The image on the cover of the post is titled “Solar panel cookers at the beach”, it is from flickr and has been released with CC BY 2.0 licence.
Among the several benefits of solar cooking of which we have talked in another page of our website (in Italian), we have not sufficiently highlighted the environmental benefits that solar cooking allows to obtain, with particular reference to the climate change mitigation potential.
As regard this, it is important to note first that solar cooking, like other plants and devices using renewable energy sources, has an energy payback time, which is the time (in terms of days or years) we need to use a solar cooker or oven in order to recover the energy that is spent for its functioning through all its lifecycle (that is mainly energy for production, transport to the place of utilization and maintenance). So only after we have reached the energy payback time we will be able to contribute to reduce the emissions of greenhouse gases (ghgs).
This article aims to give the main information for both these aspects, that is to say the potential for the reduction of greenhouse gases emissions and the estimate of the energy payback time of solar cookers and solar ovens.
Summarizing the main information of the article, we can say that:
- there is a global potential reduction of 1230 million tonnes of ghgs by replacing cooking by fuelwood with solar cooking;
- nowadays, thanks to solar cooking, we are currently avoiding the emission of 3 to 9 million tonnes of ghgs;
- other clean cooking solutions are available other than solar cooking, like those based on natural gas and lpg and also electric cooking and efficient biomass stoves. These allow to bring important sanitary benefits, but their environmental sustainability must be ascertained depending on the efficiency of the stove and on how the fuel source is provided;
- some Countries have established specific measures to promote clean cooking within their National Determined Contributions of the Paris Agreement in order to mitigate climate change;
- clean cooking at household level would allow to significantly reduce the emission of the Short-Lived Climate Forcers, which are ghgs having a low persistence in the atmosphere, so their reduction would allow to mitigate climate change in the short term;
- a home made solar panel has an energy payback time of less than two cookings;
- a factory made parabolic solar cooker has an energy payback time comprised between one to two years, depending on the yearly rate of utilization (200 or 100 meals per year).
So, hereunder we report more detailed information about the points the have been summarized right above.
The mitigation potential of greenhouse gases thanks to solar cooking
Several studies have estimated the mitigation potential of greenhouse gases that is obtainable thanks to solar cooking. According to that, first of all it is important to remember that today in the world about 3 billion people still use either wood, or coal, dung, charcoal, crop residues, lpg or kerosene to cook their meals. Some information related to the mitigation potential comes from the project “UNDP/GEF South African Solar Cooker Project (SOLCO)”, where we can find an estimation of the potential energy savings attainable at household level thanks to solar cooking. This project promoted solar cooking in South Africa as a good cooking practice in order to bring benefit to the environment and to improve the safety and the sanitary conditions with cooking at home, with particular reference to poor people. In order to promote solar cooking, a pilot production of solar cookers was started and these solar cookers were commercialized. Within the project, it was estimated that those families that used solar ovens managed to reduce traditional cooking for about 25% – 31% and managed to obtain an overall energy saving of about 38%. In order to pass from the estimation of the energy saving to the mitigation potential of greenhouse gases, we must consider other studies, such as Tucker’s “Can Solar Cookers Save the Forests” (1999), where the author, citing other researches, indicates that in the developing Countries 36% of fuelwood could be replaced by solar cooking, that is to say a potential saving of 246 million tonnes of fuelwood. From these data we can reach the estimate of the mitigation potential of greenhouse gases (ghgs) by knowing the ghgs emission factor from fuelwood (kg of CO2eq per kg of fuelwood). This information can be get from Grupp and Wentzel’s “Greenhouse Gas Emissions by Cooking With Different Fuels and the Reduction Potential of Solar Cookers” (2002). By multiplying the emission factor for the above indicated quantity of fuelwood, we get a potential reduction of 1230 million tonnes of ghgs.
But this is the estimated potential. If we want to know what is the mitigation of greenhouse gases that has been obtained nowadays thanks to solar cooking, we must first consider that now there are about 3,2 million solar cookers in use in the world by about 11,5 million people (while in the world there is still 3 billion people cooking with fuelwood, coal, dung, charcoal, crop residues, lpg and kerosene), allowing to save between 3 to 9 million tonnes of ghgs (see the page distribution of solar cookers and solar ovens on the website of Solar Cookers International).
