Building integrated solar concentrating systems: A review

© 2019 Elsevier Ltd In the building sector, concerns towards the vast energy consumption has promoted the development of renewable energy technologies. In this regards, the solar concentration devices show a promising concept for building applications. However, the solar concentrators for application in buildings have many restrictions, which are different from the traditional solar concentrators. The main objective of this paper is to present a concise review on the building integrated concentrating devices, that have their own characteristics and multiple functions. This paper made a classification based on device's functions, i.e. building integrated concentrated photovoltaic systems (BICPV), building integrated concentrating solar thermal (BICST) and building integrated concentrating solar daylighting (BICSD) and the combination of functions, i.e. BICPV/T, BICPV/D, BICST/D and BICPV/T/D. At the same time, this paper presented an elaborate introduction of the demands, types and applications of the building integrated concentrating devices and prospects/ directions/ policies about these technologies around the world. The review would provide important information for the actual engineering of building integrated concentrating devices

Novel up-conversion concentrating photovoltaic concepts

This thesis summarises a set of experiments towards the integration of concentrating optics into up-conversion photovoltaics. Up-conversion in rare earths has been investigated here. This optical process is non-linear therefore a high solar irradiance is required. High solar irradiance is achievable by solar concentration. Two concentrating approaches were investigated in this thesis: The first approach involved the concentration of the incident solar irradiance into optical fibres. An optical system with spherical lenses and dielectric tapers was designed accordingly. A solar concentration of 2000 suns was realised at the end of a single optical fibre. In addition to the total solar concentration, the spectral dependence was characterised to account for the effect of chromatic aberrations. Then, the solar concentration could be transferred into rare earth-doped fibres. For this reason, a series of experiments on double-clad erbium-doped silicate fibres was carried out. Although up-conversion in this type of fibre is minimised, the measured power dependence agrees with up-conversion via excited state absorption. In the second approach, concentrating optics were integrated in up-conversion solar cells. The role of the optics was to couple the photons transmitted by the solar cell to the rare earth up-converter. Therefore, imaging and non-imaging optics were investigated, with the latter exhibiting ideal coupling characteristics; concentration and high transmission of the incident irradiance, but also efficient collection of the up-converted emission. Out of the non-imaging optics, the dielectric compound parabolic concentrator fulfilled these characteristics, indicating its novel use in up-conversion solar cells. Two erbium-doped up-converters were utilised in this approach, beta-phase hexagonal sodium yttrium tetrafluoride (β-NaYF4:25%Er3+) and barium diyttrium octafluoride (BaY2F8:30%Er3+). The latter performed best, with an external quantum efficiency (EQE) of 2.07% under 1493 nm illumination, while the former exhibited an EQE of 1.80% under 1523 nm illumination both at an irradiance of 0.02 W/cm2. This corresponds to a relative conversion efficiency of 0.199% and 0.163% under sub-band-gap illumination, respectively, for a solar cell of 17.6% under standard AM1.5G conditions. These values are among the highest in literature for up-conversion solar cells and show the potential of the concentrating concept that can be important for future directions of photovoltaics.Engineering and Physical Sciences Research Council (EPSRC)European Community's Seventh Framework Program (FP7/2007-2013

