2
System Description, Categorization, and Comparison

Jörn Ruschenburg and Sebastian Herkel

Summary

In the first step, several ways to analyze and categorize solar and heat pump (SHP) systems are presented. A graphical tool is introduced for visualizing the essence of different system concepts. The main criteria for analysis and categorization are found on component level (characteristics of solar collectors, heat pump, sizing, etc.) as well as on system level. For example, systems may cover space heating, domestic hot water (DHW) generation, and even space cooling - possibly but not necessarily more than just one of these functions. A system categorization approach is introduced - featuring parallel, series, and regenerative interactions between solar collectors and heat pump - that is applied throughout this book.

The precondition for defining performance figures and test methods for solar and heat pump systems (see Chapter 4) is a review of the market-available systems, investigating the relevance of nonstandard components and configurations. Such a review is presented, conducted on international level and followed by analysis regarding technical solutions on both component level and system level. Within this survey, carried out by IEA SHC Task 44/Annex 38 participants from several countries, 128 market-available solar and heat pump systems were identified. Most companies offer "conventional" systems with flat-plate collectors for both space heating and preparation of DHW. Still, manifold alternatives and technological as well as market-specific particularities are found. For example, solutions with photovoltaic-thermal collectors or heat pump solutions with solar thermal energy as only source exist since years ago.

2.1 System Analysis and Categorization

The aim of this chapter is to present possible ways to analyze and categorize existing and even future solar and heat pump systems. There are five main criteria to describe a solar and heat pump system: (i) the type of heat demand to be served; (ii) the low-temperature heat source(s) of the heat pump; (iii) the form(s) of energy used to drive the system; (iv) the function and placement of storages in the system; and (v) the interactions between these components. In addition, the systems can be described by the type of components used, by the sizing of components, and by the control. Thus, the reader should be aware that there is no global way of categorization that could meet all demands.

2.1.1 Approaches and Principles

In the literature, various specifications are analyzed to describe or compare SHP systems [1]. Depending on the respective interests, independent authors focus on parameters that seldom coincide. The aspects chosen for the scope of this chapter and their usage by other authors are listed in Table 2.1.

Table 2.1 Examined parameters and their application in literature

Provenance and distribution are more organizational than technical parameters, though there might be, for example, climatic design influences when comparing North and South European systems. System functions include DHW preparation as well as space heating and cooling. A system concept is usually defined by the interaction of heat pump, collector, and storages (cf. Section 2.1.3). Heat pump and collector characteristics may refer to several properties discussed in the respective subsections.

The possibilities to set up categories are equally varied. For example, the systems can be described by the type of applied components such as flat-plate, unglazed, or evacuated tube collectors, or alternatively by the refrigerant used in the heat pump cycle. The behavior of SHP systems depends also on the location, on the sizing of the components, and on the control. So, the definition of categories is multifaceted and strongly influenced by its purpose.

In this chapter, a graphical representation is introduced to systematically analyze and compare solar and heat pump systems. Afterward, these examinations result in a categorization approach.

2.1.2 Graphical Representation of Solar and Heat Pump Systems

The visualization presented in this chapter, first published by Frank et al. [1], is similar to energy flow charts that are frequently used in building energy engineering. Instead of a whole building, it is the heating system that is illustrated centrally against white background, including energy-storing (blue objects) and energy-transforming components (orange objects). The analysis of many combined solar and heat pump systems resulted in the finding of five recurring components. They comprise collector, heat pump, and backup heater, complemented by storages, namely, one on the source side and one on the sink side of the heat pump. For these typical components, fixed positions are defined. Specifications such as the collector type may be chosen. As defined boundaries (gray background), environmental energy (green objects) enters the system from above, final or "to-be-purchased" energy - in case of electricity generating systems even "bidirectionally traded" energy - from the left side (dark gray objects), and useful energy such as DHW leaves to the right (red objects).

The information provided by the coloring is in any case an additional feature, that is, it is not essential for understanding. In theory, any heat losses would be shown leaving the system downward. However, because of the purely qualitative nature of the approach, component sizes, efficiencies, and so on are not shown, and thus no losses. A label for the manufacturer's and the concept's name is added in the lower left part.

The final step is the depiction of energy flows connecting certain components. In doing so, the figure is enhanced to become a qualitative energy flow chart. The line style refers to the carrier medium (water, brine, and refrigerant) or indicates driving energies such as solar irradiation, gas, or electricity. A simple example for a complete visualization is given by Figure 2.1.

Fig. 2.1 Introductory example for the visualization scheme

It has to be pointed out that all possible operational modes of one system (excluding defrosting) are shown simultaneously in one scheme. All components appearing in a particular system are depicted as filled and nonexistent components remain as shaded frames as placeholders for orientation and comparability purposes. This arrangement results in energy flows mostly in left-to-right and up-down directions, though, of course, exceptions of this rule are possible.

In Figure 2.2, the presented visualization scheme is applied to typical systems. Here, a simplified hydraulic scheme is used as well, ignoring details such as backup heating elements. The comparison between the figure parts gives an impression of the visualization method's capability.

Fig. 2.2 Simplified hydraulic schemes (left) and corresponding visualizations (right) of different solar and heat pump systems

2.1.3 Categorization

System concepts are defined by the way the heat pump and the solar subsystem interact. Introductory works about the topic are found in Refs [7,8]. The distinctions made in this book, fully described in Ref. [9], are shown below.

Collector and heat pump independently supply useful energy (space heating and/or DHW), usually via one or more storages. This configuration is denoted as parallel, independent from the heat pump's source(s).

The collector acts as a source of the heat pump, either as exclusive or as additional source, and either directly or via a buffer storage. This configuration is denoted as series.

The use of solar energy to warm the main source of the heat pump, in this case usually ground, is denoted as regenerative.

It is important to see that series and/or regenerative modes do not exclude each other within one system. Therefore, many system concepts are in fact combinations of these modes.

The regenerative approach - described in detail by Kjellson [10] and Meggers et al. [11] - could possibly be regarded as a subset of the series concept. There are conceptual and operational differences, though: The regenerative operation is usually applied to improve or at least to maintain the quality of the ground source for long timescales or merely to prevent solar collector stagnation. Consequently, regenerative operation usually occurs in summer, when highest solar availability and lowest heating demand concur, that is, when the heat pump is not in operation. Many systems were found on the market featuring a regenerative mode explicitly, partially without an intended series mode.

It has to be realized that parallel, series, and regenerative arrangements do not exclude each other within...