In August 2017, Hong Kong’s temperature reached record-breaking 36.6 degrees Celsius. As global climate change becomes more rampant, extreme weather patterns are expected to happen more and more often. Air conditioner is no longer a luxury, but a necessity. In fact, in the commercial sector of Hong Kong, air conditioning ranks as the largest end-use of energy accounting for 24% of the total energy consumed[1]. Researchers have been looking for ways to control the humidity, ventilation and temperature within buildings while cutting down on energy consumption. In light of this, Prof. Hongxing Yang of the Department of Building Services Engineering led a research team in developing an innovative energy-recovering Indirect Evaporative Cooler, essentially using exhaust air to pre-cool and pre-dehumidify the incurrent fresh air, so that less energy is required to bring it down to lower temperature and humidity. A trial was run in a wet market in Tung Chung, Lantau, and the cooler was proven to reduce energy consumption for cooling fresh air in air conditioning systems by 17% to 35%.
Heat wheel: contamination of fresh air, high maintenance cost
Since most people in Hong Kong work in centrally air-conditioned spaces such as offices, restaurants and shops for hours every day, indoor air quality is crucial to their health. Pollutants like carbon dioxide and bacteria may accumulate to harmful levels if not enough fresh air is introduced via the ventilation system. But in the sizzling summer heat, fresh air is much hotter and more humid than exhaust air. Instead of just letting fresh air in, it makes sense to cool it down and dehumidify it with exhaust air first, so as to reduce the workload of the air handling unit.
Traditionally, such energy-recovery systems in Hong Kong use a heat wheel – a perforated disc-like structure made of heat- and water-absorbing materials that rotates slowly between the incoming fresh air and outgoing exhaust air to facilitate the exchange of moisture and heat between the two streams of air. It does cool down and dry out the incoming air partially, but it mixes a little of the two air streams so that the fresh air could be contaminated by exhaust air. “Besides, heat wheel is made of microporous materials that create much resistance to air flow, meaning more energy is required to drive the air through. The microporous materials need to be replaced every three to five years and with moving parts like a motor, regular maintenance is required,” explained Prof. Yang.
Energy-recovering IEC: less maintenance, shorter payback period
The Indirect Evaporative Cooler (IEC) that the research team developed is an energy recovery system that cools fresh air with exhaust air via a heat exchanger within central air-conditioning system. There are two types of channels – wet channels for exhaust air and dry channels for fresh air are alternately installed side by side, separated by thin plates. Water evaporates inside the wet channels and condensation may happen inside the dry channels. When the water evaporates in the wet channels, it absorbs heat from the fresh air and cools it down. Different coatings are used inside the channels to allow the most efficient exchange of heat. Dr Yi Chen, a member of the team, explained, “A hydrophilic coating is applied to the insides of the wet channels to help water droplets attach evenly on the surface so that evaporation can happen more efficiently. On the other hand, a hydrophobic coating is applied on the insides of the dry channels so that any condensate water will be drained right away. Any water droplet on the dry channels would affect heat and mass transfer so that incoming fresh air is not cooled as efficiently.”
In terms of energy consumption, IEC consumes less energy than heat wheel because the air flow resistance in IEC is low without the use of microporous materials. Besides, IEC does not require much maintenance because neither is there any moving part nor is it driven by a motor. Unlike heat wheel, it does not use any material that requires regular replacement. Although the annual total energy recovered using IEC within a central air-conditioning system is nearly the same as that with heat wheel, an IEC takes less time to pay back because the initial capital investment is smaller and it incurs less maintenance cost.
The cooler was installed in a wet market in Tung Chung, Lantau, and was proven to reduce the energy consumption for cooling fresh air in air conditioning systems by 17% to 35%, hugely reducing the carbon footprint. In April 2019, the IEC for Efficient Energy Recovery won a gold medal in the 47th International Exhibition of Inventions of Geneva, Switzerland.
