Performance simulation of solar-boosted ocean thermal energy conversion plant

Ocean thermal energy conversion (OTEC) is a power generation method that utilizes small temperature difference between the warm surface water and cold deep water of the ocean. This paper describes the performance simulation results of an OTEC plant that utilizes not only ocean thermal energy but also solar thermal energy as a heat source. This power generation system was termed SOTEC (solar-boosted ocean thermal energy conversion). In SOTEC, the temperature of warm sea water was boosted by using a typical low-cost solar thermal collector. In order to estimate the potential thermal efficiency and required effective area of a solar collector for a 100-kWe SOTEC plant, first-order modeling and simulation were carried out under the ambient conditions at Kumejima Island in southern part of Japan. The results show that the proposed SOTEC plant can potentially enhance the annual mean net thermal efficiency up to a value that is approximately 1.5 times higher than that of the conventional OTEC plant if a single-glazed flat-plate solar collector of 5000-m² effective area is installed to boost the temperature of warm sea water by 20 K.     

海洋热能利用进展

海洋热能储量巨大,随时间变化相对稳定,具有广阔的开发利用前景。当前,海洋热能利用技术主要包括海洋温差能发电技术、海洋温差能制淡技术以及海水源热泵技术。发电技术要求海水温差不小于20℃,制淡技术要求海水温差不小于10℃,海水源热泵技术则在不同纬度地区、不同季节均能应用。本文重点分析了海洋温差能发电技术的3种循环方式,针对低温差导致低发电效率的问题,提出了利用太阳辐射加热温海水以提高温差和利用波浪能驱动泵以降低系统能耗两种提高发电效率的方法。     

Ocean thermal energy conversion by deliberate seawater salinization

Consideration is given to the possibility of ocean thermal energy conversion (OTEC) by the deliberate salinization of surface seawater. The proposed technique is similar to traditional OTEC, with one important exception: rather than cold water being brought from the bottom to the surface, the warm surface water is circulated to the bottom, cooled there, and lifted back to the surface. The entire process is driven by the induced salinity gradient at the surface. As a result, there is no need for a pumping system to bring the cold bottom water to the surface. Two methods are explored for surface salinity enhancement, namely solar evaporation and the direct addition of salt to the seawater.     

Off-design performance analysis of a closed-cycle ocean thermal energy conversion system with solar thermal preheating and superheating

This article reports the off-design performance analysis of a closed-cycle ocean thermal energy conversion (OTEC) system when a solar thermal collector is integrated as an add-on preheater or superheater. Design-point analysis of a simple OTEC system was numerically conducted to generate a gross power of 100 kW, representing a base OTEC system. In order to improve the power output of the OTEC system, two ways of utilizing solar energy are considered in this study: (1) preheating of surface seawater to increase its input temperature to the cycle and (2) direct superheating of the working fluid before it enters a turbine. Obtained results reveal that both preheating and superheating cases increase the net power generation by 20–25% from the design-point. However, the preheating case demands immense heat load on the solar collector due to the huge thermal mass of the seawater, being less efficient thermodynamically. The superheating case increases the thermal efficiency of the system from 1.9% to around 3%, about a 60% improvement, suggesting that this should be a better approach in improving the OTEC system. This research provides thermodynamic insight on the potential advantages and challenges of adding a solar thermal collection component to OTEC power plants.     

The exergy of the ocean thermal resource and analysis of second-law efficiencies of idealized ocean thermal energy conversion power cycles

We develop a fomula here to compute the maximum amount of work which can be extracted from a given combined mass of warm and cold ocean water (a quantity called the exergy of the ocean thermal resource). We then compare the second-law efficiencies of various proposed ocean thermal energy conversion power cycles to determine which best utilizes the exergy of the ocean thermal resource. The second-law efficiencies of the multicomponent working fluid cycle, the Beck cycle, and the open and closed single- and multiple-stage Rankine cycles are compared. These types of OTEC power plants are analyzed in a consistent manner, which assumes that all deviations from a plant making use of all the exergy (one with a second-law efficiency of 100%) occur because of irreversible transfer of heat across a finite temperature difference. Conversion of thermal energy to other forms is assumed to occur reversibly. The comparison of second-law efficiencies of various OTEC power cycles shows that the multistage Rankine open cycle with just three stages has the potential of best using the exergy of the ocean thermal resource.     

