Solar Ovens


    This section is about how we built a  Parabolic Solar Oven, Panel Cooker and a Box  Oven and the results of our experience, but first a brief overview of Solar cooking.
    The idea behind solar ovens is to harness the energy from the sun and use this energy to cook and prepare food and drink. The praticalities of this concept  is  somewhat dependant upon, how much sun you recieve ,how much time you have and what type of cooking you prefer. A solar oven will only work when the sun is shining, will require attention and adjustments throughout the cooking period and will require a thoughtful approach to what you will cook. However it is a free source of clean energy and in some cases where sunshine is plentiful a very useful one that just requires a change of habits for it to be both efficient and convenient

Principles

Most solar cookers work on basic principles: sunlight is converted to heat energy that is retained for cooking.


sun


Fuel: Sunlight

Sunlight is the "fuel." A solar cooker needs an outdoor spot that is sunny for several hours and protected from strong wind, and where food will be safe. Solar cookers don't work at night or on cloudy days.


Convert sunlight to heat energy

Dark surfaces get very hot in sunlight, whereas light surfaces don't. Food cooks best in dark, shallow, thin metal pots with dark, tight-fitting lids to hold in heat and moisture.

black pot absorbing sun's rays white pot reflecting sun's rays


Retain heat

A transparent heat trap around the dark pot lets in sunlight, but keeps in the heat. This is a clear, heat-resistant plastic bag or large inverted glass bowl (in panel cookers) or an insulated box with a glass or plastic window (in box cookers). Curved concentrator cookers typically don't require a heat trap.

black pot with bag black pot in box cooker


Capture extra sunlight

One or more shiny surfaces reflect extra sunlight onto the pot, increasing its heat potential.

panel cooker with sun's rays box cooker with sun's rays

parabolic cooker with sun's rays

Original source http://solarcookers.org/basics/how.html

 
 
    So now we move onto our experience when as part of our Earth and Sun Energy Learn and Create Course this summer we put together a variety of Solar Ovens for testing.
 


After a little discussion  we split into 3 groups with each group making a different model. The models we covered were Parabolic, Funnel and the Easy Lid.
 
Parabolic
 
    
This was Stoyans baby, he had sourced a highly reflective  aluminum allode material which he ordered pre cut and ready to asemble with just a drill ,5mm drill bit and nuts and bolts. Once the unit was built a tripod was erected around it to suspend the cooking vessel in the focus point of the sun.
 
 
Results
 
The Unit heated 4 litres of water with starting temperature of 23 C up to 53 C in 2 hours during a  partially cloudy part of the  day and served us with lukewarm tea for our break ,not a roaring success but when the sun broke later in the day the temperature quickly rose to 79 C. We also prepared a snack of Ufka( a light pastry cooked in milk and mixed with honey and yogurt) using the Parabolic which took 20 minutes to cook and was delicious.   
 
Improvements
 
Research indicates we could have  easily improved the performance of the cooker by using black pots with black lids  and using a bigger suspension plate to allow a clear plastic bag or glass container to envelope the black pot thereby providing a insulative layer of air. We also should have made more periodic movements during the day . We will try these methods and post results in the future.
 
Pros and Cons
 
The pros for this model are
  • it is made from durable material resistant to rain and will last a long time without needing much maintanance.  
  • Can cook as fast as a conventional electric burner.
  • Can fry foods.
The cons for this model are
  • not easy to pack away
  • not easily portable
  • more expensive than other models due to special material used
  • requires periodic realignment to the sun.
  • can cause damage to eyes due to its highly reflective nature
Comments  
 
Stoyans model has the potential to reach very high temperatures quickly and with some getting used to can provide a good alternative to the electric oven on sunny days .Its bulkiness eliminates it from becoming a camping and hiking utensil but its durable structure means it will serve its purpose for a long time .
 
