Testing of solar air collectors
Christoph Thoma, Thorsten Weick, Jens Richter, Thorsten Siems, Matthias Rommel
Fraunhofer Institute for Solar Energy Systems ISE, Heidenhofstrahe 2, D-79110 Freiburg
Corresponding Author, christoph. thoma@ise. fraunhofer. de
Solar air collectors do have only a very small market share. About 20 years back, in the beginning of solar collector developments in Europe, there was some development work carried out. But rather soon in those times it became clear that for the development of small solar thermal systems, designed only for domestic hot water in single-family-houses there were more advantages for collector systems using a liquid as heat transfer fluid. Although in most systems a water-glycol mixture is used as fluid, the simplifying term "water-collectors" will be used in this paper. The development aspects for water-collectors and air-collectors were pretty much seen in a direct competition. But solar thermal energy has made tremendous steps forward in the meantime. We think that it is necessary now to assess the possibilities for the development of solar air collectors again and in the light of the new technical possibilities that are available now. Fraunhofer ISE has therefore started to improve its existing testing facility for solar air collectors. Up to recently, it was directly installed in our solar indoor testing facility with solar simulator. That was convenient and helpful for the developments that we carried out in collaboration with the industry so far. But now we started to re-design the components of the testing facility. It is now possible to carry out test not only in the laboratory with the solar simulator, but also on our outdoor testing facility with tracker. And even testing of systems already installed in the field are made possible. The paper gives an overview on the achievements obtained so far.
Keywords: solar air collector, collector testing, air, European collector test standards, EN12975 [15]
In figure 1 some thermodynamic properties of air and water are compared to each other. The differences in the properties do have important consequences not only for the development of air - collectors, but also for testing them.
|
parameter |
air |
water |
ratio water / air |
|
density p in kg m3 |
1,185 |
998,200 |
~ 842 |
|
specific heat capacity c in kJ kg ■ K (mass-based) |
1,004 |
4,179 |
~4 |
|
specific heat capacity c in kJ m1 ■ K (volume-based) |
1,190 |
4171,478 |
~3505 |
|
thermal conductivity X in W/(m K) |
0,026 |
0,599 |
~23 |
|
Table 1: Comparison of some thermodynamic properties of air and water: |
