A hot new way to recycle

Sitting in his office, UC Berkeley Professor of Materials Science and Engineering Lane Martin listens to the tiny, whirring fan that cools his laptop. To him, the hum isn’t an annoyance—it’s the sound of a wasted resource with big potential.

Despite efforts to make generators and devices more energy-efficient, most of the power created by burning fossil fuels is ultimately wasted as heat. Unless we find a way to reduce energy losses, we could face a future of power shortages and environmental problems. With recent advances in materials that can soak up waste heat and turn it back into electrical power, new opportunities are opening up to help us save power and the planet.

The majority of the energy our laptops, smartphones, and electronics take in is wasted through the production, and subsequent loss, of heat. “All electrically powered devices require some form of electrical current to be carried through them,” explains Gabriel Velarde, a graduate student in materials science and engineering who is working on saving wasted power using tiny heat-capturing devices. “Running electricity through wires, even those that have low resistance, naturally creates and gives off heat.” And some electronics are more wasteful than others. Even the most efficient heat conversion processes only manage to convert half of the power they use into useful work. Even virtual services we enjoy, like the Internet, produce vast amounts of waste heat energy from the data centers that support them.

Over 29 trillion kilowatt hours of energy are generated in the United States each year from a mix of fossil fuel and renewable energy sources. That’s enough energy to power every household in California for 30 years. But according to Monthly Energy Review data from the US Energy Information Administration (EIA), nearly 68 percent of this energy is ultimately wasted in the form of heat. In fact, nearly half of the wasted energy comes from the process of generating the electricity itself.

Ideally, we could reclaim most of the lost energy, but a mere eight percent of the wasted heat is currently useful. Only waste heat over 650°C has enough thermal energy to be recovered and converted back into useful forms of energy, such as turning water into steam to generate electricity. The other 92 percent of heat is under 230°C and harder to reclaim and use simply because it is lower in energy.

Martin and his students are working to produce devices that could capture waste heat and use it to generate power. To do this, they are shrinking a power generating process down to the nanoscale and using thin-film materials that give out tiny bursts of electrical charge when heated. By harvesting the low temperature heat that’s lost from our car engines, kitchen appliances, and even our smart phones, we could significantly decrease our energy demands.

Where does waste heat come from?

An increasing global reliance on new technologies is driving greater power demands from already hard-pressed industries. From mining the metal ores needed to make cars to powering machinery needed to produce laptops and smartphones, energy is required at every stage of manufacturing. Many industrial processes also use heat directly, such as smelting aluminum to make drink cans or heating ovens to bake bread. In addition, many industrial processes intentionally release waste heat in the form of hot exhaust gases, energy purposely lost from cooling processes, and heat lost from hot equipment surfaces.

Every object we interact with needed energy to be manufactured and transported and will require energy for its eventual disposal after we finish using it. The majority of the energy consumed by these objects is generated in power plants, which burn natural gas, coal, and oil to produce around 80 percent of the energy used in industry across the US. Only 20 percent of the energy used in industry comes from biomass, solar, wind, or hydro-based sources. While renewable energy sources continue to grow, they will not solve all of our power problems. A seemingly endless demand for consumer goods means our ability to meet energy demands is unsustainable. By 2050, the US population is predicted to grow to 398 million people, and the industrial power used to produce the goods they want is projected to grow 70 percent by 2050. Simply producing more power is not enough—we need to save the power we are producing and wasting.

One of the challenges in the race to capture waste heat is the sheer scale of the amounts that will be produced. The EIA has predicted that global power use will grow by over a quarter in the next 20 years, and with many high-tech industries continuing to grow as well, more and more of this energy will be channeled into the electrical devices that create valuable waste heat energy.

For example, Bitcoin mining, the process to convert computer run time into digital currency, uses 3.4 gigawatts of power every six months across the world, and its power demands are predicted to keep increasing. Energy use in Bitcoin mining resulted in an estimated 3.3–16.5 million tons of CO2 emissions in the last two years alone, according to recent work published in Nature Sustainability.  Other computer-based processes also contribute to the massive global production of waste heat. “Super computers can use 10 percent of a coal power station’s output,” notes Martin. In 2015, Google used 5.7 terawatt hours of electricity for their operations, which is nearly as much electricity used by the entire city of San Francisco in the same year.

Efforts exist to increase the sustainability of power sources, but we cannot limit energy requirements as our global population grows. As the numbers of factories, electric vehicles, screens, and consumer products continues to increase, so does our need for capturing their wasted heat. Solving the problem of heat waste could be a key factor in reducing the environmental and economic impact of keeping our lifestyles powered.

Warming me, warming you

Waste heat is known to have a large impact on the health of our urban environments. The phenomenon is known as the heat island effect, and it can raise temperatures in these environments by up to 12°C. In turn, increased temperatures lead to the burning of more fossil fuels to cool homes and businesses, thus producing even more greenhouse gases. The electric power sector alone accounted for around 40 percent of total energy-related carbon dioxide emissions in the US. Martin explains that solving the waste heat problem is “about limiting the amount of resources needed to generate that heat. We could limit the fossil fuel we have to expend to get the energy we need, and to reduce that could remove millions of tons of CO2 being poured into the atmosphere.” Finding ways to reclaim some of the 68 percent of wasted heat energy has led to UC Berkeley becoming a hotbed of innovation.

Low-quality waste heat is a massive untapped energy source.
Top: In a typical heat engine, motion is generated through the expansion and contraction of a gas as heat is periodically applied. Since heat is lost in each cycle, a high quality heat source is required for useful efficiencies.
Bottom: A nanometer-thick pyroelectric film generates alternating electric current as it is heated and cooled, through changes in polarization. This system can be optimized to operate with a low quality heat source.

