Experimental Study on Low Carbonization of Green Building Based on New Membrane Structure Solar Sustainable Heat Collection

Abstract

In recent years, energy consumption has continuously been increasing, and the energy consumption proportion in buildings has risen yearly. In order to promote the carbon-neutral goal of carbon peaking, the building sector realizes green and low-carbon transformation. This paper proposes a new type of solar flat plate collector with an additional transparent cover made by the ETFE film, which is tested for thermal performance under different environmental and operational parameters. The Ansys Fluent software was used to build a three-dimensional steady-state model of the collector, which can simulate the collector components’ temperature and the mass outlet temperature under the test conditions. The collector’s instantaneous heat collection efficiency curve fitted by comparing and analyzing the theoretical, experimental, and simulated data. The instantaneous efficiency intercept was 0.72, and the heat loss coefficient was 3.94 W/(m²·K). The results show that the collector efficiency of the ETFE film structure collector is 18.6% higher, and the heat loss coefficient is 27.3% lower than that of an ordinary collector under standard mass flow conditions.

1. Introduction

With the introduction of China’s double carbon target in September 2020, the use of clean energy is gaining more and more attention. Clean energy replaces traditional fossil energy and plays a leading role in achieving energy transformation, reducing CO₂ emissions, and achieving the double carbon target. Solar energy is the main force of clean energy, with safe, renewable, nonpolluting utilization [1].

In existing green building technology solutions, solar thermal conversion is one of the simplest and most direct ways to utilize solar energy. Flat plate solar collectors are a vital component of solar thermal conversion due to the low collector efficiency and high heat loss of flat plate solar collectors under high-temperature operating conditions. Therefore, reducing the heat loss of the collector and improving the thermal performance are the main technical measures to improve the efficiency of flat plate solar collectors [2,3,4]. Hanane Dagdougui et al. [5] investigated the effect of different cover types and numbers on the top heat loss and related thermal performance of the collector. They determined the optimal mass flow rate and collector area based on a dual-objective constrained optimization model. They also concluded that a double-layer cover has a lower total heat loss coefficient than a single-layer cover. Siyu Liu [6] conducted experimental and theoretical analyses of double-layer hollow glass cover plate solar collectors. It concluded that double-layer cover plate solar collectors have 33.9% higher collector efficiency and 53.4% lower total heat loss coefficient than single-layer cover plate collectors. He Peng [7] adopted measures such as increasing the interplate spacing, coating the glass cover with a permeability-enhancing film, laser-welding the heat-absorbing plate and collector tube, and replacing the heat insulation material for the traditional flat plate solar collectors. This resulted in a 15.1% increase in the transient efficiency intercept and a 20.3% reduction in the heat loss coefficient. Ruru Xu et al. [8] compared the performance of vacuum glass and single glass cover PV/T collectors. The thermal and electrical performance of the two types of collectors in winter in Hefei was simulated using mathematical models. It found that the all-day thermal efficiency of the vacuum glass cover PV/T collector was increased by 5.68% relative to the single glass cover PV/T collector.

Flat plate solar collectors have been developed in China for more than 40 years, and scholars have established different structural forms of collectors for different application conditions, and many of them have been widely used [9,10,11,12,13,14,15,16,17,18,19]. However, for cold regions, especially in Urumqi, the cold winter, large temperature difference between day and night, and dusty weather climate characteristics lead to the application of flat plate collectors still having low collector efficiency, large heat loss, easy frost and ash accumulation, long working start-up time, and other problems. In order to improve the above-mentioned problems of traditional flat plate solar collectors in cold regions, increase the thermal conversion efficiency of the collectors, and improve the thermal performance, this paper, based on a summary of the current status of research and application of flat plate solar collectors at home and abroad and the effectiveness of ETFE film application in the construction field, combines the ETFE film with flat plate solar collectors and proposes a new structure of an ETFE membrane structured flat plate solar collector.

