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1 Thermochemical Energy Storage as A Way to Increase The Sustainability of Energy Generation Johannes Widhalm, Vienna University of Technology, Austria Thomas Fellner, Vienna University of Technology, Austria Markus Deutsch, Vienna University of Technology, Austria Andreas Werner, Vienna University of Technology, Austria Franz Winter, Vienna University of technology, Austria The Asian Conference on Sustainability, Energy and the Environment 2015 Official Conference Proceedings Abstract For the future of the energy sector it is necessary to increase the use of unused potentials by waste heat at low temperature (T < 350 C) by industrial processes under the focus of economic conditions. Achieving this objective involves, taking full advantage of all the potential arising from the separation between generation und consumption of heat. On the basis of this request, it is necessary on the one hand to store heat over a long period of time and on the other hand to transport heat from A to B. These requirements would not be fulfilled through the technology of sensible or latent heat storage systems. The storage of heat by the thermo chemical storage technology is one opportunity to reach these targets. The main advantage of this technology is that the storage density is a factor 10 higher by the TCS (MgO + H2O -> Mg(OH)2) in comparison to the sensible storage technology (water 60 C -> 90 C) and that the storage material can easily be transported by a truck. The storage and transport of heat without loss is the basic condition for an energy-, resource-, and cost-efficient future in the energy sector. Through consideration of the TCS in energetic, exergetic and economic sight it is possible to formulate well-founded conclusions. For this purpose it was necessary to depict different process configurations in the process simulation tool IpsePro. On basis of these simulations it was possible to compare the different processes and to optimize the system configurations. Keywords: Thermochemical heat storage, magnesium oxide, theoretical storage density, real storage density, economic, process integration iafor The International Academic Forum

2 Introduction Storage is one of the most important topics for the sustainability of energy generation in the world. But mostly we think about the storage of electricity. When we take a look to Figure 1 we see that also the energy in form of heat has a big share on the world energy consume. Figure 1: Energy demand of EU27 [1]. Sensible heat storage was for a long time the only available storing technology. Recently more sophisticated techniques are researched and developed, such as latent heat storage or storage through sorption. The techniques of heat storage by sensible and latent storages have a number of disadvantages for long-term heat storage. There are heat losses to the ambient over time and a relative low energy density (large storage volumes) [2]. The implementation of high efficient and compact TES systems will be the next innovation step in the future of heat storage [3]. As an alternative to the descript techniques, it is possible to store energy in a thermochemical storage material (TCSM). This system uses reversible chemical reactions A(s) + B(g) = C(s) + heat for the storage process. Reactants are interesting if they have the following characteristics: low cost, non-toxic, non-corrosive, sufficient energy storage density, cycle stability and reaction temperatures in the proper range with a fast kinetic. A large number of hydrates, hydroxides, carbonates and solvates can fulfill these requirements [4][5]. The technique of thermochemical energy storage is based on reversible chemical reactions. They use the binding energy of the molecules. The principle of the thermochemical heat storage can be seen in Figure 2 in the example of heterogeneous reaction of magnesiumoxide in a solid form plus water in gaseous form to magnesiumhydroxide in a solid form. The storage is done through the feeding of heat at a temperature level T 1 shown on the left side of the figure. Due that a chemical binding is separated in basic molecules MgO(s) and H 2 O(g). If the basic bonding react together to molecule Mg(OH) 2 (s) as before the splitting the enthalpy of reaction will be released at a temperature level of T 2.

3 Figure 2: Thermochemical heat storage cycle related to Magnesium. To control the reaction it is relevant to know the relation between the equilibrium temperature to the partial pressure of water p H2O. Figure 3 shows the equilibrium curve. On the curve the reaction MgO(s) + H 2 O(g) = Mg(OH) 2 has in both directions the same velocity. So the content of MgO and Mg(OH) 2 reaches after an infinity time the content 50:50%. For the partial pressure of 1 (log(1)=0) the connected equilibrium temperature over the curve is 265 C. If the temperature level is over this equilibrium temperature the system is in the storing mode and if it is under this temperature it s in the releasing mode. All these parameters influence the kinetic of the reaction [6]. Figure 3: Equilibrium temperature over partial pressure of water. This paper is focused on chemical reaction systems by using solid-gas reactions, but there are also other types as gas-gas and liquid-gas reactions available [7].

