Here is something I posted on CMNS that may be of interest. Decades ago the "energy payback time" for solar panels was many years in some cases, which meant that if they wore out or got dirty before 10 years elapsed, you might never recoup the energy used to manufacture and install them. However, the energy used in manufacturing has been greatly reduced, and the conversion efficiency increased, so nowadays the energy payback time is a little over a year. See:
http://c1cleantechnicacom.wpengine.netdna-cdn.com/files/2013/12/energy-payback-solar-trend.png This is in line with EIA estimates and various other sources. There are two other overhead issues here, which you also have to take into account. They are: 1. Longevity 2. Fuel energy overhead The Energy Payback Time (EPBT) is the time it takes to recoup the energy used in the manufacturing and installation of the device. Wind turbine EPBT is ~8 months, which is the shortest of any conventional electric power source. (That is what I read some years ago.) Hydroelectric dam EPBT is much longer, about 10 years. However, hydroelectric dams last for a very long time; a century or more. So you end up getting out much more energy than you put in. You have to replace the turbines during those 100 years but the EPBT for new turbines is small. The longer an energy source lasts, the better the overall energy payback is. Solar cells used to last 10 or 15 years, but nowadays with proper maintenance they last 30 years, so the overall payback is ~95%, as shown in the graph I linked to above. That is better than most energy sources. Why is it better? Because most energy sources require fuel. When you calculate the overall energy input and output of a coal, gas, or nuclear plant, you have to add in the energy used to extract, refine and transport the fuel. In the case of coal and uranium, you have to factor in the cost of disposing of the spent fuel (the ash). This adds to the overall energy cost of running the plant. It is difficult to get below 95% net payback for a system that requires fuel. The energy overhead for oil is 10% or 20% as I recall, depending on where you drill for oil, the quality of oil, and how far you transport it. For coal the overhead from mining and transport is usually around 5% as I recall, so you can never get below 95%, even if the generator plant materials and construction had zero energy overhead. Uranium production overhead is small because the fuel produces so much energy, but disposal overhead is somewhat unknown. With wind, solar or hydroelectric there is no fuel, so the fuel energy overhead is zero. The only overhead is from manufacturing, installation and eventual decommissioning and disposal. The latter is very small for solar. So the 95% net gain from today's PV solar is all gain, with no additional overhead. It is probably better than coal. Wind turbine energy overhead is about 2%, which is the best of any conventional system. In other words, wind turbines are expected to last about 400 months, or ~30 years. Actually, most of the energy overhead is for the towers, not the turbines. The towers should last even longer with proper maintenance. Of course there are other financial overhead costs. I am only talking about energy overhead. In my book I estimate that the fuel energy overhead cost of Pd-D cold fusion will be approximately 0.05% with today’s heavy water extraction techniques. Better extraction techniques have been invented, so it will actually be lower than this. It is trivial, in any case. There will be no energy cost for distribution, installation or disposal. No financial cost either. - Jed
