### Data Sources

The data underlying the feasibility calculations are compiled from a variety of authoritative sources. Information on biomass technologies, conversion efficiencies, feedstock energy contents, and production potentials is from the National Renewable Energy Laboratory (NREL), the International Renewable Energy Agency (IRENA), the U.S. Department of Energy, and peer-reviewed scientific literature. Demographic, economic, and geographic data on Native American tribes and specifically the Cocopah Tribe is compiled from the U.S. Environmental Protection Agency, the Bureau of Indian Affairs, and the CIHAD.

### Assessment Methodology

The feasibility assessment considers feedstock availability and quality, estimates power demand and plant capacity, and uses the levelized cost of energy (LCOE) and net present value (NPV) approaches to determine the economic viability of the project. LCOE is an economic measure allowing the comparison of different power sources taking into account initial capital requirements, discount rate, and costs of operation, maintenance and fuel acquisition. A general formula for LCOE is given below [11]:

$LCOE=∑t=1nIt+Mt+Ft(1+r)t∑t=1nEt(1+r)t$
(1)

In the above equation, t indexes time in years up to n, the expected lifespan of the system, I denotes investment expenditures in USD, M is operation and maintenance (O&M) expenditures in USD, F is fuel expenditures in USD, E is electricity generation in kilowatt hour, and r is the discount rate. Although the discounting of electricity is debated in the literature and practitioner community, the formula in Equation (1) is used by IRENA [12], and which was also used to obtain estimates on the cost of installation, fuel, and maintenance.

The NPV translates the LCOE to a unit-cost of electricity over the lifetime of a generating asset. Thus, the different biomass technologies will be compared on the basis of their NPVs per unit of energy generated and taking all major costs throughout the lifecycle of the plant into consideration.

## RESULTS

### Selection of Biomass Technology

Considering the comparatively small number of Cocopah living on the reservation and its modest energy needs, a small-to-medium-scale biomass plant will suffice to meet the tribe’s power demand at present and at foreseeable future. Although biomass is not an emission-free source of power, it burns cleaner than coal and oil and high-temperature biomass incineration combined with abatement measures such as scrubbers can achieve emission-reduction targets and burn the feedstock more efficiently [13].

### Feedstock Potential

The biomass energy plant on the Cocopah reservation requires a reliable and sufficient feedstock supply. Native Americans currently operate four farms, but the land tilled by those four operators only totals 32 acres [14, 15]. Additional biomass could be sourced from other nearby farms and include primarily agricultural residues from crops such as lettuce, wheat, alfalfa, and grasses. In general, agricultural residues have moisture content of 20–35% and an average calorific content of 11.35–11.55 MJ/kg (lower heating value) [12].

### Biomass Energy Technology Identification

The biomass conversion technologies for the project must be suitable for the available biomass feedstock options. Table 1 presents overviews of the typical options.

TABLE 1.

Suitability of biomass conversion technologies for agricultural residues

ProcessMethodEfficiency of current technology (energy output as% of energy input)Ability to accept dry agricultural residues
Biogas production Anaerobic digestion 10–15% (net facility efficiency) No
Landfill gas <39% (net facility efficiency) No
Biomass combustion Direct combustion via industrial stokers 70–90% (boiler efficiency)
20–30% (net facility efficiency)
Yes
Combustion CHP 60–90% (net facility efficiency) Yes
Waste-to-energy 22–28% (net facility efficiency) No
Co-firing 30–40% (net facility efficiency) Yes, also requires coal
Gasification Gasification 15–30% (net facility efficiency) Yes
Gasification CHP 20–50% (net facility efficiency) Yes
ProcessMethodEfficiency of current technology (energy output as% of energy input)Ability to accept dry agricultural residues
Biogas production Anaerobic digestion 10–15% (net facility efficiency) No
Landfill gas <39% (net facility efficiency) No
Biomass combustion Direct combustion via industrial stokers 70–90% (boiler efficiency)
20–30% (net facility efficiency)
Yes
Combustion CHP 60–90% (net facility efficiency) Yes
Waste-to-energy 22–28% (net facility efficiency) No
Co-firing 30–40% (net facility efficiency) Yes, also requires coal
Gasification Gasification 15–30% (net facility efficiency) Yes
Gasification CHP 20–50% (net facility efficiency) Yes

Based on [12, 16].

