Collector Absorber Research Articles

Thermal Modeling of Photovoltaic Thermal System with Polymer Sheet in Tube Absorber Collector

This study developed a thermal model of a photovoltaic thermal collector (PVT) to predict the performance and outlet temperature of the system. The PVT consisted of a polycrystalline photovoltaic module, a polymer collector–type sheet in a tube, and an insulator. The motivation of the present work is that the polymer materials are flexible, low cost and lightweight which are for the PVT applications. The outlet temperature of the PVT increased with the decreasing rate of mass flow into the riser because the water had sufficient time for thermal heating. One unit of polymer collector can achieve an outlet temperature of 69 °C at 500 W/m2 at a mass flow rate of 0.0063 kg/s.

Thermal analysis of a solar collector absorber plate with microchannels

Experimental and theoretical analyses were carried out to investigate the absorber plate temperature distribution for compact (thin and light-weight) solar thermal collectors. An analytic model combining convective heat transfer with axial conduction in the metal plate was developed. Forced convection experiments were then performed on an instrumented metal plate with micro-channels 0.5mm×2mm×270mm long, at various flow rates; the heat transfer fluid was Tyfocor® LS. Reynolds numbers were in the range 10–100 and fluid inlet temperatures ranged from 5 to 60°C. The predicted plate temperature profiles from the analytic model were in close agreement with the measured profiles. Thermal entry lengths were found to be significant and resulted in slight variations at the entry portion of the plate at higher flow rates. The model was used to study the effects of varying design/operating parameters and showed that axial conduction can significantly alter the temperature profile in the plate.

Influence of circumferential solar heat flux distribution on the heat transfer coefficients of linear Fresnel collector absorber tubes

The absorber tubes of solar thermal collectors have enormous influence on the performance of the solar collector systems. In this numerical study, the influence of circumferential uniform and non-uniform solar heat flux distributions on the internal and overall heat transfer coefficients of the absorber tubes of a linear Fresnel solar collector was investigated. A 3D steady-state numerical simulation was implemented based on ANSYS Fluent code version 14. The non-uniform solar heat flux distribution was modelled as a sinusoidal function of the concentrated solar heat flux incident on the circumference of the absorber tube. The k–ε model was employed to simulate the turbulent flow of the heat transfer fluid through the absorber tube. The tube-wall heat conduction and the convective and irradiative heat losses to the surroundings were also considered in the model. The average internal and overall heat transfer coefficients were determined for the sinusoidal circumferential non-uniform heat flux distribution span of 160°, 180°, 200° and 240°, and the 360° span of circumferential uniform heat flux for 10m long absorber tubes of different inner diameters and wall thicknesses with thermal conductivity of 16.27W/mK between the Reynolds number range of 4000 and 210,000 based on the inlet temperature. The results showed that the average internal heat transfer coefficients for the 360° span of circumferential uniform heat flux with different concentration ratios on absorber tubes of the same inner diameters, wall thicknesses and thermal conductivity were approximately the same, but the average overall heat transfer coefficient increased with the increase in the concentration ratios of the uniform heat flux incident on the tubes. Also, the average internal heat transfer coefficient for the absorber tube with a 360° span of uniform heat flux was approximately the same as that of the absorber tubes with the sinusoidal circumferential non-uniform heat flux span of 160°, 180°, 200° and 240° for the heat flux of the same concentration ratio, but the average overall heat transfer coefficient for the uniform heat flux case was higher than that of the non-uniform flux distributions. The average axial local internal heat transfer coefficient for the 360° span of uniform heat flux distribution on a 10m long absorber tube was slightly higher than that of the 160°, 200° and 240° span of non-uniform flux distributions at the Reynolds number of 4000. The average internal and overall heat transfer coefficients for four absorber tubes of different inner diameters and wall thicknesses and thermal conductivity of 16.27W/mK with 200° span of circumferential non-uniform flux were found to increase with the decrease in the inner-wall diameter of the absorber tubes. The numerical results showed good agreement with the Nusselt number experimental correlations for fully developed turbulent flow available in the literature.

