On loss characterisation and minimisation in positive displacement expansion machines
Lead Research Organisation:
Imperial College London
Department Name: Chemical Engineering
Abstract
The programme of research will take a three-pronged approach to characterisation and minimisation of RPE losses. For each, a preliminary study on a (valveless) gas spring will precede the investigation.
1) Reduced order modelling
2) Computational Fluid Dynamic (CFD) simulations
3) Experimental measurements
Reduced order modelling will enable a rapid iteration through multiple design parameters, and also provides a starting point for more accurate simulations and experiments. It will help to establish an intuitive sense of the most critical parameters to monitor and control during further work.
CFD simulations using OpenFOAM (an open source CFD package) will form a major part of the investigation. Experience of simulating gas springs in OpenFOAM already exists within the group. Modifications to the source code of OpenFOAM are possible, and these will be used to extend the code to include additional effects such as leakage flow and custom wall heat transfer models. Turbulence will be a key challenge of the modelling. However, a range of studies on internal combustion engines will have applicable findings, and expertise exists within Imperial College, particularly in the Mechanical Engineering department.
Experimental measurements on reciprocating expanders, including with modified valve arrangements, will provide important insights and validation of CFD modelling. The main role will be to characterise fluid movement and wall heat transfer, for comparison with computational work. Some low-temperature optical cylinder tests may also be used to assess valve inlet/exhaust flow patterns.
Some equipment already exists with Dr Markides' Clean Energy Processes (CEP) laboratory, and several researchers within the group have conducted experiments with ORC systems and expanders. A dedicated expander test rig is planned for the near future. Particularly relevant to the proposed research will be measurements undertaken on the existing gas spring rig, and its successor RPE rig. Read-across from optical-access internal combustion engine experiments may also be possible.
1) Reduced order modelling
2) Computational Fluid Dynamic (CFD) simulations
3) Experimental measurements
Reduced order modelling will enable a rapid iteration through multiple design parameters, and also provides a starting point for more accurate simulations and experiments. It will help to establish an intuitive sense of the most critical parameters to monitor and control during further work.
CFD simulations using OpenFOAM (an open source CFD package) will form a major part of the investigation. Experience of simulating gas springs in OpenFOAM already exists within the group. Modifications to the source code of OpenFOAM are possible, and these will be used to extend the code to include additional effects such as leakage flow and custom wall heat transfer models. Turbulence will be a key challenge of the modelling. However, a range of studies on internal combustion engines will have applicable findings, and expertise exists within Imperial College, particularly in the Mechanical Engineering department.
Experimental measurements on reciprocating expanders, including with modified valve arrangements, will provide important insights and validation of CFD modelling. The main role will be to characterise fluid movement and wall heat transfer, for comparison with computational work. Some low-temperature optical cylinder tests may also be used to assess valve inlet/exhaust flow patterns.
Some equipment already exists with Dr Markides' Clean Energy Processes (CEP) laboratory, and several researchers within the group have conducted experiments with ORC systems and expanders. A dedicated expander test rig is planned for the near future. Particularly relevant to the proposed research will be measurements undertaken on the existing gas spring rig, and its successor RPE rig. Read-across from optical-access internal combustion engine experiments may also be possible.
