Planning for Solar Access

ABSTRACT
    Land use planning is often no more than a strategy for getting the most economic value from a given piece of land while attempting to minimize the expense of mitigating environmental impacts. Because conventional fuels are currently inexpensive, development takes place with little consideration for energy efficiency or solar access. By identifying sites with solar access now, planners and building designers can assure that the south facing roofs and walls of future buildings will receive the solar energy necessary to satisfy the energy needs of their occupants.
    Planner Ian McHarg outlined scientific methods to map natural determinants including geology, hydrology, wildlife, soils, climate, as well as social determinants like population density, energy consumption, and transportation patterns in the late 60s. Overlays of these hand plotted maps gave planners a method of finding the most suitable sites for identified needs but were cost prohibitive for most projects. The affordability of studies done on high speed computers now allows general application of McHargian planning methods.
    At the time McHarg was developing his planning theory photovoltaics were so expensive their use was restricted to government projects. The low cost of heating with fossil fuels made solar heating expensive by comparison. Today advances in solar technology have made solar electricity and heating competitive with non-renewable energy resources. This paper will show how McHargian planning methods can be effectively used in conjunction with Geographic Information Systems (GIS) to include efficiency and renewable energy access in land use decision making.

1. INTRODUCTION
    Resource depletion and the environmental costs associated with using cheap conventional fuel make a new approach to planning necessary. Air pollution, water pollution and global warming are the consequences of short term planning methods. Any future approach to planning has to be based on the maintenance of the earth's life support system, which means that fertile soils, clean water and clean air must be sustained. It is essential that renewable sources including direct solar energy, wind and micro-hydro power are considered for use to satisfy energy needs.
    Planning is currently done in a reactive way. A developer presents a proposal and then the planners suggest mitigating measures. If the use of renewable energy sources is a goal, however, then the availability of these energy sources must be mapped so developers will include this information in their decision making.
    Computer analysis of digitized topographic maps used in conjunction with McHargian planning methods offers land use planners and building designers a reliable way to determine renewable energy access. Slope, orientation, and shading from vegetation, as well as existing structures, can be evaluated individually and combined in an overlay that will point out preferred building locations.
    Solar designers and planners have been familiar with shading studies and computer analysis for quite some time but these tools are not part of the mainstream architect's or planner's repertoire. Along with educating the mainstream about the necessity of solar access, tools need to be developed that make this kind of planning simple and economical.

2. GEOGRAPHIC INFORMATION SYSTEMS (GIS)
    The use of GIS can make consideration of renewable energy options a relatively simple process. A Geographic Information System is a computer program which can store, retrieve, analyze and display cartographic data. Satellite images and digitized maps are fed into the computer where separate layers can show individual themes. For instance, a land use study can be done using GIS to show individual themes like slope, orientation, vegetation and shading. When combined in an overlay the best solar building sites can be determined. GIS can also show features like highways, urban boundaries, lakes, rivers and floodplains.

3. SOLAR POTENTIAL
     Every day 1 million terawatt hours of solar energy strikes the earth. This is equal to the total amount of potential energy stored in the entire world oil reserve. Because solar energy is a very dispersed source it is important to identify sites that have unobstructed access. The south facing roof and walls of a 100 m2 building absorb an average of 10,000 Btu/m2/day in the Continental U.S. Solar radiation transmitted through double glazed south facing windows can satisfy all space heating needs in a building in the winter. Heat gain in the summer can be eliminated with proper shading. In combination with passive cooling strategies the need for air conditioning can be totally eliminated.
    On a clear day with unobstructed access 1 kW of energy is available per m2 of surface area. Photovoltaics have efficiencies of 10% to 15%. This means that using the low figure of 10% a 10 m2 surface area can produce 1 kW of electricity. A typical residential building with 50 m2 of south facing roof could provide 25 kWh/day in the summer and 5 to 10 kWh/day in the winter. Since the average home consumes about 15 kWh/day, a properly oriented roof can provide a net surplus of electricity. Passive solar heating and cooling can reduce residential electricity consumption to about 5 kWh/day.
    Solar energy can also heat liquid for space heating, i.e. radiant floors, and provide domestic hot water. Depending on the climate, thermal collector area equal to 10% to 30% of the floor area in conjunction with enough thermal mass can provide enough radiant heat to maintain comfort throughout the cold season. Mounting approximately 5 m2 of thermal collectors on a south facing roof can satisfy the majority of the hot water needs of the average residence.
    The obvious prerequisite for solar electricity and heat is solar access. A building in the sun can act as a life form in symbiosis with its inhabitants. A building on a north slope in shade all winter must rely on energy produced from consuming fossil or nuclear fuel. The consumption of both these fuel sources compromises the earth's ability to support life. Fig. 1 shows how north slopes are shaded in the winter and do not receive the benefits of solar gain during the coldest time of the year. The concept of solar orientation is easy to see in a section through an east-west ridge.

Figure 1. Section through east-west ridge

3 APPLYING GIS TO SOLAR ACCESS
    The following land use study was done using GIS to isolate solar building sites. The project to build an environmental education center with facilities for 100 students and 15 permanent staff is located on a generally south facing 128 acres (50 hectare) site with an 800 ft. (244 meter) elevation drop. The intent was to meet all energy needs with solar, wind, and micro-hydro power. After identifying program needs, four maps were produced that are based on a digitized topographic map, which had been generated from an aerial survey, Fig. 2. The four maps show slope, orientation, vegetation with shading, and a composite.

