Solar, Volume 3, Issue 3 (September 2023) – 10 articles

Despite abundant solar resources, Mali has remained one of the least electrified countries in the world. Besides daily life activities and the economy, the shortage of electricity has severely affected the quality of healthcare services in the country. In the absence of electrical grids, standalone photovoltaic (PV) systems could be an alternative option in Mali for the electrification of isolated community health centers. However, because standalone PV systems are highly weather-dependent, they must be properly sized according to the local weather conditions. This paper presents the optimal sizing of standalone PV systems for the electrification of community health centers in Mali. The optimization for PV systems was performed for five different locations through simulation and modeling using PVsyst, considering the autonomy of 1 to 3 days and the probability of loss of load for 1 to 5%. Furthermore, for the economic analysis, the levelized cost of electricity (LCOE), payback period and return on investment for the standalone PV systems were calculated. Through the optimization, it was found that the standalone PV systems with PV array sizes ranging from 1650 to 2400 watts, along with 606 Ah battery storage, would be suitable to supply the daily energy demand for community health centers anywhere in the country. Moreover, by only replacing the 606 Ah battery storage with 1212 Ah and 1818 Ah sizes, the PV systems would be able to help and keep the energy reserves for 2 and 3 autonomous days, respectively. Furthermore, the results show that in comparison to a LCOE of 0.94–0.98 USD/kWh for a diesel generator, the LCOE for the standalone PV system would range from 0.23 to 0.46 USD/kWh without discounted rates and from 0.33 to 0.60 USD/kWh if discounted at 6%. In addition to a lower LCOE, the saving of 46–76 tons of CO₂ during the project’s lifespan, the short payback periods and high return of investment (ROI) values make standalone PV systems a suitable electrification option for Mali. Considering the total expenses, LCOE, payback period, and ROI, standalone PV systems for community health centers were found to be economically viable in all cases for Mali. Full article

The decarbonization of the district heating (DH) sector is receiving attention worldwide. Solar energy and heat pump technologies are widely considered in existing and new DH networks. There is a need to understand the influence of solar energy on district heating experimentally. However, only a few university laboratories are focused on district heating aspects. Further, the concept of such laboratories is not adequately disseminated in the scientific literature. The main objective of this paper is to develop a conceptual design of a solar energy laboratory with a focus on district heating systems. The proposed concept forms part of the preliminary study carried out by a research group at the Tallinn University of Technology. First, a brief literature review on solar energy laboratory development is provided. Then, the conceptual design of such a laboratory is presented, along with a case study. Regardless of project size, the main components of a district heating-based solar energy laboratory are solar collectors, thermal energy storage (TES) tanks, and a control system. The proposed laboratory is expected to serve multiple roles, such as a practical laboratory to provide interdisciplinary courses for students, a research and experimental platform for researchers, and a cradle to achieve the campus green initiative. It is roughly estimated that the thermal energy output from the proposed laboratory would meet around 25% of the heat demand of the institutional building during the summer season (May, June, July, and August). It is expected that the present study will be a reference material for the development of innovative energy laboratories in educational institutions. Full article

The challenges associated with traditional cooking methods in African countries, particularly the use of firewood and charcoal, which have negative impacts on the environment, health and human and economic development and safety, are addressed in this work. Given the high annual solar irradiation on the African continent, photovoltaic-powered electric cooking alternatives, such as electric pressure cookers (EPCs), are identified as a potential efficient, clean and affordable cooking solution. This work focuses on the potential of standalone solar electric cookers for use in rural African locations, namely, if this type of solution can satisfy cooking demand. Surveys and experimental data from several households in two different countries (Rwanda and Kenya) were collected. Specifically, the researchers performed a survey regarding cooking habits and an experimental campaign to determine real energy consumption profiles of EPCs. The main results are analyzed and discussed in this work. An assessment of the solar power capability to directly supply the EPCs’ energy demand, as determined from the experimental data, is performed. The findings indicate that, for the most commonly prepared food types, using EPCs saves considerable time in comparison with traditional cooking methods. In Rwanda, time savings range from 55% to 84%, while in Kenya, the time saved varies from 9% to 64%. Results show that, even for scenarios with high installed solar capacity, storage solutions are required to enable the PV-powered EPC system to supply more than 50% of meal demand. Full article

The growth of renewables in the energy sector, e.g., in public low-voltage networks, leads to an increasing share of installed power electronic devices, e.g., inverters for photovoltaic applications. To rely on these devices, suitable analyses have to be performed. This includes studies of the device stability in the harmonic frequency range, i.e., above 50 Hz up to 2 kHz. State-of-the-art time-domain studies for harmonic stability analyses require detailed knowledge about the inverter design. Black-box studies must identify the inverter characteristics in the laboratory, which can differ depending on specific operating points, i.e., specific operating powers. This study analyzes the operating-point dependency of inverters on the critical inductance values of the network impedance, e.g., the inductances at which the inverter is expected to become unstable. Measurements are performed for three operating powers of an unknown, commercially available single-phase inverter to validate the critical inductances. Two further commercially available inverters and four simulative implementations are analyzed as well with regard to the critical inductance and the critical frequency. The results demonstrate the importance of considering a representative range of operating powers of the inverter for the small-signal stability analysis. Full article

