Performance of a natural circulation solar air heating system with phase change material energy storage

The design, construction and performance evaluation of a passive solar powered air heating system is presented. The system, which has potential applications in crop drying and poultry egg incubation, consists of a single-glazed flat plate solar collector integrated with a phase change material (PCM) heat storage system. The PCM is prepared in modules, with the modules equispaced across the absorber plate. The spaces between the module pairs serve as the air heating channels, the channels being connected to common air inlet and discharge headers. The system was tested experimentally under daytime no-load conditions at Nsukka, Nigeria, over the ambient temperature range of 19–41 °C, and a daily global irradiation range of 4.9–19.9 MJ m⁻². Peak temperature rise of the heated air was about 15 K, while the maximum airflow rate and peak cumulative useful efficiency were about 0.058 kg s⁻¹ and 22%, respectively. These results show that the system can be operated successfully for crop drying applications. With suitable valves to control the working chamber temperature, it can also operate as a poultry egg incubator.     

Augmentation of heat transfer coefficient by using 90° broken transverse ribs on absorber plate of solar air heater

An experimental investigation has been carried out to study the heat transfer coefficient by using 90° broken transverse ribs on absorber plate of a solar air heater; the roughened wall being heated while the remaining three walls are insulated. The roughened wall has roughness with pitch (P), ranging from 10–30 mm, height of the rib of 1.5 mm and duct aspect ratio of 8. The air flow rate corresponds to Reynolds number between 3000–12,000. The heat transfer results have been compared with those for smooth ducts under similar flow and thermal boundary condition to determine the thermal efficiency of solar air heater.     

Performance of a collector-cum-storage type of solar water heater

An inexpensive solar water heater of about 701 capacity, combining collection and storage, has been tested. The blackened plate (1.5 m²) of the collector-cum-storage unit of this heater absorbs solar energy and transfers it to the water stored in its enclosure (140 × 90 × 5.5 cm), the water being in direct contact with the absorber plate. The collector-cum-storage unit is enclosed in a wooden box with 10 cm thick glass wool insulation at the bottom and one glass cover.Experiments have been carried out to test the performance of the water heater under four different modes of operation: (a) water circulation with a small pump (b) natural convection conditions (c) water draw-offs taking place when the water is around 50–60°C (d) water flowing continuously past the absorber plate with flow rates of 38, 60, and 75.9 kg/hr. The day-long collection efficiency under the first two modes has been ascertained to be around 50–53 per cent for a rise in water temperature of 57-50°C. For water temperatures between 60 and 70°C, the collection efficiency is around 65-58 per cent. No appreciable difference in the collection efficiencies has been observed under the first two modes of operation. The average collection efficiency under the third mode of testing has been found to be 64.8 per cent with 202.61 of water heated from 38.5 to 58°C. In continuous flow of water past the absorber plate, a collection efficiency as high as 71.8 per cent was attained at the mass flow rate of 75.9 kg/hr, when tested under steady flow conditions. If no water is drawn off during the day, temperatures between 50 and 60°C are reached at about 11 a.m.–12 noon, 60–70°C at 12 noon–1 p.m., and 70–80°C at about 1–2 p.m., the maximum being as high at 86°C by about 3.30 p.m.In addition a theoretical calculation based on Hottel and Woertz equation for the overall heat loss coefficient between the absorber plate and the surroundings for the hourly rise in water temperature shows a very good agreement with the experimentally measured values of water temperatures.     

Performance analysis of a double-pass thermoelectric solar air collector

The thermoelectric (TE) solar air collector, sometimes known as the hybrid solar collector, generates both thermal and electrical energies simultaneously. A double-pass TE solar air collector has been developed and tested. The TE solar collector was composed of transparent glass, air gap, an absorber plate, thermoelectric modules and rectangular fin heat sink. The incident solar radiation heats up the absorber plate so that a temperature difference is created between the thermoelectric modules that generates a direct current. Only a small part of the absorbed solar radiation is converted to electricity, while the rest increases the temperature of the absorber plate. The ambient air flows through the heat sink located in the lower channel to gain heat. The heated air then flows to the upper channel where it receives additional heating from the absorber plate. Improvements to the thermal and overall efficiencies of the system can be achieved by the use of the double-pass collector system and TE technology. Results show that the thermal efficiency increases as the air flow rate increases. Meanwhile, the electrical power output and the conversion efficiency depend on the temperature difference between the hot and cold side of the TE modules. At a temperature difference of 22.8 °C, the unit achieved a power output of 2.13 W and the conversion efficiency of 6.17%. Therefore, the proposed TE solar collector concept is anticipated to contribute to wider applications of the TE hybrid systems due to the increased overall efficiency.     

