A spectrally splitting photovoltaic-thermal hybrid receiver utilising direct absorption and wave interference light filtering

https://doi.org/10.1016/j.solmat.2015.03.011Get rights and content

Highlights

  • We developed a hybrid photovoltaic thermal receiver for liner concentrators.

  • We thermally decoupled the PV cells from the thermal receiver using spectral splitting.

  • We simplified the required devices by combining thin film filters and direct absorption.

  • We fabricated a sample receiver and compared its optical performance with the model.

  • We showed that 73.3% of the light reaching the cells are in the high efficiency range.

Abstract

We have developed a novel spectrally splitting hybrid solar receiver by combining a simple dichroic filter and a liquid channel as a selective absorbing medium. The combination acts as a band pass filter for silicon solar cells. The geometry can be optimised for any linear concentrator; in this paper we have optimised it for a commercially available linear rooftop micro-concentrator. The optics of the concentrator at its focal region has been investigated using ray tracing. A simple 5-layer dichroic filter made of titanium dioxide and silicon dioxide has been designed, optimised, and fabricated with a focus placed on manufacturing simplicity. It has been shown that such a filter directs 54.5% of the concentrated light to the silicon photovoltaic cells; the Si cells considered in this paper can convert 26.1% of this energy into electricity which is significantly higher than their 20.6% efficiency under the full spectrum. This is due to the fact that 73.3% of the incident flux is within the cell's relatively high spectral response range, which can be efficiently converted into electricity. The rest of the spectrum can be collected as high temperature heat. This research shows the possibility of employing low-cost direct absorption-dichroic filtering hybrid receivers in liner concentrators.

Introduction

Increasing the efficiency of solar receivers represents a desirable pathway towards reducing the installed cost per Watt, and obtaining higher solar fractions in buildings with limited rooftop areas. Concentrating photovoltaic (CPV) systems and hybrid photovoltaic-thermal receivers can help us to achieve this. High heat loads in CPV systems are considered an opportunity to design a combined heat and power system to deliver electricity and heat concurrently.

Three main methods have been introduced to remove heat from CPV systems: (a) thermal removal, through passively or actively cooled cells [1], (b) device removal, through the use of high efficiency multi-junction cells [2], and (c) optical removal, through spectral modification of the incoming sunlight [3], [4]. A combination of these methods can also be incorporated to achieve the best outcome [5]. In the third method, sunlight is separated into two or more spectral bands with each band directed towards an appropriate receiver. For example, the most suitable spectral band for silicon cells as the most common type of PV cells is roughly between 700 nm and 1100 nm [6]. Using spectral beam splitting in concentrating photovoltaic thermal receivers (CPVT) can not only improve the total efficiency [3] of the system but also thermally decouple the thermal receiver from the cell. The latter allows the temperature of the thermal output to increase beyond the maximum operational temperature of the PV cell and to deliver high grade high temperature thermal output in addition to electricity. This may improve the market penetration of solar energy in solar cooling and industrial applications.

Application of spectral beam splitting in solar energy has been extensively studied for CPV systems, e.g. Barnett and Wang [7] designed and optimised a spectral beam splitting PV system using dichroic filtering for GaInP/GaAs, Si, and GaInAsP/GaInAs cells achieving an efficiency of 39.1% at a concentration ratio (CR) of 30. Khvostikov et al. [8] proposed dichroic filtering in combination with AlGaAs, GaAs, GaSb reaching an efficiency of 39.6%. The outcome of the research in this field has been reviewed thoroughly in [3], [4]. Applying spectral splitting in CPVT systems has been studied by Chendo et al. [9] and Hamdy and Osborn [10] in detail. They analysed the system performance over the year and showed that the reduced heat load on the PV cells improves the electric conversion efficiency of the system. Recently Jiang et al. [11] studied a CPVT parabolic trough system using dichroic filters with 240–400 °C thermal output. However the majority of research in this field is limited to theoretical analyses with a few practical realisation. These devices have been too expensive to be commercially viable for linear concentrators. Wave interference [12], [13] and selective absorption [9] filters can be used for spectral separation in CPVT systems. Wave interference filters employ a number of high and low refractive index transparent materials (multilayer thin film filters) or a transparent layer with continuously varying refractive index (rugate filters) deposited on a substrate to generate the light filtering effect. Selective absorbers use pure liquids, solution mixtures [9], [14], nano-fluids [15], [16], or solid state optical filters to filter out the desired spectral band(s).

