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Many electrical and mechanical devices, such as car engines, produce heat as a byproduct of their normal operation. It’s called “waste heat,” and its existence is required by the fundamental laws of thermodynamics. Heat-to-power technology is also other major opportunity to make use of any waste heat exiting through engine exhaust from cars, aeroplanes or ships, and the electricity generated could then be fed back into the vehicle, lessening its environmental footprint.
Data centers house servers and similar equipment that use enormous amounts of electricity. In the process of powering the performance of the equipment, that electricity is subsequently transformed into large amounts of low-grade heat. In fact, more than 98 percent of electricity used to power the electronic equipment in data centers is shed as wasted low-grade heat energy. Furthermore, additional electricity is required to cool the data center electronics to keep the equipment operating safely and at optimal performance. Heat-to-power technology could enable Data Centers, which previously could not effectively utilize their low temperature wasted heat, but now can economically convert that heat into useful power.
Thermoelectric generators allow lost thermal energy to be recovered, energy to be produced in extreme environments, electric power to be generated in remote areas and microsensors to be powered. Thermoelectric generators are expected to fulfil significant market needs including individual cars, transportation trucks and distant sensors in energy intensive industries, e.g. metal or glass production. “Over half of the energy we use is wasted and enters the atmosphere as heat,” said Boona, a postdoctoral researcher at Ohio State. “Solid-state thermoelectrics can help us recover some of that energy. These devices have no moving parts, don’t wear out, are robust and require no maintenance. Unfortunately, to date, they are also too expensive and not quite efficient enough to warrant widespread use. We’re working to change that.”
Portable electrical power is critical to consumer and military electronics, with demands expected to grow with the deployment of the Internet of Things (IoT) and distributed sensors. Battery technology is expected to satisfy some of this demand, as well as local energy harvesting combined with the enhanced energy efficiency of electronics. This heat-to-power technology is form of Green energy harvesting aims to supply electricity to electric or electronic systems from one or different energy sources present in the environment without grid connection or utilisation of batteries. The proliferation of Internet of Things devices now brings with it a demand for non-toxic, portable power sources. Future body sensors and portable devices could be worn constantly if they harnessed body heat to be energy self-sufficient. “But to do that we need to develop suitable new thermoelectric materials that are efficient at lower temperatures, non-toxic and cheap to produce.”
High-temperature thermoelectric generators are already a key source of power for space instruments. The Mars rover, Curiosity, and the interstellar space probe, Voyager 2, harness long-lasting nuclear heat. The latter has been running on this type of power for more than 40 years. Kedar Hippalgaonkar, Jianwei Xu and their co-workers at A*STAR’s Institute of Materials Research and Engineering (IMRE) think they could soon use low-grade waste heat – think car exhaust or body heat – to power devices. “An enormous amount of low-grade waste heat is being dumped into the environment”, says Hippalgaonkar. Converting this heat into electricity is a big opportunity that shouldn’t be missed.
“Thermoelectric will be able to provide you the opportunity to realize self-powered sensors fastest,” says Hippalgaonkar. Heart rate monitors for example have very modest power needs, on the scale of a few hundreds of microwatts. A material with a ZT of 1 operating with a temperature difference of roughly 10˚C at room temperature generates roughly 50 microwatts per square centimetre, and, in theory, PHAROS’s most recent material could achieve 10 microwatts per square centimetre. So, small-scale wearable themoelectric power is already tantalisingly close to reality, Hippalgaonkar says. And once its commercial promise starts to come into play, their work will only accelerate.
Military is interested in thermoelectrics for Energy transfer, energy harvesting, thermal management, and refrigeration. According to US Military, the reductions in the Department’s need for energy can improve warfighting capabilities, such as increased range, better endurance, longer time on station, and reduced requirements for resupply. Improved energy performance also can reduce the risk and effects of attacks on supply lines and enable tactical and operational superiority.
Energy Harvester
An energy harvester consists of: an energy source (which is natural or artificial); one or more transducers that convert environmental energy into electrical energy; an energy storage device (e.g., a rechargeable battery or a capacitor that stores the harvested energy); and process control electronics.
