A comprehensive review on nanofluid operated solar flat plate collectors

Abstract

The impact of population explosion and continuous upsurge on energy demand has resulted in the intimidating depletion of fossil fuel resources, increased environmental pollution, and elevated production and consumption cost. Hence, in the past two decades the demand for renewable energy has escalated. The solar energy is the most trending topic when talking about renewable energy sources, because of its ease of availability, reduced dependence on foreign fuels and negligible maintenance. This can be directly harnessed unlike other renewable energy sources. A solar flat plate collector converts the radiant solar energy from the sun into thermal energy; usually, copper or aluminium is used as heat absorbing material. However, to further enhance the performance and thermophysical properties of the heat exchanger liquids of flat plate solar collectors like radiative heat transfer and thermal conductivity, the nanofluids are used. The use of nanofluids as an innovative type of working fluids is reasonably a new development in solar flat plate collectors. They are prepared by mixing low concentration of solid particles, sized 1–100 nm with the base fluid. The objectives of this review paper is to recapitulate the investigations carried in the field of solar flat plate collectors using a range of nanofluids, the performance analysis of various flat plate collectors using numerous nanofluids and the challenges faced in developing an efficient thermal collector using nanofluids. Furthermore, the article discusses the opportunities for future research.

Introduction

The population of the world is increasing day by day and it is expected to be increased by 25% of the present population in the first half of this century due to modernization of life style and the increment in population. Following this increase, the demand of energy will be doubled in the middle of this century and it will be tripled at the end of this century [1]. But the fossil fuel resources are not enough to fulfill the expected demands. Climbing fuel prices, reduction in fossil fuel resources and increase in greenhouse gas emissions are the main reasons for the researchers to go with eco-friendly energy resources, i.e., renewable energy to meet the projected demands [2,3,4]. Renewable energy resources include Solar energy [5], Geothermal energy [6, 7], Bio-energy [8, 9], Marine energy [10], etc. Solar energy is one of the cleanest forms of renewable energy resources. So, considering environmental effects and easy availability at every place, the solar energy is considered one of the best forms of renewable energy resources [1, 10, 11].

Methods for collecting solar energy can basically be categorized as photovoltaic systems (PV) and thermal systems as shown in Fig. 1. Thermal systems convert solar energy into thermal energy, while PV systems transform solar energy into electric energy. Thermal systems can absorb over 95% of the incoming solar radiation [12]. The solar collector is a special type of heat exchanger in which heat exchanges occurs between radiant energy from a distance source and the working fluid flowing in the collector. The solar collector is classified as concentrating and non-concentrating. Non-concentrating is further subdivided into the flat plate solar collector and evacuated tube collector [13, 14]. Flat plate solar collector (FPSC) is the common type used to convert radiant energy into thermal energy by using absorber plate. The surface of absorber plate is black matte painted to absorb solar spectrum with minimum emissivity [1, 13, 15, 16]. Solar radiations strikes on the absorber plate and converted into heat energy, and then, it transferred to heat transfer fluid which is flowing through the collector’s tube. Schematic diagram of FPSC is shown in Fig. 2. The second type of flat plate collector is the direct absorption solar collector (DASC), where no absorber plate is required, and here, the incident rays fall directly on the working fluid and get absorbed [12, 17,18,19,20]. The DASC was initially proposed by Minardi and Chuang [21]. In DASC, operational fluid is allowed to flow between the top transparent surface and the bottom adiabatic surface as shown in Fig. 3. In this paper, the review is narrowed down to the research in the field of FPSCs.

Fig. 1

Types of solar energy harvesting system

Fig. 2

Schematic diagram of a flat plate solar collector (adapted from the Web site [22])

Fig. 3

Schematic diagram of a direct absorption solar collector

The main portion of the incoming solar radiations is absorbed by absorber plate in FPSCs. To minimize the heat losses through conduction, the sides and bottom of the absorber plate are fully insulated. The glass cover of the collector reduces heat losses by convection (containments of an air layer) and radiation (translucent to sun's shortwave solar radiations). But, practically it is non-transparent to the long-wave thermal radiation emitted by absorber plate [1].

Conventional thermal fluids such as water, engine oil and ethylene/propylene glycol play an important role in the various engineering processes and mechanical equipment, for example power generation, air-conditioning, chemical production, heating and cooling processes, electronic appliances, spaces, nuclear system cooling, transportation, and in microelectronics field. Thermal properties of these conventional working fluids are very low as compared to the solid, so heat transfer rate in thermal applications is comparatively low [23,24,25]. Efficiency and compactness of mechanical equipment are improved by uplifting heat transfer properties of the working fluids and it will also lead to the reduction in capital and operating cost [26].

The solids, in particularly metal form, have hundred times higher thermal conductivities as compared to the liquids. Several studies have focused on the thermal performance of solid particle suspending in liquids. The initial suspended particle size was a millimeter or micrometer [27, 28]. However, these millimeter or micrometer-sized particle in early studies cause some problems such as poor suspension, low stability and channel clogging. A way to solve these problems could be the introduction of nanometer-sized particles (1–100 nm). Suspension of nanometer-sized particles in a base fluid is called nanofluid, and the term “nanofluid” was first introduced by Choi and Eastman [29]. Nanofluids are more stable, having high thermal conductivity, which reducing the pumping power and showing better rheological properties [30,31,32].

The performance of FPSCs can be improved by using different methods [33,34,35]. However, the simplest way to increase the efficiency of FPSCs is to replace the conventional working fluid with the new class of fluid, i.e., nanofluids for increasing the rate of heat transfer from the absorber plate [16]. In the last decade, researchers have investigated both experimentally and theoretically [36, 37] the effect of concentration, diameter, preparation methods and thermophysical properties (density, thermal conductivity, specific heat capacity, viscosity) of different nanofluids on the performance of FPSCs. This paper has focused on a review of studies on operational parameters, i.e., absorber fluids (nanofluids) effects on efficiency of FPSCs.

Essential considerations for nanofluids

There are some important variables that ought to be considered for productive utilization of nanofluids in FPSCs. The first consideration is the preparation of nanofluid; it is the basic step to improve the thermal conductivity of fluids by using nanoparticles. There are two methods for preparation of nanofluids: single-step method and two-step method. In the single-step method, the preparation of nanoparticle and synthesis of nanofluids are done in a single step by physical vapor deposition (PVD) [38, 39]. In two-step method nanoparticle, nanotubes or nanofibers are produced by inert gas condensation, chemical vapor deposition and mechanical alloying techniques in dry powder form in the first step and then dispersed into base fluid in a second step [40]. As in this method preparation of nanoparticles and preparation of nanofluids are separately done, agglomeration of particles may take place during drying, storage and transportation of nanoparticles [41, 42]; graphical representation of preparation methods is shown in Fig. 4. Nanofluid synthesis is not simple as to make a mixture of liquid and solid; due to high surface-to-volume ratio, nanoparticles aggregate with the passage of time. Agglomeration of nanoparticles is not only settling and blockage in flow channel but also reduces the thermal conductivity of nanofluids. So, the stability of suspending nanoparticles in base fluid should be investigated thoroughly [43,44,45,46]. The stability of nanofluid can be improved by adding a surfactant, surface modification and ultrasonic vibration, but the most cost-effective and long-term stability method is the addition of surfactant that is non-covalent functionalization method [45]. Heating and cooling are regular processes in heat exchange systems; surfactants have the tendency to create forms in such situations. So, the addition of surfactant not only contaminates the working fluids but also decreases the thermal conductivity and provides negative effects on the viscosity of the nanofluids [31]. The second consideration is the cost of nanofluids. For this purpose, nanoparticles which have high thermal conductivity should be used for the synthesis of nanofluids. Due to the high thermal conductivity and low concentration of nanoparticles, nanofluids have a notable effect on cost reduction and enhance the heat transfer coefficient. This approach also affects the stability of nanofluids because of the low concentrations of dispersed nanoparticles, the nanofluids are more stable [45]. Another positive effect of low concentration of dispersed nanoparticles is the lower level of enhancement of the viscosity of the nanofluids which is the prime concern of pumping power and pressure drop [47, 48]. Nanofluids with higher concentrations have higher viscosities [49]. The pressure drop is another important factor associated with a fluid flowing that should be considered for the operating applications [50]. The increase in viscosity of nanofluids causes an increase in pressure drop which later refers to the pumping power requirement, which is one of the disadvantages of the nanofluids [24, 51].

Fig. 4

Classification of nanofluid preparation method

Data reduction and relevant mathematical equations

The efficacy of FPSCs in steady-state condition can be evaluated by energy balance that represents the incident solar radiation which is divided into useful energy gain, thermal losses and optical losses [52]. The first mathematical model for evaluating the efficacy of FPSCs was suggested by Hottel and Whillier [15] which was later extended by ASHRAE for making a standard for testing the efficiency of FPSCs [53].

