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‘The hottest technology for a cool problem’
The objective of Sheffield University’s project was to develop an energy efficient, novel direct-drive linear motor compressor that could replace conventional rotary induction motor compressors in domestic refrigerators. The Carbon Trust decided to award Applied Research grant funding of £197,000 to support the R&D.
Summary
Refrigeration accounts for approximately 14% of the total electricity consumption in the UK, representing a significant and growing electrical demand. The aim of the Carbon Trust-funded project was to develop an energy-efficient and highly innovative direct-drive linear motor compressor that could replace conventional rotary induction motor compressors in domestic refrigerators. Two years on, at the completion of the project, the team at Sheffield University, in collaboration with industrial partners, has successfully designed and demonstrated a prototype linear motor compressor, which it estimates can deliver significant energy savings.
Project Details  Domestic refrigeration accounts for around 14% of the total UK electricity demand, and UK households use £1.5bn worth of electricity every year on cooling and freezing food and drinks. The objective of Sheffield University’s project was to develop an energy-efficient, novel direct-drive linear motor compressor that could replace conventional rotary induction motor compressors in domestic refrigerators. Just this small technology change in standard domestic refrigeration equipment could generate useful energy and carbon savings, supporting the Carbon Trust’s mission to encourage the development of innovative technologies that can accelerate the transition to a low carbon economy.
The Carbon Trust decided to award Applied Research grant funding of £197,000 to support the R&D. At the outset of the project, the team aimed to design, optimise, fabricate and test a prototype compressor that could deliver a much higher overall efficiency, of between 85 and 95%, than standard refrigerator compressors. Crucially, the prototype also needed to be cost competitive with existing technology.
The original project partners were Sheffield University’s Electrical Machines & Drives Group, Merloni Elettrodomestici UK (a white goods manufacturer, now Indesit Company), and two specialists in magnetic materials: Magnet Applications & Hoganas. The Sheffield University team succeeded in designing and building a prototype linear motor compressor, which included a number of important innovations: 
- Novel design of linear permanent magnet motor
- Simple control mechanism with no sensors
- Neural-network-based current controller which adapts the linear motor to changes in operating conditions.
However, as the project progressed, it became clear that the resources required to develop a new compressor design had been underestimated. This was addressed by scaling back the project to focus on the linear motor design and engaging with another industrial collaborator on the new technology.
Excellent efficiencies have been achieved for the linear motor (93%) and controller (94%) even though the thermodynamic design of the compressor was not optimised. This has resulted in a useful reduction in energy use versus existing fixed-speed compressors, primarily achieved by eliminating the frictional loss of the crankshaft, reducing starting losses and facilitating variable capacity modulation. The technology may also be more energy efficient than variable-speed (capacity) rotary compressors, depending on the type of cabinet.
Dr Jiabin Wang and Professor David Howe, who led the project at the University of Sheffield, commented: "The significantly higher efficiency, the soft start/stop (low noise) capability, the variable cooling capacity and the higher reliability of direct-drive linear motor compressor technology make it eminently suitable for next-generation ‘green’ refrigerators. Its potential is already being exploited by a major refrigerator manufacturer in the Far East. However, there is scope for considerable improvement. The Carbon Trust’s support for this collaborative project has enabled us to advance the technology significantly."
Despite the anticipated improvements in performance, the high cost sensitivity in the domestic refrigerator market remains a barrier to further development and commercial up-take of this technology. Disappointingly, currently there does not appear to be sufficient ‘market pull’ to justify investment in energy-saving technologies. This also applies to variable-speed (variable capacity) compressors which have so far achieved poor adoption over existing compressors due to the price difference. To date, no patent applications have been filed for the highly innovative technology which has been developed during the project, despite its financial and carbon saving potential.
 The team at Sheffield University is currently considering applying for EU R&D (FP7) funding, which would support the required significant investment in new tooling and engineering design.
Legislative pressure to improve the energy efficiency of domestic appliances, such as labelling, and voluntary industry agreements, may also result in further investment by creating a demand from additional appliance manufacturers and consumers for improved refrigeration compressor technology.
For more information and all contact details, please visit our website at www.carbontrust.co.uk/appliedresearch
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