Abstract
The mission of this project was to provide a preliminary feasibility assessment of powering different marine carbon dioxide removal (mCDR), marine carbon capture (mCC), and marine carbon sequestration (mCS) strategies with marine energy. In this report, carbon capture (CC) refers to methods that can separate or capture carbon dioxide (CO2) from the air or ocean; carbon sequestration (CS) refers to methods that store CO2 obtained by capture methods out of the atmosphere for long periods of time; and carbon dioxide removal (CDR) refers to methods that do both. The project found that mCDR powered by marine energy and offshore wind energy available in the United States could meet global CDR scales needed by 2040 and 2050 to limit warming to 1.5°C by 2100 [1, 2]. Note that this preliminary estimate assumes that it is possible to harvest all the marine and offshore wind resources available in the United States with existing technology options, and it does not account for the power needed for monitoring these methods, as these power needs are not yet well defined and require further research. Additionally, these CDR scales will still require emissions reductions [2].
While the focus of this study was the potential of marine-energy-powered mCDR, mCC, and mCS, offshore wind energy was included to better understand the amount of CO2 that could be removed, captured, or sequestered when these methods are powered by U.S. offshore renewable energy sources, and compare the potential scales that can be achieved using marine versus offshore wind energy. Marine energy alone could still be used to reach CDR scales of about one gigaton per year, which, despite being less than necessary to reach global targets, is still a significant step toward mitigating climate change.
This preliminary feasibility study was split into two parts: (1) the viability of the mCDR, mCC, and mCS methods in general, and (2) their high-level compatibility with marine and offshore wind energy. The viability of the methods was assessed by determining their energy requirements, location specifications, scalabilities, cost, technology readiness levels, and environmental impacts via a literature review along with informed estimates based on information from literature, which largely involved unit conversions (see Section A.2 for more details). Artificial upwelling, deepocean storage, electrochemical mCDR and mCC, offshore microalgae cultivation, and seaweed farming and sinking along with their monitoring requirements were investigated because these methods all require power at sea (Figure ES-1). Figure ES-2 shows more details on which methods are mCDR, mCC, or mCS.
The high-level compatibility of the methods with marine and offshore wind energy involved estimating possible mCDR, mCC, and mCS scales that could be achieved using the marine and offshore wind energy available in the United States; note that the scales of mCC and mCS were also investigated because these methods could be used together for mCDR. The energy and location requirements determined in the literature review from part one as well as the resources of marine and offshore wind energy that could be harvested using existing technologies at these locations were used to determine these scales. These calculated scales were limited by the maximum scales reported in literature. The scales that could be reached with the marine and offshore wind energy available to these methods and the fraction of wave, tidal, current, ocean thermal, and river energy available in the locations where the mCDR, mCC, and mCS methods could be performed were recorded as a preliminary approximation of compatibility. These details highlight not only how effective the mCDR, mCC, and mCS methods could be in the United States but also which types of energy sources they could benefit from the most.
The literature review found that there are clear and significant environmental risks to scaling up biological mCDR and mCC methods such as artificial upwelling and seaweed farming and sinking, whereas certain methods of mCS or deep-sea storage of CO2, such as injection into self-sealing seabed sediments and subseafloor basalt mineral deposits are currently believed to be environmentally safe, although more research is needed to verify this [3, 4, 5, 6, 7, 8, 9, 10]. Additionally, electrochemical (eChem) mCDR methods such as adding alkalinity to the ocean and carbonate formation were found to have the potential to capture CO2 at scales above 1 gigaton of CO2 per year (GtCO2/yr) and sequester it for more than a thousand years [1, 4, 11, 12, 13, 14, 15]. Moreover, offshore microalgae farming is expected to reach a much smaller scale and mainly convert CO2 into short-lived products as a form of mCC; however, the field could benefit from collaboration with marine energy researchers because they are already using ambient marine energy, mainly wave energy, to improve yields [16, 17, 18].
To identify the most promising forms of mCDR, mCC, and mCS more clearly, thresholds were estimated for longevity of CO2 storage, possible global scale of CO2 capture, energy needs, and cost, which are detailed in Section 4.1. Overall, the eChem mCDR and deep-sea sequestration methods were best able to meet the thresholds, especially eChem methods that add alkalinity to the ocean or create carbonates from dissolved inorganic carbon and deep-sea seabed and basalt sequestration. eChem mCC that extracts pure CO2 from the ocean (referred to as “acid stripping CO2”) and sequestration in deep-sea aquifers were also promising because the pure CO2 from the relatively energy-efficient eChem mCC method could be sequestered via other methods, and aquifer sequestration is more developed and has a higher technology readiness level than the other mCS methods.
Additional research found that the eChem mCDR methods, including eChem acid stripping CO2 using aquifer sequestration as storage, could theoretically scale to capture and sequester 10 GtCO2/yr using U.S. marine and offshore wind energy, which would meet the necessary global CDR targets needed by 2040 and 2050 to limit warming to 1.5°C by 2100. Note that these methods not only were the most promising from the literature review but also had the highest scalabilities when powered by U.S. offshore renewable energy. This is possible even after the offshore renewable energy resources are used to meet coastal energy demand, highlighting the massive level of offshore energy resources that could be utilized to mitigate climate change. These results combined with the findings from the literature review show significant promise in powering mCDR and mCC combined with mCS with marine energy and offshore wind. Additionally, wave energy was found to generally be the most available marine energy source to the majority of the methods except for deep seabed sequestration, which had primarily ocean thermal energy available to it. This indicates that wave energy harvesting systems could be used at scale to power mCC, mCS, and mCDR however more research is needed to validate this.
It should be noted that removing this much CO2 from the ocean through 2100 will be a massive effort and an extreme change to the environment over a short period of time. Though it is theoretically possible to implement these ocean-based methods to significantly mitigate climate change, the ocean is a challenging place to monitor due to its complex dynamics and vast space.
Therefore, to deploy any of the mCC, mCS, and mCDR technologies described in this report, it will be essential to develop robust ocean environmental monitoring methods and research programs that span prototype deployments to small-scale pilot projects to global-scale deployment to identify and remediate undesirable outcomes. Currently anticipated monitoring requirements are listed in Table 2.
Though there will be significant challenges in implementing offshore renewable-energy-powered mCC, mCS, and mCDR, the findings from this report indicate that these technologies can enable significant progress toward mitigating climate change. Therefore, the compatibility between these technologies and offshore renewable energy must be assessed further through additional research that more closely examines how these technologies could be integrated with each other.