Design of solar-optimized neighborhoods.
A number of parameters should be considered in planning solar urban design for exploitation of solar radiation for lighting, heating and electricity generation. Neighborhoods can be designed to achieve net-zero energy consumption for a set of houses by addressing key parameters such as building shapes, density within a site, and site layout.
Building shape plays a role in governing energy consumption in buildings and can provide advantages in capturing solar energy (Ouraghi et al, 2006; Hachem et al, 2010). In the design of solar houses, two main surfaces should be optimized for solar radiation - equatorial facing roof and facades. Those surfaces serve as basis for the placement and orientation of windows and solar panels, and therefore can be manipulated to optimize solar energy utilization. Shade from adjacent surfaces in self-shading shapes (such as L shape) can affect significantly solar radiation on equatorial facing facades, and transmitted by their fenestration (Hachem et al, 2010).
Spatial characteristics of neighborhoods and land use regulations can significantly affect solar potential and energy demand of buildings. Land-use patterns influence local temperature distributions at a given location (Kim, 2009). Orientation of a building and its position with respect to neighboring buildings have a large impact on its accessibility to solar radiation, and on governing energy transfer mechanisms.
Several studies have focused on investigating the distribution of solar radiation on different surfaces in a built environment (e.g. Okeil, 2010), as well as on the availability of solar energy and its optimization, at the urban scale. Compagnon (2004) proposed a methodology for estimating the amount of solar energy available to a building of any shape, taking into account obstruction due to the surrounding landscape and associated reflections. Kampf et al (2010) have developed a methodology to minimize energy demand of buildings in an urban area using a multi objective evolutionary algorithm and to maximize incident solar irradiation whilst accounting for thermal losses.
Notwithstanding the interest in the effect of urban development on solar energy, and the various investigations conducted to optimize solar energy, several aspects are not sufficiently addressed, such as interaction between individual shapes, their density within a site, and the site layout. The current study presents an investigation of the effects of some design parameters on solar potential and energy demand of two-storey single family housing unit assemblages, located in Montreal, Canada. These design parameters include: the shape and orientation of individual units; the orientation of surfaces within a unit, and the site layout. The solar potential includes the radiation incident on near south facing facades and transmitted by the fenestrations of these facades, and the electricity generation potential by building-integrated photovoltaic system (BIPV), integrated in the near south facing roof of these units.
The objective of this research is to assess the effect of housing shapes and of their arrangement in a site layout, on the solar potential and energy demand of these houses. The general characteristics of the neighborhood are based on various sources, including guidelines of urban design and zoning bylaws (e.g. Ottawa bylaws, 1999). The density of units is assumed as low medium density, associated with a suburban area. The effect of higher density on the considered response variables is studied and presented in other works (e.g. Hachem et al, 2010). The design methodology consists of first determining the site layout, second designing the unit shapes to conform to this layout, and third combining the shapes in different configurations. Two site layouts are considered. For each site, several configurations consisting of combinations of groups of three to five units of a given shape are studied. Two basic shapes for housing units with constant floor area of 60 m2 (645.6ft2) are employed - rectangle and L shape. Variations of L shapes are explored to identify design possibilities that enhance solar radiation capture potential on near-south facing roofs and facades. The two-storey housing option adopted in this study represents one of the most common types of single family homes in Canada (Athienitis, 2007). The characteristics of the housing units are detailed in Table 1. The basic design of the units relies on passive solar design principles (Chiras, 2002) and rules of thumb (CMHC, 1998). The design ensures that the overall east-west dimension of the house - the solar faCade, is larger than the perpendicular dimension (north-south), to maximize passive solar gains in winter. For non-convex shapes, an additional parameter should be taken into consideration. This parameter is the ratio of the shading to shaded faCade widths, termed the depth ratio, - a/b, in Table 2 (L shape). The shaded faCade width and the depth ratio are determined so as to maintain a functional interior space. A geothermal heat pump with a COP of 4 is assumed to supplement the passive and active solar heating. Cooling strategies are not investigated in this study.
Table 1. Characteristics of housing units Thermal resistance values: Exterior wall: 6 RSI (R-34) Roof: 10 RSI (R-57) Slab on grade: 1.2 RSI (R-7) Thermal mass 20cm (7.8 in) concrete slab Window type Triple glazed, low-e, argon filled (SHGC=0.57), 1.08 RSI (R-6.1) Area of south Around 20%, and 40% glazing as percentage of floor area and south wall Shading Strategy Interior blind Occupants 2 adults and 2 children, occupied from 15:00 - 9:00 Set point temperatures Heating set point 21[degrees]C (70[degrees]F), cooling set point 25[degrees]C(77[degrees]F) Air infiltration rate 0,8ACH @50Pa (1.0 lb/f[t.sup.2])
Throughout the study, a hip roof is considered with tilt and side angles of 45o. The height of the low edge of the roof is kept constant at seven meters above ground level. In L shapes, the ridge of each wing runs along its center, with a triangular hip at the end of the lateral branch and a gable at the end of the main wing - see figures below.