Simulated production of electric power and desalination using Solar‐OTEC hybrid system

Ocean thermal energy conversion (OTEC) is an electric power generation method that utilizes temperature difference between the warm surface seawater and cold deep seawater of ocean. As potential sources of clean‐energy supply, OTEC power plants' viability has been investigated. However, The OTEC system has problems of low efficiency and high investment cost because the temperature difference between the surface and the deep sea is small and it has a long pipe line and high pumping cost for using cold deep water. Therefore, in this present study, the OTEC system is combined with a solar system. It evaluated the thermodynamic performance of Solar‐OTEC Convergence System for the simultaneous production with electric power and desalinated water. The performance analysis of Solar‐OTEC Convergence System was carried out as the fluid temperature, saturated temperature difference and pressure of flash evaporator under equivalent conditions. The results showed that the performance of solar‐open OTEC system is the highest at the flash evaporator pressure of 10 kPa. At this time, the system efficiency, electric power and desalination production enhancement ratios were approximately 3.9, 13.9 and 5.1 times higher than that of the base open OTEC system respectively. Also, the performance of solar‐hybrid OTEC system is the highest at the inflow fluid temperature of evaporator of 80 °C. The system efficiency, electric power and desalination production enhancement ratios were approximately 3.5, 3.5 and 14.5 times higher than that of the base hybrid OTEC system. Copyright © 2016 John Wiley & Sons, Ltd.     

Thermodynamic performance assessment of ocean thermal energy conversion based hydrogen production and liquefaction process

In the proposed study, the thermodynamic performance assessment of ocean thermal energy conversion (OTEC) based hydrogen generation and liquefaction system are evaluated. In this context, the energetic and exergetic analyses of integrated system are conducted for multigeneration. This integrated process is consisted of the heat exchangers, turbine, condenser, pumps, solar collector system, hot storage tank, cold storage tank and proton exchange membrane (PEM) electrolyzer. In addition to that, the impacts of different design indicators and reference ambient parameters on the exergetic performance and exergy destruction rate of OTEC based hydrogen production system are analyzed. The energetic and exergetic efficiencies of integrated system are founded as 43.49% and 36.49%, respectively.     

Conceptual design for combined ocean thermal energy conversion using computational fluid dynamics and heat balance analysis

To overcome the limited efficiency of ocean thermal energy conversion (OTEC), particularly in the mid-latitudes, combined OTEC (C-OTEC) could use power extracted from the latent heat of a power plant condenser. Past research in South Korea has demonstrated the feasibility of a 10 kW C-OTEC system using R134a as a working fluid. As the next phase, a 200 kW C-OTEC demonstration facility with a thermal efficiency of greater than 3% is proposed. This paper presents the engineering design process for kW-scale C-OTEC within a 100 MW-scale thermal power plant. The design process is divided into two stages. First, to predict patterns in steam flow to a connected external evaporator with a porous medium, computational fluid dynamics are calculated. The results show a conservative margin suitable for the conceptual design. Second, an iterative heat balance simulation method simultaneously evaluates the heat balance analysis of the C-OTEC design and the thermal impact of the existing power plant. The design stages are then integrated in terms of heat transference capacity.     

