Materials Today Energy
Volume 17, September 2020, 100480
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Enhancement of therminol-based nanofluids with reverse-irradiation for medium-temperature direct absorption solar collection

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Highlights

  • Therminol-based nanofluids are studied for reverse-irradiation direct absorption solar collector (RI-DASC).

  • The RI-DASC is not only energy-efficient but also fast response to solar irradiation.

  • The photo-thermal conversion efficiency is improved up to 80%.

  • The response time for nanofluids to achieve a steady-state temperature is shortened by 55.6%.

Abstract

The nanofluids-based direct absorption solar collector (DASC) is considered as the next-generation solar collection technology due to its high photo-thermal conversion efficiency. However, the key challenges for its development are the large temperature gradients inside nanofluids and the agglomeration of nanoparticles. To address these issues, this paper proposes to apply solar irradiation at the bottom surface of the DASC (i.e. reverse irradiation) rather than at the top surface, which changes the heat transfer mode from heat conduction to heat convection. Experimental test is carried out for the first time for medium-temperature solar collection (~150 °C), where titanium nitride is selected as nanoparticles and therminol as base fluid. The experimental results show that reverse irradiation contributes to a uniform temperature distribution in nanofluids and results in a 36.4% higher photo-thermal conversion efficiency compared with the top irradiation; the maximum efficiency can reach up to 80%. What's more, the response time for nanofluids to achieve a steady-state temperature is shortened by 55.6%. One week test shows that reverse irradiation significantly improves the stability of nanofluids and mitigates the agglomeration of nanoparticles. Therefore, it can be concluded that the reverse irradiation DASC is a high-efficient, a fast-response and a long lifetime technology for solar collection.

Graphical abstract

Reverse irradiation is an effective method for direct absorption solar collectors to significantly improve the photo-thermal conversion efficiency of nanofluids, shorten the response time to achieve a target temperature and enhance the stability of nanofluids.

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Introduction

With the increasing environmental pollution and energy demand, solar energy has become a research hotspot in recent years due to its huge amount and environmental friendliness [1]. It is reported that the solar irradiation arriving on the earth surface in 1 h can supply the global energy consumption for more than one year [2,3]. There are various methods to make use of the solar energy, among which photo-thermal conversion is one of the most straightforward approaches [[4], [5], [6], [7]]. In a conventional photo-thermal conversion system, solar irradiation is usually absorbed by a selective coating surface which transfers heat to working fluids. However, it has a lower photo-thermal conversion efficiency of ~50%, which is one of the major challenges for solar collection [8]. This is because the coating surface has the maximum temperature of the whole system, leading to high thermal losses to the environment [9]. Moreover, it is hard for working fluids to achieve a high temperature because of the thermal resistance between the coating surface and working fluids [10].

In 1978, Minard et al. [11] proposed the concept of direct absorption solar collectors (DASCs). A significant advantage of the DASCs is that solar energy is absorbed by working fluids directly, which can reduce heat losses in the process of energy transport. However, it still has a low photo-thermal conversion efficiency which restricts the development of the DASCs. In 1995, Choi and Eastman [12] first proposed the concept of nanofluids which dispersed nanoparticles into base fluids to form a suspension. Since then, researchers started to add nanoparticles into working fluids for enhancing the photo-thermal conversion efficiency of the DASCs. Amjad et al. [13] reported an experimentally study of silver-water nanofluids and demonstrated an enhancement of 99.7% in the photo-thermal efficiency compared with base fluid. Karami et al. [14] investigated aqueous suspensions with alkaline functionalized carbon nanotubes and remarkable improvement in the extinction coefficient was observed. Moreover, various types of nanomaterials had been investigated, such as Ag [15], Au [16], CuO [17], Fe3O4 [18], TiO2 [19], SiO2 [20], MWCNT [21], and graphene oxide nanoplatelets [22]. The geometry of nanoparticles was also studied including spheres [23], core-and-shell [24] and rods [25]. However, the above literature was mainly focused on the performance of the DASCs with an optical depth of 1.0 cm, which was very limited for practical applications. The different optical depths had a significant effect on the photo-thermal conversion efficiency of the DASCs [26]. Therefore, researchers started to investigate different optical depths including 6.0 cm [27] and 10.0 cm [28]. With the optical depth increasing, the DASCs showed a large temperature gradient inside nanofluids and the top surface had a high temperature which behaved like the conventional coating surface [29]. What's more, nanoparticles aggregation and sedimentation are also big challenges for the DASCs. These severely suppress the photo-thermal conversion efficiency of the DASCs.

