Inside a shipping container in an industrial area of Venice, the Italian startup 9-Tech is taking a crack at a looming global problem: how to responsibly recycle the 54 million to 160 million tonnes of solar modules that are expected to reach the end of their productive lives by 2050. Recovering the materials won’t be easy. Solar panels are built to withstand any environment on Earth for 20 to 30 years, and even after sitting in the sun for three decades, the hardware is difficult to dismantle. In fact, most recycling facilities trash the silicon, silver, and copper—the most valuable but least accessible materials in old solar panels—and recover only the aluminum frames and glass panes.

The startup 9-Tech operates its pilot plant out of a modified shipping container housed at the Green Propulsion Laboratory in the industrial port of Marghera in Venice.Luigi Avantaggiato

The need for recycling will only grow as the world increasingly deploys solar power. More than 1.2 terawatts of solar power has already been deployed globally. Solar panels are currently being distributed at a rate of more than 400 gigawatts per year, and the rate is expected to increase to a whopping 3 TW per year by 2030, according to a literature analysis by researchers at the National Renewable Energy Laboratory (NREL).

In an attempt to stop a mountain of photovoltaic garbage from accumulating, researchers are pursuing better recycling methods. The most advanced methods proposed so far can recover at least 90 percent of the copper, silver, silicon, glass, and aluminum in a crystalline silicon PV module. But these processes are expensive and often involve toxic chemicals. No recycling method has proven to be as cheap as landfilling, and very few operate on an industrial scale, says Garvin Heath, principal environmental engineer at NREL, who manages a group of international experts assigned by the International Energy Agency to analyze PV sustainability.

The founders of 9-Tech say they have a better way. Their process is a noisy one involving a combustion furnace, an ultrasound bath, and mechanical sorting, the vibrations of which shake the floor of the modest freight container where they have been testing their operation for nearly two years. The company uses no toxic chemicals, releases no pollutants into the environment, and recovers up to 90 percent of the materials in a solar panel, says Francesco Miserocchi, chief technology officer at 9-Tech.

Bits of silicon and glass are separated from the rest of the panel. Luigi Avantaggiato

How to Recycle Solar Panels

After the frame, glass, and junction box are removed from a PV panel, the inner, bendable layers of silicon, polymers, and metal conductors remain. Workers cut the inner layers into large sections in preparation for the oven.Luigi Avantaggiato

The company tailors its process to crystalline silicon solar panels, which make up 97 percent of the global PV market. The panels typically consist of an array of silicon wafers doped with boron and phosphorus, and topped with an antireflective coating of silicon nitride. Silver conductors are screen printed onto the wafer surface, and copper conductors are soldered onto the array in a grid pattern. To protect the materials from moisture and damage, manufacturers laminate the entire array in adhesive polymers—usually ethylene-vinyl acetate. Then they encase the laminated array in sheets of tempered glass, frame the whole thing in aluminum, seal the edges, and attach a junction box on the back.

When it’s time to recycle a panel, one of the most challenging steps is removing the polymers, which stick to everything. “It’s not just the edges or a couple of dots of glue. It’s an entire surface—several square feet—of polymer,” says Heath. The polymer can be burned off, but this releases carbon monoxide, hydrofluoric acid, and other harmful pollutants. Separating the silver conductors also proves challenging because they’re applied in a very thin layer–about 10 to 20 micrometers–that is strongly attached to the silicon. Removing them typically involves toxic reagents such as hydrofluoric acid, nitric acid, or sodium hydroxide.

The team at 9-Tech addresses these challenges in two ways. They recover the silver using ultrasound rather than toxic chemicals, and although they burn the polymers, they capture the pollutants emitted.

Layers of silicon and polymer are fed into a continuous combustion furnace, which heats the materials to over 400 °C, vaporizing the polymers. 9-Tech

The process at the company’s pilot plant starts with workers manually removing the aluminum frame, junction box, and tempered glass. This leaves a sandwich of polymers, silicon wafers, and metal conductors. Without the frame or glass, the sandwich layers bend, shattering the fragile silicon into small pieces. Workers crack the tempered glass and then send all the materials, which are mostly still in place because of the polymers, into a continuous combustion furnace. Heated to over 400 °C, the polymers vaporize, and a filter captures the pollutants. The system also captures the heat from the furnace and reuses it for energy efficiency.

