Ben_bai said it pretty well. water is good but it can cause rust, pressure, freeze, leak, etc. sand avoids most of this. the big thing is that sand can be heated much much hotter and this helps make up for its shortcomings relative to water. they likely charge and discharge with air blowing thru stainless steel network of pipes thru the sand. this would require special tuning. as someone else mentioned, the thing with water is that the temp is uniform everywhere because water transfers heat within its volume easily. sand doesn’t, so you need pipes spaced out right.
i’ve wondered about insulation but, i think you can step it out with refractory bricks first then a mineral insulation, then something like perlite or fiberglass. the hot air would interface with heat exchanger to transfer the heat to whichever fluid that would go on to heat the home/s. since the sand can be heated to 1700F or more, a kiln or quartz heating element that can exceed this temp is needed. last, a thermally isolated heat temp tolerance fan would be required. i guess a slew of sensors in the sand and at the exchanger unit and output, i can imagine wanting sensors everywhere for tuning sake.
This is called a Storage heater (https://en.wikipedia.org/wiki/Storage_heater), usually referred to in the UK as a Night Storage Heater as it took advantage of cheap electricity at night to provide heat for a house.
Fitting Night Storage radiators was a much cheaper way of fitting central heating than the normal boiler and plumbing approach, and when electricity was cheaper it made more sense. Now, it makes no sense and if you buy a house with it the first thing you have to do is put in "proper" central heating. But maybe with excess renewables that will swing back.
Any system that involves an electrical solar panel connected to a low-temperature electrical heater is likely to be better served with a solar hot water panel, by about a 10:1 ratio.
More like 4.5:1 solar is 20% or so efficient and solar thermal is only like 90% when including losses.
Anyway, the real question here costs both in equipment and labor. Solar hot water panels involve plumbing and need radiators etc they quickly pay for themselves when heating a large home but don’t scale down very well.
Running the numbers I was surprised how cost competitive the sand bucket is for something like a chicken coop. Sure the panel(s) are wasted most of the year, but you don’t exactly need an electrician to set this up either. Probably also worth considering for redundancy in some situations.
The monetary case of PV does not account for the damage that is done to the environment over the entire lifecycle. Solar hot water panels are ecologically superior. Not only are they more efficient in W/sqm. Their production is not as energy intensive (plumbing included), the recycling process is less complex, poisonous materials can mostly be avoided. If heat is the desired product, they probably beat PV by an order of magnitude in the energetic dimension.
PV’s falling prices have also been associated with falling environmental harm. When you’re talking multiple orders of magnitude you just cannot require as much in production.
IE: You can’t burn 500 gallons of gas if the end product costs 500$. That applies not just to transportation but also how much material and thus mining the raw materials you need, including refining them, the amount of chemicals you can use per panel, how much electricity you can use in production of the device including precursors etc.
The argument is valid, if energy is not subsidised. However, burning pv for resistance heating is still wasteful and should be avoided. It is only acceptable for peak production that cannot be put to better use.
It’s wasteful of electricity not necessarily resources.
One of the numbers I was looking at compared air sourced heat pump at night when it’s coldest vs this kind of resistive heat battery. Solar panels are far cheaper and better for the environment on a kWh/day basis so even if the COP is 3 (or less it gets colder at night) * 90%(losses from battery) = fewer panels you more than offset it by needing far more batteries.
Obviously solar thermal setups have advantages if you need lots of heat, but they are also wasted most of the year. If 8-10 months a year you’re only using them for hot water then annual efficiency is closer to 25% than 90%.
Are you including the maintenance costs of the water panel in your estimate?
I used to buy this argument, but then:
A) PV panels got ridiculously cheap
B) everyone I know with solar hot water has emptied their systems because the maintenance hassles were not worth dealing with
Using high grade intermittent current to produce resistive heat isn’t high on my list of efficient things to do, but unfortunately neither is maintaining hot water panels.
Might be a location thing; there are vast numbers of solar hot water systems here in W.Australia and typical maintainence is maybe just replace the tank outright and flush the lines every 20 years or so.
How 'clean' of salts, etc. is the water being put through your panels?
As you allude to, our water in WA is notably soft, maybe that helps? I mean it's full of iron but that mostly just seems to cause staining not clogging.
That said, a heat pump running at 4:1 COP coupled to 20% efficient solar panels gets you right back to the same efficiency as solar thermal, with a lot more flexibility.
I'd rather run both in parallel, and I do, as do most of the people here abouts.
