Design of a new static solar concentrator with a high concentration ratio and a large acceptance angle based on bifacial solar cells
Adnan Shariah. Emad Hasan, Design of a new static solar concentrator with a high concentration ratio and a large acceptance angle based on bifacial solar cells, Clean Energy, Volume 7, Issue 3, June 2023, Pages 509–518, https://doi.org/10.1093/ce/zkac068
Solar concentrators are used in solar photovoltaic systems to lower the cost of producing electricity. In this situation, fewer solar cells can be used, lowering the overall cost of the system. The purpose of this article is to design, construct, install and test a stationary (non-tracking) concentrating system in Irbid, Jordan. Bifacial solar cells are used in the design. Two concentrator designs (with the same concentration ratio) are experimentally tested. Conc-A has a parabolic shape in the lower part but flat reflecting walls, whereas Conc-B has a standard compound parabolic shape in all parts. The receiving solar cells are arranged in three distinct positions in each concentrator. The results reveal that the output power from both concentrators is affected by the placement of the receiving solar cells within the concentrator. It has also been found that concentrators with flat reflecting walls perform better than those with parabolic reflecting walls. Conc-A’s power collection is ~198% greater than that of a non-concentrating device. When Conc-B is used, the increase in power is ~181%.
Introduction
Electricity is one of the most crucial components of the Industrial Revolution. The daily demand for energy is rising as a result of the improving living standards of societies and the continuing growth of the population. This has led scholars and scientists to seek more economical, less-polluting and more efficient energy alternatives [ 1]. Solar energy is the primary alternative energy source since it is one of the most abundant alternative energy sources and will be available for a very long period. Solar technologies include photovoltaic (PV) panels, which convert sunlight directly into electricity, and solar concentrators, which turn sunlight directly into thermal energy. In the past two decades, technological advances have reduced the price of PV panels and increased their efficiency, resulting in a FOCUS on solar energy. New technological advances are expected to boost the use of PV systems by reducing the cost and increasing the efficiency of solar panels [ 2–4]. Perovskite solar cells [ 5], heterojunction technology [ 6], integrated PV cells in buildings [ 7], printable solar cells [ 8], bifacial cells, thin wafers and thin-film solar cells are among the new breakthroughs. Researchers are motivated to improve concentrated photovoltaic (CPV) technology employing commercially available bifacial solar modules in an effort to further drop and increase efficiency [ 9].
PV concentrators utilize lenses or mirrors to FOCUS the Sun’s rays on the solar cells. CPV systems have several advantages over non-concentrating PV systems since they may gather the same amount of radiation, if not more, with fewer solar cells. The disadvantage of the CPV is a rise in temperature, so it is essential to dissipate the heat generated by the PV cells.
Low-concentrating PV (LCPV) systems rely on fixed reflectors and receivers (called static concentrators), whereas high-concentrating PV (HCPV) systems and medium-concentrating PV (MCPV) systems rely on movable reflectors and/or receivers. The second and third types of concentrators acquire precision tracking to maintain the Sun’s light concentrated on the solar cells during the day, increasing the system’s cost, complexity and maintenance load [ 10].
The high acceptance angle of static concentrators eliminates the need for Sun tracking and enables the concentration of both diffuse and direct radiation. They have been the FOCUS of numerous studies for many years. The most prevalent LCPV concentrator geometries are paraboloid (symmetric, asymmetric, and truncated) and V-trough. In recent years, it has been the topic of various studies and a number of great review articles [ 11–17] have been published in the field.
For example, Parupudi et al. [ 18] examined three LCPV designs with one-sided solar cells, including asymmetric compound parabolic concentrating (ACPC), compound parabolic concentrating (CPC) and V-trough optical concentrators with geometric concentration ratios of 1.53, 1.46 and 1.40, respectively, and according to their measurements, the ACPC concentrator had the highest annual optical efficiency. Experimentally, Alnajideen and Gao [ 19] examined a new design consisting of two standard V-trough concentrators placed in a cross. The results indicated that the concentration ratio of this new structure is 40–60% higher than its typical V-trough solar concentrator counterpart. Butlers et al. [ 20] used a modified V-trough concentrator, combined with a heat-sink apparatus, utilizing the fact that LCPV has the ability to capture more energy than conventional Si solar cells in a basic concentration configuration.
