In general, water heating, space heating and air conditioning are the main consumers of energy in public buildings [14]. Nowadays, solar thermal technology, which converts solar energy into usable thermal energy, is generally regarded as a simple and effective way to harness solar radiation and address both the energy crisis and environmental concerns. Solar water heating systems (SWHS) are widely adopted by households worldwide due to their cost effectiveness, which is one of the most common applications of solar thermal technology [15]. They use solar energy to heat water to provide hot water for buildings. A typical solar water heating system consists of several components as follows:

Solar collectors typically consist of a set of tubes or panels that absorb solar energy and convert it into heat for water heating. Different types and designs of solar collectors are available to meet specific application requirements and building environments. The most common types include flat plate collectors (FPC), evacuated tube collectors (ETC) and parabolic trough collectors (PTC) [16]. Figure 1 displays the characteristics and innovative directions of each collector mentioned in the text.

A typical flat plate collector is designed to operate under low temperature conditions. As a result, the heated fluid it produces is primarily used in domestic applications, such as providing hot water or space heating in homes. Due to the lower cost and simple structure, design modification can be considered as an effective method to improve FPC’s performance [17]. Moreover, huge efforts have been invested in studying the effects of incorporating nanomaterials in the field of solar energy. These endeavors have yielded substantial improvements in performance, as evidenced by recent research [18]. Simultaneously, simulations to improve the feasibility of the optimization scheme [19] and the development of new materials [16] are still in progress today.

The evacuated tube collector was developed based on the flat plate collector. It maintains a high vacuum in the interlayer between the heat absorber and the glass tube. As the key element of ETC, the vacuum tube determines its higher efficiency and cost. In addition to the applications of nanofluids [20] mentioned above, the optimization of this technology is looking for breakthroughs in various aspects, including, but not limited to, an optimal reflectance angle, a reflector of PCM and the usage of energy storage medium. At an optimal angle of reflectance, solar radiation is directed onto the solar collector to enhance sunlight reflection onto the heating plate, thereby boosting the electricity generation capacity of the solar power plant [21]. Furthermore, employing reflectors enhances the irradiation received by the PV panel, yet simultaneously results in an increase in the PV module temperature. In order to mitigate the efficiency impact of high temperatures on the components, phase change materials are integrated into the reflectors [22]. Instead of adopting a single technique for improving the thermal efficiency of the system, a combination of more than one technique has been broadly chosen [21]. As a promising technology, an evacuated flat plate collector (EFPC) combines advantages of FPC and ETC. In practice, evacuated flat plate collectors (EFPC) are similar to typical FPCs, but they reduce convective heat losses from the absorber plate to the cover due to the absence of internal gas [23]. The inevitable variations in film thickness during the manufacturing process affect the stability and reliability of the absorber coating performance. Therefore, a key challenge in the field of EFPC is to create a selective coating that is both durable and has a low emissivity [24]. However, there are several drawbacks associated with selective solar absorbers, including lower durability, complex production techniques, and higher cost [25]. For instance, in the industrial mass production of these absorbers, the performance of the coatings can be significantly affected by imperfect control of deposition parameters caused by errors in layer thickness [24].

PTC is a linear imaging concentrator composed of parabolic trough-shaped reflectors and receivers for an operational temperature range of medium temperature. Currently, there are numerous pieces of research focusing on using hybrid nanofluids, which cause a high-efficiency heat transfer [26]. Higher efficiency of PTC could also be achievable via novel designs of receivers and inserted fins. Numerous theoretical and numerical investigations play a pivotal role in producing PTC for various applications. So far, a large number of theoretical and numerical studies have been used to conduct simulations of PTC to improve its performance [27]. However, not only are PTCs costly, but they also require a large land area [28]. Accordingly, to realize the full potential of parabolic trough solar collectors, it is critical to focus on optimizing land use [28]. A new type of polymer solar collector has been proposed, and experimental and numerical evaluations of its thermal characteristics have been conducted [29]. While their structures and operating principles may vary, their primary objective remains the same: to convert solar energy into heat to meet the hot water and space heating needs of buildings.

As research on solar collectors continues to deepen, the efficiency of solar collectors has significantly improved, leading to an increase in their domestic and industrial applications. Among them, the SWH system has attracted widespread attention due to its low cost, minimal impact on global warming, and long lifespan. Regional geographical conditions, especially climate conditions and solar radiation availability, have a crucial impact on the thermal performance of solar energy systems. There is a need for more research articles focusing on the specific conditions of solar thermal energy utilization in a particular region to support the specific and practical application of solar energy in buildings.

