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Optimization design of a flexible absorption device for solar energy application

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e-Polymers
From the journal e-Polymers Volume 17 Issue 3

Abstract

With the fast development of solar energy harvesting technology, various portable harvesters with excellent energy capture properties are highly desired, especially those devices used for facades and roofs of buildings. This study proposed a new energy absorption model by integrating wave-guiding polymer materials and fluorescent substance coated textiles, which aimed to address the issues of current energy shortage and excessive pollutant emissions in the ambient environment. Besides, this textile-based production was multifunctional in that it possessed the compelling features of solar radiation collection, thermal insulation and surface decoration. Compared with the conventional solar energy harvester, the as-fabricated device exhibited the merits of light weight, better mechanical flexibility and wider extended applicability. This study has achieved some progress in the area of solar energy harvesting and could have a possible informative effect on the future related research.

Keywords: flexible absorption device; fluorescent dyestuff; solar energy harvesting; thermodynamics; wave-guiding polymer material

1 Introduction

With the increasing consumption of natural energy resources, new energy-saving buildings have been developing very fast in recent decades that are alleviating the energy crisis and environmental pollution problems (1). As reported, nearly 40% of the total energy in the world is consumed in the built environment field, predominantly for heating, cooling, ventilation, and lighting (2, 3). Several studies thus focused on developing sophisticated solar energy products which can generate heat for rooms by absorbing sunlight and at the same time reduce the heat loss of the building (4). Meanwhile, fluorescent solar collectors have been considered as a promising technology to make solar energy much more effective in the built environment (5, 6).

With the light weight and soft features, textile materials used for solar energy absorption have many unique advantages compared with traditional solar collectors, especially when applied to outside walls and roofs of buildings (7). It is highly desirable to obtain higher conversion efficiency from solar energy to thermal energy that has been relying on textile-based material for decades (8, 9). In addition, traditional solar collector plates commonly have some intrinsic problems so that they are not well integrated with the outside buildings from the aesthetical perspective. Alternatively, the flexible fluorescent products could provide an innovative approach for solar energy absorption, due to the fact that they are coated with fluorescent dyestuff and thus possess the characteristics of functionality and aesthetics (10). Therefore, these textile-based materials are suitable for the potential application to the outside walls and roofs of residential buildings, which can work to supply heating and look better.

Nowadays, a tremendous amount of work has been done to improve the concentration ratio of solar thermal products in theory and even in reality, for example, by using optimized geometrical devices and matched luminescent concentrators (11, 12). Conventional geometric collectors have inevitable limitations due to the working principles of thermodynamics. That means energy harvesting inefficiency when they are exposed to concentrated diffuse light (13). Specifically, they demonstrate maximum concentration at close to the square of the refractive index of the material in which the concentrator is embedded. The assumption of thermal conversion with fluorescent concentrator was proposed by Stahlin in 1985, as these devices can absorb a special solar spectrum and emit for relatively narrow wavelength ranges (14). The preparation and selection of fluorescent dyes deserves great concern, as different fluorescent dyes can absorb, diffuse and direct sunlight from different regions of the solar spectrum, and emit light to a longer wavelength by Stokes shift (15). It should be noted that the fluorescence process occurs with releasing heat. Apart from absorbing the extra released heat, the base of the collector traps a large fraction of the emitted light by total internal reflection and subsequently converts it into thermal energy (16). Additionally, the emitted light in longer wavelengths has a stronger thermal effect compared with absorption light. Therefore, the combination of suitable absorption materials and high efficient fluorescent materials can achieve both high photo-thermal conversion efficiency and high utilization efficiency for solar energy (17). The mechanism principle of the current planar fluorescent solar collector is to utilize a fluorescent medium to improve the solar radiation transmissivity and then collect such radiation through wave-guiding materials by total reflection (18). However, planar fluorescent solar thermal collectors, widely used at present, are made of rigid and heavy material, which results in immobility.

The present paper reports a complete study using a flexible fluorescent product for solar energy harvesting, with the aim of developing a novel flexible solar thermal collector both for heating and decorating the targeted buildings. The thermal collection efficiencies of the wave-guiding material were measured and compared in cases of having and not having a fluorescent textile layer. The performance of the fluorescent textile material has been characterized and optimized after both parameters of high photo-thermal conversion efficiency and better warmth retention are considered. Furthermore, the expected solar energy harvesting performance has been confirmed by testing the relevant properties in the real ambient environment. This study has achieved some progress in the area of solar energy harvesting and could have a possible informative effect on the future related research.

