Energy Consumption Performance Analysis of Electrical Mitad at Mekelle City

The Injera baking electrical mitad is the most energy-consuming device in every household in Ethiopia. This research presents a detail engineering study on the energy-consumption performance of existing electrical mitad in Mekelle city. The research work considered thirty-one electrical mitad from different workshops in Mekelle. Depending on the number of clay used, there are three types of electrical mitad; the single clay, the double clay and the rotating type mitad. In this research, it was found out that the average power consumption of existing mitad is in the range of 3.5 KW to 3.9 KW. The specific average baking energy is 0.82, 0.73 and 0.54 kWh/kg of injera for double clay, single clay and rotating type mitad respectively while the thermal efficiency by considering baking of 6kg of Injera on each mitad is 38%, 47% and 61% respectively. In all designs, the baking temperature is in the range of 130-140 0C. Although the rotating type mitad is more energy efficient, it has technical limitations. Due to that, the single and the double clay mitad are widely used by households hence share all the market penetration of injera baking electrical mitad. These commonly used electrical mitad have an energy loss from 50-60% of the input energy.


INTRODUCTION
The household sector in Ethiopia accounts for about 89% of the total energy consumption and this sector also consumes about 96%, 15% and 33% of the total energy from biomass, petroleum and electricity respectively (GiZ, 2011). Studies show that domestic energy requirements are primarily for baking and cooking purposes; accounting for about 60% of households' energy consumption (Gebreegziabher, 2004(Gebreegziabher, , 2007RPTC, 1998), making the electric mitad as the most energyconsuming device in every household in Ethiopia.
Though the use of electricity for Injera baking is limited to urban dwellers, the Injera baking electrical mitad contributes to large energy consumption in the electricity supply system of the country (Dave, 2010). It is widely perceived that the efficiency for energy consumption of the existing electrical mitad is low arising from old design and its manufacturing defaults. The current mitad design dates back to 1960's when baking of Injera electrical mitad started with high-income groups in cities. However, since then, almost no design improvements have taken place (Dave, 2010). Furthermore, since electrical mitad is manufactured traditionally with untrained workers, it © CNCS, Mekelle University 44 ISSN: 2220-184X is expected that it will have lower energy consumption efficiency hence a wider room for improvement.
There are some studies on the existing electrical mitad (Dave, 2010;GIZ, 2011;Alula Gebresas and Asmamaw Tegegne, 2011;Awet, 2011;Gebreegziabher, 2004Gebreegziabher, , 2007RPTC, 1998) and on improved mitad design (Dave, 2010). A World Bank study report on existing injera baking mitad (Dave, 2010) discusses the different existing technologies (wood stove, electrical mitad) and some new emerging technologies (solar stove, the rotating type mitad, electro-induction mitad and automated electrical mitad in the USA). It also indicated a qualitative assessment of the energy consumption losses and improvement options of the existing designs (Alula Gebresas and Asmamaw Tegegne, 2011; Awet, 2011) have also studied the thermal conductivity of the clay material and its improvement. However, the existing studies lack detail engineering analysis on the energy consumption performance of the whole system as well as it components and indication of the different opportunities for improvement of energy consumption.
Even though it is inefficient, the apparatus has high market penetration and it is continuing at its current higher energy consumption and lower efficiency, subsequently higher monthly energy bill for the consumers and high energy 'waste' for the national grid. The utilization of efficient and improved appliances is one of the opportunities for the reduction of energy consumption and efficient utilization of the limited energy resource of the country. This will help to reduce the burden on the power plant as well as national grid, decreases monthly billing amount; these all subsequently contribute for the economic development of the country.
This paper is therefore, designed to fill the above gap and contribute for upcoming studies that intervene in the direction of developing improved injera baking electrical mitad. In line to this objective, this research has conducted a detail engineering study of the energy-consumption performance of existing electrical mitad in Mekelle city and indicated options of improvement.

METHODOLOGY
The

DESCRIPTION OF THE EXISTING ELECTRICAL MITAD
Depending on the number of clay used, there are three type of electric mitad in Mekelle city: the single clay, the double clay and the rotating type mitad, as described in figure 1

Experimental Setup
The experimental setup on each mitad is established to take temperature measurements at the different points of the mitad. The experimental set-up is similar for all types with minor difference on the rotating type; as discussed in the following section.

