THERMAL SHADING EFFECT OF CLIMBING PLANTS ON GLAZED FACADES

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1 The 2005 World Sustainable Building Conference, THERMAL SHADING EFFECT OF CLIMBING PLANTS ON GLAZED FACADES Marta Hoi Yan LAM BSc Kenneth IP PhD MSc CEng MCIBSE Andrew MILLER PhD BSc CEng MCIBSE School of the Environment, University of Brighton, Cockcroft Building, Lewes Road, Brighton, East Sussex, United Kingdom, BN1 2HQ, Keywords: shading coefficient, vegetation, deciduous climbing plants, glazed façade, shading device, bioclimatic building design Summary This paper reports on the thermal shading performance of a vertical layer of deciduous climbing plant canopy. The deciduous climbing plants trail on a metal framework which is mounted external to the glazed facade of a naturally ventilated building. This so-called bioshader provides some distinct advantages over conventional shading devices such as its foliage shades the building from excessive summer solar gains, the shedding of leaves in the winter allows beneficial solar heat gains, improvement in air quality and aesthetic enhancement of the surrounding. An experiment was setup in two university offices. Bioshader was installed external to the test room and the external and internal environmental parameters were monitored. Measured solar radiation data were used to determine the solar transmittances of single and multi-layers of leaf. A pixel recognition technique was used on photographs of the bioshader to establish the areas covered by different layers of leaf. The shading performance of the bioshader can be represented by the proposed dynamic shading coefficient over the growing period. The factors affecting the dynamic shading coefficient have been identified, these are: the plant coverage, the plant growth rate, the solar transmittances of different layers of leaf and the type of plant. The methodology for establishing the dynamic shading coefficient takes into account the leaf solar transmittances and the coverage of the plant, both of which have been derived from the results of the experiment. The outcomes of this research will allow the dynamic thermal analysis of the bioshader and add to the understanding and knowledge of using vegetation as a means of solar shading. 1. Introduction Excessive summer solar gains often causes overheating in buildings and thermal discomfort to occupants. One option to reduce solar gains is to introduce solar shading. Conventional shading devices such as sun blinds, sunscreens, awnings, projections etc. are commonly used. Recently there is a growing interest in using vegetation as shading devices especially buildings that are adopting the bioclimatic design philosophy. Comparative studies of using plants and conventional shading devices as passive solar control systems have been conducted by a number of researchers (Hoyano, 1988, Papadakis et al., 2001, Stec et al., 2005). Results showed that plants offered some significant advantages over conventional shading devices such as lower surface plant temperature and higher relative humidity of air shaded by plants. This is mainly due to the ability of vegetation to dissipate absorbed solar radiation into sensible and latent heat through the process of evapo-transpiration. It was observed that about 60% of the absorbed radiation is converted to latent heat in plants (Stec and Paassen, 2005). The current research proposed a bio-shading device using a vertical layer of deciduous climbing plant canopy that trails on a metal framework, which is mounted external to the glazed facade of a naturally ventilated building. This setup, as shown in Figure 1, is termed as a bioshader throughout this paper. The bioshader s shading effect changes with its growing and wilting seasons from spring to winter. In the winter, deciduous plants have bare branches that allow low angle solar radiation through the glazed facade into the building interior. Whereas in the summer, the dense leaf foliage absorbs solar radiation and, through the evapo-transpiration process, lowers the air temperatures

