A comparison of compound parabolic and simple parabolic concentrating solar collectors

The compound parabolic and simple parabolic solar collectors are analyzed and compared for their ability to accept non-direct radiation and for their respective reflector arc-lengths. The simple parabolic concentrator (SPC) can make use of some non-direct solar radiation if the absorber tube is intentionally enlarged so as to intercept defocussed radiation. A principal advantage of collecting non-direct radiation with a SPC rather than with a compound parabolic concentrator (CPC) is the reduced materials use in the construction of the reflector, but a principal disadvantage is the reduction of acceptance angle to about that of the CPC. However, a SPC with concentration ratio less than 10 can still collect most of the circumsolar non-direct radiation.     

Relation between concentration and acceptance in solar collectors

The maximum concentration ratio has been derived by Winston for optical systems having an angular acceptance described by a step function. In this paper a more comprehensive relation between maximum concentration ratio and angular acceptance is obtained. This relation is valid for any acceptance function; the proof is based on thermodynamics and is valid for large angles as well.

The total energies collected yearly by concentrators having different acceptance functions are compared.     

Testing of solar collectors

In order that tests on different solar collectors conducted at different times shall be comparable, it is necessary that the test define the governing equation of the collector. This involves measurements at different solar intensities, different operating temperatures and different flow rates.The paper describes the theory and the experimental procedure which involves connecting, say, four similar collectors in series and measuring the temperature rise across each collector. Since, at any instant of time, the flow rates and the solar intensity are identical for all the collectors, a number of points on the efficiency-temperature curve are obtained from which the characteristic equation of the collectors is readily established. Rules are established to limit the temperature rise permitted and a “standard” day is suggested to provide a quick comparison basis.     

A simplified nonlinear model for solar collectors

An explicit nonlinear model is presented which predicts the performance of a solar collector and includes the effects of radiative losses. This model was obtained from the empirical correlation of simulation data from a total of 3969 separate computer simulations which included 50 different collector designs and a wide range of operating conditions. The detailed computer model, which was used to generate the simulation data, included effects such as radiative losses, axial conduction, edge losses and temperature dependent fluid properties. Results are presented which show that the proposed nonlinear model agrees with the detailed computer simulation much better than does the linear Hottel-Whillier-Bliss model.     

A generalized thermal analysis of tubular solar collectors

A generalized thermal analysis of tubular solar collectors such as the CPC, CPC with bare absorber and flat-plate (all having U-tube fluid carriers) is presented. The analysis is applicable for smaller as well as larger size troughs. It has been found that earlier published results of thermal analyses of the CPC collectors are not applicable to all CPC collectors encountered in the literature. The present analysis can, however, be successfully applied to all such collectors.     

Daily efficiency of solar collectors

It is shown that the daily efficiency of flat-plate solar collectors working at constant flow rate can be evaluated with a simple algorithm when their structural features are known. It is also shown that long-term performances can be calculated starting from monthly values of global radiation.     

Engineering of solar photocatalytic collectors

This paper briefly describes the different collectors used in solar photocatalysis for wastewater treatment and, based on prior experience, the main advantages and disadvantages of each. As the tubular-shape reactor configuration is the most appropriate for handling and pumping water, the compound parabolic collector (CPC) is proposed as an interesting combination of parabolic concentrators and flat static systems and constitutes a good option for solar photochemical applications. The design of compound parabolic concentrators for solar photocatalytic applications is described in detail and 25–50 mm is proposed as the optimum photoreactor diameter, based on the optical characteristics and optimum concentration of the two photocatalytic systems (TiO₂ and photo-Fenton) that can be used with sunlight for wastewater treatment. It has been demonstrated that since aluminium is the only metal that is highly reflective in the ultraviolet spectrum of solar radiation, aluminium-based mirrors are the best option. But, especially when exposed to outdoor conditions, aluminium must be protected and, therefore, at the present time, anodised and electropolished aluminium surfaces are considered the most suitable solutions. As the photochemical reactor contain the working fluid, including the catalyst, it must transmit UV sunlight efficiently and be able to work under enough pressure to handle the high volumes resulting from the large number of collectors in an industrial treatment plant, only low-iron glass is proposed as feasible for constructing the photoreactor (collector absorbers). Finally, ray-tracing algorithm simulations are presented as a design tool for the optical configuration of a particular reactor, drawing conclusions for its improvement and assisting in final engineering decision-making.     

Optimal operation of solar collectors

Optimal operation of a solar collector is investigated in terms of exergy delivery of the collector. Using the Bliss equation for the collector, an approximate exergy equation is derived and, through this equation, the optimal mass flow rate, and optimal inlet and outlet temperatures are obtained. The approximate equation is also compared with the non-linear exergy delivery equation of the collector.     

A thermo-electric refrigerator powered by photo-voltaic solar collectors

In developing countries and remote areas where an electric supply is not available, a thermo-electric refrigerator is often needed for food and medical drugs storage. Such a refrigerator, which requires a direct current supply is suitable for matching with a photo-voltaic collector (PV). A normal automobile 12-volt lead-acid battery may be used as the storage system. This refrigerator is rugged and may be operated by unskilled people: a prototype has been built and its components are available commercially.     

Vacuum glazing units and solar collectors

Possible application of vacuum glazing (VG) units for transparent insulation of solar collectors (SC) to reduce their heat losses is considered. Taking into account the SC operating conditions, the heat insulating parameters of VG samples measuring 0.3 × 0.3 m² and 0.5 × 0.5 m² are experimentally studied and compared to those of double glazing (DG) of the same size. Basing on the experimental data, the order of the vacuum in the VG is calculated. The contribution of the vacuum gap to the total heat transfer resistance of the VG is evaluated.     