Heat transfer coefficient and friction factor correlations for rectangular solar air heater duct having transverse wedge shaped rib roughness on the absorber plate

As is well known, the heat transfer coefficient of a solar air heater duct can be increased by providing artificial roughness on the heated wall (i.e. the absorber plate). Experiments were performed to collect heat transfer and friction data for forced convection flow of air in solar air heater rectangular duct with one broad wall roughened by wedge shaped transverse integral ribs. The experiment encompassed the Reynolds number range from 3000 to 18000; relative roughness height 0.015 to 0.033; the relative roughness pitch 60.17φ^−1.0264<p/e<12.12; and rib wedge angle (φ) of 8, 10, 12 and 15°. The effect of parameters on the heat transfer coefficient and friction factor are compared with the result of smooth duct under similar flow conditions. Statistical correlations for the Nusselt number and friction factor have been developed in terms of geometrical parameters of the roughness elements and the flow Reynolds number.     

Evaluation of a jet plate solar air heater

To achieve higher heat transfer from the absorber plate to the flowing air stream with an intention to increase the amount of the collected energy, and hence, to improve the efficiency of an air-based solar collector, a unique jet impingement concept has been advanced for evaluation in the present study. To investigate the effects of various geometrical parameters such as the ‘hole’ or ‘nozzle’ diameter on the jet plate, their interspacings, the nozzle height, the distance between the absorber and the jet plate and the operational parameter such as the velocity of air impinging out of the holes/nozzles on to the back side of the absorber surface on the performance parameters of the jet impingement concept air heater, a detailed theoretical parametric analysis has been made on the design for different mass flow rates of air and different lengths of air channel. A parallel study has also been carried out on a conventional parallel plate air heater in order to compare its air temperature increment and performance efficiency with those of the jet plate air heater. The gain in air temperature increment and performance efficiency of the jet-concept air heater over that of the parallel plate air heater with duct depth 10 cm and length 2 m is 15.5°C to 2.5°C and 26.5% to 19%, respectively, for air flow rates in the range 50 to 250 kg/hm².     

Development and testing of a solar air-heater with conical concentrator

The free convection performance of a solar air heater with a cylindrical absorber centred to a conical concentrator for focusing incident solar radiation was studied. The primary objective was to heat air to higher temperatures than those obtainable in flat-plate collectors.The experiments were carried out and the data recorded in summer daytime, considering collector tilting angle and type of absorbing surface as the investigation parameters.It was found that a tilting angle under local latitude would be appropriate for collector installation. Although the efficiency of the heater at free convection conditions was very much smaller than flat-plate solar air-heaters, exit air temperatures reached up to 150 °C, which could allow utilisation in high temperature applications. A selective absorber surface improved appreciably the performance of the solar air-heater.     

Thermal performance investigation of double pass-finned plate solar air heater

In this paper, the double pass-finned plate solar air heater was investigated theoretically and experimentally. An analytical model for the air heater was presented. Numerical calculations had been performed under Tanta (latitude, 30° 47′N and longitude, 31°E) prevailing weather conditions. The theoretical predictions indicated that the agreement with the measured performance is fairly good. Comparisons between the measured outlet temperatures of flowing air, temperature of the absorber plate and output power of the double pass-finned and v-corrugated plate solar air heaters were also presented. The effect of mass flow rates of air on pressure drop, thermal and thermohydraulic efficiencies of the double pass-finned and v-corrugated plate solar air heaters were also investigated. The results showed that the double pass v-corrugated plate solar air heater is 9.3–11.9% more efficient compared to the double pass-finned plate solar air heater. It was also indicated that the peak values of the thermohydraulic efficiencies of the double pass-finned and v-corrugated plate solar air heaters were obtained when the mass flow rates of the flowing air equal 0.0125 and 0.0225 kg/s, respectively.     

