1 Introduction

In 2024, there is 750 million people in the world living without electricity access, while 2.3 billion people continued to rely on harmful and polluting cooking fuels such as charcoal, wood, agricultural waste and animal dung—the use of which is a leading cause of premature death and serious health issues in many of the poorest regions of the world [16]. Injera is a staple food in Ethiopia and is made mainly from teff and baked using an injera baking stove. The main energy sources in the country are biomass bioenergy (approximately 88%), petroleum fuel 10%, and hydropower electricity 3% [2]. Traditional injera baking stoves consume a high amount of wood for a single injera baking session, low thermal efficiency and high specific fuel consumption of 929 g/Kg. The thermal efficiency of traditional wood-fired injera baking stoves is 5–10% [1]. Three-stone open-fire stove as 80 ppm for CO and 1.10 µg/m3 for PM and have severe consequences on human health and the environment at large [14].

Studies have however found that household PM2.5 concentrations among users of LPG often remain as high as 40–60 µg/m3, presumably mainly due to the community pollution from neighbouring households using solid fuels. It is therefore stipulated here that exposure levels associated with cooking with LPG are on average 50 µg/m3 and low compared to 120 µg/m3 wood fired stove [15].

As Ethiopian rapid population growth and economic development of the last decades, the impact on the environment is much greater today than it was in the past. Natural resources are becoming scarce, and Ethiopia faces alarming rates of deforestation. In many regions, the current demand for forest resources outstrips the available woodlands, from 2001 to 2023, Ethiopia lost 504 kha of tree cover, equivalent to a 4.2% decrease in tree cover since 2000, and 244 Mt of CO₂ emissions [4]. The most important driver of deforestation, despite a recent decline, is household energy consumption followed by wood-based construction techniques, agricultural schemes, and overgrazing. Estimates from the World Bank suggest stove efficiencies for LPG at 42–70% while Firewood has been estimated at 23–40% for traditional cooking stoves and the energy Content of LPG is 45.5 MJ/KJ which is the largest of all other energy source such as fuel wood, biogas, Crop residue [3].

Ethiopia is one of the least developed countries in the world. In February 2024, the Ethiopia Country Climate and Development Reports (CCDR) was released, sharing findings regarding the increasing impact of climate change that is threatening Ethiopia’s development prospects. The report notes that annual average losses to gross domestic product (GDP) are expected to range between 1 and 1.5% of GDP and to rise to 5% by the 2040s, potentially pushing millions more Ethiopians into poverty [6]. Firewood has the highest carbon emission of 1,170.57 g/J, its take more time (9.43 min) and 354.29 g of fuel consumed to boil a litre of water. LPG took lesser time (5.43 min) to boil the water and emit 34 g/J of carbon. Hence burning firewood releases approx. 5 times more CO2 emissions than LPG [5].

Thus the objective of this study is to design and conduct an experimental investigation of injera baking stove using LPG to reduce exposure to household air pollution and environmental impact by providing clean, reliable and affordable stove to exposed populations. Here, in the present work, LPG injera baking stove is designed with circular ring burners with multiple flame ports to distribute heat to all parts of baking pan. The three concentric circular burner heads facilitate combustion and allow the distribution of heat through the plate. The burner is designed to ensure even heat distribution through all parts of the baking pan. The stove is manufactured from mild steel of 1.5 mm to withstand combustion temperature [8]. Clay Injera baking pan of 450 mm diameter and 15 mm thickness is used in this study. Hence, reducing baking pan thickness from conventional size to 46 cm diameter and 15 mm thickness will result to reduce the heating time and hence enhance performance of stove. The result of experiment showed the designed injera baking stove has been tested and can bake injera of 440 mm diameter.

2 Material and method

2.1 Material

The main materials required for this particular research work are 1.5 mm sheet metal, gypsum, clay pan, welding rod, Digital Weighing Scale, Thermal camera, LPG Cylinder, Plastic Tank for butter handling during baking. During the experiment, different instruments were used for experimental investigation as shown in Fig. 1. And explained below.

Fig. 1
figure 1

Instrument and material used

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Digital Weighing Scale: To measure the mass of the batter.