2 Experimental

2.1 Working principle of the fabricated device

From Figure 1 the detailed structure and work principle of the designed device is shown. The flexible solar thermal collector consists of a fluorescent textile layer and an absorption layer. As described previously, the fluorescent textile layer contains fluorescent materials which convert the absorbed sunlight into the emission of a specific wavelength and is also responsible for thermal insulation. The chemical reaction occurring on the fluorescent molecule is

[1]photo(e1)→photo(e2)+heat

where e2<e1. According to the thermodynamic theory, the fluorescent system operates like an optical pump by changing some incoming sunlight into heat energy. Moreover, the absorption and emission spectra of the fluorescent layer should ideally not overlap the solar collector, with the aim of increasing heat generation. This heat generation property is dependent upon the broadness of spectral peaks and the distance between their maximums, which is termed the Stokes shift. As the conversion refers to the number of photons, the value of the Stokes shift, Δγ, leads to the photo-thermal conversion efficiency (19). Hence, the energy conversed, Qs, is calculated by,

[2]Qs=QaΔγγa

where Qa is the incident optical energy, γa is the frequency of absorbed light, Δγ is the frequency shift. Part of the emitted light will be lost at the surface and the rest will be trapped through total internal reflection by the absorption layer, which also converts the concentrated light into thermal energy. This process endows the fluorescent solar collector with the ability of concentrating the maximum amount of light at its bottom surface for thermal energy generation.

Figure 1: The solar collector device was fabricated by composition of fluorescent fabric (A), adhesion film (B) and PMMA base (C). Illustration of different types of light: ①incident light ②reflected light ③absorbed by fluorescent substance ④fluorescent light ⑤absorbed by absorption layer ⑥transmitted light ⑦absorbed by material.
Figure 1:

The solar collector device was fabricated by composition of fluorescent fabric (A), adhesion film (B) and PMMA base (C). Illustration of different types of light: ①incident light ②reflected light ③absorbed by fluorescent substance ④fluorescent light ⑤absorbed by absorption layer ⑥transmitted light ⑦absorbed by material.

The critical factors to improve the properties of photo-thermal conversion include three aspects: 1, the high Stokes shift of the fluorescence coating, which can produce sufficient energy; 2, restricting the light from overflowing from the collector; 3, high absorption efficiency of the absorption layer. To solve these three problems, many efforts have been implemented for optimization of fluorescent dyestuff selection and utilization of wave-guiding polymer materials.

2.2 Fabrication process and design optimization

In order to achieve high thermal insulation properties, the textile layer was designed into the system. The fabric structure of the textile layer is selected as a non-woven structure, because the non-woven fabric is simple to fabricate and is processed in a short time. Additionally, the random arrangements of fibers in this structure contributes to the efficient absorption of incident light from different directions and high resistance of inner thermal diffusion by heat radiation (20). Moreover, the fabric is 250 g/m2 in density, and 2 mm in thickness. When it comes to the selection on fibers for the textile layer, the hollow PET fiber and non-hollow PET fiber were both selected so as to compare their thermal insulation properties. The fibers were supplied by Shaoxing sylon textile technology Co. Ltd, China.

To improve the fluorescent property of the fabric, six kinds of organic fluorescent species from Yaotax Shing technology Co., Ltd. (Shenzhen, Guangzhou,China) were used to coat the fabric surface, including YD-13 (denoted here “red”), YD-17 (denoted as “yellow”), YD-18 (denoted as “green”), YD-19 (denoted as “blue”), YD-20 (denoted as “purple”), YD-21 (denoted as “pink”). Specially, these fabrics were coated with a matrix of 5% polyvinyl acetate (PVA) in alcohol, into which fluorescent dyes were dispersed.

The coated surface of the fiber focuses incident sunlight into the fiber core, where the short wavelength sunlight can be converted into a longer wavelength (21). Through this continuous process, more absorbed energy particularly in the region of ultraviolet can be effectively utilized. Hence, the photo-thermal conversion property of the device has been improved. Moreover, it shows a reduced heat loss by convection owing to its structure. Heat loss by the emission of long-wave radiation can be prevented by a suitable low-emission coating.

The fluorescent dyestuff interaction affects the extinction at higher concentrations as well as specific solubility. Accordingly, the fluorescent dyestuff concentration of each group was 0.1 g/l in the ensuing experiments.

