How green are photovoltaics (solar cells)?
I came across an interesting analysis of how environmentally friendly the current solar cell technology really is if you take into account the energy it takes to produce them. The bottom line seems to be that currently there is only a small reduction in carbon emission for photovoltaics used in the Northern US. Here is a calculation I made based on the article and original publication. I also assumed the use of multi-crystalline silicon cell as these seem to be used widely:
CO2 emission in cell production (from publication): 53 g/kWh (30-year operation, 1,700 kWh/m2/yr insolation, 100% performance ratio) Insolation in Northern USA: 900 kWh/m2/yr Cell efficiency (multi-crystalline silicon): 13% Electricity yield of cells at 80% performance ratio: 94 kWh/m2/yr Energy yield over 20-year lifetime: 1872 kWh/m2 CO2 emission in cell production: 188 g/kWh (20-year operation, 900 kWh/m2/yr insolation, 80% performance ratio) CO2 emission from electricity generated with natural gas: 450 g/kWh CO2 emission from electricity generated with coal: 850 g/kWh
Facts about solar cells can be found here, including definitions of many terms.
The analysis shows that there is a significant carbon footprint of photovoltaic electricity production. However, it is still at least twice as environmentally friendly as electricity generated from fossil fuels.
The article makes the interesting point that the benefits of solar cells are only realized over many years (e.g. 20 years in the example above). If there was a giant worldwide effort in switching our electricity generation over to photovoltaics it would mean a huge initial increase in carbon emission before there will be an actual net reduction. Maybe photovoltaics are not the best course of action if CO2 emission is to be reduced as quickly as possible? Alternatively, the sun's energy can be used for heating, which does not involve the production of photovoltaic cells. However, investing in energy conserving technology (thermal insulation of your house, Energystar appliances, fuel-efficient cars etc) and changing wasteful behavior (idling your car, opening windows to regulate your indoor temperature, overheating or overcooling your house/apartment) are probably still the most efficient ways to reduce carbon emission.
Nuclear-generated electricity?
I was curious how nuclear generation of electricity compared, and the first few figures I found were ranged from 6 to 26 g/kWh (here is a summary of these data). This compares very favourably with the above.
Of course, the question of dealing with nuclear waste is a major problem. I don't know that any civilisation has been able to keep anything safe for 1,000's of years, and I certainly wouldn't presume that we're capable of doing that. So really, we'd need a way of recycling it (preferably not involving weapons). Plus I read that Uranium is a very limited resource. But still I find I'm tempted to think it may be good to rely more on nuclear (or nukeelar, as someone used to pronounce it).
The ancient egyptians managed
In reply to Nuclear-generated electricity? by Alexis
The ancient egyptians managed to build structures that withstood the test of time for thousands of years. Shouldn't we be able to do the same?
I really like the idea of nuclear, personally, and I definitely think that as time goes by, and the cost of recovering oil goes up, it will become much more widespread.
Nuclear as bridge!
In reply to Nuclear-generated electricity? by Alexis
I agree with both Alexis and Axel on fission nuclear! We need an urgent bridge (fission nuclear is a currently working technology) to first renewable energy and hopefully further to fusion nuclear when we manage to develop the technology. As far as waste goes - its volume is much smaller than CO2 volume and consequently should be much easier to contain. It is also a potential resource (mostly depleted uranium and plutonium)! There are people who work on utilizing it:
http://www.intellectualventures.com/OurInventions/TerraPower.aspx
A hybrid
A hybrid concentrating solar with subsequent photovoltaic conversion to electricity may reduce this footprint. Basically parabolic mirrors can focus the sunlight onto the solar cell.