Integrated life-cycle assessment of electricity-supply scenarios confirms global environmental benefit of low-carbon technologies

Integrated life-cycle assessment of electricity-supply scenarios confirms global environmental benefit of low-carbon technologies

Edgar G. Hertwich, Thomas Gibona, Evert A. Bouman, Anders Arvesen, Sangwon Suh, Garvin A. Heath, Joseph D. Bergesen, Andrea Ramirez, Mabel I. Vega, and Lei Shi

PNAS 2015 112: 6277-6282.

ABSTRACT AND AUTHOR AFFILIATIONS: http://www.pnas.org/content/112/20/6277.abstract

Note: Because of the technical nature of the paper, I chose to take excerpts or verbatim statements in this summary in order to preserve the precision of the content and meaning (see italicized sections).

Previous life cycle assessments of new low-carbon technologies indicate that they require more materials than fossil-fuel based energy sources on per unit power generated measure and therefore may lead to increase in other environmental impacts.  No studies, however, have answered what these environmental impacts may be, such as the impact on resource use or pollution creation.  Current energy-scenario models have not considered the manufacturing and material life cycle of energy technologies.

In the Significance section, the authors declare that this “paper presents, to our knowledge, the first life-cycle assessment of the large-scale implementation of climate-mitigation technologies, addressing the feedback of the electricity system onto itself and using scenario-consistent assumptions of technical improvements in key energy and material production technologies.”

In this paper, the authors compare different low-carbon technologies based on an analysis of their environment impacts and resource requirements under two scenarios:

1)      global deployment under the climate-change mitigation scenario of the International Energy Agency’s (IEA) BLUE-MAP program

2)      IEA’s baseline scenario

In this hybrid LCA of different low-carbon technologies, the authors consider the utilization of a mix of energy technologies and efficiencies in their analysis of the global production system.  They note that, to our knowledge, this analysis is the first to be based on a life-cycle inventory model that includes:

·         the feedback of the changing electricity mix

·         the effects of improvements in background technologies on the production of the energy technologies

The energy technologies considered are:

·         Concentrating solar power (CSP)

·         Photovoltaic power (PV)

·         Wind power

·         Hydropower

·         Gas- and coal-fired power plants with carbon dioxide capture and storage (CCS)

Two energy technologies not considered:

·         Bioenergy – would require a comprehensive assessment of the food system which is beyond the scope of the study according to the authors

·         Nuclear energy – authors were unable to reconcile conflicting results of competing assessments

Because they have to project their analysis into the future, for modeling of technologies for 2030 and 2050, they assumed improved production of aluminum, copper, nickel, iron and steel, metallurgical grade silicon, flat glass, zinc, and clinker.

Technological improvements in the electricity conversion are measured by improved:

·         Conversion efficiencies

·         load factors

·         next-generation technology adoption to achieve the technology performance of the scenarios

RESULTS

PART 1 – Comparison of low-carbon technologies with fossil fuel electricity generation without CCS

·         allows quantification of environmental cobenefits and tradeoffs for long-term investment decisions in the power sector

·         current state-of-the art technologies are used in this comparison

Impacts considered:

A.      greenhouse gas emissions (GHG)

B.      particulate-matter formation

§  From supporting information (SI): The particulate matter (PM) formation potential includes both direct emissions of fine particulates (<10 μm) and the formation of particulates from precursors such as SO2, NOx, sulfate, and ammonia. PM exposure has the highest human health impacts of any pollution type.

C.      aquatic ecotoxicity resulting from pollutants emitted in air and water throughout the life cycle of each technology

§  From SI: Freshwater ecotoxicity was chosen as an indicator to represent a wider suite of toxicity indicators (10), because it is fairly mature given the wide availability of toxicity data for aquatic species.

D.      eutrophication

§  From SI: Freshwater eutrophication addresses the addition of nutrients to freshwater bodies.

