# Thermal Stability and Performance Testing of Oil-based CuO Nanofluids for Solar Thermal Applications

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## Abstract

**:**

## 1. Introduction

_{2}O

_{3}/H

_{2}O nanofluids [7], and for a PV/T (photovoltaic/thermal) collector with 3.0 wt % SiO

_{2}/H

_{2}O nanofluids, a 12.8% thermal efficiency enhancement was reported [8]. A 10% improvement was reported for solar tower power collectors with 0.001 wt % graphite nanofluids [9].

^{®}VP-1 based MWCNT nanofluids showed significant agglomeration after only five thermal cycles. Mesgari [26] studied the thermal stability of plasma and acid-functionalized MWCNT nanofluids in different alkaline fluids. The results showed that the MWCNT nanofluids remained stable at 150 °C. However, there was no further investigation on long-term stability with heating cycles. Jiang investigated the nanofluids stability at medium temperatures by measuring the effective thermal conductivity over time. The results showed that the thermal conductivity decreased when temperature increased from 30 to 210 °C, which may result from nanoparticle aggregation [27]. In addition, working fluids also suffers from the effects of “heating–cooling–heating” thermal cycles due to the use of a heat exchanger. Taking this factor into account, Otanicar et al. [28] conducted an experiment to test the thermal cycles of SiO

_{2}@Au core-shell nanofluids to determine the degree of particle agglomeration generated by 200 accelerated heating cycles with temperatures ranging from 25 to 80 °C. An amount of soft agglomeration in a range of ± 20% was found, and it was possible to reversibly break up agglomerates with regular sonication. Taylor et al. [21] compared the thermal stability of uncoated and coated Ag nanodisc nanofluids. The results showed that even uncoated Ag could resist initial heating (up to 80 °C) while it rapidly agglomerated after several heating cycles. While useful, this temperature range was limited relative to systems that operated well above 100 °C.

## 2. Thermal Impulse Stability

#### 2.1. Nanofluids Preparation

- Soft aggregation of CuO nanoparticles, which occurred during storage/transportation over time at room temperature, can be broken up with about 15 min ultrasonic vibration.
- Oleic acid (the dispersant) was much less viscous and soluble with the heat transfer oil, when the suspension was heated up to 70 °C. Thus, in this step, the dispersant was added and kept warm for one hour to ensure CuO nanoparticles to fully adsorb oleic acid.
- The final heating step was also maintained at 90 °C for one hour to ensure the low viscosity of the Diphyl DT during the final stirring/sonication step.

#### 2.2. Thermal Impulse Testing

_{B}T [37]). In addition, the higher the temperature, the lower the viscosity of the base fluid (e.g., v = CT + B, where v is the viscosity of the nanofluids, T is the temperature of nanofluids, and C and B are constants) [38], which enables a faster settling rate for agglomerated particles. Lastly, heating also accelerates the decomposition of oleic acid and the oxidation of the heat transfer oil, both of which are dependent on temperature.

## 3. Optimization of Preparation Processes

#### 3.1. Experimental Study on the Optimal Dispersant

#### 3.2. Experimental Study on the Optimal Concentration of the Dispersant

_{CuO}: v

_{oil}= 1:99) nanofluids were prepared with different amounts of the dispersant oleic acid (from 0 mL to 1.5 mL with a 0.25 mL interval), each of which underwent a one hour ultrasonic vibration time and a settling time of 30 days. Because the stratification of some samples was not obvious, the transmittance, shown in Figure 8, was used to evaluate stability.

#### 3.3. Experimental Study on the Optimal Ultrasonic Time

_{CuO}: v

_{oil}= 1:99) nanofluids (using 1 mL of oleic acid) were prepared with different ultrasonic times (from 0.5 to 3 h with a 0.5 h intervals) and left to settle for 30 days. The measurement results are shown in Figure 9.

