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Use of microspheres in thermally insulating hybrid solar panels
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Energy (2018) 000–000 948–953 EnergyProcedia Procedia148 00 (2017) www.elsevier.com/locate/procedia 73rd Conference of the Italian Thermal Machines Engineering Association (ATI 2018), 12–14 September 2018, Pisa, Italy 73rd Conference of the Italian Thermal Machines Engineering Association (ATI 2018), September 2018, Pisa, Italy hybrid solar panels Use of microspheres12–14 in thermally insulating 1 Use of microspheres in thermally insulating hybrid solar panels Matteo Greppi ,Giampietro Fabbri The 15th International Symposium on District Heating and Cooling
Matteo Greppi1,Giampietro Fabbri
Assessing the feasibility of using the heat demand-outdoor D.I.N. ,Università degli Bologna,Viale Risorgimento 2, 40136 Bologna, ItalyBologna, Italy CIRI EDILIZIA E COSTRUZIONI,,Università degli Studi di Bologna ,via Lazzaretto 15/5 , 40136, temperature function forStudia dilong-term district heat demand forecast 1
CIRI EDILIZIA E COSTRUZIONI,,Università degli Studi di Bologna ,via Lazzaretto 15/5 , 40136, Bologna, Italy
1
2
2
I. Andrića,b,c*, A. Pinaa, P. Ferrãoa, J. Fournierb., B. Lacarrièrec, O. Le Correc D.I.N. ,Università degli Studi di Bologna,Viale Risorgimento 2, 40136 Bologna, Italy
Abstract a
IN+ Center for Innovation, Technology and Policy Research - Instituto Superior Técnico, Av. Rovisco Pais 1, 1049-001 Lisbon, Portugal
In this work the problem of thermally insulating a PhotoVoltaic Thermal (PVT) hybrid solar panel is investigated. In particular, a Abstract b Veolia Recherche & Innovation, 291 Avenue Dreyfous Daniel, 78520 Limay, France c PVT panel designed to be laid on the ground a tile is considered. Such a hybrid solar Kastler, tile, previously presented, Département Systèmes Énergétiqueslike et Environnement - IMT Atlantique, 4 rue Alfred 44300 Nantes, Franceis resistant, walkable andthe canproblem be usedoftothermally cover places, terraces and roof. It consists a corehybrid composed a photovoltaic cell layer placed ina In this work insulating a PhotoVoltaic Thermalin(PVT) solarof panel is investigated. In particular, contact withdesigned an aluminum sinkthe which transfers the cells Such to a water flow.solar Thistile, corepreviously is coated presented, on the upper side with PVT panel to be heat laid on ground like a heat tile isfrom considered. a hybrid is resistant, a transparent on the lowerterraces and lateral an opaque epoxy resin. Toofthermally insulate such a panel, walkable and epoxy can beresin used and to cover places, and sides roof. with It consists in a core composed a photovoltaic cell layer placedthe in insertion of microspheres thesink opaque is then considered. In particular, maximum concentration microspheres contact with an aluminum in heat whichresin transfers heat from the cells to a waterthe flow. This core is coated onofthetheupper side with Abstract with respect the on production is evaluated. Moreover, theepoxy effectresin. on the dispersed by thesuch panel, whichthe is aacceptable transparent epoxy resintoand the lowerprocess and lateral sides with an opaque Toheat thermally insulate a panel, produced bymicrospheres inserting microspheres in the opaque resin, is experimentally investigated. Some preliminary results presented insertion inare thecommonly opaque resin is then considered. In particular, thethe maximum concentration of for the are microspheres District of heating networks addressed in the literature as one of most effective solutions decreasing the showing reductions in the acceptable with to dispersed the production process is evaluated. Moreover, thehigh effect on the heat dispersed by the panel, is greenhouse gasrespect emissions from theheat. building sector. These systems require investments which are returned throughwhich the heat produced by to inserting microspheres the opaqueand resin, is experimentally preliminary resultscould are presented sales. Due the changed climateinconditions building renovation investigated. policies, heatSome demand in the future decrease, showing reductions in the dispersed heat. prolonging the investment return period. ©The 2018 Thescope Authors. Published Ltd. main of this paper isby to Elsevier assess the feasibility of using the heat demand – outdoor temperature function for heat demand ©forecast. 2018 The Authors. by Elsevier Ltd. This is an open access article under thelocated CC BY-NC-ND (https://creativecommons.org/licenses/by-nc-nd/4.0/) The districtPublished of Alvalade, in Lisbon license (Portugal), was used as a case study. The district is consisted of 665 This is an open access article under the CC BY-NC-ND license (https://creativecommons.org/licenses/by-nc-nd/4.0/) Selection and peer-review under the scientific committee ofscenarios the 73rd(low, Conference ofhigh) the Italian Thermal © 2018 The Authors. Published by responsibility Elsevierperiod Ltd. of buildings that vary in both construction and typology. Three medium, and three district Selection and peer-review under responsibility of the scientific committeeweather of the 73rd Conference of the Italian Thermal Machines Machines Engineering Association (ATI 2018). This is an open access article under the CC BY-NC-ND license (https://creativecommons.org/licenses/by-nc-nd/4.0/) renovation scenarios were developed (shallow, intermediate, deep). To estimate the error, obtained