Solar Thermal Energy Systems

Renewable energies meet an energy market that is characterized by an energy demand or rather an exergy demand, a supply by energy carriers and disposal of end products such as carbon dioxide (\(C{O}_{2}\)). Energy supply must satisfy the worldwide population growth as well as the increasing per capita energy consumption and this simultaneously without excessive consumption of resources. The potential of regenerative energies thus depends essentially on their properties, with which they can meet the market characteristics. The profile of the energy demand, its follow-up and production costs as well as the boundary conditions determine the energy market and thus define the framework conditions for the development of regenerative energy sources, which in the future must increasingly satisfy the aspects of sustainability and resource efficiency. This chapter sheds light on these aspects, introduces key terms and discusses in particular the energy market environment in which thermal solar energy operates and finally introduces the concept of sector coupling.

In this chapter the physical processes and the composition of the sun leading to the sun as radiation source will be described. From that the properties of extra-terrestrial irradiation close to the earth are derived. Depending on the orientation of a surface different amounts of irradiation is received (horizontal, tilted, tracked). The atmosphere is interacting with this radiation through absorption and scattering depending on a number of parameters (aerosols, clouds) and modifies the incoming terrestrial radiation on the surface of the earth. For collectors not only the beam radiation directly from the sun, but also diffuse components from sky and ground reflected albedo are important. Short-wave diffuse sky radiation depends on the clearness of the sky and sun position and is in general anisotropic. Models for the angular distribution of diffuse irradiation describe this behaviour. Statistical methods exist to derive the diffuse fraction of global irradiation based on the clearness index. The basic methods for measuring solar radiation for different locations are presented, with terrestrial methods and satellite measurements. Using long-term time series representative meteorological data sets for planning purposes can be constructed.

Solar collector systems always need to maximize the solar gain from the short-wave solar radiation of the sun and hence minimize optical losses. Many components and different materials are used in a solar collector to convert solar radiation into thermal heat. It is crucial to understand the principles and limitations of the interactions of transparent, reflecting, and absorbing components with the electromagnetic radiation. The concepts of spectral selectivity and optical concentration are introduced and several methods for achieving that are described.

Thermal technologies and thus also solar thermal always involve heat transfer within a body or between bodies. In some cases the heat transfer has to be enhanced in order to achieve a highly efficient optical conversion of solar radiation into heat, in other cases a minimal heat loss over the system boundaries to the environment is the target. In particular, the optical conversion of short wave solar irradiance (electromagnetic waves) to heat occurs in solids. Within the solid body the heat is released mostly volumetrically in thin layers or layer stacks causing not only temperature rise but also differential thermal elongation. This heat has to be transported via passive means towards a heat sink for utilization. At the same time, however, a body that is warmer than its surroundings loses this heat in the form of thermal radiation and convection, and as the temperature rises, this proportion increases rapidly. This conflicting feature emphasizes the excellent importance of understanding passive heat transfer mechanisms in solar thermal applications. In this context we consider as passive heat transfer processes such as heat conduction and heat radiation, which are not associated with a medium transport. Convection, however, which enhances heat transport through molecular heat conduction in fluid media by a movement of fluid volumes, will be treated in the Chap. 5. The driving mechanisms of passive heat transport are scalar quantities such as pressure and temperature, which characterize the thermodynamic state in a continuum, as well as waves, which propagate in a medium. The propagation of the waves in a solid or fluid medium, in turn, depends on its molecular thermo-physical properties and in cases of solids even its lattice structure. For this reason, any temperature difference leads unavoidably to a heat transfer changing the thermal properties of a solar thermal system, but also represents a key variable in component design. An important technical parameter here is, among others, the maximum permissible wall temperature. After an introductory description of the distinction between heat transfer and transport processes, the steady and transient heat conduction in gases, fluids and solids is covered. Successively, radiative heat transport in different configurations as well as methods to evaluate to transferred quantities are elaborated. Finally, as a result of a heat transport by conduction and radiation across interfaces mixed situations may occur, in which density differences in the adjacent fluid/gas are induced yielding to natural buoyant convective fluid motion increasing the heat transfer. Some selected cases of combined heat transfer problems are covered in the last section.

