Maximizing Roof Space For Solar PV

Compared to traditional skylights, tubular daylighting devices provide more “roofing real estate” for solar panel installations.

By Todd Maerowitz
From the April 2018 Issue

Rooftop solar photovoltaic (PV) system installations are expected to increase worldwide, over the next several years. In many countries, declining PV equipment costs are reducing end-user kWh cost equivalents to levels below the cost of utility-generated energy. A recent report from Transparency Market Research titled “Rooftop Solar PV Market – Global Industry Analysis, Size, Share, Growth Trends, and Forecast 2015 – 2023” projects that the global rooftop PV market will grow to $4.58 billion by 2023. North America currently accounts for the largest share of the global rooftop PV market, but growth rates in the Asia Pacific region are predicted to outpace all other regions to represent $1.09 billion of the global rooftop market by 2023; India and China are expected to be the main drivers of this growth.

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Figure 1: (IMAGE: Courtesy of Solatube International)

Combining tubular daylighting devices (TDD) with rooftop solar PV systems generates additional financial value for facility managers and building owners. The basis for this additional value is that TDDs make more roof space available to install more PV panels, and a larger PV system generates more energy savings for the end-user. TDDs also save money for end-users by reducing electrical light usage, deferring electric light fixture replacement.

TDDs are optical lighting solutions that bring daylight into buildings and can be an especially effective solution for interior areas where traditional skylights and windows cannot reach. Sometimes called “tubular skylights,” “light tubes,” “sun pipes,” and even “light tunnels,” TDD technology is composed of three zones that capture, transfer, and deliver daylight (see Figure 1). TDDs are designed to deliver stable light levels throughout the day and throughout the seasons to provide a consistent daytime lighting to building occupants.

TDD technology can direct light through long tubing runs that includes multiple angle bends. This allows the exterior light-collecting domes to be clustered into groups that leave more rooftop space open and available for PV panels. This property enhances the value of a PV system implementation.

Rooftop Space Limitations Using Skylights

Available rooftop space is often inadequate to install a PV system that meets all commercial end-user energy needs. A contributing factor to this limitation is that most building codes require commercial rooftops to include skylights and that building codes also require a 4-foot setback from each skylight as well as a 6-foot-wide access path to each skylight. This constraint dramatically reduces available space for PV modules.

Traditional skylights are positioned directly over the interior areas they illuminate, so their layout is generally dispersed uniformly over the roof area. This positional constraint of traditional skylights makes it necessary to fragment the PV panel layout on the roof. In Figure 2 below, the green area in the example schematic layout demonstrates this limitation. The green area in this figure is the maximum space available for a PV system with a typical traditional skylight layout. About 67% of the roof area is available for PV in this example.

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Figure 2: (IMAGE: Courtesy of Solatube International)

TDDs can be included in such a project to make more roof space available for PV panels. TDD light-collecting domes can be grouped-together, and grouped TDDs can be configured to direct light through reflective tubing to where it is needed inside the building. Unlike traditional skylights, TDDs do not need to be positioned directly over the interior area they illuminate. The rooftop domes of TDDs can be concentrated in smaller roof areas to make more space for PV panels.

The schematic layout in Figure 3 demonstrates the use of TDD with a PV installation. The green area shows the space available for a PV system with TDDs along the edges of the roof. About 79% of the roof area is available for PV modules in this example, which corresponds to an additional 17% of available area for a PV installation versus the traditional skylight example in Figure 2.

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Figure 3: (IMAGE: Courtesy of Solatube International)

The additional available area created by TDDs provides a number of benefits. A larger PV system generates additional kWh savings for the end-user; a pro-forma comparison between the schematic layouts in this article result in an additional energy savings of $253,705.

Basis For TDD Operational Savings

TDDs save electric lighting expense in a straightforward way. As daylight levels increase in the morning, interior electric lights are dimmed or switched-off, typically with an automation system. As daylight levels decrease in the evening, the lights are brightened or switched on.

Electric lighting wastes around 70% of input kWh as heat generated inside the building. HVAC equipment in needed to remove this heat energy added to the building. Deploying a TDD system enables the end-user to turn off electric lighting systems to reduce waste heat added to building interiors and reduce the corresponding HVAC energy consumption to remove the waste heat.

Electric lights have a finite operational hour lifetime. Daylighting with TDDs reduces the number of hours electric lights are used each day. This reduced electric light usage adds up and extends electric light calendar life. With a longer calendar life, costly lighting equipment replacements can be avoided or deferred to create additional savings for the end-user.

Adding TDDs to a rooftop PV implementation extend the economic benefit to a building owner with financial returns comparable to PV. Both TDD or PV systems generate similar financial return for the end user with paybacks as short as four years and rates of return up to 40% per year. Economic metric similarity makes it possible for an end-user to apply a similar economic selection decision process for both PV and TDD systems.

Also notable is that the similarity of financial return between TDD and PV technology is global. When both are deployed in the same location, anywhere in the world, they both generate similar financial returns. This is because the economic benefit from both technologies is driven by the same factors: available sunlight (same in one location), capital expense for equipment (similar for each technology throughout the world), and utility kWh energy cost (same at one facility).

Facility managers and building owners will find that combining TDD with rooftop solar PV generates additional financial value for a renewable energy project. Because TDDs make more roof space available for more PV panels, a larger solar energy system can be built to deliver more energy savings for the end-user.

Maerowitz, general manager at Solatube International, is responsible for the Vista, CA company’s international business. He previously worked in the solar PV industry for 15 years where he developed more than 500 MW of solar projects—including the largest commercial rooftop installation of its time at the Google global headquarters.

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