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Bitcoin Energy Debate examines electricity usage, mining costs, environmental impact, and blockchain efficiency, weighing renewable power, carbon footprint, scalability, and transaction throughput to clarify stakeholder claims from Tesla, Square, academics, and policymakers.

Key Points

Debate on Bitcoin mining's power use, environmental impact, efficiency, and scalability versus alternative blockchains.

✅ Compares energy intensity with transaction throughput and system outputs.

✅ Weighs renewables, stranded power, and carbon footprint in mining.

✅ Assesses PoS blockchains, stablecoins, and scalability tradeoffs.

There is a great debate underway about the electricity required to process Bitcoin transactions. The debate is significant, the stakes are high, the views are diverse, and there are smart people on both sides. Bitcoin generates a lot of emotion, thereby producing too much heat and not enough light. In this post, I explain the importance of identifying the key issues in the debate, and of understanding the nature and extent of disagreement about how much electrical energy Bitcoin consumes.

Consider the background against which the debate is taking place. Because of its unstable price, Bitcoin cannot serve as a global mainstream medium of exchange. The instability is apparent. On January 1, 2021, Bitcoin’s dollar price was just over $29,000. Its price rose above $63,000 in mid-April, and then fell below $35,000, where it has traded recently. Now the financial media is asking whether we are about to experience another “cyber winter” as the prices of cryptocurrencies continue their dramatic declines.

Central banks warns of bubble on bitcoins as it skyrockets
As bitcoins skyrocket to more than $12 000 for one BTC, many central banks as ECB or US Federal ... [+] NURPHOTO VIA GETTY IMAGES
Bitcoin is a high sentiment beta asset, and unless that changes, Bitcoin cannot serve as a global mainstream medium of exchange. Being a high sentiment beta asset means that Bitcoin’s market price is driven much more by investor psychology than by underlying fundamentals.

As a general matter, high sentiment beta assets are difficult to value and difficult to arbitrage. Bitcoin qualifies in this regard. As a general matter, there is great disagreement among investors about the fair values of high sentiment beta assets. Bitcoin qualifies in this regard.

One major disagreement about Bitcoin involves the very high demand for electrical power associated with Bitcoin transaction processing, an issue that came to light several years ago. In recent months, the issue has surfaced again, in a drama featuring disagreement between two prominent industry leaders, Elon Musk (from Tesla and SpaceX) and Jack Dorsey (from Square).

On one side of the argument, Musk contends that Bitcoin’s great need for electrical power is detrimental to the environment, especially amid disruptions in U.S. coal and nuclear power that increase supply strain.  On the other side, Dorsey argues that Bitcoin’s electricity profile is a benefit to the environment, in part because it provides a reliable customer base for clean electric power. This might make sense, in the absence of other motives for generating clean power; however, it seems to me that there has been a surge in investment in alternative technologies for producing electricity that has nothing to do with cryptocurrency. So I am not sure that the argument is especially strong, but will leave it there. In any event, this is a demand side argument.

A supply side argument favoring Bitcoin is that the processing of Bitcoin transactions, known as “Bitcoin mining,” already uses clean electrical power, power which has already been produced, as in hydroelectric plants at night, but not otherwise consumed in an era of flat electricity demand across mature markets.

Both Musk and Dorsey are serious Bitcoin investors. Earlier this year, Tesla purchased $1.5 billion of Bitcoin, agreed to accept Bitcoin as payment for automobile sales, and then reversed itself. This reversal appears to have pricked an expanding Bitcoin bubble. Square is a digital transaction processing firm, and Bitcoin is part of its long-term strategy.

Consider two big questions at the heart of the digital revolution in finance. First, to what degree will blockchain replace conventional transaction technologies? Second, to what degree will competing blockchain based digital assets, which are more efficient than Bitcoin, overcome Bitcoin’s first mover advantage as the first cryptocurrency?

To gain some insight about possible answers to these questions, and the nature of the issues related to the disagreement between Dorsey and Musk, I emailed a series of academics and/or authors who have expertise in blockchain technology.

