Integration of Phase Change Materials in Advancing Heat Exchangers for Enhanced Utilization of Variable Renewable Energy

Abstract

Variable renewable energy sources, such as solar and wind power, play a crucial role in sustainable energy systems. However, their intermittent nature poses challenges for maintaining a consistent energy supply. This chapter outlines a research initiative that focuses on the integration of phase change materials (PCMs) within heat exchangers to address these challenges. PCMs, known for their thermal energy storage capabilities, can enhance the efficiency and reliability of renewable energy systems by providing a means to store excess energy during peak generation periods. The chapter aims to explore the synergies between PCMs and heat exchangers, with the goal of optimizing the utilization of variable renewable energy. Through meticulous surveys and integration, this chapter endeavor seeks to overcome the intermittent limitations of renewable energy sources, fostering a more robust and dependable framework for sustainable energy.

1. Introduction

Renewable energy sources, such as solar and wind power, play a crucial role in reducing carbon emissions and mitigating the impacts of climate change. Unlike traditional fossil fuels, which can be burned continuously to produce a constant flow of energy, variable renewable energies (VREs) are intermittent, meaning their output fluctuates based on environmental conditions.

Solar power stands as a cornerstone in the transition toward sustainable energy. The harnessing of solar energy involves capturing the sun’s rays through photovoltaic cells or solar thermal systems, converting sunlight into electricity or heat. The abundance of sunlight, its widespread availability, and the absence of direct emissions during electricity generation make solar power an environmentally friendly and scalable energy source. However, its intermittent nature, influenced by factors, such as weather conditions and daylight variations, necessitates effective energy storage solutions to ensure a continuous power supply [1].

Wind power represents another critical facet of the renewable energy landscape. By harnessing the kinetic energy of wind through wind turbines, electricity is generated with minimal environmental impact compared to conventional energy sources. Wind power contributes significantly to reducing greenhouse gas emissions and dependency on finite resources. Despite its potential, the variability in wind speed and the unpredictable nature of wind patterns introduce challenges related to energy intermittency [2].

With the global push toward renewable energy sources, the need for efficient energy storage solutions has never been more critical [3]. The intermittent nature of these renewable sources poses significant challenges for grid stability and energy management [4, 5]. For example, during periods of high energy generation, such as sunny days with strong winds, there may be an excess of energy that cannot be effectively stored or utilized. Conversely, during periods of low generation, such as at night or during calm weather, there may be a shortfall of energy, leading to reliance on nonrenewable sources. This imbalance highlights the urgent need for innovative solutions that can store and release energy as needed and integrating phase change materials (PCMs) within heat exchangers offers a promising avenue for addressing this challenge.

The integration of renewable energy sources, into the grid has led to a growing need for effective energy storage solutions [6]. Conventional energy storage methods, such as batteries, have limitations in terms of their capacity, efficiency, and lifespan [7]. For example, lithium-ion batteries, while widely used, are expensive and have limited energy density, making them unsuitable for large-scale energy storage applications. This limitation becomes particularly evident during periods of peak energy generation, where excess energy cannot be efficiently stored for later use. As a result, there is a pressing need for innovative energy storage solutions that can address these challenges and ensure the efficient utilization of renewable energy.

The properties of PCMs to store and release thermal energy this technology offers several key advantages. Firstly, it can significantly improve the efficiency of renewable energy systems by allowing excess energy to be stored and released when needed, thereby reducing waste and increasing overall system efficiency. Secondly, integrating PCMs within heat exchangers can enhance grid stability by providing a reliable and flexible energy storage solution. This is particularly important in regions with high levels of renewable energy penetration, where grid stability can be a major concern. Finally, by enabling more efficient energy storage and utilization, the integration of PCMs within heat exchangers can help reduce carbon emissions and mitigate the impacts of climate change. Overall, this technology has the potential to make a significant impact on the renewable energy landscape, paving the way for a more sustainable and resilient energy future.

The primary objective of this research is to investigate the integration of phase change materials (PCMs) within heat exchangers for the enhanced utilization of variable renewable energy sources. This study aims to explore the feasibility and effectiveness of using PCMs to store and release thermal energy, thereby improving the efficiency and reliability of renewable energy systems. By conducting a comprehensive analysis of the integration process, including the selection of suitable PCMs, design of PCM-integrated heat exchangers, and evaluation of system performance, this research seeks to provide valuable insights into the potential benefits and challenges of this innovative technology. Ultimately, the goal is to contribute to the advancement of renewable energy systems and facilitate the transition toward a more sustainable energy future.

In this chapter, we will explore the integration of phase change materials in advancing heat exchangers for enhanced utilization of variable renewable energy. We will discuss the benefits of this integration, the methods of integrating PCMs into heat exchangers, and the impact on the overall performance of renewable energy systems. Additionally, we will examine case studies and applications where PCM-integrated heat exchangers have been successfully implemented, as well as future trends and research directions in this field.