A mechanical roller separates the copper grid after the PV materials exit the furnace.Luigi Avantaggiato

After the materials exit the oven, mechanical sieves separate the copper, glass, and silicon. Top: Luigi Avantaggiato; Bottom: 9-Tech

As the remaining material exits the furnace, a roller mechanically strips out the copper. A series of sieves sort the broken bits of glass and silicon based on thickness. The silicon pieces, still laced with silver, are immersed in a bath of organic acid and treated with ultrasound to loosen the bonds between the elements. The ultrasound works by propagating sound waves into the acid bath, resulting in alternating high- and low-pressure cycles. If the waves are intense enough, they create cavitation bubbles that mechanically interact with the material, causing the silver to dislodge from the silicon, explains Pietrogiovanni Cerchier, CEO at 9-Tech.

Finally, workers remove the silicon fragments from the ultrasound bath with a mesh net. This leaves a fine silver dust in the solution, which can be recovered by filtration or centrifuge. All told, Cerchier says, 9-Tech’s pilot plant can recover 90 percent of the silver, 95 percent of the silicon, and 99 percent or more of the copper, aluminum, and glass from a PV module. What’s more, the material is considered highly pure, which increases the types of applications for which it can be reused.

Workers at 9-Tech immerse silver-laced silicon pieces in a bath of organic acid and treat it with ultrasound to loosen the bonds between the elements. 9-Tech

The startup’s recycling process is more expensive than existing methods that recover only aluminum and glass. But the extraction of high-purity silicon, silver, and copper should offset the extra cost, Miserocchi says. Plus, it’s more efficient than mining for virgin elements. You can extract about 500 grams of silver from a tonne of solar panels, but only 165 grams of silver from a tonne of ore, he says. “A photovoltaic panel at the end of its life still has a lot to give,” says Miserocchi. “It can be considered a small mine of precious elements.”

Silver emerges from the silicon bath in a fine dust. 9-Tech

Dozens of New Ways to Recycle PV Panels

High-purity copper, glass, and silicon are recovered from 9-Tech’s PV-panel recycling process.Luigi Avantaggiato

The 9-Tech team will know more about the profitability of their method after they build a larger demonstration facility over the next 18 months. That plant, to be located in the same industrial district of Venice as the shipping container, will be able to handle up to 800 solar modules a day. Their pilot plant processes only about seven modules a day.

The company’s approach is one of many recycling methods for crystalline silicon PV panels in development. A comprehensive review published in April in the Journal of Cleaner Production identified dozens of other efforts globally, including thermal, chemical, mechanical, and optical approaches. The most common method involves grinding the silicon, metal, and polymer layers into small pieces, separating them by density, and recovering the silicon and metal with a thermal or chemical process. Other processes include laser irradiation, high-voltage pulses, optical sorting, pyrolysis, chemical solvents, etching, and delaminating with a hot knife.

Driving this innovation, in part, are regulations adopted by the European Union in 2012. The rules require all PV panel manufacturers in the EU to run take-back or recycling programs, or partner with other recycling schemes. As a result, Germany, which has the most solar power capacity in Europe, has one of the largest PV recycling systems in the world.

“A photovoltaic panel at the end of its life still has a lot to give,” says Miserocchi. “It can be considered a small mine of precious elements.”

But recycling is a high-volume business, and aside from catastrophic weather events that wipe out solar power stations, spent solar modules reach recyclers at a relative trickle. And then there’s the challenge of finding a second life for the materials after they are recovered—a supply chain that’s not well developed.

First Solar, a global PV manufacturer based in Tempe, Ariz., addressed both of these issues on a large scale by building an in-house recycling program with seven facilities in five countries. The company makes cadmium telluride thin-film solar panels that buyers can purchase with the recycling price built in. At the end of the panels’ lives, buyers send them back to First Solar for recycling into new products. The semiconductor material can be recycled up to 41 times, giving it a life-span of more than 1,200 years, according to the company. But the glass isn’t pure enough to be reused in solar modules, so the company plans to supply it to float-glass manufacturers for use in windows and doors.

