5 Integration of solar thermal collectors
In cooperation with ITW
5.1 General principles of a solar thermal system
Solar thermal technology is used to convert the solar radiation from the sun into heat. This heat can be used e. g. for the heating of domestic water, for space heating, for industrial processes or in combination with a thermally driven cooling process for solar thermal cooling. An active solar thermal system usually consists of the following major components: solar collector field (which absorbs the solar energy), heat exchanger(s), storage tank, security devices and controller.
More general information about solar thermal systems can be found in interior (See Part I – Chapter 5.2 - Active solar thermal heating. This chapter focuses entirely on the integration of the solar collector in the envelope of a historical building. Two main collector types exist: collectors using a liquid heat transfer medium (generally water with glycol for freeze protection) and collectors using air as heat transfer medium.
5.1.1 Collectors using liquid heat transfer fluid
This type of solar collector is most often used. Figure 34 shows examples for three different solar collector types all using water, or a mixture of water and anti-freeze, as liquid heat transfer fluid. The fluid transfers the heat from the collector to the store. Flat plate collectors are the most common in Europe. Evacuated tube collectors can produce higher temperatures and are more efficient at large temperature differences (between mean collector temperature and ambient), but typically they are also more expensive. Flat plate collectors consist of the absorber, the collector cover, the back and side insulation as well as the collector frame. In evacuated tube collectors the absorber is situated in an evacuated glass tube. Another type of evacuated tube collector is the so-called Sydney collector. The tubes are similar in construction to a thermos flask: they consist of an outer and an inner glass tube and between them is the vacuum insulation. The inner tube is provided with a selective absorption layer. A reflective mirror guarantees that solar radiation from different angles is caught and hardly any radiation is lost.
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Figure 34: Different solar collector types (from left to right): flat plate, evacuated tube, Sydney collector
Another solution which can be very nicely integrated in buildings is an unglazed absorber made of stainless steel. The absorber has a special geometry in a pad design for controlled draining with black chrome coating. The absorber is flexible and can be installed semicircle as shown in Figure 35. In order to prevent corrosion the heat transfer fluid must be without chlorine and chlorates, demineralised water with propylene glycol anti-freeze.
Figure
35: Residential
building in Vilanova i la
Geltrú [SES08]
General points of attention
When installing the collectors an appropriate orientation should be chosen to generate a significant yield of energy. Typically all orientations facing between east and west can be considered as appropriate. In this context it should be assured that no trees, chimneys or dormer windows cast shadow on the collector. Beside, there should be looked out if bats or birds use the roof for nesting.
When planning the installation it should be considered that the collectors and their associated equipment can have a life of typically 25 years, so a building could have several installations over the years. By carefully planning the placing and the removal of the installation, damage tot the building fabric can be limited.
Maintenance carries the potential for damage tot the fabric of the building. It is important to talk to the installer about the methods and frequency of routine maintenance. Typical maintenance periods are in the range of one tot two years.
Pipes run from the collector to the thermal store, usually a hot water tank where the heated is stored until required. These pipes need to be insulated, which increases the necessary space for the ducts and the openings to penetrate the building envelope.
Costs
In Germany typical average costs per square meter flat plate collector area are in the range of 300 to 500 EURO. For evacuated tube collectors between 500 and 700 EURO/m2 have to be paid.
5.1.2 Collectors using air as heat transfer liquid
Alternative to collectors using a liquid as heat transfer fluid, also air collectors can be used. In combination with controlled building ventilation the solar heated air can be used in an excellent way for heating. The system supports the existing heating system and improves the atmosphere throughout the whole building. With an additional installation of an air-water heat exchanger, it is also possible to prepare domestic hot water.
At the moment there exist four different types of air collectors on the market:
§ Unglazed air collectors: A dark coated steelsheet perforated with many small holes at a pitch of 2-4 cm is mounted on the façade. Air passes through the holes in the collector before it is drawn in the building to provide preheated fresh ventilation air.
