cover

Cover

Related Titles

Title Page

Copyright

About the Editors

List of Contributors

IEA Solar Heating and Cooling Programme

Acknowledgments

Part I

Chapter 1: Principles

1.1 Introduction

1.2 Solar Irradiance in Technical Applications

1.3 Quantifying Useful Solar Irradiation

1.4 Solar Thermal Applications

1.5 Calculating the Solar Contribution

1.6 Conclusions

Chapter 2: Solar Thermal Market

2.1 Introduction

2.2 Collector Types

2.3 Regional Markets

2.4 Market Trends

Links Providing Updated Market Data and Forecasts

References

Chapter 3: Thermal Solar Energy for Polymer Experts

3.1 Solar Thermal Systems and Technical Requirements

3.2 Overview of Solar Thermal Applications

3.3 Solar Thermal Collectors

3.4 Small to Medium Size Storages

3.5 Sources of Further Information

References

Chapter 4: Conventional Collectors, Heat Stores, and Coatings

4.1 Collectors

4.2 Material Properties of Insulations

4.3 Heat Store

4.4 Other Components

4.5 Analysis of Typical Combisystems

4.6 Definition of Polymeric Based Solar Thermal Systems

4.7 Life Cycle Assessment Based on Cumulated Energy Demand, Energy Payback Time, and Overall Energy Savings

4.8 Cumulated Energy Demand, Energy Payback Time, and Overall Energy Savings for Conventional and Polymeric Based Domestic Hot Water Systems

References

Chapter 5: Thermal Loads on Solar Collectors and Options for their Reduction

5.1 Introduction

5.2 Results of Monitoring Temperature Loads

5.3 Measures for Reduction of the Temperature Loads

References

Chapter 6: Standards, Performance Tests of Solar Thermal Systems

6.1 Introduction

6.2 Collectors

6.3 Solar Thermal Systems

6.4 Conclusion

Part II

Chapter 7: Plastics Market

Reference

Chapter 8: Polymeric Materials

8.1 Introduction

8.2 Material Structure and Morphology of Polymers

8.3 Inner Mobility and Thermal Transitions of Polymers

8.4 Polymer Additives and Compounds

References

Chapter 9: Processing

Chapter 9.1: Structural Polymeric Materials

9.1.1 Introduction to Polymer Processing

9.1.2 Extrusion Based Processes

9.1.3 Injection Molding

9.1.4 Thermoforming

9.1.5 Fiber Reinforced Polymer

References

Chapter 9.2: Paint Coatings for Polymeric Solar Absorbers and Their Applications

9.2.1 Outline of Content

9.2.2 General Remarks about Selective Paint Coatings

9.2.3 Preparation of Selective Paints

9.2.4 Application Techniques for Spectrally Selective Paints

9.2.5 Conclusions

References

Chapter 10: Polymer Durability for Solar Thermal Applications

10.1 Introduction

10.2 Polymeric Glazing

10.3 Polymeric Absorbers and Heat Exchangers

10.4 Conclusion

References

Chapter 11: Plastics Properties and Material Selection

11.1 Introduction

11.2 How to Select the Right Material

11.3 Material Databases

11.4 Selection Criteria

11.5 Real Life Example: Standard Collector in Plastic (1:1 Substitution)

11.6 Summary

Part III

Chapter 12: State of the Art: Polymeric Materials in Solar Thermal Applications

12.1 Solar Collectors

Acknowledgment

12.2 Small to Mid-Sized Polymeric Heat Stores

12.3 Polymeric Liners for Seasonal Thermal Energy Stores

References

Chapter 13.1: Structural Polymeric Materials – Aging Behavior of Solar Absorber Materials

13.1.1 Introduction and Scope

13.1.2 Methodology

13.1.3 Results, Discussion, and Outlook

References

Chapter 13.2: Thermotropic Layers for Overheating Protection of all-Polymeric Flat Plate Solar Collectors

13.2.1 Introduction

13.2.2 Materials and Sample Preparation

13.2.3 Physical Characterization of the Polymers

13.2.4 Results and Discussion

13.2.5 Effect of Thermotropic Layers on Collector Efficiency and Stagnation Temperatures

13.2.6 Outlook

Acknowledgments

References

Chapter 13.3: Application of POSS Compounds for Modification of the Wetting Properties of TISS Paint Coatings

