Principles of naval engineering - Historic Naval Ships Association

Chapter 25. -NEW DEVELOPMENTS IN NAVAL ENGINEERING147.162Figure 25-5.— Small craft with waterjetpropulsion.direct energy conversions that may ultimatelyhave application to the production of power forship propulsion. This interest arises from twomajor considerations. First, the Carnot cycle^which is the thermodynamic basis for our heatengines is inherently inefficient, with the theoreticalmaximum efficiency of the cycle beingTi - T2Tllimited to where Ti is the absolute temperatureat which heat flows from the source to theworking fluid and T2 is the absolute temperatureat which heat is rejected to the receiver. Sincethe temperature of the heat receiver (the ocean)cannot be lowered, the only way to improve theefficiency of an actual cycle based on the Carnotcycle is to increase the temperature of T^. Thepast few years have seen great advances in theuse of higher Tj temperatures (e.g., boilersoperating at higher pressures in order to increasethe difference between Tj and T2), butmaterials limitations eventually impose a barrierto progress in this direction.The second reason for current interest innovel energy conversions is that the actual shipboardcycles in which stored energy is convertedto thermal energy which is then converted to work°The Carnot cycle is discussed in chapter 8 of thistext.require a great deal of equipment to performthese various energy conversions. Beginningwith an inherently inefficient cycle which cannotoperate unless a great deal of heat is "wasted"because it must be rejected to a heat receiver,we must accept even greater inefficiency becauseof the mechanical losses and miscellaneous heatlosses that inevitably occur throughout the plant.There are two major approaches to the problemof direct energy conversion. One approachis to find an energy conversion which is not basedon the Carnot cycle and is therefore not limitedby the requirement that some heat be rejectedto a low temperature heat receiver. The otherapproach is to utilize a "static" heat enginewhich is based on the Carnot cycle, and thereforesubject to its limitations, but which has no movingparts and therefore no mechanical losses.The fuel cell is an example of a device that bypassesthe Carnot cycle to make a direct energyconversion. Thermoelectric converters, thermionicconverters, and magnetohydrodynamicgenerators are examples of static heat engineswhich, although operating on the Carnot cycle,come very much closer to the maximum theoreticalefficiency of the cycle by reducing or eliminatingmechanical losses.Fuel CellsA fuel cell is a battery-type device in whichchemical energy is converted directly into electricalenergy. The reaction involves a freeenergy release, without the rejection of heat to aheat sink; hence the process is independent of theCarnot cycle and free of Carnot cycle limitations.The major parts of a fuel cell (fig. 25-8) arean anode, a cathode, and an electrolyte. The fuelis fed continuously to the anode, while the oxidantis fed continuously to the cathode. The conversionfrom chemical energy to electrical energy occursas electrons, released at the anode, flowtothe cathode.Several types of fuel cells are under investigationand development. Some operate at relativelylow pressures and temperatures, others athigh pressures andtemperatures. A wide varietyof fuels have been considered for fuel cells; hydrogen,various hydrocarbons, and methanol appearto offer particular promise for many applications,while a sodium amalgam is beingconsidered for use in certain small fuel cells.The oxidants most commonly used are air andoxygen; however, peroxides, chlorine, and othersubstances have also been tried. Electrolytes635