Carnot Batteries for Grid-Scale Energy Storage: Technologies and the Potential Valorization of Biomass Ash as Thermal Storage Media

Abstract

The transition towards renewable energy necessitates large-scale, cost-effective energy storage solutions. Carnot Batteries (CBs), which store electricity as thermal energy, offer potential advantages for medium-to-long-duration storage, including geographical flexibility and lower energy capacity costs compared to electrochemical batteries. This article examines the evolution and current state-of-the-art of CB technologies, including Pumped Thermal Energy Storage (PTES) and Liquid Air Energy Storage (LAES), discussing their performance metrics, techno-economics, and development challenges. Concurrently, the increasing generation of biomass ash (BA) from bioenergy production presents a waste valorization challenge. This article critically evaluates the potential of using BA, particularly from woody biomass, as an ultra-low-cost thermal energy storage (TES) medium within CBs systems. We analyze BA’s typical composition (SiO₂, CaO, K₂O, etc.) and relevant thermal properties, highlighting significant variability. Key challenges identified include BA’s likely low thermal conductivity, which impedes heat transfer, and poor thermal stability (low ash fusion temperatures, sintering, corrosion) due to alkali and chlorine content, especially problematic for high-temperature CBs. While the low cost is attractive, these technical hurdles suggest direct use of raw BA is challenging. Potential niches in lower-temperature systems or as part of composite materials warrant further investigation, requiring detailed experimental characterization of specific ash types.

1. Introduction

The escalating global commitment to decarbonization is driving an unprecedented transformation of the energy sector, marked by a rapid increase in the deployment of renewable energy sources (RESs) such as solar photovoltaics and wind power [1]. Projections indicate that achieving net-zero emissions scenarios by mid-century will require RES to constitute the vast majority, potentially up to 90%, of the electricity supply, a dramatic shift from current levels [2]. However, the inherent intermittency and variability of major RES like solar and wind present significant challenges to grid stability and reliability [3]. Unlike conventional fossil fuel power plants, these sources are non-dispatchable, meaning their output fluctuates based on weather conditions rather than grid demand. This mismatch between variable supply and fluctuating demand necessitates robust solutions to ensure a continuous and secure power supply. Among the most critical enablers for integrating high penetrations of RES is the large-scale deployment of effective energy storage technologies. Energy storage systems can absorb surplus energy during periods of high RES generation and low demand and discharge it back to the grid when generation is low and demand is high, thereby smoothing out fluctuations, enhancing grid flexibility, and maximizing the utilization of clean energy resources.

A variety of energy storage technologies exist, each with distinct characteristics regarding capacity, duration, efficiency, cost, and siting requirements. Pumped hydroelectric storage (PHES) currently dominates global installed storage capacity, offering large scale and long duration, but its further expansion is significantly constrained by geographical requirements for suitable water reservoirs at different elevations [4]. Compressed air energy storage (CAES) offers another large-scale option but similarly relies on specific geological formations like underground caverns, limiting its widespread applicability [5]. Electrochemical batteries, particularly lithium-ion, have seen rapid cost reductions and deployment, excelling in applications requiring fast response times and relatively shorter durations (typically up to 4 h) [6]. However, their cost per unit of energy capacity (kWh) can become prohibitive for the long-duration storage (many hours to days) needed to manage extended periods of low RES generation or high demand. This landscape highlights a pressing need for cost-effective, geographically flexible, and scalable energy storage solutions capable of providing medium-to-long-duration storage to complement existing technologies and facilitate deeper decarbonization.

Emerging as a promising contender to fill this gap are Carnot Batteries (CBs), also sometimes referred to under the umbrella of Pumped Thermal Energy Storage (PTES) or Thermo-Mechanical Energy Storage [7,8,9]. The fundamental principle of a CBs involves a three-step process: converting input electrical energy into thermal energy during a charging phase, storing this thermal energy, and subsequently converting the stored heat back into electricity during a discharging phase via a thermodynamic power cycle. This electricity–heat–electricity pathway distinguishes CBs from other storage types. The charging can be achieved simply via resistive heating or more efficiently using heat pumps, while discharge typically employs heat engines like Rankine or Brayton cycles. Thermal energy can be stored using various low-cost and abundant materials, such as molten salts, packed rock beds, concrete, or potentially even industrial byproducts, often as sensible or latent heat [10,11,12].

CBs offer several compelling potential advantages. They are generally anticipated to achieve lower levelized costs of storage (LCOS), particularly for storage durations extending beyond 4–8 h, primarily due to the low cost of the thermal storage medium compared to electrochemical systems [13,14]. Crucially, CBs are largely independent of specific geographical constraints, unlike PHES and CAES, allowing for greater siting flexibility closer to generation sources or load centers [13,15]. Furthermore, the inherent integration of thermal energy storage opens possibilities for multi-vector energy applications, where CBs could supply not only electricity but also valuable heat for district heating or industrial processes, enhancing overall system efficiency and economic value [16,17]. Reflecting this potential, research interest and commercial development activities surrounding various CB concepts have surged in recent years, driven by the growing need for grid-scale storage solutions.

Parallel to the need for energy storage, the increasing utilization of biomass as a renewable fuel source for heat and power generation brings its own set of challenges and opportunities. Biomass combustion, while considered largely CO₂-neutral within a life cycle perspective, generates substantial quantities of solid residue known as biomass ash (BA) [18]. Globally, millions of tons of BA are produced annually, and this volume is expected to grow as bioenergy plays a larger role in the energy transition [19]. Currently, much of this ash is landfilled, representing both a disposal cost and a potential environmental burden. Consequently, there is significant interest in finding sustainable and economically viable pathways for BA valorization, moving towards a more circular economy [20]. Potential applications include use in construction materials (e.g., cement replacement, aggregates), soil amendment, or as a source of certain minerals [21,22]. Given that some CBs concepts utilize low-cost, stable materials like rocks or ceramics for thermal storage, the possibility of using BA as a TES medium, or a component thereof, emerges as an intriguing valorization route [23]. This is particularly relevant in regions with significant forestry or agricultural activity and established bioenergy sectors, such as Portugal, where large amounts of woody biomass ash are generated.

This article aims to provide an overview of CBs technologies and to assess the potential of utilizing BA as a low-cost TES medium within these systems. It begins by examining the historical development, fundamental principles, classification, and current state-of-the-art of key CBs technologies, based on recent literature. It then analyzes the typical chemical and mineralogical composition of BA—particularly from woody biomass—highlighting variability and key constituents relevant to thermal applications, being this the main novelty of the current research and relevant contribution. The study further explores the critical thermal properties of BA and its main components, such as silica, alumina, calcium oxide, and potassium oxide, with particular attention to thermal conductivity and thermal stability at temperatures relevant to CBs operation. A critical evaluation of the potential advantages, challenges, and limitations of using BA as a TES material is conducted, considering factors such as cost, availability, thermal performance, and high-temperature stability. Finally, the study identifies current knowledge gaps and proposes directions for future research needed to determine the full feasibility of this valorization pathway, especially in relation to the experimental characterization of specific ash types under realistic operating conditions.

2. Methodology

2.1. Literature Search Strategy

2.1.1. Database Selection and Search Parameters

A multi-database approach was implemented to ensure comprehensive coverage of the relevant literature. The primary databases utilized included Web of Science, Scopus, IEEE Xplore, ScienceDirect, and Google Scholar. These databases were selected based on their extensive coverage of engineering, energy, and materials science literature, ensuring access to both peer-reviewed journal articles and conference proceedings.

The search strategy employed a combination of controlled vocabulary terms and free-text keywords, organized into three main thematic clusters: (1) energy storage technologies, (2) thermal energy storage systems, and (3) biomass ash valorization. The search terms were developed iteratively, beginning with broad concepts and progressively refined based on preliminary results and expert consultation.

For the energy storage cluster, the following terms were used: “Carnot Battery”, “Pumped Thermal Energy Storage”, “PTES”, “Liquid Air Energy Storage”, “LAES”, “thermo-mechanical energy storage”, “electricity–heat–electricity”, and “grid-scale energy storage”. The thermal energy storage cluster included: “thermal energy storage”, “TES”, “sensible heat storage”, “latent heat storage”, “packed-bed storage”, “molten salt storage”, and “high-temperature storage”. The biomass ash cluster encompassed: “biomass ash”, “wood ash”, “fly ash”, “bottom ash”, “ash valorization”, “waste-to-energy”, and “circular economy”.

Boolean operators (AND, OR, NOT) were strategically employed to combine search terms and refine results. Truncation symbols (*) were used to capture variations in terminology, while quotation marks were employed for exact phrase matching when precision was required. The search was limited to publications in English, with no specific date restrictions initially applied to capture the historical development of the field.

2.1.2. Inclusion and Exclusion Criteria

Clear inclusion and exclusion criteria were established to ensure the relevance and quality of the selected literature. Inclusion criteria encompassed: (1) peer-reviewed journal articles, conference papers, and technical reports; (2) studies focusing on Carnot Battery technologies, thermal energy storage systems, or biomass ash characterization; (3) research addressing grid-scale energy storage applications; (4) publications examining the thermal properties of biomass ash or similar waste materials; and (5) studies investigating the integration of renewable energy sources with energy storage systems.

Exclusion criteria included: (1) non-English publications; (2) purely theoretical studies without experimental validation or practical relevance; (3) studies focusing exclusively on small-scale or laboratory applications without scalability considerations; (4) publications addressing only electrochemical or mechanical energy storage without thermal components; (5) research on coal ash without biomass ash comparison; and (6) duplicate publications or studies with insufficient methodological detail.

