Clean solar cooker as replacement for hazardous log-fires

Until today more than 3 billion people rely on open fires for cooking. Their cook stoves are predominantly based on burning firewood or charcoal and used indoors without sufficient ventilation [IEA (2016). "World Energy Outlook 2016"]. According to the WHO, the exposure to these dangerous fire smokes is linked to 4 million premature deaths each year, primarily among women and children [https://www.who.int/news-room/fact-sheets/detail/household-air-pollution-and-health]. Moreover, the collection of firewood demands up to 8 working hours per day and the deforestation damages the environment extensively. Some efforts are made to switch to cleaner cook stoves based on natural gas. While this would reduce the exposure to health risks, CO2 emissions and financial dependency on fuel supply would remain unchanged. Solar cookers therefore offer a more sustainable as well as independent solution but the acceptance of available technologies is rather low. One reason for this is the lack of storage capacity, meaning meal preparation relies on the availability of sun energy and must be conducted during daylight. This is in contrast to peoples need for reliable and always available energy supply and their preference to cook after sunset.

Solar cooker design

To overcome this challenge DLR is proposing an innovative renewable cooking system based on the reversible chemical reaction of calcium hydroxide (hydrated lime) to calcium oxide (quicklime) and water: Ca(OH)2 + ∆H ⇋ CaO + H2O
The reaction system is investigated at DLR for several years for the application of storing solar energy in concentrated solar power plants [Schaube, F., et al. (2010)] or renewable electricity and heat [Schmidt, M., et al. (2017)].

Basic idea is that calcium hydroxide is dehydrated by solar energy. At night time, the charged storage material (calcium oxide) can be filled into the cooker and mixed with very small amounts of liquid water. The exothermal back reaction takes place under the release of heat and temperatures of 200–500 °C can be used to prepare meals. The storage material can be recharged once renewable energy is available and this procedure can be repeated without any degradation. Moreover, limestone is very cheap, environmental friendly and widely available all over the world. Additionally, the regeneration process can be used to gain drinking water e.g. from rivers or wells, since the reaction doesn’t require clean water but releases pure water vapor during charging.

Durability and Costs

The key component is limestone contained in small packages. They have to be permeable for liquid and vapor water but impermeable for the lime powder and should withstand temperatures up to 600 °C. Promising materials are under investigation at this time. This leads to low purchase cost of below 10 € today with potential to drop further.

A crucial factor is the marketing strategy for the charging process. Low one-time investment costs are expected by implementing e.g. a heat storage unit into an existing photovoltaic facility. This could be funded by micro credits, CO2-certificate trade or by companies seeking emission reduction and image improvement.