The true energy cost of solar

What is the true lifecycle energy cost of solar PV?

Quantifying the carbon footprint of a solar PV system

What is the true energy cost of solar when you take into account its complete lifecycle from cradle to grave? The carbon footprint of solar PV is often misunderstood. In the solar industry we are frequently asked things like ‘don’t solar panels have a massive carbon footprint? Isn’t it true that they require more energy to manufacture than they will ever produce?’
 
The subject of carbon footprints is highly complex and whilst we can offer a simple answer – solar panels don’t have a massive carbon footprint (compared to other forms of electricity generation) and they don’t require more energy to manufacture than they will ever produce – presenting the evidence to back these statements in different scenarios is a huge challenge. The carbon footprint of a solar PV system is dependent on many variables, like panel type, system design and location, however, we can get a very good idea of the carbon footprint for a panel (and rough estimate for a system) if we look at the energy consumed by the panel of one particular manufacturer throughout it’s lifetime. In this case REC.
 
There are only a handful of solar panel manufacturers that publish information about the carbon footprint of their panels. REC are one of our suppliers of solar panels and in 2011 REC commissioned an independent Dutch research institute ECN to conduct the complete lifecycle analysis of an REC panel according to the international standard ISO 14040, hence telling us the true energy cost of solar. The results for the energy consumed throughout the lifetime of a panel were found to be the lowest in the solar industry.
 
Understanding where these findings have come from and how they can be used to help us estimate the carbon footprint of solar PV systems requires a bit of background technical knowledge and some basic
number crunching...
 
Energy payback
There is a common misconception that solar panels require more energy to manufacture and maintain than they produce over their lifetime. This belief is highly inaccurate because even in the UK the energy payback of a panel may be as little as a few years. The ECN study of REC panels based their calculations on a yearly insolation value of 1700 kWh/m2 (typical of a southern European Mediterranean location) and found that the energy payback time of REC panels was just one year! This is a vast improvement on the 1.4 years of a panel manufactured by REC in 2007, resulting from improved silicon processing and use of renewable energy to power factories.
 
The study examined the lifecycle of the panels and calculated the primary energy consumption for each step of the panel production. This included raw material extraction, recycling and all transportation up to when the panel is assembled. Generic data for installation and recycling was used to calculate the energy payback of a complete system.
 
Carbon footprint
The carbon footprint of our solar PV system is measured as the amount of CO2 equivalent greenhouse gas emissions (CO2e) per Kilowatt hour (kWh) of electricity generated by the system over its lifetime.
Analysis of the carbon footprint associated with the lifecycle of REC panels found that wafers and cells produced in Norway and assembled into panels in Singapore had a CO2 equivalent of 18g per kWh. The wafers, cells and panels that are all assembled in Singapore have a CO2 equivalent of 21g per kWh. This is lower than the estimated 30g per kWh of most panels on the market today.
The small carbon footprint of REC solar panels is partly thanks to the use of renewable energy during the manufacturing stage. The factory in Norway is powered by hydroelectric power, while in Singapore a combination of natural gas and solar is used. This enables REC to produce a panel with a much lower carbon footprint than panels manufactured at sites supplied by coal fired plants at around 30g of CO2e emissions per kWh.
 
Remember, these carbon footprints take into account equivalent carbon dioxide emissions at every stage of the system life cycle, from the production of silicon right through to installation and recycling once the system is decommissioned. Analysis also includes other essential components such as the inverter.
 
Carbon savings
The question that clients often ask is ‘what are the potential carbon savings from using solar power instead of a grid supply?’ To answer this we must compare the CO2e emissions of solar with the CO2e emissions from our grid supply. The CO2e emissions associated with a grid supply will vary between countries according to their energy mix so we will use the UK as an example.
DECC provide a CO2 equivalent (CO2e) figure for the National Grid and this is regularly reviewed and updated. At present the figure used for CRC reporting is 0.541 KgCO2e per unit or 541 gCO2e. This figure represents the equivalent CO2 emissions at the point of electricity consumption and takes into account the indirect emissions associated with processes like extraction and the refining or oil. It also highlights how low the carbon footprint of REC’s panels are by comparison – around 30 times smaller!
 
We would expect that calculating out our carbon savings it is simply case of taking our grid emissions figure and subtracting our solar emissions figure (541- 18) before multiplying by the number of Kilowatt hours (kWh) we are generating. However, we are now presented with the problem that our grid figure is based on findings for the UK and our solar figure based on data from a study in southern Europe! This means we can’t use RECs findings, apply them to the UK and expect to get an accurate answer.
 
Keeping it simple
DECC advises that when calculating the carbon footprint of on-site renewable energy, including solar PV, a factor of zero emissions should be used. Whilst this is arguably not very accurate (we have just highlighted that all panels have an associated carbon footprint), it does keep the reporting process far more manageable and simple.
 
Calculating carbon footprints is particularly challenging due to the number of variables affecting the CO2e emissions. This is perhaps why only a few solar PV manufacturers have undertaken the immense task of analysing the complete lifecycle and associated carbon footprint of their panels. The information provided by REC is valuable in helping to demonstrate that solar panels have a very low carbon footprint and an energy payback that is considerably less than most people are led to believe, but due to the number of variables affecting carbon footprints it is not possible to translate REC’s findings to a UK system. Instead we follow the advice from DECC and assume zero CO2e emissions for on-site renewable energy generation. This means we assume that for every kWh of electricity generated by a solar PV system there are CO2e savings of 0.541Kg.
 
For example, if we looked at an existing 50kWp solar PV system based in York, the estimated annual production for year 1 is 42,000kWh. This equates to 22,722Kg of CO2e savings (when we multiply by DECC’s conversion factor of 0.541). Taking panel performance degradation into consideration, we would expect the yield of the panels to reduce by 0.64% each year over the course of the system’s expected 25 year lifetime. This means the average annual generation will be 38,744kWh with consequent CO2e savings of 20,961Kg for 25 years. This brings the total CO2e savings over the system lifetime is an impressive 524,013Kg or 524 tonnes!
 
Although this figure is not 100% accurate (as no science ever is), it still gives us a very good idea of the performance of solar, highlighting that over the course of the 25+ year lifespan of a PV system we can expect to achieve considerable CO2e savings.