Thermodynamics The science of thermodynamics deals with the relationship between heat and work. It is governed by two laws, neither of which have ever been proved. On the other hand no violations of either law have ever been observed.
First Law of Thermodynamics The energy that can be extracted from a process can never be more than the energy put into the process In other words Energy out = Energy in This is essentially the law of conservation of energy, i.e. Energy can be neither created nor destroyed, it can only be converted from one form to another
The second law of thermodynamics The first law is concerned with the totality of energy in a process The second law tells us how much work we can extract from a given amount of heat. Carnot’s statement was to the effect that we cannot convert all the the available heat into work. The second law is also concerned with whether a process can occur at all. For example, Heat will always flow from a high to a low temperature A gas under pressure will expand; compression does not occur naturally
Heat Engines A heat engine is a device for extracting work from a hot fluid. For example A car engine extracts power from the combustion of fuel with air A steam steam turbine extracts power from steam Both of these function by allowing a hot fluid to expand so as to cause motion in a critical component of the engine. In the process, high grade energy is said to be degraded to lower grade energy.
An ideal heat engine The diagram on the right represents an ideal heat engine Heat is added at constant temperature to the fluid at the high temperature source The fluid flows through an expansion device where work is done, and the temperature of the fluid falls from T H to T L Heat is then rejected at constant temperature at the low temperature source.
Closed Cycle Heat Engine The cycle in the previous slide is known as an open cycle. The closed cycle here has four stages Isothermal heat addition Adiabatic expansion Isothermal heat removal Adiabatic compression Isothermal = const. Temp Adiabatic = perfectly insulated
The Carnot Engine The cycles above are examples of the Carnot engine. In the Carnot cycle all processes are reversible. In a Carnot engine, the maximum work that can be done, and hence the efficiency of the ideal engine depends on the temperatures T H and T L The efficiency of a Carnot engine is given by The temperature is in the Kelvin or absolute scale This efficiency is called the Carnot efficiency
Practical heat engines (1) The Carnot engine represents the theoretical limit and is not a practical engine. The main limitations of the Carnot engine are: The processes in all four stages are reversible. For this to be the case they must all take place infinitely slowly The work extracted on expansion is equal to the work required for compression, so no net work is extracted. A practical heat engine has a lower efficiency than a Carnot engine, but can make more effective use of the energy in the hot fluid.
Practical Heat Engines (2) Practical Heat Engines include: The Rankine cycle – basis of steam engines in power stations Otto and Diesel cycles – internal combustion engines Gas turbine These have lower efficiencies than the Carnot cycle but are permit useful work to be extracted.
The Rankine cycle This has two differences to the Carnot cycle There must be reasonable temperature differences in the boiler and condenser to ensure that heat addition and rejection occurs at an acceptable rate The turbine exhaust is completely condensed and returned to the boiler by a pump. This uses very much less energy than a compressor. These result in lower efficiencies than the Carnot cycle but permit useful work to be done.
Other cycles Otto, Diesel and Gas turbines all involve an initial compression stage, but are otherwise open cycle processes. Combined cycle gas turbine: This combines a gas turbine with a Rankine steam cycle to maximise the work extracted from the fuel. Efficiencies are much closer to Carnot efficiencies than in other practical cycle used to date.
Example Steam from a geothermal well is expanded in a Carnot engine from a temperature of 150 C to 50 C. How much work is extracted from 1kg of steam? If the steam is heated to 250 C before expansion, how much work is now extracted in relation to the extra heat added Heat capacity of steam = 1.9 kJ kg -1 K -1 0 C = 273 K
Solution Energy extracted = 1 1.9 100 = 190 kJ Efficiency After heating to 250: Energy extracted = 380 kJ Efficiency = 38%
And Finally... Work is heat and heat is work and all the heat in the universe is gonna coooool down! Yeh! That’s entropy man. Michael Flanders and Donald Swann