Thursday, May 5, 2022

The Role of Thermal Storage in a Decarbonizing World

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EPRI and the New York Power Authority explore low-cost, long-duration storage

The world is racing to achieve rapid decarbonization goals. Whether driven by entire countries like Denmark, Japan, and Canada or by cities or large corporations, the push to dramatically decrease or eliminate greenhouse gas emissions is accelerating.

A big part of the transition towards a net zero economy and society is an aggressive buildout of renewable energy generation, particularly wind and solar. For example, the International Energy Agency (IEA) recently reported that global additions of renewable power were expected to hit a record of almost 290 gigawatts in 2021. The IEA also forecast that stronger policy support and improving economics will increase renewable electricity capacity by more than 60% by 2026.

The challenge for countries, companies, utilities, and communities that increasingly rely on wind and solar electricity is keeping power flowing when the wind isn’t blowing, and the sun isn’t shining. “The need for decarbonized energy is becoming increasingly obvious. But renewable sources are intermittent,” said Scott Hume, a principal technical leader at EPRI. “The way to have these resources become useful contributors to meet demand is to have substantial energy storage.”

The Importance of Long-Duration, Low-Cost Storage

The vast majority of energy storage today is supplied by pumped hydro. Increasingly, though, investments are pouring into battery energy storage, especially lithium-ion batteries. As a result, battery storage installations and future growth forecasts have spiked. For example, the U.S. Energy Information Administration (EIA) reported that systems providing 402 megawatts of small-scale battery storage and just over one gigawatt of large-scale battery storage were operational in the U.S. at the end of 2019.

The EIA forecasts that by 2023, an additional 10 gigawatts of large-scale batteries will be installed in the U.S., while global projections put the total market size for batteries by 2028 at nearly $27 billion. But while there are plenty of headlines about batteries and the important role they will play in the energy transition—both for storing electricity for use by the grid and for electric transportation—batteries alone aren’t a sufficient solution for a decarbonized world.

Cost is one big reason a mixture of energy storage solutions is needed. Battery storage is growing at a robust rate because its costs have declined dramatically, with the National Renewable Energy Laboratory (NREL) showing a 13% drop in the price for utility-scale battery storage systems from 2020 to 2021. NREL also forecasts that battery prices will continue to decline, potentially dropping between 30% and 60% for a four-hour-duration battery system from 2020 to 2030.

Despite that progress, future energy storage solutions will need to deliver energy for much longer durations and be far cheaper. This is why EPRI is collaborating with the U.S. Department of Energy (DOE) and utilities like New York Power Authority (NYPA), Southern Company, and Salt River Project to design, develop, and test thermal energy storage (TES) solutions.

“We are looking at energy storage solutions that have the potential to be substantially cheaper than electrochemical batteries and store large quantities of energy,” said Horst Hack, an EPRI technical executive. “The grid of tomorrow is going to need storage that can provide energy for anywhere between eight and 40 hours, and it has to be very low cost. Which is why we are looking at storage technologies that use low-cost storage media, like sand, rocks, concrete, and other local materials that can be integrated with current fossil generation assets, as well as future renewable generation options.”

Thermal Energy Storage concept

Exploring the Potential of Crushed-Rock Thermal Energy Storage

A future EPRI Journal story will highlight research and demonstration initiatives focused on developing and testing TES systems using sand and concrete. In particular, EPRI is working with Southern Company and DOE to understand these storage technologies better.

Meanwhile, NYPA and EPRI are spearheading two projects focused on TES systems that utilize crushed rock as the storage medium. According to Alan Ettlinger, NYPA’s director of research, technology development, and innovation, the utility is actively pursuing the projects to better understand TES because of New York State’s Climate Leadership and Community Protection Act requirements. Signed into law in 2019, the act requires New York to reduce economy-wide greenhouse gas emissions 40% below 1990 levels by 2030 and 85% by 2050.

In support of those statewide objectives, NYPA developed a ten-year strategic plan called VISION2030, which, among other things, lays out a roadmap for decarbonizing the state’s natural gas plants and partnering with customers to achieve clean energy objectives. Energy storage will have to play an increasingly important role in the state as intermittent wind and solar generation continue to grow.

Because of safety concerns and space constraints, NYPA understands that it can’t rely on lithium-ion technology alone. “We are very interested in alternatives to lithium-ion technology because of the susceptibility of lithium-ion to thermal runaway and possible subsequent fire,” Ettlinger said. “Getting to the scale of storage that you need is not going to happen by lining up 500,000 batteries. You need some kind of alternative technology, especially in New York City, where real estate is at such a high premium.”

Integrating Storage at an Existing Natural Gas Power Plant

Thanks to a grant provided by DOE, EPRI and NYPA are currently evaluating the feasibility of a pilot project that would integrate a TES system using crushed rock at NYPA’s Zeltmann natural gas combined-cycle (NGCC) power plant in Astoria, New York. The Israeli-based company Brenmiller Energy initially developed the storage system named bGen. The design concept anticipates that the system that could ultimately be built as part of a pilot project would provide about four hours of storage or about 16 megawatt-hours.

