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High temperature solid media thermal energy storage system with high effective storage densities for flexible heat supply in electric vehicles
Accepted Manuscript High temperature solid media thermal energy storage system with high effective storage densities for flexible heat supply in electric vehicles Volker Dreißigacker, Sergej Belik PII: DOI: Reference:
S1359-4311(18)30184-4 https://doi.org/10.1016/j.applthermaleng.2018.12.026 ATE 13033
To appear in:
Applied Thermal Engineering
Received Date: Revised Date: Accepted Date:
9 January 2018 30 July 2018 4 December 2018
Please cite this article as: V. Dreißigacker, S. Belik, High temperature solid media thermal energy storage system with high effective storage densities for flexible heat supply in electric vehicles, Applied Thermal Engineering (2018), doi: https://doi.org/10.1016/j.applthermaleng.2018.12.026
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
High temperature solid media thermal energy storage system with high effective storage densities for flexible heat supply in electric vehicles Volker Dreißigacker1, Sergej Belik2 1
Institute of Engineering Thermodynamics, German Aerospace Center, Pfaffenwaldring 38-40, Stuttgart, Germany, Phone: 49-711-6862449, Fax: 49-711-6862747, e-mail: [email protected] 2 Institute of Engineering Thermodynamics, German Aerospace Center, Pfaffenwaldring 38-40, Stuttgart, Germany, Phone: 49-711-6862512, Fax: 49-711-6862747, e-mail: [email protected]
Abstract One major challenge for the successful transformation of the transportation system towards electromobility is strongly linked to its maximum range. Such range reducing impacts emerge exemplary during cold winter days, where electric energy is used from the battery to heat the interior. In spite of significant improvements in energy density of battery systems, alternative ideas for suitable thermal management concepts are necessary to solve the conflict between traction and heating. To overcome this disadvantage a novel concept based on an electrical heated sensible solid media thermal energy storage system is outlined. Central elements include a high efficient thermal insulation concept and a bypass operation system to allow high effective energy densities and simultaneously a flexible supply of thermal energy with defined specifications during thermal discharging. For this purpose, simulation studies regarding the insulation concept and the geometric design of the thermal energy storage for two feasible material options were conducted. Under a wide range of solutions, promising designs with high effective energy densities, low parasitic losses and high thermal discharging power were identified.
Keywords Solid media thermal energy storage; power-to-heat; high effective thermal storage density; electric vehicles
Introduction
With increasing number of electric vehicles, suitable thermal management concepts are needed due to the lack of thermal heat from missing combustion engines and the demand on thermal energy for heating the interior [1, 2]. Today, thermal energy is generated in electric vehicles by PTC (Positive Temperature Coefficient) heating elements [3] and powered by the vehicle´s traction battery. Under cold environmental conditions for example during winter days, the range of the vehicle can be reduced by the half, where heating powers up to 5 kW are required [4]. Thus, a successful conversion of the mobility towards electrified vehicles is highly associated with innovate solutions for thermal management. Beside the direct generation of heat through PTC-elements, thermal energy storage systems open up an alternative way for supplying heat [5, 6]. The temporal decoupled process and a wide range of thermal storage technologies allow manifold thermal management concepts for cabin climatisation as well as for preheating. Up to now, several investigations for thermal
storage systems based on Phase Change Materials (PCM) [7] for pre-heating of catalytic converters [8] and engines [9] in combustion vehicles and ice slurry [10] for cooling applications are conducted. New thermal management concepts based on thermochemical energy storage systems (solid/gas reaction) [11] are growing in the last years, allowing an alternative heat supply in electric and fuel cell vehicles. Especially for use in electric vehicles, two crucial requirements must be satisfied by the thermal energy storage system: high effective thermal storage density and high thermal discharging power. Former can be achieved by using high temperature heat, by utilization of phase change or reaction enthalpies and efficient thermal insulation designs. Latter needs high heat transport rates (conductive and/or convective) as well as sufficient heat transport areas. A promising solution to fulfil both requirements is given by sensible solid media thermal energy storage systems, when high temperature heat can be provided. Due to missing high temperature sources within an electric vehicle, an alternative supply method must be identified. In this contribution a novel concept based on electric heated solid media thermal energy storage for cabin climatisation in electric vehicles is outlined. The required high temperature is provided through direct resistant heating of the storage material (Powerto-Heat). During the drive (discharge period) the defined air temperature condition for cabin climatisation is adjusted via a bypass operation. The efficiency and the flexibility of this concept are mainly determined within the discharge mode through the geometric design and the material properties of the solid medium as well as the thermal insulation. Thus, to allow statements regarding the effective thermal energy density and the thermal outlet power, the discharge operation is investigated in detail inside this contribution.
Novel concept: electric heated solid media thermal energy storage system
The novel concept of a solid media thermal energy storage system (TES) for climatisation of electric vehicles consists on three central features: a direct electric heating of the solid medium to generate high temperature heat, a controlled bypass system to supply the cabin with specified temperature conditions (Tmix) and an efficient thermal vacuum insulation to allow high effective storage densities. Figure 1 illustrated schematically the functional principle.
Fig. 1: Heat supply in electric vehicles via an electric heated solid medium (charging mode) and a bypass system (discharging mode) Electric heated solid medium (charging period) During TES charging period (vehicle halt) the solid medium is heated up electrically to elevated temperatures - simultaneously to the charging mode of the battery - ensuring
high thermal storage densities. Although this Power-to-Heat procedure is not focused in this work, a well suited technical realisation for vehicle applications is based on direct resistance heating systems (Joule heating). This technology is widely used in different industrial applications, where various options in design and materials are feasible. Here, inside this contribution a metallic (UNS N06003) and a ceramic (SiC) material option are investigated. Both are well suited for direct electric heating systems and high temperature applications. Bypass system (discharging period) During TES discharge period the vehicle cabin is supplied with heat from the solid medium thermal energy storage system. For that purpose the mass flow of the ambient air is divided via the controlled bypass system in a way to achieve the specified conditions (Tmix) by mixing the high temperature exiting air TES-flow (TF,out) with the cold temperatures from ambient (TU). Such a solid medium storage type with a direct contact between the inventory and the passing fluid is known as regenerator [12, 13] and is widely used in high temperature processes with operation temperatures beyond 1000 °C. Installations of these regenerator-type thermal energy storage systems can be distinguished into moving and stationary beds [14, 15]. For fixed bed arrangements, the storage consists of stacked bricks of various shapes and sizes. Depending on the required thermal characteristics, different types of bricks are used. Where a substantial temperature decrease during discharging can be allowed, e.g. in steel and glass industries, checker bricks with small specific surfaces and small void fractions between 20 % and 40 % can be used. In contrast, applications requiring a low heat resistance, e.g. in regenerative thermal oxidizers (RTO) for the purification of air, honey-comb with high specific surfaces and void fractions (>60 %) are used. Beside a wide range of geometries, various material options depending on the operational temperatures and gaseous conditions are existent. Thermal insulation concept The effective thermal energy density - including the solid medium and the thermal insulation - depends strongly on the maximum electric heating temperatures and the efficiency of the thermal insulation. To increase the thermal energy density higher heating temperatures are required, but leading simultaneously to an increased demand for thermal insulation. In order to allow compact designs in spite of the high temperatures inside the solid medium a vacuum based thermal insulation [22] with extremely low heat conductivity is investigated. This insulation is limited by maximum operation temperatures of 500 °C and requires for elevated temperatures an additional insulation protection layer based on a conventional microporous material [23]. In order to identify suitable TES systems based on the mentioned concept with high effective thermal energy densities including the solid medium and the thermal insulation and with high thermal discharging powers simulation, studies regarding the discharging operation with materials allowing a direct resistance heating are conducted. For that purpose simulation models are needed for the two layered thermal insulation concept and the regenerator-type solid medium storage.