When solar radiation is converted into heat, thermal losses occur mainly due to passive heat transport modes as there are conduction through gases and solids, heat radiation and natural convection. In the latter one, already macroscopic mass transport driven by buoyancy effects can be observed. Convective heat transport is connected to momentum transport and movement of mass. Driven by mechanical pumps or ventilators the forced convective heat transport is supporting the thermal system by transporting heat with the help of a heat transfer medium from the place of optical conversion to the consumer. Effective heat and mass transport thus plays a key role in the design of a solar thermal system. Convective momentum and heat transport covers many aspects and is a-priori a nonlinear phenomenon. For example, flows can differ in the type of momentum transport, which can be forced, mixed or buoyancy-driven. In addition, the mass flow can have different flow patterns, for example laminar or turbulent. In the engineering treatment, a stability parameter plays a role in predicting the transition between the flow regimes. These regimes are even more complex in two-phase flows. Furthermore, the mass flow can be confined by walls, e.g. in tubes or between plates, often called bounded flows. On the other hand mass flow may fully include bodies as external flow for instance when an air flow occurs around a thermal storage tank. The interaction of mass flow and momentum transport within the media and with solid bodies not only leads to heat transfer but also to friction, which determines the hydraulic performance. External drives like pumps have to be dimensioned in a way that the pressure loss due to these hydraulic forces can be overcome. Last but not least, the effects of cold/heat can lead to phase changes, such as boiling, evaporation and condensation, which do not alter only the momentum characteristics but also the heat transfer characteristics of the transport. There is an extensive literature on convective momentum and energy transport in its various forms. The objective of this section is to elaborate the basic ideas of single-phase and multiphase momentum and energy transport, to show methods for classifying the flow type, and based on this, to present engineering approaches as well as relevant correlations for quantitative calculations, which can be used to carry out assessments.

Solar thermal collectors are the core components of solar thermal energy systems, converting the solar radiation into heat, which is transported to a demand location by active or passive means. Solar thermal collectors can use the whole solar spectrum, not only a specific spectral part. But converted solar radiation cannot be fully used as inevitable thermal losses from the hot absorbers to the colder environment cannot be completely avoided. Therefore, the design of solar collectors must compromise—depending on the specific application—between maximizing the optical gains and the minimization of thermal losses. Highly transparent covers and selective absorbers with small thermal emissivities are used to achieve this target. Concentration by mirror optics and consecutive reduction of receiver area is a powerful tool to obtain an even better energy balance also for very high temperatures. Stationary collector types without or with small concentration are described in their main characteristics and the internal processes described, as well as the concentrating collector types which need one- or two-axis tracking devices to be able to collect the direct solar radiation from the sun over the day. In a last part collector testing methods are described and discussed, as well as the stagnation of collectors at which the highest absorber temperatures are reached without any net energy output, as thermal losses and solar gains just cancel out.

High-temperature collectors using optical concentration and tracking, are described in this chapter. The determination and optimization of the optical performance of line-focusing and point-focusing technologies is key for reaching high temperatures. We classify therefore the collectors by their optical concentrator principle. Although also the selected heat transfer fluid has an impact on the temperature level which can be achieved, due to fluid stability and receiver material durability issues, this aspect is only treated in Chap. 10 on high temperature collector systems and fields. Here, the focus is on the main components and designs of individual collectors. As solar radiation is transformed into heat in the receiver, the design of this element is mainly responsible for minimizing the thermal losses. However, thermal losses play a minor role, the higher the optical concentration. We present receiver designs, determine the selection of materials, and define the performance. Finally, some line- and point-focusing collector types are compared with respect to their optical performance characteristics.

The storage of thermal energy is a core element of solar thermal systems, as it enables a temporal decoupling of the irradiation resource from the use of the heat in a technical system or heat network. Here, different physical operating principles are applicable, which enable the energy to be stored. Central parameters for the technical and economic evaluation of storage systems are storage capacity and length, whereby the storage materials and fluids in particular play a decisive role since they influence the charging and discharging dynamics as well as the efficiency. In the course of the chapter, various storage concepts, their basic mode of operation as well as terminology and evaluation variables are explained and presented.

