The Carnot efficiency dictates that higher efficiencies can be attained by increasing the temperature of the steam.
These processes cannot be achieved in real cycles of power plants. The Carnot efficiency is valid for reversible processes. It must be added, this is an idealized efficiency. For this type of power plant the maximum (ideal) efficiency will be: In a modern coal-fired power plant, the temperature of high pressure steam (T hot) would be about 400☌ (673K) and T cold, the cooling tower water temperature, would be about 20☌ (293K). T H is the absolute temperature (Kelvins) of the hot reservoir.T C is the absolute temperature (Kelvins) of the cold reservoir,.it is the ratio = W/Q H of the work done by the engine to the heat energy entering the system from the hot reservoir. is the efficiency of Carnot cycle, i.e.The formula for this maximum efficiency is: The efficiencies of all reversible engines ( Carnot heat engines) operating between the same constant temperature reservoirs are the same, regardless of the working substance employed or the operation details.No engine can be more efficient than a reversible engine ( a Carnot heat engine) operating between the same high temperature and low temperature reservoirs.In short, this principle states that the efficiency of a thermodynamic cycle depends solely on the difference between the hot and cold temperature reservoirs. In 1824, a French engineer and physicist, Nicolas Léonard Sadi Carnot advanced the study of the second law by forming a principle (also called Carnot’s rule) that specifies limits on the maximum efficiency any heat engine can obtain. There will always be significant energy losses.ĭirections of thermodynamic processes are subject of the second law of thermodynamics, especially of the Clausius Statement of the Second Law. But you cannot build a machine that converts heat completely into mechanical energy. In this reverse direction, there are plenty of devices that convert heat partially into mechanical energy. When we use the brakes to stop a car, that kinetic energy is converted by friction back to heat, or thermal energy. The mechanical energy has been converted to kinetic energy. The chemical energy in gasoline is converted to thermal energy, which is then converted to mechanical energy that makes the car move. But it doesn’t happen in nature.įor example, burning gasoline to power cars is an energy conversion process we rely on. In fact, such heat flow (from a colder body to a warmer system) would not violate the first law of thermodynamics, i.e. For example, when a temperature difference does exist heat flows spontaneously from the warmer system to the colder system, never the reverse. Many thermodynamic processes proceed naturally in one direction but not the opposite. There must be losses in the conversion process. For example, it is not possible to convert all the energy obtained from a coal in coal-fired power plant or from a nuclear reactor in a nuclear power plant into electrical energy. One of the areas of application of the second law of thermodynamics is the study of energy-conversion systems. So the second law is directly relevant for many important practical problems. The second law of thermodynamics places constraints upon the direction of heat transfer and sets an upper limit to the efficiency of conversion of heat to work in heat engines. However, no information about the direction of the process can be obtained by the application of the first law. The first law is used to relate and to evaluate the various energies involved in a process. The second law of thermodynamics is a general principle, that goes beyond the limitations imposed by the first law of thermodynamics. It follows, perpetual motion machines of the second kind are impossible. From this law follows that it is impossible to construct a device that operates on a cycle and whose sole effect is the transfer of heat from a cooler body to a hotter body. Reversible processes are a useful and convenient theoretical fiction, but do not occur in nature. This law indicates the irreversibility of natural processes. In a natural thermodynamic process, the sum of the entropies of the interacting thermodynamic systems increases. The entropy of any isolated system never decreases.