Funnel
 
 
The Funnel was Filips choice and was quickly put together using  only Cardboard ,tinfoil some glue and a box to hold the unit up-right. Filip and co.  also made a pot to fit the unit out of an old tin of dog food painted black with a tin lid also painted black than sealed this tin with an oven bag . For full construction instructions see http://www.solarcooking.org/plans/funnel.htm
 
Results
 
The Unit heated the tin of water with rice with starting temperature of 17 C upto 74 C in 2 hours during a  partially cloudy part of the  day.The rice was well cooked after 6 hours. 
 
Improvements
 
It could be made bigger to hold a bigger vessel,it could suspend the vessel in the focus using a tripod therefore reflecting more heat to all surfaces and we could have attended to making more movements to improve the focus. Further tests will be carried out bearing this in mind and results posted. 
 
Pros and Cons
 
The pros for this model are
  • it is made from cheap and ready available  materials  
  • it can be constructed very quickly
  • It is very portable
  • Demonstrated good results from the offset  
The cons for this model are
  • a few rainfalls will destroy it 
  • too small to provide a familly size meal (athough a bigger model would solve this).
  • requires periodic realignment to the sun.
  • can cause damage to eyes
Comments
  
The Funnel is my favourite as it demonstrated from the start how effective it can be and what potential it has. Its very easy and cheap  to build and is the ideal cooker to take camping with you as its so lightwieght and packs up nicely. We are looking forward to many a beach holiday with the funnel.    
 
Easy Lid
 
 
Ivan and Emil cracked on with the easy lid which is basically a cardboard box lined with foil with black tin floor plate and a perspex or glass screen with foil lined lid which can be tilted to reflect the sun into the box. look here for concise instruction how to assemble one  http://solarcooking.org/plans/easylid.htm
 
Results
 
The Unit heated 1 liter of water with a  starting temperature of 17 C up to 30 C in 2 hours during a  partially cloudy part of the  day. By 6.00 p.m that same day the temperature was 74C. Not remarkable by any means. We also made a fruit juice with this cooker. Sava filled a jar with  Cornellian Cherries and added some water after a few days in the sun the mix was ready for straining and with some sugar added made a very appetising juice.
 
Improvements
 
The cooking vessels we used could have been better i.e black and sealed in glass or plastic. A further box inside the outer box would also provide better insulation and the perspex we used was not totally clean. Also more attention to periodic movements of the lid would have undoubtedly increased the performance.    
 
Pros and Cons
 
The pros for this model are
  • it is made from cheap and ready available  materials  
  • it can be constructed relatively quickly 
  • It is sealed so can be used for uncovered food i.e drying ,heating
    The cons for this model are
    • a few rainfalls will destroy it.
    • lower temperatures.
    Comments  
     
    The easy lid is very easy to setup and does not require to much movement but it does not really operate well for cooking  that requires high temperatures .However with the low temperatures we were reading makes it an ideal tool for drying fruit and we had great success in drying figs and apples in the unit. The sealed box keeps the flies and insects out and a few days in the sun was more than enough for the apple chips ,3 days for the figs although if the figs had been halved than 2 days would have been enough.
     
     
    Sashkos Solar Oven Night Light
     
    We discovered a dual purpose for our cookers whilst we were relaxing around the fire at night , Sasho had the good sense to place  a candle within the ovens. The effect was very beautiful ,even more so under the influence of the home made rakia we were knocking back, and you can achieve a good enough light to work around. The funnel oven ,the most portable of the units also works as the best night lamp making it a perfect travel companion.
     
     
    Summary
     
    Our experiments with solar ovens so far have revealed a few things. Firstly these units are very easy to make and do work but to get the most out of them requires time and attention and more importantly a  changing of habits and expectations. They are by no means  an all out answer to off grid cooking but do provide a good alternative when conditions are correct . They are an easy thing to demonstrate to other people who may be interested  and making these with children is a great way to teach a variety of principles whilst having fun. For beach camping the funnel will really come into good use serving as a cooker by day and lamp by night.
    We are really looking forward to more experiments to test how useful we can make the units we built and will post updates. Thanks to all involved with the experiments.
     
     
    Further Reading and Links         
     
    An excellent site containing everything you could want to know including models, instructions and great interactive research
     
     
     
     
    Comments