Solar thermal energy as renewable energy

Term Paper, 2019

22 Pages, Grade: 2,0




List of representations

List of abbreviations

1 Introduction
1.1 Definition renewable energy

2 Solar thermal energy
2.1 Development of solar thermal energy
2.2 Value for renewable Energies
2.3 Current application possibilities
2.4 Required periphery (hardware)
2.4.1 Collectors
2.4.2 Storage
2.4.3 Heat exchanger
2.4.4 Solar controller
2.4.5 Expansion vessel
2.5 Heating with solar thermal energy
2.6 Cooling with solar thermal energy
2.7 Extensions of solar thermal energy (with heat pumps)

3 Case study Namibia (fictional)
3.1 Technical realization
3.2 Realization costs
3.3 Economic efficiency

4 Reflection and conclusion


List of representations

Figure 1: Collectors efficiency (ministry of economics Baden-Württemberg / ITW)

List of abbreviations

h Stunde

kWh Kilowattstunde

L Liter

m² Quadratmeter

W Watt

°C Grad Celsius

1 Introduction

In Module 3.5 renewable Energies, we dealt extensively with various types of renewable energies. In the foreground were the better known renewable energies such as photovoltaics and wind power. In addition, we also dealt with geothermal energy, biomass, tidal power and solar thermal energy and about the planned grid expansion in the Federal Republic of Germany. In order to better understand and implement the creation and planning of renewable energy plants, a fictitious training center was established during the module in Namibia, which aims to use different concepts of renewable energy. Based on this fictitious training center, the benefits and the possibility of a solar thermal system also in Namibia will be investigated in more detail in this term paper. Not only the possibility of heating but also the possibility of cooling will be investigated. The research question derived from this is:

Is the installation of a solar thermal system in Namibia for heating water or cooling food energetically reasonable?

In the course of this term paper, general terminology will be discussed first and then pivot to the renewable energy form of solar thermal. This is examined over the current value for the total segment of the renewable energies, the necessary hardware (periphery), up to current application possibilities. In addition, a system for a fictitious example in Namibia is examined and its economic viability is considered. Finally, the work ends in a reflection and the corresponding conclusion.

1.1 Definition renewable energy

Renewable energy is defined as coming from sustainable sources, i.e., sources that are not "used up." This includes biomass, wind power, solar energy, hydropower and geothermal energy. This contrasts with fossil fuels such as oil, coal, lignite and natural gas. The latter are burned to convert energy and are therefore not renewable (cf. Renewable Energy Agency 2019).

2 Solar thermal energy

Solar thermal energy uses the energy of the sun to convert its radiation into heat. A certain amount of solar radiation is collected on a surface in order to heat this surface (preferably a contained liquid such as water). The solar thermal considered here is the "non-concentrated" solar thermal, on large areas, since this can already reach temperatures above 200°C (cf. Quaschning 2011, p. 133). "Concentrated" solar thermal is used for light collectors to achieve much higher use efficiencies (e.g. for process heat). In the case of solar thermal efficiency, a distinction must be made between the efficiency of the collectors and the efficiency of the system itself. Figure 1 shows on the right side the efficiency of evacuated tubes and flat plate collectors. It can be seen that this is approx. 80% at 0°C temperature difference between absorber and outside air and, depending on the collector, decreases to 60% (vacuum tubes) or 40% (flat plate collectors) at 100°C temperature difference.

Abbildung in dieser Leseprobe nicht enthalten

Figure 1: Collectors efficiency (ministry of economics Baden-Württemberg / ITW)

The efficiency of the system itself depends on how much solar energy is available throughout the year. This is less in Europe than in Africa. In Europe, the sun provides the most energy in summer, when hot water for heating is hardly needed. Solar thermal systems for hot water thus come to about 50% efficiency in Europe, and solar thermal systems with hot water and heating support to about 25 - 30% efficiency (co2online 2018).

2.1 Development of solar thermal energy

The origin of solar thermal energy is very old. Already in ancient times, burning mirrors and concave mirrors were used to light fires (cf. Solar System Guide 2019). Quaschning also hinted at the already known possibility of using solar energy with the story of Archimedes around 214 BC (Quaschning 2011, p. 85).

