Energy performance enhancement in multistory residential buildings

Energy performance enhancement in multistory residential buildings

Applied Energy 116 (2014) 9–19 Contents lists available at ScienceDirect Applied Energy journal homepage: www.elsevier.com/locate/apenergy Energy p...

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Applied Energy 116 (2014) 9–19

Contents lists available at ScienceDirect

Applied Energy journal homepage: www.elsevier.com/locate/apenergy

Energy performance enhancement in multistory residential buildings Caroline Hachem a,⇑, Andreas Athienitis b, Paul Fazio c a

Department of Building, Civil and Environmental Engineering (BCEE), Concordia University, 1455 de Maisonneuve Blvd. West Montreal, Quebec, Canada Department of Building Civil and Environmental Engineering, Concordia University, Canada c Building Envelope Performance Laboratory, Centre for Building Studies, Department of Building, Civil and Environmental Engineering, Concordia University, Canada b

h i g h l i g h t s  Solar potential and energy consumption of multistory buildings is investigated.  Integration of PV panels in façades is considered due to the reduced roof surface per dwelling unit.  Apartment buildings are more energy efficient for heating and cooling than single houses.  With optimal roof design, a building of three stories can reach a net-zero energy status.  Above three floors, facades should be exploited to enhance energy production.

a r t i c l e

i n f o

Article history: Received 18 August 2013 Received in revised form 21 October 2013 Accepted 4 November 2013

Keywords: Solar energy Multistory buildings Energy demand Energy generation Building integrated Photovoltaic

a b s t r a c t This paper presents a study of energy performance enhancement methods in multistory residential buildings. The study is carried out for Montreal location, Canada (45°N). All configurations considered assume a suburban environment that allows high solar exposure and no obstruction from adjacent buildings or external surrounding objects such as trees. Energy performance is measured by the balance between energy consumption, on the demand side, and electricity production by means of integrated PV systems, on the supply side. The present study considers enhancement of the supply side by increasing electricity generation potential. Apartment buildings are designed to be highly energy efficient and to conform to passive solar design principles. The buildings investigated include – low rise (3–5 floors), mid-rise (6–9 floors) and high-rise (up to 12 floors), with eight apartments per floor. All Integration of PV systems in façades, in addition to roof surfaces, is considered, in view of the reduced availability of roof surface per dwelling unit. The results of simulations employing the EnergyPlus building simulation program indicate that apartment buildings are relatively energy efficient for heating and cooling, while allowing a high level of residential density, but their solar potential is limited. Under the present study, a building of three stories can generate about 96% of its total energy use, if the roof design is optimized for solar energy generation. Above 3 floors, additional measures are required to enhance energy production. Implementing PV systems on 50% of south façade and 80% of east and west façades surface areas, in addition to enhanced roof surface design (folded-plate), enables electricity production of up to 90% of energy use of a 4-story building reducing with increasing height to 50% for 12 stories. The study indicates that investment in advanced design of façades (such as folded-plate curtain walls) can substantially increase electricity production and achieve net zero and surplus energy status in building over eight stories high. Ó 2013 Elsevier Ltd. All rights reserved.

1. Introduction Attempts at mitigating environmental impact of human activities should target high-density urban complexes, where half of the

⇑ Corresponding author. Tel.: +1 514 8482424x7080; fax: +1 514 848 7965. E-mail addresses: [email protected], [email protected] (C. Hachem). 0306-2619/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.apenergy.2013.11.018

world’s population is concentrated [1]. Since a large portion of total energy consumption is attributable to buildings (30% of Canada’s total energy consumption, and over 50% of Canada’s electricity consumption [2]), reduction of energy consumption in buildings, and transforming them into energy producers, is a high priority objective in limiting their environmental impact. Implementation of energy efficiency measures in buildings enables reduction of energy consumption by up to 35% [3]. Energy