2. Phase change materials (PCMs)

2.1 Characteristics and properties

Phase change materials (PCMs) are substances known for their ability to store and release thermal energy during phase transitions, such as melting and solidification. This unique property makes them valuable in various applications within renewable energy systems. One key characteristic of PCMs is their high energy storage density, which allows them to store a large amount of energy per unit mass or volume compared to other thermal storage materials.

Additionally, PCMs can regulate temperatures by absorbing or releasing heat at nearly constant temperatures during phase transitions. This feature is crucial for systems requiring stable temperatures, such as in building applications. Moreover, PCMs are thermally stable over many cycles of heating and cooling, making them suitable for long-term use in energy storage applications. Many PCMs are also nontoxic and environmentally friendly, making them suitable for a wide range of applications [8]. Overall, the unique characteristics and properties of PCMs make them invaluable in enhancing the performance and efficiency of renewable energy systems.

The key characteristics and properties of PCMs make them valuable in various applications within renewable energy systems are the following:

• High latent heat capacity: PCMs possess a high latent heat capacity, allowing them to absorb or release a significant amount of energy during the phase change process without a substantial change in temperature.

• Temperature stability: PCMs maintain a nearly constant temperature during the phase transition, offering a stable thermal environment within a specific temperature range.

• Reversible phase transitions: PCMs undergo reversible phase transitions, meaning they can repeatedly change between solid and liquid states without significant degradation, ensuring long-term reliability.

• Nontoxic and environmentally friendly: Many PCMs are nontoxic, noncorrosive, and environmentally friendly, making them suitable for various applications, including those in contact with humans and ecosystems.

2.2 Thermal energy storage capabilities

Phase change materials (PCMs) are essential for thermal energy storage due to their unique ability to efficiently capture and release heat energy during phase transitions. When a PCM transitions from a solid to a liquid state (melting) or from a liquid to a solid state (freezing), it absorbs or releases a large amount of energy in the form of latent heat. This property allows PCMs to store thermal energy much more efficiently than traditional sensible heat storage materials, which only store energy through changes in temperature.

One of the key advantages of PCMs is their ability to store energy at a nearly constant temperature during the phase transition. This means that the temperature of the PCM remains relatively stable while absorbing or releasing heat, which is crucial for maintaining a consistent thermal environment. This characteristic makes PCMs particularly suitable for applications where temperature control is critical, such as in buildings, where they can be used to store excess solar heat during the day and release it at night to maintain comfortable indoor temperatures [8, 9, 10].

For instance, PCM-enhanced gypsum boards are a practical application of phase change materials (PCMs) in buildings for thermal energy storage. These boards are designed to improve the thermal performance of building envelopes by incorporating microencapsulated PCMs into the gypsum matrix.

During the day, when the outdoor temperature is higher, the PCM absorbs heat and undergoes a phase change from solid to liquid, storing the thermal energy. This process helps to reduce the amount of heat that penetrates into the building, thereby lowering the indoor temperature and reducing the load on air conditioning systems.

At night, when the outdoor temperature drops, the PCM solidifies and releases the stored heat, helping to maintain a comfortable indoor temperature without the need for additional heating. This thermal energy storage and release process can help reduce energy consumption for heating and cooling, making buildings more energy-efficient and cost-effective.

Additionally, PCMs can store a large amount of energy per unit mass or volume, making them space-efficient compared to other thermal storage materials. This high energy storage density allows for compact thermal energy storage systems, which is advantageous in applications where space is limited.

Overall, PCMs are essential for enhancing the efficiency and reliability of renewable energy systems by enabling efficient thermal energy storage. Their ability to store energy with minimal temperature change and high energy storage density makes them invaluable for a wide range of applications from building heating and cooling to industrial processes and energy grid stabilization.

2.3 Types of PCMs

There are various types of PCMs, each with unique properties and applications. Some common types include:

• Organic PCMs: Derived from organic compounds such as fatty acids, esters, and alcohols. These PCMs often have a low-melting point and are suitable for medium-temperature applications.

• Inorganic PCMs: Composed of salts, metals, or other inorganic materials. Inorganic PCMs typically have higher melting points and are well-suited for high-temperature energy storage applications.

• Eutectic mixtures: Combinations of two or more substances that, when mixed in specific proportions, create a mixture with a lower melting point than that of the individual components. Eutectic mixtures are designed for precise temperature control.

• Bio-based PCMs: Derived from renewable resources such as plant oils and fatty acids.

