Exergy Analyses of Low-Temperature District Heating Systems With Different Sanitary Hot-Water Boosters.

This paper presents an exergy-efficiency analysis of low-temperature district heating systems (DHSs) with different sanitary hot-water (SHW) boosters. The required temperature of the sanitary hot water (SHW) was set to 50 °C. The main objective of this study was to compare the exergy efficiencies of a DHS without a booster to DHSs with three different types of boosters, i.e., electric-, gas-boiler- and heat-pump-based, during the winter and summer seasons. To achieve this, we developed a generalized model for the calculation of the exergy efficiency of a DHS with or without the booster. The results show that during the winter season, for a very low relative share of SHW production, the DHS without the booster exhibits favorable exergy efficiencies compared to the DHSs with boosters. By increasing this share, an intersection point above 45 °C for the supply temperatures, at which the higher exergy efficiency of a DHS with a booster prevails, can be identified. In the summer season the results show that a DHS without a booster at a supply temperature above 70 °C achieves lower exergy efficiencies compared to DHSs with boosters at supply temperatures above 40 °C. The results also show that ultra-low supply and return temperatures should be avoided for the DHSs with boosters, due to higher rates of entropy generation.

Towards powerful magnetocaloric devices with static electro-permanent magnets.

INTRODUCTION: Magnetocaloric energy conversion represents an alternative to existing refrigeration, heat pump and energy harvesting technologies. A crucial part of a magnetocaloric device concerns the magnetic field source. It uses mainly rare-earth materials and consists of moving parts and a drive system while displaying a limited energy efficiency and unavailability of fast and variable control of the magnetic field. Recent advances in efficient heat transfer for high-frequency magnetic cooling call for new developments of magnetic field sources that can operate with high efficiency at high frequencies. OBJECTIVES: We report the concept of an electro-permanent magnetic (EPM) field source that efficiently recovers magnetic energy. In contrast to existing magnets, it allows very well-controlled operation without any moving parts. The main objective of this paper is to present a numerical and experimental study in which such an EPM was designed, built and tested. METHODS: An extensive numerical investigation of the proposed design was carried out in terms of various geometrical and operating parameters. One of the design variations was built and experimentally evaluated for its energy efficiency and temperature increase at various operating frequencies. RESULTS: We demonstrate an energy efficiency of these magnets of over 80% and operation with frequencies up to 50 Hz, which is crucial for future high-power-density and high-frequency magnetocaloric devices. CONCLUSIONS: Considering high energy efficiency at high operating frequencies, such EPMs would allow for miniaturization, making them a viable option for future compact magnetocaloric devices.

Protocol to evaluate a (magneto)caloric device with static thermal switches using a 1D numerical model.

This protocol describes the use of a simple 1D numerical model to evaluate a single-stage (magneto)caloric refrigerating device with static thermal switches. The model can be used to find appropriate values of parameters that lead to a significant refrigerating effect and COP of the device. The modeled device can comprise any type of static thermal switch in combination with any kind of magnetocaloric or electrocaloric material. Simulation parameters need to be set with care for acceptable computational time. For complete details on the use and execution of this protocol, please refer to Klinar et al. (2022).

The numerical study on performance evaluation of a thermal switch capacitor in a magnetocaloric cooling device.

Compact, solid thermal control devices offer a new way to control the intensity and direction of heat flow between the components of a system, which is crucial for both optimized performance and safety. In this work we study a thin, silicon thermal switch capacitor (TSC) used for heat transport in a magnetocaloric cooling system. A numerical model was developed to quantify the effects of various operating conditions and design parameters on the performance of a magnetocaloric device with an embedded TSC. Based on realistic material properties, a maximum cooling-power density of 4000 Wm-2 (2025 Wkg m c m - 1 ) was obtained for a zero temperature span and an operating frequency of 20 Hz. The use of the presented device was demonstrated on a battery system, motivating further experimental studies to develop a new, compact cooling device that can be directly attached to a heat reservoir, making it desirable for a variety of applications.

Ferrofluidic thermal switch in a magnetocaloric device.

Thermal switches are advanced heat-management devices that represent a new opportunity to improve the energy efficiency and power density of caloric devices. In this study we have developed a numerical model to analyze the operation and the performance of static thermal switches in caloric refrigeration. The investigation comprises a parametric analysis of a realistic ferrofluidic thermal switch in terms of the maximum temperature span, cooling power, and coefficient of performance. The highest achieved temperature span between the heat source and the heat sink was 1.12 K for a single embodiment, which could be further developed into a regenerative system to increase the temperature span. A sensitivity analysis is conducted to correlate the relationship between the input parameters and the results. We show that thermal switches can be used in caloric devices even when switching ratios are small, which greatly extends the possibilities to implement different types of thermal switches.

Condensation of water vapor from humid air inside vertical channels formed by flat plates.

Condensation of humid air is an important process in thermal and process engineering and a subject of many currently research-intensive scientific domains, such as atmospheric water harvesting and seawater desalination. The nature of (water) vapor condensation in the presence of non-condensable gas (NCG) such as air differs significantly from the case with the pure, quiescent vapor condensation. In the literature, simple models that describe the forced flow condensation of water vapor in the presence of air on a series of vertical flat plates are hard to find. Here we present a simple and computationally efficient semi-empirical correlation describing forced flow condensation from humid air inside vertical channels formed by flat plates. The correlation accounts air as a non-condensing gas, different heights of vertical plates, and different thermal-hydraulic parameters. The correlation has been experimentally validated and shows excellent agreement, as 90% of theoretically predicted values are within ±12% of experimental data.

Thermal rectification in multilayer phase change material structures for energy storage applications.

Solid-state thermal control devices that present an asymmetric heat flow depending on thermal bias directionality, referred to as thermal diodes, have recently received increased attention for energy management. The use of materials that can change phase is a common approach to design thermal diodes, but typical sizes, moderate rectification ratios, and narrow thermal tunability limit their potential applications. In this work, we propose a multilayer thermal diode made of a combination of phase change and invariant materials. This device presents state-of-the-art thermal rectification ratios up to 136% for a temperature range between 300 K and 500 K. Importantly, this design allows to switch between distinct rectification states that can be modulated with temperature, achieving an additional degree of thermal control compared with single-rectification-state devices. We analyze the relevance of our thermal diodes for retaining heat more efficiently in thermal storage elements.