An economic and environmental assessment of transporting bulk energy from a grazing ocean thermal energy conversion facility

An ocean thermal energy conversion (OTEC) facility produces electrical power without generating carbon dioxide (CO₂) by using the temperature differential between the reservoir of cold water at greater depths and the shallow mixed layer on the ocean surface. As some of the best sites are located far from shore, one option is to ship a high-energy carrier by tanker from these open-ocean or “grazing” OTEC platforms. We evaluate the economics and environmental attributes of producing and transporting energy using ammonia (NH₃), liquid hydrogen (LH₂) and methanol (CH₃OH). For each carrier, we develop transportation pathways that include onboard production, transport via tanker, onshore conversion and delivery to market. We then calculate the difference between the market price and the variable cost for generating the product using the OTEC platform without and with a price on CO₂ emissions. Finally, we compare the difference in prices to the capital cost of the OTEC platform and onboard synthesis equipment. For all pathways, the variable cost is lower than the market price, although this difference is insufficient to recover the entire capital costs for a first of a kind OTEC platform. With an onboard synthesis efficiency of 75%, we recover 5%, 25% and 45% of the capital and fixed costs for LH₂, CH₃OH and NH₃, respectively. Improving the capital costs of the OTEC platform by up to 25% and adding present estimates for the damages from CO₂ do not alter these conclusions. The near-term potential for the grazing OTEC platform is limited in existing markets. In the longer term, lower capital costs combined with improvements in onboard synthesis costs and efficiency as well as increases in CO₂ damages may allow the products from OTEC platforms to enter into markets.     

The viability and best locations for ocean thermal energy conversion systems around the world

Marine renewable energies offer alternatives to fossil and nuclear energies. Ocean thermal energy conversion (OTEC) is one of these alternatives, which also provides a range of additional products - food, air conditioning, water, pharmacheuticals included - hence the term deep ocean water applications (DOWA). It is also, unusually, a base-load system. Applications are in both developed and developing nations, but with particular application to island locations. Economics have significantly improved, due to advances in both design and materials, and OTEC/DOWA has many environmental advantages. Small (up to 1 MW) experimental units have been designed and built, and performance has been measured. These results confirm the growing practicality of OTEC/DOWA, and the next requirement is design, construction and operation of a representative scale demonstrator, typically 5 – 10 MW, to evaluate the feasibility of full scale production systems.     

一种海洋能及风能联合发电装置

文章提出了一种利用海洋温差能和风能联合发电的方法及装置。利用海洋表层的热海水加热低沸点工质,使之蒸发．送入汽轮机推动汽轮发电机组做功发电,汽轮机排出的工质乏气用海洋深层的冷海水冷凝为液态,再用热海水加热,送入汽轮机,使之蒸发,推动汽轮机发电机组做功发电,如此循环,持续发电;并且利用洋面风力发电,并用该电力驱动热泵装置．由热泵装置的媒质将工质的温度进一步提高．增大工质体积膨胀率;由热泵装置的媒质将冷海水的温度进一步降低．再用该低温海水去冷凝工质乏气,增强对工质乏汽的冷凝效果。该装置既需要用到小型透平,又需要用到风力发电装置．十分适合公司发展。     

Beyond electricity: The potential of ocean thermal energy and ocean technology ecoparks in small tropical islands

Small islands face difficult challenges to guarantee energy, freshwater and food supply, and sustainable development. The urge to meet their needs, together with the mitigation and adaptation plans to address climate change, have led them to develop renewable energy systems, with a special interest in Ocean Thermal Energy Conversion (OTEC) in tropical islands. Deep Ocean Water (DOW) is a resource that can provide electricity (through OTEC in combination with warm surface water), low temperatures for refrigeration, and nutrients for food production. In this paper we propose an Ocean Technology Ecopark (OTEP) as an integral solution for small islands that consists of an OTEC plant, other alternative uses of DOW, and a Research and Development (R&D) center. We present an application of OTEP to San Andres, a Colombian island that meets all the necessary conditions for the implementation of OTEC technology, water desalinization, and a business model for DOW. We present the main entrance barriers and a four-stage roadmap for the consolidation and sustainability of the OTEP.     

