# Energy conversion

learn.

1. P7.3. The pump of a water storage tank is powered with a 18-kW electric motor operating with an efficiency of 85%. The water flow rate is 30 l/s. The diameters of the inlet and exit pipes are the same, and the elevation difference between the inlet and outlet is negligible. The absolute pressures at the inlet and outlet are 100 and 500 kPa, respectively.

• Determine the mechanical efficiency of the pump in %.

2. P7.5 A wind turbine-generator with a 27-foot-diameter blade produces 0.4 kW of electric power. In the location of the wind turbine, the wind speed is 10 mile per hour.

• Determine the efficiency of the wind turbine-generator in %.

3. P7.6 . A hydroelectric plant operates by water falling from a 25 m height. The turbine in the plant converts potential energy into electrical energy with an assumed efficiency of 85%. The electricity is lost by about 8% through the power transmission.

• Estimate the mass flow rate of the water in kg/s necessary to power a 500 W light bulb.

4. P7.28 Steam expands in a turbine from 6,600 kPa and 300oC to a saturated vapor at 1 atm. The steam flow rate is 9.55 kg/s. If the turbine generates a power of 1 MW,

• Determine the thermal efficiency.

5. P7.57 A steam power plant is using a geothermal energy source. The geothermal
source is available at 220oC and 2,320 kPa with a flow rate of 200 kg/s. The
hot water goes through a valve and a flash drum. Steam from the flash drum
enters the turbine at 550 kPa and 428.62 K. The discharged steam from the
turbine has a quality of x4 = 0.96. The condenser operates at 10 kPa. The
water is a saturated liquid after passing through the condenser.

• Determine the thermal efficiency of the cycle.

6. P7.61 (modified) A steam power plant is using an ideal regenerative Rankine cycle. Steam
enters the high-pressure turbine at 11,000 kPa and 773.15 K, and the
condenser operates at 10 kPa. The steam is extracted from the turbine at 475
kPa to heat the water in an open heater. The water is a saturated liquid after
passing through the water heater.

• Determine the thermal efficiency.

7. P7.84 An ideal Diesel cycle has an air-compression ratio of 16 and a cutoff ratio of 2. At the beginning of the compression, the fluid pressure, temperature, and volume are 100 kPa, 300 K, respectively. The average specific heats of air at room temperature are Cp,av = 1.005 kJ/kg K, Cv,av = 0.7181 kJ/kg K.

• Utilizing the cold-air-standard assumptions, determine the thermal efficiency of the cycle.

8. P7.90 A refrigerator using tetrafluoroethane (R-134a) as refrigerant operates with a capacity of 2,500 kW. Cooling water enters the condenser at 280 K. Evaporator is at 271.92 K, and the condenser is at
299.87 K. The refrigerated space is at 280 K. Assume that the kinetic and potential energy changes are negligible.

• Determine the ideal and actual power necessary if the compressor efficiency is 80%

9. A typical H 2 –O 2 PEMFC might operate at a voltage of 0.8 V and λ=1.2. At STP, what is the efficiency in % of such a fuel cell (use HHV and assume pure oxygen at the cathode)? Plot the response of voltage and any other output variable of a Fuel Cell using MATLAB example https://www.mathworks.com/help/physmod/sps/examples/6-kw-45-vdc-fuel-cell-stack.html (Links to an external site.) .

10. You are provided with a fuel cell that is designed to operate at j=3Acm2 and P=2Wcm2.

• How much fuel cell active area (in cm 2) is required to deliver 2 kW of electrical power? (This is approximately enough to provide power to the average American home.)
• For the fuel cell described here, assuming operating on pure hydrogen fuel, how much water in kg would be produced during 24 hours of operation at the same power?

Reference eBook from library database: Fuel Cell Fundamentals

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