Long duration storage is the current rage. As an example of long term energy storage, consider the venerable alkaline AAA primary battery. It has a shelf life of ten years. If discharged at only 25mA it will deliver 1200 mAhr of charge before it is completely spent. Using its nominal 1.5V rating, it will deliver about 1.8Whr of energy. AAA cells cost around a buck ($1 USD); so, in other words, its cost of stored energy is $556 per kWhr. “How can a person afford such expensive energy?” one wonders. The obvious answer is, by using it only in tiny amounts.
Now for the $24,000 question. Will grid scale energy storage remain so expensive that people can afford it only in tiny amounts or will it become as inexpensive as, say, cheap coal or hydro power?
On September 25, 2023 the Department of Energy and Biden-Harris Administration Announced awards amounting to $325 Million For Long-Duration Energy Storage Projects. By long duration DOE means intraday (10 to 36 hours) or multiday (36 to 160+ hours) storage solutions.
According to the press release DOE funding for 15 projects will do the following:
- Increase Grid Resilience and Protect America’s Communities
- Help Advance Energy Storage Technologies,
- Enhance Clean Energy Adoption, and
- Reduce Impacts on the Grid from Climate Change-Fueled Extreme Weather Events
In an unusually candid admission, the press release says that today’s energy storage technologies are not sufficiently scaled or affordable to support the broad use of renewable energy on the grid. Yet, despite this clear rationale for a focussed effort, the press release betrays the defocussing politics.
“Funding applicants were required to submit a community benefits plan to outline how proposed projects will support community and workforce engagement, invest in the American workforce, advance energy and environmental justice, and promote diversity, equity, inclusion, and accessibility, and deliver benefits to disadvantaged communities as part of the Justice40 Initiative. These key requirements, when incorporated comprehensively into project proposals and executed upon, will help de-risk these projects to ensure that the transition to a clean energy economy benefits all Americans.”
The press release is here. Unfortunately it is short on specifics, as are documents it references, and as are the websites of the various participants. Here is a summary of the projects DOE chose:
Projects recycling EV batteries
Communities Accessing Resilient Energy Storage (CARES)
Second life sMARt sysTems (SMART)
New Li-ion batteries are far too expensive to provide inexpensive backup, but decommissioned electric vehicle batteries are widely available. Why not use them for grid storage?
Two projects propose to repurpose batteries as stored energy for two affordable housing complexes. The problem is that decommissioned batteries have varying capacity, which means varying charge/discharge characteristics. Each project approaches a solution to this problem differently. One proposes to use extensive testing to identify batteries that can be paired together; the other proposes to use control electronics (monitoring/buck-boost) to integrate battery packs from different manufacturers at varying states of health.
There is no doubt that either tactic can solve this problem at the small project scale of about 7MWhr. Yet a general rule is that complicated efforts to make widely varying input materials meet very specific objectives would submerge a grid scale effort in O&M costs.
Rechargeable Zn/MO2 Batteries
Project: STOred Rechargeable Energy Demonstration (STORED)
This project location is Oneonta and Westchester County, New York. Anyone who has read the guest blogs of the Manhattan Contrarian or the Pragmatic Environmentalist of New York will understand that energy storage is critical to New York’s idea of a clean energy future. If non-dispatchable power sources like wind and solar are ever to provide a significant portion of New York State’s electricity, storage has to be made available. This project aims to demonstrate the viability of its zinc/manganese dioxide (Zn and MnO2) batteries in large scale and long-duration. The proposed batteries will provide load management and power resilience at a scale of more than 600kW of power for more than 12 hours per discharge.
Zinc is an inexpensive anode material. Zinc anode batteries have been used in a variety of designs since the late 1800s and the alkaline electrolyte version of these batteries has a well-established industrial base and supply chain. Yet, despite these advantages, rechargeable versions of the alkaline batteries suffer from two issues. First, the alkaline electrolyte MnO2 yields a discharge product of ZnO which passivates the zinc anode inhibiting recharge. Second, Zinc anodes in cyclic operation develop dendrites. [5,6,7]
Three projects using Flow Batteries
Rural Energy Viability for Integrated Vital Energy (REVIVE),
Children’s HospitAl Resilient Grid with Energy Storage (CHARGES) and Front-of-the-meter Utilization of Zinc bromide Energy Storage (FUZES)
Flow batteries are a clever solution to the problem of matching fuel quantity against the cross-sectional area of the electrode stack. Design a stack appropriate to the power demand (MW) and then make reservoirs of electrolyte with capacity for the MWhr required. Circulate the electrolyte as needed. Because these batteries may be unfamiliar I have included a schematic in Figure 1.