#### 3.4. Orthogonal Tests

## 4. Experiment on Thermal Properties of CuO/Oil Nanofluids

#### 4.1. Experiment on the Thermal Conductivity of CuO/Oil Nanofluids

_{p}and k

_{bf}represent the coefficient of thermal conductivity of CuO particles and the coefficient of thermal conductivity of a base fluid, k

_{nf}is the coefficient of thermal conductivity of the mixture. The parameter K is related to the volume fraction and the shape of particles. When the particle is spherical, the parameter K can be written as [39]:

#### 4.2. Photothermal Conversion of CuO/oil Nanofluids

_{i}

_{+1}-T

_{i}represents the temperature difference, A is the lighting area of a collector tube, G is the average irradiance of concentrated incident rays, and Δt is the time interval. The mass and specific heat capacity of the prepared nanofluids were similar to those of the thermal conductive oil due to their low concentrations.

^{2}).

## 5. Conclusions

- Heating temperature, heating cycles, and heating time were all found to significantly influence the stability of nanofluids;
- Nanofluids tended to aggregate quickly at medium temperatures or after several thermal cycles, even if they had high stability at room temperature (e.g., more than 30 days without settling).
- According to our orthogonal test, the main factors affecting thermal impulse stability were the periodic resonication time, particle size, particle concentration, and the type of dispersant;
- The thermal conductivity of CuO/oil nanofluids with a 0.2% volum fraction was 3.8% higher than that of the pure oil base fluid, enabling its thermal efficiency in the solar collector to achieve 67.6%.

## Author Contributions

## Funding

## Conflicts of Interest

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**Figure 3.**Nanofluids-preparing process. (

**a**) The “two-step” method (additional steps are shown by dotted boxes). (

**b**) Prepared nanofluids after a 3-day settling test at room temperature. A in (

**b**) represents the modified two-step method, and (B) in (

**b**) represents a conventional two-step method discussed in the literature [36].

**Figure 4.**Spectral transmittances of CuO/oil nanofluids for a number of thermal cycles. Note: samples 1–4 experienced 5, 10, 25, and 45 thermal cycles, respectively, while sample 5 underwent no thermal cycle.

**Figure 5.**Spectral transmittances of CuO/oil nanofluids at different maximum temperatures. Note: samples 1–4 were tested at the maximum test temperatures of 80, 100, 120, and 150 °C, respectively, while sample 5 was kept at room temperature.

**Figure 6.**Spectral transmittances of nanofluids with different heating times. Note: samples 1–4 were tested for heating times of 5, 10, 30, and 60 min, respectively, while sample 5 experienced no heating.

**Figure 7.**Photo of nanofluids with different dispersants after 30 days of settling time: (

**a**) oleic acid; (

**b**) glyceryl monostearate; (

**c**) Span 80; (

**d**) Tween 40.

**Figure 8.**Spectral transmittances of CuO/oil nanofluids with different oleic acid volumes. Note: samples 1–7 had different oleic acid volumes from 0 mL to 1.5 mL with a 0.25 mL interval, respectively.

**Figure 9.**Spectral transmittances of CuO/oil nanofluids for (

**a**) different periodic resonication times and (

**b**) different settling times (10, 15, and 30 days).

**Figure 11.**(

**a**) Theoretical and tested values of thermal conductivities of nanofluids as a function of volume fraction. (

**b**) Thermal conductivity of nanofluids as a function of settling time.

**Figure 12.**Linear Fresnel solar collector: (

**a**) overview image; (

**b**) solar evacuated tube with nanofluids; (

**c**) schematic diagram.

**Figure 13.**Temperature and instantaneous thermal efficiency curves of CuO/oil nanofluids and the outer tube wall as a function of time.

