- Email: [email protected]
Innovative Solutions for Solar Thermal Systems Implemented in Buildings
Available online at www.sciencedirect.com
ScienceDirect Energy Procedia 85 (2016) 594 – 602
Sustainable Solutions for Energy and Environment, EENVIRO - YRC 2015, 18-20 November 2015, Bucharest, Romania
Innovative Solutions for Solar Thermal Systems Implemented in Buildings Ion Visaa*, Anca Dutaa* a
University of Brasov, R&D Centre: Renewable Energy Systems and Recycling,10 Institute Street, Brasov, Romani
Abstract The transition towards Nearly Zero Energy Buildings (nZEB) requires renewable energy systems to cover the thermal energy need. These have to cover the demand for domestic hot water, for heating and for cooling, in the needed amount and during the required periods, according to the building types and the implementation locations. Solar-thermal systems can play a key role in the sustainability scenario of any nZEB. However, implementing solar thermal systems beyond demonstration projects requires innovative solutions able to surpass the barriers that currently limit the widespread of these types of systems. Therefore, several prerequisites have to be fulfilled, related to functionality (efficiency), feasibility (implementation and O&M costs), architectural integration (giving use to roof, rooftops but also to suitably oriented facades), and end-users acceptance (incentives, promotion, etc.). These novel solutions were implemented in the R&D Center Renewable Energy Systems and Recycling, in the Transilvania University of Brasov and the results of infield optimized implementation confirm the viability of the novel trends presented by the paper. ©2015 TheAuthors. Authors. Published by Elsevier Ltd. is an open access article under the CC BY-NC-ND license © 2016 The Published by Elsevier Ltd. This Peer-review under responsibility of the organizing committee EENVIRO 2015. (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the organizing committee EENVIRO 2015 Keywords: Solar thermal systems; nearly zero energy buildings; energy mixes; solar thermal collector
1. Introduction The EU legal frame asks for a sustainable built environment, [1], and introduces the concept of Nearly Zero Energy Buildings (nZEB) that should be developed for any public investment starting with January 1 st, 2018 and for any new building starting with 2021. The transition towards nZEB requires a massive involvement of renewable energy systems in covering the thermal energy demand, responsible for over 65% from the total energy consumption
* Corresponding author E-mail address: [email protected]; [email protected]
1876-6102 © 2016 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the organizing committee EENVIRO 2015 doi:10.1016/j.egypro.2015.12.249
Ion Visa and Anca Duta / Energy Procedia 85 (2016) 594 – 602
595
in buildings, to cover the demand for domestic hot water, for heating and for cooling, in the needed amount and during the required time periods, according to the building types and the implementation locations. Solar-thermal systems, alone or as part of energy mixes/hybrids, can play a key role in the sustainability scenario of any nZEB, [2]. Although developed and optimized as a mature technology, solar-thermal systems implemented in the built environment are still scarce in many European regions (including Romania) and their true potential is far from being completely exploited, [3]. Implementing solar thermal systems in communities beyond demonstration projects requires innovative solutions that consider the specific features of the built environment. Therefore, several prerequisites have to be fulfilled, related to: (1) functionality and efficiency, because the available mounting area is limited; (2), feasibility, particularly focusing on the implementation and O&M costs that usually have to be directly supported by the end-users; (3) architectural integration, extended beyond roof and rooftops towards suitably oriented facades, thus raising significant aesthetic issues, [4], along with (4) end-users acceptance (incentives, promotion, etc.) and commitment. Considering these issues, the paper presents new trends in the solar-thermal systems development with a particular focus on building integration, considering their functionality and specificity but also the bottlenecks in their implementation, for: (1) domestic hot water production (DHW), using flat plate solar-thermal collectors; (2) heating and DHW, using evacuated tube or flat plate collectors mounted on rooftops or in the close neighbourhood of the buildings; (3) heating, cooling and DHW production, using small concentrating collectors (parabolic trough or dish), installed on the rooftops; associated with chillers able to use low temperature sources (adsorption chillers) these systems can cover a significant share of the cooling demand in buildings. Various novel solutions were implemented in the R&D Centre Renewable Energy Systems and Recycling, in the Transilvania University of Brasov and the infield results, [5] confirm the viability of the novel trends presented by the paper. 2. State of the art and current needs Two major and largely implemented applications are currently associated to solar-thermal systems: residential DHW (using flat plate or evacuated tubes collectors), and electricity production from wet or dry steam generated in solar-thermal plants that mainly involve large parabolic trough collectors, Fig. 1. Other applications are implemented in a lesser extent as heat production for agriculture and environmental processes (e.g. drying [6], and pollutants removal [7]), while very high temperature processes are also mentioned in solar furnaces and solar lasers for semiconductors processing, [8], using parabolic dish collectors or heliostats.
