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Contract No. DE-AC36-08GO28308

Rethinking the Future Grid: 

Integrated Nuclear Renewable 

Energy Systems

Preprint

S.M. Bragg-Sitton and R. Boardman

Idaho National Laboratory

M. Ruth and O. Zinaman

National Renewable Energy Laboratory

C. Forsberg

Massachusetts Institute of Technology

Presented at the 9th Nuclear Plants Current Issues Symposium: 

Moving Forward

Charlotte, North Carolina

December 7–10, 2014

Conference Paper

NREL/CP-6A20-63207

January 2015
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RETHINKING THE FUTURE GRID:

INTEGRATED NUCLEAR-RENEWABLE ENERGY SYSTEMS

S.M. Bragg-Sitton,

1 R. Boardman,2 M. Ruth,3 O. Zinaman,4 and C. Forsberg5

ABSTRACT

The 2013 electricity generation mix in the United States consisted of ~13% renewables 

(hydropower, wind, solar, geothermal), 19% nuclear, 27% natural gas, and 39% coal [1]. In the 

2011 State of the Union Address, President Obama set a clean energy goal for the nation: “By 

2035, 80 percent of America’s electricity will come from clean energy sources. Some folks want 

wind and solar. Others want nuclear, clean coal and natural gas. To meet this goal we will need 

them all.” The U.S. Department of Energy (DOE) Offices of Nuclear Energy (NE) and Energy 

Efficiency and Renewable Energy (EERE) recognize that “all of the above” means that we are 

called to best utilize all available clean energy sources. To meet the stated environmental goals 

for electricity generation and for the broader energy sector, there is a need to transform the 

energy infrastructure of the U.S. and elsewhere. New energy systems must be capable of

significantly reducing environmental impacts in an efficient and economically viable manner 

while utilizing both hydrocarbon resources and clean energy generation sources. 

The U.S. DOE is supporting research and development that could lead to more efficient 

utilization of clean energy generation sources, including renewable and nuclear options, to meet 

both grid demand and thermal energy needs in the industrial sector. One concept under

consideration by the DOE-NE and DOE-EERE is tighter coupling of nuclear and renewable 

energy sources in a manner that better optimizes energy use for the combined electricity, 

industrial manufacturing, and transportation sectors. This integration concept has been referred 

to as a “hybrid system” that is capable of apportioning thermal and electrical energy to first meet 

the grid demand (with appropriate power conversion systems), then utilizing excess thermal and, 

in some cases, electrical energy to drive a process that results in an additional product. 

For the purposes of the present work, the hybrid system would integrate two or more energy 

resources to generate two or more products, one of which must be an energy commodity, such as 

electricity or transportation fuel [2]. Subsystems would be integrated ‘‘behind’’ the electrical 

transmission bus. Energy flows within the system would be dynamically apportioned as 

necessary to meet grid demand via a single, highly responsive connection to the grid that 

provides dispatchable electricity while capital-intensive generation assets operate at full capacity. 

Candidate region-specific hybrid energy systems selected for further study, and figures of merit 

that will be used to assess system performance, will be presented.

 1 Nuclear Fuel Performance and Design, Idaho National Laboratory 2 Advanced Process and Decision Systems, Idaho National Laboratory

3 Strategic Energy Analysis Center, National Renewable Energy Laboratory

4 Strategic Energy Analysis Center, National Renewable Energy Laboratory 5

Department of Nuclear Science and Engineering, Massachusetts Institute of Technology
2

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1. MOTIVATION

Increasing global concerns regarding climate change have resulted in requirements to 

significantly reduce greenhouse gas (GHG) emissions in the coming decades. One solution is 

non-emitting, intermittent renewable resources, which are being added to the grid in increasing 

quantities to meet established State and Federal policy goals. This increased role of intermittent 

renewables in many regions can lead to more frequent occurrences of low or negative electricity 

prices at times of high renewable output and/or low demand, reduced baseload generation market 

size and associated baseload generator power reductions (e.g. load-following operation).

