A comparison of GaAs and Si hybrid solar power systems

Various silicon hybrid systems are modeled and compared with a Gallium Arsenide hybrid system. The hybrid systems modeled produce electric power and also thermal power which can be used for heating or air conditioning. Various performance indices are defined and are used to compare the system performance. The performance indices are: capital cost per unit electric power out; capital cost per total power out; capital cost per unit electric power plus mechanical power; annual cost per unit electric energy; and annual cost per unit electric plus mechanical work. These performance indices indicate that concentrator hybrid systems can be cost effective when compared with present day energy costs. Realistic costs and efficiencies of GaAs and Si are respectively $35,000/m² for 15 per cent efficient solar cells and $1000/m² for 10 per cent efficient solar cells based on information available at the time of this study in late 1975. The performance indices show that the limiting values for annual costs are 10.3¢/kWh and 6.8¢/kWh for Si and GaAs respectively. Results demonstrate that for a given flow rate there is an optimal operating condition for maximum photovoltaic output associated with concentrator systems. Also concentrator hybrid systems produce a distinct cost advantage over flat hybrid systems.     

A hybrid amorphous silicon photovoltaic and thermal solar collector

A hybrid amorphous silicon (a-Si) photovoltaic and thermal solar collector was developed and its performance tested. The solar cells, deposited on glass panels and having an average efficiency of 4% and a total area of 0.9 m², were bonded to the fin and tube aluminum heat-exchange plate using simple technology. This hybrid unit performed well as a thermal solar collector, heating water up to 65°C, while the electric characteristics of the photovoltaic modules showed little change. In addition to saving space this integral unit substantially reduces the balance-of-system cost of the photovoltaic generator. The transmission of light through various layers of an a-Si cell was measured and, in order to improve the thermal efficiency, a novel transparent type of a-Si cell was developed and tested in the hybrid unit. The results obtained show that it is possible to construct simple and cheap hybrid systems having good photovoltaic as well as thermal efficiencies.     

Modeling of a thermal energy storage system based on coupled metal hydrides (magnesium iron – sodium alanate) for concentrating solar power plants

Concentrating solar power plants represent low cost and efficient solutions for renewable electricity production only if adequate thermal energy storage systems are included. Metal hydride thermal energy storage systems have demonstrated the potential to achieve very high volumetric energy densities, high exergetic efficiencies, and low costs. The current work analyzes the technical feasibility and the performance of a storage system based on the high temperature Mg₂FeH₆ hydride coupled with the low temperature Na₃AlH₆ hydride. To accomplish this, a detailed transport model has been set up and the coupled metal hydride system has been simulated based on a laboratory scale experimental configuration. Proper kinetics expressions have been developed and included in the model to replicate the absorption and desorption process in the high temperature and low temperature hydride materials. The system showed adequate hydrogen transfer between the two metal hydrides, with almost complete charging and discharging, during both thermal energy storage and thermal energy release. The system operating temperatures varied from 450 °C to 500 °C, with hydrogen pressures between 30 bar and 70 bar. This makes the thermal energy storage system a suitable candidate for pairing with a solar driven steam power plant. The model results, obtained for the selected experimental configuration, showed an actual thermal energy storage system volumetric energy density of about 132 kWh/m³, which is more than 5 times the U.S. Department of Energy SunShot target (25 kWh/m³).     

