Development of thermal solar systems with unproblematic stagnation behaviour

Status

finished

Summary

The improvement in the efficiency of thermal collectors achieved in the last few years has led on the one hand to the higher solar yields sought after, and on the other hand higher thermal loads in the solar system in the case of stagnation. In particular thermal solar systems to support heating (combi systems) frequently reach the stagnation condition in the summer due to the lack of consumption. Since the overall system is exposed to very high temperature loads in the event of stagnation damage is often encountered respectively problems with the plant if the system is unfavourably or wrongly designed (damage to components as a result of a too high temperature load, a reaction from the safety valve, water hammer, degradation of the heat transfer medium, etc.). The major part of these defects can be avoided if there is sufficient knowledge of the procedures in the stagnation condition as early as the planning phase. Within the framework of this project concrete approaches and solutions were elaborated with the 7 industrial partners involved in the project to construct thermal solar plants with an unproblematic stagnation behaviour. The specific steam generating capacity was recognised as the most important variable and dimension figure to evaluate the stagnation behaviour of collectors and systems. This allows the classification of different collector types respectively leading the internal pipes and the calculation of the maximum range of steam in the system. Apart from solar irradiation, the emptying behaviour of the collector and the system has most influence on the specific maximum steam performance. The influences on the emptying behaviour were examined using numerous measurements performed on the test bench of AEE INTEC (within two measuring periods - summer half-year 2001 and 2002) on representative individual collectors (1st measuring period, 6m², 6 types) and on typical hydraulic connections of individual collectors (2nd measuring period, 24 m², 3 types, 8 variants). The category of collectors and systems with a good emptying behaviour has a specific maximum steam performance of <= 50 W for each m² collector area, with the maximum possible solar irradiation, the category of flat plate collectors and systems with a poor emptying behaviour on the other hand has a specific maximum steam performance of up to 120 W/m² (vacuum tube collector up to 140 W/m²). From this characteristic number one can calculate the maximum steam range with the help of the thermal tube losses.All of the different hydraulic connections and pipe leadings variants tested (24 m², 3 collector types, 8 variants) revealed only a slight influence on the emptying behaviour compared to that of the individual collectors. Mind you some principles are to be observed when it comes to the point of external pipe leading. The use of collectors and systems with good emptying behaviour is a pre-requisite for stagnation-resistant and thus the long operation of systems without the need for maintenance.Apart from the structure of the collectors, as well as internal and external pipe leading the correct arrangement of the check valve within the check group of hydraulic components (the sequence is as follows: check valve - connection membrane expansion vessel - return line) is an important criterion to achieve lower steam power.If the steam range calculated in a certain system is so great that temperature-sensitive construction element can be reached, then some measures suitable for practice were elaborated to restrict these. The most favourable today is the use of a favourably priced and simple stagnation cooler in front of the expansion vessel which, assembled in a suitable geodetic position, restricts the steam range without the use of auxiliary energy to uncritical zones. In the present paper the dimensioning rules common to date for the membrane expansion vessel were modified quite decisively, with due consideration to the procedures during the stagnation condition, and summarised in a way suitable for practice.Steam bubble inclusions were recognised as one of the reasons for water hammers caused by a mixture of a steam and liquid stream in the T-pieces area within the collector and in the course of the pipelines reached by the steam. In the same way condensation pockets in pipe bottoms and longer horizontal pipelines cause water hammers. Collectors with a good emptying behaviour and systems and pipes which drop down persistently minimise the probability of water hammers occurring. Although water hammers are a disturbing influence in acoustic terms no impermissible pressure loads on system components were observed in the high-frequency measurements (resolution 20 ms) discussed here.The heat transfer media used in the first measuring period (summer half-year 2001) were analysed and compared with the thermal stagnation loads measured. In this respect a clear relation between an analytically detectable deterioration of the spare alkalinity and the thermal load of the residual liquid in the case of stagnation became apparent. Autoclave test at temperatures of 160 °C, 200 °C and 235 °C over a period of 6 weeks on the two types of media used TYFOCORâ LS and TYFOCORâ L demonstrate the thermal degradation and the load limits. If we transfer this to practice the results of heat transfer medium tests confirm the necessity for the collectors to be emptied as thoroughly as possible in the event of stagnation so that the lowest possible quantity of heat transfer medium - if at all, is subjected to higher temperatures during the "empty boiling process".The results of the project were published at five international conferences as well as in two professional journals.

Project manager:
DI Robert Hausner, Ing. Christian Fink, Ing. Waldemar Wagner, Richard Riva
AEE INTEC, Arbeitsgemeinschaft ERNEUERBARE ENERGIE
Institut für Nachhaltige Technologien

Project partner:
Sonnenkraft Vertriebs - GmbH
GREENoneTEC Solarindustrie GmbH
S.O.L.I.D. Gesmb
HÖKOTECH GesmbH
Tyforop Chemie GmbH
RESOL - Elektronische Regelungen GmbH
Gebr. Tuxhorn GmbH & Co KG

Contact

Ing. Christian Fink
AEE Arbeitsgemeinschaft ERNEUERBARE ENERGIE
Feldgasse 19
A 8200 Gleisdorf
E-Mail: c.fink@aee.at