latent thermal-energy storage

Journal of Energy Storage 17 (2018) 140–152

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Pilot-scale demonstration of advanced adiabatic compressed air energy storage, Part 2: Tests with combined sensible/latent thermal-energy storage V. Becattinia , L. Geissbühlera , G. Zanganehb , A. Haselbachera,* , A. Steinfelda a b

Department of Mechanical and Process Engineering, ETH Zurich, 8092 Zurich, Switzerland ALACAES SA, 6900 Lugano, Switzerland

A R T I C L E I N F O

Article history: Received 29 November 2017 Received in revised form 6 February 2018 Accepted 6 February 2018 Available online xxx Keywords: Advanced adiabatic compressed air energy storage Thermal-energy storage Packed bed Pilot plant Simulation Phase-change material

A B S T R A C T

Experimental and numerical results from the world’s first pilot-scale advanced adiabatic compressed air energy storage plant with combined sensible/latent thermal-energy storage are presented. The combined thermal-energy storage was composed of sensible and latent units with maximum capacities of 11.6 MWhth and 171.5 kWhth, respectively. The latent thermal-energy storage consisted of a steel tank with 296 stainless-steel tubes encapsulating an Al–Cu–Si alloy as phase-change material. The combined thermal-energy storage was investigated using four charging/discharging cycles with durations of about 3 h each and air inflow temperatures of up to 566
C. The experimental results showed that the latent thermal-energy storage reduced the drop in the air outflow temperature during discharging. Minor leaks of the phase-change material were traced to the welding seams in the encapsulation as well as to holes required to insert resistance temperature detectors. Analysis of the leaked phase-change material revealed degradation and/or phase separation, which were attributed to the initial off-eutectic composition of and impurities in the phase-change material and resulted in a reduced heat of fusion. Simulations predicted the performance of the combined thermal-energy storage with good overall accuracy. Discrepancies were put down to changes in the thermophysical properties. © 2018 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/4.0/).

1. Introduction Among large-scale energy-storage technologies, advanced adiabatic compressed air energy storage (AA-CAES) has recently attracted much interest because of projected high power outputs (above 100 MW), high efficiencies (about 60–75%), and low capital costs, see Luo et al. [1], Budt et al. [2], and Sciacovelli et al. [3]. Experimental and numerical results from the first pilot-scale AACAES plant with high-temperature sensible thermal-energy storage (TES) were reported in Part 1, see Geissbühler et al. [4]. The results demonstrated the technical feasibility of both the AACAES technology and the packed-bed TES that used rocks as the storage material and air as the heat-transfer fluid (HTF). Sensible TES systems using rocks as storage material and air as HTF are of interest because experimental and numerical investigations have found them to be promising in terms of efficiencies and costs. One drawback of such TES systems is that during discharging, the temperature of the outflowing air decreases with

* Corresponding author. E-mail address: [email protected] (A. Haselbacher).

time, which is unfavorable for the turbine in an AA-CAES plant. To overcome this drawback, a relatively small amount of phasechange material (PCM) can be placed on top of the packed bed of rocks, resulting in a so-called combined sensible/latent TES. Combined sensible/latent TES has been investigated numerically and experimentally by several authors. Zanganeh et al. [5] and Geissbühler et al. [6] performed numerical and experimental analyses of a 42 kWhth laboratory-scale combined storage consisting of a packed bed of rocks and an eutectic aluminum–silicon alloy as PCM encapsulated in steel tubes. They demonstrated stabilization of the air outflow temperature around the melting temperature of the PCM. In addition, Geissbühler et al. [6] used simulations to show that 23 MWhth and 1000 MWhth combined storages have higher exergy efficiencies and lower specific material costs compared to sensible storages for a given maximum outflow temperature drop during discharging. Galione et al. [7] compared numerically several industrial-scale combined storage configurations for a 50 MWhel concentrated solar power plant and found that a storage consisting of multiple layers of sensible materials and PCMs diminishes the degradation of the thermocline and has a higher utilization factor. Zhao et al. [8] investigated numerically storage concepts with an output of 250 MWhth and found that a storage consisting of sensible

https://doi.org/10.1016/j.est.2018.02.003 2352-152X/© 2018 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/4.0/).

V. Becattini et al. / Journal of Energy Storage 17 (2018) 140–152

Nomenclature

Abbreviations AA-CAES advanced adiabatic compressed air energy storage DSC differential scanning calorimeter/calorimetry HTF heat-transfer fluid LFA laser flash analysis PCM phase-change material RTD resistance temperature detector TES thermal-energy storage Greek characters a thermal diffusivity (mm2/s) r density (kg/m3) Latin characters cp specific heat capacity at constant pressure (kJ/kg K) _ mass flow rate (kg/s) m h specific enthalpy (kJ/kg) k thermal conductivity (W/m K) T temperature (
C) t time (h) x axial coordinate (m) Subscripts bot bottom of the storage unit el electric exp experimental results lat latent unit ref reference sens sensible unit sim simulation results th thermal top top of the storage unit c charging d discharging f fusion l liquid m melting pc pre-charging s solid

141

3. Assess the thermal and mechanical stability of the latent TES tank, the encapsulation, and the PCM in response to the temperature and pressure variations during the charging/ discharging cycles.