Additionally, most current markets do not reward clean baseload power for its positive 

environmental attributes. Continuing to operate generation systems in traditional baseload

fashion as the penetration of renewable energy generation continues to increase is not a 

sustainable business practice for many baseload energy suppliers. Likewise, intermittent

renewable generation suffers from the same low or negative electricity prices at high 

penetrations as conventional generators. 

Growth of renewable and natural gas generation in some deregulated markets is contributing to 

the premature closure of nuclear plants, particularly in concert with the current low cost of 

natural gas in the United States (e.g. the Kewaunee nuclear power plant in Wisconsin was shut 

down in mid-2013 due to unfavorable economics in light of low natural gas prices [3]). 

Deregulated markets require all generation sources to compete to sell electric energy and 

services to meet grid demand. In some scenarios, nuclear technologies are more expensive than 

natural gas and renewable generation (once all subsidies are included); thus, they often cannot 

compete directly. In regulated markets utilities have the option to include costs for new, planned 

additions of nuclear capacity in their rate structures, essentially allowing cost recovery in 

advance of and throughout the construction of a new nuclear plant. However, clean baseload 

generators are also challenged in regulated markets where, in some cases, natural gas is being 

selected over new nuclear units due to the current low cost of natural gas. Despite this challenge, 

some utilities are proceeding to build new nuclear capacity as a hedge against volatile natural gas 

prices. New developments in nuclear technology, including small modular reactors, are also 

providing renewed attention toward nuclear generation, which could spur the development of 

novel hybrid energy systems.

To significantly impact GHG emissions, one must look at changes beyond the electricity sector –

the carbon footprint of non-electric energy sectors (industry, commercial, residential, and 

transportation) must also be reduced for the U.S. to meet long-term emission goals. Many zerocarbon technology options are resource limited (e.g. wind, solar); thus, a portfolio with a variety 

of options is needed. Integrated nuclear – renewable energy production technologies would 

provide additional options for thermal, electrical, and/or chemical energy to meet industrial and 

transportation sector demands. These systems would be tailored to regional resources and 

markets to dynamically optimize the use of thermal and electrical energy. Optimized operation of 

such hybrid systems would meet growing grid flexibility needs while allowing operation of both 

renewable and nuclear power sources at maximum capacity factors – hence maximizing

economic benefit. Renewable electricity supplied to the grid could thus be significantly increased 

while avoiding the need for fossil or nuclear plants to operate solely as stand-by dispatchable 

power sources. The resulting excess generation could, for example, support the production of 

significant quantities of clean transportation fuels from domestic resources.
3

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1.1 Possibilities for the Future Energy Grid

The current and anticipated future trends in the mix of energy generation sources on the grid 

point to a need for increased flexibility. The “problem statement” being addressed by the current 

work can be summarized as follows:

1. There is an overall desire to significantly reduce national GHG emissions in the 

coming decades. President Obama has called for 80% of electric power generation to 

come from “clean” energy sources by 2035 [4].

2. Non-emitting, intermittent renewable resources are being added to the grid in 

increasing numbers to meet the established state and federal policy goals – this is 

leading to an increased need for grid flexibility. The increasing uncertainty in net load6

resulting from intermittent renewable generation necessitates an increased need for 

frequency regulation and higher dispatchable generator ramp rates and ranges. See Figure 

1 for an illustrative example in which net load is the output the grid requires from nonwind generators to equalize supply and demand in the generator’s balancing area (the 

metered segment of the electric power system in which electrical balance is maintained). 

Net load is high when demand for electricity is high and/or variable generation is low. 

Net load is low when demand is low and/or variable production is high.

Frequency control is employed to maintain grid stability. Traditionally, large mechanical 

power generation, such as the turbines in nuclear, coal, or natural gas, inherently support 

the grid frequency because they use rotating machinery to generate electricity. In 

scenarios where a large portion of the generation is provided by sources that do not 

provide mechanical inertia, such as wind and solar, other solutions for grid stability are 

necessary. This challenge could be mitigated by a number of technologies, such as 

inclusion of sources that provide real or virtual inertia [5], but additional research is 

necessary to fully understand these challenges.

