Influence of the building shape on the energy performance of timber-glass buildings located in warm climatic regions

Energy 149 (2018) 496e504

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Influence of the building shape on the energy performance of timberglass buildings located in warm climatic regions

Miroslav Premrov, Maja Zigart, Vesna Zegarac Leskovar* University of Maribor, Faculty of Civil Engineering, Transportation Engineering and Architecture, Slovenia

a r t i c l e i n f o

a b s t r a c t

Article history: Available online 17 February 2018

An optimal proportion and appropriate orientation of glazing surfaces in timber-glass building play an important role due to the exploitation of solar radiation as a source of renewable energy for heating, applicable in most cases only to buildings located in cold and moderate climatic regions. However, the situation of timber-glass buildings located in warm climatic regions is completely different, since the energy demand for cooling represents a major contribution to the annual energy demand. The optimal solutions in such cases should therefore avoid overheating, which has not been extensively analysed in scientific literature discussing timber buildings. In the present study a total of 216 timber box-house models with a parametrically varied building shape (aspect ratio, horizontal and vertical extension), the thermal transmittance of the building envelope components, along with the varied glazing size and its position in the south or north façade, undergo separate analyses for locations in Athens and Sevilla. Climate conditions are intentionally selected and encompass very similar average temperatures but rather different solar radiation. The influence of the described parameters on the annual energy demand is thoroughly analysed and can serve architects as a systematic guideline in designing timber-glass buildings also for regions with warm climate conditions. © 2018 Published by Elsevier Ltd.

Keywords: Shape factor Glazing size Energy demand Timber-glass buildings Climate conditions

1. Introduction Climate changes of the last few decades do not only encourage researches into the origins of their onset but they also mean a warning and an urgent call for a need to remove their causes and alleviate the consequences affecting the environment. Many investigations have been carried out towards 100% renewable and sustainable energy solutions in many different areas [1e4]. As commercial and residential buildings consume almost 40% of the primary energy in the United States and Europe, eco-friendly solutions in residential and public building construction remain our most vital task, whose holistic problem solving requires knowledge integration, taking into account multiple and usually competitive objectives such as energy consumption, financial costs, environmental performance, renewable energy use, etc., Diakaki et al. [5]. Moreover, combining building materials with different material properties while fully respecting their possible advantages in a hybrid structural composition of buildings could be a relatively new approach in designing high energy-efficiency buildings and can

* Corresponding author.
E-mail address: [email protected] (V. Zegarac Leskovar). https://doi.org/10.1016/j.energy.2018.02.074 0360-5442/© 2018 Published by Elsevier Ltd.

lead to architecturally attractive and energy-efficient solutions at the same time. Owing to specific technological development and appropriate use, timber and glass are nowadays becoming essential construction materials as far as the energy efficiency is concerned. Only in recent decades has timber been rediscovered, partly due to the contemporary manufacture of prefabricated timber elements and partly on account of high environmental potential of this renewable natural building material. On the contrary, glass can definitely not be treated as a sustainable material and it long used to be treated as the weakest point of the building envelope from the thermal point of view. Nevertheless, dynamic evolution of the glazing in the last decades has resulted in insulating glass products with highly improved physical and strength properties, suitable for application to contemporary energy-efficient buildings, not only as a material responsible for solar gains and daylighting, but also as a component of structural resistance. However, due to significant differences in material properties, such as the thermal transmittance, the thermal expansion coefficient, strength, the modulus of elasticity, sensitivity to humidity, etc., their combined use is extremely complicated, from the energy efficiency and structural points of view, with the latter aspect residing in glazing being treated as a load-bearing structural component, [6]. However, appropriate consideration of all the features of both building materials led to the