Some of the energy harvesting technologies are solar (photovoltaic), movements (kinetic), radio-frequencies and thermal energy (thermoelectricity). This way to provide energy is further used when another energy source is not available (off-grid use) to supply small- and medium-sized electronic devices, as well as electrical systems, with power from nW to hundreds of mW. The most used energy harvesters are: thermal harvester based on the thermoelectric effect; light harvester based on the photoelectric effect; electromagnetic harvester based on induction; chemical harvester based on different reactions on the electrodes surfaces; piezoelectric harvester based on mechanical vibrations or human motion (which converts pressure or stress into electricity); radio-frequency (RF) harvester (that captures the ambient radio-frequency radiation).
Thermoelectric generators
Thermoelectric energy harvesting mainly depends on the operation of the thermoelectric generator (TEG). A TEG converts heat directly into electrical energy according to the Seebeck effect. A thermoelectric (TE) device can directly convert heat emanating from the Sun, radioisotopes, automobiles, industrial sectors, or even the human body to electricity.
A TEG is typically made by using p- and n-type doped semiconductors to create two paths that connect to metal electrodes of different temperatures, one hot, one cold. The Seebeck effect means that holes (positive electrical charge carriers) in p-type material and the electrons (negative charge carriers) in the n-type material diffuse from the hot electrode to the cold electrode, thus yielding a voltage and current flow.
Thermoelectric generators utilize thermoelectric effect like Seebeck effect, Peltier effect and the Thomson effect for energy conversion, in which an electric current is produced at the junction between two wires of different materials if they are at different temperatures. In this case, the motion of charge carriers (electrons and holes) leads to a temperature difference across this device. The voltage produced by TEGs or Seebeck generators is proportional to the temperature distance across between the two metal junctions.
A thermoelectric harvester produces green energy for energy harvesting with a multitude of advantages: maintenance-free, because of the use of highly reliable and compact solid-state device; silent and quiet; highly efficient in environmental terms because the heat is harvested from waste heat sources and converted into electricity; operation with high maximum temperatures (up to 250°C); useful scalable applications configured to harvest wide amounts of energy when necessary; possibility to harvest power from both hot surface or cold surface; green energy behaviour, being eco-friendly. A TEG device produces energy without using fossil fuel, leading to a reduction of greenhouse gas emissions.
Unlike thermodynamic PV systems or conventional heat engines (Rankine, Stirling), the energy conversion efficiency of the TEG is limited to about 5–15%. The temperature difference across the TEG system and the dimensionless thermoelectric figure-of-merit ( ZT ) have a major impact on the energy conversion efficiency. It is desirable to obtain the maximum electric output power and efficiency when a TEG system operates. In case of waste heat recovery applications, only electric output power is significant and the heat not recovered is lost. Considering that thermal energy harvesting has a reduced efficiency (5–6%), this could represent a major barrier for its extensive utilisation. An improvement in the TEG efficiency bigger than 10% has been lately obtained due to the progress of new thermoelectric materials.
The Seebeck effect occurs when a temperature difference across a conductor provides a voltage at the conductor ends. Two distinct conductors A and B are linked together to compose the junctions of a circuit. These conductors are connected electrically in series and thermally in parallel. One junction has the hot temperature Th and another one has the cold temperature Tc, with Th bigger than Tc. The Seebeck effect appears due to the thermal diffusion which provokes the motion of the charge carriers (electrons or holes) across (or against) temperature difference in the conductors.
The TEG device is composed of one or more thermoelectric couples. The simplest TEG consists of a thermocouple, comprising a pair of P-type and N-type thermoelements or legs connected electrically in series and thermally in parallel. Furthermore, the sign of the Seebeck coefficient depends on the type of carriers (electrons e− and holes h+) conducting the electric current. The p-type elements are made of semiconductor materials doped such that the charge carriers are positive (holes) and Seebeck coefficient is positive. The n-type elements are made of semiconductor material doped such that the charge carriers are negative (electrons) and the Seebeck coefficient is negative. A number of thermoelectric couples n form a TEG system wired electrically in series and sandwiched between two ceramic plates to maximise the output voltage from the TEG.
