solar accessibility


1. The sun as generator of urban form
2. From New York phantom buildings to contemporary solar laws
3. Solar Accessibility: shadow casting and solar-access rights
4. Solar Envelopes
5. Iso-Solar Surfaces
6. The Sky view factor
7. Tools and Applications
8. Final Remarks: qualitative and quantitative solar accessibility

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1. The sun as generator of urban form          [top]

Access to sunlight and daylight has always been one of the key generators of urban form. This was evident in vernacular architecture and has been the source of inspiration for many architects in recent times. It is surprisingly how modern urban land planning has forgotten simple rules of environmental design in shaping cities: whereas modern architecture has done giant steps towards more comfortable private indoor spaces focusing on daylight and hygienic issues of indoors, it seems modern vision has completely ignored the comfort on open spaces. In fact, from Bauhaus through Le Corbusier, to the International Style, nobody posed great attention to basic environmental urban design rules for aggregating buildings and for the relationship between buildings and open spaces. Furthermore, while the concept of 'solar energy' has received widespread publicity in recent times, 'solar access' is still misunderstood and underestimated. In fact, solar energy has reached professionals, whereas 'solar access' did not; for instance, urban planners and developers seem to give less attention to opportunities for improving sustainability through urban planning and shaping the urban form. Although, access to sunlight has slowly become a matter for stringent urban regulations in modern times: bylaws, such as those implemented in the planning of New York and other dense cities throughout the world, tend to guarantee access to sunlight and daylight through the immediate idea of 'obstruction angles'.


Bibliography

Littlefair P., 1998, "Passiv solar urban design: ensuring the penetration of solar energy into the city", Renewable and Sustainable Energy Reviews , 2, 303 -326.
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2. From New York phantom buildings to contemporary solar laws          [top]

Traditional solar laws are based on simple angular criteria, such as obstruction angle rules. The case of the New York's Zoning Law of 1916 is emblematic: it was enacted as a reaction against the negative environmental effects caused by excessive building height and density in central Manhattan. It stated that construction can "proceed up to a certain height; then the building must step back from the plotline at a certain angle to admit light to the streets. A Tower may then carry 25 percent of the plot area to unlimited heights" (Koolhaas, 1978). As a result, a maximum volume resulting from legal prescriptions could be built on each site. This is well represented in the renderings of Hugh Ferriss, which describe a city of maximal envelopes, "a ghost town of the future […] a collection of 2028 colossal phantom 'houses' that together form a Mega-Village" (Koolhaas, 1978).
The concept of 'solar-access right' as a property right and some measures to encourage the collection of solar energy were first introduced after the energy crisis at the end of the Seventies in many southern states of the United States of America, where the latitude permits an easy application of these methods. In fact, the New Mexico Solar Law (169, 1977), the California Solar Law - the California Solar Rights Act (AB 3250, 1978) and the California Solar Shade Control Act (AB 2321, 1978) - are the first enacted solar access laws (see for instance Erley and Jaffe, 1979; Thayer, 1981). They represent a petition system (solar access permits), in which the landowner must apply for a building permit to install solar collection devices; once the permit is issued, specific solar access maps are provided and future buildings must guarantee the solar access to the installed collectors.
In particular, the California Solar Rights Act set parameters for establishing solar easements, amended the Solar Tax Credit to include costs of easements, prohibited ordinances and private covenants which restrict solar systems, and required communities to consider passive solar and natural heating and cooling opportunities in new construction. The Solar Shade Control Act prohibited the obstruction of the skyspace by vegetation to solar collectors. Also San Francisco adopted the so called 'Proposition K' in 1981: "No building permit shall be issued for a project that will cast a shade or shadow upon any property under the jurisdiction, or designated for acquisition by, the Recreation and Park Commission." The city of Boulder, Colorado, instead, adopted the 'solar fence method'; this system defines the maximum allowable length of a shadow on a site, which is drawn by an imaginary fence of a specified height on the property boundary on December 21st (so called 'hypothetical solar fences'). The height of the fence is established for different solar zones. For the city of Boulder critical times were established at 10:00, 12:00 and 14:00 on December 21st. The fences were defined as 15 ft for low density and 25 ft for high density residential areas. Similar performance standards related to solar access issues were introduced in Europe as well. Just as an example, buildings regulations in the Netherlands establish that every living room should receive at least three hours of direct sunlight on its façade every day, from March 21st to September 21st.
These bylaws represent an easily applicable measure to ensure a minimal environmental quality. However, one of the limitations is that they only consider a rudimentary estimate of the openness of the city to the sky, without taking into account real sun paths. As a result, there have been a number of attempts to better incorporate solar access into urban planning.