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efficiency measures are not sufficient, however, to address an expected increase in future energy demand of the building sector. Coupling energy efficiency measures with increased renewable energy production techniques enables generating the amount of some or all of a building’s energy consumption, thus reducing dependence on fossil fuel. Initiatives to implement stringent energy efficiency measures and to enhance energy production are starting to take shape internationally [3,4]. Policymakers around the world are embracing the concept of net zero energy buildings, which generate energy to counterbalance their consumption, as a vital strategy to meet energy and carbon emission goals [5,6]. Net zero and surplus energy single-family houses and neighborhoods can be realized through careful design of such houses and their positions with respect to each other [7–9]. However, the density of such neighborhoods is limited, reaching a maximum of 26 units per acre, particularly if solar access principles are respected [8]. Achieving net zero energy buildings in a higher density context is more challenging. High-density conurbation has some significant economic and environmental advantages, such as efficient land use, efficient transport and infrastructure and reducing greenhouse gas emission (e.g. [10–13]) however it also has the negative effect of reducing building potential to capture and utilize solar energy. Multistory buildings can offer substantial solution in accommodating the increased density of modern cities while maintaining energy efficiency, in addition to playing an important role in the topography of these cities. These solutions are becoming widely adopted in North American cities where denser and taller housing buildings in cities and town centers play an increasing role. For instance, in Canada, multistory residential buildings represent a significant percentage of the housing stock (about 31%) and are responsible for 24% of the overall annual energy use within the residential sector [14]. Disadvantages of such buildings are associated with reduced potential for solar capture. Roof and exposed façade surfaces for active collection of solar energy for electricity and/or hot water generation are significantly reduced. Therefore there is a need to understand the performance of these buildings, and to develop strategies for increasing their potential to generate electricity. Research conducted to assess the performance of multistory buildings in general, and residential in particular, is scarce, their important role notwithstanding. Lack of data on multistory performance is a major problem in developing design strategies for such buildings [15]. Most reported studies concentrate on mapping the energy use of existing multistory residential buildings, focusing mainly on their energy use for heating, cooling, hot water and in few cases daylighting availability, as well as outlining efficiency measures that can be implemented in such buildings (e.g. [14,16–19]). Very few guidelines exist, on the architectural design of multistory buildings, to improve their energy efficiency and solar potential. The design of multistory buildings tends to encourage maximizing the ratio of floor area to envelope area [15]. This approach, while having some cost benefits and positive effect on heating/cooling consumption, is incompatible with passive solar design strategies, compromising the potential of building to gather and exploit solar energy in passive heating, daylighting, and installation of solar collectors for domestic hot water and electricity generation. To accommodate energy efficiency in multistory building design, building shape, including the design of façades and roofs, should be considered and optimized since the early design stage. Failing to do so would result in the mechanical and electrical systems having to compensate for design shortfalls [20]. On the other hand, the potential of these buildings to generate electricity by means of building integrated PV (BIPV) systems has

not been systematically studied. For instance, Pelland and Poissant [21] conducted a study of BIPV potential in Canada, however the study excluded the potential of apartment buildings. Increasing the potential of energy generation of multistory buildings is of primary importance in achieving net zero energy higher density status. The current research investigates the energy performance of multistory buildings, in terms of consumption for heating and cooling and potential electricity generation employing PV systems integrated in roofs and/or façades, at varying density levels. The study focuses on residential apartment buildings of different heights, ranging from low rise of 3-floors and up to12-floors. The study aims at improving the balance between energy consumption and generation in multistory building, employing different scenarios to increase the electricity generation of these buildings. The main contribution of the current study consists in providing insight on the design of energy efficient multistory buildings, as well as suggesting some basic guidelines of key parameters to consider in attaining net zero energy multistory building design. This study constitutes the first stage in the design and analysis of mixed-use high performance neighborhoods, and therefore it is concerned with new construction, where design parameters can be controlled and a systematic design methodology can be developed. New constructed multistory building is a fast growing sector especially in North America [22].

