Heat harvesting characteristics of building façades integrated photovoltaic /thermal-heat pump system in winter

Introduction

With the world's population growing at an alarming rate, the energy crisis has become a vital issue that must to be handled in the 21st century. Meanwhile, the environmental concerns induced by the usage of fossil fuels are worsening [[1], [2], [3]]. Based on this, it is critical to develop energetically renewable energy utilization technologies and attain energy sustainability [4]. Solar energy has been recognized as the most promising answer to meet the growing need for energy, owing to the advantages of larger reserves, no pollution, and widespread distribution.

Buildings have tremendous energy requirements to cover the bill of HVAC and domestic appliances, but they also give greater regions for solar energy gathering. As a result, numerous studies have been conducted to examine solar energy yield and related energy-saving technologies [5,6]. Abu-Hamdeh et al. [7] proposed a method to reduce the building energy consumption of a conference hall by using cooling capacity recovery of fresh air and phase change material (PCM), and the findings showed that energy consumption was reduced by 95% in winter and 34% in summer. Hosseinnia and Sorin [8] investigated the interaction between dynamic building loads, waste sources, and the time variation of renewable energy temporal variation, and proposed a novel dynamic targeting strategy to improve the performance of solar-assisted ground source heat pumps. Behzadi and Arabkoonsar [9] coupled photovoltaic/thermal systems with the heat storage tank and integrated them into a smart building energy system to offset a portion demand for the thermal/electrical power of buildings. Vassiliades et al. [10] concentrated on the thermal environment affected by active solar energy systems on the building façades and made some recommendations for combining urban practice and building-integrated solar energy systems.

With the process of urbanization, high-rise, high-density urban buildings have become the norm in architecture, and the limited area of roofs seriously limits the application and exploitation of solar energy. As a result, unique energy-saving or plus-energy technologies based on building facades have emerged as the preferable solution for high-rise buildings, attracting the attention of numerous researchers [11,12]. Ji [13] and He [14] offered a number of improved methods for energy-saving technologies, such as photovoltaic Trombe walls and multifunctional composite walls, providing fresh ideas for the development of energy-saving enclosure structures [15]. However, the Trombe wall mostly collects sunlight and serves to heat buildings in the winter, which is inefficient for indoor heat dissipation in the summer. They are integrated with building envelope structures based on the properties of phase change materials (PCM), which absorb heat during melting and release heat after solidification [6,16,17]. However, the limited thermal conductivity of phase change materials, on the other hand, restricts the heat transfer process of phase change energy storage walls. As a result, Kong's research team [18] put forward a double-layer microcapsule PCM to enhance wall heat transfer, which may lower the average temperature of phase change energy storage walls by 0.4–0.93 and 0.3–6.6 in summer and winter respectively. The utilization of fluid-directed flow inside walls to achieve heat gain control of building façades based on exterior climatic changes is referred to as dynamic insulation technology. It boasts a wide range of adaptability, high stability, and year-round energy conservation. Wang et al. [19,20] developed an exhaust insulation wall that uses directional ventilation with breathable porous materials to reduce temperature disparities between interior and outdoor environment and reduce indoor cooling demand. They have carried out thorough experimental and computational tests to demonstrate this wall's heat transmission capabilities [21,22].Although energy-efficient building envelopes can improve wall heat transfer, achieve “peak shaving and valley filling”, and lower energy consumption indicators, they do not fundamentally offset the indoor cooling/heating load and still require a significant amount of external energy input to create a comfortable building thermal environment. Therefore, in the construction industry, the “open source” of the productive envelope is also an essential development path of zero energy consumption buildings. Ji [23] presented a hybrid BIPV/T solar wall, which uses air and water as the collecting working fluids in winter and summer respectively. The results revealed that the electrical efficiency of this system is 12.5% and 7.6% respectively. Yu [24] simulated and studied the effect of the airflow path and valve opening inside the air-type BIPV/T wall on the enclosure structure's thermal/electrical capacity. In the winter, the fluid outlet temperature of a building envelope integrated solar photovoltaic/photothermal system is rather low, and cold water is solely used for inside ventilation and preheating [25]. Thus, it is difficult to utilize directly for indoor heating in buildings, but it can be combined with heat pumps and other technologies to enhance the outlet temperature [26,27]. Based on this, Zhang et al. [[28], [29], [30]] conducted various investigations on the performance of BIPV/T heat pump systems, demonstrating the positive role of heat pumps in the building electricity/heat/cold conversion process. Dai et al. [31] developed a mathematical model of a gas-ejected PV/T heat pump system, and findings revealed that the COP of this system can approach 4.0 at the ambient temperature of −10 and solar radiation of 500W/m².

Existing research on building envelopes integrated PV/T - heat pump systems mostly explored the performance placed in limited roofs of buildings, which is a mismatch with the demand for energy consumption in metropolitan high-rise structures in winter. To solve this problem of low outlet temperature, we propose a photovoltaic/thermal-heat pump system (BIPV/T-HP) installed in the building façade, and build a numerical model to investigate the heating performance in section 4. Results will give a reference for the space heating of urban high-rise buildings.