Thermodynamic cycle
Contents
Intro
A thermodynamic cycle refers to how a system can go though a number of changes and return to the same state. They are used as a means to transfer heat energy into work. Some common thermodynamic cycles are the Carnot Cycle, the Auto Cycle, the Diesel Cycle, the Sterling Cycle, the Rankine Cycle and the refrigeration cycle.
Carnot and Rankine Cycle
The Carnot Cycle is the theoretical limit as to what is possible in terms of converting a temperature difference into electricity, but it is unrealizable since it requires an infinite amount of time to extrate a finite amount of power over a finite area. Every steam plant tries to closely approximate the Carnot Cycle, but balance other considerations like power production. After considering the tradeoffs, the actual thermodynamic cycle more closely represents the Ranking cycle.
The Sterling Cycle
Modern steam plants and the associated turbines cannot easily be built on a small scale while maintaining performance in terms of efficiency cost and power production. A cheaper alternative for small scale power production is the sterling cycle. The sterling cycle is one of the first types of heat engines and the preferred method of converting solar power to electricity for dish shaped solar thermal power.
Turbines
Large scale generates are generally powered by turbines. Turbines are usually characterized by high efficiency yet a lower ratio of power output per mass. They can be found on airplanes but are rarely found in automobiles. A compromise between the piston and turbine is the quasi turbine.
Working Fluids
In a thermodynamic cycle used to produce power the turbine converts a pressure difference into mechanical energy, which is converted into electrical energy via a generator. The pressure difference can be created by obstructing a fluid flow as is the case in a wind turbine or by generated by pumps moving the liquid from the low potential sized of the condenser to a high pressure side.
Energy Losses and Gains in a Rankine Cycle
Although energy is lost by the pumps when fluid from a low pressure size (In the condenser) is pumped to he high pressure sized (storage tank) there is still an overall gain in energy in the rankine cycle because for incompressible flow the work done on a fluid is equal to the pressure multiplied by the change in volume. <math>W=P dV</math> Sine the heat source expands the fluid before in passes through the turbine the total volume of fluid that passes though the turbine is greater then the total fluid that passes though the pumps. Since the pressure difference across the turbine is approximately the same as the pressure difference across the pumps and the volume flow across the turbine is greater then at the pumps there is more energy produced by the turbine then is expended across the pumps.
Types of Working Fluids
On earth water/steam is typically used as the working fluid for power generation. For refrigeration general fluids are used that expand significantly with temperature (e.g. Freon). For heat transfer generally liquids are used because of they have a higher heat capacity than gases. And in some cases like CNC machining a coolant is used that does not undergo a thermodynamic cycle.
The working fluids used on Earth may not be appropriate for Mars or other space applications. For instance in nuclear power generations metal cooled reactors can achieve a much higher power to weight ratio then water cooled reactors or even reactors cooled by reffergerents as in the case of JIMO. Thus metal cooled reactors would offer higher performance for interplanetary propulsion systems.
For power production it may not be desirable to use water on Mars because on Mars although water exists in many places, including the bottom of craters, the polar ice caps and deep under ground it is far less plentiful then on earth. Also because of the low temperatures there could be issues with the water freezing. An alternative working fluid is carbon dioxide (carbon dioxide as a working fluid). Carbon dioxide is plentiful on mars and the main constituent of the Martian atmosphere.