No single point of failure, sun heats the water directly and provides power, with a breakout box that accepts power in from the grid (if required), exports excess for points, hopefully that gets better over time, and accepts a local generator input if the PV panels are offline for some reason when there's a local grid power outage.
This is pretty good for now, there's loose neighbourhood discussion about perhaps getting a local area battery in a sea container that can buffer ~200 standard homes to further secure the town's energy stability.
Flexibility, in rural settings, is about having options not a single point of failure | dependency.
Eg: Way up the hill it's good to have PV panels on the bore pumps and better to have these independant of the house circuits with cables in place to route power "in case" .. along with option to use a generator if needed.
Yeah, when I was looking into this a couple of years ago, thermal panels were about 4 times as efficient as photovoltaic panels, but they were also 4 times as expensive. The ratio has probably shifted in favor of photovoltaics since then. If you have very limited space on your roof solar thermal can still be a good idea, but otherwise why prefer low grade energy (heat) over high grade energy (electricity) if you can get the same capacity at the same price?
Solar hot water hasn't been viable for a decade or more; the ROI is piss poor. Efficiency falls as the water heats, and once your storage container is hot enough, the panels are useless.
At least in residential and commercial installations, you get a much higher ROI by putting in solar electric, using the electricity to power your home/facility, and dumping the excess into the grid to earn money/credit.
Resistive heat storage is also a thing these days; hundred-plus gallon tanks that will take power from the panels if it's more cost effective than returning it to the grid or the grid doesn't have the capacity to take it. That water then feeds a second water heater which brings it up to the final temperature, if necessary.
The efficiency relative to area doesn't really matter, as rooftop space is rarely at a premium.
MGA Thermal uses tiny aluminium droplets in carbon matrix material to do heat storage with 1: constant temperature storage and 2: at much higher temperatures and energy density that allows for steam generation etc.
I get that sand is inexpensive and simple and I like the idea - especially since it scales to an entire town (!)
But I can't help but think of one "technology" that could make a scheme like this MUCH more effective.
a phase change.
for example, it takes 1 calorie for water to go from 30 to 31 degrees F, but but it take 80 calories to go from solid at 32 to liquid at 32 F.
A project I remember reading about that was really interesting was a house constructed with logs that had a resin that phase changed around room temperature. The idea was that the logs would be heated by sunlight, and the resin in the logs would absorb lots of energy without easily going above room temperature. Then at night, they would slowly solidify and give off heat at room temperature. This would be a cool way to stabilize house temperatures without needing equipment.
EDIT: it was called the enertia house
I'm uncertain of the history, it seems there are a lot of different "enertia house" search hits.
Phase change heat storage is well established techology. It's in the product you buy when you want to have building scale latent heat storage. I've even seen a company trying their luck at the business model of selling heat in twenty foot equivalents, e.g. for building sites. So it's safe to assume that they knew about this option. But there's also the square-cube-law, at some point it will be cheaper to insulate a larger volume of the cheaper material than a smaller volume of a material that requires less volume per unit of energy at the heat range you aim at. It's well possible that this line is between building-scale and grid-scale storage.
The Venera probes used this idea in the 1970s. [0]
> One new idea, for additional thermal protection, was the addition of phase-change material. Lithium nitrate trihydrate melts at 30° C, absorbing a large amount of heat, due to its high latent heat of fusion.
> The sand itself will also be sustainably sourced – it’ll consist of crushed soapstone, which is a manufacturing byproduct of another local industry.
The requirement is not really construction grade sand but any substance with a largish thermal mass. Sand/dirt/crushed rock, etc. whatever you have right on your doorstep. There have been plenty of prototypes using such various materials as thermal mass.
All you need is a big container, some insulation to keep the heat in and lots of mass in whatever form. And some pipes to get heat in and out via e.g. water. The bigger the container, the smaller its surface area is relative to its volume. So these things can be quite efficient. If you make them large enough, you can store enough energy in the summer to last for months during the winter.
Awesome and very simple. There is a small community of eco housing that uses this system, in Boekel. They use solar panels in summer to heat the battery which lasts all winter.
On a diy scale, there is also this system for passive greenhouse called a climate battery which uses the same principle, storing heat underground and using it in winter. With almost zero energy we can use it to grow bananas in a greenhouse in the Netherlands.
Waste material costs over €100/ton to dispose of across most of the EU. So therefore this material costs less than nothing!
That's why the project doesn't use water (which has a 4x higher heat capacity and can be pumped allowing the heat exchangers to be far smaller and eliminating the 'temperature decreases as it discharges' problem).
