Exergy analysis of thermal energy storage options with nuclear power plants

doi:10.1016/j.anucene.2016.06.005

Annals of Nuclear Energy

Volume 96, October 2016, Pages 104-111

https://doi.org/10.1016/j.anucene.2016.06.005 Get rights and content

Highlights

•
Different thermal energy storage (TES) methods are proposed for small modular nuclear reactors.
•
Exergy and energy density analyses of TES integration with NPPs are conducted and presented.
•
Sensible heat liquid-type TES systems outperform solid-type systems.

Abstract

Storing excess thermal energy in a storage media, that can later be extracted during peak-load times is one of the better economic options for nuclear power in future. Thermal energy storage integration with light-water cooled and advanced nuclear power plants is analyzed to assess technical feasibility of different options. Various choices of storage media considered in this study include molten salts, synthetic heat transfer fluids, and packed beds of solid rocks or ceramics. Due to limitations of complex process conditions and safety requirements there are only few combinations which have potential integration possibilities. In-depth quantitative assessment of these integration possibilities are then analyzed using exergy analysis and energy density models. The exergy efficiency of thermal energy storage systems is quantified based on second law thermodynamics. This study identifies, examines, and compares different energy storage options for integration with modular NPPs, with the calculated values of energy density and exergy efficiency. The thermal energy storage options such as synthetic heat transfer fluids perform well for light-water cooled NPPs, whereas liquid storage salt show better performance with advanced NPPs as compared to other options.

Introduction

Nuclear power plants (NPPs) have negligible carbon emission rates as compared to their fossil fuel counterparts, but their inability to follow grid load demands make them economically less competitive. The reason behind this economic disposition for NPPs is because of various associated technical complexities. These technical challenges include the adequate handling of reactivity swings caused by time-varying fuel and moderator temperatures, a higher fuel-failure probability due to thermal-structural cycling, and spatial variations in xenon concentrations. Although there are presently some reactors around the world that are operating with flexible load-following capabilities, such operation is restricted to slowly-varying powers, 2–3 times a day, and only up to 80% of the fuel cycle. On the other hand, most of the fossil fueled plants can supply peak-loads by adding more fuel and, thus, can generate far more revenue during those peak hours. The use of NPPs for peak load following is quite complex due to technical constraints associated with reactor behavior. Thus, a more convenient and effective method to facilitate load following by NPPs would be to integrate energy storage. If the grid demand is reduced, then the excess reactor thermal power or plant electrical power is stored in an integrated storage device. This stored energy can be released to the grid when demand is higher than what the NPPs can produce at 100% reactor power (Forsberg and Curtis, 2013). There are many options for storing either the thermal energy from the nuclear reactor or the electricity from the turbo-generator in the power cycle, with both having their advantages and disadvantages respectively. Thermal, mechanical, and electrical energy storage are the most commonly used storage options. Thermal energy storage is the energy stored in the form of heat in well-insulated solids or liquids, as either sensible heat, stored within a single phase media, or latent heat, stored within phase change materials. Thermal energy storage options include but are not limited to molten salt, packed beds, heating oils, ionic liquids, phase change materials and steam accumulators. Mechanical energy storage is any kinetic or potential energy stored within a device and electrical energy storage resides in the buildup of electrons within systems called electric condensers, which store the charges between two parallel plates when a voltage is applied. Mechanical storage options include but are not limited to compressed air, pumped hydroelectric, flywheels, whereas electrical storage options include batteries and capacitors. Electrical energy storage has the advantage of directly storing the final usable form of energy i.e. electrical energy, but disadvantages come from the high costs and irreversibility. Mechanical energy storage processes such as pumped hydro have higher degree of reversibility but disadvantages include non-negligible energy losses and substantially large space requirements for grid scale storage. Disadvantages with thermal storage is the low efficiency of the conversion process from thermal to electrical energy but with NPPs generating large amounts of thermal heat, a thermal energy storage system becomes advantageous. Therefore, among these various options to store energy, thermal storage is economically more competitive for NPPs as compared to electrical or mechanical energy storage options. However, the adoption of a particular thermal storage option is largely dependent upon the operating and process conditions of the nuclear heat source and reactor coolant. Light-water NPPs operate at lower temperatures than Next Generation Nuclear Power (NGNP) reactors and only use pressurized water as the main coolant, whereas NGNP reactors use molten salts or high temperature gasses as the main coolant. The critical step remains how to select and develop an ideal choice of heat transfer fluid or storage media (Bejan, 1978). Currently, there are some thermal storage solutions such as molten nitrate salt, also known as storage or solar salt ( $40 % {KNO}_{3} + 60 % {NaNO}_{3}$ ) and packed bed of alumina particles which present very low technological risk and a high deployment potential. These solutions can be good candidates for some of the advanced high-temperature reactors but have some limitations for integration into light-water cooled NPPs. Therefore, other materials such as synthetic heat transfer fluids need to be explored to evaluate the options to store thermal energy for light-water NPPs. An overall comparative economic analysis can help in decision making process for storage integration, however for new materials and methods it is difficult to estimate the actual costs or effective costs if those technologies are deployed in large scale. Thus, an energy density and exergy model are used to compare different technologies and materials in this study.

