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.     

Solar thermal power generation in India—a techno–economic analysis

Limited fossil resources and environmental problems require new sustainable energy supply options, that use renewable energies and are economic at the same time. Solar Thermal Electricity (STE) generating systems are proven renewable energy technologies and often a very cost effective way to produce electricity from solar radiation.In India, the electricity demand is drastically increasing. At the same time, solar resources and large wasteland areas are widely available. These factors together make India an ideal country for the implementation of STE-technologies.In this paper, we analyze the potential and the cost-effectiveness of centralized and decentralized STE-generation in India. Comparing the levelized electricity costs (LEC) for STE with the corresponding LEC for the electricity generating options used at present, we find that STE is an economically viable technology under favorable conditions, i.e. in areas with high insolation levels and provided that capital is available at low interest rates.     

Conceptual design and techno-economic assessment of integrated solar combined cycle system with DSG technology

Direct steam generation (DSG) in parabolic trough collectors causes an increase to competitiveness of solar thermal power plants (STPP) by substitution of oil with direct steam generation that results in lower investment and operating costs. In this study the integrated solar combined cycle system with DSG technology is introduced and techno-economic assessment of this plant is reported compared with two conventional cases. Three considered cases are: an integrated solar combined cycle system with DSG technology (ISCCS-DSG), a solar electric generating system (SEGS), and an integrated solar combined cycle system with HTF (heat transfer fluid) technology (ISCCS-HTF).This study shows that levelized energy cost (LEC) for the ISCCS-DSG is lower than the two other cases due to reducing O&M costs and also due to increasing the heat to electricity net efficiency of the power plant. Among the three STPPs, SEGS has the lowest CO₂ emissions, but it will operate during daytime only.     

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。     

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.     

Economic analysis of power generation from parabolic trough solar thermal plants for the Mediterranean region—A case study for the island of Cyprus

In this work a feasibility study is carried out in order to investigate whether the installation of a parabolic trough solar thermal technology for power generation in the Mediterranean region is economically feasible. The case study takes into account the available solar potential for Cyprus, as well as all available data concerning current renewable energy sources policy of the Cyprus Government, including the relevant feed-in tariff. In order to identify the least cost feasible option for the installation of the parabolic trough solar thermal plant a parametric cost–benefit analysis is carried out by varying parameters, such as, parabolic trough solar thermal plant capacity, parabolic trough solar thermal capital investment, operating hours, carbon dioxide emission trading system price, etc. For all above cases the electricity unit cost or benefit before tax, as well as after tax cash flow, net present value, internal rate of return and payback period are calculated. The results indicate that under certain conditions such projects can be profitable.     

Low-grade heat conversion into power using organic Rankine cycles – A review of various applications

An organic Rankine cycle (ORC) machine is similar to a conventional steam cycle energy conversion system, but uses an organic fluid such as refrigerants and hydrocarbons instead of water. In recent years, research was intensified on this device as it is being progressively adopted as premier technology to convert low-temperature heat resources into power. Available heat resources are: solar energy, geothermal energy, biomass products, surface seawater, and waste heat from various thermal processes. This paper presents existing applications and analyzes their maturity. Binary geothermal and binary biomass CHP are already mature. Provided the interest to recover waste heat rejected by thermal devices and industrial processes continue to grow, and favorable legislative conditions are adopted, waste heat recovery organic Rankine cycle systems in the near future will experience a rapid growth. Solar modular power plants are being intensely investigated at smaller scale for cogeneration applications in buildings but larger plants are also expected in tropical or Sahel regions with constant and low solar radiation intensity. OTEC power plants operating mainly on offshore installations at very low temperature have been advertised as total resource systems and interest on this technology is growing in large isolated islands.     

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.     

