Wednesday, May 14, 2014

Renewable Energy Reduces Emissions

Is renewable energy (solar and wind) the best way to reduce carbon emissions?

At first sight, this question seems almost absurd and we are tempted to say: of course it is the best way!

But, is it? Before jumping to hasty conclusions let's do our homework.

This exercise is going to be a simplification, the purpose is mainly to show us that things in real life are not as simple as in the lab.

So, let's consider a country that supplies 100% of its electricity with coal plants.

According to this table (see link at the bottom of this page), these are the emissions per kWh generated with the different energy sources: **

Thus, if this country generates 100% of its energy with coal, their emissions per kWh would be ~1001 grams.

Now, let's say we install wind turbines (enough to supply 100% of the power when the turbines are producing at full capacity):

Let's say wind capacity factor at this country is 25% (in other words, turbines actually produce 25% of their plate rating on average). It is important to underline that this is not constant power: at some moments the turbines are producing at 100%, at other they produce nothing and at any other moment their output can be anywhere in between these extremes.

So, (simplifying) wind will produce 25% of the energy on an annual basis and the coal plants will produce the rest (75%).

Then we calculate the emissions that are really just a weighted average:

Annual average emissions per kWh = (25% x 12 g/kWh) + (75% x 1001 g/kWh) = 754 g/kWh.

We can see that the emissions of the system did drop, but they are still too high.

What better options do we have?

1. If we replace the coal plants with natural gas plants (which have much higher capacity factors and can be staggered since they are not wind / sun dependent) then the emissions would be:

          469 g/kWh

2. If we replace the coal plants with nuclear plants then the emissions would be:

          16 g/kWh

As we may see from the calculations above, Renewable energy investments are not the best way to reduce emissions.

Arguably, the fastest way to reduce emissions is to replace coal plants with natural gas plants, however, if the higher investment can be made (and the longer lead times are acceptable), nuclear is truly the low carbon energy solution.

Conclusion: Yes, Renewable energy reduces carbon emissions in most systems, however natural gas, nuclear and of course hydro, are better options.

Thank you.

a. In the developed world little new electrical capacity is needed and thus Renewable energy almost directly replaces some other energy source, however in the developing world substantial additional electrical capacity is required and thus a double investment would be required: the Renewable one, plus the reliable one.
b. Sure, Renewables (wind and sun) could be combined to somewhat compensate the fluctuations of the other one. Still, at any particular moment of the year we may have no sun and no wind. At another moment we may have both which could even force us to divert (or disconnect) capacity.
c. To simplify, here we are not considering the possibility of "dumping" energy into another country or using massive storage systems.


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At 3:22 PM, Blogger TheTracker said...

So the problem with the calculation here is that you are assuming that we cannot build >100% of the required nameplate capacity -- hence wind can provide only 25%, since its capacity factor is 25% (in point of fact modern turbines are more in the range of 30-35%).

If (leaving your other assumptions unchanged) one were to build four times as much wind energy as in the original example, the resulting system would be far cleaner than natural gas.

Of course, such a system would require power exchanges over a large and efficient grid, dynamic pricing of electricity, and some amount of storage capacity, all of which are difficult to model in a simple thought experiment.

Even if one could incorporate all of these complexities and others such as nuclear waste disposal, rigid power outputs, the risks of accident and proliferation, you still would have a hard time saying anything about the real world based on a hypothetical where one power source generations all of your electricity.

In the real world, the grid is mixed, and the question is not what one source will do everything in the future, but what to build more of and what to phase out.

The lessons of your thought experiment don't necessarily apply. If a person ate 100% kale, they would die of malnutrition. That doesn't tell us if we would benefit or not from more kale in our diets.


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