It is found from the research data that the low collector efficiency in cold regions is mainly due to the excessive heat transfer loss with the surrounding environment, and the top heat loss through the cover system is the largest part of the collector heat loss, so the structural modification to reduce the top heat loss of the collector is the main way to improve its thermal performance. The double-layer cover system has better thermal insulation performance than the traditional single-layer cover, which can maintain its internal high-temperature working condition during the daytime and delay its internal temperature drop at night to reduce the heat exchange loss with the surrounding environment. In previous studies, setting up a double-layer vacuum glass cover; adding TIMs (transparent insulation materials), such as transparent honeycomb, foam, fiber, and aerogel, in the air interlayer; and increasing the disturbance structure to fully transfer heat can all achieve the purpose of reducing heat loss and improving thermal performance. However, all of the above improvement options would make the collector structure more complex, the collector mass will be heavy, and the cost will be too high. In contrast, the ETFE membrane structured flat plate solar collector structure is simpler, only adding a layer of ETFE membrane structure cover, using its excellent optical properties, and adding a layer of air of the thermal resistance to achieve the same purpose, with the advantages of simple structure and low cost of transformation.

In this paper, a collector thermal performance test system is built, and the ETFE film structure collector is connected in parallel with the common single-layer glass cover collector to analyze the thermal performance difference between them through comparative tests. In addition, a steady-state heat transfer model is established in this paper and solved numerically using the Ansys Fluent software to verify the effects of different inlet mass temperatures, mass flow rates, and insulation thicknesses of collectors on the thermal performance of collectors, which provides a theoretical basis for the further improvement of the thermal performance of double-layer cover collectors in the field of building energy efficiency.

2. Materials

2.1. Membrane Structure with Flat-Plate-Type Solar Collector

The single-layer glass-covered flat plate solar collector is a commercial collector from Jinheng New Energy Co., Ltd., in Dezhou City, Shandong Province, China, on the basis of which the ETFE membrane structured flat plate solar collector was modified and tested in the thermal engineering laboratory of Xinjiang University School of Architecture and Engineering. The material parameters of the single glass cover plate solar collector were tested and provided by Jinheng New Energy Co. The main structural parameters of the collector when it leaves the factory are shown in

Table 1. The main structural parameters of the collector at the factory.

Parameters	Size (Material)	Parameters	Size (Material)
Dimension	2000 × 1000 × 110 mm	Tube	830 mm × 2 pcs
Light harvesting area Ac	1.85 m2	Drain tube	1700 mm × 8 pcs
ETFE film cover	2000 × 1000 × 2 mm	Pitch	120 mm
Distance between ETFE film cover and glass cover	30 mm	Insulation layer	Glass wool composite insulation material
Glass cover	1950 × 950 × 5 mm super white fabric tempered glass	Insulation layer thickness	40 mm
Glass cover plate heat transfer coefficient (reference value)	2.2 W/(m2·K)	Collector shell	Aluminized board
Glass cover plate and heat-absorbing plate spacing	30 mm	Weight	25 kg
Heat-absorbing surface	High selective absorption coating	Interface specification	G1/2 male thread
Heat-absorbing plate	Copper and aluminum composite plate	Number of interfaces	2 pcs
Heat collector tube	Purple copper	Workpiece capacity	0.8 L
Heat collector tube outer diameter	18 mm	Bonding agent	Ethylene/vinyl acetate copolymer (EVA)
Thickness of collector tube	0.5 mm	-

.

An ETFE film structure with a flat plate solar collector adds a transparent cover made by the ETFE film, and its structure is shown in Figure 1. This mainly consists of the ETFE film cover, glass cover, heat-absorbing plate, collector tube, heat insulation material, and collector box. The collector volume is 2000 mm (length) × 1000 mm (width) × 110 mm (height) with a light collection area of 1.85 m². The ETFE film has good optical properties with a transmittance of up to 95%. The glass cover plate is made of fabric tempered glass with a transmittance of 92%. The heat-absorbing plate adopts a tube-and-plate structure with a workpiece capacity of 0.8 L. The insulation layer is made of glass wool composite insulation. The collector shell is made of aluminum. A photo of the collector is shown in Figure 2.

2.2. Thermal Performance Test System for Collectors

The testing period of this paper is from April 25th to May 20th, 2022. The test site is the roof of the fifth floor of the complex building of Yanke Energy-Saving Technology Company in Urumqi City, Xinjiang (87°35′ E, 43°57′ N), with the collector facing due and an inclination angle of 50°.

