Experimental and numerical investigations of different

Available online at www.sciencedirect.com

ScienceDirect Energy Procedia 57 (2014) 2380 – 2389

2013 ISES Solar World Congress

Experimental and numerical investigations of different reactor concepts for thermochemical energy storage Barbara Mette*, Henner Kerskes, Harald Drück Research and Testing Centre for Thermal Solar Systems (TZS), Institute for Thermodynamics and Thermal Engineering (ITW), University of Stuttgart, Pfaffenwaldring 6, 70550 Stuttgart, Germany

Abstract This paper presents two different reactor concepts, an external and integrated reactor, for thermochemical energy stores which are operating in an open process using ambient air for the heat and mass transport. In the external reactor concept the material charging and discharging is performed in a reactor separated from the material store whereas in the integrated reactor concept the charging and discharging take place directly in the material store itself. For both reactor concepts, the heat and mass transfer inside the reactor or the material store respectively, have been analyzed in detail by means of numerical simulations. Based on the theoretical investigations a reactor design and an operation strategy have been developed. At laboratory scale an external test reactor (approx. 20 l of storage volume) has been built and experimentally tested. The experiments have proven the concept of the developed reactor design. A high thermal performance during the charging and discharging process of the thermochemical energy store has been obtained. Keywords: thermochemical energy storage; reactor designs; sorption; hydration; numerical model; experimental investigation; heat and mass transfer;

1. Introduction With an increasing share of renewables in the global energy mix efficient short and long term energy storage technologies are crucial. They will play a major role to enhance energy security and to improve energy efficiency in all areas of energy conversion, distribution and energy use. Thermochemical energy store (TCES) represents an important step towards a new generation of heat storage technology due to their high energy storage density and their almost loss free storage concept. Different system concepts of a thermochemical energy store to be integrated as a seasonal heat store in solar combisystems have been developed at ITW. A detailed process description is given e.g. in [1], [2] and [3].

* Corresponding author. Tel.: +49-711-68563499; fax: +49-711-68563503 E-mail address: [email protected].

1876-6102 © 2014 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/3.0/). Selection and/or peer-review under responsibility of ISES. doi:10.1016/j.egypro.2014.10.246

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Barbara Mette et al. / Energy Procedia 57 (2014) 2380 – 2389

To guarantee an efficient charging and discharging process of the thermochemical energy store, innovative reactor concepts adapted to the specific application and storage material are required. For this, two different reactor concepts have been investigated in detail by means of numerical and experimental investigations: an external and an integrated reactor concept. As storage material binderless zeolite 13X (13XBFK©, Chemiewerk Bad Köstritz) has been used as this material is characterized by a fast reaction kinetic, a high heat of adsorption and a comparatively high energy storage density. In the following the results of the numerical and experimental investigation on the heat and mass transport will be presented. Nomenclature A cp D E f1, f2

adsorption potential heat capacity dispersion coefficient characteristic energy D’Arcy and Forchheimer factor in the Brinkman equation FH, Fh molar enthalpy, specific enthalpy kLDF mass transport resistance 兼岌 mass flow rate n heterogeneity parameter (DubininAstakhov equation) p pressure p0 saturation pressure pw partial water vapour pressure R gas constant T temperature u fluid velocity W adsorption volume W0 maximum adsorption volume X* adsorption equilibrium xs water loading

xw z, r

water content of the air coordinates

Greek symbols g" porosity N" effective heat conductivity t" density j" viscosity Subscripts ads adsorbed ax axial eff effective f fluid rad radial s solid amb ambient R reactor hx heat exchanger

2. Theoretical base and processes for thermochemical energy storage In thermochemical energy stores reversible chemical or physical reaction are used for storing and releasing energy. In low temperature applications where the heat is required at temperatures below 100 °C water vapor adsorption on sorption materials (e. g. zeolite or silica gel) and hydration reactions of hygroscopic salts (e. g. magnesium sulphate or magnesium chloride) are one of the most studied processes for thermochemical energy storage ([2,4–7])). Equation (1) describes the basic principle of the charging and discharging process of the TCES:

