Sustainable and Circular solar PV
Solar photovoltaic (PV) technology is one of the key en- gines powering the energy transition. An important reason for PV’s large role in the energy transition is its perception as a renewable and sustainable energy technology with a low environmental impact.
However, it remains clear that production, transport, in- stallation and operation of PV systems all require the con- sumption of materials and energy, which has an environ- mental impact. We install PV to drastically reduce the high emissions of greenhouse gases associated with our largely fossil fuel-based current energy system. Although PV has a relatively small environmental impact per unit of electric- ity generated, the huge role that PV needs to play in the energy transition implies that enormous amounts of ener- gy and materials must be consumed for its manufacturing. This gives rise to two key issues to be addressed by R&I efforts. First, the environmental impact of manufacturing PV systems must be (further) minimized. Secondly, as the numbers of PV systems on the market rise, resource effi- ciency is becoming an increasingly critical factor for the long-term success of the sector. For a truly sustainable transition towards a low-carbon future, renewable energy technology must be close to 100% recyclable and leanly manufactured, i.e. circular.
The R-ladder of Circularity Strategies
In this objective, we discuss the sustainability and circular- ity of PV using the R-ladder of circularity strategies. There are six distinct strategies towards circularity: Refuse and Rethink, Reduce, Reuse, Repair and Refurbish, Recycle, and Recover. (37) Each of these steps would serve to (fur- ther) reduce key issues regarding the sustainability of PV: greenhouse gas emissions and other environmental im- pacts as well as material and resource availability. Here, we apply these strategies along the PV value chain, both to define key current issues, as well as to suggest specific research activities.
Evaluating the sustainability and environmental impact of PV
The production, operation, and disposal of any product carry with it an environmental burden. The minimization of the environmental burden for the whole lifetime re- (37) quires the selection of materials that create fewer toxic by-products, allow a longer life, are more easily recycled, are lighter and less energy intensive, and that, where pos- sible, reduce the net demand of scarce resources, where proof of concept of industrially feasible devices with alter- native materials must be approached.
Any negative environmental impact associated with the life cycle of PV systems must be minimized. Environmental im- pacts of products and services are commonly determined using Lifecycle Assessment (LCA), an ISO standardised method that builds an inventory of all material and energy flows required for the product, and consequently applies a set of methods to determine the environmental impact of the product in a multitude of impact categories. An im- portant concept in LCA studies is the functional unit, which identifies the function of the product under study. As the function of PV is to produce electricity, the environmental impact is commonly expressed for the functional unit of a kWh of electricity produced (or delivered to the grid).
Currently, LCA studies that aim to identify the environmen- tal burden associated with electricity from PV systems, commonly focus on estimation of the carbon footprint of PV electricity, and cumulative energy demand (CED) and associated energy payback time (EPBT) of PV systems. Ad- ditionally, researchers often use the term energy return on energy invested (ERoEI) to relate the cumulative energy demand to the lifetime energy yield of PV systems.
The carbon footprint is a measure which can be used to compare electricity from PV to that from other sources and is expressed in gCO2-eq/kWh. Here, one kWh is the functional unit. As PV electricity has no direct point-of-gen- eration emissions, a large share of the carbon-footprint of PV results from the direct and indirect energy use dur- ing manufacturing, and smaller parts are associated with transport, installation, operation, and other parts of the value chain.
EPBT refers to the amount of time necessary to generate an amount of electricity equivalent to the amount of pri- mary energy (PE) invested during manufacturing. ERoEI provides the ratio between produced and invested ener- gy, similar to the related financial parameter RoI. ERoEIel refers to the ratio between produced electricity and primary energy investments, while ERoEIpe relates to the ratio between equivalent primary energy output and primary energy inputs.
For a truly sustainable transition towards a low-carbon future, renewable energy technology must be close to 100 % recyclable and leanly manufactured.
When discussing carbon footprint, EPBT and ERoEI of PV systems, there are several key considerations to take note of. First, for all these environmental impact parameters, there are a set of assumptions that strongly affect their fi- nal values:
Production location: for a very substantial part, the environmental impact of PV systems and the elec- tricity they generate results from the consumption of electricity in direct and indirect manufacturing processes. LCA studies often assume systems are produced using the average electricity mix of the manufacturing country. As such, current LCA’s as- sume production in China, which has an electricity mix with large shares of coal.
Installation location: the installation location deter- mines both the annual irradiance and thus annual system yield, as well as the efficiency of the local grid. The efficiency of the local grid is the ratio kWhel/kWhPE at the point of deployment, and thus shows the efficiency of converting primary energy into electricity. The conversion to and from prima- ry energy is required to calculate EPBT and ERoEI, as for these metrics all embodied energy is consid- ered, and primary energy is a common metric to account for all forms of input energy. However, is very sensitive to the penetration of renewables in the grid, as they have a very high efficiency from primary energy to electricity. High grid efficiencies thus mean that the energy payback time will be higher. This also has its effect on the ERoEI values. As an example, Fthenakis and Leccisi (38) show that, when increases to a value of 0.7 (which is the esti- mated in California in 2030 with a penetration of renewables of 80 %) the ERoEIPE drops from around 50 currently, to just over 20 in this 2030 scenario.
Performance Ratio: the performance ratio PR de- termines the annual electricity yield, and as such affects all environmental impact parameters. The current standard for PR, assumed in LCA studies, is 85 %.
System lifetime: ERoEI and carbon footprint both rely on the lifetime electricity yield from PV sys- tems. Typically, a system lifetime of 30 years is as- sumed, commonly with one replacement of the inverter.
Finally, it should be noted that it is recommended by the European Commission to follow the Product Environmen- tal Footprint Category Rules (PEFCR) when conducting an analysis of the environmental impact of PV. These PEFCR include a broad set of environmental impact categories. Although as said above the focus of PV LCA is commonly on greenhouse gas emissions and energy demand, R&I on sustainable and circular PV should include all those impact categories. Many of the R-ladder strategies will reduce the environmental impact in multiple or all categories.