# Renewable Electricity Futures Study Executive Summary

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# Renewable Electricity Futures Study

Edited By

Hand, M.M. National Renewable Energy Laboratory 

### Baldwin, S. U.S. Department of Energy 

### DeMeo, E. Renewable Energy Consulting Services, Inc.



### Reilly, J. M. Massachusetts Institute of Technology 

### Mai, T. National Renewable Energy Laboratory 

### Arent, D. Joint Institute for Strategic Energy Analysis



### Porro, G

. National Renewable Energy Laboratory 

### Meshek, M. National Renewable Energy Laboratory 

### Sandor, D

. National Renewable Energy Laboratory



### Suggested Citations Renewable Electricity Futures Study (Entire Report) National Renewable Energy Laboratory. (2012). Renewable Electricity Futures Study. Hand, M.M.; Baldwin, S.; DeMeo, E.; Reilly, J.M.; Mai, T.; Arent, D.; Porro, G.; Meshek, M.; Sandor, D . eds. 4 vols. NREL/TP

- 6A20

- 52409. Golden, CO: National Renewable Energy Laboratory. http://www.nrel.gov/analysis/re_futures/

. Renewable Electricity Futures Study : Executive Summary Mai, T.; Sandor, D.; Wiser, R.; Schneider, T (2012). Renewable Electricity Futures Study: Executive Summary. NREL/TP

- 6A20

- 52409

- ES. Golden, CO: National Renewable Energy Laboratory.

# Renewable Electricity Futures Study Executive Summary

Trieu Mai, 1 

## Debra Sandor, 1 

## Ryan Wiser, 2 

## Thomas Schneider 1

1 

### National Renewable Energy Laboratory
2 

### Lawrence Berkeley National LaboratoryNOTICE

This report was prepared as an account of work sponsored by an agency of the United States government. Neither the United States government nor any agency thereof, nor any of their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States government or any agency thereof. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States government or any agency thereof.

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Printed on paper containing at least 50% wastepaper, including 10% post consumer waste.Rene wable Electricity Futures Study — Executive Summary iii 

### Perspective The Renewable Electricity Futures St udy (RE Futures) provides an analysis of the grid integration opportunities, challenges, and implications of high levels of renewable electricity generation for the U.S. electric system. The study is not a market or policy assessment. Rather, RE Futures examines renewable energy resources and many technical issues related to the operability of the U.S. electricity grid, and provides initial answers to important questions about the integration of high penetrations of renewable electricity technologies from a national perspective. RE Futures results indicate that a future U.S. electricity system that is largely powered by renewable sources is possible and that furt her work is warranted to investigate this clean generation pathway. The central conclusion of the analysis is that renewable electricity generation from technologies that are commercially available today, in combination with a more flexible electric system, is more than adequate to supply 80% of total U.S. electricity generation in 2050 while meeting electricity demand on an hourly basis in every region of the United States.

The renewable technologies explored in this study are components of a diverse set of clean energy solutions that also includes nuclear, efficient natural gas, clean coal, and energy efficiency. Understanding all of these technology pathways and their potential contributions to the future U.S. electric power system can inform the development of integrated portfolio scenarios . RE Futures focuses on the extent to which U.S. electricity needs can be supplied by renewable energy sources, including biomass, geothermal, hydropower, solar, and wind.

The study explores grid integration issues using models with unprecedented geographic and time resolution for the contiguous United States. The analysis (1) assesses a variety of scenarios with prescribed levels of renewable electricity generation in 2050, from 30% to 90%, with a focus on 80% (with nearly 50% from variable wind and solar photovoltaic generation); (2) identifies the characteristics of a U.S. electricity system that would be needed to accommodate such levels; and (3) describes some of the associated challenges and implications of realizing such a future. In addition to the central conclusion noted above, RE Futures finds that increa sed electric system flexibility, needed to enable electricity supply-demand balance with high levels of renewable generation, can come from a portfolio of supply- and demand-side options, including flexible conventional generation, grid storage, new transmission, more responsive loads, and changes in power system operations. The analysis also finds that the abundance and diversity of U.S. renewable energy resources can support multiple combinations of renewable technologies that result in deep reductions in electric sector greenhouse gas emissions and water use. The study finds that the direct incremental cost associated with high renewable generation is comparable to published cost estimates of other clean energy scenarios. Of the sensitivities examined, im provement in the cost and performance of renewable technologies is the most impactful lever for reducing this incremental cost. Assumptions reflecting the extent of this improvement are based on incremental or evolutionary improvements to currently commercial technologies and do not reflect U.S. Department of Energy activities to further lower renewable technology costs so that they achieve parity with conventional technologies.Rene wable Electricity Futures Study — Executive Summary iv RE Futures is an initial analysis of scenarios for high levels of renewable e lectricity in the United States; additional research is needed to comprehensively investigate other facets of high renewable or other clean energy futures in the U.S. power system. First, this study focuses on renewable -specific technology pathways and does not explore the full portfolio of clean technologies that could contribute to future electricity supply. Second, the analysis does not attempt a full reliability analysis of the power system that includes addressing sub -hourly, transient, and distributio n system requirements. Third, although RE Futures describes the system characteristics needed to accommodate high levels of renewable generation, it does not address the institutional, market, and regulatory changes that may be needed to facilitate such a transformation. Fourth, a full cost- benefit analysis was not conducted to comprehensively evaluate the relative impacts of renewable and non

- renewable electricity generation options

. Lastly, as a long- term analysis, uncertainties associated with assumptio ns and data, along with limitations of the modeling capabilities, contribute to significant uncertainty in the implications reported. Most of the scenario assessment was conducted in 2010 with assumptions concerning technology cost and performance and foss il energy prices generally based on data available in 2009 and early 2010. Significant changes in electricity and related markets have already occurred since the analysis was conducted, and the implications of these changes may not have been fully reflecte d in the study assumptions and results. For example, both the rapid development of domestic unconventional natural gas resources that has contributed to historically low natural gas prices, and the significant price declines for some renewable technologies (e.g., photovoltaics) since 2010, were not reflected in the study assumptions.

Nonetheless, as the most comprehensive analysis of U.S. high- penetration renewable electricity conducted to date, this study can inform broader discussion of the evolution of the electric system and electricity markets toward clean systems

. The RE Futures team was made up of experts in the fields of renewable technologies, grid integration, and end

- use demand. The team included leadership from a core team with members from the National Renewable Energy Laboratory (NREL) and the Massachusetts Institute of Technology (MIT), and subject matter experts from U.S. Department of Energy (DOE) national laboratories, including NREL, Idaho National Laboratory (INL), Lawrence Berkeley National Laboratory (LBNL), Oak Ridge National Laboratory (ORNL), Pacific Northwest National Laboratory (PNNL), and Sandia National Laboratories (SNL), as well as Black & Veatch and other utility, industry, university, public sector, and non- profit participant s. Over the course of the project, an executive steering committee provided input from multiple perspectives to support study balance and objectivity.

RE Futures is documented in four volumes of a single report: Volume 1

— describes the analysis approach and models, along with the key results and insights; Volume 2 describes the renewable generation and storage technologies included in the study; V olume 3 presents end

- use demand and energy efficiency assumptions; and Volume 4 discusses operational and institutional challenges of integrating high levels of renewable energy into the electric grid. The Executive Summary for RE Futures is both included in Volume 1 and presented separately here.Rene wable Electricity Futures Study — Executive Summary

v 

### Acknowledgments The Project Leaders for the Renewable Electricity Futures Study gratefully acknowledge the significant contributions from the numerous individuals on the RE Futures t eam, more than 110 individuals from more than 35 organizations as listed in Appendix D. We appreciate their thorough and thoughtful consideration of the present state and future potential of renewable electricity generation technologies, use of electricity, and electric sector operation. This report is the culmination of their contributions. We are also grateful to the members of the study’s executive steering committee, who assisted the RE Futures team in evolving and finalizing the scenarios to include and reviewed and provided comments on the analysis at various stages. We also thank the many outside individuals who reviewed the draft documents.

The ability to represent the technical aspects of future electricity generation portfolios, particularly with high levels of renewable electricity generation, requires sophisticated models operated by experienced analysts. We are grateful to Walter Short for his innovation, vision and leadership at NREL over several decades that led to development of both the Regional Energy Deployment System (ReEDS) and the strong team of analysts who use this model and other tools to provide context and insight around future electricity generation portfolios for this study and many others.

The support and guidance of management at NREL and MIT also was critical to the completion of this study. In particular, we recogni ze Robin Newmark and Bobi Garrett for their leadership.

We are grateful to the U.S. Department of Energy (DOE) Office of Energy Efficiency and Renewable Energy for sponsoring this work. We especially thank Sam Baldwin for his vision and leadership in conceiving, supporting, and contributing to this study from beginning to end. We also thank DOE’s Office of Electricity Delivery and Energy Reliability for its input and guidance on specific aspects of the analysis, as well as valuable comments and helpful suggestions to improve the content of the report. NREL’s contributions to this report were funded by the DOE Office of Energy Efficiency and Renewable Energy under contract number DE

- AC36 -08GO28308.Rene wable Electricity Futures Study — Executive Summary vi 

### List of Acronyms and Ab breviations

AC a lternat ing current

BA balancing area

Btu British Thermal Unit(s)

CAES compressed air energy storage

CC combined cycle

CCS carbon capture and storage

CO 2 carbon dioxide

CO 2 e , CO 2 eq carbon dioxide equivalent

CSP concentrating s olar p ower
CT combustion turbine

DC d irect c urrent

DOE U.S. Department of Energy

EEPS energy efficiency portfolio standard

EIA U.S. Energy Information Administration

EPA U.S. Environmental Protection Agency

EREC European Renewable Energy Council

GHG gree nhouse gases

GWEC Global Wind Energy Council

INL Idaho National Laboratory

IPCC Intergovernmental Panel on Climate Change

MW megawatt(s)

MWh megawatt -hour(s)

NREL National Renewable Energy Lab oratory

ORNL Oak Ridge National Laboratory

PNNL Pacific Northwest National Laboratory

PV photovoltaic

RE renewable electricity

RE Futures Renewable Electricity Futures Study

Re E DS Regional Energy Deployment System

RE

- ETI Renewable Electricity —Evolutionary Technology Improvement

RE

- ITI Renewable Electricity

— Incremental Technology ImprovementRene wable Electricity Futures Study — Executive Summary vii RE

- NTI Renewable Electricity

— No Technology Improvement

SNL Sandia National Laboratories

SolarDS Solar Deployment System

TW terawatt(s)

TWh terawatt -hour(s)

yr yearRene wable Electricity Futures Study — Executive Summary viii 

### Table of Contents

**Perspective**

................................................................................................................................................. iii Acknowledgments

...................................................................................................................................... v List of Acronyms and Abbreviations

....................................................................................................... vi Study Organization and Report Structure

................................................................................................ 4 Analysis Approach

...................................................................................................................................... 5 Scenario Framework ..................................................................................................................5 Renewable Resources Characterization .....................................................................................9 U.S. Electricity Grid Expansion and Operational Characterization ........................................13
Key Results

................................................................................................................................................ 14 Deployment of Renewable Energy Technologies in High Renewable Electricity Futures .....14 Grid Operability and Hourly Resource Adequacy in High Renewable Electricity Futures ....20 Transmission Expansion in High Renewable Electricity Futures ............................................25 Cost and Environmental Implication s of High Renewable Electricity Futures .......................27 Effects of Demand Growth on High Renewable Electricity Futures .......................................29

Conclusions

............................................................................................................................................... 30
Appendix . Project Participants and Contributors

................................................................................. 32

References

................................................................................................................................................. 43Rene wable Electricity Futures Study — Executive Summary ix 

### List of Figures Figure ES -1. Modeling scenario framework for RE Futures .....................................................7 Figure ES -2. Geographic distribution of renewable resources in the contiguous United States ..................................................................................................................................10 Figure ES -3. Installed capacity and generation in 2050 as renewable electricity levels increase (low

- demand, RE -ITI technology improvement) ................................................15 Figure ES -4. Renewable generation and capacity in 2050 by region under 80% RE- ITI scenario (low

- demand, RE -ITI technology improvement) ................................................17 Figure ES -5. Range of 2050 installed capacity and annual generated electricity by technology for the low-demand core 80% RE scenarios and the High- Demand 80% RE scenario ....19 Figure ES -6. Hourly dispatch stacks for the 80% RE- ITI scenario a ........................................21 Figure ES

- 7. Operating reserve requirements as renewable electricity levels increase ..........24 Figure ES -8. New transmission capacity requirements in the baseline and exploratory scenarios .............................................................................................................................26 Figure ES -9. New transmission capacity additions and conceptual location in the 80% RE

- ITI scenario ..................................................................................................................27 Fi gure ES- 10. Average increase in retail electricity rates relative to study

- specific reference/baseline scenarios ...............................................................................................28Rene wable Electricity Futures Study — Executive Summary 1 The Renewable Electricity Futures Study (RE Futures) is an initial investigation of the extent to which renewable energy supply can meet the electricity demands of the contiguous United States 1 over the next several decades. This study includes geographic and electric system operation resolution that is unprecedented for long- term studies of the U.S. electric sector

. The analysis examines the implications and challenges of renewable elect ricity generation levels

— from 30% up to 90%, with a focus on 80%, of all U.S. electricity generation from renewable technologies— in 2050. At such high levels of renewable electricity penetration, the unique characteristics of some renewable resources, specifically geographical distribution and variability and uncertainty in output, pose challenges to the operability of the U.S. electric system. The study focuses on some key technical implications of this environment, exploring whether the U.S. power sys tem can supply elect ricity to meet customer demand with high levels of renewable electricity , including variable wind and solar generation. The study also begins to address the potential economic, environmental, and social implications of deploying and integrating high levels of renewable electricity in the United States.