13.3.1 Introduction

13.3.2 Wetting of Surfaces

13.3.3 POSS Nanocomposites as Low Surface Energy Additives for Coatings

13.3.4 Anti-wetting Properties of Coatings with Smooth Surfaces – Lacquers for Polymeric Glazing

13.3.5 Anti-wetting Properties of Coatings on Rough Surfaces – TISS Paint Coatings

13.3.6 Conclusions

References

Chapter 14: Conceptual Design of Collectors

14.1 Introduction

14.2 Calculation of Collector Efficiency

14.3 Flow Optimization

14.4 Optimization of the Fluid Dynamics in Polymeric Collectors

14.5 Collector Mechanics

14.6 Conclusion

References

Chapter 15: Collectors and Heat Stores

15.1 Introduction

15.2 Solar Absorber Made of High-Performance Plastics

15.3 Flate Plate Collector with Overheating Protection

15.4 Flat Plate Collectors with a Thermotropic Layer

15.5 Solar Storage Tank with Polymeric Sealing Technology with Storage Volumes from 2 to 100 m3

Reference

Chapter 16: Durability Tests of Polymeric Components

16.1 Introduction

16.2 Twenty Years Outdoor Weathering of Polymeric Materials for use as Collector Glazing

Acknowledgments

16.3 Accelerated Lifetime Testing of a Polymeric Absorber Coating

16.4 Evaluation of Temperature Resistance of a Polymer Absorber in a Solar Collector

Acknowledgments

16.5 Determination of Water Vapor Transport through Polymeric Materials at Raised Temperatures

References

Chapter 17: Architecturally Appealing Solar Thermal Systems – A Marketing Tool in Order to Attract New Customers and Market Segments

17.1 Introduction

17.2 Architectural Integration as a Marketing Tool

17.3 Web Database

17.4 Examples

I-Box concept, Storelva/Tromsoe, Norway

Penthouse Vienna, Austria

“Home for Life” Concept House, Aarhus, Denmark

Bjoernveien 119, Oslo, Norway

Social housing in Paris, France

References

Chapter 18: Obstacles for the Application of Current Standards

18.1 Introduction

18.2 Internal Absorber Pressure Test

18.3 High-Temperature Resistance and Exposure Tests

18.4 Mechanical Load Test

18.5 Impact Resistance Test

18.6 Discontinuous Efficiency Curves

Reference

Glossary

Polymeric Materials

Abbreviations

Terms and Definitions

Solar Thermal Systems

Abbreviations

Terms and Definitions

Index

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Title Page

About the Editors

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Dr.-Ing. Michael Köhl, physicist, has been actively involved in the field of solar energy conversion since 1977. He presently works on service-life analysis of solar collectors and photovoltaic modules in the department Weathering and Reliability at Fraunhofer ISE. He was the coordinator of the EU projects SUNFACE and SOLABS and leader of Subtask 5 of the IP PERFORMANCE. Dr. Köhl is the current Operating Agent of the Task 39 “Polymeric Materials for Solar Thermal Applications”of the Solar Heating and Cooling Programme of the International Energy Agency IEA.

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Dr. scient. Michaela Meir, physicist, has been working with R&D on solar thermal and energy systems for more than 15 years, with particular focus on the development of solar collectors using polymeric materials. She is presently employed part-time by the University of Oslo and by Aventa AS. She is Chairman of the Norwegian Solar Energy Society board and leader of Subtask A “Information”of IEA SHC Task 39.

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Sandrin Saile, M.A. received her M.A. in British and North American Cultural Studies from the University of Freiburg. She joined the Fraunhofer ISE's department “Weathering and Reliability” in 2009 where she is responsible for the management and dissemination of the department's solar thermal activities, in particular the projects SCOOP and SpeedColl. Within IEA SHC Task 39 she is mainly active in Subtask A “Information” and played an active role in establishing the Solar Heating and Cooling Series.

