Study On Thermal Characteristics And Optimization Of Solar Vacuum Tube Hot Air Direct Supply System With Phase Change Materials

In China’s western plateau solar resource-rich areas,the use of solar energy for building heating is a clean and sustainable heating method.Solar heating can be divided into two main categories: liquid collectors and air collectors.Liquid collectors such as water are prone to overheating or frost damage in the low temperatures and large temperature differences on the plateau,which makes maintenance conditions backward and maintenance costs high for remote villages on the plateau.Compared to liquid solar collector systems,solar hot air systems are a more suitable way to heat a single building in remote villages on the plateau because of their lower efficiency,simpler system operation and maintenance,and faster thermal response.This paper investigates the characteristics of solar vacuum tube hot air collector systems with embedded heat storage tubes and the optimisation of their design for direct application in buildings in the Lhasa region of Tibet.In this paper,the structure and thermal process of the thermal storage type solar vacuum collector tube are analysed,and a calculation method for the useful energy and heat loss of the solar vacuum collector tube is proposed;a mathematical model for the internal flow and heat transfer of the thermal storage type solar vacuum collector single tube and the non-accumulation solar vacuum collector single tube is established,and experimental verification analysis of the model is carried out.Based on the above mathematical models,numerical simulations are used to analyse the structural parameters such as the inlet diameter of the vacuum collector,the diameter-to-value ratio(the ratio of the diameter of the phase change accumulation core d to the inner diameter of the collector D),and the physical parameters such as the melting point,thermal conductivity and latent heat value of the phase change material,as well as the influence of the mass flow rate of the inlet mass on the outlet temperature and operating duration of the collector.Further,the combination of thermal storage and non-storage solar vacuum tube hot air collectors with buildings in the Lhasa region is compared and analysed to characterise the creation of an indoor thermal environment by the solar system,and a reasonable strategy for optimising the system operation is proposed,with the following main conclusions:(1)As for the structural parameters of the thermal storage solar vacuum tube hot air collector: the size of the collector inlet has an important influence on the flow velocity and jet distance of the inlet section,and has less influence on the thermal performance of the collector.With the increase of diameter-value ratio,the flow velocity and resistance in the vacuum collector gradually increases,and the collector outlet temperature shows a trend of first falling,then rising and then falling again.When the diameter value ratio is close to 0.9,the bottom of the collector tube will appear to collect heat to form a local high temperature,which will cause high-temperature damage to the collector tube.it is recommended that the diameter value ratio of the thermal storage solar vacuum collector tube be between 0.75 and 0.8.(2)The influence of phase change material physical parameters on the thermal performance of heat storage vacuum collector: the influence is in the order of importance,phase change temperature > latent heat value > thermal conductivity,in which with the increase of phase change material temperature,the collector outlet temperature peak decreases first and then increases,and the collector operation time can be extended by about 1.5h～2h compared with the collector without adding phase change;As the latent heat value of the phase change material increases,the peak collector outlet temperature gradually decreases and then remains unchanged,and the collector operation time is extended by 1.7h～2h;the thermal conductivity of phase change material has less influence on the collector outlet temperature;as the mass flow rate of the inlet air increases,the peak collector outlet temperature gradually decreases,and the delayed operation time can be gradually reduced.(3)As regards the improvement of the thermal environment of buildings by thermal storage type solar vacuum tube hot air heating system: the thermal storage type solar vacuum tube hot air heating system effectively reduces the overheating temperature and the overheating time of the room,which plays a certain role in improving the thermal environment of the heated room,and also extends the effective heating time and improves the safety and reliability of the solar hot air heating system.In this paper,we have studied the influence of the structural parameters of the vacuum collector,the physical properties of the phase change materials and the operating parameters on the thermal performance and operating characteristics of the thermal storage type solar collector,and have initially obtained the characteristics of the thermal storage type solar vacuum tube hot air collector on the creation of the indoor thermal environment and the system optimisation strategy,providing some support for the promotion and use of such systems in the Tibetan plateau region.