Thermal accumulator with phase change

Latent heat storage tanks

Latent heat accumulators are primarily used to store thermal energy in the low temperature range (20 - 100 °C). The heat storage medium used, also known as PCM ( phase change material), absorbs heat during the phase change (usually solid-liquid) and releases it again during solidification. Latent heat accumulators are primarily used as buffer systems. The aim of research is, for example, to be able to buffer seasonal temperature fluctuations, thereby significantly increasing the efficiency of heat pumps, for example. Furthermore, heat storage systems can absorb waste heat from industrial processes and release it elsewhere, which can greatly reduce the overall energy requirements of the process.

Cylindrical latent heat accumulator

The cylindrical latent heat accumulator (Figure 1) exists in two configurations (Figure 2). The basic structure of the heat accumulator is the same in both configurations. The only difference between the two configurations is the number of copper coils responsible for the heat input. In the MK1 design, the heat input is provided by one copper coil, whereas in the MK2 design, the PCM is melted by two copper coils.

^{Figure 1: Front view of the cylindrical latent heat accumulator}

^{Figure 2: CAD model of the cylindrical latent heat accumulator. (a) - MK1. (b) - MK2.}

The inlet and outlet temperature and the mass flow of the demineralized water, which flows through the copper coil as a heat transfer medium, are recorded as test and influencing parameters. Furthermore, the temperature profile of the PCM is recorded using a probe consisting of 20 thermocouples. Using a specially developed neural network, it is also possible to investigate the phase distribution between solid and liquid PCM during the entire melting process (Figure 3).

^{Figure 3: Example of AI program output}

The cylindrical test setup is used to experimentally investigate the melting process of the heat storage medium. Due to the comparatively simple melting process, basic relationships and parameters influencing the melting process can be investigated. In addition, the generated data can serve as a reference for evaluating new simulation models. So far, three heat storage media (RT35HC, RT54HC, RT90HC) with different base materials (kerosene, fatty acid, surfactants) and phase change temperatures (melting temperature: 35 °C, 54 °C, 90 °C) have been investigated in the two configurations.

Finned heat accumulator

The finned heat accumulator (Figure 4) consists of an aluminum finned heat exchanger with copper tubes, which is installed in a plastic frame. Using the heat accumulator, the influencing variables for charging and discharging the accumulator are to be investigated in a setup that is closer to reality. As influencing variables, the mass flow and the temperature of the heat transfer medium, demineralized water, are investigated in a similar way to the cylindrical heat accumulator. Due to its compact design, the heat accumulator offers the advantage of comparatively simple generation of experimental data, which is only possible with very high effort or not at all with a numerical computer simulation. Furthermore, the heat accumulator offers the possibility of analyzing and evaluating measurement methods that serve to determine the state of charge of heat accumulators. As an example of such a measurement method, the change in pressure inside the container during the charging and discharging process is being investigated. So far, all experiments have been carried out with the PCM RT35HC from Rubitherm (melting temperature: 35 °C). Further experiments are planned with a PCM that has a phase change temperature of approx. 22 °C.

^{Figure 4: Front view of the finned heat accumulator. A pressure gauge for determining the state of charge of the storage tank can be seen at the top}