The world uses about 10 terawatt (TW) of energy (the US, about 3 TW) and by 2050 is projected to need about 30 TW. Thus the world will need about 20 TW of nonCO2 energy to stabilize CO2 in the atmosphere by mid century. For details about non CO2 energy needs for meeting climate change, see Hoffert et al., 1998. Hoffert (NYU), Rick Smalley (Rice Nobel Laureate), and Nate Lewis (CalTech) call this the ‘Terawatt (TW) Challenge,’ and whether thin film PV can meet the challenge is the subject of this study. Shockingly, it turns out that among the nonCO2 options, it is possible that solar is the only one that can (discussed below).

The primary barrier to TWscale use of PV is cost, and that will be the main focus of this study. Secondary barriers, including feedstock availability, land use, and energy payback, will also be covered. A final barrier, only touched on here, is system related: how do we use intermittent PV electricity to provide dispatchable electricity and fuel? This is described in outline, but deserves a separate in depth study.

But why is it crucial that PV be able to meet the TW Challenge? Why not other sources of nonCO2 energy? This is an important question that most policy makers do not yet agree on, and the public is another step removed from such a consensus.

In recent presentations and publications, CalTech’s Nate Lewis, 2004 has emphasized that among the renewables, only solar has a large enough resource base to meet a major fraction of the world’s energy needs. The rest of the renewables (wind, biomass, geothermal, hydro) do not have adequate global resources to do so – although they can meet a fraction of a TW each (still a very significant contribution, when one realizes that the US now uses 3 TW). But this means that solar (with about 125 000 TW of global incident sunlight) has both a huge opportunity and a huge responsibility.

To state clearly: any technology that can produce at least a TW of annual energy should be considered having met the TW Challenge and contribute to the reduction of climate change. But we go further here; we want to know how many TWs thin film PV could provide by mid century.

Key scenarios for stabilizing CO2 in the atmosphere during the 21st century turn on the viability of CO2 sequestration. Sequestration is capturing CO2 during, e. g., coal burning, piping it to a storage location, and then pumping it into special underground storage, e. g., aquifers, where it would presumably stay without leakage for millennia. This approach is unproven but important. Much work is being done to demonstrate it.

If sequestration does not work, it is almost certain that the world will need at least 10 TW of nonCO2 producing energy by mid century, and perhaps as much as 20 TW, to stabilize CO2 in the atmosphere. Even nuclear power would have difficulty meeting this without breeder reactors, due to the lack of uranium fuel for present designs. But this means that nuclear has multiple problems: proliferation of breeder reactors with plutonium fuel (with concomitant global tensions and terror issues), waste disposal, and accidents. Let us put aside nuclear as possibly too dangerous. Then the simplest scenario to stabilize CO2 by mid century is one in which PV and other renewables are used for electricity (10 TW) and to make hydrogen for transportation (10 TW); and fossil fuels (coal, natural gas, oil) are used to make residential and industrial heat (10 TW). Departures from this strategy include using coal to make gasoline; but this would mean more CO2. The challenges facing solar are: (i) economic (can renewables be cheap enough? Can storage of hydrogen on vehicles be cheap enough?) and (ii) infrastructure (we pump gasoline, not hydrogen; we use gasoline engines, not fuel cells or turbines). Under this scenario, and given the vastness of the solar resource, there could be a huge 10-20 TW demand for PV by mid century. (Recall that we only use about 10 TW worldwide today, so this is a huge amount of energy.)

But what if sequestration works? Would the need for solar be eliminated or much re­duced? Perhaps not as much as one might think. Why? Let us look a little more closely at the coal/sequestration approach.

People are excited about the potential of sequestration, because if it works, coal could be used to meet most electricity demand. Everyone knows there’s plenty of cheap coal. The cost of sequestration and rising demand might add about 50-150 % to the price of coal generated electricity, but it wouldn’t make coal use untenable. Still, even under this scenario, some renewables would be used simply because they would be cost competitive at these higher prices.

What about transportation?

Princeton’s Bob Williams has presented a unique scenario based on biomass CO2 seques­tration in which coal and biomass could be used for liquid fuels and still stabilize CO2 in the atmosphere. How? Biomass-produced CO2 sequestration could be adopted to actually remove significant CO2 from the atmosphere. This is even better than biomass being CO2 neutral; it provides a sink that can be played off against fossil fuels. Biomass used as energy or used as feedstock along with coal could make liquid fuels (e. g., gasoline). By effectively removing some CO2 from the atmosphere via the biomass, the net CO2 produced in the transportation sector (where CO2 cannot be captured) would be low enough to allow CO2 stabilization despite using a lot more coal.

Under the coal and biomass CO2 sequestration scenario, no other renewables would appear to be needed for a major fraction of the world’s energy, at least until mid century (when biomass resources would become inadequate to the task).

