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Article

An Experimental Comparative Study of Large-Sized Direct Solar Fryers for Injera Baking Applications

by
Mesele Hayelom Hailu
1,2,*,
Mulu Bayray Kahsay
1,2,
Asfafaw Haileslassie Tesfay
1,2 and
Ole Jørgen Nydal
1
1
Department of Energy and Process Engineering, Norwegian University of Science and Technology, Kolbjørn Hejes v 1B, Varmeteknisk lab, Floor No. 3, NO-7034 Trondheim, Norway
2
School of Mechanical & Industrial Engineering, Ethiopian Institute of Technology-Mekelle (EiT-M), Mekelle University, Mekelle P.O. Box 231, Ethiopia
*
Author to whom correspondence should be addressed.
Energies 2024, 17(19), 4949; https://doi.org/10.3390/en17194949
Submission received: 6 August 2024 / Revised: 16 September 2024 / Accepted: 17 September 2024 / Published: 3 October 2024
(This article belongs to the Section A: Sustainable Energy)
Browse Figures
Versions Notes

Abstract

:
This research experimentally demonstrates the practicability of using large-sized direct solar frying as an alternative technology for the predominantly biomass-based injera baking method. The system was designed and developed with fryers 40, 50, and 55 cm in diameter and two operational options: continuous mode and alternating mode. Extensive experimental testing was conducted on each prototype to demonstrate solar frying and determine the relative performance. The findings indicate that the 2 kW heating capacity of the 40 cm-sized solar fryer model conducted baking processes at a relatively lower system temperature in both application modes compared to the larger-sized fryers. As a result, this system maintained a consistent average fryer temperature distribution and shorter initial heating time, without the requirement of a reheating process during the subsequent baking cycles. The experimental testing also demonstrated that alternating-mode applications were more practical for the 40 cm-sized fryers than for the larger ones. Overall, direct solar frying is more efficient and convenient for the 40 cm-sized solar fryers. In contrast, the larger-sized systems required a larger fryer thermal storage capacity coupled with larger-size solar concentrators to maintain equivalent stable operational conditions, conversely leading to a lack of application simplicity and higher system costs.

