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Benchmarking analysis of a novel thermocline hybrid thermal energy storage system using steelmaking slag pebbles as packed-bed filler material for central receiver applications
Solar Energy 188 (2019) 644–654
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Benchmarking analysis of a novel thermocline hybrid thermal energy storage system using steelmaking slag pebbles as packed-bed filler material for central receiver applications Javier López Sanz1, Francisco Cabello Nuñez1, Fritz Zaversky
T
⁎,1
National Renewable Energy Center of Spain (CENER), Solar Thermal Energy Department, Spain
A R T I C LE I N FO
A B S T R A C T
Keywords: Concentrated solar power Thermal storage Thermocline Slag pebble By-product waste LCOE
The aim of this research is to provide the techno-economic and benchmarking analysis of a 100 MWe class molten salt central receiver system using a thermocline hybrid thermal energy storage (TES) system versus conventional two-tank storage. The techno-economic analysis used has been carried out by means of specifically developed transient simulation models to find out a robust comparison between technologies. Thermal energy from storage, annual net electricity production and Levelized Cost of Electricity are the concise and comprehensive figures of merit of this study. An exhaustive effort has been made in order to achieve a well-founded comparison between concentrated solar power plants with equivalent storage system designs in terms of capacity factor and performance. Finally, this research demonstrates that the thermocline hybrid TES system provides a reliable cost effective storage solution that achieves the same Levelized Cost of Electricity as state-of-the-art technology. This is achieved due to the competitive price of the steelmaking slag pebbles versus molten salts, a revalorized processed steel by-product waste from the metallurgical industry used as packed-bed solid filler material for thermocline tanks. The results of this research open an attractive potential for cost reduction in the concentrated solar power sector, a key competitive value for a new market opportunity.
1. Introduction There has been a great debate about the commercial upscale performance of thermocline thermal energy storage (TES) for central receiver systems. In this regard, the potential to provide cost effective solutions to improve dispatchability is compromised due to thermocline tank stability overcharge and discharge cycling. Consequences over global system efficiency and baseload power requirements under relevant operational environment are the key factors of the successful deployment of these systems. By this means, a direct hybrid thermocline-based TES system implemented in a 100 MWe molten salt central receiver system is techno-economically analyzed in its life-cycle to study its reliable decoupling of solar energy harvest and subsequently, electricity production, with the aim of achieving competitive energy costs against conventional state-of-the-art technology. As the European Strategic Energy Technology (SET) Plan promoted
(European Union, 2017), the Concentrated Solar Power technology roadmap must undertake improvements over advanced and reliable TES concepts to reduce costs and improve dispatchability as well as the environmental footprint of this energetic resource (See Fig. 1). The approach proposed in this research enhance the competitiveness of the molten salt central receiver system technology by contributing with a competitive and standardized up-scaled TES design based on hybrid storage integrating two thermocline tanks and two-tank storage. In addition, this solution proposes an added value to the storage concept by integrating and valorizing the steel by-product waste from the metallurgical industry as packed-bed solid filler material of the thermocline, facing the high environmental impact that represents the landfill of this steelmaking slag. In this way, high temperature and low-cost thermal storage material are provided leading to circular economy processes. The thermal storage concept currently applied in commercial CSP
Abbreviations: CSP, Concentrated Solar Power; DNI, Direct Normal Irradiation; EPGS, Electric Power Generation System; EYA, Energy Yield Assessment; HTF, Heat Transfer Fluid; LCOE, Levelized Cost of Electricity; O&M, Operation and Maintenance; PB, Power Block; SET, Strategic Energy Technology; SGS, Steam Generation System; TES, Thermal Energy Storage; WSC, Water Steam Cycle ⁎ Corresponding author. E-mail addresses: [email protected] (J. López Sanz), [email protected] (F. Cabello Nuñez), [email protected] (F. Zaversky). 1 Address: C/Ciudad de la Innovación, 7. Sarriguren, Navarra, Spain. https://doi.org/10.1016/j.solener.2019.06.028 Received 25 February 2019; Received in revised form 5 June 2019; Accepted 10 June 2019 Available online 26 June 2019 0038-092X/ © 2019 The Authors. Published by Elsevier Ltd on behalf of International Solar Energy Society. This is an open access article under the CC BY license (http://creativecommons.org/licenses/BY/4.0/).
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must be noticed. The amount of available thermal energy in packed-bed thermocline TES systems is represented by the temperature of the slag packed-bed and the heat transfer fluid (HTF) inside the TES system. Thus, thermal energy is stored in the form of sensible heat while the mass of packed-bed and HTF is kept constant. Consequently, when the energy received from the binomial heliostat field - receiver is greater
Nomenclature
Cp ρ k
specific heat capacity [J/kgK] density [kg/m3] thermal conductivity [J/mK]
Fig. 1. Concentrated Solar Power Thermal Electricity SET Plan 2017.
than the energy demanded by the power block, the temperature of the packed-bed TES increases until a limited maximum outlet fluid temperature, typically referred to as cut-off temperature. On the contrary, when the captured energy is lower than the energy demanded by the power block, the system consumes the energy stored in the packed-bed TES to produce electricity and consequently, the temperature of the packed-bed decrease until the cut-off temperature. Finally, buoyancy effects create thermal stratification expansion limiting the storage capacity of the tank. In conclusion, the overall performance efficiency of a packed-bed TES system is slightly lower than the two-tank molten salts TES system (Ortega-Fernández et al., 2018b). Several authors have studied and compared single thermocline storage systems performance against the conventional two-tank storage showing potential cost reductions of 20–37% for the TES System (TESS) (Libby, 2010). However, the thermocline tank has operational stability disadvantages and during discharging entails partial power block loading with its consequences in terms of energy production and energy storage efficiency. Even more for commercial storage designs for 100MWe class CSP plants that must provide high energy storage capacity, which means increasing design tank volumes or even combining multiple tanks; taking into consideration also design limitations due to seismic, structural and mechanical codes and standards for the design of
projects is the sensible heat thermal storage based on two-tanks, hot and cold, using molten salts (Ferri et al., 2008) as storage medium. The useful sensible heat is defined by the difference between two temperature levels, for central receiver systems the “cold temperature” is about 290 °C and the “hot temperature” 565 °C, in order to avoid problems like freezing, degradation or other operational issues (Ferri et al., 2008). If the fluid used as a storage medium is the same fluid that flows through the receiver, the plant configuration is called direct while the fluid used in the receiver is different from the storage medium, the plant configuration is called indirect. Nowadays at commercial scale, the indirect two-tank storage system is used in several parabolic trough collector plants (Gil et al., 2010; Relloso and Delgado, 2009), whereas the direct two-tank thermal is used in central receiver plants (Gil et al., 2010; Dunn et al., 2012). However, the two-tank storage system has a high price and its cost breakdown indicates that the molten salt inventory has the highest share of the total cost (Kelly, 2010). For this reason, the combination of the two-tank storage in a single tank may be a potential value for cost reduction. Also, filling this single tank with a cost effective solid material could replace a big amount of the molten salt inventory, achieving a cost-effective concept of thermocline tank. At this point, a brief technical reminder about thermocline tanks
Fig. 2. Methodology flow chart. 645
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role in the world’s future electricity supply. CSP is a proven technology with big growth potential in the following years, specifically, it is expected to grow 87% (4.3GW) until 2023, 32% more than over the same period between 2012 and 2017. Under an accelerated case, CSP growth could be even 60% higher (International Energy Agency, 2018). By 2050, with appropriate support, CSP could provide 11.3% of global electricity (IRENA, 2018a). Nevertheless, operational reliability, restricted access to financing and market designs that do not value TES systems continue to challenge the implementation of this technology. Among the four CSP technologies commercially available (parabolic trough, dish Stirling, linear Fresnel and central receiver), the central receiver system is the one with the highest cost reduction potential and the highest expected capacity installed worldwide (IRENA, 2018b; NREL/SolarPACES Project Database, 2019). For these reasons, the two CSP plant configurations under study will use a solar power tower cylindrical tube-wall external receiver, a field of two-axis tracking mirrors (heliostats) as collector system and will use molten salts as heat transfer fluid (HTF). Summing up the basic principles of central receiver systems operation, the solar energy absorbed by the receiver heats the HTF, which flows back to ground level and circulates to the TES system, where it can be pumped to the power generation block. In either case, electricity is produced by a dry-cooled power generation block. Therefore, the subsystem configuration of each plant under study is differentiated only by the type of thermal energy storage system used: hybrid thermocline based or conventional two-tanks. Furthermore, the European steel industry generated about 21.8 million tons of slag in 2010 resulting from steel making. About 76% of the produced steel slag was recycled in several applications such as aggregates for construction or road materials, but these sectors are unable to absorb the total amount of slag generated, so the remaining 24% were still landfilled (2.8 Mtons) or self-stored (2.2 Mtons) (Euroslag, 2010). From the environmental point of view, the landfill of slags emits heavy metals to environment which is a significant source of pollution of air, water and soil, and further adversely affects the human health, and the growth of plant and vegetation. Besides, it is currently a lost opportunity, as it represents a huge amount of available material with very interesting properties for potential recycling that is landfilled and therefore missed, adding the cost of landfilling for the steel sector. An adequate filler material for thermocline tanks should be inexpensive and widely available, have a high heat capacity per volume unit (this means a high specific heat capacity and a high density) and a low void fraction, be compatible with the fluid in the temperature range, and be non-hazardous (Brosseau et al., 2004). The produced steel slag brings together the main starting properties to meet most of
welded steel tanks (API, 2005; BSI, 2005). Therefore, higher storage capacities entail feasible, cost-effective and operational stable designs. Among others, the operational stability of thermocline tanks during short-term solar fluctuation compromises the continuous full load performance of the storage system and so, the capacity factor of the plant. Therefore, Ju et al. (2016) studied the hybridization of thermocline tanks with the conventional two-tanks concept modeling its behavior with positive conclusions. This research demonstrates that, for no solar fluctuations, the hybrid storage system can work with a high utilization ratio of the storage capacity. During short-term fluctuations, the plant can continuously generate steady power without discharge of the thermocline tank due to the buffering effect of the small hot pumping tank. While during long-term fluctuations, the combination of the packed-bed thermocline tank and the hot tank can still maximize a continuous power generation. Single thermocline thermal energy storage is more sensible against variable operating conditions and has a big impact on the power block operation conditions due to the effect of the thermocline cut-off temperature. Nevertheless, detailed transient annual behavior of each subsystem of the plant interconnected must be addressed taking into account thermal inertia, plant operational strategies or inlet fluid temperature dependence of the power block. As a result, real energy production and annual performance of a CSP plant will be achieved with which to cover conclusive financial studies which could give final conclusions to the industry. In view of the foregoing, this research addresses the following question, could the hybrid TES system overcome the performance difference against conventional state-of-the-art technology by base material cost reduction while uncertainties about its stable operation are solved?. In order to answer it, this research follows the methodology chart shown in Fig. 2. First of all, the general context about CSP and the metallurgical sector is proposed to properly orientate the reader to the potential of this research. Secondly, the hybrid TES system is explained and the benchmark study about the innovative packet-bed filler material is presented. Next, the methodology and modeling approach for each detailed plant subsystem and its interconnections are presented to finally evaluate the energy yield assessment and the benchmarking study. Then, under the concise context and techno-economic background, the results of the simulations are presented and analyzed. Final observations are given in the conclusions orientated to final users and the industry itself.
