Abstract
Current efforts to reduce carbon dioxide (CO2) emissions will not be enough to cool the rapidly warming climate. To achieve global carbon goals, capturing carbon from the atmosphere or ocean and sequestering it will be necessary in addition to emissions reductions. There are many different approaches for sequestration, one of which is marine carbon dioxide removal (mCDR). This paper focuses on the conceptual design of a system that sequesters CO2 (in the form of super-critical carbon) in the ocean via a pump directly driven by wave energy.
The most basic system concept of this CArbon Sequestration Harvesting Energy from Waves (CASHEW) device includes a wave energy converter (WEC), a pump, and a pipe. Functional requirements are laid out to guide the concept. First, the system must integrate with a carbon delivery method; however, the carbon capture and transportation to sequestration site are considered out of scope for this design problem. We will quantify the lifetime carbon cost of the system, including manufacturing, maintenance, installation, and decommissioning carbon costs. This will determine the minimum rate at which CASHEW must sequester CO2 (per unit and at scale) in order to offset carbon costs. It must be ensured that the CO2 will not resurface, both during the pumping process and once sequestered. The WEC must provide enough pressure to counteract hydrostatic pressure acting on the pipe (preventing implosion), pump the CO2 down to a depth where it is no longer less dense than sea water, avoid two-phase flow, and inject the carbon. The carbon may be injected into deep sea beds, aquifers, or deep sea basalt, each with their own depth requirements and carbon storage lifetime. The WEC, or WECs, depending on power requirements, must interface mechanically with the pumping system to transfer power. Additionally, lifetime economic costs must be kept beneath a viable threshold. Softer requirements are keeping the maximum change in ocean pH of beneath a threshold determined by marine life at that location and limiting ocean space used to what can be reasonably permitted.
The unique aspect of this design is that there is no transfer of mechanical power to electrical power. By directly driving the pump with WEC mechanical power, we reduce power losses and design a power production system well-suited for mCDR. This gives wave energy an edge over other renewables, such as offshore wind or floating solar, that require intermediary conversion to electricity. One initial concept involves integrating with existing decommissioned offshore oil rigs to address initial infrastructure needs, which will come with its own set of new requirements. Once the type of sequestration location (sea bed, aquifer, or basalt) is chosen, decisions about the location in the ocean, WEC architecture, offshore infrastructure, environmental safety checks, and economics can be quantified.