David Yermack, a financial economist at New York University, has written and lectured extensively on blockchains. In 2019, Yermack wrote the following: “While Bitcoin and successor cryptocurrencies have grown remarkably, data indicates that many of their users have not tried to participate in the mainstream financial system. Instead they have deliberately avoided it in order to transact in black markets for drugs and other contraband … or evade capital controls in countries such as China.” In this regard, cyber-criminals demanding ransom for locking up their targets information systems often require payment in Bitcoin. Recent examples of cyber-criminal activity are not difficult to find, such as incidents involving Kaseya and Colonial Pipeline.

David Yermack continues: “However, the potential benefits of blockchain for improving data security and solving moral hazard problems throughout the financial system have become widely apparent as cryptocurrencies have grown.” In his recent correspondence with me, he argues that the electrical power issue associated with Bitcoin “mining,” is relatively minor because Bitcoin miners are incentivized to seek out cheap electric power, and patterns shifted as COVID-19 changed U.S. electricity consumption across sectors.

Thomas Philippon, also a financial economist at NYU, has done important work characterizing the impact of technology on the resource requirements of the financial sector. He has argued that historically, the financial sector has comprised about 6-to-7% of the economy on average, with variability over time. Unit costs, as a percentage of assets, have consistently been about 2%, even with technological advances. In respect to Bitcoin, he writes in his correspondence with me that Bitcoin is too energy inefficient to generate net positive social benefits, and that energy crisis pressures on U.S. electricity and fuels complicate the picture, but acknowledges that over time positive benefits might be possible.

Emin Gün Sirer is a computer scientist at Cornell University, whose venture AVA Labs has been developing alternative blockchain technology for the financial sector. In his correspondence with me, he writes that he rejects the argument that Bitcoin will spur investment in renewable energy relative to other stimuli. He also questions the social value of maintaining a fairly centralized ledger largely created by miners that had been in China and are now migrating to other locations such as El Salvador.

Bob Seeman is an engineer, lawyer, and businessman, who has written a book entitled Bitcoin: The Mother of All Scams. In his correspondence with me, he writes that his professional experience with Bitcoin led him to conclude that Bitcoin is nothing more than unlicensed gambling, a point he makes in his book.

David Gautschi is an academic at Fordham University with expertise in global energy. I asked him about studies that compare Bitcoin’s use of energy with that of the U.S. financial sector. In correspondence with me, he cautioned that the issues are complex, and noted that online technology generally consumes a lot of power, with electricity demand during COVID-19 highlighting shifting load profiles.

My question to David Gautschi was prompted by a study undertaken by the cryptocurrency firm Galaxy Digital. This study found that the financial sector together with the gold industry consumes twice as much electrical power as Bitcoin transaction processing. The claim by Galaxy is that Bitcoin’s electrical power needs are “at least two times lower than the total energy consumed by the banking system as well as the gold industry on an annual basis.”

Galaxy’s analysis is detailed and bottom up based. In order to assess the plausibility of its claims, I did a rough top down analysis whose results were roughly consistent with the claims in the Galaxy study. For sake of disclosure, I placed the heuristic calculations I ran in a footnote.1 If we accept the Galaxy numbers, there remains the question of understanding the outputs produced by the electrical consumption associated with both Bitcoin mining and U.S. banks’ production of financial services. I did not see that the Galaxy study addresses the output issue, and it is important.

