Browsing by Author "Chen, Yuzhu"
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- Comparisons and optimization of two absorption chiller types by considering heat transfer area, exergy, and economy as single objective functions
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä(2024-01-06) Qiu, Yida; Wang, Jingkun; Han, Jing; Chen, Yuzhu; Wang, Jun; Lund, PeterAbsorption cooling technology is an environmentally friendly method to generate continuous chilled water making use of multiple thermal sources, such as waste heat and renewable thermal energy. In this study, two absorption chillers (nominal capacity of 400 kW) with series and parallel connections are evaluated. To research the ideal configuration of chillers after thermodynamic analysis, the structures of the chillers are optimized using the particle swarm optimization algorithm by considering the heat transfer area (HTA), exergy efficiency and total annual cost as single-objective functions. The impact of temperature differences between external and internal flows, heat exchanger efficiencies and the solution allocation ratio is estimated. The optimized HTA, coefficient of performance, exergy efficiency and total annual cost are 149.0 m2, 1.56, 29.44% and $229 119 for the series-connected chiller, and 146.7 m2, 1.59, 31.45% and $234 562 for the parallel-connected type, respectively. Under the lowest HTA condition, compared with the reference simulation results, the energy and exergy performances are improved, while the annual total cost is higher. The annual total cost is highest when maximizing the exergy efficiency, which is attributed to the increase in the HTA. The operating cost accounts for 27.42% (series type) and 26.54% (parallel type) when the annual cost is the lowest. - Energy, environmental-based cost, and solar share comparisons of a solar driven cooling and heating system with different types of building
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä(2022-07-05) Chen, Yuzhu; Hua, Huilian; Xu, Jinzhao; Wang, Jun; Lund, Peter D.; Han, Yifeng; Cheng, TanghuaTo reduce fossil fuel consumption and carbon emissions from building energy systems, a solar-based cooling and heating system is proposed here employing solar concentrating collectors, photovoltaics, double-effect absorption heat pump and thermal storage. The system is applied to five building types in a region with cold winter and hot summer. The system configuration is optimized using energy, environmental cost, and solar fraction as criteria. The results demonstrate that the solar system could produce at least 31.1% of the cooling/heating loads resulting in 73.3% and 64.2% energy and cost savings in a hospital. The coefficient of performance of the hybrid system ranges from 5.87 to 7.56 in cooling mode, and 1.22 to 1.65 for heating. The cost of devices is the most sensitive factor, and followed by the price of grid electricity. Increasing the renewable energy penetration rate could improve the energy performance, but decrease the cost saving ratio due to the lower carbon emissions. - Exergo-economic assessment and sensitivity analysis of a solar-driven combined cooling, heating and power system with organic Rankine cycle and absorption heat pump
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä(2021-09-01) Chen, Yuzhu; Xu, Jinzhao; Zhao, Dandan; Wang, Jun; Lund, Peter D.District energy systems based on renewable resources help to reduce greenhouse-gas emissions and fossil-fuel use. Here, a multi-generation energy system combining cooling, heating, and power is realized by employing organic Rankine cycle (ORC) and absorption heat pump (AHP) technologies, which enable cascading the utilization of solar heat. The AHP can operate steadily providing cooling, heating and hot water from solar thermal and geothermal sources. A modelling approach presented to evaluate the energy, exergy, economic, and exergo-economic performance of the above system. The results show that the AHP could reach a coefficient of performance (COP) between 1.38 and 2.37 depending on the mode of operation. The yearly energy and exergy efficiency of the tri-generation system is 56.5% and 9.6%, respectively. Compared to a separate system, the simple economic payback time of the tri-generation system is 3.5 years. The specific exergo-economic cost of electricity produced is 0.12 $/kWh, whereas the cost of hot water is much higher, or 0.31 $/kWh. The sensitivity analysis performed shows that the inlet and outlet temperatures of the AHP together with the yearly solar irradiance have the highest impact on the performance. This study provides a new direction on cost-effective utilization of renewable sources in district energy systems. - Exergo-environmental cost optimization of a combined cooling, heating and power system using the emergy concept and equivalent emissions as ecological boundary
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä(2021-10-15) Chen, Yuzhu; Xu, Jinzhao; Wang, Jun; Lund, Peter D.District energy systems, in particular combined cooling, heating and power (CCHP) systems, can provide cost-effective energy products, reduce fossil fuel consumption and emissions. A gas-fired CCHP system is considered here employing an internal combustion engine, whose exhaust gas is split between an organic Rankine cycle unit and absorption heat pump. A simulation model of the system is constructed and validated. An emergy based exergo-environmental cost method is proposed here to optimize the exhaust gas allocation ratio based on specific working conditions considering the equivalent emissions of the whole life-cycle chain from an ecological view. The emergy consumption in each process stage is evaluated accompanied with a sensitivity analysis. The results show that the minimize specific system cost is 310050 seJ/J when 63% of the exhaust gas flows to the ORC-unit. The ICE and AHP are responsible for >98% of the emergy consumption, dominated by the operation of the system. The sensitivity analysis shows that heating is the most sensitive of all products against key parameter variations, while electricity is the least. Increasing the service period and the operating hours, the economic performance could be improved, while the transformity of emission enlarges the cost. - Exergo-environmental cost optimization of a solar-based cooling and heating system considering equivalent emissions of life-cycle chain
