A Preliminary Assessment of Ocean Thermal Energy Conversion Resources

Abstract

Worldwide power resources that could be extracted from Ocean Thermal Energy Conversion (OTEC) plants are estimated with a simple one-dimensional time-domain model of the thermal structure of the ocean. Recently published steady-state results are extended by partitioning the potential OTEC production region in one-degree-by-one-degree “squares” and by allowing the operational adjustment of OTEC operations. This raises the estimated maximum steady-state OTEC electrical power from about 3TW(109kW) to 5TW. The time-domain code allows a more realistic assessment of scenarios that could reflect the gradual implementation of large-scale OTEC operations. Results confirm that OTEC could supply power of the order of a few terawatts. They also reveal the scale of the perturbation that could be caused by massive OTEC seawater flow rates: a small transient cooling of the tropical mixed layer would temporarily allow heat flow into the oceanic water column. This would generate a long-term steady-state warming of deep tropical waters, and the corresponding degradation of OTEC resources at deep cold seawater flow rates per unit area of the order of the average abyssal upwelling. More importantly, such profound effects point to the need for a fully three-dimensional modeling evaluation to better understand potential modifications of the oceanic thermohaline circulation.

Ocean Thermal Extractable Energy Visualization- Final Technical Report on Award DE-EE0002664. October 28, 2012

“…Presentday heat exchanger technology suggests that regions with ΔT ≥ 20°C provide a resource of potential interest. Such an assessment has been reported previously (Nihous, 2005;2007), and the theoretical assessment in this project updates and improves on that work.…”

“…The results of this model were provided to the NREL for independent validation and for incorporation into a GIS database for renewable energy resources. This visualization of quantifiable power output with high resolution is a significant achievement over past work, which relied on low resolution data and only visually reported the difference between sea surface temperature and temperature at 1000 m (Nihous, 2007). The result serves as the first step to providing the "technical" resource assessment and thus establishes the ability to assign power extraction estimates regionally.…”

“…This is because finding relevant values for ΔT requires a criterion for which to find T D , and this, in turn, requires information about the technology under consideration. The previous climatology-based work (Nihous, 2007) used temperatures from 1000 m for T D , but choice of depth is largely arbitrary.…”

“…The approach here builds on a global OTEC potential estimate of 5 TWe (Nihous, 2007) and provides a method for obtaining regional results as called out in subtask 2.2 of the SOPO. Based on earlier work, Nihous hypothesized that global OTEC sustainability would be constrained by the production of cold water in the polar regions and used a simplified one-dimensional model of global usage of cold-water resources by OTEC constrained to produce steady-state maintenance of those resources, albeit at a somewhat reduced level compared to the present.…”

“…Published estimates (Nihous, 2007) used a simplified model for the OTEC process and its interaction with the water column to scale the commonly discussed global value of ~10-20 terawatt (TWe) potential back to a more realistic value of about 5 TWe. The present study uses a completely different approach, discussed in Section 5.1, to estimate local limitations based on cold-water usage rates.…”