Solar Loops Research Articles

Dynamic performance of a façade-based solar loop heat pipe water heating system

This paper reported a dedicated study of a novel façade-based solar loop heat pipe (LHP) water heating system using both theoretical and experimental methods. This system employs a modular panel incorporating a unique loop heat pipe that is able to serve as part of the building façade or a decoration layer of the façade, thus creating a façade integrated, low cost, highly efficient and aesthetically appealing solar water heating structure. Taking into account heat balances occurring in different parts of the system, e.g., solar absorber, heat pipes loop, heat exchanger and storage tank, a dedicated computer model was developed to investigate the dynamic performance of the system. An experimental rig was also established to evaluate the performance of such a prototype system through measurement of various operational parameters, e.g., solar radiation, temperatures and flow rates of the heat pipe fluid and water. Through comparison between the testing and modelling results, the model has been approved to be able to give a reasonable accuracy for predicting the performance of the LHP system. Two types of glass covers, i.e., double glazed/evacuated tubes and single-glazing plate, were applied to the prototype. It was found that for both covers, the heat pipe fluid temperature rose dramatically at the start-up operation and afterwards remained a slow but steady growth; while the water temperature remained a steadily growing trend throughout the operational day. The temperature rise of the circulated water at 1.6l/min of flow rate was around 13.5°C in the double-glazed/evacuated tubes based system and 10°C in the single-glazing based system; correspondingly, their average solar conversion efficiencies were 48.8% and 36%, and the COPs were 14 and 10.5 respectively. In overall, the double-glazed/evacuated tubes based system presented a better performance than the single glazing based one.

Thermal Performance Analysis of Solar Collectors Installed for Combisystem in the Apartment Building

Thermal Performance Analysis of Solar Collectors Installed for Combisystem in the Apartment Building The paper focuses on the application of wood pellet and solar combisystem for space heating and hot water preparation at apartment buildings under the climate of Northern Europe. A pilot project has been implemented in the city of Sigulda (N 57° 09.410 E 024° 52.194), Latvia. The system was designed and optimised using TRNSYS - a dynamic simulation tool. The pilot project was continuously monitored. To the analysis the heat transfer fluid flow rate and the influence of the inlet temperature on the performance of solar collectors were subjected. The thermal performance of a solar collector loop was studied using a direct method. A multiple regression analysis was carried out using STATGRAPHICS Centurion 16.1.15 with the aim to identify the operational and weather parameters of the system which cause the strongest influence on the collector's performance. The parameters to be used for the system's optimisation have been evaluated.

Experimental and Numerical Investigations on a Combined Biomass-Solar Thermal System

This paper presents investigations on a market available combined biomass-solar thermal system for heating and domestic hot water preparation designed for small residential applications. The system consists of a storage integrated pellet burner and a solar thermal loop delivering heat to the same storage over a heat exchanger. The investigations were performed in a project with the final goal of the optimization of the control strategy with an emphasis on the interaction of two subsystems – biomass and solar thermal.The work carried out includes comprehensive laboratory measurements on system components, simulations based on the obtained experimental data and field tests on three installed systems. Measurements on the integrated pellet burner and the storage were performed under different steady-state and transient operating conditions and selected load profiles for both space heating and domestic hot water. The performance of the solar collectors was measured on an outdoor test rig according to the method with quasi-dynamic conditions described in EN12975:2006 chapter 6.3 [1].The simulation was carried out in TRNSYS using previously validated models for all major system components [2,3]. The experimental data from laboratory measurements were used to parameterize the simulation models using optimization based on an evolutionary algorithm (particle swarm optimization). The parameterized simulation models showed very good agreement with the experimental data. Hence, it was a reliable basis for the comparison with monitoring data and will provide a basis for future system optimization.The four installation sights for field tests were chosen to include different types of buildings, thus providing information on system performance and behavior under different operating conditions. A comparison between the simulated and the measured performance of the system will be presented and discussed. In a next phase of the project, this comparison will help to identify potentials for further optimizations of the overall system with a focus on the system controller.

