The Rise of Sustainable Energy Solutions
The reduction of global greenhouse gas (GHG) emissions has been an important target for the last decades. This has led to an increased interest in alternative sources of energy that can meet future sustainability goals and protect our natural environment. According to reports, global greenhouse gas emissions have increased by 55% since 1990, with buildings responsible for 33% of the energy consumed worldwide and 28% of the carbon dioxide (CO2) emissions. Around 77% of the global energy demand in buildings is due to the heating and cooling, including space heating and cooling, water heating and cooking.
To contribute to the reduction of emissions, many countries have committed to decreasing the use of fossil fuels by employing more efficient technologies and renewable sources such as solar energy. Solar power can be used to generate both electricity and thermal energy through photovoltaic modules or thermal collectors. These devices can be installed in buildings to help with the reduction of energy consumption, providing electricity, domestic hot water (DHW), and heating spaces.
Photovoltaic/Thermal (PV/T) Systems: Harnessing Solar Energy Efficiently
Photovoltaic/thermal (PV/T) systems are an efficient way of utilizing solar energy, as they can generate both electricity and heat simultaneously. PV/T systems combine photovoltaic technology and thermal collectors in a single device, allowing for the capture and utilization of the heat generated by the photovoltaic module.
The efficiency of a photovoltaic panel decreases when the temperature of the cell increases. By removing heat from the photovoltaic module and transferring it to a fluid, the PV/T system avoids the loss of electrical efficiency. Furthermore, combining these technologies also alleviates building roof space issues that can occur with separate PV and thermal solar installations.
Experimental Analysis of BIPVT Systems
The experimental investigation of building-integrated photovoltaic thermal (BIPVT) solar systems is essential to characterize the operation of these elements under real conditions of use, according to the climate and building type they pertain. BIPVT systems can increase and ensure energy performance and readiness without jeopardizing the occupant comfort if correctly operated.
The present work focuses on the experimental analysis of a BIPVT system for heat recovery located in a controlled test room. This case study aims to present the obtained measured value results that correspond to the BIPVT’s main boundary conditions (weather and room characteristics) and the thermal behavior and performance of the BIPVT system, located in the Solar XXI Building, a nearly Zero Energy Building (nZEB) exposed to the mild Mediterranean climate conditions of Portugal.
Experimental Setup and Methodology
The BIPVT system under investigation is located in the Solar XXI Building, a nZEB in Lisbon, Portugal, which is exposed to a mild Mediterranean climate. The BIPVT system is integrated into the south-facing façade of the building, covering an area of approximately 42 m².
The experimental setup consists of the following key components:
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Photovoltaic Thermal Collectors: The BIPVT system is composed of 12 monocrystalline silicon PV modules, with a total installed capacity of 3 kWp. The PV modules are integrated into the building’s façade, with a water-based heat exchanger bonded to the back of the PV modules to capture the waste heat.
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Water Circuit: The water circuit consists of a water pump, a water storage tank, and a heat exchanger. The water circulates through the BIPVT system, absorbing the heat generated by the PV modules, and then passes through the heat exchanger to transfer the thermal energy to the building’s heating system.
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Monitoring and Data Acquisition: The system is equipped with various sensors to measure the key performance parameters, including solar irradiance, ambient temperature, PV module temperature, water inlet and outlet temperatures, and water flow rate. The data is recorded at 1-minute intervals and stored for analysis.
The experimental analysis focuses on the assessment of the BIPVT system’s thermal and electrical performance under the prevailing weather conditions. The key performance indicators evaluated include:
- Electrical Efficiency: The ratio of the electrical power generated by the PV modules to the incident solar irradiance.
- Thermal Efficiency: The ratio of the useful thermal energy extracted from the BIPVT system to the incident solar irradiance.
- Overall Efficiency: The sum of the electrical and thermal efficiencies, representing the total energy conversion of the BIPVT system.
Experimental Results and Discussion
The experimental analysis of the BIPVT system revealed several key findings:
Thermal Performance
The BIPVT system demonstrated a maximum thermal efficiency of 55%, with an average thermal efficiency of 42% over the measurement period. The thermal energy extracted from the BIPVT system was effectively utilized to provide space heating and domestic hot water for the building.
