Solar Loops Research Articles

Contribution of thermal bremsstrahlung to microwave emission of solar flare loops

The contribution of thermal bremsstrahlung to the total microwave flux of a solar flare loop is considered. The total-flux data were obtained on the Nobeyama Radio Heliograph. The calculation of the thermal bremsstrahlung radio flux was based on determining the integrated temperature and number density of the hot flare-loop plasma from its soft X-ray flux, obtained using data from the GOES-10 and GOES-12 satellites. The effect of thermal bremsstrahlung on the total flux and the spectral index of the microwave radiation is insignificant at the burst maximum (Fth/Ftot < 3%, Δα < 0.2), while the contribution of bremsstrahlung can be substantial during the decay phase of the burst (up to 80%). This results in an appreciable decrease in the observed spectral index (to Δα ∼ 1.5). Therefore, when diagnosing the parameters of the accelerated electrons based on the characteristics of their gyrosynchrotron radiation, the most accurate results can be obtained using the emission characteristics obtained near the burst maximum.

Experimental Investigation of the Filling and Draining Processes of the Drainback System (Part 1)

Drainback systems (DBS) were developed in the Netherlands in the 1980's, due to governmental regulations regarding potable water quality. The crucial attribute of drainback systems is the draining of the working fluid from the collector - always when the solar circuit is not in operation. The draining ensures a protection against both, overheating and freezing of the circulating fluid in the solar loop. Small installation failures can disturb the desired filling/draining processes. This can lead not only to improperly operation, but even damage the DBS. Therefore the knowledge on behavior of the DBS during the filling and draining is essential for ensuring the proper operation.An experimental setup was developed in order to investigate the filling and draining processes of the drainback system. Two types of drainback systems were considered: system with drain-back reservoir and heat storage as drain-back reservoir. The first type has a separate, so-called drain-back reservoir which is located on the solar flow circle. The collector is hydraulically connected to the heat storage through an internal heat exchanger coil. In the second system type, the heat storage itself is the drain-back reservoir. The measurement equipment comprised magnetic-inductive flow meters, manometers and a data acquisition system. For additional visual assessments, almost all hydraulic components (tubes, collector's absorber, heat storage, drain-back reservoir) were made transparent and colored water was used as working fluid in the solar loop. Based on the dimensions of a real collector with a meander absorber its “imitator” was made of PVC pipes. The 100 liter heat storage and 25 liter drain-back reservoir were made of PP-H plastic.Different experiments were carried out and evaluated. The processes of filling and draining are described in detail in this paper. Results of this investigation are e.g.:•A siphon is a crucial effect for both filling and draining processes•A minimal hydrostatic height difference less than 10mm between flow and return sides is sufficient for draining•The largest share of the heat carrier drains over the solar return pipe, caused by siphoned draining•Horizontal piping without slope does not disturb the draining processExplanations of these processes will be presented in the paper.

Performance and Thermal Stability of a Polyaromatic Hydrocarbon in a Simulated Concentrating Solar Power Loop

Because polyaromatic hydrocarbons show high thermal stability, an example of these compounds, phenylnaphthalene, was tested for solar thermal-power applications. Although static thermal tests showed promising results for 1-phenylnaphthalene, loop testing at temperatures to 450 ℃ indicated that the fluid isomerized and degraded at a slow rate. In a loop with a temperature high enough to drive the isomerization, the higher melting point byproducts tended to condense onto cooler surfaces. This would indicate that the internal channels of cooler components of trough solar electric generating systems, such as the waste heat rejection exchanger, may become coated or clogged affecting loop performance. Thus, pure 1-phenylnaphthalene, without addition of stabilizers, does not appear to be a fluid that would have a sufficiently long lifetime (years to decades) to be used in a loop at temperatures significantly greater than the current 400 ℃ maximum for organic fluids. Similar degradation pathways may occur with other organic materials. The performance of a concentrating solar loop using high temperature fluids was modeled based on the National Renewable Laboratory Solar Advisory Model. It was determined that a solar-to-electricity efficiency of up to 30% and a capacity factor of 60% could be achieved using a high efficiency collector and 12 h thermal energy storage when run at a field outlet temperature of 550 ℃.

