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The impinging receiver is a new member of the cavity solar receiver family. In this paper, the transient performance of a prototype impinging receiver has been studied with the help of a Fresnel lens based solar simulator and an externally fired micro gas turbine. The impinging receiver can offer an air outlet temperature of 810 °C at an absorber temperature of 960 °C. The radiative-to-thermal efficiency is measured to be 74.1%. The absorber temperature uniformity is good but high temperature differences have been detected during the ‘cold startup’ process. The temperature changing rate of the receiver is within 3 °C/s for the startup process and 4 °C/s for the shut-down process. In order to avoid quenching effects caused by the impinging jets, the micro gas turbine should be turned off to stop the airflow when the radiative power is off., Export Date: 13 February 2018; Article; CODEN: ECMAD; Correspondence Address: Wang, W.; Department of Energy Technology, KTH Royal Institute of TechnologySweden; email: wujun@kth.se. QC 20180326

The need to mitigate the climate change has brought in the last years to a fast rise of renewable technologies. The inherent fluctuations of the solar resource make concentrating solar power technologies an application that demands full flexibility of the steam turbine component. A key aspect of this sought steam turbine flexibility is the capability for fast starts, in order to harvest the solar energy as soon as it is available. However, turbine start-up time is constrained by the risk of low cycle fatigue damage due to thermal stress, which may bring the machine to failure. Given that the thermal limitations related to fatigue are temperature dependent, a transient thermal analysis of the steam turbine during start process is thus necessary in order to improve the start-up operation. This work focuses on the calculation of turbine thermomechanical properties and the optimization of different start-up cases in order to identify the best solution in terms of guaranteeing reliable and fast start-ups. In order to achieve this, a finite element thermal model of a turbine installed in a concentrating solar power plant was developed and validated against measured data. Results showed relative errors of temperature evolutions below 2%, making valid the assumptions and simplifications made. Since there is trade-off between start-up speed and turbine lifetime consumption, the model was then implemented within a multi-objective optimization scheme in order to test and design faster start-ups while ensuring safe operation of the machine. Significant improvements came up in terms of start-up time reduction up to 30% less than the standard start-up process., QC 20181108

The present study evaluates and compares the optimum configurations for both PV-batteries and molten salt tower concentrating solar power plants that minimize the levelized cost of electricity for a suitable location for deployment of both solar technologies nearby Ouarzazate, Morocco, when considering two capacity factor objectives, namely 50% and 85%, and cost-projections for 2020 and 2030. Required target cost reduction rates for each of the main blocks in the tower plant (i.e. the solar field, the storage and the power block) are identified for guaranteeing its competitive advantage as the most economically viable solar-only technology at both capacity factor objectives investigated. It is shown that the larger the capacity factor requirement is, the more competitive the solar thermal technology would be. Specifically, the case-study shows that for an 85% capacity factor objective, tower plants would be more competitive even when considering the most pessimistic and optimistic cost projections for the solar thermal and PV-batteries sub-components, respectively. Nevertheless, it was also determined that in order to ensure being the most competitive solar-only technology at a 50% capacity factor objective by 2030, the costs of the solar field of the solar tower plants should reach values as low as 20-50 (sic)/m(2), depending on the scenario, which means approximately a three to seven fold decrease of the costs as of 2017. At last, recommendations to solar thermal technology owners and developers are provided, and a short discussion regarding the viability and limitations of using battery electric storage systems for utility-scale solar plants is presented., QC 20190905

Higher performance cavity receivers are needed to increase the competitiveness of solar power plants. However, the design process needs to be improved with more relevant experimental and numerical analyses. Thereby, the performance of four different Dish-Stirling cavities is investigated experimentally analyzing the influence of the cavity aperture diameter and shape at various operating temperatures. Temperatures inside the cavity receiver were collected together with the electrical power produced by the engine-generator. Then, a thermal system simulation was modelled and a comprehensive multi-parameter and multi-operation validation was performed. To improve this validation, the temperature distribution across the receiver tubes was analyzed in order to relate temperatures on the irradiated region with the non-irradiated one, where thermocouples can measure. The simulations were later used to obtain cavity receiver efficiencies, temperatures and loss breakdowns. The results show that the cavity receiver must be studied in optimization processes in conjunction with the other system components. Moreover, the reverse-conical cavity was found to be more efficient than a nearly cylindrical shape. Regarding the cavity receiver thermal losses, radiation and natural convection present similar contributions in the system under study. Finally, it was found that thermocouples installed on a non-irradiated region can be used to obtain peak receiver temperatures if the measurements are rectified by a correction value proportional to the DNI., QC 20180914

