Matthias Rommel

Application of transparent insulation materials in improved flat-plate collectors and integrated collector storages

Abstract Within the work on new collector designs for the Central European climate, investigations on two different collector systems using Transparent Insulation Materials (TIMs) are presented in this paper. The thermal and optical properties of a polycarbonate honeycomb material are discussed with respect to the design of improved flat-plate collectors and integrated storage collectors with TIMs. Measurements on a collector with this material proved the good collector performance in the temperature range of 80 to 140°C. But the collector is not stagnation proof, as the honeycombs start melting at 120°C. The paper also describes a collector with newly developed, temperature resistant glass capillaries, with which a stagnation temperature of 261 °C was measured. Finally, new measurements of an integrated collector storage with polycarbonate honeycomb are reported which confirm the good performance of this simplified solar domestic hot water system.

A new transparently insulated, bifacially irradiated solar flat-plate collector

Abstract A new type of transparently insulated flat-plate collector was developed. It reaches higher efficiencies at low irradiation values or high operating temperatures than any other collector type known. Both sides of its absorber are covered with transparent insulation material and both sides are irradiated. Thus, the heat losses of the collector related to the total absorber area are distinctly reduced. An optical efficiency of η0 = 0.72 and a temperature dependent U-value of U(ΔT) = (0.95 + 0.0076 ΔTK−1) W m−2 K−1 were measured with an outdoor test facility. The bifacial-absorber collector is considered to be the best option for the DHW system of the energetically self-sufficient solar house in Freiburg because of its outstanding winter performance.

Membrane Distillation for Solar Desalination

Membrane distillation (MD) is a hybrid thermal/membrane desalination process in which pure water vapour from a salt solution passes through a hydrophobic membrane, driven by a difference in temperature, and condenses on the opposite side. This chapter starts with a detailed explanation of the principles behind membrane distillation. The four main types of MD technology are then discussed and the technical advantages and disadvantages of each technology are outlined, focusing on the crucial features for coupling MD with solar thermal energy. Heat and mass transfer phenomena are examined with regard to the influences of temperature polarisation and salt concentration on process performance. Finally, available semi-commercial MD systems are briefly presented with details on solar thermally driven MD systems for the autonomous desalination of brackish and sea water.

Solar thermally driven desalination systems with corrosion-free collectors

There exist various desalination systems that work at operating temperatures of 60 to 80°C. As an example, the multi-effect ambient pressure desalination system with free convection of air may be mentioned. It is described in /1/ and /2/. Like other concepts and desalination systems, it is very well suited for operation with thermal solar collectors, thus using renewable energy resources. For solar thermally driven desalination systems, special corrosion-free collectors have to be developed. The paper describes the work carried out so far at the Fraunhofer ISE in Germany.

The importance of the sampling frequency in determining short-time-averaged irradiance and illuminance for rapidly changing cloud cover

The sampling interval is an important parameter which must be chosen carefully, if measurements of the direct, global, and diffuse irradiance or illuminance are carried out to determine their averages over a given period. Using measurements from a day with rapidly moving clouds, we investigated the influence of the sampling interval on the uncertainty of the calculated 15-min averages. We conclude, for this averaging period, that the sampling interval should not exceed 60 s and 10 s for measurement of the diffuse and global components respectively, to reduce the influence of the sampling interval below 2%. For the direct component, even a 5 s sampling interval is too long to reach this influence level for days with extremely quickly changing insolation conditions.