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Last updated: 13 July 2016
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.
Heat storage can also refer to heating a phase changing medium, typically oils, salts, or waxes to a high temperature and then storing it in a well-insulated container. Cooking can be done at a later time in the kitchen, as the hot medium is delivered on-demand via piping to the heat the cooking surface. The ARUN®100 with Thermal Storage at Ramkrishna Mission, Chennai, India stores heat by sending excess steam to the standby boiler system pressurizing the water supply. Steam used for cooking is later regenerated by flashing the pressurized tank.
- May 2016: Solar thermal excellence award - Ramakrishna Mission Students’ Home in Chennai was recently conferred with an award for Concentrated Solar Thermal Excellence (CST) for their Solar Cooking System by MNRE, Government of India. The award was given for the school's installation of the ARUN 100 institutional solar steam generating and heat storage system. Award for excellence - NYOOOZ
- March 2016: Antonia Lecouna Neumann reports: "We use erythritol as heat storage material, cheap, durable, edible and has a melting heat similar to ice. We found it superior to other alternatives although long term durability is still an issue."
- December 2015: Experimentation with sensible heat storage and lessons learned - Mik Hartwell used a recycled satellite disk and a cluster of steel rods for a heat sink, to see if he could cook a meal after the sun had set. The rods got very hot, cooking a meal and holding heat for several hours, but were difficult to work with. Read more...
- July 2015: Advances continue in molecular heat storage possibilities - Massachusetts Institute of Technology (MIT) professor, Jeffery Grossman explains how molecular heat storage research is advancing. Solar cooking is a prime candidate for this potentially game changing process to allow cooking early in the morning or in the evening. Specific molecules are able to store heat energy when exposed to sunlight, and release it at a desired time when stimulated by a catalyst. * Update January 2016: MIT researchers feel their discovery might not be suited for solar cooking.
- October 2014: Norwegian start-up tackles heat storage for solar cookers - Guro Seim, CEO of Morpho Solar, announced a breakthrough in the heat storage capabilities for solar cookers. They hope to have a production model ready within a year and a half. The breakthrough was recognized with a $8,000 USD award from the Norwegian Ministry of Petroleum and Energy this past August. The company will begin selling the cooker in Europe, and under an agreement with Cambridge, Mass.-based One Earth Designs in the United States. Read more...
- August 2104: Steam heat storage system used for institutional cooking in India - Ram Krishna Mission Student’s Home, Chennai, India wanted to adopt solar cooking system for its hostel. However the desired cooking times and the availability of sunshine was not matching up. Breakfast is cooked at 4.00 a.m. and lunch cooking starts at 7.00 a.m., while dinner is cooked after 5.00 p.m. A steam-based system, the ARUN®100, with heat storage was designed and commissioned on October 26, 2013. Read more about it at: ARUN®100 with Thermal Storage at Ramkrishna Mission, Chennai - Ajay Chandak
- April 2014: A molecular approach to solar heat storage Researchers MIT and Harvard University have posted promising results from experiments involving energizing photo-switching molecules, known as azobenzene, to store solar heat gain for extended periods. The molecules are then triggered much later to release their energy for uses requiring heat. Apparently the approach will not be effective for generating electricity, but cooking is a prime candidate. Read more at: A molecular approach to solar power.
- 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 (212 °F) 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
The late Christopher Jordan working in Cambodia, 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 preferred 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. 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
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.
Audio and video
- June 2016:
- January 2016:
- July 2014:
- February 2016: Solar Cooking with Heat Storage: Experiments using PCM and figures of merits for solar cookers (Abstract) António Lecuona, University of Carlos III-Madrid, Spain
- February 2016: Portable Stored Solar Thermal Energy for Household Use - University of Illinois at Urbana-Champaign and Sun Buckets, Inc.
- July 2015: Review on thermal energy storage with phase change materials and applications, October 2007 - Science Direct
- September 2014: Molton Salt Solar Cooker - Jitendra kumar Dash, Omprakash Parida, Swastik Tripathy and Sagar Dutta, Silicon Institute of Technology, Bhubaneswar, India
- September 2013: Development and Optimization of a Thermal-Storage Solar Cooker - David Gordon Wilson
- July 2006: Solar Cooker for Evening / Night Cooking Using Solar Heat Storage Materials - Someshwer Dutt Sharma
- July 2006: RECENT DEVELOPMENTS OF THE SOLAR COOKING SYSTEM WITH OR WITHOUT HEAT STORAGE FOR FAMILIES AND INSTITUTIONS - Eugenia Vieira Silva
Articles in the media
- May 2016: How Solar + Storage Can Be Cheaper Than Coal - Clean Technica
- January 2016: A new way to store solar heat - MIT News
- December 2015: This solar cooker will work at night too - The Times of India
- December 15, 2015 IISc scientists develop solar cooking device - Deccan Herald (New Delhi)
- April 2014: A molecular approach to solar power - today's energy solutions
- December 2013: Solar-Powered - New University
Gates Foundation UCI recipients explain their goals for improving their heat storage solar cooking system.
- November 2013: Gates Foundation supports development of solar cooker using heat storage - University of California Newsroom
- February 2012: Ten solar cookers that work at night - Engineering for Change
- August 2010: Project Report 8/10, (Extending solar cooking times) - engineeringforchange.org
- June 2008: Storing the Sun: Molten Salt Provides Highly Efficient Thermal Storage - Renewable Energy World
- ARUN 100 with Thermal Storage variation, 2014 - Ajay Chandak
- 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
- Chemical cookers - Ashok Kundapur
- Construction and Improvement of a Scheffler Reflector and Thermal Storage Device, 2010
- Night Cooking Solar Cooker Using Molten Sodium Chloride as Phase Change Material - O Parida, et al.
- 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."