Potential control strategies for an impinging receiver based dish-Brayton system have been presented for protecting the key components from the risks of overheating when the solar irradiation exceeds its design value. Two of them are selected for a detailed study: changing the effective diameter of the shading device and changing the inlet temperature. A rope-pulley shading device is developed for controlling the shading area in the center of the dish, and the change of the inlet temperature is achieved by applying a bypass at the cold side of the recuperator for reducing the heat transfer rate. Both control strategies can manage the peak temperature on the absorber surface within 1030 °C with an outlet temperature fluctuation between −4.1 and 15.1 °C, so that the impinging receiver can work for long time at any solar direct normal irradiance value. Furthermore, the temperature differences on the absorber surface are between 137.1 °C and 163.8 °C. The cases that are achieved by changing the shield effective diameter are significantly lower (11–26 °C) than the corresponding cases that are achieved by changing the inlet temperature., QC 20180530

This work presents the experimental evaluation of a novel pressurized high-temperature solar air receiver for the integration into a micro gas-turbine solar dish system reaching an air outlet temperature of 800°C. The experiments are conducted in the controlled environment of the KTH high-flux solar simulator with well-defined radiative boundary conditions. Special focus is placed on providing detailed information to enable the validation of numerical models. The solar receiver performance is evaluated for a range of operating points and monitored using multiple point measurements. The porous absorber front surface temperature is measured continuously as it is one of the most critical components for the receiver performance and model validation. Additionally, pyrometer line measurements of the absorber and glass window are taken for each operating point. The experiments highlight the feasibility of volumetric solar receivers for micro gas-turbine based solar dish systems and no major hurdles were found. A receiver efficiency of 84.8% was reached for an air outlet temperature of 749°C. When using a lower mass flow, an air outlet temperature of 800°C is achieved with a receiver efficiency of 69.3%. At the same time, all material temperatures remain below permissible limits and no deterioration of the porous absorber is found., QC 20180420

This paper presents the experimental results of the Cleanergy's C11S solar engine-generator tested in the KTH solar simulator. The paper focuses on the analysis of the thermal performance of the cavity receiver used in the C11S module. Multiple temperature measurements were taken on the tubes of the receiver, inside the cavity and on the internal surface of the cavity. These values allowed characterizing the temperature distribution all around the cavity receiver for the validation of thermal models and the estimation of the thermal losses. Moreover, this paper shows a comparison of the operating characteristics of the C11S module under the real operating conditions and the laboratory ones. It was observed that the temperatures of the receiver in the High Flux Solar Simulator (HFSS) resemble well the real temperatures. Thereby, the KTH solar lab provides proper irradiance levels to operate solar receivers at representative working conditions., QC 20190625

Laboratory-scale component testing in dedicated high-flux solar simulators is a crucial step in the developmentand scale-up of concentrating solar power plants. Due to different radiative boundary conditions available inhigh-flux solar simulators and full-scale power plants the temperature and stress profiles inside the investigatedreceivers differ between these two testing platforms. The main objective of this work is to present a systematicscaling methodology for solar receivers to guarantee that experiments performed in the controlled environmentof high-flux solar simulators yield representative results when compared to full-scale tests. In this work theeffects of scaling a solar air receiver from the integration into the OMSoP full-scale micro gas-turbine based solardish system to the KTH high-flux solar simulator are investigated. Therefore, Monte Carlo ray-tracing routines ofthe solar dish concentrator and the solar simulator are developed and validated against experimental characterizationresults. The thermo-mechanical analysis of the solar receiver is based around a coupled CFD/FEManalysislinked with stochastic heat source calculations in combination with ray-tracing routines. A geneticmulti-objective optimization is performed to identify suitable receiver configurations for testing in the solarsimulator which yield representative results compared to full-scale tests. The scaling quality is evaluated using aset of performance and scaling indicators. Based on the results a suitable receiver configuration is selected forfurther investigation and experimental evaluation in the KTH high-flux solar simulator., QC 20180226

