Low-impact land use pathways to deep decarbonization of electricity

Using the Optimal Renewable Energy (ORB) [13] and MapRE [22] models, we identified onshore wind, solar, and geothermal potential under four environmental Siting Levels (SL)—representing the least protective (SL1) to the most protective levels (SL4; figure 1). Results show significant solar PV potential, with the highest quantity and quality in the southwestern states (figure 1; Supplementary Materials (SM) figures S3A, S4A (stacks.iop.org/ERL/15/074044/mmedia)). Onshore wind resources are distributed throughout the Western U.S., with few remaining undeveloped resources in California and large concentrations of high-quality resources in New Mexico, Wyoming, Oregon, and Washington (figure 1, SM figures S3B, S4B).

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We identified resource potential for all areas within non-California states (Unconstrained case) as well as only within RESOLVE Zones, which is currently used by California planning agencies for representing resource areas (Constrained case). We treat the Unconstrained assumptions as the main case because the RESOLVE Zone construct may be altered in the future by state planning authorities since it artificially restricts resource availability.

Although we modeled suitable sites for geothermal, the amount of geothermal potential was significantly lower compared to wind and solar (SM figures S3B, S4B). Offshore wind was not included to maintain consistency with assumptions in existing California planning agency versions of RESOLVE. Also, at the time the study was conducted, publicly available potential and cost data for offshore wind along the California coast had not yet been well characterized or vetted in stakeholder processes. Based on technology costs in RESOLVE supply curve, estimated levelized costs of floating wind turbines would be high enough that the model would not have selected offshore wind over solar PV (with battery storage) or out-of-state onshore wind [25].

Using supply curves created from the resource potential estimates, RESOLVE created a generation portfolio for each Siting Level and Geographic case by minimizing the total system costs. The In-State Geographic case restricts development to within California; the Part-West case expands resource availability to Arizona, New Mexico, Nevada, Oregon, and Washington states; and the Full-West case further expands availability to Idaho, Utah, and Wyoming (figure 4; SM figure S2; SM table S2). We also explored the sensitivity to more behind-the-meter solar (High Distributed Energy Resources or High DER case; SM table S3), lower battery costs (Low Battery Cost case; SM table S4), and spatial restrictions on resource availability (i.e. within RESOLVE Zones) combined with limitations on maximum area for solar (Constrained case; SM figure S2). We produced a total of 61 portfolios or scenarios for 2050, all compliant with economy-wide emissions reductions of 80% below 1990 levels and all generating 102%–110% renewable and zero-carbon electricity by 2050 based on retail sales. Cost results are presented in terms of the annual levelized cost of serving electricity load. Reported numbers reflect the costs of the portfolio, as well as the fixed costs of distribution (including utilization cost for behind-the-meter PV), transmission (including utilization cost for distributed energy resources), demand-side management, existing generation expected to remain in service in 2050, and resources already reflected in utility plans. These existing and contracted resources total to $64.5 billion USD and are referred to as 'unmodeled' costs because they do not vary between scenarios.

2.2.1. Effects of Geography

Geographic availability of resources affects not only the generation technology mix, but also the total generation capacity required (99–124 GW in Full-West vs. 130–162 GW in In-State; figures 2(A), 4). Lower total capacity and significantly greater wind capacity is selected in the Part-West and Full-West Geographies. Grid storage—primarily 4-hr battery—decreases dramatically with increasing Geography (from 55 GW to 8.4 GW in SL1; and 64 GW to 27 GW in SL4). A portfolio with more wind will need less storage (figure 2(D)). By allowing more wind resources to be selected, increasing geographic availability reduces solar capacity by 30%–33% for Part-West and by 40%–62% for Full-West (range spans Siting Levels; SM figure S5).

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Across all scenarios, total annual costs ranged from $95 to $114 billion USD in 2050 (figure 2(B)), or an average retail rate of $0.24–$0.28 per kilowatt-hour (in 2016 USD; California's average rate in 2017 was about $0.16 per kWh). Access to out-of-state renewable energy significantly reduces cost. For SL3, expanding geography from In-State to Full-West reduces costs from $113 to $99 billion USD, or 12% (figure 2(B)).

