Utilization of phase change material (PCM) storage for heating and cooling of buildings for hot and cold climates | |
| Author | Ahmad, Adeel Waqas |
| Call Number | AIT Diss. no.ET-11-02 |
| Subject(s) | Heat storage Cooling |
| Note | A dissertation submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy, School of Environment, Resources and Development |
| Publisher | Asian Institute of Technology |
| Series Statement | Dissertation ; no. ET-11-02 |
| Abstract | Passive heating and cooling of buildings can help to reduce the electricity and fossil fuel use and thus address the global concerns of the environmental impacts of the fossil fuels. Utilization of solar energy and the night ambient (cool) temperatures are the passive ways of heating and cooling of buildings. Intermittent and time dependent nature of these sources makes thermal energy storage vital for efficient and continuous operation of these heating and cooling techniques. Latent Heat Thermal Energy Storage (LHTES) by Phase Change Materials (PCMs) is preferred over other storage techniques due to its high energy storage density and isothermal storage process. Storage of ambient cold during night time and utilize it during hot day time is commonly known as "Free cooling." Earlier studies conducted on PCM coupled solar heating and free cooling have shown their effectiveness in reducing the cooling and heating loads of the buildings. PCM based free cooling was found to reduce the ventilation load up to 90% by storing night cold in PCM and releasing it the following day. Similarly, PCM storage of 5 kg/m2 of solar collector area was found optimal to preheat the ventilation air from sunset to early morning using the solar radiation of 4 kWh/m2 during winter season. Though earlier studies have focused on either PCM based free cooling or PCM based solar air heating, little work has been done to study the possibility of using same storage unit for both heating and cooling purposes in the climates where both heating and cooling is required. The current study was aimed to evaluate the performance of the air based PCM storage unit utilizing solar energy and cool ambient night temperatures for comfort heating and cooling of a building in dry-cold and dry-hot climates. An adaptive comfort criterion was used to evaluate the comfort temperatures of the study area. The study involved theoretical modeling and experimental observations. The simulation results of free cooling, indicated that night temperatures could be used to help reduce hot ambient air within comfort levels during day time in dry hot climates for the system considered (PCM quantity = 5 kg, air flow rate = 1 Om3 /hr, length, width and height of heat exchanger =1.Sm, O.Sm, 0.005m). When the melting point of the PCM was equal to the comfort temperature of the hottest summer month (29°C) of the study area, the storage unit performance in terms of Cooling Capacity was maximized for the whole summer season (May to August), and was between 99% to 90% for the whole summer season. The simulation results for solar air heating coupled with PCM storage unit (PCM quantity = 5 kg, air flow rate = 10m3 /hr, length, width and height of heat exchanger =1.Sm, O.Sm, 0.005m), indicated that when melting point of the PCM was equal to the average comfort temperature of all the winter months (- 21°C), the performance of storage unit in terms of Heating Capacity was maximized (- 90% for all winter months December to February). The performance of the studied PCM storage unit was maximum when the melting point of PCM was - 29°C in summer and 21°C during winter season. The appropriate melting point was -27.5°C for all the year round performance. At lower melting points than 27.5°C declination in the cooling capacity was more profound as compared to the improvement in the 111 heating capacity. Therefore, the melting point of the PCM which provided maximum cooling capacity during summer season could be used for winter heating also. Detailed experimental observations and analysis of the experimental and theoretical results was conducted to assess the accuracy of the model developed. Experiments were conducted to observe the charging and discharging of PCM at varying air flow rates and at different inlet air conditions (temperature). Cold accumulated in PCM during charging process and cold extracted from PCM during discharging process was calculated by measuring the air velocity and air inlet and out temperatures from storage unit. The experimental results are presented in terms cold accumulated in PCM during charging process and cold extracted from PCM during discharging process. The comparative analysis indicated that dW'ing charging process, at lower charging air temperature the theoretical results compared well with the experimental results in terms of Coefficient of Determination (COD) or R2 which was 94% and at higher charging temperature R2 was between 60% and 64%. One reason why the theoretical and the experimental results deviated was due to sub cooling. At lower charging temperature the theoretical and experimental results compared well, because sub-cooling lasts for very sh01t time period. For higher charging temperature, where sub cooling lasted for longer duration the experimental results deviated from the theoretical results. During discharging process the statistical analysis showed that the theoretical results and experimental results compared well with each other as compared to the charging process results. For discharging process, R2 was found to be 94%, 86% and 74% respectively at the charging temperatures of 20°C, 22°C and 24°C. This study clearly illustrates the feasibility of using the same PCM storage unit for both heating and cooling applications of buildings in dry cold and dry hot climates using the passive sources of heat (solar energy) and cold (cool night ambient air). |
| Year | 2011 |
| Corresponding Series Added Entry | Asian Institute of Technology. Dissertation ; no. ET-11-02 |
| 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 N. ;Salam, P. Abdul ;Yinping, Zhang; |
| Scholarship Donor(s) | Higher Education Commission of Pakistan ; |
| Degree | Thesis (Ph.D.) - Asian Institute of Technology, 2011 |