Heat-exchange relations for unglazed transpired solar collectors with circular holes on a square or triangular pitch

The unglazed transpired solar collector is now an established solar air heater for heating outside air directly. Sample applications include pre-heating ventilation air and heating air for crop drying. The outside air in question is drawn straight from ambient, uniformly through the whole surface of a perforated blackened plate (the absorber plate) exposed to the sun. An important parameter in fixing the collector’s efficiency is the heat exchange effectiveness, ε. Once ε is known, finding the collector’s efficiency is straightforward. This paper presents measurements of ε for the case where the plate is perforated with circular holes on either a square or triangular layout, covering a range of wind speeds extending from zero to 5 m/s. These data extend the earlier measurements of Kutscher to a wider range of plate thicknesses, hole spacings (pitch), suction velocities, and to include a square layout of the holes. In the region where the two experiments overlap, agreement between the two is excellent. A new predictive model is developed that is based on breaking down the total heat transfer into contributions from each of the plate sections: the front, the hole and the back. Excellent agreement was found between modelled and measured ε; the new measured data were predicted with a 4.2% root mean squared error (RMSE) and Kutscher’s measured data is predicted within 6.3% RMSE.     

Design and optimisation of a combination solar collector–thermal engine operating on Mars

A “dynamic” solar power plant (which consists of a solar collector–thermal engine combination) is proposed as an alternative for the more usual photovoltaic cells. A model for heat losses in a selective flat-plate solar collector operating on Mars is developed. An endoreversible Carnot cycle is used to describe heat engine operation. This provides upper limits for real performances. The output power is maximized. Meteorological and actinometric data provided by Viking Landers are used as inputs. Two strategies of collecting solar energy were considered: (i) horizontal collector; (ii) collector tilt and orientation are continuously adjusted to keep the receiving surface perpendicular on the Sun’s rays. The influences of climate and of various design parameters on solar collector heat losses, on engine output power and on the optimum sun-to-user efficiency are discussed.     

The development and testing of small concentrating PV systems

Spreadsheets have been used to compare some 90 possible small PV concentrator designs that might be suitable for use at remote sites. They have apertures of about 2 m², use BP Solar LBG cells, and employ small aperture modules to reduce heat sinking and construction costs. Designs include fixed V-troughs and CPCs, single axis tracked cylindrical lens and mirror systems, and two-axis tracked spherical-symmetry systems. Performance and volume production costs were estimated. Four promising systems were constructed as prototypes:

(A) Point-focus Fresnel lenses, two-axis tracking; Cg=32×; and 69× with secondaries.

(B) Line-focus mirror parabolic troughs, one-axis tracking, Cg=20×.

(C) SMTS (‘single-mirror two-stage’), one-axis tracking, Cg=30×.

(F) Multiple line-focus mirror parabolic troughs, E–W 1/day manual tracking, Cg=6×.

The prototypes were tested at Reading, and three for up to a year’s field trial at ZSW’s test site, Widderstall, in Germany. The best system efficiencies, normalised to 25°C and excluding the end losses of linear systems, were 12.5%, 13.2%, 13.6% and 14.3% for collectors A, B, C, and F, respectively. The collectors were practical and robust, and the performances of collectors B, C and F are only 10% below the estimates in the spreadsheet calculations. The best collectors have estimated production costs between 1.5 and 1.8 US $/Wp, yielding energy costs at a good site (excluding BOS and overheads) of between 5 and 7 cents/kWh (18 and 25 cents/MJ). On the same cost basis a conventional PV array costs 4.3 $/Wp, and 18 cents/kWh (65 cents/MJ).     

A new heat-pipe type solar domestic hot water system

The performance of a new solar hot water system with an integrated heat-pipe is investigated theoretically and experimentally. The new system uses a wickless gravity assisted loop heat-pipe for the heat transfer from the collector–evaporator to the tank through a heat exchanger–condenser. A detailed heat transfer experimental study is performed, focused on the thermal behaviour of the different parts of the system. The results show that the system can reach satisfactory efficiencies which, in combination with manufacturing simplicity, absence of moving parts and good behaviour under freeze conditions, make it an interesting technological solution. Furthermore, a theoretical model for the collector is proposed and comparison with experimental results is performed. This model can be used for the optimisation of the system design.     