The challenges with recycling have inspired researchers to rethink the way crystalline solar panels are made. For example, some manufacturers are trying to reduce or eliminate the difficult-to-recover silver, replacing it with other conductive metals. And a team at NREL demonstrated in February a way to eliminate polymers in PV panels by laser welding the glass panes instead, which may do a better job sealing out moisture. That technique may lend itself to perovskite solar modules, a promising technology that is particularly susceptible to moisture and corrosion.

“Recycling shouldn’t be the only strategy,” says Heath. People should consider alternative ways to repair or reuse solar panels to extend their lives before resorting to recycling, he says.

Additional reporting by Luigi Avantaggiato

Nearly 400 exhibitors representing the boldest energy innovations in the United States came together last week at the annual ARPA-E Energy Innovation Summit. The conference, hosted in Dallas by the U.S. Advanced Research Projects Agency–Energy (ARPA-E), showcased the agency’s bets on early-stage energy technologies that can disrupt the status quo. U.S. Secretary of Energy Jennifer Granholm spoke at the summit. “The people in this room are America’s best hope” in the race to unleash the power of clean energy, she said. “The technologies you create will decide whether we win that race. But no pressure,” she quipped. IEEE Spectrum spent three days meandering the aisles of the showcase. Here are five of our favorite demonstrations.

Gas Li-ion batteries thwart extreme cold

South 8 Technologies demonstrates the cold tolerance of its Li-ion battery by burying it in ice at the 2024 ARPA-E Energy Innovation Summit. Emily Waltz

Made with a liquified gas electrolyte instead of the standard liquid solvent, a new kind of lithium-ion battery that stands up to extreme cold, made by South 8 Technologies in San Diego, won’t freeze until temps drop below –80 °C. That’s a big improvement on conventional Li-ion batteries, which start to degrade when temps reach 0 °C and shut down at about –20 °C. “You lose about half of your range in an electric vehicle if you drive it in the middle of winter in Michigan,” says Cyrus Rustomji, cofounder of South 8. To prove the company’s point, Rustomji and his team set out a bucket of dry ice at nearly –80 °C at their booth at the ARPA-E summit and put flashlights in it—one powered by a South 8 battery and one powered by a conventional Li-ion cell. The latter flashlight went out after about 10 minutes, and South 8’s kept going for the next 15 hours. Rustomji says he expects EV batteries made with South 8’s technology to maintain nearly full range at –40 °C, and gradually degrade in temperatures lower than that.

South 8 Technologies

Conventional Li-ion batteries use liquid solvents, such as ethylene and dimethyl carbonate, as the electrolyte. The electrolyte serves as a medium through which lithium salt moves from one electrode to the other in the battery, shuttling electricity. When it’s cold, the carbonates thicken, which lowers the power of the battery. They can also freeze, which shuts down all conductivity. South 8 swapped out the carbonate for some industrial liquified gases with low freezing points (a recipe the company won’t disclose).

Using liquified gases also reduces fire risk because the gas very quickly evaporates from a damaged battery cell, removing fuel that could burn and cause the battery to catch fire. If a conventional Li-ion battery gets damaged, it can short-circuit and quickly become hot—like over 800 °C hot. This causes the liquid electrolyte to heat adjacent cells and potentially start a fire.

There’s another benefit to this battery, and this one will make EV drivers very happy: It will take only 10 minutes to reach an 80 percent charge in EVs powered by these batteries, Rustomji estimates. That’s because liquified gas has a lower viscosity than carbonate-based electrolytes, which allows the lithium salt to move from one electrode to the other at a faster rate, shortening the time it takes to recharge the battery.

South 8’s latest improvement is a high-voltage cathode that reduces material costs and could enable fast charging down to 5 minutes for a full charge. “We have the world record for a high-voltage, low-temperature cathode,” says Rustomji.

Liquid cooling won’t leak on servers

Chilldyne guarantees that its liquid-cooling system won’t leak even if tubes get hacked in half, as IEEE Spectrum editor Emily Waltz demonstrates at the 2024 ARPA-E Energy Innovation Summit. Emily Waltz

Data centers need serious cooling technologies to keep servers from overheating, and sometimes air-conditioning just isn’t enough. In fact, the latest Blackwell chips from Nvidia require liquid cooling, which is more energy efficient than air. But liquid cooling tends to make data-center operators nervous. “A bomb won’t do as much damage as a leaky liquid-cooling system,” says Steve Harrington, CEO of Chilldyne. His company, based in Carlsbad, Calif., offers liquid cooling that’s guaranteed not to leak, even if the coolant lines get chopped in half. (They aren’t kidding: Chilldyne brought an axe to its demonstration at ARPA-E and let Spectrum try it out. Watch the blue cooling liquid immediately disappear from the tube after it’s chopped.)

