Nearly 840 million people still lack access to electricity, while over a billion more have an unreliable electricity connection. In this article, the three different electrification pathways-grid extension, centralized microgrids, and standalone solar-based solutions, such as pico-solar and solar home systems (SHS)-are critically examined while understanding their relative merits and demerits. Grid extension can provide broad scale access at low levelized costs but requires a certain electricity demand threshold and population density to justify investments. To a lesser extent, centralized (off-grid) microgrids also require a minimum demand threshold and knowledge of the electricity demand. Solar-based solutions are the main focus in terms of off-grid electrification in this article, given the equatorial/tropical latitudes of the un(der-)electrified regions. In recent times, decentralized solar-based off-grid solutions, such as pico-solar and SHS, have shown the highest adoption rates and promising impetus with respect to basic lighting and electricity for powering small appliances. However, the burning question is-from lighting a million to empowering a billion-can solar home systems get us there?The two main roadblocks for SHS are discussed, and the requirements from the ideal electrification pathway are introduced. A bottom-up, interconnected SHS-based electrification pathway is proposed as the missing link among the present electrification pathways.

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With almost 1.1 billion people lacking access to electricity, solar-based off-grid products like Solar Home Systems (SHS) have become a promising solution to provide basic electricity needs in un(der)-electrified regions. Therefore, optimal system sizing is a vital task as both oversizing and undersizing a system can be detrimental to system cost and power availability, respectively. This paper presents an optimal SHS sizing methodology that minimizes the loss of load probability (LLP), excess energy dump, and battery size while maximizing the battery lifetime. A genetic algorithm-based multi-objective optimization approach is utilized to evaluate the optimal SHS sizes. The potential for SHS to cater to every tier of the Multi-tier framework (MTF) for measuring household electricity access is examined. The optimal system sizes for standalone SHS are found for different LLP thresholds. Results show that beyond tier 2, the present day SHS sizing needs to be expanded significantly to meet the load demand. Additionally, it is deemed untenable to meet the electricity needs of the higher tiers of MTF purely through standalone SHS without compromising one or more of the system metrics. A way forward is proposed to take the SHS concept all the way up the energy ladder such that load demand can also be satisfied at tier 4 and 5 levels.

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Almost a billion people globally lack access to electricity. For various reasons, grid extension is not an immediately viable solution for the un(der-) electrified communities. As most of these electricity-starved regions lie in tropical latitudes, the use of off-grid solar-based solutions like solar home systems (SHS) is a logical approach. However, state-of-the-art SHS is limited in its power levels and availability. Moreover, sub-optimal system sizing leads to either over-utilization --- and therefore, faster degradation --- of the SHS battery, or under-utilization of the SHS battery, leading to higher system costs. Additionally, off-grid SHS designs suffer from a lack of reliable load profile data needed as the first step for an off-grid photovoltaic (PV) system (e.g., SHS) design. The work undertaken in this dissertation aims to analyze the technological limits and opportunities of using SHS in terms of power level, availability, and battery size, lifetime for achieving universal electrification. Firstly, the three main electrification pathways, viz., grid extension, centralized microgrids, and standalone solar-based solutions like pico-solar and SHS are analyzed for their relative merits and demerits. Then, a methodology is presented to quantify the electricity demand of the households in the form of load profiles for the various tiers of electricity access as outlined by the multi-tier framework (MTF) for measuring the household electricity access. Secondly, for the SHS application, a non-empirical battery lifetime estimation methodology is presented that can be used at the design phase of SHS for comparing the performance of candidate battery choices at hand in the form of battery lifetime. Thirdly, an optimal standalone system size is evaluated for each tier of electrification, taking into account the battery lifetime, temperature impact on SHS performance, power supply availability in terms of the loss of load probability (LLP), and excess PV energy. A genetic algorithm-based multi-objective optimization is performed, giving insights on the delicate interdependencies of the various system metrics like LLP, excess energy, and battery lifetime on the SHS sizing. This exercise concludes that meeting the electricity requirements of tiers 4 and 5 level of electrification is untenable through SHS alone. Consequently, a bottom-up DC microgrid born out of the interconnection of SHS is explored. A modular and scalable architecture for such a bottom-up, interconnected SHS-based architecture is introduced, and the benefits of the microgrid over standalone SHS are quantified in terms of lower battery sizes and the defined system metrics. On modeling the energy sharing between the SHS, it is shown that battery sizing gains of more than 40% could be achieved with inter-connectivity at tier 5 level as compared to standalone SHS to meet the same power availability threshold. Finally, a Geo-Information System (GIS)-based methodology is presented that takes into account the spatial spread of the households while utilizing graph theory-based approaches to arrive at the optimal microgrid topology in terms of network length. The research carried out in this dissertation underlines the technological limitations of SHS in aiming towards universal electrification, while highlighting the benefits of moving towards a bottom-up approach in building (rural) DC microgrids through SHS, which can enable the climb up the so-called electrification ladder. @en

