The Effect of Shielding Barriers on Solar Air Collector Gains

Energy Procedia 36 ( 2013 ) 1070 – 1075 1876-6102 © 2013 The Authors. Published by Elsevier Ltd. Selection and/or peer-review under responsibility of the TerraGreen Academy doi: 10.1016/j.egypro.2013.07.122 TerraGreen 13 International Conference 2013 - Advancements in Renewable Energy and Clean Environment The Effect of Shielding Barriers on Solar Air Collector Gains Ondrej Sikula*, Josef Plasek, Jiri Hirs Brno University of Technology, Faculty of Civil Engineering, VeveĜí 331/95, Brno 602 00, Czech Republic Abstract The paper deals with numerical simulations of the impact of location and orientation of the solar air collector located on the roof of a building and on its thermal performance. The solar collector is used to preheat an air, which then is supplied into the building. The solar collector is placed on the roof, which is shielded from barriers. To achieve the desired objective, numerical simulation of the annual operations of the collector in the software BSim was performed. Real climate data for Central Europe were used. The result of this work is to determine the effect of shielding barriers on the usable annual heat gain, which is represented by preheating of external air. © 2013 The Authors. Published by Elsevier Ltd. Selection and/or peer-review under responsibility of the TerraGreen Academy. Keywords: Solar air collector, heat gain, shielding effect, numerical simulation 1. Introduction Using renewable sources of energy is a present trend with purpose to meet the requirements of the directive [1] of the European Parliament, where the aim is to reduce gas emissions by at least 20 % below 1990 levels until 2020. One of these renewable energy sources is solar radiation, which can be successfully used for pre-heating of fresh air supplied into the air handling unit (AHU). One of the main conditions of successful air collector operation is its optimal orientation to cardinal directions taking into account negative influence of shielding surrounding which is discussed in this paper. Nomenclature Irad global solar radiation [W/m2] *Corresponding author. Tel.: +420 541 147 923; fax: +420 541 147 922. E-mail address: [email protected] Available online at www.sciencedirect.com © 2013 The Authors. Published by Elsevier Ltd. Selection and/or peer-review under responsibility of the TerraGreen Academy ScienceDirect Open access under CC BY-NC-ND license. Open access under CC BY-NC-ND license. Ondrej Sikula et al. / Energy Procedia 36 ( 2013 ) 1070 – 1075 1071 2. Air collector Air collector uses sun radiation to pre-heat outside air. Sun radiation falls on the surface of air absorber made by large glass sheet. Absorber, which adjoins the air gap where the fresh air is supplied to air handling unit is placed behind this glass surface. External dimensions are adjusted to dimension of an air handling unit, see Fig. 1 and [2]. This collector together with the AHU is placed and fixed on the roof and therefore it cannot be directed towards the sun in order to catch the maximal solar gains during the day. This operation could be the optimal as stated in [3]. The advantages are low space requirements, simple installation and easy maintenance of the air collector [4]. Scheme of air collector placing is in the Fig. 1. The usable annual solar heat gain, which is represented by preheating of external air is performed by using an operation of a building according [5]. Fig. 1. Scheme of placing of air collector on AHU unit. 3. Numerical simulation of the air collector Numerical simulation of the air collector can help us to find the best orientation of collector to cardinal directions and to evaluate negative influence of shielding by other buildings. Building simulation software (BSim, version 2000) was chosen to perform numerical simulations [6]. Software is based on heat balance method in particular zones defined by user. Multi-zone model of air collector adjoining AHU unit was used for the simulations. This 3D model is capable of solving shading without time-consuming techniques as mentioned in [7]. Real hour climatic data for year 2005 for Brno, Central Europe were used for these numerical simulations. The maximal global radiation was on 20th May 2010 at 12 p.m., Irad = 940 W/m2 and the minimal was on 20th December 2010 at 12 p.m., Irad = 81 W/m2. An influence of roof reflectivity    primary inlet  outlet Airhandling unit Solarair collector Building 1220mm  width 3670mm Roof secondary inlet Shielding barrier Solar radiation height Secondary inlet 1072 Ondrej Sikula et al. / Energy Procedia 36 ( 2013 ) 1070 – 1075 for solar radiation was not been taking into account - as states [8] - because of low snow precipitation in the climate region in question. 