Although agricultural residues could potentially be used for digestion and subsequent production of natural gas, digestion is best suited for nutrient-rich wet biomass, such as sewage, animal manure, or the organic portions of municipal solid waste (MSW). Similarly, waste-to-energy plants are well suited for combusting heterogeneous MSW. Efficiency and supply reliability (e.g., feedstock storage) also need to be taken into account because the Cocopah do not have a large and consistently available feedstock supply. Table 2 lists the technologies that are most suitable for combusting dry agricultural waste and their associated capital costs and fixed O&M costs.

TABLE 2.

Capital costs and fixed and variable O&M of the selected biomass conversion technologies

ProcessMethodEstimated investment/capital costs ($/kW installed capacity)Fixed O&M costs (% of installed cost)Variable O&M costs ($/MWh)
Biomass combustion Direct combustion via industrial stokers 1,880–4,260 3.2–4.2 3.8–4.7
Combustion with CHP 3,550–6,820 3–6 3.8–4.7§
Gasification Gasification 2,140–5,700 3.7
Gasification with CHP 5,570–6,545 3.7§
ProcessMethodEstimated investment/capital costs ($/kW installed capacity)Fixed O&M costs (% of installed cost)Variable O&M costs ($/MWh)
Biomass combustion Direct combustion via industrial stokers 1,880–4,260 3.2–4.2 3.8–4.7
Combustion with CHP 3,550–6,820 3–6 3.8–4.7§
Gasification Gasification 2,140–5,700 3.7
Gasification with CHP 5,570–6,545 3.7§

Fixed O&M costs include items such as labor, scheduled maintenance, routine component/equipment replacement (for boilers, gasifiers, feedstock handling equipment, etc.), and insurance.

Variable O&M costs depend on the output of the system and include non-biomass fuels costs, ash disposal, unplanned maintenance, equipment replacement and incremental servicing costs.

§No specific data for CHP available, thus the conventional system data are used. Data based on [[15], p.34].

As shown in Table 2, adding “combined heat and power” (CHP) to the technology substantially increases the capital cost. CHP would be beneficial if the tribe was located in a cold climate, but Yuma is in a sub-tropical desert climate with average low winter temperatures of 12°C. In addition, pyrolysis, carbonization, and torrefaction are all pre-treatment processes that can be applied to the biomass inputs before combustion or gasification; however, installing such separate treatment is an added capital and O&M cost.

### Capacity Specification

The peak monthly electricity consumption in Arizona is 4,645,000 MWh in the month of July due to air conditioning demands [17]. It is likely that homes on the Cocopah reservation actually consume less than the statewide household average, given the smaller square footage of the homes and widespread poverty. To ensure that the proposed plant can accommodate peak demand, the power plant size is extrapolated from average consumption for July. Average electricity consumption per household in Arizona in July is 1.668 MWh, which is extrapolated to a conservatively high annual consumption of approximately 20 MWh per household. For a total of 753 households on the Cocopah reservation, this yields a total upper demand bound of 15,075 MWh/year and hence an upper capacity bound of 1.72 MW.

The capacity factor, or actual energy production divided by the theoretical energy production, of a biomass plant, is around 85% [12], thus yielding a nameplate capacity of 2.02 MW. An important consideration for the size of the plant is its ability to account for daily peak hour usage. APS estimates that customers in the Somerton/Yuma region use about 3.5 kWh of electricity per hour during a summer on-peak period, and a slightly lower 3.3 kWh/hour during a winter on-peak period [18]. Given that there are 753 homes on the Cocopah reservation, the necessary rating of an 85% efficient biomass power plant that would accommodate peak usage is 3.1 MW. However, a reasonable projection of roughly 490 occupied homes (taking into account that more units may have become occupied since 2010 and additional units may be occupied over the 30-year life of the biomass system) would be consistent with the estimated nameplate capacity of 2.02 MW for the plant. If, in addition, the reservation remains connected to APS, it can cover any electricity demand above what the biomass power plant can produce and provide back-up in case of disruptions to plant operation. Utilities have solved the problem of the added cost of increased electricity supply by adopting time-variant pricing [19].

### Levelized Cost of Energy

The Federal Energy Management Program of the U.S. Department of Energy sets annual discount rates to be used in cost-effectiveness, lease/purchase, internal government investment, and asset sales. For fiscal year 2018, the real discount rate was set to 3.0% and the nominal discount rate was 2.4% [20].

