Abstract

Introduction

Methodology

PV efficienc

Data collectio

Experimental setup

Results and discussion

Effect of temperature on the power output of PV panels and TEGs

Conclusions

Nomenclature

References

Abstract

     Many factors affect the efficiency of photovoltaic panels (PV), which convert solar energy directly into electrical energy. Among these factors, temperature is one of the most important one. While some of the radiation from the sun is converted into electrical energy, part of it emerges as heat energy. This causes the photovoltaic cells to heat up and reduce their electrical efficiency. Different methods are used in the literature to reduce the temperature in PV panels. In this study, in order to reduce the adverse effects caused by the high temperature in the PV panels, 30 Thermoelectric Generators (TEG) were applied to the back surface of the PV panel to increase the PV panel output power and to produce additional electrical energy. Energy and exergy analysis made on the data obtained from both PV panels in the climatic conditions of the installation site showed that the temperature of the PV panel is reduced, and the energy and exergy efficiency is increased with the TEG application. At the end of July, August, and September, when the experiments were carried out, an average of 8.4% more electrical energy was obtained from a single PV panel with TEG, compared to the standard PV panel. Our results suggests that combination of TEG with PV panels could significantly increase the electrical energy, especially when a series of PV panels are used together.

Introduction

     Recently, investments in electricity generation from renewable energy sources, especially solar and wind energy, have been increasing. However, when the performance of photovoltaic cells in the natural field environment is compared with the version of the standard test conditions, the efficiency of the PV panels decreases with the increase in temperature due to radiation (Bel Hadj Brahim Kechiche, Hamza, and Sammouda 2016). Since the energy that cannot be converted into electricity increases the photovoltaic surface temperature, it causes damage to the PV panel module cells and leads to reduced service life of the PV panels (Enescu and Spertino 2017). When the studies in the literature about reducing the heat losses in photovoltaic panels (PV) are examined; A water-cooled nozzle spray system has been developed that significantly reduces the module cell temperature (Benato and Stoppato 2019). The factors affecting the performance of PV panels and the applications that can be made to increase the panel power are explained in detail (Fouad, Shihata, and Morgan 2017).To obtain higher efficiency by cooling the PV panel surface, the panel output power and the cell’s lifetime are increased with different cooling methods (Siecker, Kusakana, and Numbi 2017). To prevent the decrease in PV panel power output with temperature increase, new models are proposed on the bottom surface. In experiments carried out under natural conditions, it has been determined that the panel temperature can be reduced by 10°C with the evaporative cooling model and the power improvement is 5% (Haidar, Orfi, and Kaneesamkand 2021).

Conclusions

     In this study, the losses in energy production due to the increase in temperature in the PV panels, especially in the summer months, were reduced by TEG application. An increase in the PV Panel output power was achieved. According to the results obtained;

● More electrical energy was produced in the PV panel with a thermoelectric generator in July, August, and September when the experiments were carried out.

● With the application of a thermoelectric generator in the PV panel, an average of 8.4% more electrical energy was obtained per month. This indicates that when the number of PV panels combined with TEG is increased, the total energy could increase significantly

● The output power of TEGs also increases depending on the rise in air temperature and PV panel surface temperature

● Since the average temperature was higher in July, maximum energy was produced in the thermoelectric generator panel. Compared to the standard panel, the PV panel, in which a thermoelectric generator is applied, had 10.29% more electricity in July, 7.15% in August, and 7.09% more in September, respectively.

Abstract

     Energy efficiency is one of the most important issues of today. The limited reserves of fossil fuels make the use of renewable energy sources more attractive. Photovoltaic panels are one of the ways to utilize solar energy, which is one of the renewable energy sources.

     The instantaneous efficiency of photovoltaic panels is related to the cell temperature of the panels. In the current studies in the literature, there are empirical expressions that give the photovoltaic panel cell temperature, but the results obtained from these expressions do not overlap with each other. In order to obtain the instantaneous efficiencies of photovoltaic panels and, accordingly, the production values theoretically, there is a need for a global expression that gives the photovoltaic panel cell temperature. In this study, a global expression was developed that gives the photovoltaic panel cell temperature depending on the ambient temperature, solar radiation and wind speed. In addition, using the meteorological data of Kütahya for many years, expressions giving ambient temperature, solar radiation and wind speed were created.