2. Plant description The pilot-scale AA-CAES plant and its main components are described in detail in Part 1. Because the plant used to produce the results reported in this paper differs from that described in Part 1 only through the TES, this section is restricted to a description of the combined sensible/latent TES. 2.1. Combined sensible/latent TES As shown in Figs. 1 and 2, the combined sensible/latent TES actually consists of two separate storages: a sensible storage, identical to that described in Part 1, and a latent storage. The main reason why the two storages are separate is that they were designed and constructed at different times. Additional reasons that justify using two separate TES units will be given below. It is important to note that the addition of the latent storage affects the performance of the sensible storage because during charging, the compressed air flows first through the latent storage from top to bottom and then through the sensible storage from top to bottom. Therefore, the melting temperature of the PCM and thermal losses from the pipe connecting the two storages affect the temperature of the air flowing into the sensible storage. As a result, the maximum capacity of the sensible storage is 11.6 MWhth (calculated for an inflow temperature of 529
C) rather than 12 MWhth given in Part 1 (calculated for an inflow temperature of 550
C). The maximum capacity of the latent storage is 171.5 kWhth (calculated for an inflow temperature of 566
C). During the preparation of the experiments with the combined TES and accounting for the constraints imposed by the previously constructed sensible TES, three options were considered: (1) placing the encapsulated PCM directly on top of the packed bed of rocks, (2) placing the encapsulated PCM inside the air distributor, which is indicated in Fig. 2, and (3) placing the encapsulated PCM in a separate storage. Option (1) was adopted in the laboratoryscale combined storage by Zanganeh et al. [5] and Geissbühler et al. [6]. Due to the limited space available above the packed bed and the resulting poor heat transfer, this option could not be pursued. Similarly, option (2) was discarded because of the limited space available inside the distributor and because it would have required

material and PCMs at the top and bottom of a tank with an optimum configuration is more cost-competitive than other thermocline TES systems for the same design requirements and operating conditions. In summary, the literature shows that combined sensible/latent TES can be superior to sensible-only TES in terms of both efficiency and cost. While the combined sensible/latent TES has been investigated experimentally at the laboratory scale, to our knowledge it has so far not been tested at the pilot scale. Therefore, the overall goal of this paper is to investigate experimentally and numerically the performance of a combined sensible/latent TES in the pilot-scale AA-CAES plant introduced in Part 1. The specific objectives of this paper are: 1. Demonstrate the feasibility of a combined sensible/latent TES in the pilot-scale AA-CAES plant using several charging/discharging cycles. 2. Validate the quasi-one-dimensional heat-transfer model of Geissbühler et al. [6] using the experimental data collected with the combined TES.

Fig. 1. Picture of combined sensible/latent TES, with latent storage in foreground and sensible storage in background. The insulated pipes between the heater and the latent storage and between the latent and sensible storages can be seen.

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Fig. 2. Schematic drawing of the combined sensible/latent TES and locations of air inflow/outflow temperature measurements.

strengthening to withstand the weight of the encapsulated PCM. Therefore, option (3) was chosen and a separate latent storage was designed and constructed. The combined sensible/latent storage with separate sensible and latent storages is judged to have several advantages for the current pilot-scale and/or for possible future industrial-scale applications. First, the cross-sectional area of the latent storage can be chosen independently of that of the sensible storage to give good heat-transfer rates and hence melting behavior of the PCM. Second, a separate latent storage provides the possibility to switch between experiments either with only the sensible TES or with the combined TES by disconnecting and reconnecting the pipe linking the two storages. Third, at least for the current pilotscale plant, a separate latent storage was easier to construct and adapt than if the encapsulated PCM had been placed in the sensible storage, which would have required the removal of its steel cover and the pipe attached to the cover. Fourth, with a view to the possible use of industrial-scale combined storages, a separate latent storage allows the PCM to be inspected and/or replaced more easily. Periodic replacement of the PCM might be required due to corrosion reactions between the PCM and the encapsulation. For metallic PCMs and encapsulations, these reactions can lead to the growth of intermetallic layers and result in a decrease of the performance, see Yan and Fan [9], Yeremenko et al. [10], and Binder and Haussener [11], as well as potential leaking of the PCM through the encapsulation. Finally, again with a view to possible industrial-scale combined storages, a separate latent storage means that the top of the packed bed is not obstructed by the encapsulated PCM, simplifying the replacement of rocks near the top of the packed bed. These rocks might have to be replaced periodically because the higher temperatures near the top of the packed bed might cause them to degrade more quickly, see Allen et al. [12], Tiskatine et al. [13], and Becattini et al. [14]. Separating the sensible and latent storages results in at least two potential disadvantages. The first is that the pipe linking the two storages must be well insulated to avoid thermal losses. The second disadvantage is that the total surface area of separate storages is likely to be larger than that of a single storage, which will increase thermal losses unless the storages are adequately insulated. The additional insulation will increase material costs. 2.2. Latent TES For the latent TES, the eutectic metal alloy with the theoretical composition 68.5Al26.5Cu5Si (wt%) was chosen as the PCM. This choice was dictated by the following considerations. The melting temperature of the PCM must be sufficiently below the temperature of the air flowing into the latent storage during charging to allow complete melting. Based on simulations of laboratory-scale combined sensible/latent storages, Zanganeh et al. [5] recommend that the melting temperature of the PCM

