Multilayers for efficient thermal energy conversion in high vacuum flat solar thermal panels

doi:10.1016/j.tsf.2021.138869

Thin Solid Films

Volume 735, 1 October 2021, 138869

https://doi.org/10.1016/j.tsf.2021.138869 Get rights and content

Highlights

•
Selective Solar absorbers designed to work in high vacuum flat panels were realized.
•
The refractive index of the single layers was assessed by ellipsometry measurements.
•
Solar to Thermal energy conversion efficiency of 60% at 250 °C was obtained.
•
Stagnation temperature of 380 °C for 1000 W/m2 illuminating power density was achieved.
•
Thermal stability of SSA was demonstrated at 400 °C under high vacuum for four hours.

Abstract

Multilayer absorbers based on Cr₂O₃ and Cr, designed to improve the solar-to-thermal conversion efficiency at mid temperatures in high vacuum flat thermal panel, are fabricated via sputtering deposition on bulk copper substrates and characterized by thermal and optical analysis. The refractive index of the single layers has been measured and used to estimate absorber thermal efficiency at the operating temperatures. Multilayers have been produced via sputtering deposition on bulk copper substrates. The absorber multilayers can be 10% more efficient than the commercial alternative at 250 °C operating temperatures, reaching 380 °C stagnation temperature without sun concentration. The thermal stability has been checked at temperature of 400 °C in vacuum for four hours. High vacuum flat thermal collectors, equipped with the produced selective solar absorbers can obtain unprecedented performances and can give important contribution to the energy transition from fossil fuels to renewable energy for efficient heat production.

Introduction

The energy transition from fossil fuels to renewable energy is an important goal of our society and solar energy can play a key role in such transition [1]. Almost one third of the energy resources in the developed countries is nowadays used for heating and cooling [2,3], and a large fraction of this energy can be provided by highly efficient solar thermal collectors. The most efficient solar thermal collector in the low temperature range is the High Vacuum solar thermal Flat Panel (HVFP) [4] produced by TVPSolar SA [5]. The Selective Solar Absorber (SSA) is the key component in determining the efficiency in HVFP. The SSA idea was introduced at the end of 1950, and thenceforward several works have been devoted to SSA optimization. To fabricate an SSA with an emissivity curve close to the ideal one, different designs have been analyzed by several authors in the past years both for concentrated and unconcentrated applications [6,[7], [8]], such as nanomultilayers[8], ceramic and metal structrures (cermet) [9], nanocermet and self-doped nanocermet [10], multilayers [11], photonic designs [12], multilayered cermets [13]. In particular, its thermal emittance determines the radiation losses, that represent the main loss source thanks to the presence of high vacuum. As explained by Cao [9] at temperature higher than 100 °C without concentration, the thermal emittance gains importance over the thermal absorption. However, all absorbers available in commerce [14,15] and most of the absorbers present in literature [16,17] have been developed to work for concentrated solar applications [18] or for low temperature (< 100 °C) applications: in both conditions, to obtain high efficiency, the absorptance is more important than thermal emittance.

As pointed out by Moss and coworkers [19], HVFPs can be the most efficient collectors also at temperature up to 250 °C if they were equipped with an optimized SSA with thermal emittance of 0.04. Unfortunately, commercially available absorbers have such low emittance value only at room temperature. Their emittance increases to 0.1 or more at temperatures higher than 200 °C resulting in an increase of radiative power losses, with a consequent decrease in efficiency. Very few scientific studies have been performed on SSAs for unconcentrated applications. The only SSAs developed to work in unconcentrated environment and temperature higher than 100 °C, are reported in [20,21]. The SSA based on semiconductor multilayer developed in [20] only reaches 220 °C as maximum temperature and authors claim that it could optimized to reach 300 °C maximum temperature. A SSA based on metal/dielectric multilayer, reported in [21], was deposited by electron beam evaporation technique (e-beam) on a glass/metal substrate. The glass substrate was chosen to perform ellipsometric characterization avoiding the roughness contribution to the optical response. Results were very promising: a very selective emissivity curve was obtained opening the route to high efficiencies in the temperature range (150-300) °C without concentration. However, the produced samples could not be measured in operating conditions [21] and preliminary results on copper bulk substrates indicated that the temperature stability is not adequate to mid temperature applications.

