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Dr. Wei Wu
School of Energy and Environment, City University of Hong Kong, Hong Kong

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Research Keywords & Expertise

0 Novel absorption heating and cooling technologies
0 Renewable and waste energy technologies
0 Advanced heat pump technologies
0 Natural and low-GWP refrigerants
0 Net-zero-energy buildings (NZEBs)

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Journal article
Published: 26 June 2021 in Renewable Energy
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Hydrogen fuel cell vehicles pave a promising technological pathway to achieve carbon neutrality. Conventional electric air-conditioning greatly increases hydrogen consumption and thus reduces the driving range. To recover waste heat for vehicle air-conditioning, this study proposes an integrated proton exchange membrane fuel cell (PEMFC) and microchannel membrane-based absorption cooling (MMAC) system. Using a validated model, the PEMFC-MMAC system is characterized under different vital parameters. The PEMFC parameters affect the performance of both PEMFC and MMAC. The coupled performance of the PEMFC-MMAC system increases under a higher operating temperature, a higher operating pressure, or a higher doping level. The MMAC parameters mainly affect the performance of MMAC. The coupled performance of the PEMFC-MMAC system increases under a lower microchannel width or a lower microchannel height. In the covered PEMFC and MMAC parameter ranges, the combined energy efficiencies are improved by 202–273% while the equivalent power efficiencies are improved by 11.4–14.8% with heat recovery. The cooling-to-electrical ratio is 2.02–2.73, the cooling capacity per volume is 129.4–345.9 kW/m3, while the cooling capacity per mass is 0.0439–0.1132 kW/kg. Compared to the existing falling-film absorption cooling technology, the MMAC improves the compactness by 165.1%, reduces the weight by 51.3%, and enhances the COP by 2.6%. This study can facilitate the development of highly-compact, light-weight, energy-efficient, and zero-GWP technology for waste-driven air-conditioning in hydrogen vehicles.

ACS Style

Wei Wu; Chong Zhai; Zengguang Sui; Yunren Sui; Xianglong Luo. Proton exchange membrane fuel cell integrated with microchannel membrane-based absorption cooling for hydrogen vehicles. Renewable Energy 2021, 178, 560 -573.

AMA Style

Wei Wu, Chong Zhai, Zengguang Sui, Yunren Sui, Xianglong Luo. Proton exchange membrane fuel cell integrated with microchannel membrane-based absorption cooling for hydrogen vehicles. Renewable Energy. 2021; 178 ():560-573.

Chicago/Turabian Style

Wei Wu; Chong Zhai; Zengguang Sui; Yunren Sui; Xianglong Luo. 2021. "Proton exchange membrane fuel cell integrated with microchannel membrane-based absorption cooling for hydrogen vehicles." Renewable Energy 178, no. : 560-573.

Journal article
Published: 23 June 2021 in Energy Conversion and Management
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CO2-based ground-source heat pumps (GSHPs) have the potential to be very environmentally friendly, since GSHPs operate with high energy efficiency, and CO2 has no ozone depletion potential (ODP) and a low global warming potential (GWP). We developed a prototype CO2 liquid-to-air GSHP to investigate its performance potential in residential applications. Further, we developed a detailed model of the system that simulates both cooling and heating operation; the model is the primary focus of this report. The model simulates both subcritical and transcritical operation since the system regularly operates near and above the critical temperature of CO2 (30.98 °C) during heating and cooling operation. The model considered both the refrigerant-side thermodynamic and transport processes in the cycle, as well as the air-side heat transfer and moisture removal. We performed cooling tests for the prototype CO2 GSHP that included those from the International Standards Organization (ISO) 13256-1 standard for liquid-to-air heat pumps, as well as extended tests at additional entering liquid temperatures (ELTs). The model predicted the measurements within 0.5% to 6.7% for COP, 1.0% to 3.6% for total capacity, and 3.3% to 4.9% for sensible capacity. We compared the measured cooling performance to published performance data for a commercially-available R410A GSHP and found that for ELTs below 20 °C, the CO2 GSHP has a higher cooling COP and total capacity than the R410A GSHP. At the ‘standard’ cooling rating condition (ELT 25 °C), the CO2 GSHP COP was 4.14 and the R410A GSHP COP was 4.43. At ‘part-load’ conditions (ELT 20 °C) the CO2 GSHP COP was 4.92 and the R410A GSHP COP was 4.99. In the future, the model can be used to investigate methods to improve the CO2 GSHP performance to meet or exceed that of the R410A system over a wider range of ELTs; possible studies include replacing the electronic expansion valve (EEV) with an ejector, optimizing the charge, and optimizing the heat exchanger geometry and circuiting.

ACS Style

Wei Wu; Harrison M. Skye; John J. Dyreby. Modeling and experiments for a CO2 ground-source heat pump with subcritical and transcritical operation. Energy Conversion and Management 2021, 243, 114420 .

AMA Style

Wei Wu, Harrison M. Skye, John J. Dyreby. Modeling and experiments for a CO2 ground-source heat pump with subcritical and transcritical operation. Energy Conversion and Management. 2021; 243 ():114420.

Chicago/Turabian Style

Wei Wu; Harrison M. Skye; John J. Dyreby. 2021. "Modeling and experiments for a CO2 ground-source heat pump with subcritical and transcritical operation." Energy Conversion and Management 243, no. : 114420.

