DARPA-SS-27-01.pdf

DARPA-SS-27-01

Request for Information (RFI)

DARPA-SS-27-01

Solid-State Energy Conversion from Heat to Electricity

Responses Accepted: Until 4:00 p.m. (Eastern) on November 23, 2026 Point of Contact: Mike McDonald, Program Manager, DARPA/DSO Email Address: DARPA-SS-27-01@darpa.mil

The Defense Advanced Research Projects Agency (DARPA) Defense Sciences Office (DSO) is requesting information on the current state of the art and technological limitations of solid-state direct thermal-to-electric energy conversion technologies. We define solid state energy conversion as systems that convert a temperature differential directly into electrical energy without utilizing mechanical moving parts (e.g., pistons, turbines, or rotary alternators). This RFI seeks to identify the best possible solid state heat engine performance across a wide operating temperature range, factors that limit performance, and the most attractive use cases if these highly efficient solid state heat engines could be made today. This survey of limits, challenges and use cases will identify opportunities for research and development to inform potential program(s) in this area.

BACKGROUND

Much energy generation today relies on converting heat to electricity in heat engines using thermodynamic cycles (e.g., Brayton, Stirling, and Rankine cycles). These systems are well characterized and >50% efficient at utility scale (gigawatts, GW) but achieve more modest efficiencies of 30-40% in the strategic (megawatts, MW) and 15-25% in the tactical (kilowatts, kW) power ranges. Mechanical systems introduce vulnerability to friction, seal wear, vibration, lubrication and more, presenting complex maintenance requirements. In contrast, solid-state direct energy conversion uses electronic, photonic, or electrochemical transport to generate electricity directly from a heat source with no moving parts. In principle this allows for higher reliability, favorable scalability across power levels, and near instantaneous start-up and responsiveness. Energy conversion directly from heat to electricity offers the potential for high theoretical efficiencies, due to the ability to leverage operating temperatures far above mechanical creep limits, increasing the Carnot ceiling.

Direct thermal-to-electric conversion methods date to the 1800s, and development efforts surged in the mid-late 20th century, yet realized efficiencies have remained stubbornly low, on the order of 3-7% for thermoelectric devices and 5-15% for thermionic systems despite higher theoretical values.1,2 Thermoelectric generators (TEGs) produce power across a hot material junction. Since the junction conducts electricity and heat, the ratio of conductivities captured in the material’s ZT factor limits TEG performance. Thermionic energy converters (TECs) evaporate electrons across a vacuum gap, decoupling electrical from thermal conductivity. However, TECs require specialized materials with a low work function barrier to electron evaporation, operating at temperatures typically >1000 K, and maintaining an enormous temperature gradient across the vacuum gap. TECs are subject to a “space charge” limit on power density due to electron repulsion transiting the gap against further electron emission, requiring either micrometer gaps

1 Ryu, B., et al. (2023). "Best thermoelectric efficiency of ever-explored materials." iScience, Vol. 26, Issue 4, 106494.

2 Hao, M., Xiao, G., & Qiu, H. (2025). "Experimental characteristics of thermionic energy converters employing barium-dispenser cathode and semiconductor anodes." Energy, Vol. 325, 136132.

mailto:DARPA-SS-27-01@darpa.mil

<10 µm aps or introducing costly plasma to mitigate the space charge. Other direct conversion methods include thermophotovoltaic (TPV) converters, alkali metal thermal-to-electric (AMTEC) converters, and hybrid architectures bridging multiple methods.

Today solid-state energy converters’ relatively low efficiency, challenging material requirements, and modest power output have relegated them to niche applications in favor of more mature steam-based Rankine cycles, closed loop Brayton systems, or internal combustion engines. The largest application of TEGs today is in waste heat recovery and remote off-grid pipeline applications. Thermionic and AMTEC systems have lain dormant since the 1990s, despite laboratory demonstrations at 15-20% efficiency, as the demand for high-temperature conversion of nuclear power collapsed. Meanwhile improvements in TPV cell efficiency have sparked recent interest in harvesting energy from long-term thermal battery storage, but semiconductor-based TPV systems are limited to cold side operating temperatures near room temperature.3

This RFI is motivated by whether recent developments around long-standing physical limits in solid-state conversion could push realizable efficiency toward the ultimate Carnot limits. For example, do microfabricated spacers in thermionic converters enable high power density and efficiency without the complication of plasma neutralization?4 Could low or tunable work function emitter materials, or band structure and surface termination engineering, offer cooler operating regimes?5 Do hybrid approaches, such as thermionic/thermophotovoltaic devices enable more efficient architectures? Are there ways to dramatically accelerate screening for adverse material interactions in AMTEC systems? Advancements in nanomaterials, microfabrication, high-temperature structural ceramics, nanostructured coatings, wide-bandgap semiconductors, sub-micron thermal radiative engineering and more all motivate this inquiry into the new art of the possible.