Adsorptive solar powered ice maker: experiments and model

A solar adsorptive ice maker model is presented and experimentally validated. In order to determine the model parameters, the identification procedure is carried out employing an experimental data base obtained from tests carried out on two adsorptive solar-powered ice-makers using a methanol/carbon pair. The packaged component, the collector–condenser, represents the main new feature of these units. The prediction is compared with results of correlations. This allows a comparison of the collector–condenser behavior in the two units since the collector–condenser of one of these units is equipped with a radiation shield. The model is then used to study daily ice production sensitivity vis-à-vis critical physical parameters of the unit and to estimate the limits of the collector–condenser technology with flat plate collectors.     

Numerical treatment of transient diffussion in shrinking or swelling solids

The diffusion equation for solids that undergo change of volume was numerically integrated, using a frame of reference fixed to the volume average velocity of the system. Numerical solutions are reported for absorption and desorption processes in swelling and shrinking systems, respectively, with slab, cylindrical or spherical geometry. Uniform initial concentration, constant surface concentration, symmetry with respect to the centre, central axis or central plane of the system and diffusion coefficient independent on diffusant concentration were assumed. Plots in terms of dimensionless concentration of diffusant versus Fourier number based on the initial half-thickness of the system are reported, covering different concentration ranges of diffusant.     

Novel high efficient offline sensorless dual-axis solar tracker for using in photovoltaic systems and solar concentrators

In this study, a novel high accurate offline sensorless dual-axis solar tracker is proposed that can be widely used in photovoltaic systems and solar concentrators. The offline estimated data extracted from solar map equations are used by the tracker to find the sun direction where the maximum value of solar energy is captured. The solar tracker has been built, and it is experimentally verified that 19.1%–30.2% more solar energy can be captured depending on the seasons by utilizing the tracker. The contribution of this work is that the proposed offline sensorless dual-axis solar tracker not only has a very simple structure with a fabrication cost much less than sensor based solar trackers but also high accurately tracks the sun direction with a very small tracking error of only 0.43° which is less than the other sensorless and sensor based dual-axis solar trackers reported in the literature excluding the sensor based dual-axis solar trackers equipped with expensive sensors mounted on high accurate mechanical carriers. Furthermore, unlike all sensor based solar trackers, since the technique is offline, the proposed tracker does not use any feedback signal, and thus, its operation is independent from external disturbances and weather conditions such as cloudy sky.     

Design,modeling and testing of a standalone single axis active solar tracker using MATLAB/Simulink

This paper presents the design, modeling and testing of an active single axis solar tracker. The compactness of the proposed solar tracker enables it to be mounted onto the wall. The solar irradiance is detected by two light-dependent resistor (LDR) sensors that are located on the surface of the photovoltaic (PV) panel. The smart tracker system operates at different modes to provide flexibility to accommodate different weather conditions and preference for different users. The PV panel rotates automatically based on the sun irradiance during the day while at night; the system is in ‘sleep’ mode in order to reduce the energy consumption. A computer model of the standalone solar tracker system is first modeled using MATLAB?/Simulink?. The efficiency over the fixed solar panel, the power generated and the types of PV systems to achieve the required level of efficiency can be determined before actual implementation. The experimental testing shows some agreement with the simulation results.     

Analysis of a flat plate collector with fluid undergoing phase change

This paper presents a theoretical analysis of the performance of a flat plate solar collector with the heat removal fluid undergoing a phase change. The resultant efficiency expression is a modified Hottel-Whillier-Bliss equation. Numerical computations are made to investigate the effect of vaporisation and operational parameters on the collector's performance. The collector's efficiency increases with the increase in liquid length until a point is reached when the region of superheating the vapour disappears. The efficiency is higher when a heat removal fluid of high latent heat of vaporisation is used in the collector. An increase in the saturation temperature of the working fluid (with increase of pressure) in the collector reduces its efficiency.     

