This paper investigates ellipsoid-shaped macro-encapsulated phase change material (PCM) on a component scale. The selected PCM is a paraffin-based commercial material, namely ATP60; differential scanning calorimetry and transient plane source method are used to measure ATP60's thermo-physical properties. A 0.382 m(3) latent heat thermal energy storage (LHTES) component has been built and experimentally characterized. The temperature measurement results indicate that a thermocline was retained in the packed bed region during charging/discharging processes. The experimental characterization shows that increasing the temperature difference between the heat transfer fluid (HTF) inlet temperature and phase-change temperature by 20 K can shorten the completion time of discharge by 65%, and increasing HTF inlet flowrate from 0.15 m(3)/h (Re = 77) to 0.5 m(3)/h (Re = 256) can shorten the completion time of charge by 51%. Furthermore, a one-dimensional packed bed model using source-based enthalpy method was developed and validated by comparison to experimental results, showing discrepancies in the accumulated storage capacity within 6.6% between simulation and experiment when the Reynolds number of the HTF inlet flow ranges between 90 and 922. Compared with a conventional capsule shaped in 69-mm-diameter and 750-mm-long cylinders, the ellipsoidal capsule shows 60% less completion time of discharge but 23% lower storage capacity. Overall, this work demonstrates a combined experimental and numerical characterization approach for applying novel macro-encapsulated PCM geometries for heating-oriented LHTES.

Frost formation on evaporator surfaces is a well-known problem in air-source heat pump (ASHP) systems, which decreases the energy efficiency of the system and causes thermal comfort issues during defrosting. Coatings for evaporator heat exchanger surfaces are one potential way of decreasing the problems with frost and thus improve the performance of the HP. However, there is still no consensus on which type of coating that attributes more energy efficient HP performance: hydrophilic or hydrophobic coatings. This paper aims to investigate if superhydrophilic or superhydrophobic coatings, compared to an uncoated reference surface, help prolong the cycle lengths and time spent in frosting mode by performing cyclic frosting and defrosting experiments on aluminum surfaces. The study was performed on small aluminum substrates (40 x 50 x 10 mm, 1.57” x 0.20” x 0.39”). A total of five different surface coatings of the substrates were compared, including an elastomer surface and a Slippery Liquid-Infused Porous (SLIP) type surface. The substrates were subjected to cycles of frosting and active defrosting in a wind tunnel, placed in a climate chamber at The Royal Institute of Technology (KTH) in Stockholm, Sweden. The temperature and relative humidity of the air inside the climate chamber were kept at 2°C (35.6°F) and 84%, respectively, according to the Swedish standard for HP test conditions (SS-EN 14511-2:2018). Cycles of frosting and active defrosting of the substrates were achieved by means of a thermo electric cooler (TEC) and captured with images (front and top camera). The superhydrophilic surface displayed longer frosting periods than the superhydrophobic surface. Due to the active defrosting, all of the coatings displayed similar short defrosting periods, thus indicating that the length of the frosting period is the dominant time-component of the cycle length. The study found that superhydrophilic surfaces show potential for extending the length of frosting periods compared to an uncoated surface, whereas a superhydrophobic surface reduced those values compared to an uncoated surface.

