Environmental Engineering Reference
In-Depth Information
Solar and wind
The most practical technology to harness on-site solar energy is PV solar panels. PV panels
convert direct sunlight into electricity with efficiencies that depend on the technology.
Efficiency for polycrystalline silicon tops are around 14 percent; monocrystalline silicon can
go up to 19 percent; and thin film has a 10-percent efficiency in the best case scenario, but
more typically around 6 percent.
As of February 1, 2011, depending on efficiencies, the price of PV solar panels ranges
between 2 and 3 US$/watt peak (Wp). However, besides solar panels, a grid-connected solar
power system needs other components, such as distribution panels, overcurrent protection
devices, a grid-tie inverter, disconnect switches, isolation transformers, and a net metering
system (Gevorkian, 2007). Therefore, the price of a complete system is higher than just the
solar panels, and an installed industrial/commercial grid-connected solar power system costs
around $3.25/watt.
Accumulation of electricity from solar systems is not practical and still expensive. Where
available, a better choice is to connect the on-site solar system to the grid through a meter
capable of net metering, so energy produced by the PV system is used to satisfy electric needs
in house and any excess fed back to the utility grid. Net metering uses a special meter capable
of running backward when excess energy is produced at the customers' facility, so any surplus
energy can be used to offset consumption from the grid.
Net metering is not available everywhere. In the United States, about 35 states offer net
metering; it is more widespread in Europe as a form of incentive for customers to invest in
renewable energy systems. Net metering is not used only for solar systems but also for wind
or any other sources to produce renewable energy.
Wind power is an indirect form of solar power, which is harnessed with turbines that trans-
form wind kinetic energy into electricity. For industrial on-site generation, small wind tur-
bines are available with capacities of 100 kW or less that can be mounted on buildings or
free-standing towers. According to 2009 data, the cost per watt peak for small wind turbines
varies between $3 and $6 per watt peak (American Wind Energy Association [AWEA], 2009).
Small wind turbines have efficiencies in the vicinity of 30 percent and require average wind
speeds of at least 10 miles per hour. Power produced by wind turbines is sensitive to wind
variation. Similarly to pumps and fans, power generated by wind turbines is proportional to
the cube of the wind speed, so a decrease in 10 percent in the wind speed translates into a 27
percent drop of the power generated [(1
−
0.1)
3
].
Landfill gas and biogas
On a global scale, after agricultural soils, enteric fermentation, and natural gas and oil sys-
tems, landfill gas is the largest anthropogenic emission source of methane (EPA, 2006), and in
the United States, landfills are the second source of human-made methane after enteric fer-
mentation (EPA, 2009).
Landfill gas is the final result of the fermentation of biodegradable waste (i.e., paper, food
scraps, and yard waste) deposited in landfills. After waste has been dumped in the landfill,
aerobic bacteria start decomposing biodegradable materials until all oxygen is depleted. Then
anaerobic bacteria transform the end products of the aerobic fermentation into cellulose,
amino acids, and sugars. These compounds are further broken down into gases and short-chain
organic compounds that become the substrate for methanogenic bacteria.
Methanogenic bacteria produce stabilized organic matter and “landfill gas,” which contains
50 to 70 percent methane and 29 to 45 percent carbon dioxide with the presence of some
