2. Discuss the concept of thermal comfort. Treat aspects such as Metabolism, Draught, Clothing (6 credits).

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1 Exam in Sustainable Energy Utilisation, 4A1607, Time: Helping aid allowed: Course literature, lecture notes (NO solved problems), calculator. Clearly state and motivate assumptions. Do not use pens or pencils in red color. Do not hesitate to ask if you do not understand the questions, or if the questions are unclear. 1. A bus carrying 30 passengers is exposed to sunshine during its route through the city center of Stockholm. The bus is 15 x 3 x 2.5 m (length x width x height). The solar intensity is 500 W/m². The window area of the bus is 20 % of the total area exposed to the ambient. The walls have a U-value of 1.0 W/(m² K). The windows are of single pane type with a U-value of 3.5 W/(m² K). The ventilation air is taken from the ambient and the flow rate including infiltration is m³/h. The ambient temperature is 25 C. Calculate the temperature inside the bus if the bus is driven at noon (12 AM) in the city center of Stockholm during a day in the month of July (7 credits). 2. Discuss the concept of thermal comfort. Treat aspects such as Metabolism, Draught, Clothing (6 credits). 3. The concentration of CO 2 in a classroom room is initially 350 ppm which is equal to the outdoor concentration. The production of CO 2 for each person is assumed to be 18 dm³/h, the volume of the class room is 200 m³, and that the ventilation rate is 1 ACH. During the first 45 minutes, the classroom is occupied by 30 persons. After the first 45 minutes there is a 15 minute break. Calculate the concentration of CO 2 (in ppm) after the break if a) the classroom is empty during the break (2 credits) b) the classroom is occupied by 10 persons during the break (2 credits) c) What general recommendations would you give? (2 credits) 4. Hot humid air is flowing on the inside of a plastic tube. The inlet temperature is 39 C and the relative humidity is 80 %. The diameter of the plastic tube is 10 mm, and the volumetric flow rate at inlet conditions is 7 dm³/min. The heat transfer coefficient on the inside and the outside of the tube is 30 W/(m² K) and 8 W/(m² K) respectively. Neglect the heat resistance of the tube walls and the wall thickness. The ambient temperature is 23 C, the relative humidity is 60 %, and the atmospheric pressure is 1 bar. Assume that there are no gradients in velocity, water content, and temperature in the radial direction of the flowing air. Calculate the distance from the inlet of the tube to the location on the tube where condensation starts (7 credits). L Please turn over!

2 5. Discuss the choice of heat source for heat pumps in relation to inventment cost, ambient conditions (climate), and varying heat demand from the system (5 credits). 6. The fan cooling the condenser in a refrigeration unit breaks down. How will this affect the evaporation and condensing temperatures in the system? Motivate! (5 credits). 7. A conventional vapor compression system is working with Propane (R290) as refrigerant. The condenser is cooled by water (specific heat 4.2 kj/(kg K)) and the evaporator is heated by an ethylene glycol water solution (specific heat 3.49 kj/(kg K)). Mass flow rate of the water is 0.16 kg/s and water temperature change is 5 C. The mass flow rate of the ethylene glycol water solution is 0.20 kg/s. Condensing pressure is 10 bar and subcooling equals 8 C. Evaporating temperature is 5 C and superheat equals 6 C. The total isentropic efficiency of the compressor is η k =0.73. Calculate a) The heating capacity of the condenser (1 credits) b) The mass flow rate of the refrigerant (3 credits) c) The temperature change of the glycol solution (2 credits) d) The power input to the compressor (2 credits) e) The COP1 and COP2 of the system (1 credit). 8. In a condenser, condensation takes place on the inside of the tubes. The flow pattern in the tubes is assumed to be stratified, and the tubes are half full with condensed liquid. The condenser consists of five parallel tubes with a inside tube diameter of 16 mm and a tube length of 2 m. The condensation temperature is 40 C. The condenser heat load is 6.75 kw and the refrigerant is R22. Assume that the wall temperature is constant and calculate the heat transfer coefficient inside the tubes (7 credits). 9. Define what is meant by sustainability, and mention at least four different cases where the course treated Sustainable aspects (6 credits). Good Luck!

3 Tentamen i Uthållig energianvändning, 4A1607, Tid: Tillåtna hjälpmedel: Kompendier, föreläsningsanteckningar (inga lösta problem), miniräknare. Ange noga och motivera dina antaganden. Använd inte röda pennor. Tveka inte att fråga om du inte förstår frågorna eller om något är oklart.

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