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Prof. Dr. mont. Gernot M. Wallner, graduated with a “Diplomingenieur” degree in Polymer Engineering and Science at the University of Leoben (Austria) in 1994, and he obtained a PhD degree in the same field at the University of Leoben in 2000. In 2008 Prof. Wallner obtained a Venia Docendi in the field of “Functional Polymeric Materials” with special focus on solar energy applications. Since 2010, Prof. Wallner has been Deputy Head at the Institute of Polymeric Materials and Testing (IPMT) at the Johannes Kepler University Linz (JKU, Austria). Prof. Wallner is a member and leading person in several solar related working groups and committees. Since the establishment of IEA SHC Task 39 in 2006, he has been leader of the Subtask C “Materials”.

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Dr.-Ing. Philippe Papillon has been a senior expert in the field of solar thermal energy at INES (Institut National de l'Energie Solaire - CEA) since December 2005. He has been active in the field of thermal solar energy for more than 20 years, and has experience as coordinator as well as WP leader in European projects and also large national research projects. Beyond his research activities within INES, he is also an expert in European and French standardization committees, and is a member of the European Technology Platform on Renewable Heating and Cooling board. From 2006–2010 he acted as leader of the IEA SHC Task 39 Subtask B “Collectors”.

List of Contributors

Stephan Bachmann

University of Stuttgart

Institute for Thermodynamics and Thermal Engineering (ITW)

Pfaffenwaldring 6

70550 Stuttgart

Germany

Andreas Bohren

University of Applied Sciences Rapperswil HSR

Institute for Solar Technology SPF

Oberseestr. 10

8640 Rapperswil

Switzerland

Jay Burch

1617 Cole Blvd.

MS 52/2

Golden, CO 80401

USA

Stefan Brunold

University of Applied Sciences Rapperswil HSR

Institute for Solar Technology SPF

Oberseestr. 10

8640 Rapperswil

Switzerland

Jane H. Davidson

University of Minnesota

Department of Mechanical Engineering

111 Church Street SE

Minneapolis, MN 55455

USA

Harald Drück

University of StuttgartInstitute for Thermodynamics and Thermal Engineering (ITW)

Pfaffenwaldring 6

70550 Stuttgart

Germany

Ulrich Endemann

BASF – The Chemical Company

Segment Management Universal

BASF SE, E-KTE/IU-F 206

67056 Ludwigshafen

Germany

Stephan Fischer

University of Stuttgart

Institute for Thermodynamics and Thermal Engineering (ITW)

Pfaffenwaldring 6

70550 Stuttgart

Germany

Robert Hausner

AEE-Institute for Sustainable

Technologies

Feldgasse 19

8200 Gleisdorf

Austria

Steffen Jack

Bäckerstr. 50

31785 Hameln

Germany

Ivan Jerman

National Institute of Chemistry

L02 Laboratory for the Spectroscopy of Materials

Hajdrihova 19

1000 Ljubljana

Slovenia

Suanne Kahlen

Polymer Competence Center Leoben

GmbH Roseggerstr. 12

8700 Leoben

Austria

and

Borealis Polyolefine GmbH

St. Peter-Str. 25

4021 Linz

Austria

Matjaž Koželj

National Institute of Chemistry

L02 Laboratory for the Spectroscopy of Materials

Hajdrihova 19

1000 Ljubljana

Slovenia

Roman Kunic

Faculty of Civil and Geodetic

Engineering

Jamova 2

1000 Ljubljana

Slovenia

Reinhold W. Lang

Johannes Kepler University

Institute of Polymeric Materials and Testing

Altenberger Str. 69

4040 Linz

Austria

Andreas Mägerlein

BASF – The Chemical Company

Segment Management Universal

BASF SE, E-KTE/IU-F 206

67056 Ludwigshafen

Germany

Susan C. Mantell

University of Minnesota

Department of Mechanical Engineering

111 Church Street SE

Minneapolis, MN 55455

USA

Michaela Meir

University of Oslo

Department of Physics

Post Box 1048 Blindern

0316 Oslo

Norway

Axel Müller

HTCO GmbH

Rabenkopfstraße 4

79102 Freiburg i.Br.

Germany

Maria Christina Munari Probst

Ecole Polytechnique Federale de Lausanne

LE 0 04 (Building LE)

