Heat removal factor for a flat-plate solar collector with a serpentine tube

The performance of a flat-plate solar collector is investigated. The collector is of the sheet-and tube design and the tube is bonded to the absorbing plate in a serpentine fashion. Equations describing the variation of the fluid temperature in the different segments of the serpentine are derived. These equations are then used to determine the heat removal factor F_R for the collector.It is shown that for the general case of an N-bend serpentine, the heat removal factor depends on three non-dimensional groups containing the different operational and design variables of the collector. A generalized chart for estimating F_R for collectors with serpentines of arbitrary geometry and number of bends is presented.     

Derivation of efficiency factors for uneven irradiation on a fin absorber

In the equation for thermal energy output from a flat-plate solar collector (written as a function of the collector mean heat carrier temperature), both the gain and the loss terms are multiplied by the collector efficiency factor, F^′. For a concentrating collector with an uneven (non-uniform) irradiation on the absorber, the efficiency factor for the gain term, here called the optical efficiency factor, F^′_c, is different from F^′ and is a function of the irradiation distribution on the absorber. If the heat loss coefficient is assumed to be constant across the fin, the optical efficiency factor for absorbed irradiation at a certain distance from the edge of the absorber is independent of absorbed irradiation at other locations and can therefore be expressed F^′_c(x), where x is the distance from the edge of the absorber. Close to the edge, F^′_c(x)<F^′ and close to the pipe, F^′_c(x)>F^′. In this paper formulas are derived for calculating F^′_c(x) for a fin absorber with constant fin thickness. By weighting F^′_c(x) with the absorbed irradiance, S_c(x), and integrating across the absorber, an absorber average optical efficiency factor, F^′_c,a, is obtained. This value replaces F^′ in the gain term of the equation for thermal energy output. If, instead, the energy output equation is expressed as a function of the inlet temperature, F^′_c,a can be used for calculating a corresponding heat removal factor for uneven irradiation, F_R,c. Formulas for calculating the temperature distribution across the absorber for the case of uneven irradiation are also derived.     

Study and optimization of the thermal performances of the offset rectangular plate fin absorber plates, with various glazing

The low thermophysical characteristics of air used as a heat transfer fluid in the solar collectors with thermal conversion require a fully developed turbulent flow. This increases the thermal heat transfer between the absorber plate and the fluid, which clearly improves the thermal performances of the solar collector with obstacles arranged into the air channel duct. In the present work, we introduce, in solar collector, the offset rectangular plate fins, which are used in heat exchangers. An experimental investigation carried out showed the generated enhancement of thermal performance. The offset rectangular plate fins, mounted in staggered pattern, are oriented parallel to the fluid flow and are soldered to the underside of absorber plate. They are characterized by high heat transfer area per unit volume. High thermal performances are obtained with low pressure losses and in consequence a low electrical power consumption by the fan in comparison to the flat plate collector. The experimental results are all so compared by using two types of transparent cover; double and triple.     

Combined natural convection and surface radiation in solar collector equipped with partitions

A comprehensive numerical study was carried out to study the thermal performances of a solar collector equipped with partitions attached to its glass cover. This technique is used to favorise and increase the heat transfer coefficient between the working fluid, (air) and the absorber in the solar collector. A specifically developed numerical model based on the finite-volume method and the SIMPLER algorithm is used for the solution of the mass, momentum and energy transfer governing equations. The discretized equations are solved iteratively by an algorithm based on a preconditioned conjugate gradient method. Concerning the radiation exchange, we consider that the working fluid is transparent, so only the solid surfaces contribute to the radiation exchange and assumed to be diffuse-gray. A complete parametric study is made for various partitions length Lp, aspect ratio A and Rayleigh numbers Ra for Prandtl number Pr = 0.71. Results are reported in terms of isotherms, streamlines and average Nusselt-numbers along the absorber plate.     