Waste not, want not

Because of booming global technology growth and its environmental impacts, reclaiming waste heat has become a necessity worldwide. “Modern computers are energy hungry,” notes Martin, “[and] if we can get waste heat from [computers], even just 10 percent, it’s a marked impact on the system.”

To understand how waste heat may be reclaimed, it’s important to know that it can come in two different forms: high-quality heat, which is over 500°C, and low-quality heat, which comes from cooler temperatures around 100-200°C.  Engineers have long been working on reclaiming high-quality heat from large scale industrial processes. High-quality heat can be used to create steam that spins generator turbines. Until recently, however, scaling this process down for low-quality heat has not been feasible because the lower temperatures are just not energetic enough to drive the processes that turn heat into useful energy.

Reclaiming the low-quality heat given off by our gadgets, data centers, and other non-industrial sources of heat has been a challenge, but Martin and his graduate students—in collaboration with mechanical engineers at UC Berkeley—have recently figured out how to make tiny engines that can create electricity from low-quality waste heat.

Similar to industrial heat driving generator turbines to make power, the Berkeley scientists are using low-quality waste heat to warm up a special material that generates an electrical potential when heated or cooled. As the temperature changes, it becomes polarized, meaning the positive and negative charges move to opposite ends of the material. Polarization creates the electrical potential that can be harvested to create electricity.

To create a repeating cycle of changing temperatures, these so-called pyroelectric materials can be paired with relaxor ferroelectric materials, which are materials that change their properties in response to electric fields. As the pyroelectric material heats up, the ferroelectric material changes its properties, allowing the whole cycle to begin again. When these materials are compiled in layers, they create a tiny heat engine that can turn low-quality heat, such as waste heat from a laptop, into useful power.

Producing power at the nanoscale

Pyroelectric materials have been known in the scientific community for many years, but only in the last century have scientists begun to understand their physical effects. And it has only been in the last few decades that scientists have begun to shrink such materials down to the nano-scale to explore their properties in those states. Working at this tiny scale also means being able to take advantage of advances in materials and geometries that drive improved function, opening the possibility to recover waste heat energy from new sources and with ever increasing efficiencies. By using materials that are only nanometers thick, even tiny amounts of heat energy can be converted into more useful forms of energy, such as electricity.   

Finding materials with the thermal and mechanical properties to soak up and convert heat energy is a big challenge, as is figuring out how to fabricate and arrange them in a useful design. By taking advantage of new nanolithography techniques, which can create layers of films nanometers thick, it has become possible to build tiny heat engines at a scale roughly a million times smaller than the width of a human hair.

Martin and his collaborators developed a number of innovative new technologies to solve the technical challenges of reclaiming heat waste.  The films they work with are so small that even measuring temperature across a film weighing a millionth of a gram was a feat.

“These very thin materials are challenging to measure because the collected response is not only small but also requires us to know exactly what we’re looking for. Capturing energy from these materials becomes a challenge since most of the waste heat available is considered low grade [below 200°C], which a lot of materials are inefficient in converting,” explains Velarde, who is currently working on improving these materials as part of his doctoral research with Martin.

After finding pyroelectric and ferroelectric materials that have the perfect properties to convert heat into electric charge, Martin and his collaborators also had to figure out how to arrange them in the right layout. The thin films of the materials have to be assembled to work like a tiny machine that cycles through absorbing thermal energy, converting it into electrical energy, and resetting itself to begin the process again—all within a few milliseconds.

The breakthrough came in not just creating a material that could soak up heat and turn it into electric current, but in finding a way to layer the film so that it’s small enough to take advantage of low-quality heat. Despite the low-quality heat having only a few joules of energy, it’s enough to power nanoscale heat engines to produce tiny amounts of electricity.

With a way to recover small amounts of waste heat, the possibility of saving low-quality heat is becoming more real and paving the way towards saving some of the 68 percent of energy wasted from the 29 trillion kilowatt hours of energy generated in the United States. One could imagine a future with heat-recovery devices attached to every home appliance, industrial machine, or computer feeding the energy back into the grid.

Empowering the future

The human body’s heat can be harnessed to power wearable devices.

Scientists and engineers aren’t the only ones working to harvest power from waste heat. The value of free power is not lost on entrepreneurs and inventors who are creating products that can power themselves from wasted energy. Around the world, designers are already working on devices that are powered by our own body heat. Even when sitting perfectly still, the human body gives off around 100 watts in waste heat, the equivalent of the power used by an incandescent lightbulb. Products that can exploit this body heat are already in the prototype stage. One example is the MATRIX PowerWatch, which has already raised over a million dollars in crowdfunding. Despite the technological challenges, the possibility of powering our gadgets, homes, and infrastructure from the energy we let go to waste is getting closer. “Within days after our work was published, I got a call asking if it could be used to cool the computers that were mining Bitcoin and make them more efficient,” recalls Martin. While these materials have some distance to go before being turned into a widely-used heat reclamation technology, even small savings of waste energy represents a huge potential.

Using waste heat to power technologies could also have profound impacts on low-resource countries, where power generation is unreliable. Devices that reclaim heat energy from phones, or people themselves, and turn it into power for medical equipment could solve many problems for rural communities. Researchers at North Carolina State University are already testing out devices that reclaim heat from the body to power electrocardiogram equipment, suggesting opportunities for future medical devices that could work where there is simply no infrastructure to power life-saving equipment. One could imagine clothing that soaks up body heat to power a smartphone, connecting even the most rural communities to the internet. Or even shoes that turn the energy from the friction of walking into a source of light. If we can save just some of the 29 trillion kilowatt hours of power generated in the US from being wasted, we can help save our planet—one joule at a time.


Sonia Travaglini is a graduate student in mechanical engineering

Design: Dana Goodacre

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