Many 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. 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.
The United States consumes approximately 23 trillion cubic feet of natural gas annually. 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. The conventional pressure regulating devices used as city gates waste this energy. Further, due to the Joule-Thomson effect, 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. 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. 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. 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:
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.
 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.
 The higher the ratio of inlet pressure to outlet pressure, the greater the expected output.
 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.