An analysis of isothermal phase change of phase change material within rectangular and cylindrical containers

In this paper, a simple computational model for isothermal phase change of phase change material (PCM) encapsulated in a single container is presented. The mathematical model was based on an enthalpy formulation with equations cast in such a form that the only unknown variable is the PCM’s temperature. The theoretical model was verified with a test problem and an experiment performed in order to assess the validity of the assumptions of the mathematical model. With very good agreement between experimental and computational data, it can be concluded that conduction within the PCM in the direction of heat transfer fluid flow, thermal resistance of the container’s wall, and the effects of natural convection within the melt can be ignored for the conditions investigated in this study. The numerical analysis of the melting time for rectangular and cylindrical containers was then performed using the computational model presented in this paper. Results show that the rectangular container requires nearly half of the melting time as for the cylindrical container of the same volume and heat transfer area.     

A simple photo-voltaic tracking system

This paper describes a simple photo-voltaic (PV) tracking system which has been designed and manufactured using a pyramidal stand as a base. A rotating unit consisting of two pairs of modules fixed at an angle of 170° between them was installed at the upper edge of the stand. The four modules and a DC motor were connected to a bridge circuit making the system sensitive to solar tracking. The PV tracker has a DC–DC (24/2 V) converter, DC–AC (24/220 V) inverter and a battery. The modules are able to provide a maximum power of 100 W, which feeds the converter, inverter and DC motor. Wind resistance of the tracker is quite low. The total area of the four modules is 1.26 m². The inclination angle between the modules and the horizontal plane is adjustable (34°±11°) and the tracking angle is 120°. The system can track solar motion with an error of ±10°. The designed PV tracking system, with modules fixed at an angle of 170° to feed the load as well as the DC motor, exhibited it to be an efficient energy-conversion system. The fabricated system offers low wind resistance. The cost analysis data revealed that the proposed design was very economical and cost effective.     

一种新型极轴跟踪式定日镜的研究

提出一种新型采用轮胎面聚光镜极轴跟踪式定日镜。分析了极轴跟踪式定日镜的跟踪原理,设计了用于该定日镜的轮胎面并分析其聚光性能。结果表明:该定日镜聚光性能较高,能够满足生活热水、太阳炉等应用领域的要求。     

V-trough concentrator on a photovoltaic full tracking system in a hot desert climate

A V-trough concentrator with a two-axis tracker system to increase the performance of photovoltaics was designed by the authors and installed on the roof-top of the building of the National Research Institute of Astronomy and Geophysics at Helwan in South Cairo. The V-trough concentrator system comprises two flat mirrors with dimensions 50 cm × 18 cm. They are fixed with the reflecting surfaces facing each other with a separation of about 11 cm, on a wooden table of 50 cm axis length. A sample of polycrystalline and amorphous silicon solar cells were fixed into the system, and similar solar cells of each type were fixed separate to the system, to estimate the electrical gain. The measurements were performed daily at different air masses for one complete year. The temperature of the solar cells in and out of the system were measured for comparison. Also, measurements for beam and global solar radiation and other meteorological conditions were recorded. The optical losses of the system were analyzed and details of collectable energy calculations are presented. The energy gain from the isolated contribution of the V-trough concentrators is also evaluated.     

A new method for typical weather data selection to evaluate long-term performance of solar energy systems

A new method is proposed for determining typical 1-year weather data from a multi-year record for evaluation of solar energy systems. The procedure is very straightforward and can be utilised with ease when determining the long-term performance of a solar hot water system (SHWS). It is made up of a concatenation of 12 months individually selected from a multi-year database. The criterion for the selection is the minimisation of error in the monthly solar gain prediction of the system. Considering this criterion, the ‘typicality’ of the weather pattern is taken into account, in addition to its influence on the behaviour of the solar system. A comparison is made between the new method and others frequently referred to in the literature. Based on simulation results for yearly, monthly and daily power delivered, six indicators have been calculated. These indicators quantify the different behaviours of the system when ‘historical’ and typical weather data are applied. The all inclusive comparison shows that the new method for deriving typical weather data leads to an accurate evaluation of the long-term performance of a SHWS.     

