New library of phase-change materials with their selection by the Rényi entropy method.

The secret to the successful and widespread deployment of solar energy for thermal applications is effective and affordable heat storage. The ability to provide a high energy storage density and the capacity to store heat at a constant temperature corresponding to the phase transition temperature of the heat storage material (phase-change material or PCM) make latent heat storage one of the most alluring methods of heat storage. Today, it can be challenging to obtain all the published data on PCM qualities, including relevant non-thermodynamic properties in addition to thermodynamic ones. The developed new PCM library contains various types of PCMs which possess broad range of operation temperatures. This new library consists of 500 substances along with nine associated properties such as phase change temperature, solidification temperature, maximum operation temperature, density, latent heat and specific heat capacity, thermal conductivity, cycleability and ignition temperature. Furthermore, a new PCM selection method, based on calculating the Rényi entropy for a given set of selection criteria, has been proposed. The newly developed selection method requires no subjective judgements. The idea of the method is inspired by earlier applications of fractal analysis methods in many areas of research.

Constant mixing temperature test of a fin-and-tube latent heat thermal energy storage.

Thermal energy accumulation is one of the ways how to optimize heat production processes and how to balance the supply and demand of heat in distribution systems. This article presents a design of a fin-and-tube latent heat thermal energy storage (LHTES), which combines high thermal energy storage density and scalability. A computational model that used lumped heat capacities was tuned using the experimental data. The numerical model proved to be simple yet precise. A new constant mixing temperature test was designed and performed with the LHTES. Unlike standard constant flow rate charge/discharge test, this test provided valuable information about what to expect in the real-life operation conditions. From the tests and data from simulations, it was concluded that the LHTES would perform better in terms of its capacity utilization if it operated at lower output power than in the laboratory circuit. This indicates that a smaller, and thus more cost-effective, LHTES could be employed in the laboratory circuit with virtually the same utility to the system if its heat transfer characteristics were improved.

Experimental investigation and modelling of a laboratory-scale latent heat storage with cylindrical PCM capsules.

Heat storage efficiency is required to maximize the potential of combined heat and power generation or renewable energy sources for heating. Using a phase change material (PCM) could be an attractive choice in several instances. Commercially available paraffin-based PCM was investigated using T-history method with sufficient agreement with the data from the manufacturer. The introduced LHTES with cylindrical capsules is simple and scalable in capacity, charging/discharging time, and temperature level. The overall stored energy density is 9% higher than the previously proposed design of similar design complexity. The discharging process of the designed latent heat thermal energy storage (LHTES) was evaluated for two different flow rates. The PCM inside the capsules and heat transfer fluid (HTF) temperature, as well as the HTF flow rate, were measured. The lumped parameter numerical model was developed and validated successfully. The advantage of the proposed model is its computational simplicity, and thus the possibility to use it in simulations of a whole heat distribution network. The so-called state of charge (SoC), which plays a crucial role in successful heat storage management, is a part of the evaluation of both experimental and computational data.

Modelling the behaviour of thermal energy harvesting devices with phase-change materials.

This paper presents a new general theoretical model of thermal energy harvesting devices (TEHDs), which utilise phase-change materials (PCMs) for energy storage. The model's major goal is to identify a set of parameters under which these devices perform optimally, that is, attain the largest thermal buffering capacity and exchange heat with the surrounding phase as quickly as possible. For the first time, an expression for the characteristic harvesting time is developed from the constructal theory viewpoint under the optimal performance assumption, and a dimensionless criterion that characterizes PCM performance is provided. Furthermore, a new non-field solution of the energy equation governing the process of heat transfer within TEHDs with PCMs has also been derived. An expression for the effective thermal effusivity is then obtained. Finally, under a given set of boundary conditions and geometrical constraints, a novel simple technique for the optimal choice of PCMs in TEHDs has been established.