ASHRAE standards provide a method for indoor and outdoor testing of different solar collectors. Thermal performance of FPSCs can be tested by establishing the instantaneous efficiency at different values of incident radiation, ambient temperature and inlet temperature of the heat exchanging fluid. Instantaneous efficiency is defined as the ratio of useful energy gain to solar energy received by absorber plate as shown in Eq. (1)

$$\eta_{\text{i}} = \frac{{Q_{\text{u}} }}{{A_{\text{c}} G_{\text{T}} }} .$$

(1)

$$Q_{\text{u}} = A_{\text{c}} F_{\text{R}} \left[ {G_{\text{T}} \left( {\tau \alpha } \right) - U_{\text{L}} \left( {T_{\text{i}} - T_{\text{a}} } \right)} \right]$$

(2)

So, the instantaneous efficiencies can be rewritten in different expressions as represented by Eqs. (3–5).

$$\eta_{\text{i}} = \frac{{\rho VC_{\text{p}} \left( {T_{\text{i}} - T_{\text{a}} } \right)}}{{A_{\text{c}} G_{\text{T}} }}$$

(3)

$$\eta_{\text{i}} = \frac{{A_{\text{c}} F_{\text{R}} [G_{\text{T}} \left( {\tau \alpha } \right) - U_{\text{L}} \left( {T_{\text{i}} - T_{\text{a}} } \right)}}{{A_{\text{c}} G_{\text{T}} }}$$

(4)

$$\eta_{\text{i}} = F_{\text{R}} \left( {\tau \alpha } \right) - F_{\text{R}} U_{\text{L}} \frac{{T_{\text{i}} - T_{\text{a}} }}{{G_{\text{T}} }}$$

(5)

where $F_{\text{R}}$ represents the heat removal factor and its value can be calculated by using Eq. (6)

$$F_{\text{R}} = \frac{{\dot{m}C_{\text{p}} \left( {T_{\text{fo}} - T_{\text{fi}} } \right) }}{{A_{\text{c}} \left[ {I_{\text{c}} \tau \alpha - U_{\text{L}} \left( {T_{\text{fi}} - T_{\text{o}} } \right)} \right]}}$$

(6)

where $U_{\text{L}}$ is the overall losses of FPSCs, which is equal to the sum of the top, bottom and edge losses. All these losses could be calculated by using Eqs. (7–11)

$$U_{\text{L}} = U_{\text{t}} + U_{\text{b}} + U_{\text{e}}$$

(7)

$$U_{\text{L}} = \left\{ {\frac{{\frac{1}{N}}}{{\frac{C}{{T_{\text{p}} }}\left( {\frac{{T_{\text{p}} + T_{\text{a}} }}{N + f}} \right)^{0.33} + \frac{1}{{h_{\text{a}} }}}}} \right\} + \left\{ {\frac{{\sigma \left( {T_{\text{p}} + T_{\text{a}} } \right)\left( {T_{\text{p}}^{2} + T_{\text{a}}^{2} } \right)}}{{\varepsilon_{\text{p}} + 0.5N\left( {1 - \varepsilon_{\text{p}} } \right) + \frac{2N + f - 1}{{\varepsilon_{\text{g}} }} - N}}} \right\}$$

(8)

where

$$C = 365.9 \times \left( {1 - 0.00883\beta + 0.0001298\beta^{2} } \right)$$

(9a)

$$f = \left( {1 + 0.04h_{\text{a}} + 0.0005h_{\text{a}}^{2} } \right) \times \left( {1 + 0.091N} \right)$$

(9b)

$$h_{\text{a}} = 5.7 + 3.8V$$

(9c)

$$U_{\text{b}} = \frac{{k_{\text{b}} }}{{x_{\text{b}} }}$$

(10)

$$U_{\text{e}} = U_{\text{b}} \left( {\frac{{A_{\text{e}} }}{{A_{\text{c}} }}} \right)$$

(11)

Thermophysical properties of nanofluids

Using Eq. (12), the actual useful heat energy gain is calculated and it can also be calculated by taking the difference between heat energy absorbed and heat energy losses from the absorber plate as represented by Eq. (2). The specific heat of nanofluid can be calculated by using Eq. (13) [54] , where $\rho$, $C_{\text{p}}$. and $\varphi$. are density, specific heat and volume concentration of the nanoparticles, respectively, in the nanofluid suspension.

$$Q_{\text{u}} = \dot{m}C_{\text{p}} \left( {T_{\text{i}} - T_{\text{a}} } \right) = \rho VC_{\text{p}} \left( {T_{\text{i}} - T_{\text{a}} } \right)$$

(12)

$$\left( {\rho C_{\text{p}} } \right)_{\text{nf}} = \left( {\rho C_{\text{p}} } \right)_{\text{nf}} \left( \varphi \right) + \left( {\rho C_{\text{p}} } \right)_{\text{bf}} \left( {1 - \varphi } \right)$$

(13)

The nanofluid density was evaluated by the help of Pak and Cho [55] relation which is represented by Eq. (14).

$$\rho_{\text{nf}} = \rho_{\text{np}} \left( \varphi \right) + \rho_{\text{bf}} \left( {1 - \varphi } \right)$$

(14)

Thermal conductivity of the nanofluid can be evaluated by using Eq. (15) [56].

$$k_{\text{nf}} = k_{\text{bf}} \frac{{k_{\text{np}} + \left( {n - 1} \right)k_{\text{bf}} - \left( {n - 1} \right)\varphi \left( {k_{\text{bf}} - k_{\text{np}} } \right)}}{{k_{\text{np}} + \left( {n - 1} \right)k_{\text{bf}} - \varphi \left( {k_{\text{bf}} - k_{\text{np}} } \right)}}$$

(15)

Thermal conductivity of the base fluid can be calculated by using expression represented by Eq. (16) [57].

$$k_{\text{bf}} = 0.6067 \left[ { - 1.26523 + 3.704 \frac{{T_{\text{av}} }}{298} - 1.43955 \left( {\frac{{T_{\text{av}} }}{298}} \right)^{2} } \right]$$

(16)

where $T_{av}$ is the logarithmic average temperature of the base fluid which is shown in Eq. (17)

$$T_{\text{av}} = \frac{{\left( {T_{\text{out}} - T_{\text{in}} } \right)}}{{\ln \left[ {\frac{{T_{\text{out}} }}{{T_{\text{in}} }}} \right]}}$$

(17)

The viscosity of nanofluid is calculated by using Eq. (18) [58].

$$\mu_{\text{nf}} = \frac{{\mu_{\text{bf}} }}{{1 - 34.87 \left( {\frac{{d_{\text{np}} }}{{d_{\text{bf}} }}} \right)^{ - 0.3} \varphi^{1.03} }} \varphi \le 10\%$$

(18)

where $d_{\text{bf}}$ is representing the molecular diameter of the base fluid:

$$d_{\text{bf}} = 0.1\left( {\frac{6M}{{\pi N\rho_{\text{bfo}} }}} \right)^{0.33}$$

(19)

where M, N and $\rho_{\text{bfo}}$ are the molecular weight, Avogadro number and density of the base fluid at 293 K, respectively. Base fluid viscosity can be calculated by using Eq. (20) [59].

$$\mu_{\text{bf}} = 2.414 \times 10^{ - 5} \left( {10} \right)^{{\frac{247.8}{{\left( {T_{\text{av}} - 140} \right)}}}}$$

(20)

Thermodynamic relation for energy and exergy analysis

Thermal energy balance is represented by Eq. (21).

$$m_{\text{p}} C_{\text{p}} \left( {\frac{{dT_{\text{p,avg}} }}{dT}} \right) + \dot{m}C_{\text{p}} \left( {T_{\text{f,o}} - T_{\text{f,i}} } \right) = \eta_{\text{c}} IA_{\text{c}} - U_{\text{c}} \left( {T_{\text{p,avg}} - T_{\text{a}} } \right)A_{\text{c}}$$

(21)

Therefore, at steady-state condition, the thermal efficiency of flat plate solar collector can be represented by Eq. (22).

$$\eta_{\text{c}} = \frac{{\dot{m}C_{\text{p}} \left( {T_{\text{f,o}} - T_{\text{f,i}} } \right)}}{{IA_{\text{c}} }}$$

(22)

where exergies can be shown by the expression from Eq. (23).

$$\dot{E}_{{{\text{x}}{\kern 1pt} {\text{heat}}}} - \dot{E}_{{{\text{x}}{\kern 1pt} {\text{work}}}} - \dot{E}_{{{\text{x}}{\kern 1pt} {\text{mass,in}}}} - \dot{E}_{{{\text{x}}{\kern 1pt} {\text{mass,out}}}} = \dot{E}_{{{\text{x}}{\kern 1pt} {\text{dest}}}}$$

(23)

Substituting values in Eq. (23), Eqs. (24) and (25) could be obtained.

