• Energy Research

Abstract The bulk movement of fluid in natural convection solar air heater (SAH) is generated due to density variation and hence suffers from lower mass flow rates. Mounting additional surfaces/fins in SAH duct enhances the thermal characteristics, however, it comes at the cost of hydraulic performance. This limits the device usage in buildings and other high flow rate applications. In the present study, two new designs of SAH were numerically analyzed are: (a) taper flow passage for better thermal performance; (b) tapered designs incorporated with bell-mouth inlet opening for improved hydraulic performance. Parametric design analysis was conducted for large range of taper ratio and bell-mouth ratio. The results show that the tapered design is about 70% thermally more effective and about 6% higher Nusselt number per unit pressure drop than conventional SAH. A significant enhancement in hydraulic performance of more than 300% was observed when bell-shaped inlet design was integrated with the tapered designs. An independent correlation for Nusselt number variation has been developed as a function of Rayleigh number and tapper ratio. Predictions from the developed correlation is in excellent following with the numerical data.

Abstract Recent investigations reveal that curved solar air heaters (SAH) thermo-hydrodynamically performs better in comparison to flat SAH design. Further, it has been observed that down-configurations of turbulators or extended surfaces on the flat plate solar collector significantly enhance the thermal performance. However, scientific literature on thermal performance investigations with down-configurations of ribs in curved SAH are rare. In the paper, we systematically investigate using experimentally validated computational fluid dynamics model for different shapes of down-configuration of ribs. It was observed that half-trapezoidal and quarter-circular shape ribs shows maximum increase in thermal performance i.e. 17% and 16%, respectively, however frictional loss for quarter-circular ribs was observed to be less by about 10% when compared to trapezoidal shape ribs. The exergy recovery is maximum for trapezoidal and circular shape ribs and it is about 35% more than the smooth flat SAH. A new correlation has been developed for Nusselt number variation which has the form as N u = f [ R e , e r H ] where e r is the height of quarter-circle groove. Observed data from the model matches well with the prediction from the developed correlation.

Abstract In this paper, a novel design of counter flow curved double-pass solar air heater (DPSAH) is proposed, and its performance characteristics are numerically investigated and compared with various parallel designs under different flow and geometric conditions. The developed model is first experimentally validated. The hydraulic and thermal performance of various DPSAH designs (smooth curved single pass, smooth parallel curved double-pass, smooth counter curved double-pass, roughened parallel curved double-pass, and roughened counter curved double-pass) show that counter flow curved DPSAH with asymmetrically placed turbulators is thermally better compared to other designs. A maximum of 23% augmentation in thermal performance was observed. To predict the performance of the best design, new correlations for Nusselt number (Nu) and friction factor (f) are developed in terms of Reynolds number (Re) and relative roughness height (d/H). The data estimated from these correlations are in good agreement with the values of f and Nu predicted from the model.

Abstract This paper examines the possibility of integrating a photovoltaic (PV) module in a hybrid solar chimney power plant (HSCPP). Since HSCPP is a greenhouse thermal buoyancy-driven system, the surrounding high temperature environment makes PV module temperature extremely high resulting in lower electrical conversion efficiency. Various design configurations of collector duct and solar chimney are investigated using an experimentally validated numerical model to study the PV panel cooling and turbine power output. The results show that turbine power output is sensitive to diverging the chimney up to maximum static pressure recovery limit while PV module shows marginal increase in electrical efficiency. Converging the collector duct alone shows worst turbine and PV module performance. However, in case of combine designs of converging duct and divergent chimney, considerable improvement of PV panel efficiency (about 7%) was observed. The results show that about 80% of the collector area measured from the chimney axis are the most effective region for cooling the PV module where consistent temperature drop of 10–12 °C was observed. A design map vs. PV panel efficiency has been shown charting future directions for designing such energy efficient hybrid solar chimney systems.

Abstract Encapsulated phase change materials (PCM) are used to improve the electrical efficiency of photovoltaic (PV) panel by absorbing waste heat during the melting process. Previous investigators reported the melting process of PCM in a rectangular encapsulation and observed four transient regimes of heat transfer in sequence as: conduction, mixed conduction-convection, quasi-steady convection and solid-shrinking regimes. For higher heat extraction from the PV panel, longer duration of quasi-steady convection regime is desirable. However, this steady regime is suppressed in the rectangular PCM enclosure due to the nature of natural convection and consequently, the melting rate of the PCM is arrested. In this paper, we report on electrical and thermal performance of non-rectangular PCM integrated PV panels using an experimentally validated numerical model that enhances the quasi-steady regime by more than 100% compared to the conventional rectangular design. The strategic mass distribution of PCM for better thermal management was achieved with encapsulation designs having profile of right wall varying as y = a x - b 1 / n , w i t h n = 1 ( l i n e a r ) , 2 ( p a r a b o l i c ) a n d 3 ( c u b i c ) with different lower thickness ratio. Compared to conventional design, the proposed design increased the PCM melting rate by 17% due to which PV cell temperature dropped by 11.5% and consequently, electrical conversion efficiency approaches to 12%.

