Elsevier

Applied Energy

Volume 266, 15 May 2020, 114881
Applied Energy

Method to improve performance of building integrated photovoltaic thermal system having optimum tilt and facing directions

https://doi.org/10.1016/j.apenergy.2020.114881Get rights and content

Highlights

  • Periodic nature of solar insolation and ambient air temperature.

  • Optimum tilt angle of BIPV panels corresponding to different orientations.

  • Influence of water flow on performance of optimally tilted BIPV thermal system.

  • Exergy assessment of BIPV thermal system.

Abstract

A building integrated photovoltaic (BIPV) system can be installed at optimum tilt and orientation angle to maximize the electrical output. PV output is negatively correlated with the increase in the PV panel temperature. This paper aims at investigating the influence of water flow over the BIPV roof of a two-storied single-family house on the performance of the proposed optimally tilted semi-transparent BIPV thermal system having different facing directions. The periodic nature of insolation, ambient air temperature, BIPV cell temperature, slab temperature, water flow temperature and room temperature have been considered while solving energy equilibrium differential equations. The insolation values used in the energy equilibrium equations are computed employing periodic HDKR (Hay, Davies, Klucher and Reindl) model, which is based on anisotropic sky concept. Results indicate that the average BIPV cell temperature is reduced annually by approximately 10 °C and efficiency is enhanced by 6% when a water flow rate of 1.1 kg s−1 is provided over the BIPV roof. It is also observed that on the most critical day of the year, the temperature of outlet water is 11.5 °C higher than that of inlet water temperature which resulted in 4 kW extraction of exergy.

Introduction

Implementation of building integrated photovoltaic (BIPV) thermal technology is one of the steps towards achieving a net-zero energy building. BIPV refers to the concept of replacing the external cladding of the building envelope, including roofs and facades, by PV modules. Aaditya et al. [1] discussed various solar technologies that can be utilized in buildings and classified these technologies according to the mechanism of harnessing solar energy in the form of heat, light, and biomass. Tripathy et al. [2] reviewed the BIPV products and their suitable applications as different components of the buildings such as flat roof, pitch roof, curved roof, façades and skylight. Yang et al. [3] comprehensively reviewed the major development of various BIPV/T system and their impact on building performance. The energy generation of the BIPV system depends on incoming solar radiation and ambient air temperature. The most important aspect affecting the electrical output of PV modules is the amount of the incoming solar irradiation, which can be maximized by installing BIPV panel at optimum tilt angle with proper orientation [4].

Many researchers have established the relation between optimum tilt angle and latitude, which are generally adopted in various locations. Liu and Jordan isotropic sky model [5] was followed by Pour et al. [6] to evaluate the optimum tilt angle, and it was found that the fixed optimum tilt angle is approximately equal to the latitude of the location. For a further accurate determination of the optimum tilt angle, non-isotropic models were used by Klein et al. [7] considering the azimuth angle of PV panels. The computation of solar radiation by experimental setup gives the accurate optimum tilt angle for a particular location. Hafez et al. [8] reviewed the current methods to determine the optimum tilt angle and concluded that PV systems display significant improvement in performance when it is installed at the optimum yearly tilt angle. Mutlu et al. [9] proposed a model to obtain the optimum slope of roofs fitted with PV panels. Tripathy et al. [10] explored modified HDKR (Hay, Davies, Klucher and Reindl) model and determined the optimum tilt angle and insolation values, including shadow effect in urban areas by considering circular building around the BIPV system. Park et al. [11] analyzed the thermal and electrical performance of semi-transparent PV module that was designed as a glazing component of the building. It was concluded that power generation of PV module decreases by about 0.48% and 0.52% per 1 °C increase in PV cell temperature for the indoor and outdoor test conditions, respectively. In this context, several researchers investigated methods to reduce the PV cell temperature of BIPV thermal systems.

BIPV thermal system consists of photovoltaic modules, which are cooled by a suitable fluid such as air or water. The BIPV thermal system simultaneously converts solar insolation into thermal and electrical energy. The fluid flowing beneath the PV modules reduces the temperature of the PV cells and enhances the efficiency of PV modules. This also produces low-grade heat for various applications such as space heating and domestic hot water. Tiwari et al. [12] reviewed the thermal modelling of various types of hybrid photovoltaic systems using water and air as the working fluid. Kumar et al. [13] investigated the performance of thin-film cadmium telluride PV arrays integrated with roofs and façades in Malaysia. Aaditya et al. [14] assessed the real-time performance of a BIPV thermal system installed at the centre for sustainable technologies, Bangalore, India. Yang et al. [15] experimentally investigated the prototype of open-loop air-based BIPV thermal systems with single-inlet in cold climate solar house. The investigators further investigated the prototype with double-inlet and multiple inlets in their subsequent research [16], and it was noted that the novel design of introducing multiple inlets in the BIPV thermal air system enhances energy performance. Tripathy et al. [17] investigated the performance of both opaque and semi-transparent BIPVT systems for the different flow rate of air flowing through the duct. Herrando et al. [18] assessed the suitability of hybrid PVT system for the provision of electricity and hot water in the UK domestic sector. Chow et al. [19] conducted experiments on a centralized photovoltaic and hot water collector wall system and concluded that natural water circulation is preferable than forced water circulation. Bakar et al. [20] predicted the performance of the bi-fluid solar collector for a range of mass flow rate of air and water. Hachem et al. [21] investigated the impact of roof morphology on the performance of BIPVT systems. Deo et al. [22] developed a periodic thermal model of BiSPVT system. The energy-based performance assessment of a PV system only takes into account the electrical energy generated by the PV cell and it does not consider the climatic, geometric and operating parameters of a PV system. In order to precisely examine the performance of PV thermal systems, these parameters need to be considered.