Introduction

     The limited reserves of fossil fuels make renewable energy sources more attractive day by day. While the world primary energy consumption was 581.51 exajoules in 2019, this value was 557.1 exajoules in 2020 (BP, 2021). The amount of renewable energy sources in primary energy consumption in 2019 was 28.82 exajoules (5 %), and in 2020, primary energy consumption from renewable energy sources was 31.71 exajoules (5.7 %) (BP, 2021). From renewable energy sources, 2,789.2 TWh of electricity was produced in 2019 and 3,147.0 TWh in 2020 in the world. While total solar energy production was 707.9 TWh (25.4 %) in 2019, total solar energy production was 855.7 TWh (27.2 %) in 2020 (BP, 2021).

     Table 1 and Table 2 show the solar energy installed power and solar energy production values for 10 years between 2011 and 2020 (BP, 2021). When the tables are examined; It is seen that the installed PV power in the world has increased by 882 % in 10 years. Likewise, electrical energy production from PV systems has increased by 1,212 % over the 10-year period.

     Fig. 1 shows Turkey’s solar energy situation. While the total installed power was 7 MW as of the end of 2011, this value increased by 95,143 % and reached 6,667 MW at the end of 2020. Likewise, while electricity generation from solar energy was 2.86 GWh in 2011, this value increased by 378,732 % in 10 years and reached 10,834.6 GWh (BP, 2021).

Evaluations and suggestions

     In the current studies in the literature, there are empirical expressions that give the photovoltaic panel cell temperature, but the results obtained from these expressions do not overlap with each other. In order to obtain the instantaneous efficiencies of photovoltaic panels and, accordingly, the production values theoretically, there is a need for a global expression that gives the photovoltaic panel cell temperature.

     The instantaneous efficiency of the photovoltaic panel and the generation of electrical energy depending on the efficiency are related to the cell temperature of the photovoltaic panel. Within the scope of this study, a global expression that gives the photovoltaic panel cell temperature was created by using real photovoltaic plant data with the 7 most popular expressions in the literature that give the photovoltaic panel cell temperature. With the obtained expression, instantaneous photovoltaic panel temperature can be obtained depending on the ambient temperature, solar radiation and wind speed.

1- Introduction

2- Methodology

3- Results and discussion

4- Conclusion

References

Abstract

This paper presents the effect of heat exchanger design on heat pump performance based on partial load operating conditions. 3-D numerical analysis was conducted to calculate face velocity profiles for each outdoor heat exchanger design (rectangular, cylindrical, and trapezoidal) in 10 different operating conditions. Heat exchanger circuits were modified considering heat exchanger face velocity distributions, and seasonal heat pump performances were calculated with modified heat exchanger design. The maximum seasonal performance enhancement of 7.07% was achieved with a modified heat exchanger design. Air-side flow maldistribution could affect significantly refrigerant path design and heat exchanger performance as well as system performance. The analysis results also revealed that smaller refrigerant circuits at the upper part of the heat exchanger interacting with higher air velocity could further enhance the annual system performance.

