Energy and exergy efficiency of water-based photovoltaic thermal (PVT) systems: an overview

Nurul Shahirah Binti Rukman1, Ahmad Fudholi2, Ivan Taslim3, Merita Ayu Indrianti4, Intan Noviantari Manyoe5, Uce Lestari6, Kamaruzzaman Sopian7 1,2,7 Solar Energy Research Institute, Universiti Kebangsaan Malaysia, 43600 Bangi Selangor, Malaysia 3,4 Universitas Muhammadiyah Gorontalo, Indonesia 5 Universitas Negeri Gorontalo, Indonesia 6 Pharmaceutical Study Program, Faculty of Science and Technology, University of Jambi, Indonesia


INTRODUCTION
These past days has governed the adancements of technologies by sourcing solar energy as the energy input. Due to the sustainability of solar source, the renewability of the energy had contributed to the employment of them in most of recent applications. The most promising technology that made great pace in advancement is photovoltaic (PV) technology which using solar panel in converting sunlight to electrical energy. As further studies has been continued, photovoltaic thermal (PVT) system has been introduced which is the combi system of PV and heat collector system. The PV panel absorbs the solar radiation while the heat collector system acts as to remove heat from the PV to regulate the temperature of panel, avoiding extreme hot condition of panel during the operation. Thus, PVT system produces both electrical and thermal energies from one integrated system; drawing high attention from lots of studies regarding its prospects in fullfilling the energy demand. PVT system is a promising system to generate both energies due to its high reliability system with low environment impact. Primarily, the focus of study was on glazed collectors or panel; both air-based and water-based system.
The differences were the coolants used either air or water in the heat collector or cooling system. Among the application of these systems as had been used for building integrated photovoltaic thermal (BIPVT) system. Generally, a water-based PVT system system comprising with PV module, absorber collector in the designated tubes, the glass cover, and insulated container. An air-based PVT system consists of a PV panel and a thermal collector system. The system can produce electrical energy directly converted from solar radiation, while extracting heat from the PV panel and warm the air flow inside the collector; as well as for utilizing water in water-based PVT system. These explain the production of hot air and hot water from the water-based and air-based PVT systems [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15].
In the mid-1970s, a study with the focus on PVT systems, aiming to increase the PV efficiency had been carried out. In the meantime, to meet the deployment of the technology in domestic application was regarded as the main market [16]. In this review, energy and exergy efficiency of water-based PVT systems is presented, and types of water-based PVT system is described.

TYPES OF WATER-BASED PVT SYSTEMS
Utilization of water as coolants in PVT systems which also known as water-based PVT systems had been experienced great advancements. Based on the researches that had been conducted as shown in Figure 1, flow pattern of coolant in water-based PVT systems can be categorised into theree types are: (i) natural flow, (ii) force flow and (iii) hybrid system.

STUDIES CONDUCTED ON WATER-BASED PVT SYSTEMS
Afterwards, an evaluation of combined PVT systems also had been carried out by Hendrie [17] which the performances of both electrical and thermal energy had been evaluated by yielding sharp relation to the theoretical results. From the research, it had proved that PVT collector system can assured in having potential for useful and greater thermal and electrical energy gain. From his theoretical studies, he also concluded that the analytical models can be used in predicting the energy production's performances as it gave an accurate data.
An experimental study also had been conducted by Erdil et al. [18] which focused on energy generation with a PV-solar thermal hybrid system by using water as coolants. Two experimental modules had been used as the hybrid system while input/output diverse system had been installed in order to ensure constant flow circulation of water across the cavity.
First, the investigation on water-based PVT systems under continuous assortment of temperature mode with unlike constant flow rate mode by Mishra and Tiwari [19] had been compared for two panel's configuration which partially covered and fully covered PV module. The partially covered PV had been shown that the case was preferable for hot water or high thermal yield production meanwhile the one with fully covered resulted in generating higher electrical gain, hence better for primary requirement of obtaining high electrical gain.
Chow et al. [20] also had studied dissimilar water-based PVT system which resulting that unglazed PVT generated higher thermal energy than glazed PVT. Other than that, Shyam et al. [21] had concluded from a study on partially covered PV module of water-based PVT system connected in series, that the temperature reliant on module efficiency, tank and outlet water temperature have been validated for the water-based PVT system.
Other than that, exergy efficiency was investigated experimentally and mathematically which more realistic and practical view of process than energy analysis method [22]. Agrawal and Garg [23] also have studied the energy performance a water-based PVT system. Difference method had been used to simulate the performance of the water-based PVT system. Their results indicated that water-based PVT systems generated satisfactory electrical energy for domestic usages.

ENERGY AND EXERGY EFFICIENCY OF WATER-BASED PVT SYSTEMS
Exergy theory also had been used in term of attaining analysis on performance of water-based PVT system, experimentally. From their annual experimental assessment, they determined that the water-based PVT system can produce better exergy output than a unit of PV module or water collector [24]. Tiwari and Sodha [25] also had established a thermal model for a water-based PVT system and conveyed that a daily thermal efficiency of around 58% had been achieved. Dubey and Tiwari [26] have designed and verified an unsteady state system of water-based PVT system. An observation of it performed better thermal and average cell efficiency which is in accordance with previous research. Then, Tiwari et al. [27] had come to conclude that the overall exergy and thermal efficiency of PVT system is maximum at mass flow rate of 0.006 kg/s.
In 2014, a thermal model had been designed to test for electrical efficiency of a-Si PV module. Water flow had been implied on the front of panel in open loop condition for cooling the PV module. Water flow over the PV module had been concluded as improving its efficiency as this design was where the drained water, had been unused and also without integrating it with any plate water collector assembly [28]. Then, in the same year, it had been approved by Herrando et al. [29] that water is the best heat removal compared to air. A presentation of a numerical model had been studied by Boubekri et al. [30] to testify the electrical performance of a collecting hybrid water-based PVT system which it had been shown that the overall efficiency of the collector also can be influenced by the inclination angle, the mass flow rate of water and the conduction heat transfer coefficient in the adhesive layer.
Kalogirou et al. [31] also led a simulation on the industry of PVT systems with water heat extraction in three places with different latitudes, and also using different type of solar cells. The previous study had approved on the water efficiency as heat extraction and as well as latitudes that influences the energy generation. Meanwhile, as for this study which used different solar cells, it had been emerged that the electricity generation is higher for polycrystalline solar cells whereby amorphous solar cells produced slightly higher thermal contributions. Beforehand in 2006, a computer simulation was performed which concentrating on the combined effects of solar cell packing factor and water mass flow rate. These factors were the variables to study on the electrical and thermal efficiencies. It had reconfirmed that system operation at the optimum mass flow rate improved the thermal performance. However, it then lessening when a critical mass flow is surpassed [32]. Energy and exergy analysis of water-based PVT systems by different researchers as shown in Table 1.

CONCLUSION
In this review, water-based PVT systems had been summarized which comprising the studies that had been carried out previously on the advancement of this system. Production of hot water, portrayed by the thermal gain more than 50%. Though, the study on exergy is still limited and is recommended to be furthered in order to obtained useful energy generation by the system.