Wave interference filters provide more flexibility compared to selective absorbers [12]. Such filters can be broad band-pass which are made of one or two edge filters in combination. These two edge filters (a long and a short pass) can be deposited on either side of a substrate. A concern in such band pass filters is to combine them in such a way that one edge filter does not create transmission peaks in the rejection band of the other [17]. However, they normally consist of a large number of layers to produce an effective broadband filtering effect, but this results in higher cost.

In this work, we propose a hybrid photovoltaic-thermal (CPVT) receiver in a linear Fresnel concentrator incorporating a selective absorber together with a wave interference filter (a dichroic mirror), acting as a band pass filter. A simple filtering structure that is introduced is relatively facile and low cost to manufacture. The proposed configuration takes advantage of direct solar absorption that has been studied by Minradi [18] and Otanicar [19], [20] to simplify the structure of the required dichroic coating. The details of the design are given in the next section where we present an optical analysis of the whole system and discuss the advantages of using the proposed configuration.

Section snippets

Design description

In this paper, the term hybrid collector refers to the combination of a rooftop linear micro-concentrator (LMC) and a CPVT receiver installed at its focal axis. In this section, we briefly introduce the proposed hybrid receiver design and then discuss its applicability in an LMC hybrid collector.

The rooftop linear micro-concentrator (LMC)

The LMC is a one-axis solar tracking concentrator developed and commercialised by Chromasun Pty Ltd [22], [23], [24]. It comprises two sets of Fresnel reflectors, each set with 10 curved mirrors, encapsulated inside a glass canopy. The mirrors are controlled by a tracking system to focus the sunlight on a central axis 25 cm above the mirror plane. The whole collector is 3.3 m long, 1.2 m wide and 0.3 m high. The glass canopy protects the internal components from wind, dust and water. Fig. 2(a) and

Results and discussion

The optimisation of the filter started with the refined stack and ended up with the optimal stack as presented in Table 1. Fig. 8a shows the variation of ηpv as a function of s. In this figure, a value of 0.84 for the scalar factor s corresponds to the maximum PV conversion efficiency of approximately 27%, i.e. the optimal stack. It is important to note that this efficiency was calculated based on the transmitted power through the filter as described in Eq. (6).

Fig. 8b shows the effect of s on

Conclusion

This paper has presented the possibility of combining dichroic filters with direct absorbing liquids to achieve efficient spectral splitting of sunlight in a hybrid solar receiver. The receiver geometry was optimised for Chromasun's linear micro-concentrator; however it can be optimised for other types of concentrators with different sizes, such as parabolic troughs. The modelling results showed that the receiver is capable of directing 54.5% of the solar spectrum to the PV cells; 73.3% of this

Acknowledgements

This work was performed in part at the Melbourne Centre for Nanofabrication (MCN) in the Victorian Node of the Australian National Fabrication Facility (ANFF). The authors would also like to thank the Australian Renewable Energy Agency (ARENA) for funding A. Mojiri and this research (Grant Ref.: s-2006-ASI).

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      Citation Excerpt :

      The whole refractive index of filters depends on the stacking sequence, the number of deposition layers, and the splitter thickness. SiO2 is typically used as the lower refractive index dielectric materials, and the matching higher ones can be TiOx [67–70]), Nb2Ox (such as Nb2O5 [71] and Nb2O3 [72]), Ta2O5 [73], and SiN [74–76]. In addition, the selective reflection film filters can also be prepared by stacking multiple materials with a gradient refractive index, such as the new beam splitter designed by Wang et al. [77] was composed of Na3AlF6, Nb2O3, and Ge.

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