Fig. 1. Main functionalities of the solar-thermal energy conversion
596
Ion Visa and Anca Duta / Energy Procedia 85 (2016) 594 – 602
However, the transition towards sustainability requires to extend the implementation of the solar thermal systems in large scale applications and the built environment is one of the most important candidates. Thus, besides DHW, solar-thermal systems should be employed for heating and cooling. While the amount of DHW is almost constant over the year and has values that depend on the number of consumers and applications (residential, laundry, hotels, etc.), the heating and cooling demand has a rather large variation during one year and significantly depends on the implementation location. The main issues that currently limit the wide spread of solar heating and cooling are related to the buildings as such and to the solar-thermal systems: x Thermal energy demand in the building: any renewable energy system able to directly or indirectly provide thermal energy is currently more expensive as compared to the solutions based on fossil fuels. This means that, before implementing renewable-based systems, the thermal energy demand has to be reduced, at the level of Low Energy Buildings, LEB (at a total energy demand lower than 60…80 kWh/m2 per year, out of which above 60% is represented by the thermal energy), [9, 10]. Thus refurbishing (for already existent buildings) or adequate design (for new buildings) must be firstly employed. x The heating installations in the buildings: solar-thermal systems with flat plate or evacuated tube collectors are basically low enthalpy sources thus large heating surfaces are required in buildings, like floors, walls or ceilings, either in the common approach (at the surface) or by using larger thermal masses, as in the solutions based on temperate concrete. The first approach is rather easy to implement, even in older buildings, by replacing the currently used heaters with surface/ floor heating; however, the use of the more efficient temperate concrete (or alternative solutions) has to be done from the design and construction phases and is highly recommended, as proved through the experimental tests done in the R&D Institute of the Transilvania University where this solution was adopted for all the laboratory buildings and allows simultaneously heating the ceilings and floors, [5]. x Available surface for implementing the solar-thermal collectors: the DHW demand represents less than 5…10% of the total energy demand while the heating demand in a temperate climate exceeds 60%. For the Solar House in the Transilvania University, the total energy demand is 59.5 MWh/year, [2, 9], and the shares of each main functionality are given in Fig. 2. Therefore, to cover the thermal energy demand for heating, a larger implementation area is required, usually larger than what a rooftop or roof can offer. To extend the implementation area, other solutions were proposed as on suitably oriented facades, balconies or near the buildings. All these are highly visible places, therefore additional pre-requisites are formulated related to architectural acceptance, [4, 11], along with high efficiency in the working conditions. However, for temperate/cold regions, a heating solution fully based on solar-thermal systems does not represent a feasible alternative, therefore sustainable heating will imply energy mixes of solar-thermal systems with heat pumps and/or biomass heaters and a backup source for the peak demand.
21.4 2.1
76.5
Heating
DHW
Electricity
Fig. 2. Share of the heating, DHW and electricity consumption in the Solar House (Transilvania University of Brasov)
Ion Visa and Anca Duta / Energy Procedia 85 (2016) 594 – 602
x Thermal energy production: there is variability in the heating demand and this has large peak values in temperate (and cold) regions. Thus, when designing a solar-thermal system to cover a certain share of the heating demand a problem will be faced related to the thermal energy that will be produced also when not actually needed, during warm(er) periods of the year. The problem is serious because during summer the amount of solar radiation is higher than during the cold seasons, thus the energy production is large. There are technical and economic consequences: not using the thermal energy will bring the solar-thermal systems in the stagnation regime, when the collectors can be subjected to accelerated aging; additionally, not using (or throwing away) the heat results in a longer pay-back time. Many storage solutions were proposed, based on ground/boreholes, [12] or latent heat storage (including phase-change materials), [13]; however, the ideal long-term heat storage (produced during summer and consumed in winter) is far from being solved, lacking in up-scalable solutions with acceptable costs. To mitigate these consequences, solar-thermal systems should be designed to simultaneously consider solar heating (cold seasons) and solar cooling (warm periods). Even so, the initial investments make unfeasible the solutions 100% based on solar thermal systems (and almost impossible), again recommending the energy mixes. As this brief analysis show, there are certain technical and economic restrictions that currently limit the wide spread of solar thermal systems integrated in the built environment. These requires not only innovation (re-design, improving the efficiency, etc.) but also may ask for new concepts. Once these developed, a combination of feasibility, incentives and legal constraints could further support the community acceptance. 3. New trends Following the state of the art analysis, the R&D Centre Renewable Energy Systems and Recycling (RESREC) in the Transilvania University of Brasov focused on the development of renewable-based energy mixes, able to support the transition towards the nZEB status. (a) Solar-thermal systems: the L7 rooftop:
(1) Solar-thermal testing rig with tracked platform (2) Parabolic trough collectors (3) Durability testing rig in saline media (4) Photovoltaic-thermal (PVT) modules (b) In-field Solar-thermal laboratory (1) Flat plate solar thermal collectors (2) Evacuated tubes collectors (3) New solar-thermal collector
Fig. 3. Solar-thermal outdoor testing facilities in the R&D Institute of the Transilvania University:(a) Rooftop; (b) Facade
597
598
Ion Visa and Anca Duta / Energy Procedia 85 (2016) 594 – 602
Fig. 4. Indoor solar simulator with UV and VIS radiation, temperature control and tracking options
The theoretical and experimental investigations are developed in the R&D Institute of the university, where each of the 11 buildings are LEB, embedding novel architectural concepts and advanced solutions for insulation, glazing, temperate concrete, etc., [5]. The RESREC Centre is located in the Laboratory building L7, Fig. 3a, which is used as a 1:1 experimental rig, where various concepts are implemented and optimised, before being delivered towards community. Additionally, the outdoor solar-thermal laboratory (Fig. 3b) and the indoor testing rig (Fig. 4) supported to formulate new concepts for the convertors (flat plate solar thermal collectors), for the system durability and functionality (tracking), and as part of integrated, personalized solutions of energy mixes to be implemented at buildings’ or communities’ levels. 3.1. Novel flat plate solar thermal collector with un-conventional shape for facades integration To support increased architectural acceptance, a novel flat plate solar thermal collector was proposed, having isosceles trapeze shape and coloured absorber plate and/or glazing, as presented in Fig. 3b, position (3). This type of collector, with an aperture area of about 0.62m2 was designed for facades integration, allowing the development of various geometries and colours. The demonstrator, tested with black absorber plate and using water as thermal fluid proved a nominal efficiency of 62% on the indoor testing rig and about 60% in the outdoor solar-thermal laboratory, [14]. An optimised prototype, with lighter weight and improved tightness was recently developed and tested on the indoor testing rig (Fig. 4), according to ISO 9806:2013, at four input temperatures of the heating fluid, in the 25…60 0 C range. The experimental characteristic efficiency curve follows a linear equation, Eq. (1):
K ൌ K െ ܽܶכ
(1)
where: K0 represents the nominal efficiency and ܶ כis calculated as the difference between the average fluid temperature in the collector (Tm) and the ambient air temperature (Ta) divided to the total radiation, Eq. (2): ܶ כൌ
ሺ்ି்ೌ ሻ ீ
(2)
The results in Fig. 5 allows estimating a nominal efficiency of 68.3%. Considering the dimensions of the collector and the manufacturing conditions (that can be subject of further improvements) this efficiency is highly promising.
