Photovoltaics and urban design

Towns and cities present an ideal opportunity for the exploitation of PVs. They have a high concentration of potential PV sites with a heavy energy demand. At the same time the physical infrastructure can support localized electricity generation. It is estimated that installing PVs on suitable walls and roofs could generate up to 25% of total demand.

It is worth noting that solar irradiation in Malmo, Sweden is higher than in southern England and only 20% less than Florence despite the difference in latitude. The biggest potential for PV is as systems embedded in buildings. It is expected that building integrated PV will account for 50% of the world PV installations by 2010, with the percentage being significantly higher in Europe.

The wide-scale adoption of PV in the urban environment will depend on the acceptance of the visual change it will bring about, especially in historic situations. There is still a barrier to be overcome and planning policy guidance may have to be amended to create a presumption in favour of retrofitting PVs to buildings.

The efficiency of PVs in a given location will depend on several factors such as:

• compact developments with fairly consistent roof heights are ideal for roof-mounted PVs

• orientation is an important factor

• a more open urban grain may exploit the potential of façade PVs. In this case overshad-ing must be considered, especially in the context of seasonal changes in the sun's angle.

Nieuwland near Amersfoort in Holland is a new town in which building integrated PVs is a feature of many homes, producing a peak total of 1.3 MW (see Fig. 6.3).

A way of measuring the effectiveness of the urban massing to accommodate PV is the 'sky view factor' (SVF) devised by Koen Steemers and colleagues at the Martin Centre in the University of Cambridge. This gives an indication of the spacing between buildings and indicates the amount of the sky which is visible from any particular position in a city whether at

Figure 6.3 The new town of Nieuwland near Amersfoort (courtesy of Ecofys and REW)
Figure 6.4 Sky view factor for two towns: medieval Athens (left) and Grugliasco, Italy (source: The Martin Centre, Cambridge)

street or roof level. A totally unobstructed situation has a value of 1. Steemers gives two examples: a medieval part of Athens and Grugliasco in Italy. The average SVF from the streets for Athens is 0.68 and for Grugliasco, 0.82 (see Fig. 6.4).

The intensity of the grey indicates the degree of visible sky with white being the SVF of 1. In Athens the buildings are tightly packed at high density and so the streets offer few opportunities for façade building-integrated PVs (BIPVs). On the other hand, there is a relatively even overall building height which creates a good situation for roof PVs. The Italian example has a smaller overall area of roof but light shading at street level and therefore opportunities for façade BIPVs.

Surface-to-volume ratio

The surface area available for either façade or roof PV installation is largely determined by the surface-to-volume ratio. A high ratio of surface to volume suggests opportunities for façade-integrated PVs. However, in high-density situations, overshading will limit façade opportunities. On the other hand, lower values indicate opportunities for roof PVs. In the examples from Steemers the lighter the shading the more the PV opportunities. The lower the value the greater the potential for roof-mounted PVs (see Fig. 6.5).

As stated, the spacing between buildings is an important factor in determining façade PV opportunities. Figure 6.6 shows three plan orientations coupled with three height-to-width ratios. The street with west to east façades offers the least overall efficiency for solar access whereas the diagonal street offers the best overall solar opportunity. However, to sum up, wide spacing between buildings with a southerly aspect will be particularly suited to façade BIPV. Wide streets and city squares provide excellent opportunities for this PV mode. A tighter urban grain points to roof-mounted PVs.

The relationship between urban form and PV potential has been demonstrated by comparing four urban configurations all with a plot ratio of 1.7. Figure 6.7 shows how the percentage of façade area with annual irradiation of >800 kWh/m2 varies with building type and location. The tower layout (pavilions) is the least efficient whereas the terraces offer the best exposure.

BWrwl r .s

Figure 6.5 Comparative surface to volume ratios for European cities (source: The Martin Centre, Cambridge)

North Plan Sections

69%

76%

"L_r i

r

68%

73%

i_r i

Figure 6.6 Solar access and space between buildings (source: The Martin Centre, Cambridge)

In conclusion, the suitability of BIPV is dependent upon a variety of factors. For example, flat roofs are the most appropriate sites in city centres, combining flexibility with unobtrusiveness.

In considering pitched roofs, the orientation, angle of tilt and aesthetic impact all have to be taken into account.

The suitability of PVs for integrating with buildings is dependent upon a variety of factors. For example, flat roofs are the most appropriate sites in city centres, combining flexibility with unobtrusiveness. In considering pitched roofs, the orientation, angle of tilt and aesthetic impact all have to be taken into account (see Fig. 6.8).

Figure 6.8 illustrates the efficiency of average PV cells at different angles and orientation. What is evident is that PV façades may make a significant solar contribution in combination with roof-mounted cells.

Reflected light is a useful supplemental form of energy for PVs. Many façades in city centres have high reflectance values offering significant levels of diffuse light for façade PVs on opposite elevations, thus making orientation less important. In glazed curtain wall buildings solar shading is now de rigueur. Here is a further opportunity to incorporate PVs into shading devices. When office blocks are refurbished the incorporation of PVs into a façade becomes highly cost-effective.

Percentage of façade Climate and annual irradiation c. 800 kW

Pavilion-Court

Pavilion

Slab

Terrace

Pavilion-Court

Athens Torino Fribourg Cambridge Tronheim

Athens Torino Fribourg Cambridge Tronheim

Athens Torino Fribourg Cambridge Tronheim

38 11 2

Figure 6.7 Percentage of façade receiving solar radiation in four configurations (source: The Martin Centre, Cambridge)

Figure 6.8 PV efficiency according to angle and orientation (courtesy of REW, p. 242, July-Aug 2005)

Figure 6.8 PV efficiency according to angle and orientation (courtesy of REW, p. 242, July-Aug 2005)

Proven turbine

□ ISKRA turbine

Aug m

PV

Proven

wind

ISKRA

Figure 6.9 Hockerton project analysis of output (HHP electricity production kWh per month) over 8 months from PV panels, Proven and ISKRA 5 kW turbines (courtesy of the Hockerton Housing Project)

In conservation areas there are particular sensitivities. The next generation of thin film PVs look like offering opportunities to integrate PVs into buildings without compromising their historic integrity.

As a postscript to Chapters 5 and 6 an assessment of the performance of two wind turbines and roof PVs offers a useful prediction of the electricity generation which can be expected from domestic scale installations (see Fig. 6.9). It is worth repeating that the four homes are net zero CO2. Thus the scale of renewables on the site is of the order needed to meet the demands of houses that are 90% more energy-efficient than the average home if net zero CO2 is to be achieved.

Renewable Energy 101

Renewable Energy 101

Renewable energy is energy that is generated from sunlight, rain, tides, geothermal heat and wind. These sources are naturally and constantly replenished, which is why they are deemed as renewable. The usage of renewable energy sources is very important when considering the sustainability of the existing energy usage of the world. While there is currently an abundance of non-renewable energy sources, such as nuclear fuels, these energy sources are depleting. In addition to being a non-renewable supply, the non-renewable energy sources release emissions into the air, which has an adverse effect on the environment.

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