RE Futures was framed with a few important questions:

• The United States has diverse and abundant renewable energy resources that are available to contribute higher levels of electricity g eneration over the next decades . Future renewable electricity generation will be driven in part by federal incentives and renewable portfolio standards mandated in many states. 2 Practically , how much can renewable energy technologies, in aggregate, contribute to future U.S. electricity supply?

• In recent years, variable renewable electricity generation capacity in the United States has increased considerably. Wind capacity, for example, has increased from 2.6 GW in 2000 to 40 GW in 2010 , while solar capacity has also begun to grow rapidly. Can the U.S. electric power system accommodate higher levels of variable generation from wind or solar photovoltaics (PV) ?

• Overall, r enewable energy contributed about 10% of total power- sector U.S. electricity supply in 2010 (6.4% from hydropower, 2.4% from wind energy, 0.7% from biopower, 0.4% from geothermal energy, and 0.05% from solar energy). 3 Are there synergies that can be realized through combining these diverse sources, and to what extent can aggregating th eir output over larger areas help enable their integration into the power system?

1 Alaska

, Hawaii , and the U.S. Territories were not included in this study because they rely on electric grid systems that are not connected to the contiguous United States . However, both states and the territories have abundant renewable resources, and they have efforts are underway to substantially increase renewable electricity generation (see Volume 1, Text Box Introduction

- 1).
2 Some states have targets of a 20 %– 30% share of total electricity generation (see http://www.dsireusa.org/ for information on specific state standards) and are making progress toward meeting these goals.
3 These data reflect estimates for the electric power sector only, and they exclude the end use sectors (i.e., on

- site electric power supply that directly me ets customer demands). If the end

- use and electric power sectors are considered together, the percentage contribution from biomass would increase from 0.7% to 1.4%, and the contribution from solar would increase from 0.05% to 0.12%.Rene wable Electricity Futures Study — Executive Summary 2 Multiple international studies 4 have explored the possibility of achieving high levels of renewable electricity penetration, primarily as a greenhouse gas (GHG) mitigation m easure

. RE Futures presents systematic analysis of a broad range of potential renewable electricity futures for the contiguous United States based on unprecedented consideration of geographic, temporal, and electric system operation aspects

. 5

RE Futures explores a number of scenarios using a range of assumptions for generation technology improvement, electric system operational constraints, and electricity demand to project the mix of renewable technologies—including wind, PV, concentrating solar power (CSP ), hydropower, geothermal, and biomass— th at meet various prescribed levels of renewable generation, from 30% to 90% . Additional sensitivity cases are focus ed on an 80%-by-2050 scenario . At this 80% renewable generation level, variable generation from wind and solar technologies accounts for almost 50% of the total generation.

Within the limits of the tools used and scenarios assessed, hourly simulation analysis indicates that estimated U .S. electricity demand in 2050 could be met with 80% of generation from renewable electricity technologies with varying degrees of dispatchability, together with a mix of flexible conventional generation and grid storage, additions of transmission, more responsive loads, and changes in power system operations. 6 Further, these results were consistent for a wide range of assumed conditions that constrained transmission expansion, grid flexibility, and renewable resource availability . The analysis also finds that the abundance and diversity of U.S. renewable energy resources can support multiple combinations of renewable technologies that result in deep reductions in electric sector greenhouse gas emissions and water use. Further, the study finds that the incre mental cost associated with high renewable generation is comparable to published cost estimates of other clean energy scenarios. Of the sensitivities examined, improvement in the cost and performance of renewable technologies is the most impactful level fo r reducing this incremental cost.

While this analysis suggests such a high renewable generation future is possible, a transformation of the electricity system would need to occur to make this future a reality. This transformation, involving every element o f the grid, from system planning through operation, would need to ensure adequate planning and operating reserves, increased flexibility of the electric system, and expanded multi

- state transmission infrastructure, and would likely rely on the development and adoption of technology advances, new operating procedures, evolved business models, and new market rules.

4 As examples, recent d etailed studies include those prepared for Europe (ECF 2010) and Germany (SRU 2010), as well as a review of 164 global energy scenarios by the Intergovernmental Panel on Climate Change (IPCC 2011). Cochran et al. (2012) also describes several case studies of countries successfully managing high levels of variable renewable energy on their electric grids.

5 Previous, more conceptual or more

- limited analyses of high penetrations of renewable energy in the United States and globally include (but are not limite d to) Pacala and Socolow (2004); ACORE (2007); Kutscher (2007); Greenblatt (2009); GWEC/GPI (2008); Fthenakis et al. (2009); Jacobson and Delucchi (2009); Sawin and Moomaw (2009); EREC/GPI (2008); and Lovins (2011).

6 The study did not conduct a full reli ability analysis

, which would include sub

- hourly, stability, and AC power flow analysis.Rene wable Electricity Futures Study — Executive Summary 3 Key results of this study include the following:

• Deployment of Renewable Energy Technologies

o R enewable energy resources

, accessed with commercially available generation technologies, could adequately supply 80% of total U.S. electricity generation in 2050 while balancing supply and demand at the hourly level.

o All regions of the United States could contribute substantial renewable electri city supply in 2050 , consistent with their local renewable resource base

.

o Multiple technology pathways exist to achieve a high renewable electricity future. Assumed c onstraints that limit power transmission infrastructure, grid flexibility , or the use of p articular types of resources can be compensated for through the use of other resources, technologies, and approaches.

o Annual renewable capacity additions that enable high renewable generation are consistent with current global production capacities but are significantly higher than recent U.S. annual capacity additions for the technologies considered. N o insurmountable long- term constraints to renewable electricity technology manufacturing capacity, materials supply, or labor availability were identified.

• Grid Operability and Hourly Resource Adequacy

o Electricity supply and demand can be balanced in every hour of the year in each region with nearly 80% electricity from renewable resources, including nearly 50% from variable renewable generation, according to simulations of 2050 power system operations.

o Additional challenges to power system planning and operation would arise in a high renewable electricity future, including management of low-demand periods and curtailment of excess electricity generation.

o Electric sector modeling shows that a more flexible system is needed to accommodate increasing levels of renewable generation. System flexibility can be increased using a broad portfolio of supply- and demand-side options, and will likely require technology advances, new operating procedures, evolved business models, and new market rule s.

• Transmission Expansion

o As renewable electricity generation increases, additional transmission infrastructure is required to deliver generation from cost- effective remote renewable resources to load centers, enable reserve sharing over greater distances, and smooth output profiles of variable resources by enabling greater geospatial diversity.

• Cost and Environmental Implications of High Renewable Electricity Futures

o High renewable electricity futures can result in deep reductions in electric sector greenhouse gas emissions and water use.

o The direct incremental cost associated with high renewable generation is comparable to published cost estimates of other clean ener gy scenarios. Improvement in the costRene wable Electricity Futures Study — Executive Summary 4 and performance of renewable technolo gies is the most impactful lever for reducing this incremental cost.

• Effects of Demand Growth

o With higher demand growth, high levels of renewable generation present increase d

resou rce and grid integration challenges

. This report presents the analysis of some of the technical challenges and opportunities associated with high levels of renewable generation in the U.S. electric s ystem . However, the analysis presented in this report rep resents only an initial set of inquiries on a national scale. Additional studies are required to more fully assess the technical, operational, reliability, economic, environmental, social, and institutional implications of high levels of renewable electric ity
gener ation , and further explore the nature of the electricity system transformation required to enable such a future.


### Study Organization and Report Structure RE Futures was led by a team from the National Renewable Energy Laboratory (NREL) and the Massachusetts Institute of Technology (MIT). The leadership team coordinated teams of subject matter experts from U.S. Department of Energy ( DOE ) national laboratories, including Idaho National Labora tory (INL)

, Lawrence Berkeley National Laboratory (LBNL)

, NREL, Oak Ridge National Laboratory (ORNL)

, Pacific Northwest National Laboratory (PNNL) , and Sandia National Laboratories (SNL) , as well as Black & Veatch and other utility , industry , university, public sector, and non- profit participants . These expert teams explored the prospects for large

scale deployment of specific renewable generation and storage technologies, along with some of the issues and challenges associated with their integration into the electric system.

In total, this report is the culmination of contributions from more than 110 individuals at more than 35 organizations (Appendix D lists the contributors to the study). Over the course of the project, an executive steering committee pr ovided input from multiple perspectives to support study balance and objectivity. Technical reviewers from the renewable technology and electric sector industries , universities, public sector, non- profits, and other entities comment ed on a preliminary version of this report.

Most of the analysis informing the study, particularly the scenario assessment, was conducted in 2010. As a result, study assumptions concerning technology cost and performance and fossil
energy prices were generally based on data avail able in late 2009 and early 2010. Where possible, more recent published work has been refere nced ; however, the implications of these publications may not have been fully reflected in the RE Futures study assumptions. For example, both the rapid development of domestic unconventional natural gas resources that has contributed to historically low natural gas prices, and the significant price declines for some renewable technologies (e.g., photovoltaics) since 2010, were not reflected in the study assumptions. Finally, the technology projections presented in RE Futures do not necessarily reflect the current DOE estimates.

RE Futures is documented in four volumes of a single report: Volume 1

— describes the analysis approach and models, along with the key results and insights. Volume 2 describes the renewable generation and storage technologies included in the study; Volume 3 presents 2050 end- useRene wable Electricity Futures Study — Executive Summary 5 demand and energy efficiency assumptions; and Volume 4 discusses some operational and institutional challenges of integrating high levels of renewable energy into the electric grid.

This Executive Summary , which is also included in Volume 1, highlights the analysis approach and key results from RE Futures. First, it summarizes the an alysis approach, including scenario framework, renewable resources characterization, and modeling tools used to analyze the expansion of the U.S. electricity system and its operational characteristics. The key results from the analysis are then presented, including results associated with renewable technology deployment, grid operations, and economic, environmental and social implications. Finally, additional research opportunities are identified in the conclusions.


### Analysis Approach
Scenario Framework
Giv en the inherent uncertainties involved with analyzing alternative long -term energy futures, and given the variety of pathways that might lead to higher levels of renewable electricity supply, multiple future scenarios were modeled and analyzed. The scenarios examined include d the following considerations:

• Energy E fficiency: Most of the scenarios assumed adoption of energy efficiency (including electricity) measures in the residential, commercial, and industrial sectors that resulted in flat demand growth over the 40- year study period. 7

• Transportation: Most of the scenarios assumed a shift of some transportation energy away from petroleum and toward electricity in the form of electric and plug -in hybrid electric vehicles

, partially offsetting the elec tricity efficiency advances that were considered

. 8

• Grid Flexibility: Most scenarios assumed improvements in electric system operations to enhance flexibility in both electricity generation and end-use demand, helping to enable more efficient integration of variable -output renewable electricity generation.

• Transmission: Most scenarios expand the transmission infrastructure and access to existing transmission capacity to support renewable energy deployment. D istribution

level upgrades were not considered

.

• S iting and Permitting: Most scenarios assumed project siting and permitting regimes that allow renewable electricity development and transmission expansion subject to standard land -use exclusions.

