Thermal Energy Storage for Sustainable Smart Agricultural Facilities: Design, Integration, Control, Environmental Impacts, and Future Perspectives

Abstract

Smart agricultural systems need stable thermal environments for greenhouses, livestock housing, and on-farm processing. However, renewable heat sources such as solar collectors and heat pumps often cause fluctuations that challenge reliable operation. Thermal energy storage (TES)—particularly water-based sensible tanks, stratified reservoirs, and phase-change material (PCM) systems—provides an effective solution by decoupling heat supply and demand. In this review, tank-based TES technologies for agricultural applications, focusing on design, integration with renewable energy systems, and control strategies, are critically examined. Key performance aspects, including thermal stratification, state-of-charge estimation, and advanced predictive control, are analyzed to identify best practices and limitations. The review finds that sensible TES remains dominant in farm applications due to its low cost and durability, while latent (PCM/ice) and thermochemical storage provide a higher energy density and long-duration potential but are presently limited by material stability, system complexity, and cost. From an environmental perspective, TES contributes to reducing fossil fuel dependence, improving resource efficiency, lowering greenhouse gas emissions, and boosting the resilience of rural farming systems. Overall, TES is recognized as a key enabling technology for climate-smart, energy-efficient, and sustainable agricultural operations. However, remaining research gaps include long-term field validation, standardized performance metrics, and life-cycle environmental assessment.

1. Introduction

Modern indoor and controlled-environment agricultural systems—including greenhouses, livestock housing, and post-harvest handling facilities—are becoming increasingly resource-intensive. Maintaining optimal thermal conditions for plant growth, animal welfare, and processing efficiency requires considerable and often constant heating or cooling. As farming systems move toward higher productivity and automation, their energy demand continues to rise, placing pressure on both operating costs and environmental performance. To address these increasing energy needs while meeting sustainability objectives, energy-efficient system designs that decouple thermal energy supply from demand are essential.

Tank-based thermal energy storage (TES) systems offer a practical and mature solution by enabling the storage of thermal energy during periods of surplus availability or low cost and its subsequent release when demand arises [1,2] (Figure 1). By decoupling energy supply from demand in time, these systems enable peak-load shaving, improved utilization of renewable heat sources, and more stable operation of heating and cooling equipment. TES technologies have been widely implemented in buildings and industrial applications to reduce energy consumption, improve system flexibility, and enhance overall energy efficiency [3,4,5,6]. Their adaptation to agricultural systems has therefore attracted growing attention in recent years.

TES systems can operate via sensible heat storage (by raising or lowering the temperature of a storage medium), latent heat storage (using phase-change materials, PCMs), or thermochemical storage based on reversible reactions. Each technique offers clear advantages and limitations in terms of energy density, operating temperature range, cost, and system complexity [7,8,9]. In agricultural applications, tank-based TES systems are especially attractive because they can buffer shifting thermal loads, integrate renewable or waste heat sources, and stabilize indoor climates under daily or seasonal variability. For instance, recent studies in smart greenhouse applications have shown that properly sized hot- or chilled-water TES tanks can minimize annual energy consumption by approximately 15% while improving temperature stability; however, challenges continue to restrict the widespread deployment of TES in smart farm systems. These include precise sizing of storage tanks under highly variable agricultural loads, maintenance of thermal stratification, lowering of heat losses, integration with farm-scale renewable energy systems, assessment of life-cycle costs under persistent charge–discharge cycling, and practical constraints linked to space availability, insulation quality, and rural infrastructure [10]. Addressing these challenges requires not only better materials and system designs but also advanced control strategies and standardized performance assessment techniques.

Beyond their technical role, TES systems should be evaluated within a broader sustainability context. Climate change is intensifying thermal stress on agricultural systems, increasing the cooling demand in warm regions while raising heating demands in colder climates [11]. Simultaneously, agriculture lies at the heart of the energy–food–water nexus, where energy consumption directly affects the cultivation efficiency, water consumption, and greenhouse gas emissions [12]. By allowing higher shares of renewable energy, reducing fossil fuel dependence, and improving thermal efficiency, TES systems contribute directly to climate-smart agriculture and support multiple United Nations Sustainable Development Goals (SDGs), including SDG 2 (Zero Hunger), SDG 7 (Affordable and Clean Energy), SDG 12 (Responsible Consumption and Production), and SDG 13 (Climate Action) [13,14,15,16]. In this sense, TES is not solely an energy technology but a sustainability enabler for resilient and resource-efficient food systems.

A substantial number of review studies have examined thermal energy storage (TES) technologies mainly in the contexts of buildings, district heating networks, and industrial energy systems, with a strong emphasis on storage materials, numerical modeling, and large-scale renewable energy integration [3,17,18]. In these reviews, agricultural applications are usually mentioned only in passing or treated as extensions of building energy systems, without a detailed analysis of the operating conditions specific to farm environments. In practice, agricultural facilities exhibit strongly variable and season-dependent thermal loads, biological sensitivity of crops and livestock, limited space and infrastructure, and operational priorities that differ fundamentally from those of residential or industrial systems. Consequently, key challenges—such as intermittent occupancy, seasonal shifts between heating- and cooling-dominated operation, animal welfare and crop microclimate requirements, and the integration of on-farm renewable energy sources—are insufficiently addressed in the existing TES review literature [19,20]. This review directly addresses these gaps by providing a focused and systematic analysis of tank-based TES technologies tailored to agricultural contexts, with an emphasis on farm-relevant thermal loads, system integration strategies, control approaches, and sustainability implications that are not comprehensively covered in prior TES reviews.

Against that background, this review focuses on tank-based TES systems for smart agricultural facilities. It comprehensively investigates storage basics, storage media, tank design and sizing methodologies, integration with farm heating and cooling systems, modeling approaches and key performance indicators, as well as economic and environmental considerations. By synthesizing the recent literature and identifying research gaps and future directions, this review aims to support the development and deployment of efficient, durable, and scalable TES solutions that enhance sustainability and resilience in modern agricultural systems.

2. Background/Fundamentals

Tank-based thermal energy storage (TES) systems operate by storing and releasing heat, and their successful application depends on a clear understanding of the fundamental principles behind these processes. This section builds that foundation by outlining three key areas: (i) the basic mechanisms of thermal storage—sensible, latent, and thermochemical; (ii) common tank designs and system configurations; and (iii) the materials and media typically utilized for storage. Although the emphasis is on systems designed for heating, many of the concepts discussed are equally relevant to cold-storage applications such as chilled-water tanks used in smart agricultural facilities.

2.1. Thermal Storage Basics: Sensible vs. Latent vs. Thermochemical

In TES systems, heat can be stored in three main ways: sensible heat, latent heat (via phase-change materials, PCMs), and thermochemical storage (also referred to as adsorption/chemisorption). Sensible-heat storage simply raises (or lowers) the temperature of a storage medium, harnessing its heat capacity. This mechanism is widespread and well-commercialized. For example, ordinary hot-water tanks using water as the medium remain the most common form of sensible heat storage [5,21].

Latent-heat storage exploits the energy absorbed or released during a phase change (solid ⇄ liquid, or other) of a material, providing a higher energy density per unit mass relative to purely sensible storage for the same temperature swing. PCMs therefore enable more compact storage solutions for moderate temperature ranges [22]. Thermochemical storage is less mature but offers very high storage densities by reversible chemical (or adsorption) reactions. Its application in farm-scale tank systems remains more specialized [23]. Another important phenomenon in tank systems is stratification (i.e., the layering of storage fluid by temperature/density gradients) and the thermocline development in charging/discharging cycles. Proper stratification enhances the usable capacity and helps maintain distinct temperature zones, and loss of stratification (through mixing or baffles) reduces performance [24].

2.2. Tank Concepts and Typologies

Figure 2 illustrates that several tank configurations are used in TES systems depending on the application, temperature range, and required performance. Some common typologies such as stratified tanks (layer-charge) use vertical tanks wherein the warmer, less dense fluid rests above colder, denser fluid. Charging and discharging ports are located at different heights to maintain the layered temperature profile, thereby improving heat extraction and minimizing mixing [25]. Mixed (or fully mixed) tanks allow fluid mixing and are simpler in construction, but sacrifice stratification benefits. They are easier to design but may yield lower effective temperature separation and storage efficiency [26]. Shell-and-tube or internal heat-exchanger tanks incorporate internal piping or coil arrangements within the tank to charge or extract heat via a secondary fluid loop, enabling decoupling of fluid circuits and ease of integration into HVAC or process systems [27]. In tanks using granular media (rocks, pebbles, ceramic bricks) or encapsulated modules, packed-bed configurations enable a storage medium to be charged by hot fluid or air and then discharged by fluid flow through the media. These are more common in high-temperature or industrial storage but can inform farm-scale designs [28]. Encapsulated PCM module tanks integrate phase-change materials encapsulated in modules (spheres, plates, containers) inside a tank or module arrangement. The PCM modules charge/discharge at near-isothermal conditions during the phase change, enabling compact storage with a smaller ΔT compared to sensible systems [29].