Ion Visa and Anca Duta / Energy Procedia 85 (2016) 594 – 602
599
Tm: average temperature in the solar thermal collector Ta: ambient temperature G: solar radiation power Fig. 5. Characteristic efficiency curve a the novel trapeze solar-thermal collector
Coloured absorber plates were obtained based on an environmentally friendly and mature technology (cold spraying), by using aqueous inks, i.e. dispersed pigment oxides (obtained through sol-gel technique, followed by annealing). A rather broad colours pallet (light orange…red…dark brown) was obtained for absorber plates with the structure: Al/Al2O3/Fe2O3/TiO2, by simply modifying the Fe 2O3 and TiO2 concentrations in the inks. It is important to notice that the Vis absorptance is almost unchanged, regardless the concentration, while the IR/thermal emittance slightly increases (as expected) for the light and bright shades of colours. When using collectors with differently coloured absorber plates, [15], in lego -type serial or parallel connections, various arrays can be developed, giving use to facades in a novel solar architecture concept; further on, as Fig. 6 shows, trapeze and rectangular collectors can be matched, along with photovoltaic modules (hereby with triangle shape):
Fig. 6. Various solar facades embedding the trapeze solar collector
3.2. Increasing the output and durability of the flat plate solar thermal collectors through adaptive tracking As already outlined, the limited implementation area available on/near the buildings asks for good efficiencies and maximized output. Tracking the solar radiation may increase the input energy on the flat plate solar collector
600
Ion Visa and Anca Duta / Energy Procedia 85 (2016) 594 – 602
with an average of 30% (even higher during summer days), [16], thus this could represent a viable option for increasing the amount of thermal energy delivered by a solar-thermal system. However, this larger amount of heat may lead to faster reaching the stagnation regime, when the heat is not used, thus reducing the actual working time of the installation; additionally, stagnation increases the thermal stress on the convertor, with negative consequences on its durability. Therefore, in the solar-thermal conversion tracking has to be adapted to the heat consumption. A concept of adaptive tracking was proposed along with a 4-programs algorithm, differentiating between direct tracking (increasing the amount of solar radiation), reversed tracking (that preserves an almost constant temperature in the boiler, close to the threshold stagnation temperature by employing controlled tracking backwards), stand still program (during low radiation periods or during night, when the collector has a horizontal position that mitigates the wind effect), and the holyday program (when heat is not required) that keeps the collector in extreme positions, with low solar radiation, [17]. When higher temperature applications are targeted, e.g. in buildings solar cooling using adsorption or absorption chillers, small parabolic (trough) collectors can be employed, having accurate tracking mechanisms and algorithms. A novel cam-follower tracking mechanisms was developed and optimised for this type of collectors, [18]. Last but not least, durability depends on the collector quality and on the working environment. Most of the building integrated solar-thermal systems are used for DHW and are extensively implemented at the seaside, being therefore subjected to a long term contact with saline aerosols. So far, durability tests for the solar collectors are not considering this aging factor, but this is a problem that drastically limits the convertors’ performance and lifetime. Therefore, a testing rig was developed for outdoor testing in saline environment and the experiments developed on highly efficient market collectors proved that 20 days of testing lead to the development of “whit spots” of salt that entered the collectors through the most vulnerable part: the polymeric sealing (between glazing and casing), [19]. These spots are further acting as corrosion precursors that eventually destroy the selective absorber coating. 3.3. Integrating solar-thermal systems in renewable-based energy mixes Due to the seasonal and daily variability of the solar radiation, solar-thermal systems are delivering a nonconstant flux of output energy, usually not matching the variability in the demand. As already outlined, storage might be a short term option but it significantly raises the investment costs and/or is not always possible to implement in the built environment. This is why energy mixes are recommended to cover the thermal energy demand in a given building. The design methodology of an energy mix was previously detailed, [5] and mainly consist of: x accurate estimation of the thermal energy demand (heating, cooling and DHW) as global, seasonal and peak values; x estimating the renewables energy potential, e.g. solar radiation, biomass, geothermal sources, etc. For a reliable design, data should be collected over at least one year; obviously, simulated solar radiation data can be used (either based on models or e.g. using Meteonorm) but comparative investigations showed that deviations from the infield data are registered, particularly during the seasons with low solar radiation. x designing the energy mix considering the share that will be covered by using renewables. It is to notice that in an energy mix the systems have to synergistically function, thus being more performant in terms of output as compared to a simple summing up the components. This implies considering the multi-functionality of each component system and their overall match: e.g. a solar-thermal system used for DHW, heating and cooling should be complementary designed with a ground coupled heat pump used in direct and reversed modes. Following this concept, an energy mix was designed and implemented in the Solar House and, based on the lesson learned, the energy mix was implemented at an extended scale in the RESREC Centre, [2, 20]. The optimization studies were done considering the L7 laboratory building, with a total area of 1560 m 2, and yearly average energy demand of 69 kWh/m2. An energy mix consisting of a ground coupled heat pump can cover 80% of the total thermal energy demand and a solar-thermal system, covers the rest of 20%. The electricity required to power the solar-thermal system and the heat pump can be provided by a photovoltaic array. This energy mix was selected considering the output, the available solar energy, and the initial investment.