§ Perforated air collectors with glazing: The metal fabric of the collector is perforated at the back and covered with an acrylic glass at the front side. The ambient air is absorbed through the pressure differences in the collector through the perforation and is solar heated.
§ Air collectors with glazing: This type of collector can be installed on the roof or integrated in the façade or in the roof. A cross-section of a solar air collector with glazing is given in Figure 36.
§ Hybrid solar air collectors: combination of a PV module and an air collector, which make grid-independent operations possible.
Figure 36: Air collector [GRA08]
Costs
Unglazed air collectors are very cost-efficient, especially when they replace conventional cladding on the building. The extra costs will only amount to 25 €/m² in that case. The costs for air collectors with glazing are around 440 €/m² collector area (installation, air ducts, control technique included) and in addition around 45 €/m² for the connection to the warm water supply [ENE07].
Points of attention
It has to be considered that the air ducts require a substantial amount of space. The installation of the air ducts is often difficult, especially with regard to existing buildings. From an energetic point of view it is not recommendable to install ducts with big cross-sections outside the heated building envelope, because of high leakage losses [ENE07]. For buildings with an air based heating system solar air collectors are an interesting option.
Practical examples
§ Schloss Trebsen, Leipzig, Germany: Project funded by Deutsche Bundesstiftung Umwelt (DBU), investigations at the castle with consideration of the conservation of ancient monuments and an attractive architectural integration of solar technology; project duration 2006-2008. As the castle is a listed ancient monument, the façade should be maintained. Therefore the integration of solar-air collectors and PV panels are made in the exterior of the building: An energy garden was built up in front of the building, that comprises collectors and PV panels as well as energy plants like rape, sugar-beet etc. [GRA08].
External Links
§ R. Hastings. Solar Air Systems -Design Handbook, James &. James Science Publishers Ltd, London, 2/2000. ISBN 1 873936 86 9.
§ Robert S. Hastings. Solar Air Systems – Built examples, James and James-Verlag, 35-37 William Road, London NW1 3ER. ISBN 1 873936 85 0
§ IEA Task 19, Solar Air Systems www.iea-shc.org/tasks/task19_page.htm
§ Oikos - Green Building Source, Design Strategy 64: Solar air collectors; http://oikos.com/green_products/index.php
5.2 Roof integration of solar thermal collectors
Solar thermal collectors can be installed as on-roof constructions, where the existing roof tiles remain on the roof and the collectors are installed over the roof tiles on construction rails (Figure 37). This is a proven, reliable solution for all common types of roofs and roofing.
Figure 37: Examples of on-roof installation
Regarding flat roofs, special support frames can be used to reach the optimum inclination for maximum radiation input or heat output respectively (Figure 38). They are either fixed by weights or connected with the building structure. However, this installation method is much more expensive than on-roof or in-roof installations. An appropriate solution when a tilted collector might have an undesirable visual impact is shown in Figure 39: the collectors are installed horizontally, allowing the collector to be hidden from ground-level view. In order to collect a large amount of solar radiation, only the absorber fins are inclined. This can be a smart solution especially for listed buildings where the view should be kept in original state. This solution requires vacuum tube collectors with forced circulation.
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Figure 38: Evacuated tube collectors installed on an elevation for flat roofs |
Figure 39: Example for „hidden“ installation of evacuated tube collectors |
The most basic types of in-roof collectors can be integrated simply and quickly into the cladding of the roof as shown in Figure 40 and Figure 41. The individual modules with surface areas of up to 16 m² are fastened directly to the battens of the roof. Collectors can be supplied in a wide range of sizes and shapes, and made fit to the exact shape of the roof. This process is simple and helps to keep costs within limits.
The aim of the refurbishment
of the
listed building shown in
Figure 42 of the year 1885 was
the integration
of the solar thermal collector in the exterior appearance of the
building. For
this purpose the dimension of the dormer has been adjusted to the size
of the
solar thermal collectors.
