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What is the embodied carbon of a home PV system?

Matthew_Salkeld | Posted in General Questions on

Any estimates of how much carbon is embodied in all the parts and labour of a home rooftop PV system?

The carbon payback time of a PV system would vary with its embodied carbon content and the carbon emission factor of the electricity grid it is supplanting. The time to offset its embodied carbon could be much longer than the system’s energy payback time, perhaps even longer than the lifetime of the system.

I am worried about PVs made in Asia being used to supplant very low emissions (Hydro and Nuclear) power in Canada, for example.

I would love to see Embodied Carbon ratings made mandatory on all PV panels.

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  1. Matthew_Salkeld | | #1

    This emission reduction analysis of hybrids and EVs is analagous for my question.

    Hybrids reduce more emission than EVs where the grid is based on coal power.

    Nota bene Green builders that we aren't inadvertantly increasing global emissions by installing PV!!

  2. GBA Editor
    Martin Holladay | | #2

    If you do some Googling, you'll find lots of scholarly articles and magazine articles on the topic. Many researchers have performed the necessary calculations, and concluded that the carbon payback period for PV systems is relatively short -- generally 2 to 4 years. For example, in a 2013 article, "Energy payback time and carbon footprint of commercial photovoltaic systems," Mariska J. de Wild-Scholten found:

    "• Energy payback times and carbon footprints range 0.68–1.96 years and 15.8–38.1 g CO2-eq/kWh (hydropower/UCTE electricity, 1700 kWh/m2 year).
    • Assuming production in China results in similar energy payback times but increases the carbon footprint by a factor 1.3–2.1.
    • New data are used for production of monocrystalline-, multicrystalline, amorphous silicon-, micromorphous silicon-, cadmium telluride- and CIGS-PV modules.
    • The analysis is performed for roof-top photovoltaic systems excluding installation, operation and maintenance and end-of-life phase."

    See also this 2008 article by Justine Sanchez in Home Power magazine, "PV Energy Payback." Sanchez wrote:

    "It only takes an average of three years or less for PV systems to produce the energy required to offset what it took to manufacture the systems, and EPBT [energy payback time] continues to decrease each year. PV manufacturers are always seeking to reduce manufacturing costs, through reducing the amount of silicon required, improving PV cell efficiency, experimenting with new materials, and by utilizing new production methods. Most cost-reduction strategies will also reduce energy payback time. "

    1. Expert Member
      Dana Dorsett | | #3

      Mind you, the material inputs to a watt of rooftop PV have come down considerably since 2013, and even more since 2008. Most rooftop PV installed these days run ~20% efficiency panels, as compared to ~15% in 2013, and 12% in 2008.

      It's safe to assume that 30% fewer panels per kilowatt translates directly into 30% less racking.

      The thickness of silicon and the amount of silicon waste has also been reduced. Inverter efficiency has been improving and physical size has also been shrinking year on year.

      The Chinese power grid is also greener now than it was 5 or 10 years ago.

      So whatever the carbon footprint for PV was in 2008 or 2013, it's lower now, and getting lower year on year.

      Regarding the EV emissions issue, the real emissions over a lifecycle aren't that simple to calculate. In local grids where EVs on smart chargers get put to use for stabilizing the grid the improvements in grid-efficiency (less spinning reserve capacity needed, etc) can be a very real net carbon emissions reduction. As EVs get more market penetration eventually ALL car chargers will have to be grid-aware, and that controllable load enables even higher amounts of near-zero carbon variable output grid sources such as wind & PV.

      Over the lifecycle of an EV purchased in 2019 local grids where EV charging managment becomes a significant feature of grid management will increase. It takes a better crystal ball than mine (or NRCAN's) to predict the where, when, and how much those impacts will be felt, and thus it's impossible to calculate the lifecycle emissions of any EV with reasonable level of certainty right now. In Denmark EV owners can currently get paid to keep their EV plugged in to a smart charger to supply grid services. They get paid even more if they allow 2-way power flows (putting power onto the grid, not just a programmable load.) This stuff is coming to North America faster than most people realize.

    2. Matthew_Salkeld | | #6

      I will need to review all that research but Commercial systems don't apply here. Utility Scale or commercial PV systems are much much less energy intense than home systems.

      Let's not confuse Energy Payback and Carbon Payback times at all.

      My provincial Grid emits 40g CO2/kWh, whereas a Grid in China where it was produced could emit up to 800g CO2/kWh.

  3. Expert Member
    Peter Yost | | #4

    Hi Matthew -

    The biggest embodied carbon concerns for just about all buildings:

    1. concrete, particularly of the Portland cement component.

    2. refrigerants, particularly in mechanical systems and insulation.