An axial type impinging receiver has been developed for a solar dish-Brayton system. By using selective reflection cavity surfaces as a secondary concentrator, the solar irradiation is reflected and concentrated on a cylindrical absorber that is located in the center of the cavity. A modified inverse design method was applied for quickly finding possible cavity receiver designs, and a numerical conjugate heat transfer model combined with a ray-tracing model was utilized for studying the detailed performance of the impinging receivers. The ray-tracing results show that the flux distribution on the cavity and absorber surfaces can be efficiently adjusted to meet the design requirements by changing the absorber diameter, the cavity diameter, the cavity length and the offset length. A candidate receiver design was selected for detailed numerical studies, and the results show that the average outlet air temperature and the radiative-to-thermal efficiency can reach 801.1 °C and 82.8% at a DNI level of 800 W/m2. The temperature differences on the absorber can be controlled within 122.7 °C for DNI level of 800 W/m2, and 126.4 °C for DNI level of 1000 W/m2. Furthermore, the structure is much simpler than a typical radial impinging design., Export Date: 22 October 2018; Article; CODEN: ENEYD; Correspondence Address: Wang, W.; Department of Energy Technology, KTH Royal Institute of TechnologySweden; email: wujun@kth.se; Funding details: EU-FP7; Funding details: 308952; Funding text: This work was financially supported by the European Union's 7th Framework Programme (EU-FP7) project OMSoP (Grant Agreement No. 308952 ). QC 20181126

Concentrating solar power plants are an attractive option in the renewable energy generation market. The possibility of integrating relatively cheap forms of energy storage makes them a desirable solution when power generation must be readily available at any time of the day. Solar power plants typically start-up and shut down every day, so in order to maximize their profitability, it is necessary to increase their flexibility in transient operation and to initiate power generation as rapidly as possible. Two of the key components are the steam generator and steam turbine and the rates at which they can reach operational speed are limited by thermo-mechanical constraints. This paper presents an analysis of the effects of the thermal stress limitations of the steam generator and steam turbine on the power plant start-up, and quantifies their impact on the economy of the system. A dynamic model of a parabolic trough power plant was developed and integrated with a logic controller to identify start-up limitations, and subsequently the dynamic model was integrated in a techno-economic tool previously developed by the authors. The plant was analysed under two different operating strategies, namely solar-driven and peak-load. The results indicate that for steam generator hot start-ups, a 1.5% increase in peak-load electricity production would be achieved by doubling the maximum allowable heating rate of the evaporator. No useful increase would be achieved by increasing the rates beyond a limit of 7–8 K/min, as the turbine would then be the main limiting component during start-up. Similar conclusions can be drawn for the solar-driven case, for which the solar field and the energy source availability would pose the major constraint when starting up the steam generator system., QC 20180529

Parabolic trough configurations account for 95% of the current installed concentrating solar power (CSP) capacity. Certainly this technology is considered as the most mature among other CSP types. However, regardless of its maturity, the pursuit of cost competitiveness with respect to fossil fuels and other renewables is still a dire need. One way to maximize profitability and improve performance is flexibility through fast starts. In this regard, the steam turbine has been identified as a key limiting component to the start-up process. This work focuses on analyzing the influence of steam turbine start-up parameters on the overall annual performance of a CSP plant. For this, a detailed parabolic trough power plant (PTPP) performance model was developed including a control strategy to account for turbine transient start-up constraints. The PTPP model was developed in accordance to the latest state-of-the-art of the technology. As such, the first part of the results consisted of validation studies of the model with respect to the actual power plant. The results obtained in this regard showed that the model correlates to the rated performance of the power plant with maximum errors of 12% and of 14% to the dynamic operation of the power plant. The second part of this work consisted of using the validated model in a series of sensitivity studies concerning the variation of different turbine start-up parameters. Results showed that improvements of up to 1.8% in the annual electricity production are possible with only 0.3% increase in fuel consumption., QC 20190625

Concentrating solar power plants are taking an increasing share in the renewable energy generation market. Parabolic trough is one of such technologies and the most commercially mature. However, this technology still suffers from technical challenges that need to be addressed. As these power plants experience daily start-up procedures, the optimal performance in transient operation needs to be considered. This paper presents a performance based modelling tool for a gas-boosted parabolic trough power plant. The objective of the paper is to define an optimal operational strategy of the power plant start-up procedure with the aim of minimizing its fuel consumption while at the same time maximizing its electric energy output, taking into account all the thermo-mechanical constraints involved in the procedure. Heating rate constraints of the steam generator and the booster heater, and the steam turbine start-up schedule were considered. The simulation model was developed based on a power plant located near Abu Dhabi, and was validated against real operational data with a maximum integral relative deviation of 4.3% for gross electric energy production. A multi-objective optimization was performed for a typical operating week during winter and spring weather conditions. The results suggest that in order to minimize the fuel consumption and at the same time maximize the electric energy production, an evaporator heating rate of 6 K/min is an optimal value both for winter and spring conditions., QC 20190107