Review article
Published: 01 June 2021 in Renewable Energy
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Membrane-based absorption cycle is a novel technology to provide excellent heat and mass transfer performance in the main components for the convenience of small-scale applications. An overview is provided to summarize its progress and gives insights and possibilities for future development. A development trends analysis shows that the membrane-based absorption cycle is an emerging technology and attracts increasing attention in recent decades. Plate-and-frame module and hollow-fiber module are two common membrane-based modules that can improve the heat and mass transfer of absorber, desorber, and solution heat/mass exchanger. Besides, integrated components, including integrated evaporator-absorber and integrated condenser-desorber, have been proposed for further system size reduction. A thorough review of the cycle configurations indicates that the closed-type membrane-based absorption cycles could provide stationary cooling or heating capacity with better cycle performance, while the open cycles are more suitable for waste heat recovery and gas dehumidification. Apart from the conventional H2O-LiBr and NH3-H2O working fluids, ionic-liquid-based mixtures are promising candidates to overcome the existing constraints. But they also face some shortcomings, including high cost and possible high viscosity. This work is expected to facilitate the development and application of the membrane-based absorption cycle towards compact and efficient renewable/waste energy utilization.

ACS Style

Chong Zhai; Wei Wu; Alberto Coronas. Membrane-based absorption cooling and heating: Development and perspectives. Renewable Energy 2021, 177, 663 -688.

AMA Style

Chong Zhai, Wei Wu, Alberto Coronas. Membrane-based absorption cooling and heating: Development and perspectives. Renewable Energy. 2021; 177 ():663-688.

Chicago/Turabian Style

Chong Zhai; Wei Wu; Alberto Coronas. 2021. "Membrane-based absorption cooling and heating: Development and perspectives." Renewable Energy 177, no. : 663-688.

Journal article
Published: 25 May 2021 in International Journal of Refrigeration
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Microchannel membrane-based absorber plays a vital role in energy-efficient and highly-compact absorption cooling systems. In this study, the absorption performance of a microchannel membrane-based absorber is investigated based on a three-dimensional CFD model. The effect of the solution channel width on the absorption performance is analyzed firstly. Results show that the channel width has an insignificant impact on the heat/mass transfer behavior, however, the pressure drop increases by 16.6% with decreasing the solution channel width from 1.8 mm to 1.0 mm. To improve the absorption performance while reducing the flow resistance, three groove structures (trilateral groove, quadrilateral groove, and circle groove) with a certain inclination angle on the bottom of the solution channel are discussed. Results illustrate that the swirling solution interrupts the boundary layer at the membrane-solution interface, facilitating mixing between the diluted solution from the interface and the concentrated solution from the channel bottom, and thus the heat/mass transfer is enhanced, especially at a high solution velocity. The best-performing structure is circle groove, which reduces the solution pressure drop by 13.17% and improves the absorption rate by 0.57%. Comparisons among different channel thicknesses indicates that both the groove structure and thinner channel can improve the absorption rate, but the groove structure improves more and its pressure drop is lower. The results demonstrate that the groove structure is promising to improve the hydraulic and absorption performances of microchannel membrane-based absorbers.

ACS Style

Zengguang Sui; Chong Zhai; Wei Wu. Swirling flow for performance improvement of a microchannel membrane-based absorber with discrete inclined grooves. International Journal of Refrigeration 2021, 130, 382 -391.

AMA Style

Zengguang Sui, Chong Zhai, Wei Wu. Swirling flow for performance improvement of a microchannel membrane-based absorber with discrete inclined grooves. International Journal of Refrigeration. 2021; 130 ():382-391.

Chicago/Turabian Style

Zengguang Sui; Chong Zhai; Wei Wu. 2021. "Swirling flow for performance improvement of a microchannel membrane-based absorber with discrete inclined grooves." International Journal of Refrigeration 130, no. : 382-391.

Journal article
Published: 02 May 2021 in Energy Conversion and Management
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Microchannel membrane-based absorption heat pump (MMAHP) shows great potential in efficiency enhancement and size reduction towards renewable/waste energy utilization. Ionic liquid (IL) mixtures with H2O as refrigerant present excellent merits in avoiding crystallization and obtaining a high coefficient of performance (COP). This study investigates the feasibility of MMAHP using five H2O/ILs (i.e., [BMIM][BF4], [DMIM][DMP], [EMIM][DMP], [EMIM][OAc], and [EMIM][OMs]) under various working conditions, with H2O/LiBr as the baseline. However, MMAHP and IL mixtures are constrained by the high solution pressure drop due to narrow flow channels and high IL viscosities; therefore, the geometries are optimized to maximize efficiency and compactness. The results show that under a desired cooling capacity of 0.5 kW, [EMIM][OAc] and [EMIM][OMs] can yield higher COPs than other ILs and LiBr. The smallest system volume of each working pair increases with the targeted COP to satisfy the requirement of membrane liquid entry pressure (LEP). Nevertheless, [DMIM][DMP], [EMIM][DMP], and LiBr cannot achieve COPs above 0.70, 0.75, and 0.80, respectively, within the involved geometries range due to the limitation of LEP (80 kPa). Comparing the Pareto frontiers that show the trade-off between the COP and compactness, LiBr is the best choice with targeted COPs below 0.778, while [EMIM][OAc], [EMIM][OMs], and [BMIM][BF4] perform better with targeted COPs above 0.778. Meanwhile, although considering the constraint of LEP would make the Pareto frontier moving towards lower COP and smaller compactness, the MMAHP still outperforms the conventional falling-film systems in efficiency and compactness when the targeted COP is below 0.80.

ACS Style

Chong Zhai; Yunren Sui; Zengguang Sui; Wei Wu. Ionic liquids for microchannel membrane-based absorption heat pumps: Performance comparison and geometry optimization. Energy Conversion and Management 2021, 239, 114213 .

AMA Style

Chong Zhai, Yunren Sui, Zengguang Sui, Wei Wu. Ionic liquids for microchannel membrane-based absorption heat pumps: Performance comparison and geometry optimization. Energy Conversion and Management. 2021; 239 ():114213.