4 Theoretical heat storage There are lots of reversible reactions, which could be used for heat storage. An important criterion for the right choice of the TCSM is the temperature for heat storage and heat release as seen in Figure 2 given by temperature T 1 and T 2. These two temperatures are connected by the equilibrium temperature and so it s necessary to consider them with respect to this temperature. Another important factor is the specific storage content or the specific storage density. Both of them are connected to the enthalpy of reaction, which can be calculated by the following equation: H! = H!! (products) H!! (educts) Figure 4 and 5 show a certain choice of TCSM. For the considered application in this paper the temperature range is chosen from 100 C to 300 C. Figure 4: Specific storage content. Left: related to educts / right: related to product. Figure 4 shows that the theoretical specific storage content from the reaction MgO(s) + H2O(g) = Mg(OH) 2 (s) differs in relation to the reference material educts (MgO) or product (Mg(OH) 2 ). If the specific storage content is compared to sensible storage by water (heated up from 20 to 80 C) it shows that the MgO reaction has related to the educts a 7 times higher storage content and if it is related to the product the storage content is 4 times higher. Another way to compare different thermal energy storage systems can be done over the volumetric value. Figure 5: Storage density related to educts. Left: porosity = 0,5 / right: porosity = 0,7.

5 The storage density related to the educts for two different bulk porosities can be seen in the two diagrams of Figure 5. It can be seen that the bulk porosity has an important influence on the storage density. By considering the educts (MgO) of the MgO(s) + H 2 O(g) = Mg(OH) 2 (s) reaction with a bulk porosity of φ = 0,5 the storage density compared to sensible water is 13 times higher. By the use of a commercial available TCSM the bulk porosity is 0,7 and this decreases the storage density and compared to water it is 7 times higher. It can be seen that the relation of the TCSM is an important factor for the storage density. The further sheets of this paper considers only the reaction MgO(s) + H 2 O(g) = Mg(OH) 2, because of the fact that this reaction has the highest storage density in a certain temperature range C with respect to the educt water. The used MgO is in a granular form with a mean particle diameter of 355 µm and a bulk porosity φ = 0,7. Reactor system For further research on the system it is necessary to choose the right reactor system for the right material. There are a certain amount of reactor systems with advantages and disadvantages [8]. There are common technologies listed: Fluidized bed reactor Free fall reactor Rotary kiln reactor Screw reactor Screw reactors and rotary kiln reactors are complex to operate and to seal because of the rotating mechanical system. The heat transfer coefficients are lower than by the other two technologies. The free fall reactor cannot be used, because of the low residence time, which is usually between 1 and 10 s [9]. For further research a fluidized bed reactor was modeled in the process simulation program IpsePro from the company Simtech because of the following advantages: Flow able particles (behavior like liquid) High surface (Gas particle) Particle mixing (Distribution particle species) Homogeneous temperature distribution (axial and radial) High heat transfer coefficient High mass transport One of the limitations of the use of a fluidized bed reactor is the particle size over the density difference between particle and fluidization gas. The usage of the reactor for the chosen material can be checked with the Geldart diagram. The used material is in the group B and so there is no problem for the usage of this reactor for this application. For the heat transfer a bed heat exchanger is installed. First simulations by the IpsePro model show a heat transfer coefficient between W/(m²*K) [10]. Through the set of the bed height to a value of 0,85 m the pressure loss is always the same for the simulation and the diameter of the reactor is a result. It is also

6 assumed that there is no complete conversion of the TCSM. The conversion rate is set to a value of 90% for the heat storage and release. Processintegration One of the most effective ways to deliver heat to areas with high heat demand densities is the usage of a district heating system [11]. In order to supply a district heating network cost effectively and energy efficiently with waste heat from industrial processes it is necessary to transport the thermal energy by mobilized thermal chemical storage (M-TCS) [12]. To consider a TCS system correctly and completely it is also necessary to choose an application and a system with all components. This paper considers a possible application, which is shown in figure 6. This figure illustrates a part of Austria with three district heating networks shown by the blue circuits and a waste heat source shown by the grey triangle in the middle of the map [13]. Figure 6: Process application. Store: brick factory / Release: district heating network A conventional technique for heat storing could not be used through the fact that the heat must be stored and transported to different customers in a distance over 3 to 4 km. A connection from the waste heat source to the district heating network would also be a problem through the high investment costs of the piping and through the time offset between heat production and heat consumption. So it s thinkable that the thermochemical heat storage is an opportunity to use this waste heat besides electricity production by a low efficiency of 12,8 % (Cement plant Lengfurt) [14]. One possible flow sheet of the storing and releasing process is shown in figure 7. Both systems are closed systems. This means, that the gas for the fluidization is in a cycle. The green color signs the fluidization gas of the system. The main content is nitrogen, which is mixed with water. The water content is related to the process application and the kinetic of reaction. Especially the heat release (hydration) application needs three stages for the injection of water. These stages are combined with an up heating of the fluidization gas. Through this the condensation of water in the carrier gas (nitrogen) is prevented.