Technical and economic assessment of the integrated solar combined cycle power plants in Iran

Thermal efficiency, capacity factor, environmental considerations, investment, fuel and O&M² costs are the main parameters for technical and economic assessment of solar power plants. This analysis has shown that the Integrated Solar Combined Cycle System with 67 MW e solar field (ISCCS-67) is the most suitable plan for the first solar power plant in Iran. The Levelized Energy Costs (LEC) of combined cycle and ISCCS-67 power plants would be equal if 49 million $ of ISCCS-67 capital cost supplied by the international environmental organizations such as Global Environmental Facilities (GEF) and World Bank. This study shows that an ISCCS-67 saves 59 million $ in fuel consumption and reduces about 2.4 million ton in CO₂ emission during 30 years operating period. Increasing of steam turbine capacity by 50%, and 4% improvement in overall efficiency are other advantages of ISCCS-67 power plant. The LEC of ISCCS-67 is 10 and 33% lower than the combined cycle and gas turbine, respectively, at the same capacity factor with consideration of environmental costs.     

Ocean thermal energy conversion: a general introduction

The ocean thermal energy conversion (OTEC) concept is discussed with emphasis on the closed Rankine cycle using ammonia as a working fluid. The main features of OTEC, such as low efficiency high flow rates, and high capital cost are put in perspective in terms of energy cost at the bus bar. Sensitivity analyses of net output power to key design variables and to performance uncertainty are performed. It is concluded that even with a large error in estimating performance conditions, the plant produces net output power. This indicates the robust nature of current designs. Finally, cost figures of major system components are given and electricity cost based on a hypothetical capital cost is computed.     

Energy and exergy analyses of hydrogen production via solar-boosted ocean thermal energy conversion and PEM electrolysis

Energy and exergy analyses are reported of hydrogen production via an ocean thermal energy conversion (OTEC) system coupled with a solar-enhanced proton exchange membrane (PEM) electrolyzer. This system is composed of a turbine, an evaporator, a condenser, a pump, a solar collector and a PEM electrolyzer. Electricity is generated in the turbine, which is used by the PEM electrolyzer to produce hydrogen. A simulation program using Matlab software is developed to model the PEM electrolyzer and OTEC system. The simulation model for the PEM electrolyzer used in this study is validated with experimental data from the literature. The amount of hydrogen produced, the exergy destruction of each component and the overall system, and the exergy efficiency of the system are calculated. To better understand the effect of various parameters on system performance, a parametric analysis is carried out. The energy and exergy efficiencies of the integrated OTEC system are 3.6% and 22.7% respectively, and the exergy efficiency of the PEM electrolyzer is about 56.5% while the amount of hydrogen produced by it is 1.2 kg/h.     

Low pressure solar thermal converter

The current development of solar power converters with air as working fluid focuses mostly on concentrating collectors combined with hot-air engines, and on very low temperature solar tower concepts. Whilst concentrating collectors and Stirling engines need complex technology, solar tower converters have very low efficiencies and require large installations. Pressurized containers as energy converters offer the advantage of simplicity, but appear not to have been investigated in detail. In order to assess their performance potential, an idealised thermal pressure converter was analysed theoretically. Two improvements to increase the initially low efficiency derived from theory were found. Neglecting losses, maximum theoretical efficiencies ranged from 6.7% for a temperature difference of 60 K to 17.7% for a difference of 195 K. The low pressure solar thermal converter appears to offer development potential for low-tech solar energy conversion.     

Estimating the manufacturing cost of purely organic solar cells

In this paper we estimate the manufacturing cost of purely organic solar cells. We find a very large range since the technology is still very young. We estimate that the manufacturing cost for purely organic solar cells will range between $50 and $140/m². Under the assumption of 5% efficiency, this leads to a module cost of between $1.00 and $2.83/W_p. Under the assumption of a 5-year lifetime, this leads to a levelized cost of electricity (LEC) of between 49¢ and 85¢/kWh. In order to achieve a more competitive COE of about 7¢/kWh, we would need to increase efficiency to 15% and lifetime to between 15-20 years.     

