Organic Rankine cycle technology boosts onsite power projects

Paul Boughton

A novel waste heat recovery process that couples a gas engine to traditional organic Rankine cycle technology is boosting efficiency at power generation sites. Sean Ottewell reports.

GE Energy and energy developer ECOS have announced they plan to demonstrate an innovative, industrial waste-heat recovery system that they say will dramatically increase the efficiency and output of a 7.2MW biogas power plant in the eastern Slovenia town of Lendava, near the border with Hungary.

GE's new pilot Organic Rankine cycle (ORC) waste-heat recovery system for gas engines is designed to make onsite power plants that use natural gas, landfill gas and other waste gases more cost-attractive to build as countries install more cogeneration and renewable energy capacity to enhance energy security and lower regional emissions.

Executives from GE Energy and ECOS signed the new project agreement during a celebration of the fifth anniversary of GE's European global research facility in Munich. GE's new ORC system will allow ECOS to capture more waste heat created by its 7.2MW Bioplinarna Lendava biogas plant. The extra thermal power will be used to produce steam, which in turn will help generate enough electricity to support 300 European homes without using additional fuel.

"Our goal is to demonstrate that dramatic improvements in energy efficiency and output can be achieved in gas engine power plants through enhanced waste-heat recovery," said ECOS director and owner Joze Pavlinjek. "We believe our collaboration with GE will help lead to the expanded use of cogeneration and renewable biogas solutions in support of European Union directives designed to improve regional energy security and combat global climate change."

The pilot ORC system will be installed on one of the three GE ecomagination-certified Jenbacher J420 biogas engines that have powered ECOS' Bioplinarna Lendava plant since June 2008. The ORC technology will boost the Jenbacher unit's electrical efficiency by an estimated five per cent.

Landfill gas and other renewable biogas projects are among the prime candidates for ORC systems, especially in countries - including those in Europe - that offer high electricity feed-in tariffs.

"Countries around the world want to increase the use of renewable biofuels to meet their energy security and environmental requirements," said Prady Iyyanki, ceo of GE's Jenbacher gas engine business. "Pairing GE's ORC technology with gas engines represents an important innovation in energy efficiency, allowing existing and future onsite power plants around the world to generate extra electricity without consuming additional fuel or creating more emissions."

The ORC is so named for its use of an organic, high molecular mass fluid with a liquid-vapour phase change or boiling point than water to create steam for electricity generation. Because of this lower temperature, however, the choice of working fluid here is crucial. It is the thermodynamic characteristics of this fluid that determine how successful the process is.

Optimal characteristics of the ideal working fluid include: an insentropic saturation vapour curve; low freezing point, high stability temperature; high heat of vaporisation and density; low environmental impact; safety; good availability; and low cost.

Refrigerants and hydrocarbons are two commonly used working fluids. However, due to technical waste-heat recovery constraints, there had been few gas engine-based ORC applications.

GE says its new gas engine-ORC technology is a milestone for the global energy industry because for the first time all of the waste heat from an engine's exhaust gas and cooling cycle can be fully captured and utilised to drive the power plant's enhanced steam-creation process.

Thomas Frey, research scientist at the alternative energy lab of the global research centre, explains: "Thousands of megawatts of heat are wasted through stacks, chimneys and coolers into the atmosphere every day via refineries, steel mills, cement plants, furnaces and power plants. The latter have only an average electrical efficiency of 33 per cent in the US. The rest is thermal heat - a huge untapped source of energy. Experts have estimated that low-grade heat worth billions of dollars is wasted every year. Even a significant impact on carbon dioxide emissions could be made, if only a fraction of that heat could be recycled to save fossil fuels rather than rejecting it to the atmosphere. However, cost effective waste heat recovery systems for power production didn't exist so far. This is exactly what motivated our waste heat recovery technology team at GE Global Research Munich to have a fresh look at an old technology: ORC. These systems have been known for more than a hundred years and operate very similarly to the conventional steam-based Rankine cycle - which is the basis of every conventional coal plant. The big difference is that ORCs don't rely on high temperatures from burning fossil fuels but can use much lower heat input temperatures."

In May GE also unveiled its organic regenerator (ORegen) waste heat recovery system developed with GE Oil & Gas. When coupled with a simple-cycle gas turbine, the ORegen system generates electricity from waste heat while consuming no additional fuel or water and avoiding associated carbon dioxide emissions.

Therefore, heat recovery now offers a great opportunity to conserve fuel by productively using waste energy to reduce overall plant energy consumption and simultaneously decrease carbon dioxide emissions. For example, when an ORegen unit is joined to GE Oil & Gas' PGT25 gas turbine, it can provide up to an additional 25 per cent more power on top of the output of the turbine itself.

This breakthrough recently received GE's ecomagination certification, in which a product is evaluated for its ability to significantly and measurably improve a customer's environmental and operating performance. It's the first ecomagination certified product to originate from GE's global research centre in Munich.

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