Thermal camera: To measure heat distribution throughout baking pan.

LPG Cylinder: To supply fuel for combustion.

Baking Pan: The material on which the baking of injera is carried out.

Plastic Tank: for batter handling during baking.

Sefied: Is used to remove injera from the baking pan.

2.2 Design of LPG injera baking stove

These design parameters are based on the following data indicated on Table 1.

Table 1 Input data for design of injera baking LPG stove
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The time taken to bake a single injera is 3 min (Dengeto, A. & Aman, A., [9]) and an idle time of 2 min is recorded between successive baking cycles.

The total time is the time taken during a single baking session and calculated as:

$${\text{T}}t{\text{otal }} = {\text{ T}}inh{\text{eating }} + \,n\, * tbak{\text{ing }} + \, (n - { 1})$$
(1)

Input energy can be calculated by multiplying the volume of gas consumed with the calorific value of the gas by measuring the volume of gas consumed. The power consumption of this clay mitad is 3.5 kw, including losses.

The Power needed, for 1 h of baking time is: 3600*3500 = 12.6 MJ/hr

$${P}_{in}={Q}_{gas}\times {Cal}_{gas}$$
(2)
$${Q}_{gas}=\frac{\text{Power Input}}{\text{Calorific value of biogas}}$$
$${Q}_{gas} =\frac{12.6\text{ MJ}/\text{hr}}{25560\text{MJ}/m3}= 0.49 L/hr$$

2.3 Description of injera baking LPG stove

Injera baking LPG Stove which has been considered in the present study is demonstrated in Fig. 2. The stove consists of a burner head where the LPG fuel is burned by mixing with air. The major components of the stoves are a burner, stand, gas, and air supply line. Its burner is a multi-holed circular burning flame port head type to allow uniform distribution of heat. The designed LPG stove burner has a 10 cm diameter radially drilled cylinder in the center and three concentric circle burner heads placed at a distance of 7 cm from each other and are used for the combustion of LPG and to achieve even heat distribution through the baking pan. The distance between the baking pan and the stove burner is 30 mm, and the gap between each of the ports is 5 mm.

Fig. 2
figure 2

LPG injera baking stove

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The cylindrical bottom part of the stove allows air supply to the burner head and is made of 2 mm thick sheet metal. The upper part of the stove and air inlet sections are insulated using 50 mm thick gypsum and covered with metal sheets. The injera baking cover is a standard item made of aluminium with a wooden holder at the edge. The pan seat is on the top of the burner to mount the baking pan. The baking seat is insulated by a 50 mm gypsum Insulator.

Clay Injera baking pan of 450 mm diameter and 15 mm thickness with thermal conductivity of 5 W/m.K and specific heat capacity 0.83 kJ/kg.k is used. Clay has the properties of retaining heat for a long time. Reducing baking pan thickness will increase thermal conductivity hence reduce the heating time to enhance performance of stove. The thermophysical properties of clay \(=1.045 \frac{W}{{m}^{2}.k}\), \(C_{p} = 830{\raise0.7ex\hbox{$j$} \!\mathord{\left/ {\vphantom {j {kg.k}}}\right.\kern-0pt} \!\lower0.7ex\hbox{${kg.k}$}}\) and\(\rho =\frac{1900 kg}{{m}^{3}}\). Thermo-physical property of injera baking stove is indicated in Table 2.

Table 2 Geometry and thermo-physical property of injera baking stove [13]
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To ensure the best performance of LPG stove, the diameter of the burner head is made large enough equal to baking pan diameter to allow the distribution of heat. Gas flow control is connected to the gas bottle outlet valve on the LPG cylinder. It reduces the gas pressure to a usable level as it passes from the cylinder to a piece of burner, designed to establish system pressure and prevent the supply system from overconsumption of LPG.