Studentship Projects
| Project Reference | Relationship | Related To | Start | End | Student Name |
|---|---|---|---|---|---|
| EP/N509486/1 | 01/10/2016 | 31/03/2022 | |||
| 1855524 | Studentship | EP/N509486/1 | 30/11/2016 | 31/08/2020 | Michael Simpson |
| Description | Reciprocating-piston expanders are being considered for a range of clean energy applications, including small- to medium-scale organic Rankine cycles for conversion of low-grade heat to power, and pumped thermal energy storage to integrate intermittent renewables. At large scale, turbines remain the dominant technology, but at scales of 10 - 100s of kilowatts, reciprocating-piston expanders can offer high efficiency at large pressure ratios, robust part-load performance, and affordable manufacture. Although there is a wealth of experience in reciprocating pistons for internal combustion engines and compressors, the particular loss mechanisms which influence reciprocating expanders have individual characteristics, as a consequence of operating without combustion, at different temperatures and valve timings. The primary achievement of the research to date has been to develop and begin to validate a lumped-mass reciprocating-piston model by means of experimental and CFD results. The model has been found to replicate the cylinder pressure and temperature to a high degree of accuracy, and captures trends in gas-wall heat transfer, though with slight discrepancies in instantaneous magnitudes. Surrogate model-based optimisation has been demonstrated to find part-load performance of a given expander, and to optimise an expander geometry and valve timings for a set of operating conditions. The lumped-mass model has been extended to incorporate frictional losses, and a time-resolved exergy calculation has been introduced to attribute losses to particular thermodynamic processes occurring within the model. Organic Rankine cycle (ORC) system studies have investigated the role of piston expanders in multiple applications, such as recovering heat from stationary internal combustion engines and from biomass gasification plants. These analyses have helped to identify promising working fluids and confirm the scenarios under which such systems can be economically viable. For both applications, the piston expander was compared and contrasted with screw expanders, while a comparison has also been made between piston expanders and low-power radial inflow turbines for a fluctuating low-grade heat source. A detailed study has also been made of the potential for ORC engines alongside combined heat and power engines in supermarkets, identifying an attractive business case for a particular configuration for each of the >30 buildings analysed. A further set of investigations has looked at piston compressors and expanders in compressed air energy storage applications, in which piston machines offer interesting possibilities for reversible compressor/expanders that operate at high pressure ratios. Parametric studies and optimisation approaches have been used to understand and map performance of the machines over a wide range of operating conditions. |
| Exploitation Route | The reciprocating-piston design and optimisation model is of interest to a range of researchers in the fields of organic Rankine cycle engines, refrigeration, thermo-mechanical energy storage and several others, as it provides a capability to simulate devices which may not be widely available at present, or to conduct investigations that would be hazardous, costly and/or time-consuming to undertake experimentally. Similarly, there are a range of potential industrial applications for such a model, with the designers and manufacturers of both devices and systems. For example, an ORC engine manufacturer could use the tool to help identify the optimum working fluid, expander type and cycle design for their product, taking into account the effect of different operating conditions of the expander on its efficiency. Equally, a technology developer in the fields of compressed air or pumped thermal energy storage could use the model to predict and optimise the compression and expansion efficiency, helping to deliver high overall round-trip efficiency for the energy storage system. |
| Sectors | Energy |
| Description | Under this award a range of modelling capabilities have been developed for optimising energy technologies at both device (compressor/expander) and system level. These modelling and optimisation approaches have been described in a number of publications, and have informed feasibility and design studies for a UK thermal and compressed air energy storage startup which is looking to deliver electricity storage at low cost with a low environmental footprint. Capabilities developed alongside the main research findings have also supported work by the Dearman Engine Company (now Clean Cold Power) to develop zero-emission refrigeration units for transport applications, in particular helping to verify and correct analysis of efficiency, emissions and heat leakage rates for refrigerated transport. Use of thermo-mechanical systems for low-carbon transport and electricity storage has also been a useful point of engagement with the general public, helping to capture the attention of children and adults at Imperial College outreach events. |
| First Year Of Impact | 2020 |
| Sector | Energy |
| Impact Types | Economic |
| Title | Compressed air energy storage model |
| Description | A thermodynamic modelling framework for thermal and compressed air energy storage systems has been created in Matlab. The model allows a range of architectures to be simulated rapidly, combining heat exchangers, compressors, expanders, thermal and pressurised air storage in a flexible manner, and calculating outputs such as energy stored for a given system design, roundtrip efficiency and temperatures and pressures throughout the system. The model requires input data on compressor and expander performance, heat exchanger effectiveness and pressure drops and motor and pump efficiencies. The model has been validated against prior work in the field. |
| Type Of Material | Computer model/algorithm |
| Year Produced | 2020 |
| Provided To Others? | No |
| Impact | The model has helped to inform feasibility and design studies for a UK thermal and compressed air energy storage company that is seeking to develop a modular low-cost design for electricity storage. The company will shortly deploy a prototype system with its first customer. |
| Title | Lumped-mass model for simulation and optimisation of reciprocating-piston devices |