should be about 20
C below the air inflow temperature during charging. Because the design temperature of the air inflow during charging was 550
C, suitable melting temperatures were considered to be between 520 and 530
C. Among PCMs with melting temperatures in this range, we restricted our choice to metal alloys because of their high thermal conductivity, small overcooling during solidification, and small volume change during melting, see Kenisarin [15]. Among suitable metal alloys with high heats of fusion, see Kenisarin [15], we selected 68.5Al26.5Cu5Si because of unpublished laboratory-scale experience with this material. The thermophysical properties of this PCM were reported by Gasanaliev and Gamataeva [16] as a melting temperature of 525
C, a heat of fusion of 365 kJ/kg, and a density of 2938 kg/m3. The elemental composition and the temperature-dependent properties of the alloy used in the experiments will be presented in Section 3. Based on prior experience with a laboratory-scale combined storage by Zanganeh et al. [5] and Geissbühler et al. [6], we elected to encapsulate the PCM in AISI316 stainless-steel tubes. Because the laboratory-scale combined storage had a circular cross-section, the length of the tubes was not constant. This did not present any problems because only 68 tubes were needed. For the pilot-scale storage, however, tubes of variable length would have increased the cost and complicated the assembly of the storage. As a result, the PCM in the pilot-scale combined storage was encapsulated in tubes of equal length and therefore the latent storage had a square cross section. To determine the cross section and height of the latent storage as well as the number and diameter of the tubes encapsulating the PCM, we used the quasi-one-dimensional heattransfer model of Geissbühler et al. [6]. The storage consists of two nested shells made from AISI304 stainless steel panels. The inner shell consists of a central body with a square cross-section of 0.75 m by 0.75 m and a height of 0.65 m as well as two truncated pyramids that connect the central body to the inlet/outlet pipes. The outer shell fixes the insulation, made of microporous panels of powdered silica (MICROBIFIRE 1000 from Bifire S.R.L.) with a thickness of 15 cm. The properties of the insulation material are given in Part 1. The pipe connecting the latent storage to the sensible storage is made of AISI304 stainless steel and wrapped with several layers of microporous insulation, ceramic wool, and rock wool to reduce heat losses. The PCM was encapsulated in 296 tubes made of AISI316 stainless steel. The tubes had a length of 73 cm, an inner diameter of 3.2 cm, and a thickness of 0.15 cm. The tubes were closed with caps made of AISI316 stainless steel with a thickness of 0.15 cm. The length of the tubes was chosen to be 2 cm shorter than the dimensions of the inner shell to allow for thermal expansion. The tubes were arranged into 16 layers, with the tubes in each layer resting on and oriented at 90
with respect to the tubes in the layer below. The tubes in layers with same orientation were staggered by half a tube diameter, as shown in the inset in Fig. 3, resulting in 8 layers each with 18 and 19 tubes. The gaps between the outermost tubes in a layer and the inner shell were stuffed with strips of insulation material (felt), as can be seen from Fig. 4. These strips ensured that the tubes in a layer were touching at the caps, prevented the tubes from rolling, and eliminated bypass flows. The tubes in the bottommost layer were resting on a perforated plate of S235 JR steel with a thickness of 1 cm and a void fraction of 0.16. The tubes were filled while the PCM was liquid. A suitable void space was left while filling the tubes to allow expansion of the PCM upon solidification. Prior to filling, eight of the tubes were internally coated with a protective layer that was shown to prevent the formation of intermetallic layers in small-scale experiments, see Binder and Haussener [11]. The coated tubes were included in the pilot-scale experiments to assess the

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Table 1 Actual composition of PCM in wt.% as obtained from supplier. In addition to the elements shown, minor impurities of Cr, Ni, Ca, Na, and Sr were also detected. Al

Cu

Si

Mg

Ti

Fe

Mn

Zn

67.100

25.861

6.508

0.265

0.083

0.049

0.035

0.019

3. PCM composition and properties

Fig. 3. Drawing of the latent TES with positions of resistance temperature detectors (RTDs). The inset shows a detail view of the staggered tubes. Note: triangles and circles indicate that an RTD is measuring the PCM and air temperatures, respectively.

Table 1 shows that the actual composition of the PCM obtained from the supplier (Fonderia EMI S.R.L.) differed from the eutectic composition. As a result, the thermophysical properties of the supplied PCM were expected to differ from those reported by Gasanaliev and Gamataeva [16]. Because the thermophysical properties of the PCM can have a significant impact on the performance of the latent storage, the thermophysical properties of the supplied PCM were measured and are reported in the following. 3.1. Density The density was determined at room temperature by linear measurements using three disks with average diameter and thickness of 12.51 mm and 2.92 mm, respectively, and found to be 3246 kg/m3 with an estimated relative error of 0.2%. The measured density is thus about 10% higher than that reported by Gasanaliev and Gamataeva [16]. The difference is ascribed to the different compositions. 3.2. Specific heat capacity, heat of fusion, and melting temperature range

Fig. 4. Picture of interior of latent storage, partially filled with the steel tubes that encapsulate the PCM. Some of the RTDs that were used to measure the air and PCM temperatures are visible.

effectiveness of the coating under operating conditions that are representative of industrial-scale AA-CAES plants. 2.3. Sensors The temperatures in the two TES units were measured by resistance temperature detectors (RTDs). Seven RTDs were placed on the centerline of the latent TES as shown in Fig. 3. As indicated in the figure, two RTDs measured the temperature of the PCM. These RTDs were inserted into the solid PCM by drilling holes through the encapsulations. The remaining five RTDs measured air temperatures. Due to the limited number of data-acquisition channels, measuring temperatures in the latent TES meant using fewer RTDs in the sensible TES compared to the experiments described in Part 1. (See Fig. 5 of Part 1 for the locations of the RTDs in the sensible TES.) The RTDs in the sensible TES that were disconnected for the tests with the combined TES were chosen such that the axial resolution of the thermocline was not affected. The air mass flow rate was measured as described in Part 1.

The specific heat capacity was determined by TOPEM1, a temperature-modulated differential scanning calorimetry (DSC) technique introduced by Mettler Toledo, see Schawe et al. [17]. The measurements were performed on a PCM disk with a mass of 56.5 mg in the temperature ranges 100–500
C and 545–700
C using a heating rate of 1
C/min. Sapphire was used as the reference material. The heat of fusion and melting temperature range were determined by classical DSC in the temperature range 500–550
C using a sample of 2.3 mg with a heating rate of 5
C/min. Both TOPEM1 and classical DSC were performed using 30 mL alumina crucibles in Ar flows of 50 ml/min with a Mettler Toledo DSC3+, previously calibrated using Zn, In, and Al. Fig. 5(a) shows the measured specific heat capacity for the solid and liquid phases. The specific heat capacity of the solid PCM increases with temperature, in agreement with the behavior of the solid metals forming the alloy, i.e., Al, Cu, and Si, see Chase [18]. At about 350
C, the specific heat capacity exhibits an unexpected change in slope. This can be attributed to a second-order phase transition, e.g., a solid–solid phase transition (for which the Gibbs free energy and its first derivatives are continuous but its second derivatives are discontinuous), which would not occur in an eutectic alloy. After the melting phase, the specific heat capacity decreases with temperature. The decrease is in agreement with the study of Bergman and Komarek [19], who report that the specific heat capacity of liquid alloys generally decreases with increasing temperature. The average specific heat capacity of the solid PCM in the temperature range 100–500
C is cp;s ¼ 1:004 kJ/kg. For the average specific heat capacity of the liquid PCM, only the temperature range 545–566
C is considered to reflect the maximum temperature reached in the experiments. The average specific heat capacity of the liquid PCM is then cp;l = 1.103 kJ/kg. Fig. 5(b) shows the measured heat flows during melting and solidification. We first discuss the heat-flow curve obtained during