A( s) - x H 2O( g )

A © x H 2O( s) - FH

(1)

The TCES is discharged by bringing the solid reactant A in contact with water vapor (H2O(g)). The solid takes up the water to form the product A© x H 2O( s ) and heat of adsorption or hydration (FH) is released. This process is reversible. When heat is supplied, the product A© x H 2O( s ) is decomposed into

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its elements water vapor and the reactant A (charging of the TCES). A quasi-loss free storage of the energy over long periods of time is possible if the reactant A is stored separately from the water vapor. For high energy storage densities a high water uptake of the material and a high heat of adsorption/hydration is required. For a high thermal power output during discharging of the store a fast reaction kinetic of the material is desirable. In recent times, much research is being done in the area of material development and material testing for thermochemical energy storage. New material classes such as microporous aluminophosphates (AlPO), silicoaluminophophate (SAPO), metal organic frameworks (MOF) or composite materials of hygroscopic salts incorporated into a mesoporous carrier substance are subject of intense research ([8– 12]). The positive developments and great advances in the material research promise novel and high performance storage materials in near future. In the presented work, a new binderless zeolite of type 13X (bead size 1.6 – 2.5 mm, 13XBFK©, Chemiewerk Bad Köstritz, [13] has been used as storage material for the application under investigation. The zeolite is commercially available and characterized by a fast reaction kinetic and a high water uptake [13,14]. The reactor designs under investigation, an external and integrated reactor, are shown in Fig. 1 and Fig. 3. In the external reactor concept the reactor is separated from the storage material reservoir. A material transport (a vacuum conveying system is used in the TCES under investigation) is required to transport the material from the material reservoir to the reactor and vice versa. With the separation between the material reservoir and the reactor, the heat and mass transport is reduced to only a small part of the total storage material amount at a time. The thermal heat capacities and heat losses especially during the regeneration process are reduced. Furthermore, the high temperature during material regeneration is restricted to the reactor only. In the integrated reactor concept the material stays inside the material storage reservoir. As no material transport is required this means less material stress as well as non-technical or energetic effort for the transport. To minimize pressure losses and thermal capacities during the charging and discharging of the store a segmentation of the storage reservoir is performed. Only one segment is flown through by the air flow at a time which means that the adsorption and desorption process is also reduced to a small part of the total storage material. 3. Experimental and numerical investigation of heat and mass transfer in the different reactor concepts To analyze the heat and mass transport inside the reactor numerical models of the developed reactor designs have been setup in the finite-element-software COMSOL Multiphysics (V.4.3a, [15]). The model parameters have been obtained by experimental investigation in a sorption analyzer (adsorption equilibrium) and in a fixed bed reactor (reaction kinetic and heat transport). The experimental method, the determination of the model parameters and the validation of the numerical model will not be described here. Information on this can be found in [16] and [17]. 3.1. Governing equations for the numerical model The velocity profile inside of the reactor is calculated with the extended Brinkman equation (eq. (2) and (3a,b,c)). The heat and mass transport inside the reactor is described with a quasi-homogenous model, which means that no difference is made between the solid and fluid phase temperature (eq. (4) and (5)). The effective transport parameters for the radial and axial heat conduction (Nrad and Nax) and diffusion (Drad and Dax) are calculated with the so-called 追 -model, [18].

