An Innovative Heating Solution for Sustainable Agriculture: A Feasibility Study on the Integration of Phase Change Materials as Passive Heating Elements

Abstract

In this study, we investigate an innovative option for the ecological management of agricultural land. The focus is on the use of phase change materials (PCMs) for passive temperature regulation in greenhouses and fruit crop fields in order to reduce yield losses due to unforeseen late frost events. The use of PCMs represents a novel approach to enhancing crop growth and extending growing seasons without relying on conventional energy-intensive methods, providing a stable microclimate that can protect plants from cold stress. This passive regulation of temperature helps to reduce the need for fossil fuel-based heating systems, thereby lowering greenhouse gas emissions and operational costs. The application of PCMs in agricultural settings is particularly innovative as it leverages naturally occurring temperature variations to create a self-sustaining, low-maintenance solution that aligns with the principles of sustainable farming. This approach not only improves energy efficiency but also contributes to the resilience of agricultural practices in the face of climate variability. This study focuses on the possible use of PCMs in passive heating modules for the protection of potted plants in greenhouses. Various PCMs such as paraffin, beeswax, and shea butter were tested. Experiments were then conducted, using one kind of paraffin-based PCM, in a specially designed module. In addition, an FEM simulation model (CFD) was built and tested. The model was used to perform detailed analyses of the heat transfer efficiency, fluid dynamics, and overall performance of the modules. The model can also be used for optimization purposes (e.g., efficiency improvements).

1. Introduction

The use of fossil fuels has serious negative impacts on the environment. These impacts include increased carbon dioxide emissions, air pollution, and far-reaching consequences for biodiversity and human health. The transition to renewable energy and sustainable practices is therefore essential to overcome these challenges and protect the health of our planet and its inhabitants.

By 2021, Austria aimed to have 36.4% of its gross final energy consumption coming from renewable sources [1]. One of the major technological challenges in utilizing renewable energy is harmonizing the fluctuating and uncontrollable supply profiles of solar and wind power with consumer energy demands, ensuring a cost-effective and consumption-oriented supply of electricity and heat [2]. Thermal energy storage, in particular, plays a crucial role in achieving a renewable and efficient energy supply, given that the heating and cooling sector accounts for nearly 25% of global energy production [3]. Thermal energy storage technologies can be categorized into three primary systems: sensible, latent, and thermochemical. Latent energy storage systems make use of phase change enthalpies during phase transitions. Here, a constant temperature is maintained throughout the melting process of pure substances, like fatty acids, paraffines, and salt hydrates.

In agriculture, cold winters present the challenge of protecting plants from frost damage. Frost occurs when water vapor condenses from saturated air and freezes on solid surfaces at low temperatures, posing a significant threat to most fruit crops, particularly during spring when bud formation increases frost sensitivity [4]. Frost damage can cause substantial delays in leaf and fruit development, leading to considerable yield losses [5]. Therefore, effective frost protection is essential to ensure stable yields and high fruit quality. All frost protection methods aim to prevent or replace radiant heat losses. The choice of appropriate frost protection technology depends on various factors, including acquisition and operating costs, personnel requirements, and availability. Conventional methods such as heaters and aerators have high energy consumption (e.g., heating oil, electricity), while irrigation systems entail significant investment costs and risks of flooding and soil siltation [4].

The susceptibility of most fruit crops to frost-induced damage is well documented. Of notable concern is the occurrence of spring frost, which poses a significant threat to fruit crops. While deciduous trees generally exhibit resilience to winter frost, their vulnerability escalates markedly during the formation of buds in spring [6]. Hence, implementing strategies to shield nascent buds from sub-zero temperatures becomes imperative for safeguarding crop yields.

