| Thermodynamics |
Assignment Narrative and Questions:
The residential (household) sector in Singapore contributes about 7% of Singapore’s CO2 emissions, with air conditioning and refrigeration accounting for most of the emissions in that sector. (Air conditioning and refrigerators in households use electricity, which, in Singapore, is mainly generated using combined cycle power plants running on natural gas, which is a fossil fuel producing CO2 when combusted in the power plants.)
There are about 1.4 million households in Singapore. You may assume that each household uses an air conditioning system that consumes 1 kW of electricity for an average of 8 hours a day,
Question 1
a. What could be the cost savings and CO2 emissions reductions for each household of switching to fans to provide thermal comfort at home? (Hint: Need to calculate the electricity consumption and bill for each household based on information provided above, and find out how much electricity would be consumed by fans at home)
b. What are possible reasons why such a switch may or may not be realistic for Singapore households. [Qualitative answer only. Maximum 3 reasons]
c. Assuming that the average Coefficient of Performance (COP) of residential air conditioners is 3, how much heat (in kW) is rejected to the ambient by one air conditioner in the residential sector during hours of operation?
d. Assuming that the airflow used to take away this heat is 0.2 m3 /s, estimate the temperature rise that the air will experience? (Hint: Use the 1st Law of Thermodynamics and ideal gas properties of air)
e. Calculate the CO2 emissions resulting from air conditioning electricity use for the residential sector in Singapore. (Note: You will have to find out what is the Grid Emission Factor (GEF, in kg CO2/kWh) for electricity generation in Singapore.)
f. Suggest possible ways for the GEF for power generation to be reduced. [Qualitative answer only.
Question 2. (Hint: Use the 2nd Law of Thermodynamics).
a. Assuming that the air conditioner operates between 15 °C and 40 °C, what is the maximum theoretical COP of the air conditioning system?
b. If the rejected heat from the air conditioner at 40°C is used to provide the heat input to a reversible heat engine, what is the maximum theoretical thermal efficiency and power output (kW) of the heat engine, assuming that the ambient temperature is 30°C.
c. As an alternative to b., the rejected heat can be “upgraded” using a heat pump to provide hot water for homes (e.g. for showers/baths). If the heat pump delivers hot water at 50°C, estimate how much hot water each household can produce daily from its air conditioning use.
d. Calculate the rate of entropy generation (kW/K) for the air conditioning system resulting from
(i) the heat rejection of the air conditioner taking place between 40°C (refrigerant temperature) and 30°C (ambient temperature) and
(ii) the heat addition from the cooled room taking place between 15°C (the temperature at which the air conditioner is receiving heat) and 25°C (desired room temperature)
e. Discuss in practical (engineering) terms, how entropy generation arising from heat transfer across finite temperature differences can be reduced. [Maximum 3 ways]. Can such entropy generation be reduced to zero? Briefly explain your answer.
Question 3
a. Besides CO2 emissions from electricity use, non-CO2 Greenhouse Gases (GHGs) are emitted when hydrofluorocarbon (HFC) refrigerants are leaked or released into the atmosphere from air conditioners and refrigerators that use such refrigerants. Suggest/Describe several ways to significantly reduce non- CO2 GHGs emissions from air conditioning. [Qualitative answer only. Maximum 3 ways]
b. It is proposed to use district cooling systems to provide chilled water for air-conditioning to homes in Singapore. However, such centralised cooling systems typically have to operate at a lower refrigeration temperature (e.g. 5°C) to counter losses in distributing chilled water over a wide area.
Discuss if it is possible for district cooling systems to be more efficient than small household air conditioning units. (Hint: Estimate the theoretical COP reduction from operating at a lower temperature, balanced against the higher efficiencies of larger chiller systems used in district cooling systems especially those with cooling storage, and the additional losses from using such a district cooling system).
