Bring Boring Batteries Back to the Forefront.
To battle the climate crisis, it will be critical to effectively eliminate carbon emissions from electricity production. Wind and solar power are two of the most affordable ways to aggressively reduce how much carbon is emitted as a result of electricity production. One thing that is limiting how much wind and solar we can add to the world’s energy grids is how variable wind and solar are. In our current world of how energy is produced and consumed we have a hard time handling the variability of wind and solar. A big part of that challenge is that we haven’t had an affordable way to match production with demand.
Fortunately for consumers, as well as the environment, battery technology is starting to get cheap enough to capture energy surpluses and store the extra until the grid needs that power. According to a 2019 paper from a research group at MIT we could move our energy grid to 95% wind and solar produced power if battery prices go below $150/kWhr. For reference, when that study was published in battery prices were less than $180/kWhr and prices are on track to keep getting lower. Even better news for a sustainable future is that this study focuses on storing energy in the form of electricity. Why is that good news for sustainability? There are some boring ways to store energy that are even cheaper.
Hot water and ice. No seriously, hot water and ice can store a pretty decent amount of energy, I will go into the math in a minute, but I wanted to give the high level. Water can store a lot of heat energy as a solid, liquid, or gas, in part because of how individual water molecules interact with each other. Why you might ask did the MIT study ignore storing energy in the form water at a set temperature? Simply put flexibility. If I store a certain amount of energy in a battery, I can do almost anything with that energy, because that energy is in the form of electricity. Electricity is great, I can power a motor, a computer, a light, or a heater. Storing that same amount of energy in the form of hot water I can only really do one thing, warm something with that hot water (there are some more nuanced things, but I want to keep things short).
To the physics
A block of ice that is 1 cubic meter in volume can store about 90 kWhrs of cooling potential. That cooling potential comes in the form of what is called the heat of fusion, or the energy needed to change ice back into water. For reference a Test Model S with a 90 kWhr battery can drive over 270 miles on a single charge.
A tank of hot water at 80 deg C (176 F for us ‘Muricans) can store about 258 kJ/kg, That means that a hot water tank that can store 1 cubic meter of 80 deg hot water is able to store about 69 kWhrs of heat energy.
(Density of 80 deg water 0.9718 grams/cubic cm, specific heat of water at 20 C = 4.18 kJ/kgK, specific heat of water at 80 C =4.18 kJ/kgK, 1kWhr= 3,600 kJ
Energy Add = Energy gram at 80 C-Energy/gram at 20C=258kJ/kg
(1 m^3)*(1,000,000 cm^3/m^3)*(0.9718 gram/cm^3)*(Energy add)=250,724 kJ
Sounds pretty awesome right? In 2 cubic meters of water I can store more energy than any electric car on the market (Feb 2020). Even better water is cheap, so what’s the catch? Two kind of biggies and one detail that should not be ignored usefulness, storage life, and cost of system. It is impressive to store all of this energy in the form of hot water and ice, but consumers need energy for other uses. A thermal battery is most useful when helping control the temperature of something, it is less useful in powering my TV. Storing energy as heat has tradeoffs in how long you can store energy. Heat is always moving around, trying to even out as much as possible, this means the design of a thermal storage system needs to balance the cost of insulation. Costs for thermal storage systems are certainly lower than their battery counterparts, but its not like you can just put down a water tank and call it a day. A good thermal battery system would need to be integrated into a building’s heating and cooling system. Integrating the thermal battery system into an existing heating and cooling systemwould certainly add complexity and as a result cost. What would be better would be promoting installing thermal storage systems into new construction with purpose built gear.
The good news is thermal storage doesn’t need to be perfect to be helpful. Over half of the electricity used by American households goes to heating and cooling, thermal storage could represent a massive shift on how that electricity gets used. https://www.eia.gov/energyexplained/use-of-energy/electricity-use-in-homes.php Even if thermal storage was only used in brand new construction and massive renovations, we could avoid spending billions of dollars on battery infrastructure.
I hope this was interesting, questions and comments are always welcome.
Some follow up thoughts that I couldn’t quite fit in
I want to be clear about my comments about the MIT study, I agree with their choice to limit their research to electrical batteries. Thermal storage will be an important tool in greening our economy, but there are too many unanswered questions on how the economics would work. For example, thermal storage is already being used by some folks as a way to get the most out of rooftop solar panels, when batteries are topped off make ice and hot water that might save your energy demand when its cloudy, that’s great for off grid scenarios, but for situations where buildings are connected to the power grid it gets more complicated with more stakeholders producing and consuming the energy. In some markets with lots of surplus wind, consumers are already able to get no cost electricity if they receive that energy between 9 PM and 6 AM. Electricity that can be instantly available from batteries have clearer economics than storing ice for later.
Looking at thermal storage as just electricity offset is an imperfect image. A lot of the energy use in the US and other countries comes from energy in the form of heating using natural gas and oil. The average home uses 90 million BTUs of heat energy, I believe that includes electricity use as well as burning things to make heat ( I don’t think it includes people’s cars)
1 BTU is equal to 1060 Joules, with that equivalency 90 million BTUs translates to 9.54E10 Joules/year in kilowatt hours that’s 26,500 kWhrs of energy (so if we look at it from that perspective home energy use is 2.5x greater than the electricity alone would indicate) That being said a hot water tank the size of the TARDIS (or about 4 cubic meters) can store 276 kWhrs of heat energy, or the equivalent of more than 3 days of heating needs
If we look at the larger energy picture we can see that homes consume about 2,200 kilowatt hours of energy in various forms.
As I mentioned earlier thermal storage is limited in what can be done once the electricity is converted into heating or cooling something, but for the larger grid that’s not necessarily a bad thing. Let’s look at things from the really big picture, if every home has batteries and thermal storage, grid operators can start to plan around periods of potentially near zero grid demand from much of the market. That means surplus energy produced during that time period can be put into super long term storage like hydrogen from water. One of the big reasons why the energy sector hasn’t embraced making hydrogen from water is that it is a relatively energy intensive way to produce hydrogen when compared to hydrogen made from natural gas. In a grid where customers are better able to go to zero demand from the power grid for hours on end, surpluses can be rerouted to things that aren’t efficient for next day use, but could be useful for seasonal storage.
Honestly I wish I knew how cost effective a well implemented thermal storage, lithium ion battery hybrid system could be, but the number is complicated and would require a stupid amount of research
What I think is mechanically possible and what I would like to see is an energy wall for future homes. Imagine a pre-fabricated wall that is prewired for batteries and thermal storage, with easy hookups to take advantage of the energy walls storage abilities. This energy wall might also have plumbing and the core electrical distribution system, now when you build a house, you might only need to hire the plumber/electrician to connect to the main grid. If the wall was built using strong standards it would mean that long term maintenance and improvements would be easier for future home owners. This may turn into a post