Ocean Thermal Energy Conversion Resources Research Articles

Growth of ocean thermal energy conversion resources under greenhouse warming regulated by oceanic eddies

The concept of utilizing a large temperature difference (>20 °C) between the surface and deep seawater to generate electricity, known as the ocean thermal energy conversion (OTEC), provides a renewable solution to fueling our future. However, it remains poorly assessed how the OTEC resources will respond to future climate change. Here, we find that the global OTEC power potential is projected to increase by 46% around the end of this century under a high carbon emission scenario, compared to its present-day level. The augmented OTEC power potential due to the rising sea surface temperature is partially offset by the deep ocean warming. The offsetting effect is more evident in the Atlantic Ocean than Pacific and Indian Oceans. This is mainly attributed to the weakening of mesoscale eddy-induced upward heat transport, suggesting an important role of mesoscale eddies in regulating the response of thermal stratification and OTEC power potential to greenhouse warming.

Site selection of Ocean Thermal Energy Conversion (OTEC) plants for Barbados

Tropical climates provide ideal year-round marine thermal gradients that meet the requirements for the operation of commercial Ocean Thermal Energy Conversion (OTEC) plants. This study assesses Barbados' OTEC resources and viability at several locations around the island. The island's coastline was divided into four quadrants and four proposed plant locations were selected within each quadrant based on the shortest distance to the coast. These locations were ranked via a multi-criteria decision analysis using the AHP-TOPSIS methodology. The results of the analysis ranked east coast plant locations highest, with the north, south and west coasts following in that order. The results of this study can guide OTEC development as well as marine planning and investment on the island.

Estimation of Ocean Thermal Energy Conversion Resources in the East of Malaysia

The Malaysian Government has set a target of achieving 20% penetration of Renewable Energy (RE) in the energy mix spectrum by 2025. In order to get closer to the target, Ocean Thermal Energy Conversion (OTEC) aligned with solar PV, biogas and biomass energy sources must be evaluated and comprehended. Hybrid OTEC systems consisting of energy and water production are currently under research and validation. Therefore, for the construction of a commercial OTEC plant, 1 MW or 2.5 MW, the choice of a strategic location or potential site is vital. In this paper, oceanographic data such as seawater temperature, depth, salinity and dissolved oxygen obtained from the Japan Oceanographic Data Center (JODC) for Semporna, Tawau, Kudat, Pulau Layang-Layang and Pulau Kalumpang in Sabah, Malaysia, are reported. The RE available from the Exclusive Economic Zone (EEZ) on the coast of Sabah was estimated based on the JODC data obtained. There were no remarkable differences in temperatures between the five sites, which were reported as approximately 27 °C at the surface and 7 °C at depths below 600 m. The surface salinities below 100 m at those sites were slightly lower than the deeper waters, where the salinity increased up to approximately 34.5 PSU. Dissolved oxygen data from the Pulau Kalumpang site showed a slight increment to approximately 4.7 mL/L at depth intervals below 50 m, before declining steadily to approximately 1.7 mL/L along with the depth. The temperature-salinity profiles of the Malaysian sites were congruent with those of Palau, Kumejima and Okinawa, but not with that of Fiji, where the salinity profile showed a distinct variation at the relative depth (below 200 m). Estimates of RE using two different methods were used to prove the potential of OTEC in Malaysia.

A novel modelling approach to the identification of optimum sites for the placement of ocean thermal energy conversion (OTEC) power plant: application to the tropical island climate of Mauritius

The geographical location of Mauritius near the warm tropical waters of the Indian Ocean coupled to the vast exclusive economic zone approximating 2.3 million square kilometers, encourage the promotion of ocean thermal energy conversion (OTEC) systems. Technological advancements have enabled offshore structures to pump the cold water lying at 1000 m depth in the seawater column to the surface, and through the temperature difference set up with the warm surface layer, drives a turbine and generates electricity. In this study, a model has been developed to compute the temperature difference between the deep (1000 m) and surface (20 m) seawater layers around Mauritius. An algorithm has been implemented to determine the net power generated from a proposed OTEC power plant, acquired through the processing of sea surface temperature satellite images, at a resolution of 1 km. The spatial and temporal variations of the net power generated has been observed by splitting the annual data into four monsoonal time frames. Results show that the south-western region of Mauritius possesses high OTEC resources, with annual mean daily net power generation capacity of about 95 MW, representing about 20% of the peak power demand of the island. Moreover, the bathymetry of the southern region is propitious due to deep cold water availability at a proximity of less than 5km from the coastline. The energy and exergy efficiencies of the OTEC system are found to be 1.9 and 22.8%, respectively. A cost–benefit analysis indicates that profits of the order of 4.5 times the initial investment can be generated.

