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    Nocturnal passive cooling by transpired solar collectors
    (Pergamon-Elsevier Science Ltd, 2021) Bokor, Balazs; Akhan, Hacer; Eryener, Dogan; Horvath, Miklos
    A novel use of the commercially available transpired solar collector is presented in the current paper. The reliable solar air heating system loses heat to the night sky if mounted on a building roof so it can be used as a passive cooling system based on thermal radiation. The collector plate cools down below ambient temperature and has the potential to cool the air as it is drawn through the perforations by a fan. A model has been elaborated for the cooling process based on heat transfer between the system components and energy balance equations. A method has been developed in order to choose the most suitable equivalent sky temperature model, as the radiative heat flow to the sky is the driving force of the cooling process and thus its accuracy is of utmost importance. The model has been validated by a series of field measurements carried out using a 5 m(2) setup in Edirne, Turkey. It has been found that the collector plate cools down up to 4.3 K below ambient temperature and it has the potential to cool air by up to 4.0 K. The system reached a maximum cooling performance of 66.5 W/m(2), while the average cooling performance was 34.6 W/m(2). It has been found that the collector plate cools down below ambient temperature an hour before sunset and does not reach ambient until one hour after sunrise under clear sky. A new Nusselt number correlation has been developed for the convection heat transfer between a perforated plate and the transpiring air flow.
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    Nocturnal radiation: new opportunity in building cooling
    (Elsevier Science Bv, 2017) Bokor, Balazs; Kajtar, Laszlo; Eryener, Dogan
    Free cooling systems are gaining increased attention as air conditioners are responsible for a remarkable amount of the electric peak load in summer. Several studies have shown that night ventilation of buildings reduces peak loads of the following day. However, their effect can be increased by a free cooling system based on nocturnal long wave radiation, which cools the air below ambient temperature. A perforated, corrugated metal plate which is used in transpired solar air heaters, functions as a radiating shield in this case. The air can reach temperatures below ambient as it passes the plate further increasing the free cooling's potential. A mathematical model has been set up to describe the cooling process. Based on governing equations diagrams are developed to show the correlation between the transpired radiant plate's temperature drop, as well as its cooling rate as a function of the long wave radiant heat flux and the air flow rate. Four European capitals are chosen to illustrate the cooling potential of nocturnal radiative air cooling. Building simulation is carried out using RETScreen 4 Clean Energy Project Analysis software. Results show that locations with drier climate reach higher nocturnal cooling performance as the moisture content of the atmosphere influences the radiant heat transfer. (C) 2017 The Authors. Published by Elsevier Ltd.
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    PEM fuel cell performance with solar air preheating
    (Pergamon-Elsevier Science Ltd, 2020) Uzun, Alper; Bokor, Balazs; Eryener, Dogan
    Proton Exchange Membrane Fuel Cells (PEMFC) have proven to be a promising energy conversion technology in various power applications and since it was developed, it has been a potential alternative over fossil fuel-based engines and power plants, all of which produce harmful by-products. The inlet air coolant and reactants have an important effect on the performance degradation of the PEMFC and certain power outputs. In this work, a theoretical model of a PEM fuel cell with solar air heating system for the preheating hydrogen of PEM fuel cell to mitigate the performance degradation when the fuel cell operates in cold environment, is proposed and evaluated by using energy analysis. Considering these heating and energy losses of heat generation by hydrogen fuel cells, the idea of using transpired solar collectors (TSC) for air preheating to increase the inlet air temperature of the low-temperature fuel cell could be a potential development. The aim of the current article is applying solar air preheating for the hydrogen fuel cells system by applying TSC and analyzing system performance. Results aim to attention fellow scholars as well as industrial engineers in the deployment of solar air heating together with hydrogen fuel cell systems that could be useful for coping with fossil fuel-based power supply systems. (C) 2020 The Authors. Published by Elsevier Ltd on behalf of Hydrogen Energy Publications LLC.
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    The Potential of Solar Air Heating in the Turkish Industrial Sector
    (Budapest Univ Technology Economics, 2019) Bokor, Balazs; Akhan, Hacer; Eryener, Dogan; Kajtar, Laszlo
    Transpired solar collector (TSC) systems are simple solutions for the preheating of ventilation air with solar energy. Their performance is a function of several environmental factors, so the climatic conditions of the location play an important role. In this paper, the effect of different climatic zones on the thermal performance of the TSC is investigated. To exclude other sources of influence, the same reference industrial building is examined in four Turkish locations (Antalya, Istanbul, Ankara and Sivas) representing different climatic conditions. RETScreen simulation is carried out for all four regions to obtain the drop of conventional heating requirement in case absorber azimuth of 0 degrees, 45 degrees and 90 degrees. To illustrate the performance, temperature rise, heating energy savings and annual solar fraction are presented. Generally, it can be stated that a location with cold climate and high solar radiation at the same time benefits most from the use of a TSC system. A mathematical correlation has been found showing the solar fraction's dependence on solar radiation and heating degree days. Finally, simulation results have been compared to a set of measurement data from an industrial building's TSC system near Istanbul.
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    Theoretical and experimental analysis on the passive cooling effect of transpired solar collectors
    (Elsevier Science Sa, 2017) Bokor, Balazs; Akhan, Hacer; Eryener, Dogan; Kajtar, Laszlo
    The present study provides results about the experimental performance of the transpired solar collector's passive cooling effect for the first time. In order to see the cooling performance of the transpired solar collector, a slope-adjustable experimental setup was built on the campus of Trakya University, Engineering Faculty, Edirne, Turkey. Solar radiation, ambient temperature, absorber temperature, cavity and back plate temperatures were monitored during summer period for different collector tilts. A physical and a mathematical model have been created to describe the heat transfer processes in the collector. The models were used to evaluate the measured data. It has been found that a natural airflow comes to be through the perforated plate, which acts similarly as in open-end double layer roofs. This airflow discharges the heat from the plenum decoupling the back plate from the exposed perforated plate. The temperature of the back plate, which represents the roof under the transpired collector, is significantly lower compared to that of the exposed roof on a typical sunny day. Similarly, the heat gain of the back plate is remarkably lower than the solar radiation received on the exposed roof. It has been found that the passive cooling effect of the transpired solar collector is increasing with rising intensity of radiation, as the heat transfer coefficient between the plenum and the back plate decreases with increasing solar radiation. Due to the natural character of the airflow in the plenum, wind heat losses are strongly dependent on the airflow's characteristics. Convective losses of the perforated plate have been defined by using two methods energy balance equations and a referenced heat loss correlation. (C) 2017 Elsevier B.V. All rights reserved.