In parabolic trough concentrators, a parabolic-shaped mirror focuses sunlight on a receiver tube at the focal point of the parabola [ 21]. Mallick et al. [ 22] developed an asymmetric CPC that did not require imaging. Their experiments revealed a 62% improvement in maximum power output when compared to a comparable non-concentrating PV panel. Mokri [ 23] performed experiments on CPV technology and its economic potential, as well as the potential for this technology to bring the cost of energy to a level comparable to that of oil-based resources.
Solar concentrators have also been used in hybrid electric/thermal systems (PVT), such as [ 24] and [ 25], where both electric energy and heat could be obtained from the system. Masood et al. [ 26] gave a comprehensive review of the applications of CPCs in hybrid PVT systems. Katardjiev [ 27] introduced a novel method to concentrate both direct and indirect sunlight by combining wave-guiding and refractive optics. The design displayed a transmittance of 90% at acceptance angles of 70%.
Bifacial solar panels are a new product in the PV industry that have just recently become commercially available. When used as conventional solar panels, the power output can be increased by ≤30% depending on the kind of ground (concrete, green field, white gravel, sand, etc.), the height of the installation and the tilt angle of the panels [ 28–31]. Because such panels receive solar radiation from both sides, developers could design solar PV concentrators that reflect solar energy onto both sides of the solar cell.
Design parameters for LCPV concentrators
The compound parabolic concentrator and V-trough solar concentrator are the two main shapes utilized to manufacture LCPV solar concentrators for conventional monofacial silicon solar cells, in addition to their asymmetrical and truncated shapes. Typically, the concentration ratio characterizes the light-concentration process. The most common definition of a concentration ratio (CR) is the ratio between the aperture area and the receiver area. This is called the geometric concentration ratio (Cg). When coupled with the reflectivity of the surface, it is referred to as the effective concentration ratio (Ceff). LCPV concentrators can reflect all incident radiation to the receiver over large ranges of incidence angle. Limits specify the concentrator acceptance angle (θ). All radiation within the acceptance angle is reflected to the receiver. Fig. 2 illustrates the structure of a typical V-shaped and paraboloid LCPV concentrator and the relevance of the acceptance angle limit. The same definition can be used for compound parabolic concentrators.
Schematic diagram of a typical V-shaped and paraboloid LCPV concentrator.
Geometric and effective CR can be expressed as follows [ 37]:
Technology
Luminescent Solar Concentrators (LSCs) are composed of coloured panels of plastic material that have a special characteristic: they can capture sunlight and concentrate it along their edges, where it is intercepted by small photovoltaic cells and converted into electricity. Developed in collaboration with MIT in Boston, the key advantage of these LSCs is that they can produce electricity even in low light conditions, and can be incorporated into architectural structures as transparent elements.
Context
Current photovoltaic technologies rely on panels that are more or less opaque, and perform best in direct light conditions; as such, the only way to integrate these into buildings and architectural structures is to fix them at a certain angle on walls and roofs, avoiding Windows and any other element that must remain transparent. The characteristics of our Luminescent Solar Concentrators mean that they can produce electricity even in low light conditions. and can also be used where transparent surfaces are required.
The goal we set ourselves when we started developing LSCs was to create a system that could produce electricity in an efficient manner, even in conditions characterised by diffuse and low light. We were also looking for a material that was light and photostable, and perhaps also easy to produce in various forms, so that we could integrate the technology directly into architectural elements. We obtained these characteristics by adding special luminescent pigments developed in our laboratories to sheets of transparent plastic. When added to the material during the production phase, these photoactive substances can absorb part of the solar radiation and convert it to higher wavelengths: from the Band between ultraviolet and visible light to the Band between visible and infra-red light. At the same time, the light is transported inside the plastic sheet and concentrated in the edges, via an internal reflection mechanism. Here, we have added rows of small photovoltaic cells that intercept the solar radiation which in turn have been intercepted by the pigments and concentrated inside the plastic plates, transforming this into electricity. We have developed Luminescent Solar Concentrators in a range of colours, but the most efficient of these. now ready for industrial development. have proved to be yellow and red. Meanwhile, we are also working on a range of neutral colour versions.
