Investigation and improvement of a pcm-air heat exchanger to reduce energy use in buildings

Abstract

Energy demand for cooling buildings has risen in recent years due to increased external temperatures and occupants’ demands for perceived increased comfort provided by air-conditioning systems. An alternative method to provide cooling without using energy is by ventilation in locations where ambient conditions are favourable. This thesis investigated one ventilative cooling (VC) system. The investigated system is a mechanical ventilation system which uses PCM thermal storage in the ventilation path and utilise night cool air for solidifying the PCM which in turn cools recirculated or external air during its melting phase. The project has analysed in detail the operational performance of the system and proposed improvements in the control system and heat transfer of the PCM encapsulation panel. The methodology used for the investigation was (a) collection of system and field data from an operational system and their analysis to understand its performance using simple statistical methods as well as Dynamic Thermal Modelling (DTM) and Computational Fluid Dynamics (CFD). Based on the operational performance analysis, proposals for improvements were formulated for the control system and the PCM encapsulation panel and were investigated using DTM, CFD and a purposely built experimental rig. Analysis of the detailed monitoring of the operational space with the system installed in retrofit and thermal/CFD analysis indicates that the system can provide acceptable thermal comfort throughout at seating occupant level (0.7 m from the floor) in the moderate weather summer conditions of south and west England using adaptive thermal comfort limits. They also indicate that indoor air quality is maintained. Proposed improvements in the control system by changing the airflow set points can increase the permanence of temperatures within the set point range by 10%. This was implemented in the DTM model so that designers can model the system more accurately and it is easy to implement in existing systems. Improvements for the PCM panel proposed a new design of its encapsulation and variations of its packing in the thermal battery which offers double the heat transfer between the panel’s surface and the air and is capable of holding 30% more PCM material for the same space requirement. With more material more energy can be stored allowing longer duration (up to 9 hrs during laboratory tests) until complete melting was reached. The results obtained from this research using a combination of research methods (analysis of operational performance, computational models and laboratory test) indicate that mechanical ventilation systems with PCM integrated in the air path can provide the cooling demand required in non-domestic buildings in temperate climates reducing drastically operational energy environmental impacts and costs.