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Figure 40: In-roof integration of solar thermal collectors with preservation of building appearance |
Figure 41: In-roof integration of solar thermal collectors with preservation of building appearance [THE08] |
Figure 42: Roof integration of solar thermal colletors, source: Solifer |
Practical examples
§ English Heritage. Small-scale solar thermal energy and traditional buildings. 2008. page 7, figure 8: ground mounted collector at the Keeper’s Cottage, Woolbeding, West Sussex. http://www.english-heritage.org.uk/upload/pdf/17999-SolarThermal_08.pdf
§ Zukunft Haus. Renewable Energy, examples residential buildings: Coswig, Gartenstraße. http://www.zukunft-haus.info/de/projekte/erneuerbare-energien/beispiele-wohngebaeude/coswig-gartenstrasse.html
Points of attention
It is recommended not to fix collectors to thatch roofs, where the material is organic, as the thickness of the thatch decreases over time [HER08].
Before the installation a structural survey should be carried out. The weight of the collectors and fixing framework as well as additional snow and wind loads due to the installed collector will be borne by the roof rafters which must be capable of supporting the collector and its supporting framework [HER08].
During the installation it is usually normal for tiles and slates to get broken. Replacements for roofs with stone or old handmade tiles can be expensive and difficult to find. Drilling through them will render them unusable. Therefore it is advisable to investigate what type of roof covering is present and how to get replacements before undertaking any work [HER08].
Flat roof coverings have a life of 10 to 15 years before replacement. Thermal solar collectors usually have lifetimes of around 25 years. It is advisable to plan the installation at the same time as re-covering of the roof.
Another possibility for the use of solar thermal energy in historic buildings is to position the collectors elsewhere. If it is not acceptable to mount collectors to the roof, or if it is not physically possible to accommodate them, they can be positioned on another building or on the ground, if there is enough space. For an example please refer to [HER08], page 7, ground mounted collector at the Keeper’s Cottage.
5.3 Façade integration of solar thermal collectors
Façade-integrated collectors are becoming more and more popular. An advantage of façade-integrated collectors consists in a rather even irradiation of sunlight over the year, which is due to their vertical installation. This is very interesting for solar combisystems as a lot of irradiation can be used in winter, when the highest heat demand occurs for space heating. Further arguments for installing solar thermal collectors on the façade are that there is often not enough space on the roof or no suitable oriented roof area is available. This is typically the case for multi-family buildings with a relative high number of floors.
Façade-integrated collectors fulfil several functions: first of all, they function as a solar thermal collector, they improve the building’s thermal insulation, and they act as a weatherskin for the façade through the glazing and are at the same time a structural element of the façade. Besides, they contribute to lowering heat losses since the absorbers warm up even at low levels of sunlight in winter, thereby reducing the temperature difference between the internal space and the outer wall of the building. This multi-purpose use of building components may result in a considerable cost reduction.
In fact façade-integrated collectors have become a new element in architectural design and are available in a variety of colours, shapes and materials (Figure 43). Integration can be on the entire façade as shown in Figure 43 or they can be installed without attracting attention like it is shown in Figure 44 and Figure 45.
Façade collectors can be installed in lightweight or massive building structures. In principal there exist two different integration modes:
§ Ventilated façade collectors: The collector is installed in front of a façade with a ventilation gap (non-bearing ventilated façade).
§ Façade-integrated collectors: The collector is directly mounted on the façade without thermal separation between absorber and building envelope (non-ventilated façade). In this case insulation is a component of both the building and the collector.
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Figure 43: Façade integration of coloured absorbers with preservation of original building appearance |
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Figure 44: Example of façade integration of solar thermal collectors with preservation of building appearance [AEE08] |
Figure 45: Façade integration of collectors looking like a shutter without attracting attention [AEE08] |
Points of attention
It is important to stress that façade-integrated collectors have to meet the same requirements as other façade components. Special designed façade collectors are on the market which can be integrated into the building shell without condensation concerns. However, up to now there exist no standards of façade integration. Solar thermal collectors for façade integration have to comply with building standards that are valid for façade components.
