Contribution of shading in improving the energy performance of

718 2nd PALENC Conference and 28th AIVC Conference on Building Low Energy Cooling and Advanced Ventilation Technologies in the 21st Century, September 2007, Crete island, Greece

Contribution of shading in improving the energy performance of buildings

T. Nikolaou, G. StavrakakisTechnical University of Crete, Greece

I. SkiasSKIAS engineering T.O., Greeceengineering T.O., Greece T.O., Greece

D. KolokotsaTechnological Educational Institute of Crete , Greece

while increasing the heating loads due to loss of benefi-cial solar gains. (Dubois, 1997).One study (Treado et al., 1984) showed that various types of shading devices increase the heating load and reduce the cooling load; the net energy savings only oc-cur if the reduction in cooling energy use exceeds the increase in heating energy use. According to this study the shading strategy depends on the climate where the building is erected. Another study (Hunn et al., 1993) showed that exterior shading devices and adsorbing glass are net energy losers in heating-dominated climates and that interior devices perform better than exterior fixed devices because they shade the entire window while providing additional insulation to the windowpanes. A study of energy characteristics and savings potentials in office buildings in Greece (Santamouris et al., 1994) showed that an appropriate shading of buildings could provide a significant reduction of cooling loads. Accord-ing to this study it is possible to reduce the total cooling load of the air conditioned buildings by approximately 7% by employing a more efficient shading strategy.Several studies have been performed using building en-ergy simulation programs to calculate annual cooling energy savings from the installation of interior or/and exterior shading devices (Loutzenhiser et al., 2007). A study (Hunn et al., 1993) using DOE-2.1 found a cool-ing energy savings up to 30% in Northern US climate with the installation of external shading devices. An-other study (Florides et al., 2000) using TRNSYS found cooling load reduction up to 20% for Cyprus using in-ternal shading devices. In this research the impact of fixed external shading de-vices on cooling loads and annual energy consumption is investigated for 10 office buildings located in Athens using TRNSYS simulation programme.

2. SHADING DEVICES UNDER INVESTIGATION

Fixed shading devices are generally used on the external face of glazing since they lower direct radiation from

ABSTRACT

The effect of shading in reducing the energy consump-tion of office buildings in Athens is investigated in this article. The aim of this paper is to demonstrate the role of shading devices in the improvement of energy effi-ciency of urban buildings especially in Southern Eu-rope. The first phase of research consisted of detailed data collection from 10 office buildings in the region of Athens, employing questionnaires, in-situ visits as well as interviews. Afterwards, a thermal building model was developed in TRNSYS 16 simulation pro-gram. The model takes into account the weather data of Athens and the collected data of the 10 office buildings and calculates the anticipated annually thermal /cooling loads and the total annual energy consumption for each of them. The post retrofit heating/cooling loads and the total annual energy consumption were calculated and compared to the pre-retrofit ones. After an appropriate processing of the simulation results, the energy savings due to the implementation of shading devices and the overall efficiency of this retrofit are estimated, allowing us to draw conclusions for the shading factor contribu-tion in energy saving techniques for urban buildings.

1. INTRODUCTION

The building sector requires large amounts of energy both for cooling and heating. The cooling loads due to solar gains represent about half of the global cooling loads for residential and non-residential buildings. The decrement of cooling loads, in order to achieve energy conservation in buildings, is very important especially for the Mediterranean urban areas. There is an interest to study shading devices and their impact on building energy use because shading systems represent a great retrofit opportunity at relatively low investment costs. (Dubois, 1998). Studies of the shad-ing impact on annual energy use have demonstrated that shading devices reduce the cooling demand in buildings

PALENC 2007 - Vol 2.indd 718 7/9/2007 1:24:20 µµ

7192nd PALENC Conference and 28th AIVC Conference on Building Low Energy Cooling and Advanced Ventilation Technologies in the 21st Century, September 2007, Crete island, Greece

reaching the internal ambient, dissipating the heat out-side. They are more efficient than internal fixed shading devices which dissipate the heat to the air gap between the shading device and the glazing. (Datta, 2001), (Of-fiong and Ukpoho, 2004). The shading devices being investigated in this study are the horizontal overhangs and vertical wingwalls.