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temperature of the peak only indicates that the sample is completely molten but does not correspond to the melting temperature, see Boettinger et al. [20]. Consequently, the melting temperature range was extracted from Fig. 5(b) using two considerations. First, for alloys the melting process is considered to begin at the temperature at which the heat-flow signal departs from the baseline rather than at the on-set temperature, see Boettinger et al. [20]. Second, the temperature at which the heatflow signal returns to the baseline is not significant since it is affected by experimental conditions (such as the heating rate and sample mass) and is therefore merely indicative of thermal lags within the instrument and the sample, see, e.g., He et al. [21]. Based on these considerations, we extract the melting temperature range of the PCM to be 509–527
C, corresponding to the range between the temperature at which the heat-flow signal departs from the baseline and the peak temperature. We now move on to discuss the heat-flow curve obtained during the exothermic solidification process. The two small peaks at temperatures of about 518
C and 530
C are attributed to the solidification of carbides that originate from the impurities and to the formation of new secondary phases upon solidification because of phase separation, see Mehling and Cabeza [22, pp. 65–66]. The large peak at about 511
C, which is caused by the solidification of the eutectic phase, indicates a marked hysteric behavior of the PCM with a temperature difference of about 16
C between the melting and solidification peaks. This behavior could be caused by a genuine subcooling effect or by an apparent hysteresis introduced by the sample size and the heating and cooling rates used in the experiments, see Mehling and Cabeza [22, p. 66] and Kheradmand et al. [23]. Because of the uncertainty associated with the experimentally determined solidification temperature range, in the simulations the experimentally determined melting temperature range is used for both melting and solidification. Compared to the values reported by Gasanaliev and Gamataeva [16], we measured a relatively large melting temperature range with a mid-value of 518
C rather than a distinct melting temperature of 525
C and a heat of fusion that is about 10% larger. Fig. 5. Measured (a) specific heat capacity and (b) heat flow during melting and solidification of the PCM.

the endothermic melting process, which when integrated results in a heat of fusion of 404 kJ/kg. The relatively broad heat-flow curve as well as the presence of two peaks and the plateau at temperatures of about 535–540
C are ascribed to the material impurities and to the non-uniform structure resulting from the offeutectic composition, see Table 1. The first peak at a temperature of about 510
C is attributed to a solid–solid phase transition. The second peak corresponds to the melting of the eutectic phase and reaches a maximum at about 527
C. The plateau at temperatures of about 535–540
C is hypothesized to be caused by the superposition of the hightemperature tail of the second peak and a third peak at about 540
C. The obscured third peak might be caused by the melting of carbides that originate from the impurities. The maximum heat flow associated with the hypothesized third peak is thought to be small because the carbide amounts are expected to be small. A detailed analysis of the secondary phases present in the PCM, which requires that the binary and ternary phase diagrams of Al, Cu, and Si be analyzed, was beyond the scope of this study. Because of the broad heat-flow curve during melting, we infer that the melting process does not occur at a distinct temperature but rather within a temperature interval, and hence we refer to a melting temperature range. In the absence of a sharp and narrow peak of the heat-flow curve (as expected for an eutectic alloy), the

3.3. Thermal diffusivity and thermal conductivity The thermal diffusivity was determined using laser-flash analysis (LFA) with a NETZSCH LFA 457 MicroFlash. Three samples were used with an average mass of 1163.2 mg and an average diameter and thickness of 12.51 mm and 2.92 mm, respectively. Measurements were taken between 50 and 450
C in 50
C increments. The maximum temperature was chosen to be below the melting range to avoid softening or melting of the PCM. Three measurements were performed per sample at each temperature. For all measurements, the Cowan model with pulse correction from the NETZSCH software was used. The thermal conductivity of the sample was calculated from kðTÞ ¼ rcp;s ðTÞaðTÞ;

ð1Þ

where r is the measured density at room temperature (assuming negligible expansion during the measurements), cp, s(T) is the temperature-dependent specific heat capacity measured by DSC, and a(T) is the temperature-dependent thermal diffusivity measured using LFA. Fig. 6 shows the average measured thermal diffusivity and calculated average thermal conductivity as a function of temperature. The averages at each temperature were computed from the three measurements per sample for three samples. The vertical bars indicate the standard deviation of the average values of the three samples. From Fig. 6(a), the average thermal diffusivity is seen to have a maximum at about 300
C, which is about 11% larger

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Table 2 Measured and calculated thermophysical properties of PCM. Quantity

Value

Units

DTm Dhfus




cp;s cp;l

509–527 404 1.004 1.103

k

151

C kJ/kg kJ/kg K kJ/kg K W/mK

3246

kg/m3

r

coefficient between the air and the tubes encapsulating the PCM. The heat-transfer coefficient for the perforated plate is calculated using the correlation of Tomi c et al. [26] with a pitch-to-diameter ratio of 2.2. At the lateral walls, natural convection and conduction losses through the insulation are considered while the thermal inertia of the metal sheets is neglected. At the top and bottom of the tank, adiabatic conditions are assumed. Grid- and time-refinement studies were performed to ensure that the results were numerically accurate. The temperature-dependent thermophysical properties of the insulation materials and the rocks are given in Part 1. The temperature-independent thermophysical properties for the PCM and the encapsulation material are those reported in Table 2 and by Geissbühler et al. [6], respectively. 5. Experimental conditions

