Focusing on the commonly neglected low-grade waste heat, the team from City University of Hong Kong discovered a method to harvest the lost energy.
In addition to saving energy and developing renewable energy, recovering energy from waste heat is a workable approach to alleviate problems such as energy shortage and global warming. This team recognised that Organic Rankine Cycle (ORC) has been commonly applied to generate electricity using low-grade heat. Furthermore, the team noticed that the low-grade heat produced by air-conditioning systems has been generally omitted, but is applicable to be the heat source for ORC. During summer, air-conditioning is critical to maintain a comfortable indoor environment. Air-conditioning is well known for its high energy consumption, occupying 30% of electricity used by buildings. Therefore, it is conducive to have ORC to recover part of the energy.
Thermal efficiency is dependent on heat source temperature. For a heat source temperature of around 60 to 80 ℃, the thermal efficiency is expected to be lower than 5%. Despite the low efficiency, the system can recover a substantial amount of energy when applied on a large scale. Prior to implementation, simulations were done to seek the design with an ideal balance between performance and economy of scale. Then, the designed system was tested in a commercial building. As a result, 3% of energy consumed by air-conditioning was recovered (~189,000 kWh out of ~5,744,000 kWh), which can allow a 3/4 HP air-conditioner to function for approximately 230,000 hours. Therefore, in spite of its low efficiency, this is a highly useful and practical idea for energy saving.
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