heat demand values were Engineering Association (ATI 2018). Selection responsibility of the scientific committee of the Conference of the Italian Thermal comparedand withpeer-review results fromunder a dynamic heat demand model, previously developed and 73rd validated by the authors. Keywords: hybrid solar panels; thermal insulation; microsphere. Machines Engineering Association 2018). The results showed that when only(ATI weather change is considered, the margin of error could be acceptable for some applications (the error in annual demand was lower than 20% for all weather scenarios considered). However, after introducing renovation Keywords: solar panels; thermal insulation; microsphere. scenarios,hybrid the error value increased up to 59.5% (depending on the weather and renovation scenarios combination considered). 1.The Introduction value of slope coefficient increased on average within the range of 3.8% up to 8% per decade, that corresponds to the decrease in the number of heating hours of 22-139h during the heating season (depending on the combination of weather and 1. Introduction considered). Onresearches the other hand, intercepton increased for 7.8-12.7% per decade (depending on of the Inrenovation the last scenarios few decades different havefunction been focused the optimization (exergetic and structural) coupled scenarios). The values suggested could be used to modify the function parameters for the scenarios considered, and hybrid (electrical and thermal) solar panels mainly adopted for sloped roofs`integration in residential areas (Zondag improve thefew accuracy of heat demand estimations. In the Hybrid last decades different focused on the the optimization and structural) of [1,2]). panels that use waterresearches as workinghave fluidbeen are preferred, since can provide(exergetic higher thermal efficiencies,
hybrid (electrical and thermal) solar panels adopted for of sloped roofs`integration in residential areas has (Zondag as reported by J.Michael [3] and R.Goic [4].mainly Recently the use epoxy resins to encapsulate solar panels been © 2017Hybrid The Authors. by Elsevier Ltd. [1,2]). panelsPublished that use water as working fluid are preferred, since the can provide higher thermal efficiencies, under responsibility of the Scientific Committee of The 15th International Symposium on District Heating and 1 reported by J.Michael [3] and R.Goic [4]. Recently the use of epoxy resins to encapsulate solar panels has been asPeer-review * Corresponding author. Cooling.
E-mail address: [email protected]
1
* Corresponding author. Forecast; Climate change Keywords: Heat demand; 1876-6102 E-mail © 2018address: The Authors. Published by Elsevier Ltd. [email protected] This is an open access article under the CC BY-NC-ND license (https://creativecommons.org/licenses/by-nc-nd/4.0/) Selection peer-review under responsibility of the scientific 1876-6102and © 2018 The Authors. Published by Elsevier Ltd. committee of the 73rd Conference of the Italian Thermal Machines Engineering Association (ATI 2018). This is an open access article under the CC BY-NC-ND license (https://creativecommons.org/licenses/by-nc-nd/4.0/) Selection and©peer-review under responsibility the scientific 1876-6102 2017 The Authors. Published byofElsevier Ltd. committee of the 73rd Conference of the Italian Thermal Machines Engineering 1876-6102 © 2018 The Authors. Published by Elsevier Ltd. Association 2018). Peer-review(ATI under responsibility of the Scientific Committee of The 15th International Symposium on District Heating and Cooling. This is an open access article under the CC BY-NC-ND license (https://creativecommons.org/licenses/by-nc-nd/4.0/) Selection and peer-review under responsibility of the scientific committee of the 73rd Conference of the Italian Thermal Machines Engineering Association (ATI 2018). 10.1016/j.egypro.2018.08.090
Matteo Greppi et al. / Energy Procedia 148 (2018) 948–953
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studied [5]. Epoxy resins have been employed to protect the panels from atmospheric agents and make them robust enough to be stepped on. The cover glass used in classical solar panel has been replaced by a transparent resin which provides comparable optical properties but also improves the mechanical features and allows easier cleaning and maintenance. A first prototype, described in [6], is specifically designed for boats. The encapsulating resin protects the panel from moist air and saline water. A hybrid solar panel has been subsequently designed [7] to be laid on the ground like a tile. In this hybrid solar tile a transparent epoxy resin is used to coat the photovoltaic cell layer, while an opaque one is employed to coat the lateral and lower sides of the underlying heat exchanger. Such a tile can be used to cover terraces, flat roofs and pavements to obtain a full exploitation of the horizontal covers from an exergetic and functional point of view. In this study the problem of improving the thermal insulation of a hybrid solar tile is considered, in order to reduce the dispersed heat power. To this aim, the insertion of ultralight air microspheres in the opaque resin is evaluated. Some experimental tests are presented, that have been performed in order to determine the maximum concentration of the microspheres acceptable with respect to the production process and the effect obtainable by inserting microspheres in the opaque resin on the heat dispersed by the tile.