Design of an ocean thermal energy plant ship to produce ammonia via hydrogen

The 43°F (24°C) temperature difference that exists between surface water and deep water at selected sites in tropical oceans can be used to drive a heat engine to produce electric power, electrolyze water, and produce ammonia from the resulting hydrogen plus nitrogen from the air. A baseline design has been developed for a 100-MWe Ocean Thermal Energy Conversion (OTEC) plant-ship that would produce 313 tons per day of ammonia. The cost estimates for this design have been extrapolated to 500-MWe plant-ships to produce ammonia (for fertilizers and chemicals) or liquid hydrogen for shipment to the U.S. It is judged that ammonia will be producible at competitive cost ($96/short ton in 1975 dollars) by the sixth and subsequent plant-ships in the mid-1980s. This production by OTEC/ammonia plants would conserve supplies of natural gas or other fossil fuels now used to produce ammonia on shore. For the longer term (1990s), liquid hydrogen from OTEC plants should become competitive as demands for this clean fuel and efficient ways for employing it in larger markets (fuel cells, transportation, etc.) come to maturity.     

Characteristic of local boiling heat transfer of ammonia and ammonia / water binary mixture on the plate type evaporator

Power generation using small temperature difference such as ocean thermal energy conversion (OTEC) and discharged thermal energy conversion (DTEC) is expected to be the countermeasures against global warming problem. As ammonia and ammonia/water are used in evaporators for OTEC and DTEC as working fluids, the research of their local boiling heat transfer is important for improvement of the power generation efficiency. Measurements of local boiling heat transfer coefficients were performed for ammonia /water mixture (z = 0.9−1) on a vertical flat plate heat exchanger in a range of mass flux (7.5–15 kg/m² s), heat flux (15–23 kW/m²), and pressure (0.7–0.9 MPa). The result shows that in the case of ammonia /water mixture, the local heat transfer coefficients increase with an increase of mass flux and composition of ammonia, and decrease with an increase of heat flux.     

Renewable ocean energy in the Western Indian Ocean

Several African countries in the Western Indian Ocean (WIO) endure insufficiencies in the power sector, including both generation and distribution. One important step towards increasing energy security and availability is to intensify the use of renewable energy sources. The access to cost-efficient hydropower is low in coastal and island regions and combinations of different renewable energy sources will play an increasingly important role. In this study the physical preconditions for renewable ocean energy are investigated, considering the specific context of the WIO countries. Global-level resource assessments and oceanographic literature and data have been compiled in an analysis of the match between technology-specific requirements for ocean energy technologies (wave power, ocean thermal energy conversion (OTEC), tidal barrages, tidal current turbines, and ocean current power) and the physical resources in 13 WIO regions Kenya, Seychelles, Northern Tanzania and Zanzibar, Southern Tanzania, Comoros and Mayotte, Northern-, Central-, and Southern Mozambique, Western-, Eastern-, and Southern Madagascar, Réunion, and Mauritius. The results show high potential for wave power over vast coastal stretches in southern parts of the WIO and high potential for OTEC at specific locations in Mozambique, Comoros, Réunion, and Mauritius. The potential for tidal power and ocean current power is more restricted but may be of interest at some locations. The findings are discussed in relation to currently used electricity sources and the potential for solar photovoltaic and wind power. Temporal variations in resource intensity as well as the differences between small-scale and large-scale applications are considered.     

Analysis of optimization in an OTEC plant using organic Rankine cycle

This study quantified the effects of evaporation temperature, condensation temperature, and the inlet- and outlet-temperature differences of deep cold seawater and warm seawater on the performance of an ocean thermal energy conversion (OTEC) plant using an organic Rankine cycle (ORC), and also investigated the optimal operations required for the performance. A finite-temperature-difference heat transfer method is developed to evaluate the objective parameter, which is the ratio of net power output to the total heat transfer area of heat exchanger in the system, and R717, R600a, R245fa, R152a, and R134a were used as the working fluids. The optimal evaporation and condensation temperatures were obtained under various conditions for maximal objective parameters in an OTEC system.The results show that R717 performed optimally in objective parameter evaluation among the five working fluids, and that R600a performed better than other fluids in thermal efficiency analysis. The optimal seawater temperature differences between the inlet and outlet of the evaporator and condenser are proposed. Furthermore, the influences of inlet temperatures of warm and cold seawater in the ORC are presented for an OTEC plant. The simulation results should enable the performance of an ORC system to be compared when using various organic working fluids.     