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.

Selective Coatings for New Concepts of Parabolic Trough Collectors

The CSP technology based on parabolic trough solar collector for large electricity generation purposes is currently the most mature of all CSP designs in terms of previous operation experience and scientific and technical research and development. The current parabolic trough design deals with a maximum operating temperature around 400°C in the absorber collector tube but some recent designs are planned to increase the working temperature to 600°C increasing the performance by 5-10% to attain the improved productivity that the market demands .These systems are expected to be working during 20-25 years.One of the key points of the receiver is the stack of layers forming the selective absorber coating. Typical optical values for this coating are >95% of absorbance and 10% of emittance at 400°C. In the HITECO EU funded project a new modular concept of collector is being tested. With this new design the coating has to fulfill new requirements as the collector will be working at 600°C and in a low vacuum of 10-2 mbar. The authors from IK4-Tekniker have collaborated in the HITECO project building a sputtering system to upscale these new coatings to 4 meters long tubes for the application in a set of experimental collectors, being the project coordination done by ARIES Ingeniería y sistemas S.A.However, there is a great lack of knowledge about the performance of the absorber coatings during the whole life of the receiver. So, it is necessary to establish characterization methods and standards to guarantee the performance, durability and quality control of high temperature coatings. For this purpose, a new EU funded project called NECSO, aims to design accelerated ageing protocols to test the evolution of these coatings under more aggressive conditions in the new designs.

Flat-plate collector construction and system configuration to optimize the thermosiphonic effect

Thermosiphon systems heat potable water or heat transfer fluid and use natural convection to transport it from the collector to storage. This type of technology is applied extensively in countries with good sunshine potential. One such example is Cyprus, which is currently the leading country in the world with respect to the application of solar water heaters for domestic applications, with more than 93% of the houses equipped with such a system. The performance of such a system depends on many factors including the collector construction and the arrangement of the system, mainly with respect to the distance between the top of the solar collector and the bottom of the storage tank and the solar collector slope, which affects both the energy collected and the hydrostatic pressure of the system. A typical system in Cyprus uses 3 m2 of collectors, 160 l storage, its collectors are usually inclined at 45° from horizontal and has 15 mm copper riser tubes and header tubes with a diameter of 28 mm. The collector absorber plate is also made from copper. The main objective of this paper is to investigate through modeling and simulation possible configurations, which will optimize the performance of the system. For this purpose, a number of riser and header tube diameters were considered ranging from 6 mm to 35 mm, slopes from 20° to 90° and distances between the top of the collector to the bottom side of the storage tank ranging from ±15 cm. The system is modeled using TRNSYS and simulated with the Typical Meteorological Year (TMY) of Nicosia, Cyprus. The results showed that the best-optimized system is obtained for small header and riser pipe diameters and very close performance is obtained for various combinations. Therefore, the decision on the optimum system should depend on cost issues, which are currently very important because of the increased price of copper and operational problems depending on the hardness of the water in the area of installation, which could cause scale deposits that could clog the riser pipes. The optimum slope is found to be equal to the latitude plus 10°, i.e., 45°, although a smaller slope does not affect the performance a lot, and the optimum distance between the top of the collector and the bottom of the storage tank is −15 cm. These findings should prove valuable for the collector and systems designers and manufacturers.

Enhancement of latent heat storage in a rectangular cavity: Solar water heater case study

The energy production provided by a heat excess or a discontinuous source (solar radiation, waste heat, etc.) involves the utilization of a thermal storage systems. In this work, an experimental study of a storage system using paraffin as phase change material (PCM) has been done. This system takes the form of two rectangular cavities incorporating behind the absorber of a flat plat solar collector. Measurements were performed during different weather conditions and illustrate that the PCM contributes to increase the performance of the solar collector at night. An analysis of the temperature stratification inside the PCM-filled cavities was also carried out. Theoretical solid–liquid of phase change material model is used to evaluate the PCM melted volume fraction, liquid–solid interfaces, PCM temperature and melting/solidification flow in the PCM-filled cavity used in the present experimental study.

