Ethical Dimensions of Renewable Energy and Sustainability Systems

Case 2: PV is Toxic & Pollutes


Case 2: PV is Toxic & Pollutes

Video: Si and CdTe PV (1:05)

Si and CdTe PV
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Researchers for some time have been looking at the lifecycle assessment of photovoltaic modules. And they've really focused on two core materials in those modules, the first being silicon a semiconductor that is incorporated into crystalline cells, crystalline wafers, that ultimately are incorporated into a module. And then a thin film technology that makes use of cadmium telluride, cadmium telluride being the absorber of light, and it is deposited and then incorporated into a module as well. And when we are looking at these, we ultimately break them apart into their constituent elements. So, in the case of silicon, we're really only following the path of silicon through its lifecycle. In the case of cadmium telluride we're following the path of cadmium, and parallel to that but separate, tellurium. So let's first start thinking about why we would be looking at these. And so this really gets into the goals and scope of a lifecycle framework.

Credit:  J. Brownson © Penn State University is licensed under CC BY-NC-SA 4.0

Video: Broadening Goals and Scope to Include Toxics (2:08)

Broadening Goals and Scope to Include Toxics
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The goals and scope for this particular type of lifecycle assessment was really set up such that the scope would encompass the whole arc from cradle to grave, in this case, where we would have the acquisition and processing of raw materials, a gate towards the technical processing of materials that ultimately become cells and photovoltaic modules. And then going through a phase of use, a stage of use, and then ultimately ending up in a stage of disposal. Some of the earliest papers are really focused especially on stages of raw materials processing and technical processing, where we have silicon, for example, examined in terms of, in this case, the goals of the life cycle assessment. Silicone is being examined in terms of greenhouse gas emissions, in terms of emissions of metals associated with the processing, and looking at the energetic impact, the amount of energy that is required to manufacture these photovoltaic modules, such that we can understand in the use phase what is going to be the return, the payback of energy coming from using a photovoltaic module in the sun when its producing energy that is carbon free. So we look at silicon for these examples and then we similarly follow cadmium and tellurium for those same goals, the goals of understanding the emissions of greenhouse gasses, the emissions of metals, and especially when we know a metal like cadmium is a toxic metal under given concentrations. We want to know, is that toxic metal getting out into the environment, is it impacting society and the biome that is supporting us in some way, and if so how much? Alright. So we're looking at understanding those, and again, understanding the energetic impact of making a cadmium telluride photovoltaic module.

Credit:  J. Brownson © Penn State University is licensed under CC BY-NC-SA 4.0

Video: Difference in Comparative Emissions (4:45)

Difference in Comparative Emissions
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When we're looking at the lifecycle assessment, we had stated how we first create these goals in scope, and from that phase, really stems everything else, including the important three of the inventory analysis, the impact assessment, and the interpretation. And these all feedback together, but when we use the example of photovoltaic modules, we understand that we're inventorying our mass flows and our energy flows and our goals already were determined, that we're looking at, what are the greenhouse gas emission? What are the pollutant emissions that we traditionally think of as pollutants, which were metals, volatile organic carbons, VOCs, and other emissions like oxide of nitrogen (nox), oxides of sulfur (sox), these things are associated with processing, they are associated with energy. So we inventory the flows, the emissions or the outflows, that are associated with the technical processing stage and with the raw material processing stage of a photovoltaic technology. And we then want to understand the impact that those will have on society, on the environment, that is tied to those emissions, whether its a local impact or a global impact. And we want to interpret those impacts and interpret how we have defined our inventory analysis, according to the goals that we've set and according to the scoping that we've set within the lifecycle framework. Now when we're doing an interpretation, we can think of different ways of interpreting our results. We can interpret our results relative to internal technologies. So we could be comparing the inventory analysis from silicon photovoltaic module relative to that of a typical cadmium telluride photovoltaic module. And we might see some strong differences, one between the other, in that internal analysis, right? But we could also do an interpretation where we are interpreting the impact of a photovoltaic module relative to a comparative impact that would be made from generating energy, based on coal-based combustion or petroleum combustion, and what are the emissions associated with that in the generation of energy, understanding that photovoltaic modules, ultimately, are going to be used to generate carbon-free electricity. When we make that comparison, when we interpret what is happening on that much larger scale, we end up seeing that regardless of the technology choice, regardless of silicone or cadmium telluride in the dominant technologies, we find that the actual emissions impact, say, of cadmium in a cadmium telluride module, the cadmium emissions associated with a cad-tell module are so minuscule compared to the emissions of cadmium that come out from the combustion of coal. Because coal is essentially plants, right? They're plants that have been consolidated, they've been compressed, and they've been concentrated. And so while plants have trace metals in them, by the time they've been concentrated over millions of years and then combusted, they're actually very concentrated with metals that do not burn but are emitted as particulates in the combustion process. And those cadmium emissions can be much higher than the emissions associated with the entire lifecycle of a cadmium telluride photovoltaic module. And there's certainly more than what you would see in silicon considering that silicon that isn't even made of the element cadmium. So, again this all ties together with the inventory analysis, the impact assessment and the interpretation relative to the goals and scope of the lifecycle assessment. And it's really that scoping that kind of dials us in to either talking internally about different technologies or thinking broadly about how our one technology, photovoltaic modules, compares and contrasts with an alternate technology like the combustion of coal to generate an equivalent unit of electricity.

