Available online at ScienceDirect. Energy Procedia 49 (2014 ) SolarPACES R. Yogev a, A.

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1 Avaiabe onine at ScienceDirect Energy Procedia 49 (2014 ) SoarPACES 2013 PCM storage system with integrated active heat pipe R. Yogev a, A. Kribus a * a Schoo of Mechanica Engineering, Te-Aviv University, Te-Aviv 69978, Israe Abstract The use of the atent heat of phase change materias (PCM) is considered a promising approach to store heat at a neary constant temperature for direct steam generation (DSG), but the poor therma conductivity of commony avaiabe storage materias imposes severe imitations on storage performance. A new method is proposed to overcome the imitations of the ow therma conductivity. The approach is to physicay decoupe the evaporator pipes from the PCM, thus aowing independent sizing of each component. The therma ink between the two components is done via evaporation and condensation of a heat transfer fuid (HTF), according to the principe of a heat pipe. Pumping the iquid HTF provides active contro of the heat pipe operation. The new concept is modeed and compared to the conventiona design of conduction based PCM annuus around the steam pipe. An exampe case shows a significant advantage in performance of the active heat pipe configuration due to its reduced therma resistance The Authors. Pubished by Esevier Ltd. This is an open access artice under the CC BY-NC-ND icense ( The Authors. Pubished Esevier Ltd. Seection and peer review by by the the scientific conference committee committee of SoarPACES of SoarPACES 2013 under 2013 responsibiity under responsibiity of PSE AG. of PSE AG. Fina manuscript pubished as received without editoria corrections. Keywords: Phase change materias (PCM); Latent heat therma storage (LHTS); Heat pipe; Concentrating soar power (CSP) 1. Introduction One of the major chaenges of soar energy production is the intermittency in the avaiabiity of soar radiation. The need for a reiabe energy source past sunset and during sporadic coudy periods makes it difficut to rey on soar power pants as a major source. This probem coud be greaty mitigated by introducing a heat storage system into arge scae soar therma pants. Numerous soutions have been proposed and investigated [1] incuding various types of chemica and therma storage systems. Latent heat therma storage (LHTS) is one promising approach, * Corresponding author. Te.: ; E-mai address: kribus@tauex.tau.ac.i The Authors. Pubished by Esevier Ltd. This is an open access artice under the CC BY-NC-ND icense ( Seection and peer review by the scientific conference committee of SoarPACES 2013 under responsibiity of PSE AG. Fina manuscript pubished as received without editoria corrections. doi: /j.egypro

2 1062 R. Yogev and A. Kribus / Energy Procedia 49 ( 2014 ) where heat from the soar fied is used to met a store of phase change materia (PCM) during charging. In discharge, the PCM is soidified, thus deivering its heat of phase transition to the therma process. This type of storage provides high therma capacity per unit mass, and is advantageous for steam production (e.g., in a Rankine cyce power pan due to the isotherma nature of the process. In a power pant that requires superheat, a separate unit is added with sensibe heat storage (using, for exampe, concrete or moten sa to provide heat for superheating of the steam [2]. In this work it is assumed that such a sensibe heat unit exists but it is not anayzed here. A major inhibition for the PCM storage unit is the typicay ow therma conductivity of storage materias avaiabe at high temperatures. The ow therma conductivity eads to a steep decrease with time of the power extracted from storage, and compensating for this effect requires pant operation at siding pressure mode and reduced thermodynamic efficiency [3]. Severa methods were proposed to overcome this probem, incuding additives in the PCM to enhance its therma conductivity, and various fin geometries [4,5]. Nomencature A Area (m 2 ) Subscripts d Steam pipe diameter (m) C Heat capacity (J kg -1 K -1 ) 0 Process start g Earth s gravity constant (m s 2 ) Liquid HTF h Latent heat (J kg 1 ) v HTF phase transition k Therma conductivity (W m 1 K 1 ) m Meting m Mass (kg) p PCM m Mass fow rate (kg s 1 ) s Steam property q Heat fux (W m 2 ) sat Steam saturation R Therma resistance (m 2 KW 1 ) sp Steam pipe T Temperature (K) struc Structura materias t Time (s) v Vapor HTF x Steam quaity ( ) w PCM sab wa y Vertica coordinate aong PCM sab (m) wo Outer pipe wa z Horizonta coordinate aong steam pipe (m) wi Inner pipe wa Greek Therma power reduction rate ( ) δ HTF fim thickness (m) μ Dynamic viscosity (kg m 1 s 1 ) ξ Soid/iquid boundary position (m) ρ Density (kg m 3 ) τ Typica discharge time (s) In this work, a different approach is presented to improving the therma performance of the storage. We propose to reduce the impact of the ow therma conductivity of the PCM by physicay decouping the areas for heat transfer on the steam side and the PCM side. This aows independent sizing, producing a arger heat transfer area and a shorter path for conduction on the PCM side. The transport between the two decouped eements is done with evaporation and condensation of a heat transfer fuid (HTF), which is equivaent to a heat pipe mechanism. An additiona eement of the proposed soution is active operation by pumping of the iquid HTF, eading to a controabe and reversibe operation for charge and discharge modes. A simiar soution with evaporation and condensation was proposed [6] but it was based on passive refux without active pumping. A passive refux heat pipe was aso proposed for soar receivers in space power appications [7] and terrestria dish-stiring systems [8]. Here we anayze the operation of the proposed storage ony in discharge, which is the more probematic mode in terms of maintaining the desired output power. A mode is presented of the storage modue with active heat pipe and

