SUMMARY AND CONCLUSION

Transcription

1 5 SUMMARY AND CONCLUSION Energy in the form of heat is required for diverse applications in various sectors including domestic, agricultural, commercial and industrial sectors. As there is a gap between demand and supply of thermal energy, it is essential not only to design and fabricate novel evacuated tubular systems but also to evaluate the thermal performance of existing evacuated tubular systems so as to match the demand and supply of heat energy. In this connection, the present investigation was carried out with specific objectives in connection with design, development and experimental evaluation of the thermal performance of the evacuated tube assisted systems. The framed objectives of the present investigation included (i) measurement of the dimensions of the components of evacuated tube assisted solar thermal systems, (ii) assessment of the optical and thermal characteristics of the components of evacuated tube assisted solar thermal systems, (iii) estimation of the thermal performances of evacuated tubular solar hot water systems of different dimensions, (iv) evaluation of the thermal characteristics of evacuated tubular solar hot water systems with different coatings on tubes, (v) evaluation of the thermal characteristics of evacuated tubular solar hot water systems with different reflectors, (vi) assessment of the thermal characteristics of evacuated tube assisted solar air heating systems and (vii) assessment of the thermal characteristics of evacuated tube based solar cooling systems 214

2 All these objectives have been materialized in the present investigation by adopting standard procedures. The results of the experimental investigations have been documented in the thesis. The generated database of results on the thermal characteristics and performances of the evacuated tube coupled solar thermal systems would be beneficial to the manufactures, marketing executives, researchers and end - users so as to produce, promote and apply this solar thermal technology effectively. Experimental investigations on solar components and collectors The measurements on components and collectors were made by using calibrated instruments. It was found that the thickness of glass of the evacuated tubes of current concern was 1.6 mm each. The inner and outer tube diameters were 47 and 58 mm respectively. The length of the same evacuated tubes was found to be 1500 mm each. All these dimensions were found to be the specific dimensions prescribed in standards. It was also found that the thicknesses of the glass cover, absorber sheet, cover foil, bottom sheet, support retaining the glass cover and the channel section of the flat plate collectors of current concern were 4.37, 0.02, 0.01, 0.76, 1.20 and 1.60 mm respectively. The diameter of the riser and header were and mm respectively. The dimensions of the flat plate collectors were found to be 1860, 1240 and 2120 mm respectively. All these dimensions were found to adhere the specifications prescribed in Indian Standards (BIS, 2003). These dimensions of components would have been selected so as to have simplicity in design, easiness in fabrication and good thermal performance. As has been mentioned earlier, evacuated tubes and reflectors are the two integral components of evacuated tubular collectors. By the by, the evacuated tubes 215

3 were tested in connection with the estimation of their stagnation temperatures. It was noticed that Cu / SS / Al - N / Al coated evacuated tube had the stagnation temperature of C, where as Al - N / Al / Black chrome coated evacuated tube had the stagnation temperature of C and stagnation temperature of Al - Zn coated evacuated tube had the 0 C. The evacuated tube coated with Cu / SS / Al - N / Al was found to have relatively higher stagnation temperature than that of the same stagnation temperatures of all other tubes of current concern and so this tube would be preferred in the fabrication of evacuated tubular collectors. In fact, the reflectors were tested in connection with the estimation of their reflectances. It was noticed that plain aluminium, corrugated aluminium, corrugated stainless steel, plain anodized aluminium and corrugated anodized aluminium had the average reflectance of 74.2%, 77.3%, 71.3%, 79.2% and 82.3% respectively. While comparing the reflectances of plain and corrugated reflectors of the same materials, it was observed that the corrugated reflector had higher values of reflectances. As corrugated anodized aluminium reflector had the highest reflectance among all the reflectors of current concern, corrugated anodized aluminium reflector would be utilised in all evacuated tubular collector based solar thermal systems for their effective applications. In the case of flat plate collectors, the glass cover, absorber plate and insulation material are the integral components. All these components were tested and the results have been documented. In all the glass covers examined, the plain and toughened glass covers in solar collectors were found to be free from bubbles and rough surfaces. The transmittance was found experimentally in the two square meter plain and toughened glass covers for five times and the average value was taken. While the transmittance of plain glass cover was 77%, the transmittance of toughened glass cover was 82%. The enhancement of transmittance could be correlated with the chemical nature of 216