In designing a TES tank, key features include attaining good thermal stratification, minimizing heat losses (via insulation), ensuring compatibility of charge/discharge flow rates, and selecting the proper tank geometry and materials to support agricultural system integration.

2.3. Common Media: Water, Brine, Eutectic Salts, PCMs, Ice Slurry, Refrigerated Water + Antifreeze

Selecting the storage medium is a crucial decision for TES tanks in agricultural applications because the medium impacts the cost, energy density, temperature range, safety, maintenance, and integration. Water remains the most common sensible-heat medium due to its availability, high specific heat capacity (~4.18 kJ/kg·K), low cost, and safe handling. Hot-water tanks are widely used for heating applications including solar thermal, CHP, and building HVAC [5]. Brine/antifreeze solutions for moderate-temperature systems (including low-temperature heat pumps or chilled-water loops), brine (glycol–water mixtures), or antifreeze solutions enable lower-freezing-point operation and safety in colder environments. Eutectic salts/salt hydrates are used in latent storage—they have defined melting/freezing points and relatively high latent enthalpy, and they can be designed for specific temperature ranges. For example, eutectic salt hydrates are increasingly studied for medium-temperature TES [30]. PCMs (organic, inorganic, eutectic), as noted above, store latent heat during a phase change. They are also used in the encapsulated form inside tanks or modules. Key challenges include their low thermal conductivity, supercooling, phase separation, and cost [31]. Although ice slurry/refrigerated water + antifreeze are more commonly associated with cold energy storage (CES) applications, chilled-media tanks are also relevant for agricultural facilities with significant cooling demands, such as post-harvest storage, dairy processing, or animal cooling systems. Storage using chilled water or ice slurry enables temporal shifting of refrigeration compressor loads and exploitation of off-peak electricity. The underlying thermodynamic principles are analogous to those of thermal (heat) storage, but applied in the reverse temperature domain [32]. Thermochemical media or salt sorption beds may be used in niche applications, but their adoption in farm-scale tank systems remains limited [33].

For agricultural smart-farm applications, selecting a medium will depend on the farm’s thermal demand profile (heating vs. cooling), temperature range of the process, space constraints, integration with heat pumps or solar thermal, maintenance capabilities, and cost. Ensuring that the tank medium and typology can deliver the required temperature range with minimal losses and robust cycling is critical.

3. Methodology

This study systematically reviews the peer-reviewed literature related to thermal energy storage (TES) systems, with a particular emphasis on tank-based configurations and their applications in energy-efficient and sustainable agricultural systems. The review process was designed and reported in accordance with the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA 2020, Supplementary Materials) guidelines to ensure methodological transparency, reproducibility, and comprehensive coverage of the existing literature [34]. The primary objective was to identify, screen, and synthesize experimental, numerical, analytical, techno-economic, and review studies addressing sensible heat storage, latent heat storage using phase-change materials (PCMs), and ice-based thermal energy storage technologies relevant to livestock housing, greenhouses, food processing, dairy operations, and cold storage facilities.

The published literature was collected primarily from major scientific databases, including Scopus, Web of Science (Core Collection), ScienceDirect, IEEE Xplore, and Google Scholar, as well as publisher platforms such as MDPI and SpringerLink, following established best practices for conducting systematic reviews in energy and sustainability research [34,35,36,37]. The search strategy employed combinations of keywords and Boolean operators applied to titles, abstracts, and author-provided keywords [36,38]. The main search terms included “thermal energy storage”, “TES”, “stratified TES tank”, “sensible heat storage”, “latent heat storage”, “phase change material”, “PCM encapsulated tank”, “ice storage tank”, and “TES control strategies”, combined with application-oriented terms such as “agriculture”, “smart farming”, “livestock”, “greenhouse”, “food processing”, “dairy”, and “cold storage”. Only peer-reviewed journal articles published in English were considered, with no restriction on publication year, allowing both classical and recent studies to be included in order to provide a comprehensive overview of TES technologies, performance characteristics, and applications in agricultural energy systems [34,37].

The initial database search resulted in a total of 488 records. All retrieved references were imported into reference management software, where duplicate entries were identified and removed through a combination of automated and manual procedures, as recommended by the PRISMA guidelines [34,36]. The remaining records were subjected to a screening process based on titles and abstracts to evaluate their relevance to the scope of the review. Studies were excluded at this stage if they did not address TES technologies, focused exclusively on non-agricultural or non-agri-food applications, or lacked relevance to tank-based TES systems, in line with standard systematic review screening protocols [36,38].

Full-text versions of the remaining articles were subsequently assessed for eligibility using predefined inclusion and exclusion criteria [34,37]. Studies were included if they investigated sensible heat storage, latent heat storage using PCMs, or ice-based thermal energy storage systems implemented in tank configurations and demonstrated applicability to agricultural or agri-food energy systems. Experimental, numerical, analytical, techno-economic, and review studies were all considered eligible, consistent with previous systematic reviews in the fields of thermal energy storage and sustainable energy systems [37,38]. Exclusion criteria comprised non-tank storage concepts such as geological or borehole storage, conference proceedings, theses, technical reports, patents, non-English publications, and studies with insufficient technical or methodological detail [34,36].

The inclusion and exclusion criteria were defined to ensure that the selected studies were directly relevant to tank-based TES technologies under practical agricultural operating conditions. Non-tank storage concepts (e.g., borehole or geological storage) were excluded because their design, time scales, and spatial requirements differ fundamentally from farm-scale applications. Conference proceedings and non-peer-reviewed sources were excluded to ensure methodological rigor and reproducibility of the synthesized findings.

After full-text eligibility assessment, a total of 196 studies met all inclusion criteria and were selected for final synthesis. These studies formed the basis for qualitative and comparative analysis [34,37]. Most of the data and information included in this review were extracted directly from the selected publications, focusing on the TES technology type, storage medium, system configuration, energy density, operating temperature range, cycle life or thermal stability, cost information where available, and specific agricultural applications [37,38].

The qualitative synthesis was conducted by systematically grouping the selected studies according to the (i) TES storage principle (sensible, latent, ice-based), (ii) tank configuration and storage medium, (iii) agricultural application type (e.g., greenhouse, livestock housing, cold storage), and (iv) system integration and control strategy. Within each group, studies were comparatively analyzed to identify recurring design approaches, performance ranges, technological limitations, and reported sustainability outcomes, rather than merely summarizing individual results.

Potential sources of selection bias should be acknowledged. The review focused on peer-reviewed journal articles published in English and indexed in major scientific databases, which may have excluded relevant studies published in other languages or as technical reports. In addition, database-specific indexing practices and keyword selection may have influenced the visibility of certain publications. Nevertheless, the use of multiple complementary databases and broad search terms was intended to minimize such bias and ensure a comprehensive literature coverage.

The methodological quality of the included studies was assessed qualitatively by examining the clarity of system descriptions, appropriateness of experimental or modeling methodologies, transparency of assumptions and boundary conditions, and relevance to sustainable and smart agricultural energy applications, as commonly adopted in energy-focused systematic reviews [37,38]. This combined systematic and comprehensive approach ensured a thorough and up-to-date synthesis of TES research without restricting the review to a narrowly predefined set of studies. The overall study selection process, including identification, screening, eligibility assessment, and final inclusion, is summarized using a PRISMA flow diagram, which indicates that 488 records were initially identified and 196 studies were ultimately included in the systematic review [34] (Figure 3).

4. Technologies and Materials

Tank-based thermal energy storage (TES) systems can be broadly categorized by the storage mechanism and the materials or structures used. In this section, data reviewed include the major technology classes, like sensible heat, latent heat (PCMs), and thermochemical storage, and also highlight supporting design features such as internal heat-exchangers and thermal insulation, as shown in

Table 1. Comparison of tank-based thermal energy storage technologies relevant to energy-efficient and sustainable agricultural applications.