2.4 Experimental work

The experiment was conducted to bake injera in Addis Ababa, Ethiopia using 12 kg LPG at room temperature of 24 ℃ is used. The baking pan was heated with an LPG flame and temperature was recorded at every 5-min interval by an infrared thermal imaging camera. The laboratory test performed to evaluate the stove performance was based on Control cooking test CCT [8] and the parameter explained in Tables 1 and 2 are used. Steady-state operation conditions and the applicable heat transfer equations were used to calculate the heat transfer in each unit. Burner flame stability test was conducted before baking starts. The test ensured the flame is stable and no flash back or flame lift as shown in Fig. 3.

Fig. 3
figure 3

Flame test

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Baking pan surface smoothing is required to remove injera from the baking pan easily as shown in Fig. 4.

Fig. 4
figure 4

Pan smoothing

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Injera baking pan is heated for 20 min to attain a working temperature of 234 ℃. The operation temperature was 234 ℃ to obtain a well-baked injera. After attaining working temperature, the thin dough is poured into a pan by container. The weight of the batter and injera were measured as shown in Fig. 5. The batter was prepared by mixing water with powdered teff, and this batter was fermented by adding yeast for 2–3 days to bake injera. [7].

Fig. 5
figure 5

Batter weighing

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2.5 Energy analysis of baking stove

For this study, total energy is the amount of energy used to bake a fixed quantity of Injera; it includes both the energy utilized and lost during baking process. Whereas, the utilized energy is the amount of energy used during the baking of Injera, not including any of the losses during the baking process. The average energy required for Injera baking includes the energy that is necessary to raise the temperature of batter from room temperature to boiling point and evaporate the required amount of water during the baking process. This useful baking energy it could be estimated in the form of sensible heat for heating of the batter from room temperature to water boiling temperature and latent heat responsible for evaporating some of the water content of the batter. The following assumptions are made in order to calculate the amount of utilized energy by a baking stove, the average mass of Injera and moisture loss for every single Injera is constant and the difference in mass between the baked Injera and the initial batter is equal to the mass of moisture loss during baking.

The design of the injera baking LPG stove starts with finding the energy demand using values illustrated in Table 3. Useful energy is a combination of sensible and latent heat used to evaporate water in the batter at the boiling point of water.

Table 3 Measurement values
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The thermal energy required to bake a single injera (E injera) is calculated based on Eq. (3)

$${\text{E}}_{{{\text{injera}}}} = {\text{ m}}_{{{\text{batter}}}} {\text{Cp}}\left( {{\text{Tboil}} - {\text{Troom}}} \right) + \left( {{\text{m}}_{{{\text{batter}}}} - {\text{m}}_{{{\text{injera}}}} } \right){\text{hfg}}$$
(3)

where minjera is the weight of injeras baked (kg), Cp is the specific heat capacity of water (kJ/kg K), Tboil is the boiling temperature of water (℃), Tbatter is the temperature of the batter (C), mevap is the amount of water evaporated during injera baking (kg), hfg is the latent heat of vaporization of water (kJ/kg K). The local boiling temperature of water in Addis Ababa is 92 \(^\circ{\rm C}\).

$$\begin{gathered} {\text{E}}\_{\text{injera}} = 0.{46}*{ 4}.{187 }*\left( {{92} - {24 }} \right) + \left( {0.{46} - 0.{32}} \right)*{226}0 \hfill \\ \,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\, = \,\,{447}.{4} {\text{kJ}} \hfill \\ \end{gathered}$$
(4)
$${\text{Rate of Heat Transfer to Injera Baking pan}}, Q = {\text{K}}A\Delta {\text{T}}/{\text{X}}$$
(5)

where: Q̇ = Rate of heat input (W), k = Thermal conductivity (W/m.K), x = material thickness parallel to heat flow (m), ∆T = Temperature change (K), and A contact area normal to the direction of heat flow (m2). From Eq. 11, it can be concluded that if the thickness of the injera baking pan decreases, the rate of heat flow to the pan increases. In this study thickness of the pan is decreased from a 20 cm conventional thickness clay pan to a 15 cm clay pan. The diameter of the baking pan decreased from 56 to 46 cm to improve heat flux and performance of LPG Stove. The rate of heat transfer per unit area is heat flux, and the average heat flux on a surface is expressed in Eq. 9.

$$\dot{q}\, = \,\frac{{\dot{Q}}}{A}{\raise0.7ex\hbox{$W$} \!\mathord{\left/ {\vphantom {W {m^{2} }}}\right.\kern-0pt} \!\lower0.7ex\hbox{${m^{2} }$}}$$
(6)

For energy consumption analysis additional positions are required, these are:

  • ✓ At the centre tip of the Aluminium lid cover.

