A study of upgrading and storage of solar thermal energy by solid adsorption chemical heat pumps | |
| Author | Wipawadee Wongsuwan |
| Call Number | AIT DISS. no. ET-03-01 |
| Subject(s) | Solar thermal energy Heat pumps |
| Note | A disse1iation submitted in partial fulfillment of the requirements for the degree of Doctor of Engineering, School of Environment, Resources and Development |
| Publisher | Asian Institute of Technology |
| Series Statement | Dissertation ; no. ET-03-01 |
| Abstract | Heat pumps transport heat from a region of low temperature to a region of high temperature and thus upgrade heat from renewable energy resources and waste heat to provide heating and cooling. Over 4,600 industrial heat pumps are currently used in the industrial sector in Europe, North America and Japan, and are commonly based on vapor compression or vapor absorption cycles. In recent years, chemical heat pump (CHP) or solid-vapor sorption system based on physical rather than chemical transformation are increasingly being considered as a viable option. In a solid-vapor sorption system, the compressor is replaced by an adsorber. The coupling uses solar thermal energy with CHP for any suitable application has the potential to improve efficiencies and reduce the fossil energy usage. This dissertation reports the study on the performance of zeolite-water solid adsorption chemical heat pump coupled to a solar water heater for energy storage and upgrading purpose. The research was carried out by conducting theoretical and experimental studies on a solar thermal system, solid adsorption system, and the coupled solar - solid adsorption system. The performance of a 12 m2 forced circulation solar water heating system by using the Transient System Simulation program (TRNSYS) and the Artificial Neural Network (ANN) to estimate the useful energy delivered and temperature of hot water produced was predicted. The hot water generated is the input for the evaporator of solid adsorption system. Experimental studies were carried out at various climatic conditions. A comparison of the experimental observations with the predictions by the two tools indicate that the maximum mean absolute error (MAE) and the root mean square error (RMSE) using TRNSYS were l.5°C and l.7°C respectively, while, in the case of ANN prediction, higher discrepancies were obtained. Overall, TRNSYS was found to predict better the performance of the SWH as compared to ANN, while ANN had the additional disadvantage that experimental observations were necessary over a wide range of operating conditions for training. Mathematical models were developed for solid adsorption heat pump system (NaX zeolite-water), solid-gas (ammonia) system, coupled solar-solid gas system, and the coupled solar-solid adsorption (zeolite water) heat pump system. The simulation results of the system · performance are indicated by the temperature profile of adsorber and peripheral components, coefficient of amplification, power production for heating, specific heat production, volumetric heat production, overall system performance, and exergetic efficiency. The variation of heating power of adsorber, fraction of adsorbent mass in adsorber, heat transfer fluid flow rate, insolation in a day, solar collectors area, storage tank volume, solar hot water or water withdrawal to load flow rate, cycle time and number of cycles in a day, size of adsorber and peripheral components were also studied. The results from the mathematical models also considered the influence of evaporation _ and regeneration temperature. The coefficient of amplification (COA) and the power production for heating (PPH) of the solid adsorption system are influenced by evaporation temperature and the heat source or regeneration temperature. The operation at higher evaporation temperature (55°C) gives a larger increase of adsorber temperature during adsorption period than that of lower evaporation temperature. Higher COA and PPH was obtained for a regeneration temperature range (164 °C - 174 °C). The simulation results also showed that the amount of heat accumulated in storage tanks and heat supplied to load increase with solar collector area and vary with climatic conditions. Heat input to the adsorber accounts for up to 20% of the total heat involved.Experimental studies on a coupled solar-solid adsorption system were carried out to compare and verify the theoretical predictions. The solid adsorption system was operated as an intermittent cycle. Hot water from a solar water heater circulated through the evaporator to provide heat of evaporation at temperatures of between 40 - 60°C. Hot water was also withdrawn from the coupled system to increase the storage capacity of the solar water heater, and to raise the solar output temperature to the adsorption-temperature of adsorption system. The results from the mathematical models were compared to the experimental results. The adsorption cycle was drawn on the Clapeyron's diagram, and was compared with the theoretical cycle. The calculated temperature profiles of adsorbent bed, inlet and otitlet heat transfer fluid, and power input and output for five cycles are comparable to the experimental profiles. Similarity between temperature and power profiles was observed in most of cycles. Most of the peaks during the heating period was similar, though a small deviation was found in the cooling period. By operating a number of adsorption cycles in a day, COA and PPH of the coupled solar-solid adsorption system ranged from 1.34 - 1.66, and from 1.28 - 1.50 kW, respectively. Compared to previous studies for single-effect adsorption system using the same adsorber, the COA was found to increase from about 1.50 to 1.66 in the present study, primarily due to its operation at higher evaporation temperature. The calculated results agree well with the experimental results. The mean bias error (MBE) and root mean square error (RMSE) of the power evolution obtained were 0.061 kW and 0.919 kW, respectively. The results for the coupled solar water heating system with solid adsorption system indicate that this system provides a higher efficiency than the adsorption system without the coupling, and is also environmental friendly. The low temperature output of the flat plate solar water heater can be increased to the higher level (80 - 110°C) by the adsorption heat pump. The tube-in-tube evaporator combined with the water tank gave a better performance than the case of evaporator without the water tank. This improves the COA of the adsorption system. About 17 % of the useful energy gained by the solar collectors is used for the evaporation of the working fluid. The coupled system that operated with more cycles and with load withdrawal provided 10-30% higher overall efficiency. For the application of the coupled solar/solid adsorption in a hotel, the optimum combination for energy storage and upgrading by CHP is by using waste heat at 160 - 180°C to regenerate the adsorber during its heating and desorption period. This temperature level can be supplied by hot flue gas from a conventional boiler. By applying the coupling used in this study, storage of waste energy as well as upgrading of solar thermal is possible. The temperature upgrading to the load in the range 47 - 50°C can be easily supplied by the solid adsorption system. By this approach, solar energy and waste heat can be utilized together to satisfy hot water requirements in the commercial and industrial sector. The study clearly illustrates the feasibility to couple renewable energy source to a chemical heat pump, and thus upgrade heat and store energy for useful purposes. Such systems will be useful for many applications in tropical locations where solar energy is available throughout the year. |
| Year | 2003 |
| Corresponding Series Added Entry | Asian Institute of Technology. Dissertation ; no. ET-03-01 |
| 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, Sivanappan; |
| Examination Committee(s) | Bhattacharya, Sribas C.;Attapol Noomhorm; |
| Scholarship Donor(s) | Government of France; |
| Degree | Thesis (Ph.D.) - Asian Institute of Technology, 2003 |