2.2.1.1. Effects of environmental siting levels

Siting Level constraints also affect the generation mix and the total generation capacity. The amount of (available and selected) wind capacity declines with more land protections in the Part-West and Full-West scenarios. Selected utility-scale solar capacity increases with more land protections from SL2 to SL4 for all Geographic cases (figure 2(A)). By reducing wind availability and requiring more solar capacity, more land protections also increase the need for battery storage by about 14% (In-State), 20% (Part-West), and 230% (Full-West) between SL1 and SL4 (figure 2(D)). Siting Level constraints affect the geographic distribution of selected capacity across states. For example, reduced wind capacity in New Mexico and Wyoming is replaced by increased solar in California, Arizona, Nevada, and Utah in SL3 and SL4 in Full-West (SM figure S5).

Siting Levels are also a key determinant of total costs. All else equal, more land conservation increases the total resource cost. For In-State cases, costs increase 7.5% between SL1 and SL4 (figure 3(A)), or from $106 to $114 billion (figure 2(B)). The marginal impact of each successive Siting Level can vary widely. For example, in the In-State cases, SL2 has a modest incremental cost increase over SL1 ($1.4 billion, or 1.3%), while the impacts of SL3 and SL4 are more significant ($6.2 and $8 billion, or 5.6% and 7.5%, respectively; figures 2(B) and 3(A)).

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2.2.1.2. The interaction of Geography and Siting Levels

While increasing siting protections increases total costs, expanding geography reduces total costs. These two trends can be combined to produce portfolios that satisfy both land use and cost objectives, achieving siting protections at low cost. Generally, procuring renewable electricity from more states can offset the cost increase associated with increasing land protections. SL1 In-State incurs nearly the same cost as SL3 in Part-West and is actually 6.5% more expensive than SL3 in Full-West (figure 3(B)). Thus, it is more cost effective to achieve SL3 in the out-of-state scenarios than to achieve SL1 In-State. Total costs are nearly the same for the most protective case (SL4) in Full-West and the least protective case (SL1) in the In-State Geography (figure 3(B)).

2.2.1.3. Effects of lower battery cost and high behind-the-meter PV adoption

Overall, sensitivity analyses increasing behind-the-meter PV (High DER case) and reducing battery storage costs (Low Battery Cost case; SM figure S2) do not significantly alter the generation mix, the distribution of selected capacity between states (SM figure S9–S12), or the total costs (figure 3).

Lowering battery costs decreases the overall cost of portfolios, particularly for In-State scenarios (figure 3), but does not change the relative cost-effectiveness across scenarios, does not shift the technology mix, and does not significantly change the quantity of batteries selected (SM figures S9–S12). This indicates that the quantity of batteries selected is determined more by the mix of other available resources rather than cost. Perhaps the most significant effect of lower battery costs is that no additional pumped hydro storage is selected (SM figure S11).

Increasing residential and commercial BTM solar resources by about 35% by 2050 (SM table S3) has the effect of reducing selected utility-scale solar capacity by 5–5.5% in SL3, primarily in California (SM figures S9A–S12A), but had only minor or negligible impacts on the geographic distribution of selected resources (SM figures S9B–S12B). While costs to utilities are lower in the High DER cases, the total cost (including $2.2 billion USD of rooftop PV borne by homeowners and businesses) goes up.

Transmission requirements

In order to assess the environmental impacts of each portfolio, we selected potential wind and solar project areas to satisfy each portfolio's technology-specific generation requirements. This step is necessary for evaluating land use trade-offs and impacts as these can vary significantly by location, and power plants have specific siting requirements that make them more likely to be sited in some areas over others.

Results show that for the In-State Geography, increasing land protections causes solar development to shift from Southern to Northern California (figure 4). In Part-West, wind expands to New Mexico and the Oregon-Washington border. With increasing land protections, solar continues to shift northward in California, and wind shifts from New Mexico to the Pacific Northwest. In the Full-West scenarios, with increasing land protections, Wyoming and New Mexico wind selection reduces and is replaced by wind in the Pacific Northwest and Idaho.

