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Abstract The increasing penetration of PV and wind energy in several markets is forcing the power plants (both fossil and renewable) to operate with a high level of flexibility. In this scenario, the load-following capability of CSP power plants can play a fundamental role. The Thermal Energy Storage (TES) system allows exploiting solar energy after sunset and adjusting the power output according to the power demand variability. The paper aims to investigate different CSP plant configurations operating in island-mode to fulfill the power demand of a mid-size town located in the Upington region (South Africa). The power block is based on a steam Rankine cycle with superheated steam temperature 550°C, coupled with a molten salt direct storage system. Two solar fields are compared: Parabolic Trough Collectors (PTCs) and Central Receiver System (CRS). In order to analyze the load-following capability, two different electric loads (with the same annual energy demand of 360 GWh) are considered: the first one is typical of an industrial city, whilst the second one is the power demand pattern of a residential district. A numerical model of the two selected CSP technologies has been developed with the software Trnsys® and the optimization tool GenOpt. The simulation results show that the CSP plant based on CRS exhibits a higher load-following capability: both industrial and residential power demands are met with a lower aperture area and lower investment costs. The PTCs result to be affected by a strong efficiency variation between summer and winter: the load-following capability is good for the residential load profile, whilst is poor for the flat industrial load. [ABSTRACT FROM AUTHOR]

Micro gas turbine applications in concentrating solar field systems is already on industrial stage. The peculiarity of these systems is the possibility to use fossil fuels when solar power source is lacking. It is preferable that the system works in solar-only mode for long time; however, owing to the efficiency loss which occur for low radiation levels, a fuel integration is necessary. This work presents a system which allows to operate with constant efficiency, without the use of fuel for over one fifth of the nominal power rate. It is based on a regenerated micro gas turbine in closed loop configuration. The proposed system includes the solar tower, the heliostats field, the regenerator and a low temperature heat exchanger which cools the working fluid. Finally, two more devices, for the actuation of the proposed control are included: an auxiliary compressor and a bleed valve. The use of air as working fluid has been analyzed, with different values of the base cycle pressure (inlet pressure of the main compressor), which are needed for varying the mass flow flowing in the system. The control of the mass flow rate is mandatory to regulate the gas turbine power, by keeping almost constant the maximum temperature of the thermodynamic cycle when the incident solar radiation changes. In particular, the auxiliary compressor admits fresh air in the cycle when the thermal power received by the sun increases, while the bleed valve discharges it in the atmosphere, when the thermal power decreases. Therefore, the thermodynamic cycle is unchanged and guarantees constant net system efficiency for all the operations conditions. Particular attention is given to the receiver thermal incident flux, heliostat field and solar tower design. The current results are compared with the annual efficiency and energy production of an existing plant in hybrid configuration (solar-fuel).The analysis has been carried out on a commercial gas turbine having a power of 100 kW, sited on Seville town. For the heliostat field analysis, the open source code Solar PILOT has been used, while for the entire plant the code Thermoflex has been employed. [ABSTRACT FROM AUTHOR]

This work proposes a hybrid system based on a micro-gas turbine plant integrated with a solar field either to provide a part of the heat addition or to totally replace the combustor in the case that the only solar heat input is enough to ensure that the working fluid reaches the turbine inlet temperature. The system can operate in a pure combustion mode when solar irradiance is weak or completely absent during some hours in the day or seasons. The analysis is based on the simulation of the overall plant by the Thermoflex software, a modular program that allows assembling model with several components. In particular, many calculations have been performed to simulate the solar-assisted cogenerating plant in different operating conditions in order to identify the optimum sizing of the solar system which reduces the annual costs or the fuel consumption and, consequently, the CO2 emissions. Numerical results are compared with the data available from standard micro gas turbine plant. Finally, an economical evaluation by calculation the SPB (simple pay back) is obtained for all test cases to evaluate the benefit from Italian State incentives. [ABSTRACT FROM AUTHOR]

This work presents a preliminary thermodynamic assessment of three different supercritical CO 2 (sCO 2 ) power cycles integrated in a high temperature solar tower system, working up to 800 °C. An indirect cycle configuration is considered with KCl-MgCl 2 molten salt as heat transfer fluid (HTF) in the solar receiver and a two tanks thermal energy storage (TES) system. The most promising cycle configuration is selected, optimizing the cycle turbine inlet temperature to achieve the best compromise between cycle and receiver efficiency. An estimate of the yearly energy yield of the proposed power plant is finally performed, indicating the possibility of reaching solar-to-electric efficiency of about 17.5%. [ABSTRACT FROM AUTHOR]

