Comparative Analysis of Thermodynamic Performances of a Linear Fresnel Reﬂector Photovoltaic/Thermal System Using Ag/Water and Ag-CoSO 4 /Water Nano-Fluid Spectrum Filters

: Renewable energy represented by solar energy has played an important role in the transformation of the energy consumption structure of the world. This study describes and studies a hybrid photovoltaic/thermal (HPVT) system using a linear Fresnel reﬂector concentrator (LFRC) and nano-ﬂuid spectrum ﬁlter (NFSF). The method of designing the HPVT system is provided. Ag/water and Ag-CoSO 4 /water NFSFs used for this HPVT system were prepared and experimentally tested. Optical analysing results indicate that the optical efﬁciencies of the PV cell module (PVCM) and NFSF are 0.7556 and 0.9053 under the condition of 0.3 ◦ solar tracking error (STE), demonstrating good adaptable capacity to the STE. Moreover, the operating performances of the HPVT system using different NFSFs are compared. The comparison results demonstrate that compared with the Ag/water NFSF, the introduction of CoSO 4 can enhance the thermal performance but decrease the photovoltaic efﬁciency of the HPVT system. When the Ag-CoSO 4 /water (1.2 mg–3 g/130 mL) NFSF is used, the photovoltaic and thermal efﬁciencies of the whole HPVT system are 0.1366 and 0.4259, and the overall exergy efﬁciency is 0.209. The exergy efﬁciency of the HPVT system will be improved if the NFSF temperature is increased appropriately or if the external convection heat transfer coefﬁcient and environment temperature are reduced.


Introduction
Since entering the 21st century, human society has been facing serious resource shortages and environmental pollution problems. The proposal of the carbon neutrality goal is beneficial for the development of human society as well as that of the natural environment. In order to realise the objective of carbon neutrality, the transformation of energy consumption structure is undoubtedly imperative. Renewable energy [1,2], represented by solar energy [3,4], has played an important role in the transformation of energy consumption structure. At present, the main solar energy application methods are photovoltaic (e.g., photovoltaic power [5,6], photovoltaic hydrogen production [7], hybrid photo-thermal and photo-electric utilisation [8]) and photothermal utilisations [9,10]. And, solar concentrators are usually needed for many solar systems to improve the incident radiation intensity, for instance, parabolic trough [11,12], dish [13], annular Fresnel [14] and multi-mirror [15].
Single-crystal silicon cells are currently the most mature commercial photovoltaic cells, whose performances are spectral-dependent [16]. To address this issue, researchers proposed the concept of a hybrid photovoltaic/thermal (HPVT) system [17]. Generally, there are two beam splitting approaches for HPVT systems. One is the coating beam splitter (CBS) [18,19], and the other is the nano-fluid spectrum filter (NFSF) [20,21]. When the HPVT system utilises sunlight concentrating and spectrum splitting synchronously, its energy efficiency will increase. Currently, solar HPVT technology has been considered to be an effective approach to increase overall solar utilisation efficiency.
Lots of studies about CBS or NFSF photovoltaic/thermal systems have been carried out. Jiang et al. [11] integrated the parabolic trough and CBS in an HPVT system and studied the optical behaviour of the system. The advantages of the optical behaviour of the HPVT system were revealed. Crisostomo et al. [20] built a test device and launched experimental works on an HPVT system utilising NFSF. Han et al. [21] simulated the working process of an HPVT system using Ag-CoSO 4 /water NFSF to estimate the technical feasibility of the HPVT system. Otanicar et al. [22] compared the behaviours of two HPVT systems using CBS and NFSF, respectively. The analysis results showed that the CBS can bring higher efficiency to the HPVT system. A research review on NFSF for different kinds of HPVT systems was carried out by Goel et al. [23]. A lot of typical NFSF-related works can be seen in that review article. An et al. [24] estimated an HPVT system using Cu 9 S 5 NFSF via an experimental method. The results indicate that the overall efficiency increased by about 18% in contrast with the system without NFSF. Hjerrild et al. [25] employed water-based Ag-SiO 2 /carbon NFSF to enhance the operation performance of an HPVT system. The research results show that the HPVT system had a 30% increase in overall efficiency compared with that only using water. Parsa et al. [26] studied the effects of the preparation methods of NFSF on the operation performance of an HPVT system based on fluid spectrum splitting. Ag NFSFs with different concentrations were prepared using the one step and two step methods, respectively. The results show that the Ag NFSF prepared using the one step method had better stability. In contrast with the two step method, when the Ag NFSF prepared using the one step method was utilised, the HPVT system could have larger energy and exergy outputs. Zhang et al. [27,28] proposed an HPVT system using Ag NFSF for the building integrated with photovoltaics. The HPVT system utilised Ag NFSF to regulate the operating modes to adapt to the changes in the external environment. The relevant theoretical analysis and experimental results revealed relatively good agreement. In addition, the feasibility of an HPVT system used in home heating was also investigated. Abdelrazik et al. [29] carried out an experimental study on the effect of Ag/water NFSF on the operation performance of an HPVT system. The impacts of optical path length and concentration of Ag nano-particles on the optical behaviour of Ag/water NFSF were studied and the relevant correlation was obtained through experiments. More HPVT system studies can be found in the relevant references [30,31].