Luminescent Solar Concentrators. Eni’s research | Eni Video Channel
Industrial integration
As a technology, Luminescent Solar Concentrators have been conceived, designed and developed to be installed with ease within other structures, particularly in the construction sector. The sector of reference is BIPV (Building Integrated PhotoVoltaics), which includes all the photovoltaic systems that can be considered an integral part of the building envelope. Specifically, LSCs can be used to create transparent surfaces with photovoltaic properties. This principle has been applied in the Eni Ray Plus® technology that is at the heart of Domal Smart Windows. presented at Milan Design Week 2018. which use the sun’s rays to power an automatic temperature control system. Consisting of a sheet of double glazing composed of two LSCs, these Windows feature a Venetian blind inside them, which automatically adjusts according to the response provided by the internal and external light and heat sensors, reducing or increasing sun exposure in response to variations in temperature. The entire system is connected to a battery which is powered by the electricity generated by the LSCs, with no need to be connected to the mains power. Other interesting areas of application within the construction sector include motorway and railway anti-noise barriers and public transport shelters. Elsewhere, greenhouses for flower growing, nurseries and agriculture represent another promising sector. in this instance, panels with pigments that absorb only the solar radiation not used by plants for photosynthesis (for example, green) can be used.
The different uses of LSCs. Eni’s research | Eni Video Channel
Environmental impact
Luminescent Solar Concentrators belong to the category of technologies that boast more than one advantage at the same time. Due to their characteristics and their versatility, LSCs can be used to increase the energy savings of buildings, whilst simultaneously improving their habitability. The use of this solution in intelligent Windows through Eni’s Ray Plus® technology can reduce energy consumption by 50 to 75%. The yellow colour of these mean that they also improve luminous efficiency. by converting the light spectrum into wavelengths that are more agreeable to the human eye, increasing visual comfort. A key aspect to be taken into consideration is that these improvements can be achieved simply by replacing the old Windows, with no need to resort to major building work, and without requiring a connection to the electrical grid.
Selected contents on this issue.
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Materials and Methods
2.1 Modeling and Design
In this work, we optimized a photonic LSC (PLSC) for an InGaP-Si tandem PV design (Figure 1A) because 30% efficiency is possible with this bandgap combination but is cost prohibitive because of the expensive III-V semiconductor growth process (Essig et al., 2016; Phelan et al., 2021). III-V cells are approximately three orders of magnitude costlier than silicon cells on a per area basis (19,320/m 2 versus 44.2/m 2 ) so we propose to reduce the area coverage of the costly III-V’s by including inexpensive luminescent concentrators (∼5/m 2 ) (Horowitz et al., 2018; Wu et al., 2018; Smith et al., 2020). The cost impact of replacing the III-V cell area with would be immense: assuming an LSC concentrator of 100x and a 34% module, we could have high efficiency photovoltaics at less than 450/W. We first optimized a nanophotonic design that maximizes luminophore emission into total internal reflection modes. We designed a structure consisting of alternating high and low dielectric layers as this can maximize emission into high angles for a given design wavelength. Gutmann et al. (2012) showed through simulations that trapping efficiencies of 99.7% are possible with a 1D photonic crystal of alternating quarter wavelength layers of n = 1.5 and n = 2.0. In our design, we investigate two combinations of refractive indices (n = 1.5/n = 2.1 and n = 1.5/n = 2.4) and use CdSe/CdS quantum dots as our emitters, which have demonstrated near unity quantum yields and excellent stability properties (Hanifi et al., 2019). Our choice for luminophore wavelength (624 nm) and top subcell (InGaP) was driven by the near unity quantum yield of CdSe/CdS quantum dots at that wavelength and the fact that InGaP (Eg = 1.84 eV) is a near ideal tandem partner for silicon (Yu et al., 2016). Figure 1B shows a schematic of our modeled structure, which consists of a set of five high-low refractive index pairs above and below a central layer consisting of the CdSe/CdS quantum dots. Instead of using a Bragg Reflector composed of quarter wavelength layers as was demonstrated previously (Goldschmidt et al., 2010), we chose to vary the thicknesses of the layers not only to maximize the photoluminescence trapping, but also to maximize transparency for all wavelengths greater than the CdSe/CdS emission, allowing this to be used in a high efficiency, multijunction structure.