Nowadays, a commercially available SSA (Mirotherm from Alanod) is mounted in HVFPs, providing very good efficiency up to 150 °C operating temperatures. In the present study we present SSAs based on metal (Cr) and dielectric (Cr₂O₃) multilayer deposited on Cu industrial substrate via DC magnetron sputtering, designed to work at temperatures higher than 150 °C. They have been characterized in operating conditions by absorptance and thermal emittance measurements. SSA thermal stability was also verified at 400 °C under vacuum for 4 hours.

Section snippets

Experimental and theoretical details

The most important properties of a SSA are the spectrally averaged absorptivity $\bar{α}$ (also referred to as solar absorptance) and the spectrally averaged emissivity $\bar{ε} (T)$ (also referred to as thermal emittance). They are defined and calculated according to the following Eqs. [22]: $\bar{α} = \frac{\int_{0 μ m}^{\infty μ m} α (λ) S (λ) d λ}{\int_{0 μ m}^{\infty μ m} S (λ) d λ}$ $\bar{ε} (T) = \frac{\int_{0}^{\infty} ε (λ) E_{b b} (λ, T) d λ}{\int_{0}^{\infty} E_{b b} (λ, T) d λ}$ where S(λ) (Wm⁻²μm⁻¹) is the solar radiation spectrum and E_bb(λ, T) (Wm⁻²μm⁻¹) is the blackbody radiation spectrum depending on the radiation wavelength

Results and discussions

Using the refractive indices reported in the Section 2.2, it is possible to predict the reflectivity of multilayer as function of the layer thicknesses. Then, using Eqs. (3), (1) and (2) is possible to calculate the spectrally average emissivity and absorptivity. In Fig. 4 a) we report the result for two coatings designed to work at Operating Temperatures (OT) higher than 150 °C (coating A: OT = 200 °C and coating B: OT = 300 °C). The reported thermal emittance is extremely low. To obtain such

Conclusions

The use of Cr based multilayers to produce selective solar absorbers paves the route to the application of high vacuum flat panels as very efficient thermal solar collectors at temperature up to 250-300 °C. The multilayers are fabricated via sputtering deposition techniques and their fabrication process can be easily transferred to industrial deposition systems.

The presented selective solar absorbers have a very low emittance preserving a large absorptance and reaching stagnation temperature

Credit author statement

Davide De Maio: Investigation, Data Curation, Software, Writing - Original Draft Carmine D'Alessandro: Investigation, Data Curation Antonio Caldarelli: Investigation, Daniela De Luca: Data Curation, Visualization Emiliano Di Gennaro: Writing - Review & Editing, Supervision Maurizio Casalino: Methodology, Software Mario Iodice: Methodology, Formal analysis, Writing - Review & Editing , Mariano Gioffr`e: Resources, Validation Roberto Russo: Writing - Review & Editing, Supervision, Funding

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

The Ph.D. grant of two of the authors (DDL, AC) is funded by the PON2014-2020 “Dottorati innovativi con caratterizzazione industriale, XXXIV ciclo” program. The Ph.D. grant of one of the authors (DDM) is funded by the CNR-Confindustria “Dottorati di Ricerca Industriali” program XXXIV ciclo.