In contrast with previous studies, the main novelty of this paper is the proposal of a novel HPVT system using a linear Fresnel reflector concentrator (LFRC) and NFSF simultaneously. Ag/water and Ag-CoSO 4 /water NFSFs used for this HPVT system were prepared and experimentally tested. The performances of the HPVT system using the two NFSFs will be evaluated and compared. The HPVT system design will be provided in Section 2, and the preparation and tests of Ag/water and Ag-CoSO 4 /water NFSFs will be provided in Section 3. Sections 4 and 5 will show the optical analysis and thermodynamic evaluation results, respectively. And, Section 6 highlights the conclusions and summary of this paper. The current study aims to extend the research contents of solar HPVT technologies, and the results presented in this paper will have certain reference value for further research works on solar HPVT systems in the future.

HPVT System
This section presents the design of the proposed HPVT system. Figure 1 shows the diagrammatic sketch of the HPVT system using LFRC and NFSF, which consists of an LFRC field, photovoltaic cell module (PVCM), NFSF and some bracing structures. The explanations of the width and length directions of the HPVT system are also presented in Figure 1 (see the black dash arrow lines and corresponding explanatory texts). As presented in Figure 1, the NFSF is arranged below the PVCM through bracing structures. The NFSF is a nano-fluid flowing channel, which serves as the spectrum splitting and heat utilisation devices simultaneously. It can be seen that the incident light will be reflected to the NFSF firstly via the LFRC. After that, part of the light is absorbed by the NFSF, and the rest passes through the NFSF and arrives at the PVCM. The two parts of light obtained via the NFSF and PVCM will be utilised for thermal application and generating power, respectively. utilisation devices simultaneously. It can be seen that the incident light will be reflected to the NFSF firstly via the LFRC. After that, part of the light is absorbed by the NFSF, and the rest passes through the NFSF and arrives at the PVCM. The two parts of light obtained via the NFSF and PVCM will be utilised for thermal application and generating power, respectively.  Figure 2 shows the structural design diagrammatic sketch of the LFRC. As presented in Figure 2, w is the width of the NFSF with the installation height of f. The LFRC is composed of many plane mirrors. Taking the right half part of the LFRC as the example, for the ith plane mirror, the dip angle and width are recorded as βi and Di. γi represents the angle between the reversed prolongation of the reflected light at the ith mirror end and the ith mirror line. Si represents the distance from the left end of the ith mirror to the Xaxis origin.
For the first mirror, the relevant formulae are as follows:   Figure 2 shows the structural design diagrammatic sketch of the LFRC. As presented in Figure 2, w is the width of the NFSF with the installation height of f. The LFRC is composed of many plane mirrors. Taking the right half part of the LFRC as the example, for the ith plane mirror, the dip angle and width are recorded as β i and D i . γ i represents the angle between the reversed prolongation of the reflected light at the ith mirror end and the ith mirror line. S i represents the distance from the left end of the ith mirror to the X-axis origin. The equations for the ith mirror are as follows:  For the first mirror, the relevant formulae are as follows: The equations for the ith mirror are as follows: Then, the geometric concentrating ratio of the LFRC is as follows: Based on Equations (1)-(10), the parameters of the HPVT system using LFRC and NFSF are calculated and the results can be seen in Table 1.