Modelling
For identification of designs with high effective thermal energy densities, small thermal losses and high discharging powers, two thermal simulation models are used. The first one describes the two-layered thermal insulation concept and the second one the temporal and spatial TES characteristics. For clarification of the fundamental equations explained in the following, figure 2 illustrates schematically the TES system including central values and boundary conditions.
Fig. 2: Schematic illustration of the TES model with the two-layered thermal insulation
3.1 Thermal insulation For a cylindrical containment shape with up to two different insulations layers, the heat flow from the inner core Ti via conduction to the surface of the containment TW and via convective heat transport W [16] to the environment TU can be expressed:
(1)
Here, Ins represents the thermal conductivity of the insulation layers 1 and 2, L the length and r the radius of the cylinder at the inner surface with index i, between the two insulation layers with index and at the outer surface AW. For the areas A at the bottom and the top of the containment a comparable formulation can be used, where s stands for the insulation thickness of layer 1 and 2, respectively.
(2)
The total thickness of the thermal insulation, thus the required insulation volume is given by:
(3) Based on these equations the total insulation thickness can be determined by specifications of the maximum temperature limit of insulation layer 2 (T) as well as the outer surface temperature (TW).
3.2 Sensible solid media thermal energy storage system The temporal temperature characteristic of a sensible solid media thermal energy storage system is determined by the inlet conditions, material properties, geometric parameters of the inventory option and the containment. During discharging procedure, the ambient air mass flow passes the thermal energy storage system, receives heat from the previously electric heated solid medium in direct contact and is mixed at the outlet in bypass operation to the specified temperature (see fig. 1). For modelling the discharge procedure of such a regenerator-type solid media heat storage system, a heterogeneous porous continuum approach is used, providing the temporal and spatial temperature distribution for the gaseous (F) and the solid phase (S) [17]. With well justified simplifications regarding the neglection of radial gradients across the flow direction through a uniform flow distribution at the entrance of the solid medium and low heat losses over the surrounding walls, the resulting one dimensional heat balances in time t and space z can be written: Fluid (4)
Solid (5) With the associated boundary conditions: Fluid (6) Solid (7) (8)
Here, TF and TS represent the heat transfer fluid and solid medium temperature, w the fluid velocity, the void fraction, aV and aW the specific heat transfer surface of the
inventory and the surrounding casing relating to the total storage volume (fluid and solid). Effective heat conduction coefficients for the fluid F,z,eff and solid phase S,z,eff are determined by [18, 19] and a real gas model by Lemmon for air [20] is used to calculate the density F and the specific heat capacity cP. The total regenerator heat transfer coefficient k is calculated according to Hausen [14, 21]. The coefficient accounts in a compact way the heat resistance inside the inventory considering the thermal conductivity of the solid, its shape and the time as well as the heat transfer coefficient of the fluid for the investigated channel-shaped bricks as suggested in [14]. This allows calculating the effective heat transfer, which is limited by the solid phase or the fluid phase depending on the material properties, wall thickness and duration or the convective heat transfer coefficient. Thermal losses through the thermal insulation are regarded by kW inside the heat balance equations and by the solid phase boundary conditions. The partial differential equations in (4) and (5) are solved numerically by using a central and a backward finite-difference-method in space for the solid and for the fluid phase respectively. Subsequently, the resulting set of differential algebraic equations including the boundary conditions is solved in time with a commercial simulation tool (Matlab).
Results
Design studies are performed in order to obtain promising solutions for the heat supply in cabine climatisation with high effective storage density and high thermal discharging power. In a first step, an efficient thermal insulation concept is elaborated based on a two-layered design. In a second step, variation studies regarding the geometric parameters of the inventory and the containment are conducted to identify solutions with high storage utilizations via the bypass operation during thermal discharging (see fig. 1). For these simulation steps specifications and material properties must be defined. Essential thermal boundary conditions and target values (Tmix, TW) are listed in table 1. Thermal storage capacity: Q [kWh]
2.5
Thermal discharging power [kW]
5
Discharge duration [min]
30
Mixing temperature: Tmix [°C]
60
Ambient temperature: TU [°C]
-10
Outer surface temperature: TW [°C]
60
Table 1: Boundary conditions and target values The discharge duration of 30 min in this contribution is based on the Worldwide Harmonized Light Duty Test Procedure (WLTP). Inside the simulation studies, a metallic (UNS N06003) and a ceramic (SiC) material option are investigated. Both materials are well suited for direct electric heating
operation and used in high temperature applications for example as heating elements. Here, temperature averaged material properties between 100 °C and 1000 °C are used, which are summarized in table 2. UNS N06003
SiC
Density: S [kg/m3]
8350
2960
Specific heat capacity: cS [J/kgK]
570
1095
Heat conductivity: S [W/mK]
20
71.5
Table 2: Material properties for the solid media Likewise, averaged values for air are used in the same temperature range.
4.1 Thermal insulation concept To achieve low heat losses, an efficient and compact vacuum insulation is used limited by maximum operating temperatures of 500 °C [22]. Beyond these temperatures an additional thermal insulation layer based on a microporous material [23] must be integrated, which serves as thermal protection and allows operating temperatures of up to 1000 °C. Both materials show comparable material densities of 280 kg/m3, whereat the heat conductivity of the vacuum insulation at a temperature of 500 °C (0.003 W/mK) is by the factor 12.5 lower compared to the microporous material (0.038 W/mK). Based on the specifications for thermal storage capacity, outer surface and ambient temperature (table 1) as well as for the selected inventory materials (table 2), simulation studies are conducted to determine the insulation thickness for different electric heating temperatures (Tmax). Inside these calculations, an inventory void fraction of 25 % and an inventory length to diameter ratio L/Di of 1 are assumed exemplarily. The resultant maximum effective gravimetric and volumetric storage densities including the required insulation volume and mass are illustrated in figure 3 and figure 4.
