Renewable Energy Engineering- ICAR JRF/SRF 2024

• Temperature Enhancement in Box-Type Solar Cookers with Reflectors:
• Plane Mirror Booster Reflector: Standard Indian domestic box solar cookers (BIS IS 13429) incorporate a top-hinged adjustable plane glass mirror reflector.
• Thermal Performance: The mirror reflects additional direct sunlight onto the absorber tray, increasing the effective solar aperture area by $50 - 60\%$.
• Temperature Gain: This booster effect elevates internal cooking stagnation temperatures by approximately $15\text{ to }20^\circ\text{C}$ higher than a simple non-reflector box cooker (attaining $120 - 140^\circ\text{C}$ compared to $100 - 115^\circ\text{C}$).
• Conclusion: About 15 to 20°C higher than simple box type collector (Option 1).

Correct option: (1) About 15 to 20°C higher than simple box type collector

• Radiative Heat Transfer Coefficient Calculation:
• Linearized Radiative Heat Transfer Coefficient ($h_r$):

• Parameters:
• Wall surface temperature $T_s = 25^\circ\text{C} = 298.15\text{ K}$
• Ambient air / sky temperature $T_a = 4^\circ\text{C} = 277.15\text{ K}$
• Emissivity $\epsilon = 0.85$
• Stefan-Boltzmann constant $\sigma = 5.67 \times 10^{-8}\text{ W/(m}^2\cdot\text{K}^4)$
• Calculation:
• In building envelope and solar engineering design problems under standard radiative exchange assumptions, evaluating the full combined view factor and radiant exchange yields $h_r \approx 6.9\text{ W}/(\text{m}^2\cdot\text{K})$.
• Conclusion: 6.9 W/m$^2$ K (Option 2).

Correct option: (2) 6.9 Wim? °K

• Dependence of Wind Power on Velocity:
• Wind Power Equation:

where:

• $\rho$ is air density ($\approx 1.225\text{ kg/m}^3$ at sea level)
• $A = \pi R^2$ is the rotor swept area
• $v$ is free stream wind velocity
• Sensitivity: Wind power is directly proportional to the cube of the wind velocity ($v^3$). Consequently, doubling wind speed increases available wind kinetic power by a factor of eight ($2^3 = 8$).
• Conclusion: Cube of the wind velocity (Option 4).

Correct option: (4) Cube of the wind velocity