Chilldyne

The system is leakproof because Chilldyne’s negative-pressure system pulls rather than pushes liquid coolant through tubes, like a vacuum. The tubes wind through servers, absorbing heat through cold plates, and return the warmed liquid to tanks in a cooling distribution unit. This unit transfers the heat outside and supplies cooled liquid back to the servers. If a component anywhere in the cooling loop breaks, the liquid is immediately sucked back into the tanks before it can leak. Key to the technology: low-thermal-resistance cold plates attached to each server’s processors, such as the CPUs or GPUs. The cold plates absorb heat by convection, transferring the heat to the coolant tube that runs through it. Chilldyne optimized the cold plate using corkscrew-shaped metal channels, called turbulators, that force water around them “like little tornadoes,” maximizing the heat absorbed, says Harrington. The company developed the cold plate under an ARPA-E grant and is now measuring the energy savings of liquid cooling through an ARPA-E program.

Salvaged mining waste also sequesters CO₂

Phoenix Tailings’ senior research scientist Rita Silbernagel explains how mining waste contains useful metals and rare earth elements and can also be used as a place to store carbon dioxide.Emily Waltz

Mining leaves behind piles of waste after the commercially viable material is extracted. This waste, known as tailings, can contain rare earth elements and valuable metals that are too difficult to extract with conventional mining techniques. Phoenix Tailings—a startup based in Woburn, Mass.—extracts metals and rare earth elements from tailings in a process that leaves behind no waste and creates no direct carbon dioxide emissions. The company’s process starts with a hydrometallurgical treatment that separates rare earth elements from the tailings, which contain iron, aluminum, and other common elements. Next the company uses a novel solvent extraction method to separate the rare earth elements from one another and purify the desired element in the form of an oxide. The rare earth oxide then undergoes a molten-salt electrolysis process that converts it into a solid metal form. Phoenix Tailings focuses on extracting neodymium, neodymium-praseodymium alloy, dysprosium, and ferro dysprosium alloy, which are rare earth metals used in permanent magnets for EVs, wind turbines, jet engines, and other applications. The company is evaluating several tailings sites in the United States, including in upstate New York.

The company has also developed a process to extract metals such as nickel, copper, and cobalt from mining tailings while simultaneously sequestering carbon dioxide. The approach involves injecting CO₂ into the tailings, where it reacts with minerals, transforming them into carbonates—compounds that contain the carbonate ion, which contains three oxygen atoms and one carbon atom. After the mineral carbonation process, the nickel or other metals are selectively leached from the mixture, yielding high-quality nickel that can be used by EV-battery and stainless-steel industries.

Better still, this whole process, says Rita Silbernagel, senior research scientist at Phoenix Tailings, absorbs more CO₂ than it emits.

Hydrokinetic turbines: a new business model

Emrgy adjusts the height of its hydrokinetic turbines at the 2024 ARPA-E Energy Innovation Summit. The company plans to install them in old irrigation channels to generate renewable energy and new revenue streams for rural communities. Emily Waltz

These hydrokinetic turbines run in irrigation channels, generating electricity and revenue for rural communities. Developed by Emrgy in Atlanta, the turbines can change in height and blade pitch based on the flow of the water. The company plans to put them in irrigation channels that were built to bring water from snowmelt in the Rocky Mountains to agricultural areas in the western United States. Emrgy estimates that there are more than 160,000 kilometers of these waterways in the country. The system is aging and losing water, but it’s hard for water districts to justify the cost of repairing them, says Tom Cuthbert, chief technology officer at Emrgy. The company’s solution is to place its hydrokinetic turbines throughout these waterways as a way to generate renewable electricity and pay for upgrades to the irrigation channels.

