Theoretical and experimental investigation of a solar-biomass hybrid air heating system for drying applications | |
| Author | Leon, Mathias Augustus |
| Call Number | AIT Diss. no.ET-07-04 |
| Subject(s) | Air heaters Solar heating Solar thermal energy |
| Note | A dissertation submitted in partial fulfilment of the requirements for the degree of Doctor of Engineering in Energy Technology, School of Environment, Resources and Development |
| Publisher | Asian Institute of Technology |
| Series Statement | Dissertation ; no. ET-07-04 |
| Abstract | The overall objective of this research is to design, develop and investigate the performance of a renewable energy-based (solar-biomass) hybrid air heating system. The system consists of an unglazed transpired solar collector (UTC), a rock bed thermal storage, and a biomass gasifier stove with heat exchanger, to supply hot air at a required temperature and flow rate for a daily load fraction exceeding 90%. From a review of literature, an air heating system, aimed at reducing the weather dependency and improving the temperature and flow rate stability without a conventional back-up heater was designed. Among the various types of solar collectors, thermal storage, biomass stoves and heat exchangers that are generally used, specific designs were chosen and analysed. Based on the analyses, individual components of the air heating system were designed and developed. The components were coupled, and detailed experimentation carried out on the integrated system. A mathematical model was developed to predict the thermal performance of unglazed transpired solar collectors for a wide range of design and operating conditions. The influence of porosity, airflow rate, solar radiation, and solar absorptivity/thermal emissivity on the collector efficiency, heat exchange effectiveness, air temperature rise and useful heat delivered were studied. The observations indicate that solar absorptivity, collector pitch and approach velocity have the strongest effect on collector heat exchange effectiveness as well as efficiency. The effect of thermal emissivity and porosity on heat exchange effectiveness seems to be moderate. The results of the model were used to develop nomograms, to assist in the design and analyses of the thermal performance of UTC. It also enables the prediction of the thermal performance of a UTC for a given set of operating conditions. Fifteen sets of experiments were carried out on the transpired solar collector, to compare the results with the model predictions, and to validate the model. The experimental results correspond reasonably well with the model predictions especially at high flow rates. At 150 m3/h, the experimental results are in closer agreement with the model predictions, with 9.1 % reduction in temperature rise compared to model prediction. At lower flow rates of 90 and 120 m3 /h, the air temperature rise is 22.3% and 14.0% lower than the model predictions due to wind effect on the absorber, which has not been incorporated in the model. For the same reason, collector efficiency predicted by the model corresponds closely with experimental results at high airflow rates. A deviation of only 6.1% was observed for an airflow of 150 m3/h, while the deviations were 8.9% and 14.5% for 120 and 90 m3/h respectively. A rock bed was designed and fabricated for the proposed air heating system, and its performance analysis carried out. The vertical rock bed, with a volume of 0.7 m3 , contains round white pebbles of 25 mm size. The thermal performance of the rock bed was analysed using parameters such as airflow rate, charging time and bed temperature profile to estimate the capacity of the bed to meet specific load conditions. Time step simulation calculations utilizing the component models available from TRNSYS (TRaNsient SYstem Simulation Program of the University of Wisconsin, USA) were performed for this analysis. Results were used to find the relationship between the charging airflow rate and charging time during charging, and hot air supply duration as a function of bed temperature during extraction. From this relationship, it was found that to get a steady supply of hot air at 55°C overnight (i.e., for 16 hours) at a flow rate of 90 m3/h, the rock bed needs to be charged to 73 °C. A biomass gasifier stove-heat exchanger unit was also designed and fabricated to supply 3.27 kW of heat to the rock bed. The gasifier stove is of cross-flow type, which operates on natural draft, and uses wood chips as fuel, which is consumed at an average rate of 3 kg/h. The cross-flow fin-and-tube type heat exchanger, designed to supply hot air at 120°C at a flow rate of 134 kg/h ensures the supply of clean air, free of soot and other products of combustion from the biomass stove into the rock bed. Average overall heat transfer coefficient for the heat exchanger was estimated at 9.98 W/m2 -K. The air heating system does not use conventional auxiliary heaters (which generally use fossil or electrical energy), but instead utilises part of the rock bed to supply supplementary heat. The UTC supplies the required hot air during the day (to meet the load), and the stove-heat exchanger unit supplies hot air to the rock bed (charging), also during the daytime. The rock bed stores the thermal energy during the daytime, and supplies heat during off-sunshine hours - both during day and night. The performance of the integrated system was analysed for six load conditions and two weather conditions. Charging and extraction were simulated in TRNSYS for different load conditions and the bed temperature profile and charging/extraction time plotted for comparison with experimental results. The air heating system could supply hot air at 55°C at a flow rate of 90 m3/h continuously during day and night, at both sunny/partially cloudy, and fully overcast weather conditions when the solar collector may not be operated. Temperature of delivery air could be controlled within ±3°C for upto 18 hrs in 24-hour system operation. An average daily load :fraction of 90% was obtained for the optimum load conditions 55°C/90 m3/h and 60°C/70 m3/h by the air heating system. In three of the total twelve experiments, daily load fraction exceeded 90%. The air heating system was tested by drying 22 kg of red chilly, by supplying hot air at 60°C and 90 m3 /h to a drying chamber loaded with chilli. The product was dried from an initial moisture content of 76.7% (w.b) to a final moisture content of 8.4% over a period of 32.5 hours of continuous drying. The system provided a load fraction of 91 .6% during the initial 24-hours of operation, and contributed to a reduction in drying time of about 66% in comparison with open sun drying. The temperature of hot air supplied by the air heating system was stable at 60±3°C for over 24 hours during the 32.5 hour duration of drying. An evaluation of the air heating/drying system developed (based on an evaluation procedure developed within this study) indicated that the system offers the best performance in comparison with a cabinet type solar dryer and open sun drying. The improvement in quality of dried chilli in comparison with chilli dried in the open sun as well as commercial drying was also distinctly recognised. Using only renewable solar and biomass energy for its operation, the air heating system thus presents itself as a reliable alternate to other renewable energy-based dryers with conventional backup heaters, for medium-scale drying of food products. Additional contributions from the present study include new data on UTC performance under tropical conditions for drying applications, nomograms for use by collector/dryer designers to design and analyse UTC performance for drying applications, and a comprehensive procedure for evaluation of renewable energy-based dryers. This dissertation is the first reported study on the performance of an air heater with integrated UTC, biomass stove and thermal storage. It is also the first reported study of a UTC-based solar-biomass hybrid air heating system without conventional backup heater that can provide a daily load fraction of over 90% for day and night operation. |
| Year | 2007 |
| Corresponding Series Added Entry | Asian Institute of Technology. Dissertation ; no. ET-07-04 |
| Type | Dissertation |
| School | School of Environment, Resources, and Development (SERD) |
| Department | Department of Energy and Climate Change (Former title: Department of Energy, Environment, and Climate Change (DEECC)) |
| Academic Program/FoS | Energy Technology (ET) |
| Chairperson(s) | Kumar, S.; |
| Examination Committee(s) | Athapol Noomhorm ;Surapong Chirarattananon ;Dutta, Animesh ;Mujumdar, Arun S. ; |
| Degree | Thesis (Ph.D.) - Asian Institute of Technology, 2007 |