601
Ion Visa and Anca Duta / Energy Procedia 85 (2016) 594 – 602
All the thermal functionalities can be covered over the year by this energy mix and an excess heat is generated by the 50m2 of flat plate solar thermal collectors, as Fig. 7 shows. For a regular residential environment, this leaves room for further optimization, in terms of tuning the STS:HP power ratio, or adding new functionalities (e.g. pond, pool, etc.) [kWh/year] 100000 75000 65817 50000 25000 0
9865 16454
4244
Heating
DHW STS Useful
18601
6440
367 Cooling STS Excess
Electricity
HP
Fig. 7. Thermal energy components produced by the energy mix: solar-thermal system (STS 20%) and heat pump (HP 80%) and the electricity consumption to power the mix
When designing solar-thermal systems or energy mixes involving these, one has to outline that the costs are important issue in the relation with the end-users; these costs significantly vary with the building energy efficiency. For the 80:20 STS-HP energy mix a simulation based on common market costs was done, considering two scenarios that cover 50% and, respectively, 70% from the total energy demand by using renewable energy systems (RES), as presented in Fig. 8.
Fig. 8. RES Initial investment according to the energy efficiency of the building
602
Ion Visa and Anca Duta / Energy Procedia 85 (2016) 594 – 602
The financial effort for implementing renewables in a larger extent (70%) is not much higher as compared to the 50% share, if the building is LEB; however, costs are almost double for a building that has an average energy consumption of 110 kWh/m2/year (corresponding to what today is considered to be a “well refurbished” building). 4. Conclusions The paper reviews the main barriers that limit the large scale implementation of solar-thermal systems in the built environment, beyond the DHW production. Addressing issues like efficiency, durability, architectural integration and costs, the papers presents novel, integrated solutions at conceptual and applicative levels. These solutions can be further refined when stepping on to the next level: sustainable communities, where the integrated approach building – community allows the design of efficient and affordable sustainable energy mixes.
Acknowledgements We hereby acknowledge the project EST in URBA, PNII-PCCA Type 2 contract no. 28/2012 financially supported by the Romanian National Research Council.
References [1] The Directive 2010/31/EU of the European Parliament and of the Council of 19 May 2010 on the energy performance of buildings, Official Journal of the European Union 53 (2010), eur-lex.europa.eu/ LexUriServ/LexUriServ.do?uri=OJ:L:2010:153:0013:0035:EN:PDF (last access 20/08/2015) [2] Visa I., Moldovan M.D., Comsit M., Duta A., Improving the renewable energy mix in a building toward the nearly zero energy status, Energy and Buildings, Vol. 68(A), 2014, pp. 72–78 [3] Kalogirou S.A., Chapter 3 - Solar Energy Collectors, in Solar Energy Engineering (Second Edition), edited by Soteris A. Kalogirou, Academic Press, Boston, 2014, pp. 125-220 [4] Munari Probst M., Roecker C. Towards an improved architectural quality of building integrated solar thermal systems (BIST). Solar Energy, Vol. 81, 2007, pp. 1104–16 [5] Visa I., Duta A., The Built Environment In Sustainable Communities, in Sustainable Energy in the Built Environment – Steps Towards nZEB, Springer Proceedings in Energy, 2015, p. 3-30 [6] Pirasteh G., Saidur R., Rahman S.M.A., Rahim N.A., A review on development of solar drying applications, Renewable and Sustainable Energy Reviews, Vol. 31, 2014, pp. 133-148 [7] El-Sebaii A.A., El-Bialy E., Advanced designs of solar desalination systems: A review, Renewable and Sustainable Energy Reviews, Vol. 49, 2015, pp. 1198-1212 [8] Li B., Oliveira F.A.C., Rodríguez J., Fernandes J.C., Rosa L.G., Numerical and experimental study on improving temperature uniformity of solar furnaces for materials processing, Solar Energy, Vol. 115, 2015, pp. 95-108 [9] Moldovan M., Visa I., Burduhos B., Energetic autonomy for a Solar House, Environmental Engineering and Management Journal Vol. 10, 2011, pp. 1283–1290 [10] Methodology of calculation of the energy performance of building, Mc-001/1,2, 3 (2006), 4 (2009) (Romanian language). [11] COST Action TU1205 – Overview of BISTS state of the art, models and applications, COST Office, 2015 [12] Gao L., Zhao J., Tang Z., A Review on Borehole Seasonal Solar Thermal Energy Storage, Energy Procedia, Vol. 70, 2015, pp. 209-218 [13] Aydin D., Utlu Z., Kincay O., Thermal performance analysis of a solar energy sourced latent heat storage, Renewable and Sustainable Energy Reviews, Vol. 50, 2015, pp. 1213-1225 [14] Visa I., Duta A., Comsit M., Moldovan M., Ciobanu D., Saulescu R., Burduhos B., Design and experimental optimisation of a novel flat plate solar thermal collector with trapezoidal shape for facades integration, Applied Thermal Engineering, Vol. 90, 2015, pp. 432-443 [15] Duta A., Isac L., Milea A., Ienei E., Perniu D., Coloured Solar-thermal Absorbers – A Comparative Analysis of Cermet Structures, Energy Procedia, 48, 2014, pp. 543-553 [16] Burduhos B.G., Visa I., Neagoe M., Badea M., Modeling and optimization of the global solar irradiance collecting efficiency, International Journal of Green Energy, Vol. 12, 2015, pp. 743-755 [17] Neagoe M., Visa I., Burduhos B., Moldovan M., Thermal Load based Adaptive Tracking for Flat Plate Solar Collectors, Energy Procedia, Vol. 48, 2014, pp. 1401-1411 [18] Ciobanu D., Visa I., Tracking system type cam mechanisms for parabolic trough collector, Acta Technica Napocensis, 2007, pp. 115-120 [19] Ciobanu D., Visa I., Duta A., Solar Thermal Collectors Outdoor Testing in Saline Environment, Energy Procedia, Vol. 48, 2014, pp. 707-714 [20] Moldovan M., Visa I, Neagoe M, Burduhos B. (2014) Solar heating & cooling energy mixes to transform low energy buildings in nearly zero energy buildings. Energy Procedia, Vol. 48, 2014, pp. 924-937