Some important points have to be considered during planning and dimensioning of façade-integrated collectors: shading of the collectors by other building components, thermal bridging between the absorber or the piping and timber structures should be avoided. Furthermore the connection between collector and wall requires special attention from the point of view of structural calculation and building physics [BMV01].
Rules of thumb
Concerning the application of solar façades for solar domestic hot water systems the area of a façade-integrated collector must be increased by a factor of 1.5 as compared to a 45° tilted collector in order to achieve 40 % solar fraction [BMV01]. In case of higher solar fractions are desired, the relation between the two areas increases even more [BER02].
For solar space heating systems the relation is different. The higher the solar fraction of the system, the less extra collector area is necessary. Therefore, the optimum application for façade-integrated collectors are solar combisystems for domestic hot water and space heating [BER02].
In order to avoid overheating inside the building in the case of high irradiation, the following minimum insulation requirements for walls with a collector are recommended [BMV01]:
§ Massive construction: 5 - 8 cm collector insulation
§ Lightweight construction: 10 cm building insulation and 5 cm collector installation
With these insulations it can be ensured that the temperature increase for walls with a collector is smaller or equal to 1 K in comparison to a room without collector.
The annual global irradiation on the façade is some 30 % less than on 45° tilted surfaces. However, façade-integrated collectors benefit from increased irradiation through reflection from snow and they have less problems with snow covering than surfaces tilted under 45°. Taking these factors into account, annual global irradiation on a vertical surface is only 24 % less than on a 45° tilted surface [BMV01].
Concerning coloured absorbers an enlargement of the collector area from 20 % up to 70 % depending on the colour used is necessary due to the reduction of the thermal performance of the collectors caused by the coloured absorber. [STA01].
External links
§ Results of the EU project NEGST (new generation of solar thermal systems), workpackage 3: integration in buildings: http://www.swt-technologie.de/html/publicdeliverables2.html
§ http://www.english-heritage.org.uk/upload/pdf/17999-SolarThermal_08.pdf
§ Built for the sun – solar thermal collectors as architectural elements – Renewable Energy World; March-April 2007
§ www.brita-in-pubs.com (BIT = Brita in pubs information tool) BRITA = Bringing Retrofit Innovation to Application, demonstration of retrofit projects, computer tool for the first planning phase
§ http://www.helm.org.uk/server/show/nav.8578 - Guidance Library of Historic Environment
§
Fundamentals
of direct façade integration
http://www.nachhaltigwirtschaften.at/publikationen/forschungsforum/013/teil2.en.html
§ http://www.aee-intec.at/0uploads/dateien19.pdf - Façade integrated solar thermal collectors
§ http://www.aee-intec.at/0uploads/dateien32.pdf - Colourface - Planungsrichtlinien für farbige Fassadenkollektoren (planning guidelines for coloured façade collectors); Herausgeber: AEE INTEC; Autoren: Thomas Müller, Irene Bergmann, Robert Hausner, Karl Höfler, Werner Nussmüller; 42 Seiten, 53 farbige Abbildungen Gleisdorf, 2004
5.4 Semitransparent solar active façades
Semi-transparent solar-active glass façades are an interesting new development because solar integration in the façade does not necessarily consist of opaque elements any more. Transparent or semi-transparent elements offer new possibilities for architectural design and provide more daylight entrance.
The solar active glass façade shown in Figure 46 (the solar collector is the lower central part of the window) consists of a solar collector integrated into a conventional double-glassed window. To improve the collector efficiency reflector stripes are properly arranged. The absorber covers one half of the window area and therefore diffuse and direct irradiation can still enter a room behind the facade. Overheating of the room in summer can be avoided by an adapted geometrical position of the absorber fins. This semi-transparent solar thermal collector, which is developed by the company Robin Sun in France, was developed for domestic hot water and space heating.
