Thermochemical Energy Storage: Materials, concepts for reactors and process integration under economic boundary conditions.

Johannes Widhalma, Thomas Fellnera, Andreas Wernera, Markus Haidera, Franz Wintera

aInstitute for Energy Systems and Thermodynamics, Vienna University of Technology, 1060 Vienna, Austria

E-Mail: 2johannes.widhalm@tuwien.ac.at

Abstract: The possibilities for thermochemical energy storage have been screened within a 1-year exploration study. The focus of this work was the evaluation of thermochemical materials (TCM), based on literature search and own basic experiments. The analysis started at reaction pairs like CaO / Ca(OH)2 (CaCO3) or MgO / Mg(OH)2 (MgCO3). Materials were evaluated according to chemical equilibrium as well as reaction kinetics. Additionally different reactor concepts (fixed bed, pipe-reactor and fluidized bed) were modelled basically. Models have been developed by using “IPSEpro”, whereby parametric studies have been done with these models. An economic analysis describes the financial boundary conditions for the possibilities of thermochemical storeage (TCS) to substitute conventional heat sources.

1. Search and development of compounds suitable for TCSThe study of literature and the results of a BS-thesis done at IET have shown, that most investigated reaction pairs are metal oxides / hydroxides (or carbonates) like CaO / Ca(OH)2 (CaCO3), MgO / Mg(OH)2 (MgCO3). These materials show acceptable storage density of about 2 MJ/kg in case of using gaseous water for hydration, especially at MgO. In particular at hydration with steam the energy demand for evaporation has to be considered, because storage density could be halved if evaporation energy can’t be recovered.

Figure 1: Overview of TCM [2]

2. Reactor modellingThe process simulation program “IPSEpro” is used for the process simulation of the thermochemical reactor. MgO is used as TCM, the following reaction equation

is used.

2.1. Reaction kineticBy the reaction of gaseous with solid materials the following processes are important for the reaction velocity:

Diffusion of the reactants through the gas phase to the particle surface Diffusion into the pores of the particles Adsorption to the surface of the particle Reaction with the solid Desorption from the particle surface Diffusion from the pores into the gaseous phase

Reaction experiments are necessary to determine the reaction rate. The described processes above are represented in the overall reaction rate. The identification of the individual processes is difficult. By variation of the reaction conditions, it is possible to obtain information about the individual processes. It is therefore necessary to use the overall reaction rate for the models. The following equations are used for the reaction velocity by the variables of frequency factor, E activation energy and R gas constant. The equation of Arrhenius is supplemented with values from experiments by Ishitobi et al. [1] for dehydration.

For the hydration the equation by Kato et al. [3] is used.

These processes occur simultaneously with the heat transfer between the gas phase and the particles.

2.2.Two parameter modelTo obtain a flexible model, which is applicable for different TCM with different kinetics and for different reactors a two parameter model for the reactor is used. This model consists of a reaction volume , a non reaction volume and a bypass flow , which is shown in Figure 2. Parameter α gives the possibility to vary the non reaction volume. Simultaneously it is possible to vary the bypass flow by the parameter ß.

Figure 2: Reactor with non reaction volume and bypass flow

The result of the Figure 2 is the equation for the conversion X of the TCM in relation to the two parameters and as well as the reaction time t and the reaction rate constant k.

Through the connection of the kinetic model with the “two parameter model” it is possible to calculate the conversion for different reactors (fixed bed, pipe reactor and fluidized bed) by variation of the parameters and .

2.3.Heat transfer and pressure lossFigure 3 shows a simple design of a fixed bed reactor for the hydration process. A gas flow (light blue arrows) enters the reactor at the bottom flange and is distributed through a nozzle floor. The outlet of the gas is performed at the top flange. The gas leaves the reactor after the flow through the nozzle floor and the TCM bed. The TCM (orange arrows) inserts the reactor through a screw at the top side of the bed and leaves it at the bottom of the bed. The produced heat is transferred by a heat exchanger.The model calculates pressure loss and heat transfer. The pressure loss is calculated for a fixed bed reactor, pipe reactor or a fluidized bed. There are three main category groups of heat transfer in a reactor with gas-solid reactions. They are shown in Figure 3:

1. heat transfer between the gaseous phase and the particles of TCM,2. heat transfer from the gaseous phase and from the particles to the bed heat exchanger,3. heat transfer from the gaseous phase and the particles to the reactor wall.

Figure 3: Fixed bed reactor in the sectional view with heat transfer points

3 . Process integrationThe reactor model is designed in “IPSEpro” as a reactor with two storage tanks for the TCM. Hydration and dehydration take place in the same reactor. The combination of hydration and dehydration in one thermochemical storage station increases the complexity of the system. This is shown by a simplified flowsheet of a district heating station in Figure 4.This design has the advantage that it is not necessary to transport the TCM, but also the disadvantage of higher heat loss through thermal conduction to the ambient.

Figure 4: Simplified flowsheet, solid lines - hydration, dashed lines – dehydration

A parametric study for the hydration of the illustrated system in Figure 4 was carried out, for a district heating station with a power of 1 MW. Some results are displayed in Figure 5. One of the following parameter bypass massflow (ß), non reaction volume (α), reaction temperature (T) or the residence time (t) has been varies while the other parameters were fixed in value. The reference points are represented by the variables αref, ßref, Tref or tref.

Figure 5: Parameter study for hydration (αref = 0,1; ßref = 0,95; Tref = 95 °C; tref = 8050 s)

The results of Figure 5 based on a conversion rate of 66,66%. The consideration of the variation by the different parameters α, ß, T and t shows that the biggest gradient occurs by temperature line. Parameter ß has the smallest influence on the reactor volume. This is seen through the low gradient of the ß–line. The increase of temperature by 5% increases the reactor volume by 40%.

The conclusions from the simulations are, that the influence of the temperature T is a main parameter for size of the reactor and thus to the investment costs of the reactor. Considering Figure 1 & Figure 5 together it becomes clear that for a waste heat stream (heat storage) or a heat demand the level of the temperature is important. The kinetics and the size of the reactor is affected thereby.

4. Economic analysisEconomic analysis concentrates on the one hand on the cost of the production and continuous regeneration of the storage material, as given in the following:

Investment costs of the station Operation and maintenance Material (TCM, carrier gas and demineralised water) Transport costs for the TCM Heat production costs for the dehydration (waste heat from industry) Recycling costs (TCM, carrier gas and demineralised water)

On the other hand case studies are under progress taking under consideration different boundary conditions like season, price of electricity, demand on district heat, net topology and peak load suppliers. The analysis should return size and power of TCS-systems as well as the location to be installed.

References[1] Hirokazu Ishitobi, K. U. (2011). Dehydration and hydration behavior of metal-salt-modified materisals for chemical heat pumps. Japan, Tokio.[2] Lipp, F. W. (2013). Literaturstudie und thermodynamische Berechnung für thermochemische Speichermaterialien. Austria, Wien.[3] Yukitaka Kato, N. Y. (1996). Kinetic study of the hydration of magnesium oxide for a chemical heat pump. Japan, Tokio.

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