The bGen TES system being evaluated for NYPA’s natural gas plant has three components: a heat exchanger, crushed rocks to store heat, and a steam generator. One of its main benefits is very low cost – crushed rocks are inexpensive.

The storage system can be charged by heat (either flue gas or steam) produced at the combined-cycle plant, by electricity, or by a hybrid of the two. For example, high-temperature steam produced by the plant’s turbine can be piped into the storage system, where its heat is stored in the crushed rocks. Alternatively, electricity can be used to power industrial heaters embedded in the crushed rocks to charge the system, either from renewable energy or from the power plant itself. This research has shown that a hybrid solution employing both steam and electrical charging provides the optimal integration with NGCC power plant applications. In either case, the Brenmiller TES system can then discharge the energy it has stored to power the plant’s steam turbine to generate electricity.

The versatility in how the storage system is charged is important, particularly given how much variable renewable energy is being added to the grid in New York. “In many markets right now, there is a significant amount of variable renewable power generation, and there are times of the day when electricity prices are very low because of the mismatch between generation and electric demand,” Hack said. “Electric prices are low or even negative in some areas, and if you can develop a system to capitalize on that, there are advantages, both financial and environmental.” In the New York Independent System Operator market, the most financially advantageous time to produce electricity is in the evening hours.

Not only does this technology have the potential to increase revenues, but it also delivers much-needed flexibility to the grid – which will only become more important as the amount of intermittent renewable energy increases. There are also potential operational benefits for the existing natural gas power plants where TES is integrated. This is particularly true if the plants incorporate carbon capture and storage (CCS) technology. “If you have carbon capture on any fossil asset, coal or gas, you don’t want to be ramping the gas plant up and down for demand purposes because the post-combustion carbon capture plant doesn’t work efficiently if it’s ramped up and down hourly,” Hume said. “If we decarbonize with CCS, we will need storage to help levelize operation.” Even where CCS is not present, ramping a coal or gas plant too often can damage or accelerate the degradation of the asset’s equipment.

Other potential benefits of incorporating TES into a combined-cycle gas plant are faster starting time, a lower minimum load, and an increased maximum load. For example, at times when a gas plant is responding to peak demand, steam from the storage system can supplement steam produced by the plant’s heat recovery steam generator. This can increase the thermal energy generated and up the plant’s nominal capacity.

combined cycle gas plant

Lessons Learned at Combined Heat and Power Project

After the initial evaluation phase of the Zeltmann project is complete, a potential future step would be to actually integrate the system at the plant. This project would be five times larger than an existing 1.7-megawatt Brenmiller storage system located at a solar power plant in Israel.

Already, though, NYPA and Brenmiller Energy are in the process of commissioning a smaller Brenmiller TES system with a different use than that envisioned at Zeltmann. Instead of being integrated with an existing natural gas power plant, the storage system is being paired with a microturbine as part of a combined heat and power (CHP) system at the State University of New York (SUNY) at Purchase.

The project, which began in 2017 and was delayed along the way due to COVID-19, is being commissioned this month and was attractive to NYPA because of its hybrid features. “The CHP paired with the Brenmiller technology offers a level of efficiency improvement because of the stored energy,” said Steven Wilkie, a senior research and technology development engineer at NYPA. “You can store the heat and use it separately from the operation of the turbine or the limitations of recovered heat. Also, other technologies we were looking at, at the time, did not have the dual electric and thermal charging capability.”

Also attractive to NYPA was the modular configuration of the CHP and Brenmiller storage pairing. The microturbine being used is 200 kilowatts, and the storage capacity is 450 kilowatts/400 kilowatt-hours. “That could be scaled up with any size CHP,” Wilkie said.

Having reached the project’s commissioning phase, NYPA has learned several lessons that will help shape and improve its approach to future TES projects. One is simply that utility involvement is essential when it comes to integrating storage with existing or new assets. “With both batteries and energy storage, the main lesson we have learned is that there is a learning curve with technology providers in understanding the nuances of New York’s codes and installation issues,” Ettlinger said. “Almost without exception, we have had to provide assistance or oversight because we have experience with siting and integration and other issues.”

Other lessons learned are common to any advanced R&D project, including challenges with certification testing and the nationwide supply chain and personnel hurdles related to COVID-19. Nevertheless, NYPA believes the research is critical to advancing the storage technologies necessary to achieve the state’s decarbonizing objectives. “When the Climate Leadership and Community Protection Act came out, I was sitting next to our former CEO. And he said, ‘We don’t have the technology to reach those goals today,’” Ettlinger said.

Though the CEO’s comment was specific to the situation in New York at the time, the reality is that the observation was true for the entire utility industry. As the world transitions to a larger proportion of variable generation, the need for advanced new storage technologies becomes more acute. That is why it’s so important to continue pursuing thermal energy storage innovation and advances in New York and elsewhere. It’s how technology can play a meaningful role in achieving ambitious decarbonization targets.

“I think thermal energy storage has a significant role to play in achieving those goals, and that requires advancements with the technology,” Ettlinger said. “Every step forward is a step towards that end, and this is part of that effort.”

EPRI Technical Experts:

Horst Hack, Scott Hume, Andrew Maxson

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