Considering the efficiency of solar absorption, the black poly(methylmethacrylate) (PMMA) base was adopted to absorb the thermal energy. Combining the fluorescent fabric and PMMA material can overcome the problems mentioned above on the wave guide property. The application of higher refractive index materials is an obvious way of decreasing the wave guide losses. In addition, the PMMA base stands with refractive indexes in the order of 1.49. Serving as the absorption layer, it also helps to increase the absorption of basement according to the principle of total reflection.

Copolyamides adhesion agent which was supplied by Shijiazhuang jinghua lining cloth Co., Ltd, China was selected to assemble these two parts through a lamination process by consideration of its low melting point. Considering the PMMA material’s limited heat resistance and thermal stability, the laminated temperature and time was set at 140°C and 80 s, respectively. After the lamination process, the fluorescent fabric was combined with the PMMA base. The size of the solar collection device is 20×25 mm and the total average thickness is 2.5 mm. In order to explore the conversion property of the solar collector, the device had to be measured in the natural environment. Specifically, the device was sealed into an insulated box, the volume of which is 4.2×10-3 m3.

2.3 Characterization methods

The morphology of each of the fabric samples was examined by a Hitachi scanning electron microscope (Model “S-4800”). Additionally, their fluorescent spectra were measured by a fluorescence spectrometer.

The absorption spectrum of each of the samples was measured by a ultraviolet-visible spectrophotometer. The transmission of the fabric is responsible for the solar utilization. The whole solar energy can be assumed, except the emergent light. The emergent light includes the reflection part of the initial incident light R(λ) and escape part in other ways E(λ). Therefore, via

[3]η(λ)=1-R(λ)-E(λ),

the spectral utilization efficiency η(λ) can be determined. The energy of the sunlight received by earth varies according to the wavelength, whose value can be obtained by the ASTM reference spectra (AM1.5). Therefore, the theatrical photo-thermal utilization percentage of the absorption layer can be calculated depending on the wavelength (22) by

[4]P(λ)=∫η(λ)AM1.5G(λ)dλ∫AM1.5G(λ)dλ,

where η(λ) means the spectral utilization efficiency, and AM1.5G(λ) means the solar spectrum in the condition of AM1.5. The solar energy harvesting property can be measured forthwith by comparing the temperature in and outside the insulation box, which was heated by the fluorescent solar collector. The temperature measuring element, whose resolution is 0.1°C, was installed both in indoor and outdoor environments. The solar collector and temperature measuring element outside had been ensured the same environment of coincident sun exposure. Additionally, the dates of measurement were in July, and observation time was from 11 a.m. to 3 p.m. roughly. The solar energy transformation efficiency of the flexible fluorescent product (p) can be calculated by

[5]p=cmΔTS⋅t

where c, m, ΔT are the specific heat capacity, mass, temperature variable of the air in the insulated box, S is the surface area of the absorption device and t is the measuring time, respectively.

3 Results and discussion

3.1 Thermal conductivity of textile layer

The comparison of thermal insulation properties between the non-woven material made by hollow PET fiber and stuffed PET fiber is shown in Table 1. The thermal conductivity of the textile layer used by the hollow PET fiber can be decreased to nearly half, compared with non-hollow PET fiber. Therefore, the selection of the hollow PET fiber helps to improve the thermal insulation property of the device, because the hollow structure of fibers lets the radiation through, but blocks the convection and conduction heat losses.

Table 1:

Thermal conductivity of the non-woven material made by hollow PET fiber and non-hollow PET fiber.

Non-woven material made by hollow PET fiberNon-woven material made by non-hollow PET fiber
Thermal conductivity (W/m2×°C)5.8910.13

3.2 Fluorescent properties of the textile layer

The fluorescent medium strongly absorbs light in a certain range of the solar spectrum and then emits it at longer wavelengths, where it is weakly reabsorbed. The fluorescent properties of fabrics coated by different fluorescent dyes were compared through the fluorescent spectra of each of the samples shown in Figure 2.

Figure 2: Normalized absorption spectra of samples coated by different fluorescent dyestuff, including red dyestuff (A), yellow dyestuff (B), green dyestuff (C), blue dyestuff (D), purple dyestuff (E), pink dyestuff (F).
Figure 2:

Normalized absorption spectra of samples coated by different fluorescent dyestuff, including red dyestuff (A), yellow dyestuff (B), green dyestuff (C), blue dyestuff (D), purple dyestuff (E), pink dyestuff (F).

