have been harvested sustainably, and pellets. Advanced biomass-fuel appliances use pellets, which are produced by compressing woody material that may include waste wood and sawdust, agricultural wastes, wastepaper, and other organic materials. Some pellet-fuel appliances can also burn corn kernels, nutshells, and wood chips. Pellet stoves use electricity to run fans, controls, and pellet feeders.

One of the concerns about solid-fuel combustion for home heating is air pollution. In areas where wood stoves are prevalent, wood smoke is a major source of fine particulates and gaseous pollutants, including nitrogen oxides, carbon monoxide, and organics. The mandatory smoke-emission limit set by the U.S. Environmental Protection Agency (EPA) for wood stoves is 7.5 grams of smoke per hour for noncatalytic stoves and 4.1 g/h for catalytic stoves.18 Modern noncatalytic stoves have improved fireboxes to achieve high combustion efficiency. The most efficient wood-burning appliances also use catalytic converters to achieve nearly complete combustion of the feedstock and to reduce harmful emissions. Stoves are available with EPA-certified emissions as low as 1 g/h. Stoves require homeowner maintenance and catalyst replacement, however, to retain their high efficiencies and low emissions.

In summary, modern solid-fuel stoves are efficient and clean compared to the fireplaces of the past. The economics of using a stove to combust biomass products depends on the fuel being displaced and the distance from home to supplier.

CONCLUSION

A future characterized by a large penetration of renewable electricity represents a paradigm shift from the current electricity generation, transmission, and distribution system. There are many reasons why renewable electricity represents such a shift, including the spatial distribution and intermittency of some renewable resources, as well as issues related to greatly increasing the scale of deployment. Wind and solar—two renewable-energy resources with the potential for large near-term growth in deployment—are intermittent resources that have some of their resource bases located far from demand centers. The transformations required to incorporate a significant penetration of additional renewables include transformation in ancillary capabilities, especially the expansion of transmission



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