7 The efficiency gains assumed are described in Volume 3. They do not represent an upper bound of energy efficiency, i.e., they were not as large as estimated by NAS/NAE (2010).
8 The flat demand (low

- demand) projection included this increase in demand from the transportation sect or, whereas the business

- as

- usual demand (high

- demand) projection did not

. The contribution of biofuels to the transportation sector is n ot quantified in RE Futures.Rene wable Electricity Futures Study — Executive Summary 6 In all the scenarios analyzed , only currently commercially available technologies (as of 2010) were considered , together with their incremental or evolutionary improvements despite the longterm (2050) timeframe

, because the focus of this study was on grid integration and not on the potential advances of any indi vidual technologies. 9 Technologies such as enhanced g eothermal

s ystems ; o cean energy technologies (e.g., wave, tidal, current or ocean thermal); floating offshore wind technology; and others that are currently under development and pilot testing— and which show significant promise but are not yet generally commercially available—were not included.

More than two dozen scenarios were modeled and analyzed in this study as outlined in Figure ES -1 and detailed below. The number and diversity of scenarios allowed an assessment of multiple pathways that depend ed on highly uncertain future technological, institutional, and market choices. The framework included scenarios with specific renewable electricity generation levels to enable exploration of some of the techni cal issues associated with the operation of the U.S. electricity grid at these levels. 10 This scenario framework does not prescribe a set of policy recommendations for renewable electricity generation in the United States, nor does it present a vision of what the total mix of energy sources should look like in the future. Further, the framework does not intend to imply that one future is more likely than another.

9 RE Futures did not allow new nuclear plant s , fossil technologies with CCS

, as well as gasified coal without CCS (integrate d gasification combined cycle) to be built in any of the scenarios presented in this report. Existing n uclear (and integrated gasification combined cycle) units, however, were included in the analysis , as were assumptions f or the retirement of those units.
10 The scenarios were not constructed to find the optimal GHG mitigation or clean energy pathway (e.g., to minimize carbons emissions or the cost of mitigating these emissions)

. In addition, because the scenarios included specific renewable generation levels, they were not designed to explore how renewable technologies might economically deploy under certain technology advancement projections without the generation constraints

.Rene wable Electricity Futures Study — Executive Summary 7 Figure ES -1

. Modeling scenario framework for RE Futures Dotted lines indicate that the 80% RE exploratory scenarios are the same as the 80% RE

- ITI and 80% RE

- ETI scenarios

.

**Low**

**- Demand Baseline**  **S cenario** 
A Low

- Demand Baseline s cenario was designed to reflect a largely conventional generation system as a point of comparison, or reference, for the high- penetration renewable electricity scenarios. The Low

- Demand Baseline scenario assumes that a combination of emerging trends— including policies and legislation dealing with codes and standards, innovation in energy efficiency , and the green building and supply chain movements—drive the adoption of energy efficiency measures in the residential, commercial, and industrial sectors (see Volume 3 for details)

. 11 Substantial adoption of electric and plug- in hybrid electric vehicles was also assumed. In aggregate, these low-demand assumptions resulted in overall electricity consumption that exhibits little growth from 2010 to 2050. Existing state policies (e.g., renewable portfolio standards) and existing federal policies (e.g., investment tax credits, production tax credit s, tax depreciation rules ) were assumed to continue only as allowed under existing law, with no

11 In addition to these trends, the primary historical drivers of electricity demand, population growth

, and economic growth

, were also considered in the construction of the scen ario. While investment costs of these efficiency measures were not considered in the scenario development, findings from other studies generally indicate that such measures can be considered cost

- effective or cost

- competitive.Rene wable Electricity Futures Study — Executive Summary 8 extensions. Expiration dates for existing federal policies vary, but generally are 2017 or earlier. 12 In combination with incremental technology improvements, these assumptions result in low levels of renewable electricity generation in the Low

- Demand Baseline scenario.

**E**

**xplo r**  **atory S**  **cenarios**
A series of “ exploratory scenarios ,” in which the proportion of renewable electricity in 2050 increased in 10% increments from 30% to 90%, was evaluated. The primary purpose of these exploratory scenarios was to assess how increased levels of renewable electricity might impact the generation mix of renewable and non- renewable resources, the extent of transm ission expansion in these cases, and the use of various forms of supply- and demand- side flexibility to enable a match between electricity supply and demand. These exploratory scenarios were evaluated under two distinct sets of renewable electricity techno logy advancement assumptions : Incremental Technology Improvement (ITI) and Evolutionary Technology Improvement (ETI).

**C ore 80%**  **RE S**  **cenarios** Further analysis was performed on six core 80% RE scenarios, each of which met the same 80%

- by-2050 renewable electricity penetration level and each of which was designed to elucidate the possible implications of certain technological, institutional, and market drivers. 13 Three scenarios explored the impacts of future renewable energy technology advancements of cur rently commercial technologies and the resulting deployment of different combinations of renewable energy technologies 14 :

• The RE – No Technology Improvement (80% RE- NTI) scenario simply assumed that the performance of each renewable technology was maintaine d at 2010 levels for all years in the study period (2010–2050).

• The RE – Incremental Technology Improvement (80% RE- ITI) scenario reflected only partial achievement of the future technical advancements that may be possible (B lack & V ea tch 2012).

• The RE – Evolutionary Technology Improvement (80% RE- ETI) scenario reflected a

more -complete achievement of possible future technical advancements (Volume 2). The RE -ETI scenario is not designed to be a lower bound and does not span the full range of possible future s; further technical advancements beyond the RE

- ETI are possible. 15

Three additional scenarios explored the impacts of different electricity system constraints based on assumptions that limit ed the building of new transmission, reduced system flexibility to

12 Similarly, indirect incentives for conventional energy technologies

— sometimes delivered through the tax code without sunset provisions

— were assumed to be maintained as allowed under existing law. These same renewable and conventional technology policy as sumptions were consistently applied to all the other scenarios as well.
13 The specific assumptions used for these scenarios are discussed in Chapter 1.
14 Although the methods used in RE Futures to project the future cost of each renewable electricity technology differ to some degree by technology, the resulting forecasts are largely based on anticipated scientific and engineering advancements rather than on learning

- curve

- based estimates that are endogenously driven by an assumed learning rate applied to cumulative production or installation. In reality, costs may decline in part due to traditional learning and in part due to other factors, such as research and development investment, economies of scale in manufacturing, component, or plant size, and reduc tions in material costs

. 15 Indeed, current DOE initiatives are focused on achieving substantially better cost and performance in many cases

, with a target of achieving parity with conventional technologies.Rene wable Electricity Futures Study — Executive Summary 9 manage the variability of wind and solar resources, and decreased renewable resource availability :

• The Constrained Transmission scenario evaluated how limits to building new transmission might impact the location and mix of renewable resources used to m eet an 80%-by-2050 future.

• The Constrained Flexibility scenario sought to understand how institutional constraints to and concerns about managing the variability of wind and solar resources, in particular, might impact the resource mix of achieving an 80%-by-2050 future.

• The Constrained Resources s cenario posited that environmental or other concerns may reduce the developable potential for many of the renewable technologies in question, and evaluated how such constraints could impact the resource mix of r enewable energy supply.

**High**

**- Demand S**  **cenarios**
The scenarios described above—the Low

- Demand Baseline s cenario , the exploratory scenarios, and the six core 80% RE scenarios— were based on the low-demand assumptions, with overall electricity consumption that exhibits little growth from 2010 to 2050. To test the impacts of a higher-demand future, a scenario with the 80%- by -2050 renewable electricity generation but a higher end- use electri city demand was evaluated , with demand in 2050 30% higher than in the low- demand scenarios

. 16 A corresponding reference scenario , the High

- Demand Baseline scenario, with the same higher demand was also evaluated

. 17

**Alternative Fossil S cenarios**  Finally, given uncertainties in the future cost of fossil energy sources , the analysis included 80%- by-2050 RE scenarios in which: (1) the price of fossil energy (coal and natural gas) was both higher and lower than otherwise assumed in the other scenarios and (2) fossil energy technologies 18 experienced greater technology improvements over time than assumed in the other scenarios. Corresponding reference scenarios with these alternate fossil energy projections were also evaluated.

Renewable Resources Chara cterization
The United States has diverse and abundant renewable resources, including biomass, geothermal, hydropower, ocean, solar, and wind resources. Solar and wind are the most abundant of these

resources. These renewable resources are geographically c onstrained but widespread

— most are distributed across all or most of the contiguous states ( Figure ES -2). Within these broad resource types, a variety of commercially available renewable electricity generation technologies have been deployed in the United States and other countries, including stand-alone biopower, co- fired

16 The low

- demand scenarios assume an annual growth rate of 0.17%; the high

- demand scenarios assume an annual growth rate of 0.84%. Details on end

- use energy demand assumptions are provided in Volume 3 of this report.
17 For comparison, all high renewable electricity scenarios require a baseline or reference scenario that uses the same high

- level assumptions regarding electricity demand.
18 Consistent with the study’s focus on commercially available renewable generation technologies, emerging fossil and nuclear technologies, such as carbon

- capture and sequestration and modular nuclear plants, were not included.Rene wable Electricity Futures Study — Executive Summary 10 biopower (in coal plants), hydrothermal geothermal, hydropower, distributed PV, utility- scale PV, CSP, 19 onshore wind, and fixed -bottom offshore wind.

Figure ES -2

. Geographic distributi on of renewable resources in the contiguous United States The United States has potential ocean energy and enhanced geothermal resources; however, these

technologies were not modeled and therefore the resource potential is not included in this figure

.

Whi le o nly commercially available biomass, geothermal, hydropower, solar PV, CSP, and windpowered systems were considered in the modeling analysis—only incremental and evolutionary advances in renewable technologies were assumed —the study describes a broad range of commercial and emerging renewable energy technologies in Volume 2, including the following 20 :

19 In this report, CSP refers to concentrating solar thermal power. Concentrating photovoltaic technologies were not considered in the modeling analysis.
20 The renewable resource characterizations described below and used in the models are based on historical climatic
average resource patterns and have standard land area exclusions applied. A fter accounting for these standard exclusions, the aggregate renewable generation resource is many times greater than current electricity demand.Rene wable Electricity Futures Study — Executive Summary 11 • Biomass power (Chapter 6, Volume 2) is generated by collecting and combusting plant matter and using the heat to drive a steam turbine. Biomass r esources from agricultural and forest residues, although concentrated primarily in the Midwest and Southeast, are available throughout the United States. While biomass supply is currently limited, increased supply is possible in the future from increased production from energy crops and advanced harvesting technologies. DOE (2011) provides an estimate of 696 –1,184 million annual dry tonnes of biomass inventory potential (of which 52%–61% represents dedicated biomass crops) in 2030. 21 The estimated biomass fe edstocks correspond toroughly 100 GW of dedicated biopower capacity. Biopower can be generated from standalone plants, or biomass can be co -fired in traditional pulverized coal plants.

• Geothermal power (Chapter 7, Volume 2) is generated by water that is heated by hot underground rocks to drive a steam turbine. Geothermal resources are generally concentrated in the western United States, and they are relatively limited for hydrothermal technologies (36 GW of new technical resource potential), which rely on natural hot water or steam reservoirs with appropriate flow characteristics . Only commercially available hydrothermal technologies were included in the modeling analysis. Although not modeled, emerging technologies, including enhanced geothermal systems , engineered hydrothermal reservoirs, geopressured resources, low temperature

resources, or co -production from oil and gas wells , could expand the geothermal resource potential in the United States by more than 500 GW.

• Hydropower (Chapter 8, Volu me 2) is generated by using water— from a reservoir or run-of- river —to drive a hydropower turbine. Run-of-river technology could produce electricity without creating large inundated areas, and many existing dams could be equipped to generate electricity. The future technical potential of run-of- river hydropower from within the contiguous United States is estimated at 152–228 GW. Only new run-of-river hydropower capacity was considered in RE Futures modeling, and existing hydropower p lants were assumed to continue operation. Other hydropower techn ologies, such as new generation at non- powered dams and constructed waterways, have the potential to contribute to future electricity supply, but they were not modeled in this study.

• Ocean technologies (Chapter 9, Volume 2 ) ar e not broadly commercially available at this time, and therefore were not modeled in this study , but both U.S. and international research and development programs are working to reduce the cost of the technologies. Ocean current resources are best on the U.S. Gulf and South Atlantic Coasts; wave energy resources are strongest on the West Coast. All resources are uncertain; preliminary estimates indicate that the U.S. wave energy technical potential is on the order of 2,500 TWh/y r. Other ocean technologies, including ocean thermal energy conversion technologies and tidal technologies, may also contribute to future electricity supply.