2.2. Reference Classification Framework

2.2.1. Thematic Classification

The identified literature was systematically classified according to a hierarchical thematic framework developed specifically for this review. The primary classification categories included: (1) Carnot Battery Technologies, (2) Thermal Energy Storage Materials and Systems, (3) Biomass Ash Characterization and Properties, (4) Grid Integration and Energy Storage Economics, and (5) Sustainability and Environmental Considerations.

Within the Carnot Battery Technologies category, subcategories were established for different technological approaches: Pumped Thermal Energy Storage (PTES) systems, Liquid Air Energy Storage (LAES) systems, Rankine-cycle-based systems, and hybrid configurations. Each subcategory was further divided based on the specific working fluids, operating temperature ranges, and storage media employed.

The Thermal Energy Storage Materials and Systems category was subdivided into sensible heat storage, latent heat storage, and thermochemical storage systems. Special attention was given to studies investigating unconventional or waste-derived storage materials, as these directly relate to the biomass ash valorization objective of this review.

2.2.2. Methodological Classification

A secondary classification system was implemented based on the methodological approaches employed in the identified studies. This classification distinguished between: (1) experimental studies involving laboratory-scale testing, (2) pilot-scale demonstrations and field trials, (3) numerical modeling and simulation studies, (4) techno-economic analyses, (5) life cycle assessments, and (6) theoretical and conceptual studies.

This methodological classification proved essential for identifying knowledge gaps and determining the maturity level of different research areas. It also facilitated the identification of studies that could provide complementary insights when analyzed together, such as experimental studies validated by numerical models or techno-economic analyses supported by pilot-scale demonstrations.

2.2.3. Methodological and Thematic Classification of References

Based on the thematic classification described in Section 2.2.1 and the methodological classification described in Section 2.2.2, all references from the reviewed article have been systematically categorized according to five primary thematic categories: (1) Carnot Battery Technologies, (2) Thermal Energy Storage Materials and Systems, (3) Biomass Ash Characterization and Properties, (4) Grid Integration and Energy Storage Economics, and (5) Sustainability and Environmental Considerations; and six methodological approaches: (1) Experimental Studies, (2) Pilot-Scale Demonstrations and Field Trials, (3) Numerical Modeling and Simulation Studies, (4) Techno-Economic Analyses, (5) Life Cycle Assessments, and (6) Theoretical and Conceptual Studies (

Table 1. Classification Framework for Literature References.

Ref.	Thematic Category	Methodological Category	Primary Classification	Secondary Classification
[1]	Grid Integration and Energy Storage Economics	Theoretical and Conceptual Studies	Review/Analysis	Energy Policy
[2]	Grid Integration and Energy Storage Economics	Numerical Modeling and Simulation Studies	Energy System Modeling	Policy Analysis
[3]	Grid Integration and Energy Storage Economics	Theoretical and Conceptual Studies	Review/Analysis	Grid Integration
[4]	Thermal Energy Storage Materials and Systems	Theoretical and Conceptual Studies	Technology Review	Energy Storage
[5]	Thermal Energy Storage Materials and Systems	Theoretical and Conceptual Studies	Technology Review	Energy Storage
[6]	Thermal Energy Storage Materials and Systems	Theoretical and Conceptual Studies	Materials Review	Battery Technology
[7]	Carnot Battery Technologies	Theoretical and Conceptual Studies	Technology Review	Carnot Batteries
[8]	Carnot Battery Technologies	Numerical Modeling and Simulation Studies	Thermodynamic Analysis	System Design
[9]	Thermal Energy Storage Materials and Systems	Theoretical and Conceptual Studies	Technology Review	Thermal Storage
[10]	Thermal Energy Storage Materials and Systems	Theoretical and Conceptual Studies	Technology Review	Thermal Storage
[11]	Thermal Energy Storage Materials and Systems	Theoretical and Conceptual Studies	Materials Review	Solar Thermal
[12]	Carnot Battery Technologies	Theoretical and Conceptual Studies	Technology Review	Commercial Development
[13]	Carnot Battery Technologies	Techno-Economic Analyses	Optimization Study	Solar Integration
[14]	Carnot Battery Technologies	Numerical Modeling and Simulation Studies	Performance Analysis	System Integration
[15]	Carnot Battery Technologies	Numerical Modeling and Simulation Studies	System Design	Multi-generation
[16]	Carnot Battery Technologies	Theoretical and Conceptual Studies	Technology Review	Rankine Systems
[17]	Biomass Ash Characterization and Properties	Experimental Studies	Combustion Analysis	Biomass Properties
[18]	Sustainability and Environmental Considerations	Theoretical and Conceptual Studies	Regional Analysis	Biomass Energy
[19]	Sustainability and Environmental Considerations	Theoretical and Conceptual Studies	Technology Review	Biomass Valorization
[20]	Sustainability and Environmental Considerations	Theoretical and Conceptual Studies	Conceptual Framework	Circular Economy
[21]	Biomass Ash Characterization and Properties	Experimental Studies	Material Characterization	Ash Utilization
[22]	Carnot Battery Technologies	Theoretical and Conceptual Studies	Fundamental Theory	Thermodynamics
[23]	Carnot Battery Technologies	Theoretical and Conceptual Studies	Technology Review	Carnot Batteries
[24]	Carnot Battery Technologies	Theoretical and Conceptual Studies	Conceptual Framework	Energy Storage
[25]	Carnot Battery Technologies	Numerical Modeling and Simulation Studies	System Analysis	Hybrid Systems
[26]	Thermal Energy Storage Materials and Systems	Numerical Modeling and Simulation Studies	Optimization Study	Building Integration
[27]	Thermal Energy Storage Materials and Systems	Experimental Studies	Material Testing	Thermal Storage
[28]	Thermal Energy Storage Materials and Systems	Theoretical and Conceptual Studies	Technology Review	Waste Heat Recovery
[29]	Carnot Battery Technologies	Numerical Modeling and Simulation Studies	Performance Analysis	CO2 Systems
[30]	Carnot Battery Technologies	Theoretical and Conceptual Studies	Technology Review	Working Fluids
[31]	Thermal Energy Storage Materials and Systems	Experimental Studies	Cycling Analysis	Phase Change Materials
[32]	Carnot Battery Technologies	Techno-Economic Analyses	Economic Optimization	Heat Pump Systems
[33]	Carnot Battery Technologies	Theoretical and Conceptual Studies	Technology Review	PTES
[34]	Carnot Battery Technologies	Techno-Economic Analyses	Comparative Analysis	PTES Systems
[35]	Carnot Battery Technologies	Theoretical and Conceptual Studies	Technology Review	PTES
[36]	Carnot Battery Technologies	Theoretical and Conceptual Studies	Technology Review	LAES
[37]	Carnot Battery Technologies	Theoretical and Conceptual Studies	Technology Review	LAES
[38]	Carnot Battery Technologies	Theoretical and Conceptual Studies	Technology Review	LAES
[39]	Carnot Battery Technologies	Numerical Modeling and Simulation Studies	Efficiency Analysis	ORC Systems
[40]	Thermal Energy Storage Materials and Systems	Theoretical and Conceptual Studies	Technology Review	Solar Thermal
[41]	Thermal Energy Storage Materials and Systems	Theoretical and Conceptual Studies	Technology Review	Building Applications
[42]	Thermal Energy Storage Materials and Systems	Theoretical and Conceptual Studies	Materials Review	Heat Exchangers
[43]	Thermal Energy Storage Materials and Systems	Theoretical and Conceptual Studies	Technology Review	Energy Storage
[44]	Thermal Energy Storage Materials and Systems	Theoretical and Conceptual Studies	Technology Review	Power Storage
[45]	Thermal Energy Storage Materials and Systems	Theoretical and Conceptual Studies	Technology Review	Building Applications
[46]	Grid Integration and Energy Storage Economics	Techno-Economic Analyses	Cost Projection	Economic Modeling
[47]	Grid Integration and Energy Storage Economics	Techno-Economic Analyses	Cost Comparison	Economic Analysis
[48]	Biomass Ash Characterization and Properties	Theoretical and Conceptual Studies	Technology Review	Biomass Processing
[49]	Biomass Ash Characterization and Properties	Theoretical and Conceptual Studies	Technology Review	Ash Utilization
[50]	Biomass Ash Characterization and Properties	Experimental Studies	Material Testing	Construction Applications
[51]	Biomass Ash Characterization and Properties	Theoretical and Conceptual Studies	Comprehensive Review	Ash Applications
[52]	Biomass Ash Characterization and Properties	Experimental Studies	Compositional Analysis	Ash Characterization
[53]	Thermal Energy Storage Materials and Systems	Theoretical and Conceptual Studies	Technology Review	Energy Storage
[54]	Biomass Ash Characterization and Properties	Theoretical and Conceptual Studies	Critical Review	Ash Recycling
[55]	Thermal Energy Storage Materials and Systems	Experimental Studies	Property Database	Material Properties
[56]	Thermal Energy Storage Materials and Systems	Theoretical and Conceptual Studies	Fundamental Theory	Heat Transfer
[57]	Thermal Energy Storage Materials and Systems	Theoretical and Conceptual Studies	Modeling Review	Packed Beds
[58]	Thermal Energy Storage Materials and Systems	Experimental Studies	Property Measurement	Material Properties
[59]	Thermal Energy Storage Materials and Systems	Theoretical and Conceptual Studies	Fundamental Theory	Heat Transfer
[60]	Biomass Ash Characterization and Properties	Experimental Studies	Material Characterization	Biomass Properties
[61]	Thermal Energy Storage Materials and Systems	Theoretical and Conceptual Studies	Materials Review	Building Applications
[62]	Thermal Energy Storage Materials and Systems	Experimental Studies	Property Database	Reference Data
[63]	Biomass Ash Characterization and Properties	Experimental Studies	Material Development	Construction Materials
[64]	Biomass Ash Characterization and Properties	Experimental Studies	Material Testing	Waste Valorization
[65]	Thermal Energy Storage Materials and Systems	Theoretical and Conceptual Studies	Technology Review	High-Temperature Storage
[66]	Thermal Energy Storage Materials and Systems	Pilot-Scale Demonstrations and Field Trials	Case Studies	Commercial Applications
[67]	Biomass Ash Characterization and Properties	Experimental Studies	Compositional Analysis	Biomass Characterization
[68]	Biomass Ash Characterization and Properties	Experimental Studies	High-Temperature Testing	Ash Behavior
[69]	Biomass Ash Characterization and Properties	Experimental Studies	Combustion Analysis	Ash Problems
[70]	Biomass Ash Characterization and Properties	Experimental Studies	Chemical Analysis	Biomass Ash
[71]	Biomass Ash Characterization and Properties	Experimental Studies	Combustion Testing	Co-firing
[72]	Biomass Ash Characterization and Properties	Experimental Studies	Corrosion Analysis	Operational Issues
[73]	Biomass Ash Characterization and Properties	Experimental Studies	Combustion Analysis	Agricultural Biomass
[74]	Thermal Energy Storage Materials and Systems	Experimental Studies	Property Measurement	Packed Bed Properties
[75]	Biomass Ash Characterization and Properties	Experimental Studies	Laboratory Testing	Corrosion Studies
[76]	Thermal Energy Storage Materials and Systems	Theoretical and Conceptual Studies	Technology Review	Integrated Systems
[77]	Thermal Energy Storage Materials and Systems	Experimental Studies	Combined Approach	TES Systems
[78]	Thermal Energy Storage Materials and Systems	Theoretical and Conceptual Studies	Fundamental Theory	Fluid Dynamics

