IEA SHC Task 42 / ECES Annex 29 WG A1: Engineering and Processing of PCMs, TCMs and Sorption Materials

An overview on the recent results on the engineering and characterization of sorption materials, PCMs and TCMs investigated in the working group WG A1 "Engineering and processing of TES materials" of IEA SHC Task 42 / ECES Annex 29 (Task 4229) entitled "Compact Thermal Energy Storage" is presented

Reduction of supercooling in paraffin phase change slurry by polyvinyl alcohol

Dispersed phase change materials (PCM) tend to supercool due to absence of nucleation promoting surfaces. Surfactants stabilizing phase change slurries (PCS) offer a useful surface to catalysing nucleation at the particle interface. Temperature dependent X-ray diffraction (XRD) and differential scanning calorimetry (DSC) analysis were used to investigate the crystallization of n–octadecane-water-slurries stabilized by common used surfactants (Triton X100, Tween60 and Span60) as well as by several polyvinyl alcohols (PVA) differing in degree of hydrolyses and polymerization. With PVA supercooling of these phase change slurries is reduced from 12 K down to 2 K independent of the particle size, whereas other surfactants reduce supercooling by 2 K. The results are discussed by taking into account nucleation acceleration through PVA crystals which are partially formed at the paraffin-water interface. The results show that PVA combines the two functionalities to act as surfactant and as nucleating agents in paraffin slurries

Characterisation and evaluation of a new phase change enhanced working solution for liquid desiccant cooling systems

Desiccant solutions play an essential role in desiccant cooling systems to absorb moisture from the process air. This paper presents the characterisation of a new working solution for liquid desiccant cooling systems. The new working solution was prepared through dispersion of micro-encapsulated phase change materials (MPCMs) into lithium chloride (LiCl) desiccant solutions to ensure that the dehumidification process was achieved under a low temperature condition and to improve thermal capacity and moisture removal efficiency of the mixture. The properties of the new solution, including density, enthalpy-temperature relationship, particle size distribution, thermal conductivity, and vapour pressure were characterised through either experimental tests or theoretical analysis. It was shown that the density and thermal conductivity of the new working solution slightly decreased with the increase of the mass fraction of the MPCMs in the mixture. The thermal capacity of the new working solution substantially increased in the melting temperature range of the MPCMs used. The vapour pressure of the new working solution decreased due to the existence of the MPCM particles. It is expected that the dehumidification efficiency of adiabatic dehumidifiers can be potentially improved when using this new working solution due to the decreased vapour pressure and increased thermal capacity of the phase change enhanced desiccant solution

Cycling stability of D-mannitol when used as phase change material for thermal storage applications

d-Mannitol is a potential phase change material to store thermal energy for process heat applications between 120 °C and 200 °C. This study presents thermal cycling stability tests of d-mannitol and its effect on melting enthalpy. Short-term cycling tests were carried out in differential scanning calorimeter in combination with air and nitrogen/vacuum as atmosphere. Results show that melting enthalpy decreases in contact with oxygen, decreases less in vacuum and stays almost constant in nitrogen atmosphere over the performed cycles. Afterwards, a long-term cycling test in nitrogen atmosphere was performed. Here melting enthalpy decreases about 9% during 500 cycles. If this effect cannot be overcome, the material is unsuitable for most applications. Fourier-transform infrared spectroscopy was carried out to evaluate chemical stability after the cycling tests. A possible hypothesis for a degradation mechanism of d-mannitol is presented. It assumes that degradation products might be produced during thermal treatment. By additional oxidation the production of further degradation products might be triggered. It is likely that more degradation products were produced in the long-term cycling test than in the short-term cycling test in nitrogen leading to a decrease of melting enthalpy

Characterization of an n-octadecane PCS in a 0.5 m³ storage tank test facility

Due to the increasing share of renewables in the energy system, research into thermal storage systems is becoming increasingly important. Phase change slurries (PCS) consisting of a phase change material (PCM) within a carrier fluid can be used as thermal storage. The advantage of phase change slurries is that they can be pumped through a hydraulic system independent of the state-of-matter of the emulsified phase change material. In this work a PCS containing 35 wt% Parafol 18 97 (Sasol Germany, n-octadecane, Tm = 27.3°C) as the PCM was characterized under realistic thermomechanical stress in a 0.5 m³ storage tank test facility. Measurements showed that at a temperature difference of 7 K (20°C – 27°C) the PCS was able to store 2.6 times more heat than water. Furthermore, the influence of PCS on the energy demand of a pump was investigated

Characterization of an N-octadecane PCS in a 0.5 m3 storage tank test facility

Due to the increasing share of regenerative energy, research into thermal storage systems is becoming increasingly important. Phase change slurries (PCS) can be used as a thermal storage system and consist of a phase change material (PCM) within a carrier fluid. This enables the pumping of the PCS in a hydraulic system independently of the state of matter of the PCM. In this contribution a PCS with 35 wt.% Parafol 18-97 (Sasol Germany, n-octadecane, Tm = 27.3 °C) as a PCM was characterized under realistic thermomechanical stress in a storage tank test facility at a scale of 0.5 m3. At a temperature difference of 7 K (20 °C â 27 °C) it was possible to store 2.57 times more heat in the PCS in comparison to water. Furthermore, the influence of PCS on the energy demand of a pump was investigated

Experimental and simulative characterization of a fin and tubes heat exchanger with PCM for process heat applications

Thermal storage is one option to make solar or conventional process heat applications more efficient. Phase change material (PCM) storages offer the potential to keep constant temperatures for different time periods. In this paper a fin and tubes heat exchanger filled with the sugar alcohol D-mannitol was characterized regarding its usability as PCM storage in the temperature range between 100 and 250 °C. D-mannitol melts at ca. 166 °C. Different heating and cooling experiments were carried out. The specific heat capacity of the PCM as well as the latent heat was determined. The heat flow into the storage was determined at different mass flow rates for heating and cooling processes. During the measurements thermal losses to the ambient were very high leading to high uncertainties. Due to the viscosity of D-mannitol natural convection between the fins seems to have no significant influence on heat transfer during melting of the PCM. Some results led to the assumption that the heat distribution in the heat exchanger was not even. A simulation model was created and a first validation was carried out. Geometry improvement with the aim of more even heat distribution and higher charge and discharge power will be carried out by using the simulation model and further measurements