On the ocean heat budget and ocean thermal energy conversion

Ocean water covers a vast portion of the Earth's surface and is also the world's largest solar energy collector. It plays an important role in maintaining the global energy balance as well as in preventing the Earth's surface from continually heating up because of solar radiation. The ocean also plays an important role in driving the atmospheric processes. The heat exchange processes across the ocean surface are represented in an ocean thermal energy budget, which is important because the ocean stores and releases thermal energy. The solar energy absorbed by the ocean heats up the surface water, despite the loss of heat energy from the surface due to back‐radiation, evaporation, conduction, and convection, and the seasonal change in the surface water temperature is less in the tropics. The cold water from the higher latitudes is carried by ocean currents along the ocean bottom from the poles towards the equator, displacing the lower‐density water above and creating a thermal structure with a large reservoir of warm water at the ocean surface and a large reservoir of cold water at the bottom, with a temperature difference of 22°C to 25°C between them. The available thermal energy, which is the almost constant temperature water at the beginning and end of the thermocline, in some areas of the oceans, is suitable to drive ocean thermal energy conversion (OTEC) plants. These plants are basically heat engines that use the temperature difference between the surface and deep ocean water to drive turbines to generate electricity. A detailed heat energy budget of the ocean is presented in the paper taking into consideration all the major heat inputs and outputs. The basic OTEC systems are also presented and analyzed in this paper. Copyright © 2011 John Wiley & Sons, Ltd.     

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 impact of different grid regulatory scenarios on the development of renewable energy on islands: A comparative study and improvement proposals

Electricity generation costs are typically higher on islands than in mainland regions, primarily due to the costs associated with conventional primary energy transportation. However, at the same time, islands are commonly granted with significant renewable energy potential in terms of wind, solar radiation and marine energy, among others, varying by case.This article is focused on the impact that the grid regulatory framework has had on several islands from both the technical and economical points of view, with respect to renewable energy development. A comparison among the studied islands is carried out. Additionally, the possible differences between each island (or archipelago) and the rest of the corresponding country on the mainland are analysed to determine to what extent the peculiarities of the islands have been taken into account in the regulations.Our objective is to analyse whether the renewable energy developments on certain islands have taken place because of certain favourable scenarios or by promoting specific actuations, which could be applicable on other islands to promote similar developments. As a result of the study, strategic key ideas are identified to increase the renewable energy percentage of the electricity generation and energy consumption mix on islands.     

An overview of ocean renewable energy resources in Korea

Korea relies on imported fossil fuels to meet its energy consumption demands. As such, there is a need to investigate alternative energy resources such as renewable energy. In this paper, assessments of the potential of various ocean renewable energy resources in the sea around Korea; potential sources of energy including wave energy, tidal energy, tidal current energy and ocean thermal energy. Tidal energy and tidal current energy are likely to play an important role in meeting the future energy needs of Korea, whereas the potentials of wave energy and ocean thermal energy for the same are relatively low. The level of technical development and the renewable energy market in Korea is currently in an early stage. The government will have to be more aggressive in the promotion of renewable energy to achieve sustainable development in Korea.     

The promise of clean energy

The world's energy system is at least a 1.5 trillion dollars market dominated by fossil fuels, where small changes can have a large influence on efforts to reach sustainability. Renewable energy sources are key to achieving this goal. Excluding traditional biomass, in 2001 renewables represented 4.4% of primary energy consumption, unevenly distributed between developed and developing countries. Environmental problems at local, regional and global levels, as well as external dependency and security of supply will persist if we rely on an energy future based on fossil fuels. Solutions encompass extending the life of fossil fuel reserves and expanding the share of renewable in the world energy system through top down and bottom up policies, described in this paper.     

Near zero energy islands in the Mediterranean: Supporting policies and local obstacles

Based on a recent technical–economical analysis on the island of Pantelleria, a policy feasibility study for a complete upgrading of the energy system of this Mediterranean Island is carried out. Pantelleria, situated between Sicily and Africa, owns a large potential in terms of renewable energy resources, although there are some obstacles in turning it into a Near Zero Energy system. Starting from a deep energy system audit, the study proposes the project for a near zero energy island, through the efficient transformation of the different existing natural energy resources into electrical energy and heat: the solar, the wind-based and the geothermal systems. In this way, the island can be turned into an almost autonomous system. The main difficulties connected to the implementation of the project can be identified in the national energy policies as well as in the specific local situation, characterized by a strong private monopole on generation and distribution of electrical energy which has no incentive for supporting the costs connected to the energy requalification of the island. On the other hand, the local administrations, involved in the project through bottom-up European policies, do not have the cultural and economic tools to go on with the implementation.     