Materials | Mean Diameter, (nm) | Specific Surface, (m^{2}/g) | Density, (g/cm^{3}) | Thermal Conductivity, (W/(m × K)) |
---|---|---|---|---|

Diphyl DT | - | - | 1.035 | 0.1554 |

CuO | 60 | 60 | 6.4 | 76.5 |

Dispersants | Oleic Acid | Glyceryl Monostearate | Span 80 | Tween 40 |
---|---|---|---|---|

Chemical formula | C_{18}H_{34}O_{2} | C_{21}H_{42}O_{4} | C_{24}H_{44}O_{6} | C_{22}H_{42}O_{6} |

Density (20 °C) | 0.891 g/mL | 0.985 g/cm^{3} | 0.994 g/mL | 1.10 g/mL |

Boiling point | 350–360 °C | 476.9 °C | 579.3 °C | — |

Melting point | 13.4 °C | 80 °C | 11 °C | 0.1 °C |

Hydrophile lipophile balance (HLB) | 1.0 | 3.8 | 4.3 | 5.6 |

Purity | AR | CP | CP | CP |

Level | Diameter (A) | Concentration (B) | Dispersant (C) | Periodic Resonication Time (D) |
---|---|---|---|---|

1 2 3 | 100 nm 60 nm 30 nm | 3 vol% 2 vol% 1 vol% | Oleic acid | 2 h 0.5 h 0 h |

Oleic acid + Span 80 | ||||

Span 80 |

Items | A | B | C | D | Results |
---|---|---|---|---|---|

1 | 1 | 1 | 1 | 1 | 19 |

2 | 1 | 2 | 2 | 2 | 16 |

3 | 1 | 3 | 3 | 3 | 12 |

4 | 2 | 1 | 2 | 3 | 16 |

5 | 2 | 2 | 3 | 1 | 32 |

6 | 2 | 3 | 1 | 2 | 24 |

7 | 3 | 1 | 3 | 2 | 15 |

8 | 3 | 2 | 1 | 3 | 24 |

9 | 3 | 3 | 2 | 1 | 50 |

k_{1} | 47 | 50 | 67 | 101 | |

k_{2} | 72 | 72 | 82 | 55 | |

k_{3} | 89 | 86 | 59 | 52 | |

$\overline{{k}_{1}}$ | 15.7 | 16.7 | 22.3 | 33.7 | |

$\overline{{k}_{2}}$ | 24 | 24 | 27.3 | 18.3 | |

$\overline{{k}_{3}}$ | 29.7 | 28.7 | 19.7 | 17.3 | |

Range | 14 | 12 | 7.6 | 16.4 |

_{i}(i = 1, 2, 3) and $\overline{{k}_{i}}$ (i = 1, 2, 3) represent ranges and average ranges at different levels.

Parameter | Value |
---|---|

Mirror width W (mm) | 800 |

Mirror length L (mm) | 2200 |

Q_{1} (mm) | 475 |

Center distance S (mm) | 950 |

Collector height H (mm) | 1800 |

Tilt angle (°) | 32 |

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## Share and Cite

**MDPI and ACS Style**

Yang, M.; Wang, S.; Zhu, Y.; Taylor, R.A.; Moghimi, M.A.; Wang, Y.
Thermal Stability and Performance Testing of Oil-based CuO Nanofluids for Solar Thermal Applications. *Energies* **2020**, *13*, 876.
https://doi.org/10.3390/en13040876

**AMA Style**

Yang M, Wang S, Zhu Y, Taylor RA, Moghimi MA, Wang Y.
Thermal Stability and Performance Testing of Oil-based CuO Nanofluids for Solar Thermal Applications. *Energies*. 2020; 13(4):876.
https://doi.org/10.3390/en13040876

**Chicago/Turabian Style**

Yang, Moucun, Sa Wang, Yuezhao Zhu, Robert A. Taylor, M.A. Moghimi, and Yinfeng Wang.
2020. "Thermal Stability and Performance Testing of Oil-based CuO Nanofluids for Solar Thermal Applications" *Energies* 13, no. 4: 876.
https://doi.org/10.3390/en13040876