Mathematical modeling and thermal performance analysis of unglazed transpired solar collectors

Unglazed transpired collectors or UTC (also known as perforated collectors) are a relatively new development in solar collector technology, introduced in the early nineties for ventilation air heating. These collectors are used in several large buildings in Canada, USA and Europe, effecting considerable savings in energy and heating costs. Transpired collectors are a potential replacement for glazed flat plate collectors. This paper presents the details of a mathematical model for UTC using heat transfer expressions for the collector components, and empirical relations for estimating the various heat transfer coefficients. It predicts the thermal performance of unglazed transpired solar collectors over a wide range of design and operating conditions. Results of the model were analysed to predict the effects of key parameters on the performance of a UTC for a delivery air temperature of 45–55 °C for drying applications. The parametric studies were carried out by varying the porosity, airflow rate, solar radiation, and solar absorptivity/thermal emissivity, and finding their influence on collector efficiency, heat exchange effectiveness, air temperature rise and useful heat delivered. Results indicate promising thermal performance of UTC in this temperature band, offering itself as an attractive alternate to glazed solar collectors for drying of food products.The results of the model have been used to develop nomograms, which can be a valuable tool for a collector designer in optimising the design and thermal performance of UTC. It also enables the prediction of the absolute thermal performance of a UTC under a given set of conditions.     

A prototype photovoltaic/thermal system integrated with transpired collector

Building-integrated photovoltaic/thermal (BIPV/T) systems may be utilized to produce useful heat while simultaneously generating electricity from the same building envelope surface. A well known highly efficient collector is the open-loop unglazed transpired collector (UTC) which consists of dark porous cladding through which outdoor air is drawn and heated by absorbed solar radiation. Commercially available photovoltaic systems typically produce electricity with efficiencies up to about 18%. Thus, it is beneficial to obtain much of the normally wasted heat from the systems, possibly by combining UTC with photovoltaics. Combination of BIPV/T and UTC systems for building facades is considered in this paper - specifically, the design of a prototype façade-integrated photovoltaic/thermal system with transpired collector (BIPV/T). A full scale prototype is constructed with 70% of UTC area covered with PV modules specially designed to enhance heat recovery and compared to a UTC of the same area under outdoor sunny conditions with low wind. The orientation of the corrugations in the UTC is horizontal and the black-framed modules are attached so as to facilitate flow into the UTC plenum. While the overall combined thermal efficiency of the UTC is higher than that of the BIPV/T system, the value of the generated energy - assuming that electricity is at least four times more valuable than heat - is between 7% and 17% higher. Also, the electricity is always useful while the heat is usually utilized only in the heating season. The BIPV/T concept is applied to a full scale office building demonstration project in Montreal, Canada. The ratio of photovoltaic area coverage of the UTC may be selected based on the fresh air heating needs of the building, the value of the electricity generated and the available building surfaces.     

An improved thermal and electrical model for a solar photovoltaic thermal (PV/T) air collector

In this paper, an attempt is made to investigate the thermal and electrical performance of a solar photovoltaic thermal (PV/T) air collector. A detailed thermal and electrical model is developed to calculate the thermal and electrical parameters of a typical PV/T air collector. The thermal and electrical parameters of a PV/T air collector include solar cell temperature, back surface temperature, outlet air temperature, open-circuit voltage, short-circuit current, maximum power point voltage, maximum power point current, etc. Some corrections are done on heat loss coefficients in order to improve the thermal model of a PV/T air collector. A better electrical model is used to increase the calculations precision of PV/T air collector electrical parameters. Unlike the conventional electrical models used in the previous literature, the electrical model presented in this paper can estimate the electrical parameters of a PV/T air collector such as open-circuit voltage, short-circuit current, maximum power point voltage, and maximum power point current. Further, an analytical expression for the overall energy efficiency of a PV/T air collector is derived in terms of thermal, electrical, design and climatic parameters. A computer simulation program is developed in order to calculate the thermal and electrical parameters of a PV/T air collector. The results of numerical simulation are in good agreement with the experimental measurements noted in the previous literature. Finally, parametric studies have been carried out. Since some corrections have been down on thermal and electrical models, it is observed that the thermal and electrical simulation results obtained in this paper is more precise than the one given by the previous literature. It is also found that the thermal efficiency, electrical efficiency and overall energy efficiency of PV/T air collector is about 17.18%, 10.01% and 45%, respectively, for a sample climatic, operating and design parameters.     

