[Later note, Sep 2015, it’s clear – as can be seen in the later comments that follow the article – there is a difference between a number of papers that cannot be explained by ‘improved efficiencies in manufacturing’ or ‘improved solar-electricity conversion efficiencies’. The discrepancies are literally one group making a large mistake and taking “energy input” to be electricity input rather than fuel to put into power stations to create electricity – or the reverse. I suspect that the paper I highlight below is making the mistake, in which case this article is out by a factor of 3 against solar being a free lunch. In due course, I will try to fight through all the papers again to get to the bottom of it. I also have not been able to confirm that any of the papers really account for building all the new factories that manufacture the solar panels (instead perhaps they are just considering the marginal electricity use to make each solar cell).]
There are lots of studies of the energy and GHG input into production of solar panels. I’ve read some and wanted to highlight one to look at some of the uncertainties.
Lu & Yang 2010 looked at the energy required to make, transport and install a (nominal) 22 KW solar panel on a roof in Hong Kong – and what it produced in return. Here is the specification of the module (the system had 125 modules):
For the system’s energy efficiency, the average energy efficiency of a Sunny Boy inverter is assumed 94%, and other system losses are assumed 5%.
This is a grid-connected solar panel – that is, it is a solar panel with an inverter to produce the consumer a.c. voltage, and excess power is fed into the grid. If it had the expensive option of battery storage so it was self-contained, the energy input (to manufacture) would be higher (note 1).
For stand-alone (non-rooftop) systems the energy used in producing the structure becomes greater.
Here’s the pie chart of the estimated energy consumed in different elements of the process:
A big part of the energy is consumed in producing the silicon, with a not insignificant amount for slicing it into wafers. BOS = “balance of system” and we see this is also important. This is the mechanical structure and the inverter, cabling, etc.
The total energy per meter squared:
- silicon purification and processing – 666 kWh
- slicing process – 120 kWh
- fabricating PV modules – 190 kWh
- rooftop supporting structure – 200 kWh
- production of inverters – 33 kWh
- other energy used in system operation and maintenance, electronic components, cables and miscellaneous – 125 kWh
Transportation energy use turned out pretty small as might be expected (and is ignored in the total).
Therefore, the total energy consumed in producing and installing the 22 kW grid-connected PV system is 206,000 kWh, with 29% from BOS, and 71% from PV modules.
What does it produce? Unfortunately the data for the period is calculated not measured due to issues with the building management system (the plan was to measure the electrical production, however, it appears some data points have been gathered).
Now there’s a few points that have an impact on solar energy production. This isn’t comprehensive and is not from their paper:
- Solar cells rated values are taken at 25ºC, but when you have sunlight on a solar cell, i.e., when it’s working, it can be running at a temperature of up to 50ºC. The loss due to temperature is maybe 12 – 15% (I am not clear how accurate this number is).
- Degradation per year is between 0.5% and 1% depending on the type of silicon used (I don’t know how reliable these numbers are at 15 years out)
- Dust reduces energy production. It’s kind of obvious but unless someone is out there washing it on a regular basis you have some extra, unaccounted losses.
- Inverter quality
Obviously we need to calculate what the output will be. Most locations, and Hong Kong is no exception, have a pretty well-known solar W/m² at the surface. The angle of the solar cells has a very significant impact. This installation was at 22.5º – close to the best angle of 30º to maximize solar absorption.
Lu & Yang calculate:
For the 22 kW roof-mounted PV system, facing south with a tilted angle of 22.5, the annual solar radiation received by the PV array is 266,174 kWh using the weather data from 1996 to 2000, and the annual energy output (AC electricity) is 28,154 kWh. The average efficiency of the PV modules on an annual basis is 10.6%, and the rated standard efficiency of the PV modules from manufacturer is 13.3%. The difference can be partly due to the actual higher cell operating temperature.
The energy output of the PV system could be significantly affected by the orientations of the PV modules. Therefore, different orientations of PV arrays and the corresponding annual energy output are investigated for a similar size PV system in Hong Kong, as given in Table 3. Obviously, for the same size PV system, the energy output could be totally different if the PV modules are installed with different orientations or inclined angles. If the 22 kW PV system is installed on vertical south-facing facade, the system power output is decreased by 45.1% compared that of the case study.
So the energy used will be returned in approximately 7.3 years.
Energy in = 206 MWh. Energy out = 28 MWh per year.
Let’s say we put that same array on a rooftop in Germany, the poster-child for solar takeup. The annual solar radiation received by the PV array is about 1000 KWh per m², about 60% of the value in HK (note 2).
Energy in = 206 MWh. Energy out in Germany = 15.8 MWh per year (13 years payback).