2. Methodology 2.1. General The objective of this research is to investigate the effect of increasing residential density in multistory buildings on the overall solar potential and energy use of these buildings. Solar potential relates primarily to the potential of roofs and façades to capture solar radiation for the generation of electricity, employing BIPV systems. Montreal (45°N) is adopted as the pilot location of the study, to represent northern cold climate of mid-latitude. Buildings are designed to conform to passive solar design principles and to be highly energy efficient (e.g. insulation of 7 K m2/W for walls and 10 K m2/W for roofs, triple glazing low-e argon fill windows, airtight construction, etc.). Details of the characteristics of these apartment buildings are given in Table 1. Moreover, the multistory buildings investigated are assumed to have maximum solar exposure, with no shading from surrounding elements or buildings. Total energy consumption in units is obtained by assuming energy use for appliances, for lighting and for domestic hot water (DHW) based on energy use in energy efficient houses and net zero energy houses (NZEH) [23]. Lighting energy consumption in rear apartments, where daylighting is limited, is based on the Comprehensive Energy Use Database of the Office of Energy Efficiency of Natural Resources Canada [24]. This database relies on data collected from surveys and other sources (manufacturers, electricity distribution companies, government surveys, etc.) [25]. Elevators are considered in buildings with 5 floors and above. Average energy consumption of elevators is based on 8 h of elevator operation per day [26]). A heat pump with a coefficient of performance (COP) of 4 is assumed to supplement the passive and active solar heating systems. This COP rating falls within the range of commercially available heat pumps (about 3.5 to 4) [27]. Simulations are performed employing the Energyplus software [28], to compute energy generation of the BIPV systems integrated on the south and near south facing surfaces (see below) and heating and cooling energy consumption.

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Table 1 Energy related characteristics of apartments. Thermal resistance values

Exterior wall: 7 K m2/W Roof: 10 K m2/W Slab on grade (for ground floor): 1.2 K m2/W Slab perimeter: 7 K m2/W

Thermal mass

20 cm concrete slab on grade (ground floor) 15 cm concrete floor slabs (in all apartments except ground floor)

Window type

Triple glazed, low-e, argon filled (SHGC = 0.57), 1.08 RSI

Area of south glazing North windows (in north apartments) East or west windows

30% of south façades 12% of the north façades 4% of the total floor area of the apartment (maximum suggested for cold climate, [29])

Shading strategy Shading control

Interior blinds Blinds shut at indoor air temperature of 22 °C

Occupants Setpoint temperatures Infiltration rate Ventilation rate

2 Adults and 2 children, occupied from 17:00–8:00 Heating set point 21 °C, cooling set point 25 °C 0.6ACH @ 50 Pa 0.35ACH

Assumptions for electrical loads Lighting (front apartments) Lighting in rear apartments DHW Major appliances Minor appliances

3 kW h/m2/yr [30] 14 kW h/m2/yr [24,25] 2.75 kW h/day/person) [30] 1600 kW h/yr ([31] 1100 kW h/yr [32]

Fig. 1. Schematic of floor plan of basic apartment building.

2.2. Design of apartment blocks Increased density can be achieved using apartment building with increasing number of stories. The basic apartment building is designed with 8 apartments per floor (basic module, Fig. 1), and a core area that serves for functions such as elevators and stairs. 1 Acre with a long layout can fit about 2½ such buildings. Buildings ranging from 3 to 12 stories are designed and simulated, in this study. A fixed floor area of 120 m2 is considered for all apartments. The interior spaces of all residential units are partitioned to fit a family of four persons. The floor area is based on the need to reduce costs by having as compact a design as possible. For the equatorial facing apartments, the layout of the interior space ensures that the living area and the kitchen are adjacent to the equatorial-facing façade. Since the study is conducted for the northern hemisphere, the term ‘‘south facing’’ is employed hereunder to refer to ‘‘equatorial facing’’ in the more general context. Design of south window area is based on sensitivity analysis of the effects of window size on heating and cooling loads. Window area larger than 30% of the south façade leads to a large cooling load especially in the upper floors, which may exceed the heating

load. A south window area of 30% of the façade is therefore selected. The area of east and west windows is assumed constant (4% of the floor area of the apartment unit) based on the maximum non-south window area suggested for houses in cold climate [29]. North facing windows in the northern apartments are assumed at 12% of the north facing wall area. This assumption is in order to obtain minimum functional windows (a window per room). Increasing this area to about 18% of the north wall area does not affect the heating load (less than 3%), however it can affect cooling load significantly.

