Numerična in eksperimentalna analiza aktivnih elastokaloričnih regeneratorjev

Elastocaloric cooling technology is considered an environmentally friendly alternative to vapor-compression cooling technology because it uses environmentally benign materials and has the potential to achieve high efficiency. It is based on the elastocaloric effect in shape-memory materials when subjected to sufficient mechanical stress. One of the main challenges of elastocaloric technology is to develop an effective and durable elastocaloric regenerator. A tube-based elastocaloric regenerator loaded under compression has high potential to solve this issue. A comprehensive experimental and numerical analysis of the thermo-hydraulic properties of the tube-based elastocaloric regenerator was conducted. We concluded that the tube-based geometry provides a good balance between heat transfer properties and viscous losses. In the second part, an active elastocaloric regenerator was built and experimentally tested for its cooling and heating properties. The regenerator demonstrated sustained dynamic operation and achieved record-breaking performance. It attained a maximum temperature span of 31.3 K and a specific cooling/heating power of 4,400 W kg-1 of the elastocaloric material. Finally, a new 1D numerical model was developed and validated against the experimental results. The model was used to compare fatigueresistant tube-based and parallel-plate active elastocaloric regenerators, leading to a better understanding of the potential and limitations of this technology.Elastokalorična tehnologija hlajenja se uveljavlja kot okolju prijazna alternativa parnokompresijski tehnologiji hlajenja, saj temelji na okolju neškodljivih trdninskih materialih, hkrati pa ima potencial doseči visoko učinkovitost. Temelji na elastokaloričnem učinku v materialih z oblikovnim spominom, ko so ti podvrženi zadostni mehanski obremenitvi. Eden ključnih izzivov elastokalorične tehnologije je razvoj učinkovitega in vzdržljivega elastokaloričnega regeneratorja. Tlačno obremenjeni cevni elastokalorični regenerator izkazuje velik potencial za dosego tega cilja. V tem delu je bila izvedena obsežna eksperimentalna in numerična analiza termo-hidravličnih lastnosti cevnega regeneratorja. Pokazali smo, da cevni regenerator predstavlja dober kompromis med lastnostmi prenosa toplote in viskoznih izgub. V drugem delu je bil aktivni cevni elastokalorični regenerator izdelan in eksperimentalno ovrednoten z vidika hladilnih in grelnih karakteristik. Regenerator je dokazal trajno-dinamično delovanje (preko 300 000 obratovalnih ciklov brez poškodb) ter rekordne karakteristike. Dosegel je največji temperaturni razpon 31,3 K in specifično moč 4400 W kg-1 elastokaloričnega materiala. Razvit je bil tudi nov 1D numerični model elastokaloričnega regeneratorja, ki je bil ovrednoten z eksperimentalnimi rezultati. Z uporabo numeričnega modela smo optimizirali in primerjali tlačno obremenjeni cevni elastokalorični regenerator in natezno obremenjeni elastokalorični regenerator iz vzporednih plošč, pri pogojih ki omogočata trajno dinamično obratovanje, kar omogoča boljše razumevanja potenciala in omejitev te tehnologije