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development of a new type of structures, the so-called timber-glass buildings, [7]. In such buildings an optimal proportion and appropriate orientation of the glazing surfaces play an important part due to the exploitation of solar radiation as a source of renewable energy within a passive use of energy for heating. A great number of studies treat the influence the glazing size and building geometry exert on the energy demand of buildings located in cold climate conditions [8e14]. They all point out a strong correlation between the final energy use for heating and the shape of the building located in cold climates. A general suggestion was that a cold climate may increase the impact of the building shape on the transmission heat losses and consequently on the energy need for heating. Thus the optimum form of the building has a minimum external surface and usually a rectangular shape. Moreover, in such cases an optimal proportion and appropriate orientation of the glazing surfaces usually play an important role due to the exploitation of solar radiation as a source of renewable energy for heating, most often applicable only to buildings located in cold and moderate climatic regions. With regard to the latter, it is sometimes important to simultaneously investigate the solar heat transfer through the south-orientated glazing in order to avoid overheating, which is particularly relevant to climate conditions with relatively warm summers. Mingfang [15] defines southern orientation of a building as the optimum from the viewpoints of the solar heat gains in the winter and the solar heat control in the summer, while pointing out that the optimal building proportion to ensure solar control is a rectangular floor plan. On the other hand, buildings located in warm climatic regions face a completely different situation. The energy demand for cooling is usually the main contributor to the total annual energy demand owing to a higher solar heat transfer through the glazing. Bouden [16] investigated the appropriateness of glass curtain walls for the Tunisian local climate. The influence of windows on the energy balance of apartment buildings in Amman, Jordan, was analysed in the study performed by Hassouneh et al. [17]. The impact of relative compactness (RC) on the building's annual cooling energy demand and the total annual energy demand was dealt with by Al Anzi et al. [18], whose research involved a prototypical building with over 20 floors based in Kuwait. The results of the latter study indicate that the energy use decreases as the relative compactness increases. An interesting research into energy efficiency design of residential building in the Mediterranean region including energetic, economic and environmental points of view was presented by Jaber and Ajib [19]. The windows were separately placed in the east-, south-, west- and north-oriented façades. The research outcome pointed to a 28% annual energy consumption saving achievable by choosing the best orientation, the optimum window size and the optimum U-value. For the purpose of the present research our interest goes to studies related to warm climate conditions in Europe, where a slight energy need for heating may occur, in addition to that for cooling. A number of interesting analyses have been carried out with a focus on defining the optimal architectural geometry of the building, e.g. the impact of windows on the heating and cooling demand, Inanici and Demirbilek [20]. Depecker et al. [9] studied the relationship between the shape and energy requirements during the winter season in two French localities with different climate conditions. They found no correlation between the energy consumption of a building and its shape in mild climates. Albatici and Passerini [21] were encouraged to research new indicators of the energy performance in mild and warm climate conditions relative to the building shape. Thus, they presented heating requirements of buildings with different shapes placed in the Italian territory, which confirms that compactness is more important in cold localities than in warm regions.

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The parametric study of a cost-optimal energy efficient office building in Serbia with very hot summers was presented in Ref. [22]. Hot summers of Belgrade climate imply that cost-optimal solutions have close-to-minimal window-to-wall ratio (AGAW) at the southern façade and significantly larger AGAW value at the northern façade. Studies on the impact of climate change on energy use in buildings in the different parts of the world were reviewed in Ref. [23], where it was pointed out that the most significant adverse impact of climate change on energy use in the built environment would occur in the hot summer and warm winter climate zone where building energy use is dominated by cooling need, which is important also for our study. Badescu et all [24]. analysed the first Romanian passive office building constructed in 2009 in warm climate region in Bragadiru, 10 km south of Bucharest. They proved that the overheating rate and the cooling load are higher for a passive building than for a standard building and suggested that an active cooling system should be used when passive buildings are implemented in the Romanian climate. Florides et all [25]. investigated the importance of the roof insulation on a typical model hollow-brick house in the Nicosia area, which resulted in a reduction up to 45.5% of the cooling load and up to 75% of the heating load. On account of great many parameters having a considerable influence on the energy behaviour of buildings, the presented studies usually treat only a few simple building models, mostly within selected climate regions. Identifying the building parameters which significantly impact the energy performance is a complex and an important step towards enabling the reduction of the heating and cooling energy loads in the design stage, with a special focus on implementing passive design techniques. Regarding theoretical facts about the heat transfer through the building envelope components, multiple parameters have to undergo a careful analytical or semi-analytical investigation via mathematical optimization or sensitivity methods which are correlated with several passive design parameters, such as the shape of the building, its orientation, the size and orientation of the glazing to suit the given climate conditions. Having started in 1984, Radford [26] used multicriteria optimization of a prism-shaped multi-storey office building in Australia applying the criteria of minimum thermal load ratio, minimum capital cost and maximum net usable area. The purpose of the analysis of Marino et al. [27] was to verify the existence of an optimal size of the window surface, a size allowing for minimum overall energy consumption, and the variations that this optimum might undergo if the climate conditions, insulation features of the façade or luminaries input power change. The study was performed on an office building whose structure and configuration represent a typical reference case of the Italian building stock. Nevertheless, the authors suggested highlighting the existence of even more design factors calling for further investigation within future research, such as the influence exerted by the position and shape of the window. In the study of Aksoy and Inalli [28], a transient heat transfer problem in the building envelope with or without insulation is solved by using the finite difference method for a city located in a cold region of Turkey. The treated buildings with different shapes are placed on the ground with the azimuth angles ranging from 00 to 900. Zhang et al. [29] provide a method with a ModellingeSimulationeOptimization framework simultaneously considering the solar radiation, the shape coefficient and the space efficiency. The method is applicable to a free-form building design that receives more solar radiation through the shape optimization and takes into account the other two objectives mentioned above. The method has been tested for severe cold China climate conditions. Several approaches relying on sensitivity analysis have been recently developed to balance between a number of parameters