The useful life of a combustion stoker and a gasifier is typically between 20 and 30 years [3, 12]. IRENA lists the average prices for a variety of feedstocks and transportation costs, and states that the typical cost for U.S. local agricultural residues (20–35% moisture content) is US$1.73–4.23/GJ, or US$20–50/metric ton, including collection, premiums paid to farmers, and transportation [12]. Given that all of the commercial farms considered in this feasibility study are located no more than five miles away from the Cocopah Reservation, IRENA’s estimated rate for cost of feedstock will be used in the LCOE calculation.

The LCOE calculation, furthermore, assumes that 1 metric ton of agricultural feedstock produces approximately 1 MWh of electricity, equivalent to an efficiency of about 20% for converting feedstock to electricity without CHP. Considering the peak demand of 15,075 MWh/year, this translates into 15,075 metric tons of feedstock per year at a cost of US$301,500–753,750/year [21]. The LCOE analysis was carried out for the two most suitable biomass energy technologies—gasifiers and combustion stokers. It includes scenarios for the low and high end of capital expenditure costs, respectively, as well as different discount rates and financing costs. Table 3 shows the high and low estimates for the technology-specific variables in the LCOE formula for combustion stokers and gasifiers, while Table 4 shows the resulting LCOE estimates for the two technologies based on different assumptions. TABLE 3. Investment, O&M, and fuel costs of combustion stokers and gasifiers TechnologyInvestment cost ($/kW)Fixed + variable O&M costs ($/kWh)Fuel costs ($/ton)
Combustion stoker 1,880–4,260 0.0119–0.0287 20–50
Gasifier 2,140–5,700 0.0123–0.0266 20–50
TechnologyInvestment cost ($/kW)Fixed + variable O&M costs ($/kWh)Fuel costs ($/ton) Combustion stoker 1,880–4,260 0.0119–0.0287 20–50 Gasifier 2,140–5,700 0.0123–0.0266 20–50 Based on [21] Mott MacDonald Group, 2011. TABLE 4. LCOE for combustion stoker and gasifier using low and high end of cost estimates of investment, O&M, and fuel as well as 3 and 5% discount rates ScenarioLCOE ($/kWh) at 3% discount rateLCOE ($/kWh) at 5% discount rate Stoker, low end of assumed ranges Investment ($) 3,797,600
O&M ($/year) 178,808 Fuel ($/year) 301,500
LCOE ($/kWh) 0.0443 0.0475 Stoker, high end of assumed ranges Investment ($) 8,605,200
O&M ($/year) 432,271 Fuel ($/year) 753,750
LCOE ($/kWh) 0.1069 0.1140 Gasifier, low end of assumed ranges Investment ($) 4,322,800
O&M ($/year) 185,462 Fuel ($/year) 301,500
LCOE ($/kWh) 0.0465 0.0501 Gasifier, high end of assumed ranges Investment ($) 11,514,000
O&M ($/year) 401,198 Fuel ($/year) 753,750
LCOE ($/kWh) 0.1144 0.1239 ScenarioLCOE ($/kWh) at 3% discount rateLCOE ($/kWh) at 5% discount rate Stoker, low end of assumed ranges Investment ($) 3,797,600
O&M ($/year) 178,808 Fuel ($/year) 301,500
LCOE ($/kWh) 0.0443 0.0475 Stoker, high end of assumed ranges Investment ($) 8,605,200
O&M ($/year) 432,271 Fuel ($/year) 753,750
LCOE ($/kWh) 0.1069 0.1140 Gasifier, low end of assumed ranges Investment ($) 4,322,800
O&M ($/year) 185,462 Fuel ($/year) 301,500
LCOE ($/kWh) 0.0465 0.0501 Gasifier, high end of assumed ranges Investment ($) 11,514,000
O&M ($/year) 401,198 Fuel ($/year) 753,750
LCOE ($/kWh) 0.1144 0.1239 The LCOE for a 30-year lifetime using local agricultural residues and a small nameplate capacity of 2.02 MW are US$0.0443–0.1140 per kWh for combustion stokers and US$0.0465–0.1239 per kWh for gasifiers. They are within the range of IRENA’s estimates of US$0.06–0.24 per kWh.[[12], Key Findings].