The study investigates the effects of the design parameters on two major response variables-solar potential and energy demand for heating and cooling. Solar potential includes radiation incident on near-south facing facades, solar radiation transmitted by ground floor windows in these facades and PV electricity production potential. The parameters, whose effects on the response variables are investigated, consist of the geometric shape of the housing units, and the site layout with corresponding arrangement of the units. The shape parameters include, in addition to the basic shapes, several variations to the geometry of the L shape (Table 2).
The rectangular and L shapes are selected in this study because they can be considered as the basic shapes for passive solar design. Other shapes can be derived from combination / variation of these shapes. The L shape consists of a main wing and an attached branch. The main wing is assumed to be oriented east-west, so as to have the long faCade facing south. The branch can be attached at either the west end (W configuration) or at the east end (E configuration). A depth ratio (a/b) of 1/2 is adopted throughout this study. This ratio is selected in order to minimize the shade cast on the main wing, while maintaining a functional plan (Hachem et al, 2010). A depth ratio of 1/2 can reduce transmitted radiation by the windows of the south facing main wing of the L shape by up to 7%. A depth ratio of 1 may result in reduction of the transmitted radiation by up to 27% (Hachem et al, 2010).
The wings of L shape and of its variants are assumed as south facing, in this paper. L variants are characterized, in addition to the depth ratio, by the angle [beta] - the deviation from 90o of the angle between the wings of the L (Table 2). Two values of [beta] are considered in this study - 30 [degrees] and 60 [degrees] . An additional shape, termed hereunder Obtuse-angle (O), can be considered a special L variant with [beta]=70[degrees].
[TABLE 2 OMITTED]
[FIGURE 1 OMITTED]
Roofs. A photovoltaic system is assumed to cover the total area of all south and near-south facing roof surfaces. This area includes the triangular portions of hip roofs in L-shape and its variants, and the two near south facing surfaces in obtuse-angle roofs (Fig. 1). This assumption is employed to compare the potential of the BIPV systems of the different housing shapes. In practice, a percentage of the roof area is used for the mounting structure and other technical considerations. Future roof models are planned to take into consideration such details.
[FIGURE 2 OMITTED]
Two site layouts are studied. Site I is characterized by a straight road, while site II layout incorporates a semi-circular road. In site II the curved road is south facing (i.e. the center lies south of the arc). The circular road is selected to represent an extreme case of curved road design option corresponding to a cul de sac street design option. The housing units are positioned with respect to the shape of the roads in both sites. The distance between detached adjacent units varies from 4m (13.1ft) to7m (23ft). This distance changes according to the curvature of the road and the shape of the unit. Shapes featured in site I include rectangle, L shape and L variant V-W30 (Fig. 2).
Three configurations are designed for site II. The configuration shown in Fig. 3a consists of rectangular units. A combination of L shape and its variants is shown in Fig. 3b. The layout of Fig. 3c includes three central units of obtuse-angle shape, and two L variants, U1 and U5, which are identical to the corresponding units of layout 3b, and this in an attempt to optimize facade orientation for solar radiation.
[FIGURE 3 OMITTED]
Weather data. This study is performed for Montreal, Canada (45[degrees] N Latitude). Two design days, a sunny and cold winter day (in January) - WDD, and a sunny and hot summer design day (in July) - SDD, are used to represent two extreme sunny days, to study the incident solar radiation on the facades and roofs of all configurations. Additionally, a whole year weather data set is used to estimate the annual electricity production of the PV system installed on near south-facing roof surfaces. The weather files of the building simulation program EnergyPlus are used for the simulations (EnergyPlus, 2010).
EnergyPlus Solar Radiation Computations. The hourly direct solar radiation is computed using the EnergyPlus program. The computation is based on the ASHRAE model of clear sky (ASHRAE, 2003) applied to Montreal area (45 [degrees] N) and accounts for the shadow cast on exposed surfaces. The shading algorithm accounts for self-shading geometries.
Slab on grade model. The slab program (EnergyPlus, 2010), is used to compute the temperature of the outside surface of the slab (in contact with the ground). Taking into account the slab and ground properties, the slab program produces average monthly temperature of the slab, which is input in EnergyPlus to carry out the simulations.