Fig 1. Air microspheres
2. The tile structure Two hybrid solar tiles have been created. Both are composed of a layer of high efficiency ( 22.8%) mono-crystalline photovoltaic cells by SunPower placed on an aluminum heat sink. The cells are soldered together and then mechanically connected to the heat sink through a thermally conductive silver and aluminum based paste produced by Prolimatech (thermal conductivity equals to 10.2 W/mK), which ensures a satisfactory thermal contact. The heat sink consists in an aluminium block with internal parallel 6.5 mm diameter channels disposed in three staggered rows as shown in Fig. 2. Inlet and outlet plena have also been created in the aluminium block. A water flow passes through the channels, absorbing the heat captured by the cell layer. The photovoltaic cells and the heat sink are all enclosed in an epoxy resin case obtained by cold polymerization. The upper cover, created with a transparent resin (PlexiFluid by Prochima), allows the solar radiation to reach the cells, while the lower cover, made of an opaque resin, avoids heat losses though radiation. The opaque resin has also been employed to realize some feet which allow tubes and cables to pass under the tile. The tiles were made by casting the opaque resin in a mold, placing the heat exchanger with the overlying photovoltaic cells in the still liquid resin on appropriate supports and subsequently casting the transparent resin. For
Author/ Energy Procedia 00 ( 950
Matteo Greppi et al. / Energy Procedia 148 (2018) 948–953
Fig. 2. Tiles` cross section
Matteo Greppi et al. / Energy Procedia 148 (2018) 948–953
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one tile (tested sample) the opaque resin has been mixed with microspheres containing air, for the other one (reference sample) the pure resin has been employed. Fig. 3 shows the obtained tiles.
Fig. 3. The two thermophotovoltaic tiles: without microspheres (gray; reference sample) and with microspheres in the lower resin (white; tested sample).
3. Experimental analysis The insertion of microspheres in the opaque resin produces the advantage that the obtained mixture, once hardened, has a higher thermal resistance and a lower weight, but also causes the disadvantage that the mixture is more viscous than the resin alone. The mixture viscosity increases with the concentration of microspheres. Before casting the opaque resin some tests have been performed by preparing mixture with different concentration and individuating the maximum microspheres` concentration value which allows the mixture to be adequately cast. This value has resulted to be 15 % of the entire mixture volume .The total weight of the tile, thanks to the insertion of the microspheres, passes from 10.1 kg to 9.5 kg. After having created the hybrid solar tiles, an experimental apparatus has been assembled. In order to compare the heat dispersion properties of the tiles, the one with microspheres and the one with pure resin have been put under the same operating conditions. Since the heat convection from the lower tile surface and the air can be affected by several parameters and cannot be easily controlled, the air under the tile has been maintained stagnant focusing the analysis on the heat dispersed through the tile feet. The upper and lateral sides of both tiles have then been coated by thermally insulating extruded polystyrene panels (having thermal conductivity equal to 0,033 W/mK and thickness equal to 20 mm) that also trapped the air under the tiles (Fig. 4).
