Enhancing Value of LED Commercial Lighting Investment with Tubular Daylighting Devices

Light output of LED equipment decreases with use, so expensive replacements or design augmentations are required to maintain target interior light levels. Adding tubular daylighting devices (TDDs) to a building lighting design can reduce the replacement frequency of LED lighting equipment as well as reduce the energy cost to light a facility. Another benefit is that full spectrum visible light provided by TDDs improves human performance, which can increase productivity and profit. This article discusses the basis for these benefits and the financial return on investment by adding TDDs to a building LED lighting design.

LED Lifetime

Image courtesy of Hong Kong Double Light Electronics Technology Co.

Light Emitting Diodes (LED) have become a well-known and reliable technology. LED provides major advantages over conventional technologies such as incandescent, halogen, and compact fluorescent lamp (CFL) lighting. LED is more energy-efficient than these conventional technologies—a critical edge considering that more than 50% of the total cost of lighting stems from energy consumption. In the next few years, the shift toward LED technology is likely to accelerate.  Navigant Research projects that close to 74% of all new and replacement light-source unit sales will be based on LEDs by 2023.

According to claims by various manufacturers, LED lighting has a life expectancy in the range of 35,000 to 100,000 hours. This is roughly 30-100 times longer than incandescent bulbs and 4-10 times longer than compact fluorescents. Lifetime estimates are vague as they are conducted at the component level rather than at the system/luminaire level. The brightness of an LED decreases over time. This decrease is typically quoted as a L70 value, meaning the number of hours that the light will keep at least 70% of its original output.

The L70 lifespans quoted by manufacturers are generally based upon projected lumen depreciation of the LED chip and not all components of the luminaire. Complete LED luminaires incorporate a number of components—including power supplies and lenses whose light delivery capability can degrade with time. The plastic lens cover can become cloudy over time or the gel-filled capacitors in power supplies can dry out and fail. Because the performance of these additional components decline, actual hours of luminaire life can be significantly less than the specified L70 for the individual LED chip. L70 values claimed by cheaper manufacturers have sometimes been made without independent validation, so the 50,000 or 100,000-hour lifetimes stated by such suppliers may be wishful thinking.

LED lighting is sometimes installed in environments that exceed the maximum ambient temperature specified by the manufacturer or with noncompatible control systems. These issues result in shorter lifetimes and some consumers report that large LED systems purchased needed replacement long before the lifetime hours quoted by the supplier. High-powered LED lights have not been in the market for 20 years yet, so it is difficult to verify the accuracy of manufacturer or consumer claims.

Integrating TDDs with LED Lighting

Tubular daylighting device (TDD).

FIGURE 1: Tubular daylighting device (TDD).
Image courtesy of Solatube International

Tubular daylighting devices, or TDDs, are high-performance lighting solutions that bring daylight into buildings 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 or deliver light. 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. Light levels delivered by TDDs are stable throughout the day and throughout the seasons to provide a consistent light environment to building occupants. TDDs are sealed, require no power to operate, do not have moving parts and do not require maintenance.

LED Light Energy Savings

TDD deployments can avoid energy consumption needed to power LED lighting during daylight hours. The savings mechanism is straightforward. As sunlight levels increase in the morning, the LED lights are dimmed or switched-off, typically with an automation system. As sunlight levels decrease in the evening, the LEDs are brightened or switched on. A TDD deployment reduces the energy consumed by LED lighting.

Deferred LED Retrofit Savings

Energy analysis software uses this LED energy savings as the primary economic benefit of combining TDDs with LEDs. Such software typically ignores the additional economic benefit of deferred LED replacement expense. These additional benefits increase economic performance.

Manufacturers rate LEDs for a finite number of operational hours before their light output degrades to a level where the LEDs need to be replaced. A lighting replacement is often performed once the light level drops below 70% of the original specification (L70). The L70 is more of a performance specification than a product specification. For example, the LED system designer can specify more than the required amount of light output (e.g. 130%). Then, to keep the initial light level at 100% of specification, the designer adds special power supplies or lighting control systems to adjust the light level as LED output declines over time. System designs with L70 values of 100,000 hours are possible this way.

