Solar Photovoltaics (SPV) technology is growing in popularity as an indispensable electricity generation option for the future. Easy to install and low maintenance, SPV panels are primarily installed to offset the buildings’ operational energy/greenhouse gas (GHG) emissions. SPV is also required to decarbonize the electricity grids and to move toward the Government of Canada’s goal of zero-carbon emissions by 2050.
However, a considerable amount of GHGs are still emitted throughout the SPV life cycle, which could potentially represent a significant portion of the total carbon footprint. Fortunately, this risk can be ascertained by quantifying the embodied carbon impacts and provisions to offset the associated environmental impact made through recycling of components, for example. Nevertheless, limited data availability from manufacturers is one of the key challenges to accurately quantifying the embodied carbon of SPV systems.
Background
Solar power is harnessed using SPV technology that converts sunlight (solar radiation) into electricity via semiconductors. When the sun hits the semiconductor within the PV cell, electrons are freed and bus bars collect the running electrons resulting in an electric current.
Types of Photovoltaic Technology
There are three types of photovoltaic cell technologies that dominate the market: monocrystalline silicon, polycrystalline silicon, and thin-film (e.g., cadmium telluride).
Crystalline silicon modules are the most widely used and account for up to 90% of the global PV market. Crystalline modules are reliable, efficient, and produce the most power per square meter, especially during peak hours.
Monocrystalline modules are the most efficient rooftop-mounted systems, and a good choice if you don’t have much space for a PV installation or want maximum power production. However, the ecological impact of silicon is high as a result of its energy-intensive fabrication, metallurgical process, and purification of silicon. Silicon purification is the process of reducing silicon dioxide (SiO2) to silica (Si), where oxygen is removed through a reaction with carbon (C) in a large arc furnace, in addition to toxic compounds emitted through the process (e.g., chlorine).
Polycrystalline modules are almost as efficient as monocrystalline, and they are less expensive, so these are recommended for those on a tighter budget or with a little more space for the modules.
Thin-film PV modules require less energy and fewer raw materials during their life cycle due to the relatively simple production technologies. Thin-film PV may cost less than crystalline, but it’s not as efficient. Amorphous silicon (a-Si), cadmium telluride (CdTe), and copper indium selenide (CIS)/copper indium gallium selenide (CIGS) are the major technologies.
Thin film such as Cadmium Telluride (CdTe), works well in low-light conditions and in high temperatures. Over the course of a year, it can create more power than crystalline PV per rated capacity. However, since the rating capacity is lower for the same area, you’ll need a lot more room on the rooftop to achieve the same results.
Solar Photovoltaic and Sustainable Building Design
The primary reason for installing rooftop SPV is to offset/reduce the operational energy consumption of buildings, which directly influences operational carbon emissions. When SPV are known to be included on sustainable building projects, they can influence the roof design, massing, and the orientation of the building as well as the placement of rooftop mechanical items. This is done so that the panels can be installed in locations to best harvest the sunlight with minimal shading while maximizing the power output of the modules to minimize upfront installation costs. Building orientation is also a key consideration for SPV application in sustainable design.
There are a few knock-on effects of installing SPV. For instance, a small amount of building footprint is required to house the inverter and additional connections, or a larger amount is required if a battery bank is to be installed. This in turn adds an additional quantity of building materials to the project, which will increase the embodied carbon of the base building.
When including SPV on a sustainable building, another factor is considering which building systems it will be serving or offsetting. Since SPV produces electricity it can directly offset the electrical use of the building; however, consideration toward selecting non-traditional mechanical systems is required . When a zero-energy building is desired, SPV is the most beneficial when mechanical systems are selected to utilize electricity instead of natural gas. This in turn greatly reduces the operational carbon of the sustainable building.
Life Cycle Period of Typical SPV Panels
The operational life of SPV panels guaranteed by manufacturers is typically 25 years, though this is slightly higher for newer production with improved technology.
However, studies show that many users exchange their PV installations before the theoretical end of life, after an average of 17 years of use, in order to obtain better energy yields, resulting from continuous technology improvements.
SPV has a shorter life span than a building. Going by the effective/average life cycle of a PV system, it is expected to be replaced 3.5 times throughout a building’s 60-year typical service life.
Entuitive’s SPV Study: Methodology and Assumptions
Entuitive conducted a study to determine the whole life carbon impacts of rooftop SPV. To present global warming potential (GWP) values in a relatable way, the study compares embodied and operational carbon for four cities in Canada: Toronto, Vancouver, Calgary, and Regina. The SPV cells scenarios include monocrystalline and polycrystalline panels (also known as modules). These are selected as the most applicable modules in Canada, based on availability and efficiency.
While PV panels typically require accessory equipment, the study includes PV modules, inverters, and module mounting structure frames for rooftop panels. Ancillary rooftop equipment is excluded from the study scope. This study assumes a 25–year power warranty period as the guaranteed service life of a PV panel based on most Environmental Product Declarations (EPDs). The rooftop mounting structure assumes 25 years while the inverter and control boxes are warranted for a 10-year life span. Therefore, an assumption is made that the inverter will be replaced at least once before the study’s end of life (i.e., 25 years).
Embodied Carbon Impacts
Embodied carbon refers to carbon dioxide (CO2) emissions emitted during the manufacturing, transportation, construction, replacement, and end-of-life stages of a product. Carbon is frequently used as shorthand for either carbon dioxide (CO2) or carbon dioxide equivalents (CO2-e), which includes both CO2 and other significant greenhouse gas (GHG) emissions that contribute to GWP. This means that they tend to trap heat in our atmosphere. The life cycle carbon analysis is modeled using the OneClick LCA tool.
