Drain Back Systems in Laboratory and in Practice

Abstract

Drain Back systems with ETC collectors are tested and analyzed in a Danish -Chinese cooperation project. Experiences from early work at DTU, with drain back, low flow systems, was used to design two systems: 1) One laboratory system at DTU. 2) One demonstration system in a single family house in Sorö Denmark. Detailed monitoring and modelling/validation of the system in the DTU lab is done, to be able to generalize the results, to other climates and loads by simulation and to make design optimizations. The advantage with drain back, low flow systems, is that the system can be made more simple with less components and that the performance can be enhanced. Also problems with long term degradation of glycol collector loops are totally avoided. A combination of the drain back and system expansion vessel was tested successfully. It is very important to achieve a continuous slope for the pipes in the collector loop to have a safe reliable operation. The components should also be designed and marked so that only one correct mounting option is possible, like forward and return pipes to/from the collector of slightly different sizes or color. Adapted installer education and training is a very important step to have success with drain back systems. Practices used in glycol systems may give serious failures.

Drainback Solar Thermal Systems in Switzerland – Market Overview and Main Barriers

“…The most important is the lack of qualified professionals (Suter et al, 2003. Combined systems using DB technology with evacuated solar collectors have been studied experimentally and by simulation. Recommendations have thus been drawn up for a correct installation of this type of system: assembly of pipes on a continuous slope; rapid filling/starting of the installation is advantageous in order to reduce pumping energy; the risk of frost formation of the external pipes is important only under certain extreme temperature conditions (Perers et al, 2014(Perers et al, , 2015. More recently, Botpaev, (2017) conducted a series of experimental work on the fundamental principles of DB systems working with water.…”

Drainback solar thermal systems: A review

“…Other drawbacks mentioned by these authors are the low UV stability, the higher risk of sagging (depending on mechanical properties) and the performance of the fittings, not always made in an appropriate material. Corrugated stainless steel pipes might be used too; however their flexibility also increases the risk of water pockets creation and therefore problems with draining and filling of the system (Perers et al, 2015). As with plastic, the risk of sagging can be reduced by decreasing the spacing between the supports , obviously increasing the costs.…”

“…Independently from the configuration considered, the device initiating the draining should never be obstructed. Boye-Hansen and Furbo (1995) and Perers et al (2015) noticed during their experiments that the downcomer was connected too low, close to the HTF level in the drain back volume. As a result, rising water blocked the connection in the middle of the draining phase, creating two air volumes -one in the DB volume and one in the collectors.…”

“…A tank-in-tank design is another option to cope with this issue (Kutscher, 1985;Perers et al, 2015). Some authors also proposed the use of an additional DB tank in this configuration, notably in the case of high buildings, in order to minimize the pump power at the start of the system (Pei, 1982;Perers et al, 2015).…”

“…This also helps air venting in a glycol system and might be standard for all collectors in the near future. ETC may also be utilized in DB systems, with heat pipes (Perers et al, 2015) or with the manifold located at the bottom in case of direct flow (Frissora, 1981). Perers et al (2015) also reported that monitored ETC did not show signs of degradation after hundreds of "dry" stagnation cycles.…”