Enhancing Performance of Building Integrated Concentrating Photovoltaic systems

Buildings both commercial and residential are the largest consumers of electricity. Integrating Photovoltaic technology in building architecture or Building Integrated Photovoltaics (BIPV) provides an effective means for meeting this huge energy demands and provides an energy hub at the place of its immediate requirement. However, this technology is challenged with problems like low efficiency and high cost. An effective way of improving the solar cell efficiency and reducing the cost of photovoltaic systems is either by reducing solar cell manufacturing cost or illuminating the solar cells with a higher light intensity than is naturally available by the use of optical concentrators which is also known as Concentrating Photovoltaic (CPV) technology. Integrating this technology in the architecture is referred as Building integrated Concentrating Photovoltaics (BICPV). This thesis presents a detailed performance analysis of different designs used as BICPV systems and proposes further advancements necessary for improving the system design and minimizing losses. The systems under study include a Dielectric Asymmetric Compound Parabolic Concentrator (DiACPC) designed for 2.8×, a three-dimensional Cross compound parabolic concentrator (3DCCPC) designed for 3.6× and a Square Elliptical Hyperbolic (SEH) concentrator designed for 6×. A detailed analysis procedure is presented showcasing the optical, electrical, thermal and overall analysis of these systems. A particular issue for CPV technology is the non-uniformity of the incident flux which tends to cause hot spots, current mismatch and reduce the overall efficiency of the system. Emphasis is placed on modelling the effects of non-uniformity while evaluating the performance of these systems. The optical analysis of the concentrators is carried out using ray tracing and finite element methods are employed to determine electrical and thermal performance of the system. Based on the optical analysis, the outgoing flux from the concentrators is predicted for different incident angles for each of the concentrators. A finite element model for the solar cell was developed to evaluate its electrical performance using the outputs obtained from the optical analysis. The model can also be applied for the optimization of the front grid pattern of Si Solar cells. The model is further coupled within the thermal analysis of the system, where the temperature of the solar cell is predicted under operating conditions and used to evaluate the overall performance under steady state conditions. During the analysis of the DiACPC it was found that the maximum cell temperature reached was 349.5 K under an incident solar radiation of 1000 W/m2. Results from the study carried on the 3DCCPC showed that a maximum cell temperature of 332 K is reached under normal incidence, this tends to bring down the overall power production by 14.6%. In the case of the SEH based system a maximum temperature of 319 K was observed on the solar cell surface under normal incidence. An average drop of 11.7% was found making the effective power ratio of the system 3.4. The non-uniformity introduced due to the concentrator profile causes hotspots in the BICPV system. The non-uniformity was found to reduce the efficiency of the solar cell in the range of 0.5-1 % in all the three studies. The overall performance can be improved by addressing losses occurring within different components of the system. It was found that optical losses occurred at the interface region formed due to the encapsulant spillage along the edges of the concentrator. Using a reflective film along the edge of the concentrating element was found to improve the optical efficiency of the system. Case studies highlighting the improvement are presented. A reflective film was attached along the interface region of the concentrator and the encapsulant. In the case of a DiACPC, an increase of 6% could be seen in the overall power production. Similar case study was performed for a 3DCCPC and a maximum of 6.7% was seen in the power output. To further improve the system performance a new design incorporating conjugate reflective-refractive device was evaluated. The device benefits from high optical efficiency due to the reflection and greater acceptance angle due to refraction. Finally, recommendations are made for development of a new generation of designs to be used in BiCPV applications. Efforts are made towards improving the overall performance and reducing the non-uniformity of the concentrated illumination

Design and performance evaluation of a low concentrating line-axis dielectric photovoltaic system