Innovative Compact Solar Air Conditioner based on Fixed and Cooled Adsorption Beds and Wet Heat Exchangers

In this work, a new all-in-one compact solar air conditioner concept is presented. The system is mainly based on a new DEC process which utilises fixed and cooled adsorption beds operating in a batch process and two wet heat exchangers. The proposed innovative adsorption bed is a fin and tube heat exchanger commonly used in the air conditioning sector, wherein the spaces between the fins are filled with silica gel grains. The main feature of this component is to allow simultaneous dehumidification and cooling of air. Furthermore, since the component hosts a considerable amount of adsorption material, solar energy can be efficiently stored in the desiccant media in terms of accumulated adsorption capacity. This potential can be used when regeneration heat is not available, strongly reducing the need for thermal storage in the solar loop. The indirect evaporative cooling process, operated downstream to the dehumidification, is realized by two wet plate heat exchangers connected in series. The process can be operated at relatively low temperature, allowing supply air temperature to the room of about 20°C.A prototype of the compact solar air conditioner specifically developed for residential application is presented. The main features of the system as well as the thermodynamic cycle are first described. Monitoring results are presented by means of most commonly used performance indicators showing several advantages which can be obtained using the proposed solution.

Development and Test Results of a Calorimetric Technique for Solar Thermal Testing Loops, Enabling Mass Flow and Cp Measurements Independent from Fluid Properties of the HTF Used

Thermal heat transfer fluids (HTF) used in solar collectors (e.g. synthetic oils) are known to age and degrade [1]. This degradation is impossible to control, affecting the fluid heat transfer capacity and thus the ability of measuring the performance of an HTF heating device (e.g. a solar collector) based on known specific heat values.Collector testing is also crucially dependent on an accurate measurement of HTF mass flow rate. Such measurement relies on flow meters suitable for the accuracy, operating temperature and flow range requirements of the testing procedures, often an expensive and demanding component in particular when no-intrusive measurements are to be done ia a close circuit.For power measurement purposes, as those performed in solar collector testing procedures, a direct measurement of the product between specific heat and mass flow rather than a separate measurement of both quantities is suitable. A calorimetric technique delivering this direct measurement is thus a suitable strategy to overcome such difficulties with acceptable (and even higher) measurement accuracy.Solutions of this kind have already been proposed [2,3]. In this paper we revisit and improve the solution presented in [2] and demonstrate its usefulness in a solar collector testing loop, for temperatures up to 200°C. A calorimeter prototype was thoroughly tested and calibrated with water as HTF (enabling accurate independent measurement of specific heat and mass flow rates values). Calorimeter calibration results where then used in the testing with thermal oil whose specific heat values were previously known from manufacturer and independent laboratory measurements.A comparison of Cp measured by the calorimeter with the value given by the HTF manufacturer is used to test the calorimeter capacities. The agreement achieved was very good. It is noted that the technique can be easily implemented in any high temperature loops, large or small.

Variable-volume Storage Systems for Solar Heating and Cooling System: A Case Study for Different Italian Climates

This paper analyzes different control strategies for the thermal storage management in Solar Heating and Cooling systems (SHC) for different Italian climates. This novel thermal storage system consists in a variable volume storage tank system, which includes three separate tanks and a number of mixers and diverters. Such devices are managed through two different control strategies, based on combinations of series/parallel charging and discharging approaches. Thus, it is possible to vary the thermal storage capacity as a function of the combinations of solar radiation availability and user thermal/cooling energy demands. The system allows one to either increase or reduce the number of active tanks when the occurring mismatch between the solar energy supply and the user demand is either high or low, respectively. In addition, the surplus of solar energy is used through a heat exchanger included in the solar loop for the production of Domestic Hot Water (DHW). This novel variable-volume storage system, in all the proposed configurations, is also compared with a constant-volume storage system from the energetic and economic points of view. In addition, in order to determine the set of the synthesis/design variables which maximize the system profitability, a parametric analysis is implemented. A case study developed for an office building located in different Italian climatic areas is also presented. Simulation results show that the analyzed SHC systems system configurations may be profitable for all those cases and weather locations in which a sufficiently high solar fraction is achieved.

