08-05-2013, 04:42 PM
Distributed generation
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Abstract
Distributed generation, defined as generation located at or near the load centers, is being recognized as an environment friendly, reliable, and secure source of power which not only has minimal negative social impacts but also serves to promote social welfare. This paper aims at bringing out the reason for the current interest in distributed generation, its salient features from an economic and social perspective and the challenges to be faced while increasing its share in the electricity generation mix. The paper to identify the distributed energy resources available and proposes methods to tap them. It also studies the social consequences of wide spread deployment of distributed systems and their accommodation into the new liberalized energy market. But in order to be a credible alternative generation paradigm, distributed generation will have to overcome significant technical, economic, regulatory and environmental hurdles.
Introduction
When Thomas Edison built the Pearl Street Power station to provide the first electric service to customers in New York City he was essentially following a strategy that today would be called distributed generation – building power generation within the localized area of use.
As the young industry grew, many industrial facilities built their own power plants both to serve their own needs and to sell to customers around them, another example of distributed generation. Rapid technological development led to larger and more efficient generating plants built farther and farther from the end-user. Large regional power transmission networks delivered this power to the local distribution systems and finally to the end-user. The industry was regulated so that these changes could occur efficiently without wasteful duplication of facilities, and the economic role of distributed
generation became much more limited.
Since the 1970s, however, large central nuclear and coalfired power stations have become increasingly expensive and more difficult to site and to build. At the same time, technological development has improved the cost and performance of smaller, modular power generation options – from 300 megawatt (MW) gas-fired combined cycle power plants down to individual customer generation of as little as a few kilowatts. The industry is also restructuring to allow customers to
competitively select the optimum combination of energy resources to meet their needs.
Under the current centralized system, electricity is mainly produced at large generation facilities, shipped through the transmission and distribution grids to the end consumers. However the recent quest for the efficiency and reliability and reduction of green house gas emissions led to explore possibilities to alter the current generation paradigm and increase its overall performance. In this context, one of the best methods to complement or even replace the existing system is distributed generation where the energy is produced next to the point of use.
Although they represent a small share of the electricity market they play a key role for applications in which reliability is crucial, as a source of emergency capacity, and as an alternative to expansion of a local network, in developed economies where uninterrupted power supply is essential. In developing countries like India, where the generation is inadequate to meet the demand, reliability and energy security are of lesser importance. Developing country can tap the potential of DG to extend their present generation capacity in an environment friendly and cost friendly manner.
DERs mainly constitute non-conventional and renewable energy sources like solar PV, wind turbines, fuel cells, small-scale hydro, tidal and wave generators, micro-turbines etc. These generation technologies are being preferred for their high energy efficiency (micro-turbine or fuel cell based CHP systems), low environmental impact (PV, wind, hydro etc.) and their applicability as uninterruptible power supplies to power quality sensitive loads.
Distributed generation:
Distributed generation is an electric power source connected directly to the distribution network or on the customer site of the meter. It generates electricity from many small energy sources. Most countries generate electricity in large centralized facilities, such as fossil fuel (coal, gas powered), nuclear, large solar power plants or hydropower plants. These plants have excellent economies of
scale, but usually transmit electricity long distances and negatively affect the environment. Distributed generation allows collection of energy from many sources and may give lower environmental impacts and improved security of supply.
Distributed generation plants are mass-produced, small, and less site-specific. Their development arose out of:
• concerns over perceived externalized costs of central plant generation, particularly environmental concerns,
• the increasing age, deterioration, and capacity constraints upon T&D for bulk power,
• the increasing relative economy of mass production of smaller appliances over heavy manufacturing of larger units and on-site construction, and
Along with higher relative prices for energy, higher overall complexity and total costs for regulatory oversight, tariff administration, and metering and billing.
Capital markets have come to realize that right-sized resources, for individual customers, distribution substations, or micro grids, are able to offer important but little-known economic advantages over Central Plants. Smaller units offered greater economies from mass-production than big ones could gain through unit size. These increased values due to improvements in financial risk, engineering flexibility, security, and environmental quality of these resources can often more than offset their apparent cost disadvantages. DG, vis-à-vis Central Plants, must be justified on a life-cycle basis. Unfortunately, many of the direct, and virtually all of the indirect, benefits of DG are not captured within traditional utility cash-flow accounting.
Some of them are discussed below:
A. Reciprocating Engines
Reciprocating engines are the most common technology used for distributed generation. They are a proven technology with low capital cost, large size range, fast start-up capability, relatively high electric conversion efficiency (up to 43% for large diesel systems) and good operating reliability.
