Exploring High Efficiency, High Temperature Modular Power Generation with Dr. Michael Ohadi
Originally published October 17, 2017. This blog was updated January 8, 2018.
Dr. Michael Ohadi discusses his background, how he came to ARPA-E, and how high efficiency, compact power systems could be a game changer for a range of industries.
Tell us a little about your background. How does your previous experience provide insight into your work at ARPA-E?
Mike: My focus at ARPA-E includes heat/mass transfer enhancement and process intensification using innovative designs, materials, and manufacturing techniques. Currently, I am on leave from my role as a professor of mechanical engineering and co-founder of the Center for Environmental Energy Engineering at the University of Maryland, College Park (UMD). During a previous leave from UMD, I assisted in the creation of an engineering school to serve the engineering needs of the energy sector in the United Arab Emirates.
I’ve always been interested in fundamental research that carries big practical implications. Since early in my academic career, I’ve had support from both industry and government in pursuit of this kind of work, and for over 20 years, I have participated in an industrial consortium in Advanced Heat/Mass Exchangers and Process Intensification techniques, with member companies from the United States, Europe and Asia. I earned my Ph.D. in Mechanical Engineering from the University of Minnesota, and I am a fellow of the American Society of Mechanical Engineers and the American Society of Heating, Refrigeration, and Air-Conditioning Engineers (ASHRAE).
What brought you to ARPA-E?
Mike: I am interested in high temperature power systems because of their higher efficiency and potentially smaller size. In 2001, my colleague and I wrote an article for the Experimental Thermal and Fluid Science journal in support of this field. Now, nearly two decades later, I am delighted to say the community has made the kind of progress in materials and manufacturing techniques needed to put future high-temperature systems within reach.
Because ARPA-E focuses on early-stage research that isn’t mature enough for private-sector investment, I feel I am in the right place to foster innovation in thermal/fluid systems design and manufacturing/fabrication disciplines.
On a side note, I was a former principal researcher on an ARPA-E IDEAS project—which paved the way for the ARID program—and then went on to join an ARID project team. Now, it is my turn now to work with the technical community, take advantage of the infrastructure that ARPA-E provides, and define next-generation energy systems that can run at higher temperatures while being more efficient, compact and affordable.
You’re leading an upcoming workshop that explores innovative, high efficiency, modular electricity generation systems—why is ARPA-E potentially interested in this technology area?
Mike: We’re looking at modular power systems that can run at higher temperatures and, as a result, achieve higher efficiency. We’re also interested in leveraging supercritical working fluids that require attractively smaller pressure ratios, which reduces the need for many stages of turbomachinery, enabling a substantially smaller footprint for these systems while yielding better efficiency and environmental sustainability. ARPA-E advances research that has the potential to radically improve U.S. economic prosperity, national security and environmental well-being. If we can build energy systems that run at an average temperature of 1000°C, it may be possible to reach greater than 60% efficiency compared to today’s 35% at 550°C for a comparable modern fossil fuel power plant.
Modular power—typically smaller systems that can be deployed in more places—is advantageous because it can avoid the costs and energy losses (usually 5-7%) of transmitting and distributing electricity over long distances while also lowering capital costs. Modular power provides greater flexibility to the grid, promotes energy security and could be an enabling technology for compact modular nuclear reactors, possibly underground. Right now, the biggest obstacle to a below-ground system is the size of the power generation unit, so a small, compact, modular unit would be complementary to existing technology. Other applications could include power for standalone natural gas to electricity, conversion of flared gas to useful local power production sites, commercial ships, aerospace, defense (e.g. remote operating bases) and concentrating solar power.
Any knowledge gained from this workshop or forthcoming efforts would complement ongoing initiatives at the Department of Energy like DOE’s investment into larger scale (10+ MW) supercritical cycle power plants. In addition, smaller scale technology advances could enhance systems being developed under these efforts such as materials, bearings and seals, and manufacturing capabilities.
Dr. Michael Ohadi answering questions following his Fast Pitch at the 2017 ARPA-E Energy Innovation Summit
You mentioned DOE’s efforts in developing power cycle technology with a specific focus on supercritical CO2-based cycles. Will your envisioned effort also focus on supercritical cycles or are other power cycles with different kinds of working fluids also of interest? Where would these solutions fit into a broader or longer term solution?
Mike: First, let’s define what supercritical cycles are. Supercritical cycles are made up of supercritical fluids, which exist at a combination of temperature and pressure that puts them above a critical point—when the fluid is in equilibrium between a liquid/gas phases. Operating beyond the critical point means that, in the supercritical region, the fluid is operating in a single phase. Systems using supercritical fluids benefit from this as they don’t require equipment that has to accommodate both liquid and gas phases like boilers and condensers. Because of this, supercritical systems can be much simpler and more compact.
As for why CO2 cycles are preferred—generally, CO2 is a popular choice because it has excellent thermophysical properties, such as its high heat capacity. Through this workshop and forthcoming discussions, we want to encourage the community to overcome the unique technical challenges of using supercritical CO2 for small systems under 1 MW output. We want to explore if it is possible to develop power cycles based on other fluids that achieve the affordable, high efficiency targets I envision, including closed-loop Brayton cycles (such as ones with air or argon as the working fluid) or perhaps even cycles using liquid metal working fluids. We are excited to see what creative ideas the community has along these lines.