Introduction

Energy and environmental issues are critical problems due to rapid increases in energy systems around the world. The International Energy Agency has reported that energy systems such as air conditioning and heat pump systems account for almost 700 million metric tons of CO2 equivalent direct (7%–19%) and indirect emissions (74%) per year, which is responsible for global warming and ozone depletion. Direct emissions can be reduced by introducing low global warming potential (GWP) refrigerants, and indirect emissions can be controlled using a more efficient system [1]. The finned tube heat exchanger is widely used as a condenser or evaporator of heat pump systems, and it is one of the most important components of the systems and plays a significant role in the system size and performance. Therefore, it is important to design more efficient, lightweight, cost-effective, and low power consuming finned tube heat exchangers [2,3]. Heat exchanger performance degradations are associated with maldistribution of refrigerant and face air velocity distribution, particularly for downstream rows as airflow velocity decreases significantly on the lower side [4]. Although it is difficult to measure the air velocity profile experimentally [5], it strongly influences heat exchanger performance [6]. Non-uniform flow distribution has many disadvantages: produces wake in flow [7], changes flow pattern [8], decreases overall heat transfer coefficient [9], and reduces system capacity [10]. Consequently, it also increases pressure drop [11] across the heat exchanger hence increases required pumping power [12–15]. Performance deterioration due to non-uniform flow is not limited to finned tube heat exchangers with louver fins [9] and plate fins [16,17], but also occurs in concentric tube [14], and microchannel condensers of automotive air conditioning systems [18]. These examples highlighted the problem of non-uniform flow distribution in different applications. However, these studies are limited to rated load conditions for specific heat exchanger design. No study has been found in literature that covers face velocity distribution for outdoor heat exchanger under partial load operating conditions. Therefore, it is important to evaluate face velocity profiles under off-design condition.

1- Introduction

2- Experimental investigation

3- Procedure and Calculations

4- Results and discussion

5- Conclusion

References

Abstract

Nanofluids have attracted huge attention because of their effective physical and thermal properties. One of many applications of nanofluids is the enhancement of the thermal performance of heat exchangers. In the current study, an experimental investigation has been conducted for studying the effects of graphene nanofluids on the convective heat transfer in a vertical shell and tube heat exchanger. Graphene flakes were prepared using graphite foam that is derived from sugar as a raw material. The prepared Graphene flakes have been characterized using scanning electron microscopy, X-ray diffraction, atomic force microscopy, and Raman spectroscopy. The graphene nanofluid has been used in the tube side of the heat exchanger to enhance its heat transfer performance. Different parameters such as nanofluids’ concentration, flow rate and inlet temperature were studied and their effects on heat transfer coefficient and thermal efficiencies are discussed. The results show that using of graphene/water nanofluids enhances the thermal performance of the vertical shell and tube heat exchanger. A maximum increase in the heat transfer coefficient of 29% was achieved using 0.2% graphene/water nanofluids. Furthermore, the mean thermal efficiency of the heat exchanger was enhanced by 13.7% by using graphene/ water nanofluid.

Introduction

Heat exchangers are widely used by various types of industries to exchange heat between different fluids for waste heat recovery and utilities cost reduction. The thermal and physical properties of the heat transfer fluids are crucial factors that dedicate the efficiency of heat exchangers. During the last two decades, nanofluids have attracted a huge attention of researchers due to its enhanced thermal properties and flow characteristics [1]. These advantages make nanofluids promising heat transfer fluids to be used for heat transfer enhancement. Many experimental studies have been conducted for the evaluation of heat transfer characteristics of nanofluids in different types of heat exchangers [2–7]. These types of heat exchangers include plate heat exchangers [2], double pipe heat exchangers [3], and micro heat exchangers [6]. However, in open literature, similar studies on shell and tube heat exchangers are scarce. Thermal performance of a shell and tube heat exchanger was analytically investigated by Shahrul et al. using nanofluids of four different types of nanoparticles, Fe3O4, ZnO, TiO2, CuO, and Al2O3 [8].

Gas-gas plate heat exchanger is an important component to remove heat generated from electronic devices in the cabinet cooling system. The counter-flow plate heat exchanger is usually used due to its higher heat transfer performance than the cross-flow plate heat exchanger. However, in the cabinet cooling system, the overall dimensions for the heat exchanger is limited. Therefore, it is necessary to consider the overall dimensions of the system during heat exchanger design, but the traditional thermal design method of heat exchanger doesn’t consider the effect of system parameters. In this paper, a cross-flow plate heat exchanger is proposed to improve the cooling performance of cabinet cooling system, and is compared with the counter-flow plate heat exchanger. The ε-NTU method and effectiveness-thermal resistance method are applied to evaluate the performance. It is found that for large width of cabinet cooling system, the system with cross-flow plate heat exchanger has higher cooling performance and lower thermal resistance than the system with counter-flow plate heat exchanger. When the width is 700 mm, the cooling capacity of the two systems are 176.13 W/K and 138.95 W/K, respectively. The dimensionless thermal resistance can characterize the irreversibility of the heat transfer at constant mass flow rate.