The concept of placing hydrokinetic turbines in waterways isn’t new, but until recent years, connecting them to the grid wasn’t practical. Emrgy’s timing takes advantage of the groundwork laid by the solar power industry. The company has five pilot projects in the works in the United States and New Zealand. “We found that existing water infrastructure is a massive overlooked real estate segment that is ripe for renewable energy development,” says Emily Morris, CEO and founder of Emrgy.

Pressurized water stores energy deep underground

Quidnet Energy brought a wellhead to the 2024 ARPA-E Energy Innovation Summit to demonstrate its geoengineered energy-storage system.Emily Waltz

Quidnet Energy brought a whole wellhead to the ARPA-E summit to demonstrate its underground pumped hydro storage technique. The Houston-based company’s geoengineered system stores energy as pressurized water deep underground. It consists of a surface-level pond, a deep well, an underground reservoir at the end of the well, and a pump system that moves pressurized water from the pond to the underground reservoir and back. The design doesn’t require an elevation change like traditional pumped storage hydropower.

Quidnet’s system consists of a surface-level pond, a deep well, an underground reservoir at the end of the well, and a pump system that moves pressurized water from the pond to the underground reservoir and back.Quidnet Energy

It works like this: Electricity from renewable sources powers a pump that sends water from the surface pond into a wellhead and down a well that’s about 300 meters deep. At the end of the well, the pressure from the pumped water flows into a previously engineered fracture in the rock, creating a reservoir that’s hundreds of meters wide and sits beneath the weight of the whole column of rock above it, says Bunker Hill, vice president of engineering at Quidnet. The wellhead then closes and the water remains under high pressure, keeping energy stored in the reservoir for days if necessary. When electricity is needed, the well is opened, letting the pressurized water run up the same well. Above ground, the water passes through a hydroelectric turbine, generating 2 to 8 megawatts of electricity. The spent water then returns to the surface pond, ready for the next cycle. “The hard part is making sure the underground reservoir doesn’t lose water,” says Hill. To that end, the company developed customized sealing solutions that get injected into the fracture, sealing in the water.

This year, the sun will reach solar maximum, a period of peak magnetic activity that occurs approximately once every 11 years. That means more sunspots and more frequent intense solar storms. Here on Earth, these result in beautiful auroral activity, but also geomagnetic storms and the threat of electromagnetic pulses (EMPs), which can bring widespread damage to electronic equipment and communications systems.

Yilu Liu

Yilu Liu is a Governor’s Chair/Professor at the University of Tennessee, in Knoxville, and Oak Ridge National Laboratory.

And the sun isn’t the only source of EMPs. Human-made EMP generators mounted on trucks or aircraft can be used as tactical weapons to knock out drones, satellites, and infrastructure. More seriously, a nuclear weapon detonated at a high altitude could, among its more catastrophic effects, generate a wide-ranging EMP blast. IEEE Spectrum spoke with Yilu Liu, who has been researching EMPs at Oak Ridge National Laboratory, in Tennessee, about the potential effects of the phenomenon on power grids and other electronics.

What are the differences between various kinds of EMPs?

Yilu Liu: A nuclear explosion at an altitude higher than 30 kilometers would generate an EMP with a much broader spectrum than one from a ground-level weapon or a geomagnetic storm, and it would arrive in three phases. First comes E1, a powerful pulse that brings very fast high-frequency waves. The second phase, E2, produces current similar to that of a lightning strike. The third phase, E3, brings a slow, varying waveform, kind of like direct current [DC], that can last several minutes. A ground-level electromagnetic weapon would probably be designed for emitting high-frequency waves similar to those produced by an E1. Solar magnetic disturbances produce a slow, varying waveform similar to that of E3.

How do EMPs damage power grids and electronic equipment?

Liu: Phase E1 induces current in conductors that travels to sensitive electronic circuits, destroying them or causing malfunctions. We don’t worry about E2 much because it’s like lightning, and grids are protected against that. Phase E3 and solar magnetic EMPs inject a foreign, DC-like current into transmission lines, which saturates transformers, causing a lot of high-frequency currents that have led to blackouts.

How do you study the effects of an EMP without generating one?

Liu: We measured the propagation into a building of low-level electromagnetic waves from broadcast radio. We wanted to know if physical structures, like buildings, could act as a filter, so we took measurements of radio signals both inside and outside a hydropower station and other buildings to figure out how much gets inside. Our computer models then amplified the measurements to simulate how an EMP would affect equipment.