21 To be conservative, for each modeled year, the analysis used feedstock estimates from Walsh et al. (2000) and Milbrandt et al. (2005), which are consistent with the low end of the DOE (2011) estimate for 2030, and did not assume any increase in resource over time; on the other hand, the analysis also did not include potential future growth in demand for biomass from the fuel sector. Maximum biopower capacity deployment w as assumed to be roughly 100 GW in this study, with 27% from dedicated biomass crops.Rene wable Electricity Futures Study — Executive Summary 12 • Solar resources (Chapter 10, Volume 2) are the most abundant renewable resources

. They extend across the entire United States , with the highest quality resources concentrated in the Southwest. The technical potential of u tility

- scale PV and CSP technologies is estimated to be approximately 80,000 GW and 37,000 GW, respectively, in the United States. Distr ibuted rooftop PV technologies are more limited, with approximately 700 GW available. PV technologies convert sunlight directly to electricity while CSP technologies collect high temperature heat to drive a steam turbine.

• Wind resources (Chapter 11, Volum e 2) on land are abundant, extending throughout the United States, and offshore resources provide additional options for coastal and Great Lakes regions. Onshore and fixed-bottom offshore technologies are currently commercially available. 22 Floating platform offshore wind technologies that could access high- quality wind resources in deeper waters are less mature and were not considered in the modeling. Wind technical resource estimates exceed 10,000 GW in the contiguous United States

. Renewable resource supp ly var ies by location and , in most cases, by the time of day and season. The electricity output characteristics of some renewable energy technologies also vary substantially, potentially introducing electric system operation challenges. A key performance

c haracteristic of generators in general is their degree of dispatchability , specifically the ability of operators to control power plant output over a range of specified output generation levels. Conventional fossil plants are considered dispatchable, to varying degrees . Several renewable generator types, including biopower, geothermal, and hydropower plants with reservoir storage, are also considered dispatchable technologies in that system operator s have some ability to specify generator output, if needed. Concentrating solar power with thermal storage can similarly be considered a dispatchable technology but is limited by the amount of storage. The output from run-of- river hydropower is generally constant over short time periods (minutes to hours) but

va ries over longer periods (days to seasons). Several emerging ocean technologies, such as

ocean

- current, may also provide fairly constant output and, in some cases, may be able to offer some level of dispatchability.

W ind and solar PV have little dispatchability

— the output from these sources can be reduced, but not increased on demand. An additional challenge is the variability and uncertainty in the output profile of these resources, with wind and solar having limited predictability over various time scale s. High levels of deployment of these generation types can therefore introduce new challenges to the task of ensuring reliable grid operation. However, it deserves note that the requirement for balanced supply and demand must be met on an aggregate basis —t he variability and uncertainty of any individual plant or load entity does not ultimately define the integration challenge associated with high levels of variable renewable generation

.
The analysis presented here focuses on electricity generation technology deployment, system operation al challenges , and implications associated with specified levels of renewable generation , which represent the total annual renewable electricity generation from commercially available biomass, geothermal, hydropower, solar, and wind electricity generating technologies.

22 Although there are no offshore wind power plants operating in the United States, a number of projects have been proposed. In addition, offshore wind is widely deployed in Europe.Rene wable Electricity Futures Study — Executive Summary 13 U.S. E lectricity Grid Expansion and Operational C haracterization RE Futures employs two key models to characterize U.S. electricity grid operations with high levels of renewable generation. The NREL Regional Energy Deployment System (ReEDS) model explores the adequacy of the geographically diverse U.S. renewable resources to meet electricity demand over future decades . T he ABB model, GridView , explores the hou rly operation of the U.S. grid with high levels of variable PV and wind generation. 23 The linked-butseparate use of the two mod els, ReEDS and GridView, allows for a rich assessment of the technical, geographic, and operational aspects of renewable energy deployment. 24

ReEDS is the analytical backbone of the study, providing estimates of the type and location of conventional and renewable resource development; the transmission infrastructure expansion requirements of those installations; and the composition and location of generation, storage, and demand-side technologies needed to maintain balance between supply and demand. ReEDS is unique among national, long -term capacity expansion models for its highly discretized regional structure and statistical treatment of the im pact of variable wind and solar resources on capacity planning and dispatch. GridView was used to supplement the ReEDS analysis by modeling the hourly operation of the power system in 2050 for a subset of the high renewable scenarios. As one of the commerc ially available production cost models used by utilities, systems operators , and industry experts, GridView enables a more detailed exploration of the operational implications of a system with high levels of renewable electricity penetration through the us e of an hourly time step, a more accurate representation of thermal generation ramp

- rate limits, and a more realistic representation of transmission power flows compared with ReEDS.

These models were used to investigate a broad portfolio of supply- and demand-side options available to increase the flexibility of the electric system, including: having dispatchable renewable or conventional generators available to supply needed electricity when there is insufficient electricity generation from variable renewa ble plants; having the ability to provide

reserves or change electricity demand through demand response (interruptible load) or transportation electric loads; deploying storage technologies for added system flexibility; and expanding the electric system tr ansmission infrastructure to move more distant electricity supply to meet the load. Geospatial diversity was also taken into account, since it can assist in the integration of variable renewable generation because wind and solar plants that are located far apart generally have a combined output profile that is less variable than the individual plant profiles. Further, wind and solar resources may be uncorrelated or even anti- correlated depending on location; if so, combining their outputs would then further reduce aggregate variability.

23 The NREL Solar Deployment Systems (SolarDS) model was also used in RE Futures to represent rooftop PV deployment.
24 In assessing high penetrations of variable renewable electricity, RE Futures addressed some aspects of electric system adequacy through statistical treatments of reserve requirements and hourly dispatch analysis; however, the analysis did not include a complete assessment of power system reliability (addressing such issues as stability, contingencies, and AC power flows). Similarly, RE Futures is not a fully detailed renewable energy integration study. Such studies typically seek to understand the impacts of variable and uncertain wind and solar generation on the operations of regional electric power systems and networks, relying o n high time

- resolution data and using methods that range from statistical analysis and production cost modeling to power flow simulations and steady state and transient grid analysis. RE Futures assessed electric system integration issues on a broader, national level, and the modeling tools used considered the variability and uncertainty of some renewable technologies, but not to the level of detail typical in integration studies.Rene wable Electricity Futures Study — Executive Summary 14 

### Key Results Deployment of Renewable Energy Technologies in High Renewable Electricity Futures
R enewable energy resources

, accessed with commercially available renewable generation technologies, could adequately supply 80% of total U.S. electricity generation in 2050 while balancing supply and demand at the hourly level

. Figure ES -3 presents estimated 2050 capacity and generation, by technology, for the exploratory scenarios. 25 Generation from wind and PV technologies is varia ble , with lower capacity factors and relatively limited di s patchability . The growing deployment of this variable generation in these scenarios, increasing with renewable electricity penetration (from 20% in the b aseline s cenario to as high as 90% at the othe r end), drives the need for a growing amount of aggregate electric generation capacity in order to meet demand. Specifically, adequate capacity from dispatchable resources is required to ensure delivery of necessary generation year-round.

C ommercially available renewable technologies were deployed in the modeling to varying degrees in the exploratory scenarios, in part to exploit geographic and temporal diversity in achiev ing high renewable electricity penetration levels . Onshore wind was found to contr ibute most significantly in these exploratory scenarios, with offshore wind becoming an increasingly important player as higher renewable electricity levels were achieved. Among the solar technologies, PV (utility -scale and rooftop, combined) was generally found to play a moresizable role than CSP under the relatively lower renewable penetration scenarios . E lectricity supply from CSP was projected to grow more rapidly under the higher renewable penetration scenarios, in part because CSP with thermal storag e provides added dispatch ability . Both dedicated and co -fired biomass were also found to contribute significantly to the renewable energy mix, with a shift from co- firing to dedicated biomass plants as renewable electricity penetration levels increase d. Geothermal and hydropower were found to contribute proportionately less than the other renewable energy sources, especially under the highest renewable electricity scenarios considered, due to assumed resource and cost constraints. 26 However, even for this limited set of geothermal and hydropower resources , capacity expansion was substantial relative to recent trends, and much of the estimated available resource potential was accessed . E nhanced geothermal systems , ocean energy, and floating platform offshore wind energy were not considered, but these technologies may offer large resource potential, additional diversity , and regional advantages if technological advancements enable commercialization

.

25 Deployment results shown in Figure ES -3 used the renewable electricity incr emental technology improvement (RE

- ITI) assumptions. Results for the RE

- ETI scenarios are included in Chapter 2.
26 The assumptions used in the analysis were particularly constraining on geothermal technologies, for which advanced technologies, such as enhanced geothermal systems that can tap large quantities of energy inside the earth, were not considered in the grid modeling. The modeling analysis focused on currently commercial technologies only.Rene wable Electricity Futures Study — Executive Summary 15 (a) Capacity mix in 2050 for the exploratory scenarios

(b) Generation mix in 2050 for the exploratory scenarios Figure ES

- 3. Installed capacity and generation in 2050 as renewable electricity levels increase (low

- demand, RE

- ITI technology improvement)Rene wable Electricity Futures Study — Executive Summary 16 All regions of the United States could contribute substantial renewable electricity supply in 2050 , consistent with their local renewable resource base

. Figure ES -4 presents the modeled location of renewable electricity generation and capacity by 2050 for one 80%-by-2050 RE scenario (80% RE

- ITI) . It also compares total regional electricity generated in 2050 to regional electricity demand (based on low-demand assumptions). In the scenario shown, wind energy supply was significant in most regions but was most prominent in the Great Plains, Great Lakes, Centr al, Northeast, and Mid Atlantic regions (with a large fraction of wind generation coming from offshore resources in the Northeast and Mid Atlantic regions). Solar energy was found to deploy most substantially in the Southwest (dominated by CSP), followed by California and Texas (CSP and PV) , and then by the Florida and the Southe ast regions (dominated by PV). Biomass supply was most significant in the Great Plains, Great Lakes, Central, and South e ast regions. Hydropower supply was most significant in the North west, but hydropower was also a sizable contributor in California, the North east, and the Southeast. Geothermal was found to deploy primarily in California and the Southw est.Rene wable Electricity Futures Study — Executive Summary 17 Figure ES

- 4. Renewable generation and capacity in 2050 b y region under 80% RE

- ITI scenario (low

- demand, RE

- ITI technology improvement )Rene wable Electricity Futures Study — Executive Summary 18 Multiple technology pathways exist to achieve a high renewable electricity future. Assumed constraints, which limit power transmission infrastructure, grid flexibility , or the use of particular types of resources can be compensated for through the use of other resources, technologies, and approaches

. The renewable energy resource base of the United States is both abundant and diverse. As a result, a central finding of the analysis is that there are many possible ways to achieve high renewable penetration level s.
For example, the technology improvement scenarios included in the six core 80% RE scenarios (Impact of Technology Improvement scenarios) sh owed that technologies that are currently at earlier stages of commercialization (e.g., solar) could achieve greater deployment if significant technology improvements were realized in the future. In contrast, if these improvements were not realized, curren tly more commercially mature technologies (e.g., onshore wind) could deploy to a greater extent. Also , a set of scenarios included in the core 80% RE scenarios explored the impacts of limits on building new transmission, constraints on the flexibility of the electric system to manage the variability of wind and solar resources, and constraints on the developable potential for many renewable technologies (Impact of System Constraints scenarios) . The mix of renewable resources deployed and th e deployment of flexible supply- and demand- side technologies were significantly impacted in these scenarios. In particular, when new transmission builds we re constrained, greater deployment was observed for resources located closer to load centers, includ ing PV, offshore wind, and biomass. A future where the flexibility of the electric system was limited resulted in a shift of renewable electricity supply away from variable wind and PV technologies and to ward more dispatchable options, particularly CSP with thermal storage, and to storage technologies. When the assumed availability of renewable energy supply was reduced —due to siting or permitting challenges, for example—the contributions from the most resource -constrained technologies (biopower, geothermal, and hydropower) declined , while more abundant wind and solar resources wer e used to a greater degree. Figure ES -5 shows the range in 2050 capacity and generat ion by technology among the six low -demand 80%-by-2050 RE scenarios examined. Although the type and quantity of renewable technologies deployed in these scenarios varied significantly, estimated direct electric sector aggregate cost

wa s relatively insensitive to most of these variations

. 27

Finally, the analysis found that the renewable resource base in the United States was sufficient to support 80% renewable electricity generation by 2050 in a higher demand growth scenario. Figure ES -5 also shows 2050 d eployment results for the High -Demand 80%-by-2050 RE scenario, which features a much greater amount of solar capacity compared with the low-demand scenarios

.

