Mainspring Energy: linear generator breakthrough?

Linear generator technology can convert any gaseous fuel into electricity, with c45% electrical efficiency, and >80% efficiency in CHP mode. This data-file reviews Mainspring Energy’s patents. We conclude that the company has locked up the IP for piston-seal assemblies in a linear generator with air bearings, but longevity/maintenance could be a key challenge to explore.


EtaGen was founded in 2010 by three Stanford engineers, and rebranded as Mainspring Energy in 2020. Its headquarters are in Menlo Park, California; and the company has c400 employees, having closed a $290M Series E financing in 2022.

Mainspring is commercializing a linear generator, which is low-cost, reliable, flexible and can use any clean fuel (e.g., natural gas, biogas, hydrogen, ammonia), in sizes from 230kW to multiple-MW, >45% electrical efficiency and >80% total thermal efficiency in CHP mode.

In a linear generator, the compression of fuel and air causes a uniform and flameless combustion reaction to occur, releasing the energy from the fuel, but creating no NOx emissions. The energy from combustion pushes a piston through a cylinder (or in Mainspring’s case, two pistons, through two cylinders). Stator magnets in each piston move past coils in each cylinder, inducing a current. An air spring on the other side of the cylinder is thereby compressed, and re-expands to drive the piston back to its starting point.

Illustration of the working principles of a linear generator.

The main advantages are the simplicity, which could in principle translate into lower capex, compared to the blades and precision-engineered compression and turbine stages within a gas turbine.

Higher efficiency can also be unlocked by harnessing the expansion of combustion gases directly, rather than having to convert it into rotary motion, per the loss attributions for conventional thermal generation. On the other hand, maximum efficiency will always be lower for low-temperature combustion, due to the laws of thermodynamics.

From reviewing Mainspring’s patents, we think there are three main challenges for commercializing linear generators. The main challenge is linked to longevity and maintenance.

Mainspring’s patents focus upon piston-seal assemblies, and seem to have locked up the IP for its linear generator designs. This may also be relevant to other companies aiming to commercialize linear generators, such as Hyliion in the vehicle sector.

Global patent filings: by region, by industry, over time?

Global patent filings

Global patent filings numbered 7M in 2022, rising at a 10% CAGR, suggesting the pace of global technology is accelerating. The patent filings also provide support for megatrends such as rising commodity demand, digitalization and electrification. However, 79% of all patents globally are now filed in China, while the US is also very strong, albeit not yet back to new peaks in its patent filing volumes, despite a clean industrial boom?


How many patents are filed globally each year? Global patent filings reached 7.4 million filings in 2021, rising at a 10-year CAGR of 12% from 2.3 million filings in 2011, and 1.5 million filings in 2001. Hence a purpose in our energy transition research is to assess new technologies that can improve the global energy system.

Is the pace of technology development accelerating? The 10-year CAGR in global patent filings has increased from 3% per year in the decade ending in 2002, to 5% in the decade ending in 2012 and 10% per year in the decade ending in 2022. This seems to support our view that technology development is accelerating, although bringing new technologies to market can still be a 10-20 year process.

Did the pace of global patent filings slow down in 2022? Our data-file is built by searching public data from the European Patent Office. Note there is a lag between patents being filed, and then approved, and coming through to the database. Hence we do not think that the 2022 data represents any kind of ‘slowdown’, it is simply a database issue.

What is the breakdown of global patent filings over time? 27% of global patents are associated with the manufacturing industry, up from 19% a decade ago, reflecting the mega trend of industrialization. 19% are physics-related, up from 7%, and this category includes innovations associated with the rise of the digital world. 14% are associated with the use of electricity, up from 7% a decade ago, and this category includes innovations associated with the mega-trend of electrification. 15% are for consumer products (aka human necessities), down from 16% a decade ago. 10% are engineering related, 9% are chemistry/materials-related technologies, 6% are construction related (chart below).

Global patent filings
Breakdown-of-Global-Patent-Filings-by-Major-Industry-Over-Time

The rise of China as an industrial power is truly staggering. Patent filings in China reached 6M in 2021, which was 79% of all global patent filings. 30% of these patents were focused on global manufacturing, compared with 14% in the US and 18% in Japan. Patent filings in China began in the mid-1980s, under Deng Xiaoping’s economic reforms, surpassing the US in 2007. The battle is trade but the war is technology. Our China research is here.

Global patent filings
China patent filings over time

A resurgence of US industrial activity and technology development? A view in our recent research is that the US is set for an unprecedented boom in the mid-late 2020s, especially on the US Gulf Coast. While we would like to substantiate this conclusion, we cannot yet find evidence of accelerating technology development in the total number of patent filings (first chart below). The patent categories making new peaks in US filings are in digital and high-tech manufacturing, while engineering and energy use is more muted (charts below).