Consider some quick statistics which relate to the issue of outputs. The total market for global financial services was about $20 trillion in 2020. The number of Bitcoin transactions processed per day was about 330,000 in December 2020, and about 400,000 in January 2021. The corresponding number for Bitcoin’s digital rival Ethereum during this time was about 1.1 million transactions per day. In contrast, the global number of credit card transactions per day in 2018 was about 1 billion.2

Bitcoin Value Falls
LONDON, ENGLAND - NOVEMBER 20: A visual representation of the cryptocurrencies Bitcoin and Ethereum ... [+] GETTY IMAGES
These numbers tell us that Bitcoin transactions comprise a small share, on the order of 0.04%, of global transactions, but use something like a third of the electricity needed for these transactions. That said, the associated costs of processing Bitcoin transactions relate to tying blocks of transactions together in a blockchain, not to the number of transactions. Nevertheless, even if the financial sector does indeed consume twice as much electrical power as Bitcoin, the disparity between Bitcoin and traditional financial technology is striking, and the experience of Texas grid reliability underscores system constraints when it comes to output relative to input.  This, I suggest, weakens the argument that Bitcoin’s electricity demand profile is inconsequential because Bitcoin mining uses slack electricity.

A big question is how much electrical power Bitcoin mining would require, if Bitcoin were to capture a major share of the transactions involved in world commerce. Certainly much more than it does today; but how much more?

Given that Bitcoin is a high sentiment beta asset, there will be a lot of disagreement about the answers to these two questions. Eventually we might get answers.

At the same time, a high sentiment beta asset is ill suited to being a medium of exchange and a store of value. This is why stablecoins have emerged, such as Diem, Tether, USD Coin, and Dai. Increased use of these stable alternatives might prevent Bitcoin from ever achieving a major share of the transactions involved in world commerce.

We shall see what the future brings. Certainly El Salvador’s recent decision to make Bitcoin its legal tender, and to become a leader in Bitcoin mining, is something to watch carefully. Just keep in mind that there is significant downside to experiencing foreign exchange rate volatility. This is why global financial institutions such as the World Bank and IMF do not support El Salvador’s decision; and as I keep saying, Bitcoin is a very high sentiment beta asset.

In the past I suggested that Bitcoin bubble would burst when Bitcoin investors conclude that its associated processing is too energy inefficient. Of course, many Bitcoin investors are passionate devotees, who are vulnerable to the psychological bias known as motivated reasoning. Motivated reasoning-based sentiment, featuring denial,3 can keep a bubble from bursting, or generate a series of bubbles, a pattern we can see from Bitcoin’s history.

I find the argument that Bitcoin is necessary to provide the right incentives for the development of clean alternatives for generating electricity to be interesting, but less than compelling. Are there no other incentives, such as evolving utility trends, or more efficient blockchain technologies? Bitcoin does have a first mover advantage relative to other cryptocurrencies. I just think we need to be concerned about getting locked into an technologically inferior solution because of switching costs.

There is an argument to made that decisions, such as how to use electric power, are made in markets with self-interested agents properly evaluating the tradeoffs. That said, think about why most of the world adopted the Windows operating system in the 1980s over the superior Mac operating system offered by Apple. Yes, we left it to markets to determine the outcome. People did make choices; and it took years for Windows to catch up with the Mac’s operating system.

My experience as a behavioral economist has taught me that the world is far from perfect, to expect to be surprised, and to expect people to make mistakes. We shall see what happens with Bitcoin going forward.

As things stand now, Bitcoin is well suited as an asset for fulfilling some people’s urge to engage in high stakes gambling. Indeed, many people have a strong need to engage in gambling. Last year, per capita expenditure on lottery tickets in Massachusetts was the highest in the U.S. at over $930.

High sentiment beta assets offer lottery-like payoffs. While Bitcoin certainly does a good job of that, it cannot simultaneously serve as an effective medium of exchange and reliable store of value, even setting aside the issue at the heart of the electricity debate.

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SM-1A Nuclear Plant Decommissioning details the US Army Corps of Engineers' removal of the Fort Greely reactor, Cold War facility dismantling, environmental monitoring, remote-site power history, and timeline to 2026 under a deactivated nuclear program.

Key Points

Army Corps plan to dismantle Fort Greely's SM-1A reactor and complete decommissioning of remaining systems by 2026.

