Comprehensive energy analysis of a photovoltaic thermal water electrolyzer
Graphical abstract
Introduction
Photovoltaic (PV) technologies employ the photovoltaic effect to convert solar energy into electricity. PV technologies are rapidly becoming an important component of the energy landscape. In fact PV is the third most important renewable energy source in terms of globally installed capacity [1]. To further exploit the potential of PV technologies, the energy output of these technologies needs to be increased. Several strategies have been employed to enhance the electrical conversion efficiency of PV cells, e.g., by the use of new materials [2], [3] and through development of sophisticated designs [4], [5]. Despite these efforts, the electrical conversion efficiency, the fraction of incoming solar radiation that is converted to electricity, of PV cells currently on the market is still less than 20% [6]. Another strategy to improve the electrical conversion efficiency is to reduce the temperature of the PV cell, as the efficiency is known to decrease at higher temperatures [7]. This rise in temperature mainly originates from the energy that is not converted into electricity and dissipated as heat. Hence, active and passive cooling methods have been employed to remove waste heat, hereby increasing efficiency, and consequently energy output [8].
One approach to further increase the overall energy utilization of PV technologies is to convert the energy removed as waste heat into another useful form of energy, e.g., use of hybrid photovoltaic/thermal (PVT) systems [9]. A PVT system combines a PV module that converts the solar energy into electricity, and a solar thermal collector module that absorbs some of the waste heat for later use (thereby also reducing the temperature of the PV module) [10], [11]. The overall energetic efficiency (fraction of incoming solar radiation that is converted to electricity and useful heat energy) of PVT systems ranges from 53% to 68% based on the location and time [12]. Along with the advantage of a high overall energy efficiency, PVT systems also produce more energy per unit surface area [13] and reduce space and installation cost compared to a separate PV panel and a solar heat collector system.
In PVT systems the captured heat energy is typically used for domestic hot water and/or room heating. The need for these applications, however, is intermittent and seasonal, leading to stored energy being wasted. For instance, the overall energy output of PVT systems will be higher when the ambient temperature is high (due to higher level of solar radiation), but the need for hot water and heating during these times will be lower. As a result, a need exists for alternative ways to store and use the waste heat of PVT systems, especially ways that are less subject to intermittency and seasonal requirements. To this end, we propose an integrated system – “a photovoltaic thermal water electrolyzer (PVTE)” – which comprises a PV cell positioned on top of a planar micro-water electrolyzer. This system not only produces the same output as typical PVT systems, i.e., electricity and heat energy, but also produces hydrogen, an environmentally benign and a sustainable energy carrier that can be stored and used for applications such as power generation when solar power is not available or transportation [14]. Naturally, hydrogen storage (e.g., by compression) would require additional energy, but that energy cost is not included in the analyses reported here.
Fig. 1(a) provides a schematic illustration of the PVTE system, while the design and operation of the electrolyzer is illustrated in Fig. 1(b). Part of the electrical energy generated by the PV cell is used for water electrolysis, which is a promising approach for hydrogen production, while the remaining electrical energy is supplied to the grid, similar to conventional PVT systems [6], [9]. The excess heat dissipated from the PV cell is captured by the water electrolyzer, which functions as a heat sink, and causes the temperature of the electrolyte in the electrolyzer to increase and approach the temperature of the PV cell. The higher temperature of the electrolyte in the electrolyzer increases the efficiency of the electrolysis reaction and also reduces the over-potentials for H2 and O2 gas evolution at the electrodes [15]. As a result, the dissipated heat energy is utilized to not only produce hydrogen, but also increase the efficiency of the electrolysis. Additionally, the electrolyte in electrolyzer functions as a heat-transport fluid, and the electrolyte at the elevated temperature (exiting the electrolyzer) is circulated to transfer heat to an insulated water tank, similar to the scheme used for domestic hot water and/or room heating in conventional PVT systems. The produced hydrogen can be used, for example, to operate a fuel cell for power generation, or as a fuel for a fuel cell-based car.
In this paper, we present a comprehensive analysis of the overall efficiency of the proposed PVTE configuration, and compare its performance to that of the PV panel alone. We also estimated the efficiency of the electrolysis process within the PVTE system and compared it to that of a stand-alone electrolyzer. COMSOL Multiphysics® software was used to predict the temperatures of the electrolyte and the PV cell for various values of ambient temperature and solar flux. We also used the simulations to optimize the chamber thickness of the electrolyzer and the flow velocity of the electrolyte with the objective of maximizing the electrical efficiency, thermal efficiency, and the efficiency of water electrolysis. To demonstrate the practical feasibility of the PVTE system, we estimated the hourly and monthly efficiency values of the various processes and the power output using solar irradiation and temperature data for Phoenix, Arizona from the year 2010. The high temperatures in combination with the abundant availability of solar power in Phoenix make this location an attractive location for employing a PVTE.