The integration of a Concentrated Solar Power (CSP) plant with cooling energy production is investigated in the present paper. The power block is based on a steam Rankine cycle fully driven by two types of solar concentration devices: i) Parabolic Trough Collector (PTC) solar field and ii) Heliostat field with Central Receiver (HCR). Storage tanks allow the plant to operate also during nighttime hours. Cooling production is carried out by steam-driven double-effect absorption chillers feeding a district cooling network. The system was designed to operate in "island mode" to cover both the electrical and the cooling demand for a town of about 50,000 inhabitants. A typical location in the desert region of the Saudi Arabia Kingdom was selected as a case study. Commercial software and in-house computer codes were combined together to predict annual CSP plant performance. Results of annual plant operation on a one hour basis are presented and discussed. CSP plant showed a good capability of load-following operation, providing at the same time a relevant fossil fuel saving. [ABSTRACT FROM AUTHOR]

Decreasing water resources and steadily rising water demand drive research towards new approaches for safe and reliable water supply for the municipal, agricultural and industrial sectors. One solution for fresh water provision is sea water desalination using solar thermal energy. This paper describes the coupling of a Multi Effect Distillation (MED) plant with a Clausius-Rankine cycle powered by a solar central receiver system. A steady state model of an MED plant is developed and a correlation for the Gained Output Ratio (GOR) as function of heating steam temperature, specific seawater massflow and specific heat transfer surface of the desalination plant is deduced. This correlation is integrated into a model of a central receiver plant, comprising the heliostat field, a molten salt receiver system, two-tank storage and the steam cycle. A showcase simulation for the location Al-Kosseir, Egypt is performed to show the trade-off between electricity and water production for a cogeneration situation. [ABSTRACT FROM AUTHOR]

Solar thermal technology for the production on electricity is one of the current technological challenges. In concentrating solar power (CSP) plants, in order to achieve high power it is required to use a high operating temperature to reach high conversion efficiencies. The majority of today's commercial solar thermal power plants are based on the parabolic trough collector technology with operating temperature around 400 °C. However, the technology of solar tower is used in order to maximize the efficiency of the CSP plants. This technology reaches an operating temperature higher than 1000 °C and the development of high temperature receivers that work in this temperature ranges is still in its early stages. The fundamental problems observed are related to materials durability and reliability. The main objective of this paper has been to develop testing methods for solar receivers which guarantee their reliability and durability under demanding working conditions of high solar concentrating technology. Based on a revision of published or draft Standards, a qualification test methodology for durability tests has been developed. [ABSTRACT FROM AUTHOR]

The choice of heliostat aspect ratio has previously been shown to have an effect on the wind loading coefficients experienced by the structure due to an attacking atmospheric wind. Knowing the wind loads on heliostats is important in order to allow for adequate sizing of all components. The preferred method for determining such loadings is experimental wind tunnel studies, with Computational Fluid Dynamics (CFD) being less popular. A CFD model capable of predicting the mean wind loadings on a heliostat structure subjected to an attacking atmospheric wind and turbulence intensity profile was developed and validated using experimental results available in the open literature. The commercial software package ANSYS Fluent V15.0 was used together with the ANSYS Workbench Design Explorer toolset to study the influence of aspect ratio on the wind loading coefficients. [ABSTRACT FROM AUTHOR]

This paper presents an advanced methodology for the detailed modeling of the heat transfer and fluid dynamics phenomena in solar tower receivers. It has been carried out in the framework of a more ambitious enterprise which aims at modeling all the complex heat transfer and fluid dynamics phenomena present in central solar receivers. The global model is composed of 4 sub-models (heat conduction, two-phase flow, solar and thermal radiation and natural convection) which are described. Results of the numerical model obtained so far are also presented and discussed [ABSTRACT FROM AUTHOR]