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The adsorption equilibrium X* is approximated using the Dubinin-Astakhov equation which is based on the micro pore filling theory of Polanyi [19] (eq. (6) and (7)). The adsorption enthalpy FHads as a function of the water loading of the zeolite xs is calculated from the adsorption isotherms by applying the van’t Hoff equation (eq. (8)). The adsorption rate is modeled by a linear driving force (LDF) approach ([20] eq. (9)). In the LDFapproach, the mass transport resistant from the fluid phase into the micro pores of the zeolite pellet is reduced to an overall mass transport resistant kLDF. Momentum equation: 挺賑肉肉擢擢通肉岫追岻擢椎噺伐血怠憲捗岫堅岻伐血態憲捗岫堅岻態髪岾堅峇擢佃

追

with the terms: 血怠噺なのど

岷怠貸敵岫追岻峅鉄 "挺賑肉肉鳥濡鉄

敵岫追岻典

擢

""""""血態噺なばの

岷怠貸敵岫追岻峅鉄 "諦肉敵岫追岻典

Energy equation (fixed bed):

""

"怠擢

"追擢追

岾

擢脹追堅擢追峇

Mass equation: 怠擢掴香岫堅岻葱噺擢痛

擢

髪

追擢追

鳥濡

挺賑肉肉

,

挺肉

噺にど ̋®ı岫ぬの糾など貸戴迎結待岻 "" 項劇噺" 項建" 岫月銚鳥鎚" 捲鎚岻

範香岫堅岻貢捗潔椎捗髪盤な伐香岫堅岻匪貢鎚盤潔椎鎚髪捲鎚潔椎銚鳥鎚" 匪飯

擢脹銚掴擢佃

擢

擢佃

岾&追堅

(2)

擢追

擢掴葱擢追

Adsorption equilibrium:

伐憲捗岫堅岻貢捗潔椎捗

峇髪

擢

擢佃

&銚掴

擢掴葱擢佃

擢脹擢佃

髪貢鎚盤な伐香岫堅岻匪

伐憲捗岫堅岻

擢掴葱擢佃

髪

諦濡

諦肉

擢

擢痛

盤な伐香岫堅岻匪

擢掴濡擢痛

隙茅噺貢銚鳥鎚岫劇岻糾激

(3a,b,c)

(4)

(5)

(6)

With the adsorption volume W and the adsorption potential A: 凋津

椎

"畦" 噺 " 迎直劇"Øº 岾轍峇

激噺激待 ̋®ı 峙伐岾峇峩 , 帳

椎葱

(7)

From experimental investigation the following parameters have been obtained W0 = 341.03 ml/g;

E = 1192.3 kJ/kg; n = 1.55

Adsorption enthalpy: 擢椎伐茎銚鳥鎚噺迎直岾エ葱岻"峇

Adsorption rate:

擢岫怠脹

(8)

掴濡

項捲鎚噺倦挑帖庁岷隙茅岫劇喧栂岻伐捲鎚峅項建

(9)

3.2. Numerical investigation of the integrated reactor design A concept of an open sorption store integrated in a building’s heating system has already been elaborated in the MonoSorp project [1]. The sorption store has been designed as an integrated reactor. This concept is now developed further in the SolSpaces project [21]. Based on the experience gained from the MonoSorp project a segmentation of the sorption store is performed to reduce heat capacities and heat losses during adsorption and desorption. A schematic drawing of the developed integrated reactor design of the thermochemical energy store is depicted in Fig. 1 (a, b). The air is entering the reactor through an air flow channel located in the central part of the TCES. The reactor is separated in four horizontal and several vertical segments, each segment

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consisting of two layers of zeolites and air gaps for the fluid flow. The arrows in Fig. 1 (b) indicate the flow direction of the air. From the inner flow channel the air is flowing into the two air gaps at the bottom and top of the activated segment. It is passing through the beds of zeolite and then entering the air gap in the center of the segment where the two air flows are merged together. The air is leaving the segment through an air duct which is located at the edge of the thermochemical energy store. Air dampers in each segment control the air flow in the segmented energy store. This means that only one segment is activated for adsorption or desorption at a time.