This thesis presents a detailed investigation of the design optimisation and performance analysis of a dielectric concentrator for building façade integration at high latitudes (>55°). Considering the seasonal variation of the sun’s position at these latitudes, a concentrating photovoltaic (CPV) system with stationary concentrators of large acceptance angle and low concentration ratio is a suitable alternative to conventional flat plate photovoltaic (PV) modules. A well designed dielectric asymmetric compound parabolic concentrator (ACPC) is a suitable choice to achieve optimum range of acceptance angles and concentration ratio for building façade integration in the Edinburgh and higher latitudes. A theoretical study of the optical performance shows that a truncated dielectric ACPC with acceptance half-angles of 0o and 55o (termed as DiACPC-55) is the optimum design, when compared to the dielectric ACPC designs with acceptance half angles of (0o and 66o) and (0o and 77o) in Edinburgh and higher latitudes. An increase in the range of acceptance angles is achieved by truncating the concentrator profile. Ray tracing simulations show the DiACPC-55 exhibits the widest range of acceptance angles compared to the other designs. The maximum optical efficiency of the DiACPC-55 is found to be 83%. In addition it is found to have a better intensity distribution at the receiver and a higher total annual energy collection, compared to the other designs. Thermal modelling of a CPV system with the DiACPC-55 concentrator shows that the solar cell and rear plate temperature can reach up to 41.6oC for 1000 W/m2 irradiance, when operating with an average ambient temperature of 10oC. The maximum power ratio of the CPV module (fabricated using the DiACPC-55 concentrator) to a similar non-concentrating counterpart is found to be 2.32, when characterised in an indoor controlled environment using a solar simulator. An average electrical conversion efficiency of 9.5% is measured for the entire range of acceptance angles. The optical loss analysis shows that incident light can escape from the parabolic sides and concentrator-encapsulation interface. The outdoor characterisation of the CPV module with the DiACPC-55 concentrator shows that a maximum power ratio of 2.22 can be achieved on a sunny day. In comparison, a maximum power ratio of 1.9 is observed on a rainy day. These results reveal that the designed dielectric concentrator is capable of collecting 68% of the diffuse radiation. The maximum electrical conversion efficiency of the CPV module in outdoor condition is found to be 9.4%. Module degradation due to the delamination of the solar cell is observed in the long term investigation study, which reduces the module efficiency to 8.6% on a clear sunny day. The fabricated CPV system with the DiACPC-55 concentrator is found to be £190.3/m2 cheaper than similar sized conventional glass-glass laminated modules. Therefore the cost of the CPV module is found to be £0.53/Wp cheaper than the conventional glass-glass laminated modules for building facade integration at high latitudes

Design and characterisation of a novel translucent solar concentrator

This thesis begins with an investigation into the optical performances of the Crossed Compound Parabolic Concentrator (CCPC) for photovoltaic application and introduces the novel concept of a Translucent Integrated Concentrated Photovoltaic (TICPV). The use of solar concentrators in BIPV enables a reduction in the cost of generating photovoltaic electricity lending to yet another field of research known as Building Integrated Concentrated Photovoltaics (BICPV). The potential of BICPV as the most promising technologies for future electricity supply is examined by the design, optimisation and testing of the main component of the TICPV, a novel static nonimaging transparent 3-D concentrator coined the Square Elliptical Hyperboloid (SEH), for the use in building fenestrations. The SEH concentrator was designed and optimised via ray-tracing technique. A preliminary investigation into the optical efficiencies of 160 SEH concentrators of varying geometries was conducted and from this 20 concentrators were chosen and studied in more detail using the developed optical model with the aim of obtaining an optimised SEH concentrator out of these 20. The optimisation process proved to be far from straightforward, however, after careful consideration, five SEH concentrators with the best optical performances, each with different heights, were chosen. These concentrators were fabricated and used to assemble five separate TICPV modules. Subsequent to carrying out the simulation, the five optimised TICPV modules were examined in different environments (indoor and outdoor). The results of the indoor test, where the TICPV modules are exposed to direct radiation from a solar simulator, provided clear validation of the optical model; the results of the outdoor test added further to the validation and confirmed the power output of the TICPV modules when exposed to both direct and diffuse radiations. The TICPV modules are developed in a way such that they collect sunlight during most of the hours throughout the day, allowing the generation of electrical power whilst maintaining the level of transparency of the fenestration. It was found that the TICPV modules are capable of saving more than 60% of the solar cells used in conventional flat PV systems. The designed TICPV modules simultaneously provide solar energy generation and optimised day lighting. The TICPV module designed in this thesis provides a viable solution to coping with the increasing energy demands and will create a new age of energy efficient buildings reducing the carbon footprint of both existing buildings and buildings of the future

