How Green is Solar???

Thin Film CdTe under the Spotlight. Dr. Mae-Wan Ho

Solar power tops the list of renewable energies in the world, both in installed 
capacity and rate of growth. Solar photovoltaic – generating electricity from 
sunlight – has been growing the fastest, driven by technological advances that 
bring prices down, as much as by government policies such as feed-in tariffs [1] 
(World Reached 25 % Renewable Energy Capacity, SiS 49). But how green is solar 
energy? A particular worry is the toxicities of metals such as cadmium that goes 
into the new thin film PVs.

The Silicon Valley Toxics Coalition based in San Jose California in the United 
States carried out a survey early in 2010 comparing environmental health and 
safety practices, recycling policies and sustainability among 14 major solar 
panel manufacturers that represent about a quarter of the global solar panels 
market share [2, 3]. German companies scored the highest overall: Calyxo, 
Solarworld and Sovello; Yingli in China came fourth. US-based First Solar and 
Abound Solar followed.  There was no response from several major manufacturers 
including Sharp, Miasolé, Best Solar and Solyndra.

The survey also found that:

- Half the companies surveyed supported mandatory take-back and recycling at the 
end of the solar panels; life, but many need to start setting aside money for 
the programmes 
- About two-thirds of the companies surveyed failed to perform life-cycle analysis 
of their products 
- The risk of chemicals used during solar panel manufacture is not assessed by the 
majority of responding manufacturers 

It is important to perform life-cycle analysis as well as environmental and 
health impacts of renewable energy options, and end of life recycling should be 
mandatory, especially for toxic components, such as cadmium or cadmium 
telluride.

Life cycle analysis

Researchers led by Vasilis Fthenakis at Columbia University, New York, in the 
United States, have been carrying out life-cycle analysis of solar panels ([4] 
Solar Power Getting Cleaner Fast, SiS 39). In the most recent comprehensive 
update, the analysis start from raw material acquisition, through material 
processing, manufacturing, use, decommissioning, treatment/disposal and 
recycling, if any, accounting for the material and energy inputs, and waste 
effluents to air, water or ground [5].

The material and energy inputs and outputs during life cycles of silicon 
photovoltaics (Si PVs) and thin-film cadmium telluride (CdTe) PVs were 
investigated in detail based on actual measurements from PV production plants 
between 2004 and 2006. 

They used the most up-to-date data from researchers on life-cycle inventory 
(LCI) for producing crystalline silicon modules in Western Europe under the 
framework of the Crystal Clear, a large European Integrated Project focusing on 
crystalline silicon technology, co-funded by the European Commission and the 
participating countries. The LCI data for CdTe thin-film technology were taken 
from the production data of First Solar’s plant in Perrysburg, Ohio, United
States.

The typical thickness of multi- and mono-Si PV is 270–300 mm, and that of 
ribbon-Si is 300–330 mm; 72 individual cells of 156 cm2 (125 cm x 125 cm) 
comprise a module of

1.25 m2 for all Si-PV types. The conversion efficiency of ribbon-, multi-, and 
mono-Si module is 11.5, 13.2, and 14.0 percent, respectively. As of 2006, First 
Solar’s 25-MWp plant manufactures frameless, double-glass, CdTe modules of 1.2 m 
by 0.6 m rated at 9 percent efficiency with ~3mm thick active layer, a hundredth 
the thickness of the silicon PVs.

An important part of the solar panel installation that has not been considered 
in previous life-cycle analysis is the ‘balance of system’ (BOS), all components 
other than the PV panels needed for supporting the panels and for their 
operation.

Silicon modules require an aluminium frame of 3.8 kg/m2 for mounting, while a 
glass backing performs the same functions for the CdTe PV. For rooftop PV, the 
BOS typically includes inverters, mounting structures, cable and connectors. 
Large-scale ground-mounted PVs require additional equipment and facilities, such 
as grid connections, office facilities, and concrete. The mass of this was 9 to 
10 times the panels for a 3.5 MW installation in Springville, AZ.

Energy payback time and greenhouse emissions

The most frequently measured life-cycle metrics of PV system environmental 
analyses are the energy payback time (EPBT) and the greenhouse-gas (GHG) 
emissions. EPBT is the period required for a renewable energy system to generate 
the same amount of energy that we used to produce the system itself. Calculating 
the primary energy equivalent requires knowledge of the country-specific fuels, 
feedstock and technologies used to generate energy.

The EPBTs are shown in Figure 1. As can be seen, CdTe PVs have the lowest values 
by far, one year in Europe and slightly over one year in the US. 

Figure 1  Energy payback time for different solar PVs

The greenhouse-gas (GHG) emissions during the lifecycle of a PV system are 
estimated as an equivalent of CO2 using an integrated time period of 100 years; 
the major emissions included as GHG emissions are CO2 (global warming potential, 
GWP = 1), CH4 (GWP = 23), N2O (GWP = 296), and chlorofluorocarbons (GWP = 
4600–10 600).

Figure 2  Greenhouse gas emissions for solar PVs compared with some non-
renewable sources

Again, CdTe PVs emissions at 24 g per kWH come out ahead of silicon PVs at 37 gm 
per kWh. These are way below those of coal natural gas and petroleum. I have 
omitted the GHG emissions of nuclear given in the authors’ graph at 24 g, the 
same as CdTe PVs. This value was way below that resulting from a detailed 
realistic analysis, which put nuclear GHG emissions at between a minimum of 85 g 
and a maximum of 130 g per kWh ([6] The Nuclear Black Hole, SiS 40).

Electricity and fuel use during the PV materials and module production are the 
main sources of the GHG emissions for PV lifecycles. Upstream electricity-
generation methods also play an important role in determining the total GHG 
emissions. For instance, the GHG emission factor of the average US electricity 
grid is 40 percent higher than that of the average Western European (UCTE), 
resulting in higher GHG estimates for the US – produced modules. It is clear 
that improvements in material and energy use and recycling will enhance the 
environmental profiles.

A recent, major improvement is a recycling process for the sawing slurry, the 
cutting fluid that is used in the wafer cutting. This recycling process recovers 
80–90 percent of the silicon carbide and polyethylene glycol that was previously 
wasted, and decreases the EPBTs of these technologies by 10 percent.

Other waste gases were compared. Emissions of NOx and SOx from CdTe PVs at 42 
mg/kWh and 79 mg/kWh are about half of those of the silicon modules. 

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