Phase changes will probably work well for structures that would otherwise be used for insulation anyway - in the case of this particular idea though, I think heat is moved in and out using another medium, air in this case, so a phase change would complicate things greatly (hard to bubble air through lava). But yeah, something like a phase changing fluid (gas <-> liquid) will probably work well, as it does in air conditioners right now.
Now what would be suitable material with such phase change around 80 to 110 degree temperature? As that is what effective heating needs. Water with the safe change at 0, is clearly too low as then you could just run ground source or air source heatpumps...
> But I can't help but think of one "technology" that could make a scheme like this MUCH more effective.
I think this is one of those things that drive home the point that there are fundamental differences between physics and engineering.
The article states that the thermal silos are heated with excess energy from the power grid. This alone tells you right from the start that efficiency is not the primary requirement.
Sand is inert, doesn't decompose or degrade, is readily available, is easy to work with, and has no moving parts. You can make it work in a silo, or digging a well to fill it with sand. In fact, geothermal heat pumps are already used extensively in residential buildings to regulate temperature. You just have to drill a hole in the ground that's deep enough, run a water pipe through it to heat/cool the water, and run that water through your building to heat/cool the environment. The nifty trick of Polar Night Energy is that they introduce the extra step of actively heating the thermal source with cheap energy supplied by the electrical power grid.
This sort of argument is like complaining that a Formula 1 car is far more efficient than a Volkswagen Golf. Yes it is,but that's a mute point.
I can't endorse this perspective enough. The amount of energy storage we need is staggering and ever-growing. We've someone convinced ourselves that the 'baseline' is consuming with abandon millennia worth of stored energy and anything even slightly less responsive than that is too inconvenient. Given those parameters, we need any and all energy storage options and efficiency is not a priority. Tesla powerwalls were never going to power the world, but giant caverns full of sand might.
sand battery size is "13m tall and 15m wide". Assuming most voluminous possible shape that's 13m * 15m * 15m = 2925 cubic meters of sand (100% fill, no account for insulation, etc.)
Dry sand density is about 1600kg/m3
Total weight of sand would be 2925m3 * 1600kg/m3 = 4.7Mkg (4.7kt)
4.7Mkg of sand has a heat capacity of 830J/kg * 4.7Mkg = 3.9 * 10^9 joules / degree C (it takes this much energy to heat up entire battery by one degree C)
So from this, we get that 100MWh of energy would heat up the battery by (3.6 * 10^11 J) / (3.9 * 10^9 J/C) or about 100C.
If we include a different shape (a cylinder), and account for a thick insulation needed, this becomes closer to 200C of temp diff.
I guess it checks out... It is going to be more difficult to estimate heat loss.
But sand is quite expensive so my question is, why sand and not water? Water has 5 times higher specific heat per weight, about 3 times per volume. Water is way cheaper than sand and much easier to find, transport and extract energy from. The only real problem with water is you can only heat it up to 100C.
> But sand is quite expensive so my question is, why sand and not water?
The article says they will use a byproduct from a local industry, perhaps it's available for cheaper.
"The sand itself will also be sustainably sourced – it’ll consist of crushed soapstone, which is a manufacturing byproduct of another local industry. This material can apparently conduct heat even better than regular old sand.".
It's the not the cost of obtaining the water that makes water less viable, it's the cost of storing superheated water. Water has this pesky ability to expand 1600 times larger when it goes over 212 degrees Fahrenheit. That means that if not handled carefully you get deadly steam explosions.
Hot sand / crushed rock doesn't have the same problem. If you read in the linked article it says
> with the sand heated to somewhere around 500-600 degrees Celsius (932-1112 °F).
That would be extraordinarily hard to do with water as you'd need significant containment and safety measures.
If that water has to come from underground, yes, it would be cheaper and easier to just grab surface stone and crush it for material to make a heat battery.
Also, water tends to make for a horrible heat battery as it is much more thermal-emissivity than rock particles. You need all sorts of additional insulation to retain the heat, whereas the sand will insulate itself.
I can think of several reasons I would choose sand:
-Order of magnitude smaller coefficient of thermal expansion.
-No real risk of phase change - freezing or boiling.
-No problems with corrosion/scale in high temperature. On one hand regular water contains minerals which can build up on the heat exchanger element, on the other demineralized water sucks in carbon dioxide and oxygen from the atmosphere, causing corrosion of steel parts. You would need an airtight container to alleviate this.
Sand is great because it's largely inert in a huge range of temperatures.
I’ve got a few trucks in which I could transport tons and tons of and but I’d struggle to move more than a few hundred gallons of water at a time. I’d guess most transportation outfits are similar and costs in accordance. Point: sand.
> -Order of magnitude smaller coefficient of thermal expansion.
