Correlations for natural convective heat exchange in CPC solar collector cavities determined from experimental measurements

A detailed experimental study was undertaken to analyse the natural convective heat transfer in CPC cavities, a complex function of collector orientation, geometrical aspect ratios and thermal boundary conditions at the enclosure walls. Results are reported for CPC solar collectors with full-, three quarter- and half-height reflectors, C_R = 2 and a 100 mm wide flat plate absorber. Experiments were conducted using a purpose built solar simulator under controlled lab environment employing realistic boundary and thermal conditions. The effects of simultaneous tilting of the solar collectors about both transverse and longitudinal axes, truncation of the reflector walls and inlet water (collector heat removal fluid) temperature on the natural convective heat flow characteristics inside the CPC cavity have been determined. It is concluded that the correlations developed for prediction of natural convection characteristics in rectangular, annuli and V-trough enclosures are not appropriate for application to CPC solar collectors with divergence ranging from 150% to 300%. Based on the experimental data a correlation is presented to predict the natural convection heat loss from the absorber plate of solar collectors for a range of water inlet temperatures.     

Optimization of solar-powered Stirling heat engine with finite-time thermodynamics

A mathematical model for the overall thermal efficiency of the solar-powered high temperature differential dish-Stirling engine with finite-rate heat transfer, regenerative heat losses, conductive thermal bridging losses and finite regeneration processes time is developed. The model takes into consideration the effect of the absorber temperature and the concentrating ratio on the thermal efficiency; radiation and convection heat transfer between the absorber and the working fluid as well as convection heat transfer between the heat sink and the working fluid. The results show that the optimized absorber temperature and concentrating ratio are at about 1100 K and 1300, respectively. The thermal efficiency at optimized condition is about 34%, which is not far away from the corresponding Carnot efficiency at about 50%. Hence, the present analysis provides a new theoretical guidance for designing dish collectors and operating the Stirling heat engine system.     

System analysis of a multifunctional PV/T hybrid solar window

The work presented in this article aims to investigate a PV/T hybrid solar window on a system level. A PV/T hybrid is an absorber on which solar cells have been laminated. The solar window is a PV/T hybrid collector with tiltable insulated reflectors integrated into a window. It simultaneously replaces thermal collectors, PV-modules and sunshade. The building integration lowers the total price of the construction since the collector utilizes the frame and the glazing in the window. When it is placed in the window a complex interaction takes place. On the positive side is the reduction of the thermal losses due to the insulated reflectors. On the negative side is the blocking of solar radiation that would otherwise heat the building passively. This limits the performance of the solar window since a photon can only be used once. To investigate the sum of such complex interaction a system analysis has to be performed. In this paper results are presented from such a system analysis showing both benefits and problems with the product. The building system with individual solar energy components, i.e. solar collector and PV modules, of the same size as the solar window, uses 1100 kW h less auxiliary energy than the system with a solar window. However, the solar window system uses 600 kW h less auxiliary energy than a system with no solar collector.     

An experimental investigation of the heat losses of a U-type solar heat pipe receiver of a parabolic trough collector-based natural circulation steam generation system

The performance of a parabolic trough collector (PTC)-based steam generation system depends significantly on the heat losses of the solar receiver. This paper presents an experimental study of the heat losses of a double glazing vacuum U-type solar receiver mounted in a PTC natural circulation system for generating medium-temperature steam. Field experiments were performed to determine the overall heat losses of the receiver. Effects of wind, vacuum glass tube, radiation, and structural characteristics on the heat losses were analyzed. The thermal efficiency of the receiver was found to be 0.791 and 0.472 in calm and windy days, respectively, at a test temperature of about 100 °C, whereas the thermal efficiencies became 0.792 and 0.663, respectively, while taking the receiver element into consideration. The heat losses were increased from 0.183 to 0.255 kW per receiver for the two cases tested. It was shown that neither convection nor radiation heat losses may be negligible in the analysis of such U-type solar receivers.     