Fig.4 Test setup
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The insulated tiles have been put on a board cooled by a forced air flow on the lower surface. The water inlet has been connected to a thermostatic water bath (RB_5A by Techne). The circuit has been closed with a water circulator RIO_C by KSB (adjustable with three variable velocity from 0.018 kg/s to 0.045 kg/s). Connections have been made with polyamide pipes (characterized by high flexibility, thermal and chemical resistance). A flow meter (UCC by Parker; flow range from 0.01 kg/s to 0.4 kg/s with an accuracy of +/-2 %) has been inserted in the circuit to detect the mass flow rate. T-type thermocouples by Omega (accuracy +/-0.75 %) were used to measure the water inlet and outlet temperatures and the temperature of the board surface in contact with the tile feet. They were all connected to a digital data acquisition interface, which also measured the frequency signal from the flow meter to determine the mass flow rate in the loop. A dedicated LabView® interface has been created to easily collect and process the measured data. The tiles have been supplied by water entering at the following temperature values: 50 °C, 55 °C, and 60 °C. Measurements have been carried out on the experimental apparatus under steady state conditions. On the basis of the measurements, the heat power lost by the water and dispersed has been calculated: Qd = Q cp (Tin – Tout) where Q is the mass flow rate [kg/s] and cp the specific heat [J/kg K]. Since the uncertainty in measuring temperatures and mass flow rate is +/-0.75 % and +/-2 %, respectively, the uncertainty in determining Qd is +/-2,7%. Moreover, an equivalent thermal conductance from water to board (G t) has been evaluated by dividing the dispersed heat by the temperature difference between the average water temperature T wm and the board temperature Tb. Results are reported in Tab.1.
Table 1. Results Thermal conductance Gt [W/K]
Decremental heat rate [%]
Tb [⁰CC]
Q [kg/s]
Dispersed thermal power [W]
59.81
33.2
0.0305
48.5
1.8226
- 18.1
54.70
54.85
31.5
0.0339
42.5
1.8199
- 16.2
49.76
49.88
29.2
0.0376
37.8
1.8261
- 9.2
__ __ __
Prototypes
Tin [⁰CC]
Tout [⁰CC]
Hybrid tile with microspheres
60
59.62
55 50
Twm [⁰CC]
Reference
60
59.50
59.75
32.9
0.0273
57.2
2.1358
hybrid
55
54.61
54.80
31.6
0.0303
49.4
2.1293
tile
50
49.70
49.85
30.5
0.0329
41.3
2.1344
In the studied case the heat dispersed by the tile with microsphere is 9.2 % to 18.1% lower than that lost by the other one. The conductance Gt of the reference tile is about 2.13 W/K and 18.33 % more than the conductance of the tested tile (about 1.8 W/K). Such values can be compared with the results of a one-dimensional analysis in order to determine the thermal conductivity of the opaque resin. In particular, to obtain a conductance of 2.13 W/K between water and board, a thermal conductivity of 1.6 W/mK must be assumed in the one-dimensional analysis. This value is compatible with the range declared by the producer (1.4 W/mK to 1.7 W/mK) of the pure resin. Moreover, to obtain a conductance of 1.8 W/K, a thermal conductivity of 1.35 W/mK must be assumed. This is therefore the apparent thermal conductivity of the resin with air microspheres under the experimental conditions created. It is 15.63 % less than the thermal conductivity of the pure resin.
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4. Conclusions In the work presented the effect on the dispersed heat produced by inserting air containing microspheres in the case material of a hybrid solar tile has been investigated. A maximum microsphere concentration of 15 % has been found acceptable with respect to the production process. The results obtained demonstrate that the use of microspheres provides a noticeable improvement in the tile insulation properties. In particular, in the studied cases, a more than 15 % reduction in the resin thermal conductivity has been observed. Another caused advantage regards the structure lightening, which is particularly useful for the specific use as floor covering. A 8.3% reduction in weight has been obtained with a 15% microsphere concentration. Further analysis will be performed to identify materials that, in addition to enhanced thermal insulation properties, provide improved structural characteristics. 5. References [1] H.A. Zondag, Flat-plate PV-Thermal collectors and systems: A review, Renewable and Sustainable Energy Reviews 12 (2008): 891–959. [2] H. A. Zondag, D. W. De Vries, The yield of different combined PV-thermal collector designs, Solar Energy 74(3) (2003) 253-269. [3] Tiwari A, Sodha M., Performance evaluation of hybrid PVT water/air heating system: A parametric study, Renewable Energy 31 (2006) 2460- 2474. [4] J. J. Michael, Iniyan S., R.Goic, Flat plate solar photovoltaic–thermal (PV/T) systems: A reference guide, Renewable and Sustainable Energy Reviews 51(2015): 62-88. [5] G.Fabbri, M.Greppi Experimental analysis of PVT panels for industrial applications, Energy Procedia 45 (2014): 308-314. [6] G. Fabbri, M. Greppi, Experimental characterization of a new hybrid thermophotovoltaic industrial prototype, Energy Procedia 81 (2015): 117-121. [7] G. Fabbri, M. Greppi, Experimental characterization of a hybrid industrial solar tile, Energy Procedia 126 (2017): 621 – 627.