On the other hand the thermal expansion of water does not matter because water, you know, is a liquid -- it conforms to whatever vessel you put it in.
> -No real risk of phase change - freezing or boiling.
In a vessel that is meant to store thousands of tonnes of water that is hot and that is very well insulated to store energy for months, there is no real danger that the water will freeze. On the boiling side, water make it easy to monitor the temperature and you just stop adding energy if it starts boiling. Your kettle can do that reliably, we can do this for a huge battery.
> No problems with corrosion/scale in high temperature
It is hard to put 5 thousand tonnes of sand in a container and ensure it is dry. There is always going to be water that will be evaporating when you heat the sand in the middle and condensing on the sides that are cold.
There is no possibility of scale when you do not heat water to boiling.
> Sand is great because it's largely inert in a huge range of temperatures.
Another point, and I'd love to be corrected here, is that with a container of water you're going to get a ton of convection currents leading to a much sharper heat gradient at the edges, resulting in significant heat losses with the same amount of insulation.
At a guess, and I confess I'm not capable of running the numbers, this offsets the much higher temperature delta of sand.
Interesting! Wonder if it's also due to ease / lower risk of containment. And sand doesn't expand if you accidentally let it freeze, which is again nice from an "it won't rupture" perspective.
As to cost, the article does note that they're reusing crushed soapstone from a local byproduct, so maybe that helps reduce the cost?
> Many solid materials, such as sand, can be heated to temperatures well above the boiling point of water. Sand-based heat storages can store several times the amount of energy that can be stored in a water tank of a similar size; this is thanks to the large temperature range allowed by the sand. So, it saves space and it allows versatile use in many industrial applications.
So perhaps they're also specifically targeting future applications where they would need to supply > 100C heat.
More water takes more space, and perhaps the higher temperatures make it easier to reuse the heat? And you probably don't want your water to get above 100 degrees C, because then you need to deal with pressure.
Sand is also a really good insulator if I'm not mistaken. That could also be a factor somehow.
I cannot explain the physics, but a big rock that has basked in the sun is really warm for a long time after sunset. A bucket of water loses its temperature faster.
The higher density of rock probably plays a big role.
Water can store many times the amount of energy per volume or mass, per degree Celcius, than rock.
What you see is that rock has much lower thermal conductivity. It can be hot inside but it is not as good at transferring that heat outside. It means when you have hot rock it will stay hot for longer than equivalent amount of water. That because water emits that energy faster.
But put that rock and water in a well insulated vessel and you will find that the properties of the insulation and the vessel will start dominating the process and what counts now is how much energy you can store in the material inside.
If it’s going to function as an energy source doesn’t it need to run a turbine so it can’t be water? The article said it’s going to just use heat directly so I guess they could use water.
But if ever the need arose it needs to be higher than 100C so it can generate steam for the turbine? Maybe I’m way off, I’m just a guy.
I was about to write just that. Also, desertification is a problem, so they could possibly get free raw material from many places, paying only shipping, and doing something good at the same time.
Here's a smaller scale water battery that heats an entire house year round. The water tank itself is placed in the center of the house across all 3 floors and actually makes for a quite nice design element as well. The energy is generated in the warm seasons and the volume of water is large enough to last through the entire winter. It's a German vid but subtitles are quite accurate.
Doesn't this suffer the problem that you're then putting heat into the device, which is in your house, during the warm season when you want your house to stay cool?
Think of the water tank as being in but insulated from the house.
In the hot summer the hot air within the house is cooled by heat pumping the heat into the water tank which is slowly over weeks bought up from cold to mean summer tempreture.
In the winter the heat is extracted from the water and transfered to the house.
The house and the water mass are out of phase by six months.
It's an interesting concept. Cheap. Doesn't take up a lot of space. Could be driven by a solar furnace.
My concern is whether the cyclone particle separator will do a good enough job - gas turbine blades don't like being blasted with grit. I expect they'd need a heat exchanger to be sure to get only clean gas driving the turbines.
This is great technology, simple and effective. I’ve spent many hours reverse engineering to see how effective and expensive it might be. I’ve found it’s very cost effective but heat can be hard to calculate. i like that they did a prototype. i think i’ll do one at some point. for an individual house it makes more sense to improve my insulation but i think ill still build a small version for fun.
How long ago was this? The idea seems simple enough and would really benefit from the increasing glut of renewables. Even better, unlike a sand battery, you can potentially cycle it for several months of summer, by offsetting consumption by just a few hours feels like it could be economically viable.
Then again, the competition is general purpose batteries which can be used all year and probably require less maintenance.