2.1 OverhangsOverhangs are devices that block direct solar radiation from entering a window during certain times of the day or the year. These are desirable for reducing the cool-ing loads and avoid uncomfortable lighting in perimeter rooms due to excessive contrast. They are more effective on south-facing windows. (Yanda and Jones, 1983).

2.2 WingwallsWingwalls or vertical fins are the most appropriate shading devices for eastern and western facing open-ings which receive sun radiation at low angles, and for southeastern and southwestern openings in combination with horizontal shading. Wingwalls which increase the ventilation effectiveness of the building may also be uti-lized for shading if they are properly designed.

2.3 Benefits and drawbacksAppropriately designed external shading systems can ef-fectively control the sun’s direct radiation and partially block diffuse and reflected radiation. The role of shad-ing devices is to improve thermal and visual comfort by reducing overheating and glare. Shading the build-ing envelope and apertures directly reduces the need for cooling, because a shaded facade will still be subject to the influence of external air temperature and reflected and diffuse solar radiation, but will be free of the influ-ence of the direct radiation. A shaded wall, therefore, conducts less heat to the building interior and so will lead to a lower cooling load.The drawbacks of the external shading systems are that in most of the cases the heating load of the building in-creases and the associated reduction of daylight levels in the space sometimes may lead to a higher electrical lighting load. Besides, they usually have an impact on the aesthetic character of the building.

3. BUILDING DATA COLLECTION

For the needs of this research we collected analytical data for 10 office buildings in the area of Athens. Questionnaires were filled by the responsible individual of each build-ing while in-situ measurements were conducted as well. The following information has been collected from each building:

-General information about the building and the occu-pants (owner, construction year, the heated surface, the daily occupation profile)-Specific information about the building envelope (ori-entation, walls, ceiling, floors, openings, type of glazing and shading)-Detailed information about the heating, cooling and lighting systems (power, set points, operation profile) and information about the office equipment.-The annual energy consumption of the building. The con-sumed electricity and the quantity of fuel were recorded from the building energy bills. (Santamouris et al. 1994)

4. THE SIMULATION MODEL IN TRNSYS

For the simulation of the thermal behavior of the 10 buildings, the simulation software TRNSYS (TRNSYS 16, 2004) is used. TRNSYS is a transient systems simu-lation program with a modular structure and is used in the present study for deriving the results. Each module contains a mathematical model for a system compo-nent. The TRNSYS engine calls the system components based on the input file and iterates at each time-step un-til the system of equations is solved.

4.1 The simulation parametersThe simulation parameters (TRNSYS routines-TYPES) are listed in Table 1. Table 1. Simulation Parameters

Parameter TRNSYSType

Comments

Climatic Data 109-TM2 Athens (Greece)Building Data 56 From the data of the

10 buildings buildingsPsychrometrics 33eEffective sky tem-perature for long-wave radiation exchange

69b

Overhang and Wingwall Shading

34

Outputs QHEATQCOOL

Heating and Cool-ing Demand for the building

The analytical data of the 10 buildings are input to the model using TRNbuild. TRNbuild.TRNbuild..

4.2 Validation of the modelIn order to verify the reliability of the model, the actual energy consumption of the 10 buildings is compared to the outputs of the simulation model. The model, though, yields only the Heating and Cooling Demand



for the building. Therefore, in order to calculate the to-tal energy consumption that would be comparable to the electricity bills of the buildings, the annual energy con-sumption for lighting and equipment was assessed and the total energy consumption was calculated according to the following equation:Ε� (KWh) = Q (KWh) = Qh/nh + Qc/nc + Elight + Eequipwhere::Qh, Q Qc: Heating and Cooling Demand for the and Cooling Demand for theand Cooling Demand for the Cooling Demand for theCooling Demand for the Demand for theDemand for the for thefor the thethe

building (from the outputs of the model) (from the outputs of the model)from the outputs of the model))nh, n nnc: Efficiency factor of the heating and cool-

ing system accordinglyElight: Annual energy consumption for lightingEequip: Annual energy consumption of electrical

appliances and equipmentTable 2 presents the actual energy consumption of each building and the energy consumption calculated from the simulation model. The average difference between simulated and actual values is +2.12 kWh/m2 and it is also systematic, as depicted in Figure 1, by the satisfac-tory linear regression between them.Table 2. Total annual energy consumption

Building Actual(kWh/m2)

Simulated(kWh/m2)

1 137.40 116.382 108.57 103.573 47.17 41.004 36.00 37.105 153.85 166.356 29.85 28.257 130.00 142.968 238.53 224.549 56.26 58.0810 109.20 107.41

Figure 1. Annual Energy Consumption – Experimental values versus TRNSYS calculations.