On the correlations between collector efficiency factor and material content of parallel flow flat-plate solar collectors

The collector efficiency factor F^′, besides the collector heat loss coefficient U_L, characterizes the thermal quality of a solar collector. As F^′ is strongly influenced by the tube distance w and the absorber plate thickness δ, F^′ is also correlated with the material content of absorber plus tubing. Due to the future mass production of collectors and to the restricted copper resources (in the literature, a range until 2026 is given), the role of material savings can be expected to become more and more important. This paper focuses on the correlations between F^′ and the material content of absorber and tubing for flat-plate collectors with the fin-and-tube geometry. The correlations between w, δ, F^′ and material content are presented in a new type of nomograph. This nomograph indicates the values of w and δ that minimize the material content (for a given F^′). For a typical absorber with F^′=0.90, material savings of 25% can theoretically be achieved without any deterioration of F^′, by reducing the absorber plate thickness and the tube distance. The resulting plate thickness is below 0.1 mm; the respective tube distance will be about 7 cm. Practical restrictions are discussed. In a sensitivity analysis, the influence of different parameters on F^′ is investigated. The most important parameters are w, U_L,δ and the Reynolds number. The technique chosen for contacting tube and absorber has only a minor influence on F^′.     

Design and test of non-evacuated solar collectors with compound parabolic concentrators

The intermediate range of concentration ratios (1.5X–10X) which can be achieved with CPCs without diurnal tracking provides both economic and thermal advantages for solar collector design even when used with non-evacuated absorbers. The present paper summarizes more than 3 yr of research on non-evacuated CPCs and reviews measured performance data and critical design considerations. Concentrations in the upper portions of the practical range (e.g. 6X) can provide good efficiency (40–50 per cent) in the 100–160°C temperature range with relatively frequent tilt adjustments (12–20 times per year). At lower concentrations (e.g. 3X) performance will still be substantially better than that for a double glazed flat plate collector above about 70°C and competitive below, while requiring only semi-annual adjustments for year round operation. In both cases the cost savings associated with inexpensive reflectors, and the optimal coupling to smaller, simple inexpensive absorbers (e.g. tubes, fins, etc.) can be as important an advantage as the improved thermal performance.The design problems for non-evacuated CPC collectors are entirely different from those for CPC collectors with evacuated receivers. For example, heat loss through the reflector can become critical, since ideal CPC optics demands that the reflector extend all the way to the absorber. Recent improvements in reflector surfaces and low cost antireflection coatings have made practical a double-glazed non-evacuated CPC design. It is calculated that a 1.5X version of such a collector would have an optical efficiency η_o = 0.71, a heat loss coefficient U = 2.2 W/m²°C and a heat extraction effciency factor F′ ≥ 0.98, while requiring no tilt adjustments.     

Numerical and experimental study of a solar equipped with offset rectangular plate fin absorber plate

A mathematical model that allows the determination of the thermal performances of the single-pass solar air collector with offset rectangular plate fin absorber plate is developed. The model can predict the temperature profile of all the components of the collector and of the air stream in the channel duct. Into the latter are introduced the offset rectangular plate fins, which increase the thermal heat transfer between the absorber plate and the fluid. The offset rectangular plate fins, mounted in a staggered pattern, are oriented parallel to the fluid flow and are soldered to the underside of the absorber plate. They are characterized by high heat transfer area per unit volume and generate the low pressure losses. The experimental results of the air stream temperature will be compared with the results obtained by the theoretical model suggested.     

Heat extraction efficiency of a concentric glass tubular evacuated collector

The results of detailed measurements and calculations of the properties of Sydney University/Nitto Kohki evacuated collector tubes have been used to develop a formula for the instantaneous heat extraction efficiency η of a collector panel incorporating the evacuated tubes. The instantaneous efficiency depends on ambient temperature, mean fluid temperature in the collector, solar flux and the design of the manifold used to extract heat from the glass absorber tubes. Manifold design determines the mean temperature difference between absorber tube surface and mean fluid temperature for given operating conditions, and strongly affects the efficiency η of a collector panel. Neither changes in the number of evacuated tubes per unit area of collector, nor variations in solar flux, significantly alter the efficiency decrement Δ η₀ associated with a particular manifold design. Calculated efficiencies agree well with experimental results for collector panels incorporating manifolds of various designs. The formula for efficiency η allows detailed analysis of the relative importance of various energy loss mechanisms in a collector.     