Numerical simulation of concrete hollow bricks by the finite volume method

Numerical thermal analysis has become a powerful tool for hollow brick design concerning energy saving issue. In the present paper it was employed to understand the heat transfer performance of 240 × 115 × 90 concrete hollow bricks with 71 different configurations. The general commercial computational fluid dynamics (CFD) soft FLUENT^® was used in the research. A comprehensive investigation on the effect of enclosure configurations with the same void volume fraction on equivalent thermal conductivity (ETC) was conducted. The enclosure numbers in parallel and vertical directions of heat transfer were investigated in details. The effect of enclosure staggered form is also discussed carefully. In the research, ETC data was compared with radiation and nature convection and without them. In addition, temperature and velocity vector distributions were also illustrated in order to clarify heat transfer mechanism. It can be concluded from this research that ETC values are dependent on the combining effect of heat conduction in all domains, natural convection within enclosure and radiation on inner surface. ETC values decrease with the increasing enclosure numbers in the parallel direction of heat transfer and increase in the vertical direction of heat transfer, vise versa. Therefore, increasing enclosure numbers in the parallel direction becomes more favorable to decrease ETC than that in the vertical direction of heat transfer. Secondly, the relative enhancement of radiation heat transfer on ETC ranged from 7.41% to 25.39%, and the nature convection from near zero to 22.50%, depending on the enclosure numbers and their arrangement configurations. Finally, the increasing enclosures in the parallel direction can decrease ETC values from 11.59% to 20.94% for enclosure vertical aligned bricks. Under the given conditions the smallest ETC is 0.32619 W/(m K) for enclosure staggered form with three enclosures in length direction and eight in width direction, which is only 23.30% of the solid concrete brick in the present research.     

Transient analysis of modified cuboid solar integrated-collector-storage system

A novel solar water heating system, modified cuboid solar integrated-collector-storage (ICS) system with transparent insulation material (TIM) has been designed and developed, which combines collection and storage in a single unit and minimizes the nocturnal heat losses. A comprehensive study has been carried out to evaluate the heat transfer characteristics inside the enclosure of the system to enhance the collection and storage of solar energy. The transient behavior of the modified-cuboid solar integrated-collector-storage system is investigated numerically to evolve optimum configuration. The optimum design for the system is obtained by carrying out a numerical parametric study with different geometry parameters like the depth of the cuboid (d = 2, 5, 8, and 12 cm), and inclination angles (10°, 20°, 30°, and 50°). The inside heat transfer coefficient of the ICS system, stratification factor and water temperature distribution inside the enclosure have been predicted by numerical simulation. Average heat transfer coefficient at the bottom surface of absorber plate is 20% higher for depth of 12 cm as compared to the 2 cm depth of cuboid section, after 2 h of heating. The stratification factor also increases from 0.02 to 0.065 as depth of the system increases from 2 cm to 12 cm. There is a marginal effect of inclination angles of the system on the convection in the enclosure. As the inclination angle increases from 10° to 50°, the average heat transfer coefficient increases from 90 W/m² K to 115 W/m² K. But the stratification factor is comparatively high for lower inclination angles. With the optimum design parameters, a field experimental set-up was built and the numerical model was validated for efficient heat collection and storage in a modified cuboid ICS system. The model is in good agreement with the experimental results.     

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.     

Impact of different internal convection control strategies in a non-evacuated CPC collector performance

Over the last decade the technological advances observed in solar collector materials, namely better spectrally selective absorber coatings and ultra clear glass covers, contribute to performance improvements and translate into higher operational temperature ranges with higher efficiency values.While the use of Evacuated Tube Collectors (ETCs) is becoming widespread in the thermal conversion of solar energy, non-evacuated solar collectors still hold advantages at manufacturing, reliability and/or cost levels, making them interesting and competitive for a large range of applications, in particularly, in temperature ranges up to 80 °C. However, these advantages have not prevented the major drawback of these collectors when compared to ETCs: thermal losses due to internal convection which prevent their general use in the range of operating temperatures up to 150 °C.Insulation, double glazing or selective coatings can be used in non-evacuated collectors to reduce heat losses. To prevent internal convection losses in these solar collectors, different control strategies have been studied, such as the adoption of different inert gases within the collector cavity, physical barriers reducing air flow velocities over the absorber or cover surfaces or the use of concentration.In the present article, an assessment of adopting such internal convection control strategies in a CPC collector is presented. Each of the presented strategies is assessed in terms of the resulting collector optical and thermal characterization parameters and yearly collector yield. For this purpose, an integrated tool allowing the design, optical and thermal characterization of CPC collectors was developed. The results obtained provide valuable guidelines for anyone wishing to implement any of these strategies in a new collector design.     