27 S ee individual technology chapters in Volume 2 for a discussion of the specific scenarios that lead to high and low estimates for each technology individually; Volume 1 provides more discussion of the operational and cost implications these scenarios.Rene wable Electricity Futures Study — Executive Summary 19 (a) 2050 installed capacity by technology (b) 2050 contribution to total generated electricity

Figure ES

- 5. Range of 2050 installed capacity and annual generated electricity by technology for the low

- demand core 80% RE scenarios and the High

- Demand 80% RE scenario

Annual renewable capacity additions that enable high renewable electricity are consistent with current global production capacities but are significantly higher than recent U.S. annual capacity additions for the technologies considered . N o insurmountable long

- term constraints to renewable electricity technology manufacturing capacity, materials supply, or labor a vailability were identified. The analysis showed that achieving high renewable electricity future s would require a sustained increase in renewable capacity additions. In the core 80% RE scenarios, average annual renewable capacity additions of 19 –22 GW/yr from 2011–2020 were estimated, increasing to a maximum rate of 32–46 GW/yr from 2041–2050. Given recent historical experience with U.S. renewable electricity capacity additions (11 GW in 2009 and 7 GW in 2010), 28 achieving these rates of deployment may pose challenges as production ramps up

, including those related to materials availability, equipment manufacturing capacity, labor needs, and project development and siting processes. However, no insurmountable long- term technical constraints to renewable tech nology manufacturing capacity, materials supply, or labor availability were identified; better informed siting practices and regulations can mitigate potential constraints related to project development and siting processes (see Chapter 3 and Volume 2).

G rowth in renewable capacity additions in the United States and globally over the last decade has been considerable , and it demonstrat es the ability to scale manufacturing and deployment at a rapid pace. The wind power additions required in the scenarios, f or example, were substantial,

28 The challenges associated with the rates of deployment presented here depend on technology. For example, renewable installations in the United States in recent years were dominated by new wind technologies; therefore, achieving the deployment rates envisioned in the scenarios for wind energy would likely be less challenging from an industry growth perspective compared with other technologies.Rene wable Electricity Futures Study — Executive Summary 20 but historical growth in manufacturing and installation suggests that manufacturing need not be a major constraint to the continued growth that would be necessary to meet an 80%-by-2050 generation level. The biopower and geothermal additions resulting from the scenario modeling, al though greater than recent historical trends, are similarly unlikely to place undue strain on supply chains. The estimated rate of PV deployment is particularly high, but PV manufacturing

an d deployment are highly scalable, and worldwide PV production capacity has been growing rapidly and is already comparable to the deployment rates projected in high renewable scenarios presented here for the United States . Moreover, many of the renewable technologies are based on common materials that are not supply

- constrained. Even for PV, which does use some materials that may be supply-constrained, worldwide production capacity is already sizable and that capacity continues to expand rapidly. In addition , alternate approaches exist to reduce dependence on supply

- constrained materials if necessary. While a comprehensive analysis of industry scale -up, including labor demands and access to critical materials, is beyond the study scope, the initial analysis d id not identify any insurmountable technical challenges associated with industry scale-up at the technology deployment levels considered.

Grid Operability and Hourly Resource Adequacy in High Renewable Electricity Futures Electricity supply and demand can be balanced in every hour of the year in each region with nearly 80% of electricity from renewable resources, including nearly 50% from variable renewable generation, according to simulations of 2050 power system operations

. 29 Although a full reliability a ssessment i s beyond the scope of this analysis, hourly production simulation did consider unit commitment with imperfect forecasts, DC optimal power flow, and thermal generator flexibility limits (e.g., ramp rates and minimum generation levels). Figure ES -6 shows nationwide dispatch by generator type during the annual peak coincident load ( Figure ES -6[a] ) and during the lowest coincident load of the year ( Figure ES -6[b]) in 2050 for a high renewable electricity scenario. The operational simulations did not project any hours of unserved load during the peak load hour, lowest coincident load hour, or any other hour of the year. 30

29 Although the capacity expansion modeling (ReEDS) planned for renewable resources to contribute 80% of annual generation in 2050, the hourly operational model (GridView) simulated roughly 75%, in part due to a lack of a renewable generation requirement. GridView model dispatch decisions were based on the variable cost of generation and did not consider the renewable or non

- renewable nature of the generation source.
30 The electric system is a complex system of systems that operates on many time scales ranging from milliseconds to years; ultimately, analyses must be conducted to address all of the potential operating aspects of future electricity generation systems as they e volve. Electric system operations are described in detail in Volume 4.Rene wable Electricity Futures Study — Executive Summary 21 (a) Summer peak load in 2050

(b) Lowest coincident load in 2050 Figure ES

- 6. Hourly dispatch stacks for the 80% RE

- ITI scenario a

a The solid black line representing “load” includes charging of electric vehicles. The broken line representing “shifted load” represents “load” minus storage. The Gas CT category includes a small number of oil

- fired units. The unit types are ordered (subjectively) from least variable or flexible (at the bottom) to most variable (at the top). 0 100 200 300 400 500 600 700 800

Jul-16 00:00 Jul-16 12:00 Jul-17 00:00 Jul-17 12:00 Jul-18 00:00 Jul-18 12:00 Jul-19 00:00 Jul-19 12:00Power (GW)

Date Curtailment Wind PV Gas CT Gas CC Hydropower CSP Biopower Coal Geothermal Nuclear Shifted Load Load

0 100 200 300 400 500 600 700 800

Apr-27 00:00 Apr-27 12:00 Apr-28 00:00 Apr-28 12:00 Apr-29 00:00 Apr-29 12:00 Apr-30 00:00 Apr-30 12:00Power (GW)

Date Curtailment
Wind
PV Gas CT Gas CC Hydropower CSP Biopower Coal Geothermal
Nuclear Shifted Load
LoadRene wable Electricity Futures Study — Executive Summary 22 Additional challenges to power system planning and operation would arise in a high renewable electricity future, including management of low-demand periods and curtailment of excess electricity generation . The hourly analysis also found that, in contrast to today’s fossil- fuel

dominated electricity system for which the time of peak load (e.g., summer afternoons ) is of most concern, operational challenges for high renewable generation scenarios were most acute during low-demand periods (e.g., spring evenings) when the abundance of renewable supply relative to demand would force thermal generators to cycle or ramp down to their minimum generation levels. 31 During low-demand periods in today’s system and in the baseline scenario

, most of the peaking needs are met with hydropower and combined cycle units; combustion turbines are needed but to a much lesser extent than in the summer. Although the load characteristics in 2050 are similar in the baseline scenario and the high renewable scenarios, during low-demand periods in the latter (e.g., Figure ES -6[b] ), there was enough aggregate renewable electricity to fully serve load , causing the net load (load minus variable wind and PV generation) to be much more variable compared to the rest of the year. This increased variability in net load creates challenges associated with gre ater power plant cycling and ramping.

A primary challenge of variable renewable energy integration at higher levels of penetration is the need at times to curtail excess electricity, particularly during periods with low electricity demands. 32 The hourly dis patch analysis estimated that overall in 2050, 8%–10% of wind, solar, and hydropower generation would need to be curtailed under an 80%-by-2050 RE scenario. Curtailments reduce capacity factors and introduce uncertainty in electricity sales , thereby negatively impacting plant economics. A variety of technical and institutional approaches could be applied to reduce these levels of curtailment. First, additional transmission capacity in congested corridors would help alleviate congestion and reduce curtailment. Second, increasing the size of reserve -sharing groups could help reduce the number of inflexible generators online to provide spinning reserves; curtailment of renewable generation could be reduced if fewer plants operate at minimum levels. Third, the flexibility of the thermal fleet could be improved , or market structures could be implemente d to encourage the operation of more flexible generators. Fourth, additional energy storage and controllable loads could be used to improve system flexibility. Finally, new or existing industries could take advantage of the low- cost electricity available during seasons or times when curtailment would have occurred, and the resulting increased demand could then consume electricity that otherwise would have been c urtailed.

Electric sector modeling shows that a more flexible system is needed to accommodate increasing levels of renewable generation. System flexibility can be increased using a broad portfolio of supply- and demand -side options and will likely require technology advances, new operating procedures, evolved business models, and new market rules

. As renewable electricity generation increased from 20% in the baseline scenario to 90 % in the exploratory scenarios, the annual contribution from variable generation (wind and solar PV) grew from 7% to 48% in 2050 . At

31 Peak load still requires management and will be challenging for the same reasons it is today, but in addition, management of low

- demand periods and curtailment will be required with h igh variable renewable electricity.
32 This situation parallels the use of combustion turbines in conventional systems, which are typically used just a few hundred hours per year to meet summer peak loads and are largely idle much of the rest of the year. A s such, both the conventional and the high renewable electricity systems operate with excess capacity most of the time. While the high renewable system generates power with the excess capacity as long as resources are available, the conventional system sim ply leaves the excess capacity idle.Rene wable Electricity Futures Study — Executive Summary 23 this high level of variable generation, ensuring a real- time balance between electricity supply and demand is more challenging The variability and uncertainty associated with these high levels of wind and PV penetration were found to be manageable through the application of adequate flexible generation capacity, the use of grid storage and demand- side technologies, the expansion of transmission infrastructure , and greater conventional plant dispatch flexibility, including significant daily ramping of fossil generators . (Dispatchable renewable technologies, like conventional technologies, do not impose significant additional challenges to system operability, and they are also used to help manage wind and solar PV integration.)

The RE Futures analysis considers reserves necessary for reliable electric system operation s, spanning a wide range of timescales (from long-term planning rese rves to short- term operating

reserves ). T he same capacity reserve margin requirements were satisfied across all scenarios despite the relatively low capacity values of variable resources and their increasing deployment as renewable penetration increased. P artly t o satisfy planning reserve requirements, greater aggregate capacity was needed in high penetration renewable scenarios (see Figure ES-3 ).

Additional operating reserves were also found to be required in high variable renewable generation systems and were accommodated through the availability of conventional power plants, storage technologies, and demand-side practices. The analysis included multiple components of operating reserves, namely contingency reserves, frequency regulation reserves, and reserves associated with imperfect forecasts of wind and PV generation. Figure ES-7 shows how operating reserve requirements increased as renewable deployment increased and the different options used to meet these requirements in the exploratory scenarios. 33 Although operating reserve requirements increase with wind and PV deployment (due to greater forecast errors), because the dispatch of existing conventional power plants declines to accommodate additional wind and PV generation, these existing conventional units were found to be more available to satisfy the necessary operating reserve requirements. In other words, a high renewable electricity future would reduce the energy-providing role of the conventional fleet and increase its reserve -providing role.

The analysis found use of storage to be an attractive option to increase electric system flexibility due to the ability of storage to shift load to better correlate with output from variable generators, reduce curtailments by storing excess generation in times of low demand, and provide firm capacity for a variety of reserve services. In the core 80% RE scenarios, for example, storage deployment was found to increase from approximately 20 GW in 2010 to 100–152 GW in 2050. Demand -side options were also found to play a significant role in meeting the integration challenges of a high renewable electricity future. For example, in the core 80% RE scenarios, 28–48 GW of demand-side interruptible load were deployed in 2050, compared with just 15.6 GW deployed in 2009.

33 In Figure ES

- 7, the total contribution to operating reserves exceeds the requirement due to the fact that only one time (summer peak) is shown, while certain reserve

- types (e.g.

, interruptible load) are annual in natu re and deployed to serve other times not shown.Rene wable Electricity Futures Study — Executive Summary 24 (a) 2050 operating reserve requirement during the summer peak by reserve type (b) 2050 c ontributions toward total operating

reserve requirement by technology type
Figure ES

- 7. Operating reserve requirements as renewable e lectricity levels increase

The RE Futures analysis suggests that variable generation levels of up to nearly 50% of annual electricity can be accommodated when a broad portfolio of supply- and demand- side fle xibility resources is available at a level substantial ly higher th an in today’s electricity system. A broad portfolio of flexible supply- and demand- side resources and options were made available in the scenario modeling , and were relied upon particularly in the high renewable generation scenarios, including:

• Maintaining sufficient capacity on the system for planning reserves

• Relying on demand

- side interruptible load, conventional generators (particularly natural gas generators), and storage to manage increased operating reserve requirements

• Mitigating curtailment with st orage and controlled charging of electric vehicles

• Operating the system with greater conventional power plant ramping

• Relying on the dispatchability of certain renewable technologies (e.g., biopower, geothermal, CSP with storage and hydropower)

• Leveraging the geospatial diversity of the variable resources to smooth output ramping

• Transmitting greater amounts of power over longer distances to smooth electricity demand profiles and meet load with remote generationRene wable Electricity Futures Study — Executive Summary 25 Achieving the system flexibility required to integrate high levels of renewable generation will require some combination of technology advances, new operating procedures, evolved business models, and new market rule s. Although the analysis does not examine how these mechanisms could be implemented , it does describe the power system flexibility characteristics needed for the integration of high levels of renewable generation.