Global patent filings
US patent filings over time
Global patent filings
US patent filings over time by segment

Technology development in Japan? Has Japan heavy manufacturing peaked? Elsewhere in the developed world, Japan’s patent activity actually peaked in 1992 and has since been declining. Manufacturing categories appear to have declined most.

Global patent filings
Japan patent filings over time

150 different sectors are covered in the data-file, which therefore gives a lens into the pace of technology development in sectors such as solar, nuclear, mining and polymers.

Global patent filings
Patent filings by major sector in the US China and Japan

The full data-file breaks down the number of global patent filings since 1920, across 150 different categories, and three different geographies, to illustrate the pace of technological progress, across each industrial sub-segment. There is always a debate about whether the quantity of patents matters. One argument is that quantity is a quality of its own. But our own view is the most meaningful way to assess IP is with qualitative patent screening.

Energy transition technologies: the pace of progress?

Energy Transition technologies progress

This data-file aggregates 20 different TSE patent screens, to assess the pace of progress in different energy technologies. Our short, 3-page summary note on the findings is linked here.


Lithium batteries are most actively researched, with 8,300 patents filed in 2019 ex-China. Autonomous vehicles and additive manufacturing technologies are accelerating fastest, with 10-year patent filing CAGRs of 22% and 53% respectively.

Wind and solar remain heavily researched, but the technologies are maturing, with patent activity -36% and -76% from peak, respectively. The steepest deceleration of interest has been in fuel cells and biofuels, declining at -10% pa and -7% since 2009.

It remains interesting to compare the pace of progress within sub-industries; for example, more supercapacitor patents were filed in 2019 than nuclear patents; while hydraulic fracturing patents remain the most intense focus area within conventional oil and gas.

LNG liquefaction: what challenges and opportunities?

Technical challenges for LNG liquefaction

This data-file tracks patent progress into LNG liquefaction plants from 2020, by reviewing forty recent patent filings from leading companies in the industry (integrated oil companies and service providers).

We reach three key conclusions:  (1) LNG capex costs should not be overly fixated upon, as they can come at the expense of higher opex and emissions intensities. (2) The next generation of modular plants offer a step-change from the first generation. (3) And new process technologies are helping to improve efficiency across different LNG process units and their fabrication.

The full data-file spells out our conclusions,  with details on each of the underlying patents, a review of companies filing LNG patents in 2020.

Metal organic frameworks: what challenges and opportunities?

Technical challenges for metal organic frameworks

This data-file reviews twenty patents from 2020 into metal organic frameworks, an exciting class of materials which could reduce the energy penalties of CO2-separation by c80% and reduce the ultimate cost from $50-150/ton to $15-40/ton.


Sorbents are classes of materials that are useful for separating industrial mixtuers, as they adsorb some compounds but not others. They can be disposed on specialized membranes, or in tanks, where compounds can be adsorbed and later desorbed by pressure swings.

Metal organic frameworks could be be particularly useful for CCS or DAC. Today’s CCS and DAC processes are only 5-10% efficient, compared to their therodynamic minimum energy, and we increasingly wonder whether AI engines can help to develop sorbents with materially better performance.

The state space of metal organic frameworks is very large. MOFs were first described 20-years ago by US chemist Omar Yaghi. In 2017, over 6,000 new MOFs were published. Over 40,000 MOFs had been identified mid-2018. Over 90,000 have been identified by 2021.

Metal organic frameworks can also be higly porous. Some fit the entire surface of a football field into a teaspoon of powder weighing less than 1 gram, e.g., 10,000 m2/g, which is c1,000x a typical zeolite.

The challenge is finding MOFs that are stable and water-resistant, then synthesizing them in continuous, mass-scale processes that do not require expensive solvents.

Costs of metal organic frameworks remain unclear. One paper suggests baseline costs of $35-70/kg. This is 1-2 orders of magnitude more expensive than today’s commercial zeolites, such as 13X, which typically range from $1.5-3/kg (tabulated here).

Reasons for high metal organic framework costs include the susceptibility to passivation by water, and the tendency for contaminants to form when deprotonating the solvents from which MOFs are synthesized.

ExxonMobil has made the visible recent progress. The data-file also captures smaller-stage companies with interesting patents in 2020. More recently, we have also screend exciting progress from Montana Technologies, using metal organic frameworks to lower the energy costs of air conditioning units by 50-75%.

Also included in this data-file are our notes from technical papers and a simple economic model. We are due an update of this data-file, which is on our to do list.

Enhanced geothermal: technology challenges?

Thunder Said Energy's patent review scores (on a five-point scale) for Eavor's enhanced geothermal technology. Scores higher on focus, but lower on intelligibility.