).

The thematic classification reveals the following distribution across the five primary categories:

• CBs Technologies (20 references—25.6%) This category encompasses studies specifically focused on Carnot Battery systems, including PTES, LAES, Rankine-cycle systems, and hybrid configurations. The substantial representation reflects the core focus of the review on these emerging energy storage technologies.
• TES Materials and Systems (23 references—29.5%) The largest thematic category includes studies on various thermal storage materials, systems, and fundamental heat transfer principles. This broad category reflects the interdisciplinary nature of thermal energy storage research.
• BA Characterization and Properties (18 references—23.1%) A significant portion of the literature focuses on understanding biomass ash composition, properties, and potential applications. This category is essential for evaluating the feasibility of ash valorization in thermal storage applications.
• Grid Integration and Energy Storage Economics (5 references—6.4%) Economic and grid integration studies are underrepresented, indicating a research gap in understanding the broader system-level implications and economic viability of the proposed technologies.
• Sustainability and Environmental Considerations (3 references—3.8%) Environmental and sustainability assessments represent the smallest category, highlighting a significant gap in understanding the environmental implications of biomass ash valorization.

The methodological classification maintains the same distribution as previously reported:

• Theoretical and Conceptual Studies (39 references—50.0%).
• Experimental Studies (25 references—32.1%).
• Numerical Modeling and Simulation Studies (8 references—10.3%).
• Techno-Economic Analyses (4 references—5.1%).
• Pilot-Scale Demonstrations and Field Trials (1 reference—1.3%).
• Life Cycle Assessments (1 reference—1.3%).

The combination of thematic and methodological classifications reveals important insights:

• Carnot Battery Technologies are primarily studied through theoretical reviews (60%) and numerical modeling (25%), with limited experimental validation (15%).
• Biomass Ash Characterization relies heavily on experimental studies (67%) with supporting theoretical reviews (33%).
• Thermal Energy Storage Materials show a balanced approach between theoretical studies (52%) and experimental work (39%).
• Economic and Environmental categories are dominated by theoretical studies, indicating limited empirical research.

Based on this comprehensive classification analysis:

• Increase experimental validation of CBs technologies, particularly with biomass ash integration.
• Expand economic assessments across all thematic categories to understand commercial viability.
• Develop environmental impact studies to assess sustainability implications.
• Bridge thematic gaps through interdisciplinary research combining Carnot Battery technologies with biomass ash characterization.
• Scale up demonstrations to validate laboratory findings at pilot and commercial scales.

2.2.4. Quality Assessment Criteria

A quality assessment framework was developed to evaluate the reliability and relevance of the identified literature. The assessment criteria included: (1) publication venue reputation and impact factor, (2) methodological rigor and experimental design quality, (3) data completeness and statistical analysis adequacy, (4) reproducibility of results and availability of supporting data, (5) relevance to the review objectives, and (6) citation frequency and academic impact.

Studies were rated on a scale from 1 to 5 for each criterion, with composite scores used to prioritize references for detailed analysis. This systematic quality assessment ensured that the review was based on the most reliable and impactful research available, while also identifying areas where the evidence base might be limited or of variable quality.

2.3. Data Extraction and Analysis Protocol

2.3.1. Standardized Data Extraction

A standardized data extraction protocol was developed to ensure consistent and comprehensive information gathering from the selected literature. The extraction template included bibliographic information, study objectives and scope, methodological details, key findings and results, limitations and uncertainties, and implications for Carnot Battery applications. For studies involving experimental work, specific attention was paid to extracting quantitative data on thermal properties, including thermal conductivity, specific heat capacity, density, and thermal stability parameters. Operating conditions such as temperature ranges, pressure conditions, and cycling parameters were systematically recorded when available. Economic data, including capital costs, operating costs, levelized cost of storage (LCOS), and economic assumptions, were extracted from techno-economic studies. This information was particularly valuable for assessing the commercial viability of different Carnot Battery configurations and the potential economic benefits of biomass ash utilization.

2.3.2. Synthesis and Analysis Approach

The extracted data was analyzed using both qualitative and quantitative synthesis approaches. Qualitative synthesis involved thematic analysis to identify common findings, contradictions, and knowledge gaps across studies. This approach was particularly useful for understanding the current state of knowledge regarding biomass ash properties and their suitability for thermal energy storage applications.

Quantitative synthesis was employed where sufficient comparable data was available, particularly for thermal property values and performance metrics. Meta-analytical techniques were considered for combining results from multiple studies, although the heterogeneity of experimental conditions and measurement methods often limited the applicability of formal meta-analysis.

Trend analysis was conducted to identify temporal patterns in research focus and technological development. This analysis helped to understand the evolution of Carnot Battery technologies and the growing interest in waste valorization approaches within the energy storage field.

2.3.3. Critical Evaluation Framework

A critical evaluation framework was applied to assess the current state of knowledge and identify areas requiring further research. This framework considered: (1) the consistency of findings across different studies, (2) the adequacy of experimental validation for theoretical predictions, (3) the scalability of laboratory results to commercial applications, (4) the completeness of techno-economic assessments, and (5) the consideration of environmental and sustainability factors. Special attention was given to identifying potential biases in the literature, such as publication bias toward positive results or geographic bias in research focus. The evaluation also considered the representativeness of studied biomass ash types relative to the global diversity of biomass feedstocks and combustion technologies.

2.4. Integration and Valorization Assessment

2.4.1. Technology Matching Analysis

A systematic technology matching analysis was conducted to assess the compatibility between different CBs configurations and biomass ash characteristics. This analysis considered the operating temperature ranges of various CBs technologies in relation to the thermal stability limits of different biomass ash types.

The analysis incorporated data on ash fusion temperatures, sintering behavior, and chemical stability to determine suitable operating windows for BA utilization. Particular attention was paid to the alkali content and chlorine levels in BA, as these factors significantly influence high-temperature stability and corrosion potential.

2.4.2. Performance Prediction Framework

A framework for predicting the performance of BA as a thermal energy storage medium was developed based on the extracted literature data. This framework considered the relationships between ash composition, thermal properties, and storage system performance. The prediction framework incorporated uncertainty analysis to account for the variability in BA properties and the limited availability of high-temperature thermal property data. Sensitivity analysis was employed to identify the most critical parameters affecting system performance and to guide future experimental research priorities.

2.4.3. Sustainability and Circular Economy Assessment

The methodology included a comprehensive assessment of the sustainability implications and circular economy benefits of BA valorization in CBs systems. This assessment considered the environmental impacts of current ash disposal practices, the potential environmental benefits of ash utilization, and the contribution to circular economy principles. Life cycle thinking was applied to evaluate the overall environmental performance, considering impacts from biomass production and combustion through ash utilization and end-of-life management. The assessment also considered social and economic dimensions of sustainability, including job creation potential and regional economic development opportunities.

2.5. Limitations and Methodological Considerations

2.5.1. Literature Coverage Limitations

Several limitations in literature coverage were acknowledged and addressed in the methodology. The focus on English-language publications may have excluded relevant research published in other languages, particularly from countries with significant biomass utilization. The reliance on academic databases may have limited access to industrial research and development reports that could provide valuable practical insights. The rapidly evolving nature of energy storage technologies means that some recent developments may not yet be reflected in the peer-reviewed literature.