This paper presents the simulation of the novel prototype of a heterojunction perovskite solar cell (PSC) based on C_SGeI₂Br/C_SGeI₃. The device consists of two absorber layers (C_SGeI₂Br, C_SGeI₃), an electron transport layer (ETL) chosen as TiO₂ and a hole transport layer (HTL) given as poly(3-hexylthiophene) (P3HT). Within the simulation, the effects of thickness, doping and defect density in each absorber layer and different back contact metal electrodes on electrical parameters (efficiency, short circuit current, open circuit voltage, and fill factor) are evaluated. In addition, the contribution of the HTL (doping density and thickness), temperature, shunt and series resistance were also checked on the same electrical parameters. The simulations are conducted in standard test conditions with the irradiation normalized as 0.1 W/cm² using the SCAPS-1D platform. The maximum efficiency obtained within the simulation of this device was about 31.86%. For this device, the thickness of the CSGeI₃ layer should be around $900 \mathrm{nm},$ while that of the CsGeI₂Br should be around $100 \mathrm{nm}$ to facilitate optimal absorption of the incident photons. The doping density in the absorber layer is such that in CsGeI₃ should be around ${10}^{18}{ \mathrm{cm}}^{-3}$ and around ${10}^{16}{ \mathrm{cm}}^{-3}$ in the CsGeI₂Brlayer. The defects densities in both layers of the perovskite materials should be around ${10}^{14}{ \mathrm{cm}}^{-3}$. Concerning the HTL, the thickness and the doping density of the P3HT should be around $50 \mathrm{nm}$ and ${10}^{18}{ \mathrm{cm}}^{-3}$, respectively. In terms of the back contact electrode, the work function of the metal should be at least equal to 5 eV, corresponding to gold (Au) metal. The series resistance due to the connection of the cell to the external load should be very small, while the shunt resistance due to the leakage current in the solar cell should be high. Furthermore, the operating temperature of the new PSC should be maintained at an ambient level of around 25 °C in order to deliver high efficiency. Full article

The necessity of affordable and durable building-integrated photovoltaics has gained widespread importance for the renewable energy transition involving electrification and decarbonization in climate-neutral cities that possess many public health co-benefits. Although the PV market is dominated by polycrystalline and monocrystalline silicon solar cells of the first generation, there is an impetus in the research lately for more sophisticated solar cell architectures with higher efficiency, longer lifetime, and less use of raw materials in an eco-design approach. To accelerate building integration of the next generation of photovoltaics and the associated climate change mitigation benefits, we propose in this work a holistic novel approach to the requirements and associated parameters for the emerging and innovative PV structures, spanning from intrinsic cell properties to panels effect in the urban environment. Within this framework, and supported by building simulation, the improvement of cells’ efficiency is revealed as an important parameter for their wider PV building and urban deployment as well as a major improvement in covering the building energy needs with minimized thermal impact in the urban environment. By analyzing the lab-reported values and the timeline of emerging and novel tandem solar cells, we propose the 30% BIPV efficiency of the eco-designed BIPV products as a central milestone to be attained before 2030 for a sustainable urban transformation. Full article

In this paper, we analyse 40 years of maximum wind speed and wave height data to identify potential sites for solar photovoltaic (PV) systems floating on seas and oceans. Maximum hourly wave height and wind speed data were segregated into 5 distinct categories. These categorisations were then combined at the nearest wind speed and wave height grid point for each sea location, generating a comprehensive wind–wave map via a geographic information system (GIS) visualisation. We find that regions around the equator are generally calm, i.e., free from strong winds and large waves. The most favourable locations are around the Indonesian archipelago, and the Gulf of Guinea on the west coast of tropical Africa. Our analysis indicates the huge potential of floating solar PV systems in calm tropical maritime regions, capable of generating about one million terawatt-hours per year in regions that rarely experience waves larger than 6 m or winds stronger than 15 m/s. This study furthers our understanding of alternative renewable energy options, emphasising the promising potential of offshore floating solar PV systems in the global energy transition. Full article

