ENERGY EFFIC

IENT BUILDING ENVELOPES FOR CHENNAI, INDIA

Vijayalakshmi, M.M.,Ä Natarajan, E.# and Shanmugasundaram, V@

Ä Civil Engg. Dept., Sathyabama University, Chennai – 119, India.#Institute for Energy Studies, Anna University, Chennai – 25, India.

@ Department of Civil Engineering, Anna University, Chennai – 25, India.

ABSTRACT

Buildings are mainly designed to provide comfortable shelter for the occupants with minimalvariation in indoor conditions and minimal requirements for space conditioning by external means. Aconsiderable share of energy is required for space conditioning, especially if the building is located inregions of harsh (cold or hot) climatic conditions. If the building envelope is properly oriented and designedwith optimum proportions, the solar energy incident on the building envelope can be effectively controlledfor promoting winter heating and for preventing summer cooling. In this paper, the total solar radiation onvarious building orientations in the Climate of Chennai in India is analyzed to identify optimum orientationand optimum aspect ratios of a rectangular building for solar control. The extent of increased solarradiation on the outer surface of the buildings with non-optimum orientation and aspect ratios is alsodiscussed. Relative values of thermal delays and decrement factors required for various building surfacesare suggested.

Key words: Solar control, building, envelope, optimum aspect ratio

1. INTRODUCTION

Buildings consume considerable energy for maintainingcomfortable living conditions. In general, the amount of energyused for maintaining comfortable living conditions in a buildingduring its life time is considerably higher than the energyconsumed during its construction. For instance, it has beenreported that in Sweden the energy consumption for buildings is 4%for materials, 3% for the building process, 0.5% fortransportation and 92.5% for daily maintenance for an assumeddepreciation period of forty years [1].

A considerable share of energy is required for spaceconditioning, especially if the building is located in regions ofharsh (cold or hot) climatic conditions. In the average Americanhome, for example, 50% – 70% of total energy consumption isattributed to space heating and cooling [2]. In Hong Kong, wherethe climate is subtropical with summer from March to early

Ävijinat@hotmail.com, vijnat@gmail.com

November, the air-conditioning accounts for 50 - 60% of the totalelectricity use [3].

Solar radiation is one of the main factors that determine thecooling loads, particularly in the hot climates. The mosteffective method to control the solar heat gain is to design thebuilding envelope so as to avoid unwanted solar heat in summerwithout much interference with the arrival of solar heat in thewinter when it is required.

In this work, the total solar radiation on the variousbuilding surfaces are calculated for various months of the yearfrom the climatic data of Chennai. The total soar radiation on thewhole external surface of the rectangular building of differentorientation and different aspect ratios are analysed to identifyoptimum values. Relative values of thermal delays and decrementfactors required for various building surfaces are suggested.

2. TOTAL SOLAR RADIATION ON BUILDING SURFACES

The metropolitan city of Chennai is the capital of Tamil Naduin India and situated on 13°04’ North Latitude and 80°17’ EastLongitude. It is located in the peninsular part of South India.The minimum average temperature is 25°C and the maximum average is32°C. The average relative humidity is between 60% and 75%. Theaverage wind velocity is between 0.4 m/s and 1.2 m/s. Theclimatic data for this analysis is obtained from the handbook ofsolar radiation data for India [4] which presents details based oncareful measurement made over long periods at various radiationstations in the country, operated and maintained by the IndianMeteorological Department.

The total solar radiation on any vertical wall of a buildingcan be calculated as a sum of direct radiation, diffused radiationand reflected radiation from the ground surface and in general canbe written as [5, 6, 7, 8]

where qj is the total solar radiation per unit area on jth wall indaytime (kWh/m2/day), IDj is the intensity of the direct solarradiation falling normal to the jth

vertical wall, IDH is theintensity of the direct solar radiation falling normal to thehorizontal surface and IdH is the intensity of diffused radiationon the horizontal surface. is the reflective coefficient ofground surface and it is taken as 0.2 in this study. The aboveintegration (summation) is evaluated from sunrise to sunset foreach wall and for each month.

The total solar radiation on the roof (horizontal surface) ofbuilding can be calculated as:

where qH is the total solar radiation per unit area on the roof indaytime (kWh/m2/day) and the limits of integration is over thetime from sunrise to sunset. The daily average total solarradiation during various months of the year are calculated andsummarised in the Fig. 1.

3. BUILDING FORM AND ASPECT RATIOS

The length of the rectangular building along the east to westdirection is taken as X and length along north to south directionas Y and the height as Z. The ratio of X to Y is taken as aspectratio ‘x’ and aspect ratio ‘z’ is the ratio of Z to Y.

The total solar radiation incident on the rectangularbuilding surface can be calculated as

where qE, qW, qN, and qS are the total solar radiation on east,west, north and south wall respectively. A1 is the area of eastand west wall, A2 is the area of the north and south wall and A3 isthe area of horizontal roof. If the areas are deduced in terms ofaspect ratios x and z and the volume of the building (V) and then bysubstitution the above equation becomes,

Since the cubic shape has the minimum external surface for agiven volume among the practically possible shapes, it is taken asthe reference shape for the building. For the building of cubicform of same volume (for which x =1 and z =1), the total solarradiation incident on its surface (called as Qo) is given as,

The relative solar radiation on the building surface isdefined as ratio of solar radiation on the actual buildingshape(Q) to that on the cubic shape of same volume(Qo) and it isexpressed as:

The above ratio is minimum, if and .

