Thermal Performance Analysis Of Solar Heating Device With Phase Change Thermal Insulation In The Straw Fermentation System

The development of biogas technology is the established energy policy of "building a new socialist countryside". It is also the core technology to solve the rural energy demand decided by renewable energy development strategy during the National Twelfth Five-Year Plan period. However, there are many bottleneck problems to promote the actual use of biogas technology, especially the phenomenon that biogas production efficiency was reduced or no biogas yield occurred when anaerobic methane fermentation device runs at the relatively low ambient temperature during the winter and spring, a serious impediment to the promotion of biogas technology in rural areas. At present, the one-piece traditional fermentation devices such as digesters etc are used in rural areas, and solar greenhouses, solar cell, solar water heaters etc are used widely to heating these fermentation devices. On the one hand, the solar flux density is small in winter. On the other hand, a larger volume of traditional fermentation equipment will result in a larger heat load. Hence, the corresponding cost will increase, losing the value of promoting this kind of technology.Here, a solar heating tank with phase change thermal insulation is designed based on this two-phase fermentation system, utilizing the heating tank as a stable heat source for the system. In the phase change thermal insulation layer of the heat storage tank is filled with phase change material 54 paraffin, making use of its character of having a small temperature fluctuation in the process of releasing the latent heat to ensure the stability of temperature of heat storage tank. What’s more, the electric resistance wires are also decorated in the phase change thermal insulation layer, to complement the latent heat dissipated at the night. By transferring the heat dissipation of the heat storage tank to the phase change thermal insulation layer, the water temperature can be maintained stably.Firstly, the system thermal load for 1m3 fermentation tank, consisting of the heat dissipation of tank and the heat load of Hydrolysis acidification liquid recycled to fermentation tank, can be calculated based on the system operating parameters to be 262W. And the solar collector area is calculated based on the system thermal load to be 2.75m2.Secondly, a 1 m3 thermal storage tank volume with phase change thermal insulation layer is built, which is matched with solar collector to meet the system heat load. The 30mm phase change thermal insulation layer of the heat storage tank is filled with phase change material 54 paraffin, making use of its character of having a small temperature fluctuation in the process of releasing the latent heat to ensure the stability of temperature of heat storage tank. What’s more, the electric resistance wires are also decorated in the phase change thermal insulation layer, to complement the latent heat dissipated at the night. By transferring the heat dissipation of the heat storage tank to the phase change thermal insulation layer, the water temperature can be maintained stably.Thirdly, the latent heat of paraffin can release for nearly 18 hours under the natural cooling process. The paraffin can be held in the melting temperature at 54.5℃ by heating the phase change thermal insulation layer for 7 hours, and the water temperature can be maintained at 55℃. Through numberical simulation method, three different cases:replacing the phase change thermal insulation layer with the same thickness polyurethane, the paraffin layer without heating, heating the paraffin layer for 7 hours every day are analyzed comparatively. The results show that the water temperature without the phase change thermal insulation layer drops faster from 56.5℃ to 44.5℃and the exergy decreases 24.9% versus 5.8%when with phase change thermal insulation layer.Finally, in this paper, a long-term stable heat storage process can be achieved by a heat storage tank with the phase change thermal insulation layer. And it proposes an effective research idea of the combination with the solar stable storage technology and biogas fermentation technology.