Unglazed Transpired Solar Collectors Research Articles

Flat Unglazed Transpired Solar Collector: Performance Probability Prediction Approach Using Monte Carlo Simulation Technique

Engineering applications including food processing, wastewater treatment, home heating, commercial heating, and institutional heating successfully use unglazed transpired solar collectors (UTCs). Trapping of solar energy is the prime goal of developing an unglazed transpired solar collector. The UTC is usually developed in and around the walls of the building and absorbs the solar energy to heat the air. One of the key challenges faced by the UTC designer is the prediction of performance and its warranty under uncertain operating conditions of flow variables. Some of the flow features are the velocity distribution, plate temperature, exit temperature and perforation location. The objective of the present study was to establish correlations among these flow features and demonstrate a method of predicting the performance of the UTC. Hence, a correlation matrix was generated from the dataset prepared after solving the airflow over a perforated flat UTC. Further, both strong and weak correlations of flow features were captured through Pearson’s correlation coefficient. A comparison between the outcomes from a linear regression model and that of computational simulation was showcased. The performance probability for the UTC was interlinked with correlation matrix data. The Monte Carlo simulation was used to predict the performance from random values of the flow parameters. The study showed that the difference between the free stream value of temperature and the value of temperature inside the UTC’s chamber varied between 15 and 20 °C. The probability of achieving system efficiency greater than 35% was 55.2%. This has raised the hope of recommending the UTC for drying and heating where the required temperature differential is within 20 °C.

Techno-economic analysis of an exhaust air heat pump system assisted by unglazed transpired solar collectors in a Swedish residential cluster

Solar heating technologies hold a significant potential to supplement or replace the fossil fuel-driven heating systems in residential and industrial applications. This paper presents a techno-economic study aiming to assess the use of Unglazed Transpired Solar Collectors (UTSC) coupled with an energy system assisted by Exhaust Air Heat Pump (EAHP) in cold climates applied to a residential building cluster. The performance of the system and its components is assessed for different sizes of solar collector field. In addition, a rule-based algorithm is developed to manage the airflow into the UTSC, and a comparative analysis is carried out with conventional flow control. The existing EAHP assisted energy system of a multifamily building cluster in Sweden is modelled by using a simulation software TRNSYS, and the effects of the UTSCs integration on the performances of the energy system are evaluated. Results show that the integration of UTSCs has a small but positive impact on the overall system performance. Moreover, the developed control based on the variation of the collector airflow rate for UTSC is an effective control strategy to increase the seasonal performance factor of the overall system and to maximize the savings.

Numerical Simulation of Flow over a Flat Unglazed Transpired Solar Collector (UTC) and it’s Performance Prediction

An unglazed transpired solar collector is a system that can leverage the abundant solar energy for various purposes. The solar collector is available in flat or corrugated form and is seen to be installed as an exterior layer of building facades. The cladding thus made absorbs radiation from the sun and heats up air being sucked by fan and flowing through perforations. In this research the focus has been to understand the correlation of plate temperature, exit temperature, the velocity distribution in the chamber and perforation location when air flows past an unglazed transpired solar collector (UTC). The establishment of correlations was carried out in the dataset of flow variables obtained after solving the problem using Navier-Stokes (NS) equations along with standard two-equation (k-ε) turbulence models and Shear Stress Transport (SST ) k-ω models for turbulent flow. The same problem was also solved using NS equation using laminar model. An attempt has also been made to compute Pearson’s correlation coefficient of any two variables to understand their strong and weak correlations. A linear regression analysis was done through an open source software Rstudio for a dataset produced during the computational modeling using a commercial CFD solver, Ansys® Fluent. At the end a Monte Carlo simulation has been done to predict the likelihood of using the flat UTC for drying as well as to understand the dependency of system efficiency on plate exit temperature, suction velocity and freestream temperature.