Station 18

1015 Lausanne

Switzerland

Fabian Ochs

University of Stuttgart

Institute for Thermodynamics and Thermal Engineering (ITW)

Pfaffenwaldring 6

70550 Stuttgart

Germany

Boris Orel

National Institute of Chemistry

L02 Laboratory for the Spectroscopy of Materials

Hajdrihova 19

1000 Ljubljana

Slovenia

Philippe Papillon

Institut National de l'Energie Solaire

Commissariat à l'Energie Atomique et aux Energies Alternatives

50 avenue du Lac Leman

73377 Le Bourget du Lac

France

Markus Peter

dp2 – Intelligent use of Energy

Mengeweg 2

59494 Soest

Germany

Lidija Slemenik Perše

National Institute of Chemistry

L02 Laboratory for the Spectroscopy of Materials

Hajdrihova 19

1000 Ljubljana

Slovenia

Micha Plaschkes

MAGEN-ECOENERGY

Product and Process Development

Kibutz Magen-85465

Israel

Christoph Reiter

Ingolstadt University of Applied Sciences

Esplanade 10

85049 Ingolstadt

Germany

John Rekstad

University of Oslo

Department of Physics

Post Box 1048 Blindern

0316 Oslo

Norway

Katharina Resch

University of Leoben

Institute of Materials Science and Testing of Plastics

Otto-Glöckel-Str. 2

8700 Leoben

Austria

Florian Ruesch

University of Applied Sciences Rapperswil HSR

Institute for Solar Technology SPF

Oberseestr. 10

8640 Rapperswil

Switzerland

Sandrin Saile

Fraunhofer Institute for Solar Energy Systems ISE

Department Weathering and Reliability

Heidenhofstr. 2

79110 Freiburg

Germany

Karl Schnetzinger

Advanced Polymer Compounds

Kurzheim 22

8793 Gai

Austria

Ingvild Skjelland

Aventa Solar/Aventa AS

Trondheimsveien 436a

0962 Oslo

Norway

Elke Streicher

University of Stuttgart

Institute for Thermodynamics and Thermal Engineering (ITW)

Pfaffenwaldring 6

70550 Stuttgart

Germany

Beate Traub

University of Stuttgart

Institute for Thermodynamics and Thermal Engineering (ITW)

Pfaffenwaldring 6

70550 Stuttgart

Germany

Christoph Trinkl

Ingolstadt University of Applied Sciences

Esplanade 10

85049 Ingolstadt

Germany

Jens Ullmann

Institute for Thermodynamics and Thermal Engineering (ITW)

Pfaffenwaldring 6

70550 Stuttgart

Germany

Helmut Vogel

University of Osnabrück

Albrechstr. 30

49076 Osnabrück

Germany

Gernot M. Wallner

Johannes Kepler University

Institute of Polymeric Materials and Testing

Altenberger Str. 69

4040 Linz

Austria

Karl-Anders Weiß

Fraunhofer Institute for Solar Energy Systems ISE

Department Weathering and Reliability

Heidenhofstr. 2

79110 Freiburg

Germany

Claudius Wilhelms

University of Kassel

Kurt-Wolters-Str. 69

34125 Kassel

Germany

Christoph Zauner

AIT Austrian Institute of Technology

Giefinggasse 2

1210 Vienna

Austria

Christoph Zimmermann

Institute for Thermodynamics and Thermal Engineering (ITW)

Pfaffenwaldring 6

70550 Stuttgart

Germany

Wilfried Zörner

Ingolstadt University of Applied Sciences

Esplanade 10

85049 Ingolstadt

Germany

IEA Solar Heating and Cooling Programme

The Solar Heating and Cooling Programme was founded in 1977 as one of the first multilateral technology initiatives (“Implementing Agreements”) of the International Energy Agency. Its mission is to “advance international collaborative efforts for solar energy to reach the goal set in the vision of contributing 50% of the low temperature heating and cooling demand by 2030.”

