The ever-increasing global energy demand, coupled with pressing environmental concerns, has intensified the focus on energy conservation, improving energy efficiency, and developing renewable energy sources. In this context, latent heat thermal energy storage (LHTES) technology presents a powerful solution to address the mismatch between energy supply and demand in both time and space. Phase Change Materials (PCMs) are at the core of this technology, offering high storage density and near-isothermal operation during charge and discharge cycles. Integrating PCM-based storage with solar energy systems, particularly photovoltaic/thermal (PV/T) collectors, represents a highly promising pathway to enhance overall solar utilization efficiency, reduce energy consumption, and mitigate environmental impact.
A critical challenge in this field is the development of PCMs with optimal thermophysical properties. While organic PCMs like paraffin wax are chemically stable and inexpensive, they suffer from low thermal conductivity and relatively low latent heat, necessitating large heat exchange areas. Inorganic salt hydrates, on the other hand, offer high volumetric energy density but are often plagued by issues like supercooling and phase separation. Barium hydroxide octahydrate (Ba(OH)2·8H2O) is an attractive inorganic PCM with a phase change temperature of approximately 78°C and a high latent heat of fusion (~264 kJ·kg-1), making it suitable for medium-temperature solar thermal applications. However, its practical application has been hindered by significant supercooling and poor thermal conductivity.
This article details the development of a novel composite PCM based on Ba(OH)2·8H2O, engineered to overcome these limitations. The composite’s heat release characteristics were then experimentally evaluated within a novel cascade thermal storage solar PV/T collector. The performance of this integrated solar system demonstrates significant potential for delivering stable, high-temperature thermal output while simultaneously cooling photovoltaic modules to improve electrical efficiency.
1. Development and Thermophysical Characterization of the Composite PCM
The primary goal was to formulate a stable Ba(OH)2·8H2O-based composite with minimal supercooling and enhanced thermal conductivity, without substantially compromising its latent heat capacity.
1.1 Mitigation of Supercooling
Supercooling, where a liquid cools below its freezing point without solidification, is a major drawback of many salt hydrates. It prevents heat release at the intended phase change temperature. The addition of effective nucleating agents can provide sites for crystal growth, thereby reducing supercooling. Based on preliminary research indicating the efficacy of certain barium salts, barium carbonate (BaCO3) was selected as a nucleating agent. The cooling curves of pure Ba(OH)2·8H2O and the composite with 1.0 wt% BaCO3 were compared. The pure material exhibited a supercooling degree of 3–5°C, whereas the composite with BaCO3 showed negligible supercooling. Furthermore, the phase change plateau for the composite was longer and more distinct, confirming the effectiveness of BaCO3 in promoting consistent nucleation.
1.2 Enhancement of Thermal Conductivity
Low thermal conductivity slows down the charge and discharge rates of a PCM. To address this, nanotechnology was employed. Metallic nanoparticles are known to significantly improve the thermal conductivity of host materials. Three different nanoparticles—iron (Fe), nickel (Ni), and aluminum (Al)—were dispersed into the 1.0% BaCO3/Ba(OH)2·8H2O base mixture using ultrasonic agitation. Comparative analysis of their cooling curves revealed that nano-iron additives resulted in a more stable phase change plateau and a faster cooling rate post-solidification, indicating improved heat transfer.
To determine the optimal concentration, composites with varying mass fractions of nano-iron (0.02%, 0.05%, 0.08%, 0.1%, 0.2%, and 0.3%) were prepared. Their thermal performance was initially screened via cooling curve analysis. Composites with 0.08%, 0.1%, and 0.2% nano-iron showed promising characteristics: negligible supercooling, a stable plateau, and a rapid temperature drop. These three candidates were then subjected to detailed thermophysical analysis.
The phase change temperature and latent heat were measured using Differential Scanning Calorimetry (DSC). The thermal conductivity was measured using a transient plane source method. The results are summarized in the tables below.
| Sample Composition | Phase Change Temperature (°C) | Latent Heat (J·g-1) |
|---|---|---|
| 1.0% BaCO3 / Ba(OH)2·8H2O (Base) | 78.33 | 288.8 |
| Base + 0.08% nano-iron | 79.57 | 275.1 |
| Base + 0.1% nano-iron | 78.87 | 280.2 |
| Base + 0.2% nano-iron | 79.91 | 277.1 |
| Sample Composition | Thermal Conductivity (W·m-1·K-1) |
|---|---|
| 1.0% BaCO3 / Ba(OH)2·8H2O (Base) | 1.225 |
| Base + 0.1% nano-iron | 1.358 |
| Base + 0.2% nano-iron | 1.368 |
The composite with 0.2% nano-iron offered the best combination of properties. Its thermal conductivity increased by 11.7% compared to the base material (from 1.225 to 1.368 W·m-1·K-1>), while retaining a high latent heat of 277.1 kJ·kg-1 and a stable phase change temperature near 80°C. The formulation 0.2% nano-iron / 1.0% BaCO3 / Ba(OH)2·8H2O was therefore selected as the optimal composite PCM for further system integration.