Figure 1. Reactions in the electrode stack at the electrode surfaces produce a current through the load. A compensating ionic current flows through the membrane to complete the circuit. In contrast to the limited capacity of a typical battery the flow battery has electrolyte tanks of a size designed for energy capacity.
The technology provider for REVIVE proposes to place vanadium redox flow batteries at five rural locations. The batteries have nominal discharge ratings of 700kW to 3.6MW and discharge capabilities of up to 20 hours. So, these are not large batteries by any means and not long duration.
What one finds when researching vanadium flow batteries is, first, they have been around for a surprisingly long time. The idea was first advanced by NASA in the 1970s. Sumitomo Electric advertises that they began developing these batteries in 1985 and commercialized them in 2001; second, there are a stunning number of companies involved.
One would think that such a long history of development and so many competitors would have led to broad deployment already. However, the issue here may be one of cost. I’ll examine this issue more fully in the Discussion section. Meanwhile, Figure 2, using data from the U.S. Geological Survey, shows the cost of vanadium pentoxide per thousand kilograms over the past twenty years or so. The average price is $27 (nominal USD) per kilogram. Vanadium is not cheap and its price is volatile. Much like lithium, the deposits are widespread and of low grade. In fact, a leading source of vanadium is petroleum refining; another is uranium mining.
Figure 2. Price of vanadium (V2O5) over the past 40 years. Data is from the USGS.
Two other teams propose to install zinc/bromide flow batteries for grid backup. The CHARGES project will install a 34.4MWh behind-the-meter, zinc bromide flow battery system as backup for a children’s hospital.This project will allegedly replace diesel generators with cleaner, and allegedly more cost-effective resources. Is the cost claim credible without subsidies?
Similarly the FUZES project proposes to place ahead of the meter zinc/bromide flow batteries at sites in Oregon and Wisconsin. These batteries provide a mere 10 hours of duration. So, they aren’t really long duration or grid-scale storage.
Columbia Energy Storage Project
Columbia County, Wisconsin is the site of a project proposing a closed-loop CO2-based energy storage system. Energy Dome will supply the technology for the project. They have already demonstrated their scheme at a scale of 2.5MW. The idea is to use excess electrical energy to compress CO2 from a large storage dome to a supercritical fluid held in high pressure vessels. When energy is needed the high pressure CO2 is heated and allowed to expand through a turbine back into the dome. Thus, the working fluid remains in a closed circuit.
The project plan calls for using a brownfield site of a coal-fired thermal plant that should be closed in 2026. This site will provide facilities, electrical substation probably, to lower the project cost. There are few details on the technology suppliers site to evaluate the scheme.
Polar heat pumping
Healy, Alaska is a small town along the highway connecting Fairbanks to Anchorage. The area currently gets electrical energy from a coal-fired plant that is soon to retire. Apparently the replacement will be wind energy, but to firm-up this non-dispatchable source, the POLAR project seeks to develop and deploy a Pumped Thermal Energy Storage system. In effect, this system is a very large heat pump that converts excess wind energy into heat stored in inexpensive concrete blocks. This stored energy is then converted back into electricity using a heat engine.
Multiday Iron air Demonstration (MIND)
I find this project to be the most interesting among the lot because it represents true long duration storage – 100 hours at 10MW. The battery chemistry is iron-air which at least promises low-cost materials. The proposed energy storage system is larger in terms of energy than the Hornsdale battery (1000 MWhr vs. 194 MWhr) though it is smaller in terms of power (10MW vs. 150MW). These batteries are proposed to be sited at retiring coal plants in Minnesota and Colorado which will reduce project costs by reusing facilities like transmission lines and substations.
Only one project can be called a long-duration project. Most plan for small power output or small stored energy or both. There are few details at this time to analyze the potential for success and what constitutes a success isn’t all that clear. There are some glaring issues. For example, though zinc anode batteries offer many advantages, including low material cost, they are prone to developing dendrites during the recharge cycle and a loss of capacity. Vanadium electrolyte may have an “infinite” life as claimed, but it is surprisingly expensive. At the peak price shown in Figure 2, the cost of a kWhr of electrolyte capacity would exceed $450 (USD); that is, exceeds the cost of lithium batteries in whole by a factor of three.
The two projects making use of thermo-mechanical cycles are vague. One project tries to store energy as heat in a subarctic location. This doesn’t seem like a good choice. The other involves large quantities of stored CO2 which upon a catastrophic release would risk smothering people and animals. It could arguably present a bigger hazard than fire or explosion.
However, a general principle of making and delivering compressed gasses is that one usually reduces the work to accomplish this goal by throwing away the heat it produces. In this CO2 project the objective is not to compressed gas, but to store energy efficiently and that different goal demands something useful has to be done with the heat produced. Let’s just say that schemes to store energy in compressed gasses abound but none appear effective.