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which influence building behaviour using passive energy strategies. The approach adopted by Yildiz and Arsan [30] determines the most significant parameters for buildings in hot-climates in Turkey, based on the design of an existing apartment building in Izmir. Hemsath et al. [31] presented a methodology to evaluate the building form in order to compare the energy consumption of geometric variations and material considerations through two types of sensitivity analyses, comparing geometric and material sensitivity. The results indicate that both the vertical and horizontal geometric proportions are equally sensitive as certain material aspects related to the building energy use. Rodrigues et al. [32] presented the analysis of six geometry-based building indices to determine their adequacy in eight different climate regions in Europe. For each of the locations, three residential building design programs were applied as building specifications, using two algorithms to randomly generate and assess the thermal performance of the three sets of 500 alternative building models. Finally, the authors concluded from the presented results that none of the indices have a strong correlation in all of the analysed locations and building design programs. A significant correlation can only be encountered in specific design programs or climate regions. A review of sensitivity analysis methods in building energy analysis and the practical guidance based on the advantages and disadvantages of different sensitivity analysis methods in assessing the building thermal performance were presented by Tian [33]. In spite of a number of different studies presented there is still a lack of investigations on the energy behaviour of light timber buildings with enlarged glazing areas located in warm climate conditions. Owing to a relatively low thermal capacity of timber resisting elements there is a risk of strong overheating of such buildings in the summer period. Optimal solutions for such cases should therefore primarily avoid overheating, which raises a question whether it can be fully avoided? Is it better to design single or two-storey house models? Architects and investors are still reluctant to design light timber buildings with enlarged glass areas in southern European regions with warm climate conditions, in fear of increased overheating as a result of primarily southoriented glazing. The present study will try to provide answers to such questions through separate analyses of the buildings with the same south- and north- orientated glazing sizes but different thermal transmittance values of the building envelope elements. Variations will be performed with the Passive House Planning Package programme (PHPP) [34] and pre-programmed Excel files, for the purpose of multi-criteria parametric analysis. In order to assess the extent to which building geometry influences the energy efficiency of timber buildings in warm European regions, the current study analyses twelve differing single and twostorey models of timber-glass single-family houses with a varying building aspect ratio, located in two selected European capitals with warm European climate conditions (Sevilla and Athens). With a focus laid on the influence the glazing size and its orientation exert on the energy demand, the locations are selected upon the criterion of very similar average temperatures and fairly different solar radiation. The current paper considers the building energy performance from the viewpoint of energy need for space heating and cooling, which is similarly analysed in comparable scientific literature and it also represents the basis for standardization of buildings energy performance. This paper doesn't deal with the precise dimensioning of the heating and cooling systems. However, the additional simplified analysis of peak case scenario for one model variation is presented at the end of the study with the aim to demonstrate the necessity of cooling devices enabling the acceptable indoor temperature for buildings located in warm climate regions. The main benefit of the present study in comparison to previous similar researches performed on light timber-frame buildings is a