1. Introduction

Injera (also written as Enjera in some studies) is a staple food indigenous to Ethiopia and Eritrea, forming a crucial part of the daily meal to most of the 133 million population [1] in these two countries. The traditional method of baking injera involves the use of a large-sized clay pan locally known as ‘mitad’ in Ethiopia. These pans typically measure 500 to 600 mm in diameter and 20 to 30 mm in thickness [2,3]. In rural areas, where the majority of the population resides, biomass is predominantly used as a fuel source for baking injera [4,5,6,7]. In urban areas, a combination of electricity and biomass is commonly utilized due to limited access to electricity [7]. However, biomass-based baking processes are inefficient, pose an indoor air pollution hazard, and are becoming expensive due to the limited availability or unsustainable use of biomass resources in the country [8,9,10], while electricity access is very low [11], as the majority of the population lives off-grid.
The injera baking process is energy-intensive. Studies have indicated that households are a major energy-consuming sector in Ethiopia [5,8,9,10,11], with the energy primarily consumed for baking and cooking purposes [10,11,12,13,14,15,16,17,18]. Conventional baking processes also discussed in detail in the authors’ previous work [2] and other studies [12,13], mostly employ a traditional large-sized clay pans with relatively low thermal conductivity, leading to highly inefficient energy utilization due to their high specific energy consumption [2,3,19]. The baking process occurs in the temperature range of 130 to 220 °C, at an average energy consumption of 1 MJ/kg of injera, and with a baking time of 180 to 240 s [2,19]. Subsequently, this process requires a higher amount of heat than typical boiling. This makes the injera baking process a highly energy-consuming, labor-intensive, and unsustainable activity in terms of the energy consumption profile of the country [14,15,16,17,18,20,21]. Therefore, the development of an alternative intervention technology for the process of injera baking will make a significant contribution to the energy consumption profile and socio-economic aspects of the country.
In terms of injera baking applications, the reviews by Mulugeta Tadesse [12] and K.A. Adem and D.A. Ambie [13] presented the currently available technologies that have so far been developed and reported on. In this regard, there are a variety of technologies that employ either electricity, biomass, gaseous, and liquid fuels, or other alternative energy resources, such as solar energy.
Solar energy-based technologies are some of the most promising alternative interventions in the conventional energy-intensive injera baking processes, specifically biomass-based applications in off-grid communities in rural areas. These technologies are commonly named “solar fryers”. In general, any solar fryer system will have three main components: a solar collector, where the solar radiation is captured; a receiver, where the captured solar radiation is concentrated onto the receiver surface and converted into heat energy; and a fryer (or baking plate), where suitable food is cooked/baked. These solar fryers can be classified in different ways, as discussed in many studies [22,23,24,25,26,27,28,29,30], with classification methods based on the way the solar radiation is captured, the availability and configuration of an intermediate thermal energy storage component, and the way thermal energy is extracted as final useful energy for the baking process.
From an application perspective, solar fryers can be classified as either a direct system, also analogously named direct solar fryer systems, or an indirect system, also analogously named indirect solar fryer systems. The difference between these systems is the presence or absence of an intermediate thermal energy storage component. The direct system does not have such an intermediate component. It directly utilizes the captured solar radiation in the solar collector, which is concentrated onto the heating surface while the food is baked on the baking surface of the solar fryer, without the use of an intermediate thermal storage component in the system. Such a direct solar fryer system utilized in injera baking applications were developed and reported by A. Gallagher [31] and the authors’ previous work [32].
The indirect system uses an intermediate thermal energy storage system between the receiver, where the intercepted solar radiation is converted into heat energy, and the target application, where the heat energy is utilized as the useful final energy. Such indirect systems usually have different system configurations and operational options. One common configuration is that the thermal energy can be exchanged and used directly at the application (i.e., on the baking plate), without the requirement of thermal energy transportation. Another variation in the system is that the thermal energy harvested at the receiver surface is stored in the form of sensible heat (for example, storing a hot heat transfer oil or any solid thermal storage material) or latent heat by using phase change materials (PCMs) and is transported to the final application target (i.e., the baking plate). Such indirect solar fryer systems for injera baking applications were developed and reported on in the authors’ previous work [33], as well as in works by Abdulkadir et al. [34], Asfafaw Haileselassie Tesfay et al. [35], and T. Jemal, O. Fatoba, S. Shimels et al. [36].
The direct system is suitable for and restricted to outdoor baking activities, while the indirect system is intended for indoor applications. The direct system is simpler, more cost-effective, and thermally more efficient but offers lower operational flexibility to the user, as it is limited to outdoor applications and the availability of sufficient direct solar radiation. The indirect system is more costly and thermally less efficient but offers better operational flexibility to the user as compared to the direct system.
Most frying processes employ smaller fryer sizes with diameters of around 30 cm. However, traditional injera pans are 50 to 60 cm in diameter, while in some commercial applications, their size can be in the order of 40 cm. Unlike many frying methods, the injera baking process is, therefore, a process conducted on a relatively larger fryer size. While many designs of the direct system type exist for cooking applications or baking of smaller-sized staple food; Gallagher’s model [31] and that in the authors’ previous work [32] are the only large-sized direct solar fryers that meet the requirement of injera baking applications, with the latter being the largest size developed and reported so far. Both studies demonstrated solar frying for injera baking application, and the performances of the respective models were evaluated experimentally. However, as both experimental demonstrations were conducted at different design and site conditions, it was inconvenient to make comparative performance studies. Therefore, this work experimentally demonstrates the possibility of solar frying by using large-sized solar fryers for injera baking applications, thereby extending the above works by studying the performance of solar fryers of different sizes and comparing their performance in terms of temperature development in the initial heating, baking, and reheating processes for varying site solar radiation potential.

2. Materials and Methods

2.1. Design of the Direct Solar Fryer System

2.1.1. Geometrical Design

The solar fryer system utilizes a parabolic shape satellite dish with a 1.8 m rim diameter and a 68.4 cm focal length, laminated with a high reflectivity aluminum sheet (MIRO High Reflectance 95) to function as a solar collector. The system was designed and developed in two operational modes. The first system, here referred to as “the solar fryer system with a continuous application mode” or, in short, “the continuous mode solar fryer”, enables heating and baking to be conducted simultaneously. In this mode, heating occurs on one side of the fryer plate (here referred to as the heating or illumination surface), while baking takes place on the opposite side (here referred to as the baking surface), enabling both processes to happen concurrently. A schematic representation of this system is presented in Figure 1.
The second mode, here referred to as “the solar fryer system with an alternating application mode” or in short “the alternating mode solar fryer” enables heating and baking to be conducted alternatively on the same surface of the solar fryer by positioning the fryer in the focal area during charging and turning the fryer 180 degrees for the subsequent baking. The schematic representation of this system is indicated in Figure 2.
From an operational perspective, the choice between continuous- and alternating-mode systems depends on whether the baking activity is occurring under shading from the collector dish. In the continuous mode, the plate remains exposed to the incoming solar radiation on the collector during both the heating and baking phases. In contrast, the alternating-mode system is equipped with a dish rotation mechanism, which enables the dish–plate assembly to reorient under the collector shading during the baking phases. As a result, the alternating-mode system offers greater user operational comfort and flexibility compared to the continuous-mode solar fryer system.
All models of the solar fryer system feature a baking plate made of aluminum, which is locally produced through a casting and machining process. Aluminum is chosen for its suitability as a baking utensil due to its ease of local manufacturing and good thermal conductivity, hence its ability to distribute heat evenly and quickly across the plate. The fryer system includes a two-axis manual dish-tracking mechanism to adjust for daily and seasonal changes in the sun’s direction. Adjustments were needed every few minutes. The dish concentrates the incoming solar radiation onto the fryer plate, which serves as a large-area receiver. Baking is then started once the plate has been heated to a sufficient temperature. The baking plate is available in different diameters of 40, 50, and 55 cm, with varying thicknesses depending on the application mode.