Applying these conditions, we get the following values of x and zfor which the relative heat transfer is minimum.

and

4. RESULTS AND DISCUSSIONS

The external surfaces of building have different solar heatgain, since the direct solar radiation is of a strong azimuthcharacter. The roof of the building is heated by solar radiationfrom sunrise to sunset. The maximum TSR on the surface of roof is880 W/m2 during the summer, but it is 593 W/m2 during winter. Itindicates that the TSR on the roof surface considerably varies

from summer to winter season. Thus, maximum solar heat gain inthe building is due to the roof and it emphasizes the need for thecareful thermal design of the roof of the building to avoidundesirable heat during the summer season [9,10].

The east and west walls are mutually shaded during the day.When the east wall receives maximum solar heat between 9am and10am, the west wall receives maximum solar heat about 4pm. Themaximum TSR is about 500 W/m2 during summer and 400 W/m2 duringwinter. When the maximum TSR on the east and west wall increasesby 25% during the summer, the same on the roof increases by 35%.East wall should not delay the morning heat load to reach theindoor in the afternoon, which will worsen the indoor comfort.Hence east wall must use material with sufficiently long thermaldelay [11, 12].

The north wall receives relatively less (200 W/m2) TSR duringwinter month compared to summer month (300 W/m2). But the TSR onthe south wall is 427 W/m2 during winter and 234 W/m2 during thesummer month. It is mainly due to variation of the solar azimuthfrom the winter to the summer. It receives direct solar radiationas long as (September to April) sun is south of the local latitude(13°N), in addition to diffused and reflected solar radiation.Hence, the total solar radiation received during this period isrelatively high compared to heat received during other months (Mayto August) when it receives only diffused and reflected solarradiation.

It may be noted that the south wall is receiving relativelymore solar heat during the winter month when it is the mostwelcome and the north wall is receiving high solar heat during thesummer when it is unwanted. Hence it is desirable to have betterheat conductance (hence poor heat resistance) for the southernwall and poorer heat conductance for the north wall. The southwall should be provided with wall material with short time lag andlow decrement factor to effectively use the solar heat during thewinter for warming up the living space of the house. The northwall may be built with heavy wall structural mass so that it hassufficiently long time lag and low decrement factor so as to avoidunwanted heat entering into the house.

The average daily total solar radiations on various buildingsurfaces in kWh/m2 is shown in Fig. 1. The north wall receives thelowest (1.958 kWh per day) daily average total solar radiation,but it receives solar heat during undesirable summer months. Themagnitude of daily average total solar radiation on the east andwest walls is about the same magnitude. The daily average totalsolar radiation received by the roof is about 3 times as that bythe northern wall. Its peak value occurs at about noon. Thus, theroof of the building not only demands for higher thermalresistance to reduce the heat transfer but also better thermalmass to delay the heat transfer into late night.

The energy consumption of buildings depends on the buildingforms. In hot areas, the solar radiation is one of the mainfactors that influence the cooling loads. The shape andorientation of the building decide the amount of solar radiationreceived by the building surface. In cold areas, if the externalsurface of building is large, more energy is required for heating,and hence the optimum form of the building should have the minimumexternal surface area.

For different values of aspect ratios of the building, thevalue of the relative solar incident radiation Q/Qo (which is theratio of solar radiation on a rectangular building to that on acubic building of the same volume) is computed and shown inFig. 2. It shows that the solar radiation on the surface of therectangular building is considerably (20 to 72%) more than thecubic building of the same volume, if x is less than one. For anyvalue of x, the solar radiation on the surface decreases as theheight increases, because the roof contribution considerablydecreases in comparison with walls. The solar radiation on thebuilding surface is minimum if the ratio x and z are about 1.25.Under such conditions, the solar radiation on the rectangularbuilding is almost same as that on the cubic building of the samevolume. If the value of x is equal to 1.25 (i.e., if the buildingis oriented along east to west axis), Q/Q0 varies from 1.0 to 1.12for z values of 0.75 to 4.

If these values differ from their optimum values, the totalsolar radiation on the surface increases and it may lead toincreased discomfort in the non-air conditioned building and

increased cooling load in air-conditioned buildings. The increasedsolar radiation load may go upto 20 % if the aspect ratios aremore than their optimum values. The same may be much higher if itis less than their optimum values.

For any length to width ratio (x), the incident solarradiation decreases with increase in height to width ratio (z).When the value of height to width ratio is more than 1.25, thevariation in annual average daily incident solar radiation persquare meter with the length to width ratio becomes insignificant.If the value of z is more than two, the average daily incidentsolar radiation per square meter of building surface area becomesconstant at 3.03 kWh/day.

5. CONCLUSIONS

The major heat gain in the buildings located in the tropicalzone is due to the solar heat. The judicial design of the buildingwith suitable orientation and aspect ratios can considerablyreduce the solar heat incident on the building surfaces. For theclimate of Chennai, the building orientation with long surfacesfacing north and south direction is found to be optimum and thelength and height aspect ratio of 1.25 is recommended. The eastand north wall must be designed to have high thermal time delayand low decrement factor to alleviate the unwanted solar heatgain. The thermal design of roof of the building must be givenutmost importance as it receives high solar heat gain throughoutthe year. The usual way of constructing the building with similarouter walls on all sides of the building does not seem to besuitable. Depending upon the functional requirements of thedifferent spaces of the building, individual walls must beprovided with suitable structural mass and resistance so as toimprove the thermal performance of the building without muchassistance from the external resources.

Reference

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Fig. 1 Average total daily Solar Radiation on different surfacesof the building for various months of the year.

z = Z/Y

Fig Relative solar Radiation on Building Surface

0.901.001.101.201.301.401.501.601.701.80

0 0.5 1 1.5 2 2.5 3 3.5 4Ratio of North side wall length to South side wall length (x = X/Y)

Q/Qo

0.250.50.7511.251.51.7522.252.52.7533.253.53.754

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