Solar thermal cooling system consisting of a parabolic trough collector field and a steam jet ejector chiller

Status

completed

Summary

Up to now, mostly sorptive cooling technologies are used for solar cooling applications. An alternative offers the steam jet ejector technology. The steam jet ejector chiller (SJEC) has the potential of high operational availability and low investment costs because of the simple process layout of a SJEC and the possibility to use water as motive steam medium and as refrigerant. The main advantages of the SJEC are a short response time (i.e. a good dynamic behavior), good part-load behavior and a simple system concept. The coefficient of performance (COP) of the SJEC is higher for low cooling water temperatures. For the operation of the SJEC motive steam is needed that can be produced using parabolic trough collectors (PTC). Contrary to flat plate collectors or evacuated tube collectors, PTCs can generate heat efficiently at temperatures significantly higher than 100°C.

A number of possible heat transfer fluids have been analyzed in the first part of the project. No fluid could be identified that can be used in both steam jet ejector chiller and parabolic trough collector without problems. Therefore, it was decided to separate the collector loop from the chiller by means of a heat exchanger for the rest of project duration. That means that water can be used as refrigerant and motive steam medium in the SJEC. On the other hand, the parabolic trough collectors are operated with ammonia water mixture (approx. 14%). This ensures freezing protection and at the same time makes direct steam generation in the collector possible. The advantages of the ammonia-water-mixture are its low viscosity, its high evaporation enthalpy and heat capacity and its low cost. Disadvantages are the health hazard (most relevant at high concentrations) and the incompatibility with non-ferrous metals such as copper, brass, aluminum etc. Towards steal ammonia even acts as a corrosion inhibitor. Therefore it is used in very low concentrations in power stations.

The collector, which was further developed for direct steam generation, was tested using the ammonia water mixture as heat transfer fluid. First experiences with the operation with this medium could be gained. Afterwards, the collector construction was improved in several ways.

In direct steam generation mode, problems measuring the heat capacity were encountered. The reason for this were that the flow rates are extremely small in direct steam generation mode if only one collector is tested, and the fact that one has to know the exact state of the steam (or steam liquid mixture) at the outlet of the collector to be able to set up an energy balance. If the steam is superheated, this is not a problem. However, the heat transfer between pipe and superheated steam is not good. Therefore it makes more sense to operate the collector with wet steam. If ammonia-water-mixture is used rather than plain water, the state of the steam is defined by the temperature and the pressure which are measured. For the test of the collector field with 10 troughs, this method was used in order to determine the collector efficiency.

The construction of the parabola was reengineered entirely compared to the prototype collector from the previous project. The new construction consists of deep-drawn aluminum segments. Using optical measuring equipment (otherwise used for large parabolic troughs in power plants) the contour accuracy of the mirror has been verified. Both the design concept and the manufacturing method have been assessed positively. The new concept can be manufactured more simply, faster and less expensively than the previous construction made from glass. In addition, the weight of the collector was reduced significantly.

Another improvement is the new selective receiver coating which has good optical properties and is available at low cost even for prototypes. However, there is still need for further development to optimize the coating and to ensure the long-term temperature stability. Contrary to the original planning, the receiver glass cover tube was not evacuated but filled with a noble gas. Evacuating the tube proved to be not feasible with the means available within this project. However, the noble gas filling reduces the heat losses of the collector significantly.

A collector array with 10 troughs including the tracking system was built at the test facility and was tested successfully. A reliable operation at a flow temperature of 200°C could be shown. The response time of the collector array is extremely short. Only less than 2 minutes after activation of the system, a temperature of 200°C is reached at the collector outlet. The measured thermal efficiency at this temperature was between 50 and 58%.

A concept for the operation of the overall system was elaborated and a fully automated steam jet ejector chiller with a cooling capacity of 5 kWth was planned and designed accordingly. The SJEC was connected to the parabolic trough collector field at the test facility of AEE INTEC in Austria in early summer of 2009.

The SJEC is connected to the parabolic trough collector field using two heat exchangers. In the first heat exchanger the ammonia-water steam is condensed, the heat is transferred to a hot water circuit for steam production. The ammonia-water condensate is then cooled further and thereby the feed water for the steam generator is preheated. For heat rejection a dry cooling with spraying function is used. Thanks to the spraying, cooling water temperature can be below ambient temperatures. The spraying is not in continuous operation but is turned on depending on the difference between wet bulb temperature and ambient temperature.

Both parts of the system (PTC and SJEC) have worked satisfactorily during the test period. The concept as has proven to work in practice. However, there is still need for optimization for both subsystems. Some parts of the collectors need to be improved regarding long term stability.

For the chiller, the next steps would be towards a reduction of size and components. The goal for the future would be a standardized chiller, which can be further developed in cooperation with an industry partner for series production.

Project management

DI Dagmar Jähnig
AEE - Institut für Nachhaltige Technologien

Project collaborator

Ing. Waldemar Wagner, DI Robert Hausner
AEE - Institut für Nachhaltige Technologien

Project or cooperation partner

• Dr.-Ing. Christian Dötsch, Dr. Peter Noeres, Dr. Clemens Pollerberg
  Fraunhofer Institut UMSICHT
• Univ.-Doz. Dipl.-Chem. Dr. Rudolf Pietschnig
  Karl-Franzens-Universität Graz, Institut für Chemie
• Ing. Richard Matthias Knopf
  Button Energy Energietechnik GmbH, Wien
• Dr. DI Manfred Peritsch
  Innovation Management Group
• Dr.-Ing. Klaus Hennecke, Dr.-Ing. Eckhard Lüpfert, Stefan Wilbert, Heiko Schenk
  Deutsches Zentrum für Luft- und Raumfahrt (DLR), Institut für Technische Thermodynamik, Köln, Deutschland

  Georg Knopf
  Button Energy Energietechnik GmbH, Wien

• Knopf Glastechnik, Wien
  Knopf Design, Wien
  Ing. Gerald W. Jungreithmayr
• Solution Solartechnik GmbH, Sattledt
  Klaus Reisner
• Reisner Kältetechnischer Anlagenbau, Holzwickede, Deutschland
• Thermodynamik, Köln

AEE INTEC, AEE - Institut für Nachhaltige Technologien
Dipl.-Ing. Dagmar Jähnig
Feldgasse 19, A-8200 Gleisdorf
Tel.: +43 (3112) 5886-28
Fax: +43 (3112) 5886-18
E-Mail: d.jaehnig@aee.at