3. The increased role of intermittent renewables in many regions can lead to more 

frequent occurrences of low or negative prices, reduced baseload generator market 

size, and associated baseload generator output reductions. This scenario can lead to 

decreased capital deployment efficiencies and declining business cases for baseload 

technologies and renewables alike. See Figure 2 for an illustrative example.

4. The carbon footprint of all energy segments of the U.S. economy must be 

significantly reduced if long-term emission goals are to be met. Utilization of lowcarbon resources for heat and electricity presents a potential solution for decarbonization

of all energy services, utilizing fossil fuel and biomass resources for the production of 

clean transportation fuels and higher value products in the chemical commodities 

manufacturing sector.

 6 Net load is the remaining demand that must be met by conventional generation sources after variable generation is subtracted 

from the total load (demand).
4

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Figure 1. System load, wind generation, and net load for a two-week period in April. This plot 

uses Electric Reliability Council of Texas (ERCOT) grid data from 2005 along with 15-GW of 

spatially diverse simulated wind data from the same year [5].

(a) (b)

Figure 2. (a) Distribution of California electricity prices in 2012 [6] and (b) simulated dispatch in 

California for four Spring days (24-hr period) showing how electricity generation changes with different 

fractions of electricity produced by PV. The PV fraction is defined as the fraction of all electricity 

produced by PV over the full year. PV production peaks in June and is at its lowest in December [7].

Several potential solutions could be implemented in the future energy grid while providing grid 

flexibility. Each of these solutions has associated costs, limitations and region-specific 

implications that have been characterized to varying degrees. For a more detailed discussion on 

grid flexibility options, see relevant reports from NREL [8] and North American Electric 

Reliability Corporation (NERC) [9]. 

-5,000

0

5,000

10,000

15,000

20,000

25,000

30,000

35,000

PV Penetration and Hour

Generation (MW)

PV

Gas

Turbine

Pumped

Storage

Hydro

Combined

Cycle

Imports

Coal

Nuclear

Wind

Geo

Exports

 Base 2% 6% 10% 

 (no PV)

0

5000

10000

15000

20000

25000

30000

35000

40000

45000

50000

1-Apr 8-Apr 15-Apr

MW

Load Wind Net Load

Variation in wind output increases net load 

ramp rate (Increases in this period from 4,052 

MW/hour to 4,560 MW/hour)
5

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1. Modifications to system operations. Increased frequency of grid dispatch allows for 

decisions to be made closer to real time, yielding more economically efficient solutions. 

Improved wind and solar forecasting decreases the uncertainty in net load (the difference 

between the load and the variable generation; hence, generation requirement from 

dispatchable generators) resulting in reduced utilization of expensive peaking capacity. 

Expanding balancing area coordination efforts to larger geographic areas leads to 

increased access to dispatchable capacity, resulting in increased overall grid flexibility.

2. Expansion of high-voltage transmission infrastructure. Expansion of transmission 

infrastructure can increase interconnections with adjacent balancing areas, enable virtual 

grid-scale electricity storage, and decrease congestion (and associated congestion pricing) 

in electricity markets.

3. Enrollment of demand-side resources. Enabled by innovative information and 

communications technologies, coordinated utilization of demand response, distributed 

generation and storage resources across the residential, commercial and industrial sectors 

can help to provide flexibility to the bulk power sector. This is a novel approach to grid 

flexibility that must be enabled by appropriate regulation, market rules and associated 

business and investment models. 

4. Add grid-scale storage. Grid-scale storage can be “charged” when generation is greater 

than load and “discharged” when the system has more load than generation. Today, 

pumped hydropower storage is commonly used; however, that resource does not meet the 

projected need and has limited locations where it can be built [10]. Other options under 

development include compressed air energy storage, hydrogen storage, conversion of 

excess electricity to methane, battery storage, and fly wheels.

