With an estimated global share of greenhouseemissions of 35%, the building sectormust be the target of significant transformation. These next decades represent one of the best opportunities for a giant energy consumer to become an efficient energy prosumer. Of the many technical possibilities available that can help in this objective, building integrated photovoltaic systems is becoming one of the most salient because of their versatility of application and simplicity of operation. However, to achieve their projected market growth, BIPV products must overcome technical, financial, and social barriers.

The interrelationship between these barriers creates requirements that are sometimes in conflict with each other. Research suggests that the most important parameters for the financial viability of a BIPV system (studied in North America) are its electrical performance and its effective lifetime. However, the social acceptance of BIPV systems, among architects and other consumers, increases when photovoltaic modules are provided with a variety of colors, resulting in losses in their electrical output. Furthermore, a BIPV system can present an installation layout in which its modules operate at high values of temperature, which hinders their useful lifetime.

Significant research has been done to tackle these requirements, striving to find a balance in which each, however contradictory, is met. This thesis seeks to add to this body of research through modeling and experimental efforts focused on creating techniques and concepts to improve the aesthetics and thermal behavior of photovoltaic modules.

After an introductory chapter, Chapter 2 presents an overview of the efforts done so far to improve aesthetics and passively cool photovoltaic modules. The challenges and barriers that remain from a technical perspective are outlined, and the base modeling approach deployed to tackle them is introduced. Chapter 3 presents how this model can be complemented by auxiliary algorithms to find ways to provide color directly to c-Si solar cells by using interference optical filters. The chapter also provides insight into ways of stabilizing color and the beneficial impact that a color optical filter has on the operating temperature of a c-Si solar cell.

Chapter 4 expands on the findings of Chapter 3 and discusses that the application of the color optical filter on the front glass of a PV module provides better benefits, particularly in terms of better color saturation and more vivid hues. In addition, it provides guidelines for how colorimetry can be added to the modeling effort to improve the quality of color matching and color stability of a color photovoltaic module based on optical filters. Furthermore it demonstrates that it is possible to achieve colorful designs with relative DC energy losses below 10%.

Chapter 5 argues that optical filters can also be designed to provide thermal control to photovoltaic modules. Furthermore, this thermal control can be achieved by taking advantage of the harmonic reflectance produced by simple designs. It shows that the analysis of a thermal management solution must always consider their benefits related to extended lifetime. A simple optical thermal filter, despite its lossy nature (in terms of electrical output), can still provide a net benefit in terms of energy yield when this benefit is accounted for.

Chapter 6 presents the huge cooling potential provided by phase change material (PCM). This work is entirely experimental and demonstrates that a single type of PCM can be used in different locations, with different installation layouts, to provide substantial temperature reductions and increase the electrical output of photovoltaic modules with great consistency, even during the wintermonths.

Chapter 7 presents the novel concept of a photovoltaic chimney, developed with the purpose of studying the potential use of the thermal energy produced by photovoltaic modules. The concept was analyzed using common modeling approaches to calculate mass flow and heat production, offering quick ways to create sensitivity analysis for earlier stages of design. These initial stages of the model can be used to have insight into the quality (or lack thereof) of the heat produced and its potential use to improve the ventilation of buildings.

Chapter 8 completes this dissertation, highlighting its main conclusions and providing details on potential areas that can drive future research.
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In this section, we described the various device architectures in production for crystalline-silicon PV as well as the various absorber materials being used and developed for thin-film PV. We also explained why multijunction PV technology is considered the future of PV technology. We identified four major technological challenges (Fig. 1) for future mass production and deployment of PV technology in order to achieve the goal of climate neutrality and indicated how optics could help resolve these challenges. The four main challenges (Fig. 1) are: • Sustainable production of PV systems; • Higher energy conversion efficiencies; • Higher energy output and operational lifetime; • Integration of PV systems in their environment.

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The ongoing development of photovoltaics into terawatt scale poses a number of challenges where the optics and photonics communities can contribute. An international consortium recently compiled a roadmap that elaborates on these challenges.

