Talk:Net energy gain

message: Has anyone done the calculation to see if there is an actual power gain over the life time of a photovoltaic cell,when applied against the whole installation including batteries, control electronics etc.Confusedenviron 08:09, 5 April 2006 (UTC)
replies: text: Yes. Several somebodies have done such calculations over the last decade or two and have always come up with the same answer: Yes, there is a positive NEG for photovoltaics. The typical range is currently (2008) two to three years, but this utterly depends on location and cell orientation (e.g. on-house facade plants are worse than rooftop inclined panels). For a recent study see: Fthenakis V., Alsema E. Photovoltaics Energy payback times, Greenhouse Gas Emissions and External Costs: 2004 – early 2005 Status. Progress in Photovoltaics: Research and Applications, 2006, Vol. 14, p. 275-280 http://www.clca.columbia.edu/papers/Photovoltaic_Energy_Payback_Times.pdf. For some reason it has almost become an urban myth that PV has negative NEG. This misinformation seems very long-lived, and I imagine it must be welcome news for some people, otherwise it would die out. At most, I could imagine (but have never seen information to back this up) that in the very beginning of solar cells, i.e. space exploration in 1958, the cells were fabricated in such small numbers and without regard for production economy, that negative could have NEG resulted. For current cells and terrestrial use in well-designed power plants this is utterly not true.--83.77.173.240 (talk) 18:56, 30 May 2008 (UTC)
replies: text:  Yes, many times.  NREL has a discussion of energy payback on their website. Geoffrey.landis (talk) 18:53, 28 August 2008 (UTC)
replies: text:  Net energy gain is a factor without any unit. It is a multiplier for the invested energy that can be used at the end. Any timeframe such as 2-3 years is not a net energy gain, but the energy payback time. This parameter is not very precise, as it does not tell you how long the whole installation will provide energy after the payback time has been reached. There is a difference if you have a payback time of 4 years and two different generators: one with an operating time of 5 year in average and another one with a lifetime of 50 years. The best concept I have seen so far in energy assessment is Bruce Hannon's energy discounting = energetic net present value. --Gunnar (talk) 16:40, 21 December 2018 (UTC)
thread_title:  Photovoltaics 

message: I find this article extremely confusing.  Can anyone clarify why non-renewable fuels always have a NEG below 1?  Why does taking just the extraction cost raise the NEG above 1?
:If you count the energy contained in non-renewable fuels, its NEG can't be above 1--you can't extract more than there is in it. (There is some debate, whether nuclear energy is renewable, since possible supplies are so big.) If you don't count it, its NEG can be well above 1, and an energy source will probably not be economically viable, if its NEG isn't significantly higher than 1.
::Of course, that's a silly way of defining net energy gain, and not the way that it is ever defined in the real world.  Taken to the limit, using that definition says that every energy source has a net energy gain less than 1, since energy is conservedGeoffrey.landis (talk) 18:53, 28 August 2008 (UTC)
thread_title:  Non-renewable fuels 

message: "Given that the solar constant (which assumes rays perpendicular to the surface) is 1367 W / m2, one should probably scale up the payback time by a factor of 5 or so for real-life photovoltaic systems."
This looks wrong not only because the payback time ought to be scaled down but because a major part of those 1367 W/m2 are radiated in the low infrared that can, due to fundamentel laws of physics, not be efficiently used for electricity generation. This might be entirely different at locations where temperature is very low, such as for a satellite in (outer) space.
Does anyone has a firmer and more detailed knowlege about this? 84.160.246.75 10:32, 25 September 2005 (UTC)
thread_title:  Solar constant 

message: * Net energy gain A lot of dubious statements and confusion, I don't even know where to start with this one. --Sinus 20:41, 10 November 2005 (UTC)
thread_title:  From PNA/Physics 

message: Is there any such thing as an NEG-ratio? Shouldn't it just be energy ratio?, a search on google showed the only place on the net with the term NEG-ratio was wikipedia, kind of suspicious eh? If NEG-ratio exists what is the formula? It should be stated on the page, or referenced.sbandrews 17:43, 23 September 2006 (UTC)
thread_title: NEG-ratio

message: My understanding is that large modern wind generator systems pay back the energy used to build them in about 3 months, best among general-purpose expandable renewable sources -- seems like they deserve special mention.-69.87.199.117 13:03, 18 June 2007 (UTC)
replies: text: The newest commercial-scale wind turbines have an excellent energy payback, but as with any form of electrical power generation technology, much of the energy consumed in their construction is in the form of petroleum, a form of energy which happens to be getting particularly scarce, expensive, and insecure. The basic problem is that all forms of electric power generation require transportation in their manufacture and construction, but at the moment we lack practical large-scale methods to use electricity to power transportation. Transportation remains overwhelmingly dependent on liquid hydrocarbon fuels from petroleum (with a small but growing fraction from biofuels). Wind power, hydropower, nuclear power, and just about every other electric power generating technology remain net consumers of petroleum. There are some schemes to address this problem, including the Pickens Plan, the hydrogen economy, plug-in hybrids, electric vehicles, and telecommuting. Large-scale implementation of one or more of these schemes might allow electric power generation technology to actually pay back its petroleum investment. Another way to look at the problem is to ask whether we can use wind power to build more wind power (or nuclear power to build more nuclear power, etc.). A form of energy which consumes a different and unsustainable form of energy is itself not yet sustainable. This is not an objection to wind power by any means, but rather it points out that until we electrify our economies in general and transportation in particular, "net energy gain" analysis may be a rather crude and potentially misleading first-order analysis that ignores the relative scarcity of limiting resources such as petroleum. As an extreme analogy, imagine burning paper money to stay warm in winter - considering only energy would overlook some other important aspects of that exchange. Not all forms of energy are equal. Energy is a way to equate different forms of energy, but only certain kinds of energy conversions are practical due to thermodynamic and engineering constraints. --Teratornis (talk) 18:29, 21 September 2008 (UTC)
replies: text: When we refer to a process as producing a gain, we imply that the output is in some way equivalent to the input. If we cannot actually substitute the output for the input, then our notion of "gain" is somewhat hypothetical. --Teratornis (talk) 18:43, 21 September 2008 (UTC)
thread_title:  Wind 

message: I propose deleting this line: From a theoretical perspective, if the energy content of non-sustainables is taken into account, they will always have a NEG-ratio below one;  and also deleting the several sentences later which attempt to clarify and explain this.
While this is technically correct if you use the terms as defined by whoever wrote this, it is misleading enough (as noted by the discussions above) that I think including it in the article is counterproductive, and I don't think that the "theoretical" definition here is actually representative of the way the term is actually used.  Geoffrey.landis (talk) 19:25, 28 August 2008 (UTC)
thread_title:  Propose deleting a misleading line 

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