What are the sources of exergy destruction for both systems

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Reference no: EM133931921

Assignment:

Our campus buildings are typically heated with steam that is generated at a central facility. Let us assume that steam is generated at 45 bar and 265 °C in a boiler that is 90% efficient and runs on coal (heat value ≈30100 kJ/kg). We will analyze only the steam generation unit in the utility facility and for the purposes of the analysis, this system has a boiler, liquid pump and a throttle valve. The pump is used to pressurize the condensate returning from the buildings at 1 bar and 21 °C to the boiler pressure at 45 bar, and has an isentropic efficiency of 45%. The pump requires electricity to run and the central facility pays for the electricity required to run the pump. The throttle valve is at the outlet of the boiler and used to decrease the pressure of the steam down to 13 bar before piping it to the buildings. The total heat provided to the campus buildings is 2.11 PJ over the course of a nominal heating season.

You have devised a plan to replace the throttle valve with a steam turbine such that you can generate some of the electricity you need to run the pump instead of just wasting (potentially) useful steam energy in the throttle valve. The turbine has an isentropic efficiency of 75% and is connected to an electric generator with efficiency 90%, and replacing the throttle valve with the turbine will incur some start-up or capital expenditure. You must use an exergy and cost analysis to see whether this is a good idea or not. Answer the following questions, and note that this list of questions will guide you through your analysis:

(a) How much coal (in metric tons) is required during a nominal heating season for both systems?

(b) How much electricity can you produce with the modified system that uses the turbine?

(c) What is the cost incurred from burning coal to heat the campus buildings during the heating season if the cost of coal per metric ton is $97.14 (c. 2021 pricing)?

(d) What is the total cost for both systems if the cost of electricity is $0.10/kWh (dollar per kilo-watthour, current 2022 rates) and the excess (amount not needed by the pump) electricity generated can be sold back to the grid?

(e) What are the sources of exergy destruction for both systems?

(f) What are the 2nd law efficiencies for the two systems if the specific exergy of coal is equal to its heating value and the dead state is at 21 °C and 1 bar?

Reference no: EM133931921

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