Luminescent solar concentrators for building integrated photovoltaics: opportunities and challenges

This review examines the application of luminescent solar concentrators (LSCs) for building integrated photovoltaics (BIPV), both in terms of opaque façade elements and as semi-transparent windows. Many luminophores have been developed for LSC applications, and their efficiencies examined in lab-scale (<25 cm$^2$) devices. This analytical review illustrates, using ray-tracing simulations, the technical challenges to maintaining efficiency when scaling these energy conversion devices to pilot- (1000 cm$^2$) and commercial-scale (100 000 cm$^2$) modules. Based on these considerations, ambitious but feasible target efficiencies for LSCs based on ideal quantum dot (QD) luminophores are suggested as follows – for opaque and semi-transparent (50% average visible transmission), respectively: (i) 11.0% and 5.5% for lab-scale devices; (ii) 10.0% and 5.0% for pilot-scale modules; and (iii) 9.0% and 4.5% for commercial-scale modules. It is worth noting though, that the QD design requirements – particularly with regard to the overlap integral between the absorption and emission spectrum – become very critical as the LSC area increases. Whereas it is difficult to see opaque LSCs successfully competing against standard flat-plate photovoltaic modules for building integration, the application of semi-transparent LSCs as power-generating window elements has potential. Therefore, an economic analysis of the inclusion of LSCs into commercial glazing elements is presented and the potential for novel technologies – such as down-conversion (quantum-cutting) and controlling the direction of emitted light – to move this technology towards application is also discussed

Impact of energy fluctuation on permeate quality in autonomous and directly coupled renewable energy powered nanofiltration and reverse osmosis systems

Autonomous membrane systems provide a unique opportunity to overcome challenges of lacking or dysfunctional water supply, sewage and electricity infrastructure which is the case in many rural areas worldwide1. Membrane technology provides a unique advantage where water is available yet through predominantly dissolved contaminants such as TDS, fluoride, arsenic, uranium, nitrate and many other inorganic as well as organic contaminants not usable. Coupling membrane processes directly to renewable energies such as wind or photovoltaics is important to realise robust and decentralised systems for remote areas. However this poses particular challenges in terms of system operation, maintenance, as well as water quality2. Following several years of laboratory studies as well as field work with real waters the impact of such fluctuation has been studied for short term operation with a unique system3,4. To do so, the nature of fluctuations for both wind and solar resources was investigated to understand the impact on the membrane system5,6. This information was then transferred into suitable experimental protocols to study the amplitude, frequency and intermittency of fluctuations in a systematic manner7. In the process the resulting operation – and the safe operating window – was determined as a function of minimum power requirements2. Short term energy buffering was investigated via super-capacitor banks8. Please click Additional Files below to see the full abstract

Quantifying barriers to monovalent anion transport in narrow non-polar pores

The transport of anionic drinking water contaminants (fluoride, chloride, nitrate and nitrite) through narrow pores ranging in effective radius from 2.5 to 6.5 Å was systematically evaluated using molecular dynamics simulations to elucidate the magnitud

Photonic crystal-driven spectral concentration for upconversion photovoltaics

International audienceThe main challenge for applying upconversion (UC) to silicon photovoltaics is the limited amount of solar energy harvested directly via erbium-based upconverter materials (24.5 W m(-2)). This could be increased up to 87.7 W m(-2) via spectral concentration. Due to the nonlinear behavior of UC, this could increase the best UC emission by a factor 13. In this paper, the combined use of quantum dots (QDs)for luminescent down-shiftingand photonic crystals (PCs)for reshaping the emissionto achieve spectral concentration is shown. This implies dealing with the coupling of colloidal QDs and PC at the high-density regime, where the modes are shifted and broadened. In the first fabricated all-optical devices, the spectral concentration rises by 67%, the QD emission that matches the absorption of erbium-based upconverters increases by 158%, and the vertical emission experiences a 680% enhancement. Remarkably, the PC redshifts the overall emission of the QDs, which could be used to develop systems with low reabsorption losses. In light of this, spectral concentration should be regarded as one of the main strategies for UC photovoltaics

Limitation of room temperature phosphorescence efficiency in metal organic frameworks due to triplet-triplet annihilation

The effect of triplet-triplet annihilation (TTA) on the room-temperature phosphorescence (RTP) in metal-organic frameworks (MOFs) is studied in benchmark RTP MOFs based on Zn metal centers and isophthalic or terephthalic acid linkers (ZnIPA and ZnTPA). The ratio of RTP to singlet fluorescence is observed to decrease with increasing excitation power density. Explicitly, in ZnIPA the ratio of the RTP to fluorescence is 0.58 at 1.04 mW cm(−2), but only 0.42 at (the still modest) 52.6 mW cm(−2). The decrease in ratio is due to the reduction of RTP efficiency at higher excitation due to TTA. The density of triplet states increases at higher excitation power densities, allowing triplets to diffuse far enough during their long lifetime to meet another triplet and annihilate. On the other hand, the shorter-lived singlet species can never meet an annihilate. Therefore, the singlet fluorescence scales linearly with excitation power density whereas the RTP scales sub-linearly. Equivalently, the efficiency of fluorescence is unaffected by excitation power density but the efficiency of RTP is significantly reduced at higher excitation power density due to TTA. Interestingly, in time-resolved measurements, the fraction of fast decay increases but the lifetime of long tail of the RTP remains unaffected by excitation power density. This may be due to the confinement of triplets to individual grains, leading decay to be faster until there is only one triplet per grain left. Subsequently, the remaining “lone triplets” decay with the unchanging rate expressed by the long tail. These results increase the understanding of RTP in MOFs by explicitly showing the importance of TTA in determining the (excitation power density dependent) efficiency of RTP. Also, for applications in optical sensing, these results suggest that a method based on long tail lifetime of the RTP is preferable to a ratiometric approach as the former will not be affected by variation in excitation power density whereas the latter will be

Renewable energy powered membrane technology:Experimental investigation of system performance with variable module size and fluctuating energy

Integration of renewable energy and membrane filtration technologies such as nanofiltration (NF) and reverse osmosis (RO) can provide drinking water in places where freshwater is scarce and grid electrical connections are unavailable. This study investigated a directly-connected photovoltaic-powered membrane system under fluctuating solar conditions. Specifically, two configurations of NF/RO membranes with the same membrane area were investigated: a) 1 × 4″ module, which contained one 4″ NF/RO element; and b) 3 × 2.5″ module, which contained three 2.5″ NF/RO elements in series. A high fluoride brackish water ([F − ] = 56.2 mg/L, total dissolved solids [TDS] = 4076 mg/L) collected from northern Tanzania was treated by different membranes in the two configurations. Performance indicators such as flux, specific energy consumption, and permeate F − concentration were monitored over a 60-min period of energy fluctuation that are part of a typical solar day. The results showed that the overall performance of the 1 × 4″ module was superior to that of the 3 × 2.5″ module. This is because the performance of a 3 × 2.5″ module degraded significantly from the first element to the third element due to the increased feed concentration and the decreased net driving pressure. Three 1 × 4″ modules (BW30, BW30LE and NF90) and one 3 × 2.5″ module (BW30) were able to meet the drinking water guideline for fluoride. During cloud periods, the transient permeate F − concentration exceeded the guideline value due to insufficient power, however the cumulative permeate F − concentration was always well below the guideline. The photovoltaic-powered membrane system equipped with the above modules provides a promising solution for addressing drinking water problems in remote and rural areas. </p