Reported studies on the carbon footprint of solar power range from 12 – 213 g CO2e / kWh. The average carbon footprint of common solar panels is estimated at 78.6 g CO2e / kWh at 4.5 kWh / m2, which is typical for the midwest and east cost of the United States. Compared to the average US power grid emissions, the most commonly sold technology, Monocrystalline Solar Photovoltaics (PV), emits 5.5 times less carbon than the average US Grid per kWh generated.

The cradle to grave carbon footprint of 5 mainstream solar power generation technologies is presented in the figure above, which is based on averages of the cradle to grave studies presented in Kommalapati et al. (2017). Given recent advances in manufacturing and Solar PV efficiency, the carbon footprints of the Solar PV technologies presented are expected to significantly decrease over time (Bracquene et al., 2018). Each technology is explained later in this article, and each of the selected values is presented in the Data and Assumptions Section.

The carbon footprint of solar power is highly dependent upon the availability of sunlight. The carbon footprint of Solar PV installed in Los Angeles has a 33% lower carbon footprint compared to the same solar panels installed around Chicago (Based on solar irradiance values stated by the US Department of Energy – NREL PVWatts Calculator, 2020). However, even countries with a low solar irradiance, such as Northern Europe and Canada, solar PV will still emit 3.6 times less carbon compared to the average US Grid at just 3 kWh / m2.

See the figure below to see how much sunlight is available where you live (Provided by Sengupta et al., 2018). Darker colors indicate more solar energy availability. The carbon footprint of Solar PV has been adjusted to the color of Iowa, and therefore darker colors will have a lower carbon footprint than listed above. To get a more precise estimate of your solar resource, see the PVWatts Calculator, which is a free tool provided by the US Department of Energy – National Renewable Energy Laboratory.

Carbon Footprint of Monocrystalline Solar PV

Monocrystalline Solar PV cells utilize a single crystal structure to capture the energy from sunlight. As opposed to Polycrystalline Solar PV, monocrystalline cells take advantage of increased efficiencies realized by the single crystal structure as opposed to an array of different crystal structures that prevent the free flow of electrons (US Department of Energy, 2019).

The main advantage of Monocrystalline Solar PV is the increased efficiency over Polycrystalline Solar PV, however this efficiency gain results in an increased cost per kW capacity. Advances in Monocrystalline Solar PV manufacturing have led to a nearly 90% market share for monocrystalline Solar PV produced in 2020, compared to an approximate market share of 25% in 2016 (Colville, 2020). In addition, 95% of all Solar PV panels use monocrystalline or polycrystalline silicon technology (US Department of Energy, 2019).

The maximum theoretical efficiency of Monocrystalline Solar PV is 32%, due to light spectrum bandgap limitations (US Department of Energy, 2019). Monocrystalline Solar PV efficiencies available on the market range from 16.5 – 22.6% (Svarc, 2020). The most efficient Monocrystalline Solar PV panels require 37% less space compared to the least efficient, which has real-world impacts both on how much solar PV can fit on a roof, for example, and on how much land is required for commercial-scale solar PV, which often competes with agricultural land.

Kommalapati et al. (2017) reports cradle to grave LCA results from 9 studies that modeled Monocrystalline Solar PV. Values ranged from 12 – 180.3 g CO2e / kWh generated. The average system emits 85.3 g CO2e / kWh with a standard deviation of 58.3 g CO2e / kWh, The median system emits 88.7 g CO2e / kWh. System efficiencies range from 12.8 – 14.4%. When these values are adjusted for solar irradiance, the irradiance-adjusted average is 78.6 g CO2e / kWh at 4.5 kWh / m2 / day. See the Data and Assumptions Section for the full list of studies included.

Carbon Footprint of Polycrystalline Solar PV

Polycrystalline Solar PV cells utilize multiple crystal structures to capture the energy from sunlight. Compared to Monocrystalline Solar PV, the polycrystalline systems will produce less energy on average because the multiple crystal structures prevent the free flow of electrons (US Department of Energy, 2019). The multiple crystal structures can easily be observed in the Polycrystalline Solar PV photo above.

The main advantage of Polycrystalline Solar PV is price. If space is not a significant concern, Polycrystalline Solar PV systems may save money. However, Consumer Ecology cautions that when land use change is required, such as cutting down trees or converting natural lands to Solar PV generation systems, the most efficient Solar PV system should be chosen to minimize disruption to the environment, including induced carbon emissions from land use change.

Monocrystalline Solar PV efficiencies on the market range from 15 – 19.5% (Svarc, 2020). The most efficient Monocrystalline Solar PV panels require 50% less space compared to the least efficient Polycrystalline Solar PV panels.

