Analysis and design consideration of mean temperature differential Stirling engine for solar application

This article presents a technical innovation, study of solar power system based on the Stirling dish (SD) technology and design considerations to be taken in designing of a mean temperature differential Stirling engine for solar application. The target power source will be solar dish/Stirling with average concentration ratio, which will supply a constant source temperature of 320 °C. Hence, the system design is based on a temperature difference of 300 °C, assuming that the sink is kept at 20 °C. During the preliminary design stage, the critical parameters of the engine design are determined according to the dynamic model with losses energy and pressure drop in heat exchangers was used during the design optimisation stage in order to establish a complete analytical model for the engine. The heat exchangers are designed to be of high effectiveness and low pressure-drop. Upon optimisation, for given value of difference temperature, operating frequency and dead volume there is a definite optimal value of swept volume at which the power is a maximum. The optimal swept volume of 75 cm³ for operating frequency 75 Hz with the power is 250 W and the dead volume is of 370 cm³.     

Simulation, construction and testing of a two-cylinder solar Stirling engine powered by a flat-plate solar collector without regenerator

In this research, a gamma-type, low-temperature differential (LTD) solar Stirling engine with two cylinders was modeled, constructed and primarily tested. A flat-plate solar collector was employed as an in-built heat source, thus the system design was based on a temperature difference of 80 °C. The principles of thermodynamics as well as Schmidt theory were adapted to use for modeling the engine. To simulate the system some computer programs were written to analyze the models and the optimized parameters of the engine design were determined. The optimized compression ratio was computed to be 12.5 for solar application according to the mean collector temperature of 100 °C and sink temperature of 20 °C. The corresponding theoretical efficiency of the engine for the mentioned designed parameters was calculated to be 0.012 for zero regenerator efficiency. Proposed engine dimensions are as follows: power piston stroke 0.044 m, power piston diameter 0.13 m, displacer stroke 0.055 m and the displacer diameter 0.41 m. Finally, the engine was tested. The results indicated that at mean collector temperature of 110 °C and sink temperature of 25 °C, the engine produced a maximum brake power of 0.27 W at 14 rpm. The mean engine speed was about 30 rpm at solar radiation intensity of 900 W/m² and without load. The indicated power was computed to be 1.2 W at 30 rpm.     

Thermodynamic analysis of a Stirling engine including dead volumes of hot space, cold space and regenerator

This paper provides a theoretical investigation on the thermodynamic analysis of a Stirling engine. An isothermal model is developed for an imperfect regeneration Stirling engine with dead volumes of hot space, cold space and regenerator that the regenerator effective temperature is an arithmetic mean of the heater and cooler temperature. Numerical simulation is performed and the effects of the regenerator effectiveness and dead volumes are studied. Results from this study indicate that the engine net work is affected by only the dead volumes while the heat input and engine efficiency are affected by both the regenerator effectiveness and dead volumes. The engine net work decreases with increasing dead volume. The heat input increases with increasing dead volume and decreasing regenerator effectiveness. The engine efficiency decreases with increasing dead volume and decreasing regenerator effectiveness.     

Improving load matching characteristics of a thermosyphonic solar system by thermostatically controlled circulation

The load matching characteristics of a thermosyphonic solar water heater can be improved by utilizing a thermostatic flow control (TFC). Simulation of the performance of a typical thermosyphonic domestic solar heater, with and without a TFC, was used to evaluate the yearly requirement of auxiliary energy to meet four different loads. The amount of yearly auxiliary energy required to fully match the load demands is used as a measure of the matching improvement.Results indicate that, for load temperatures of 40°, 60°, and 70°C, the thermostatic flow controller improves the system's (multi- or single-pass) performance, while the common single-pass system without the thermostatic flow controller is the best choice for a 50°C load temperature.     