Fig. 6. (a) Measured thermal diffusivity and (b) calculated thermal conductivity of the PCM. Symbols indicate the average values of three measurements per sample for three samples (nine measurements per temperature) and the vertical bars indicate the corresponding standard deviations. The dashed lines indicate the polynomial regression of the measured values.

than its value at 50
C. The decrease in the thermal diffusivity at temperatures higher than 350
C might be linked to the secondorder phase transition to which we attributed the change in slope of the specific heat capacity in Fig. 5(a). Fig. 6(b) shows that the thermal conductivity at 450
C is about 35% larger than its value at 50
C. The increase of the conductivity with temperature is consistent with the behavior of aluminum alloys, see Drezet et al. [24]. The average thermal conductivity in the temperature range 50–450
C is k ¼ 151 W=mK. The measured and calculated thermophysical properties are summarized in Table 2. 4. Numerical model The numerical model of the combined sensible/latent TES in the cavern is based on the model of Geissbühler et al. [6], modified as described in Part 1. The boundary conditions for the simulations presented here are the measured air mass flow rate and inflow/ outflow temperatures to/from the storages, as indicated in Fig. 2. The sensible TES is treated as described in Part 1, assuming a 15% bypass flow with respect to the measured air mass flow rate. The latent TES is treated as described by Geissbühler et al. [6] using the correlation of Žukauskas [25] with Crow = 1.0 for the heat-transfer

Since the behavior of the cavern was already investigated through the ambient-temperature tests presented in Part 1, only high-temperature tests were performed with the combined sensible/latent storage. Furthermore, because the behavior of the cavern was already discussed extensively in Part 1, it will not be discussed further here. The tests with the combined sensible/latent storage consisted of four charging/discharging cycles. Before the first charging phase, a so-called pre-charging was performed to approach steady cycling conditions more quickly, as described in Part 1. The durations of the pre-charging, charging, and discharging phases are listed in Table 3. During the pre-charging phase, the inflow temperatures to the latent and sensible storages, indicated in Fig. 2 by Tair,top,lat and Tair,top,sens, respectively, and the mass flow rate varied as shown in Fig. 7. In this and subsequent figures, light blue, white, and gray backgrounds indicate pre-charging, charging, and discharging phases, respectively. Fig. 8 shows the air inflow and outflow temperatures of the latent and sensible storages during the charging/discharging cycles. During the charging phases of each cycle, the heater was used at full power. However, because of the thermal inertia of the heater and the pipes and because of the aforementioned thermal losses from the pipe connecting the heater and the latent storage, Tair,top,lat and therefore also Tair,top,sens are seen not to have reached their maximum values immediately during the second, third, and fourth charging phases. In the first charging phase, Tair,top,lat and Tair,top,sens attained their maximum values because of the preceding pre-charging phase.

Table 3 Operating conditions of the tests. Dtpc, Dtc, and Dtd denote the durations of the precharging, charging, and discharging phases, respectively. Cycle

Dtpc (h)

Dtc (h)

Dtd (h)

_ air,top,lat/sens m,T

1 2 3 4

70:36 – – –

1:30 1:29 1:31 1:32

1:30 1:31 1:27 1:29

Figs. 7 and 8 Fig. 8 Fig. 8 Fig. 8

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During charging, the mass flow rate is dependent on the compressor, while during discharging, it is controlled through outlet valves. 6. Results and discussion 6.1. Cycling experiments

Fig. 7. Measured air mass flow rate and inflow temperatures to the sensible and latent TES during the pre-charging phase.

6.1.1. Pre-charging phase Fig. 9 shows the comparison between the measured and computed temperature profiles in the latent and sensible TES units at several times during the pre-charging phase. In this and the following figures, the melting range DTm of the PCM is indicated by a red background. First, we find that good agreement between experimental and numerical results for both TES units is achieved, as for the sensible-only TES in Part 1, meaning that the heattransfer model can accurately predict the behavior of a pilot-scale combined sensible/latent TES. Second, we note that the temperature evolution over time differs between the two TES units. A thermocline develops in the sensible TES, but the latent TES shows little or no thermal stratification. The lack of stratification in the latent TES is already visible after 10 h of pre-charging, when the average measured temperature is 357
C and the difference between the highest and lowest measured temperatures is only about 10
C. With the exception of the time when the PCM melts, after 10 h this difference is never larger than 22
C. We attribute the lack of stratification partly to the slow heating rate during the pre-charging phase. For example, Fig. 7 shows that between 9:27 and 10:00, the temperature of the air entering the latent TES varies between 363
C and 350
C. The average air inflow temperature is therefore close to the abovementioned average measured temperature of 357
C after 10 h. The

Fig. 8. Measured air temperature at the top (inlet during charging) and bottom (inlet during discharging) of the latent TES, measured air temperature at the top (inlet during charging) and bottom (inlet during discharging) of the sensible TES, and measured air mass flow rate during the four consecutive charging/discharging cycles.