References (34)

A.J. Chapman et al.
Prioritizing mitigation efforts considering co-benefits, equity and energy justice: fossil fuel to renewable energy transition pathways
Appl. Energy
(2018)
A. Buonomano et al.
Experimental analysis and dynamic simulation of a novel high-temperature solar cooling system
Energy Convers. Manage.
(2016)
M. Bello et al.
Achievements in mid and high-temperature selective absorber coatings by physical vapor deposition (PVD) for solar thermal application-a review
J. Alloys Compd.
(2020)
D. Yang et al.
Enhanced thermal stability of solar selective absorber based on nano-multilayered AlCrSiO films
Sol. Energy Mater. Sol. Cells
(2020)
C. Wang et al.
Solar thermal harvesting based on self-doped nanocermet: structural merits, design strategies and applications
Renew. Sustain. Energy Rev.
(2020)
A.B. Khelifa et al.
Growth and characterization of spectrally selective Cr2O3/Cr/Cr2O3 multilayered solar absorber by e-Beam evaporation
J. Alloys Compd.
(2018)
X.-L. Qiu et al.
Optical design, thermal shock resistance and failure mechanism of a novel multilayer spectrally selective absorber coating based on HfB2 and ZrB2
Sol. Energy Mater. Sol. Cells
(2020)
Y. Ning et al.
NiCr–MgF2 spectrally selective solar absorber with ultra-high solar absorptance and low thermal emittance
Sol. Energy Mater. Sol. Cells
(2020)
H.C. Barshilia et al.
Deposition and characterization of TiAlN/TiAlON/Si3N4 tandem absorbers prepared using reactive direct current magnetron sputtering
Thin Solid Films
(2008)
T.K. Tsai et al.
Spectral properties and thermal stability of CrN/CrON/Al2O3 spectrally selective coating
Thin Solid Films
(2016)

K. Xu et al.

A review of high-temperature selective absorbing coatings for solar thermal applications

J. Materiomics

(2020)

R.W. Moss et al.

Performance and operational effectiveness of evacuated flat plate solar collectors compared with conventional thermal, PVT and PV panels

Appl. Energy

(2018)

G. Contoux et al.

Deposition process study of chromium oxide thin filmsobtained by d.c. magnetron sputtering

Thin Solid Films

(1997)

M.F. Al-Kuhaili et al.

Optical properties of chromium oxide thin films deposited by electron-beam evaporation

Opt. Mater.

(2007)

R. Boidin et al.

Pulsed laser deposited alumina thin films

Ceram. Int.

(2016)

Fleiter, T., Elsland, R., Rehfeldt, M., Steinbach, J., Reiter, U., Catenazzi, G., Jakob, M., Rutten, C., Harmsen, R.,...

L. Kranzl, M. Hartner, A. Müller, G. Resch, S. Fritz, T. Fleiter, A. Herbst, M. Rehfeldt, P. Manz, and A. Zubaryeva,...

Cited by (15)

