Siemens and Siemens-like Processes for Producing Photovoltaics: Energy Payback Time and Lifetime Carbon Emissions

Headnote

Polysilicon photovoltaics will play a signifi cant role in meeting the world's shortfall in electrical energy this century. The photovoltaic industry relies on high-purity silicon produced in the Siemens process. New Siemens-like processes (which convert metallurgical silicon to trichlorosilane and deposit purifi ed silicon through the decomposition of silane) and metallurgical processes for producing solar silicon are under development. Their energy payback time and lifetime carbon emissions are reviewed. The history of development of Siemens and Siemens-like processes is summarized.

INTRODUCTION

Substantial growth in the renewable energy sector will occur this century. By 2050 the world's oil production will be half of what it is today.1 Capping CO2 in the atmosphere at 450 ppmv will require its sequestration. Estimates are that with the cap the world's shortfall in electrical energy will be 1.4 terawatt-year (TWy) in 2030.2

Polysilicon photovoltaics (PVs) are expected to dominate the market for the next 15 years.3 Feltrin and Freundlich analyzed the availability of resources for the different PVs.2 They concluded that only silicon and dye sensitized TiO2 have suffi cient ore reserves to produce electrical energy at the TWy level. Attention is focused on the role Siemens and Siemens-like processes will play in meeting demand for purifi ed silicon for PVs, their energy payback time (EPBT) and carbon footprint.

DISCUSSION

Siemens Process

The history of the Siemens process, too extensive to present here, is contained in the patent literature. For those wanting to explore the literature, a list of patents is provided in Tables I and II.

The traditional multistep Siemens process in Figure 1 produces high purity polysilicon involving:

1. Hydrochlorination of metallurgical silicon (m-Si) forming trichlorosilane (HSiCl3 or TCS)

2. Purifi cation of TCS by distillation

3. TCS decomposition to produce purified silicon

4. Recycle H2(g) and unreacted TCS

The Siemens process meets standards for both the electronics and photovoltaic industries.4 Trichlorosilane is formed in a fl uidized bed reactor (FBR) at temperatures of 583 K to 623 K and at 50 bar. Distillation columns are used to remove SiCl4 and other high boilers from TCS that is subsequently decomposed on silicon rods heated to 1,373 K (Figure 2).4

The containment envelope is cooled to minimize deposition of silicon on its walls. Bell jars have been replaced...