Fig. 3: Maximum effective gravimetric thermal energy densities for various electrical heating temperatures
The effective gravimetric storage densities (figure 3) increase with growing electrical heating temperatures up to a maximum value depending on the material option. This behaviour results from increasing gravimetric energy densities with higher electrical heating temperatures inside the solid medium only and with simultaneously increasing insulation demand to achieve the fixed outer surface temperature of 60 °C. Comparing both materials, higher effective gravimetric storage densities are achieved for SiC caused by a higher specific heat capacity. In contrast, higher effective volumetric storage densities are visible in figure 4 for the metallic option resulting from a significantly higher material density. Here, the highest values are reached at the maximum operating temperature of the efficient vacuum insulation (500 °C). Above this limit the microporous material layer must be integrated requiring considerable larger insulation thicknesses due to the higher heat conductivity.
Fig. 4: Maximum effective volumetric thermal energy densities for various electrical heating temperatures Both exemplary results show, that the electrical heating of a solid media is a promising option to achieve high effective storage densities in spite of the required thermal insulation demand. But under operating conditions - for example a stand still period of several hours – the effects of thermal losses compared to the stored thermal energy must be regarded. Therefore, simulation runs are conducted based on specifications listed in table 1 and dimensions deduced from the maximum effective gravimetric energy densities illustrated in figure 3. This results in a mass of 22.5 kg with a volume of 17.4 dm3 by heating temperatures of 850 °C for the metallic material option and 13.8 kg with a volume of 14.2 dm3 by heating temperatures of 725 °C for the SiC material option. Therefore, the temporal characteristics of the thermal energy related to the initial values are shown in figure 5 over a stand still period of 12 h.
Fig. 5: Temporal characteristics of the stored thermal energy over a stand still period of 12 h for exemplary designs Both materials show comparable heat losses of about 30 % over the stand still period of 12 hours. Here, for the considered case of cabin climatisation with a thermal power of 5 kW (table 1), the discharging duration will be reduced from 30 minutes to 21 minutes due to heat losses after the stand still period. Including these effects moderate high effective storage densities are still available in spite of the long stand still period and the continuously cold ambient conditions of -10 °C. Further improvements in performance can be achieved by adaptions in the geometric design, by maximum temperatures as well as by more efficient thermal insulation options. These exemplary illustrated results related to the maximum effective thermal energy densities point the potential of electrical heated solid media thermal energy storages. However, to achieve a high thermal utilization of the solid medium during discharge operation defined as the ratio of usable to maximum stored thermal energy, promising inventory designs with high heat transport rates need to be identified.
4.2
Thermal discharge operation
During discharge operation ambient air (TU) is heated up via the bypass system (fig. 1) to supply the vehicle cabin with the defined temperature conditions (Tmix). For this purpose, the total mass flow is splitted to compensate the temporal decrease of the TES fluid outlet temperature including heat losses and thus to achieve a constant mixing temperature over the specified discharging duration. Based on the thermal insulation concept in chapter 4.1, additional variation studies regarding the geometric parameters of the inventory (void fraction , specific heat transfer surface aV, diameter Di) and the electrical heating temperatures (Tmax) are conducted to identify solutions with high thermal utilizations during discharge operation. For that, the inventory length L, the thermal insulation thickness and the temporal split of the total mass flow passing the TES are determined iteratively to fulfil the specification of constant mixing temperatures over the discharging operation and the outer surface temperatures (table 1). Additionally, a further target value must be
defined, given by the maximum mass flow passing the TES at the end of discharging. Here, this value related to the total mass flow is set to 80 %. The resulting effective gravimetric and volumetric thermal energy densities for both materials are shown in figure 6 and figure 7. Those results include a wide range of inventory variations concerning the specific heat transfer surface aV (50 – 800 m2/m3), the void fraction (20 – 60 %), the diameter Di (0.05 – 0.3 m) and the electrical heating temperature Tmax (400 – 1000 °C).
Fig. 6: Effective gravimetric and volumetric thermal energy density including the thermal utilization during discharging mode for the metallic inventory option
Fig. 7: Effective gravimetric and volumetric thermal energy density including the thermal utilization during discharging mode for the ceramic inventory option
The results show for both materials a wide range of solutions which differ from their attainable effective thermal energy densities. Relating to the electrical heating temperatures - illustrated in grey coloured dots – increasing gravimetric thermal energy densities are visible up to a maximum value. For the metallic inventory option a maximum value of 105 Wheff/kg is calculated at heating temperatures of 900 C, for the SiC option of 165 Wheff/kg at 800 C. In contrast, the maximum volumetric thermal storage density is reached for both materials at 500 C. This behaviour - comparable with results shown in figure 3 and figure 4 - is caused by increasing gravimetric energy densities with higher electrical heating temperatures for the solid medium only, an increasing thermal insulation demand to fulfil the outer surface temperature condition and the temperature limitations of the efficient vacuum insulation. The wide range of solutions for one specific electric heating temperature results from the geometric variations studies regarding the specific heat transfer areas, the void fractions and the diameters of the inventory options. To fulfil the specifications for constant mixing temperatures via the bypass operation and to achieve high thermal utilizations inside the TES, a sufficient heat transfer area must be offered. In contrast, geometric combinations with low specific surfaces require high storage length, resulting in low thermal utilizations as well as low thermal energy densities. In summary, it turns out that promising designs with efficient heat transport and high effective storage densities include inventory solutions with specific heat transfer surface of at least 200 m2/m3, void fractions of less than 30 % and length to diameter ratios of about 1 to minimize heat losses. In addition, it has been found that the heat transfer between the fluid and solid phase is mainly dominated by the convective heat transfer coefficient due to high heat conductivities for both materials within the entire temperature range. In spite of these geometric limitations a multitude of feasible solutions is visible in figure 6 and figure 7, allowing high thermal utilization factors and high thermal storage densities during discharging operation. For one exemplary solution the temporal ratio of mass flow passing the TES and the temporal fluid temperature characteristics are shown in figure 8. This selected SiC solution includes a bed diameter of 200 mm, a bed length of 230 mm, an inventory option with a specific heat transport surface of 400 m2/m3 and a void fraction of 25 % as well as electric heating temperature of 600 °C.
Fig. 8: Temporal characteristics of the mass flow ratio passing the TES and of the fluid outlet temperatures During discharging a partial flow of the cold air enters the TES via the bypass operation, is heated up to elevated temperatures while passing the storage material and is mixed afterwards with the ambient mass flow. Due to temporal decreasing TES outlet temperatures an increasing share of the total mass flow passing the TES is necessary to fulfil the defined specifications of constant mixing outlet temperatures. For this illustrated characteristic a TES thermal utilization of 87.1 % is achieved, leading to effective thermal energy densities of 142.5 Wh/kg and 187.3 kWh/m3 respectively. The maximum pressure drop during discharging caused by the air passing the inventory option reaches a value of 6.8 mbar. The exemplary results for this configuration demonstrate the high potentials of electrical heated solid media thermal energy storages associated with efficient thermal insulation concepts and the bypass operation. Further improvements to increase the effective volumetric and gravimetric energy density can be achieved by using Al2O3 due to high densities compared to SiC with sufficient high thermal conductivity, by combinations of different material options (metallic with ceramic) and by new designs with adapted flow guides to reduce thermal losses. Additionally investigations regarding the electrical charging process are necessary to identify additional potentials as well as design limitations.