Preparation and Tests of NFSFs
The NFSFs used in this HPVT system are Ag/water nano-fluid and Ag-CoSO 4 /water nano-fluid. The main reason for choosing CoSO 4 is that CoSO 4 has better light propagation characteristics compared with other cobalt salts [32]. Silver nano-particles are synthesised by reducing AgNO 3 with sodium citrate. The preparation process of Ag nano-particles is briefly presented in Figure 3a, and the reactor for the preparation of Ag nano-particles is shown in Figure 3b. After the reactions, the light green solution should be repeatedly cleaned three times with a high-speed centrifuge, and Ag nano-particles can be obtained after drying. In Figure 3a, M in "0.0588 M" stands for the unit of mol/L. nano-fluid. The main reason for choosing CoSO4 is that CoSO4 has better light propagation characteristics compared with other cobalt salts [32]. Silver nano-particles are synthesised by reducing AgNO3 with sodium citrate. The preparation process of Ag nano-particles is briefly presented in Figure 3a, and the reactor for the preparation of Ag nano-particles is shown in Figure 3b. After the reactions, the light green solution should be repeatedly cleaned three times with a high-speed centrifuge, and Ag nano-particles can be obtained after drying. In Figure 3a, M in "0.0588 M" stands for the unit of mol/L.  The results of weighing the prepared Ag nano-particles show that the nano-particle mass obtained in a single experiment is about 0.030 g. A part of the Ag nano-particles is taken out to be dissolved in ethanol for the TEM tests. Figure 4 illustrates the TEM photo of the Ag nano-particles. The test results show that the prepared nano-particles are quasicircular and have a uniform particle size distribution. The formation of nano-crystals can be divided into two kinds of processes, which are the nucleation and crystal growth, The results of weighing the prepared Ag nano-particles show that the nano-particle mass obtained in a single experiment is about 0.030 g. A part of the Ag nano-particles is taken out to be dissolved in ethanol for the TEM tests. Figure 4 illustrates the TEM photo of the Ag nano-particles. The test results show that the prepared nano-particles are quasi-circular and have a uniform particle size distribution. The formation of nano-crystals can be divided into two kinds of processes, which are the nucleation and crystal growth, respectively. During the particle growth stage, the crystal structure will evolve in different directions due to different reaction temperatures and environments. Generally speaking, in a low-temperature environment, it is beneficial for the adsorption of coated molecules on nano-particles, thus limiting the growth rate of particles. However, in unsaturated solutions, the crystal dissolution is inevitable. As the dissolution progresses, the dissolution rate will gradually decrease or even completely stop. Therefore, the remaining undissolved crystals can stably exist in the unsaturated solutions. A size test of the Ag nano-particles was conducted and the results can be seen in Figure 5. According to Figure 5, the average particle size of the prepared Ag nano-particles is about 70 nm. in a low-temperature environment, it is beneficial for the adsorption of coated molecules on nano-particles, thus limiting the growth rate of particles. However, in unsaturated solutions, the crystal dissolution is inevitable. As the dissolution progresses, the dissolution rate will gradually decrease or even completely stop. Therefore, the remaining undissolved crystals can stably exist in the unsaturated solutions. A size test of the Ag nanoparticles was conducted and the results can be seen in Figure 5. According to Figure 5, the average particle size of the prepared Ag nano-particles is about 70 nm.  Ag/water NFSFs prepared in the current study included 1.2 mg/50 mL, 1.2 mg/70 mL, 1.2 mg/90 mL, 1.2 mg/110 mL and 1.2 mg/130 mL. For the Ag-CoSO4/water NFSF, CoSO4 of different masses was added in the Ag/water 1.2 mg/130 mL nano-fluid. Ag/water NFSFs with different concentrations were prepared by mixing Ag nano-particles with water using an ultrasonic cleaning machine. After the Ag/water NFSFs had been prepared, the Ag-CoSO4 NFSF was obtained through stirring the Ag/water and CoSO4 crystal in the directions due to different reaction temperatures and environments. Generally speaking, in a low-temperature environment, it is beneficial for the adsorption of coated molecules on nano-particles, thus limiting the growth rate of particles. However, in unsaturated solutions, the crystal dissolution is inevitable. As the dissolution progresses, the dissolution rate will gradually decrease or even completely stop. Therefore, the remaining undissolved crystals can stably exist in the unsaturated solutions. A size test of the Ag nanoparticles was conducted and the results can be seen in Figure 5. According to Figure 5, the average particle size of the prepared Ag nano-particles is about 70 nm.  