A simplified method for evaluating thermal performance of unglazed transpired solar collectors under steady state

Abstract Due to the advantages of low investment and high energy efficiency, unglazed transpired solar collectors (UTC) have been widely used for heating in buildings. However, it is difficult for designers to quickly evaluate the thermal performance of UTC based on the conventional methods such as experiments and numerical simulations. Therefore, a simple and fast method to determine the thermal performance of UTC is indispensable. The objective of this work is to provide a simplified calculation method to easily evaluate the thermal performance of UTC under steady state. Different parameters are considered in the simplified method, including pitch, perforation diameter, solar radiation, solar absorptivity, approach velocity, ambient air temperature, absorber plate temperature, and so on. Based on existing design parameters and operating conditions, correlations for the absorber plate temperature and the heat exchange effectiveness are developed using dimensional analysis and data fitting, respectively. Results show that the proposed simplified method has a high accuracy and can be employed to evaluate the collector efficiency, the heat exchange effectiveness and the air temperature rise. The proposed method in this paper is beneficial to directly determine design parameters and operating status for UTC.

Thermodynamic investigation on an innovative unglazed transpired solar collector

Renewable energy represents an attractive solution to fulfil two requirements: indoor air quality and energy efficiency. Passive solar systems are easy to implement and effective in areas with high solar potential. The Unglazed Transpired Solar Collector (UTSC) is made of metal cladding with perforations, installed at several centimetres from a building wall, creating thus a cavity, allowing to preheat the fresh air. Several measurements were performed on an innovative perforated solar wall model. This study is a preliminary approach of an analysis on the importance of the orifice shape of the perforated panel as a heat transfer influencing parameter. Both the fluid dynamics and thermal behaviours were investigated. The more complex dynamics of the lobed flows results in a better heat transfer rate. Changing the geometry of the perforations will increase on one hand the orifice’s perimeter and it would generate complex fluid dynamics, resulting in higher mixing between the primary flow and the ambient and thus a higher efficiency of heat recovery of these devices. The comparison of a conventional UTSC with a new geometry with innovative perforation leads to interesting results, with almost 40% increase in thermal efficiency.

Large scale test of a novel back-pass non-perforated unglazed solar air collector

This paper describes large scale tests conducted on a novel unglazed solar air collector system. The proposed system, referred to as a back-pass solar collector (BPSC), has on-site installation and aesthetic advantages over conventional unglazed transpired solar collectors (UTSC) as it is fully integrated within a standard insulated wall panel. This paper presents the results obtained from monitoring a BPSC wall panel over one year. Measurements of temperature, wind velocity and solar irradiance were taken at multiple air mass flow rates. It is shown that the length of the collector cavities has a direct impact on the efficiency of the system. It is also shown that beyond a height-to-flow ratio of 0.023 m/m³/hr/m2, no additional heat output is obtained by increasing the collector height for the experimental setup in this study, but these numbers would obviously be different if the experimental setup or test environment (e.g. location and climate) change. An equation for predicting the temperature rise of the BPSC is proposed.

Experimental study of performance of Photovoltaic–Thermal Unglazed Transpired Solar Collectors (PV/UTCs): Energy, exergy, and electrical-to-thermal rational approaches

Photovoltaic panel associated with Unglazed Transpired Collector (PV/UTC) can convert solar energy into thermal and electrical energy. In the present paper, a UTC capable of combing with PV panels is designed, constructed and tested at Shahid Bahonar University in Kerman, Iran. The performance of the PV/UTC and UTC systems are evaluated based on the simple first law, first law defined as a function of electrical-to-thermal ratio number, and the second law efficiencies. The obtained results showed that mounting PV panel on the UTC can result in photovoltaic cooling, depending on the mass flow rate value of the air passed through the transpired plate. A critical radiation level based on the useful exergy gain is also presented and it is drawn that the greater number of PV panels would cause a decrease in critical radiation level. Also, the results show that the electrical-to-thermal rational and exergetic analyses are very important to design PV/UTC systems.

Exergy analysis of Unglazed Transpired Solar Collectors (UTCs)

The Unglazed Transpired Solar Collector (UTC) is a suitable device for preheating outside air directly. They are used mostly in preheating ventilation air as well as in heating air for crop drying. The thermal performance of UTC promises its popular usage in near future. A number of large scale installations have been used all over the world since 1990s. In this study, first, the exergy efficiency of UTC is derived. Based on this efficiency, optimization was performed on 200 different cases and finally, a simple but useful correlation was proposed to predict the optimum working temperature. After finding the optimum working temperature, an economic analysis may be performed to determine the most effective and least expensive geometry.