$$\sum \left( {1 - \frac{{T_{\text{a}} }}{{T_{\text{sur}} }}} \right)\dot{Q}_{\text{s}} - \dot{w} + \sum \dot{m}_{\text{in}} \varPsi_{\text{in}} - \sum \dot{m}_{\text{out}} \varPsi_{\text{out}} = \dot{E}_{{{\text{x}}{\kern 1pt} {\text{dest}}}}$$

(24)

$$\sum \left( {1 - \frac{{T_{\text{a}} }}{{T_{\text{sur}} }}} \right)\dot{Q}_{\text{s}} - \dot{m}\left( {h_{\text{out}} - h_{\text{in}} } \right) - T_{\text{a}} \left( {S_{\text{out}} - S_{\text{in}} } \right) = \dot{E}_{{{\text{x}}{\kern 1pt} {\text{dest}}}}$$

(25)

In Eqs. (24, 25), $\dot{Q}_{\text{s}}$ is representing the total rate of energy absorbed by the absorber plate area, Eq. (26).

$$\dot{Q}_{\text{s}} = I_{\text{t}} \left( {\tau .\alpha } \right) = G_{\text{s}} A_{\text{c}}$$

(26)

The change in enthalpy and entropy generation due to nanofluid in solar collector is presented by Eq. (27).

$$\Delta h = h_{\text{out}} - h_{\text{in}} = C_{\text{p,nf}} \left( {T_{\text{f,out}} - T_{\text{f,in}} } \right)$$

(27)

Change in entropy is represented by Eq. (28).

$$\Delta S = S_{\text{out}} - S_{\text{in}} = \dot{m}C_{\text{p,nf}} \ln \frac{{T_{\text{f,out}} }}{{T_{\text{f,in}} }} - \frac{{\dot{m}}}{{T_{\text{a}} }}\frac{{T_{\text{a}} }}{{T_{\text{f,in}} }}\frac{\Delta p}{\rho }$$

(28)

Substituting Eqs. (27) and (28) in Eq. (25), the expression for exergy loss is given by:

$$\dot{E}_{{{\text{x}}{\kern 1pt} {\text{dest}}}} = \left( {1 - \frac{{T_{\text{a}} }}{{T_{\text{sur}} }}} \right)I_{\text{t}} \left( {\tau .\alpha } \right)A_{\text{c}} - \dot{m}C_{\text{p,nf}} \left( {T_{\text{f,out}} - T_{\text{f,in}} } \right) + \dot{m}C_{\text{p,nf}} T_{\text{a}} \times \ln \frac{{T_{\text{f,out}} }}{{T_{\text{f,in}} }} - \dot{m}\frac{{T_{\text{a}} }}{{T_{\text{f,in}} }}\frac{\Delta p}{\rho }$$

(29)

where $\dot{E}_{{{\text{x}}{\kern 1pt} {\text{dest}}}}$ is the exergy loss rate and calculated by Eq. (30) in terms of temperature and entropy.

$$\dot{E}_{{{\text{x}}{\kern 1pt} {\text{dest}}}} = T_{\text{a}} \cdot S_{\text{gen}}$$

(30)

where S_gen is represented by Eq. (31).

$$S_{\text{gen}} = \dot{m}C_{\text{p,nf}} \ln \frac{{T_{\text{f,out}} }}{{T_{\text{f,in}} }} - \frac{{\dot{m}}}{{T_{\text{a}} }}\frac{{T_{\text{a}} }}{{T_{\text{f,in}} }}\frac{\Delta p}{\rho } - \frac{{G_{\text{t}} A_{\text{c}} }}{{T_{\text{a}} }} \left\{ {1 - \frac{4}{3}\left( {\frac{{T_{\text{a}} }}{{T_{\text{s}} }}} \right) + \frac{1}{3}\left( {\frac{{T_{\text{a}} }}{{T_{\text{s}} }}} \right)^{4} } \right\} + \frac{{\left( {\tau .\alpha } \right)G_{\text{t}} A_{\text{c}} }}{{T_{\text{a}} }} \left\{ {1 - \frac{4}{3}\left( {\frac{{T_{\text{a}} }}{{T_{\text{s}} }}} \right) + \frac{1}{3}\left( {\frac{{T_{\text{a}} }}{{T_{\text{s}} }}} \right)^{4} } \right\}$$

(31)

where exergy efficiency η_ex is represented by Eq. (32).

$$\eta_{\text{ex}} = 1 - \frac{{T_{\text{a}} . S_{\text{gen}} }}{{\left( {1 - \frac{{T_{\text{a}} }}{{T_{\text{sur}} }}} \right)\dot{Q}_{\text{s}} }}$$

(32)

Pressure drop and pumping power can be calculated by Eqs. (33) and (34), respectively.

$$\Delta p = f\frac{{\rho V^{2} }}{2}\frac{\Delta l}{d}$$

(33)

where f represents friction factor and its value in laminar flow can be equated to 64/Re.

$${\text{Pumping}}\;{\text{power}} = \left( {\frac{{\dot{m}}}{{\rho_{\text{nf}} }}} \right) \times \Delta p$$

(34)

Error evaluation in energy and exergy analysis

Errors of exergy and energy efficiencies are calculated from Eqs. (35) and (36), respectively.

$$\Delta \eta_{\text{ex}} = \frac{{\Delta \dot{I}}}{{\dot{E}_{{{\text{x}}{\kern 1pt} {\text{heat}}}} }} + \frac{{\dot{I}\dot{E}_{{{\text{x}}{\kern 1pt} {\text{heat}}}} }}{{\dot{E}^{2}_{{{\text{x}}{\kern 1pt} {\text{heat}}}} }}$$

(35)

$$\Delta \eta_{\text{gen}} = \frac{{\Delta \dot{q}_{\text{a}} }}{{\dot{G}_{\text{c}} }} + \frac{{\dot{q}_{\text{a}} \dot{G}_{\text{c}} }}{{\dot{G}_{\text{c}}^{2} }}$$

(36)

where each of the error components can be calculated by using Eqs. (37–40).

$$\Delta \dot{E}_{{{\text{x}}{\kern 1pt} {\text{heat}}}} = \left( {\frac{\Delta T}{{T_{\text{s}} }} + \frac{{T_{\text{a}} \Delta T}}{{T_{\text{s}}^{2} }}} \right)A_{\text{c}} \left( {\tau .\alpha } \right)G_{\text{c}} + \left( {1 - \frac{{T_{\text{a}} }}{{T_{\text{s}} }}} \right)A_{\text{c}} \left( {\tau .\alpha } \right)G_{\text{c}}$$

(37)

$$\Delta \dot{I} = \dot{T}_{\text{a}} \Delta \dot{S}_{\text{gen}} + \dot{S}_{\text{gen}} \Delta T$$

(38)

$$\Delta \dot{S}_{\text{gen}} = \left( {\frac{\Delta p}{{T_{\text{f,in}} \rho }} + C_{\text{p}} \ln \frac{{T_{\text{f,in}} }}{{T_{\text{f,out}} }} + C_{\text{p}} \ln \frac{{T_{\text{f,out}} + T_{\text{f,in}} }}{{T_{\text{a}} }}} \right)\Delta \dot{m} + G_{\text{t}} A_{\text{c}} \left( {\tau .\alpha } \right)\frac{\Delta T}{{T_{\text{a}}^{2} }} + \dot{m}C_{\text{p}} \left( {\frac{1}{{T_{\text{f,out}} }} + \frac{1}{{T_{\text{f,in}} }} + \frac{2}{{T_{\text{a}} }} + \frac{{T_{\text{f,out}} + T_{\text{f,in}} }}{{T_{\text{a}}^{2} }}} \right)\Delta T + A_{\text{c}} \left( {\tau .\alpha } \right)\left( {\frac{1}{{T_{\text{s}} }} + \frac{1}{{T_{\text{a}} }}} \right)\Delta G_{\text{c}}$$

(39)

$$\Delta \dot{q}_{\text{a}} = C_{\text{p}} \left\{ {\frac{{\Delta \dot{m}\left( {T_{\text{f,out}} - T_{\text{f,in}} } \right) + 2\dot{m}\Delta T}}{{A_{\text{C}} }}} \right\}$$

(40)

Bejan number can be calculated from Eq. (41).

$$B_{\text{e}} = \frac{{\dot{S}_{{{\text{gen}}\Delta {\text{T}}}} }}{{\dot{S}_{{{\text{gen}}\Delta {\text{T}}}} + \dot{S}_{{{\text{gen}}\Delta {\text{P}}}} }}$$

(41)

To enhance the performance of the solar collector, it needs to enhance the absorption of solar radiations and reduce the heat losses (by conduction, convection and radiation) from the absorber plate to the surroundings. Efficiency can also be improved by improving the heat transfer rate between the absorber plate and the operational fluid, and then finally to the end user [60]. Therefore, use of nanofluid as working fluid in solar collector is one of the ways to enhance the efficiency of the flat plate solar collector [60,61,62,63,64,65].

