A portable refrigerator is installed in a campervan. It maintains a temperature of 4C inside the cold compartment, and has a COP that is one third of the reversible limit. The ambient temperature is 28 C. The refrigerator draws 15 W of power and has an external area of 1.2 m (a) (b) (c) What is the rate of heat extraction from the cold compartment of the refrigerator? What is the thermal resistance of the refrigerator? Assuming planar conduction, what is the overall average R-value? (e) The refrigerator is powered by a portable petrol-powered electric generator with an efficiency of 19%. What fuel energy input (in watts) is required to power the generator when it is running only the refrigerator? (d) We will define the 'combined efficiency' of this overall system to be the cooling power (rate of heat extracted) from the refrigerator, divided by the fuel energy input. Calculate the combined efficiency of this system. Calculate the theoretical limit on the combined efficiency of this system, assuming the engine is effectively extracting heat from a thermal reservoir at 500 C.