Chicago/Turabian Style

Chong Zhai; Yunren Sui; Zengguang Sui; Wei Wu. 2021. "Ionic liquids for microchannel membrane-based absorption heat pumps: Performance comparison and geometry optimization." Energy Conversion and Management 239, no. : 114213.

Journal article
Published: 28 April 2021 in Energy
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Absorption refrigeration system (ARS) is significant for renewable/waste energy utilization to mitigate global warming. To select the best-performing ARS, four systems, namely falling-film horizontal tube ARS, falling-film vertical tube ARS, plate heat exchanger ARS, and microchannel membrane-based ARS, are compared systematically. System models have been established with validated accuracies to evaluate the coefficient of performance (COP) and volumetric cooling effect (qv). The results show that under a design driving power of 0.5 kW, the COP increases with the tube outer diameter/length in falling-film ARS and channel width in plate heat exchanger ARS and microchannel membrane-based ARS. qv decreases with tube outer diameter/length in falling-film ARS and channel width/height in plate heat exchanger ARS and microchannel membrane-based ARS. With geometry optimization, microchannel membrane-based ARS provides the highest COP of 0.855 with qv = 385 kW/m3, followed by plate heat exchanger ARS of 0.846, falling-film horizontal tube ARS of 0.832, and falling-film vertical tube ARS of 0.801. Meanwhile, microchannel membrane-based ARS also produces the maximum qv of 1147 kW/m3 with COP = 0.840, followed by plate heat exchanger ARS of 714 kW/m3, falling-film horizontal tube ARS of 391 kW/m3, and falling-film vertical tube ARS of 197 kW/m3. Thus, microchannel membrane-based ARS is advantageous in both efficiency and compactness among four ARSs. This work aims to facilitate absorbers/desorbers structure design towards compact and efficient ARS.

ACS Style

Chong Zhai; Wei Wu. Performance optimization and comparison towards compact and efficient absorption refrigeration system with conventional and emerging absorbers/desorbers. Energy 2021, 229, 120669 .

AMA Style

Chong Zhai, Wei Wu. Performance optimization and comparison towards compact and efficient absorption refrigeration system with conventional and emerging absorbers/desorbers. Energy. 2021; 229 ():120669.

Chicago/Turabian Style

Chong Zhai; Wei Wu. 2021. "Performance optimization and comparison towards compact and efficient absorption refrigeration system with conventional and emerging absorbers/desorbers." Energy 229, no. : 120669.

Review article
Published: 18 April 2021 in Renewable and Sustainable Energy Reviews
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Thermal energy storage (TES) technology is playing an increasingly important role in addressing the energy crisis and environmental problems. Various TES technologies, including sensible-heat TES, latent-heat TES, and thermochemical TES, have been intensively investigated in terms of principles, materials, and applications. A bibliometric study between 2000 and 2019 is conducted to show the evolution of TES technology and to predict future trends. While the existing studies are focused on basic TES, advanced/hybrid TES technologies have attracted increasing interest and demonstrated outstanding merits in overcoming the disadvantages of basic TES. To promote the advanced/hybrid TES technologies, a review is conducted to summarize the progress in advanced storage cycles, hybrid storage materials, and hybrid storage systems. The comprehensive literature review indicated that latent-heat TES has been the focus in the past years. While thermochemical TES and its hybrid TES technologies show the greatest research potential and become an emerging hot topic. Each advanced/hybrid TES technology has a certain improvement over basic TES, such as increasing the energy storage density or energy storage efficiency, reducing the charging temperature, enhancing the thermal conductivity of the sorbents, stabilizing the discharging temperature, or improving the performance of the integrated systems. Apart from the published results, some potential advanced/hybrid TES technologies are put forward to further enrich the TES family and to improve the TES performance. This work aims to facilitate the advancement of advanced/hybrid TES technologies.

ACS Style

Zhixiong Ding; Wei Wu; Michael Leung. Advanced/hybrid thermal energy storage technology: material, cycle, system and perspective. Renewable and Sustainable Energy Reviews 2021, 145, 111088 .

AMA Style

Zhixiong Ding, Wei Wu, Michael Leung. Advanced/hybrid thermal energy storage technology: material, cycle, system and perspective. Renewable and Sustainable Energy Reviews. 2021; 145 ():111088.

Chicago/Turabian Style

Zhixiong Ding; Wei Wu; Michael Leung. 2021. "Advanced/hybrid thermal energy storage technology: material, cycle, system and perspective." Renewable and Sustainable Energy Reviews 145, no. : 111088.

Journal article
Published: 14 March 2021 in Energy Conversion and Management
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Distributed energy systems based on proton exchange membrane fuel cell (PEMFC) pave a promising technological pathway to achieve carbon neutrality. The waste heat from PEMFC can be well recovered for cooling/heating, but additional systems are required if the available waste heat is not enough to meet the building cooling/heating demands. To achieve a dynamic supply–demand match efficiently and flexibly, this study proposes a novel PEMFC distributed energy system integrated with a hybrid-energy heat pump (HEHP). It can flexibly switch between a single absorption heat pump under sufficient waste heat and a hybrid absorption-compression heat pump under deficient waste heat. The integrated PEMFC-HEHP system is characterized and optimized using a validated model under different working conditions and hybrid configurations. The equivalent power density and fuel cell efficiency can be improved by 18.5% and 7.9% with supply temperatures of −10–10 °C for cooling, and be improved by 54.8% and 20.3% with supply temperatures of 40–60 °C for heating. Optimization yields a maximum equivalent power density of 4.341 kW/m2 in cooling mode and 5.357 kW/m2 in heating mode, compared to 3.290 kW/m2 of the individual PEMFC system. A higher absorption fraction enhances the PEMFC equivalent power while a higher compression fraction improves the HEHP cooling/heating capacity, the hybrid configuration needs to be determined by comprehensively considering the required cooling/heating loads and the available waste heat. The results can facilitate the better development of PEMFC distributed energy systems.

