Integrated collector storage solar water heaters

The Integrated Collector Storage Solar Water Heater (ICSSWH) developed from early systems comprised simply of a simple black tank placed in the sun. The ICSSWH, by its combined collection and storage function suffers substantial heat losses to ambient, especially at night-time and non-collection periods. To be viable economically, the system has evolved to incorporate new and novel methods of maximising solar radiation collection whilst minimising thermal loss. Advances in ICS vessel design have included glazing system, methods of insulation, reflector configurations, use of evacuation, internal and external baffles and phase change materials.     

Heat retaining integrated collector storage solar water heater with asymmetric CPC reflector

A novel integrated collector storage solar water heater (ICSSWH) was designed, optically analysed and experimentally studied. The unit was based around a heat retaining ICS vessel design consisting of two concentric cylinders mounted horizontally inside a stationary truncated asymmetric compound parabolic concentrating (CPC) reflector trough. The annulus between the cylinders was partially evacuated and contained a small amount of water, which changed phase at low temperature, producing a vapour and creating a thermal diode transfer mechanism from the outer absorbing surface to the inner storage vessel surface. The absorbing outer vessel surface covered with selective absorber film and was partially exposed to solar radiation. The remaining vessel surface area (including the vessel ends) was thermally insulated to improve heat retention during the night. Curved reflectors with a high reflectance along with high transmittance glazing were also used to improve effective operation of the ICS system. The thermal behaviour of the ICS system was compared to that of a Flat Plate Thermosiphonic Unit (FPTU). The experimental results showed that the ICS system is as effective during daily operation as it is during the night. Furthermore, the thermal loss coefficient during night gives similar values between the ICS system and FPTU.     

Transient behaviour of collector/storage solar water heaters for generalised demand patterns

This communication presents a simple transient model for predicting the thermal performance of collector/storage solar water heaters for generalised demand patterns. These heaters consist of either (i) an insulated rectangular metallic tank whose top surface is blackened and suitably glazed (i.e. a built-in storage solar water heater) or (ii) an insulated open shallow tank with black bottom.inner sides and a glass plate at the surface in contact with the water (i.e. a shallow solar pond water heater). The time dependence of the water temperature for the withdrawal of hot water from the system at constant flow rates constantly or intermittently has been explicitly evaluated. Numerical results for the operation of the system in industrial and community service applications are discussed.     

Performance of collector/storage solar water heaters: Arbitrary demand pattern

This paper presents a simple transient model for predicting the thermal performance of two novel water heaters which combine both collection and storage of solar energy in a single configuration. The proposed model is applicable for demand patterns, characteristic of various domestic and industrial applications. The model takes into account the appropriate heat transfer processes during sunshine and off-sunshine hours and predicts, for a particular case, the time dependence of water temperature which is in close agreement with experimental observations. The model also predicts the variation of water temperature for the withdrawal of hot water at constant flow rate. Furthermore, it determines the time dependence of flow rate corresponding to extraction of hot water at constant temperature; this is a desirable situation in process heating and other applications.     

Performance enhancement of an integrated collector storage hot water system

Integrated collector storage (ICS) systems offer a solution to reduce the height of the conventional flat-plate thermosiphon type collectors. The initial system developed had an aperture area of 1.77 m², a receiver diameter of 200 mm, a concentration ratio of 1.47 and total water storage volume of 65 litres. The main disadvantage of the ICS systems comes from their design, i.e., because the collector absorber is also the storage cylinder it is not possible to insulate it properly and therefore there are significant losses during the night. The main cause of these losses is the convection currents created during the night, circulating around the top glass cover. Another disadvantage of the system is its draw-off characteristics. Because the water cylinder/absorber is horizontal there is very little stratification of the water in the cylinder. It is suggested that a primary 110 mm diameter cylinder is introduced at the space between the main cylinder and the glass. The cold water is introduced directly to the primary cylinder, which feeds the main cylinder. With this modification the convection currents are drastically reduced due to the obstruction created by the primary vessel, thus reducing the night thermal losses. Also as the cold water is introduced first to the primary cylinder there is no direct mixing of the two streams thus greatly improving the system draw-off characteristics. This modification creates an 8% increase in the total cost of the system, which is reasonable, if the above benefits are considered     