How carbon pricing changes the relative competitiveness of low-carbon baseload generating technologies

There is wide public debate about which electricity generating technologies will best be suited to reduce greenhouse gas emissions (GHG). Sometimes this debate ignores real-world practicalities and leads to over-optimistic conclusions. Here we define and apply a set of fit-for-service criteria to identify technologies capable of supplying baseload electricity and reducing GHGs by amounts and within the timescale set by the Intergovernmental Panel on Climate Change (IPCC). Only five current technologies meet these criteria: coal (both pulverised fuel and integrated gasification combined cycle) with carbon capture and storage (CCS); combined cycle gas turbine with CCS; Generation III nuclear fission; and solar thermal backed by heat storage and gas turbines. To compare costs and performance, we undertook a meta-review of authoritative peer-reviewed studies of levelised cost of electricity (LCOE) and life-cycle GHG emissions for these technologies. Future baseload electricity technology selection will be influenced by the total cost of technology substitution, including carbon pricing, which is synergistically related to both LCOE and emissions. Nuclear energy is the cheapest option and best able to meet the IPCC timetable for GHG abatement. Solar thermal is the most expensive, while CCS will require rapid major advances in technology to meet that timetable.     

Thermal characteristics and performance of salt gradient solar ponds

A mathematical model with various parameters such as effective absorptivity-transmitivity product and total heat loss factor, including ground losses and angle of refraction, which are related to the physical properties and dimensions of the pond, is developed to study the thermal behaviour of salt gradient solar ponds at different operational conditions. A linear relation is found between the efficiency of the solar pond and the function (ΔT/H ). The convective heat loss, the heat loss to the atmosphere due to evaporation through the surface of the pond and ground heat losses have been accounted for in finding out the efficiency of the pond. The dependence of the thermal performance of the solar pond on the ground heat losses is investigated and minimized using low cost loose and insulating building materials such as dry dunes and, Mica powder and loose asbestos at the bottom of the pond. The ground heat losses are considerably reduced with the asbestos (loose) and the retention power of solar thermal energy of the pond increases.     

THE THERMAL CHARACTERISTICS AND ECONOMIC ANALYSIS OF A SOLAR POND COUPLED LOW TEMPERATURE MULTI STAGE DESALINATION PLANT PART II: ECONOMIC ANALYSIS

This paper presents the thermal performance and economic feasibility of matching the SGSP with the MSF destilation plant with a daily product water output of 1000 m³/day. The analysis are based on the assumption that the solar pond is to be used as the sole heal source (thermal energy) for the distillation plant. The thermal simulation of the MSF desalination process was predicted by using a mathematical model based on stage by stage calculations taking into account the variations in fluid properties and flow conditions. The generated simultaneous equations of the mass and energy balances were combined and arranged in a matrix form and then translated into algorithm to predict process variables such as temperature and flash evaporation rates.

The paper discusses optimisation of the size of the pond and the number of stages for three different storage zone temperatures taking into account the large variation in quantity of energy supplied by the pond between summer and winter. One result is that oversizing the pond, leading to some rejection of the heat collected during the summer (which is referred to as peak clipping), will result in a higher utilisation factor of the desalination plant and a reduction in the summer/winter yield ratio. Optimum peak clipping days, leading to the minimum product water cost, for each storage zone temperature and performance ratio is presented.

The sensitivity analysis of the various factors affecting the overall water costs show that the capital costs comprise about two thirds (2/3) of the total desalinated water costs. This demonstrates and re-emphasises the inherent and basic fact that solar desalination is a capital intensive enterprise. Each 1% increase in interest rate increases solar pond thermal energy costs by about 13–15% and desalinated water costs from SP/MSF combination by about 10–13%.     

Technical and economical system comparison of photovoltaic and concentrating solar thermal power systems depending on annual global irradiation

Concentrating solar thermal power and photovoltaics are two major technologies for converting sunlight to electricity. Variations of the annual solar irradiation depending on the site influence their annual efficiency, specific output and electricity generation cost. Detailed technical and economical analyses performed with computer simulations point out differences of solar thermal parabolic trough power plants, non-tracked and two-axis-tracked PV systems. Therefore, 61 sites in Europe and North Africa covering a global annual irradiation range from 923 to 2438 kW h/m² a have been examined. Simulation results are usable irradiation by the systems, specific annual system output and levelled electricity cost. Cost assumptions are made for today's cost and expected cost in 10 years considering different progress ratios. This will lead to a cost reduction by 50% for PV systems and by 40% for solar thermal power plants. The simulation results show where are optimal regions for installing solar thermal trough and tracked PV systems in comparison to non-tracked PV. For low irradiation values the annual output of solar thermal systems is much lower than of PV systems. On the other hand, for high irradiations solar thermal systems provide the best-cost solution even when considering higher cost reduction factors for PV in the next decade. Electricity generation cost much below 10 Eurocents per kW h for solar thermal systems and about 12 Eurocents/kW h for PV can be expected in 10 years in North Africa.     

