All the talk about the exciting design opportunities afforded by LEDs is often hampered by two words: thermal management. While incandescent lamps can exhaust heat through radiation, says Michael Gershowitz, Bridgelux’s director of technical marketing, LEDs produce little infrared heat. As a result, they must be cooled through conduction, or risk, diminished performance in luminance, lifespan, and color stability.
But time and market demand have a way of advancing technology to the point where thermal management may soon be just an afterthought rather than an obstacle in luminaire design. With each generation of LEDs, efficacy continues to rise exponentially; LEDs today are more than 50 percent efficient—that is, they convert more energy into light than they do into heat. Most original equipment manufacturers (OEMs) have moved away from simply retrofitting conventional fixtures in order for them to use LEDs, and instead are creating luminaires tailored and optimized for LED output. And LEDs can now “withstand higher temperatures longer without depreciating light and lumens,” says Mark Hand, Acuity Brands Lighting’s director of indoor new product development and technology.
Improvements to LED technology and fixture design aren’t the only things that manufacturers can boast about. Thermal management strategies have been improving and expanding as well. Though passive and active cooling are still the primary options, other innovative technologies are entering the game.
Passive is Aggressive
More than 90 percent of LED fixtures employ passive cooling. With this strategy, heat is conducted away from the LED assembly by direct physical contact with a heat sink, a material with high thermal conductivity. Aluminum—often extruded or cast in a circular shape with fins—is the standard material used for a heat sink because of its light weight, cost effectiveness, and ease of manufacture, says Christopher Reed, Xicato’s strategic partners manager. For now, manufacturers prefer passive heat sinks for their reliability. “At day one, their performance is going to be very similar, if not the same, as 50,000 hours later or 20 years later,” Reed says.
But even a tried-and-true solution can be improved. Along with heat sink optimization, in which designers finesse different fin thicknesses and spacing, passive solutions have taken a new form with pin-fin heat sinks. Instead of an extruded profile, pin-fins resemble an upside-down round table with a copious number of legs, and the diode affixed to the tabletop surface.
Pin-fins, Reed says, allow heated air to rise and then flow out unencumbered around the fins. This attribute is advantageous in fixtures with rotatable heads such as tracklights. “When you tilt a standard fin that’s been extruded, the air can’t flow up on the same orientation or axis as gravity because it actually runs into the fin,” Reed says.
Lighting manufacturers have also begun turning fixture bodies into integrated thermal solutions. With this strategy, the trim around a luminaire in an insulated ceiling system can serve double duty as the finishing trim and as the heat sink. Similarly, the fins of such sculptural fixtures as Cree’s Aeroblades streetlight function as more than just eye candy.
As LEDs become more efficient, the size and weight required for a heat sink will decrease, says Mark McClear, Cree’s vice president of applications engineering. In turn, material and shipping costs will decrease. “Everything goes in the right direction when LEDs get more efficient,” McClear says.
The ease of passive thermal management is often countered by the bulkiness of the heat sink. This size can be problematic for small fixtures, inadequate for sealed outdoor luminaires, and not feasible at all for high-lumen downlights. By forcibly circulating air instead of relying on natural convection, active cooling “allows you to dissipate a lot more heat in a lot smaller space,” Hand says.
Because active cooling inherently has more moving parts and costs more, its adoption by OEMs has been slow. However, recent advancements have made active strategies more efficient and reliable.
Even the de facto active solution—the fan—has made strides. As Xicato’s Reed explains, the long-standing configuration options use either a sleeve bearing—typically constructed out of plastic or polymer-based materials and susceptible to wear from dust buildup and time—or a more durable metal ball-bearing system.
To eliminate the friction between the fan bearing and shaft, cooling design company Sunon, in Brea, Calif., introduced MagLev, which uses magnetic levitation. To counter dust buildup, the company also developed a two-way rotational technology that counter-spins fans for the first few seconds of operation, which “peels off dust from the fan blades,” Reed says.
Sound is another source of complaints with active solutions. In rooms where track fixtures are installed, for example, the noise created by active solutions—which currently ranges from 15 to 30 decibels each, on average—can build up noticeably. “It sounds like a beehive,” Reed says. Farmington, Conn.–based Ebm-Papst offers a fan that operates at 7 decibels, which is the quietest product that Reed has seen to date.