Simulation model of a CPC collector with temperature-dependent heat loss coefficient

We describe a mathematical model for the optical and thermal performance of non-evacuated CPC solar collectors with a cylindrical absorber, when the heat loss coefficient is temperature-dependent. Detailed energy balance at the absorber, reflector and cover of the CPC cavity yields heat losses as a function of absorber temperature and solar radiation level. Using a polynomial approximation of those heat losses, we calculate the thermal efficiency of the CPC collector. Numerical results show that the performance of the solar collector (η vs. ΔT_f(0)/I_coll) is given by a set of curves, one for each radiation level. Based on the solution obtained to express the collector performance, we propose to plot efficiency against the relation of heat transfer coefficients at absorber input and under stagnation conditions. The set of characteristic curves merge, then, into a single curve that is not dependent on the solar radiation level. More conveniently, linearized single plots are obtained by expressing efficiency against the square of the difference between the inlet fluid temperature and the ambient temperature divided by the solar radiation level. The new way of plotting solar thermal collector efficiency, such that measurements for a broad range of solar radiation levels can be unified into a single curve, enables us to represent the performance of a large class of solar collectors, e.g. flat plate, CPC and parabolic troughs, whose heat loss functions are well represented by second degree polynomials.     

A comparison of optical performance between evacuated collector tubes with flat and semicylindric absorbers

In this article, concepts of solar irradiance ratio and absorbed energy factor on the surface of the evacuated collector tube absorbers were presented respectively. For evacuated collector tubes with flat and semicylindric absorbers, we used a solar simulator as a light source, measured separately distribution of the solar irradiance ratio that varies with incident angles on various points on the absorber surface in a glass-covered tube, and gave their three-dimensional regressive equations correspondingly. Experimental measurement of solar irradiance ratio and solar absorptance of coatings on absorber surfaces was carried out. On this basis, rules of absorbed energy factors on absorbers in two shapes that vary with incident angles were analyzed and studied. According to clear-day model, the daily absorbed energy and its annual changes of single evacuated collector tubes with absorbers in two shapes placed under 40° northern latitude, 40° inclined angle and south orientation were calculated and compared. The results show that the annual absorbed energy of evacuated collector tube with a semicylindric absorber is 15.9% higher than that with a flat absorber. In addition, optimized incident angles for the absorber in two shapes of evacuated collector tubes operated in a whole year were tentatively investigated.     

Theoretical and experimental study of efficiency factor,heat transfer and thermal heat loss coefficients in solar air collectors with selective and nonselective absorbers

The thermal heat performance of a solar air collector depends strongly on the thermal heat loss and the efficiency factor. In order to increase these performances, it is necessary to use a solar air collector which is well insulated and where the fluid flow is fully developed turbulent flow. It needs a high heat transfer between the absorber plate and the fluid to decrease the absorber‐plate temperature and hence the heat loss by radiation from the absorber to the ambient. This increases the efficiency factor. In the present paper, the heat loss and efficiency factor are treated for solar air collectors with selective and nonselective absorber plate. It is shown that the selectivity of the absorber plate cannot play an important role in a well‐insulated solar collector with a fanned system which permits a fully developed turbulent flow and, in consequence, high heat transfer. Copyright © 1999 John Wiley & Sons, Ltd.     

Thermal and mechanical performance of sealed,gas-filled,flat plate solar collectors

The study includes calculations for both the thermal performance and the mechanical behaviour of a gas-filled, flat plate solar collector without external gas expansion, i.e., a collector with varying gas volume and gas pressure and movement in both cover glass and absorber. Classical theories for the thermal performance are combined with a finite-element method to investigate which factors have an impact from the mechanical stress point of view.This article describes major results for collectors with copper and aluminium absorbers combined with different inert gases. It is shown that a collector may be designed which uses less material than a standard collector but achieves at least the same thermal performance, by using a thinner collector and a thinner absorber and a suitable gas filling other than air. If copper is used in absorber and tubes, a 0.15 mm thick absorber together with a tube-to-tube distance of 103 mm results in the same performance as a 0.3 mm absorber with a 144 mm tube-to-tube distance, but the former will use 25% less material. The use of copper can be further reduced if the absorber is made of aluminium and the tubes are made of copper. The factor of safety for thick (>0.5 mm) aluminium absorbers is, however, not as large as it is for copper absorbers.     

