June 18, 2021

Supercritical CO₂ enables Power Generation from Waste Heat

5 min read
Siemens Energy has licensed Echogen Power Systems’ patented technology to use supercritical carbon dioxide (sCO2)...

Siemens Energy has licensed Echogen Power Systems’ patented technology to use supercritical carbon dioxide (sCO2) as the working fluid in a closed-loop power cycle to collect waste heat from the source and convert it to electrical power.

Founded in 2007, Echogen, located about 40 miles south of Cleveland, in downtown Akron, Ohio, is working in commercial sCO2 systems. Supercritical CO2 has several properties (which we’ll discuss in a moment) that make it particularly suited for this application. In an interview with EE Times, Timothy Held, Echogen CTO, said that unlike classical steam-based systems, sCO2 cycles can start and operate autonomously, with zero water consumption and zero risk of freezing, which is especially important in remote cold-weather sites.

Several types of machines, electrical equipment, and industrial processes produce huge amounts of thermal energy, which in most cases escapes into the environment, unused. Often, this waste heat can be recovered and used for other purposes. When this occurs, we talk about “waste heat.”

Depending on the specific application, waste heat can be recycled back into the existing manufacturing process, transferred from one process to another within the same facility, or converted into mechanical or electrical power. These processes are alternatively referred to as waste heat transfer (WHT), waste heat to mechanical drive (WH2MD), and waste heat to power (WHP). The first two options are usually economical, but not always practical, inasmuch as the product must be used locally. WHP has the advantage of creating a product — electrical power — which can be used directly onsite, or can be easily transferred to another location or to the grid.

Waste heat to power
WHP creates electricity by heating a fluid at high pressure and then expanding the fluid through a turbine to power an electric generator. A typical waste heat system is based on the Rankine cycle, where a turbine is moved by a pressurized water vapor (steam), which has been boiled by the waste heat stream. The steam is then cooled off and condensed to liquid water, pumped back to high pressure, and begins the process again. This system takes heat from an external source and converts it to power, or work, through a closed-loop thermodynamic system.

The industrial sector, which in 2009 accounted for more than 30 percent of all United States energy consumption, represents the greatest potential source for WHP application. About 30 percent of this consumed energy is wasted as thermal losses due to inefficiencies. Another electric generating capacity is available for recovery in non-industrial applications, such as exhaust from natural gas pipeline compressor drives, landfill gas engines, refineries, steel mills, glass furnaces, thermal oxidizers and cement kilns.

Supercritical CO2
Supercritical carbon dioxide is a fluid state of carbon dioxide that is reached when it exceeds its critical temperature and critical pressure. In this state, carbon dioxide assumes intermediate properties between a gas and a liquid. A first benefit of this solution is high density, which means a very small footprint. Moreover, sCO2 is a non-flammable, non-toxic, non-corrosive, thermally stable working fluid. Due to these properties, sCO2 can interact more directly with the heat source, eliminating the need for a secondary thermal loop, and further reducing the total installed cost of the system. Figure 1 shows how the entire system operates on the following main five steps:

  • liquid CO2 is pumped above its critical pressure, thus obtaining sCO2
  • CO2 is preheated in the recuperator
  • recovered waste heat is added at the waste heat exchanger
  • high-enthalpy CO2 is expanded in the turbine, driving the generator
  • CO2 is cooled (by means of air or water) in the recuperator and then condensed to a liquid.
Figure 1: the principle of operation of sCO2-based WHP. Click to enlarge the image above.

Supercritical fluids offer advantageous properties of both liquid and gas. In particular, CO2 is widely chosen because it becomes supercritical above relatively low temperature and pressure: 31˚C and 74 bar, respectively. Held pointed out that the outstanding properties of supercritical CO2 allow obtaining a compact, closed-loop system with minimal operating and maintenance support. Unlike the traditional steam-based cycle, it is very flexible, allowing integration with a wide variety of exhaust temperatures, from low-speed diesels to simple cycle gas turbines. “Finally, it allows achieving high efficiency in thermal-to-electric power conversion, remaining competitive from an economical point of view,” said Held.

Echogen Power Systems
With 39 issued patents and over 30 pending, Echogen has progressed from multi-kW demonstration units to multi-MW commercial products, including its 8 MW EPS100 heat engine. Echogen licensed its technology to Germany’s Siemens Energy.  In February 2021 Siemens announced it will deploy the heat-to-energy system developed by Echogen to convert the waste heat from a TC (TransCanada) Energy natural gas pipeline into 9.3 megawatts of electricity.

According to Held, the company has been working for over 10 years to bring its technology to market, refining its system with projects and model simulations demonstrating its know-how and the system feasibility. Held said the goal is to show it will be able to generate the amount of power the company said it would; that validation will allow Echogen to leverage it into other opportunities.

By using supercritical CO2 instead of traditional water and steam, Held said that Echogen’s closed-loop system achieves higher efficiency while featuring a smaller size than traditional steam-based systems. In this specific installation, the heat is generated by a natural gas compressor station that pushes gas through the pipeline, thus delivering a huge amount of waste heat in the environment. Siemens Energy’s interest is primarily focused on oil and gas applications, since companies operating in this sector need support to decarbonize their operations.

Figure 2: an Echogen heat recovery system is loaded for transport. Click to enlarge the image above

Echogen has also developed an energy storage technology they hope will be shortly adopted by the renewable energy industry. Using a substrate material, such as regular silica sand, Held said that this technology is able to capture and hold the heat generated by a wind turbine, solar PV farm or another high-power source. The stored heat can later be reused and converted back into electricity. This energy-storage system could fulfill one of renewable energy’s greatest challenges: how to keep providing energy when the wind strength is not enough or the sun is not shining.

“Over the last 14 years, we have been developing technology in two main areas. One of them is basically the conversion of heat into power (electricity) through a power cycle that uses supercritical carbon dioxide (sCO2) as the working fluid. The other one, where we have spent the last three years, is energy storage. That’s where we really see the big needs of global energy, a big opportunity where production and consumption system is going to be heading”, said Timothy Held.

The post Supercritical CO₂ enables Power Generation from Waste Heat appeared first on EETimes.

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