The cost of storing energy as heat is a fraction of the cost of storing it as electricity. Methods for long-duration, big-scale storage already exists.
The ability to store excess energy is key in transitioning from a fossil-powered energy system to a system built on sustainable methods of energy generation. This article is a call-out to all the inventive minds and resourceful investors out there. There should many opportunities in this space over the next many years.
To find the golden pot at the end of this rainbow, let me start with a brief run-through of the main storage solutions.
ELECTRICITY can be stored in batteries, by using pumped hydro, in compressed air and liquids, and using fly-wheels.
Batteries are good for meeting short term peak demand, but though billions are spent on inventing and improving ways to store electrons, no solution promises costs that will allow for utility-scale energy to be stored for months, weeks, or even just days. For more detailed insights into battery storage costs, see this excellent analysis from Lazard.
Pumped hydro, where excess energy is used for pumping water from a lower point to a higher point is the most efficient way of large-scale storage of excess energy. But as with batteries pumped hydro is only suited for storing electricity. Further, it does for natural reasons require a topography that allows you to pump the water upwards. For that reason alone, it’s not an option in many geographies.
Compressed air and fly-wheels are the biggest representants of other solutions for storing electricity. But so far they are struggling with even worse cost profiles than batteries.
HEAT on the other hand is much, much cheaper to store using low-tech materials such as water, rock/concrete/stones, or molten salt.
Water is great for low-cost storage of heat below 100ºC/212ºF – which is the temperature level needed for applications like processing of dairy products, beer brewing, turning salt water into potable water (ie desalination), and district heating.
The low-cost of storing heated water for a very long time is especially relevant for district heating powered by solar panels since the need for heat is least when the sun shines the most. For this, a Danish municipality has built a 200,000m3 water heat storage pit capable of storing heat for several months of usage. At another Danish district heating facility, EON uses Heliac’s solar collectors for harvesting the sun’s energy. The produced heat is stored in a standard 400m3 water tank sufficient to cover several weeks of local heat demand.
Boreholes and geothermal, where heated water runs through tubes dug or drilled into the ground thereby heating up the ground and where the heat is discharged by reversing the process, is another cost-efficient way of storing heat at temperatures well-suited for being boosted by heat pumps (see Generation for the role of heat pumps).
Rocks, stones, and concrete are inexpensive, abundant materials capable of storing temperatures up to 400-800ºC/750-1500ºF. Presently, many of the existing solutions still have room for optimizing how they charge and discharge. Concrete is challenged by the use of different components with different thermal properties which it isn't all straightforward to make work. Rocks and stones are challenged by the energy demand needed for making hot air travel to colder areas in the storage. However, compared to the challenges battery development is facing, to me these challenges seem relatively trivial.
Molten salt and other phase change materials can store similar temperatures as rocks and concrete. Though these solutions are already cost competitive in several instances, they too should have room for significant improvements given the limited focus they have had up to recently.
My guess is that up to now less money has been spent on developing all the possible heat storage solutions combined than what has been spent on developing almost any of the present methods for storing electricity. This is probably also why the Department of Energy in the US in 2018 launched sizeable support programs supporting the development of high-temperature heat storage technologies. I'm not aware the European Commission has any similar initiatives underway.
The cost of heat is probably the main reason why heat has received so little attention up to now: Storing heat as heat for heat purposes only does not make financial sense if the cost of producing this heat is higher than the cost of generating the same heat using fossil fuels. When heat production without storage can’t compete with fossil fuels, then a solution that includes storage will, of course, be an even worse proposition. And proposing to store heat for electricity production – where most of the stored energy is lost in the conversion into electricity - could be among the world’s least fundable ideas.
Going forward it's likely that heat will get a much more prominent place in energy and climate discussions. Partly because carbon taxes increase the cost of fossil-powered heat generation significantly (re. Cost of Heat). Partly because new low-cost methods for sustainable heat generation, like Heliac’s collectors for concentrated solar heat, start to emerge. Partly because the market potential for cheap energy is close to infinite. And finally, because the three reasons in combination will incentivize smart brains and strong investors around the world to develop even better solutions.
With unsubsidized energy from renewables already below the cost of natural gas, things may change fast, especially if agile first movers and their potential investors believe they will be able to reap good profits before the dinosaurs of the energy world join the party. Still, things could move even faster if political focus and policies were better and if large-scale consumers of heat realize this opportunity as well. This I will discuss in "The Credibility Challenge".
Storage is great, but even for situations where suitable storage solutions haven't yet been developed, renewable heat may still provide competitive, clean energy. All this requires is standard heat exchangers. This is the topic of the next article "Integration".
Interested in reading more? Please see the links to my other articles below. Additionally, a 'Like’ from you will also be much appreciated as this should help direct more attention at the many business and climate opportunities the market for heat production offers.
Thank you for reading,
Heat Storage Pits, Ramboll, 2017
Lazard's Levelized Cost of Storage, Lazard, 2017
Thermal Energy Storage, IRENA, 2013
I have spent the better part of 20 years investing in cleantech startups. During my career I have probably seen at least 3,000 business proposals, including Heliac's which I was introduced to in 2016 when I headed Climate-KIC Nordic's accelerator program. I found -and still find - Heliac's solution to be by far the best new solution I've ever come around, which is why I joined the company in early 2017.
Disclaimer: I have not double-checked all my sources and I am not an expert in all areas mentioned in the articles. I may therefore have reached conclusions that wiser men and women may know to be inaccurate. If so, I trust they will let me know, so I can become a bit wiser too.