Solar Cooking After Dark  -- A Stored Heat Solar Cooker

Mik has been working on a way to collect and store solar energy during the daytime so that it can be used after sunset to cook meals.

Mik is looking for ideas to solve some of the technical issues that still remain, so if you have any ideas, please use the comments section at the end to pass them on.

Clearly the high temperatures generated by this device pose some serious safety issues -- so be very careful!


The full story from Mik:

stored heat solar cooker

Solar cooking after dark

In 2002 I cooked dinner for three with solar energy 3-4 hours *after* the sun was gone. The device also desalinated sea-water (by distilling the steam) and can be used as an efficient aluminum-smelter (as I discovered by mistake).

With the exception of the following, all materials were salvaged from scrap.

- a cheap sheet-metal barrel 
- the sleeve/piston arrangement for holding the item to be heated
- 20 m (65 ft) of square-section steel rods,

This approach could help get around the problem of cooking after dark without burning fuel, and could provide huge benefits:

- relieving deforestation pressure by not burning trees
- reducing emissions from burnt wood, coal and petroleum by-products
- releasing several hours daily to productive activity by making wood-collecting unnecessary

Think about it.



EYE PROTECTION: Always wear good dark glasses. I always wore two sets
of sunglasses, one behind the other! This looks hilarious, but if
you're building something like this people are going to be laughing at
you anyway  Aligning the mirrors means you'll be looking at the area
where the sunlight focuses and it is very, very bright. Even if you've
blacked the receiving part it's still a lot like looking into the sun.

SKIN PROTECTION: Use sunblock. Wear long sleeves, a hat and gloves. If
you have long hair keep it tied up under the hat. Stray light will
sunburn you in a surprisingly short time if you're working in
sunlight. This can be very uncomfortable on a hot day - but make the
sacrifice in the name of Science, The Trees, The Greater Good - or
whatever - just do it.

PROTECTING OTHERS: Tell everyone to keep away from the concentrator.
Especially children and others unfamiliar with the dangers involved in
concentrating sunlight will want to come close and touch it. You can
'Secure the perimeter' by putting down 'NO ENTRY' tape around the
device. Demonstrating the power of the device by holding the tip of
long, think  wooden stick into the focused beam for two seconds is
enough to drive the point home to most people - it bursts into flames
instantly. Always keep a water bath close by and be careful!.

PETS: A large dog could re-align the dish if it could reach the rim
with its paws. Cats are curious and do not have nine lives. Keep
animals clear.

STORAGE: Cover the dish with something when not in use, and make sure
it's in the shade. I used a cotton sheet. Angle the dish to face north
in the northern hemisphere or south in the southern hemisphere. A
cautionary tale: One day after testing I place the dish in shadow
under the eaves, angled it to point up and covered it with a sheet.
Next morning I discovered the cover sheet had blown off (secure it
well!) and the sun's low angle reflected off the dish onto the large
glass sliding door and cracked it. Oops. I was lucky - something could
have caught fire.

LOADING and UNLOADING: Only attempt this after you've rotated the dish
so all the mirrors are in shade. Touching the core or anything near it
could result in permanent injury.

GAUGING TEMPERATURE: A good thermometer will guide you some of the
way. Mine only went to about 500 C (930 F). A small chunk of aluminum
will melt at 660 C (1220 F). Above that you can guestimate the
temperature of steel from the color (try Googling 'temperature steel
color' for charts to guide you).


The following pictures and descriptions show the stored heat cooker design and how its used

Mid afternoon - Core at maximum temperature

The concentrator is a 1,3m (4ft) diameter scrap satellite receiver dish and stand. The dish is covered with (glued on) bits of scrap glass mirror from the local glazier.

Iron locator rods attach a galvanized sheet metal sleeve and open-ended piston arrangement I had made at the local metalworkers. The piston contains a 10x10 bundle of 1 cm (0.4") square-section steel rods, all 20 cm (8") in length. The bundle is secured inside the piston with fencing wire, and the one end (blackened with soot) is presented to the focused sunlight. At the other end of the piston is a handle to grab when moving the heated steel core.


stored heat solar cooker
Diagram showing the heat storage capsule. 
Click on diagram for full size.


A rudimentary locking mechanism (not shown) keeps the dish in place. Constant attention is required in tracking the sun.

After a few hours the steel rods are glowing yellowish-orangish (visible in the shade). My guess at the temperature: somewhere around 900 C (1 650 F)


Things that didn't work so well....

My first experiment with a heat storage ballast was an aluminum heat-sink from a scrap computer. This was presented to the focal point inside the sleeve-piston arrangement on a summer day with the sun near its zenith. After about 45sec I was alerted by the sound of breaking glass - molten aluminum was dripping onto the concentrator's mirrors and shattering them. Back to the drawing board (and back to the local glazier for some more scrap mirror bits).

The glass wool insulation was the most heat-resistant I could find, but wasn't good enough to handle the orange-yellow steel. To avoid fire I had to let the heated steel core cool before storing it in the insulating barrel (see next images).


Insulating Barrel

An old coffee tin accepts the heated steel core. In this picture I was experimenting with basalt-type rocks. No good.

Note the electronic thermometer. The temperature probe pierces both barrel and inner coffee tin.


Insulating barrel - ready for storing the hot core

After letting it cool a bit so the glass wool doesn't burn too much, the hot steel core is placed in the barrel to keep it nice and toasty until needed.


Core in the barrel

With the hot steel core packed into the thermal insulation and a big earthenware pot as a weight on the lid, the temperature can be monitored until dinner-time.

I found that letting the core cool to a temperature of somewhere around 450 C (850 F) was workable as it didn't instantly vaporize the glass wool.


This will become dinner

Pot contains my own recipe - based on ground beef, onions, and veritable cornucopia of exotic spices.

My kids call it 'Ruskum-Snusk'. (No, I'm not releasing the recipe).


Placing the pot on the solar 'stove'

Stainless steel pot containing food is resting on the hot core. The core is now at about 300 C (570 F) after lying in the insulating barrel for 3-4 hours.


Cooking with stored heat

Where the rubber meets the road - it's late afternoon, about half an hour to sunset. The sun has been gone for a few hours (house is in shadow).

The meat mix was cooked using only the heat from the rods (and tasted great according to some sources). However, there was not enough heat left in the rods to boil the rice we were supposed to have with it.

Most obvious areas for Improvement:

A good high-temperature insulator that can around 900-1 000 C (1 650 - 1 830 F). I wanted to try ash, but ran out of time before I could try that.

Better insulating barrel - While experimenting I inserted the thermometer into the center and heat leaked out through the holes. Furthermore, the lid was too loose, and the central coffee can rested on metal - another thermal bridge.

Nice to have: Thermometer that goes beyond the 500 C (932 F) limit mine had. I used aluminum pieces to tell me the temperature was above 660 C (1 220 F) where it melts, and above that guessed at the steel's temperature by its color.


My Solar Cooking Laboratory

Solar concentrator - Background, right of center
Insulating barrel - lower left.

This where I cooked dinner for three with solar energy that was stored in steel rods for 3-4 hours. This approach could help get around the problem of cooking after dark without burning fuel.

Think about it.


Mik Hartwell

You can email Mik at: 



This is a really interesting project, and one with a lot of potential for being helpful. 

Some first thoughts on sizing, heat storage materials, and insulation...



Comments and Suggestions ...

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January 7, 2012