Conclusions
For improvements in range of electric vehicles during cold winter days a novel thermal management concept based on an electric heated sensible solid media thermal energy storage system is introduced. The storage system includes a thermal efficient insulation concept and a bypass operation system allowing high effective thermal energy densities and a flexible supply of heat for various specifications. Based on exemplary specifications for heating the interior a wide range of feasible solutions were identified with low parasitic losses, with high thermal utilization of up to 90 % and with materialdependent effective volumetric and gravimetric thermal energy densities of more than 400 kWh/m3 and 160 Wh/kg respectively. Although, alternative thermal storage technologies such as PCM or thermochemical systems with suitable materials offer elevated storage densities regarding the storage medium, these require in contrast to solid media storages an additional effort for heat transfer structures due to the indirect contact between storage and heat transfer medium. Those effects lead to significantly decreasing effective storage densities and to reduced operational flexibilities due to limitations in achievable specific heat transport surfaces apart from further open issues relating material stability, thermal resistance inside the storage medium and housing concept. In comparison with state-of-the-art lithium-ion batteries as today´s energy source for heating with effective electric energy densities in a range between 100 and 150 Wh/kg [24], the solid media thermal energy storage system shows competitive first results as a novel thermal management concept in electric vehicles. With additional improvements in thermal storage density by using alternative materials with high specific heat capacity and density as well as by new design concepts with adapted flow guides and supplemented by the high potential for low cost and long-term stable configurations, a promising approach for the successful transformation of the transportation system towards electromobility is outlined. Current works for further development on this field contains electro-thermal modelling and simulation studies for resistance heating systems both for direct and indirect electrical heating of the storage mass [25] as well as the elaboration of an experimental device including the efficient thermal insulation concept and the bypass operation system. References [1] C. Kuper, M. Hoh, G. Houchin-Miller, J. Fuhr: Thermal Management of Hybrid Vehicle Battery Systems; EVS24, Stavanger, Norway, 2009. [2] M. Jung, A. Kemle, T. Strauss, M. Wawzyniak: Innenraumheizung von Hybridund Elektrofahrzeugen; ATZ, 396-401, 2011. [3] S. Pischinger, P. Genender, S. Klopstein, D. Hemkemeyer: Aufgaben beim Thermomanagement von Hybrid- und Elektrofahrzeugen; ATZ, 54-59, 2014. [4] H. Grossmann: Pkw – Klimatisierung – Physikalische Grundlagen und technische Umsetzung; Berlin Heidelberg: Springer, 2013. [5] ECES 18 Annex, Transportation of Energy by Utilization of Thermal Energy Storage Technology, 2006-2009. [6] F. Schüppel: Optimierung des Heiz- und Klimakonzepts zur Reduktion der Wärmeund Kälteleistung im Fahrzeug; TU Berlin, 2015.
[7] A. Ugurlu, C. Gokcol: A review on thermal energy storage systems with phase change materials in vehicles; Electronic Journal of Vocational Colleges, 2012. [8] E. Korin, et al.: Improving cold-start functioning of catalytic converters by using phase-change materials; No. 980671. SAE Technical Paper, 1998. [9] M. Gumus: Reducing cold-start emission from internal combustion engines by means of thermal energy storage system; Applied thermal engineering 29(4), 652660, 2009. [10] Y. Kata: Thermal energy storage in vehicles for fuel efficiency improvement. Proc. Effstock: 14-17, 2009. [11] M. Dieterich, I. Bürger, M. Linder: Open and closed metal hydride system for high thermal power applications: preheating vehicle components; International Journal of Hydrogen Energy, 1–13., 2017. [12] R. H. Turner: High temperature thermal energy storage, Philadelphia, Pa.: Franklin, 1978. [13] I. Dincer, M.A. Rosen: Thermal energy storage, Systems and Applications; John Wiley & Sons, Chichester (England), 2002. [14] H. Hausen: Wärmeübertragung im Gegenstrom, Gleichstrom und Kreuzstrom. Springer-Verlag, 1976. [15] W. Heiligenstaedt: Wärmetechnische Rechnungen für Industrieöfen. 4. Aufl. Düsseldorf: Verl. Stahleisen, 1966. [16] S.W. Churchill, H.H.S. Chu: Correlating equations for laminar and turbulent free convection from a vertical plate; International Journal of Heat and Mass Transfer 18, 1323-1329, 1975. [17] K. A. R. Ismail, R. Stuginsky: A parametric study on possible fixed bed models for pcm and sensible heat storage; Applied Thermal Engineering 19, 757-788, 1999. [18] N. Wakao, S. Kaguei: Heat and mass transfer in packed beds; Gordon and Braech, New York, 1982. [19] R. Bauer, E. U. Schlünder: Effective radial thermal conductivity of packings in gas flow. Part II: Thermal conductivity of the packing fraction without gas flow; Int. Chem. Eng. 18, 189-204, 1978. [20] E. Lemmon, et al.: Thermodynamic properties of air and mixtures of nitrogen, argon and oxygen from 60 to 2000 K at Pressures to 2000 MPa; J. Phys. Chem. Ref. Data 29, 331-385, 2000. [21] F. W. Schmidt, A. J. Willmott: Thermal energy storage and regeneration; McGrawHill Book Company, 1981. [22] J. H. Kerspe: Vielfältige Eigenschaftsprofile von gestützten Vakuum-Isolierungen Einsatzmöglichkeiten für das aktive und passive Thermomanagement in Fahrzeugen; VDI-Fachkonferenz: Thermomanagement für elektromotorisch angetriebene Fahrzeuge, Deutschland, 2014. [23] http://www.silca-online.de/en/thermal-insulation/insulation-boards/silcapor.html [24] M. M. Thackeray, C. Wolverton, E. D. Isaacs: Electrical energy storage for transportation – approaching the limits of, and going beyond, lithium-ion batteries; Energy Environ. Sci. 5, 7854-7863, 2012. [25] B. Sergej, V. Dreißigacker, M. Dieterich, W. Kraft: Next Generation Car Thermal energy storage systems: Power-to-Heat concept in solid media storage for high storage densities; EVS30 Proceedings, Stuttgart, Germany, 2017.