ACS Style

Wei Wu; Chong Zhai; Yunren Sui; Houcheng Zhang. A novel distributed energy system using high-temperature proton exchange membrane fuel cell integrated with hybrid-energy heat pump. Energy Conversion and Management 2021, 235, 113990 .

AMA Style

Wei Wu, Chong Zhai, Yunren Sui, Houcheng Zhang. A novel distributed energy system using high-temperature proton exchange membrane fuel cell integrated with hybrid-energy heat pump. Energy Conversion and Management. 2021; 235 ():113990.

Chicago/Turabian Style

Wei Wu; Chong Zhai; Yunren Sui; Houcheng Zhang. 2021. "A novel distributed energy system using high-temperature proton exchange membrane fuel cell integrated with hybrid-energy heat pump." Energy Conversion and Management 235, no. : 113990.

Review article
Published: 05 March 2021 in Renewable and Sustainable Energy Reviews
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Advancements in residential net-zero energy buildings (NZEBs) could significantly reduce energy consumption and greenhouse gas emissions. NZEB design considerations broadly categorize into energy infrastructure connections, renewable energy sources, and energy-efficiency measures. There is a lack of systematic literature review focused on recent progress in residential NZEBs. Therefore, this work provides an overview of each category including recent developments (last ≈ 10 years), aiming to provide references and support of wider and more successful implementation of residential NZEBs throughout the globe. The discussed energy infrastructure connections include electrical grids, district heating/cooling networks, and energy storage options including vehicle-to-home and hydrogen storage. Renewable energy sources considered here are solar photovoltaic and solar thermal, wind, and biomass including micro combined heat and power (CHP) systems. The final category detailed is energy-efficiency measures, which include improved building envelope designs, efficient HVAC systems, efficient domestic hot water systems, and phase change material integration. Within these categories there are many technology options, which makes selecting the ‘best’ configuration more difficult but allows design flexibility to adapt to local climates and other considerations (i.e. building codes, energy resources, costs). This paper provides references and highlights technology options to achieve residential NZEBs throughout the world.

ACS Style

Wei Wu; Harrison M. Skye. Residential net-zero energy buildings: Review and perspective. Renewable and Sustainable Energy Reviews 2021, 142, 110859 .

AMA Style

Wei Wu, Harrison M. Skye. Residential net-zero energy buildings: Review and perspective. Renewable and Sustainable Energy Reviews. 2021; 142 ():110859.

Chicago/Turabian Style

Wei Wu; Harrison M. Skye. 2021. "Residential net-zero energy buildings: Review and perspective." Renewable and Sustainable Energy Reviews 142, no. : 110859.

Journal article
Published: 04 February 2021 in International Journal of Refrigeration
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Absorption heat pump (AHP) is significant for renewable/waste energy utilization to alleviate the energy crisis. Compact and energy-efficient AHP is essential for wider applications, and absorbers and desorbers play critical roles. To select the best-performing components for this purpose, heat and mass transfer characteristics are compared for different kinds of absorbers/desorbers, including microchannel membrane-based heat exchanger, plate heat exchanger, falling-film horizontal tube, and falling-film vertical tube. Heat and mass transfer models for each absorber/desorber have been established with validated accuracies. A wide range of geometrical parameters of absorbers/desorbers have been considered to obtain the variations of heat and mass transfer rates of each kind of absorber/desorber. Results indicate that the heat transfer coefficient and sorption rate (absorption/desorption) show a decreasing trend as the volume increases for all absorbers/desorbers. The microchannel membrane-based structure yields overwhelming heat and mass transfer performance in both absorbers and desorbers. The maximum heat transfer coefficient and sorption rate are up to 8.6 kW m−2 K−1 and 0.0150 kg m−2 s−1 for the microchannel membrane-based absorber and 4.2 kW m−2 K−1 and 0.0225 kg m−2 s−1 for the microchannel membrane-based desorber. This work provides a thorough comparison among various absorbers and desorbers and identifies their heat/mass transfer rate domains, facilitating structural design towards compact and efficient AHPs.

ACS Style

Chong Zhai; Wei Wu. Heat and mass transfer performance comparison of various absorbers/desorbers towards compact and efficient absorption heat pumps. International Journal of Refrigeration 2021, 127, 203 -220.

AMA Style

Chong Zhai, Wei Wu. Heat and mass transfer performance comparison of various absorbers/desorbers towards compact and efficient absorption heat pumps. International Journal of Refrigeration. 2021; 127 ():203-220.

Chicago/Turabian Style

Chong Zhai; Wei Wu. 2021. "Heat and mass transfer performance comparison of various absorbers/desorbers towards compact and efficient absorption heat pumps." International Journal of Refrigeration 127, no. : 203-220.

