DAWN OF SOLAR PV COOKING
Humans need food and water for survival. Cooking is ubiquitous in human life as a daily ritual. It is considered to be an important influence in human evolution for survival and sustenance. Charles Darwin considered ‘the art of making fire’ as probably ‘the greatest discovery’, except language, ever made by man. By cooking, we get to eat the food that gives us the energy and nutrients to grow and develop, be healthy and active, to move, work, play, think, and learn.
TRANSITION IN COOKING FUELS
Over the years, human civilization has evolved by leaps and bounds with the development of science and technology, and so are the types, techniques, and technologies of cooking. The options available as the sources of heat energy for the purpose of cooking are numerous today. When looked at closely, one could witness a significant transition in the form of heat energy source for household cooking; starting with use of solid fuel (traditional biomass) since stone age to liquid fuel (kerosene) to gaseous fuel (LPG) to now most modern electricity as cooking fuel (electric resistive and induction-based cookstoves).
The transition towards more modern and clean cooking solutions is desirable and is also taking place, however, the pace in terms of its reach to most of the rural households around the world remains slow. Moreover, there are households that use LPG along with biomass fuels. The ‘energy ladder model’ conceptualizes fuel switching in three distinct phases. The first phase is characterized by universal reliance on biomass. In the second phase of fuel switching households are hypothesized to move to ‘transition’ fuels, such as kerosene, coal, and charcoal in response to higher incomes, urbanization, and biomass scarcity. The third and final phase of fuel switching is characterized by households switching to modern energy, such as LPG, natural gas, or electricity for cooking. Yet, a large number of households simultaneously use a variety of cooking fuels sometimes spanning both upper and lower levels on the energy ladder.
CURRENT SCENARIO AND ACCESS TO CLEAN ENERGY FOR COOKING
Today, around 2.8 billion people in the world lack access to clean cooking (Source: www.iea.org). A large number of people die prematurely each year as a result of smoke caused by burning solid fuels or from combustion of kerosene or coal. Children below five years of age can develop risks of acute lower respiratory infections due to the exposure (Source: Asante et al. 2016). They have adverse environmental impacts too; they emit black carbon that has a global warming potential 4,000 times greater than that of carbon dioxide over a 20-year span. Access to clean energy cooking is essential for economic growth, human development, and environmental sustainability. Women, in particular, would gain by reducing the time spent in gathering fuel and cooking, and thus avoiding household air pollution. UN’s sustainable energy goal (SDG 7) aims to ensure access to affordable, reliable, sustainable, and modern energy for all. Hence, it’s crucial to enhance energy efficiency and invest in technologies that convert energy into electricity using renewable sources.
As the cookstoves have evolved over centuries, their efficiencies have also increased. The efficiency of three stone cookstove is only 7%. The LPG cookstove has an efficiency typically in the range of 30%–40% while the induction stoves have gone up to 74% (Figure 1). Before we start looking at solar energy options for cooking, let’s ask ourselves the following questions: How much minimum energy is required for cooking, considering that we do very efficient cooking? Is it possible to increase efficiency further, leading to further reduction in cost?
The total energy required (Q) for cooking is given by the following expression:
Q= Energy required for cooking food + energy required for heating vessel + energy lost due to evaporation of water + other energy losses
Q= ∑ mingredient ∆T +mvessel Cvessel ∆T +mvapourescaped Cv + Qlosses
Here, m represents mass, C specific heat capacity, and dT temperature difference and Q energy.
From the above equation, theoretically, the estimated energy required for cooking 100 g rice comes to only about 30 Wh. Theoretical calculations do not take heat losses in account. In our experiments, energy required for cooking the same amount of rice in pressure cooker on induction cookstove was measured to 76.5 Wh but in well-insulated vessel measured to about 35 Wh. This indicates that there is significant scope of minimizing the heat losses. For designing an efficient cooking solution, the cooking process is to be understood in detail, as depicted in Figure 2. In the process of cooking, loss of heat during heat generation, heat transfer and during cooking needs to be avoided or minimized.