Selecting the appropriate type of PCM depends on the specific requirements of the application, including temperature range, thermal conductivity, and cycling stability. The integration of these diverse PCMs into heat exchanger designs holds the potential to revolutionize thermal energy storage in renewable energy systems.

Selecting the right phase change material (PCM) is crucial for integrating PCMs in advancing heat exchangers for enhanced utilization of variable renewable energy (VRE). Several key factors should be considered in the selection process. Firstly, the melting point of the PCM should align with the temperature range of the VRE source and the intended application. PCMs with melting points around 60–70°C are often suitable for solar energy storage. Secondly, the latent heat of fusion, which denotes the energy absorbed or released during the phase change process, is critical. PCMs with higher latent heat values can store more energy per unit volume or mass. Additionally, thermal conductivity is important for efficient heat transfer between the PCM and the heat transfer fluid, maximizing the storage and release of thermal energy. It is also essential to choose a chemically stable PCM that will not degrade or react with other materials in the heat exchanger over its operating temperature range. Compatibility with the encapsulation material, heat transfer fluid, and other components of the heat exchanger is another crucial consideration to prevent issues, such as leakage or degradation. Cost is also a factor, including both the material cost and any additional costs associated with encapsulation or integration. Lastly, the availability of the PCM is important to ensure that it can be readily sourced for maintenance and repair purposes. Considering all those factors, the most suitable PCM can be selected for integration into heat exchangers for enhanced utilization of VRE [8, 9, 10, 11, 12].

The three types of phase change materials (PCMs) are suitable for integrating into heat exchangers for enhanced utilization of variable renewable energy (VRE).

Paraffin wax is a commonly used PCM with a melting point range of approximately 45–70°C, making it suitable for storing thermal energy from VRE sources, such as solar or wind power. It has a high latent heat of fusion, allowing it to store a significant amount of thermal energy per unit volume or mass. However, paraffin wax has relatively low thermal conductivity compared to other PCMs, which can affect the heat transfer efficiency in heat exchangers [8, 9]. Nonetheless, its chemical stability and cost-effectiveness make it a practical choice for large-scale applications. Paraffin wax offers a high latent heat of fusion ranging from 150 to 250 J/g, making them ideal for applications requiring high energy storage capacity per unit mass. However, their thermal conductivity is relatively low, typically around 0.2–0.5 W/m K, which can limit heat transfer efficiency in heat exchangers.

For instance, paraffin wax could store 200 J of thermal energy per gram when it undergoes a phase change from solid to liquid. This property makes it well-suited for applications where space is limited but high energy storage is required, such as in portable electronics or building materials [13]. Paraffin wax has a thermal conductivity of 0.3 W/m K, and a melting point of 60°C, which means it is not as efficient at transferring heat compared to other materials suitable for applications where slow and controlled heat transfer is desired in building materials and thermal energy storage systems [14].

On the other hand, salt hydrates, such as sodium sulfate decahydrate or magnesium sulfate heptahydrate, have melting points in the range of 30–100°C, offering flexibility for different VRE applications. They have high latent heat of fusion values and good thermal conductivity, making them effective PCMs for storing and releasing thermal energy. However, salt hydrates can absorb moisture from the air, leading to potential issues with caking and degradation over time. Despite this, their availability and cost-effectiveness make them attractive PCM candidates for medium to large-scale applications [11, 12, 15]. For instance, sodium sulfate decahydrate has a lower latent heat of fusion, ranging from 50 to 150 J/g, but exhibits higher thermal conductivity values, typically between 0.5 and 2.5 W/m K. This makes them more suitable for applications requiring rapid heat transfer. For example, sodium sulfate decahydrate can store 100 J of thermal energy per gram during its phase change, but it can transfer this energy more rapidly than paraffin wax due to its higher thermal conductivity suitable for applications where fast heat transfer is important, such as in heat exchangers or thermal management systems [16]. Sodium sulfate decahydrate has a melting point of 32°C, which means it remains solid at temperatures below this point and transitions to a liquid state above it. It is suitable for applications where higher operating temperatures are required, such as in industrial heat storage or waste heat recovery systems [4, 13, 14, 16].

Bio-based PCMs, derived from renewable sources, offer environmental benefits and sustainability. They have melting points in the range of 5–100°C, providing a wide range of options for different VRE applications [17]. Bio-based PCMs have moderate to high latent heat of fusion values, making them suitable for efficient energy storage [18]. However, they generally have lower thermal conductivity compared to salt hydrates but higher than paraffin wax [19]. Their biodegradability and nontoxic nature make them environmentally friendly options, but they may have lower chemical stability compared to traditional PCMs, requiring proper sealing and encapsulation [20]. For example, eutectic mixtures, such as potassium nitrate-sodium nitrate, offer a unique advantage with a sharp melting point, allowing for precise temperature control. They have a latent heat of fusion ranging from 150 to 300 J/g and moderate thermal conductivity values of around 0.5–1.5 W/m K. Bio-based PCMs, derived from sources, such as palm oil, offer a balance between latent heat of fusion (around 100–200 J/g) and thermal conductivity (approximately 0.1–0.5 W/m K). However, they may be more susceptible to degradation over time compared to synthetic PCMs.

