System performance

In the previous subsections, we have listed a certain number of criteria that should fulfill a working fluid to be considered as good fluid for an ORC system. But, the most important thing remaining is to know how could perform the system using a particular fluid in realistic conditions. The easiest and fastest way is to simulate the system using a computer program and a database of thermodynamic properties of the fluid. Using the first and second law of thermodynamics [12], the performance of the system can be evaluated under diverse working conditions for different working fluids.

For simplicity, mass, energy and exergy balance equations for any control volume at steady state can be written respectively as follows neglecting the kinetic and potential energy changes:

У m = У m

4-^ out in

(1)

Q + W = У m hout — У m h.

out out n n

(2)

Eh + W = У E — У E +1

h out n

(3)

Where the subscriptions in and out represent the inlet and exit states, m is the mass flow rate,

Q and W are the net heat and work inputs, h is the enthalpy, Eh is the net exergy transfer by

heat and I is the rate of irreversibility. From the above equations, the heat input or rejected, the pump work, the turbine work and many other derived characteristics as thermal efficiency, exergy efficiency and exergy destruction are obtained. Parametric studies performed at various operating conditions for different potential working fluids then show us which one yields better performance.

Most authors while selecting a working fluid for an organic Rankine cycle consider the cycle thermal efficiency as the most determinant parameter. This is not for sure the best way. A very efficient plant can be bulky, expansive and unsafe. On the other hand, it is not easy to find an ideal fluid that fulfils all the desired criteria mentioned in the previous section. One may have low vapour specific volume, good heat transfer properties but present a low critical temperature for example. Since, the ideal working fluid does not exist a compromise should be found among the desired criteria. A general methodology is proposed here. It comprises three steps [1, 13]: (1) data collection, (2) data analysis and (3) decision. At the first step, the operating conditions are set and all the data: thermophysical, safety, environmental and calculated properties concerning different candidates are collected. The data are then analyzed at the second step. At this phase, the criteria are classified starting from the most critical ones and the process is done sequentially. At each sub-step, all the fluids are screened using the criterion and before going to the next criterion, the fluids are put in two groups: those which fulfil the criteria and those which do not. At the last step, the best fluid should come out followed by some alternatives. Usually, it is not easy to make up the decision, and some authors propose to combine several criteria into one that could be used as an “objective criterion” [14]. Different steps described above are illustrated in Table 5.

Table 5: steps involved in the selection of the working fluid

Steps

Sub-steps

Step 1: Data collection

Operating conditions

Evaporating and condensing parameters, components’ efficiencies, desired power output, etc.

Thermophysical, safety and environmental properties

Molecular weight, Boiling temperature, Critical parameters, ASHRAE Classification, ODP, GWP, ALT, etc.

Calculated properties

Efficiencies, Heat input, Mass flow rate, Specific volume, etc.

Step 2: Data analysis

Cycle analysis

T-s/P-h diagram, Critical parameters, Operating conditions

Cycle pressures analysis

Condensing pressure, Evaporating pressure ( 0.1-2.5 MPa)

Thermal stability and compatibility with materials

working fluids/components’ materials/lubricants

System performance analysis

Thermal efficiency, Exergetic efficiency, Heat input, specific work, Irreversibility, Mass flow rate, Power output, Optimum criteria, etc

Size-Cost of the system

Specific volumes, volume flow rates

Safety data analysis

flammability, toxicity, radioactivity, etc

Environmental data analysis

(ODP, GWP, ALT)

Availability and Cost of the fluid

(market availability, prices: €/kg )

Step 3: Decision

Classification

Fluid 1, fluid 2, fluid 3

4. Conclusion

The choice of the most suitable working fluid for a peculiar application is a critical step when designing an organic Rankine cycle system. It is not easy to find a working fluid that could

fulfill all the characteristics of an ideal working fluid. Various criteria that should satisfy a good fluid were given. These include: appropriate critical parameters, moderate cycle pressures, low specific volumes, high efficiency, non-toxicity, non-corrosiveness, easy purchase, non-ozone depleting and no contribution to the global warming.

We attempted in this work to propose a general methodology for selecting working fluids.

The selection according to this methodology can be made in three steps: (1) data collection,

(2) data analysis and (3) decision.

References

[1] O. Badr, S. D Probert and P. W. O’Callaghan, Selecting a working fluid for a Rankine-cycle engine, Applied Energy 21 (1985) 1-42.

[2] W. B. Stine and M. Geyer, Power cycles for electricity generation, In power from the sun, 2001. <http://www. powerfromthesun. net/chapter12/chapter12.new. htm> [July 2008]

[3] R. L. Powell, CFC phase-out: Have we met the challenge? Journal of Fluorine Chemistry 114 (2002) 237-250.

[4] G. Angelino and P. Colonna di Paliano, Multicomponent working fluids for organic Rankine cycles (ORCs), Energy 23 (1998) 449-463.

[5] M. J. Lee, D. L. Tien, C. T. Shao, Thermophysical capability of ozone-safe working fluids for an organic rankine cycle system, Heat Recovery Systems & CHP 13 (1993) 409-418.

[6] V. Maizza, A. Maizza, Working fluids in non-steady flows for waste energy recovery systems, Applied Thermal Energy 16 (1996) 579-590.

[7] H. Yamaguchi, X. R. Zhang, K. Fujima, M. Enomoto, N. Sawada, Solar energy powered Rankine cycle using supercritical CO2, Applied Thermal Engineering 26 (2006) 2345-2354.

[8] Ezzat Wali, Working fluids for solar, rankine cycle cooling systems, Energy 5 (1980) 631-639.

[9] L. Calderazzi and P. Colonna di Paliano, Thermal stability of R-134a, R141b, R13I1, R7146, R125 assiociated with stainless steel as a containing material, International Journal of Refrigeration 20 (1997) 381-389.

[10] G. Angelino, C. Invernizzi, Experimental investigation on the thermal stability of some new zero — ODP refrigerants, International Journal of Refrigeration 26 (2003) 51-58.

[11] Kyoto Protocol to the United Nations Framework Convention on Climate Change, United Nations, 1998.

[12] Yunus A. Cengel, Michael A. Boles, Thermodynamics: An Engineering Approach, 4th edition, McGraw-Hill, 2002.

[13] U. Drescher, D. Bruggemann, Fluid selection for the organic Rankine cycle (ORC) in biomass power and heat plants, Applied Thermal Engineering 27 (2007) 223-228.

[14] H. D. Madhawa Hettiarachchi, Mihaljo Golubovic, William M. Worek, Yasuyuki Ikegami, Optimum design criteria for an organic Rankine cycle using low-temperature geothermal heat sources, Energy 32 (2007) 1698-1706.