Whether it is coal, oil, gas or nuclear power, 80% of the worlds electricity is derived from heat sources and almost all of the energy conversion processes used convert the thermal energy into electrical energy involve an intermediate step of converting the heat energy to mechanical energy in some form of heat engine. To satisfy this need a wide range of energy conversion systems has been developed to optimise the conversion process to the available heat source. Despite over 250 years of development since James Watt’s steam engine was first fired up, the best conversion efficiency achieved today is only around 60% for combined cycle steam and gas turbine systems. Efficiencies in the range of 35% to 45% are more common for steam turbines, 20% to 30% for piston engines and as low as 3% for OTEC ocean thermal power plants. This page describes some thermodynamic aspects of a variety of representative heat engines. More detailed descriptions of these engines can be found on other pages on this site via the links below. The efficiency of heat engines was first investigated by Carnot in the 1824 and expanded upon by Clapeyron who provided analytical tools in 1834 and Kelvin who stated the Second Law of Thermodynamics in 1851 and finally by Clausius who introduced the concept of entropy in 1865. The Thermodynamic System Every thermodynamic system exists in a particular state which is defined by the properties of its components such as heat, temperature, pressure, volume, density, entropy and phase (liquid, gas etc) at a given point in time. Thermodynamics concerns the conversions between heat and other forms of energy in the system and the related energy flows. In a thermodynamic cycle, energy is applied in one form to change the state of the system and energy is then extracted in a different form to return the system to its original state. In a heat engine, the energy is applied in the form of heat to change the state of a working fluid and then extracted in the form of mechanical work to return the working fluid to its initial state. In other words, a heat engine is a system in which energy is interchanged between an energy conversion system and its surroundings. It is important to note that though the working fluid in a heat engine may work in a closed cycle, the “system” and the “state of the system” are defined to include both the physical “engine” as well as the working environment or surroundings. Heat Engines Heat engines employ a range of methods to apply the heat and to convert the pressure and volume changes into mechanical motion. From the Gas Laws PV = kN T where P is the pressure, V the volume and T the temperature of the gas and k is Boltzmann’s constant and N is the number of molecules in the gas charge. Putting energy in the form of heat into a gas will increase its temperature, but at the same time the gas laws mean that the gas pressure or volume or both must increase in proportion. The gas can be restored to its original state by taking this energy out again but not necessarily in the form of heat. The pressure and / or volume change can be used to perform work by moving a suitably designed mechanical device such as a piston or a turbine blade. The greater the temperature change, the more energy which can be extracted from the fluid The Heat Engine as Part of a System Heat engines enable heat energy to be converted to kinetic energy through the medium of a working fluid. The diagram opposite shows the system heat flow. Heat is transferred from the source, through working fluid in the heat engine and into the sink, and in this process some of the heat is converted into work. Heat engine theory concerns only the process of converting heat into mechanical energy, not the method of providing the heat, the combustion process. Combustion is a separate conversion process and is subject to its own efficiency losses. In some practical systems such as steam turbines these two processes are physically separate, but in internal combustion engines, which account for the majority of engines, the two processes take place in the same chamber, at the same time. Entropy The concept of entropy is useful for understanding system energy conversions, energy flows and the workings of heat engines. The word “entropy” comes from the Greek “transformation”. Although entropy was first defined for thermodynamic applications, the concept has been used in other branches os science, notably electrochemistry and communications. There are thus many definitions of entropy some of which are contradictory or confusing. The following three examples are consistent and used in the context of heat engines. Entropy a measure of the disorder of a system. Entropy a measure of the amount of energy which is unavailable to do work. Entropy S is a state variable for a reversible (loss free) process whose change at any point in the cycle is defined as: dS = dQ/T Where Q is the heat in Joules entering the system at any point in the cycle and T is the temperature in °K at the point of heat entry An example is the temperature of an enclosed volume of gas being raised by heat from an energy source or reservoir. As the temperature of the gas increases the disorder or kinetic energy of its molecules increases which means that its entropy has increased. This is accompanied by a change of state of the gas whose volume or pressure to increases depending on the nature of the enclosure. Second Law of Thermodynamics The second law concerns changes in entropy. It can be stated in different forms as follows; The entropy of an isolated system which is not in equilibrium will tend to increase over time, approaching a maximum value when the system is in equilibrium In any cyclic process the entropy will either increase (or in ideal system remain the same). Clausius Inequality Clausius’ theorem is another way of stating the Second Law. Thus: ∫ dQ/T
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