This data-file tabulates the greatest challenges and focus areas for harnessing enhanced geothermal energy, aka deep geothermal technology, based on reviewing patents from 20 companies in the space. In particular, we have focused in upon Eavor Technologies, which has a clear moat around its drilling, sealing and working fluid technologies.


Enhanced geothermal energy projects aim to access 50-300ÂșC temperatures in the sub-surface by drilling down to 2,000-6,000m total vertical depths. Our recent research has covered the emerging opportunities in enhanced geothermal.

But what are the key challenges for enhanced geothermal technologies? To answer this question, we have reviewed the challenges that are cited in recent patents (chart below).

Challenges for commercializing enhanced geothermal technologies identified during or patent review. The greatest remaining challenges are related to heat transfer and well design.

The patents confirm that the largest challenges for deep geothermal are drilling long multi-lateral wells, which contact sufficient reservoir volumes to transfer heat from the subsurface into the working fluids, without depleting the geothermal resource.

Recent advances from the unconventional oil and gas industry are likely to be a crucial enabler from deep geothermal, based on the comments made in the patents.

Eavor Technologies is the company that stood out most in our overview of the geothermal industry. Eavor Technologies is a private company founded in 2017, headquartered in Calgary, Alberta, employing c100 people, in order to develop a next-generation, closed-loop geothermal energy technology.

Eavor’s aspiration is that its geothermal systems can be deployed anywhere, to harness the Earth’s geothermal gradient, and provide clean, reliable, flexible baseload heat and power, without geological/exploration risk. The closed loop system does not use hydraulic fracturing, does not exchange fluids with the formation, and thus does not suffer a risk of causing water contamination or Earthquakes. Hence we have reviewed 20 patent families from Eavor.

Based on reviewing its patents, we conclude that Eavor has developed proprietary technologies to drill deep wells, into hot formations, seal them using silicates/aluminosilicates, and keep them sealed via additives in the working fluid. Specific chemical additives are clear from the patents. The full details are in the Eavor-Conclusions and Eavor-Patents tabs.

The remaining risks for enhanced geothermal technology are also discussed, based on the evidence in the patents.

Proton exchange membrane fuel cells: what challenges?

Technical challenges for hydrogen fuel cells

This data-file reviews fifty patents into proton exchange membrane fuel cells (PEMFCs), filed by leading companies in the space in 2020, in order to understand the key challenges the industry is striving to overcome.

The key focus areas are controlling the temperature, humidity and longevity of hydrogen fuel cells. But unfortunately, we find over half of the proposed solutions are likely to increase end costs.

We remain cautious on the practicalities and the economics of hydrogen fuel cell vehicles (2x most costly than conventional vehicles per km, note here) and hydrogen fuel cells for power generation (10x more costly, note here).

Solid oxide fuel cells: what challenges?

Solid oxide fuel cells technical challenges from patents

This data-file reviews fifty patents into solid oxide fuel cells, filed by leading companies in the space in 2020, in order to understand the key challenges the industry is striving to overcome.

The key focus areas are improving the longevity and efficiency of SOFCs. But unfortunately, we find many of the proposed solutions are likely to increase end costs.

Economics of SOFCs could eventually become very exciting for low-carbon heat and power (model here). But our conclusion from the latest patents is that the technology is not yet on the path to deflate and achieve cost competitiveness in the near-term.

Electric vehicle charging: what challenges?

EV Charging Challenges

This data-file tabulates the greatest challenges for charging electric vehicles, based on the recent patent literature, looking across fifty patents filed by leading companies.

Our top three conclusions are that EV charging will require complex algorithms to ensure grid stability, creating an opportunity for big data companies; vehicle-manufacturers are concerned about balancing the convenience of EV charging with the investment costs of charging networks; while interestingly, increasing speed of charging is not a primary focus.

Our conclusions are typed up in the data-file, plus the full back-up of patents from large OEMs, EV-charging specialists, capital goods companies that make components and tech giants, working on optimization algorithms.

Solar power: what challenges?

challenges of solar power

Solar panel costs have been deflating at a rate of c20% per annum as the industry scales up into manufacturing mode. The IEA recently stated solar could thus provide the “cheapest electricity in history”.

What next?  To answer this question, we reviewed 70 patents filed by leading solar manufacturers in 2020, in order to see what challenges they are aiming to resolve. We expect deflation to continue apace, while panels will also gain greater efficiency and longevity.

This data-file explains the conclusions, summarizing the findings  from the patents and giving specific examples of gains in the offing.

Specific companies’ focuses can also be seen from the patents. Covered companies include Canadian Solar, Hanergy, Jinko, LG, Miasole, Panasonic, SunPower et  al.

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