Spatial brightness distribution of hard X-Ray emission along flare loops

The work is devoted to the simulation of recently discovered hard X-ray and gamma radiation sources localized near the top of solar flare loops. The calculations were performed in the context of a model of a flare magnetic loop inhomogeneous with respect to the magnetic field. Two cases of injection were considered: isotropic and anisotropic along the loop axis. The distributions of electrons along the loop are found by solving the nonstationary relativistic kinetic equation in the Fokker-Planck form. Based on the calculated electron distribution functions, the spatial brightness distribution of hard X-ray and gamma radiation has been calculated. Radiation characteristics are compared for different sets of injection parameters.

Thermoeconomic analysis of storage systems for solar heating and cooling systems: A comparison between variable-volume and fixed-volume tanks

The paper investigates different control strategies for the thermal storage management in SHC (Solar Heating and Cooling) systems. The SHC system under investigation is based on a field of evacuated solar collectors coupled with a single-stage LiBr–H2O absorption chiller; auxiliary thermal energy is supplied by a gas-fired boiler. The SHC is also equipped with a novel thermal storage system, consisting in a variable volume storage tank. It includes three separate tanks and a number of mixers and diverters managed by novel control strategies, based on combinations of series/parallel charging and discharging approaches. The aim of this component is to vary the thermal storage capacity as a function of the combinations of solar radiation availability and user thermal/cooling energy demands. The system allows one to increase the number of active tanks when the time shift between solar energy and user demand is high. Conversely, when this time shift is low, the number of active tanks is automatically reduced. In addition, when the solar energy in excess cannot be stored in such tanks, a heat exchanger is also used in the solar loop for producing DHW (Domestic Hot Water).The analysis is carried out by means of a zero-dimensional transient simulation model, developed by using the TRNSYS software. In order to assess the operating and capital costs of the systems under analysis, an economic model is also proposed. In addition, in order to determine the set of the synthesis/design variables which maximize the system profitability, a parametric analysis was implemented. The novel variable-volume storage system, in both the proposed configurations, was also compared with a constant-volume storage system from the energy and economic points of view. The results showed that the presented storage system allows one to save up to 20% of the natural gas used by the auxiliary boiler only for very high solar fractions. In all the other cases, marginal savings are achieved by the variable-volume storage tanks system.

Performance and thermal charging/discharging features of a phase change material assisted heat pump system in heating mode

At low ambient temperature, air-source heat pump suffers from decrease in both heating capacity and coefficient of performance (COP), and increase in compressor's pressure ratio. A parallel triple-sleeve energy storage exchanger with phase change material (PCM) for storing thermal heat was designed to ensure the reliable operation of a heat pump under various weather conditions and enhance the system performance at low ambient temperature. The innovative device also includes a solar thermal collector loop for utilizing free solar energy. Thermal heat can be transferred into and stored by the PCM using water as the heat transfer medium. Controlled experiments were carried out to investigate the performance and the simultaneous thermal charging and discharging behavior of this enhanced heat pump system. Transient operating characteristics, including the temperatures, pressures and heat transfer rate, were analyzed. The COP value in an operating mode with a constant heat transfer fluid water temperature and flow rate increased until the system entered into the stable operating stage. The final COP can reach up to 3.9 when the three heat transfer mediums (water/PCM/refrigerant) achieved the steady-state. The experimental results show an interesting phenomenon that the heat transfer process between water and refrigerant with a PCM interlayer was a dynamic balance. The PCM temperature and the difference of the water temperature at the inlet and outlet of the evaporator regularly fluctuated around some balance points. In this study, the measured PCM temperatures in the dynamic steady-state were 12.8 and 14.2 °C. The difference of the water temperature at the inlet and outlet of the evaporator balanced around 2.8 °C. The findings in the research implied more study is needed to explore the PCM charging/discharging mechanisms and improve the operation of the PCM assisted thermal system.