Experimental studies on FPSCs by using nanofluids

Yousefi et al. [61, 66, 67] investigated the effect of Al₂O₃/water, MWCNT/water and pH variation with MWCNT/water nanofluids on thermal performance of 2 m² FPSC. To enhance the stability of nanofluid, Triton X-100 surfactant was used at 0.2 mass% and 0.4 mass% concentration of nanofluids. The flow rates were 1–3 L min⁻¹. Experiments were performed by following the ASHRAE standard 93-86. The experimental results revealed that for Al₂O₃/water nanofluid [66], the maximum enhancement in thermal efficiency of FPSC was noted as 28.3% at 0.2 mass% concentration with a surfactant. For MWCNT/water nanofluid [61], it was observed that at 0.2 mass% concentration with surfactant the values of $F_{\text{R}} \left( {\tau \alpha } \right)$ and $F_{\text{R}} U_{\text{L}}$ factor were improved by 47.76% and 71.7%, respectively. Moreover, there are effects of different pH values (3.5, 6.5 and 9.5) on the MWCNT/water and the performance of FPSCs. The results showed that increasing or decreasing the values of pH from the isoelectric point gave a positive effect on the thermal performance of FPSCs and the isoelectric point for MWCNT was noticed about 7.4.

The effect of using Cu-synthesized/EG nanofluid as heat transfer fluid on the effectiveness of 0.67 m² FPSC was examined experimentally by Zamzamian et al. [68]. Cu nanoparticle with the outer diameter of 10 nm and the mass concentrations of 0.2–0.3% was used. Nanofluid was synthesized without surfactant, and no information about stability was presented. The flow rate of nanofluid varied between 0.5 and 1.5 L min⁻¹, and ASHRAE 93-2003 standard was used for the efficiency calculation. They used copper absorber plate with a black coating, aluminium frame and four riser tubes having 0.96 m length and 20 mm diameter. The efficiency of FPSC was decreased with a decrease in flow rate and increased with the increase in Cu-synthesized/EG nanofluid concentration. Due to nanofluid, absorbed energy parameter $F_{\text{R}} \left( {\tau \alpha } \right)$ was increased for FPSC and this parameter was highest at 1.5 L min⁻¹. The removal energy parameter $F_{\text{R}} U_{\text{L}}$ was decreased with the increase in nanofluid concentration. So, an optimum point for FPSC efficiency could be reached for 0.3 mass% Cu-synthesized/EG nanofluid at 1.5 L min⁻¹ flow rate as shown in Fig. 5.

Fig. 5

Variations of collector efficiency versus reduced temperature for the Cu/EG nanofluid at different concentrations and the base fluid (reprint of the publication of Zamzamian et al. [68] with the permission from Elsevier Publisher)

The effect of CuO/water nanofluid on the performance and efficiency of FPSC was investigated experimentally by Moghadam et al. The volume fraction of CuO nanoparticles with a mean diameter of 40 nm was set 0.4. The mass flow rate of nanofluid in the experimental observation was varied from 1 to 3 kg min⁻¹. No surfactant was used for the preparation of nanofluid. Aluminium-based FPSC having 1.51 m² area, 22 mm and 10 mm header and riser diameter, respectively, was used for experimental investigation. An electric pump was used for forced circulation in the test setup, where the tank capacity for fluid storage was kept 20 L in this study. ASHRAE 93-2003 standard was followed for this study. Experimental results revealed that CuO/water nanofluid had improved the collector efficiency about 21.8% at 1 kg min⁻¹ flow rate as shown in Fig. 6. This improved efficiency was 16.7% higher than that of water when compared with CuO/water nanofluid and water alone as presented in Fig. 7.

Fig. 6

Efficiency of solar collector for CuO/water nanofluid at different flow rates (reprint of the publication of Moghadam et al. [69] with permission from Elsevier Publisher)

Fig. 7

Efficiency of solar collector for water and CuO/water nanofluid (reprint of the publication of Moghadam et al. [69] with the permission from Elsevier Publisher)

A FPSC of 2 m² area and absorber plates containing 8 parallel copper strips with 8 mm inner diameter was fabricated by He et al. [70]. Cu–H₂O nanofluid with mass fractions of 0.1 mass% and 0.2 mass% and particle sizes 25 nm and 50 nm was prepared by a two-step method. To enhance the stability of nanofluid, SDBS surfactant was added and the pH value of nanofluid was adjusted at 8 by HCl and NaOH of analytical grades. Thermal efficiency was evaluated by ASHRAE standard 86-93. Experiments were conducted from 9:00 to 16:00 h. The flow rate of nanofluid was maintained by an external pump at 140 L h⁻¹ where the tank capacity used was at 100 L. Heat gain, frictional resistance coefficient and water temperature were also investigated in that study. The experimental results revealed that FPSC efficiency was increased up to 23.83% by using Cu–H₂O nanofluids (25 nm. 0.1 mass%) as shown in Fig. 8. With the increase in nanoparticle size, the efficiency of the FPSC decreases as shown in Fig. 9. The highest temperature and the highest heat gain of water in Cu/water nanofluids (25 nm. 0.1 mass%) were 12.24% and 24.52%, respectively. The frictional resistance coefficient increment rate was less than 1%.

Fig. 8

Thermal efficiency of flat plate solar collector with Cu–water nanofluid at different mass concentrations (reprint of the publication of He et al. [70] with the permission from Elsevier Publisher)

Fig. 9

Thermal efficiency of flat plate solar collector with Cu–water nanofluid at different sizes of Cu nanoparticles (reprint of the publication of He et al. [70] with permission from Elsevier Publisher)

Said et al. used TiO₂–H₂O nanofluid for improving the thermal efficiency of FPSC. The schematic diagram of the experimental setup is shown in Fig. 10. Nanofluids were prepared by using TiO₂ nanoparticles of diameter 20 nm and 40 nm. The mass flow rates were varied from 0.5 to 1.5 kg min⁻¹, and the volume fractions were 0.1% and 0.3%. The sedimentation and thermophysical properties of the nanofluids were improved by using PEG 400 dispersant. Thermal efficiency was enhanced by 76.6% for 0.1 vol% and 0.5 kg min⁻¹ flow rate, whereas the exergy efficiency was improved by 16.9% for 0.1 vol% and 0.5 kg min⁻¹ flow rate. Pumping power and pressure drop of the nanofluid were close to those of base fluid for the studied volume fraction of the nanoparticles.

Fig. 10

Schematic drawing of the test setup of Said et al. (reprint of the publication of Said et al. [64] with permission from Elsevier Publisher)

Michael and Iniyan [71] prepared the copper oxide/water nanofluid from copper acetate and conducted experiments to study the effect of nanofluid on the performance of a 2.184 m² FPSC as shown in Fig. 11. The stability of the CuO nanoparticles was checked with the addition of SDBS and Triton X-100 surfactant, where SDBS showed better stability after 3 days. SDBS was selected with 0.05 vol% concentration. Thermal performance of FPSC was investigated both in forced circulation and in thermosyphon circulation. The flow rate of natural (thermosyphon) circulation was considered 100 L per day (LPD), and the maximum efficiency enhancement was 6.3% as shown in Fig. 12. This enhancement of efficiency could be further improved with the effectiveness of the nanofluid.

Fig. 11

Schematic and experimental diagrams of Michael and Iniyan (reprint of the publication of Michael and Iniyan [71] with permission from the Elsevier Publisher)

Fig. 12

Efficiency of flat plate solar collector against reduced temperature parameter (reprint of the publication of Michael and Iniyan [71] with the permission from the Elsevier Publisher)

Thermal efficiency and performance characteristics of FPSC having area 1.59 m² were investigated experimentally by Meibodi et al. as shown in Fig. 13 [72]. SiO₂/ethylene glycol (EG)–water was selected as nanofluid for the study. Experiments were conducted under ASHRAE standard 86-93 with volume fractions of 0.5%, 0.75% and 1% nanoparticles, and the mass flow rates of the nanofluids were 0.018, 0.032 and 0.045 kg s⁻¹. Although SiO₂ nanoparticles have low thermal conductivity than the other considered nanoparticles, still they showed noticeable enhancement in thermal efficiency when suspended in EG–water as presented in Fig. 14. It was noticed that with the variation of the volume concentrations from 0 to 1%, the enhancement in efficiency of FPSC was varied from 4 to 8%. It was also observed that the efficiencies at the concentrations of 0.75% and 1% were very close to each other, so it was suggested to select 0.75% concentration for its enhanced stability as nanofluid due to low particle loading.

