PNNL

Abstract

Tropical islands and coastal communities in the United States and internationally suffer from high energy costs, unreliable electrical supplies, poverty, and underemployment, which are all being exacerbated by climate change. Multi-use Ocean Thermal Energy Conversion (OTEC) systems could align with the goals and values of these underserved and remote communities as scalable platforms that can provide baseload power, freshwater, and food, as well as potentially support new industries. The cold, nutrient-rich deep seawater needed for OTEC’s power cycles has many promising byproduct applications such as seawater air conditioning (SWAC) for residential and industrial cooling, enhancing aquaculture farms (e.g., cooling and nutrients for seaweed, shellfish, finfish), producing freshwater through desalination, and generating efuels such as ammonia and hydrogen. Developing multi-use OTEC systems for islands and remote communities could help meet the United Nations’ Sustainable Development Goals #7 (Affordable and Clean Energy) and #13 (Climate Action). With support from the U.S. Department of Energy’s (DOE) Water Power Technologies Office (WPTO), the Pacific Northwest National Laboratory (PNNL) assessed the feasibility and design considerations for developing the first U.S. multi-use OTEC platform based upon the existing onshore OTEC system run by the Natural Energy Laboratory of Hawaii Authority (NELHA) in Kona, Hawaii. The project team examined the multiple uses of OTEC water by conducting an OTEC resource characterization around Kona, exploring design considerations for OTEC and various additional uses, examining OTEC’s potential environmental effects and modeling the cold water return, engaging with the local community to educate and gather feedback, and considering tradeoffs among multiple uses. The outcomes of the analyses indicate that there is ample thermal differential between the warm surface water off Kona to operate an OTEC plant. The major environmental concern for the development of an OTEC plant revolves around the need to discharge the cold water to an appropriate depth after the OTEC heat exchange process. To address this need a new open source OTEC discharge water model was developed and linked to the resource characterization model to demonstrate the depth and location where the discharge should be sited. The project team partnered with Hawaii Sea Grant to engage with community leaders and the public and trained a group of outreach and engagement professionals in the topic area and visited and engaged with many of the tenants and management of the NELHA site, gauging their knowledge, interest, and long-term vision for the existing (and possible future) OTEC plant at NELHA. Following the analysis of each potential use of OTEC process water in addition to power production, the challenges of combining each use and the tradeoffs required was examined. The results show that there are several viable pathways to combine onshore OTEC power production with other uses, most notably SWAC and industrial cooling, including cooling for aquaculture operations in warm climates. Seawater desalination also shows promise in collaboration with open cycle OTEC power production. Other future uses that have promise for offshore OTEC power production include the extraction of critical minerals from seawater and production of efuels. The report concludes with several recommendations for future work to address the most pressing data and information needs around OTEC development, as well as the project team’s next steps.