Figure 2. Digitized GIS Topographic information

3.1 Slope
    The soil and geology of the site determines what slope will allow construction using a conventional foundation. In this case Yorkville soils were present, which are very unstable, and made building on more than a 10% slope structurally challenging and very expensive. Consequently the slopes of less than 10% were isolated in an overlay, Fig. 3. The degree of slope is expressed in different levels of hatching, i.e. 10% of slope or less showed up as white space, slopes over 50% showed up as black space.

3.2 Orientation
     A level or south facing slope provides the best solar access for heat gain and solar electricity in the winter when the sun is low above the horizon. Consequently areas that are level or slope within 15 degrees east or west of due south were isolated. These areas show up on the overlay as white space, east or west facing slopes are cross hatched, Fig. 4. North slopes would have shown as black and no building would be allowed in those areas. There were no north slopes on the site.

Figure 3. Slope

Figure 4. Solar Exposure (Orientation)

3.3 Vegetation and Shading
    Shading caused by vegetation can partially or totally obstruct solar access. It was therefore important to identify the climax height of the site's different plant communities and show the areas that will be shaded for a predetermined portion of the day during the winter months. Grassy areas or those without vegetation are shown on the overlay as white space. Vegetation is shown in a shade of gray. The shadow cast by the vegetation in December and January for an hour-and-a-half either side of solar noon is shown hatched, see Fig. 5.

Figure 5. Vegetation and Shading

3.4 Composite
    The composite overlays the three previous maps to reveal the best building sites for solar access, see Fig. 6. Unsuitable building sites show up as dark areas. The lighter the area, the better the building site. GIS eliminates the arbitrary nature of building site selection and provides a scientific basis for decision making. The composite also includes features like existing buildings, roads, trails, property lines, viewsheds, streams, ponds and numerous other characteristics of the site that are very important in land use planning.

Figure 6. Composite of all plots

4. WIND POWER
    Wind speed data is usually recorded at airports, but that data is very unreliable since the wind's energy potential is site specific and dependent on geography. The application of GIS to analyze the relationship between geography and average wind speed would allow more accurate predictions about the wind power potential of a specific site. An average wind speed of 12 miles/hr. (19 km/hr.) is necessary to justify the expense of a wind energy system. Wind energy compliments solar energy well in most areas because high winds usually coincide with storms.

5. MICRO-HYDRO POWER
    Micro-hydro power generation is an inexpensive compliment to solar energy. The overall size and cost of solar electric installations can be reduced when hydro electricity is available for use during overcast and rainy periods. Hydro potential is directly related to the geography of an area and the amount of precipitation and soil saturation. GIS can be used to evaluate specific watersheds for runoff and the difference in elevation from where water can be collected and where a turbine can be located.
    The amount of hydro energy available is a function of the change in elevation, called head, and the gallons per minute available at the turbine. Approximately 1 kWh/day is available when the product of the head in feet and gallons per minute equals 200 (head in meters x liters/meters = 250). For instance, 10 gallons/min. (38 liters/min.) falling 20 feet (6.5 meters) for 24 hrs./day will produce approximately 1 kWh of electricity. Mountainous areas with high rainfall should be analyzed by GIS to determine hydro potential.
    A hidden benefit of micro-hydro is that water, which would otherwise cause soil loss on steep slopes, is directed through pipes and its force is used to produce energy rather than cause erosion.

6. CONCLUSION
    The tremendous potential of renewable energy could be realized if GIS was used as a proactive planning tool. If planning agencies identified renewable energy sites through the use of GIS and other tools and made that information available to prospective builders and developers, then the likelihood of solar, wind and water energy use would increase.
    The GIS industry is constantly growing and expects to steadily increase its presence in a multitude of professions. But while use of GIS appears to become a commonplace tool in disciplines such as forestry, catchment management, geology and mining, it is not widely used by building professionals and in the renewable energy field. Solar and renewable energy professionals and advocates should let GIS software providers know how important it is to develop programs specific to renewable energy applications. The development of such programs would not only benefit the GIS industry economically but more importantly open new doors for sustainable development practices.

6. REFERENCES

1. Itami, Robert M., Raulings Robert J. SAGE Introductory Guidebook, DLSR, 1993.
2. Wright, Robert M., Hoinkes, Rodney M. Proceedings of CAAD Futures '93, Elsevier Publishers B.V., 1993.
3. Outlook '97, GIS World, Pearson, 1997
4. Stephen Heckeroth. Solar Charging. January/February Solar Today, American Solar Energy Society,1996
5. Heckeroth, Stephen, McLees, Christiane. Passive Solar Design. Proceedings of The 19th National Passive Solar Conference, American Solar Energy Society, 1994.
6. Swan, Christopher C., Suncell - Energy, Economy & Photovoltaics, Sierra Club Books, 1986.

Steve Heckeroth & Christiane McLees Homestead Enterprises: Solar Design and Renewable Energy Products Box 151 Albion, CA 95410 phone/FAX 707-937-0338     EMail Us!    solar@renewables.com