2.1.2. Thermal Design

The thermal design of the solar fryer takes into account two main factors: the solar radiation potential at the application site and the injera baking energy requirements. The input power to the frying plate depends on the solar irradiation, the collector aperture area and the optical quality of the system. This solar radiation is concentrated and converted into the thermal energy of the fryer, to provide heat for baking. The heat is transferred throughout the frying plate by thermal conduction, to give near homogeneous temperatures as required by the baking process.
An average injera is made of 70% water and 30% ‘teff’ flour by mass. The diameter is traditionally preferred to be near 60cm, in our experimental cases the plate diameters were 40, 50 and 55 cm. During the baking process, the dough is heated from room temperature of 25 °C to the water boiling temperature of 100 °C, causing approximately 25% of its moisture content to evaporate. Boiling temperatures of water depend on the elevation of the test site, at Mekelle University, Ethiopia, it is close to 100 °C. Consequently, the baking energy requirement includes both the thermal energy needed to raise the dough’s temperature and the latent heat required to evaporate the 25% water content. The thermal energy storage requirement for the baking plate is then calculated based on these two energy needs.

2.2. Experimental Methods and Materials

The prototype for the alternating-mode solar fryer system is shown in the photographs in Figure 3. The baking plate is put into the heating position by rotating the dish–plate assembly toward the incoming solar radiation, as shown in Figure 3a. The collector’s direction is continuously adjusted by using the two-axis manual tracking mechanism. Once sufficient plate temperature is reached, the dish–plate assembly is released to rotate by itself to a position where the plate is at the lowest point and ready for baking under the shade of the reflector, as shown in Figure 3b. The baking phase is then conducted at this position. Once baking is completed, the dish–plate assembly is returned to the heating position. Similar prototypes for the continuous mode solar fryer system were also developed and tested. In the continuous-mode system, the dish–plate assembly is placed in the heating position during the heating and baking phases; thus, the plate is continuously heated at its bottom surface and the baking activity is conducted at the plate’s top surface.
Extensive experimentation was conducted on the two models to evaluate the thermal performance of each system through temperature, mass, and solar radiation measurements. The experimental setup is presented in Figure 4. The temperature measurement was taken at the illumination and baking surface of the fryer plate for the continuous-mode solar fryer system. Similarly, for the alternating-mode solar fryer system, the temperature measurement was conducted at the illumination (or baking) surface and the backside of the plate. Solar radiation measurements using the SPN1 Sunshine Pyranometer (Manufacturer Delta-T Devices, Cambridge, UK) were conducted, thereby measuring the global and diffuse irradiance. Mass measurements of the dough and the baked injera were also taken. This experimental testing was conducted at the Solar Demonstration Center of Mekelle University, Tigray Region of Ethiopia (latitude: 13°28′34.46″; longitude: 39°29′2.12″). When conducting the solar radiation measurements, the data are prone to different errors, generally classified as equipment and operational errors [37]. Equipment errors are inherent to the type of pyranometer used. As the equipment (i.e., both the pyranometer and data logger) were new and used for the first time at the application site, their factory level errors as provided by the manufacturer were applicable in analyzing the measurement accuracy. In this regard, the SPN1 Pyranometer matches or exceeds the ISO First Class standard and the WMO Good Quality standard for a solar pyranometer in all respects, apart from the spectral response, which is accurate to ±10% over 0.4 to 2.7 μm (SPN1 technical sheet). Operational errors are usually application-specific cases, and therefore, occur due to different reasons. The most common operational errors are categorized and listed by Younes et. al. [37]. As this study had a short-period radiation measurement application, the most commonly encountered error was due to outlier data (i.e., relatively very high or low positive or negative readings compared to the expected data or adjacent data). These errors were removed and replaced with average values from adjacent readings.

