NREL is a national laboratory of the U.S. Department of Energy, Office of Energy 

Efficiency & Renewable Energy, operated by the Alliance for Sustainable Energy, LLC.

Contract No. DE-AC36-08GO28308

Navajo Generating Station and 

Clean-Energy Alternatives: 

Options for Renewables 

D.J. Hurlbut, S. Haase, C.S. Turchi, and 

K. Burman

National Renewable Energy Laboratory

Produced under direction of the U.S. Department of the Interior by the 

National Renewable Energy Laboratory (NREL) under Interagency 

Agreement R11PG30024 and Task No WFJ5.1000.

Technical Report

NREL/TP-6A20-54706

June 2012
NREL is a national laboratory of the U.S. Department of Energy, Office of Energy 

Efficiency & Renewable Energy, operated by the Alliance for Sustainable Energy, LLC.

National Renewable Energy Laboratory

15013 Denver West Parkway

Golden, Colorado 80401

303-275-3000 • www.nrel.gov

Contract No. DE-AC36-08GO28308

Navajo Generating Station and 

Clean-Energy Alternatives: 

Options for Renewables 

D.J. Hurlbut, S. Haase, C.S. Turchi, and 

K. Burman

National Renewable Energy Laboratory

Prepared under Task No. WFJ5.1000

Technical Report

NREL/TP-6A20-54706

June 2012
NOTICE

This manuscript has been authored by employees of the Alliance for Sustainable Energy, LLC (“Alliance”) under 

Contract No. DE-AC36-08GO28308 with the U.S. Department of Energy (“DOE”).

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.

Cover Photos: (left to right) PIX 16416, PIX 17423, PIX 16560, PIX 17613, PIX 17436, PIX 17721

Printed on paper containing at least 50% wastepaper, including 10% post consumer waste.
iii

Table of Contents

List of Figures............................................................................................................................................ iv

List of Tables.............................................................................................................................................. iv

1 Introduction .......................................................................................................................................... 1

1.1 Benefits............................................................................................................................... 1

1.2 Cost ..................................................................................................................................... 3

1.3 Alternatives Other Than Renewables ................................................................................. 5

2 Framework and Analytical Assumptions........................................................................................... 6

2.1 Size of the Puzzle................................................................................................................ 6

2.2 Separating the Puzzles........................................................................................................ 7

3 Assessment of Resource Options ..................................................................................................... 9

3.1 Renewable Energy Credits................................................................................................ 10

3.2 Solar.................................................................................................................................. 11

3.2.1 Solar Energy Capability on Tribal Land................................................................... 12

3.2.2 State Potential ........................................................................................................... 14

3.2.3 Summary ................................................................................................................... 15

3.3 Wind.................................................................................................................................. 16

3.3.1 Wind Energy Potential on Tribal Land..................................................................... 17

3.3.2 State Potential ........................................................................................................... 18

3.3.3 Summary ................................................................................................................... 20

3.4 Geothermal........................................................................................................................ 21

3.4.1 Geothermal Energy Potential in Arizona .................................................................. 22

3.4.2 Summary ................................................................................................................... 23

3.5 Biomass and Small Hydro ................................................................................................ 23

3.5.1 Summary ................................................................................................................... 24

3.6 The Renewable Portfolio .................................................................................................. 26

4 Special Cases for Solar Alternatives ............................................................................................... 28

4.1 Solar Augmentation at an Existing Navajo GS Unit ........................................................ 28

4.1.1 Solar-Augment Results for Navajo GS..................................................................... 30

4.1.2 Summary ................................................................................................................... 31

4.2 On-Site Distributed Solar Power ...................................................................................... 32

4.2.1 Methodology, Tools, and Assumptions.................................................................... 33

4.2.2 Results....................................................................................................................... 33

4.2.3 Summary ................................................................................................................... 34

5 Summary and Next Steps.................................................................................................................. 35
iv

List of Figures

Figure 1. Qualitative relationship of major benefits currently provided by Navajo GS..... 2

Figure 2. Comparison of capital costs in 2010 for various types of new generating 

units......................................................................................................................... 4

Figure 3. Comparison of capacity factors for various types of new generating units ........ 4

Figure 4. Conceptual illustration of reserve margin in utility planning.............................. 7

Figure 5. Relationship between DNI and electricity generated ........................................ 12

Figure 6. Renewable resources after WREZ screening .................................................... 15

Figure 7. Most likely benefit opportunities for solar........................................................ 16

Figure 8. Screened wind resource potential on Arizona Indian reservations ................... 17

Figure 9. Average annual wind speed areas in Arizona at 80 meters hub height............. 19

Figure 10. Most likely benefit opportunities for wind...................................................... 20

Figure 11. Geothermal favorability in the Southwest....................................................... 21

Figure 12. Most likely benefit opportunities for geothermal............................................ 23

Figure 13. Comparison of likely benefits from solar, wind, and geothermal ................... 27

Figure 14. Solar power tower............................................................................................ 29

Figure 15. Solar parabolic trough ..................................................................................... 29

Figure 16. Satellite image of Navajo GS showing land potentially suitable for CSP 

development.......................................................................................................... 30

Figure 17. CAP pumping stations included in PV analysis.............................................. 32

Figure 18. Average daily load profiles for Mark Willmer and Hassayampa pumping 

stations.................................................................................................................. 33

List of Tables

Table 1. Estimated Solar Capability on Arizona Indian Reservations.............................. 13

Table 2. Biopower and Small Hydro Potential on Arizona Indian Reservations ............. 25

Table 3. Solar-Augment Results for Power Tower Technologies.................................... 31

Table 4. Solar-Augment Results for Parabolic Trough Technologies.............................. 31
1

1 Introduction 

In January 2012, the National Renewable Energy Laboratory delivered to the Department 

of the Interior the first part of a study on Navajo Generating Station (Navajo GS) and the 

likely impacts of best available retrofit technology (BART) compliance options under the 

Clean Air Act’s regional haze provisions. That document establishes a comprehensive 

baseline for the analysis of clean energy alternatives, and their ability to achieve benefits 

similar to those that Navajo GS currently provides.

This analysis is a supplement to NREL's January 2012 study. It provides a high level 

examination of several clean energy alternatives, using background established by the 

previous analysis. To be clear, this analysis is not intended to justify any particular BART 

outcome, nor is its purpose to support arguments for or against retiring Navajo GS. The 

factors addressed here are not likely to drive those threshold decisions in any case. 

However, if the ultimate outcome is retirement, then the task would be to identify a 

portfolio of generation resources that could provide the benefits Navajo GS is providing 

today. This analysis is an initial characterization of renewable energy options that would 

be available for a replacement portfolio, under a conceptual scenario in which the 

decision to retire the coal plant has already been made based on factors outside the ones 

addressed here. 

None of the alternatives discussed in this analysis can happen quickly. It is assumed here 

that, if there were a decision to replace Navajo GS, the development of any alternative 

resource (or portfolio of resources) would occur at the end of a staged transition plan 

designed to reduce economic disruption. This glide path necessarily would most likely 

take several years and would need to take into account changes to the Navajo GS site 

lease, tribal development plans, coal supply contracts, the value of utility partners’ 

investments in the coal plant that are not yet depreciated, and the outcomes of EPA 

rulemakings relating to air emissions. We assume that replacing the federal government’s 

24.3% ownership share of Navajo GS would be a cooperative responsibility of both the 

U.S. Bureau of Reclamation (USBR) and the Central Arizona Water Conservation 

District (CAWCD), and that at a minimum the replacement strategy must be sufficient to 

ensure that the Central Arizona Project (CAP) can economically meet all of its water 

delivery obligations.

1.1 Benefits

The January 2012 study described a wide and complex array of benefits provided by 

Navajo GS. Figure 1 provides a visual summary of those benefits. The aim of this 

supplement is to provide an initial assessment of alternative generating technologies, and 

to describe how each alternative’s benefits are likely to qualitatively map to the array of 

benefits currently provided by Navajo GS.