Ref.	Technology	Storage Medium	Energy Density (kWh/m3)	Cost Range (USD/ton)	Cycle Life/Cycle Stability (Cycles)	Typical Operating Temp. (°C)	Suitable Agricultural Applications
Liu et al. [39]	Stratified Sensible	Hot water (solar heated)	81.3	NR	NR	20–60	Heating, cooling, solar thermal
Lugolole et al. [40]	Stratified Sensible	Sunflower oil + water + small pebbles	25–26	900–2500	100	180–200	NR
Wang et al. [41]	Stratified Sensible	Hot water	17.4	NR	NR	60–75	Heating buildings
Dolgun et al. [42]	Stratified Sensible	Water (hot and cold)	46.47 (heating and cooling)	NR	10,000	0–40 (heating); −40–0 (cooling)	Seasonal heat load management
Kumar et al. [43]	PCM-Encapsulated (Latent)	Water + polyurethane foam	20–25	NR	NR	35–50	NR
Liang et al. [44]	PCM-Encapsulated (Latent)	Phase-change emulsion + water	45.5	NR	NR	5–40	Heat transfer and energy performance studies
Meng and Zhang [45]	PCM-Encapsulated (Latent)	Paraffin + water	43.6	NR	NR	54–64	NR
Castro-Vizcaíno et al. [46]	PCM-Encapsulated (Latent)	Monoethylene glycol + water + eutectic (mainly sodium carbonate)	NR	NR	NR	−40–100	Food preservation
Munir et al. [47,48]	PCM-Encapsulated (Latent)	Paraffin wax	63.56	NR	NR	69–71	Milk pasteurization
Bianco et al. [49]	PCM-Encapsulated (Latent)	Microencapsulated paraffin wax	11.1–14.4	NR	NR	5–40	Residential heating
Hunt et al. [50]	PCM-Encapsulated (Latent)	Paraffins (N-pentadecane, hexadecane, octadecane, N-hexadecane)	11.32	1500–4000	5000	10–18	Coastal/island cooling
Oró et al. [51]	PCM-Encapsulated (Latent)	Spherically encapsulated PCM (PK6) + water	23.25	NR	>2000	−2–13	NR
Hoseini Rahdar et al. [52]	PCM-Encapsulated (Latent)	PCM + ice + water–glycol (25% glycol)	42.8	2000–5000	2000–5000	−5–9	Exergetic, economic, environmental studies
Altuntas et al. [53]	Ice Storage	Ice + water + 30% ethylene glycol	NR	NR	NR	0–5	Commercial building cooling
Ezan et al. [54]	Ice Storage	Water + PCM + ethylene glycol–water (40% glycol)	85.2	NR	5000–20,000	−15–25	NR
Dincer [55]	Ice Storage	Ice + chilled water + eutectic salts	85.0	NR	~5000–20,000	0–5	Building heating, cooling, AC
Yan et al. [56]	Ice Storage	Ice + brine (35 wt% ethylene glycol) + water-based alumina nanofluid	85.2	NR	5000–20,000	−5–7	Cold storage performance evaluation

Ref. = references; NR = not reported. Reported energy densities depend on assumed temperature span and system configuration. Energy density, cost range, cycle life, and temperature range are reported for the primary storage medium in each TES system. When multiple materials are used, supporting materials contribute minimally to thermal storage and are therefore not included in these metrics.

. The discussion emphasizes pros and cons for each, with a view to applicability in smart agricultural facilities.

The comparison in Table 1 reveals clear trade-offs among sensible, PCM-based, and ice-based tank thermal energy storage technologies for agricultural applications. Sensible heat storage systems, typically based on hot or chilled water, exhibit lower to moderate energy densities (≈17–81 kWh/m³) but benefit from a simple design, operational robustness, and ease of maintenance, making them well suited to smallholder and resource-constrained farm environments. In contrast, PCM-based systems provide a higher energy density and improved temperature control, supporting applications such as greenhouse climate regulation, food preservation, and milk processing, although higher material costs and system complexity may limit their adoption in rural settings. Ice-based storage technologies offer the highest energy density (≈85 kWh/m³) and are particularly effective for cooling-dominant, industrial-scale operations such as cold storage and dairy chilling, but require sub-zero operation and more advanced control strategies. Overall, the results indicate that no single TES technology is universally optimal; rather, selection should be guided by the farm scale, dominant thermal demand, and trade-offs between energy density, cost, system complexity, durability, and long-term maintainability.

4.1. Sensible Heat Storage

Sensible heat storage (SHS) involves raising (or lowering) the temperature of a storage medium where the phase remains fixed. The stored thermal energy Q may be expressed as

Q = m c_p ΔT

(1)

where m is mass, c_p is the specific heat capacity, and ΔT is the temperature difference. SHS is the most mature and commercially adopted TES method [23].

Common media for SHS include water—often used for low-temperature (<100 °C) applications owing to the high c_p (~4.18 kJ/kg·K), low cost, and ease of handling [21]. Solids such as concrete, rocks, ceramic bricks are used for moderate to high temperature ranges and often in packed-bed or embedded heat-storage systems [57,58,59]. As shown in Figure 4, sensible heat storage tanks are relatively low-cost (especially water or natural rock/stone media) and have a simple design with well-understood operation, high reliability, good thermal conductivity in many cases (especially solid media), and wide availability of materials [60,61]. Sensible heat storage has a lower energy density per unit volume or mass compared to latent or thermochemical storage for a given temperature span. It requires a large ΔT to store significant energy (which may conflict with agricultural thermal constraints). In this system, stratification/mixing losses may reduce the effective capacity unless properly managed (especially in vertical tanks) [62]. In the case of concrete or rock-based media, construction complexity, weight, space, and structural support become considerations. For example, concrete-based SHS systems are promising but often at temperatures up to 500–600 °C in the framework [63].

For farm applications (e.g., space heating of barns or greenhouses), SHS with water tanks or moderate-temperature rock/brick beds may be quite suitable given the moderate temperature ranges (e.g., 20–60 °C) and lower cost sensitivity. Also, due to its low cost, simple maintenance, and use of non-toxic, food-compatible materials, sensible heat storage is well suited for farms, especially in greenhouses and livestock housing, where it supports renewable heat integration, minimizes fossil fuel dependence, and improves energy efficiency under rural operational constraints.

4.2. Latent Heat Storage (Phase-Change Materials, PCMs)

Figure 5 describes how latent heat storage systems store energy via the phase change of a material (typically solid ↔ liquid), while maintaining a near-constant temperature during the phase transition, offering a higher storage density for a given temperature range [5]. PCM types and examples include organic paraffins (e.g., C_nH_2n+2)—stable, non-corrosive, good cycling behavior, but have lower thermal conductivity and relatively low volumetric heat storage compared to salts [64]. Inorganic salt hydrates (e.g., CaCl₂·6H₂O, Na₂SO₄·10H₂O) and eutectic mixtures have a higher latent heat per unit volume and wider melting/freezing temperature ranges, but have issues such as phase separation, supercooling, corrosion, and cycle stability [65], as explained in Table 2. PCMs provide significantly higher stored energy per unit mass or volume compared to sensible media for a given ΔT, because of the latent-heat term L. For agricultural heating, the PCM transition temperature also must match the operational range of the heating loop (e.g., 40–60 °C). Mismatch reduces effectiveness. PCMs often need to be encapsulated (micro- or macro-encapsulation) to manage leakage, volume change, and corrosion and to improve thermal conductivity, as shown in Figure 6. For instance, microencapsulated paraffins were subjected to >5000 cycles with minimal degradation [64]. Many PCMs have low conductivity, which can limit charge/discharge rates—in practice, fins, heat pipes, or composite enhancement materials may be used [66,67,68,69].

Figure 5. PCM heating–cooling behavior and phase-change cycle. (a) PCM melting–solidification temperature profile. (b) PCM phase-change cycle (solid ↔ liquid).

Figure 5. PCM heating–cooling behavior and phase-change cycle. (a) PCM melting–solidification temperature profile. (b) PCM phase-change cycle (solid ↔ liquid).

Figure 6. PCM nano- and micro-encapsulation [70].

Figure 6. PCM nano- and micro-encapsulation [70].

Table 2. Phase-change material (PCM) classes and key thermophysical properties relevant to energy-efficient and sustainable thermal management in agricultural systems.

Ref.	PCM Class	Example Materials	Melting Temp. (°C)	Latent Heat (kJ/kg)	Thermal Conductivity (W·m−1·K−1)		Advantages	Drawbacks
					Solid	Liquid
Munir et al. [47]	Organic	Paraffin wax	69–71	260	0.2	0.2	Stable latent heat storage; suitable for agricultural heating	Low thermal conductivity; slow melting/solidification
Moradi et al. [71]	Organic	Paraffin capsules	61	213	0.4	0.15	Enables night-time heating; effective solar energy shifting	Output temperature fixed to PCM; complex system design
Balachandran et al. [72]	Organic	Paraffin wax	118	195	0.22	NR	Supports continuous drying under low solar radiation	Low conductivity; requires accurate IoT calibration
Poonia et al. [73]	Organic	Polyethylene glycol	17–23	NR	NR	NR	Maintains temperature up to 7 h after sunset; suitable for remote areas	Seasonal PCM switching required; higher system cost
Moon and Kim [74]	Inorganic	Sodium sulfate decahydrate	32.4	254	0.544	NR	Enables energy shifting and storage	Poor charging consistency; phase segregation
Batlles et al. [75]	Inorganic	Bischofite	58.2	116.9	NR	NR	Long-duration cooling; reduced storage volume	Limited discharge heat transfer efficiency
Tafone et al. [76]	Inorganic	KNO3–LiNO3	132	167.3	0.5	0.5	High TES density; improved heat-pump performance	High-temperature system uncertainty; design sensitivity
Lombardo et al. [77]	Inorganic salt hydrates	Magnesium sulfate heptahydrate	−5.81 to −3.87	227.0	0.620	NR	High latent heat; low supercooling; good cycling stability	Relatively high cost; low conductivity

Ref. = references; NR = not reported. Conductivity columns denote solid and liquid-phase thermal conductivity, respectively. Advantages and drawbacks are summarized based on performance in agricultural or low-temperature TES applications.

Table 2. Phase-change material (PCM) classes and key thermophysical properties relevant to energy-efficient and sustainable thermal management in agricultural systems.

