Turboexpanders: Harnessing the Hidden Potential of Our Natural Gas Distribution System

turbineabstractMany of the innovations in green energy involve the recapture of otherwise wasted energy.  Regenerative breaking systems on hybrid automobiles recapture the kinetic energy inherent in the motion of the vehicle.  In a conventional automobile, as the brakes are applied, friction in the braking system converts this kinetic energy to waste heat.  But in a hybrid, a portion of this energy is converted to electricity and stored in batteries for future use.  Since this energy would otherwise be wasted, this is essentially free energy.[1] Similarly waste heat recovery systems recover energy that would otherwise be wasted from power generation facilities.  Most conventional power generation facilities covert approximately half of the energy in the fuel into electricity.  The remainder is lost as waste thermal heat.  Waste heat recovery systems recapture this heat so it can be put to good use, increasing the efficiency of power generation facility.  These are but two examples of innovative methods of recapturing otherwise wasted energy.  There is another unharnessed form of energy rushing through our cities and countryside every day: our natural gas distribution pipelines.

ngpipesThe United States consumes approximately 23 trillion cubic feet of natural gas annually.[2] This gas is transported around the country in a system of pressurized pipelines.  Generally, the long distance pipelines operate at higher pressures to efficiently move the gas over long distances while local city networks operate at lower pressures that are more appropriate for consumer use.  The junctions between the high pressure long distance network and low pressure local network are often referred to as “city gates.”  At these city gates a pressure regulating device, such as a control valve or a regulator, drops the pressure to the appropriate pressure for the low pressure network.

There is energy contained in the pressure of the high pressure gas in the form of enthalpy[3].  The conventional pressure regulating devices used as city gates waste this energy.  Further, due to the Joule-Thomson effect[4], the gas cools as it flows through the pressure letdown device.  Typically, some gas is used to power boilers to heat the gas to offset this temperature loss resulting in further energy costs.

Turboexpanders:  Capturing the Lost Energy

Turboexpanders, also referred to as expansion turbines, provide a way to capture the energy otherwise lost in the gas pressure letdown process.  Turboexpanders have a range of applications, but for the purpose of this discussion we are referring primarily to the application of turboexpanders to natural gas pressure letdown facilities.  In these applications, an axial flow turbine is placed in line between the high pressure and low pressure pipes.  As the gas flows from the high pressure pipe into the turboexpander, the gas spins the turbine, which can in turn spin a generator producing electricity.   The effect is similar to what one would observe by pointing the nozzle of an aerosol can at a pinwheel.  As the gas rushes from the high pressure can to the low presure atmosphere, it spins the pinwheel.  Thus, by replacing a conventional control valve or regulator with a turboexpander, the energy in the pressure of the high pressure gas (or the enthalpy), that would otherwise be lost, can be converted to electricity.  Simultaneously, the turboexpander is reducing the pressure of the gas to the appropriate pressure for the local gas network.

Turboexpanders also cool the gas via the Joule-Thomson effect.  But modern turboexpander installations utilize efficient methods for coping with the temperature loss.  Turboexpanders are often coupled with a second power generator such as a fuel cell or conventional fuel combusting generator.  This secondary generator produces waste heat that is typically lost.  However, this waste heat can be used to offset the cooling effect of the turboexpander.  This symbiosis between the turboexpander and secondary generator increases the net efficiency of the entire system.

History of Turboexpanders

Turboexpanders are not new technology.  They have been around for over a century.  But the application of turboexpanders to natural gas pressure let down facilities only began in the early 1980’s.  In 1983, San Diego Gas and Electric was among the first to install a turboexpander in a natural gas letdown station.[5] Subsequent installations were made in the mid 1980’s in Memphis, Tennessee, Stockbridge, Georgia and Hamilton, New Jersey.

There are recent turboexpander projects that incorporate a secondary power generation source thereby increasing both overall output and efficiency.

  • In October of 2008, Enbridge opened a combination turboexpander and fuel cell facility in Toronto, Canada.[6] The facility produces 2.2 megawatts.
  • In January of 2009, a project was announced to install turbo expanders throughout London’s natural gas distribution system.[7] This project combines turboexpanders with biofuel burning generators. The project is projected to produce 20 megawatts.

Hurdles of Turboexpander Installation in Gas Pressure Letdown Facilities

Several hurdles face turboexpander projects.  The capital costs of engineering and installing a turboexpander can be high.  Each turboexpander must be custom engineered for a specific application.  However, the capital costs of turboexpanders do not increase proportionally with expected output.  Turboexpanders with large output capabilities cost substantially less on a per-kilowatt basis that smaller turboexpanders.  As the production capacity of a turboexpander increases, the per-kilowatt capital cost decreases somewhat exponentially.  The capital cost hurdles thus become less imposing for larger installations.

The utilization of a high capacity turboexpander requires a gas pressure letdown facility with the capacity to drive a large turbine.  Several factors bear on the size turbine a gas pressure letdown facility can drive, which in turn bear on the amount of energy any gas pressure letdown facility can be expect to produce.  Generally, these factors are:

  1. Inlet pressure
  2. Outlet pressure[8]
  3. Inlet temperature
  4. Outlet temperature
  5. Volume of flow[9]
  6. Gas makeup

Additionally, turboexpanders generally produce power outputs ranging from several hundred kilowatts to several megawatts.  This scale power production can be small for some natural gas utilities companies making it challenging to garner their interest in turboexpander projects.

Recent innovations that combine turboexpanders with secondary generation systems improve efficiency and energy output, helping to overcome these hurdles and expanding the scope of feasible applications.