REQUESTED INFORMATION

DARPA seeks information on the current state of the art and technological limitations of solid-state direct thermal-to-electric energy conversion technologies. This request is intentionally use-case and heat-source agnostic to consider a broad range of technologies and identify opportunities with the potential for significant return on investment. Responses may address one, several, or all of the following three questions:

• Current Capability: What are the most efficient solid-state converters realizable today?

o Distinguish if the device currently exists in an integrated system, or if the key constituent pieces exist but have not yet been demonstrated in a device. For key constituent pieces that do not yet exist, but could with focused effort, see the “Technology Potential” section below.

o Define the operating point (i.e., hot side temperature, cold side temperature, and power output), the efficiency calculation method, any limits on the operating temperature range both for hot side and cold side, and the device size, weight and

3 MIT & NREL (2022). "Thermophotovoltaic efficiency of over 40% using ultra-high-temperature selective emitters." Nature, Vol. 604, pp. 287–291.

4 Campbell, G., et al. (2021). "Review of Modern Micron-Gap Vacuum Thermionic Converters and High-Temperature Materials." Journal of Energy Conversion and Management, Vol. 245, 114532.

5 Bellucci, S., et al. (2026). "Next-Generation Materials for Solid-State Direct Energy Conversion: Perovskite Oxides and Functionalized CVD Diamond." Advanced Energy Materials, Vol. 16, Issue 4, 2500123.

power (SWaP).

• Technology Potential: What performance improvements may be achievable, and what evidence indicates that current technical barriers can be overcome?

o Be quantitative, and if available, use data to show how these challenges limit performance today and how a solution might improve performance.

• Applications: What applications would most benefit from improved solid-state thermal-to-electric conversion?

o Identify the current pain points from existing solutions to your suggested use case, and what quantitative thresholds in efficiency, output power, or SWaP would make a solid-state system the clear favorite over existing options.

DARPA recognizes candidate technologies may differ substantially in physical scale, operating temperature range, power density, and efficiency. This RFI will inform the potential for future DARPA programs in this area across a range of technical approaches and use cases. For example, systems requiring cold-side temperatures near 300 K may be suitable for terrestrial use, while systems that can operate with higher cold-side temperatures may enable efficient radiative cooling for space systems.

Respondents are encouraged to submit technical information, models, and/or test data. Please identify the current technology readiness level (TRL) of demonstrated performance and projected improvements. DARPA welcomes responses on low-TRL concepts and mature technologies.

SUBMISSION FORMAT

Responses to this RFI should be succinct and provide actionable insight. Page limits for each section appear below. Format specifications for responses include 12-point font, single-spaced, single-sided, 8.5 by 11-inch paper with 1-inch margins in MS Word or Adobe PDF format.

A. Cover Sheet (1 page): Provide the following information.

1. Response title

2. Technical point of contact name, organization, telephone number, and email address

B. Technical Description (5 pages)

C. Bibliography/References (2 pages)

D. Description of relevant prior research in this area, experimental capabilities (if applicable), and/or list of relevant publications (1 page)

E. Graphic Overview Slide (Optional): A single PowerPoint slide that graphically depicts the main ideas of the response is optional but highly encouraged (1 slide)

SUBMISSION INSTRUCTIONS AND CONTACT INFORMATION

Respondents should email all unclassified responses to this RFI to DARPA-SS-27- 01@darpa.mil. To the maximum extent possible, respondents should send non-proprietary information. Respondents are responsible for clearly identifying any page conveying proprietary information with a label such as “Proprietary” or “Company Proprietary.” DO NOT INCLUDE

ANY CLASSIFIED INFORMATION IN THE RFI RESPONSE TO THE UNCLASSIFIED

EMAIL ADDRESS.

DARPA will accept responses any time from the publication of this RFI until the date and time listed above. DARPA encourages respondents to respond early.

Send all technical and administrative correspondence and questions regarding this RFI to the email address in the first paragraph above. Emails sent directly to the DARPA Program Manager may result in a delayed response or no response at all.

ELIGIBILITY

DARPA invites those engaged in related research activities to participate and appreciates responses from all capable and qualified sources, including, but not limited to, universities, university-affiliated research centers (UARC), Federally Funded Research and Development Centers (FFRDC), private or public companies, and Government research laboratories.

DISCLAIMERS AND IMPORTANT NOTES

• This is an RFI issued solely for information and new program planning purposes; it does not constitute a formal solicitation for proposals. In accordance with Federal Acquisition Regulation 15.201(e), responses to this RFI are not offers and cannot be accepted by the Government as such.

• Responses do not bind DARPA to any further actions related to this topic, including requesting follow-on proposals from respondents to this RFI.

• Submission to this RFI is voluntary and is not required to propose to a subsequent solicitation (if any) on this research topic.

• DARPA will not provide reimbursement for costs incurred in responding to this RFI.

• Respondents are advised that DARPA is under no obligation to acknowledge receipt of the information received or provide feedback to respondents with respect to any information submitted under this RFI.

• DARPA will disclose submission contents only for the purpose of review. Submissions may be reviewed by the Government (DARPA and other Government partners);

FFRDCs; and Scientific, Engineering and Technical Assistance (SETA) support contractors.

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