Solar Electric Design Basics
Solar electric system design is very region-specific, as the ideal angle and orientation varies based on geographic location. Shading is a universal concern, as are practical considerations such as which type of solar collector to choose for a project based on engineering needs and project budget. This is far from an authoritative guide for solar designers, but is intended to help homeowners and others researching a solar investment.
Note: Most of these principles apply equally to solar thermal systems as well, though the influence of various factors on production will be different based on technology.
- Angle and Orientation
- Shading (Solar access)
- Practical considerations (project budget, collector performance)
Angle and Orientation
Obviously, if you plan on collecting solar energy, you need to orient said collectors towards the sun. In the northern hemisphere, that means orienting solar collectors as close to south as possible. Due to magnetic declination, when doing a compass reading you want to adjust for “true” south. In Maine and New Hampshire, that is about 15 degrees west of south, or 195° on a compass.
Now, if your roof faces east or west, it is not the end of the story. Grid tied solar photovoltaic arrays will produce as much electricity as they possibly can whenever the sun is available (compared to an off-grid array, which has an output limited to the battery’s storage capacity). This means that an easterly-oriented array will tend to produce a bit more electricity in the morning hours, and a westerly-oriented array will tend to produce a bit more in the afternoon. For most rooftop slopes, being off-axis up to 45 degrees will result in a production loss of under 10%. More extreme west or east facing installations face production loss of 30% or more.
Roof angle (slope) is also important, as the sun’s position in the sky varies throughout the year. In the summer, the sun is higher in the sky (a shallower pitch is better), and in the winter, the sun is lower in the sky (a steeper pitch is better). While advanced technology such as dual-axis solar trackers allows arrays to be at an ideal angle/orientation at all times, for practical considerations rooftop fixed-mount arrays tend to offer the best economic performance (more on that in a moment).
Additionally, a great tool for understanding the movement of the sun throughout the year is via the The Nebraska Astronomy Applet Project’s Motions of the Sun Simulator.
Shading (Solar Access)
Of great concern in solar electric system design is solar access, or, the effect of shading by trees or other obstructions on the array. Ideally, we like to see the solar window of 9am-3pm completely shade-free on a prospective array site.
While experienced solar designers can get a sense of a site’s viability by eye, the only reliable way to assess solar access for a site is with a professional tool such as a SunEye or Solar Pathfinder. These tools take a fish-eye photo of the open sky, and then super-impose the sun’s arc (adjusted for longitude) onto the open sky photo. The results can be interpreted by software to develop a percentage figure of available solar access at a site; generally greater than 80% is required and we prefer to see access that is 90% or more.
Technological advancements like micro-inverters, DC optimizers, as well as bypass diodes and dual-MPPT on string inverters means that arrays are more shade-tolerant than they have been in the past. However, there is no replacement for simple access to the sun. For the record, should trees need to be removed prior to a solar installation, in most situations the environmental benefits of the solar panels will far outweigh the toll of having the trees removed (especially so if the trees are used as biomass for heating or sawn into useful lumber).
While the above two sections are hard and fast physics, it’s impossible to close a section on solar electric system design without talking about the ‘real world.’ In the real world, homeowners and businesses face choices about where to purchase their energy and what kind of solar array makes sense. So when we discuss solar with a prospective customer, we need to know a lot about them in order to design a system. For example:
- Is a rooftop array feasible? If so, how much roof space is there? If not, is a ground mount feasible?
- Beyond pure production economics, what other factors are important? Aesthetics? Carbon reduction?
- What is the rate currently paid for electricity? How much electricity is used per month?
- Has this building had efficiency improvements? Is there room to decrease their need for energy inputs?
- How many people live in the home? How much hot water do they use? How is that hot water heated?
In the real world, solar electric systems are designed around three aspects: solar potential (determined by physical size of viable solar roof or ground options), load (how much solar is necessary to meet current/future demand), and budget.
For example, a homeowner that has limited viable solar roof space faces a different set of decisions than a homeowner that has a large south-facing barn in an open field. A seasoned solar designer does not have a ‘one size fits all’ strategy nor are they pre-disposed towards a single technology. For example, a solar hot water system may be the better investment for the homeowner with limited space (two 4×8 solar thermal collectors can provide 80% or better of a home’s domestic hot water supply) while the homeowner with vast open roof space may prefer the simplicity of a single technology and oversize a PV array to meet their domestic hot water needs (using an efficient electric tank or heat pump water heater).
Similarly, we are often asked the pros/cons of installing a ground mounted array vs. a fixed rooftop array. Does a perfectly oriented fixed ground mount array outperform a non-ideal solar array? Yes. Is it necessarily the better choice? No. Walking through the nuances of system electric system design is a key part of our work as we help prospective solar customers understand their options.
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