A two-stage thermoelectric cooling system is to be designed to maintain a small insulated chamber at 0°F when ambient temperature is 90°F. You may assume a cold-plate temperature of –15°F, intermediate-plate temperature of 60°F, and a warm-plate temperature of 105°F. Estimated cooling load is 120 BTU per hr. You may assume cylindrical elements having a length of 1.00 cm and a diameter of 1.00 cm. Electrical contact resistance r may be assumed as 0.0001 ohm-cm². Each stage is designed for maximum refrigerating capacity. Determine: (a) The number of couples required for each stage; (b) The required output capacity of the d-c power supply unit.

I know how to find thermal conductance= U electrical resistance= R fig of merit= Z I can also find I current and q cooling capacity but the I dn't know which temperature to take and which load to take to find the no of couple?