Thermodynamics

Chapter 11 | 649do calculations to see if it would be feasible. (You may makeoptimistic assumptions for the analysis.)(a) Sketch the system showing how you will drive it andhow step 3 will be accomplished.(b) Determine what pressure will be required (step 2).(c) Estimate (guess) how long step 3 will take and whatsize will be needed for the piston–cylinder to provide therequired air changes and temperature.(d) Determine the work required in step 2 for one cycleand per hour.(e) Discuss any problems you see with the concept of yourdesign. (Include discussion of any changes that may berequired to offset optimistic assumptions.)11–127 Solar or photovoltaic (PV) cells convert sunlight toelectricity and are commonly used to power calculators, satellites,remote communication systems, and even pumps. Theconversion of light to electricity is called the photoelectriceffect. It was first discovered in 1839 by Frenchman EdmondBecquerel, and the first PV module, which consisted of severalcells connected to each other, was built in 1954 by BellLaboratories. The PV modules today have conversion efficienciesof about 12 to 15 percent. Noting that the solar energyincident on a normal surface on earth at noontime is about1000 W/m 2 during a clear day, PV modules on a 1-m 2 surfacecan provide as much as 150 W of electricity. The annual averagedaily solar energy incident on a horizontal surface in theUnited States ranges from about 2 to 6 kWh/m 2 .A PV-powered pump is to be used in Arizona to pump waterfor wildlife from a depth of 180 m at an average rate of 400L/day. Assuming a reasonable efficiency for the pumping system,which can be defined as the ratio of the increase in thepotential energy of the water to the electrical energy consumedby the pump, and taking the conversion efficiency of the PVcells to be 0.13 to be on the conservative side, determine thesize of the PV module that needs to be installed, in m 2 .11–128 The temperature in a car parked in the sun canapproach 100°C when the outside air temperature is just 25°C,and it is desirable to ventilate the parked car to avoid such hightemperatures. However, the ventilating fans may run down thebattery if they are powered by it. To avoid that happening, it isproposed to use the PV cells discussed in the preceding problemto power the fans. It is determined that the air in the carshould be replaced once every minute to avoid excessive rise inthe interior temperature. Determine if this can be accomplishedby installing PV cells on part of the roof of the car. Also, findout if any car is currently ventilated this way.Solar panelsSolar energySunSolar-poweredexhaust fanPV panelWaterFIGURE P11–127PV-poweredpumpFIGURE P11–12811–129 A company owns a refrigeration system whoserefrigeration capacity is 200 tons (1 ton of refrigeration 211kJ/min), and you are to design a forced-air cooling system forfruits whose diameters do not exceed 7 cm under the followingconditions: The fruits are to be cooled from 28°C to anaverage temperature of 8°C. The air temperature is to remainabove 2°C and below 10°C at all times, and the velocity ofair approaching the fruits must remain under 2 m/s. The coolingsection can be as wide as 3.5 m and as high as 2 m.Assuming reasonable values for the average fruit density,specific heat, and porosity (the fraction of air volume in a box),recommend reasonable values for (a) the air velocity approachingthe cooling section, (b) the product-cooling capacity of thesystem, in kg · fruit/h, and (c) the volume flow rate of air.