Numerical investigation of building integrated solar thermal collectors under diverse conditions

ABSTRACT The scope of this study is the investigation of the thermal performance of building-integrated solar flat collectors with a uniform and multiple riser structure. The effect of dynamic operating parameters such as the environmental temperature, solar radiation, the inclination angle and further model modifications to the fluid inlet, outlet and riser are studied. Numerical calculations were carried out using Finite Element (FE) analysis. Three-dimensional transient models were developed to calculate the thermal performance of the investigated objects. The study revealed increased efficiency for the multiple-riser configuration. Under all boundary conditions, the highest fluid temperatures occur in the south orientation during autumn and winter and in the west during spring and summer. The thermal assessment comparison between the roof installed and vertical solar collectors were performed, and the differences throughout the slope angles were distinguished. The results showed that a building-integrated solar system acted as a shield barrier, and provided heat to the building structure.

Fluid velocity [m/s] x i I th component of the used coordinate system [-] x j J th component of the carrier fluids velocity [-] 1. Introduction The use of solar thermal collectors dates back to the '60s.In states of southern Europe and the Middle East like Cyprus, Greece, Italy, Spain and Israel, solar thermal collectors' use to produce domestic hot water is part of everyday life (Kalogirou 2009).The percentage of installed solar thermal collectors per inhabitant in some cases exceeds one m²/capita, such as in the case of Cyprus (Kalogirou 2005).The use of solar thermal collectors has proven to be economically viable, which is why today this technology is flourishing in other parts of central and northern Europe, such as Austria and Germany (Eicker et al. 2014).The use of these systems extends to other applications, such as industrial heat production, space heating, etc. (Kylili et al. 2018).
Although these systems are highly efficient, it is a fact that their design rationale was not developed significantly in recent decades.There are evident changes in the use of materials, emphasising properties that enhance durability and improve heat transfer (Jiandong, Hanzhong, and Susu 2015), however in terms of the geometry and connectivity of the solar collectors, research has not evolved.The use of finite elements and computational fluid dynamic (CFD) modelling could intensify the improvement of solar thermal collectors' design, emphasising improving their thermal performance and allowing their integration into the building shell.This is a significant challenge for solar thermal collectors.Unlike Building Integrated Photovoltaics (BIPV), solar thermal systems have inherent difficulties in being integrated into the building shell (Lamnatou et al. 2017); hence to date, their integration applications are minimal.
Solar thermal collectors have gained significant attention and have been widely utilised in southern European and Mediterranean countries, leveraging the abundant solar resources and aiming to meet the increasing demand for renewable energy sources.
Several studies have demonstrated the successful implementation of solar thermal collector systems in the region, primarily for domestic hot water production, space heating, and industrial process heat (Papamanolis 2016).The Mediterranean climate, characterised by long periods of sunshine and high solar radiation, provides favourable conditions for the efficient operation of solar thermal collectors, resulting in substantial energy savings and reduced carbon emissions (Arabzadeh, Jokisalo, and Kosonen 2019).Researchers have explored various types of solar thermal collectors, including flat-plate collectors, evacuated tube collectors, and concentrating collectors, to optimise performance and maximise energy yield in these geographic areas (Papadopoulos et al. 2017).Policy frameworks and incentives, such as feed-in tariffs, tax incentives, and renewable energy targets, have played a crucial role in promoting the widespread adoption of solar thermal collectors in southern European and Mediterranean countries, further accelerating the transition towards clean and sustainable energy systems (Kashani, Izadkhast, and Asnaghi 2014;Mazloum and Ghanem 2022) This study attempts to shed light on the object of the optimised design of solar thermal systems installed in the building shell, using finite elements and Computational Fluid Dynamic (CFD) modelling.In this work, three different solar thermal systems were numerically tested concerning their energy performance.The study was carried out on the rationale of integrating the collectors into the building in a vertical layout.The work's assumptions allowed the simulation of the building's real conditions, considering the simultaneous consumption of hot water and the possibility of integration into different orientations of the shell for different seasons of the year.The work resumes calculating temperature and energy profiles produced by various systems, resulting in the optimal conditions and data for the design of building-integrated solar thermal systems.