7 Figure 7: Flow sheet of the TCS-Application. The following figure shows the simulation results of the system, which is shown in Figure 7. It can be seen that the storage density is decreasing compared to the theoretical storage density (see Figure 5). If the condenser heat could be used in a district heating network by the heat storage system the storage density can be increased crucial. To identify the losses of this system an energy and exergy analysis is done. Figure 8: Calculation results for MgO + H 2 O = Mg(OH) 2 shown in a Figure 5

8 Energy & exergy analysis The shown process from Figure 7 is simulated and the results are used to implement an energy, exergy and economic analyze of the whole system. The thermochemical storage process is optimized to highest energy efficiencies. This is equal to a high storage density of the TCSM. Another optimization point is the residence time of the particle in the reactor. This parameter does not influence the efficiency of the whole process, but truly the investment costs for the economic analyze. This will be described clearly in the next section. The following two figures show the whole cycle of the TCS process in one glace from the view of energy and exergy. The two main boundary conditions for this process are the transport temperature of the TCSM, whit 25 C and the conversation rate with a value of 90 %. All shown percentage values in the sankey diagram of Figure 9 are related to the input energy of the storage reactor (625,3 kw = 100 %). The energy efficiency of the whole system is 35,87 % (without condenser heat). This value is not equal to the usable heat value of the heat output reactor with 33,6 %, because of the fact that the whole input energy of heat exchangers 80,2 %, blowers 13,5 % and injectors 0,9 % results only to 94,6 %. The values of the reactor losses are not shown through the low losses of the reactor itself. The other losses are from the filters 0,6 %, storage & transport 18,1 % and condenser 41,5 %. The energy analysis shows, that the fully or partially use of condenser heat increases the whole efficiency of the system up to 80,1%. The second big share of the losses is given by the storage & transport of the TCSM. It is assumed that the material is transported by only 25 C. That means that the sensible losses especially by the heat storage system (330 C to 25 C) are enormous through the loss of sensible energy. It could be practicable to use special trucks and storage facilities to avoid the loss of sensible energy. To compare the TCS application with electricity production an exergy analysis is done and can be seen in Figure 10. The shown percentage values in the sankey diagram are related to the input exergy of the storage reactor (299,7 kw = 100%). The exergy efficiency is calculated with 10,1 %, if the heat of the condenser can be used fully or partially the efficiency increases up to 18,5 %. An interesting point is that the exergy efficiency is very low in comparison to the energy efficiency. This is describable by the fact that the heat is converted from a temperature level of approximately 350 C to a level of approximately 85 C. Compared to the electricity production of a similar process it is in a similar range [14]. The whole input exergy is delivered from one heat exchanger 88% and the blowers 28,4 %. In contrast to the energy analysis the losses in the exergy analysis are especially high in the reactors 59,7 % and the losses in the condenser has a low value 9,8 %. The other losses are placed in the other process components heat exchangers 3,7 % injectors 10,3 %, storage & transport 10 % and blowers 10,6 %.

9 Figure 9: Sankey diagram of the energy analysis

10 Figure 10: Sankey diagram of the exergy analysis

11 Economic analysis There are several possibilities to evaluate the profitability of an investment for an industrial application. One of them is the investment calculation. The different methods of the investment calculation provide an opportunity to get significant results. The results of the investment calculation give a decision support for the usage of thermochemical energy storage application. To rate the quality of an investment a dynamic method of investment calculations is used in this paper. This method considers the date of incoming and outgoing payments [15][16]. Through the use of the net present value method the necessary heat price per MWh is calculated. There are three main parts of cost: direct investment costs fix costs (including: repair, maintenance and personal costs) variable costs (including: operation, TCSM, heat and transport costs) The direct investment costs are calculated over the costs of the different components. The following components are used in the system: Reactor Filter Heat exchanger Blower Pumps Storage tank Injectors The costs for pipe works, assembling and engineering are assumed with 20 % of the components costs. The whole sum of the components costs, pipe work costs and engineering costs amount to the direct investment costs. It is assumed that the investment costs are paid from the company`s own funds. The depreciation period is taken as equal to the systems lifetime and the average operation time of both systems (heat storage and heat release) is assumed 20 years. Other assumptions are: yearly maintenance costs are 2,5 % of the direct investment costs yearly repair costs are 2 % of the direct investment costs inflation and escalation rates are assumed to be zero no influence from the CO 2 price market energy transport capacity of a truck is 19,2 MWh th /truck