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä(2022-04-15) Chen, Yuzhu; Li, Xiuxiu; Hua, Huilian; Lund, Peter D.; Wang, JunSolar-driven energy systems can effectively reduce the fossil fuel use and pollutant emissions in the built environment. A solar-based cooling and heating (SCH) system employing solar thermal collectors, photovoltaics, a double-effect absorption heat pump, and an electric boiler/chiller is proposed to meet the energy demand of a community. The system and its components are optimized by simultaneously minimizing the specific exergo-environmental cooling/heating costs over the life-cycle and maximizing the cost saving ratio. Compared to the conventional exergo-economic and exergo-environmental optimization methods without life-cycle equivalent emissions, the results show that the system resulting from the proposed method has higher specific costs, or, 1.10 $/kWh for cooling and 2.77 $/kWh for heating, and the corresponding cost saving ratio is >0.02%-unit lower. The coefficient of performance of the hybrid system in the cooling and heating modes are 4.5 and 1.04, respectively. The specific heating cost shows the highest sensitivity against parameter changes. Increasing the capacity of the heat storage and price of grid power would increase the cost saving benefit, while increasing other parameters such as investment cost would decrease the saving ratio. - Integrated performance analysis of a space heating system assisted by photovoltaic/thermal collectors and ground source heat pump for hotel and office building types
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä(2021-05) Chen, Yuzhu; Hua, Huilian; Wang, Jun; Lund, Peter D.Utilizing the solar and geothermal energy in space heating systems can save fossil fuels and reduce emissions. In this study, compound parabolic concentrator (CPC) photovoltaic/thermal collectors (PV/T), absorption and ground source heat pumps are integrated to a novel space heating system for use in hotel and office buildings. A full simulation model of the system is constructed and validated by using the Engineering Equation Solver software. The heating system is analyzed against its energy, environmental, economic, and flexibility performance using multiple variables including the PV coverage ratio, ambient temperature, solar beam irradiance, and electricity price. The results show that integrating solar and geothermal energy can be effective to supplement the space heating demand. The best integrated performance point of the office building gives a 32% energy saving ratio, 9% annual cost savings ratio, 23% emission reduction ratio, and 67% excess adjustable ratio compared to a pure ground heat pump system. This research provides new directions of integrated performance evaluation of heating systems assisted by renewable energy resources. - Multi-objective optimization of a solar-driven trigeneration system considering power-to-heat storage and carbon tax
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä(2022-07-01) Chen, Yuzhu; Hu, Xiaojian; Xu, Wentao; Xu, Qiliang; Wang, Jun; Lund, Peter D.Advanced solar driven tri-generation systems are highly relevant to reduce emissions and increase energy security. Here, solar collectors and photovoltaics are coupled to a tri-generation system to produce multiple final energy forms simultaneously for an office building. The excess solar electricity is employed for cooling/heating through a power-to-heat conversion employing thermal energy storage. Comprehensive optimization is performed to maximize the energy, environmental, and economic benefits, and the carbon tax is included to monetize the emissions. A coupled decision-making method is then used to choose the ideal scheme from the optimized sets of system configuration accompanied with a sensitivity analysis against key parameters. Compared to the conventional system, the proposed system improves the energy performance by 41.4% and the environmental benefits by 41.7% with the highest solar energy utilization rate. The economic performance improves in the best case by 14.4% only, but with the lowest utilization rate of solar energy. The ideal solution covers 30%, 54%, and 62% of the electricity, cooling, and heating loads, respectively, and the corresponded energy, environmental, and economic performance improves by 29.1%, 34.6%, and 7.7%, respectively. The sensitivity analysis shows that the economic performance is more sensitive to the electricity price than to the carbon tax. - Multi-objective optimization of an integrated energy system against energy, supply-demand matching and exergo-environmental cost over the whole life-cycle
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä(2022-02-15) Chen, Yuzhu; Xu, Zhicheng; Wang, Jun; Lund, Peter D.; Han, Yifeng; Cheng, TanghuaAn integrated energy system (IES) can yield several benefits in energy, environmental impacts, cost, and flexibility over a separate system, although the initial cost may be higher. An IES using gas turbine, solar photovoltaics (PV), heat pumps, electrical cooling, and energy storage units is proposed here to satisfy the electricity, cooling, and heating demands of a residential building. A multi-objective optimization approach is used to find the best solutions considering energy, supply-demand matching and exergo-environmental economic indices with life cycle assessment (LCA) in following electric mode. The maximum benefit from the IES studied is reached with a system yielding 53.08% for energy savings, 99.88% matching, and 43.50% cost savings. The ideal scheme selected by the TOPSIS method has a higher annual total cost than the cost with conventional method, but has a better cost saving ratio, 41.81%. A sensitivity analysis shows that a higher PV use would decrease the fuel consumption, but it would reduce the matching and economic performance. Similar to the effect of natural gas price, the off-grid electricity price has higher impact on the cost saving ratio, but lower influence on the specific exergo-environmental cost. - Optimization of a weather-based energy system for high cooling and low heating conditions using different types of water-cooled chiller