Ref.	PCM Class	Example Materials	Melting Temp. (°C)	Latent Heat (kJ/kg)	Thermal Conductivity (W·m−1·K−1)		Advantages	Drawbacks
					Solid	Liquid
Munir et al. [47]	Organic	Paraffin wax	69–71	260	0.2	0.2	Stable latent heat storage; suitable for agricultural heating	Low thermal conductivity; slow melting/solidification
Moradi et al. [71]	Organic	Paraffin capsules	61	213	0.4	0.15	Enables night-time heating; effective solar energy shifting	Output temperature fixed to PCM; complex system design
Balachandran et al. [72]	Organic	Paraffin wax	118	195	0.22	NR	Supports continuous drying under low solar radiation	Low conductivity; requires accurate IoT calibration
Poonia et al. [73]	Organic	Polyethylene glycol	17–23	NR	NR	NR	Maintains temperature up to 7 h after sunset; suitable for remote areas	Seasonal PCM switching required; higher system cost
Moon and Kim [74]	Inorganic	Sodium sulfate decahydrate	32.4	254	0.544	NR	Enables energy shifting and storage	Poor charging consistency; phase segregation
Batlles et al. [75]	Inorganic	Bischofite	58.2	116.9	NR	NR	Long-duration cooling; reduced storage volume	Limited discharge heat transfer efficiency
Tafone et al. [76]	Inorganic	KNO3–LiNO3	132	167.3	0.5	0.5	High TES density; improved heat-pump performance	High-temperature system uncertainty; design sensitivity
Lombardo et al. [77]	Inorganic salt hydrates	Magnesium sulfate heptahydrate	−5.81 to −3.87	227.0	0.620	NR	High latent heat; low supercooling; good cycling stability	Relatively high cost; low conductivity

Ref. = references; NR = not reported. Conductivity columns denote solid and liquid-phase thermal conductivity, respectively. Advantages and drawbacks are summarized based on performance in agricultural or low-temperature TES applications.

The PCMs summarized in Table 2 demonstrate that latent thermal energy storage can be tailored to diverse agricultural applications through appropriate material selection. Organic PCMs, particularly paraffin-based materials, are generally well suited to smallholder and rural farming systems due to their chemical stability, non-corrosive behavior, and compatibility with food and livestock environments, although their relatively low thermal conductivity can limit charging and discharging rates. In contrast, inorganic and salt-hydrate PCMs offer a higher latent heat and energy density, making them attractive for space-constrained or large-scale applications such as controlled-environment agriculture, cold storage, and process heating. However, these materials often require careful system design to address challenges related to phase segregation, corrosion, and long-term durability, potentially increasing system complexity and maintenance requirements. Overall, the comparison highlights trade-offs between energy performance, cost, and operational robustness, indicating that PCM selection in agricultural systems should be guided by the farm scale, dominant thermal demand, and available technical resources.

Latent heat storage systems have a compact size for a given storage capacity and moderate ΔT, near the isothermal storage during phase change, which simplifies control and improves usable energy. These systems have a higher cost than basic sensible media (depending on material) with challenges of stability over many cycles (especially salt hydrates) and system integration (encapsulation, compatibility with fluids). The temperature range is limited to the melting/freezing point and latent heat, so selection is more constrained. It needs excellent heat transfer management to fully exploit the latent capacity [78].

PCM-based storage offers a higher energy density and better temperature stability, and its application in farm environments is constrained by a higher material cost, encapsulation durability, and maintenance complexity. Vigilant material selection and containment are needed to confirm the safety, leakage prevention, and compatibility with food and livestock operations.

4.3. Thermochemical Storage

Thermochemical energy storage (TCES) stores thermal energy via reversible chemical or sorption reactions (adsorption/absorption or chemisorption). During charging, heat drives the reaction (e.g., drying of a sorbent, or endothermic reaction), and during discharge, the reaction reverses, releasing heat [79,80] (Figure 7). TCES offers very high theoretical energy densities, lower heat losses (since energy is stored in chemical bonds), and long-term (seasonal) storage potential. However, this system remains at an earlier technology readiness level compared to sensible and latent storage. For farm applications, this storage system may be more suited for seasonal storage of waste heat or solar heat, rather than daily cycling, unless the system is specifically engineered for it [23,81]. In

Table 3. Thermochemical energy storage (TCES) materials, reaction mechanisms, and operating conditions relevant to long-duration and sustainable thermal energy storage.

Ref.	Material/System	Mechanism	Reaction Type	Operating Temperature (°C)	Notes
Solé et al. [82]	MgCl2·6H2O ⇌ MgCl2·H2O + 5H2O	Dehydration–hydration	Reversible solid–gas chemisorption	150 (charging)/30–50 (discharging)	Promising performance; diffusion limitations and partial irreversibility reported
Wang et al. [83]	Shell-and-tube-based TCES reactor	Gas–solid thermochemical heat release	Exothermic adsorption reaction	23–38	Heat exchanger-based configuration; performance depends on geometry and material packing
Wu and Long [84]	Carbonate system (CaO/CaCO3)	Endothermic decomposition and exothermic recombination	Reversible solid–gas reaction (calcination ↔ carbonation)	700–1000	High-temperature storage with high energy density; challenges include low efficiency and poor reversibility
Wu and Long [84]	Metal hydrides	Hydrogen absorption–desorption cycle	Reversible solid–gas reaction involving H2	250–500	Promising high-temperature TCES materials with high chemical heat storage density
Liu et al. [85]	NH3 ⇌ N2 + H2	Ammonia synthesis and decomposition	Endothermic (decomposition)/exothermic (synthesis)	350–650	Excellent reversibility; automatic gas separation; abundant and cost-effective reactants
Han et al. [86]	Zeolite 13X	Dehydration–hydration	Physical adsorption/desorption	180 (charging)/25–30 (discharging)	High sorption capacity; fast heat release and good cycling stability

Ref. = references. Operating temperatures correspond to charging (endothermic) and discharging (exothermic) modes, where applicable. “Promising” reflects performance trends reported in the referenced studies, including reversibility, storage density, and material stability.

, some thermochemical storage materials and reaction mechanisms, reaction types, and operating conditions are presented.

TCES offers very high volumetric and gravimetric storage densities—often cited as several times higher than latent or sensible thermal storage systems—and the potential for minimal standby losses when well-insulated, making it suitable for long-term storage (days to weeks or even months) [87,88,89]. Despite these advantages, these systems face significant challenges, including a more complex system design (reactor beds, sorbent handling, fluid/heat exchange), high capital cost and lower commercial maturity compared to sensible/latent options, and unresolved issues such as heat and mass transfer limitations, reaction kinetics, sorbent degradation or attrition, and durability under repeated cycling [87,90]. Although thermochemical storage provides a high energy density and long-term storage potential, its current complexity and lower technology readiness limit near-term adoption in typical farm settings. Such systems may be better suited for centralized or pilot-scale agricultural facilities with technical support rather than small or medium-sized farms. A comparison of storage mechanisms and system parameters of sensible storage, latent (PCM) storage, and thermochemical storage (TCES) is given in

Table 4. Comparison of thermal energy storage mechanisms and system parameters.

Parameter	Sensible Storage	Latent (PCM) Storage	Thermochemical Storage (TCES)
1. Storage Mechanism	Temperature change (ΔT) of material [32,91]	Phase change (solid ⇌ liquid) [92,93]	Reversible chemical/sorption reaction [79,87]
2. Typical Energy Density (kWh/m3)	20–100 [91,94]	50–200 (depending on PCM) [92,93]	200–1000 [89]
3. Operating Temperature Range	0–400 °C [32]	−10 to 200 °C [92]	40–600 °C [81]
4. Heat Loss During Storage	High [91]	Medium [92]	Very low [79]
5. Storage Duration	Hours–days [90]	Hours–days [90]	Weeks–months [88,90]
6. Charging/Discharging Rate	Fast (convective limits) [32]	Moderate–fast (depends on encapsulation) [93]	Limited by reaction kinetics and mass transfer [80]
7. Round-trip Efficiency	40–90% [32]	70–95% [91]	30–90% (depending on reaction) [81]
8. Technology Readiness Level (TRL)	High (commercial) [90]	Medium–High (commercial PCMs available) [90]	Low–Medium (pilot stage) [90]
9. Material/Equipment Cost	Low [79]	Medium [79]	High [79]
10. Key Challenges	Large storage volume [95]	Leakage risk, cycling durability, material stability [93]	Kinetics, degradation, complex design [87,90]
11. Suitable Applications	Domestic heating, hot water [23]	HVAC, greenhouses [23]	Seasonal storage, solar heat [23]

.

Table 4 indicates that sensible and latent TES technologies are presently the most practical options for agricultural applications, as they combine a high technological maturity, relatively low cost, and straightforward operation. However, these systems are mainly suited for short-term thermal buffering rather than long-duration storage. In contrast, thermochemical energy storage provides a substantially higher energy density and the ability to store heat over extended periods with minimal losses, but its technical complexity and lower maturity currently restrict its use in typical farm settings. This comparison reveals an important gap in the existing TES literature: limited application-oriented guidance that connects storage mechanisms with the specific thermal demands, operational constraints, and sustainability goals of agricultural systems—an issue that this review explicitly seeks to address.