4 toughened glass cover. It would be worth mentioning that the toughened glass cover was specially prepared so as to have enhanced transmittance of solar radiation that would be required for solar collectors. The reflectance was also found experimentally in the two square meter plain and toughened glass covers for five times and the average value was taken. While the reflectance of plain glass cover was 15%, the reflectance of toughened glass cover was 11%. It would be again worth mentioning that the toughened glass cover was specially prepared so as to have reduced reflectance of solar radiation that would be required for solar collectors. While the absorption in plain glass cover was 8%, the absorption in toughened glass cover was 7%. It was reported that the toughened glass covers were highly resistant to breakage both from thermal cycling and natural events. The optical reflectance of black coated absorber plate with variations in wave length of radiation was found by using UV - Vis spectrophotometer. The optical absorptance and thermal emittance of the black coated absorber plate was calculated by using suitable formulae. The optical absorptance was found to be 95%, whereas the thermal emittance was found to be only 5% at the operating temperatures. In the absorbers of current concern, it was noticed that the black chrome selective surfaces were deposited by electro-deposition process. It was already reported that the black chrome selective surfaces that were deposited by electro - deposition process had good thermal stability. So, black chrome selective surfaces could be used in absorber plates, as they had high optical absorptance, low thermal emittance and good thermal stability. The structure of the deposits on the absorber plate was studied by X - ray diffractometer with the usage of suitable radiations. In general, it was reported that the stable selective surface would be made by electroplating a layer of bright nickel on the absorber plate. Then, an extremely thin layer of chromium oxide on the nickel substrate would be 217

5 electro deposited Black chrom [Cr - Cr 2 O 3 ] coatings for very high temperatures. The results from the X - ray diffractometer showed, in addition to the well defined peak for nickel as substrate, peaks for Cr 2 O 3 and chromium particles. This clearly established the composite nature of the coating that was composed of Cr 2 O 3 and chromium particles on nickel substrate. Neither there was blistering and rupture nor peeling of the coated surfaces on the absorber plates of collectors during the temperature test on the absorber plates. In the solar collectors examined, it was noticed that only the rock wool was used insulation material. It was reported that the bulk density of the rock wool used in solar collector was ± 15%. While the sulphur content was 0.01%, the chloride content was %. The moisture content was 0.27%. Thermal insulation of insulation material was 0.96 m 2 0 C/W. As per the specification of Bureau of Indian Standards, the thermal resistance and conductance of rock wool were found to be satisfactory. The experimental part was carried out on two different samples of solar collectors. While the plain glass was used in one solar collector, toughened glass cover was used in another solar collector. The copper material was used for absorber plate in one solar collector, whereas the aluminium material was used for absorber plate in another solar collector. The aluminium material was used for angle sections, channel sections and bottom sheets in both these collectors. In the case of solar collector with toughened glass cover and copper absorber plate, the outlet temperatures of the water were found to increase by 6.4 to C over the inlet water temperature of 30 0 C. They were 7.0 to 7.6, 5.8 to 6.6 and 6.0 to C when the inlet temperatures of the working fluid were 40, 50 and 60 0 C respectively. The corresponding average thermal efficiencies were 69, 68, 64 and