The Future of Turboexpanders

Turboexpanders provide a simple means of recapturing otherwise wasted energy from our natural gas distribution grid.  Recent innovations coupling turboexpanders with other forms of power generation substantially increase efficiency and bolster the feasibility of further turboexpander development.  While turboexpanders may only capture a few megawatts as a time, the widespread deployment of turboexpanders could serve an important function in the greater agenda of a more efficient and greener energy system.

[1] There is some energy cost to this system.  The system has weight which must increases the energy required to accelerate the vehicle.  On a more macro scale, the system is more complex and requires more energy to construct than conventional braking systems.  But the consensus is that the net effect of the application of regenerative braking systems is a more efficient vehicle.

[2] http://tonto.eia.doe.gov/dnav/ng/ng_cons_sum_dcu_nus_a.htm

[3] http://actamont.tuke.sk/pdf/2004/n3/27pozivil2.pdf

[4] http://en.wikipedia.org/wiki/Joule-Thomson_effect

[5] http://www.ingaa.org/File.aspx?id=6210

[6] http://www.marketwire.com/press-release/Enbridge-Inc-TSX-ENB-913121.html

[7] http://www.guardian.co.uk/environment/2009/jan/06/gas-energy-pressure

[8] The higher the ratio of inlet pressure to outlet pressure, the greater the expected output.

[9] The higher the flow rate, the greater the expected output.  Turboexpanders generally have a band of flow rate in which they can efficiently operate.  Efficient sizing of the turboexpander requires an analysis of seasonal flow rate fluctuations.

18 thoughts on “Turboexpanders: Harnessing the Hidden Potential of Our Natural Gas Distribution System”

  1. Very good article. TransAlta Energy Systems in Canada developed a small 750kW turboexpander in the late 1980s, which they subsequently sold to Hawker Siddeley Canada. A prototype, eventually paired with a Perkins 4 cylinder natural gas engine, was installed at British Gas’s regional HQ at Altrincham near Manchester. This ran as a technology demonstrator. The turbine design was not good but HS could not get any of the industrial small turbine manufacturers in Europe interested in developing the concept. At around the same time, a couple of larger units, based on cryogenic expanders and rated at several MW each, were installed by the Dutch gas utility on the gas ring main round Amsterdam.
    The greatest problem with developing this market in Europe at the time was that the “city gates” were mainly relatively far from the cities and off the major electricity grids. The necessary investment was not just in the capital cost of the turboexpander and generator and associated hardware but also in making a connection, sometimes over 10km or more to a power line. In those days of relatively low energy costs – who remembers the “dash for gas”? – the turboexpander/generator was not seen as a viable option. Perhaps someone should look at it again.

  2. i am the student Metallurgy & materials engineering, University of The Punjab Lahore, Pakistan. I also devise this kind of electricity generation process dated 1st january 2009 using centrifugal compressor and radial inflow turbine to get electricity from pressurized pipelines of natural gas. then I came to know that rest of the world is also doing the same with turbo expander. I want some more information about its design and its suppliers.

  3. Turboexpanders coupled with modern technology to produce electricity are beginning to be looked at seriously by but a few companies, those mainly with innovative engineers. Using the turboexpander in main gas pipeline pressure letdown has become viable for both the production of electricity for small scale use and the extraction of NGL liquids for use in the “clean fuel” applications.

  4. I am the student Chemical engineering, Dawood College Of Engineeriong and Technology,Karachi Pakistan. I have a project of using turbo expander at the inlet of gas station, to utilize the pressure at throttling vave.I want some more information about its design and its suppliers.

  5. We have a project in Pakistan with SNGPL where we have 60 nm3/hr gas flow at 70 bar with required let down to 10 bar. At this moment we have quotes from various companies and would like to make a techno financial feasability of our project. Siemens have quoted a modified steam turbine to use as a gas expander ( so far lowest capex cost )and are intending to use a 1.5 MWe gas turbine to heat the gas. At the outlet we intend to use the gas to produce refrigeration in an ammonia absorber unit. Would you be able to do the engineering approach,prepare a techno financial feasability for our intended project ?

    Regards, Suhail Husain.

  6. In above article several process variables like inlet/outle pressure & temp. have considered. but my question is that how does turboexpander control system work to control mentioned above variables? any comments or information is appreciated.
    best regards

  7. Bestnaft: I would recommend that you contact one of the turboexpander suppliers listed in the comment above to get more information about how variables in a specific application can be controlled.

  8. Dear Mr.Rheuban,
    I’m a chemical engineering student at Sharif University of Technology,Iran.
    I’m really interested in this subject because of its relevance to my MSc thesis and I was wondering if you happen to know any specific professor and(or) university that are currently working on this matter.
    thanks a lot

  9. I working a student @ Strathclyde University(Msc.Sustainable Eng.Energy Systems and Enviroment.)
    I will be formerly launching a completely new way of extracting heat from sorrounding air and converting it into Electricity or Mechanical power.Generating in the order of megawatts.I’m glad because such a way of sourcing energy will completely transform our world.The turboexpander is at the heart of this process and once it comes on board,the manufacturers of The turboexpanders will struggle to cope with demand.Please join me in awaiting for this new noble phenomena.

  10. The Technology Introduction to common men is excellent.
    The use of most efficient energy trnsfomation to useful applications are endless.
    City gates are now near to energy consumers, and there are still excellent co-gerenation aspects of combining non renewable and renewables.
    Example is 20 MW cogeneration from biomass in london.
    Please , keep it up.

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