PhD Oral Exam - Efstratios Dimitrios Rounis, Building, Civil and Environmental Engineering

When studying for a doctoral degree (PhD), candidates submit a thesis that provides a critical review of the current state of knowledge of the thesis subject as well as the student’s own contributions to the subject. The distinguishing criterion of doctoral graduate research is a significant and original contribution to knowledge.

Once accepted, the candidate presents the thesis orally. This oral exam is open to the public.

Abstract

Open-loop air-based building integrated photovoltaic thermal (BIPV/T) systems have the potential to become integral elements of net-zero or near net-zero building design. In addition to on-site electricity generation, BIPV/T systems offer various options for producing useful heat when coupled with the building’s HVAC system. Furthermore, when properly designed, they can fulfil building envelope functions for heat, moisture and air transfer, thus replacing the building envelope exterior layer, while offering superior architectural value compared to racked or building-applied photovoltaic (PV) systems.

Despite their potential, these systems comprise a very small share of PV applications and are not yet a mature technology. This can be attributed to several factors. Firstly, there are no standardised design guidelines, with the majority of realized systems being custom designs, not easily repeatable or scalable, and not adherent to established building techniques. Secondly, the documented performance of PV/T and BIPV/T systems is highly variable due to varying prototype designs and inconsistent testing conditions. Finally, the modelling of such systems is equally inconsistent, especially with regard to the wind-driven and channel flow convective heat transfer, elevating uncertainties in the prediction of a system’s performance, further reducing confidence in such applications. The latter usually results in non-optimal thermal utilization, and overheating of the PV panels that will affect their durability.

The main objective of this thesis is to set the foundation for a novel design and modelling approach for BIPV/T systems that can lead to increased share of applications of power generating envelopes (facades and roofs) for both new building constructions and retrofits, as well as enhanced system performance and durability.

This research consists of two main parts. The first part focuses on BIPV/T design considerations and investigates the adoption of curtain wall design techniques, modified for the BIPV/T systems. The purpose of this part is to address the lack of design standardization of BIPV/T and set the foundation for the adoption of common building practices in BIPV/T design, also incorporating concepts of modularity and prefabrication. To this end, the design, development and the indoor experimental testing of a modular BIPV/T curtain wall prototype is presented. The prototype was conceived as part of a large façade application and was built using commercially available curtain wall mullion extrusions and frameless semi-transparent, glass-on-glass PV (STPV) modules with two levels of transparency. Furthermore, several thermal enhancement techniques deemed suitable for building integrated systems were incorporated, namely multiple air intakes, using transparent instead of opaque PV modules, as well as a specially built flow re-direction component. The prototype was tested in an indoor solar simulator facility under conditions representative of full-scale demonstration projects. It was found to have comparable or better thermal performance (26-32%) compared to other systems in literature (17-32%), with potential for further improvement if optimized in terms of its geometry and flow rate.

The second part of this work presents a novel approach for the modelling of convective phenomena, which takes into consideration the interlinked nature of wind-driven and channel flow-driven convection of air-based BIPV/T systems. Indeed, part of the reason for the lack of standards for BIPV/T systems is the lack of a widely accepted method for modelling convective phenomena. The key parameters that have been found to affect the thermal performance of a BIPV/T system, including the environmental (or boundary) conditions, are formulated into dimensionless groups and correlated to the ratio of wind-driven convective heat transfer over the system’s heat recovery. This correlation was verified through solar-simulator testing of a modular BIPV/T system under varying environmental conditions, flow rate, channel aspect ratio and PV module opacity. Outlet air temperature predictions from the proposed modelling approach showed very good agreement with the experimental results (within ±0.4°C), as well as superior performance compared to commonly used modelling approaches (which have accuracy of ±3.0°C). Literature has shown that use of said approaches could result in up to more than 10°C error in the predicted outlet air temperature, resulting in poor thermal utilization and possible overheating of the PV panels.

This methodology can be tailored to individual systems via calibration through key temperature monitoring and can be instrumental in the optimal control and heat utilization for a coupled BIPV/T-HVAC system. In addition, it yields increased durability and performance of the PV installation through incorporation of more efficient cooling strategies, through accurate outlet air temperature and PV temperature predictions, respectively.