A handful of startups are taking a second look at thermal storage with the rise of variable renewables. It is ~50x cheaper per kilowatt hour than lithium-ion batteries. Any mass can store heat, often with minimal physical change. Electrochemical batteries are complex devices requiring pricey materials and elaborate manufacturing processes that increase cost. But batteries can efficiently store electricity while heat is expensive to convert back into electricity. Thermal storage can beat batteries by providing heat or operating for longer durations.
Industrial heat makes up 26% of global final energy usage, providing ample opportunity for thermal storage.
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The Nuclear Engineering Department at MIT and the startup Rondo Energy popularized the idea of using resistors (toaster wire) integrated with bricks for heating. It is substantially cheaper than charging with hot air, unlocks faster charging, and allows continuous operation of the system.
The downsides are that the brick form factors are slightly different, and the wire limits the maximum temperature to around 1500C. Customers aren't bothered. Rondo is building a 90 gigawatt hours/year factory to meet demand. Remarkably, a small startup founded in 2020 is setting up a larger facility than any existing lithium-ion battery factory, highlighting the scalability of bricks.
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Steam delivers roughly half of industrial heat consumption. And most steam temperatures are below 400C. The process for making steam from hot bricks is to blow air through the bricks and then use the hot air to produce steam in a heat recovery boiler. Facilities like natural gas combined cycle power plants already use these boilers. The moderate temperatures and simple equipment make steam the obvious market entry point, especially in sunny regions closer to the equator.
The economics push towards on-site renewables in most cases. Grid electricity is usually expensive. Negative wholesale electricity prices stir excitement, but these prices are location-dependent and don't include transmission and delivery charges. Those charges often make up the majority of an electricity bill. Utilities fight hard to deny customers direct access to wholesale markets. And the equipment to handle grid electricity is expensive. A transformer can cost as much as the entire storage system, doubling the installed cost.
Hooking up solar panels directly to the storage system removes inverter and utility interconnection costs. Wind power could contribute in some instances, but most facilities won't have cost-effective wind resources nearby, and the turbine spacing requires a much larger footprint.
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Solar PV plus brick storage could be very cheap. A solar farm produces electricity that costs anywhere from $25-$40/MWh when connected to the grid. $20-$30/MWh for off-grid could be a reasonable assumption. That equals $5.75-$9/MCF of gas or $140-$210/ton of coal. Those prices are already cheaper than much of the world's energy prices at trading hubs. On-site electricity is more competitive than it appears because the efficiency of a thermal storage system is often higher than fossil fuel boilers, and delivery costs for fossil fuel add significantly to end-user costs.
This was super informative, thanks for sharing! I thought the idea of using a giant pile of gravel as a thermal battery was fascinating and also a bit hilarious as a mental picture. “That pile of...
This was super informative, thanks for sharing!
I thought the idea of using a giant pile of gravel as a thermal battery was fascinating and also a bit hilarious as a mental picture. “That pile of rocks? No, it’s not leftover mess from construction, it’s, uh, a thermal battery!”
The author has a great writing style. He can explain the rather complicated concepts here in a simple and straightforward way, a skill that I am envious of.
A while ago I fell into a YouTube rabbit hole of seeing videos of “Earthships,” which are detached homes that are intended to be as closed of an ecological loop as is reasonable. In addition to...
A while ago I fell into a YouTube rabbit hole of seeing videos of “Earthships,” which are detached homes that are intended to be as closed of an ecological loop as is reasonable. In addition to managing heat through positioning windows strategically to catch the sun and vents to optimize airflow, one of the other big strategies is just to build with a lot of mass, usually with rammed earth filled into and around old truck tires. And then some of the bigger ones will also be attached to an enormous cistern of water (with a green roof and some gravel and limestone as the base to filter the water soaking in). That water ends up being a huge thermal battery, holding tons of heat in and also as an evaporative cooler when it gets too hot.
Thinking small, the large hearth didn't just keep your home from burning down, it also insured that after the fire died it would help keep the house warm for quite some time. I envision a future...