4.3 Shading devicesIn order to investigate the contribution of the shading devices in reducing the cooling load and the total energy

consumption for cooling/heating for the 10 office build-ings, each opening is modeled individually employing the TYPE 34 component. This component computes the solar radiation on a vertical receiver shaded by an over-hang and/or wingwall. Based on previous research stud-ies (Florides et al., 2002), (Raeissi and Taheri, 1998) the southern oriented openings are chosen to be shaded by overhangs and the eastern and western oriented open-ings by wingwalls. It should be noted that not all studied buildings have openings at all these orientations.The dimensions of the shading devices were chosen ac-cording to the relevant literature (Florides et al., 2002), (Florides et al., 2002),Florides et al., 2002), et al., 2002),et al., 2002), al., 2002),al., 2002),., 2002), (Jorge et al., 1993), (Raeissi and Taheri, 1998), (El-Re-Jorge et al., 1993), (Raeissi and Taheri, 1998), (El-Re- et al., 1993), (Raeissi and Taheri, 1998), (El-Re-et al., 1993), (Raeissi and Taheri, 1998), (El-Re- al., 1993), (Raeissi and Taheri, 1998), (El-Re-al., 1993), (Raeissi and Taheri, 1998), (El-Re-., 1993), (Raeissi and Taheri, 1998), (El-Re-Raeissi and Taheri, 1998), (El-Re- and Taheri, 1998), (El-Re-and Taheri, 1998), (El-Re- Taheri, 1998), (El-Re-Taheri, 1998), (El-Re-, 1998), (El-Re-El-Re--Re-Re-faie and El-Asfouri, 1988). For these simulations, the and El-Asfouri, 1988). For these simulations, theand El-Asfouri, 1988). For these simulations, the El-Asfouri, 1988). For these simulations, theEl-Asfouri, 1988). For these simulations, the-Asfouri, 1988). For these simulations, theAsfouri, 1988). For these simulations, the, 1988). For these simulations, theFor these simulations, the overhang is assumed to be located 0.5 m above the win-dow and extended 1 m both sides of the window. For the wingwalls the top and bottom extension is assumed to be 0.5 m and the left - right gap 0.3 m. To investigate the optimal overhang length and wingwall projection for each window of the 10 buildings a number of simu-lations are performed. The overhang length varies from 0.5 - 2 m and the wingwall projection from 0.5 - 2 m. As soon as the dimensions of the shading devices were chosen, the heating and cooling loads of each one of the 10 buildings are calculated.

5. RESULTSRESULTS

Having shaded the southern openings by overhangs and the eastern and western ones by wingwalls of appro-priate dimensions, the model is run again in order to calculate the new anticipated heating and cooling loads of each building as well as its total annual energy con-sumption. In all cases, a reduction in cooling loads is observed as well as a small increase in heating loads and a reduction of annual energy consumption, except to some cases where a small increase is found.

5.1 The effect of overhangsShading the southern oriented opening by overhangs, an average of 18% reduction in cooling loads and an av-erage of 8.7% reduction in annual energy consumption are found. Table 3 presents the reduction percentage in cooling load and annual energy consumption of each studied building. In Figures 2 and 3 the cooling load and the total annual consumption of each building, with or without shading, are depicted.Table 3. Cooling load and energy consumption reduction with overhangs.

Bldg Southernopenings

(%)

Reduction of cooling load

(%)

Reduction of en-ergy consumption

(%)1 44.7 18.6 7.23 90.8 16.7 7.7


7212nd PALENC Conference and 28th AIVC Conference on Building Low Energy Cooling and Advanced Ventilation Technologies in the 21st Century, September 2007, Crete island, Greece

4 59.2 32.9 10.97 42.7 0.3 0.28 64.3 21.5 17.5

Figure 2. Effect of overhangs on cooling load.

Figure 3. Effect of overhangs on energy consumption.