Fig. 13

Schematic drawing of the experimental test setup used by Meibodi et al. (reprint of the publication of Meibodi et al. [72] with the permission from the Elsevier Publisher)

Fig. 14

Efficiency of solar collector in different nanofluid concentrations and mass flow rates (reprint of the publication of Meibodi et al. [72] with permission from the Elsevier Publisher)

By using the same setup of the preceding work, Said et al. [64] investigated the effect of SWCNTs suspended in water on the thermophysical properties of the fluid. To enhance the stability, SDS was used as a dispersant, and the ration used for SDS/SWCNT particles was 1:1 where the nanofluid was found stable up to 30 days at the specified ratio. The concentrations for SWCNTs having 1–3 µm length and 1–2 nm diameter were 0.1 vol% and 0.3 vol%, and their flow rates were maintained at 0.5, 1.0 and 1.5 kg min⁻¹ for the investigation. Thermal conductivity was increased linearly with the enhancement of concentration and temperature, while specific heat and viscosity were increased with the concentration but decreased with the increase in temperature. Energy and exergy efficacies were enhanced by 95.12% and 26.25% as compared to the water data. The low exergy efficiency shows that the used FPSC requires a considerable improvement.

The effect of using 15 nm Al₂O₃/H₂O nanofluid on the exergy efficiency of 1.51 m² FPSC were studied by Shojaeizadeh et al. [73]. The effect of different parameters like inlet temperature, ambient temperature, volume concentration and the mass flow rate on FPSC’s exergy efficiency were investigated and found the optimum values for all these parameters. In that investigation, ASHRAE standard 93-2003 was used. To improve the stability of Al₂O₃/water nanofluid, SDBS was used as the surfactant and 0.090696%, 0.094583%, 0.10293%, 0.11057%, 0.117686%, 0.1244%, 0.13082%, 0.137% and 0.1423% were the volume concentrations of the nanofluids for that study with the flow rate maintained between 0.00727 and 0.01598 kg s⁻¹. By introducing nanofluid in FPSC, the maximum exergy efficiency was increased about 1% and also the corresponding optimum values of inlet temperatures and the mass flow rates were decreased by 2% and 68%, respectively. Exergy efficiency was increased with the increase in solar radiation, and it was maximum at the low concentration of nanofluid.

Vakili et al. [74] investigated experimentally the effect of graphene nanoplatelet nanofluid on FPSC for domestic hot water system (Fig. 15). European Standard EN 12975-2 was used for those experiments. A 60 × 60 cm² collector was used for four different types of nanofluids including base fluid. The mass fraction and mass flow rate for that study were 0.0005, 0.001, 0.005 mass% concentration and 0.0075, 0.015, 0.225 kg s⁻¹, respectively. FPSC efficiency increased with flow rate, and the optimum flow rate was 0.015 kg s⁻¹; the increase in flow rate beyond this caused a decrease in collector efficiency. The zero-loss efficiency for 0.005 mass%, 0.001 mass% and 0.005 mass% was 83.5%, 89.7% and 93.2%, respectively, whereas this zero-loss efficiency for base fluid was 70% as shown in Fig. 16.

Fig. 15

Schematic diagram of Vakili et al. (reprint of the publication of Vakili et al. [74] with the permission from the Elsevier Publisher)

Fig. 16

Effect of mass concentrations on efficiency of flat plate solar collector at flow rate of 0.015 kg s⁻¹ (reprint of the publication of Vakili et al. [74] with the permission from the Elsevier Publisher)

Using the experimental setup shown in Fig. 17, the impact of GNP on the efficiency of FPSC was investigated experimentally and theoretically by Ahmadi et al. [75]. Nanofluids with mass concentrations of 0.01% and 0.02% and GNP having a structural length of less than 100 nm were prepared by a two-step method. Colloidal stability was tested with different pH values to prevent aggregation and sedimentation, and pH value 11.5 was selected for this study. The tests were performed from 9:00 AM to 4:00 PM under ISO 9806 test standard. Efficiency of the collector (0.47 × 0.27 m) was increased 12.19% and 18.87% at 0.01 mass% and 0.02 mass% nanoparticle concentrations, respectively, as shown in Fig. 18. The thermal conductivity of nanofluid with 0.02 mass% also increased 13% as compared to water data. The outlet temperature of the water heater reached 71 °C for 0.02 mass% nanofluid which is appropriate for household use.

Fig. 17

Schematic diagram for experimental setup of Ahmadi et al. (reprint of the publication of Ahmadi [75] with the permission from the Elsevier Publisher)

Fig. 18

Thermal efficiency of flat plate solar collector for water and graphene–water nanofluids at different concentrations (reprint of the publication of Ahmadi et al., [75] with the permission from the Elsevier Publisher)

The effect of SiO₂/H₂O nanofluid on the efficiency of a square (1 m²) FPSC was investigated experimentally by Noghrehabadi et al. [76]. Nanofluid of mass fraction 1% without surfactant is used in this study. Tests were performed under ASHRAE standard 93 with different flow rates between 0.35 and 2.8 L min⁻¹. Pumping power and pressure drop were not considered as high mass fraction concentrations were used for the study. However, collector efficiency was enhanced with the application of nanofluid and increased with the enhancement of flow rate.

Verma et al. [77] used 0.375 m² solar collector for testing the effect of MgO/H₂O nanofluid of 40 nm diameter on the performance of a FPSC as shown in Fig. 19. Nanofluids were synthesized with concentrations of 0.25, 0.5, 0.75, 1.0, 1.25 and 1.5 mass% in the presence of CTAB surfactant, and the flow rates were 0.5, 1.0, 1.5, 2.0 and 2.5 L min⁻¹. MgO/H₂O nanofluid was stable for 50 h in the tank, and after that, sedimentation started. The parameters which were analyzed in the study were the thermal conductivity, energy efficiency, Bejan number, pumping power, entropy generation and reduction in the area of FPSC. Maximum thermal efficiency was observed at 0.75 vol%, and it was 9.34% at the flow rate of 1.5 L min⁻¹ as shown in Fig. 20. At the same concentration and flow rate, energy efficiency was 32.23%. Bejan number reached about 0.97 for the optimum concentration and flow rate. Pumping power loss of 0.75 vol% and 1.5 vol% was 6.84% and 12.84%, respectively, higher than the data for water alone. Economically, by using this nanofluid the surface area of the FPSC was reduced about 12.5% compared to the data for water alone.

Fig. 19

Schematic setup of Verma et al. (reprint of the publication of Verma et al. [77] with the permission from the Elsevier Publisher)

Fig. 20

Effect of different concentrations of MgO/H₂O nanofluid on thermal efficiency against reduced temperature parameter (reprint of the publication of Verma et al. [77] with the permission from the Elsevier Publisher)

Vincely and Natrajan [78] studied the performance of a 2 m² FPSC using graphene oxide (GO)-based nanofluid. No surfactant was used, and it was prepared by ultrasonication of GO nanoparticles in a base fluid with mass concentrations of 0.005, 0.01 and 0.02. Nanofluid was stable more than 60 days with no sedimentation. Thermal efficiency and heat transfer coefficient values were evaluated for nanofluid under laminar condition. A 7.3% improvement in thermal efficiency was noticed for GO nanofluid compared to the base fluid at a mass fraction of 0.02% and the flow rate of 0.0167 kg s⁻¹ as represented in Fig. 21. Maximum heat removal factor for the same flow rate and concentration was noted as 28.3%. Similarly the increase in h values for GO nanofluid at the mass fractions of 0.005, 0.01 and 0.02 was 8.03%, 10.93% and 11.5%, respectively.

Fig. 21

Effect of mass flow rate of GO nanofluid on efficiency (reprint of the publication of Vincely and Natarajan [78] with the permission from the Elsevier Publisher)

Kim et al. [79] studied the efficiency of an U-tube solar collector (Fig. 22) using Al₂O₃/H₂O nanofluid with nanoparticle sizes of 20, 50 and 100 nm. Thermal conductivity behavior of nanofluid was increased with the increase in concentration but decreased with the increase in particle size. At the ambient inlet temperature, the efficiency of the collector was 24.1% higher than the base fluid at the concentration 1.0 vol% and flow rate 0.047 kg s⁻¹ for 20 nm nanoparticles which is 5.6% and 9.7% higher when compared with the data at the volume concentration of 1.5% and 0.5%, respectively, as shown in Fig. 23. For an equal concentration of Al₂O₃/water nanofluid, the highest efficiency for the collector was 72.4% at the nanoparticle size 20 nm, which is 3.05% and 5.32% higher than the data of the nanoparticle of sizes 50 nm and 100 nm, respectively.