Ag/water NFSFs prepared in the current study included 1.2 mg/50 mL, 1.2 mg/70 mL, 1.2 mg/90 mL, 1.2 mg/110 mL and 1.2 mg/130 mL. For the Ag-CoSO4/water NFSF, CoSO4 of different masses was added in the Ag/water 1.2 mg/130 mL nano-fluid. Ag/water NFSFs with different concentrations were prepared by mixing Ag nano-particles with water using an ultrasonic cleaning machine. After the Ag/water NFSFs had been prepared, the Ag-CoSO4 NFSF was obtained through stirring the Ag/water and CoSO4 crystal in the Ag/water NFSFs prepared in the current study included 1.2 mg/50 mL, 1.2 mg/70 mL, 1.2 mg/90 mL, 1.2 mg/110 mL and 1.2 mg/130 mL. For the Ag-CoSO 4 /water NFSF, CoSO 4 of different masses was added in the Ag/water 1.2 mg/130 mL nano-fluid. Ag/water NFSFs with different concentrations were prepared by mixing Ag nano-particles with water using an ultrasonic cleaning machine. After the Ag/water NFSFs had been prepared, the Ag-CoSO 4 NFSF was obtained through stirring the Ag/water and CoSO 4 crystal in the container using a magnetic stirrer. Using spectrometers, the optical properties of different NFSFs were experimentally tested and the results are shown in Figure 6. The specifications of the spectrometers used in the tests were NIRQuest and QE Pro. The experimental results in Figure 6 were directly derived from the experimental data. It can be seen from the relevant experimental results that when Ag nano-particles are added in water, a clear absorption peak appears at the wavelength of 435 nm. CoSO 4 has the ability to further regulate the absorption range of Ag/water NFSF. It cannot only regulate the absorption characteristics of Ag/water NFSF, but can also enhance the absorption in the infrared band of the Ag/water NFSF. errors are caused by the characteristics of the instrument itself and may include the zero drift error, sensitivity error and linearity error. The sampling errors are caused by the processes of sample acquisition, preparation and operation, and mainly include the sample heterogeneity error, preparation error and operational error. According to the data provided by the instrument manufacturers and previous experimental research experience values, the total error of the spectral property test results of NFSFs can be smaller than 8%. The average transmittance and absorptivity values of different NFSFs in the spectrum range of λ1~λ2 can be calculated using the following equations: where τnfsf and αnfsf are the spectral transmittance and absorptivity of the NFSF. The average transmittance and absorptivity values of different NFSFs are calculated based on the optical property test results of different NFSFs presented in Figure 6, as well as the solar spectral irradiance data and the calculation results, which are shown in Table 2. For the test results of the optical properties of different NFSFs in Figure 6, the experimental errors mainly include the instrument errors and sampling errors. The instrument errors are caused by the characteristics of the instrument itself and may include the zero drift error, sensitivity error and linearity error. The sampling errors are caused by the processes of sample acquisition, preparation and operation, and mainly include the sample heterogeneity error, preparation error and operational error. According to the data provided by the instrument manufacturers and previous experimental research experience values, the total error of the spectral property test results of NFSFs can be smaller than 8%.
The average transmittance and absorptivity values of different NFSFs in the spectrum range of λ 1~λ2 can be calculated using the following equations: where τ nfsf and α nfsf are the spectral transmittance and absorptivity of the NFSF. The average transmittance and absorptivity values of different NFSFs are calculated based on the optical property test results of different NFSFs presented in Figure 6, as well as the solar spectral irradiance data and the calculation results, which are shown in Table 2.

