Efficient solar cooking that stores heat in sand

66 pointsposted 2 months ago
by gsf_emergency_6
(sciencedirect.com)

43 Comments

infinet

2 months ago

That traditional biomass stove in paper's figures is perhaps the worst stove imaginable. Its efficiency could below 10%. Better designed stove has efficiency above 30% and has much less indoor air pollution.

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.

buckle8017

2 months ago

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.

elektronika

2 months ago

> 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.

kragen

2 months ago

Do you have a direct-DC-powered workshop?

PunchyHamster

2 months ago

I'd like to see comparison between that and just an induction stovetop + some batteries.

kragen

2 months ago

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.

bikelang

2 months ago

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.

MomsAVoxell

2 months ago

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 ..

bikelang

2 months ago

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.

kumarvvr

2 months ago

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.

ortusdux

2 months ago

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

PunchyHamster

2 months ago

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.

kragen

2 months ago

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.

21asdffdsa12

2 months ago

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.

paddleon

2 months ago

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.

IAmBroom

2 months ago

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.

ch4s3

2 months ago

The fertile zone around the Nile was Rome’s bread basket, not the modern desert. It’s still a major agricultural area.

fakedang

2 months ago

Rome's breadbasket was more than the Nile. Tunisia and parts of modern Algeria were also extremely green even during the late Roman period

IAmBroom

2 months ago

"Carthago delenda est"... Although apparently the "sewing with salt" was hyperbole, Northern Africa was far more fertile then.

loloquwowndueo

2 months ago

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!

Legend2440

2 months ago

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.

dv_dt

2 months ago

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.

taneq

2 months ago

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? :)

kragen

2 months ago

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).

dv_dt

2 months ago

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

MomsAVoxell

2 months ago

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 ..

actionfromafar

2 months ago

This should scale up for heating a house at night, too.

gsf_emergency_6

2 months ago

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.

gsf_emergency_6

2 months ago

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...

dagss

2 months ago

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..

mrgaro

2 months ago

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.

actionfromafar

2 months ago

Especially if panels continue to drop in price, a heat pump will just add needless complexity.

PunchyHamster

2 months ago

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.

k7sune

2 months ago

I thought it would be heat pump related