FIGURE 1. (A) Schematic of the Photonic Luminescent Solar Concentrator (PSLC) multijunction design. High energy light is absorbed and concentrated by a PLSC for conversion by an InGaP subcell while low energy light (λ 700 nm) is transmitted to a Si subcell. (B) Schematic of the simulated PLSC. A central layer of thickness tem containing the dipole emitters is surrounded by 5 pairs of high (thigh, nhigh) and low (tlow, nlow) refractive index layers on each side. The layer thicknesses and the high refractive index (nhigh) are varied. (C) Results of the FDTD optimization. The collection efficiency of the PLSC (contour color) is plotted against the thickness of the high refractive index layer and the thickness of the low refractive index layer. The left plot shows the optimization when the refractive index of the high index layer is 2.1 and the emitter thickness is 15 nm. The right plot shows the optimization when the refractive index of the high index layer is 2.4 and the emitter thickness is the same thickness of the low index layers (perfectly periodic). The area within the overlayed black contour shows where the transmission to the bottom junction is 90%.
This design was adapted into a finite-difference time-domain (Lumerical FDTD) simulation as shown in Supplementary Figure S1. Three dipole sources were placed at the center of the luminophore layer to represent a random quantum dot emission and perfectly matched layer (PML) boundaries were used at the top and side interfaces to monitor the escaped and trapped light, respectively. The simulation width was set at 20 μm to accurately capture the escaped and collected light fractions. The fraction of collected photoluminescence was determined by dividing the light collected by monitors within the stack by the total light in the system (light trapped in the stack and light escaped from the stack). The thicknesses of the high (thigh) and low refractive index layers (tlow) were each varied to determine the best luminescence trapping for a given refractive index combination (1.5/2.1 versus 1.5/2.4) and for a given emitter layer thickness (tem).
The total modeled efficiency of the multijunction cell with a 100x PLSC was determined by a modified detailed balance calculation and is described in the Supplementary Material (Warmann et al., 2017; Eisler et al., 2019). First, the light transmission, reflection, and absorption of the PLSC was calculated using the open-source optical simulation software OpenFilters (Larouche and Martinu, 2008), which uses the transfer matrix method to determine the Fresnel coefficients of a planar multilayer stack. We input the refractive indices and layer thicknesses of a given design and determined the transmission, reflection, and absorption for varying CdSe/CdS optical density and number of repeating layers. We modeled the periodic structures (tem = tlow) and assumed every low index layer had quantum dot luminophores as in Figure 1A. The imaginary refractive index, k, of the low index layers was calculated using absorption data (Figure 2A) of the synthesized CdSe/CdS quantum dots and was modified for different optical densities. The resulting spectra were used to modify the AM1.5G solar flux and determine the photon flux to each subcell. The Si subcell photon flux is simply the transmission-modified AM1.5G solar flux. Because the PLSC downshifts the absorbed light, the InGaP subcell flux was calculated by summing the absorption-modified photon flux and creating a normal distribution with that number of photons centered at the emission wavelength of the CdSe/CdS quantum dots. This flux was multiplied by the quantum yield and the photoluminescence collection efficiency raised to the power of the number of reabsorption events, as defined in (Olson et al., 1981). This combined calculation accounts for the quantum dot losses, escape losses, and reabsorption losses. The modeled power generated by each subcell is determined by a modified detailed balance calculation that uses the external radiative efficiency (ERE) and fraction of ideal short circuit current to more realistically estimate the photovoltaic conversion losses (Warmann et al., 2017; Eisler et al., 2019). Here, we assumed EREs of 5% and 1% and fraction ideal current of 90% and 96% for the InGaP and Si subcells, respectively, as based on the current records (Warmann et al., 2017; Green et al., 2022). The given power of each subcell was scaled by the respective concentration factor (100x for InGaP, 1x for Si), summed, and then divided by the incident power of the Sun to yield the modeled multijunction cell efficiency for this design.