Section snippets

Nuclear power plants considered

Firstly, different NPPs which can be considered as potential candidates for TES integration will be briefly described with the sufficient details in the process system conditions. For this analysis three NPP designs are selected – light water-cooled small modular reactors (LW-SMR), the modular high-temperature gas-cooled reactor (MHTGR) and pebble-bed fluoride-salt-cooled high-temperature reactor (PB-FHR). The schematics of each type are shown in Fig. 1, Fig. 2, Fig. 3, and Table 1 shows key

Thermal energy storage options

Sensible TES is more robust and established than any other thermal energy storage systems (Hasnain, 1998, Herrmann and Kearney, 2002). It has been shown by Bindra et al. (Bindra et al., 2013) that sensible heat storage have much higher exergy efficiency for high energy density storage design as compared to other mechanisms. So, in this work integration studies are only performed with sensible TES. The temperature difference between cold state and hot state, and thermal capacity of the media

Exergy and energy density quantification model

The disadvantage of the heat transfer in the heat exchanger between the RC and HTF for storage is that it reduces the top temperature during storage and in turn exergy efficiency of the system. The heat exchange process between HTF and RC is an important component of the model which involves indirect heat exchange to the HTF of the TES system. Due to the presence of additional resistances in the heat exchanger, the actual storage inlet temperature $T_{in}$ is lower than the RC outlet temperature and

Results

This study evaluates the exegetic performance of different materials and methods to store thermal energy of NPPs. The analysis was conducted for thermal storage integration with existing and future generation NPPs based on the plant design data from literature (Doster et al., 2012, Ball, 2004, Andreades and Peterson, 2014). The study reveals that there are various possible options to store thermal energy of next generation NPPs efficiently, but the options for existing NPPs are limited. Low

Conclusions

This paper provides a conceptual presentation on the integration of NPPs with technically robust thermal energy storage solutions. Although a more detailed analysis with entire condensate and feed water system integrated to the energy storage solution is required for more accurate analysis, for the conceptual presentation a simpler approach based on energy density and exergy analysis is adopted. Therminol and Dowtherm are commercially available robust technological solutions for this purpose

Acknowledgments

The material presented is based upon work partly supported by the U.S. Department of Energy via Idaho National Laboratory under Prime Contract No. DE-AC07-05ID14517. First author, Jacob Edwards, was supported by NRC graduate fellowship program.

References (18)

D.E. Beasley et al.
Transient response of a packed bed for thermal energy storage
Int. J. Heat. Mass Transf.
(1984)
H. Bindra et al.
Thermal analysis and exergy evaluation of packed bed thermal storage systems
Appl. Therm. Eng.
(2013)
H. Bindra et al.
Sliding flow method for exergetically efficient packed bed thermal storage
Appl. Therm. Eng.
(2014)
S.M. Hasnain
Review on sustainable thermal energy storage technologies, part I: heat storage materials and techniques
Energy Convers. Manage.
(1998)
R.J. Krane
A second law analysis of the optimum design and operation of thermal energy storage systems
Int. J. Heat Mass Transf.
(1987)
B. Zalba et al.
Review on thermal energy storage with phase change: materials, heat transfer analysis and applications
Appl. Therm. Eng.
(2003)
C. Andreades et al.
Technical Description of the “Mark 1” Pebble-Bed Fluoride-Slt-Cooled High-Temperature Reactor (PB-FHR) power plant
(2014)
S. Ball
(2004)
A. Bejan
Two thermodynamic optima in the design of sensible heat storage units for energy storage
J. Heat Transf.
(1978)

There are more references available in the full text version of this article.