| Description | The thermodynamic model that has been developed is of a reciprocating-piston device, which could be a compressor or expander. By calculating the fluid flows and various loss mechanisms in the device, it allows predictions to be made of the performance of the system on the basis of readily-available geometric measurements, and draws on physical principles rather than relying on extensive experimental data for capturing the various phenomena that take place. It therefore offers a useful design tool for researchers wishing to understand device performance and manufacturers seeking to design and optimise new and existing systems. The principles and equations underpinning the model have been published in several conference articles and a journal paper (doi:10.1016/j.apenergy.2018.12.086). Extensions to the model since its first publication have added the ability to track cycle-resolved exergy destruction (destruction of the ability to do work) and assign these losses to particular mechanisms. This extends the insight available to the designer as to how to address and resolve these losses. There are no legal or ethical constraints affecting the sharing of the model. |
| Type Of Material | Computer model/algorithm |
| Year Produced | 2017 |
| Provided To Others? | Yes |
| Impact | The model has been used in two main areas: piston expanders for organic Rankine cycle (ORC) engines, and compressors for compressed air energy storage. In the former, piston expanders are not yet a mature technology for ORC applications, so there is very little experimental data available. The model therefore allows exploratory studies to be conducted, for example to assess different working fluids or to compare piston expanders against other devices. A journal paper has been published on off-design performance of an ORC system with a piston expander, drawing on the ability of the model to provide predictions at on- and off-design conditions. Multiple conference papers have been presented, including on the model itself, on piston expander performance maps, comparing piston expanders against screw expanders, and against turbines, and comparing ORC and CO2 cycles. |
| Description | Dearman Engine Company |
| Organisation | Dearman |
| Country | United Kingdom |
| Sector | Private |
| PI Contribution | The collaboration has been undertaken to increase the technology readiness of Dearman's liquid air/nitrogen engine for mobile refrigeration applications. My colleagues and I have provided expertise in heat transfer, modelling and analysis of test data in order to scrutinise the modelling and testing work previously carried out by the company, identify flaws and recommend improvements. This has helped to increase the fidelity of their modelling work and provide confidence to third-party stakeholders in the performance of the Dearman engine and the associated system. Ongoing work is looking in greater depth at a possible enhancement to the technology, which could deliver greater system efficiency. This work involves further modelling and validation of these models with an experimental campaign. |
| Collaborator Contribution | In return, Dearman have supplied test data and simulation tools, and granted access to their experimental test facilities. They have supplied both current and future system designs and detailed schematics, as well as answering a range of in-depth questions about their hardware and modelling approaches. We have worked closely with Dearman to understand the technical challenges they face and how applied research can be directed to produce maximum benefit. Dearman have also provided advice on how to overcome practical challenges in our own experimental work, for example regarding export of electricity to the grid. |
| Impact | The main outcomes have been two technical reports, on the environmental performance of the Dearman engine relative to the incumbent diesel technologies, and on the efficiency of the system under different operating conditions. The latter was accompanied by two brief memoranda on the testing campaigns carried out by Dearman, expressing an independent view on the validity and accuracy of the results. A further outcome has been the award of internal funding from Imperial to extend the collaboration, with the support of two PDRAs and two academics. This project has only recently begun and has not yet delivered outcomes. |
| Start Year | 2017 |
| Company Name | CHEESECAKE ENERGY LIMITED |
| Description | Cheesecake Energy Ltd (CEL) is developing energy storage technology to support the charging of electric vehicles and allowing for deeper integration of renewable energy such as solar at lower cost than market-leading alternatives. Their breakthrough system uses thermal energy storage and compressed air to achieve costs that are 30-40% lower than those of the cheapest batteries currently available. CEL's technology uses electricity to charge the system by powering an electric motor to drive an air compressor which produces high-pressure air, and heat. This high-temperature heat is captured and stored separately in a thermal store, alongside the high pressure compressed air held in an air tank. The battery can then be discharged by running the process in reverse by using the heat in the thermal store to re-heat the high pressure air, and expanding it through the expanders, which turns a generator to create electricity. |
| Year Established | 2016 |
| Impact | CEL has developed a design for low-cost energy storage and has filed ten patent applications around the technology, while building a team of 18 people and establishing a demonstrator system. CEL has been awarded funding as part of the UK Government's BEIS Longer Duration Energy Storage Demonstration (LODES) competition. The funding will accelerate development and commercialisation of CEL's eTanker system. CEL raised £1.1M in Seed funding to fuel the development of its manufacturing capabilities and support product development of its eTanker storage system. The round was led by Imperial College Innovation Fund alongside prominent investors including Perivoli Innovations, and other angel investors. |
| Website | http://www.cheesecakeenergy.com |
| Description | Future Commuter outreach event at Imperial College |
| Form Of Engagement Activity | Participation in an activity, workshop or similar |
| Part Of Official Scheme? | No |
| Geographic Reach | Local |
| Primary Audience | Public/other audiences |
| Results and Impact | A very well-attended Imperial College outreach event on the subject of the future of transport and commuting presented an opportunity to share findings on work on energy storage and how it can help to enable electric vehicle charging. Small-scale hardware demonstrations helped to make the discussion more tangible. |
| Year(s) Of Engagement Activity | 2020 |
| URL | https://www.imperial.ac.uk/events/95797/imperial-lates-future-commuter/ |