During the discharging phases, air flows from the cavern into the sensible storage. Therefore, Tair,bot,sens is approximately equal to the air temperature in the cavern, see Fig. 8, and decreases with time due to the expansion of the air in the cavern. From the sensible storage, air enters the latent storage with a temperature of Tair,bot,lat that also decreases with time, see Fig. 8. The air mass flow rates during the charging and discharging _ c and m _ d , respectively, are shown in Fig. 8. phases, denoted by m

Fig. 9. Temperature profiles as a function of the vertical position for latent TES (top) and sensible TES (bottom) at several times during the pre-charging phase. Note: The shaded red area indicates the melting range of the PCM; the RTD measuring T11 failed at t = 60.2 h; the symbols in the bottom plot represent the air temperature measured by the RTDs located inside the sensible TES, as shown in Fig. 5 of Part 1. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

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lack of stratification is attributed only partly to the slow heating rate because, as will be shown in Section 6.1.2, limited stratification is observed also during the cycling phase in which the latent TES is exposed to higher heating rates. As a result, the lack of stratification is put down also to the heat transfer between the air and the PCM being effective, which is supported by the simulations that predict the differences between the air and PCM temperatures never to exceed 5
C with the exception of the time when the PCM melts. At 27 h, the measured temperatures of layers 11 and 14 are within the melting range, suggesting that the PCM located in the bottom half of the tank is in the melting phase, whereas the measured temperatures of layers 2, 5, and 8 are already higher than the melting range. However, the model predicts the PCM in layers 2, 5, 8, 11, and 14 to be in the melting process. After 70 h of pre-charging, the average measured temperature in the latent TES is 561
C and the difference between the highest and lowest measured temperatures is 7
C. Since the latent TES has been charged for 44:49 h with an average air inflow temperature of 553
C, see Fig. 7, the storage can be considered to be fully charged. At the end of pre-charging, the thermal energies stored in the latent and sensible TES units, computed from the simulation results, are 167.4 kWhth and 4.5 MWhth, respectively, corresponding to 98% and 39% of their maximum storage capacities. Fig. 9 shows that at the end of the pre-charging phase, the temperature of the air flowing out of the latent TES is 557
C, while that of the air flowing into the sensible TES reaches only 530
C. The difference is ascribed (1) to thermal losses from the pipe connecting the two storages, and (2) to air losses from the steel cover of the sensible TES, which were discussed already in Part 1. Since the behavior of the sensible TES was thoroughly studied in Part 1, it will not be discussed further. 6.1.2. Cycling phase Fig. 10 shows the computed and measured PCM and air temperatures as a function of time during the four charging/ discharging cycles for layers 2, 5, 8, and 14. As indicated in Fig. 3, for layer 2 the measured temperature is that of the PCM, whereas for layers 5, 8, and 14 the measured temperature is that of the air. We make three observations from Fig. 10. First, we observe that the measured PCM temperatures in layer 2 at the ends of the discharging phases decrease with successive cycles. At the end of the first discharging phase, the temperature of the PCM has only just entered the melting range and therefore the PCM has only just begun solidifying. At the ends of the second and third discharging phases, the temperature of the PCM is well within the melting range and hence the PCM is partially solidified. At the end of the fourth discharging phase, the PCM temperature is below the melting range, indicating that the PCM is fully solidified. The trend of decreasing PCM temperatures at the ends of the discharging phases is visible not only in the measurements for layer 2, but also in the simulations for layers 5, 8, and 14. For example, the simulations for layer 14 predict that the PCM temperature barely enters the melting range during the charging phases of the third and fourth cycles. As a consequence of the effective heat transfer between the PCM and the air, the measured air temperatures at the ends of the discharging phases for layers 5, 8, and 14 decrease with each successive cycle just like the measured PCM temperatures. The second observation relates to the agreement between the measurements and simulations. The overall agreement is reasonably good in the sense that general trends, such as the abovementioned decreases of the PCM and air temperatures at the ends of the discharging phases, are predicted by the simulations with good accuracy. Conversely, the agreement between measured and simulated PCM and air temperatures in a specific layer at a specific

Fig. 10. Temperature evolution during the four charging/discharging cycles as a function of time at four locations in the latent TES.

time is highly variable. On the one hand, in layer 14 the discrepancies between the simulated and measured air temperatures over the four charging–discharging cycles are very small. On the other hand, in layer 2 the maximum discrepancy between the measured and simulated PCM temperatures is about 43
C. Interestingly, the simulated air temperatures approximate the measured PCM temperatures in layer 2 more closely than the simulated PCM temperatures. The discrepancies between the measurements and the simulations might be due to approximations in the model, uncertainties in the experimental conditions, and uncertainties in the thermophysical properties. The effect of uncertainties in the thermophysical properties on the simulated temperatures will be quantified through a sensitivity analysis in Section 6.3. The third and final observation concerns thermal stratification. In Section 6.1.1, it was shown that during the pre-charging phase, there is little or no stratification in the latent TES. The lack of stratification is reflected in Fig. 10 through all layers being at the same temperature at the beginning of the first discharging phase. At the end of the first discharging phase, however, the simulated air temperatures in layer 14 are already about 43
C lower than those in layer 2. At the end of the fourth discharging phase, the difference between the air temperatures in these two layers has grown to 83
C. The development of thermal stratification can be seen more clearly in Fig. 11, which presents the temperature profiles as a function of the axial coordinate at the ends of the charging and discharging phases for each cycle. The profiles at the ends of the charging phases show that the portion of the latent TES with temperatures above the melting range decreases with each cycle. Similarly, the profiles at the ends of the discharging phases

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Fig. 11. Temperature profiles in the latent TES as a function of the axial coordinate at the ends of the charging and discharging phases.