Exergy is considered as an accurate tool for performance evaluation of BIPV thermal system from the thermodynamic point of view. This is evaluated by considering the parameters of BIPV thermal system such as wind velocity, ambient air temperature and PV cell temperature [23]. Saloux et al. [24] defined exergy as a qualitative aspect of energy which is the available share of energy. To calculate the exergy of a BIPV thermal system, both the first law and second law of thermodynamics are followed [25]. Joshi et al. [26] thoroughly reviewed the photovoltaic and photovoltaic thermal system based on their electrical, thermal and exergetic performance. Evola et al. [27] evaluated the exergy of BIPV/T system provided with rectangular stainless-steel ducts to enhance heat transfer between the thermal absorber and the water passage. In their investigation, it was concluded that an optimum water inlet temperature can maximize the overall exergy efficiency of the BIPV/T system. Daghigh et al. [28] assessed a photovoltaic–thermal combi collector theoretically and experimentally in weather conditions of Sanandaj city, Iran. The energy and exergy obtained from collector during the experiment were found to be 125 W m−2 and 119 W m−2, respectively. Gupta et al. [29] evaluated the exergy of a two-storied BIPV thermal system without considering the periodic nature of insolation parameters and thermal exergy of the lower story room.

In the open literature, several researchers dealt with the performance assessment of south-facing BIPV system installed in urban areas without considering the periodic nature of insolation and concluded that efficiency of BIPV system is inversely proportional to the PV cell temperature. In urban application of BIPV systems, it is not always possible to ensure a south-facing orientation. Hence, for more realistic cases, the influence of different facing directions of BIPV panel on the performance of BIPV system should be investigated. Moreover, certain measures may be adopted to reduce the PV cell temperature and to enhance the efficiency of the BIPV system. In the present investigation, the performance evaluation of BIPV thermal system provided with water flow over PV modules for different facing directions of BIPV panels has been discussed by considering the actual (periodic) nature of climatic parameters combined with optimum tilt angle of BIPV panel. The most suitable combination of optimum tilt angle and orientation of the BIPV panel allowing maximum energy output is investigated. This study assumes a semi-transparent BIPV thermal system, integrated within the roof system of a two-storied single-family house. The periodic behaviour of insolation, ambient air temperature, inlet water temperature, PV cell temperature, intermediate slab temperature, and room temperature are considered while solving energy equilibrium differential equations. This model is employed to determine the warmest day of the year. The influence of water flow on the efficiency of optimally tilted BIPV thermal system has been investigated to find the optimum mass flow rate of water for different facing directions of the BIPV roof. The exergy assessment of BIPV thermal system is also carried out for different months of the year.

Section snippets

Problem overview

In this investigation, the influence of water flow on the performance of optimally tilted semi-transparent BIPV thermal system with different orientations is evaluated on the warmest day of the year (15th May) as the performance is most affected on this day due to highest PV cell temperature. The above warmest day is determined by using the present mathematical model and optimum tilt angle of the PV panels is calculated by using periodic HDKR model. The BIPV system is integrated as a roof of a

Mathematical formulation

A number of assumptions are made to derive energy equilibrium equations for the considered semi-transparent BIPV thermal system. The assumptions are (i) Heat transfer through PV roof and side walls is along normal to the surface, (ii) The properties of air and wall materials do not change with the change in temperature and time, (iii) A thin film of water is considered over BIPV roof without any storage effect, (iv) The system is considered under quasi-steady state, (v) Climatic parameters such

Results and discussions

To solve the periodic energy equilibrium equations of the BIPV system, it is required to express insolation and ambient air temperature values in terms of Fourier approximation. Fig. 2 shows the Fourier approximation of solar insolation values calculated from HDKR model at Delhi (ϕ = 28.70°, 15th May 2018), India. The variation of solar insolation with time is plotted for the different number of Fourier series harmonics (n). It is observed that the solar insolation values converge after the

Conclusion

BIPV system is a successful technology that can be suitably integrated within a building structure, as a stage towards reaching a net-zero energy status. The BIPV panels should installed at optimum tilt angle and a proper orientation, in order to maximize the incident solar radiation and consequently enhance the efficieny of the BIPV system. In this investigation, a semi-transparent BIPV system is integrated with the roof of a two-storied sample building situated at New Delhi, India (ϕ=28.70°).

CRediT authorship contribution statement

Somil Yadav: Conceptualization, Visualization, Investigation, Methodology, Validation, Writing - original draft. S.K. Panda: Conceptualization, Visualization, Investigation, Methodology, Supervision, Writing - review & editing. Caroline Hachem-Vermette: Conceptualization, Visualization, Investigation, Methodology, Supervision, Writing - review & editing.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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