It’s a tough challenge to balance daylight with the heat that accompanies it. This tradeoff often comes with the price tag of increased energy costs to cool the space.
Neall Digert, Ph.D., MIES, vice president of Product Enterprise, Solatube International Inc., Vista, CA
Warehouses present a particular challenge because of their interior volume and the difficulty of maintaining electric lighting systems that are located at a high ceiling plane. Traditional daylighting systems—windows and skylights—are often used to reduce the use of electric lighting and the associated maintenance costs. While that strategy almost always works, it frequently introduces unwanted additional heat. An option that breaks this daylight/heat partnership is use of tubular daylighting devices (TDD).
To illustrate this, let’s take a look at one of the hottest places on earth. Saudi Arabia’s largest Pepsi distributor, Saudi Industrial Projects Co. (SIPCO), is located in Jeddah, where summer temperatures can spike to 109 F (43 C).
When management sought to replace the facility’s 400-W, metal-halide lighting system with daylighting, the objective was to attain adequate light levels without heat gain. Optically-advanced TDDs were the perfect solution. They provided abundant daylight (producing an average 200 lux throughout the year) without introducing heatin to the 129,166-sq.-ft (12,000 sq. m) warehouse building. This reduced SIPCO’s electricity use by 648,240 kWh/yr., by eliminating the use of electric lighting and the heat gain that is associated with the electric fixtures.
As SIPCO discovered, thermal performance of the daylighting system is crucial because it affects occupant comfort and productivity as well as building costs.
Daylighting and thermal issues
When it comes to thermal performance, the three daylighting options—windows, skylights, and TDDs—all perform differently on the two key issues: solar heat gain and conductive transfer of heat or cold.
Solar heat gain is infrared energy transmitted from outside to inside, through a window or skylight, that can heat a building’s interior. Conductive transfer occurs when surfaces touch and heat or cold are transferred. If a window or skylight allows conduction of either from the outside environment to the inside, the space becomes too hot or too cold.
To compensate, the indoor temperature is often adjusted, increasing heating and cooling utility costs. This increased load placed on the HVAC system can even require equipment with increased heating and cooling capacity, further increasing costs
A more efficient solution is installing TDDs for daytime lighting and energy-efficient LEDs for nighttime. With less lighting-related heat produced, and reduced conductive heat transfer, the air conditioning and heating systems don’t have to be as large and/or work as hard.
Gauging daylighting system performance
To assess a daylighting system’s performance, you’ll want to look at three key factors: visible transmittance (VT) rating; U-factor; and solar heat gain coefficient (SHGC).
In general, you want a:
- High VT for maximum light transmittance
- Low U-factor, meaning minimal heat conduction through the system
- Low SHGC, indicating minimal solar gain.
The most meaningful measure of total system performance is the light to solar heat gain (LSG) ratio. It reflects the amount of usable light to solar heat transmitted into a space. The higher the LSG, the better.
When evaluating daylighting options, an LSG of 1.0 to 1.5 is generally considered a high-performing daylighting product. But with modern innovations in daylighting technologies, LSGs of 3.0 and higher are now feasible.
Interpreting the data
To ensure you select the best daylighting option for your project, it’s imperative to consider total system performance, not a single factor or rating. For example, let’s say you’re considering a TDD because it has low U-factor and low SHGC. If you don’t also consider VT, you’re overlooking light transmittance, the main purpose of the system. A low VT could be the result of tubing material that isn’t very reflective or a system design that’s inefficient at capturing daylight.
Assessing performance based on LSG ensures you’re not selecting good thermal performance, for example, at the expense of light transmission.
Achieving high LSG with TDDs
High LSG performance ratings are typically achieved only with TDDs engineered to maximize light capture and output while minimizing heat gain/heat loss caused by radiation (air transfer) or conduction (surface transfer).
Special dome materials, shapes, lenses and reflectors maximize light capture and reject heat at the rooftop level. Reflective tubing with built-in cooling properties transfers large amounts of daylight to the interior with minimal light loss and without heat. This improves VT and SHGC measures.
Thermal breaks also improve performance. Often manifested as dome rings, ceiling rings or expansion joints, they prevent conduction of heat between metal surfaces. This improves U-factor performance.
Systems that incorporate daylight collectors are even more effective because they gather light that would normally pass right over the dome. Collectors reflect more daylight down into the system, even amplifying it for a VT that exceeds 1.0, or more than 100%. This also means the system can deliver an LSG that approaches a value of 5.0.
Understanding and applying rating systems can help you select a daylighting solution that provides optimal light output and thermal performance. The sky is no longer the limit…it’s where your opportunity for better lighting begins.
Neall Digert, Ph.D., MIES, is vice president of Product Enterprise for Solatube International. With more than 25 years in the energy/lighting/daylighting design and research fields, he works to build public awareness of new optical daylighting technologies, guide future product developments and refinements, develop new global sales and marketing strategies, and pioneer new design and application tools and protocols to support the successful integration of optical daylighting products into today’s commercial buildings.