Solar panels are great at lowering your energy bills and saving carbon. But are the environmental benefits worth it when you take into account how much energy goes into manufacturing them? The following guest article aims to answer this tricky question.
I bought a couple of solar photovoltaic cell (PVC) panels last year. They’re generating electricity off-grid, so I’m not eligible for the Feed-In-Tariff (FIT). Which is perhaps as well, since this article isn’t about solar subsidy. Instead, it’s about a question often raised when the subject of solar panels comes up: does manufacturing a solar panel use more energy than the panel produces over its lifetime?
It’s an important question, because solar PVC is often referred to as a “clean” technology – one that generates electricity without producing environmentally-harmful greenhouse gas emissions. Solar panels do this at point of use of course, but they are complex pieces of kit with many components, and energy is required in their manufacture: mining the raw materials, processing, assembling etc. As the world’s electricity system is still primarily fossil fuelled, the energy required to manufacture the panel will probably have come from burning fossil fuel, which means greenhouse gas emissions – the very thing a solar panel is supposed to avoid.
The easiest way to determine whether there is a clean energy payback from solar PVC is to work out the “Energy Yield Ratio” (EYR, the ratio of energy delivered over a panel’s lifetime to the energy required to manufacture it). If the panel’s EYR is greater than 1 (or “unity”, the break-even point), the panel will generate more energy than was required to manufacture it and is therefore an environmentally sustainable energy source. But if the EYR is less than unity, more energy was required to manufacture the panel than it will produce, and it cannot be labelled as a sustainable energy technology under current production methods.
So what is the EYR of a typical solar PVC panel in the UK? To make this calculation, first you need to know how much energy was required to produce the panel. Then to calculate how much energy the panel itself will produce, you need to know the average sunlight intensity at the point of use and the conversion efficiency of the panel (how much of the light energy hitting the panel can be converted into electrical energy). You also need to know how many years the panel is expected to last for. Fortunately this information is readily available. The only thing to bear in mind is the units: power is the rate at which energy is generated and has the unit Watts, which is joules per second. Energy produced or consumed is generally measured in kilowatt hours. For example, if a solar panel has a power rating of 100 Watts, over the course of an hour at maximum output it will generate 100 watt hours of energy, which is 0.1 kilowatt hours (kWh). Over 10 hours at this output it will generate 1000 watt hours of energy, which is 1kWh.
To treat each of the above points in turn:
- To produce typical silicon solar PVC panels requires around 420 kilowatt hours of energy per square metre (kWh/m2). (1) This is roughly the same amount of energy an old-style 100W incandescent light bulb would consume in 6 months.
- Average light energy intensity in the UK on a south-facing roof is around 110 Watts per square metre (W/m2). (2)
- The conversion efficiency of typical solar PVC panels used in the UK is about 12%. (3)
So 110W/m2 of light energy at a conversion efficiency of 12% means a solar panel has an energy rating of 13.2W/m2 – over an hour, it will produce 13.2Wh/m2. Average day length in the UK is about 12 hours – average over the year – so in a day it will produce around 158.4Wh/m2 of electrical power. Over the course of a year this adds up to just under 58kWh/m2. Over the 25 year life of the panel it will therefore generate around 1450kWh/m2 – around four times as much energy as was required to produce the panel, giving an ERY of 4.
So even in the UK, which receives sunlight intensity of only around 60% of that at the equator and where average daylight hours over the year are comparatively short, solar PVC is still an environmentally sustainable source of energy. And it’s expected that production processes will continue to become more efficient in the future, so it’s probable that the EYR will continue to increase.
(1) http://www.nrel.gov/docs/fy04osti/35489.pdf
(2) http://www.inference.phy.cam.ac.uk/sustainable/book/tex/sewtha.pdf
(3) http://www.inference.phy.cam.ac.uk/sustainable/book/tex/sewtha.pdf
About the author
Jonathan Hood
Jonathan currently works in international low-carbon energy policy. Previously he has worked in international climate change and has a long-term interest in climate change and conservation.
This article was originally posted on the Department of Energy & Climate Change blog. Read the original article and join the discussion here.