The test system mainly consists of a flat plate solar collector, plate heat exchanger, heat storage tank, make-up water tank, circulation pump, flow regulating valve, total radiation meter, anemometer, flow sensor, temperature sensor, and data acquisition instrument (Figure 3). Among them, ETFE membrane structured and ordinary single-layer glass-cover-plate-type solar collectors are arranged in parallel. The total radiation meter is thermoelectric, model RS485; the anemometer model is TSI 8360, the data acquisition instrument model is Agilent 34970A; and the temperature sensor is a copper concentrate T-type thermocouple. The parameters of each type of the test instrument are shown in

Table 2. Experimental test instrument parameters.

Instrument	Uncertainty Percentage	Measurement Range	Type (Model)
Thermocouple	±0.1%	−200–350 °C	Copper–copper T type Thermocouple
Solar irradiator	±0.5%	0–2000 W/m2	RS485
Flowmeter	±1%	0.03–3 m3/h	YNRC
Anemometer	±3%	0.5–45 m/s	TSI 8360
Data collector	-	-	Agilent 34970A

.

A total of 40 thermocouples are arranged in the collector cover, collector tube, water inlet and outlet, and water inlet and outlet of the heat storage tank. The circulating water pump is Longchamp LPS40-6S with a rated power of 100 W and a total of three gears, combined with a flow adjustment valve to adjust the mass flow of the collector-tested mass. The collector circulating mass is water. Figure 4 shows the collector thermal performance test site.

3. Methodology

3.1. Test Method

The tests in this paper mainly refer to the international standard ISO 9806-2017, “Solar Energy—Solar Collectors—Test Methods”, and the national standard GB/T 4271-2007, “Test Methods for Thermal Performance of Solar Collectors”. The thermal performance test conditions of the collectors are quasi-steady-state test conditions, in which the solar irradiance during the test period is greater than 700 W/m² and the wind speed is not higher than 4 m/s. The test equipment and instrumentation lap are in accordance with the requirements of the above standards.

3.2. Thermal Performance Index

At this stage, most of the testing methods for various types of solar collectors are established based on the heat balance equation model under quasi-steady-state conditions. Under quasi-steady-state conditions, the solar radiation energy absorbed by the collector per unit of time is equal to the sum of beneficial energy output from the collector, the energy lost by the collector, and the change in heat capacity of the collector itself during the same time. Based on this principle, the heat balance equation under experimental conditions is established [20,21,22,23].

Under quasi-steady-state test conditions, the beneficial heat gained by the collector per unit time, that is, the valuable energy of the circulating mass Q, is:

$$Q=m·c_f\Delta t$$

(1)

In the above equation, m is the collector heat transfer mass flow rate, kg/s; c_f is the specific heat capacity of the collector heat transfer mass, J/(kg °C); and ∆t is the collector heat transfer mass import and export temperature difference, °C.

The instantaneous efficiency of the collector is the ratio of the actual valuable energy obtained by the collector to the solar radiation energy received by the surface. Based on the collector area A_a and introducing the heat transfer factor F_R, the instantaneous efficiency is expressed as a function of the normalized temperature difference $T_i^{*}$ as,

$${\eta }_{\mathrm{i}}=\frac{Q_u}{{\mathrm{A}}_{\mathrm{a}}G}=F_R{\left(\tau \alpha \right)}_e-F_{R}U{T}_i^{*}$$

(2)

In the above equation, Q_u is the collector effective utilization energy, W; A_a is the collector light harvesting area, m²; G is the solar irradiance, W/m²; F_R is the collector heat transfer factor; (τα)_e is the effective utilization absorption rate product; U is the collector total heat loss coefficient W/(m²·°C); and $T_i^{*}$ is the normalized temperature difference based on the collector work mass inlet temperature, (m²·°C)/W. The expression for the normalized temperature difference $T_i^{*}$ is,

$$T_i^{*}=\frac{t_i-t_a}{G}$$

(3)

In the above equation, t_i is the collector mass inlet temperature, °C; and t_a is the ambient temperature, °C.

The physical meaning of the heat transfer factor is the ratio of the collector’s actual energy gain to the collector’s useable energy gain, assuming that the surface temperature of the heat-absorbing plate is equal to the collector’s mass inlet temperature.

The curve of the instantaneous efficiency should be obtained by curve fitting using the least-squares method.

