Efficient solar cooking that stores heat in sand(sciencedirect.com)
sciencedirect.com
Efficient solar cooking that stores heat in sand
https://www.sciencedirect.com/science/article/pii/S266711312500035X
60 comments
If you can directly use the DC from the panels (ideally ~250vdc) then literally anything to make them more efficient is worse then just more panels.
Applications that directly use DC from PV arrays is cheap, ac grid tied solar... not so much.
Applications that directly use DC from PV arrays is cheap, ac grid tied solar... not so much.
> Applications that directly use DC from PV arrays is cheap
Direct DC is very underrated in America. Almost everyone I know with solar panels is grid tied and they're missing out. Antique belt drive shop tools are cheap, relatively easy to restore and maintain, and lend themselves to solar conversion (just add DC motor). Only downside is that you can only work while the sun's shining.
Direct DC is very underrated in America. Almost everyone I know with solar panels is grid tied and they're missing out. Antique belt drive shop tools are cheap, relatively easy to restore and maintain, and lend themselves to solar conversion (just add DC motor). Only downside is that you can only work while the sun's shining.
Do you have a direct-DC-powered workshop?
I'd like to see comparison between that and just an induction stovetop + some batteries.
Batteries cost US$80/kWh. 700° sand costs about US$1.25/kWh, though that depends on how far the sand has to be trucked. A normal stove burner is about 2kW and requires about 20 minutes to cook a meal for a person, so about 0.7kWh. Either of these is probably affordable in Ghana, where the national income is about US$2500 per person per year, i.e., US$200 per person per month. But the cost of the battery would be significant for an individual (one or two weeks of income), and the cost of the sand isn't.
Sand batteries are pretty well known in the passive solar greenhouse world. Even in cold northern climates they’ve been proven to store enough heat for year round use to keep tropical fruits producing.
I have used a spare (older) Kelly Kettle as a sand battery while camping up some damp mountains in my neighborhood (Austria) and it has been consistently terrific to charge it up, and bring it inside to heat up the tent. The other (newer) Kelly Kettle serves boiling water purposes too, though, of course .. but otherwise the sand-kettle is real convenient, and easy to recharge as well ..
I do the same thing on canoe/rafting trips as well. Load my metal cook pot full of sand and stick it on the edge of the fire once we’ve finished dinner. Keeping it under the tent fly is enough to keep things a little more toasty all night. Especially nice with how chilly it can get camping next to water.
A system similar to this can also be used for cooling rooms and buildings.
A lot of the world's population lives on the tropical belt, in hot conditions. A lot of them use air conditioning, and a lot more are going to start using it in coming decades.
A simple system, that cools a room down about 5 to 10 degrees Celsius, would be a great way to reduce dependence on fossil fuels.
A lot of the world's population lives on the tropical belt, in hot conditions. A lot of them use air conditioning, and a lot more are going to start using it in coming decades.
A simple system, that cools a room down about 5 to 10 degrees Celsius, would be a great way to reduce dependence on fossil fuels.
I've seen very simple DIY versions of this (stove coil directly wired to a panel) for home heating. At current panel pricing, I wonder if there is a config with a reasonable ROI.
https://www.youtube.com/watch?v=X6KOWGN6C28
https://www.youtube.com/watch?v=X6KOWGN6C28
For an iterated version of this design, there's also this project posted a few weeks ago.
https://news.ycombinator.com/item?id=45703666
https://news.ycombinator.com/item?id=45703666
That's a very poorly thought out design: https://news.ycombinator.com/item?id=45789850
No worries, iteration is good and more iteration is better! I look forward to seeing how this new build performs. If I can link to a better set of (preferably tested) instructions I'd be happy to.
if you just want to store day's energy for night's heating a big ole tank of water is far easier solution to integrate with whatever house heating system you have (tho it performs vastly better on underfloor heating just because of the lower temperatures needed).
Just pump, mixing valve, and some heating coils. You could even just heat the water directly on the solar heat panels but frankly using electric panels is a bit more flexible as it is less plumbing, and power for the rest of the house.
Block of sand have more sense if you need to store it for longer and/or have industrial source of over 100C heat that can be used to fill it up.
Just pump, mixing valve, and some heating coils. You could even just heat the water directly on the solar heat panels but frankly using electric panels is a bit more flexible as it is less plumbing, and power for the rest of the house.
Block of sand have more sense if you need to store it for longer and/or have industrial source of over 100C heat that can be used to fill it up.
