A research team at the City University of Hong Kong has designed a novel hybrid-energy heat pump (HEHP) system that enables a gradual transition from an absorption cycle to a compression cycle. In addition, the novel system uses refrigerant/ionic liquids (ILs) as working fluids to eliminate crystallization constraints.
“Our research introduces a novel HEHP that integrates both absorption and compression heat pump cycles, allowing for seamless transitions between the two types. This flexibility makes the system adaptable to variations in solar irradiation and cooling demand, thereby maximizing energy efficiency,” corresponding author Wei Wu told pv magazine. “Moreover, the use of crystallization-free ionic liquids as working fluids eliminates the crystallization constraints associated with traditional absorption cycles, enhancing the energy efficiency and system flexibility across a broader temperature range.”
Wu further explained that the need for such a solution stems from issues with thermally driven absorption heat pumps. “The building sector accounts for 20-30% of global energy consumption and carbon emissions, with cooling demands responsible for over 10% of global electricity use,” he said. “Thermally-driven absorption heat pumps can significantly reduce electricity consumption, but they are hindered by issues such as low reliability, limited applicability, low efficiency, and crystallization limitations.”
In their most recent study, the team simulated the novel system at a primary school with different ILs and in different locations across China. However, Wu said that the group is currently planning further research to explore the scalability of this system for larger commercial and industrial applications. “This includes optimizing the control strategy of hybrid configurations and assessing the feasibility of integrating the HEHP with renewable energy sources in different geographic regions with varying climates. In addition, low-cost and high-reliability working fluids will be explored for scalable and affordable applications,” he said.
Image: City University of Hong Kong,
The system includes two parallel sub-cycles: an absorption sub-cycle driven by a thermal compressor supplied with solar heat, and a compression sub-cycle driven by an electric compressor. They share a common refrigerant loop, allowing the refrigerant flow to be dynamically split between the absorption and compression sub-cycles to match solar availability and cooling demand. The system operates in three modes: pure absorption mode when solar irradiation is sufficient to meet the cooling demand; hybrid absorption–compression mode when solar energy is available but insufficient; and pure compression mode when solar irradiation is low or absent.
After setting up the system, the team tested the performance of different ILs with it, namely two water (H2O)/ILs, four ammonia (NH3)/ILs, two hydrofluorocarbon (HFC)/ILs, and two heavy fuel oil (HFO)/ILs. The systems with different ILs were designed to accommodate a typical primary school with an area of 4,680 m2 and to operate on weekdays between 8:00 AM and 6:00 PM. A dynamic cooling load was modeled using hourly simulations for four cities: Beijing, Shanghai, Hong Kong, and Singapore. Those cities have solar intensities of 0.149 kW/m2, 0.132 kW/m2, 0.140 kW/m2, and 0.186 kW/m2, respectively.
“The cycle using NH3/[DMIM][DMP] was identified as the most suitable alternative due to its high electrical coefficient of performance (COP) of 19.2 and its significantly high compactness,” Wu added. “The HEHP cycle employing NH3/[DMIM][DMP] is designed for Hong Kong, with the energy efficiency of the solar absorption sub-cycle ranging from 0.31 to 0.50. As the solar collector area rises, the COP rises from 6.8 to 19.8, while the unit cooling potential decreases from 1.75 kWh/m2/day to 0.64 kWh/m2/day.”
Concluding, he added that the levelized cooling cost initially decreases before increasing, reflecting the interplay between higher initial costs and reduced operation costs, with the lowest value of $0.075/kWh occurring at a solar collector area of 600 m2. “With a relative improvement in demand met ratio of 27.7% to 47.5% and a reduction in electricity consumption of 39.4–110.0 MWh/year, the HEHP cycle demonstrates both high efficiency and flexible applicability for sustainable building cooling,” he concluded.
The system was presented in “A flexible hybrid-energy heat pump using efficient ionic liquids for sustainable solar cooling,” published in Applied Energy. Researchers from City University of Hong Kong and City University of Hong Kong Shenzhen Research Institute, Shenzhen, China, have participated in the study.
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