2. Context Concentrated solar power (CSP) is expected to play an important
Fig. 3. Hybrid thermal energy storage system configuration. 646
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guarantee 9 h of storage and 110 MWe of gross electrical output. However, this solution presents several disadvantages, among others, for each thermocline tank, is necessary to guarantee two minimum submergence level above the pump’s suction bell (one in the top of the tank due to the hot pump and other at the bottom where the cold pump takes suction) which involve a large useless dead volume of molten salts and so, high direct system costs. Therefore, to propose to the market a cost competitive solution to store 9 full load hours (110 MWe gross power) a thermocline hybrid TES system was selected. This system comprises two thermocline tanks using steelmaking slag pebbles as packed-bed solid filler material connected to two molten salt tanks acting as “pumping tanks”, where all molten salts pumps would take suction (see Fig. 3). In this way, the minimum molten salt level is only necessary for the pumping tanks and the thermocline tank’s height is then not limited by the maximum length of the pump’s shaft, reducing direct costs without affecting operational stability. As a summary, the main parameters of the thermocline tank are shown in Table 1.
Table 1 Parameters of the thermocline tank storage. Parameter
Value
Thermocline tank diameter Thermocline tank height Maximum increase of outlet temperature in the charge Maximum drop of outlet temperature in the discharge Void fraction Particle size
20 m 20 m 30 °C 30 °C 0.4 1.5 cm
these criteria. In short, the slag pebble not only suits the necessary characteristic to be an interesting solid filler material for thermocline tanks for CSP storage systems, but it also has a positive environmental impact recycling the steelmaking slag. Therefore, the potential use of the slag produced in Europe represents a new source to be partially absorbed by CSP. The possibility of integrating high temperature and low-cost storage materials represents a competitiveness opportunity needed to analyze for CSP sector and depends on the availability in the market of cost-efficient storage solutions.
3.2. Packed-bed filler material: sintered slag pebbles The steel slag used in this research conforms to most the basic criteria for solid filler materials being a promising cost reduction path for CSP applications. Nevertheless, the CSP sector must undertake improvements in reliable sub-systems; therefore, special attention must be given when proposing new advanced TES designs and materials. It is envisaged that subject to chemical composition, the raw steel slag mechanical and chemical performance could have an impact on the operational stability and safety of the storage system. Therefore, the chemical compatibility between the steel slag and the molten salts as well as the mechanical performance under cyclic conditions must be taken into account. In order to ensure reliability, initial chemical studies were conducted in the H2020 RESLAG project between molten salts and the raw metallurgical by-product waste revealing modifications on the thermophysical properties of the salts (Ortega-Fernández et al., 2018a). In consequence, a production process was designed where the gross steel slag is treated by a fine milling, compression and sintered process improving the mechanical resistance to thermal cycling of the final slag pebbles and its chemical stability up to 1000 °C under static test conditions, cycling the slag pebbles in an air atmosphere. It has also been observed that this production process also avoids entrainment of fine solid steel particles, breakdown or any change over the composition of the molten salts under static test conditions where the slag pebbles with the molten salts were cycled between 290 and 550 °C. Therefore, the chemical stability of the slag under cycling conditions is guaranteed, ensuring the operational stability and safety of the storage system. The media properties of the sintered slag pebbles have been characterized within the H2020 RESLAG project as it is described below, where temperature is in degrees Celsius.
3. The hybrid thermal energy storage system The hybrid thermocline based TES system design has been optimized to reach competitive costs against the conventional two-tank molten salts TES for a central receiver plant with 110 MW of design turbine gross electrical output and 9 full load hours of storage. The final configuration of the hybrid system integrates two “pumping tanks” and two thermocline tanks that store nine full load hours to power an electrical gross output of 110 MWe. By this means, the optimized costeffective solution selected combines two tank concepts: conventional molten salts tanks and thermocline tanks filled with steel making slag pebbles interconnected as is shown in Fig. 3. 3.1. Design of the hybrid TES system as a combination of thermocline tanks and molten salt tanks Due to the large amount of thermal energy necessary for a 110MWe CSP plant’s storage system, firstly, it is necessary to determine the maximum diameter and height of a single thermocline tank. For the conservative design of the thermocline, the conditions of a conventional hot molten salt tank have been considered, using ASTM A240 Grade 347H as structural material with a maximum stress allowable of 112 MPa at 575 °C (maximum operation temperature plus a safety offset of 10 °C) obtained from ASME Part II (ASME, 2010), following the codes and standards for the design of welded steel tanks (API, 2005; BSI, 2005) as well as seismic regulation. A finite element analysis (FEA) was performed in order to verify that the dimensions of the thermocline tanks are admissible and the stress is lower than the maximum allowable. The analysis concluded that the thermocline tank is structurally feasible with a maximum diameter of 20 m and the maximum height is also structurally limited up to 20 m. In this tanks the upper and bottom part is used as a distributor, for this reason, it is necessary 0.4 m of a top and bottom gap before the packed bed, thus, the packed bed high is 19.2 m which has a void fraction of 0.4. Secondly, it is necessary to define the discharging mass flow of the TES system at full load. For this purpose, the IPSEpro software (SimTech, 2013) was used to model a dry-cooled Rankine cycle with reheat of the steam before the medium-pressure turbine. The molten salt mass flow needed in the steam generation system (SGS) is around 670 kg/s while the charge mass flow of the TES system is 730 kg/s, defining a reasonable mass flow through the receiver at design conditions of around 1400 kg/s. Knowing the maximum feasible storage capacity per thermocline tank, at least 3 thermocline tanks would be necessary in order to
Cp = 827.69 + 0.3339·T k = 1.6792 − 5.3237·10−4 ·T The density of the slag pebbles is characterized as non-temperature dependent and with a fixed value of 2850 kg/m3 and the considered packed-bed void fraction has been set to 0.4 as indicated in Table 1. 3.3. Comparison of the sintered slag pebbles against other packed-bed filler materials The filler materials to be compared with the slag pebbles are the quartzite rock and the taconite. According to Pacheco et al. (2002), these materials are potential low-cost candidates as a solid storage medium in a packed-bed as well as compatible with molten salts in isothermal and cyclic conditions. One of the most interesting properties to pay attention to is the heat capacity per volume, which determines 647
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(LCOE) as the final figure of merit.
the thermal energy that can be stored for a specific tank size. The physical properties and price of these materials are shown in Table 2, which summarizes the comparison between these materials, highlighting the interest of the slag pebble material due to its higher ratio between heat capacity per volume of the slag and costs.
4.1. Energy yield assessment modeling approach 4.1.1. General modelling approach The modeling approach used for the EYA consists of effectively coupling detailed optical simulations of the heliostat solar field with also detailed and full-transient thermal simulations of the molten salts tube-based external receiver, hybrid thermocline based or conventional two-tank TES system and, finally, the power block, all of them controlled by a realistic Control System as seen in Fig. 4. Relevant aspects in the operation of the plant otherwise neglected, as transient TES system behavior and operational limitations, transient plant processes as preheating/shut-down-heating and secure cloud passing operating modes, and more, are taken into account increasing the quality of the EYA. Special attention has been taken into account to evaluate the thermodynamic performance of the hybrid TES concept on the power block operation and on the plant performance versus conventional technology. A compromise between high accuracy and limited computational costs has been deployed to cover this knowledge, providing a powerful solution with minimal computing time for the whole plant simulation. This has been possible by reducing other complex plant systems (solar field, receiver, power block) from initial accurate simulations to well-grounded efficiency matrices. Consequently, the annual performance model for each objective solar power plant is split into the following subsystems: heliostat solar field, central receiver, TES system, HTF system, control system and power block which covers steam generation system, water & steam cycle and electric power generation. Since the optical and thermal simulations are essentially different in the underlying physical principles and suitable methodologies, completely different approaches are used. In addition, every main plant system model has been interconnected in an accurate way. Finally, the study considers well-operated power plants without problems which force to stop the plant (molten salts freezing, deposition of slag particles in pumps or valves, etc.).
3.4. Hybrid thermal energy storage system benefits The main expected benefits of the hybrid TES system configuration are:
• The thermal storage capacity of the thermocline tanks is maximized: •
• •
•
the useful volume inside the thermocline tanks increases when placing the pumps outside the tanks since the minimum level of immersion of the pumps is not necessary, which leads to increase the storage capacity of these tanks. Higher efficiency, operational stability and lifetime: the pumping tanks operate as a thermocline buffer system under short-term solar fluctuations avoiding the partially and frequently charge-discharge process of the thermocline. This operation reduces local disturbances avoiding thermocline degradation and improves the performance of the storage system (Ju et al., 2016). In addition, the materials are subjected to lower mechanical stress increasing the lifetime of the components. The thermocline disturbance is reduced: it is caused by the absence of pumping disturbance effect on the molten salts flow inside the packed bed. The pumping system reduces direct and O&M costs: o Easiness of installation, maintenance and protection of the molten salts pumps in the pumping tanks against placing the pumps in the thermocline tank. o Thermocline tanks are higher than pumping tanks; therefore, shorter and cheaper pump shafts are needed, which helps to reduce the mechanical stress in the pumps’ shaft due to lower vibration, increasing the life cycle of the component. The hybrid design reduces direct and O&M costs: the inclusion of the pumping tanks mitigates the direct and O&M costs of a thermocline leakage failure. In the event of a tank leak, the molten salt can be transferred to another tank so the thermocline can be repaired. Thus, the drain tank dimensions and costs can be reduced.