Bibliography

Erley D., Jaffe M., 1979, Site planning for solar access: a guidebook for residential developers and site planners, American Planning Association, U.S. Dept. of Energy, U.S. Govt. Print. Off., Washington.

Koolhaas R., 1978, Delirious New York: A retroactive Manifesto for Manhattan, Oxford University Press, New York.

Thayer R.L., 1981, Solar access, "it's the law!": a manual on California's solar access laws for planners, designers, developers, and community officials, Institute of Governmental Affairs: Institute of Ecology, University of California, Davis.
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3. Solar Accessibility: shadow casting and solar-access rights          [top]

The subroutine that governs shadowing on a DEM, together with the dynamic calculation of sun positions, allows the detection of solar envelopes (such as defined by R.L. Knowles and by Capeluto and Shaviv) and of the volume described by the obstruction rule.
The approach is to compute 'shadow volumes', that is, the upper surface of the volume of air that is in shadow over a given urban DEM. This can be done by repeatedly shifting the DEM and reducing its height. This algorithm is very simple and impressively fast (it allows processing acres of city at a time, something unthinkable with traditional geometric models).
The algorithm described above computes shadows for an arbitrary lighting angle; the next stage is to add a procedure to calculate shadows from the sun for any given latitude, time of year, and time of day, using the usual astronomical formulae (Szokolay, 2004). This allows the possibility of performing simulations for a whole day or number of days. As initial assumption for the shadow-casting during a whole day we just considered the hours of sun chosen for all altitudes higher than 5 degrees above the horizon. That means 7 hours for winter solstice, 11 for the equinoctials, and 15 for the summer solstice.
The first operation made possible by this subroutine is the dynamic representation of shadowing. Different images can be produced and subsequently animated in Matlab, creating short movies which give the architect or planner an idea of the shadowing conditions of an urban site during an entire day.
From the previous codes, simple quantitative parameters can be derived. For instance we can bring all the hourly frames of a single day together. Summing all black and white shadowing images, each made of 0s and 1s, it is possible to obtain greyscale images which have values in the range 0 - n, where n is the number of sun positions considered for the day (equivalent to the number of hours of sun, if the sun positions are taken at hourly intervals). We call the resulting map as the 'Mean Shadowing Map'. The resulting image portrays in an elementary way the number of hours of shadow for each pixel (this value is simply given by the resulting value of that pixel). Contours can be added to improve visualisation: a standard function in Matlab allows the detection of value changes on images - such as between a pixel with x and x+1 hours of sun. The resulting map defines portion of space where there are 1, 2, 3, …, hours of shadows.
Some applications of the technique deal with the computation of generated shadows on the surroundings. This helps for the environmental prediction of the impact of new design projects on urban spaces where the ecological status quo might be compromised. The proposed tools reveal themselves to be very useful in comparative studies where computations can envision better and worse solutions. For example, we can very quickly count the open areas that are under shadow at every hour of the day; we can differentiate between areas that are permanently in shadow and areas that are constantly illuminated.


Bibliography

Ratti C., Richens P., 2004, "Raster analysis of urban form", Environment and Planning B: Planning and Design, 31:2, 297-309.
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4. Solar Envelopes          [top]