Hence, a eutectic mixture can store 250 J of thermal energy per gram during its phase change, providing a balance between energy storage capacity and temperature control systems [13, 14, 16].

Overall, when selecting a PCM, it should consider the specific requirements of their application. For example, applications requiring high energy storage capacity per unit mass may favor organic PCMs, while those requiring rapid heat transfer may benefit from inorganic PCMs. Eutectic mixtures may be preferred for applications requiring precise temperature control, while bio-based PCMs offer a sustainable and environmentally friendly alternative.

3. Fundamentals of heat exchangers

Heat exchangers are devices designed to transfer heat between two or more fluids, or between a fluid and a solid surface while keeping them physically separated. They play a critical role in various industrial processes and heating, ventilation, and air conditioning (HVAC) systems. The types of heat exchangers and the basic principles of heat transfer involved.

3.1 Types of heat exchangers

• Shell and tube heat exchangers: This is one of the most common types of heat exchangers, consisting of a shell (outer vessel) with multiple tubes running through it. One fluid flows through the tubes, while the other fluid flows over the tubes, facilitating heat transfer between the two fluids.

• Plate heat exchangers: These heat exchangers consist of multiple thin plates arranged in a way that allows for heat transfer between two fluids. The plates have large surface areas, which enable efficient heat exchange in a compact space.

• Finned tube heat exchangers: In this type, extended surfaces (fins) are attached to the outside of tubes to increase the surface area for heat transfer. This design is often used in air-cooled heat exchangers.

• Double pipe heat exchangers: This is a simple type of heat exchanger consisting of two concentric pipes through which the hot and cold fluids flow in opposite directions, allowing for heat transfer between them.

Plate heat exchangers and shell and tube heat exchangers are both important for the enhanced utilization of variable renewable energy (VRE) due to their specific characteristics and capabilities, which make them well-suited for this application.

Plate heat exchangers are essential for enhancing the utilization of variable renewable energy (VRE) due to their high efficiency, compactness, and flexibility. Research indicates that plate heat exchangers can achieve high heat transfer coefficients, resulting in efficient heat transfer performance, which is crucial for maximizing the utilization of VRE sources [13]. Additionally, the compact nature of plate heat exchangers makes them suitable for installations where space is limited, such as in renewable energy systems integrated into existing infrastructure [14]. Plate heat exchangers also offer flexibility in design and operation, allowing for easy modification to accommodate changing system requirements [16]. This flexibility is important for adapting to the variable nature of VRE sources.

Shell and tube heat exchangers are also important for the enhanced utilization of VRE, offering versatility, durability, and scalability. Shell and tube heat exchangers are versatile and can handle a wide range of temperatures and pressures, making them suitable for VRE applications where the characteristics of the energy source vary. Additionally, shell and tube heat exchangers are durable and can withstand harsh operating conditions, providing reliable performance over their lifespan. Furthermore, shell and tube heat exchangers can be easily scaled up for larger systems, making them suitable for applications where the energy output of VRE sources is significant [4, 16, 21, 22]. In conclusion, both plate heat exchangers and shell and tube heat exchangers play a crucial role in maximizing the utilization of VRE and advancing renewable energy systems.

The results in Figure 1 indicated that fin-tube heat exchangers and double pipe heat exchangers were not suitable for use as heat stores with PCM. The study found that heat exchanger 1 (double pipe heat exchanger with PCM) had a very low heat transfer area, while heat exchanger five (plate heat exchanger with PCM) had a very low ratio of PCM heat capacity over empty heat exchanger heat capacity. On the other hand, heat exchangers two (double pipe heat exchanger with PCM embedded in a graphite matrix), three (double pipe heat exchanger with fins and PCM), and four (compact heat exchanger with PCM) showed better values and were more promising for real applications. Heat exchanger four (compact heat exchanger with PCM) exhibited the highest average thermal power with values above 1 kW for both charging and discharging tests at cases with larger temperature differences between PCM and water. Heat exchanger two (double pipe heat exchanger with PCM embedded in a graphite matrix) showed higher values for average power per unit area and per average temperature gradient, ranging from 700 to 800 W/m² K, which were one order of magnitude higher than the ones presented by the second-best heat exchanger. Overall, the study concluded that fin-tube heat exchangers and double pipe heat exchangers were not suitable for integrating PCM in heat storage systems, while other configurations showed more promise for real applications.