### Net Present Value

The LCOE estimates do not yet include NPV analysis. The total investment costs for each proposed biomass plant for a 30-year lifespan and a high and low estimate are shown in Table 5 (in current $), Table 6 (discounted at 3%) and Table 7 (discounted at 5%), respectively. TABLE 5. Total investment cost (current$) for each proposed type of biomass plant using high and low estimates for the 30-year life of the plant

Stoker lowStoker highGasifier lowGasifier high
Capital ($) 3,797,600 8,605,200 4,322,800 11,514,000 O&M ($) 5,364,246 12,968,127 5,563,845 12,035,925
Fuel ($) 9,045,000 22,612,500 9,045,000 22,612,500 Total ($) 18,206,846 44,185,827 18,931,645 46,162,425
Stoker lowStoker highGasifier lowGasifier high
Capital ($) 3,797,600 8,605,200 4,322,800 11,514,000 O&M ($) 5,364,246 12,968,127 5,563,845 12,035,925
Fuel ($) 9,045,000 22,612,500 9,045,000 22,612,500 Total ($) 18,206,846 44,185,827 18,931,645 46,162,425
TABLE 6.

Total investment cost (discounted at 3%) for each proposed type of biomass plant using high and low estimates for the 30-year life of the plant

Stoker lowStoker highGasifier lowGasifier high
Capital ($) 3,797,600 8,605,200 4,322,800 11,514,000 O&M ($) 3,504,720 8,472,700 3,635,127 7,863,648
Fuel ($) 5,909,533 14,773,833 5,909,533 14,773,833 Total ($) 13,211,853 31,851,733 13,867,460 34,151,481
Stoker lowStoker highGasifier lowGasifier high
Capital ($) 3,797,600 8,605,200 4,322,800 11,514,000 O&M ($) 3,504,720 8,472,700 3,635,127 7,863,648
Fuel ($) 5,909,533 14,773,833 5,909,533 14,773,833 Total ($) 13,211,853 31,851,733 13,867,460 34,151,481
TABLE 7.

Total investment cost (discounted at 5%) for each proposed type of biomass plant using high and low estimates for the 30-year life of the plant

Stoker lowStoker highGasifier lowGasifier high
Capital ($) 3,797,600 8,605,200 4,322,800 11,514,000 O&M ($) 2,748,720 6,645,063 2,850,998 6,167,389
Fuel ($) 4,634,794 11,586,985 4,634,794 11,586,985 Total ($) 11,181,114 26,837,248 11,808,592 29,268,374
Stoker lowStoker highGasifier lowGasifier high
Capital ($) 3,797,600 8,605,200 4,322,800 11,514,000 O&M ($) 2,748,720 6,645,063 2,850,998 6,167,389
Fuel ($) 4,634,794 11,586,985 4,634,794 11,586,985 Total ($) 11,181,114 26,837,248 11,808,592 29,268,374

The costs for each plant are high, between US$18.2 million and US$44.2 million for combustion stokers and US$18.9 million and US$44.2 million for gasifiers in current dollars (see Tables 6 and 7 for the discounted costs). This is a substantial investment and the Cocopah will likely need to borrow some or all of it by partnering with an outside organization or taking out an independent loan. As noted, Native American tribes are reluctant to enter into a financial relationship with a non-tribal entity, so it is most likely that the Cocopah would choose to borrow directly from a bank, incurring additional loan financing costs.