4.4. Heat Exchangers, Internal Baffling, and Stratification

Tank performance is strongly influenced by internal design features, particularly heat-exchanger placement, fluid port configurations, baffling to promote or maintain stratification, and minimization of mixing. For example, stratified tanks maintain a temperature gradient (hot at top, cold at bottom), enabling higher exergy extraction by locating the supply/return appropriately, while mixed tanks see full mixing and thus lower effective ΔT [3]. Internal heat-exchanger coils or tubes allow separate circuits (e.g., heat pump loop vs. storage loop) and facilitate decoupled charging/discharging; however, comparative modeling shows that internal exchangers may degrade stratification relative to external ones, reducing charging efficiency [26,96]. Baffles or diffuser plates reduce fluid momentum and mixing, preserving stratification—studies on diffuser design show that an optimized diffuser geometry significantly improves stratification and reduces thermocline thickness [97]. In an agricultural context, where space may be limited and temperature swings moderate, maintaining stratification and efficient charging/discharging is crucial to maximize storage utilization and avoid unnecessary losses [98].

4.5. Thermal Insulation and Losses

Even the best-designed thermal energy storage (TES) tank will suffer heat losses to the ambient unless it is properly insulated. Key factors influencing these losses include

• Standing losses (or standing loss rate)—that is, the heat lost over time while the tank is idle. This is especially important in agricultural settings, where the tank may stand idle longer (e.g., overnight) and thus lose stored energy [3].
• Insulation quality, tank orientation, ambient temperature, and heat-exchanger/piping insulation—the insulation’s thickness and thermal conductivity and the tank’s exposure to ambient conditions, which strongly affect overall losses [99].
• Impact of losses—these losses reduce the usable stored energy, lower the round-trip efficiency of the TES system, and may erode its economic viability. In the context of hot-water storage, standing loss is the amount of energy lost through the tank walls and piping to the ambient [100].

In arrayed farm-scale systems, where the storage tank might be installed in an unconditioned barn or shed, accounting for these insulation and loss effects is particularly important. High-quality insulation, minimizing exposure to large ambient temperature swings, and careful design of the tank’s positioning will reduce heat loss and enhance overall system performance.

5. Tank Design and Sizing for Farm Applications

Designing and sizing tank-based thermal energy storage (TES) for agricultural facilities requires matching the storage capacity and power to highly variable farm loads, balancing cost/footprint constraints, and ensuring reliable integration with HVAC, heat pumps, refrigeration, and renewable sources.

5.1. Sizing Methods: Load Profiling, Degree-Hours, and Hourly Demand Curves

TES sizing typically begins with a load-profiling exercise to quantify the hourly (or sub-hourly) heating demand over representative days or seasons. Common practical approaches include

• Hourly demand curve integration (most accurate): integrate the farm’s measured or modeled hourly heat demand over the sizing period to produce the required energy (kWh) and peak power (kW). Use of full hourly profiles supports simulation-based sizing and optimization [101].
• Degree-hours (or degree-days) methods: for simpler estimates, degree-hour approaches translate a temperature-based demand index into energy requirements and are helpful in early design stages or for rule-of-thumb sizing [102].
• Load profiling with statistical/representative days: for seasonal crops or livestock cycles, choose representative daily/weekly profiles (peak/off-peak) to size for typical and extreme conditions (common in biomass-TES and district heating sizing studies) [41].

5.2. Rules of Thumb vs. Simulation-Based Sizing

Rules of thumb (simple ratios, capacity expressed as 4–8 h of plant load) are useful early in concept development and for quick economic screening, but they risk under/over-sizing when farm loads are highly variable. Industry practice still uses many heuristic rules for preliminary sizing [103]. Simulation-based sizing (dynamic system simulation using TRNSYS v18.00.001 (Thermal Energy System Specialists, USA); Modelica (OpenModelica v1.23.0, Open Source Modelica Consortium); EnergyPlus v9.6.0, or in-house tools) uses the full temporal load profile, equipment COP curves, and control strategies to find the optimal tank volume and power ratings. These methods produce more robust designs and enable techno-economic optimization (CapEx vs. OpEx vs. emissions) [104]. Accurate TES sizing is critical for sustainable farm energy systems, as undersized storage limits renewable energy utilization while oversized tanks increase material use, embodied emissions, and capital cost. Simulation-based sizing supports sustainability goals by minimizing life-cycle environmental impacts, improving resource efficiency, and reducing the reliance on fossil-fuel backup systems, thereby enhancing the long-term resilience of agricultural operations under variable climate and energy conditions. Figure 8 depicts the TES sizing workflow and rules-of-thumb vs. simulation-based sizing.

5.3. Fundamental Sizing Equations and Sample Calculation

For phase-change materials (PCMs), the stored energy includes latent heat:

Q = m L + m c_p ΔT

(2)

where L is the latent heat of fusion (kJ·kg⁻¹) [105].

Worked example (sensible water tank): Suppose a greenhouse needs 200 kWh_th of thermal energy per day to cover night heating (typical example for moderate-size greenhouse systems).

Compute the water mass needed if we design a tank with ΔT = 40 K and use c_p = 4.18 kJ·kg⁻¹·K⁻¹.

Step-by-step:

• Convert energy to kJ: 200 kWh × 3600 kJ/kWh = 720,000 kJ.
• Rearranged: m = Q/(c_p ΔT) = 720,000/(4.18 × 40).
• Compute denominator: 4.18 × 40 = 167.2.
• Mass: m = 720,000/167.2 ≈ 4306.22 kg (4.3 tons).
• Volume (water): V ≈ 4.31 m³ (since 1 kg water ≈ 1 L).

This simple template illustrates how design ΔT has a large effect on the required tank volume; increasing ΔT reduces mass/volume but may be constrained by system temperatures and component limits. Use hourly simulation to refine such estimates and to size associated pumps and heat-exchanger power [106].

5.4. Stratification Management (Diffusers, Baffles, Volume Segmentation)

Effective exploitation of the usable capacity relies on preserving thermal stratification (hot top, cold bottom) so that the temperature difference is available at higher exergy. Practical measures include inlet diffusers and ports at different heights to introduce charge/discharge flows gently and reduce momentum-driven mixing; an optimized diffuser geometry (radial, nonequal-diameter, elbow-type) has been shown experimentally and numerically to improve stratification and reduce the thermocline thickness [107,108]. Similarly, baffles and flow straighteners (plates or perforated diffusers) can dissipate inlet kinetic energy and preserve layering; small-scale experiments and CFD studies demonstrate significant stratification gains with appropriately designed baffles [109,110]. In volume segmentation and multi-tank banks, using multiple smaller tanks or internal segmented volumes (e.g., cascade of tanks) can retain stratification more easily than a single large mixed tank and allow staged charging/discharging strategies (Figure 9). This is commonly practiced in district heating and biomass-TES designs [102].

Designers should specify inlet velocities, diffuser discharge patterns, and port heights during early design and validate the stratification with transient simulation (CFD or simplified layered models) since stratification directly affects the retrieved usable energy and system COP [104]. Maintaining stratification improves the storage efficiency, reduces energy losses, and supports low-emission, resource-efficient thermal management in agricultural facilities.

5.5. Integration with Building Envelope, HVAC, Heat Pumps, Refrigeration Systems, and Renewables

Thermal energy storage (TES) must be sized and controlled within a whole-system design framework to maximize the efficiency and economic performance in agricultural applications. When integrated with heat pumps and solar thermal collectors, TES can store surplus heat during high-generation periods and enable time-shifting of heat-pump operation to off-peak hours, improve the overall COP by stabilizing compressor operation and aligning source temperatures with demand profiles. Several integrated design studies and pilot greenhouse systems highlight these benefits [111]. In systems using biomass boilers or combined heat and power (CHP), TES helps the plant operate near its optimal load while the storage tank absorbs short-term fluctuations and supports peak shaving, a trend commonly reported in biomass-TES sizing research [41]. Farms with simultaneous heating and cooling needs can further enhance system-level efficiency by coupling TES with refrigeration processes—for example, using waste heat recovery to charge storage or deploying coordinated hybrid TES–heat pump configurations—though such systems require carefully synchronized control to avoid operational conflicts and maximize economic value [111]. Because control strategies (rule-based, predictive, and model-predictive control) strongly influence both the required storage capacity and dispatch performance, TES sizing and control should be co-optimized using dynamic, system-level simulation approaches [103].

5.6. Physical Constraints: Footprint, Weight, Placement, Insulation, Frost Protection

Farm environments impose several practical constraints that significantly influence the selection, placement, and design of thermal storage tanks. Structural considerations are particularly important, as solid-media or concrete tanks impose substantial loads that require an adequate soil-bearing capacity and strong foundations, whereas water-based tanks occupy a larger volume but involve far fewer structural challenges [112]. The decision to place tanks indoors or outdoors also affects system performance. Outdoor installations demand higher levels of insulation, weather protection, and in some cases, frost mitigation while indoor placement minimizes standing thermal losses but competes with valuable operational space within barns or greenhouses [113]. In systems operating at low temperatures, frost protection becomes compulsory. Chilled loops or cold circuits may require antifreeze brines, glycol solutions, or mechanical freeze-protection measures to prevent damage to pipes and tanks exposed to harsh outdoor conditions [114].

