3. a) i. Define thermal energy reservoir. [ 3 marks] ii. Name 2 thermal energy reservoirs. [2 marks] b) Natural gas liquefaction process is based on refrigeration cycles, which take warm, pretreated natural gas and cool it through cryogenic heat exchangers into a liquid product. The classical cascade process is one of such process that utilizes several refrigeration cycles having refrigerants that vaporize at different but constant temperatures. Consider the first cascade stage that cool the natural gas to 35C by using R134a as the refrigerant and cycle parameters as shown in Fig. Q3(b). Calculate: i. the mass flow rate of the refrigerant. [13 marks] ii. the minimum power input for the same refrigeration load. [7 marks]