Phonon phase stability,structural, mechanical,electronic, and thermoelectric properties of two new semiconducting quaternary Heusler alloys CoCuZrZ (Z = Ge and Sn)

For meeting the energy demand, the development of new and novel thermoelectric (TE) materials for power generation is very vital. In this draft, we have theoretically investigated two new quaternary CoCuZrZ (Z = Ge and Sn) Heusler alloys for their structural, mechanical, electronic, and TE properties. In the energy minimization process, the alloys are found to be non-magnetic in the ground state. Based on calculated phonon dispersion curves, formation energy, and elastic constants, we propose that both CoCuZrGe and CoCuZrSn are stable. Furthermore, the mechanical properties indicate that CoCuZrGe (CoCuZrSn) has a brittle (ductile) nature. The electronic properties examined in Perdew-Burke-Ernzerhof (PBE), PBEsol, and modified Becke-Johnson (mBJ) potential, all predict that reported systems are narrow-gap semiconductors (SCs). In addition, the temperature dependent TE properties have been studied by calculating the electronic thermal conductivity (κ), Seebeck coefficient (S), power factor (PF) and electrical conductivity (σ/τ). The obtained positive value of S conveys the materials as p-type SCs, with a maximum value of 26.2 μV/K for CoCuZrGe and 28 μV/K for CoCuZrSn. The σ/τ, κ, and PF show increasing trends with rising temperature. The PF is found to be 1.55 × 10¹² WK⁻²m⁻¹s⁻¹ for CoCuZrGe and 1.38 × 10¹² WK⁻²m⁻¹s⁻¹ for CoCuZrSn. The proposed semiconducting Heusler alloys may receive attention for a range of TE and spintronic applications.     

A solid state thermogalvanic cell harvesting low-grade thermal energy

A high performance polymer electrolyte thermogalvanic cell, which converts thermal energy to electrical energy directly, is transformed from a proton exchange membrane fuel cell. The transform is realized by connecting the anode and cathode chamber with a gas tube and filling hydrogen to both chambers. Provided a heat flux through the cell, hydrogen is consumed in the cold side and regenerated in the hot side while circulating in two chambers during operation. The Seebeck coefficient is 0.531 mV K^?1 at a cold side temperature of 60.0 °C and the maximum power density could reach up to 20 μW cm^?2 with a temperature difference of 15.3 °C between two electrodes.     

Momentum and Heat Transfer Characteristics for the Flow of Power-Law Fluids over a Semicircular Cylinder

In this study, the two-dimensional steady flow of power-law fluids past a semicircular cylinder (flat face oriented upstream) has been investigated numerically. The governing equations (continuity, momentum, and energy) have been solved in the steady symmetric flow regime over the range of the Reynolds number (0.01 ≤ Re ≤ 25), power-law index (0.2 ≤ n ≤ 1.8), and Prandtl number (0.72 ≤ Pr ≤ 100). Extensive new results reported here endeavor to elucidate the role of power-law index (0.2 ≤ n ≤ 1.8) on the critical Reynolds number denoting the onset of flow separation (Re ^c ) and of vortex shedding (Re _c ). In shear-thinning fluids, both of these transitions are seen to be delayed than that in Newtonian and shear-thickening fluids. Furthermore, the influence of the Reynolds and Prandtl numbers, power-law index on drag phenomenon, and heat characteristics of semicircular cylinder have been studied in the steady flow regime. Finally, the present numerical values of the critical Reynolds numbers and the average Nusselt number have been correlated by simple forms which are convenient for interpolating these results for the intermediate values of the governing parameters in a new application.     

Maximising the energy output of a PVT air system

Simultaneously generating both electricity and low grade heat, photovoltaic thermal (PVT) systems maximise the solar energy extracted per unit of collector area and have the added benefit of increasing the photovoltaic (PV) electrical output by reducing the PV operating temperature. A graphical representation of the temperature rise and rate of heat output as a function of the number of transfer units NTUs illustrates the influence of fundamental parameter values on the thermal performance of the PVT collector. With the aim of maximising the electrical and thermal energy outputs, a whole of system approach was used to design an experimental, unglazed, single pass, open loop PVT air system in Sydney. The PVT collector is oriented towards the north with a tilt angle of 34°, and used six 110 Wp frameless PV modules. A unique result was achieved whereby the additional electrical PV output was in excess of the fan energy requirement for air mass flow rates in the range of 0.03–0.05 kg/s m². This was made possible through energy efficient hydraulic design using large ducts to minimise the pressure loss and selection of a fan that produces high air mass flow rates (0.02–0.1 kg/s m²) at a low input power (4–85 W). The experimental PVT air system demonstrated increasing thermal and electrical PV efficiencies with increasing air mass flow rate, with thermal efficiencies in the range of 28–55% and electrical PV efficiencies between 10.6% and 12.2% at midday.     

