System analysis of a multifunctional PV/T hybrid solar window

The work presented in this article aims to investigate a PV/T hybrid solar window on a system level. A PV/T hybrid is an absorber on which solar cells have been laminated. The solar window is a PV/T hybrid collector with tiltable insulated reflectors integrated into a window. It simultaneously replaces thermal collectors, PV-modules and sunshade. The building integration lowers the total price of the construction since the collector utilizes the frame and the glazing in the window. When it is placed in the window a complex interaction takes place. On the positive side is the reduction of the thermal losses due to the insulated reflectors. On the negative side is the blocking of solar radiation that would otherwise heat the building passively. This limits the performance of the solar window since a photon can only be used once. To investigate the sum of such complex interaction a system analysis has to be performed. In this paper results are presented from such a system analysis showing both benefits and problems with the product. The building system with individual solar energy components, i.e. solar collector and PV modules, of the same size as the solar window, uses 1100 kW h less auxiliary energy than the system with a solar window. However, the solar window system uses 600 kW h less auxiliary energy than a system with no solar collector.     

Correlations for natural convective heat exchange in CPC solar collector cavities determined from experimental measurements

A detailed experimental study was undertaken to analyse the natural convective heat transfer in CPC cavities, a complex function of collector orientation, geometrical aspect ratios and thermal boundary conditions at the enclosure walls. Results are reported for CPC solar collectors with full-, three quarter- and half-height reflectors, C_R = 2 and a 100 mm wide flat plate absorber. Experiments were conducted using a purpose built solar simulator under controlled lab environment employing realistic boundary and thermal conditions. The effects of simultaneous tilting of the solar collectors about both transverse and longitudinal axes, truncation of the reflector walls and inlet water (collector heat removal fluid) temperature on the natural convective heat flow characteristics inside the CPC cavity have been determined. It is concluded that the correlations developed for prediction of natural convection characteristics in rectangular, annuli and V-trough enclosures are not appropriate for application to CPC solar collectors with divergence ranging from 150% to 300%. Based on the experimental data a correlation is presented to predict the natural convection heat loss from the absorber plate of solar collectors for a range of water inlet temperatures.     

Thermal performance of a novel rooftop solar micro-concentrating collector

Concentrating solar thermal systems offer a promising method for large scale solar energy collection. Although concentrating collectors are generally thought of as large-scale stand-alone systems, there is a huge opportunity to use novel concentrating solar thermal systems for rooftop applications such as domestic hot water, industrial process heat and solar air conditioning for commercial, industrial and institutional buildings. This paper describes the thermal performance of a new low-cost solar thermal micro-concentrating collector (MCT), which uses linear Fresnel reflectors, and is designed to operate at temperatures up to 220 °C. The modules of this collector system are approximately 3 m long by 1 m wide and 0.3 m high. The objective of the study is to optimise the design to maximise the overall thermal efficiency. The absorber is contained in a sealed enclosure to minimise convective losses. The main heat losses are due to natural convection inside the enclosure and radiation heat transfer from the absorber tube. In this paper we present the results of a computational and experimental investigation of radiation and convection heat transfer in order to understand the heat loss mechanisms. A computational model for the prototype collector has been developed using ANSYS–CFX, a commercial computational fluid dynamics software package. The numerical results are compared to experimental measurements of the heat loss from the absorber, and flow visualisation within the cavity. This paper also presents new correlations for the Nusselt number as a function of Rayleigh number.     

Thermal and mechanical performance of sealed,gas-filled,flat plate solar collectors

The study includes calculations for both the thermal performance and the mechanical behaviour of a gas-filled, flat plate solar collector without external gas expansion, i.e., a collector with varying gas volume and gas pressure and movement in both cover glass and absorber. Classical theories for the thermal performance are combined with a finite-element method to investigate which factors have an impact from the mechanical stress point of view.This article describes major results for collectors with copper and aluminium absorbers combined with different inert gases. It is shown that a collector may be designed which uses less material than a standard collector but achieves at least the same thermal performance, by using a thinner collector and a thinner absorber and a suitable gas filling other than air. If copper is used in absorber and tubes, a 0.15 mm thick absorber together with a tube-to-tube distance of 103 mm results in the same performance as a 0.3 mm absorber with a 144 mm tube-to-tube distance, but the former will use 25% less material. The use of copper can be further reduced if the absorber is made of aluminium and the tubes are made of copper. The factor of safety for thick (>0.5 mm) aluminium absorbers is, however, not as large as it is for copper absorbers.     