Two types of benefits are unaffected by the choice of generating alternative, because they 

relate directly to retiring Navajo GS and not to the choice of alternatives. The first benefit 

involves health and other environmental factors that may be associated with shutting 

down the mine that supplies coal to Navajo GS. As discussed in the January 2012 study, 

there has been no detailed epidemiological study of the health impacts on the nearby 
2

Navajo and Hopi communities. Nevertheless, the potential health and environmental 

improvements that may result from closing Navajo GS and the coal mine would not 

depend on whether the replacement power came from wind, solar, or any other non-coal 

resource.

Another type of benefit not shown in Figure 1 relates to the cost of power. No 

alternative—renewable or conventional—would cost less than Navajo GS as it currently 

operates. As detailed in the January 2012 report, most of the plant’s capital costs have 

been depreciated, and its operating costs are among the lowest in the region. Therefore, 

the decision to seek alternatives would be driven by BART compliance, not by lower 

generating costs alone. 

The task of screening alternatives on the basis of cost is reserved for Phase 2. Reliable 

cost estimates would require more specific guidance from the Department of the Interior 

with respect to siting constraints, timing, and other policy objectives. Independent 

determinations on such factors are beyond the scope of this analysis. 

Figure 1. Qualitative relationship of major benefits currently provided by Navajo GS

Benefits to utility 

partners

Benefits relating to Bureau 

of Reclamation and Central 

Arizona Water 

Conservation District

Development benefits to tribes

(including non-Indian localities)
3

Figure 1 is a qualitative depiction of the types of benefits currently provided by Navajo 

GS. They are grouped by beneficiary type, as described in the January 2012 report: 

USBR and CAWCD, the power plant’s five utility partners, and tribes and localities who 

enjoy the development benefits of having the plant as part of their local economies. Note 

that the figure is not intended to imply any quantitative comparison of the benefits; that 

detailed analysis is reserved for Phase 2. The intent here is to identify the types of 

benefits provided by Navajo GS, and how alternatives would most likely map to that 

array of benefits.

• For USBR and CAWCD, the primary benefit is providing power for the Central 

Arizona Project; related is the sale of surplus power to others as a source of revenue 

for the Lower Colorado River Basin Development Fund. We include in this type of 

benefit any tribal infrastructure project that is financed through the fund.

• For the utilities, the primary benefit is a supply of electricity to serve their retail 

customers. An added benefit to these utilities is the fact that Navajo GS is one of 

several sources of baseload power.

• Tribal and local development benefits primarily accrue to the Navajo and Hopi tribes, 

as well as to the city of Page, AZ. Some of the alternatives described here are found 

on other reservations. However, for this analysis we distinguish between benefits that 

historically have been limited to the Navajo and Hopi tribes, and future benefits that 

may extend to other Arizona tribes.

1.2 Cost

While a more precise analysis of cost is reserved for Phase 2, some elements affecting 

cost can be compared generally based on current data. Three primary factors shape an 

alternative’s all-in cost: the fixed cost of the capital equipment and its installation 

(sometimes referred to as overnight costs), the variable cost of operating the plant (mostly 

the cost of fuel), and plant’s productivity (commonly represented as the plant’s capacity 

factor).1 The perfect resource would have low capital costs, low variable costs, and a high 

capacity factor. Practically, any alternative involves a tradeoff with respect to at least one 

of these primary cost factors.

• Coal and nuclear plants have high capital and other fixed costs, but they also generate 

relatively more electricity over which those fixed costs can be spread. Per unit of 

electricity generated, the cost of coal is normally lower than the cost of natural gas.

• Natural gas plants have relatively low fixed costs, but their operating costs are 

affected by the price of natural gas, which can be volatile. Currently natural gas fuel 

costs are low—as of this writing, near $3 per mmBtu—but as recently as 2008 they 

were four times that level.

 1 A unit’s capacity factor indicates how much of its capacity is used over a given period of time. A unit 

running at full capacity all the time has a capacity factor of 100%. It would have a 50% capacity factor if it 

ran at half capacity all the time, or if it ran at full capacity half the time and was idle half the time.
4

*Integrated gasification combined cycle

Figure 2. Comparison of capital costs in 2010 for various types of new generating units

(Compiled by NREL from various sources. Circles indicate average values; bars indicate averages plus and 

minus one standard deviation. For supporting data, see http://www.nrel.gov/analysis/tech_costs.html.)

*Integrated gasification combined cycle

Figure 3. Comparison of capacity factors for various types of new generating units

(Compiled by NREL from various sources. Circles indicate average values; bars indicate averages plus and 

minus one standard deviation. For supporting data, see http://www.nrel.gov/analysis/tech_costs.html.)

 $-

 $2,000

 $4,000

 $6,000

 $8,000

$/kw

0%

20%

40%

60%

80%

100%
5

• Solar and wind power have negligible variable costs, but their capacity factors are 

low due to the variability of sunshine and wind. Capital costs for wind are relatively 

low. Capital costs for solar are higher, but have declined significantly in recent years 

due to technological improvements and excess supply in the world market.

Figure 2 provides a generic comparison of capital costs for various types of new 

generating units—renewable as well as nuclear and fossil fuels. The data are based on 

information as of 2010, and represent the plausible range of costs for each technology 

(adjusted for inflation to 2012 equivalent values). Since the time the numbers in this table 

were compiled, solar photovoltaic (PV) plant costs have continued to decline. 

Capacity factors can significantly affect how the all-in cost of one alternative compares to 

another. Figure 3 compares the capacity factors of the technologies shown in Figure 2. 

Note, however, that capacity factors for renewable technologies are very site-specific 

even within Arizona, which is why a full comparative analysis of the all-in cost of 

alternatives is reserved for Phase 2. 

1.3 Alternatives Other Than Renewables

Due to time and budget constraints, this overview sets aside for Phase 2 three potential 

alternatives: advanced coal with carbon capture and sequestration, modular nuclear, and 

natural gas. Their exclusion from this analysis is intended to be without prejudice to the 

merits of those options. All three can and should be examined in the next phase of this 

project. Focusing the present analysis on renewable energy alternatives enabled the study 

team to leverage off other work currently being done at NREL.
6

2 Framework and Analytical Assumptions

NREL’s January 2012 report suggests a number of threshold issues that frame how to 

approach the question of alternatives. 

• Is it necessary to find one single alternative that provides all the benefits that Navajo 

GS currently provides by itself?

• Should the federal government differentiate between the public interests (power for 

CAP, tribal economic development) and the utility interests that would be at stake in 

a post-Navajo GS world?

• Should the federal government continue to provide CAP with nearly all of its 

electricity from a single source?

More simply, solving the puzzle of Navajo GS alternatives begins by asking “alternatives 

to do what?” Without first deconstructing what a generating alternative (or a portfolio of 

alternatives) would need to accomplish, the analysis can quickly find itself on a circular 

path, such that the only “alternative” to Navajo GS is Navajo GS itself. 

2.1 Size of the Puzzle

The January 2012 report supports the conclusion that finding alternatives to Navajo GS is 

not one single puzzle, but several overlapping puzzles. One is a 2,250-megawatt puzzle in 

which a large amount of generating capacity is centrally located. Navajo GS is the 

largest-capacity coal plant operating in the Western Interconnection, and it ranks fourth—

behind only the Palo Verde and Diablo Canyon nuclear plants and the Grand Coulee 

hydroelectric plant—in terms of total electricity generated during a typical year.2

Transmission infrastructure supporting it is commensurately large and designed to 

accommodate one single injection point on the grid. Operating such a large plant means 

significant local economic benefits in terms of employment, direct payments to local 

governments, and secondary economic impacts, in this case focused on the Navajo 

Nation, the Hopi Tribe, and the nearby community of Page, AZ.