Energy modeling of photovoltaic thermal systems with corrugated unglazed transpired solar collectors – Part 1: Model development and validation

Building-integrated photovoltaic–thermal (BIPV/T) systems with unglazed transpired solar collectors (UTCs) can provide a key solution for on-site electricity and thermal energy generation. Although the energy saving potential of this technology is significant, no systematic thermal analysis model has been developed for optimal system design and integration with building operation. This paper is the first of two companion papers focused on modeling and performance analysis of BIPV/T systems with UTC. In Part 1, energy models are presented for two configurations: UTC only and UTC with PV panels, to predict the cavity exit air temperature and plate surface temperature with weather (incident solar radiation, ambient air temperature, dew point temperature and wind speed) and design (airflow rate or suction velocity and geometry) parameters used as inputs. Nusselt number and effectiveness correlations, representing both the exterior and interior convective heat transfer processes, have been obtained from experimentally validated, three-dimensional, Reynolds-Averaged Navier–Stokes (RANS), Computational Fluid Dynamics (CFD) simulations, using high resolution grids and the ReNormalization Group Methods k–ε (RNG k–ε) turbulence closure model. The energy models were validated with measurements in an outdoor test-facility. Good agreement was observed between the model prediction and the experimental data, with the root mean square error (RMSE) being within 1°C for the UTC-only model and within 2°C for the model of UTC with PV modules. In the companion paper, Part 2, the effects of important parameters on system performance are demonstrated based on information from the literature and simulations using CFD and energy models. The optimal geometry is investigated for both configurations and the performance curves, under different levels of solar radiation, wind speed and suction velocity, are presented to provide guidelines for system design.

Energy modeling of photovoltaic thermal systems with corrugated unglazed transpired solar collectors – Part 2: Performance analysis

This paper is the second of two companion papers focused on energy modeling and performance analysis of building-integrated photovoltaic thermal (PV/T) systems with corrugated unglazed transpired solar collectors (UTCs). In Part 1, energy models are presented for two configurations: UTC only and UTC with PV panels. The models predict the energy output of the system for different weather and system design conditions and are validated using measured data from an outdoor test facility. In this paper (Part 2), the system performance is evaluated based on data drawn from the literature and simulations with Computational Fluid Dynamics (CFD) and energy models. The analysis includes parameters that are unique for this system, such as the corrugation geometry and the collector orientation. Validated, high resolution CFD simulations are used to study the impact of plate orientation and incident turbulence intensity, based on the comparison of exterior and interior Nusselt (Nu) number and the cavity exit air temperature, as well as the PV surface temperature when UTCs are integrated with PV panels. It is found that for configurations with UTC only, both exterior and interior convective heat transfer is enhanced in the ‘vertical’ installation, while similar results were obtained for increased incident turbulence intensity levels. However, only minor influences from these two parameters are observed for UTCs with PV panels. The energy model is used to investigate the optimal geometry for both configurations. It is found that parameters such as slope length and corrugation wavelength have the most significant impact on UTC performance while the wavelength and PV panel height have the largest effect for UTCs with PV panels.

An evaluation of heat transfer and effectiveness for unglazed transpired solar air heaters

The use of Unglazed Transpired Solar Collectors (UTSCs) is considered to be one of the most effective methods of reducing HVAC loads in buildings. The operation of the UTSC is relatively simple. A perforated absorber plate is installed in a location where it is exposed to solar radiation. Air is drawn through the perforations, and into the fresh air intake of an HVAC system. This pre-warmed air could make a significant contribution towards decreasing the energy used for heating. Unfortunately, commercially available UTSC products are no longer geometrically similar to the products used to produce correlations used by designers for estimating heat loss and effectiveness. The intent of the current investigation, therefore, is to numerically investigate modern UTSC system performance. Numerical development was successful, and the model was validated by comparison to existing experiments and correlations. An experimental investigation, however, is still required to provide formal validation. Correlations for heat loss and effectiveness have been developed.

