 water-to-water heat pump system. The experimental results showed that the revised expander had a much higher efficiency compared to the primary one, reaching a maximum of 77% at the revolution speed of 867 rpm, and a maximum recovered expansion work of 242 W at the revolution speed of 770 rpm. However, further improvement of the expander is necessary because the measured parameters of the prototype do not yet approach the design values.

Experimental performance of a piston expander in a small- scale organic Rankine cycle

Volumetric expanders are suitable for more and more applications in the field of micro- and small-scale power system as waster heat recovery or solar energy. This paper present an experimental study carried out on a swatch-plate piston expander. The expander was integrated into an ORC test-bench using R245fa. The performances are evaluated in term of isentropic efficiency and filling factor. The maximum efficiency and power reached are respectively 53% and 2 kW. Inside cylinder pressure measurements allow to compute mechanical efficiency and drown P-V diagram. A semi-empirical simulation model is then proposed, calibrated and used to analyse the different sources of losses.

Experimental and computational investigation of a MEMS-based boiler for waste heat recovery

Thermodynamically limited processes make waste heat abundant in availability. An Organic Rankine Cycle (ORC) steam powered micro system designed to scavenge waste heat from various sources (transportation, industries or solar) is presented. The key boiler component is fabricated and characterized in this work. The system design has been inspired by the various efforts implemented in development of micro heat recovery devices and engines. The complete system consists of three individual micro components (1) boiler, (2) free piston expander and (3) superheater. Specifically, design, fabrication techniques, test setup and results of the miniaturized boiler are presented in this paper. A key design feature of the boiler is the inclusion of capillary channels for fluid flow from the surrounding reservoirs to the heated area. The pressurized steam is created by the boiler as a result of phase transformation of the working fluid. This pressurized steam can be utilized to drive another MEMS device (PZT membranes, turbines, thermoelectric, etc.) to generate power. In this upgraded boiler design, radial capillary channels and a thin film glass steamdome were considered to improve the operating efficiency. These inclusions enhanced capillary flow, energy absorption via phase change, mass flow rate and operating pressure. The power inputs of 1.8W and 2.7W were selected to simulate and characterize the boiler performance based on real-world waste heat source temperatures. For these power inputs, the maximum power absorption efficiency demonstrated by the boiler via phase change of the working fluid was approximately 88%. The peak operating pressure demonstrated by the boiler was 8.5kPa. These thermally efficient characteristics of the boiler make it a potential future device for waste heat scavenging.