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What is Distributed Generation (DG)?
Distributed generation refers to a variety of small, modular power-generating technologies that can be combined with energy management and storage systems and used to improve the operation of the electricity delivery system, whether or not those technologies are connected to an electricity grid. Implementing DG can be as simple as installing a small electricity generator to provide backup power at an electricity consumer's site. Or it can be a more complex system, highly integrated with the electricity grid and consisting of electricity generation, energy storage, and power management systems. DG systems range in size and capacity from a few kilowatts to 50 MW. They comprise a portfolio of technologies, both supply-side and demand-side, that can be located at or near the location where the energy is used.
What are the benefits of Distributed Generation?
The Connecticut Energy Advisory Board’s 2000 “Energy Policy Report” noted that "Distributed generation has the potential to provide site-specific reliability and transmission and distribution (‘T&D’) benefits including:
- increased reliability, shorter and less extensive outages
- lower reserve margin requirements
- improved power quality
- reduced lines losses
- reactive power control
- mitigation of transmission and distribution congestion, and
- increased system capacity with reduced T&D investment”
For more information on the benefits of DG: CEAB 2000 Energy Policy Report
Combined Heat and Power DG Systems
Conventional electricity generation processes, which are capable of converting only about a third of the potential energy in fuel into usable energy, are inherently inefficient. In applications that utilize separate heat and power systems, total system efficiency typically approaches only 45%. However, the use of CHP in commercial and industrial applications can provide a tremendous opportunity, as system efficiencies approaching 85% can be attained.
Recent advancements have resulted in the development of new technologies and systems for CHP applications. For instance, improvements in electricity generation technologies – namely advanced combustion turbines and engines – have led to the development of new configurations that reduce system size but increase output efficiency. The prospects for economical onsite generation improve dramatically when waste heat from electricity generation can be used to offset costs associated with space heating, water heating, and air conditioning needs.
Reciprocating Engine
Reciprocating engines, also called internal combustion engines, are a widespread and well-known technology. Electric efficiencies of 25 to 50 % make them an economic CHP technology for a variety of applications. Depending on the ignition source, reciprocating engines are categorized in one of two ways: 1) spark ignited engines are typically fueled by gasoline or natural gas; and 2) compression-ignited engines are typically fueled by diesel fuel, heavy oil, or a combination of oil and gas. Reciprocating engines range in size from a few kW to several MW. Advantages of reciprocating engines include low capital costs, easy start-up, proven reliability, good load-following characteristics, and good heat recovery. Applications in power generation include prime power generation, peak-shaving, back-up power, premium power, remote power, and standby power.
Combustion Gas Turbine
Combustion turbines (CTs) use the expansion of hot combustion gases to drive a rotating power turbine. CTs have been developed using technology from jet airplane engines. Technological advancements have helped them evolve into compact and efficient prime movers for power generation. CTs are most commonly fueled by natural gas, although they are capable of utilizing a broad range of gaseous and liquid fuels. Although CTs represented just 20% of the power generation market 20 years ago, they now claim approximately 40% of new capacity additions. CTs are economic for CHP in sizes ranging from five to several hundred MW. Heat dissipation associated with gas turbine use is a concern for applications in which the surplus heat cannot be utilized. Additionally, interconnected applications must be synchronous to the system.
Steam Turbine
Steam turbines are the most versatile and oldest prime mover technology used for electricity generation. They are widely used in the U.S. and Europe for CHP applications. Steam turbines require a source of high-pressure steam that is produced in a boiler or heat recovery steam generator to drive a turbine. Boiler fuels include fossil and renewable fuels, such as coal, oil, natural gas, wood, and municipal waste. Steam turbine applications are very compatible with existing sources of waste high-pressure steam. Unlike combustion gas turbines, they can also directly utilize solid fuels such as coal and biomass in boilers to create steam. However, for DG applications (smaller scale applications) standalone steam turbine systems can be more capital intensive and less efficient than other combustion-based DG technologies.
Microturbine
As their name implies, microturbines are very small combustion turbines that range in size from 20 to 250 kW. Microturbine technology evolved from automotive and truck turbochargers, auxiliary power units for airplanes, and small jet engines. Microturbines typically operate at high speed (70,000 to 100,000 rpm) and drive a high-speed generator directly. The high frequency power must be rectified and inverted to 60 Hz using complex power electronics. Although they have yet to reach commercial maturity, microturbines are expected to offer numerous potential advantages compared to other technologies for small-scale power generation. Advantages include few moving parts, compact size, lightweight, relatively high efficiency, and low emissions. Waste heat recovery can be used with the microturbine systems to achieve efficiencies greater than 80%.
Fuel Cell
Fuel cells refer to a class of technologies that convert fuel to electricity via an electrochemical process. Unlike a battery, the chemical input is not stored in the system, but is fed continuously into the fuel cell. The chemical input to the fuel cell takes place in the form of hydrogen and oxygen. Any of various fuels, including natural gas, methanol, ethanol, and gasoline, can be reformed to provide the hydrogen necessary for the fuel cell.
Fuel cells are named according to the electrolyte they utilize.
Renewable Energy DG Systems
Power generation systems that use renewable resources - the sun, wind, organic matter, and geothermal energy - have some advantages over traditional fossil-fuel-powered generation systems. For example, most renewable power technologies do not produce greenhouse gases and emit far less pollution than does burning oil, coal, or natural gas to generate electricity. With the exception of biomass technologies, renewable energy utilizes free fuel sources. The use of indigenous renewable energy sources also provides a secure and stable source of energy.