This study focuses on achieving climate neutrality in European cities by integrating solar energy technologies and nature-based solutions. Through an examination of current practices, emerging trends, and case examples, the study explores the benefits, challenges, and prospects associated with this integration in urban contexts. A pioneering approach is presented to assess the urban heat and climate change mitigation benefits of combining building-integrated photovoltaics and nature-based solutions within the European context. The results highlight the synergistic relationship between nature-based components and solar conversion technology, identifying effective combinations for different climatic zones. In Southern Europe, strategies such as rooftop photovoltaics on cool roofs, photovoltaic shadings, green walls, and urban trees have demonstrated effectiveness in warmer regions. Conversely, mid- and high-latitude European cities have seen positive impacts through the integration of rooftop photovoltaics and photovoltaic facades with green roofs and green spaces. As solar cell conversion efficiency improves, the environmental impact of photovoltaics is expected to decrease, facilitating their integration into urban environments. The study emphasizes the importance of incorporating water bodies, cool pavements, spaces with high sky-view factors, and effective planning in urban design to maximize resilience benefits. Additionally, the study highlights the significance of prioritizing mitigation actions in low-income regions and engaging citizens in the development of social photovoltaics-positive energy houses, resilient neighbourhoods, and green spaces. By adopting these recommendations, European cities can create climate-neutral urban environments that prioritize clean energy, nature-based solutions, and the overall wellbeing of residents. The findings underscore the need for a multidisciplinary approach combining technological innovation, urban planning strategies, and policy frameworks to effectively achieve climate neutrality. Full article

Passivated, selective contacts in silicon solar cells consist of a double layer of highly doped polycrystalline silicon (poly Si) and thin interfacial silicon dioxide (SiO₂). This design concept allows for the highest efficiencies. Here, we report on a selective laser activation process, resulting in highly doped p⁺⁺-type poly Si on top of the SiO₂. In this double-layer structure, the p⁺⁺-poly Si layer serves as a layer for transporting the generated holes from the bulk to a metal contact and, therefore, needs to be highly conductive for holes. High boron-doping of the poly Si layers is one approach to establish the desired high conductivity. In a laser activation step, a laser pulse melts the poly Si layer, and subsequent rapid cooling of the Si melt enables electrically active boron concentrations exceeding the solid solubility limit. In addition to the high conductivity, the high active boron concentration in the poly Si layer allows maskless patterning of p⁺⁺-poly Si/SiO₂ layers by providing an etch stop layer in the Si etchant solution, which results in a locally structured p⁺⁺-poly Si/SiO₂ after the etching process. The challenge in the laser activation technique is not to destroy the thin SiO₂, which necessitates fine tuning of the laser process. In order to find the optimal processing window, we test laser pulse energy densities (H_p) in a broad range of 0.7 J/cm² ≤ H_p ≤ 5 J/cm² on poly Si layers with two different thicknesses d_{poly Si,1} = 155 nm and d_{poly Si,2} = 264 nm. Finally, the processing window 2.8 J/cm²≤ H_p ≤ 4 J/cm² leads to the highest sheet conductance (G_sh) without destroying the SiO₂ for both poly Si layer thicknesses. For both tested poly Si layers, the majority of the symmetric lifetime samples processed using these H_p achieve a good passivation quality with a high implied open circuit voltage (iV_OC) and a low saturation current density (J₀). The best sample achieves iV_OC = 722 mV and J₀ = 6.7 fA/cm² per side. This low surface recombination current density, together with the accompanying measurements of the doping profiles, suggests that the SiO₂ is not damaged during the laser process. We also observe that the passivation quality is independent of the tested poly Si layer thicknesses. The findings of this study show that laser-activated p⁺⁺-poly Si/SiO₂ are not only suitable for integration into advanced passivated contact solar cells, but also offer the possibility of maskless patterning of these stacks, substantially simplifying such solar cell production. Full article

Perovskite solar cells have become a popular alternative to traditional silicon solar cells due to their potential to provide high-efficiency, low-cost, and lightweight solar energy harvesting solutions. However, the multilayer architecture of perovskite solar cells demands careful investigation of the interaction and interfacing between the various layers, as they play a crucial role in determining the overall performance of the cell. In this context, the present work aims at analyzing the coupling between a spongy transparent electron-transporting layer (ETL) and perovskite in a formulation CsPbI${}_3$:EuI${}_2$. The ETL used in this work is a transparent mesoporous TiO${}_2$ layer called “gig-lox” (grazing incidence angle geometry–local oxidation), which has been optimized to boost the interfacing with the perovskite for achieving a highly interconnected blend of materials. The gig-lox TiO${}_2$ ETL shows a high surface wettability with respect to the perovskite solution, especially after pre-annealing at 500 ${}^{°}$C, and this enables the perovskite material to deeply infiltrate throughout it. The surface wettability of the gig-lox TiO${}_2$ has been estimated by contact angle measurements, while the deep infiltration of the perovskite material has been demonstrated through X-ray diffraction and transmission electron microscopy analyses. Thanks to the achieved deep infiltration, the photo-generated charge injection from the perovskite into the mesoporous oxide is enhanced with respect to the use of a planar compact oxide, as shown by the photoluminescence measurements. The mainstay of the approach resides in the ETL that is deposited by a solvent-free sputtering method and is up-scalable for high industrial throughput. Full article