Frontiers in multi-benefit value stacking for solar development on working lands

MBVS is the combination of energy and non-energy land uses on working lands to augment the total sum of benefits achieved through land management. The MBVS framework brings together the concept of value-stacking from the field of energy and the concept of multiple-benefit conservation [14] from the fields of natural resources management and food systems science (see figure 1).

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Value-stacking is an established concept in the field of energy, where compatible energy applications such as solar generation and battery storage are co-located to increase profitability and provide multiple grid services. Multiple-benefit conservation is an emerging concept in natural resources management and food systems science, where land is intentionally managed for ecological and societal co-benefits [14, 15]. Currently, value-stacking does not consider agricultural and conservation co-location and multi-benefit conservation does not include energy co-location. In the context of increased solar deployment, mounting water constraints and growing conservation needs, this is an opportune time to employ the MBVS framework to systematically explore how energy and non-energy uses of working lands can be co-optimized from technical and policy standpoints. To clarify, we use the phrase 'energy uses' to mean energy production directly on the land such as through solar PV installations. Bioenergy crops grown on working lands are considered crop production for the purposes of this article, even though they are used to generate energy subsequently. Further, while we do not consider wind energy generation, exploring the compatibility of competing renewable energy technologies on working lands represents a useful future expansion of the MBVS framework.

MBVS can thus be of two types:

• (a)  
  vertical stacking, where multiple land use activities are integrated (or layered) at the field level; and
• (b)  
  horizontal stacking, where multiple land uses are optimized across an operation or region to maximize feasibility, efficiency, and profitability.

We identify three distinct but interlinked scales at which MBVS can be conceptualized for solar development (see figure 2): field (a crop field/livestock pasture or paddock), operational (an entire farm/ranch) and regional (an agricultural region containing many farms/ranches). Distinct decision-makers and policies operate at each scale. At the field level, a farmer or rancher can vertically stack a solar project with agricultural activities (e.g. livestock grazing, row cropping) or nature conservation (e.g. reduced soil erosion, native plant restoration, wildlife habitat). For instance, a farmer could combine agrivoltaics with battery storage, where a portion of the solar panels are dedicated to on-site fertilizer production. The revenue streams for the farmer would include crop sales, the sale of solar energy and ancillary grid services, and the sale of excess fertilizer to nearby farms.

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Additionally, through horizontal stacking, solar can be integrated into a farmer's income diversification strategy at the operational scale. For instance, the introduction of water use constraints (such as due to the Sustainable Groundwater Management Act in California) could result in fallowing of a portion of an existing farm. The landowner could then pursue low-impact solar development with pollinator habitat on the fallowed land with continued crop production on the remaining land. Together, the field and operational levels constitute landowner decision-making. At the regional scale, solar facility siting can be optimized for competing land use benefits (e.g. food production, nature conservation, and amenity values). All three scales are influenced by national and international policies, commitments to climate change mitigation, and clean energy goals.

Assessing solar development opportunities through the MBVS framework enables stakeholders to intentionally consider: (a) the extent to which a range of co-located energy and non-energy activities could be compatible rather than competing, shifting from single-purpose land use planning to more complex and layered multi-use landscapes, (b) how solar development can support the long-term viability of working lands at three distinct scales, and (c) the central role of farmers and ranchers at the field and operational levels in deciding the mix of uses on their private lands. This perspective can inform decision-making by landowners, solar developers, local governments, and policymakers balancing energy and non-energy land uses.

Table 1 shows examples of MBVS on working lands by land use type (croplands/rangelands/other). The examples are chosen for variety and are drawn from both academia and industry, so this table is by no means a comprehensive list. Key gaps in research and policy that were identified while compiling table 1 are described in the following section. Some important insights from table 1 are as follows:

• (a)  
  For croplands, based on geography and crop type, prior work on integrating PV systems has focused on: (a) utilizing unused farmland for PV systems, such as land between grape trellises [16]; (b) identifying co-benefits of tracker-based PV systems placed over shade-intolerant crops, such as reduced evapotranspiration or increased frost protection [17]; and (c) integrating PV systems with shade-tolerant crops to enhance plant yield [6].
• (b)  
  MBVS can increase land productivity on rangelands too. Andrew et al (2021) find that lower herbage mass in pastures with PV systems was offset by higher forage quality, resulting in similar lamb production to open pastures [18]. Hassanpour Adeh et al (2018) find that PV systems covering a sheep pasture significantly benefitted soil moisture, water efficiency, and biomass production under the panels [19].
• (c)  
  Both research and demonstration projects highlight the increased economic resilience of working lands with MBVS. For instance, Makhijani et al [13] found that for a lettuce farm in Maryland where less than 10% of the land was under PV systems, farm profitability increased about 2.5 times in years of favorable grain prices and about 4 times when grain prices were unfavorable. They find similar results for dairy farms with PV systems on a portion of the land.
• (d)  
  While examples of stacking conservation with PV systems are limited, some work has highlighted opportunities to minimize adverse impacts on animal species under solar farms [20], although further research is needed.
• (e)  
  Overall, prior work has repeatedly pointed to the relative nascence of our understanding of the co-benefits and disadvantages of solar generation stacked with existing uses on working lands [21]. With some exceptions, prior work has also focused on stacking PV systems with just one non-energy use, rather than the multiple uses often present in such landscapes.