4.1.2. Heliostat solar field modelling The heliostat solar field model or optical model is based on an accurate Monte Carlo ray-tracing method which solves the complete solar field performance. The heliostat field layout has been previously optimized as a complex multi-leveled problem (García-Barberena et al., 2016). Once the heliostat field, the tower, and the receiver are fully optimized and defined, the complete three-dimensional model is constructed using Tonatiuh, an open-source program for the detailed simulation of complex optical systems, which estimations have demonstrated highly accurate results in the simulation of heliostat systems (Blanco et al., 2011). The optical model is simulated for a large number of sun positions, covering all possible combinations of azimuth and elevation along a year, to fully describe the solar field optical performance. Each heliostat is placed in its geometric position in the solar field considering also the aiming point of each heliostat to accurately reproduce the solar flux shape reaching the receiver panels’ surface (Cardoso et al., 2017). From this accurate simulation for each solar position, a lookup table, from now on called Solar Field Efficiency Matrix is obtained, providing a powerful solution with minimal computing time for the whole plant simulation. This efficiency matrix is an essential input for the receiver
4. Methodology and modeling approach Equivalent power plant designs in terms of capacity factor are the departure point from where to start the performance evaluation of each power plant configuration. Also, it is crucial to properly interconnect technical and financial approaches for a robust comparison. Therefore, annual net electricity production is the key figure of merit which will be connected with the financial model. The techno-economic modelling approach implemented consists of detailed full-transient performance and economic models of CSP plants based on a simulation library (developed by CENER) that has turned out to be an extremely valuable tool and has been validated against commercial power plant performance (Hernández et al., 2015; García-Barberena et al., 2017). With concise EYA results and system cost analysis, a comprehensive sensitivity analysis was developed in order to cover uncertainties. The final benchmarking analysis will be done taking Levelized Cost of Electricity Table 2 Physical properties and price of slag pebbles, taconite and quartzite rock. Material
Density (kg/m3)
Specific heat capacity (J/kgK)
Heat capacity per volume (kJ/m3K)
Price (€/t)
Taconite Chang et al. (2015) Quartzite Xu et al. (2012) Slag pebble
3200 2500 2850
800 830 970
2560 2075 2765
178.73 10.01 100
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Fig. 4. Simulation Blocks.
one-year transient simulation, representing the operation results of the binomial heliostat field – receiver over time. The tube-based cylindrical external receiver is modeled assuming one representative tube per panel since all the tubes in a panel behave quite similar to their neighbors. Each receiver tube is discretized along its longitudinal and thickness dimensions to secure an accurate representation of the temperatures and solar fluxes along the tube in the external and internal surfaces. For each node, the transient energy and mass balances in temperature-dependent molten salts and steel tube
thermal simulation, representing the thermal input into the receiver as a function of the sun position and DNI. 4.1.3. Receiver modelling approach On the thermal model side, the simulation of the receiver is based on an object-oriented mixed differential algebraic and discrete equation-based model developed in Modelica language and simulated with the commercial software Dymola. The thermal model provides the dynamic thermal behavior and operation of the receiver along with
Fig. 5. IPSEpro Scheme: detailed configuration of the components of the power generation block that comprise the steam generation system (SGS), the water and steam cycle (WSC) and the electric power generation system (EPGS) components. 649
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models are solved. Each node receives the solar flux corresponding to its position in the receiver obtained from the optical receiver flux map. A one-dimensional radial heat transfer model is used to solve the radiation, convection and conduction mechanisms. The description, references and validation of this model can be found in (García-Barberena et al., 2017). From this accurate simulation, a 3D lookup table, from now on called Receiver Efficiency Matrix is obtained. This receiver efficiency matrix depends on wind velocity, ambient temperature and concentrated solar flux received from the solar field.
optimized design has been modeled using GateCycle and IPSEpro software (Fig. 5) to finally generate a group of 3D lookup table to provide computational efficiency. These lookup tables contain the power block gross power and its parasitic consumptions, as a function of mass flow, ambient temperature and inlet temperature of molten salt. 4.1.6. Ambient model The ambient model is connected to the Heliostat Solar Field, Receiver, TES system and the power block model providing the meteorological conditions to be simulated. Basically, it is a Typical Meteorological Year (TMY) which comprises the time series of the most relevant meteorological variables for a complete year. Specifically, the TMY contains the time series for the Direct Normal Irradiation (DNI), ambient temperature, atmospheric pressure, wind velocity and direction, and relative humidity. For this research, these time series are considered to be representative of the long term most probable meteorological conditions of De Aar location. The annual DNI of the TMY is 2712 kWh/m2 (Hirsch, 2017). It must be noticed that the location has been selected in order to reach a competitive energy yield assessment taking into account its high annual direct beam radiation. It is not the objective of this research to singularize the solution to a specific location or energy market boundary conditions, just to give a competitive comparison between technologies to final users and the industry itself.
4.1.4. Thermal energy storage modelling approach The TES model structure depends on the solar power plant configuration. For molten salt two-tank TES modeling, a fully transient storage tank validated model (Zaversky et al., 2013) has been used. Basically, this model divides the storage tank model into model subunits, categorized by the three basic modes of heat transfer, conduction, convection and radiation. On the other hand, the dynamic modeling of the thermocline storage system combines the previously presented model for the pumping tanks and a successive energy balance of thermocline storage tank slices (considering in a slice the solid and the fluid) with an infinitesimal height (dh ) stated in an explicit finite difference method. The thermocline model employed has been also successfully validated against experimental and theoretical data and it is explained and referenced in detail in (Hernández Arriaga et al., 2015). The solid particles of a slice of the packed-bed are assumed to have uniform temperature and the heat transfer by radiation is neglected. Therefore, it is necessary to consider the temperature distribution along the thermocline tank at each time step of the simulation process. The molten salts outlet temperature at the end of each step of the discharge process impacts directly on the power block production. This effect is evaluated accurately in order to correctly compare the hybrid TES storage system with the conventional two-tank molten salts TES. The pressure drop in the packed-bed can be estimated using the Ergun equation (Ergun, 1952). The result shows that the power consumed associated with this pressure drop is negligible if it is compared against the total parasitic consumption of the molten salts pumping in the CSP plant. In this way, the complexity of the thermocline model is reduced and thus, the computing time of the whole plant model.
4.1.7. Control modelling approach The annual performance model is completed with a detailed control and operation strategy module, able to represent the routine operation of the plant. The operation strategy implemented in the model is based on the simulation of different operation modes typically used in Solar Towers (Abraham and De Meyer, 2017) with some minor simplifications to make the model computationally efficient. The operating profile assumed aims at maximizing the power generation through maximizing the energy from the solar field, minimizing energy dumping associated with a full state of charge in the storage system and so, minimizing the impact of lower power block efficiency associated with part loads. The demand profile is not considered in order to not singularize the solution to a specific market, thus offering an equitable comparison between the proposed TES technologies. The operation strategy implemented switches among different operation modes such as preheating, filling, start-up, normal operation, cloud passing, shutdown and overnight hold mode. Within each of the operation modes, different operating conditions are used to represent the realistic operation of the plant. The switching among these modes depends on the current operating conditions during the transient simulation such as heliostat wind protection; receiver solar flux excess or
4.1.5. Power block modelling approach The power block model is based on a performance interpolation matrix representing the performance of the steam generation system (SGS), water and steam cycle (WSC) and electric power generation system (EPGS) with a design turbine gross output of 110 MWe. An
Fig. 6. Temperature distribution across the packed-bed at the end of the discharge and charge processes. 650
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molten salts overheating taking actions through solar field defocusing or mass flow controller among others. The solar fluxes onto the receiver during the receiver heating up and cool down are considered to be uniform (constant irradiance) and a smooth transition from this solar flux to the complete field flux map is used for partial focusing. When the solar resource is available during long periods, the normal operation mode is set and the hot molten salts coming from the receiver flows through the TES system and the power block, charging each thermocline tank from top to bottom with the same molten salt mass flow. When there is a long-term solar absence (overnight hold or cloud passing mode), each thermocline tank is discharged with half of the molten salt mass flow required by the power block. If the thermocline tanks are charged, the operation must try to avoid a long period without using them in order to avoid loss of stratification, which leads to a loss of stored energy. The thermocline tanks charge-discharge process is controlled by the temperature of the molten salts. The charge process ends when the temperature in the bottom of the tank increases 30 °C and immediately the discharge process starts. On the other hand, the discharge process ends when the outlet temperature at the top of the tank drops 30 °C and the cycle starts again. Fig. 6 shows the temperature along the packedbed axial axis at the end of the charge-discharge cycling process. The xaxis of the chart starts at 0, at the top of the tank. It can be observed that the difference of the temperature distribution at the end of the discharge or charge process between a cycle and the previous one decrease while the number of cycle increases until the charge-discharge temperature and time convergence. Once the discharging convergence time is observed, the maximum useful energy stored in the thermocline tanks is reached. The use of pumping tanks as shock absorbers of the partial cycling of the thermocline stabilizes the maximum energy stored.
•
4.2. Benchmarking assessment Levelized Cost of Electricity (LCOE) is the final figure of merit used in the benchmarking analysis to compare on average costs basis the two solar thermal power plants proposed. LCOE determine the energy generation costs, from building to operating a generation plant, ergo represent how much money must be made per unit of electricity to recover the lifetime costs of the system. Thus, the LCOE takes into account the following inputs: plant capital costs, financing costs, incomes (yearly net electricity production), expenses (fix and variable operation and maintenance costs, insurance costs, etc.) and taxes (rate, sales taxes, Tax Credit Expiration Period, etc.). In addition, the lower ratio between M&O and investment costs for CSP projects ensures minimum lifetime costs fluctuations that could affect the stability of the LCOE output. This means that the capital cost of generation capacity governs the calculation, unless currency or financial instabilities of less regulated markets occurs, turning the LCOE parameter in a trustful figure of merit to compare both power plants configurations. Even more, if we take into account the exhaustive previous design work made on reaching an equivalent capacity factor for each plant under study, a capacity factor that must be almost equal in order to equally compare two renewable energy power plants. For financial or tax credits it is needed to use the same assumptions in both power plant calculations in order to compare them under the same conditions. Therefore, the financial model parameters are shown in Table 3:
4.1.8. Simulation level of detail The simulations’ level of detail has been adjusted by the discretization of the receiver tubes, the complexity of the materials’ properties (temperature dependence), temporal resolution and TES system discretization among others. For the current modeling approach, the level of detail is as follows:
4.2.1. Benchmarking sensitivity analysis Economic assumptions uncertainties must be considered while performing an LCOE calculation over the whole lifecycle of a plant taking into account the plausible scenarios and their effects on the most sensitive parameters of the calculation. For this reason, a sensitivity analysis has been done taking into account the variation of referenced operation and maintenance costs from a real two-tank molten salts power plant (Turchi, 2010) for the hybrid TES based plant case. In addition, the variation of the storage material cost per ton for the steel slag pebbles and molten salts (Parrado et al., 2016) was also taken into account as it summarizes Table 4. The steelmaking slag cost per ton is the one derived from the commercial product developed within RESLAG project which comprises from manufacturing to transportation. The price variation of the slag
• Time step: To ensure proper consideration of the transient effects • •
axial direction are used, no internal convection resistance, no conduction resistance on the pipe and only two nodes in the conduction resistance of the insulation layer. Material properties: temperature-dependent properties for molten salts, receiver pipes material and slag pebbles (see Section 3.2). Constant properties are used for the absorber coating and insulation material.