BIPV model and computation. A simple model roof integrated PV system based on constant electrical conversion efficiency ([eta]) of 12%, is used. This efficiency is based on a nominal PV efficiency of 16% and a PV system performance ratio of 0.75 ([eta] =16*0.75). The PV performance ratio is the ratio of the actual system yield (kWh/kW) to the reference yield. Reference yield refers to the insolation in the plane of the PV module (kWh/[m.sup.2]). The performance ratio accounts for all PV system losses, including electrical wiring losses and to PV operation under non-optimal conditions (Poissant et al., 2003).
The analysis evaluates the effect of shape and site layout on solar potential, including solar irradiation (incident and transmitted), the BIPV energy production, and the energy demand for heating and cooling. An effect is measured as the change from the reference case. The reference case used to evaluate the shape effect on the solar radiation is a south facing rectangle, and the reference site is the site layout with straight road (site I).
PRESENTATION AND ANALYSIS OF RESULTS
In the absence of shading, solar irradiation on a surface depends primarily on the orientation of a surface relative to the south. The effect of surface rotation from the south on solar potential is analyzed by 1) computing the ratio of transmitted radiation by windows of a surface at a given orientation angle to that transmitted by south facing windows and 2) by analyzing the electricity generation (per [m.sup.2]) of roof surface relative to the electricity generated by a south facing roof. The effect of orientation of the rectangular unit on the energy demand for heating and cooling is determined as well.
The effect of faCade rotation on transmitted radiation by south facing windows is illustrated in Figure 4a. The ratio of transmitted radiation by the rotated faCade to that of a south facing faCade is plotted against the angle of rotation towards the east or west, for the two design days. In WDD the best performance is associated with the south orientation and it is reduced by 50% when the orientation angle approaches 60[degrees] west or east of south. By contrast, for the SDD, the transmitted radiation increases with increasing rotation angles, particularly towards west (up to 80%). The shift towards west may be explained by the high solar intensity in the afternoon coupled with extended duration of the position of the sun in the western orientation.
For electricity generation, the results are similar to the effect on faCade irradiation. For the SDD, the west orientation yields the best performance (Fig. 4b). This is due to the long daylight period, with high solar intensity in the afternoon. For the WDD, the best performance of the BIPV is associated with the south orientation and it declines sharply, when the orientation exceeds 30 [degrees] of true south, (Fig. 4b).
The annual heating and cooling loads for rectangular units are determined as function of their orientation from due south (Fig 4c). The results indicate that both heating and cooling loads increase with increased angles of rotation. Heating loads are converted to electricity consumption using a COP of 4, associated with a typical geothermal heat pump. Comparison between total energy use for heating and cooling and the energy production of these rectangular units is presented in the graph of Figure 4d. The cooling demand of the west oriented units is slightly larger than the east oriented units (Fig 4c and 4d). This can be explained by the larger solar heat gain of these west oriented units in the summer, as explained above and shown in Figure 4a.
[FIGURE 4 OMITTED]
Effect of shape
Solar irradiation. The shape effect is computed as the difference between the corresponding response (incident or transmitted radiation) of south and near south facing facades of the studied shapes and that on the south facing faCade of the rectangular shape. Two main observations are highlighted:
* The south facing faCade of the main wing of L and V shapes (faCade b, Table 2), is mostly affected by the angle ([beta]) between the wings and by the depth ratio of mutually shading facades. The radiation on the south facing faCade of the main wing is reduced with increasing values of a/b (Hachem et al, 2010), and increased with increasing values of [beta].
* Rotation of the south facing facade of the branch of L shape causes an increase of the incident radiation on facade, for the SDD and decrease of this radiation for the WDD. This effect of rotation follows the same principle detailed above in Fig. 4a.
Electricity generation. The electricity generated by the BIPV of south and near south facing roof surfaces of each unit shape is compared to electricity generated by the reference case. The main observations of the shape effect on electricity generation for sites I and II are as follows:
* The south facing roof of the main wing of L and V shapes (faCade b, Table 2), is not significantly affected by the angle between the wings ([beta]) and by the depth ratio of mutually shading facades.
* The rotation of the branch by 30[degrees] ([beta]=30[degrees]) in the L variant produces an increase of about 13% of electricity generation per [m.sup.2] of the rotated hip roof, for the SDD, and 9% reduction per [m.sup.2] for the WDD. This effect is presented in Figure 4b.
* The comparison of the total electricity production by the roof indicates an increase of the annual generation of L variant shapes ranging from 20% to 35% relative to the reference case (rectangle). This effect is attributed to the larger south facing roof area (31[m.sup.2] (333.6 f[t.sup.2])) for the L variant, in comparison with 25.6[m.sup.2] (275.4f[t.sup.2]) for the rectangular shape).