Highlights New thermal management concept for heat supply in electric vehicles is presented Electric heated solid medium storage allows high thermal storage densities Bypass operation system offers flexible heat supply with high thermal power Solid media design options with efficient heat transport are identified Compact designs based on a vacuum thermal insulation concept are elaborated
S1359-4311(18)30184-4 https://doi.org/10.1016/j.applthermaleng.2018.12.026 ATE 13033
To appear in:
Applied Thermal Engineering
Received Date: Revised Date: Accepted Date:
9 January 2018 30 July 2018 4 December 2018
Please cite this article as: V. Dreißigacker, S. Belik, High temperature solid media thermal energy storage system with high effective storage densities for flexible heat supply in electric vehicles, Applied Thermal Engineering (2018), doi: https://doi.org/10.1016/j.applthermaleng.2018.12.026
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
High temperature solid media thermal energy storage system with high effective storage densities for flexible heat supply in electric vehicles Volker Dreißigacker1, Sergej Belik2 1
Institute of Engineering Thermodynamics, German Aerospace Center, Pfaffenwaldring 38-40, Stuttgart, Germany, Phone: 49-711-6862449, Fax: 49-711-6862747, e-mail: [email protected] 2 Institute of Engineering Thermodynamics, German Aerospace Center, Pfaffenwaldring 38-40, Stuttgart, Germany, Phone: 49-711-6862512, Fax: 49-711-6862747, e-mail: [email protected]
Abstract One major challenge for the successful transformation of the transportation system towards electromobility is strongly linked to its maximum range. Such range reducing impacts emerge exemplary during cold winter days, where electric energy is used from the battery to heat the interior. In spite of significant improvements in energy density of battery systems, alternative ideas for suitable thermal management concepts are necessary to solve the conflict between traction and heating. To overcome this disadvantage a novel concept based on an electrical heated sensible solid media thermal energy storage system is outlined. Central elements include a high efficient thermal insulation concept and a bypass operation system to allow high effective energy densities and simultaneously a flexible supply of thermal energy with defined specifications during thermal discharging. For this purpose, simulation studies regarding the insulation concept and the geometric design of the thermal energy storage for two feasible material options were conducted. Under a wide range of solutions, promising designs with high effective energy densities, low parasitic losses and high thermal discharging power were identified.
Keywords Solid media thermal energy storage; power-to-heat; high effective thermal storage density; electric vehicles
Introduction
With increasing number of electric vehicles, suitable thermal management concepts are needed due to the lack of thermal heat from missing combustion engines and the demand on thermal energy for heating the interior [1, 2]. Today, thermal energy is generated in electric vehicles by PTC (Positive Temperature Coefficient) heating elements [3] and powered by the vehicle´s traction battery. Under cold environmental conditions for example during winter days, the range of the vehicle can be reduced by the half, where heating powers up to 5 kW are required [4]. Thus, a successful conversion of the mobility towards electrified vehicles is highly associated with innovate solutions for thermal management. Beside the direct generation of heat through PTC-elements, thermal energy storage systems open up an alternative way for supplying heat [5, 6]. The temporal decoupled process and a wide range of thermal storage technologies allow manifold thermal management concepts for cabin climatisation as well as for preheating. Up to now, several investigations for thermal
storage systems based on Phase Change Materials (PCM) [7] for pre-heating of catalytic converters [8] and engines [9] in combustion vehicles and ice slurry [10] for cooling applications are conducted. New thermal management concepts based on thermochemical energy storage systems (solid/gas reaction) [11] are growing in the last years, allowing an alternative heat supply in electric and fuel cell vehicles. Especially for use in electric vehicles, two crucial requirements must be satisfied by the thermal energy storage system: high effective thermal storage density and high thermal discharging power. Former can be achieved by using high temperature heat, by utilization of phase change or reaction enthalpies and efficient thermal insulation designs. Latter needs high heat transport rates (conductive and/or convective) as well as sufficient heat transport areas. A promising solution to fulfil both requirements is given by sensible solid media thermal energy storage systems, when high temperature heat can be provided. Due to missing high temperature sources within an electric vehicle, an alternative supply method must be identified. In this contribution a novel concept based on electric heated solid media thermal energy storage for cabin climatisation in electric vehicles is outlined. The required high temperature is provided through direct resistant heating of the storage material (Powerto-Heat). During the drive (discharge period) the defined air temperature condition for cabin climatisation is adjusted via a bypass operation. The efficiency and the flexibility of this concept are mainly determined within the discharge mode through the geometric design and the material properties of the solid medium as well as the thermal insulation. Thus, to allow statements regarding the effective thermal energy density and the thermal outlet power, the discharge operation is investigated in detail inside this contribution.
Novel concept: electric heated solid media thermal energy storage system
The novel concept of a solid media thermal energy storage system (TES) for climatisation of electric vehicles consists on three central features: a direct electric heating of the solid medium to generate high temperature heat, a controlled bypass system to supply the cabin with specified temperature conditions (Tmix) and an efficient thermal vacuum insulation to allow high effective storage densities. Figure 1 illustrated schematically the functional principle.
Fig. 1: Heat supply in electric vehicles via an electric heated solid medium (charging mode) and a bypass system (discharging mode) Electric heated solid medium (charging period) During TES charging period (vehicle halt) the solid medium is heated up electrically to elevated temperatures - simultaneously to the charging mode of the battery - ensuring
high thermal storage densities. Although this Power-to-Heat procedure is not focused in this work, a well suited technical realisation for vehicle applications is based on direct resistance heating systems (Joule heating). This technology is widely used in different industrial applications, where various options in design and materials are feasible. Here, inside this contribution a metallic (UNS N06003) and a ceramic (SiC) material option are investigated. Both are well suited for direct electric heating systems and high temperature applications. Bypass system (discharging period) During TES discharge period the vehicle cabin is supplied with heat from the solid medium thermal energy storage system. For that purpose the mass flow of the ambient air is divided via the controlled bypass system in a way to achieve the specified conditions (Tmix) by mixing the high temperature exiting air TES-flow (TF,out) with the cold temperatures from ambient (TU). Such a solid medium storage type with a direct contact between the inventory and the passing fluid is known as regenerator [12, 13] and is widely used in high temperature processes with operation temperatures beyond 1000 °C. Installations of these regenerator-type thermal energy storage systems can be distinguished into moving and stationary beds [14, 15]. For fixed bed arrangements, the storage consists of stacked bricks of various shapes and sizes. Depending on the required thermal characteristics, different types of bricks are used. Where a substantial temperature decrease during discharging can be allowed, e.g. in steel and glass industries, checker bricks with small specific surfaces and small void fractions between 20 % and 40 % can be used. In contrast, applications requiring a low heat resistance, e.g. in regenerative thermal oxidizers (RTO) for the purification of air, honey-comb with high specific surfaces and void fractions (>60 %) are used. Beside a wide range of geometries, various material options depending on the operational temperatures and gaseous conditions are existent. Thermal insulation concept The effective thermal energy density - including the solid medium and the thermal insulation - depends strongly on the maximum electric heating temperatures and the efficiency of the thermal insulation. To increase the thermal energy density higher heating temperatures are required, but leading simultaneously to an increased demand for thermal insulation. In order to allow compact designs in spite of the high temperatures inside the solid medium a vacuum based thermal insulation [22] with extremely low heat conductivity is investigated. This insulation is limited by maximum operation temperatures of 500 °C and requires for elevated temperatures an additional insulation protection layer based on a conventional microporous material [23]. In order to identify suitable TES systems based on the mentioned concept with high effective thermal energy densities including the solid medium and the thermal insulation and with high thermal discharging powers simulation, studies regarding the discharging operation with materials allowing a direct resistance heating are conducted. For that purpose simulation models are needed for the two layered thermal insulation concept and the regenerator-type solid medium storage.