separate analysis of two opposite situations; various sizes of glazing are placed first only in the south façade, which is followed by the same sizes of glazing placed in the north façade, with a simultaneous parametrical analysis of the influence of the building horizontal shape at a varying aspect ratio and that of the vertical building shape at a varied number of storeys. 2. Influence of the building form on the energy demand For the purpose of adequate explanation of the numerical results obtained in Section 3 it is necessary to state first basic facts about the energy flows in buildings following the theoretical backgrounds with physical interpretations of a time-dependant thermo-dynamical analysis of buildings [35,36] and possible calculation simplifications to be used in the current numerical study. The building shape can have a significant influence on the energy demand of buildings, generally in dependence on the given climate conditions. The presentation of adequate explanation based on the obtained numerical results and theoretical correlations therefore needs to be preceded by the overview of basic theoretical facts regarding energy flows in buildings. Energy efficiency of buildings according to EN ISO 13790 [37] calls for consideration of the energy demand for heating (Qh) and cooling (Qc). The building is therefore analysed as a thermal system consisting of the transmission heat losses (Qt), ventilation heat losses (Qv), internal heat gains (Qi) and solar heat gains (Qs). The energy need is generally calculated in the form normed per 1 m2 of usable floor area (ATFA):

DQ ¼ (Qt þ Qv) e hG$ (Qs þ Qi) / ATFA

(1)

The utilization factor hG represents in an approximate manner a part of energy gains which cannot be accumulated in the building and thus predominantly depends on the type of the load-bearing structural system taking into account also the impact of the building's thermal inertia. Therefore the surplus heat, e.g. excess solar gains (Qs), is not or only partially usable for heating. It is thus of the utmost importance to consider this fact, whereby the best and most accurate method is seen in the use of dynamic simulation methodology. In a simple calculation methodology, in the quasistationary method according to EN ISO 13790 [37] for example, the heat gains are thus reduced with the utilization factor hG as the fraction of free heat gains (QG¼QsþQi) that can be used for space heating in the form depending on a ratio between the heat gains (QG) and the heat losses (QL¼QtþQv):

hG ¼ (1 e (QG / QL)5) / (1 e (QG / QL)6)

(2)

Based on different temperatures of the building and its surroundings we can distinguish between two opposite heat flow scenarios; the heating (DQ ¼ Qh) and the cooling (DQ ¼ Qc) period. In the heating periods of the year when the average outdoor temperature is generally lower than the prescribed or chosen indoor temperature, the sum of all heat flows in the building is usually negative. Energy flows in the summer period have opposite directions, which calls for the need for cooling to avoid overheating in a building. However, the shape of the building usually has a considerably different influence on the energy demand in the heating and cooling periods. The transmissions heat losses Qt are calculated for every building element of the heat-exchanging envelope in the form of: Qt ¼ A$U$fT $Gt / ATFA

(3)

where A is the building envelope area, U is the building envelope thermal transmittance, fT is the reduction factor for reduced

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temperature difference and Gt is the temperature difference time integral (heating degree hours). Respecting Eq. (3) it can be concluded that an enlarged building envelope area (A) generally increases the transmission losses and consequently has a negative impact on the energy demand for heating. The solar heat gains Qs are calculated using: Qs ¼ r$g$Aw$Gs / ATFA

(4)

where r is the total shading reduction factor, g is the degree of solar energy transmitted through the glazing normal to the irradiated surface, Aw is the window area (rough opening) and Gs is the total radiation during the considered period (heating or cooling). The radiation is separately treated for all four basic orientations of the glazing. With reference to Eq. (4), it can be concluded that an enlarged glazing area (Aw) increases the solar gains, which is positive in the heating period Eq. (1), but not in summer time. On the other hand, enlarged glazing also has a negative impact in both periods on the transmission losses; their increase is due to the thermal transmittance of the windows which surpasses that of the wall elements. The latter is an interesting situation mostly depending also on the given climate conditions (temperature difference and solar potential in the heating and summer periods). One of the most important characteristics which significantly influence the utilization of the solar gains through the glazing in the both periods is the thermal capacity of the building which directly affects the phase difference and the energy behaviour of the building. The behaviour of massive concrete or masonry buildings in such cases is therefore completely different than that of light timber buildings. A parameter which can produce a significant impact on the amount of solar gains and consequently on the total energy demand for heating and cooling is the glazing-to-wall area ratio (AGAW), described as the ratio between the total area of glazing (Agl) to the total area of the wall (Awall): AGAW ¼ Agl / Awall

(5)

