Ask the Expert: Organic Rankine Cycle (ORC)

Do you have a question for our ORC experts?

Here are some questions our experts have already answered. Don't see yours? Fill out the form and you will get an emailed response.

Please note: Questions posed might be posted, so do not include any sensitive questions that might reveal your company's intellectual property. 

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Question: Can you talk about CO2 as a working fluid? I have seen articles recently about supercritical CO2 as a working fluid.

Answer: You are correct that CO2 can be used as a working fluid in a thermodynamic cycle that has been given the name: Supercritical CO2 Cycle System. There’s considerable interest in this power cycle in the US with funding by DOE but also in other countries. However, this cycle is more appropriately called a Brayton Cycle (i.e., the cycle that is the basis for gas turbine propulsion systems). The cycle can achieve very high cycle efficiencies when it is powered by very high (700° to 900°C) heat source temperatures. The operating pressures are between 1100 psia and 4,000 psia across the compressor and turbine. Of course, a more traditional Reverse Rankine Cycle (vapor compression cycle); the cycle that is used for refrigeration has CO2 as a working fluid at very much lower operating pressures.

Question: What is the best-in-class $/kW for just the ORC? What are the largest available commercial ORC systems?

Answer: This linked technical paper is a reference that can provide very good and current survey of ORC costs. From our experience, these costs are representative of what we have found from competitors. The wide range of costs at power ratings less than 200 kWe is evidence of the variability of costs with source temperature and environment temperatures but also whether the manufacturer has a standard, mass produced system that is offered “AS IS” or whether the systems are custom designed to satisfy unique requirements.

Question: My question is about the possibility of using this technology in turbocharger in ICE. Also, do you think is suitable if it is inserted in solar powered cycles small or micro scale apps?

Answer: Your application is interesting: using the ORC to capture solar energy and then use the recovered power to provide compressed air (i.e., similar to the purpose of a turbocharger) to an IC or even a gas turbine engine. The issue may be the periodic availability of the solar-sourced energy. The 12 hour -ON and 12 hour-OFF cycle will require some means of energy storage. This is certainly possible, and I refer you to this linked technical paper that Concepts NREC prepared for a conference. The paper presents what we believe to be an attractive solution to the integration of energy storage and solar-sourced energy, although in this paper the solar energy is utilized along with heat recovery from a gas turbine, and only implicitly indicates that a steam cycle or ORC could also be used to recover even more energy from the GT’s exhaust waste heat (see Figure 9 in the paper).