SOLAR THERMAL COOKING: MORE LIMITATIONS THAN ADVANTAGES
When it comes to use of solar energy for cooking, there are two technology routes: converting sunlight directly into heat for cooking (solar thermal route) or converting sunlight into electricity, and then using it for generating heat for cooking (solar PV route). However, over the span of 250 years (in 1767, Horace de Saussure, a Swiss physicist, invented the first solar oven), solar thermal has primarily been explored for cooking and hence solar cooking has become synonymous with solar thermal cooking. Solar PV technology has always been considered expensive and unviable. Here we shall discuss how and why the solar PV technology route for cooking can begin now and overtake the solar thermal route for cooking.
Any modern cooking solution needs to fulfill the 5C criteria of: (i) convenience, (ii) control, (iii) cost-effectiveness, (iv) clean, and (v) compactness. The comparison between various fuels on these parameters is given in Figure 3. A cooking solution must provide convenience of indoor cooking, should provide good control over cooking temperature, must be cost-effective for users, must be clean from health and sustainability perspective, and compact for installation and use. Most of the solar thermal cooking solutions available today do not satisfy one or more of these criteria. Although one of the advantages of solar thermal cooking is that it is simple to build and of low-cost.
Solar thermal cooking is practically cumbersome as the place for heat generation (in outdoors, under sunlight) and the place of cooking (indoors, inside kitchen) are different, and the transfer of heat is not easy. Also, concentration of sunlight requires optics to do the same and the structure to hold optics adds to the cost. The life of reflectors, cleaning of reflectors, and impact of wind on output remain significant challenges even today. Since one can only concentrate parallel rays, diffused sunlight which is nearly 15%–20% of total sunlight becomes useless. For achieving sunlight concentration, movement of sun needs to be followed with sun tracking devices and that adds to the overall cost. Moreover, storing heat in thermic fluids (generally requires 7–8 L per 5 member family) and recovering it again for cooking requires an elaborate set- up. Despite a long history of development, solar thermal cookstoves have not been adopted in a significant way. By the end of 2014, only 6.4 MWth capacity was in operation in India with 9,200 m2 of installed collector area. There has been a 15% reduction in the market growth of new installed capacity in the year 2013/14. Hence, given the limitations, solar thermal cooking solutions such as box-type solar cookers, parabolic dish cookers, Scheffler dish cookers are not very convenient for users. In the modern world, since time is precious, quick cooking solutions that would be able to provide quick supply of energy to achieve the required temperatures in short time, that is, (80–100 °C for boiling and 180–240 °C for frying) are needed.
SOLAR PV COOKING: A NEW DAWN
Can solar PV cooking solution overcome the problem inherent to solar thermal cooking? Can solar PV cooking fulfill the 5C criteria? The answer is ‘yes’, to both. Solar PV technology converts sunlight into electricity, which not only can be used for cooking directly inside kitchen, but can also be stored easily in batteries. Therefore, it can provide the convenience of cooking indoors as well as during nonsunshine hours. Use of electricity coupled with inductive or resistive principle for cooking can give good control over the rate of heat energy supplied. One can cook in low, medium or higher power with just a push of button. Solar PV cooking solutions can fulfill all domestic cooking needs and can be customized as per the user’s choice and needs. Usage of induction cookstoves with PV modules and batteries can provide the required power to cook all types of Indian cuisines, both vegetarian and non-vegetarian, on 24×7 basis, matching the performance of LPG-based cooking.