* The comparison of the average electricity generation of all configurations of site II to the rectangular configuration indicates that the obtuse-angle configuration is the best configuration when energy generation per unit area is considered (Table 3). The total production averaged by units, shows however that L variant configuration exceeds that of the rectangle by 33% and of the obtuse-angle by 10%, annually.
Table 3. Comparison of electricity generation of the configurations of site II (Fig. 3) PV PV generation generation of L of variant Obtuse-angle relative to relative to Shape Rectangle Obtuse-angle Rectangle L variants SDD 1.02 0.98 1.04 1.02 (/[m.sup.2]) WDD 1.04 0.97 1.07 1.03 (/[m.sup.2) Annual 1.04 1.02 1.06 1.02 (/[m.sup.2) Annual 1.33 1.10 1.34 1.00 (/total area)
Effect of shape on energy demand. The shape of units affects significantly the energy demand for heating and cooling. L shape, L variant (V-30W) and obtuse angle shape require 7%, 8% and 2% respectively, more heating energy than the reference case. The cooling load of L variant exceeds that of the reference case by about 19%, while obtuse angle and L shape require 8% and 4%, respectively, more cooling energy than the reference. The increase of heating demand of the L shape and its variations can be explained by: 1) the decrease of the solar gain in the winter due to the shade cast by the wings, and the rotation of the wings in the L variations; 2) the larger area of the building envelope of these shapes as compared to the rectangular shape, for the same floor area. In the summer the cooling load is increased due to the increase of solar radiation on the rotated wings, in addition to the large envelope area, mentioned above. Comparison of the cooling and heating consumption, computed using a COP of 4, of all L variants is shown in Figure 6a. Comparison of the total energy use for heating and cooling with the energy generation associated with each unit is presented in Figure 6b.
[FIGURE 6 OMITTED]
Table 4 presents a comparison of the average heating and cooling load for each configuration of site I and site II to the corresponding average energy load of the rectangular configuration. While the heating load does not dramatically change, the cooling load can increase by up to 37% and 23 % for L variant, in site I and site II, respectively. This increase of energy demand in L variant configurations is however counterbalanced by an increase of energy generation: L variant produces 20% and 33% more electricity in site I and site II, respectively, as compared to the corresponding rectangular configuration.
Table 4. Average energy demand for all configurations relative to the rectangular configuration Site Site I Site II Configuration Heating Cooling Configuration Heating Cooling of load load of load load L shape 1.07 1.11 L variant 1.05 1.22 L-VW30 1.05 1.37 Obtuse angle 0.99 1.23
Effect of site layout
Solar irradiation. The incident radiation on the near south facing facades of individual units and transmitted by their windows, in each assemblage is compared first to isolated units to assess the effect of shade from adjacent units. In site I, the units are positioned in straight layout facing south; therefore the south facing facades are not affected by adjacent units, whereas east and/or west facades are completely shaded. In site II, the main effect, as compared with site I is due to the rotation of units relative to the south, along the curved layout. In addition, some units in these configurations cast shadows on facades of adjacent units. The reduction of incident WDD radiation on the south facing faCades ranges from about 4% on the central units, to up to 30% for some adjacent units (e.g.U2 and U4 in Fig.3b), in comparison to the isolated south facing units. This reduction depends on the distance between the units, and whether they are totally or partially shadowed.
Electricity generation. The site layouts are compared for the two shapes shared by both sites - rectangles and L variants. The performance of these configurations, in terms of average electricity generation per unit area and of the total annual generation averaged per units, is compared for each of the site II shapes to the corresponding configurations of site I. The results are presented in Table 5. The site layout has no significant overall effect on the electricity generation per unit area. A maximum reduction of about 3% is observed in the generation of the detached rectangle configuration in site II as compared with the similar configuration in site I.
Table 5. Site layout effect on average electricity generation and on energy demand Site II/Site I Energy generation Energy demand Design SDD WDD Annual Annual Annual Annual period per total heating cooling area demand demand Rectangles 1.02 0.91 0.97 0.97 1.07 1.39 L variant 0.98 0.97 0.99 1.04 1.07 1.24
Energy demand. The arrangement of units with respect to each other in a site can result in shading different surfaces of a specific unit. An additional effect is, as mentioned above, the orientation of each individual unit. To isolate the adjacency effect from the effect of orientation, only the central due south unit in a site is compared to the corresponding isolated unit. The results indicate that in general the heating load increases for all units in site I and II wile cooling load decreases, as compared to the corresponding isolated units. The increase in heating load reaches 12 % and 22% for the rectangular shape in site I and site II, respectively. L shape heating load increases by 15% in site II as compared to 12% in site I. One reason for this effect is the shade cast on the east and west facades, in all configurations, and partially on south facing facades in site II.
