Solar Access in Tropical Cities
PLEA2005 - The 22 nd Conference on Passive and Low Energy Architecture. Beirut, Lebanon, 13-16 November 2005 4/5 The cylindrical projection is rotated 90° clockwise for better visualization. The dark zones are obstructed and the top of the rectangle is the northern zone (azimuth 0°). A sky zone is considered to be obstructed if its central point azimuth is between the initial and the final horizontal obstruction angle AND if the altitude angle is lower than the vertical obstruction angle. Each rectangle represents the obstruction of one of the test cells. The sky radiance and luminance modelling required some considerable processing capacity, since it multiplies 8760 radiation inputs by 145 zones, generating a table of over a million cells. Therefore, they were calculated in a different spreadsheet and only the final result was imported. That means that, in order to insert other climatic data, it would be necessary to repeat the process and import them again. The cell parameters – dimensions, U-values for glazing and opaque, WWR, internal loads, mass factor, etc – are defined in Sheet 5 - Temperature and air conditioning. Changing these values may alter significantly the results. Therefore, a precise hypothesis of the surrounding buildings is necessary for satisfactory calculations. Should the software be implemented in any city or town, this part of the spreadsheet should be accessed only by specialists from public administration and not by regular users. Total energy consumption increase or reduction is given separately for each final use (heating, cooling, lighting) in absolute numbers in the graphs in the top right sector. Below the data input there are tables that summarize the relative variations in energy consumption. At the bottom of the screen, graphs show the variation for each cell, which allows the study of different obstruction orientation. Thresholds for energy consumption increase have not been suggested, since it is a political matter. Planner should have in mind that the more restrictive the thresholds are, the lowest densities will be. 4. EXPLORING THE MODEL 4.1 A theoretical example In order to explore the potential of the spreadsheet, a pilot study was made using the building shown in fig. 7 B (0,84) N 2 (12,72) C (84,84) 3 (72,72) 1 (12,12) A (0,0) Figure 7: Example building 4 (72,12) D (84,0) The building is a 60m wide square oriented along the north-south axis and it is located 12 m far from the edge in order to simulate an infinite horizontal obstruction. First simulated height was 12 m. For the first simulations, test cells dimensions were set to 6x6x3 m. In later simulations they were reduced to a cubic proportion of 3x3x3 m. The other parameters for the simulations are given in the tab. I Table I: Parameters for the pilot study Parameter Value Parameter Value WWR 0,5 Glazing SF 0,4 Air Ch Hour QI 1,0 0 W·m -2 Glazing 0,7 Light transm. U –walls* 3,6 Illuminance levels Light power 300 lux 12 W·m -2 U – glazing* 4,9 AC efic. 10,4 kJ·W -1 U – roof * 2,0 Mass factor 0,5 Absorption 0,5 * U values in W·m -2 ·°C -1 The study results showed that air conditioning consumption was very low in this situation and, surprisingly, heating was very high (fig. 8). Figure 8: Example building’s simulation results The tolerance for heating was 4°C and neutral temperatures in Sao Paulo are around 24°C, which means a heating base temperature of approximately 19°C. That may be too high a standard for Brazil, where most houses don’t have heating and a temperature of 14°C to 15°C is perceived as cold but acceptable. Moreover, this room has no internal loads at all and those are important heat sources in most buildings. However, it can still be noted how heating consumption was not as affected by the obstruction as the others. There was only a 10% increase. That is because most heating usage takes place at night, when obstruction helps to avoid long-wave heat losses to the sky. This compensates a bit for the obstruction in the early morning. Since the comparative results were reasonable, parameters for calculating heating were kept. It was shown that, for São Paulo’s climate, cold stress should be a concern in spaces with low internal loads. It was also clear that obstruction influence on this case is stronger in the lighting consumption, which drives the 31% increase in total result.
