Photovoltaic Rooftop Market Rooftop solar photovoltaic (PV) systems are becoming a preferred source of energy worldwide. Declining PV equipment costs are reducing end-user kWh cost equivalents to levels below the cost of utility-generated energy in many countries. The future of the global rooftop solar PV market appears to be very bright and promising.
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.
PV solution providers are scaling-up to accommodate rapid market growth, but declining equipment costs decrease PV solution provider per-project revenue and profit. Increased price competition between solution providers exacerbates this problem, because offering a lower price (with lower-margin) is often needed to win the deal. Incorporating TDDs into a PV system design reduces this negative impact on profitability as discussed in the following sections of this article.
Tubular Daylight Devices Tubular Daylighting Devices, or TDDs, are high-performance 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. TDDs are 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. The components in each zone incorporate designs that consistently deliver the maximum amount of “good” visible light while excluding damaging UV light as well as excluding excess heat from infrared light. TDDs are designed to deliver stable light levels throughout the day and throughout the seasons to provide a consistent daytime lighting environment to building occupants. TDDs are sealed, require no power to operate, do not have moving parts, and do not require maintenance.
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 available for PV panels. This property can enhance the value of a PV system implementation as discussed in the following sections of this article.
Roof Space Limitations with Traditional Skylights Available rooftop space is often inadequate to install a PV system that meets all commercial end-user energy need. Contributing factors to this limitation are 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. The photo above is a commercial rooftop installation with traditional skylights that demonstrates this constraint.
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. The green area in the example schematic layout to the right demonstrates this limitation. The green area 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.
Utilizing TDDs to Make More Roof Space for PV Panels TDD light-collecting domes can be grouped-together as shown in the photo below. The second photo below demonstrates how 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.
Reducing the Installed Cost/Watt of PV Roofs with traditional skylights generally require asymmetrical PV layouts and such asymmetrical designs tend to cost more due to the additional racking, wire and labor required. Large, symmetrical PV arrays tend to have a lower total cost per installed watt for this reason. Incorporating TDDs into a PV design enables the design of more-symmetrical PV panel layouts which reduces system cost. Additional cost savings can increase the economic benefit to both the end-user as well as profit margin to the system provider.
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. HVAC equipment consumes energy to remove this waste heat from a building interior. 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.
PV and TDD Financial Return Similarity Adding TDDs to a rooftop PV implementation extends economic benefit to the building owner with financial returns comparable to PV. The figure to the right compares financial returns between typical rooftop PV and TDD deployments. 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 decision process for both PV and TDD systems. Economic metric similarity also makes it possible for the PV solution provider to communicate a TDD value proposition to an end-user in a manner similar to PV.
Global Economic Benefit Parity 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 benifit 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).
Summary Combining TDDs with rooftop solar PV systems generates additional financial value to both the PV solution provider and the end-user. The basis for this additional value is that TDDS make more roof space available to install more PV panels, more symmetrically. More-symmetrical PV panel arrays decrease PV system cost. A larger PV system generates more revenue for the PV solution provider and more energy savings for the end-user. TDDs save money for end-users by reducing electrical light usage, deferring electric light fixture replacement and reducing HVAC energy usage. The financial returns from a solar PV or TDD investment are similar.