Modeling a thermocell with proton exchange membrane and hydrogen electrodes

Direct conversion of thermal energy to electric energy with thermoelectric generators is an attractive technique to recover low-temperature heat. Thermoelectric generators based on galvanic cells (thermocells) provide promising results with respect to the Seebeck coefficient. In this study, based on the theory of non-equilibrium thermodynamics, we simulated a thermocell with hydrogen gas electrodes and a proton exchange membrane. We calculated a maximum power density of 1461 $\text{mW}/{\text{m}}^2$ and a thermal efficiency of 2% relative to the Carnot efficiency for a cell operating with the same gas composition at both the anode and the cathode, but fully saturated at the anode. We predict a Seebeck coefficient in the range of 0.7–1.8 mV/K, higher than those of classical thermoelectric generators. The thermocell presented here provides promising values regarding the Seebeck coefficient.     

On the Validity of the Isothermal Model for Natural Convection Flow around Blocks Submitted to Volumetric Heat Generation in a Horizontal Channel

This work presents numerical results of natural convection in a horizontal channel provided with heating blocks periodically mounted on its lower adiabatic surface. The upper surface of the channel is maintained cold at a constant temperature. The parameters of the study are the ratio of solid blocks to fluid thermal conductivities (0.1 ≤ k* = k _s /k _a ≤ 200), the Rayleigh number (10⁴ ≤ Ra ≤ 10⁷), and the relative blocks height (1/8 ≤ B ≤ 1/2). Two models are considered in this study depending on whether the blocks are submitted to uniform heat generation (model 1), or maintained isothermal (model 2) at the average temperature calculated using model 1. The effect of the thermal conductivities ratio and the other controlling parameters on the validity of the isothermal model is examined. It is found that when multiple steady solutions are possible, some of the solutions obtained with the isothermal model may not reproduce the results of the model with blocks submitted to volumetric heat generation, even at very large conductivities ratio.     

Critical factors affecting the photovoltaic characteristic and comparative study between two maximum power point tracking algorithms

This paper presents the characterization and the modeling of the electric characteristics of currentvoltage and power–voltage of the photovoltaic (PV) panels. The philosophy behind digital simulation of solar energy systems is that experiments which normally should be done on real systems under high assembling costs can be done numerically in a short time on a computer, thus saving time and investments. The electric parameters of PV cells and the optimal electric quantities of PV panels have been analyzed (voltage and power) according to the meteorological variations (Temperature, solar irradiation …). The obtained results show that the diode parameters of the PV cells depend on solar irradiation: the current saturation increases with solar irradiation. This induces a decrease of the optimal voltage with solar irradiation; when the solar irradiation varies from 600 W/m² to 1000 W/m². By taking into consideration all the modeling results, the electric behavior of the cells association in parallels or in series, as well as the aging of a PV panel have been analyzed. Moreover, a comparative study between two types of MPPT techniques that are used in photovoltaic systems to extract the maximum power have been introduced which are Perturb and Observe (P &O) and Incremental Conductance (INC).     