Optimization of sensible heat storage with phase change materials

V magistrskem delu je predstavljena analiza izboljšave shranjevanja toplote v senzibilnem hranilniku toplote z vgradnjo modulov s fazno spremenljivo snovjo. Izvedle so se meritve pri različnih pogojih delovanja, čemur je sledila validacija simulacijskega modela, ki je bila izvedena s programskim orodjem TRNSYS. Nato je bila v omenjenem programskem orodju izvedena primerjava senzibilnega hranilnika toplote z in brez vgrajenih modulov s fazno spremenljivo snovjo pri različnih deležih polnitve hranilnika toplote ter temperaturah shranjevanja toplote. Ugotovljeno je bilo, da vgradnja fazno spremenljivih snovi v senzibilne hranilnike toplote poveča gostoto shranjene energije, kar posledično omogoča daljši čas zagotavljanja želene izstopne temperature vode. Izkazalo se je, da so bolj učinkovite polnitve z višjim deležem fazno spremenljive snovi, njihova uporaba pa je bolj primerna v ožjih temperaturnih območjih shranjevanja toplote.In master’s thesis an analysis of improving the thermal energy storage in sensible storage tank with integration of modules with phase change materials is presented. A series of measurements has been carried out under variable conditions, followed by a validation of simulation model that was performed with a software package TRNSYS. In mentioned software package a comparison between sensible heat storage with and without built-in modules with a phase change material was made at different shares of filling material and temperatures of heat storage. It has been found out that with the implementation of phase change materials in the sensible heat storage the density of saved energy is increased. Consequently, the heat storage can provide desired outlet temperature of water for a longer period of time. It was also concluded that higher filling percentage of a phase change material is more efficient and their usage is more appropriate in narrow temperature ranges of storing thermal energy

Parametric analysis of fatigue-resistant elastocaloric regenerators

Elastocaloric cooling has recently shown high potential as an environmentally friendly alternative to vapor-compression technology. Here, we have studied and analyzed the geometric characteristics of two active elastocaloric regenerators (AeCRs) that were proved to have high application potential, i.e., a shell-and-tube AeCR loaded in compression and a parallel-plate AeCR loaded in tension, with the goal of maximizing their cooling performance. For this purpose, a previously developed and experimentally verified 1D numerical model was used. We focused only on the geometries and operating conditions that allow for durable, i.e., buckling-free operation in compression and fatigue-resistant operation in tension. The results show that although the applied strain of the parallel-plate AeCR loaded in tension needs to be limited (below 2%) to ensure fatigue-resistant operation, it outperforms (in terms of cooling power and COP at 15 K of temperature span) the shell-and-tube AeCR, which due to buckling issues suffers from a poorer heat-transfer geometry, but can withstand higher strains due to compressive loading. At the maximum strain of 2%, the optimum parallel-plate AeCR can generate a maximum cooling power of 1825 W (corresponding to 7075 W kg−1 of elastocaloric material) and a COP of 9.15 at a zero-temperature span. On the other hand, due to a higher applied strain (3%) the optimum shell-and-tube AeCR can generate a higher maximum temperature span at zero cooling power (up to 50 K) but has limited cooling performance at lower temperature spans. In addition, the layering of the shell-and-tube AeCR was investigated for the first time to improve its performance. This study shows the crucial impact of the heat-transfer geometry (heat-transfer area and hydraulic diameter), which needs to be further improved in compression-loaded AeCRs to improve their efficiencies (without compromising the buckling stability). The study also shows the importance of the applied strain, which needs to be at least 2% or more to achieve a high cooling performance of the AeCR. The obtained results should serve as guidelines for designing powerful and efficient AeCRs in the future

Thermo-hydraulic evaluation of oscillating-flow shell-and-tube-like regenerators for (elasto)caloric cooling

The development of novel regenerators for caloric cooling applications requires a detailed evaluation of their thermo-hydraulic properties. Structures similar to shell-and-tube heat exchangers are one of the most promising geometries for elastocaloric technology since they exhibit high thermal performance and can be applied under compressive loading to overcome the limited fatigue life of elastocaloric materials normally experienced in tension. However, thermo-hydraulic properties of shell-and-tube-like structures at the conditions relevant for caloric cooling applications (oscillating counter-flow regime at low Reynolds numbers

Elastokalorično hlajenje: pregled razvoja in nadaljnji izzivi pri razvoju regenerativnih elastokaloričnih naprav

