Thermochemical energy storage and conversion: A-state-of-the-art review of the experimental research under practical conditions

Abstract

Thermal energy storage and conversion aims to improve the high inefficiency of the industrial processes and renewable energy systems (supply versus demand). Chemical sorption processes and chemical reactions based on solid–gas systems are a promising way to store and convert thermal energy for heating or cooling applications and, thereby to increase the efficiency of the processes and to reduce the greenhouse effect. Although more efforts are required to bring this technology to the market, some important breakthrough have been made regarding to system efficiency. Over the last two decades, the experimental research in this field has increased largely to validate these advances under practical conditions. Therefore, this paper gives a state-of-art review of performances obtained under practical conditions by the different prototypes built over the last two decades. In addition, the main advantages and disadvantages of solid–gas chemical sorption processes and chemical reactions are summarized.

Introduction

The increased demand for energy, the rise in the price of fuel associated with the depletion of fossil fuels, and the growth of CO₂ emissions all require the development of more energy-efficient processes and a shift from non-renewable energy sources to renewable energy sources. In this sense, thermal energy storage and conversion (TESC) can increase the thermal energy efficiency of a process by reusing the waste heat from industrial processes, solar energy or other sources. Furthermore, considering that in 2004 the heating and cooling demands of the industrial, commercial and domestic sector accounted for between 40–50 % of the total global 320 EJ (7,639 Mtoe) final energy demand [1], TESC could contribute to substantial energy savings and a reduction in CO₂ emissions [2].

Heat is the form of energy most widely used in industry and power plants for driving the processes or producing electricity, either through steam or in fired furnaces. As a consequence of the work produced, most of this heat is degraded to a lower level and is released to the environment through cooling water, cooling towers, flue gases or other means (Fig. 1). This waste heat is left unused due to its relatively low grade. TESC not only allows the waste heat to be re-used, it also allows the heat to be upgraded by means of a chemical heat pump. Furthermore, when the thermal demand is located at a distance from the supply, this heat could be transported [4].

Even in diesel or gasoline engines, TESC can be used to recover the waste heat lost through the radiator or the exhaust. For instance, in vehicles this waste heat accounts for 60 % of the fuel energy [5]. In fact, only 20 % of fuel energy is used to power the vehicle. Therefore, recovering this heat and reusing it for heating or cooling applications would significantly increase the efficiency of the engine.

TESC may also be able to increase the potential of solar energy. It can be used to eliminate the time gap between energy supply and energy demand. This is between seasons, between day-time and night-time or even between sunny and cloudy days. For instance, for space heating and domestic hot water in households and offices, the surplus of energy generated during the summer period could be stored and used in the winter, when the demand exceeds the solar supply [6], [7], or, in a solar power plant, the surplus of thermal energy generated during the day could be stored and used at night or on cloudy days to produce electricity [8], [9].

There are several ways to store thermal energy [10] by sensible heat [11], by latent heat [12], by sorption process (physical or chemical) or by chemical reactions. Of all these ways, chemical sorption processes (chemisorption) and chemical reactions based on solid–gas systems show the highest potential for energy savings and CO₂ emissions reduction. In both cases, heat is not stored directly as sensible or latent heat but by way of a reversible chemical process or reaction which is usually carried out in a chemical heat pump (CHP) system (closed systems).

S′+HEAT⇔S+G

The basic CHP system comprises a solid–gas reactor coupled with a condenser/evaporator. The working principle is illustrated in Fig. 2 in a Clausius–Clapeyron's diagram, wherein the solid–gas (S/G) and liquid–gas (L/G) equilibrium lines are given by the equation:

$$\ln (P_{eq})=\frac{\mathrm{\Delta }H}{RT}+\frac{\mathrm{\Delta }S}{R}$$

The chemical heat pump system operates in two successive phases (thermal conversion) or with a time gap (thermal storage mode): charging phase (also named regeneration, decomposition or desorption) and discharging phase (also named production, synthesis or sorption). The charging phase occurs at high pressure (P_h). Heat at high temperature (T_h) is supplied to the reactor and the solid (S') decomposes. The gas (G) released from the decomposition is condensed by rejecting heat from the condenser at medium temperature (T_m). By contrast the discharging phase (reverse phase) occurs at low pressure (P_l). The liquid evaporates by absorbing heat at low temperature (T_l) and the synthesis heat is released at T_m. In both phases the synthesis and decomposition occurs when the salt is removed from its equilibrium of temperature (equilibrium temperature drop, ΔT) and pressure (equilibrium pressure drop, ΔP) for heat and mass transfer [13], [14], [15], [16].

The basic cycle shown in Fig. 2 can be used to produce refrigeration at T_l and/or to produce heat at T_m. However the two phases can be interchanged to operate the cycle as a heat transformer and thus upgrade heat from T_m to T_h. In this case the charging phase occurs at low pressure (P_l) and the discharging phase occurs at high pressure (P_h). Furthermore, in CHP systems that implies the use of non-condensable gases or that uses a condensable gas, but where the safety problem regarding the high pressure becomes an issue (legislators), the evaporator/condenser is replaced by another reactor [17], [18], [19]. In sorption processes, the CHP systems using two reactors are also known as resorption systems.

The advantages of chemical sorption processes and chemical reactions lie in the fact that they offer high energy density to the materials involved (important when the volume is limited), can cover a wide range of working temperatures (from −50 to over 1000 °C), and allow for long-term storage due to the negligible heat loss. Moreover, sorption and chemical heat pumps or heat transformers have several advantages to that of traditional vapor-compression heat pumps (mechanical energy). In addition to the benefit of being driven by waste heat, they utilize refrigerants with zero ozone depletion and global warming potentials, i.e. no chlorofluorocarbons (CFC) and hydrochlorofluorocarbons (HCFC), they are noise and vibration free, and long lasting [17], [20].

The aim of this paper is to provide a state-of-the-art review of the experimental research on sorption processes and chemical reactions based on solid–gas systems over the last two decades. The paper gives the performances of the current materials in small and pilot scale test rigs under practical conditions.