    Worry about those long before worrying about the PV system.


    1. Expert Member
      Dana Dorsett | | #5

      >"2. refrigerants, particularly in mechanical systems and insulation."

      When a house is insulated primarily with closed cell foam blown with HFC245fa the lifecycle hit from the just the insulation exceeds the carbon footprint of the rest of the construction materials combined, including the foundation concrete. The 100 year global warming potential of HFC245fa is about 1000x CO2. In most new construction it's possible (and desirable) to completely design out refrigerant-blown insulation from the assemblies.

      The R410A commonly used in refrigerant systems and heat pumps/air conditioners has a 100-year global warming potential of about 2000x CO2.

      Under the 2016 Kigali Amendment to the Montreal Protocol both HFC245fa and R410A are to be phased out by at least 80% by the year 2030. For sprayed insulation there is already some movement toward using HFO1234ze, with a 100 year global warming potential of about 6x CO2 (instead of ~1000x).

      For R410A in refrigerant systems it's entirely clear where it's going. For some air conditioning and food refrigerators/freezers propane (GWP= 3.3) and isobutane (GWP=3) are becoming common in signatory countries in Europe & Asia, and HFO1234ze (GWP=4) products have also showed up in Europe, primarily in automotive air conditioning applications. Late in 2017 the US EPA approved the use of propane, butane, ethane, and isobutane in home refrigerators and freezers, so it's possible that the next refrigerator you buy will use one or a combination of these lower-impact refrigerants rather than R410A.

      While it's possible to use propane in home heating heat pumps & split system air conditioning at comparable performance to R22 or R410A, the higher refrigerant volumes required for whole-house systems combined with the fact that propane is highly combustible makes it a difficult safety-engineering & market hurdle to overcome. R32 is being pushed in some quarters, but while not as extreme as R410A, at ~675x CO2 it can hardly be considered "low GWP" compared to propane at 3.3x CO2. (That's a 200x difference between the two refrigerants.)

      Despite being a primary leader in it's drafting, the US has not yet ratified the Kigali Amendment. It doesn't appear to be a priority for the current administration, even though the HVAC and refrigeration industry in the US is lobbying for ratification. Without ratification of the Kigali Amendment it's most likely that development & manufacture of replacement refrigerants will happen outside the US.

      So yes, it makes far more sense to pay more attention to the refrigerants used in the refrigerator and air conditioner than sweating the minutae of differences in carbon footprint of the PV based on where it was manufactured.

      1. Matthew_Salkeld | | #7

        Are you assuming the refrigerant always dissipates to the atmosphere or can it be captured end of life (assuming it hasn't leaked out already)? Isn't recovery in ODS requirements?

        What quantities of ODS are we talking about in fridges and air conditioners? I am amazed the amount in fridge would eclipse the emissions of the rest of the house.

        1. Expert Member
          Dana Dorsett | | #9

          The realistic assumptions are a leakage & fujitive loss rate of about 3% per year for split space conditioning systems that have field-installed connections and field charging/recharging of the system (mini-splits, heat pumps and central AC) , 1% per year for hermetically sealed systems (like window AC, dehumidifiers, or refrigerators). Recovery at end of life is possible, but not well regulated worldwide.

          Refrigerants used as foaming agents for insulation have varying rates of escape, but there is NO recovery (ever), and 100% will eventually get into the environment. So when HFC245fa or other HFC closed cell polyurethane is the primary insulation in a house (a house built with polyurethane foam SIPs, for instance), on a lifecycle basis ALL of the refrigerant escapes, with a much higher global warming effect than all of the energy & transportation that went into the rest of the building materials. With hydrocarbon blown foams like EPS or polyiso that is not the case.

    2. Matthew_Salkeld | | #8

      While I don't disagree, it is still possible that the PV system causes more emissions over its life than it saves for a given grid.

      Cement and refrigerants don't generate electricity, nor claim to help reduce your carbon emissions. Different issue.

      1. Expert Member
        Dana Dorsett | | #10

        >"While I don't disagree, it is still possible that the PV system causes more emissions over its life than it saves for a given grid."

        Theoretically maybe, in reality not so much. (In low-sun / high-hydro or geothermal Norway & Iceland, maybe.) Like other site-sourced generation, distributed PV also reduces the amount of grid infrastructure capacity that needs to be built, and that infrastructure also has a carbon footprint. Carbon accounting at that level of detail gets pretty complicated.

        >"Cement and refrigerants don't generate electricity, nor claim to help reduce your carbon emissions. "

        The marketing claims of SIP & ICF vendors often make those sorts of claims alone with other claims of "green-ness".

        Concrete and insulation can and often DO reduce the amount of electricity used by a building, so while different it's not completely orthogonal. A kilowatt hour saved is equivalent to an kilowatt hour generated.

