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1. Directly use a compressor to pressurize the waste steam and send it into the boiler. For over a hundred years, heat engines that use water vapor as a working fluid have basically followed the same pattern: the waste steam after doing work enters a heat exchanger where it is cooled to become condensed water, which is then pumped back into the boiler to be reheated. Now let’s try to use a compressor to send the uncooled waste steam directly into the boiler and see what happens: Steam with pressure p1 and temperature t1 enters the turbine to generate power; the waste steam discharged by the turbine has a pressure of p2 and a temperature of t2, with p1 > p2 and t1 > t2. Now we will use a compressor to raise the pressure of the waste steam to p1 and see what occurs (relevant knowledge from engineering thermodynamics is needed for this; those without such knowledge can simply look at the results). ) (Also: Don’t ask me why I insist on increasing the pressure, or why we can’t just feed the waste steam directly into the boiler through a pipe – it’s like asking why water can’t flow directly uphill; this belongs to another realm of intelligence, and I refuse to answer.) From the enthalpy-entropy diagram, the waste steam is compressed by the compressor from point (p2, t2) to a point on the isobaric line at p1, namely (p1, t’). It is clear that, regardless of the magnitude of the entropy increase, t’ is much greater than t2; moreover, the increase in enthalpy during the compression process is significantly larger than the decrease in enthalpy during the work output process of the turbine. In other words, feeding the exhaust steam into the boiler without cooling and increasing its pressure will incur a cost far greater than the output power of the turbine. 2. Use a compressor with intercooling to boost the pressure of the waste steam. In practical engineering applications, compressors almost always come equipped with intercooling. Haha, how could I, a graduate in thermal engineering, not know that the temperature of a gas rises when it is compressed? (BTW: Many popular science articles attribute the high temperatures encountered by spacecraft as they re-enter the atmosphere to frictional heating; in reality, it is the compression of air in front of a fast-moving object that causes a sharp rise in temperature – this process is known as adiabatic stagnation, so friction plays no role at all.) Below we will analyze the situation where a compressor with intermediate cooling, which is used in engineering practices, is used to pressurize waste steam, under the assumption that water vapor does not turn into liquid water during the cooling process and that the gas does not lose its latent heat of vaporization; otherwise, such a discussion would be meaningless. Simple analysis: 1) The pressure increase is excessive, with the exhaust pressure of the turbine rising from 0.0053 MPa to pressures in the tens or twenties of megapascals in the boiler; there are too many compression stages involved, resulting in low efficiency. 2) The compression process proceeds from point (p2, t2) along the x=1 dryness line to the p1 isobaric line. It can be seen that the saturated temperature corresponding to the end point of this compression process, p1, is already quite close to t1; there is little room for an increase in enthalpy during the boiler heating process, which means that the boiler plays almost no role in this process. Most of the work you do is used to drive the compressor. The overall system net output is low. 3) I made a few rough sketches on the temperature-entropy diagram, but realized that it was impossible to compare it with the Rankine cycle. This was because it wasn’t possible to qualitatively compare the ratio T2/T1 between the average heat release temperature and the average heat absorption temperature of the two cycles. Later, I intended to use actual parameters to perform calculations on the temperature-entropy diagram, in order to determine the values of T1 and T2 for both cycles. However, right after making one stroke, I suddenly realized that the Rankine cycle is an ideal cycle; what’s the point of comparing it using real-world parameters? Therefore, I ultimately gave up. In the problem statement, someone mentioned that “at this point, the thermal efficiency of the Rankine cycle is quite low” (my phone isn’t suitable for inserting images, so please bear with me). I would greatly appreciate any guidance on this matter. Practically speaking, if one wishes to make the most of the heat in waste steam without losing the latent heat of vaporization, the existing design of steam turbines must be altered: 1) a working fluid with a temperature far higher than the saturation temperature should be selected, along with multi-stage compression and intercooling. 2) Increase t1 as much as possible to improve system efficiency; otherwise, it is not cost-effective. 3) Similar to steam turbines, reheat and regenerative cycles can also be applied to new turbines. 3. Situation of gas turbines Based on the above analysis, we consider the situation of gas turbines; obviously, gas turbines meet the three conditions mentioned above. Wait, can the exhaust gases from the gas turbine be fed back into the turbine again? With less oxygen, how can the fuel burn? Hmph, does a gas turbine have to use gas? Just check out the versatile Baidu – Gas turbine cycle – Closed cycle: \"The working fluid is reused; the fluid discharged by the turbine is not cooled in the atmosphere, but rather is cooled by a cooler before being drawn back into the compressor to participate in the cycle again (Figure 5) (as written in the original text; I don’t know where the figure is).\" The compressed gas refrigerant is heated in a gas boiler (or heater). In a closed-loop system, the working fluid can be air or other gases. The main disadvantages of closed-cycle systems include large sizes and high costs of heat exchangers, including gas boilers, as well as a low T3 temperature, which results in their limited use. If a gas-cooled reactor is used as the heat source for heating the gaseous working fluid (such as He or N2), it constitutes a nuclear-powered closed-cycle system, whose efficiency can be higher than that of nuclear power plants utilizing steam turbines. ” 4. Summary 1) It is not feasible to send the waste steam directly to the boiler (heater) without cooling, as the cost of compressing the gas back to its initial pressure is far higher than the work it can deliver. 2) It is acceptable for the waste steam to be cooled without losing its vaporization latent heat (without condensing into water). However, it is not suitable for steam turbines that are currently used on a large scale and use water vapor as the working fluid. The most reasonable approach at present remains to condense the waste steam into water, pump it to a higher pressure, and feed it into the boiler.