6. Integration of Thermal Energy Storage in Smart Farm Systems

The integration of thermal energy storage (TES) into smart farm infrastructures enables the coordinated control of heating, cooling, and energy use, thereby improving efficiency, stability, and operational flexibility. In smart agricultural settings, TES systems are typically managed through multi-layer control architectures. At the supervisory level, an energy management system (EMS) oversees global optimization—evaluating price signals, weather forecasts, and farm operation schedules—while local controllers regulate pumps, valves, and heat exchangers to maintain desired temperatures in TES units such as hot-water tanks, chilled-water tanks, or phase-change material (PCM) modules [92,115]. IoT-based sensing networks support this hierarchy by continuously monitoring temperature profiles, flow rates, and the state of charge (SOC) of storage tanks, enabling data-driven and adaptive operation [116,117].

TES charging and discharging in smart farms can follow multiple control strategies. Rule-based strategies (e.g., time-of-day charging) store heat during off-peak hours or when renewable energy is in access. More advanced predictive strategies incorporate solar radiation forecasts, ambient temperature predictions, and energy price projections to schedule the optimal charging of thermal tanks [118]. Model predictive control (MPC) has gained particular attention due to its ability to handle system constraints and anticipate future thermal demands, providing an improved performance in dynamic agricultural environments such as greenhouses and dairy farms [119].

In terms of demand response, TES enables smart farms to shift thermal loads away from high-tariff periods by preheating or precooling storage tanks, thereby reducing electricity costs and mitigating peak loads on rural grids. Applications include greenhouse heating, where thermal energy stored during the daytime (via solar collectors or heat pumps) is discharged at night to maintain stable temperatures [120]; produce storage, where chillers operate during low-cost periods to charge cold-storage tanks [121]; and milk cooling, where TES buffers the sharp cooling loads immediately after milking [118]. Additionally, TES facilitates combined heating and cooling arrangements—for example, recovering waste heat from refrigeration systems and storing it for domestic or process heating needs [120].

Successful TES integration requires appropriate interface components, including pumps for circulation, valves for flow control, and heat exchangers for transferring charge between the TES medium and farm equipment. Accurate sensing of thermal parameters ensures reliable SOC estimation, prevents overcharging or thermal losses, and supports automated decision-making [116,117]. Overall, TES serves as a key enabler of flexible, efficient, and renewable-integrated smart farming systems. By decoupling thermal supply from demand, TES improves the part-load efficiency of farm energy systems and reduces the peak electrical demand, which is particularly critical for emission reduction and grid stability in rural agricultural contexts.

7. Performance Modeling and Simulation of Thermal Energy Storage Systems

Performance analysis of thermal energy storage (TES) systems in agricultural and smart-farm environments relies heavily on robust modeling and simulation frameworks. A variety of modeling approaches are used depending on the required level of detail and computational complexity. Simplified 0D and 1D tank models are widely applied to estimate charging–discharging behavior, temperature evolution, and energy balances under varying load conditions [32]. More detailed analyses employ computational fluid dynamics (CFD) to capture stratification, flow mixing, and buoyancy-driven convection inside hot-water or chilled-water tanks, offering higher accuracy for system optimization [122]. Intermediate-complexity methods such as the lumped capacitance approach and resistor–capacitor (RC) thermal network models provide a balance between resolution and computational efficiency, making them suitable for dynamic control studies in smart farms [123]. In more integrated assessments, co-simulation environments that couple buildings, greenhouses, or farm facilities with TES systems allow researchers to evaluate interactions between storage, HVAC operation, and environmental conditions [124].

A wide range of software tools is available for TES modeling. TRNSYS and Modelica are commonly used for detailed dynamic simulations of thermal storage tanks and integrated energy systems, while EnergyPlus is often applied in agricultural building studies that incorporate TES-enhanced HVAC operation [125]. MATLAB (R2023a, MathWorks Inc., Natick, MA, USA)/Simulink remains a standard tool for control-oriented modeling, MPC design, and system-level optimization [126].

To evaluate system performance, researchers rely on several key performance indicators (KPIs). As shown in

Table 5. Key performance indicators (KPIs) for thermal energy storage (TES) tanks and corresponding measurement methods.

KPI	Definition/Purpose	Measurement Method	Ref.
1. Energy Density (kWh/m3)	Amount of energy stored per unit volume of storage medium.	Calculate from measured ΔT (sensible) or latent heat and density (latent). Use tank geometry for volume.	[127]
2. Charge/Discharge Power (kW)	Rate at which TES is charged or discharged; indicates response capability.	Real-time measurement of mass flow, inlet/outlet temperatures: Q = m cp (Tin − Tout) or latent heat rate.	[128]
3. Round-Trip Efficiency (%)	Ratio of delivered (discharged) energy to stored (charged) energy; indicates losses.	Monitor total input and output energies across a full cycle; use metering of heat flows and electrical input.	[127]
4. Storage Capacity (kWh)	Total energy that the storage system can hold.	Integrate heat transfer over charging phase; from cumulative heat curves or measurement of mass and ΔT or phase change.	[129]
5. State of Charge (SOC)	Percentage (%) of energy stored relative to maximum; useful for control and monitoring.	Monitor via temperature profiling (multiple thermocouples), interface front sensors (PCM/ice), or stratification measurement cards.	[127]
6. Stratification Index/Thermal Stratification Degree	Metric of layering quality in sensible TES tank (hot vs. cold zones); better stratification ⇒ better performance.	Place vertical array of thermocouples, calculate temperature variance or stratification index.	[130]
7. Heat Loss Rate (W or W/m2K)	Rate of thermal energy leakage from the TES system; impacts standby losses.	Monitor temperature decay over time with no load; calculate U-value from ambient conditions or use insulated tank test.	[129]
8. Phase-Change Completion/Solid Fraction (%)	For latent TES (PCM, ice): % of material melted or frozen; indicates usable storage.	Use interface sensors, electrical conductivity of material (ice/water), ultrasonic/optical sensors, weight change.	[131]
9. Melting/Freezing Time (h)	Time required for storage to reach full charge or discharge state; important for sizing and response.	Monitor temperature plateau, interface movement, mass or energy change over time.	[132]
10. Cycle Life (# cycles)	Number of charge/discharge cycles the system can operate before significant degradation; impacts lifetime cost.	Long-term cycling tests; track changes in thermal capacity, interface damage, fouling, mechanical wear.	[127]
11. Exergy Efficiency (%)	Quality of the stored and released energy relative to ambient; important for system integration performance.	Calculate using exergy formulas from mass flow, temperature measurements of HTF and ambient.	[133]
12. Internal Heat Transfer Coefficient	Describes heat transfer inside tank.	Nusselt correlations or curve-fitting helped to measure heat flux data.	[134,135]
13. Supercooling Degree (°C)	Temperature below freezing before nucleation.	Minimum water temperature measured before freezing starts (water/ice systems).	[136,137,138]
14. Reaction conversion rate	Fraction of the thermochemical material that has reacted during charging or discharging.	Determined from mass change using TGA/DSC or in situ measurements of reacted mass or released gas.	[139,140]
15. Sorption capacity	Amount of water (or sorbate) the material can adsorb/desorb per unit mass; determines energy density in sorption TES.	Measured from adsorption isotherms using gravimetric vapor sorption or DVS/TGA tests under controlled humidity and temperature.	[139,141]
16. CO2 Emissions Avoided (kg CO2/kWhth)	Reduction in greenhouse gas emissions due to TES-enabled load shifting or renewable heat utilization.	Compare baseline fossil-based heating/cooling emissions with TES-integrated operation using emission factors.	[17,142,143]
17. Energy Payback Time (years)	Time required for TES system to offset the embodied energy of materials and installation.	Ratio of embodied energy to annual energy savings from TES operation.	[32,51,144,145]

Ref. = references. All KPIs apply to every TES technology; applicability depends on storage mechanism and system design. Cycle Life (# cycles) = Cycle life measured as the number of charge–discharge cycles (or operation cycles) until failure or degradation.

, these include the round-trip efficiency, which measures the total recoverable energy; delivered temperature stability, important for maintaining greenhouse or storage-room climates; and the coefficient of performance (COP) impact when TES is coupled with heat pumps or chillers [118]. Additional KPIs such as storage utilization, energy savings, peak load reduction, and economic indicators like the payback time or life-cycle cost help quantify operational and financial benefits in smart-farm settings [95].

Validation of TES models is typically performed using either experimental testbeds or numerical verification. Experimental validation involves controlled laboratory-scale tanks, PCM modules, or pilot-scale farm installations to measure temperature distribution, heat losses, and charging–discharging performance [146]. Numerical validation compares model predictions with high-fidelity CFD simulations or benchmark datasets to ensure stability, accuracy, and reliability across different operating conditions [122].

In terms of sustainability, these approaches enable the design of TES systems that minimize thermal losses, reduce oversizing, and improve overall system efficiency across varying agricultural load conditions. By supporting optimized control strategies and technology selection, these tools contribute to lower energy consumption, reduced operational emissions, and improved resilience of farm energy systems under climate and market variability.