Thermoelectric generator (TEG): if ΔT is kept between the hot and cold sides of the device, an external circuit can be supplied by the voltage resulting at the TEG output terminals, providing power to the external electrical load. A single TEG generates power from 1 to 125 W. The use of more TEGs in a modular connection may increase the power up to 5 kW and ΔTmax could be bigger than 70°C.
DC-DC converter (Boost, Buck-Boost, Buck, Sepic, or Cuk converter), which is a power electronic circuit designed for voltage conversion (to convert a DC source from one voltage level to another voltage level); since the output voltage of the TEG is low or is not constant, it is necessary to provide a DC-DC converter; its role is to increase the output voltage obtained in the TEG (which depends on the number of TEGs in series and on the TEG features) corresponding the requirements of the external load.
The efficiency of the thermoelectric energy harvesting system is defined as the ratio of the electrical energy output (used or stored) to the total energy input. This efficiency also contains the electrical efficiency of TEGs, the heat exchangers efficiency, as well as the efficiency of the DC-DC converter. The total energy input especially depends on the energy obtained from the hot source. Also, the total energy input depends to a lesser extent on the mechanical energy needed to operate the thermoelectric energy harvesting system (e.g., pressure losses in the heat exchangers or cooling of the cold heat sink).
Flexible Tech Harvests Body Heat to Power Health Wearables
In a paper in Applied Energy, the researchers report significant enhancements to the flexible body heat harvester they first reported in 2017. The harvesters use heat energy from the human body to power wearable technologies—think of smart watches that measure your heart rate, blood oxygen, glucose, and other health parameters—that never need to have their batteries recharged. The technology relies on the same principles governing rigid thermoelectric harvesters that convert heat to electrical energy.
Flexible harvesters that conform to the human body are highly desired for use with wearable technologies. Superior skin contact with flexible devices, as well as the ergonomic and comfort considerations to the device wearer are the core reasons behind building flexible thermoelectric generators, or TEGs, says corresponding author Mehmet Ozturk, a professor of electrical and computer engineering at North Carolina State University. The performance and efficiency of flexible harvesters, however, currently trail well behind rigid devices, which have been superior in their ability to convert body heat into usable energy.
“The flexible device reported in this paper is significantly better than other flexible devices reported to date and is approaching the efficiency of rigid devices, which is very encouraging,” Ozturk says. The proof-of-concept TEG originally reported in 2017 employed semiconductor elements that were connected electrically in series using liquid-metal interconnects made of EGaIn—a nontoxic alloy of gallium and indium. EGaIn provided both metal-like electrical conductivity and stretchability. Researchers embedded the entire device in a stretchable silicone elastomer.
The upgraded device employs the same architecture but it significantly improves the thermal engineering of the previous version, while increasing the density of the semiconductor elements responsible for converting heat into electricity. One of the improvements is an improved silicone elastomer—essentially a type of rubber—that encapsulates the EGaIn interconnects.
“The key here is using a high thermal conductivity silicone elastomer doped with graphene flakes and EGaIn,” Ozturk says. The elastomer provides mechanical robustness against punctures while improving the device’s performance. “Using this elastomer allowed us to boost the thermal conductivity—the rate of heat transfer—by six times, allowing improved lateral heat spreading,” he says. Ozturk adds that one of the strengths of the technology is that it eliminates the need for device manufacturers to develop new flexible, thermoelectric materials because it incorporates the very same semiconductor elements used in rigid devices. Ozturk says future work will focus on further improving the efficiencies of these flexible devices.
An inexpensive thermoelectric device harnesses the cold of space without active heat input, generating electricity that powers an LED at night, researchers report.
While solar cells are an efficient source of renewable energy during the day, there is currently no similar renewable approach to generating power at night. Solar lights can be outfitted with batteries to store energy produced in daylight hours for night-time use, but the addition drives up costs.