2.3. Comparative Performance Assessment

The thermal performance of the direct solar fryer systems was evaluated by assessing the following four comparative factors: temperature, time, site solar energy potential, and injera quality.

2.3.1. Temperature Achievement

The maximum temperature reached by the fryer plate was measured to determine the system’s effectiveness in generating the required heat before initiating the baking cycles and during the baking–reheating phases. The starting temperature depends on the intensity of the solar radiation at the application site and the time of the specific testing. For a given heating time, the maximum temperature of the fryer will be higher with higher solar radiation intensity compared to lower radiation intensity. Therefore, a measure of such a maximum temperature was taken as one of the relative performance indicators for the solar fryer system.

2.3.2. Time for Initial Heating and Baking–Reheating Cycles

The time taken for the fryer plate to reach the desired temperature from a cold start was recorded, providing insight into the efficiency of the system’s heat-up phase. The time taken to reach such a starting temperature depends on the solar radiation intensity, which in turn, depends on the application site, the time of the specific testing, and the fryer size. A high solar radiation intensity will lead the system to reach a given maximum temperature under a shorter heating time as compared to lower radiation conditions. However, such a heating time should also be equivalent to the time needed in a conventional baking system. Similarly, the duration of each baking cycle and the time required to reheat the system between cycles were also analyzed. This helps to clarify how well the system maintains temperature and handles subsequent baking sessions without a significant plate temperature drop. Therefore, the measures of time for the initial heating phase and during the baking–reheating cycles were also taken as performance indicators.

2.3.3. Input Solar Energy

The amount of solar energy captured by the system was quantified to evaluate how this input solar energy affects the maximum temperature reached by the fryer, the time for the initial heating, and the baking–reheating cycles. Mathematically, the amount of solar energy captured by the system is given by the product of the average solar radiation measured during the experimental testing at the specific site and the effective collector aperture area that intercepts and concentrates the radiation fluxes into the receiver surface (i.e., the heating surface of the fryer plate), as given by Equation (1):
Available solar power = average solar radiation × effective aperture area of the collector
The actual solar energy reaching the receiver surface is then the product of the available solar energy and the effective optical efficiency (ηeff,opt) of the solar collector. This effective optical efficiency is the combined effect of the inefficiencies that arise from the geometrical inaccuracy of the collector (i.e., a parabolic-shaped dish) and its reflecting surface (i.e., strips of reflecting sheets mounted on the surface of the parabolic collector), the manual tracking system, and the optical properties of the reflecting sheet on the collector and receiver surface. This actual solar energy is converted into heat flux at the receiver surface, leading to heat transfer across all components and heat losses from the surfaces of the fryer system. As the energy conversion process and the inefficiencies discussed above are the same for all fryer models, comparative studies are preferably carried out by comparing the resulting temperature development and operation time of each testing case with the respective available solar power (or energy) during the specific testing.

2.3.4. Injera Quality Assessment

While determining the performance of the system, the temperature level and heating time was determined by experimental iterations that could provide a standard-quality injera, as studied by T. Girma et al. [38]. The quality of the baked injera was determined qualitatively by physical inspection of its size (thickness), color, eye formation, underside appearance, texture, and taste.

2.4. Estimating the Capital Cost of the Direct Fryer System

From an Ethiopian perspective, the solar fryer system and its main components do not have established market prices; as a result, it was difficult to pinpoint the actual investment cost of such a system. However, an overview of the required capital cost is presented by considering the cost of the input materials that constitute the system and the possible local manufacturing cost for the system. The material costs were taken from a widely available online retailer, Merkato [39]. The following components contributed to the overall capital cost of the direct solar fryer system:
  • Supporting structure: Human-made RHS and CHS pipes and sheet metal.
  • Collector dish: A collector dish made of four to six strips is widely available and has a lower cost; however, such a dish is liable to geometrical inefficiencies. A dish manufactured from one complete sheet (i.e., without a strip) is more effective for solar concentrator applications. However, it is not commonly available in the market and has a higher cost.
  • Solar reflecting sheet: This material is rarely available in the market, thus incurring a higher cost. Glass mirrors, which are widely and cheaply available, can also be used as a replacement; however, they come with reduced efficiency.
  • Fryer plate: This component is locally manufactured by casting and machining.
  • Insulation material: A small amount of insulation material is used for alternating-mode solar fryer systems.
  • System manufacturing cost: A workshop activity for assembling the support structure, laminating the reflective material into the collector dish, and assembling the whole system are considered components of the system manufacturing cost.