The NPV analysis compares the savings that would result from reduced electricity costs (in the form of cash flow over 30 years) with the dollar amount that would result from accrued interest in an investment over the same period. It is assumed that only the capital needed to build the plant are financed, while fuel and O&M costs are covered by the tribe without financing. Non-use compounding interest rates of 3 and 5% are used to calculate the compounded value of the investment over time and compare it to the fiscal savings from biomass energy at 3 and 5% discount rates. The fiscal savings from the biomass plant were calculated using an electricity price of US$0.13412/kWh, which is the rate that APS charges for customers on the Premier Choice Large plan. This figure is likely to increase year-over-year by a factor of about 3.1%, so savings will increase more each year, but to be conservative, the current electricity price is used here [22]. Tables 812 show the NPV and adjusted LCOE for each biomass conversion technology, using high and low estimates. TABLE 8. Total investment cost for each proposed type of biomass plant over 30 years, before and after financing Stoker—lowStoker—highGasifier—lowGasifier—high Total before interest 18,206,846 44,185,827 18,931,645 46,162,425 Total after interest on installment cost (5%) 21,748,206 52,210,827 22,963,005 56,900,025 Stoker—lowStoker—highGasifier—lowGasifier—high Total before interest 18,206,846 44,185,827 18,931,645 46,162,425 Total after interest on installment cost (5%) 21,748,206 52,210,827 22,963,005 56,900,025 TABLE 9. NPV and adjusted LCOE (with financing) for a combustion stoker using low estimates Stoker biomass plantReturn on alternative investment/saving Capital ($) 3,797,600 3,797,600 3,797,600 3,797,600
O&M ($) 5,364,246 5,364,246 5,364,246 5,364,246 Fuel ($) 9,045,000 9,045,000 9,045,000 9,045,000
Total cost ($) 18,206,846 18,206,846 18,206,846 18,206,846 Installment cost + financing at 5% interest 21,748,206 21,748,206 N/A N/A Electricity generation (kWh/year) 15,074,850 15,074,850 Discount rate/interest rate (%) 3 5 3 5 LCOE ($/kWh) 0.0443 0.0475
Savings/kWh ($) 0.08982 0.08662 0 0 Savings/year ($) 1,354,023 1,305,784 0 0
Term (years) 30 30 30 30
NPV ($) 4,791,243 −1,675,113 7,123,380 17,093,671 Adj. LCOE (with financing) ($/kWh) 0.0560 0.0620
Stoker biomass plantReturn on alternative investment/saving
Capital ($) 3,797,600 3,797,600 3,797,600 3,797,600 O&M ($) 5,364,246 5,364,246 5,364,246 5,364,246
Fuel ($) 9,045,000 9,045,000 9,045,000 9,045,000 Total cost ($) 18,206,846 18,206,846 18,206,846 18,206,846
Installment cost + financing at 5% interest 21,748,206 21,748,206 N/A N/A
Electricity generation (kWh/year) 15,074,850 15,074,850
Discount rate/interest rate (%) 3 5 3 5
LCOE ($/kWh) 0.0443 0.0475 Savings/kWh ($) 0.08982 0.08662 0 0
Savings/year ($) 1,354,023 1,305,784 0 0 Term (years) 30 30 30 30 NPV ($) 4,791,243 −1,675,113 7,123,380 17,093,671
Adj. LCOE (with financing) ($/kWh) 0.0560 0.0620 TABLE 10. NPV and adjusted LCOE (with financing) for a combustion stoker using high estimates Stoker biomass plantReturn on alternative investment/saving Capital ($) 8,605,200 8,605,200 3,119,200 3,119,200
O&M ($) 12,968,127 12,968,127 5,879,192 5,879,192 Fuel ($) 22,612,500 22,612,500 123,480 123,480
Total cost ($) 44,185,827 44,185,827 9,121,872 9,121,872 Cost + financing at 5% interest 52,210,827 52,210,827 N/A N/A Electricity generation (kWh/year) 15,074,850 15,074,850 Discount rate/Interest rate (%) 3 5 3 5 LCOE ($/kWh) 0.01069 0.1140
Savings/kWh ($) 0.02722 0.02012 0 0 Savings/year ($) 410,337 303,306 0 0
Term (years) 30 30 30 30
NPV ($) −44,168,032 −47,548,271 20,927,05 45,546,576 Adj. LCOE (with financing) ($/kWh) 0.1333 0.1470
Stoker biomass plantReturn on alternative investment/saving
Capital ($) 8,605,200 8,605,200 3,119,200 3,119,200 O&M ($) 12,968,127 12,968,127 5,879,192 5,879,192