Integrating TDDs with LED can reduce long-term lighting expense. System designs with longer L70 values are often more expensive, because of additional cost for more lighting capacity, special power supplies and/or more control components. Combining TDDs with a simple LED design can be more economical than implementing a complex LED design without TDDs.

TDDs reduce the number of hours that LEDs are operated each day. Over the long term this reduction can defer, or even avoid, expensive LED equipment replacements. The diagrams to the right depict an example of LED light output degradation over time with, and without, TDDs.

FIGURE 2a: LED light output degradation over time without TDDs. FIGURE 2b: LED light output degradation over time with TDDs.

Assuming a L70 of 50,000 hours and a 12-hour-per-day lighting requirement and a typical sunlight source, an LED installation will require one replacement over a 20-year time period. When combined with TDDs and the right type of power supply, no LED replacements are needed over the same 20-year time period.

Light bulb's contribution to heat transfer.

FIGURE 3: Turning-off LEDs systems reduces the waste heat added to building interiors.
Image courtesy teachengineering.com.

TDDs help reduce HVAC heat energy.

FIGURE 4: TDDs enhance the value of a HVAC investment.
Image courtesy twentyonecelsius.com.

LED Heat Energy Savings

A TDD deployment saves additional kWh energy by reducing HVAC operational hours to remove waste heat created by electric lighting.  An LED system wastes around 70% of input kWh as heat generated inside the building. LEDs are more efficient than most lighting devices in converting electricity into light, but like all electric lighting devices, LEDs convert a portion of the kWh energy they consume into heat.

As depicted in the conceptual figure to the right (with incandescent light), HVAC equipment removes heat energy added to the building due to electric light operation. Turning-off LEDs systems reduces the waste heat added to building interiors and the corresponding HVAC energy consumption to remove it.

In addition to energy savings, less HVAC use extends HVAC calendar life—thereby deferring the date of expensive HVAC equipment maintenance or replacement. TDDs enhance the value of a HVAC investment in this way. It is important to note that HVAC savings with TDDs occur in warm climates where HVAC is primarily used to cool the building during occupied hours.

Firm Economic Benefits

Financial returns of using TDDs.
TABLE 1: Financial returns.
Lifetime financial contributions of TDDs
FIGURE 5: Lifetime contribution to savings.

With the right site conditions TDD deployments can generate internal rates of return (IRR) over 40% and paybacks shorter than four years. The table and figure below details the financial returns at a large facility. The Equivalent kWh Cost of deploying TDDs can be substantially less than utility rates—in this case five-times less. The “Equivalent kWh Cost” is the TDD system Purchase Price divided by the lifetime kWh savings.

Deploying a TTD solution in conjunction with an LED deployment saves money, is an attractive short-term investment and an even-more attractive long-term investment.

Soft Economic Benefits

TDDs provide additional economic benefit from improved human performance. These benefits can be several-times larger than the firm economic benefits discussed here, but calculation of these benefits requires a qualitative assessment of the value.

For example, if light is an operational requirement, a facility without light ceases operation during power failures. A TDD deployment enables building occupants to continue functions that do not require electrical power. A few examples are uninterrupted business meetings, continued customer selection of merchandise to purchase, continued manual assembly in factories and uninterrupted shipment packing in warehouses. This benefit increases productivity for business operations where electrical power is unreliable.

Demonstrating environmentally sustainable business practice increases customer loyalty. Earth-friendly messaging from green building certifications often justifies a higher price to the consumer with an associated higher profit to the seller. Most green building certification systems require delivery of daylight to building interiors. Using TDDs to decrease a corporation’s carbon footprint by reducing use of utility energy produced with fossil fuels is another way to communicate sustainable business practice to customers and employees.


Incorporating TDDs into a building design enhances the value of an LED investment. TDDs deliver this result by reducing electric lighting and HVAC energy expense as well as deferring LED replacement costs. Additionally, the potential to increase workplace productivity can be a significant contributor to increased company profit. Adding TDDs to an LED design is an economically attractive choice.

About the Author

Todd Maerowitz is general manager, Solatube International, Inc.


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