At the same time, a Navajo GS alternative is also a 547-megawatt puzzle. This is the size 

of the federal government’s share of the plant, which is dedicated to the operation of 

CAP. USBR is charged with managing this share of the plant in a manner consistent with 

federal policy. This share of the power is provided at cost to CAWCD, which was created 

to manage the delivery of CAP water to tribes, municipalities, agricultural users, and 

others. What CAP does not need out of this share is sold to other utilities. Revenues from 

surplus power sales flow into the Lower Colorado River Basin Development Fund, which 

among other things helps facilitate the terms of water rights settlements with several 

Arizona Indian tribes.

 2 Market data used in this analysis are compiled by the energy information service of SNL Financial LC. 

SNL Energy. http://www.snl.com
7

Figure 4. Conceptual illustration of reserve margin in utility planning 

The 1,703 megawatts owned by the five utility partners constitute five separate reserve 

margin puzzles. For each utility, losing its share of Navajo GS will have some impact on 

its reserve margin and (depending on what it uses for replacement) its overall supply 

costs. 

A utility’s reserve margin is the total amount of capacity it has at its disposal in excess of 

its forecasted annual peak demand, as illustrated in Figure 4. A reserve margin that is too 

small means unexpected events such as a line outage or a generator failure are more 

likely to lead to a blackout or some other disturbance to service. Utilities in the West 

routinely project how they expect their reserve margins to change over time by 

forecasting load growth and factoring in planned retirements of old generators. The 

addition of new capacity is usually timed to prevent the utility’s projected reserve margin 

from dwindling to a critical benchmark level. 

Navajo GS constitutes between 3.5% and 7.6% of its utility partners’ generation fleets. 

This is the amount of capacity each utility would need to replace with some alternative or 

another in the event that Navajo GS were no longer available. The timing would depend 

on when the utility’s reserve margin (without Navajo GS) might fall below the minimum 

benchmark level.

2.2 Separating the Puzzles

The three types of benefits described above—benefits related to USBR and CAWCD, 

benefits related to tribal and local economic development, and benefits to the coal plant’s 

utility partners—are not fundamentally linked, apart from the fact that one large facility 

currently provides them all. The value of baseload capacity depends on usage patterns 

where the electricity is consumed, not on where the power is generated. Similarly, the 

fact that Navajo GS is a baseload plant is not a necessary condition of the employment 

and economic benefits that accrue to the Navajo and Hopi tribes. 

500

600

700

800

900

1000

1100

2010 2020 2030 2040

MW

Planning year

Available capacity

Capacity without

Navajo GS

After capacity

addition

Forecasted peak

load

Reserve 

margin

As margin shrinks, reliability is 

harder to maintain.

With new capacity, reliability 

is easier to maintain.
8

USBR and CAWCD do not require 2,250 megawatts of generating capacity for CAP and 

the Development Fund; they now meet those needs with only 547 megawatts. The 

Arizona Water Settlements Act does not restrict revenues from surplus power sales to 

baseload power only. 

Utilities need baseload capacity, but they have operational flexibility with regard to 

where it is located; it need not be on the Navajo reservation. Indeed, Navajo GS came 

into existence because by the 1970s baseload generators no longer had to be 

geographically near the load they served. Technological advances in supercritical steam 

technologies in the 1960s enabled utilities to partner on large projects, such as Navajo 

GS, that could be optimally located anywhere in the region, and not necessarily on a 

utility’s own network.

This analysis of alternatives will unbundle the array of current benefits and examine the 

alternatives with respect to:

• Options for clean energy generation on the Navajo or Hopi reservations (as well as on 

other Arizona tribal lands), but not constrained to 2,250 megawatts of baseload 

capacity

• Clean energy baseload options for the five partner utilities, but not constrained to 

2,250 megawatts in one location and not limited geographically to the Navajo or Hopi 

reservations

• Clean energy options for operating CAP and providing revenues for the Development 

Fund, but not constrained to 2,250 megawatts of capacity and not limited 

geographically to the Navajo or Hopi reservations.
9

3 Assessment of Resource Options

This section provides a high-level overview of the renewable energy options relevant to 

the benefits currently provided by Navajo GS. The assessments that follow indicate 

where a given clean energy technology can be most productive; i.e, where a developer is 

likely to find the sunniest and windiest areas with the fewest physical obstacles to 

development. 

Ownership structures are outside the scope of the factors assessed here. While we include 

assessments of how much solar power or wind power might be found on a reservation, 

the analysis does not consider whether the tribe itself owns the project. We assume here 

that the question of who owns a project will not appreciably change its cost or 

productivity. 

Similarly, we make no assumption about whether the resource might be dedicated to 

providing CAP power, or how ownership might be structured to provide revenues into the 

Lower Colorado River Basin Development Fund. Those questions are more appropriately 

addressed once a preferred technological path has been identified.

State-level resource assessments draw on previous work conducted by NREL for the 

Western Renewable Energy Zone (WREZ) Initiative. The Western Governors’ 

Association began the WREZ Initiative in 2008 with support from the Department of 

Energy.3 In the first phase of the initiative, NREL conducted detailed screenings of wind, 

solar and geothermal resource potential across the entire Western Interconnection.

In addition, NREL conducted a geographic information system (GIS) analysis of 

renewable energy potential on all U.S. Indian reservations for DOE. This analysis draws 

on that work for its estimation of renewable potential available on tribal lands in 

Arizona.4

There are a number of other on-going planning efforts related to renewable energy 

development that may impact the siting, viability, and suitability of potential NGS 

generating alternatives. These efforts need to be considered in any Phase 2 analyses. 

• The Bureau of Land Management’s (BLM) is conducting a Programmatic 

Environmental Impact Statement (PEIS) for solar energy development in Arizona, 

California, Colorado, New Mexico, and Nevada. Included in the PEIS is BLM’s plan 

to create up to 18 solar energy zones across the region, including three proposed 

zones in Arizona, two in California, and four in Nevada.

• The BLM is conducting a focused analysis for solar energy development in Arizona 

entitled the Restoration Design Energy Project (RDEP). The RDEP seeks to identify 

the best location for solar energy development in Arizona. This process is on-going, 

 3 “Western Renewable Energy Zones – Phase 1 Report.” (June 2009). Western Governors’ Association. 

Accessed March 6, 2012 (WREZ Phase 1): http://www.westgov.org/component/joomdoc/doc_download/

5-western-renewable-energy-zones--phase-1-report 4 NREL. Unpublished analysis conducted for the DOE Tribal Energy Program.
10

and the draft EIS was released on February 17, 2012. USBR has successfully worked 

with BLM to include five potential solar energy sites located along the CAP for 

inclusion in the RDEP draft EIS.

• USBR is actively working to identify land it owns that is suitable for renewable 

energy development. Other potential USBR locations which could play into future 

NGS scenarios include several sites near Yuma, Arizona, as well as several potential 

locations along the All American Canal in Imperial County, CA.

• The Desert Renewable Energy Conservation Plan (DRECP) is a multi-agency, multiyear effort focused on identifying the best sites for renewable development in the 

Mohave Desert.

• The BLM Renewable Energy Program has more than 200 applications, totaling over 

60,000 MW of potential projects in BLM lands in the west. As of December 2011, 

BLM has issued permit approvals for over 6,600 MW of renewable energy projects in 

the West. Several of these projects are already under construction. There are multiple 

proposed projects in Arizona, Nevada, and California.

3.1 Renewable Energy Credits

A renewable energy credit (REC) is an accounting device associated with one megawatthour (MWh) of electricity produced from renewable resources. It represents the value of 

the electricity’s renewable energy attributes, separate from the electricity’s work value. 

Unlike physical electricity that must be used in real time, RECs may be banked for use at 

a later time. A REC’s economic value comes from its usefulness as a compliance 

mechanism for state renewable energy requirements, and from direct voluntary consumer 

demand for green power.

