## 1 Introduction  
There is a growing concern over energy supply and ecological environment conservation in the world. Utilizing a renewable, safe, and eco-friendly energy resource to cater the energy demand of buildings has undoubtedly been regarded as an encouraging solution to achieve sustainable development for buildings. The Tibet plateau has a peculiar environment, which is characterized by high altitude of over 4000 m, low air temperatures, and transparent and rarified atmosphere with minimal water and dust content. This region has inexhaustible resources of solar energy for annual solar radiation intensity of approximately 8400 MJ/m\({}^{2}\) and annual sunshine duration of more than 3000 h [1]. However, the ecological environment is fragile, and conventional fossil energy sources such as coal, oil, and natural gas are very scanty in Tibet. The energy consumption for cooking and heating is dominated by traditional biomass fuel, such as dung, firewood, and crop straw in this region. Excessive depletion of biomass fuel is responsible for eco-environmental degradation [2; 3; 4]. It is urgent to develop the non-polluting and renewable energy sources, such as solar energy, to sustain the development of buildings and protect the fragile ecosystem in Tibet [5; 6]. Solar buildings utilizing the locally abundant solar energy resources are the most suitable strategy to improve the indoor thermal environment of residential buildings [7; 8]. The reported results of local solar radiation estimation [9] and developed passive solar technologies [10; 11; 12; 13] make it possible to utilize solar energy to cater to the energy needs of building heating. A passive solar building is considered to be a promising form suitable for Tibet, which is characterized by a simple structure, high heat collection efficiency, and low construction cost [14; 15; 16]. [PERSON] et al. [17] and [PERSON] et al. [18] investigated the influence of conventional passive design methods on the indoor thermal environment. [PERSON] and [PERSON] et al. [15; 19] analyzed the effect of external window types and thermal performance of the envelope on the indoor thermal environment of passive solar building. However, it is worth noting that the above research is based on the assumption that the building interior space is a single space. Although assuming the building interior space as a homogeneous whole can simplify the building design, this hypothesis ignores the difference of the thermal environment between different rooms, making it challenging to achieve passive solar building optimization design. The indoor thermal environment of passive solar buildings is established mainly by the amount of solar energy collected through passive solar technologies and indoor auxiliary heat. Published research on the indoor thermal environment of multi-space buildings indicated that the indoor air temperature of each independent room was different [20]. If the collected solar energy is distributed evenly in every room of the building, the low indoor air temperature of the main room will inevitably occur [21]. Therefore, according to the thermal environment requirement of different rooms, differentially distributing indoor thermal energy is an effective way to improve the efficiency of thermal energy utilization and to raise the indoor air temperature of the main room. The rational layout of the passive solar building with temperature buffers can lead to less heating consumption than the linear layout of the traditional buildings [22]. The temperature buffer is usually served by auxiliary rooms such as kitchens, bathrooms, and storage rooms. Research on the dwelling in Lhasa indicated that the average temperature in south-facing rooms was higher than that in north-facing rooms, and the indoor air temperature of the traditional dwelling was low and fluctuated sharply [23]. As mentioned above, the arrangement of indoor space had a significant effect on the solar energy utilizing efficiency and the indoor thermal environment of passive solar buildings.