The original publication’s Fig. 5 image lacks some of its labels. On the top right, the label for the solid black line is BCF^ while the label of the broken blue line is BCF^. The correct label for the solid black line should be BCfmin^ while the broken blue line should be BCF = 1^. Figure 5 image with the wrong label (top image) and with the correct label (bottom image) is shown here. The original publication was corrected.(Figure Presented.).@en

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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The use of batteries is indispensable in stand-alone photovoltaic (PV) systems, and the physical integration of a battery pack and a PV panel in one device enables this concept while easing the installation and system scaling. However, the influence of high temperatures is one of the main challenges of placing a solar panel close to a battery pack. Therefore, this paper aims to select a suitable battery technology considering the temperature of operation and the expected current profiles. The methodology for battery selection is composed of a literature review, an integrated model, the design of an application-based testing, and the execution of the aging test. The integrated model was employed to choose among the battery technologies, and to design a testing procedure that simulated the operational conditions of the PV-battery Integrated Module (PBIM). Two Li-ion pouch cells were tested at two representative temperatures while applying various charging/discharging profiles. After the testing, the LiFePO4 (LFP) cells showed better performance when compared to LiCoO2 batteries (LCO), where for instance, the LCO cells capacity tested at 45∘C, faded 2,45% more than the LFP cells at the same testing conditions. Therefore, LFP cells are selected as a suitable option to be part of the PBIM.@en

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. @en

The past few years have seen strong growth of solar-based off-grid energy solutions such as Solar Home Systems (SHS) as a means to ameliorate the grave problem of energy poverty. Battery storage is an essential component of SHS. An accurate battery model can play a vital role in SHS design. Knowing the dynamic behaviour of the battery is important for the battery sizing and estimating the battery behaviour for the chosen application at the system design stage. In this paper, an accurate cell level dynamic battery model based on the electrical equivalent circuit is constructed for two battery technologies: the valve regulated lead-acid (VRLA) battery and the LiFePO₄ (LFP) battery. Series of experiments were performed to obtain the relevant model parameters. This model is built for low C-rate applications (lower than 0.5 C-rate) as expected in SHS. The model considers the non-linear relation between the state of charge (SOC) and open circuit voltage (V_OC) for both technologies. Additionally, the equivalent electrical circuit model for the VRLA battery was improved by including a 2nd order RC pair. The simulated model differs from the experimentally obtained result by less than 2%. This cell level battery model can be potentially scaled to battery pack level with flexible capacity, making the dynamic battery model a useful tool in SHS design.

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Given the complementary nature of photovoltaic (PV) generation and energy storage, the combination of a solar panel and a battery pack in one single device is proposed. To realize this concept, the PV Battery-Integrated Module (PBIM), it is fundamental to analyze the system architecture and energy management. This paper focuses on selecting a suitable architecture among the different options, while also indicating the control strategy that the converters must follow to ensure appropriate performance. Also, several modes of operation for the complete system are introduced to implement energy management. For the selected DC architecture, two case studies, viz. off-grid and peak-shaving for a grid-tied system, were employed to characterize the response of the model demonstrating its utility to perform maximum power-point tracking, excess solar power curtailment, and battery charging and discharging. The proposed control and system architecture prove to be feasible for a PV battery-integrated device such as PBIM.@en