The generation alternatives examined here would have the ability to earn RECs.5 How 

this would affect an alternative generation scenario would depend on the project’s 

ownership structure, but in any case, RECs add a new element to potential scenarios for 

CAP power.

For example, a 100-megawatt PV installation near the Mark Willmer pumping station 

would generate in excess of 200,000 megawatt-hours per year, along with a 

corresponding number of RECs. If the project were structured so that the pumping station 

received the power and the RECs were sold separately to someone other than CAWCD, 

the revenues could amount to between $200,000 and $300,000 per year.6

 5 The Western Renewable Energy Generation Information System tracks and issues RECs for the electricity 

produced by wind, solar, and other eligible resources that have registered. More than 2,500 generators were 

registered with WREGIS as of March 2012: http://www.wregis.org 6 In March 2012, the market price of RECs in California was between $0.98 and $1.50 per REC (SNL 

Energy). Although solar RECs trade at much higher prices in New England and the mid-Atlantic Coast 

area, currently the West has no separate market for solar RECs.
11

3.2 Solar

Arizona has some of the most productive solar potential in the United States. Because 

solar power increases in the morning and tapers off in the evening, it is generally better 

suited as a daily load following resource than as an around-the-clock baseload resource. 

Capital costs tend to be high—$5 to $6 per watt for dry-cooled concentrated solar power 

(CSP) systems and $3 to $5 per watt for PV, compared to $1 to $4 per watt for a new coal 

plant.7 Since 2006, however, total installation costs for PV in Arizona have fallen by 

14%.8

A standard measure of the quality of an area’s solar potential is direct normal insolation 

(DNI), which indicates the average amount of sunlight falling on a typical square meter 

of ground during a given period of time. Higher DNI means more electricity can come 

from the same equipment. Figure 5 shows the degree to which more sunshine (as 

measured in DNI) translates into more energy generated. An area with an average annual 

DNI of 7.5 kilowatt-hours per square meter per day will yield about 7% more electricity 

than an area with DNI of 6.5.

Variable costs for solar are negligible, whereas fuel and other variable costs for Navajo 

GS are around $19 per MWh.9 On the other hand, solar capacity factors are much lower 

than for Navajo GS. A CSP plant near Tucson can achieve a capacity factor of 30% (and 

up to 41% with thermal storage), as can a PV installation with single-axis tracking 

located near Tucson. By comparison, the Navajo GS capacity factor is typically 89%. A 

more precise comparison of overall costs (taking into account differences in capital costs, 

variable costs, and capacity factors) is not possible without knowing the specific options 

for siting, due to how differences in the quality of solar resources might affect a project’s 

financial pro forma.

NREL has performed a number of screening analyses for solar resource in Arizona and 

other western states. The estimates of solar resource potential used here are taken from 

some of those prior screening analyses. Typically, the screening excludes uneven terrain 

such as hills and mountains. Solar collector fields typically require large, flat areas to 

keep development costs low. National parks, wilderness areas, and other areas where 

development is precluded by law are also screened out.

For this discussion, we distinguish between solar capability and solar potential. 

“Capability” refers to the land area capable of siting solar power. However, solar projects 

actually built within an expansive area of capability would likely not cover the entire 

area. A single project tends to be geographically dense within a limited footprint, and a 

larger area of capability would mean more choices for siting a project. “Potential” refers 

to the amount of capacity likely to be developed within a given area of capability.

 7 “Utility-Scale Energy Technology Capital Costs.” National Renewable Energy Laboratory. Accessed 

February 29, 2012: http://www.nrel.gov/analysis/tech_costs.html), 8 State-specific summary data on PV costs are available via NREL’s interactive Open PV tool. Accessed 

February 29, 2012: http://openpv.nrel.gov/visualization/index.php 9 SNL Financial. Accessed February 29, 2012: http://www.snl.com
12

Figure 5. Relationship between DNI and electricity generated

[Line plot calculated using NREL’s System Advisor Model (https://sam.nrel.gov/). Reference points are 

based on 100-megawatt PV systems near Phoenix (DNI 6.9), Tucson (DNI 7.2), and Daggett CA 

(DNI 7.6).]

3.2.1 Solar Energy Capability on Tribal Land

Nearly every Indian reservation in Arizona has areas with very high DNI. Some have 

conditions more suited to development, however, and these “best of the best” areas may 

hold a competitive edge with respect to siting a solar facility that can be cost effective. 

Table 1 shows the quality and quantity of screened solar capability on Arizona 

reservations, ranked by DNI. The table suggests that a utility-scale PV installation would 

be about as productive on the Tohono O’odham Reservation as it would be on the 

Colorado River Reservation, but that the Tohono O’odham Reservation might offer 

roughly 10 times as many technically feasible siting options. The table indicates gross 

solar capability and does not take into account wildlife habitat or uniquely local 

constraints such as cultural resources, proximity to sacred sites, availability of roads, 

transmission access, or proximity to water. The estimates also do not reflect nontechnical considerations that could affect the ability to find a purchaser for the power.

 220,000

 230,000

 240,000

 6.8 7.0 7.2 7.4 7.6

MWh/yr,

100 MW DC

installation

DNI (kwh irradiance per square meter per day)
13

Table 1. Estimated Solar Capability on Arizona Indian Reservations

Reservation

Average annual DNI

(kwh irradiance per 

square meter per day)

Estimated utility-scale 

capability

(MW)a

Yavapai 7.30 4 

Tohono O’odham 7.28 389,043 

Colorado River 7.25 36,251 

Hualapai 7.24 28,018 

San Carlos 7.20 36,640 

Maricopa (Ak-Chin) 7.19 3,747 

Gila Bend 7.19 46 

Hopi 7.17 142,607 

Fort Mojave 7.17 4,100 

San Xavier 7.14 10,211 

Pascua Yaqui 7.13 125 

Payson (Yavapai-Apache) Community 7.13 - 

Gila River 7.11 53,573 

Fort Yuma (Quechan) 7.10 5,847 

Cocopah 7.07 1,038 

Fort Apache 7.04 13,125 

Havasupai 7.01 5,439 

Fort McDowell 7.01 2,919 

Camp Verde 6.99 49 

Navajo 6.99 1,235,874 

Salt River 6.98 7,090 

Kaibab 6.92 9,564 

a Areas with a terrain slope greater than 3% were screened out, as were areas such as wetlands, urban 

areas, water features, and protected federal lands. Remaining areas smaller than 1 km2 were also 

screened out.

The Tohono O’odham Reservation has an estimated 389 gigawatts of solar capability 

with very high DNI. Currently Tucson Electric Power, one of the Navajo GS partners, 

plans to purchase power from a 35-megawatt PV facility on a site yet to be determined.10

The reservation is also close to the CAP pumping stations upstream from Tucson. This 

proximity could reduce the cost of transmission upgrades for projects built to serve CAP 

directly. The Tohono O’odham Nation is also close to transmission connecting to the 

Palo Verde Hub, which would provide a path to the California market.

The Gila River Reservation has almost 54 gigawatts of capability close to the CAP 

pumping stations near Phoenix. The productivity potential of these resources is only 

 10 Tilghman, C., UniSource Energy Director of Renewable Resources and Programs (29 March 2012). 

Email correspondence on file with author.
14

slightly less than those on the Tohono O’odham Reservation. DOE has awarded $210,000 

to the Gila River Indian Communities to study the feasibility of commercial-scale solar 

and biomass generation for export.11

The Colorado River Reservation has some of the most productive solar capability and is 

just 10 miles from the largest CAP pumping station near Lake Havasu.

The Hopi and Navajo reservations also have significant solar capability. The Navajo 

Tribal Utility Authority (NTUA) is exploring the feasibility of large-scale solar, including 

possible development on a reclaimed portion of the Black Mesa coal mining complex.12

One factor that might affect the cost of a project is the distance from the site to a 

transmission tie-in near the current Navajo GS site. 