Solar panels can pretty easily produce way over 100°, like 1000°. All you need is a nichrome wire. 1000° sand has about 3× as much energy as 100° water by mass, about 7× by volume. Water is cheaper but harder to contain.
Just imagine the machinery you could power beneath the dessert, if you store the heat of the day.. Imagine for example salt water pits near the coast, where the heat creates fresh water mist, that rises and distills int he cold desert nights. To then be pumped onwards with sundriven steampumps, to places where the water is needed. No moving parts, just light redirecting glasfiber and infrastructure created beneath the sands creating geysers of fresh water miles away.
Just imagine the incalculable wealth if you could store the energy of the sun in, say, plants, which would release the fresh mist during the night, build the health of the soil, and enable mobile life forms to carry the energy around to move it to other locations.
North Africa was the breadbasket of Rome, filled with life and water. Then we turned it into a desert.
North Africa was the breadbasket of Rome, filled with life and water. Then we turned it into a desert.
Cute, but there are many times in my life when I want greater power density than I can find in a crop field - or more readily converted potential energy. An ear of corn won't run my flashlight.
The fertile zone around the Nile was Rome’s bread basket, not the modern desert. It’s still a major agricultural area.
"Carthago delenda est"... Although apparently the "sewing with salt" was hyperbole, Northern Africa was far more fertile then.
Awesome! Then imagine people wearing full-body solar-powered suits that capture and recycle body moisture and fluids, purify them and store them in a tank for later use. Would be fantastic!
r/UnexpectedDune
I was expecting some sort of cool focused-light cooker like a solar oven, but it's basically just an electric cooker powered by solar cells.
Because of their simplicity solar-cell based systems have become lower-cost and easier to install and maintain for water heating as panel + electric water heater, vs a piped direct solar thermal heating system. Higher "efficiency" for the direct thermal system, but overall system costs are lower for panel + heat.
I still love seeing the interplay with different combinations of physical systems and clever things humans figure out. Including with solar panels + other system items.
I still love seeing the interplay with different combinations of physical systems and clever things humans figure out. Including with solar panels + other system items.
Is this a good point to mention that simple solar heating circuits work better if you use diodes in series (for a fairly constant voltage drop across the solar cells) than if you just use a resistive element? :)
Diode heating does a better job of "MPPT tracking" than a resistive wire, but diodes are orders of magnitude more expensive, and they only work at low temperature (typically up to 175°, though SiC diodes can go a little higher). Resistive wires can easily work up to 500° or 1000°, which is potentially much better for sensible-heat thermal energy storage like this sand system. It might be better to run your resistive wire with a 3¢ microcontroller running an MPPT algorithm and controlling a 60¢ power MOSFET, something like a PSMN4R0 (https://www.digikey.com/en/products/detail/nexperia-usa-inc/...), which is rated to control almost 3000 watts (30 volts at 95 amps).
You hint at something that is interesting to think about - that question might be: is there a different charge controller, heating element controller pairing that is possible with much less complexity and cost than using an inverter to ac and back to driving a resistive element.
would it be worth a system cost of dedicating some panels to just that kind of control
would it be worth a system cost of dedicating some panels to just that kind of control
I have a solar oven on my wishlist for Christmas this year, I have a perfectly great spot for it, and generally think that if I can use it to bake bread, even if it takes a bit longer, its gonna get some serious usage ..
This should scale up for heating a house at night, too.
Yep! focussed light anything is a hassle really-- mirrors have to be maintained and positioned. For anything larger than a family, mirrors have to be unrealistically large..
There was a concentrated light power station in north of Vegas, but it bankrupted the company that built it. They didn't think about storage at the time
https://en.wikipedia.org/wiki/Crescent_Dunes_Solar_Energy_Pr...
>As of 2023, it is operated by its new owner, Vinci SA, and in a new contract with NV Energy, it now supplies solar energy _at night only_, drawing on [molten salt] thermal energy stored each day.
There was a concentrated light power station in north of Vegas, but it bankrupted the company that built it. They didn't think about storage at the time
https://en.wikipedia.org/wiki/Crescent_Dunes_Solar_Energy_Pr...
>As of 2023, it is operated by its new owner, Vinci SA, and in a new contract with NV Energy, it now supplies solar energy _at night only_, drawing on [molten salt] thermal energy stored each day.
Sorry my wording was misleading. SolarReserves lived and died on the thermal storage hill. The point about mirrors being a hassle is still relevant, though:
>When the plant finally opened in 2015, solar panels had left the Concentrated Solar Power technology in the dust [desert sand?] in terms of efficiency and cost.
https://jpt.spe.org/becoming-obsolete-how-high-tech-solar-pl...