Hydrogen production by coupling pressurized high temperature electrolyser with solar tower technology

Solar hydrogen production by coupling of pressurized high temperature electrolyser with concentrated solar tower technology is studied. As the high temperature electrolyser requires constant temperature conditions, the focus is made on a molten salt solar tower due to its high storage capacity. A flowsheet was developed and simulations were carried out with Aspen Plus 8.4 software for MW-scale hydrogen production plants. The solar part was laid out with HFLCAL software. Two different scenarios were considered: the first concerns the production of 400 kg/d hydrogen corresponding to mobility use (fuel station). The second scenario deals with the production of 4000 kg/d hydrogen for industrial use. The process was analyzed from a thermodynamic point of view by calculating the overall process efficiency and determining the annual production. It was assumed that a fixed hydrogen demand exists in the two cases and it was assessed to which extent this can be supplied by the solar high temperature electrolysis process including thermal storage as well as hydrogen storage. For time periods with a potential over supply of hydrogen, it was considered that the excess energy is sold as electricity to the grid. For time periods where the hydrogen demand cannot be fully supplied, electricity consumption from the grid was considered. It was assessed which solar multiple is appropriate to achieve low consumption of grid electricity and low excess energy. It is shown that the consumption of grid electricity is reduced for increasing solar multiple but the efficiency is also reduced. At a solar multiple of 3.0 an annual solar-to-H₂ efficiency greater than 14% is achieved at grid electricity production below 5% for the industrial case (4000 kg/d). In a sensitivity study the paramount importance of electrolyser performance, i.e. efficiency and conversion, is shown.     

Hybrid photovoltaic–thermal systems for domestic heating and cooling—A theoretical approach

Hybrid photovoltaic–thermal (PV/T) collectors consist of a thermal collector in which a photovoltaic laminate is attached as a thermal absorber. In addition to their improved electrical efficiency, due to the decrease of the photovoltaic temperature, PV/T collectors have also thermal efficiency, which is the main issue of this research. The theoretical study of a photovoltaic–thermal system for domestic heating and cooling has led to the result that the system can cover a remarkable percentage of the domestic heating and cooling demands. Furthermore, the performance of the PV/T collector with respect to the variation of geographical region as well as to different total surface areas of the system was also investigated. According to the results in both cases, the variation of the above-mentioned parameters has notably affected the solar coverage percentage of the system. As a conclusion it could be claimed that the photovoltaic–thermal technology offers a remarkable solution to the problem of domestic heating and cooling, hence contributing to the reduction of energy consumption in houses.     

Solar electricity generation—A comparative view of technologies, costs and environmental impact

The implementation of solar power plants in regions of high insolation is a promising option for an environmentally compatible electricity supply strategy. Today, approximately 80% of the solar generated electricity is provided by solar thermal power plants, while 20% is supplied by photovoltaic systems. Decision-makers have the choice among the following solar technologies: (1) parabolic trough; (2) central receiver; (3) paraboloidal dish; (4) solar chimney; (5) solar pond; (6) photovoltaic cells. This article compared present solar electricity technologies from the point of view of system analysis, taking into consideration their performance, costs and environmental impact. The study shows that the different approaches cover a wide range from units of a few Watts to utiilty-size plants and from isolated to grid-connected systems. It also shows that there is still a need for intense efforts to integrate those technologies into the existing electricity supply scheme. If a continuous development and market introduction is achieved, solar power plants will contribute substantially to the reduction of global CO₂-emissions. A practical tool for decision-makers is presented that facilitates a first estimate of the performance and costs of such plants under local conditions.