Question

4. (30 pt) A laboratory freezer maintains biological samples at -80 °C. The laboratory air temperature is 18 °C. Heat transfers from the lab air to the freezer contents through the freezer walls at a rate of 0.05 kW. Electricity costs $0.12/kWh. a) Identify the relevant cycle for this problem, and draw a schematic of the system that clearly shows where each energy flow comes from and goes to. (2 pt) b) How much cooling must the freezer supply to maintain the samples at -80 °C? (1 pt) c) Determine the minimum daily cost of operating the freezer. (14 pt) d) A real -80°C lab freezer might have a coefficient of performance of 0.7. Determine the daily cost of operating an actual lab freezer. (4 pt) S0.206/da)y Suppose that your lab freezer is about to die, and you must buy a new one. You can either replace the current freezer with an identical model, or you can pay an extra S500 for a high efficiency model with the same coefficient of performance but better insulation, so that the heat transfer through the freezer walls is 0.03 kW. If the lifetime of the freezer is 15 years, which freezer has a lower total cost? Assume 365 days/yr. (9 pt) e) Basic model is cheaper

Answer 1