(durations in the order of 15–30 min), a time step of 5 min has been used for the complete year simulation. Discretization of receiver tubes: Each tube is longitudinally discretized in 9 longitudinal nodes and in 3 nodes along its thickness achieving grid independence. Transport system pipes models: The rest of the pipes of the transport system, including the riser and downcomer, the cold supplies and hot discharges are simulated with one single node in the longitudinal direction. Simplified heat transfer models in the Table 3 Financial model parameters. General Incomes Investment Debt
Equity Expenses
Taxes
Useful life [years] Yearly inflation rate [%] (OECD, 2019) Yearly net electricity production [kWh/year] Plant capital cost [€] Debt [% total investment] Debt repayment period [years] Yearly debt interest [%] Grace period - Interest-only period [years] Equity [% total investment] Yearly rate of return on equity capital [%] O&M costs p.u. of net electricity [€/kWh] Insurance cost [%] Decommissioning Cost Tax rate [%] Yearly Depreciation Rate [%]
651
25 2.20 To be defined in the following sections To be defined in the following sections 100 15 8 3 0 12 To be defined in the following sections 0.5 0 35 6.67
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5.2. Systems cost results
Table 4 Sensitivity analysis. Annual O&M costs (€/kWh)
Best
Design
Worst
Hybrid TES CSP Plant Two-tank storage CSP Plant
0.02951 0.02951
0.02980 0.02951
0.03010 0.02951
Storage medium material costs (€/t) Slag pebbles Molten salts
Best 75 752
Design 100 940
Worst 200 1034
Table 6 summarizes the cost breakdown of the different cases under study according to references applied for each plant configuration (Bhargav et al., 2013; Kolb et al., 2007), (Kelly, 2010) and (Turchi and Heath, 2013). 5.2.1. Thermal energy storage system direct costs Special attention must be considered to the comparison of the direct costs of each TES system configuration. Taking into account (Kelly, 2010) and (Turchi and Heath, 2013), among CENER confidential information, each direct cost are shown in Fig. 7. The hybrid thermal energy storage system proposed is economically competitive against the state-of-the-art. The utilization of four tanks (two pumping tanks plus two thermocline tanks) is compensated by the cost reduction due to the smaller inventory of molten salts required by the hybrid configuration. Considering the dimension of the thermocline tanks, the void fraction and the slag pebbles density, the total amount of molten salts required is 14,910 tons in addition to 20,630 tons of slag pebbles. Therefore, the amount of molten salt is reduced by 31.4%, a difference of 6820 tons between the hybrid and the state-of-the-art solution that represent the high potential of cost reduction of this configuration.
Table 5 EYA comparison. Parameter
State-of-theart
Hybrid TES CSP Plant
Units
Available Solar Energy Used Solar Energy Dumped Solar Energy Receiver Incident Solar Energy Average Optical Efficiency Receiver Thermal Energy Output Average Receiver Efficiency Thermal Energy from Storage Gross PB Energy Production from Receiver Gross PB Energy Production from Storage Gross PB Energy Production Parasitic Energy consumption Net Electricity Production Average Thermal-to-Gross Electrical PB Efficiency Average Solar-to-Net Electrical Efficiency Capacity Factor
2363.76 2282.25 81.51 1300.46 56.98 1143.34 87.92 459.36 246.85
2363.76 2282.25 81.51 1300.46 56.98 1143.34 87.92 456.57 246.85
GWh GWh GWh GWh % GWh % GWh GWh
165.77
164.57
GWh
412.62 80.69 331.93 36.09
411.42 80.69 330.73 35.98
GWh GWh GWh %
14.54
14.49
%
6. Conclusions
37.89
37.75
%
An exhaustive effort has been made in order to achieve a wellfounded comparison between CSP power plants with equivalent thermal energy storage system design in terms of capacity factor and performance, to achieve robust results focusing on cost analysis. From this, the following conclusions can be obtained.
5.3. Benchmarking results Finally, the costs analysis results in terms of LCOE comparing both technologies are shown in Table 7 and charted in Fig. 8:
material is motivated due to conservative and risk assumptions over an innovative thermal energy storage material.
• The
5. Techno-economic and benchmarking analysis results 5.1. Energy yield assessment
•
A summary of the annual results obtained from the Energy Yield Assessment of both power plants is provided in Table 5. The hybrid thermocline based TES plant (Hybrid TES) results are quite similar to the State-of-the-Art plant (two-tank storage) due to the thermocline hybridization with pumping tanks which allows the same capacity factor and operational stability for both plants.
•
hybrid TES system achieves the same Levelized Cost of Electricity as the state-of-the-art two-tank technology of central receiver systems due to the competitive price of steelmaking slag pebbles versus molten salts. The hybrid TES energy production is close to the one produced by a plant with conventional storage due to hybrid TES configuration and operational strategy. The cut-off temperature in thermocline charge/discharge cycling maximizes the cycle efficiency. The addition of pumping tanks prevents the partial charge-discharge process of the thermocline, which is directly related to thermocline charge-discharge times, improving its transient operational
Table 6 Estimated cost ([M€]) for each plant components. Plant part
State-of-the-art CSP Plant [M€] Best case
Site Improvement Heliostat Field Tower Receiver TESS WSC + EPGS SGS Direct Costs Contingencies Total Direct Costs Indirect Costs Total costs
10.56 126.91 31.00 71.02 43.38 150.05 40.84 473.76 33.16 506.93 102.92 609.84
Hybrid TES CSP Plant[M€]
Design case
Worst case
Best case
Design case
Worst case
47.30
49.27
42.21
44.62
47.63
477.69 33.44 511.13 103.77 614.90
479.66 33.58 513.23 104.20 617.43
472.59 33.08 505.67 102.67 608.34
475.01 33.25 508.26 103.19 611.45
478.02 33.46 511.48 103.84 615.33
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Fig. 7. TES direct costs benchmarking analysis results: two-tank molten salts TES system (2 Tanks) versus hybrid TES (2TT + 2PT). Table 7 LCOE benchmarking results. LCOE [€/kWh]
Best
Design
Worst
Hybrid TES Two-tank storage
0.16701 0.16685
0.16799 0.16803
0.16914 0.16862
validation of Tonatiuh at Mini-Pegase CNRS- PROMES facility. Renew. Energy 1–8. Brosseau, D., Edgar, M., Kelton, J.W., Chisman, K., Ray, D., Emms, B., 2004. Testing of thermocline filler materials and molten-salt heat transfer fluids for thermal energy storage systems in parabolic trough power plants. In: Solar Energy. ASME, pp. 587–595. BSI, 2005. EN14015-2004 Specification for the design and manufacture of site built, vertical, cylindrical, flat-bottomed, above ground, welded, steel tanks for the storage of liquids at ambient temperature and above, British Standards Institution. Cardoso, J.P., Mutuberria, A., Marakkos, C., Schoettl, P., Osório, T., Les, I., 2017. New functionalities for the tonatiuh ray-tracing software. In: SolarPACES Concentrating Solar Power and Chemical Energy Technologies, pp. 150002. Chang, Z.S., Li, X., Xu, C., Chang, C., Wang, Z.F., 2015. The design and numerical study of a 2MWh molten salt thermocline tank. Energy Procedia 779–789. Dunn, R.I., Hearps, P.J., Wright, M.N., 2012. Molten-salt power towers: newly commercial concentrating solar storage. Proc. IEEE 100, 504–515. https://doi.org/10.1109/ JPROC.2011.2163739. Ergun, S., 1952. Fluid flow through packed columns. Chem. Eng. Prog. 48, 89–94. European Union, 2017. The Strategic Energy Technology (SET) Plan, Joint Research Centre. https://doi.org/10.2777/48982. Euroslag, 2010. Euroslag - Statistic Data (Blast furnace and steel slags) 2010 [WWW Document]. Stat. 2010. https://doi.org/10.1016/j.vaccine.2007.01.091. Ferri, R., Cammi, A., Mazzei, D., 2008. Molten salt mixture properties in RELAP5 code for thermodynamic solar applications. Int. J. Therm. Sci. 47, 1676–1687. https://doi. org/10.1016/j.ijthermalsci.2008.01.007. García-Barberena, J., Mutuberria, A., Palacin, L.G., Sanz, J.L., Pereira, D., Bernardos, A., Sanchez, M., Rocha, A.R., 2017. Coupled optical and thermal detailed simulations for the accurate evaluation and performance improvement of molten salts solar towers, in: AIP Conference Proceedings. https://doi.org/10.1063/1.4984545. García-Barberena, J., Mutuberria Larrayoz, A., Sánchez, M., Bernardos, A., 2016. State-ofthe-art of heliostat field layout algorithms and their comparison. Energy Procedia 93, 31–38. https://doi.org/10.1016/j.egypro.2016.07.146. Gil, A., Medrano, M., Martorell, I., Lázaro, A., Dolado, P., Zalba, B., Cabeza, L.F., 2010. State of the art on high-temperature thermal energy storage for power generation. Part 1—Concepts, materials and modellization. Renew. Sustain. Energy Rev. 14, 31–55. https://doi.org/10.1016/j.rser.2009.07.035. Hernández Arriaga, I., Zaversky, F., Astrain, D., 2015. Object-oriented modeling of molten-salt-based thermocline thermal energy storage for the transient performance simulation of solar thermal power plants. Energy Procedia 879–890. https://doi.org/ 10.1016/j.egypro.2015.03.116. Hirsch, T., 2017. SolarPACES guideline for bankable STE yield assessment, IEASolarPACES. International Energy Agency, I., 2018. Market Report Series: Renewables 2018 English ES. IRENA, 2018a. Global Energy Transformation. A roadmap to 2050, Microscopy Research and Technique. IRENA, 2018b. Renewable Power Generation Costs in 2017. Ju, X., Xu, C., Wei, G., Du, X., Yang, Y., 2016. A novel hybrid storage system integrating a packed-bed thermocline tank and a two-tank storage system for concentrating solar power (CSP) plants. Appl. Therm. Eng. 92, 24–31. https://doi.org/10.1016/j. applthermaleng.2015.09.083. Kelly, B., 2010. Advanced Thermal Storage for Central Receivers with Supercritical Coolants. Lakewood. Kolb, G.J., Jones, S.A., Donnelly, M.W., Gorman, D., Thomas, R., Davenport, R., Lumia, R., 2007. SANDIA REPORT Heliostat Cost Reduction Study. Libby, C., 2010. Solar Thermocline Storage Systems. OECD, 2019. OECD Inflation [WWW Document]. https://doi.org/10.1787/eee82e6e-en. Ortega-Fernández, I., Grosu, Y., Ocio, A., Arias, P.L., Rodríguez-Aseguinolaza, J., Faik, A., 2018a. New insights into the corrosion mechanism between molten nitrate salts and ceramic materials for packed bed thermocline systems: a case study for steel slag and Solar salt. Sol. Energy 173, 152–159. https://doi.org/10.1016/j.solener.2018.07. 040. Ortega‐Fernández, I., Uriz, I., Ortuondo, A., Hernández, A.B., Faik, A., Loroño, I., Rodríguez‐Aseguinolaza J., 2018b. Operation strategies guideline for packed-bed thermal energy storage systems. Int. J. Energy Res. https://doi.org/10.1002/er.4291.