✅ Built for remote Arctic radar support during the Cold War

✅ High costs beat diesel; program later deemed impractical

✅ Reactor parts removed; residuals monitored; removal by 2026

The US Army Corps of Engineers has begun decommissioning Alaska’s only nuclear power plant, SM-1A, which is located at Fort Greely, even as new US reactors continue to take shape nationwide. The $17m plant closed in 1972 after ten years of sporadic operation. It was out of commission from 1967 to 1969 for extensive repairs. Much of has already been dismantled and sent for disposal, and the rest, which is encased in concrete, is now to be removed.

The plant was built as part of an experimental programme to determine whether nuclear facilities, akin to next-generation nuclear concepts, could be built and operated at remote sites more cheaply than diesel-fuelled plants.

"The main approach was to reduce significant fuel-transportation costs by having a nuclear reactor that could operate for long terms, a concept echoed in the NuScale SMR safety evaluation process, with just one nuclear core," Brian Hearty said. Hearty manages the Army Corps of Engineers’ Deactivated Nuclear Power Plant Program.

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He said the Army built SM-1A in 1962 hoping to provide power reliably at remote Arctic radar sites, where in similarly isolated regions today new US coal plants may still be considered, intended to detect incoming missiles from the Soviet Union at the height of the Cold War. He added that the programme worked but not as well as Pentagon officials had hoped. While SM-1A could be built and operated in a cold and remote location, its upfront costs were much higher than anticipated, and it costs more to maintain than a diesel power plant. Moreover, the programme became irrelevant because of advances in Soviet rocket science and the development of intercontinental ballistic missiles.

Hearty said the reactor was partially dismantled soon after it was shut down. “All of the fuel in the reactor core was removed and shipped back to the Atomic Energy Commission (AEC) for them to either reprocess or dispose of,” he noted. “The highly activated control and absorber rods were also removed and shipped back to the AEC.”

The SM-1A plant produced 1.8MWe and 20MWt, including steam, which was used to heat the post. Because that part of the system was still needed, Army officials removed most of the nuclear-power system and linked the heat and steam components to a diesel-fired boiler. However, several parts of the nuclear system remained, including the reactor pressure vessel and reactor coolant pumps. “Those were either kept in place, or they were cut off and laid down in the tall vapour-containment building there,” Hearty said. “And then they were grouted and concreted in place.” The Corps of Engineers wants to remove all that remains of the plant, but it is as yet unclear whether that will be feasible.

Meanwhile, monitoring for radioactivity around the facility shows that it remains at acceptable levels. “It would be safe to say there’s no threat to human health in the environment,” said Brenda Barber, project manager for the decommissioning. Work is still in its early stages and is due to be completed in 2026 at the earliest. Barber said the Corps awarded the $4.6m contract in December to a Virginia-based firm to develop a long-range plan for the project, similar in scope to large reactor refurbishment efforts elsewhere. Among other things, this will help officials determine how much of the SM-1A will remain after it’s decommissioned. “There will still be buildings there,” she said. “There will still be components of some of the old structure there that may likely remain.”

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Washington Grid Resilience Grant funds DOE-backed modernization to harden Washington's electric grid against extreme weather, advancing clean energy, affordable and reliable electricity, and community resilience under the Bipartisan Infrastructure Law via projects and utility partnerships.

Key Points

A $23.4M DOE grant to modernize Washington's grid, boost weather resilience, and deliver clean, reliable power.

✅ Targets outages, reliability, and community resilience statewide.

✅ Prioritizes disadvantaged areas and quality clean energy jobs.

✅ Backed by Bipartisan Infrastructure Law and DOE funding.

Washington state has received a $23.4 million Grid Resilience State and Tribal Formula Grant from the U.S. Department of Energy (DOE) to modernize the electric grid through smarter electricity infrastructure and reduce impacts due to extreme weather and natural disasters. Grid Resilience State and Tribal Formula Grants aim to ensure the reliability of power sector infrastructure so that communities have access to affordable, reliable, clean electricity.

“Electricity is an essential lifeline for communities. Improving our systems by reducing disruptive events is key as we cross the finish line of a 100% clean electricity grid and ensure equitable benefits from the clean energy economy reach every community,” said Gov. Jay Inslee.