Although the solar collector is the key component in a solar thermal system, depending on the temperature range needed and the heat transfer fluid found useful for this, many other system components like controls, pumps, heat exchangers, valves etc. are needed. When we investigate the application in buildings, room heating and hot water are the dominant applications, both restricted to a low temperature range up to 60 °C. The solar cooling and climatization of buildings are a rather special application which needs higher temperatures in the collector circuit, therefore it is treated in a separate Chap. 13. A large variety of system concepts are found nowadays, with passive and active heat transport to the consumer, which are described after a classification in this chapter. The components of a collector circuit are described and specified. The planning and dimensioning of a solar thermal collector systems has many aspects and can use simple rule-of-thumb design, but generally utilizes numerical simulation tools, where the time series of temperatures and heat flows in a solar thermal system are modelled for all individual components. These design rules and the use of simulation tools are detailed in the last sections of the chapter.

Based on the characteristics of individual collector units, a solar field has to be assembled which is sufficiently large to generate the required thermal power at a sufficiently high temperature. The heat transfer fluid is pumped through a solar field, transporting the heat from the collectors to the demand side heat exchanger or directly to the consumer. Additional optical losses may occur in a solar field due to mutual shading and blocking. Thermal losses, especially at nighttime, and hydraulic pressure losses to be overcome by pumping power, shall be minimized. Changes in the thermophysical properties of single-phase fluids as well as phase change of two-phase fluids have to be considered. The basics of solar thermal loop control are presented, boundary conditions for the operation and maintenance of the components are illustrated, and finally some line- and point-focusing systems are compared with respect to their operating characteristics.

Solar heat can be used for electricity production by means of coupling a collector technology with an energy conversion cycle. In the chapter, at first the principles of the thermal cyclic process are elaborated, and then different ideal single and two-phase cyclic processes are discussed in terms of thermodynamics, plant arrangement as well as optimization options and limitations. In order to categorize and analyse power plants technically and economically rating parameters are used, which are introduced in a next subchapter. Economic performance and competitiveness are essential for all electricity generation systems; therefore, cost structures of thermal solar power plants are elaborated and options for optimization are indicated. Complete solar thermal power station concepts, divided into plants based on non-concentrating and concentrating fields, are presented further with respect to their architecture, operational aspects as well as their prospects and limitations. Also, the aspects for hybridisation and cogeneration are addressed.

The application of solar thermal systems to cover the heat demand of industries is aa relatively new and developing field. The heat demand in industry worldwide is huge and is needed to drive many different processes. Temperature requirements, demand profiles and integration complexities shown in the chapter vary from case to case. For temperatures above the typical 50–80 ℃ of a solar thermal hot water system, improved medium temperature collectors have been developed. For these collectors the thermal efficiency at higher temperatures between 100 ℃ and 400 ℃ is achieved by vacuum technologies, improved transparent covers or by optical concentration. A direct and strong economic competition to renewable supply of heat is the optimization of heat recovery potentials using a well-known tool like Pinch technology. Energy efficiency and renewable energies have to be considered in parallel to arrive at the best solution. Up to now the standardization level is low, leading to individual engineering solutions. The philosophies cover direct supply of solar thermal heat to individual processes looking for the best match of technology and process, to the integration of solar thermal heat into existing supply and distribution systems. A classification and characterization of the solar integration using different types of heat exchangers is shown. This is also demonstrated and described in some detail for real installations.

Solar heat can also be used as a thermal drive to operate refrigeration and air conditioning systems. Starting from the definition of refrigeration and air conditioning, a quantification of the power demand and a description of different methods for refrigeration and cooling is given. In particular, thermally driven refrigeration processes such as closed-loop absorption and adsorption processes are presented, their operation and plant design are explained, and their limitations are discussed. A further class of air-conditioning is formed by open sorption-based systems, the basic ideas and performance of which are discussed. Finally, a comparison of the technical options is given.