Modeling of the warm water displacement from stratified tanks of integrated collector storage solar water heaters

The numerical modeling of warm water displacement from an inclined tank of an integrated collector storage solar water heater was performed, connection schemes of two heaters and the location of a supply and an extraction pipes were compared. It was shown that the parallel connection of heaters provides the stability of the stratification and a higher heat pickup than the series one, and the diagonal layout of the pipes in a stratified tank is preferable. The picture of thermocline dissipation was studied and the strong influence of the displacing fluid temperature on the degree of stratification was shown.     

Experimental investigation of a domestic solar water heater with solar collector coupled phase-change energy storage

Phase change materials (PCMs) have good properties such as high thermal capacity and constant phase change temperature. Their potential use in solar energy storage is promising. Tests of exposure and constant flow rate are performed to investigate the thermal performance of a domestic solar water heater with solar collector coupled phase-change energy storage (DSWHSCPHES). Due to the low thermal conductivity and high viscosity of PCM, heat transfer in the PCM module is repressed. The thermal performance of the DSWHSCPHES under exposure is inferior to that of traditional water-in-glass evacuated tube solar water heaters (TWGETSWH) with an identical collector area. DSWHSCPHES also performs more efficiently with a constant flow rate than under the condition of exposure. Radiation and initial water temperature have impacts on system performance; with the increase of proportion of diffuse to global radiation and/or initial water temperature, system performance deteriorates and vice versa.     

Three dimensional numerical and experimental study of forced convection heat transfer on solar collector surface

In this study, experimental and three dimensional numerical work was carried out to determine the average heat transfer coefficients for forced convection air flow over a rectangular flat plate. Three dimensional numerical simulations were obtained using a commercial finite volume based fluid dynamics code called Fluent 6.3. The experiments were performed for mass transfer using the naphthalene sublimation technique. The results were presented in terms of heat transfer parameters using the analogy between heat and mass transfer. All the experimental results are correlated within an accuracy of ± 12%.     

太阳能热水器和电热水器的环境和经济效益分析与评价

运用生命周期方法（LCA）对家用太阳能热水器进行了系统、全面的环境影响评价。主要分析了太阳能热水器的用材、生产和运输3个阶段，分别计算3个阶段的单位能耗和环境影响，并与电热水器进行了比较。结果表明，太阳能热水器具有显著的节能和环保效应。     

Short communication simple transient thermal model for solar colector/storage water heaters

This note presents a simple transient model for predicting the thermal performance of some novel solar water heaters which combine both collection and storage of solar energy. These heaters consist of either (i) an insulated rectangular tank whose top surface is blackened and suitably glazed, or (ii) an insulated open shallow tank with black bottom/inner sides and a top glass cover (shallow solar pond). the heaters are adequately covered with an insulation during the night to reduce the heat losses. the proposed model is based on different characteristic equations during sunshine and off-sunshine hours. It is seen that the model predicts the water temperature in close agreement with the experimental observations and earlier theoretical investigations.     