>When the plant finally opened in 2015, solar panels had left the Concentrated Solar Power technology in the dust [desert sand?] in terms of efficiency and cost.
https://jpt.spe.org/becoming-obsolete-how-high-tech-solar-pl...
In buildings with water heating this is already commonly done, accumulating heat in water tanks. Size of water tanks is dimensioned after how much heat you have to store.
Electric heating with water heating is sometimes used in Northern Europe at least, often with a heat pump.
Ultimate would be solar panels on the roof, heat pump to multiply the electricity 3x-5x and water tank storage to last 24 hours.... Never recoup the investment though..
Electric heating with water heating is sometimes used in Northern Europe at least, often with a heat pump.
Ultimate would be solar panels on the roof, heat pump to multiply the electricity 3x-5x and water tank storage to last 24 hours.... Never recoup the investment though..
At least in Nordics (I'm from Finland) heat pumps are rapidly replacing other forms of heating. One can get a big enough heat pump for a 200m^2 house (including heating hot water) for around 10-15k, with a few thousand more for installation price.
Adding 10-15kWp of solar panels to the roof is around 6k more. It's definitively a no-brainer as it will recoup the investment in 5-10 years.
Adding 10-15kWp of solar panels to the roof is around 6k more. It's definitively a no-brainer as it will recoup the investment in 5-10 years.
Especially if panels continue to drop in price, a heat pump will just add needless complexity.
Depends on the location, but around here solar for heating is completely useless.
In Germany (which is farther south than the nordics and gets far more sunlight), solar panels are already insufficient for heating half of the year. On a typical single-family home, you will get at most 10kW peak power solar on the roof, which you can reach in the summer months when there are no clouds. In winter, those 10kWp will generate at most 5kWh of energy per day. Which is a factor of 4 to 5 below the 20 to 30kWh per average day for heating (with generous insulation). The farther north you go, the worse this gets. Half of the nordics get essentially no sun at all in winter, and are quite a bit colder than Germany.
So you need something other than the sun to heat your home in winter. A heat pump can double, maybe triple the solar energy you might get on sunny winter days, but that doesn't usually cut it. So you need grid electricity, wood or fossil fuels. And when electricity prices are as low as in the nordics (around or below 20ct/kWh), heat pumps are totally viable.
Adding solar can be sensible for cooling in the summer months, and maybe a bit of hot water, and heating in late spring, early autumn. But for winter? Totally useless.
And while you could do long-term storage, that will cost you several arms and legs, tons of space and a huge maintenance hassle. And if anything should go wrong with your storage, you have no heat all winter and better have an emergency plan...
In Germany (which is farther south than the nordics and gets far more sunlight), solar panels are already insufficient for heating half of the year. On a typical single-family home, you will get at most 10kW peak power solar on the roof, which you can reach in the summer months when there are no clouds. In winter, those 10kWp will generate at most 5kWh of energy per day. Which is a factor of 4 to 5 below the 20 to 30kWh per average day for heating (with generous insulation). The farther north you go, the worse this gets. Half of the nordics get essentially no sun at all in winter, and are quite a bit colder than Germany.
So you need something other than the sun to heat your home in winter. A heat pump can double, maybe triple the solar energy you might get on sunny winter days, but that doesn't usually cut it. So you need grid electricity, wood or fossil fuels. And when electricity prices are as low as in the nordics (around or below 20ct/kWh), heat pumps are totally viable.
Adding solar can be sensible for cooling in the summer months, and maybe a bit of hot water, and heating in late spring, early autumn. But for winter? Totally useless.
And while you could do long-term storage, that will cost you several arms and legs, tons of space and a huge maintenance hassle. And if anything should go wrong with your storage, you have no heat all winter and better have an emergency plan...
There's a surprising amount of earth architecture in Germany. The walls are your storage ... not enough to last all winter, and not enough to make your house actually "warm", but enough to provide a baseline that smooths over energy availability issues.
Of course, it works even better with higher levels of insolation, since the exterior surfaces of the walls receive more energy during the day.
Of course, it works even better with higher levels of insolation, since the exterior surfaces of the walls receive more energy during the day.
In Norway heating is needed during winter, and there is 0-6 hours of daylight depending on how far north you go.
Going north from wherever in the world you think about, there exists a point where heat pumps become profitable.
(Eventually you drop the solar panel and only go heat pump though.)
Going north from wherever in the world you think about, there exists a point where heat pumps become profitable.
