The ABCs of ORCs

Organic Rankine cycle systems are a good fit for companies with biomass waste streams to produce combined heat and power, and, in some cases, sell power to the grid.
By Ron Kotrba | December 29, 2017

A fundamental principle of distillation, whether for beverage alcohol production or petroleum refining, is that most liquids have different boiling points. Alcohol boils at 173 degrees Fahrenheit, and water at 212 degrees, so to make alcohol, distillers employ temperatures between those two points in order to evaporate the alcohol, and then condense it back into liquid form—sans water. Now, imagine standing outside on a cool autumn day with a cold, closed jar of unknown liquid. It’s 58 degrees outside, and the jar is much colder than that. When you open the jar and expose the liquid to the 58-degree air, it starts to boil. 

This is the basis of the organic Rankine cycle (ORC)—a thermodynamic cycle using an organic, high-molecular mass fluid instead of water to produce power from low-grade heat. “ORC is literally a refrigeration cycle in reverse,” says John Fox, managing director of ElectraTherm. Fox is former CEO of ElectraTherm, a small Reno, Nevada-based startup that experienced significant growth from 2010-’16. The company was acquired one year ago by Bitzer SE, a privately held German compressor manufacturer, and Fox was asked to stay on as managing director after the acquisition. Fox says ElectraTherm now has 65 systems worldwide in 11 countries with 750,000 fleet hours. In explaining how ORCs work, Fox says, “In refrigeration systems, you buy kilowatts and transfer heat. It’s just the opposite for ORCs. Here you transfer heat and make kilowatts—you do the cycle in reverse.”

In an ORC system, the heat contained in the gases from combustion or gasification of fuel is transferred via a closed thermal oil loop. “The heart of the system is the ORC turbine, which feeds with mechanical power a generator for the production of electric power,” says Filippo Vescovo, deputy manager for Turboden Turkey. Turboden, an Italian firm owned by Mitsubishi Heavy Industries, has installed 355 turbogenerators in 38 countries, according to Vescovo.

In the Rankine cycle, water is heated in a boiler to create steam or high-pressure vapor that, as Fox explains, is released across an expansion device—turbines or screw expanders. “It spins something and out comes kilowatts,” he says. High-grade heat—400 to 600 degrees—is needed to drive the steam cycle. But ORC systems make use of low-grade heat due to the low boiling point of the organic fluid.

ElectraTherm uses a refrigerant in its ORC systems that boils at 58 degrees. “If it wants to boil at 58 degrees, then it really wants to boil at 250 degrees,” he says. There are many applications for 500-degree heat, but far fewer for 200-degree waste heat.

At the heart of ElectraTherm’s ORC systems are twin screw expanders. “It’s a male and a female screw, like a piston, rotating against each other,” Fox says. High pressure goes through the screws and, as it moves from high to low pressure, the screws spin. “That’s the expansion device,” he says. “We like the twin screw expanders because we can run dual phase flow—liquid and vapor at the same time. You could never do this through a turbine as it would damage the blades—you never want impingement of liquid on the turbine blades.”

Antonio Mendes Nazare, vice president of sales and marketing for Paris-based ORC designer and manufacturer Aqylon, says to generate power from heat sources, either technology—traditional steam turbines or ORCs—could be used. “Depending on the features of the project, however, one of these technologies would certainly fit better than the other,” Nazare says. “For high powers above 10 or 15 megawatts of electricity, the steam turbines make a lot of sense. But for lower capacities, the ORCs are bringing better economics and technical figures.”

Advantages Over Steam
Beyond their ability to operate with much lower heat sources, Nazare notes several advantages ORCs have compared to steam turbines. One is that ORCs have a high efficiency at partial loads. “Even if the heat input decreases by more than 50 percent, or increases by 20 percent, the ORC follows the heat and remains in a close range of efficiency while steam turbine efficiency drops dramatically, or even stops, with slight load variations,” he says.

