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====== Solar heating check: practical experiences with 3500 solar thermal systems ======

This article is based on a paper published in the 56th volume of the Research Group - Energy efficiency and renewable energy: Conflict of goals or synergy?. The original article by// Peter Kafke// , is availabe in German in the proceedings, which can be accessed [[https://passipedia.de/medien/medien/veroeffentlichungen/uebersicht_protokollbaende/protokollband_56|here]]. \\
More English articles about 56th volume of Research Group are [[https://passipedia.org/phi_publications#nr56_energy_efficiency_and_renewable_energy|here]]


===== Introduction =====

The Federation of German Consumer Organisations (Verbraucherzentrale Bundesverband) provides a Solar Heating Check, in which solar thermal systems are inspected on site for a small fee, with funding provided by the Federal Ministry of Economics. More than 3500 operators have taken advantage of this offer so far. The main aim of the programme is to ensure in terms of consumer protection that customers who have spent 3,000 to 10,000 € on a solar thermal system get the appropriate value for their money. However, from the extensive experiences gained, it is possible to obtain information relating to the problems that frequently occur with solar energy utilisation in practice and how these problems can be solved.


===== What is the Solar Heating Check? =====


Fig. 1 is a schematic illustration which a client may find in the report after commissioning a Solar Heating Check by the consumer organisation. 


The green dots represent the places where the consultant measured temperatures in the system. These temperatures are measured using simple temperature data loggers; highly accurate data for research purposes is not collected here since the Solar Heating Check has to remain affordable for the consumer. The measurements take several days, which must include at least one sunny day.


Fig. 2 similarly shows an excerpt from a report of a Solar Heating Check for a customer by way of example.


[{{:picopen:pb56-6-1.png?600|Fig. 1: Part of a Solar Heating Check report}}]


[{{:picopen:pb56-6-2.png?600|Fig. 2: Section from a Solar Heating Check report}}]


In the report, green ticks indicate which aspects are in order and do not need to be changed. In contrast, the yellow triangles with an exclamation mark indicate the areas in which abnormalities have occurred that may need to be addressed.





Fig. 3 shows the graphical representation of the temperature measurements. The dashed line represents the global radiation measured at the nearest weather station. The red line is the flow temperature, the blue line represents the return flow temperature. In the example, it can be seen that the system is turned slightly to the west from the southern direction, because the highest collector temperature only occurs after the maximum global radiation. From such a diagram, it is often possible to read out quite a bit about the functioning of the system. 


[{{:picopen:ak56_6_fig_3.png?600|Fig. 3:	Section from a Solar Heating Check report}}]


===== Overview of results =====

Although the Solar Heating Check is not designed to carry out statistical evaluation of the systems, it is possible to draw general conclusions from the results. As part of the Solar Heating Check, the consumer advice organisation also conducts surveys among operators, which are also included in the report. Some conclusions from the evaluations and surveys can be seen in the following figures. 


From Fig. 4 it can be seen that the majority of users would give a rating of 1 to 3 in terms of satisfaction with their solar heating system, i.e. the users are largely satisfied.



[{{:picopen:ak56_6_fig_4.png?600|Fig. 4: Results of a survey on satisfaction with the system before the Solar Heating Check}}]


Unfortunately, this only partly matches the energy consultants' assessment. This starts with the fact that it is often difficult to gain an understanding of the system. As can be seen in Fig. 5, documentation is only available for 34 % of the systems with hot water generation only. This figure is as high as 41 % for systems with combined space heating support and hot water generation, i.e. the more expensive systems. The latter are presumably planned and executed more carefully by the technicians.


[{{:picopen:ak56_6_fig_5.png?600|Fig. 5: Results of a survey asking whether documentation for the system is available}}]


The functioning of the system could also be improved. The information in Fig. 6 refers to the first 1800 of the systems examined by the consumer organisations. Of these, 48 %, almost half, had problems. 