Year round performance studies on a built-in storage type solar water heater at Jodhpur, India

An improved solar water heater (capacity 901) made up of a 112×80×10 cm rectangular tank which performs the dual function of absorbing heat and storing the heated water has been designed and a prototype tested in Jodhpur. The performance tests carried out at the Central Arid Zone Research Institute, Jodhpur, indicate an efficiency factor reaching as high as 70 per cent. The year-round performance tests show that this heater can supply 901 of water at a mean temperature of 50 to 60°C in winter and 60 to 75°C in summer (measured at 4:00 p.m.). The performance tests also indicate that sufficient hot water can be obtained in the early morning if the heater is covered with an insulation blanket overnight or if the hot water is stored in an insulated tank.A performance equation for this type of heater, where the inputs are the solar intensity, ambient air temperature and geometry and material specifications of the heater, has also been developed. With this performance equation the optimum gap depth, i.e. the distance between upper and lower plate of the heater, has been found to be 10·0 cm.     

Evaluation and optimization of solar desiccant wheel performance

This paper presents the evaluation and optimization of a solar desiccant wheel performance. A numerical model is developed to study and discuss the effect of the design parameters such as wheel thickness, wheel speed, regeneration to adsorption area ratio, wheel porosity, and the operating parameters such as air flow rate, inlet humidity ratio of the air and regeneration air temperature on the wheel performance. It is also used to draw the performance curves of the desiccant wheel to quantify the optimum design parameters for certain operating conditions.Also, an open test loop for the desiccant wheel is constructed with appropriate control devices and measuring instruments. A perforated plate solar air heater of 2 m² area, together with an electric heater, is used as a source of energy to regenerate the desiccant material. The experimental tests are used to validate the numerical model and to evaluate the performance of the solar system and the desiccant wheel under actual conditions of Cairo climate (30° latitude).Comparison between numerical and experimental results shows good agreement between them, especially at low flow rates of air. Numerical results show that there is a maximum value of each design parameter at each operating condition, and above that no remarkable changes in the wheel performance are noticed. The results also show that there is an effective range of the air flow rate, due to which wheel performance becomes inefficient. This range is found to be between 1 and 5 kg/min. The performance curves of the wheel, which help to determine the humidity reduction ratio, are drawn for wheel speeds between 15 and 120 rev/h, dimensionless wheel thickness between 0.15 and 0.5, air flow rate equal to 1.9 and 4.9 kg/min, and regeneration temperature equal to 60 and 90 °C. These curves show that there is an optimum value of the wheel speed for each wheel thickness to obtain the best wheel performance for certain operating conditions.Experimental results show that the perforated plate solar air heater of 2 m² area can share about 72.8% of the total regeneration energy required at 1.9 kg/min air flow rate and 60 °C the regeneration air temperature. This value decreases to about 13.7% at a flow rate equal to 9.4 kg/min and regeneration temperature equal to 90 °C. The perforated plate solar air heater area required to completely fulfill the regeneration energy during the daytime is also calculated.     

Manufacturing and testing of a gamma type Stirling engine

In this study, a gamma type Stirling engine with 276 cc swept volume was designed and manufactured. The engine was tested with air and helium by using an electrical furnace as heat source. Working characteristics of the engine were obtained within the range of heat source temperature 700–1000 °C and range of charge pressure 1–4.5 bar. Maximum power output was obtained with helium at 1000 °C heat source temperature and 4 bar charge pressure as 128.3 W. The maximum torque was obtained as 2 N m at 1000 °C heat source temperature and 4 bar helium charge pressure. Results were found to be encouraging to initiate a Stirling engine project for 1 kW power output.     

