The Power Cycle of the Future

power cycle
Manuel A. Fontan, ERS, for Zondits

Steam has an important role in our everyday lives, although we don’t always realize all that it’s doing for us. It can be found virtually in any industry or technology that requires heat, moisture, sterilization, and other crucial processes. Steam can be used to heat the water in our homes, and in some regions, provide space heating via heat exchangers and other heat transfer equipment. But perhaps one of the most important applications of steam is in the electricity sector.

According to the International Energy Agency (IEA), in 2014 around 81.1% of the world’s consumption came from fossil fuels. Around the same time period, the Organization for Economic Cooperation and Development (OECD) – a group consisting of some of the world’s biggest economies, notably without Brazil, China, India and Russia – accounted for an estimate of 49,084 MWh of fossil-fuel generation, while the rest of the world recorded around 80,124 MWh. Notably, most of this fossil-fuel generation uses a Rankine cycle, which enables steam-operated heat engines to produce power.  It is important to note that the capacity of the OECD countries is likely to decrease with energy efficiency and/or renewables, while developing countries will likely urbanize using cheap energy fuels given that most lack the infrastructure, funds, or strong leadership to support these green measures.

Additionally, the fresh water that these facilities consume is interdependent with the amount of energy being produced, and this is often overlooked. This relationship is known as the water-energy nexus, and the amount of resources involved is immense. The water requirement can range from 60 gal/MWh to 480 gal/MWh, depending on the demand and cooling technology (open-loop, closed-loop w/cooling tower, or air-cooling). This can translate up to roughly 62 billion gallons of fresh water needed worldwide to operate fossil fuel-based generation at any given hour.

Recently, the Environmental Protection Agency (EPA) finalized the Steam-Electric Power Generating Effluent Guidelines, the first federal regulations on power plants for compliance with the Clean Water Act. This 2015 rule will limit toxic metals such as lead and mercury from being discharged into wastewater streams and will potentially save between $451 and $566 million. But this doesn’t necessarily assess the water intake dilemma of power stations. Water scarcity is becoming a real threat, and with a definite energy demand increase in the upcoming years, this will only aggregate concerns for our communities. Fresh water is becoming increasingly scarce, such that the World Wildlife Fund (WWF) predicts that by 2025 75% of the world’s population will suffer water shortages. Although technological improvements have been made in every device and equipment imaginable in our society, every few years we read about new optimization initiatives in the marketplace and companies seemingly line up to invest high amounts of capital, even if it means just a one-point upgrade of efficiency in the thermodynamic cycle. To be frank, generating 80% of the global electricity by heating water and pressurizing steam to drive a turbine should not be the most used concept today, at least in developed countries.

In July 2016, the U.S. Department of Energy (DOE) announced that it will invest around $6 million on a supercritical CO2-based power-cycle, an innovative and potentially disrupting technology that replaces steam with supercritical carbon dioxide (S-CO2) as the turbine’s working fluid. This concept is able to provide around 50% of cycle efficiency in comparison with 38% in steam Rankine cycles. It is not the first time the DOE has invested in the technology. In early 2015 US Secretary of Energy Dr. Ernest Moniz mentioned that Fiscal Year 2016 would have a mercialization.

What is Supercritical CO2?

The state where the carbon dioxide adopts properties of both gas and liquid phases is called the supercritical point. This occurs when the fluid has both its temperature and pressure above its critical point (around 88°F and 1,100 psi). These conditions enables the fluid to have big changes in its density with small changes in temperature or pressure, allowing it to be ‘’tailored’’ relatively easy to site specifics, hence increasing thermodynamic efficiency.

It seems counterintuitive to advocate for a greenhouse gas (GHG) that green technologies and environmentalists are trying to avoid and eliminate altogether, but S-CO2 is not an ordinary working fluid. It is nonflammable and noncorrosive and has the capability to generate power with a single pressure value, whereas steam turbines need several pressure stages in order to produce power. This is a benefit that can decrease design, operation, and maintenance costs, as well as complexity. A conceptual design between an S-CO2 turbine and steam turbine is illustrated below.

 

S-CO2 Turbine
Source: U.S. Department of Energy

Since the supercritical fluid is linked to numerous DOE programs, given the flexibility it has with thermal sources (fossil fuels, solar thermal, geothermal, and nuclear), the Federal entity sees great potential and has established its own tech team to promote commercial acceptance. There are some firms ahead of the curve in the United States such as: General Electric, Sandia National Laboratories, and Echogen, which all have been funded at some point during their R&D efforts by the DOE and are expanding initiatives to enable its entrance to the marketplace. One of the most impressive prototypes from these organizations is the 10 MW S-CO2 turbine by GE, which is the size of a desk and is powerful enough to supply around 10,000 homes. Moreover, the GE Global Research team announced that they plan to extend such technology to 500 MW turbines with the insights gathered in the process. Needless to say, there are several S-CO2 power cycle designs that are being researched and there is a consensus among scientists and engineers that a closed-loop Brayton cycle is the predominant configuration in R&D.

What Lies Ahead for Supercritical CO2?

In order for supercritical CO2 power cycles to be adopted at a commercial level, there are several challenges to overcome for good market performance. According to the Electric Power Research Institute (EPRI), some of these hurdles include high-pressure boundaries in the primary heater and piping, operation capabilities to satisfy demand loads, and a recuperator heat exchanger, which is generally installed in the exhaust pipe to capture waste heat and can be one of the  main components delivering high efficiency, to name a few.

A holistic analysis is necessary to determine how much water can be saved in each S-CO2 closed Brayton cycle configuration, but it is a great start and will soon be a realistic option for developing countries looking to densify cities. It is necessary to increase awareness on cutting-edge technologies that improve energy and water efficiency, as well as GHG emission reduction. Furthermore, a possible innovation in cooling technologies for power generation can enable new design capabilities and definitely help market transformation in the near future.

 

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