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Figure 46: Solarglass of Robin Sun, France |
A smart solution is the new development of the semi transparent façade shown in Figure 47 and Figure 48. This construction was developed by Prof. Stefan Behling, Head of “Lehrstuhl 2 für Baukonstruktion und Entwerfen”, and his team as well as the German company Schott Rohrglas. The construction consists of a combination of a glass façade with an evacuated tube collector. The evacuated tubes are integrated as additional elements and the profile of the façade is used as header for the collector. This kind of construction shows a comparable thermal performance like façade collectors do (with respect to the effective absorber area). The effective U-value of the façade is lower than by using the same glazing without solar-active elements. This offers new possibilities of the reduction of the energy demand as well as for architectural design. Another advantage of the construction is that the solar façade acts at the same time as shading device and therefore provides a considerable potential of cost reduction in case external shading has to be foreseen anyway.
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Figure 47: Example of a semi transparent façade [UNI08] |
Figure 48: Example of a semi transparent façade [UNI08] |
External links
§ http://www.swt-technologie.de/html/publicdeliverables2.html; EU Project NEGST, workpackage 3 (Integration in buildings); WP3.D2 Inventory of existing requirements and guidelines.
§ http://www.renewables-made-in-germany.com/en/solar-thermal/
§ J.M. Robin, B. Flament, C. Vasile, A new solution for the architectural integration, Eurosun 2004
§
H.
Kerskes, W. Heidemann, H. Müller-Steinhagen Investigation of a Solar
active
glass façade
Eurosun 2004,
§ ibk2, University of Stuttgart, Germany, http://www.uni-stuttgart.de/ibk2/index_f.html
5.5 References
[SES08] Swiss Energie Solaire SA, Switzerland, www.energie-solaire.com
[GRA08]
Solar-Luft-System zur
Erwärmung von Gebäuden mit extrem großer Speichermasse; Untersuchungen
am
Schloss Trebsen in Leipzig unter Berücksichtigung des Denkmalschutzes
und einer
architektonisch ansprechenden Integration von Solartechnik; more
information on
the website of the manufacturer Grammer Solar:
http://www.grammer-solar.de/images/funde/Informationstafel080429.pdf
[ENE07] Energieeffizient sanieren. 2007, ISBN 978-3-934595-72-9
[THE08] Thermomax / ESTIF, http://www.estif.org/pictures/gallery/public-buildings/
[HER08] English Heritage. Small-scale solar thermal energy and traditional buildings. 2008. http://www.english-heritage.org.uk/upload/pdf/17999-SolarThermal_08.pdf
[AEE08] AEE INTEC / ESTIF, http://www.aee-intec.at/
[BMV01]
Nachhaltig
wirtschaften konkret, Forschungsforum; Façade-integrated thermal solar
installations, system and building physics fundamentals and
implementation of
results within the sub-program “Building of Tomorrow”; BMVIT
Bundesministerium
für Verkehr, Innovation und Technologie; 3/2001
http://www.nachhaltigwirtschaften.at/(en)/publikationen/forschungsforum/013/teil1.html
[BER02]
Systemtechnische und
bauphysikalische Grundlagen für die Fassadenintegration von thermischen
Sonnenkollektoren ohne Hinterlüftung, Endbericht, Projekt im Rahmen der
Programmlinie Haus der Zukunft, Impulsprogramm Nachhaltig Wirtschaften
im
Auftrag des BMVIT, 2002, (209 Seiten), Bergmann, I., Weiss, W.
http://www.aee-intec.at/0uploads/dateien18.pdf
[STA01]
Façade integrated
solar thermal collectors, Irene Stadler, AEE INTEC, Industry Workshop
und
Experts Meeting der Task 26 des Solar Heating and Cooling Program der
Internationalen Energieagentur (IEA-SHC), 03.04.2001
http://www.aee-intec.at/0uploads/dateien19.pdf
[UNI08] University of Stuttgart, Germany, http://www.uni-stuttgart.de/ibk2/index_f.html