PhD Thesis defense by Mr Nikolaos Savakis
09/19/2019
Phd Title «Investigation, development and experimental evaluation of thermal management techniques for photovoltaics with the aim to enhance their energy efficiency and ecological footprint»
Tuesday 17 July 2021, at: 12:00,
Venue: Teleconference
https://tuc-gr.zoom.us/j/87443344555?pwd=RVh0b1hSc21kWWhaWTNDcSs5YnJDQT09
Meeting ID: 874 4334 4555
Password: 349656
Supervisor: Theocharis Tsoutsos
Seven-membered Examination Committee:
- Professor Theocharis Tsoutsos
- Professor Konstantinos Kalaitzakis
- Professor Mavromatakis Fotios
- Professor Mihalis Lazaridis
- Professor Dionysia Kolokotsa
- Professor Agis Papadopoulos
- Professor Georgios Papadakis
Abstract
Recently, the penetration of Renewable Energy Sources (RES) in the final energy consumption is considered as a key measure to curb climate change impacts and support the transition from a fossil-fuelled energy system to clean energy. To this end, the exploitation of solar energy became one of the most promising options in order to respond to the ever-growing energy needs globally. Among several solar applications, photovoltaic (PV) technology has been widely deployed and is anticipated to have a primary role in realising the vision of sustainability. To further strengthen its growth, it is essential the control of specific parameters, that will lead to increasing the PV systems’ efficiency.
These aspects include the solar irradiance levels, the PV cell technology, as well as the PV cell operating temperature, which is partly transformed into thermal energy. This thermal energy provokes PV module temperature rise and reasonable energy losses. The high operating temperature of PVs do not only limit energy conversion efficiency but also affects their lifetime. Consequently, heat removal from the PV module’s surfaces is considered of crucial importance to maximize PV efficiency. Therefore, potentially applicable cooling manners, have been introduced and investigated by several researchers. Among these techniques, taking advantage of Phase Change Materials (PCMs) has earned remarkable attention, since they can absorb an increased quantity of thermal energy at a consistent temperature range of phase alteration and by extension, serve for heat deposition and temperature handling. To contribute to this research area, this PhD thesis is focused on the theoretical analysis, design, and experimental evaluation of the overall performance of PV+PCM systems during their operation by deepening insights into this passive approach under actual Mediterranean conditions. The prior step before the analysis was the development of a general methodology adopted for the PV+PCM system design and evaluation. This methodological approach is based on a reasonable modification of the energy balance model for typical PV modules. Following, an extensive and intensive experimental process under real field conditions took place in two distinct phases, and on different experimental set-ups, to clearly understand the operational aspects of the proposed method (e.g., seasonality, efficiency, reliability, etc.).
During the 1st stage of this study, a modified photovoltaic system (PV+PCM system) combining a conventional photovoltaic module (PV) with a selected type of Phase Change Material (PCM) is assessed in the Mediterranean climate during a year-long period. This type of analysis allowed us to recognise the core operational aspects (e.g., seasonal performance, stability) of such a system and serve a deeper understanding of the potential faults and underestimations. According to the results of this study, the operating temperature difference could arise up to 26,1 °C, so the yearly power generation of the new system increases by 5,7%. To explore the evolutionary possibilities of PV+PCM systems, the present study focuses on the development of an alternative design approach regarding the PCM container. The peculiarity of this approach relies on the combination of the main advantages of a heat sink (i.e., enhanced heat transfer with natural convection) and a heat absorber (i.e., heat absorption). In this research, the inclusion of Phase Change Materials (PCMs) through an alternative type of enclosure in tubular shape was proposed and investigated as an option of mitigating the PV operating temperature to enhance their efficiency and lifetime. Two PVs incorporating different PCMs (PV+PCM systems) and a conventional PV module (reference case) were experimentally tested to assess their energy performance under Mediterranean conditions. As PCMs Paraffins RT27 and RT31 were selected. The results indicated that a peak temperature decrement of 6,4 °C and 7,5 °C could be observed by using 260 g of PCM27 and PCM31, respectively. Hence, PV+PCM27 and PV+PCM31 systems exhibited increased energy generation by 4,19% and 4,24%, respectively. The proposed configuration of PCM enclosures took advantage of the synergistic effect of wind, as demonstrated by the recorded daily temperature profiles of the PV+PCM27 and PV+PCM31 systems, even after the time of complete PCMs' melting.
In addition, the actual data collected from a PV+PCM system, constructed and operated under the Mediterranean climate conditions (Greece), were further exploited to estimate its environmental performance using the Life Cycle Assessment (LCA) methodology. The integration of PCM cooling increases PV’s total environmental footprint by ⁓11,7%, but at the same time also increases PV panel’s lifespan and electricity output. In that sense, reasonable environmental gains through displacing fossil-fuel-dependent electricity from Greece’s energy mix may appear. In conclusion, the results of the present dissertation refer to substantial findings on the benefits arising from the proposed passive cooling method (using PCM) for PV modules and complement the existing literature on the thermal and energy response of PV+PCM systems in field conditions.
