The Solar Pathfinder (starting at $260) is a simpler device consisting of a plastic dome that overlays a sun-path template appropriate for the site. Once the device is leveled and oriented true south, the reflection of objects on the dome can be traced on the template with a grease pencil. Alternately, the Pathfinder can be aligned with magnetic south, and a photo of the reflection can be imported into the SPA modeling software. The software will automatically account for the site’s magnetic declination.
At the site, the assessor can manually calculate the shade derating each month by counting the shaded numbers on the template. Each sun-path on the template shows a number for each half-hour increment of the day. The numbers are the percentage of the day’s total solar radiation occurring during that half-hour window. During the middle of the day, when solar radiation passes through less atmosphere, it is more intense and the percentages are higher. In the early morning and late afternoon, the percentages are much smaller. The procedure is less precise and more arduous than with the SunEye, but gives the site assessor a fairly solid idea of the site’s potential.
Many solar professionals will qualitatively observe the reflections on the diagrams of the Solar Pathfinder to compare the shading at various locations around the same site. For accurate assessment, the site assessor will take a digital photo and import it into the Solar Pathfinder Thermal Assistant Software ($189), which allows users to input various details about the proposed system (number of collectors, collector model, collector tilt and azimuth, size of storage tank, etc.) and the hot water use in the home (i.e., average daily hot water use, delivery temperature) to estimate the system’s monthly and annual production.
Many states’ incentive programs require the use of the Solar Pathfinder or SunEye to verify that a project is properly situated and worthy of funding. Most solar heating professionals use one of these two tools for meeting these requirements and for quantifying the benefit of an SWH system.
As you plan, you can take an approach similar to the pros. Start by using aerial maps and other online tools to get an idea of the site’s overall suitability. If you want more accuracy, consider investing in a solar siting tool. (Local organizations may have the shade-assessment tools available for the public to borrow or rent.)
If you are planning to install the system yourself and have limited knowledge of SWH, you will do well to use an engineered system that has received OG-300 Certification through the Solar Rating & Certification Corporation (SRCC) or the International Association of Plumbing and Mechanical Officials (IAPMO).
These systems have been reviewed and approved by a third party in accordance with national standards and are required to have thorough installation and operation manuals, alleviating some of the challenges that arise with installing and also integrating a new SWH system with your existing water heating system. Specifications for system equipment are included, which will help you determine where this equipment can be placed—or whether it will fit at all.
Finally, you could use the shading assessment to estimate the system output. You could make an investment in a Solar Pathfinder and Solar Pathfinder Thermal Assistant software, purchase another solar heating modeling software, or use free resources for estimating system performance (see "Methods", in HP157).
These approaches are valuable for solar professionals, whose success depends upon providing productive systems for their clients and minimizing trips to the job site. For do-it-yourself projects, these approaches help prevent very unpleasant surprises and maximize the production of a solar heating system.
Vaughan Woodruff is a contractor, engineer, and educator. He owns Insource Renewables, a design/build and consulting firm in his hometown of Pittsfield, Maine.