Journal article
Published: 22 January 2021 in International Journal of Heat and Mass Transfer
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Solar absorption cooling is a promising approach to address the energy and environmental issues. Large-size and low-efficiency are two big challenges for wider applications of solar absorption cooling, while membrane-based microchannel absorbers are potential for effective enhancement in compactness and efficiency. To investigate the local heat and mass transfer characteristics of membrane-based microchannel absorbers, a two-dimension coupling model is developed and validated. The influences of key parameters and entrance effect on the absorption rate are analyzed in detail. Results show that the average absorption rate increases by 40% as the solution channel thickness decreases from 1 mm to 0.15 mm, but the pressure drop in the solution channel increases nearly exponentially. Meanwhile, as solution inlet velocity increases from 0.002 m/s to 0.02 m/s, the average absorption rate increases by 56%. Considering both pressure drop and absorption rate, the recommended solution inlet velocity and channel thickness are approximately 0.004 m/s and 0.5 mm, respectively. In addition, an enhancement strategy (channel baffles) is proposed to improve the performance of microchannel absorbers. The average absorption rate of the baffled module increases to 0.0053 kg/(m2•s), which is 20% higher than that without baffles. As for the compactness, the volumetric cooling capacity with baffles is 230% higher than that of a conventional falling-film absorber. Given a similar volumetric cooling capacity, the pressure drop of the baffled absorber is approximately 50% lower as compared to the non-baffle absorber. This study can facilitate the in-depth analysis and optimal design of membrane-based microchannel absorbers towards compact solar absorption cooling systems.

ACS Style

Zengguang Sui; Wei Wu; Tian You; Zhanying Zheng; Michael Leung. Performance investigation and enhancement of membrane-contactor microchannel absorber towards compact absorption cooling. International Journal of Heat and Mass Transfer 2021, 169, 120978 .

AMA Style

Zengguang Sui, Wei Wu, Tian You, Zhanying Zheng, Michael Leung. Performance investigation and enhancement of membrane-contactor microchannel absorber towards compact absorption cooling. International Journal of Heat and Mass Transfer. 2021; 169 ():120978.

Chicago/Turabian Style

Zengguang Sui; Wei Wu; Tian You; Zhanying Zheng; Michael Leung. 2021. "Performance investigation and enhancement of membrane-contactor microchannel absorber towards compact absorption cooling." International Journal of Heat and Mass Transfer 169, no. : 120978.

Journal article
Published: 08 January 2021 in Applied Thermal Engineering
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H2O/LiBr absorption refrigeration is promising in renewable energy utilization but suffers from large size and crystallization. Therefore, geometry optimization of plate heat exchanger (PHE) absorbers plays a vital role in increasing the compactness whilst maintaining a high efficiency. It is the first time that an absorption refrigeration cycle using a PHE absorber with H2O/[DMIM][DMP] was modeled considering solution pressure drop and heat transfer, taking the conventional H2O/LiBr as a baseline. Results showed that solution pressure drop in absorber had a more pronounced effect on the cycle performance than that in desorber. Parametric studies indicated that the required absorber length, overall heat transfer coefficient, and pressure drop all decreased with the increase of the three PHE geometries (channel width, channel height, and plate number), which caused the absorber volume and cycle coefficient of performance (COP) to increase accordingly. H2O/[DMIM][DMP] could perform comparably to H2O/LiBr with an absorber temperature difference below 3 °C and an absorber volume over 0.00015 m3. The Pareto diagram of H2O/[DMIM][DMP] under an absorber temperature difference of 5 °C showed that the cycle COP increased from 0.700 to 0.724 with the absorber volume being increased by 16.6%, but increasing the COP from 0.724 to 0.748 required a volume increase of 71.46%. Meanwhile, the Pareto frontier formed by optimal designs presented the trade-off between absorber compactness and cycle efficiency, which needs careful balance at the design stage. To achieve a targeted COP of 0.748 on the Pareto frontier, the optimal geometry set had the smallest volume of 0.001 m3, with 0.05 m width, 0.01 m height, 0.0816 m length, and 24 plates. This study was expected to provide guidance for geometries optimization of PHE absorbers by balancing the absorber volume and cycle COP towards compact and efficient absorption refrigeration.

ACS Style

Chong Zhai; Zengguang Sui; Wei Wu. Geometry optimization of plate heat exchangers as absorbers in compact absorption refrigeration systems using H2O/ionic liquids. Applied Thermal Engineering 2021, 186, 116554 .

AMA Style

Chong Zhai, Zengguang Sui, Wei Wu. Geometry optimization of plate heat exchangers as absorbers in compact absorption refrigeration systems using H2O/ionic liquids. Applied Thermal Engineering. 2021; 186 ():116554.

Chicago/Turabian Style

Chong Zhai; Zengguang Sui; Wei Wu. 2021. "Geometry optimization of plate heat exchangers as absorbers in compact absorption refrigeration systems using H2O/ionic liquids." Applied Thermal Engineering 186, no. : 116554.

Journal article
Published: 04 December 2020 in Energy Conversion and Management
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The basic absorption thermal energy storage cycle suffers from low energy storage efficiency and density, while the conventional H2O/salt working fluids risk crystallization problems. To achieve crystallization-free thermal batteries with improved energy storage performance, a hybrid compression-assisted absorption thermal energy storage cycle using H2O/1,3-dimethylimidazolium dimethylphosphate is proposed. The fluid property is modeled using non-random two liquid equations and the hybrid cycle is modeled using heat/mass transfer/conservation equations; both models are validated against measurement data with good accuracies. The effect of pressure ratio on the transient behaviors and energy storage density/efficiency is investigated to characterize the performance enhancement. Besides, different hybrid cycles are compared with the basic cycle to identify the best-performing thermal battery. Results show that, with both generation and absorption enhancement, the concentration difference is significantly enlarged from 0.230 to 0.480 by the hybrid cycle with a pressure ratio of 2.0. Meanwhile, with energy storage efficiencies maintained at 0.715–0.794, the energy storage density is effectively increased from 110.3 to 199.2 kWh/m3. Through valve switching, three hybrid cycles are realized, i.e., hybrid cycle with charging/discharging compression, hybrid cycle with charging compression, and hybrid cycle with discharging compression. Comparisons indicate that the hybrid cycle with discharging compression is the most energy-efficient, with the highest energy storage efficiency of 0.816, compared to 0.794 of the hybrid cycle with charging/discharging compression and 0.790 of the hybrid cycle with charging compression. In terms of energy storage density, the hybrid cycle with discharging compression performs quite close to the hybrid cycle with charging/discharging compression, only slightly lower by 1.6–4.3%. However, compared to the hybrid cycle with charging compression, the energy storage density of the hybrid cycle with discharging compression is significantly enhanced by 11.4–52.0%. In summary, the hybrid cycle with discharging compression is the best cycle comprehensively considering the energy storage efficiency and density. This study aims to provide theoretical supports and suggestions for the development of advanced thermal battery cycles.