Thermal performance of an n-pass solar air heater

A mathematical model is developed for the prediction of the thermal performance for an n-pass solar air collector. The thermal efficiency and the temperature rise in teh collector are increased by preheating the air within the collector before it comes into contact with the absorber plate. The thermal losses to the ambient, through the bottom plate, are minimized due to the lower bottom plate temperature. The analysis describes how the governing equations are manipulated with the aid of the boundary conditions to obtain a set of equations which is solved numerically. In order to demonstrate the capability of the model, the analysis is applied to investigate the effect of increasing the number of air passes within the collector on the thermal efficiency and the temperature rise for a range of air mass flow rates. The results indicate that the thermal efficiency of the collector increases rapidly and approaches the optical efficiency of the collector as the mass flow rate is increased. In general, the model can be used to analyze the effect of various design parameters on the thermal performance.     

Modeling and simulation of ray tracing for compound parabolic thermal solar collector

A ray tracing model for the compound parabolic collector (CPC) is presented in this work. The pertinent parameters for the compound parabolic thermal solar collector are analyzed and calculated, and the ray tracing model is further investigated. The ray tracing model is validated by comparing our ray tracing model results with a commercial optical software. Each ray is traced by the CPC model, so the incident angle is calculated when solar ray enters the absorption tube. The ray tracing model was applied to the thermal efficiency analysis of the CPC, and the thermal performance results obtained by the model and test results were compared.     

Simulation and optimisation studies on a solar parabolic collector: an experimental investigation

The present paper has focused on parametric optimisation of a solar parabolic collector. This work has been performed in two phases. In the first phase, simulation studies have been done to identify the influential parameters. Based on simulation results, an experimental set-up of the parabolic collector has been fabricated using the materials selected by priority-score method. In the second phase, experiments are conducted according to Taguchi's L₉ orthogonal array. Parabolic collector parameters, namely reflector materials, absorber materials, positions of the absorber tube and angle of the absorber tube, are optimised with the consideration of multi-responses such as temperature, enthalpy, optical efficiency and thermal efficiency. The obtained experimental data are analysed using the desirability functional analysis approach and optimal levels of input parameters have been identified. Analysis of variance also has been performed to know the contribution of influential parameters on the responses.     

Performance of a double pass solar air collector

Double pass counter flow solar air collector with porous material in the second air passage is one of the important and attractive design improvement that has been proposed to improve the thermal performance. This paper presents theoretical and experimental analysis of double pass solar air collector with and without porous material. A mathematical model has been developed based on volumetric heat transfer coefficient. Effects of various parameters on the thermal performance and pressure drop characteristics have been discussed. Comparison of results reveals that the thermal efficiency of double pass solar air collector with porous absorbing material is 20-25% and 30-35% higher than that of double pass solar air collector without porous absorbing material and single pass collector respectively.     

Extension of the Hottel-Whillier model to the analysis of combined photovoltaic/thermal flat plate collectors

The well known Hottel-Whillier model for thermal analysis of flat plate collectors is extended to the analysis of combined photovoltaic/thermal collectors in a manner, such that, with simple modification of the conventional parameters of the original model, all of the existing relations and supporting information available in the literature still apply. Beyond the basic assumptions of the original model, it is only necessary to assume that the local electrical conversion efficiency of the solar cell array (absorber) is a linear decreasing function of the local absorber temperature over its operating temperature range. Based on the extended model, examples of both thermal and electrical performance of a combined collector as a function of collector design parameters are presented and discussed.     

Field study of various air based photovoltaic/thermal hybrid solar collectors

In the present work a comparative study for thermal and electrical performance of different hybrid photovoltaic/thermal collectors designs for Iraq climate conditions have been carried out. Four different types of air based hybrid PV/T collectors have been manufactured and tested. Three collectors consist of four main parts namely, channel duct, glass cover, axial fan to circulate air and two PV panels in parallel connection. The measured parameters are, the temperature of the upper and the lower surfaces of the PV panels, air temperature along the collector, air flow rate, pressure drop, power produced by solar cell, and climate conditions such as wind speed, solar radiation and ambient temperature. The thermal and hydraulic performances of PV/T collector model IV have been analyzed theoretically based on energy balance. A Matlab computer program has been developed to solve the proposed mathematical model.The obtained results show that the combined efficiency of collector model III (double duct, single pass) is higher than that of model II (single duct double pass) and model IV (single duct single pass). Model IV has the better electrical efficiency. The pressure drop of model III is lower than that of models II and IV. The root mean square of percentage deviations for PV outlet temperature, and thermal efficiency of model IV are found to be 3.22%, and 18.04% respectively. The calculated linear coefficients of correlation (r) are 0.977, 0.965 respectively.     