A big problem for steam turbines is their loss of efficiency over time as a result of water’s long-term effect on metal equipment. “For ORCs, there is no erosion of the metallic parts and blades caused by water,” Nazare says. “Therefore, the first-day efficiency of ORCs remains for their lifetime, while the efficiency of steam turbines gets significantly degraded with time.” Furthermore, the water used in a steam turbine must be demineralized using a costly water treatment system. “Moreover,” Nazare says, “steam turbines imply large quantities of water consumption. Depending on the site or country, this can become a heavy economic issue. ORC modules don’t have any fluid consumption, as all the required fluids circulate inside a closed loop.”

Another advantage of ORC systems is that fewer, or in many cases no, operators are required to run them thanks to their automatic, continuous operation—not the case for steam turbines. “Because of its high turbine rotation rate and high pressure, the steam turbine requires permanent presence of operators to ensure the safety of the site,” Nazare adds. Vescovo says ORC turbines work at lower pressure levels and turbine speeds, implying lower mechanical stress in moving parts. This, he says, equates to lower maintenance costs and requires less effort and skill for operation.

There is a big difference in the restart time between the two types of systems as well. Nazare says it can take days to restart a steam turbine whereas an ORC system restarts in minutes. “At a site where the grid stops regularly or frequently, this makes a real impact on the revenues,” he says.

Last, over five to 10 years, ORC efficiencies and related costs are more attractive for small-capacity power generation systems. “We can say that the capex for an ORC solution is slightly higher than for a steam turbine solution,” Nazare says. “However, it is largely compensated by the difference in the opex, as the opex of an ORC solution is two to three times lower than for a steam turbine solution. Over a project lifetime, the internal rate of return is in favor of the ORC solution.”

Ideal Applications
“We always like solving problems,” Fox says, adding that the value proposition of his ORC units goes up when an ElectraTherm system can fix a situation. “For instance, like with what we did in Maryland with chicken manure.” Last June, ElectraTherm announced the sale and shipment of one of its ORC systems to a farm in Maryland that will combust chicken litter and use the heat to produce electricity for on-site consumption. Due to environmental regulations concerning run-off into the Chesapeake Bay, spreading the manure was no longer a viable option. Tipping fees to get rid of the waste were eating away at the bottom line. “We put all that into a return on investment for the client,” Fox says. “We’re solving the tipping fees and transportation problems by taking in a waste on the frontend, and getting Btu out of the backend.” 

Nazare says there are thousands of ORCs installed all over the world and hundreds of them in biomass installations. “There are a few manufacturers of ORCs, and most of them are addressing different specific sizes, temperatures, applications or markets,” he says. “The main difference Aqylon has—apart from the fact that we are addressing all markets, temperatures and applications—is probably its unique technology.” He says not only does Aqylon’s patents address the price/efficiency ratio, but also containerizing all its high-temperature ORCs. “This brings not only easier transportation and faster on-site installation, but mainly significant savings on the civil work,” Nazare says, “as we just need a concrete base or even some concrete plots.”

ORC systems are a great fit for small-scale, localized, distributed power generation, particularly for companies whose processes create a waste stream—either waste fuel like sawdust or chicken manure, or waste heat from existing combustion practices—in order to reduce “behind-the-fence” expenditures on retail power or, in some circumstances, sell power back to the grid. “We’re the low-temperature, smaller guys,” Fox says. “We manufacture 35, 65, 110-kw systems that provide the benefits of combined heat and power (CHP). Besides making electricity, with an ORC system you can heat water for barns, chicken coops, office buildings, and dry wood chips or other biomass materials, thereby raising your efficiency. The more bites to the apple you get, the more valuable your waste heat is.”

Fox says sometimes it doesn’t make economic sense for companies to buy an ORC system for the purpose of selling power to the grid as a money-making venture, unless there are biomass power incentives at play. “For us, we sell kilowatts,” he says. “So the value proposition is, the higher the kilowatt price, the better the return—and in places like Japan, Alaska or Europe, it works. But if you go into Oregon, it might only be 5 cents a kilowatt. The economics are challenged in the U.S. Our major biomass pull has been from the U.K. and its renewable heat incentives.”

In 2015, ElectraTherm had zero units sold in the U.K. In 2016, the company sent a dozen systems over. “In one year, 0 to 80 percent of my business was in the U.K., and it’s all incentive-driven,” Fox says. “But that’s where the market is. Incentives work.”

Author: Ron Kotrba
Senior Editor, Biomass Magazine