Thermodynamics

6–76 In tropical climates, the water near the surface of theocean remains warm throughout the year as a result of solarenergy absorption. In the deeper parts of the ocean, however,the water remains at a relatively low temperature since thesun’s rays cannot penetrate very far. It is proposed to takeadvantage of this temperature difference and construct apower plant that will absorb heat from the warm water nearthe surface and reject the waste heat to the cold water a fewhundred meters below. Determine the maximum thermal efficiencyof such a plant if the water temperatures at the tworespective locations are 24 and 3°C.24°COCEAN3°CSOURCET HCarnotHESINK60°F800 Btu/minFIGURE P6–75EPumpBoilerCondenser6–77 An innovative way of power generation involves theutilization of geothermal energy—the energy of hot waterthat exists naturally underground—as the heat source. If asupply of hot water at 140°C is discovered at a locationwhere the environmental temperature is 20°C, determine themaximum thermal efficiency a geothermal power plant builtat that location can have. Answer: 29.1 percent6–78 An inventor claims to have developed a heat enginethat receives 700 kJ of heat from a source at 500 K and produces300 kJ of net work while rejecting the waste heat to asink at 290 K. Is this a reasonable claim? Why?·W net, outTurbineFIGURE P6–76Chapter 6 | 3216–79E An experimentalist claims that, based on his measurements,a heat engine receives 300 Btu of heat from asource of 900 R, converts 160 Btu of it to work, and rejectsthe rest as waste heat to a sink at 540 R. Are these measurementsreasonable? Why?6–80 A geothermal power plant uses geothermal waterextracted at 160°C at a rate of 440 kg/s as the heat sourceand produces 22 MW of net power. If the environment temperatureis 25°C, determine (a) the actual thermal efficiency,(b) the maximum possible thermal efficiency, and (c) theactual rate of heat rejection from this power plant.Carnot Refrigerators and Heat Pumps6–81C How can we increase the COP of a Carnotrefrigerator?6–82C What is the highest COP that a refrigerator operatingbetween temperature levels T L and T H can have?6–83C In an effort to conserve energy in a heat-engine cycle,somebody suggests incorporating a refrigerator that will absorbsome of the waste energy Q L and transfer it to the energysource of the heat engine. Is this a smart idea? Explain.6–84C It is well established that the thermal efficiency of aheat engine increases as the temperature T L at which heat isrejected from the heat engine decreases. In an effort toincrease the efficiency of a power plant, somebody suggestsrefrigerating the cooling water before it enters the condenser,where heat rejection takes place. Would you be in favor ofthis idea? Why?6–85C It is well known that the thermal efficiency of heatengines increases as the temperature of the energy sourceincreases. In an attempt to improve the efficiency of a powerplant, somebody suggests transferring heat from the availableenergy source to a higher-temperature medium by a heatpump before energy is supplied to the power plant. What doyou think of this suggestion? Explain.6–86 A Carnot refrigerator operates in a room in which thetemperature is 22°C and consumes 2 kW of power whenoperating. If the food compartment of the refrigerator is to bemaintained at 3°C, determine the rate of heat removal fromthe food compartment.6–87 A refrigerator is to remove heat from the cooled spaceat a rate of 300 kJ/min to maintain its temperature at 8°C.REFRIG.–8°C300kJ/minW in,minFIGURE P6–87·25°C