Field testing of a spectrum-splitting transmissive concentrator photovoltaic module
Graphical abstract
Introduction
To fill the growing demand for more secure, reliable, and cost effective thermal and electric power, public and business interests are shifting towards renewable energy systems [1]. This energy demand includes both electrical power [2] and process heat [3]. As ∼33% of solar power incident on photovoltaics (PV) is in the form of infrared (IR) light and wasted as a heat byproduct, there has been a movement over the past 15 years towards hybrid photovoltaic-thermal (PV/T) systems that output both electricity and heat while more efficiently converting the entire solar spectrum [4,5]. Significant research has been done on topping mode designs in which PV components absorb all wavelengths and waste heat is captured through a cell cooling system [[6], [7], [8], [9]]. However, this architecture generally cannot yield heat outputs at temperatures above 80 °C, and therefore cannot satisfy the heat requirements for most commercial and industrial applications [3]. Furthermore, topping PV/T suffers from reduced PV cell efficiency due to elevated cell temperatures. An alternative is to use spectrum-splitting PV cells that selectively absorb ultraviolet and visible (UV–vis) light for energy conversion, while either reflecting or transmitting IR wavelengths [[10], [11], [12], [13], [14], [15], [16]]. Transmissive photovoltaics have been developed in the past for applications such as building and window-integrated PV, with 1%–13% [17]; However, by utilizing concentrated sunlight and high-efficiency III–V based photovoltaics and focusing transmitted light into a dedicated thermal receiver, the total efficiency of the system can be drastically improved [18]. This configuration offers several advantages, such as the thermal decoupling of the PV and thermal systems, allowing the PV components to operate at low temperatures and thermal components at high temperatures [19]. Parabolic troughs can be used to concentrate sunlight onto such a hybrid transmissive receiver, but troughs generally achieve lower concentration than two-axis parabolic mirrors, which can achieve concentration in excess of 1000 suns [20,21]. Such concentrations are critical for high-efficiency concentrator photovoltaic (CPV) designs in which the cells operate at >500 suns with thermal system stagnation temperatures of up to 450 °C [22]. Operation at high concentrations allows for both increased PV cell cost effectiveness ($/W) and higher process heat temperatures. Furthermore, high temperature output heat enables use in industrial processes, the largest consumers of heat [23]. The shared PV and thermal collector infrastructure in concentrator PV/T (CPV/T) designs also enable cost reductions, facilitating commercialization in previously unavailable markets [24]. In light of this, we here present a transmissive CPV (tCPV) module designed for high solar concentrations with applications in industrial process heating and electricity production. For the first time, we demonstrate a working two-axis transmissive CPV solar collection system and detail its performance on-sun.
The system herein, similar to PV/T systems, seeks to optimize performance within the trade-space of electrical performance, thermal performance, and PV thermal management. Most importantly, unlike topping modes, specular transmissivity must be achieved in the tCPV/T module. Previous reports have demonstrated transmissive GaAs-substrate III–V triple junction PV cells that absorb light of wavelength λ < 870 nm for electricity conversion, while transmitting IR light for thermal collection [25]. These bare cells achieved up to 24.9% full-spectrum efficiency under 1-sun, and 29.5% efficiency at 500x concentration [25]. We here demonstrate a module that encapsulates, protects, cools, and electrically connects these cells for outdoor use on a concentrator dish and 2-axis tracker. Along with the cells, all module subsystems are designed to be transparent, including substrates, superstrates, encapsulant, and active cooling systems [26]. Non-transparent module components, such as copper wire electrodes, were utilized sparingly to reduce shadowing while maintaining acceptable series resistance. The work described here constitutes the experimental results of our fifth and sixth PV module prototype iterations, which are referred to as “Module 5” and “Module 6”.
We introduce three metrics for describing the PV efficiency of the tCPV module, as shown in Table 1; the cell efficiency, , describes the power conversion efficiency (PCE) of a bare PV cell in air. The cell area is defined as 0.29 cm2 and excludes the regions covered by busbar. Module efficiency, , describes the collective PCE of all working cells in the module, and accounts only for sunlight incident on the cell area, given that light spilled outside of the cell area is directed to the thermal receiver by design. Receiver efficiency, , describes the total PCE of the module aperture and is dependent on the number of cells in the module and their spacing. Because the cells are only designed to convert light of wavelength λ < 870 nm, we define solar spectrum light of wavelength λ < 870 nm as “in-band” light, and similarly define solar spectrum light of wavelength λ > 870 nm as “out-band” light. Therefore, each efficiency metric can be further specified as full-spectrum (AM1.5D) or in-band (λ < 870 nm). All electrical output power numbers represent two-terminal, max-power-point measurements. One sun measurements are taken at 25 °C and use an AM1.5D calibrated multi-zone solar simulator to define input power. High flux measurements use real outdoor testbed conditions for temperature and input flux, as described further in the text. The key figure of merit for our tCPV system is in-band module efficiency, , which most directly speaks to the electrical quality of the module in the context of a tCPV system. We also highlight the out-band transmission (λ > 870 nm) through the cell area as a key metric.
Section snippets
Physical design
Fig. 1A shows an exploded CAD view of the module. The module contains an opto-electronic stack, microfluidic cooling system, and aluminum housing. The opto-electronic stack is composed of a quartz superstrate, PDMS encapsulant layer, CPV cells, sapphire substrate, and 34AWG copper electrodes embedded in the superstrate and substrate via InPb solder. The module optical aperture is a 75 mm diameter circular opening at the top of the module, and each CPV cell is 5.5 mm × 5.5 mm x 0.45 mm. Cells
Testing methods
The module was tested under 1 sun using a solar simulator (AM1.5D Spectrum, TS Space) as well as outdoors in San Diego using a two-axis tracked 2.72 m2 concentrating dish collector (45° rim angle, 1.5 m focal length), shown in Fig. 3. To achieve different fluxes incident on the module, the mirror was selectively taped to reduce the incident solar concentration, as shown in Fig. 3A. A close-up image of the illuminated module is shown in Fig. 3B, in which concentrated light strikes the cells and
Test results and characterization
The module underwent more than 60 h of outdoor concentrated testing, spanning 11 individual test days. Fig. 5a plots the power flow for one full-day (∼7 h) test of the system at an average concentration of 130 suns, with the three distinct power output streams of PV electrical power, PV cooling power, and transmitted optical power. The black line represents the total input power incident on the module aperture, calculated from DNI incident on the mirror calibrated for the mirror and module. In
Future prospects
The key measurements of module 6 are summarized in Table 3, along with projections of future module performance based on design improvements being implemented in upcoming iterations. A key limitation of module 6 was the occurrence of cell failures at high concentration. These cell failures were primarily caused when the solder bonds between cells and copper electrodes were damaged from heating and expansion. It is noted that this failure mechanism is largely the result of manual prototyping
Conclusions
In conclusion, a novel spectrum-splitting CPV module is designed, built, and tested to convert in-band light (λ < 870 nm) to electricity and to transmit out-band light (λ > 870 nm) to a thermal receiver.
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The PV and thermal receiver components are thermally decoupled, allowing for optimal cell efficiency at low cell temperatures and heat collection at temperatures far exceeding that of the cells for use in a wide range of applications.
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The CPV module was tested and validated outdoors at an average
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These authors contributed equally to the writing and assembly of this manuscript.