Transmission Expansion in High Renewable Electricity Futures As renewable electricity generation increases, additional transmission infrastructure is required to deliver generation from cost

- effective remote renewable resources to load centers, enable reserve sharing over greater distances, and smooth output profiles of variable resources by enabling greater geospatial diversity

. Many of the system flexibility resources and options described above can benefit from transmission infrastructure enhancements to enable the transfer of power and sharing of reserves over large areas to accommodate the variability of wind and solar elect ricity generation in combination with variability in electricity demand. With high penetrations of variable generation, net load (load minus variable generation) in a specific region can show dramatic ramps. T ransmission between regions help s reduce ramps in net load because it allows system operators to access a larger pool and more diverse mix of variable generation, with some smoothing of output profiles and demand profiles over larger geographic areas. Figure ES -8 shows projected new transmission capaci ty deployed over the 40-year study period for the exploratory scenarios. Demands for new transmission capacity are much greater in the higher renewable generation scenarios than in lower renewable generation scenarios , outstripping the effects of the low -demand assumption, reductions in transmission use by conventional fossil generation (freeing up the lines for renewable generation), and deployment of renewable resources that are proximate to load centers (e.g., PV and offshore wind). 34 T he increase in transmission needs as renewable electricity supply grows, for all 80%-by-2050 renewable electricity scenarios, r esult in an average annual projected transmission and interconnection investment that is within the recent historical range for t otal investor- owned utility transmission expenditures in the United States (i.e., $2 billion/yr to $9 billio n/yr from 1995 through 2008) (Pfeifenberger et al. 2009).

34 The analysis assumed that the existing transmission infrastructure is operational throughout the study period and did not consider maintenance needs for the existing transmission lines or other infrastructure. In addition, the analysis did not consider distribution

- level maintenance or upgrades.Rene wable Electricity Futures Study — Executive Summary 26 Figure ES

- 8. New transmission capacity requirements in the b aseline and exploratory scenarios

Ex isting total transmission capacity in the contiguous United States is estimated at 150

– 200 million MW

- miles 35

New transmission in the high renewable electricity scenarios was found to be concentrated in the middle and southwestern regions of the United States, mainly to access the high-quality wind and solar resources in those regions and to deliver generation from those resources to load centers. For example, Figure ES -9 presents a conceptual map of new transmissio n infrastructure needed in an 80%-by-2050 scenario. As shown in Figure ES -9 and quantified in Figure ES -8, the current isolation of the three asynchronous interconnections— Western Electricity Coordinating Council ( WECC ), Electric Reliability Council of Texas ( ERCOT ), and Eastern Interconnection— was greatly reduced in many of the high renewable electricity scenarios through the expansion of AC

- DC

- AC interties. This expansion enabled the East to have greater access to the high

- quality renewable resources loca ted in the western United States, although the hourly simulations and DC transmission power flow analysis suggests that these east

- west transmission linkages were used bi

- directionally to manage temporal variations in electricity supply and demand. Expandi ng interties between the three asynchronous interconnections was found to be desirable in many of the high renewable scenarios; however, results from the Constrained Transmission scenario showed that an 80%-by-2050 RE scenario was achievable even when such expansion was not allowed.

Significant institutional obstacles, including constraints in siting new transmission lines, cost allocation concerns with transmission projects, and coordination between multiple governing entities, currently inhibit transmissi on expansion. The mechanisms to overcome these obstacles were not explored in the study, but the analysis demonstrates that additional long- distance transmission capacity can be an important characteristic of high renewable electricity futures.

35 The ReEDS model assumed 150 million MW

- miles of existing inter

- BA transmission capacity ; the 200 million MW

- mile estimate is from Homeland Security Infrastructure D atabase (2008) and other sources. 0 20,000 40,000 60,000 80,000 100,000

0 50 100 150 200 250Baseline30% RE40% RE50% RE60% RE70% RE80% RE90% REAC-DC-AC Intertie Capacity (MW)Million MW-Miles

Percent RE New Transmission New IntertieRene wable Electricity Futures Study — Executive Summary 27 Figure ES

- 9. New transmission capacity additions and conceptual location in the 80% RE

- ITI scenario

Cost and Environmental Implications of High Renewable Electricity Futures High renewable electricity futures can result in deep reductions in electric sector greenhouse gas emissions and water use. Direct environmental and social implications are associated with the high renewable futures examined, including reduced electric sector air emissions and water use resulting from reduced fossil energy consumption, an d increased land use competition and associated issues. At 80% renewable electricity in 2050, annual generation from both coal- fired and natural gas

- fired sources was reduced by about 80% , resulting in reductions in annual greenhouse gas emissions of about 80% (on a direct combustion basis and on a full life cycle basis) and in annual power sector water use of roughly 50%. At 80% renewable electricity, gross land- use impacts associated with renewable generation facilities, storage facilities, and transmissi on expansion totaled less than 3% of the land area of the contiguous United States. 36

The direct incremental cost associated with high renewable generation is comparable to published cost estimates of other clean energy scenarios. Improvement in the cost a nd performance of renewable technologies is the most impactful lever for reducing this incremental

cost. The retail electricity price implication s estimated for the 80%-by-2050 RE scenarios are comparable to those seen in other studies with similarly trans formative electricity futures

, as shown on Figure ES-10. Low carbon and clean energy scenarios, evaluated by the U.S. Energy

36 Net land

- use impacts, considering the implications of reduced conventional generation, and land

- use impacts based on disrupted lands, are both expected to be smaller. As an example of the latter case, disrupted land would generally be less than 5% of gross land area for wind generation facilities.Rene wable Electricity Futures Study — Executive Summary 28 Information Administration (EIA) and the U.S. Environmental Protection Agency (EPA), with avoided carbon emissions trajectories similar to the core 80% RE scenarios showed increases in average retail electricity prices (relative to their own reference scenarios) in 2030 of $9

– $26/MWh, rising to $41–$53/MWh by 2050. These studies generally considered a portfolio of clean generation tec hnology options, including renewable, nuclear, and low emissions fossil. The estimated incremental price impacts of the core 80% RE scenarios are comparable to these estimates.

Figure ES

- 10 . Average increase in retail electricity rates relative to study- specific reference/baseline scenarios EIA 2011a and 2011b document analysis of clean energy scenarios. EIA 2009, EPA 2009, EIA 2010, and EPA 2010 report on analysis of several low carbon emissions scenarios.

As with these other clean generation scenarios that would represent a nearly wholesale transformation of the U.S. electricity system , the h i gh renewable generation scenarios examined show a direct incremental cost relative to the continued evolution of today’s conventional generation- dominated system. Higher electricity price s associated with the high renewable scenarios are driven by replacement of existing generat ion plants with new generators (mostly renewable); additional balancing requirements reflected in expenditures for combustion turbines,
stor age, and transmission; and the assumed higher relative capital cost of renewable generation, compared to conventional technologies, assumed in the analysis. The increased capital investments associated with these drivers, compared to the baseline scenario, were not fully offset by cost savings associated with lower fossil energy consumption. The incremental cost does not include investments in energy efficiency implied by low electricity demand assumptions, or the savings in avoided generation resulting from these investments. Further, the incremental cost estimate does not consider indirect societal costs associated with the scenarios (e.g., associated with the greenhouse gas emissions described above), or economy- wide impacts

. $0 $10 $20 $30 $40 $50 $60

2010 2020 2030 2040 2050Real 2009$/MWh

Year Increase in retail electricity price relative to reference/baseline Core 80% RE (ReEDS) EIA 2009c EPA 2009a EIA 2010b EPA 2010 EIA 2011a EIA 2011bRene wable Electricity Futures Study — Executive Summary 29 A dvancements in renewable tec hnologies, reflected by technology cost and performance improvement assumptions, had the greatest impact on the incremental cost of the high renewable generation scenarios. For example, the low end of the range of incremental electricity price shown in Fig ure ES

- 10 reflects the scenario with the highest assumed renewable technology improvement (RE -ETI), while the high end reflects the lowest technology improvement scenario (RE

- NTI). 37 Assumed system constraints had more modest impact on direct incremental co sts; scenarios reflecting constraints to transmission expansion, renewable resources, and grid flexibility all had similar costs, which fell well within the bounds identified in Figure ES -10. Finally, incremental costs we re largely insensitive to differences in projections for fossil fuel prices and fossil technology improvements.

The lower renewable generation levels examined in the exploratory scenarios showed lower incremental 2050 retail electricity prices. For example, the 30% RE scenario und er highest technology improvement assumptions (RE-ETI) showed no price increase in 2050 relative to the baseline scenario (which used RE

- ITI assumptions). This result suggests that significant expansion of renewable generation beyond the 2010 level (10% of total generation) could be achieved with little or no incremental cost, assuming evolutionary improvements in renewable technologies.

T here are significant inherent uncertainties with respect to future electricity demand, technology improvements, fossil energy prices, social and institutional choices, and regulatory and legislative actions related to the scenarios examined that, in turn, contribute to significant uncertainty in the implications reported above. Further, there are a variety of indirect (or downstream) implications that may result from the direct electric sector cost, environmental, and social implications identified. For example, incremental investment in generation capacity and associated infrastructure will have implications related to eco nomic activity and employment in the energy industry. Reductions in fossil energy consumption will have environmental implications beyond air emissions, including implications related to water quality, terrestrial and marine contamination, and waste disposal, not only associated with electricity generation facilities but also for activities related to fuel extraction and transportation. Further, air emissions reductions will have implications for human health and climate change. Identification, and in some cases quantification, of these indirect implications is an active area of wide -ranging research. This analysis does not attempt to evaluate these indirect impacts of high renewable electricity futures. Further research is critically needed to systematicall y assess the relative impacts of different forms of energy supply in the context of a robust comprehensive framework that assesses both direct and indirect impacts. Such research could inform national energy policy decisions as well as local siting and permitting processes related to proposed generation facilities and supporting infrastructure.

Effects of Demand Growth on High Renewable Electricity Futures With higher electricity demand growth, high levels of renewable generation present increased

resource and grid integration challenges. RE Futures did not explicitly evaluate the cost effectiveness of energy efficiency adoption compared with supply-side options. However, the

37 The RE

- ETI assumptions are based on evolutionary improvements to currently commercial technologie s and do not reflect DOE activities to further lower renewable technology costs so that they achieve parity with conventional technologies.Rene wable Electricity Futures Study — Executive Summary 30 analysis suggests that under a high -demand scenario, greater and more rapid deployment of renewable and other supply- and demand-side technologies would be required. For example, wh ile 32–46 GW/yr of renewable capacity additions were estimat ed from 2041 to 2050 in the low- demand core 80% RE scenarios, approximately 66 GW/yr would be needed during the same time period under a more-traditional, higher- demand trajectory . The analysis also found that in the 80%-by-2050 renewable electricity high-demand scenario, variable resources (wind and PV) were deployed to a greater extent in absolute and percentage terms than they were in the low

demand scenarios due to the greater resource available for wind and solar generation compared with other forms of renewable generation . As a consequence, additional flexible supply - and demand-side technologies, such as storage facilities, natural gas combustion turbine power plants, and interruptible load , were deployed and g reater transmission expansion was needed to connect remotely located renewable resources of all types.

Higher end- use electricity demand increased the environmental impacts from the electric sector, such as greater greenhouse gas emissions, water use for thermoelectric cooling, and land use. In addition, higher demand growth also resulted in a greater increase in electric ity prices. For example, in the High- Demand 80% RE scenario, the average annual retail electricity price increased by 1.3% per year (2011–2050, in real dollar terms) compared with 1.1% per year in the (lo w-demand) 80% RE- ITI scenario. 38 The increase in reta il electricity prices driven by higher demand growth is not restricted to the high renewable penetration scenarios ; it is evident under the baseline scenario as well . In particular, the average annual retail electricity price increased by 0.6% per year (2011– 2050, in real dollar terms) in the High

- Demand Baseline scenario compared with 0.3% per year in the Low

- Demand Baseline scenario. While these results indicate that higher demand growth would lead to greater electricity price increases, they also demonst rate that the direct incremental costs associated with high renewable generation levels actually decreased under higher demand growth.



