Monday, July 23, 2007

Hydro, not so Green?

We tend to think of hydro-electric power as a totally clean form of energy, but there is a carbon footprint here too. According to figures obtained by Vincent St. Louis, who is a scientist at the Canadian University of Alberta, man-made reservoirs (of which one quarter are used for hydrolectric power production) release one billion tonnes of CO2 annually on top of another 70 million tonnes of methane, which has a global warming capacity of around 100x that of CO2. The problem is that bacteria break down plant materials submerged under their huge reservoirs into greenhouses gases. 80% of electricity is provided by Hydropower in Brazil, as an example of a tropical country, and Norway produces almost 100% of its electricity from hydroelectric sources. Canada and Switzerland, too, use the technology on a large scale since both are well provided for by rivers.

[The above figure that methane has 100x the global warming potential of CO2 might be disputed since it is often cited that it is nearer 20x. However, that is the average taken over 100 years, if equal volumes of methane and CO2 were emitted into the atmosphere, allowing that methane is oxidised in the troposphere to CO2 over a period of about 12 years. It is the "instantaneous radiative warming factor" that matters, since this would account for the relative effects of a steady release of the two gases, rather than a one-off emission].

The UN has decided to evaluate whether some countries might actually be better off by constructing coal and gas-fired power stations, which is an especially touchy point when heavy financial investments have been pledged, and certainly for the developing nations, e.g. Brazil. According to St.Louis, over a period of 100 years, hydro-dams will account for 7% of the global warming from all human activities. A typical example is the 250 MW Balbina dam in Brazil, which was created by flooding 2,500 square kilometers of Amazonian rainforest, the emissions from which are reckoned at 25% - 38% higher than from a coal-fired power station of equivalent capacity.

The degree of the emissions depend on the area and the depth of the reservoir, and on the nature of the underlying vegetation, and the deeper the better it would seem. However, each dam needs to be evaluated on its individual basis. Philip Fearnside, a conservation biologist at the National Institute for Amazon Research in Manaus, has concluded that the problem of the Balbina and Tucurui dams (the latter with 20x the generating capacity, but a dam area some 300 km^2 less than Balbina) is worse than previously thought, and that an average tropical hydropower plant emits four times as much carbon during the first four years of its life than a comparable fossil-fuel fired power station. Others have argued that the large emissions are caused by poor system design, and could be improved.

The jury is out still, since "The big issue is what would have happened if the reservoir hadn't been there," so sums-up Mike Acreman, a professor at the UK Centre for Ecology and Hydrology in Wallingford, Oxfordshire. "You can't go to oine and measure the methane coming off the surface and say that that was definitely caused by the hydropower scheme." Nonetheless, he does concede, "Perhaps hydropower is not as green as we thought. A lot of these tropical hydropower schemes would have been made by simply flooding a forest. There would have been a lot of trees and plants, and you need to think about what happens to all that carbon."

To my mind there are two issues here. Global warming and the decline in world oil and gas reserves. In the UK we make most of our electricity from gas, along with coal and nuclear, but hydropower is renewable unlike these finite and rapidly dwindling resources. I think we will need as much hydropower as possible, since the depletion of fuel resources will most likely decimate human civilization ahead of climate change.

Related Reading.
"Hidden dangers," by David Adam, The Guardian:


Anonymous said...

I see several problems with this line of argument. First the carbon captured by organic material in a man made lake comes from the earth's atmosphere. Thus it does not incease the ammount of carbon tied up in the carbon cycle. Second there are things that can be done to control methane emissions. Plant material that would go under water when an impounded lake is filled, can be removed, and burned. Ditto for aquatic plants growing in the lake. But lake managers have a slightly different take on this issue. They argue that aquatic plants tend to support wildlife. By the way, we have the same carbon issues with natutal lakes and wet lands. Should we drain Everglades to limit carbon emissions? Hydro power not green? That is downright crazy.

Professor Chris Rhodes said...

Hi Charles!

Fair comment! As I say, in my opinion we should use all the hydropower we can get. On balance, and against that backdrop of oil and gas running short, it is a good option, and fully renewable too! I found the matter interesting and thought I would share it on here. If the UN are involved in it, I think we might hear more on the subject? But like you, I hope there will not be a "culling" of hydroelectric power. It is interesting that it is developing nations that depend more on this form of generation than the industrialised nations, and I wonder if there is some political agenda underlying this present "concern"?



Unknown said...

It should be noted that hydropower is absolutly necissary for dispatchable power. The only thing besides dam/pumped hydro that can be dispatched quickly is natural gas, diesel, and some types of coal power plants.

With power sources that act as negative load rather than baseload (wind, sometimes solar) the need is even more pronounced. The only reason Denmark can utilize so much wind is the tie in with the Sweedish hydroelectric grid.

Professor Chris Rhodes said...

I agree with you completely, and it would be plain daft to curb hydro-power. I see how well Norway especially and Canada and Switzerland too use it, and we should follow that lead. It is fully renewable whatever "faults" it may have.

I think the emissions problem is negligible compared to the benefits from it and the argument that the dams will become silted-up is surely not tenable as they could be dug-out, couldn't they.

I saw this story and found it interesting but worrying if the climate change movement might be hijacked to actually remove something so beneficial, using an excuse based on emissions. It worries me if the UN are taking it seriously.


Liam said...

Sorry Charles, but I don't buy your argument.

In response to your first point, yes the carbon volume in the atmosphere stays the same, but it's not in the same form. You're trapping CO2 and emitting methane, which is a much nastier greenhouse gas.

As for your second point, your 'solution' doesn't cater for areas where you have marked dry and rainy seasons. In the dry season, when the water level is low, plants grow down to the waters' edge trapping CO2. Then in the wet season, they get covered, rot, and release methane. It's not a once-off that can be solved by clearing vegetation, it's a continuous methane emitting system.

No we shouldn't drain the everglades, but maybe we should reconsider our views of how green hydro power is. (It can be green, but it often isn't).

Unknown said...

Do you have a reference for the 100x warming power of CH4 compared to CO2? Thanks.

Professor Chris Rhodes said...

I worked it out basically from the definition for Global warming potential in:

along with some factors for atmospheric lifetimes for CH4 and CO2 (the latter is quite variable, between 50 and 200 years), but from the data in that link, if the warming potential (relative to CO2) for methane is 7.6 (over 500 years), 25 (100 years), 72 (20 years) - or the other set of data) a value of 100 is reasonable for zero-years, i.e. instantaneous radiative forcing by a given MASS of each.

Professor Chris Rhodes said...

Hi Theo,

just an update:

if you look at that link and the equation for GWP (x), and then reduce it for t=o (TH=0), you get:

GWP(CH4) = aCH4/aCO2 x MWCO2/MWCH4 = .53/.015 x 44/16 = 97.2. i.e. about 100. The point is that it's one kg of each gas that is assumed to be added to the atmosphere, and these concentrations thus divide out except that the values for "a(x)" are usually quoted in units of W/m^2.ppmv (i.e. by volume).

Since the volume of a kg of a gas depends on the MW of its molecules you need the correction factor as shown. i.e the volume of a kg of CO2 is less than that of a kg of CH4 by a factor of 44/16.

Both gases will over time be removed from the atmosphere, with lifetimes of about 12 years for methane and 50 - 200 years for CO2 depending on where it is, but I am assuming a kind of steady-state situation where both gases are continually emitted, as seems to be the case, although the amounts of each may vary over time. However, the latter case is complex and goes beyond this simple definition of GWP.

The point is that in terms of assessing the relative GWP of gases relative to CO2, it is important to note the time interval that is being quoted.




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