What gaps currently exist in this technology area and what are some areas for advancement? How do these technical challenges and developments differ from the other ongoing DOE-funded projects you previously mentioned?
Mike: One major unique challenge in this area is temperature! We are targeting a minimum of 700°C, and ideally even higher (1000°C) operating temperatures on the “hot side” of the power cycle, which will be required to achieve our efficiency targets. This goal would require the use high-temperature alloys that are difficult to manipulate, as well as more advanced manufacturing techniques to enable these materials to be cost effective. In addition to high temperatures, these systems are likely to operate at high pressures as well—perhaps as much as 300 bars or roughly 4,500 psi on the high pressure side of the cycle – to create the conditions necessary for the fluid to enter the supercritical regime.
Another unique challenge for us is that of scale; at less than 1 MW, turbomachinery becomes a challenge as compared to DOE’s other 10+MW initiatives. For example, turbomachinery for supercritical cycles under 1 MW can fit in the palm of your hand and must rotate at incredibly high speeds (RPM) to generate the required pressure. Because of the small size, manufacturing tolerances are critical—even small manufacturing defects can lead to very high leakage and losses. For similar reasons, new high temperature seals must be developed. Because of the high RPM we are aiming for, innovative and robust bearings (stabilizers to avoid friction) will also become crucial to withstanding higher temperatures.
In general, ARPA-E aims to bring together different technical communities to solve big energy challenges and fill the gaps or “whitespace” in a field. I envision several communities uniting to solve this challenge: power generation, materials, manufacturing and fabrication and, lastly, controls and risk/reliability. At ARPA-E, we strongly encourage scientists from different disciplines and technology sectors to come together for interdisciplinary and cross-sector collaboration spanning organizational boundaries. We have seen that this enables and accelerates the achievement of extremely hard-to-reach outcomes, making the impossible possible! I am looking forward to collaborating in this area.
Post-workshop update January 2018
Can you give us some brief highlights from the workshop or new things you learned from the research community?
We entered the workshop open to a number of potential approaches and asked the community to help us prioritize them. Given our interest in high temperature, high efficiency modular power generation applications, the participants encouraged us to prioritize natural gas fueled modular power, similar to the focus of ARPA-E’s INTEGRATE program. The workshop community felt that there was a potential need for this application, and that these technologies could translate into other, lower temperature power markets, such as mid to high-temperature waste heat recovery, biomass-fired systems, and advanced nuclear reactors.
Once the target application became clear, workshop participants discussed requirements and came up with a set of target system-level metrics for potential research in this space. In order to achieve the level of transformative change ARPA-E supports, a modular power system would need the following characteristics:
- An overall thermodynamics cycle efficiency (ratio of net power produced over the corresponding net energy input for the cycle) of 60%, at the megawatt or less scale
- A capital cost goal of $1,600/kWe (at production scale of ~100k units/yr)
- A modular design, such that the full system can be easily deployed where needed and otherwise easy to mobilize. In fact, one suggestion was the core system should easily fit in one or more conventional 40ft trailers that can be easily integrated together
- A minimum lifetime of 25 years and full automation
- 24/7 continuous operation to achieve an acceptable payback period
- Utilizes dry cooling for its waste heat dissipation to minimize water requirements
- To achieve high efficiencies the turbine inlet temperature may have to be in excess of 700°C
Once we had an idea of target metrics, we were able to dig deeper into technical discussions including the advantages of direct-fired vs indirect cycle architectures, and whether both should be considered of interest. Because the working fluid is preserved in an indirect cycle, and because direct cycle configurations present additional challenges, the workshop community strongly encouraged us to prioritize indirect cycle configurations.
We also discussed potential working fluids for the indirect cycle architecture, because we wanted to understand whether or not ARPA-E should be interested in noble gas working fluids, and if systems leveraging them had any chance of hitting the aggressive system level targets. The workshop community encouraged us to allow them, and we heard many interesting concepts that supported their guidance. Noble gasses might allow for higher temperature operation than supercritical-CO2-based, though with a potentially larger footprint and more costly materials.
Finally, as I suggested before the workshop, we would like to allow for component level innovations as part of the research in this space (for example, high temperature heat exchangers, turbomachinery, seals, and bearings). During the workshop, it quickly became clear that this would require a robust system analysis to help set ranges for operating conditions and performance goals for each of the components, such that overall system targets could be realized.
Going forward, what would focused research on this topic area look like?
After the workshop, it became clear that a potential program would be broader in scope than just sCO2 cycles and I would envision and welcome concepts for non-sCO2 cycles that can meet system metrics. Overall, the objective is to utilize innovative designs, materials, and manufacturing techniques to realize next generation high efficiency modular power systems.
I could envision a two-phased approach, where the first phase aims to fill the innovation pipeline with promising component technologies and system design concepts that are leveraged in the second phase to enable full systems meeting the ambitious performance and cost targets. In that case, we could start by seeking innovative cycle designs and components that could support the overall system goals. We would also need to develop new detailed analyses of these cycles to guide the technical community about both the range of operating conditions and performance/cost goals for each individual component, and help component technology developers in justifying the potential impact of their concept towards system targets. All of this would set up the opportunity in later stages of a program to build transformational new systems based on these new technologies developed in the early stages.
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