Results
3.1 Luminescent Concentrator Design Space
The first goal of this work was to demonstrate a photonic luminescent solar concentrator (PLSC) design that would minimize photoluminescence escape losses while still being suitable for incorporation into a multijunction design. Figure 1C shows the fraction of collected light as a function of the thicknesses of the low and high refractive index layers resulting from our FDTD simulations; the left figure shows the optimization for nhigh = 2.1 and a thin emitting layer (tem = 15 nm) while the right figure shows the optimization for nhigh = 2.4 and a periodic structure (tem = tlow). Additional optimizations are included in the supplementary material (Supplementary Figure S3). The highest collection efficiencies (maximum = 95%) are seen in for nhigh = 2.4. High collection efficiency (80%) is also seen for nhigh = 2.1, mostly for very thin emitter thicknesses (tem). Both of these trends are expected given that evanescent photoluminescent emission couples most effectively into another layer when there is a high refractive index contrast and the emitter is at the interface (Lukosz and Kunz 1977). For both optimizations, the optimal collection efficiencies lie on a diagonal pattern because of the constructive and destructive interference wavelengths. The diagonals that show a maximum light trapping represent where the reflection for the photoluminescence emission (∼620 nm) is the highest (Supplementary Figure S2). This trend is consistent with other photonic LSC work as the nearfield photoluminescence trapping is maximum for a structure whose reflectivity maximum is designed for the emission wavelength (Gutmann et al., 2012).
There are multiple designs with high photoluminescence trapping (90%) for both nhigh = 2.1 and nhigh = 2.4. However, the final design must also be transparent for longer wavelengths for effective use in a multijunction architecture. Figure 1C also includes black contour lines showing where the AM1.5G-weighted transmission of longer wavelengths (700–1,100 nm) for the Si subcell is 90%; all designs within each contour have transmissions ≥ 90%. This adds a significant constraint on the design space. While previous designs have used a Bragg stack comprised of quarter wavelength layers (Gutmann et al., 2012), the quarter wavelength thicknesses for our design (tlow = 104 nm, thigh = 65–74 nm) do not meet the requirements for high transmission of longer wavelengths. Instead, the optimum for both high collection and high long wavelength transparency occurs for thicker high index layers and thinner low index layers. Because we grew the structure using Si3N4 (n = 2.1) and SiO2 (n = 1.5) and would use a thin emitting layer in our characterization, we chose our stack to have tlow = 100 nm and thigh = 210 nm thicknesses. This corresponded to a photoluminescence trapping of 85% and a long wavelength transmission of 90%, minimizing the optical losses to each subcell.
3.2 Characterization of Luminescent Concentrator Test Structure
After fabricating the structure as outlined in Section 2.2, we determined the angular emission (Figure 3) and reabsorption (Figure 4) for quantum dots on a control substrate (plain class coverslip) and our PLSC half structure. The first characterization focuses on how the PLSC structure affects the photoluminescence escape losses. Figure 3A shows photographs of the CdSe/CdS quantum dots on a plain glass coverslip (control) and on the PLSC half structure under black light illumination. Qualitatively, the PLSC half structure is guiding significantly more of the quantum dot photoluminescence into oblique angles: while the photoluminescence (red light) is seen at both the top and sides of the control structure, indicating emission at shallow and oblique angles, photoluminescence is only seen at the sides of the PLSC half structure, indicating that the light is mostly emitted into modes that are directly guided to the edge of the substrate. Figure 3B also shows the full BFP images which show the projected photoluminescence intensity versus the polar angle (read from the center outward as normal to oblique angles) and the azimuthal angle (read around the circle). We include a dotted line representing the total internal reflection angle of an air-glass interface (42°); signal within the dotted line represents light emitted within the escape cone while signal outside the dotted line represents light that is guided. There is a significant difference in the control and PLSC half structure emission patterns. The PLSC half structure exhibits almost no emission at low angles while the control structure has significant emission at angles where light would escape. Figure 3B also shows the extracted intensity as a function of polar angle for each structure, which illustrates the significant change in emission from shallow to very oblique angles. To quantitatively determine the percent increase in trapping, we integrate the intensity with respect to polar angle ( ∫ I sin ( θ ) d θ ) and determine the fraction of light that is trapped (θ ≥ critical angle) and escaped (θ critical angle). Overall, the fraction of photoluminescence trapping increases by 40% when the multilayer structure is included, a significant improvement over the traditional LSC design.