The San Carlos Apache Tribe has also received DOE funding to study the feasibility of 

solar generation on its reservation.13

3.2.2 State Potential

The WREZ Initiative estimated Arizona’s developable export-quality solar potential to be 

around 20 gigawatts.14 This estimate of potential is the result of significantly more 

rigorous screening than was applied to the estimates of solar capability on Indian 

reservations, however. 

First, the WREZ analysis used a higher resource quality threshold, excluding areas where 

the DNI was less than 7.25 kwh irradiance per square meter per day. Second, the 

maximum slope screen was 2%, which was flatter and more restrictive than the 3% slope 

screen applied to the analysis of the reservations. The WREZ analysis applied the same 

land use exclusions, but then applied a density screen to the remaining areas to eliminate 

small, isolated resource pockets. This resulted in the solar resource areas shown in Figure 

6, equivalent to 565 gigawatts of capability. Finally, the WREZ analysis mathematically 

discounted these remaining areas by 96.5% to approximate unknown development 

limitations and the amount of solar capacity that might actually happen. 

Two solar energy zones (SEZs) under study by the Bureau of Land Management (BLM) 

are located in the Arizona West renewable energy zone hub (AZ_WE). One SEZ is 

located 40 miles from the largest CAP pumping station; the other is 30 miles from a 

pumping station west of Phoenix. These SEZs are on federally-owned land and are part 

of an initiative by the U.S. Department of the Interior to accelerate the development of 

renewable energy resources.

 11 DOE Tribal Energy Program, http://energy.gov/downloads/tribal-energy-program-february-2012-awardproject-descriptions. 12 “Four Corners Sustainable Futures Initiative: Phase 1 Preliminary Report.” (2011). Northern Arizona 

University. Accessed October 2011: http://www.fourcorners.nau.edu/docs/4Corners-WhitePaper.pdf 13 DOE Tribal Energy Program. 14 WREZ Phase 1, p. 23.
15

Figure 6. Renewable resources after WREZ screening

(Source: WREZ Phase 1 report. Circles indicate relative amount of a zone’s potential energy; they do not 

indicate the zone’s boundary.)

The southern Arizona WREZ hub (AZ_SO) includes significant portions of the Tohono 

O’odham Reservation. The northeast hub (AZ_NE) includes solar resources on the Hopi 

Reservation and the Navajo Reservation.

3.2.3 Summary

Solar power is Arizona’s most abundant renewable energy alternative to Navajo GS. 

Most types of benefits currently provided by the coal plant can be met to some extent by 

solar power, specifically:

• Tribal economic development benefits related to siting projects on Indian 

reservations, possibly including but not limited to the Navajo Nation and Hopi Tribe

• Electricity to power CAP pumping stations

• Peak-period energy for the Navajo GS utility partners (although most of them can and 

are pursuing similar options independently).
16

*Depending on project ownership structure

Figure 7. Most likely benefit opportunities for solar

A benefit solar cannot easily provide is baseload capacity. While it is technically possible 

for CSP to provide some baseload energy with the addition of on-site thermal storage, 

doing so would entail significant additional cost. Solar Reserve’s Crescent Dune project, 

for example, which is under construction near Tonopah, NV, will be the first commercial 

application of CSP with molten salt in the United States. Developers anticipate that this 

plant will be able to store energy for 10 to 15 hours and significantly reduce generation 

intermittency.15 

Surplus power sales to others might also be difficult, unless project costs continue to 

decline significantly. At current costs, the purchase price that would be needed to recover 

a solar project’s capital costs might be too high to attract buyers. Unbundling RECs and 

selling them separately from the power generated could potentially provide a revenue 

stream for the Development Fund.

3.3 Wind

While Arizona has some commercial-quality wind resource areas, they tend to be less 

productive than wind areas in neighboring New Mexico. Of the 3.7 gigawatts of screened 

wind potential that the WREZ initiative identified in Arizona, only 5% was Class 4 or 

 15 SolarReserve, project developer. Press release. February 8, 2012: http://www.solarreserve.com/who-weare/newsroom/
17

better. In comparison, New Mexico had an estimated 13 gigawatts that was Class 4 or 

better.16

Wind power is generally lower in cost than any other utility-scale renewable energy 

source. Capital costs are currently $1 to $2 per watt, somewhat lower than for coal but 

somewhat higher than for a natural gas unit. Wind power has no fuel cost, which reduces 

its overall cost relative to coal and natural gas even further. 

Unlike coal, however, wind power is variable, and its output is generally uncontrollable. 

Forecasting can reduce the cost and the difficulty of managing wind’s variability, and 

utilities have begun to incorporate wind forecasts into their day-ahead and real-time 

operations.17

Also unlike coal, wind power in most cases can earn RECs, which enhance project 

revenues. 

3.3.1 Wind Energy Potential on Tribal Land

Of all the Arizona tribes, Navajo Nation has the largest and the most productive 

developable wind resource areas. NTUA is currently assessing major wind projects that, 

in all, could amount to more than 600 megawatts of wind power.

Figure 8. Screened wind resource potential on Arizona Indian reservations

Figure 8 shows the six reservations in Arizona with the largest amount of wind potential. 

The Navajo Reservation leads both in terms of quantity and quality. It has nearly 1.8 

gigawatts of wind potential, 500 megawatts of which NREL estimates to have a potential 

capacity factor of 35% or higher.

 

16 WREZ. The WREZ analysis estimated capacity factors of 32% and 36% for Class 3 and Class 4 wind 

areas. 

17 Rogers, J.; Porter, K. (March 2011). Central Wind Power Forecasting Programs in North America By 

Regional Transmission Organizations and Electric Utilities: Revised Edition, NREL/SR-5500-51263. 

Golden, CO: National Renewable Energy Laboratory. 
18

A number of wind projects currently under development or study would provide some 

development benefits to Arizona tribes. On the Navajo Reservation, NTUA and Edison 

Mission Energy have partnered to build the 85-megawatt Boquillas Ranch wind farm.18

The project is currently in the permitting phase. Navajo Nation is also studying a possible 

multi-resource site on a reclaimed portion of the Black Mesa coal mining complex. The 

Black Mesa project would include up to 250 megawatts of wind and solar power.

On the western side of the Grand Canyon, the Hualapai Tribe is conducting a feasibility 

study for a wind project of up to 150 megawatts.19

3.3.2 State Potential

In contrast to the rigorous screening used for Arizona’s abundant and highly productive 

solar potential, the WREZ analysis applied more liberal criteria to assess the state’s 

relatively limited wind resources. Most of the capability is in the north central part of the 

state, either near or on the Navajo reservation. Some wind capability exists in northwest 

Arizona.

The WREZ analysis estimates Arizona’s export-quality wind resources at around 3.7 

gigawatts.20 Wind speed models based on turbines at a hub height of 50 meters estimate 

that most of this amount has a capacity factor of around 28%. Recent models of 

productivity for turbines at 80 meters and 100 meters hub height estimate annual capacity 

factors in northeast Arizona around 35%.

Arizona currently has about 230 megawatts of wind power operating in the northern part 

of the state. Eight other projects in planning or permitting would add another 1,800 

megawatts, including a 500-megawatt project near Lake Mead in northwest Arizona.

 18 Salt River Project, Navajo Tribal Utility Authority, Edison Mission Energy selected by SRP for new wind 

project in Coconino County. Press release. July 27, 2011. 19 DOE Tribal Energy Program. Presentation. October 2010. http://apps1.eere.energy.gov/tribalenergy/pdfs/

17_hualapai.pdf

20 The area that passed all the WREZ screens amounted to about 15 gigawatts. This amount was discounted 

by 75% to account for unknown limitations to development, and to estimate the amount that might actually 

be developed.
19

Figure 9. Average annual wind speed areas in Arizona at 80 meters hub height
20

3.3.3 Summary

Arizona wind power can provide a low-cost alternative to some power currently provided 

by Navajo GS. Because most of the state’s best resources are in the northern part of the 

state, it is unlikely that wind could be developed economically near CAP pumping 

stations. Wind projects near the current site of Navajo GS could move power to CAP via 

transmission currently used to carry power from the coal plant. 