Thinking small, the large hearth didn't just keep your home from burning down, it also insured that after the fire died it would help keep the house warm for quite some time. I envision a future where we return to having large brick hearths in every home for similar reasons. A wood pellet stove would be great for emergency heating, and using it as a thermal battery to run refrigerant lines through could be a massive win, if requiring some engineering efforts.
There's a bunch of reasons that would be not desirable. For one, burning fires indoors is not ideal for respiratory health or for keeping the house clean. It never was, people just tolerated it...
There's a bunch of reasons that would be not desirable. For one, burning fires indoors is not ideal for respiratory health or for keeping the house clean. It never was, people just tolerated it because the alternative was to freeze to death. But in hot climates, cooking was traditionally done outside, in an open air gazebo type structure instead of in an enclosed space. You didn't start fires inside unless the weather was nasty. You know how in older movies they always showed that a house was dirty by having a butler put on a white glove and run it over a surface to show how black it was? That's from an era where people were heating their houses by burning coal. The dust settles on everything which means you had to dust constantly or everything would be covered in soot. I have friends who live in a restored brownstone in Pittsburgh from that era and when they were hanging up pictures coal dust would sprinkle down on their heads because so much of it had piled up in the rafters and the attic from those days. ALL of that was going into peoples' lungs.
Secondly, transporting the fuel for the fire and stoves introduces its own energy demand issues. Moving energy from place to place with wires is extremely efficient relative to any other way of getting it from point A to point B.
And lastly, it's one more thing that's a hassle to child and pet proof. You sort of childproof open flames by default in a traditionally managed household because there is basically always someone working in the kitchen since food prep and preservation is so labor intensive without modern appliances. But even then, accidents did happen and they happened quite frequently. Fires were a constant danger in pre-modern households.
There are more modern ways you can get at similar advantages to a hearth. Something like an AGA range is basically a stove that functions as a home heater. You get much less control over the heat on any given thing, but most home cooks don't actually need that sort of control anyway. But it's gonna be a hard sell for if you live somewhere that the weather is highly variable. Maybe some sort of efficient heat-exchanging mechanism can help manage home temperature (for example, if you need to cook for a big party in the middle of summer and would prefer not to have it bake your house).
Finding ways to flesh out an idea seems a better discussion than lecturing on the dangers of open fires. I imagined a pellet stove placed inside the hearth, using the brick thermal mass to soak up...
Finding ways to flesh out an idea seems a better discussion than lecturing on the dangers of open fires.
I imagined a pellet stove placed inside the hearth, using the brick thermal mass to soak up and dispense heat evenly.
Perhaps the brick hearth, ideally in a central location and 2-3 stories tall could also create a beneficial stack effect when "cooling"? Probably requires dampers and might not work with active cooling (refrigerant lines @vord suggests). Can we think of a way to make that work?
Modern pellet stoves are what I was refering to for deploys. They have intake and exhaust pipes, minimizing emissions in the home. Fairly small ones can heat up to 2500 sqft for up to 3 days....
Modern pellet stoves are what I was refering to for deploys. They have intake and exhaust pipes, minimizing emissions in the home. Fairly small ones can heat up to 2500 sqft for up to 3 days.
While heatpumps are generally superior over 35F for heating comfort and efficiency, these are fantastic backup options for long winter cold spells, especially north of Virginia.
I envision a home with heat pumps as primary heating/cooling, but when the temp drops below say 20F, a wood pellet kicks on for a few hours, the heat pump redistributes through the house (and defrosts the outside unit)), and a sizeable hearth helps keep the home warm after as well.
In milder climates, where cooling is the primary concern, having a large thermal mass is still useful, as keeping it at room temp would allow for longer periods of AC shutoff.
I have some relevant experience due to how my dad chose to heat his house. Although well-insulated, it was built in the early 1970’s with all-electric heat, which turned out not to be a good...
I have some relevant experience due to how my dad chose to heat his house. Although well-insulated, it was built in the early 1970’s with all-electric heat, which turned out not to be a good choice.
