The energy payback time (EPBT) of a power generating system is the time required to generate as much energy as is consumed during production and lifetime operation of the system. The past decade the energy payback time for solar PV systems has been reduced drastically.
Energy payback time and improvements in production technology
Due to improving production technologies the energy payback time has been decreasing constantly since the introduction of PV systems in the energy market. In 2000 the energy payback time of PV systems was estimated as 8 to 11 years and in 2006 this was estimated to be 1.5 to 3.5 years for crystalline silicon silicon PV systems and 1–1.5 years for thin film technologies (S. Europe). These figures fell to 0.75–3.5 years in 2013, with an average of about 2 years for crystalline silicon PV and CIS systems.
Factors that influence energy payback time
The energy payback time is influenced by the following three factors:
- The materials used in the system
- The solar cell efficiency
- The location and related irradiation
1. Energy payback time and materials used
Over the past years, manufacturers of silicon wafers have been able to reduce the thickness of the wafer and therefore reduce the costs of the solar cell significantly. One enabler for thinner wafers are the automatic soldering machines, which put a lot less pressure on solar cells than manual soldering.
Below you can see that over the past decade, wafer thicknesses have been reduced from 300µm in 2004 to 180µm in 2014. Fraunhofer collected the thickness of the wafer and the silicon usage in gram/Wp in one simple table, great overview:
2. Energy payback time and solar cell efficiency
When solar cell efficiency increases, more power is generated with the same amount of materials. Below you can see the clear trend in Fraunhofer's overview 'Development of Laboratory Solar Cell Efficiencies':
3. Energy payback time and related irradiation
As you can see from the handy Fraunhofer over 'Energy Pay-Back Time of Multicrystalline Silicon PV Rooftop Systems' below, the energy payback time in Europe varies between approximately 1 and 2.5 years. The energy payback time in Northern Europe is 2.1 years compared to 1.2 years in Southern Europe.
If you want more information on this topic, have a look at the The Fraunhofer annual Photovoltaic Report. It has excellent data on energy payback time. You can download the 2015 report here.