First‐principles spectroscopic screening of hybrid perovskite (CH3CH2NH3PbI3) with fundamental physical properties: A potential photovoltaic absorber

Herein, we have discussed the ethyl‐ammonium based hybrid perovskite (viz. CH₃CH₂NH₃PbI₃ or EAPbI₃) as the potential candidate material for the development of photovoltaic devices having low processing cost and high power conversion efficiency (PCE). To address the stability and environmental issues due to leaching of lead from MAPbI₃, we urge to replace cation CH₃NH₃⁺ (MA⁺) with an appropriate cation CH₃CH₂NH₃⁺ (EA⁺) and hope that the EAPbI₃ perovskite would prove to be a stable and eco‐friendly photovoltaic absorber (PVA) material yielding high PCE. We have investigated physical properties like energy bandgap, electron density distribution and optical coefficients by FP‐LAPW+lo and density functional theory (DFT). The present study reveals that EAPbI₃ has a direct energy bandgap of 1.55 eV with absorption coefficient exceeding 2 × 10⁴ per cm, which confirms its suitability as PVA material. The dependence of thermoelectric (TE) coefficients on chemical potential and carrier concentration at various temperatures has also been discussed. We have also carried out the calculations of spectroscopic limited maximum efficiency (SLME) parameter (30.5%), and the thermodynamic (TD) properties in the realm of quasi‐harmonic approximation. A detailed investigation on some of the properties of EAPbI₃ perovskite relevant to PVA material is being done for the first time, the present study may motivate researchers for more comprehensive theoretical and experimental investigations in search of stable and economically and environmentally viable PVA materials.     

Transient helium heat transfer phase I—Static coolant

Transient heat-transfer data have been obtained for flat heating surfaces in static liquid and supercritical helium. Measurements start 2(10)^?5 s after step power inputs, and cover a heat flux range of 0.05–20 W/cm², pressures from 0.09–0.3 MPa, and four different heater orientations. Initial heat-transfer coefficients, being limited primarily by the Kapitza resistance, are 10–100 times greater than steady state, and the time to reach steady state varies from 10^?5 to 1s. For heat flux below the steady state peak nucleate boiling limit the temperature follows calculations based on pure conduction to the steady state nucleate boiling level. Above that limit the transient conduction period leads to an apparent metastable nucleation period followed by a transition to film boiling.     

Performance evaluation of solar dryer system for optimal operation: mathematical modelling and experimental validation

An analytical moisture diffusion model which considers the influence of external resistance to mass transfer is developed. The methodology to determine constant and variable moisture diffusion coefficients, D_eff is proposed. A laboratory model of mixed-mode solar dryer is constructed to perform 16 experiments for different performance dependent variables under simulated indoor conditions. The potatoes (Solanum tuberosum) of Kufri Safed variety have been chosen as the test food product. The range of variables investigated is absorbed thermal energy (150–750 W/m²); air mass flow rate (0.009–0.022 kg/s); loading density (1.08–4.33 kg/m²) and sample thickness (5–18 mm). The efficiency results have been analysed to identify the value of each process variable leading to optimal operation of dryer. The study reveals that dryer with sample thickness of 8 mm and loading density of 4.33 kg/m² can operate optimally for absorbed energy of 450 W/m² and air mass flow rate of 0.017 kg/s.     

Industrial application of PV/T solar energy systems

Hybrid photovoltaic/thermal (PV/T) systems consist of PV modules and heat extraction units mounted together. These systems can simultaneously provide electrical and thermal energy, thus achieving a higher energy conversion rate of the absorbed solar radiation than plain photovoltaics. Industries show high demand of energy for both heat and electricity and the hybrid PV/T systems could be used in order to meet this requirement. In this paper the application aspects in the industry of PV/T systems with water heat extraction is presented. The systems are analyzed with TRNSYS program for three locations Nicosia, Athens and Madison that are located at different latitudes. The system comprises 300 m² of hybrid PV/T collectors producing both electricity and thermal energy and a 10 m³ water storage tank. The work includes the study of an industrial process heat system operated at two load supply temperatures of 60 °C and 80 °C. The results show that the electrical production of the system, employing polycrystalline solar cells, is more than the amorphous ones but the solar thermal contribution is slightly lower. A non-hybrid PV system produces about 25% more electrical energy but the present system covers also, depending on the location, a large percentage of the thermal energy requirement of the industry considered. The economic viability of the systems is proven, as positive life cycle savings are obtained in the case of hybrid systems and the savings are increased for higher load temperature applications. Additionally, although amorphous silicon panels are much less efficient than the polycrystalline ones, better economic figures are obtained due to their lower initial cost, i.e., they have better cost/benefit ratio.     