Overview of studies in the numerical analysis of solar thermal collectors
The scientific studies conducted in the numerical assessment of solar thermal plate collectors address different solar collectors under diverse boundary conditions.In recent years, the leading practice in this field is the application of computational fluid dynamics (CFD) models to define the thermal performance of the investigated models.A variety of multi-dimensional models have been developed using various tools and CFD models such as Ansys-Fluent, OpenFoam, COMSOL, Solidworks, Star-CCM, Matlab etc.
Some studies assume the use of an absorber made of copper.(Fertahi et al. 2018;Gunjo, Mahanta, and Robi 2017;Long et al. 2020) The initial temperature of the thermal medium is also considered.
The investigated studies result in the extraction of the following information: . the effect of the system geometry (e.g.absorber thickness, the air gap between glass and absorber), the mass flow rate and the materials (e.g. of absorber or pipes) on the system efficiency (Mintsa Do Ango, Medale, and Abid 2013; Wang et al. 2015).
Tables 1 and 2 summarise the boundary conditions, the physics and the geometry of the conducted numerical studies in the field of solar thermal collectors assessment.

Boundary conditions
In this study, a numerical model for the simulation of building-integrated solar thermal collectors' thermal performance was developed.The model was based on the finite element method, and the flow simulation engine of SolidWorks was employed (Dassault Systèmes SOLIDWORKS Corp., n.d.).The analysis was performed for four calendar months (January, March, August and November) for Nicosia, Cyprus, delivering results for all seasons of the year in the four orientations (North, South, West and East) (EU SCIENCE HUB, n.d.).The fluid circulation rate was set based on ISO 12975, and all systems were simulated with forced convection considering a 50-watt typical pump with 8 h of operation per day, with approximately electricity consumption of 1,6 kWh.In this numerical model, the water consumption profile was considered (Department for Environment Food & Rural Affairs 2008) as shown in Figure 1.The specific fluid rate at a given time was subtracted from the system, thus accounting for heat removed from the system, therefore water consumption.In Table 3, the boundary conditions of the simulated models are provided.

Geometry and mesh
Two geometries of solar thermal collectors were investigated, a uniform riser in the size of commercial solar thermal collectors and a group of interconnected risers with an area of 0.4 × 0.4 m² each.
Additionally, for the latter geometry, the risers' connectivity was modified, delivering a third alternative geometry.As shown in Table 4, the investigated geometry's main elements were the absorber, the risers, the casing, the thermal insulation, the protective glass, the air between the glass and the absorber, and the tank reservoir.An overview of the investigated geometries and information and drawings of the investigated domain mesh are provided in Table 4.The curvature level regarding the domain mesh was set to be at 0.5548, and the tolerance criterion value was 0.0005 m.The mesh preset was set at level two, which corresponds to the cell density in regions.Based on a preliminary study with different presets, this refinement level was decided, according to which this mesh domain delivered reliable results in a shorter time Table 5.
The solar collector models were incorporated vertically into a building masonry structure.A thermal assessment was performed, and the heat transferred through the masonry structure was investigated.Information regarding the building-integrated solar collector system materials is provided in Table 6, and an overview of the model is shown in Figure 2. Also, the velocity profile of each system is illustrated in Figures 3-5.

Governing equations
The physics employed for the numerical analysis was based on transient heat transfer principles and fluid dynamics.The absorber was particularly subjected to non-steady solar radiation, and heat transfer due to conduction was assumed between the absorber and the riser.Thermal losses due to the convection of the fluid in the riser were also considered.The governing equations employed were the mass, momentum and energy conservation laws, based on the Navier-Strokes approach, for closed-loop forced circulation, expressed as follows: Concerning the solid regions of the model, heat conduction was assumed:

Numerical model validation
The numerical model employed in this study was validated with the use of experimental data published by (Souliotis 2021).The geometry and boundary conditions described in (Souliotis 2021) were developed, as well as the physics of the numerical model implemented in this study was applied.The boundary conditions of the validation model were the specified riser input fluid temperature, the ambient temperature and the solar radiation.For the validation of the numerical (N) and experimental values (E), the root mean square deviation formula was incorporated.