Fig. 8. LCOE benchmarking analysis results for Hybrid TES and two-tank molten salts TES CSP plants taking into account storage material and O&M costs scenarios.
effectiveness and affording a stable response in terms of full power block loads and so, energy production. Finally, it is relevant to notice that critical issues over operational stability while using slag pebbles were addressed within this research. Nevertheless, further experimental upscale studies are needed (thermal ratcheting, filler loading/unloading process, etc.) to present the hybrid storage solution as a reliable key competitive value prepared for the needed cost reduction path in the concentrated solar power sector. Acknowledgments This work was supported by the European Union’s Horizon 2020 research and innovation programme under Grant Agreement No. 642067. References Abraham, O., De Meyer, J., 2017. Optimisation in Plant Operations for a 100 MW Central Receiver CSP Plant with Focus on the Plant Operating Strategies. API, 2005. 650 Welded steel tanks for oil storage, American Petroleum Institute. Office of the Federal Register, Washington D.C. ASME, 2010. Boiler & Pressure Vessel Code, Section II Part D, Properties, The American Society of Mechanical Engineers. New York. Bhargav, K.R., Gross, F., Schramek, P., 2013. Life cycle cost optimized heliostat size for power towers. Energy Procedia 40–49. https://doi.org/10.1016/j.egypro.2014.03. 005. Blanco, M., Mutuberria, A., Monreal, A., Albert, R., 2011. Results of the empirical
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Available at: https://solarpaces.nrel.gov/ [accessed 30 January 2019]. Turchi, C., 2010. Parabolic Trough Reference Plant for Cost Modeling with the Solar Advisor Model (SAM). Turchi, C.S., Heath, G.A., 2013. Molten salt power tower cost model for the system advisor model (SAM). Energy. https://doi.org/10.2172/1067902. Xu, C., Wang, Z., He, Y., Li, X., Bai, F., 2012. Sensitivity analysis of the numerical study on the thermal performance of a packed-bed molten salt thermocline thermal storage system. Appl. Energy 92, 65–75. https://doi.org/10.1016/j.apenergy.2011.11.002. Zaversky, F., García-Barberena, J., Sánchez, M., Astrain, D., 2013. Transient molten salt two-tank thermal storage modeling for CSP performance simulations. Sol. Energy 93, 294–311. https://doi.org/10.1016/j.solener.2013.02.034.
Pacheco, J.E., Showalter, S.K., Kolb, W.J., 2002. Development of a Molten-Salt thermocline thermal storage system for parabolic trough plants. J. Sol. Energy Eng. 124, 153–159. https://doi.org/10.1115/1.1464123. Parrado, C., Marzo, A., Fuentealba, E., Fernández, A.G., 2016. 2050 LCOE improvement using new molten salts for thermal energy storage in CSP plants. Renew. Sustain. Energy Rev. 57, 505–514. https://doi.org/10.1016/j.rser.2015.12.148. Relloso, S., Delgado, E., 2009. Experience with molten salt thermal storage in a commercial parabolic trough plant; Andasol 1 commissioning and operation. 15th SolarPaces, Berlin, Germany. SimTech, 2013. IPSEpro Process Simulator Program Modules and Model Libraries [WWW Document]. Solarpaces -NREL Project Database, 2019. NREL/SolarPACES Project Database. [ONLINE]
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Benchmarking analysis of a novel thermocline hybrid thermal energy storage system using steelmaking slag pebbles as packed-bed filler material for central receiver applications Javier López Sanz1, Francisco Cabello Nuñez1, Fritz Zaversky
T
⁎,1
National Renewable Energy Center of Spain (CENER), Solar Thermal Energy Department, Spain
A R T I C LE I N FO
A B S T R A C T
Keywords: Concentrated solar power Thermal storage Thermocline Slag pebble By-product waste LCOE
The aim of this research is to provide the techno-economic and benchmarking analysis of a 100 MWe class molten salt central receiver system using a thermocline hybrid thermal energy storage (TES) system versus conventional two-tank storage. The techno-economic analysis used has been carried out by means of specifically developed transient simulation models to find out a robust comparison between technologies. Thermal energy from storage, annual net electricity production and Levelized Cost of Electricity are the concise and comprehensive figures of merit of this study. An exhaustive effort has been made in order to achieve a well-founded comparison between concentrated solar power plants with equivalent storage system designs in terms of capacity factor and performance. Finally, this research demonstrates that the thermocline hybrid TES system provides a reliable cost effective storage solution that achieves the same Levelized Cost of Electricity as state-of-the-art technology. This is achieved due to the competitive price of the steelmaking slag pebbles versus molten salts, a revalorized processed steel by-product waste from the metallurgical industry used as packed-bed solid filler material for thermocline tanks. The results of this research open an attractive potential for cost reduction in the concentrated solar power sector, a key competitive value for a new market opportunity.
1. Introduction There has been a great debate about the commercial upscale performance of thermocline thermal energy storage (TES) for central receiver systems. In this regard, the potential to provide cost effective solutions to improve dispatchability is compromised due to thermocline tank stability overcharge and discharge cycling. Consequences over global system efficiency and baseload power requirements under relevant operational environment are the key factors of the successful deployment of these systems. By this means, a direct hybrid thermocline-based TES system implemented in a 100 MWe molten salt central receiver system is techno-economically analyzed in its life-cycle to study its reliable decoupling of solar energy harvest and subsequently, electricity production, with the aim of achieving competitive energy costs against conventional state-of-the-art technology. As the European Strategic Energy Technology (SET) Plan promoted
(European Union, 2017), the Concentrated Solar Power technology roadmap must undertake improvements over advanced and reliable TES concepts to reduce costs and improve dispatchability as well as the environmental footprint of this energetic resource (See Fig. 1). The approach proposed in this research enhance the competitiveness of the molten salt central receiver system technology by contributing with a competitive and standardized up-scaled TES design based on hybrid storage integrating two thermocline tanks and two-tank storage. In addition, this solution proposes an added value to the storage concept by integrating and valorizing the steel by-product waste from the metallurgical industry as packed-bed solid filler material of the thermocline, facing the high environmental impact that represents the landfill of this steelmaking slag. In this way, high temperature and low-cost thermal storage material are provided leading to circular economy processes. The thermal storage concept currently applied in commercial CSP
Abbreviations: CSP, Concentrated Solar Power; DNI, Direct Normal Irradiation; EPGS, Electric Power Generation System; EYA, Energy Yield Assessment; HTF, Heat Transfer Fluid; LCOE, Levelized Cost of Electricity; O&M, Operation and Maintenance; PB, Power Block; SET, Strategic Energy Technology; SGS, Steam Generation System; TES, Thermal Energy Storage; WSC, Water Steam Cycle ⁎ Corresponding author. E-mail addresses: [email protected] (J. López Sanz), [email protected] (F. Cabello Nuñez), [email protected] (F. Zaversky). 1 Address: C/Ciudad de la Innovación, 7. Sarriguren, Navarra, Spain. https://doi.org/10.1016/j.solener.2019.06.028 Received 25 February 2019; Received in revised form 5 June 2019; Accepted 10 June 2019 Available online 26 June 2019 0038-092X/ © 2019 The Authors. Published by Elsevier Ltd on behalf of International Solar Energy Society. This is an open access article under the CC BY license (http://creativecommons.org/licenses/BY/4.0/).
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must be noticed. The amount of available thermal energy in packed-bed thermocline TES systems is represented by the temperature of the slag packed-bed and the heat transfer fluid (HTF) inside the TES system. Thus, thermal energy is stored in the form of sensible heat while the mass of packed-bed and HTF is kept constant. Consequently, when the energy received from the binomial heliostat field - receiver is greater
Nomenclature
Cp ρ k
specific heat capacity [J/kgK] density [kg/m3] thermal conductivity [J/mK]
Fig. 1. Concentrated Solar Power Thermal Electricity SET Plan 2017.