One clear goal of all this is to realize a goal of reducing the cost of long duration storage by 90%”, or what is stated elsewhere as $0.05 (USD) per kWhr of delivered energy. Let’s examine basic finance.
Consider this hypothetical situation. Each kWhr of energy storage is fully used each day. Energy is purchased at $0.02 per kWhr at night, and sold each day for $0.05. Efficiency each direction is 80%. Thus, we have gross profit of $0.015 per kWhr or, on an annualized basis, $5.48 per annum. What first cost, capital cost essentially, and interest rate can we withstand to just break even? In other words, what interest rate and first cost make it possible to deliver energy at $0.05 per kWhr.
An interest rate is a measure of a person’s time preference for money. An interest rate discounts the distant future and accentuates the near future. An interest rate of zero is not remotely reasonable as it implies a person can wait forever to be repaid, even beyond their lifetime – a ludicrous suggestion. A reasonable interest rate is 4% per annum. Let’s see what principal amount allows us to break even in 30 years – long past the life of most energy system components.
The factor that converts a present value to an annualized amount is (A/P) or vise versa (P/A) and is a function of interest rate and duration. At 4% per annum and 30 years, the conversion factor is 0.0578 or 17.3 respectively. Thus, $5.48 times 17.3 converts an annualized value to a present value, a first cost in this case, of $94.72. In this incredibly low bar scenario, with no O&M costs and no depreciation or taxes, we cannot allow the capital cost or first cost to exceed $94.72 per kWhr of capability or a delivery cost from storage of $0.05 per kWhr is never possible. A person can explore other interest rates, and time periods by looking up conversion factors on a suitable table like this one.
Using the average cost of vanadium over the past twenty years, and a factor of 7.2 to turn V2O5 price into cost of electrolyte, the cost of electrolyte alone precludes ever delivering stored energy at $0.05 per kWhr – its alleged infinite life matters not at all.
These projects may change form significantly as DOE says “Selection for award negotiations is not a commitment by DOE to issue an award or provide funding. Before funding is issued, DOE and the applicants will undergo a negotiation process, and DOE may cancel negotiations and rescind the selection for any reason during that time.”
Nonetheless, I don’t see how any but one of the projects actually demonstrates long duration storage. Most provide a couple of days storage at most, at small power, and without guarantee that a facility relying on them for backup wouldn’t have to resort to rationing. In several cases the technology supplier has already built projects to the proposed scale, meaning that these are not really “technology demonstrations” in the ordinary use of that term. The lack of truly long term projects in this list may reflect that there were almost no long duration proposals to choose from.
A large number of recent essays or analyses have shown that a goal of 100 hours of replacement energy, though called long duration, is far below adequate capability to avoid rolling blackouts and grid collapse. Some estimates run above 1,000 hours. Projects cited here are too small and too burdened with politics to “accelerate the development of long-duration energy storage (LDES) technologies”.
Alas, we await true demonstrations of capability and cost.
1-One of the most grating aspects of government projects is the perceived need for cute acronyms – in this case even including FUZES. Fuzes are what set off bombs. Long ago our energy projects had fun names, like, say “Project Gasbuggy”, that really did employ a fuze.
2-Sumitomo has made a nice video about how these batteries operate, even so it is full of marketing blather https://www.youtube.com/watch?v=TSsqCazP1V0
3-Flow batteries for grid-scale energy storage: Guiding future research pathways, by
Nancy W. Stauffer, January 25, 2023, https://energy.mit.edu/news/flow-batteries-for-grid-scale-energy-storage/
4-The two MIT researchers highlighted in  make a perfectly reasonable point that the low capital cost of a battery system can be negated by a high lifetime cost. However, even if the only cost of a battery system is its fixed first cost, if that cost is very high then time preference of money, reasonable interest rates in other words, make a long pay-back period untenable.
5-Zhicheng Xu, et al, Review of zinc dendrite formation in zinc bromine redox flow battery, Renewable and Sustainable Energy Reviews Volume 127, July 2020, 109838
6-Hyeon Sun Yang, et al, Critical rate of electrolyte circulation for preventing zinc dendrite formation in a zinc–bromine redox flow battery, Journal of Power Sources Volume 325, 1 September 2016, Pages 446-452
7-Yuchuan Shi, et al, An Overview and Future Perspectives of Rechargeable Zinc Batteries, Small 2020, 16, 2000730, DOI: 10.1002/smll.202000730
8-Elements of a thermal storage system were discussed in the WUWT guest blog about why is energy difficult to store.