For manufacturers still leery of fans, Nuventix in Austin, Texas, offers a pulsing silicon membrane technology that moves air from the LED to a heat sink, which in turn can be reduced to as little as one-third the size of a standalone passive solution. The synthetic jet technology, or SynJet, “has no rotational or frictional parts,” says Nuventix’s senior vice president of sales and marketing Tom Dalton. Instead it uses power from the LED driver—about 0.35W to 1W, he estimates—to pulse the membrane 50 times per second. These pulsations create a turbulent airflow that results in “better heat extraction in terms of convection” than the laminar airflow created by a fan, Reed says. The technology, which SynJet guarantees for five years, stretches the silicon membrane to about 10 percent of its limits, Nuventix’s Dalton says.
Though some manufacturers remain wary of active cooling solutions’ moving parts, most agree that active strategies are more efficient than passive ones. “The more air you can move and do it quietly, the better off you’ll be,” Gershowitz says.
Hybrid Heat Pipes
Heat pipes, an established cooling mechanism for computers that combines attributes from active and passive solutions, have also garnered newfound interest from OEMs and lighting manufacturers. Composed essentially of a sealed metal tube filled with a thermally conductive liquid, heat pipes wick heat away from an LED and take it to a remote cooling mechanism. Though the system’s efficacy decreases as the distance between the heat source and heat sink increases, heat pipes do enable the components to be disparate. This separation is a great design opportunity, Gershowitz says. For example, heat pipes in a compact pendant fixture could transfer heat to a creative ceiling canopy. Designers could “come up with some very attractive solutions,” he says.
FrigoDynamics has even imbued this technology with new tricks. By threading heat pipes through passive sheet metal heat sinks strategically shaped in geometries to maximize cooling, the German company has created an efficient hybrid cooling technology. “The heat pipe is the element that takes heat from one side of the heat sink to the other,” Reed says. “It’s very efficient at moving heat around and getting a very homogenous temperature across your current heat sink.”
A Bright Future
Technologies that have yet to prove their mettle in practice are also attracting interest from curious manufacturers. Sandia Corp. has been working at length on the Sandia Cooler, a one-piece metal fan unit that spins on an air bearing, eliminating the potential for a physical bearing to fail.
Conversations on thermal management strategies would be remiss without mention of the thermal interface material, which fills the microscopic voids between the metal LED backing and its heat sink. Choosing a thermally conductive pad material to fill the air gap is critical to transferring heat from the LED. Phase-change materials are one plausible, effective, and tidy alternative to plasticor a metal-filled compound, Gershowitz says.
Cambridge Nanotherm also focuses on these intricacies at the microscopic level. The U.K.-based company recently introduced a nano-ceramic dielectric layer for an LED’s printed circuit board (PCB) with a thermal conductivity of 7 watts per meter Kelvin (W/mK). Metal-backed PCBs (MBPCBs) typically use epoxy fillers with a thermal conductivity between 1W/mK and 3W/mK. Nanotherm says its MBPCBs conduct heat away from the LED die 20 percent more efficiently than aluminum-backed PCBs on the market.
With the rate at which LED efficacy is rising—Cree’s McClear foresees efficacies reaching as high as 65 percent by 2020—in addition to the advancements in thermal management strategies, cooling LEDs may no longer be a concern in luminaire design. Acuity’s Mark Hand predicts that low-power LEDs, strategically distributed to self-diffuse heat, may even forgo a dedicated thermal management strategy. “On the upper end of the scale, where people will try and get more lumens out of a smaller space, they will start looking at thermally conductive polymers” that can be injection-molded into multiple LED components, he says.
Bridgelux’s Gershowitz thinks thermal management will continue to have some relevance despite the gains in LED efficacy. “Every time there’s a step forward … rather than looking to reduce the power or reduce the thermal solution … [OEMs are] staying with the same thermal solution and putting in a larger light source.” The good news: The rate of advancements in thermal management technology will allow OEMs and lighting manufacturers to expand solid-state lighting into more markets without fear of burning up.