ACS Style

Wei Wu; Zhixiong Ding; Yunren Sui; Michael Leung. Comparative dynamic performance of hybrid absorption thermal batteries using H2O/1,3-dimethylimidazolium dimethylphosphate. Energy Conversion and Management 2020, 228, 113690 .

AMA Style

Wei Wu, Zhixiong Ding, Yunren Sui, Michael Leung. Comparative dynamic performance of hybrid absorption thermal batteries using H2O/1,3-dimethylimidazolium dimethylphosphate. Energy Conversion and Management. 2020; 228 ():113690.

Chicago/Turabian Style

Wei Wu; Zhixiong Ding; Yunren Sui; Michael Leung. 2020. "Comparative dynamic performance of hybrid absorption thermal batteries using H2O/1,3-dimethylimidazolium dimethylphosphate." Energy Conversion and Management 228, no. : 113690.

Journal article
Published: 16 November 2020 in Applied Energy
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With worsening of global warming, environmental pollution, and energy crisis, the effective storage of renewable/waste energy has become a widely focused research topic. As an emerging thermal battery technology, absorption thermal energy storage aims to utilize low-grade energy for flexible applications (e.g., cooling, heating, dehumidification), which facilitates the matching between the energy supply and the energy demand. However, the current absorption thermal battery cycle suffers from high charging temperature, slow charging/discharging rate, low energy storage efficiency, or low energy storage density. To further improve the storage performance, a hybrid compression-assisted absorption thermal energy storage cycle is proposed in this work. Four thermal battery cycles, with/without compression in the charging/discharging processes, have been designed for comparisons. Dynamic characteristics and storage performance have been comparatively investigated by simulation using an experimentally validated model. Results show that the cycles with auxiliary compression can achieve a higher energy storage efficiency and density with a faster charging/discharging rate under a lower charging temperature. With a charging temperature of 80 °C, the energy storage efficiency and density are as high as 0.67 and 282.8 kWh/m3 for the proposed compression-assisted cycle, while they are only 0.58 and 104.8 kWh/m3 for the basic cycle. Moreover, the average charging and discharging rates of the compression-assisted cycle are 6.78 kW and 4.88 kW, respectively, which are also enhanced significantly compared to 1.88 kW and 1.27 kW of the basic cycle. This study could facilitate the development of absorption thermal battery with lower charging temperatures.

ACS Style

Zhixiong Ding; Wei Wu. A hybrid compression-assisted absorption thermal battery with high energy storage density/efficiency and low charging temperature. Applied Energy 2020, 282, 116068 .

AMA Style

Zhixiong Ding, Wei Wu. A hybrid compression-assisted absorption thermal battery with high energy storage density/efficiency and low charging temperature. Applied Energy. 2020; 282 ():116068.

Chicago/Turabian Style

Zhixiong Ding; Wei Wu. 2020. "A hybrid compression-assisted absorption thermal battery with high energy storage density/efficiency and low charging temperature." Applied Energy 282, no. : 116068.

Journal article
Published: 16 October 2020 in Energy Conversion and Management
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Distributed energy systems (DESs) are supposed to help alleviate energy crisis and environmental issues. However, the effects of operation strategy on their design and performance have not been fully understood yet. Based on the genetic algorithm, this research aims to conduct the design optimization and performance analysis of the DES in shopping malls, hotels, and office buildings under seven operation strategies, that is, following thermal/electric energy load (FTL and FEL), following hybrid load with no/maximal surplus energy, and with no thermal startup threshold (FHL-ns, FHL-ms, and FHL-nst), and following monthly/seasonal electric–thermal load ratio (FMLR and FSLR). Results show that: (1) The optimal capacities of power generation unit are 623-1 782 kW, while the optimal electric cooling ratios are 0.5-0.9. (2) The optimal primary energy saving ratio, exergy difference, equivalent CO2 and PM2.5 emission reduction ratios, total cost saving ratio, and operation and maintenance cost saving ratio are respectively up to 0.298, 0.114, 0.495, 0.912, -0.166, and 0.221. (3) The optimal strategy for the above commercial buildings is respectively FMLR, FMLR, and FHL-nst, and the comprehensive performance is negatively related to the coal-to-gas consumption ratio. Overall, results can offer a reference for selecting the optimal strategy for DESs in commercial buildings.

ACS Style

Qingmei Wen; Gang Liu; Wei Wu; Shengming Liao. Genetic algorithm-based operation strategy optimization and multi-criteria evaluation of distributed energy system for commercial buildings. Energy Conversion and Management 2020, 226, 113529 .

AMA Style

Qingmei Wen, Gang Liu, Wei Wu, Shengming Liao. Genetic algorithm-based operation strategy optimization and multi-criteria evaluation of distributed energy system for commercial buildings. Energy Conversion and Management. 2020; 226 ():113529.