Solar wind energy input during prolonged, intense northward interplanetary magnetic fields: A new coupling function

[1] Sudden energy release (ER) events in the midnight sector auroral zone during intense (B > 10 nT), long-duration (T > 3 h), northward (N = Bz > 0 nT) IMF magnetic clouds (MCs) during solar cycle 23 (SC23) have been examined in detail. The MCs with northward-then-southward (NS) IMFs were analyzed separately from MCs with southwardthen-northward (SN) configurations. It is found that there is a lack of ER/substorms during the N field intervals of NS clouds. In sharp contrast, ER events do occur during the N field portions of SN MCs. From the above two results it is reasonable to conclude that the latter ER events represent residual energy remaining from the preceding S portions of the SN MCs. We derive a new solar wind–magnetosphere coupling function during northward IMFs: ENIMF = a N −1/12 V 7/3 B 1/2 + b V |Dstmin|. The first term on the right-hand side of the equation represents the energy input via “viscous interaction,” and the second term indicates the residual energy stored in the magnetotail. It is empirically found that the magnetotail/magnetosphere/ionosphere can store energy for a maximum of ∼4 h before it has dissipated away. This concept is defining one for ER/substorm energy storage. Our scenario indicates that the rate of solar wind energy injection into the magnetotail/ magnetosphere/ionosphere for storage determines the potential form of energy release into the magnetosphere/ionosphere. This may be more important to understand solar wind–magnetosphere coupling than the dissipation mechanism itself (in understanding the form of the release). The concept of short-term energy storage is also applied for the solar case. It is argued that it may be necessary to identify the rate of energy input into solar magnetic loop systems to be able to predict the occurrence of solar flares.

Turbulent cross-field transport of non-thermal electrons in coronal loops: theory and observations

A fundamental problem in astrophysics is the interaction between magnetic turbulence and charged particles. It is now possible to use \emph{Ramaty High Energy Solar Spectroscopic Imager (RHESSI)} observations of hard X-rays (HXR) emitted by electrons to identify the presence of turbulence and to estimate the magnitude of the magnetic field line diffusion coefficient at least in dense coronal flaring loops.} {We discuss the various possible regimes of cross-field transport of non-thermal electrons resulting from broadband magnetic turbulence in coronal loops. The importance of the Kubo number $K$ as a governing parameter is emphasized and results applicable in both the large and small Kubo number limits are collected.} {Generic models, based on concepts and insights developed in the statistical theory of transport, are applied to the coronal loops and to the interpretation of hard X-ray imaging data in solar flares. The role of trapping effects, which become important in the non-linear regime of transport, is taken into account in the interpretation of the data.} For this flaring solar loop, we constrain the ranges of parallel and perpendicular correlation lengths of turbulent magnetic fields and possible Kubo numbers. We show that a substantial amount of magnetic fluctuations with energy $\sim 1%$ (or more) of the background field can be inferred from the measurements of the magnetic diffusion coefficient inside thick-target coronal loops.

Magneto-seismology of solar atmospheric loops by means of longitudinal oscillations

AbstractThere is increasingly strong observational evidence that slow magnetoacoustic modes arise in the solar atmosphere. Solar magneto-seismology is a novel tool to derive otherwise directly un-measurable properties of the solar atmosphere when magnetohydrodynamic (MHD) wave theory is compared to wave observations. Here, MHD wave theory is further developed illustrating how information about the magnetic and density structure along coronal loops can be determined by measuring the frequencies of the slow MHD oscillations. The application to observations of slow magnetoacoustic waves in coronal loops is discussed.

Review of long-term fault detection approaches in solar thermal systems

Abstract This paper presents an overview, assessment and comparison of automated fault detection methods that check if solar heating systems are functioning correctly. Fault detection in solar thermal systems is important to minimize the time when the system is not functioning well, thereby ensuring an optimal energy (and economic) yield. During the past decades many systems have been monitored, mainly for scientific or demonstration projects by logging measurement data which was subsequently analysed by an expert. Automation of fault detection is necessary to reduce costs and minimize experts’ time needed for analysis of a system. An overview of existing fault detection approaches is given; these are evaluated and compared with a multi-criteria analysis. The only commercially available automated method, the Input–Output Controller, detects faults causing more than 20% energy loss in the solar loop. The function control approach is cheap without a heat meter, and only relies on few sensors to check how several components in the solar loop are functioning with algorithms. The approach developed at Kassel University checks how well a solar plant is functioning both with plausibility checks and with energy balances based on simulations. This method includes a larger part of the solar heating system and therefore requires more measurement equipment. Further research and application of several fault detection methods should improve the effectiveness and costs of these methods.