5. Enroll dispatchable generation to operate flexibly. While all dispatchable technologies 

are, by definition, equipped to vary output to meet load, they may be limited by certain 

technical constraints (i.e. maximum turn-downs, ramp rates). Additionally, there are very 

limited zero-carbon options for flexible generation. Plants that are designed to provide 

flexible generation (i.e. gas combustion turbines) are expensive to operate and require 

high energy and ancillary service prices to remain financially viable. Flexible operation 

of baseload technologies is an option, although such plants may experience technical 

limitations on their ability to provide flexibility. Furthermore, this operational mode can 

result in reduced capital deployment efficiencies, increased operation and maintenance 

costs, and potentially shortened plant life times. The potential impact of load-following 

operation on the operational lifetime of a nuclear plant and reliability of the nuclear fuel 

requires additional study. Alternately, one could curtail renewable generators when load 

is insufficient. This action would be appropriate if the emissions benefits are equivalent 

between baseload (i.e. nuclear) and intermittent (i.e. wind, solar) generators.

6. Develop a new operational paradigm: industrial-scale, integrated energy systems 

with internally managed resources. The proposed operational system would integrate 

generation sources behind the electrical bus. An internally managed, integrated system 

configuration offers the opportunity to operate baseload generation sources in a “loaddynamic” fashion rather than “load-following,” enabling the plant to:
6

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i) Reliably and flexibly provide electricity to meet grid demand.

ii) At times of low electricity demand, provide excess thermal energy input to alternate 

applications (maintaining the baseload plant at its nameplate operating capacity), thus 

minimizing cycling of baseload systems (e.g. the nuclear reactor) and maximizing 

capital deployment efficiency.

1.2 Postulated Benefits of Hybrid Energy Systems

Preliminary studies indicate that tightly coupled hybrid systems may provide a number of 

benefits, including:

• Reduced greenhouse gas emissions in the coming decades, thereby enabling progress 

toward the Administration’s goals for reduced emissions;

• Increased energy conversion efficiency through deployment of advanced integration, 

control, and heat/process management technologies that allow generators, grid operators, 

and energy consumers to optimally utilize assets and maximize system reliability, 

electricity supply stability, and profitability;

• High reliability of electricity supply and consistent power quality by economically 

providing flexibility and other ancillary services to the grid;

• High penetration of renewable energy by transforming the grid infrastructure to provide 

grid-scale energy storage and dispatch;

• Reduced fossil fuel dependence for the transportation sector via expansion of clean 

energy sources that can be used by plug‐in vehicles, hydrogen fuel cell vehicles, and for

biofuel and synfuel production;

• Reduced fresh water withdrawals and consumption through higher efficiency

thermodynamic power cycles, increased utilization of wind turbines and solar 

photovoltaics, desalination of seawater, and other productive utilizations of low-grade 

heat; and

• Conversion of U.S. natural resources to desirable, high value products that enhance the 

nation’s economic gain in domestic and international markets.

1.3 Challenges to Hybrid Energy Systems Development

The energy generation sources considered in tightly coupled, integrated energy systems are not 

novel, but integration in this manner is a novel approach to achieving the goal of low-carbon, 

reliable energy supply. Design, development, and deployment of tightly coupled integrated 

energy systems face numerous challenges. These challenges can generally be grouped as 

follows: 

1) Integration Value: Possibility for integration to increase the value of system 

components; added risk of integration relative to improvement in efficiency and energy 

availability; market structures that do not necessary monetize the value of grid services 

that might be provided by an integrated system.
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2) Technical: Novel subsystem interfaces; ramping performance; advanced instrumentation 

and control for reliable system operation; safety risk assessments; commercial readiness 

of the technology and operational risks.

3) Financial: Business model; cost and arrangement of financing and risk/profit taking 

agreements; shifts in cultural values and associated market evolution trends for various 

products; assurance of high capital utilization efficiency.