Airflow and thermal analysis of flat and corrugated unglazed transpired solar collectors

The flow structure and convective heat transfer mechanisms in Unglazed Transpired solar Collectors (UTCs) are crucial to their performance. High-resolution, 3-dimensional, steady, Reynolds-Averaged Navier–Stokes (RANS), Computational Fluid Dynamics (CFD) simulations have been used to analyze the convective heat transfer processes for both flat and corrugated UTCs. The performance of five potentially suitable turbulence closure models: Standard k–ε, Renormalization Normal Group k–ε (RNG k–ε), Realizable k–ε, Shear Stress Transport k–ω (SST k–ω) and Reynolds Stress Model (RSM), has been evaluated. Two scenarios have been considered: a flat UTC under free stream approaching flow and a corrugated UTC subjected to a plane wall jet flow. The results were compared against experimental data from the literature and those obtained using a full-scale experimental set-up in a solar simulator, in terms of the velocity and turbulent kinetic energy of the airflow as well as the plate surface and air temperature. The validation study showed that the RNG k–ε model has the best overall performance for both UTC geometries at reasonable accuracy and computing cost. A parametric analysis has been conducted using the RNG k–ε model at 3m/s approaching flow velocity, for different suction flow rates (0.01 and 0.06m/s) and free stream turbulence intensities (0.1% and 20%). The results showed that in UTCs, it is the suction velocity, rather than the suction ratio of Vs/U∞, that has the most profound impact on the boundary layer development and thermal efficiency. For flat UTCs, the presence of perforations is more significant than the level of turbulence intensity in the approach flow. However, for corrugated UTCs, the incident turbulence intensity plays a more important role in the system performance than the perforation dimensions due to the turbulent interaction between the corrugated geometry and the incoming flow. Although a high suction rate can help maintain the asymptotic boundary layer profiles and, hence simplify the energy analysis of UTCs, it is not applicable in practice due to the requirement of large fan power. These outcomes will not only further advance the development of UTCs and optimize their performance but also provide insights for the development of simplified thermal analysis models for use in building energy simulation.

A full 34 factorial experimental design for efficiency optimization of an unglazed transpired solar collector prototype

The goal of this study was to model and optimize the thermal performance of an unglazed transpired solar collector (UTC) prototype using a full factorial experiment with four factors (hole diameter, absorber coating, irradiation and mass flow rate) at three levels. A quadratic polynomial model for efficiency was shown to explain 95.47% of the variance of thermal output. Residuals analysis and analysis of variance were used to validate the best-fit model. Finally, the model was optimized using response surface representations. An optimal combination of levels of the four parameters was obtained to provide a collector efficiency of 70–80%. The chosen experimental methodology provided accurate characterization of the parameters that have the greatest impact on UTC performance.

Evaluation of Turbulence Models for Airflow and Heat Transfer Prediction in BIPV/T Systems Optimization

Airflow patterns and convective heat transfer mechanisms in unglazed transpired solar collectors (UTCs) integrated with Photovoltaic-Thermal systems are crucial to their performance. Computational Fluid Dynamics (CFD) simulations can be a valuable tool for the design and analysis of these hybrid building-integrated solar energy systems while recent advances in CFD approaches and turbulence models provide a great potential for improving prediction accuracy. This study performs 3-dimensional steady Reynolds Averaged Navier-Stokes (RANS) CFD simulations and evaluates the performance of five turbulence closure models, potentially suitable for modeling UTCs in terms of accuracy and computing cost. These models include four two-equation models (Standard k-ɛ, Renormalization Group Methods k-ɛ, Realizable k-ɛ and Shear Stress Transport k-ω) and one Reynolds Stress Model. A flat UTC under free stream approaching flow conditions and a corrugated UTC subjected to a plane wall jet flow were considered. The predicted air velocity, air temperature, and turbulent kinetic energy were compared against experimental data from the literature and those obtained using a specially designed full-scale experimental set-up in a solar simulator. The results show that although the Reynolds Stress Model can provide more detailed flow features at an increased computational cost compared to other models, it does not necessarily result in better performance. Among the two- equation models, the Renormalization Group Methods k-ɛ has the best overall performance for both cases studied while the Standard k- ɛ and Shear Stress Transport k-ω models can also provide acceptable prediction accuracy. A BIPV/T system with two PV panel arrangements across a UTC under the same coverage was investigated using the CFD models developed in the present study. It was found that BIPV/T systems integrated with UTCs can result in electrical efficiency up to 1.2% higher, for the cases considered.