Also, providing the right temperature for crop roots stimulates plant development and flower production, increasing productivity [7]. The optimal root temperature varies from plant to plant. The ideal root temperature for tomato plants is located between 15 °C and 25 °C [8]. It is located between 17 °C and 24 °C for leaf lettuce [9]. For other crops such as tobacco, cotton, corn, and peas, the ideal root temperature is close to 32 °C, 25 °C, 20 °C, and 10 °C, respectively [10]. Apples are less sensitive to frost than cherries, plums, and pears.

Various methods can be used to protect plants against late frost events: paraffin candles, solid fuel stoves, and mobile and fixed gas heaters. These solutions are costly, difficult to implement, and polluting. Furthermore, the effectiveness of these methods in rugged terrain is difficult to assess [11]. An effective method is frost irrigation, where up to 40 m³ of water is sprayed over the trees, per hour and per hectare. During freezing, the temperature of the floral organs is maintained slightly above 0 °C. This is currently the only frost protection method that leverages the advantages of latent heat energy, using water as a PCM. This approach has several advantages over other protection methods. Irrigation has lower greenhouse gas emissions compared to the use of fossil fuels, irrigation systems are cheaper to operate, and parts of the irrigation system can be used at other times of the growing season to avoid water stress [12]. The disadvantage of this method is that it requires significantly more water, up to 400 m³ per hectare per night. For this reason, it can only be used if there is sufficient water at the installation site.

A promising alternative solution to protect plants against late frost events is the use of passive PCM heating elements (e.g., fatty acids, paraffines, or salt hydrates). PCM heating elements are movable, passively temperature-controlled objects with high energy storage capabilities within a predefined temperature range. They passively “charge” and “discharge” based on ambient temperature fluctuations, providing an efficient and sustainable solution for frost protection. They can capture solar energy during the day and release it in the colder nighttime hours [13]. PCM heaters can be used both outdoors and indoors. In greenhouses, where plants are cultivated over extended periods, they could help further reduce the need for fossil fuel-based heating solutions [14,15,16,17,18,19]. They might also be used to heat plant roots. By lowering carbon emissions and promoting sustainable farming practices, the incorporation of PCMs could enhance the thermal efficiency of heating systems and offers a viable alternative to traditional root zone heating methods [20].

In Section 2, novel PCM-based concepts for the outdoor protection of fruit trees and indoor protection of potted plants (incl. roots) against late frost events are introduced. Application-specific criteria for these PCM-based concepts are outlined, including material selection and design parameters. The experimental setups are described in detail, along with the FEM model (CFD) that was developed iteratively, using the experimental results obtained from the initial tests. Experimental and simulation results are presented and discussed in Section 3. In the Conclusion, we provide a summary of the study and outline future work. In particular, the FEM model developed in this study will enable better assessment of PCM requirements to protect plants against late frost events in various outdoor as well as indoor configurations.

2. Materials and Methods

2.1. Phase Change Materials: The Basics

Latent heat storage refers to the storage of heat during a phase transition. In this usage, “latent” refers to a hidden store of heat that is not immediately felt. An additional supply of heat at the phase boundary causes no further temperature increases in the storage substance. After the phase change is complete, the temperature in the storage material keeps rising [21]. This indicates that energy is stored and released by changing the medium’s aggregate state with the corresponding enthalpy. This implies that heat can be retained for extended periods of time with minimal loss. Figure 1 illustrates the difference in heat storage capability between sensitive heat (blue curve) and latent heat/PCM (red curve) materials. As ambient temperatures rise, the PCM undergoes an endothermic transition from solid to liquid, efficiently absorbing and storing excess heat. Conversely, as the temperature drops, the PCM facilitates an exothermic transition from liquid to solid, releasing the stored heat. The melting temperature range, dT, is determined by the nature of the PCM substance. A significantly higher amount of thermal energy can be stored in PCM (∆Q PCM >> ∆Q sensitive).

2.2. PCM Selection for Experimental Tests

The following criteria must be considered to select the right PCM for a given application:

• Operating temperature range;
• Availability of the material;
• Handling of the material;
• Environmental compatibility of the material.

