Thermodynamics

Chapter 15 | 781cal reactions, the irreversibility is due to uncontrolled electron exchangebetween the reacting components. The electron exchange can be controlledby replacing the combustion chamber by electrolytic cells, like car batteries.(This is analogous to replacing unrestrained expansion of a gas in mechanicalsystems by restrained expansion.) In the electrolytic cells, the electronsare exchanged through conductor wires connected to a load, and the chemicalenergy is directly converted to electric energy. The energy conversiondevices that work on this principle are called fuel cells. Fuel cells are notheat engines, and thus their efficiencies are not limited by the Carnot efficiency.They convert chemical energy to electric energy essentially in anisothermal manner.A fuel cell functions like a battery, except that it produces its own electricityby combining a fuel with oxygen in a cell electrochemically withoutcombustion, and discards the waste heat. A fuel cell consists of two electrodesseparated by an electrolyte such as a solid oxide, phosphoric acid, ormolten carbonate. The electric power generated by a single fuel cell is usuallytoo small to be of any practical use. Therefore, fuel cells are usuallystacked in practical applications. This modularity gives the fuel cells considerableflexibility in applications: The same design can be used to generate asmall amount of power for a remote switching station or a large amount ofpower to supply electricity to an entire town. Therefore, fuel cells are termedthe “microchip of the energy industry.”The operation of a hydrogen–oxygen fuel cell is illustrated in Fig. 15–36.Hydrogen is ionized at the surface of the anode, and hydrogen ions flowthrough the electrolyte to the cathode. There is a potential differencebetween the anode and the cathode, and free electrons flow from the anodeto the cathode through an external circuit (such as a motor or a generator).Hydrogen ions combine with oxygen and the free electrons at the surface ofthe cathode, forming water. Therefore, the fuel cell operates like an electrolysissystem working in reverse. In steady operation, hydrogen and oxygencontinuously enter the fuel cell as reactants, and water leaves as the product.Therefore, the exhaust of the fuel cell is drinkable quality water.The fuel cell was invented by William Groves in 1839, but it did notreceive serious attention until the 1960s, when they were used to produceelectricity and water for the Gemini and Apollo spacecraft during their missionsto the moon. Today they are used for the same purpose in the spaceshuttle missions. Despite the irreversible effects such as internal resistance toelectron flow, fuel cells have a great potential for much higher conversionefficiencies. Currently fuel cells are available commercially, but they arecompetitive only in some niche markets because of their higher cost. Fuelcells produce high-quality electric power efficiently and quietly while generatinglow emissions using a variety of fuels such as hydrogen, natural gas,propane, and biogas. Recently many fuel cells have been installed to generateelectricity. For example, a remote police station in Central Park in NewYork City is powered by a 200-kW phosphoric acid fuel cell that has anefficiency of 40 percent with negligible emissions (it emits 1 ppm NO x and5 ppm CO).25°CREACTANTS(CH 4 , air)Exergy = 818 MJ(100%)Adiabaticcombustionchamber1789 KPRODUCTSExergy = 530 MJ(65%)FIGURE 15–35The availability of methane decreasesby 35 percent as a result of irreversiblecombustion process.2e – O 2Load2e – 2e –O 2H 2H 2Porous2H + PorousanodeH 2 OcathodeFIGURE 15–36The operation of a hydrogen–oxygenfuel cell.