Injection power cycle applied in OTEC power plants

Ocean Thermal Energy Conversion (OTEC) power plants use the natural temperature differences found in the ocean of tropical regions to drive turbines that create electricity.A thermal compressor (injector) applied in a power cycle allows extension of expansion of a working fluid. Therefore, the unique feature of the Injection Power Cycle (IPC) is that the maximal temperature difference of the working fluid in a power cycle is greater than the temperature difference between a heat source and a heat sink. This results in the increase of pressure drop and technical work of the working fluid during its expansion.Furthermore, the OTEC power can be used:• for seawater desalination and production of enough potable water to support large populations located near the coast and• to produce energy carriers such as Hydrogen (stored in HYDRNOL: Organic Liquid Storage), which can be shipped to areas not close to OTEC resources. This could be fuel for thermal power plants, as well as for internal combustion vehicles, such as gas turbines and reciprocating engines. The ability to store and transport hydrogen within a liquid carrier at standard (room) temperature and atmospheric pressure provides many advantages over current hydrogen storage methods. The need for large, heavy storage tanks is eliminated. Major changes in the fuelling infrastructure of the world are avoided. The risk for explosion is minimized since the hydrogen is stored within a molecule and not in its elemental form. Furthermore, it is less flammable than gasoline or diesel. Also, HYDRANOL is cost effective when compared to traditional (fossil) fuels.

OTEC Potential in the Indonesian Seas

Abstract The potency of sea temperature difference between the surface and subsurface below the thermocline that could be converted into electrical energy has been investigated using ocean temperature datasets during January 2009 to December 2009. This study focused on seeking potential site of Ocean Thermal Energy Conversion (OTEC) in the Indonesian seas where most water has potential temperature gradient from surface to the bottom. Most potential OTEC resources are located in South Kalimantan and North Sulawesi waters within depth down to 500 m. Furthermore, this study emphasizes for optimizing OTEC utilization in a shallower depth (500 m) should be performed in the future.

An assessment of global Ocean Thermal Energy Conversion resources under broad geographical constraints

Rates of Ocean Thermal Energy Conversion (OTEC) are assessed with a high-resolution (1° × 1°) ocean general circulation model when broad geographical restrictions are imposed on the OTEC implementation area. This may correspond to practical or legal limitations, such as the cost of long submarine power cables or the extent of Exclusive Economic Zones. Because some environmental effects predicted under large-scale OTEC scenarios exhibit a strong asymmetry among major oceanic basins, numerical experiments where the OTEC domain is restricted to such specific areas are also conducted. Results suggest that in all cases, a rate of about 0.2 TW per Sverdrup of OTEC deep cold seawater is sustained when overall OTEC net power peaks. At that juncture, temperature profiles in the OTEC implementation areas are affected in similar ways, while the strength of the Thermohaline Circulation roughly doubles. Overall geographical constraints simply defined by distance to shore, given the model's 1° horizontal resolution, produce global OTEC net power maxima of 12–14 TW. In such cases, OTEC net power density approximately increases in inverse proportion to the OTEC implementation area. Limiting OTEC development to the Indo-Pacific yields results similar to the global case with a maximum proportional to the implementation area (12 TW), but simulations restricted to the Atlantic behave quite differently. In the latter case, OTEC net power peaks a little over 5 TW. It is estimated that producing half the predicted power maxima would substantially limit large-scale environmental temperature changes in each case.

