Constant flow solar pond collector/storage system

This paper presents a periodic analysis of the process of heat extraction by the brine layer circulating at constant flow rate through the bottom convective zone of a solar pond. Explicit expressions for the transient rate of heat extraction and the temperature at which heat can be extracted, as a function of time, depths of convective as well as non-convective zones and the flow rate, are derived. Extensive analytical results for the optimum performance of a pond during its year round operation are presented. In a pond with an upper convective zone depth of 0.2 m optimum heat extraction efficiencies of 24 per cent, 29 per cent and 32 per cent corresponding to heat extraction temperatures of 89, 55 and 42°C are predicted for water flow rates of 2 × 10^?4, 5 × 10^?4 and 10^?3 kg/s m², respectively. The load levelling in the extracted heat flux as well as in its temperature improves as the flow rate is lowered and the non-convective zone is over sized. An increase in the total depth of the solar pond improves the load levelling in extraction temperature, but influences the extracted heat flux differently; shifts its maximum to winter months and deteriorates the load levelling. The variability in flow rate required for the maintenance of constant temperature of the heat extraction zone is also investigated. It is found that the required variability is less for higher temperatures of the heat extraction zone and larger depths of the non-convective zone.     

Reflecting bottom solar pond

This communication presents an investigation of a three zone solar pond with a diffusely reflecting bottom. The dynamic performance and its optimization as a large scale solar energy collection and long term storage system have been discussed. Numerical computations, corresponding to the data of solar radiation and ambient air temperature of New Delhi during 1974, have been made for two modes of operation, viz (i) constant flow rate of heat removal fluid, and (ii) zone of heat retrieval remaining at constant temperature. At a typical flow rate of 10^?3kg/m²s, optimal heat retrieval efficiencies of 32.4% (at non-convective zone depth, l₂, of 1.25 m), 25.2% (at l₂ = 0.75 m) and 21.5% (at l12 = 0.6 m) are predicted for bottom reflectivities of 0.0, 0.5 and 0.8, respectively. Furthermore, the variability in flow rate required to keep the temperature of the heat extraction zone constant is less for higher temperatures of the heat extraction zone, and the effect of R is found to be insignificant. In the mode of heat retrieval at constant temperature, the calculations of Sodha et al. [Energy Res.5, 321 (1981)] for the case of a completely absorbing surface with no bottom reflection (α = 1.0, R = 0.0) have yielded the optimal heat retrieval efficiency of 35.5 and 26.5% at extraction temperatures of 40 and 100°C, respectively. However, for the realistic case of a partially absorbing bottom (α = 0.9 and R = 0.0), these efficiencies are predicted to go down to 32.5 and 23.5%, respectively. The effect of bottom reflectivity on heat collection efficiency has further revealed the fact that the heat collection efficiency decreases with an increase in bottom reflectivity. For a typical value of R(= 0.6) corresponding to the realistic case of a partially absorbing bottom (α = 0.9), optimum efficiencies decrease further to 22.3 and 12.4% at collection temperatures of 40 and 100°C, respectively.     

Performance assessment of a magnesium chloride saturated solar pond

This paper deals with the experimental investigation of a magnesium chloride saturated solar pond and its performance evaluation through energy and exergy efficiencies. The solar pond system is filled with magnesium chloride containing water to form layers with varying densities. A solar pond generally consists of three zones, and the densities of these zones increase from the top convective zone to the bottom storage zone. The incoming solar radiation is absorbed by salty water (with magnesium chloride) which eventually increases the temperature of the storage zone. The high-temperature salty water at the bottom of the solar pond remains much denser than the salty water in the upper layers. Thus, the convective heat losses are prevented by gradient layers. The experimental temperature changes of the solar pond are measured by using thermocouples from August to November. The densities of the layers are also measured and analysed by taking samples from at the same point of the temperature sensors. The energy and exergy content distributions are determined for the heat storage zone and the non-convective zone. The maximum exergy destructions and losses appear to be 79.05 MJ for the heat storage zone and 175.01 MJ for the non-convective zone in August. The energy and exergy efficiencies of the solar pond are defined as a function of solar radiation and temperatures. As a result, the maximum energy and exergy efficiencies are found to be 27.41% and 26.04% for the heat storage zone, 19.71% and 17.45% for the non-convective zone in August, respectively.     