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Energy (2018) 000–000 948–953 EnergyProcedia Procedia148 00 (2017) www.elsevier.com/locate/procedia 73rd Conference of the Italian Thermal Machines Engineering Association (ATI 2018), 12–14 September 2018, Pisa, Italy 73rd Conference of the Italian Thermal Machines Engineering Association (ATI 2018), September 2018, Pisa, Italy hybrid solar panels Use of microspheres12–14 in thermally insulating 1 Use of microspheres in thermally insulating hybrid solar panels Matteo Greppi ,Giampietro Fabbri The 15th International Symposium on District Heating and Cooling
Matteo Greppi1,Giampietro Fabbri
Assessing the feasibility of using the heat demand-outdoor D.I.N. ,Università degli Bologna,Viale Risorgimento 2, 40136 Bologna, ItalyBologna, Italy CIRI EDILIZIA E COSTRUZIONI,,Università degli Studi di Bologna ,via Lazzaretto 15/5 , 40136, temperature function forStudia dilong-term district heat demand forecast 1
CIRI EDILIZIA E COSTRUZIONI,,Università degli Studi di Bologna ,via Lazzaretto 15/5 , 40136, Bologna, Italy
1
2
2
I. Andrića,b,c*, A. Pinaa, P. Ferrãoa, J. Fournierb., B. Lacarrièrec, O. Le Correc D.I.N. ,Università degli Studi di Bologna,Viale Risorgimento 2, 40136 Bologna, Italy
Abstract a
IN+ Center for Innovation, Technology and Policy Research - Instituto Superior Técnico, Av. Rovisco Pais 1, 1049-001 Lisbon, Portugal
In this work the problem of thermally insulating a PhotoVoltaic Thermal (PVT) hybrid solar panel is investigated. In particular, a Abstract b Veolia Recherche & Innovation, 291 Avenue Dreyfous Daniel, 78520 Limay, France c PVT panel designed to be laid on the ground a tile is considered. Such a hybrid solar Kastler, tile, previously presented, Département Systèmes Énergétiqueslike et Environnement - IMT Atlantique, 4 rue Alfred 44300 Nantes, Franceis resistant, walkable andthe canproblem be usedoftothermally cover places, terraces and roof. It consists a corehybrid composed a photovoltaic cell layer placed ina In this work insulating a PhotoVoltaic Thermalin(PVT) solarof panel is investigated. In particular, contact withdesigned an aluminum sinkthe which transfers the cells Such to a water flow.solar Thistile, corepreviously is coated presented, on the upper side with PVT panel to be heat laid on ground like a heat tile isfrom considered. a hybrid is resistant, a transparent on the lowerterraces and lateral an opaque epoxy resin. Toofthermally insulate such a panel, walkable and epoxy can beresin used and to cover places, and sides roof. with It consists in a core composed a photovoltaic cell layer placedthe in insertion of microspheres thesink opaque is then considered. In particular, maximum concentration microspheres contact with an aluminum in heat whichresin transfers heat from the cells to a waterthe flow. This core is coated onofthetheupper side with Abstract with respect the on production is evaluated. Moreover, theepoxy effectresin. on the dispersed by thesuch panel, whichthe is aacceptable transparent epoxy resintoand the lowerprocess and lateral sides with an opaque Toheat thermally insulate a panel, produced bymicrospheres inserting microspheres in the opaque resin, is experimentally investigated. Some preliminary results presented insertion inare thecommonly opaque resin is then considered. In particular, thethe maximum concentration of for the are microspheres District of heating networks addressed in the literature as one of most effective solutions decreasing the showing reductions in the acceptable with to dispersed the production process is evaluated. Moreover, thehigh effect on the heat dispersed by the panel, is greenhouse gasrespect emissions from theheat. building sector. These systems require investments which are returned throughwhich the heat produced by to inserting microspheres the opaqueand resin, is experimentally preliminary resultscould are presented sales. Due the changed climateinconditions building renovation investigated. policies, heatSome demand in the future decrease, showing reductions in the dispersed heat. prolonging the investment return period. ©The 2018 Thescope Authors. Published Ltd. main of this paper isby to Elsevier assess the feasibility of using the heat demand – outdoor temperature function for heat demand ©forecast. 2018 The Authors. by Elsevier Ltd. This is an open access article under thelocated CC BY-NC-ND (https://creativecommons.org/licenses/by-nc-nd/4.0/) The districtPublished of Alvalade, in Lisbon license (Portugal), was used as a case study. The district is consisted of 665 This is an open access article under the CC BY-NC-ND license (https://creativecommons.org/licenses/by-nc-nd/4.0/) Selection and peer-review under the scientific committee ofscenarios the 73rd(low, Conference ofhigh) the Italian Thermal © 2018 The Authors. Published by responsibility Elsevierperiod Ltd. of buildings that vary in both construction and typology. Three medium, and three district Selection and peer-review under responsibility of the scientific committeeweather of the 73rd Conference of the Italian Thermal Machines Machines Engineering Association (ATI 2018). This is an open access article under the CC BY-NC-ND license (https://creativecommons.org/licenses/by-nc-nd/4.0/) renovation scenarios were developed (shallow, intermediate, deep). To estimate the error, obtained heat demand values were Engineering Association (ATI 2018). Selection responsibility of the scientific committee of the Conference of the Italian Thermal comparedand withpeer-review results fromunder a dynamic heat demand model, previously developed and 73rd validated by the authors. Keywords: hybrid solar panels; thermal insulation; microsphere. Machines Engineering Association 2018). The results showed that when only(ATI weather change is considered, the margin of error could be acceptable for some applications (the error in annual demand was lower than 20% for all weather scenarios considered). However, after introducing renovation Keywords: solar panels; thermal insulation; microsphere. scenarios,hybrid the error value increased up to 59.5% (depending on the weather and renovation scenarios combination considered). 