Afriend offers to sell you an experimental stirling engine to power your house. the engine is guaranteed to last 15 years and is fueled by burning natural gas with a combined combustion and heat transfer efficiency of 92.0% and a generation efficiency of 97.0%. your friend tells you that the stirling engine has an efficiency of 55.0%. operations and maintenance costs total $0.01/kwh. assume natural gas costs will be $7.50/mbtu for the next 15 years and an interest rate of 6% and financing for 15 years. assume your house requires 22 kwh per day and you currently pay $0.18/kwh and you anticipate this will remain constant for the next 15 years. a. what is the overall efficiency of the stirling engine system? b. what is the cost of fuel per kwh? c. if you are willing to spend 80% of your current electrical costs on this system, what is the maximum you should pay for the stirling engine system?