Fig. 22

Schematic diagram of test setup Kim et al. (reprint of the publication of Kim et al. [79] with the permission from the Elsevier Publisher)

Fig. 23

Variation of thermal efficiency of solar collector as a function of nanoparticle size (reprint of the publication of Kim et al. [79] with the permission from the Elsevier Publisher)

Experimental study on 0.375 m² FPSC for different nanofluids such as SiO₂–H₂O, TiO₂–H₂O, Al₂O₃–H₂O, CuO–H₂O, graphene/water and MWCNT–water was conducted by Verma et al. [80]. The methodology used in the present study is presented in Fig. 24. The effect of nanofluids on exergy efficiency, entropy generation, Bejan number and thermal efficiency of FPSC were calculated following ASHRAE standard 93-2003. Triton X-100 surfactant, volume fractions of 0.25, 0.50, 0.75, 1.0, 1.5% and the mass flow rates of 0.01 to 0.05 kg s⁻¹ were used for this study. Experiments showed that at volume fraction of 0.75% and flow rate of 0.025 kg s⁻¹ the exergy efficiency of MWCNT/water nanofluid was enhanced by 29.32%, whereas 21.46%, 16.67%, 10.86%, 6.97% and 5.74% enhancements were obtained for graphene/H₂O, CuO/H₂O, Al₂O₃/H₂O, TiO₂/H₂O and SiO₂/H₂O, respectively, as represented in Fig. 25. Maximum drop in entropy generation was observed for MWCNTs which is 65.55% followed by 57.89%, 48.32%, 36.84%, 24.49% and 10.04% for other nanofluids accordingly as mentioned in the previous results above. Thermal efficiency of FPSC was improved by 23.47%, 16.97%, 12.64%, 8.28%, 5.09% and 4.08% for MWCNT/water, graphene/H₂O, CuO/H₂O, Al₂O₃/H₂O, TiO₂/H₂O and SiO₂/H₂O, respectively.

Fig. 24

Experimental methodology of Verma et al. (reprint of the publication of Verma et al. [80] with the permission from the Elsevier Publisher)

Fig. 25

Experimental efficiency of solar collector against reduced temperature (reprint of the publication of Verma et al. [80] with the permission from Elsevier Publisher)

A (0.8 × 0.7 m²) FPSC with metal porous foam-filled channel was used to find thermal efficiency of SiO₂/water nanofluid experimentally (Fig. 26) by Jouybari et al. [81]. Nanoparticles of sizes 7, 20 and 40 nm were used to synthesize nanofluid with volume fractions of 0.2%, 0.4% and 0.6%. Thermal efficiency of nanofluids was examined by ASHRAE standard 93-2003. Using nanofluid in metal porous foam channel, the maximum improvement in thermal efficiency of FPSC was 8.1%. Based on experiments, it was found that the thermal efficiency was improved with the increase in concentration than flow rate. Due to the use of porous media, the pressure drop of nanofluid was increased. For the consideration of pressure drop and heat transfer enhancement, a performance evaluation criterion (PEC) was evaluated for both the nanofluids and the porous media. Results reveal that the PEC value for nanofluid at the concentrations of 0.2–0.6% and at the flow rate of 0.5 L min⁻¹ was enhanced from 1.07 to 1.34. Finally, the effect of nanoparticle size on the performance was investigated by authors, and the results showed (Fig. 27) that the efficiency curve slope parameter reduced with the decrease in nanoparticle size.

Fig. 26

Schematic experimental setup of Jouybari et al. (reprint of the publication of Jouybari et al. [81] with the permission from the Elsevier Publisher)

Fig. 27

Effect of nanoparticle size on flat plate solar collector’s efficiency (reprint of the publication of Jouybari et al. [81] with the permission from the Elsevier Publisher)

Sharafeldin et al. [65] conducted an experimental study using test setup as shown in Fig. 28. They used WO₃/H₂O nanofluid as the working fluid to find out the thermal efficiency of 2.009 × 1.009 m² FPSC. Nanofluids were prepared with the volume fractions of 0.0167%, 0.0333% and 0.0666%, and the mass flux rates were 0.0156, 0.0183 and 0.0195 kg s⁻¹. Stability of nanofluid was checked with zeta potential, where the nanofluids were found stable for 7 days. ASHRAE standard 93 was used for thermal efficiency. Results showed that the thermal performance of collector reached 71.87% for 0.0666 vol% concentration of nanofluid and at 0.0195 kg s⁻¹ mass flux as shown in Fig. 29. Similarly, 13.48% increase in the absorbed energy parameter was noticed for the same concentration and flow rate.

Fig. 28

Schematic test setup of Sharafeldin et al. (reprint of the publication of Sharafeldin et al. [65] with the permission from the Elsevier Publisher)

Fig. 29

Efficiency of solar collector at highest flow rate of 0.0195 kg s⁻¹. (reprint of the publication of Sharafeldin et al. [65] with the permission from the Elsevier Publisher)

The efficiency of FPSC and U-tube collector using Al₂O₃/H₂O nanofluid was investigated experimentally by Kang et al. [82]. In this study energy savings, CO₂ and SO₂ generated were calculated and compared with water data. Based on the experimental results, it was noted that the performance of collectors improved with the enhancement of the concentration of Al₂O₃/water nanofluid. Three concentrations were used in this study with the volume percentages of 0.5, 1.0 and 1.5%. The maximum efficiency of FPSC and U-tube collector for 20 nm particle size of Al₂O₃/H₂O nanofluid was 74.9% and 72.4%, respectively, at the volume fraction of 1.0% and the flow rate of 0.047 kg s as shown in Fig. 30. These improvements were 14.8% and 10.7% higher than those of water data for FPSC and U-tube collector, respectively. The coal, carbon dioxide and sulfur dioxide generated were 189.99, 556.69 and 2.03 kg, respectively, less than those of using water. The electricity and its cost reduced by using nanofluid were 1546.56 kWh and 540.4 US dollar, respectively, for Germany.

Fig. 30

Thermal efficiency of flat plate solar collector (adapted from the publication of Kang et al. [82] with the permission from the publisher Elsevier)

Stalin et al. [83] investigated the efficiency of the FPSC theoretically and experimentally by using CeO₂/H₂O nanofluid. The FPSC area of 2 m² and 100 L per day capacity were fabricated for experimental study. Nanofluid with average particle size of 25 nm, volume concentration of 0.01% and the flow rate from 1 to 3 L min⁻¹ was considered to carry out experiments as per ASHRAE standard 93. The efficiency improvement in FPSC by using CeO₂/H₂O nanofluid was 78.2% at the flow rate of 2 L/min⁻¹ which is 21.5% higher than that of water data used at the same flow rate. The same enhancement in efficiency of FPSC was observed theoretically with an error of ± 7.5%. It was noticed that by increasing the flow rate between 2 and 3 lpm, the collector efficiency was reduced 4.4% due to thermal properties of the nanofluid. Thus, based on the theoretical and experimental results it was observed that using CeO₂/water nanofluid the collector area can be reduced 25.2% as compared to water used as heat transfer liquid.

Sunder et al. [84] fabricated a FPSC test setup of area 2 m² (Figs. 31, 32) and observed that the thermal performance of FPSC can enhanced by passive heat transfer method and the most effective passive method is to improve the thermal conductivity of working fluid and its flow rate. Sunder et al. used Al₂O₃/H₂O nanofluid with particle size less than 20 nm and volume concentrations of 0.1% and 0.3% and SDBS as surfactant, and observed that the nanofluid was stable for 6 months. Four different flow rates 0.033, 0.05, 0.066 and 0.083 kg s⁻¹ were considered with twisted tape inserts of twist ratios 5, 10 and 15. The collector’s efficiency was increased with the increase in mass flow rate and volume concentrations of nanoparticles. ASHRAE standard 93-86 was used for the experiments. Results showed that the heat transfer of collector enhanced for nanofluids at volume concentrations of 0.1 and 0.3% at 0.083 kg s⁻¹ flow rate was 9.4% and 22%, respectively, compared to water data. The heat transfer was further increased 37.73% and 52.80% for the volume concentrations of 0.1 and 0.3%, respectively, for the collector with twisted tape H/D = 5 as compared to collector without twisted tape. The maximum friction loss was observed 1.25 times for 0.3 vol% nanofluid and twisted tape ratio H/D = 5 as compared to water data. Thermal effectiveness was 58% for the plane collector and it was increased to 76% with the use of twisted tape ratio H/D = 5.

Fig. 31

Schematic test setup of Sunder et al. (reprint of the publication of Sundar et al. [84] with the permission from the Elsevier Publisher)

Fig. 32

a Photograph of a twisted tape inserts, b FPSC with twisted tape inserts inside the tubes (reprint of the publication of Sundar al. [84] with the permission from the Elsevier Publisher)

Sharafeldin and Gróf [85] conducted an experimental study to determine the efficiency curves of FPSC with the use of CeO₂/water nanofluid. Stability of nanofluid is very low. In this case, three volume fractions of 0.0167, 0.0333 and 0.0666% with three mass flow rates of 0.015, 0.018 and 0.019 kg s⁻¹ were used to find the efficiency of FPSC of 2.027 m² area. ASHRAE standard 93-2003 was followed for experiments. Based on the experimental results, it was found that the maximum efficiency of collector against reduced temperature parameter was equal to 10.74% at the nanofluid with volume fraction of 0.066% and the mass flow rate of 0.019 kg s⁻¹. The change in absorbed energy parameter was between 3.51 and 10.74%, and the recorded removal energy parameter was between 30.61 and 191.8%. The heat removal factor as a function of mass flow rate is represented in Fig. 33.