8. Control and Sensing in Smart Thermal Energy Storage Systems

Advanced control and sensing technologies play a critical role in enabling the smart operation of thermal energy storage (TES) systems in agriculture. One of the most important functions is state-of-charge (SOC) estimation, which reflects the amount of usable thermal energy remaining in hot-water or chilled-water tanks. Several SOC estimation approaches have been reported, ranging from temperature-stratification-based methods, which infer available energy by measuring vertical temperature gradients inside the tank [147], to more advanced energy- and entropy-based estimators that model thermodynamic properties to improve accuracy under dynamic charging–discharging conditions [148]. Accurate SOC estimation is essential for predictive control, load shifting, and optimal coordination with heat pumps, chillers, or on-site renewable energy sources. Figure 10 illustrates how sensible, PCM, and ice TES systems respond during charging and discharging in terms of the temperature and state of charge.

Effective SOC monitoring also depends on sensor placement strategies. Proper placement of temperature sensors along the height of a storage tank enables the precise detection of thermocline movement and thermal stratification, which strongly influences the heat recovery efficiency [149]. Inadequate sensor spacing or poor vertical coverage can lead to errors in SOC estimation, particularly when stratification weakens due to mixing or high mass-flow rates. Recent studies have explored optimal sensor configurations using sensitivity analysis and model-based optimization to balance accuracy and cost [147].

Smart TES installations also integrate fault detection and diagnostics (FDD) to ensure a reliable operation. Common faults include tank leakage, loss of stratification due to mixing or sensor degradation, and pump or valve failures that affect charging cycles. Data-driven and model-based FDD methods have been applied to thermal storage systems to identify abnormal temperature patterns, unexpected flow conditions, or deviations from predicted SOC trajectories [150]. Such diagnostic tools are particularly valuable in remote farm settings where continuous human monitoring is difficult.

To support integrated control, TES systems in smart farms rely on communication standards and secure data exchange within the farm’s energy management system (EMS). Protocols such as Modbus, BACnet, and MQTT are commonly used for sensor communication and real-time monitoring [151]. As farms become increasingly digitalized, cybersecurity has become an essential consideration, with risks related to unauthorized data access, manipulation of control signals, and disruption of automation processes. Best practices include encrypted data communication, secure authentication, and regular network auditing to safeguard EMS operations [152].

Overall, the combination of accurate sensing, smart control algorithms, fault diagnostics, and secure communication forms the cornerstone of genuine TES integration in next-generation smart farming systems. From an environmental standpoint, advanced sensing and control of TES systems reduce energy waste by preventing overcharging, unnecessary heat losses, and inefficient cycling. Reliable SOC estimation and fault diagnostics improve the system lifetime and operational stability, lowering material replacement needs and embodied emissions. In digitally managed smart farms, secure and adaptive TES control supports higher renewable energy utilization while enhancing resilience in remote and resource-constrained agricultural settings.

9. Economic and Environmental Analysis

9.1. CapEx and OpEx Drivers

The economic performance of thermal and cold energy storage systems is strongly shaped by both capital expenses (CapEx) and operating expenses (OpEx). CapEx is mainly influenced by tank construction materials, phase-change materials (PCMs) or sensible storage media, insulation quality, and auxiliary components such as pumps, valves, and controls. For example, steel tanks with high-density insulation can significantly increase upfront costs, while PCM-enhanced systems often require additional encapsulation or heat exchangers, further raising the initial investment [95,153].

OpEx is dictated by maintenance needs, thermal losses through the insulation, and electricity costs associated with charging/discharging. Systems integrated with heat pumps or chillers may also experience varying operating costs depending on the efficiency and tariff periods. Thus, both CapEx and OpEx must be evaluated together to understand the total financial burden across the system’s lifetime.

9.2. Life-Cycle Assessment (LCA)

Life-cycle assessment (LCA) provides a structured approach to evaluating the environmental impacts of thermal energy storage (TES) systems across material production, installation, operation, and end-of-life stages. In agricultural applications, direct environmental impacts mainly arise from material-intensive components such as storage tanks, insulation, and phase-change materials. In contrast, indirect benefits are primarily associated with a reduced peak electricity demand and improved integration of renewable energy during operation [145,154].

While several studies suggest that operational benefits can offset initial embodied impacts, agriculture-specific LCA evidence remains limited, and reported outcomes are highly context-dependent. Uncertainties related to material durability, PCM degradation, and end-of-life management persist. Consequently, the sustainability performance of TES in agricultural systems should be evaluated on a case-by-case basis rather than assumed to be universally positive.

9.3. Levelized Cost of Storage (LCOS) for Thermal Energy Storage

The concept of a levelized cost of storage (LCOS), originally developed for electrical energy storage, is now widely applied to assess the economics of thermal energy storage (TES) systems. In the thermal domain, the LCOS typically incorporates several key parameters, including the initial capital investment, annual operation and maintenance expenses, expected system lifetime, round-trip thermal efficiency, and the useful thermal energy delivered each year [155,156].

TES technologies usually benefit from lower capital costs and longer operational lifespans compared to electrochemical batteries, making their LCOSs particularly competitive in applications such as district or building-level heating. Research shows that TES achieves significantly lower LCOS values when integrated with strategies like peak-load reduction or time-of-use shifting, or when operated alongside high-efficiency heat pumps to raise the overall system coefficient of performance (COP) [156]. For example, Ref. [155] demonstrated that these operational strategies can substantially improve the cost-effectiveness of thermal storage in heating networks.

9.4. Incentives and Tariff Structures for Thermal Storage

The economic feasibility of thermal energy storage is highly dependent on the local energy pricing framework, with several tariff and policy mechanisms playing critical roles. These include

• Time-of-use (TOU) electricity pricing, where charging TES during low-price periods reduces operating cost.
• Demand charges, which reward shifting heating-related electrical loads away from peak hours.
• Seasonal tariffs, particularly relevant where winter heating electricity prices are elevated.
• Incentives for renewable energy utilization, encouraging TES to store surplus solar or other renewable heat.
• Grid flexibility or demand-response programs, where TES provides thermal load shifting.
• Energy efficiency rebates, supporting the adoption of high-efficiency heating and storage technologies.

In many areas, thermal storage systems set up for building or agricultural heating can gain shortened payback periods by shifting heat production to off-peak hours and reducing the peak electricity demand. Policy mechanisms that reward load flexibility or renewable integration have been shown to reduce payback durations significantly—for example, from more than a decade to 3–5 years, depending on the TES capacity, insulation quality, and control strategy [118,142,157].

From a life-cycle viewpoint, tank-based TES systems provide environmental benefits that extend beyond direct cost savings. Although initial embodied energy and material impacts contribute to CapEx, these are typically offset over the system lifetime through reduced fossil fuel use, improved renewable energy utilization, and a lower peak electricity demand. When evaluated using the levelized cost of storage (LCOS), TES often demonstrates a favorable long-term sustainability performance due to its long service life, high cycle stability, and low operational emissions compared with electrochemical storage. Additional environmental co-benefits include reduced grid congestion, lower indirect CO₂ emissions, and enhanced energy resilience in agricultural systems, supporting climate-smart and resource-efficient farming practices.

10. Case Studies and Applications of Thermal Energy Storage in Agriculture

Thermal energy storage (TES) has been increasingly applied across agricultural systems, particularly in greenhouse heating, dairy processing, and integrated renewable energy farms. These applications demonstrate TES’s ability to improve heating flexibility, reduce fossil-fuel consumption, and support greater utilization of solar thermal and geothermal resources. One representative example is the hybrid renewable energy system installed at the Purme Social Farm in Yeoju, South Korea, where solar thermal collectors, a water-based tank TES (TTES), and a borehole seasonal thermal energy storage (BTES) unit were deployed. Long-term monitoring showed strong thermal stratification, a high renewable heat contribution, and substantial reductions in conventional heating demand [158,159,160]. Similar trends were observed in a citrus greenhouse in South Korea, where a 2905 m³ BTES coupled with solar collectors achieved an estimated solar fraction of approximately 80% and demonstrated long-term thermal stability [161,162]. Economic modeling of BTES for greenhouse heating has also produced favorable results. For example, studies on mid-size (≈1300 m²) greenhouses have shown that optimizing borehole numbers and storage volumes can result in payback periods in the range of 5–7 years [163,164,165,166].

Other regions have implemented TES to support greenhouse climate control and reduce cooling/heating costs. In Malaysia, a TES-integrated cooling system for lowland greenhouses demonstrated substantial savings—up to 60–70% annual electricity reduction—compared with standalone air-conditioning systems [167,168]. Broader reviews confirm that both sensible storage (water tanks, underground reservoirs) and latent storage (PCM-based systems) help buffer temperature fluctuations, improve microclimate stability, and shift thermal loads to off-peak energy periods [92,169,170,171].

TES has also gained traction in agro-industrial processes, particularly in dairy operations where heat is required sporadically for pasteurization, mixing, and cleaning-in-place cycles. Phase-change material (PCM)-enhanced TES systems incorporated into water-jacketed dairy equipment have demonstrated the ability to maintain stable thermal conditions and reduce the peak energy demand, supported by both laboratory experiments and CFD simulations [172]. Large dairy facilities have also examined the integration of TES with heat pumps and CO₂-based thermal systems, reporting reduced energy consumption and lower greenhouse gas emissions [142,173].