The member countries of the Programme collaborate on projects (referred to as “Tasks”) in the field of research, development, demonstration (RD&D), and test methods for solar thermal energy and solar buildings.

A total of 47 such projects have been initiated to date, 38 of which have been completed. Research topics include:

In addition to the project work, several special activities – Memorandum of Understanding with solar thermal trade organizations, statistics collection and analysis, conferences and workshops – have been undertaken. An annual international conference on Solar Heating and Cooling for Buildings and Industry was launched in 2012. The first of these conferences, SHC2012, was held in San Francisco.

Current members of the IEA SHC are:

Australia Finland Singapore
Austria France South Africa
Belgium Italy pain
Canada Mexico Sweden
Denmark Netherlands witzerland
European Commission Norway United States
Germany Portugal

Further information:

For up to date information on the IEA SHC work, including many free publications, please visit the web site www.iea-shc.org

Acknowledgments

The process of compiling this handbook was a collaborative effort involving the experience and expertise of many people. While the handbook consists of separate chapters with individual authors responsible for their own contents, it is also the product of a progressive development process to which all participating Task 39 experts (see, for example, http://www.iea-shc.org/about/members/task.aspx?Task=39) contributed either during discussions in experts meetings or by sharing their experience and the results of numerous funded research projects.

As it is not possible to thank all of the many people involved, we hereby acknowledge the essential support from funding agencies and the industry as a whole. Furthermore, we thank the Task 39 experts for their active and inspiring work – their contributions greatly added to the vivid and dynamic nature of Task 39 and naturally also to the contents of this book.

As is the case with every publication, the final result would have not been feasible without the help of a great number of people working hard behind the scenes. We would like to take this opportunity to acknowledge their passion and endurance, which have allowed the project to come to fruition. The editors and authors are very grateful to Bente Flier, Lesley Belfit, Dr. Frank Weinreich, and the rest of the Wiley-VCH team for their patient and constructive support throughout the project as well as to Sarah Greuter and Raphael Präg for their tireless assistance in preparing the manuscript.

Michael Köhl, Freiburg, Germany
Sandrin Saile, Freiburg, Germany
Michaela Meir, Oslo, Norway
Philippe Papillon, Chambery, France
Gernot M. Wallner, Linz, Austria

Part I

1

Principles

Markus Peter

1.1 Introduction

Apart from fossil and nuclear energy sources, so-called renewable energy is available. The already located and predicted resources of fossil fuels and fissile materials evidently underline that in a limited system like the earth only renewable energy can assure a long-lasting existence. Three categories of renewable energies, based on different primary sources, are available:

Within the ability and experience of humankind only these energy sources are inexhaustible.

Renewable energy like heat in the upper lithosphere or wind is a mixture of two or more primary sources. In the case of heat in the upper lithosphere geothermal and solar energy are relevant, for wind the rotation of earth, planetary motion, and solar irradiance are important.

The largest flux of energy available on earth is solar irradiation. The total power emitted by the sun is approximately 380 × 1018 MW. This corresponds to 62.6 MW m−2 related to a surface calculated from the diameter of the sun. Even sources with high capacity too, the potential of geothermal and tidal energy are orders of magnitude smaller than that of solar irradiation.1

The energy emitted by the sun results from different fusion processes (mainly the fusion of 4H+ to 1He). Owing to the distance between the sun and earth (approximately 1.5 × 108 km) the electromagnetic radiation arrives at the outer atmosphere highly diluted, with a power density of approximately 1367 W m−2. This so-called solar constant, measured outside the atmosphere, has a fluctuation range of about ±3.5% that is mainly caused by the variation of the distance between the sun and the earth, sun activity, and sunspots (Figure 1.1).

Figure 1.1 Geometrical proportions between the sun and the earth.

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The fraction of energy reaching the earth is only 0.045 · 10−6% of the amount emitted by the sun. Related to this the actual worldwide primary energy consumption is less than 0.01%.