Performance of a non-metallic unglazed solar water heater with integrated storage system

The performance of a new design of non-metallic unglazed solar water heater integrated with a storage system has been studied. In this system, the collector and storage were installed in one unit. All parts of the system have been fabricated from fiberglass reinforced polyester (GFRP) using a special resin composition that provides good thermal conductivity and absorptivity. The storage tank has a capacity of 329 l. The design of the storage system was sandwich construction, with the core material made out of polyurethane foam, which combines stiffness and lightness of structure with very good thermal insulation. The width and length of the absorber plat were 1.4 and 1.8 m, respectively. The performance of the system has been investigated by two methods. In the first method, the storage tank was filled up with water the night before the test. The tank was then drained during the night, refilled and made ready for the next day’s test. The tests were repeated under varied environmental conditions for several days. The maximum water temperature in the storage tank of 63 °C has been achieved for a clear day operation at an average solar radiation level of 700 W m⁻² and ambient temperature of 30 °C. The decrease of water temperature with and without the thermal diode is 10 and 20 °C, respectively. In the second method, the testing was of the same way, but in this case without draw-off or draining of the hot water from the storage tank. All data readings were recorded from sunrise to sunset over the same period. The temperature was recorded for several days and ranges of 60–63 °C were obtained in the storage tank. A system efficiency of 45% was achieved at an average solar radiation level of 635 W m⁻² and ambient temperature of 31 °C.     

A solar air heater with composite–absorber systems for food dehydration

Development of appropriate technologies for conversion of solar radiation to thermal energy is essential for food preservation. A solar air heater, comprising two absorber systems in a single flat-plate collector, was designed on the principles of psychrometry. The heater was integrated to a drying chamber for food dehydration. This collector design offered flexibility in manual adjustment of the thermal characteristics of the solar dryer. The performance of the dryer was evaluated by drying fresh samples of mango (Mangifera indicus). Both fresh and dried mango samples were analysed for moisture content (MC), pH and ascorbic acid. During the dehydration period, meteorological measurements were made. The air heater converted up to 21.3% of solar radiation to thermal power, and raised the temperature of the drying air from about 31.7 °C to 40.1 °C around noon. The dryer reduced the MC of sliced fresh mangoes from about 85% (w/w) to 13% (w/w) on wet basis, and retained 74% of ascorbic acid. It was found that the dryer was suitable for preservation of mangoes and other fresh foods.     

Performance calculations for closed-loop air-to-water solar hybrid heating systems with and without a rock bed in the solar air heater

A transient analysis has been carried out on a hybrid solar water heater which comprises a rock bed air heater with optimum design parameters in conjunction with an air-to-water transverse fin shell-and-tube heat exchanger (mixed air and unmixed water type) in which cold water from the storage tank receives heat from the hot air coming out of the air heater which flows in the shell at right angles to the water flowing in the tubes. The system's performance has been evaluated for typical winter weather conditions in Delhi for different combinations of flow rates of air and water for different volumes of the water storage tank. No hot water is assumed to be withdrawn from the tank to serve the load. A comparative analysis of the system's performance with and without a rock bed in the air heater reveals about 11°C higher temperature of storage tank in the former at 50 kg/h m² air flow rate. With both the air heater types, although the system performance was observed to increase with the rates of air and water flow, no significant improvement in system performance was achieved at .     

Technical note Best connection scheme of collector modules of thermosyphon solar water heater operated at high temperatures

Large scale thermosyphon solar water heater for high temperature applications is simulated by the use of the Transient Simulation Program (TRNSYS). A daily hot water load of 1500 l/day and 2500 l/day at 80°C was assumed. The hot water is consumed daily from 08·00–17·00 h. A back-up electric auxiliary heater was added to the system in two schemes: first, located inside the storage tank with a thermostat; second, outside the tank connected to the heating system between the tank and the facilities. The collector modules were connected in five different schemes: first, all collectors were connected in series in one line, or collectors were connected in two, three, four or five parallel lines each consisting of many collectors. The results showed that the best connection is when the 20 collectors, comprising the system, are connected in two parallel lines each consisting of 10 collectors. It was found that the monthly and yearly useful energy from the system was higher when the auxiliary water heater was added to the system outside the storage tank.     

