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Electrocaloric (EC) and barocaloric (BC) materials undergo reversible thermal changes in response to changes in applied electric field and pressure, respectively. These materials could potentially be exploited in novel solid-state cooling systems that may replace current vapour-compression systems, which are environmentally harmful, noisy, and relatively energy inefficient. In this work, I studied EC effect and BC effects in multicaloric organic materials, which promise large caloric effects near room temperature. The dissertation is structured as follows. Chapter 1 introduces the background for conventional refrigeration and traditional caloric materials. Chapter 2 then surveys the literature on EC materials and BC materials, as well as the literature on EC prototype devices. Chapter 3 reviews the experimental and modelling methods used for this work. These include sample preparation methods, dielectric spectroscopy and ferroelectric polarisation measurements, calorimetry and infrared imaging, dilatometry and Landau models. Chapter 4 describes the study of EC effects in two dabco-based organic salts, namely [Hdabco][BF4] and [AH][ReO4], where dabco is 1,4-diazabicyclo[2.2.2]octane and AH is a variant of dabco, 1-azabicyclo[2.2.1]heptanium. Experiments and modelling demonstrates that [Hdabco][BF4] shows giant EC effects (isothermal entropy change |∆S| = 15.5 J K-1 kg-1 for |∆E| = 12 kV cm-1) that are one order-of-magnitude larger than those observed in traditional EC oxides such as BaTiO3 (|∆S|= 2.1 J K-1 kg-1 for |∆E| = 4 kV cm-1). [AH][ReO4] shows smaller EC effect of |∆S|~ 7.5 J K-1 kg-1 for |∆E| = 11.2 kV cm-1, but displays better mechanical integrity and operates closer to room temperature. It is concluded that dabco-based organic salts exhibit promising performance due to their large EC effects, non-toxicity and great tunability via chemical alterations. However, electrical leakage and breakdown remains an important challenge to be overcome for their use as EC working bodies. Chapter 5 describes the study of BC effects in the aforementioned dabco-based organic salts. BC effects in these materials have the advantage to be driven using hydrostatic pressure instead of electric field, thus avoiding issues related to electrical leakage and breakdown. Three compositions were selected for BC studies, namely [Hdabco][BF4], [Hdabco][ClO4] and [Hdabco][ReO4]. Among these, [Hdabco][ClO4] shows the largest reversible BC effects, |∆S| = 73.2 J K-1 kg-1 for |∆p| = 1200 bar, which compare well with those reported in state-of-the-art BC materials. Notably, BC effects in [Hdabco][BF4] largely outperform EC effects in the same compound, thus demonstrating that pressure is a useful driving parameter for leaky organic ferroelectrics. Chapter 6 describes BC studies in ureasil polymeric materials. These compounds show large changes in entropy when transforming from liquid to solid. By exploiting a gelation method, the liquid to solid phase transition in these compounds is transformed to a gel-to-solid phase transition, which is desirable for some caloric applications. By driving these transitions using pressure, very large BC effects of |ΔS| = 263 J K−1 kg−1 for |∆p| = 1200 bar are found, which are similar to those observed in commercial vapour-compression refrigerants, e.g. R134a. Moreover, the studied polymers have other advantages, in terms of being stretchable, non-toxic, inexpensive and have great tunability of transitions temperatures. Finally, chapter 7 summarises the main results of this work and discusses interesting avenues for future work.

Heat pumps (HPs) are an excellent solution to supply heating and cooling for indoor space conditioning and domestic hot water (DHW) production. Conventional HPs are typically electrically driven and operate with a vapour-compression thermodynamic cycle of refrigerant fluid to transfer heat from a cold source to a warmer sink. This mature technology is cost-effective and achieves appreciable coefficients of performance (COP). The HP market demand is driven up by the urge to improve the energy efficiency of building heating systems coupled with the increase of global cooling needs for air-conditioning. Unfortunately, the refrigerants used in current conventional HPs can have large greenhouse or ozone-depletion effect. Alternative gaseous refrigerants have been identified but they present some issues regarding toxicity, flammability, explosivity, low energy efficiency or high cost. However, several non-vapour-compression HP technologies have been invented and could be promising alternatives to conventional systems, with potential for higher COP and without the aforementioned refrigerant drawbacks. Among those, the systems based on the so-called “caloric effects” of solid-state refrigerants are gaining a lot of attention. The caloric effects are large entropy and adiabatic temperature changes caused by the application or removal of an external field in certain specific solid materials. There are 4 main caloric effects: magnetocaloric, elastocaloric, electrocaloric and barocaloric. Each of them is characterized by the nature of the field and the response that induces the entropy and adiabatic temperature change: variation of the magnetic field, uniaxial mechanical stress, electrical field or hydrostatic pressure, respectively. A HP cycle can be based on these caloric effects and several heating/cooling prototypes were developed and tested over the last few decades. Although not mature technologies yet, some of these caloric systems are well suited to become new efficient and sustainable solutions for indoor space conditioning and DHW production. This paper aims to raise awareness in the building community about these innovative caloric systems. It sheds some light on the recent progress in that field and compares the performance of caloric systems with that of conventional vapour-compression HPs for building applications.Presentation of this paper at the CLIMA 2022 conference can be found here: https://youtu.be/oAjfT2_HEYo

cited By 2; International audience; In the framework of solid-state cooling technologies, four kinds of caloric effects, i.e., magnetocaloric (MC), electrocaloric (EC), barocaloric (BC) and elastocaloric (eC) effects, have been compared from the viewpoint of environmental issues and application cases. This field is primarily developed with the intention of protecting the environment. Currently, alternative caloric materials (called “good” materials) with properties such as being environmentally friendly, low-cost and practicable (low stimulus field) need to be found. All current caloric effects/materials are investigated for the common objective of cooling. Due to the variety of stimuli and intrinsic properties (different caloric performances for different dimensions) of caloric materials, they can exhibit unique advantages for various application cases. A “good” caloric material for large-scale cooling application is still rare and requires more research. © 2016 Elsevier Ltd