Heat storage is fairly self-descriptive. It can be done by adding a heavy dark colored mass, usually bricks, or a phase-changing material, often salt, to the cooking area of the solar cooker. The mass is able to heat up before the food is added, and stay hot while the food is cooking. Then it gives back the heat, stabilizing the cooking temperature, and extending cooking time into the evening. Alternately a mass or phase-change material can be heated during the day alone and then used for cooking after dark or possibly the next day.
- 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.
Types of heat storage
- Sensible heat storage: This means holding heat in a material without changing its phase when heat is added or removed. Rocks and bricks are examples, which 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. Salt, Erythritol, and citric acid may be candidates. Another latent heat storage approach is to chemically change a medium, usually under intense heat, and then when it is changed back to it's original form, heat for cooking is released. Research is being done using quicklime(CaO) as this type of medium.
Sensible heat storage examples
Solar Rice Cooker
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 dark-colored 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 dark-colored rocks, bricks, or sand to use however.
Basically then, additional heat is retained for cooking by adding thermal mass, with dark 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.
Heat retention solar oven
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.
Scheffler heat storage system (sensible)
Research is also continuing with incorporating a Scheffler reflector with a heat storage unit. The Scheffler reflector has shown to be an effective way to concentrate heat input to the storage device, with minimal loss of temperature due to not requiring an heat exchanging mechanism. Intense sunlight is focussed at an absorption plate attached to an insulated concrete block located inside of the kitchen wall. The block has a system of imbedded metal rods to help evenly distribute the heat within the storage device. It will be important to cover the absorption plate at night, and use insulated covers over the cook plates on top of the storage block when they are not being used for cooking. Early testing has shown cooking in the evening will be possible hours after the sun has set, and potentially early the next morning as well. Read more about the research: Construction and Improvement of a Scheffler Reflector and Thermal Storage Device, November, 2010 - Jason Rapp
Latent heat storage examples
Wilson Solar Grill
Students at MIT are working on a case study for a new type of solar powered outdoor grill. Based on the technology from MIT professor David Wilson, this grill would collect thermal energy from the sun and store it to allow cooking times for up to twenty five hours at temperatures above 230°C (450°F). Wilson’s technology uses a Fresnel solar reflector to harness the sun’s energy to melt down a container of lithium nitrate. The Lithium Nitrate serves as a solar battery. Due to its phase change reaction, the thermal energy is able to be stored for longer periods of time and at higher temperatures, by means of latent heat storage. Heat is then redistributed through convection, which allows for outdoor cooking. A Solar Grill Prototype for a Greener Tomorrow, August, 2011 - BarbequeLovers.com
Quicklime heat storage
The use of quicklime(CaO) for heat storage is not a new concept. Under intense heat, water vapor is released from calcium hydroxide and quicklime is created. Then at a later time, water can be added back to the quicklime, releasing its heat for cooking. It has been proposed that a community Scheffler reflector would have the capacity to 'charge' CaO units for approximately twenty-five families each day. Families could pick-up their unit of CaO and return to their homes where the CaO would be placed in a special cooker. Cooking heat is controlled by regulating the amount of water added back to the CaO. Benefits of such an approach will allow cooking to happen when the user wishes, inside the home and at night. The materials involved are widely available and nontoxic. Read more: Development and Testing of a Regenerative Rechargable Solar Stove System
Sometimes it is desirable to keep fresh foods cool, rather than focussing on gathering and storing heat for cooking purposes. A simple evaporative cooler can provide a solution. The Pot-in-pot cooler uses two clay pots, one inside the other, separated by a sand barrier. The sand is saturated with water and the cooler is covered with a wet towel. As the water slowly evaporates, heat is taken from the clay pots, keeping perishable food inside cool and fresh for some time.
Articles in the media
- February 2012: Ten solar cookers that work at night - Engineering for Change
- 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
- 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
- Construction and Improvement of a Scheffler Reflector and Thermal Storage Device, November, 2010 - Jason Rapp
- Stored Energy Solar Cookstove for Rajasthan, India.
- Heat-retention cooking
- Solar Rice Cooker
- Solar Cooker for Evening / Night Cooking Using Solar Heat Storage Materials, July, 2006 - Someshwer Dutt Sharma
- Wolfgang Scheffler
- Michael Götz
- Dieter Seifert
- Development and Testing of a Regenerative Rechargable Solar Stove System - Tewodros Eshetu, Shimeles Desalegn, and Dr. A.Venkata Ramayya
- Solar Cooker Storage - Christopher Jordan
- Molten Salt Solar Cooker, Silicon Institute of Technology, Bhubaneswar, India
- The Heat Retention Solar Oven by Ronald L. Conte Jr.
- Heat Retention Cooking vs. Solar Cooking - Mike Bridgwater
- Development and Testing of a Regenerative Rechargable Solar Stove System
- 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."