Thesis Summary: Theoretical and Experimental Investigation of a Solar-Biomass Hybrid Air Heating System for Drying Applications

The overall objective of this research was to design, develop, and investigate the performance of a renewable energy-based (solar-biomass) hybrid air heating system. The system consists of an unglazed transpired solar collector (UTC), a rock bed thermal storage, and a biomass gasifier stove with heat exchanger, to supply hot air at a required temperature and flow rate for a daily load fraction exceeding 90%. From a review of literature, an air heating system aimed at reducing the weather dependency and improving the temperature and flow rate stability without a conventional back-up heater was designed. Among the various types of solar collectors, thermal storage, biomass stoves, and heat exchangers that are generally used, specific designs were chosen and analyzed. Based on the analyses, individual components of the air heating system were designed and developed. The components were coupled, and detailed experimentation was carried out on the integrated system.

Effect of Unglazed Transpired Collector on the Performance of a Polycrystalline Silicon Photovoltaic Module

The electrical power generated by a polycrystalline silicon photovoltaic (PV) module mounted on an unglazed transpired solar collector (UTC) has been studied and compared to that of a PV module without UTC for a quantitative analysis of electrical output and its role in reducing the simple payback periods of photovoltaic electrical systems. A 75W polycrystalline silicon PV module was fixed on an UTC in front of the ventilation fan, and effectiveness of cooling by means of the forced ventilation at the rate of 160CFM was monitored. The temperature reduction under forced ventilation was in the range of 3-9°C with a 5% recovery in the electrical output power on a typical day of the month of February 2005. The simulated and measured electrical power outputs are in reasonable agreement with root-mean-square error of 2.40. The life cycle assessment of a hypothetical PV system located at Daejeon, South Korea and consisting of 3kW PV modules fixed on a 50m2 UTC shows that with a possible reduction of 3-9°C in the operating temperatures, the system requires three 75W fewer PV modules. The simple payback period of PV system is reduced from 23yearsto15years when integrated into an UTC air heating system.

A field study of the wind effects on the performance of an unglazed transpired solar collector

An experimental study was carried out on a working unglazed transpired solar collector (UTSC) to determine what effects ambient wind has on its performance. The monitoring system included instruments to measure temperatures, collector outlet flow rates, solar radiation, wind speed, and wind direction; as well as an ultrasonic anemometer placed near the centre of the collector. Efficiency was defined as the fraction of incident solar heat flux that went to preheating the transpired air. Our observations indicate a high degree of turbulence near the wall which feeds the near wall region. This is supported by observations of efficiency which decrease monotonically with increasing turbulence intensities. It was also observed that peak efficiencies did not occur at the lowest wind speeds. Both these findings seem to contradict existing laminar boundary layer models for UTSC performance.

A CFD heat transfer analysis of the transpired solar collector under no-wind conditions

Due to the advantages of low investment and high energy efficiency, unglazed transpired solar collectors (UTC) have been widely used for heating in buildings. However, it is difficult for designers to quickly evaluate the thermal performance of UTC based on the conventional methods such as experiments and numerical simulations. Therefore, a simple and fast method to determine the thermal performance of UTC is indispensable. The objective of this work is to provide a simplified calculation method to easily evaluate the thermal performance of UTC under steady state. Different parameters are considered in the simplified method, including pitch, perforation diameter, solar radiation, solar absorptivity, approach velocity, ambient air temperature, absorber plate temperature, and so on. Based on existing design parameters and operating conditions, correlations for the absorber plate temperature and the heat exchange effectiveness are developed using dimensional analysis and data fitting, respectively. Results show that the proposed simplified method has a high accuracy and can be employed to evaluate the collector efficiency, the heat exchange effectiveness and the air temperature rise. The proposed method in this paper is beneficial to directly determine design parameters and operating status for UTC.