[{{:picopen:ak56_6_fig_6.png?600|Fig. 6: Representation of the most frequent sources of problems; the red line corresponds to 5 % (50%???)of the examined systems. The red bars are particularly critical functional errors.}}]


The points marked in red are particular noteworthy as these are especially relevant for the functioning: 


  * Problems with the fluid, often due to frequent overheating in summer
  * Leakage
  * Insufficient venting, which impairs the flow and in turn promotes excessive temperatures
  * Hydraulic problems
  * Problems with the temperature sensor, the function of which is a prerequisite for proper control
  * Problems with the control system itself


An example of a problem with the collector itself is shown in Fig. 7 on the left (defective vacuum tube in the middle of the left collector field), while unsuitable thermal insulation, which is neither UV-resistant nor adequately protected, can be seen on the right in Fig. 7. The thermal insulation of the pipes also inside the building is sometimes inadequate; in some places this is missing altogether. Uninsulated plugs of storage tanks are frequently encountered. 


{{:picopen:pb56-6-7a.jpg?300|}} {{:picopen:pb56-6-7b.jpg?200|}}


Fig. 7: Pictures showing problematic cases on the roof


Overall, the frequency of defects in solar heating systems is comparable to that of other comparable technical systems, but they are noticed less often. At least 7 % of the solar heating systems examined by us did not work at all, the temperature reached remained below 40 °C even with high levels of solar irradiation. As shown, the users are often satisfied despite this since the defective solar system is not noticed, because the heating system takes over its function. 


Experience has shown that systems that are regularly maintained have a higher availability on average. As Fig. 8 shows, only about one third of the solar thermal systems are maintained; for two thirds of the systems, maintenance is neither carried out by the operator nor via a maintenance contract.


[{{:picopen:ak56_6_fig_8.png?600|Fig. 8: Results of a survey on whether maintenance takes place}}]


===== Performance review of the systems =====
 


[{{:picopen:ak56_6_fig_9.png?600|Fig. 9: Survey results on post-heating shutdown in summer}}]


If the post-heating continues to run in summer, the user will not notice if the solar heating system is not working or is only working to a limited extent. Fig. 9 shows how often the post-heating is shut down in the summer. 


In the case of systems solely for hot water generation, the post-heating is not switched off in summer in over two thirds of the cases; in the case of systems providing heating support, the rate is slightly more favourable. Switching this off in summer is a simple and cost-free way to check the basic functioning of the solar thermal system.


[{{:picopen:ak56_6_fig_10.png?600|Fig. 10: Heat meter}}]


Functioning of the systems can be even better checked by means of a heat meter costing approximately 300 €, but in the majority of cases this is not available (Fig. 10). In these systems, only the temperature is measured, the volume flow is not measured. Errors in the solar circuit (due to overheating of degraded fluid, insufficient pressure, poor venting), which are not so uncommon, cannot be detected in this way. A minimum pressure sensor would offer support so that it becomes apparent at an early stage if the pressure in the solar heating system falls and the circuit doesn't work properly.

===== Conclusion =====

Simple systems in particular have a good chance of functioning properly. With fewer components, the probability of errors is lower, and fewer pumps, pipes and storage tanks cause fewer losses. With simple systems, the price of the generated kilowatt hour is also better, although at present it is generally higher than the heat price for post-heating which is always available.


Favourable conditions for a solar thermal system are a good installation situation (unshaded, south-facing roof, short pipe distances) and a hot water consumption that is not too low. When selecting a specific system, the Solar Keymark should be used as a guide in order to obtain a high quality. High-quality storage tanks are also important for a good net yield.


It is crucial for the functionality of the system to ensure that the solar circuit is operating. In short, this means that the system must be vented, the pressure must be maintained, the condition of the heat transfer medium must be good, it must be possible to check the available volume flow and it must be checked regularly. Maintenance also improves the situation somewhat. 


A simple but very effective functional check is to switch off the post-heating in summer. Installing a heat meter and checking the yield regularly is even better.


===== See also =====


[[:phi_publications:nr.56_energy_efficiency_and_renewable_energy]]

[[basics:energy_and_ecology:primary_energy_renewable_per]]

[[basics:energy_and_ecology:renewable_sources_of_energy]]