Building owners and managers looking for reliable ‘green power’ are turning to turnkey rooftop photovoltaic systems and power purchase agreements that allow them to lower their energy costs while reducing their carbon footprint.

Such an arrangement was announced in April by Tremco Roofing and Building Maintenance, which has joined Smart Energy Capital, a solar finance, development and advisory firm, to give building managers flexibility in choosing and financing a customized rooftop photovoltaic system that is right for them. The systems are designed to ensure smooth functionality between the roof and the PV equipment.

SEC finances the systems and will sell power to the customer under a long-term power purchase agreement. It also helps customers get the full benefit of all applicable state and federal financial incentives, such as the solar investment tax credit.

“This is an excellent option for those who want to move toward ‘green power’ at minimal risk,” said Brian Weisman, managing partner of SEC. “They do not need to commit any up-front capital, nor do they have to deal with the hassles of building and maintaining the PV system. The PPA is a hedge against future electricity rate increases.

“The combination of our finance and solar industry experience together with Tremco’s roofing and service capabilities will positively impact the growth of the market for distributed solar generation.”

Ralph Velasquez, director of Tremco Sustainable Technologies Group, said the relationship is beneficial in many ways. “Customers can select the rooftop photovoltaic system that best matches their requirements. They do not need to worry about installation, maintenance, upgrades or other concerns common to PV systems. That’s all handled for them.”

Photovoltaic systems convert sunlight directly into electricity. They work whenever the sun is shining, when sunlight striking the semiconductor material in a solar panel frees electrons and captures them in an electric current. The more intense the sunlight striking the panel, the greater the amount of electricity produced.

According to the Solar Energy Business Association of New England (SEBANE), the solar cell is the basic block of PV technology. Solar cells are aggregated together to form a PV module or panel. One or more panels are ganged together and connected to an inverter, which converts the direct current (DC) produced by panels into the alternating current (AC) used by electrical devices in the United States and supplied by electric utilities.

PV Electricity Production

Electricity production from PV systems is a function of PV panel (or arrays of PV panels) orientation and DC to AC conversion losses. In Massachusetts, according to SEBANE, an average 1-kilowatt (DC) of PV, at the optimal orientation and tilt for maximum annual production, can produce between 1,000 and 1,500 kilowatt-hours of electricity annually.

Of course, PV systems only produce electricity when the sun is shining. However, this is not a problem for grid-connected installations where any electricity demand that exceeds on-site production is automatically met by electricity delivered by the serving utility, day or night. For businesses, PV-generated power is available during daylight hours when businesses usually experience their highest electricity consumption.

System Sizing

Larger systems are somewhat more cost effective than smaller systems due to economies of scale associated with system design, installation, and interconnection. A good rule of thumb, says SEBANE, when sizing systems is that 1 kilowatt of PV requires 100 square feet of unobstructed roof area.

PV systems are easily installed on the flat roofs typical of commercial buildings, using racking systems for panel mounting. The panels can be installed on different roof surfaces, including shingles or membranes, tar, and pea gravel.

Effective placement of modules requires areas of unobstructed roof surface. “Unobstructed” means without chimneys and other roof vents, rooftop HVAC systems, and hatchways that cannot be blocked. Also, building systems and architectural elements on the roof (such as chillers and parapet walls) that can shade nearby PV modules are considered obstructions in that they prevent installation of PV panels in those shaded areas.

Siting for Maximum Production

Solar panels generate electricity at their rated output intermittently, only when the sun is shining. And because the sun moves across the sky at varying heights from sunrise to sunset, and from season to season, the amount of electricity generated by a module varies during the daylight hours and over the course of the year.

PV installations typically are “stationary” and do not follow the track of the sun. Furthermore, they are generally “fixed” installations that are not adjusted to account for changes in sun angle from season to season. (A cost-effective design that increases performance by tracking the sun’s movements and/or seasonal adjustments have not yet been invented.)

Therefore, to maximize the production of electricity, SEBANE says the design of individual PV installations must consider (and optimize) the factors of shading, orientation, and module tilt.

Shading

The system design should avoid placing solar panels in any area that is shaded at any point during the day. The only exceptions are up to 90 minutes after sunrise in the morning and before sunset in the afternoon.

The most common features that cause shading are trees, other buildings, and telecommunications or HVAC systems. PV systems are designed to avoid panel-to panel shading except near sunrise or sunset. South-facing is best to maximize the panel’s annual power production, but you can still get up to 95 percent of optimal production even if your roof faces Southeast or Southwest.

Tilt

For maximum annual generation at latitudes in Massachusetts, SEBANE says a solar array should be installed at about a 33-degree angle to the ground. For maximum summer generation, a solar array should be installed at about an 18-degree angle to the ground. Even if you place modules flat on a roof, they will produce up to 80 percent of optimal generation. Most designs allow a slight angle to promote array self-cleaning and cooling of the panels, which improves their performance.

Installation Considerations

Beyond the questions of system orientation and tilt, the existing condition of the roof, including its structural integrity, is perhaps the most important planning consideration with regards to installation.

PV systems can be installed on any type of roof, with necessary care taken to insure that any penetrations of the waterproof membrane do not result in leaks.

Installation over common EPDM rubber membrane roofs has proven to be very effective. If the building roof is older and will need to be replaced in the foreseeable future, it may be sensible to replace it in conjunction with the PV installation to avoid the trouble and expense of removing and reinstalling the PV system later. There also may be economies in completing both jobs at once.

Regulatory Considerations

Electric grid: While use of PV for off-grid electricity generation is cost-effective in areas where it is impractical or uneconomical to connect to the electric grid, electricity customers with renewable energy generation systems are allowed to interconnect with the grid and purchase whatever additional power they need from their electric distribution company, according to SEBANE.

Some states allow customers with PV systems of a designated size to sell excess power back to their utility and receive a credit for power produced.

This practice is called “net metering.” The customer is billed for the “net” electricity purchased from the utility over the entire billing period, which is the difference between the amount of electricity delivered from the power grid and the electricity generated by the PV system.

Utilities are prohibited from imposing special fees on these customers, such as backup charges and demand charges, additional controls, or liability insurance, as long as the generation facility meets established interconnection standards and all relevant safety and power quality standards. ❑