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.     

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.     

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.     

华东沿海海洋温差发电系统的优化设计

海洋温差能是一种可再生的绿色能源,储藏量大,资源稳定。海洋温差发电是利用深层、表层海水的温度差,以高温海水为热源,使液态工质气化推动发电机发电,以低温海水为冷源,使气态工质液化的不断循环的过程。基于能源的可持续发展考虑,可以利用风能、太阳能等可再生能源来优化设计海洋温差发电系统。华东沿海海域有着丰富的太阳能和风能资源,利用太阳能可以提高表层海水与深层海水的温差,利用风力转化装置可以提高和调整汽轮机的转速,保证发电系统持续稳定的发电。利用太阳能、风能对海洋温差发电系统进行优化设计,不仅避免和解决了当前海洋温差发电技术上的一些难点,还扩大了应用温差能资源的海域范围。     

Algerian renewable energy assessment: The challenge of sustainability

Algeria plays a very important role in world energy markets, both as a significant hydrocarbon producer and as an exporter, as well as a key participant in the renewable energy market. Due to its geographical location, Algeria holds one of the highest solar potentials in the world. This paper presents a review of the present renewable energy situation and assesses present and future potential of renewable energy sources (RESs) in Algeria. This paper also discusses the trends and expectation in solar systems applications and the aspects of future implementation of renewable energies in the Middle East and North Africa (MENA) region status. The problem related to the use of RES and polices to enhance the use of these sources are also analysed in this paper. In addition the available capacity building, the technical know-how for each RES technology and localizing manufacturing of renewable energy equipments are defined. The co-importance of both policy and technology investments for the future Algerian markets of RES and competitiveness of the solar/wind approach is emphasized. Some examples of policy significantly impacting Algerian markets are reviewed, and the intention of the new Algerian RES initiative is discussed.     

Assessing the potential of renewable energy sources in Turkey

To meet Turkey’s growing energy demand, the installed electric power capacity of 27.8 GW in 2001 has to be doubled by 2010 and increased fourfold by 2020. The difference between Turkey’s total primary energy supply (TPES) of from its own sources and total final consumption (TFC) is projected grow from 1 quad (1.06–2.06) in 1999 to 5.71 quads (2.79–8.5) in 2020 (1 QUAD=293.071 TWh). Turkey’s limited amount of fossil fuels has a present average ratio of proved reserves of 97.38 quads to production rate of 3.2 quads yr⁻¹ of about 30 years. Turkey’s reliance on fossil fuel-based energy systems to meet the growing demand is most likely to exacerbate the issues of energy insecurity, national environmental degradation, and global climate change in increasing proportions. Economically-feasible renewable energy potential in Turkey is estimated at a total of ca. 1.69 quads yr⁻¹ (495.4 TWh yr⁻¹) with the potential for 0.67 quads yr⁻¹ (196.7 TWh yr⁻¹) of biomass energy, 0.42 quads yr⁻¹ (124 TWh yr⁻¹) of hydropower, 0.35 quads yr⁻¹ (102.3 TWh yr⁻¹) of solar energy, 0.17 quads yr⁻¹ (50 TWh yr⁻¹) of wind energy, and 0.08 quads yr⁻¹ (22.4 TWh yr⁻¹) of geothermal energy. Pursuit and implementation of sustainability-based energy policy could provide about 90 and 35% of Turkey’s total energy supply and consumption projected in 2010, respectively. Utilization of renewable energy technologies for electricity generation would necessitate about 23.2 Mha (29.8%) of Turkey’s land resources.     