4) Regulatory: Projected environmental regulations; deregulation or re-regulation of 

electrical and other energy markets; licensing of a co-located, integrated system; 

involvement of various regulatory bodies for each subsystem and possible “interface” 

issues.

5) Timeframe: Resolution of issues/challenges within the timeframe established based on 

external motivators for these systems (e.g. EPA carbon pollution standards); possibility 

of hybrid implementations at the rate market forces influence build-out of renewable 

resources; possibility for grid stability issues to drive alternative solutions that create 

alternative long-lasting capital investments/inertia.

1.4 Engaging Stakeholders: Foundational Workshop

In July 2014 a Foundational Workshop was held at Idaho National Laboratory (INL) under the 

leadership of principal investigators at INL, the National Renewable Energy Laboratory (NREL) 

and the Massachusetts Institute of Technology (MIT) [11]. The workshop focused on five 

primary objectives:

1. Identify and refine priority region-specific opportunities for integrated nuclear-renewable 

energy systems in the U.S.;

2. Select Figures of Merit (FOM) to rank and prioritize candidate systems;

3. Discuss development needs for enabling technologies;

4. Identify analysis requirements, capabilities and gaps to estimate FOM for integrated 

system options;

5. Identify experimental needs to develop and demonstrate nuclear-renewable energy 

systems.

The workshop accomplished the goal of initiating an inter-laboratory, university, and industry 

team for integrated energy systems development; additional stakeholder engagement will ensure 

proposed energy systems are relevant and marketable to a potential customer. Significant 

progress was made at the three-day workshop toward identification of regional system 

configurations, prioritization of options for evaluation, and definition of key figures of merit 

necessary to evaluate the potential technical and financial performance of those systems.
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2. ENERGY SYSTEMS DESIGN AND ANALYSIS

Figure 3 illustrates a manner in which traditionally baseload power systems (Primary Heat

Suppliers) could be directly integrated with renewable generation sources. Nuclear generation is 

the selected thermal energy resource for the current study, recognizing the benefits of nuclear 

technology as a high energy density, non-CO2 emitting, reliable energy supply that has 

demonstrated an excellent safety record. The thermal energy that is constantly produced can be 

dynamically apportioned between the conventional power generation train and a selected 

industrial heat user (list provided in Figure 3 is not comprehensive; other applications may be 

appropriate). Operation in this manner allows the system to reliably meet grid demand, manage 

the intermittency of the renewable resource, and maximize use of the thermal energy produced 

by the capital-intensive nuclear subsystem. Optimal system designs will vary across regions due 

to available resources, market conditions, and infrastructure for storage and delivery of resources 

and products. In addition to standard electricity transmission, one alternate delivery option could 

be production of hydrogen for fuels upgrading, fertilizer production, or combustion in fuel cell 

vehicles or in plants that provide peak power generation needs and that help manage power 

quality conditions.

Figure 3. Conceptual Integrated Energy System.

2.1 Priority Regional Energy Systems

Integrated Nuclear-Renewable Hybrid Energy Systems must be tailored to regional resources 

and markets to dynamically optimize the use of thermal and electrical energy. Definition, 

prioritization and analysis of key options based on selected figures of merit is necessary to 

identify energy systems that have the greatest likelihood for success in a specified region. 

Results of these studies will feed a technology development roadmap; a roadmap is targeted for 

completion in late 2015. 
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Nuclear-renewable energy systems can be organized into five subsystem categories: thermal 

energy generation (i.e. nuclear reactor); power conversion (electricity generation); renewable 

resources and related systems; industrial processes; and interface or storage technologies. By 

definition, each system in this study must have a nuclear reactor; however, the other subsystems 

vary depending on the region’s resources and market opportunities. For the purposes of the 

current study, technologies of interest should be capable of deployment at a commercial scale by 

2035 to assist the nation in meeting the GHG emissions goals. This timeframe requires either 

utilization of nuclear plants that are currently operating (or recently shuttered) or reactor 

technologies that can be licensed today. Advanced reactor technologies have the potential to 

improve future options; however, they are unlikely to be available soon enough to meet the 

preferred timeline unless there is a national priority to develop such technologies. Hence, all of 

the currently proposed regional cases would incorporate a nuclear subsystem utilizing light water 

reactor technology. A large number of industrial processes are available for possible integration, 

as are interface and storage technologies. These options are detailed in the Integrated NuclearRenewable Energy Systems Workshop Report [11].

