After years of pushing for green energy, which some viewed as a niche technology and others as an existential need; our society is finally at a point where it is mainstream. A good chunk of power in many nations today comes from solar and wind energy. Sure, there are other sources like geothermal and wave energy, but we have scaled solar and wind to a level never seen before. Make no mistake - this is a success story. We do not always market it that way in our social discourse because climate change demands far more innovation for us to continue maintaining our quality of life. But progress is progress.
As with any new technological revolution, a new set of challenges organically arise with its widespread adoption, usually because of its idiosyncrasies that were not addressed in the first revolution. Then a secondary revolution happens, which makes the technology more reliable, less expensive and finally making it mainstream (until the next “new” tech revolution). With renewable energy, we are entering the secondary revolution phase in renewable energy, and I’ll focus on a specific idea in this post. Welcome to this week’s Climatonomics!
With renewable energy, we have a vexing problem that came because of its adoption everywhere: Base load and energy storage. The problem is simple to understand, but hard to solve. Solar and wind energy production is heavily time-dependent. The sun shines only during the day, and mostly during summers, when there is less cloud cover.
The wind does not blow all the time, and depending on geography, can also be seasonal and changing throughout the day. It’s much more unpredictable than solar because it relies on complex global weather patterns that are challenging to understand.
Electricity demand is also time dependent, but does not change as wildly. Do you have a big manufacturing plant in your city? Prepare for industrial power demand to be high between 9am-5pm, but low elsewhere, because people are not home. In the evening, prepare for the opposite: household power demand increases, because people are back from work and would use their lights, TV and so on. Industrial power demand is usually minimal. There are also seasonal demands. Cities that have centralized heating systems in colder regions will see an enormous demand for energy in winter and nearly none in summer. If we plot this electricity supply and demand with time for solar, we can end up with a supply-demand mismatch illustrated in the chart below. Similar plots exist for wind power.
There is excess power produced during the day, and none at night, but power demand surges because of night-time lighting and appliances. This includes heating for homes when it’s coldest, usually at night. Notice that while the power demand goes down, it never hits rock bottom. There is always a minimum level of power that we have to guarantee for a city to function: the base load. The base load is typically a large fraction of the total power consumed, around 70-80%, so it is non-trivial and the mismatch is a serious issue.
Gas, Batteries and Coal plants…
The problem with renewables in our electricity grid is its intermittent nature and how to handle base loads. There are two strategies:
Have fossil fuel power plants that can operate 24/7 regardless of the weather. This is usually coal worldwide, with the US slowly switching over to natural gas. When the renewable supply is low, these plants are “back ups” that are fired up to meet the base load capacity. How often this can happen is difficult to predict, and energy companies have sophisticated math models to price their energy in the market based on supply and demand.
If the renewables generate excess power when demand is low, store it in batteries. The batteries provide electricity back to the grid when power demand rises and renewables are offline (at night, or in winter).
Option (1) is commonplace and is a feature of almost all power grids worldwide that use renewables. But this is still not clean energy as a significant amount of fossil fuels are still burned, as we see in the chart for the north-eastern USA.
Here is why Option (2) is possibly the true, long-term solution. But energy storage is a frustratingly hard problem, because any long-term storage involves losses in energy. If you are generating excess solar and wind power in summer but need them in several months later in winter, we have few great choices for now. Lithium-Ion batteries are the most energy dense technology we have currently, but lithium is expensive and mining it causes severe damage to ecosystems. Plus, supply is limited and there is widespread recognition that lithium batteries consume a lot of resources to mine, are green only when they are used, have significant safety hazards and extremely polluting if not recycled correctly. Even the original inventor of the Lithium battery has stated that we need better batteries. In short, between consumer electronics, electric cars and power grids, lithium batteries are viable neither environmentally nor financially.
Batteries Made Of Sand Show Promise…
Recently, BBC reported on a startup from Finland that claims to have found a cheap and inexpensive solution to the battery conundrum: Storing energy in sand. They call it the “sand battery”. The idea is deceptively simple and, on some level, surprising that it hasn’t been done before.
Sand can store a lot of heat. Anyone who has walked on a sandy beach during evenings knows that even though the sun has set, the sand is still quite hot. Scientifically, sand has a very low specific heat, i.e. the amount of energy required to heat sand is low. Sand also has a low heat transfer coefficient, meaning that it does not release its heat easily to the surroundings. Both these properties are attractive if you see sand as a heat storage medium. Plus, it’s dense, and can be heated to extremely high temperatures. In comparison, water can only be heated up to 100 degree Celsius, before it vaporizes. This places an upper limit on how much heat can be stored.
But sand has no such limitation, is not hazardous, not toxic…. and widely available. In fact, the startup says it can be heated up to even 1000 degrees Celsius. For example, In 5 hours and 30 minutes, a kilogram of sand may cool from 104 degrees Fahrenheit to 68 degrees Fahrenheit, with no external insulation.