Just because something seems like should work doesn't mean it will. Air conditioners are simple and cheap to install. An insulated swimming pool in the basement is not.
1 cubic meters of water chilled 10C below ambient is 40MegaJoules of energy.
Or in other words: 11kW-hrs of cooling, comparable to an entire Tesla Powerwall.
We aren't talking about entire swimming pools here. Just a few cubic meters of water. Shift the temperature delta as you see fit but... It's actually very space efficient.
Air conditioners have a COP of about 3 though. So really its more like 3.5kwh of storage. If you want to go lower temperature to store more then your COP drops.
Typical residential air conditioning is using 4kW of power. So your 40MJ of energy would be used up in 40MJ/4kW=10000 seconds, or just 3 hours. And its only theoretical, because it assumes 100% efficiency in storage and conversion
> Air conditioners are simple and cheap to install
Today. In my chats with the sort of home improvement types that seem to be in this thread, we're less concerned with today and more concerned with the cost of things in the future, as societies include "climate taxes" (in the form of actual taxes, subsidies, or whatever else) in the cost of things.
I'm hoping someone recreates it. It's just a few fans, a tub of water, a pump, and a slightly modified heat pump/air conditioner unit that can safely run down to 50F or so.
It's already viable for stadiums and other public events.
This has been around for a long time. The Detroit VA hospital used it, I believe with tanks under the parking structure. I also believe there were problems related to the maintenance of the tanks.
But the important aspect is that this is a cost saving technique, not an energy saving one. You’ll spend more energy because of the thermal losses of the tanks, but hopefully it is more than offset by the cheaper electricity rate.
Energy is (sometimes) free, or even negative priced (!!!!).
As it turns out: it's economically infeasible to turn off solar panels, wind, nuclear and sometimes Hydro (depending on water rights, it may be illegal to store water/energy at a water dam).
In all of these cases, the energy is 0 cost or even negative cost.
You're confused, negative prices are not driven by techincal limitations. Solar, and especially hydro, and even nuclear can shut down just fine.
Solar and Wind instead drive prices negative because they have subsidized contracts. Each country is slightly different in implementation, between Feed-in-Tarif vs Feed-in-Premium. In either method there will exist prices which are negative to the market but positive to the producer.
Thus if you cannot plan long term to rely on negative prices. Those FIT schemes around the world are transforming into FIP, and the FIP premiums are getting lower and lower. Negative pricing in electricity markets will go away in a decade or two. This isn't magic or even special, solar is succeeding which means the subsidies are getting weakened.
Curtailing solar production is not a big deal. How many existing plants have grid operator directed shutoffs depends on your market. In Japan as of 2024 all new non-rooftop solar has it. Prior to 2020ish only Kyushu and Kansai regions required it. Now Tepco, Hokkaido, and Touhoku require it for new contracts. There are still a couple old contracted plants getting developed this year, but those are rarities.
And that's only Japan, which itself does not have a super strong duck curve yet: http://jepx.org/ Regions like California have had extreme duck curves for ages. While duck curves are a big worry for internet commentors, they've been points of discussion for grid operators are a lot longer. Hence the move to Feed in Premiums which will slowly make the duck curve a solar operator's problem and not a tragedy of the commons situation.
> In either method there will exist prices which are negative to the market but positive to the producer.
I'm talking about running air conditioners extra hard during low market prices (which includes free and/or negative priced periods of energy), and then storing that cooling power in single-digit cubic meters of water.
Negative market prices of electricity absolutely applies to this case.
Since you burn the wood really fast, you need to delay that heat release. That's where sand comes in as a cheap and good enough material. There are also systems that heat water and store it in a tank, which also make sense if you want hot water in your house too.
This is basically how houses in central Europe work. The walls and floors are built with masonry (bricks or concrete) and wrapped in a thick layer of insulation. This results in a building with a very high thermal mass.
The result is that it takes a long time for the inside of the building to change temperature.
The other day we had a warm sunny spring day (5C / 40F), so I opened all the windows and did some spring cleaning. I set the thermostat to 16C as I didn't want the heating coming on while I was doing this.
I forgot about this, and the next day when I woke up wondered why the temperature in the house was only 20.5C (68F) as usually we have it set to 22C (72F) - then I remembered I effectively turned off the heating 18 hours ago, and forgot to turn it back on.
100 MWh is 8 tons of "oil", according to a google search. Oil is probably about the density of water, so 1 cubic meter is about 1 ton of oil, and this structure is about 2000 cubic meters, so oil has 200 times the fuel density of this battery.
