Summary 'Environmental Aspects of Solar Cell Modules'

 In this report we summarize and update the results of a study project on the environmental aspects of photovoltaic solar cell technology. Four major types of solar cell modules, based on respectively multicrystalline silicon, amorphous silicon, cadmium telluride and copper indium selenide are reviewed with special attention to future expected technology developments.
    For each module type an assessment is made of the potential environmental impacts in case of large scale implementation of the technology. In principle the entire module life cycle is taken into consideration: from resource mining, via module production and module utilization until module decommissioning and waste handling.
    In the report for each module type the following aspects are discussed: energy requirements and energy pay-back time, material requirements and resource depletion, environmental emissions, waste handling, possibilities for recycling of modules, occupational health and safety and external safety.

The environmental aspects of four major solar cell technologies have been reviewed with special attention for future expected technology developments. Cell technologies investigated are multicrystalline silicon (mc-Si), amorphous silicon (a-Si), cadmium telluride (CdTe) and CuInSe2 (CIS). The following aspects were considered: energy requirements and energy pay-back time, material requirements and resource depletion, environmental emissions, waste handling, possibilities for recycling of modules, occupational health and safety and external safety.
     Although the energy pay-back time of the present-day mc-Si and a-Si modules is relatively high, around 4 to 4.5 years for frameless modules under Dutch irradiation conditions, this pay-back time is still considerably shorter than the expected technical lifetime of the module (15-30 years). Moreover, very good prospects exist for reduction of energy requirements by future technology developments, resulting in energy pay-back times well below 1.5 years for all module types (under Dutch irradiation conditions; below 1 year for global average irradiation).
    It is remarkable that thin film technologies (a-Si, CdTe, CIS) do not score significantly better (in some cases even worse) as wafer-based mc-Si technology. This mainly caused by the superior efficiency of mc-Si cells.
     Note that frames and support structures can add substantially to the energy requirements and may double the energy pay-back time of the total PV system (compared to modules only). Therefore serious attention is necessary for designs of array support structures which have a low energy requirement.
    From our analyses of the material flows we conclude that for the immediate future (and within the considered system boundaries) there are no reasons for concern regarding the material requirements and emissions of solar cell modules. Only if large scale deployment of modules - with annual production levels of several GW's - becomes probable there are some points which need closer attention, namely:
 * resource depletion of silver (mc-Si modules);
 * resource depletion of indium (CIS modules)
 * waste management and recycling possibilities for decommissioned modules (mc-Si, CdTe, CIS).
     Although there is still a considerable range of uncertainty in our emission estimates the risks from cadmium or selenium use in CdTe respectively CIS modules seem acceptable in comparison with some existing products or services like NiCad batteries or coal-fired electricity production.

    Regarding occupational health and safety and external safety the only significant risks are found in the storage and handling of explosive and/or toxic gasses, i.c. silane in a-Si production and H2Se in a certain CIS deposition process.
    With proper safety measures in place silane risks seem to be well manageable, but use of hydrogen selenide gas should be avoided.

Finally, table 7.1 presents a qualitative comparison of these cell types on the aspects mentioned above.

    We can see that there is not one single cell type that scores good or excellent on all considered aspects, although future a-Si technology, seems to be the most "environmentally friendly" technology, with mc-Si as a good second. CIS and CdTe score less well because of problems related to the use of heavy metals, some of which are rather scarce. However, these problems should not be considered as a major bottle-neck for the immediate future. Therefore they should not be used as a reason for ruling out one or more of the considered solar cell technologies from further R&D efforts.

Table 7.1: Qualitative comparison of the investigated solar cell technologies. Present respectively future indicates the assumed technology status with regard to module production, emission control technology and recycling. Scores for present technology are based on the worst case results described in previous chapters, while scores for future technology are based on both base case (70%) and best case results (30%). Note that effects of increasing production volumes, leading for example to increasing emissions, are not considered between present and future technology.
 

	mc-Si		a-Si		CdTe		CuInSe2
	present	future	present	future	present	future	present	future
Energy Pay-Back1	+/-	++	-	++	++	+++	+	+++
Resource depletion	+/-	+	++	++	+	+	-	-
Emissions	+	+	++	++	-	+/-	+/-	+
Health & Safety2	+	+	+/-	+/-	+	+	+/-	+/-
Recyclability	-	+/-	++	++	-	-	-	-

 

Notes:

1) - = 4-5 yr, +/- = 3-4yr, + = 2-3 yr, ++ = 1-2 yr, +++ = 0-1 yr under Dutch irradiation conditions;

2) Refers to occupational health & safety, and to external safety aspects, not to public health aspects.

 Points which deserve further attention both from manufacturers and researchers are: the energy requirements of modules (and module frames and supports), the use of heavy metals, gas safety issues and module recycling possibilities.