Figure 1.

Normalized thermal power (W/m² K) for charging (melting) and discharging (solidification) processes at two different temperature differences between water and PCM and for the five heat stores studied [23].

Fin-tube heat exchangers (HX) and double pipe heat exchangers are not always the most suitable for integrating phase change materials (PCMs) due to several reasons:

Limited heat transfer area: Fin-tube heat exchangers have a limited heat transfer area per unit volume, which can restrict the amount of PCM that can be effectively integrated, especially if a large amount of thermal energy storage is required. For stance, research by Duffie and Beckman [24] indicates that the heat transfer area of fin-tube heat exchangers is limited by the surface area of the fins, which can restrict the amount of PCM that can be effectively integrated.

Complexity of integration: Integrating PCMs into fin-tube heat exchangers or double pipe heat exchangers can be complex and may require modifications to the existing design, which can increase costs and fabrication time. A study by Cabeza et al. [25] discusses the challenges and complexities involved in integrating PCMs into existing heat exchanger designs, including the need for modifications and potential increases in cost and fabrication time.

Limited thermal conductivity: Fin-tube heat exchangers and double pipe heat exchangers typically have lower thermal conductivity compared to other types of heat exchangers. This can limit the rate at which thermal energy is transferred to or from the PCM, affecting the overall efficiency of the system. Research by Kalogirou [26] highlights that fin-tube heat exchangers and double pipe heat exchangers typically have lower thermal conductivity compared to other types of heat exchangers, which can limit the rate of heat transfer to or from the PCM.

Temperature limitations: Fin-tube heat exchangers and double pipe heat exchangers may not be suitable for applications requiring high-temperature thermal energy storage as the materials used in their construction may not be able to withstand the high temperatures. Studies by Farid et al. [27] and Zalba et al. [28] suggest that the materials used in the construction of fin-tube heat exchangers and double pipe heat exchangers may not be suitable for high-temperature thermal energy storage applications involving PCMs.

Space constraints: Fin-tube heat exchangers and double pipe heat exchangers may have space constraints that limit the amount of PCM that can be integrated, especially in applications where space is limited. Research by Tyagi et al. [29] discusses the space constraints associated with fin-tube heat exchangers and double pipe heat exchangers, which can limit the amount of PCM that can be integrated, especially in applications with limited.

Efficiency concerns: The design of fin-tube heat exchangers and double pipe heat exchangers may not be optimized for efficient heat transfer with PCMs, leading to lower overall system efficiency. A study by Alva et al. [30] investigates the efficiency of heat transfer in fin-tube heat exchangers and double pipe heat exchangers when integrated with PCMs, highlighting potential efficiency concerns compared to other heat exchanger designs.

Overall, those studies provide evidence supporting the notion that while fin-tube heat exchangers and double pipe heat exchangers are commonly used in various applications, they may not always be the most suitable choice for integrating PCMs due to their design limitations and potential efficiency concerns.

4. Basic principles of heat transfer

1. Conduction: Heat is transferred through the solid walls of the heat exchanger (e.g., tube walls) by conduction. The rate of heat transfer by conduction is influenced by the thermal conductivity of the material and the thickness of the walls.

2. Convection: Heat transfer between a solid surface and a fluid (liquid or gas) flowing over it is primarily by convection. Convection can be natural (due to density differences) or forced (aided by external means such as fans or pumps).

3. Radiation: In some cases, heat transfer can also occur through electromagnetic radiation, where heat is emitted or absorbed by surfaces at different temperatures.

The heat transfer equation plays a crucial role in understanding the integration of phase change materials (PCMs) in advancing heat exchangers for enhanced utilization of variable renewable energy (VRE). The basic heat transfer equation governing the heat transfer process is given by:

$$Q=mc\Delta T$$E1

where Q is the amount of heat transferred, m is the mass of the substance, c is the specific heat capacity of the substance, and ΔT is the temperature change.

For PCMs undergoing phase change, an additional term is included to account for the latent heat of fusion (L):

$$Q=mc\Delta T+mL$$E2

In the context of heat exchangers, this equation is used to calculate the amount of heat transferred between the PCM and the heat transfer fluid or solid surface. By controlling the flow rate of the heat transfer fluid and the surface area of contact between the PCM and the fluid, the efficiency of heat transfer can be optimized.

The integration of heat exchangers (HX) and phase change materials (PCMs) is governed by several key equations that describe the heat transfer processes within the system. One critical aspect of this integration is the determination of the heat transfer rate between the PCM and the heat exchanger surface, which can be described using Fourier’s law of heat conduction.

Furthermore, the Fourier’s law of heat conduction is relevant for understanding heat transfer within the PCM:

$$Q=-kAdx/dT$$E3

where q is the heat flux, k is the thermal conductivity of the material, A is the cross-sectional area for heat transfer, and dxdT is the temperature gradient.