Light crude oil has an API gravity higher than 31.1° (i.e., less than 870 kg/m3)
Medium oil has an API gravity between 22.3 and 31.1° (i.e., 870 to 920 kg/m3)
Heavy crude oil has an API gravity below 22.3° (i.e., 920 to 1000 kg/m3)
Extra heavy oil has an API gravity below 10.0° (i.e., greater than 1000 kg/m3)
I've been obsessed, mildly, with Feolite. It could be brought back for these sorts of applications to great effect. It's got the volumetric heat capacity of water and the ability to withstand much greater temperatures without any annoying phase transitions. It was very popular in the UK, where they went in early on charging for electricity on a rate varying by hour. The basic formula is easy, but there's a lot of secret ingredients to get the heat capacity way, way up.
Speaking of home uses and phase transitions, they have made inroads into making materials with a phase transitioned tuned to household temperatures. Say you have aimed for seventy-two Fahrenheit. The temperature gets higher than that? The material melts, absorbing ambient heat. Then, as the temperature dips below seventy-two Fahrenheit, it freezes, releasing heat. It would make a fantastic sort of "heat capacitor," designed to deal with rapid, small-scale temperature fluctuations, while larger heat batteries could deal with fluctuations over days, weeks, and so on.
Only problem is that these phase transition materials tend to be terrifyingly flammable, so far. Like soaking all of your drywall panels in candle wax before putting them up.
I was thinking of ways to get around the peak solar generation being when you don't need it for heating and genuinely really heartening to hear about solutions like this.
The article doesn't mention the costs or complexity in building each unit but definitely implies it's cheaper than storing it batteries, which makes sense if it's essentially a heating element buried in sand.
Wonder what the smallest viable size would be, could you have one for each street / block of flats? One of the comments mentions burying them for better insulation which I'm assuming they didn't want to do just for the prototypes?
>The article doesn't mention the costs or complexity in building each unit but definitely implies it's cheaper than storing it batteries, which makes sense if it's essentially a heating element buried in sand.
Batteries are more expensive than nuclear power so yes, anything else is better.
The issue is that we can't both electrify everything and use legacy heating applications. Going back the the 20th century idea of central heating/cooling is great until you realize the costs involved with adding all the plumbing to buildings which were never designed with it in mind.
Peak solar is peak A/C usage in the summer, which is typically more expensive than heating. In colder times more energy is used at night, but I think less energy is still used at night in the winter than in the day in summer.
You could be right, insulation is a lot more effective at keeping houses warm than cold though (greenhouse effect) and heat pumps can dump their waste heat inside.
Still, if you got this kind of temperature gradients, and I happen to have lived in such a place, you also got at least 100mm mineral wool insulation or eqivalent. Which is poor by the way, and people do invest into 200 or sometimes 300mm. Which makes CO2 and moisture control more of a problem than heating. You basically either heat the outside via ventilation anyway, or invest into heat exchangers. Since complex HVAC systems come with maintenance burden, people just put up 50-100KW-ish heaters, be it natgas or wood and declare the problem fixed.
It's more effective volume wise. I did the math, some time ago here are the rough numbers.
Sand has way less heat capacity then water per kg (about half).
Water can be heated to 95C with standard unpressurized vessel. Sand in this application is heated to 600C.
Sand is denser then water (kg/m3).
For the same heat energy stored this comes out to about 2.5x more volume of water(95C) compared to sand(600C).
Water and Sand are both dirt-cheap.
Hot water can be managed with standard plumbing equipment.
Sand needs some high temperature piping (hot air to water heat-exchanger, resistive heat tho heat up the sand).
How well both contain the heat is primarily dependent on the isolation. Which favors the smaller footprint of sand, but needs to isolate a higher temperature difference...
One advantage of heating water over sand is that you can heat it up with high temperature heat pumps which currently have CoPs ranging between 2.4 to 5.8 [1]. So for every kW of electrical energy you put in you get at least 2.4kW of thermal energy out.
So yes, the volume of 95C water would be much greater than that of 600C sand, but if volume wasn't an issue you could do it much more efficiently. Alternatively, you could use battery storage for just the electrical capacity required and not the (much higher) thermal capacity which may be more cost effective when you look at the conversion.
The temperature also matters. If you need at least 50 C water to run district heating, about half of the energy stored in near boiling water cannot be utilized. This is much less with 600 C sand.
And 50C is too low. That is minimum temperature of the heated tap water(Legionella and other diseases). And preferably you want some higher. And then as distance increases there are losses and other people using the heat. So temperature you need is actually quite high and in very cold days can be over 100C...