The maximum UV absorption wavelength of pink dyestuff is 321 nm, while the maximum of emission spectra is 374 nm. Also the maximum of the emission spectra is higher than the other dyestuffs, at 376 nm. More importantly, the band distance of the two wavelength peaks is the widest. According to the value of the Stokes displacement and variation frequency illustrated in Table 2, the conversion property of the fabric coated by pink dyestuff is stronger than those of other coated samples. By contrast, the conversion property of yellow dyestuff is the weakest, with the least variation frequency.

Table 2:

Fluorescent property of samples.

Stokes displacement value (nm)Δγ (Hz)
Red521.34×1014
Yellow451.16×1014
Green471.16×1014
Blue531.34×1014
Purple531.32×1014
Pink551.36×1014

According to Tributsch’s original work (23), a characteristic “luminescence gap” exceeding 1×1014 Hz is a precondition for efficient fluorescent light collection and has been recognized as an evolutionary adaptation contributing to the optical heat pump. Therefore, it indicated that fluorescent properties of different samples coated by each dyestuff are similar and all meet the requirements. Moreover, the sample coated with pink dyestuff matched the expected functions best and was selected for the ensuing experiments.

3.3 SEM morphology

Figure 3 shows the surface SEM micro-graphs of untreated materials and materials coated with pink dyestuff, and the changes within the surface morphology after the treatment processes. The fiber surface shows almost no significant change and the insoluble fluorescent particles, whose molecular size is on the nanoscale, were clearly observed. This result indicates that the effect of the treatment was mainly on the surface structure of the fibers.

Figure 3: SEM examination micrographs of ramie fibers (magnification×600); (A) untreated material (B) material coated by pink dyestuff.
Figure 3:

SEM examination micrographs of ramie fibers (magnification×600); (A) untreated material (B) material coated by pink dyestuff.

3.4 Optical properties

In Figure 4 the comparison between the coated textile layer and the untreated textile layer on reflectivity and transmissivity is presented. With transmission of sunlight to the absorption layer increasing, the utilization efficiency of solar collector has been promoted after modification. To be specific, the transmission efficiency of the sample coated with pink dyestuff is approximate to that of the untreated sample in the range of ultraviolet, but it has been improved dramatically in the range of 350–700 nm. This was in agreement with scanning electron microscopy (SEM) analysis of coated fibers. Therefore, the utilization efficiency of the visible part has been improved markedly by fluorescent dyestuff coating.

Figure 4: Comparison of reflectivity (A) and transmissivity (B) of untreated hollow PET textile layer (a), hollow PET textile layer coated by pink dyestuff (b).
Figure 4:

Comparison of reflectivity (A) and transmissivity (B) of untreated hollow PET textile layer (a), hollow PET textile layer coated by pink dyestuff (b).

Figure 5 indicates the solar energy utilization percentage of both samples after calculation. The solar energy utilization percentage of the coated sample was improved markedly more than the untreated material. Specifically, 89% of the UV part energy can be utilized by the coated textile layer in comparison to 60.15% for the untreated textile layer. The solar energy utilization proportion of both textile layers was much lower in the visible light part compared with the UV light part. The photo-thermal utilization percentages of visible light by untreated hollow PET textile layer and coated textile layer are 24.48% and 63.82%, respectively. Therefore, it is demonstrated that the fluorescent fabric made by pink dyestuff was suitable to be manufactured for the solar collection device, as solar energy utilization proportion was significantly improved.

Figure 5: Comparison of solar energy utilization percentage of untreated hollow PET textile layer (A), hollow PET textile layer coated by pink dyestuff (B).
Figure 5:

Comparison of solar energy utilization percentage of untreated hollow PET textile layer (A), hollow PET textile layer coated by pink dyestuff (B).

3.5 Solar harvesting efficiencies

As mentioned above, a temperature measuring system was built to simulate the solar energy harvesting performance of the flexible fluorescent solar collector in the real ambient environment. Additionally, the photo-thermal conversion property between the flexible absorption device for solar energy application and pure black PMMA base had been compared. Specifically, the textile layer of the device was coated by pink dyestuff.

According to Figure 6, it indicates that both of the temperatures detected by the measuring system began to increase gradually from the start in spite of some fluctuation. The heating effect of the flexible absorption device was superior compared with the pure black PMMA base. To be specific, the mean temperature difference with and without the assistance of fluorescent textile layer is 17.1°C. Moreover, the maximum temperature difference between the flexible fluorescent solar collector and ambient environment was 29.4°C. However, that was only 12.4°C for the pure black PMMA base. Therefore, the solar energy harvesting of the pure black PMMA base had been significantly improved after combined with the fluorescent textile layer. Moreover, the warmth keeping property of the device had been enhanced simultaneously.