Global analysis of the techno-economic potential of renewable energy hybrid systems on small islands

Globally, small islands below 100,000 inhabitants represent a large number of diesel based mini-grids. With volatile fossil fuel costs which are most likely to increase in the long-run and competitive renewable energy technologies the introduction of such sustainable power generation system seems a viable and environmental friendly option. Nevertheless the implementation of renewable energies on small islands is quite low based on high transaction costs and missing knowledge according to the market potential.Our work provides a global overview on the small island landscape showing the respective population, economic activity, energy demand, and fuel costs for almost 1800 islands with approximately 20 million inhabitants currently supplied by 15 GW of diesel plants. Based on these parameters a detailed techno-economic assessment of the potential integration of solar PV, wind power, and battery storage into the power supply system was performed for each island. The focus on solar and wind was set due to the lack of data on hydro and geothermal potential for a global island study. It revealed that almost 7.5 GW of photovoltaic and 14 GW of wind power could be economically installed and operated on these islands reducing the GHG-emissions and fuel consumption by approximately 50%. In total numbers more than 20 million tons of GHG emissions can be reduced by avoiding the burning of 7.8 billion liters of diesel per year. Cost savings of around 9 USDct/kWh occur on average by implementing these capacities combined with 5.8 GWh of battery storage. This detailed techno-economic evaluation of renewable energies enables policy makers and investors to facilitate the implementation of clean energy supply systems on small islands. To accelerate the implementation of this enormous potential we give specific policy recommendations such as the introduction of proper regulations.     

Renewable energy for sustainable development in Africa: a review

Renewable energy usage in Africa has been reviewed using South Africa, Egypt, Nigeria and Mali as case studies. The various national energy policies of these countries were analysed and areas that require attention to achieve sustainability were highlighted. On the overall, the success of sustainable development in Africa lies in addressing the imminent energy crisis in the continent. Excessive usage of fuel wood is already creating considerable environmental problems especially in the Sahel. Africa has all the potentials to solve its energy problems if appropriate infrastructural support can be provided for harnessing the abundant renewable resources in the continent, and if skills are pooled together and experiences shared in addressing the key issues.     

Renewable energy: Progressing towards a net zero energy island, the case of Reunion Island

For the last two decades, economic development in Reunion Island has led to major structural changes. The latter have been characterized by an increase in energy demand per person. This demand is mostly related to a high population growth (1.55% per year). Reunion currently has 833,000 inhabitants. The population will rise to 1 million in 2030. Like most ultraperipheral regions of the European Union, Reunion is heavily dependent on imported fossil fuels for its energy production. The total primary energy consumption amounted to 1352 ktep in 2009 and 87.1% is imported energy. The development of various renewable energies such as solar energy, biomass, ocean energy, etc. is thus of priority concern to aim to achieve energetic independence. Just like other French overseas territories, Reunion policies have widely invested in Renewable Energy Sources (RES) since 2000. This paper aims at presenting the current status, the major achievements of policies and the future objectives in the deployment of renewable energy programmes. The perspective of a net zero energy island versus the pressure of the population is analysed. The barriers to penetration of RES in a small-scale territory are also discussed.     

Hydropower in Turkey for a clean and sustainable energy future

Over the last two decades, global electricity production has more than doubled and electricity demand is rising rapidly around the world as economic development spreads to emerging economies. Not only has electricity demand increased significantly, it is the fastest growing end-use of energy. Therefore, technical, economic and environmental benefits of hydroelectric power make it an important contributor to the future world energy mix, particularly in the developing countries. This paper deals with policies to meet increasing energy and electricity demand for sustainable energy development in Turkey. Turkey has a total gross hydropower potential of 433 GWh/year, but only 125 GWh/year of the total hydroelectric potential of Turkey can be economically used. By the commissioning of new hydropower plants, which are under construction, 36% of the economically usable potential of the country would be tapped. Turkey's total economically usable small hydropower potential is 3.75 GWh/year.     

Estimation of the wave energy conversion efficiency in the Atlantic Ocean close to the European islands

This paper presents an evaluation of the efficiency of twelve state of the art wave energy converters in the Atlantic Ocean, in the vicinity of the most important European islands and archipelagos (Iceland, Archipelago of Azores, Madeira Archipelago and Canary Islands). An analysis of the wave conditions in the target areas was first performed by considering a 10-year interval (2004–2013) of wave data provided by the European Centre for Medium-Range Weather Forecasts. For this reason, twenty reference points, all located in water depths of about 50 m, were defined. In order to provide a general picture of the wave potential and also to highlight the presence of some hot spots, several wave parameters, such as significant wave height, mean wave direction and wave power, were evaluated. Then, for every nearshore area, based on the bivariate distributions of the sea states occurrences and also on the power matrix of each device, the performances of each wave energy converter were estimated in terms of the expected electrical power. The results of the present work provide valuable information for the future wave farm projects, which could become in the near future a reliable and effective way to produce energy in island environments.     