Simulation-based long-term fault detection for solar thermal systems

This paper discusses a simulation-based fault detection approach for detecting faults during the operation of low-temperature solar thermal systems. An effective fault detection approach for solar thermal systems can ensure a quick response and reparation of occurring malfunctions that cause a reduced solar energy yield. Several fault detection methods can be applied, e.g. manual fault finding, algorithms, spectral analysis or simulation-based fault detection.The simulation-based fault detection approach is based on an hourly, daily or monthly comparison of measured and simulated energy yields. The method was applied and evaluated for three field test systems with different hydraulics and a collector area between 15 and 1290m2. Several faults were successfully detected. Faults in the solar loop can be detected as long as the fault causes a large enough energy loss in comparison to the uncertainties of the simulated and measured energy yield. Fault diagnosis turns out to be more difficult and is expected to greatly improve by integrating the simulation-based fault detection approach in a larger framework that includes data management and algorithm-based fault detection.

Magnetography of Solar Flaring Loops with Microwave Imaging Spectropolarimetry

We have developed a general framework for modeling gyrosynchrotron and free–free emission from solar flaring loops and used it to test the premise that 2D maps of source parameters, particularly the magnetic field, can be deduced from spatially resolved microwave spectropolarimetry data. We show quantitative results for a flaring loop with a realistic magnetic geometry, derived from a magnetic-field extrapolation, and containing an electron distribution with typical thermal and nonthermal parameters, after folding through the instrumental profile of a realistic interferometric array. We compare the parameters generated from forward-fitting a homogeneous source model to each line of sight through the folded image data cube both with the original parameters used in the model and with parameters generated from forward-fitting a homogeneous source model to the original (unfolded) image data cube. We find excellent agreement in general, but with systematic effects that can be understood as due to the finite resolution in the folded images and the variation of parameters along the line of sight, which are ignored in the homogeneous source model. We discuss the use of such 2D parameter maps within a larger framework of 3D modeling, and the prospects for applying these methods to data from a new generation of multifrequency radio arrays now or soon to be available.

KINETIC ALFVÉN TURBULENCE AND PARALLEL ELECTRIC FIELDS IN FLARE LOOPS

This study investigates the spectral structure of the kinetic Alfven turbulence in the low-beta plasmas. We consider a strong turbulence resulting from collisions between counterpropagating wavepackets with equal energy. Our results show that (1) the spectra of the magnetic and electric field fluctuations display a transition at the electron inertial length scale, (2) the turbulence cascades mainly toward the magnetic field direction as the cascade scale is smaller than the electron inertial length, and (3) the parallel electric field increases as the turbulent scale decreases. We also show that the parallel electric field in the solar flare loops can be 10(2)-10(4) times the Dreicer field as the turbulence reaches the electron inertial length scale.

Modeling the effect of plasma density on the dynamics of the microwave spectrum of solar flaring loops

The energetic electron distribution dynamics and radioemission have been numerically simulated in order to interpret three characteristic types of the frequency spectrum slope dynamics between 17 and 34 GHz in different parts of solar flaring loops observed with the Nobeyama Radioheliograph. In all types, the spectrum slope decreases at the rise phase of microwave burst. The behavior is qualitatively different at the decay phase of the burst: the spectrum slope increases immediately after the burst maximum in the first type; continues decreasing or stabilizes, not reaching zero, in the second type; and continues decreasing to zero and becomes positive in the third type. The parameters at which the calculated dynamics of the frequency spectrum slope in different parts of the magnetic loop most strictly corresponds to the observed dynamics have been found. It has been indicated that the first type of the spectrum slope dynamics is related to an increase and decrease in the radio source optical thickness at the rise and decay phases, respectively. The second type is caused by the flattening of the electron energy spectrum that continues at the decay phase. The third type is related to a gradual flattening process of the electron energy spectrum, combined with a large ratio of the plasma density to the magnetic field strength (n 0 /B) within a loop, and is explained by the Razin effect.