Thermal conduction effects on the kink instability in coronal loops

Context. Heating of the solar corona by nanoflares, which are small transient events in which stored magnetic energy is dissipated by magnetic reconnection, may occur as the result of the nonlinear phase of the kink instability (Hood et al. 2009). Because of the high temperatures reached through these reconnection events, thermal conduction cannot be ignored in the evolution of the kink instability. Aims. To study the effect of thermal conduction on the nonlinear evolution of the kink instability of a coronal loop. To assess the efficiency of loop heating and the role of thermal conduction, both during the kink instability and for the long time evolution of the loop. Methods. Numerically solve the 3D nonlinear magnetohydrodynamic equations to simulate the evolution of a coronal loop that is initially in an unstable equilibrium. The initial state has zero net current. A comparison is made of the time evolution of the loop with thermal conduction and without thermal conduction. Results. Thermal conduction along magnetic field lines reduces the local temperature. This leads to temperatures that are an order of magnitude lower than those obtained in the absence of thermal conductivity. Consequently, different spectral lines are activated with and without the inclusion of thermal conduction, which have consequences for observations of solar corona loops. The conduction process is also important on the timescale of the fast magnetohydrodynamic phenomena. It reduces the kinetic energy released by an order of magnitude. Conclusions. Thermal conduction plays an essential role in the kink instability of coronal loops and cannot be ignored in the forward modelling of such loops.

Modeling and experimental validation of the solar loop for absorption solar cooling system using double-glazed collectors

Solar cooling applied to buildings is without a doubt an interesting alternative for reducing energy consumption in traditional mechanical steam compression air conditioning systems. The study of these systems should have a closely purely fundamental approach including the development of numerical models in order to predict the overall installation performance. The final objective is to estimate cooling capacity, power consumption, and overall installation performance with relation to outside factors (solar irradiation, outside temperature...). The first stage in this work consists of estimating the primary energy produced by the solar collector field. The estimation of this primary energy is crucial to ensure the evaluation of the cooling capacity and therefore the cooling distribution and thermal comfort in the building. Indeed, the absorption chiller performance is directly related to its heat source. This study presents dynamic models for double glazing solar collectors and compares the results of the simulation with experimental results taken from our test bench (two collectors). In the second part, we present an extensive collector field model (36 collectors) from our solar cooling installation at The University Institute of Technology in St Pierre, Reunion Island as well as our stratified tank storage model. A comparison of the simulation results with real scale solar experimental data taken from our installation enables validation of the double glazing solar collector and stratified tank dynamic models.

Congruency between Biophysical Data from Multiple Platforms and Molecular Dynamics Simulation of the Double-Super Helix Model of Nascent High-Density Lipoprotein

The predicted structure and molecular trajectories from >80 ns molecular dynamics simulation of the solvated Double-Super Helix (DSH) model of nascent high-density lipoprotein (HDL) were determined and compared with experimental data on reconstituted nascent HDL obtained from multiple biophysical platforms, including small angle neutron scattering (SANS) with contrast variation, hydrogen-deuterium exchange tandem mass spectrometry (H/D-MS/MS), nuclear magnetic resonance spectroscopy (NMR), cross-linking tandem mass spectrometry (MS/MS), fluorescence resonance energy transfer (FRET), electron spin resonance spectroscopy (ESR), and electron microscopy. In general, biophysical constraints experimentally derived from the multiple platforms agree with the same quantities evaluated using the simulation trajectory. Notably, key structural features postulated for the recent DSH model of nascent HDL are retained during the simulation, including (1) the superhelical conformation of the antiparallel apolipoprotein A1 (apoA1) chains, (2) the lipid micellar-pseudolamellar organization, and (3) the solvent-exposed Solar Flare loops, proposed sites of interaction with LCAT (lecithin cholesteryl acyltransferase). Analysis of salt bridge persistence during simulation provides insights into structural features of apoA1 that forms the backbone of the lipoprotein. The combination of molecular dynamics simulation and experimental data from a broad range of biophysical platforms serves as a powerful approach to studying large macromolecular assemblies such as lipoproteins. This application to nascent HDL validates the DSH model proposed earlier and suggests new structural details of nascent HDL.