than the energy demanded by the power block, the temperature of the packed-bed TES increases until a limited maximum outlet fluid temperature, typically referred to as cut-off temperature. On the contrary, when the captured energy is lower than the energy demanded by the power block, the system consumes the energy stored in the packed-bed TES to produce electricity and consequently, the temperature of the packed-bed decrease until the cut-off temperature. Finally, buoyancy effects create thermal stratification expansion limiting the storage capacity of the tank. In conclusion, the overall performance efficiency of a packed-bed TES system is slightly lower than the two-tank molten salts TES system (Ortega-Fernández et al., 2018b). Several authors have studied and compared single thermocline storage systems performance against the conventional two-tank storage showing potential cost reductions of 20–37% for the TES System (TESS) (Libby, 2010). However, the thermocline tank has operational stability disadvantages and during discharging entails partial power block loading with its consequences in terms of energy production and energy storage efficiency. Even more for commercial storage designs for 100MWe class CSP plants that must provide high energy storage capacity, which means increasing design tank volumes or even combining multiple tanks; taking into consideration also design limitations due to seismic, structural and mechanical codes and standards for the design of
projects is the sensible heat thermal storage based on two-tanks, hot and cold, using molten salts (Ferri et al., 2008) as storage medium. The useful sensible heat is defined by the difference between two temperature levels, for central receiver systems the “cold temperature” is about 290 °C and the “hot temperature” 565 °C, in order to avoid problems like freezing, degradation or other operational issues (Ferri et al., 2008). If the fluid used as a storage medium is the same fluid that flows through the receiver, the plant configuration is called direct while the fluid used in the receiver is different from the storage medium, the plant configuration is called indirect. Nowadays at commercial scale, the indirect two-tank storage system is used in several parabolic trough collector plants (Gil et al., 2010; Relloso and Delgado, 2009), whereas the direct two-tank thermal is used in central receiver plants (Gil et al., 2010; Dunn et al., 2012). However, the two-tank storage system has a high price and its cost breakdown indicates that the molten salt inventory has the highest share of the total cost (Kelly, 2010). For this reason, the combination of the two-tank storage in a single tank may be a potential value for cost reduction. Also, filling this single tank with a cost effective solid material could replace a big amount of the molten salt inventory, achieving a cost-effective concept of thermocline tank. At this point, a brief technical reminder about thermocline tanks
Fig. 2. Methodology flow chart. 645
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role in the world’s future electricity supply. CSP is a proven technology with big growth potential in the following years, specifically, it is expected to grow 87% (4.3GW) until 2023, 32% more than over the same period between 2012 and 2017. Under an accelerated case, CSP growth could be even 60% higher (International Energy Agency, 2018). By 2050, with appropriate support, CSP could provide 11.3% of global electricity (IRENA, 2018a). Nevertheless, operational reliability, restricted access to financing and market designs that do not value TES systems continue to challenge the implementation of this technology. Among the four CSP technologies commercially available (parabolic trough, dish Stirling, linear Fresnel and central receiver), the central receiver system is the one with the highest cost reduction potential and the highest expected capacity installed worldwide (IRENA, 2018b; NREL/SolarPACES Project Database, 2019). For these reasons, the two CSP plant configurations under study will use a solar power tower cylindrical tube-wall external receiver, a field of two-axis tracking mirrors (heliostats) as collector system and will use molten salts as heat transfer fluid (HTF). Summing up the basic principles of central receiver systems operation, the solar energy absorbed by the receiver heats the HTF, which flows back to ground level and circulates to the TES system, where it can be pumped to the power generation block. In either case, electricity is produced by a dry-cooled power generation block. Therefore, the subsystem configuration of each plant under study is differentiated only by the type of thermal energy storage system used: hybrid thermocline based or conventional two-tanks. Furthermore, the European steel industry generated about 21.8 million tons of slag in 2010 resulting from steel making. About 76% of the produced steel slag was recycled in several applications such as aggregates for construction or road materials, but these sectors are unable to absorb the total amount of slag generated, so the remaining 24% were still landfilled (2.8 Mtons) or self-stored (2.2 Mtons) (Euroslag, 2010). From the environmental point of view, the landfill of slags emits heavy metals to environment which is a significant source of pollution of air, water and soil, and further adversely affects the human health, and the growth of plant and vegetation. Besides, it is currently a lost opportunity, as it represents a huge amount of available material with very interesting properties for potential recycling that is landfilled and therefore missed, adding the cost of landfilling for the steel sector. An adequate filler material for thermocline tanks should be inexpensive and widely available, have a high heat capacity per volume unit (this means a high specific heat capacity and a high density) and a low void fraction, be compatible with the fluid in the temperature range, and be non-hazardous (Brosseau et al., 2004). The produced steel slag brings together the main starting properties to meet most of
welded steel tanks (API, 2005; BSI, 2005). Therefore, higher storage capacities entail feasible, cost-effective and operational stable designs. Among others, the operational stability of thermocline tanks during short-term solar fluctuation compromises the continuous full load performance of the storage system and so, the capacity factor of the plant. Therefore, Ju et al. (2016) studied the hybridization of thermocline tanks with the conventional two-tanks concept modeling its behavior with positive conclusions. This research demonstrates that, for no solar fluctuations, the hybrid storage system can work with a high utilization ratio of the storage capacity. During short-term fluctuations, the plant can continuously generate steady power without discharge of the thermocline tank due to the buffering effect of the small hot pumping tank. While during long-term fluctuations, the combination of the packed-bed thermocline tank and the hot tank can still maximize a continuous power generation. Single thermocline thermal energy storage is more sensible against variable operating conditions and has a big impact on the power block operation conditions due to the effect of the thermocline cut-off temperature. Nevertheless, detailed transient annual behavior of each subsystem of the plant interconnected must be addressed taking into account thermal inertia, plant operational strategies or inlet fluid temperature dependence of the power block. As a result, real energy production and annual performance of a CSP plant will be achieved with which to cover conclusive financial studies which could give final conclusions to the industry. In view of the foregoing, this research addresses the following question, could the hybrid TES system overcome the performance difference against conventional state-of-the-art technology by base material cost reduction while uncertainties about its stable operation are solved?. In order to answer it, this research follows the methodology chart shown in Fig. 2. First of all, the general context about CSP and the metallurgical sector is proposed to properly orientate the reader to the potential of this research. Secondly, the hybrid TES system is explained and the benchmark study about the innovative packet-bed filler material is presented. Next, the methodology and modeling approach for each detailed plant subsystem and its interconnections are presented to finally evaluate the energy yield assessment and the benchmarking study. Then, under the concise context and techno-economic background, the results of the simulations are presented and analyzed. Final observations are given in the conclusions orientated to final users and the industry itself.
2. Context Concentrated solar power (CSP) is expected to play an important
Fig. 3. Hybrid thermal energy storage system configuration. 646
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guarantee 9 h of storage and 110 MWe of gross electrical output. However, this solution presents several disadvantages, among others, for each thermocline tank, is necessary to guarantee two minimum submergence level above the pump’s suction bell (one in the top of the tank due to the hot pump and other at the bottom where the cold pump takes suction) which involve a large useless dead volume of molten salts and so, high direct system costs. Therefore, to propose to the market a cost competitive solution to store 9 full load hours (110 MWe gross power) a thermocline hybrid TES system was selected. This system comprises two thermocline tanks using steelmaking slag pebbles as packed-bed solid filler material connected to two molten salt tanks acting as “pumping tanks”, where all molten salts pumps would take suction (see Fig. 3). In this way, the minimum molten salt level is only necessary for the pumping tanks and the thermocline tank’s height is then not limited by the maximum length of the pump’s shaft, reducing direct costs without affecting operational stability. As a summary, the main parameters of the thermocline tank are shown in Table 1.
Table 1 Parameters of the thermocline tank storage. Parameter
Value
Thermocline tank diameter Thermocline tank height Maximum increase of outlet temperature in the charge Maximum drop of outlet temperature in the discharge Void fraction Particle size
20 m 20 m 30 °C 30 °C 0.4 1.5 cm
these criteria. In short, the slag pebble not only suits the necessary characteristic to be an interesting solid filler material for thermocline tanks for CSP storage systems, but it also has a positive environmental impact recycling the steelmaking slag. Therefore, the potential use of the slag produced in Europe represents a new source to be partially absorbed by CSP. The possibility of integrating high temperature and low-cost storage materials represents a competitiveness opportunity needed to analyze for CSP sector and depends on the availability in the market of cost-efficient storage solutions.
3.2. Packed-bed filler material: sintered slag pebbles The steel slag used in this research conforms to most the basic criteria for solid filler materials being a promising cost reduction path for CSP applications. Nevertheless, the CSP sector must undertake improvements in reliable sub-systems; therefore, special attention must be given when proposing new advanced TES designs and materials. It is envisaged that subject to chemical composition, the raw steel slag mechanical and chemical performance could have an impact on the operational stability and safety of the storage system. Therefore, the chemical compatibility between the steel slag and the molten salts as well as the mechanical performance under cyclic conditions must be taken into account. In order to ensure reliability, initial chemical studies were conducted in the H2020 RESLAG project between molten salts and the raw metallurgical by-product waste revealing modifications on the thermophysical properties of the salts (Ortega-Fernández et al., 2018a). In consequence, a production process was designed where the gross steel slag is treated by a fine milling, compression and sintered process improving the mechanical resistance to thermal cycling of the final slag pebbles and its chemical stability up to 1000 °C under static test conditions, cycling the slag pebbles in an air atmosphere. It has also been observed that this production process also avoids entrainment of fine solid steel particles, breakdown or any change over the composition of the molten salts under static test conditions where the slag pebbles with the molten salts were cycled between 290 and 550 °C. Therefore, the chemical stability of the slag under cycling conditions is guaranteed, ensuring the operational stability and safety of the storage system. The media properties of the sintered slag pebbles have been characterized within the H2020 RESLAG project as it is described below, where temperature is in degrees Celsius.
3. The hybrid thermal energy storage system The hybrid thermocline based TES system design has been optimized to reach competitive costs against the conventional two-tank molten salts TES for a central receiver plant with 110 MW of design turbine gross electrical output and 9 full load hours of storage. The final configuration of the hybrid system integrates two “pumping tanks” and two thermocline tanks that store nine full load hours to power an electrical gross output of 110 MWe. By this means, the optimized costeffective solution selected combines two tank concepts: conventional molten salts tanks and thermocline tanks filled with steel making slag pebbles interconnected as is shown in Fig. 3. 3.1. Design of the hybrid TES system as a combination of thermocline tanks and molten salt tanks Due to the large amount of thermal energy necessary for a 110MWe CSP plant’s storage system, firstly, it is necessary to determine the maximum diameter and height of a single thermocline tank. For the conservative design of the thermocline, the conditions of a conventional hot molten salt tank have been considered, using ASTM A240 Grade 347H as structural material with a maximum stress allowable of 112 MPa at 575 °C (maximum operation temperature plus a safety offset of 10 °C) obtained from ASME Part II (ASME, 2010), following the codes and standards for the design of welded steel tanks (API, 2005; BSI, 2005) as well as seismic regulation. A finite element analysis (FEA) was performed in order to verify that the dimensions of the thermocline tanks are admissible and the stress is lower than the maximum allowable. The analysis concluded that the thermocline tank is structurally feasible with a maximum diameter of 20 m and the maximum height is also structurally limited up to 20 m. In this tanks the upper and bottom part is used as a distributor, for this reason, it is necessary 0.4 m of a top and bottom gap before the packed bed, thus, the packed bed high is 19.2 m which has a void fraction of 0.4. Secondly, it is necessary to define the discharging mass flow of the TES system at full load. For this purpose, the IPSEpro software (SimTech, 2013) was used to model a dry-cooled Rankine cycle with reheat of the steam before the medium-pressure turbine. The molten salt mass flow needed in the steam generation system (SGS) is around 670 kg/s while the charge mass flow of the TES system is 730 kg/s, defining a reasonable mass flow through the receiver at design conditions of around 1400 kg/s. Knowing the maximum feasible storage capacity per thermocline tank, at least 3 thermocline tanks would be necessary in order to
Cp = 827.69 + 0.3339·T k = 1.6792 − 5.3237·10−4 ·T The density of the slag pebbles is characterized as non-temperature dependent and with a fixed value of 2850 kg/m3 and the considered packed-bed void fraction has been set to 0.4 as indicated in Table 1. 3.3. Comparison of the sintered slag pebbles against other packed-bed filler materials The filler materials to be compared with the slag pebbles are the quartzite rock and the taconite. According to Pacheco et al. (2002), these materials are potential low-cost candidates as a solid storage medium in a packed-bed as well as compatible with molten salts in isothermal and cyclic conditions. One of the most interesting properties to pay attention to is the heat capacity per volume, which determines 647
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(LCOE) as the final figure of merit.
the thermal energy that can be stored for a specific tank size. The physical properties and price of these materials are shown in Table 2, which summarizes the comparison between these materials, highlighting the interest of the slag pebble material due to its higher ratio between heat capacity per volume of the slag and costs.
