Point of View

Good wood energy

April 26, 2010 

— North Carolina has new markets for renewable energy. One result is that Duke Energy has recently proposed to "co-fire" wood in its coal plants in its move to increase use of renewable fuels. This prospect concerns many, including the state Environmental Management Commission, which is preparing a report about the use of wood to generate renewable energy.

Although wood is one of humanity's oldest energy resources and one relied upon by more than 2 billion persons today, it is not part of most renewable energy discussions, which typically concentrate on solar and wind. In recent decades, however, advanced wood combustion (AWC) is providing an increasing stream of clean, economical and sustainable energy in many nations, supported by peer-reviewed science and life-cycle analyses.

AWC is not Grandpa's smoky wood stove; it is clean burning and highly energy efficient. Because wood is the product of plant growth, recurring environmental, economic and social benefits can be assured, if we manage wood and forests with care, wisdom and efficiency.

Advanced wood combustion (defined as clean combustion of wood with high energy-conversion efficiency) will not solve the energy crisis, but wood is abundant and can help us transition to renewables. Wood is called the "renewable heavyweight" in Europe because of its abundance and because it can be deployed with off-the-shelf technologies. In North Carolina too, woody biomass is abundant and is relatively inexpensive compared with long-term trends in fossil fuel prices.

However, the promise of wood energy is being betrayed by North Carolina's first big plans for wood energy in renewable markets. While Duke Energy may paint its proposal to burn coal with wood as a step toward a renewable future, an inescapable problem arises from the proposal's extremely wasteful use of wood.

A few details can illustrate the point. Duke Energy's wood-fired electric plants will capture but 25 to 35 percent of wood's inherent energy as electricity and will therefore send the majority of wood's energy up the cooling towers as waste heat. In other words, for every three to four trees that will be grown for several decades, logged, transported and combusted, the equivalent of only one will produce "renewable energy."

This waste of wood and energy is so large that it mocks the meaning of renewability. The large scale of these electric plants has concerned the Environmental Management Commission about the regional impacts on the state's forests.

The promise of wood energy lies not with stand-alone electric plants but with some of the most exciting of the wood-energy industries, those community-scale facilities with advanced wood combustion. AWC systems convert 70 perecnt to over 90 percent of the energy stored in wood to heat and cooling, and sometimes combine thermal with secondary electrical power (combined heat and power, or CHP). A quarter to a third of North Carolina's energy budgets are devoted to heating and cooling, and so there is tremendous opportunity for wood energy to heat and cool large buildings, malls and campuses, hospitals, schools and prisons, and to create some electrical power as well.

Wood is far too valuable to waste, and thousands of these high-efficiency AWC facilities are being installed across Europe and the United States; in urban areas such as in St. Paul, Minn., and on college campuses such as the University of South Carolina in Columbia. A new report, out this month from the New York state energy agency, reviews this industry's impressive technological progress in boiler design, air-pollution control and energy efficiency.

As the N.C. Utilities Commission contemplates Duke Energy's proposal to co-fire wood and coal, it must consider how the state can make the most of the wood our forests can sustainably produce. From the perspective of renewable energy, community economics and the forest environment, the efficiency with which wood's energy is captured is as important as the promotion of wood energy itself.

Low-efficiency systems that send wood into stand-alone electric plants should not be counted as a renewable in the new market. The efficiency of wood-energy systems needs to be central to the strategy of wood-energy development, if only because efficiency will help ensure the renewability of energy supply over the long-term.

Two final points are clear. First, the physics of wood-energy systems makes wood well suited to local-scale heating and cooling, and secondarily electrical generation. If wood's thermal energy is not valued and is sent up cooling towers, wood will be wasted and in very large amounts. Second, the renewables such as wind, solar, geothermal and biomass each has strengths and weaknesses, and each needs to be developed as a piece that fits into a larger renewable puzzle.

To achieve energy independence and back away from fossil fuels, we need all the renewable energy that we can sustainably muster, including that from wood-energy systems with high efficiency.

Daniel deB. Richter is professor of soils and forest ecology and director of graduate studies for Duke University's Interdepartmental Program in Ecology.

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