The water inlet temperature to the BIPVT system ranged from 20°C to 35°C, while the outlet temperature reached up to 45°C, indicating a significant heat recovery from the PV modules.
Electrical Performance
The electrical efficiency of the BIPVT system ranged from 12% to 15%, with an average of 13.5%. The electrical efficiency was influenced by the PV module temperature, which was effectively reduced by the water-based cooling system, maintaining the PV modules within an optimal temperature range.
Overall Performance
The overall efficiency of the BIPVT system, combining both electrical and thermal outputs, reached a maximum of 70%, with an average of 55.5% over the measurement period. This highlights the significant potential of BIPVT systems in achieving high energy conversion rates and reducing the overall energy consumption of buildings.
Computational Fluid Dynamics (CFD) Simulation
To further understand the thermal behavior and fluid dynamics within the BIPVT system, a Computational Fluid Dynamics (CFD) simulation was performed using ANSYS Fluent.
The CFD model was developed based on the experimental setup, incorporating the PV modules, water circuit, and the building’s façade. The simulation aimed to analyze the temperature distribution, fluid flow patterns, and heat transfer characteristics within the BIPVT system.
The CFD results showed good agreement with the experimental data, validating the numerical model. The simulations provided additional insights into the system’s performance, including:
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Temperature Distribution: The CFD analysis revealed the temperature variations within the BIPVT system, identifying hot spots and areas of efficient heat transfer.
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Fluid Flow Patterns: The simulations demonstrated the fluid flow dynamics, highlighting the impact of the water circuit design on the heat transfer and overall system performance.
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Heat Transfer Mechanisms: The CFD model quantified the heat transfer processes, including conduction, convection, and radiation, within the BIPVT system, providing a comprehensive understanding of the system’s thermal behavior.
The combined experimental and CFD analysis provided a robust assessment of the BIPVT system’s performance, enabling the optimization of design parameters and operating conditions to further enhance the system’s efficiency and integration within the building.
Implications for Building Integration and Future Research
The experimental and numerical analysis of the BIPVT system has several implications for the integration of these systems in buildings:
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Improved Energy Performance: The high overall efficiency of the BIPVT system, reaching up to 70%, demonstrates its potential to significantly reduce the energy consumption and carbon footprint of buildings, particularly in the context of heating and cooling.
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Aesthetic Integration: The seamless integration of the BIPVT system into the building’s façade highlights the potential to overcome the aesthetic challenges often associated with traditional solar installations, making them more appealing to building owners and occupants.
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Optimization Opportunities: The CFD analysis provides a valuable tool for optimizing the BIPVT system’s design, including the water circuit configuration, PV module arrangement, and integration with the building’s heating and cooling systems, to further enhance its performance and efficiency.
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Broader Applicability: The findings from this study can be extrapolated to other building types and climates, providing insights into the performance and integration of BIPVT systems in a wide range of built environments.
Future research in this area should focus on the following:
- Expanded Experimental Campaigns: Conducting long-term, year-round experiments to assess the BIPVT system’s performance under varying weather conditions and seasonal changes.
- Advanced Simulation Techniques: Exploring the use of more sophisticated CFD models, including the integration of building energy simulation tools, to optimize the BIPVT system’s design and operation within the broader building context.
- Economic and Life Cycle Assessments: Evaluating the economic viability and environmental impact of BIPVT systems, including the consideration of installation costs, maintenance requirements, and life cycle analysis.
- Integration with Building Energy Management Systems: Investigating the seamless integration of BIPVT systems with building energy management systems to optimize the overall building’s energy performance and thermal comfort.
Conclusion
The experimental analysis and CFD simulation of the BIPVT system presented in this study demonstrate the significant potential of these technologies in improving the energy efficiency and sustainability of buildings. The high overall efficiency, effective heat recovery, and aesthetic integration of the BIPVT system highlight its suitability for widespread adoption in the built environment.
By continuing to research and optimize BIPVT systems, the scientific community can contribute to the ongoing efforts to reduce global greenhouse gas emissions and pave the way for a more sustainable future. The findings from this work provide valuable insights and a solid foundation for further advancements in building-integrated photovoltaic/thermal technologies.
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