The hot water storage tank is used to store the heated water from the solar collectors. Typically, the tank is insulated to minimize heat loss and has a volume sufficient to meet the daily hot water demand. Various innovative designs have been proposed to improve the design of hot water storage tanks.

Heat loss is a major concern for these tanks, as the mixing of hot and cold fluids exacerbates heat loss and reduces system efficiency. To address this issue, thermal stratification structures, such as diffusers, baffles, membranes and fabrics, have been invented. These structures not only reduce energy loss but also help maximize energy collection from solar heaters. Recent studies have shown that the use of diffusers can reduce shadowing losses in solar thermal utilization [30], while passive baffles can suppress natural convection within the tank, thereby reducing stagnant heat loss [31]. Additionally, fabric membrane materials exhibit higher heat transfer coefficients, poor thermal insulation properties and higher light transmittance performance [32], all of which are significantly influenced by solar radiation intensity. These findings are crucial for the design and optimization of solar thermal systems, and ongoing improvements continue to enhance this design [33]. Research has shown that the initial temperature significantly affects thermal stratification, while higher initial temperatures can reduce the range of initial mixing and improve the output rate of hot water [34]. With the aim of improving engineering accuracy, methods utilizing artificial neural network (ANN) models to calculate the accuracy of the overall conductance, equivalent to thermal resistance, have been proposed [35]. Furthermore, the presence of a baffle inside the tank has been found to have a significant impact on natural convection by altering the flow, ultimately reducing heat loss from jacketed baffled solar storage tanks [31].

The piping system connects the solar collectors to the hot water storage tank, allowing heated water to flow from the collectors to the tank. Valves and pumps are typically used in the piping system to control the flow of water. Introducing obstacles of various shapes in the fluid path is one method to increase the efficiency of SWHS (Solar Water Heating Systems) [15].

In addition to solar water heating systems, solar thermal technology can be integrated into buildings for other applications. For example, solar air heating systems use solar thermal energy to heat air and transfer it to the interior of a building for space heating. Solar floor heating systems use solar thermal energy to transfer heat through radiant floor panels, further enhancing indoor comfort. All these applications require solar collectors as the key component for capturing solar energy. And these diverse applications require the use of solar collectors as the fundamental component for efficient solar energy capture. Being the most widely used components of solar equipment, solar collectors require optimization to maximize performance and minimize costs [36]. Currently, nanofluids have been utilized in heat pipes powered by solar energy, allowing for efficient capture of solar energy and simultaneous fast transfer of the collected thermal energy to its desired applications [37].

Through extensive testing, several thermal efficiency improvement techniques have been developed and evaluated. In particular, in building integrated PV thermal (BIPV/T) systems, the incorporation of innovative flow deflectors, semi-transparent PV technology and multiple inlets have demonstrated significant improvements, achieving thermal efficiencies of up to 33% [38]. Heat pumps have emerged as a promising solution to meet carbon reduction targets in the residential sector. However, challenges such as electricity consumption and low supply temperatures have hindered their widespread adoption. To overcome these limitations, researchers have sought to improve the efficiency of heat pumps in residential buildings through the integration of phase change materials (PCMs) and building-integrated photovoltaics (BIPVs). PCMs possess the capability to absorb and store significant amounts of latent heat at a constant phase transition temperature, facilitating passive heat storage and temperature control [39]. With proper selection of parameters, a solar air heater using a paraffin–wax–aluminum compound as a thermal storage material encapsulated in a cylinder has a better performance [40]. In order to adapt to various climatic conditions in different regions, researchers have made numerous attempts and have confirmed the effectiveness of these methods. For example, the performance of a solar heater designed for the winter conditions of the city of Baghdad was improved by adding aluminum chips, paraffin wax and nano-SiC [41].

According to the existing literature, a high-quality SWHS should possess superior heat transfer capabilities while minimizing frictional losses. By introducing obstacles to the flow of fluids, vortices are created, facilitating effective liquid mixing and enhancing heat transfer. These obstacles not only alleviate pressure drop penalties but also promote increased heat transfer rates [15]. As examples, the effect of obstruction enhancers is given in [19], and the effect of utilizing a helical enhancer in an absorber pipe is described in [42].