Thermal analysis of constant flow partitioned solar pond

This paper presents the thermal analysis of the process of heat extraction by circulating water layer through the convective zone of a partitioned solar pond. The observed variation of atmospheric air temperature and solar intensity is assumed periodic. Explicit expressions for the transient rate and temperature at which heat can be extracted by circulation of water at constant flow rate, are derived. Numerical computations corresponding to solar heat flux and atmospheric air temperature measurement at New Delhi during the year 1974 have been made, and the optimization of the flow rate as well as the depth of the convective-non-convective zones in the pond have been investigated. The optimum heat retrieval efficiency of 27.5%, 34% and 40% corresponding to heat retrieval temperatures of 97°C, 60.5°C and 45.5°C, respectively, are predicted for water flow rates of 2 × 10^?4, 5 × 10^?4 and 10^?3 kg/s.m², respectively. The load levelling in retrieved heat flux improves as flow rates are lowered, and the non-convective zone is oversized. With the non-convective zone depth near optimum, an increase in the depth of the heat extraction zone considerably influences the retrieved heat flux; it shifts its maximum to winter months and deteriorates the load levelling. The variability in flow rate required for the maintenance of constant temperature of the heat extraction zone is also investigated. It is found that the required variability is less for higher temperatures of the extraction zone and larger depths of non-convective zone.     

Heat extraction from salinity-gradient solar ponds using heat pipe heat exchangers

This paper presents the results of experimental and theoretical analysis on the heat extraction process from solar pond by using the heat pipe heat exchanger. In order to conduct research work, a small scale experimental solar pond with an area of 7.0 m² and a depth of 1.5 m was built at Khon Kaen in North-Eastern Thailand (16°27′N102°E). Heat was successfully extracted from the lower convective zone (LCZ) of the solar pond by using a heat pipe heat exchanger made from 60 copper tubes with 21 mm inside diameter and 22 mm outside diameter. The length of the evaporator and condenser section was 800 mm and 200 mm respectively. R134a was used as the heat transfer fluid in the experiment. The theoretical model was formulated for the solar pond heat extraction on the basis of the energy conservation equations and by using the solar radiation data for the above location. Numerical methods were used to solve the modeling equations. In the analysis, the performance of heat exchanger is investigated by varying the velocity of inlet air used to extract heat from the condenser end of the heat pipe heat exchanger (HPHE). Air velocity was found to have a significant influence on the effectiveness of heat pipe heat exchanger. In the present investigation, there was an increase in effectiveness by 43% as the air velocity was decreased from 5 m/s to 1 m/s. The results obtained from the theoretical model showed good agreement with the experimental data.     

Enhancing the thermal efficiency of solar ponds by extracting heat from the gradient layer

An alternative method of heat extraction from salinity-gradient solar ponds is investigated with the aim of increasing the overall energy efficiency of collecting solar radiation, storing heat and delivering this heat to an application. In this alternative method, heat is extracted from the non-convecting gradient layer of a solar pond as well as, or instead of, from the lower convective zone (LCZ). A theoretical analysis of combined gradient-layer and LCZ heat extraction is conducted to obtain expressions for the variation of temperature with depth in the pond, and the temperature gradient with depth. The dependence of the overall energy efficiency of the pond on thickness of the gradient-layer, temperature of delivered heat, and various combinations of gradient layer and LCZ heat extraction rates, including the limiting cases of gradient-layer heat extraction only, and LCZ heat extraction only, is then explored. This theoretical analysis suggests that heat extraction from the gradient layer has the potential to increase the overall energy efficiency of a solar pond delivering heat at a relatively high temperature by up to 50%, compared with the conventional method of heat extraction solely from the LCZ. The potential gain in efficiency using gradient-layer heat extraction is attributed to the lowering of heat losses by conduction to the upper convective (surface) zone that can be achieved with this method. Experimental investigations are proposed to test the predictions of the theoretical analysis in practice, and assess the impact of a number of idealized assumptions made on the findings reported here.     

A fully coupled,transient double-diffusive convective model for salt-gradient solar ponds

A fully coupled two-dimensional, numerical model that evaluates, for the first time, the effects of double-diffusive convection in the thermal performance and stability of a salt-gradient solar pond is presented. The inclusion of circulation in the upper and lower convective zone clearly shows that erosion of the non-convective zone occurs. Model results show that in a two-week period, the temperature in the bottom of the solar pond increased from 20 °C to approximately 52 °C and, even though the insulating layer is being eroded by double-diffusive convection, the solar pond remained stable. Results from previous models that neglect the effect of double-diffusive convection are shown to over-estimate the temperatures in the bottom of the solar pond. Incorporation of convective mixing is shown to have profound impacts on the overall stability of a solar pond, and demonstrates the need to actively manage the mixing and heat transfer to maintain stability and an insulating non-convective zone.     