A numerical comparison among different water-based hybrid nanofluids on their influences on natural convection heat transfer in a triangular solar collector for different tilt angles

Natural convection inside a triangular solar collector is investigated numerically for different nanofluids and hybrid nanofluids in this study. The individual effects of Al₂O₃–water, carbon nanotubes (CNT)–water, and Cu–water nanofluids are observed for different solid volume fractions of nanoparticles (0%–10%). Three types of hybrid nanofluids are prepared using different ratios of Al₂O₃, CNT, and Cu nanoparticles in water. A comparison is made varying the Rayleigh numbers within laminar range (10³–10⁶) for different tilt angles (0°, 30°, 60°, and 90°) of the solar collector. The inclined surface of the triangular solar collector is isothermally cold and the bottom wall (absorber plate) is isothermally hot, whereas the vertical wall with respect to the absorber plate is considered adiabatic. Average Nusselt numbers along the hot wall for different parameters are observed. Streamlines and isotherm contours are also plotted for different cases. Dimensionless governing Navier–Stokes and thermal energy conservation equations are solved by Galerkin weighted residual finite element method. Better convective heat transfer is found for higher Rayleigh number, solid volume fraction, and tilt angle. In the case of hybrid nanofluid, increasing the percentage of the nanoparticle that gives better heat transfer performance individually results in enhancing natural convection heat transfer inside the enclosure.     

Effects of physical parameters on natural convection in a solar collector filled with nanofluid

Solar energy is one of the best sources of renewable energy with minimal environmental impact. A numerical study has been conducted to investigate the natural convection inside a solar collector having a flat‐plate cover and a sine‐wave absorber. The water‐alumina nanofluid is used as the working fluid inside the solar collector. The governing differential equations with boundary conditions are solved by the penalty finite element method using Galerkin's weighted residual scheme. The effects of physical parameters on the natural convection heat transfer are simulated. These parameters include the number of wave λ and non‐dimensional amplitude A of the sinusoidal corrugated absorber. Comprehensive average Nusselt number, average temperature, and mean velocity field for both nanofluid and base fluid within the collector are presented as functions of the parameters mentioned above. Comparison with previously published work is made and found to be in excellent agreement. The numerical results show that the highest heat transfer rate is observed for both the largest λ and A. In addition, the design for enhancing the performance of the collector is determined by examining the above‐mentioned results. © 2012 Wiley Periodicals, Inc. Heat Trans Asian Res; Published online in Wiley Online Library (wileyonlinelibrary.com/journal/htj). DOI 10.1002/htj.21026     

Experimental and numerical investigation on solar concentrating characteristics of a sixteen-dish concentrator

The concentrated solar flux distributions of a sixteen-dish concentrator (SDC) were measured applying a thermal infrared imager in combination with water-cooled Lambert target, and predicted using a Monte Carlo ray tracing method (MCRT). A slope error of 2.2 mrad is detected by comparing the experimental and numerical results. Then, a two-stage concentrator system, formed by the SDC in tandem with a three-dimensional compound parabolic concentrator (3D CPC–SDC), is constructed based on the geometrical optics approach. The interception performances and the energy concentration ratio images (ECR) are presented for both the SDC and the 3D CPC–SDC. The results show that the ECR profiles of the SDC depend on the receiver sizes, whereas that of the 3D CPC–SDC is rather steady because most sunlight enters the receiver via several reflections with the 3D CPC mirror. The 3D CPC–SDC is capable of increasing the geometric concentration ratio (GCR) at the expense of a little interception efficiency.     

Experimental study of an inexpensive solar collector cum storage system for agricultural uses

This paper presents an experimental investigation of an Augmented Integral Rock System. Experiments have been performed on such a solar air heater fabricated at the Institute. Experimental observations of fluid temperature, energy storage and other measures of system performance with variation of air mass flow rate, number of glazings and depth of rock bed are presented. The heat decay characteristics of the rock bed, while using a night insulation cover in addition to glass covers, are also reported in detail.     