An Assessment of Global Ocean Thermal Energy Conversion Resources With a High-Resolution Ocean General Circulation Model

Global rates of ocean thermal energy conversion (OTEC) are assessed with a high-resolution (1 deg × 1 deg) ocean general circulation model (OGCM). In numerically intensive simulations, the OTEC process is represented by a pair of sinks and a source of specified strengths placed at selected water depths across the oceanic region favorable for OTEC. Results broadly confirm earlier estimates obtained with a coarse (4 deg × 4 deg) OGCM, but with the greater resolution and more elaborate description of key physical oceanic mechanisms in the present case, the massive deployment of OTEC systems appears to affect the global environment to a relatively greater extent. The maximum global OTEC power production drops to 14 TW, or about half of previously estimated levels, but it would be achieved with only one-third as many OTEC systems. Environmental effects at maximum OTEC power production are generally similar in both sets of simulations. The oceanic surface layer would cool down in tropical OTEC regions with a compensating warming trend elsewhere. Some heat would penetrate the ocean interior until the environment reaches a new steady state. A significant boost of the oceanic thermohaline circulation (THC) would occur. Although all simulations with given OTEC flow singularities were run for 1000 years to ensure stabilization of the system, convergence to a new equilibrium was generally achieved much faster, i.e., roughly within a century. With more limited OTEC scenarios, a global OTEC power production of the order of 7 TW could still be achieved without much effect on ocean temperatures.

Estimates of global Ocean Thermal Energy Conversion (OTEC) resources using an ocean general circulation model

Abstract Global Ocean Thermal Energy Conversion (OTEC) resources are assessed for the first time with an ocean general circulation model (OGCM). Large-scale OTEC operations are represented with fluid sources and sinks of prescribed strength in global (4° × 4°) MITgcm simulations. Preliminary steady-state (time-asymptotic) results show similarities, but also significant differences with earlier one-dimensional (1-D) studies. It is confirmed that global OTEC resources are likely limited by OTEC flow effects on the stability of the vertical oceanic thermal structure. Such a limit is several times greater in a full three-dimensional context, however, with an estimated maximum annual OTEC net power production of about 30 TW. The significant OTEC flow rates corresponding to maximum net power output would result in a strong boost of the oceanic thermohaline circulation (THC). In contrast to simple 1-D analyses, the present simulations of large-scale OTEC operations also show a persistent cooling of the tropical oceanic mixed-layer. This would be balanced by a warming trend in the higher latitudes, which may practically limit OTEC deployment to smaller flow rates than at maximum net power output. An annual OTEC net power production of about 7 TW, for example, could be achieved with little effect on the oceanic temperature field.

Mapping available Ocean Thermal Energy Conversion resources around the main Hawaiian Islands with state-of-the-art tools

This paper aims to demonstrate how the evaluation of Ocean Thermal Energy Conversion (OTEC) resources can benefit from currently available high-resolution ocean models. The case of waters around the main Hawaiian Islands is presented because of its relevance to the future development of OTEC. OTEC resources are defined here by ocean temperature differences between water depths of 20 and 1000 m, with little loss of generality. Using state-of-the-art tools like the HYCOM+NCODA (1/12°) model affords the possibility to track changes on a daily basis over a wide area (e.g., 17 °N to 24 °N and 153 °W to 162 °W). An examination of numerical data over a time period of 2 years reveals interesting geographical patterns. It is found that average OTEC temperature differences are consistently higher (by about 1 °C) west of the islands, whereas the amplitude of the yearly cycle globally decreases from north to south as expected. Better OTEC resources in the lee of the islands are attributed to the narrow eastward-flowing Hawaiian Lee Counter Current. All other things being equal, a change of 1 °C in the resource typically would amount to a 15% variation in net OTEC power output.

An estimate of Atlantic Ocean thermal energy conversion (OTEC) resources

An interhemispheric box model of the Atlantic thermohaline circulation (THC) is modified by replacing the tropical box with two vertically resolved sub-domains. Seawater flows from large-scale ocean thermal energy conversion (OTEC) are allowed in one of the tropical sub-domains. Under present conditions and standardized OTEC operations, steady-state net power production density would reach a maximum of about 80 kW/km 2 (corresponding to 1.8 TW) with a cold seawater withdrawal per unit area of about 14 m/yr. This maximum reflects the impact of large OTEC flows on the oceanic thermal structure, although the THC would not be significantly affected. It is larger than a recently suggested worldwide value of the order of 30 kW/km 2 because of the relative strength of the Atlantic THC. Under asymmetric high-latitude warming scenarios potentially representative of current climatic trends, a substantial weakening or a reversal of the THC are possible. In the former case, recoverable OTEC resources could practically vanish. In the latter case, the emergence of a stronger reverse THC eventually could boost OTEC resources. Such events are hypothetical and would unfold over centuries, but the mere possibility of their occurrence challenges the accepted notion that OTEC resources are forever renewable.

A Preliminary Assessment of Ocean Thermal Energy Conversion Resources

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.