Drainback solar thermal systems: A review

Abstract

Although solar drainback systems have been used for a long time, they are still generating questions regarding smooth functioning. This paper summarizes publications on drainback systems and compiles the current knowledge, experiences, and ideas on the technology. The collective research exhibits a lack of scientific publications dedicated to the drainback technology, however a significant number of patents have been published, detailing innovative technical solutions towards improvements and reliability. Based on the evaluation of drainback hydraulics, a detailed classification of this technology has been developed, with a brief description of each hydraulic typology. The operating modes have been split into three stages: filling, operation, and draining, which have been studied separately. A difference in the minimal filling velocities for a siphon development in the solar loop has been discovered in various reports. Specific features of the operation mode have been described. For the draining, existing mechanisms to initiate the emptying process have been identified and categorised. Finally, stateof-the-art hydraulic components for drainback systems have been established, with emphasis on their requirements. Based on those findings, the authors suggest potential future research paths in order to fill the knowledge gap and disseminate the drainback technology.

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

“…However, attention must be paid to disadvantages such as the need for adequate slope of the piping and the use of appropriate components to ensure that no water will remain in the installation when the circulation pump stops. A summary of the advantages and disadvantages of DB systems can be found in (Botpaev et al, 2016).…”

“…Very few scientific studies concerning DB technology have been recorded in the public domain. The contributions encountered are mainly technology and DB market reviews (Berner, 2008(Berner, , 2004Botpaev et al, 2016;Botpaev and Vajen, 2014). The technology is often presented as safe in stand-by, frost protected, low maintenance and easy to install.…”

Influence of Sector Coupling on Solar Thermal Energy: A scenario analysis of the German energy system

“…For example, in Denmark cost/performance ratio of solar thermal applications could be significantly improved by means of so-called drain back systems [1] where hydraulic system components are reduced to a minimum and hydraulic set-up is less prone to errors ( Figure 1). .…”

A Standard-Based Method to Simulate the Behavior of Thermal Solar Systems with a Stratified Storage Tank

“…The water temperature in the top portion of the solar field is reported to grow by 4-6 • C per minute, so that in only a quarter of an hour steam bubbles appear in the flow. In this respect, solutions with draining features like the so-called "drain-back" systems are a possible alternative [9] that, however, introduces other types of practical challenges such as loop design to ensure the necessary slope, pump sizing to provide the initial head required to fill the system, and suitability to cold climates.…”