Therefore, the type of benefits wind power could provide include:

• Economic development benefits to Navajo Nation (and possibly the Hopi Tribe) 

related to siting projects on tribal lands near Navajo GS, and some benefits for the 

Hualapi Tribe, but very little potential for similar benefits by other tribes

• Electricity to power CAP pumping stations via lines currently serving Navajo GS

• Low-cost renewable energy for sale to others 

• Low-cost renewable energy for the Navajo GS utility partners (although most of them 

can and are pursuing similar options independently).

*Depending on project ownership structure

Figure 10. Most likely benefit opportunities for wind
21

3.4 Geothermal

Of all the Navajo GS owners, NV Energy has the greatest potential for using geothermal 

power as a baseload alternative. NV Energy is the utility serving most of Nevada, and the 

nation’s most expansive geothermal resource play covers a large portion its network in 

northern Nevada, including some potential sites on tribal lands. Figure 11 shows the areas 

that are particularly favorable to geothermal power—areas where subsurface heat is 

relatively high at relatively shallow depth.

NV Energy’s ownership position in Navajo GS is equivalent to 250 MW of baseload 

capacity, which historically has served Las Vegas and the rest of southern Nevada. A 

major transmission line currently under construction would, for the first time, provide 

southern Nevada access to northern Nevada’s rich geothermal resources. With the new 

line, NV Energy could replace all of the baseload capacity it currently has at Navajo GS 

with geothermal power from northern Nevada.

Figure 11. Geothermal favorability in the Southwest

(Source: U.S. Geological Survey, “Assessment of Moderate- and High-Temperature Geothermal Resources 

of the United States,” 2008.)
22

Unlike wind or solar, geothermal power is well suited to be a baseload power resource. 

Many plants operating in northern Nevada and in California’s Imperial Valley (another 

area with significant geothermal development) have capacity factors comparable to that 

of Navajo GS.21

Units tend to be small, however. Most operating in Nevada and California today are 

between 20 and 100 megawatts in nameplate capacity. NV Energy, for example, would 

need five 50-megawatt geothermal plants to replace the baseload power it currently gets 

from Navajo GS.

Currently Sierra Pacific Power (NV Energy’s northern network) has about 346 

megawatts of geothermal power operating, spread among more than a dozen individual 

plants. On a nameplate basis, this amounts to one-third of Sierra Pacific’s baseload of 

about 1 gigawatt.22 NV Energy’s southern network (Nevada Power) has a baseload of 

around 2 gigawatts. The company has plans to retire by 2025 a coal plant of which it 

owns 50%, equivalent to 261 megawatts of baseload capacity.23 NV Energy included 474 

megawatts of geothermal capacity in Sierra Pacific’s 2010 integrated resource plan, and 

by 2011 nearly three-quarters of that was in operation. Going beyond NV Energy’s own 

resource planning, the Sierra Pacific transmission queue for interconnection requests 

includes 15 proposed geothermal projects, constituting more than 730 megawatts of 

capacity. 

3.4.1 Geothermal Energy Potential in Arizona

Arizona itself lacks a significant base of proven geothermal resource potential. 

Geothermal favorability maps even suggest a “cool spot” below the Navajo and Hopi 

reservations that would limit development in the vicinity of Navajo GS and its 

transmission substation.

Nevertheless, Arizona utilities are currently seeking geothermal resources in California to 

complement the solar resources connecting to their own networks. The Imperial Valley, 

which already has transmission interconnections with Arizona near Yuma, has been an 

area of particular commercial interest. Salt River Project (SRP), the managing partner of 

Navajo GS, already purchases some geothermal power generated in the Imperial 

Valley.24

 21 SNL Energy, database of electric generating units. 22 Hurlbut, D., Geothermal Energy and Interconnection: The Economics of Getting to Market, NREL/TP6A20-54192. Golden, CO: National Renewable Energy Laboratory. 23 NV Energy. Sierra Pacific Power Company Integrated Resource Plan. (2010). Filing before the Public 

Utilities Commission of Nevada. 24 Salt River Project, SRP to purchase 49 MW of geothermal energy. News release. Dec. 12, 2011. 

http://www.srpnet.com/newsroom/releases
23

Figure 12. Most likely benefit opportunities for geothermal

3.4.2 Summary

Geothermal power could provide some of the baseload energy benefits that Navajo GS 

currently provides (Figure 12). NV Energy has sufficient geothermal resources in its own 

service territory to make up for the loss of its share of the coal plant. Knowing whether 

Nevada’s geothermal resources could also serve load in Arizona, however, would require 

more detailed studies of network power flows that take into account the major 

transmission lines due to be completed in 2013.

Aside from NV Energy, other Navajo GS utility partners are independently pursuing 

deals to secure baseload geothermal power. For any partner, completely replacing its 

share of the coal plant with geothermal would require deals with many geothermal plants.

Because geothermal within the borders of Arizona is limited, it would not be able to 

provide the economic development benefits for Arizona tribes that Navajo GS currently 

provides. 

3.5 Biomass and Small Hydro

Arizona’s potential for generating electricity from biomass resources and small 

hydroelectric resources is limited, both statewide and on tribal lands. Table 2 shows the 

resource estimates for Arizona reservations based on NREL’s GIS analysis. Note that the 
24

grand total for all reservations for biopower and hydropower is still just 7% of what 

Navajo GS generated in 2010.

The WREZ analysis estimated that Arizona could generate up to 2.4 million MWh per 

year through biopower. It also identified about 72 megawatts of small hydroelectric 

potential, which would produce about 347,000 MWh per year assuming a 55% capacity 

factor.

The interconnection-wide WREZ analysis treated biopower and small hydro as local 

rather than regional electric generating resources. Stakeholder discussions concluded that 

because these resources tend to be small and highly dispersed, they generally would not 

be competitive with larger wind and solar projects in a regional market for renewable 

power. Nevertheless, these resources could be a source of electricity for the communities 

in which they may be located. Those benefits are different from those provided by 

Navajo GS, however.

3.5.1 Summary

Biopower and small hydroelectric power cannot provide the same benefits as Navajo GS. 

They provide different types of benefits that are local rather than regional in nature.
25

Table 2. Biopower and Small Hydro Potential on Arizona Indian Reservations

Biopower from 

Solid Residuesa

(MWh/yr)

Biopower from 

Gaseous Residuesb

(MWh/yr)

Hydropower 

Generation 

Potential 

(MWh/yr)

Camp Verde 26 1 - 

Cocopah 1,161 25,403 - 

Colorado River 17,814 34 106,505 

Fort Apache 14,148 182 115,435 

Fort McDowell 190 74,323 14,108 

Fort Mojave 565 50 3,050 

Gila Bend 1 - - 

Gila River 26,922 459 47,987 

Havasupai 153 4 5,692 

Hopi 745 62 1,860 

Hualapai 580 16 897 

Kaibab 14 4 452 

Maricopa (Ak-Chin) 11,100 9 313 

Navajo 103,018 1,755 369,000 

Pascua Yaqui 415 10 - 

Payson (Yavapai-Apache) Community - - - 

Salt River 3,495 59,395 17,910 

San Carlos 12,211 76 49,442 

San Xavier 575 293 2,638 

Tohono O’odham 7,512 296 - 

Yavapai 639 8 - 

All Arizona reservations 201,284 162,382 735,289 

Navajo GS net generation (2010) 16,429,593

a Forest, crop, primary mill, and urban wood residues. Generation estimated assuming 1.1 MWh/bone dry 

ton of residue.

b Landfill and domestic wastewater residues. Generation estimated assuming 4.7 MWh/tonne of CH4

produced by the residues.
26

3.6 The Renewable Portfolio

The foregoing discussion indicates that neither solar, wind, nor geothermal power alone 

can replace all the types benefits currently provided by Navajo GS. They might in 

aggregate, however. Figure 13 juxtaposes the resource-specific benefit graphics from 

Figure 7, Figure 10, and Figure 12. When considered together, they have the technical 

capability to cover the entire spectrum of current benefits, plus some tribal economic 

development benefits that the coal plant does not provide. A diversified renewable 

portfolio to provide some of CAP’s power needs could expand tribal economic 

development benefits beyond the Navajo Nation and the Hopi Tribe to include tribes 

nearer to CAP pumping stations.