Fuel ($) 22,612,500 22,612,500 123,480 123,480 Total cost ($) 44,185,827 44,185,827 9,121,872 9,121,872
Cost + financing at 5% interest 52,210,827 52,210,827 N/A N/A
Electricity generation (kWh/year) 15,074,850 15,074,850
Discount rate/Interest rate (%) 3 5 3 5
LCOE ($/kWh) 0.01069 0.1140 Savings/kWh ($) 0.02722 0.02012 0 0
Savings/year ($) 410,337 303,306 0 0 Term (years) 30 30 30 30 NPV ($) −44,168,032 −47,548,271 20,927,05 45,546,576
Adj. LCOE (with financing) ($/kWh) 0.1333 0.1470 TABLE 11. NPV and adjusted LCOE (with financing) for a gasifier using low estimates Gasifier biomass plantReturn on alternative investment/saving Capital ($) 4,322,800 4,322,800 4,322,800 4,322,800
O&M ($) 2,850,998 2,850,998 2,850,998 2,850,998 Fuel ($) 4,634,794 4,634,794 4,634,794 4,634,794
Total cost ($) 11,808,592 11,808,592 11,808,592 11,808,592 Cost + financing at 5% interest 22,963,005 22,963,005 N/A N/A Electricity generation (kWh/year) 15,074,850 15,074,850 Discount rate/Interest rate (%) 3 5 3 5 LCOE ($/kWh) 0.0465 0.0500
Savings/kWh ($) 0.08762 0.08412 0 0 Savings/year ($) 1,320,858 1,268,096 0 0
Term (years) 30 30 30 30
NPV 2,926,402 −3,469,256 5,179,525 15,287,926
Adj. LCOE with Financing ($/kWh) 0.0598 0.0666 Gasifier biomass plantReturn on alternative investment/saving Capital ($) 4,322,800 4,322,800 4,322,800 4,322,800
O&M ($) 2,850,998 2,850,998 2,850,998 2,850,998 Fuel ($) 4,634,794 4,634,794 4,634,794 4,634,794
Total cost ($) 11,808,592 11,808,592 11,808,592 11,808,592 Cost + financing at 5% interest 22,963,005 22,963,005 N/A N/A Electricity generation (kWh/year) 15,074,850 15,074,850 Discount rate/Interest rate (%) 3 5 3 5 LCOE ($/kWh) 0.0465 0.0500
Savings/kWh ($) 0.08762 0.08412 0 0 Savings/year ($) 1,320,858 1,268,096 0 0
Term (years) 30 30 30 30
NPV 2,926,402 −3,469,256 5,179,525 15,287,926
Adj. LCOE with Financing ($/kWh) 0.0598 0.0666 TABLE 12. NPV and adjusted LCOE (with financing) for a gasifier using high estimates Gasifier biomass plantReturn on alternative investment/saving Capital ($) 11,514,000 11,514,000 11,514,000 11,514,000
O&M ($) 12,035,925 12,035,925 12,035,925 12,035,925 Fuel ($) 22,612,500 22,612,500 22,612,500 22,612,500
Total cost ($) 46,162,425 46,162,425 46,162,425 46,162,425 Cost + financing at 5% interest 56,900,025 56,900,025 N/A N/A Electricity generation (kWh/year) 15,074,850 15,074,850 Discount rate/Interest rate (%) 3 5 3 5 LCOE ($/kWh) 0.1144 0.1239
Savings/kWh ($) 0.01972 0.01022 0 0 Savings/year ($) 297,276 154,065 0 0
Term (years) 30 30 30 30
NPV −51,073,283 −54,531,669 6,832,676 30,807,205
Adj. LCOE with financing ($/kWh) 0.1497 0.1681 Gasifier biomass plantReturn on alternative investment/saving Capital ($) 11,514,000 11,514,000 11,514,000 11,514,000
O&M ($) 12,035,925 12,035,925 12,035,925 12,035,925 Fuel ($) 22,612,500 22,612,500 22,612,500 22,612,500
Total cost ($) 46,162,425 46,162,425 46,162,425 46,162,425 Cost + financing at 5% interest 56,900,025 56,900,025 N/A N/A Electricity generation (kWh/year) 15,074,850 15,074,850 Discount rate/Interest rate (%) 3 5 3 5 LCOE ($/kWh) 0.1144 0.1239
Savings/kWh ($) 0.01972 0.01022 0 0 Savings/year ($) 297,276 154,065 0 0
Term (years) 30 30 30 30
NPV −51,073,283 −54,531,669 6,832,676 30,807,205
Adj. LCOE with financing ($/kWh) 0.1497 0.1681 The NPV analysis shows that under the assumptions made, the investment in a biomass facility for residential space-heating purposes is not profitable under any of the tested conditions. For both, the combustion stoker and the gasifier, the project is closest to viability under the low input costs, 3% discount rate over 30 years and 3% non-use interest rate, while both still result in losses of US$2.1–2.5 million over 30 years. However, considering a zero-interest loan to finance the construction of the plant would make the low-cost stoker option cost-competitive.

