Changes: Heat storage

Revision as of 23:48, 9 March 2011

Heat storage is fairly self-descriptive. It is used so that:

Cooking may be done at night.
Cooking may continue if the sun goes behind clouds.
The cooker's temperature does not drop too much when cold food is added.
One doesn't have to "worry" about the above.

It can be done by means of:

Sensible heat storage. This means holding heat in a material without changing its phase when heat is added or removed. Rocks and bricks may become hot, but remain solid. Oil may become hot, but remains liquid. Much more heat can be stored in oil than in water since water can only be raised to 100° C without pressurizing it.

Latent heat storage. This is usually accomplished by using solar heat to melt a special material, and then when the heat is needed it is drawn from said material. As it re-solidifies, it releases this heat. A very large amount of heat is stored this way, and the temperature during melting or solidification remains constant. The material must melt at a "reasonable" temperature, hot enough to begin and complete the cooking of food, but also low enough to be attainable using solar energy. It must also be reasonably non-toxic, stable, easy to work with, and of course affordable. Erythritol and citric acid may be candidates.

Insulation. Simply retaining the heat that has already been created. See: Heat-retention cooking

Christopher Jordan working in Cambodia, has experimented with various styles of solar cookers appropriate for use in locations fairly close to the equator. He became interested in finding ways to extend the available cooking time to early evening. He prefers a simple 'V' shape cooker with fairly tall reflector sides. See: Solar Rice Cooker. The design allows space for blackened rocks or bricks to be placed under the cooking pot. Acting like a thermal flywheel, the rocks will soak up heat all day, and give it back in the evening. However, regardless of the style cooker, the rocks need to be insulated to retain their heat, once the sun nears the horizon. He did some experimentation with rock salt as a storage medium, but found it too viscous to be practical. Each locale should each have a viable source of blackened rocks, bricks, or sand to use however.

Basically then, additional heat is retained for cooking by adding thermal mass, with blackened bricks, oil in vacuum tubes, or phase-changing salts, which give back their heat once the sun begins to set. Or, heat can be retained by adding insulation. This has typically been done by removing cook pots from the oven and placing them in Heat-retention cooking baskets to allow the food to continue cooking with its own heat. In some areas where solar cooking has been introduced, the residents have embraced the heat retention cooking ideas over the solar cooking solution due to cultural concerns.

A solar cooker that economically adds both mass and super-insulation as part of the design, is possibly best suited to a solar box cooker approach. This will require a dimensionally larger cooker to fit enough insulation and mass. See: Heat Retention Solar Oven Also initial heat-up times will increase to reach working cooking temperatures. The transparent glass panel will need an insulated cover to be placed when solar gain is not available. However, the ovens may be able to reach higher temperatures than are typically achieved, and hold that heat into the evening, and possibly overnight. An oven may take a couple days to reach maximum output. Because of the higher temperature potential, and the prospect that ovens be left out, always on, in all-weather conditions, oven construction materials will need to be non-combustible and weatherproof.

External links

The Heat Retention Solar Oven by Ronald L. Conte Jr.
Heat Retention Cooking vs. Solar Cooking - Mike Bridgwater
August 2010: Project Report 8/10, (Extending solar cooking times) - engineeringforchange.org
July 2010: Thermal performance of a solar cooker based on an evacuated tube solar collector with a PCM storage unit - Science Direct
July 2010: Heat transfer performance analysis of a solar flat-plate collector with an integrated metal foam porous structure filled with paraffin - Science Direct
July 2010: Design, development and performance evaluation of a latent heat storage unit for evening cooking in a solar cooker - Science Direct
July 2010: Solar cooker with latent heat storage systems: A review -Science Direct
November 2009: Wolfgang Scheffler discussed thermal storage in an interview: "We did some samples. We did a small iron which we used in Portugal in some installations for many years. Because it's small, 50 kilograms of iron is just a few litres, you know, it's like six litres of iron, so it's quite small, so it still cools down a lot. Like when you have 450 degrees in the evening, in the morning you have 300 degrees. If you increase the mass of iron, like what you talked about, like 300, 400 kilograms, then that drop is only 50 degrees overnight. But then the next day you still have a lot of energy to cook and to use."[1]

June 2008: Storing the Sun: Molten Salt Provides Highly Efficient Thermal Storage - Renewable Energy World

April 2008: A new solar cooker that can even cook dinner - Business Standard

Liquid Tin Heat Storage for Scheffler Parabolic Cookers - Dr. Michael Götz

BSR Solar Technologies

Solar heat storage at Barli Development Institute for Rural Women

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 ==See also==
+*[[Heat-retention cooking]]
 *[[Solar Rice Cooker]]*'''July 2006:''' [[Media:Somesh sharma.pdf|Solar Cooker for Evening / Night Cooking Using Solar Heat Storage Materials]] - ''[[Someshwer Dutt Sharma]]''
 *[[Wolfgang Scheffler]]