Introduction
Distributed Generation (DG) refers to the generation of electrical power from multiple and usually small energy sources. The US is facing energy consumption problems that include energy shortages, rising fuel cost, the depletion of resources and environmental pollution. DG of Heat and Power is an effective way alleviating energy demands by incorporating new power generation technologies into the private sector. This means that citizens need to do their part in helping solve the energy problems that face them by purchasing such devices as small scale wind turbines and photovoltaic panels for home or private use. These technologies can greatly reduce the amount energy that needs to be provided from fossil fuels when incorporated into household power supply. In addition to providing energy from renewable sources, these technologies do so without environmental impact. The Western Governors Associations describes DG as “an affordable, efficient, clean and reliable piece of the puzzle for meeting energy needs” and goes to describe the benefits as “a way to literally save billions in capital investment, reduce power cost, reduce security vulnerabilities, improve reliability and power quality, cut greenhouse gas emissions and cut other pollutants.”
Small-Scale Wind Turbines
Small wind turbines are a viable option in distributed generation. Generally residential scale turbines range from 1 to 10KW in size. As the turbine size increases, so does cost effectiveness. A small turbine maybe cheaper initially but cost more per KW than a larger one. With costs of about $5,000 per KW, turbines are within reach of homeowners and the commercial sector.
Figure 2: Small Turbine for a residence
Wind turbines are effective if the average annual wind speed is above 10 mph. This ensures that the turbine will be operating at a reasonable efficiency for most of the year.
The most important consideration for a prospective wind turbine user is site selection. A wind turbine should be sited so that the average wind speed is the highest possible and that nothing blocks the predominant wind direction. This means placing the turbine as high as possible. A building or a stand of trees will create large areas of turbulent air on the leeward side so a turbine should be sited with this in mind. The topography of the location is also important. Turbines should be placed on ridge lines or on hills where wind will be forced up to them.
Figure 1:Wind Turbine for a commercial building
Though wind turbines have a small footprint, they can be tall and unsightly. Some communities have strict codes that prevent towers from being erected, limiting the residents to smaller roof-mounted turbines. These are usually much smaller and may generate less than one kilowatt.
A small wind turbine is similar to a larger one in design. Utilizing a wind driven rotor to produce work on a shaft, the shaft speed is usually increased through a gear box to drive a generator. Because the turbines are relatively small, a tail fin is usually used to point the rotor in the direction of the wind. This is unlike larger scale turbines which use computer controlled servo motors and wind indicators to aim the rotor.
A wind turbine is an effective way to reduce the user’s dependence on grid power. To be practical, the user should live in an area with 10mph plus average annual wind speed and a relatively high cost of electricity. This will ensure a good return on the initial investment of the turbines with the sale of power back to the grid at premium prices.
The turbine is usually connected directly to house and to the grid through an inverter and the user’s standard power meter. The inverter converts the DC voltage supplied by the turbine to AC line current used by the house and grid. When the wind speed is low, the user buys electricity from the grid to supply the power requirements not produced by the turbine. When the wind speed is normal, if user generates more power than they use, their power meter essentially runs backwards. This either directly offsets your power bill or creates credit with the power company depending on the quantity of energy produced.
Solar Power and Renewable Distributed Electricity and Heat Generation
Photovoltaics (PV) are materials that convert light energy provided by the sun directly into electricity. A solar panel or multiple panels that make up a photovoltaic array are growing in popularity significantly in recent years due to high conventional fuel costs. These solar panels are popular in hard to reach or very isolated areas where being tied to a grid is too difficult. These systems generally collect all of the energy during the day and store excess energy in batteries for night time use. This is fairly uncommon; around 90% of solar energy panels are grid tied systems where the solar power system it connected to the local mains electric grid. This is beneficial because when insufficient electricity is produced by the solar panel system, energy can be drawn from the grid for use. Inversely, when an excess of electricity is produced by the solar panels energy can be fed and sold to the grid. Therefore these systems can be used not only for supplying electricity, but also for financial gain.
Photovoltaic cells are made up of semiconductor material, usually silicon. This semiconductor, when placed in sunlight is bombarded with photons which excite the electrons around the silicon molecules. An electric field is induced through the semiconductor to create a flow of the excited electrons out of the silicon and into metal contacts allowing the electricity to be used elsewhere.
Single crystalline silicon is actually a terrible conductor because all of its valence electrons are held into place by bonds. Silicon has four valence electrons and forms four covalent bonds with four other silicon molecules to form the valence octet. The common way of freeing up electrons from these bonds and creating the electric field in the cell is through doping. Doping is essentially the implantation of impurities into the silicon to break up the crystalline structure and free up electrons. Doping with phosphorous is called N-type (negative-type) because phosphorous has 5 valence electrons and results in free, un-bonded electrons. Doping silicon with boron is called P-type doping. The boron only has 3 valence electrons and results in being attracted to the electrons. When N and P-type silicon are in contact with each other the excited free electrons from the N-type silicon are attracted to the P-type silicon creating a heavy flow of electrons from the N-type side to the P-type side. This is the built in electric field that is necessary to draw current.
Figure 3: Photovoltaic Concept
The operation of photovoltaic cells is costless, but the initial cost of the cells is quite the opposite causing its limited use. Some ways to decrease the initial costs are to use polycrystalline silicon or amorphous silicon because of the reduced cost of production, although these panels are less efficient at producing power than doped single crystalline silicon. Another more creative way to cut costs on solar panels is to integrate the solar panels into the actual structure of buildings on the roof, walls and windows so the panels displace the costs of the materials that would have been used.
Figure 4: Example of home roof paneling
Another very usable way of using or storing solar energy is by heating water. This system consists of solar collector and some sort of water circulation system. The circulation can be powered either by a pump or passively with a thermosyphon system. Thermosyphons are very attractive because they do not require an outside source of energy to power the circulation but a major drawback of this type of system is that the storage tank needs to be above the thermo collector. It operates with the principle that the hot fluid from the collector rises to the storage tank and circulates with the fluid in the storage tank. Pumping systems are convenient in that the storage tank can be located anywhere. This solar heating is important because households very high energy expenditure on water heating.
Figure 5: Example of macro use of photovoltaics
References
[1] Wind and Hydropower Technologies, US Department of Energy. <http://www.eere.energy.gov/windandhydro/windpoweringamerica/ne_history_modern.asp>
[2] Clean and Diversified Energy Initiative, Western Governors' Association. <http://www.aceee.org/chp/wga.pdf>
[3] Building-integrated photovoltaic, Wikipedia<http://en.wikipedia.org/wiki/Building-integrated_photovoltaic>
[4] Anatomy of a Solar Cell How Stuff Works <http://science.howstuffworks.com/solar-cell3.htm>
[5] Small Wind Electric Systems, US Department of Energy <http://www.eere.energy.gov/consumer/your_home/electricity/index.cfm/mytopic=10880
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Nick Farrell
Jake Barry
Eliot George
