The recent pandemic due to SARS-CoV-2 has brought to light the need for strategies to mitigate contagion between human beings. Apart from hygiene measures and social distancing, air ventilation ...highly prevents airborne transmission within enclosed spaces. Among others, educational environments become critical in strategic planning to control the spread of pathogens and viruses amongst the population, mainly in cold conditions. In the event of a virus outbreak – such as COVID or influenza – many school classrooms still lack the means to guarantee secure and healthy environments.
The present review examines school contexts that implement air ventilation strategies to reduce the risk of contagion between students. The analysed articles present past experiences that use either natural or mechanical systems assessed through mathematical models, numerical models, or full-scale experiments. For naturally ventilated classrooms, the studies highlight the importance of the architectural design of educational spaces and propose strategies for aeration control such as CO2-based control and risk-infection control. When it comes to implementing mechanical ventilation in classrooms, different systems with different airflow patterns are assessed based on their ability to remove airborne pathogens considering parameters like the age of air and the generation of airflow streamlines. Moreover, studies report that programmed mechanical ventilation systems can reduce risk-infection during pandemic events.
In addition to providing a systematic picture of scientific studies in the field, the findings of this review can be a valuable reference for school administrators and policymakers to implement the best strategies in their classroom settings towards reducing infection risks.
Hydrogenated nanocrystalline silicon oxide (nc-SiOx:H) films have demonstrated a unique combination of low parasitic absorption and high conductivity. Here, we report on the use of n-type nc-SiOx:H ...as front surface field (FSF) in rear-emitter silicon heterojunction (SHJ) solar cells exhibiting excellent electrical cell parameters at a thickness down to only 5 nm. Using a seed layer, we are able to maintain excellent electrical performance (high fill factor (FF) and open circuit voltage (VOC)), while enhancing layer transparency for maximizing short circuit current (JSC). These results, together with the short deposition time (< 100 s), make the (n)nc-SiOx:H FSF attractive for reducing production costs in industrial applications. The best device, with the optimized (n)nc-SiOx:H FSF layer, shows VOC of 731 mV, FF of 80.6%, JSC of 38.3 mA/cm2 and a power conversion efficiency of 22.6%.
•Hydrogenated nanocrystalline silicon oxide is applied as front surface field in SHJ cells.•High transparency and low parasitic absorption to enhance photocurrent density.•The effect of layer thickness and seed/contact layer on cells parameters is investigated.•Excellent electrical performance for ultra-thin nanocrystalline silicon oxide layers (10–5 nm).•Short deposition time (<100 s) is attractive for reducing production costs in industrial applications.
The Breathing Wall behaviour under variable boundary conditions is described by an analytical model based on a one-dimensional porous domain crossed by air and subject to third type steady periodic ...boundary conditions. To the best of the authors’ knowledge, its experimental validation is not provided in literature.
In this work, a new model is derived considering Dirichlet steady periodic boundary conditions. The model is experimentally validated testing a 1 m2 no-fines concrete sample in the Dual Air Vented Thermal Box apparatus, specially improved to replicate dynamic thermal conditions. The experiments show that increasing the air flow velocity across the Breathing Wall from 0 to 12 mm/s enhances thermal coupling between the two environments, namely reduces the wall thermal capacity, with a decrease in the penetration time from 4.3 h to 3 h.
The model shows a very good agreement with experimental data when predicting temperature distribution across the domain, with error averages and standard deviations within the thermocouple accuracy after calibration, assumed to be 0.15 ∘C. The lesser yet good agreement concerning conduction heat flux density is explained in terms of accuracy in the measurement of the boundary conditions and critical issues in the heat flow measure itself (i.e. probe thermal resistance, thermal contact, emissivity mismatch).
•In Breathing Walls ventilation air moves across permeable layers.•Steady periodic model for heat and mass transfer under 1st type boundary conditions.•Laboratory apparatus for experimental validation of the model on a permeable concrete sample wall.•Good agrement between model and experiments in terms of temperature profile and flux.•At increasing air velocity the Breathing Wall thermal capacity decreases.
Breathing Walls are envelope components, based on porous materials, crossed by a natural or forced airflow. Since they behave both as recovery heat exchangers and active insulation, reducing the ...conductive heat flux, they represent a promising envelope technology, allowing to reduce energy consumption in buildings.
From the modeling point of view, an analytical model can be found in literature, describing heat and mass transfer across Breathing Walls in steady state conditions. However, to the best of the authors' knowledge, the model lacks an exhaustive experimental validation. Therefore, in this paper, the novel laboratory apparatus named Dual Air Vented Thermal Box developed at Politecnico of Milano is presented. The apparatus is used to experimentally investigate the steady state behaviour of a 1 m2 Air Permeable Concrete sample, crossed by an airflow at different velocities up to 12 mm/s.
The temperature profile inside the sample, measured in different positions, is compared with the model predictions. While in the central portion of the wall a very good agreement is found, the experimental results at the top and at the bottom of the wall suggest a non-uniform velocity field entering the sample. A qualitative confirmation of this hypothesis is provided by CFD simulations on the apparatus, clearly showing a mixed convection regime on both sides of the wall. The results lead to state the validity of the one-dimensional analytical model in literature, although a careful application should take into account adjusted boundary conditions, consisting in an airflow velocity possibly variable with height.