Section snippets
Dimension and material properties
In this work, we analyze a unit PV cell (16 cm × 16 cm) with 16 water electrolyzers underneath it. An illustration of the module with the various layers and thicknesses are shown in Fig. 2(a). The PV cell size was chosen such that the cell can be fabricated on a 6 in. silicon wafer. The solar cell portion of this module is sandwiched between two layers of ethylene vinyl-acetate (EVA) covered on top with a protective glass layer and on the bottom with a backing layer (Tedlar) to separate the cell
COMSOL simulation and temperature distribution
We first estimated the change in the temperature throughout the PV module and the electrolyzer using FEA simulations. The velocity of the fluid, as expected, has a significant influence on the temperature distribution within these structures. Fig. S1 (see supplementary information) shows the steady state temperature distributions in a 0.4 mm thick electrolyzer chamber operated at two values of the electrolyte velocity within the chamber, 0.17 mm/s and 0.68 mm/s (75.6 μL/min and 302.4 μL/min), while
Conclusion
This paper reported an energetic analysis of a photovoltaic system that integrates a photovoltaic (PV) module with an electrolyzer, referred to as a photovoltaic thermal water electrolyzer (PVTE). Active cooling of the PV module by the electrolyte in the electrolyzer reduces the temperature of the PV module and consequently increases the electrical efficiency. The heat absorbed by the electrolyte enhances the reaction kinetics of the electrolysis process leading to more efficient production of
Acknowledgements
We acknowledge support for this work (MEO and RGN) as part of the ’Light-Material Interactions in Energy Conversion’ Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Basic Energy Sciences under Award Number DE-SC0001293. We also acknowledge financial support from the National Science Foundation under awards CMMI 03-28162 and CMMI 07-49028 to Nano-CEMMS, a Nano Science & Engineering Center (NSEC) on Nanomanufacturing, for PJAK and AVD.
References (36)
- et al.
A review of solar photovoltaic technologies
Renew Sustain Energy Rev
(2011) - et al.
Recent developments in high-efficiency Ga0.5In0.5P/GaAs/Ge dual- and triple-junction solar cells: steps to next-generation PV cells
Sol Energy Mater Sol Cells
(2001) - et al.
Photovoltaic modules and their applications: a review on thermal modelling
Appl Energy
(2011) - et al.
On the temperature dependence of photovoltaic module electrical performance: a review of efficiency/power correlations
Sol Energy
(2009) - et al.
Cooling of photovoltaic cells under concentrated illumination: a critical review
Sol Energy Mater Sol Cells
(2005) - et al.
Photovoltaic thermal (PV/T) collectors: a review
Appl Therm Eng
(2007) Solar thermal collectors and applications
Prog Energy Combust Sci
(2004)A review on photovoltaic/thermal hybrid solar technology
Appl Energy
(2010)- et al.
Optimizing limited solar roof access by exergy analysis of solar thermal, photovoltaic, and hybrid photovoltaic thermal systems
Appl Energy
(2014) - et al.
Impact of hydrogen on the environment
Int J Hydrogen Energy
(2011)
Recent progress in alkaline water electrolysis for hydrogen production and applications
Prog Energy Combust Sci
A thermal model for photovoltaic systems
Sol Energy
Simulation and experimental validation of heat transfer in a novel hybrid solar panel
Int J Heat Mass Transf
Annual exergy evaluation on photovoltaic-thermal hybrid collector
Sol Energy Mater Sol Cells
Cost studies on terrestrial photovoltaic power-systems with sunlight concentration
Sol Energy
Performance analysis of photovoltaic-thermal collector by explicit dynamic model
Sol Energy
Study of a hybrid solar-system solar air heater combined with solar-cells
Energy Convers Manage
Energy and exergy analysis of photovoltaic-thermal collector with and without glass cover
Appl Energy
Cited by (37)
A review of solar hybrid photovoltaic-thermal (PV-T) collectors and systems
2023, Progress in Energy and Combustion ScienceEstimating the global technical potential of building-integrated solar energy production using a high-resolution geospatial model
2022, Journal of Cleaner ProductionCitation Excerpt :The solar energy market is dominated by the economically most profitable PV panels, although the hybrid PV/T collectors also represent a noticeable part of the solar industry. The main advantage of these PV/T collectors (combination of PV cells and a heat absorber) related to standalone PV panels is that they utilize the waste heat generated by overheating cells and transform it to useful and consumable thermal energy (Oruc et al., 2016; Shin et al., 2020). In the meantime, remarkably lower cell temperature and enhancing electric efficiency (ηelec) can be achieved by exhausting heat from the module (Diwania et al., 2020).
Review on poly-generation application of photovoltaic/thermal systems
2022, Sustainable Energy Technologies and Assessments