Comparative study of fluid-in-metal manifolds for heat extraction from single ended evacuated glass tubular collectors

An investigation is reported of heat transfer between the glass absorber tubes of all-glass evacuated collectors and fluid-in-metal manifolds designed for heat extraction from the glass asborber tubes. The heat transfer is studied using a novel solar simulator which heats a panel of glass tubes electrically to simulate solar input to a collector panel. Measurement of the temperatures at various points on the glass tubes and on the manifolds gives a measure of the efficiency of heat transfer for each manifold under various operating conditions and allows calculation of the efficiencies η₀ of collectors incorporating the manifolds. The effect of fluid temperature, collector inclination and fluid flow rate has been investigated for four manifold designs of increasing simplicity. Experimental results for the manifolds are compared with calculations of heat transfer. Potential lifetime problems for the manifolds are also discussed. The simplest manifold design is shown to have good prospects for high-temperature (>100°C) heat extraction.     

Modelling of parabolic trough direct steam generation solar collectors

Solar electric generation systems (SEGS) currently in operation are based on parabolic trough solar collectors using synthetic oil heat transfer fluid in the collector loop to transfer thermal energy to a Rankine cycle turbine via a heat exchanger. To improve performance and reduce costs direct steam generation in the collector has been proposed. In this paper the efficiency of parabolic trough collectors is determined for operation with synthetic oil (current SEGS plants) and water (future proposal) as the working fluids. The thermal performance of a trough collector using Syltherm 800 oil as the working fluid has been measured at Sandia National Laboratory and is used in this study to develop a model of the thermal losses from the collector. The model is based on absorber wall temperature rather than fluid bulk temperature so it can be used to predict the performance of the collector with any working fluid. The effects of absorber emissivity and internal working fluid convection effects are evaluated. An efficiency equation for trough collectors is developed and used in a simulation model to evaluate the performance of direct steam generation collectors for different radiation conditions and different absorber tube sizes. Phase change in the direct steam generation collector is accounted for by separate analysis of the liquid, boiling and dry steam zones.     

Heat Transfer by Nanofluid with Different Nanoparticles in a Solar Collector

The buoyancy flow and heat transfer characteristics inside a solar collector having the flat‐plate cover and sinusoidal corrugated absorber are analyzed numerically. The water‐based nanofluid with alumina and copper nanoparticles is used as the working fluid inside the solar collector. The governing partial differential equations with proper boundary conditions are solved by the finite element method using Galerkin's weighted residual scheme. The behavior of both nanoparticles related to performance such as temperature and velocity distributions, radiative and convective heat transfers, mean temperature, and velocity of the nanofluid is investigated systematically. This performance includes the solid volume fraction, namely ?₁ and ?₂, with respect to Al ₂ O ₃ and Cu nanoparticles. The results show that the better performance of heat transfer inside the collector is found by using the highest ?₂ than ?₁. The result of this study expresses a good agreement with the theoretical result available in the literature. © 2013 Wiley Periodicals, Inc. Heat Trans Asian Res, 43(1): 61–79, 2014; Published online in Wiley Online Library ( wileyonlinelibrary.com/journal/htj ). DOI 10.1002/htj.21061     

Theoretical and Experimental Studies of the Ultra-Thin-Channel Solar Water Collector