Biomass Power
Biomass electricity conversion technologies convert renewable biomass fuels into electricity (and heat) using a variety of different technologies, including modern boilers, gasifiers, turbines, generators, and other methods. Electricity from biomass also can be produced from a variety of fuels, including residues from the wood and paper products industries, residues from food production and processing, trees and grasses grown specifically to be used as energy crops, and gaseous fuels produced from solid biomass, animal wastes, or landfills. Current U.S. biomass power plants have a combined capacity of 7000 MW, and use approximately 60 million tons of biomass fuels (primarily wood and agricultural wastes) to generate 37 million kWh of electricity annually.
Solar Photovoltaic
Solar photovoltaic (PV) systems, which convert sunlight directly into electricity, offer many advantages as generation systems, both as a supply side option and as a demand-side management option. Solar PV is the most modular and operationally simple of the clean, distributed power technologies. Its benefits include the ability to provide peak period power, distribution benefits (reduced strain on distribution infrastructure), environmental benefits, reduced fuel price risk, and local economic development. PV technology has several niche and broader applications, including:
- Grid attached residential and commercial
- Communication (e.g., to power a remote switch tower)
- Consumer goods (power for cell phones, watches, etc.)
- Off grid (developing world)
- Off grid/remote (industrialized nations)
- Central power stations (typically 100 kW or larger)
Wind Power
Wind energy systems transform the kinetic energy of the wind into mechanical or electrical energy that can be harnessed for practical use. Mechanical energy is most commonly used for pumping water in rural or remote locations. Wind turbines generate electricity for homes and businesses and for sale to utilities. The most economical application of electric wind turbines is in groups of large machines (700 kW and up), called "wind power plants" or "wind farms." Wind plants can range in size from a few megawatts to hundreds of megawatts in capacity. Wind power plants are "modular," which means they consist of small individual modules (the turbines) and can easily be made larger or smaller as needed. Turbines can be added as electricity demand grows. Today, construction of a 50 MW wind farm can be completed in 18 months.
What are some of the barriers to DG development?
Despite the potential benefits of more widespread DG development, DG has been slow to gain a firm foothold in most commercial and industrial energy markets. Even in regions like SW CT, where DG could provide potential relief to significant electricity T&D constraints, DG has not been widely adopted by end-users. Other frequently mentioned relevant barriers to DG development include the following:
- Relatively immature technology, lack of commercial availability, and associated high capital costs make DG uneconomical for many applications.
- The technical ability of the T&D grid to support DG. The SW CT distribution and transmission system may have a finite ability to support DG interconnection due to engineering limitations.
- Certain DG technologies (e.g. non-renewables) may be difficult to site due to emission concerns.
Noise restrictions, local zoning restrictions, and other permitting issues can make it difficult to site certain DG technologies.
- Uncertainties about natural gas infrastructure and supply, as well as unproven reliability and O&M costs, create added risks for DG developers.
- Technical requirements associated with interconnection, such as integrated controls, protective relaying, and the ability of the existing electricity distribution infrastructure to support DG, create challenges for DG developers.
- An inability on the part of regional transmission organizations to verify environmental and other attributes from small generators prevents DG operators from capturing a full benefits stream.
- Potential external costs associated with DG development, such as gas infrastructure modifications, upgrades to the electrical system, siting and permitting, and real estate, are likely to affect DG development.
- Wind and solar are intermittent energy sources, e.g. not generating electricity when there are no wind or solar resources.
What are the emissions from DG systems?
Emissions of greenhouse gases and criteria pollutants from DG technologies range from zero (renewables) to quite high when used at high capacity and/or in high quantities (see table). Consequently, the expansion of DG may lead to higher levels of pollution unless states can create a framework that recognizes and encourages clean and renewable technologies.
|
Fuel Cells |
Gas-Fired Engine |
Diesel Engine w/ SCR |
Micro Turbine |
Small Gas Turbine |
Photovoltaic |
Wind Turbine |
Electric Efficiency (LHV) |
40-70% |
25-45% |
30-50% |
20-30% |
25-40% |
15-30% |
20-46% |
Typical Capacity (kW) |
200 |
1000 |
1000 |
25 |
4600 |
5000 |
1500 |
NO x (lb/MWh) |
0.03 |
0.50 |
4.70 |
0.44 |
1.15 |
0.00 |
0.00 |
SO 2 (lb/MWh) |
0.006 |
0.007 |
0.454 |
0.008 |
0.008 |
0.000 |
0.000 |
PM-10 (lb/MWh) |
0.00 |
0.03 |
0.78 |
0.09 |
0.08 |
0.00 |
0.00 |
CO 2 (lb/MWh) |
1078 |
1376 |
1432 |
1596 |
1494 |
0 |
0 |
Key:
NO x = Nitrogen oxides |
PM = Particulate Matter |
LHV= Lower Heating Value |
SO 2 = Sulfur dioxide |
CO 2 = Carbon dioxide |
SCR= Selective Catalytic Reduction |
Source: Emissions data from Joel Bluestein, Energy and Environmental Analysis, Inc. The emissions data for the gas-fired engine assume a rich-burn engine with a three-way catalyst. Available at www.epa.gov/globalwarming/greenhouse/greenhouse18/distributed.html.
Sources and Additional Information:
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