Analysis of wind flow around a parabolic collector (2) heat transfer from receiver tube

Parabolic collectors of commercial solar thermal power plants are subject to variable convection heat transfer from the receiver tube. In the present study heat transfer from a receiver tube of the parabolic trough collector of the 250 kW solar power plants in Shiraz, Iran, is studied taking into account the effects of variation of collector angel of attack, wind velocity and its distribution with respect to height from the ground.The governing equations for the two-dimensional steady state wind flow include continuity, momentum and energy equations and RNG-based k–ε model for turbulence scheme. Finite volume discretization method is used to solve the governing equations with wall function boundary condition and the SIMPLE approach is employed to iterate for the pressure correction and convergence of the velocity field. The momentum equation contains buoyancy force when the buoyancy effect is high and force convection effect is low.Computation is carried out for various wind velocities and different collector orientations with respect to wind direction. For solution of the energy equation, temperature of the receiver tube is taken as 350 K and ambient temperature is assumed to be 300 K. Various recirculation and temperature fields were observed around the receiver tube for different flow conditions. Effect of collector orientation on the average Nu number for the receiver tube was found negligible when the wind speed is low (Re⩽4.5×10⁵ based on the collector aperture). But when the wind velocity is high (Re>4.5×10⁵), the collector effect on the variation of Nu around the glass cover of the absorber tube is considerable.     

Modeling and optimizing parabolic trough solar collector systems using the least squares support vector machine method

Investigating the complicated thermal physics mechanisms of the parabolic trough solar collector systems plays a vital role in efficiently utilizing the solar energy. In this paper, the least squares support vector machine (LSSVM) method is developed to model and optimize the parabolic trough solar collector system. Numerical simulations are implemented to evaluate the feasibility and efficiency of the LSSVM method, where the sample data derived from the experiment and the simulation results of two solar collector systems with 30 m² and 600 m² solar fields, and the complicated relationship between the solar collector efficiency and the solar flux, the flow rate and the inlet temperature of the heat transfer fluid (HTF) is extracted. Some basic rules, such as the solar collector efficiency increases with the increase of the solar flux and the flow rate of the heat transfer fluid, and decreases with the increase of the inlet temperature of the HTF, are obtained, which indicates the LSSVM method is competent to optimize the solar collector systems. As a result, the new approach will provide meaningful data for developing the parabolic trough solar thermal power plant in China.     

An experimental investigation of a natural circulation heat pipe system applied to a parabolic trough solar collector steam generation system

A U-type natural circulation heat pipe system is designed and applied to a parabolic trough solar collector for generating mid-temperature steam. Thermal performance of the heat pipe system is investigated experimentally. A detailed heat transfer analysis is performed on thermal behaviors of the system, especially the solar collector. The results show that the system can generate mid-temperature steam of a pressure up to 0.75 MPa. The thermal efficiency is found to be 38.52% at discharging pressure of 0.5 MPa during summer time.     

A MCRT and FVM coupled simulation method for energy conversion process in parabolic trough solar collector

A coupled simulation method based on Monte Carlo Ray Trace (MCRT) and Finite Volume Method (FVM) is established to solve the complex coupled heat transfer problem of radiation, heat conduction and convection in parabolic trough solar collector system. A coupled grid checking method is established to guarantee the consistency between the two methods and the validations to the coupled simulation model were performed. Firstly, the heat flux distribution on the collector tube surface was investigated to validate the MCRT method. The heat flux distribution curve could be divided into 4 parts: shadow effect area, heat flux increasing area, heat flux reducing area and direct radiation area. The heat flux distribution on the outer surface of absorber tube was heterogeneous in circle direction but uniform in axial direction. Then, the heat transfer and fluid flow performance in the LS-2 Solar Collector tube was investigated to validate the coupled simulation model. The outlet temperatures of the absorber tube predicted by the coupled simulation model were compared with the experimental data. The absolute errors are in the range of 1.5–3.7 °C, and the average relative error is less than 2%, which demonstrates the reliability of the coupled method established in this paper. At last, the concentrating characteristics of the parabolic trough collectors (PTCs) were analyzed by the coupled method, the effects of different geometric concentration ratios (GCs) and different rim angles were examined. The results show the two variables affect the heat flux distribution. With GC increasing, the heat flux distributions become gentler, the angle span of reducing area become larger and the shadow effect of absorber tube become weaker. And with the rim angle rising, the maximum value of heat flux become lower, and the curve moves towards the direction φ = 90°. But the temperature rising only augments with GC increasing and the effect of rim angle on heat transfer process could be neglected, when it is larger than 15°. If the rim angle is small, such as θ_rim = 15°, lots of rays are reflected by glass cover, and the temperature rising is much lower.     