2 The current on-going research aims to evaluate the environmental performance of the bioshader. This paper reports mainly on the experimental measurements of the thermal shading effect and the development of the dynamic shading coefficient for the bioshader. In the summer, the dense foliage of the bioshader blocks the high angle sun Bioshader deciduous climbing plants grown on a robust framework External stainless steel framework In the winter, the bioshader sheds off its leaves; this allows low angle solar radiation entering the building Openable window Plant container Figure 1 A vertical section through the bioshader system 2. Bioshader Experiment An experiment was set up at the School of the Environment, University of Brighton, to evaluate the bioshader s thermal shading performance. The aim of the experiment is to measure and evaluate the bioshader s shading effect to an occupied office. The results were compared with an identical office, acting as the control room, which has no bioshader. The following sections outline the physical setup of the experiment, the selection of deciduous climbing plant, the equipment and experimental layout, the test and control rooms details, the measurement and data collection procedure. 2.1 Plant Selection The selection of plant has taken into consideration the local and regional climatic conditions. Brighton is in the southern part of England, where the average temperature is a few degrees warmer than most parts of the United Kingdom, with earlier springs, warmer summers and milder winters. The lowest winter temperature is rarely below 5ºC and there are constant onshore breezes with occasional strong winds. These factors together with the experimental site conditions were used as some of the plant selection criteria established by Lam et al. (2002). A number of possible deciduous climbing plants were identified among which the deciduous climbing plant Virginia Creeper (Panthenocisus quinquefolia) was considered appropriate. Its unique large 5-oval leaflets characteristic makes it a good shader. Also it is tall, fast spreading, and requires little maintenance. An added aesthetic effect of Virginia Creeper is that its leaves change to a bright crimson colour before they shed off in the autumn.

3 2.2 Experimental Rooms The selected building is naturally ventilated with large windows facing south west and the rooms are subjected to excessive summer solar gains. Two identical offices located next to each other were selected as the test and the control rooms. They are located on the first floor and each room measures 3 m (W) by 3 m (L) by 3 m (H). There are two windows in each room; each window measures 1.2 m by 2.1 m. Stainless steel metal frames, provided by Mendip Manufacturing Agency Limited, were mounted external to the test room windows. The grid size of metal frame is 150 mm by 200 mm. Virginia Creeper plants were trailed onto these frames as shown in Figure 2. Both the test and control rooms were occupied by one member of staff during normal working hours. Bioshaders on metal framework outside the test room windows The windows of the control room without any solar shading Figure 2 Virginia Creeper trained onto the metal framework outside the windows of the test room. 2.3 Experimental Setup Four Virginia Creeper plants in four containers were evenly placed under the metal framework in front of the test room windows as shown in Figure 1 and 2. The internal dimensions of each plant container are 400mm (length) by 300mm (width) by 600mm (height) and they were tailored made from lined plywood. A layer of 2.5mm aggregates was put at the bottom of the containers to prevent the compost from clogging up and blocking the drainage holes. N Corridor temp and RH Internal temp and RH Internal temp and RH Central data logging unit Test Room Air flow Control Room Air flow Solar flux energy sensors Solar flux energy sensor Figure 3 Gap temp and RH Plan showing locations of monitoring equipment Dome solarimeter External temp and RH

4 The locations of the monitoring equipment and extract fans are shown in Figure 3. The monitoring parameters incl ude both the external and internal solar radiations, air temperatures and relative humidities. All measuring and monitoring equipment in the control and test rooms were connected to a central logging system in the adjacent room. The Agilent A unit which allows up to 40 channels of data logs was used to log the data. All sensors were linked to the central data logger through junction boxes. Two extractor fans were installed to provide controlled mechanical ventilation. The air velocity, air temperature and relative humidity are monitored with sensors installed in the air ducts. The location of the dome solarimeter (GS1), solar energy flux sensors (ES2), external relative humidity and air temperature sensors (RHT2nl) are also shown in Figure 3. The indoor solar flux sensors were secured by means of moveable clamps so that the flux sensors can be positioned anywhere across the windows. The external dome solarimeter was fixed vertically onto the wooden plant container. 2.4 Measurements The well established Virginia Creeper were planted from their pots into the troughs on the 13 th May 2003 and the monitoring equipment were completely installed in June 2003, The data logging started from the 25 th June 2003, and the experiment ended on the 31 st August A total number of 14 months data, recorded at every two-minute interval throughout the experimental period, were collected. The test and control rooms were also fitted with door sensors for the det ection of room occupancy. As the thermal environments of the rooms would be affected w hen doors were opened, data of the days with occupancy were excluded in the analysis. The plants were pruned twice over the experimental period. The first time was on 4 Aug 2003 and the second pruning was on the 24 th May 2004 to provide a good growth start for the new summer season. Apart from data recorded from the data logger, photos were taken every other day to monitor the growth rate of the plants. These regular photographic shots were taken during the summer season of year The solar reductions by various leaf layers were monitored on sunny days throughout the summer. th 3. Results and Discussion The collected data were organised and analysed. Three key areas of results - the leaf solar transmittances, the bioshader growth coverage and the dynamic shading coefficient of the bioshader - are summarised in the following sections. 3.1 Leaf Solar Transmittance The total solar radiation data measured by the dome solarimeter and the solar flux sensors were adjusted to take into account their vertical positions. The vertical and horizontal components of the external solar radiations were derived from generic equations by Sukhatme (1996) using known latitude, declination, slope of the bioshader, wall azimuth and hour angle. A series of experiments were carried out to find the solar transmittances through single or multi layers of Virginia Creeper leaves. The process involved repeated measurements of external solar radiations on the surface of the leaf and transmitted radiation behind the leaf or layers of leaf. The transmittances are calculated, using Equation (1) as shown below: T I I n n = (1) Ext Whe re: T n = Solar transmittance with n th layers of leaf I Ext = External incident solar radiation perpendicular to the leaf canopy [W/m 2 ] I n = Solar radiation measured by the solar flux sensor behind n th layers of leaf [W/m 2 ]