(a)

(b)

(c)

Fig. 1: 3D-view of the integrated reactor design of the thermochemical energy store (a); Vertical cut through the thermochemical energy store: the arrows indicate the flow direction through the activated segment (b); calculated velocity profile in the y-z plane (vertical cut through one segment) at x = L/2 (c)

The geometry of the segments has been derived from three dimensional fluid flow calculations with the finite element software COMSOL Multiphysics. An important issue during the designing process is a homogenous fluid flow through the zeolite bed. Only by this, a constant heat release over the complete adsorption process can be achieved. In Fig. 1, c the calculated velocity profile in the y-z plane of one segment at x = L/2 is depicted. The mass flow rate of the air at the flow channel inlet has been set to 兼岌捗噺200 kg/h. It can be seen that the flow velocity is slightly higher in the region with the reduced bed height (left side of the reactor in Fig. 1, c). However, a quite homogenous velocity profile in the fixed bed is obtained with flow velocities in the range of 0.35 to 0.5 m/s. For the calculation of the heat and mass transport during adsorption the complexity of the threedimensional model was reduced to a two-dimensional model as this saves considerable calculation time. For the y-z-plane at x = L/2 the heat and mass transport in the fixed bed of zeolite was calculated only. The fluid flow in this plane was recalculated by applying a pressure boundary condition at the fixed bed inlet and outlet (cf. Fig. 2 left). The pressure field from the 3D-flow simulation was used. The following boundary conditions have been applied: ‚ the air enters the fixed bed at a temperature of 30 °C and a water content of 7.5 g/kg (50 % relative humidity at 20 °C), ‚ at the beginning of the adsorption process the storage material has a water loading of 84 g/kg (desorption at 180 °C, water content of the airflow of 6.2 g/kg); the initial temperature of the storage material is 20 °C, ‚ heat losses of the reactor to the ambient are neglected as they strongly depend on the physical implementation of the reactor design such as construction and insulation materials. With equation (6) and (7) the maximum water uptake of the material is calculated to 238 g/kg. By applying equation (8) the mean adsorption enthalpy is determined to 3470 kJ per kg adsorbed water. Under these adsorption/desorption conditions the energy storage density of zeolite is 158.3 kWh/m³ (bulk


density of zeolite 13XBFK of approximately 690 kg/m³). In Fig. 2 (left) the velocity profile calculated with the 2D model is depicted. A good agreement with the 3D flow simulation is obtained which proofs the applicability of the 2D model. In the middle and right figure the temperature and water loading of the zeolite is depicted at the fixed bed outlet at position b1 and b2. After heating up the material to the adsorption temperature, a maximum temperature of 56 °C is obtained in the fixed bed. Assuming a similar temperature profile across the whole segment a thermal power of 1.46 kW is provided by the airflow.

Fig. 2: 2D-flow simulation: calculated velocity profile in the y-z plane at x = L/2 (left); temperature (middle) and water loading of zeolite (right) at the fixed bed outlet at position b1 and b2 and average temperature on the boundary of the fixed bed outlet.

Over a long period of time, the water loading of the material at the outlet (position b 1 and b2) remains unchanged as the water vapor of the airflow is completely adsorbed by the storage material in the upper layers. The fast increase of water loading after 0.7 (b1) and 1 h (b2) together with the steep decrease in temperature after 1 (b1) and 1.3 h (b2) is attributed to the fast reaction kinetic of the zeolite 13XBFK. The decrease in temperature and the increase in water loading start earlier at position b 1 compared to position b2. This can be explained by the flow profile through the fixed bed. In regions with a higher flow velocity the adsorption is completed earlier resulting in earlier temperature decrease and water loading increase. These results point out the importance of a uniform flow profile in the fixed bed reactor. Despite the slightly inhomogeneous fluid flow a high mean temperature T M can be maintained over almost the entire adsorption. 3.3. Numerical investigation of the external reactor design The concept of the external reactor design has been elaborated within the CWS project [3]. The external reactor design is characterized by a separation of the material reservoir and the reactor (cf. Fig. 3). The reactor itself consists of a containment for the storage material of approximately 20 l (L x B x H = 0.08 x 0.5 x 0.5 m³) with material funnels at the top and bottom, an air to water heat exchanger (not depicted in the 3D-view) and flow channels at the air inlet and outlet. The reactor is filled with storage material from top and emptied through the material outlet at the bottom. The air is entering the reactor from the side. Flow channels at the inlet and outlet ensure a homogenous fluid flow over the reactor width and height. To keep the pressure loss across the reactor as low as possible the reactor has been designed with a large surface area for the fluid flow and a short reactor length. For the external reactor design a fixed bed operation during adsorption has been investigated. In the operation mode the zeolite is in fixed position during the whole adsorption process. When complete adsorption is achieved and the temperature at the air outlet decreases the material is removed from the reactor and the reactor is refilled from top with new storage material.

