Systems that make use of evaporative cooling and ventilation, as well as optimized vapor compression refrigeration, are on their way to the market, thanks to new research.
It’s an irony of global warming that the more humans do to keep themselves cool against rising temperatures, the more they will probably be adding to the problem by burning fossil fuels to power air conditioners. But, because conventional air conditioning technology now only achieves roughly 35% of theoretically maximum efficiency, there is still room to improve the situation.
Recently, researchers from Tsinghua, supervised by Doctor Wang Baolong and Professor Shi Wenxing, have developed a series of intelligent hybrid systems, one of which can achieve a peak 85.7% reduction in climate impact. This work has been done in partnership with one of the largest residential air conditioner manufacturers in the world, Gree.
An efficient vapor compression air conditioner (white) works in tandem with an evaporative membrane installed within an environmentally responsive ventilator and photovoltaic power.
The system combines improvements to the energy intensive process of conventional vapor compression refrigeration technology, with calibrated input from less energy intensive water evaporation and ventilation cooling, as well as solar power.
“An environmentally responsive fresh air ventilator with evaporative cooling was developed to diminish the operating hours of the vapour compression air conditioner,” explains Shi. When the air outside is humid, for example, then conventional vapor compression refrigeration is the best option. But when the conditions are hot and dry, water evaporation can remove heat from the air by using heat energy to turn water into vapor. The Tsinghua/Gree system uses a wet membrane to do this, although spraying water mist into hot air or onto fine pipes through which hot air passes can achieve the same result.
When the air outside is cool and dry, using a fan to bring in outside air can also be a efficient means to cool a room than recycling the air inside it via a conventional air conditioning system. In the early morning or in the evening, for example, the air outdoors is sometimes cooler. Even if the air is warmer, it may still have a cooling effect as long as the air is dryer and can create an evaporative effect with more humid air inside, adds Shi.
In April, 2021, one design by the team was selected from more than 2,100 entries as the winner of the Global Cooling Prize.
To achieve the gains needed to win the Global Cooling Prize, the team not only used solar power for their system, but also a super-efficient vapor compressor. This utilized a gas-injected compression cycle with relay cooling, a rotary compressor with triple suction ports and two condensers and evaporators.
A schematic diagram of an ultra-efficient air conditioner with smart evaporative cooling ventilation and photovoltaic power.
Local climate-aware cooling
The elegance of the Tsinghua/Gree team’s innovation comes from its integration of the three cooling methods with intelligent control of the input from each to suit outdoor conditions, says Shi.
However, outdoor conditions will vary by season, time of day and of course local climate, so a hybrid system featuring smart evaporative cooling ventilation, alongside a conventional air conditioner, was tested in three cities; Nanjing (in a wet climate zone), Beijing (semi-wet) and Lanzhou (semi-dry/dry).
The system achieved the biggest energy efficiency improvement in arid Lanzhou (221.9%), and the least in humid Nanjing (12.3%).
“Different versions of our systems could be manufactured to suit different climates and economic conditions,” explains Wang. “A simplified version may be suitable for hot and dry underdeveloped areas, while a system incorporating a hybrid heat pump might be developed for regions with heating demands. It also accords with the goal of "Providing Sustainable Cooling for All" proposed by the UN.”
The Global Cooling Prize was a one-off contest launched in November 2018. It was sponsored by the Government of India and sustainability advocacy groups, the Rocky Mountain Institute and Mission Innovation. It attracted entries from 96 countries. At the award ceremony in April, organizing committee chairman Iain Campbell said he believed the Tsinghua/Gree system has “become an important part of the global climate change solution.”
It may well help China meet its emissions targets, agrees Shi, who points out that China is the largest country in regards to the manufacturing of room air conditioners, exceeding 150 million units in 2019. Hybrid air conditioning units are currently being commercialized with Gree.
(L-R): Cui Mengdi, Yang Zixu, Wang Baolong, Jiang Yi, Shi Wenxing, Xiao Hansong and Zhao Jiaan.
Editors: Guo Lili, Sangeet, John Olbrich
Reference: Yang, Z., Zhao, J.,Wang, B., Zhuang, R., Li, X. et al Experimental performance analysis of hybrid air conditioner in cooling season. Building and Environment 204, 108160 (2021)
