A combined optimisation concet for the design and operation strategy of hybrid-PV energy systems[edit | edit source]
Abstract: This paper presents a method to jointly determine the sizing and operation control of hybrid-PV systems. Hybrid energy systems use different energy sources such as solar and wind energy and diesel gensets. They are an economical option in areas remote from the grid. In this context the correct and cost-effective system sizing as well as efficient system operation are important. The problem becomes complicated through uncertain renewable energy supplies and load demand, non-linear characteristics of some components, and the fact that optimum operation strategies and optimum sizing of hybrid system components are interdependent. The outlined approach finds an optimum operation strategy for a hybrid system by carrying out a search through possible options for the system operation control. The search is conducted over some time period using estimated weather and demand data and long-term system component characteristics. The costing of the operating strategies is evaluated and component sizes are changed by the designed algorithm according to optimum search rules. As a result an optimum system configuration is chosen by the algorithm together with an optimum operation strategy for a given site and application requirement.
- With this method the interdependency of hybrid operation strategies and system sizing can be incorporated. Operation strategies are selected by searching through possible settings for the system operation control, considering the non-linear characteristics of some components. The operation control and sizing selection method is based on genetic optimization techniques.
Abstract: The overall electrical efficiency of the photovoltaic (PV) module can be increased by reducing the temperature of the PV module by withdrawing the thermal energy associated with the PV module. In this communication an attempt has been made to develop analytical expression for electrical efficiency of PV module with and without flow as a function of climatic and design parameters. The four different configurations of PV modules are considered for the present study which are defined as; case A (Glass to glass PV module with duct), case B (Glass to glass PV module without duct), case C (Glass to tedlar PV module with duct), case D (Glass to tedlar PV module without duct). Further, experiments were carried out for all configurations under composite climate of New Delhi.
- It is found that the glass to glass PV modules with duct gives higher electrical efficiency as well as the higher outlet air temperature amongst the all four cases. The annual effect on electrical efficiency of glass to glass type PV module with and without duct is also evaluated. The annual average efficiency of glass to glass type PV module with and without duct is 10.41% and 9.75%, respectively.
Expanding photovoltaic penetration with residential distributed generation from hybrid solar photovoltaic and combined heat and power systems[edit | edit source]
Abstract: The recent development of small scale combined heat and power (CHP) systems has provided the opportunity for in-house power backup of residential-scale photovoltaic (PV) arrays. This paper investigates the potential of deploying a distributed network of PV + CHP hybrid systems in order to increase the PV penetration level in the U.S. The temporal distribution of solar flux, electrical and heating requirements for representative U.S. single family residences were analyzed and the results clearly show that hybridizing CHP with PV can enable additional PV deployment above what is possible with a conventional centralized electric generation system. The technical evolution of such PV + CHP hybrid systems was developed from the present (near market) technology through four generations, which enable high utilization rates of both PV-generated electricity and CHP-generated heat. A method to determine the maximum percent of PV-generated electricity on the grid without energy storage was derived and applied to an example area. The results show that a PV + CHP hybrid system not only has the potential to radically reduce energy waste in the status quo electrical and heating systems, but it also enables the share of solar PV to be expanded by about a factor of five.
- Here it shows that a PV + CHP hybrid system not only has the potential to radically reduce energy waste to 16% from the status quo of 65% for thermal electrical generation, but it also enables the share of solar PV to be expanded without the use of large amounts of storage technology.
Design of Novel Compound Fresnel Lens for High-Performance Photovoltaic Concentrator[edit | edit source]
Abstract: Author(s) present a new design of compound Fresnel-R concentrator which is composed of two lenses: a primary lens (Fresnel lens) that works by total internal reflection at outer sawteeth but refraction at inner sawteeth, and a ringed secondary lens that works by refraction. In contrast to previous Fresnel lens concentrators, this design increases the acceptance angle, improves the irradiance uniformity on the solar cell, and reduces the aspect ratio significantly. Meanwhile several sawteeth of the primary Fresnel lens can correspond to a same ring of secondary lens, which will efficiently lower the complexity of designing and manufacturing. Moreover, in order to reduce the influence of manufacturing tolerances and to increase the optical efficiency further, the central part of the bottom of the secondary lens which directly adhered to the solar cell is designed as a cone-shaped prism to collect the sunlight that does not reach the solar cell. Finally, Author(s) provide simulations and analyses of the design method an optical efficiency more than 80% and an aspect ratio smaller than 0.5 can be achieved.