Among concentrating solar power technologies, solar tower power plants currently represent one of the most promising ones. Direct steam generation systems, in particular, eliminate the usage of heat transfer fluids allowing for the power block to be run at greater operating temperatures and therefore further increasing the thermal efficiency of the power cycle. On the other hand, the current state of the art of these systems does not comprise thermal energy storage. The lack of storage adds to the already existing variability of operating conditions that all concentrating power plants endure due to the fluctuating nature of the solar supply. One way of improving this situation is increasing the operating flexibility of power block components to better adapt to the varying levels of solar irradiance. In particular, it is desirable for the plant to achieve fast start-up times in order to be available to harness as much solar energy as possible. However, the start-up speed of the whole plant is limited by the thermal inertia of certain key components, one of which is the steam turbine. This paper studies the potential for power plant performance improvement through the increase of steam turbine flexibility at the time of start-up. This has been quantified by carrying out power plant techno-economic studies in connection with steam turbine thermo-mechanic behavior analysis. Different turbine flexibility investigations involving the use of retrofitting measures to keep the turbine warmer during offline periods or changing the operating map of the turbine have been tested through multi-objective optimization considering annual power performance and operating costs. Results show that reductions of up to 11% on the levelized cost of electricity are possible through the implementation of these measures., Not duplicate with DiVA 1131018QC 20230202

As part of an effort towards the future smart energy system, integration of different distributed generation technologies is proposed in literature. These technologies include heat pumps, gas boilers, combined heat and power (CHP) plants, solar photo-voltaic (PV) and so on. Some of these technologies couple different energy carriers in which case the independent analysis of each network could lead to unrealistic results. Optimization of heat pumps and CHP plants in coupled electricity and heating network, for example, needs consideration of both networks’ parameters in order to get results that are optimal in both networks. The first step in such optimization process is to have a load flow model (as an equality constraint) for the two coupled networks. Even though many researchers tried to address optimization of energy mixes at a district level, they did not consider the details of network parameters. Too little has been done to investigate the effect of different distributed generation technologies on the operational parameters of different energy networks. This paper deals with a pseudo-dynamic simulation of a district energy system that consists of coupled electricity and heating networks. The details of transmission line and pipe parameters together with the coupling devices are modelled using an extended energy hub approach. A network of six energy hubs with different distributed generation technologies such as heat pump, gas boiler, CHP and Solar PV is considered in the simulation. Time series data for demands and generations at different hubs are used on hourly basis. The CHP and heat pumps are scheduled to operate in certain period of the year while the PV output follows the annual solar radiation. Annual pseudo-dynamic load flow simulation is done to see how the operational parameters and power losses in the network vary with hourly changes in demands, generations and loading of coupling technologies., QC 20190319

Energy systems at district/urban level are getting more complex and diversified from time to time. Different energy carriers are coupled each other to meet various types of energy demands. The conventional way of analyzing energy networks independently does not reflect the true nature of the coupled networks. One of such a promising coupled multi-carrier energy system (MCES) is the combination of district heating and electricity networks. The coupling between these two networks is increasing due to the integration of co– and poly-generation technologies at the distribution networks. Recent literatures tried to address a load flow analysis for lightly coupled networks by formulating case-specific load flow models. This paper presents a more general and flexible tool developed using Matlab® which can be used to conduct the load flow analysis of highly coupled electricity and heating networks. An energy hub concept is extended further to formulate a general model in which local generations and detailed network parameters of MCES can be taken into account. Coupled heating and electricity networks are modeled in detail for illustration. The flexibility and generality of the model are then tested by considering case studies with different network topologies (tree and meshed). A comparison is also made with a model used in recent literature. The results show that the proposed model is more accurate. The main contribution of this paper can be summarized by the following five points: (1) Coupling matrices are used to relate network power flow equations of different energy carriers; (2) Hybrid hydraulic head and pipe flow equations are used to develop the hydraulic model which can be applied for both types of tree and meshed heating networks with the possibility of pumping units; (3) A general thermal model that relates steady state temperature drops and mass flow rates, even during change of flow direction, is developed for the heating network; (4) The electricity network is, QC 20220126

Within the Nordic countries, distributed heat and power supply technologies, like domestic scale heat pumps and photovoltaics, are challenging the current centralized district energy infrastructure. An increasing number of customers decide to disconnect from the traditional heating network by comparing the bill to the potential economic savings which can be generated by a residential heat pump system. However, this approach can be considered valid only on a short-term perspective. This paper presents a new approach to compare the techno-economic performance of alternative technologies, based on their lifetime average cost of generation. The proposed analysis is able to determine the optimal energy infrastructure at urban district level. Within this solution, operators, city planners and users will have a solid reference for their decision making process on resources investment. From a first step analysis of a few Swedish case studies, it was found that a district heating based system is more techno-economically efficient compared to the distributed alternative. By comparing the district heating production cost to its final price, a significant profit margin for the utility was qualitatively highlighted. Thus, from a customer perspective, on the medium run, the district heating tariff can be adapted and the estimated savings from switching to a residential heat pump system can be nullified., QC 20181101

