Hydroelectric power has long been left out of renewable energy counts, on the assumption that it creates some greenhouse gas emissions as vegetation caught in damned rivers rots. But that may be about to change, with the results of new research just published by Dr. Jonathan Cole in Nature Geoscience finding that hydroelectric power reservoirs are responsible for only about a sixth of the carbon dioxide and methane previously attributed to them.
An international team of scientists has amassed the largest data set to date on greenhouse gas emissions from hydroelectric reservoirs. The new analysis of 85 globally-distributed hydroelectric reservoirs revealed that these systems emit only 48 million metric tons of carbon annually, much lower than the earlier estimates of 321 million metric tons.
In addition, most of the emissions only occur when reservoirs are newly built, as construction leaves behind decomposing vegetation and soil organic matter. But as they age, carbon and methane emissions decline, with cold-water systems stabilizing much more rapidly than warm water hydroelectric power stations.
But even so, including both cold and warm systems, all hydroelectric reservoirs represent only 16% of the greenhouse gas emissions from all man-made reservoirs such as for municipal water supplies.
Along with geothermal power, hydroelectricity is one of the oldest methods of generating renewable energy, and at this point, represents 85% of “renewable” energy produced, and 20% of all energy. Yet most policies requiring the production of additional renewable energy do not include hydro power.
Although some states in the Northeast and Northwest do allow various percentages of hydro power to count towards renewable energy requirements, most states disallow hydro, because of the much higher emissions found by earlier research.
The new findings could really shake up the world of renewable energy policy.
Utility-scale hydro-electricity does not count towards the renewable energy standards (RES) mandate in California. Any new hydroelectric power must not require any appropriation or diversion of water from a watercourse, so utility-scale hydro-electric power will never be built in California, which has a 30 MW upper limit. Only future wave energy off-shore would qualify for its strict renewable rules.
But if its old existing hydro power were included in the count of California’s renewable energy supplies for example, the state would almost meet its 2020 mandate to get 33% from renewables. Although the state currently gets 18% from true renewables (solar and wind, geothermal and bio), if the 14.5% it gets from hydro were included, then the total is already 32.5% from renewables.
The study was funded by the Swedish Foundation for International Cooperation in Research and Higher Education and The Cary Institute of Ecosystem Studies, in upstate New York. States in the Northeast with an abundance of cold water are already a little more lenient in allowing hydroelectric power to count towards state renewable energy targets than are the states in the hot and parched West. (Most of the South lacks renewable policy)
The state of Maine, one of the first to pass the original Renewable Portfolio Standards allows (30%) hydropower to count and including it gets 55% from renewable energy. Tiny, cold, wet Vermont allows hydro up to 200 MW to count towards its renewables policy, six times what California allows. According to the NOAA, both the Northeast and the Northwest will see greatly increasing precipitation events with climate change, so encouraging hydro power there makes policy sense.
This study would seem to back up the wisdom of that regional variation in policy, with the finding that it is warmer water that is largely responsible for the (now much lower) greenhouse gas emissions that are estimated to come from hydro power.
But since the emissions estimate itself is so much lower, this study might also influence policy in warmer regions – at least in states that have adequate water resources – if the findings by Dr. Cole and his team wind up being replicated by other researchers.