Astrophysical jets and solar loops in the lab

Astrophysical jets and solar loops in the lab

Astrophysical jets and solar loops in the lab

Astrophysical jets and solar loops in the lab

Extraction of Solar Coronal Magnetic Loops with the Directional 2D Morlet Wavelet Transform

We present an automated extraction method based on the continuous wavelet transform (CWT) for analyzing solar magnetic loops. The aim of the work is to extract, from the images taken from solar EUV telescopes, the traces of bright loops presumably shaped by the magnetic field of the solar corona. The technique is that of wavelet analysis, using the two-dimensional Morlet wavelet, because of its efficiency in detecting oriented features, which allows us to follow closely the curvature of the loops. Next, we segment the wavelet modulus image and we threshold it, both globally and locally (i.e., adaptively), in order to eliminate the remaining noise. Altogether, our method performs well, it is robust and fast, and could be used as a standard tool for analyzing large data sets expected from missions like SDO.

Evidence of wave harmonics in a brightened magnetic network observed from Hinode/EIS

We study intensity oscillations in the upper chromosphere to search acoustic wave harmonics in a low-atmospheric solar cavity associated with a brightened magnetic network near the south pole of the Sun. We use a wavelet tool to produce power spectrum of He II 256.32 Å intensity oscillation as observed by EUV Imaging Spectrometer (EIS) onboard Hinode spacecraft. We detect a periodicity of ∼14.33 min with a probability of 99%–100%. Using wavelet filtering and reconstruction techniques, we detect other statistically significant periodicities of ∼8.51 min and ∼5.51 min with the probabilities 98% and 97% respectively. These observed periodicities provide the first possible signature of acoustic wave harmonics in a low-atmospheric cavity-canopy system. The ratio of the period of first harmonic ( P 1 = 14.33 min) to the period of second harmonic ( P 2 = 8.51 min) is 1.68. The difference of this period ratio from 2.0 may be the first signature of a density stratification in low-atmospheric solar cavities, complementing such period ratio differences of acoustic wave harmonics have been reported previously in the solar loops as a diagnostic of their density stratification. We seismologically estimate the density scale height as ∼0.57 Mm in the cavity–canopy loop interface. This scale height is low compared to the hydrostatic scale heights estimated at wide range of temperatures that correspond to the considerable formation of He II ion in the lower solar atmosphere. The departure of density scale height estimated by the principle of MHD seismology from the values of hydrostatic scale heights estimated at wide temperature range (4.2 < Log T e < 5.2), probably indicates non-hydrostatic conditions above the cavity–canopy interface.

Physics of ion acceleration in the solar flare on 2005 September 7 determines γ-ray and neutron production

Relativistic neutrons were observed by the neutron monitors at Mt. Chacaltaya and Mexico City and by the solar neutron telescopes at Chacaltaya and Mt. Sierra Negra in association with an X17.0 flare on 2005 September 7. The neutron signal continued for more than 20 min with high statistical significance. Intense emissions of γ -rays were also registered by INTEGRAL, and during the decay phase by RHESSI. We analyzed these data using the solar-flare magnetic-loop transport and interaction model of Hua et al. [Hua, X.-M., Kozlovsky, B., Lingenfelter, R.E. et al. Angular and energy-dependent neutron emission from solar flare magnetic loops, Astrophys. J. Suppl. Ser. 140, 563–579, 2002], and found that the model could successfully fit the data with intermediate values of loop magnetic convergence and pitch-angle scattering parameters. These results indicate that solar neutrons were produced at the same time as the γ -ray line emission and that ions were continuously accelerated at the emission site.