Integrating latent heat thermal energy storage (LHTES) units into building heating systems has been increasinglyinvestigated as a heat load management technology. A conventional LHTES integration method for heat pumpbased heating systems is to connect the heat pump’s condenser for charging the LHTES unit. This integratinglayout however usually leads to increased electricity input to the heating system. To underline this issue andprovide solutions, this paper presents three new LHTES integrating layouts where the LHTES unit is connectedwith the de-superheater of the main heat pump (Case 2), the condenser of a cascaded booster heat pump cycle(Case 3), or a combination of using both the de-superheater and the booster cycle (Case 4). In the context of amulti-family house in Stockholm, a quasi-steady state heating system model was developed to evaluate the newintegrating layouts, which were benchmarked against the baseline heating system without storage (Case 0) andthe conventional integrating layout using the main heat pump condenser (Case 1). Hourly electric power input tothe heating system was modelled for calculating the performance indicators including the heating performancefactor, the operational expense and justifiable capital expense, and the indirect CO2 emissions. Two load shiftingstrategies were simulated for an evaluation period of Week 1, 2019: 1) charge during off-peak hours (8 pm to 6am) and 2) charge during daytime hours (10 am to 7 pm). The simulation results of the off-peak charging strategyshow that, in Cases 2–4, the heating performance factor is 22%-26% higher than Case 1 and the operational expense can be reduced by 2%-5% as compared with Case 0. The savings in the operational expense can justifythe capital expense of 11 k-25 k Swedish Krona (SEK) for the LHTES systems in Cases 2–4 assuming a 15-yearoperation. Furthermore, the advantage of using the daytime charging strategy is principally the mitigation of CO2 emissions, which is up to 14% lower than the off-peak charging strategy. In summary, higher energy efficiencyfor heating is validated in the three new proposed integration layouts (Cases 2–4) against the condensercharging layout.

Latent heat thermal energy storage has been receiving increasing interests in residential heating applications. In this paper, a numerical heat transfer model was built with finite element method for a cylindrically encapsulated latent heat storage prototype and used for investigating its thermal performance optimization measures. The model was validated against four sets of experimental results for both charge and discharge, as the difference in accumulated storage capacity between simulation and experiment is less than 4%. Transient storage inlet boundary conditions were set in simulation for discharge considering the thermal output from the coupled radiators. The results of the optimization analyses show that: 1) reducing the capsule diameter from 69 mm to 15 mm shortens the completion time of charge and discharge by up to 70%, however, at the expense of 23% decrease in total storage capacity; 2) using parabolic or linear time-increasing heat transfer fluid flowrate profiles than a time-constant one extends around twofold the useful discharge timespan; 3) increasing the storage vessel diameter from 0.6 m to 0.7 m and to 0.8 m prolongs the useful discharge timespan from 2 hrs to the recommended 3 hrs, though the further enlargement to 0.8 m results in a lower state of charge after 3 hrs due to increase in unexploited storage capacity. From the numerical optimization study, we proposed a storage design adjustment of using 15 mm-diameter phase change material capsules in a 0.7 m-diameter cylindrical storage vessel, coupled with a parabolic flow strategy, to improve the storage on-peak discharging performance.

Experimental and Numerical Investigation of a Latent Heat Thermal Energy Storage Unit with Ellipsoidal Macro-encapsulationTianhao Xu, Emma Nyholm Humire, Silvia Trevisan, Monika Ignatowicz, Samer Sawalha, Justin NW Chiu*Department of Energy Technology, KTH Royal Institute of Technology, 100 44, Stockholm, SwedenAbstractIn this paper, a novel type of macro-encapsulated phase change material (PCM)shaped in ellipsoidis investigated on a component scale.The encapsulated PCM is a paraffin-based commercial material (ATP60); differential scanning calorimetry and transient plane source method are used to measure the thermo-physical properties of the PCM. A numerical model and a 0.382 m3latent heat thermal energy storage (LHTES) prototype have been built and experimentally characterized. The temperature measurements indicate that thermocline is retainedin the packed bed region. The charge/discharge can be accelerated by65% with20 K increase intemperature difference between the phase-change temperature and heat transfer fluid (HTF)inlet temperature. Increasing HTF inlet flowrate from 0.15 m3/h (Re=77) to 0.5 m3/h (Re=256) shortenscompletion time of charge by51%. Furthermore, a one-dimensional packed bed using source based enthalpy method was constructed and validatedwithin6.6% errorfor aReynolds numberof 90 to 922. Compared with a conventional cylindrical PCM capsule, the ellipsoidal encapsulation shows60% increase in discharge speed but 23%lower storage capacity. This work demonstrates a combined experimental and numerical characterization approach for a novel ellipsoidal PCM encapsulationgeometry.