The device developed by Raman and Stanford University scientists Wei Li and Shanhui Fan sidesteps the limitations of solar power by taking advantage of radiative cooling, in which a sky-facing surface passes its heat to the atmosphere as thermal radiation, losing some heat to space and reaching a cooler temperature than the surrounding air. This phenomenon explains how frost forms on grass during above-freezing nights, and the same principle can be used to generate electricity, harnessing temperature differences to produce renewable electricity at night, when lighting demand peaks.
Raman and colleagues tested their low-cost thermoelectric generator on a rooftop in Stanford, California, under a clear December sky. The device, which consists of a polystyrene enclosure covered in aluminized mylar to minimize thermal radiation and protected by an infrared-transparent wind cover, sat on a table one meter above roof level, drawing heat from the surrounding air and releasing it into the night sky through a simple black emitter. When the thermoelectric module was connected to a voltage boost convertor and a white LED, the researchers observed that it passively powered the light. They further measured its power output over six hours, finding that it generated as much as 25 milliwatts of energy per square meter.
Since the radiative cooler consists of a simple aluminum disk coated in paint, and all other components can be purchased off the shelf, Raman and the team believe the device can be easily scaled for practical use. The amount of electricity it generates per unit area remains relatively small, limiting its widespread applications for now, but the researchers predict it can be made twenty times more powerful with improved engineering — such as by suppressing heat gain in the radiative cooling component to increase heat-exchange efficiency — and operation in a hotter, drier climate.
“Our work highlights the many remaining opportunities for energy by taking advantage of the cold of outer space as a renewable energy resource,” says Raman. “We think this forms the basis of a complementary technology to solar. While the power output will always be substantially lower, it can operate at hours when solar cells cannot.”
Radioisotope Thermoelectric generators for Deep Space Missions
Radioisotope power systems are generators that produce electricity from the natural decay of plutonium-238, which is a non-weapons-grade form of that radioisotope used in power systems for NASA spacecraft. Heat given off by the natural decay of this isotope is converted into electricity, providing constant power during all seasons and through the day and night. RTG can generate hundreds of watts to power multiple spacefaring instruments.
Radioisotope thermoelectric generator or RTG have been used to power many deep space missions from the Cassini orbiter around Saturn, the New Horizon probe to the outer Solar System, the Curiosity rover on Mars and the veteran Voyager probes. Because an RTG has no moving parts and doesn’t require regular maintenance, it is well suited for powering gadgets that can’t be attended to for long durations.
They offer the key advantage of operating continuously, independent of sunlight, for a long time. They have little or no sensitivity to cold, radiation or other effects of the space environment. Radioisotope electrical power and heating systems enable science missions that require greater longevity, more diverse landing locations or more power or heat than missions limited to solar power systems, says NASA.
PwrCor Now Commercializes its Disruptive Heat-to-Power Technology Breakthrough
PwrCor, Inc. (OTCQB:PWCO), is an advanced technology company focused on clean, renewable energy solutions while reducing the overall cost of energy for power users. Low-grade and ultra-low-grade heat (150-212F), which up until now could only be discarded and wasted, can be cost-effectively utilized to generate electrical power, providing cost savings and improving bottom line performance for corporations globally.
The technology breakthrough exceeds cycle efficiencies of competing conventional power cycles and now enables PwrCor to exploit applications and markets, such as Data Centers, which previously could not effectively utilize their low temperature wasted heat, but now can economically convert that heat into useful power.
Based on a proprietary thermodynamic cycle, PwrCor’s technology breakthrough achieves unparalleled thermal efficiencies for a closed cycle piston engine operating at low temperature. As Tom Telegades, CEO of PwrCor, stated, “The engineering enhancement that was developed, and that PwrCor is now exclusively commercializing, is truly an engineering and scientific breakthrough in thermodynamics. At the operating temperatures currently being addressed by PwrCor’s technology, the efficiencies now exceed that of most thermodynamic power cycles, including the Rankine Cycle, used in many applications, as well as the Organic Rankine Cycle used in Waste-Heat-to-Power, Geothermal, and Solar Thermal power applications.”