  • ✓ At the bottom of the baking stove.

  • ✓ At the sides of the baking stove.

2.5.1 Heat transfer analysis for injera baking stove

The heat source of electric injera baking stove is the current flow through wire inserted into the bottom surface of the stove. Generally, heat flows from the baking plate to the different stove components and the product injera as shown in Fig. 6.

Fig. 6
figure 6

Heat flow through the stove

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2.5.2 Heat transfer from baking surface

During the baking process heat is transferred from the baking surface to the lid cover and surrounding through convection and radiation, respectively. The transferred heat through convection and radiation is lost heat. To determine the lost energy, it required to know their heat transfer coefficients.

2.5.3 Heat losses from the baking stove

The heat transfer mechanism of the Injera baking stove model is shown in Fig. 8. Considering an overall energy balance on the baking stove, the heat loss is given as follows.

Considering an overall energy balance on the Mitad system, the heat loss is

$${\text{Q}}_{{{\text{losses}}}} = {\text{ Q}}_{{{\text{rad}},{\text{top}}}} + {\text{ Q}}_{{{\text{conv}},{\text{top}}}} + {\text{ Q}}_{{{\text{rad}},{\text{bottom}}}} + {\text{ Q}}_{{{\text{conv}},{\text{bottom}}}} + {\text{ Q}}_{{{\text{rad}},{\text{side}}}} + {\text{ Q}}_{{{\text{conv}},{\text{side}}}}$$
(7)
$${\text{Q}}_{{{\text{cov}}}} = {\text{Ah}}\left( {{\text{T}}_{{\text{f}}} - {\text{ T}}_{{\text{s}}} } \right) = {\text{Ah}}\Delta {\text{T}}$$
$${\text{Q}}_{{{\text{rad}}}} = \varepsilon \sigma {\text{A}}\left( {{\text{Ts}}^{{4}} - {\text{ T}}\infty^{{4}} } \right)$$

2.5.4 Heat transfer from baking surface of the baking stove to the lid cover

For free convection heat transfer the Nusselt number relates the Rayleigh number and other parameters. In addition to that, the Nusselt number is defined as function of Rayleigh number, prandtl number, geometric shape and boundary condition. The air properties are used at film temperature in order to determine the convective heat transfer coefficient between baking surfaces of the baking stove to the lid cover. Convective heat transfer coefficient from the baking surface to the lid cover is given by [13]. hcvbc ¼ Nu k l ð1Þ where, Nu = Nusselt number. k = Thermal conductivity of evaporated water [W/m.K].

The Nusselt number for horizontal plate and uniform surface temperature for the calculated interval, the recommended correlation for the heated top surface is given as follows.

$${h}_{c}= Nu\frac{k}{l}$$
(8)

Nu = Nusselt number, k = thermal conductivity of evaporated water \(\left( {{\raise0.7ex\hbox{$W$} \!\mathord{\left/ {\vphantom {W {m.K}}}\right.\kern-0pt} \!\lower0.7ex\hbox{${m.K}$}}} \right)\)

2.5.5 Heat transfer from bottom surface

Heat transfer through this surface is by conduction through the insulation to the plate casing and consequently by convection and radiation from the bottom casing surface to the surrounding. Overall heat transfer coefficient could be simply calculated by using thermal resistance concept:

2.5.6 Heat transfer from side

The energy lost from the side of the baking stove may be calculated using similar procedure with the bottom surface. The overall heat transfer coefficient could be simply calculated by using thermal resistance concept:

The LPG stove was tested in LPG a room temperature of 24 ℃ and wind speed of 5 m/s. Steady-state operation conditions and the applicable heat transfer equations were used to calculate the heat transfer in each unit. Then, the convective heat transfer (\({h}_{D}\)) coefficient and convective heat loss (\({q}_{D}\)) for cylindrical shape were calculated according to Eq. (11).