Development of reflective low concentrated photovoltaic/ thermal system

This work aims to investigate the performance of a new design for CPV/T system using 3D flat sided (3D V-trough) concentrators named squared (SAC), Hexagonal (HAC), Octagonal (OAC) and Circular (CAC) inlet and exit Aperture concentrators, with an effective cooling facility that keeps PV temperature within the Normal operating range. Novel mathematical optical models were generated, to the HAC and OAC geometries and validated using OPTISORKS software, to calculate the geometrical concentration ratio ($GCR$) and actual concentration ratio ($ACR$ by the inlet aperture area (A$_i$$_n$) a function of aperture width (W$_i$$_n$) and number of reflections (n), and material reflectivity ($p$) and at any concentrator side angle (Ψ), consequently the optical performance. Results showed that the optimum concentrator side angels for GCR of 2, 4, 6, 8 and 10 are 35°, 30°, 20°, 20° and 15°, respectively for all investigated geometries. Also COMSOL Multiphysics software was used for thermal modelling. Optical, thermal and electrical investigation results highlight that the designed CPV/T system is beneficial enough and feasible to be used in generating electrical and thermal powers for domestic use, as one useful package of energy with high output compared with the flat PV modules which generate only electrical power

Intelligent windows for electricity generation: A technologies review

Buildings are responsible for over 40% of total primary energy consumption in the US and EU and therefore improving building energy efficiency has significant potential for obtaining net-zero energy buildings reducing energy consumption. The concurrent demands of environmental comfort and the need to improve energy efficiency for both new and existing buildings have motivated research into finding solutions for the regulation of incoming solar radiation, as well as ensuring occupant thermal and visual comfort whilst generating energy onsite. Windows as building components offer the opportunity of addressing these issues in buildings. Building integration of photovoltaics permits building components such as semi-transparent façade, skylights and shading devices to be replaced with PV. Much progress has been made in photovoltaic material science, where smart window development has evolved in areas such as semi-transparent PV, electrochromic and thermochromic materials, luminescent solar concentrator and the integration of each of the latter technologies to buildings, specifically windows. This paper presents a review on intelligent window technologies that integrate renewable energy technologies with energy-saving strategies contributing potential solutions towards sustainable zero-energy buildings. This review is a comprehensive evaluation of intelligent windows focusing on state-of-the-art development in windows that can generate electricity and their electrical, thermal and optical characteristics. This review provides a summary of current work in intelligent window design for energy generation and gives recommendations for further research opportunities

Smart window photovoltaic concentrator for energy generation and solar control

Central to the global mission on reducing societies carbon footprint is the commitment of governments and international institutions to set energy reduction targets. In this regard, buildings are responsible for large energy loads. Due to the necessity to create thermal and visual comfort, vast energy is consumed to satisfy internal cooling, heating, and lighting loads. The two main strategies to reduce buildings energy consumption are renewable energy technologies and energy efficient building planning. Building Integrated PV systems (BIPV) are devices capable to generate electricity while replacing building materials and reduce electricity costs, protect the building from weather acting as a building envelope and offering aesthetically pleasing features to the building. Windows play key role in the building energy consumption allowing for sunlight and heat to enter the building. Some commercial technologies offer solar control functions using reversible photochromic, thermochromic or electrochromic mechanisms. However, only few offer an automated system able to respond to dynamic changes of the environment while producing onsite energy. The research presented in this thesis covers the details of the design and development of a novel lightweight solar concentrator for “smart window” applications. The smart window design was conceived to automatically control the solar radiation entering buildings and generate clean electricity at the same time, thus compensating artificial lighting, cooling, and heating loads. To achieve the dual functionality of the smart window two novel thermotropic membranes were developed and characterised using two gelling agents and 3 polymers. Transmittance levels of 95% in clear state and 40% when in light scattering state were achieved. A ray tracing model was validated against experimental indoor tests with 8% deviation. Indoor tests comparing between 2% wt. HPC & 1.5 % wt. GGF and 6% wt. HPC & 1.5 % wt. GGF membranes reported efficiency values of 3.7% and 5.1% and MPP values of 0.018W and 0.024W, respectively. Outdoor tests showed that the automated solar control function allows sunlight to pass through the smart window during the morning and the evening hours but block the sun when irradiation levels surpass 600 W/m2. The study concludes, however, that in order to produce a more efficient device the membrane reflectivity of the smart window should be close to 90%