ScienceDirect Energy Procedia 85 (2016) 594 – 602
Sustainable Solutions for Energy and Environment, EENVIRO - YRC 2015, 18-20 November 2015, Bucharest, Romania
Innovative Solutions for Solar Thermal Systems Implemented in Buildings Ion Visaa*, Anca Dutaa* a
University of Brasov, R&D Centre: Renewable Energy Systems and Recycling,10 Institute Street, Brasov, Romani
Abstract The transition towards Nearly Zero Energy Buildings (nZEB) requires renewable energy systems to cover the thermal energy need. These have to cover the demand for domestic hot water, for heating and for cooling, in the needed amount and during the required periods, according to the building types and the implementation locations. Solar-thermal systems can play a key role in the sustainability scenario of any nZEB. However, implementing solar thermal systems beyond demonstration projects requires innovative solutions able to surpass the barriers that currently limit the widespread of these types of systems. Therefore, several prerequisites have to be fulfilled, related to functionality (efficiency), feasibility (implementation and O&M costs), architectural integration (giving use to roof, rooftops but also to suitably oriented facades), and end-users acceptance (incentives, promotion, etc.). These novel solutions were implemented in the R&D Center Renewable Energy Systems and Recycling, in the Transilvania University of Brasov and the results of infield optimized implementation confirm the viability of the novel trends presented by the paper. ©2015 TheAuthors. Authors. Published by Elsevier Ltd. is an open access article under the CC BY-NC-ND license © 2016 The Published by Elsevier Ltd. This Peer-review under responsibility of the organizing committee EENVIRO 2015. (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the organizing committee EENVIRO 2015 Keywords: Solar thermal systems; nearly zero energy buildings; energy mixes; solar thermal collector
1. Introduction The EU legal frame asks for a sustainable built environment, [1], and introduces the concept of Nearly Zero Energy Buildings (nZEB) that should be developed for any public investment starting with January 1 st, 2018 and for any new building starting with 2021. The transition towards nZEB requires a massive involvement of renewable energy systems in covering the thermal energy demand, responsible for over 65% from the total energy consumption
* Corresponding author E-mail address: [email protected]; [email protected]
1876-6102 © 2016 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the organizing committee EENVIRO 2015 doi:10.1016/j.egypro.2015.12.249
Ion Visa and Anca Duta / Energy Procedia 85 (2016) 594 – 602
595
in buildings, to cover the demand for domestic hot water, for heating and for cooling, in the needed amount and during the required time periods, according to the building types and the implementation locations. Solar-thermal systems, alone or as part of energy mixes/hybrids, can play a key role in the sustainability scenario of any nZEB, [2]. Although developed and optimized as a mature technology, solar-thermal systems implemented in the built environment are still scarce in many European regions (including Romania) and their true potential is far from being completely exploited, [3]. Implementing solar thermal systems in communities beyond demonstration projects requires innovative solutions that consider the specific features of the built environment. Therefore, several prerequisites have to be fulfilled, related to: (1) functionality and efficiency, because the available mounting area is limited; (2), feasibility, particularly focusing on the implementation and O&M costs that usually have to be directly supported by the end-users; (3) architectural integration, extended beyond roof and rooftops towards suitably oriented facades, thus raising significant aesthetic issues, [4], along with (4) end-users acceptance (incentives, promotion, etc.) and commitment. Considering these issues, the paper presents new trends in the solar-thermal systems development with a particular focus on building integration, considering their functionality and specificity but also the bottlenecks in their implementation, for: (1) domestic hot water production (DHW), using flat plate solar-thermal collectors; (2) heating and DHW, using evacuated tube or flat plate collectors mounted on rooftops or in the close neighbourhood of the buildings; (3) heating, cooling and DHW production, using small concentrating collectors (parabolic trough or dish), installed on the rooftops; associated with chillers able to use low temperature sources (adsorption chillers) these systems can cover a significant share of the cooling demand in buildings. Various novel solutions were implemented in the R&D Centre Renewable Energy Systems and Recycling, in the Transilvania University of Brasov and the infield results, [5] confirm the viability of the novel trends presented by the paper. 2. State of the art and current needs Two major and largely implemented applications are currently associated to solar-thermal systems: residential DHW (using flat plate or evacuated tubes collectors), and electricity production from wet or dry steam generated in solar-thermal plants that mainly involve large parabolic trough collectors, Fig. 1. Other applications are implemented in a lesser extent as heat production for agriculture and environmental processes (e.g. drying [6], and pollutants removal [7]), while very high temperature processes are also mentioned in solar furnaces and solar lasers for semiconductors processing, [8], using parabolic dish collectors or heliostats.