Fig. 33

Values of heat removal factor as a function of mass flux rate (reprint of the publication of Sharafeldin and Gróf [85] with the permission from the Elsevier Publisher)

Farajzadeh et al. [86] studied numerically and experimentally the thermal performance of FPSC (1.85 m²) using Al₂O₃–water (20 nm 0.1%), TiO₂–water (15 nm 0.1%) and their mixture with equal concentration ratio. The nanofluids were prepared in a two-step method using CTAB as a surfactant. ASHRAE standard 93 and open-source computational fluid dynamics (CFD) software were used for experimental and numerical investigations. Different volume flow rates of 1.5, 2.0 and 2.5 L min⁻¹ were considered. Based on the experimental results, an enhancement of the thermal efficiencies of Al₂O₃–water, TiO₂–water and their mixture at 0.1 mass% was observed as 19%, 21% and 26%, respectively, compared to the data of the standard working fluid (water). The thermal efficiency of the mixture was increased as compared to the single nanofluid as shown in Fig. 34. With further increase in the concentration of the mixture from 0.1 to 0.2 mass%, the thermal efficiency was enhanced 5%. Since TiO₂ nanoparticles are expensive than Al₂O₃ nanoparticles, using their mixture is more economical with better efficiency.

Fig. 34

Efficiency of solar collector with individual and mixture forms of nanofluids (reprint of the publication of Farajzadeh et al. [86] with the permission from the Publisher Elsevier)

Mirzaei et al. [87] investigated the effect of Al₂O₃–water (20 nm 0.1 vol%) nanofluid on the thermal efficiency of 1.51 m² FPSC at different flow rates of 1, 2 and 4 L min⁻¹. ASHRAE standard 86-93 were considered for this study. This study was conducted to find the optimum operation condition for Al₂O₃–water and the standard working fluid. Results reveal that adding nanoparticles in base fluid enhanced the thermal efficiency of FPSC. Thermal efficiency also increased with the increase in flow rate and there is an optimum flow rate at which efficiency was maximum. Optimum flow rate for Al₂O₃–water nanofluid in this study was 2 L min⁻¹ at which the thermal efficiency was 23.6% as compared to water data as represented in Fig. 35. There is no information recorded in this case about the stability of the used nanofluid.

Fig. 35

Efficiency of nanofluid at different flow rates (reprint of the publication of Mirzaei et al. [87] with the permission from the Publisher Elsevier)

Akram et al. [88] investigated the thermal efficiency of FPSC experimentally. GNP was covalently functionalized using one-pot technique. Zeta potential reflects that nanofluids were stable for 45 days after preparation and no sedimentations were counted. Three mass flow rates of 0.0133, 0.0200 and 0.0260 kg s⁻¹ and three concentrations of 0.025, 0.075 and 0.1 mass % were used in this experimental work. The highest thermal efficiency of FPSC was 78% at 0.1 mass% and 0.0260 kg s⁻¹ which is 18.2% higher than the base fluid as represented in Fig. 36.

Fig. 36

Thermal efficiency of flat plate solar collector at 0.0260 kg s⁻¹ (reprint of the publication of Akram et al. [88] with the permission from the Springer nature Publisher)

Theoretical studies on FPSCs using nanofluids

The effect of using Al₂O₃, TiO₂, CuO and SiO₂ nanoparticles dispersed in water on flat plate solar collector was theoretically analyzed by Alim et al. [89]. In that study, the main parameters analyzed were exergy destruction, entropy generation, pressure drop and heat transfer enhancement. Exergy destruction and entropy generation rate were recorded as the functions of nanoparticle volume concentration (1–4%) and flow rates (1–4 L min⁻¹) as presented in Fig. 37. It can be realized that the entropy generation drops with the rises in volume fraction and flow rate. This happened because with the growth of the heat flux on the absorber plate, the irreversibility turned out as the governing effect. Based on the results, it was concluded that the heat transfer feature improved with the increase in volume fraction of the nanoparticles. The evaluated friction factor of metal oxide nanofluids was close to that of the base fluid (water). Among all these nanoparticles, the CuO nanofluid reduced the entropy generation by 4.34% and improved the heat transfer coefficient by 22.15% theoretically. Due to high volume concentration of nanoparticles, about 1.58% penalty in pumping power was noticed.

Fig. 37

Effect of nanofluids on entropy generation and exergy destruction at different a volume concentrations and b flow rates (reprint of the publication of Alim et al. [89] with the permission from the Elsevier Publisher)

Faizal et al. [90] analyzed the effect of different concentrations (0.2%, 0.4%) of MWCNT/water nanofluid on the reduction in the size of the FPSC. The analysis was based on Yousefi et al. [66] and Foster et al. [5] data. Different flow rates, mass fractions and surfactants in nanofluid were considered in this study. However, only a single equation (Eq. 42) was used to analyze the decrement in the size of FPSC, where no methodology was presented clearly. Analysis showed that 37% decrement in the size of the FPSC was possible when MWCNT/H₂O nanofluid was used as compared to water data.

$$A_{\text{c}} = \frac{{\dot{m}C_{\text{p}} \left( {T_{\text{o}} - T_{\text{i}} } \right)}}{{\eta_{\text{c}} G_{\text{T}} }}$$

(42)

Furthermore, Faizal et al. [91] analyzed the performance of the collector and obtained the possible reduction in size, cost and embodied energy by utilizing Al₂O₃, TiO₂, CuO and SiO₂ nanoparticles dispersed in the base fluid for synthesizing nanofluid. The flow rates between 1 and 3.8 L min⁻¹ and the volume fraction of 3% were considered. Based on the calculations, it was observed that the thermal efficiency of the collector was enhanced by 38.5% using CuO, while it was 28.8% for other metal oxide nanoparticles as compared to water for the same concentration. Reduction in areas of collector was calculated as 21.5, 21.6, 22.1 and 25.6%, by using nanofluids of Al₂O₃, SiO₂, TiO₂ and CuO, respectively, as it is shown in Fig. 38. The Estemated reduction in masses of 1000 units are 8618, 8625, 8857 and 10,239 kg for Al₂O₃, SiO₂, TiO₂ and CuO, respectively. The average values for the embodied energy and CO₂ were predicted 220 MJ and 170 kg, respectively. However, volume concentration used in this study was higher than that of the previous study conducted by Faizal et al. [90] where MWCNT/water with low concentration provided higher efficiency and a notable reduction in size.

Fig. 38

Percentage of size reduction of flat plate solar collector by using different nanofluids (reprint of the publication of Faizal et al. [91] with the permission from the Elsevier Publisher)

By the employed second law of thermodynamics, the effect of SWCNT, SiO₂, TiO₂ and Al₂O₃ nanofluids on the performance of a 1.51 m² FPSC was analyzed by Said et al. [92] and found similar pattern of performance enhancement. Entropy generation analysis is important for the operation of a system at higher temperature. The power output of a system can be increased by minimization of entropy generation, and the entropy generation of SWCNT/water nanofluid was minimum compared to that of oxide-based nanofluid. Therefore, exergy output value of the SWCNT/water was higher than that of the oxide-based nanofluid in the referred investigation. The exergy destruction with respect to different flow rates and concentrations are presented in Fig. 39. In both the cases, the exergy destruction was lower for SWCNT/water nanofluid compared to those of other nanofluids. Furthermore, heat transfer coefficient, pressure drop and pumping power of nanofluids in FPSC were numerically investigated. SWCNT/water was selected as the best nanofluid than the metal oxide nanofluids. Results revealed that SWCNT/water reduced entropy generation by 4.34% and the enhanced heat transfer coefficient was 15.33% when compared with the water data obtained theoretically. The effect of pumping power and pressure drop was considered negligible as the pumping power penalty of using SWCNT/water in FPSC was found to be 1.20%.

Fig. 39

Exergy destruction as a function of a volume fraction, b volume flow rate (reprint of the publication of Said et al. [92] with the permission from the Elsevier Publisher)

Mahian et al. [93] analytically analyzed the performance of a mini-channel-based FPSC. They used four different nanofluids including Al₂O₃/H₂O, TiO₂/H₂O, Cu/H₂O and SiO₂/H₂O with particle size 25 nm. The analysis was based on the first and second laws of thermodynamics for turbulent flow with volume concentration of 4% and mass flow rate from 0.1 to 0.5 kg s⁻¹. According to the first law of thermodynamics, the results reveal that Al₂O₃/H₂O nanofluid showed the highest heat transfer coefficient value and the minimum value were obtained for SiO₂/H₂O. Entropy generation rate for all the nanofluids used in this study is presented in Fig. 40, and it was clear from the investigation that nanofluids instead of water lead to a reduction in entropy generation rate. As volume concentration of nanofluids was increased, the entropy generation reduced. The analysis of the second law of thermodynamics revealed that the Cu/H₂O nanofluid produced the lowest entropy generation, and it was also noticed that as TiO₂/H₂O nanofluid had less thermal conductivity than Al₂O₃/H₂O, the entropy generation of TiO₂/H₂O was lower than that of Al₂O₃/H₂O. Pressure drop decreased with the increase in volume fraction except SiO₂/H₂O nanofluid as it had low density than other nanofluids.