The present work deals with the techno-economic analysis of a novel combined power cycle consisting of a molten-salt solar tower power plant with storage supported by additional heat provided from the exhaust of a topping gas-turbine unit. A detailed model has been elaborated using in house simulation tools that simultaneously encompass meteorological, demand and required dispatch data. A range of possible designs are evaluated for a suitable location with both good solar resource and vast natural gas resources in order to show the trade-offs between the objectives of achieving low carbon-intensive and economically competitive designs. These were compared against more conventional integrated solar combined cycles of equivalent capacity factors. It is shown that the novel concept is worth further investigating as it is able to outperform the more conventional cycle while simultaneously offering additional flexibility to grid-operators., QC 20190905

Concentrating solar power plants are an attractive option in the renewable energy generation market. The possibility of integrating relatively cheap forms of energy storage makes them a desirable solution when power generation must be readily available at any time of the day. Solar power plants typically start-up and shut down every day, so in order to maximize their profitability, it is necessary to increase their flexibility in transient operation and to initiate power generation as rapidly as possible. Two of the key components are the steam generator and steam turbine and the rates at which they can reach operational speed are limited by thermo-mechanical constraints. This paper presents an analysis of the effects of the thermal stress limitations of the steam generator and steam turbine on the power plant start-up, and quantifies their impact on the economy of the system. A dynamic model of a parabolic trough power plant was developed and integrated with a logic controller to identify start-up limitations, and subsequently the dynamic model was integrated in a techno-economic tool previously developed by the authors. The plant was analysed under two different operating strategies, namely solar-driven and peak-load. The results indicate that for steam generator hot start-ups, a 1.5% increase in peak-load electricity production would be achieved by doubling the maximum allowable heating rate of the evaporator. No useful increase would be achieved by increasing the rates beyond a limit of 7–8 K/min, as the turbine would then be the main limiting component during start-up. Similar conclusions can be drawn for the solar-driven case, for which the solar field and the energy source availability would pose the major constraint when starting up the steam generator system.

Parabolic trough power plants using thermal oil as heat transfer fluid are the most mature concentrating solar power technology and state of the art. To further increase their efficiency and lower costs, molten salts can be used as heat transfer fluid. This results in higher operating temperature differences for improved cycle efficiencies and enables direct thermal energy storage at lower costs due to omission of the oil-to-salt heat exchanger and the need for smaller storage sizes. As a variety of salts are available to choose from, this study uses a multi-objective optimization to identify the most suitable heat transfer fluid for three locations in South Africa, Spain and Chile. The lowest values for the levelized costs of electricity (LCOE) can be found in Chile using Solar Salt as heat transfer fluid (75.0 $/MWhe). Generally, Solar Salt offers the lowest LCOE values followed by thermal oil and Hitec. The results also suggest that the choice of the heat transfer fluid is dependent on the direct normal irradiance (DNI) at each location. Thermal oil is competitive with Solar Salt in small systems at locations with low DNI values, whereas Hitec can be cheaper than thermal oil in large systems at locations with high DNI. Furthermore, it is also investigated at which freeze alert temperature set point the activation of the freeze protection system is optimal. The results indicate that this temperature should be chosen close to the solar field inlet temperature for small systems, while it can be lowered significantly for large systems to reduce electricity consumption from the freeze protection system.

Liberalized electricity market conditions and concentrating solar power technologies call for increased power plant operational flexibility. Concerning the steam turbine component, one key aspect of its flexibility is the capability for fast starts. In current practice, turbine start-up limitations are set by consideration of thermal stress and low cycle fatigue. However, the pursuit of faster starts raises the question whether other thermal phenomena can become a limiting factor to the start-up process. Differential expansion is one of such thermal properties, especially since the design of axial clearances is not included as part of start-up schedule design and because its measurement during operation is often limited or not a possibility at all. The aim of this work is to understand differential expansion behavior with respect to transient operation and to quantify the effect that such operation would have in the design and operation of axial clearances. This was accomplished through the use of a validated thermo-mechanical model that was used to compare differential expansion behavior for different operating conditions of the machine. These comparisons showed that faster starts do not necessarily imply that wider axial clearances are needed, which means that the thermal flexibility of the studied turbine is not limited by differential expansion. However, for particular locations it was also obtained that axial rubbing can indeed become a limiting factor in direct relation to start-up operation. The resulting approach presented in this work serves to avoid over-conservative limitations in both design and operation concerning axial clearances., QC 20171121

