June 23, 2020
7离网制冷原型:决赛from EWB-USA’s Chill Challenge
贡献者：F. Andrew Dowdy, Christine Broda-Bahm
Draft animals and wood fires count among the power sources for new, extremely low-cost technology proposed in a worldwide design contest for off-grid refrigeration. In September of 2019, the non-profit organization Engineers Without Borders USA (EWB-USA) issued the Chill Challenge, soliciting proposals to develop proof-of-concept prototypes for refrigerators and ice makers. The call received 43 proposals from 36 teams representing universities, companies, NGOs, and individuals across the globe. Seven were selected for grants totaling $300,000 for research and development.
“Affordable refrigeration in off-grid communities will be more than a cool drink. It will, in a small way, change the world because of wider benefits such as improved nutrition and education,” said Larry Bentley, who along with F. Andrew Dowdy, is spearheading the challenge on behalf of EWB-USA, with support from an Open Philanthropy grant.
Through their work in off-grid communities, Mr. Dowdy and Mr. Bentley realized that refrigeration was a difficult challenge. Access to refrigeration is key to a number of the UN’s Sustainable Development Goals. Refrigeration contributes to health and nutrition, provides economic opportunities for farmers and businesses, and reduces the burden on women. However, it remains scarce in many economically developing regions. There may be as many 2 billion people on the planet without access.
Part of the problem may be that traditional refrigerators are power hogs. In the global West, the appliances count among the top five energy consumers in the home. In underserved communities, electricity from a central grid can be spotty, and home systems can typically handle only small loads. Solar home systems are increasingly common, providing light and TV and charging phones. Refrigeration, however, is a much more energy-intensive process, and has proved to be a bigger hurdle.
There have been several initiatives to develop efficient off-grid refrigerators, including the Global LEAP Awards by the Efficiency for Access Coalition. The Chill Challenge is intended to complement those efforts, with a slightly different approach to the problem.
Second, is the inclusion of community sources of refrigeration. Rather than smaller household refrigerators, the challenge has solicited proposals for either a larger (150-250 liter) fridge or a community-sized (100-1000 kg/day) icemaker. The fridge is sized to be suitable for small commercial applications, such as shops or bars, which tend to be the earliest adopters of refrigeration. Similarly, the icemaker could provide a very flexible source of refrigeration for households or various income producing activities.
Finally, the initiative is not limited to vapor compression refrigeration. The Chill Challenge is open to all cooling technologies, including thermally driven processes. Design parameters can be found on EWB-USA’s site atChill Challenge Detailed Design Parameters。
Vapor compression refrigeration, powered by solar PV panels, is the most widespread off-grid refrigeration technology in use today. However, it is difficult to meet the Chill Challenge affordability targets using this technology. The two vapor compression proposals receiving grants are for ice makers, which offer certain economies of scale over refrigerators, and can address “dark day” challenges by storing ice, rather than using battery storage.
- 太阳能冷却工程将使用由Solar PV提供动力的5个Selfchill®冷却单元来构建ICEMAKER。这些是使用R-600A（异丁烷）作为具有低全球变暖电位（GWP）的制冷剂的直流直接驱动蒸汽压缩单元。负担能力的关键是将这些单元与本地生产的制冷柜组合，以提供灵活，低成本的制冷选择。该装置将使用智能手机连接进行监控，并将启用薪酬融资。
- 普渡大学proposes to offset the cost of an icemaker by using a combined heating and cooling vapor compression system to make ice and to dry crops. The unit will make 100-150 kg of ice/day, using R-290 (propane) as a low GWP refrigerant and solar PV as an energy source. The research will evaluate food drying and storage practices, including their prices and seasonality. Potential applications of the technology will then focus on locations that produce food items that command high value and/or are available throughout the year.
Solar Thermal Sorption
太阳能制冷,用吸附过程been investigated for many years, but commercial success has been elusive. A big challenge is that the coefficient of performance (COP) for thermal sorption systems is well below that of vapor compression systems. The low COP’s of thermal systems, combined with the falling cost of solar PV panels has favored vapor compression systems for off-grid applications. However, the costs of evacuated-tube solar collectors have also fallen rapidly. Furthermore, thermal systems may be able to address “dark-day” contingencies by storing hot water, which could prove cheaper than battery storage required for vapor compression.
- Xergy Corporationwill build an adsorption refrigerator, driven by solar thermal energy, that uses hydrogen and metal hydrides as the working pair. The unit will employ an advanced heat exchanger design, which the company believes will result in an efficiency of 70%. If successful, the team believes the refrigerator would provide a simple, easy to maintain solution for remote off-grid communities.
- Imperial College Londonproposes to use hot water from solar collectors to drive a diffusion absorption refrigeration (DAR) process. The basic process, used for decades in propane and kerosene refrigerators, has proved to be simple and reliable with no moving parts, but COP’s, using ammonia and water as the working pair, are typically 0.2 or less. The team expects to significantly improve performance by using alternative working fluid pairs and optimized unit design.
Biomass Thermal Sorption
- 普渡大学建议测试由厨师炉子的热量驱动的“冷藏电池”。这基本上是间歇吸附制冷系统 - 热源将产生制冷剂，这可以在以后的时间放电，以提供按需冷却。虽然该装置设计用于厨师炉灶，但可以使用其他热源。
- 新叶动态技术will use its GreenCHILLTM值technology to build a 1000 kg/day ice-maker that can be powered by any number of biomass sources. This is an adsorption system, using R-717 (ammonia), that the company has already deployed in more than 25 locations in India. The system will need an estimated 105 kg of wood equivalent biomass per day to produce 1000 kg of ice and will use electrical power, provided by solar PV, to run auxiliary equipment.
“Night sky radiation” is a well-known phenomenon that was used by ancient Persians to produce ice, and was tested in “roof-pond” houses in the 1970s for residential cooling. In recent years, researchers have developed surfaces that enhance this effect by radiating heat at infrared wavelengths that can pass through the atmospheric window into space. At the same time, the surfaces are highly reflective of sunlight which allows them to maintain temperatures below ambient while exposed to full sun.
- Arup Engineeringproposes to build a completely passive cooling box using this phenomenon for cooling and using phase change materials for cold storage. If successful, this could lead to the development of a refrigerator that operates without electricity or refrigerant, a simple and inexpensive cooling solution for many communities in the developing world.
“Successful solutions to our Chill Challenge will be a game-changer for millions of people and we are extremely proud to be catalyzing new ideas and inspiring out of the box thinking on this vital development tool,” says Cathy Leslie, CEO of EWB-USA.
For more information, please seewww.ewb-usa.org/chill-challenge。
F. Andrew Dowdy is a retired foreign service officer and engineer, now working on energy access, innovation and historical research.
Christine Broda-Bahm is Senior Writer at Engineers Without Borders – USA.