Discussion
Here we have demonstrated that a photonic luminescent solar concentrator can be optimized simultaneously for high concentration and for use in a spectrum-splitting, multijunction InGaP-Si cell. By moving beyond the Bragg stack design, we showed that high photoluminescence trapping (80%) and high transparency to lower wavelengths (90%) is possible, with the best designs for layers with high refractive index contrast (i.e., nlow = 1.5, nhigh = 2.4). We experimentally verified a significant reduction in two of the major LSC loss mechanisms, namely photoluminescence escape and the reabsorption within the LSC plane, in our PLSC half structure. Because the nanophotonic design changes the angular emission light pattern of the luminophores, the ratio of photoluminescence in guided modes (θ ≥ critical angle) to total photoluminescence increased by 40% and the effective absorption coefficient in the LSC plane is reduced by four orders of magnitude with the inclusion of the photonic element. Finally, we showed that 30% efficiency cells are possible even with the less optimal refractive index contrast (nlow = 1.5, nhigh = 2.1). Despite having a lower photoluminescence trapping of 74%, this design shows a significant increase in efficiency over a single junction Si cell, demonstrating the potential in multijunction cell designs.
The development of near unity quantum yield luminophores and sophisticated nanophotonic structures could finally usher in the next generation of high efficiency and inexpensive spectrum-splitting photovoltaics. While embedding luminophores within a photonic crystal inherently makes PLSC designs more complex than the traditional LSC design, there have been significant advancements in nanofabrication, such as nanoimprint lithography (Shneidman et al., 2018; Wang et al., 2018) and polymer coextrusion (Kazmierczak et al., 2007; Li Z et al., 2020), that could enable these designs to be inexpensive and scalable. Our work, along with the growing literature of photonic luminescent solar concentrator designs, demonstrates the need to develop nanophotonic design elements to realize a luminescent solar concentrator with a significant (100x) concentration. Further, the design principles discussed in this work can be extended to any number of junctions or any multijunction design, such as a series of luminescent solar concentrators (Imenes and Mills 2004), creating a pathway for 50% efficiency solar modules that can support sustainable energy generation.
The Rockingham PV Trough Concentrator System. Case Study
Researchers at the Australian National University (ANU) have been developing photovoltaic (PV) concentrator technology since 1995. Funding has come from a number of sources, including Western Power from 1995 to 1997. In early 1998 Western Power joined a consortium with ANUTECH (the commercial arm of the ANU) and Solahart to apply for federal government funding for a 20kW pre-production system using the ANU technology. The Rockingham concentrator PV system was constructed by ANU and Solahart and commenced in November 1999. The system was connected to the public grid in July 2000.
TABLE 1: The Rockingham PV Trough Concentrator System, basic features
Concentrating Photovoltaic Power Plant, Dallas. Case Study
In March 1995, the Texas Utility Electric Company installed a 100 kW photovoltaic power plant at its new Energy Park in Dallas, Texas. The 100 kW plant consists of four east-west rows of collector modules, each 104 m in length and containing 72 photovoltaic modules. The system is connected to the public grid via DC to AC inverter and a transformer.
TABLE 2: Concentrating Photovoltaic Power Plant, Dallas, basic features
Web Sites
Arzon Solar. integrated high-concentration photovoltaic (IHCPV) systems.
SolarTec. SolarTec International AG focuses on Concentrating Photovoltaics (CPV), as well as, worldwide planning, turnkey realization, and management of PV power plants.
SolFocus. the company’s mission is to provide innovative solar energy solutions which are competitive with fossil fuels. SolFocus has an expanding portfolio of products and technologies including solar concentrator photovoltaic (CPV) systems, intelligent tracking systems for CPV and flat panel PV systems.
Alanod. Manufacturer of aluminium reflective sheets for different purposes. Part of Alanod’s portfolio are also mirrors for solar concentrators.
Absolicon Solar Concentrator. Absolicon´s main product FOCUS is solar concentrators shaped as parabolic troughs.
Edmund Optics. Edmund Optics offers a variety of solar optical components supporting photovoltaic technologies. From large Fresnel or glass condenser lenses to a variety of cold coated optics, many of our components have been used in photovoltaic applications for years.