Fig. 1. Main functionalities of the solar-thermal energy conversion
596
Ion Visa and Anca Duta / Energy Procedia 85 (2016) 594 – 602
However, the transition towards sustainability requires to extend the implementation of the solar thermal systems in large scale applications and the built environment is one of the most important candidates. Thus, besides DHW, solar-thermal systems should be employed for heating and cooling. While the amount of DHW is almost constant over the year and has values that depend on the number of consumers and applications (residential, laundry, hotels, etc.), the heating and cooling demand has a rather large variation during one year and significantly depends on the implementation location. The main issues that currently limit the wide spread of solar heating and cooling are related to the buildings as such and to the solar-thermal systems: x Thermal energy demand in the building: any renewable energy system able to directly or indirectly provide thermal energy is currently more expensive as compared to the solutions based on fossil fuels. This means that, before implementing renewable-based systems, the thermal energy demand has to be reduced, at the level of Low Energy Buildings, LEB (at a total energy demand lower than 60…80 kWh/m2 per year, out of which above 60% is represented by the thermal energy), [9, 10]. Thus refurbishing (for already existent buildings) or adequate design (for new buildings) must be firstly employed. x The heating installations in the buildings: solar-thermal systems with flat plate or evacuated tube collectors are basically low enthalpy sources thus large heating surfaces are required in buildings, like floors, walls or ceilings, either in the common approach (at the surface) or by using larger thermal masses, as in the solutions based on temperate concrete. The first approach is rather easy to implement, even in older buildings, by replacing the currently used heaters with surface/ floor heating; however, the use of the more efficient temperate concrete (or alternative solutions) has to be done from the design and construction phases and is highly recommended, as proved through the experimental tests done in the R&D Institute of the Transilvania University where this solution was adopted for all the laboratory buildings and allows simultaneously heating the ceilings and floors, [5]. x Available surface for implementing the solar-thermal collectors: the DHW demand represents less than 5…10% of the total energy demand while the heating demand in a temperate climate exceeds 60%. For the Solar House in the Transilvania University, the total energy demand is 59.5 MWh/year, [2, 9], and the shares of each main functionality are given in Fig. 2. Therefore, to cover the thermal energy demand for heating, a larger implementation area is required, usually larger than what a rooftop or roof can offer. To extend the implementation area, other solutions were proposed as on suitably oriented facades, balconies or near the buildings. All these are highly visible places, therefore additional pre-requisites are formulated related to architectural acceptance, [4, 11], along with high efficiency in the working conditions. However, for temperate/cold regions, a heating solution fully based on solar-thermal systems does not represent a feasible alternative, therefore sustainable heating will imply energy mixes of solar-thermal systems with heat pumps and/or biomass heaters and a backup source for the peak demand.
21.4 2.1
76.5
Heating
DHW
Electricity
Fig. 2. Share of the heating, DHW and electricity consumption in the Solar House (Transilvania University of Brasov)
Ion Visa and Anca Duta / Energy Procedia 85 (2016) 594 – 602
x Thermal energy production: there is variability in the heating demand and this has large peak values in temperate (and cold) regions. Thus, when designing a solar-thermal system to cover a certain share of the heating demand a problem will be faced related to the thermal energy that will be produced also when not actually needed, during warm(er) periods of the year. The problem is serious because during summer the amount of solar radiation is higher than during the cold seasons, thus the energy production is large. There are technical and economic consequences: not using the thermal energy will bring the solar-thermal systems in the stagnation regime, when the collectors can be subjected to accelerated aging; additionally, not using (or throwing away) the heat results in a longer pay-back time. Many storage solutions were proposed, based on ground/boreholes, [12] or latent heat storage (including phase-change materials), [13]; however, the ideal long-term heat storage (produced during summer and consumed in winter) is far from being solved, lacking in up-scalable solutions with acceptable costs. To mitigate these consequences, solar-thermal systems should be designed to simultaneously consider solar heating (cold seasons) and solar cooling (warm periods). Even so, the initial investments make unfeasible the solutions 100% based on solar thermal systems (and almost impossible), again recommending the energy mixes. As this brief analysis show, there are certain technical and economic restrictions that currently limit the wide spread of solar thermal systems integrated in the built environment. These requires not only innovation (re-design, improving the efficiency, etc.) but also may ask for new concepts. Once these developed, a combination of feasibility, incentives and legal constraints could further support the community acceptance. 3. New trends Following the state of the art analysis, the R&D Centre Renewable Energy Systems and Recycling (RESREC) in the Transilvania University of Brasov focused on the development of renewable-based energy mixes, able to support the transition towards the nZEB status. (a) Solar-thermal systems: the L7 rooftop:
(1) Solar-thermal testing rig with tracked platform (2) Parabolic trough collectors (3) Durability testing rig in saline media (4) Photovoltaic-thermal (PVT) modules (b) In-field Solar-thermal laboratory (1) Flat plate solar thermal collectors (2) Evacuated tubes collectors (3) New solar-thermal collector
Fig. 3. Solar-thermal outdoor testing facilities in the R&D Institute of the Transilvania University:(a) Rooftop; (b) Facade
597
598
Ion Visa and Anca Duta / Energy Procedia 85 (2016) 594 – 602
Fig. 4. Indoor solar simulator with UV and VIS radiation, temperature control and tracking options
The theoretical and experimental investigations are developed in the R&D Institute of the university, where each of the 11 buildings are LEB, embedding novel architectural concepts and advanced solutions for insulation, glazing, temperate concrete, etc., [5]. The RESREC Centre is located in the Laboratory building L7, Fig. 3a, which is used as a 1:1 experimental rig, where various concepts are implemented and optimised, before being delivered towards community. Additionally, the outdoor solar-thermal laboratory (Fig. 3b) and the indoor testing rig (Fig. 4) supported to formulate new concepts for the convertors (flat plate solar thermal collectors), for the system durability and functionality (tracking), and as part of integrated, personalized solutions of energy mixes to be implemented at buildings’ or communities’ levels. 3.1. Novel flat plate solar thermal collector with un-conventional shape for facades integration To support increased architectural acceptance, a novel flat plate solar thermal collector was proposed, having isosceles trapeze shape and coloured absorber plate and/or glazing, as presented in Fig. 3b, position (3). This type of collector, with an aperture area of about 0.62m2 was designed for facades integration, allowing the development of various geometries and colours. The demonstrator, tested with black absorber plate and using water as thermal fluid proved a nominal efficiency of 62% on the indoor testing rig and about 60% in the outdoor solar-thermal laboratory, [14]. An optimised prototype, with lighter weight and improved tightness was recently developed and tested on the indoor testing rig (Fig. 4), according to ISO 9806:2013, at four input temperatures of the heating fluid, in the 25…60 0 C range. The experimental characteristic efficiency curve follows a linear equation, Eq. (1):
K ൌ K െ ܽܶכ
(1)
where: K0 represents the nominal efficiency and ܶ כis calculated as the difference between the average fluid temperature in the collector (Tm) and the ambient air temperature (Ta) divided to the total radiation, Eq. (2): ܶ כൌ
ሺ்ି்ೌ ሻ ீ
(2)
The results in Fig. 5 allows estimating a nominal efficiency of 68.3%. Considering the dimensions of the collector and the manufacturing conditions (that can be subject of further improvements) this efficiency is highly promising.