Introduction

Plate heat exchangers are widely used in energy consuming and handling industries [1]. Generally, plate heat exchangers are used for liquid–liquid heat exchange [2]. Imran et al. [3] optimized liquid–liquid plate heat exchanger. The overall dimensions of the heat exchanger were independently considered in their research. Miao et al. [4] numerically studied liquid–liquid plate heat exchangers. The heat exchanger was counter-flow plate heat exchanger. Two methods were utilized to get simulation results, respectively. The results of grey-box method were better. Kumar et al. [5] experimentally studied the liquid–liquid plate heat exchangers and obtained correlation based on their experimental data, in which the overall dimensions of the measured heat exchangers were constant. Yang et al. [6] obtained correlation based on their experimental data and empirical correlations in open literature. The overall dimensions of heat exchangers in their work were varied. Counter-flow plate heat exchanger also could be applied for two-phase heat exchange [7] and the heat exchangers in organic Rankine cycle could be counter-flow plate heat exchangers for better performance [8]. However, due to the primary heat transfer surfaces and light weight the plate heat exchanger was developed for gas–gas heat exchange [9] and even for liquid–gas heat exchange [10]. Cross-flow arrangement was convenient for liquid–gas plate heat exchanger [11]. High temperature gas–gas heat exchangers were mainly applied for the microturbine recuperated cycle system [12] and air preheater of the fossil fuel power system [13]. Recuperators for micro gas turbine was reviewed by Xiao et al. [14] recently. Wang et al. [15] improved the performance of air preheater by optimizing geometrical parameters of primary surface.

1. Introduction

2. Materials and methods

3. Results

4. Conclusions

Appendix A. Supplementary data

References

Abstract

The thermal system design strongly depends on material selection. Nanofluids offer design flexibility and finetuning of properties by incorporation of nanoparticles in base fluids. This flexibility is guided by particle-particle communication, which may be beneficial in creating ballistic routes in heat transfer but also detrimental due to affecting nanofluid properties. The transition of nanofluids to industrial use requires application-based examinations. For this purpose, different types of nanofluids were investigated in this work in terms of their thermal efficiency in a flat plate solar collector (FPSC) and some figure-of-merits (FOMs), under laminar and turbulent flow conditions. Investigation of both aims at clarifying the correlation between FOMs and FPSC thermal efficiency, and further reporting on the validity of FOMs in assessing thermal efficiency. Results indicate that nanofluids’ eligibility as a heat transfer fluid depends on the flow condition, since a base fluid could outperform a nanofluid under turbulent flow. Nanofluid type and nanoparticle shape affects thermal performance, as suspensions of nanoplatelets/nanotubes in low concentrations (< 0.04 vol%/0.25 vol%) are shown to outperform certain spherical metal-oxide nanoparticles (< 3 vol%), according to some FOMs. It is shown that performance evaluation criteria (PEC), overall energetic efficiency, and energy ratio (ER) do not capture FPSC thermal efficiency trends, e.g., for graphene nanoplatelet nanofluid, as Mouromtseff number-based comparisons do for laminar and turbulent conditions. It must be highlighted that the FOM type to indicate thermal efficiency should be chosen depending on the application, and simultaneous consideration of thermal and hydrodynamic characteristics is required.

Introduction

In today’s world, the main emphasis in energy industry lies on the concept of “sustainability” in energy applications. Plenty of Rooms at the Bottom are available, as stated by Richard P. Feynmann [1], at the enhanced surface-to-volume ratio of nanoparticles which bring about unique characteristics to base fluids when mixed with nanoparticles. Nanofluids [2] or nano-enhanced Heat Transfer Fluids (ne-HTFs) have been under research in a variety of heat transfer-based systems. This has been by necessity due to the poor thermal characteristics of most conventional HTFs, e.g., water, ethylene glycol, mineral oils, brines, etc. compared to solids. This fact highlighted the need for their replacement by ne-HTFs as a potential way to improve thermal performance of solar collectors, electronic cooling systems, nuclear reactor cooling schemes, refrigerators, and so forth.