I did a quick calculation using 13.3% module efficiency (rated performance at 25ºC), a 15% loss due to the high temperature of the module being in the direct sunlight (when it is producing most of its electricity), an inverter & cabling efficiency of 90% and a 0.5% loss per year of solar efficiency. Imagine no losses from dust. Here is the year by year production – assumes 1000 kWhr solar radiation annually and 150 m² PV cells:
Here we get to energy payback at end of year 14.
I’m not sure if anyone has done a survey of the angle of solar panels placed on residential rooftops but if the angle is 10º off its optimum value we will see very roughly something towards a 10% loss in efficiency. Add in some losses for dust (pop quiz – how many people have seen residents cleaning their solar panels on the weekend?) What’s the real long term energy efficiency of a typical economical consumer solar inverter? It’s easy to see the energy payback moving around significantly in real life.
Efficiency Units – g CO2e / kWh and Miles per Gallon
When considering the GHG production in generating electricity, there is a conventional unit – amount of CO2 equivalent per unit of electricity produced. This is usually grams of CO2 equivalent (note 3) per KWh (a kilowatt hour is 3.6 MJ, i.e., 1000J per second for 3,600 seconds).
This is a completely useless unit to quote for solar power.
Imagine, if you will, the old school (new school and old school in the US) measurement of car efficiency – miles per gallon. You buy a Ford Taurus in San Diego, California and it gets you 28 miles per gallon. You move to Portland, Maine and now it’s doing 19 miles per gallon. It’s the exact same car. Move back to San Diego and it gets 28 miles per gallon again.
You would conclude that the efficiency metric was designed by ..
I’m pretty sure my WiFi router uses just about the same energy per GBit of data regardless of whether I move to Germany, or go and live at the equator. And equally, even though it is probably designed to sit flat, if I put it on its side it will still have the same energy efficiency to within a few percent. (Otherwise energy per GBit would not be a useful efficiency metric).
This is not the case with solar panels.
With solar panels the metric you want to know is how much energy was consumed in making it and where in the world most of the production took place (especially the silicon process). Once you have that data you can consider where in the world this technology will sit, at what angle, the efficiency of the inverter that is connected and how much dust accumulates on those beautiful looking panels. And from that data you can work out the energy efficiency.
And from knowing where in the world it was produced you can work out, very approximately (especially if it was in China) how much GHGs were produced in making your panel. Although I wonder about that last point..
The key point on efficiency in case it’s not obvious (apologies for laboring the point):
- the solar panel cost = X KWh of electricity to make – where X is a fixed amount (but hard to figure out)
- the solar panel return = Y KWhr per year of electricity – where Y is completely dependent on location and installed angle (but much easier to figure out)
The payback can never be expressed as g CO2e/KWh without stating the final location. And the GHG reduction can never be expressed without stating the manufacturing location and the final location.
Moving the Coal-Fired Power Station
Now let’s consider that all energy is not created equally.
Let’s suppose that instead of the solar panel being produced in an energy efficient country like Switzerland, it’s produced in China. I can find the data on electricity production and on GHG emissions but China also creates massive GHG emissions from things like cement production so I can’t calculate the GHG efficiency of their electricity production. And China statistics have more question marks than some other places in the world. Maybe one of our readers can provide this data?
Let’s say a GHG-conscious country is turning off efficient (“efficient” from a conventional fossil-fuel perspective) gas-fired power stations and promoting solar energy into the grid. And the solar panels are produced in China.
Now while the energy payback stays the same, the GHG payback might be moving to the 20 year mark or beyond – because 1 KWh “cost” came from coal-fired power stations and 1 KWh return displaced energy from gas-fired power stations. Consider the converse, if we have solar panels made in an (GHG) energy efficient country and shipped to say Arizona (lots of sun) to displace coal-fired power it will be a much better equation. (I have no idea if Arizona gets energy from coal but last time I was there it was very sunny).
But if we ship solar panels from China to France to displace nuclear energy, I’m certain we are running a negative GHG balance.
Putting solar panels in high latitude countries and not considering the country of origin might look nice – and it certainly moves the GHG emissions off your country’s balance sheet – but it might not be as wonderful as many people believe.
It’s definitely not free.
Other Data Points
How much energy is consumed in producing the necessary parts?
This is proprietary data for many companies.
Those very large forward-thinking companies that might end up losing business if important lobby groups took exception to their business practices, or if a major government black-listed them, have wonderful transparency. A decade or so ago I was taken on a tour through one of the factories of a major pump company in Sweden. I have to say it was quite an experience. The factory workers volunteer to take the continual stream of overseas visitors on the tour and all seem passionate about many aspects including the environmental credentials of their company – “the creek water that runs through the plant is cleaner at the end than when it comes into the plant”.