Ion Visa and Anca Duta / Energy Procedia 85 (2016) 594 – 602
599
Tm: average temperature in the solar thermal collector Ta: ambient temperature G: solar radiation power Fig. 5. Characteristic efficiency curve a the novel trapeze solar-thermal collector
Coloured absorber plates were obtained based on an environmentally friendly and mature technology (cold spraying), by using aqueous inks, i.e. dispersed pigment oxides (obtained through sol-gel technique, followed by annealing). A rather broad colours pallet (light orange…red…dark brown) was obtained for absorber plates with the structure: Al/Al2O3/Fe2O3/TiO2, by simply modifying the Fe 2O3 and TiO2 concentrations in the inks. It is important to notice that the Vis absorptance is almost unchanged, regardless the concentration, while the IR/thermal emittance slightly increases (as expected) for the light and bright shades of colours. When using collectors with differently coloured absorber plates, [15], in lego -type serial or parallel connections, various arrays can be developed, giving use to facades in a novel solar architecture concept; further on, as Fig. 6 shows, trapeze and rectangular collectors can be matched, along with photovoltaic modules (hereby with triangle shape):
Fig. 6. Various solar facades embedding the trapeze solar collector
3.2. Increasing the output and durability of the flat plate solar thermal collectors through adaptive tracking As already outlined, the limited implementation area available on/near the buildings asks for good efficiencies and maximized output. Tracking the solar radiation may increase the input energy on the flat plate solar collector
600
Ion Visa and Anca Duta / Energy Procedia 85 (2016) 594 – 602
with an average of 30% (even higher during summer days), [16], thus this could represent a viable option for increasing the amount of thermal energy delivered by a solar-thermal system. However, this larger amount of heat may lead to faster reaching the stagnation regime, when the heat is not used, thus reducing the actual working time of the installation; additionally, stagnation increases the thermal stress on the convertor, with negative consequences on its durability. Therefore, in the solar-thermal conversion tracking has to be adapted to the heat consumption. A concept of adaptive tracking was proposed along with a 4-programs algorithm, differentiating between direct tracking (increasing the amount of solar radiation), reversed tracking (that preserves an almost constant temperature in the boiler, close to the threshold stagnation temperature by employing controlled tracking backwards), stand still program (during low radiation periods or during night, when the collector has a horizontal position that mitigates the wind effect), and the holyday program (when heat is not required) that keeps the collector in extreme positions, with low solar radiation, [17]. When higher temperature applications are targeted, e.g. in buildings solar cooling using adsorption or absorption chillers, small parabolic (trough) collectors can be employed, having accurate tracking mechanisms and algorithms. A novel cam-follower tracking mechanisms was developed and optimised for this type of collectors, [18]. Last but not least, durability depends on the collector quality and on the working environment. Most of the building integrated solar-thermal systems are used for DHW and are extensively implemented at the seaside, being therefore subjected to a long term contact with saline aerosols. So far, durability tests for the solar collectors are not considering this aging factor, but this is a problem that drastically limits the convertors’ performance and lifetime. Therefore, a testing rig was developed for outdoor testing in saline environment and the experiments developed on highly efficient market collectors proved that 20 days of testing lead to the development of “whit spots” of salt that entered the collectors through the most vulnerable part: the polymeric sealing (between glazing and casing), [19]. These spots are further acting as corrosion precursors that eventually destroy the selective absorber coating. 3.3. Integrating solar-thermal systems in renewable-based energy mixes Due to the seasonal and daily variability of the solar radiation, solar-thermal systems are delivering a nonconstant flux of output energy, usually not matching the variability in the demand. As already outlined, storage might be a short term option but it significantly raises the investment costs and/or is not always possible to implement in the built environment. This is why energy mixes are recommended to cover the thermal energy demand in a given building. The design methodology of an energy mix was previously detailed, [5] and mainly consist of: x accurate estimation of the thermal energy demand (heating, cooling and DHW) as global, seasonal and peak values; x estimating the renewables energy potential, e.g. solar radiation, biomass, geothermal sources, etc. For a reliable design, data should be collected over at least one year; obviously, simulated solar radiation data can be used (either based on models or e.g. using Meteonorm) but comparative investigations showed that deviations from the infield data are registered, particularly during the seasons with low solar radiation. x designing the energy mix considering the share that will be covered by using renewables. It is to notice that in an energy mix the systems have to synergistically function, thus being more performant in terms of output as compared to a simple summing up the components. This implies considering the multi-functionality of each component system and their overall match: e.g. a solar-thermal system used for DHW, heating and cooling should be complementary designed with a ground coupled heat pump used in direct and reversed modes. Following this concept, an energy mix was designed and implemented in the Solar House and, based on the lesson learned, the energy mix was implemented at an extended scale in the RESREC Centre, [2, 20]. The optimization studies were done considering the L7 laboratory building, with a total area of 1560 m 2, and yearly average energy demand of 69 kWh/m2. An energy mix consisting of a ground coupled heat pump can cover 80% of the total thermal energy demand and a solar-thermal system, covers the rest of 20%. The electricity required to power the solar-thermal system and the heat pump can be provided by a photovoltaic array. This energy mix was selected considering the output, the available solar energy, and the initial investment.