Performance analysis of a spectrally selective concentrating direct absorption collector

Spectrally selective liquid filters are proposed as a means of efficiently utilizing majority of solar radiation incident on a given area. This paper reports on the outdoor testing done to verify the thermal performance of a spectrally selective liquid under concentrated light. The design consists of a Fresnel lens based concentrator that focuses radiation on to a glass absorber. Collector thermal performance was measured using Copper Sulfate solution as the heat collection fluid. Effect of various parameters on efficiency is also reported. It was found that parameters such as solution flow rate through the absorber and optical concentration as measured by the spot size have an insignificant impact on the thermal performance.

Study of an efficient compact solar water heater

Solar passive water heaters are potential candidates for enhancing heat transfer. The purpose of this study was the design, construction, outdoor testing of a passive, simple and low-cost compact solar water heater (CSWH) system. As a new design, the black colored sands immersed into the water storage tank established the main portion of the collector absorber section. The water storage tank was manufactured from galvanized sheet of 0.0015m in thickness and the volume of 1.45×0.56×0.17m3. The effective area of the collector was 0.67m2. In all experiments results, the collector averaging daily efficiencies achieved higher than 70%. The performance of the novel CSWH system was simulated by using thermal network analysis. The simulation results showed that the collector absorber operated as a gray material. During one interest test, the peak temperature value of the stored water into the tank reached about 90°C.

Cross-Sectional Temperature Field of a Solar Collector’s Absorber: Development of the Model / Saules Kolektora Absorbera Šķērsgriezuma Temperatūras Lauks Apaļas Caurulītes Gadījumā – Fināla Modelis

In the work, the temperature field model is developed for the absorber of a round-pipe collector. As distinguished from previous models when the temperature of liquid was assumed to be constant over the entire pipe crosssection, the results obtained clearly show the temperature variations in the absorber’s cross-section. In the work, optimal values are found in the work for geometrical parameters of the collector (i.e. the plate thickness and the pipe diameter) that allow the highest possible temperature of liquid to be achieved.

Modeling and co-simulation of a parabolic trough solar plant for industrial process heat

In the present paper a tri-dimensional non-linear dynamic thermohydraulic model of a parabolic trough collector was developed in the high-level acausal object-oriented language Modelica and coupled to a solar industrial process heat plant modeled in TRNSYS. The integration is performed in an innovative co-simulation environment based on the TLK interconnect software connector middleware. A discrete Monte Carlo ray-tracing model was developed in SolTrace to compute the solar radiation heterogeneous local concentration ratio in the parabolic trough collector absorber outer surface. The obtained results show that the efficiency predicted by the model agrees well with experimental data with a root mean square error of 1.2%. The dynamic performance was validated with experimental data from the Acurex solar field, located at the Plataforma Solar de Almeria, South-East Spain, and presents a good agreement. An optimization of the IST collector mass flow rate was performed based on the minimization of an energy loss cost function showing an optimal mass flow rate of 0.22kg/sm2. A parametric analysis showed the influence on collector efficiency of several design properties, such as the absorber emittance and absorptance. Different parabolic trough solar field model structures were compared showing that, from a thermal point of view, the one-dimensional model performs close to the bi-dimensional. Co-simulations conducted on a reference industrial process heat scenario on a South European climate show an annual solar fraction of 67% for a solar plant consisting on a solar field of 1000m2, with thermal energy storage, coupled to a continuous industrial thermal demand of 100kW.