As mentioned in Section 1, there are several mostly numerical studies on the optimal size of glazing placed on different façades of the building in various climate conditions. In Ref. [14], an attempt at a more systematic approach was made, with the model of a building being performed in many variations of timber construction systems. An extensive parametric analysis served as a basis for the generalisation of the problem concerning the optimal glazing area size (AGAWopt) dependence on one single variable, the Uwall -value which becomes the only variable parameter for all contemporary prefabricated timber construction systems, independently of their type. Another important parameter often used to determine solar access of a building, assuming that the latter is of a given height and optimally oriented, is the aspect ratio (AR), a ratio between the building's length and depth (AR ¼ L/W), Fig. 1. Most of the numerous studies discussing the influence of the aspect ratio variation on the energy use in buildings treat only cold climate conditions, as in Chiras [38] where the ideal aspect ratio for a rectangular-shaped solar house design ranges from 1.3 to 1.5. A sensitivity study in Ref. [27], carried out for the cold region of Turkey, shows that masonry (hollow brick) buildings with a square shape offer more advantages, with the most suitable orientation angles being 00 and 800 for buildings with the aspect ratio 2/1 and 1/2, respectively. In the study by McKeen and Fung [39], the energy consumption at a varying aspect ratio changes significantly between the 13 simulated ten-storey models in multi-unit residential buildings in five selected Canadian cities (Vancouver, Calgary, Toronto, Montreal and Halifax). It was finally concluded that compared to a building

Fig. 1. Schematic presentation of the analysed timber-frame box house models.

with a less efficient aspect ratio, such as 4.2:1, the reduction in the energy consumption by more than 15% is possible in many scenarios. However, it was also noted that the optimal aspect ratio for the heating efficiency is not necessarily optimal for the cooling, especially when taking into account also a different U-value of the glazing and the shading device, [40]. The optimal aspect ratio allows buildings to receive more solar gains in winter and more shading in summer, thus decreasing the demand for heating and cooling. The impacts of the glazing-to-wall area ratio (AGAW) and that of the aspect ratio (AR) on the energy demand of buildings are usually interconnected and demand a highly sensitive consideration in the analysis, as seen in Wei et al. [41]. In their analysis for the cold climate region in China they included the number of floors, and overall scales, besides the above parameters. Both, global sensitivity analysis and statistical machine learning methods were used to examine the relationship between the building-form parameters and three output performance indicators (heating, cooling and electricity) in office buildings. It was found out that the number of floors is the most influential variable relevant to the annual heating energy demand, whereas the overall building scale proves the strongest influence on cooling and electricity. The results of our study considering the case of vertical extension of box-models can also be compared to those in Ref. [42], where the influence of atrium on the energy performance of a massive hotel building in Belgrade was presented. Owing to low thermal capacity, the interdependence between the AGAW and the AR is particularly noticeable in lightweight timber-frame buildings. The suggested method in Ref. [14] was further implemented on different timber-glass single-storey box house models varying the aspect ratio and the building shape along with the depth ratio of the self-shading, [8]. Nonetheless, the analysis was performed only for the cities with relatively strong winter conditions (Helsinki, Munich and Ljubljana) and solely for the south-oriented glazing. Another analysis treating single and two-storey timber-frame box models in six selected European cities, with the south-oriented glazing having the AGAW value of 35% and the U-value of the thermal envelope of 0.10 W/m2K, was presented in Ref. [43]. It was concluded that the building shape (the aspect ratio and the number of floors in this case) can have an important influence on the energy behaviour of buildings and strongly depends on the given climate conditions. Finally, it was suggested that there are still a lot of parameters to be analysed, with most of them systematically included in the numerical study for the warm European climate conditions, in Section 3 of this paper.

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3. Numerical study 3.1. Simulation model 3.1.1. Parametric variation of the thermal transmittance (U-values) The presented numerical research is based on a case study considering box models of single and two-storey houses built in the prefabricated timber-frame structural system with different thermal transmittance values of the thermal envelope wall and roof elements, systematically combined with the windows (triple or double glazing): - U1 ¼ 0.1 W/m2K with triple glazing Ug ¼ 0.51 W/m2K, - U2 ¼ 0.2 W/m2K with double glazing Ug ¼ 1.10 W/m2K, - U3 ¼ 0.3 W/m2K with double glazing Ug ¼ 1.10 W/m2K. The U-values are parametrically selected at a step of 0.1 W/m2K, starting with the analysis in the case of the minimal thermal transmittance of the external wall elements (U1 ¼ 0.1 W/m2K), widely used especially in northern and central Europe as a passive standard of timber-framed building elements. The second selected U-value (U2 ¼ 0.2 W/m2K) is usually the maximal prescribed Uvalue for the external wall elements in northern and central Europe, i.e. a low-energy standard of timber-framed building elements. Double glazing is normally used in the above case. With the last selected value (U3 ¼ 0.3 W/m2K), which is usually not allowed in northern or central European countries, we try to investigate the case of warm climate conditions, with higher maximal U-values prescribed, according to the regulations for Greece [44,45] and Spain [46]. Greek regulations, for example, (zone B for Athens) define the maximal prescribed thermal transmittance of the external wall elements at 0.45 W/m2K, with the U-value of windows being even 3.00 W/m2K. In order to prevent building overheating through the glazing we decided to take the Double glazing with Ug ¼ 1.10 W/m2K. The selected values of the thermal transmittance of external walls for all treated combinations with the floor slab and windows are presented in Table 1.