It was Knowles (1974) who refined the obstruction angle rule and introduced the well-known concept of 'solar envelope' (SE): a 3-dimensional surface, on a given site, that does not obstruct more than n hours of sun onto adjacent sites. Using Knowles' words, the solar envelope on a given site is defined as "the volumetric limits of building that will not shadow surroundings at specified times" (Knowles, 1981). In other words, given a certain site in an urban context, the solar envelope defines the maximum built height that can be reached on that site without compromising the neighbouring buildings' solar accessibility. The latter is defined as the minimum number of hours of sun, during critical periods of the day and the year.
The idea was later extended by Capeluto and Shaviv (1997), who distinguished between 'solar rights envelopes' (SRE) and 'solar collection envelopes' (SCE) - the former resembling Knowles' initial definition, and the latter examining the total number of sun-hours collected (as opposed to cast) by a particular urban 3-dimensional surface. In particular, the SCE defines the lowest possible surface to locate windows and solar collectors so that they are not obstructed by neighbouring buildings, during a given period of the year. This is, in a certain sense, a symmetrical parameter to the SRE: it describes the overshadowing of a neighbourhood on a given site. The SRE and SCE can be interpreted as the upper and lower boundary of a 'solar volume', which represents the portion of space where new developments could be allowed without reducing the solar access of the neighbouring buildings while guaranteeing sufficient solar access to themselves. While seductive from a theoretical standpoint, they are difficult to compute on large urban sites.
Initially, solar envelopes were very difficult to compute and physical models and artificial sun have been applied for simulating solar paths and shadows. Today, solar envelope calculation can be more accurate if we refer to the presented technique based on the image processing of DEMs. In a very fast way we can also increase the accuracy of cut-off times, and defining thresholds on demand. For instance, we can decide to produce solar envelops that take into account all hours of the day, or we can chose to guarantee solar accessibility just for the central hours of a day, where actually the peak of solar irradiation occur; moreover, we can refine the discrete intervals at which we generate solar volumes (hourly steps, or 30 minutes' steps, or even less if needed). See some examples here.


Bibliography

Capuleto I., Shaviv G.E., 1997, "Modelling the urban grids and fabric with solar rights considerations", Solar World Congress, ISES International Energy Society, Korea.

Capuleto I., Yezioro A., Shaviv G.E., 2003, "Climatic aspects in urban design - a case study", Building and Environment, 38, 827-835.

Knowles R.L., 2003, "The solar envelope: its meaning for urban growth and form", Energy and Buildings, 35, 15-25.

Knowles R.L., 1981, Sun Rhythm Form, MIT Press, Cambridge, MA.

Knowles R.L., 1974, Energy and Form, MIT Press, Cambridge, MA.

Pereira F.O.R., Nome Silva C. A., Turkienikz B., 2001, "A methodology for sunlight urban planning: a computer-based solar and sky vault obstruction analysis", Solar Energy, 70:3, 217-226.

Ratti C., Baker N., Steemers K., 2005, "Energy consumption and urban texture", Energy and Buildings, 37:7, 762-776.

Ratti C., Morello E., 2005, "SunScapes: extending the 'solar envelopes' concept through 'iso-solar surfaces'", Proceedings of the 22nd International Conference on Passive and Low Energy Architecture, Beirut, Lebanon.

Ratti C., Richens P., 2004, "Raster analysis of urban form", Environment and Planning B: Planning and Design, 31:2, 297-309.

Szokolay S.V., 2004, Introduction to architectural science: the basis of sustainable design, Elsevier, Architectural Press, Amsterdam, Boston.
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5. Iso-Solar Surfaces          [top]

Solar envelopes at present do not take into account energy considerations. They are defined in terms of discrete numbers of hours of sun or shadow, with little reference to actual radiation or illumination levels. However, an hour of sun at midday has different effects than an hour of sun at dawn. The idea of energy-weighting solar envelopes was hinted at in seminal work by Knowles, but later abandoned due to computational difficulties. In fact, Knowles suggests to weight incident rays of light considering different sun's altitudes during the day, in order to approximately quantify the amount of energy that gets through the atmosphere (1981, p.57).
This limitation mentioned above could be overcome using new techniques for urban analysis, based on the image processing of DEMs. We introduce the concept of 'iso-solar surfaces': 3-dimensional geometric envelopes which receive equal amounts of solar energy.
Two types of iso-solar surfaces can be defined: iso-solar rights surfaces (ISRS) and iso-solar collection surfaces (ISCS).
- Iso-solar rights surfaces are defined as 3-dimensional geometric envelopes which describe the maximum height for buildable volumes to preserve a given amount of irradiation on adjacent sites. Increasing values of radiance can be chosen, each of which will correspond to decreasing buildable heights.
- Iso-solar collection surfaces define the lowest possible surface that will collect a given amount of solar radiation. This parameter is symmetrical to the latter and describes the effect of shadowing on a given site by neighbouring buildings.


Bibliography

Compagnon R., 2004, "Solar and daylighting availability in the urban fabric", Energy and Buildings, 36, 321-328.