Credit:  J. Brownson © Penn State University is licensed under CC BY-NC-SA 4.0

Video: Difference in Cradle to Gate (4:42)

Difference in Cradle to Gate
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In the lifecycle assessment of photovoltaic modules, we do see studies that are effectively cradle-to-gate studies, where we're really trying to understand what is the raw material processing from mining and mineral processing to ultimately deliver a refined commodity that can then be turned into a technical material, into a photovoltaic module. And then once it's actually made and sent to market, that's the gate, because it's actually a gate transitioning from the technical processing stage to the youth stage. So that cradle-to-gate arc is gonna be pretty unique from different module technologies. So a silicon module technology will have a raw material processing and technical processing stages that will be pretty different from thin film processing for cadmium telluride. Not the least of which because silicon will be mined or will be acquired through quartz or sand quarries, whereas cadmium and tellurium will be mined from different ore bodies elsewhere in the world. So let's start with cadmium. Cadmium will come from, is essentially a byproduct of the mining of zinc ores. And so we'll see a mine for zinc, zinc sulfide effectively, will have additional materials tied in. The geology is such that there are additional minerals tied in and cadmium will be one of those minerals. It will be refined out of the ore body as a by-product, as, excuse me, a co-product. Let's just define those two things, a co-product versus a by-product. When I'm processing a raw material especially a mineral reserve, I'm going to have certain things I'm mining for directly, in this case, it could be zinc in a zinc ore. That's my primary product but there will be other things that are derived in the processing of a zinc ore. One of those things will be cadmium. And because I can sell cadmium, because I can find it useful to the technologies in society and it's not a waste product, it is called a co-product. Now if I have something that is derived from the mineral processing stage, the raw materials processing state of mineral processing, if I can't use it if I don't have a use for it, and effectively I have to leave it at the site of processing, it's effectively a by-product. It's not something that I was desiring, but I go anyway in the process of refining that. Following those flows becomes very important because sometimes we find that there are unintended consequences of acquiring a certain mineral. We'll have certain by-products that might be toxic or that might affect the environmental chemistry of the surrounding area. So we need to be very aware of what are our co-products and our by-products and our primary products from mineral processing. The raw material stage, that raw material processing stage then goes to the technical processing stage. Technical processing is where we're actually at the plant taking a commodity metal cadmium, commodity metal tellurium, finding a way to combine them together into a cadmium telluride thin film. That can be documented fairly easily. There will be potential by-products that come from that, emissions that come from that that are by-products, and we need to know what those are and we need to inventory those in our inventory analysis. And then later understand it in our impact assessment and interpret how we can react to that or if we want to change that in the future and in future lifecycle assessments. And that really ties together a cradle-to-gate process where we're talking about the stages of raw materials processing and technical processing. Because once we go past that gate after technical processing to the youth stage, we're going to have that similar period of 25 to 40 years of a photovoltaic module that has been set, that has been installed and is generating carbon-free energy for that period. So that similarity there is in a way common to all technologies and so we can retain some of our focus into the areas of lifecycle assessment that we feel have the strongest impact on the environment and on society.

Credit:  J. Brownson © Penn State University is licensed under CC BY-NC-SA 4.0