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This work is a long-term, interannual, and experimental study conducted in multiple locations. It studies the effects of phase change materials (PCMs) on photovoltaic modules’ performance by reducing their operational temperature. Two PV modules were manufactured so that PCM slabs could be mechanically attached to their backside, ensuring contact with the related photovoltaic active area. Experiments were conducted in Delft, Netherlands, from 2019 until 2021 and in Catania, Italy, during the winter and start of spring of 2023. The experiment also considered two installation layouts: building integrated (Delft) and standard rack-mounted (Catania). The measurements showed that the PCM provides significant cooling under both locations, with a temperature reduction of up to 15 °C. In Delft, thermal control could be obtained for most of the sunny hours of the day, even during the summer months. In Catania, the module with PCM presented, on occasion, higher temperatures than its standard counterpart, primarily due to winter-time environmental conditions. However, the PCM provided sufficient thermal control on all conditions, ensuring increased energy yield. This increase ranged from 2.1 to 2.5 % in Delft and 1.3–1.6 % in Italy.

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This work presents a practical approach to designing an optical filter for thermal management for photovoltaic modules. The approach emphasizes the practicality of manufacturing over optical performance. Simulation work demonstrates that, for an interdigitated back contact solar cell architecture, complete rejection of infrared radiation offers limited thermal benefits requiring highly complex optical filter designs. An alternative approach consists of reducing thermalization losses by providing reflectance at lower wavelength values. An optical filter design that fulfills this requirement is possible using simple structures based on two materials and taking advantage of the harmonics present in quarter wavelength optical thickness designs. The filter is later optimized for angular performance via second-order algorithms, resulting in a device consisting of only 15 thin-film layers. Performance simulations on two locations, Delft (the Netherlands) and Singapore, estimate a temperature reduction of 2.20°C and 2.45°C, respectively. In a single year, the optical loss produced by the filter is not compensated via temperature reduction. However, improvements in the annual degradation rate show that in Singapore, the overall effect of the filter on the lifetime DC energy yield is positive.

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Herein, the application of a comprehensive modeling framework that can help optimize the design of multilayered optical filters for coloring photovoltaic (PV) modules is presented based on crystalline silicon solar cells. To overcome technical issues related to the implementation of color filters (CFs) on PV modules, like glare and color instability, colorimetry metrics, such as the hue, chroma, luminance color space, and the quantitative concept of difference between two colors are extensively deployed. It is showcased in this work that designing colored modules with high hue and chroma stability is possible by using a front-side texturing with edged geometry, like V-shaped grooves and inverted pyramids, while obtaining colors with relatively high luminance values, indicating good brightness. Furthermore, it is argued that adapting the rear surface of the front glass with a random textured layout where the CF is applied can improve color and luminance stability without significant loss of chroma while eliminating glare. Finally, the models can be used to optimize the number of layers for a given CF, reducing unnecessary optical losses. Compared to a standard PV module, performance simulation of optimized, bright-colored PV modules predicts relative energy yield losses ranging from 7% to 25%.

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When integrated into urban environments, photovoltaic (PV) systems usually present operational temperatures that are significantly higher than those shown by rack-mounted systems. High operating temperatures are associated with reduced reliability of PV modules and significantly impact the electrical performance of solar cells. Utilizing the heat produced on PV modules or reducing operating temperatures can bolster their application within the building sector. We present the three main concepts studied to achieve these goals. First, a PV is a chimney concept that allows the use of the heat generated by the modules. Simulations for a PV chimney installed on a building in the Netherlands showed that although the heat quality produced inside its cavity was low, the potential use of the air mass flow for ventilation applications is promising. Additionally, we present two passive cooling solutions that can reduce the operating temperatures of PV modules: Optical filters and phase change materials. Experimental measurements in Delft showed that these solutions reduce the operating temperature of PV modules between 4 °C to 20 °C, particularly under high irradiance hours.