Nanofluids thermal conductivity and heat transfer coefficient have been among the most investigated characteristics [3–5] in determining ne-HTFs’ potential in various heating and cooling processes. Hydrodynamic and colloid state points of view show per contra that high viscosity of nanofluids cause increments in pressure drop, pumping power, operation cost, and nanoparticle sedimentation concerns to increase; as well as sometimes thermal performances no better than HTFs in turbulent flow and fully developed flow conditions [3,6–8]. Although the convective heat transfer coefficient and Nusselt number are important performance indicators, various types of figure-of-merits (FOMs) have been proposed to compare the heat transfer characteristics of HTFs. One of the most earliest FOM definition is the Mouromtseff number (Mo) [9] that has been used to compare heat transfer capabilities of HTFs in laminar and turbulent flow conditions. Another FOM is defined as the ratio of viscosity enhancement and thermal conductivity enhancement coefficients [10], valid in the case of fully developed laminar flow. Sekrani et al. [11] suggested two different types of FOMs to compare the heat transfer capability of nanofluids as the performance evaluation criterion, i.e., PEC, [12] and overall energetic efficiency.

1. Introduction

2. Numerical model description

3. Aerodynamic and sea wave loadings

4. Fragility analysis of wind turbine

5. Conclusions

Acknowledgements

References

Abstract

To more effectively extract the vast wind energy in marine areas, offshore wind turbines have been constructed with slender tower and large rotor. External vibration sources such as aerodynamic, sea wave and seismic loadings can threaten the safety of these energy infrastructures. It is important to evaluate the reliability of offshore wind turbines subjected to external vibration sources. Previous research works on the wind turbine fragility analyses only considered the fragility of the tower by assuming the wind turbine was in the parked condition with the blade mass lumped at the top of the tower. The study of the fragility of the blade which is one of the most important components of a wind turbine has not been reported. In the present study, a detailed three-dimensional (3D) finite element (FE) model of the NREL 5 MW wind turbine is developed in ABAQUS, and the tower and blades are explicitly modelled to realistically estimate the aerodynamic loads and structural behaviours of the wind turbine. The uncertainties of the structural mass, stiffness and damping are taken into account to develop the probabilistic wind-induced demand models for the tower and blades. The dynamic behaviours of the wind turbine subjected to the simultaneous aerodynamic and sea wave loadings are investigated in a probabilistic frame and the fragility curves for both the tower and blades under the parked and operating conditions are derived and discussed.

Introduction

Wind energy as one of the renewable energies is becoming a main contributor to the new electricity generation. The growth and expansion of wind farms increased rapidly in the past decade. As reported by the Global Wind Energy Council (GWEC), the worldwide installed capacity of wind power at the end of 2018 reached 591 GW, with an increase of 9.6% compared to that at the end of 2017 [1]. Due to the fact that the power generated by the wind turbine is proportional to the rotor area and cube of wind speed, multi-megawatt wind turbines with slender tower and large rotor are widely constructed in the state-of-the-art designs. These wind turbines are very flexible and lightly damped 38 since they are normally manufactured by the light-weight high-strength materials. They are thus susceptible to external vibration sources such as aerodynamic and sea wave loadings, which are experienced constantly during the whole lifetimes by offshore wind turbines. Moreover, many wind farms are located in the regions of high seismic activities such as western of United States, Japan and China [2], seismic loading is another possible vibration source during their lifetimes in these regions. The excessive vibrations may slow down the conversion of wind energy to electricity, reduce the 44 fatigue life of the structural components or even lead to the structural collapse in extreme conditions.

1. Introduction

2. Experimental apparatus and procedure

3. Modeling and calculation methodology

4. Experimental results and discussion

5. Conclusion

Acknowledgments

References

Abstract

Freeze concentration (FC) by suspension crystallization is a complex process due to the combination of a scrapedsurface heat exchanger, crystallizer, and wash column. Following the study of a three-in-one structure of a multipass FC published earlier this year, modeling and experiments of the heat and mass transfer of this freeze concentrator are presented in this paper. The experimental assessment of the system performance, including the measured values of the heat transfer coefficient, ice production rate, and energy efficiency as well as their correlation with the concentration ratio and partition coefficient are presented in this article.

