Experimental Analysis PVP Coated Silver Nanofluid Properties for Application in Photovoltaic/Thermal (PVT) Collectors

Nowadays, supplying energy for the global population has turned into a prominent issue for countries engendering the consumption of huge amounts of fossil fuels which leads to some serious environmental problems. Among the renewable energy technologies, solar collectors can play major role to improve the efficiency, in air conditioning utility systems by minimum pollution. In photovoltaic/thermal (PVT) solar collectors, which are currently considered as the most advanced type to produce electricity and heat simultaneously, working fluid absorbs Energy from photovoltaic (PV) module engendering to decrease temperature of PV module and increase the electricity efficiency and also provide permissible amount of heat for other residential applications. Meanwhile, utilizing nanofluid as the working fluid in collector, regarding that the nanofluid has enhanced thermal properties relative to the base fluid, leads to a higher collector efficiency. In this research, PVP coated silver nanofluid was prepared in three volume concentration being 250, 500 and 1000 ppm by two-step method. To assess the stability of nanofluid the zeta potential is calculated which is obtained -41.6 V. Also, the prominent thermal properties of the nanofluid were analyzed regarding PVT solar collector applications. According to the results, thermal conductivity of the PVP coated silver nanofluid, improves the properties of base fluid, to the extent that thermal conductivity coefficient grows up 50% in some temperatures and increased from 0.594 for base fluid to 1.098 W/mK by escalation of concentration to 1000 ppm. Thus, PVP coated silver nanofluid can be deemed as the vital working fluid to improve the performance of PVT solar collectors. DOI: 10.14302/issn.2643-2811.jmbr-20-3476 Corresponding author: A. Naderi, School of Mechanical Engineering, Imam Ali University, Tehran, Iran, Email: a.naderi.iut@gmail.com


Introduction
The growth of global population and the ascending trend of energy needs in societies, as well as the limitations and problems of existing energy resources, the use of renewable energy can be deemed as a vital issue for researchers. Because of the availability in most parts of the world and its capability to be provided without any environmental pollution, solar energy is a substantial alternative for fossil fuels rather than other sources of renewable energy, such as wind, water, and biomass. Meanwhile, the use of photovoltaic/thermal (PVT) solar collectors to supply thermal and electrical energy simultaneously would be a significant component in installation systems [1]. PVT solar collectors ( Figure 1) includes two main parts being an electric panel and a heat absorber, which convert solar radiation into electrical and thermal energy at the same time [2]. Due to the structure of this type of collectors, the working fluid moving on the back side of the photovoltaic panels, absorbs thermal energy from the photovoltaic (PV) panel that reduces the surface temperature of the PV panel rising its electrical efficiency. The working fluid which gained the extra heat from PV panel, can be utilized for diverse purposes such as solar dryers and heating the water consumption [3,4]. Therefore, the thermal properties of the working fluid are considered prominent issue which should be studied. The enhancement of thermal conductivity engendering to more heat is dissipated from the photovoltaic panel and more energy is obtained by working fluid. Also, in recent years, with the advancement of nanotechnology, numerous researchers have studied the use of nanofluids to improve heat transfer in engineering applications [5][6][7][8][9][10]. Thus, by application of the properties of nanofluids in PVT solar collectors, the performance of this solar system can be improved.
The main step in using nanofluids as the working fluid in PVT solar collectors is to examine the properties of them. Stability, thermal properties and radiant properties that should be considered for any nanofluid. Khan et al. review some significant researches about the properties of diverse nanofluids and determine the effect of some parameters such as particle size on the performance of them [11]. Over the past decade, many researches have been done on the properties of nanofluids for use in solar collectors [12].
In separate studies, Karami

Materials and Methods for Preparing Nanofluid
In this study, Silver nanoparticle coated with

Results and Discussion
TEM & X-Ray

Density
In this research, Pak Chu equation is utilized to calculate the density of nanofluids [13]. (Equation (1)) In Equation (1), φ is volume fraction of nanofluid, ρ f and ρ p are density of base fluid and PVP coated silver nanoparticles, respectively; and ρ nf is density of nanofluid which is demonstrated in Table 2.

Specific Heat Capacity
The specific heat capacity of working fluids having striking impact on thermal performance of PVT collectors can be calculated by equation (2).

…(2)
According to the physical properties of base fluid and silver nanoparticle, specific heat capacity of nanofluids are indicated in table 3.
The trend of specific heat capacity in terms of temperature for different volume concentration of nanofluids is illustrated in figure 6. Furthermore, the changes of C p in terms of volume concentration at 25 and 40 °C are shown in figure 7.
As it can be seen in figure 6 and 7, by growth of volume concentration of nanofluids, the specific heat capacity decrease significantly. Due to noticeable different between the base fluid's and silver nanoparticle's heat capacity and also low concentration of nanofluids, the trend of C p for nanofluids follow the pattern of base fluid's curve. Thus, the higher concentration of nanofluids, the more decrement is occurred in specific heat capacity. According to the results, the specific heat capacity of 500 and 1000 ppm silver nanofluid decrease compared to the deionized water, which the former is 5% and the latter is 10% lower than base fluid. The C p of water is 17 times higher than silver, so the more amount of silver nanoparticle the more decrease would be occurred in the specific heat capacity of nanofluid.

Thermal Conductivity
Experimental researches indicates that fluids including nanoparticles, have more thermal conductivity coefficient and lower specific heat capacity than base fluids [41]. In current study, transient hot wire method has been utilized for measuring thermal conductivity of nanofluids in temperature range of 25 to 55°C by using In order to better analyses of thermal conductivity, the increment of thermal conductivity