Microsoft Word - Cengel and Boles TOC _2-03-05_.doc

Chapter 11 | 649

do calculations to see if it would be feasible. (You may make
optimistic assumptions for the analysis.)
(a) Sketch the system showing how you will drive it and
how step 3 will be accomplished.
(b) Determine what pressure will be required (step 2).
(c) Estimate (guess) how long step 3 will take and what
size will be needed for the piston–cylinder to provide the
required air changes and temperature.
(d) Determine the work required in step 2 for one cycle
and per hour.
(e) Discuss any problems you see with the concept of your
design. (Include discussion of any changes that may be
required to offset optimistic assumptions.)

11–127 Solar or photovoltaic (PV) cells convert sunlight to
electricity and are commonly used to power calculators, satel-
lites, remote communication systems, and even pumps. The
conversion of light to electricity is called the photoelectric
effect. It was first discovered in 1839 by Frenchman Edmond
Becquerel, and the first PV module, which consisted of sev-
eral cells connected to each other, was built in 1954 by Bell
Laboratories. The PV modules today have conversion efficien-
cies of about 12 to 15 percent. Noting that the solar energy
incident on a normal surface on earth at noontime is about
1000 W/m^2 during a clear day, PV modules on a 1-m^2 surface
can provide as much as 150 W of electricity. The annual aver-
age daily solar energy incident on a horizontal surface in the
United States ranges from about 2 to 6 kWh/m^2.
A PV-powered pump is to be used in Arizona to pump water
for wildlife from a depth of 180 m at an average rate of 400
L/day. Assuming a reasonable efficiency for the pumping sys-

tem, which can be defined as the ratio of the increase in the

potential energy of the water to the electrical energy consumed

by the pump, and taking the conversion efficiency of the PV

cells to be 0.13 to be on the conservative side, determine the

size of the PV module that needs to be installed, in m^2.

11–128 The temperature in a car parked in the sun can

approach 100°C when the outside air temperature is just 25°C,

and it is desirable to ventilate the parked car to avoid such high

temperatures. However, the ventilating fans may run down the

battery if they are powered by it. To avoid that happening, it is

proposed to use the PV cells discussed in the preceding prob-

lem to power the fans. It is determined that the air in the car

should be replaced once every minute to avoid excessive rise in

the interior temperature. Determine if this can be accomplished

by installing PV cells on part of the roof of the car. Also, find

out if any car is currently ventilated this way.

11–129 A company owns a refrigeration system whose

refrigeration capacity is 200 tons (1 ton of refrigeration  211

kJ/min), and you are to design a forced-air cooling system for

fruits whose diameters do not exceed 7 cm under the follow-

ing conditions: The fruits are to be cooled from 28°C to an

average temperature of 8°C. The air temperature is to remain

above 2°C and below 10°C at all times, and the velocity of

air approaching the fruits must remain under 2 m/s. The cool-

ing section 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 approach-

ing the cooling section, (b) the product-cooling capacity of the

system, in kg · fruit/h, and (c) the volume flow rate of air.

Sun

Water

PV-powered

pump

PV panel

FIGURE P11–127

Solar energy

Solar panels

Solar-powered

exhaust fan

FIGURE P11–128