2.2.1. Design of roofs Two variations of roof design are explored in this study, with a view to maximizing electricity generation potential using BIPV systems – gable roof and folded-plate roof systems (Fig. 2). The basic roof is a gable roof with a tilt angle of 45°. BIPV system is assumed to cover the south facing roof surface. The area of this surface constitutes about 70% of the floor area. The portion of the electricity generating surface area to the total residential area is inversely proportional to the number of stories – Fig. 3.

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Fig. 2. 3-D view of a mid-rise building, (a) with gable roof and (b) with folded-plate roof.

Fig. 3. Relation between the south facing roof area of gable roof and the total floor area for all studied apartment buildings.

2.2.1.1. Folded-plate. Folded-plate roof design is employed as an option to improve the potential of the roof to integrate PV systems. Folded-plate roof design refers in this paper to the shape of the roof, not necessarily to the structural system. This roof design is composed of triangular plates with various orientations. The basic shape of one unit (see Fig. 4a) consists of two sets of eight plates meeting at a horizontal east–west ridge. The southern half of the basic unit has two side plates facing south and the central plates rotated 30° east and west. Fig. 4a shows two basic units, which cover a single apartment unit. An apartment building of 8 apartments and one service zone requires 3 rows of 9 basic units such as shown in Fig. 4b. Folded-plate roof design enables obtaining various orientations for the same rectangular plan roof and has significantly higher south facing surface area than the gable roof [33]. Diverse orientations of the PV system enables spread of peak generation times relative to solar noon, which can be economically beneficial [34]. In addition, this roof design allows reducing the overall height of the roof as compared to a gable roof with 45° tilt angle.

Fig. 5. Relation between BIPV area covering south façade and east and west façades and the total floor area for all studied apartment buildings.

2.2.2. Design of façades In order to increase the potential of generation of multistory buildings, this study explores a number of scenarios where PV systems are integrated on south façades, alone or in combination with east and west façades. The basic PV system is assumed to cover 50% of the south façade area, starting from the third floor up, and 80% of the east and west façades. The combined area of the east and west façades constitutes about 2/3 of the total south façade area. The reduced window area on the east and west façades enables a total BIPV area that is comparable to that of the south façade. Relation of the BIPV area integrated in south, east and west façades to the total floor area of all studied buildings, is illustrated in Fig. 5. Comparison between trends in Figs. 3 and 5 shows that while the ratio of BIPV area to total floor area of the roof decreases with increasing height of the building, it increases for facades.

4- folded-plate basic unit

30°W

30°E

(a)

(b)

Fig. 4. Folded-plate roof design, (a) 2 basic units and (b) the total roof of the apartment building.

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The potential of increased generation through modification of the basic PV system integrated in façades is also explored and results are given in Section 3. Enhancement of the potential of façades to generate electricity can be achieved by employing folded-plate curtain walls as investigated in Hachem et al. [35]. Such design would increase the potential of south façades to generate electricity by up to 250%, as compared to a flat façade. 2.3. Simulations EnergyPlus building simulation software [28] is employed in the simulations using annual weather data for Montreal, Canada (latitude 45°N) [36]. SketchUp/OpenStudio [37] is employed to generate geometric data for EnergyPlus. Each apartment is modeled as a single conditioned zone. The Conduction Finite Difference algorithm is selected as the heat balance algorithm. This solution technique employs a one-dimension finite difference method to represent the construction elements such as walls, floors and roofs. A short time step of 10 min is used in the simulations. 2.3.1. Weather data The weather files of EnergyPlus are used for the simulations [36]. The weather data file, which is based on CWEC – Canadian Weather for Energy Calculations, provides hourly weather observations. These observations represent an artificial one-year period, specifically intended for building energy calculations. The data collected for this year includes different meteorological data such as hourly values for solar radiation, ambient temperature, wet bulb temperature, wind speed, wind direction and cloud cover. Two design days, a sunny cold winter day (in January) – WDD, and a sunny hot summer design day (in June) – SDD, are used to represent two sunny days with extreme temperatures. This is particularly significant for the analysis of the variation in electricity generation timing of BIPV systems on east, west and south oriented façades. Additionally, a whole year weather data set is used to estimate the annual electricity generation of all configurations of BIPV systems, as well as the energy demand for heating and cooling of apartment units. 2.3.2. Solar radiation and shading calculations Hourly direct solar radiation is computed by EnergyPlus, based on the ASHRAE model of clear sky [38] applied to Montreal (45°N). This model is the default model used by EnergyPlus to estimate the hourly clear-day solar radiation for any month of the year. The solar radiation accounts for direct beam and diffuse radiation, as well as for radiation reflected from the ground and adjacent surfaces. The shading algorithm handles shade that may be cast by some surfaces on others, such as in the case of folded-plate, where some surfaces with integrated PV may be partly shaded at certain times of day. 2.3.3. BIPV modeling The Equivalent One-Diode Model (or ‘‘TRNSYS PV’’ model) employed in EnergyPlus is selected to perform electricity generation