Other solutions of clean cooking and their diffusion in the world
Solar cooking is certainly not the only clean cooking solution allowing to substitute the cooking from fuelwood, coal, kerosene, dung and charcoal, which are particularly critical not only for their emission of greenhouse gases, but also for the emissions of particulate matter and for their impact on families, especially on women and children, which are more exposed to the emissions from cooking.
Among the clean cooking solutions, the International Energy Agency (IEA), in its Energy Access Outlook 2017, includes also natural gas, lpg and electric cooking. According to that, it is important to note that while their benefit on the health of women and children is clear, their capability to reduce the emissions of greenhouse gases should be evaluated taking into consideration their emission factor for ghgs and comparing it with the emission factor from the other fuel source that has been substituted. The IEA 2017 Outlook is important also because it highlights the current trend of diffusion of clean cooking in the world and the potentialities for its further diffusion.
The commitments of the Countries in the world for promoting solar cooking
Greenhouse gases other than CO2: the importance to reduce the emissions of short-lived climate forcers (or pollutants)
Two months before COP24 in Katowice took place, IPCC published a Special Report about the impacts related to a temperature global rise of 1,5° C above the preindustrial levels and about the possible future scenarios for the trend of the emissions of greenhouse gases. This special report highlights the importance to reduce the emissions not only of CO2, but also of other greenhouse gases that are less persistent in the atmosphere, but in the short term contribute heavily on the rise of temperature and thus on climate change. For this reason they are called short-lived cimate forcers or pollutants (SLCF o SLCP). In particular, they are methane, hydrofluorocarbons and black carbon. Since they are less persistent in the atmosphere, reducing their emissions would give an important contribution to reduce the global temperature rise yet in the short term.
” “Though CO2 dominates long-term warming, the reduction of warming Short-Lived Climate Forcers…can in the short term contribute significantly to limiting warming to 1.5°C,” the report notes.
Globally, up to 25% of black carbon emissions come from residential cooking, heating, and lighting. In many Asian and African countries, residential use can account for as much as 60-80% of black carbon emissions. The report finds with high confidence that “reductions of black carbon and methane would have substantial co-benefits, including improved health due to reduced air pollution.””
Similar considerations have been reported in this article from the Climate & Clean Air Coalition. All these articles also highlights the importance to act now and the importance of cooperation at global level in order to reduce the potential rise of global temperatures.
Life cycle analysis and energy payback time of solar cookers and solar ovens and how they relate to the potential reduction of the emissions of greenhouse gases
We have already said that solar cooking allows to reduce the emissions of greenhouse gases. But we haven’t said that solar cooking has an impact on the environment too, first emitting some greenhouse gases. So, how can these two things coexist?
In order to understand that, we must specify both how do they emit ghgs and how can they contribute to reduce the overall emissions of ghgs. And in order to specify that, we must consider the Life Cycle Analisis (LCA) of solar cookers and solar ovens and we also must introduce the concept of Energy PayBack Time (EPBT).
So, starting with EPBT, we can say that it is the time we need to recover the energy we spend for the functioning of cooker or oven. That is to say, the energy we spend in order to produce the cooker or oven, to transport it to the place of utilization and to maintain it over its lifetime. Considering that the consumption of non renewable energy determines the emission of greenhouse gases, the EPBT represents the time for which we need to use the solar cooker or oven before we start to reduce these emissions.
For example, in a dedicated webpage (in Italian) of our website we have talked about the energy payback time of the photovoltaic solar modules, which is comprised between 1 and 3 years.
Considering now Life Cycle Analysis, we can say that it evaluates the impact of a product or system over its life cycle. Here we don’t want to describe in detail the method and the technical standards which define the criteria that should be respected in order to estimate the environmental impacts of a product or system over its life cycle. Anyway, simply speaking we can say that when we make a life cycle analysis it is important to specify the boundaries of the system and in particular what are the starting point and the ending point of the analysis. The most accurate analysis starts from the recovery of the raw materials (or otherwise the secondary raw materials) necessary for the manufacturing of the product and ends up with the disposal or recycling of the product itself.