Modelling
For identification of designs with high effective thermal energy densities, small thermal losses and high discharging powers, two thermal simulation models are used. The first one describes the two-layered thermal insulation concept and the second one the temporal and spatial TES characteristics. For clarification of the fundamental equations explained in the following, figure 2 illustrates schematically the TES system including central values and boundary conditions.
Fig. 2: Schematic illustration of the TES model with the two-layered thermal insulation
3.1 Thermal insulation For a cylindrical containment shape with up to two different insulations layers, the heat flow from the inner core Ti via conduction to the surface of the containment TW and via convective heat transport W [16] to the environment TU can be expressed:
(1)
Here, Ins represents the thermal conductivity of the insulation layers 1 and 2, L the length and r the radius of the cylinder at the inner surface with index i, between the two insulation layers with index and at the outer surface AW. For the areas A at the bottom and the top of the containment a comparable formulation can be used, where s stands for the insulation thickness of layer 1 and 2, respectively.
(2)
The total thickness of the thermal insulation, thus the required insulation volume is given by:
(3) Based on these equations the total insulation thickness can be determined by specifications of the maximum temperature limit of insulation layer 2 (T) as well as the outer surface temperature (TW).
3.2 Sensible solid media thermal energy storage system The temporal temperature characteristic of a sensible solid media thermal energy storage system is determined by the inlet conditions, material properties, geometric parameters of the inventory option and the containment. During discharging procedure, the ambient air mass flow passes the thermal energy storage system, receives heat from the previously electric heated solid medium in direct contact and is mixed at the outlet in bypass operation to the specified temperature (see fig. 1). For modelling the discharge procedure of such a regenerator-type solid media heat storage system, a heterogeneous porous continuum approach is used, providing the temporal and spatial temperature distribution for the gaseous (F) and the solid phase (S) [17]. With well justified simplifications regarding the neglection of radial gradients across the flow direction through a uniform flow distribution at the entrance of the solid medium and low heat losses over the surrounding walls, the resulting one dimensional heat balances in time t and space z can be written: Fluid (4)
Solid (5) With the associated boundary conditions: Fluid (6) Solid (7) (8)
Here, TF and TS represent the heat transfer fluid and solid medium temperature, w the fluid velocity, the void fraction, aV and aW the specific heat transfer surface of the
inventory and the surrounding casing relating to the total storage volume (fluid and solid). Effective heat conduction coefficients for the fluid F,z,eff and solid phase S,z,eff are determined by [18, 19] and a real gas model by Lemmon for air [20] is used to calculate the density F and the specific heat capacity cP. The total regenerator heat transfer coefficient k is calculated according to Hausen [14, 21]. The coefficient accounts in a compact way the heat resistance inside the inventory considering the thermal conductivity of the solid, its shape and the time as well as the heat transfer coefficient of the fluid for the investigated channel-shaped bricks as suggested in [14]. This allows calculating the effective heat transfer, which is limited by the solid phase or the fluid phase depending on the material properties, wall thickness and duration or the convective heat transfer coefficient. Thermal losses through the thermal insulation are regarded by kW inside the heat balance equations and by the solid phase boundary conditions. The partial differential equations in (4) and (5) are solved numerically by using a central and a backward finite-difference-method in space for the solid and for the fluid phase respectively. Subsequently, the resulting set of differential algebraic equations including the boundary conditions is solved in time with a commercial simulation tool (Matlab).
Results
Design studies are performed in order to obtain promising solutions for the heat supply in cabine climatisation with high effective storage density and high thermal discharging power. In a first step, an efficient thermal insulation concept is elaborated based on a two-layered design. In a second step, variation studies regarding the geometric parameters of the inventory and the containment are conducted to identify solutions with high storage utilizations via the bypass operation during thermal discharging (see fig. 1). For these simulation steps specifications and material properties must be defined. Essential thermal boundary conditions and target values (Tmix, TW) are listed in table 1. Thermal storage capacity: Q [kWh]
2.5
Thermal discharging power [kW]
5
Discharge duration [min]
30
Mixing temperature: Tmix [°C]
60
Ambient temperature: TU [°C]
-10
Outer surface temperature: TW [°C]
60
Table 1: Boundary conditions and target values The discharge duration of 30 min in this contribution is based on the Worldwide Harmonized Light Duty Test Procedure (WLTP). Inside the simulation studies, a metallic (UNS N06003) and a ceramic (SiC) material option are investigated. Both materials are well suited for direct electric heating
operation and used in high temperature applications for example as heating elements. Here, temperature averaged material properties between 100 °C and 1000 °C are used, which are summarized in table 2. UNS N06003
SiC
Density: S [kg/m3]
8350
2960
Specific heat capacity: cS [J/kgK]
570
1095
Heat conductivity: S [W/mK]
20
71.5
Table 2: Material properties for the solid media Likewise, averaged values for air are used in the same temperature range.
4.1 Thermal insulation concept To achieve low heat losses, an efficient and compact vacuum insulation is used limited by maximum operating temperatures of 500 °C [22]. Beyond these temperatures an additional thermal insulation layer based on a microporous material [23] must be integrated, which serves as thermal protection and allows operating temperatures of up to 1000 °C. Both materials show comparable material densities of 280 kg/m3, whereat the heat conductivity of the vacuum insulation at a temperature of 500 °C (0.003 W/mK) is by the factor 12.5 lower compared to the microporous material (0.038 W/mK). Based on the specifications for thermal storage capacity, outer surface and ambient temperature (table 1) as well as for the selected inventory materials (table 2), simulation studies are conducted to determine the insulation thickness for different electric heating temperatures (Tmax). Inside these calculations, an inventory void fraction of 25 % and an inventory length to diameter ratio L/Di of 1 are assumed exemplarily. The resultant maximum effective gravimetric and volumetric storage densities including the required insulation volume and mass are illustrated in figure 3 and figure 4.