But this is not the last word on this key question.

Williams and Lewis have proposed an innovative idea for using solar energy to make carbon fuels, i. e., as an alternative to biomass in the sequestration scenario. If successful, such an approach would significantly increase the demand for PV despite sequestration’s success.

How could PV produce carbon fuels? Lewis suggests an in situ, electrochemical approach mimicking photosynthesis. An electrochemical solar cell would be used to combine CO2 taken from the air with hydrogen from water splitting, producing a hydrocarbon liquid fuel. His suggested in situ approach, similar to but more complex than existing efforts to split water electrochemically with in situ PV, would require long term research and one or more major breakthroughs.

Perhaps we could afford the wait. Under Williams’ plan for biomass sequestration, analysis suggests that biomass resources would become inadequate about mid century. Thus develop­ment of in situ PV synthesis of carbon fuels could take until then and still play a major role.

However, there may be simpler and shorter term ways to harness PV to remove some CO2 from the atmosphere and make hydrocarbons. Just as there is a way to use any PV electricity to split water using an (ex situ) electrolyzer, there may be ways to simply use flat plate PV electricity to make carbon fuels. How? Carbon dioxide may be absorbed from the atmosphere (as it is by leaves in photosynthesis) or taken from the emissions of coal plants, and hydrogen from water can be combined with it to make hydrocarbons. Because concentrating atmospheric CO2 takes too much energy, the large area of a leaf or of flat plate PV can be used to capture CO2 from the air economically. Physical and chemical processes would have to be developed to capture the CO2. But the rest of the processes are already being done: splitting the water with an electrolyzer; chemically synthesizing the hydrocarbon fuel from the CO2 and the resulting hydrogen (e. g., running a methanol fuel cell backwards), and the DC PV energy. On the PV side, the challenge is purely economic: can PV be inexpensive enough? But it would not have to be directly competitive with gasoline prices; it would compete (under the sequestration scenario) with biomass, instead.

Thus even under the sequestration scenario, many TW of PV might be of value. Of course, without sequestration (which seems more probable), solar would be essential. And under either scenario, sequestration or not, we should establish that PV can meet the TW Challenge from an economic and materials availability standpoint.

It may be that developing PV to produce hydrocarbons (instead of hydrogen) will be impor­tant for transportation, even if sequestration fails. The cost of using PV to make a liquid fuel instead of hydrogen is approximately only the added cost of the CO2 processing, since splitting water is part of both approaches. But some of this added cost would be offset by the ease with which liquid fuels can be transported and stored (versus hydrogen) and the presence of the whole liquid fuel infrastructure in transportation. Then there would be further cost savings during use in vehicles, as the weight advantage of liquid fuels is significant. In fact, many policy makers are skeptical about whether the use of hydrogen for transportation is feasible at all, due to these factors. One way or another, PV as the source of nonCO2 energy for making hydrocarbons or hydrogen could play a critical role in the transportation sector.

There is another collateral value to making hydrogen or hydrocarbons – storage. Photo­voltaic is intermittent. Measures must be taken to smooth the delivery of solar energy to meet fluctuating demand, especially at night, during cloudy periods, or for seasonal extremes. Two methods of doing this exist: storage and long distance transmission. Storage could be accom­plished by making hydrogen or hydrocarbons for conversion back to electricity. Other supply and demand mismatches could be minimized by improving long distance transmission. Na­tional and supranational grids could be upgraded to support large proportions of intermittent PV and wind; and to best use fossil fuels, storage, and other renewables. And multicontinent (even multihemispheric) transmission (e. g., Colbert and Smalley, 2002) could also fill gaps. Some of these infrastructure adjustments will be necessary to bring on the solar age.

As always, the bottom line is PV cost. Solar can meet energy needs globally from a resource standpoint. What would it have to cost to do it cost effectively? Each market has a different system cost and competitive price. And competitive prices will change with time, especially in a carbon constrained world, or if oil becomes more expensive. This study will establish that thin film PV costs can be brought down to about 5 c/kWh. (There are pathways that would take these costs even lower, but they are more speculative.) A cost of 5 c/kWh seems adequate to provide energy (electricity and transportation fuel) at prices not-too-different from today’s prices.

The need for solar by mid century could be 10-20 TW. ‘Solar is the only big number out there,’ as Lewis would say. We have to find ways to use that resource. Thus it is incumbent on the PV community to rethink how it can meet the challenge, including facing issues such as reaching ultralow costs; managing explosive growth (with any materials bottlenecks that might occur); and producing, deploying, and using PV systems on an unheard of scale.

There may be other ways to make low cost PV than thin films (e. g., wafer silicon or concentrating PV), but thin films are paradigmatically designed for low cost; and they are the focus of this study.

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