Thermodynamic investigation of a concentrating solar collector based combined plant for poly-generation

The detailed thermodynamic evaluation for combined system assisted on solar energy for poly-generation are studied in this paper. This poly-generation cycle is operated by the concentrating solar radiation by using the parabolic dish solar collector series. The beneficial exits of this integrated plant are the electricity, fresh-water, hot-water, heating-cooling, and hydrogen while there are different heat energy recovery processes within the plant for development performance. A Rankine cycle with three turbines is employed for electricity production. In addition to that, the desalination aim is performed by utilizing the waste heat of electricity production cycle in a membrane distillation unit for fresh-water generation. Also, a PEM electrolyzer sub-component is utilized for hydrogen generation aim in the case of excess power generation. Finally, the hot-water production cycle is performed via the exiting working fluid from the very high-temperature generator of the cooling cycle. Moreover, based on the thermodynamic assessment outputs, the whole energy and exergy efficiencies of 58.43% and 54.18% are computed for the investigated solar plant, respectively.     

Comparative studies on thermal performance of water-in-glass evacuated tube solar water heaters with different collector tilt-angles

To performance comparative studies, two sets of water-in-glass evacuated tube solar water heater (SWH, in short) were constructed and tested. Both SWHs were identical in all aspects but had different collector tilt-angle from the horizon with the one inclined at 22° (SWH-22) and the other at 46° (SWH-46). Experimental results revealed that the collector tilt-angle of SWHs had no significant influence on the heat removal from solar tubes to the water storage tank, both systems had almost the same daily solar thermal conversion efficiency but different daily solar and heat gains, and climatic conditions had a negligible effect on the daily thermal efficiency of systems due to less heat loss of the collector to the ambient air. These findings indicated that, to maximize the annual heat gain of such solar water heaters, the collector should be inclined at a tilt-angle for maximizing its annual collection of solar radiation. Experiments also showed that, for the SWH-22, the cold water from the storage tank circulated down to the sealed end of tubes along the lower wall of tubes and then returned to the storage tank along the upper wall of solar tubes with a clear water circulation loop; whereas for the SWH-46, the situation in the morning was the same as the SWH-22, but in the afternoon, the cold water from the storage tank on the way to the sealed end was partially or fully mixed with the hot water returning to the storage tank without a clear water circulation loop, furthermore, such mixing became more intense with the increase in the inlet water temperature of solar tubes. This indicated that increasing the collector tilt-angle of SWHs had no positive effect on the thermosiphon circulation of the water inside tubes. No noticeable inactive region near the sealed end of solar tubes for both systems was observed in experiments.     

Optimal design and placement of serpentine heat exchangers for indirect heat withdrawal,inside flat plate integrated collector storage solar water heaters (ICSSWH)

Parameters that affect the temperature at which service hot water (SHW) is offered by an immersed tube heat exchanger (HX), inside a flat plate Integrated Collector Storage Solar Water Heater (ICSSWH), are examined numerically, by means of Computational Fluid Dynamics (CFD) analysis. The storage water is not refreshed and serves for heat accumulation. Service hot water is drawn off indirectly, through an immersed serpentine heat exchanger. For the intensification of the heat transfer process, the storage water is agitated by recirculation through a pump, which goes on only when service water flows inside the heat exchanger. Three main factors, which influence the performance, are optimized: The position of the HX relative to tank walls, the HX length and the tube diameter. All three factors are explored so that to maximize the service water outlet temperature. The settling time of the optimum configuration is also computed. Various 3-D CFD models were developed using the FLUENT package. The heat transfer rate between the two circuits of the optimum configuration is maintained at high levels, leading to service water outlet temperatures by 1–7 °C lower than tank water temperatures, for the examined SHW flow rates. The settling time is retained at sufficient law values, such as 20 s. The optimal position was found to lay the HX in contact with the front and back walls of the tank, with an optimum inner tube diameter of 16 mm, while an acceptable HX length was found to be about 21.5 m.     

Study of three‐dimensional natural convection and entropy generation in an inclined solar collector equipped with partitions

Three‐dimensional natural convection in an inclined solar collector equipped with partitions has been investigated numerically. The presence of partitions improves the performances of the collector by increasing the heat transfer near the absorber. A parametric study was done for various partitions length and Rayleigh numbers, while Prandtl number and inclination angle were fixed at 0.71 and 45°, respectively. Results are reported in terms of isosurfaces of temperature, isotherms, particles trajectories, velocity vector projection, average Nusselt number along the absorber plate and entropies generation contours.     