Whether a diversified renewable portfolio could match the amount of benefits in each 

category would depend on the number of projects developed, the economics of various 

options compared to other factors, and on the business arrangements linking them to the 

CAP and to the Development Fund.

With regard to the utility partners’ benefits in particular, renewable energy alternatives to 

replace some or all of their Navajo GS shares might not require federal leadership. The 

utilities are already evaluating solar, wind, and geothermal resources that could address 

their portion of the puzzle at least in part. 

What is unclear is whether a utility partner would respond to the retirement of Navajo 

GS—and, consequently, to the end of the partnership agreement—by incrementally 

increasing its renewable energy procurements by the amount of its ownership share. A 

utility’s typical path forward would be to regard future capacity loss as an anticipated 

change in its reserve margin and to add new capacity on a least-cost basis. The factors 

guiding its procurement of renewable resources would probably not change. Therefore 

the total amount of renewables needed probably would not change if Navajo GS were no 

longer part of the generation fleet.
27

Types of benefits provided by Navajo GS

Solar:

• Energy (but not baseload energy)

• CAP power (possible REC sales to others)

• Tribal development, including tribes in addition to 

Navajo and Hopi

Wind:

• Energy (but not baseload energy)

• CAP power and possible market sales to others

• Tribal development, mostly Navajo

Geothermal (Nevada, Imperial Valley):

• Energy, including baseload energy

• Unlikely for CAP, market sales to others

• Unlikely for Arizona tribal development

Figure 13. Comparison of likely benefits from solar, wind, and geothermal
28

4 Special Cases for Solar Alternatives

NREL has been engaged in separate analyses that relate to the question of clean energy 

alternatives to Navajo GS. Two of these analyses are summarized here as additional 

information. These are not necessarily recommended technologies, because such a 

recommendation would require more detailed analysis. They are included here for 

reference and for the reader’s convenience, and include:

• Installation of concentrating solar power augmentation at one of the existing Navajo 

GS units

• On-site PVs to provide some of the power for CAP pumps.

4.1 Solar Augmentation at an Existing Navajo GS Unit

Concentrating solar power (CSP) differs from PV solar power in that CSP uses the sun’s 

heat to drive a thermal power cycle. This reliance on thermal energy means CSP plants 

can be backed up with natural gas and can supply steam to augment fossil-fired power 

plants. Such “solar-augment” allows CSP to be combined with existing or new fossil 

power plants. 

Solar-augment of a fossil power plant offers several advantages:

• It takes advantage of a pre-existing steam power block, electrical substation, and 

other ancillary equipment

• It takes advantage of pre-existing transmission and grid interconnection

• Location next to an existing power plant likely minimizes environmental and viewshed concerns

• Solar variability is mitigated by fossil fuel use. 

These features combine to reduce some of the cost and risk associated with the solar 

project, and also may shorten project development timelines. As risk is reduced, indirect 

costs associated with financing costs and project contingencies may also decrease. 

In 2009, the Electric Power Research Institute (EPRI) completed studies examining the 

best ways to integrate CSP steam into coal-fired and natural gas combined cycle (NGCC) 

power plants.25 This work was followed by a joint NREL/EPRI study that examined the 

solar-augment potential of coal and gas plants in the United States.26 Navajo GS was one 

of the plants included in the study.

 25 Libby, C. Project Manager (April 2010). Solar Augmented Steam Cycles for Coal Plants: Conceptual 

Design Study. Report 1018648. Work performed by Electric Power Research Institute, Palo Alto, CA.

Libby, C. Project Manager (July 2009). Solar Augmented Steam Cycles for Natural Gas Plant: Conceptual 

Design Study. Report 1018645. Work performed by Electric Power Research Institute, Palo Alto, CA. 26 Turchi, C.; Langle, N.; Bedilion, R.; Libby, C. (2011). Solar-Augment Potential of U.S. Fossil-Fired 

Power Plants. NREL/TP-5500-50597. Golden, CO: National Renewable Energy Laboratory. February 

2011.
29

The potential to hybridize Navajo GS by augmenting the plant with solar-generated steam 

was evaluated following a procedure developed by NREL and EPRI. Qualitatively the 

plant is a good candidate for solar augment because of its attractive solar resource and the 

proximity of flat, vacant land. The protocol developed by NREL and EPRI seeks to 

maximize solar integration while maintaining high solar-use efficiency and the ability to

run the power cycle with and without solar input. The best case integration of solar 

thermal energy utilizes power tower technology because of its better match to coal plant 

steam conditions. 

For Navajo GS, developing the largest contiguous plot of suitable land near the plant 

would allow for a solar-augment potential of approximately 8% of annual energy 

generation of one of the 750 MW coal units, assuming the coal plant runs at a 90% 

capacity factor. The design-point solar contribution is as high as 26%. That is, a power 

tower with thermal storage could achieve up to 26% solar contribution, although this 

system would require roughly three times more land area than this analysis assumed 

would be available. The 8% annual and 26% design-point values are based on the use of 

molten salt power tower CSP technology. Parabolic troughs would have a lower solaraugment due to the lesser steam conditions provided by troughs. 

This analysis was a high-level assessment based on properties that are typical for coal and 

CSP plants. A more detailed, site-specific analysis is necessary if development is to be 

considered. 

Figure 14. Solar power tower

Source: NREL/PIX 02183

Figure 15. Solar parabolic trough

 Source: NREL/PIX 16604
30

Figure 16. Satellite image of Navajo GS showing land potentially suitable 

for CSP development

4.1.1 Solar-Augment Results for Navajo GS

The limiting factor for the amount of solar-augment at Navajo GS depended on the CSP 

technology employed. For parabolic troughs, the maximum augment was limited by the 

ability of the steam cycle to accept relatively low-temperature steam. Turbine pressure 

rises as one incorporates lower quality steam and tries to offset the steam quality by 

increasing mass flow through the turbine to maintain plant output. This restriction limited 

the amount of solar steam that could be accepted by each unit. By the time the third unit 

was brought into play, the land area was exhausted, but it had nearly reached its steam 

turbine limit as well.

For power towers the limiting factor was land. The analysis only used the largest land 

parcel (1,220 acres). Integrating all solar energy from this land parcel into Unit 1 at 

Navajo GS would yield a 7.6% solar contribution to Unit 1 on an annual basis. The 

design point solar-energy contribution is as high as 26%; that is, when the solar field is a 

full power it can represent 26% of the energy output from Unit 1. The annual contribution 

is much lower because the capacity factor for the CSP plant is less than 30%, compared 

to the assumed coal capacity factor of 90%. The addition of thermal storage would 

increase the CSP capacity factor, but would also require additional land. Additional 

power tower solar could be integrated into Navajo GS if one chooses to construct 

multiple solar fields on different parcels of land. The solar plant could include thermal 

energy storage to increase the solar contribution of one coal unit or distribute the augment 

across all three coal units. Ultimately the amount of solar that could be integrated will be 

limited by the estimated 3,232 acres of suitable land within 3 km of the plant.
31

Table 3. Solar-Augment Results for Power Tower Technologies 

Unit

Nameplate

Capacity 

(MW)

Augment 

Potential 

(MWe)

Annual Solar 

Generation 

Contribution

Solar Use 

Efficiency Limiting Factor

1 750 198.5 7.6% 35.4% steam turbine

2 750 0 0.0% - land*

3 750 0 0.0% - land*

*Only the largest contiguous land parcel is used.