The thermal stability of the selected composite was verified through 100 repeated melting-freezing cycles. After cycling, the phase change temperature remained stable, the supercooling degree was only 0.66°C, the latent heat was 270.8 J·g-1 (a decrease of less than 2.3%), and the thermal conductivity was 1.366 W·m-1·K-1, demonstrating excellent cyclic stability for use in a solar system.
2. Experimental Solar PV/T System with Cascade Thermal Storage
To evaluate the practical performance of the developed composite PCM, a dedicated experimental solar PV/T system featuring cascade thermal storage was designed and constructed. The core innovation lies in the integration of multiple PCMs at different phase change temperatures and a water buffer to create a temperature-gradient, or cascade, storage unit. This design optimizes energy collection and delivery.
2.1 System Design and Components
The experimental setup primarily consists of a crystalline silicon photovoltaic panel, a cascade phase-change thermal storage collector, a constant temperature water tank with a circulation pump acting as a heat sink/source, and data acquisition instrumentation.
Photovoltaic/Thermal (PV/T) Panel: A standard photovoltaic panel is modified to function as a PV/T collector. Water flows through channels bonded to the back of the panel. This serves a dual purpose: it absorbs waste heat from the solar cells, thereby lowering their operating temperature and increasing electrical conversion efficiency (as efficiency decreases by ~0.4-0.5% per °C temperature rise), and it preheats the water using this recovered thermal energy. The preheated water is then fed into the primary thermal storage collector.
The energy balance for the PV/T panel can be described as:
$$ Q_{total} = Q_{elec} + Q_{th,PV/T} $$
where \( Q_{total} \) is the total incident solar energy, \( Q_{elec} \) is the electrical energy output, and \( Q_{th,PV/T} \) is the thermal energy collected by the water coolant. The thermal energy gain can be approximated by:
$$ Q_{th,PV/T} = \dot{m} c_p (T_{out,PV} – T_{in,PV}) $$
where \( \dot{m} \) is the mass flow rate, \( c_p \) is the specific heat capacity of water, and \( T_{in,PV} \) and \( T_{out,PV} \) are the inlet and outlet temperatures of the PV/T panel coolant.
Cascade Phase-Change Thermal Storage Collector: This is the heart of the thermal storage solar system. It is based on an evacuated tube collector architecture but integrates sealed cylindrical PCM containers. The collector houses three distinct thermal storage media arranged to create a temperature gradient:
- Low-Temperature Zone: Contains paraffin wax (melting point ~50-60°C), receiving the preheated water from the PV/T panel.
- Intermediate Buffer: A layer of water acts as a sensible heat buffer and separator.
- High-Temperature Zone: Contains the developed Ba(OH)2·8H2O composite PCM (melting point ~78°C), positioned to receive the hottest fluid.
Each PCM is encapsulated in a sealed alloy container with an integrated copper U-tube heat exchanger. This design prevents PCM leakage and can withstand pressure. The U-tube from the PV/T panel runs sequentially through these three zones. During charging (daytime), the fluid gets heated stepwise, storing energy in each PCM at its respective phase change temperature. During discharge (nighttime or on demand), cold water flows in the reverse direction, extracting heat first from the high-temperature PCM, then from the buffer, and finally from the low-temperature PCM, thereby delivering hot water at a sustained high temperature.
The total energy stored in the cascade collector (\( E_{store} \)) is the sum of sensible and latent heat contributions:
$$ E_{store} = \sum_{i=1}^{n} \left[ m_i c_{p,i} (T_{m,i} – T_{initial}) + m_i \Delta h_{f,i} \right] + m_w c_{p,w} \Delta T_w $$
where \( m_i \), \( c_{p,i} \), \( T_{m,i} \), and \( \Delta h_{f,i} \) are the mass, specific heat, melting point, and latent heat of the i-th PCM, respectively; \( m_w \) and \( \Delta T_w \) are the mass and temperature rise of the water buffer; and \( T_{initial} \) is the initial temperature.
