One thing we have learnt this week – solar energy payback

DSCN1669One thing we have learnt this week is that solar energy payback is now much faster than people think.  As we wrote in our book all energy sources have to have an energy investment made before you get an energy return.

Energy scientists use a measure called the Energy Return on Energy Invested (EROEI), which can be applied to any energy source. To obtain energy (from, for example, firewood) some energy has to be expended: in planting and growing the tree, making the tools to cut the tree down, the physical labour involved, and getting the firewood back to where it is used. This is fine when there is a net gain in energy, that is, the fire gives you more heat than all the energy you put into the process. If the firewood produced five times more energy than what had been expended, the net energy gain, or EROEI, would be 5:1. With concentrated forms of energy like oil, the EROEI ratio
can be as high as 100:1, because even with all the effort of drilling, pumping and refining, there is still a colossal payback in terms of the energy recovered.

In actual fact as we go onto say the EROEI has been falling for oil and gas as the easy to extract stuff has been exploited.  Conventional oil and gas give a return ratio of less than 20 x now.  Wind returns vary depending on the study, but are generally better than oil, coal and nuclear.  Solar energy payback has always been thought to be slow reducing its EROEI over its lifetime.  Generally it was thought to be about 4 years.  Of course solar energy payback depends on the location, but this was a conservative figure for Northern Europe and the US.  A couple of climate change scientists have now determined the payback could be as low as 2 years or in the best way of looking at (and new to me) 1000kW of generation per 1kWp of system for systems since 2010.  In other words if my 1Kw system generates 500W a year it will take two years to payback the energy to make it.  In the Southern USA this could be less than a year.   For me personally this means both my old and news systems are now positive energy producers having made that solar energy payback.  The EROEI would now be roughly 12.5 (assuming a system lasts 25 years) perhaps 25:1 or greater in some places.  Incidentally the same authors have looked at wind power and even from the earliest days when turbines were unreliable and blades flew off its been a net energy generator.

The authors did the analysis to work out whether the above renewables were generating enough energy to provide for the production of new renewables without recourse to climate change generating fossil fuels.  They conclude that solar probably has since 2012 and wind energy has always more than compensated for the energy needed to make new turbines.  Two slight cautions.  The original paper is not free and so I do not know whether the solar energy payback includes replacing an inverter over this 25 year period and whether the wind is onshore or offshore or both.  There is also the decline in output from PV systems to be taken into account although this is very low, the oldest PV system in the UK has showed a decline 2.5% over 20 years.  In actual fact PV modules will work well for 40 years and are fully recyclable making the solar energy payback even better than quoted above.

Neil

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