Laboratory for Manufacturing and Sustainability

In contrast to widely-used electricity generation technologies, photovoltaic (PV) systems produce little or no environmental pollution at the point of use, contributing to their market status as an environmentally-preferable product. However, there are numerous materials and energy inputs that go into the fabrication of the components of PV systems that may carry significant environmental burdens. A life-cycle perspective helps to compare the net environmental benefits of a particular generation. In this effort, we systematically examine design options from feedstock to integration in order to identify current and future opportunities for minimizing the environmental impact of PV systems.

We use a combination of process-based and economic input-output life cycle assessment (EIOLCA) to capture both a breadth and depth of information. We decompose a PV system into a set of design and manufacturing choices at each step of the process: 1) feedstock – electronic and solar grade silicon, 2) diffusion – conventional furnace (CFP) and rapid thermal processing (RTP), 3) silicon growth – multicrystaline silicon using electromagnetic casting (EMC) or directional solidification, single crystalline silicon using Czochralski or float zone crystal growth, and amorphous silicon, 4) cell encapsulation and covering – standard ethylene vinyl acetate (EVA) versus mixtures containing additives for adhesion strength and standard low-iron glass versus cerium-doped glass 5) module construction – traditional framed module and building integrated frameless glass laminates, 6) integration – sloped roof, flat roof, Building Integrated PV (BIPV), ground mounted, 7) construction – new or retrofit, 8) heat recovery, 9) insolation maximization – tracking or flat plate, and 10) energy storage – grid connected, electrochemical battery, or micro-hydro using a pre-existing agricultural infrastructure.

We find that 1) carbon intensities for best case systems are an order of magnitude lower than coal; 2) carbon intensities for best case and conventionally designed systems are still higher than wind or hydro; 3) significant opportunities exist in further development of solar-grade silicon feedstock, float zone crystal growth rapid thermal processing, and high durability encapsulants; 4) there are significant drawbacks in employing ground-based installations, including 30-50% increases in air pollutant emissions; 5) in many cases, the efficiency gains realized by using tracking devices do not translate into financial or environmental benefits; 6) the emissions from the manufacturing of batteries for stand alone systems are significant, increasing toxic material releases by 100 fold.

When the best choices are made throughout the system’s life cycle, environmental burden reduction of 25% can be achieved for carbon intensity, while increasing economic value.