4.1. Energy yield assessment modeling approach 4.1.1. General modelling approach The modeling approach used for the EYA consists of effectively coupling detailed optical simulations of the heliostat solar field with also detailed and full-transient thermal simulations of the molten salts tube-based external receiver, hybrid thermocline based or conventional two-tank TES system and, finally, the power block, all of them controlled by a realistic Control System as seen in Fig. 4. Relevant aspects in the operation of the plant otherwise neglected, as transient TES system behavior and operational limitations, transient plant processes as preheating/shut-down-heating and secure cloud passing operating modes, and more, are taken into account increasing the quality of the EYA. Special attention has been taken into account to evaluate the thermodynamic performance of the hybrid TES concept on the power block operation and on the plant performance versus conventional technology. A compromise between high accuracy and limited computational costs has been deployed to cover this knowledge, providing a powerful solution with minimal computing time for the whole plant simulation. This has been possible by reducing other complex plant systems (solar field, receiver, power block) from initial accurate simulations to well-grounded efficiency matrices. Consequently, the annual performance model for each objective solar power plant is split into the following subsystems: heliostat solar field, central receiver, TES system, HTF system, control system and power block which covers steam generation system, water & steam cycle and electric power generation. Since the optical and thermal simulations are essentially different in the underlying physical principles and suitable methodologies, completely different approaches are used. In addition, every main plant system model has been interconnected in an accurate way. Finally, the study considers well-operated power plants without problems which force to stop the plant (molten salts freezing, deposition of slag particles in pumps or valves, etc.).
3.4. Hybrid thermal energy storage system benefits The main expected benefits of the hybrid TES system configuration are:
• The thermal storage capacity of the thermocline tanks is maximized: •
• •
•
the useful volume inside the thermocline tanks increases when placing the pumps outside the tanks since the minimum level of immersion of the pumps is not necessary, which leads to increase the storage capacity of these tanks. Higher efficiency, operational stability and lifetime: the pumping tanks operate as a thermocline buffer system under short-term solar fluctuations avoiding the partially and frequently charge-discharge process of the thermocline. This operation reduces local disturbances avoiding thermocline degradation and improves the performance of the storage system (Ju et al., 2016). In addition, the materials are subjected to lower mechanical stress increasing the lifetime of the components. The thermocline disturbance is reduced: it is caused by the absence of pumping disturbance effect on the molten salts flow inside the packed bed. The pumping system reduces direct and O&M costs: o Easiness of installation, maintenance and protection of the molten salts pumps in the pumping tanks against placing the pumps in the thermocline tank. o Thermocline tanks are higher than pumping tanks; therefore, shorter and cheaper pump shafts are needed, which helps to reduce the mechanical stress in the pumps’ shaft due to lower vibration, increasing the life cycle of the component. The hybrid design reduces direct and O&M costs: the inclusion of the pumping tanks mitigates the direct and O&M costs of a thermocline leakage failure. In the event of a tank leak, the molten salt can be transferred to another tank so the thermocline can be repaired. Thus, the drain tank dimensions and costs can be reduced.
4.1.2. Heliostat solar field modelling The heliostat solar field model or optical model is based on an accurate Monte Carlo ray-tracing method which solves the complete solar field performance. The heliostat field layout has been previously optimized as a complex multi-leveled problem (García-Barberena et al., 2016). Once the heliostat field, the tower, and the receiver are fully optimized and defined, the complete three-dimensional model is constructed using Tonatiuh, an open-source program for the detailed simulation of complex optical systems, which estimations have demonstrated highly accurate results in the simulation of heliostat systems (Blanco et al., 2011). The optical model is simulated for a large number of sun positions, covering all possible combinations of azimuth and elevation along a year, to fully describe the solar field optical performance. Each heliostat is placed in its geometric position in the solar field considering also the aiming point of each heliostat to accurately reproduce the solar flux shape reaching the receiver panels’ surface (Cardoso et al., 2017). From this accurate simulation for each solar position, a lookup table, from now on called Solar Field Efficiency Matrix is obtained, providing a powerful solution with minimal computing time for the whole plant simulation. This efficiency matrix is an essential input for the receiver
4. Methodology and modeling approach Equivalent power plant designs in terms of capacity factor are the departure point from where to start the performance evaluation of each power plant configuration. Also, it is crucial to properly interconnect technical and financial approaches for a robust comparison. Therefore, annual net electricity production is the key figure of merit which will be connected with the financial model. The techno-economic modelling approach implemented consists of detailed full-transient performance and economic models of CSP plants based on a simulation library (developed by CENER) that has turned out to be an extremely valuable tool and has been validated against commercial power plant performance (Hernández et al., 2015; García-Barberena et al., 2017). With concise EYA results and system cost analysis, a comprehensive sensitivity analysis was developed in order to cover uncertainties. The final benchmarking analysis will be done taking Levelized Cost of Electricity Table 2 Physical properties and price of slag pebbles, taconite and quartzite rock. Material
Density (kg/m3)
Specific heat capacity (J/kgK)
Heat capacity per volume (kJ/m3K)
Price (€/t)
Taconite Chang et al. (2015) Quartzite Xu et al. (2012) Slag pebble
3200 2500 2850
800 830 970
2560 2075 2765
178.73 10.01 100
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Fig. 4. Simulation Blocks.
one-year transient simulation, representing the operation results of the binomial heliostat field – receiver over time. The tube-based cylindrical external receiver is modeled assuming one representative tube per panel since all the tubes in a panel behave quite similar to their neighbors. Each receiver tube is discretized along its longitudinal and thickness dimensions to secure an accurate representation of the temperatures and solar fluxes along the tube in the external and internal surfaces. For each node, the transient energy and mass balances in temperature-dependent molten salts and steel tube
thermal simulation, representing the thermal input into the receiver as a function of the sun position and DNI. 4.1.3. Receiver modelling approach On the thermal model side, the simulation of the receiver is based on an object-oriented mixed differential algebraic and discrete equation-based model developed in Modelica language and simulated with the commercial software Dymola. The thermal model provides the dynamic thermal behavior and operation of the receiver along with
Fig. 5. IPSEpro Scheme: detailed configuration of the components of the power generation block that comprise the steam generation system (SGS), the water and steam cycle (WSC) and the electric power generation system (EPGS) components. 649
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models are solved. Each node receives the solar flux corresponding to its position in the receiver obtained from the optical receiver flux map. A one-dimensional radial heat transfer model is used to solve the radiation, convection and conduction mechanisms. The description, references and validation of this model can be found in (García-Barberena et al., 2017). From this accurate simulation, a 3D lookup table, from now on called Receiver Efficiency Matrix is obtained. This receiver efficiency matrix depends on wind velocity, ambient temperature and concentrated solar flux received from the solar field.
optimized design has been modeled using GateCycle and IPSEpro software (Fig. 5) to finally generate a group of 3D lookup table to provide computational efficiency. These lookup tables contain the power block gross power and its parasitic consumptions, as a function of mass flow, ambient temperature and inlet temperature of molten salt. 4.1.6. Ambient model The ambient model is connected to the Heliostat Solar Field, Receiver, TES system and the power block model providing the meteorological conditions to be simulated. Basically, it is a Typical Meteorological Year (TMY) which comprises the time series of the most relevant meteorological variables for a complete year. Specifically, the TMY contains the time series for the Direct Normal Irradiation (DNI), ambient temperature, atmospheric pressure, wind velocity and direction, and relative humidity. For this research, these time series are considered to be representative of the long term most probable meteorological conditions of De Aar location. The annual DNI of the TMY is 2712 kWh/m2 (Hirsch, 2017). It must be noticed that the location has been selected in order to reach a competitive energy yield assessment taking into account its high annual direct beam radiation. It is not the objective of this research to singularize the solution to a specific location or energy market boundary conditions, just to give a competitive comparison between technologies to final users and the industry itself.
4.1.4. Thermal energy storage modelling approach The TES model structure depends on the solar power plant configuration. For molten salt two-tank TES modeling, a fully transient storage tank validated model (Zaversky et al., 2013) has been used. Basically, this model divides the storage tank model into model subunits, categorized by the three basic modes of heat transfer, conduction, convection and radiation. On the other hand, the dynamic modeling of the thermocline storage system combines the previously presented model for the pumping tanks and a successive energy balance of thermocline storage tank slices (considering in a slice the solid and the fluid) with an infinitesimal height (dh ) stated in an explicit finite difference method. The thermocline model employed has been also successfully validated against experimental and theoretical data and it is explained and referenced in detail in (Hernández Arriaga et al., 2015). The solid particles of a slice of the packed-bed are assumed to have uniform temperature and the heat transfer by radiation is neglected. Therefore, it is necessary to consider the temperature distribution along the thermocline tank at each time step of the simulation process. The molten salts outlet temperature at the end of each step of the discharge process impacts directly on the power block production. This effect is evaluated accurately in order to correctly compare the hybrid TES storage system with the conventional two-tank molten salts TES. The pressure drop in the packed-bed can be estimated using the Ergun equation (Ergun, 1952). The result shows that the power consumed associated with this pressure drop is negligible if it is compared against the total parasitic consumption of the molten salts pumping in the CSP plant. In this way, the complexity of the thermocline model is reduced and thus, the computing time of the whole plant model.