High vacuum flat plate photovoltaic-thermal (HV PV-T) collectors: Efficiency analysis
2023, Applied Energy
Through a numerical model developed in MATLAB, we investigate the performance of a novel hybrid flat plate photovoltaic-thermal collector under high-vacuum (HV PV-T) to optimize the solar-to-thermal energy conversion and efficiently meet the thermal loads of industrial processes up to 150 °C along with additional production of electrical energy. In the proposed design, the photovoltaic (PV) cell is positioned directly above the Selective Solar Absorber (SSA) in a multi-layered PV-SSA structure.
The performance analysis of the system has been first carried out by considering the theoretical Shockley-Queisser limit of the electrical efficiency, different values of energy bandgap (0.66 eV $\leq E_{g} \leq 3.00 eV$ ), emittance ( $0.1 \leq ɛ \leq 1$ ), and working temperature ( $25 °$ C $\leq T_{P V} \leq 175 ° C$ ) of the PV layer, and secondly by focusing on a wide variety of actual semiconductive materials. We analyzed the specific case of high bandgap materials, such as CdTe, CdS, and GaAs reported in previous publications. The analysis performed shows that full exploitation of the incident solar radiation with HV-PVT collectors can produce about 12.2% of electrical efficiency while 76.4% of the incident power remains available for thermal conversion at 100 °C. Moreover, obtaining the same annual energy using stand-alone solutions (i.e., a combination of the best PV and solar thermal collectors commercially available) would require up to 50% of the collector area more than the proposed HV PV-T collector.
Erratum: Enhancing the fuel saving and emissions reduction of light-duty vehicle by a new design of air conditioning worked by solar energy (Case Studies in Thermal Engineering (2022) 30, (S2214157X22000442), (10.1016/j.csite.2022.101798))
2023, Case Studies in Thermal Engineering
Increasing the demand of using fossil fuel is started appear in the recent decades, especially in the transport sector such as diesel engines. Normally, air conditioning (a/c) increases the fuel consumption about 20% in engines due to extra load needs for a/c operation. Therefore, the manufactures search more towards control strategies to reduce the fuel consumption by developing more efficient a/c system. In this study, a new design of a/c system working by solar energy was used to reach the requirement of energy and improve the fuel economy of vehicles. The a/c system was linked with flexible photovoltaic cell (PV) to compensate the electricity that generated from vehicle engine. Furthermore, the fuel consumption from a/c system (operated with and without solar energy) was measured for two vehicles. In addition, the temperature of cooling and heating was measured inside and outside room of the car. The a/c system was tested in the saloon car for both conditions of cooling and heating and. It was found that the a/c system working with solar energy reduced the engine fuel consumption by 25% in comparison with conventional a/c system. The results from a/c system showed that the cooling temperature inside cabin of car through hot day is 20 °C within half a minute, while the temperature outside is 40 °C. In contrast, the heating temperature was reach to 49.2 °C for long period of time compared to the cooling temperature. The harmful engine emissions carbon monoxide (CO), carbon dioxide (NO_X) and total hydrocarbons (THCs) were reduced when a solar a/c system presented in comparison with conventional design of a/c.
Thermal treatment of stainless steel substrates along with a thin silicon layer for high temperature solar thermal applications
2023, Solar Energy
Solar energy harvesting in solar thermal systems using different solar absorber coatings on collectors has been widely studied. Here, we incorporate a single layer Si (∼250 nm) onto one of the most common and cost-effective collector materials, i.e., stainless steel (SS304) to improve its optical performance by thermal treatment. During the heating cycle, the samples were heated at 900 °C for short durations and suddenly quenched to room temperature in air. A high solar absorptance of 0.92 along with an emittance of 0.37 were obtained after heat-treatment. The reflectance studies showed reduced uniform reflectance (<10%) over the entire UV–Vis-NIR region and high reflectance in the far IR region. The morphology, composition, and crystal structures were studied in detail. The Si layer and the quenching process aided in the formation of a highly irregular microstructure consisting islands of Cr-Fe-Mn-Si oxides and silicides (with trace amount of Ni) which in turn increases surface roughness. This irregular array of islands of varying sizes and varying gaps between them helps in increasing absorptance due to multiple reflections, scattering and diffraction effects. Similar studies carried out on SS202 substrates resulted in different surface morphology and hence different absorptance (0.84), suggesting that the surface morphology and absorptance are highly dependent on the substrate composition. The initial thermal stability data on heat-treated Si/SS304 suggests stability up to 700 °C in air for 1000 hrs. Our studies indicate that thermal treatment of Si/SS304 for a short duration is a plausible solution to develop cost effective high temperature solar thermal collectors.
