Heat Pump design enhancementwith phase change materials (PCMs) - a case study on Assemblin AB : Improving Defrost Efficiency with Phase Change Materials in anAir Water Heat Pump System

Sammanfattning

Phase change materials (PCMs) can store and release large amounts of latent heat during phase transitions, and are therefore promising for thermal energy storage (TES) applications. This study explores the integration of PCMs in an existing air–water heat pump (AWHP) system in Uppsala. Defrosting is required regularly due to frost buildup decreasing the air flow, which affects the heat pump performance. The purpose is to improve energy efficiency, reduce electricity costs and reduce the defrosting time using a PCM-TES. The case study consisted of two models, Business As Usual (BAU) model which is the current situation of the heat pump system, and a PCM model where the PCM is incorporated. The BAU case was modeled in Mathwork’s software Simulink using the tool package Simscape. However, due to limitations in Simulink, the main calculations for the PCM case were done manually. PCM-TES tanks were dimensioned based on the required masses, containing encapsulated PCM spheres (of diameter 4 cm), with a packing factor of 64%. Three PCMs were chosen, one inorganic and two organic, based on their thermal properties and price. PCM-1, 2 and 3 all had melting points close to 24 °C, and varied in latent heat. The used HTF was water. The total PCM masses were sized to cover three full defrost cycles per day, which corresponds to a TES capacity of 210 MJ. Heat transfer processes considered thermal conduction in the PCM spheres and convection to the water during the discharging conditions of 15–30 °C and a mass flow of 2.55 kg/s. Through manual calculations, the PCM mass, TES volumes, number of spheres and the heat transfer area were estimated. Key technical performance indicators include the TES capacity, consisting of both PCM and water, the discharging power, that should meet approximately 16 kW, and the defrosting time. The BAU case, which represents the current AWHP system with a water-TES, has a peak power demand of 16.11 kW and a defrost time of 73 minutes per cycle, requiring approximately 70 MJ. This is replaced by the PCM-TES in the PCM case. The water-TES has a capacity of 98 MJ, and the PCM-TES 210 MJ, representing a 114% increase in storage capacity. Integration of PCM-2 reduced defrosting time by 44.5%, while both PCM-1 and 3 resulted in longer defrost times than for the current system. PCM-3 has the shortest payback time (3.3 years), but also has a short lifespan (5.1 years). PCM-1 has a lifespan of 26 years and a payback time of 11 years, while PCM-2, with a potentially unlimited lifespan, has the longest payback time of 29 years. The cost analysis was conducted using market prices and included only the PCM cost, manufacturers are anonymized for confidentiality. The analysis shows that the choice of PCM affects the economic viability and depends on its lifespan, cost and thermal properties. Implementing a PCM-TES allows more flexible operation and lowers the electricity costs due to off-peak charging. Future work could include broader data collection, more comprehensive thermal performance analysis, testing of more PCMs as well as real life testing.