| 1. |
- Rong, X., et al.
(author)
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Experimental study on a multi-evaporator mutual defrosting system for air source heat pumps
- 2023
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In: Applied Energy. - : Elsevier Ltd. - 0306-2619 .- 1872-9118. ; 332
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Journal article (peer-reviewed)abstract
- Air source heat pumps (ASHPs) are prone to frost when heating in a low-temperature and high-humidity environment, which deteriorates the heating performance of the unit. In this study, a new multi-evaporator mutual defrosting (MEMD) system was proposed to overcome the disadvantages of traditional defrosting methods: intermittent heating and inefficient defrosting. To validate the performance of the proposed defrosting technology, comparative tests were conducted in various outdoor environmental conditions. The experimental results showed that the MEMD system could continuously heat water during the defrosting period. In five experimental conditions, the MEMD system exhibited a lower water temperature drop range (2.1–2.8 °C) than that of a traditional reverse-cycle defrosting (RCD) system (6.0–7.3 °C). Due to the effective utilization of heat production during the heating period, the effective heat power (qe) of the unit increased by 0.7–1.4 kW, and the heat loss coefficient (HLC) of frosting and defrosting increased by an average of 6 % in the five experimental conditions, effectively reducing the heating capacity loss of the unit caused by defrosting. While defrosting, the MEMD system was able to utilize the remaining evaporators to absorb heat from the air and then deliver it to the defrosting evaporator. The equivalent defrosting energy efficiency (COPd) of the MEMD system was 17.5 % greater than that of the RCD system on average. During the heating and defrosting cycle, the energy saved when defrosting could increase the cycle coefficient of performance (CCOP) of heating by 3.7 %.
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| 2. |
- Liu, L., et al.
(author)
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Forecasting Power Output of Photovoltaic System Using A BP Network Method
- 2017
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In: Energy Procedia. - : Elsevier Ltd. - 1876-6102. ; 142, s. 780-786
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Journal article (peer-reviewed)abstract
- The characteristics of intermittent and stochastic of solar energy has brought great challenges to power grid system in terms of operation and regulation. Power forecasting is an important factor for optimal schedule of power grid system and assessing the working performance of PV systems. In order to forecast the power output of a PV system located in Ashland at 24-hour-ahead for higher efficiency, a back propagation (BP) neural network model is proposed. Before designing the model, correlation analysis is done to investigate the relationship between power output and solar irradiance and ambient temperature, which are key parameters affecting the power output of PV systems. Based on a correlation analysis, the model admitted the following input parameters: hourly solar radiation intensity, the highest, the lowest daily and the average daily temperature, and hourly power output of the PV system. The output of the model is the forecasted PV power output 24 hours ahead. Based on the datasets, the neural network is trained to improve its accuracy. The best performance is obtained with the BP neural network structure of 28-20-11. The analysis of the error indicator MAPE shows that the proposed model has great accuracy and efficiency for forecasting the power output of photovoltaic systems.
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| 3. |
- Shao, S., et al.
(author)
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Enlarging Regional Disparities in Energy Intensity within China
- 2020
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In: Earth's Future. - : John Wiley and Sons Inc. - 2328-4277. ; 8:8
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Journal article (peer-reviewed)abstract
- As energy saving and emission reduction become a global action, the disparity in energy intensity between different regions is a new rising problem that stems a country's or region's energy-saving potential. Here we collect China's provincial panel data (1995–2017) of primary and final energy consumption to evaluate China's unequal and polarized regional pattern in energy intensity, decompose the inequality index into contributing components, and investigate possible driving factors behind the unequal pattern both regionally and structurally, for the first time. The results show that China's interprovince disparities in energy intensity increase and are exacerbated by the enlarging disparities in energy intensity between the least developed and most developed regions of China. The causes for this phenomenon are as follows: (i) rather loose regulatory measures on mitigating coal consumption; (ii) inferior energy processing technology in areas specializing in energy-intensive industries; (iii) increasing interregional energy fluxes embodied in trade; and (iv) separate jurisdictions at provincial administrative levels. These factors can synthetically result in unintended spillover to areas with inferior green technologies, suggesting an increasingly uneven distribution of energy-intensive and carbon-intensive industries and usage of clean energy. The results reveal the necessities of regional coordination and cooperation to achieve a green economy. ©2020. The Authors. Earth's Future published by Wiley Periodicals LLC on behalf of American Geophysical Union
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| 4. |
- Wei, P., et al.
