The methodology is based an engineering model as shown in chart below that can illustrate the basic working approach.
2.1 Size and Specification of Baking Pan (Mitad)
A baking pan is a flat and circular pan commonly about 50 to 60 cm in diameter and traditionally used over large clay hearths to bake Injera. The baking pan ‘Mitad’ considered in this case was 5mm thick and 500mm in diameter. Due to reduced thickness, it has high thermal conductivity than the one which is available in the local market. Direct contact of the pan with the hot heat storage salt solution can cause cracking of the baking pan; therefore, the baking pan is separated from the heat transfer fluid by thin sheet metal cover.
2.2 Energy Required for Injera Baking pan
To measure the energy utilized in baking Injera, the initial mass of batter, and the total amount of Injera produced from this batter were measured. Thus, the mass of water vapor can be obtained by reducing the mass of Injera produced from the initial mass of batter. It is assumed that the energy utilized to bake the Injera is the energy required in raising the temperature of the batter from room temperature to the boiling point of water which is called sensible heat, plus the energy required to evaporate water which is called latent heat. Therefore, the utilized energy is:
𝐸𝑢𝑡𝑖𝑙𝑖𝑧𝑒𝑑 = 𝑚𝑏𝑎𝑡𝑡𝑒𝑟 × 𝐶𝑃𝑤𝑎𝑡𝑒𝑟 × (𝑇 𝑏𝑜𝑖𝑙 − 𝑇𝑟𝑜𝑜𝑚 ) + (𝑚𝑏𝑎𝑡𝑡𝑒𝑟 − 𝑚𝐼𝑛𝑗𝑒𝑟𝑎 ) × ℎ𝑣𝑎𝑝𝑜𝑟𝑖𝑧𝑎𝑡𝑖𝑜𝑛 (1) [8]
Where:
𝑚𝑏𝑎𝑡𝑡𝑒𝑟- The mass of the batter expected for one injera= 400 (g)
𝑇 𝑏𝑜𝑖𝑙-the boiling temperature of water = 94℃
𝑇𝑟𝑜𝑜𝑚 − 𝑡ℎ𝑒 𝑟𝑜𝑜𝑚 𝑡𝑒𝑚𝑝𝑒𝑟𝑎𝑡𝑢𝑟𝑒 𝑖𝑛 𝑡ℎ𝑒 𝑏𝑎𝑘𝑖𝑛𝑔 𝑝𝑎𝑛 𝑟𝑜𝑜𝑚, ≅ 25℃
𝐶𝑃 − 𝑡ℎ𝑒 ℎ𝑒𝑎𝑡 𝑐𝑎𝑝𝑎𝑐𝑖𝑡𝑦 𝑜𝑓 𝑤𝑎𝑡𝑒𝑟 = 4.187𝑘𝐽/𝑘𝑔. K 𝐼𝑛𝑗𝑒𝑟𝑎
𝑡ℎ𝑒 𝑚𝑎𝑠𝑠 𝑜𝑓 𝑡ℎ𝑒 𝐼𝑛𝑗𝑒𝑟𝑎 𝑝𝑟𝑜𝑑𝑢𝑐𝑒𝑑 = 320𝑔
ℎ𝑣𝑎𝑝𝑜𝑟𝑖𝑧𝑎𝑡𝑖𝑜𝑛 − 𝑡ℎ𝑒 ℎ𝑒𝑎𝑡 𝑜𝑓 𝑣𝑎𝑝𝑜𝑟𝑖𝑧𝑎𝑡𝑖𝑜𝑛 𝑜𝑓 𝑤𝑎𝑡𝑒𝑟
ℎ𝑓𝑔 = 2260𝑘𝐽/𝑘𝑔
2.3 Specification of thermal storage
The main important properties of thermal storage container material include; Excellent corrosion resistance at high temperature, high degree of chemical compatibility between the container material and PCM salt, Good mechanical properties like strength, creep and thermal fatigue resistance. For solar salt (60wt%NaNO3/40wt%KNO3) PCM storage corrosion resistance of stainless steels is better than that of carbon steels and other metals. Since, Stainless steel is an alloy of iron carbon and chromium, as it exposed to solar salt, chromium oxide layer is formed on the surface of the tank wall which prevents the material from corrosion. The small pits observed in the material is due to the rupture in this passive layer. Due to this passive layer the propagation of corrosion through formation of pits is minimum for stainless steel compared to other metals [9]. Due to this reason, stainless steel is used as PCM storage tank material for long-term utilization solar energy.
Table 1: properties of materials [10].
Material
|
Melting point (℃)
|
Rate of corrosion (𝑚𝑔⁄𝑐𝑚2. 𝑦𝑟)
|
Stainless steel
|
1400-1455
|
0.3004
|
Aluminum
|
660.4
|
0.9423
|
Copper
|
1085
|
10.9869
|
2.4 parabolic collector size specification
Several parameters are used to describe solar concentrating collectors. Given below are brief descriptions of some of these parameters: The aperture area (Aa) is the area of the collector that intercepts solar radiation and the absorber area (𝐴𝑎𝑏𝑠) is the total area of the absorber surface that receives the concentrated solar radiation. It is also the area from where useful energy can be extracted. The Concentration Ratio C is defined as the ratio of the aperture area to the absorber area and can be written as:
The instantaneous thermal efficiency of a solar concentrator may be calculated from an energy balance on the absorber. The useful thermal energy delivered by a concentrator is given by:
𝑞𝑢 = 𝜂𝑜𝐼𝑏𝐴𝑎 − 𝑈𝐿(𝑇𝑎𝑏𝑠 − 𝑇𝑎)𝐴𝑎𝑠 (3) [13] [14]
At higher operating temperatures the radiation loss term dominates the convection losses and the energy balance equation may be written as
𝑞𝑢 = 𝜂𝑜𝐼𝑏𝐴𝑎 − 𝑈𝐿(𝑇𝑎𝑏𝑠4 − 𝑇𝑎4)𝐴𝑎𝑏𝑠 (4) [15]