Some theoretical and experimental aspects of built-in solar storage

The possibility of using a built-in storage solar water heater in Jordan has been investigated. Experimental and theoretical results were obtained using a 90 × 90 × 10 cm solar heater which was tilted at an angle of 30° to the horizontal.It was found that the efficiency of the built-in storage solar heater may reach as high as 78% with a maximum increase in the water temperature of 70°C and at low cost compared to the conventional flat plate collector. It is therefore expected that such solar system may be used to provide reasonably sufficient hot water in Jordan. Analytical solution, based on the assumption that the water temperature equals that of the plate, was used to solve the governing equations. While a numerical solution was used to solve the equations under the condition that the plate temperature is not equal to that of the water.It was found that the assumption of the temperature of the plate (T_p) is equal to the temperature of water (T_w) is only justifiable at early hours of operation in the morning.     

Solar assisted heat pump

The paper describes a series solar heat pump, using Freon 11 as the working fluid. The heat pump is specifically designed for use in a tropical climate where the normal daytime ambient of above 25°C permits the evaporator to be operated at a high temperature (15–50°C depending on solar input). The use of Freon 11 permits conventional reciprocating refrigeration compressors to be used at elevated temperatures without exceeding design pressure limits. A single unit acts as the evaporator and solar collector. When solar insolation is low the evaporator pressure automatically drops so that energy is received from the atomsphere. However the C.O.P. and output are so low in this mode that the system cannot correctly be termed dual source. The water cooled condenser operates in the temperature range of 35–90°C, the heated water representing the useful output of the system. Operation in the air conditioning mode is not possible due to the large specific volume of Freon 11 at low temperatures. A theoretical analysis is presented to describe the system operation, and the experimental results are shown to agree well with the computer simulation. Average values of C.O.P. of between 2.5 and 3.5 were obtained for the small prototype developed with high side storage temperatures of up to 80°C.     

Optimizing the tilt angle of solar collectors

Solar collectors need to be tilted at the correct angle to maximize the performance of the system. In this paper, the annual solar fraction of the system (the fraction of energy that is supplied by solar energy) is used as an indicator to find the optimum inclination angles for a thermosyphon solar water heater installed in northern and southern parts of Jordan. Calculations are carried out using the powerful computer program TRNSYS (Transient System Simulation). The system is assumed to operate with a daily hot water load of 150 l at 55°C flowing during the day according to the widely used Rand consumption profile. The results show that the optimum inclination angle for the maximum solar fraction is about φ+(0→10°) for the northern region (represented by Amman) and about φ+(0→20°) for the southern region (represented by the town of Aqaba). These values are greater than those for maximum solar radiation (which is commonly used as an indicator) at the top of the collector by about 5 to 8°.     

Thermal response of solar and conventional school buildings to design- and human-driven factors

The objectives of this work are (I) to describe the design and building technology details of a solar school built in a continental semiarid region of central Argentina, (II) to show data from energy, hygrothermal and environmental monitoring, and (III) to compare the behavior of a solar and a conventional school building in response to design and human management factors. Both buildings are located on 36° 24′ latitude and 63° 25′ longitude. With respect to the solar school, good outdoor climate conditions during the study period in winter, plus unnecessary auxiliary heating, have led to indoor temperature increasing beyond 24 °C under real use conditions. Similarly, overheating of functional areas in autumn and spring was the result of temperate outdoor conditions, uncontrolled use of auxiliary heating and internal gains. Even without incoming sunlight, indoor temperature reached 27 °C on some days in summer, while outdoor temperature peaked to 35 °C. Based on the parameters of ISO 7730, more than 10% of people experienced discomfort when temperature exceeded 25 °C. In contrast, an average indoor temperature of 16 and 21 °C was recorded, respectively, for the conventional and the solar building. However, the consumption of natural gas for heating was 50% less in the solar school. Furthermore, this consumption could also be smaller if we take into account that a large proportion of people expressed discomfort at noon because of overheating during the winter time. This demonstrated once more that proper use of heating devices is a very important factor to be considered in solar designs. The work allowed a good validation of methods to assess energy gains and losses during the winter.     