Drawing on table 1, we highlight several research gaps:

• (a)  
  Evaluating feasibility of multi-benefit stacks at scale and by geography: First, as table 1 shows, agrivoltaics are a growing area of research but there is limited work on bundling non-agricultural land uses like conservation with solar development, perhaps because solar development is a high-impact activity. Some research has investigated nature conservation under solar arrays [8, 40, 41], such as conservation of Kit Fox habitat in California's San Joaquin Valley [20], but examples are limited. With the growing deployment of solar projects on working lands, there is thus a need to identify (a) conservation approaches that can be stacked with solar PV, and (b) conservation approaches that can be stacked with both agriculture and solar PV, either horizontally or vertically.Second, many 'stacked' solutions, even in agrivoltaics, are currently either demonstrative academic projects or small-scale applications on farms that can afford them. There is thus a need to evaluate existing and new stacked solutions to identify the most feasible, scalable, and financially viable multi-benefit value stacks by geography.Third, as the last column in table 1 illustrates, vertical stacking at the field level is more commonly considered than horizontal stacking in terms of how landowners allocate their land at the operational level and how their decisions collectively impact regional land use. This relative paucity of horizontal stacking approaches could be due to the lack of an accepted conceptual framework, the complexity of accounting for multiple ecological and economic variables, and the recency of solar generation expansion on working lands. Nevertheless, it is a key gap that needs further research and analysis.
• (b)  
  Bundling multi-benefit payments: Despite growing global interest in payments for ecosystem services (PES), few studies have explored the legal and technical possibilities for landowners to 'bundle' distinct PES with other multi-benefit stack components. In the US, for example, new funding opportunities to combine solar with ecosystem services are beginning to address this need [42], but similar efforts will be needed in other contexts.
• (c)  
  Centering landowner decision-making: Private landowners often decide whether to put solar arrays on their lands based on factors like profit and viewshed, and in the face of climate-related challenges around water constraints and habitat conservation. Prior work on siting solar capacity has often relied on geospatial optimization and stakeholder analysis to identify least-conflict lands [3] at the regional, state and national scales. The critical variable of landowner decision-making at the field and operational levels thus needs more attention–how does a landowner decide the mix of land uses on their property, and what are the factors that drive their decisions? Such research can offer a more nuanced qualitative understanding of the key factors that influence decision-making in ways that complement quantitative spatial studies.
• (d)  
  Accounting for climate-driven shifts in working lands: Extreme heat and water scarcity are already impacting working lands and can strongly influence landowner decision-making. For example, landowners may enter long-term land leases with solar developers when they anticipate land fallowing due to decreased water availability [43]. Research on how climate-driven shifts will impact working lands can assist local and state government agencies as they engage stakeholders from the energy, agriculture, and conservation communities. This process would ensure that multiple goals can be achieved: habitat conservation, renewable energy development, protection of prime farmland from conversion, and food security.

From the policy perspective, we find several opportunities:

• (a)  
  Integrated land use and energy planning: Prior work has repeatedly pointed to the need for increased cross-sector coordination across policymakers, the solar industry, and farmer groups and improved alignment of policy incentives across energy planning, land use planning, water security, and nature conservation [44]. For instance, improved coordination can facilitate the provision of adequate transmission capacity to support utility-scale solar generation. It can also help account for future water availability, land fallowing, and conservation of high-quality habitat in solar siting.
• (b)  
  Simplify how farmers and ranchers can unlock multiple, value-stacked income streams: Many working lands are privately-owned, which means that land productivity is directly tied to the livelihoods and well-being of landowners. Access to value-stacked income streams via novel agricultural technology (AgTech) approaches, ecosystem service markets [45], and private sector sustainable sourcing initiatives could thus enable farmers and ranchers to increase and diversify their income. For example, rancher cooperatives could be created to streamline the establishment of solar leases by livestock operations, while maintaining grazing for improved soil health. This could unlock new income from carbon offsets.
• (c)  
  Develop multi-purpose environmental policies, wherein national, state, regional and local policies work in tandem towards the optimal combination of energy and non-energy services. For example, government agricultural subsidies could provide financial and technical support for piloting and deploying integrated solar-cropping systems, reducing the financial burden and risk to small farmers and ranchers.