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After selecting project areas, we used least-cost-path analysis to spatially model interconnection (gen-tie) transmission corridors required to interconnect selected areas to the existing network. With these spatially-explicit infrastructure build-outs, we assessed each portfolio's impact on natural and working lands through a 'Strategic Environmental Assessment'.

Environmental Exclusion Categories that were used to identify resource potential under each Siting Level were also used as general environmental metrics to explore overall impacts to natural and working lands. Without land protections, new solar and wind projects are likely to have sizable land impacts. Results show significant overlap (> 50%) between selected project areas and Categories 2, 3, and 4 (figure 5(A); see SM Methods and SM tables S9–S12 for datasets). Results suggest that applying these Environmental Exclusions could significantly shape the build-out of wind and solar power plants and that the lack of protections beyond legally protected areas (SL1) leaves open the potential to considerably impact natural lands.

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The overlap of selected project areas in SL1 scenarios with Category 2 lands ('Administratively Protected') is considerable: one-third of In-State solar, one-third of Part-West wind, and half of Full-West wind (figure 5(A)). Overlap with Category 3 lands ('High Conservation Value' including prime agricultural land) is greater: one-fourth of solar in all Geographies and half to three-fourths of wind in all Geographies under SL1 and SL2 (figure 5(A)). For Category 4 (areas with 'Landscape Intactness'), the overlap is also significant for SL1–SL3 scenarios: one-third of In-State and Part-West solar and half of wind in all Geographies (figure 5(A)). Generally, impacts are higher for SL1 and SL2 in the out-of-state Geographies.

Specific ecological metrics were chosen to explore focal species or habitats emphasized in public energy planning. Impacts to these specific ecological criteria—Critical Habitat, Important Bird Areas, Eagle Habitat, Sage Grouse Habitat, Big Game Habitat, Wetlands, and Wildlife Linkages—are considerably lower than the general Environmental Exclusion Categories (figure 5(A)). This suggests that ecological siting considerations are likely to be dominated by other factors, such as sensitive grassland birds and The Nature Conservancy portfolio areas (SM tables S9–S12), and may be emerging causes of land use conflicts. Impacts to these ecological impact metrics are greater in the Part-West and Full-West Geographies (figure 5(A)).

2.4.1. Agricultural impacts of generation infrastructure

However, we find that technology choices, resource costs, and the spatial build-out to achieve California's climate targets by 2050 are highly sensitive to the level of environmental siting protections and the availability of western regional resources. Importantly, we also find that the spatially-explicit portfolios have different impacts on natural and working lands. These results are broadly consistent with previous studies examining land use siting constraints on renewable energy development that found system cost increases of 2.3%–13% [15, 19] and that land availability is one of the most significant determinants of system cost increases [15].

The large overlap of selected project areas in SL1 and SL2 with Environmental Exclusion Categories suggests that developers may face siting challenges for a sizable majority of projects because a large percentage of desirable sites also have environmental and social value. However, impacts can be largely avoided in SL3 and SL4. These findings underscore the importance of integrated land use and energy planning and effective screening tools early in the project development cycle.

Between 35% to 50% of selected solar project areas in the In-State scenarios are sited on existing agricultural lands, with the majority being prime farmland in SL1 and SL2. One-third to half of all selected wind and solar is sited on rangelands. Thus, agrivoltaics [27, 28], or land uses that integrate agriculture and solar PV generation, have the potential to not only reduce conflicts between farmland and solar development, but actually promote synergistic, higher land-use-efficiency landscapes. This is because agrivoltaic systems, particularly in arid and semi-arid ecosystems, can increase both crop productivity as well as electricity generation due to micro-climate effects [28]. Wind farms are inherently well suited for integration in agricultural landscapes. Wind-friendly farming and ranching practices and wildlife-friendly wind farm design and operations on rangelands will be important in reducing or avoiding siting conflicts.