Current status and prospects of marine renewable energy applied in ocean robots

The power supply for ocean robots has always been an important issue since it has a fatal influence on the endurance of these vehicles. However, the marine renewable energy (MRE) has huge potential and can provide the possibility to solve this problem between essential endurance and finite energy in ocean robots. This paper starts with brief introduction of marine energy resource and ocean robots and presents significance of improving ocean robots' endurance, through comparison of their performance characteristics. MRE applied in ocean robots developed or under development, including energy conversion and driving principle, is reviewed, such as solar, wind, tides, waves, thermal energy, etc. Many challenges and difficulties are also discussed in energy exploitation and utilization related to ocean robots. Finally, the prospect for the future development of related technologies is proposed in this paper.     

Thermal properties of the Florida Current as related to Ocean Thermal Energy Conversion (OTEC)

The thermal properties of the Florida Current are presented and analyzed for the available ocean thermal energy. For a cold water intake depth equal to or greater than 600 m, potential sites for Ocean Thermal Energy Conversion (OTEC) power plants appear to exist in the Straits of Florida and, to a lesser extent, off the coasts of Georgia and South Carolina. The maximum thermal differences occur on the continental shelf because of the geostrophic motion of the Gulf Stream. An estimate of the total available ocean thermal energy from the Florida Current, delivered in the form of electricity, is 3.5 × 10¹² kWh yr⁻¹. For a cold water suction depth of 600 m or more, seasonal variability in the ocean thermal resource is approx. ±35 per cent of average annual output.     

Hydrogen production through an ocean thermal energy conversion system operating at an optimum temperature drop

Storage of electrical energy produced from an ocean thermal energy conversion (OTEC) system is considered to be extremely essential, since the conversion process could take place in a remote offshore area and distant from the actual utilization sites. Energy conversion from an OTEC system into hydrogen energy, which is used for power generation through fuel cells, is an important approach of storing such energy for further utilizations. In this paper, a technical analysis of hydrogen production through an OTEC system coupled with a polymer electrolyte membrane electrolyser (PEM), which is developed by the Japanese international clean energy network using hydrogen conversion (WE-NET), is performed. The analysis is conducted at an optimum temperature drop between the working fluid and seawater, δT_op. Furthermore, the analysis is carried out at various temperature differences between the surface and deep sea water, ΔT. The calculated results demonstrated the significance of temperature drop and temperature difference on the electrical power output and conversion efficiency. Moreover, the actual rate of hydrogen production varied from 2.5 N m³/h to 60 N m³/h as ΔT raised from 5 °C to 25 °C, respectively.     

A new hybrid ocean thermal energy conversion–Offshore solar pond (OTEC–OSP) design: A cost optimization approach

Solar thermal electricity (STE) generation offers an excellent opportunity to supply electricity with a non-CO₂ emitting technology. However, present costs hamper widespread deployment and therefore research and development efforts are concentrated on accelerated cost reductions and efficiency improvements. Many focus on the latter, but in this paper we rather focus on attaining very low levelised electricity costs (LEC) by designing a system with very low material cost, while maintaining appreciable conversion efficiency and achieving low maintenance cost. All investigated designs were dimensioned at a 50 MW scale production. Calculated LECs show that a new proposed hybrid of ocean thermal energy conversion with an offshore solar pond (OTEC–OSP) may have the lowest LEC of 0.04 €/kWh. Addition of a floating offshore solar pond (OSP) to an OTEC system increases the temperature difference in the Rankine cycle, which leads to an improved efficiency of 12%, while typical OTEC efficiencies are 3%. This higher efficiency leads to much lower investments needed for power blocks, while the OSP is fabricated using very low-cost plastic foils. The new OTEC–OSP design can be located in many sunny coastal areas in the world.