Detailed studies on sputter-deposited Hf and HfC thin films for solar energy devices
2023, Solar Energy Materials and Solar Cells
Thin films of hafnium and hafnium carbide were deposited by pulsed magnetron sputtering of a Hf cathode in an atmosphere of argon and acetylene (C₂H₂) at room temperature. Different flow rates of C₂H₂ gas were injected into the chamber to form distinct HfC_x compounds and to analyze changes in their structural, chemical, morphological and optical properties in the UV-VIS-NIR regions. The analyzed samples are flat, with an average roughness of less than 1 nm. The refractive index of HfC_x films shows a weak dependence on the chosen C₂H₂ flow rates, whereas the extinction coefficient decreases significantly with increasing gas flow rate, making the use of this material attractive in many applications. The accurate knowledge of the complex refractive index of this material is essential to fill the gap in the current literature and to accurately design optical structures for energy conversion purposes, such as selective solar absorbers (SSAs), selective emitters (SE), dielectric mirrors (DM). We have designed and optimized SSAs based on HfC_x, Si₃N₄, and SiO₂ that maximize the solar-to-thermal energy conversion efficiency in high vacuum solar thermal panels.
Modeling of energy and exergy efficiencies in high vacuum flat plate photovoltaic–thermal (PV–T) collectors
2023, Energy Reports
This work deals with the performance evaluation of novel flat photovoltaic–thermal (PV–T) modules under vacuum. Through a 1D (dimensional) steady-state-energy-balance numerical model developed in MATLAB, two different layouts are studied: the first consisting of a photovoltaic (PV) cell installed just below the glass encapsulating the flat panel, and the second where the PV cell is placed on the selective solar absorber (SSA). In both cases the thermal and electrical efficiencies have been evaluated at different SSA operating temperatures, in the range of 323 K to 423 K. The analysis has been conducted at different energy bandgap ( $E_{bg}$ ) of the PV cell and assuming a variable transmittance or emittance of the PV cell, depending on the design. The two systems efficiency comparison has been carried out at the same operating temperature. Overall, this work highlights the importance of high vacuum insulation, which guarantees the reduction of convective thermal losses, and shows that the maximum energy is produced for PV cells with $E_{bg}$ $\approx$ 1.5-1.7 eV, depending on layout and operating temperature, by including the thermal output in the PV–T optimization. The energy and exergy efficiencies obtainable using the proposed PV–T systems are considerably improved compared to the results previously reported in the literature.
Low emissivity thin film coating to enhance the thermal conversion efficiency of selective solar absorber in high vacuum flat plate collectors
2023, Thin Solid Films
Citation Excerpt :
Previous works [7–19] have made progress in improving the optical properties of a solar absorber for mid-high temperature applications: in [16] Ag–Al2O3 nanocermet SSA coatings prepared at different Ag contents on copper, silicon, and glass substrates exhibited α = 0.93 in the visible region and ε = 0.04 – 0.05 at 82 °C in the infrared region of the solar spectrum, but the thermal stability test at 400 °C showed degradation in the vacuum annealed coatings due to size decrease of the Ag nanoparticles.
Industrial heat and cooling applications are an essential fraction of the overall energy demand, mainly produced by fossil fuels. Solar thermal energy production can satisfy such a need by adopting the High Vacuum Flat Plate Collectors (HVFPCs) and increasing their efficiency. The absorptance and emittance of Selective Solar Absorbers (SSAs) determine the thermal efficiency of HVFPCs. Being the absorptance already maximized, the thermal emittance of the absorber should be minimized to increase further the operating temperature of the collector and its efficiency. This research aims to reduce the thermal emittance of commercially available Selective Solar Absorber by depositing a thin silver film on the aluminium substrate. So, in this work, the thermal stability of a silver coating has been investigated, and a diffusion barrier layer has been adopted to stabilize the coating performance up to 360 °C. The low-emissive layer of Ag and a diffusion barrier of CrO_x guarantees a decrease of 11% in thermal emittance at 200 °C of commercially available SSA deposited on aluminium. Further emittance reduction can be obtained by depositing a thin Ag film on both sides of the aluminium substrate before the SSA deposition, proving to be a promising way to enhance the efficiency of HVFPCs.

View all citing articles on Scopus

View full text

Multilayers for efficient thermal energy conversion in high vacuum flat solar thermal panels

Highlights

Abstract

Introduction

Section snippets

Experimental and theoretical details

Results and discussions

Conclusions

Credit author statement

Declaration of competing interest

Acknowledgements

Appl. Energy

Energy Convers. Manage.

J. Alloys Compd.

Sol. Energy Mater. Sol. Cells

Renew. Sustain. Energy Rev.

J. Alloys Compd.

Sol. Energy Mater. Sol. Cells

Sol. Energy Mater. Sol. Cells

Thin Solid Films

Thin Solid Films

J. Materiomics

Appl. Energy

Thin Solid Films

Opt. Mater.

Ceram. Int.