4. Results

4.1. Effective Transmission Absorption Area of the Collector

In order to determine the heat collection efficiency of the collector, it is necessary to know the solar radiation energy absorbed by the heat-absorbing plate and the heat loss of the collector. The heat loss of the collector can be seen in Figure 5; Q_t, Q_e, and Q_b are the top, side, and bottom heat losses of the collector, respectively. For this purpose, the effective transmission rate of solar radiation by the transparent cover and the effective absorption rate of the heat-absorbing plate, that is, (τα)_e, the effective transmission absorption product, should be calculated first.

Regarding the effective transmittance absorption product and radiation heat exchange of multilayer transparent cover plate, numerous scholars have made a lot of derivations, including the ray-tracing method, net radiation flow method, and embedding method [24,25,26,27]. However, they do not widely use these methods due to the complicated calculation process. Therefore, this manuscript adopts a different method to determine the effective transmittance of the multilayer cover and the effective absorption rate of the heat-absorbing plate. The derivation is limited to the case of vertical sunlight irradiation. The solar radiation schematic of the double-layer cover collector is shown in Figure 6, where 2, 1, and P represent the collector’s two transparent cover plates and the heat-absorbing plate, respectively.

The effective transmittance τ* of the transparent cover refers to the transmittance produced by considering the projected radiation in the transparent cover after absorption and multiple reflections inside the cover. As the membrane structure with a flat-plate-type solar collector cover for ultrawhite fabric tempered glass and ETFE film, the surface reflectivity of their respective two surfaces is the same, that is, ρ = ρ’; then there are:

$${\tau }^{*}=\frac{{\left(1-\rho \right)}^2{\tau }_i}{1-{\rho }^2{\tau }_i^2}$$

(4)

In the above equation: ρ is the surface reflectance of the transparent cover, and τ_i is the primary transmittance of the transparent cover due to absorption.

Set the two surfaces of the transparent cover be 1 and 1’, the effective reflectivity ρ* of surface 1, is the ratio of the reflected radiation of 1 to the incident radiation when the projected radiation falls on the cover, where the reflected radiation includes the sum of the primary reflection and the reflection after transmission. The expression for the effective reflectance of the two transparent covers of the collector is

$${\rho }^{*}={\rho }^{*\prime }=\rho \left[1+\frac{{\left(1-\rho \right)}^2{\tau }_i^2}{1-{\rho }^2{\tau }_i^2}\right]$$

(5)

In the above equation, ρ* and ρ*′ are the effective reflectance of the surface 1 and 1’, respectively.

The following relationship can be easily deduced from Figure 6:

$$G_{\mathrm{in},\mathrm{P}}={\tau }_1^{*}{\tau }_2^{*}G+J_{1\downarrow }+{\tau }_1^{*}{J}_2$$

(6)

$$J_{1\downarrow }={\rho }_1^{*}{J}_P$$

(7)

$$J_2={\rho }_2^{*}\left(J_{1\uparrow }+{\tau }_1^{*}{J}_{\mathrm{P}}\right)$$

(8)

$$J_{1\uparrow }={\rho }_1^{*}\left(J_2+{\tau }_2^{*}G\right)$$

(9)

$$J_{\mathrm{P}}={\rho }_{\mathrm{P}}\left({\tau }_1^{*}{\tau }_2^{*}G+J_{1\downarrow }+{\tau }_1^{*}{J}_2\right)$$

(10)

In the above equation, G_in,P is the transmitted irradiance falling on the collector absorber plate P, W/m²; G is the total solar irradiance, W/m²; τ^1* and τ^2* are the effective transmittance of the first and second cover plates of the collector; J_1↑ and J_1↓ are the effective radiation irradiance of the two sides of the first cover plate of the collector; J₂ and J_P are the effective radiation irradiance of the second cover and heat-absorbing plate of the collector; ρ^1* and ρ^2* are the effective reflectance of the first and second cover of the collector, respectively; and ρ_P is the reflectance of the heat-absorbing plate of the collector to solar radiation.

It is noted that effective radiation irradiance includes its radiation and the sum of reflected radiation. Because of the temperature, the collector transparent cover and heat-absorbing plate radiation energy almost do not contain a wavelength of less than 3 μm radiation. Hence, the above formula for effective radiation irradiance is essentially only reflected radiation irradiance.