Abstract Previous investigations have mainly focused on enhancing the thermal performance of a natural convection solar air heaters (SAH) at the expense of hydraulic performance by incorporating protruded surfaces and, thereby compromising on air mass flow rate significantly due to considerable pressure drop. Low mass flow rate of SAH makes it unsuitable for wide applications. In a first, design investigations using experimentally validated numerical model of SAH is reported that enhances flow rate by more than 100% in comparison to conventional flat plate SAH design. Integration of bell-shaped designs at the inlet of SAH adds to the ram-air effect that converts dynamic pressure into static pressure thereby manifesting into an excellent enhancement of air flow rate as well as in heat transfer associated with less hydraulic losses. The high-flow SAH was further investigated for building application using unsteady first-law of thermodynamic equation and it was observed to be 33% efficient over conventional design. An independent correlation for Nusselt number variation with Rayleigh numbers and bell mouth ratio of the form Nu ( R a c o s θ ) m ( h / R ) n has been developed and found to be in good agreement with the data.

Abstract Trombe wall is one of the sustainable architectural designs in the building that offers significant opportunities for energy saving and contribute to the low green-house emissions. Naturally or buoyancy driven dual solar air heaters (SAH) are barely used in building sector with Trombe wall due to its low mass flow rates and low efficiency. In this paper, a new concept of integrating high flow naturally driven dual heating system of Trombe wall integrated with SAH has been numerically investigated, optimized and an efficient design configuration is proposed. Total seven independent configurations namely type-1 to 7 have been investigated wherein type-1 and 2 is SAH coupled with a vertical chimney, type-3 has SAH coupled with Trombe wall, and type-4 to 7 are SAH coupled with Trombe wall having bell-mouth inlet opening for improved hydraulic performance for rapid space heating. It is observed that if Trombe wall is integrated suitably with bell-mouth inlet, SAH along with chimney, it can significantly increase the mass flow rates of heated air by about 2.5 times the convectional design. It is also observed that with increase in mass flow rates, system shows significant exergy gain of 142% when compared to conventional design. Time needed to heat a given space from a specific initial temperature are also calculated using first law of thermodynamics with mass flow values taken from the model and compared with conventional design. It was noted that to heat a given space to a desired temperature, new system of dual SAH takes about 70% less time. The proposed coupled Trombe wall-SAH device has shorter payback period and could save tons of greenhouse gas emissions annually compared to the conventional fossil fuel-based heating systems. Thermal comfort of the occupant can be enhanced with dual SAH-Trombe wall system resulting in significant energy saving.

Abstract A new concept of bifacial PV/PCM (BIF-PV/PCM) system with sandwiched thermal energy storage enclosure has been investigated that possesses 1.21 times power output density and 7.39 times total energy utilization density per unit land area compared to the conventional PV system. Based on the melting morphology and thermo-electric performance of the initially rectangular PCM enclosure, an optimized bifurcated non-rectangular design of the enclosure is proposed to enhance the incident radiation utilization ability by 87% more than a simple one PV/PCM system. Enhanced utilization of solar radiation manifests into 105% more melting compared to conventional PV/PCM system. With the strategic mirror reflection and bypassing the solar radiation towards the rear PV panel and strategic design of PCM enclosure to aid convection-driven melting, electric power output has been increased significantly (about 77%) compared to similar conventional PV/PCM system. The study revealed that overall system efficiency could approach about 74% if suitable strategies are adopted, as demonstrated in this paper. Results are discussed in terms of energy utilization efficiency, exergy efficiency, power conversion efficiency, tracking melting front morphology, and its effect on heat transfer characteristics, etc. This investigation has been carried out with the help of an experimentally calibrated numerical model that accurately mimics the melting morphology of PCM and other heat transfer characteristics of the system. The findings of this study would help design and develop a more efficient BIF-PV/PCM system to meet exponentially increasing energy needs.

Abstract Recent research has shown that flat plate double-pass solar air heaters (DPSAH) exhibit higher thermal performance compared to conventional flat plate single-pass solar air heaters (SPSAH). However, scientific literature on design and performance evaluation of a curved DPSAH is scarce. In this paper, we systematically investigated various designs of DPSAH and reported its performance characteristics using a validated numerical model. Higher outlet air temperature by about 5 °C was observed when the DPSAH absorber plate is located at the mid of the insulating wall and transparent glass cover. Furthermore, putting asymmetric semi-circular roughened surfaces shows better performance than symmetric circular shapes as the reattachment of vortices with the absorber plate is more frequent in the former case. Two new correlations were developed for Nusselt number ( N u ) and friction factor ( f ) as a function of Reynolds number ( R e ) and relative roughness height ( d H ) . The values of N u and f obtained from the developed correlations agree well with data from the model.

Abstract As conventional sources of energy are depleting fast, solar energy based devices can only be mass adopted in future if suitable design innovations are conceptualized and investigated to improve its extremely low energy conversion efficiency. In a first, apart from two important components: a collector and chimney, a third component called bell-mount inlet is proposed, investigated and optimised to enhance system thermal performance. Results show that with suitable design changes in collector, chimney and integrating an efficient bell-mouth at the inlet can substantially increase the air velocity by about 270% and hence drastically enhance the turbine power output compared to the conventional design. Further investigations on physics behind such a high increase in air velocity revealed that total pressure potential or the static pressure recovery becomes high and uniform along the chimney height in the new design, which is entirely absent in the conventional design. The lab-scale design was further analysed by scaling to a 50 kW power plant. Results show that the proposed new system can produce electrical power up to 1738 kW for the same collector area and chimney height of the conventional design. The proposed design is compact, efficient and a step towards making the solar device more viable.