Introduction

Freeze concentration (FC) is a nonthermal processing technology of liquid food, in which a portion of the water in the aqueous solution is frozen and converted into relatively pure ice crystals, after which it is removed from the liquid phase to concentrate the remaining solution. It can be used to concentrate or pre-concentrate heat-sensitive aqueous solutions, such as milk (Habib and Farid, 2007; Sanchez et al., 2011), fresh fruit juices (Orellana-Palma et al., 2017; Petzold et al., 2013; Bayindirli et al., 1993), other liquid foods (Moreno et al., 2014a), and biological solutions (Moreno et al., 2014b). Studies have shown that compared with evaporation concentration, FC has the advantage of producing less thermal denaturation of the solution, and thus, it can better maintain the original flavor, nutrition, and color of liquid foods (Benedetti et al., 2015; Moreno et al., 2014c; Miyawaki et al., 2016a). Moreover, the latent heat of water freezing is almost one seventh of the latent heat of water evaporation (Lide and Haynes, 2010), which offers potential for energy savings for de-watering of aqueous solutions.

Ice crystals can be formed from aqueous solutions in two ways: progressive crystallization (Miyawaki et al., 2005, 2016b; Zambrano et al., 2018) and suspension crystallization (Huige and Thijssen, 1972; Qin et al., 2007). In the former, the water freezes on the cooling surface, forming an ice layer progressively and a concentrated liquid phase. Thus, it is also known as layer crystallization. This method of FC is called progressive FC. When the ice layer extends to the entire vessel to form an ice block, it is known as block FC (Moreno et al., 2014c; Zambrano et al., 2018). The advantage of this technology is the low equipment cost and simple operation management. However, the ice layer has a poor heat transfer coefficient of less than 0.1 kW m 2 K 1 (Qin et al., 2003a; Pronk et al., 2010; Hasan et al., 2017), and a huge cooling surface area is required for practical applications. In addition, the ice layer tends to entrain liquid sacs and causes severe solute loss (Miyawaki et al., 2016a, 2016b; Samsuri et al., 2015).

Targeting the heat transfer performance of geothermal pile-foundation heat exchanger in ground source heat pump system (GSHP), the physical models of 3-U pile-foundation heat exchanger and heat exchanger group were established. CFD software was used to simulate heat transfer processes and heat transfer performance was analyzed both in cooling and heating mode. The simulation results indicated that the higher thermal conductivity of pile-foundation heat exchanger contributed to the higher heat transfer efficiency than soil. Heat transfer flux per meter of the pile-foundation heat exchanger gradually decreased with time went on. After operating for ten years, the average soil temperature increased by 2.96 K in non-equilibrium condition and decreased by 0.61 K in equilibrium condition. The equilibrium condition of cooling and heating load was beneficial to operation system’s safety and efficiency. The experimental values of temperature differences were 2.2 K, 2.5 K and 3.5 K, and the heat transfer flux were stable at 58.1 W·m − 1, 65.9 W·m − 1 and 46.2 W·m − 1 in three schemes separately. The maximum difference value was 8.4% for temperature difference between experiment and simulation. The simulation results corresponded well with experimental data, indicating the reliability of simulation. The study results were approximate to the actual situation and can be used as theoretical basis for design and application of pile-foundation heat exchanger in GSHP system.