What did you learn about protecting buildings from damage by EMPs?

Liu: When constructing buildings, definitely use rebar in your concrete. It’s very effective as a shield against electromagnetic waves. Large windows are entry points, so don’t put unshielded control circuits near them. And if there are cables coming into the building carrying power or communication, make sure they are well-shielded; otherwise, they will act like antennas.

Have solar EMPs caused damage in the past?

Liu: The most destructive recent occurrence was in Quebec in 1989, which resulted in a blackout. Once a transformer is saturated, the current flowing into the grid is no longer just 60 hertz but multiples of 60 Hz, and it trips the capacitors, and then the voltage collapses and the grid experiences an outage. The industry is better prepared now. But you never know if the next solar storm will surpass those of the past.

This article appears in the June 2024 issues as “5 Questions for Yilu Liu.”

Solar panels are built to last 25 years or more in all kinds of weather. Key to this longevity is a tight seal of the photovoltaic materials. Manufacturers achieve the seal by laminating a panel’s silicon cells with polymer sheets between glass panes. But the sticky polymer is hard to separate from the silicon cells at the end of a solar panel’s life, making recycling the materials more difficult.

Researchers at the U.S. National Renewable Energy Lab (NREL) in Golden, Colo., say they’ve found a better way to seal solar modules. Using a femtosecond laser, the researchers welded together solar panel glass without the use of polymers such as ethylene vinyl acetate. These glass-to-glass precision welds are strong enough for outdoor solar panels, and are better at keeping out corrosive moisture, the researchers say.

A femtosecond laser welds a small piece of test glass.NREL

“Solar panels are not easily recycled,” says David Young, a senior scientist at NREL. “There are companies that are doing it now, but it’s a tricky play between cost and benefit, and most of the problem is with the polymers.” With no adhesive polymers involved, recycling facilities can more easily separate and reuse the valuable materials in solar panels such as silicon, silver, copper, and glass.

Because of the polymer problem, many recycling facilities just trash the polymer-covered silicon cells and recover only the aluminum frames and glass encasements, says Silvana Ovaitt, a photovoltaic (PV) analyst at NREL. This partial recycling wastes the most valuable materials in the modules.

“At some point there’s going to be a huge amount of spent panels out there, and we want to get it right, and make it easy to recycle.” —David Young, NREL

Finding cost-effective ways to recycle all the materials in solar panels will become increasingly important. Manufacturers globally are deploying enough solar panels to produce an additional 240 gigawatts each year. That rate is projected to increase to 3 terawatts by 2030, Ovaitt says. By 2050, anywhere from 54 to 160 million tonnes of PV panels will have reached the end of their life-spans, she says.

“At some point there’s going to be a huge amount of spent panels out there, and we want to get it right, and make it easy to recycle,” says Young. “There’s no reason not to.” A change in manufacturing could help alleviate the problem—although not for at least another 25 years, when panels constructed with the new technique would be due to be retired.

In NREL’s technique, the glass that encases the solar cells in a PV panel is welded together by precision melting. The precision melting is accomplished with femtosecond lasers, which pack a tremendous number of photons into a very short time scale--about 1 millionth of 1 billionth of a second. The number of photons emitted per second from the laser is so intense that it changes the optical absorption process in the glass, says Young. The process changes from linear (normal absorption) to nonlinear, which allows the glass to absorb energy from the photons that it would normally not absorb, he says.

The intense beam, focused near the interface of the two sheets of glass, generates a small plasma of ionized glass atoms. This plasma allows the glass to absorb most of the photons from the laser and locally melt the two glass sheets to form a weld. Because there’s no open surface, there is no evaporation of the molten glass during the welding process. The lack of evaporation from the molten pool allows the glass to cool in a stress-free state, leaving a very strong weld.

A femtosecond laser creates precision welds between two glass plates.David Young/NREL

In stress tests conducted by the NREL group, the welds proved almost as strong as the glass itself, as if there were no weld at all. Young and his colleagues described their proof-of-concept technique in a paper published 21 February in the IEEE Journal of Photovoltaics.