6 percent. The overall thermal efficiency was found and it was 70%. The heat losses were also found and it was 3.32 W/m 2. In the case of solar collector with plain glass cover and aluminium absorber plate, the outlet temperatures of the water were found to increase by 6.4 to C over the inlet water temperature of 30 0 C. They were 6.1 to 6.6, 5.3 to 5.8 and 4.8 to C, when the inlet temperatures of the working fluid were 40, 50 and 60 0 C respectively. The corresponding average thermal efficiencies were 64, 62, 54 and 51 percent. The overall thermal efficiency was found and it was 65%. The heat loss was also found and it was 4.10 W/m 2. By consolidating all the results, it can be stated that (i) all the dimensions of components and collectors were as per specifications prescribed in Indian Standards, (ii) Cu / SS / Al - N / Al coated evacuated tube had relatively higher stagnation temperature than all other evacuated tubes with other chemical coatings, (iii) corrugated anodized reflector had relatively higher reflectance than all other reflectors, (iv) collector with copper fins and toughened glass cover had better thermal performance than all other collectors. On the basis of the generated database, it could be concluded that Cu / SS / Al - N / Al coated evacuated tube, toughened glass cover, anodized aluminium reflector and copper fins would be preferred in solar heating and cooling systems so as to have enhanced thermal performances in the application sectors. Experimental investigations on evacuated tube assisted water and air heating systems Evacuated tubular collector integrated solar water heating systems were designed as per standard specifications. They were fabricated with suitable materials as per the requirements of heat energy of the end users. All these solar water heating systems with different dimensions, coatings on evacuated tubes and materials of 219

7 reflectors were tested by adopting the procedures set by Ministry of New and Renewable Energy (MNRE), Government of India. In addition, novel evacuated tubular collector integrated solar air heating systems (pilot scale and enlarged scale) were designed. They were fabricated with suitable components so as to have effective generation of hot air for the benefits of end users. Both these pilot scale and enlarged scale solar air heating systems were tested in the field by adopting standard procedures. As far as measurements of dimensions of components were concerned, all the measurements were carried out by using calibrated measuring instruments. It was found that all these dimensions of components were found to adhere the specifications prescribed in Indian Standards. All the heating systems such as heating systems with variations in dimensions (50 lpd, 100 lpd, 200 lpd and 300 lpd), heating systems with variations in materials of reflectors (aluminiuum band, stainless steel band, anodized aluminium band, plain anodized aliminium reflector, corrugated anodized aluminium reflector, corrugated aluminium reflector and corrugated stainless steel reflector), heating systems with variations in coatings in evacuated tubes (Al - Zn, Al - N / Al - Black chrome and Cu / SS / Al - N / Al coating ) and heating systems with variations in shape (T - shape and H - shape) were tested. As an initial part of the experimental work, 50 lpd, 100 lpd, 200 lpd and 300 lpd solar water heating systems were designed and fabricated. Of course, it was reported in a MNRE project report that these 50, 100, 200 and 300 lpd systems were generally preferred by common public for domestic applications. In the case of 50 lpd system, the temperature elevation of working fluid varied from 16.2 to C with the 220

8 corresponding thermal efficiency variation of 28 to 49%. As far as 100 lpd system was concerned, the temperature elevation of working fluid ranged between 16.4 and C with the corresponding thermal performance variation of 28 to 48%. It was noted in the 200 lpd system that minimum temperature elevation was C and the maximum temperature elevation of working fluid was C with the corresponding thermal efficiency variation of 28 to 47%. It was also noted in 300 lpd system that minimum temperature elevation was C and the maximum temperature elevation of working fluid was C with the corresponding thermal efficiency variation of 33 to 43%. It was obvious from the results pertaining to the thermal performances of evacuated tubular hot water systems with different dimensions that there were mostly trivial variations in their thermal performances. As an integral part of the experimental work, the evacuated tubular hot water system without the reflector was tested. In addition, the evacuated tubular hot water system with aluminium band, stainless steel band, anodized aluminium band, plain anodized aluminium reflector, corrugated anodized aluminium reflector, corrugated aluminium reflector and corrugated stainless steel reflector was tested. The experimental results revealed that the thermal efficiency of evacuated tube based hot water system without reflector was the lowest while comparing with the thermal performance of evacuated tube based hot water system with all other reflectors. It was noted in the aluminium band reflector integrated heating system, the temperature elevation of working fluid ranged between 13.7 and C with the corresponding thermal performance variation of 26 to 40%. It was also noted in the stainless steel band integrated heating system that the minimum temperature elevation was C and the maximum temperature elevation was C with the thermal efficiency variation of 30 to 53%. It was noticed in the anodized aluminium band integrated heating system that 221