Optical properties and stability of water-based nanofluids mixed with reduced graphene oxide decorated with silver and energy performance investigation in hybrid photovoltaic/thermal solar systems

The present work investigates the effect of the nanoparticles concentration on the optical and stability performance of a water-based nanofluid in solar photovoltaic/thermal (PV/T) systems experimentally and numerically. A novel nanofluid is formulated with the inclusion of the reduced graphene oxide decorated with silver (rGO-Ag) nanoparticles in water. Five different concentrations of nanoparticles in the range from 0.0005 to 0.05 wt% is suspended in water to prepare the samples. Optical properties are measured using UV-Vis. The UV-Vis absorption analysis reveals that all samples show consistent optical absorption coefficient (α) at higher value (more than 3 cm⁻¹) in the range of 1.5 to 4 eV. The application of optical filtration (OF) using water/rGO-Ag nanofluid in hybrid PV/T system presented more solar energy absorption through the OF. The hybrid system shows better performance at concentrations less than 0.0235 wt% compared to the PV system without integration with optical filtration. The hybrid solar PV/T system with OF using water/rGO-Ag nanofluid is able to produce thermal energy with efficiencies between 24% and 30%.     

Simulation of high temperature thermal energy storage system based on coupled metal hydrides for solar driven steam power plants

Concentrating solar power plants can achieve low cost and efficient renewable electricity production if equipped with adequate thermal energy storage systems. Metal hydride based thermal energy storage systems are appealing candidates due to their demonstrated potential for very high volumetric energy densities, high exergetic efficiencies, and low costs. The feasibility and performance of a thermal energy storage system based on NaMgH₂F hydride paired with TiCr_1.6Mn_0.2 is examined, discussing its integration with a solar-driven ultra-supercritical steam power plant. The simulated storage system is based on a laboratory-scale experimental apparatus. It is analyzed using a detailed transport model accounting for the thermochemical hydrogen absorption and desorption reactions, including kinetics expressions adequate for the current metal hydride system. The results show that the proposed metal hydride pair can suitably be integrated with a high temperature steam power plant. The thermal energy storage system achieves output energy densities of 226 kWh/m³, 9 times the DOE SunShot target, with moderate temperature and pressure swings. In addition, simulations indicate that there is significant scope for performance improvement via heat-transfer enhancement strategies.     

Thermal modelling of hybrid photovoltaic thermal (HPVT) integrated-biogas plants

In this paper, the energy balance equations for the different components of hybrid photovoltaic thermal integrated-biogas plant have been written for quasi-steady state conditions to develop a thermal model. An analytical expression for slurry temperature has been obtained as a function of design and climatic parameters namely mass of the slurry, mass flow rate of fluid in collector, number of collectors, solar intensity, ambient temperature etc. Numerical computations have been carried out for climatic conditions of Srinagar, India. Based on mathematical computations it has been observed that the optimum slurry temperature (∼37°C) is achieved for a given set of design parameters of biogas plant and hybrid collectors (M _S = 2000, [(m)\dot]_f = 0.05 kg/s\dot m_f = 0.05 kg/s, L = 25 m). It is also observed that the peak slurry temperature decreases with increase in mass of the slurry as expected. Equivalent CO₂ credits earned by hybrid biogas plant for optimised parameters have also been evaluated.     