        1. Matthew_Salkeld | | #11

          This is my concern like Norway for Canada. Some provinces like Quebec where I live and Manitoba are almost 100% Hydro Power (incredibly fortunate). You also have me wondering if my heat pump will save any GHGs displacing the hydro power since it is bound to release some or all R410a over time. When will CO2 refrigerant become the standard I wonder.

          PV decreases main grid infrastructure capacity?? How so? PV doesn't generate here a third of the year and snow can cling onto even steeply inclined panels for months at a time. I hear the opposite observation actually. The essential grid infrastructure plus peaking natural gas generators (backup for the intermittency) are subsidizing PV and wind systems and their owners on the backs of regular rate payers. And the cost and carbon footprint of the essential infrastructure is not being apportioned or shared with the renewable systems. The analysis is too simplistic and in the end that serves no one.

          The PV Energy Payback times being thrown around are global averages. For high latitudes safely double them.

          The PV issue to me is very very climate and grid specific.

          I also suggest we manufacture PV in hydro powered places and send them over to the coal-fired markets.

          1. GBA Editor
            Martin Holladay | | #12

            You wrote, "Snow can cling onto even steeply inclined [PV] panels for months at a time."

            That's not true. If the panels are steeply inclined, your statement is only accurate if you substitute "days" for "months." In most cases, snow on steeply inclined panels is gone in three days or less.

          2. Expert Member
            Dana Dorsett | | #16

            >"When will CO2 refrigerant become the standard I wonder."

            Probably never, for space heating heat pumps, but we'll see. Because CO2 doesn't undergo a phase change it the refrigeration cycle it's only really efficient at high temperature differences. The pressure is also many times higher than what's needed for hydrocarbon or HFC refrigerants, making it unsuitable for systems than need field connections of the refrigerant plumbing. While there have been some experimentation with automotive AC using CO2 refrigeratant, it's better suited to water heating, where then incoming water temperatures are dramatically lower than the output temperature of the heat pump, sustaining a large delta-T.

            The very few combi-systems that have been developed with CO2 compressors for space heating and hot water have had considerably lower efficiency than hydrocarbon & HFC refrigerant systems, and are only efficient in homes with moderate to high domestic hot water use and low heat loads. There is room for improvement on these systems, but it seems unlikely to ever meet the efficiency of HC/HFC heat pumps.

            >"PV decreases main grid infrastructure capacity?? How so? PV doesn't generate here a third of the year and snow can cling onto even steeply inclined panels for months at a time. I hear the opposite observation actually."

            Ya hear rong (or they're telling it wrong). Peak grid capacity requirements are all about sustained peaks during warmer outdoor temperatures when the transformers and wires aren't air-cooled very well. That basically translates to what's needed to support peak air conditioning loads, which occur during daylight hours. Even modest amounts of local battery storage can reduces the grid capacity requirement by quite a bit compared to PV alone.

            I'm curious as to the theory or observations you may have heard on how distributed PV can create a need for increased grid capacity. In the utility business the "non-wires alternatives" solutions is a hot buzz topic, which is how to increase throughput while reducing peak capacity requirements without brute-forcing it with higher capacity wires and subtations. Distributed generation (including PV, which delivers power during the daily peak loads in most regional and local grids) and local storage along with smart controllable loads (aka "demand response") is a large part of those solutions.

        2. Matthew_Salkeld | | #14

          Martin not so. Here in Ottawa our winter 2015/2016 saw snow cling for over two months. I worked on some NZE projects at the time. Losses due to snow and dirt in Ottawa average 11% over modelled estimates of annual production, that figure derived from hundreds of measured systems.

          I have also seen dry snow cling to 70 degree tilts.

          Remember snow is now snow/rain/ice rain in any imaginable sequence and combination. It rains during our winter almost once a week then temperatures can whipsaw down to -25 in a day or two.

          Where do you live?

          1. GBA Editor
            Martin Holladay | | #15

            I live at 1800 ft. elevation in the northeast corner of Vermont, with snowfall exactly equal to Jay Peak. I had a 10-foot pile of snow this past winter beside my garage.

            The key to winter snow clearing is obviously tilting your PV array as close to vertical as your mounts allow. To some extent, our disagreement depends on one's definition of "steeply inclined." I usually clear my panels with a broom, but sometimes I can't. Weather conditions that don't allow clearing of panels for "months" are hard for me to imagine, even though I'm used to temperatures down to -40 (either F or C, your choice) and lots of snow.

    3. Matthew_Salkeld | | #13

      I read last year that mitigating the refrigerant problem is the first priority and greatest opportunity in emission reductions.

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