3.1.2. Parametric variation of the glazing size in the south and north façades The size of glazing with a parametrically selected value of: - AGAW1 ¼ 35%, - AGAW2 ¼ 45%, - AGAW3 ¼ 55%, according to Eq. (4), the size of glazing is used separately for the south- and the north-oriented façades. Overheating of the simulation models in the summer is reduced with external shading devices. Summer shading calculation for all analysed building shapes is based on the temporary shading factor of z ¼ 37%, which means that 63% of the glazing surface is shaded in the cooling season. The reduction factor was calculated on account of the type of shading devices selected - blinds with exterior position according to DIN V 18599-2 [47] and activation factor of use without any automatic control.

3.1.3. Parametric variation of the model geometry Three groups of rectangular models are parametrically analysed using a systematic variation of the building aspect ratio (AR ¼ L/W), ranging from 0.83 to 1.69, with a doubled horizontal (Model group B) and vertical extensions (Model group C), schematically presented in Fig. 1: - Model group A: single-storey models with a constant occupied floor area ATFA ¼ 81 m2 and a heated volume of V ¼ 243 m3. - Model group B: single-storey models with a doubled constant occupied floor area ATFA ¼ 162 m2 and a heated volume of V ¼ 486 m3. - Model group C: two-storey models with a doubled basic occupied floor area of each storey ATFA ¼ 81 m2 and a heated volume per storey of V ¼ 243 m3.

3.1.4. Climate data The study analyses the energy performance for two different European cities with relatively warm climate conditions (Athens and Sevilla). For the purpose of underlining the influence of the glazing size and its orientation on the energy demand, the locations were selected upon the criterion of very similar average annual temperatures, in addition to average annual temperatures in the heating period, and fairly different solar radiation. As seen in Table 2, the values defining total annual solar radiation and total solar radiation in the heating period on the south-facing vertical surface are essentially higher for Sevilla as compared to Athens. The situation is practically opposite with northern orientation of the glazing in which case the solar radiation for Athens is higher. Climate information from PHPP V8.5 [34] was used for calculations; it is presented in Table 2. 3.1.5. Ambient temperature For the heating period a constant minimal ambient temperature of Tint,min ¼ 200C is taken into account in all calculations. For the cooling period the maximal considered temperature is Tint,max ¼ 250C.

Table 2 Climate data for Athens and Sevilla.

Latitute ( ) Longitude ( ) Altitude (m) Average annual temperature ( C) Average annual temperature in the heating period ( C) Length of the heating period (days/an.) Total annual solar radiation Gs on the south vertical surface (kWh/m2) Total solar radiation Gs on the south vertical surface in the heating period (kWh/m2) Total annual solar radiation Gs on the north vertical surface (kWh/m2) Total solar radiation Gs on the north vertical surface in the heating period (kWh/m2)

Athens

Sevilla

38.0 23.7 147 18.6 9.0

37.4 6.0 17 18.2 8.9

65.6 1100

67.6 1364

151

221

444

374

41

36

Table 1 Parametrically selected U-values of the thermal envelope elements and the windows. Uwall, Uroof [W/m2K]

U1 U2 U3

0.100 0.200 0.300

Ufloor [W/m2K]

0.135 0.250 0.350

Windows Ug [W/m2K]