Introduction

With the development of society and improvement of people’s living standards, energy consumption has been growing apace. Nowadays, urban heating has been accounting for the largest proportion of the total building energy consumption in China [1]. For the purpose of saving energy and protecting environment, geothermal energy, as one of renewable energies, has attracted increasing interest around the world. One of the main technologies of using geothermal energy is ground source heat pump system (GSHP), which has been widely studied and applied since the early 20th century. In contrast to traditional air conditioning system, GSHP system has a lot of significant advantages. By extracting heat into and injecting heat from soil, GSHP system can operate stably by avoiding the impact of weather changes on system performance, meeting the requirements of sustainable devel-opment strategy. Without material exchanges with atmosphere, the pollution of waste heat, vapor and noise can be reduced. Moreover, GSHP system has a great advantage in investment and maintenance costs [2]. However, the disadvantages of GSHP system cannot be ignored neither. To invest a new system, a large area of land is needed and drilling holes costs additional investment. In 1994, steel pipes in pile foundation were applied to buildings for the first time, and then the concept of pile-foundation heat exchanger was proposed [3]. Consequently, geothermal pile-foundation heat exchangers in GSHP system with pipes laying inside the foundation and fixing via concrete are applied, contributing to declining outlays, saving ground area and reducing thermal interference between heat exchanger [4]. Compared with horizontal pile-foundation heat exchanger, vertical heat exchangers had better performance in heat transfer efficiency and land consumption [5].

1- Introduction

2- Proposed methodology

3- Architecture of hybrid wind power forecasting model

4- Cases study and analysis

5- Conclusion

References

Abstract

Given the intermittency and randomness of wind energy, the mass grid connection of wind power poses great challenges in power system and increases the threat in power system balance. Wind power forecasting can predict the fluctuation of output wind power in wind farms, which can effectively reduce wind power uncertainty. Improving the accuracy of wind power is indispensable for enhancing the efficiency of wind power utilization. To improve the forecasting accuracy, this research proposed a novel wind power forecasting method based on singular spectrum analysis and a new hybrid Laguerre neural network. First, singular spectrum analysis was used to analyze the wind power series, which decomposes the series into two subsequences, namely, trend and harmonic series and noise series. Then, Laguerre neural network and new Laguerre neural network were proposed to build the hybrid forecasting model optimized by the opposition transition state transition algorithm. The two decomposed signals were used for forecasting the future wind power value by using a forecasting model. Finally, the proposed hybrid forecasting method was investigated with respect to the wind farm in Xinjiang, China. Prediction performance results demonstrated that the proposed model has higher accuracy than the Laguerre neural network, hybrid Laguerre neural network, hybrid Laguerre neural network with singular spectrum analysis, hybrid Laguerre neural network with opposition transition state transition algorithm and singular spectrum analysis, and other popular methods.

Introduction

Efficient, clean, and low carbon has become the mainstream of the world’s energy development direction. Wind energy, as one of the most popular renewable clean energies, has developed rapidly in recent years. The total installed capacity of 14 years (from 2005 year to 2018 year) is reported in Fig. 1. It can be seen from Fig. 1 that the wind power installed capacity maintains sustained growth state in recent years, indicating that wind energy utilization remains rapidly increasing. The overall capacity of all wind turbines installed worldwide by the end of 2018 reached 597 GW [1]. Given the intermittency and randomness of wind energy, the mass grid connection of wind power poses great challenges in power system and increases the threat in power system balance. Wind power forecasting is a technology for predicting the fluctuation of output wind power in wind farms, which can effectively reduce wind power uncertainty. In other words, the accuracy of wind power forecasting is the basis of wind system optimization scheduling and efficient accommodation [2]. Improving the forecasting accuracy of wind power is indispensable for enhancing the efficiency of wind power utilization [3].

This work presents an experimental and theoretical evaluation of the thermal performance of a square straight diffusion-bonded stainless steel compact heat exchanger. A one-dimensional steady-state thermal model was proposed to predict the thermal characteristics of the heat exchanger. To validate the model and to study the thermal behavior of the heat exchanger, an experimental test apparatus was developed. The heat exchanger was tested in several combinations of Reynolds ranging from 2600 to 7500, representing transition to turbulent regimes. The temperatures were varied from 70 C to 80 C for the water and from 25 C to 42 C for the air, at the inlet of the heat exchanger, respectively. A good agreement between the experimental data and the analytical model was obtained.