Experimental investigations of the performance of a solar air collector with latent heat thermal storage integrated with the solar absorber

The paper deals with experimental investigations of the performance of a solar air collector with latent heat thermal storage integrated with the solar absorber. The main purpose of heat storage in solar thermal systems is to store heat when the supply of solar heat exceeds demand and release it when otherwise. A number of heat storage materials can be used for this purpose; the phase change materials among them. Short-term latent heat thermal storage integrated with the solar absorber can stabilize the air temperature at the outlet of the collector on cloudy days when solar radiation intensity incident on a solar collector fluctuates significantly. Two experimental front-and-back pass solar air collectors of the same dimensions have been built for the experimental investigations. One collector had a conventional solar absorber made of a metal sheet while the solar absorber of the other collector consisted of containers filled with organic phase change material. The experimental collectors were positioned side by side during the investigations to ensure the same operating conditions (incident solar radiation, outdoor temperature).

A Critical Review on Enhancement in System Performance of Flat Plate Hybrid PV/T Solar Collector System

The combined efficiency i.e. electrical and thermal of a simple flat plate solar hybrid PV/T water/air collector system is low. Therefore above systems are commercially not viable till date. To make these systems commercially viable, development in the system configurations can be done by modifying PV absorber shape, size and materials, and also by attaching various types of diffuse reflectors or flat concentrators to the sides of commercial PV module. PV absorber surface design with various configurations such as simple open air channel, single pass with rectangular tunnel, spiral flow absorber collector, thin metallic sheet suspended at the middle of air channel, fins at the back wall of an air duct etc., are used in a simple flat plate solar hybrid PV/T water/air collector systems to enhance its system performance at one sun concentration and above. This article reviews a simple flat plate hybrid PV/T water/air collector systems with and without PV absorber surface design with various configurations as discussed above. Finally effect of attaching a flat concentrators or diffuse reflectors with unglazed and glazed surface on performance of these systems is also discussed and analyzed.

An Effective Simulation Model to Predict and Optimize the Performance of Single and Double Glaze Flat - Plate Solar Collector Designs

This paper outlines and formulates a compact and effective simulation model, which predicts the performance of single and double glaze flat-plate collector. The model uses an elaborated iterative simulation algorithm and provides the collector top losses, the glass covers temperatures, the collector absorber temperature, the collector fluid outlet temperature, the system efficiency, and the thermal gain for any operational and environmental conditions. It is a numerical approach based on simultaneous guesses for the three temperatures, T p plate collector temperature and the temperatures of the two glass covers T g1 , T g2 . A set of energy balance equations is developed which allows for structured iteration modes whose results converge very fast and provide the values of any quantity which concerns the steady state performance profile of any flat-plate collector design. Comparison of the results obtained by this model for flat-plate collectors, single or double glaze, with those obtained by using the Klein formula, as well as the results provided by other researchers, is presented.

Experimental Analysis of Flat Plate Solar Air Collector Efficiency

In the present study, a flat plate solar air collector is used for research purposes. The absorber of solar collector made by steel plate with an area of 2×1m 2 and thickness of 0.5mm in the form of window shade has been developed for increasing the air contact area. The surface of absorbent plate was covered by black paint. To insulate the collector, the glass wool with the thickness of 5cm was used. The experiments on the efficiency were conducted for a week during which the atmospheric conditions were almost uniform and data was collected from the collector. The results of the collector efficiency in the natural and forced convection were evaluated and their graphs were plotted. The results showed that the collector efficiency in forced convection was lower, but the low temperature difference between inlet and outlet of the collector decreased its heat loss. In addition, the average air speed in forced convection was about 21% higher than the natural convection.

Photothermal response of CVD synthesized carbon (nano)spheres/aqueous nanofluids for potential application in direct solar absorption collectors: a preliminary investigation

Direct-absorption solar collectors have the potential to offer an unlimited source of renewable energy with minimal environmental impact. Unfortunately, their performance is limited by the absorption efficiency of the working fluid. Nanoparticles of functionalized carbon nanospheres (CNS) have the potential to improve the photothermal properties of the working fluid. CNS are produced by the pyrolysis of acetylene gas in a tube-based electric furnace/chemical vapor deposition apparatus. The reaction takes place at 1000°C in the presence of nitrogen gas without the use of a catalyst. The synthesized CNS were examined and characterized using field-emission scanning electron microscopy, transmission electron microscopy, X-ray diffraction spectroscopy, Raman spectroscopy, thermal gravimetric analysis, and ultraviolet-visible analysis. The CNS powders with a mean particle size of 210 nm were then functionalized using tetraethylammonium hydroxide ([C2H5]4 N[OH]) and used to produce a series of aqueous nanofluids with varying mass content. The photothermal response of both the nanofluids and films composed of CNS were investigated under 1000 W/m(2) solar irradiation.