DYNAMIC MAGNETOGRAPHY OF SOLAR FLARING LOOPS

We develop a practical forward fitting method based on the SIMPLEX algorithm with shaking, which allows the derivation of the magnetic field and other parameters along a solar flaring loop using microwave imaging spectroscopy of gyrosynchrotron emission. We illustrate the method using a model loop with spatially varying magnetic field, filled with uniform ambient density and an evenly distributed fast electron population with an isotropic, power-law energy distribution.

Topological constraints on solar loop plasma instabilities and Alfvén waves

Abstract Inhomogeneous plasmas-solar instabilities-are investigated by using the techniques of classical differential geometry for curves, where the Frenet torsion and curvature describe completely the motion of a curve. In our case, the Frenet frame changes in time and also depends upon the other coordinates, taking into account the inhomogeneity of the plasma. The exponential perturbation method, so commonly used to describe cosmological perturbations, is applied to the magnetohydrodynamic (MHD) plasma equations to find modes describing Alfvén wave propagation in the medium of planar loops. Stability is investigated in the imaginary axis of the spectra of complex frequencies ω, i.e. $$ \Im $$ m (ω) ≠ 0. A pratical guide for experimental solar physicists is given by computing the twist of force-free solar loops, which generalizes the Parker formula relating the twist to the Frenet torsion. In our expression the twist of the solar loops also depends on the abnormality of the normal vector of the frame.

Parametric interaction of coronal loops with p modes

Parametric interaction of coronal loops with p modes

THERMAL FRONTS IN FLARING MAGNETIC LOOPS

We examine the existence of thermal fronts in solar flaring magnetic loops. In the past, two opposite conclusions have been made from the analysis of the evolution of a system in which hot electrons are injected into a coronal magnetic loop. One, made on the basis of analytical solutions, claims that this leads to the formation of ion-acoustic turbulence and subsequently a thermal front develops, while the opposing view obtained later on the basis of detailed numerical modeling shows no evidence of thermal fronts. In our study a one dimensional Vlasov solver is implemented to solve the distribution functions for each particle species fa(r, v, t). The numerical simulations are electrostatic since we examine motions along the field lines only. The results presented here show that earlier numerical results only showed the absence of thermal fronts because of the restricted size of the initial hot-electron region. If larger hot regions are simulated then thermal fronts do indeed form. This dependence on the system size is also explained. These results therefore confirm some of the theory presented by Levin and Melnikov and Brown et al. (i.e., the hot electrons are confined). The net effect of thermal fronts is that the electron energy is confined (mi/me)1/2 times longer than the estimate based on the free streaming of hot electrons.

Stability and mode analysis of solar low-coronal structures using irreversible energy principles

Low coronal highly anisotropic structures as loops and prominences are relatively stable configurations with densities hundreds of times higher and temperatures hundreds of times lower than their surrounding corona. Magnetohydrodynamics (MHD) open systems, generally driven by coupled mechanical and thermal perturbations, can go beyond the linear state configurations into new stationary states known as nonlinear equilibria. Thus, a crucial requirement for any theoretical model that intends to describe these far-from-equilibrium states is to give an account of the stability and evolution of the numerous nonlinear thermodynamic stationary states that can arrive. Observations obtained with high spatial and time resolution instruments of the new spacecraft generation describe a wide spectrum of configurations which sustain fast and slow magnetoacoustic oscillations or propagating MHD waves that are ducted by the magnetic fields of the low β-corona media. Measurements of characteristic periods, speeds and damping times as well as different theoretical models that intend to describe non-dissipative damping mechanisms and leakage processes of systems with high-Reynolds number provide us with new diagnostic tools to reveal unknown or more accurate solar physics parameters. We summarize some theoretical and observational results that give an account of the coronal seismology state-of-art. We present a thermodynamic stability criterion to describe the modes and stability of coronal structures. We give some results of its application to coronal seismology. This allows us to discuss the feasibility of wave- and flow-based models for solar loops and to offer a different explanation to the presence of frequencies associated with helioseismological p-modes at the altitude of the corona.