Fig. 40

Variation of entropy generation rate for different nanofluids with different volume fractions: am = 0.1 kg s⁻¹; bm = 0.5 kg s⁻¹ (reprint of the publication of Mahian et al. [93] with the permission from the Elsevier Publisher)

Mahian et al. [47] conducted an analytical study to examine the effect of SiO₂/H₂O nanofluid on FPSC. SiO₂/water nanofluid with pH values of 5.8 and 6.5, and particle sizes of 12 and 16 nm with volume concentration of 1% were used to analyze the pressure drop, heat transfer coefficient and entropy generation in a FPSC. Results showed that the highest heat transfer coefficient and collector efficiency were obtained from Brinkman theoretical model instead of experimental value as represented in Fig. 41. It was also noticed that at 16 nm particle size, the increase in pH value caused an increase in entropy generation, and at 12 nm particle size, the increase in pH value had decreased the entropy generation.

Fig. 41

Thermal efficiency of solar collector for different cases (adapted from the publication of Mahian O et al. [47] with the permission from ASME)

Shojaeizadeh and Veysi [94] conducted a study dealing with exergy efficiency optimization for Al₂O₃/H₂O nanofluid in FPSC using mathematical optimization (SQP) method. This study accounts for exergy efficiency optimization for two uncontrollable parameters ambient temperature and solar radiation. Furthermore, two cases were considered for this study which were open and closed loop. In open loop, fluid temperature at the inlet of solar collector was independent of storage tank, while in closed loop the storage tank was considered. Both cases were operated for the base fluid and Al₂O₃/H₂O nanofluid. The results of this study revealed that the optimum exergy efficiency for the collector inlet temperature, nanoparticle volume concentration and mass flow rates decreased exponentially with the increase in $T_{\text{a}} /G_{\text{t}}$ values.

Using the test setup from their previous work, Said et al. (2016a, 2016b) investigated the energy and exergy analysis of Al₂O₃/H₂O nanofluid, pH-treated nanofluid (Al₂O₃, 13 nm) [95] and the varied diameters of (13 nm, 20 nm) nanoparticles [96] for FPSC. The volume fractions of 0.1% and 0.3% and the mass flow rates of 0.5 to 1.5 kg min⁻¹ were used for the investigation. Nanofluids were stable for more than 30 days. ASHRAE standard 93-2003 was used for the experiments. For pH-treated Al₂O₃/water nanofluid, the energy efficiency of FPSC was enhanced by 83.5% at 0.3 vol% and 1.5 kg min⁻¹ flow rate. Exergy efficiency was improved by 20.3% at 0.1 vol% and 1 kg min⁻¹ flow rate. It could be noted that the thermal efficiency was 50% higher compared to the available data from the literature for the same nanofluid. For the different diameter sizes (13, 20 nm) of nanoparticles, the energy efficiency for 13 nm nanoparticles was higher than that of 20 nm nanoparticles as shown in Fig. 42. Energy efficiency enhanced for 13 nm particles was 73.7% at 0.1 vol% and 1.5 kg min⁻¹.

Fig. 42

Variation in energy and exergy efficiencies against testing period (reprint of the publication of Said et al. [96] with the permission from the Elsevier Publisher)

Hajabdollahi and Premnath [97] performed thermoeconomic modeling for FPSC using Al₂O₃/water nanofluid. They used particle swarm algorithm to carry out optimization of FPSC’s total annual cost (TAC), and efficiency at different design parameters like mass flow rate, number of the tube, collector length, collector width, insulation thickness and particle volume concentration was considered for optimization. Al₂O₃/water nanofluid gave higher collector efficiency at a low flow rate. Based on analysis, it was observed that all the design parameters except the number of the tube should be at a lower magnitude for Al₂O₃/water nanofluid-based FPSC. The number of the tubes between 5 and 8 with the diameter less than 10 mm was considered best for obtaining higher efficiency of the collectors. Results showed that using of Al₂O₃/water nanofluid, the total annual cost can be reduced by 3.5% along with the increase in efficiency by 2%.

In a numerical study, Moghadam et al. [98] examined the three-dimensional aluminum/water nanofluid-based FPSC at 30° inclination angle by using ANSYS Fluent software. Al₂O₃/water nanofluid with various volume concentrations 0–4% and particles of diameter 25–100 nm was considered. The coefficient of heat transfer from convection to conduction increased with the increase in Reynolds number and decreased with the increase in Richardson number and particle volume fraction. Results showed that heat transfer coefficient increased between 45 and 58% when nanofluid was introduced. Simulation also showed that entropy generation promptly rises with the increases in Reynolds number and decreased with the increase in Richardson number and nanofluid concentration. Pressure drop was considerable when Richardson number was increased, at the particle size of 25 nm. The pressure drop value was lowered even the Richardson number was considered constant. Compared to the previous literature, the efficiency of FPSC has improved by 2% in that study.

Hawwash et al. [99] conducted numerically and experimentally the research on the performance of FPSC using alumina nanofluid. Alumina nanofluid with the surfactant Triton X-100 in the range of 0.1–3 vol% was used for the study. ASHRAE standard and ANSYS 17 were used for the experimental and numerical investigations, respectively. Aluminum/water nanofluid enhanced FPSC’s efficiency by 3–18% at the low to high temperature differences. Pressure drop was increased by 28 Pa with the increase in volume concentrations from 0.1 to 3%. The FPSC efficiency was also affected with the flow rate, and 5.5 L min⁻¹ flow rate was considered the best. The thermal efficiency of FPSC increased with the volume concentration and it increased up to 0.5%, and after that, further increase in concentration caused a negative effect on the performance (Table 1).

Table 1 Previous studies on the use of nanofluids in FPSCs

Challenges and difficulties in using nanofluids

Using nanofluid in flat plate solar collector faces many difficulties and challenges. The following challenges and difficulties are observed during the present survey.

• 1. The major drawback of using nanoparticle is their high cost of procurement and/or manufacturing cost.

• 2. The stability of nanoparticles is its major problem; it has the characteristics to agglomerate over a period of time.

• 3. An increase in viscosity and pressure drop is observed due to the addition of NPs in the base fluid which results a higher pumping power.

• 4. Preparation of nanofluid is a complex, time-consuming and noneconomic process.

• 5. The nanofluids are corrosive and toxic in nature, and inhalation of NPs may cause severe respiratory disorders.

• 6. The payback period is higher due to a higher operating cost.

• 7. It is unfavorable to add surfactants at higher temperatures.

• 8. Long-term usage of nanofluids in the solar collectors is not feasible since it results in erosion of walls.

Conclusions

This review paper focuses on the latest development in solar energy harvesting technology, namely flat plate solar collector (FPSCs). Nanofluids are one of the advanced types of working fluids which are synthesized by colloidal dispersion of nanoparticles in the base fluids (ethylene glycol, engine oil, water). The use of nanofluid as absorber fluid in FPSC has been studied since last two decades. Nanofluids showed promising enhancement in thermal efficiency of FPSCs due to its abnormal enhancement in thermal conductivity. From the previous investigations, it is observed that:

• 1. DI water was used as base fluid for the synthesis of the nanofluids in most of the previous research reported in the literature. However, few studies were reported on nanofluids with the base fluid of ethylene glycol and engine oil for the varied nanoparticle concentration, viscosity, temperature and thermal conductivity to design the thermal systems.

• 2. Optimum values of particle size, surfactants and pH values have a positive impact on the thermal efficiency of FPSCs, and further increase or decrease from optimum values has a negative effect.

• 3. For the proper dispersion of nanoparticles and long-term stability, surfactants were used, but those surfactants had negative effects on the thermophysical properties of the base fluids and nanofluids.

• 4. Most of the researchers had focused on using metal- and metal oxide-based nanofluids for experimental investigations on FPSCs. Only very few researchers reported the results with carbon nanotubes (CNTs) and graphene (GNP).

• 5. No preceding experimental investigations addressed the effect of using hybrid nanofluids on thermal performance of FPSCs. Hybrid nanofluids are a combination of two or more nanoparticles having better thermophysical properties with low cost.

• 6. At higher temperature, the higher efficiency of flat plate solar collector was reported as compared to water data.

• 7. Nanofluid was considered as a single-phase fluid for numerical simulation to predict thermal conductivity and the effects of the other different parameters. Two-phase mixture models are needed to be done more for nanofluid (two-phase fluid)-based solar collectors.

Future scope of work

• 1. Investigation is needed for the synthesis of covalently functionalized nanoparticles for the better stability and thermal performance of their nanofluids.

• 2. As CNTs and GNPs exhibit higher specific surface area, high thermal conductivity and good mechanical strength, further studies should be carried out to explore them for intensive future application.

• 3. Experiments can be performed with various types of hybrid nanofluids.

• 4. Experiments can be performed for different absorber plate materials to investigate the effects of nanofluids on different materials.

• 5. The experiments can be performed for the various types of solar collectors.