1.The Introduction value of slope coefficient increased on average within the range of 3.8% up to 8% per decade, that corresponds to the decrease in the number of heating hours of 22-139h during the heating season (depending on the combination of weather and 1. Introduction considered). Onresearches the other hand, intercepton increased for 7.8-12.7% per decade (depending on of the Inrenovation the last scenarios few decades different havefunction been focused the optimization (exergetic and structural) coupled scenarios). The values suggested could be used to modify the function parameters for the scenarios considered, and hybrid (electrical and thermal) solar panels mainly adopted for sloped roofs`integration in residential areas (Zondag improve thefew accuracy of heat demand estimations. In the Hybrid last decades different focused on the the optimization and structural) of [1,2]). panels that use waterresearches as workinghave fluidbeen are preferred, since can provide(exergetic higher thermal efficiencies,
hybrid (electrical and thermal) solar panels adopted for of sloped roofs`integration in residential areas has (Zondag as reported by J.Michael [3] and R.Goic [4].mainly Recently the use epoxy resins to encapsulate solar panels been © 2017Hybrid The Authors. by Elsevier Ltd. [1,2]). panelsPublished that use water as working fluid are preferred, since the can provide higher thermal efficiencies, under responsibility of the Scientific Committee of The 15th International Symposium on District Heating and 1 reported by J.Michael [3] and R.Goic [4]. Recently the use of epoxy resins to encapsulate solar panels has been asPeer-review * Corresponding author. Cooling.
E-mail address: [email protected]
1
* Corresponding author. Forecast; Climate change Keywords: Heat demand; 1876-6102 E-mail © 2018address: The Authors. Published by Elsevier Ltd. [email protected] This is an open access article under the CC BY-NC-ND license (https://creativecommons.org/licenses/by-nc-nd/4.0/) Selection peer-review under responsibility of the scientific 1876-6102and © 2018 The Authors. Published by Elsevier Ltd. committee of the 73rd Conference of the Italian Thermal Machines Engineering Association (ATI 2018). This is an open access article under the CC BY-NC-ND license (https://creativecommons.org/licenses/by-nc-nd/4.0/) Selection and©peer-review under responsibility the scientific 1876-6102 2017 The Authors. Published byofElsevier Ltd. committee of the 73rd Conference of the Italian Thermal Machines Engineering 1876-6102 © 2018 The Authors. Published by Elsevier Ltd. Association 2018). Peer-review(ATI under responsibility of the Scientific Committee of The 15th International Symposium on District Heating and Cooling. This is an open access article under the CC BY-NC-ND license (https://creativecommons.org/licenses/by-nc-nd/4.0/) Selection and peer-review under responsibility of the scientific committee of the 73rd Conference of the Italian Thermal Machines Engineering Association (ATI 2018). 10.1016/j.egypro.2018.08.090
Matteo Greppi et al. / Energy Procedia 148 (2018) 948–953
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Author/ Energy Procedia 00 (2018) 000–000
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studied [5]. Epoxy resins have been employed to protect the panels from atmospheric agents and make them robust enough to be stepped on. The cover glass used in classical solar panel has been replaced by a transparent resin which provides comparable optical properties but also improves the mechanical features and allows easier cleaning and maintenance. A first prototype, described in [6], is specifically designed for boats. The encapsulating resin protects the panel from moist air and saline water. A hybrid solar panel has been subsequently designed [7] to be laid on the ground like a tile. In this hybrid solar tile a transparent epoxy resin is used to coat the photovoltaic cell layer, while an opaque one is employed to coat the lateral and lower sides of the underlying heat exchanger. Such a tile can be used to cover terraces, flat roofs and pavements to obtain a full exploitation of the horizontal covers from an exergetic and functional point of view. In this study the problem of improving the thermal insulation of a hybrid solar tile is considered, in order to reduce the dispersed heat power. To this aim, the insertion of ultralight air microspheres in the opaque resin is evaluated. Some experimental tests are presented, that have been performed in order to determine the maximum concentration of the microspheres acceptable with respect to the production process and the effect obtainable by inserting microspheres in the opaque resin on the heat dispersed by the tile.