(Eventually you drop the solar panel and only go heat pump though.)
tripling the heating/cooling power isn't "needless complexity".
Full solar heating pretty much works if either you have some truly massive roof area or live in place with no winters. Here solar power is at most 1/4 of summer one, at time where you need it most.
Remember, even with free panels, roof space is limited.
Full solar heating pretty much works if either you have some truly massive roof area or live in place with no winters. Here solar power is at most 1/4 of summer one, at time where you need it most.
Remember, even with free panels, roof space is limited.
I am annoyed how water-water pumps are consistently much more expensive than water-air despise the fact complexity doesn't really change, only the type of heat exchanger (they are just less popular).
Hybrid system where the fluid could be sent to radiator (at night, or in summer when you want to cool) or some solar heat panels on the roof wouldn't be that much expensive, if not for pump costs.
Hybrid system where the fluid could be sent to radiator (at night, or in summer when you want to cool) or some solar heat panels on the roof wouldn't be that much expensive, if not for pump costs.
It even scales up to a whole town: https://polarnightenergy.com/news/worlds-largest-sand-batter...
For reasons I don’t understand, American cities seem allergic to installing new municipal steam or hot water utilities, even though things like cogeneration were an obvious use case for it, and now things like solar heat storage.
Steam and hot water pipes are extremely expensive to install, far worse than electricity, fibre, water or sewage.
You need to be more leak-proof than cold water pipes, because loss of pressure with steam and hot water is much more of a problem than with cold water and cannot easily be solved by just adding more cheap water. Pipe materials have to be more resistant to corrosion because higher temperatures and pressures make them corrode so much faster than with cold water. Closed hot water/steam circuits also mean that there won't be a protective limescale coating on the inside. You need insulation that you can bury and which will last for at least 40 years, which is even more expensive than the pipes. And the insulation will double the pipe diameter. And the insulated pipes have a larger keepout area that needs to be kept free of rocks, other pipes and mechanical strain because the insulation is soft and sensitive to those things. Since usually the pipes aren't operated in summer, and since generally thermal variance is far higher than with cold water, thermal expansion needs to be taken into account, so you need expansion corners, sliding sections, different valve constructions that are tight in all temperatures, etc.
And even with perfect insulation, you will loose approximately 30 to 40% of heat in your piping. So all of this is only viable if you don't care about the cost of the heat, your consumers can (be forced to or persuaded to) accept at least 30% higher prices per kWh compared to their local boiler, not to mention the capital cost.
There are only some areas in Europe even, where those kinds of installations take place: Densely packed inner cities with largely rented-out flats in appartment buildings. There, the landlords/owners avoid the cost and risk of a local boiler and don't care about the running cost of heat, because they don't pay for it. In smaller towns, like in the example, mostly public buildings like schools use those kinds of district heating systems, because the municipality doesn't care as much about cost of the heat, and more about cost of maintenance of a hundred local boilers vs. one centralized system. And in the end, it's taxpayers' money, so they don't actually care that much, headlines and opening ceremonies are more important than that.
Individual home owners usually do have their local systems, which can be run cheaper than what district heating will charge you. And since city density is lower and home ownership is more widespread in the US, district heating is even less competitive there.
You need to be more leak-proof than cold water pipes, because loss of pressure with steam and hot water is much more of a problem than with cold water and cannot easily be solved by just adding more cheap water. Pipe materials have to be more resistant to corrosion because higher temperatures and pressures make them corrode so much faster than with cold water. Closed hot water/steam circuits also mean that there won't be a protective limescale coating on the inside. You need insulation that you can bury and which will last for at least 40 years, which is even more expensive than the pipes. And the insulation will double the pipe diameter. And the insulated pipes have a larger keepout area that needs to be kept free of rocks, other pipes and mechanical strain because the insulation is soft and sensitive to those things. Since usually the pipes aren't operated in summer, and since generally thermal variance is far higher than with cold water, thermal expansion needs to be taken into account, so you need expansion corners, sliding sections, different valve constructions that are tight in all temperatures, etc.
And even with perfect insulation, you will loose approximately 30 to 40% of heat in your piping. So all of this is only viable if you don't care about the cost of the heat, your consumers can (be forced to or persuaded to) accept at least 30% higher prices per kWh compared to their local boiler, not to mention the capital cost.