Experimental testing of SiNx/SiO2 thin film filters for a concentrating solar hybrid PV/T collector

Achieving high temperature thermal outputs from concentrating photovoltaic/thermal (PV/T) systems presents a challenge in that the performance of the PV cells declines with increasing temperature. Spectral beam splitting is an attractive approach to address this conflict by thermally decoupling the PV and thermal receivers, allowing the PV cells to operate at low temperature and the thermal receiver to operate at high temperature. In this study, SiN_x/SiO₂ multilayer thin film filters were designed and fabricated to act as beam splitting devices in a 10 sun, linear Fresnel mirror-based, concentrating PV/T solar collector. In this collector, reflected light is directed to a silicon PV cell whilst the transmitted light is directed to a thermal receiver. Plasma-enhanced chemical vapor deposition (PECVD) was used to fabricate the filters which were designed to obtain maximum hybrid output. The resulting devices have high reflectance (greater than 95%) for light between 713 and 1067 nm and high transmittance (greater than 90%) for sunlight outside that reflection window. The concentration of process gases in the PECVD reactor was varied in order to reduce undesired absorption at short wavelengths –lower than 650 nm– by the SiN_x layers. Indoor testing was carried out for the filters in a system which consists of a Si PV cell, a thermal sensor, and a solid-state plasma light source (6500 K black body spectrum). This study tested filter performance for various angles of incidence (AOI) between 20 and 45°. The experimental results indicate that the PV cells, illuminated with the reflected light from the filters, operate on average at 9.2% absolute higher efficiency than the same cells without the filter. Furthermore, for the best filter, in terms of relative percentage, the measured hybrid output (weighted by a worth factor of electrical vs. thermal energy) is ∼9% higher than the electrical output of a PV cell stand-alone system exposed to the same light source. This paper represents the first study of a hybrid PV/T solar collector using SiN_x/SiO₂ thin film filters and demonstrates the feasibility of such systems. This study also indicates that this type of system can utilize 85.6% of the incoming solar spectrum based on the measured optical properties of the filters.     

Experimental investigation of natural convection heat exchange within a physical model of the manifold chamber of a thermosyphon heat-pipe evacuated tube solar water heater

The high capital costs associated with heat-pipe evacuated tube solar water heating systems can be reduced by replacing forced circulation with thermosyphon circulation. Currently research on thermosyphon heat-pipe evacuated tube solar water heaters is limited. An experimental investigation of the natural convective heat exchange regime that exists within the manifold chamber of a proprietary heat-pipe evacuated tube solar water was undertaken. This paper presents experimental data from a heat-pipe Evacuated Tube Solar Water Heater (ETSWH) subjected to the Northern Maritime Climate at the University of Ulster’s outdoor solar testing facility located at the Jordanstown campus. The thermal performance of this across solar noon (±30 min) was experimentally determined to be comparable to two physical laboratory 10 pin-fin model manifolds constructed to the same dimensions and geometry as the manifold chamber of the heat-pipe ETSWH when operated under steady laboratory conditions. When the surface temperatures of the pin-fins (simulated condensers) in the model manifold were normalised with respect to the lowest most pin-fin in the array the influence of buoyant flow was observed. Similarly to related studies in this field it was found that normalised surface temperatures on downstream pin-fins do not increase monotonically as would be expected if no interactions occur. It was found that at the pin-fin diameter to pitch used in the model manifold that normalised surface temperatures decrease at certain points in the array due to the action of buoyant flow generated from upstream pin-fins which increased heat transfer. Two-dimensional Particle Imaging Velocimetry (2D-PIV) was used to visualise the thermosyphon fluid flow regime. It was observed that the fluid flow regime varied across the model due to interactions between the fluid, chamber walls and pin-fins.