Uf [W/m2K]

j

Uw

0.51 1.10 1.10

0.73 1.60 1.60

0.035 0.035 0.035

0.75 1.30 1.30

average

[W/m2K]

g 0.52 0.60 0.60

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3.1.6. Calculation procedure The calculation of the heating and cooling demand was performed in accordance with DIN EN ISO 13790 [37], by the software program PHPP V8.5 [34]. PHPP is a certified software program designed for planning low-energy and passive houses. It is based on the energy flux equations from EN ISO 13790 standard [37] described in Section 2 as well as on other European standards. The results of our research work are computed by the use of monthly steady-state EN ISO 13790 [37] computing method whose calculations prove to be more accurate than those provided by the annual computing method. Since the paper treats the light timberframe structures the minimal possible value for spec. capacity (spec. capacity ¼ 60 Wh/K per m2 of the usable area) of the building structure was selected as the input data to simulate the influence of the building thermal capacity. Additionally programmed MS Excel files were developed to achieve multi-parametrical analysis with 432 calculated models from Fig. 1. Next to PHPP [34] there are many other software tools widely used for the purposes of building energy simulations, such as for example Energy Plus [48]. The later is recognized as dynamic simulation modelling software and is especially important for energy analysis of buildings with higher heat storage capacity. In our case where the timber-glass buildings with low heat storage capacity are treated, the results obtained by dynamic simulation modelling would not differ drastically from those obtained by steady-state simulation as the energy requirement for heating and cooling is considered on the annual level. 3.2. Results and discussion The obtained numerical results for the U1 envelope values (see Table 1) are graphically presented in Fig. 2, separately for the south-

501

and north-oriented glazing. The results show the annual energy demand for heating (Qh) and cooling (Qc) along with the total annual energy demand for heating and cooling (Qtotal), considering all three parametrically selected AGAW values (Qh_35 for AGAW1 ¼ 35%, Qh_45 for AGAW2 ¼ 45%, Qh_55 AGAW3 ¼ 55%). As evident from the presented results for the south orientation of the glazing (Fig. 2a), the energy need for heating (Qh) is estimated to be extremely low. Thus, the findings relative to the total energy need (Qtotal) depend basically on findings for cooling (Qc). The increasing size of the glazing with the increased length of the south façade (L) lead to a deduction that the energy need for cooling increases in an almost exponent dependence on the increasing aspect ratio (AR) for all models. Comparison of the results for Athens and Sevilla points to a close similarity for the heating period, with differences noticed in the cooling demand. The reason lies in a higher total annual solar radiation Gs on the south-facing vertical surface at Sevilla (1364 kWh/m2, Table 2), since the value at Athens reaches only 1100 kWh/m2. The above fact results in increased solar gains in the summer period for Sevilla, Eq. (4) and to enlarged energy demand for cooling Eq. (1). The analysis focused only on the influence the building shape has on the annual energy demand, for both cities, leads to a conclusion that the single-storey building (Model group B) behaves evidently better than the two-storey one (Model group C), with both model groups having the same occupied floor area. By comparing only single-storey models it becomes evident that Models B demonstrate lower energy demand than Models A. A comparison based only on the results for Models A and C (vertical extension) shows that Models C indicate a slightly lower energy demand. This finding, even though obtained for slightly different climate data, proves to be in good accordance with the results in Ref. [42].

a.)

b.)

Fig. 2. Energy demand for U1 ¼ 0.100 W/m2K with the glazing orientation to the south (a) and north (b).

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The total energy demand (Qtotal) for all models in both cities can be significantly decreased if the glazing is oriented to the north (Fig. 2b). In this case the cooling demand evidently decreases due to lower solar radiation on the north vertical surface (Table 2, Eq. (4)) while the heating demand slightly increases, as compared to Fig. 2a. Since the difference in the annual solar radiation between south and north orientations is higher for Sevilla (Table 2), the above described impact is more essential for this particular city. The total energy demand for Sevilla is consequently lower than that for Athens (Fig. 2b), which is the opposite of the case with the southoriented glazing (Fig. 2a). Moreover, the graphs in Fig. 2b and a suggest that the building form (aspect ratio, horizontal and vertical extension) along with the glazing size (AGAW) have evidently smaller impact on the total energy demand at northward glazing orientation than in the case of its southward placement, which is particularly evident when observing the results for Sevilla. On the contrary, Models B and C display almost no difference in the total energy demand with north-oriented glazing. The results for the doubled thermal transmittance value U2¼0.2 W/m2K are graphically presented in Fig. 3. These are given solely for the informative purpose and basically confirm the already presented facts for the U1 value, with a significant remark referring to the energy demand which is generally higher for all analysed building models. A more detailed commentary on the results will be presented for the U3 value (Fig. 4) which is closely nearing the maximal prescribed thermal transmittance values for the considered locations. The presented results point out that the heating demand at northward glazing orientation evidently increases due to higher transmission losses (Eq. (3)), which is in accordance with the previous example of the U2 value. The case (Fig. 4b) is worth noting since the heating demand tends to be nearly independent of the aspect ratio or the AGAW value in all three treated model groups, for