In this paper, a Fresnel lens based high flux solar simulator (HFSS) is developed for concentrating solar power research and high temperature material testing. In this design, each commercially available 7 kW(e) xenon-arc lamp is coupled with a silicone-on-glass Fresnel lenses as the optical concentrator, and 12 lamp-lens units are distributed in a circular array. In total, the power of the solar simulator can reach 84 kWe. A ray tracing model has been developed based on the real arc-emitter shape and the Fresnel lens optics for predicting the optical performance of the HFSS design. The testing result shows that the ray tracing model can predict the flux distribution on the focal plane accurately but a bit conservative in the center region. The flux distribution on the focal plane appears axisymmetric with a peak flux of 7.22 MW/m(2), and 19.7 kW of radiative power in total is delivered on a 280 mm diameter target. (C) 2017 Elsevier Ltd. All rights reserved., QC 20170524

The dispatchability offered by thermal energy storage (TES) in concentrated solar power (CSP) and solar hybrid plants based on such technology presents the most important difference compared to power generation based only on photovoltaics (PV). This has also been one reason for recent hybridization efforts of the two technologies and the creation of Power Purchase Agreement (PPA) payment schemes based on offering higher payment multiples during daily hours of higher (peak or priority) demand. Recent studies involving plant-level thermal energy storage control strategies are however to a large extent based on pre-determined approaches, thereby not taking into account the actual dynamics of thermal energy storage system operation. In this study, the implementation of a dynamic dispatch strategy in the form of a TRNSYS controller for hybrid PV-CSP plants in the power-plant modelling tool DYESOPT is presented. In doing this it was attempted to gauge the benefits of incorporating a day-ahead approach to dispatch control compared to a fully pre-determined approach determining hourly dispatch only once prior to annual simulation. By implementing a dynamic strategy, it was found possible to enhance technical and economic performance for CSP-only plants designed for peaking operation and featuring low values of the solar multiple. This was achieved by enhancing dispatch control, primarily by taking storage levels at the beginning of every simulation day into account. The sequential prediction of the TES level could therefore be improved, notably for evaluated plants without integrated PV, for which the predicted storage levels deviated less than when PV was present in the design. While also featuring dispatch performance gains, optimal plant configurations for hybrid PV-CSP was found to present a trade-off in economic performance in the form of an increase in break-even electricity price when using the dynamic strategy which was offset to some extent by a reduction in upfron, QC 20171208

This paper presents the performance improvements implemented in the KTH high-flux solar simulator to deliver a total power on target closer to the working conditions of real CSP systems. Therefore, additional rectifiers were installed in the power conversion unit of the high-power lamps as well as the back reflector was coated providing more favorable spectral reflectance properties. The results of a single lamp/lens-combination show that the power on target in an aperture of 280mm in diameter was increased from 831W to 1446W while the peak flux was increased from 675kW/m² to 905kW/m²., QC 20170807

In the operation and maintenance of concentrating solar power plants, high operational flexibility is required in order to withstand the variability from the inherent solar fluctuations. However, during the development phases of a solar thermal plant, this important objective is overlooked as a relevant factor for cost reduction in the long term. This paper will show the value of including flexibility aspects in the design of a concentrating solar power plant by breaking down their potential favorable impact on the levelized cost of electricity (LCOE) calculations. For this, three scenarios to include flexibility as a design objective are analyzed and their potential impact on the LCOE is quantified. The scenarios were modeled and analyzed using a techno-economic model of a direct steam generation solar tower power plant. Sensitivity studies were carried out for each scenario, in which the level of improvement due to each scenario was compared to the base case. Then, the results obtained for each scenario were compared for similar levels of LCOE and flexibility improvements. In general, all scenarios were beneficial on power plant performance. Improvements on the LCOE in the range of 3-4% were obtained with different distributions of costs and annual electricity for each case., QC 20170823

Concentrating solar power plants, even though they can be integrated with thermal energy storage, are still subjected to cyclic start-up and shut-downs. As a consequence, in order to maximize their profitability and performance, the flexibility with respect to transient operations is essential. In this regard, two of the key components identified are the steam generation system and steam turbine. In general it is desirable to have fast ramp-up rates during the start-up of a power plant. However ramp-up rates are limited by, among other things, thermal stresses, which if high enough can compromise the life of the components. Moreover, from an operability perspective it might not be optimal to have designs for the highest heating rates, as there may be other components limiting the power plant start-up. Therefore, it is important to look at the interaction between the steam turbine and steam generator to determine the optimal ramp rates. This paper presents a methodology to account for thermal stresses limitations during the power plant start up, aiming at identifying which components limit the ramp rates. A detailed dynamic model of a parabolic trough power plant was developed and integrated with a control strategy to account for the start-up limitations of both the turbine and steam generator. The models have been introduced in an existing techno-economic tool developed by the authors (DYESOPT). The results indicated that for each application, an optimal heating rates range can be identified. For the specific case presented in the paper, an optimal range of 7-10 K/min of evaporator heating rate can result in a 1.7-2.1% increase in electricity production compared to a slower component (4 K/min)., QC 20170822