Chicago/Turabian Style

Qingmei Wen; Gang Liu; Wei Wu; Shengming Liao. 2020. "Genetic algorithm-based operation strategy optimization and multi-criteria evaluation of distributed energy system for commercial buildings." Energy Conversion and Management 226, no. : 113529.

Journal article
Published: 08 October 2020 in Journal of Cleaner Production
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Distributed energy systems have been proved as an effective solution to energy depletion and environmental pollution. However, the decision making on the distributed energy system planning is difficult because multiple evaluative criteria and alternative systems are involved in the evaluation process. The study, therefore, is aimed at proposing a multicriteria comprehensive evaluation framework for the DES plan. First, the evaluation criteria system is established to offer a fundamental premise for the fair evaluation, based on numerous researches on DES evaluation. In the established criteria system, technology, economy, environment, and socio-politics are scrutinized. Second, the subjective weights, objective weights and the comprehensive weights of criteria are determined by the fuzzy analytic hierarchy process method, Shannon entropy method and a comprehensive method, respectively. The fuzzy analytic hierarchy process method combines the triangular fuzzy number and the analytic hierarchy process method. The triangular fuzzy number incorporates the decision-makers’ uncertainties during evaluation and thus improves the efficiency of decision making. The comprehensive method combines the single-objective optimization and Jaynes’ maximal entropy principle method. Third, the grey relational analysis method is used to achieve the alternatives’ ranking with and without preferences. The proposed framework is employed to evaluate various distributed energy systems for a cultural industrial park in Changsha of China. Results show that the system that integrates water source heat pump with the improved Combined Cooling, Heating & Power (WSHP + CCHP) is optimal in the integrated evaluation and the evaluations with technology priority and economy priority, with a grey relational degree of 0.7475, 0.7239, and 0.5764, respectively. The three systems that exploit renewable energy, i.e. the system that integrates ground source heat pump with the improved Combined Cooling, Heating & Power (GSHP + CCHP), the system that integrates water source heat pump with the improved Combined Cooling, Heating & Power (WSHP + CCHP) and the system that ground source heat pump with water source heat pump (GSHP + WSHP), have a better ranking than the other two systems in the evaluation with socio-political priority. The case study of the park demonstrates the proposed framework’s feasibility of multi-criteria comprehensive evaluation for industrial park-level distributed energy systems. The study hopes to provide some references for the multicriteria evaluation of distributed energy systems.

ACS Style

Qingmei Wen; Gang Liu; Wei Wu; Shengming Liao. Multicriteria comprehensive evaluation framework for industrial park-level distributed energy system considering weights uncertainties. Journal of Cleaner Production 2020, 282, 124530 .

AMA Style

Qingmei Wen, Gang Liu, Wei Wu, Shengming Liao. Multicriteria comprehensive evaluation framework for industrial park-level distributed energy system considering weights uncertainties. Journal of Cleaner Production. 2020; 282 ():124530.

Chicago/Turabian Style

Qingmei Wen; Gang Liu; Wei Wu; Shengming Liao. 2020. "Multicriteria comprehensive evaluation framework for industrial park-level distributed energy system considering weights uncertainties." Journal of Cleaner Production 282, no. : 124530.

Journal article
Published: 10 August 2020 in Energy Conversion and Management
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Solar cooling plays an important role in low-carbon societies. The existing solar cooling systems commonly suffer from low energy efficiency or low applicability. To solve this problem, a novel solar-powered flexible HEHP (hybrid-energy heat pump) is proposed. Firstly, thermodynamic and TRNSYS models are established with verified accuracies. Using the models, the steady-state HEHP performance is investigated under various operating modes and working parameters. In addition, transient characterization shows that the solar-based modes operate longer than the non-solar mode with RAHP (absorption ratio) ranging in 0.3–1.0. With enhanced cooling capacities, the HEHP mode generally operates at high cooling loads. The seasonal performance is compared between FPC (flat plate collector) and ETC (evacuated tube collector), concluding that the ETC-based HEHP performs much better than the FPC-based HEHP. The collector area greatly affects the system performance; as the collector area increases from 0 to 300 m2, the load fraction of the solar-based modes significantly increases from 0 to 0.69 for FPC and from 0 to 0.93 for ETC, the seasonal COP (coefficient of performance) respectively rises from 5.5 to 7.2 and from 5.5 to 8.9, while the energy saving ratio (ESR) respectively reaches 21.2% and 31.8%. As for the tank volume, it has a much smaller influence; with the specific tank volume increasing from 0.01 to 0.06 m3/m2, the ESR slightly varies from 15.1% to 17.3% for FPC and from 25.1% to 28.4% for ETC. This study aims to facilitate the development, design and operation of the solar HEHP cooling system.

ACS Style

Wei Wu; Michael Leung. Transient and seasonal performance evaluation of a novel flexible heat pump for solar cooling. Energy Conversion and Management 2020, 223, 113269 .

AMA Style

Wei Wu, Michael Leung. Transient and seasonal performance evaluation of a novel flexible heat pump for solar cooling. Energy Conversion and Management. 2020; 223 ():113269.

Chicago/Turabian Style

Wei Wu; Michael Leung. 2020. "Transient and seasonal performance evaluation of a novel flexible heat pump for solar cooling." Energy Conversion and Management 223, no. : 113269.