Athens and Sevilla. On the contrary, the findings are not the same at the southoriented glazing (Fig. 4a) where the heating demand is essentially lower and almost linearly decreases with the increasing aspect ratio for all three selected AGAW values. On the other hand, the cooling demand increases almost exponentially with the increasing aspect ratio. The reason lies in the enlarged size of the glazing (Agl) placed on the increased south façade length (L) at a constant AGAW value (Eq. (5)). Consequently, the solar gains (Qs) through the windows rapidly increase in the summer period, Eq. (4), leading to a subsequently higher cooling demand (Qc), Eq. (1). On the whole, it can be said that the increasing U-value results in a more substantial increase in the heating demand (Qh) than in that for cooling, for both orientations of the glazing. It is important to stress that the buildings should be designed to enable thermal comfort over the whole year, but also to depict the peak worst case scenario. In this sense we additionally controlled the maximal ambient (room) temperature during all months for Athens, the square Model A2 with U1 ¼ 0.1 W/m2K and with AGAW ¼ 35% on the south façade (Fig. 5). The shading device was used according to description in subsection 3.1.2. The method applied is based on a dynamic single zone building model that calculates an annual temperature curve for a scenario without active cooling. The temperatures are sorted by magnitude to produce an annual temperature/duration curve. A line of best fit in the vicinity of the temperature threshold of interest gives the percentage of time in which interior temperatures exceed the desired limit. This method allows determination of the acceptability of the interior temperature without hourly climate data and with only a small number of entries [34]. The results of the simulation show that the maximal interior temperature, when no air-conditioning is used, varies between 20  C and 22  C for winter (November to April) and between 29  C

a.)

b.)

Fig. 3. Energy demand for U2 ¼ 0.200 W/m2K with the glazing orientation to the south (a) and north (b).

M. Premrov et al. / Energy 149 (2018) 496e504

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a.)

b.)

Fig. 4. Energy demand for U3 ¼ 0.300 W/m2K with the glazing orientation to the south (a) and north (b).

Fig. 5. Maximal ambient temperature for Athens, Model A2, AGAW ¼ 35%, U1, south.

and 36  C for the summer period (June to September). The obtained results confirm the findings from the study [25] performed on a hollow-brick typical house model for Nicosia and prove that additional air-condition systems are necessary in the summer time, even if shading devices are used, to ensure a suitable living comfort ambient temperature. 4. Conclusions It can be generally concluded from the presented study that the building shape has an important influence on the energy behaviour of timber-framed buildings located in warm European climate conditions. Thus, a total of 216 box models for each climate condition were parametrically tested through variations of the most important parameters (thermal transmittance, glazing size and its orientation, aspect ratio, horizontal and vertical extension of the building). Identifying the building parameters which significantly impact the energy performance is a complex and an important step

towards enabling the reduction of the heating and cooling energy loads in the design stage, with a special focus on implementing passive design techniques like exploitation of solar gains. To point out the latter fact the climate conditions were selected upon the criterion of very similar average temperatures and fairly different solar radiation. The results for the two chosen cities lead to a conclusion that in both analysed glazing positions, the southern and the northern, the energy demand increases with the aspect ratio. The comparison of the results for the south- and north-oriented glazing shows that the building form (aspect ratio, horizontal and vertical extension) has an evidently smaller impact on the total energy demand in the case of the northern glazing position. The difference increases with the smaller U-value of the building envelope. The total energy demand, which mostly consists of that for cooling, is lower at the northward orientation of the glazing for all three selected U-values of the building envelope. What is more, increasing the glazing size in both façades caused an almost linear increase in the cooling load, which accords with the findings in Ref. [19], for the Jordanian climate conditions. The given conclusions are opposite to those based on the same models exposed to strong winter conditions [43], where the increased aspect ratio with the glazing placed in the south façade mostly decreases the energy demand due to an increase of potential solar gains during the heating period. Consequently, the designers are less limited in choosing a proper building shape of timber buildings in warm European climate conditions, as compared to cold ones, which is particularly relevant to northoriented glazing. The results of the study aim at provision of best-practice design for new buildings, while for existing buildings it is usually quite difficult to carry out a complete change of glazing size and

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