In this paper, the effects of cavity surface materials on the radiative flux distribution of solar cavity receivers have been studied with the help of a ray-tracing methodology. Three metallic substrate materials (Inconel 600, austenitic stainless steel 253 MA and Kanthal APM) and two coating materials (Pyromark® 2500 coating and YSZ TBC coating) were selected as the candidate cavity surface materials. The results show that the flux distribution and the total optical efficiency are much more sensitive to the absorptivity on the cylindrical surface than on the bottom. By using high absorptivity coating on the cylindrical surface and low absorptivity coating on the bottom, the radiative flux on the bottom can be controlled at a low level, and it can help to reduce the cavity length for an impinging receiver with jets on the cylindrical surface. Furthermore, the radiative flux distribution on the cylindrical surface can also be tailored to meet various design requirements by applying different coating designs on the cylindrical surface., QC 20170103

Among concentrating solar power technologies, solar tower power plants currentlyrepresent one of the most promising ones. Direct steam generationsystems, in particular, eliminate the usage of heat transfer uids allowing forthe power block to be run at greater operating temperatures and thereforefurther increasing the thermal eciency of the power cycle. On the otherhand, the current state of the art of these systems does not comprise thermalenergy storage. The lack of storage adds to the already existing variability ofoperating conditions that all concentrating power plants endure due to theuctuating nature of the solar supply. One way of improving this situationis increasing the operating exibility of power block components to betteradapt to the varying levels of solar irradiance.In particular, it is desirable for the plant to achieve fast start-up times inorder to be available to harness as much solar energy as possible. However,the start-up speed of the whole plant is limited by the thermal inertia ofcertain key components, one of which is the steam turbine. This paperstudies the potential for power plant performance improvement through theincrease of steam turbine exibility at the time of start-up. This has beenquantied by carrying out power plant techno-economic studies in connectionwith steam turbine thermo-mechanic behavior analysis. Dierent turbineexibility investigations involving the use of retrotting measures to keep theturbine warmer during oine periods or changing the operating map of the turbine have been tested through multi-objective optimization consideringannual power performance and operating costs. Results show that reductionsof up to 11% on the levelized cost of electricity are possible through theimplementation of these measures, which in turn has a favorable impact onpower plant protability., QC 20170814

The present study quantifies the reduction in the levelized cost of electricity (LCoE) and capital expenditure (CAPEX) of a dish Stirling power plant (DSPP) through an increase in localization and unit production volume. Furthermore, the localization value of the plant is examined to determine how much investment is brought into the local economy. Ouarzazate, Morocco, was chosen as the location of the study due to the country's favorable regulatory framework with regards to solar power technologies and its established industry in the concentrating solar power (CSP) field. A detailed techno-economic model of a DSPP was developed using KTH's in-house modelling tool DYESOPT, which allows power plant evaluation by means of technical and economic performance indicators. Results on the basis of LCoE and CAPEX were compared between two different cases of production volume, examining both a minimum and maximum level of localization. Thereafter, the DSPP LCoE and localization value were compared against competing solar technologies to evaluate its competitiveness. In addition, a sensitivity analysis was conducted around key design parameters. The study confirms that the LCoE of a DSPP can be reduced to values similar to solar photovoltaic (PV) and lower than other CSP technologies. Furthermore, the investment in the local economy is far greater when compared to PV and of the same magnitude to other CSP technologies. The competiveness of a DSPP has the potential to increase further when coupled with thermal energy storage (TES), which is currently under development., QC 20170823