The effective transmittance of solar radiation in the double-layer cover of the collector can be obtained through the joint calculation of Equation (6) to Equation (10).

$$\tau =\frac{G_{\mathrm{in},\mathrm{P}}}{G}=\frac{{\tau }_1^{*}{\tau }_2^{*}}{1-{\rho }_1^{*}{\rho }_2^{*}-{\rho }_{\mathrm{P}}\left({\tau }_1^{*2}{\rho }_2^{*}-{\rho }_1^{*2}{\rho }_2^{*}+{\rho }_1^{*}\right)}$$

(11)

The effective absorption rate of collector heat absorbing plate a.

$$a=\frac{a_{\mathrm{P}}{G}_{\mathrm{in},\mathrm{P}}}{G}=\frac{{\tau }_1^{*}{\tau }_2^{*}{a}_{\mathrm{P}}}{1-{\rho }_1^{*}{\rho }_2^{*}-{\rho }_{\mathrm{P}}\left({\tau }_1^{*2}{\rho }_2^{*}-{\rho }_1^{*2}{\rho }_2^{*}+{\rho }_1^{*}\right)}$$

(12)

Hence, it is only necessary to know the primary transmittance τ_i and surface reflectance ρ of the transparent cover and the reflectance ρ_P of the heat-absorbing plate to find the effective transmittance absorption product of the collector using the above equation.

Using Equations (4) to (12), the effective transmittance and effective absorbance of the transparent cover can be found, and thus, the effective transmittance absorption product of the collector.

According to the above formulas, the effective transmittances of the ETFE film and fabric tempered glass are 0.95 and 0.92, respectively, after combining relevant information and actual measurement data. The surface reflectances are 0.02 and 0.07, respectively. The reflectance of the heat-absorbing plate is 0.08. The absorption rate is 0.861, the effective transmission absorption product is 0.759. In the ordinary single-layer glass cover plate solar collector transparent cover, the effective transmission rate is 0.925, the effective absorption rate is 0.851 heat-absorbing plates, and the effective transmission absorption product is 0.787.

4.2. Collector Heat Loss

The heat-absorbing plate absorbs solar radiation energy and converts it into heat energy, which is the highest temperature component of the collector [28,29,30,31,32,33,34]. Its temperature is higher than the ambient temperature, so it dissipates heat to the outside world through radiation, conduction, and convection. Collector heat loss mainly includes top, side, and back heat loss. The collector thermal network diagram is shown in Figure 7.

In Figure 7, t, δ, λ, and h are temperature, thickness, thermal conductivity, and convective heat transfer coefficient, respectively; the corner markers w, r, and conv represent wind-dominated convective heat transfer, radiant heat transfer, and convective heat transfer, respectively; and the corner markers a, ETFE, c, p, e, and b represent the environment, ETFE film cover, glass cover, heat-absorbing plate, collector side wall, and collector bottom surface, respectively.

As the collector side temperature is much lower than the collector plate temperature, with the slight surrounding environment temperature difference, the side and bottom heat loss in convective heat transfer and radiation heat transfer compared with the heat transfer is much smaller and can ignore its impact. The following formula can approximate the collector side and back heat loss coefficient.

Due to the improved performance of insulation materials, few improvements and performance enhancements have been made to flat plate solar collectors for side and bottom heat losses in recent years. Compared with the side and bottom heat losses, the top heat loss of the collector is the highest and has the highest impact on the collector’s thermal performance. This manuscript also focuses on the effect of increasing the ETFE film structure on the top heat loss and collector efficiency.

$$R_{\mathrm{t}}=\frac{1}{U_{\mathrm{t}}}=\frac{1}{h_{\mathrm{w}}+h_{\mathrm{r},\mathrm{ETFE}-\mathrm{a}}}+\frac{{\delta }_{\mathrm{ETFE}}}{{\lambda }_{\mathrm{ETFE}}}+\frac{1}{h_{\mathrm{conv},\mathrm{g}-\mathrm{ETFE}}+h_{\mathrm{r},\mathrm{g}-\mathrm{ETFE}}}+\frac{{\delta }_{\mathrm{c}}}{{\lambda }_{\mathrm{c}}}+\frac{1}{h_{\mathrm{conv},\mathrm{P}-\mathrm{g}}+h_{\mathrm{r},\mathrm{P}-\mathrm{g}}}$$