simulations of the BIPV systems. The TRNSYS model employs a four-parameter empirical model to predict the electrical performance of PV modules [39]. The current–voltage characteristics of the diode depend on the PV cell’s temperature. The model automatically calculates parameter values from input data, including short-circuit current, opencircuit voltage, current at maximum power, etc. [40]. For this study, the PV array is selected from EnergyPlus database to provide approximately 12.5% efficiency, under standard conditions. The electrical conversion efficiency decreases by some 0.45% for each °C increase of cell temperature from the temperature under standard conditions. For Montreal, the annual potential of PV electricity generation of south facing surfaces at latitude tilt angle is about 1200 kW h per kW peak of installed PV [41]. 3. Results Results are presented in term of heating and cooling loads, on the demand side, and electricity generation, on the supply side. Heating and cooling energy load are defined as the total energy demand, for heating and for cooling, over a year. Energy consumption for heating and cooling is obtained assuming a heat pump of coefficient of performance (COP) of 4. Balance between supply and overall electricity consumption, including energy use for domestic hot water, appliances and lighting, is presented as well. 3.1. Heating and cooling loads Heating and cooling loads in apartment buildings are lower than in detached two-storey single-family houses, with identical total floor area. Energy use of such houses is presented in [8,9]. Heating load decreases by up to 50% in apartment buildings with 8 stories and above, as compared to the detached houses. Cooling load is reduced by 65% in low-rise apartment buildings as compared to the detached house, and it increases with increasing number of stories. In high-rise buildings cooling load is similar to detached houses – see Table 2. Fig. 6 displays the average annual heating and cooling energy demand per apartment, for all studied apartment buildings. In general, cooling load is an order of magnitude lower than heating load, for the studied location. Table 2 presents heating and cooling energy demand of 3–12 storey apartment buildings as compared with detached houses, based on data analyzed in [8]. The Table displays as well the ratio of average heating and cooling loads of apartment buildings to the values of detached houses in a neighborhood of houses along an east–west road, with minimum distance between houses [9]. It should be noted that the heating and cooling loads of these houses, simulated within a neighborhood, are different than for isolated houses [9]. Average heating energy demand for each floor of buildings of varying heights is presented in Fig. 7. It can be observed that the heating load in each of the top four floors does not significantly differ among buildings over five stories high, as indicated by the dot-

Table 2 Average annual heating and cooling energy demand in different configurations and in detached houses. Detached houses in a neighborhood [33]

Apartment buildings Average energy demand (kW h/yr) Comparison to detached houses

Average annual heating energy demand (kW h/yr)

Average annual cooling energy demand (kW h/yr)

2630

149

Heating Cooling

3 Floors 1724 54

4 Floors 1595 79

5 Floors 1521 98

6 Floors 1456 111

7 Floors 1408 121

8 Floors 1377 128

9 Floors 1360 136

10 Floors 1346 141

11 Floors 1341 144

12 Floors 1333 148

Heating Cooling

0.66 0.36

0.61 0.53

0.58 0.66

0.55 0.74

0.54 0.81

0.52 0.86

0.52 0.91

0.51 0.94

0.51 0.97

0.51 0.99

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Fig. 6. Average annual heating and cooling loads per apartment.

Fig. 7. Average heating load of each floor in all apartment buildings.