The low cost and relative abundance of solar enable large shares to be selected across all scenarios. However, in the absence of dispatchable generation, solar electricity must be complemented by wind or balanced by storage. Wind resources tend to be higher value in a high-variable-renewables system and are preferred over solar combined with batteries even in the low-battery cost sensitivity scenarios. However, compared to solar, wind is more limited in the more protective scenarios because of the relative rarity of low-conflict sites with high wind speeds. Wind resources are generally more spatially heterogeneous, while also having lower land use efficiencies when considering turbine spacing, making it more sensitive to land use restrictions. Thus, in wind-limited scenarios, additional selected solar capacity requires battery storage, driving up costs.

Although further research is required, results suggest that enough low-impact onshore wind and solar resources are available to meet the increased clean energy demand of all Western states. After accounting for the 41 GW of wind capacity in California's portfolio, 59 GW of wind potential remains in the eight Western states in SL3, and 89 GW if Montana and Colorado are included. The total load in these remaining seven states is currently roughly equal to California's. Assuming other states adopt a similar zero-carbon electricity target and pursue electrification, the technology mix and capacity requirements could closely resemble California's. Thus, a rough estimate of the wind and solar resource needed to achieve ambitious targets in all nine states would be double the requirements presented here for California, and still within estimates of remaining resource availability under SL3.

Increasing geographic availability of renewable resources can offset the cost increases due to lower impact siting within California. When California has access to out-of-state resources, we find that protecting areas with 'High Conservation Value' (SL3) and high 'Landscape Intactness' (SL4) can be achieved at a 2%–8% cost savings compared to avoiding just 'Legally Protected' (SL1) areas when development is limited to within California.

This suggests that if California increases regional renewable resource sharing or allows significant out-of-state procurement, standards for permitting regional projects will be critical to ensure that greater land protections in California do not lead to leakage of ecological impacts. Impacts to Wildlife Linkages under SL3 remain, which points to the importance of design and operational practices that can minimize unavoidable impacts to wildlife.

Although gen-tie and bulk transmission land requirements are a small fraction of the total (< 5%), transmission projects are known to have disproportionate siting impacts due to landscape fragmentation [29], have long lead times for permitting and construction [30], and suffer from interstate permitting and cost allocation uncertainties [31–33].

In High DER scenarios, 12%–14% of California's 2050 demand can be met with rooftop solar. These scenarios still require 100–145 GW of utility-scale capacity in SL3 across all Geographies, or 5,180–8,740 km² of land, which would double the recent historical rate of urbanization in California [34]. Nonetheless, these scenarios are limited in assuming development of 25% of technical rooftop PV potential in California [35] and do not include other land-sparing systems (e.g. floatovoltaics).

As other states pursue equally ambitious climate goals, increased competition for the best sites may change resource availability and introduce more land competition and conflict, leading to inefficiencies if not adequately coordinated. For example, compared to states independently pursuing bilateral contracts within or outside of the state, a regional electricity market could lower transaction costs and help develop the best renewable energy sites across the region that serves the demand of the whole region [36]. Coordinated interstate transmission planning could expedite the lengthy process of transmission development. Offshore wind resources can enable access to much needed wind generation in In-State or most land protective (SL4) cases.

For jurisdictions planning to use bio-electricity, the source of feedstocks creates additional land concerns that could be similarly anticipated through a planning framework such as the one proposed in the present study. Bioenergy may also play a synergistic role in future deep decarbonization pathways. As jurisdictions pursue economy-wide climate goals, they may look towards 'Natural Climate Solutions' (NCS) for achieving avoided or negative emissions [37, 38]. Some of these NCS are also sources of sustainable biomass feedstock (e.g. forest thinning for wildfire management [39]). A bioenergy market for these feedstocks could create a positive feedback cycle for sustaining NCS activities.

In the U.S., many exclusions in SL2 have been incorporated through local, state, and federal land use policy specific to renewable energy. Regulatory mechanisms for achieving SL3-4 could include a combination of land use policy or zoning changes. Non-regulatory mechanisms can include adopting the framework laid out in this study—incorporating environmental spatial data into long-term energy and transmission planning to send market signals to prioritize low-impact development.