Saturated solar ponds: 1. Simulation procedure

The mass and energy balances on the upper convective zone, nonconvective zone, and lower convective zone of a saturated solar pond are written to yield a set of nonlinear partial differential equations. These are solved numerically to predict the thermal performance of the pond over a long period of time for various initial and boundary conditions. This model considers external parameters such as hourly variation of incident solar radiation, ambient temperature, air velocity, and relative humidity. Temperature and concentration dependence of density, thermal conductivity, specific heat, and mass diffusivity are taken into account. Heat transfer modes considered between the upper convective zone and the ambient are convection, evaporation, and radiation. Ground heat losses from the lower convective zone are also considered. This model is used to study the development of temperature and concentration profiles inside a saturated solar pond. This model can also be used to predict the long-term performance of a saturated solar pond for various heat extraction temperatures and rates.     

Use of porous baffles to enhance heat transfer in a rectangular channel

An experimental investigation was carried out to measure module average heat transfer coefficients in uniformly heated rectangular channel with wall mounted porous baffles. Baffles were mounted alternatively on top and bottom of the walls. Heat transfer coefficients and pressure loss for periodically fully developed flow and heat transfer were obtained for different types of porous medium (10, 20, and 40 pores per inch (PPI)) with two window cut ratios (B_h/D_h=1/3 and 2/3) and two baffle thickness to channel hydraulic diameter ratios (B_t/D_h=1/3 and 1/12). Reynolds number (Re) was varied from 20,000 to 50,000. To compare the effect of foam metal baffle, the data for conventional solid-type baffle were obtained for (B_t/D_h=1/3). The maximum uncertainties associated with module Nusselt number and friction factor were 5.8% and 4.3% respectively. The experimental procedure was validated by comparing the data for the straight channel with no baffles (B_h/D_h=0) with those in the literature [Publications in Engineering, vol. 2, University of California, Berkeley, 1930, p. 443; Int. Chem. Eng. 16 (1976) 359]. The use of porous baffles resulted in heat transfer enhancement as high as 300% compared to heat transfer in straight channel with no baffles. However, the heat transfer enhancement per unit increase in pumping power was less than one for the range of parameters studied in this work. Correlation equations were developed for heat transfer enhancement ratio and heat transfer enhancement per unit increase in pumping power in terms of Reynolds number.     

Saturated solar ponds: 2. Parametric studies

The effects of following parameters on the performance of saturated solar ponds are studied: thickness of upper convective zone, nonconvective zone, and lower convective zone; starting time of the pond; water table depth below the pond; ground thermal conductivity; transmissivity of salt solution; incident radiation; ambient air temperature, humidity, and velocity; thermophysical properties of salt solution; pond bottom reflectivity; convection, evaporation, radiation, and ground heat losses; temperature and rate of heat removal; type of salt. Magnesium chloride and potassium nitrate salt ponds located at Madras (India) are considered for the parametric study. A comparison is also made with an unsaturated solar pond.     

The influence of the heat extraction mode on the performance and stability of a mini solar pond

The performance and stability of a 0.64 m² mini solar pond were experimentally and numerically studied. The experimental results showed that the daily average temperatures of the mini pond increased quickly to reach 54 °C in the lower convective zone after only 20 days. A 1 D numerical model to predict the temperature and the salinity in the mini pond was developed. The results obtained from the numerical calculation were compared with our experimental results and good agreement was found. Therefore a new method of heat extraction from the non convective zone was numerically investigated in order to enhance the mini pond performance. Using this new method of heat extraction, the numerical results showed that the mini pond efficiency could be considerably improved. However, the method was found to reduce the stability of the lower interface.     

Heat extraction methods from salinity-gradient solar ponds and introduction of a novel system of heat extraction for improved efficiency