PwrCorTM is a completely ‘green’ technology that uses no fossil fuels, does not operate via combustion, has no emissions, and does not process any working fluids that are flammable, harmful to the environment, or costly to replace. PwrCorTM is scalable, modular, and runs relatively silently, all within a small footprint.
The advancements afford a substantial increase of the potential maximum cycle efficiency of PwrCor’s proprietary power cycle while also realizing higher actual operating efficiency with its latest engine design. The design enhancements are expected to reduce the overall cost per kW output of the PwrCor engine, and will have little impact to the size of the engine, resulting in significantly greater power output within the same general footprint.
PwrCor is currently engaged in discussions with leading companies in such industries as fuel cells and reciprocating engines, and on additional project initiatives in oil and gas, solar thermal, and data centers, all of which have enormous amounts of wasted ultra-low-grade heat that can now be converted to additional power, contributing to higher profits. The technology cost-effectively converts heat to mechanical power or electricity, and represents a breakthrough for those corporations which can now profit from converting wasted heat into electrical power.
Cotton fibres can become conduits for ions, atoms, or molecules, and generate electricity.
Wood seems an unlikely material for the forefront of an energy revolution. But materials scientist Tian Li believes it could unlock electricity from a source of energy that is both sustainable and ubiquitous: our own body heat. The key lies in the remarkable fibrous structure of cellulose, the main component of wood, paper and cotton. These nanoscale fibres are “a beautiful structure” and a marvel of natural molecular engineering, says Li, a postdoctoral researcher at the University of Maryland. When treated with a chemical bath, the fibres can become conduits for ions, atoms or molecules with a net electrical charge. By exploiting how ions move in the presence of heat, they can generate electricity, even from small temperature differences, recovering energy that would otherwise be lost to the atmosphere.
Li and her colleagues at the University of Maryland developed a process for soaking slabs of common American basswood in a chemical bath of sodium hydroxide (T. Li et al. Nature Mater. 18, 608–613; 2019). This extracts the lignin, a natural polymer intertwined around the cellulose fibres, which gives wood its brownish hue. The treatment also breaks the hydrogen bonds along the remaining cellulose nanofibres and forms a crystalline structure with highly aligned molecular strands, along which sodium and hydroxide ions can freely travel.
When placed against a heat source, the nanostructure of the cellulose allows the sodium ions to drift away from the source more easily than the hydrogen ions, creating an electric potential that can later be tapped to release electricity. This treated cellulose can harvest heat energy from sources just 5 °C warmer than its surroundings.
Li envisions shirts made of similarly treated cotton, which is nearly pure cellulose, that could harvest body heat, storing it in a battery to recharge phones, or charging them wirelessly in pockets. “Previously, when people talked about wearable flexible devices, they put this device on top of a substrate,” she says. But in this case, a separate surface is not needed. “Your cotton t-shirt can be a device itself.” Wood could also potentially tap into other industrial sources. It is estimated that the global amount of low-grade wasted heat from industrial processes and other sources is enough to power more than 6,000 typical one-gigawatt nuclear reactors.
Thermoelectric generators for Military
In 2014, GMZ Energy successfully demonstrated a 1,000W TEG designed for diesel engine exhaust heat recapture. With the effort involved in transporting fuel to a battle site, diesel can cost the U.S. military upwards of $10.50 per liter ($40 per gallon). So using that fuel more efficiently will save the Department of Defense significant amounts of money, says Scott Rackey, GMZ’s vice president of business development.
Cheryl A. Diuguid, CEO of GMZ, said: “With the successful demonstration of GMZ’s 1,000W TEG solution, we are excited to move to the next phase of this program and begin testing in a Bradley Fighting Vehicle. In addition to saving money and adding silent-power functionality for the U.S. Military, this TEG can increase fuel efficiency for most gasoline and diesel engines. We look forward to implementing our low-cost TEG technology into a broad array of commercial markets, including long-haul trucking, heavy equipment, and light automotive.”
“GMZ’s patented half-Heusler material is uniquely well suited for military applications. The 1000W TEG features enhanced mechanical integrity and high-temperature stability thanks to GMZ’s patented nanostructuring approach. GMZ’s TEG also enables silent generation, muffles engine noise, and reduces thermal structure,” claims GMZ.