5.2 The effect of wingwallsShading the eastern and western openings by wingwalls, an average of 7.6% reduction in cooling loads and an av-erage of 1.4% reduction in annual energy consumption are found. Table 4 below presents the reduction percent-age in cooling load and annual energy consumption of each studied building. In Figures 4 and 5 the cooling load and the total annual consumption of each building, with or without shading, are depicted. It should be noted that in three cases (buildings 5, 6 and 10) the annual energy consumption is increased by a tiny percentage and this is because the increase of heating load after the employ-ment of the shading devices is greater than the reduction in the cooling load. We should mention though that in all these three cases the percentage of shaded openings is relatively small compared to the remaining buildings.

Table 4. Cooling load and energy consumption reduc-tion with wingwalls.Bldg Western

openings(%)

Eastern openings

(%)

Cooling load reduction

(%)

Energy consumptionreduction (%)

1 30.72 36.78 14.9 4.52 27.72 29.65 19.5 2.8

5 13.52 - 5.7 -0.86 40.79 9.08

4.0 -0.97 35.23 82.67

3.7 2.48 58.14 -

4.8 3.89 35.84

6.5 0.010 48.17 -

1.4 -0.3

Figure 4. Effect of wingwalls on cooling load.

Figure 5. Effect of wingwalls on energy consumption.

6. CONCLUSIONS

In this paper, the contribution of two types of external shading systems, overhangs and wingwalls, to the re-duction of cooling loads and annual energy consumption of 10 office buildings in the area of Athens is studied. The simulations are performed by employing a dynamic model in TRNSYS 16. This simulation is validated by comparison with the real data for the energy consump-tion of the ten office buildings. For the shading of the buildings the use of overhangs for the southern oriented openings and the use of wingwalls for the eastern and western oriented openings are inves-tigated. By a series of simulations and according to the literature data, the appropriate dimensions of the shad-ing devices for each building alone are determined.The simulation results show that in the overhangs case we achieve an average of 18% in cooling load reduc-tion. The annual energy reduction is found to be 8.7%. 8.7%.In the case of shading the building openings by the



employment of wingwalls, the cooling load reductionwingwalls, the cooling load reductionthe cooling load reduction is 7.6% in average and the reduction in annual energy7.6% in average and the reduction in annual energy in average and the reduction in annual energy consumption 1.4%. In three cases, a small increase in 1.4%. In three cases, a small increase inIn three cases, a small increase in annual energy consumption is observed due to the fact that in these buildings the shading of small sized open-ings result in a great increase of heating load and a small reduction of cooling load.Driven from the results of this study it may be con-cluded that the shading openings with the employment of overhangs results in greater energy savings than theoverhangs results in greater energy savings than theresults in greater energy savings than the shading with wingwalls. wingwalls. The conclusion of this study is that the shading of the openings in office buildings located in areas where the climatic conditions are similar to that of Athens consti-tutes an efficient intervention for cooling loads reduc-tion and energy saving. With the appropriate design of With the appropriate design ofWith the appropriate design of the shading devices a reduction of the investment cost can be achieved as well as a reduction of the operating cost of cooling systems in an office building.

ACKNOWLEDGMENT

The work described in this paper has been supported by the ARXIMHDES project of the Operational Programme for Education and Initial Vocational Training (EPE-AEK II) under the 3rd European Community Support Framework for Greece and Hellenic National resources.

REFERENCES

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gain through a glazing unit with external and internal shading screens, Applied Thermal Engineering, 27, pp. 528-538.Offiong A., Ukpoho A., (2004), An analysis of Solar gain through externally shaded window of buildings, Renewable Energy 29, pp. 131-150.Raeissi S., Taheri M., (1998) Optimum Overhang Dimensions for En-ergy Saving, Building and Environment, Vol. 33, No. 5, pp. 293-302.Santamouris M., Argiriou A., Daskalaki E., Balaras C., Gaglia A., (1994), Energy Characteristics and Saving Potential in Office Building, Solar Energy Vol.52, No.1, pp. 59-66.Treado S., Barnett J., Remmert W., (1984), Effectiveness of Solar Shading for an Office Building, NBS Building Science Series 161, Washington, National Bureau of Standards.TRNSYS, A transient simulation program, version 16. (2004) Solar Energy Laboratory, University of Wisconsin, USA.Yanda R., Jones R., (1983) Shading effects of finite width over-hang on windows facing towards the equator, Solar Energy, Vol. 30, No 2, pp. 171-180.


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Contribution of shading in improving the energy performance of