3. Results and Discussions

This research analyzed and presented the thermal performance of large-sized solar fryers operating in two modes: the continuous and the alternating modes. The performance of these models was evaluated in terms of the respective heating and baking temperature profiles. The relative performance of the system is discussed in the following sections.

3.1. Performance Evaluation of the Alternating-Mode Solar Fryer Systems

The temperature profile of the heating–baking process for the 40 cm-sized alternating-mode solar fryer system is shown in Figure 5. In this experimental demonstration, 14 baking cycles were successfully conducted. During the experiment, the average available solar energy from the solar collector with a 1.8 m rim diameter was about 2 kW.
In the experimental demonstration, the baking plate achieved an average temperature of 170 °C, with the maximum temperature at the plate illumination surface reaching about 180 °C after an initial heating time of 16.5 min. Each subsequent baking cycle lasted an average baking time of 4 min, during which the plate’s average temperature dropped by 20 °C. During each reheating phase, the plate’s average temperature was again raised by 20 °C, with a reheating time of 5 min.
Another experimental performance evaluation of the 40 cm-sized alternating-mode solar fryer model is shown in Figure 6. In this experimental testing, 16 baking cycles were successfully conducted. This system exhibited behavior similar to that of the system indicated in Figure 5, with comparable fryer plate temperature development, initial heating time, baking time, and reheating time as the performance indicators. However, the system was set to operate at a slightly higher temperature because the average solar radiation in that specific experimental time was lower compared to the previous one. Therefore, it can be noted that whenever lower solar radiation conditions exist, the operational conditions of the system are compromised, in terms of the maximum temperature reached and the time for the initial heating and baking–reheating cycles, compared to the operational conditions in high-solar-radiation conditions.
Similar experimental tests were conducted on the 50 cm- and 55 cm-sized alternating-mode solar fryer systems. While the 40 cm model exhibited stable operational conditions, as shown in Figure 5 and Figure 6, the larger-sized models demonstrated slower operational conditions due to the requirement of a longer initial heating time and the time needed for the baking–reheating cycles. Additionally, these systems experienced higher surface heat losses due to their larger size. As a result, the larger-sized fryers operating in the alternating mode were characterized by lower system performance indicators.

3.2. Performance Evaluation of the Continuous-Mode Solar Fryer Systems

The performance of the 40 cm continuous-mode solar fryer, in terms of temperature development during its heating, baking, and reheating phases, is illustrated in Figure 7. The system was tested in two experiments conducted on different dates under different solar radiation conditions. It can be noted that higher solar radiation in Experiment 2 led to a shorter initial heating time (i.e., 25 min) compared to Experiment 1 (i.e., 30 min). Due to this solar radiation variation, the baking process in Experiment 2 was set to operate at a relatively lower temperature than the fryer temperature in Experiment 1. Therefore, it can be noted that high solar radiation conditions enabled a relatively lower operational temperature level while lower radiation conditions led to higher operational temperature levels in the system. By considering such a compromise, the overall time taken was equivalent in both experimental tests. Whenever lower solar radiation conditions existed, the operational conditions took 60 min for baking 10 injera in Experiment 2 and 90 min for baking 15 injera in Experiment 1. Therefore, the system showed consistent temperature development throughout the baking period.
The temperature development of the continuous-mode 50 cm-sized fryer is shown in Figure 8. In this specific test, the average solar radiation during the initial heating phases was 950 W/m2, and as a result, the system achieved a maximum temperature of 170 °C in a shorter initial heating time. As the test progressed into the baking and reheating phases, cloud cover at the site affected the intensity of solar radiation, leading to irregular temperature development in the fryer plate. However, in general, it was observed that such a large-sized system failed to provide sufficient stored thermal energy for the subsequent baking and reheating phases compared to the 40 cm-sized solar fryer, despite having a similar high solar radiation intensity of 950 W/m2.
Similarly, Figure 9 illustrates the temperature development in a continuous-mode 55 cm-sized solar fryer system. It is evident that this system also failed to provide sufficient stored thermal energy, leading to a decrease in the average fryer temperature during the subsequent baking phases. Additionally, the system experienced a greater temperature drop during baking and required a higher temperature increase during the reheating cycles, resulting in longer baking and reheating times compared to the 40 cm-sized solar fryer system.