Development of a solar-assisted dryer and evaluation of energy requirement for the drying of onion

A solar-assisted forced convection dryer was developed to study the effect of airflow rate (2.43, 5.25, 8.09 kg/min), air temperature (55, 65, 75 °C), and fraction of air recycled (up to 90%) on the total energy requirement of drying of onion slices. The dryer was provided with a flat plate solar air heater having both the corrugations and triangular fins to the absorber plate. For drying of onion slices from initial moisture content of about 86% (wet basis) to final moisture content of about 7% (wet basis), the energy required per unit mass of water removed during without using recirculation of air was found between 23.548 and 62.117 MJ/kg water. The percent energy contribution by the solar air heater, electrical heater, and blower was found between 24.5% and 44.5%, 40.2% and 66.9%, and 8.6% and 16.3%, respectively. The savings in total energy due to fraction of air recycled were determined at 65 and 75 °C air temperature for the above three airflow rates. The maximum saving in total energy up to 70.7% was achieved by recycling of the exhaust air. The energy required per unit mass of water removed was found between 12.040 and 38.777 MJ/kg water. The percent energy contribution by the solar air heater, auxiliary heater, and blower was found between 22.4% and 40.9%, 33.6% and 62.6%, and 11.2% and 37.2%, respectively.     

The use of moderate vacuum environments as a means of increasing the collection efficiencies and operating temperatures of flat-plate solar collectors

It is shown that evacuating a flat-plate solar collector to a pressure 1–25 torr results in elimination of the natural convection heat loss from the absorber for absorber-to-cover spacings up to 15 cm. This mode of heat transfer then reduces to pure conduction through the air space between the absorber and the cover. The effect of this reduction on the total upward heat loss from the collector is considered for a variety of collector operating conditions and is shown to be especially pronounced for collectors employing wavelength-selective surfaces (high absorptance for solar radiation, but low emittance for the energy re-radiated by the absorber). Computer simulations of collector performance for the Dallas, Texas area indicate that the combination of a moderate vacuum and a selective surface (α = 0·90, = 0·15) can increase daily energy collection as much as 278 per cent over that obtained with a non-vacuum collector using a flat-black (α = = 0·95) surface and can make it possible to operate at a temperature of 150°C with a daily energy collection efficiency of more than 40 per cent. The theoretical predictions are supported by the results of twelve experiments with a no-load solar tester. At an absorber-to-cover spacing of 7·5 cm, the steady-state temperature of a moderately selective absorber (α = 0·75, = 0·3) was increased from 115°C at atmospheric pressure to 179°C at a pressure of 25 torr.     

Exergy‐based performance analysis of packed‐bed solar air heaters

This paper presents an experimental investigation of the thermal performance of a solar air heater having its flow channel packed with Raschig rings. The packing improves the heat transfer from the plate to the air flow underneath. The dimensions of the heater are 0.9 m wide and 1.9 m long. The aluminium‐based absorber plate was coated with ordinary black paint. The characteristic diameter of the Raschig rings, made of black polyvinyl chloride (PVC) tube, is 50 mm and the depth of the packed‐bed in flow channel is 60 mm. Energy and exergy analyses were applied for evaluating the efficiency of the packed‐bed solar air heater. The rate of heat recovered from the packed‐bed solar air heater varied between 9.3 and 151.5 W m^?2, while the rate of thermal exergy recovered from the packed‐bed solar air heater varied between 0.04 and 8.77 W m^?2 during the charging period. The net energy efficiency varied from 2.05 to 33.78%, whereas the net exergy efficiency ranged from 0.01 to 2.16%. It was found that the average daily net energy and exergy efficiencies were 17.51 and 0.91%, respectively. The energy and exergy efficiencies of the packed‐bed solar air heater increased as the outlet temperature of heat transfer fluid increased. Copyright © 2004 John Wiley & Sons, Ltd.     

