Natural gas, because of its clean burning nature, has become a very popular fuel for the generation of electricity. In the 1970s and 1980s, the choices for most electric utility generators were large coal or nuclear powered plants. However, due to economic, environmental and technological changes, natural gas has become the fuel of choice for new power plants built since the 1990s. In fact, the Energy Information Administration (EIA) estimates that between 2009-2015, 96.65 gigawatts (GW) of new electricity capacity will be added in the U.S. Of this, over 20 percent, or 21.2 GW, will be natural gas additions. The graph below shows how, according to the EIA, natural gas-fired electricity generation is expected to account for 80 percent of all added electricity generation capacity by 2035.
Source: EIA Annual Energy Outlook 2010
There are many reasons for this increased reliance on natural gas to generate our electricity. While coal is the cheapest fossil fuel for generating electricity, it is also the dirtiest, releasing the highest levels of pollutants into the air. The electric generation industry, in fact, has traditionally been one of the most polluting industries in the United States. Regulations surrounding the emissions of power plants have forced these electric generators to come up with new methods of generating power, while lessening environmental damage. New technology has allowed natural gas to play an increasingly important role in the clean generation of electricity. Click on the link for more information on the environmental benefits of natural gas, including its role as a clean energy source for the generation of electricity.
Steam Generation Units
Natural gas can be used to generate electricity in a variety of ways. The most basic natural gas-fired electric generation consists of a steam generation unit, where fossil fuels are burned in a boiler to heat water and produce steam that then turns a turbine to generate electricity. Natural gas may be used for this process, although these basic steam units are more typical of large coal or nuclear generation facilities. These basic steam generation units have fairly low energy efficiency. Typically, only 33 to 35 percent of the thermal energy used to generate the steam is converted into electrical energy in these types of units.
|A Centralized Gas Turbine Generation Station
|Source: National Energy Technology Laboratory, DOE
Centralized Gas Turbines
Gas turbines and combustion engines are also used to generate electricity. In these types of units, instead of heating steam to turn a turbine, hot gases from burning fossil fuels (particularly natural gas) are used to turn the turbine and generate electricity. Gas turbine and combustion engine plants are traditionally used primarily for peak-load demands, as it is possible to quickly and easily turn them on. These plants have increased in popularity due to advances in technology and the availability of natural gas. However, they are still traditionally slightly less efficient than large steam-driven power plants.
Combined Cycle Units
Reciprocating Engine System
Many of the new natural gas fired power plants are known as ‘combined-cycle’ units. In these types of generating facilities, there is both a gas turbine and a steam unit, all in one. The gas turbine operates in much the same way as a normal gas turbine, using the hot gases released from burning natural gas to turn a turbine and generate electricity. In combined-cycle plants, the waste heat from the gas-turbine process is directed toward generating steam, which is then used to generate electricity much like a steam unit. Because of this efficient use of the heat energy released from the natural gas, combined-cycle plants are much more efficient than steam units or gas turbines alone. In fact, combined-cycle plants can achieve thermal efficiencies of up to 50 to 60 percent.
|A Proposed Natural Gas Combined Cycle Power Plant in New York
|Source: New York Power Authority
Until recently, methods of generating power have been discussed in the context of large, centralized power plants. However, with technological advancements, there is a trend towards what is known as ‘distributed generation’. Distributed generation refers to the placement of individual, smaller sized electric generation units at residential, commercial, and industrial sites of use. These small scale power plants, which are primarily powered by natural gas, operate with small gas turbine or combustion engine units, or natural gas fuel cells.
Distributed generation can take many forms, from small, low output generators used to back up the supply of electricity obtained from the centralized electric utilities, to larger, independent generators that supply enough electricity to power an entire factory. Distributed generation is attractive because it offers electricity that is more reliable, more efficient, and cheaper than purchasing power from a centralized utility. Distributed generation also allows for increased local control over the electricity supply, and cuts down on electricity losses during transmission. Below is a discussion of the various forms of natural gas-fired distributed generation.
Natural gas is one of the leading energy sources for distributed generation. Because of the extensive natural gas supply infrastructure and the environmental benefits of using natural gas, it is one of the leading choices for on-site power generation. There are a number of ways in which natural gas may be used on-site to generate electricity. Fuel cells, gas-fired reciprocating engines, industrial natural gas-fired turbines, and microturbines are all popular forms of using natural gas for on-site electricity needs.
Industrial Natural Gas Fired Turbines
Industrial natural gas-fired turbines operate on the same concept as the larger centralized gas turbine generators discussed above. However, instead of being located in a centralized plant, these turbines are located in close proximity to where the electricity being generated will be used. Industrial turbines – producing electricity through the use of high temperature, high pressure gas to turn a turbine that generates a current – are compact, lightweight, easily started, and simple to operate. This type of distributed generation is commonly used by medium and large sized establishments, such as universities, hospitals, commercial buildings and industrial plants, and can achieve efficiency up to 58 percent.
In contrast with distributed generation the heat that would normally be lost as waste energy can easily be harnessed to perform other functions, such as powering a boiler or space heating. This is known as Combined Heat and Power (CHP) systems. These systems make use of heat that is normally wasted in the electric generation process, thereby increasing the energy efficiency of the total system.
In addition, on-site natural gas turbines can be used in a combined cycle unit, as discussed above. Due to the advantages of these types of generation units, a great deal of research is being put into developing more efficient, advanced gas turbines for distributed generation.
For more information on natural gas as a fuel for generating electricity, click here to see a study from the Natural Gas Supply Association.
|Gas Fired Microturbine
|Source: Oak Ridge National Laboratory
Microturbines are scaled down versions of industrial gas turbines. As their name suggests, these generating units are very small, and typically have a relatively small electric output. These types of distributed generation systems have the capacity to produce from 25 to 500 kilowatts (kW) of electricity, and are best suited for residential or small scale commercial units.
Advantages to microturbines include a very compact size (about the same size as a refrigerator), a small number of moving parts, light-weight, low-cost, and increased efficiency. Using new waste heat recovery techniques, microturbines can achieve energy efficiencies of up to 80 percent.
Natural Gas-Fired Reciprocating Engines
|Gas Fired Reciprocating Engine
|Source: National Energy Technology Laboratory, DOE
Natural-gas fired reciprocating engines are also used for on-site electric generation. These types of engines are also commonly known as combustion engines. They convert the energy contained in fossil fuels into mechanical energy, which rotates a piston to generate electricity. Natural-gas fired reciprocating engines typically generate from less than 5 kW, up to 7 megawatts (MW), meaning they can be used as a small scale residential backup generator to a base load generator in industrial settings. These engines offer efficiencies from 25 to 45 percent, and can also be used in a CHP system to increase energy efficiency.
Fuel cells are becoming an increasingly important technology for the generation of electricity. They are much like rechargeable batteries, except instead of using an electric recharger, they use a fuel, such as natural gas, to generate electric power even when they are in use. Fuel cells for distributed generation offer a multitude of benefits, and are an exciting area of innovation and research for distributed generation applications.