Outside the atmosphere electromagnetic radiation from the sun consists of wavelengths in a range 10−20 to 104 m. However, solar radiation is mainly emitted in wavelengths between 0.2 and 5 µm. Approximately 90% of the radiation is emitted with wavelengths between 0.3 and 1.5 µm, reaching from the near UV-B to UV-A, visible light, and near-infrared. The sun radiates similarly to a black body with a temperature of approximately 5800 K. For most of the radiation the atmosphere is practically opaque; however, an optical window that is transparent for wavelengths in the range 0.29–5 µm enables radiation with a total power of approximately 1000 W m−2 to pass. While even further diluted while passing through the atmosphere, the aforesaid optical window enables more than 90% of visible light within the solar spectrum to reach earth's surface. In the range 0.38–0.78 µm visible light represents almost 50% of the transmitted energy. This range is most important for the biosphere and also for technical use.

Figure 1.2 gives the spectral distribution of solar radiation outside the atmosphere. The range shown contains approximately 95% of the radiation power of the solar spectrum.

Figure 1.2 Spectral distribution of solar radiation outside the atmosphere.

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Extraterrestrial solar radiation (outside the atmosphere) is the sole direct radiation coming from the direction of the sun. Direct solar radiation is characterized by the capability to cause shadow and the possibility to be concentrated. On entering the atmosphere, part of this direct radiation is reflected (scattered) or absorbed by aerosols, dust, and diverse molecules (e.g., H2O and O3). The measure of extinction within the atmosphere depends on the amount and kinds of particles and the length of the path. In this respect and for calculating of the solar irradiation that is available on earth, the so-called air mass is an important figure. For solar radiation with perpendicular (normal) incidence an air mass of 1 is defined. An air mass of 1.5 corresponds to an incidence angle, θZenith of 48.19 ° (Figure 1.3).

Figure 1.3 Path of solar radiation through the atmosphere.

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The scattered part of the direct radiation becomes diffuse radiation. Different to direct radiation, diffuse radiation (idealised) has no defined direction and for this reason is not capable of causing shadow and cannot be concentrated.

The total irradiance that is available on earth is given by:

(1.1) equation

where

G is the global solar irradiance,
Gb is the direct (beam) solar irradiance,
Gd diffuse solar irradiance.

The ratio between the radiation that is reflected to outer space by the atmosphere or after striking earth's surface and the radiation reaching the atmosphere is called albedo. The albedo ranges from less than 10% for forest areas to approximately 90% for fresh snow. Mainly depending on properties of the atmosphere and changes of earth's surface (e.g., ice/snow), the average albedo is around 30%. Besides the different amounts of solar energy reaching the diverse areas of the globe, particularly high values of the albedo in the Arctic and Antarctic regions are driving forces of the climate. Changes of the albedo, for example, due to melting of ice or snow, seem to cause significant climatic changes.

1.2 Solar Irradiance in Technical Applications

The energy emitted by the sun introduces different forms of appearance of solar energy on earth. Evaporation, rainfall, and melting of ice, ocean current and waves, movements and heating of the atmosphere and earth's surface, as well as the production of biomass are typical forms where solar energy appears, often indirectly. Photovoltaics and solar thermal systems represent almost direct ways of using solar irradiance by involving technical equipment. While photovoltaic is the direct conversion of sunlight into electricity, the thermal utilization of solar energy is versatile and characterized by numerous different applications. Solar thermal systems are applicable even where heat up to several hundreds degree centigrade is needed, for instance in industrial processes. Compared to photovoltaics, thermal applications often show advantages in efficiency and the comparatively simple feasibility of storage, even on large scales.

As described above, the fraction of extraterrestrial radiation hitting earth's surface, and hence technical equipment for using solar irradiance, depends on the air mass, that is, on the length of the optical path through the atmosphere in combination with the cloudiness and haze. After passing through the atmosphere the available irradiance consists of a direct and a diffuse part. In general, technical applications might use both direct and diffuse irradiance. In particular constructions, such as concentrating collectors or facilities with heliostats and central receivers, only direct irradiance can be utilized.

1.3 Quantifying Useful Solar Irradiation

For solar thermal applications and photovoltaic systems, but also with regard to passive use of sunlight in buildings, the amount of irradiation on surfaces with different orientations can be calculated. With respect to detailed performance predictions, particularly for solar thermal systems, the fractions of direct and diffuse irradiance on any surface have to be determined separately. As discussed, in the case of concentrating systems, knowledge of the direct irradiation is most important.