601
Ion Visa and Anca Duta / Energy Procedia 85 (2016) 594 – 602
All the thermal functionalities can be covered over the year by this energy mix and an excess heat is generated by the 50m2 of flat plate solar thermal collectors, as Fig. 7 shows. For a regular residential environment, this leaves room for further optimization, in terms of tuning the STS:HP power ratio, or adding new functionalities (e.g. pond, pool, etc.) [kWh/year] 100000 75000 65817 50000 25000 0
9865 16454
4244
Heating
DHW STS Useful
18601
6440
367 Cooling STS Excess
Electricity
HP
Fig. 7. Thermal energy components produced by the energy mix: solar-thermal system (STS 20%) and heat pump (HP 80%) and the electricity consumption to power the mix
When designing solar-thermal systems or energy mixes involving these, one has to outline that the costs are important issue in the relation with the end-users; these costs significantly vary with the building energy efficiency. For the 80:20 STS-HP energy mix a simulation based on common market costs was done, considering two scenarios that cover 50% and, respectively, 70% from the total energy demand by using renewable energy systems (RES), as presented in Fig. 8.
Fig. 8. RES Initial investment according to the energy efficiency of the building
602
Ion Visa and Anca Duta / Energy Procedia 85 (2016) 594 – 602
The financial effort for implementing renewables in a larger extent (70%) is not much higher as compared to the 50% share, if the building is LEB; however, costs are almost double for a building that has an average energy consumption of 110 kWh/m2/year (corresponding to what today is considered to be a “well refurbished” building). 4. Conclusions The paper reviews the main barriers that limit the large scale implementation of solar-thermal systems in the built environment, beyond the DHW production. Addressing issues like efficiency, durability, architectural integration and costs, the papers presents novel, integrated solutions at conceptual and applicative levels. These solutions can be further refined when stepping on to the next level: sustainable communities, where the integrated approach building – community allows the design of efficient and affordable sustainable energy mixes.
Acknowledgements We hereby acknowledge the project EST in URBA, PNII-PCCA Type 2 contract no. 28/2012 financially supported by the Romanian National Research Council.
References [1] The Directive 2010/31/EU of the European Parliament and of the Council of 19 May 2010 on the energy performance of buildings, Official Journal of the European Union 53 (2010), eur-lex.europa.eu/ LexUriServ/LexUriServ.do?uri=OJ:L:2010:153:0013:0035:EN:PDF (last access 20/08/2015) [2] Visa I., Moldovan M.D., Comsit M., Duta A., Improving the renewable energy mix in a building toward the nearly zero energy status, Energy and Buildings, Vol. 68(A), 2014, pp. 72–78 [3] Kalogirou S.A., Chapter 3 - Solar Energy Collectors, in Solar Energy Engineering (Second Edition), edited by Soteris A. Kalogirou, Academic Press, Boston, 2014, pp. 125-220 [4] Munari Probst M., Roecker C. Towards an improved architectural quality of building integrated solar thermal systems (BIST). Solar Energy, Vol. 81, 2007, pp. 1104–16 [5] Visa I., Duta A., The Built Environment In Sustainable Communities, in Sustainable Energy in the Built Environment – Steps Towards nZEB, Springer Proceedings in Energy, 2015, p. 3-30 [6] Pirasteh G., Saidur R., Rahman S.M.A., Rahim N.A., A review on development of solar drying applications, Renewable and Sustainable Energy Reviews, Vol. 31, 2014, pp. 133-148 [7] El-Sebaii A.A., El-Bialy E., Advanced designs of solar desalination systems: A review, Renewable and Sustainable Energy Reviews, Vol. 49, 2015, pp. 1198-1212 [8] Li B., Oliveira F.A.C., Rodríguez J., Fernandes J.C., Rosa L.G., Numerical and experimental study on improving temperature uniformity of solar furnaces for materials processing, Solar Energy, Vol. 115, 2015, pp. 95-108 [9] Moldovan M., Visa I., Burduhos B., Energetic autonomy for a Solar House, Environmental Engineering and Management Journal Vol. 10, 2011, pp. 1283–1290 [10] Methodology of calculation of the energy performance of building, Mc-001/1,2, 3 (2006), 4 (2009) (Romanian language). [11] COST Action TU1205 – Overview of BISTS state of the art, models and applications, COST Office, 2015 [12] Gao L., Zhao J., Tang Z., A Review on Borehole Seasonal Solar Thermal Energy Storage, Energy Procedia, Vol. 70, 2015, pp. 209-218 [13] Aydin D., Utlu Z., Kincay O., Thermal performance analysis of a solar energy sourced latent heat storage, Renewable and Sustainable Energy Reviews, Vol. 50, 2015, pp. 1213-1225 [14] Visa I., Duta A., Comsit M., Moldovan M., Ciobanu D., Saulescu R., Burduhos B., Design and experimental optimisation of a novel flat plate solar thermal collector with trapezoidal shape for facades integration, Applied Thermal Engineering, Vol. 90, 2015, pp. 432-443 [15] Duta A., Isac L., Milea A., Ienei E., Perniu D., Coloured Solar-thermal Absorbers – A Comparative Analysis of Cermet Structures, Energy Procedia, 48, 2014, pp. 543-553 [16] Burduhos B.G., Visa I., Neagoe M., Badea M., Modeling and optimization of the global solar irradiance collecting efficiency, International Journal of Green Energy, Vol. 12, 2015, pp. 743-755 [17] Neagoe M., Visa I., Burduhos B., Moldovan M., Thermal Load based Adaptive Tracking for Flat Plate Solar Collectors, Energy Procedia, Vol. 48, 2014, pp. 1401-1411 [18] Ciobanu D., Visa I., Tracking system type cam mechanisms for parabolic trough collector, Acta Technica Napocensis, 2007, pp. 115-120 [19] Ciobanu D., Visa I., Duta A., Solar Thermal Collectors Outdoor Testing in Saline Environment, Energy Procedia, Vol. 48, 2014, pp. 707-714 [20] Moldovan M., Visa I, Neagoe M, Burduhos B. (2014) Solar heating & cooling energy mixes to transform low energy buildings in nearly zero energy buildings. Energy Procedia, Vol. 48, 2014, pp. 924-937