Fig. 8. Average cooling load of each floor in all apartment buildings.

ted lines (less than 2%). Heating load in the last floor increases by about 20% as compared to the floor below, due to the unheated attic. Average annual cooling energy demand of each floor, for all studied apartment buildings are presented in Fig. 8. The results show that in buildings up to 5 stories high the cooling load increases with floor level. For taller buildings, cooling demand increases sharply up to the fourth floor, and subsequently the gradient moderates, reaching a maximum and then decreasing gradually to the last but one floor. In the last floor there is an increase in cooling load due to the roof effect. Similarly to heating load, there is no significant change in cooling load of the top four floors among buildings over five stories high (dotted lines in Fig. 8). The rear apartments have an increased heating load of about 47% as compared to the front south facing apartments. Cooling load in such apartments is significantly lower than the front apartments (by 65%).

3.2. Electricity generation The electricity generation potential of the studied apartment buildings are presented in this section. This includes the electricity generation by roofs – gable and folded-plate, as well as generation by south, east and west façades. The potential effect of proposed improvements to electricity generation by the façades is studied as well. 3.2.1. Electricity generation and overall consumption Electricity generation by a gable roof is limited, reaching a maximum of 76% of the total energy consumption of a 3-storey building, and about 19% of consumption by a 12-storey building. Energy consumption for heating and cooling is obtained assuming a heat pump of COP 4. The total energy consumption by apartments and the energy generation by a gable roof for all studied buildings are presented in Fig. 9. Energy use accounts for heating and cooling, domestic hot water, appliances, and energy used by the service zone including the elevators’ activity.

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Fig. 9. Energy consumption and energy generation by a gable roof.

Fig. 10. Energy use versus energy generation by a folded-plate roof, south façade and east and west façades. Fig. 11. Comparison of energy generation by the PV systems of studied configurations to total energy consumption.

Replacing the gable roof by a folded-plate option enables to generate the approximate energy need (97%) of a 3-storey apartment building (Fig. 10), and about 24% of the total energy consumption of 12-storey building. Folded-plate roof enable to generate about 27% more electricity than the gable roof. This is mostly due to the larger area of the south-facing roof surface. The electricity generation of the second and third rows of the folded-plate roof (see Fig. 4) is reduced by about 10% as compared to the first exposed row. This effect can be mitigated by slightly elevating each successive row to reduce shading and increase exposure. 3.2.2. Electricity generation by façades The basic design of façades for electricity generation assumes PV panels integrated in 50% of the total area of the south façades, and 80% of the east and west façades, starting from the third floor and up. As the total combined area of the east and west façades of the apartments is 2/3 that have the south façade, the area covered by PV systems of the south façade and of the side façades is roughly equal. BIPV system integrated on the south façade generates annually about 64% of the electricity generated by the same area of a south facing roof with a near optimal tilt angle (45° in this study), while east and west façades generate in general about 76% of the south façade generation. The electricity generated by the BIPV integrated in the south façade increases with higher apartment buildings and can constitute about 15% of the total energy use of a 12-storey building. This value is comparable to the fraction generated by the gable roof, for the same building (19%). The combined generation of east and west façades constitutes a further 12% of the total energy use of the 12storey building. The combined energy generation by folded-plate roof design, by south façade, and combined east and west façades of all studied buildings, is presented in Fig. 10 against total consumption.

Fig. 11 presents the contribution to electricity generation of the various electricity producing components (roofs, façades) as percentage of energy consumption. The graph shows that the combined generation of the façades is larger than that of the roof above the 8th floor with gable roof and above the 10th floor with folded-plate roof. It can be observed as well that the combined energy generation by folded-plate roof and façades can supply 8% more electricity than the consumption of a 3-storey building, while it constitutes about 90% of the total consumption of the 4-storey building and 50% of the 12-storey building. 3.2.3. Shift of peak An important effect of implementing BIPV system on the east and west façades is the time shift of electricity generation significantly towards morning and afternoon, respectively. For instance the peak generation in a summer design day (SDD) is at around 10 AM for the east façade and 4 PM, for the west façade. This is in addition to the increase of generation of these façades as compared to the south façade during the SDD (see Fig. 12a). Fig. 12a and b show the generation by all studied façades and by a gable roof of a 12-storey building, for the SDD and the WDD. For the winter design day (WDD), the south façade generation is larger than the generation of the roof, per unit area. The combined generation of the façades is about 50% higher than the generation of the roof, and it peaks at around 11 AM and 2 PM (Fig. 12b). 3.2.4. Additional analysis An additional analysis is carried out to find the effect of increasing generation potential by façades on the overall energy balance of different buildings. The analysis shows the limitation of the roof, even with increased potential generation. The roof potential becomes comparatively small for higher apartment buildings. In

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Fig. 12. Electricity generation of different BIPV components covering different areas of east, south and west facades, and south facing roof of a 12-storey building, (a) on SDD and (b) on WDD.