Ultra-thin-channel solar water collector efficiency (UCSWC) was investigated theoretically and experimentally. An ultra-thin-channel solar water collector was constructed using several flat plates with an ultra-thin fluid channel formed using an adjustable flexible silicon frame inserted between the absorber plate and bottom plate. The advantages of the ultra-thin-channel solar water collector are low absorber plate temperature and low total water mass flow rate, resulting in considerable collector efficiency improvement with high outlet fluid temperature and low pump power requirement. A simple and general modeling method was developed to predict the collector efficiencies and mean temperatures of the glass cover, absorber plate and fluid. Good agreement was achieved between the calculated and experimental values. The superior collector efficiencies of the UCSWC are obtained as 82.2% and 75.5% for the inlet temperatures 30°C and 70°C, respectively, operating at a total fluid mass flow 8.3 × 10^?3 kg/s and solar radiation incident of 1100 W/m².     

Experimental study of heat transfer and flow friction insolar air heaters with and without selective absorbers

High thermal performances have been needed for the use of solar air collectors andcompact heat exchangers. The thermal performance improvements have prompted us to lead thiscomparative study on the solar air heaters with and without selective absorbers. Rectangular finsare soldered staggered on the collectors back. The interstices are let between two consecutive finslocated in the same rows. The fluid flow undergoes constrictions followed one after the other byexpansions. A turbulent fluid flow is developed, that permits the improvement of the thermalheat transfer of these collectors in comparison to the flat-plate. For the same fin configurations,the thermal heat transfer coefficient has been evaluated with selective or non-selectiveabsorber-plate. The results obtained with the fin configurations with black-painted absorber(non-selective absorber), are compared with those of the fin configuration with coppersunabsorber-plate (selective absorber). The aim of this study is to show that the nature of theabsorber-plate (selective or nonselective) has no important effect on the thermal performances ofthe fanned system collector. Only a trivial increase is noted on behalf of the selective absorbercollector.     

New heat transfer factors for flat plate solar collectors

New expressions for the heat transfer factor F_R based on inlet, outlet and mean fluid temperature are derived for use with the Hottel-Whillier collector model by the use of restrictive heat transfer assumptions. The new factors are simple algebraic functions which are shown to be equivalent to the generally accepted exponential relationships in realistic collector applications. Their utility is demonstrated through a set of equations by which efficiency curves based on inlet, outlet or mean fluid temperatures may be converted to an alternate basis if the flow rate and the slope and intercept of one efficiency curve are known.     

Elastomer–metal–absorber: development and application

A new principle of a solar collector that consists of appropriately shaped metal form plates as absorbers and clipped in elastomer fluid pipes, the so-called elastomer–metal–absorber, will be presented. The advantages are its freeze resistance, its seawater suitability and new possibilities for simplified collector installation and system techniques. The design parameters including a detailed analysis of the thermal resistance between the absorber and fluid will be discussed, where special regard is given to the development of an appropriate elastomer material with high thermal conductivity as one of the key items. The first development steps have shown that absorbers with a high thermal performance may be constructed. Finally, the idea of application of the principle of the elastomer–metal–absorber to metal roofs and façades will be presented. This idea is followed up within a development project.     

HIGH EFFICIENCY EVACUATED FLAT-PLATE SOLAR COLLECTOR FOR PROCESS STEAM PRODUCTION

A flat-plate solar collector for process steam production was developed. The operating temperatures are in the range between 100 and 150°C. The boiling collector can be used for process heat supply in the industry and for solar cooling applications as well. It operates as a system with controlled influx of liquid. Instabilities of the two-phase flow in the internal evaporator have been successfully suppressed and the design of the system has been investigated. We constructed a prototype collector based on a commercially available evacuated flat-plate collector. To realise high thermal efficiencies at temperatures up to 150°C, the thermal losses of the absorber have been drastically reduced using an ultra low emissive selective absorber, a low pressure krypton filling (50 hPa) in the collector casing, and a highly reflecting aluminium foil between absorber and rear side. The prototype collector was dynamically tested at our outdoor test facility and showed very high efficiencies of more than 60% at 100°C steam temperature and of 45% at 150°C steam temperature (T_amb=15°C). The operation behaviour of the prototype was always stable and the steam mass quality showed excellent values of nearly 100%.