Efficient single glazed flat plate photovoltaic-thermal hybrid collector for domestic hot water system

This paper deals with the design of a single glazed flat plate Photovoltaic-Thermal (PV-T) solar collector. First, the thermal and electrical performances of several single glazed flat plate PV-T concepts based on water circulation are investigated, using a simple 2D thermal model, then different ways of improvement are presented. It mainly consists in focusing on the heat transfer between PV cells and fluid, and also on the optical properties of materials. Thus, the most appropriate concept configuration has been identified and suitable material properties have been selected. A prototype collector has been designed, built and tested. A high thermal efficiency was reached at zero reduced temperature. For this level of thermal efficiency, the corresponding electrical efficiency has is lower than efficiency of a standard PV panel using the same technology. However, this solar PV-T collector is reaching, in these standard conditions, the highest efficiency level reported in the literature.     

Hybrid photovoltaic and thermal solar-collector designed for natural circulation of water

The electricity conversion-efficiency of a solar cell for commercial application is about 6–15%. More than 85% of the incoming solar energy is either reflected or absorbed as heat energy. Consequently, the working temperature of the solar cells increases considerably after prolonged operations and the cell’s efficiency drops significantly. The hybrid photovoltaic and thermal (PVT) collector technology using water as the coolant has been seen as a solution for improving the energy performance. Through good thermal-contact between the thermal absorber and the PV module, both the electrical efficiency and the thermal efficiency can be raised. Fin performance of the heat exchanger is one crucial factor in achieving a high overall energy yield. In this paper, the design developments of the PVT collectors are briefly reviewed. Our observation is that very few studies have been done on the PVT system adopting a flat-box absorber design. Accordingly, an aluminum-alloy flat-box type hybrid solar collector functioned as a thermosyphon system was constructed. While the system efficiencies did vary with the operating conditions, the test results indicated that the daily thermal efficiency could reach around 40% when the initial water-temperature in the system is the same as the daily mean ambient temperature.     

Energy performance of water hybrid PV/T collectors applied to combisystems of Direct Solar Floor type

The integration of photovoltaic (PV) modules in buildings allows one to consider a multifunctional frame and then to reduce the cost by substitution of components. In order to limit the rise of the cell operating temperature, a photovoltaics/thermal (PV/T) collector combines a solar water heating collector and PV cells. The recovered heat energy can be used for heating systems and domestic hot water. A combination with a Direct Solar Floor is studied. Its low operating temperature level is appropriate for the operating conditions of the mono- or poly-crystalline photovoltaic modules which are selected in that study. However, for a system including a glass covered collector and localised in Mâcon area in France, we show that the annual photovoltaic cell efficiency is 6.8% which represents a decrease of 28% in comparison with a conventional non-integrated PV module of 9.4% annual efficiency. This is obviously due to a temperature increase related to the cover. On the other hand, we show that without a glass cover, the efficiency is 10% which is 6% better than a standard module due to the cooling effect.Moreover, in the case of a glazed PV/T collector with a conventional control system for Direct Solar Floor, the maximum temperature reached at the level of the PV modules is higher than 100 °C. This is due to the oversize of the collectors during the summer when the heating needs are null, i.e. without a heated swimming pool for example. This temperature level does not allow the use of EVA resin (ethylene vinyl acetate) in PV modules due to strong risks of degradation. The current solution consists of using amorphous cells or, if we do not enhance the thermal production, uncovered PV/T collector. Further research led to water hybrid PV/T solar collectors as a one-piece component, both reliable and efficient, and including the thermal absorber, the heat exchanger and the photovoltaic functions.     