Optimization of a Microchannel Heat Sink with Varying Channel Heights and Widths

The optimal tapered channel design of a microchannel heat sink is obtained by a combined optimization procedure that includes a model of a three-dimensional microchannel heat sink and a simplified conjugate-gradient method. The objective function to be minimized is the overall thermal resistance with the number of channels N, channel-width ratio β, height-tapered ratio Λ _y , and width-tapered ratio Λ _z as the design parameters. It is shown that the thermal resistance in all its relationships with the individual parameters exhibits a decrease followed by an increase. The thermal resistance is sensitive to the variations in the channel number, channel-width ratio, or width-tapered ratio while less sensitive to the height-tapered ratio. Optimization results show that for a given pumping power (0.5 W), the optimal design variables are N = 78, β = 0.78, Λ _z  = 0.59, and Λ _y  = 0.81 with a corresponding minimum overall thermal resistance of 0.087 K W^?1. These optimal design variables produce a 37.6% decrement in thermal resistance compared to the initial parallel-channel design estimate (N = 71, β = 0.85, Λ _z  = 0.99, and Λ _y  = 0.99). Additionally, as the pumping power increases, the optimal thermal resistance decreases and the corresponding optimal values of N increase; whereas, β, Λ _z , and Λ _y decrease.     

Comparative performance analysis of a grid connected PV system for hydrogen production using PEM water,methanol and hybrid sulfur electrolysis

This paper presents comparative performance analysis of photovoltaic (PV) hydrogen production using water, methanol and hybrid sulfur (SO₂) electrolysis processes. Proton exchange membrane (PEM) electrolysers are powered by grid connected PV system. In this system design, electrical grid is considered as a virtual energy storage system (VESS) where the surplus of PV production can be injected and subsequently taken to support the electrolyser. Methanol (ME) and hybrid sulfur (HSE) electrolysis are compared to the conventional water electrolysis (WE) in term of operating cell voltage. Based on the experimental results reported in the literature, semi-empirical models describing the relationship between the hydrogen production rate and the electrolyser cell power input are proposed. Furthermore, power and hydrogen management strategy (PHMS) is developed. Case study is carried out to show the impact of each type of electrolysis on the system component sizes and evaluate the hydrogen production potentialities. Results show that the use of ME allows to produce 65% more hydrogen than with using WE. Moreover, the amount of hydrogen produced is almost double in the case of HSE. At Algiers city, based on a grid connected PV/Electrolyser system, it is possible to produce about 25 g/m² d and 29 g/m² d of hydrogen, respectively, through ME and HSE compared to 15 g/m² d of hydrogen when using WE.     

Morphology-Driven Emissivity of Microscale Tree-like Structures for Radiative Thermal Management

Spectral emissivity of surface materials has a strong impact on thermal properties of systems that are exposed in the ambient environment. While the solar spectrum heating up the surface ranges from 200 to 2,500 nm, the atmospheric transmission spectrum allowed for infrared cooling ranges from 8 to 14 µm. However, conventional surface materials have emissivity values that are either high or low throughout the spectrum. For example, ceramic materials are typically emissive and metallic materials are typically reflective and not emissive. Here, we show that surface materials with artificial periodicities can have a selectively controlled emissivity and that the surface morphology can transform ceramic materials to be reflective or metallic materials to be emissive. As a model system, we use microscale tree-like structures, or briefly micro-trees, to demonstrate wide variations of morphology-driven emissivity spectra. Our computation based on the rigorous coupled-wave analysis shows that optimal designs of micro-trees can act as a nearly perfect reflector or a black body depending on the spectral range and offer radiative cooling or heating capabilities beyond the limits of conventional materials. For cooling, metallic micro-trees provide a surface temperature 10 K lower than that of bare metallic surfaces in a normal ambient condition, and for heating, ceramic micro-trees provide a surface temperature 8 K higher than that of bare ceramic materials. The morphology-driven emissivity of micro-trees can offer a net cooling power of 136 W/m² or a net heating power of 12 W/m² depending on the application without requiring any active devices, and these results guide optimal designs of artificial materials for thermal management.