(13)

$$h_{\mathrm{w}}=5.7+3.8v$$

(14)

$$h_{\mathrm{r},\mathrm{ETFE}-\mathrm{a}}=\sigma {\epsilon }_{\mathrm{ETFE}}\left(T_{\mathrm{ETFE}}^2+T_{\mathrm{a}}^2\right)\left(T_{\mathrm{ETFE}}+T_{\mathrm{a}}\right)$$

(15)

$$h_{\mathrm{r},\mathrm{P}-\mathrm{g}}=\frac{\sigma \left(T_{\mathrm{P}}^2+T_{\mathrm{g}}^2\right)\left(T_{\mathrm{P}}+T_{\mathrm{g}}\right)}{\frac{1}{{\epsilon }_{\mathrm{P}}}+\frac{1}{{\epsilon }_{\mathrm{g}}}-1}$$

(16)

In the above equation, R_t is the thermal resistance at the top of the collector, (m²·K)/W; U_t is the heat loss at the top of the collector, W/(m²·K); v is the wind speed, m/s; ε is the emissivity; and σ is the Stefan–Boltzmann constant, 5.67 × 10⁻⁸ W/(m²·K⁴).

The convective heat transfer coefficients between the ETFE film and the fabric tempered glass cover and between the glass cover and the heat-absorbing plate were calculated using the Nusselt number Nu.

$$h_{\mathrm{conv},\mathrm{g}-\mathrm{ETFE}}=h_{\mathrm{conv},\mathrm{P}-\mathrm{g}}=\frac{kNu}{L}$$

(17)

In the above equation, k is the thermal conductivity of air, W/(m·K); and L is the thickness of the air interlayer, m.

Combination of the experimental test data, the theoretical heat loss of the collector, is calculated by substituting the above equation.

Table 3. Thermal resistance at the top of the collector composed of different ETFE membrane structure air interlayer thicknesses.

Air Interlayer Thickness	Top Thermal Resistance Rt (t)	Top Heat Transfer Coefficient Ut (t)	Total Heat Transfer Coefficient U	The Reduction Compared to Ordinary Collectors
10 mm	0.2908	3.4388	5.5388	16.47%
20 mm	0.3347	2.9877	4.0877	27.42%
30 mm	0.3786	2.6413	3.7413	35.84%

shows the thermal resistance of the top of the collector composed of different ETFE membrane structures’ air interlayer thicknesses.

According to Duffie and Beckman [35], the effect of the distance between the transparent cover and the heat-absorbing plate on the total loss coefficient of the collector was simulated and calculated. When the distance between the transparent cover and the heat-absorbing plate is 30 mm, the total heat loss coefficient is very low, and the total heat loss coefficient decreases fastest. Later, as the spacing increases, the total heat loss coefficient decreases slowly. That is, the influence of spacing on the total heat loss coefficient decreases. When the spacing increases to 65 mm, the total heat loss coefficient reaches the minimum value.

By combining theoretical calculations with actual engineering difficulties, 30 mm is the spacing between the glass cover and the heat-absorbing plate of the two collectors, and the thickness of the air interlayer in the ETFE membrane structure was set as 30 mm.

4.3. Experimental Test Results

Figure 8 and Figure 9 show typical daily climatic parameters and collector test inlet and outlet water temperatures, respectively. The test results show that the difference between the inlet and outlet work mass temperatures of ETFE membrane structure collectors is enormous. The maximum outlet work mass temperature of ETFE membrane structured collectors is 62.11 °C, while the inlet work mass temperature is 54.05 °C. The maximum difference between the inlet and outlet work mass temperatures of ETFE membrane structured collectors is 9.32 °C, and the maximum difference between the inlet and outlet work mass temperatures of ordinary collectors is 8.29 °C. It is because ETFE membrane collectors have lower heat loss, which produce less heat exchange with the outside world than standard collectors, and are less affected by the environment.

After measuring multiple sets of data for collector inlet and outlet temperatures, ambient temperature, solar irradiance, and collector flow rate, the data satisfying the steady state were selected for processing. Moreover, the resulting instantaneous efficiencies were fitted using the least-squares method. Figure 10 shows the instantaneous efficiency curve of two collectors at the standard mass flow rate.

