Capturing Wasted Energy from Natural Gas
by: Chris Cote
Sept 2011
There is enough energy currently being wasted in the United State at gas pressure reduction stations to power about 10 million homes—and for mere pennies. So, why aren’t we using it?
Much natural gas is found in remote or distant regions (like Canada), and needs to travel thousands of miles from the wellhead to more populated areas (like Southern California) where gas consumers live. An elaborate 272,000+-mile natural gas pipeline grid pushes gas downstream using compressor stations every 40 to 100 miles or so. Compression stations are used to increase the pressure, often to a linepack of 1,000 psi to 1,750 psi. Energy in the form of enthalpy is contained in this highly pressurized gas.
As the gas nears populated areas, however, this tremendous pressure needs to be reduced to acceptable levels, and eventually to as low as 1/4 psi for residential use. That’s where letdown stations come into play (also known as citygates, gate stations, farm taps, or reducing stations, etc.). Though a necessary part of the process, it’s the series of letdown stations in the distribution system where all the potential energy from pressure is wasted due to the use of reduction-throttling valves.
The Challenges
This wasted energy isn’t a new discovery. Many attempts have been made to capture wasted energy with turboexpanders, a complex centrifugal or axial flow turbine. Turboexpanders were originally designed for other applications and have been used for over a century but, in the 1980s, axial flow turbines were placed in natural gas pipelines between the high-pressure and low-pressure pipes. As the gas flows from the high-pressure pipe into the turboexpander, it spins the turbine, which in turn spins a generator to produce electricity. 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 reduces the pressure of the gas for the local gas network.
The challenges associated with utilizing turboexpanders in the natural gas pipeline letdown stations are primarily cost related. For one, the equipment must be custom-engineered for each specific application. Turboexpanders are complex machines that generally operate efficiently in a specific band of flow-rate volume. Efficient sizing of the turboexpander requires an analysis of seasonal flow-rate fluctuations. Maintaining a turboexpander is also relatively expensive as the delicate, rapidly spinning blades can be damaged by moisture or foreign elements. Perhaps, most significantly, the capital cost doesn’t increase proportionally with the expected output. The larger the output, the lower the cost on a per-kilowatt basis. Therefore, an acceptable payback can only be obtained at the higher pressure stations—those which are furthest from the cities. Yet, a huge part of the unharnessed energy is found closer to the cities at smaller letdown stations.
In 1983, San Diego Gas and Electric attempted to recover this energy with a turboexpander, as did other companies in later attempts in other states. Even many European companies, including the owner of most of the UK’s gas pipeline network (the National Grid), have or have considered installing turbine generating systems, but couldn’t justify the cost to capture this wasted energy without being subsidized. More complicated systems with fuel cells have been tried in Canada but, once again, the cost was just too great.
The Benefits
Giving up is one option, however, the benefits of utilizing the wasted energy at gas reduction stations are compelling and numerous—particularly from an environmental stance. Not only is the energy readily available and cheap (assuming there’s a cost-effective way to capture it), but it’s a form of baseload power that’s available 24/7, rain or shine. It’s the epitome of distributed energy: located where the need is the greatest, both close to and in cities. Moreover, no fuel is used to generate this energy; it’s simply the pressure from the gas. So, there are no emissions and no CO2.
Another potential by-product of using such wasted energy to generate electricity is the creation of extreme cooling—which can be a challenge at gas letdown stations, but would be a huge benefit to end users of gas further down the distribution line (such as manufacturers who can use the cooling to eliminate air conditioners).
The Solution
The good news is that when there’s a need, often and eventually there is a way. Thanks to dedicated innovators and engineers, equipment has recently been commercialized that solves the challenges of the turboexpander. One solution is an extremely robust expander with very low RPMs (1/5th the speed of turboexpanders) that is highly efficient, simple, flexible & easy to install. This equipment allows gases, dry or wet steam, hot water, impurities & contaminates directly into the machine without hurting the machine. It allows changes in flow rates and pressure so it is flexible enough to allow one design to handle multiple applications. The initial cost of under $2,000/kW is lower than the cost of turbo systems and the operating cost of less than 25 cents/kW is substantially lower than the cost of operating a turbo system. It’s also flexible enough to use either a synchronous or an induction generator. And, as a direct-expansion device, the generator isn’t limited to gas, and may be designed for various applications, including natural gas letdown, wet or dry steam, geothermal liquid/vapor, as well as hot air and hot exhaust gas streams.
Simply search online for “gas letdown generators,” to find viable solutions other than turboexpanders available to capture wasted energy. Some are even worth the cost. Langson Energy has a prototype running in its R&D shop in Carson City, NV. With new, economical technology available, wasted energy no longer has to be wasted when it comes to gas pressure reduction systems—or other systems.