The federal funding for energy resilience will enhance and expand ongoing current grid modernization and resilience efforts throughout the state. For example, working directly with rural and typical end-of-the-line customers to develop resilience plans and collaborating with communities and utilities, including smart city efforts in Spokane as examples, on building resilient and renewable infrastructure for essential services.

“This is a significant opportunity to supplement our state investments in building a robust, resilient electric grid that supports our long-term vision for clean, affordable and reliable electricity – the foundation for economic growth and job creation that strengthens our communities and keeps Washington globally competitive. It shows once again that we are maximizing the federal funding being made available by the Biden-Harris Administration to invest in the country’s infrastructure,” said Washington State Department of Commerce Director Mike Fong.

Across the border, British Columbia's clean energy shift adds regional momentum for resilient, low-carbon power.

Goals include:

Reducing the frequency, duration and impact of outages as climate change impacts on the grid intensify while enhancing resiliency in historically disadvantaged communities.
Strengthening prosperity by expanding well-paying, safe clean energy jobs accessible to all workers and ensuring investments have a positive effect on quality job creation and equitable economic development.

Building a community of practice and maximizing project scalability by identifying pathways for scaling innovations such as integrating solar into the grid across programs.

“The Grid Resilience Formula Grants will enable communities in Washington to protect households and businesses from blackouts or power shutdowns during extreme weather,” said Maria Robinson, Director, Grid Deployment Office, U.S. Department of Energy. “Projects selected through this program will benefit communities by creating good-paying jobs to deliver clean, affordable, and reliable energy across the country.”

DOE has also announced $34 million for grid improvements to bolster reliability nationwide.

“An innovative, reliable, and efficient power grid is vital to Washington’s continued economic growth and for community resilience especially in disadvantaged areas,” said U.S. Rep. Strickland, Co-Lead of the bipartisan Grid Innovation Caucus. “The funding announced today will invest in our energy grid, support good-paying jobs, and means a cleaner, more energy-efficient future.”

Funded through the Bipartisan Infrastructure Law and administered by DOE’s Grid Deployment Office, with related efforts such as California grid upgrades advancing nationwide, the Grid Resilience State and Tribal Formula Grants distribute funding to states, territories, and federally recognized Indian Tribes, over five years based on a formula that includes factors such as population size, land area, probability and severity of disruptive events, and a locality’s historical expenditures on mitigation efforts. Priority will be given to projects that generate the greatest community benefit providing clean, affordable, and reliable energy.

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COVID-19 Impact on U.S. Wind Industry: disrupting wind power projects, tax credits, and construction timelines, risking rural revenues, jobs, and $35B investments; AWEA seeks Congressional flexibility as OEM shutdowns like Siemens Gamesa intensify delays.

Key Points

Pandemic disruptions threaten 25 GW of projects, $35B investment, rural revenues, jobs, and tax-credit timelines.

✅ 25 GW at risk; $35B investment jeopardized

✅ Rural taxes and land-lease payments may drop $8B

✅ AWEA seeks Congressional flexibility on tax-credit deadlines

In one of the latest examples of the havoc that the novel coronavirus is wreaking on the U.S. economy and the crisis hitting solar and wind sector alike, the American Wind Energy Association (AWEA) -- the national trade association for the U.S. wind industry -- yesterday stated its concerns that COVID-19 will "pose significant challenges to the American wind power industry." According to AWEA's calculations, the disease is jeopardizing the development of approximately 25 gigawatts of wind projects, representing $35 billion in investments, even as wind additions persist in some markets amid the pandemic.

Rural communities, where about 99% of wind projects are located, in particular, face considerable risk. The AWEA estimates that rural communities stand to lose about $8 billion in state and local tax payments and land-lease payments to private landowners. In addition, it's estimated that the pandemic threatens the loss of over 35,000 jobs, and the U.S. wind jobs outlook underscores the stakes, including wind turbine technicians, construction workers, and factory workers.