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä(2022-08-01) Chen, Yuzhu; Xu, Jinzhao; Wang, Jun; Lund, Peter D.A weather-based energy system consisting of a tri-generation unit, photovoltaics, and water-cooled chiller is proposed here for improving the energy and environmental performance. Together with capacities of other devices, the cooling ratio, and capacities of different types of chiller are optimized to find the ideal system configuration setting the energy and cost savings and renewable energy use as the objectives. In addition, daily and monthly operating modes with the optimal system composition are analyzed followed by a sensitivity analysis. The results show that increasing energy saving ratio improves the cost saving benefits, but the renewable energy penetration rate would decrease due to lower grid electricity consumption. The ideal system configuration in ratio optimization process has a higher gas turbine and chiller capacity resulting in higher performance than the system with capacity optimization process, or 2.7%, 1.6%, and 0.2%-unit higher of the considered indices. When increasing the specific cost of the chiller, cost saving ratio of the ideal system decreases, while the impacts on the energy savings and renewable energy use are lower. The analysis indicates that the price of grid electricity is the most sensitive factor, while the influence of carbon cost is slight. - Thermo-ecological cost optimization of a solar thermal and photovoltaic integrated energy system considering energy level
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä(2022-09) Xu, Jinzhao; Wang, Jun; Chen, Yuzhu; Xu, Zhicheng; Lund, Peter D.Integrating renewable resources into traditional tri-generation systems helps to reduce fossil fuel use and emissions. A solar thermal and photovoltaic assisted integrated energy system is proposed here using high- performance cooling approaches to provide cooling, heating, and electricity. To find the best system configurations with a focus on the ecological performance, the specific thermo-ecological cost of the energy products considering energy level is optimized employing the cumulative exergy consumption over the whole life-cycle. The results show that the ideal specific costs for cooling, heating and electricity demands are 8.699, 7.129, and 1.970 J/J, respectively. Compared to the method without the energy level consideration, the specific cost of the hybrid system is 0.47 J/J higher due to the lower energy level of water products. Moreover, the specific thermo-ecological cost of natural gas has higher impacts on the performance of hybrid system than the other parameters. - Thermodynamic and Economic Performance Assessment of Double-Effect Absorption Chiller Systems with Series and Parallel Connections
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä(2022-12) Hu, Jianke; Teng, Kai; Chen, Yuzhu; Wang, Jun; Lund, PeterAbsorption cooling technologies converting excess heat and renewable heat resources to cooling energy have shown progress in recent years. In this study, two 400 kW LiBr solution absorption chiller types with series and parallel connected are analyzed over a range of parameter values to better understand their applicability for different uses. Thermodynamic models for the components were constructed and validated. The performance of the chillers related to heat transfer, energy, exergy, and economy performance was comprehensively analyzed. The operating performance was investigated by considering the external variables, including inlet cooling water, chilled water, and inlet steam temperatures and the solution allocation ratio. The results indicate that the parallel connected chiller reaches higher energy and exergy performance than the series-connected chiller, but the heat transfer and economic performance was lower. The coefficient of performance and the exergy efficiency of the parallel chiller were for the reference system 1.30 and 24.42%, respectively. Except for the exergy efficiency, the inlet steam and inlet chilled water temperature had positive impact on the heat transfer, energy, and economic performance, while the inlet cooling water temperature trends the opposite. The sensitivity analysis on solution allocation ratio showed that a higher ratio decreases the heat transfer and economic performance, but considering the energy and exergy performance, a suitable allocation ratio would be 0.54. - Thermodynamic performance analysis and modified thermo-ecological cost optimization of a hybrid district heating system considering energy levels
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä(2021-06-01) Chen, Yuzhu; Hua, Huilian; Wang, Jun; Lund, Peter D.Utilization of the renewable resources in district heating systems can reduce the use of fossil fuels, operating costs and protect the environment. In this study, an integrated hybrid system consisting of concentrating photovoltaic/thermal collectors (PV/T), geothermal (GSHP) and absorption (AHP) heat pumps is considered for district heating. The thermodynamic performance of the system at various conditions is explored through detailed simulations. A modified thermo-ecological cost (TEC) method is used to optimize the structure of the PV/T by considering contributions of different flows. The results show that a higher solar irradiance level and a higher PV coverage ratio have a positive impact on the thermal performance of the hybrid system. The TEC-based optimization shows that a 66% PV coverage ratio of PV/T yields a minimum thermo-ecological heating cost of 6.86 J/J, which is slightly lower than cost with a conventional method. Based on the sensitivity analysis, other key parameters except the operating time and the PV coverage ratio have a negative influence on the economic performance of the district heating system, because of the increasing cumulative exergy consumption of the GSHP or PV/T.