3.3. Input Solar Energy Comparison

The solar radiation potential at the site during the experimental testing of the 55 cm-diameter continuous-mode solar fryer model is shown in Figure 10. The solar radiation in Experiment 1 was recorded at 5-s intervals, while in Experiment 2, it was recorded at 2-s intervals. This allowed Experiment 2 to capture the solar radiation variations at a smaller time scale. For ease of observation, the solar radiation data in Experiment 2 are represented by the average data generated from curve fitting of the actual data. It is evident from Figure 9 that the temperature development in the fryer during the two experiments followed the solar radiation patterns at the site on the specified test date.
During the initial heating phases, the solar radiation in Experiment 1 was almost constant and higher (approximately 950 W/m2) compared to the conditions in Experiment 2 (which increased from 860 to 950 W/m2). As a result, both models reached their respective maximum temperatures to start the subsequent baking phases. However, on that specific testing day, the solar radiation in Experiment 2 kept increasing, compared to the radiation in Experiment 1, resulting in a higher fryer temperature and shorter overall baking time compared to the conditions in Experiment 1, which resulted in a lower fryer temperature and longer baking time. Similar conditions were observed in other experimental tests of different solar fryer models.
Overall, it was observed that the availability of high solar radiation at the application site led to a shorter initial heating time and hence it was possible to continue cooking even at relatively lower fryer plate temperatures. Furthermore, such solar radiation intensity also led to smaller effects on the temperature drop of the fryer plate, leading to shorter reheating times and a lower requirement for the fryer plate temperature to rise during the subsequent reheating phases. On the contrary, if the available solar radiation intensity was relatively lower, the performance of the fryer plate was lower, as indicated by relative performance indicators of a lower plate temperature, longer heating time, a higher temperature drop at the end of each baking phase, longer reheating times, and a higher temperature rise requirement for the plate during the reheating processes.

3.4. Baking Quality Assessment of the Solar Fryers

The quality of the baked injera is shown in Figure 11. The injera exhibited excellent quality in terms of physical properties, such as size (thickness), color, eye formation, underside appearance, texture, and taste. Overall, the quality standard was found to be comparable to the injera baked using conventional methods.

3.5. Estimated Capital Cost of the Direct Solar Fryer System

The estimated capital costs of the direct solar fryer system are outlined in Table 1. These costs pertain to the direct fryer system with a 1.8 m-rim-diameter collector and a 40 cm-diameter fryer plate. However, the cost increase when the fryer plate size increases to 50 and 55 cm in diameter is small compared to the total cost. Additionally, other costs, such as installation and operational costs, are considered minimal in comparison to the capital cost, as the system is intended for small-scale household applications. Solar fryers utilize cost-free sunlight as an energy source, resulting in negligible energy costs, unlike conventional cooking methods that rely on biomass, electricity, or gas, which accrue additional energy costs. Furthermore, maintenance costs for solar fryers are generally low, with regular cleaning and occasional repairs (e.g., fixing broken reflectors or seals) being the primary operational expenses. Therefore, the capital cost can be considered the main component of the total investment required for the system.
These costs are approximately 4-8 times higher than the costs of a conventional electric stove for injera baking applications. These investment costs pose a challenge to the economic feasibility of the solar fryer system application. Although these investment costs are high, the system can be competitive in terms of overall economic benefits, which take into account not only the initial investment cost but also other relative factors, such as operational costs, fuel savings, environmental impact, health benefits, economy of scale, shared household utilization and government support and incentives. Furthermore, the economic feasibility of such a solar energy-based system is better evaluated based on the system’s feasibility on annual cycles, as solar radiation potential varies throughout the year.