Capital cost and economic viability of thermosyphonic solar water heaters manufactured from alternate materials in India

Many companies in India manufacture solar water heaters but these are not becoming popular in the domestic sector because of their high cost. The Ministry of Non-Conventional Energy Sources (MNES), New Delhi is recommending flat-plate collectors with copper (Cu) risers, headers and plate. Therefore, their cost is high. Long term studies have been carried out at the Central Arid Zone Research Institute, Jodhpur, to reduce the cost by replacing copper tubes with galvanised steel (G.S.) tube and copper plate with aluminium (Al) plate. The aluminium plate is wrapped over the G.S. tube by a special wire wound technique so that good contact of plate with risers and headers has been maintained. In this paper performance and testing of solar water heaters having G.S.–Al fin, Cu–Al fin and Cu–Cu fin in flat-plate collectors have been compared. It has been found that performance of all the three heaters is almost similar. The heater can provide 100 litres of hot water at an average temperature 62.0°C at 4 pm that can be retained to 50.4°C when average tap water temperature was 23.9°C. The efficiency of the heater is 51.9%. The cost of the heater with G.S.–Al collector is only Rs. 8,000.00 while it is Rs. 10,250.00 for solar water heaters with Cu–Cu collectors. The payback period of a solar water heater with G.S.–Al collector has been worked out by considering 10% compound annual interest, 5% maintenance cost, 5%, inflation in fuel prices and maintenance cost. The payback period varies between 2.92 years to 4.53 years depending upon which fuel it replaces. The payback periods are in increasing order with respect to fuels: electricity, firewood, LPG, charcoal, and kerosene.     

A two-pass solar air heater

The performance of a solar heater depends on the losses from the collector surfaces. The losses through the bottom and sides of the heater may be reduced or almost eliminated by the use of adequate insulation. In an attempt to reduce the losses from the glass cover of the simple two-glass cover air heater, a unit was constructed in which provision was made for the air to pass between the glass panes before passing through the blackened metal collector (two pass). It was found that the outer glass cover temperatures under these conditions were significantly lower (4–10°F over the day) and much nearer atmospheric temperatures compared to those when the collector was operated in the conventional single-pass manner. Consequently, efficiencies of the order of 10–15 per cent higher, were obtained. It was also found that smaller separations between the two glass panes yielded better results with the two-pass mode of operation.     

Energy and exergy efficiencies of a hybrid photovoltaic–thermal (PV/T) air collector

In this communication, an attempt has been made to evaluate exergy analysis of a hybrid photovoltaic–thermal (PV/T) parallel plate air collector for cold climatic condition of India (Srinagar). The climatic data of Srinagar for the period of four years (1998–2001) has been obtained from Indian Metrological Department (IMD), Pune, India. Based on the data four climatic conditions have been defined. The performance of a hybrid PV/T parallel plate air collector has been studied for four climatic conditions and then exergy efficiencies have been carried out. It is observed that an instantaneous energy and exergy efficiency of PV/T air heater varies between 55–65 and 12–15%, respectively. These results are very close to the results predicted by Bosanac et al. [Photovoltaic/thermal solar collectors and their potential in Denmark. Final Report, EFP Project, 2003, 1713/00-0014, www.solenergi.dk/rapporter/pvtpotentialindenmark.pdf].     

Comparative study of transparent insulation materials cover systems for integrated-collector-storage solar water heaters

The thermal performance of transparently insulated integrated-collector-storage solar water heaters is investigated theoretically as well as experimentally for a comparative study of cover systems having transparent insulation materials devices placed between the top glazing and the absorber. The data on solar transmittance, heat loss reduction characteristics and solar collector-storage efficiencies of various configurations is generated for the system performance comparisons. These hot water systems exhibit average (diurnal basis) solar collection and storage efficiencies in the range of 20–40% at a collection temperature of 40–50°C. The performance of water heaters with cover system having absorber-perpendicular configuration of TIM excel over absorber-parallel TIM covers. The effect of variation in the temperature of heat collected and cost of cover systems on the performance comparisons is also discussed.     

An optimum performance of flat-type solar air heater with porous absorber

This investigation is concerned with the design and performance of a flat-type solar air heater in which air flows perpendicularly from the transparent cover to a porous absorber plate. The design phase involved a stability analysis to determine the critical distance (maximum allowable distance) between the absober and transparent cover, for suppressing convection currents, at various environmental and operating conditions. These results are useful to designers of solar collectors of the proposed type. In addition, the thermal performance of this solar heater at its optimum design conditions was computed for a wide range of system parameters illustrating the contribution of conduction and radiative modes of heat transfer. The results indicate that the best operating efficiency can be obtained when running the collector with a mass flow rate of m > 40 kg/m².h. Furthermore, the collector thermal performance is superior than channel type solar air heaters operating under similar conditions and much simpler than honeycomb porous bed solar air heaters.     