9 the temperature elevation of working fluid varied from 8.4 to C with the corresponding thermal efficiency variations of 30 to 44%. It was also noticed in the plain anodized aluminium reflector based heating system that the temperature elevation of working fluid ranged between 15.6 and C with the corresponding thermal performance variation of 26 to 51%. It was observed in the corrugated anodized aluminium reflector integrated heating system, the minimum temperature elevation was C and the maximum temperature elevation of working fluid was C with the thermal efficiency variation of 32 to 53%. It was also observed in the corrugated aluminium reflector system that the temperature elevation of working fluid varied from 18.1 to C with the corresponding thermal efficiency variation of 32 to 42%. As far as corrugated stainless steel reflector system was concerned, the temperature elevation of working fluid ranged between 9.0 and C with the corresponding thermal performance variation of 34 to 46%. It was obvious from the results related to thermal performances of evacuated tubular hot water systems that were attached with reflectors of different materials that there were substantial variations in their thermal performances. It was observed that plain reflector integrated hot water systems had the relatively lower thermal performance than that of the corrugated reflector integrated hot water systems. On the basis of generated database, it was experimentally perceived that the solar hot water system with corrugated aluminium reflectors had better thermal efficiency than that of the system with all other reflectors So, it is recommended to utilize corrugated anodized aluminium absorber as reflecting material in the evacuated tube based hot water system so as to have its effective utilization. As an another integral part of experimental work, solar water heating systems were fabricated with evacuated tubes of Al / Zn, Al - N / Al - Black chrome and Cu / SS / Al - N / Al coatings. It was noted in the Al - Zn coating absorber integrated 222

10 system that minimum temperature elevation was C and the maximum temperature elevation of C with the thermal efficiency variation of 30 to 42%. It was also noted in the of Al - N / Al - Black chrome coating absorber integrated system that the temperature elevation of working fluid varied from 8.2 to C with the corresponding thermal efficiency variation of 36 to 48%. As far as Cu / SS / Al - N / Al coating system was concerned, the temperature elevation of working fluid ranged between 17.0 and C with the corresponding thermal performance variation of 35 to 54%. So, it is recommended to utilize Cu / SS / Al - N / Al coating based evacuated tubular collector in the evacuated tube based hot water system so as to have enhanced thermal performance. As a final part of the experimental work, T - shape and H - shape solar water heating systems were fabricated. The thermal performances of all of these heating systems were also experimentally estimated. It was noted in the T - shape system that the temperature elevation of working fluid ranged between C and C with the thermal efficiency variation of 28 to 45%. It was also noted in the H - shape system that the temperature elevation of working fluid varied from 16.5 to C with the corresponding thermal efficiency variations of 36 to 52%. It was obvious from the results related to thermal performance that H - shape system had better thermal performance than that of the T - shape system. So, it is recommended to utilize H-shape heating system so as to reap the benefits such as enhanced thermal performance and reduced thermal losses. In the present investigation, evacuated tubular air heating systems with different dimensions were designed, fabricated and tested. A pilot scale and enlarged version of air heaters with (i) aluminium air carrier without black coating, (ii) copper air carrier 223

11 without black coating, (iii) aluminium air carrier with black coating and (iv) copper air carrier with black coating were designed and fabricated. Incidentally, the developed air heaters were tested by using these air carriers namely non - coated aluminium tube, coated aluminium tube non - coated copper tube and coated copper tube as air carriers. In addition, they were tested not only with coated aluminium tube and reflector but also with the same coated copper tube and reflector. All the influencing parameters during the experimental period were monitored. It was noticed in non - coated aluminium tube based air heater that outlet temperature of working fluid ranged between 39.3 to C with the corresponding temperature elevation of 0.9 to C. It was also noted in the coated aluminium tube based air heater that minimum temperature elevation was C and the maximum temperature elevation was C with the outlet temperature of working fluid variation of 38.5 to C. It was noticed in non coated copper tube with reflector based air heater that the outlet temperature of working fluid ranged between 37.4 to C with the corresponding temperature elevation of 2.1 to C. It was also noticed in coated copper tube based air heater that the temperature elevation of working fluid varied from 2.9 to C with the corresponding outlet temperature variations of 38.2 to C. It was observed that in coated aluminium tube with reflector based air heater that outlet temperature of working fluid ranged between 39.2 to C with the corresponding temperature elevation of 3.0 to C. It was also observed in the coated aluminium tube with reflector based air heater that minimum temperature elevation was C and the maximum temperature elevation of C with the outlet temperature of variation of working fluid in the range of 40.3 to C. 224