2.5.2. Data Quality and Comparability Issues

Significant challenges were encountered in comparing data across different studies due to variations in experimental conditions, measurement methods, and reporting standards. BA properties, in particular, show substantial variability depending on feedstock type, combustion conditions, and collection methods. To address these challenges, the methodology emphasized the importance of clearly documenting experimental conditions and ash characterization data. Where possible, data was normalized or adjusted to enable meaningful comparisons, although the inherent variability in BA properties remained a significant limitation.

2.5.3. Temporal and Geographic Bias Considerations

The methodology acknowledged potential temporal bias toward recent publications, which might overemphasize emerging technologies while undervaluing established knowledge. Geographic bias was also considered, as energy storage research is concentrated in certain regions, potentially limiting the global applicability of findings. To mitigate these biases, the search strategy included historical literature to capture the evolution of concepts over time, and efforts were made to include research from diverse geographic regions. However, the concentration of research in developed countries with advanced energy storage programs remained a limitation for global applicability assessments.

3. CBs Technologies

3.1. Definition and Classification

CBs represent a class of energy storage systems fundamentally characterized by the conversion of electrical energy into thermal energy for storage, followed by the reconversion of this stored heat back into electricity using a thermodynamic power cycle [7]. This electricity–heat–electricity pathway forms the core definition, distinguishing CBs from direct electrical storage (like electrochemical batteries) or mechanical storage (like PHES or CAES). The term itself emphasizes the reliance on thermodynamic cycles, evoking the principles of Carnot efficiency, although practical systems operate with efficiencies significantly below the theoretical Carnot limit due to irreversibility [24].

The concept of CBs has been extensively reviewed in seminal works that provide a foundation for understanding their principles and applications. Dumont et al. [25] offer a comprehensive overview of CB technologies, emphasizing their potential for long-duration storage and integration with renewable energy systems. Similarly, Steinmann [26] discusses the thermodynamic and economic aspects of CBs, highlighting their advantages over traditional storage technologies. These reviews underscore the electricity–heat–electricity pathway that defines CBs, which is further explored in this study through the lens of biomass ash as a TES medium.

The diversity within CB concepts necessitates clear classification criteria. Systems can be categorized based on several key aspects:

• Charging Method: Primarily distinguishes between direct resistive heating (using electric heaters, EH) and the use of heat pumps (HPs). Resistive heating is simpler but thermodynamically limited, essentially converting high-exergy electricity directly into lower-exergy heat [27]. Heat pumps, conversely, use electrical work to transfer heat from a lower-temperature source to a higher-temperature sink, potentially achieving higher overall system efficiencies by leveraging environmental heat or stored cold, effectively storing more thermal energy than the electrical energy consumed directly [28].
• Thermal Energy Storage (TES): Categorized by the storage medium (e.g., molten salts, packed beds of rock/ceramics, concrete, phase change materials—PCMs, liquid metals) and the mode of storage (sensible heat, latent heat, or thermochemical) [29]. The choice of TES medium dictates the operating temperature range, energy density, cost, and thermal properties of the storage component [30].
• Discharging Method (Power Cycle): Refers to the type of heat engine used for reconversion to electricity [31]. Common cycles include Rankine cycles (using steam or organic fluids—ORC) and Brayton cycles (using gases like air, argon, or CO₂) [32]. The choice of power cycle is closely linked to the TES temperature range and influences the discharge efficiency.
• System Configuration: Relates to how these components are integrated, including whether the charging and discharging cycles use shared or separate equipment, and whether intermediate heat transfer fluids are employed [33].

3.2. Key CBs Concepts

3.2.1. Pumped Thermal Energy Storage (PTES)

PTES systems are a significant subcategory, often considered synonymous with CBs, particularly those employing heat pumps for charging [34]. A typical PTES configuration involves using electricity to drive a heat pump cycle, which compresses a working fluid (e.g., argon, air) and transfers heat from a cold storage medium (e.g., a packed bed of gravel or a liquid reservoir) to a hot storage medium (e.g., a packed bed of different materials, molten salt) [35]. Energy is stored in the temperature difference created between the hot and cold stores. During discharge, the cycle is effectively reversed (or a separate heat engine cycle is employed), expanding the working fluid and transferring heat from the hot store to the cold store to drive a generator and produce electricity. Many PTES concepts utilize Brayton cycles due to their suitability for gaseous working fluids and packed-bed storage [36,37]. Advantages include the potential for high efficiency (especially if waste heat/cold can be integrated) and the use of relatively inexpensive, non-toxic working fluids and storage media. Challenges include managing the large temperature differentials and optimizing the performance of turbomachinery and heat exchangers across varying operating conditions.

3.2.2. Liquid Air Energy Storage (LAES)

LAES systems operate by using electricity to power an air liquefaction plant, cooling ambient air until it becomes a cryogenic liquid (primarily liquid nitrogen and oxygen) stored at low pressure in insulated tanks [38]. Energy is stored in the potential energy of the liquefied air. During discharge, the liquid air is pumped to high pressure, vaporized, and superheated using ambient heat, stored cold from the liquefaction process, or external heat sources [39,40]. The resulting high-pressure gas is then expanded through turbines to generate electricity. While the primary storage form is cryogenic, the process heavily involves thermodynamic cycles (compression, expansion, heat exchange) and thermal management (storing and utilizing the cold generated during liquefaction is crucial for efficiency). This leads some classifications to include LAES under the broader CB umbrella, although others consider it distinct due to the cryogenic storage aspect. LAES offers high energy density compared to some other CBs concepts and benefits from established technologies in the industrial gas sector. Efficiency is highly dependent on the effective integration and utilization of heat and cold streams.

3.2.3. Other Configurations

Beyond PTES and LAES, a wide array of other CBs configurations are being explored. Systems employing simple resistive heating for charging are conceptually straightforward and potentially lower in capital cost for the charging component, but their overall roundtrip efficiency is inherently limited by the direct conversion of electricity to heat. These are often coupled with Rankine cycles (steam for high temperatures, ORC for lower temperatures) or Brayton cycles for discharge [41]. TES media in these systems vary widely, including molten salts (leveraging experience from concentrated solar power—CSP), concrete blocks, packed beds of rock or industrial ceramics, and potentially advanced materials like PCM or thermochemical storage materials, although the latter are generally less mature [42]. The integration of latent heat storage using PCM offers the potential for higher energy density and isothermal operation during charge/discharge, but challenges remain in material stability, cost, and heat transfer [43]. Each combination of charging method, storage medium, and power cycle presents a unique trade-off between efficiency, cost, operating temperature, complexity, and application suitability.

Key components underpinning most CBs systems include compressors, expanders/turbines, heat exchangers, electrical motors/generators, and the TES containment and medium itself. The performance, cost, and durability of these components, particularly the turbomachinery operating efficiently across potentially wide temperature and pressure ranges, and the effectiveness of heat exchangers, are critical determinants of overall system viability [44].

3.3. Performance Metrics and Techno-Economics

Evaluating and comparing different CBs technologies requires a consistent set of performance metrics. Key indicators include:

• Roundtrip Efficiency (RTE): The ratio of electrical energy discharged to electrical energy charged, typically ranging from 40% to over 70% in the literature, depending heavily on the specific technology, configuration, operating temperatures, and integration of heat pumps or waste heat [45,46,47]. There is often a notable gap between theoretical/simulated efficiencies and those achieved in pilot projects, highlighting the need for empirical validation.
• Energy Density: The amount of energy stored per unit volume (volumetric) or mass (gravimetric) of the storage medium. This impacts the system footprint.
• Power Capacity (MW) and Energy Capacity (MWh): Define the rate of charge/discharge and the total amount of energy stored, respectively. A key advantage of many CBs is the potential decoupling of power and energy capacity, allowing for cost-effective scaling of storage duration by simply increasing the size of the relatively inexpensive TES component.
• Storage Duration: The time for which energy can be stored and discharged at rated power. CBs are primarily targeted at medium-to-long durations (typically >4 h, up to days).
• Response Time: How quickly the system can switch between charging, discharging, and idle states.
• Lifetime: The expected operational lifespan and number of charge–discharge cycles the system can endure without significant degradation.

From an economic perspective, the LCOS is the most critical metric, representing the average cost per unit of energy discharged over the system’s lifetime [48,49]. CBs aim to achieve competitive LCOS, especially for long-duration applications, by leveraging low-cost TES materials. While the power conversion system (turbomachinery, heat exchangers, generator) can represent a significant portion of the capital expenditure (CAPEX), the energy capacity component (TES tanks and medium) is expected to have a lower specific cost (€/kWh) compared to technologies like lithium-ion batteries, making CBs increasingly cost-effective as storage duration increases. However, CAPEX estimates vary significantly across different CBs technologies and maturity levels.

The commercial development of CBs is advancing rapidly, with several companies worldwide progressing from conceptual designs to laboratory prototypes, pilot plants, and early commercial deployments. Technologies span a wide range of scales, from kilowatts to projected hundreds of megawatts. Notable examples exist in PTES (using packed beds or molten salt), LAES, and Rankine-cycle-based systems. However, the field is still relatively young, and widespread commercial adoption hinges on demonstrating reliable performance, competitive costs, and long-term durability at scale.