Heat has generally been successfully extracted from the lower convective zone (LCZ) of solar ponds by two main methods. In the first, hot brine from the LCZ is circulated through an external heat exchanger, as tested and demonstrated in El Paso and elsewhere. In the second method, a heat transfer fluid circulates in a closed cycle through an in-pond heat exchanger, as used in the Pyramid Hill solar pond, in Victoria, Australia. Based on the experiences at the El Paso and Pyramid Hill solar ponds, the technical specifications, material selection, stability control, clarity maintenance, salt management and operating strategies are presented. A novel method of extracting heat from a solar pond is to draw the heat from the gradient layer. This method is analysed theoretically and results of an experimental investigation at Bundoora East, RMIT, are presented. An in-pond heat exchanger made of polyethylene pipe has been used to extract heat for over 2 months. Results indicate that heat extraction from the gradient layer increases the overall energy efficiency of the solar pond by up to 55%, compared with conventional method of heat extraction solely from the LCZ. The experimental results are compared with the theoretical analysis. A close agreement has been found. From this small-scale experimental study, convection currents were found to be localised only and the density profiles were unaffected. An experimental study using an external heat exchanger for brine extraction and re-injection at different levels within the gradient layer still needs to be conducted to determine the effect of the heat extraction from the non-convective zone (NCZ) on the stability of the salinity gradient (both vertically and horizontally) and an economic analysis needs to be conducted to determine the economic gains from increased thermal efficiency.     

Experiment on heat storage characteristic of microencapsulated phase change material slurry

Heat storage experiment by natural convection in rectangular enclosures heated from bottom has been conducted with fluid slurry composed of microencapsulated phase change material (PCM). The microencapsulated PCM is prepared by in-situ polymerization method, where the core materials are composed of several kinds of n-paraffin waxes (mainly nonadecane) and the membrane is a type of melamine resin. Its slurry mixed with water is used in this study, and shows a peak value in the specific heat capacity with latent heat at the temperature of about T=31 °C. The influences of the phase change material on heat storage and the heat transfer process, as well as effects of PCM mass concentration C_m on the microcapsule slurry, temperature of heat storage T_H and a horizontal enclosure height H are also investigated. Transient heat transfer coefficient α, heat storage capacity Q and completion time of heat storage t_c are discussed.     

Effects of porous media on thermal and salt diffusion of solar pond

Laboratory and field experiments were carried out along with numerical simulations in this paper to study the effects of porous media on thermal and salt diffusion of the solar ponds. From our laboratory experiments simulating heat transfer inside a solar pond, it is shown that the addition of porous media to the bottom of a solar pond could help enhance its heat insulation effect. The experiment on salt diffusion indicates that the upward diffusion of the salt is slowed down when the porous media are added, which helps maintain the salt gradient. Our field experiments on two small-scaled solar ponds indicate that when porous media are added, the temperature in the lower convective zone (LCZ) of the solar pond is increased. It is also found that the increase in turbidity is repressed by porous media during the replenishment of the salt to the LCZ. Thermal diffusivities and conductivities of brine layers with porous media such as pebble and slag were also respectively measured in this paper based on the unsteady heat conducting principles of a semi-infinite body. These measured thermal properties were then used in our numerical simulations on the effect of porous media on thermal performance of a solar pond. Our simulation results show that brine layer with porous media plays more positive role in heat insulation effect when thermal conductivity of the ground is big. On the other hand, when the ground has a very small thermal conductivity, the performance of solar pond might be deteriorated and total heat storage quantity of solar pond might be reduced by brine layer with porous media.     

Heat transfer in a crater bed

The aim of this work is to study heat transfer in a laboratory scale crater bed, which was set up from a cylindrical acrylic/quartz tube, using sand as the bed particle. The bed employs a downward gas jet from a nozzle which causes the particles to ascend fountain-like into the freebroad, leaving a crater on the bed surface. After reaching a certain height, these particles will descend again to the bed surface and move into the crater, where the cycle or circulation pattern starts again. The study had been separated into three parts. Firstly, the void fraction of the bed fountain zone was studied by direct measurement of the ascending sand weight within the specific volume. Secondly, the convection heat transfer coefficients between the fountain zone and the external surface of the gas inlet tube were determined by measuring the quantity of heat loss from an electrical heater that was wrapped on the outside surface at desired positions of the gas inlet tube. Thirdly, the radiation heat transfer coefficients were evaluated by heat balance of LPG combustion in the crater bed. From experimental results, the void fraction of the fountain zone could be approximated as a dilute bed (>0.98). For convective heat transfer coefficients, the value found experimentally varied from 80–260 W/m² K depending on the experimental conditions, showing an increase when the gas velocity increases, and a decrease along the height of the gas inlet tube. Radiation heat transfer coefficients, the values of which are (within the experimental temperature range), the same order as the convective mode, increase when the bed temperature is increased and when the bed particle diameter is decreased. Empirical correlations for both bed voidage and heat transfer coefficients are proposed. The combined model, gas and particle convection and the published data on radiation heat transfer, showed good prediction when compared with experimental data.     