Optimization of solar DHW system including PCM media

The use of phase change materials (PCMs) to increase solar domestic hot water (SDHW) system efficiency has been already studied by different ways. Some studies place the storage material in the water tank, others directly in the solar thermal collector. However both of them show that the effectiveness of such a use is not relevant. This paper is devoted to a new approach: the PCM is placed in the heat transfer fluid solar loop from the SDHW system. This configuration is studied under different weather conditions and system parameters. On the contrary to previous results, this parametric study highlights a significant increase of the system efficiency due to the PCM. Then, a genetic algorithm allows proposing an optimized system configuration.

SDO AIA and EVE observations and modelling of solar flare loops

We present imaging and spectroscopic observations of an isolated C1-class solar flare, obtained with the Atmospheric Imaging Assembly (AIA) and Extreme ultraviolet Variability Experiment (EVE) on the Solar Dynamics Observatory (SDO). We obtain excellent agreement between the peak flare temperatures estimated using the EVE spectra with those obtained from GOES and, most importantly, from the ratio of the 94 A and 131 A AIA channels, which are found to be dominated by Fe xviii and Fe xxi .T hese results confirm that these two AIA bands can be reliably used to provide temperature diagnostics for the peak and gradual phases of solar flares. The flare kernels, probable sources of chromospheric evaporation, are seen as strong localised emission in the AIA bands at the footpoints of flare loops. The flare loops are close to isothermal during the gradual phase. We have run several hydrodynamic simulations (using the HYDRAD code) to study the cooling of the flare loops. We find good overall agreement between observed and predicted electron temperatures and densities when a gradual increase and decrease of the heating is assumed.

Magnetohydrodynamics in solar and space physics

Because of its proximity, our Sun provides a unique opportunity to perform high resolution observations of its outer layers throughout the whole electromagnetic spectrum. We can also theoretically model most of the fascinating physical phenomena taking place on the Sun, as well as their impact on the solar system.Many of these phenomena can be properly studied within the framework of magnetohydrodynamics. More specifically, we assume a fully ionized hydrogen plasma and adopt the more comprehensive two-fluid magnetohydrodynamic approximation. For problems such as the solar wind or magnetic loops in the solar corona, which are shaped by a relatively strong mean magnetic field, the reduced magnetohydrodynamic approximation is often used.We will review the basic features of both two-fluid and one-fluid magnetohydrodynamics, and focus on two particular applications: the turbulent heating of coronal active regions and the dynamics of the solar wind.

Optimal Collector Type and Temperature in a Solar Organic Rankine Cycle System for Building-Scale Power Generation in Hot and Humid Climate

The objective of this paper is to determine the optimal solar collector type and temperature of a building-scale power generation system employing solar organic Rankine cycle (ORC) engine for a geothermal air-conditioned net zero-energy building (NZEB) in a hot and humid climate. In the authors' previous work, 11 fluids have been suggested to be employed in solar ORCs that use low-temperature or medium-temperature solar collectors. In this paper, the system requirements needed to maintain the electricity demand of a commercial building have been compared for the 11 suggested fluids. The solar collector loop, building, and geothermal air conditioning system are modeled using TRNSYS with the required input for the ORC system derived from the previous study. The commercial building is located in Pensacola of Florida and is served by grid power. The building has been equipped with two geothermal heat pump units and a vertical closed loop system. The performance of the geothermal system has been monitored for 3 weeks. Monitoring data and available electricity bills of the building have been employed to calibrate the building and geothermal air conditioning system simulation. Simulation has been repeated for Miami and Houston in order to evaluate the effect of the different solar radiations on the system requirements.

