Experimental characterization of a solid industrial by-product as material for high temperature sensible thermal energy storage (TES)
Introduction
Once a material is considered a waste, it can follow two different paths of treatment: incineration or disposition into the ground, and recovery or recycling. The first path represents a serious damage to the environment. Also, due to the current waste management programme, it represents an effort on money and on time for industry: to make an annual waste declaration, to contract special haulage contractor, etc. Because of that, the alternative path, consisting on the reuse of waste (becoming a by-product) is the wanted option for industries.
Another important waste in industry is the heat coming from production processes. This heat normally goes to the ambient. But, depending on the temperature, this heat can be stored and used later for other applications. For example, in high temperatures (above 100 °C), it can be used in: cogeneration, energy efficiency measures, passive heat recovery, solar cooling [1], etc.
Three reversible methods to store heat or cold are described in [2], sensible heat, latent heat and chemical processes. The properties that define the suitability of a material to store sensible heat are mainly thermal capacity (Cp ρ), conductivity (for long term storage), diffusivity (for short term storage), and thermal stability along the thermal cycles. Also, high availability, non-combustibility, non-toxicity, low capital costs, safety, good processability and long lifetime even at high temperatures have to be considered [3], [4], [5], [6]. Until now, sensible storage has been tested experimentally mainly with materials such as high-temperature concrete [7] and castable ceramics due to their low cost and high thermal capacity [8]; in fact, there is a test facility in Stuttgart since May 2008 where concrete is used as high temperature solid storage media [4]. Other studies propose amorphous materials coming from hazardous wastes [9] or by-products from the copper industry, the steel industry and mineral industry [10] as high temperature storage materials. In a range from 150 to 200 °C an exhaustive screening of materials for long and short term heat storage was realized together with proposing a methodology to select materials depending on their potential in sensible energy storage [3]. However, for industrial waste materials thermal properties, there is a lack of information.
The objective of this paper is to evaluate the suitability of a solid by-product coming from the potash industry to be used for industrial sensible heat recovery in a range temperature from 100 to 200 °C. The development of a thermal storage system demands the selection of the storage medium, the definition of geometric parameters and the choice of an operation strategy [6]. Therefore, a complete analysis of thermophysical properties was done at laboratory scale (composition, specific heat capacity, thermal stability, conductivity and density measured) and then, in a high temperature pilot plant of the University of Lleida, thermal cycles were performed with the storage material. This material has been selected for its low price, its availability and its similar thermal characteristics with other sensible materials found in the literature. Thus, this type of by-product is expected to be a good option to store waste heat from industrial processes due mainly to its low cost and its availability.
Section snippets
Materials
The material used to perform this characterization is a by-product from the potash industry. The original shape of this waste is a granulated material with a particle size between 1 and 2 mm. It was first tested in this form. Then, in order to increase its thermal conductivity, different ways to shape it were considered. In other thermal storage materials, authors propose metal additives, metal fins or graphite to increase it [2], but these solutions represent an increase of the system cost. So,
Laboratory characterization
The characterization at laboratory scale of both salts consists on the analyses of specific heat, thermal stability, conductivity, diffusivity, and density, as well as the composition. Also, a corrosion test was performed.
Salt composition was analysed by means of a P analytical XRD difractometer with Cu kα radiation (0.154 nm wavelength) under 40 mA and 45 kV. The XRD pattern was acquired at 20 °C.
The salt thermal stability was studied using a thermogravimetric analysis (TGA), at a heating rate of 5
Pilot plant experimentation
The high temperature pilot plant facility used consists of an electrical boiler to heat the heat transfer fluid (HTF), an air heat exchanger to cool down the HTF temperature when it is needed, a TES tank containing the salt and a piping circuit to connect all the elements.
Laboratory characterization results
The solid by-product from the potash industry was identified by X-ray Diffraction (XRD) analysis as crystalline sodium chloride (NaCl) as can be seen in its XRD patterns in Fig. 3. NaCl peaks correspond to the black lines.
Fig. 4 shows salt thermal stability obtained by TGA where the mass losses of the salt when heated up to 800 °C at a constant heating rate. With this measurement the mass losses during heating are associated with the stability of the material. Depending on the temperature the
Conclusions
A by-product from potash industry was tested for its suitability as TES material using sensible heat. The initial presentation of this material was in grain (Salt A). In order to increase its thermal properties, it was added a 17 wt% of water and then dried as a cheap way to compact it (Salt B). This material is composed mainly by NaCl.
At laboratory scale, TGA analysis shows good thermal stability. Moreover, its average specific heat capacity is 0.738 kJ/kg K. Density and conductivity were also
Acknowledgements
The work was partially funded by the Spanish Government (Project ENE2011-22722 and ULLE10-4E-1305). The authors would like to thank the Catalan Government for the quality accreditation given to their research group GREA (2009 SGR 534) and research group DIOPMA (2009 SGR 645), and to Iberpotash. Antoni Gil would like to thank the Col·legi d’Enginyers Industrials de Catalunya for his research appointment. Laia would like to thank the Spanish Government for her research fellowship
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