Impact of different internal convection control strategies in a non-evacuated CPC collector performance

Over the last decade the technological advances observed in solar collector materials, namely better spectrally selective absorber coatings and ultra clear glass covers, contribute to performance improvements and translate into higher operational temperature ranges with higher efficiency values.While the use of Evacuated Tube Collectors (ETCs) is becoming widespread in the thermal conversion of solar energy, non-evacuated solar collectors still hold advantages at manufacturing, reliability and/or cost levels, making them interesting and competitive for a large range of applications, in particularly, in temperature ranges up to 80 °C. However, these advantages have not prevented the major drawback of these collectors when compared to ETCs: thermal losses due to internal convection which prevent their general use in the range of operating temperatures up to 150 °C.Insulation, double glazing or selective coatings can be used in non-evacuated collectors to reduce heat losses. To prevent internal convection losses in these solar collectors, different control strategies have been studied, such as the adoption of different inert gases within the collector cavity, physical barriers reducing air flow velocities over the absorber or cover surfaces or the use of concentration.In the present article, an assessment of adopting such internal convection control strategies in a CPC collector is presented. Each of the presented strategies is assessed in terms of the resulting collector optical and thermal characterization parameters and yearly collector yield. For this purpose, an integrated tool allowing the design, optical and thermal characterization of CPC collectors was developed. The results obtained provide valuable guidelines for anyone wishing to implement any of these strategies in a new collector design.     

Energy matrices analyses of hybrid photovoltaic thermal (HPVT) water collector with different PV technology

In this paper, an attempt has been made to evaluate and compare the energy matrices of a hybrid photovoltaic thermal (HPVT) water collector under constant collection temperature mode with five different types of PV modules namely c-Si, p-Si, a-Si (thin film), CdTe and CIGS. The analysis is based on overall thermal energy and exergy outputs from HPVT water collector. The temperature dependent electrical efficiency has also been calculated under composite climate of New Delhi, India.It is observed that c-Si PV module is best alternative for production of electrical power. Maximum annual overall thermal energy and exergy is obtained for c-Si PV module. The maximum and minimum EPBT of 1.01 and 0.66 years on energy basis is obtained for c-Si and CIGS respectively, whereas on exergy basis maximum EPBT of 5.72 years is obtained for a-Si and minimum of 3.44 in obtained for CIGS PV module. EPF and LCCE increase with increasing the life time of the system.     

Optical efficiency of a PV–thermal hybrid CPC module for high latitudes

The advantage of PV–thermal hybrid systems is their high total efficiency. By using concentrating hybrid systems, the cost per energy produced is reduced due to simultaneous heat and electricity production and a reduced PV cell area. In this article, the optical efficiency of a water-cooled PV–thermal hybrid system with low concentrating aluminium compound parabolic concentrators is discussed. The system was built in 1999 in Älvkarleby, Sweden (60.5° N, 17.4° E) with a geometric concentration ratio of C=4 and 0.5 kW_p electric power. The yearly output is 250 kWh of electricity per square metre solar cell area and 800 kWh of heat at low temperatures per square metre solar cell area. By using numerical data from optical measurements of the components (glazing, reflectors, and PV cells) the optical efficiency, η_opt, of the PV–CPC system has been determined to be 0.71, which is in agreement with the optical efficiency as determined from thermal and electrical measurements. Calculations show that optimised antireflection-treated glazing and reflectors could further increase the electric power yield.     

Experimental investigation of thermal performance of a double-flow solar air heater having aluminium cans

This study experimentally investigates a device for inserting an absorbing plate made of aluminium cans into the double-pass channel in a flat-plate solar air heater (SAH). This method substantially improves the collector efficiency by increasing the fluid velocity and enhancing the heat-transfer coefficient between the absorber plate and air. These types of collectors had been designed as a proposal to use aluminium materials to build absorber plates of SAHs at a suitable cost. The collector had been covered with a 4-mm single glass plate, in order to reduce convective loses to the atmosphere. Three different absorber plates had been designed and tested for experimental study. In the first type (Type I), cans had been staggered as zigzag on absorber plate, while in Type II they were arranged in order. Type III is a flat plate (without cans). Experiments had been performed for air mass flow rates of 0.03 kg/s and 0.05 kg/s. The highest efficiency had been obtained for Type I at 0.05 kg/s. Also, comparison between the thermal efficiency of the SAH tested in this study with the ones reported in the literature had been presented, and a good agreement had been found.     