The development of wind projects is heavily reliant on the earning of tax credits, and debates over a Solar ITC extension highlight potential impacts on wind. However, in order to qualify for the current credits, project developers are bound to begin construction before Dec. 31, 2020. With local and state governments implementing various measures to stop the spread of the virus, the success of project developers' meeting this deadline is dubious, as utility-scale solar construction slows nationwide due to COVID-19. Addressing this and other challenges, the AWEA is turning to the government for help. In the trade association's press release, it states that "to protect the industry and these workers, AWEA is asking Congress for flexibility in allowing existing policies to continue working for the industry through this period of uncertainty."

Illustrating one of the ways in which COVID-19 is affecting the industry, Siemens Gamesa, a global leader in the manufacturing of wind turbines, closed a second Spanish factory this week after learning that a second of its employees had tested positive for the novel coronavirus.

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India Thermal Power Coal Imports surged 17.6% as CEA-monitored plants offset weaker CIL and SCCL supplies, driven by Saubhagya-led electricity demand, regional power deficits, and varied consumption across Uttar Pradesh, Bihar, Maharashtra, and Gujarat.

Key Points

Fuel volumes imported for Indian thermal plants, tracked by CEA, reflecting shifts in CIL/SCCL supply, demand, and regional power deficits.

✅ Imports up 17.6% as domestic CIL/SCCL deliveries lag targets

✅ Saubhagya-driven demand lifts generation in key beneficiary states

✅ Industrial slowdowns cut usage in Maharashtra, Tamil Nadu, Gujarat

The receipt of imported coal by thermal power plants, where plant load factors have risen, has shot up by 17.6 per cent during April-October. The coal import volumes refer to the power plants monitored by the Central Electricity Authority (CEA), and come amid moves to ration coal supplies as electricity demand surges, a power update report from CARE Ratings showed.

Imports escalated as domestic supplies by Coal India Ltd (CIL) and another state run producer- Singareni Collieries Company Ltd (SCCL) dipped in the period, after earlier shortages that have since eased in later months. Rate of supplies by the two coal companies to the CEA monitored power stations stood at 80.4 per cent, indicating a shortfall of 19.6 per cent against the allocated quantity.

According to the study by CARE Ratings, total coal supplied by CIL and SCCL to the power sector stood at 315.9 million tonnes (mt) during April-October as against 328.5 mt in the comparable period of last fiscal year.

The study noted that growth in power generation during the April-October 2019, with India now the third-largest electricity producer globally, was on account of higher demand from Pradhan Mantri Sahaj Bijli Har Ghar Yojana or Saubhagya Scheme beneficiary states. Providing connection to households in order to achieve 100% per cent electrification has in part helped the sector avert de-growth, as part of efforts to rewire Indian electricity and expand access.

Large states namely Uttar Pradesh, Bihar, Punjab, West Bengal and Rajasthan have recorded over five per cent growth in consumption of power. These states along with Odisha, Madhya Pradesh and Assam accounted for 75 per cent of the beneficiaries under the Saubhagya Scheme (Household Electrification Scheme). The ongoing economic downturn has led to a sharp fall in electricity demand from industrialised states. Maharashtra, which is also the largest power consuming state in India, recorded a decline in consumption of 5.6 per cent.

Other states namely Tamil Nadu, Telangana, Gujarat and Odisha too recorded fall in power consumed, echoing global dips in daily electricity demand seen later during the pandemic. These states house large clusters of mining, automobile, cement and other manufacturing industries, and a decline in these sectors led to fall in demand for power across these states. - The demand-supply gap or power deficit has remained at 0.6 per cent during the April-October 2019. North-East reported 4.8 per cent of power deficit followed by Northern Region at 1.3 per cent. Within Northern Region, Jammu & Kashmir and Uttar Pradesh accounted for 65 per cent and 30 per cent respectively of the regions power supply deficit.

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Near-Field Thermophotovoltaics captures radiated energy across a nanoscale gap, using thin-film photovoltaic cells and indium gallium arsenide to boost power density and efficiency, enabling compact Army portable power from emitters via radiative heat transfer.

