In organic Rankine cycle, an organic fluid is used in the place of water to reduce the cost of heating done in the boiler. The organic fluid used has a lower boiling point as compared to water due to which it requires less heat in the boiler to produce the organic fluid steam and it also requires less energy at condenser for cooling (S. Quoilin 2011). Less heat requirement means low fuel-burning which reduces the cost of operating the Rankine cycle. The organic fluid selected for the organic Rankine cycle is evaluated on strict selection criteria and the fluid performing the best is select as working fluid. The selection criteria of the organic fluid consist of thermal properties that influence the performance of organic fluid or are very critical for the basic working of the organic Rankine cycle.
Following is the list of thermal properties and why they are important for the evaluation of the organic fluid of organic Rankine cycle
1. Heat Capacity
The heat capacity of the fluid is defined as the amount of heat required to increase the temperature of the fluid by one degree. It is measured as joule per kelvin or centigrade. It’s a critical thermal property of the fluid in this case as a material with high heat capacity will require more heat to increase its temperature and at the same time will deliver more heat for a decrease in its temperature.
2. Thermal Conductivity
Fluid ability to transfer heat rather it is moving into the fluid or moving out of the fluid. Fluid for ORC should have a high thermal conductivity in order to make the heat transfer process at the heat exchanger or boiler efficient.
3. Viscosity
The viscosity of the fluid defines the resistance made by the fluid to change the state of rest or continuous motion of one molecule with respect to the other. The viscosity of the organic fluid should be low for the ORC case so the pump does not work too hard.
4. Chemical Stability
Chemical stability of the organic fluid used in the organic Rankine cycle is also the thermal stability of the fluid. The chemical stability of the organic fluid of the organic Rankine cycle means the ability of the fluid to resist decomposition, deterioration, and any chemical reaction with container or tube material at high temperature during the process and its useful life. The chemical stability of the organic fluid of the organic Rankine cycle has to be excellent because organic fluid with low chemical stability will decompose and react inside the organic Rankine cycle forcing the system to stop or reduced performance.
5. Low Freezing Point
Long with its ability to resist high temperatures the ability of organic fluid of organic Rankine cycle to resist freezing inside the turbine or condenser is also very important. The sudden expansion of organic fluid inside the turbine or condenser of the organic Rankine cycle can force the organic fluid to freeze thus forcing the system to stop. The freezing point of organic fluid of the organic Rankine cycle should be lower than any point in the complete organic Rankine cycle.
6. Heat of Vaporization
The organic fluid used in an organic Rankine cycle should have the ability to absorb a high amount of energy before it gets evaporated. High latent heat of the organic fluid will help it to absorb more energy from the boiler of the organic Rankine cycle which will in return help to reduce the mass flow rate of the organic fluid of the organic Rankine cycle. This also helps to reduce the size of the pumping and related system of organic Rankine cycle thus saving the initial cost of installation of the organic Rankine cycle.
7. Density
The density of the material defines the material ability to hold heat and it defines the organic fluids' ability to deliver high quality and quantity of the heat. Greater the density of the organic fluid of the organic Rankine cycle greater will be the heat absorb, maintain, and delivered by the organic fluid or organic Rankine cycle.
8. Pressure
The organic fluid used for the organic Rankine cycle should be able to resist the pressure applied onto it during the working of the organic Rankine cycle. The organic fluid used for the organic Rankine cycle should not decompose or deterioration and any chemical reaction with container or tube material at high pressure.
9. Environmental aspect
The organic fluid used in an organic Rankine cycle should have minimum impact on the environment. The impact on the environment includes the energy and resources required to obtain the required organic fluid of the organic Rankine cycle. The environmental aspect also includes the decomposition and disposal of the organic fluid after its useful life.
10. Safety
The organic fluid used for the organic Rankine cycle should be safe to handle and safe to work with. The organic fluid used for organic Rankine cycle should not be harmful to human and surrounding in its natural state and at normal temperature
11. Availability
The organic fluid used for the organic Rankine cycle should be easily available as a good quantity of the fluid is required in the organic Rankine cycle.
12. Cost
The cost of the organic fluid used for the organic Rankine cycle should below in order to keep the cost of operation of the organic Rankine cycle lower than standard Rankine Cycle.
Organic Fluids of Organic Rankine Cycle
Selection of the Organic fluid for use in the organic Rankine cycle is one of the most important steps in analysis and optimization of ORC. So the selection of the fluid is done on the basis of the work done in the literature and the fluid thermal properties mention above. Some of the organic fluids used in previous work are Iso-butane, n-pentane, Iso-pentane, methanol, ethanol, R123a, R245fa and dichloro-trifluoro-ethane (HCFC-123). Work of kamyar darvish on selecting the optimum fluid for organic Rankine cycle is show that the Iso butane and R245fa is much better as compared to n-pentane and Iso-pentane whereas the work of Yamamoto (1999) show that dichloro-trifluoro-ethane is better than methanol, ethanol, and water. Comparing the thermal and physical properties of dichloro-trifluoro-ethane, Isobutane and R245fa shows that the dichloro-trifluoro-ethane cannot perform better as it has very high density. Iso butane has lower boiling temperature and latent heat but R245 has higher density and high critical temperature.
|
Fluid |
Density Kg/m^3 |
Boiling point K |
Latent Heat KJ/Kg |
Critical Temp K |
Critical Pressure MPa |
Freezing point K |
Safe |
|
R245fa |
5.71 |
288.4 |
196 |
427 |
3 .65 |
167 |
Non flame able |
|
Iso butane |
2.44 |
261 |
165 |
407 |
3.64 |
114 |
Nonflame able |
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