In order to improve the photo-thermal conversion efficiency of the traditional DASCs, key factors had been studied including the absorbing properties of nanoparticles [30], optical boundary conditions [[31], [32], [33]] and the volume temperature homogeneity [34]. Lenert et al. [30] investigated carbon-coated absorbing nanoparticles to optimize the DASCs, which showed a photo-thermal conversion efficiency exceeding 35%. Delfani et al. [33] showed that a black internal bottom surface contributed to a higher photo-thermal conversion efficiency of ~50% compared with a reflective internal bottom surface (~40%). There are various ways to get a uniform temperature field in nanofluids, including mechanical stirring [35] and outside magnetic [36,37]. Wang et al. [35] evaluated the solar harvesting performance of the ZnO–Au/oil nanofluid with a stirring bar to reach a uniform temperature field, which exhibited a 240% enhancement in the photo-thermal conversion efficiency compared to the base fluid. With an external rotating magnetic field, a forced convection nanofluid absorption system was achieved, where α-Fe2O3 magnetic nanoparticles served as the nano-rotor [37]; the photo-thermal conversion efficiency was improved by 14.5%. However, these methods are either costly or complicated for practical applications. Most recently, Wang et al. [38] proposed to achieve a uniform temperature field in nanofluids by simply changing the solar irradiation direction (i.e. reverse irradiation); experimental studies were carried out with ZrC-water as nanofluids for low temperature applications (~45 °C), which showed not only the improved photo-thermal conversion efficiency, but also the relieved sedimentation of nanoparticles.

The above literature review identifies the development gaps for the DASCs: (1) the addition of nanoparticles into working fluids could enhance the absorbance for solar irradiation, hence improving the photo-thermal conversion efficiency; however, the improvement is limited due to a large temperature gradient inside nanofluids and the nanoparticles aggregation; (2) a couple of measures have been taken to achieve a uniform temperature distribution inside nanofluids and to mitigate nanoparticles aggregation, but most of them are either costly or complicated for practical applications; (3) reverse irradiation is a simple way to address the above issues, while previous literature was mainly focused on low-temperature solar collection for domestic heating.

Considering that a medium-temperature heat source (~150 °C) is widely required to improve power generation in the conventional power plants (such as Brayton cycle [39]) and energy storage plants (such as cryogenic energy storage [40]), this paper aims to experimentally investigate the performance of the reverse irradiation DASC (denoted as RI-DASC) for medium temperature applications (~150 °C). Therminol is selected as the base fluid and TiN as the nanoparticle which has a better irradiation absorption performance and is more stable compared with ZrC [41]. Photo-thermal conversion efficiency of the RI-DASC is analyzed under various working conditions, including absorption depths, irradiation intensities and nanoparticle concentrations. Comparisons are also made with the traditional DASC. The RI-DASC not only shows a high photo-thermal conversion efficiency but also has a fast response to the target temperature.

Section snippets

The experimental setup

Fig. 1 shows the schematic diagram of the test rigs. The traditional DASC, as shown in Fig. 1(a), consists of a solar simulator, a sample stage and a data acquisition system. The irradiance of the solar simulator (test scope 0–20000 W/m2; stability ± 3.5%) arriving at nanofluids is set at 1000–4000 W/m2 (denoted as 1–4 suns) by adjusting the power of a lamp; light intensity meter (DT-1308) is utilized to quantify the solar irradiation intensity. The sample stage is composed of a quartz beaker

Absorption coefficient

The extinction coefficient of nanofluids (i.e. absorptivity), α(λ), can be calculated from the spectrum transmittance by the Beer-Lambert law [42]:T(λ)= exp(α(λ)d)where T(λ) is the transmittance of nanofluids and d represents the optical depth of 1.0 cm.

The solar weighted absorption coefficient, Am(x), is used to evaluate the fraction of solar energy absorbed by the nanofluids at a given penetration distance of x and can be calculated as follows [43]:Am(x)=Sm[1eα(λ)x]dλSm(λ)dλwhere Sm(λ)

Nanoparticles characterization

The composition and crystal structure of TiN nanoparticles are shown in Fig. 3(a). Five characteristic peaks are observed at the 2-theta degree of 36.6°,42.6°,61.8°,74.1° and 77.9°, which corresponds to the crystal faces (1 1 1), (2 0 0), (2 2 0), (3 1 1) and (2 2 2), respectively (PDF: 38–1420). From the Scherrer equation (D = 0.89λ/βcosθ), where λ = 0.154056 nm, β = 0.0032, and θ = 42.6°/2 = 21.3°, the particle size is estimated to be 30.046 nm.

According to the transmission electron

Conclusions

Nanofluids-based direct absorption solar collection (DASC) is an effective way for solar collection. However, it has a large temperature gradient inside nanofluids and significant nanoparticles agglomeration, which limit its photo-thermal conversion efficiency. To address these issues, this paper, for the first time, investigates reverse irradiation for direct absorption solar collection (RI-DASC) for medium temperature applications (~150 °C). Therminol is selected as the base fluid and TiN as

Credit author statement

Wei Yu, Xiaohui She and Huaqing Xie contributed to the conception of the study, designed the work that led to the submission, and played an important role in interpreting the results. Kongxiang Wang, Yan He, Ankang Kan, Pengyu Liu and Debing Wang contributed significantly to perform the data analyses and wrote the manuscript. All the experimental work in this article was finished by Kongxiang Wang, Pengyu Liu and Lingling Wang, who contributed equally to this article. Xiaohui She and Wei Yu

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

The work was supported by National Natural Science Foundation of China (No. 51590901 & 51876112), Shanghai Municipal Natural Science Foundation (No. 17ZR1411000), the Key Subject of Shanghai Polytechnic University (Material Science and engineering; No. XXKZD1601 and A10GY19H10-g01) and an IGI/IAS Global Challenges Funding (IGI/IAS ID 3041).

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