Fig. 3: Maximum effective gravimetric thermal energy densities for various electrical heating temperatures
The effective gravimetric storage densities (figure 3) increase with growing electrical heating temperatures up to a maximum value depending on the material option. This behaviour results from increasing gravimetric energy densities with higher electrical heating temperatures inside the solid medium only and with simultaneously increasing insulation demand to achieve the fixed outer surface temperature of 60 °C. Comparing both materials, higher effective gravimetric storage densities are achieved for SiC caused by a higher specific heat capacity. In contrast, higher effective volumetric storage densities are visible in figure 4 for the metallic option resulting from a significantly higher material density. Here, the highest values are reached at the maximum operating temperature of the efficient vacuum insulation (500 °C). Above this limit the microporous material layer must be integrated requiring considerable larger insulation thicknesses due to the higher heat conductivity.
Fig. 4: Maximum effective volumetric thermal energy densities for various electrical heating temperatures Both exemplary results show, that the electrical heating of a solid media is a promising option to achieve high effective storage densities in spite of the required thermal insulation demand. But under operating conditions - for example a stand still period of several hours – the effects of thermal losses compared to the stored thermal energy must be regarded. Therefore, simulation runs are conducted based on specifications listed in table 1 and dimensions deduced from the maximum effective gravimetric energy densities illustrated in figure 3. This results in a mass of 22.5 kg with a volume of 17.4 dm3 by heating temperatures of 850 °C for the metallic material option and 13.8 kg with a volume of 14.2 dm3 by heating temperatures of 725 °C for the SiC material option. Therefore, the temporal characteristics of the thermal energy related to the initial values are shown in figure 5 over a stand still period of 12 h.
Fig. 5: Temporal characteristics of the stored thermal energy over a stand still period of 12 h for exemplary designs Both materials show comparable heat losses of about 30 % over the stand still period of 12 hours. Here, for the considered case of cabin climatisation with a thermal power of 5 kW (table 1), the discharging duration will be reduced from 30 minutes to 21 minutes due to heat losses after the stand still period. Including these effects moderate high effective storage densities are still available in spite of the long stand still period and the continuously cold ambient conditions of -10 °C. Further improvements in performance can be achieved by adaptions in the geometric design, by maximum temperatures as well as by more efficient thermal insulation options. These exemplary illustrated results related to the maximum effective thermal energy densities point the potential of electrical heated solid media thermal energy storages. However, to achieve a high thermal utilization of the solid medium during discharge operation defined as the ratio of usable to maximum stored thermal energy, promising inventory designs with high heat transport rates need to be identified.
4.2
Thermal discharge operation
During discharge operation ambient air (TU) is heated up via the bypass system (fig. 1) to supply the vehicle cabin with the defined temperature conditions (Tmix). For this purpose, the total mass flow is splitted to compensate the temporal decrease of the TES fluid outlet temperature including heat losses and thus to achieve a constant mixing temperature over the specified discharging duration. Based on the thermal insulation concept in chapter 4.1, additional variation studies regarding the geometric parameters of the inventory (void fraction , specific heat transfer surface aV, diameter Di) and the electrical heating temperatures (Tmax) are conducted to identify solutions with high thermal utilizations during discharge operation. For that, the inventory length L, the thermal insulation thickness and the temporal split of the total mass flow passing the TES are determined iteratively to fulfil the specification of constant mixing temperatures over the discharging operation and the outer surface temperatures (table 1). Additionally, a further target value must be
defined, given by the maximum mass flow passing the TES at the end of discharging. Here, this value related to the total mass flow is set to 80 %. The resulting effective gravimetric and volumetric thermal energy densities for both materials are shown in figure 6 and figure 7. Those results include a wide range of inventory variations concerning the specific heat transfer surface aV (50 – 800 m2/m3), the void fraction (20 – 60 %), the diameter Di (0.05 – 0.3 m) and the electrical heating temperature Tmax (400 – 1000 °C).
Fig. 6: Effective gravimetric and volumetric thermal energy density including the thermal utilization during discharging mode for the metallic inventory option
Fig. 7: Effective gravimetric and volumetric thermal energy density including the thermal utilization during discharging mode for the ceramic inventory option
The results show for both materials a wide range of solutions which differ from their attainable effective thermal energy densities. Relating to the electrical heating temperatures - illustrated in grey coloured dots – increasing gravimetric thermal energy densities are visible up to a maximum value. For the metallic inventory option a maximum value of 105 Wheff/kg is calculated at heating temperatures of 900 C, for the SiC option of 165 Wheff/kg at 800 C. In contrast, the maximum volumetric thermal storage density is reached for both materials at 500 C. This behaviour - comparable with results shown in figure 3 and figure 4 - is caused by increasing gravimetric energy densities with higher electrical heating temperatures for the solid medium only, an increasing thermal insulation demand to fulfil the outer surface temperature condition and the temperature limitations of the efficient vacuum insulation. The wide range of solutions for one specific electric heating temperature results from the geometric variations studies regarding the specific heat transfer areas, the void fractions and the diameters of the inventory options. To fulfil the specifications for constant mixing temperatures via the bypass operation and to achieve high thermal utilizations inside the TES, a sufficient heat transfer area must be offered. In contrast, geometric combinations with low specific surfaces require high storage length, resulting in low thermal utilizations as well as low thermal energy densities. In summary, it turns out that promising designs with efficient heat transport and high effective storage densities include inventory solutions with specific heat transfer surface of at least 200 m2/m3, void fractions of less than 30 % and length to diameter ratios of about 1 to minimize heat losses. In addition, it has been found that the heat transfer between the fluid and solid phase is mainly dominated by the convective heat transfer coefficient due to high heat conductivities for both materials within the entire temperature range. In spite of these geometric limitations a multitude of feasible solutions is visible in figure 6 and figure 7, allowing high thermal utilization factors and high thermal storage densities during discharging operation. For one exemplary solution the temporal ratio of mass flow passing the TES and the temporal fluid temperature characteristics are shown in figure 8. This selected SiC solution includes a bed diameter of 200 mm, a bed length of 230 mm, an inventory option with a specific heat transport surface of 400 m2/m3 and a void fraction of 25 % as well as electric heating temperature of 600 °C.
Fig. 8: Temporal characteristics of the mass flow ratio passing the TES and of the fluid outlet temperatures During discharging a partial flow of the cold air enters the TES via the bypass operation, is heated up to elevated temperatures while passing the storage material and is mixed afterwards with the ambient mass flow. Due to temporal decreasing TES outlet temperatures an increasing share of the total mass flow passing the TES is necessary to fulfil the defined specifications of constant mixing outlet temperatures. For this illustrated characteristic a TES thermal utilization of 87.1 % is achieved, leading to effective thermal energy densities of 142.5 Wh/kg and 187.3 kWh/m3 respectively. The maximum pressure drop during discharging caused by the air passing the inventory option reaches a value of 6.8 mbar. The exemplary results for this configuration demonstrate the high potentials of electrical heated solid media thermal energy storages associated with efficient thermal insulation concepts and the bypass operation. Further improvements to increase the effective volumetric and gravimetric energy density can be achieved by using Al2O3 due to high densities compared to SiC with sufficient high thermal conductivity, by combinations of different material options (metallic with ceramic) and by new designs with adapted flow guides to reduce thermal losses. Additionally investigations regarding the electrical charging process are necessary to identify additional potentials as well as design limitations.