Optical Characteristics Evaluation
To investigate the optical performance of this HPVT system, the optical modelling of the HPVT system was carried out. The NFSF used in the HPVT system was assumed to be the Ag/water (1.2 mg/130 mL)+3 g CoSO 4 nano-fluid. The average absorption rate and average transmittance of the NFSF were 53.52% and 46.48%, respectively. The reflectance of the LFRC was assumed to be 100%, and the incident sunlight intensity was set to be 1000 W/m 2 . Using the ray tracing method, the optical processes of the HPVT system were simulated.
The two-dimensional distributions of light intensity on the NFSF bottom surface and the lower PVCM surface are shown in Figure 7. The relevant data in Figure 7 were derived from the results based on the ray tracing optical simulations of the HPVT system. The introductions of the width and length positions of the NFSF as well as those of the PVCM can be seen in Figure 1. The results of Figure 7 show that the light intensity distributing condition on the NFSF bottom surface is relatively uniform, and that on the lower PVCM surface it is approximately unimodal. The energy flux received by the PVCM and that absorbed by the NFSF are 240.7 W and 331.54 W, respectively. Thus, the optical efficiency of the whole HPVT system is 0.9138.   In order to verify the adaptive capacity of the HPVT system using LFRC and NFSF to the solar track error (STE), the sunlight concentrating process under different STEs is simulated through the ray tracing simulation method. Figure 8 shows the light intensity distributing conditions on the NFSF bottom surface of the HPVT system when the STE changes by 0~0.3 • . Table 3 shows the optical efficiencies of the PVCM, the NFSF and the whole HPVT system under different STEs. The relevant results in Table 3 were calculated using the data based on the ray tracing optical simulations of the HPVT system when the STE changes. In the current study, for the optical evaluation of the HPVT system, the optical efficiency is defined as the ratio of the actual energy flux value on a device (or a subsystem) to the theoretical value. It can be seen from Table 3 that with the rise in the STE, the optical efficiencies of the PVCM, the NFSF and the whole HPVT system all decrease. This is because when the STE rises, the optical loss in the PVCM as well as that in the NFSF becomes larger, thus increasing the total optical loss in the whole HPVT system. This results in reductions in relevant optical efficiencies. When the STE is 0.3 • , the optical efficiencies of the PVCM and NFSF are 0.7556 and 0.9053, and that of the whole HPVT system is 0.8357, revealing relatively good adaptive capacity of the HPVT system to the STE.

Thermodynamic Evaluation
The thermodynamic evaluation results of the HPVT system are introduced in this section. The formulae for the thermodynamic estimate of the HPVT system using LFRC and NFSF are as follows:

Thermodynamic Evaluation
The thermodynamic evaluation results of the HPVT system are introduced in this section. The formulae for the thermodynamic estimate of the HPVT system using LFRC and NFSF are as follows: V oc,c,bs = hcV oc / λ 2 E g + n f k B T pv ln(CR)/e (13) I sc,c,bs = CR · I sc,bs (14) P pv,c,bs,m = FF · V oc,c,bs · I sc,c,bs (16) η pv = P pv,c,bs,m /Q pv,bs (18) η pv,sys = P pv,c,bs,m /Q in (19) η ex,sys = η pv,sys + η th,sys 1 − T 0 T nfsf (24) where P pv,c,bs,m and η pv stand for the output power and photoelectric efficiency of the PVCM. η th,sys and η pv,sys are the thermal efficiency and photovoltaic efficiency of the whole HPVT system. η ex,sys is the exergy efficiency of the HPVT system. Q r and Q con are the radiation and convective heat losses of the NFSF. The explanations of other parameters can be seen in the nomenclature part of the paper. For the initial conditions of the thermodynamic estimate of the HPVT system, the PVCM area is 0.5 m 2 , V oc is 0.706 V, FF is 0.828, n f is 1.28 and T pv is 30 • C. The total incident solar energy flux Q in is 14,089.5 W. The average transmittance and emissivity of the upper and lower