For direct irradiation, equations mainly referring to geometrical relations are available. For diffuse irradiation, geometric basics have been combined with correlations and models derived from empirical investigations and findings. In general the solar irradiance available on a horizontal surface ranges from around 100 W m−2 for a totally dull sky with only diffuse sunlight to more than 1000 W m−2 under clear sky conditions. Between these values the available amount of irradiance depends on the position of the sun in the sky, the cloudiness, and extinction processes within the atmosphere. In Central Europe, during a year roughly half of the irradiation is direct while the other half is diffuse irradiation. During winter the fraction of diffuse irradiance might reach 70%; however, approximately 75% of the irradiation reaches Central Europe between April and September. The amount of solar irradiation on one square meter horizontal surface per year is approximately 900 kWh in Northern Europe, around 1100 kWh in Central, and about 1400 kWh in Southern Europe. Within the sunbelt of the globe the yearly solar irradiation may exceed 2200 kWh m−2. Notably, independent of the location or time within the year the irradiance perpendicular (normal) to a surface might exceed 1000 W m−2. This is valid for all areas of the globe, whether having lavish sunshine like Africa or lower irradiation like regions at higher latitudes, for example, in Central or Northern Europe. Evidently, the differences in annual irradiation on a horizontal surface in various regions are caused mainly by the angle of incidence at the locations, the hours of sunshine, and the fraction of diffuse irradiance.

As the availability of solar irradiation on the globe is different between regions, the average ambient temperature differs as well. A rough classification reveals a correlation between regions with high solar irradiation and high ambient temperatures and regions with lower solar irradiation and lower ambient temperatures. Nevertheless, exceptions in both directions are common. For example, according to Meteonorm 5.1, with approximately 3.3 °C the average ambient temperature of Davos in Switzerland is quite low, while the irradiation on a horizontal surface reaches 1380 kWh m−2 h−1 – a fairly high value – for Central Europe.2

With respect to solar thermal collectors, as well as the irradiance the ambient temperature, inducing heat losses, is an important parameter.

1.4 Solar Thermal Applications

In combination with solar thermal systems the available amount of solar irradiation enables an economic use of solar energy and significant savings of conventional energy sources. In particular in a temperature range below 130 °C, which is the main focus for solar collectors equipped with polymers, space heating, domestic hot water preparation, and approximately 50% of the heat that is demanded by industry can be supported.

However, solar thermal systems are used over a much wider temperature range. On the basis of different operating temperatures in IEA-SHC Task 33, Solar Heat for Industrial Processes (http://www.iea-shc.org/task33/), three different fields of thermal applications for solar energy have been defined (Figure 1.4):

Figure 1.4 Global solar irradiation on a horizontal surface in Central Europe and differentheat demands in households and industry in schematic manner.

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Low temperature, below 20 up to 250 °C:

Medium temperature, 250–400 °C:

High temperature, from 400 up to approximately 1700 °C:

In all solar thermal applications solar irradiance is absorbed and converted into thermal energy. With respect to the particular ways of utilizing solar irradiance, passive and active use can be distinguished. A typical example for passive use is heating of buildings with solar irradiance entering the house through windows. This well-known kind of solar heating is particularly effective if the windows are equipped with glass. As glass is much more transparent to visible light and other high-energy wavelengths emitted by the sun, than for infrared radiation emitted by the floor, walls, and furniture in a house that have been heated by absorbing solar irradiance, solar energy is trapped. This so-called greenhouse effect is also present considering the globe, with the earth's surface as absorber and the atmosphere as transparent cover; technically it is applied and optimized in all kinds of solar thermal collectors featuring transparent covers.

