Comprehensive energy analysis of a photovoltaic thermal water electrolyzer

doi:10.1016/j.apenergy.2015.11.078

Applied Energy

Volume 164, 15 February 2016, Pages 294-302

https://doi.org/10.1016/j.apenergy.2015.11.078 Get rights and content

Highlights

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A Photovoltaic Thermal Water Electrolyzer (PVTE) configuration is reported.
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The PVTE system was modeled to determine optimal geometry and operating conditions.
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The overall efficiency increased with the velocity of heat-transfer fluid.
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The max improvement in power output for the PVTE compared to a PV alone is in the afternoon.
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A PVTE (instead of a standalone electrolyzer) exhibits 2.5 times more hydrogen production.

Abstract

The use of photovoltaic thermal (PVT) technologies enables improvement in the electrical efficiency of a photovoltaic (PV) module by reducing the temperature of the PV module via active waste heat removal. In current PVT systems, the removed heat is mainly used for specific applications, such as water and/or room heating, but their need is intermittent and seasonal. For a more efficient and versatile use of the removed waste heat, we propose a new architecture where the PV module is integrated with a dual-functional electrolyzer that removes the waste heat by active cooling and produces hydrogen via electrolysis. The excess heat from the PV cell is utilized to enhance the reaction kinetics of the electrolysis process (due to an increase in temperature) inside an electrolyzer, which is located below the PV module. In this paper, we used finite-element analysis (FEA) simulations to optimize the geometry and operating conditions of an electrolyzer to maximize overall energetic efficiency and hydrogen production. To evaluate the practical feasibility of the approach, we performed a comprehensive energy analysis of the PVTE system using data from Phoenix, AZ. The energetic efficiency of the proposed PVTE system was calculated to be 56–59%, which is comparable to those of current PVT systems. Additionally, the integration of the electrolyzer with the PV module led to an almost 2.5-fold increase in hydrogen production compared to a stand-alone electrolyzer operated at ambient temperature. The analyzed hybrid approach potentially represents a viable and useful alternative for utilization of waste heat energy from PV cells. This approach may further increase the use of photovoltaic technologies as a renewable energy source.

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.

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Comprehensive energy analysis of a photovoltaic thermal water electrolyzer

Highlights

Abstract

Graphical abstract

Introduction

Section snippets

Dimension and material properties

COMSOL simulation and temperature distribution

Conclusion

Acknowledgements

Renew Sustain Energy Rev

Sol Energy Mater Sol Cells

Appl Energy

Sol Energy

Sol Energy Mater Sol Cells

Appl Therm Eng

Prog Energy Combust Sci

Appl Energy

Appl Energy

Int J Hydrogen Energy

Prog Energy Combust Sci

Sol Energy

Int J Heat Mass Transf

Sol Energy Mater Sol Cells

Sol Energy

Sol Energy

Energy Convers Manage

Appl Energy