12 As far as non coated aluminium tube integrated air heater was concerned, the outlet temperature of working fluid ranged between 46.2 to C with the corresponding temperature elevation of 9.8 to C. It was noted in the coated aluminium tube integrated solar air heater that minimum temperature elevation was C and the maximum temperature elevation of C with the outlet temperature of working fluid variation of 48.1 to C. It was also noted in non coated copper tube integrated solar air heater that the temperature elevation of working fluid varied from 11.5 to C with the corresponding outlet temperature variations of 49.8 to C. It was noticed in coated copper tube integrated solar air heater that the outlet temperature of working fluid ranged between 43.7 and C with the corresponding temperature elevation of 8.7 to C. It was also noticed in the coated aluminium tube integrated air heater with reflector that the minimum temperature elevation was C and the maximum temperature elevation was C with the outlet temperature of working fluid variation of 48.5 to C. In the case of coated copper tube integrated solar air heater with reflector, the temperature elevation of working fluid varied from 13.2 to C with the corresponding outlet temperature variations of 45.3 to C. The evacuated tube systems of current concern showed higher thermal performances than those of conventional flat plate collector based hot air system. The thermal performance of evacuated tube integrated solar air heater (pilot scale) was found to vary from 14.4% to 41.4%. The same thermal performance of enlarged version of evacuated tubular air heater was found to range between 13.2% and So, it is recommended to utilize (i) black coated copper tube as air carrier, (ii) anodized aluminium reflector and (ii) Cu / SS / Al - N / Al coating based evacuated tubular collector in the evacuated tube based hot air system so as to have enhanced thermal performance. It is also recommended to 225

13 utilise evacuated tube assisted solar collector instead of conventional flat plate air heater so as to have elevated thermal performance. As the thermal analyses showed that the evacuated tubes, evacuated tube based solar water heating system and evacuated tube based solar air heating system had satisfactory thermal efficiencies, it could be concluded that the evacuated tube based solar water and air heater would be effectively used for low and medium temperature applications in energy-intensive sectors so as to have effective renewable energy utilization, effectual environment protection and sustainable development. Experimental investigations on evacuated tube assisted cooling systems In the present investigation, the test samples were flat plate and evacuated tubular collector integrated solar cooling systems. Both these cooling systems were tested in the field by adopting standard procedures. Experimental optimization of mass adsorbent was executed with standard size of collector area. In fact, the temperature at the cooling chamber with respect to the variation of masses of adsorbent was noted periodically and the average value was taken. It was found that 3.0 Kg was the optimized mass of the adsorbent in connection with the generation of low temperature in the cooling chamber. The optimized mass of the adsorbent was used in the cooling system and the temperatures at adsorbent bed, evaporator, condenser and cooling chamber were measured with variations in incident solar radiation due to their influence in the coefficient of performance of the cooling system. 226

14 In the case of evacuated tubular collector integrated solar cooling system, the temperature increase at adsorbent bed was found to be C, which was mainly due to the temperature rise of working fluid until C. While the temperature enhancement in condenser was C, the temperature enhancement in evaporator was noted to be C. The temperature reduction of C was observed in the cooling chamber with the corresponding variation in solar radiation. As far as flat plate collector integrated solar cooling system was concerned, the temperature increase at adsorbent bed was found to be C, which was mainly due to the temperature rise of working fluid until C. While the temperature enhancement in condenser was C, the temperature enhancement in evaporator was noted to be C. The temperature reduction of C was observed in the cooling chamber with the corresponding variation in solar radiation. Hence, the evacuated tubular collector connected cooling system had better thermal performance than the flat plate collector connected cooling system. The coefficient of performance of solar cooling systems was calculated by using suitable formulae. It was found that the coefficient of performance of flat plate integrated solar cooling system was It was also found that the coefficient of performance of evacuated tubular collector integrated solar cooling system was The enhancement in coefficient of performance of evacuated tubular collector integrated solar cooling system could be correlated with enhanced incident solar radiation on the collector (due to its spherical shape), enhanced transmittance of the glass cover due to the usage of borosilicate glass, increased absorptance of selective 227