The precursor of today's solar collectors dates back to the 18th century by Horace-Bénédict de Saussure1. A simple wooden box with a black bottom and glass cover represented the first solar collector. Inside the collector, a temperature of 87°C was measured (Solaranlage Ratgeber 2019). In the middle of the 19th century, Augustin Mouchot2 took up Saussure's investigations again and developed them further to a solar steam engine. Further designs to tap solar energy (including the use of electrical energy) were deemed uneconomical by the French government (Mouchot, Griese, Weber 1987).

The use for heating and furthermore for cooling has only become more energetically viable in recent years.

2.2 Value for renewable Energies

The value for renewable energies is enormous. The figure of the global energy scenario (Agentur für Erneuerbare Energien 2011, P. 32 Fig. 23), makes clear by how much the use of solar energy will increase by the year 2100. Here, not only electrical energy is included, but also thermal energy. Furthermore, it is also evident that fossil fuels, which are not among the renewable forms of energy, will gradually decrease. This is certainly due to the fact that they are finite and people will have to gradually switch to using renewable energies even more than they already do. Apart from that, from the point of view of global warming, it is already elementary to replace the old energy sources and to provide almost 100% from ecological energy for consumption.

The figure of the development of the final energy consumption of private households (AG Energiebilanzen 2018) maps the historical development of the final energy consumption of private households from 1990 to 2016. Here, too, it can be clearly seen that the share colored in light green (renewable heat) is successively increasing. This type of heat is expected to continue to be a more cost-effective alternative than fossil fuels for private households. It should also be noted here that the Federal Republic of Germany strongly promotes heating with solar thermal energy for private households, businesses and public institutions (co2online 2017a). Thus, beyond the original basic subsidy, the innovation subsidy, the combination subsidy (e.g. with heat pump), the heat network bonus and still other additional bonuses can be promoted. Basically, the following solar thermal use is promoted:

- Systems for the exclusive preparation of hot water
- Systems for the exclusive heating support
- Systems for combined hot water preparation and heating support
- Systems for solar cooling
- Systems for the generation of process heat
- Extension of systems by up to 40 m² collector area
- Simultaneous installation of a gas or oil condensing boiler
- Connection of the system to a heating network
- Optimization of the heating system

It can be stated that the overall value for renewable energies seems to be very high, since solar energy is available "almost" unlimited (at least for a very long time) and does not cause CO2 emissions.

2.3 Current application possibilities

The application possibilities within solar thermal energy are manifold. First and foremost, the use of solar heat for heating liquids respectively hot water preparation either for use for showering / bathing or hot water preparation for heating rooms or buildings. Depending on the size of the building area, solar thermal can also be purchased to support the existing heating system. Public swimming pools use the possibility of solar thermal to bring the water to a certain temperature and to save heating costs. "Concentrated" solar thermal can be used to generate for process heat. Another possibility is to use solar thermal to cool food or medicine in warm areas (e.g. Africa). Decentralizing the solar thermal system to heat multiple residential units is also an option.

2.4 Required periphery (hardware)

The individual components for the construction of a solar thermal system are explained in more detail in this chapter.

2.4.1 Collectors

The collectors are divided into flat plate collectors, evacuated tube collectors and storage collectors. The latter are used for direct storage of the heated water without an external storage system. The problem with these collectors is the low temperature in winter. This can cause the water to freeze and the collectors cannot be used.

Flat plate collectors are the most widely used collectors in Europe. This is due on the one hand to the favorable conditions, but also to the fact that the temperature difference between the absorber and the outside air is lower than in Africa, for example. As a result, the efficiency (cf. chapter 2, fig. 1) between flat plate and evacuated tube collectors is marginally different (in Europe). They are composed of a housing, a transparent cover and the absorber. The heat is transferred from the absorber to the water, which flows through the absorber in tubes. The figure Processes in a flat-plate collector (Quaschning 2011, p. 101, Figure 3.15) shows the individual processes in detail. It can be seen that the collectors are to be set up in such a way that the sun can shine in directly (30-35° inclination). Heat losses occur on the glass pane through absorption and reflection, as well as on the absorber itself. The useful output is therefore also related to the materials used (cf. Quaschning 2011, p. 98 ff.).

The heat losses that occur due to convection (air movements) can be significantly reduced by using a vacuum. The vacuum tube collector makes use of this principle. There are two different designs for these: Heatpipe and continuous heat transfer tube.

In the heat pipe, a temperature-sensitive fluid is heated in the vacuum tube so that it can rise to transfer heat to the carrier fluid (e.g. water). Afterwards, the temperature-sensitive liquid condenses and runs back into the collector surrounded by the vacuum (Quaschning 2011, p 105, Figure 3.19, Design and operating principle of the vacuum tube collector).