Conjugate-mixed convection heat transfer in a lid-driven enclosure with thick bottom wall

Conjugate heat transfer by mixed convection and conduction in lid-driven enclosures with thick bottom wall has been studied by a numerical method. The enclosure is heated from the bottom wall isothermally. Temperature of the top moving wall, which has constant flow speed, is lower than that of the outside of bottom wall. Vertical walls of the enclosure are adiabatic. Governing parameters are solved for a wide range of Richardson numbers (0.1 ≤ Ri ≤ 10), ratio of height of bottom wall to enclosure height (0.1 ≤ h/H ≤ 0.5) and thermal conductivity ratio (0.01 ≤ λ_f/λ_s ≤ 10). Obtained results showed that heat transfer decreases with increasing of λ_f/λ_s ratio, Richardson number and thickness ratio of the wall. Flow strength is affected for only higher values of λ_f/λ_s ratio.     

Experimental study of the convective heat transfer from in-line and staggered configurations of two wall-mounted cubes

The paper reports on the experimental investigation of the effects of the relative obstacle position on the convective heat transfer from a configuration of two wall-mounted cubes located in a fully developed turbulent channel flow. Both in-line and staggered arrangements were studied for various streamwise (S_x/H) and spanwise (S_z/H) distances. Distributions of the local heat transfer coefficient (h) were obtained from infrared thermography and local convective heat flux analyses. Laser Doppler anemometry measurements and flow visualisations were performed to document the flow and turbulence fields around the cubes. The results showed a large variation in the distribution of the local convective heat transfer for the various in-line and staggered configurations studied. While the in-line arrangements were featured by symmetric flow pattern and heat transfer distributions, the staggered arrangements showed distinct asymmetric pattern for certain combinations of S_x/H and S_z/H. Flow reattachment caused typically a monotonic decay of the convective heat transfer. On the other hand, flow separation caused distinct heat transfer extrema at the cube faces. In addition, the effect of vortex shedding on the convective heat transfer of the downstream cube was studied with a fast-responding heat flux sensor. Despite distinct variation in the distribution of the time-averaged heat transfer coefficient, the cube-averaged heat transfer coefficients appeared to be independent of the relative placement of the two cubes.     

Laminar heat transfer and friction characteristics of microencapsulated phase change material slurry in a circular tube with twisted tape inserts

It is novel and better method that microencapsulated phase change material (MPCM) slurry and the tube with twisted tape inserts are adopted together to enhance convective heat transfer. In this paper, numerical analyses were carried out to study laminar heat transfer and friction characteristics of MPCM slurry in a circular tube with twisted tape inserts. It is found that the MPCM slurry in the tube with twisted tape insert leads to the best performance of convective heat transfer for the bigger apparent specific heat and the intensive swirl flow. Furthermore, the modified average Nusselt number increases with decreasing bulk Stefan number, twisted ratio and increasing Re, while the friction factor increases with increasing Re and decreasing twisted ratio. It is also found that the heat transfer enhanced efficiency increases with increasing Re and decreasing Ste_b, and the heat transfer enhancement effects of twisted tape for low Ste_b slurry are better than that for high Ste_b slurry. Moreover, the thermal-hydraulic performance ratios increase to a peak, then decrease gradually with increasing Re for different twist ratio. The performance ratio increases with decreasing the twist ratio only in a definite Re range, and the Re range decreases with decreasing twist ratio.     

Modeling of forced convective heat transfer of a non-Newtonian nanofluid in the horizontal tube under constant heat flux with computational fluid dynamics

In this paper, convective heat transfer effect on the non-Newtonian nanofluid flow in the horizontal tube with constant heat flux was investigated using computational fluid dynamics (CFD). For this purpose, non-Newtonian nanofluid containing Al₂O₃ and Xanthan aqueous solution as a liquid single phase with two average particle sizes of 45 and 150 nm and four particle concentrations of 1, 2, 4 and 6 wt.% and two concentrations of Xanthan aqueous solutions (0.6,1.0 wt.%) were used. Effect of particle size and concentration of Xanthan solution on convective heat transfer coefficient was investigated in different Reynolds numbers (500 < Re < 2500) for various axial locations of tube. The results showed that heat transfer coefficient and Nu number of non-Newtonian nanofluid increased with increasing concentration of Xanthan solution. By applying the modeling results, an equation was obtained for Nusselt number prediction using the dimensionless numbers. The results showed that the correlated data were in very good agreement with predicted data. The maximum error was around 5%.