$${h}_{D}=\frac{{Nu}_{D}k}{D}$$
(9)
$${q}_{D}={h}_{D}A({T}_{s}-{T}_{\infty })$$

2.5.7 Heat lost in the side of baking stove

The pan holding the upper part of the stove is cylindrical and made of mild steel with gypsum insulation as shown in Fig. 7. Assuming conduction heat transfer is the mode of heat transfer prevailing in the system, the following equation was used to quantify the heat absorbed in these units:

$$\text{Qcyl}=\frac{({T}_{hot}-{T}_{cold})}{\frac{ln(\frac{{r}_{2}}{{r}_{1}})}{2\pi KsL}+\frac{ln(\frac{{r}_{3}}{{r}_{2}})}{2\pi \text{Kg}L}+\frac{ln(\frac{{r}_{4}}{{r}_{3}})}{2\pi {k}_{s}L}}$$
(10)

where Thot is the temperature on the hot side (ºC), Tcold is the temperature on the cold side (ºC) and r1, r2, r3,r4 are the concentric radius of the internal diameter of different layers. Ks and kg are the thermal conductivity of steel and gypsum, respectively (W/mK). hi and h0 are the convective heat transfer coefficient. The length of the component is represented by L [10].

Fig. 7
figure 7

Cylindrical part of Stove

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3 Result and discussion

3.1 Injera quality and heat distribution

The heat was uniformly distributed in the pan, resulting in well-baked injera. weight, size, and texture of injera were also acceptable [7]. Injera size and weight are 44 cm and 320 g respectively which is similar to the market-sold injera. A picture of injera baked during the baking cycle is illustrated in Fig. 8. The baked injera is similar in texture. It is also removed from the baking pan surface easily (without sticking) and the small bubbly structures (eyes) were similar to those obtained in conventional baking pans. The quality injera is baked due to uniform heat distribution through baking pan.

Fig. 8
figure 8

Injera

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Heat distribution on the plate is depicted in Fig. 9. The thermal image of heat distribution through baking pan was captured for 20 min in the 5-min interval until the baking pan reached the optimum temperature for baking injera is 234 °C. According to the measured data of this kind of pan, the difference between the highest value and the lowest value of different measuring points in the pan is less than 3.3 °C which will increase the quality of Injera while the previous temperature difference in the baking surface is 55 °C as shown in Table 4. The result shows there is significant improvement in heat distribution and hence the new LPG injera baking stove can bake quality injera.

Fig. 9
figure 9

Heat distribution

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Table 4 Comparison of Temperature distribution on baking pan surface [11]
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3.2 Baking time and heating time

The average time to bake one injera was 3 min for the LPG stove is 3 min as shown in Table 5.

Table 5 Heating time, baking time, reheat time
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The time required to heat and bake injera is one of performance indicators of a stove. The time taken to heat the baking pan to the temperature of 234 ℃ was 20 min as shown in Table 6 while it is 25 min to heat to bake a single injera in a three-stone fire as shown in Table 7. The result shows reduction in heating time and improvement in efficiency of injera baking stove.

Table 6 Heating time
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Table 7 Performance comparison of this and traditional biomass stove (12)
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The time taken to bake one injera was 3 min. A reheat time of 2 min was recorded between each injera as shown in Table 5.

4 Conclusions

Design Manufacturing and Experimental Investigation of LPG Injera baking stove was carried out. Maximum temperature variation at points of the pan recorded is 3.3 ℃. Based on the result, the newly designed stove enables uniform heat distribution to the baking pan. A full-size injera 440 mm in diameter was baked successfully. The baked injera is good in texture and removed from the baking pan surface easily (without sticking on the baking pan).The study showed the improvement in design of burner and selection of efficient baking pan enabled LPG application for Injera baking applications where electricity is not accessible. Thus, LPG technology can improve the lives of millions of people living in rural societies and meet efforts to combat climate change and meet sustainable development goal (SDG7). LPG Injera baking Stove economic analysis is supposed to be researched further and disseminated to all rural and urban community with minimum cost to be affordable by low income community.