This equation describes how heat is conducted through the PCM, influencing the rate of temperature change and phase transition within the PCM. By selecting PCMs with high thermal conductivity and designing heat exchangers with optimized surface areas, the efficiency of heat transfer can be improved, enhancing the utilization of VRE in renewable energy systems [13].

The integration of phase change materials (PCMs) in advancing heat exchangers for enhanced utilization of variable renewable energy (VRE) relies on the basic principles of heat transfer. Heat exchangers are devices designed to transfer heat between two or more fluids, or between a fluid and a solid surface to achieve desired thermal conditions. The integration of PCMs enhances this process by leveraging their ability to store and release thermal energy during phase transitions.

One of the fundamental principles of heat transfer in heat exchangers is the concept of thermal conductivity. This property determines the rate at which heat is transferred through a material. PCMs with high thermal conductivity can efficiently transfer heat between the heat source (such as solar or wind energy) and the heat exchanger, ensuring that thermal energy is effectively captured and stored.

Another important principle is heating capacity, which refers to the amount of heat energy required to raise the temperature of a substance by a certain amount. PCMs have a high heat capacity during phase change, allowing them to store large amounts of thermal energy without a significant increase in temperature. This property is crucial for storing excess energy from VRE sources during peak production periods.

Furthermore, the design and configuration of heat exchangers play a crucial role in maximizing the utilization of PCMs. For example, the use of finned heat exchanger surfaces can increase the surface area available for heat transfer, enhancing the efficiency of PCM melting and solidification processes. Additionally, the integration of PCM capsules or containers within the heat exchanger can ensure efficient heat transfer between the PCM and the heat transfer fluid.

In summary, the integration of PCMs in advancing heat exchangers for enhanced utilization of VRE relies on principles of heat transfer, including thermal conductivity, heat capacity, and efficient heat exchanger design. By leveraging these principles, heat exchangers can effectively store and release thermal energy, optimizing the utilization of VRE sources and improving the overall efficiency of renewable energy systems.

The equation for calculating the energy storage capacity is:

$$E=m·L$$E4

where E is the thermal energy storage capacity, m is the mass of the PCM, and L is the latent heat of fusion of the PCM.

Furthermore, the overall heat transfer coefficient between the PCM and the heat exchanger surface can be determined using the following equation, which considers both the convective heat transfer coefficient (h) and the conductive heat transfer resistance (R):

$$U=1/\left(1/h+R\right)$$E5

By using these governing equations, can optimize the design of the HX-PCM integration to maximize energy efficiency and enhance the utilization of variable renewable energy sources.

5. Integrating phase change materials

Integrating phase change materials (PCMs) into heat exchangers offers significant benefits, enhancing their efficiency and effectiveness in various applications. One key advantage is the ability of PCMs to store and release large amounts of thermal energy during phase change processes. This capability enables heat exchangers to store excess energy during periods of high availability, such as sunny or windy days, and release it when needed, improving overall energy utilization and grid stability. Additionally, PCMs can help regulate temperatures by absorbing or releasing heat at constant temperatures during phase transitions. This feature can help maintain a more stable temperature profile in heat exchangers, reducing temperature fluctuations in the system. Moreover, integrating PCMs can improve the efficiency of heat exchangers by reducing the energy required for heating or cooling processes, leading to lower energy consumption and operating costs. Furthermore, PCMs have high energy storage densities compared to conventional thermal storage materials, allowing for more compact designs of heat exchangers and energy storage systems.

As shown above (Figure 2), the collector frame provides structural support for the system, holding components, such as the collector box, which houses the heat exchanger and PCM capsules. The centrifugal blower regulates airflow, aiding in heat exchange processes. Polyvinyl chloride (PVC) connection pipes facilitate fluid flow, while the gate valve controls fluid flow rates. The conical inlet and exit sections help direct airflow efficiently. The glass cover protects components and allows for sunlight penetration. Anemometers measure wind speed, pyranometers measure solar radiation, and digital temperature sensors monitor temperatures. Integrating these parts optimizes energy capture and utilization, enhancing the overall performance of renewable energy systems.

Figure 2.

(a) Schematic diagram of the experimental setup; (b) a comparison of instantaneous thermal efficiency between v-corrugated and flat plate solar air heater with and without PCM when mass flow rate = 0.062 kg/s [17].

