How would you build a solar stove that works at night & the next morning?

[pskpi]

Hey everyone,

I just came across a competition that was run nearly 10 years ago and I was just wondering if anyone here has any interesting ideas on how it could be done.

These were the criteria:

Store 4kWhr for 18 hours.

Temperature for cooking at least 200C.

Powered by sunlight.

Locally available easy repair & maintenance.

Cost less than $200.

At that price I'm guessing that a PV and battery setup is out of the question and you would need to use some sort of heat storage material like an oil, salt, sand or stone (maybe a combination of a liquid and solid or something else?). One of the issues I see is that you're probably going to need a lot of the heat storage material to actually store the heat and with the weight you'll get the issue with moving your thermal storage from outside (where you collect the heat) and inside (where you cook).

You could use insulated pipes to pump a heat transfer fluid but it starts to get more complicated and expensive. Is there a way to transfer heat from something like a parabolic concentrator to the storage vessel?

The stove in this competition was targeted towards households in rural villages so instead of building an individual setup for each house, what about a larger centralised setup for something like 50 households where the 'heat' could be stored in one place and then collected or delivered when a person needs it? You wouldn't need as much individual storage and you don't need to supply each house with it's own concentrator setup. I have no idea how this could be made to work though.

What do you think, with such a tight budget is it actually possible?

Thanks for reading.

 

[Baluncore]

A terracotta oven, with a triple wall, and a double insulated door.
The inner wall cavity filled with a material having a melting point (phase transition) to store the heat overnight. The outer wall cavity filled with vermiculite for insulation. (Expanded clay).

Heated by a parabolic concentrator made from metal foil or shim, that directs heat into the oven through the open door.

What phase transition material should be used to store the thermal energy and regulate the temperature?
The m.p. of salt is too high; NaCl = 800.7°C; KCl = 770°C;
Sulfur is too low at S = 115.21°C;
Lead and Tin are toxic but have the right values; Pb =327.46 °C; Tin, Sn =231.93 °C;
Recycled solder 60/40 Sn-Pb = 188°C is too low.
But recent Sn solders would be close to the required temperature.

 

[256bits]

Some PCM ( phase change material ) would be preferable.
Eutectic is a good choice.

Organic PCM.
HPDE - I suppose it is classified as organic
has a high heat of fusion - approx 200 kJ/kg,
( and a specific heat capacity of 1300-2400 J/kg which we can disregard )
So for a 4kWhr energy content, that's about only 70 pounds of the stuff. I think that's right.
Only problem is that the melting point ( around 150 C ) is below 200 C.
Perhaps another thermoplastic has a higher and suitable melting temperature.

Here is a list of some thermoplastics.
https://www.plastikcity.co.uk/useful-stuff/material-melt-mould-temperatures
Note - it gives the melting temp of HPDE as 210 to 270 C range which makes sense as thermoplastics do not have a definite melting point per se. Except the difference in what I quoted previously. Some more in depth investigation would be required.

 

[pskpi]

Very interesting with the heat storage material ideas. I didn't think about low temperature metals or plastics (would plastics still function after years of heating cycles?)

For the weight of the plastic that you mentioned, does that take into account that the usable temperature has to be over 200C.

How do you calculate the amount of material that you need? If you're running a stove using the heat of fusion of tin then you would need 245kg of the stuff. (14,400kJ is 4kWhr, heat of fusion for tin is 7kJ/mol which is 119g so (14400/7)*0.119 right?).

What do you think about the per household vs centralised whole village idea?

 

[hutchphd]

From wikipedia:
A commonly used thermal salt is the eutectic mixture of 60% sodium nitrate and 40% potassium nitrate, which can be used as liquid between 260-550 °C. It has a heat of fusion of 161 J/g,[2] and a heat capacity of 1.53 J/(g K).[3]

So storage of 4kWhr would require ~100 kg molten salt. Obviously need some solar concentrator to do the job.

 

[Baluncore]

What do you think about the per household vs centralised whole village idea?