Key Points

A nanoscale TPV method capturing near-field photons for higher power density at lower emitter temperatures.

✅ Nanoscale gap boosts radiative transfer and usable photon flux

✅ Thin-film InGaAs cells recycle sub-band-gap photons via reflector

✅ Achieved ~5 kW/m2 power density with higher efficiency

With the addition of sensors and enhanced communication tools, providing lightweight, portable power has become even more challenging, with concepts such as power from falling snow illustrating how diverse new energy-harvesting approaches are. Army-funded research demonstrated a new approach to turning thermal energy into electricity that could provide compact and efficient power for Soldiers on future battlefields.

Hot objects radiate light in the form of photons into their surroundings. The emitted photons can be captured by a photovoltaic cell and converted to useful electric energy. This approach to energy conversion is called far-field thermophotovoltaics, or FF-TPVs, and has been under development for many years; however, it suffers from low power density and therefore requires high operating temperatures of the emitter.

The research, conducted at the University of Michigan and published in Nature Communications, demonstrates a new approach, where the separation between the emitter and the photovoltaic cell is reduced to the nanoscale, enabling much greater power output than what is possible with FF-TPVs for the same emitter temperature.

This approach, which enables capture of energy that is otherwise trapped in the near-field of the emitter is called near-field thermophotovoltaics or NF-TPV and uses custom-built photovoltaic cells and emitter designs ideal for near-field operating conditions, alongside emerging smart solar inverters that help manage conversion and delivery.

This technique exhibited a power density almost an order of magnitude higher than that for the best-reported near-field-TPV systems, while also operating at six-times higher efficiency, paving the way for future near-field-TPV applications, including remote microgrid deployments in extreme environments, according to Dr. Edgar Meyhofer, professor of mechanical engineering, University of Michigan.

"The Army uses large amounts of power during deployments and battlefield operations and must be carried by the Soldier or a weight constrained system," said Dr. Mike Waits, U.S. Army Combat Capabilities Development Command's Army Research Laboratory. "If successful, in the future near-field-TPVs could serve as more compact and higher efficiency power sources for Soldiers as these devices can function at lower operating temperatures than conventional TPVs."

The efficiency of a TPV device is characterized by how much of the total energy transfer between the emitter and the photovoltaic cell is used to excite the electron-hole pairs in the photovoltaic cell, where insights from near-light-speed conduction research help contextualize performance limits in semiconductors. While increasing the temperature of the emitter increases the number of photons above the band-gap of the cell, the number of sub band-gap photons that can heat up the photovoltaic cell need to be minimized.

"This was achieved by fabricating thin-film TPV cells with ultra-flat surfaces, and with a metal back reflector," said Dr. Stephen Forrest, professor of electrical and computer engineering, University of Michigan. "The photons above the band-gap of the cell are efficiently absorbed in the micron-thick semiconductor, while those below the band-gap are reflected back to the silicon emitter and recycled."

The team grew thin-film indium gallium arsenide photovoltaic cells on thick semiconductor substrates, and then peeled off the very thin semiconductor active region of the cell and transferred it to a silicon substrate, informing potential interfaces with home battery systems for distributed use.

All these innovations in device design and experimental approach resulted in a novel near-field TPV system that could complement distributed resources in virtual power plants for resilient operations.

"The team has achieved a record ~5 kW/m2 power output, which is an order of magnitude larger than systems previously reported in the literature," said Dr. Pramod Reddy, professor of mechanical engineering, University of Michigan.

Researchers also performed state-of-the-art theoretical calculations to estimate the performance of the photovoltaic cell at each temperature and gap size, informing hybrid designs with backup fuel cell solutions that extend battery life, and showed good agreement between the experiments and computational predictions.

"This current demonstration meets theoretical predictions of radiative heat transfer at the nanoscale, and directly shows the potential for developing future near-field TPV devices for Army applications in power and energy, communication and sensors," said Dr. Pani Varanasi, program manager, DEVCOM ARL that funded this work.

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