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•A novel laboratory setup to test Breathing and conventional walls is presented.•Steady state boundary conditions are achieved using a mixed control strategy.•A permeable no-fines concrete wall is tested at different air flow rates crossing.•Measured and analytically calculated temperature profiles show good agreement.•Adjusted boundary conditions regarding inlet velocity are proposed for the literature model.
The optical modelling for optimizing high-efficiency c-Si solar cells endowed with poly-SiOx or poly-SiCx carrier-selective passivating contacts (CSPCs) demands a thorough understanding of their ...optical properties, especially their absorption coefficient. Due to the mixed phase nature of these CSPCs, spectroscopic ellipsometry is unable to accurately detect the weak free carrier absorption (FCA) at long wavelengths. In this work, the absorption coefficient of doped poly-SiOx and poly-SiCx layers as function of oxygen and carbon content, respectively, was obtained for wavelengths (300–2000 nm) by means of two alternative techniques. The first approach, photothermal deflection spectroscopy (PDS), was used for layers grown on quartz substrates and is appealing from the point of view of sample fabrication. The second, a novel inverse modelling (IM) approach based on reflectance and transmittance measurements, was instead used for layers grown on textured c-Si wafer substrates to mimic symmetrical samples. Although the absorption coefficients obtained from these two techniques slightly differ due to the different used substrates, we could successfully measure weak FCA in our CSPCs layers. Using an in-house developed multi-optical regime simulator and comparing modelled reflectance and transmittance with measured counterparts from symmetrical samples, we confirmed that with increasing doping concentration FCA increases; and found that the absorption coefficients obtained from IM can now be used to perform optical simulations of these CSPCs in solar cells.
•We use two techniques, PDS and IM to extract absorption coefficients of poly-SiOx and poly-SiCxCSPCs.•Inverse modelling (IM) approach is a novel method based on reflectance and transmittance measurements.•Using GenPro4, the modelled reflectance and transmittance have been compared with measured counterparts.•We also study the effect of changing oxygen, carbon and doping concentrations in our CSPCs.
In this work we develop a rear emitter silicon solar cell integrating carrier-selective passivating contacts (CSPCs) with different thermal budget in the same device. The solar cell consists of a ...B-doped poly-Si/SiOx hole collector and an i/n hydrogenated amorphous silicon (a-Si:H) stack acting as electron collector placed on the planar rear and textured front side, respectively. We investigate the passivation properties of both CSPCs on symmetric structures by optimizing the interdependency among annealing temperature, time and environment. The optimized B-doped poly-Si/SiOx reaches a saturation current density of ~10 fA/cm2 on n-type wafers and an implied open circuit voltage (iVOC) of 716 mV. Furthermore, the i/n a-Si:H stack shows an effective carrier lifetime above 4 ms and iVOC of ~705 mV for cell-relevant layers thickness. After a post-deposition annealing in H2, lifetime is above 10 ms and iVOC = 708 mV. Finally, we optimize the optoelectronic properties of indium-based transparent conductive oxide (Indium Tin Oxide ITO and hydrogenated indium oxide IO:H) to reduce parasitic absorption with a gain in short circuit current density of 0.23 mA/cm2. In conclusion, the optimized layer stacks are implemented at device level obtaining a device with VOC = 704 mV, fill factor of 73.8%, a short circuit current of 39.7 mA/cm2 and 21.0% aperture-area conversion efficiency.
•High & low thermal budget passivating contacts are combined in hybrid Si solar cells.•Boron-doped poly-Si shows excellent J0 < 15 fA/cm2 and iVoc of 716 mV.•Study of the interdependency of deposition/annealing parameters on i/n a-Si:H stack.•21.0% efficiency achieved with rear B-doped poly-Si and front i/n a-Si: H.
•Definition of a numerical transient model of a historical building (case study).•Evaluation of the thermal energy needs and adaptive comfort analysis of the case study.•Overall retrofit of the ...historical building considered.•Energy, economic and environmental analysis of absorption and vapor compression heat pump systems, proposed for the retrofit of the case study.
In this work a numerical investigation of the energy needs of the Italian historical building Castle of Zena (XXIII century) and a feasibility study for the retrofit of its HVAC plant is presented.
About the building envelope behaviour two types of numerical analyses have been performed. First of all, free floating conditions (no temperature control) have been considered, in order to evaluate the building envelope performance through the adaptive comfort approach. During almost the 50% of the summer season the internal temperature is above the upper limit of the comfort range, therefore a cooling plant is needed. Secondly, an ideal temperature control has been considered, in order to calculate the annual energy needs for space heating (around 164kWh/m2/year) and cooling (around 5kWh/m2/year).
Regarding the HVAC retrofit, due to the historical constraints, it has been decided to use a fan coil emission system linked to a heat pump appliance. Four combination have been analysed, combining two type of heat pump (compression and absorption) with two type of heat source (air and water). Neglecting the control, emission, storage and distribution sub-system energy needs, there are two systems with the lowest primary energy consumption: the EHP-WS and the GAHP-WS (around 130kWh/m2/year), followed by the GAHP-AS (around 147kWh/m2/year) and the EHP-AS (around 180kWh/m2/year). From the economic point of view the EHP-WS has the shortest pay-back time, 7 years, also thanks to the use of the existing well. However, excluding the water source, only the GAHP is economically feasible. Lastly, a significant reduction of greenhouse gas emission (CO2) could be obtained replacing the EHP-AS with the GAHP-AS (−26%), the EHP-WS (−28%) or the GAHP-WS (−34%).
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