Common Setup for all Mitad
The measurement points for all mitad are indicated in figure 2. Temperature measurement has been taken at the following points, for all mitad:

Additional Test Setup for Single and Double Clay types
In addition to the above measurement points, temperature measurement for the single clay and the

Additional Test Setup for Rotating type Mitad
Similarly the measurement points at the different section of the rotating type mitad are:

Material and Equipment used in the Experiment
The other experimental setup also includes computer for recording of thermocouple data, balance to measure the weight of the injera and the data logger to connect the thermocouple with the

Modeling the Baking Process Heat Transfer Mechanism of Mitad
The energy transfer from heating element to the baking clay, side and bottom insulations is through conduction heat transfer. On the other hand the stove experienced a considerable natural convection and radiation heat losses from its surfaces to the surrounding room.  process and the remaining heat is transferred loses to the atmosphere. The heat loss of the system has tried to reduce by using an insulation in tis side and bottom part. The governing equation for the three types of heat transfer mechanism have represented by basic heat transfer equations.
From basic heat transfer principle, the thermal and electrical resistance network of the mitad by considering the mitad construction components from left to right (horizontal plane sectioning) is represented in figure 7.

The Useful Energy of the Baking Process
The average energy required for Injera baking implies 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 if estimated in the form of sensible heat for heating-up 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 net useful energy in Injera baking has modeled mathematically in equation1.

Heat Losses of the Baking Process
The heat transfer mechanism of the mitad is modeled in Figure 8. Considering an overall energy balance on the Mitad system, the heat losses has calculated using eq.2: The governing equation for the Convection heat transfer and the radiation heat transfer mechanism can be represented by basic heat transfer equations (Adrian, et al., 2003;Frank, et al., 2011;John, et al., 2008;Incropera, et al., 2007;Yunus, 2002) as:

Input Energy of the Baking Process
The heat generated by the heating system, Qheat is the input energy for the baking process. This input energy is a function of voltage (V) and the current (I) flowing across the heating element, this relation is given in eq.4.

Thermal Efficiency of the System
The system thermal efficiency is the ratio of net useful energy utilized to the gross energy supply, which is computed by using equation 6.

Thermal efficiency=
Usefull energy utilized Gross input energy required

RESULTS AND DISCUSSION
In this research, the energy consumption characteristics of the existing electrical mitad in Mekelle city is determined and described by determining its baking power, baking temperature, baking process temperature profile, useful energy consumption, energy loss and thermal energy efficiency. These results are discussed below.

Baking Power of the Existing Mitad
The measurements taken to investigate the baking power of the existing mitad by considering thirty-one electrical mitad from different workshops in Mekelle and Mekelle University main campus student cafeteria as presented in table 2. The line voltage was in the range of 207V to 214 volts, therefore, for an average line voltage of 210V, the average power requirement, the Baking Power, of existing electrical mitad in Mekelle city is in the range of 3.53kW to 3.95kW. Therefore, as per the existing rating of the conventional equipment, the electrical mitad is the most energyconsuming device in every household.

Baking Temperature
An investigation of the baking temperature of the electrical mitad is an important indicator about the energy consumption behavior of the equipment. This temperature reading is taken from the top  surface of the baking pan. The temperature profile for the baking process in the single clay type mitad is indicated in figure 9. It can be observed that the baking temperate is in the range of 130 -140 0 C. The remaining curves in figure 9 shows the room temperature (near mitad) and the batter temperature. Similar temperate range has been observed in other type of mitad i.e. the double clay and rotating type mitad.

Baking Process Temperature Profile
The baking process temperature profile is another important indicator about the energy consumption behavior of the equipment. The temperature profile of the mitad is determined into two conditions: with no load and with load conditions.

No Load Temperature Measurements
This was done to determine the energy requirements for heating-up, heating rate and time. The heating up temperature development of the three type of mitad is indicated in figure 10.
It can be clearly understood that the rotating type electrical mitad has a faster heating-up period than the others. This faster heating rate is attained as the heat source in this type of mitad is sandwiched between the two baking clays as a result it would likely has minimal losses hence contributing to faster heating up of both clays. The thickness of the clays, which are 14mm for the main clay (heating element holder) and 10mm for dependent clay have also contributed to relatively faster heat up of the clay surfaces. The single clay type mitad has better heating-up speed than the double clay mitad.
The average energy consumption for the baking process to start is summarized in table 3. It has been observed that the double layer mitad has the slowest response time and the highest energy consumption while the rotating type has attained the required temperature with in short period of time and relatively smaller energy consumption. Therefore, analogous to the temperature development, the rotating type mitad consumed the least energy (for two baking clay) as compared to the other type of mitad.