Evaluation of small wind turbines in distributed arrangement as sustainable wind energy option for Barbados

The island of Barbados is 99% dependent on fossil fuel imports to satisfy its energy needs, which is unsustainable. This study proposes a 10 MW distributed wind energy scheme using micro wind turbines (WT) of horizontal (HAWT) and vertical axis (VAWT) configurations. These units are rated less than 500 W, and the scheme is hereafter referred to as mWT10. mWT10 is compared to the proposed 10 MW medium WT farm by the Barbados Light & Power Company (BL&P). The economic bottom line is the levelized cost of electricity (LCOE). The results highlight the BL&P proposal as the best economic option at BDS$0.19 per kWh, while that of both mWT10 configurations exceeds the conventional cost of BDS$0.25 by two to nine times. This is attributed to significantly higher relative installation and operational costs. However, the financial gap between mWT10 LCOE and the retail price of electricity is much smaller due to a large fuel surcharge passed on to each customer. Annual additional benefits of using wind energy include: greenhouse gas emissions savings of 6–23 kt of carbon dioxide; and anavoided fuel costs of BDS$1.5–5.3 million.

The distributed mWT10 using HAWTs competes directly with the BL&P farm, however, it provides these benefits without the visual or ecological impacts of the larger machines. Conversely, VAWTs have features that favour a visually discrete and widely repeatable scheme but suffer relatively high costs. Therefore, this study illustrates the great potential of small wind turbines to be competitive with conventional wind farms, thus challenging the small wind industry to meet its potential by producing reliable and robust machines at lower cost.     

The potential for renewable energy in industrial applications

To date, insufficient attention has been paid to the potential of renewable energy resources in industrial applications. Our analysis suggests that up to 21% of final energy demand and feedstock-use in the manufacturing industry sector could be of renewable origin by 2050, a five-fold increase over current levels in absolute terms. This estimate is considerably higher than other recent global scenario studies. In addition, if a 50% share of renewables in power generation is assumed, the share of direct and indirect renewable energy use rises to 31% in 2050. Our analysis further suggests that bioenergy and biofeedstocks can constitute three-quarters of the direct renewables use in this sector by 2050. The remainder is roughly evenly divided between solar heating and heat pumps. The potential for solar cooling is considered to be limited.While low-temperature solar process heat can reach cost-effectiveness today in locations with good insolation, some bioenergy applications will require a CO₂ price even on the longer term. Biomass feedstock for synthetic organic materials will require a CO₂ price up to USD 100/t CO₂, or even more if embodied carbon is not considered properly in CO₂ accounts. Future fossil fuel prices and bioenergy prices in addition to the development of feedstock commodity markets for biomass will be critical. Decision makers are recommended to pay more attention to the potential for renewables in industry. Finally, we propose the development of a detailed technology roadmap to explore this potential further and discuss key issues that need to be elaborated in such a framework.     

Multi-objective optimization model for sustainable Indonesian electricity system: Analysis of economic,environment, and adequacy of energy sources

This paper presents a multi-objective optimization model for a long-term generation mix in Indonesia. The objective of this work is to assess the economic, environment, and adequacy of local energy sources. The model includes two competing objective functions to seek the lowest cost of generation and the lowest CO₂ emissions while considering technology diffusion. The scenarios include the use of fossil reserves with or without the constraints of the reserve to production ratio and exports. The results indicate that Indonesia should develop all renewable energy and requires imported coal and natural gas. If all fossil resources were upgraded to reserves, electricity demand in 2050 could be met by domestic energy sources. The maximum share of renewable energy that can be achieved in 2050 is 33% with and 80% without technology diffusion. The least cost optimization produces lower generation costs than the least CO₂ emissions, as well as the combined scenario. Total CO₂ emissions in 2050 are five to six times as large as current emissions. The least CO₂ emissions scenario can reduce almost half of the CO₂ emissions of the least cost scenario by 2050. The proposed multi-objective optimization model leads some optimal solutions for a more sustainable electricity system.