Exergoeconomic analysis of glazed hybrid photovoltaic thermal module air collector

In this paper, an exergoeconomic analysis has been carried out and on the basis of this analysis it has been concluded that in terms of energy saving the glazed hybrid photovoltaic thermal (PVT) module air collector offers a greater potential compared to PV module. The experimental validation for glazed hybrid PVT module air collector has also been performed and it has been observed that there is a good agreement between the theoretical and experimental values with correlation coefficient in range of 0.96–0.99 and root mean square percentage deviation in range of 2.38–7.46. The experiments have been carried out on clear days during the month July 2010 to June 2011. For the validation of theoretical results with experimental results, a typical day of winter month (December 08, 2010) and summer month (April 11, 2011) has been considered. An experimental uncertainty for December and April month is 11.6% and 2.1% respectively. The annual overall thermal energy and exergy gain are 1252.0 kWh and 289.5 kW h respectively. The annual net electrical energy savings by glazed hybrid PVT module air collector is 234.7 kW h.     

Empirical study on thermal performance through separating impacts from a hybrid PV/TE system design integrating heat sink

A hybrid system design integrating a thermoelectric (TE) module has recently represented the advanced photovoltaic (PV) prototype with promoted efficiency for utilizing solar energy from the surroundings. Our present work during development of such a hybrid PV/TE system evaluates the thermal behaviors and the cooling performance associated with when integrating TE and heat sink modules. It has been noticed that a more effective structure through combining a heat sink with a TE module profits heat dissipation by cooling down the whole cell by ~ 8 °C, wherein the TE module itself demonstrates the cooling performance by ~ 27% enhancement in addition to its conventional role for electricity generation. Therefore, the PV/TE with a proper design can be used as a passive method for improving the cell efficiency as well as alleviating hot spot, which is typically occurring when the cell is unevenly heated during its operation. These results could be useful for further advancement on stability of power generation of a hybrid PV/TE system and may also be important for developing high-powered light emit diode.     

Experimental investigation on the thermal efficiency and performance characteristics of a flat plate solar collector using SiO2/EG–water nanofluids

Application of nanofluids in thermal energy devices such as solar collectors is developing day by day. This paper reports the results of experiments on a flat plate solar collector where the working fluid is SiO₂/ethylene glycol (EG)–water nanofluid with volume fractions up to 1%. The thermal efficiency and performance characteristics of solar collector are obtained for mass flow rates between 0.018 and 0.045 kg/s. The curve characteristics of solar collector indicate that the effects of particle loading on the thermal efficiency enhancement are more pronounced at higher values of heat loss parameter. The results of this work elucidate the potential of SiO₂ nanoparticles to improve the efficiency of solar collectors despite its low thermal conductivity compared to other usual nanoparticles.     

Hot air balloon engine

This paper describes a solar powered reciprocating engine based on the use of a tethered hot air balloon fuelled by hot air from a glazed collector. The basic theory of the balloon engine is derived and used to predict the performance of engines in the 10 kW to 1 MW range. The engine can operate over several thousand metres altitude with thermal efficiencies higher than 5%. The engine thermal efficiency compares favorably with the efficiency of other engines, such as solar updraft towers, that also utilize the atmospheric temperature gradient but are limited by technical constraints to operate over a much lower altitude range. The increased efficiency allows the use of smaller area glazed collectors. Preliminary cost estimates suggest a lower $/W installation cost than equivalent power output tower engines.     

Hybrid photovoltaic/thermal (PV/T) solar systems simulation with Simulink/Matlab

The purpose of this work consists in thermodynamic modeling of hybrid photovoltaic–thermal (PV/T) solar systems, pursuing a modular strategy approach provided by Simulink/Matlab.PV/T solar systems are a recently emerging solar technology that allows for the simultaneous conversion of solar energy into both electricity and heat. This type of technology present some interesting advantages over the conventional “side-by-side” thermal and PV solar systems, such as higher combined electrical/thermal energy outputs per unit area, and a more uniform and aesthetical pleasant roof area. Despite the fact that early research on PV/T systems can be traced back to the seventies, only recently it has gained a renewed impetus. In this work, parametric studies and annual transient simulations of PV/T systems are undertaken in Simulink/Matlab. The obtained results show an average annual solar fraction of 67%, and a global overall efficiency of 24% (i.e. 15% thermal and 9% electrical), for a typical four-person single-familiar residency in Lisbon, with p-Si cells, and a collector area of 6 m². A sensitivity analysis performed on the PV/T collector suggests that the most important variable that should be addressed to improve thermal performance is the photovoltaic (PV) module emittance. Based on those results, some additional improvements are proposed, such as the use of vacuum, or a noble gas at low-pressure, to allow for the removal of PV cells encapsulation without air oxidation and degradation, and thus reducing the PV module emittance. Preliminary results show that this option allows for an 8% increase on optical thermal efficiency, and a substantial reduction of thermal losses, suggesting the possibility of working at higher fluid temperatures. The higher working temperatures negative effect in electrical efficiency was negligible, due to compensation by improved optical properties. The simulation results are compared with experimental data obtained from other authors and perform reasonably well.The Simulink modeling platform has been mainly used worldwide on simulation of control systems, digital signal processing and electric circuits, but there are very few examples of application to solar energy systems modeling. This work uses the modular environment of Simulink/Matlab to model individual PV/T system components, and to assemble the entire installation layout. The results show that the modular approach strategy provided by Matlab/Simulink environment is applicable to solar systems modeling, providing good code scalability, faster developing time, and simpler integration with external computational tools, when compared with traditional imperative-oriented programming languages.     