With Load Temperature Measurements
The temperature profile measurement is done while conducting the baking process on each type of mitad. Figure  The non-uniformity of the heating likely occurs as a result of the inaccuracy of the manufacturing process of the mitad itself. Such manufacturing error includes the difference in the depth of the groove the electrical resistor is embedded and the difference in density of the resistor rolling across its length. Furthermore, it indicates that even though the baking surface has reached the required baking temperature at early period of the heating up process (i.e. after 1200sec from starting); the temperature of the heating element is increasing which means the system is consuming additional power from the source irrelevantly (though some power will be consumed for increasing the temperature of the batter during the baking process). As a result, that additionally consumed energy is being dissipated in the form of different losses such as side loss, bottom loss and top losses. and rotating type mitad respectively. The general approach in these mitad is also similar to that of the singly clay mitad. As compared to the single clay mitad, it can be observed that the double clay mitad reaches the baking temperature relatively slower (i.e. 24 minutes) while it also consumes higher energy. Furthermore, heating non-uniformity and temperature increment during baking process shows similar behavior with that of the single layer mitad, but with slower rateIn the rotating type mitad, the non-uniformity of heating in a given clay (i.e. either the dependent clay or the heating clay) is not significant as compared to the other two types of mitad discussed above.
The baking surface attains the required baking temperature (130 O C) faster as compared to the other two mitad, with the time requirement of 15minutes. In this type of mitad, the problem of different heating patterns of the two baking clays was identified, the first clay (heating element holder) heats-up faster than the second (dependent clay), which created an overheating problem to the first clay and forces to turn OFF the power source for a while during the testing period. Even though this problems can be resolved by using proper thickness (i.e. proportionally varied thickness) of the two clays so that it will deliver uniform heating on the baking surface, but it is not widely addressed by the mitad manufacturers.
As the baking process is started (i.e. opening of the coverlid and staring of baking), the motion has resulted measurement irregularities in thermocouples, as indicated in figure 11, seen after the power is turned off or starting of baking in each curve for the three type of mitad.

Energy Consumption Performance
The energy consumption of the different mitad is measured by undertaking a baking process of 8 to 13 pcs injera. The measurement data from the performance testing is indicated in   be seen that while all mitad has relatively the same power rating (i.e. 3.5KW), the rotating type mitad consumes the least energy for heating up (1.10 KWh) as well as for the baking process (1.90KWh) for 13pcs of injera as compared to the other type of mitad. The time for heat up and baking is much shorter for the rotating type than the other type of mitad.
The summary of the analysis on the energy consumption of the three type of mitad is indicated in table 5. The heat loss on the double clay type is 2.35 times higher than the rotating type and about 1.5 times higher than single clay type. The single clay type has better performance than double type, however, the heat loss with respect to rotating type is about 1.5 times higher. The heat loss from rotating type is less than the other types, this implies it has a good benefit in view of energy conservation but it has technical complexity during operation.
The total useful energy, that is the sum of total Sensible heat gain and Latent heat gains, will only depend on the number of Injera baked, not on the type of mitad used. The average specific useful energy consumption is an important parameter for designing of any injera baking system and this value is determined to 480 KJ/pc injera (with a variation of ±2 %) or 1180 KJ/Kg injera (with a variation of ±5 %).
Since, the number of Injera or total masses of Injera baked at each types of the mitad are different, the energy consumption comparison shall be normalized for the same number or amount of injera baked. Thus for purpose of normalized comparisons, the initial energy consumption data is normalized for a consumption of 12 pcs or 6 kg of injera, as presented in difficult to get aesthetically attractive injera. There was also a problem of unequal heating of both clays hence it created additional challenge to operate both clays steadily. The energy consumption analysis presented in table 5 is an average energy consumption characteristic of a typical mitad, with an average mitad capacity of 3.57, 3.52 and 3.62 KW for double clay, single clay and the rotating type mitad respectively. As different mitad have slightly varying capacity, the performance analysis characteristics will show slight variation but the general trend for all performance parameters remain the same. A similar extensive analysis on a double clay, single clay and rotating type mitad with slightly different capacity of 3.71, 3.74 and 3.62KW respectively have a thermal efficiency of 38.3, 49.5 and 64.2% respectively.
The energy consumption performance indicators of a given mitad will also depend on the life cycle utilization of the mitad i.e. older mitad will have lower performance than new mitad. Even though life-cycle energy-consumption performance analysis gives more accurate indicator, it is expected that the mitad performance will deteriorate as the respective mitad gets older. Therefore, the performance indicator such as the baking efficiency from the above analysis are expected to be the maximum limitations. Therefore, the above analysis and its resulting performance indicators are justifiable from cost perspective. Similarly as the number (pcs) or amount (Kg) of injera-baked increases, the thermal efficiency of a given mitad will also increase as the heating up energy consumption is the same. In general, it can be noted that the energy consumption performance of existing mitad design is low, with average efficiency of 40, 50 and 65% for double clay, single clay and rotating type mitad respectively.