PLEA2005 - The 22 nd Conference on Passive and Low Energy Architecture. Beirut, Lebanon, 13-16 November 2005 5/5 4.2 Some further studies In order to evaluate cell parameters influence on energy consumption, some more studies were carried on using the 3x3x3 m room. Most parameters were kept from previous simulations, verifying mainly the influence of wall window ratios and internal loads changes. The simulations were made for unobstructed test cells in cardinal orientations. The fig. 9 shows some comparative results. Energy consumption (MWh/year) 120,0 90,0 60,0 30,0 0,0 91,7 Qi=0 WWR = 0,17 85,1 Qi=0 WWR = 0,50 88,6 Qi=0 WWR = 1,00 58,9 Qi=17 WWR = 0,17 56,1 Qi=17 WWR = 0,50 63,8 Qi=17 WWR = 1,00 107,3 105,4 Qi=67 WWR = 0,17 Qi=67 WWR = 0,50 113,5 Qi=67 WWR = 1,00 Lighting Heating Air conditioning Figure 9: Graph showing the performance of unobstructed test cells varying WWR and QI Energy consumption on unobstructed sites depends much on internal loads. The optimised heating-cooling solution for Sao Paulo’s climate was reached at 17 W/m². Lighting in this case was not much of a problem, even though consumption was already high with a 0,17 WWR. It can be concluded, also, that in unobstructed sites, balanced glazing and opaque areas lead more efficient solutions. Taking the most efficient case as an example, it is possible to evaluate obstruction effect on performance. The angle was increased from 0° to 75° in a 15° step. The results are shown in fig. 10. Energy consumption (kWh/year) 80,0 70,0 60,0 50,0 40,0 30,0 20,0 10,0 0,0 0 15 30 45 60 75 Obstruction angle (°) Air Conditioning Heating Lighting Total Figure 10: Relationship between obstruction angle and energy consumption for a 3x3x3 m test cell with 0,5 WWR and a 17 W/m² internal load. One of the first conclusions that can be drawn from this graph is that the behaviour of the total energy consumption is determined by the increase in lighting. It also can be noted that the line is quite flat until the 45° mark and then it starts to rise at a growing rate. Air conditioning reduction tends to get more significant between 30° and 60°, but it does not compensate for the lighting consumption after 45°. Heating is an almost flat line and does not get much affected by obstruction. Reasons for that were discussed in the previous item. 5. CONCLUSIONS This method proved to be valid for studying solar access in tropical city. It accounts for overheating problems and it points towards integrated approach on the matter. Some more testing, however, should be done in order to evaluate obstruction effects on different kinds of buildings. Data from the actual urban system, and from the constructions composing it, should be gathered, adding precision to the model. Even though there is much to be done, it is already possible to state that solar access (or protection) should be a concern in urban planning for tropical cities. 6. ACKNOWLEDGEMENT We would like to thank IAG-USP, (MASTER and Laboratório de Micrometeorologia) for the climatic data and CNPq for financing this research. REFERENCES J. D. Kendrick, Guide to recommended practice of daylight measurement, CIE, Vienna (1989). , A. Brunger, F. Hooper, Anisotropic sky radiance model based on narrow field of view measurements of short-wave radiance. Solar Energy, Pergamon. v. 51, n.1 (1993) 53-64  N. Igawaa, H. Nakamura, All Sky Model as a standard sky for the simulation of daylit environment. Building and Environment, Pergamon, v. 36, (2001) 763-770  B. Givoni, Passive and low energy cooling of buildings. John Wiley & Sons. New York. (1994) 263  R. BRANDÃO,. Acesso ao sol e à luz natural: avaliação do impacto de novas edificações no desempenho térmico, luminoso e energético do seu entorno. São Paulo, USP (2004) 156  R. G. Hopkinson, P. Petherbridge, J. Longmore, Iluminação Natural, Fundação Calouste Gulbenkian. Lisboa (1975) 776 p.  I. Frame, S. Birch. Daylight Software. Anglia Polytechnic, Londres, (1991) Software  A. Frota, S. Schiffer, Manual de conforto térmico, São Paulo, Nobel, (1995) 243  M. Alucci, Conforto térmico, conforto luminoso e conservação de energia elétrica: procedimentos para desenvolvimento e avaliação de projetos de edificações, São Paulo, USP (1992) 225  RORIZ, Maurício. (2001) Consumo de energia no condicionamento térmico de edificações.; um método de avaliação. Anais 6 ENCAC, São Pedro, ANTAC (2001) CD-ROM