4. Conclusions

This research experimentally demonstrated the feasibility of using solar frying for injera baking applications by utilizing large-sized solar fryers. The experimental testing evaluated the relative performance of different solar fryer systems under varying site solar radiation conditions. It was found that the 40 cm-sized solar fryer models allowed for the baking process to be carried out at relatively lower temperatures compared to the larger-sized solar fryers, in both modes of application. Consequently, the 40 cm-sized fryer matches the 2 kW heating capacity from the 1.8 m parabolic collector, hence this solar fryer maintained a consistent average baking plate temperature distribution throughout the subsequent baking phases. Furthermore, the continuous-mode 40 cm-sized solar fryer system facilitated uninterrupted baking without the requirement of additional reheating phases between the baking cycles, enabling a continuous baking process.
Comparing the relative performance of the two application modes, the alternating mode is more convenient for the 40 cm medium-sized solar fryers than the larger-sized models. For larger-sized solar fryers used in alternating modes, such systems need to be heavier (i.e., larger fryer thermal storage capacity), coupled with larger-size solar concentrators. This added complexity results in higher system costs and reduced simplicity due to the combination of a larger fryer and solar concentrator.
Furthermore, it is evident that the alternating-mode solar fryer system consumes more energy compared to the continuous-mode system due to the longer operational time for initial heating, baking, and intermediate reheating phases. Consequently, the alternating-mode system was accompanied by lower thermal efficiency, as indicated by the relative temperature profile and time during the initial heating, baking, and reheating phases. Similarly, larger-sized fryers were accompanied by lower thermal efficiency due to a longer operational time as compared to the 40 cm-sized solar fryer systems with the same operational mode. The reduced thermal efficiency in larger-sized fryer systems is further attributed to the need for a larger baking plate (55 cm in diameter) in order to meet the larger injera size requirement. While larger solar fryers meet the traditional injera size requirements, they are less efficient and have longer baking times. In contrast, the 40 cm-sized solar fryers are more efficient but may face social acceptance issues due to their smaller injera size.
The direct solar fryer system is suitable for single-household use but is more cost-effective when shared among 3 to 4 households. However, it is not practical for larger shared use. In conclusion, these direct solar fryer systems present a promising alternative technology, particularly in rural areas where biomass fuels are commonly used. Ethiopia, with its abundant sunshine throughout the year, is well-suited for solar frying applications, as demonstrated by the experimental tests showing solar radiation intensity of approximately 900 W/m2. Solar frying for injera baking can be a sustainable method that reduces reliance on biomass fuels, like firewood or charcoal, thus helping to mitigate deforestation and air pollution. Once the initial investment in a solar fryer system is covered, its operational costs are minimal, making it a cost-effective solution in the long term. Moreover, solar frying can improve health by reducing indoor air pollution and respiratory issues associated with traditional cooking fuels.
This study focused on the technical feasibility of using a direct solar fryer system for baking injera. Additionally, the study estimated the investment cost for the application of a direct solar fryer system. However, future research on such systems can explore the overall economic benefits, taking into account not only the initial investment cost but also other relative factors, such as operational costs, fuel savings, environmental impact, health benefits, economy of scale, shared household utilization, and government support and incentives.

Author Contributions

M.H.H. contributed to the conceptualization, methodology, software, validation, formal analysis, investigation, resources, data curation, and writing—original draft preparation. M.B.K., A.H.T. and O.J.N. contributed to the writing—review and editing and supervision. All authors have read and agreed to the published version of the manuscript.

Funding

The authors would like to acknowledge and thank the Norwegian Agency for Development Cooperation NORAD under the Enpe 2013–2020 project (EnPe capacity 5: Capacity Building in Renewable Energy Education and Research) for providing the funding for this study.