When we choose the boundaries, it could happen that some of the data are not available for the specific context of our study, so we should integrate the missing data with some taken from literature or from databases. When we talked about the energy payback time right above, we have talked about production, transport and maintenance. All these processes are usually part of a life cycle analysis, but while the first two are nearer to the beginning of the life of a product, maintenance interests it throughout all its utilization time. A last important thing to note relating to the life cycle analysis is that when we study the environmental impact of a product over its lifetime, we can decide to focus on several impact indicators other than ghgs emissions, such as for example the emissions of particulate matter (PM), the acidification potential, the eutrofication potential, the terrestrial and acquatic ecotoxicity potential.
For what concerns solar cooking, there aren’t a lot of studies analysing the life cycle of solar cookers or solar ovens, but there are some of them, which are well prepared and detailed, such as for example the one described in the article “Sustainability assessment of home made solar cookers for use in developed countries”, which concentrates on homemade solar cookers and solar ovens. These cookers and ovens have been realized using recovered materials so they have a lower environmental impact than those that are factory made. The article then also estimates the potential social and environmental benefits of solar cooking in developed Countries, choosing Spain as a reference example. The article does not explicit the energy payback time, but it estimates the potential reduction of greenhouse gases (and the environmental benefits for other impact indicators) in the hypothesis that solar cooking partially substitute cooking with a microwave oven. The best results are obtained for the panel cooker, due to the lower environmental impact of its construction, that allows to obtain reductions in the emission of ghgs going from -14% (worst case, using the solar panel for 65 days and with a lifetime of one month) up to -64% (best case, using the solar panel for 240 days and with a lifetime of eight months).
And as regards the energy payback time, it isn’t explicited in the article, but we can try to estimate it for the solar panel. For this oven, the article indicates a primary energy demand of 14,6 MJ (see figure 4 at page 18 of the article). This represents the energy that is consumed for the manufacturing and utilization of the oven throughout all its lifetime. So in order to determine the energy payback time, we must estimate how much energy can be saved for every cooking and how many times in a year we can use the oven. For simplicity, we can take as a reference the energy that is necessary to boil 2 L of water. About that, in the project KiloWattene from the Italian research institute of ENEA, we find that gas stoves consume about 2,23 kWh of energy (see this page in Italian, searching for the table that is present in the chapter “Risultati: consumo di energia primaria, emissioni di CO2, costi di esercizio”); 2,23 kWh correspond to about 8,03 MJ, so the energy payback time for the solar oven would be very short. Indeed, it would be sufficient to use it for preparing two meals and its primari energy demand would have been extensively recovered!
And what about a solar cooker or solar oven that is factory made? At the moment we do not have studies expliciting the value of the energy payback time for factory made solar cookers. Nonetheless, we have some information about the energy that is needed to manufacture, transport and maintain a solar cooker. About that, the study “Life Cycle Assessment and Environmental Impact Evaluation of the Parabolic Solar Cooker SK14 in Madagascar” indicates an energy consumption of 609,3 MJ for the production of the reflective paraboloid of a parabolic solar cooker. Indeed, then in the article it is not clear how the energy consumption have been calculated neither for the other parts of the cooker, nor for the painting production nor for its transport to the place of utilization, since the corresponding energy consumption are referred to a single meal. Moreover it is not clear why the study have hypothesized that when the solar cooker cannot operate it is substituted by cooking with coal, considering that the same study takes into consideration also cooking with fuelwood, which determines lower emissions of greenhouse gases. Nonetheless, we can try to calculate the energy consumption for the aforementioned processes of production of the other parts of the cooker, for the painting production and for the transport to the place of utilization,by applying the same factor of proportionality in order to determine the overall consumption starting from the data relative to a single meal. According to that, if we consider that the energy consumption for the production of the reflective material is 609,3 MJ and that the corresponding value relative to a single meal is 0,04 MJ, we have that the factor of proportionality is 15’225,75. So, knowing that the energy consumption for the other processes and relative to a single meal are about 0,1 MJ, we have that the energy consumption for these processes is equal to 15’22,57 MJ, for a total energy consumption of 2131,87 MJ. So, if we remember that in a single meal we can save about 8,03 MJ of energy thanks to solar cooking, we understand that the energy payback time is reached after having cooked 266 meals. So, if with solar cooking we can prepare 100 meals per year, the energy payback time will be of about 2,5 years, if we manage to prepare 200 meals (that is common in tropical regions), the energy payback time will be slightly more than one year.