The Greenhouse Effect
Although the bulk of photovoltaic devices today are used for purely practical and economic reasons, a potential benefit of photovoltaics is that PV is one of the most environmentally benign of any electricity generating source. The environmental impact of electricity generation, particularly the greenhouse effect, adds an important reason for examining photovoltaics. A brief overview of the greenhouse effect is given below.
The Earth's temperature is a result of an equilibrium established between the incoming radiation from the sun and the energy radiated into space by the Earth. The outgoing radiation emitted by the Earth is strongly affected by the presence and composition of the Earth's atmosphere. If we had no atmosphere, as on the moon, the average temperature on the Earth's surface would be about -18°C. However, a natural background level of 270 ppm carbon dioxide (CO2) in the atmosphere absorbs outgoing radiation, thereby keeping this energy in the atmosphere and warming the Earth. The atmosphere causes the Earth's temperature to be about 15°C on average, 33°C above the moon's. Carbon dioxide absorbs strongly in the 13-19 µm wavelength band and water vapour, another atmospheric gas, absorbs strongly in the 4-7 µm wavelength band. Most outgoing radiation (70%) escapes in the "window" between 7-13 µm.
Human activities are increasingly releasing "anthropogenic gases" into the atmosphere, which absorb in the 7-13 µm wavelength range, particularly carbon dioxide, methane, ozone, nitrous oxides and chlorofluorocarbons (CFC's). These gases prevent the normal escape of energy and potentially will lead to an increase in terrestrial temperature. Present evidence suggests "effective" CO2 levels will double by 2030, causing global warming of 1~4°C. This would lead to changes in wind patterns and rainfall and as a result may cause the interior of continents to dry out and cause the Earth's oceans to rise. Further increases in the release of anthropogenic gases would, of course, cause more severe effects.
Correlation of the rise in atmospheric carbon dioxide concentration (blue line) with the rise in average temperature (red line)[1].
The graph above only goes to the mid-90s since that was the time we started writing the PVCDROM. At the time there was considerable discussion about whether the warming was a trend or a statistical fluctuation. There was some hope that the average temperatures would go down again to the level of the statistical average. In the in the intervening years the temperature of the earth has continued to rise as shown in the graph below:
Average surface temperature of the earth. Temperatures are continuing their upward rise.[2][3]
Clearly, human activities have now reached a scale where they are impacting on the planet's environment and its attractiveness to humans. The side-effects could be devastating and technologies with low environmental impact and no "greenhouse gas" emissions are likely to be of increasing importance over the coming decades. Since the energy sector is the major producer of "greenhouse gases" via the combustion of fossil fuels, technologies such as photovoltaics, which can substitute for fossil fuels, must increasingly be used [4].
References
- Citekey <span data-scayt_word="Kerr1995" data-scaytid="12">Kerr1995</span> not found
- GISS Surface Temperature Analysis, , 2010.
- "Global temperature change", Proceedings of the National Academy of Sciences, vol. 103, issue 39, pp. 14288 - 14293, 09/2006.
- Citekey <span data-scayt_word="Blakers1991" data-scaytid="14">Blakers1991</span> not found
Comments
The Greenhouse Effect
The Greenhouse Effect
by stas » 12 Jul 2010, 21:31
Reason: Edit
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Re: The Greenhouse Effect
by stuart » 17 Jul 2010, 17:27
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Re: The Greenhouse Effect
by psyfr3ak » 27 Oct 2010, 13:02
after reading the introduction about greenhouse effect i have a question. the PV parks will contribute against the global warming directly, by absorbing the radiation and converting to electricity?
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Re: The Greenhouse Effect
by stuart » 04 Nov 2010, 14:57
psyfr3ak wrote:Hello,
after reading the introduction about greenhouse effect i have a question. the PV parks will contribute against the global warming directly, by absorbing the radiation and converting to electricity?
No, the amount of land covered by solar is too small. Even if all our electricity was generated by photovoltaics the change in reflectivity would be marginal compared to crop coverage etc.
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Re: The Greenhouse Effect
by halfmoonhalf » 26 Jul 2011, 20:55
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Re: The Greenhouse Effect
by mofogie » 27 Jul 2011, 16:03
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Re: The Greenhouse Effect
by stuartb » 02 Aug 2011, 21:42
mofogie wrote:I would like to see more evidence that human activities are contributing to Greenhouse effect, significantly. Intuitively, even one volcanic eruption must release far more of these gases than a millions of fossil fuel engines. The earth has seen global warming in past ages, when humans werent even around. Also if humans are contirbuting that much, how much is due to energy consumption? There are many other sources, like animal waste from cattle. If anyone has some rough figures of these slices of the pie, would be interesting to read.
There are many articles covering the sources of greenhouse gas emissions. The preponderance of evidence is that man made sources have changed the balance dramatically leading to higher average temperatures across the globe. A large fraction of the greenhouse gas emission results from energy generation and photovoltaics provide a way of reducing emissions
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Re: The Greenhouse Effect
by Vasey0001 » 06 Aug 2011, 04:49
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Re: The Greenhouse Effect
by kmarti11 » 18 Sep 2011, 20:03
I have a question regarding the "anthropogenic gases" absorbing certain micro-wavelengths of light. You explained that 70% of heat radiation escapes through the 7-13 micro-meter wavelength window, but I was wondering: Is there was a particular range of wavelengths within this window through which significantly more radiation escapes? . A window, within a window essentially.
Also of the gases, which exhibit the highest thermal absorptive properties? I had believed that I'd heard that methane is actually worse then CO2 in contributing to global warming.
Thank you.
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Re: The Greenhouse Effect
by stuartb » 30 Sep 2011, 19:58
kmarti11 wrote:Hi,
I have a question regarding the "anthropogenic gases" absorbing certain micro-wavelengths of light. You explained that 70% of heat radiation escapes through the 7-13 micro-meter wavelength window, but I was wondering: Is there was a particular range of wavelengths within this window through which significantly more radiation escapes? . A window, within a window essentially.
Also of the gases, which exhibit the highest thermal absorptive properties? I had believed that I'd heard that methane is actually worse then CO2 in contributing to global warming.
Thank you.
Yes there are wavelengths with greater transmissivity. Atmospheric scientists take advantage of the regions of higher tramissivity to shoot lasers into space but I am not sure of the details.
Molecule for molecule methane is a more effective greenhouse gas than CO2. The concern with CO2 is that there is so much of it and it is relatively long lasting.
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