These characteristics, combined with the engines' ability to start up during a power outage, make them the main choice for emergency or standby power supplies. They are by far the most commonly used power generation equipment under 1 MW. The main drawbacks of reciprocating engines are noise, costly maintenance and high emissions, particularly of nitrogen oxides. These emissions can be reduced, with a loss of efficiency, by changing combustion characteristics. Catalytic converters are a proven emissions-control technology. Larger systems can use selective catalytic reduction (SCR) to reduce emissions. Particulate emission control is usually necessary with diesel engines
B. Gas Turbines
Small industrial gas turbines of 1- 20 MW are commonly used in CHP (combined heat and power) applications. They are particularly useful when higher temperature, steam is required than can be produced by a reciprocating engine. The maintenance cost is slightly lower than for reciprocating engines, but so is the electrical conversion efficiency. Gas turbines can be noisy. Emissions are somewhat lower than for engines, and cost-effective NOx emissions-control technology is commercially available.
C. Micro turbines
One of the most striking technical characteristics of micro turbines is their extremely high rotationalspeed. The turbine rotates up to 1, 20,000 rpm and the generator up to 40,000 rpm. Individual units range from 30-200 kW but can be combined readily into systems of multiple units. Low combustion temperatures can assure very low NOx emissions levels. They make much less noise than an engine of comparable size. Natural gas is expected to be the most common fuel, but landfill gas, or biogas can also be used . The main disadvantages of micro turbines at this stage are its short track record and high costs compared with gas engines.
D. Fuel Cells
Fuel cells are compact, quiet power generators that use hydrogen and oxygen to make electricity. The transportation sector is the major potential market for fuel cells, and car manufacturers are making substantial investments in research and development. Power generation, however, is seen as a market in which fuel cells could be commercialized much more quickly. Fuel cells can convert fuels to electricity at very high efficiencies (35%-60%), compared with conventional technologies. Their efficiency limits the emissions of greenhouse gases. As there is no combustion, other noxious emissions are low.
E. Photovoltaic Systems
Unlike the other DG technologies discussed above, photovoltaic systems are a capital-intensive, renewable technology with very low operating costs. They generate no heat and are inherently small-scale. These characteristics suggest that PV systems are best suited to household or small commercial applications, where power prices on the grid are highest. Operating costs are very low, as there are no fuelling costs. PV systems also are widely used in developing countries, serving rural populations that have no other access to basic energy services. PV systems can be used to provide electricity for variety of applications in households, community lighting, small businesses, agriculture, healthcare, and water supply . The other half of existing PV capacity is on-grid, mostly as distributed generation. Most on-grid PV installations to date have enjoyed very large investment subsidies or favorable prices for the electricity they generate . The economic viability of PV systems is much higher when they can displace an extension to a distribution line.
F. Wind
Wind generation is rapidly growing in importance as a share of worldwide electricity supply. About 4.2 GW of capacity was installed during the year 2000.Wind power is sometimes considered to be distributed generation, because the size and location of some wind farms makes it suitable for connection at distribution voltages
Challenges to be faced:
To sustain the forecast penetration rate, the architecture of the electricity sectors needs to be altered. The current infrastructures were not originally built to accommodate a large proportion of distributed generation. For the moment, only necessary adjustments are undertaken in order to accommodate these new capacities.
Over the long run, however, increasing significantly the share of distributed generation will necessarily mean revamping the whole physical and regulatory architecture of the electricity network and more precisely the distribution network.
Technical constraints:
The first difficulties to overcome are related to technical improvements
necessary to ensure high system reliability with distributed generation. The following
section gives an overview of the technical issues caused by distributed generation.
The issues can be classified as follows:
Capacity:
Adding distributed generators at the distribution level can significantly impact the amount of power to be handled by the equipment (cables, lines, and transformers). In order to avoid overload problems, reinforcement work will have to be undertaken. The critical piece will often be the transformers (converting medium voltage to low voltage or high voltage to medium voltage) if power generated exceeds by far consumption, power will have to flow back from the low voltage network to the medium voltage network or from the medium to the high voltage network and be directed to other consumption areas. The transformer will have to be able to handle this reverse flow i.e. being able to convert it back and have specification to cope with potential oversupply. This is of major issue at peak hours: at that time both continuous and peaking distributed generators will
operate to cash in the price premium. Production forecast from peaking distributed generators is key while determining the specifications of the equipment, as capacities will be added when the total power flow is already significant.