The Fundamental Limitations of Renewable Energy
"Many people still think that it will not be long before renewable energy such as solar and wind becomes outright cheaper than fossil fuels, thereby leading to a rapid expansion of the thin orange slither in the graph below. This is an ideologically very attractive notion, but, as discussed in this article, it is questionable whether this is in fact physically possible.
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Renewable energy advocates often point to the total energy fluxes of the Earth (below) and proclaim that renewable energy resources are essentially boundless. Yes, it is true: we are surrounded by incredible amounts of diffuse renewable energy (e.g. solar radiation and wind). Unfortunately, however, this energy is useless to us unless it is concentrated into forms such as electricity or fuels.
The reason behind this is called the second law of thermodynamics which states that energy must flow from a concentrated form to a more diffuse form in order to do work. Our entire society was built on the work performed through transforming concentrated fossil energy to diffuse heat and, in order to compete, renewable energy technologies also need to deliver such concentrated energy."
http://theenergycollective.com/schalk-cloete/257351/fundamental-limitations-renewable-energy?utm_source=feedburner&utm_medium=feed&utm_campaign=The+Energy+Collective+%28all+posts%29
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Because however you look at it, the large scale sources we intend to exploit - wind and solar - are profoundly intermittent. Solar produces most of its output within 4-6 hours each day, depending on location, and wind produces most of its output over less than 30% of the time. The German 2012 wind output graph shows the problem:
To displace coal or nuclear power in Germany, they need to be able to clip off the peaks above about 7,500 and store the energy so as to fill in the majority of the gaps in production below 5,000.
The cost of doing that is what determines the feasibility of the whole enterprise, which is one reason I have begun watching fuel cells so closely.
Last edited Wed Aug 7, 2013, 01:55 PM - Edit history (1)
Just as food for thought, here is the status of world final energy consumption by source.
This is the concept behind calls for energy efficiency (a strategy that is anathema to the coal and nuclear industry because it slashes their profits).
The energy wasted from thermal sources is a very significant factor in understanding the issue of what energy source is doing what. Primary energy measures the total amount of energy that a fuel source yields - no matter whether it is powering our lives (ie electricity or or propulsion for autos) or whether it is waste heat being transferred to our waterways from nuclear plants or heat causing NO2* emissions off the hot engine block of an internal combustion.
An alternative (and most say better) way of looking at the production and use of energy is to measure what is needed and consumed by the actual work being accomplished. For example, an average internal combustion engine (ICE) powered car ejects 85% of the energy content of the gasoline it consumes as heat and only uses 15% for motive power. When we look for alternatives to gasoline do we think biofuels, and duplicate the efficiencies of the gasoline powered ICE or do we focus on batteries and electric motors that have far better efficiencies - typically using 90% of the input energy for locomotion?
Writ large, what does that mean? Take a look at this flow chart and note that the "rejected energy" comprised 58.1 quads of the total 95.1 quads of primary energy used in the US last year. How much was actually used to do the work of the nation? Only 37 quads.
If we look more closely at the various sectors we can see where the major opportunities for energy efficiency improvements are to be found:
Sector: Gross - Useful Energy; Rejected Energy (proportion of useful to rejected)
Transportation: 26.7 - 5.6; 21.1 (21 : 79)
Electric Generation: 38.10 - 12.40; 25.70 (33 : 67)
In sectors where the heat value of the energy is useful we see much higher efficiency
Industrial: 23.9 - 19.1; 4.77 (80 : 20)
Commercial: 8.29 - 5.39; 2.90 (65 : 35)
Residential: 10.60 - 6.9; 3.72 (65 : 35)
Now let's look at the Solar, Wind and Hydro Subset of Electric Generation. These produce electricity directly with insignificant primary energy lost as heat in the generation phase, however they do incur line losses of about 7%.
SolarWindHydro: 4.07 - 3.78; 0.285 (93 : 7)
Let's compare that to
Nuclear: 8.05 - 2.62; 5.43 (33 : 67)
In the US, the our fleet of nuclear reactors (what is it, down to 99 and falling fast?) might have produced 8.05 quads of primary energy, but at about 35% efficiency at the busbar and a further 7% line loss, (8.05q x 0.35 = 2.82q x 0.93) that only equals 2.62 quads actually delivered to the end user for work.
3.78q > 2.62q
See also: http://www.nawindpower.com/e107_plugins/content/content.php?content.11788#utm_medium=email&utm_source=LNH+07-19-2013&utm_campaign=NAW+News+Headlines
Oh yes, and then there is this.