Journal article
Published: 06 June 2020 in Energy and Buildings
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The window with a roller blind inside can have huge impacts on solar heat gains and the air exchange between indoor and outdoor environments, but its potential value in terms of improving both building energy efficiency and indoor air quality is poorly understood. This study examines cooling energy consumption and indoor PM2.5 exposure concentrations from indoor and outdoor sources in Hong Kong dwellings under seven different window and roller blind control strategies. Dwellings with windows facing south were simulated for a selected week in April using EnergyPlus. The modelled control strategies (including conventional rule-based control and Genetic-Algorithm-based predictive control) accounted for thermal comfort and indoor and outdoor environmental conditions. A novel coupled-window-and-interior-roller-blind zone was developed in EnergyPlus and used to capture the impact of shading on ventilation performance. Results indicate that dwellings under the predictive window and roller blind control strategy used 55% less cooling energy than those with windows closed and roller blinds open, while meeting the World Health Organisation standard for PM2.5 exposure (25 µg/m3). Comparisons between control strategies also show that shading windows (which were open) in the early morning led to an increase in cooling energy consumption, whilst opening windows at night and shading them with roller blinds in the first half of the night offered the co-benefits of cooling through increased ventilation and the ability to reduce indoor PM2.5 exposure.

ACS Style

Xuyang Zhong; Zhiang Zhang; Wei Wu; Ian Ridley. Comprehensive evaluation of energy and indoor-PM2.5-exposure performance of residential window and roller blind control strategies. Energy and Buildings 2020, 223, 110206 .

AMA Style

Xuyang Zhong, Zhiang Zhang, Wei Wu, Ian Ridley. Comprehensive evaluation of energy and indoor-PM2.5-exposure performance of residential window and roller blind control strategies. Energy and Buildings. 2020; 223 ():110206.

Chicago/Turabian Style

Xuyang Zhong; Zhiang Zhang; Wei Wu; Ian Ridley. 2020. "Comprehensive evaluation of energy and indoor-PM2.5-exposure performance of residential window and roller blind control strategies." Energy and Buildings 223, no. : 110206.

Journal article
Published: 05 June 2020 in Solar Energy
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Recently there have been many reports on the application of phase change materials (PCM) in solar water heating system (SWHS). However, there are some doubts and different views in this field, since the sensitivity of the system to the PCM design parameters and external conditions. In this study, a typical SWHS with a PCM storage tank is proposed to overcome the existing problems. The mathematical model is established and validated against experimental data. Annual simulations have been done under a typical user load and different PCM design parameters (phase change temperature, PCM usage amount) among six cities which have different meteorological conditions. The results indicate that the application of PCM in SWHS can reduce the power consumption of electric auxiliary heater (EAH) and the heat loss, but the benefits vary widely by regions and PCM design parameters. The highest power saving rate is up to 31% in Lhasa which has extremely abundant solar energy, while for the heat loss saving rate the highest value is 9.9% in Guangzhou which is located in the hot summer and warm winter zone. Besides, a parametric study has been conducted to explore the power saving potentials with different PCM design parameters in different regions, as well as to discuss the optimal phase change temperature and the PCM usage amount. This study aims to provide theoretical references and suggestions for the design of the PCM storage tank in SWHS.

ACS Style

Zhixiong Ding; Wei Wu; Youming Chen; Yantong Li. Dynamic simulation and parametric study of solar water heating system with phase change materials in different climate zones. Solar Energy 2020, 205, 399 -408.

AMA Style

Zhixiong Ding, Wei Wu, Youming Chen, Yantong Li. Dynamic simulation and parametric study of solar water heating system with phase change materials in different climate zones. Solar Energy. 2020; 205 ():399-408.

Chicago/Turabian Style

Zhixiong Ding; Wei Wu; Youming Chen; Yantong Li. 2020. "Dynamic simulation and parametric study of solar water heating system with phase change materials in different climate zones." Solar Energy 205, no. : 399-408.

Journal article
Published: 01 May 2020 in Journal of Building Engineering
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The energy recovery ventilator (ERV) is a type of mechanical equipment that provides ventilation into the building while reducing the energy required to condition the ventilation air. A computer modelling approach was used to assess energy consumption (the energy used to run the air-conditioning and ERV) and indoor PM2.5 exposure concentrations from both indoor and outdoor sources in Hong Kong high-rise residential flats under various ERV control strategies. Three varying inputs, including ventilation mode (energy-exchange, bypass or a hybrid of both), filter efficiency and ventilation rate, were used to develop different ERV control strategies. The estimated energy consumption and indoor PM2.5 exposure concentration were monetised using per-occupant cost functions, in order to allow a direct combination between the two. Results show that the ERV switching between energy-exchange and bypass modes according to outdoor weather conditions saves 43% of the annual energy cost compared with that operating in bypass mode, while the ERV operating in energy-exchange mode saves 28%. Integrating an air filter with a high PM2.5 removal efficiency into the ERV causes a reduction in the annual exposure cost with increased ventilation. By combining energy and exposure costs, this study has been able to determine the most cost-effective ventilation rate of the ERV, which costs an occupant approximately HK$ 2692 over the course of a year. Varying the ventilation rate as a function of outdoor temperatures, outdoor humidity ratios or outdoor enthalpy further reduces the combined cost compared with operating the ERV at the most cost-effective ventilation rate.

ACS Style

Xuyang Zhong; Wei Wu; Ian A. Ridley. Assessing the energy and indoor-PM2.5-exposure impacts of control strategies for residential energy recovery ventilators. Journal of Building Engineering 2020, 29, 101137 .

AMA Style

Xuyang Zhong, Wei Wu, Ian A. Ridley. Assessing the energy and indoor-PM2.5-exposure impacts of control strategies for residential energy recovery ventilators. Journal of Building Engineering. 2020; 29 ():101137.

Chicago/Turabian Style

Xuyang Zhong; Wei Wu; Ian A. Ridley. 2020. "Assessing the energy and indoor-PM2.5-exposure impacts of control strategies for residential energy recovery ventilators." Journal of Building Engineering 29, no. : 101137.