This is the first time a femtosecond laser has been used to test glass-to-glass welds for solar modules, the authors say. The cost of such lasers has declined over the last few years, so researchers are finding uses for them in a wide range of applications. For example, femtosecond lasers have been used to create 3D midair plasma displays and to turn tellurite glass into a semiconductor crystal. They’ve also been used to weld glass in medical devices.

Prior to femtosecond lasers, research groups attempted to weld solar panel glass with nanosecond lasers. But those lasers, with pulses a million times as long as those of a femtosecond laser, couldn’t create a glass-to-glass weld. Researchers tried using a filler material called glass frit in the weld, but the bonds of the dissimilar materials proved too brittle and weak for outdoor solar panel designs, Young says.

In addition to making recycling easier, NREL’s design may make solar panels last longer. Polymers are poor barriers to moisture compared with glass, and the material degrades over time. This lets moisture into the solar cells, and eventually leads to corrosion. “Current solar modules aren’t watertight,” says Young. That will be a problem for perovskite cells, a leading next-generation solar technology that is extremely sensitive to moisture and oxygen.

“If we can provide a different kind of seal where we can eliminate the polymers, not only do we get a better module that lasts longer, but also one that is much easier to recycle,” says Young.

Twenty homes scattered across Canada and the northern United States are keeping warm this winter using prototypes of the latest iteration in residential heating systems: cold-climate heat pumps.

Heat pumps aren’t common in homes at this latitude, because historically they haven’t worked well in subzero temperatures. But heat-pump manufacturers say they now have the technology to heat homes just as efficiently in bitter cold as they do in milder winter temperatures.

To prove it, eight manufacturers are publicly testing their prototypes in the Cold-Climate Heat Pump Technology Challenge, hosted by the U.S. Department of Energy in partnership with Natural Resources Canada. The companies’ task is to demonstrate a high-efficiency, residential air-source heat pump that can perform at 100 percent capacity at -15 °C. Companies can choose to further test their machines down to -26 °C.

Heat-pump manufacturers Bosch, Carrier, Daikin, Johnson Controls, Lennox, Midea, Rheem, and Trane Technologies have each passed the laboratory phase of the challenge, according to the DOE. They are now field-testing their prototypes in homes in 10 northern U.S. states and two Canadian provinces, where furnaces and boilers burning fossil gas, fuel oil, or propane are more commonly used.

Companies that complete the challenge won’t receive cash prizes. But the DOE will help them expand into cold-climate markets by engaging with stakeholders in those regions, a DOE spokesperson told IEEE Spectrum. The challenge will conclude later this year, and prototypes will likely be ready for commercialization in 2025.

How heat pumps beat the cold

Advances in the technology came primarily through improvements in one key heat-pump component: the compressor. Heat pumps work by moving and compressing fluids. In the winter, the systems draw heat from outside the home, most commonly from the air. (There is heat in the air even in subzero temperatures.) An outdoor heat exchanger, or coil, absorbs the heat into the heat-pump system.

The outdoor air passes over a heat exchanger containing a fluid, or refrigerant, that has a very low boiling point. A common refrigerant, called R410a, boils at -48.5 °C. The refrigerant boils and evaporates into a vapor, and a compressor increases its temperature and pressure. The superheated vapor then moves through an indoor coil, where fans blow air across it, moving heat into the home. In the summer, the system reverses, moving heat from inside the building to the outside, and cooling the home.

“They couldn’t get the lab any colder than [-30 °C], so we had to cut the power to get the heat pump to turn off.” —Katie Davis, Trane Technologies

The colder the temperature outside, the harder heat pumps must work to extract and move enough heat to maintain the home’s temperature. At about 4 °C, most air-source heat pumps currently on the market start operating at less than their full capacity, and at some point (usually around -15 °C), they can no longer do the job at all. At that point, an auxiliary heat source kicks on, which is less efficient.

But advancements in compressor technology over the past five years have addressed that issue. By controlling the compressor motor’s speed, and improving the timing of when vapor is injected into the compressor, engineers have made heat pumps more efficient in colder temperatures.

For example, Trane Technologies, headquartered in Dublin, “played with the vapor compression cycle” so that it gets an extra injection of refrigerant, says Katie Davis, vice president of engineering and technology in Trane’s residential business. “It’s works a little like fuel injection,” she says. When the system begins to lose its capacity to heat, the system injects refrigerant to give it a boost, she says.