Solar road technology provides an opportunity to harvest the vast, albeit dispersed, photovoltaic (PV) energy, while maximizing the land utilization. Deriving experience from the pioneering 70-m solar bike path installed in the Netherlands, this paper highlights the operational challenges and performance parameters using the first-year measured data. The theoretically predicted energy yield is compared with the measured energy yield. Based on the best performing module, the benchmark annual energy yield is set to 85&#x2013;90&#x00A0;kWh/m<formula><tex>$^2$</tex></formula> specific to the installation site. It is shown that this value can be bettered by about 1.5 times if different cell technology such as monocrystalline is used. With different installation sites around the world, thermal behavior as well as annual energy yield changes. Theoretical proof is offered that it is not unreasonable to expect an annual energy yield in the upwards of 150&#x00A0;kWh/m<formula><tex>$^2$</tex></formula> with solar road energy harvesting technology. For example, the annual yield is found to be 213&#x00A0;kWh/m<formula><tex>$^2$</tex></formula> if the same model is simulated for a solar road PV installation in India, which increased further with the use of monocrystalline to almost 300&#x00A0;kWh/m<formula><tex>$^2$</tex></formula>.

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The rapid increase in the adoption of Solar Home Systems (SHS) in recent times hopes to ameliorate the global problem of energy poverty. The battery is a vital but usually the most expensive part of an SHS; owing to the least lifetime among other SHS components, it is also the first to fail. Estimating battery lifetime is a critical task for SHS design. However, it is also a complex task due to the reliance on experimental data or modelling cell level electrochemical phenomena for specific battery technologies and application use-case. Another challenge is that the existing electrochemical models are not application-specific to Solar Home Systems. This paper presents a practical, non-empirical battery lifetime estimation methodology specific to the application and the available candidate battery choices. An application-specific SHS simulation is carried out, and the battery activity is analyzed. A practical dynamic battery lifetime estimation method is introduced, which captures the fading capacity of the battery dynamically through every micro-cycle. This method was compared with an overall non-empirical battery lifetime estimation method, and the dynamic lifetime estimation method was found to be more conservative but practical. Cyclic ageing of the battery was thus quantified and the relative lifetimes of 4 battery technologies are compared, viz. Lead-acid gel, Flooded lead-acid, Nickel-Cadmium (NiCd), and Lithium Iron Phosphate (LiFePO₄) battery. For the same SHS use-case, State-of-Health (SOH) estimations from an empirical model for LiFePO₄ is compared with those obtained from the described methodology, and the results are found to be within 2.8%. The relevance of this work in an SHS application is demonstrated through a delicate balance between battery sizing and lifetime. Based on the intended application and battery manufacturer's data, the practical methodology described in this paper can potentially help SHS designers in estimating battery lifetimes and therefore making optimal SHS design choices.

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Solar Home Systems (SHSs) can fulfil the basic energy needs of the globally unelectrified population. With costs as one of the biggest barriers for SHS uptake, optimizing the system size with energy needs is crucial. Where most solutions focus only on the present needs, this work also addresses the future energy needs. The methodology includes extensive mapping of the current electricity needs in rural Cambodia through data analysis on existing SHSs in the field. Additionally, a 2-month field research was carried out in Cambodia to assess the qualitative state of electricity usage and investigate the future (2021) energy needs. A data analysis was performed on 111 SHSs (100 Wp, 1200 Wh).
SHS users were found to have a mean energy consumption of 310 Wh/day, with σ = 159 Wh. Most energy was consumed at night. The field research showed a clear demand for more energy and more appliances. The appliances attached to SHS in the future will be more diverse in power consumption and usage duration, resulting in a wide variety of energy consumption and high power peaks, causing fast and deep battery discharges. Three load profiles are presented. Solutions are discussed that can be applied to ensure the SHSs fit with future energy needs.@en

Developing technological solutions for the developing nations is more than simply re-sizing a prevalent solution from the developed world. Not only the environmental conditions, but also the technology usage varies greatly between the developing nations in sunnier latitudes around the world and the developed nations of north-western Europe. This paper sheds light on the various technical and non-technical factors to be considered for designing a standalone PV system for low-income households in emerging economies. The significance of the battery in a low-cost PV system is examined and the role of power electronics in such a system design is highlighted. Finally, the paper proposes a system design methodology. This paper presents an overview of the current initiatives in the Solar Home System landscape, and identifies gaps that can be potentially filled by technology.@en