The concentrating solar power (CSP) industry is working intensively on cost reductions to increase the economic competitiveness of CSP plants. Efforts include new power plant concepts, optimised component manufacturing, as well ashigher steam parameter and these indeed lead to cost reductions over the last years. However, further improvements are required and this paper analyses the impact of supercritical steam parameters in CSP plants. A few decades ago the coal industry moved from subcritical Rankine cycle power plants, currently all CSP plants operate at subcritical conditions, to supercritical steam parameters to improve cycle efficiency. Many supercritical coal plants are now in commercial operation with significant engineering experience available in regards to plant design, construction and operation. The CSP industry can use this expertise to reproduce the benefits in their plants but some challenges also exist. Currently, supercritical CSP steam has only been shown at demonstration scale and the upscale to the smallest turbine size for supercritical parameters, being 250 MWe steam turbine capacity, is very significant. This paper compares three different 250 MWe (net) solar tower scenarios, one with subcritical and two with supercritical steam parameters. One supercritical power plant scenario is based on a novel high temperature stable molten salt that allows steam parameter of 620 °C at 280 bar and the second uses current molten salt and natural gas to reach supercritical conditions. The analysis shows that the net plant cycle efficiency can be raised from 41.3% in a subcritical to 44.2% in a supercritical concept, which translates into a levelised cost of electricity reduction of 4.3%. [ABSTRACT FROM AUTHOR]

Small molten salt modular towers linked together to feed into a large power block, including storage, offer the potential to significantly reduce the cost of solar thermal energy. This is primarily through the significant increase in solar field and receiver efficiency that are achieved while still retaining the benefit of scale in the power block. Such towers would use cavity type receivers that are inherently more efficient than an external receiver. This paper examines the potential for a new concept for a cavity receiver, suitable for molten salt,which can increase efficiency and reduce metal hot-spot temperatures. By distributing the tubes within the volume of the cavity and arranging for the cooler inlet tubes to take the highest flux, the metal temperatures can be reduced close to the outlet salt temperature. The proposed design concept has the potential to solve a number of design issues that increase the cost of receiver systems. The paper provides a first-stage, simplified, theoretical analysis to show how receiverefficiency (from a radiative perspective) and hot-spot temperatureare affected by the number of heat transfer layers and the degree to which each layer blocks the radiation. The work shows promising results that needs to be taken forward in a number of areas. [ABSTRACT FROM AUTHOR]

Coal power stations have been hybridised with concentrated solar thermal (CST) fields which producefeedwater or with turbine bleed steam (TBS) heating from directlinear Fresnel to steam technology. This paper assesses solar hybridisation of boiler based steam power plants, whichpreheat boiler combustion air with a novel high temperature CST system based on a solid particle receiver (SPR). This new method of preheating has the potential to increase the solar share of the overall system, improve fuel saving and therefore produce a higher solar to electric conversion efficiency. These benefits result from theSPR solar systems higher operating temperature and integrated thermal storage. The integrated thermal storage also allows a buffered response time for handling transients in the intermittent solar resource. Analysis indicates that air-solarisation of coal plants can result in significantly higher solar to electric conversion efficiency than existing solar hybridisation options. Solarisation by TBS decreases power cycle efficiency due to bleed steam reduction, while solarisation by air-preheating increases the power system efficiency, primarily due to enhanced boiler efficiency brought about by reduced stack losses. The air solarisation option proposed in this paper has beencompared to current TBS with Fresnel based technology. The comparison was conducted by modelling both systems and analysing the thermodynamic heat and mass balance of the steam cycle and boiler using EBSILON®Professional software. Annual simulation tools, which calculate the performance of the solar field, receiver, storage (when applicable) and other system components, were used to model the output of the solar technologies. These tools, coupled with available economic data and cost models for the newly developed solar components, were used to calculate the levelized cost of energy of the compared hybridisation options. It was calculated that the levelized cost of the solar electricity produced by the SPR system was approximately 59%theelectricity produced by the Fresnel hybridisation. [ABSTRACT FROM AUTHOR]

In the last years more experience has been achieved with long time operation of solar tower receivers increased in size. The insulation of these receivers shows higher complexity and importance compared to small prototypes [1,2]. Both insulation of tubular cavity receiver and inner insulation of piping are safety-relevant components as they protect e.g. support structures and pressurized tubes and connections against overheating. Besides safety, operating reliability is one of the most important requirements to meet as down times, maintenance and repairs cause high costs. This paper describes the new development of the insulation for a tubular cavity receiver with air as heat transfer medium, where costs and thermal heat loss should be reduced. The prototype has been designed, built and tested for 100 operating hours at receiver outlet temperatures of up to 800 °C at the test facility Plataforma Solar de Almería (PSA), Spain. The results of a thermal model, of material tests and the evaluation of the thermal heat loss are presented. [ABSTRACT FROM AUTHOR]