Solar thermal enhanced oil recovery (S-EOR) is an advanced technique of using concentrated solar power (CSP) technology to generate steam and recover oil from maturing oil reservoirs. The generated steam is injected at high pressure and temperature into the reservoir wells to facilitate oil production. There are three common methods of steam injection in enhanced oil recovery - continuous steam injection, cyclic steam stimulation (CSS) and steam assisted gravity drainage (SAGD). Conventionally, this steam is generated through natural gas (NG) fired boilers with associated greenhouse gas emissions. However, pilot projects in the USA (Coalinga, California) and Oman (Miraah, Amal) demonstrated the use of S-EOR to meet their steam requirements despite the intermittent nature of solar irradiation. Hence, conventional steam based EOR projects under the Sunbelt region can benefit from S-EOR with reduced operational expenditure (OPEX) and increased profitability in the long term, even with the initial investment required for solar equipment. S-EOR can be realized as an opportunity for countries not owning any natural gas resources to make them less energy dependent and less sensible to gas price fluctuations, and for countries owning natural gas resources to reduce their gas consumption and export it for a higher margin. In this study, firstly, the market potential of S-EOR was investigated worldwide by covering some of the major ongoing steam based EOR projects as well as future projects in pipeline. A multi-criteria analysis was performed to compare local conditions and requirements of all the oil fields based on a defined set of parameters. Secondly, a modelling approach for S-EOR was designed to identify cost reduction opportunities and optimum solar integration techniques, and the Issaran oil field in Egypt was selected for a case study to substantiate the approach. This modelling approach can be consulted to develop S-EOR projects for any steam flooding based oil fiel, QC 20170823

Parabolic trough power plants using thermal oil as heat transfer fluid are the most mature concentrating solar power technology and state of the art. To further increase their efficiency and lower costs, molten salts can be used as heat transfer fluid. This results in higher operating temperature differences for improved cycle efficiencies and enables direct thermal energy storage at lower costs due to omission of the oil-to-salt heat exchanger and the need for smaller storage sizes. As a variety of salts are available to choose from, this study uses a multi-objective optimization to identify the most suitable heat transfer fluid for three locations in South Africa, Spain and Chile. The lowest values for the levelized costs of electricity (LCOE) can be found in Chile using Solar Salt as heat transfer fluid (75.0 $/MWhe). Generally, Solar Salt offers the lowest LCOE values followed by thermal oil and Hitec. The results also suggest that the choice of the heat transfer fluid is dependent on the direct normal irradiance (DNI) at each location. Thermal oil is competitive with Solar Salt in small systems at locations with low DNI values, whereas Hitec can be cheaper than thermal oil in large systems at locations with high DNI. Furthermore, it is also investigated at which freeze alert temperature set point the activation of the freeze protection system is optimal. The results indicate that this temperature should be chosen close to the solar field inlet temperature for small systems, while it can be lowered significantly for large systems to reduce electricity consumption from the freeze protection system., QC 20190420

This paper presents the characterization of the first Fresnel lens-based High-Flux Solar Simulator (HFSS) showing the evaluation of the total thermal radiative power dependent on the aperture radius at the focal plane. This result can be directly applied to calculate the thermal power input into any solar receiver tested in the KTH HFSS. Three measurement setups were implemented and their results combined to assess and verify the characterization of the solar simulator: a thermopile sensor measuring radiative flux, a CMOS camera coupled with a Lambertian target to obtain flux maps, and a calorimeter to measure the total thermal power within an area of 300×300 mm. Finally, a Monte Carlo analysis was performed to calculate the total uncertainties associated to each setup and to combine them to obtain the simulator characterization. The final result shows a peak flux of 6.8 ± 0.35 MW/m2 with a thermal power of 14.7 ± 0.75 kW within an aperture of 180 mm in diameter at the focal plane, and a thermal-electrical conversion efficiency of 25.8 ± 0.3%. It was found very good repeatability and a stable energy output from the lamps during the experiments., QC 20171215

Concentrating solar power plants, even though they can be integrated with thermal energy storage, are still subjected to cyclic start-up and shut-downs. As a consequence, in order to maximize their profitability and performance, the flexibility with respect to transient operations is essential. In this regard, two of the key components identified are the steam generation system and steam turbine. In general it is desirable to have fast ramp-up rates during the start-up of a power plant. However ramp-up rates are limited by, among other things, thermal stresses, which if high enough can compromise the life of the components. Moreover, from an operability perspective it might not be optimal to have designs for the highest heating rates, as there may be other components limiting the power plant start-up. Therefore, it is important to look at the interaction between the steam turbine and steam generator to determine the optimal ramp rates. This paper presents a methodology to account for thermal stresses limitations during the power plant start up, aiming at identifying which components limit the ramp rates. A detailed dynamic model of a parabolic trough power plant was developed and integrated with a control strategy to account for the start-up limitations of both the turbine and steam generator. The models have been introduced in an existing techno-economic tool developed by the authors (DYESOPT). The results indicated that for each application, an optimal heating rates range can be identified. For the specific case presented in the paper, an optimal range of 7-10 K/min of evaporator heating rate can result in a 1.7-2.1% increase in electricity production compared to a slower component (4 K/min).