Review of Component Models for the Simulation of Combined Solar and Heat Pump Heating Systems

Combined solar and heat pump systems have a large potential to reduce CO2 and other emissions from the sector of domestic hot water preparation and space heating. Subtask C of the IEA SHC Task 44/HPP Annex 38 deals with modeling and simulation of these systems. A review of component models for the simulation of combined solar and heat pump heating systems has been carried out with a special focus on the particularities of the combination of solar thermal collectors with heat pumps. Some of these particularities are effects of water vapor condensation on the surface of collector absorbers operated below the dew point, refrigeration cycles using more than one heat exchanger for the evaporator in order to use solar heat in addition to a conventional heat source, the effect of higher source temperature for the heat pump when solar heat is used, or the regeneration of ground heat sources with heat from solar collectors.

Exergy analysis of an air solar heater

This paper presents an exergetic analysis of a downward flat plate collector, an unglazed selective absorber collector and an artificial rough surface collector. The analysis is based on mass, energy and exergy balance equations. The results show that for the Reynolds number of Re = 4000, the daily exergy output rate varies inversely with the mass flow rate per unit area, while the exergy efficiency of the artificial rough surface collector is four times greater than that of a conventional solar air heater. In addition, there is maximum dimensionless exergy loss in the downward flat plate collector already having the smallest thermal efficiency.

Movement and mechanical stresses in sealed, flat plate solar collectors

► Mechanical stresses occur in sealed, gas-filled solar collectors. ► Short glass to absorber distances, long tubes and large areas give low stresses. ► There are no strict rules regarding selection of material, thicknesses, etc. ► Beware: strengthening of one part can weaken the construction. This article describes movements and mechanical stresses in sealed, gas-filled, flat plate solar collectors. The advantage of a sealed space between a collector absorber and glass cover is that it (i) eliminates the influence of humidity condensation and dust and (ii) it lowers the heat losses when the enclosed space is filled with a suitable gas at normal pressure. However as the solar collector temperature varies, volume and pressure changes cause movement and mechanical stresses. In this study, the finite element method was used to determine movements and mechanical stresses. Several geometries were analyzed and it was found that the stresses in the investigated collectors are a factor 2–4 below the critical stress levels. Furthermore, it was found to be possible to reduce the stresses and improve the factor of safety by (i) using a larger area and/or reducing the distance between the glass and the absorber and/or (ii) changing the length and width so the tubes are longer.

A Novel Integrated Direct Absorption Self-Storage Solar Collector

The present work introduces a novel integrated solar collector with direct absorption of solar radiation and self-energy storage. The proposed system has compact size without the need for any connecting pipes. The working fluid absorbs the incident solar radiation directly and passes it to the storing fluid container that has been implemented within the core of the novel system. A mathematical model has been presented to describe the thermal behavior of the proposed system. Moreover, an expression has been derived to predict the system efficiency, outlet temperature, and utilized heat transfer. Simultaneously, a comparison of the performance of the novel solar system with that of the conventional solar collector reveals that the performance of the proposed system is much better than that of the conventional collector. The proposed direct absorption collector has a collector efficiency factor, F′ = 1 which is 20% more than that of well-manufactured conventional collectors. This yields about 20% increase in the heat removal factors and efficiency of the integrated system as compared with that of the conventional collectors over the entire range of overall heat loss coefficients and for the same operating conditions regarding working fluid mass flow rate, inlet fluid temperature, ambient temperature, and incident solar radiation. However, it is found that the performance of the integrated collector is more sensitive to the variation in the overall heat loss coefficient and working fluid mass flow rate as compared with the conventional collectors.