Design and experimental evaluation of a solar dryer for commercial high-quality hay production

Design, experimental functional performance and economic evaluation of an energy efficient commercial-type solar energy dryer for production of high-quality hay, especially for the export market, are presented. The newly developed solar hay dryer consists of a solar collector with aluminum absorber plate and spaced fins, a drying shed with perforated metal grate floor above the ground level, swing-away plywood frames and polyethylene curtains for effectively sealing the hay stack during drying operations, an insulated duct, and a crawl space below the floor where a 3-hp in-line centrifugal fan is housed for air circulation by suction. In late August and in early September, 1996, 160 small rectangular bales of alfalfa hay with about 25% bromegrass were successfully dried from 33% initial moisture content to 13% moisture and from 25% to 11% moisture in 4 and 3 days, respectively, under average weather conditions in Saskatoon, Canada. The air temperature rise above ambient was 13–15 °C during peak bright sunshine hours in August and 10–13 °C in September. Ambient relative humidities ranged from 30–90%. Unlike field-cured hay, the hay produced by the dryer was of high-quality and remained green in colour and attractive after drying. Compared to field drying or conventional natural gas drying system, the payback period on investment in full-scale solar hay drying system may be just one to two years.     

Review on solar water heater collector and thermal energy performance of circulating pipe

The effect of thermal conductivity of the absorber plate of a solar collector on the performance of a thermo-siphon solar water heater is found by using the alternative simulation system. The system is assumed to be supplied of hot water at 50 °C and 80 °C whereas both are used in domestic and industrial purposes, respectively. According to the Rand distribution profile 50, 125 and 250 l of hot water are consumed daily. The condition shows that the annual solar fraction of the planning functions and the collector's configuration factors are strongly dependent on the thermal conductivity for its lower values. The less dependence is observed beyond a thermal conductivity of 50 W/m °C for the solar improper fraction and above 100 W/m °C for the configuration factors. In addition, the number of air ducts and total mass flow rate are taken to show that higher collector efficiency is obtained under the suitable designing and operating parameters. Different heat transfer mechanisms, adding natural convection, vapor boiling, cell nucleus boiling and film wise condensation is observed in the thermo-siphon solar water heater with various solar radiations. From this study, it is found that the solar water heater with a siphon system achieves system characteristic efficiency of 18% higher than that of the conventional system by reducing heat loss for the thermo-siphon solar water heater.     

Investigation of thermal performance of-double pass-flat and v-corrugated plate solar air heaters

In this paper, the double pass flat and v-corrugated plate solar air heaters are investigated theoretically and experimentally. Analytical models for the air heater with flat and v-corrugated plates are presented. Numerical calculations have been performed under Tanta (latitude, 30^° 47^′ N) prevailing weather conditions. The theoretical predictions indicated that the agreement with the measured performance is fairly good. Comparisons between the measured outlet temperatures of flowing air, output power and overall heat losses of the flat and v-corrugated plate solar air heaters are also presented. The effect of mass flow rates of air on pressure drop, thermal and thermo hydraulic efficiencies of the flat and v-corrugated plate solar air heaters are also investigated. The results showed that the double pass v-corrugated plate solar air heater is 11-14% more efficient compared to the double pass flat plate solar air heater. It is also indicated that the peak values of the thermo hydraulic efficiencies of the flat and v-corrugated plate solar air heaters are obtained when the mass flow rate of the flowing air is 0.02 kg/s.     

Performance evaluation of solar air heaters having v-down discrete rib roughness on the absorber plate

This paper presents results of a study of the performance of solar air heaters with 60 ° v-down discrete rectangular cross-section repeated rib roughness on the air flow side of the absorber plate. A detailed investigation has been carried out using a mathematical model to study the effects of various ambient, operating and design parameters on the thermal efficiency and effective efficiency (based on the net gain after taking account of the pumping power) of such air heaters. The study shows that, at air mass flow rates less than about 0.04 kg s⁻¹ per m² of the absorber plate, roughened duct solar air heaters provide significant performance advantage over the smooth duct air heater. The thermal and effective efficiencies differ only marginally at low flow rates. With the increase in the flow rate, the difference between the thermal and effective efficiencies increases because of the increase in the pumping power. At the mass flow rate of about 0.045 kg s⁻¹ m⁻², the effective efficiencies of the roughened and smooth duct solar air heaters are practically the same. The results of the study are presented in the form of design plots.