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The Dark Side of Trees

July 2, 2013

Solar is in our near future and it’s in the news. Its huge potential impact on the existing grid and power industry was recently discussed on Grist. Likewise, Solar Today, the magazine of the American Solar Energy Society (ASES), published an article in their May/June edition on preserving solar access on properties with solar array installations.

Clearly if you’ve spent a few tens of thousands implementing a large photovoltaic (PV) array on your home or business, you’ll want to protect the solar energy it’s there to collect. But in most places, including my home city of Portland, Oregon, height restrictions on buildings don’t currently apply to trees. In fact our city government protects its tree citizens as much or more as it does its human constituents. (I am an avid “tree hugger” myself and happy to live in our fair green city.) Indeed, solar farms exact a heavy cost on surrounding ecosystems, as is well expressed in a recent article posted on Red Lodge Clearinghouse.

Spherical Geometry for Solar

Solar access depends on spherical geometry while height restrictions are Cartesian based.

For now, lets assume solar panels are great, that they should have precedence over neighbors’ buildings and trees. Even so, imposing flat height restrictions to protect solar access is an oversimplified approach to the much larger and more complex problem of “rights to light” or “rights to view” laws. That’s because height restrictions are a Cartesian coordinate solution to the spherical geometry problem of view and solar access.

In other words, a height restriction places a plane at a fixed height above a site using a local, flat earth view of the world. However from that same local point of view, solar position and field of view are actually windows onto a sphere surrounding the viewer. If, for example, your neighbor plants a 50 foot tree on the far side of his lot where it casts no shade on your PV array, why should you impose a restriction on him based on the twenty foot tree limit you need on your side of his lot?

A flat height restriction on your neighbor does not accurately reflect account your solar access needs.

A flat height restriction on your neighbor does not accurately account for your solar access needs.

For these reasons, the concept of “solar envelopes” developed in the 1980s by Ralph Knowles of the University of Southern California School of Architecture, is a much more interesting and effective solution to the problem of solar and view access. Dr. Knowles defined shadow “fences” or lines which could not be shaded by adjacent structures or trees. He then geometrically projected these fences outward to restrict the edges and heights of adjacent structures. Or trees.

Solar envelopes may be used to concentrate building volumes to the south or center of their sites, thus allowing greater access to the sun for their neighbors to the north. It’s easy to see why this is a better approach for urban planning that will increasingly need to accommodate solar access.

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