4. Influence of air collector orientation Eight numerical simulations in BSim software by one hour step were performed in order to determine influence of air collector orientation on usable heat gains for a heating by air of buildings. In this step the simulations were performed without the influence of shielding. Simulations does not include months from May to September because during this time pre-heating of fresh supplied air is not necessary. Calculation was done for four main cardinal (N, E, S, W) and also for four (NW, NE, SE, SW) ordinal directions. The most suitable orientation of solar air collector is to the south, where obtained heat gains can vary from 11.9 kWh/(m2ͼmonth) in December up to 35.6 kWh/(m2ͼmonth) in March. Conversely, the less suitable orientation of collector is to the North, where heat gains vary from 2.4 kWh/(m2ͼmonth) in December to 12.8 kWh/(m2ͼmonth) in March. Immediate efficiency of air collector range up to 26 %, however the average long-term efficiency is lower and does not exceed 13 %. Fig. 2. Solar radiation heat gains from air collector in chosen months of the year. Table 1 Heat gains per 1 m2 of solar air collector face area from solar radiation without shielding kWh/(m2·month). Month in year North orientation East orientation South orientation West orientation January 3.1 5.3 17.8 5.2 February 5.6 8.8 20.6 8.5 March 10.6 19.0 35.6 16.7 April 12.8 20.5 33.0 19.1 October 6.6 11.9 32.2 12.9 November 3.6 5.2 17.3 6.3 December 2.4 3.7 11.9 3.7 Year sum 44.7 74.4 168.3 72.3 Ondrej Sikula et al. / Energy Procedia 36 ( 2013 ) 1070 – 1075 1073 5. Influence of air collector shielding The most suitable orientation of solar air collector is to the South, which is proved by the table above and also in [9]. In reality, different obstacles (elevated floors, lift machine rooms and other constructions of the building) can be found near to the solar air collector. They shield the collector and decrease its heating power. Next part of the paper focuses on quantification of this effect and also on decreasing of annual heat gain of collector. a) b) Fig. 3. (a) Scheme of shielding barrier and division of roof into partial areas; (b) Relative heat gains with south shielding. As a typical example building with flat roof with dimensions 12 x 12 m was chosen. This flat roof was virtually divided according to [9] into sixteen particular elements marked from 1A to 4D – see Fig. 3a. Consequently, there was placed elevated floor as a shielding barrier with dimensions 6 x 12 x 3 m with orientation to east, south and west. Orientation of air collector in all versions was optimal, therefore to the south. As a reference value – 100 % of solar heat gains was taken value 25.0 kWh/(m2ͼmonth) belonging to south orientation of collector. This heat gain is in good agreement with findings specified in [10], which was calculated for similar purposes with using climatic data in Slovakia - Central Europe. Results of numerical simulations in particular elements when shielding barrier is placed on east, south and west sides are shown in the Figures 3b, 4a and 4b. a) b) Fig. 4. (a) Relative heat gains with west shielding; (b) Relative heat gains with east shielding. Shielding barrier 1074 Ondrej Sikula et al. / Energy Procedia 36 ( 2013 ) 1070 – 1075 In case of solar air collector oriented to the south, the smallest influence of shielding from the east was in columns A and B, when shielding was from west, the smallest influence was in columns C and D. Conversely, the worst solution is to place collector oriented to the south near barrier shielding from the south (sector 4B and 4C). The best possible orientations of solar air collector to cardinal directions when shielding from east were obtained from the results of numerical simulations – see Fig. 5a and 5b. a) b) Fig. 5. (a) Optimal orientation of air collector to cardinal directions with south shielding; (b) Relative heat gains by this orientation. The most suitable