4.1.7. Control modelling approach The annual performance model is completed with a detailed control and operation strategy module, able to represent the routine operation of the plant. The operation strategy implemented in the model is based on the simulation of different operation modes typically used in Solar Towers (Abraham and De Meyer, 2017) with some minor simplifications to make the model computationally efficient. The operating profile assumed aims at maximizing the power generation through maximizing the energy from the solar field, minimizing energy dumping associated with a full state of charge in the storage system and so, minimizing the impact of lower power block efficiency associated with part loads. The demand profile is not considered in order to not singularize the solution to a specific market, thus offering an equitable comparison between the proposed TES technologies. The operation strategy implemented switches among different operation modes such as preheating, filling, start-up, normal operation, cloud passing, shutdown and overnight hold mode. Within each of the operation modes, different operating conditions are used to represent the realistic operation of the plant. The switching among these modes depends on the current operating conditions during the transient simulation such as heliostat wind protection; receiver solar flux excess or
4.1.5. Power block modelling approach The power block model is based on a performance interpolation matrix representing the performance of the steam generation system (SGS), water and steam cycle (WSC) and electric power generation system (EPGS) with a design turbine gross output of 110 MWe. An
Fig. 6. Temperature distribution across the packed-bed at the end of the discharge and charge processes. 650
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molten salts overheating taking actions through solar field defocusing or mass flow controller among others. The solar fluxes onto the receiver during the receiver heating up and cool down are considered to be uniform (constant irradiance) and a smooth transition from this solar flux to the complete field flux map is used for partial focusing. When the solar resource is available during long periods, the normal operation mode is set and the hot molten salts coming from the receiver flows through the TES system and the power block, charging each thermocline tank from top to bottom with the same molten salt mass flow. When there is a long-term solar absence (overnight hold or cloud passing mode), each thermocline tank is discharged with half of the molten salt mass flow required by the power block. If the thermocline tanks are charged, the operation must try to avoid a long period without using them in order to avoid loss of stratification, which leads to a loss of stored energy. The thermocline tanks charge-discharge process is controlled by the temperature of the molten salts. The charge process ends when the temperature in the bottom of the tank increases 30 °C and immediately the discharge process starts. On the other hand, the discharge process ends when the outlet temperature at the top of the tank drops 30 °C and the cycle starts again. Fig. 6 shows the temperature along the packedbed axial axis at the end of the charge-discharge cycling process. The xaxis of the chart starts at 0, at the top of the tank. It can be observed that the difference of the temperature distribution at the end of the discharge or charge process between a cycle and the previous one decrease while the number of cycle increases until the charge-discharge temperature and time convergence. Once the discharging convergence time is observed, the maximum useful energy stored in the thermocline tanks is reached. The use of pumping tanks as shock absorbers of the partial cycling of the thermocline stabilizes the maximum energy stored.
•
4.2. Benchmarking assessment Levelized Cost of Electricity (LCOE) is the final figure of merit used in the benchmarking analysis to compare on average costs basis the two solar thermal power plants proposed. LCOE determine the energy generation costs, from building to operating a generation plant, ergo represent how much money must be made per unit of electricity to recover the lifetime costs of the system. Thus, the LCOE takes into account the following inputs: plant capital costs, financing costs, incomes (yearly net electricity production), expenses (fix and variable operation and maintenance costs, insurance costs, etc.) and taxes (rate, sales taxes, Tax Credit Expiration Period, etc.). In addition, the lower ratio between M&O and investment costs for CSP projects ensures minimum lifetime costs fluctuations that could affect the stability of the LCOE output. This means that the capital cost of generation capacity governs the calculation, unless currency or financial instabilities of less regulated markets occurs, turning the LCOE parameter in a trustful figure of merit to compare both power plants configurations. Even more, if we take into account the exhaustive previous design work made on reaching an equivalent capacity factor for each plant under study, a capacity factor that must be almost equal in order to equally compare two renewable energy power plants. For financial or tax credits it is needed to use the same assumptions in both power plant calculations in order to compare them under the same conditions. Therefore, the financial model parameters are shown in Table 3:
4.1.8. Simulation level of detail The simulations’ level of detail has been adjusted by the discretization of the receiver tubes, the complexity of the materials’ properties (temperature dependence), temporal resolution and TES system discretization among others. For the current modeling approach, the level of detail is as follows:
4.2.1. Benchmarking sensitivity analysis Economic assumptions uncertainties must be considered while performing an LCOE calculation over the whole lifecycle of a plant taking into account the plausible scenarios and their effects on the most sensitive parameters of the calculation. For this reason, a sensitivity analysis has been done taking into account the variation of referenced operation and maintenance costs from a real two-tank molten salts power plant (Turchi, 2010) for the hybrid TES based plant case. In addition, the variation of the storage material cost per ton for the steel slag pebbles and molten salts (Parrado et al., 2016) was also taken into account as it summarizes Table 4. The steelmaking slag cost per ton is the one derived from the commercial product developed within RESLAG project which comprises from manufacturing to transportation. The price variation of the slag
• Time step: To ensure proper consideration of the transient effects • •
axial direction are used, no internal convection resistance, no conduction resistance on the pipe and only two nodes in the conduction resistance of the insulation layer. Material properties: temperature-dependent properties for molten salts, receiver pipes material and slag pebbles (see Section 3.2). Constant properties are used for the absorber coating and insulation material.
(durations in the order of 15–30 min), a time step of 5 min has been used for the complete year simulation. Discretization of receiver tubes: Each tube is longitudinally discretized in 9 longitudinal nodes and in 3 nodes along its thickness achieving grid independence. Transport system pipes models: The rest of the pipes of the transport system, including the riser and downcomer, the cold supplies and hot discharges are simulated with one single node in the longitudinal direction. Simplified heat transfer models in the Table 3 Financial model parameters. General Incomes Investment Debt
Equity Expenses
Taxes
Useful life [years] Yearly inflation rate [%] (OECD, 2019) Yearly net electricity production [kWh/year] Plant capital cost [€] Debt [% total investment] Debt repayment period [years] Yearly debt interest [%] Grace period - Interest-only period [years] Equity [% total investment] Yearly rate of return on equity capital [%] O&M costs p.u. of net electricity [€/kWh] Insurance cost [%] Decommissioning Cost Tax rate [%] Yearly Depreciation Rate [%]
651
25 2.20 To be defined in the following sections To be defined in the following sections 100 15 8 3 0 12 To be defined in the following sections 0.5 0 35 6.67
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5.2. Systems cost results
Table 4 Sensitivity analysis. Annual O&M costs (€/kWh)
Best
Design
Worst
Hybrid TES CSP Plant Two-tank storage CSP Plant
0.02951 0.02951
0.02980 0.02951
0.03010 0.02951
Storage medium material costs (€/t) Slag pebbles Molten salts
Best 75 752
Design 100 940
Worst 200 1034
Table 6 summarizes the cost breakdown of the different cases under study according to references applied for each plant configuration (Bhargav et al., 2013; Kolb et al., 2007), (Kelly, 2010) and (Turchi and Heath, 2013). 5.2.1. Thermal energy storage system direct costs Special attention must be considered to the comparison of the direct costs of each TES system configuration. Taking into account (Kelly, 2010) and (Turchi and Heath, 2013), among CENER confidential information, each direct cost are shown in Fig. 7. The hybrid thermal energy storage system proposed is economically competitive against the state-of-the-art. The utilization of four tanks (two pumping tanks plus two thermocline tanks) is compensated by the cost reduction due to the smaller inventory of molten salts required by the hybrid configuration. Considering the dimension of the thermocline tanks, the void fraction and the slag pebbles density, the total amount of molten salts required is 14,910 tons in addition to 20,630 tons of slag pebbles. Therefore, the amount of molten salt is reduced by 31.4%, a difference of 6820 tons between the hybrid and the state-of-the-art solution that represent the high potential of cost reduction of this configuration.
Table 5 EYA comparison. Parameter
State-of-theart
Hybrid TES CSP Plant
Units
Available Solar Energy Used Solar Energy Dumped Solar Energy Receiver Incident Solar Energy Average Optical Efficiency Receiver Thermal Energy Output Average Receiver Efficiency Thermal Energy from Storage Gross PB Energy Production from Receiver Gross PB Energy Production from Storage Gross PB Energy Production Parasitic Energy consumption Net Electricity Production Average Thermal-to-Gross Electrical PB Efficiency Average Solar-to-Net Electrical Efficiency Capacity Factor
2363.76 2282.25 81.51 1300.46 56.98 1143.34 87.92 459.36 246.85
2363.76 2282.25 81.51 1300.46 56.98 1143.34 87.92 456.57 246.85
GWh GWh GWh GWh % GWh % GWh GWh
165.77
164.57
GWh
412.62 80.69 331.93 36.09
411.42 80.69 330.73 35.98
GWh GWh GWh %
14.54
14.49
%
6. Conclusions
37.89
37.75
%
An exhaustive effort has been made in order to achieve a wellfounded comparison between CSP power plants with equivalent thermal energy storage system design in terms of capacity factor and performance, to achieve robust results focusing on cost analysis. From this, the following conclusions can be obtained.
5.3. Benchmarking results Finally, the costs analysis results in terms of LCOE comparing both technologies are shown in Table 7 and charted in Fig. 8:
material is motivated due to conservative and risk assumptions over an innovative thermal energy storage material.
• The
5. Techno-economic and benchmarking analysis results 5.1. Energy yield assessment
•
A summary of the annual results obtained from the Energy Yield Assessment of both power plants is provided in Table 5. The hybrid thermocline based TES plant (Hybrid TES) results are quite similar to the State-of-the-Art plant (two-tank storage) due to the thermocline hybridization with pumping tanks which allows the same capacity factor and operational stability for both plants.
•
hybrid TES system achieves the same Levelized Cost of Electricity as the state-of-the-art two-tank technology of central receiver systems due to the competitive price of steelmaking slag pebbles versus molten salts. The hybrid TES energy production is close to the one produced by a plant with conventional storage due to hybrid TES configuration and operational strategy. The cut-off temperature in thermocline charge/discharge cycling maximizes the cycle efficiency. The addition of pumping tanks prevents the partial charge-discharge process of the thermocline, which is directly related to thermocline charge-discharge times, improving its transient operational
Table 6 Estimated cost ([M€]) for each plant components. Plant part
State-of-the-art CSP Plant [M€] Best case
Site Improvement Heliostat Field Tower Receiver TESS WSC + EPGS SGS Direct Costs Contingencies Total Direct Costs Indirect Costs Total costs
10.56 126.91 31.00 71.02 43.38 150.05 40.84 473.76 33.16 506.93 102.92 609.84
Hybrid TES CSP Plant[M€]
Design case
Worst case
Best case
Design case
Worst case
47.30
49.27
42.21
44.62
47.63
477.69 33.44 511.13 103.77 614.90
479.66 33.58 513.23 104.20 617.43
472.59 33.08 505.67 102.67 608.34
475.01 33.25 508.26 103.19 611.45
478.02 33.46 511.48 103.84 615.33
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Fig. 7. TES direct costs benchmarking analysis results: two-tank molten salts TES system (2 Tanks) versus hybrid TES (2TT + 2PT). Table 7 LCOE benchmarking results. LCOE [€/kWh]
Best
Design
Worst
Hybrid TES Two-tank storage
0.16701 0.16685
0.16799 0.16803
0.16914 0.16862
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Fig. 8. LCOE benchmarking analysis results for Hybrid TES and two-tank molten salts TES CSP plants taking into account storage material and O&M costs scenarios.
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