In our case, we need a PCM with a melting temperature slightly over 0 °C, a small melting range (which is beneficial for applications requiring precise temperature control), and as large a phase transition enthalpy as possible. Mixtures of various PCMs with different melting points could be used to adjust the melting point. However, PCM mixtures usually have lower phase transition enthalpies and a broader melting range [21].

Table 1. Data sheet of salt hydrate-based SP15_gel and paraffin-based RT5HC [22,23].

	SP15_gel	RT5HC
Melting range [°C]	15–17	5–6
Congealing range [°C]	15–13	6–5
Heat storage capacity [kJ/kg] latent-sensible −2 °C to 13 °C	160 44	250 70
Specific heat capacity [kJ/kgK]	2	2
Density solid (at −15 °C) [kg/L]	1.40	0.88
Density liquid (at 20 °C) [kg/L]	1.30	0.76
Heat conductivity [W/mK]	0.60	0.20
Volume extension [%]	7	13
Flashpoint [°C]	-	115
Max. operation temperature [°C]	30	45

displays the material parameters of possibly suitable PCMs from Rubitherm. These PCMs have transition temperatures of 5 °C and 17 °C, respectively.

Equation (1) can be used to compute the quantity of heat (Q) “contained” in a given RT5HC PCM sample. In this equation, “m” (kg) stands for the mass of the sample and “Cp” is the heat storage capacity (kJ/kg) of the sample in the relevant temperature range (−2 °C to 13 °C). The paraffin-based RT5HC PCM actually comes with a heat storage capacity of 250 kJ/kg between −2 °C and 13 °C. It also has a smaller melting range than the PCM SP15_gel. Furthermore, organic PCMs are easier to handle and test. They are also less expensive than inorganic PCMs and easier to store [23].

$$Q=m Cp$$

(1)

To confirm the data provided in Table 1, we heated a 250 mL sample of PCM RT5HC to 20 °C before cooling it down to −10 °C. The cooling curve shows how the sensible heat dissipates over a period of around 30 min until the sample reaches the solidification temperature of 6 °C. During the liquid–solid phase transition, the latent heat stabilizes the temperature for 200 min. The freezing point is reached after 300 min.

Given the unique challenges associated with food processing applications, the phase transition characteristics, heat storage capacity, and thermal stability of biological PCMs including shea butter and beeswax were also evaluated. We heated several 250 mL samples of biological PCMs up to 80 °C (these PCMs have higher melting points) before cooling them down to −10 °C. The measurement results are shown in Figure 2.

Beeswax experiences a phase transition between 63 °C and 50 °C. Bringing down this temperature range by at least 40 °C would require specific additives, which would undermine the inherent benefits of using natural waxes, such as their biodegradability, non-toxicity, and sustainability. Therefore, natural beeswax is unsuitable for our intended applications. The cooling curve of shea butter also reveals it to be unsuitable for late frost protection applications, as its cooling curve is simply too irregular.

Among the various PCMs tested, paraffin RT5HC is, therefore, the most suitable for late frost protection applications. This material meets the necessary melting temperature criteria and exhibits an adequate heat storage capacity.

2.3. Movable Passive Heating Elements (Concepts) for Protection against Late Frost Events

PCM-based concepts for the protection of potted plants in greenhouses (root zone heating systems are essential for raising crop quality and yield [20]) and for the protection of outdoor free-standing fruit trees are presented in Figure 3, respectively. The system for the protection of potted plants features a double-walled container designed to house PCM. The container can subsequently be filled with plant soil. This arrangement allows for precise temperature regulation of the soil in the plant’s root zone. The system for the protection of fruit trees is made up of two half-shell modules that can be easily positioned around the plants.