solar air collector orientation is to the south. In case of shielding barrier from the south side, it is best to avoid placing of solar air collector in rows 3 and 5 in this case; it is six metres from shielding barrier. If it is not possible to place air collector more than 6 m from barrier, then it is recommended to slew collector slightly to the southeast, eventually to the southwest, see Fig. 5a. This move can increase usable solar heat gains of air collector in unfavourable sectors 4B and 4C up to 16 %. Figure 5a shows interesting result, when in sections 3B and 4B it is more convenient to slew solar air collector to the southeast then to southwest even though difference between these orientations is less than 1 %. The small difference can be caused by bigger afternoon clouds comparing with morning weather conditions and also by influence of higher afternoon temperature, which decreases solar heat gains. 6. Conclusion The paper deals with quantification of usable solar heat gains collected by new solar air collector with respect to its orientation to cardinal directions and to shielding of surrounding barriers. Simulations have taken into account heat gains which are transported by using a preheated external air into a building. Totally, more than one hundred numerical simulations were performed during entire heating season with real climatic data. Obtained results show that in case of non-shielded vertically placed collector, its optimal orientation is to the south. In case of placing solar air collector near south orientated shielding barrier, heat gains can decrease by 32 % of original value. In this position it is better to slew solar air collector to south-east and to slightly eliminate unfavourable influence of south shielding. From the results follows that in case of south orientation of air collector, shielding by obstacle from east, or west is important only when the collector is placed very close to the wall. Obtained results allow to choose Ondrej Sikula et al. / Energy Procedia 36 ( 2013 ) 1070 – 1075 1075 optimal position of solar air collector during its design and to maximise using of this renewable energy from sun radiation, which is from 11.9 to 35.6 kWh/(m2ͼmonth) (relative to the face area of the collector) in Central Europe. Acknowledgements This paper was supported by the project CZ.1.07/2.3.00/30.0039 of Brno University of Technology, and also with a support of the Specific research at universities Reg. No. FAST-J-13-2098. References [1] Directive 2010/31/EU of the European Parliament and of the council of 19 May, 2010 on the energy performance of buildings. [2] Merka V., Sikula O., Solar air collector integrated in ventilation units. Utility model Reg. Nr. 23111, Aplication Nr. 201125046. Prague Czech Republic 2011. [3] Koussa M., Haddadi M., Saheb D., Malek A., Hadji S., Sun Tracking Mechanism Effects on Flat Plate Photovoltaic System Performances for Different Step Time and Main Parameters Affecting the Obtained Gains: Case of North Africa and Mediterranean Site, Energy Procedia, Volume 18, 2012, Pages 817-838, ISSN 1876-6102. [4] Sikula O., Merka V., Plasek J., Simulation of the influence of shielding surrounding on an efficiency of solar air collector, SOLARIS 2011, 5th International Conference on Solar Radiation and Daylighting, Brno University of Technology, Czech Republic ISBN: 978-80-214-4306-8. [5] Sikula O., Plasek J., Hirs J, Numerical Simulation of the Effect of Heat Gains in the Heating Season, Energy Procedia, Volume 14, 2012, Pages 906-912, ISSN 1876-6102. [6] Wittchen K., B. Johnsen K., Grau K., User guide BSim 2000, version 2.1. [7] Ampatzi E, Knight I, Modelling the effect of realistic domestic energy demand profiles and internal gains on the predicted performance of solar thermal systems, Energy and Buildings. ISSN 0378-7788. [8] Thevenard D., Haddad K., Ground reflectivity in the context of building energy simulation, Energy and Buildings, Volume 38, Issue 8, August 2006, Pages 972-980, ISSN 0378-7788. [9] Sikula O., Merka V., Hirs J., Plasek J. Maximization of Solar Gains of an Air Collector by Simulations, Applied Mechanics and Materials, ISSN: 1660-9336, 2012. [10] Katunsky D., Lopusniak M., Impact of shading structure on energy demand and on risk of summer overheating in a low energy building, Energy Procedia, Volume 14, 2012, Pages 1311-1316, ISSN 1876-6102.