A basic solar PV rice cooker is a combination of a PV module, directly coupled with DC resistive coil designed to match the impedance of coil and panel for maximum power transfer. In this simple arrangement, the heating coil (generally made up of Nichrome or Constantan) is embedded with the vessel and PV module, directly supplying power to the coil during day time. A 40 Watt PV module coupled with 6.5 Ohm nichrome resistive coil can cook rice for 3–4 people, twice in a day. The cost of such cookstove would be about Rs 2,000. This arrangement not only matches performance of a boxtype solar thermal cooker and cost, but also provides the convenience of indoor cooking. A 350–400 Watt PV module, with 1 kW induction cookstove and 1,300 Wh battery storage would be sufficient for 2-member family to cook thrice a day. Design can be customized to fulfill entire cooking needs for different family sizes. The exact choice of PV panel power depends on the location and number of sunshine hours at that place and the exact size of battery depends on how much back up time one requires.
ONGC launched a ‘Solar Chulha Challenge’ at national level to come up with a solar cookstove that can cook three meals a day for five-member family. The IIT Bombay team won the competition with its innovative prototype: SIX Cook-Stove (Solar Intelligent Cooking with Storage). The induction cookstoves available in market work on AC power. Internally, in the induction cookstove, the 230 V AC power gets converted into 325 V DC power, which is then converted into 20 kHz high frequency AC power (See Figure 4). High frequency AC power is required for induction of eddy currents in the vessel, which is responsible for generating heat. In solar PV cookstoves, power generation and storage occur in the form of DC; therefore, it makes sense to design the power control circuitry based on DC. Circuit configuration is more suitable for solar PV cookstoves and will be more efficient as well (Figure 4). A 48 V DC power system, which will also be safe to operate, is a good choice. Globally, PV module prices have come down significantly and PV modules are now available at less than $0.45 per Watt-peak (Rs 30) Similarly, the prices of batteries are going down. Currently, leadacid batteries are available at nearly $70 per kWh (Rs 4,700) and Li-ion batteries with $125 per kWh (Rs 8,350). Induction circuits and power electronics control circuits are not expensive and can be manufactured in $20–$25. With these costs, solar PV cooking solutions for domestic needs can be made in Rs 30,000– Rs 60,000, depending on consumer needs, family size, and backup time.
Annualized Life Cycle Costing (ALCC) is a good tool to compare the cost of various alternatives available for the same purpose. For a 5-member household, 3 units of electricity per day, derived theoretically as well as measured experimentally, is sufficient in solar PV cooking. A solar PV cookstove with 800 W panel and a 4 kWh lead-acid battery storage can provide this much energy. Considering a slightly overdesigned cookstove with 1,000 W solar panel (@Rs 28/W) and 5 kWh lead-acid battery storage (@Rs 4,700/kWh), the ALCC for solar PV cookstove comes to about Rs 8,000. A life-cycle of 20 years with battery replacement every five years is assumed. Doing the ALCC analysis for the same effective energy consumption from LPG cookstove (equivalent of 15 cylinders per year), and considering cost of LPG cylinder as Rs 750, the annualized cost comes to nearly same amount, Rs 8,000. Thus, it appears that the solar PV cooking solution, even in today’s economic terms is competitive with LPG solutions. With further expected reduction in the cost of battery and PV module and increase in efficiency of cooking, the author believes that it can become one of the preferred cooking solutions. An additional benefit of this solution can be to use the stored electrical energy to power the home-lighting systems and other appliances as well, thus, finding solution for electricity scarcity in underserved areas.
Cooking solutions are becoming convenient, quick, and efficient, but ensuring the availability of the same as clean, reliable, and affordable remains a humongous task for world leaders and policymakers. The thrust on clean energy scenario is at its peak at present and the need of the hour. The emergence of solar PV cooking can be most viable solution to address cooking needs as it fits well in 5C parameters of convenience, control, cost-effective, clean, and compact. Since the solar PV cooking solutions fits well to fulfill the cooking needs, both in urban as well as in rural areas, therefore, if promoted well, it can grow significantly and it will not be far when it becomes one of the most adopted cooking methods. It is the dawn of solar PV cooking.
Dr Chetan S Solanki, Professor, Department of Energy Science and Engineering, IIT Bombay PI, Solar Urja through Localization for Sustainability (SoULS) and National Center for Photovoltaic Research and Education (NCPRE)