(author)
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Numerical study on thermal energy storage tube filled by metal foam with gradient porosities
- 2019
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In: IOP Conference Series. - : Institute of Physics Publishing.
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Conference paper (peer-reviewed)abstract
- Thermal energy storage has attracted more and more attentions due mainly to its ability of peak load shifting. Shell-and-tube configuration is a typical heat exchanger for thermal energy storage. To enhance phase change heat transfer, open-cell metal foam has been involved in various kinds of shell-and-tube heat exchangers. To further improve the overall thermal performance of a shell-and-tube heat exchanger, metal foams with gradient porosities were inserted into the shell side. Positive and negative gradients in porosity were studied for comparison. Numerical model was developed based on the finite volume method and three sets of numerical simulations were performed. Transient melting front and melting fraction were illustrated for comparison. Results demonstrated that the positive gradient in porosity outperformed the other two kinds of configurations, resulting in a 17.5% reduction in full melting time.
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| 5. |
- Gao, Y., et al.
(author)
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Cooling pitch cabinets in wind turbines using a pulsating heat pipe : A case study
- 2023
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In: Case Studies in Thermal Engineering. - : Elsevier Ltd. - 2214-157X. ; 50
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Journal article (peer-reviewed)abstract
- As the electric capacity of wind turbine increases, heat dissipation in pitch cabinets becomes challenging owing to the limited space and rotating conditions. To cool down the pitch cabinet more effectively and allow heat dissipation, we designed and implemented a pulsating heat pipe (PHP) in this study. We designed PHP parameters and conducted performance tests to compare the cooling performance of the fabricated PHP with that of an air-based cooling system in a 1.5 MW wind turbine. The results demonstrated steady PHP operation under rotating conditions (17.3 rpm). At a heat load of 1000 W, the evaporator outlet temperature was only 76.1 °C. However, increasing the ambient temperature adversely affected PHP operation, resulting in higher temperature and thermal resistance. The heat-pipe-based cooling system lowered the insulated gate bipolar transistor (IGBT) temperature by about 20.4 °C in relation to the air-based cooling system, while being suitable under varied conditions. Additionally, the system could successfully operate when the heat load of IGBT was 2350 W, corresponding to a 7 MW electric capacity of the wind turbine. Reducing the manufacturing cost of the heat pipe would further enhance the applicability of this system for pitch cabinet IGBT cooling, such as decreasing payback period.
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| 6. |
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| 7. |
- Liu, G., et al.
(author)
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Experimental and numerical studies on melting/solidification of PCM in a horizontal tank filled with graded metal foam
- 2023
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In: Solar Energy Materials and Solar Cells. - : Elsevier B.V.. - 0927-0248 .- 1879-3398. ; 250
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Journal article (peer-reviewed)abstract
- Although solar energy is a clean and abundant resource, it has an unstable nature. It is demonstrated that latent thermal energy storage (LTES) systems have been an excellent way to utilize solar energy fully and widely. However, LTES has the problem of insufficient thermal conductivity. For this reason, it is inevitable to consider effective methods to intensify the thermal conductivity of LTES system. In the current study, experiment and numerical simulation are used to study the influence of non-uniform metal foams on heat transfer during phase transition. In this study, a horizontal shell-and-tube LTES test system is established. Moreover, the phase change melting rate of radially filled metal foams with different porosity gradients is compared. According to the numerical simulation results of phase interface, velocity field and temperature field, natural convection can accelerate the melting of PCM. However, there is no distinct effect on the solidification process. When the equivalent porosity is 0.94, the optimal combination (melting process is 0.84-0.92-0.99 and solidification process is 0.87-0.94-0.97), compared with the uniform structure, can shorten the total consumption time by 9.7% and 6.2%, respectively.
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| 8. |
- Liu, L., et al.
