This is especially important in solar thermal power plants, since they are usually built in areas with little access to cooling water. In all thermodynamic cycles, it is beneficial to use as much of the heat produced as possible, since this increases the efficiency of the cycle as a whole. Hilger, in Comprehensive Renewable Energy, 2012 3.18.4.3 Off-Heat Usage The cooled gases are discharged to the atmosphere through the chimney ( Fig. 5.6, points 4–5), that is to say, the heat from the exhaust gases is totally utilized, decreasing their temperature to the lowest possible value. In this device, hot exhaust gases and high-pressure water of the Rankine cycle are exchanging heat ( Fig. ![]() The crucial component is the recovery boiler, in which both cycles are connected. Therefore, the electric efficiency is higher than having a single cycle in operation (may exceed 55%, considering that a gas turbine rarely exceeds 40%, and the most common values are around 35%). 5.6.īoth power cycles are coupled in such a way that the heat absorption occurs at high temperature in the gas turbine, and the heat transfer occurs at low temperature in the steam cycle. The most common way is coupling a Brayton cycle with a Rankine cycle, as shown in Fig. Two thermodynamic cycles are joined together in such a way that the heat generated by one of the cycles is partially or totally used as heat that is absorbed by the other cycle. Pilar Monsalvete Álvarez de Uribarri, in Urban Energy Systems for Low-Carbon Cities, 2019 5.1.5 Combined heat and power cycle and its use for district heating networks The result is the so-called vapor–compression refrigeration cycle. ![]() The expander is replaced by a throttling valve, which destroys the potential energy of pressure and reduces the refrigerant's pressure from p high to p low but makes the system more reliable and simple. Solution: The expansion process in the turbine is sacrificed in favor of a more thermodynamically effective process: throttling. The advantage of the net work production is surpassed by the losses. ![]() Although this process provides work for the compressor, the price to be paid in terms of efficiency and difficult operation conditions does not justify the presence of the expander–turbine. The expansion involves a low-quality liquid–vapor mixture and therefore the turbine's efficiency is extremely low. Solution: The compression should occur in the superheated vapor region.Ģ. Due to the presence of the liquid phase, the liquid droplets may damage the compressor blades (erosion and pitting). The adiabatic compression between T low and T high occurs in the liquid–vapor region, which is why it is called wet compression. There are two major drawbacks to the reversed Carnot cycle with phase-change refrigerant:ġ. Reversed Carnot refrigeration cycle with liquefaction and vaporization of the refrigerant.
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