Parabolic-trough solar collectors and their applications

This paper presents an overview of the parabolic-trough collectors that have been built and marketed during the past century, as well as the prototypes currently under development. It also presents a survey of systems which could incorporate this type of concentrating solar system to supply thermal energy up to 400 °C, especially steam power cycles for electricity generation, including examples of each application.     

Artificial neural network analysis of Moroccan solar potential

An artificial neural network (ANN) model is used to forecast the annual and monthly solar irradiation in Morocco. Solar irradiation data are taken from the new Satellite Application Facility on Climate Monitoring (CM-SAF)-PVGIS database. The database represents a total of 12 years of data from 1998 to 2010. In this paper, the data are inferred using an ANN algorithm to establish a forward/reverse correspondence between the longitude, latitude, elevation and solar irradiation. Specifically, for the ANN model, a three-layered, back-propagation standard ANN classifier is considered consisting of three layers: input, hidden and output layer. The learning set consists of the normalised longitude, latitude, elevation and the normalised mean annual and monthly solar irradiation of 41 Moroccan sites. The testing set consists of patterns just represented by the input component, while the output component is left unknown and its value results from the ANN algorithm for that specific input. The results are given in the form of the annual and monthly maps. They indicate that the method could be used by researchers or engineers to provide helpful information for decision makers in terms of sites selection, design and planning of new solar plants.     

A regime-dependent artificial neural network technique for short-range solar irradiance forecasting

Solar power can provide substantial power supply to the grid; however, it is also a highly variable energy source due to changes in weather conditions, i.e. clouds, that can cause rapid changes in solar power output. Independent systems operators (ISOs) and regional transmission organizations (RTOs) monitor the demand load and direct power generation from utilities, define operating limits and create contingency plans to balance the load with the available power generation resources. ISOs, RTOs, and utilities will require solar irradiance forecasts to effectively and efficiently balance the energy grid as the penetration of solar power increases. This study presents a cloud regime-dependent short-range solar irradiance forecasting system to provide 15-min average clearness index forecasts for 15-min, 60-min, 120-min and 180-min lead-times. A k-means algorithm identifies the cloud regime based on surface weather observations and irradiance observations. Then, Artificial Neural Networks (ANNs) are trained to predict the clearness index. This regime-dependent system makes a more accurate deterministic forecast than a global ANN or clearness index persistence and produces more accurate predictions of expected irradiance variability than assuming climatological average variability.     

Optimum exergy efficiency of solar collectors

An optimum exergy efficiency is derived for flat-plate solar collectors as a ratio of exergy delivery of the collector to the maximum output exergy obtainable. It is a function of the optimum mass flow rate through the collector, which itself is obtained through an optimization of the exergy delivery of the collector.     

Calculation of the top loss coefficient by the network method and applications to solar collectors

A method is presented for calculating the top loss coefficient of a flat-plate collector using the radiation network method with the superposition of convection heat resistances. This general method allows the inclusion of all paths of radiative heat transfer. Any number and combination of covers can be handled with ease. Results are presented for wide ranges of collectors and ambient temperatures for different configurations of glazings. Representative results are compared with the results derived from a commonly accepted approach.     

Experimental performance of three solar collectors

Simultaneous testing of solar collectors is important for the determination of accurate comparative performance data. Three flat-plate solar collectors were tested for over six months: a water trickle collector, a typical collector with double glazing, and a thermal trap collector. The first two collector types have been previously tested by other investigators, but the development of the thermal trap collector is unique to New Mexico State University, where in 1964 work was initiated on this type of collector. The thermal trap collector employs a transparent solid (methyl methacrylate) adjacent to the fluid cooled collector plate. It is found that by the use of this transparent solid, which has a high transmittance of short wavelengths combined with a low transmittance of long wavelengths and a small thermal conductivity, high temperatures can be achieved.

The comparative collector tests were performed for a variety of operational conditions. The collector efficiencies were experimentally determined, and analysis of the collector losses was accomplished. The thermal trap collector was found to have a higher operational efficiency than the other collector types and is capable of collecting solar energy for a longer period of time each day. At operating temperatures above 145°F, the thermal trap collector is more than twice as efficient as the water trickle collector.     

Characterization of solar flat plate collectors

Characterization of solar collectors is based on experimental techniques next to validation of associated models. Both techniques may be adopted assuming different complexities. In this work, a general methodology to validate a collector model, with undetermined associated complexity, is presented. It serves to characterize the device by means of critical coefficients, such as the film (convection) transfer coefficient, plate absortance or emmitance. The first step consists of identifying those significant parameters that match the selected model with the experimental data, via nonlinear optimization techniques, applied to steady state conditions. Second, new correlations must be adopted, in those terms where it is necessary (i.e. film coefficient equations). Finally, the overall model must be checked in transient regime. To illustrate the technique, a tailor-made prototype flat plate solar collector has been analyzed. An intermediate complex collector model has been proposed (2D finite-difference method). Both steady and transient states were analyzed under different operating conditions. Parameter identification is based on Newton's method optimization. For parameter approximation, exponential regression functions through multivariate analysis of variance is proposed among many other alternatives. Results depicted a robustness of the overall proposed method as starting point to optimize models applied to solar collectors.