Distributed Generation
Distributed generation (DG) is the strategic placement of small power generating units (5 kW to 25 MW) at a customer's facility. DG represents an innovative and efficient way to both generate and deliver electricity, since it creates energy right where it's going to be used. Technological improvements now allow power generation systems to be built in smaller sizes with high efficiency, low cost, and minimal environmental impact.
DG can serve as a supplement to electricity generated by huge power plants and delivered through the electric grid. Located at a customer's site, DG can be used to manage energy service needs or help meet increasingly rigorous requirements for power quality and reliability. Distributed generation technologies include industrial gas turbines, reciprocating engines, fuel cells and microturbines.
Please scroll down for more information on each of these applications. If you would like to discuss your facility's individual energy needs -- and the rebates or other financial incentives associated with these technologies -- with a Florida City Gas account executive, email us at gasadvantage@aglresources.com.
Industrial Gas Turbines Many industrial energy customers can save money on their electric bills by generating their own power with natural gas-fueled turbines. The popularity of gas turbines is growing among industrial plant operators, as well as large commercial energy users and institutional facilities (such as hospitals and universities). When heat from the gas turbine's exhaust is recovered to produce high-pressure steam or other useful thermal energy, the system is called combined heat and power (CHP), or cogeneration.
Major process industries, primarily chemicals, paper and oil and gas, have been using large turbine CHP systems (over 25 MW) for some time. But the economics of smaller turbine systems (1 to 10 MW) have recently become more attractive. Improvements to existing gas turbines include higher power ratings and efficiencies, advanced controls, lower emissions of nitrogen oxides (NOx) and lower lifecycle costs.
Gas turbines can be operated in three ways:
1. Simple cycle turbines use an air-compression section, a burner and a power turbine driving a load, such as a generator for producing electricity. The turbine's high temperature exhaust heat can be used to produce process steam.
2. Recuperated turbines incorporate a heat exchanger, which recovers heat from the turbine exhaust to preheat the compressed air before it enters the burner.
3. Combined-cycle turbine systems use exhaust heat to produce steam to drive a second turbine, which produces additional electricity. Most combined-cycle systems are larger than industrial-scale plants.
Today's turbines reach efficiencies of 30 to 40 percent or greater; in systems with heat recovery, overall thermal efficiencies of 70 to 80 percent are common, and 90 percent is achievable. Turbine efficiency can also be evaluated by heat rate (Btu/kWh), a measure of fuel consumption per unit of output. The higher a turbine's efficiency, the lower its heat rate.
Air emissions from gas turbines have decreased substantially. Early systems used water or steam injection to reduce flame temperature and control NOx emissions. Recently, manufacturers introduced "dry" (no water or steam) low-NOx combustors, which reduce NOx emissions to 25 ppm and can achieve 15 ppm.
Reciprocating Engines Reciprocating engines are the fastest-selling, lowest-cost form of DG. They can be used in a variety of applications due to their small size, low unit cost and useful thermal output. Reciprocating engines are available commercially in sizes from .5 kW to 6.5 MW, making them suitable for a wide range of commercial, industrial and institutional applications. These applications include continuous power generation, peak shaving, back-up power, standby power and mechanical drive use.
Reciprocating engines also offer heat recovery potential. They make up a large portion of the cogeneration market in the United States.
Fuel Cells A fuel cell operates much like a battery. Instead of generating power through combustion, a fuel cell uses an electrochemical process to extract the hydrogen from natural gas and convert it directly into electricity and hot water. Since fuel cells don't burn gas, they operate virtually pollution-free. The "greenhouse gases" that are normally a by-product of electric generation -- like carbon dioxide, nitrogen oxide and sulfur dioxide -- are substantially reduced if not eliminated.
In addition to being environmentally friendly, fuel cells are also very energy efficient. For example, fuel cells produce electricity at efficiencies of 40 to 60 percent, compared to 30 to 35 percent for conventional boilers. The efficiency of a fuel cell that re-uses its waste heat can be as high as 80 percent or more.
A fuel cell has three basic components: a fuel processor that produces a hydrogen-rich fuel mixture; a power section that combines this mixture with oxygen to produce electricity; and the power conditioner that converts the resulting direct current (DC) to alternating current (AC). The by-products of this process -- water and heat -- can be used for other operations.
Microturbines Microturbines are essentially miniature jet engines that are connected to small electric generators. They have very sophisticated electronic systems, which allow them to provide safe and efficient operation by consistently monitoring themselves.
Microturbines are easy to install, have low emissions, and are located at your place of business. In areas where natural gas rates are low in comparison to electric rates, it may be more economical to use a microturbine instead of relying on electricity from the grid. For the most part, however, microturbines are used for avoiding the high demand charges associated with using electricity during peak time periods. Microturbines are particularly cost effective when the waste heat from the exhaust is captured and used to offset the energy needed to heat or cool a building or preheat a boiler.
In order to create electricity, microturbines burn natural gas. As the gas flame heats the incoming air, the air expands and flows over a turbine with blades, forcing the blades to spin. The spinning action (torque) spins the shaft of a generator, which converts the mechanical torque to an electrical output. This electrical output is converted to a usable voltage and frequency. The exhaust heat, which is about 600 degrees Fahrenheit, is captured and used for HVAC, water heater, or preheat boiler applications. Like most turbines that burn natural gas, microturbines are environmentally friendly and produce very low emissions.
Distributed Generation Fallacies
A number of fallacious arguments often appear in discussions of DG . Since many of these arguments appear "reasonable" at first glance, it is useful to address them before presenting recommendations for integrating DG in utility system planning.
Fallacy 1 Current natural gas shortages make DG a bad option. Wrong. Using natural gas for onsite generation and capturing the resulting waste heat actually stretches current supplies further. Instead of sending gas to a utility turbine where half or more of the gas energy is lost as waste heat, it should be sent to a DG system where only 15 percent or so is lost to waste heat. Gas savings can also be achieved by switching to oil at dual-fuel central power plants where the added benefit of more effective pollution controls will also reduce the overall level of emissions.
Fallacy 2 DG isn't economical except for large industrial customers. Ten years ago, this was true; it no longer is. System costs make DG an attractive option for 15 - 25 percent of customer energy use in commercial and industrial sectors. Improvements in equipment costs are likely to increase this potential to as much as 30 percent by the end of this decade.
Fallacy 3 DG can't make a big enough difference to have an impact on the system. Reducing system loads by even a small amount when the system is at or near capacity has a substantial impact on system reliability. DG contributions can significantly improve transmission and distribution system reliability.
Fallacy 4 DG causes safety problems for utilities. A great deal of effort over the last several years has resulted in a final set of industry standards that addresses safety and other interconnection issues.
Fallacy 5 The DG market isn't mature enough to provide substantial benefits to the electric system. DG opponents often point to the high current cost of fuel cells to suggest that DG cannot provide an economic solution to today's energy problems. The DG market includes a variety of technologies which vary in their development stages. As pointed out above, engines and turbines have been used for decades and can easily be integrated in the current electric system. At this moment thousands of companies are using DG to lower their electric bills and improve reliability - DG is not the distant technology option portrayed by some media reports.
-Jerry Jackson, Energy Pulse, 9/12/2003