Probably not relevant to the specific problem at hand because sand that's getting heated to 600 degrees is going to quickly boil off any residual moisture. As a warning, though, so that people don't experience the pain I got to experience, a pile of sand is really good at holding onto water internally and freezing when it gets cold. We had to replace our sewer line a while back and for various reasons were taking care of filling the hole and redoing the concrete ourselves. The sand guys left a big pile in the driveway for us and as soon as winter came the whole pile turned into a single giant rock despite having been out in the hot sun previously.
Exactly to your point, though, one of the great things about using sand for this application instead of water is that you can probably just shut the thing off for maintenance without having to worry about draining all of the sand out. If it freezes up due a bit of residual moisture content it's not going to expand nearly the same way that a silo full of water would, and it should be easy enough to thaw out just by putting some heat into it.
A quick caveat/clarification: It's only true if you're pushing the system over the 100°C mark. Otherwise a volume of liquid water--with its greater latent heat-capacity--will outclass the same volume of sand.
Water's heat-capacity is 4.186 J/g°C, while estimates for sand run towards ~0.830 J/g°C. If we also assume the sand is 1.6x denser, then our below-boiling water still comes out ahead at ~3.15x the joules per volume.
There are hints [0] this system tops out around 600°C.
I think the original plan was to convert the heat back into electricity with a turbine. So the higher temperature of sand would greatly improve thermodynamic efficiency.
>I think the original plan was to convert the heat back into electricity with a turbine.
Is that just speculation or did you read it somewhere? IIRC the original motivation of PNE was a bunch of engineers at uni speculating on how to build the perfect building for engineers, and making it self-sufficient would require handling its own heating, which they originally thought would be best done with a big hot-water tank to store the heat. No turbine was suggested, IIRC.
In addition to what others have said, isn't one of the weird properties of water that it tends to take in energy easily, but not give it back so easily? I've never really understood how that works, but I think I've leqrned that at some stage. Hence why it's used in cooling so much. Somebody jump in and tell me how wrong I am, or if I'm on a track that doesn't lead completely nowhere.
Water vaporizes, and at that point blows up just about every container you can build around it. As will ice when it cools down. Sand is just sand, very little difference, very unreactive from way under freezing temps to about 1300 degrees.
And you might vent steam, but you should probably take into account that while water < 45 degrees or so is pretty innocent, steam will strip flesh from bone starting at 180 degrees or so, it won't "just" burn you.
That’s the other bizarre thing about water/ice: most things expand as you heat them, but very few things expand as you cool them. Water has maximum density at 4C, so even before you freeze it it’s already starting to expand as you cool it.
ah OK, I get your point :) It doesn't really matter though because water when liquid doesn't cause any problem when expanding or contracting, its level in a vessel will just slightly change. It's only the ice that can fuck up pipes etc
Note here is also that district heating uses water that is heated to 65-115C.
Which means that you have rather little of delta to work with. And at upper end it becomes somewhat risky to have large container of water that is beyond boiling in normal pressure...
With sand you can use very simple heat-exchangers. No need to use exotic heat pumps that require extra energy...
Been curious if you can't do this with houses generally. Set a temperature range of say 21-25c. Drive the temperature up when there is excess wind/solar (was primarily thinking of EU) then let it fall off.
Obviously works better the better insulated a house is. Has the advantage of turning everything in your home into a thermal battery with the only real cost being furnace controller, potentially even just software. At one point I meant to do the math to figure out the rough storage/efficiency for an average home but never got around to it.
I'm not suggesting adding anything. Homes have lots of stuff in them. Everything in your house becomes a thermal battery - tables, chairs, beds, walls, counters, clothes and so on. Just allow a range of temperatures and when energy is very low cost drive the indoor temperature to the upper range.
Ideally you have a brick/stone home with good exterior insulation.
Would it make sense to actually store it underground, or are the gains minimal when compared to the costs?
In general, I'm impressed that there's so much research and investment in this field (storing excess renewable energy). I think most people don't realize how much and that the problem of the sun not shining and wind not blowing all the time will be solved much more quickly than most people think is possible.
You can DIY this on the cheap by connecting solar panels directly a $10 hot water heating element and burying it in sand. You will need to pair the right spec element with your panels
It feels that Europe is so far ahead the US in matters of environmental sustainability. Why is that? Is it more dense than the US, so they need to be more careful with resources?
Europe is more politically progressive (in some ways) than the USA, and sustainability is a progressive standpoint. Political identity politics in the USA means that even if you have a ranch in rural Texas and are directly observing the negative effects of fracking on your local environment, you still support the people that are trying to make more of that happen because if you don't, well, you must hate pickup trucks and beer and Christians too (drawing on personal experience as a Texan).
