Organic rankine cycle

Using the Organic Rankine Cycle (ORC) allows even relatively low-grade byproduct heat to be economically converted to valuable electric power.

08/18/2015


Figure 1: Simplified diagram of an organic Rankine cycle plant for heat recovery and electric generation. Courtesy: TurbodenIncreasingly, industrial and institutional energy users look for ways to extract more useful energy from each fuel dollar spent. A key solution is to utilize byproduct heat from industrial processes. One challenge has always been low-grade waste heat. By using the Organic Rankine Cycle (ORC), even relatively low-grade byproduct heat can be economically converted to valuable electric power.

Rankine Cycle a foundation of power plants

The Rankine Cycle, developed by the Scottish engineering genius William Rankine, is the fundamental principle used in the design of vapor engines such as steam powered reciprocating and turbine engines. It describes the role of heat in the phase change between liquid and vapor, and the potential of this phase change to generate mechanical power for electric generation. For decades, mechanical engineers have used the Rankine Cycle calculations to design turbogenerators.

The principal limitation on the use of water as the base fluid in an engine is the need for relatively high heat levels to accomplish the phase change, because of the high boiling point of water. Thus if a heat exchanger is used to collect process waste heat, the temperature of the heat must be significantly above the boiling point of water. The water-steam phase change is not usable for lower temperature sources.

Lower temperature working fluid

However, an interesting option is the use of what is called the Organic Rankine Cycle (ORC), where the working fluid has a phase change at a lower temperature. A variety of organic liquids have much lower boiling points, and thus can develop useful pressures to spin a turbine using much lower temperature thermal sources. In recent decades, the ORC has been developed to generate electric power from heated liquids from solar ponds, geothermal sources, industrial process fluids, and from the exhaust streams of conventional steam cycles.

The thermal efficiency of the ORC is often quite low, but when it is being used in combination with a thermal stream that would otherwise be wasted, this may not be a difficulty. Especially given the increasingly high cost of central-station electricity, and the need to extract maximum energy from purchased fuels, the ORC will increasingly be adopted by industrial and institutional energy users.

Italian firm an industry leader

Figure 2: An organic Rankine cycle plant is used for heat recovery and electric generation at this cement manufacturing plant in Morocco. Courtesy: TurbodenOne of the leaders in the development and promotion of ORC technology is Turboden, a group company of Mitsubishi Heavy Industries. The company is headquartered in Brescia, Italy and markets this technology worldwide. Turboden has manufactured turbogenerators using ORC technology since 1980, and offers systems ranging in size from 200 kWe output up to 3 MWe. They have extensive experience using byproduct heat streams from industries including cement-making, glass furnaces, forest products, and electric generation using exhaust streams from engines and combustion turbines.

Clotilde Rossi de Schio is Manager for Sales and Development for Turboden in North America. She points out that these turbogenerators take advantage of the lower temperature limit for economic energy extraction. "In order to give a general indication, a ballpark figure can be 100 C [212 F] for liquid sources and about 140 C [284 F] for exhaust gases." This is significantly lower than is practical with a water/steam cycle.

Wide range of industrial heat sources

A wide range of industries have potential for electric generation using byproduct heat. Rossi points out, "Typical industries that are looking for heat recovery options are energy intensive industries such as cement, steel, aluminum, refractories, glass and chemicals. Another attractive source is as a bottoming [final heat recovery] cycle for gas turbines and gas engines." She adds, "Any heat source in the form of gas, liquid or steam can be a good candidate for heat recovery, independent of where the heat is coming from."

She notes, "Turboden utilizes different families of organic fluids, depending on client needs and preferences (e.g. non-flammable) and on thermodynamic properties. They can be siloxanes, hydrocarbons or refrigerants. They are typically used at operating temperatures far from their cracking points, therefore we can consider their lifetime to be 20+ years."

Proven reliability

Because of the relatively long experience with this technology, and the fact that these turbogenerators operate at relatively low temperatures, they are highly reliable. Rossi indicates that Turboden's experience is that with all installed units, average availability is greater than 98%.

Figure 3: A view of an installed Turboden ORC heat recovery application for electric generation. Courtesy: TurbodenShe explains that units can be operated automatically and are normally unmanned. An annual service of one week per year of predictive maintenance is recommended. Turboden's website includes a calculator to provide an estimate of the power generation potential of various thermal streams, with the input of character, volume and temperature of the stream.

GE Clean Cycle Technology

Another manufacturer that has commercialized ORC technology for heat recovery or other applications is General Electric, with their Clean Cycle technology. This product is a turbine-powered ORC generation package rated at 50 kWe, and scalable to 1 MWe or more with multiple units. It is particularly suitable for heat recovery from engine or turbine exhausts.

Alternative to the turbine

Another intriguing application was described by Michael Newell, CEO of Ener-G-Rotors at a recent Technology & Market Assessment Forum, sponsored by the Energy Solutions Center. The New York firm has developed a unique rotary technology for extracting energy using the ORC with a method alternative to a turbine design. According to Newell, the product is nearing commercialization. Its advantage is significantly lower cost per kWe of capacity than with the use of turbines in smaller scale applications.

According to Newell, their ORCA unit will be rated at about 50kWe and will be able to generate electricity from waste heat at a cost of three to four cents per kWh. He indicates this will offer a two-year payback in many applications, and has the potential to reduce CO2 emissions by 28 tons per year.

Newell cites the forest products and paper industries as two examples of industrial operations that have large volumes of low-grade steam that would be highly suitable for Ener-G-Rotors applications. The company is having conversations with a large number of industries for possible field applications.

Heat recovery to lower emissions

Heat recovery is receiving increased attention as a means of getting better use of the energy dollar as well as reducing carbon emissions. The Organic Rankine Cycle is an interesting option, with a growing number of applications for industrial, utility and institutional energy users who have lower temperature byproduct heat streams. The commercial offerings continue to increase in size and number. Your consulting engineer can help your make an assessment of its suitability for your application.

MORE INFO

Ener-G-Rotors 

General Electric Clean Cycle 

Ormat Technologies, Inc. 

Turboden 

This article originally appeared in the Gas Technology Summer 2015 issue.



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