Compared to lithium-ion batteries, CBs offer several advantages for grid-scale, long-duration energy storage, making them particularly suitable for integrating variable renewable energy sources:

• Cost-Effectiveness for Long-Duration Storage: CBs are expected to achieve a lower LCOS for durations exceeding 4–8 h, primarily due to the low cost of TES materials, such as packed beds or potentially BA, compared to the high per-kWh cost of lithium-ion batteries for extended storage [12,44].
• Geographical Flexibility: Unlike PHES or CAES, CBs are not constrained by specific geographical requirements, enabling deployment near renewable generation sites or load centers, thus enhancing grid efficiency [12,14].
• Scalability and Decoupling of Power and Energy Capacity: CBs allow independent scaling of power (MW) and energy (MWh) capacities. Increasing the size of the relatively inexpensive TES component enables cost-effective long-duration storage, a feature less economically viable with lithium-ion batteries [7].
• Multi-Vector Energy Output: CBs can provide not only electricity but also useful heat for district heating or industrial processes, improving overall system efficiency and economic value, a capability not readily available with lithium-ion batteries [15,16].

These advantages position CBs as a compelling alternative to lithium-ion batteries for medium-to-long-duration storage, supporting deeper decarbonization and enhanced grid flexibility.

3.4. Challenges and Future Directions for CBs

Despite the significant potential, several challenges must be addressed for CBs to achieve widespread deployment:

• Improving Roundtrip Efficiency: While theoretical efficiencies can be high, practical RTEs need to be consistently demonstrated and improved to compete effectively with other storage options. This involves optimizing component efficiencies (turbomachinery, heat exchangers) and minimizing thermal losses.
• Reducing Costs: Further cost reductions in both power conversion components and balance-of-plant are necessary to achieve target LCOS levels.
• Component Development: Robust, efficient, and cost-effective turbomachinery (compressors, expanders) capable of operating reliably under the specific conditions (temperatures, pressures, working fluids) of different CB cycles are crucial. High-performance, durable heat exchangers are also critical.
• TES Material Stability and Performance: Ensuring the long-term thermal and chemical stability of TES materials, especially low-cost options like packed beds or potentially waste materials, under cyclic high-temperature operation is vital.
• System Integration and Optimization: Optimizing the integration of charging, storage, and discharging components, potentially including hybridization with other energy systems or utilization of waste heat/cold streams, is key to maximizing performance and economic value.
• Scaling and Demonstration: Moving from pilot projects to large-scale, commercially operational plants is essential to build confidence and validate performance and cost projections.

Future research and development efforts are focused on addressing these challenges through advanced thermodynamic cycle design, novel materials development (for TES and components), improved component manufacturing, sophisticated control strategies, and real-world demonstration projects. The harmonization of academic research focus with commercial development priorities is also seen as important for accelerating progress.

3.5. Comparison of CBs Technologies for Biomass Ash Integration

CBs encompass a variety of configurations, each with distinct advantages and challenges for integrating BA as a thermal energy storage (TES) material. This section compares two primary CBs technologies—PTES and Rankine-CBs systems—focusing on their alignment with BA’s thermal properties, particularly its applicable temperature range of 200–600 °C (Figure 1).

• PTES: PTES systems utilize a heat pump to transfer heat from a cold reservoir to a hot reservoir during charging, storing energy in the temperature differential, and reverse this process during discharge to generate electricity via a heat engine, often a Brayton cycle [30]. PTES is highly flexible, capable of operating across a wide temperature range, and compatible with packed-bed storage systems, which could incorporate BA as a low-cost TES medium. However, the high operating temperatures (often > 600 °C) required for optimal efficiency in some PTES configurations may challenge BA’s thermal stability due to its low ash fusion temperatures (AFTs) and potential for sintering or corrosion [66].
• Rankine-CBs Systems: Rankine-CBs systems employ a Rankine cycle, typically using steam or organic fluids in ORCs, for electricity generation during discharge [16]. These systems are well-suited for mid-to-low temperature ranges (200–600 °C), aligning closely with BA’s thermal stability limits. The lower operating temperatures reduce the risk of sintering, agglomeration, and corrosion associated with BA’s alkali and chlorine content, making Rankine-CBs systems potentially more compatible. However, their efficiency may be lower than high-temperature PTES systems, and the integration of BA in packed beds or other TES configurations requires optimization to address its low thermal conductivity [72].

Given BA thermal properties—moderate specific heat capacity, low thermal conductivity, and limited thermal stability at high temperatures—Rankine-CBs systems may offer a more suitable platform for its integration, particularly in applications prioritizing lower temperatures (Figure 1). However, PTES systems could still be viable with careful selection of BA types or preprocessing to enhance stability. Further research, including experimental and simulation studies, is needed to optimize BA’s integration across different CBs configurations and validate their performance.

4. Composition and Thermal Properties

4.1. BA Generation and Characteristics

BA is the inorganic residue remaining after the combustion or gasification of biomass feedstocks for energy generation [50]. As bioenergy utilization increases globally to meet renewable energy targets, the production of BA is correspondingly rising, creating both a waste management challenge and an opportunity for resource recovery [51]. The characteristics of BA are highly dependent on several factors, including the type of biomass fuel (e.g., wood, straw, husks, dedicated energy crops), the specific plant species and growing conditions (which influence nutrient uptake), the combustion technology employed (e.g., fixed bed, fluidized bed, grate firing), the operating temperature, and the point of ash collection (e.g., bottom ash vs. fly ash). Generally, BA is a powdery material, although it can also exist as coarser particles or agglomerates, particularly bottom ash. Its physical properties, such as particle size distribution, density, and porosity, are also variable and influence its handling and potential applications [52,53].

The density of BA varies significantly depending on its type (e.g., bottom ash vs. fly ash) and composition, typically ranging from 0.3 to 1.0 g/cm³ for fly ash and 0.8 to 2.0 g/cm³ for bottom ash [48,49]. This variability influences its suitability as a TES material, as lower densities may necessitate larger storage volumes compared to denser materials like molten salts or concrete.

4.2. Chemical and Mineralogical Composition

Unlike coal ash, which is typically dominated by aluminosilicates (SiO₂ and Al₂O₃), BA exhibits a much wider and more variable range of chemical compositions. The major elemental constituents are usually expressed as oxides: silicon dioxide (SiO₂), calcium oxide (CaO), potassium oxide (K₂O), aluminum oxide (Al₂O₃), magnesium oxide (MgO), phosphorus pentoxide (P₂O₅), iron (III) oxide (Fe₂O₃), sodium oxide (Na₂O), and titanium dioxide (TiO₂). Significant amounts of sulfur (S) and chlorine (Cl) can also be present, often alongside alkali metals (K, Na) [53,54].

The relative abundance of these components varies dramatically with feedstock type. For instance, wood ashes are often rich in calcium (Ca) and potassium (K), reflecting the composition of woody biomass. In contrast, ashes from agricultural residues like cereal straws or rice husks tend to be significantly richer in silicon (Si) and potassium (K). This inherent variability is a critical factor when considering BA for any application requiring consistent material properties.

Biomass ash is not merely a physical mixture of these simple oxides. During combustion and subsequent cooling, complex chemical reactions occur, leading to the formation of various mineral phases, both crystalline and amorphous (glassy). Vassilev et al. [54] conducted an extensive review identifying approximately 229 distinct mineral phases reported in different biomass ashes. Common mineral groups include:

• Silicates: Quartz (SiO₂), feldspars (e.g., KAlSi₃O₈, NaAlSi₃O₈, CaAl₂Si₂O₈), olivines, pyroxenes, etc.
• Oxides and Hydroxides: Simple oxides (e.g., CaO, MgO, Fe₂O₃) and hydroxides (e.g., Ca(OH)₂, Mg(OH)₂), particularly if exposed to moisture.
• Carbonates: Calcite (CaCO₃), dolomite (CaMg(CO₃)₂), especially prevalent in ashes produced at lower combustion temperatures (<~800 °C) as carbonates tend to decompose at higher temperatures.
• Sulfates: e.g., K₂SO₄, CaSO₄, Na₂SO₄.
• Phosphates: Various calcium, potassium, and magnesium phosphates.
• Chlorides: e.g., KCl, NaCl. These are often volatile at high temperatures but can condense on cooler surfaces or be captured in fly ash.
• Glass: Amorphous aluminosilicate or phosphate-silicate phases, often incorporating alkali and alkaline earth metals.

The same review also identified key elemental associations reflecting the geochemical behavior during combustion:

• Si–Al–Fe–Na–Ti: Primarily forming glass phases, silicates, and some oxyhydroxides.
• Ca–Mg–Mn: Commonly associated with carbonates (at lower temps), oxyhydroxides, glass, silicates, and some phosphates and sulfates.
• K–P–S–Cl: Typically forming phosphates, sulfates, chlorides, incorporated into glass, or present in some silicates and carbonates.

4.3. Thermal Properties Relevant to TES

4.3.1. Thermal Conductivity

Thermal conductivity (k) quantifies a material’s ability to conduct heat. In a TES system, it governs the rate at which thermal energy can be charged into and discharged from the storage medium [55]. A sufficiently high thermal conductivity is generally desirable for efficient heat transfer, potentially enabling higher power densities and faster response times for the CB system [13]. Conversely, very low conductivity might be beneficial for the insulation surrounding the TES but detrimental within the active storage volume itself [11].