Binary conversion cycles for concentrating solar power technology

It is recognized that the temperature potential of concentrated solar energy is much higher than needed by standard conversion cycles. High temperature solar receivers are in the development stage hopefully leading to the use of solarized gas turbines or of solar combined cycles. These systems are analyzed and taken as a reference standard. Binary alkali-metal steam cycles are shown to be intrinsically more efficient than combined cycles owing to their fully condensing nature. Even at top temperatures of about 600 °C typical for steam cycles the binary cycle allows, in principle, a significant efficiency gain (49.5% against 43% of a steam cycle). However, the binary high temperature systems are investigated featuring either a direct vaporization of the metal within the receiver or a liquid receiver cooling loop with the working fluid vaporized in a proper heat exchanger.With reference to the second option, the computed efficiency is 56% at a top cooling loop temperature of 1000 °C (the same efficiency is attained in a direct vaporization loop at 720 °C). A 60% thermal efficiency is within the potential of the technology. The above figures can be compared with a combined cycle efficiency of 50% at 1200 °C turbine inlet temperature.Available alkali metals are reviewed for the use of working fluid: potassium being the best known fluid but rubidium (or cesium) offering, in perspective, a better overall performance. Material problems connected with the containment of alkali metals at high temperature are reviewed. Experimental evidence suggests that up to 800–850 °C stainless steel is an adequate material, while for higher temperatures, up to 1200 °C, refractory metals should be used.With reference to heat storage the availability of appropriate high temperature substances either as liquids or as melting solids, storing energy as sensible or as latent heat respectively, is discussed.Finally the critical issue of metal vapour turbine design is considered. The results of a number of computations are presented giving the basic geometrical data of some potassium, rubidium and cesium expanders. Rotor diameters tend to be comparatively large. With reference to a 50 MW overall plant output the maximum tip diameter is 3.9 m for a potassium and 2.8 m for a rubidium turbine.     

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.     

Design and fabrication of low cost solar water heaters

We designed two types of very low cost solar water heaters which do not need a water supply connection. The first one consisted of two plastic bowls, one inside the other with 5 cm thickness of insulation in between. A transparent plastic cover was tied around the smaller vessel. It was found that water placed inside the smaller vessel had its temperature raised by 18°C from the ambient temperature within four hours when the average insolation was about 600 kWh/m². The second one consisted of two earthen vessels in place of plastic bowls. The water temperature rise was 20°C under the same radiation. If either of the heaters was covered with cotton-wool insulation in the afternoon, the temperature remains 13°C above the ambient early next morning. The cost of the material was around US$ 4.50 for the plastic bowl heater while this is around US$ 3.50 for the earthen vessel water heater.     

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.     

Design and performance of a large-size solar water heater

The paper reports the design details of a solar water heater suitable for the large, intermittent demands for hot water by hospitals and hostels. It employs a flat-plate collector consisting of a wire-tied aluminium fin of 28 gauge with galvanized iron pipes of 19 mm diam spaced at 10 cm centres. The unit is adjusted to give maximum efficiency per unit cost under Indian conditions. Various arrangements for connecting the collectors, such as cascade, series, series parallel, and true parallel, were experimentally studied. This revealed that for a solar water heating system having a large number of absorber banks, the true parallel arrangement yields maximum efficiency and economy.The system is fully-automatic and heats 600 l. of water up to 55°C in winter months at Roorkee. Heat losses at night may be compensated for, if required, by an auxiliary electric heater provided in the storage tank and controlled by a low-cost radiation-sensing device. A simple electric circuit controls the tank mean temperature of water in the storage tank.Required collector areas based on meteorological observations for various water capacities and water temperatures are given for several Indian cities.