Design considerations for flat-plate-photovoltaic/thermal collectors

Several potentially useful features in the design of photovoltaic/thermal (PV/T) collectors are explored in order to determine their effectiveness and interaction. Based on a computer simulation of flat-plate PV/T collectors that is applicable to a wide range of designs, the present work focuses on air-type collectors employing single crystal silicon PV cells. Features explored center on two main areas: increasing the solar absorptance and reducing the infrared emittance. The results of the simulations can be summarized as follows: for PV cells covering greater than approximately 65% of the total collector area, a selective absorber actually reduces the thermal efficiency when used with a gridded-back cell. The requirements for the low emissivity coating are an infrared emissivity of less than 0.25 and a solar transmissivity of greater than 0.85. The optimum combination for an air PV/T was found to consist of gridded-back PV cells, a nonselective secondary absorber, and a high-trans-missivity/low-emissivity cover above the PV cells.     

Theoretical and experimental analysis on efficiency factors and heat removal factors of Fresnel lens solar collector using different cavity receivers

The efficiency and heat removal factors are useful parameters for evaluating the thermal performance of concentrating solar collectors. In this work, the efficiency factors and heat removal factors of Fresnel lens solar collectors using different kinds of point-focus cavity receivers were obtained both theoretically and experimentally. An experimental unit was built, whereby eight kinds of cavity receivers, namely: conical, spherical, cylindrical, hemispherical, positive cone frustum, reverse cone frustum, heteroconical and domical, were tested and analyzed. It is found that the theoretical results agree well with the test results. For the point-focus Fresnel lens solar collector, the conical cavity receiver showed the best thermal performance, with a highest theoretical heat removal factor of 0.868. Effect of conical cavity parameters on the efficiency and heat removal factors were studied. Results showed that under given operation conditions, the optimum aperture diameter of the cavity, the optimum inside diameter of the receiver tube and the optimum vertex angle of the cross section through the symmetric axis of the receiver are 80 mm, 15 mm and 60°, respectively. For better thermal performance, the geometrical concentration ratio of the studied Fresnel lens solar collector should be more than 500.     

The daily performance of a solar pond integrated with solar collectors

The present study deals with heat storage performance investigation of integrated solar pond and collector system. In the experimental work, a cylindrical solar pond system (CSPS) with a radius of 0.80 m and a depth of 2.0 m and four flat plate collectors dimensions of 1.90 m × 0.90 m was built in Cukurova University in Adana, Turkey. The CSPS was filled with salty water of various densities to form three salty water zones (Upper Convective Zone, Non-Convective Zone and Heat Storage Zone). Heat energy collected by collectors was transferred to the solar pond storage zone by using a heat exchanger system which is connected to the solar collectors. Several temperature sensors connected to a data acquisition system were placed vertically inside the CSPS and at the inlet and outlet of the heat exchanger. Experimental studies were performed using 1, 2, 3 and 4 collectors integrated with the CSPS under approximately the same condition. The integrated solar pond efficiencies were calculated experimentally and theoretically according to the number of collectors. As a result, the experimental efficiencies are found to be 21.30%, 23.60%, 24.28% and 26.52%; the theoretical efficiencies to be 23.42%, 25.48%, 26.55% and 27.70% for 1, 2, 3 and 4 collectors, respectively. Theoretical efficiencies were compared with the experimental results and hence a good agreement is found between experimental and theoretical efficiency profiles.