glass surfaces of the NFSF are 0.91 and 2 × 10 −7 T 2 nfsf + 5 × 10 −5 T nfsf + 0.05, respectively. The convective heat transfer (CHT) coefficient between the NFSF and air is 5 W/(m 2 ·K). The reflectivity of the LFRC is 0.95. The environment temperature is 20 • C. Figures 9 and 10 present the relevant thermodynamic evaluation results of the HPVT system. Figure 9 shows the maximum output power and Q pv,bs of the PVCM when different NFSFs are used. Here, the maximum output power refers to the maximum output power of the PVCM of the HPVT system under the fixed-parameter condition when the HPVT system operates stably. The initial parameter values for the fixed-parameter condition were introduced above. It can be seen that compared with the other three NFSFs, the HPVT system with Ag/water (1.2 mg/130 mL) has the largest output power of 2499.7 W, and the radiation flux transferred to the PVCM is 7205.9 W. When the Ag-CoSO 4 /water NFSF is used, as the thermal performance of the HPVT system is improved, the maximum output power and Q pv,bs of the PVCM decrease.   Figure 9 shows the maximum output power and Qpv,bs of the PVCM when different NFSFs are used. Here, the maximum output power refers to the maximum output power of the PVCM of the HPVT system under the fixed-parameter condition when the HPVT system operates stably. The initial parameter values for the fixed-parameter condition were introduced above. It can be seen that compared with the other three NFSFs, the HPVT system with Ag/water (1.2 mg/130 mL) has the largest output power of 2499.7 W, and the radiation flux transferred to the PVCM is 7205.9 W. When the Ag-CoSO4/water NFSF is used, as the thermal performance of the HPVT system is improved, the maximum output power and Qpv,bs of the PVCM decrease.   Figure 10 are the efficiency values of the HPVT system under the fixed-parameter condition when the HPVT system operates stably, and the initial parameter values for the fixed-parameter condition were mentioned above. The results indicate that the introduction of CoSO4 will enhance the heat absorption of the NFSF and increase the thermal efficiency of the system. But, as a result, the output electric power as well as the photoelectric efficiency of the PVCM will be reduced, leading to a decrease in exergy efficiency of the HPVT system. When Ag/water (1.2 mg/130 mL) is used, ηpv,sys, ηth,sys and ηex,sys are 0.1774, 0.311 and 0.23. And, when Ag/water (1.2 mg/130 mL)+3 g CoSO4 is used, ηpv,sys, ηth,sys and ηex,sys are 0.1366, 0.4259 and 0.209.
The thermodynamic estimate of the HPVT system using Ag/water (1.2 mg/130 mL+3 g CoSO4) NFSF was carried out. The two key effective investigated parameters were the thermal efficiency ( th,sys η ) and exergy efficiency ( ex,sys η ) of the HPVT system. Figure 11 shows the changes in radiant heat loss and convective heat loss in the NFSF, thermal efficiency and exergy efficiency of the HPVT system at different NFSF temperatures ( Figure  11a), different CHT coefficients and different environment temperatures (Figure 11c). The energy and efficiency results presented in Figure 11 also refer to the performance param- Figure 10. Efficiencies of the HPVT system with different NFSFs. Figure 10 presents different efficiencies of the HPVT system with different NFSFs. Similarly, the efficiencies in Figure 10 are the efficiency values of the HPVT system under the fixed-parameter condition when the HPVT system operates stably, and the initial parameter values for the fixed-parameter condition were mentioned above. The results indicate that the introduction of CoSO 4 will enhance the heat absorption of the NFSF and increase the thermal efficiency of the system. But, as a result, the output electric power as well as the photoelectric efficiency of the PVCM will be reduced, leading to a decrease in exergy efficiency of the HPVT system. When Ag/water (1.2 mg/130 mL) is used, η pv,sys , η th,sys and η ex,sys are 0.1774, 0.311 and 0.23. And, when Ag/water (1.2 mg/130 mL)+3 g CoSO 4 is used, η pv,sys , η th,sys and η ex,sys are 0.1366, 0.4259 and 0.209.