Fig 1. Air microspheres
2. The tile structure Two hybrid solar tiles have been created. Both are composed of a layer of high efficiency ( 22.8%) mono-crystalline photovoltaic cells by SunPower placed on an aluminum heat sink. The cells are soldered together and then mechanically connected to the heat sink through a thermally conductive silver and aluminum based paste produced by Prolimatech (thermal conductivity equals to 10.2 W/mK), which ensures a satisfactory thermal contact. The heat sink consists in an aluminium block with internal parallel 6.5 mm diameter channels disposed in three staggered rows as shown in Fig. 2. Inlet and outlet plena have also been created in the aluminium block. A water flow passes through the channels, absorbing the heat captured by the cell layer. The photovoltaic cells and the heat sink are all enclosed in an epoxy resin case obtained by cold polymerization. The upper cover, created with a transparent resin (PlexiFluid by Prochima), allows the solar radiation to reach the cells, while the lower cover, made of an opaque resin, avoids heat losses though radiation. The opaque resin has also been employed to realize some feet which allow tubes and cables to pass under the tile. The tiles were made by casting the opaque resin in a mold, placing the heat exchanger with the overlying photovoltaic cells in the still liquid resin on appropriate supports and subsequently casting the transparent resin. For
Author/ Energy Procedia 00 ( 950
Matteo Greppi et al. / Energy Procedia 148 (2018) 948–953
Fig. 2. Tiles` cross section
Matteo Greppi et al. / Energy Procedia 148 (2018) 948–953
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Author/ Energy Procedia 00 (2018) 000–000
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one tile (tested sample) the opaque resin has been mixed with microspheres containing air, for the other one (reference sample) the pure resin has been employed. Fig. 3 shows the obtained tiles.
Fig. 3. The two thermophotovoltaic tiles: without microspheres (gray; reference sample) and with microspheres in the lower resin (white; tested sample).
3. Experimental analysis The insertion of microspheres in the opaque resin produces the advantage that the obtained mixture, once hardened, has a higher thermal resistance and a lower weight, but also causes the disadvantage that the mixture is more viscous than the resin alone. The mixture viscosity increases with the concentration of microspheres. Before casting the opaque resin some tests have been performed by preparing mixture with different concentration and individuating the maximum microspheres` concentration value which allows the mixture to be adequately cast. This value has resulted to be 15 % of the entire mixture volume .The total weight of the tile, thanks to the insertion of the microspheres, passes from 10.1 kg to 9.5 kg. After having created the hybrid solar tiles, an experimental apparatus has been assembled. In order to compare the heat dispersion properties of the tiles, the one with microspheres and the one with pure resin have been put under the same operating conditions. Since the heat convection from the lower tile surface and the air can be affected by several parameters and cannot be easily controlled, the air under the tile has been maintained stagnant focusing the analysis on the heat dispersed through the tile feet. The upper and lateral sides of both tiles have then been coated by thermally insulating extruded polystyrene panels (having thermal conductivity equal to 0,033 W/mK and thickness equal to 20 mm) that also trapped the air under the tiles (Fig. 4).