In the lab portion of the DOE’s heat pump challenge, Trane’s unit operated at 100 percent capacity at -15 °C and kept running even as the lab’s temperature dropped to -30 °C, although no longer at full capacity. “They couldn’t get the lab any colder than that, so we had to cut the power to get the heat pump to turn off,” Davis says.

Vapor-injection compressor technology has been around for years, but until recently, had not been optimized for heat pumps, Davis says. That, plus the introduction of smart systems that enable the indoor and outdoor units to communicate with each other and the thermostat, has enabled heat pumps to take on colder weather.

Heat pumps can reduce emissions and cut energy costs

The DOE is pushing for wider adoption of heat pumps because of their potential to reduce greenhouse gas emissions. Such systems run on electricity rather than fossil fuels, and when the electricity comes from renewable sources, the greenhouse gas savings are substantial, the DOE says. Because heat pumps transfer heat rather than generate it, they are significantly more efficient than traditional heating systems, the agency says.

A two-year study published 12 February in the journal Joule supports the DOE’s claim. The study found that if every heated home in the United States switched to a heat pump, home energy use would drop by 31 to 47 percent on average, and national carbon dioxide emissions would fall by 5 to 9 percent, depending on how much electricity is provided by renewable energy. Those figures are based on heat pumps that draw heat from an air source (rather than ground or water) and includes both homes that pull heat through ductwork, and homes that are ductless.

The energy savings should lower bills for 62 to 95 percent of homeowners, depending on the efficiency and cold-climate performance of the heat pump being installed. How well a home is insulated and the type of heating system being replaced also makes a big difference in energy bills, the study found. For households that are currently heating with electric resistance heat, fuel oil, or propane, heat pumps could save thousands of dollars annually. For natural gas, the savings are less and depend on the price of natural gas in the local area.

Some homeowners are hesitant to switch to heat pumps because of what’s known as “temperature anxiety.”

Cold-climate heat pumps will likely boost energy savings for homeowners, but will require higher up-front costs, says Eric Wilson, a senior research engineer at the National Renewable Energy Laboratory in Golden, Colo., and an author of the paper. “It’s generally well known that heat pumps can save money, but there’s a lot of confusion around whether they’re a good idea in all climates,” he says. His study and the DOE’s cold-climate heat pump challenge will help provide a clearer picture, he says.

The DOE is one of several government entities trying to expedite adoption of residential high-efficiency heat pumps. Nine U.S. states earlier this month pledged to accelerate heat-pump sales. Their pledge builds on an announcement in September from 25 governors, who vowed to quadruple heat-pump installation in their states by 2030. The U.S. federal government also offers tax credits and states will be rolling out rebates to offset the cost of installation.

So far, the efforts seem to be working. In the U.S., heat pumps outsold furnaces for a second year in a row in 2023, according to data released 9 February by the Air-Conditioning, Heating, and Refrigeration Institute, in Arlington, Va.

Europe is making a similar push. The European Commission has called for expedited deployment of heat pumps, and recommended that member states phase out the use of fossil-fuel heating systems in all buildings by 2035. Many European countries are subsidizing residential heat pump installation by offering grants to homeowners.

But some homeowners are hesitant to switch to heat pumps because of what’s known as “temperature anxiety.” It’s like electric-vehicle range anxiety: Homeowners are concerned about getting stuck in a cold house.

And some just like the feel of old fashioned heat. “Folks who have furnaces say they really like the way that hot heat feels when it’s coming out,” says Davis at Trane. “Heat pumps put out warm heat and it’s going to do a good job heating your home, but it’s not that hot heat that comes out of a furnace.”

Trane’s cold-climate heat pump—the one entered into the DOE’s challenge—is currently heating the home of a family in Boise, Idaho, Davis says. “We’ve had excellent feedback from our customer there, who said their energy bills went down,” she says.

Heat Pump Challenge Specs

To pass the DOE’s field test, heat pumps must operate at -15 °C with the following specifications:

• Operate at 100 percent heating capacity without relying on backup electrical resistance heat
• Demonstrate 40 percent greater efficiency than current heat pumps on the market
• Function in homes that distribute air through ductwork
• Draw heat from the air (rather than the ground or water)