The heat transfer during discharge for the composite PCM can be modeled considering conduction through the solidified layer and convection to the heat transfer fluid (HTF). The one-dimensional transient conduction equation is:
$$ \frac{\partial T}{\partial t} = \alpha \frac{\partial^2 T}{\partial x^2} $$
where \( \alpha = k / (\rho c_p) \) is the thermal diffusivity. The enhanced thermal conductivity \( k \) of our composite directly increases \( \alpha \), leading to faster discharge rates. The boundary condition at the PCM-U-tube interface involves convective heat transfer:
$$ -k \frac{\partial T}{\partial x}\bigg|_{interface} = h (T_{interface} – T_{HTF}) $$
where \( h \) is the convective heat transfer coefficient.
2.2 Experimental Procedure for Discharge Characterization
The discharge performance of the integrated solar system was tested under controlled conditions. The collector was first charged using natural solar irradiation from 9:00 AM to 2:00 PM. In the evening, the discharge test was initiated. A constant temperature water tank, set at 20°C, supplied water to the system inlet at a fixed flow rate of 90 L·h-1 (simulating a typical demand). Temperature sensors (PT1000) recorded the temperatures at key points: the inlets and outlets of the collector, and within the different PCM zones. The outlet water temperature over time was the primary indicator of the system’s thermal discharge capability.
3. Results and Discussion: Discharge Performance of the Solar System
The discharge curve, depicting the outlet water temperature and the temperature of the composite PCM zone over time, is the critical result. Under the test conditions (inlet water at 20°C, flow rate 90 L·h-1), the solar system demonstrated exceptional heat release stability and capacity.
The composite Ba(OH)2·8H2O PCM exhibited a clear and stable temperature plateau during discharge, confirming the effective elimination of supercooling. Its temperature remained steady near its phase change point for an extended period as it released its latent heat.
The performance of the overall cascade storage solar system was remarkable. The outlet water temperature remained above 70°C for the first 20 minutes of the discharge test. This high-temperature delivery is a direct benefit of the cascade design, where the high-temperature PCM zone (with the composite) provides the initial thermal boost. Subsequently, the outlet temperature decreased gradually but remained above 50°C for more than 60 minutes. This extended period of usable hot water supply highlights the high energy density and effective sequential discharge of the cascade storage unit.
The total thermal energy delivered (\( E_{delivered} \)) during the test can be calculated by integrating the heat transfer rate over time:
$$ E_{delivered} = \int_{0}^{t} \dot{m} c_{p,w} [T_{out}(\tau) – T_{in}] d\tau $$
where \( T_{out}(\tau) \) is the time-varying outlet temperature. The long duration above 50°C indicates a high value for \( E_{delivered} \), showcasing the system’s effectiveness in storing solar energy. The integration of the composite PCM with enhanced thermal conductivity ensured a rapid and responsive heat transfer rate from the storage medium to the water, contributing to the high initial outlet temperatures.
This solar system successfully addresses two key objectives: (1) it provides a stable and sustained supply of hot water at temperatures suitable for domestic use, and (2) by integrating with the PV/T panel, it actively cools the photovoltaic cells, leading to higher electrical energy generation. The synergy between the high-performance composite PCM and the cascade storage architecture is crucial for this dual benefit.
4. Conclusion
This work successfully developed and characterized a high-performance composite phase change material based on Ba(OH)2·8H2O for medium-temperature solar thermal storage. The key material-level achievements are:
- Elimination of Supercooling: The addition of 1.0 wt% BaCO3 as a nucleating agent reduced the supercooling degree from 3-5°C to nearly negligible levels.
- Enhanced Thermal Conductivity: The dispersion of 0.2 wt% nano-iron particles increased the thermal conductivity by 11.7%, from 1.225 to 1.368 W·m-1·K-1, facilitating faster charge/discharge rates.
- Retained High Energy Density: The optimized composite maintained a high latent heat of 277.1 kJ·kg-1 and exhibited excellent thermal stability over 100 cycles.
The composite PCM was integrated into a novel cascade thermal storage solar PV/T system. The experimental evaluation of this complete solar system demonstrated its outstanding practical performance:
- The cascade design, utilizing multiple PCMs and a water buffer, enabled the delivery of high-temperature water (>70°C) initially, followed by a prolonged supply of usable hot water (>50°C for over 60 minutes).
- The composite PCM discharged its latent heat isothermally and reliably, validating the success of the material modifications in a real application.
- The overall solar system effectively stores solar energy and can provide thermal energy on demand while simultaneously improving the electrical output of the photovoltaic components through active cooling.
This research provides a comprehensive solution, from material engineering to system design, for efficient solar energy utilization. The developed composite PCM and the cascade storage collector concept represent significant steps forward in creating more efficient, reliable, and practical solar systems for combined heat and power generation in residential and commercial applications. Future work may focus on long-term durability testing, optimization of the cascade temperature grades, and economic analysis of the integrated solar system.