E.       life cycle use of key materials (like aluminum, iron, copper, and cement), nonrenewable energy, and land on a per unit of electricity produced

§  From SI: These materials are used for their structural and conductive properties. The production of these materials causes high environmental impacts.  Allwood et al. find that iron and steel, aluminum, and cement together cause about 50% of anthropogenic CO2 emissions from the industrial sector, although plastic (5%) and paper (4%) are also important. However, plastic and paper have much shorter average lifetimes and their use in energy technologies is not particularly high. The environmental significance of copper is related to its toxicity.

§  From SI: Cement was quantified because its production causes substantial environmental impacts; not all limestone is calcinated and concrete may contain different amounts of cement. As a result, cement is a superior indicator for environmental impact over limestone and concrete.

§  From SI: We quantified total non-renewable primary energy as an additional resource indicator called cumulative energy demand. We calculated the indicator by multiplying the amount of fuel extracted with the higher heating value of fossil fuels and the producible heat from uranium ore using current best technology, including recycling of the plutonium.

§  From SI: We also quantified total land occupation measured in land area multiplied by the time that this land area is occupied.

§  From article: As an indicator of potential habitat change, we use the area of land occupied during the life cycle of each technology (Fig. 1E).

Technology Comparison per Unit Generation

See Figure 1.  In one set of data, the unit environmental impacts in terms of A. greenhouse gas, B. particulate matter, C. ecotoxicity, D. eutrophication, and E. land occupation.  These measurements were made for PV, concentrating solar power, hydropower, wind power, coal, and natural gas.  The impacts are further disaggregated for sub-categories of each technology.  As the authors noted at the beginning of this section, Comparative LCA indicates that renewable energy technologies have significantly lower pollution-related environmental impacts per unit of generation than state-of-the-art coal-fired power plants in all of the impact categories considered

Some noted results:

·         Natural gas combined cycle

o   produces little eutrophication

o   has a low land occupation area (see Figure 1 for units)

o   between coal and renewable technologies on ecotoxicity and greenhouse gas emissions

o   high on particulate matter emissions

·         wind

o   requires more bulk material (iron, aluminum, copper, and cement) than coal- and gas-based electricity

·         PV

o   requires more bulk material (iron, aluminum, copper, and cement) than coal- and gas-based electricity

·         For fossil fuel-based power systems, materials contribute a small fraction to total environmental impacts, corresponding to <1% of GHG emissions for systems without CCS and 2% for systems with CCS.

·         For renewables, however, materials contribute 20–50% of the total impacts, with CSP tower and offshore wind technologies showing the highest shares (SI Appendix, Fig. S1).

·         CCS reduces CO2 emissions of fossil fuel-based power plants but increases life-cycle indicators for particulate matter, ecotoxicity, and eutrophication by 5–60% (Fig. 1 B–D). Both postcombustion and precombustion CCS require roughly double the materials of a fossil plant without CCS (Fig. 1 G–J). The carbon capture process itself requires energy and therefore reduces efficiency, explaining much of the increase in air pollution and material requirements per unit of generation.

·         High land-use requirements are associated with hydropower reservoirs, coal mines, and CSP and ground-mounted PV power plants. The lowest land use requirements are for NGCC plants, wind, and roof-mounted PV. See article for definitions of land use measurement.

·         The current technologies used in the production of renewable systems consume 0.1–0.25 kWh of nonrenewable energy for each kWh of electricity produced (Fig. 1F). The situation is different for fossil fuel-based systems, for which the cumulative energy consumption reflects the efficiency of power production and the energy costs of the fuel chain and, if applicable, the CCS system.

PART 2 - we show the potential resource requirements and environmental impacts of the evaluated technologies within the BLUE Map scenario and compare these results with those of the Baseline scenario. The analysis assumes implementation and utilization of low-carbon technology prescribed by the BLUE MAP scenario:

The BLUE Map scenario posits an increase in the combined share of solar, wind, and hydropower from 16.5% of total electricity generation in 2010 to 39% in 2050.