The protection module for free-standing trees is shown in more detail in Figure 4. The exterior casing is made of sheets of copper, chosen for its excellent solar energy absorption properties and thermal conductivity. Copper ensures efficient heat transfer from the absorber surface throughout the PCM, thereby enabling uniform distribution and utilization of the stored heat [24]. Additionally, the module is equipped with heat-conductive plates to further facilitate thermal energy transfer. The module contains 690 mL or 0.52 kg of PCM (Rubitherm, Berlin, Germany, RT5HC). The module’s heat storage capacity amounts to 131 kJ, or 36.71 Wh. The heat requirement for the targeted application is roughly 20–40 Wh/m² [25]. The module meets this requirement. However, freezing caused by wind or advection, as well as cold air phenomena resulting from thermal immersion, are additional factors [4] that might prevent PCM-stored heat from fully reaching the plants. Given the complexity of these phenomena, we initially focused on studying the first concept only (i.e., the modules for potted plant protection). The experimental results obtained, as well as the CFD models developed, which we present later, will subsequently allow us to study and optimize this concept for the protection of fruit trees as well.

2.4. PCM-Based Temperature Control Module for Potted Plants: Fabrication and Test

The two versions of the fabricated and tested modules for the protection of potted plants are shown in Figure 5 and Figure 6. The first version was fully 3D-printed using food-safe polylactide (PLA) filaments. For the second version, we used internal metal walls (a galvanized steel sheet) to improve the thermal conduction and durability. The overall heat capacity is the same for both versions.

The thermal conductivity of PLA was experimentally measured using a Heat Transfer Analyzer Model 2104 from Isomet, Manassas, VA, USA. These measurements were then used to determine the heat transfer coefficient (W/m²·K) of the material for simulation purposes. To visualize thermal energy flows, we opted for CFD simulations, with the following initial conditions: PCM temperature = 22 °C; ambient temperature = −10 °C. The simulations were performed using Autodesk CFD 2024. Figure 7 (left) shows the simulation model (including the so-called “Fluid Body” in red), whereas Figure 7 (right) shows the location where temperature is specifically computed for comparison purposes with the experimental data. In the fabricated modules, this corresponds to the location where a temperature sensor was used to measure temperature over time.

The PCMs’ parameters were obtained from the manufacturer’s data sheet [23]. The simulation model was adjusted to take the so-called PCM partial enthalpy at various temperatures into account (see Figure 8). These PCM partial enthalpy values at the appropriate temperature were used as material parameters in the form of specific heat values (J/kg K).

During the experimental tests, the modules were filled with 1700 mL PCM RT5HC, resulting in a heat storage capacity of 323 kJ (approximately 90 Wh). They were heated to 22 °C (thermal preconditioning) before being placed in a climate chamber at −10 °C (see Figure 9).

3. Results

3.1. Potted Plant Protection Modules

The experimental results from the tests described in Section 2.4 are presented in Figure 10. In the first case (100% PLA module), the thermal response was characterized by distinct stages of sensible and latent heat dissipation. The dissipation of sensible heat as the temperature decreased from 22 °C to the solidification range was observed to take approximately 60 min. During this period, the material exhibited a linear cooling trend that is typical of sensible heat loss. As the system entered the liquid–solid phase transition, the temperature remained stable between 6 °C and 4 °C, indicative of the latent heat being released during crystallization. This phase transition lasted for approximately 326 min, demonstrating the material’s significant capacity to maintain a near-constant temperature, a behavior that underscores the thermal buffering potential of the PLA module. This latent heat stabilization is critical for applications requiring prolonged thermal regulation without active energy input. Following the completion of the phase change, the temperature of the module continued to drop, ultimately reaching 0 °C after a total duration of 497 min. This final cooling phase highlights the completion of the solidification process and the resumption of sensible heat dissipation. The extended duration of thermal regulation observed in this experiment underscores the effectiveness of the 100% PLA module in maintaining thermal stability over extended periods, making it a promising candidate for applications where passive temperature control is essential.