Values assume 90% fossil capacity factor and 26% CSP capacity factor.

Source: Turchi et al., 2011.

Table 4. Solar-Augment Results for Parabolic Trough Technologies

Unit

Nameplate 

Capacity

(MW)

Augment 

Potential 

(MWe)

Annual Solar 

Generation 

Contribution

Solar Use 

Efficiency Limiting Factor

1 750 55.8 2.1% 26.9% steam turbine

2 750 55.8 2.1% 26.9% steam turbine

3 750 46.7 1.8% 30.9% land*

*Only the largest contiguous land parcel is used.

Values assume 90% fossil capacity factor and 26% CSP capacity factor.

Source: Turchi et al., 2011.

4.1.2 Summary

Adding solar-augment to one of the three units, while continuing to operate the other two 

as usual, would potentially preserve the same types of benefits currently provided by 

Navajo GS, all other factors held the same. Land availability may limit the relative degree 

to which solar-augment could contribute to a plant as large as Navajo GS.

An alternative scenario is that the other two units are retired, with USBR and CAWCD 

retaining a majority share of the remaining solar-augmented unit. In this case, the 

remaining benefits would include:

• Economic development benefits to Navajo Nation and the Hopi Tribe (but at a 

reduced level)

• Electricity to power CAP pumping stations

• Energy for sale to others, and possibly the additional sale of RECs 

• Baseload power for any utility partner retaining a minority share of the solaraugmented unit.

It is possible that a solar-augmented coal unit could earn RECs calculated from the 

amount of solar energy used to generate power. Whether these RECs would have value 

would depend on state policies governing the types of resources eligible to meet 

renewable energy requirements. 
32

4.2 On-Site Distributed Solar Power 

NREL is currently conducting a study for USBR that examines the potential for using 

distributed PV to offset some of the power needed for water pumping stations along the 

CAP aqueduct. Distributed (or on-site) generation reduces the amount of power supplied 

from the transmission system, thereby eliminating long-distance line losses and reducing 

transmission-related costs. The methodology here assumes excess solar power would be 

sold, and that current electricity supplies would balance out variations in PV production 

so that pumping schedules were unchanged.

The pumping loads at Mark Willmer and Hassayampa Pumping Stations were selected 

for the analysis. Figure 17 shows the two stations in relation to the screened solar 

potential areas identified in the WREZ analysis. The Mark Willmer station is the largest 

on the CAP system, and is the point of withdrawal from the Colorado River. BLM’s 

proposed Brenda solar energy zone is located 40 miles southeast of the station’s 

withdrawal point. The much smaller Hassayampa station is about 30 miles northwest of 

Phoenix, and about 30 miles north of BLM’s proposed Gillespie solar energy zone. 

Figure 18 shows the average daily load profiles of the two stations. The average load in 

2010 for Mark Willmer was around 180 megawatts, with a peak load of 287 megawatts.

The total annual energy consumption is 1,572,621 MWh per year. Hassayampa’s average 

load in 2010 was around 33 megawatts, with a peak load of 55 megawatts. The total 

annual energy consumption is 288,272 MWh per year.

Figure 17. CAP pumping stations included in PV analysis

Mark 

Willmer 

Hassayampa

Station
33

4.2.1 Methodology, Tools, and Assumptions

NREL used the software optimization tool HOMER27 to model three different PV system 

sizes: 20, 50, and 100 megawatts. HOMER contemporaneously matched the hourly 

production of a solar PV system with the hourly demand of the pumping plant. The PV 

system is modeled as a horizontal continuous adjustment tracking system with a lifetime 

of 25 years. The HOMER model evaluates the opportunities from the perspective of the 

Bureau of Reclamation owning and operating the PV plant with capital and operating and 

maintenance (O&M) costs. 

Figure 18. Average daily load profiles for Mark Willmer and Hassayampa pumping stations

The model used PV capital costs of $3 per watt and replacement costs of $2.50 per watt. 

O&M costs were assumed to be 0.1% of the capital cost every year. Economic 

calculations used a 6% annual real discount rate and a project lifetime of 25 years. 

In the model, energy from the PV system offset the use of power from Navajo GS to meet 

pumping loads. We used CAP’s cost of power from Navajo GS—$0.03 per kilowatthour—as the value of offset power. Excess production from the PV system that would be 

sold on the market as green power was valued at $0.10 per kilowatt-hour. 

4.2.2 Results

A 100-megawatt PV system would provide 13% of Mark Willmer’s annual energy needs 

and about 54% of Hassayampa’s. A system of that size would yield some excess power at 

Hassayampa during the summer months. The estimated value of this excess, based on the 

assumptions used in the model, was around $3.2 million.

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However, the effective cost of a 100-megawatt PV system (levelizing capital and O&M 

costs to an equivalent cost per kilowatt-hour value) would have increased energy costs at 

both pumping stations. The model indicated that levelized costs would increase 23% at 

Mark Willmer and 41% at Hassayampa.

The analysis indicated that the economics are not favorable for PV as long as the cost of 

energy at the pumping stations is only $0.03/kWh. Power costs would have to increase to 

around $0.12/kWh before PV would be economical (assuming PV’s capital costs were 

spread out over 25 years). Other factors that could affect economic viability in the future 

include further reductions in PV system costs, and an increase in the cost of power from 

Navajo GS.

The economics of integrating PV into the pumping stations located to the east of 

Hassayampa may be more favorable. Additional opportunities may be available for using 

PV during peak hours, thereby freeing up additional excess NGS power for possible sale 

during summer peak hours, when excess NGS power will have the highest market value. 

4.2.3 Summary

On-site PV could provide power directly to CAP pumping stations. Near-site PV located 

on tribal lands would also provide economic development benefits to the tribe. The Mark 

Willmer station, for example, is just a few miles outside the Colorado River Reservation. 

However, the economics of a project probably would not be favorable under the 

conditions modeled in this analysis.

Benefits not addressed by this alternative would include:

• Economic development benefits to Navajo Nation and the Hopi Tribe

• Baseload power for any of the current utility partners.
35

5 Summary and Next Steps

Based on this preliminary screening, the following options should be evaluated in 

additional detail in Phase 2:

• Distributed wind and solar generation potential located across the Navajo and Hopi 

reservations, as well as the reservations of CAP water-using tribes

• Distributed wind and solar on non-tribal lands located throughout Arizona, Nevada, 

and California, including on lands owned by USBR, BLM, the Department of 

Defense, and private landowners

• Solar-augment at NGS, with possible tribal ownership or partial ownership of the 

CSP plant

• Geothermal from northern Nevada and from California’s Imperial Valley

• Additional analysis of PV integration at CAP’s pumping plants located east of 

Hassayampa, with an analysis of the potential to optimize PV use during summer 

peak hours, thereby freeing up additional NGS excess power for sale on the open 

market during peak hours

• Clean coal and CCS located on the Navajo/Hopi reservation

• Small, modular nuclear

• Natural gas generation.

The Phase 2 analysis should evaluate and compare the costs, benefits, and impacts of the 

various alternative generation scenarios against several alternative scenarios for Navajo 

GS, including:

• Baseline (business as usual) conditions 

• Resulting plant operating and production costs from potential additional required 

control technologies from BART and MATS

• Shutdown scenario

• Intermediate solutions such as scaling back generation in one unit and/or shutting 

down one unit.

For each of the scenario comparisons, the Phase 2 analysis should compare impacts on 

jobs, emissions, CAP water costs, tribal benefits, visibility, public health, and other 

impacts and benefits associated with Navajo GS.