Theoretical aspects for optimization of solar radiation concentrators with plane facets

The experience of the 25 years operation of two concentrators of solar energy (CSE) of 2.8 and 5.0 m diameter manufactured by gluing plane facets of thin glass onto metal radio antennae convincingly demonstrates their technical and economic efficiency. Besides (or in addition to) simplicity of manufacturing and the possibility to achieve temperatures up to 2000 K and more, there is one more undoubted advantage of such concentrators: it is possible to get a focal spot with rather uniform distribution of the irradiance. The problem of determining the plane facets’ optimum configuration and the structure of their arrangement on the CSE surface which guarantees forming a focal spot of the given dimension is solved by this work. Perfect geometric configuration of the facets and paraboloid is assumed. The working zone of the facets reflecting the incident solar radiation at the circular focal spot is approximately an ellipse. Trapezoid is an optimum configuration for the facets taking up a minimum area and ensuring the maximum use of CSE surface. A system of equations is obtained for calculation of a CSE structure with annular packing of the facets. The results of calculations for the 1.8 m and 15 m diameter concentrators are presented.     

Free cooling of a building using PCM heat storage integrated into the ventilation system

This article presents a study of the free cooling of a low-energy building using a latent-heat thermal energy storage (LHTES) device integrated into a mechanical ventilation system. The cylindrical LHTES device was filled with spheres of encapsulated RT20 paraffin, a phase-change material (PCM). A numerical model of the LHTES was developed to identify the parameters that have an influence on the LHTES’s thermal response, to determine the optimum phase-change temperature and to form the LHTES’s temperature-response function. The last of these defines the LHTES’s outlet-air temperature for a periodic variation of the inlet ambient-air temperature and the defined operating conditions. The temperature-response function was then integrated into the TRNSYS building thermal response model. Numerical simulations showed that a PCM with a melting temperature between 20 and 22 °C is the most suitable for free cooling in the case of a continental climate. The analyses of the temperatures in a low-energy building showed that free cooling with an LHTES is an effective cooling technique. Suitable thermal comfort conditions in the presented case-study building could be achieved using an LHTES with 6.4 kg of PCM per square metre of floor area.     

Measured and modelled improvement in solar energy yield from flat plate photovoltaic systems utilizing different tracking systems and under a range of environmental conditions

This work is performed to investigate the effect of using different sun tracking mechanisms on the flat plate photovoltaic system performances and the main parameters affecting the amount of their electrical energy output as well as those affecting their gains compared to the traditional fixed photovoltaic systems. To this end, five configurations of sun tracking systems and two traditional fixed panels have been considered. The sun tracking systems effect on the PV system performances is improved by using the hourly data collected over 18 days for different seasonal sky conditions. The daily cumulative electrical energy produced by the different systems have been quantified separately for each sky state and the corresponding electrical gains have then been compared to those experienced with two traditional fixed photovoltaic systems. It is found that for a completely clear day, the highest obtained gains are those related to the two-axis sun tracker systems, which decrease gradually from the inclined to the vertical rotating axis when the same optimum slope is applied and from the seasonal to the yearly optimum slope if the same rotating axis is considered. On the other hand, for the partially clear days, the gain amounts are mainly dependant on the clearness index and on the seasonal variation of day length values. For a completely cloudy day, the results show that all considered systems produced closely the same electrical energy and the horizontal position of the photovoltaic panel presented the best performance.     

Off-axis aberration correction surface in solar energy application

A new fixed aberration correction surface for applications in solar energy is proposed. The surface equation was derived based on the theory of focusing heliostat and the technique of canting the mirror array at a particular incident angle. The performance of the optimized new surface is simulated and compared with that of a spherical surface for two different ranges of incident angles, i.e., 0°–33° and 33°–57°. The comparison result shows that there are considerable improvements: not only is the size and concentration of the sun’s image improved, but it is also possible to shift the hot spot from noon time to early morning and late afternoon.     

A new conception of an adsorptive solar-powered ice maker

An experimental data base obtained from specific tests carried out on an adsorptive solar-powered ice maker using methanol/carbon pair and equipped with the collector–condenser technology is analysed and the part of the evaporator in the condensation of the methanol vapour during the desorption phase is highlighted. Consequently, a new adsorptive solar-powered ice maker equipped with a single heat exchanger playing alternatively the role of condenser and evaporator is conceived. When working as a condenser, this heat exchanger is simply cooled by natural convection by means of fins. The estimated heat transfer area of the fins needed to reach the performance of the tested machine (COPs≈12%) is 1.3 m². This new conception of the machine leads to a 30 kg lightening of the metallic mass and enables to equip the collector with airing shutters, in order to improve its nocturnal cooling, which is the limiting factor of the collector–condenser technology. A notable reduction of the manufacturing cost and more than 10% improvement of performance are expected.