Conclusions
For improvements in range of electric vehicles during cold winter days a novel thermal management concept based on an electric heated sensible solid media thermal energy storage system is introduced. The storage system includes a thermal efficient insulation concept and a bypass operation system allowing high effective thermal energy densities and a flexible supply of heat for various specifications. Based on exemplary specifications for heating the interior a wide range of feasible solutions were identified with low parasitic losses, with high thermal utilization of up to 90 % and with materialdependent effective volumetric and gravimetric thermal energy densities of more than 400 kWh/m3 and 160 Wh/kg respectively. Although, alternative thermal storage technologies such as PCM or thermochemical systems with suitable materials offer elevated storage densities regarding the storage medium, these require in contrast to solid media storages an additional effort for heat transfer structures due to the indirect contact between storage and heat transfer medium. Those effects lead to significantly decreasing effective storage densities and to reduced operational flexibilities due to limitations in achievable specific heat transport surfaces apart from further open issues relating material stability, thermal resistance inside the storage medium and housing concept. In comparison with state-of-the-art lithium-ion batteries as today´s energy source for heating with effective electric energy densities in a range between 100 and 150 Wh/kg [24], the solid media thermal energy storage system shows competitive first results as a novel thermal management concept in electric vehicles. With additional improvements in thermal storage density by using alternative materials with high specific heat capacity and density as well as by new design concepts with adapted flow guides and supplemented by the high potential for low cost and long-term stable configurations, a promising approach for the successful transformation of the transportation system towards electromobility is outlined. Current works for further development on this field contains electro-thermal modelling and simulation studies for resistance heating systems both for direct and indirect electrical heating of the storage mass [25] as well as the elaboration of an experimental device including the efficient thermal insulation concept and the bypass operation system. References [1] C. Kuper, M. Hoh, G. Houchin-Miller, J. Fuhr: Thermal Management of Hybrid Vehicle Battery Systems; EVS24, Stavanger, Norway, 2009. [2] M. Jung, A. Kemle, T. Strauss, M. Wawzyniak: Innenraumheizung von Hybridund Elektrofahrzeugen; ATZ, 396-401, 2011. [3] S. Pischinger, P. Genender, S. Klopstein, D. Hemkemeyer: Aufgaben beim Thermomanagement von Hybrid- und Elektrofahrzeugen; ATZ, 54-59, 2014. [4] H. Grossmann: Pkw – Klimatisierung – Physikalische Grundlagen und technische Umsetzung; Berlin Heidelberg: Springer, 2013. [5] ECES 18 Annex, Transportation of Energy by Utilization of Thermal Energy Storage Technology, 2006-2009. [6] F. Schüppel: Optimierung des Heiz- und Klimakonzepts zur Reduktion der Wärmeund Kälteleistung im Fahrzeug; TU Berlin, 2015.
[7] A. Ugurlu, C. Gokcol: A review on thermal energy storage systems with phase change materials in vehicles; Electronic Journal of Vocational Colleges, 2012. [8] E. Korin, et al.: Improving cold-start functioning of catalytic converters by using phase-change materials; No. 980671. SAE Technical Paper, 1998. [9] M. Gumus: Reducing cold-start emission from internal combustion engines by means of thermal energy storage system; Applied thermal engineering 29(4), 652660, 2009. [10] Y. Kata: Thermal energy storage in vehicles for fuel efficiency improvement. Proc. Effstock: 14-17, 2009. [11] M. Dieterich, I. Bürger, M. Linder: Open and closed metal hydride system for high thermal power applications: preheating vehicle components; International Journal of Hydrogen Energy, 1–13., 2017. [12] R. H. Turner: High temperature thermal energy storage, Philadelphia, Pa.: Franklin, 1978. [13] I. Dincer, M.A. Rosen: Thermal energy storage, Systems and Applications; John Wiley & Sons, Chichester (England), 2002. [14] H. Hausen: Wärmeübertragung im Gegenstrom, Gleichstrom und Kreuzstrom. Springer-Verlag, 1976. [15] W. Heiligenstaedt: Wärmetechnische Rechnungen für Industrieöfen. 4. Aufl. Düsseldorf: Verl. Stahleisen, 1966. [16] S.W. Churchill, H.H.S. Chu: Correlating equations for laminar and turbulent free convection from a vertical plate; International Journal of Heat and Mass Transfer 18, 1323-1329, 1975. [17] K. A. R. Ismail, R. Stuginsky: A parametric study on possible fixed bed models for pcm and sensible heat storage; Applied Thermal Engineering 19, 757-788, 1999. [18] N. Wakao, S. Kaguei: Heat and mass transfer in packed beds; Gordon and Braech, New York, 1982. [19] R. Bauer, E. U. Schlünder: Effective radial thermal conductivity of packings in gas flow. Part II: Thermal conductivity of the packing fraction without gas flow; Int. Chem. Eng. 18, 189-204, 1978. [20] E. Lemmon, et al.: Thermodynamic properties of air and mixtures of nitrogen, argon and oxygen from 60 to 2000 K at Pressures to 2000 MPa; J. Phys. Chem. Ref. Data 29, 331-385, 2000. [21] F. W. Schmidt, A. J. Willmott: Thermal energy storage and regeneration; McGrawHill Book Company, 1981. [22] J. H. Kerspe: Vielfältige Eigenschaftsprofile von gestützten Vakuum-Isolierungen Einsatzmöglichkeiten für das aktive und passive Thermomanagement in Fahrzeugen; VDI-Fachkonferenz: Thermomanagement für elektromotorisch angetriebene Fahrzeuge, Deutschland, 2014. [23] http://www.silca-online.de/en/thermal-insulation/insulation-boards/silcapor.html [24] M. M. Thackeray, C. Wolverton, E. D. Isaacs: Electrical energy storage for transportation – approaching the limits of, and going beyond, lithium-ion batteries; Energy Environ. Sci. 5, 7854-7863, 2012. [25] B. Sergej, V. Dreißigacker, M. Dieterich, W. Kraft: Next Generation Car Thermal energy storage systems: Power-to-Heat concept in solid media storage for high storage densities; EVS30 Proceedings, Stuttgart, Germany, 2017.
Highlights New thermal management concept for heat supply in electric vehicles is presented Electric heated solid medium storage allows high thermal storage densities Bypass operation system offers flexible heat supply with high thermal power Solid media design options with efficient heat transport are identified Compact designs based on a vacuum thermal insulation concept are elaborated