I've poked around on this topic here and there when bored and never found many good explanations for why this is the case. Probably a million weird little reasons, historical, economic, cultural. IMO the country is just too damn big to try to claim a single cultural identity, and it's resulted in absurd caricatures of polarized political identity, fostered as well by a two party system where it's basically impossible to be represented well per your values. In the USA you can't vote for someone that's promoting reducing government spending without also voting for someone that's attacking trans people's right to exist, or at least in the same party as such a person. Or someone that's trying to take away your wife's healthcare rights. Same way you can't vote for reducing oil dependence without also voting for, idk, taking everyone's guns away.
Much of the US has access to cheap natural gas. And a good amount of oil. The greater the cost of energy the more it drives innovation in energy usage as well as incentivizes projects that would have a longer pay back period where energy is cheaper. Although Norway has abundant energy but socialized the profits and is using them to advance their usage of renewables and electrification. In the US we privatize the gains and only socialize the losses.
Civil engineering in general is more advanced in Europe. The oil crisis in the 1970 and the subsequent use of thermal insulation made "Bauphysik" (building physics) an integral part of the planing process. In the US you just rely on more heating/cooling power instead.
Just as an example: >=16cm thermal insulation + heat pump or solar thermal energy + double or triple pane windows have been standard for new single family homes since at least 2005 in Austria.
It's also that the infrastructure of the US is very young. In Europe a 100 year old building is shockingly common. And they are still in use. Mostly because even back then (some) housing was already build in a lasting manner. And not being able/willing to tear them down gives you a baselevel of sustainability.
The other thing is that we used up most of "our" fossil energy reserves decades ago and what we want to burn today requires imports, which requires a reasonably stable world economy, a reasonably strong geopolitical position and ... well ... even the last conservative governments have woken up, that those assumptions may not be true any longer
It's mostly centered around cost. Fuel (of all types) is so cheap in the US that efficiency was not an ultra high priority for most people. Ironically, the US has now gotten extremely competent at drilling for natural gas and the price is going to stay pretty much fixed where it is for the next 50+ years. Most of the waste from the early days has also been reduced and funneled into new petrochemical production processes that used to be reliant on oil.
If you are going to stay in one place for a long time, renewable solutions to problems like hot water and residential electricity are worth the investment, but with the migratory nature of the US worker, it's often cheaper for a household to pay for fuel for 5~ years rather than invest in a renewable solution that they will never reap the monetary benefits from.
From what I've experienced since moving from the US to Europe in 2022, it's a lot more common for people to stay in the same houses for an extended period of time. Especially here where I live in the Netherlands, you can commute to work almost anywhere in the country in about 2 hours by car. This allows people to keep the family home.
The fact I find most fascinating is that they have a local district heating system reusing heat produced from local industry. That’s efficiency on another level.
I think that's pretty common, no? Here in NL our district heat comes from a chemical refinery a few clicks away. For them the heat is an annoying waste product.
Here in Brno, Czech Republic, 25% of the city wide heating is provided by the municipal waste incinerator.
There are also a few cities that get heat into their networks from nuclear power plants and something similar is planned in the future for Brno as well.
It isn't totally new, nothing under the sun etc etc, but honestly it's closer to a hot water tank than it is to CSP. Which makes sense, since the original conception of PNE was a hot water tank IIRC.
CSP stores heat as a means to produce electricity, the PNE sand battery stores heat as a means to provide heat. CSP's molten salt was heated by pumping it through (solar) heated pipes, which meant it had to have the correct fluid properties etc. The PNE sand is heated in-place via electricity which means it can be basically any old crap, as long as it's inert at 600 degrees or less.
In other words, CSP differs drastically by the energy input method, the energy output method, and the composition and requirements of the storage medium.
Basically all of the PNE tech is centuries old, the point is that together in this specific configuration they're an incredibly cheap heat storage medium - resistive heating, insulation, sand, all cheap.
In 2012 Vast Solar commenced its 1.2 MW Performance Validation Project which was supported by funding from the then Australian Solar Institute.
This CST project was completed on 22 October 2019.
Scaled up second stage is being built out now (2024):
Vast said on Friday that it has partnered up with global design and manufacturing firm Contratos y Diseños Industriales (CYD) to take the next step forward on its VS1 project, the 30MW/288MWh plant in South Australia.
Yep, combined with district heating that works with simple heat exchangers, it is pretty decent system. So you either pump power to sand or take heat from it and put it heating network.
