Linear Stirling Engine with a Buffer Tube
Lead Research Organisation:
University of Oxford
Department Name: Engineering Science
Abstract
Domestic CHP systems are an obvious way of both generating electricity with a high efficiency and reducing strain on the grid and local distribution systems. The move to electric and plug-in hybrid vehicles will place additional demand on the grid and overload the local electrical distribution system that can currently only cope with about 10% of households recharging vehicles. With domestic CHP systems a different mind-set is needed as it is necessary to consider the electricity generation to be a by-product of the heating demand, as the electricity can be exported. Although a small domestic boiler might have a rating of 12 kW the heating demand in the summer is of course much smaller and this leads to a lower power (but high efficiency) requirement for the Stirling engine. Consider the following example which assumes a baseline efficiency for a conventional boiler of 90%:
For an electrical output of 1 kW the 'indicated power' of the Stirling engine would need to be 1.3 kW (to allow for losses that are mostly electrical losses in the generator and power electronics). With a pessimistic 28% efficiency assumption (Net W[e] out/Heat in), this will require a heat input of 3.6 kW, with 4 kW of fuel energy. The waste heat from the engine will provide 2.3 kW for domestic heating, and in a conventional boiler this would have required 2.6 kW of fuel energy. So, 1kW of electricity has been generated from an increased fuel energy consumption of 1.4 kW (= 4.0 - 2.6); an overall electrical efficiency of 71% assuming the heat is needed. This is about double the efficiency of a conventional power plant, once allowance is made for the grid transmission efficiency.
The ultimate aim is for a Stirling engine with an electrical output of at least 1 kW, but as a demonstration unit the current work will produce a Stirling engine with an electrical output of 100 W. This smaller size has been chosen because we have a moving magnet motor of this rating that can be used as a generator. This will avoid the need to scale-up the motor design and will give a significant reduction in the project cost. This 100 W system will be large enough to install pressure transducers, thermocouples and displacement transducers, and the experimental data can be used to validate the modelling, so that there will be confidence in the model predictions of the larger engines. The smaller size will also reduce the manufacturing costs. Electrical heating will facilitate accurate measurements of the heat input, and avoid the need to develop a combustion system. Longer term, a catalytic combustion system would operate at a sufficiently low temperature so as to make NOx emissions negligible, and be suitable for a range of gaseous fuels.
The attraction of CHP systems has already led to small linear Stirling generators being developed (e.g Sunpower/Microgen and Infinia/QEnergy systems). Although the idea has been well demonstrated these technologies have not been successful due to high ownership costs and reliability issues. The low cost manufacture of conventional displacer configurations is extremely challenging. A very significant benefit of the research proposed here will be the demonstration of a new engine configuration that radically simplifies the design and manufacture of the displacer - a key component. The cost reductions possible will greatly enhance the prospects of Stirling CHP systems.
The US Department of Energy has recently funded several Stirling engine projects for domestic CHP (https://arpa-e.energy.gov/?q=news-item/department-energy-announces-18-new-projects-accelerate-technologies-efficient-residential). Although the mass market is envisaged to be domestic CHP there are other niche markets for silent power generation that can be exploited, and these would support greater costs associated with small scale manufacture. Examples of this include auxiliary power generation on yachts and military applications.
For an electrical output of 1 kW the 'indicated power' of the Stirling engine would need to be 1.3 kW (to allow for losses that are mostly electrical losses in the generator and power electronics). With a pessimistic 28% efficiency assumption (Net W[e] out/Heat in), this will require a heat input of 3.6 kW, with 4 kW of fuel energy. The waste heat from the engine will provide 2.3 kW for domestic heating, and in a conventional boiler this would have required 2.6 kW of fuel energy. So, 1kW of electricity has been generated from an increased fuel energy consumption of 1.4 kW (= 4.0 - 2.6); an overall electrical efficiency of 71% assuming the heat is needed. This is about double the efficiency of a conventional power plant, once allowance is made for the grid transmission efficiency.
The ultimate aim is for a Stirling engine with an electrical output of at least 1 kW, but as a demonstration unit the current work will produce a Stirling engine with an electrical output of 100 W. This smaller size has been chosen because we have a moving magnet motor of this rating that can be used as a generator. This will avoid the need to scale-up the motor design and will give a significant reduction in the project cost. This 100 W system will be large enough to install pressure transducers, thermocouples and displacement transducers, and the experimental data can be used to validate the modelling, so that there will be confidence in the model predictions of the larger engines. The smaller size will also reduce the manufacturing costs. Electrical heating will facilitate accurate measurements of the heat input, and avoid the need to develop a combustion system. Longer term, a catalytic combustion system would operate at a sufficiently low temperature so as to make NOx emissions negligible, and be suitable for a range of gaseous fuels.
The attraction of CHP systems has already led to small linear Stirling generators being developed (e.g Sunpower/Microgen and Infinia/QEnergy systems). Although the idea has been well demonstrated these technologies have not been successful due to high ownership costs and reliability issues. The low cost manufacture of conventional displacer configurations is extremely challenging. A very significant benefit of the research proposed here will be the demonstration of a new engine configuration that radically simplifies the design and manufacture of the displacer - a key component. The cost reductions possible will greatly enhance the prospects of Stirling CHP systems.
The US Department of Energy has recently funded several Stirling engine projects for domestic CHP (https://arpa-e.energy.gov/?q=news-item/department-energy-announces-18-new-projects-accelerate-technologies-efficient-residential). Although the mass market is envisaged to be domestic CHP there are other niche markets for silent power generation that can be exploited, and these would support greater costs associated with small scale manufacture. Examples of this include auxiliary power generation on yachts and military applications.