The fitted curve of the ETFE membrane structured flat-plate-type solar collector is Equation (18).

$${\eta }_i=0.7203-3.7403T_i^{*}$$

(18)

The fitted curve for an ordinary single-layer glass-covered flat plate solar collector is Equation (19).

$${\eta }_i=0.6314-4.3504T_i^{*}$$

(19)

The instantaneous intercept efficiency of the ETFE film structured flat plate solar collector is 14.1% higher than that of an ordinary single-layer glass flat plate solar collector. The heat loss coefficient of the ETFE film structured flat plate solar collector is 3.94 W/(m²·K), while that of the ordinary collector is 5.42 W/(m²·K), and the heat loss coefficient is reduced by 27.3%. The instantaneous efficiency and intercept of the collector are increased with the gradual mass flow rate increases, as shown in Figure 11. The increase gradually decreases due to the mass flow rate from 0.006 to 0.016 kg/(m²·s). In the process, the mass flow state gradually changes from laminar flow to turbulent flow. The convective heat transfer coefficient between the mass and the tube wall gradually increases; thus, the convective heat transfer coefficient between the mass and the tube wall steadily increases, enhancing the convective heat transfer strength between the mass and the tube wall. Therefore, the instantaneous efficiency rises gradually. However, with the further increase of the work mass flow rate, the work mass flow state gradually stabilizes, and the work mass flow rate is too large to cause the heat transfer process of the work mass to be insufficient. Therefore, the instantaneous efficiency gradually stabilizes again.

As seen from Figure 12, the instantaneous efficiency of the collector gradually decreases, and the decrease rate increases as the inlet temperature of the mass increased under the standard mass flow rate. It is due to the fact that the steady-state test conditions during the ambient temperature are stable at about 25 °C. As the inlet temperature of the mass gradually increases, the temperature difference between the mass and the environment gradually increases, resulting in a gradual increase in the heat loss of the collector and a gradual decrease in the actual heat absorbed by the mass, hence a steady reduction in the instantaneous efficiency.

The heat transfer factor of a collector is a dimensionless parameter that integrates the heat transfer performance of the collector’s heat-absorbing plate and the convective heat transfer coefficient between the working fluid and the heat-absorbing plate to affect the thermal performance of the collector. Figure 13 shows the variation of the collector heat transfer factor with a mass flow rate under the work mass flow rate increasing and the increase rate gradually decreasing. In addition, the work mass flow rate gradually increases, and the collector outlet work mass temperature gradually decreases. Then, the heat transfer process becomes less and less adequate, resulting in a gradual decrease in the error caused by using the outlet work mass temperature instead of the average temperature of the heat absorption plate.

5. Discussions

5.1. Calculation Conditions

Combined with the parameter information provided by the collector manufacturer, in this paper, aluminum is the numerical calculation of flat plate solar collector shell material. The heat-absorbing plate and collector tube material are copper, the insulation material is polyurethane, the heat transfer medium is water, and the transparent cover plate is ETFE film and tempered glass. The physical parameters of the materials are shown in

Table 4. Table of physical properties of materials for simulation.

Parameters	Aluminum	Copper	ETFE Film	Glass	Polyurethane	Air
Density	2770	8800	1750	2220	30	1.23
Specific thermal capacity	875	420	1050	890	1700	1006.43
Thermal conductivity	177	401	0.32	1.20	0.04	0.02
Reflectance	0.08	0.08	0.02	0.07	-	-
Transmittance	-	-	0.95	0.92	-	-

.

According to the mathematical model built in Section 4 of this paper, for the collector, adiabatic boundary conditions are used for the sides and back side, and mixed boundary conditions are used for the top. The collector tube and the heat-absorbing plate adopt the fluid–solid coupling boundary conditions. The simulation calculation assumes that the material physical parameters do not vary with temperature and that the air only has density variation. Water is an incompressible fluid. The discrete ordinates radiation model is used for the calculation, and the solar radiation is loaded using the solar ray tracing model. The ambient temperature can be set to 20 °C, the solar irradiance is 800 W/m², and the wind speed is 2 m/s during the collector operation. Figure 14 shows the mesh division of the numerical model of the collector.

