Planning and Installing Solar Thermal Systems

A Guide for Installers, Architects and Engineers

German Solar Energy Society (DGS)

• Publisher: Routledge
• Year: 2010
• ISBN:
  9781136541315
• Edition: 2
• Topic: Biological Sciences
• Subtopic: Ecology
• Index: Biological Sciences

Solar thermal systems available today offer efficiency and reliability. They can be applied in different conditions to meet space- and water-heating requirements in the residential, commercial and industrial building sectors. The potential for this technology and the associated environmental benefits are significant.

This fully updated edition of 2004's bestselling guide offers clear guidance on planning and installing a solar thermal system, crucial to the successful uptake of this technology. All major topics for successful project implementation are included. Beginning with resource assessment and an outline of core components, it details solar thermal system design, installation, operation and maintenance for single households, large systems, swimming pool heaters, solar air and solar cooling applications. Details on how to market solar thermal technologies, a review of relevant simulation tools and data on selected regional, national and international renewable energy programmes are also provided.

In short, the book offers comprehensive guidance for professionals who wish to install solar thermal technology and is a highly valued resource for architects and engineers alike who are working on new projects, electricians, roofers and other installers, craftsmen undertaking vocational training and anyone with a specialized and practical interest in this field.

Published with DGS

1 Solar Radiation and Arguments for its Use

1.1 Solar Radiation

1.1.1 Solar Energy

The most important supplier of energy for the earth is the sun. The whole of life depends on the sun’s energy. It is the starting point for the chemical and biological processes on our planet. At the same time it is the most environmentally friendly form of all energies, it can be used in many ways, and it is suitable for all social systems.

In the core of the sun a fusion process takes place in which pairs of hydrogen nuclei are fused into helium nuclei. The energy thus released is radiated into space in the form of electromagnetic radiation. As the sun is 148 million km from the earth, it radiates only a tiny fraction of its energy to the earth. In spite of this, the sun offers more energy in four hours than the human race uses in a whole year.

The age of the sun is estimated by astrophysicists to be about 5 billion years. With a total life expectation of 10 billion years the sun will be available as an energy source for another 5 billion years. Hence from our human perspective the sun offers an unlimited life.

Figure 1.1 The sun: basis of all life on earth

1.1.2 Astronomical and Meteorological Bases

On the outer edge of the earth’s atmosphere the irradiated power of the sun is virtually constant. This irradiated power or radiation intensity falling on an area of one square metre is described as the solar constant. This constant is subject to small variations influenced both by changes in the sun’s activity (sunspots) and by differences in the distance between the earth and the sun. These irregularities are mostly found in the ultraviolet range; they are less than 5%, and hence not significant in application of the solar constant for solar technology. The average value of the solar constant is given as I₀ = 1.367 W/m² (watts per square metre).

Even based on the astronomical facts alone, the amount of solar energy available on the earth is very variable. It depends not only on the geographical latitude, but also on the time of day and year at a given location. Because of the inclination of the earth’s axis, the days in summer are longer than those in winter, and the sun reaches higher solar altitudes in the summer than in the winter period (Figure 1.2).

Figure 1.2. The sun’s path at different times of the year at central European latitude (London, Berlin)

Irradiated Power, Irradiance, Heat Quantity

When we say that the sun has an irradiance, G, of for example 1000 W/m², what is meant here is the capability of radiating a given irradiated power, ϕ (1000 W), onto a receiving surface of 1 m² (10.76 ft²). The watt is the unit in which power can be measured. If this power is referred, as in this case, to a unit area, then it is called the irradiance.

When the sun shines with this power of 1000 W for 1 hour it has performed 1 kilowatt-hour of work (1 kWh) (Work = Power × Time).

If this energy were converted completely into heat, a heat quantity of 1 kWh would be produced.

Irradiated power,	ϕ(W)
Irradiance,	G(W/m^2)
Heat quantity,	Q(Wh, kWh)

Figure 1.3 shows the sequence over a day of the irradiation in London on a horizontal receiving surface of 1 m² (10.76 ft²) for four selected cloudless days over the year. It is clear from the graph that the supply of solar radiation, even without the influence of the weather or clouds, varies by a factor of about ten between summer and winter in London. At lower latitudes this effect decreases in strength, but at higher latitudes it can be even more pronounced. In the southern hemisphere the winter has the highest irradiations, as shown in Figure 1.4, which shows the sequence over a day of the irradiation in Sydney on a horizontal receiving surface of 1 m² on three average days over the year.

Figure 1.3. Daily courses and daily totals for irradiation in London

Figure 1.4. Irradiation on three different days in Sydney, Australia

Even when the sky is clear and cloudless part of the sun’s radiation comes from other directions and not just directly from the sun. This proportion of the radiation, which reaches the eye of the observer through the scattering of air molecules and dust particles, is known as diffuse radiation, G_dif. Part of this is also due to radiation reflected at the earth’s surface. The radiation from the sun that meets the earth without any change in direction is called direct radiation, G_dir. The sum of direct and diffuse radiation is known as global solar irradiance, G_G (Figure 1.5).

Figure 1.5. Global solar irradiance and its components

Unless nothing else is given, this always refers to the irradiation onto a horizontal receiving surface.¹

When the sun is vertically above a location the sunlight takes the shortest path through the atmosphere. However, if the sun is at a lower angle then the path through the atmosphere is longer. This causes increased absorption and scattering of the solar radiation and hence a lower radiation intensity. The air mass factor (AM) is a measure of the length of the path of the sunlight through the earth’s atmosphere in terms of one atmosphere thickness. Using this definition, with the sun in the vertical position (elevation angle, γ_S = 90°), AM = 1 (AM = 1/sin γ_S).

Figure 1.6 shows the respective highest levels of the sun on certain selected days in London and Berlin. The maximum elevation angle of the sun was achieved on 21 June with γ_S = 60.8°, and corresponded to an air mass of 1.15. On 22 December the maximum elevation angle of the sun was γ_S = 14.1°, corresponding to an air mass of 4. At lower latitudes, all elevation angles will increase: for example, at a latitude of 32° (north or south), the highest elevation angle will be 80.8° and the lowest angle will be 34.1°.

Figure 1.6 Sun’s level at midday within the course of a year in London and Berlin (latitude: 52°N)

The sun’s radiation in space, without the influence of the earth’s atmosphere, is described as spectrum AM 0. As it passes through the earth’s atmosphere, the radiation intensity is reduced by:

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Table of contents

• Cover
• Half Title
• Title Page
• Copyright
• Contents
• Foreword
• Chapter 1: Solar radiation and arguments for its use
• Chapter 2: Components of solar thermal systems
• Chapter 3: Systems for single-family houses
• Chapter 4: Installation, commissioning, maintenance and servicing
• Chapter 5: Large-scale systems
• Chapter 6: Solar concentrating systems
• Chapter 7: Solar heating of open-air swimming pools
• Chapter 8: Solar air systems
• Chapter 9: Solar cooling
• Chapter 10: Electronic media and software within the area of solar thermal energy
• Chapter 11: Marketing and promotion
• Appendix A: Glossary
• Appendix B: Relevant UK solar regulations and technical standards
• Appendix C: Nomenclature
• References and further information
• Index