Planned Impact
Economic Benefits:
The development of more energy efficient technologies is vital for economic growth. In the longer term this project will lead to the development of new business, not only in the area of microCHP but also in adjacent applications such as deployable power systems and power generation from biomass.
Funding by US Energy Department's ARPA-E programme DE-FOA-0001198: GENerators for Small Electrical and Thermal Systems (GENSETS) supports our market assessment of the opportunities. We will target initially for systems with an output of order 1 kW(e). This program has $25m of funding. The business case for rollout in the US starts at 1M/year units for the domestic markets alone with a target cost of $3,000/system.
The wider benefit to the UK is that the project will bring new business and the opportunity for increasing employment in the UK. A key aspect of this is that the benefit will be long term - the areas of new business are central the world's continuing struggle to supply its energy needs whilst dealing with the problems of emissions and sustainability. The area of technology is one where the UK has a good track record - growth in this area would help better exploit the UK's existing skills.
Social Benefits:
The development of microCHP will help to meet the electricity requirements of the UK and other countries. By reducing the requirement from other sources there will be a social benefit as there will be less pressure to build new power stations with associated disruption that this generally causes. These is also a potential benefit to the householder in that they will either reduce their electricity bill or they will earn some income from any electricity exported to the grid
Environmental Benefits:
There is a very significant environmental benefit. The generation of electricity in microCHP systems offers a high return in the sense that the extra gas consumption is very nearly matched by an output of electricity i.e. 1.4 kW of extra fuel use can generate ~1 kW of electricity. In 2015 there were ~ 27 million households in the UK. If each household is generating 1 kWe for even 3 hours a day then it is equivalent to ~ 3 GW of continuous power generation - a typical power station generates ~ 2 GW. Licensing of any IP would be by Oxford University Innovation, who already license technology and know-how generated by the Oxford Cryogenics Engineering Group, notably to NGAS and Honeywell Hymatic.
The market for small generators (a few kW) is dominated by spark ignition engines. They are cheap to manufacture but are noisy and generate significant NOx. It is probably only a matter of time before legislation is introduced to control their emissions and use, especially 2-stroke engine powered units. NOx reduction is proportionately more expensive to achieve on small engines. In the long term a low cost Stirling generator with a NOx free catalytic burner that uses low temperature 'combustion' will become more competitive in other mass markets.
There is also a significant environmental benefit to be gained from the development of Stirling generators that use catalytic burners. Catalytic burners are able to operate at much low temperatures than conventional burners with very low NOx production. With reasonably clean fuel the only emissions will carbon dioxide and water. This approach will therfore be fundamentally aligned with any clean environment legislation.
The development of more energy efficient technologies is vital for economic growth. In the longer term this project will lead to the development of new business, not only in the area of microCHP but also in adjacent applications such as deployable power systems and power generation from biomass.
Funding by US Energy Department's ARPA-E programme DE-FOA-0001198: GENerators for Small Electrical and Thermal Systems (GENSETS) supports our market assessment of the opportunities. We will target initially for systems with an output of order 1 kW(e). This program has $25m of funding. The business case for rollout in the US starts at 1M/year units for the domestic markets alone with a target cost of $3,000/system.
The wider benefit to the UK is that the project will bring new business and the opportunity for increasing employment in the UK. A key aspect of this is that the benefit will be long term - the areas of new business are central the world's continuing struggle to supply its energy needs whilst dealing with the problems of emissions and sustainability. The area of technology is one where the UK has a good track record - growth in this area would help better exploit the UK's existing skills.
Social Benefits:
The development of microCHP will help to meet the electricity requirements of the UK and other countries. By reducing the requirement from other sources there will be a social benefit as there will be less pressure to build new power stations with associated disruption that this generally causes. These is also a potential benefit to the householder in that they will either reduce their electricity bill or they will earn some income from any electricity exported to the grid
Environmental Benefits:
There is a very significant environmental benefit. The generation of electricity in microCHP systems offers a high return in the sense that the extra gas consumption is very nearly matched by an output of electricity i.e. 1.4 kW of extra fuel use can generate ~1 kW of electricity. In 2015 there were ~ 27 million households in the UK. If each household is generating 1 kWe for even 3 hours a day then it is equivalent to ~ 3 GW of continuous power generation - a typical power station generates ~ 2 GW. Licensing of any IP would be by Oxford University Innovation, who already license technology and know-how generated by the Oxford Cryogenics Engineering Group, notably to NGAS and Honeywell Hymatic.
The market for small generators (a few kW) is dominated by spark ignition engines. They are cheap to manufacture but are noisy and generate significant NOx. It is probably only a matter of time before legislation is introduced to control their emissions and use, especially 2-stroke engine powered units. NOx reduction is proportionately more expensive to achieve on small engines. In the long term a low cost Stirling generator with a NOx free catalytic burner that uses low temperature 'combustion' will become more competitive in other mass markets.
There is also a significant environmental benefit to be gained from the development of Stirling generators that use catalytic burners. Catalytic burners are able to operate at much low temperatures than conventional burners with very low NOx production. With reasonably clean fuel the only emissions will carbon dioxide and water. This approach will therfore be fundamentally aligned with any clean environment legislation.
| Description | We have found that the delays resulting form Covid are greater than anticipated at the time. Manufacture of the components is now nearing completion and safety critical items have been pressure tested. |
| Exploitation Route | Once we have a demonstration system operating we will contact UK Space Agency, Thales and Rolls-Royce |
| Sectors | Energy,Other |
| Description | As yet there are no 'findings' but the design work has improved our understanding of Stirling machines and commercial enquiries have come from 3rd parties Covid has led to much slower design and manufacture and a key post-doc (Amin Abolghasemi) Leaving to work for Universal Quantum. It is hoped that manufacture will be completed before the end of the project, but testing will have to be funded internally. |
| First Year Of Impact | 2020 |
| Sector | Energy |