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For the calculation of the heat and mass transport a two dimensional model of the reactor has been implemented in the simulation software COMSOL (cf. Fig. 3, left).

Fig. 3: 2D- and 3D-view of the external reactor design of the thermochemical energy store.

A homogenous air flow over the reactor width and height with a total mass flow of 200 kg/h has been assumed. The same boundary conditions as for the integrated reactor design have been applied for the heat and mass transport simulation. In Fig. 4 the temperature (left) and the water loading of the zeolite (right) at four different axial positions in the reactor (l1 = 20 mm, l2 = 40 mm, l3 = 60 mm, l4 = 80 mm) are depicted. The adsorption front moving in axial direction through the reactor is clearly visible. At the beginning of the adsorption the heat released is used to heat up the storage material to the adsorption temperature. After that, during almost the complete adsorption a high temperature increase of the airflow of approximately 26 K is achieved. This corresponds to a thermal power output of 1.46 kW.

Fig. 4: Temperature profile (left) and water vapour uptake of zeolite (right) during water vapour adsorption of zeolite 13XBFK at different axial position in the fixed bed reactor

If - for technical use - a minimum temperature at the reactor outlet of 45 °C is required, 98 % of the energy stored in the material can be used. This high value reaffirms the very good adsorption behaviour of the zeolite under investigation which is characterized by a very fast reaction rate and a fast heat release. 3.4. Experimental investigation of the external reactor design For a proof of concept of the developed external reactor design a laboratory prototype of the reactor has been built and experimentally tested on a test rig. The reactor prototype and a schematic drawing of the setup of the test rig are depicted in Fig. 5. The reactor has a cross flow section for the air flow of 0.53 x 0.53 m² and a bed length of 0.085 m. This corresponds to a storage material mass of 16.5 kg inside of the reactor. For the discharge of the thermochemical energy store (adsorption), ambient air is drawn from the surrounding, is first passed through an air to air heat exchanger to preheat the air flow and is then entering


the reactor. In the reactor the exothermic adsorption takes place and the air is heated up to the adsorption temperature. Via the airflow the heat of adsorption is transported to the air to oil heat which is located directly downstream of the reactor. The air is then passed through the air to air heat exchanger to preheat the incoming air and then released to the ambient.

Fig. 5: Prototyp of the external reactor (left) and schematic drawing of the experimental test rig (right)