A numerical investigation has been conducted on the heat transfer and hydrodynamic flow characteristics in a horizontal tube with trapezoidal dimples on its surface, using water as the working fluid and applying a constant heat flux on the outer surface of the tube [43].

Another factor that can make a difference to thermal performance is the use of perforations. Perforations have the potential to further improve the overall thermal performance by reducing excessive longitudinal vortex interactions in the mainstream without significantly decreasing impingement to the thermal boundary layer [44].

The employment of effective building performance simulation can reduce the energy consumption and carbon emissions, enhancing the quality and productivity of indoor spaces while also promoting innovation and technological advancements in the construction industry [45]. Due to factors such as cost and time efficiency, parameter optimization, ethics and safety, building energy simulation (BES) plays an important role in prediction and forecasting [46]. Only when models of emerging technologies are credible and accurate will they gain traction with stakeholders [11]. The use of simulation in academic research has increased, driven by recent technological advances [47]. Table 1 presents a brief comparison of the previously conducted studies on the numerical values of thermal efficiency in simulation tests.

To assess the suitability and accuracy of each simulation software, researchers have compared the most popular building performance (bp) tools, namely TRNSYS (a software package for simulating the behavior of transient systems), EnergyPlus (a building energy simulation program used for simulating the energy use of buildings and their HVAC systems) and IDA ICE (a building performance simulation software that offers detailed simulations of building energy use, indoor climate conditions and thermal comfort). The results demonstrated that all tools were highly accurate in the absence of phase change materials (PCM), while IDA ICE was recommended when PCM was present [48]. To overcome the limitations of using a single simulation tool to predict system performance, the researchers developed a co-simulation framework between the BES tool (EnergyPlus) and the CFD tool (Ansys Fluent, a computational fluid dynamics software used for simulating fluid flow, heat transfer and other related phenomena). The utilization of the co-simulation framework helped them obtain more accurate predictions compared to existing BES tools [49]. To economically optimize the system design, the researchers developed a proxy model and compared its economic cost. First, they designed a solar-assisted air-source heat pump (SAASHP) system according to the national design standards for solar thermal systems in China using TRNSYS software. Second, they used the artificial neural network toolbox of MATLAB and the results of GenOpt (a generic optimization program used for optimizing complex systems and processes) to predict the most economical solar fraction design values for solar-assisted air source heat pump systems [50].

A two-dimensional temperature-based finite-volume numerical simulation model was developed and experimentally validated to analyze PCM energy storage and temperature regulation [54]. The results demonstrated the potential for optimizing the use of solar energy to drive heat pumps while storing thermal energy in PCMs for radiant floor heating. What is more, a hybrid solar–ground-source heat pump system (HSGSHPS) was implemented, which comprised a ground-source heat pump system (GSHPS) and a solar-assisted ground-source heat pump system (SAGSHPS) for heating and cooling a building. The study demonstrated that the optimized use of this system can effectively save electrical energy. Furthermore, the researchers designed a borehole heat exchanger (BHE) and borehole thermal energy storage (BTES) that are compatible with the two heat pump units [51]. They analyzed the operating efficiency of the hybrid solar–ground-source heat pump system and confirmed that solar space heating can significantly improve the performance of solar-assisted ground-source heat pump (SAGSHP) systems.

Additionally, flow pattern maps for horizontally aligned pipes for a direct expansion solar-assisted heat pump system (DX-SAHP) were presented based on another mathematical model [52]. Researchers have also developed a thermal simulation model that accounted for the hysteresis effect of phase change materials. For heating residential water, a solar-assisted heat pump system was constructed, and the mathematical model was verified using experimental data [55]. In addition, a dynamic model for the solar water heating mode of the indirect solar-assisted heat pump (i-SAHP) systems was presented, which can be used for the design and evaluation of solar heat pump systems [53]. These optimized integrations led to improved energy efficiency in the solar thermal system, resulting in a cost-effective and efficient heating system for residential buildings.

Through simulation, designers can evaluate the effectiveness of various building components, systems and configurations. This empowers them to pinpoint areas for enhancement, enhance energy efficiency and reduce the environmental footprint. By analyzing simulation outcomes, designers can make informed decisions and integrate inventive design approaches that result in more sustainable and resource-efficient building solutions.