The figure comparing the instantaneous thermal efficiency between v-corrugated and flat plate solar air heaters with and without PCM at a mass flow rate of 0.062 kg/s provides insightful comparisons. The inclusion of PCM shows a significant improvement in thermal efficiency compared to systems without PCM. This enhancement can be attributed to the PCM’s ability to store and release thermal energy, effectively regulating temperature fluctuations and improving overall energy utilization. Furthermore, the comparison between v-corrugated and flat plate designs indicates that the former generally outperforms the latter in terms of thermal efficiency. This is likely due to the increased surface area for heat exchange in v-corrugated designs, leading to more efficient heat transfer. The selected mass flow rate of 0.062 kg/s allows for a meaningful comparison as it influences the rate of heat transfer and, consequently, the thermal efficiency of the systems. Overall, this figure highlights the importance of PCM integration and collector design in enhancing the efficiency of solar air heaters, providing valuable insights for the optimization of solar thermal systems [4, 14, 16, 17, 18, 19, 21, 22].

The integration of phase change materials (PCMs) in advancing heat exchangers for enhanced utilization of variable renewable energy (VRE) involves a carefully designed system setup. A flat plate heat exchanger serves as the core component, featuring inlet and outlet ports for the heat transfer fluid. Inside the heat exchanger, channels facilitate the flow of the fluid with evenly distributed PCM capsules (e.g., paraffin wax) contained within. These capsules are encapsulated to prevent mixing with the fluid. The system includes a control system responsible for regulating the flow rate of the heat transfer fluid and maintaining the temperature within the desired range. Energy from the VRE source (e.g., solar panels, wind turbines) flows to the heat transfer fluid, causing the PCM to melt and store thermal energy. When additional heat is required, the PCM solidifies, releasing the stored energy to the heat transfer fluid. This integration offers benefits, such as improved energy efficiency and grid stability, making it a promising solution for sustainable energy systems [31].

There are several methods to integrate PCMs into heat exchangers, each with its advantages and considerations. One common method is encapsulation, where PCMs are encapsulated within containers or capsules and then integrated into heat exchangers. This approach ensures that the PCM remains contained and does not mix with the fluid in the heat exchanger, preventing contamination. However, proper selection of encapsulation materials is crucial to ensure compatibility with the PCM and heat exchanger operating conditions. Another method is direct contact, where the PCM is in direct contact with the heat transfer fluid (e.g., water, air) in the heat exchanger. As the PCM undergoes phase change, heat is transferred directly to or from the fluid, enabling efficient heat exchange. Proper design is essential to prevent leakage and ensure uniform heat transfer. Additionally, PCMs can be immersed in a heat transfer fluid (e.g., water) within a heat exchanger, allowing for simpler integration compared to encapsulation. However, care must be taken to select PCMs and fluids that are compatible to avoid issues, such as corrosion or degradation. Overall, integrating PCMs into heat exchangers offers a promising pathway to enhance energy efficiency, improve system performance, and promote the integration of renewable energy sources in various applications.

For instance, to integrate paraffin wax into a flat plate heat exchanger for enhanced utilization of variable renewable energy (VRE), several steps must be taken. Firstly, paraffin wax with suitable properties must be selected. For instance, paraffin wax with a melting point between 45°C and 70°C and a high latent heat of fusion is ideal for solar energy storage applications. Next, the flat plate heat exchanger needs to be designed to accommodate the paraffin wax. This involves creating channels for the flow of the heat transfer fluid and ensuring the heat exchanger can withstand the operating temperatures and pressures [15, 17, 18, 19]. The paraffin wax should then be encapsulated to prevent it from mixing with the heat transfer fluid. Encapsulation materials should have good thermal conductivity to facilitate heat transfer [32]. Once integrated, a control system is necessary to regulate the flow rate of the heat transfer fluid and maintain the temperature within the desired range for efficient heat exchange with the paraffin wax [33]. In conclusion, integrating paraffin wax into a flat plate heat exchanger requires careful consideration of PCM properties, heat exchanger design, encapsulation, and control systems. Proper integration can significantly enhance the utilization of VRE [34].

6. Enhanced utilization of variable renewable energy

Variable renewable energy sources, such as solar and wind power, face challenges due to their intermittent nature, which can lead to grid instability and difficulty in matching supply with demand. The variability of these sources poses challenges for grid operators in maintaining a reliable electricity supply. However, phase change materials (PCMs) offer a promising solution to mitigate these challenges. They investigated the heat transfer performance of PCMs in a regenerative heat exchanger. Varying Reynolds number, hot fluid temperature, and PCM types revealed increased effectiveness, suggesting potential for designing more efficient and higher-capacity heat exchangers [35].

Researched, Mehling and Cabeza [36] focused on optimizing multiteam plate fin heat exchangers (MPFHE) with four operating streams. Utilizing the non-sorting genetic algorithm (NSGA-II), the study aimed to enhance thermal-hydraulic performance, resulting in up to a 1.5% increase in heat transfer and an 11.1% reduction in power consumption in cryogenic units, promoting efficiency and sustainability. Also, the research explored thermal energy storage using nano-enhanced phase change materials in various plate geometries. Particularly, the sawtooth profile exhibited superior performance, reducing melting time by up to 12% [37].