Table 1 provides an overview of the energy system configurations proposed to stakeholders for 

eight regions around the continental U.S., specifying the renewable resource, industrial process, 

and interface/storage components. Stakeholders have been engaged to aid in the refinement of 

these proposed cases, such that some of the listed cases could be modified prior to detailed 

analysis. Dynamic analyses planned for early in fiscal year 2015 will focus on two distinct cases

selected based on stakeholder feedback. One case would be sited in the West Texas region, 

utilizing land-based wind and nuclear generation sources to produce electricity and to support a 

natural gas to liquid fuels plant (see Figure 4; note that this could also be applicable to the 

Mountain West region, although the market and supporting transmission infrastructure would 

differ in each region). The second case would be sited in the Southwest (Arizona), utilizing solar 

photovoltaic and nuclear generation sources to produce electricity and potable water, in light of 

significant interest and feedback from energy leaders in that region.

Table 1. Renewable resource, industrial process, and interface/storage proposed for each region.

Region Renewable Resource Industrial Processes Interface / Storage

Mountain-West Wind – Land-based Natural gas to liquid fuels Battery storage

Pacific Northwest Wind – Land-based or 

off-shore

Bauxite to aluminum Battery storage

Southern California Tidal power Desalination Battery storage

Southwest Concentrating solar and 

geothermal power

OR

Solar photovoltaic

Hydrogen for 

transportation

Hydrogen (via high temperature 

electrolysis) and compressed air 

energy storage OR

Water desalination

Gulf Coast Offshore wind power Petroleum refining Hydrogen (via high temperature 

electrolysis) and thermal storage

Southeast None* Biofuel production Hydrogen (via high temperature 

electrolysis) and thermal storage

Industrial Midwest / 

Northeast

Wind power Natural gas to ethylene Pumped hydroelectric storage

Agricultural 

Midwest

Wind power Ammonia / fertilizer 

production

Hydrogen (via high temperature

electrolysis)

*The Southeast Region does not have a renewable resource because the renewable resource used is biomass for the 

industrial process.
10

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Figure 4. Example regional case that could be applicable to West Texas or the Mountain West.

2.2 Key Figures of Merit for System Evaluation

Many figures of merit (FOM) can be defined to evaluate the technical, economic, environmental, 

and social benefits of hybrid energy systems integration versus “business as usual” plant 

operations. Capital project decisions in the U.S. have historically focused on “return on 

investment” (ROI). These ROI evaluations take technical, economic, regulatory, and safety risks 

into consideration. Project technical risk is generally mitigated by performing scaled 

demonstration of components and subsystems to confirm component operation and integration. 

Safety risks are mitigated by demonstrating that plant operations meet authorized risk probability 

thresholds upon completing a probabilistic risk assessment (PRA). Economic risk can be 

evaluated based on sensitivity of cash flows tied to the projected future of natural resources and 

market preferences. Initial regulatory risk can be reduced by incorporating well-known, currently 

licensed subsystems and by siting the plant in a location that has previously been approved for 

the types of plants considered in the integrated system of interest (e.g. nuclear, chemical, etc.). 

Future regulatory risks can be mitigated by designing the plant with flexibility to adapt to 

evolving society values.