Data Availability Statement

The original data presented in this study are included in the article; further inquiries can be directed to the corresponding author.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. Geometrical description of the direct solar fryer with continuous heating and baking application modes. A more detailed description of the geometrical design of the system is also reported in the authors’ previous work [32].
Figure 1. Geometrical description of the direct solar fryer with continuous heating and baking application modes. A more detailed description of the geometrical design of the system is also reported in the authors’ previous work [32].
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Figure 2. Geometrical description of the direct solar fryer with alternating heating and baking application modes. A more detailed description of the geometrical design of the system is also reported in the authors’ previous work [32].
Figure 2. Geometrical description of the direct solar fryer with alternating heating and baking application modes. A more detailed description of the geometrical design of the system is also reported in the authors’ previous work [32].
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Figure 3. The direct solar fryer prototype with an alternating application mode: (a) the system in the heating mode and (b) the fryer in the baking mode.
Figure 3. The direct solar fryer prototype with an alternating application mode: (a) the system in the heating mode and (b) the fryer in the baking mode.
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Figure 4. Experimental setup for temperature, solar radiation, and mass measurements.
Figure 4. Experimental setup for temperature, solar radiation, and mass measurements.
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Figure 5. Experimental results of the heating and baking temperatures for the 40 cm-diameter solar fryer system with alternating heating and baking application modes. The experiment was conducted on 23 February 2017, during which 14 injera were baked. Similar experimental tests were also conducted on 20–22 February 2017.
Figure 5. Experimental results of the heating and baking temperatures for the 40 cm-diameter solar fryer system with alternating heating and baking application modes. The experiment was conducted on 23 February 2017, during which 14 injera were baked. Similar experimental tests were also conducted on 20–22 February 2017.
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Figure 6. Experimental results for the heating and baking temperatures of the 40 cm-diameter solar fryer system with alternating heating and baking application modes. The experiment was conducted on 18 November 2017, during which 16 injera were baked. Similar experimental tests were conducted on 20–22 February 2017.
Figure 6. Experimental results for the heating and baking temperatures of the 40 cm-diameter solar fryer system with alternating heating and baking application modes. The experiment was conducted on 18 November 2017, during which 16 injera were baked. Similar experimental tests were conducted on 20–22 February 2017.
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Figure 7. Experimental results for the heating and baking temperatures of the 40 cm-diameter solar fryer system with continuous heating and baking application modes. Experiment 1 was conducted on 1 March 2017, during which 10 injera were baked. Experiment 2 took place on 18 June 2017, with 15 Injera baked during the testing.
Figure 7. Experimental results for the heating and baking temperatures of the 40 cm-diameter solar fryer system with continuous heating and baking application modes. Experiment 1 was conducted on 1 March 2017, during which 10 injera were baked. Experiment 2 took place on 18 June 2017, with 15 Injera baked during the testing.
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Figure 8. Experimental results for the heating and baking temperatures of the 50 cm-diameter solar fryer system with continuous heating and baking application modes. The experiment was conducted on 11 February 2017, during which 14 injera were baked. Similar experimental tests were also conducted on 10 and 13 February 2017.
Figure 8. Experimental results for the heating and baking temperatures of the 50 cm-diameter solar fryer system with continuous heating and baking application modes. The experiment was conducted on 11 February 2017, during which 14 injera were baked. Similar experimental tests were also conducted on 10 and 13 February 2017.
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Figure 9. Experimental results for the heating and baking temperatures of the 55 cm-diameter solar fryer system with continuous heating and baking application modes. Experiment 1 was conducted on 16 March 2017, during which 10 injera were baked. Experiment 2 was conducted on 17 June 2017, with which 10 injera were baked during the testing.
Figure 9. Experimental results for the heating and baking temperatures of the 55 cm-diameter solar fryer system with continuous heating and baking application modes. Experiment 1 was conducted on 16 March 2017, during which 10 injera were baked. Experiment 2 was conducted on 17 June 2017, with which 10 injera were baked during the testing.
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Figure 10. Measurement results for the solar radiation intensity during an experimental test of the 55 cm-diameter sized solar fryer system with continuous heating and baking application modes. Experiment 1 was conducted on 16 March 2017. Experiment 2 was conducted on 17 June 2017.
Figure 10. Measurement results for the solar radiation intensity during an experimental test of the 55 cm-diameter sized solar fryer system with continuous heating and baking application modes. Experiment 1 was conducted on 16 March 2017. Experiment 2 was conducted on 17 June 2017.
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Figure 11. Results of the qualitative assessment of the solar frying process in terms of injera quality (i.e., physical property of size (thickness), color, eye formation, and underside appearance).
Figure 11. Results of the qualitative assessment of the solar frying process in terms of injera quality (i.e., physical property of size (thickness), color, eye formation, and underside appearance).
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Table 1. Estimated capital costs of a direct solar fryer system.
Table 1. Estimated capital costs of a direct solar fryer system.
Cost ComponentDescriptionQuantityPrice (USD) 1
Supporting structureRHS pipe, 50 mm × 50 mm × 1.5 mm × 6 m2 pc43.33
CHS pipe, 50 mm × 1.5 mm2 m11.67
Sheet metal, 0.88 mm0.4 m29.17
Reflective materialReflective glass, 4 mm thick5.34 m270.5
Collector dish1.8 m rim diameter, 64.8 cm focal length1 pc37.5
Fryer plateCasting, 45 cm diameter, 3 cm thick1 pc37.5
Machining process1 pc41.67
Insulation materialLocally available materialLS4.17
ManufacturingSupporting structureLS25.0
Reflective material laminationLS25.0
System assemblyLS25.0
Sub total330.51 USD
1 Exchange rate to Ethiopian birr: 120 birr = 1 USD [40].
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Hailu, M.H.; Kahsay, M.B.; Tesfay, A.H.; Nydal, O.J. An Experimental Comparative Study of Large-Sized Direct Solar Fryers for Injera Baking Applications. Energies 2024, 17, 4949. https://doi.org/10.3390/en17194949

AMA Style

Hailu MH, Kahsay MB, Tesfay AH, Nydal OJ. An Experimental Comparative Study of Large-Sized Direct Solar Fryers for Injera Baking Applications. Energies. 2024; 17(19):4949. https://doi.org/10.3390/en17194949

Chicago/Turabian Style

Hailu, Mesele Hayelom, Mulu Bayray Kahsay, Asfafaw Haileslassie Tesfay, and Ole Jørgen Nydal. 2024. "An Experimental Comparative Study of Large-Sized Direct Solar Fryers for Injera Baking Applications" Energies 17, no. 19: 4949. https://doi.org/10.3390/en17194949

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