Fig.4 Test setup
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The insulated tiles have been put on a board cooled by a forced air flow on the lower surface. The water inlet has been connected to a thermostatic water bath (RB_5A by Techne). The circuit has been closed with a water circulator RIO_C by KSB (adjustable with three variable velocity from 0.018 kg/s to 0.045 kg/s). Connections have been made with polyamide pipes (characterized by high flexibility, thermal and chemical resistance). A flow meter (UCC by Parker; flow range from 0.01 kg/s to 0.4 kg/s with an accuracy of +/-2 %) has been inserted in the circuit to detect the mass flow rate. T-type thermocouples by Omega (accuracy +/-0.75 %) were used to measure the water inlet and outlet temperatures and the temperature of the board surface in contact with the tile feet. They were all connected to a digital data acquisition interface, which also measured the frequency signal from the flow meter to determine the mass flow rate in the loop. A dedicated LabView® interface has been created to easily collect and process the measured data. The tiles have been supplied by water entering at the following temperature values: 50 °C, 55 °C, and 60 °C. Measurements have been carried out on the experimental apparatus under steady state conditions. On the basis of the measurements, the heat power lost by the water and dispersed has been calculated: Qd = Q cp (Tin – Tout) where Q is the mass flow rate [kg/s] and cp the specific heat [J/kg K]. Since the uncertainty in measuring temperatures and mass flow rate is +/-0.75 % and +/-2 %, respectively, the uncertainty in determining Qd is +/-2,7%. Moreover, an equivalent thermal conductance from water to board (G t) has been evaluated by dividing the dispersed heat by the temperature difference between the average water temperature T wm and the board temperature Tb. Results are reported in Tab.1.
Table 1. Results Thermal conductance Gt [W/K]
Decremental heat rate [%]
Tb [⁰CC]
Q [kg/s]
Dispersed thermal power [W]
59.81
33.2
0.0305
48.5
1.8226
- 18.1
54.70
54.85
31.5
0.0339
42.5
1.8199
- 16.2
49.76
49.88
29.2
0.0376
37.8
1.8261
- 9.2
__ __ __
Prototypes
Tin [⁰CC]
Tout [⁰CC]
Hybrid tile with microspheres
60
59.62
55 50
Twm [⁰CC]
Reference
60
59.50
59.75
32.9
0.0273
57.2
2.1358
hybrid
55
54.61
54.80
31.6
0.0303
49.4
2.1293
tile
50
49.70
49.85
30.5
0.0329
41.3
2.1344
In the studied case the heat dispersed by the tile with microsphere is 9.2 % to 18.1% lower than that lost by the other one. The conductance Gt of the reference tile is about 2.13 W/K and 18.33 % more than the conductance of the tested tile (about 1.8 W/K). Such values can be compared with the results of a one-dimensional analysis in order to determine the thermal conductivity of the opaque resin. In particular, to obtain a conductance of 2.13 W/K between water and board, a thermal conductivity of 1.6 W/mK must be assumed in the one-dimensional analysis. This value is compatible with the range declared by the producer (1.4 W/mK to 1.7 W/mK) of the pure resin. Moreover, to obtain a conductance of 1.8 W/K, a thermal conductivity of 1.35 W/mK must be assumed. This is therefore the apparent thermal conductivity of the resin with air microspheres under the experimental conditions created. It is 15.63 % less than the thermal conductivity of the pure resin.
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4. Conclusions In the work presented the effect on the dispersed heat produced by inserting air containing microspheres in the case material of a hybrid solar tile has been investigated. A maximum microsphere concentration of 15 % has been found acceptable with respect to the production process. The results obtained demonstrate that the use of microspheres provides a noticeable improvement in the tile insulation properties. In particular, in the studied cases, a more than 15 % reduction in the resin thermal conductivity has been observed. Another caused advantage regards the structure lightening, which is particularly useful for the specific use as floor covering. A 8.3% reduction in weight has been obtained with a 15% microsphere concentration. Further analysis will be performed to identify materials that, in addition to enhanced thermal insulation properties, provide improved structural characteristics. 5. References [1] H.A. Zondag, Flat-plate PV-Thermal collectors and systems: A review, Renewable and Sustainable Energy Reviews 12 (2008): 891–959. [2] H. A. Zondag, D. W. De Vries, The yield of different combined PV-thermal collector designs, Solar Energy 74(3) (2003) 253-269. [3] Tiwari A, Sodha M., Performance evaluation of hybrid PVT water/air heating system: A parametric study, Renewable Energy 31 (2006) 2460- 2474. [4] J. J. Michael, Iniyan S., R.Goic, Flat plate solar photovoltaic–thermal (PV/T) systems: A reference guide, Renewable and Sustainable Energy Reviews 51(2015): 62-88. [5] G.Fabbri, M.Greppi Experimental analysis of PVT panels for industrial applications, Energy Procedia 45 (2014): 308-314. [6] G. Fabbri, M. Greppi, Experimental characterization of a new hybrid thermophotovoltaic industrial prototype, Energy Procedia 81 (2015): 117-121. [7] G. Fabbri, M. Greppi, Experimental characterization of a hybrid industrial solar tile, Energy Procedia 126 (2017): 621 – 627.