Performance optimization of evacuated tube collector for solar cooling of a house in hot climate

Evacuating the space connecting cover and absorber significantly improves evacuated tube collector (ETC) performance. So, ETCs are progressively utilised all over the world. The main goal of current study is to explore ETC thermal efficiency in hot and severe climate like Kuwait weather conditions. A collector test facility was installed to record ETC thermal performance for one-year period. An extensively developed model for ETCs is presented, employing complete optical and thermal assessment. This study analyses separately optics and heat transfer in the evacuated tubes, allowing the analysis to be extended to different configurations. The predictions obtained are in agreement with experimental. The optimum collector parameters (collector tube length and diameter, mass flow rate and collector tilt angle) are determined. The present results indicate that the optimum tube length is 1.5 m, as at this length a significant improvement is achieved in efficiency for different tube diameters studied. Finally, the heat generated from ETCs is used for solar cooling of a house. Results of the simulation of cooling system indicate that an ETC of area 54 m², tilt angle of 25° and storage tank volume of 2.1 m³ provides 80% of air-conditioning demand in a house located in Kuwait.     

The Influence of Collector Fluid Inlet Temperature on the Performance of a Solar-Assisted Absorption System Using Step-Finned Flat-Plate Collector

In the present work, an extensive analysis is developed for an evaluation of the thermal performance of a solar-powered H₂O/LiBr absorption cooling system using a step-fin flat-plate collector (SFC). The performance parameters, namely, collector efficiency factor, heat removal factor, and collector efficiency, for the SFC is derived. A system simulation model has been developed to analyze the system performance—that is, to identify an operating criterion as a function of the collector fluid inlet temperature (T _FI). It has been observed from the results that the performance of the system depends strongly on T _FI. Simulation results show that the system operates optimally (maximum coefficient of performance) at an optimal T _FI. When the system runs at this optimal value of T _FI, minimum collector material is required. Thus, when using SFC in place of a rectangular-fin flat-collector, thirty-five percent or more collector material can be saved. However, it has been observed that the effect of thermal conductivity on the plate volume of SFC has a marginal effect.     

Energetic performance evaluation of a corrugated channel solar air heater: Experimental investigation,mathematical modeling,and solution procedure

The cost of manufacturing and electricity consumption are key considerations in encouraging the adoption of solar air heaters (SAH) in sunny and low-income areas. These parameters can have a significant impact on promoting the use of these devices. In this study, solar air heating was designed with the objective of achieving the highest possible efficiency/cost ratio. It is a corrugated channel SAH whose structure is equipped with two barriers perforated with a sufficient number of holes for a good airflow distribution. A new model was developed to evaluate the qualitative parameters that describe the thermos-energetic behavior of a heating system. These parameters were measured using experimental data obtained under real operating conditions. The thermal model assumes a uniform temperature for the glass, absorber, and insulation of the collector, while the temperature of the circulating air is assumed to vary linearly along the collector. To ensure that these assumptions were valid, the collector was cut into a number of 0.1 m sections in the direction of flow. By comparing the numerical results with the experimental data, the model was validated and then used to calculate the temperature profiles of the different elements of the collector, as well as to estimate the impact of certain operational parameters on its thermal performance. Relative percentage error values, between the numerical and the experimental results, of 1.7517%, 1.0750%, 0.8577%, 2.2371%, and 2.3637% for absorber plate temperature, outlet airflow temperature, useful power, thermal, and effective efficiencies, respectively, are recorded.     

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.     

An evaluation on thermal performance of CPC solar collector

The main objective of this work is the investigation and improvement of thermal performance of evacuated CPC (Compound Parabolic Concentrator) solar collector with a cylindrical absorber. Modified types of this solar collector are always combined with the evacuated glass envelop or tracking system. The conventional stationary CPC solar collector has been compared with the single axis tracking CPC solar collector in outlet temperature, net heat flux onto the absorber and thermal efficiency. Numerical model has been analyzed based on the irradiation determined actually and the results have been calculated to predict the thermal efficiency. Based on the comparison of the measured and calculated results, it is concluded that the numerical model can accurately estimate the performance of solar collectors. The result shows the thermal efficiency of the tracking CPC solar collector is more stable and about 14.9% higher than that of the stationary CPC solar collector.     

波纹丝网型太阳能空气集热器热性能特性研究

提出一种新型波纹丝网型太阳能空气集热器,根据集热器的传热关系,用Matlab建立数学模型,并搭建实验系统进行测试,验证模型的准确性.利用模型研究关键结构参数和运行参数对集热器热性能的影响规律.结果表明:出口温度和集热量的模拟值与实验值最大偏差分别为2.3 K和443％,当温度系数介于0.01～0.05时,波纹丝网型太阳...     