That would be necessary as someone will need to maintain the solar tracking every 5 minutes or so, unless some form of mechanical solar tracking (heliostat) was implemented.
https://en.wikipedia.org/wiki/Heliostat

My conceptual design would avoid high tech materials unless they could be recovered from a waste stream. Lead from car batteries, solder from circuit boards, reflectors from drink cans or foil wrapping.

A standardised modular design would enable each region to produce and exchange-trade standard parts that would rapidly become a commodity with a distributed replacement parts industry.

 

[anorlunda]

Consider a parabolic trough for the collector. I reduces the need for the collector to move tracking the sun.

https://en.wikipedia.org/wiki/Parabolic_trough

 

[hmmm27]

Why can't they just have cold cereal, like everybody else in the world.

Okay, so about 20kg of sodium nitrite (the stuff that makes packaged meat pink) through a phase change will hold 4kWh of heat @270C.

Dewar flask, buried to the neck in the ground for safety, filled with a polytube arrangement and say 30kg of sodium nitrite.

The transfer medium is air, convection driven : insulated pipe comes out of the flask from the top of the polypipes, through the oven/stovetop, and back in, into the bottom of the pipes.

Flask is capped by glass for IR reabsorption, and that's the target of the solar concentrator.

Alternate :
External concentrator target ; fluid thermal medium ; enclosed flask (with the same in and out piping); valves to determine whether the system is heating the thermal storage from the concentrator, or the cooktop from thermal storage. Requires a pump.

 

[256bits]

Very interesting with the heat storage material ideas. I didn't think about low temperature metals or plastics (would plastics still function after years of heating cycles?)

They may degrade over time, but it was given as a expansion of the list of materials that could be used with a phase change. The amount, weight wise, needed is reasonable to be portable. But volume wise, due to the low density would make for a large unit.
Metals and ceramics are heavy making them mostly immovable, but more compact.

You do have a trade off of desirable features of what may be available in supply and cost, taking into account the design temperature and amount of necessary stored heat. Density, specific heat, heat of fusion, practicality, longevity, are some things to consider, as well as the ergonomics of the design. Plastics may give off some fumes, some liquids, if not most, are corrosive, and/or poisonous, and when used around food some of these features will write the material off the list.

A good discussion that you have brought up in PCM's and energy storage mediums.

 

[pskpi]

With a trough, wouldn't a stationary one be much less efficient than one that is tracked?

Yes, simple standardised parts where replacements could be made locally is totally how it should be done. Will it be possible to get away with only one axis tracking if the target is places that are between 30N and 30S latitudes?

While a centralised setup offers advantages over having one outside each house you end up with the issues of how to move the heat storage from the concentrator area to the households and how to charge them up. For the latter you either have to fill up empty insulated containers with the 'charged' liquid (seems dangerous unless a trained person does it with some sort of safe release & seal system) or have the energy storage container be permanently sealed and each one gets it's own mini concentrator as part of a larger array (though if they are well insulated and sealed this could solve the transport issue).

How did you get the figure of 20kg for sodium nitrite? It has a heat of fusion of 177kJ / kg so you need 80kg right (14400 / 177)?

 

[hmmm27]

How did you get the figure of 20kg for sodium nitrite? It has a heat of fusion of 177kJ / kg so you need 80kg right (14400 / 177)?

No idea where I came up with that figure... perhaps it was for 1kWh... "show your work", indeed.

 

[Baluncore]

While a centralised setup offers advantages over having one outside each house you end up with the issues of how to move the heat storage from the concentrator area to the households and how to charge them up.

You need communal cooking areas close to groups of homes.

With a trough, wouldn't a stationary one be much less efficient than one that is tracked?

The ideal would be a flat mirror that tracked the Sun, with a parabolic (or elliptical) reflector fixed with the oven at the focus. A trough would heat the axial line near the central axis, which would need a special insulation to pass incident radiation while blocking the IR heat loss.

The most efficient would be a parabolic mirror that tracks the Sun on one axis, parallel with the Earths axis, adjusted gradually through ±23° for season, with the oven heat input door towards the East for the reflected evening sun.