(author)
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Evaluating the benefits of Integrating Floating Photovoltaic and Pumped Storage Power System
- 2019
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In: Energy Conversion and Management. - : Elsevier Ltd. - 0196-8904 .- 1879-2227. ; 194, s. 173-185
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Journal article (peer-reviewed)abstract
- Floating Photovoltaic systems have developed very fast in recent years. Compared to individual Floating Photovoltaic systems, further advantages, such as grid connectivity and energy storage, can be obtained when Floating Photovoltaic operates collaboratively with Pumped Storage Power Systems. This paper proposed an Integrated Floating Photovoltaic-Pumped Storage Power System and quantitatively assessed the potential of the integrated system in electricity generation and conservation of water and land resource. The study developed a coordinated operation model for the Integrated Floating Photovoltaic-Pumped Storage Power System, which employed a dual-objective optimization, namely to maximize the benefits of electricity generation and to minimize the energy imbalance at the same time. The dual-objective optimization was solved using the genetic algorithm method. Other benefits of the Integrated Floating Photovoltaic-Pumped Storage Power System, namely conservation of water and land resource, were also assessed. The proposed methodology was applied to a 2 GW Floating Photovoltaic farm and a 1 GW Pumped Storage Power System. Results indicated that the Integrated Floating Photovoltaic-Pumped Storage Power System has a great potential for gaining the benefits of electricity generation (9112.74 MWh in a typical sunny day averagely) and reducing energy imbalance (23.06 MW aggregately in one day). The coordinated operation provides the possibility to achieve a higher generation benefits without affecting the reliability of the grid, while the optimization method plays a key role of efficient coordination. In addition, the system would help to save 20.16 km 2 land and 19.06 million m 3 water a year due to the reduction in evaporation loss. The synthetic benefits greatly improve the economic and environmental feasibility of photovoltaic systems in reality.
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| 9. |
- Liu, L., et al.
(author)
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Forecasting the occurrence of extreme electricity prices using a multivariate logistic regression model
- 2022
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In: Energy. - : Elsevier Ltd. - 0360-5442 .- 1873-6785. ; 247
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Journal article (peer-reviewed)abstract
- Extreme electricity prices occur with a higher frequency and a larger magnitude in recent years. Accurate forecasting of the occurrence of extreme prices is of great concern to market operators and participants. This paper aims to forecast the occurrence probability of day-ahead extremely low and high electricity prices and investigate the relative importance of different influencing variables. The data obtained from the Australian National Electricity Market (NEM) were employed, including historical prices (one day before and one week before), reserve capacity, load demand, variable renewable energy (VRE) proportion and interconnector flow. A Multivariate Logistic Regression (MLgR) model was proposed, which showed good forecasting capability in terms of model fitness and classification accuracy with different thresholds of extreme prices. In addition, the performance of the MLgR model was verified by comparing with two other models, i.e., Multi-Layer Perceptron (MLP) and Radical Basis Function (RBF) neural network. Relative importance analysis was performed to quantify of the contribution of the variables. The proposed method enriches the theories of electricity price forecast and advances the understanding of the dynamics of extreme prices. By applying the model in practice, it will contribute to promoting the management of operation and establishment of a robust energy market.
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| 10. |
- Liu, Z., et al.
(author)
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Solar harvest : Enhancing carbon sequestration and energy efficiency in solar greenhouses with PVT and GSHP systems
- 2023
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In: Renewable energy. - : Elsevier Ltd. - 0960-1481 .- 1879-0682. ; 211, s. 112-125
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Journal article (peer-reviewed)abstract
- It is universally acknowledged that climate change brings widespread attention to solar greenhouse plant carbon sequestration. Suitable technologies in solar greenhouses were, are, and will be play a leading role in this vital transition. The primary aim of this research is to examine the energy efficiency and carbon sequestration potential of a solar-assisted ground-source heat pump (SAGSHP) heating system. This hybrid system, which integrates a horizontal ground-source heat pump (GSHP) system with PVT and heat storage, can efficiently fulfill the heating demands of a greenhouse and function as a positive energy building. Four plants include cucumber, tomato, cowpea, and lettuce were selected to compare the carbon absorption effects. Results show that the hybrid system outperforms conventional systems, with a coefficient of performance (COP) of 6.71 during peak hours and PVT efficiency over 57.88%, which effectively meet the heat load of the greenhouse and keep the indoor heat comfortable. In addition, for the carbon sequestration potential of four plants, tomato exhibited the highest photosynthetic carbon sequestration of 3522 kgCO2·m−2. Cowpea showed the strongest daily carbon sequestration capacity at 26.86 gCO2m−2d−1 and better economic income. Through the application of this enhanced solar greenhouse, people can enhance the utilization of solar energy, establish flexible interaction between energy and information flow, and make a promising option for sustainable building design.
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