Knowles R.L., 1981, Sun Rhythm Form, MIT Press, Cambridge, MA.

Muneer T., 1997, Solar Radiation and Daylight Models for Energy Efficient Design of Buildings, Architectural Press, Oxford.

Noble D., Kensek K., 1998, "Computer generated solar envelopes in architecture", The Journal of Architecture, 3, 117-127.

Tregenza P.R., 1987, "Subdivision of the sky hemisphere for luminance measurements", Lighting Research and Technology, 19:1, 13-14.
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6. The Sky view factor          [top]

As emerged from the study of solar envelopes, not only the indent beam radiation coming from the sun is important when dealing with solar accessibility. In most urban environments also the diffuse radiation from the sky plays a major role. This can be measured with solar geometry formulae or roughly estimated through the Sky View Factor (SVF).
SVFs are a measure of the openness of the urban texture to the sky. If the entire sky vault is visible, the SVF equals 1, hence the exposition of the vantage point to the direct solar radiation is maximal. In fact, the SVF defines the amount of the sky which is visible from each vantage point on the urban texture. For instance, the SVF is a precise estimate of illuminance in urban areas. To obtain urban maps with the SVFs, we distribute several points (illumination sources) on the hemisphere. Depending on the distribution of these points we can refer to different sky models to calculate the sky view factor. To determine the distribution of radiation points on the sky vault we can considerate for example a uniform overcast sky or the CIE overcast sky which follows the function ? = arcsin (z). The second sky is brighter near to the zenith and has lower spots approaching the horizon.
Obviously, the SVF represents a very simplistic indicator for daylight measurements, because it is orientation-insensitive and thus can not predict where the solar radiation exactly comes from. Although, SVFs work well at the larger urban scale where spatially averaged results matter and the city shape itself can be reduced into a unique texture.
SVF reveals itself to be a useful parameter when dealing with the thermal radiation exchanges. It computation can explain some mechanisms related to the Urban Heat Island (UHI) generation


Bibliography

Watson, I., Johnson, G., 1987, "Graphical estimation of sky view-factors in urban environments", Journal of Climatology, 7, 193-197.
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7. Tools and Applications          [top]

A set of tools and applications can be developed in order to quantify the amount of received sunlight and daylight on the urban texture. For instance we propose:

- Shadow Casting
- Mean Shadow Density
- Solar Accessibility on open spaces
- Solar Envelopes as defined by P.L. Knowles
- Solar Rights Envelopes for every day of the year
- Solar Collection Envelopes for every day of the year
- Superimposition of projects to Solar Envelopes
- Iso-Solar Rights Surfaces
- Iso-Solar Collection Surfaces
- Total Incident Solar Radiation on the urban texture


Bibliography

Compagnon R., 2004, "Solar and daylighting availability in the urban fabric", Energy and Buildings, 36, 321-328.

Muneer T., 1997, Solar Radiation and Daylight Models for Energy Efficient Design of Buildings, Architectural Press, Oxford.

Tregenza P.R., 1987, "Subdivision of the sky hemisphere for luminance measurements", Lighting Research and Technology, 19:1, 13-14.
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8. Final Remarks: qualitative and quantitative solar accessibility          [top]

"The functional time period for a solar envelope must be specified in response to a clear purpose. Time periods seem easier to specify for energy purposes than for quality of life. As society increasingly values solar access as a right, however, we may find that our purposes for solar access are more clearly qualitative. This discussion relates useful periods of solar access to energy, which requires definition of the term "useful" in relation to the technology of energy conversion and to the movement of the sun" (Knowles, 1981).
Guaranteeing solar accessibility is fundamental not only for the possibility to install more solar collectors on the roofs and facades of urban buildings, but also for ameliorating psycho-physiological conditions in open and closed spaces. In fact, both aspects of energy performance and environmental quality are important, but the way for matching them is not equivalent. This was clearly shown in the description of solar envelopes versus iso-solar surfaces, where the former count the hours of direct radiation incident on a horizontal surface and the latter compute also the amount of gained energy. Moreover, through ISSs we move the focus from the concept of 'sunlight', used for the calculation of solar envelopes, to the concept of 'skylight', which enables the quantification of energy coming from the unobstructed sky vault.


Bibliography

Knowles R.L., 1981, Sun Rhythm Form, MIT Press , Cambridge, MA.
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