15 coating, reduced heat losses due to the presence of vacuum and effective heat transfer to the working fluid. CONSOLIDATED RESULTS AND CONCLUSION In the case of experimental investigations on solar components and collectors, all the dimensions of components and collectors were as per specifications prescribed in Indian Standards. It was found that Cu / SS / Al - N / Al coated evacuated tube had relatively higher stagnation temperature than all other evacuated tubes with other chemical coatings. It was also found that corrugated anodized reflector had relatively higher reflectance than all other reflectors. The experimental investigation also revealed that solar collector with copper fins and toughened glass cover had better thermal performance than all other collectors. So, it could be concluded that Cu / SS / Al - N / Al coated evacuated tube, toughened glass cover, anodized aluminium reflector and copper fins would be preferred in solar heating and cooling systems so as to have enhanced thermal performances in the application sectors. As far as the experimental investigations on evacuated tube assisted solar fluid heating systems were concerned, all the dimensions of components and collectors were as per specifications prescribed in Indian Standards. The research revealed that the thermal performance of water heating system with Cu / SS / Al - N / Al coated evacuated tubes, water heating system with corrugated anodized reflector and water heating system with H - shape was better than that of all other interrelated water heating systems. The research also revealed that air heating system with coated copper carrier and anodized aluminium reflector had better thermal performance than that of all other interrelated air heating systems. As the thermal analyses showed that the evacuated tubes, evacuated tube based solar water heating system and evacuated tube based solar 228

16 air heating system had satisfactory thermal efficiencies, it could be concluded that the evacuated tube based solar water and air heater would be effectively used for low and medium temperature applications in energy-intensive sectors so as to have effective renewable energy utilization, effectual environment protection and sustainable development. In the case of evacuated tube assisted solar cooling systems, all the dimensions of components and collectors were as per specifications prescribed in prescriptions of commercial systems. The research revealed that the evacuated tubular collector connected cooling system could provide better cooling effect than the flat plate collector connected cooling system. As the thermal analyses showed that the evacuated tube assisted cooling system had satisfactory thermal efficiencies, it could be concluded that the evacuated tube based solar cooling would be effectively used so as to have effective renewable energy utilization, effectual environment protection and sustainable development. FUTURE SCOPE OF RESEARCH Solar thermal technology has very broad avenues for applied researches. On the basis of the generated database pertaining to the thermal characteristics of solar heating and cooling systems, the future scope of research has been presented. Evacuated tubular collectors may be used in the application sectors for the generation of hot air, production of desalinated water and cooking of food and the thermal performance of the evacuated tube assisted fluid heating, desalination and cooking systems may be estimated experimentally. Evacuated tubular collector based solar cooling systems may be developed not only with variations in adsorbents and refrigerants but also with variations in 229

17 design, development and deployment on the basis of the state-of- the-art technology. Nano coating on reflectors may be applied so as to increase the reflectance and hence the thermal performance of the solar heating and cooling systems. Evacuate tubes with nano coatings (with maximum absorptance and minimum reflectance) may be utilized so as to augment the thermal performance of solar heating and cooling system. Nano coating on evacuated tubes may be applied so as to reduce the reflectance and hence increase the thermal performance of solar heating and cooling systems By materialising these researches, the thermal characteristics and performances of evacuated tubes, evacuated tube based solar fluid (water and air) heating system and evacuated tube based solar cooling system can be further increased favourably. As a consequence, these solar thermal gadgets shall be effectively utilised energy-intensive sectors so as to reap the benefits like effective renewable energy utilization, effectual environment protection and economic development. 230