## DISCUSSION

The present study evaluated the feasibility of a biomass-to-energy project to serve the Cocopah Reservation’s energy needs while maintaining a high level of tribal sovereignty. However, each project requires a careful and information-intensive assessment that takes into account local contexts and tribal objectives. The case study assessment found that none of the tested scenarios for a 2.2 MW nameplate capacity plant yield cost-competitive results, although the stoker option comes close under the lost cost scenarios and discount/interest rates of 3%. If the loan for installing the plant could be obtained at zero interest, the projects would achieve cost-competitiveness, a proposition that would be further buttressed by future rises in utility power charges [2326]. Projecting future utility rates depends on many factors, including policy and market forces as well as changes in the energy mix. The Energy Information Agency (EIA) foresees short-term increases in electricity rates for residential, commercial and industrial customers of between 0.8–2.9% between 2018 and 2020 [27]. Under this assumption, the low-cost stoker option could become a more robust choice, depending on whether the underlying economic factors would also influence the operating costs of the biomass facility. In light of the tribe’s interest in energy autonomy and economic development, the results indicate the need for a careful decision-making process that takes not only investment returns into account, but also opportunities for social and economic improvements.

It is also noted, that the Cocopah reservation’s energy demand consisted solely of residential energy usage, while many tribes operate commercial facilities, such as casinos, hotels, etc. that require larger, stable energy supplies. These could potentially create opportunities for economies of scale that could positively affect the calculations. There are also other industrial uses of biomass-based energy that could be beneficial for the Cocopah, for example, in agricultural processing. And as market conditions change and technological innovation continue to evolve, the calculations would change as well and lead to different conclusions. It is also noted that the analysis did not take environmental abatement costs, such as emission controls, into consideration due to lack of detailed information. Air pollution control measures should be considered as part of a comprehensive environmental impact review and to avoid environmental justice concerns. Also not addressed in the study is the issue feedstock-related slagging and fouling—common problems in pure biomass combustion plants—that can be addressed through different designs such as unstaged and air-staged co-combustion but need to be evaluated with respect to their influence on facility design and cost.

## CONCLUSION

This analysis examined options for a small, rural, and remote Native American tribes to diversify and decarbonize their energy supply, especially in light of objectives such as tribal sovereignty and local economic development. Biomass energy has advantages over solar and wind power and can also be custom-tailored to accept a variety of inputs as feedstock and meet the residential energy demand of a smaller tribal community. While the tested scenarios were not yet cost-competitive in this specific context, there are additional factors such as energy independence, local employment opportunities, and climate benefits to consider. Given the often limited administrative capacity and expertise of small, isolated tribes to initiate and manage such projects, it could be helpful for geographically-proximate tribes to join forces in an intertribal consortium to share technical, administrative, and financial resources. Supportive policies, such as zero-interest loans, by state and federal governments and agencies are also critical as are fewer bureaucratic hurdles for tribes to navigate the renewable energy development landscape.

## CASE STUDY QUESTIONS

1. What are some of the reasons that biomass-to-energy project may be a suitable/unsuitable energy supply choice for rural tribal reservations.

2. In assessing the feasibility of a biomass-to-energy project what are the key factors that should be considered in the analysis and why?

3. Which factors have dominant impacts on the results of the feasibility assessment? How could they be addressed to make biomass-to-energy projects more feasible?

4. What are the short-term and longer-term factors most likely to influence utility energy prices and do these factors also influence the cost of biomass-to-energy plants.

5. Which geographic regions and local economic contexts are most suitable for biomass-to-energy projects for Native American tribes? Is the case study illustrated in this paper widely generalizable to other locations and why or why not?

6. How does the NPV analysis influence the cost of the biomass-to-energy project and are there additional cost factors that should be taken into account or be excluded from the analysis?

7. The Cocopah Tribe project is only considering residential energy demand. How does the inclusion of additional infrastructure such as casinos and resorts or other industrial process uses affect the scoping of the project and necessary back-up planning in case of disruptions in biomass-based energy generation?

8. How could state and federal governments assist Native American tribes to assess renewable energy projects in general and biomass projects in particular? How can they, furthermore, help with overcoming the cited obstacles of financing, knowledge and expertise, and promote tribal sovereignty at the same time?

## AUTHOR CONTRIBUTIONS

LD conceived the study and conducted the background research. WA provided research input and overall guidance. TS provided additional input and guidance. LD and TS drafted the manuscript. LD, WA, and TS edited and agreed on the submitted version of the manuscript.

None to declare.

## COMPETING INTERESTS

The authors have declared that no competing interests exist.

## SUPPLEMENTARY MATERIALS

Teaching Notes S1. Docx.

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