### Conclusions The RE Futures study assesses the extent to which future U.S. electricity demand could be supplied by commercially available renewable generation technologies—including wind, utilityscale and rooftop PV, CSP, hydropower, geothermal, and biomass—under a range of assumptions for generation technology improvement, electric system operational constraints, and electricity demand . W ithin the limits of the tools used and scenarios assessed, hourly simulation analysis indicates that estimated U.S. electricity demand in 2050 could be met with 80% of generation from renewable energy technologies with varying degrees of dispatchability together with a mix of flexible conventional generation and grid storage , additions of transmission, more responsive load s, and foreseeable changes in power system operations . While the analysis was based on detailed geospatially rich mo deling down to the hourly timescale, the study is subject to many limitations both with respect to modeling capabilities and the many assumptions required about inherently uncertain variables, including future technological advances, institutional choices, and market conditions. Nonetheless, the analysis shows that r ealizing this significant transformation of the electricity sector would require:

38 To isolate the effect of demand growth, the High

- Demand 80% RE Scenario is compared with the 80% RE

- ITI scenario s ince they both relied on the same technology improvement projection and used the same assumptions related to transmission, system flexibility, and renewable resources.Rene wable Electricity Futures Study — Executive Summary 31 • Sustained build- up of many renewable resources in all regions of the United States

• Deployment of an appropriate mix of renewable technologies from the abundant and diverse U.S. renewable resource supply in a way that accommodates institutional or operational constraints to the electricity system, including constraints to transmission expansion, system flexibility, a nd resource accessibility

• Establishment of mechanisms to ensure adequate contribution to planning and operating reserves from conventional generators, dispatchable renewable generators, storage, and demand-side technologies

• Increasing the flexibility of the electric system through the adoption of some combination of storage technologies, demand -side options, ramping of conventional generation, more flexible dispatch of conventional generators, energy curtailment, and transmission

• Expansion of transmission i nfrastructure to enable access to diverse and remote resources and greater reserve sharing and balancing over larger geographic areas

. These general requirements indicate that many aspects of the electric system may need to evolve substantially for high levels of renewable electricity to be deployed. Significant further work is needed to improve the understanding of this potential evolution, such as the following:

• A comprehensive cost -benefit analysis to better understand the economic and environmental implications of high renewable electricity futures relative to today’s electricity system largely based on conventional technologies and al ternative futures in which other sources of clean energy are deployed at scale

• Further investigation of the more complete set of issues around all aspects of power system reliability because RE Futures only partially explores the implications of high penet rations of renewable energy for system reliability

• Improved understanding of the institutional challenges associated with the integration of high levels of renewable electricity , including development of market mechanisms that enable the emergence of flexi ble technology solutions and mitigate market risks for a range of stakeholders, including project developers

• Analysis of the role and implications of energy research and development activities in accelerating technology advancements and in broadening the p ortfolio of economically viable future renewable energy supply options and supply- and demand- side flexibility tools.Rene wable Electricity Futures Study — Executive Summary 32 

### Appendix. Project Participants and Contributors The National Renewable Energy Laboratory coordinated preparation of the RE Futur es report, which is the culmination of contributions from more than 110 individuals and more than 35 organizations representing industry, utilities, universities, electric system operators and transmission organization, energy regulators, the government, and other sectors. The Project Leaders regret the inadvertent omission of any project participants and contributors , whether their input to the project involved contributing to project design, conducting analysis, authoring or contributing content, reviewing manuscript drafts, or producing the report.

**Project Sponsor**
Sam Baldwin U.S. Department of Energy

**Project Leaders**
Maureen Hand National Renewable Energy Laboratory Edgar DeMeo Renewable Energy Consulting Services, Inc. James Newcomb National Renewable Energy Laboratory , formerly Gian Porro National Renewable Energy Laboratory John M. Reilly Massachusetts Institute of Technology

**Project Steering Committe e Members**  Doug las Arent , Chair National Renewable Energy Laboratory Jessica Bian North American Elect ric Reliability Corporation Aaron Bennett * North American Electric Reliability Corporation Mark Brownstein Environmental Defense Fund Nils Mellquist Deutsche Asset Management Clark W. Gellings Electric Power Research Institute Kurt J. Haeger Xcel Energy Jo hn (Skip) Laitner American Council for an Energy- Efficient Economy Douglas Larson Wester n Interstate Energy Board James Lyons Novus Energy Partners Ben Paulos Energy Foundation Katie McCormack * Energy Foundation Mark O'Malley University College Dublin Dale W. Osborn Midwest Independent System Operator Frederick Weston The Regulatory Assistance Project

**Project Steering Committee Observers** David Andrejcak Federal Energy Regulatory Commission Sasan Jalali Federal Energy Regulatory Commission Dharmendra (Dave) Sharma Federal Energy Regulatory Commission Monica Taba Federal Energy Regulatory Commission Mary Beth Tighe Federal Energy Regulatory Commission Eric Sedina Federal Energy Regulatory CommissionRene wable Electricity Futures Study — Executive Summary 33 Sam Baldwin U.S. Department of Energy Larry Mansueti U.S. Department of Energy David Meyer U.S. Department of Energy John Schnagle U.S. Department of Energy Matt Clouse U.S. Environmental Protection Agency

* Alternate for organization represented.
**Report Editors**
Maureen Hand National Renewable Energy Labor atory Sam Baldwin U.S. Department of Energy Edgar DeMeo Renewable Energy Consulting Services, Inc. John M. Reilly Massachusetts Institute of Technology Trieu Mai National Renewable Energy Laboratory Douglas Arent National Renewable Energy Laboratory Gian Porro National Renewable Energy Laboratory Mike Meshek National Renewable Energy Laboratory Debra Sandor National Renewable Energy Laboratory

**Report Authors and Contributors** For each volume, chapter, or appendix, lead authors are shown in bold . Authors are listed in italics . All others listed were contributors to the report.

Volume 1. Exploration of High

- Penetration Renewable Electricity Futures For the suggested citation of Volume 1, see the title page of this volume.

**Executive Summary** Trieu Mai National Renewable Energy Laboratory Debra Sandor National Renewable Energy Laboratory Ryan Wiser Lawrence Berkeley National Laboratory Thomas Schneider National Renewable Energy Laboratory

**Introduction, Ch**  **apter 1–**  **5, and Conclusions** Trieu Mai National Ren ewable Energy Laboratory Ryan Wiser Lawrence Berkeley National Laboratory Gregory Brinkman National Renewable Energy Laboratory Paul Denholm National Renewable Energy Laboratory Donna J. Hostick Pacific Northwest National Laboratory Debra Sandor National Renewable Energy Laboratory Galen Barbose Lawrence Berkeley National Laboratory Naim Darghouth Lawrence Berkeley National Laboratory Easan Drury National Renewable Energy Laboratory Erik Ela National Renewable Energy Laboratory Jeffrey Logan National Renewable Energy Laboratory Matthew Mowers National Renewable Energy Laboratory, formerly Walter Short National Renewable Energy Laboratory, formerly Patrick Sullivan National Renewable Energy LaboratoryRene wable Electricity Futures Study — Executive Summary 34 Lan Trinh ABB Ethan Warner National Renewable Energy Laboratory Mary Werner National Renewable Energy Laboratory Jinxiang Zhu ABB

Appendix A. Implications of High

- Penetration Renewable Electricity Futures Ryan Wiser Lawrence Berkeley National Laboratory Trieu Mai National Renewable Energy Laboratory Naim Da rghouth Lawrence Berkeley National Laboratory Paul Denholm National Renewable Energy Laboratory Garvin Heath National Renewable Energy Laboratory Gian Porro National Renewable Energy Laboratory Adam Schlosser Massachusetts Institute of Technology Ken Strzepek Massachusetts Institute of Technology Jonathan Baker Massachusetts Institute of Technology , formerly Galen Barbose Lawrence Berkeley National Laboratory Jordan Macknick National Renewable Energy Laboratory Andrew Mills Lawrence Berkeley National L aboratory Jon Pietruszkiewicz Black & Veatch Debra Sandor National Renewable Energy Laboratory

Appendix B. Model s Used in RE Futures Gregory Brinkman National Renewable Energy Laboratory Trieu Mai National Renewable Energy Laboratory

Appendix C. Methods for Estimating Life Cycle GHG Emissions for RE F utures Scenarios and Discussion of Limitations Garvin Heath National Renewable Energy Laboratory Stacey Dolan National Renewable Energy Laboratory, formerly John Burkhardt National Renewable Energy Laboratory, formerly Elliot Cohen National Renewable Energy Laboratory, formerly Margaret Mann National Renewable Energy Laboratory Patrick O’Donoughue National Renewable Energy Laboratory Pamala Sawyer National Renewable Energy Laboratory, formerly Martin Vorum National Renewable Energy Laboratory Ethan Warner National Renewable Energy Laboratory

Appendix D. Project Participants and Contributors The Project Leaders compiled and reviewed this appendix of project participants and contributors.Rene wable Electricity Futures Study — Executive Summary 35 Volume 2. R enewable Electricity Generation and Storage Technologies For suggested citations of Volume 2 and the chapters within it, see the title page of Volume 2.

**Introduction** Debra Sandor National Renewable Energy Laboratory

**Chapter 6. Biopower Technologies** Richard Bain National Renewable Energy Laboratory Paul Denholm National Renewable Energy Laboratory Garvin Heath National Renewable Energy Laboratory Trieu Mai National Renewable Energy Laboratory Suzanne Tegen National Renewable Energy Laboratory Mark Downing Oak Ridge National Laboratory Scott Haase National Renewable Energy Laboratory Jordan Macknick National Renewable Energy Laboratory Margaret Mann National Renewable Energy Laboratory Dave O'Connor Electric Power Research Institute Robert Perlack Oak Ridge National Laboratory

**Chapter 7. Geothermal Technologies**  Chad Augustine National Renewable Energy Laboratory Paul Denholm National Renewable Energy Laboratory Garvin Heath National Re newable Energy Laboratory Trieu Mai National Renewable Energy Laboratory Suzanne Tegen National Renewable Energy Laboratory Katherine Young National Renewable Energy Laboratory Arlene Anderson U.S. Department of Energy Jordan Macknick National Renewable Energy Laboratory Clay Nichols U.S. Department of Energy Greg Stillman U.S. Department of Energy

**Chapter 8. Hydropower Technologies** Douglas G. Hall Idaho National Laboratory Norman A. Bishop Knight Piésold and Co. Glenn F. Cada Oak Ridge National Laboratory Trieu Mai National Renewable Energy Laboratory Stephen R. Brown HDR|DTA Jordan Macknick National Renewable Energy Laboratory

**Chapter 9. Ocean Energy Technologies** Robert Thresher National Renewable Energy Laboratory Paul Denholm National Renewable Energy Laboratory George Hagerman Virginia Polytechnic Institute and State University Garvin Heath National Renewable Energy Laboratory Sean O’Neil Ocean Renewable Energy Coalition Joshua Paquette Sandia National LaboratoriesRene wable Electricity Futures Study — Executive Summary 36 Debra Sandor National Renewable Energy Laboratory Suzanne Tegen National Renewable Energy Laboratory

**Chapter 10. Solar**  **Energy Technologies** Easan Drury National Renewable Energy Laboratory Robert Margolis National Renewable Energy Laboratory Paul Denholm National Renewable En ergy Laboratory Alan C. Goodrich National Renewable Energy Laboratory Garvin Heath National Renewable Energy Laboratory Trieu Mai National Renewable Energy Laboratory Suzanne Tegen National Renewable Energy Laboratory Michael Coddington National Renewable Energy Laboratory Barry Friedman National Renewable Energy Laboratory Jordan Macknick National Renewable Energy Laboratory Mark Mehos National Renewable Energy Laboratory Craig Turchi National Renewable Energy Laboratory

**Chapter 11. Wind**  **Energy Technologies** Jamie Chapman Vestas Wind Systems/Texas Tech University Eric Lantz National Renewable Energy Laboratory Paul Denholm National Renewable Energy Laboratory Fort Felker National Renewable Energy Laboratory Garvin Heath National Renewable Energy Laboratory Trieu Mai National Renewable Energy Laboratory Suzanne Tegen National Renewable Energy Laboratory Jordan Macknick National Renewable Energy Laboratory Craig Mataczynski RES Americas Andrew Oliver RES Americas Paul Veers Sandia National Laboratories /National Renewable Energy Laboratory Lawrence D. Willey General Electric/Clipper Windpower Ryan Wiser Lawrence Berkeley National Laboratory