- An effective way to reduce the cost is to cut down the amount of the semiconductor material by means of combination with concentrating optics.The Fresnel lens has been used as a concentrator in photovoltaic field.
- One of the purposes of our work is to design nonimaging Fresnel lenses used in concentrating photovoltaic systems (CPVs) with a high concentration factor but its aspect ratio maintains a relatively small value.
- Design of primary Fresnel lens and design of secondary lens of CPV system. The solar cell is adhered at the bottom of the secondary lens directly, making it simple to seal against moisture and prevent misalignment.
- The primary Fresnel lens has six TIR sawteeth and two refractive sawteeth.The secondary lens has three aspherical rings. The first three TIR sawteeth of primary Fresnel lens correspond to the first ring of secondary lens and the secondary three TIR sawteeth correspond to the secondary ring.
- Full internal reflection is the working principal
- Fresnel concentrator,the aspect ratio can be less than 0.5 and the optical efficiency of the optical system can be obtained more than 80%
Parametric analysis of a coupled photovoltaic/thermal concentrating solar collector for electricity generation[edit | edit source]
Abstract: The analysis of the combined efficiencies in a coupled photovoltaic (PV)/thermal concentrating solar collector are presented based on a coupled electrical/thermal model. The calculations take into account the drop in efficiency that accompanies the operation of PV cells at elevated temperatures along with a detailed analysis of the thermal system including losses. An iterative numerical scheme is described that involves a coupled electrothermal simulation of the solar energy conversion process. In the proposed configuration losses in the PV cell due to reduced efficiencies at elevated temperatures and the incident solar energy below the PV bandgap are both harnessed as heat. This thermal energy is then used to drive a thermodynamic power cycle. The simulations show that it is possible to optimize the overall efficiency of the system by variation in key factors such as the solar concentration factor, the band gap of the PV material, and the system thermal design configuration, leading to a maximum combined efficiency of ∼ 32.3% for solar concentrations between 10–50 and a band-gap around 1.5–2.0 eV.
- This work extends the concept of a hybrid PV/thermal system at high concentration ratios and temperatures by creating a coupled electrothermal model of the entire system.
- Electrical model of a PV cell inclusion of concentrated solar irradiance and coupling to a detailed heat transfer model.
- The overall energy balance relies on the PV efficiency requiring a coupled iterative approach with the PV modeling equations of thermal & electrical parts.
- Performance analysis depending of band-gap, mass-flow rate, concentration ration.
- Following values for the empirical parameters in this calculation following the discussion in K=0.05, m=1.02, n=0.98, and A=1.
Analysis of Potential Conversion Efficiency of a Solar Hybrid System With High-Temperature Stage[edit | edit source]
Abstract: The analysis is given of hybrid system of solar energy conversion having a stage operating at high temperature. The system contains a radiation concentrator, a photovoltaic solar cell, and a thermal generator, which could be thermoelectric one or a heat engine. Two options are discussed, one (a) with concentration of the whole solar radiation on the PV cell working at high temperature and coupled to the high-temperature stage, and another (b) with a special PV cell construction, which allows the use of the part of solar spectrum not absorbed in the semiconductor material of the cell ("thermal energy") to drive the high-temperature stage while the cell is working at ambient temperature. The possibilities of using different semiconductor materials are analyzed. It is shown that the demands to the cell material are different in the two cases examined: in system (a) with high temperature of cell operation, the materials providing minimum temperature dependence of the conversion efficiency are necessary, for another system (b) the materials with the larger band gap are profitable. The efficiency of thermal generator is assumed to be proportional to that of the Carnot engine. The optical and thermal energy losses are taken into account, including the losses by convection and radiation in the high-temperature stage. The radiation losses impose restrictions upon the working temperature of the thermal generator in the system (b), thus defining the highest possible concentration ratio. The calculations made show that the hybrid system proposed could be both efficient and practical, promising the total conversion efficiency around 25–30 % for system (a), and 30–40 % for system (b).