Numerical investigation of a two-stage air-cooled absorption refrigeration system for solar cooling: Cycle analysis and absorption cooling performances

Two-stage air-cooled ammonia–water absorption refrigeration system could make good use of low-grade solar thermal energy to produce cooling effect. The system simulation results show that thermal COP is 0.34 and electrical COP is 26 under a typical summer condition with 85 °C hot water supplied from solar collector. System performances under variable working conditions are also analyzed. Circular finned tube bundles are selected to build the air-cooled equipment. The condenser should be arranged in the front to get an optimum system performance. The mathematical model of the two-stage air-cooled absorber considering simultaneous heat and mass transfer processes is developed. Low pressure absorber should be arranged in front of middle pressure absorber to minimize the absorption length. Configuration of the air-cooled equipment is suggested for a 5 kW cooling capacity system. Temperature and concentration profiles along the finned tube length show that mass transfer resistance mainly exists in liquid phase while heat transfer resistance mainly exists in cooling air side. The impacts on system refrigeration capacities related to absorption behaviors under variable working conditions are also investigated. Both cycle analysis and absorption performances show that two-stage air-cooled ammonia–water absorption chiller is technically feasible in practical solar cooling applications.     

Theoretical and experimental analysis on efficiency factors and heat removal factors of Fresnel lens solar collector using different cavity receivers

The efficiency and heat removal factors are useful parameters for evaluating the thermal performance of concentrating solar collectors. In this work, the efficiency factors and heat removal factors of Fresnel lens solar collectors using different kinds of point-focus cavity receivers were obtained both theoretically and experimentally. An experimental unit was built, whereby eight kinds of cavity receivers, namely: conical, spherical, cylindrical, hemispherical, positive cone frustum, reverse cone frustum, heteroconical and domical, were tested and analyzed. It is found that the theoretical results agree well with the test results. For the point-focus Fresnel lens solar collector, the conical cavity receiver showed the best thermal performance, with a highest theoretical heat removal factor of 0.868. Effect of conical cavity parameters on the efficiency and heat removal factors were studied. Results showed that under given operation conditions, the optimum aperture diameter of the cavity, the optimum inside diameter of the receiver tube and the optimum vertex angle of the cross section through the symmetric axis of the receiver are 80 mm, 15 mm and 60°, respectively. For better thermal performance, the geometrical concentration ratio of the studied Fresnel lens solar collector should be more than 500.     

Experimental study of a compact PCM solar collector

The performance of a compact phase change material (PCM) solar collector based on latent heat storage was investigated. In this collector, the absorber plate–container unit performs the function of both absorbing the solar energy and storing PCM. The solar energy was stored in paraffin wax, which was used as a PCM, and was discharged to cold water flowing in pipes located inside the wax. The collector's effective area was assumed to be 1 m² and its total volume was divided into 5 sectors. The experimental apparatus was designed to simulate one of the collector's sectors, with an apparatus-absorber effective area of 0.2 m². Outdoor experiments were carried out to demonstrate the applicability of using a compact solar collector for water heating. The time-wise temperatures of the PCM were recorded during the processes of charging and discharging. The solar intensity was recorded during the charging process. Experiments were conducted for different water flow rates of 8.3–21.7 kg/h. The effect of the water flow rate on the useful heat gain (Qu) was studied. The heat transfer coefficients were calculated for the charging process. The propagation of the melting and freezing front was also studied during the charging and discharging processes. The experimental results showed that in the charging process, the average heat transfer coefficient increases sharply with increasing the molten layer thickness, as the natural convection grows strong. In the discharge process, the useful heat gain was found to increase as the water mass flow rate increases.     

Improvement of PV module optical properties for PV-thermal hybrid collector application

Photovoltaic-thermal collectors (or PV-T collector) are hybrid collectors where PV modules are integrated as an absorber of a thermal collector in order to convert solar energy into electricity and usable heat at the same time. In most of the cases, the hybrid collectors are made by the superposition of a PV module on the thermal absorber of a solar collector. In this paper, the approach is different and is to analyze thermal and optical properties related to both PV and solar thermal functions in order to identify an optimum combination leading to a maximum overall efficiency. Indeed, although these two functions do not exploit the same range of radiation wavelengths, thermal and PV functions are not so complementary due to photo-conversion thermal dependency. In this context, an alternative PV cell lamination has been developed with increased optical and thermal performance. The improvements were evaluated around 2 mA/cm² in terms of current density in comparison to a standard module encapsulation. Based on this technique, a real size PV-T module has been built and tested at Fraunhofer solar test facilities. The results show a global efficiency of the PV-T collector above 87% (79% thermal efficiency plus 8.7% electrical efficiency, based on the absorber area).     