So… how about using it as a battery?
This article and video from the BBC explains the concept well. The engineers claim that if we heat sand in large quantities, keep it insulated to prevent loss of heat… it can store this heat for several months. In Finland, where the average temperature is around 10 degree Celsius, the country needs heating for most of the year.
The startup, Polar Night Energy, plans to meet this demand by connecting the sand battery to the electricity grid, where the wind power supplies excess electricity to heating coils that become red hot, and these coils heat the air inside the massive building where sand is stored. The heated air is then passed through the sand using a system of ducts so that it is heated uniformly. This would be trivial with a liquid, but due to sand’s low heat transfer capacity, uniform heating is a challenge, and has to be engineered into the system to function properly.
The company has already built a pilot plant in the small town of Kankaanpää in Finland (picture below) in an ominous looking black building. Heck, considering how many Scandinavian noir shows that exist, this probably has something to do with how Finns see things… but I digress.
The “battery” contains 100 tonnes of sands is also connected to the centralized district heating grid, so the sand, which has been hot for months since summer, now feeds the heat to the city. As the video shows, its heating an entire town and even its swimming pool - an impressive feat for a new startup in the energy sector.
There has been a wave of interest in the company since the BBC featured their article, and they claim that they can replicate the system anywhere in the world, since it only needs sand. It gets better: considering the sad state of pristine sand mining, which I wrote about last week, this technology is unlikely to place extra burden. Why? Sand for retaining heat is less sensitive to its quality, so the company can recycle sand from construction waste and rubble - a quantity that is plentiful, cheap and environmentally friendly.
Polar Night Energy is on track with a larger plant for a city and there is a lot of excitement, because it looks like we finally seem to have a technology that seems to get everything right: cost, effectiveness and simplicity. The goldilocks zone of scientific progress.
But there is reason to be cautiously optimistic.
Every Battery is an Energy Storage Device For the Power Grid… But is The Opposite True?
With all the hype, and the company’s claim that it is universally scalable, it is worth building some realistic expectations. Storing heat in sand is not a new idea: the native people in the Sahara desert and elsewhere have been known to heat bags of sand in the sun and use it at night to heat their homes, due to large differences between day and night temperatures in the desert. The idea itself is not radical, but the implementation is.
Recall the problem of base load: A Lithium-Ion battery is extremely energy-dense. Still, an average Tesla car battery weighs over 1000 pounds to power it for hundreds of miles. In the energy grid, these batteries can store the energy during day and release it back at night, but needing several thousand pounds of expensive Lithium. These batteries in the grid are around 80-90% efficient.
The sand battery has not accomplished this so far, and some estimates put its energy conversion efficiency much lower. Why? The sand battery needs a network of fans and ducts to keep heat circulating through hundreds of tonnes of sand, and this takes energy. Unlike Lithium-Ion, sand is extremely low in energy density.
The lower the energy density of the material, the more energy it takes to convert it to a different form. In this case, heat to electricity. At some point, the cost of doing it becomes high enough that it is not commercially feasible. Where exactly this point is for sand batteries, only Polar Night Energy knows. Therefore, the first applications for this technology is to ensure district heating, and not electricity generation for base load, which is still ruled by coal, gas and Lithium-Ion batteries. So is this really a battery? For heat, yes. But in ways we think of batteries today in the power grid… No. It can be, but not today - you have to generate power to be a battery.
Even in the context of heating, I remain skeptical about how scalable it is to a large population. The prototype in Kankaanpää is for a town of barely 12,000 people, and it still needs 100 tonnes of sand! How much will we need for New York City, with 8 million inhabitants (Their website mentions NYC as an application area since they have a centralized district heating system, like Finland)? A simple extrapolation takes us to millions of metric tons of sand. Covered by a steel insulated chamber with fans, vents and ducts…. All of this is a new infrastructure that must be built, not to mention finding the space put in all that sand. Densely populated urban areas do not have that luxury.
If Sand is plentiful, and land is cheap, this looks promising. As the company mentions, this is a nordic solution at heart: Small populations, Lot of land, and an abundance of energy, with long cold winters that make heating a matter of survival. Even if it never generates electricity, this tech can still be a game changer in the Nordics.
But elsewhere, like the megalopolises of North America and Europe? I think the real engineering challenge for the company is yet to come. Exciting times ahead.
Maybe we can find use for all that rubble from construction waste after all. What do you think? Comment below!
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Sand! What an incredible idea -- except for the needed space. Bummer. Still, I applaud these new ideas. I'm sure there will be ONE that'll work.
Really interesting, thank you. Storage is definitely the big challenge with wind and solar so it's interesting to look at what's coming in storage technology.