12 Figure 11: Results of the economic analyze. Figure 11 shows the results of the economic analysis. Without using the condenser heat the heat price has a value of /MWh th over the operation hours per year of the heat release systems. The both cake diagrams show the distribution of the costs for 2210 and 3250 operation hours of the heat release system. The transport costs have a small influence on the whole heat price. The operation costs represent the greatest value, but they are only changeable through the use of another reactor system. The energy analysis shows that it is a crucial advantage if the condenser heat can be used. Through this enormous influence the heat price can be decreased to /MWh th. The dashed red line shows the possible market price from today (45 60 /MWh th )[17]. By the usage of the condenser heat we have a competitive system today. The system without condenser heat is not competitive against other technologies for the heat production. If the fuel costs of conventional heat production techniques are rising, the thermochemical storage technique could be more competitive in the future. The influence of the CO 2 certificate costs is not considered, because of the running process to modify the consisting system by the European commission. The calculation of a system with heat production by natural gas 233,3 (kg CO 2 )/MWh th [18] and the market price 7,02 /t CO2 [19] results in extra costs of 1,63 /MWh th. This means that the CO 2 certificate costs have no important influence on the market price of heat.

13 Conclusion A brickyard factory as heat storage location and three district heating networks as heat release locations are considered. A conceptual mobilized thermal energy storage system was evaluated in a technical way (energy and exergy analysis) and economic way. According to these analyses, the following conclusions are made: By the comparison of different TCSM it is on the one hand important that the values are related to the form of the storage material (educts or products, bulk density) and at the other hand that the whole system is considered. The M-TCS system can raise the global amount of heat without using fuels, through the increasing of heat production by waste heat sources. Cost of heat could be in a range from 88 to 108 /MWh th and could be decreased if the condenser heat could be used from 39 to 49 /MWh th. Certain applications which use condenser heat have potential for realization. The transport costs with approximately 8 % have no important influence on the whole system. The costs of CO 2 certificates have no important influence on the economic feasibility.

14 References Energieverbrauch für EU 27 aus 2009 Quelle IEA Database N Tsoukpoe K. E., Liu H., Nolwenn P., Lingai L.: A review on long-term sorption solar energy storage. Renewable and Sustainable Energy Reviews 13, , 2009 DLR H. Kerskes, H. Drück (2011): Energetic and economic aspects of seasonal heat storage in single and multifamily houses, ESTEC 2011, October 2011, Marseille, France Kerskes, Henner; Bertsch, Florian; Mette, Barbara; Wörner, Antje; Schaube, Franziska (2011): Thermochemische Energiespeicher: Thermochemical Energy Storage. In: Chemie Ingenieur Technik 83 (11), S W.E. Wentworth, E. Chen (1976): Simple thermal decomposition reactions for storage of solar thermal energy. Solar Energy, Vol. 18, No. 3, pp Yukitaka Kato, Norimichi Yamahita, Kei Kobayashinand, Yoshi Yoshizawa (1996): Kinetic study of the hydration of magnesium oxide for a chemical heat pump. In: Applied thermal engineering Vol. 16, No 11, pp T. Yan, R.Z. Wang,T.X. Li, L.W. Wang, T. Ishugah (2015): A review of promising candidate reactions for chemical heat storage. Renewable and Sustainable Energy Reviews, No. 0, Vol. 43, pp H.A. Zondag, Alex Kalbasenka, Martijn van Essen, Lucas Bleijendaal, Roelof Schuitema, Wim van Helden, Lucienne Krosse: First studies in reactor concepts for Thermochemical Storage P. Pardo, Z. Anxionnaz-Minvielle, S. Rougé, P. Cognet, M. Cabassud, Ca(OH)2/CaO reversible reaction in a fluidized bed reactor for thermochemical heat storage, Solar Energy, Volume 107, September 2014, Pages , ISSN X, A Levenspiel, Daizo KuniiOctave (1991): Fluidization Engineering Second Edition, ISBN: S.F.Nilsson, C. Reidhav, K. Lygnerud, S. Werner (2008): Sparse district-heating in Sweden. Applied Energy, 85(7), pp Li. Hailong, W. Wang,Y. Jinyue, E. Dahlquist (2013): Economic assessment of the mobilized thermal energy storage (M-TES) system for distributed heat supply.applied Energy, Vol. 104, No. 0, pp Bewertung des Potentials für den Einsatz der hocheffizienten KWK und Effizienten Fernwärme und Fernkälte Versorgung (EEG / IET / Ecofüs) -> Map online in autumn 2015

15 Brandstätter R. (2008): Industrielle Abwärmenutzung, Beispiel & Technologien. Grundlagen der Investitionsrechnung: Eine Darstellung anhand einer Fallstudie, 2007 Oldenbourg Wissenschaftsverlag GmbH D. Brennan (1998): Process Industry Economics: an International Perspective, ISBN: Wien Energie (2012): Geschäftsbericht Wien Energie European commission (July, 2006): Economics and Cross-Media Effects CO2 Zertifikat Preis, Contact