5 Outliners discarded Outliners discarded Solar transmittance Solar transmittance Number of readings Number of readings Data used for analysis Figure 4 Solar transmittance data of 2 layers leaf on the left graph, and 3 layers of leaf on the right graph Examples of measured solar transmittance results for 2 and 3 layer leaves are shown in Figure 4. The calculated mean solar transmittances of one to five layers of leaves are summarised in Table 1. Table 1 The solar transmittances of one to five layers of Virginia Creeper leaf Leaf layer 1-leaf 2-leaves 3-leaves 4-leaves 5-leaves Mean solar transmittance Growth Rate and Coverage The Virginia Creeper continued to grow after they were re-planted in the troughs at the end of May Over the month of June 2003, the plants grew vigorously with significant increase in leaf coverage. The changes in the leaf area and leaf layer density were recorded with regular photographic shots. Pruning No leaf ls Pixe leaf 2-leaves 3-leaves 4-leaves 5-leaves M ay 24-M ay 03-J un 13-J un 23-J un 03-J ul 13-J ul 23-J ul 02-A ug 12-A ug 22-A ug 01-S ep 11-S ep 21-S ep 01-O ct 11-O ct 31-Oct 21-O ct Figure 5 The pixels of one to five layers of leaf of the test room window

6 The bioshader leaf layer coverage was estimated by differentiating the leaf colours on the photographs. The procedure involved the use of colour recognition facility in the Adobe Photoshop software to determine the colour variation of different leaf layers and their corresponding areas in the photographs. The numbers of colour pixels representing different layers of leaf were extracted by the software, which were subsequently converted into areas. Each photograph taken over summer 2003 was analysed using the method described above. The area of each leaf layer was calculated and used to establish the overall leaf coverage of the bioshader. Figure 5 shows the coverage by different layers of leaf as calculated from the number of pixels in the photographs. Each vertical line represents the proportion of window areas covered by different layers of leaf including the gaps without leaf on that particular day. The bioshader was pruned on 2 nd August 2003 when it had grown to its maximum state covering almost the whole window with dense foliage. The dotted line in Figure 5 shows the time when pruning occurred. The area without leaf coverage increases after pruning, while the areas of the denser four to five leaf layers decrease. The plant coverage soon recovered during its vigorous growth over August and by early September 2003, it reached its maximum foliage again. 3.3 Dynamic Shading Coefficient The dynamic shading coefficient of the bioshader can be defined as the fractional reduction of the incident solar radiation through the bioshader on a particular day. Since the bioshader changes its proportion of different layers of leaf and their coverage over the growing season, the dynamic shading coefficient would therefore be non-linear and time dependent. In order to establish this dynamic shading coefficient for the bioshader covering the entire window area, a cumulative solar transmittance representing the weighted shading effect of individual leaf layer is used. This dynamic shading coefficient is represented by Equation (2) as shown below: SC = n t [ A k ( 1 T k )] (2) k = 0 Where: SC t n A k T k = Overall shading coefficient for the bioshader at a certain date t = The maximum number of leaf layers of the canopy th = Area covered by n layer leaf, A 0 represents the gap area with no leaf = Solar transmittance of area covered by n th layer of leaf, T 0 equals to 1 for the gap with no leaf Shadin g coefficient Pruning A Projected growth B 0 29/04/ /05/ /06/ /06/ /07/ /08/ /08/ /09/ /10/ /10/ /11/2003 Figure 6 The overall shading coefficient of the bioshader for the left window of the test room