A comparison of solar absorption system configurations

Solar thermal driven cooling systems for residential applications are a promising alternative to electric compression chillers, although its market introduction still represents a challenge, mainly due to the higher investment costs. The most common system configuration is an absorption chiller driven by a solar thermal system, backed up by a secondary heating source, normally a gas boiler. Heat storage in the primary (solar) circuit is mandatory to stabilize and extend the operation of the chiller, whereas a cold storage tank is not so common.This paper deals with the selection of the most suitable configuration for residential cooling systems with solar energy. In Spain, where cooling needs are usually higher than heating needs, the interest of a reversible heat pump as auxiliary system and a secondary cooling storage are analyzed.A complete TRNSYS model has been developed to compare a configuration with just hot storage (of typical capacity 40 L/m² of solar collector surface) and a configuration with both, hot and cool storages. The most suitable configuration is very sensible to the solar collector area. As the collector area increases, the advantages of a cool storage vanish. Increasing the collector area tends to increase the temperature of the hot storage, leading to higher thermal losses in both the collector and the tank. When the storage volume is concentrated in one tank, these effects are mitigated. The effect of other variables on the optimal configuration are also analyzed: collector efficiency curve, COP of the absorption chiller, storage size, and temperature set-points of the chillers.     

Experimental study of the performance of a solar collector by closed-end oscillating heat pipe (CEOHP)

An experimental flat plate solar collector operating in conjunction with a closed-end oscillating heat pipe (CEOHP) offers a reasonably efficient and cost effective alternative to conventional solar collector system that use heat pipes. The CEOHP system described in this study relies on the natural forces of gravity and capillary action and dose not require an external power source. The flat plate collector consisted of a 1 mm thick sheet of black zinc covered by a glass enclosure with a collecting area of 2.00 × 0.97 m² , an evaporator located on the collecting plate, and a condenser inserted into a water tank. A length of 0.003 ID copper tubing was bent into multiple turns at critical points along its path and used to channel the working fluid throughout the system. R134a was used as the working fluid. Efficiency evaluations were conducted during daylight hours over a two month period and included extensive monitoring and recording of temperatures with type-K thermocouples placed at key locations throughout the system. The results confirmed the anticipated fluctuation in collector efficiency dependant on the time of day, solar energy irradiation, ambient temperature and flat plate mean temperature. An efficiency of approximately 62% was achieved, which correlates with the efficiency of the more expensive heat pipe system. The CEOHP system offers the additional benefits of corrosion free operation and absence of freezing during winter months.     

Numerical simulation of natural convection of nanofluid in a square enclosure: Effects due to uncertainties of viscosity and thermal conductivity

The present study aims to identify effects due to uncertainties in effective dynamic viscosity and thermal conductivity of nanofluid on laminar natural convection heat transfer in a square enclosure. Numerical simulations have been undertaken incorporating a homogeneous solid–liquid mixture formulation for the two-dimensional buoyancy-driven convection in the enclosure filled with alumina–water nanofluid. Two different formulas from the literature are each considered for the effective viscosity and thermal conductivity of the nanofluid. Simulations have been carried out for the pertinent parameters in the following ranges: the Rayleigh number, Ra_f = 10³–10⁶ and the volumetric fraction of alumina nanoparticles, ? = 0–4%. Significant difference in the effective dynamic viscosity enhancement of the nanofluid calculated from the two adopted formulas, other than that in the thermal conductivity enhancement, was found to play as a major factor, thereby leading to contradictory results concerning the heat transfer efficacy of using nanofluid in the enclosure.     

A solar-powered traveling-wave thermoacoustic electricity generator

Traveling-wave thermoacoustic electricity generator is a new external-combustion type device capable of converting heat such as solar energy into electric power. In this paper, a 1 kW solar-powered traveling-wave thermoacoustic electricity generation system is designed and fabricated. The system consists of a traveling-wave thermoacoustic electricity generator, a solar dish collector and a heat receiver. In the preliminary tests, using electric cartridge heaters to simulate the solar energy, a maximum electric power of 481 W and a maximum thermal-to-electric efficiency of 15.0% were achieved with 3.5 MPa pressurized helium and 74 Hz working frequency. Then, after integrating the traveling-wave thermoacoustic electricity generator with the solar dish collector and the heat receiver, the solar-powered experiments were performed. In the experiments, a maximum electric power of about 200 W was obtained. However, due to the solar dish collector problems, the heating temperature of the receiver was much lower than expected. Optimizations of the collector and the heat receiver are under way.