There are only some areas in Europe even, where those kinds of installations take place: Densely packed inner cities with largely rented-out flats in appartment buildings. There, the landlords/owners avoid the cost and risk of a local boiler and don't care about the running cost of heat, because they don't pay for it. In smaller towns, like in the example, mostly public buildings like schools use those kinds of district heating systems, because the municipality doesn't care as much about cost of the heat, and more about cost of maintenance of a hundred local boilers vs. one centralized system. And in the end, it's taxpayers' money, so they don't actually care that much, headlines and opening ceremonies are more important than that.
Individual home owners usually do have their local systems, which can be run cheaper than what district heating will charge you. And since city density is lower and home ownership is more widespread in the US, district heating is even less competitive there.
> And even with perfect insulation, you will loose approximately 30 to 40% of heat in your piping.
At least here in Finland the norm is losses in the range of 5-20%, with the upper end of the scale for smaller scale networks with smaller diameter piping and low flow rate. In the larger cities losses are closer to the low end of that scale.
In the summer when the consumption is very low (essentially only hot tap water production) losses can rise up to 50%.
At least here in Finland the norm is losses in the range of 5-20%, with the upper end of the scale for smaller scale networks with smaller diameter piping and low flow rate. In the larger cities losses are closer to the low end of that scale.
In the summer when the consumption is very low (essentially only hot tap water production) losses can rise up to 50%.
lots of industrial processes produce waste heat that can't easily be turned into energy, so the comparison isn't to a boiler, but to not having the heat.
It is true that the heat can be used if it is there anyways. But usually not in a big city-wide network. Instead a more localized, larger consumer is far better, because running the hot water network is far too expensive. For example, large producers of heat like data centers, dairy processing or chemical plants around here deliver their heat to public swimming pools, schools or greenhouses that are intentionally built nearby.
Even the grandparent's article says so if you read carefully: "A large portion of the town’s own buildings, including the municipal school, town hall, and library, are connected to the district heating network.". They didn't even attach all of the public buildings. Not to mention about the rest of the town.
Even the grandparent's article says so if you read carefully: "A large portion of the town’s own buildings, including the municipal school, town hall, and library, are connected to the district heating network.". They didn't even attach all of the public buildings. Not to mention about the rest of the town.
That's exactly what I was thinking of. Cogeneration plant goes up right next to datacentre, and then various municipal services are built near that - including other factories, high density commercial and high density residential that can take advantage of cogeneration and use district heat. In particular commercial and industrial uses actually do need heat in the summer.
In some areas, natural-gas fired cogeneration only is really necessary in the winter during dark months/cold weather - which is an ideal situation to be producing district heat.
In some areas, natural-gas fired cogeneration only is really necessary in the winter during dark months/cold weather - which is an ideal situation to be producing district heat.
There are district heating and cooling in cities where it makes sense, both Minneapolis and St Paul, plus the University of Minnesota all have district heating and district cooling systems. The Minneapolis and U of M systems are operated by Cordia Energy and the St Paul system is operated by Evergreen Energy.
This is the coldest large metro area in the lower 48 states, so it’s economical to do district heating and cooling here.
This is the coldest large metro area in the lower 48 states, so it’s economical to do district heating and cooling here.
These heating districts don't cover a very large area. It's impressive in its own right, but if you're using them as an example, it just shows that it only works at very specific scales.
Agreed, you need a fairly dense concentration of buildings with heating and cooling requirements in the tens of millions of BTUs per hour for it to make economic sense.
Right, but quite a few of such places do indeed exist (and new ones are being built).
hot water also have pretty high losses, because you need to keep it cycling constantly to keep the heat up (you don't want a case where opening a tap in the morning means letting up the hot water from the 30m length of pipe from the street to your house).
So paradoxically, if your heat is not "free" (cogenerated with electricity) it might be far more efficient to have a boiler in each house (and definitely a heat pump), than to push the heat that far away.
So paradoxically, if your heat is not "free" (cogenerated with electricity) it might be far more efficient to have a boiler in each house (and definitely a heat pump), than to push the heat that far away.
In a modern system, you'd pull the heat from the hot water using a heat exchanger or a heat pump. (I'm also assuming a closed-cycle.)
It sounds like communism / socialism / marxism to people that are unable to define what those things are.
I thought it would be heat pump related
The article talks about a similar device developed by Cal Poly in 2015: https://solar.lowtechmagazine.com/2023/08/direct-solar-power...
That being said, the more I read it, the more I like this PV steam cooker. It is simple and easy to scale up by adding more PV panels and more sands. Although Ghana is close to the equator, I wonder whether it worth to steer the PV panels during the day.
Edit, efficiency measured by energy transferred to boiling water or cooking vs energy released from burning biomass.