Due to the variable nature of the photovoltaic generation, energy storage is imperative, and the combination of both in one device is appealing for more efficient and easy-to-use devices.Among the myriads of proposed approaches, there are multiple challenges to overcome to make these solutions realistic alternatives to current systems. This paper classifies and identifies previous efforts to achieve integrated photovoltaic storage devices. Moreover, the gaps and future perspectives are analysed to give an overview of the field, paying attention to low-power devices (<10 W) as well as high-power devices (>10 W). We focus on devices that combine solar cells with supercapacitors or batteries, providing information about the structure, materials used, and performance. On the one hand, small power devices must concentrate on including power electronics to increase the efficiency of the system as well as ensuring a safe operation; likewise, more attention should be drawn to validate the feasibility of novel concepts by carrying out more realistic and standardised tests, including long-term testing. On the other hand, high-power devices must be researched thoroughly to evaluate the impact of high temperatures on energy storage and solar module ageing; furthermore, optimum system sizing is a relevant topic that deserves attention and its relation to modular solutions. This critical literature review serves as a guide to understand the characteristics of the approaches followed to integrate photovoltaic devices and storage in one device, shedding light on the improvements required to develop more robust products for a sustainable future. KEYWORDS battery, one device, PV-storage integration, solar-battery integration, solar energy, supercapacitor 1 INTRODUCTION Solar photovoltaic (PV) energy generation is highly dependent on weather conditions, making solar power intermittent and many times unreliable. Moreover, energy demand is widespread during the day, and solar energy is available for few hours, provoking a mismatch between demand and supply. These two issues are the driving force behind the use of energy storage (ES) devices, which help decrease the fluctuations from the generation side but also provide the possibility of performing ancillary services. Consequently, it is fundamental to find the most appropriate energy storage device for particular applications and operational conditions.According to the characteristics of ES devices, the criterion that defines when the energy is stored and utilised may vary, even for the same component. Some ES devices can supply power during extended periods (hours, days), while others are more suitable for shorter periods of operation (seconds, minutes). For instance, rapid changes in PV power due to rapidly changing illumination conditions can be smoothed using supercapacitors (SCs); they deliver power when solar supply is scarce, so the load is still satisfied. For devices with lower self-discharging values like electrochemical cells (batteries), the electrical energy produced by...

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Off-grid solar home systems (SHSs) currently constitute a major source of providing basic electricity needs in un(der)-electrified regions of the world, with around 73 million households having benefited from off-grid solar solutions by 2017. However, in and of itself, state-of-the-art SHSs can only provide electricity access with adequate power supply availability up to tier 2, and to some extent, tier 3 levels of the Multi-tier Framework (MTF) for measuring household electricity access. When considering system metrics of loss of load probability (LLP) and battery size, meeting the electricity needs of tiers 4 and 5 is untenable through SHSs alone. Alternatively, a bottom-up microgrid composed of interconnected SHSs is proposed. Such an approach can enable the so-called climb up the rural electrification ladder. The impact of the microgrid size on the system metrics like LLP and energy deficit is evaluated. Finally, it is found that the interconnected SHS-based microgrid can provide more than 40% and 30% gains in battery sizing for the same LLP level as compared to the standalone SHSs sizes for tiers 4 and 5 of the MTF, respectively, thus quantifying the definite gains of an SHS-based microgrid over standalone SHSs. This study paves the way for visualizing SHS-based rural DC microgrids that can not only enable electricity access to the higher tiers of the MTF with lower battery storage needs but also make use of existing SHS infrastructure, thus enabling a technologically easy climb up the rural electrification ladder.

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