Introduction

In the petroleum and process industries, shell and tube or variations represent about 35% of the total of the heat exchangers used, being by far the most popular technology [1,2]. Although these types of heat exchangers are reliable and robust, their large volumes and footprint area make them not appropriate to be used in some applications [1,3]. On the other hand, compact heat exchangers are highly efficient, as their main characteristic is their large heat transfer surface area for a fixed volume. Although these devices have evolved considerably lately to new efficient solutions, they still deserve a great deal of research around the world. Compact heat exchangers have been developed for applications where requirements of small weight and space are mandatory, as encountered in aerospace, naval and automotive fields. In many heat exchangers especially the compact ones, hot and/or cold streams may flow through non-circular cross-section ducts, i.e., triangular or rectangular, among other geometries. The lengths of these ducts are usually small. The equipment may operate in several regimes, varying from laminar to turbulent. Advanced heat exchangers, like the printed circuit (PCHE), are compact devices characterized by a large heat transfer surface to volume ratio, which presents high effectiveness and low terminal temperature difference. Generally, the compact heat exchanger is fabricated from a large number of plates with channels, chemically etched or water–jet machined [4,5]. After a stacking process, a diffusion bonding technique is applied for fabricating the cores.

1- Introduction

2- Mathematical model and calculation conditions

3- Gas–solid heat transfer characteristics of packed binary-size particles

4- Gas–solid heat transfer characteristics of packed quintuple-size particles

5- Comparative study of the heat transfer characteristics of packed multi-size particles with the same mean diameter

6- Conclusions

References

Abstract

Real industrial particles generally have a wide size distribution. Therefore, the gas–solid heat transfer characteristics of packed multi-size particles should be studied. A mathematical model of gas–solid heat transfer for packed multi-size particles is established. This model includes gas–solid convection heat transfer and intraparticle and interparticle conduction. The cooling processes of packed binary- and quintuple-size particles ranging from 10 mm to 60 mm under different conditions are investigated. The EDEM software is used to obtain the porosities of different cases. Results show that the presence of small particles in the packed multi-size particles reduces porosity and increases specific surface area, thereby benefiting the gas–particle heat transfer process. The temperature of large particles is always higher than that of small particles during particle cooling. Particle–particle conduction helps in the cooling process of large particles, and the maximum heat flux ratio of interparticle conduction to gas–solid convection for large particles reaches 0.196. The volumetric heat transfer coefficient of the packed multi-size particles varies with time. The initial heat transfer coefficient is the average value weighted by mass fractions, and the limit of the final value is that of the large particle under the actual porosity. The proposed dimensionless volumetric heat transfer coefficient can be a general description of gas–solid heat transfer characteristic of various packed multi-size particles. Its time variation can be well described by an exponential correlation, and the variation rate is related to the variance of particle size in each case.

Introduction

Fixed and moving beds with particles are widely used in industrial applications, such as waste heat recovery [1], gas separation [2], and chemical looping combustion [3]. The gas–solid heat transfer that occurs between the flowing gas and packed particles plays a vital role in determining the performance of such devices [4]. Numerous studies have been conducted to investigate the gas– solid heat transfer characteristics of various packed particles in the last decades [5,6], and information regarding packed mono-size spherical particles has been successfully achieved. Ranz and Marshall [7] theoretically studied the heat transfer process of gas flows over a single sphere and derived a relation for predicting the heat transfer intensity of a single sphere in Nusselt form depending on the Prandtl and Reynolds numbers. Wakao et al. [8] improved an expression on the basis of the experimental data of packed sphere beds. Their result had the same form as but different coefficient from the result of Ranz’s study, which excluded the porosity. Kunii and Levenspiel [9] proposed a heat transfer expression in an improved form, including the porosity of the packed beds. Similar studies were conducted by Gnielinski [10] and Achenbach [11], and corresponding correlations were established to extend the range of the Reynolds number, porosities, and Prandtl number. These expressions perform well in predicting the gas–solid convective heat transfer of packed mono-size spherical particles under various conditions. However, the effects of heat conduction inside a sphere should be considered when the particle size is large. Jeffreson [12] proposed a modified effective heat transfer coefficient for gases and spheres by incorporating the effects of intraparticle conduction, which is based on a theoretical solution of the heat conduction process inside a sphere. Furthermore, Kye et al. [13] derived the expression of a gas–solid volumetric heat transfer coefficient on the basis of the representative elementary volume method of porous medium.