Kommalapati et al. (2017) reports cradle to grave LCA results from 7 studies that modeled Polycrystalline Solar PV. Values ranged from 38 – 189 g CO2e / kWh generated. The average system emits 102.6 g CO2e / kWh with a standard deviation of 61.0 g CO2e / kWh, The median system emits 98.9 g CO2e / kWh. System efficiencies range from 11.86 – 16.5%. When these values are adjusted for solar irradiance, the irradiance-adjusted average is 100.3 g CO2e / kWh at 4.5 kWh / m2 / day. See the Data and Assumptions Section for the full list of studies included.

Carbon Footprint of Thin Film Solar PV

Thin Film Solar PV panels are a family of technologies, but each of the thin film technologies is not reliant upon rigid silicon crystals to capture solar energy. Rather, thin films create a thin layer of a highly absorptive semiconducter that is deposited onto a substrate such as glass, plastic, or metal, which makes manufacturing much simpler (US Department of Energy, 2019). Some thins films, such as Amorphous Silicon, can be bent into different shapes, which one day may allow for the creation of the solar shingles shown above, or building siding.

The main advantage of Thin Film Solar PV panels is their flexibility and theoretical cost to manufacture. The main disadvantage of thin film solar is that efficiencies have not been on par with Monocrystalline and Polycrystalline Solar PV. Energy Sage (2020) reports an average thin film efficiency of 11%, which means that thin film solar will require more space than the crystalline Solar PV technologies. There is significant research and development in the field of Thin Film Solar PV, with perhaps the most promising candidate being Pervoskite Solar PV, which has already obtained efficiencies of over 24% (US Department of Energy, 2019). In one recent development, Stanford researchers created a manufacturing process to create 15.5% efficient perovskite modules at an estimated cost of 10% of the cost of crystalline Solar PV (Bellini, 2020).

The most common Thin Film Solar PV technology sold is Cadmium Telluride (US Department of Energy, 2020A). Kommalapati et al. (2017) reports cradle to grave LCA results from 6 studies that modeled Cadmium Telluride Solar PV. Values ranged from 16 – 51 g CO2e / kWh generated. The average system emits 28.3 g CO2e / kWh with a standard deviation of 13.8 g CO2e / kWh, The median system emits 22.3 g CO2e / kWh. System efficiencies range from 9 – 10.9%. When these values are adjusted for solar irradiance, the irradiance-adjusted average is 29.5 g CO2e / kWh at 4.5 kWh / m2 / day. See the Data and Assumptions Section for the full list of studies included.

Carbon Footprint of Central Receiver Concentrated Solar Power

Central Receiver Concentrated Solar Power works by reflecting sunlight to a central receiver, which is then used to superheat materials such as molten salt (US Department of Energy, 2020B). The superheated substrate can then be used to either generate steam to generate electricity, or used directly as a heat source for industrial processes or other heating. The superheated substrate can be used to generate energy for 12 or more hours after the sun has stopped shining, which makes Central Receivers an attractive alternative to battery storage in order to utilize solar energy at night.

The main disadvantage of Central Receivers is the cost. The US Department of Energy (2020B) estimates a 2018 price of $0.098 / kWh, which is close to the average cost of electricity in the US of $0.136 / kWh (EIA, 2020). Central Receivers operate best in full sun conditions, and thus they are limited in geographic range to deserts and arid regions. With advancements in Central Receiver Concentrated Solar Power research and development, the technology could become part of a mainstream solution to lower the carbon intensity of the US power grid.

Kommalapati et al. (2017) reports cradle to grave LCA results from 7 studies that modeled Central Receiver Concentrated Solar Power. Values ranged from 18 – 213 g CO2e / kWh generated. The average system emits 100.3 g CO2e / kWh with a standard deviation of 83.8 g CO2e / kWh, The median system emits 60 g CO2e / kWh. See the Data and Assumptions Section for the full list of studies included.

Carbon Footprint of Parabolic Trough Concentrated Solar Power

Parabolic Trough Concentrated Solar Power works by reflecting sunlight into a linear receiver tube (US Department of Energy, 2020C). The receiver tube is usually filled with water to generate steam, which can then be used to run a turbine to generate electricity. Parabolic Troughs can also store energy for later use.

The main disadvantage of Parabolic Troughs is the cost. The current cost of Parabolic Troughs is approximately twice as high as as conventional electricity (US Department of Energy, 2020D). Parabolic Troughs operate best in full sun conditions, and thus they are limited in geographic range to deserts and arid regions.

Kommalapati et al. (2017) reports cradle to grave LCA results from 7 studies that modeled Parabolic Trough Concentrated Solar Power. Values ranged from 15 – 185 g CO2e / kWh generated. The average system emits 94.4 g CO2e / kWh with a standard deviation of 76.0 g CO2e / kWh. The median system emits 90 g CO2e / kWh. See the Data and Assumptions Section for the full list of studies included.

