Energy density and storage capacity cost comparison of conceptual solid and liquid sorption seasonal heat storage systems for low-temperature space heating

Sorption heat storage can potentially store thermal energy for long time periods with a higher energy density compared to conventional storage technologies. A performance comparison in terms of energy density and storage capacity costs of different sorption system concepts used for seasonal heat storage is carried out. The reference scenario for the analysis consisted of satisfying the yearly heating demand of a passive house. Three salt hydrates (MgCl2, Na2S, and SrBr2), one adsorbent (zeolite 13X) and one ideal composite based on CaCl2, are used as active materials in solid sorption systems. One liquid sorption system based on NaOH is also considered in this analysis. The focus is on open solid sorption systems, which are compared with closed sorption systems and with the liquid sorption system. The main results show that, for the assumed reactor layouts, the closed solid sorption systems are generally more expensive compared to open systems. The use of the ideal composite represented a good compromise between energy density and storage capacity costs, assuming a sufficient hydrothermal stability. The ideal liquid system resulted more affordable in terms of reactor and active material costs but less compact compared to the systems based on the pure adsorbent and certain salt hydrates. Among the main conclusions, this analysis shows that the costs for the investigated ideal systems based on sorption reactions, even considering only the active material and the reactor material costs, are relatively high compared to the acceptable storage capacity costs defined for different users. However, acceptable storage capacity costs reflect the present market condition, and they can sensibly increase or decrease in a relatively short period due to for e.g. the variation of fossil fuels prices. Therefore, in the upcoming future, systems like the ones investigated in this work can become more competitive in the energy sector.This project receives the support of the European Union, the European Regional Development Fund ERDF, Flanders Innovation & Entrepreneurship and the Province of Limburg. TU/e has received funding from European Union’s Horizon 2020 research and innovation programme under grant agreement Nº 657466 (INPATH-TES). The results of this study can contribute to the development of educational material within INPATH-TES

Sorption heat storage for long-term low-temperature applications: A review on the advancements at material and prototype scale

Sorption heat storage has the potential to store large amounts of thermal energy from renewables and other distributed energy sources. This article provides an overview on the recent advancements on long-term sorption heat storage at material- and prototype- scales. The focus is on applications requiring heat within a temperature range of 30–150 °C such as space heating, domestic hot water production, and some industrial processes. At material level, emphasis is put on solid/gas reactions with water as sorbate. In particular, salt hydrates, adsorbents, and recent advancements on composite materials are reviewed. Most of the investigated salt hydrates comply with requirements such as safety and availability at low cost. However, hydrothermal stability issues such as deliquescence and decomposition at certain operating conditions make their utilization in a pure form challenging. Adsorbents are more hydrothermally stable but have lower energy densities and higher prices. Composite materials are investigated to reduce hydrothermal instabilities while achieving acceptable energy densities and material costs. At prototype-scale, the article provides an updated review on system prototypes based on the reviewed materials. Both open and closed system layouts are addressed, together with the main design issues such as heat and mass transfer in the reactors and materials corrosion resistance. Especially for open systems, the focus is on pure adsorbents rather than salt hydrates as active materials due to their better stability. However, high material costs and desorption temperatures, coupled with lower energy densities at typical system operating conditions, decrease their commercial attractiveness. Among the main conclusions, the implementation within the scientific community of common key performance indicators is suggested together with the inclusion of economic aspects already at material-scale investigations.This project receives the support of the European Union, the European Regional Development Fund ERDF, Flanders Innovation & Entrepreneurship and the Province of Limburg. TU/e has received funding from European Union’s Horizon 2020 research and innovation programme under grant agreement No 657466 (INPATH-TES). The results of this study can contribute to the development of educational material within INPATH-TES

COMBINED SPIV-PLIF AND ORTHOGONAL PLIF MEASUREMENTS: MIXING IN A PULSED JET IN CROSSFLOW

A simultaneous SPIV-PLIF measurement in water is used to study a pulsed jet in a crossflow. In the longitudinal plane of symmetry of the flow, the velocity fields are measured using Stereoscopic Particle Image Velocimetry (SPIV) and the concentration fields with Planar Laser Induced Fluorescence (PLIF). At the same time, a PLIF measurement is carried out in an orthogonal plane. To take into account the PLIF error sources (spatio-temporal laser intensity variations and out-of-plane fluorescence due to SPIV particles seeding), a special data processing technique has been developed. It allows the study of mixing at an early stage of the development of a pulsed jet in a crossflow. The studied jet is characterized by the Reynolds number Rej = 500 based on the mean jet velocity Uj = 1.67 cm/s, velocity ratio R = 1, sinusoidal forcing amplitude ratio Aj = 2, and the stroke ratio Sr = 2.23

Experimental and numerical investigation of the vortex formation process behind a heated cylinder

This paper describes the three-dimensional (3D) vortex formation process behind a heated cylinder at low Reynolds numbers. Both experimental and numerical techniques are used, including an electrochemical tin-precipitation visualization method, a two-dimensional (2D) high resolution particle velocimetry technique, and a 3D spectral element method. The wake flow is simultaneously investigated in two perpendicular planes using a dual-plane configuration. It appears that for Reynolds number around 100 and Richardson number larger than 1.0 thermal plumes occur in the far wake. Correspondingly distinct counterrotating vortices, with a spanwise wavelength of two cylinder diameters, are shown in the near wake. Furthermore, a difference in the flow motion for in-plume and out-of-plume positions is observed. The vortex shedding process for the out-of-plume positions is quite similar to the one for an unheated cylinder, which is a 2D flow. At in-plume positions, it is observed that the flow moves upward from the lower to the upper half of the wake. From the calculated temperature field, a region of unstable thermal stratification in gravity direction is observed at the in-plume positions. This indicates that the upward motion at the in-plume positions is induced by buoyancy when the temperature gradient is large enough

Nanoscale Heat Transfer in Carbon Nanotubes - Sugar Alcohol Composite as Heat Storage Materials

Nanoscale carbon structures such as graphene and carbon nanotubes (CNTs) can greatly improve the effective thermal conductivity of thermally sluggish heat storage materials, such as sugar alcohols (SAs). The specific improvement depends on the heat transfer rate across the carbon structure. Besides, the heat transfer rate is further dependent on the material and the CNT diameter. In this paper, molecular dynamics simulations are applied to graphene/CNT-SA interfacial systems. Using erythritol and xylitol as model materials, we find the cross-plane thermal contact conductance to decrease as the CNT diameter decreases, with an exception for CNT­(7,7). A phonon mode analysis is carried out to explain the general decreasing trend. The larger phonon mode mismatch observed between the molecules on both sides of smaller diameter CNTs is found to be a finite size effect of the confinement, instead of an interfacial effect. From the molecular collision point of view, a higher molecular density promotes heat transfer. In the case of CNT­(7,7), the effective density of molecules enclosed in the CNT is found to be much higher than that of CNT­(8,8). This may be the cause of the higher heat transfer rate across CNT­(7,7). Molecular orientations and hydrogen bond structures of the molecules inside the CNTs are investigated to demonstrate the finite size effect of the confinement. For graphene-SA composites, five model materials are considered and their cross-plane thermal contact conductance values fall into a narrow range