Figures of merit may be binary (go/no-go); quantitative, tied directly to project goals and/or plant 

design criteria; or subjective, falling on a continuum from low to high social value. The proposed 

FOM can roughly be divided into four categories that may be used to measure the value 

proposition for a proposed system based on 1) environmental, 2) economic, 3) technical/design, 

and 4) socio-political criteria. A preliminary set of FOM were presented to stakeholders for 

review and input. Stakeholder input via an informal survey resulted in top-ranking FOM from 

each of the four FOM categories. Although there was an insufficient number of stakeholder 

participants to provide statistically significant results, the following FOM were clearly important 

in reviewing the performance of regional cases:

1. Financial pro forma analyses – need a good business case that supports industry, 

economy, and service-providers
11

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2. Environmental greenhouse gas emissions – needs to be environmentally responsible

3. Provides / establishes national energy security – energy independence, diversity

4. Near-term deployability

5. Grid reliability

The FOM will be further refined and quantified where possible as the dynamic analysis of each 

case is refined. The current high-ranking FOM will be suitable for down-selection of regional 

cases and optimization of the system configurations. More details on the proposed FOM can be 

found in the Integrated Nuclear-Renewable Energy Systems Workshop Report [11].

3. PATH FORWARD

Significant research is required to reduce development risk for advanced energy systems. Design, 

development, and deployment of tightly coupled integrated energy systems face numerous 

challenges that must be addressed. Technical challenges to tight system integration can be 

addressed via system modeling and simulation and hardware testing. Additional challenges can 

be categorized as financial (demonstration of a feasible business model), regulatory, and time 

available to meet the demands for non-emitting energy solutions. Leadership by the DOE 

national laboratories in these early development phases is necessary to ensure the advancement 

of novel concepts that have the potential to significantly enhance the performance, reliability and 

sustainability of future energy systems in the U.S. and abroad. As technology gaps are reduced 

and a clear implementation path is defined, technology can and should be transitioned to industry 

leadership for prototype development and eventual commercialization. A Technology 

Development Roadmap for Integrated Energy Systems will be issued in 2015.

REFERENCES

1. U.S. Energy Information Administration, http://www.eia.gov/electricity/.

2. Ruth, M.F., O.R. Zinaman, M. Antkowiak, R.D. Boardman, R.S. Cherry, and M.D. Bazilian. 

(2014). “Nuclear-renewable hybrid energy systems: Opportunities, interconnections, and 

needs,” Energy Conversion and Management, Vol. 78, p. 684-694.

3. M.L. Wald. (2013). “As Price of Nuclear Energy Drops, a Wisconsin Plant is Shut,” The New 

York Times, May 7, 2013.

4. http://www.whitehouse.gov/the-press-office/2011/01/25/remarks-president-state-unionaddress

5. Denholm, P. and M. Hand. (2011). “Grid flexibility and storage required to achieve very high 

penetration of variable renewable electricity,” Energy Policy, Vol. 39, p. 1817-1830. 

6. California ISO. (2012).“Binding Real Time Pre Dispatch Analysis Spreadsheet,” accessed 

August 2014, http://www.caiso.com/Documents/BindingRealTimePreDispatchAnalysis

Spreadsheet-FERCOrderNo-764MarketChangesStrawProposal.xls.

7. Denholm, P., R. M. Margolis and J. Milford. (2008). “Production Cost Modeling for High 

Levels of Photovoltaics Penetration” NREL/TP-581-42305.

8. Cochran, J., M. Miller, O. Zinaman, et al. (2014). Flexibility in 21st Century Power Systems. 

21st Century Power Partnership. NREL/TP-6A20-61721. 

http://www.nrel.gov/docs/fy14osti/61721.pdf.
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This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications.

9. NERC. (2010). Flexibility Requirements and Metrics for Variable Generation: Implications 

for System Planning Studies. http://www.nerc.com/files/IVGTF_Task_1_4_Final.pdf.

10. Electric Power Research Institute. (2003). EPRI-DOE Handbook of Energy Storage for 

Transmission and Distribution Applications. Electric Power Research Institute, Concord, CA.

11. Bragg-Sitton, S., et al. (2014). Integrated Nuclear-Renewable Energy Systems: Foundational 

Workshop Report, INL/EXT-14-32857 Rev. 1, NREL/TP-6A20-6278.