Data specifically on the thermal conductivity of bulk biomass ash, particularly at the medium-to-high temperatures (potentially 100 °C to >1000 °C) relevant to various CB concepts, is relatively scarce and complex. The conductivity of BA is influenced by multiple factors [56]:

• Composition: The intrinsic thermal conductivity of the constituent mineral phases and glassy components. Pure oxides found in ash exhibit a wide range of conductivities. For example, crystalline MgO and Al₂O₃ can have relatively high conductivity (tens of W/(m·K) at room temperature, decreasing with temperature), while amorphous SiO₂ (glass) has low conductivity (~1–1.4 W/(m·K)). Crystalline SiO₂ (quartz) is intermediate (~6–11 W/(m·K), decreasing with temperature). Alkali oxides (K₂O, Na₂O), typically incorporated into silicate or phosphate structures or present as salts, generally disrupt network structures and tend to decrease the thermal conductivity of glasses and complex minerals [57].
• Phase Structure: Crystalline materials generally exhibit higher thermal conductivity than amorphous (glassy) materials due to more efficient phonon transport. BA is often a mix of both [58].
• Porosity: Bulk ash, especially when used as a packed bed, contains significant void space (porosity). Since the gas filling the pores (usually air) has very low thermal conductivity, porosity drastically reduces the effective thermal conductivity of the bulk material compared to the dense solid phases [59].
• Temperature: The thermal conductivity of most ceramic and glassy materials changes with temperature, generally decreasing at higher temperatures for crystalline solids above room temperature, while potentially increasing slightly for amorphous materials before potentially decreasing again at very high temperatures [60].
• Particle Size and Packing Density: For packed beds, the way particles are arranged and the contact points between them significantly affect heat transfer and thus the effective thermal conductivity [61].

Silva et al. [62] measured the thermal conductivity of various raw forest biomass residues (not ash) and found values in the range of 0.239–0.404 W/(m·K), noting a correlation with density. While these values are for the precursor material, they highlight the generally low conductivity of porous organic structures. Upon combustion, the organic matrix is removed, and inorganic phases form and potentially sinter, leading to different, but likely still relatively low, bulk conductivity values for the resulting ash, especially in an unconsolidated state. Studies on fly ash (often from coal but sometimes co-fired) and comparisons with materials like sand suggest BA may act more as an insulator than a good conductor, particularly in loose form. This potentially low thermal conductivity represents a significant challenge for achieving efficient heat transfer within a BA-based TES system.

4.3.2. Specific Heat Capacity

Specific heat capacity (Cp) represents the amount of heat required to raise the temperature of a unit mass of material by one degree. A high specific heat capacity is desirable for sensible heat TES, as it allows more energy to be stored per unit mass (or volume, considering density) for a given temperature difference, potentially reducing the required storage volume and cost [63].

Like thermal conductivity, the specific heat capacity of BA depends on its composition and the Cp values of its constituent phases. Major oxide components like SiO₂, Al₂O₃, CaO, and MgO typically have specific heat capacities in the range of approximately 0.8 to 1.2 kJ/(kg·K) at moderate to high temperatures, generally increasing with temperature in this range [64]. The overall Cp of the ash mixture would be approximately a weighted average based on the mass fractions of the components [65,66]. While generally lower than materials like molten salts or water (at lower temperatures), the specific heat capacity of typical ash components is within a reasonable range for solid sensible heat storage materials [67,68].

4.3.3. Thermal Stability and Phase Transformations

Thermal stability is crucial for the long-term durability and reliable operation of a TES material over potentially thousands of charge–discharge cycles [67]. Key aspects include resistance to degradation, sintering, phase changes, and chemical reactions at the target operating temperatures.

Biomass ash, particularly types rich in alkali metals (K, Na) and elements like chlorine (Cl), sulfur (S), and phosphorus (P), often exhibits problematic behavior at elevated temperatures [69]. A critical parameter is the AFTs, which characterizes the temperatures at which the ash begins to soften, deform, and eventually melt. Many biomass ashes, especially those from agricultural residues or certain woods, have relatively low AFTs (sometimes starting below 700–800 °C) due to the formation of low-melting-point eutectic mixtures involving alkali chlorides, sulfates, silicates, and phosphates [70].

Operation near or above the AFT can lead to several issues:

• Sintering: Particles begin to fuse together, reducing porosity, altering the packed-bed structure, potentially increasing thermal conductivity locally but hindering fluid flow if a heat transfer fluid is used, and making material handling difficult [71].
• Agglomeration and Slagging: More severe fusion leading to the formation of large, hard deposits (slag).
• Phase Changes: Minerals present may undergo phase transformations at specific temperatures, potentially associated with volume changes or alterations in thermal properties [72].
• Volatilization: Some components, particularly alkali chlorides, can volatilize at high temperatures, potentially leading to deposition and corrosion elsewhere in the system [73].
• Corrosion: Molten salt phases formed within the ash can be highly corrosive to metallic containment materials, heat exchangers, and other system components [74].

These stability issues, driven largely by the presence of alkali metals and chlorine, represent a major obstacle to using many types of raw biomass ash in high-temperature TES applications, potentially limiting the maximum safe operating temperature well below that desired for efficient CB power cycles.

To support the discussion on BA’s thermal properties,

Table 2. Comparison of thermal properties of biomass ash with traditional TES materials.

Material	Thermal Conductivity (W/(m·K))	Specific Heat Capacity (kJ/(kg·K))	Fusion/Max Operating Temperature (°C)
BA	0.239–0.404 [58]	0.8–1.2 [60]	700–800 [66]
Molten Salts	0.5–1.5 [10]	~1.5 [10]	>1000 [11]
Concrete	1.0–1.8 [11]	0.8–1.0 [11]	~600–800 [11]

compares its thermal conductivity, specific heat capacity, and AFTs with those of traditional TES materials, such as molten salts and concrete, based on available literature [10,11,58]. As shown, BA exhibits lower thermal conductivity (0.239–0.404 W/(m·K)) compared to molten salts (~0.5–1.5 W/(m·K)) and concrete (~1.0–1.8 W/(m·K)), which may limit heat transfer rates. Its specific heat capacity (0.8–1.2 kJ/(kg·K)) is comparable to concrete but lower than molten salts (~1.5 kJ/(kg·K)). Additionally, BA’s AFTs, often below 700–800 °C for certain types, is significantly lower than the operating temperatures of molten salts (>1000 °C), posing challenges for high-temperature CBs applications. These comparisons, also visualized in Figure 2, highlight BA’s limitations and underscore the need for experimental validation of specific BA types under CBs-relevant conditions.

To fully assess the suitability of BA as a TES material, long-term thermal cycling tests (>100 cycles) are critical to evaluate its durability and degradation behavior under operational conditions. Such tests would provide essential data on corrosion, sintering, agglomeration, and phase transitions driven by alkali metals (K, Na) and chlorine content, which are known to cause issues in high-temperature environments [65,66]. While initial assessments indicate challenges with low AFTs, specific degradation data from extended cycling are currently unavailable in this study. Literature on related materials, such as biomass-derived fly ash, suggests that prolonged exposure to cyclic temperatures can exacerbate these issues, necessitating rigorous experimental validation [49]. Future research should prioritize comprehensive thermal cycling experiments to quantify the long-term stability of BA and its interactions with containment materials and heat transfer fluids in CBs systems.

5. Potential of BA as TES Material in CB

5.1. Matching Ash Properties with TES Requirements

The successful integration of any material into a CBs’ TES system hinges on its ability to meet stringent performance, stability, and economic criteria [17]. As outlined previously, ideal TES materials should offer appropriate thermal properties (conductivity, heat capacity), excellent stability under cyclic thermal stress, low cost, widespread availability, and minimal environmental impact. BA, as a high-volume industrial byproduct, primarily attracts interest due to its potential low cost and availability. However, its suitability must be rigorously assessed by comparing its intrinsic properties, detailed in Section 3, against these demanding TES requirements.

5.2. Advantages

The primary driver for considering BA in TES applications is economic and environmental. Its potential advantages include:

• Low Cost: As a waste product from biomass combustion, BA often incurs disposal costs. Utilizing it as a TES material could potentially represent a negative cost feedstock, significantly lowering the capital expenditure associated with the energy capacity component (kWh) of a CBs, which is crucial for achieving cost-competitiveness in long-duration storage applications.
• High Availability: The growing bioenergy sector ensures a large and increasing supply of BA globally, suggesting good scalability if the technical challenges can be overcome.
• Waste Valorization: Using BA in TES aligns with circular economy principles, transforming a waste stream into a valuable component for renewable energy infrastructure, thereby avoiding landfilling and associated environmental impacts.

5.3. Challenges and Limitations

Despite the compelling cost and availability arguments, utilizing raw BA as a TES medium faces significant technical hurdles stemming directly from its inherent properties:

• Compositional Variability: The highly variable chemical and mineralogical composition of BA, dependent on feedstock and combustion conditions, is a major impediment [69]. This variability makes it difficult to predict and guarantee consistent thermal performance (conductivity, heat capacity) and, more critically, thermal stability across different batches or sources of ash. Reliable TES operation demands predictable material behavior [75].
• Low Thermal Conductivity: As discussed in Section 4.3.1, the effective thermal conductivity of bulk BA is expected to be relatively low, likely acting more as an insulator than a conductor, especially in porous packed-bed configurations [76]. This low conductivity can severely limit the rate of heat transfer during charging and discharging, potentially reducing the power density (MW rating relative to storage size) and responsiveness of the CBs system [65]. Overcoming this might require complex and potentially costly heat exchanger designs embedded within the storage medium.
• Poor Thermal Stability: Perhaps the most critical challenge is the often poor thermal stability of BA at the medium-to-high temperatures (frequently > 500–600 °C) targeted by many efficient CBs cycles [74,77]. The presence of alkali metals (K, Na), chlorine, sulfur, and phosphorus often leads to low AFTs, resulting in sintering, agglomeration, slagging, and the formation of corrosive molten phases. Sintering alters the bed structure, impedes heat transfer and potential fluid flow, while molten salts can aggressively corrode containment materials and heat exchangers, compromising system lifetime and safety. This instability may restrict the use of untreated BA to lower-temperature CBs applications (e.g., those using ORC cycles), which generally have lower power cycle efficiencies.
• Potential Environmental Concerns: While valorization avoids landfilling, the potential for leaching of heavy metals or soluble salts from the ash during operation or at end-of-life needs careful assessment to ensure no secondary environmental issues are created.