- It is a two-stage hybrid converter including solar cell with optical or thermal concentrator combined with a heat-toelectric/ mechanic energy converter, which is denoted as thermalgenerator (TG).
- The general theoretical analysis of the system which is actually a coupled thermal and photovoltaic converter was performed where the thermal converter was assumed to be the Carnot engine, and the photovoltaic converter having the maximum ideal efficiency not depending on temperature which, of course, is not realistic.
- It can be said that around 80% of concentrated solar radiation energy will be transformed to heat within a cell, and may be used for a heat-to-electric energy conversion by the second stage of a hybrid system—a TG.
- The system is proportional to that of the Carnot engine, and the coefficient K<1 shows how close the TG efficiency to the ideal one is.
- Efficiency estimation the values of K=0.5 or 0.6 for the Carnot cycle can be used. On the other hand, the second stage can be a real heat engine which has the efficiency close to that of Carnot cycle, with parameter K of the same order. The mechanical energy produced by the heat engine could be converted into electricity with effectivity of 90–95%, so the overall efficiency can be characterized by that of the Carnot cycle.
Abstract: Many studies have found that the photovoltaic (PV) cell temperature plays an important impact on the solar-to-electricity conversion efficiency. Different cooling liquids like air and water have been introduced to pass across the PVs to reduce the cell temperature, and thus increase the electrical efficiency. In this paper, the refrigerant R134a is used as the cooling liquid and the PV/thermal (PV/T) panel is coupled with a heat pump system acting as the evaporator, which is expected to achieve a better cooling effect and energy performance due to its low boiling temperature. Two different kinds of PV/T panels, glass vacuum tube (GVT) type and flat plate (FP) type, are proposed for the study on the energy performance comparison. The results show that the GVT PV/T panel has an average thermal efficiency of 0.775 and an average electrical efficiency of 0.089 (based on the reference efficiency of 0.12), which is 73.4% and 1.1% higher than that of the FP PV/T panel respectively, with the solar radiation varying from 200 W/m2 to 1000 W/m2. The GVT PV/T heat pump system has an average COP of 5.6, 9.8% higher the FP PV/T heat pump system. The GVT PV/T heat pump system has a better energy performance than the FP PV/T heat pump system.
- The performance comparison of the GVT and FP PV/T heat pump systems is carried out in this study. Result follows.
- The thermal efficiency increases by 0.02 for the GVT PV/T panel and 0.03 for the FP PV/T panel with the increase in solar radiation by 100 W/m2. The GVT PV/T panel has an average thermal efficiency of 0.775, 73.4% higher than that of the FP PV/T panel of 0.447.
- The electrical efficiency decreases by 0.004 for the GVT PV/T panel and 0.005 for the FP PV/T panel with the increase in solar radiation by 100 W/m2. The GVT PV/T panel has an average electrical efficiency of 0.089, slightly higher than that of the FP PV/T panel of 0.088.
Abstract: Hybrid photovoltaic/thermal (PV/T) solar systems can simultaneously provide electricity and heat, achieving a higher conversion rate of the absorbed solar radiation than standard PV modules. When properly designed, PV/T systems can extract heat from PV modules, heating water or air to reduce the operating temperature of the PV modules and keep the electrical efficiency at a sufficient level. In this paper, Author(s) present TRNSYS simulation results for hybrid PV/T solar systems for domestic hot water applications both passive (thermosyphonic) and active. Prototype models made from polycrystalline silicon (pc-Si) and amorphous silicon (a-Si) PV module types combined with water heat extraction units were tested with respect to their electrical and thermal efficiencies, and their performance characteristics were evaluated. The TRNSYS simulation results are based on these PV/T systems and were performed for three locations at different latitudes, Nicosia (35°), Athens (38°) and Madison (43°). In this study, Author(s) considered a domestic thermosyphonic system and a larger active system suitable for a block of flats or for small office buildings. The results show that a considerable amount of thermal and electrical energy is produced by the PV/T systems, and the economic viability of the systems is improved. Thus, the PVs have better chances of success especially when both electricity and hot water is required as in domestic applications.