The elastocaloric cooling, utilizing latent heat associated with martensitic transformation in shape-memory alloys, is being considered in the recent years as one of the most promising alternatives to vapour compression cooling technology. It can be more efficient and completely harmless to the environment and people. In the first part of this work, the basics of the elastocaloric effect (eCE) and the state-of-the-art in the field of elastocaloric materials and devices are presented. In the second part, we are addressing crucial challenges in designing active elastocaloric regenerators, which are currently showing the largest potential for utilization of eCE in practical devices. Another key component of elastocaloric technology is a driver mechanism that needs to provide loading for active elastocaloric regenerators in an efficient way and recover the released energy during their unloading. Different driver mechanisms are reviewed and the work recovery potential is discussed in the third part of this work

Improved thermal energy storage for heating and cooling of buildings

One of the great challenges in the energy sector represents retrofit of residential buildings where 3/4 of buildings in Europe are residential. To reduce energy consumption and increase the use of renewables in existing residential buildings a holistic approach of retrofit with interconnected technological system is needed. In the present paper energy toolkit based on the synergetic interaction between technologies integrated in the system for holistic retrofit of residential buildings which is under development within HEART project (HORIZON 2020) is presented. In this project step towards self-sufficient heating and cooling of building is made with an increase in on-site consumption of self-produced energy in PV from solar energy, where produced electrical energy is used also for heat pump operation. In this case thermal energy storage plays an important role for storing heat or cold for time when solar energy is not available. Improvement of sensible thermal energy storage with implemented cylindrical modules at the top of the heat storage tank and filled with phase change material is investigated experimentally. 43 litres of paraffin with phase change temperature between 27 °C and 29 °C was used in a system, what represented 15 % of total volume of heat storage tank. The results from experiment shows that thermal energy storage unit with integrated modules filled with phase change material can supply desired level of water temperature for twice as long at smaller temperature level as sensible thermal energy storage what is the consequence of higher energy density that can be stored during phase change. The advantage of phase change materials is in thermal energy storage for applications that needs narrow temperature range of supplying and storing thermal energy what is the subject matter of consideration in the case of HEART project

Numerical Modeling of Shell-and-Tube-like Elastocaloric Regenerator

Elastocaloric cooling is considered an environmentally friendly future alternative to vapor-compression technology. Recently, a shell-and-tube-like elastocaloric regenerator loaded in compression has demonstrated record-breaking heat-pumping performance and fatigue-resistant operation. The aim of this work is thus to present a new 1D numerical model to simulate and optimize the operation of an elastocaloric regenerator with a shell-and-tube-like design. In the first part of this work, the superelastic and elastocaloric properties of a single NiTi tube, which serve as input data for the numerical model, were determined through experimental characterization and phenomenological modeling. In the second part, the results of the numerical model were compared with the experimentally obtained results. Relatively good agreement was found regarding the temperature span, cooling and heating power, and COP values, which indicates that the developed numerical model could be used for accurate optimization of shell-and-tube-like elastocaloric regenerators. Finally, the effects of operating conditions and hysteresis losses on the performance of the shell-and-tube-like elastocaloric regenerator are modeled and discussed. This work shows that the shell-and-tube-like elastocaloric regenerator with this configuration can achieve a maximum temperature span of more than 50 K at zero-thermal-load conditions and a maximum cooling/heating power of up to 4000 W·kg−1 and COP of about 4 (at zero temperature span)

High-performance cooling and heat pumping based on fatigue-resistant elastocaloric effect in compression

Refrigeration is vital to the modern societyhowever, it is one of the most polluting sectors in the world. Despite decades of development, our standard vapor-compression technology remains relatively inefficient and still uses harmful refrigerants. Elastocaloric cooling has shown significant potential as an environmentally benign alternative to the vapor-compression technology, but one of the biggest challenges to be solved is its limited fatigue life. Our new design of an elastocaloric regenerator made of compression-loaded Ni–Ti tubes enables buckling- and fatigue-resistant operation and record performance with commercially relevant cooling and heat-pumping characteristics. In terms of maximum specific cooling/heating performance metrics, it surpasses all previously developed caloric devices and demonstrates the enormous potential of compression-loaded elastocaloric devices for a wide range of cooling and heat-pumping applications