Fig. 13. Energy generation by different rates of improvement of south façade BIPV systems as percentage of the total energy consumption.

contrast, increasing the potential of south, east and west façades becomes very advantageous for high-rise buildings (>9 floors). The sections below present a detailed analysis of hypothetical increase of the potential of façades to generate electricity, employing BIPV systems. The annual electricity generation per unit area of PV system integrated on the south façades is about 64% as of that integrated within a south-facing surface of a gable roof of near optimal tilt angle (45° in this study). For the base case, BIPV covers 50% of the south façade, starting from the third floor up. Incremental increases of the electricity generation of the base case by 30%, 60%, 100% and 150% are investigated. This improvement is based on a study by [35], in which it is found that generation of the façades can be greatly improved by adopting certain façade design such

as folded-plate curtain walls. According to the geometric design and proportioning of the plates, their tilt and orientation angles, the electricity generation can be increased by up to 250%, as compared to a flat south façade, where PV system covers 50% of its total area (similar to the base case in this study). The results show that by increasing the generation of the south façade by 150%, this façade alone can supply up to 35% of the energy use of a 12-storey building. Fig. 13 presents the comparison of energy generation by different options of improvement of south façade BIPV systems to the total energy consumption. Applying similar incremental generation increases to the east and west façades, the combined generation of all façades, corresponding to the base case (PV coverage of 50% of south façade and 80% of east and west façades) and to the improved cases

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4. Summary and conclusion

Fig. 14. Energy generation by different rates of improved south, east and west façades BIPV systems as percentage of the total energy consumption.

are presented in Fig. 14. It can be observed that a net zero energy and positive energy status can be achieved by improving the potential of these façades by 150%, for buildings over 9 stories high. The trend of the influence of the building height (number of stories) on energy balance for façades is the opposite of the trend for roofs (increase instead of decrease). The curves for the combined electricity generation of folded-plate roofs with the base case and with improved façades are presented in Fig. 15. It can be observed from Figs. 14 and 15 that investment in façade exploitation offers the potential of high returns, while improving roof design is of limited potential.

This study provides an analysis of energy consumption and energy generation of multistory residential buildings and investigates various methods to increase energy generation potential to match consumption. The design of these multistory building incorporates energy efficiency measures and passive design strategies (e.g. high insulation, triple glazed low-e argon fill windows, large south facing window areas, very tight construction). The basic multistory building considered in this study is composed of 8 apartments per storey, laid out in two rows, and a service zone. Buildings of 3–12 stories are investigated. The study adopts design strategies to maximize solar energy capture and assumes energy demand data as proposed in the literature for mid-latitude cold climate locations (Montreal, Canada). Electricity generation is obtained by exploring various scenarios of PV systems, integrated in roofs and façades. Gable roof with PV system integrated on the complete south portion of the roof constitutes the basic roof design case. A variant folded-plate design consists of plates oriented 30° east and west from south. The PV system is assumed to cover south and near south facing plates. Integration of PV systems in façades is considered in order to augment roof electricity generation. PV is integrated in the south façade alone or in south, east and west façades. Additional analysis is conducted to explore the effect of hypothetical improvement in the generation potential of the façades. A comparison of energy generation to energy use, in all apartment buildings and all studied scenarios is then conducted. Table 3 below summarizes the main findings of the study. The energy intensity of apartments is included based on the assumptions of energy efficiency and passive

Fig. 15. Electricity generation of folded-plate roof with different façade scenarios as percentage of the total energy consumption of buildings.