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Building Integrated Photovoltaic systems can produce a significant portion of the energy demand of urban areas. Despite their potential, they remain a niche technology that architects and project engineers still find esthetically limited. The dark blue or black color of standard photovoltaic panels is considered inappropriate for restoration projects of historic buildings and represents a major constraint on the development of new projects. This work will provide insight into how the use of optic filters can offer new pathways for architectural acceptance of photovoltaic panels. Optic filters selectively reflect or transmit light by interference and can be designed and fabricated using cost-effective and industrially compatible processes. By using in-house developed ray tracing software coupled with TCAD Sentaurus, more than 400 colors were obtained, and their impact on the opto-electrical performance of interdigitated back-contacted solar cells was studied. Results show a maximum efficiency loss of 1.6% absolute at the perpendicular incidence of light on the range of obtained colors when compared with a standard dark blue solar cell. Simulations for different angles of incidence showed that the current reduction on the standard device could be modeled using a cosine relationship. The colored cells, however, deviated significantly from this relationship. We propose that the angular behavior of any cell (colored or standard) could be simulated by modifying the effective irradiance with scaling factors equal to the ratios of the photogenerated current at any angle with respect to the value at normal incidence. We demonstrate that this approach accurately models the effect of the color filter and allows for an easy transition from a bare cell to an encapsulated one. Due to the spectral effect of the filter, we developed both a spectrally resolved optical model and a two-dimensional finite volume transient thermal model. In case of the optical model, we demonstrate an accuracy in the prediction of the reflectance produced by the color with values of mean bias error (MBE) between 2.0% and 3.9%. As for the thermal model, it was validated by first analyzing a standard model under conditions of nominal operating cell temperature and then comparing its results with published scientific literature. Later, we compare its prediction against 2 weeks of measurements. In both cases the thermal model proves an adequate accuracy, yielding differences below 1.5°C with respect to other scientific works and an MBE value of 0.89°C as well as a root-mean-square error value of 2.10°C for the entire measurement period. With the validated models, we studied the effect of the encapsulation on the color perception. We present two options of color filters. The first one produces relatively low reflectance losses and presents relative annual direct current (DC) energy losses of up to 6.4% for Delft, in the Netherlands, and up to 5.9% for Alice Springs in Australia. However, this first option has very poor color brightness. The second studied filter produces highly saturated bright colors. Improving brightness can increase the annual DC relative losses up to 13.7% and 13.5% for Delft and Alice Springs, respectively. Overall, we demonstrate that colored filters based on multilayer optical stacks are a versatile option for coloring cells that allow a good compromise between esthetics and performance.

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This work presents the concept of a photovoltaic (PV)-powered solar chimney. We modeled and experimentally studied the integration of a PV system within a naturally ventilated façade (NVF), attempting to use the inherent cavity as a ventilation channel to transfer heat. Thermodynamic models were created to study the thermal and, therefore, the electrical performance of a PV system installed at different positions within the cavity of the NVF. An experimental setup of the PV chimney was manufactured to validate the computational models. Results show low root mean square error (RMSE) values for the prediction of the mass flow and the temperature of the different materials considered in the chimney. A basic sensitivity analysis was performed to find the best position of the PV modules within the chimney for a three-story household in the Netherlands. Optimization showed that with a cavity depth of 0.2 m with PV modules located at the front layer, the electric annual yield is maximized. For the same cavity depth, placing the modules in the middle significantly increases heat flow production, albeit with a reduction on electrical performance.

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We report on an advanced modelling approach to accurately predict the energy yield of custom environment / urban integrated photovoltaic systems (E/UIPV). Several submodels are here presented, and their mutual interaction discussed. The flexibility of our software platform allows to exhaustively simulate custom horizons in combination with rigid/flexible PV modules and in presence of albedo component. In this respect, a modelling example predicts AC-side yield with <1% error on annual basis with respect to actual data. In addition, our platform can also deal with colored/bifacial modules and soiling losses for PV energy yield potential or performance prediction.

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