Exergetic optimization of a solar photovoltaic thermal (PV/T) air collector

In this paper, an exergetic optimization has been developed to determine the optimal performance and design parameters of a solar photovoltaic thermal (PV/T) air collector. A detailed energy and exergy analysis has been carried out to calculate the thermal and electrical parameters, exergy components, and exergy efficiency of a typical PV/T air collector. The thermal and electrical parameters of a PV/T air collector include solar cell temperature, back surface temperature, outlet air temperature, open‐circuit voltage, short‐circuit current, maximum power point voltage, maximum power point current, etc. An improved electrical model has been used to estimate the electrical parameters of a PV/T air collector. Furthermore, a new equation for the exergy efficiency of a PV/T air collector has been derived in terms of design and climatic parameters. A computer simulation program has been also developed to calculate the thermal and electrical parameters of a PV/T air collector. The results of numerical simulation are in good agreement with the experimental measurements noted in the previous literature. Moreover, the simulation results obtained in this paper are more precise than the one given by the previous literature, and the new exergy efficiency obtained in this paper is in good agreement with the one given by the previous literature. Finally, exergetic optimization has been carried out under given climatic, operating, and design parameters. The optimized values of inlet air velocity, duct length, and the maximum exergy efficiency have been found. Parametric studies have been also carried out. Copyright © 2010 John Wiley & Sons, Ltd.     

Optimization of a solar photovoltaic thermal (PV/T) water collector based on exergy concept

In this paper, the optimization of a solar photovoltaic thermal (PV/T) water collector which is based on exergy concept is carried out. Considering energy balance for different components of PV/T collector, we can obtain analytical expressions for thermal parameters (i.e. solar cells temperature, outlet water temperature, useful absorbed heat rate, average water temperature, thermal efficiency, etc.). Thermal analysis of PV/T collector depends on electrical analysis of it; therefore, five-parameter current–voltage (I–V) model is used to obtain electrical parameters (i.e. open-circuit voltage, short-circuit current, voltage and current at the point which has maximum electrical power, electrical efficiency, etc.). In order to obtain exergy efficiency of PV/T collector we need exergy analysis as well as energy analysis. Considering exergy balance for different components of PV/T collector, we obtain the expressions which show the exergy of the different parts of PV/T collector. Some corrections have been done on the above expressions in order to obtain a modified equation for the exergy efficiency of PV/T water collector. A computer simulation program has been developed in order to obtain the amount of thermal and electrical parameters. The simulation results are in good agreement with the experimental data of previous literature. Genetic algorithm (GA) has been used to optimize the exergy efficiency of PV/T water collector. Optimum inlet water velocity and pipe diameter are 0.09 m s⁻¹, 4.8 mm, respectively. Maximum exergy efficiency is 11.36%. Finally, some parametric studies have been done in order to find the effect of climatic parameters on exergy efficiency.     

V型多通道平板太阳能集热器的热性能研究

提出一种V型多通道平板太阳能集热器,对其建立稳态传热模型,利用Matlab软件编程进行求解,并进行实验测试验证模型的准确性.利用已验证的传热模型,模拟分析V型多通道平板太阳能集热器的结构和物性参数对其热性能的影响,结果表明当透明盖板和吸热体发射率变大时,集热器的集热效率会呈下降趋势;当V型槽顶角、吸热体长度及空气夹层厚...     

Thermal performance of three solar air heaters

This paper reports the thermal analysis and the results of thermal performance tests of solar air heaters with V-corrugated absorber plates. Six collector units were used to test three collector types: (a) flow over the absorber, (b) flow on both sides of the absorber, and (c) flow under the absorber. Paired simultaneous testing was used for collectors of the same type to check thermal performance differences which might occur due to construction differences or testing variables. Predictions of collector thermal performance, when using temperatures and environmental data from the tests, were in good agreement with the corresponding measured thermal performance. The predicted performance, particularly with solar glass, indicated very good thermal efficiency for vee corrugated absorber plates.     

Thermal advantages for solar heating systems with a glass cover with antireflection surfaces

Investigations elucidate how a glass cover with antireflection surfaces can improve the efficiency of a solar collector and the thermal performance of solar heating systems. The transmittances for two glass covers for a flat-plate solar collector were measured for different incidence angles. The two glasses are identical, except for the fact that one of them is equipped with antireflection surfaces by the company SunArc A/S. The transmittance was increased by 5–9%-points due to the antireflection surfaces. The increase depends on the incidence angle. The efficiency at incidence angles of 0° and the incidence angle modifier were measured for a flat-plate solar collector with the two cover plates. The collector efficiency was increased by 4–6%-points due to the antireflection surfaces, depending on the incidence angle. The thermal advantage with using a glass cover with antireflection surfaces was determined for different solar heating systems. Three systems were investigated: solar domestic hot water systems, solar heating systems for combined space heating demand and domestic hot water supply, and large solar heating plants. The yearly thermal performance of the systems was calculated by detailed simulation models with collectors with a normal glass cover and with a glass cover with antireflection surfaces. The calculations were carried out for different solar fractions and temperature levels of the solar heating systems. These parameters influence greatly the thermal performance associated with the antireflection surfaces.