7 Figure 6 and Figure 7 show the overall shading effect of the bioshader for the test room s left and right windows respectively over the summer period The two dotted lines from A to B in the graphs are the projected growth if there were no pruning. The results show that the bioshaders achieve a maximum shading coefficient of 0.55 over the growing period. Shading coefficient A Pruning Projected growth B 0 29/04/ /05/ /06/ /06/ /07/ /08/ /08/ /09/ /10/ /10/ /11/2003 Figure 7 The overall shading coefficient of the bioshader for the right window of the test room 4 Conclusion and future work With the growing trend towards the use of bioclimatic design features in buildings, there is an increasing scope for using vegetation as climatic modifiers in the built environment. The bioshader described in this paper has demonstrated the successful use of such design option. The current research has developed a bioshader design suitable for office buildings with glazed façade. The proposed design has been applied to two offices and their thermal performances were monitored and subsequently analysed. A methodology for the calculation of dynamic shading coefficient was also developed and applied. The overall shading performance of the bioshader can be represented by the proposed dynamic shading coefficient over the growing period. A number of factors affecting the dynamic shading coefficients have been identified, these include: the plant coverage, the plant growth rate, the solar transmittances of different layers of leaf and the type of plant. Calculations from the experimental results show that the leaf solar transmittances are between 0.43 to 0.14 for one to five leaf layers. A pixel recognition technique was used to determine the areas covered by different leaf layers using photographs taken for the bioshader. By applying the results of the leaf solar transmittances and the leaf area coverage, the overall dynamic shading coefficient of the bioshader was calculated. This dynamic shading coefficient, which takes into account the influencing factors identified, was shown to vary over the growing season and have a maximum value of The experimental measurement forms part of the process towards the validation of the previously developed theoretical model (Ip et al. 2004), which takes into account the microclimate of the bioshader with different leaf coverage and different leaf layers. Future work will also include the use of dynamic shading coefficient in thermal analysis software to evaluate the environmental performance of the bioshaders when they are applied to different buildings and climates.

8 5 References Hoyano, A. 1988, Climatological uses of plans for solar control and the effects on the thermal environment of a building. Energy and buildings, 11, pp Ip, K., Lam, M. H. Y. & Miller, A. 2004, Bioshaders for sustainable buildings. CIB World Building Congress 2004, May 2-7. Toronto, Ontario, Canada. Lam, M. H. Y., Ip, K. & Miller, A. 2002, Experimental modelling of deciduous climbing plants as shading devices. Sustainable Building 2002, 3rd International Conference on Sustainable Buildings. Oslo, Norway. Papadakis, G., Tsamis, P. & Kyritsis, S. 2001, An experimental investigation of the effect of shading with plants for solar control of buildings. Energy and Buildings, 33, pp Stec, W. J. & Van Paassen, A. H. C. 2005, Symbiosis of the double skin facade with the HVAC system. Energy and Buildings, 37, pp Stec, W. J., Van Paassen, A. H. C. & Maziarz, A. 2005, Modelling the double skin facade with plants. Energy and Buildings, 37, pp Sukhatme, S. P. 1996, Solar energy: principles of thermal collection and storage. Tata McGraw-Hill.