Integration of Dye-Sensitized Solar Cells, Thermoelectric Modules and Electrical Storage Loop System to Constitute a Novel Photothermoelectric Generator

This study self-develops a novel type of photothermoelectric power generation modules. Dye-sensitized solar cells (DSSCs) serve as the photoelectric conversion system and a copper (Cu) heat-transfer nanofilm coating on both sides of the thermoelectric generator (TEG) acts as a thermoelectric conversion system. Thus module assembly absorbs light and generates electricity by DSSCs, and also recycles waste heat and generates power by the TEG. In addition, a set of pulsating heat pipes (PHP) filled with Cu nanofluid is placed on the cooling side to increase cooling effects and enhance the power generation efficiency. Results show that when the heat source of thermoelectric modules reaches 90 degrees C, TEG power output is increased by 85.7%. Besides, after thermoelectric modules are heated by additional heat source at 80 degrees C, the electrical energy generated by them can let a NiMH cell (1.25 V) be sufficiently charged in about 30 minutes. When photothermoelectric modules is illumined by simulated light, the temperature difference of two sides of TEG can reach 7 degrees C and the thermoelectric conversion efficiency is 2.17%. Furthermore, the power output of the thermoelectric modules is 11.48 mW/cm2, enhancing 1.4 % compared to merely using DSSCs module.

Three dimensional particle-in-cell simulation of particle acceleration by circularly polarised inertial Alfven waves in a transversely inhomogeneous plasma

The process of particle acceleration by left-hand, circularly polarised inertial Alfven waves (IAW) in a transversely inhomogeneous plasma is studied using 3D particle-in-cell simulation. A cylindrical tube with, transverse to the background magnetic field, inhomogeneity scale of the order of ion inertial length is considered on which IAWs with frequency 0.3ωci are launched that are allowed to develop three wavelength. As a result time-varying parallel electric fields are generated in the density gradient regions which accelerate electrons in the parallel to magnetic field direction. Driven perpendicular electric field of IAWs also heats ions in the transverse direction. Such numerical setup is relevant for solar flaring loops and earth auroral zone. This first, 3D, fully kinetic simulation demonstrates electron acceleration efficiency in the density inhomogeneity regions, along the magnetic field, of the order of 45% and ion heating, in the transverse to the magnetic field direction, of 75%. The latter is a factor of two times higher than the previous 2.5D analogous study and is in accordance with solar flare particle acceleration observations. We find that the generated parallel electric field is localised in the density inhomogeneity region and rotates in the same direction and with the same angular frequency as the initially launched IAW. Our numerical simulations seem also to suggest that the “knee” often found in the solar flare electron spectra can alternatively be interpreted as the Landau damping (Cerenkov resonance effect) of IAWs due to the wave-particle interactions.

THEn-DISTRIBUTION OF ELECTRONS AND DOUBLE LAYERS IN THE ELECTRON-BEAM-RETURN-CURRENT SYSTEM OF SOLAR FLARES

We investigate processes in the electron-beam-return-current system in the impulsive phase of solar flares to answer a question about the formation of the n-electron distribution detected in this phase of solar flares. An evolution of the electron-beam-return-current system with an initial local density depression is studied using a three-dimensional electromagnetic particle-in-cell model. In the system the strong double layer is formed. Its electric field potential increases with the electron beam flux. In this electric field potential, the electrons of background plasma are strongly accelerated and propagate in the return-current direction. The high-energy part of their distribution at the high-potential side of the strong double layer resembles that of the n-distribution. Thus, the detection of the n-distributions, where a form of the high-energy part of the distribution is the most important, can indicate the presence of strong double layers in solar flares. The similarity between processes in solar flare loops and those in the downward current region of the terrestrial aurora, where the double layers were observed by FAST satellite, supports this idea.