**Chapter 12. Energy Storage**  **Technologies** Paul Denholm National Renewable Energy Laboratory Steven J. Fernandez Oak Ridge National Laboratory Douglas G. Hall Idaho National Laboratory Trieu Mai National Renewable Energy Laboratory Suzanne Tegen National Renewable Energy Laboratory Chaitanya Baone Oak R idge National Laboratory Norman A. Bishop Knight Piésold Stan ton W. Hadley Oak R idge National Laboratory John D. Kueck Oak R idge National Laboratory

**Appendix E. Supplementa**  **l Information for Biopower Technologies**
Richard Bain National Renewable Energy LaboratoryRene wable Electricity Futures Study — Executive Summary 37 **Appendix F. Supplemental Information for Wind Energy Technologies**  Jamie Chapman Vestas Wind Systems/Texas Tech University Eric Lantz National Renewable Energy Laboratory Donna Heimiller National Renewable Energy Laboratory Jordan Macknick National Renewable Energy Laboratory

Volume 3. Electricity Use in 2050 For the suggested citations of Volume 3, see the title page of Volume 3

.
**Introduction, Chapters 12** 

**– 20**  **, and Conclusions** Donna J. Hostick Pacific Northwest National Laboratory David B. Belzer Pacific Northwest National Laboratory Stanton W. Hadley Oak R idge National Laboratory Tony Markel National Renewable Energy Laboratory Chris Marnay Lawrence Berkeley National Laboratory Michael Kintner

- Meyer Pacific Northwest National Laboratory Daryl R. Brown Pacific Northwest National Laboratory Gonçalo Cardoso Lawrence Berkeley National Laboratory Peter Chan Lawrence Berkeley National Laboratory James A. Dirks Pacific Northwest National Laboratory Steven Doig Rocky Mountain Institute Trucy Phan Lawrence Berkeley National Laboratory

**Appendixes G–N**  Donna J. Hostick Pacific Northwest Natio nal Laboratory David B. Belzer Pacific Northwest National Laboratory Stanton W. Hadley Oak R idge National Laboratory Tony Markel National Renewable Energy Laboratory Chris Marnay Lawrence Berkeley National Laboratory Michael Kintner

- Meyer Pacific Northwest National Laboratory Daryl R. Brown Pacific Northwest National Laboratory Gonçalo Cardoso Lawrence Berkeley National Laboratory Peter Chan Lawrence Berkeley National Laboratory Trucy Phan Lawrence Berkeley National Laboratory

Volume 4. Bulk Electric Power System Operations Strategies and Transmission Planning For the suggested citations of Volume 4, see the title page of Volume 4

. **Introduction, Chapters 21**

**– 25, and Conclusions**  Michael Milligan National Renewable Energy Laboratory Erik Ela National Renewable Energy Laboratory Jeff Hein National Renewable Energy Laboratory, formerly Thomas Schneider National Renewable Energy Laboratory Gregory Brinkman National Renewable Energy Laboratory Paul Denholm National Renewable Energy Laboratory Jay Caspary Southwest Power Pool Lynn Coles National Renewable Energy LaboratoryRene wable Electricity Futures Study — Executive Summary 38 Brendan Kirby Consult Kirby Trieu Mai National Renewable Energy Laboratory Andrew Mills Lawrence Berkeley National Laboratory J. Charles Smith Utility Wind Integration Group Lan Trinh ABB

**Report Coordinators, Technical**  **Editors**
**, Report**  **Designers, and Report Producers**  Mike Meshek, Report Coordinator National Renewable Energy Laboratory Debra Sandor, Report Coordinator National Renewable Energy Laboratory Jørn Aabakken National Renewable Energy Laboratory Joshua Bauer National Renewable Energy Laboratory Linda Bevard PWT Communications Michelle Dorsett PWT Communications Devin Egan National Renewable Energy Laboratory Josh Finn Black & Veatch Bill Gillies National Renewable Energy Laboratory Scott Gossett National Renewable Energy Laboratory Sara Havig National Renewable Energy Laboratory Al Hicks National Renewable Energy Laboratory Jennifer Josey National Renewable Energy Laboratory Carol Kerstner National Renewable Energy Labora tory Michelle Kubik National Renewable Energy Laboratory Cezanne Murphy- Levesque PWT Communications Anthony Lopez National Renewable Energy Laboratory Mary Lukkonen National Renewable Energy Laboratory Jordan Macknick National Renewable Energy Laboratory Alexis Powers National Renewable Energy Laboratory Stephanie Price National Renewable Energy Laboratory Billy Roberts National Renewable Energy Laboratory Rachel Sullivan National Ren ewable Energy Laboratory Mark Swisher The Office of Mark Swisher Lisa J. Teman All the Right Words Patricia Weis -Taylor PWT Communications

**Interim Review Meeting Facilitators** Jennifer Downes-Angus Energetics Incorporated Richard Scheer Energetics Incorpor ated Peggy Welsh Energetics IncorporatedRene wable Electricity Futures Study — Executive Summary 39 **Report Reviewers** Comments provided by reviewers represent only the opinions of the individual reviewers, and they do not imply agreement with the conclusions of the report. Reviewers did not help to draft the chapters they reviewed. Their participation is not meant to imply that they or their respective organizations either agree or disagree with the findings of the effort.

Mark Ahlstrom WindLogics Arlene Anderson U.S. Department of Energy Jay Apt Carnegie Mellon University William Ball SOCO Mike Bahrman ABB Floyd Barwig New York State Department of Public Service Roger Bedard private consultant David Bellman American Electric Power, formerly Jessica Bian North American Electr ic Reliability Corporation Doug Blankenship Sandia National Laboratories Matt Bowen U.S. Department of Energy John Boyes Sandia National Laboratories, retired Gerry Braun University of California, Davis Marilyn Brown Georgia Institute of Technology Ron Brown American Forest & Paper Association Mark Brownstein Environmental Defense Fund Caitlin Callaghan AAAS Fellow at the U.S. Department of Energy Echo Cartwright New York Independent System Operator Gerry Cauley North American Electric Reliability Corporati on Luis Cerezo PGS Steven G. Chalk U.S. Department of Energy Robert Charles Sargent & Lundy Kerry Cheung AAAS Fellow at the U.S. Department of Energy Helena Chum National Renewable Energy Laboratory Charlton Clark U.S. Department of Energy Steve Clemmer Union of Concerned Scientists Matt Clouse Environmental Protection Agency Jessica Collins Xcel Energy James Critchfield U.S. Environmental Protection Agency Matt Crozat U.S. Department of Energy Mike DeAngelis Sacramento Municipal Utility District Paul DeC otis Long Island Power Authority John DeFusco Babcock & Wilcox Company Chris DeMarco University of Wisconsin– Madison Joseph (Phil) Dipietro U.S. Department of Energy Ron DiPippo University of Massachusetts Dartmouth Wayne Dyok Tetra Tech Joseph Eto Lawrenc e Berkeley National Laboratory Phil Farese National Renewable Energy Laboratory Ronald Flood APS (Arizona Public Service)Rene wable Electricity Futures Study — Executive Summary 40 Suresh Garimella Purdue University Bobi Garrett National Renewable Energy Laboratory Clark Gellings Electric Power Research Institute Eric Gimon AAAS Fellow at the U.S. Department of Energy Avi Gopstein U.S. Department of Energy Jason Grumet Bipartisan Policy Center Bryan Hannegan Electric Power Research Institute Zia Haq U.S. Department of Energy Liangying Lynn Hecker MISO (Midwest Independent Transmission System Operator) Mike Henderson ISO New England Michael Hogan European Climate Foundation Douglas Hollett U.S. Department of Energy Brian Holmes University College Cork John Holmes University of Oxford, U.K., and European Academies Science Advisory Council Energy Programme (EASAC

- EP) John Huckerby Aotearoa Wave and Tidal Energy Association Keith Jamison Energetics Incorporated Henry Jeffrey University of Edinburgh Thomas Johansson Lund University Susanne Jones Pacific Northwest National Laboratory Frederic Jouve EDF Jordan Kislear U.S. Department of Energy Elmar Kriegler Potsdam Institute for Climate Impact Research Robert Kondziolka Salt River Project Sven Kullander Uppsala University, Sweden, and EASAC- EP John “Skip” Laitner Am erican Council for an Energy Efficient Economy Doug Larson Western Interstate Energy Board Mark Lauby North American Electric Reliability Corporation Elliott Levine U.S. Department of Energy Joshua Linn Massachusetts Institute of Technology, formerly Rober t Litwin Pratt & Whitney Peter Lund Helsinki University, Finland, and EASAC- EP John Lushetsky U.S. Department of Energy, formerly Seungwook Ma U.S. Department of Energy/American Association for the Advancement of Science Enzo Marano The Ohio State Universi ty Ralph Masiello KEMA Jim McCalley Iowa State University Jeff Mechenbier PNM (Public Service of New Mexico) Amir Mikhail Clipper Windpower Rick Miller HDR|DTA JoAnn Milliken U.S. Department of Energy Natalie Mims Rocky Mountain Institute, formerly Roy Mink Mink GeoHydroRene wable Electricity Futures Study — Executive Summary 41 Fred Moore Dow Chemical Joe Morabito Alcatel -Lucent Andrew Munro Grant County Public Utility District Kelly Murphy Electric Utility Consultants, Inc. (EUCI) Jay Nathwani U.S. Department of Energy Robin Newmark National Renewable Energy Laboratory Bradley Nickell Western Electricity Coordinating Council David Olsen Western Grid Group Michael Ornetzeder Austrian Academy of Sciences, Austria, and EASAC

EP Dale Osborn Midwest ISO Christina Palmero New York State Department of Public Service W illiam Parks U.S. Department of Energy Jeff Peterson New York State Energy Research & Development Authority John Peterson U.S. Department of Energy, formerly Cedric Philibert International Energy Agency Brian Polagye University of Washington Alojz Poredos University of Ljubljana, Slovenia, and EASAC- EP Michael Power University College Dublin Mirko Previsic RE Vision Consulting Veronika Rabl Vision & Results Valerie Reed U.S. Department of Energy Brent Rice National Renewable Energy Laboratory Giorgio Rizzoni The Ohio State University Curt Robinson Geothermal Resources Council David Rosner Bipartisan Policy Center Jurriaan Ruys McKinsey & Company Kent Saathoff Electric Reliability Council of Texas Doug Sabo U.S. Bureau of Reclamation Mike Sale MJ Sales Associates Elizabeth Salerno American Wind Energy Association John Scahill Thermal Biofuels Consultants Ferd Schuth Max

- Planck -Institut fur Kohlenforschung, Germany & EASAC

- EP Timothy Skone U.S. Department of Energy J. Charles Smith Utility Variable Genera tion Integration Group Paul Smith University College Dublin Steven J. Smith Pacific Northwest National Laboratory Neil Snyder National Renewable Energy Laboratory Jeb Stenhouse U.S. Environmental Protection Agency Don Stevens Pacific Northwest National La boratory John Stough Exelon Morgan Summers U.S. Department of Energy Fred Tornatore TSS Consultants Jessica Trancik Massachusetts Institute of TechnologyRene wable Electricity Futures Study — Executive Summary 42 Rachel Tronstein U.S. Department of Energy Elaine Ulrich U.S. Department of Energy Eugenijus Uspuras Lithuanian Energy Institute, Lithuania, and EASAC- EP Jan S. Vaagen IFT University of Bergen, Norway, and EASAC- EP Carl VanSant HCI Partners Uday Varadarajan Climate Policy Initiative Luis Vega University of Hawaii Vijay Vittal Arizona State University James Walker enXco, Inc. Joy Wang Georgia Institute of Technology Yu Wang Georgia Institute of Technology Mike Weedall Bonneville Power Administration Carl Weinberg Pacific Gas and Electric Company, retired John Weyant Stanford University Colin Williams U. S. Geological Survey Marshall Wise Pacific Northwest National Laboratory Xiaojing Xun Georgia Institute of Technology Jose Zayas U.S. Department of Energy Ken Zweibel The George Washington UniversityRene wable Electricity Futures Study — Executive Summary 43 

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NREL is a national laboratory of the U.S. Department of Energy Office of Energy Efficiency and Renewable Energy Operated by the Alliance for Sustainable Energy, LLC NREL/TP-6A20-52409-ES • December 2012

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