Metal hydrides based high energy density thermal battery

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Highlights

  • The principle of the thermal battery using advanced metal hydrides was demonstrated.

  • The thermal battery used MgH2 and TiMnV as a working pair.

  • High energy density can be achieved by the use of MgH2 to store thermal energy.

Abstract

A concept of thermal battery based on advanced metal hydrides was studied for heating and cooling of cabins in electric vehicles. The system utilized a pair of thermodynamically matched metal hydrides as energy storage media. The pair of hydrides that was identified and developed was: (1) catalyzed MgH2 as the high temperature hydride material, due to its high energy density and enhanced kinetics; and (2) TiV0.62Mn1.5 alloy as the matching low temperature hydride. Further, a proof-of-concept prototype was built and tested, demonstrating the potential of the system as HVAC for transportation vehicles.

Introduction

Thermal energy storage (TES) is one of the most promising approaches for harnessing and utilizing thermal energies, such as solar energy and industrial waste heat. TES is generally classified by different forms of storing heat such as: sensible heat, latent heat, and chemical energy [1]. Of all these forms, the use of thermo-chemical energy has received growing interest due to its intrinsic high energy–density [2]. In particular, a metal hydride based TES system is very appealing due to the fact that many metals and alloys can combine with hydrogen to form metal hydrides under favorable temperature and hydrogen pressure conditions. The basic idea of using metal hydrides for thermal engineering applications was originated in the 1970’s, when Libowitz [3], [4] suggested that the hydrogenation and dehydrogenation reactions could be used for TES. It was recognized that the substantial heats of reaction associated with hydrogenation (exothermic) and dehydrogenation (endothermic) reactions can be utilized for various practical purposes such as thermal storage, heat pumps, and heating and cooling systems [5]. There is considerable renewed interest in the use of metal hydrides for TES, including reports by Ono et al. [6], Yonezu et al. [7], and a recent review by Muthukumar et al. [8].

A common thread of the past research reports on metal hydride based thermal applications has been that most, if not all, utilize intermetallic AB5 and/or AB2 alloys as hydride materials [8], which offer the advantages of excellent kinetic behavior and cyclic stability. However intermetallic metal hydrides suffer drawbacks including: (1) high cost of the misch metals; and (2) low hydrogen capacity (typically less than 1.8 wt.% hydrogen capacity), and hence low energy density, particularly low gravimetric energy density. The cost and weight of the intermetallic alloys are considered a major limitation, impeding their application in electric vehicles.

Bogdanovic et al. [9], [10], [11] suggested that Mg-based hydride systems could be used for TES because these systems cover a wide temperature range from 250 °C to 550 °C and have high thermal energy density, up to 2257 kJ/kg. However, pure MgH2, is known to have poor kinetic rates for both the dehydrogenation and hydrogenation reactions. Fortunately, recent progresses on hydrogen storage materials have shown that several nano-catalyzed Mg materials [12], [13], [14], [15], [16], [17] are capable of absorbing a significant amount of hydrogen at room temperature. These findings have paved the way to practical application of Mg-based hydrides.

In this paper, we introduce the concept of a high-energy–density thermal battery based on exploiting the differences between hydrogen equilibrium pressures of Mg-based hydrides and intermetallic alloys (from AB2, AB5 or BCC families) at relevant temperatures. The present research focused on a specified on-board application to develop a novel heating, ventilation, and air conditioning (HVAC) system for electric vehicles (EV). However, the methodology of the thermal battery could be extended to a much broader range of applications, including other transportation vehicles such as long haul trucks, stationary HVAC, solar thermal energy storage systems, and waste heat recovery and storage systems.

Section snippets

Concept

As shown in Fig. 1, the thermal battery utilizes a thermodynamically-coupled pair of metal hydrides: one of which is designated as the high temperature (HT) hydride because it will provide heat. The HT hydride has a low equilibrium pressure and can release heat (exothermic) when it absorbs hydrogen; and it decomposes only at moderately higher temperatures. The other hydride is designated as the low temperature (LT) hydride because it will provide cooling (endothermic) when it dehydrogenates.

Selection of the hydride materials

As mentioned above, the selection of the two hydride materials must have complimentary thermodynamic properties to enable discharging and charging of the thermal battery in desired temperature ranges. Specifically, for discharging of the thermal battery, the equilibrium pressure of the LT hydride must be high enough at the ambient temperature (e.g. −10 to 30 °C) so that the HT hydride metal can be hydrogenated at temperatures at or above the ambient temperature. For charging of the thermal

Materials preparation

Catalyzed MgH2 was prepared by co-milling MgH2 (Sigma–Aldrich) with TiMn2 (Sigma–Aldrich) using a high energy planetary ball mill. Details of the milling method and properties of the as-milled product can be found in our previous publication [16]. After milling, 50 g of TiMn2 catalyzed MgH2 was mixed with 5 wt.% expanded graphite (ENG). Then, samples of the HT hydride materials, with and without graphite, were loaded into two HT-HBs. For the LT-HB, 190 grams of as-received TiMn1.5V0.62 alloy

Hydride materials

For the purpose of demonstrating the concept, the present work focused on verifying and testing the functionality of the system using the selected hydride materials. Extensive research efforts, including optimization of the hydride materials, have been reported elsewhere [12], [16], [19]. Initially, in order to verify thermodynamic compatibility of the two hydride candidate materials, hydrogen equilibrium pressures of MgH2 and TiMnV were characterized using a Sievert-type apparatus (PCT-Pro

Summary

In this work, the concept of a thermal battery based on advanced metal hydrides was introduced. A laboratory prototype was designed, fabricated, and tested. The results successfully demonstrate the feasibility of the thermal battery using advanced metal hydrides including the high temperature hydride, MgH2, and the low temperature hydride, TiMnV, as a working pair with matching thermodynamic properties. The performance of the concept-demonstration-unit showed both high heating/cooling power and

Acknowledgement

This research was supported by the U.S. Department of Energy (DOE) under contract number DE-AR0000173.

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