Abstract
Increasing energy prices make space heating more expensive every year in The Organisation for Economic Co-operation and Development (OECD) member countries. Thermochemical heat storage systems (THSS) can be used to reduce residential energy consumption for space heating and to control humidity. Utilizing compressed thermochemical pellets as heat storage materials is a way to increase volumetric energy storage capacity and to improve the performance of the THSS. In this work, expanded natural graphite (ENG), activated carbon (AC), strontium bromide, and magnesium sulphate were mixed in different mass ratios and compressed under applied pressures in a range of 0.77 to 5.2 kN⋅mm−2 to form composite pellets with a diameter of 12 and 25 mm, respectively, and a thickness from 1.5 to 25 mm. These pellets were characterized using thermogravimetric analysis and differential scanning calorimetry. Cyclic tests of hydration at 20 °C and dehydration at 85 °C were conducted to investigate changes in the surface morphology and the heat and mass transfer characteristics of the composite pellets. The permeability and thermal conductivity of the composite pellets were also measured. It was found that the structural stability of the pellets was enhanced by increasing the compression pressure. Utilizing AC and ENG in the composite mixture enhanced the porosity, thermal conductivity, and the permeability of the pellets.
Highlights
Global energy demand is in constant increase during the last decades [1]
We present our research results for activated carbon (AC)–expanded natural graphite (ENG) composite materials with enhanced thermal conductivity and porosity for increased heat conduction and vapor sorption kinetics
SrBr2 ·6H2 O (SBH), SBH/G+C(1:1), and MgSO4 ·7H2 O (MSH)/G+C(1:1) composite pellets prepared under different compression pressures were compared, and changes in the porosity of the pellets with the increasing compression
Summary
Global energy demand is in constant increase during the last decades [1]. Due to the climatic change and the rise in the use of air conditioning systems, there is a sharp increase in the energy demand on space heating and cooling [2]. The THSS material’s geometrical structure and physical properties, such as the porosity and the thermal conductivity, should be improved to maintain the heat and mass transfer rates over a long period of operation (> 10 years) Materials such as silica gels or silica-aluminophosphate (SAPO) are thought as promising supportive materials, and zeolite has attracted particular attention due to its high porosity [25]. Supportive materials with lower heat storage capacities will reduce the overall energy storage density; the compression of the composite mixture helps keep the volumetric energy storage density similar to pure salt hydrates, while improving structural integrity. The utilization of composite materials may help tackle THSS long term energy storage challenges, including salt hydrate deliquescence and geometrical deformations of the salt. The individual potential of the MSH, SBH, AC, and ENG as THS materials has been investigated [30,31,32,33,34,35,36,37], to the best of our knowledge there are no examples of using an AC–ENG mixture as a supportive matrix in the THS pellets or an investigation into the effect of pressure
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