Stefan Hiebler

Heat transfer enhancement in water when used as PCM in thermal energy storage

Efficient and reliable storage systems for thermal energy are an important requirement in many applications where heat demand and supply or availability do not coincide. Heat and cold stores can basically be divided in two groups. In sensible heat stores the temperature of the storage material is increased significantly. Latent heat stores, on the contrary, use a storage material that undergoes a phase change (PCM) and a small temperature rise is sufficient to store heat or cold. The major advantages of the phase change stores are their large heat storage capacity and their isothermal behavior during the charging and discharging process. However, while unloading a latent heat storage, the solid–liquid interface moves away from the heat transfer surface and the heat flux decreases due to the increasing thermal resistance of the growing layer of the molten/solidified medium. This effect can be reduced using techniques to increase heat transfer. In this paper, three methods to enhance the heat transfer in a cold storage working with water/ice as PCM are compared: addition of stainless steel pieces, copper pieces (both have been proposed before) and a new PCM-graphite composite material. The PCM-graphite composite material showed an increase in heat flux bigger than with any of the other techniques.

PCM-module to improve hot water heat stores with stratification

Hot water heat stores with stratification are a common technology used in solar energy systems and reuse of waste heat. Adding a PCM module at the top of the water tank would give the system higher storage density, and compensate heat loss in the top layer. The work presented here includes experimental results and numerical simulation of the system using an explicit finite-difference method. Experiments and simulations were carried out using different cylindrical PCM modules. With only 1/16 of the volume of the store being PCM, 3/16 of water at the top of the store was held warm for 50% to 200% longer and the average energy density was increased by 20% to 45%. Furthermore, these 3/16 of water were reheated by the heat from the module after being cooled down in only 20 min.

Verification of a T-history installation to measure enthalpy versus temperature curves of phase change materials

Phase change materials (PCM) are able to store thermal energy in small temperature intervals very efficiently due to their high latent heat. Accurate knowledge of the enthalpy as a function of temperature, or the storage capacity at each temperature, is the key to design any application. Conventional methods for thermal analysis however often lack sufficient accuracy or sample size to be applied to PCM. The T-history method is a simple method to determine the storage capacity of PCM and allows the use of large sample sizes. The experimental setup and methods of data analysis have been significantly improved in recent years. In this paper, a proper methodology to verify the correct setup and data analysis method of a T-history installation using standard materials with known properties is described and tested. The implementation of the T-history method has been done at the ZAE-Bayern. Three standard materials, gallium, water and hexadecane, were measured, as well as two commercial PCM, RT27 and sodium acetate trihydrate graphite compound (SAT+G). The obtained results confirm that the T-history installation can be used to analyse different PCM.

Thermal performance of sodium acetate trihydrate thickened with different materials as phase change energy storage material

The use of phase change materials (PCMs) in energy storage has the advantage of high energy density and isothermal operation. Although the use of only non-segregating PCMs is a good commercial approach, some desirable PCM melting points do not seem attainable with non-segregating salt hydrates at a reasonable price. The addition of gellants and thickeners can avoid segregation of these materials. In this paper, sodium acetate trihydrate is successfully thickened with bentonite and starch. Cellulose gives an even better thickened PCM, but temperatures higher than 65 °C give phase separation. The mixtures would show a similar thermal behavior as the salt hydrate, with the same melting point and an enthalpy decrease between 20% and 35%, depending on the type and amount of thickening material used.

Modeling of subcooling and solidification of phase change materials

Phase change materials (PCM) are able to store thermal energy in small temperature intervals very efficiently due to their high latent heat. Particularly high storage capacity is found in salt hydrates. Salt hydrates however often show subcooling, thus inhibiting the release of the stored heat. In the state of the art simulations of PCM, the effect of subcooling is almost always neglected. This is a practicable approach for small subcooling, but it is problematic for subcooling in the order of the driving temperature gradient on unloading the storage. In this paper, we first present a new algorithm to simulate subcooling in a physically proper way. Then, we present a parametric study to demonstrate the main features of the algorithm and a comparison of computed and experimentally obtained data. The new algorithm should be particularly useful in simulating applications with low cooling rates, for example building applications.

Calibration of a T-History calorimeter to measure enthalpy curves of phase change materials in the temperature range from 40 to 200??C

Thermal energy storage using phase change materials (PCMs) provides high storage capacities in small temperature ranges. For the design of efficient latent heat storage, the enthalpy curve of a PCM has to be measured with high precision. Measurements are most commonly performed with differential scanning calorimetry (DSC). The T-History method, however, proved to be favourable for the characterization of typical PCMs due to large samples and a measuring procedure close to conditions found in applications. As T-History calorimeters are usually individual constructions, performing a careful calibration procedure is decisive to ensure optimal measuring accuracy. We report in this paper on the calibration of a T-History calorimeter with a working range from 40 to 200??C that was designed and built at our institute. A three-part procedure, consisting of an indium calibration, a measurement of the specific heat of copper and measurements of three solid?liquid PCMs (stearic acid, dimethyl terephthalate and d-mannitol), was performed and an advanced procedure for the correction of enthalpy curves was developed. When comparing T-History enthalpy curves to literature data and DSC step measurements, good agreement within the uncertainty limits demanded by RAL testing specifications was obtained. Thus, our design of a T-History calorimeter together with the developed calibration procedure provides the measuring accuracy that is required to identify the most suitable PCM for a given application. In addition, the dependence of the enthalpy curve on the sample size can be analysed by comparing results obtained with T-History and DSC and the behaviour of the bulk material in real applications can be predicted.

Poly(ethylene-co-1-tetradecylacrylate) and poly(ethylene-co-1-octadecylacrylate) copolymers as novel solid–solid phase change materials for thermal energy storage

Poly(ethylene-co-1-tetradecylacrylate) (EcoTDA) and poly(ethylene-co-1-octadecylacrylate) (EcoODA) copolymers were synthesized using poly(ethylene-co-acrylic acid) (EcoA) copolymers with 5 and 20% acrylic acid contents, respectively, and fatty alcohols (1-tetradecanol and 1-octadecanol) as novel polymeric solid–solid phase change materials. Chemical structure, thermal property, and crystalline morphology of the copolymers were characterized by using Fourier transform infrared spectroscopy, differential scanning calorimetry (DSC), and polarized optical microscopy, respectively. Poly(ethylene-co-1-tetradecylacrylate) and poly(ethylene-co-1-octadecylacrylate) copolymers have two reversible phase transitions in DSC thermograms, that is, they have two morphologically different phases to transform from solid to amorphous connected each other. The change in surface morphology is only by color tone after the low-temperature phase transition as it was discrete to single color amorphous after the high-temperature phase transition according to microscopy. This property makes them resistant to flow above the first transition where thermal energy is stored for utility. Microindentation test proved that the hardness and reduced modulus values are considerably low after the solid–solid phase transition, but it is still measurable as a solid material.