From the absorber of a solar collector the heat is in most cases removed through a heat transfer medium, while in a few cases solar irradiance is directly absorbed by the heat transfer fluid. Commonly used fluids for heat transfer in solar heating systems for houses are water or mixtures of water and antifreeze/anticorrosion, in particular applications air or specific oils are employed. On leaving the collector, the heat is either transferred directly to the consumer and consuming devices, such as radiators or floor heating, or it is transferred to a heat store. Heat stores are introduced to decouple the supply of energy from the sun and other heat sources from the demands. Heat stores enable a level, high availability of energy at times without equal demand and vice versa; furthermore, they can supply energy with a power exceeding that of the original energy source. The capacity of heat stores range from several hours up to some days or a few weeks. In the case of pool heating, the pool water is a store with high capacity. A particular approach is the construction of seasonal stores designed to keep solar heat from summer for space heating and domestic hot water preparation during winter.

In some industrial applications and in the case of floor or wall heating an instantaneous supply of heat directly from a solar thermal system to the consumer might be possible.

In most applications the connection between the collector and the store or directly to the consumer is made by insulated pipes – in the case of air as heat transfer medium in the form of air ducts. Unless the motion of the heat transfer fluid is caused by gravity, introduced by differences in the density of the fluid between the collector and the store or consumer due to temperature differences, a circulation pump (or fan) has to be present in the collector loop.3 A controller determines the starting and stopping of this device as well as other temperature or time dependent operations. Thus, apart from the collector and the store the controller is another important component of many solar heating systems.

In cases where the amount of solar energy is not sufficient, auxiliary heating might be installed.

1.5 Calculating the Solar Contribution

The amount of heat delivered by a solar heating system can either be measured or calculated. Measurements are common for billing, system investigation and optimization, and, furthermore, for trouble-shooting in exiting set-ups. Calculation and numerical simulation of solar thermal systems are helpful to support general decisions during planning and design and in general cases of building and (re)construction.

Besides calculation “by hand”, personal computers with simple spreadsheet programs and programs for numerical system simulation are available. Owing to interactions of the components in solar heating systems and the dependency of system performance on different transient parameters, for example, weather conditions and loads, the applicability of static calculations is limited. For several decades numerical simulation has been a well-established method that can take various system designs, load structures, and climatic conditions into account.

In cases where solar energy is not the only source supplying heat, the fraction of energy delivered by the sun and the amount of energy delivered by other sources are of interest. To define the fraction of energy delivered by the solar part of a heating system and thus, for example, the savings of conventional energy, depending on varying system designs, different methods are proposed. As a general and simple approach, universal for all kinds of solar heating systems, the fraction of energy delivered by the solar part, fsol can be calculated as:

(1.2) equation

where

fsol is the solar fraction of a given system,
Qload is the total heat load to which fsol is referred to,
Qaux is energy supplied by heat sources different from solar.

In many cases Qaux is the auxiliary heat delivered to the system if the solar contribution is not sufficient.

In this definition the kind of auxiliary heating and its individual performance are included and not taken into account separately. The solar fraction fsol typically accounts for net energy and allows a rough and easy understandable calculation of the solar contribution. In the literature, other performance indicators are defined, such as the fractional energy savings fsav. Particularly for solar heating systems for houses, detailed information is given in the results of IEA-SHC Task 26.

1.6 Conclusions

The energy content of solar irradiation that is available on earth is several orders of magnitude larger than world's energy consumption of today. Even in countries not situated in the sunbelt of the globe, the most of the needed energy can be provided by solar irradiance using available techniques.

With respect to Europe almost half of the final energy consumption is connected to the production of heat, the major part for heating of buildings. In the European Union currently approximately 75% of energy consumption in buildings is related to space heating and the preparation of domestic hot water. Besides the housing sector, considering industry within Europe, approximately img of energy consumption is in the form of heat. About 30% of this heat is used at temperatures below 100 °C – which is easy to provide with solar thermal systems.

Notes

1. Solar energy approximately 5.6 × 106 EJ a−1; geothermal energy approximately 9.7 × 102 EJ a−1; tidal energy approximately 9.4 × 101 EJ a−1.

2. Meteonorm is a global meteorological database for applied climatology, developed by Meteotest in Bern, Switzerland (http://meteonorm.com/).

3. Circulation caused by gravity (thermosiphon effect) can only be achieved in cases where the store or the consumer is located above the collector. Today, for most technical applications in Central or Northern Europe the opposite is the case.