He decided to save money with a wood-burning stove that we used for many years. Wood-burning stoves do send all the smoke outside if designed right, but this can still be a lot of nasty smoke outdoors, depending on weather conditions. (Nowadays I don’t like it when the neighbor barbecues, and even biking past a house that’s using their fireplace is a little annoying. This wouldn’t be appropriate technology for California.)
An iron stove is itself a fair bit of thermal mass. Also, houses with full basements already have a lot of mass. For new construction, insulating the basement better on the outside should help too.
For existing housing, there’s another way to use thermal mass that avoids large installation costs. My mother’s house now has multiple electric heaters, where each heater is a box full of bricks. The bricks are just sitting in them loose, but they’re not going anywhere because they’re heavy. They’re installed by putting them in one at a time, which avoids the installer having to lift anything too heavy.
It’s just regular resistive heating, but it allows using electricity at night to heat during the day. My dad installed them a long time ago because they were subsidized by the electric company.
It seems like heating a box of bricks is more suitable for colder climates. They are nice to have during shorter power outages, too.
Heat pumps came later and unlike resistive heating, they can be more than 100% efficient. Perhaps they could be combined somehow, but unlike resistive heating they get less efficient as the temperature difference increases, so it’s not great for heating a box of bricks to high temperatures. Plus they also work for air conditioning.
One thing about having a lot of thermal mass is that if it’s the wrong temperature, it takes a lot of energy to get it to the right temperature. That means setting a thermostat and not changing it much; it’s no good if you want a heating system that can respond quickly to adjustments.
An old-fashioned form of solar heating is to design so that sunlight through windows hits a brick wall or floor. However, this only makes sense in winter, and has the downside that it’s heating during the day when it’s least needed. In summer, you want to keep the heat out, so now, new construction uses low-e windows that reflect infrared radiation instead of passing it through.
From the blog post:
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This was super informative, thanks for sharing!
I thought the idea of using a giant pile of gravel as a thermal battery was fascinating and also a bit hilarious as a mental picture. “That pile of rocks? No, it’s not leftover mess from construction, it’s, uh, a thermal battery!”
The author has a great writing style. He can explain the rather complicated concepts here in a simple and straightforward way, a skill that I am envious of.
A while ago I fell into a YouTube rabbit hole of seeing videos of “Earthships,” which are detached homes that are intended to be as closed of an ecological loop as is reasonable. In addition to managing heat through positioning windows strategically to catch the sun and vents to optimize airflow, one of the other big strategies is just to build with a lot of mass, usually with rammed earth filled into and around old truck tires. And then some of the bigger ones will also be attached to an enormous cistern of water (with a green roof and some gravel and limestone as the base to filter the water soaking in). That water ends up being a huge thermal battery, holding tons of heat in and also as an evaporative cooler when it gets too hot.
Thinking small, the large hearth didn't just keep your home from burning down, it also insured that after the fire died it would help keep the house warm for quite some time. I envision a future where we return to having large brick hearths in every home for similar reasons. A wood pellet stove would be great for emergency heating, and using it as a thermal battery to run refrigerant lines through could be a massive win, if requiring some engineering efforts.
There's a bunch of reasons that would be not desirable. For one, burning fires indoors is not ideal for respiratory health or for keeping the house clean. It never was, people just tolerated it because the alternative was to freeze to death. But in hot climates, cooking was traditionally done outside, in an open air gazebo type structure instead of in an enclosed space. You didn't start fires inside unless the weather was nasty. You know how in older movies they always showed that a house was dirty by having a butler put on a white glove and run it over a surface to show how black it was? That's from an era where people were heating their houses by burning coal. The dust settles on everything which means you had to dust constantly or everything would be covered in soot. I have friends who live in a restored brownstone in Pittsburgh from that era and when they were hanging up pictures coal dust would sprinkle down on their heads because so much of it had piled up in the rafters and the attic from those days. ALL of that was going into peoples' lungs.