The following measured values are determined (cf. Fig. 5): ‚ Tamb, xamb: ambient temperature and water content of the air ‚ Tf,0, Tf,1, Tf,1’: supply air temperature, discharge air temperature in front and behind the air to air heat exchanger ‚ TR,0, TR,1, TR,2, TR,3: temperature inside of the reactor (center) at L = 0 mm, 10 mm, 35 mm, 80 mm ‚ TR,1*, TR,2*, TR,3*: Temperature inside of the reactor (upper right corner) at L = 10 mm, 35 mm, 80 mm ‚ xf,0, xf,1: water content of the supply and discharge air ‚ 兼岌捗 : mass flow rate of the air 岌 : thermal power transmitted in the air to oil heat exchanger (calculated) ‚ 芸朕掴 During adsorption, the air was drawn from the ambient at a temperature of 21 °C with a relative humidity of 45 % (water content of 7.0 g/kg). The air flow in the system was set to a maximum mass flow rate of 125 kg/h. The zeolite filled in the reactor was previously dried in an oven at 180 °C (water vapor content of the air of 7.8 g/kg) and then cooled down to ambient temperature in a hermetically sealed container. The thermal oil temperature in the oil circuit is controlled by a thermostat to a constant temperature of 30 °C (at the heat exchanger inlet). In Fig. 6 the temperatures and the water content of the air flow measured during the adsorption experiment are depicted. The adsorption front moving through the reactor can be clearly seen by the temperature increase and decrease at the axial distributed temperature sensors inside of the reactor. A maximum temperature increase of 25 K is obtained, corresponding to a thermal power of 860 W. After one hour of adsorption the temperature in the first measuring plane, after 3 and 4.7 hours in measuring plane 2 and 3 start to decrease. After 5.5 hours the adsorption is completed. In the near wall region the temperature decrease starts earlier compared to the central part of the reactor. This is due to a higher bed porosity and higher flow velocity of the air flow. As a result the adsorption front in the near wall region is moving faster through the reactor. The water content of the air flow leaving the reactor already starts to increase after 3.5 hours of adsorption. The early begin and the significant flatter gradient of the curve compared to the results of the numerical model can be explained by the inhomogeneous fluid flow. In near wall regions the adsorption is completed earlier compared to the central part of the reactor resulting in an increase of the mean water

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content of the airflow.

Fig. 6: Experimental results of the adsorption experiment performed with the prototype of the external reactor design: temperature measured inside the fixed bed reactor and at different positions in the test rig; water vapour content of the supply and discharge air

A thermal power of approximately 500 W is transferred to the oil circuit in the air to water heat exchanger. This is 60 % of the thermal power released inside of the reactor. An insufficient heat insulation of the reactor is the main reason for this comparatively low value. The experimental results are in good agreement with the numerical results. The temperature increase calculated with the numerical model is 26 K (water content of the air flow of 7.5 g/kg) compared to 25 K (water content of the air flow of 7 g/kg) in the experiment. The steep temperature decrease in the experiment confirms the fast reaction rate of the zeolite under investigation. The slower decrease in temperature compared to the results of the simulation is mainly caused by a slower mass flow rate of the air (125 kg/h in the experiment compared to 200 kg/h in the simulation). This leads to a slower cooling of the material as the heat transport by convection is lower. The experimental investigation of the prototype reactor reveals a high thermal performance of the developed reactor design and confirms the results of the numerical investigation. 4. Summary and Conclusion An integrated and an external reactor design for thermochemical energy storage have been investigated. As storage material zeolite 13XBFK has been used as this material is characterized by a fast reaction kinetic and comparatively high energy storage density. By means of numerical simulation the heat and mass transport during the exothermic reaction (adsorption) has been studied. A prototype of an external reactor design has been experimentally tested on a laboratory test rig. The experimental result has proven the concept of the developed reactor design and the validity of the numerical model. With the integrated and external reactor design two different reactor concepts are available which can be chosen according to the application purpose. The numerical simulations and the physical testing have demonstrated that both reactor designs offer a high thermal performance. For a low power heating system, a sufficient thermal power output can be provided. However, the experimental and numerical results have revealed that a homogenous fluid flow through the storage material is of great importance. Only by this, it can be ensured that a high thermal power output together with a high temperature increase can be maintained over the complete adsorption process. This places high demands on the reactor design and requires an in-depth understanding of the physical process inside of the reactor. The numerical models together with physical testing build an excellent base for the development process and allow a targeted designing of the reactor. To date, a prototype of the integrated reactor design is under construction for experimental


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