DE technologies include the full array of renewables suitable for deployment on local networks such as photovoltaics, biogas and biomass cogeneration, and geothermal, wind, wave, tidal and small-scale hydroelectric power. Cogeneration through "combined heat and power" (CHP) and trigeneration through "combined cooling, heat and power" (CCHP) are also considered as distributed energy technologies.
DER deployment has experienced steady growth over the years while more and more power is now being provided by IPP''s. Benefits of DE include low transmission and distribution costs, improved accessibility, improved energy efficiency, security and reliability, as well as a reduction in environmental impact. The challenges to full-scale adoption of DE include political, legislative, finance and economic, as well as technical barriers.
The number of countries implementing policy measures and targets aimed at improved distributed energy deployment has been observed to be on the increase, especially regarding renewable energy sources; there is also an increase in the diversity of these policies. Table 1 shows a global coverage of regions with renewable energy policies in the years 2004, 2013 and 2014.
Reciprocating Engines: reciprocating engines are a mature technology that is largely proliferated mainly due to their lower capital investment costs, fast start- up capabilities and higher energy efficiencies when combined with heat recovery systems. Most reciprocating engines run either on fuel or natural gas with an
increasing number of engines running on biogas produced from biomass and landfill waste. Most of the reciprocating engines are used as back-up or stand-by generators; some are used as peaking generators and as continuous generators. Reciprocating engines, however, do not perform well in terms of noise, maintenance and emissions.
Gas Turbines: gas turbines are widely used for electricity generation. They mostly run on natural gas and have lower emission levels. They are widely used as continuous generators; with some being used as standby generators and peaking generators. Gas turbines are widely used in cogeneration.
Fuel cells: instead of converting mechanical energy into electrical energy, fuel cells are built to convert chemical energy of a fuel into electricity; and usually either natural gas or hydrogen is used as fuel. Fuel cells continue to be a major field of research and considerable effort is put in cutting down capital costs and improving efficiency which are the two main drawbacks to this technology.
Renewable sources: renewables have been utilised as distributed energy resources; renewable energies range from photovoltaics, wind, thermal energy etc. These sources only qualify as distributed generation if they satisfy the definition criteria. Distributed generation is thus not identical as renewable energy. For instance, offshore wind farms are not considered as "true" distributed generation.
Storage: a distributed energy resource is not limited to electricity generation but may also include a device to store distributed energy. Distributed energy storage systems (DESS) applications include numerous types of battery, pumped hydro, compressed air, and thermal energy storage.
In addition to the technologies listed in the forgoing sections, are interface or interconnection technologies; these consist of both hardware and software equipment that makes up the physical link (or electrical connection) between distributed energy resource and the outside electrical power system (usually the local electric grid); and it can also include monitoring, control, metering, and dispatch of the distributed energy resource unit. They include inverters, transformers, power meters, transfer switches, and information and telecommunication technology.
With the persistence load shedding affecting the whole sub-region it makes sense at least in the short term to encourage small businesses and individuals to invest in energy generation either for own use or for distribution to other users having energy deficits. This will improve security and reliability of energy availability and supply in terms of:
- Back up generation: the use of distributed generation as backup or standby supply will prevent operational failures during peak hours or when there are network problems; in fact backup generators have been installed at critical locations such as hospitals and shopping malls; and increasingly many small businesses are investing in backup generators.
- Fuel diversity: the fact that distributed generators use the energy that is optimally available to them leads to a fuel diversity which in turn helps mitigate higher fuel prices, diminishing fossil fuel resources, and variations in whether and climatic patterns affecting wind, solar, hydro and oceanic energy resources, such as in the case of a river supplying a hydro-power station drying up. Distributed generation also taps into previously unused energy resources such as waste heat and landfill biogas.
Traditionally most centralised energy systems are based on fossil fuel (coal, oil, natural gas) power stations and are thus associated with large green-house gases emissions. Distributed generation can mitigate the impact in terms of avoidance of emissions associated with transmission and distribution losses, increasing of thermal efficiencies through cogeneration and trigeneration, and through distributed renewable energy.
Adopting distributed energy systems also reduces the investments required for transmission and distribution infrastructure; the cost for transmission infrastructure can vary between 4% - 15% of the total cost and between 27% - 34% for the distribution infrastructure [5] . Distributed generation can be used as a way to bypass the transmission and distribution networks.
Transmission and distribution costs account for up to 30% of the cost of delivered electricity on average; the lowest cost being attained by industrial clients receiving electricity at high to medium voltages and the highest cost by small customers receiving electricity from the distribution network at lower voltages. The higher costs for transmission and distribution are mainly a result of losses made up of:
• Line losses: these are energy losses in the transmission and distribution lines mainly due to line heating, the Joule effect, and for very high voltage, corona discharge losses, which are losses due to the ionization of a fluid or gas surrounding a conductor that is electrically charged. The ratio for developing countries is reported to vary between 11.6% and 20.7% [5] .
• Unaccounted for electricity: is attributed to energy theft and distribution loss deviations, as well as errors arising from meter measurements, power flow modelling, and statistical load profiles. Transmission over vast distances is prone to illegal connections as can be seen in the image, in Figure 2, of an
• Conversion losses: these are losses associated with the changing of the characteristics of the power flow to fit the specifications of the network, such as magnetic losses in transformers for stepping up or down of the voltage or in inverters for conversion from alternating current (AC) to direct current (DC) or vice versa; or conversion of the waveform from sinusoidal waveform to a different waveform such as triangular or square wave.
Figure 3 shows electricity generation efficiencies for different technologies. It can clearly be seen that fossil powered power plants have about 40% - 50% conversion efficiencies. These figures can be raised to about 80% - 90% through combined heat and power (CHP), or combined cooling, heat and power (CCHP). Figure 4 shows a schematic representation of a trigeneration plant. Both CHP and CCHP especially on a small to medium size scale are considered distributed energy resources.
Adoption of distributed energy systems promotes deregulation of the electricity market as it entails the involvement of many players; this in turn leads to flexibility
in the market with potential benefits of efficient and reliable delivery, competitive pricing, and employment creation. Deregulation has brought with it the new concept of "prosumer", ―both producer and consumer, whereby consumers can also take on the role of producers.
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