دانلود رایگان مقاله بازیابی حرارت زباله صنعتی با ذخیره سازی انرژی حرارتی رسانائی گرمای نهان

abstract

The aim of this work is to present the experimental performance of a latent heat thermal energy storage. A demonstrator devoted to recover waste heat in food processing industry is investigated. The storage is composed of an expanded natural graphite matrix impregnated with paraffin wax. This kind of composite material has been studied in previous works and appears to be one of the best solutions for the applications requiring a high heat transfer density, defined as the ratio of requested thermal power and stored energy. An investigation of the thermal performance of the storage during cooling and heating phases is presented. The results show that the storage is able to save 6 kWh, which represents 15% of the energy of the process and delivers a thermal power larger than 100 kW, as planned during the design phase. Differences appear between the performances in heating and cooling. Some assumptions on the causes of this phenomenon are proposed, such as the change of viscosity of the heat transfer fluid, the heat losses through the external casing, or the variation of the thermal contact resistance within the heat exchanger containing the storage material. Finally, an economical approach is performed, showing a manufacturing cost of 260 €/kWh and a payback period within 500 days for this application.

4. Conclusion

A latent heat thermal energy storage system adapted to a sterilization process has been designed, manufactured and tested. This storage is based on an expanded natural graphite matrix, impregnated with a phase change material. The system is well adapted to industrial applications with short-term cycles, including set ramp temperatures during heating and cooling phases. The demonstrator reaches an efficiency of 15%, which represents an energy storage of about 6 kWh. As planned, it is able to deliver an average thermal power of 100 kW during heating and cooling phases, corresponding to heating/cooling rates of 6.5 K/min, required by the sterilization process. Differences on the heat transfer coefficients have appeared between heating and cooling phases. These differences are the consequence of some identified phenomena. The first one is the change of viscosity of the heat transfer fluids between hot and cold phases. The second one concerns the heat losses which have appeared to be significant and especially when the system is hot. The third one is the temperature difference between the two heat transfer fluids, much higher at the beginning of the cooling phase than at the beginning of the heating phase. Finally, the thermal contact resistance between the tubes and the ENG/PCM composite material plays a role in the heat transfer. During the operating time of the storage, the average composite material temperature remains at a constant temperature, slightly above 80 C, showing that the heat storage with the system is mainly by latent heat. After several tests, a good condition of the demonstrator has been observed. Moreover, an economical study of the manufacturing of a 1.2 MW industrial storage has been performed. A cost of about 260 €/kWh and a payback period within 500 days have been calculated. These results are encouraging for an industrial use, but the long-term stability of the composite material still needs to be investigated to confirm the viability of the concept.