Results
In this section, a brief overview of all investigated systems is presented.The roof-installed systems are exposed to significantly higher solar radiation due to their position.The fluid temperature difference gap between uniform and multiple riser configurations is narrowed Table 7 presented slope 0°simulation results.The maximum average fluid temperature difference is marked under autumn conditions between the uniform riser and multi-riser configuration.Identical thermal behaviour between multi-riser setups is obtained, further designating that the fluid temperature gap is much lower than the slope of 90°.Even though slope 0°displayed inferior thermal performance compared to slope 45°, it achieved the maximum fluid temperature of 54 [°C], which was considered the highest of all.According to the slope 45°simulation results in Table 8, the optimal   orientation for maximum thermal performance is south.Concerning all seasons Table 9 presents the results obtained for slope 90°, and the south orientation provided the best performance under autumn, winter, and spring conditions and the east orientation for the summer season.
Regarding the BIST results presented in Figure 12, the maximum heat flux occurred in winter conditions and the lowest in spring with a value of 141.55 [W/m 2 ] and −56.03 [W/m 2 ] respectively.The full set of data for this numerical study considering all the hourly average fluid temperature, hourly heat flux, hourly solar radiation and ambient temperature for all investigated dynamic conditions is provided.(Georgiou et al. 2021).

Numerical model validation
The numerical and experimental temperature profile comparison is presented in Figure 6 which indicated a good agreement between the two.The calculated value from the root mean square deviation formula (RMSD) for the experimental and numerical cases is 5.01% which is considered acceptable (Pawar and Sobhansarbandi 2020).The simulated results of the fluid temperature profiles are presented in Figures 7 and 8 under two scenarios, including and excluding water consumption.The first case involving water consumption resulted in lower fluid temperatures since the hot water was being removed from the tank reservoir; thus, the temperature decreased.A higher fluid temperature is achieved when water consumption is not considered.The consumption of hot water represents realistic conditions; to this end, this parameter's inclusion resulted in more reliable results.
In winter conditions, the temperature difference between the two scenarios was higher due to the lower environmental temperatures and the reduced heat transfer due to solar radiation.In contrast, the temperature difference in summer conditions is lower since the solar collector is exposed to prolonged solar radiation times, and the ambient temperature is considerably higher.

Assessment of uniform and multiple risers
The thermal analysis of the fluid temperature regarded the three solar collector model configurations are shown in Figure 9.The uniform riser system delivered the lowest fluid temperature; in contradiction, the multiple riser model configurations proved to be more efficient with multiriser configuration one to have achieved the highest fluid temperature even higher when compared to multi-riser configuration two.The improvement of thermal performance in a multiple riser model was attainable because of the larger area of the riser, which resulted in higher efficiency of solar radiation absorption.It should be stated that the overall dimensions of both systems' absorbers are the same.Thus, although it does not require more space, the multiple risers' solution results in a more effective system.The fluid flow pattern is a critical factor for collector systems, affecting thermal performance and should be considered.

The impact of the orientation on thermal performance
The thermal performance simulation results presented in Figures 10 and 11 considered the orientation under diverse weather conditions.The best thermal performance regarded winter and summer conditions in the south and west orientation, respectively.The lowest fluid temperature is observed to occur in the north orientation in all cases; consequently, it can be extracted that the north orientation for the proposed system should be avoided.The west and east orientation thermal performances were identical.A slight time shift is observed in winter conditions.Still, a considerably bigger occurred in summer, which indicated that a solar collector with a tracking system could increase the overall efficiency.These results come as no surprise and occur due to the solar angle, which delivers better outcomes for these two orientations than the south orientation.However, in the south, the solar exposure time is higher.

Assessment of roof installed system
In the study, solar thermal collectors' performance on the buildings' roofs for slope angles of 0°and 45°was investigated.The thermal performance simulation results for January and July are presented in Figure 9.In summer conditions, the slope's fluid temperature is 0°, and 45°is considerably higher than 90°due to the significant increase in solar radiation.Contrary to the summer situation's performance difference, the results under winter conditions showed that vertical collectors have identical thermal performance with roof-installed solar collectors.Despite that, the roof-installed systems with slopes 0°and 45°outperform the integrated solar collectors, but there is great potential since the difference in fluid temperatures is not much apart.