The aim of this paper is to provide a comparative energetic and economic assessment of two solar tower power configurations; the molten salt (MS) and the direct steam generation (DSG) technologies in order to study the appropriate configuration for Algerian climate conditions. For this, Tamanrasset has been chosen as representative site to simulate the proposed solar tower power plant configurations. The first section of this paper analyses the conversion of the solar energy into electric energy. The NREL's SAM software (Solar Advisor Model) is used to evaluate the energetic performances of the two plant configurations in the proposed site and also to study their economic feasibility in the second section. [ABSTRACT FROM AUTHOR]

Abstract: A test bench located at Jabal Hafeet Mountain in Abu Dhabi has been prepared. Four pyrheliometers located at different altitudes from 340 m to 1035 m above mean sea level continuously recorded the Direct Normal Irradiance (DNI) with high accuracy. From the analysis of these measurements, a model simulating the attenuation of direct irradiance with distance of propagation, within 100 m above the surface (approximate height of a tower) was designed. A general shape of the relationship between the atmospheric optical depth from the top of the atmosphere to the ground reference and the extinction coefficient of the irradiance from the ground reference to the target was derived and validated with the use of ground measurements. This relation was used to estimate the DNI at two stations with mean path of sunlight from the reference station equals to 0.3km and 0.6km respectively. The comparison of the estimated and measured DNI at these two stations resulted in favorable bias and standard deviation values (bias of 0.3% and 0.4%, and standard deviation equals to 6% and 8% for the two stations respectively). Additionally, the applicability of this method in other locations was discussed. [Copyright &y& Elsevier]

Abstract: This paper deals with the development of a control design model for a 1MW Solar Tower equipped with a heat storage facility. This model is precise enough to achieve a good prediction of the responses but is also simple enough to avoid computational burden. The paper presents the assumptions and equations used for the different components of the plant. The behavior of the model developed in Matlab/Simulinktm is qualitatively validated by closed loop simulations. The control used for these simulations is also given. It consists of two levels, the upper level being an automaton whose outputs are the set points of the lower level controllers. [Copyright &y& Elsevier]

Abstract: Solar tower based plants are seen as a promising technology to reduce the cost of electricity from solar radiation. This paper assesses the design and overall yearly performances of two different solar tower concepts featuring two commercial plants running in Spain. The first plant investigated is based on Direct Steam Generation and a cavity receiver (PS-10 type). The second plant considers an external cylindrical receiver with molten salts as heat transfer fluid and storage system (Gemasolar type). About the optical assessment performed with DELSOL3, a calibration of heliostat aim points was performed to match available flux maps on the receiver. Moving to results, the PS-10 type has higher optical performances both nominal design and yearly average. This is due both to the field size and orientation which guarantee a higher efficiency and to the receiver concept itself. About power production, the molten salts allow higher temperature and consequently conversion efficiency than PS-10. The solar-to-electricity efficiency is equal to 18.7% vs. 16.4% of DSG cavity plant. The obtained results are strictly related to the set of assumptions made on each plant component: when available real plant data where used. The two solar tower plants results were also compared to corresponding commercial linear focus plants featuring the same power block concept. Gemasolar type shows a higher solar-to-electricity efficiency compared to a parabolic trough plant with storage (18.7% vs. 15.4%) because of the higher maximum temperatures and, consequently, power block efficiency. PS-10 is better than a linear Fresnel DSG (16.4% vs. 10.4%) because of the higher optical performances. [Copyright &y& Elsevier]

Abstract: In this work the transient simulations of four hybrid solar tower power plant concepts with open-volumetric receiver technology for a location in Barstow-Daggett, USA, are presented. The open-volumetric receiver uses ambient air as heat transfer fluid and the hybridization is realized with a gas turbine. The Rankine cycle is heated by solar-heated air and/or by the gas turbine's flue gases. The plant can be operated in solar-only, hybrid parallel or combined cycle-only mode as well as in any intermediate load levels where the solar portion can vary between 0 to 100%. The simulated plant is based on the configuration of a solar-hybrid power tower project, which is in planning for a site in Northern Algeria. The meteorological data for Barstow-Daggett was taken from the software meteonorm. The solar power tower simulation tool has been developed in the simulation environment MATLAB/Simulink and is validated. [Copyright &y& Elsevier]