In the second case (PLA–metal module), the dissipation of sensible heat as the temperature dropped from 22 °C to the solidification range also occurred over a span of approximately 60 min. This similarity in initial cooling behavior between the two modules suggests that the presence of the metal component does not significantly alter the rate of sensible heat loss prior to the phase change. During the liquid–solid phase transition, latent heat stabilized the temperature between 6 °C and 4 °C for about 343 min, slightly longer than the 326 min observed for the 100% PLA module. This extended period of thermal stabilization indicates that the addition of metal marginally enhances the module’s capacity to regulate temperature during the phase change process. Finally, the temperature of the PLA–metal module reached 0 °C after 507 min, which is 10 min longer than the time required by the 100% PLA module. This slight improvement in performance is consistent with the expected behavior of composite materials, where the integration of metal could potentially enhance thermal conductivity. The performance of the PLA–metal module, while slightly better than that of the PLA-only module, underscores the limitations imposed by the inherent thermal conductivity of the PCM utilized in the study. The minimal difference in performance between the two modules suggests that variations in module design, specifically through the incorporation of metal, have a limited impact on the overall passive heating capability. This observation highlights that the thermal conductivity of the PCM is a more critical factor than the choice of insulation and housing materials in determining the effectiveness of passive thermal management systems [21].

3.2. CFD Simulations

The computed temperature distributions in the modules at different times (revealing thermal gradients and heat flow patterns during phase transitions) are shown in Figure 11 and Figure 12. The simulated temperature curves at the reference measurement point are shown in Figure 13 and further demonstrate the accuracy of the numerical model. The simulation results exhibit a strong correlation with the experimental data, particularly in the key thermal events. Remarkably, the simulation accurately reproduces the time required for cooling from 22 °C to 6 °C (approximately 60 min), aligning closely with the experimental observations. This phase is critical, as it involves the dissipation of sensible heat, which sets the stage for the subsequent phase transition. Moreover, the model precisely captures the time required for the liquid–solid phase transition (which is the most critical in our case) and the remaining time until reaching 0 °C. This agreement between the experimental and numerical results not only validates the computational approach but also highlights its potential as a reliable tool for predicting thermal behavior in PCM-based systems.

The simulation results provide detailed insights into the spatial distribution of heat absorption and release within the modules, offering valuable information for optimizing thermal management designs. Figure 11 and Figure 12 illustrate regions of effective heat absorption and dissipation, clearly identifying the thermal behavior throughout the cooling and phase transition processes. These figures highlight areas within the module where heat absorption is most pronounced, particularly during the initial cooling phase. The regions with the highest core temperature are identified, indicating zones where the material stores the maximum amount of thermal energy before the onset of phase transition. Understanding these areas is crucial, as they represent the sections of the module that are most thermally active and, therefore, most influential in the overall heat management performance. The simulation also highlights the most critical areas of the heat exchanger, where heat dissipation is most effective. These critical regions are essential for optimizing the module’s design, as they are key to enhancing the efficiency of thermal regulation. By identifying where heat is released most efficiently, the simulation provides a roadmap for targeting design improvements, such as enhancing thermal conductivity or adjusting the material composition in these regions to maximize performance.

4. Conclusions

In this study, we demonstrated that it should, in principle, be possible to use PCMs to protect plants against late frost events. Especially, using specially designed modules for the protection of potted plants, we managed to maintain the temperature of the plant and its roots above 5 °C for almost 6 h, while the surrounding temperature was −10 °C.

Additionally, we developed and validated a numerical simulation model (CFD) that yielded simulation results closely matching the measured data, demonstrating its utility in designing and optimizing PCM modules. The high degree of accuracy observed in these simulations suggests that the model could be extended to explore different configurations, materials, and operational conditions, providing valuable insights for the development of more efficient thermal management systems in future studies. This model will allow for the optimization of the modules and the numerical testing of other types of modules, such as the one proposed for protecting fruit trees.

Future research will also focus on optimizing PCM integration and exploring advanced materials to improve the performance and sustainability of late frost protection modules. In future studies, we will notably examine PCM behaviors throughout an entire winter season and accordingly refine our simulation models.