Data and Assumptions

Selected studies from Kommalapati et al. (2017) are either cradle to grave, or 3 additional studies (marked with asterisks in below table) did not include End of Life emissions, which should not significantly effect the lifecycle emissions. The following assumptions were made:

  • Solar irradiance value for the Nomura et al. (2001) is proxied at 1,427 kWh / m2, which is reported on several other studies included in Kommalapati et al. (2017).
  • Solar irradiance values for Switzerland were estimated based on the average values for central Switzerland. The assumed value is 3.4 kWh / m2, based on SolarGIS (2020).
  • The Oliver and Jackson (2000) study in Switzerland reported values from 2 systems. These values were averaged into 1 single value.
 
Solar PV studies were normalized to 4.5 kWh / m2 / day (1,643 kWh / m2 / yr), which is typical of the midwest and east coast of the United States. To obtain the carbon footprint of Solar PV at a different solar irradiance, simply divide the carbon footprint of the solar PV system of interest by the ratio of solar irradiance, with 1,643 kWh / m2 / yr as the denominator. For example, the solar irradiance of Los Angeles is 6.13 kWh / m2 / day (US Department of Energy NREL – PVWatts Calculator, 2020), or 2,237 kWh / m2 / yr, and the ratio is: 2,237 / 1,643 = 1.36. For Monocrystalline Solar PV, the conversion is: 78.6 g CO2e / kWh / 1.36 = 57.6 g CO2e / kWh in Los Angeles. Solar irradiance values were not listed for Concentrated Solar Power, and thus this conversion is not available.
 
The studies included in Kommalapati et al. (2017) date between 1998 – 2012, and thus the results may not be fully representative of recent advances in manufacturing and efficiency of Solar PV and Concentrated Solar Power. Bracquene et al., (2018) estimates that the carbon intensity per generation capacity of solar PV has come down by as much as 3.5 times between 2000 and 2017. A recent review by Constantino et al. (2018) lists an average carbon intensity of 56.0 g CO2e / kWh based on 16 systems modeled up to 2015, including 2 of its own results, which was normalized by Consumer Ecology to 4.5 kWh / m2 solar irradiance. However, Solar PV systems were not differentiated, and system boundaries were not clearly described in this review, and thus the cradle to grave carbon intensity could be higher if missing life cycle stages are added. The two results modeled by Constantino et al. (2018), were between 73.3 – 83.1 g CO2e / kWh, normalized to 4.5 kWh / m2, agree with the results presented from Kommalapati et al. (2017) for Monocrystalline Solar PV. Results from Constantino et al. (2018) were based on the literature review mentioned above and include cradle to end of operations. These results were not included to avoid bias from a non-systematic literature review.
 
The full list of studies included is shown below. Full references are available in Kommalapati et al. (2017), but the reference numbers from the cited studies are included in the table. An Asterisk denotes that the study did not include End of Life disposal.


References

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Bracquene, E., Peeters, J. R., Dewulf, W., & Duflou, J. R. (2018). Taking evolution into account in a parametric LCA model for PV panels. Procedia CIRP, 69, 389-394.

Colville,F. (November, 2020). Monocrystalline cells dominate solar photovoltaic industry, but technology roadmap is far from certain. LaserFocusWorld. See Link to Source

Constantino, G., Freitas, M., Fidelis, N., & Pereira, M. G. (2018). Adoption of photovoltaic systems along a sure path: A life-cycle assessment (LCA) study applied to the analysis of GHG emission impacts. Energies, 11(10), 2806.

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Kommalapati, R., Kadiyala, A., Shahriar, M., & Huque, Z. (2017). Review of the life cycle greenhouse gas emissions from different photovoltaic and concentrating solar power electricity generation systems. Energies, 10(3), 350.

Sengupta, M., Y. Xie, A. Lopez, A. Habte, G. Maclaurin, and J. Shelby. 2018. “The National Solar Radiation Data Base (NSRDB).” Renewable and Sustainable Energy Reviews  89 (June): 51-60.

SolarGIS. (2020). Solar Resource Maps of Switzerland. See Link to Source

Svarc, J. (September, 2020). Most Efficiency Solar Panels 2020. Clean Energy Reviews. See Link to Source

US Department of Energy. (2019). PV Cells 101: Primer on the Solar Photovoltaic Cell. See Link to Source

US Department of Energy. (2020A). Cadmium Telluride. See Link to Source

US Department of Energy. (2020B). Concentrating Solar-Thermal Power. See Link to Source

US Department of Energy. (2020C). Linear Concentrator System: Concentrating Solar-Thermal Power Basics. See Link to Source

US Department of Energy. (2020D). Parabolic Trough. See Link to Source

US Department of Energy – National Renewable Energy Laboratory (NREL). (2020). PVWatts Calculator. Version 6.1.3. See Link to Source