DARPA’s Materials for Transduction (MATRIX) program is seeking new materials for energy transduction ( conversion of energy from one form into another) such as communications antennas (radio waves to electrical signals), thermoelectric generators (heat to electricity) and electric motors (electromagnetic to kinetic energy) that would result in new capabilities or significant size, weight, and power (SWAP) reduction for military devices and systems.
UConn, Army Research Lab Collaborate on New Portable, Pyroelectric Renewable Energy Technology
An alternative approach is to start with an energy-dense fuel and combust to release heat, which is then delivered to a thermal energy-conversion material such as thermophotovoltaics (TPVs), thermoelectrics (TEs), or pyroelectrics (PEs). The energy density of methanol is 44 MJ/kg, compared to ∼0.5 MJ/kg for a Li-ion battery; therefore, with an overall conversion of efficiency of 3%, combustion-based systems would exceed the energy density of a Li-ion battery.
UConn’s Associate Dean for Research and Industrial Partnerships, S. Pamir Alpay, and Yomery Espinal ’18 PhD (ENG) have published a paper on a novel portable pyroelectric technology in Cell Reports Physical Science with support from the Army Research Laboratory. Pyroelectric energy research is focused on how to generate energy from heat that would otherwise be wasted in a catalytic chemical reaction. The Army Research Lab (ARL) is particularly interested in this technology because it can provide more power with less weight, which is important for soldiers carrying heavy bags.
When pyroelectric materials are heated, their polarization changes, leading to an electron flow that generates energy. These materials are commonly used in household devices like motion sensor lights, which detect body heat to determine when someone is near. Anytime there is a catalytic reaction, heat is generated. These devices harness that heat and use it as energy. For example, a combustion engine in a car produces heat that, with this kind of technology, could be used to power the electrical functions of the car that otherwise rely on battery power.
The technology proposed in this publication is portable and has an extended lifetime. It uses on-chip combustion of methanol, a high-energy fuel, to harness energy from the reaction. The pyroelectric material converts waste heat from the reaction to usable power. Vapor of a high-energy fuel, in this case methanol, is combusted on a thin, 440 nanometer film on platinized silicon wafers. The device converts the heat from this reaction into pyroelectric power. Nanostructured iridium oxide is the top electrode and combustion catalyst. Iridium is a dense, corrosion and heat-resistant metal making it an excellent candidate for this application. Iridium oxide is first activated at temperatures as low as 105 degrees Celsius and fully catalyzes methanol to carbon dioxide at 120 degrees Celsius.
This is an advantage compared to platinum-based catalysts, which do not achieve full conversion until 150 degrees Celsius. This means less heat must be applied to the device for it to be fully effective. This on-chip combustion technology has a 90% combustion efficiency rate. This technology would be significantly more powerful than lithium-ion batteries, the common rechargeable batteries used in electronics. The energy density of methanol is 22 times greater than a lithium-ion battery.
Market growth
The market for thermoelectric energy harvesters will reach over $1.1 billion by 2026, according to report by idtech. A large number of car companies, including Volkswagen, VOLVO, FORD and BMW in collaboration with NASA have been developing thermoelectric waste heat recovery systems in-house, each achieving different types of performance but all of them expecting to lead to improvements of 3-5% in fuel economy while the power generated out of these devices could potentially reach up to 1200W.
Wireless sensors powered by thermogenerators in environments where temperature differentials exist would lead to avoiding issues with battery lifetime and reliability. It would also lead to the ability to move away from wired sensors, which are still the solution of choice when increased reliability of measurement is necessary. Some applications have low enough power demands to operate with small temperature differentials, as small as a few degrees in some cases. These types of developments increase adoption trends.
In Consumer applications, the type of solution that thermogenerators provide varies: it could be related to saving energy when cooking by utilising thermo-powered cooking sensors, powering mobile phones, watches or other consumer electronics, even body sensing could become more widespread with wearables such as sensory wristbands, clothing or athletic apparel that monitor vitals such as heart rate, body temperature, etc.
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