Secondly, transporting the fuel for the fire and stoves introduces its own energy demand issues. Moving energy from place to place with wires is extremely efficient relative to any other way of getting it from point A to point B.
And lastly, it's one more thing that's a hassle to child and pet proof. You sort of childproof open flames by default in a traditionally managed household because there is basically always someone working in the kitchen since food prep and preservation is so labor intensive without modern appliances. But even then, accidents did happen and they happened quite frequently. Fires were a constant danger in pre-modern households.
There are more modern ways you can get at similar advantages to a hearth. Something like an AGA range is basically a stove that functions as a home heater. You get much less control over the heat on any given thing, but most home cooks don't actually need that sort of control anyway. But it's gonna be a hard sell for if you live somewhere that the weather is highly variable. Maybe some sort of efficient heat-exchanging mechanism can help manage home temperature (for example, if you need to cook for a big party in the middle of summer and would prefer not to have it bake your house).
Finding ways to flesh out an idea seems a better discussion than lecturing on the dangers of open fires.
I imagined a pellet stove placed inside the hearth, using the brick thermal mass to soak up and dispense heat evenly.
Perhaps the brick hearth, ideally in a central location and 2-3 stories tall could also create a beneficial stack effect when "cooling"? Probably requires dampers and might not work with active cooling (refrigerant lines @vord suggests). Can we think of a way to make that work?
Modern pellet stoves are what I was refering to for deploys. They have intake and exhaust pipes, minimizing emissions in the home. Fairly small ones can heat up to 2500 sqft for up to 3 days.
While heatpumps are generally superior over 35F for heating comfort and efficiency, these are fantastic backup options for long winter cold spells, especially north of Virginia.
I envision a home with heat pumps as primary heating/cooling, but when the temp drops below say 20F, a wood pellet kicks on for a few hours, the heat pump redistributes through the house (and defrosts the outside unit)), and a sizeable hearth helps keep the home warm after as well.
In milder climates, where cooling is the primary concern, having a large thermal mass is still useful, as keeping it at room temp would allow for longer periods of AC shutoff.
I have some relevant experience due to how my dad chose to heat his house. Although well-insulated, it was built in the early 1970’s with all-electric heat, which turned out not to be a good choice.
He decided to save money with a wood-burning stove that we used for many years. Wood-burning stoves do send all the smoke outside if designed right, but this can still be a lot of nasty smoke outdoors, depending on weather conditions. (Nowadays I don’t like it when the neighbor barbecues, and even biking past a house that’s using their fireplace is a little annoying. This wouldn’t be appropriate technology for California.)
An iron stove is itself a fair bit of thermal mass. Also, houses with full basements already have a lot of mass. For new construction, insulating the basement better on the outside should help too.
For existing housing, there’s another way to use thermal mass that avoids large installation costs. My mother’s house now has multiple electric heaters, where each heater is a box full of bricks. The bricks are just sitting in them loose, but they’re not going anywhere because they’re heavy. They’re installed by putting them in one at a time, which avoids the installer having to lift anything too heavy.
It’s just regular resistive heating, but it allows using electricity at night to heat during the day. My dad installed them a long time ago because they were subsidized by the electric company.
It seems like heating a box of bricks is more suitable for colder climates. They are nice to have during shorter power outages, too.
Heat pumps came later and unlike resistive heating, they can be more than 100% efficient. Perhaps they could be combined somehow, but unlike resistive heating they get less efficient as the temperature difference increases, so it’s not great for heating a box of bricks to high temperatures. Plus they also work for air conditioning.
One thing about having a lot of thermal mass is that if it’s the wrong temperature, it takes a lot of energy to get it to the right temperature. That means setting a thermostat and not changing it much; it’s no good if you want a heating system that can respond quickly to adjustments.
An old-fashioned form of solar heating is to design so that sunlight through windows hits a brick wall or floor. However, this only makes sense in winter, and has the downside that it’s heating during the day when it’s least needed. In summer, you want to keep the heat out, so now, new construction uses low-e windows that reflect infrared radiation instead of passing it through.