5.4. Potential Application Niches and Mitigation Strategies

Given the significant challenges, the direct use of untreated, unselected BA in high-temperature CBs appears problematic. However, potential niches or mitigation strategies might exist:

• Lower-Temperature Systems: BA might be more suitable for CBs operating at lower temperatures (e.g., <500 °C), where issues related to sintering and melting of alkali salts are less severe. This could align with systems using (ORCs for discharge, although these typically have lower efficiencies than high-temperature cycles.
• Composite TES Materials: BA could potentially be used as a low-cost filler material within a composite TES medium. For example, incorporating BA into a matrix of a more stable and conductive material (like concrete, ceramics, or graphite) might improve the overall properties, although this adds complexity and cost.
• Ash Selection: Not all biomass ashes are equally problematic. Ashes from certain feedstocks (e.g., specific wood types) combusted under controlled conditions might have lower concentrations of detrimental elements like K and Cl, resulting in higher AFTs and better stability. Careful selection and characterization of specific ash sources could identify more suitable candidates.
• Pre-treatment: Various pre-treatment methods could potentially improve BA properties. Washing or leaching can remove soluble alkali salts (like KCl), increasing the AFTs and reducing corrosion potential, although this adds processing steps, cost, and generates a potentially problematic liquid effluent. Thermal treatment or blending with additives (e.g., kaolin) are other possibilities explored in combustion contexts to manage ash behavior, which might be adaptable for TES preparation.

5.5. Thermal Conductivity Analysis

The issue of thermal conductivity warrants specific emphasis due to its direct impact on TES performance [78]. The likely low effective thermal conductivity of bulk BA poses a fundamental challenge to achieving rapid charge and discharge rates. In packed-bed TES systems, heat transfer typically occurs via conduction through the solid particles and contact points, convection via any heat transfer fluid flowing through the bed, and radiation at higher temperatures [79]. Low solid conductivity limits the conduction pathway. If a heat transfer fluid (like air or CO₂) is used, the heat transfer between the fluid and the solid particles becomes critical, and low solid conductivity can create significant temperature gradients within the particles, limiting the effective heat transfer rate [80].

The variability in composition further complicates conductivity analysis. Ashes rich in SiO₂, particularly in amorphous forms, are expected to have lower conductivity than ashes potentially richer in crystalline phases like CaO or MgO (though the latter are less common as dominant phases in many ashes) [52]. High porosity will invariably decrease the effective bulk conductivity regardless of the solid phase composition. Designing a TES system using low-conductivity BA would likely require strategies such as using smaller particle sizes (increasing surface area but potentially increasing pressure drop), incorporating highly conductive fins or embedded heat exchangers throughout the storage volume, or accepting lower charge/discharge rates (reducing power density).

Accurate data on the effective thermal conductivity of specific, relevant BA types (e.g., Portuguese woody BA) as a function of temperature, packing density, and composition is currently lacking and represents a critical knowledge gap for assessing feasibility.

5.6. Environmental Considerations of Using BA in CBs

The utilization of BA as a TES material in CBs, if confirmed its viable use, may offer significant environmental benefits, primarily through waste valorization and the promotion of a circular economy (

Table 3. Environmental benefits and challenges of using BA in CBs.

Aspect	Benefit	Challenge
Carbon Footprint	Reduces landfill disposal	Preprocessing energy impacts
Waste Valorization	Promotes circular economy	Compositional variability
Comparison with Molten Salts	Potentially lower environmental impact	Limited LCA data

). By repurposing BA, a byproduct of bioenergy production, CBs can reduce the environmental burden associated with ash disposal in landfills, which often incurs costs and potential ecological risks. However, a comprehensive assessment of the carbon footprint of BA—from biomass cultivation, ash production, preprocessing, and application in CBs—is essential to fully understand its environmental impact compared to traditional TES materials like molten salts.

Preliminary analysis suggest that BA can contribute to lower greenhouse gas emissions by avoiding landfill disposal and leveraging an existing waste stream. However, preprocessing steps, such as washing to remove alkali salts or thermal treatment to enhance stability, may introduce additional energy consumption and emissions. These factors must be quantified through a life cycle assessment (LCA) to evaluate the net environmental benefit of BA relative to molten salts, which require energy-intensive production processes but offer established stability and performance.

Future research should focus on conducting a detailed LCA to compare the environmental impacts of BA-based TES with conventional materials, considering all stages from raw material extraction to end-of-life management. Such an analysis will provide a holistic view of BA’s sustainability in CBs applications, strengthening the case for its use as a low-cost, environmentally friendly TES medium.

Figure 3 outlines the life cycle of BA, highlighting key stages from production to CBs application.

5.7. Economic Analysis of Using BA in CBs

The primary economic advantage of BA as a TES material lies in its low cost as a byproduct of bioenergy production, potentially reducing the LCOS for CBs compared to traditional materials like molten salts [12]. By repurposing BA, disposal costs can be avoided, further enhancing its economic appeal. However, preprocessing steps—such as screening to remove impurities, sintering to improve thermal stability, or blending with other materials—may introduce additional costs that could partially offset these advantages.

The economic feasibility of BA depends on factors such as the scale of operation, regional availability, and specific preprocessing requirements. For instance, washing to remove alkali salts may increase costs due to water and energy use, while thermal treatments could add energy-intensive steps [52]. Comprehensive techno-economic analyses are needed to quantify the total cost of ownership for CBs systems using BA, including CAPEX, operational costs, maintenance, and end-of-life management. Such studies should compare BA-based TES with conventional materials to determine whether the raw material cost advantage translates into overall economic benefits. Future research will focus on these analyses to validate BA’s cost-competitiveness in CBs applications. Figure 4 illustrates the performance metrics of CBs using BA compared to other storage technologies.

5.8. Comparison of BA with Other Low-Cost TES Materials

To evaluate the potential of BA as a TES material, it is useful to compare its properties with other low-cost TES media, such as rocks, waste ceramics, and industrial slag, in terms of thermal stability and economic performance.

Table 4. Comparison of BA with other low-cost TES materials in terms of thermal and economic properties.

Material	Thermal Conductivity (W/(m·K))	Specific Heat Capacity (kJ/(kg·K))	Max Operating Temperature (°C)	Cost (USD/ton)
Biomass Ash	0.239–0.404 [58]	0.8–1.2 [60]	700–800 [66]	~0–10 [52]
Rocks	1.5–3.0 [9]	0.8–1.0 [9]	>1000 [9]	10–50 [11]
Waste Ceramics	1.0–2.0 [11]	0.8–1.2 [11]	800–1200 [11]	50–100 [11]
Industrial Slag	0.5–1.5 [52]	0.8–1.2 [52]	>1000 [52]	20–80 [52]

summarizes these comparisons based on available literature [9,11,52].

• Rocks (e.g., basalt, granite): Rocks are widely used in packed-bed TES systems due to their high thermal stability (>1000 °C), moderate specific heat capacity (~0.8–1.0 kJ/(kg·K)), and low cost (~USD 10–50/ton). However, their thermal conductivity (~1.5–3.0 W/(m·K)) is higher than BA’s, facilitating better heat transfer but requiring robust containment systems.
• Waste Ceramics: Recycled ceramics, such as those from industrial processes, offer good thermal stability (~800–1200 °C) and moderate thermal conductivity (~1.0–2.0 W/(m·K)). Their cost is typically low (~USD 50–100/ton), but preprocessing (e.g., crushing) may increase expenses. Compared to BA, ceramics exhibit better stability but lack the waste valorization benefit.
• Industrial Slag: Slag from steel or other industrial processes has high thermal stability (>1000 °C) and moderate specific heat capacity (~0.8–1.2 kJ/(kg·K)). Its thermal conductivity (~0.5–1.5 W/(m·K)) is comparable to BA, but its cost (~USD 20–80/ton) varies by region and preprocessing needs. Like BA, slag supports waste valorization but may pose handling challenges due to heavy metal content.

BA’s primary advantage lies in its near-zero cost as a byproduct and its circular economy potential. However, its lower thermal stability (AFT ~700–800 °C) and low thermal conductivity (0.239–0.404 W/(m·K)) limit its applicability compared to these alternatives, particularly in high-temperature CBs. Future research should explore composite TES materials combining BA with more stable media to leverage its economic benefits while mitigating technical drawbacks.

5.9. Proposed TES Structural Designs for BA

To address the challenges of using BA as a TES material, such as low thermal conductivity and thermal stability, tailored TES system designs are required. This section proposes conceptual designs for sensible and latent heat storage systems incorporating BA, illustrated in Figure 5 and Figure 6, respectively.

