 As we move from chapter 9 into chapter 8, we are transitioning from analyzing gas power cycles to analyzing vapor power cycles. That can be confusing because both gas and vapor can refer to the same phase of matter, but vapor, in vapor power cycle, doesn't refer to the vapor phase, it refers to the vaporization process. In a gas power cycle, the working fluid remains a gas the entire time. In a vapor power cycle, the working fluid vaporizes and condenses. If our working fluid was water, we would start with liquid water and we would add heat to it until it boils, and as a result of going from liquid water to vapor water, we have a lot more expansion that we can take advantage of in the power production process. If you consider an ideal gas undergoing an isobaric heating process, we are lucky to get two or three factors of expansion as we heat it. If we had water going through an isobaric heat addition process until it went from liquid water to entirely vapor, we would have one or two thousand times the expansion. More expansion means more potential for power, and that's what the vapor power cycle excels at. We can produce a lot of power and we can do it relatively efficiently. The downside of the vapor power cycle is, A, that it's heavy. I mean, it doesn't exactly bode well for portable applications because you have to carry around a bunch of water. B, it's also very slow to react. A gas power cycle can throttle up and down very quickly in order to accommodate changes in power requirements. A vapor power cycle has a lot more inertia. It takes a while for it to throttle up or throttle down. In trying to establish an idealized model of a vapor power cycle, we can start by considering the Carnot vapor power cycle. In the Carnot vapor cycle, we would have isothermal heat addition followed by isentropic expansion and then isothermal heat rejection and isentropic compression. However, since we're trying to cross the dome, an isothermal heating and cooling process means that we end up either with our isothermal heat addition process going across the dome or the isothermal heat rejection process going across the dome. Neither is particularly advantageous. When the heating process is the transition across the dome, we end up with a mixed stream in our turbine. We have condensation occurring on the turbine blades, which is bad for heat and weight distribution reasons. Plus, our pump has to try to handle a mixed stream, which pumps don't like. If we had our cooling process occurring across the dome, we would have to maintain our heating process at a very high temperature and pressure in order to accommodate keeping the cooling process going across the dome. What is more practical and useful for us is going to be replacing the isothermal heat addition and isothermal heat rejection with isobaric processes, which leads us to the Rankine cycle. The Rankine cycle consists of isentropic compression, unless we're given enough information to deduce that it's not isentropic, isobaric heat addition, isentropic expansion, and then isobaric heat rejection. If it sounds a lot like the Brayton cycle, that's because it is a lot like the Brayton cycle. In fact, the big difference between the two is that the Rankine cycle deals with water and we are also going to be splitting our stream periodically because we have the advantage of external heat addition as opposed to mixing our fuel with the working fluid. An assumption that we're going to be establishing with our Rankine cycle, unlike our Brayton cycle, is that the outlet of our condenser is a freshly condensed substance. So in a simple cycle like this, we assume that the outlet of the condenser and inlet to the pump is a saturated liquid. It's a waste of effort to try to sub-cool it. It just goes across the dome and then enters the pump. Let's try an example.