I think that the importance of the ORC comes from its flexibility to use energy sources of lower of low-temperature  such as biomass combustion, industrial waste heat, geothermal heat and solar collectors. 

Dear Nabil,

Generally, the efficiency of the ORC is not as high as the conventional Rankine cycle. However, ORC is considered as a superior for low to medium heat sources. So, for low or medium temperature energy sources, it is almost impossible to generate high temperature steam to run a turbine. The efficiency of the ORC strongly depends on the working fluid used which its selection depends on the heat source level and the system configuration. The higher the source temperature, the higher efficiency can be achieved if the aproperiate working fluid is selected.

Dear Khaled,

Thank you for answering my question. But, is there a certain fluid families that recommended to be used to obtain high efficiency?

could you please provide a good review article about ORC... I am really interested to know more about it.

Thank you in advance and Regards

Dear Nabil

Many researchers reported that there is no a unique working fluid which is the best for all the applications and heat source levels. There are some working fluids possess high performance however their effect on the environment restricts their use. Again, the heat source temperature plays a vital role in the working fluid selection. As an example for a heat source temperature of about 150C, R245fa is one of the recommended fluids if the environmental point of view is considered. Regarding to the review articles, once I am back I will provide you with them.

Regards  

Ignoring aspects such as environment-friendliness and thermal stability at operating temperature, theoretically, the working fluid plays no part in the cycle efficiency. However, considerations that lower the cycle efficiency only relate to how much it deviates from a reversible process. Irreversibility arises from heat transfer across a finite temperature difference which is determined not only by the heat exchange equipment, but the fluid's heat transfer coefficient also, which depends mainly on the thermal conductivity of the fluid. As for irreversibility due to finite pressure drop, it depends on the velocity (a consequence of the dimensions of the flow passages) but also on the viscosity. Therefore it would be best to choose a fluid with the highest thermal conductivity and least viscosity. Specific heat of various candidate fluids varies little, and hence can be ignored.

I think that the importance of the ORC comes from its flexibility to use energy sources of lower of low-temperature  such as biomass combustion, industrial waste heat, geothermal heat and solar collectors. 

Biomass combustion is by no means a low temperature source, though I agree with the others. We have combusted biomass in a Swirling Fluidized Bed and reached temperatures of 900 C. We have also generated steam at 10 bar. The only reason for not going to higher pressures and therefore, higher temperatures is that it was a laboratory unit.

Dear Khaled,

Thank you again, and I wish you all the best

Regards

Dear Professor Vijay,

Thank you very much indeed for your cooperation and this interesting information.

Best Regards

Dear Gamal,

Thanks a lot for your cooperation .

Regards

As explained in the previous messages, your efficiency will depend of the heat source temperature. The maximum cycle performance you get reach is determined by the carnot efficiency (1-Tc/Th). However, as already mentionned, many irreversabilities decrease this optimum and these losses depend of the technology used for each component. Experimentally (because there is always a difference with the theoritical expectations), low-temperature ORC are typically around 5 to10% for the net efficiency. Higher values (~20-25%, even up to 30%) can be reached at nominal point with high-temperature heat source (300-350°C at the expander supply).

It must be noted that the efficiency is not the only criteria you must account for while choosing a fluid. Toxiticy, GWP, ODP, flammability, cost, density, ... are among many other properties you must keep in mind.

Dear Remi,

Thank you very much, really wonderful information. If you have got good article regarding this , could you please pass it to me.

Best Regards

Regarding the experimental data, or the fluid selection criteria?

Dear Remi,

Actually, I am interested in ORC and I like to know more about it , so I will appreciate if you can provide any articles you think are useful.

Regards

I would advice you to have a look to S. Quoilin's PhD thesis (http://orbi.ulg.ac.be/bitstream/2268/96436/1/PhD_Thesis_Dissertation.pdf). His manuscript covers many relevant subjects of ORC systems, from the fluid selection to modeling methods and control aspects . You can also have a look to the KCORC website which provides many useful literature reference (http://www.kcorc.org/en/)

Dear Nabil

I find this article is interesting and may be useful for you

http://docs.lib.purdue.edu/cgi/viewcontent.cgi?article=1721&context=iracc

Regards

Saleh Etaig

Dear Remi,

Thank you very much indeed for your generous cooperation.

Best Regards

Dear Mr Etaig,

Thanks a lot for forwarding this interesting article.

Regards

I learned this from professor I.K.Smith, City University London.

The maximum efficiency possible from heat recovered from a hot fluid stream, which cools as it gives up its heat, in which all the processes are reversible is given by the expression:

 Eta-cycle = 1 - To/(T-To) x ln(T/To)

 Where To is the ambient temperature and T is the initial fluid temperature.  This expression is obtained by assuming that power is produced through an infinite number of infinitesimal Carnot Cycles, each operating over the temperature difference between the local source temperature, as the heat source cools, and the sink temperature, which is assumed to be constant.

 I send you a diagram generated by the above formula.

Dear Bo Ivar,

Thank you for participating in this discussion.

Regards

Dear Nabil,

Regarding the review papers I promised you last week, here are some useful ones which cover wide sides of ORCs and their applications. Also, the selection of the working fluid is generally covered in these articles.

'Low-grade heat conversion into power using organic Rankine cycles – A review of various applications', 2011, by Tchanche et al.

'A review of working fluid and expander selections for organic Rankine cycle', 2013, by Bao and Zhao.

'Techno-economic survey of Organic Rankine Cycle (ORC) systems', 2013, by Quoilin et al.

'Organic Rankine cycle performance evaluation and thermoeconomic

assessment with various applications partI:Energy and exergy

performance evaluation', 2016, by Gang Li.

I hope this may help.

I attach a plot showing the approximative ORC-efficiency of what you can get of a system with todays technique.

The type of fluid is important. It effects the performance to some because of many things, do you expand in the superheated region or not ....

How much energy must you spend on pumps in the system ...

Of great importance is: What size will you get on for instance a Screw Expander..

Please find enclosed a graph showing that the choice of working media has a great effect on the performance of Screw Expanders.

Please note that I have corrected the formula for generating the graph ORCbest.png.