5.2. Simulation Result

Figure 15 shows the experimental and simulated curves of the inner and outer temperatures of the transparent cover of the ETFE membrane structured collector and the common collector. The results show that the inside cover temperature of the ETFE membrane structure collector is higher, with a maximum temperature of 55.9 °C, under the same environmental and operating parameters. This is due to the lower heat loss at the top of the double transparent cover compared with the single transparent cover, which reduces the convective heat exchange with the external environment. For the inside temperature of the transparent cover, the ETFE membrane structure with a glass cover is much less influenced by the environment than the single glass cover.

When the thermal conductivity of the insulation material is around 0.01 to 0.09 W/(m·K), and the range of thickness is 20–60 mm, the changes in the thermal collector efficiency are shown in Figure 16. When the thermal conductivity of the insulation material becomes more significant, or the thickness of the insulation material becomes smaller, the heat collection efficiency of the collector will show a trend of continuous decline. However, the decline rate will continue to decrease. When λ = 0.01 W/(m²·K) and H = 60 mm, the heat collection efficiency is 67.67%. When λ = 0.09 W/(m²·K) and H = 30 mm, the heat collection efficiency is 60%. The above results show that if the collector’s thermal performance needs to be improved, it is necessary to choose insulation materials with low thermal conductivity and large thickness.

5.3. Simulation Validation

The mathematical model mainly simulates the inlet and outlet mass temperatures and the temperature of each component of the two flat plate solar collectors. This verifies the model’s accuracy by comparing it with the experimental results. The deviation of the simulated results from the experimental results is expressed as root mean square deviation (RMSD).

$$RMSD=\sqrt{\frac{1}{n}{{\displaystyle {\sum }_{i=1}^n\left[\left(X_{sim,i}-X_{\mathit{exp}.i}\right)/X_{\exp .i}\right]}}^2}$$

(20)

The RMSD values between the simulated and experimental results are given in

Table 5. RMSD values of simulated data and experimental data.

RMSD (%)	ETFE Membrane Collector	General Collector
Collector outlet mass temperature	0.65	0.81
Temperature on the inside of the cover	4.57	4.31
Temperature on the outside of the cover	1.24	0.98
Thermal storage tank temperature	1.46	1.82

, ranging from 0.65% to 4.57%. The RMSD value for the outside temperature of the collector cover is significant, 4.57%, probably because the solar irradiance changes more frequently in cloudy weather. There are some peaks between the experimental records and the simulated results. In summary, the mathematical model can accurately predict the temperature and thermal performance of both collectors.

6. Conclusions

In this paper, we establish the mathematical model of the ETFE membrane structured flat plate solar collector, and its thermal performance is tested by experiments and numerical simulation, considering the current situation of low carbon transition in the building field. The effects of different operating and structural parameters on the collector’s thermal efficiency are studied, and the results are as follows:

• The effective transmission absorption product of flat plate solar collectors with an ETFE membrane structure was theoretically derived, and the effect of different thicknesses of the ETFE membrane structure on the heat loss of the collector was analyzed. The double-layer transparent cover has better thermal insulation performance than the single-layer glass cover, and the convective heat exchange with the outside world is weaker, which can better maintain the internal temperature of the collector.
• Compared with ordinary single-layer glass flat plate solar collectors, ETFE film structured flat plate solar collectors have higher heat collection efficiency and lower heat loss coefficient. Under the standard mass flow rate, when the collector inlet mass temperature is equal to the ambient temperature, the maximum instantaneous heat collection efficiency of the ETFE film structured flat plate solar collector based on the light collection area is 72%. Its total heat loss coefficient is 3.94 W/(m²·K). The heat collection efficiency is 18.6% higher than that of ordinary flat plate collectors, and the heat loss is 27.3% lower.
• The instantaneous collector efficiency gradually increases by increasing the collector mass flow rate from 0.006 to 0.02 kg/(m²·s). However, the incremental magnitude gradually decreases, which is caused by the insufficient heat transfer due to the change of the mass flow state from laminar flow to turbulent flow in the collector tube after the mass flow rate increases.
• The numerical simulation verifies the mathematical model’s accuracy and proves high credibility. It is also deduced that when the thermal conductivity of the insulation material becomes more significant, or the thickness of the insulation material becomes smaller, the collector’s heat collection efficiency will show a continuous decrease trend, but the decrease rate will decrease. If the collector’s thermal performance needs to be improved, we should choose insulation materials with low thermal conductivity and large thickness.