Now let’s picture a solar PV company which has just built its new factory next to a new coal-fired power station in China. You are the CEO or the marketing manager. An academic researcher calls to get data on the energy efficiency of your manufacturing process. Your data tells you that you consume a lot more power than the datapoints from Siemens and other progressive companies that have been published. Do you return the call?
There must be a “supplier selection” bias given the data is proprietary and providing the data will lead to more or less sales depending on the answer.
Perhaps I am wrong and the renewables focus of countries serious about reducing GHGs means that manufacturers are only put on the approved list for subsidies and feed-in tariffs when their factory has been thoroughly energy audited by an independent group?
In a fairly recent paper, Peng et al (2013) – whose two coauthors appear to be the same authors of this paper we reviewed – noted that mono-silicon (the solar type used in this study) has the highest energy inputs. They review a number of studies that appear to show significantly better energy paybacks. We will probably look at that paper in a subsequent article, but I did notice a couple of interesting points.
Many studies referenced are from papers from 15 years ago which contain very limited production data (e.g. one value from one manufacturer). They comment on Knapp & Jester (2001) who show much higher values than other studies (including this one) and comment “The results of both embodied energy and EBPT are very high, which deviate from the previous research results too much.” However, Knapp & Jester appeared to be very thorough:
This is instead a chiefly empirical endeavor, utilizing measured energy use, actual utility bills, production data and complete bill of materials to determine process energy and raw materials requirements. The materials include both direct materials, which are part of the finished product such as silicon, glass and aluminum, and indirect materials, which are used in the process but do not end up in the product such as solvents, argon, or cutting wire, many of which turn out to be significant.
All data are based on gross inputs, fully accounting for all yield losses without requiring any yield assumptions. The best available estimates for embodied energy content for these materials are combined with materials use to determine the total embodied and process energy requirements for each major step of the process..
..Excluded from the analysis are (a) energy embodied in the equipment and the facility itself, (b) energy needed to transport goods to and from the facility, (c) energy used by employees in commuting to work, and (d) decommissioning and disposal or other end-of-life energy requirements.
Perhaps Knapp & Jester got much higher results because their data was more complete? Perhaps they got much higher results because their data was wrong. I’m suspicious.. and by the way they didn’t include the cost of building the factory in their calculations.
A long time ago I worked in the semiconductor industry and the cost of building new plants was a lot higher than the marginal cost of making wafers and chips. That was measured in $ not kWh so I have no idea on the fixed/marginal kHr cost of making semiconductors for solar PV cells.
One other point to consider, the GHG emissions of solar panels all occur at the start. The “recovered” GHG emissions of displaced conventional power are year by year.
Solar power is not a free lunch even though it looks like one. There appears to be a lot of focus on the subject so perhaps more definitive data in the near future will enable countries to measure their decarbonizing efforts with some accuracy. If governments giving subsidies for solar power are not getting independent audits of solar PV manufacturers they should be.
In case some readers think I’m trying to do a hatchet job on solar, I’m not.
I’m collecting and analyzing data and two things are crystal clear:
- accurate data is not easily obtained and there may be a selection bias with inefficient manufacturers not providing data into these studies
- the upfront “investment” in GHG emissions might result in a wonderful payback in reduction of long-term emissions, but change a few assumptions, especially putting solar panels into high-latitude energy-efficient countries, and it might turn out to be a very poor GHG investment
Environmental payback time analysis of a roof-mounted building-integrated photovoltaic (BIPV) system in Hong Kong, L. Lu, H.X. Yang, Applied Energy (2010)
Review on life cycle assessment of energy payback and greenhouse gas emission of solar photovoltaic systems, Jinqing Peng, Lin Lu & Hongxing Yang, Renewable and Sustainable Energy Reviews (2013)
Empirical investigation of energy payback time for photovoltaic modules, Knapp & Jester, Solar Energy (2001)
Note 1: I have no idea if it would be a lot higher. Many people are convinced that “next generation” battery technology will allow “stand-alone” solar PV. In this future scenario solar PV will not add intermittancy to the grid and will, therefore, be amazing. Whether or not the economics mean this is 5 years away or 50 years away, note to the enthusiasts to check the GHG used in the production of these (future) batteries.
Note 2: The paper didn’t explicitly give the solar cell area. I calculated it from a few different numbers they gave and it appears to be 150m², which gives an annual average surface solar radiation of 1770 KWh/m². Consulting a contour map of SE Asia shows that this value might be correct. For the purposes of the comparison it isn’t exactly critical.
Note 3: Putting of 1 tonne of methane into the atmosphere causes a different (top of atmosphere) radiation change from 1 tonne of CO2. To make life simpler, given that CO2 is the primary anthropogenic GHG, all GHGs are converted into “equivalent CO2”.