• Sensible Heat TES Design: Figure 5 depicts a packed-bed TES system for sensible heat storage, where BA particles are contained in an insulated tank. Hot air or another heat transfer fluid (HTF) flows through the bed during charging, transferring heat to the BA, and reverses during discharge. To mitigate BA’s low thermal conductivity (~0.239–0.404 W/(m·K)), embedded heat exchanger tubes or conductive fins are proposed to enhance heat transfer. The design operates below BA’s ash fusion temperature (~700–800 °C) to prevent sintering, making it suitable for lower-temperature CBs, such as those using ORCs.

• Latent Heat TES Design: Figure 6 illustrates a latent heat TES system, where BA is blended with PCMs, such as sodium nitrate, to leverage both sensible and latent heat storage. The composite material is encapsulated in a modular tank with integrated heat exchanger pipes to improve heat transfer, addressing BA’s conductivity limitations. This design targets temperatures within BA’s stable range (200–600 °C), enhancing energy density compared to sensible heat systems.

These designs aim to capitalize on BA’s low cost while mitigating its technical limitations through optimized system configurations. Experimental validation and detailed modeling are needed to refine these concepts and ensure their feasibility in CB applications.

5.10. Impact of BA Properties on CBs Performance

The physical properties of BA, particularly its low thermal conductivity and limited thermal stability, significantly influence the operating performance of CBs, including RTE and response speed. This section provides approximate quantitative estimates based on BA’s common components (e.g., SiO₂, CaO, K₂O) and their impact on CBs performance, acknowledging the variability in BA composition [49,50].

• RTE: BA’s low thermal conductivity (0.239–0.404 W/(m·K)) reduces heat transfer rates within the TES system, potentially lowering RTE compared to materials like molten salts (~0.5–1.5 W/(m·K)). For a typical CBs with an RTE of 40–70% using conventional TES [41], BA could reduce RTE by approximately 5–10% due to increased thermal losses and slower heat transfer, particularly in packed-bed systems. The presence of SiO₂ (low conductivity) dominates this effect, while CaO may slightly improve conductivity but is less prevalent.
• Response Speed: The low thermal conductivity and moderate specific heat capacity (0.8–1.2 kJ/(kg·K)) of BA result in slower thermal response times, as heat penetration into BA particles is limited. This could increase charging/discharging times by 10–20% compared to higher-conductivity materials, affecting CBs’ ability to respond to rapid grid demands. K₂O and alkali salts may exacerbate this by causing sintering at temperatures near the AFT (~700–800 °C), altering bed structure and further impeding heat transfer.

These estimates highlight the trade-offs of using BA in CBs. Mitigation strategies, such as integrating conductive fins or blending BA with PCMs, could improve performance. Experimental studies focusing on specific BA compositions are needed to refine these estimates and optimize CBs designs.

5.11. Density and Container Size Considerations for Biomass Ash TES

The density of BA, ranging from 0.3 to 1.0 g/cm³ for fly ash to 0.8–2.0 g/cm³ for bottom ash [48], is generally lower than that of traditional TES materials like molten salts (~1.8–2.2 g/cm³) or concrete (~2.2–2.4 g/cm³) [11]. Combined with BA’s moderate specific heat capacity (0.8–1.2 kJ/(kg·K)), this lower density implies a larger storage volume is required to achieve the same energy storage capacity. For example, to store 1 MWh of thermal energy with a temperature differential of 400 °C, BA may require 1.5–2 times the volume of molten salts, depending on its density and packing efficiency.

The increased container volume could raise capital costs due to larger tanks and insulation requirements. However, BA’s near-zero material cost (~USD 0–10/ton) compared to molten salts (~USD 500–1000/ton) or concrete (~USD 50–100/ton) [11,52] may offset these expenses, particularly for long-duration storage where TES material costs dominate. Techno-economic analyses are needed to quantify this trade-off, considering factors such as container design, insulation, and regional BA availability. Optimizing BA’s packing density through pelletization or blending with denser materials could further reduce volume requirements, enhancing its economic viability.

5.12. Convective Heat Transfer Coefficients for Biomass Ash TES

The low thermal conductivity of biomass ash (BA, 0.239–0.404 W/(m·K)) necessitates enhanced heat exchanger designs to ensure efficient heat transfer in TES systems. This section estimates convective heat transfer coefficients for BA in a hypothetical packed-bed TES system using classical correlations, acknowledging the variability in BA’s properties [57].

Consider a packed bed of BA particles (average diameter ~1 mm, porosity ~0.4) with air as the HTF at 400 °C, flowing at a velocity of 0.5 m/s. Using the Wakao and Kaguei correlation for packed beds [57], the convective heat transfer coefficient (h) can be estimated as:

$$h={\displaystyle \frac{k_f}{d_p}}\bullet \left(2.0+1.1\bullet {Re}^{0.6}\bullet {Pr}^{1/3}\right)$$

(1)

where (k_f) is the fluid thermal conductivity (~0.04 W/(m·K) for air at 400 °C), (d_p) is the particle diameter (0.001 m), (Re) is the Reynolds number (~50 for the given conditions), and (Pr) is the Prandtl number (~0.7 for air). This yields (h approximately 100–150 W/(m²·K)), which is relatively low compared to denser materials like rocks (~200–300 W/(m²·K)) due to BA’s higher porosity and smaller particle size.

This estimate suggests that larger or more complex heat exchangers are needed to compensate for BA’s limited convective heat transfer, potentially increasing system costs. Strategies such as using smaller BA particles or higher HTF velocities could increase (h), but may raise pressure drops or energy consumption. Experimental measurements of BA-specific heat transfer coefficients under CBs-relevant conditions are recommended to refine these estimates.

6. Conclusions

CBs represent a diverse and rapidly developing field of energy storage technologies poised to address the critical need for cost-effective, long-duration storage solutions essential for integrating high penetrations of intermittent renewable energy sources into the grid. Encompassing concepts like PTES, LAES, and various configurations employing resistive heating or heat pumps coupled with different thermal storage media and power cycles, CBs offer potential advantages including geographical flexibility and the possibility of multi-vector energy provision (combined heat and power). Significant progress has been made in recent years, with several pilot and demonstration projects emerging globally. However, challenges remain, primarily concerning the need to improve RTE, reduce CAPEX, and demonstrate long-term reliability and durability at scale. Continued innovation in component design (turbomachinery, heat exchangers) and system integration is crucial for realizing the full potential of CBs in future decarbonized energy systems. BA, a high-volume byproduct of bioenergy production, presents an intriguing possibility as an ultra-low-cost TES medium, aligning with circular economy principles. Its composition is highly variable, strongly dependent on the biomass feedstock and combustion conditions, but typically consists of oxides and mineral phases involving Si, Ca, K, Al, Mg, P, Fe, Na, S, and Cl. While its low cost and availability are significant advantages, its properties present substantial challenges for TES applications, particularly at the medium-to-high temperatures relevant to many CBs concepts. Key drawbacks include: (i) inherent compositional variability, making performance prediction difficult; (ii) likely low effective thermal conductivity, especially in bulk form, potentially hindering efficient heat transfer; and (iii) poor thermal stability, characterized by low ash fusion temperatures, sintering, agglomeration, and potential corrosion issues, primarily driven by alkali metals and chlorine content.

Based on the analysis presented, the direct utilization of raw, unselected biomass ash as a primary TES medium in medium-to-high temperature CBs appears highly challenging. The critical issues of compositional variability, likely low thermal conductivity, and poor thermal stability (sintering, melting, corrosion) significantly limit its applicability in demanding, high-cycle operational environments. While the economic and waste valorization incentives are strong, the technical hurdles associated with material properties, particularly thermal stability and conductivity, currently outweigh the benefits for many CBs designs aiming for high efficiency (which often implies high temperatures).

The potential for BA might be more realistic in specific niches, such as lower-temperature CBs systems where stability issues are less pronounced, or potentially as a low-cost filler component within engineered composite TES materials designed to mitigate its drawbacks. Furthermore, careful selection of specific ash types with inherently better properties (e.g., low alkali/chlorine content) or the application of pre-treatment methods (like washing or thermal treatment) could improve suitability, but these strategies require further investigation regarding their effectiveness, cost-implications, and environmental impact.

To definitively ascertain the feasibility and potential scope for utilizing biomass ash in CBs TES, targeted research is necessary to address the current knowledge gaps. Key recommendations include:

• Characterization: Detailed experimental characterization of specific BA types relevant to potential deployment regions (e.g., Portuguese woody BA) is needed. This must include accurate measurement of effective thermal conductivity, specific heat capacity, density, and particle properties as a function of temperature, composition, and packing density under conditions relevant to CBs operation.
• Thermal Stability Assessment: Rigorous testing of long-term thermal cycling stability is crucial. This should involve evaluating sintering behavior, phase changes, potential reactions with containment materials or heat transfer fluids, and mechanical integrity over thousands of cycles at target operating temperatures.
• Pre-treatment and Modification Studies: Investigation into the technical feasibility, effectiveness, and economic viability of pre-treatment methods (e.g., washing, thermal conditioning, pelletization) or the development of composite materials incorporating BA to improve thermal conductivity and stability.
• System-Level Modeling and Analysis: Techno-economic modeling of complete CBs systems incorporating BA-based TES (considering its specific properties, including potentially low conductivity and temperature limitations) is required to assess the overall impact on performance (efficiency, power density) and LCOS compared to systems using conventional TES materials.
• Pilot-Scale Demonstration: If promising results emerge from laboratory-scale studies, pilot-scale testing of TES modules using selected or treated BA would be necessary to validate performance and durability under more realistic operating conditions.