- It shows that the electrical production of the system employing polycrystalline solar cells is more than that employing the amorphous ones, but the solar thermal contribution is slightly lower. A non-hybrid PV system produces about 38% more electrical energy, but the present system covers also, depending on the location, a large percentage of the hot water needs of the buildings considered. The derived TRNSYS results give an account of the energy and cost benefits of the studied PV/T systems with thermosyphon and forced water flow.
Abstract: Approximately 90 per cent of the world's power is generated by heat engines that use fossil fuel combustion as a heat source and typically operate at 30–40 per cent efficiency, such that roughly 15 terawatts of heat is lost to the environment. Thermoelectric modules could potentially convert part of this low-grade waste heat to electricity. Their efficiency depends on the thermoelectric figure of merit ZT of their material components, which is a function of the Seebeck coefficient, electrical resistivity, thermal conductivity and absolute temperature. Over the past five decades it has been challenging to increase ZT > 1, since the parameters of ZT are generally interdependent. While nanostructured thermoelectric materials can increase ZT > 1, the materials (Bi, Te, Pb, Sb, and Ag) and processes used are not often easy to scale to practically useful dimensions. Here Author(s) report the electrochemical synthesis of large-area, wafer-scale arrays of rough Si nanowires that are 20–300 nm in diameter. These nanowires have Seebeck coefficient and electrical resistivity values that are the same as doped bulk Si, but those with diameters of about 50 nm exhibit 100-fold reduction in thermal conductivity, yielding ZT = 0.6 at room temperature. For such nanowires, the lattice contribution to thermal conductivity approaches the amorphous limit for Si, which cannot be explained by current theories. Although bulk Si is a poor thermoelectric material, by greatly reducing thermal conductivity without much affecting the Seebeck coefficient and electrical resistivity, Si nanowire arrays show promise as high-performance, scalable thermoelectric materials.
- Efficiency depends on the thermoelectric figure of merit ZT of their material components, which is a function of the Seebeck coefficient, electrical resistivity, thermal conductivity and absolute temperature
- Si nanowire arrays show promise as high-performance, scalable thermoelectric materials.
- The main advantage of using Si nanowires for thermoelectric applications lies in the large difference in mean free path lengths between electrons and phonons at room temperature
- It is possible to achieve ZT= 0.6 at room temperature in rough Si nanowires of 50nm diameter that were processed by a wafer-scale manufacturing technique
- Achieving broadband impedance of phonon transport, we have demonstrated that the EE Si nanowire system is capable of approaching the limits of minimum lattice thermal conductivity in Si.
Comparative Study on Exergy Efficiency of Solar Photovoltaic/Thermal (PV/T) System[edit | edit source]
Abstract: A brief comparative study is presented regarding different exergy efficiency correlations of solar photovoltaic/thermal (PV/T) system that have been proposed in the literature. Performance evaluation results of a certain PV/T hybrid system are found to be inconsistent with each other when using different correlations of exergy efficiency. It is mainly due to that no consensus has been reached in the calculation of thermal exergy, solar radiation exergy and other causal factors. Also, the issues that need to be further investigated are briefly discussed. This study will be beneficial to the design and performance assessment of PV/T hybrid system.
- Through the electricity and heat co-generation, the overall output is higher for a given PV/T collector than the outputs of two separated PV and solar thermal systems placed side-by-side.
- Unlike PV system, PV/T system uses the thermal energy available on the PV panel, the thermal energy gain can be utilized as useful energy and hence, the desirable exergy of PV/T system becomes the sum of the electrical exergy and thermal exergy
- Mathematical model of exergy analysis.
- Exergy has been analyzed for both natural & force cooling system
- At low solar radiation intensity, they show that the exergy efficiency of PV/T system equals the electrical efficiency of the reference conditions.
- Exergy annalysis by considering the temperature difference between the cell and ambient on the premise that there is no heat loss gives higher value
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