Table 3 Summary. Studied scenarios Density (Number of units per building) Intensity of energy consumption Ratio of energy generation to energy use Gables roofs Folded-plate (improved roof) South façade (PV integrated on 50% of the façade) Combined east and west façades (PV integrated on 80% of the façades) Combined façades and gable roof Combined façades and improved roof Improved generation of façades Façade improvement by 30% Façade improvement by 150% Improved generation of façades + improved roof Façade improvement by 30% Façade improvement by 150%

Apartment buildings Height dependent (24–96 units in this study) 68 kW h/m2 76% for 3 floors building – 19% for 12 floors building 96% for 3 floors building – 24% for 12 floors building 5% for 3 floors building – 12% for 12 floors building 5% for 3 floors building – 12% for 12 floors building 87% for 3 floors building – 46% for 12 floors building 108% for 3 floors building – 51% for 12 floors building 22% for 3 floors building – 57% for 12 floors building 44% for 3 floors building – 109% for 12 floors building 120% for 3 floors building – 80% for 12 floors building 140% for 3 floors building – 133% for 12 floors building

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design adopted in this study. The main results found in this research are discussed below. The study shows that the average energy required for heating decreases with increasing number of floors. For instance the heating energy demand averaged per apartment in a 3-storey building is about 30% larger than for a 12-storey building. Average heating demand beyond a certain height (8 floors) becomes relatively stable. Cooling load increases with increasing number of stories, stabilizing above 10 stories. Cooling load under the studied climatic conditions, however, is an order of magnitude less than the heating load. Energy consumption of residential units in apartment buildings is, in general, lower than in detached houses (within a neighborhoods). For instance, the average heating load is reduced by up to 50% in a 12-storey building as compared to the heating load of a single-family 2-storey house, with identical floor area. Multistory buildings have a limited roof area. Employing the total south facing roof area of a gable roof would generate about 76% of the total energy use of a 3-storey building. Replacing this roof by a folded-plate roof design supplies 97% of the total energy consumption of such a building. As the number of floors increases, the energy production/consumption balance decreases; generation by the roof integrated PV systems represents only a 19% for gable roof and 27% for folded-plate roof of a 12-storey building. Results of this study clearly indicate the potential benefits in investing in exploitation of façades for electricity generation. As roof contribution to the energy balance decreases with increasing height, the potential contribution of façades increases. In this study the PV system on façades is implemented starting from the third floor up. A PV system on 50% of the south façade of a 12-storey building can generate about 15% of its energy use. Combining this with electricity generated by PV system implemented on 80% of the east and west façades can increase the potential of this building to produce up to 27% of the energy consumption, in addition to the roof contribution (19% of the total energy use of the 12-storey building, for a gable roof). By exploring all three façades and a gable roof, the electricity produced can supply up to 50% of the energy use of a 12-storey building, and 90% to 60% of energy use of mid-rise apartments (4 floors–8 floors). Energy generation by façades can be further enhanced through geometric manipulations of these façades to increase their effective surface areas and optimize their tilt and orientation angles. For instance, employing folded-plate curtain wall systems [35] can increase the potential of the façades by 250% as compared to flat façade. Assuming that the potential of the default façades adopted in this study is improved by 150%, the output electricity generated by these façades can supply the need of a 9-floor building and about 8% more than it is needed for the 12-floor building, this in addition to the roof contribution (32% and 24%, respectively for folded-plate roof). This indicates that improvement of façade design is an important key to achieve net zero energy multistory buildings. This result is in conflict with the conventional view that energy efficient buildings require minimizing envelope surface. While this convention may hold for energy conservation, it collapses when the building envelope becomes an energy generator, which can more than counterbalance the negative effect that it may have on heating and cooling loads. Another important result of using east and west façades to integrate PV systems is the significant time shift of electricity generation, obtained as compared to south facing BIPV system. Although the total annual generation by these façades is lower by 24% compared to the south façade, east and west BIPV systems enable a spread of the timing of peak electricity generation over up to 6– 8 h. This feature can be beneficial in reducing mismatch between demand and supply of the grid.

Design of land use and configurations within the land would play an important role in achieving optimal density and energy generation configurations. In the case of mix of different building types (low and high rise), the land design and positioning of the buildings should be optimized, to minimize shading on south, east and west façades. In all cases, policies and regulations affect the available options.

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