Energy Loss Analysis
The energy losses that are extracted from the gross and the useful energy consumption analysis discussed in the above sections for each of the existing design types is revised and presented in has found out that electrical mitad usually function for about 3 to 5 years without the requirement of maintenance. Therefore, the wide perception of minimal maintenance cost over that of energy saving in mitad manufacturers and users is wrongly perceived and widely practiced. rotating type mitad respectively. Therefore for a higher thermal efficiency requirement (for example 80%), the electrical mitad design shall have a power rating in the range of 1.6-1.9 KW.
As presented with the temperature profile of the different components of the mitad in figure 11, most of this heat is also lost through the bottom insulation. Therefore, the utilization of effective insulation material is expected to reduce energy loss of the system and hence enabling to design lower power rating of the electrical mitad (i.e. 1.6 to -1.9 KW capacity).

CONCLUSION AND RECOMMENDATION
From the assessment study of the existing mitad designs in Mekelle city, the electrical mitad which has a power consumption of 3.5kW to 3.9kW is the most energy-consuming device in every household. The specific average baking energy is 0.82 kWh/kg, 0.73 kWh/kg, and 0.54 kWh/kg for double clay, single clay and rotating type mitad respectively. The thermal efficiency comparisons by considering baking of 6kg of Injera on each of mitad is 38%, 47%, and 62% respectively for double clay, single clay and rotating type of existing designs.
Because of its technical limitation, the energy efficient rotating type mitad does not have wider use thus, those widely electrical mitad in every household are either the single or the double clay mitad. These widely used electrical mitad have energy loss from 50-60% of the input energy. This loss is significant. Even without design improvement options, the existing single clay mitad would have an energy saving of at least 35 KWh/per year per person over that of the double clay mitad.
Therefore, the energy saving that can be obtained by just choosing the existing single clay mitad over that of the double clay mitad would have significant impact at every household in particular and the national grid in general. However the awareness for energy saving is very low at all level, from the mitad manufacturers to end user. This needs be addressed with implementation of appropriate policies and standardization of more efficient mitad at local manufacturing workshops.
The significant losses in the current electric mitad can also be minimized by applying different design improvements. The possible options for the improvement of the electric mitad would be attained from the reduction of the thickness of the baking clay, thermal conductivity improvement works of the baking clay, the insulation thickness increment, selecting a better and cheap thermal insulation material and proper dimensioning of the electrical resistor. An improvement in one or a combination of the above options would significantly reduce the losses and improve the efficiency of the mitad. Other improved innovative system, such as metal plate based baking system, solar stove etc. may also improve the overall performance of the injera baking process.

ACKNOWLEDGEMENTS
The : Temperature of ambient air (baking room) hr,b-a : Radiation heat coefficient from bottom insulation to ambient air hc,b-a : Convection heat coefficient from bottom insulation to ambient air hr,l-a : Radiation heat coefficient from covering lid to ambient air hc,l-a : Convection heat coefficient from covering lid to ambient air hr,p-l : Radiation heat coefficient from hot clay to aluminum covering lid hc,p-l : Convection heat coefficient from hot clay to aluminum covering lid ΔXclay : Thickness of hot clay (Baking pan) ΔXinsu : Thickness of Insulation from the bottom of the mitad Kclay : Thermal conductivity of hot clay (baking pan) Kinsu : Thermal conductivity of insulation material, Qrad,topheat loss through radiation from top surface of the mitad (cover lid) Qconv,topheat loss through convention from top surface of the mitad (cover lid) Qrad,bottomheat loss through radiation from bottom surface of the mitad (insulation bottom) Qconv,bottomheat loss through radiation from bottom surface of the mitad (insulation bottom) Qrad,sideheat loss through radiation from side surface of the mitad (insulation side) Qconv,sideheat loss through radiation from side surface of the mitad (insulation side) Eusefuluseful energy requirement mbatter -is the mass of the batter expected for one Injera Tboil -is the boiling temperature of water Troom -is the room temperature in the baking pan test room Cp.batter -is the heat capacity of batter (mixture of water and Teff flour) minjera -is the mass of the Injera baked hvaporization -is the heat of vaporization of water (hfg) Qconv -Rate of convection heat transfer h -Convection heat transfer coefficient (W/m 2 o C) A -Surface area where convection takes place (m 2 ) ΔT -Temp. difference between the fluid and the wall surface ( o C) Ts -Body surface temperature ( o C or K) T∞ -Surrounding or ambient temperature ( o C or K) ε -Emissivity of the surface and has a value between 0 and 1, for a perfect reflector ε = 0 and for a perfect emitter a so called "black body", ε =