Cited by (54)

Peridynamic analysis of thermal behaviour of PCM composites for heat storage
2024, Computer Methods in Applied Mechanics and Engineering
One possibility to utilize excess energy from electricity generation or other industrial processes is to use thermal energy storage systems based on phase change materials (PCM). These systems can accumulate and release significant amounts of heat energy during the phase transitions. The volume and properties of PCM undergo rapid changes during the transitions, creating strong physical non-linearities and geometric discontinuities. Describing this complexity with local formulations of the physical processes, i.e., formulations with differential equations, makes the numerical solutions of the corresponding problems challenging. This paper presents a numerical approach for the analysis of such physically complicated systems based on Peridynamics. The approach is illustrated by modelling and analysing the thermal behaviour of a PCM. The computational challenges associated with simulating composite structures with large differences in the heat conductivity of their components are addressed by investigating the influence of different types of peridynamic kernels and their numerical implementation. The results demonstrate that the solution is significantly influenced by the definition of the average thermal conductivity of the peridynamic bonds. The proposed modelling framework provides the base for developing a precise thermo-hydro-mechanical description of PCM composites that can fully represent the complex physical behaviour of thermal energy storage systems.
Coupled performance evaluation method of heat exchanger in a lead-cooled fast reactor based on life-cycle cost and exergy analysis
2024, Annals of Nuclear Energy
The present study attempts to fill a gap in the thermal–hydraulic and economic performance evaluation of HE. By combining the life-cycle cost (LCC) analysis and the exergy analysis, a net exergy saving cost (NESC) evaluation method is established to determine the overall performance of the HE by converting the exergy loss into economic cost. The comparison with traditional research methods shows the comprehensiveness of the NESC evaluation method. In addition, the simulation results based on the numerical studies state that the thermal economy of the twisted tube HE initially increases and then decreases with the pitch. Based on the comparison of NESC for the HEs made of different materials, it is believed that T91 steel exhibited the best thermal economy. The method can be applied to optimize the design of other equipment or systems to reduce their energy loss and improve their economic performance.
Nuclear microreactors and thermal integration with hydrogen generation processes
2024, Nuclear Engineering and Design
Nuclear microreactors offer reliable, low-carbon dispatchable power and heat for various end use applications. Although the direct electricity end uses are straightforward, the feasibility of microreactors’ integration for thermal end use has not been analyzed in literature in sufficient detail. Delivering process heat generated by nuclear microreactors to supply the high temperatures essential for hydrogen production has been proposed as one cogeneration option that can further aid in the alleviation of climate change, since hydrogen can replace carbon-emitting fuels such as gasoline, diesel or natural gas. This review provides a novel perspective on the intersection of microreactors and process heat use by investigating hydrogen production technologies, microreactor designs and process heat integration options. A comprehensive overview of hydrogen production methods including electrolysis and thermochemical conversions of hydrocarbons and water is presented by classifying the methods based on process temperatures and maturity. Additionally, an in-depth summary detailing the reactor type, power output, and maximum operating temperature of many prospective microreactor designs has been created. Finally, heat transfer options for integrating microreactors to hydrogen production systems were evaluated. The intermediate heat exchanger (IHX) assessment considers IHX material, IHX type, and heat transfer media utilized within the apparatus.
Model predictive control of a Lab-Scale thermal energy storage system in RELAP5-3D
2024, Nuclear Engineering and Design
