A cascade nanofluid-based PV/T system with optimized optical and thermal properties

Highlights

• A novel configuration of PV/T hybrid system operated with nanofluids is proposed.

• Proposed electrical model was in accordance with the experimental results.

• Electrical efficiency of the PV/T system could be improved by ∼8.6%.

• Overall efficiency is proportional to volume fraction of nanofluid based coolant.

• The proposed PV/T is highly recommended at high solar concentration (C > 100).

Abstract

In recent years, nanofluids have been proposed as efficient coolant fluids and as a means to filter sunlight for PV/T systems. In the present study, a new, cascading nanofluid-based PV/T configuration with separate channels is proposed, where one channel controls the optical properties while the other enhances heat removal from the PV cells. That is, the first nanofluid, optical nanofluid, acts as a liquid optical bandpass filter above the PV cells while the second, thermal nanofluid, removes heat from the back of the PV cells. Both nanofluids have been designed to work together to improve the overall electrical and thermal performance of the PV/T hybrid system. The proposed PV/T system has been investigated for two designs – with separate channels (D-1) and with a double-pass design (D-2). In addition these designs were simulated for both GaAs- and Si-based PV cells at various concentration ratios. The simulation results show that the best nanofluid-based optical filter can transmit ∼82% of the desired spectrum to GaAs or Si PV cells. In concentrated systems it was found that the separate channel system (D-1) outperformed the double-pass design (D-2) by ∼8.6%, in terms of the electrical efficiency of GaAs (at C = 45) and Si (at C = 30). The overall efficiency of the D-1 system with GaAs (at C = 160) and Si (at C = 100) have been improved by ∼5.8% and ∼4.6%, respectively, by increasing the volume fraction of the thermal nanofluid from 0.001 to 1.5%. Overall, it was found that the proposed PV/T configuration with separate channels has potential for further development in high-concentration (C > 100) solar systems.

Introduction

Prior to the industrial revolution, humanity was almost entirely dependent on renewable energy resources (biomass, wind, solar, and hydro) to supply energy for a majority of human activity (e.g. heating, cooking, and transportation). After the massive expansion in coal, oil, and natural gas usage over the last 150 years, many economies are planning (and investing in) a return to renewable energy. According to the Renewables 2015 Global Status Report [1], renewable energy represented approximately 59% of net addition to global power capacity in 2014. Solar energy, which is by far the most abundant type of renewable energy, can be used to generate heat and electricity – and could potentially be used to meet a majority of total energy demand.

Photovoltaic (PV) cells are able to convert sunlight directly into electricity by exciting the electrons of semiconducting materials. However, because only a portion of the solar spectrum has enough energy to excite the electrons, PV cells have limited efficiency. The efficiency of well-designed silicon cells can exceed 50% for incoming light between 700 nm and 1100 nm, but is low or zero at shorter or longer wavelengths, respectively [2], [3]. Absorbed light outside 700–1100 nm becomes heat, which could reduce the efficiency of the cell or, in the extreme, cause damage.

Solar thermal collectors, on the other hand, can utilize the entire solar spectrum through a black absorber. Conventional thermal absorbers consist of a solid surface with a heat transfer fluid flowing over them to remove heat. In recent years, advanced volumetric absorbers have been proposed as a means to absorb the light directly inside the working fluid, saving a heat transfer step and (potentially) losses incurred from having the highest temperature on the outer boundary [4], [5]. Even with these advancements, heat is still much less fungible and transportable compared when to electricity.

Considering the aforementioned limitations of using PV and thermal solar technologies, scientists and engineers have long sought to bring them together with hybrid PV/T technology [6]. The hybrid PV/T collector is more efficient than stand-along PV or thermal systems [7] because it can be designed to utilize nearly 80% of the incoming solar energy [8]. The interested reader can peruse the large body of work on PV/T collectors in the review papers published by Chow [8], Michael et al. [9], and Al-Shamani et al. [10]. Overall, it can be concluded that researcher have put a lot of effort into the optics, geometry, and configuration for PV/T systems, but little effort to date has gone into optimizing the working fluid(s) inside the system.