The impact of the building-integrated solar collector on the building performance
The results considering the implications of BIST on the building wall thermal performance are presented in Figure 12.It is possible to conclude that the BIST acts as a heat source on the building wall justified by the outwards heat flux.When comparing the case of no solar collector integration, the outwards heat flux values are presented, meaning that heat escapes the building.Due to the water's heat capacity and insulation, the BIST system acts as a thermal mass, releasing heat even when there is no solar radiation present.Depending on the environmental conditions, BIST's heat is considered an advantage because it contributes to the overall indoor temperature regulation.It minimises heat released into the environment by acting as a shield barrier and supplying heat.However, as shown  in Figure 12, there is also heat flux inwards the building in the summer season.Consequently, BIST system implementation has both advantages and disadvantages for the building.Positively, it adds to the building's existing heat supply, hence improving thermal comfort in general.Improved  energy efficiency is a result of the BIST system's ability to keep heat from escaping the structure.The enhanced heat retention of the BIST system, on the other hand, negatively impacts the building's cooling loads.It hinders the heat's natural dissipation, making it harder to maintain cooler indoor temperatures in the summer.The building's cooling loads are adversely impacted as a result.Given a comparison of the heat flux values between the two situations, it is clear that the BIST system regularly produces heat flux values that are substantially greater.

Discussion
This study examined a plate solar collector system's thermal performance integrated into a building shell under diverse weather conditions.Three transient models were developed based on (FEM) in the CFD tool (Dassault Systèmes SOLIDWORKS Corp., n.d.), one model with a uniform riser, and two models with multiple risers assembled.The dynamic operating parameters considered during the simulation were the solar collector's inclination angle, environmental temperature and solar radiation intensity.
Numerical simulation results illustrated each model's fluid temperatures during north, south, west and east orientations.The multiple riser models demonstrated superior thermal performance over the uniform riser configuration, which proved to be a beneficial resolution for improving overall efficiency.The inlet and outlet modifications on the multiple riser model showed a non-significant decrease in fluid temperature, therefore displaying an inferior thermal performance.A critical component of the solar collector system is a riser, which was investigated and revealed to impact the efficiency directly.The water consumption factor was considered in all the simulations, and a preliminary numerical simulation presented the impact of water consumption on the accuracy of the results Figures 13-16.
A crucial outcome of building-integrated solar collectors' feasibility and performance is delivered using numerical simulations of various slopes.The building-integrated solar collectors act as a thermal mass and potentially can contribute to the heating and cooling of a building during different seasons.
The novelty of this paper lies in the investigation of building-integrated solar flat collectors with both uniform and multiple riser structures, and their thermal performance under various operating parameters and weather conditions.The study utilises numerical simulations and finite element analysis to evaluate the efficiency and fluid temperatures of different collector configurations.The findings demonstrate the superiority of multiple riser models in terms of thermal performance and highlight the potential of building-integrated solar collectors to contribute to the heating and cooling of buildings throughout different seasons.
Further investigation can be conducted to optimise the design and arrangement of multiple risers in building-integrated solar collectors.This could involve exploring different geometries, sizes, and orientations of the risers to maximise thermal performance and overall efficiency.Also future research can focus on the integration of building-integrated solar collectors with heating, ventilation, and air conditioning (HVAC) systems.This would involve studying the feasibility and effectiveness of utilising the thermal mass of the collectors for heating and cooling purposes, and developing control strategies to optimise energy management within the building.
Solar collector; flat plate collector; building integrated solar thermal collector; transient heat transfer; finite elements Nomenclature e Specific internal energy [J/kg] H Total energy of flowing fluid [J/kg] h Thermal enthalpy [J] P Pressure [Pa] QH Specific heat release or absorption per unit volume [J/(kg•K)] q i Heat flux [w/m 2 ] R Gas constant [8.3145J/mol•K] T Temperature [°C] S i Mass-distributed external force per unit mass [N/kg] u i I th component of the carrier fluids velocity [-] u j J th component of the carrier fluids velocity [-] u

Figure 12 .
Figure 12.Total daily Heat flux [W/m 2 ] Inwards and Outwards of the building wall -Building Integrated Solar Collector Vs No Solar Collector.

Figure 13 .
Figure 13.Temperature distribution [K] Outer surface of the collector end in the area in contact with the building envelope -System 2A -August -South Orientation.

Figure 14 .
Figure 14.Temperature distribution [K] Outer surface of the collector end in the area in contact with the building envelope -System 2A -March -South Orientation.

Figure 15 .
Figure 15.Temperature distribution [K] Outer surface of the collector end in the area in contact with the building envelope -System 2A -November -South Orientation.

Table 1 .
Boundary conditions that are considered for the numerical simulation.

Table 2 .
Numerical Simulation model characteristics.