The thermodynamic estimate of the HPVT system using Ag/water (1.2 mg/130 mL+3 g CoSO 4 ) NFSF was carried out. The two key effective investigated parameters were the thermal efficiency (η th,sys ) and exergy efficiency (η ex,sys ) of the HPVT system. Figure 11 shows the changes in radiant heat loss and convective heat loss in the NFSF, thermal efficiency and exergy efficiency of the HPVT system at different NFSF temperatures (Figure 11a), different CHT coefficients and different environment temperatures (Figure 11c). The energy and efficiency results presented in Figure 11 also refer to the performance parameters of the HPVT system under the fixed-parameter condition when the HPVT system achieves stable operation. It can be seen that when the NFSF temperature changes from 338 K to 363 K, η ex,sys increases linearly from 0.194 to 0.218, while η th,sys decreases from 0.431 to 0.422. With the CHT coefficient rising from 1 W/(m 2 ·K) to 11 W/(m 2 ·K), η th,sys and η ex,sys both decrease due to the increase in the convective heat loss in the NFSF. When the CHT coefficient is 11 W/(m 2 ·K), η th,sys and η ex,sys are 0.4055 and 0.2055. When the environment temperature rises from 289 K to 299 K, η th,sys increases slightly from 0.4247 to 0.4278, while η ex,sys decreases from 0.2136 to 0.202. Hence, when the water-based Ag NFSF is used, the overall thermodynamic performance of the HPVT system could be improved by appropriately rising the NFSF temperature as well as properly decreasing the environment temperature and the CHT coefficient between the NFSF and air.

Conclusions
In order to realise the objective of carbon neutrality, the transformation of the energy consumption structure is undoubtedly imperative for the world. Renewable energy represented by solar energy has played an important role in the transformation of the energy consumption structure. Solar HPVT technology has been considered to be an effective ap- Figure 11. Thermodynamic performances of the HPVT system at different NFSF temperatures (a), different CHT coefficients (b) and different environment temperatures (c).

Conclusions
In order to realise the objective of carbon neutrality, the transformation of the energy consumption structure is undoubtedly imperative for the world. Renewable energy represented by solar energy has played an important role in the transformation of the energy consumption structure. Solar HPVT technology has been considered to be an effective approach to increase overall solar utilisation efficiency. The current study proposes and studies a novel solar HPVT system using LFRC and NFSF. The method of designing the HPVT system is given. Ag/water and Ag-CoSO 4 /water NFSFs used for this HPVT system were prepared and experimentally tested. Based on the optical property test results, the spectrum splitting performances of different Ag/water and Ag-CoSO 4 /water NFSFs were analysed. The results reveal that the average absorption rate and average transmittance of the Ag/water (1.2 mg/130 mL)+3 g CoSO 4 NFSF are 53.52% and 46.48%. The optical characteristics of the HPVT system were investigated based on the ray tracing method. The optical analysing results indicate that when the STE is 0.3 • , the optical efficiencies of the PVCM and the NFSF are 0.7556 and 0.9053, which shows a good adaptable capacity of the HPVT system to the STE. Moreover, the thermodynamic performances of the HPVT system using different NFSFs were compared. The comparison results demonstrate that compared with the Ag/water NFSF, the introduction of CoSO 4 can enhance the thermal performance but decrease the photovoltaic efficiency of the whole HPVT system. When Ag-CoSO 4 /water (1.2 mg-3 g/130 mL) is used, the photovoltaic and thermal efficiencies of the HPVT system are 0.1366 and 0.4259, and the exergy efficiency is 0.209. The exergy efficiency of the HPVT system will increase if the NFSF temperature properly rises or if the external convection heat transfer coefficient and environment temperature decrease. The current study aimed to extend the research contents of solar HPVT technologies, and the results presented in this paper will have certain reference value for further research works on solar HPVT systems in the future.
The proposed solar HPVT system using NFSF could be applied in the field of electric power generation combined with thermal utilisations (e.g., heat supply, desalination, etc.). As the LFRC has the advantages of small wind-resistance and simple structure, the HPVT system will be suitable for large-scale applications in the future.
However, there are still some limitations for this study. As the preparations and tests of Ag/water and Ag-CoSO 4 /water NFSFs were preliminary in this study, for further research works, more detailed tests on different NFSFs should be carried out. In addition, the interaction analysis between structural and optical parameters of the HPVT system should be conducted. Moreover, the experimental set-up of the HPVT system can be built and a series of experiments should be carried out to validate the operation performances as well as the technical feasibility of the HPVT system using different NFSFs.