In the version with the continuous "heat transfer tube, the heat transfer fluid runs directly through the collector." (Quaschning 2011, p. 105).

The vacuum cannot be maintained forever, so individual tubes must be replaced after a certain period of use. The initial costs of evacuated tube collectors are higher than for flat-plate collectors, but maintenance by replacing individual tubes is cheaper than for flat-plate collectors. There, only the whole collector can be replaced. Evacuated tube collectors consume less floor space for the same output, which is due to the effectiveness of the vacuum.

2.4.2 Storage

In the case of the storage tank, a distinction must also be made between different storage options. The drinking water storage tank is used for hot water, i.e. cooking, bathing or showering. This holds about 300 - 500 liters and is often installed in single-family houses.

The buffer tank is used when heat is to be stored in order to make it available at later times. For example, during the day the heat is buffered in order to be able to operate the heating with it at night. This storage tank is mainly used to support (or completely) the heating. It holds between 750 - 1500 liters for a single-family house.

The combined storage tanks combine all the storage tanks and are a space-saving alternative. "They are heat storage for the solar thermal system, buffer for the boiler and serve to heat the drinking water." (co2online 2017b).

2.4.3 Heat exchanger

In solar thermal systems there are two circuits with fluids. One circuit consists of water with an additive (usually antifreeze). This circuit is used to absorb the heat in the collector. The antifreeze is used to prevent freezing and to improve heat absorption. In the heat exchanger the absorbed heat from the "solar circuit" is exchanged with the "service water", i.e. the water for cooking or bathing (drinking water circuit). The heat exchanger can be located directly in the solar storage tank (internal heat exchanger) or on the supply pipes (external heat exchanger). The internal variant can lead to scaling problems after a certain runtime, but it is more cost-effective (cf. co2online 2017b).

2.4.4 Solar controller

The solar controller controls the entire circuit. It permanently controls the temperature in the collector and the temperature in the storage tank. At a difference of approx. 4-8°C, it signals the pump that the water from the collector must be transported to the heat exchanger in order to supply the higher heat to the useful circuit. If the temperature in the storage tank is higher than that in the collector, the pump is switched off again.

2.4.5 Expansion vessel

This vessel is connected to the solar circuit and serves the solar fluid to expand due to the heat. It is intended to prevent lines or pipes from being damaged.

2.5 Heating with solar thermal energy

The greatest benefit of a solar thermal system is achieved when heating is combined with hot water preparation. This is mainly due to the fact that a larger heat storage tank is installed and can therefore store the thermal energy for longer. If the system is to support heating and hot water preparation, a combined storage tank is required, as there are a total of three different water circuits (heating water, drinking water and solar circuit).

In Central Europe, solar thermal systems are sized to support the regular heating system. In spring and autumn, the heating demand can be largely covered by solar energy (in summer as well, but there only the hot water preparation is interesting; not the heating) (cf. Quaschning 2011, p. 94f.). In winter, it is no longer possible to provide the required energy from the sun, so that the regular heating system provides the required thermal energy.

Quaschning also states that it is possible "to cover the complete heating energy demand by the sun. This then requires a seasonal storage system that stores heating energy from the summer for the winter." (Quaschning 2011, p. 95). This storage is titled seasonal storage and is a stratified storage to avoid turbulence between the cold and hot water. It takes advantage of physics in that the storage tank holds heat much longer through different layers as the heat successively rises to the top. However, the storage tank must be much larger in order to keep the heat for a very long time, and for purely economic reasons, such systems are not installed in single-family homes. In addition, the boiler and the house must be optimally insulated, which is usually not the case.

It is worth mentioning the possibility of decentralization of the heating system with several parties. Thus, in a new residential area to be developed (e.g., several single-family houses), an even larger dimensioned seasonal storage system could be realized (cf. Fisch 1998, p. 81 Fig. 7, System diagram of the solar system with long-term heat storage).


1 geneva naturalist

2 french high school teacher of mathematics

Excerpt out of 22 pages


Solar thermal energy as renewable energy
University of Bremen  (Institut Technik und Bildung)
Erneuerbare Energien
Catalog Number
ISBN (Book)
Übersetzung der Hausarbeit (V1243213) in Englisch.
Renewable energy, solar thermal energy, heating, sun, cooling
Quote paper
Anonymous, 2019, Solar thermal energy as renewable energy, Munich, GRIN Verlag,


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