Identification and Development of Novel Optics for Concentrator Photovoltaic Applications

Concentrating photovoltaic (CPV) systems are a key step in expanding the use of solar energy. Solar cells can operate at increased efficiencies under higher solar concentration and replacing solar cells with optical devices to capture light is an effective method of decreasing the cost of a system without compromising the amount of solar energy absorbed. CPV systems are however still in a stage of development where new designs, methods and materials are still being created in order to reach a low levelled cost of energy comparable to standard silicon based photovoltaic (PV) systems. This work outlines the different types of concentration photovoltaic systems, their various design advantages and limitations, and noticeable trends. Comparisons on materials, optical efficiency and optical tolerance (acceptance angle) are made in the literature review as well as during theoretical and experimental investigations. The subject of surface structure and its implications on concentrator optics has been discussed in detail while highlighting the need for enhanced considerations towards material and hence the surface quality of optics. All of the findings presented contribute to the development of higher performance CPV technologies. Specifically high and ultrahigh concentrator designs and the accompanied need for high accuracy high quality optics has been supported. A simulation method has been presented which gives attention to surface scattering which can decrease the optical efficiency by 10-40% (absolute value) depending on the material and manufacturing method. New plastic optics and support structures have been proposed and experimentally tested including the use of a conjugate refractive-reflective homogeniser (CRRH). The CRRH uses a reflective outer casing to capture any light rays which have failed total internal reflection (TIR) due to non-ideal surface topography. The CRRH was theoretically simulated and found to improve the optical efficiency of a cassegrain concentrator by a maximum of 7.75%. A prototype was built and tested where the power output increase when utilising the CRRH was a promising 4.5%. The 3D printed support structure incorporated for the CRRH however melted under focused light, which reached temperatures of 226.3°C, when tested at the Indian Institute of Technology Madras in Chennai India. The need for further research into prototyping methods and materials for novel optics was also demonstrated as well as the advantages of broadening CPV technology into the fields of biomimicry. The cabbage white butterfly was proven to concentrate light onto its thorax using its highly reflective and lightweight wings in a basking V-shape not unlike V-trough concentrators. These wings were measured to have a unique structure consisting of ellipsoidal pterin beads aligned in ladder like structures on each wing scale which itself is then tiled in a roof like pattern on the wing. Such structures of a reflective material may be the answer to lightweight materials capable of increasing the power to weight ratio of CPV technology greatly. Experimental testing of the large cabbage white wings with a silicon solar cell confirmed a 17x greater power to weight ratio in comparison to the same set up with reflective film instead of the wings. An ultrahigh design was proposed taking into account manufacturing considerations and material options. The geometrical design was of 5800x of which an optical efficiency of either ~75% with state of the art optics should produce and effective concentration of ~4300x. Relatively standard quality optics on the other hand should give an optical efficiency of ~55% and concentration ratio ~3000x. A prototype of the system is hypothesised to fall between these two predictions. Ultrahigh designs can be realised if the design process is as comprehensive as possible, considering materials, surface structure, component combinations, anti-reflective coatings, manufacturing processes and alignment methods. Most of which have been addressed in this work and the accompanied articles. Higher concentration designs have been shown to have greater advantages in terms of the environmental impact, efficiency and cost effectiveness. But these benefits can only be realised if designs take into account the aforementioned factors. Most importantly surface structure plays a big role in the performance of ultrahigh concentrator photovoltaics. One of the breakthroughs for solar concentrator technology was the discovery of PMMA and its application for Fresnel lenses. It is hence not an unusual notion that further breakthroughs in the optics for concentrator photovoltaic applications will be largely due to the development of new materials for its purpose. In order to make the necessary leaps in solar concentrator optics to efficient cost effective PV technologies, future novel designs should consider not only novel geometries but also the effect of different materials and surface structures. There is still a vast potential for what materials and hence surface structures could be utilised for solar concentrator designs especially if inspiration is taken from biological structures already proven to manipulate light