Pilot and demonstration-scale testbeds further illustrate TES’s potential benefits on farms. For example, the Nexus Farm project implemented a 5680 L above-ground water tank with solar collectors for greenhouse heating, achieving an approximately 30% reduction in heating energy compared to conventional approaches [174,175]. Additional testbeds—including PCM-enhanced greenhouse structures, solar-assisted water tanks, and underground TES systems—have shown improvements in temperature uniformity, renewable energy use, and overall energy efficiency [176].

Table 6. Summary of thermal energy storage (TES) case studies in the literature including transferable building-sector examples.

Location/Project	TES Capacity	Application	Reported Savings/Performance	Ref.
1. IKEA Retail Building, Castellón, Spain	15 m3 PCM (RT-31) latent storage	HVAC cooling load shifting	Reduced on-peak demand by 640 kW with an additional energy use of 20.5%; lower overall power consumption compared to a conventional system	[94]
2. Johns Hopkins University, Maryland, USA	10,074 m3 chilled-water storage tank	Campus cooling	Peak electrical demand reduced by 5–7 MW	[177]
3. Friedrichshafen, Germany	12,000 m3 water storage tank	District heating	Reduced seasonal heat losses from the thermal storage system	[178]
4. Aalto New Campus Complex (ANCC), Otaniemi, Finland	Large-scale ground thermal storage (equivalent volume ≈ 4 × 106 m3)	GSHP-based energy system	Improved long-term energy balance and demand–supply matching under variable climatic conditions	[179]
5. Office building, Kuala Lumpur, Malaysia	NR	Building air-conditioning system	Achieved approximately 4% higher energy efficiency compared to a non-storage system	[180]
6. Commercial building, Hong Kong	612 m3 chilled-water storage tank	Building cooling	Reduced energy consumption of the chilled-water plant	[181]
7. Marstal Solar District Heating Plant, Denmark	75,000 m3 pit TES	Seasonal solar heat storage	Achieved 40–50% annual fossil fuel reduction	[182]
8. CSP plants, Spain	PCM storage (0.97–1.11 m3 per module)	Environmental performance assessment	Demonstrated reduced environmental impacts compared to alternative configurations	[183]
9. HVAC laboratory, Boulder, CO, USA	7.31 m3 ice-on-coil storage tank	Thermal comfort control	Achieved cost savings under optimal control strategies with ~10% agreement	[184]
10. Building sector, India	1.51 m3 cool thermal energy storage tank	Building cooling/heating	Enabled chiller operation near +5 °C set-point temperature	[185]
11. Masdar Institute Solar Platform, Abu Dhabi, UAE	≈5.77 m3 modular concrete TES pilot	TES performance testing	Demonstrated stable thermophysical properties of storage materials	[186]

Ref. = references; NR = not reported. Although many case studies originate from the building and district energy sectors, they demonstrate mature TES designs, control strategies, and performance benchmarks that are directly transferable to agricultural applications such as greenhouses, livestock housing, and on-farm processing facilities.

describes several case studies investigated in different regions of the world.

Table 6 shows that latent, sensible, and seasonal TES systems have been successfully applied in building and district energy systems, delivering meaningful reductions in peak demand, improvements in energy efficiency, and lower reliance on fossil fuels. Although most of these examples are drawn from non-agricultural settings, they offer practical and well-tested insights into storage design, system sizing, and control strategies that can be readily adapted to agricultural facilities with comparable thermal behavior, such as greenhouses, livestock buildings, and on-farm processing units. Chilled-water and PCM-based systems, in particular, appear well suited for cooling-dominated agricultural applications, whereas large water tanks and pit TES systems provide useful guidance for meeting seasonal heating demands in energy-intensive farming operations. At the same time, the limited availability of long-term, field-scale TES demonstrations in agricultural contexts points to an important research gap, highlighting the need for targeted validation of these established TES concepts under farm-specific operating conditions and sustainability requirements.

Across these studies, a consistent trend emerges in which TES can enhance energy efficiency, increase renewable heat utilization, and strengthen the resilience of agricultural energy systems. Nonetheless, many implementations remain pilot-scale or limited to simulation studies, indicating the need for broader commercial deployment and long-term field evaluation [118,169].

11. Challenges, Limitations, and Safety Considerations

Despite the clear benefits of thermal energy storage (TES) in agricultural systems, several technical and operational problems still limit extensive adoption. From a technical perspective, thermal losses, particularly in inefficient insulated tanks or underground systems, remain a major obstruction and can significantly reduce the effective storage capacity over time [95]. Systems utilizing phase-change materials (PCMs) raise additional concerns, including material degradation, subcooling, and reduced performance after repeated freeze–thaw cycles [187]. In water-based or brine-based storage units, long-term operation may also lead to corrosion, fouling, and mineral scaling, which increase maintenance requirements and reduce the equipment lifetime [188].

Operationally, TES deployment on farms needs advanced control and forecasting capabilities, as inaccurate scheduling of charging or discharging can decline efficiency or disrupt farm thermal processes. Integrating TES with legacy heating systems, older boilers, or low-efficiency distribution networks can be composite, often requiring retrofits or hybrid operation strategies [189]. Weather and load forecasting errors—common in agricultural settings—may reduce predicted savings or impede temperature control in sensitive applications such as greenhouses or dairy processing.

Safety considerations also impose design and regulatory limitations. Hot-water tanks and pressurized vessels must comply with pressure and temperature safety standards, while some organic PCMs pose fire hazards due to flammability and must be housed in proper containment systems [190,191]. Environmental risks related to PCM leakage and end-of-life disposal must also be considered, as improper handling can lead to soil or water contamination. Although TES itself does not entail hazardous refrigerants, systems that interact with heat pumps or refrigeration units may introduce refrigerant leakage risks, requiring adherence to environmental and safety regulations such as F-gas standards [192].

Finally, the scalability of TES remains a complication. Small farms may face cost and space barriers, whereas large industrial farming operations require high-capacity storage and more advanced control architectures to manage dynamic heating and cooling loads [193,194,195,196]. From a long-term sustainability perspective, improving the material durability, recyclability of PCMs, and modular system design is essential to reduce environmental impacts and ensure economically viable deployment across diverse farm scales.

12. Research Gaps and Future Directions

Regardless of the explained benefits, several gaps still limit the sustainable and widespread adoption of thermal energy storage (TES) in agriculture. From a life-cycle perspective, there is a shortage of long-term studies on the environmental performance of TES materials, particularly the durability, recyclability, and end-of-life management of PCMs, insulation, and tank materials. Eco-friendly, farm-relevant PCMs with high cycling stability remain underdeveloped.

Adoption barriers are largely technical and economic. Reliable state-of-charge estimation under variable agricultural loads remains complicated, particularly in stratified tanks. In addition, high upfront costs, limited space availability, and the absence of low-cost, robust sensors and simplified IoT solutions hinder deployment on small and medium-sized farms.

Policy and standardization gaps further constrain execution. The lack of standardized performance metrics, testing protocols, and sustainability indicators (e.g., life-cycle emissions, energy payback time) complicates a comparison across TES technologies. Clear policy frameworks, incentives, and inclusion of TES within agricultural energy standards are required to speed up market uptake.

Addressing these sustainability, adoption, and policy gaps is vital for positioning TES as a scalable and climate-resilient solution in future smart agricultural systems.

13. Conclusions

Thermal energy storage (TES) provides a clear pathway toward sustainable agriculture by enabling the efficient use of renewable heat, reducing fossil-fuel dependence, and stabilizing thermal conditions in greenhouses, livestock housing, and agro-processing facilities. Among the currently available technologies, tank-based sensible heat storage remains the most practical and widely adopted TES solution due to its low cost, durability, and ease of integration. PCM-based and ice-based latent TES offer higher-energy-density and compact solutions, particularly where space is limited or cooling-intensive applications exist, while thermochemical TES shows long-duration potential but is currently constrained by material stability, system complexity, and economic feasibility.

Proper sizing, thermal stratification, and intelligent control enhance energy efficiency, reduce operating costs, and lower greenhouse gas emissions. Policy support, including targeted incentives, standardized performance metrics, and integration of TES into energy-efficiency and renewable-heat strategies, can further accelerate adoption, particularly in rural and resource-constrained agricultural settings. For practical implementation, simple and robust TES designs integrated with solar thermal systems or heat pumps, alongside basic monitoring and control, are recommended.

Looking forward, the most critical research priorities include

• Long-term field validation of TES systems across diverse agricultural settings.
• Development of standardized performance metrics for farm-scale TES applications.
• Deeper integration with IoT-based monitoring, predictive control, and energy management platforms.
• Comprehensive cost–benefit and life-cycle assessments across different climates and farm types.
• Design of scalable, modular TES systems suitable for smallholder farms.

Addressing these priorities will support a wider adoption, maximize the sustainability benefits, and reinforce TES as a cornerstone of resilient, low-carbon agricultural systems, thereby providing actionable guidance for both researchers and practitioners.