show that the portion of the latent TES with temperatures below the melting range increases with each cycle. In other words, in each cycle a smaller fraction of the PCM melts and solidifies and therefore the latent TES behaves more like a sensible TES. The results presented above indicate a decrease in the performance of the latent TES with each cycle. The decrease in performance is ascribed to four factors, in descending order of importance: (1) the thermal losses from the pipe connecting the latent and sensible TES units and the air leakages from the cover of the sensible TES, (2) the experimentally measured mass flow rates being smaller than the nominal mass flow rate used during the design of the latent TES, (3) the experimentally measured average mass flow rates during the discharging phases being smaller than those during the charging phases, and (4) the nonconstant air inflow temperature during charging caused by the start-up times of the heater. The effect of these factors was assessed through simulations, the results of which are not shown for brevity. Despite the reduction in its performance, the latent TES can effectively reduce the decrease in the air outflow temperature

during discharging. This may be deduced by comparing the measured air inflow and outflow temperatures during discharging, i.e., Tair,bot,lat and Tair,top,lat. During discharging, these temperatures can be interpreted as the air outflow temperatures from a sensibleonly and from a combined-sensible/latent TES, respectively. Fig. 12 shows that Tair,bot,lat drops by about 99
C during the first discharging phase, while the drop in Tair,top,lat is reduced to 35
C thanks to the latent heat released by the PCM. Therefore, the combined TES as used in an AA-CAES plant can provide smaller variations in the temperature of the air flowing to the turbine, which should be beneficial for the overall plant efficiency. 6.2. Post-experiment inspection and analysis After the experimental campaign, we inspected the latent TES to assess the thermal and mechanical stability of the tank and the tubes encapsulating the PCM. The inner and outer shells and the insulation were observed to have withstood the thermal cycling well. After opening the inner shell, we inspected the tubes and found solidified pieces of PCM such as those shown in Fig. 13. Based

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pieces weighed between 2.8 mg and 3.7 mg and are denoted by A, B, and C in the following. Fig. 14 compares the measured heat flows during melting for the PCM samples after thermal cycling (indicated by the solid lines)

Fig. 12. Measured air temperature at the inlet and outlet of the latent TES during the first discharging phase.

Fig. 13. Pictures of leaked PCM after experiments.

on the locations of these pieces, we concluded that the PCM had leaked from either the welding seams or the two RTD insertion points. On one tube, a small hole was found in a welding seam. The PCM pieces were collected and found to correspond to about 0.2% of the total PCM mass. While a more detailed analysis of the PCM, the encapsulation, and the growth of intermetallic layers is underway, the recovered PCM pieces provided a valuable first opportunity to assess the effect of thermal cycling on the PCM. To this end, the DSC analysis described in Section 3.2 was repeated for three PCM pieces that differed in color and texture. The samples extracted from these

Fig. 14. Measured heat flows during melting for PCM samples after thermal cycling (solid lines) and comparison to measured heat flow during melting before thermal cycling (dashed line).

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to the measured heat flow during melting before thermal cycling (indicated by the dashed line). We make four observations. First, it is noted that all heat-flow curves after cycling are qualitatively similar and that they differ from the heat-flow curve before cycling. This indicates that the phase composition of the PCM changes upon cycling and that degradation and/or phase segregation occur. Second, compared to the heat-flow curve before cycling, the heat-flow curves after cycling show a larger low-temperature peak and a smaller eutectic peak. The low-temperature peak after cycling could be caused by the same solid–solid phase transition as before cycling (see Section 3.2) or phase transitions of Al–Fe compounds. These compounds could have originated from the dissolution of stainless steel from the encapsulation into the liquid PCM. Corrosion processes would also explain the decrease of the eutectic peak through the loss of the eutectic phase and the formation of Al–Fe compounds. Third, by integrating the heat-flow curves, the heats of fusion of samples A, B, and C are found to be 374 kJ/kg, 386 kJ/kg, and 326 kJ/ kg respectively. Relative to the heat of fusion prior to cycling, this corresponds to reductions of 7.4%, 4.5%, and 19.3%, respectively. Finally, we observe that the heat-flow curves of samples A and B are closer to each other than either of them is to the heat-flow curve of sample C. This suggests that samples A and B have a very similar phase composition and that it differs from that of sample C. Therefore, at least two alloys appear to have formed during thermal cycling. In addition to the DSC analysis, the density was determined as for the PCM before cycling for two samples each extracted from the same PCM pieces from which samples A and C were extracted. The average density of the four samples was found to be only 0.5% smaller than the density of the PCM before cycling. The standard deviation of the densities of the four samples was 1.4% of the average density. The DSC results show that the PCM is not stable under thermal cycling. The lack of stability is attributed to the off-eutectic composition of and impurities in the PCM as well as to corrosion phenomena between the liquid PCM and the stainless-steel

encapsulation. We assume that other thermophysical properties, such as the mid-temperature of the melting range, the melting range, and the specific heat capacity, are also affected by thermal cycling. Changes in the thermophysical properties may have contributed to the decrease in the performance of the latent TES in addition to the factors listed in Section 6.1.2. 6.3. Sensitivity study To assess the degree to which changes in the thermophysical properties of the PCM might explain the discrepancies between the experimental and numerical results, a sensitivity study was performed. In the sensitivity study, the simulations of the precharging and cycling phases were repeated by varying one thermophysical property at a time relative to its value before thermal cycling, see Table 2. Variations in the following properties were studied: heat of fusion, specific heat capacity, mid-temperature of the melting range, and melting range. For the specific heat capacity, the variations were applied to both the solid- and liquidphase specific heat capacities. The effect of the PCM melting range was investigated in two ways. First, by keeping the range constant and varying the mid-temperature of the range between 512 and 524
C in increments of 3
C. Second, by varying the range by 25% and 50% with respect to its value before cycling and keeping its mid-temperature constant. The results of the sensitivity study are presented in Fig. 15 in terms of Tair,top,lat during the fourth discharging phase. It is immediately apparent that variations in the heat of fusion have the largest impacts and that variations in the specific heat capacity, the mid-temperature of the melting range, and the melting range have limited impacts. It is interesting to observe that the simulations and experiments agree well when the heat of fusion is decreased by 20% relative to its value prior to thermal cycling. That such large decreases are plausible is clear from Section 6.2, in which post-experiment analysis of the PCM showed decreases in the heat of fusion of up to 19.3%. Because a decrease of 20% is larger than the mean of the

Fig. 15. Results of sensitivity study in terms of Tair,top,lat during the fourth discharging phase. “Ref.” indicates the values before thermal cycling.