The following are quantified and compared between the two scenarios:
requirements of bulk materials (and comparison with annual production levels of these materials)

Environmental pressures over time

Some noted results:

·         The required up-front investment in renewable generation capacity would require a combined investment of bulk materials of 1.5 Gt over the period 2010–2050, which is more than the total use of these materials in the Baseline scenario. The difference in material demand displayed in Fig. 2 G–J shows that the initial demand for iron and cement is mainly associated with wind and CSP installations whereas it is mainly PV driving additional copper demand.  The BLUE Map scenario has a lower material demand associated with conventional coal-fired power plants without CCS, which is partly offset by the material demand from coal-fired power plants with CCS. The most important contributor to the material demand from coal-fired power plants is associated with producing and transporting the ∼500 kg of coal required per MWh of electricity generated.

·         The BLUE Map scenario would be able to keep the emissions of particulate matter and ecotoxicity stable despite the doubling of annual electricity generation from 18 petawatt hours per annum (PWh/a) to 36 PWh/a for the technologies investigated.

·         Compared with the situation in 2010, a substantial reduction in GHG emissions (from 9.4 Gt CO2 eq. to 3.4 Gt CO2 eq.) and eutrophication would be achieved (SI Appendix, Fig. S4) in the BLUE MAP scenario.

·         The difference in pollution between the BLUE Map and Baseline scenarios would grow dramatically over time (Fig. 2) whereas the additional required material investment would rise only moderately.

·         For the BLUE Map scenario, the higher material requirement per unit of renewable electricity and a projected increase in energy demands cause a substantial increase in material use (SI Appendix, Fig. S4). The overall material requirement per unit of electricity produced would be 2.3 kg/MWh compared with 1.2 kg/MWh for the Baseline scenario.

·         The authors add, “That increase appears manageable in the context of current production volumes, the long lifetime of the equipment, and the ability to recycle the metals.”

·         Compared with material production levels in 2011, the construction and operation of the 2050 electricity system envisioned in the BLUE Map scenario would require less than 20% of the cement, 90% of the iron, 150% of the aluminum, and 200% of the copper, all relative to their respective 2011 production quantities (Table 1). Meeting copper demand could be problematic due to declining ore grades, and it would result in potential increases in the environmental costs of copper production.

·         Displacing fossil fuels through the widespread deployment of solar and wind energy could limit air and water pollution (Fig. 2).

·         Over the study period (2010–2050), emissions of GHG connected to the power plants investigated are 62% lower in BLUE Map than they are in the Baseline Scenario whereas the particulate matter is 40% lower, freshwater ecotoxicity is almost 50% lower, and eutrophication is 55% lower.  Furthermore, both cumulative energy consumption and land use are reduced.

DISCUSSION

In this section, the authors give a broad summary of their goal for this study and what they have achieved, reiterating the novelty of this hybrid LCA that allows incorporation of how increasing use of renewable energy sources may impact and, in some cases, mitigate the potential environmental impacts of the manufacture and material use of new renewable energy technologies.  They also note that their analysis leads to further questions and, in the discussion section, they offered some brief responses for thought (see article).

CONCLUSIONS

·         Our analysis indicates that the large-scale implementation of wind, PV, and CSP has the potential to reduce pollution-related environmental impacts of electricity production, such as GHG emissions, freshwater ecotoxicity, eutrophication, and particulate-matter exposure.

·         The pollution caused by higher material requirements of these technologies is small compared with the direct emissions of fossil fuel-fired power plants.

·         Bulk material requirements appear manageable but not negligible compared with the current production rates for these materials. Copper is the only material covered in our analysis for which supply may be a concern.

MATERIALS AND METHODS

Methods for calculations, modeling software, assumptions, information use, and information sources are summarized.  For detailed descriptions and definitions, see supporting information documents.  Spreadsheets use for calculations and tabulation of information are given in the PNAS website.