By integrating PCMs into energy systems, excess thermal energy generated from renewable sources during periods of high availability can be stored. This stored energy can then be released when renewable energy generation is low, helping to balance supply and demand and stabilize the grid.

One of the key benefits of PCMs is their ability to store and release large amounts of thermal energy during phase change processes, such as melting and solidification. Emmi and Bottarelli [19] delved into performance enhancement strategies for heat exchangers, focusing on nanofluids for increased thermal conductivity. Latent thermal energy storage (LTES) heat exchangers, however, lack an analytical method for determining outlet temperature, as addressed by Beyne et al. [17] with an analytical model validated numerically.

This capability allows PCMs to act as a buffer, absorbing excess energy during peak generation periods and releasing it during periods of low generation. This can help reduce strain on the grid and improve overall energy efficiency. Additionally, PCMs can enable load shifting, allowing excess energy generated during off-peak hours to be stored and used during peak demand periods. This can further enhance the utilization of variable renewable energy sources and reduce reliance on fossil fuels. By shifting focus to triple concentric-tube heat exchangers, investigated superior single-phase flow heat transfer characteristics. The study categorized recent publications based on experimental, numerical, and analytical approaches, highlighting advantages, such as shorter length requirements for equivalent performance [38]. Comparing different heat exchanger designs, Kumari and Ghosh [14] evaluated the efficiency of concentric and hairpin heat exchangers for latent heat energy storage using PCMs. Numerical analysis revealed improved heat transfer with reduced diameter and increased length of the high-temperature fluid (HTF).

In phase change heat exchangers, introduced phase-change materials to expand heat dissipation buffer space. Simulations showed that incorporating inner ring ribs into the heat exchange copper tube significantly improved heat transfer efficiency [32].

Overall, integrating PCMs into energy systems offers a promising pathway to enhance the utilization of variable renewable energy sources. By providing a reliable and efficient means of energy storage, PCMs can help address the challenges posed by intermittency and grid instability, paving the way for a more sustainable and resilient energy future.

7. Future trends and research directions

The future of advanced heat exchangers lies in the exploration of emerging technologies and materials that can enhance their performance and efficiency. One promising area is the development of advanced materials with improved thermal conductivity and heat transfer properties. Nanotechnology, for instance, offers the potential to engineer materials at the nanoscale to achieve superior heat transfer characteristics.

Another trend is the integration of smart technologies, such as sensors and control systems, into heat exchangers to optimize their performance and energy efficiency. These technologies enable real-time monitoring and adjustment of heat exchanger operations, leading to more efficient heat transfer processes.

Furthermore, the use of additive manufacturing (3D printing) techniques allows for the rapid prototyping and production of complex heat exchanger designs. This approach opens up new possibilities for designing heat exchangers with optimized geometries for enhanced heat transfer performance.

In terms of research directions, further investigations are needed to explore the potential of phase change materials (PCMs) in improving the energy efficiency of heat exchangers. Studies could focus on developing novel PCM-based heat exchanger designs and materials to maximize heat storage and transfer capabilities.

Additionally, there is a need to explore the integration of renewable energy sources, such as solar and wind power, into heat exchanger systems. Research in this area could lead to the development of hybrid systems that utilize both conventional and renewable energy sources to achieve higher energy efficiency.

Overall, future research in advanced heat exchangers should focus on leveraging emerging technologies and materials to enhance their performance, efficiency, and sustainability. By addressing these challenges, researchers can pave the way for the development of next-generation heat exchangers that play a crucial role in the transition to a more sustainable energy future.

8. Conclusions

In conclusion, this chapter has highlighted the importance of advanced heat exchangers in enhancing the utilization of variable renewable energy sources. The integration of phase change materials (PCMs) into heat exchangers offers significant benefits, including improved heat transfer efficiency and energy storage capacity.

Fundamental principles of heat exchangers, including different types and basic heat transfer mechanisms, were discussed. The chapter also explored various methods of integrating PCMs into heat exchangers, such as encapsulation and direct contact, emphasizing their role in enhancing energy storage and heat transfer processes.

Studies and applications showcased successful integration of PCMs in heat exchangers, leading to performance improvements and energy savings. Numerical analyses and experimental studies provided valuable insights into optimizing heat exchanger designs for improved heat transfer efficiency.

Future trends and research directions were also discussed, highlighting the importance of exploring emerging technologies and materials for advanced heat exchangers. Continued innovation in heat exchanger design is crucial for effectively integrating renewable energy sources into energy systems.