Increased renewable energy penetration of an electrical grid generally creates more supply fluctuations. Traditional base-load systems, such as nuclear power, can be retrofitted with thermal energy storage (TES) systems to meet supply demand fluctuations. These systems can store energy during low electricity demand times to allow the reactor to operate at constant power while simultaneously fluctuating electricity output. The control of these systems is of vital importance and Model Predictive Control (MPC) is a leading control mechanism for optimized production plans with forecast information. This study combines high fidelity RELAP5-3D models with MPC to control an experimental benchtop system that approximates expected nuclear power thermal-hydraulics. RELAP5-3D is a high-fidelity code utilized for thermal–hydraulic phenomenon simulations for nuclear applications. The United States Nuclear Regulatory Commission (NRC) has supported the use of this code for licensing purposes. The NRC requires in depth testing before nuclear plants can be built or changed. The use of MPC with RELAP5-3D is used here to demonstrate the potential of a lab-scale TES coupled with a high-fidelity thermal–hydraulic model for use in the NRC licensing process.
Design and performance analysis of deep peak shaving scheme for thermal power units based on high-temperature molten salt heat storage system
2024, Energy
The transition to renewable energy production is imperative for achieving the low-carbon goal. However, the current lack of peak shaving capacity and poor flexibility of coal-fired units hinders the large-scale consumption of renewable energy. This study takes a 670 MW coal-fired unit as the research object and proposes eight design schemes for molten salt heat storage auxiliary peak shaving system. And through simulation calculations using Ebsilon software, the thermal performance, peak shaving capacity, environmental performance, and investment cost of each scheme were compared and analyzed. The results show that the molten salt heat storage auxiliary peak shaving system improves the flexibility of coal-fired units and can effectively regulate unit output; The combination of high-temperature molten salt and low-temperature molten salt heat storage effectively overcomes the problem of limited working temperature of a single type of molten salt, and can further improve the peak shaving capacity of coal-fired units, and the overall efficiency of operation is not low; Choosing LiNaK carbonates as high parameter molten salt and Hitec as low parameter molten salt has greatly expanded the operating range of the unit; Upgrading the combined molten salt solution with the existing low pressure cylinder zero output pipeline of the power plant can further improve the thermal performance, peak shaving performance, and environmental performance of the thermal power molten salt coupling system, with a peak shaving depth of up to 90.2 %; Combined with the zero output technology of low-pressure cylinder, when heat storage, the intermediate pressure cylinder exhaust steam extraction and electric heater heating molten salt. When releasing heat, use combined molten salt to heat bypass water supply. During heat release, a combination of molten salt is used to heat the bypass water supply. This scheme is the best flexible peak shaving transformation plan for the unit studied in this article, which can recover the initial investment within five years and meet the requirements of technical transformation difficulty.
A proposal for advanced supplementary technologies and a hybrid system with gas-cooled fast reactor concept ALLEGRO
2023, Nuclear Engineering and Design
A coupling of advanced reactors with high-capacity energy storage and other innovative non–nuclear energy technologies is a way to increase the flexibility and utilisation factor of the future nuclear systems and to diversify its energy outputs. In this paper, the basic layout of a hybrid system for a gas-cooled fast reactor is proposed. The system is composed of a high-temperature thermal energy storage unit coupled with a supercritical CO₂ conversion cycle and both low-temperature and high-temperature hydrogen production and utilization. As a reference GFR reactor, the ALLEGRO concept is considered. The paper describes state-of-the-art and overviews potentially applicable technologies. Selection and justification of concepts of the individual parts of the hybrid system including operational conditions is also given. The R&D needs necessary for the potential implementation of the system are identified and the activities ongoing in the Czech Republic and especially in Centrum Výzkumu Řež are also described.

View all citing articles on Scopus

View full text

Exergy analysis of thermal energy storage options with nuclear power plants

Highlights

Abstract

Introduction

Section snippets

Nuclear power plants considered

Thermal energy storage options

Exergy and energy density quantification model

Results

Conclusions

Acknowledgments

Int. J. Heat. Mass Transf.

Appl. Therm. Eng.

Appl. Therm. Eng.

Energy Convers. Manage.

Int. J. Heat Mass Transf.

Appl. Therm. Eng.

Technical Description of the “Mark 1” Pebble-Bed Fluoride-Slt-Cooled High-Temperature Reactor (PB-FHR) power plant

Two thermodynamic optima in the design of sensible heat storage units for energy storage

J. Heat Transf.