Figure 6: Measuring of the photo-thermal conversion property of the flexible fluorescent solar collector by comparison the temperature of the box which heating by solar collector (A), pure black PMMA base (B), and outdoor environment (C).
Figure 6:

Measuring of the photo-thermal conversion property of the flexible fluorescent solar collector by comparison the temperature of the box which heating by solar collector (A), pure black PMMA base (B), and outdoor environment (C).

If the amount of solar radiation was sustained as a constant value, the detected air temperature below the position of the flexible fluorescent device would increase firstly and then reach a stable value. However, the intensity of the solar radiation began to decrease after 13:00 p.m. in the natural environment; also the detected temperature (A in Figure 6) began to decrease at 14:00 p.m., which lasted for 3 h. As can be seen from Figure 6, the plotted line A exhibited a monotonous increasing trend in the first 3 h. It is noted that the solar energy transformation efficiency can be calculated in accordance with the amount of the raised absorbed energy of the air in the thermal insulated box. As the main temperature increased 13°C per hour, the solar energy transformation efficiency of flexible fluorescent product is approximately 1.9 J/h·m2 in the first 3 h.

It has to be admitted that the photo-thermal conversion property of the flexible fluorescent products for solar energy absorption is limited compared with current solar collectors. Nevertheless, the feasibility and application prospect of this device have been approved. Furthermore, the applicability of similar solar collector products has been greatly enhanced as a variety of colors can be used.

4 Conclusion

In this paper, we introduced a new type of flexible absorption product for solar energy application which possessed a number of advantages. The fluorescent fabric made of hollow PET fiber and a PMMA base were combined tightly through a lamination process and actualized the multi-function of high solar energy harvesting and high thermal insulation.

Additionally, the photo-thermal conversion efficiency of pink dyestuff was proved to be superior to any other dyestuff and the Stokes displacement value was almost the best case for the utilization for the fluorescent concentrator. Through a fluorescent coating process, a higher solar collection efficiency of the fabric can be achieved by improving the transmission efficiency of ultraviolet in solar energy. The theoretical photo-thermal utilization percentage of the hollow PET material can be enhanced to be 66.66% after pink dyestuff coating. Apart from that, the expected thermal collection property of the flexible absorption device has been verified by testing the solar energy harvesting performance in the regular natural environment. The solar energy transformation efficiency of the flexible fluorescent product can reach approximately 1.9 J/h·m2 through calculation. If matched with an appropriate circulation system with air or water as medium, the expected function of heating for residental buildings can be achieved.

To sum up, the experimental results demonstrated that the as-developed flexible fluorescent products for solar energy absorption exhibited better photo-thermal efficiency. Due to their comparatively lightweight, good mechanical flexibility and favorably low cost, they have broad prospects for future applications in the areas of outside wall thermal insulation material and roofs of energy-converting textiles. Moreover, the decorative function of flexible fluorescent product made by textile layer has been great improved depending on different dyestuffs. This has made some advances in the field of flexible solar collector research, and it will definitely help to produce cost effective and efficient solar based photo-thermal conversion systems.

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Received: 2016-3-23
Accepted: 2016-4-24
Published Online: 2016-5-27
Published in Print: 2017-5-1

©2017 Walter de Gruyter GmbH, Berlin/Boston

This article is distributed under the terms of the Creative Commons Attribution Non-Commercial License, which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.

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  2. In this Issue
  3. Guest editorial
  4. Electrospinning, fibers and textiles: a new driving force for global development
  5. Full length articles
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https://doi.org/10.1515/epoly-2016-0072
Keywords for this article
flexible absorption device; fluorescent dyestuff; solar energy harvesting; thermodynamics; wave-guiding polymer material
Creative Commons
BY-NC-ND 3.0

Articles in the same Issue

  1. Frontmatter
  2. In this Issue
  3. Guest editorial
  4. Electrospinning, fibers and textiles: a new driving force for global development
  5. Full length articles
  6. Study on uniformity of a melt-blown fibrous web based on an image analysis technique
  7. Structure and properties of polytetrafluoroethylene (PTFE) fibers
  8. Modeling of braiding parameter impact on pore size and porosity in a tubular braiding fabric
  9. Optimization design of a flexible absorption device for solar energy application
  10. Synthesis and structure analysis of polyesteramides modified with bio-based diaminopentane hexanedioic salt
  11. Insight into the precursor nanofibers on the flexibility of La2O3-ZrO2 nanofibrous membranes
  12. Fabrication of superhydrophobic nanofiber fabric with hierarchical nanofiber structure
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