Abstract: Optical and thermal simulation of a new up-down arranged dual-receiver for solar tower plant is presented in this paper. The top receiver is an external absorber type to serve as the boiling section, the bottom receiver is a cavity type to serve as the superheating section. The heliostat field is divided into two parts respectively for boiling and superheating section, it is quick and simple to control the heat flux distribution on both section. Then multi-aiming strategy is used for avoiding appearance of heat spot. For cavity receiver, a optimized layout for tubes is to increase convective heat transfer coefficient in the high heat flux religions. The concept of this new receiver is illustrated by a 10 MWe solar power plant that produces main steam at 513.5°C and pressure of 10.12MPa. Finally, this dual-receiver has a thermal efficiency of 91%. [Copyright &y& Elsevier]

Abstract: Concentrating solar power (CSP)-biomass hybrids plants are becoming increasingly interesting as a low cost option to provide dispatchable renewable energy since the first reference plant commenced operation late 2012, 22.5MWe Termosolar Borges in Spain. The development of such project is a complex task with not only one but two energy sources required to make the project successful. The availability of several studies but only one reference plant worldwide is proof of that. This paper investigates the hybridisation of a biomass power plant with a molten salt solar tower system. The benefit of this combination is a high cycle efficiency as both the steam generators can provide steam at 525°C and 120bar to the steam turbine. A case study approach is used to provide technical, economic and environmental benefits of a 30MWe CSP-biomass plant with 3h thermal storage in Griffith, New South Wales. At this site such a plant could provide annually 160,300MWh of electricity with an annual average electricity price of AU$155/MWh. Compared to a standalone CSP plant with 15h of thermal storage the hybrid plant investment is 43% lower, providing a possibility to fast-track CSP implementation in countries where CSP is struggling to enter the market due to low wholesale electricity prices, such as Australia. [Copyright &y& Elsevier]

Abstract: Introducing solar energy into the gas turbine of Combined Cycle systems (CC) offers significant advantages over other solar power plant concepts. A promising way to introduce solar power is solar preheating of the compressor discharge air before it enters the combustor of the gas turbine using a receiver built consisting of several metallic tubes. The main challenge during the design of such a receiver is the low solar flux capability caused by the limited convective heat transfer. To ensure a sufficient efficiency, the receiver is usually located in a cavity. The heat losses by conduction through the (insulated) cavity walls can be reduced by increased insulation thickness. Heat losses by radiation and convection to ambient depend strongly on the aperture area, receiver inclination and the receiver temperature. The receiver temperatures can be reduced by increasing the cavity si ze, as the flux distribution gets more homogenous and the overall flux density is reduced. Following heat losses by radiation and convection to ambient are reduced. On the other hand additional costs for the cavity walls have to be considered. The paper deals with the comparison of two different strategies to increase the receiver efficiency of a cavity receiver used to heat the compressed air of a 4.7 MWel turbine from 330°C up to 800°C, with a mass flow of 15.9kg/s at 10 barabs. The influence on the levelized cost of energy (LCOE) of different receiver sizes and one design option (transparent covering of aperture) for reducing the convections losses were analyzed and compared. The thermal and hydraulic layout was done for a thermal heat of 8.4MWth and 250 mbar pressure drop using thermal finite element (FE) models considering the local heat flux distribution, heat transfer to working fluid, radiation exchange between components and ambient as well as conduction and convection losses of the cavity. As the convective losses play a significant role, CFD models were used to evaluate the convective heat losses. [Copyright &y& Elsevier]

Abstract: This paper presents an advanced methodology for the detailed modeling of the heat transfer and fluid dynamics phenomena in solar tower receivers. It has been carried out in the framework of a more ambitious enterprise which aims at modeling all the complex heat transfer and fluid dynamics phenomena present in central solar receivers. The global model is composed of 4 sub- models (heat conduction, two-phase flow, thermal radiation and natural convection) which are described. Results of the numerical model obtained so far are also presented and discussed [Copyright &y& Elsevier]

Abstract: The objective of this research work is to provide a effective assistance to all those who have to make decisions concerning the planning and implementation of projects in the renewable energy. Indeed, The integration in the Moroccan power grid of electrical power via Concentrating Solar Power (CSP) units using parabolic mirrors or solar tower is expected to have a significant impact on the reliability of the electrical system. In this context. Therefore, in this work, firstly, we assess the reliability of the Moroccan grid at hierarchical level HL1 (HL1: load covering under the assumption of infinite node) by use of a non-sequential Monte Carlo simulation in which CSP units (with parabolic through mirrors and with Solar tower) are introduced. Secondly objective of this paper is to assess the reliability of the Moroccan grid with an expected common integration of 14% wind and 14% solar energy. [Copyright &y& Elsevier]