The efficiency of PV cells decreases when the cell temperature increases as a result of the absorption of photons at energy levels below the cell bandgap [11]. The combination of PV and thermal systems can prevent the increase of the PV cell temperature, while at the same time harvesting another useful output. Standard PV/T system uses water [12] or air [13] flowing behind the PV cells to remove this heat for thermal applications. Optical methods [14], spectral beam-splitting systems [15], [16], and spectral filtering with waste heat recovery system [17] have been proposed as techniques to intercept unwanted radiation before it lands on the PV cells.

Recently, a wide range of selective transmitter materials was studied for feasibility as selective solar transmitters [18].

Since the PV and thermal receivers are physically separated in this system, they can operate at significantly different temperature.

Pure fluids can serve as ‘selective’ filters [19], [20] and coolants [21] in PV/T systems [9], but recent advancements in nanomaterials have provided a means to substantially improve upon conventional pure fluids. In particular, nanofluids [22] promise tenability of optical properties [23], [24], [25], [26] and more attractive thermal properties than pure fluids [27], [28].

Zhao et al. [29] presented the ideal nanofluid for a hybrid double-pass PV/T solar collector. In Zhao's work the same nanofluid was used as both an optical filter and a coolant fluid. The theoretical results show that increasing the mass flow rate improves the thermal efficiency of the thermal unit, and the electrical efficiency of the PV cells is not influenced by variations in the mass flow rate. At high solar concentration, this does not hold true because decreasing the mass flow rate increases the working fluid temperature. This leads to an increase in the solar cell temperature, which consequently decreases the electrical efficiency.

Taylor et al. [30], [31] conducted a theoretical study on the optimization of nanofluid-based optical filters in PV/T systems and investigated the various combinations of base fluids, nanoparticle materials, nanoparticle shapes, and volume fraction to discover a set of potential nanoparticle-based fluid filters. The optical properties were numerically modeled for optimal performance in five PV cell materials, namely Si, Ge, InGaP, InGaAs, and CdTe. The results show that nanofluid-based optical filters can achieve the same level of control as conventional optical filters, although the some of the materials may be difficult to fabricate with the necessary geometric tolerances. To obtain the desired optical properties, very low volume fractions (on the order of 0.001%) were found to be for optimum PV/T filters, which makes nanofluids potentially inexpensive spectrally selective optical filters.

Otanicar et al. [11] applied the resulting nanofluid-based optical filters developed by Taylor et al. [30] to concentrated PV/T systems and compared the output performance with different conventional thin-film-based optical fluid filters. The results demonstrate that nanofluid-based filters have a slightly lower overall efficiency than conventional thin-film filters. Otanicar et al. [11] did not consider the heat removed from the PV cells in calculating the overall efficiency of the PV/T system – an unexploited source of thermal energy.

Cui and Zhu [32] experimentally explored the effect of nanofluids in a PV/T system. In their study, MgO-water nanofluids were used as a coolant and applied to the top of a silicon photovoltaic panel to cool down solar cells and collect heat. The results show that the increase of both particle volume fraction and nanofluid film thickness decreases the transmittance of visible light, which leads to a reduction in the output power of solar cells in the PV/T system.

After reviewing references [29], [33], [34], [35], [36], the authors have found that a gap exists in the literature in that no studies have investigated the use of separate nanofluids for the optical filtering and for the cooling process. In fact, using the same nanofluid for optical filtering and cooling in PV/T systems is disadvantageous since it imposes conflicting requirements in the particle volume fractions and materials, as is shown in Fig. 1b. Increasing the volume fraction of the nanoparticles will enhance the thermal conductivity of the nanofluid and promote heat transfer between the nanofluid and PV cells, but will considerably degrade the optical properties of the nanofluid filter. Similarly, a good nanofluid-based optical filter is obtained at a low volume fraction and no measureable change is possible in the thermal properties. Therefore, a new PV/T design is proposed herein which uses two different nanofluids as shown in Fig. 1a. The first nanofluid – the ‘optical nanofluid’ – is optimized to obtain the best liquid optical filter (high transmittance at the visible spectrum and high absorbance at the UV–IR spectrum). The second nanofluid – the ‘thermal nanofluid’ – is designed to enhance heat removal from the PV cells. To verify the benefit of the proposed PV/T system configuration, a comparative analysis is conducted between a separate and a double-pass channel configuration. Overall, this study presents a rigorous study of an improved PV/T design – a design which opens up a new approach for hybrid solar collectors.