Comparing the configurations of site II to their corresponding in site I, it is noted that the heating load for both rectangular and L variant configurations increase by 7% in site II. The cooling load increases by up to 39% in the rectangular configuration of site II.
SHIFT OF PEAK ELECTRICITY GENERATION
An important result of the interaction of site layout and configurations is the shift of peak electricity generation. A significant shift of the profile of the electricity generation is obtained by the BIPV of different units. A maximum shift of 3 hours is obtained in site I. In site II the rotation of whole units in addition to the rotation of individual faCade, increase the gap between the time of the peak electricity generation. A difference of peak time of up to 6 hours is observed in the configurations of site II. Figure 7 presents the daily variation of electricity generation of configurations of site II. Electricity generation profiles for rectangular units for a WDD is presented in Figure 7a. The graph of Figure 7b show the effect of L variant configurations of site II on the electricity generation profile of the hip roof of different units for winter design days.
[FIGURE 7 OMITTED]
SUMMARY AND DISCUSSION
* This study evaluates the effects of several parameters affecting solar response and energy demand for heating and cooling of housing units' assemblages. The objective is to optimize the design of urban settlements for the exploitation of solar irradiation that can be used for passive heating and daylight, or conversion to electricity. The study demonstrates that, under a predetermined density, shape and site layout closely interact. Rectangular shape, considered as optimum for energy efficiency, is not optimal from the point of view of solar capacity in all site layouts. The main results are as follows:
* Two important factors should be considered in the design of geometrical shapes of buildings: the orientation of individual facades or of the whole building, and self-shading effect on south facing facades and roofs. Solar irradiation on facades and roofs is significantly affected by the rotation from the true south. Rotation to the west may increase faCade irradiation by some 70% and roof BIPV electricity generation by some 25% for the summer design day under the conditions assumed in this study, while the corresponding reduction for winter is some 50% and 30%, respectively. In L shape and its variations, shading of the main wing (oriented east-west) by the branch is proportional to the depth ratio, the ratio of branch to main wing lengths, and inversely proportional to the angle enclosed by the wings. Different shapes and orientations affect the total exploitable area of roof surfaces for PV integration. Maximizing the south roof area is extremely important, given that floor plan area is usually determined by residential needs. As a result, the total energy generation can be increased by up to 35% annually relative to the rectangular shape.
* Heating and cooling demands are affected by the shape of units as well. The optimal shape is a south facing rectangle. L shape and L variant require about 7-8% more heating demand. Rotation of part of the branch of L does not affect significantly the energy demand. Energy demand of isolated units is in general less than the same units studied in a neighborhood (about 12% difference for both rectangular and L shapes). One reason for this effect is the shade cast on the east and west facades, in all configurations, and partially on south facing facades in site II.
* Some configurations of building shape are more suitable for implementation in a specified site layout than others. For instance, the L variant configuration, employed around a curved road, yields 33 % more electricity generation than the rectangular configuration, used in the same layout. The energy demand for heating of this configuration is only 5% higher than the rectangular configuration.
* An important effect of site layout on electricity generation is the shift in peak generation time on surfaces of different orientations of roof surfaces. A difference as large as six hours of peak generation of different units can be achieved by the implementation of these variations in a specific site layout. Shifting peak generation time towards peak demand time can lower net energy cost and also reduce net peak demand from the grid.
The analysis presented in this study indicates a systematic integration of solar efficiency considerations in the design of residential assemblages from the earliest stages of design. A trade-off should be made to balance between energy generation and energy consumption for heating and cooling. Future investigations should include the effect of land use on design and locations of solar collectors. Such investigations can affect important decisions including the housing types and concentration, and the design of solar systems, as building integrated or stand alone.
The first author would like to thank the Natural Sciences and Engineering Research Council of Canada (NSERC) for its financial support through a CGS D2 Alexander Graham Bell Graduate Scholarship.
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Caroline Hachem Andreas K. Athienitis, PhD, PE Paul Fazio, PhD, PE Student Member ASHRAE Member ASHRAE
Andreas K. Athienitis, and Paul Fazio are Professors in the Department of Building, Civil and Environmental Engineering, Concordia University, Montreal, Quebec, Canada. Caroline Hachem is a PhD candidate.
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|Author:||Hachem, Caroline; Athienitis, Andreas K.; Fazio, Paul|
|Date:||Jul 1, 2011|
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