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measured decreases of 10.4%, we assume that the decrease in the heat of fusion is not the sole explanation for the discrepancies between the experimental and numerical results. Additional results, not presented for brevity, show that changes of up to 20% of the heat-transfer coefficient and the PCM thermal conductivity have small impacts, while changes of up to 20% of the PCM density have significant impacts. In particular, a decrease of 20% in the density relative to the value measured before cycling led to significantly improved agreement between the simulations and experiments. This result appears irrelevant because we reported in Section 6.2 that our measurements did not find significant changes in the density after thermal cycling. However, in our numerical model the PCM density appears only as a multiplier of the PCM volume, so relative variations in the PCM density are equivalent to relative variations in the PCM volume. Variations in the PCM volume could have been caused by some tubes having been filled with less PCM and/ or by tubes having been shorter than specified. Nevertheless, it is very unlikely that these two causes can account for a 20% decrease in the PCM volume. The sensitivity study shows that the thermophysical properties of the PCM have a significant impact on reducing the discrepancies between simulations and experiments. Variations in the heat of fusion of the PCM that give improved agreement are plausible given the measured changes reported in Section 6.2, but it seems unlikely that they are the sole explanation for the discrepancies. Instead, it seems likely that uncertainties in the total volume of the PCM are an important factor also. Although other factors that are not included in the model, such as radial effects and the thermal inertia of the steel support structure, might have contributed to the discrepancies, they are believed to be less important. 7. Summary, conclusions, and further work A pilot-scale AA-CAES plant with a combined sensible/latent TES was built in an unused tunnel. The combined sensible/latent TES differs from the sensible-only TES presented in Part 1 through a latent TES with maximum storage capacity of 171.5 kWhth. The latent TES consists of a steel tank containing 296 stainless-steel tubes encapsulating an Al–Cu–Si alloy. The combined sensible/latent TES was investigated using four charging/discharging cycles with durations of about 3 h each and air charging temperatures of up to 566
C. The main conclusions are: 1. The latent TES reduced the decrease in the air outflow temperature during discharging, confirming prior work on combined sensible/latent TES at the laboratory scale and demonstrating the potential of sensible/latent TES as an attractive option for industrial-scale high-temperature storage. 2. The performance of the latent TES decreased with each cycle. The decrease was traced primarily to thermal losses from the pipe connecting the latent and sensible TES units, to air leakages from the cover of the sensible TES, and to mass flow rates that were smaller in the experiments than during the design of the latent TES. 3. Although minor leaks of the PCM were detected after the experiments, the latent TES structure and the PCM encapsulation exhibited good thermal and mechanical stability to temperature and pressure variations. The leaks were traced to the welding seams between the tubes and the caps that encapsulated the PCM and to the holes drilled through the tubes to insert RTDs into the PCM. 4. The PCM exhibited degradation and/or phase segregation upon thermal cycling, resulting in a decrease of its heat of fusion. These changes were attributed to the initial off-eutectic composition of and impurities in the PCM as well as to corrosion phenomena

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between the liquid PCM and the stainless-steel encapsulation at high temperatures. 5. Simulations predicted the behavior of the combined sensible/ latent TES with good overall accuracy. The discrepancies were attributed primarily to changes in the heat of fusion of the PCM due to thermal cycling and uncertainties in the volume of the PCM contained in the latent TES. From the experience gathered with the pilot-scale combined sensible/latent TES, it is clear that further work should focus on the PCM and the encapsulation. For the PCM, it is imperative that eutectic compositions with minimal impurities can be guaranteed when ordering quantities commensurate with pilot-scale or industrial-scale applications. For the encapsulation, further work must address the large-scale production of leak-proof encapsulations and continue the development of coatings that prevent corrosion phenomena. Acknowledgments The authors gratefully acknowledge funding by the Swiss National Science Foundation under the National Research Program 70 (grant no. 407040_153776), the Swiss Federal Office of Energy (SI/501001-01), the Commission for Technology and Innovation through the Swiss Competence Center for Energy Research on Heat and Electricity Storage, and the European Union under the 7th Framework Program (SFERA-II, grant no. 312643). The authors are grateful to Luciano Serio for the engineering support during the design, building, and operational phases of the combined TES, to Mattia Fransioli and Vittorio Lo Vaglio for the support during the design of the storage, to Elke Hempel and Urs Jörimann for performing DSC and TOPEM1 analysis on the PCM and for helpful discussions, and to Mihai Stoica for valuable inputs on metal physics. References [1] X. Luo, J. Wang, M. Dooner, J. Clarke, Overview of current development in electrical energy storage technologies and the application potential in power system operation, Appl. Energy 137 (2015) 511–536. [2] M. Budt, D. Wolf, R. Span, J. Yan, A review on compressed air energy storage: basic principles, past milestones and recent developments, Appl. Energy 170 (2016) 250–268. [3] A. Sciacovelli, Y. Li, H. Chen, Y. Wu, J. Wang, S. Garvey, Y. Ding, Dynamic simulation of adiabatic compressed air energy storage (A-CAES) plant with integrated thermal storage-link between components performance and plant performance, Appl. Energy 185 (2017) 16–28. [4] L. Geissbühler, V. Becattini, G. Zanganeh, S. Zavattoni, M. Barbato, A. Haselbacher, A. Steinfeld, Pilot-scale demonstration of advanced adiabatic compressed air energy storage, Part 1: Plant description and tests with sensible thermal-energy storage, J. Energy Storage 17 (2018) 129–139. [5] G. Zanganeh, R. Khanna, C. Walser, A. Pedretti, A. Haselbacher, A. Steinfeld, Experimental and numerical investigation of combined sensible-latent heat for thermal energy storage at 575
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