For this study, the life cycle of embodied carbon consists of four main stages:
Product stage (A1-A3)
Construction stage (A4 -A5)
Maintenance, repair, and materials replacement stage (B2 -B5)
End of life (C1 -C4)
Why is reporting embodied carbon of SPVs important?
It is broadly accepted that SPV does not emit CO2 at the point of electricity generation; however, GHG emissions are emitted during the manufacturing and construction phases. This is because the manufacturing of modules and material processing involves the use of chemical substances and associated potential emissions, which are not environmentally neutral, accounting for nearly all the life cycle GWP. For example:
The electricity grid used for the manufacturing of PV modules can be predominantly sourced from fossil fuels (i.e., coal, natural gas, etc.).
Toxic gases and heavy metals can be emitted directly from material processing and PV manufacturing, and indirectly from generating the energy used at both stages.
Most industry PV modules, including monocrystalline silicon PV modules utilized in Canada, are manufactured in China powered by a high fossil fuel electricity grid. Also, carbon emissions associated with the transregional transportation of the panels must be accounted for.
Key results:
Vancouver has the least embodied carbon impact compared to other cities studied.
Toronto has the highest embodied impact, with approximately 5% increase for monocrystalline and 6% increase for polycrystalline scenarios compared to the Vancouver reference cases.
In most scenarios, the production and use of polycrystalline shows a significant reduction in embodied carbon emissions, up to a 20% decrease compared to monocrystalline scenarios.
Accounting for embodied carbon emission per square meter, an average of 10 kgCO2e/m2/year and 12 kgCO2e/m2/year are reported for polycrystalline and monocrystalline respectively.
Operational Carbon Impacts
IES VE software is used to estimate the annual electricity production for each module. The PV tilt angle for Calgary and Regina is modeled as 45°, while Toronto and Vancouver are modeled with 35° and 34° due to the lower altitude. The first-year power drops are reported by the manufacturers to be 1% (Monocrystalline) and 1.5% (Polycrystalline). The power degradation for Mono– and Poly crystalline are assumed to be -0.55%/yr and 0.70%/yr, respectively, as per manufacturer warrantied power. The operational carbon savings are estimated by displacing grid electricity. The grid GHG rate varies by province as well as over time due to the decarbonization policies.
The table below presents the study assumptions for the future grid GHG intensity.
Key results:
The operational carbon savings strongly depend on the grid GHG emissions rate, therefore the decarbonization policies can amplify the importance of the embodied carbon of SPVs over the avoided carbon emissions by using PV panels.
The grid GHG intensity of Alberta and Saskatchewan is projected to stay above the national average and therefore provides greater opportunity to utilize PV panels.
Cities with higher yearly PV generation such as Regina, SK (1,361 kWh/kW), and Calgary, AB (1,292 kWh/kW) mostly benefit from operational carbon savings from installation of SPVs. For example, Regina yearly PV potential is estimated as 1.3 times higher than Vancouver.
Estimated avoided operational carbon emissions based on each grid scenario
CARBON PAYBACK CALCULATIONS
The study uses a carbon payback approach in order to understand the whole life carbon performance. The payback period is expressed as the relationship between associated embodied carbon and avoided operational carbon emissions for the 25-year service life. In other words, we identified how long it takes for the carbon savings to exceed the invested embodied carbon in the SPV modules.
Key takeaways:
The grid GHG intensity in BC (Vancouver) and ON (Toronto) are significantly low due to the electricity generation by hydroelectricity and nuclear power plants.
Low grid GHG intensity of BC and ON directly impact the operational avoided carbon during the lifespan of PV panels, therefore, their carbon pay time is over 25 years and cannot offset embodied carbon. Implementing a PV rooftop system is not a net carbon benefit.
On the other hand, the AB (Calgary) and SK (Regina) grids are mainly dependent on fossil fuels. In these cases, the carbon payback time is approximately two years for both monocrystalline and polycrystalline based on the current policies scenario.
Conclusions: Study Main Takeaways
To achieve full grid decarbonization, rooftop SPV will likely play a vital role, and therefore the associated embodied carbon emissions are unavoidable. However, given that solar electricity generation displaces the grid electricity, if the grid is already clean then the carbon avoidance is low (e.g., Vancouver and Toronto scenarios). The recycling of SPV components, in addition to advances in technology, must be considered as important elements to further reduce embodied carbon emissions associated with their production and use.
The summary below highlights the main takeaways of this study:
Minimal differences are reported for the cities’ embodied carbon due to them having the same manufacturing source. However, the main difference is the transportation stage.
While the monocrystalline modules result in higher embodied carbon in comparison to polycrystalline, a much higher operational carbon savings is reported for monocrystalline due to the improved efficiency of solar cells to produce more electricity per kWh.
Rooftop SPV installation will favour cities with current electricity grids more dependent on fossil fuels, such as Calgary, in addition to higher PV yearly potential based on latitude.
Consideration should be given to balancing the PV carbon payback time with current grid GHG intensity to maximize SPV benefits for a specific city and project location.
Pure silicon is the most valuable and recoverable material in the c-Si family, and when recycled it lowers the embodied carbon associated with the new production line.
Environmental benefits of recycling translate to avoided raw materials extraction and production to offset potential GWP impact.
We must work with the PV manufacturing industry to enhance the service life of PV installations to reduce replacement frequency (less material extraction and manufacturing) and ensure components can be readily recycled and recovered at the end of their useful life.