3 R. Yogev and A. Kribus / Energy Procedia 49 ( 2014 ) the thermodynamic cyce. A representative exampe of storage and cyce performance is presented, showing a potentia to improve the thermodynamic performance of the entire pant compared to previous storage soutions. 2. Concept and method 2.1. Storage configuration The active heat pipe storage system comprises water/steam pipes and PCM capsues as separate eements not in physica contact with each other. This aows assigning a much arger heat transfer area to the PCM side, whie maintaining a reativey sma amount of high pressure steam pipes. Fig. 1 shows schematicay the major components of the system. Fig. 1 Storage system scheme in discharge mode The PCM capsues have a generay fat shape, with high surface area and ow thickness, providing a short path for conduction heat transfer through the PCM. Both the PCM capsues and the steam pipes are inside a seaed container fied with a heat transfer fuid (HTF) in a state of saturation, with iquid and vapor in equiibrium. A circuation system is provided in the container with a pump, pipes, and a contro system in order to circuate the iquid HTF to the surfaces of either the PCM capsues or the steam pipes. During charging, the HTF pressure is adjusted such that its saturation temperature is sighty higher than the PCM meting temperature, but ower than the saturation temperature of the steam arriving from the soar coector fied. The pump is operated to provide saturated iquid HTF over the steam pipes. The HTF evaporates on the steam pipes and condenses over the PCM capsues. During discharge, the HTF pressure is adjusted such that its saturation temperature is sighty ower than the PCM meting temperature, but higher than the saturation temperature of the water to be evaporated. The pump is operated to provide saturated iquid HTF over the PCM capsues. The HTF evaporates on the PCM capsues and condenses over the pipes. In both modes the vapor HTF fows from side to side as needed by free convection, over a arge cross section area that does not restrict its fow, in contrast to traditiona heat pipes that have a restrictive sma cross section area. The active heat pipe storage requires a suitabe HTF that provides the needed saturation temperature at reasonabe pressure near atmospheric pressure, to avoid the need of a high-pressure container. For the standard soar Rankine cyce with PCM meting temperature around 300 C, we examined the performance of the proposed storage modue using Thermino VP-1 as HTF. This materia is perhaps not idea for a rea system, since it is fammabe,

4 1064 R. Yogev and A. Kribus / Energy Procedia 49 ( 2014 ) toxic and is chemicay unstabe in high temperatures. However, it is used here as an exampe for the anaysis of the thermodynamic behavior, whie for future practica appications, other materias shoud be sought Mode setup The mode is based on severa simpifying assumptions, the main ones being: constant PCM meting temperature; a HTF is at saturation temperature; no presence of incondensabe materias in the HTF container; negigibe pressure drop in the HTF fow; one-dimensiona heat transfer in the PCM capsue and in the pipe; quasi-steadystate; and negigibe heat oss to the environment. The physica decouping of the PCM from the steam pipes aows separate formuation of the heat transfer processes on the steam side and on the PCM side. These two separate sets of formuae are ater couped to sove the compete probem. The mode described here refers ony to the discharge process of the storage PCM capsue heat transfer During storage discharge, iquid HTF at saturation conditions is provided at the top of the PCM sab creating a faing fim. This iquid fim is cooer than the PCM, causing the PCM to soidify whie the iquid evaporates. The heat then fows through three therma resistances in series: the faing fim, the capsue wa, and the soidified PCM ayer. The therma resistance of the capsue wa is represented in the initia condition. A momentum baance on the faing fim produces a differentia equation for the faing fim thickness δ y [9]: 3 δ 1 3μ = y δ gρ ( ρ ρ ) v k hv [ T T ( y, ] An energy baance on the PCM eads to the equation for the advancement of the PCM moving soidification front [3]: ξ k p [ Tm Tw ( y, ] ρ h p m = (2) t ξ ( y, The heat extracted from the PCM equas the heat transferred to the fim, eading to a couping of equations (1) and (2) via the PCM modue s externa wa temperature : v w ( ) (1) δ ( y, k p Tm Tw ( y, = δ ( y, k p + ξ ( y, k T + ξ ( y, k v (3) This competes the set of equations that describe the position of the moving soidification front in the PCM as a function of time ξ () t and the thickness of the HTF fim as a function of vertica position and time δ ( y,. In practice, when substituting typica parameter vaues, the change in fim thickness is very sma and has a negigibe effect on the rate of heat transfer. This has been vaidated for severa sets of data [10]. Therefore, the mode can be simpified using the approximation of uniform therma resistance of the fim. This reduces equations (1) and (2) to: [ T T ( y, ] ξ m w (4) q p ( = ρ phm = t ξ ( / k p + R( R( is the therma resistance of the iquid fim, which depends on time due to the variations in HTF temperature during discharge that affect its therma conductivity. With this approximation, the amount of HTF evaporated from one face of a singe sab is:

5 R. Yogev and A. Kribus / Energy Procedia 49 ( 2014 ) m v = A q ( h p v (5) 2.4. Water-Steam pipe heat transfer Heat transfer in the steam pipes consists of three therma resistances in series: free convection condensation of the HTF on the externa part of the pipe, conduction through the pipe wa, and forced convection boiing within the pipe. We use Nusset's correation for the heat transfer coefficient in condensation on horizonta pipes [11]: gρ qs ( z) = ( ρ ρ ) h k [ T T ( z) ] v d v wo μ v wo 3 (6) Though dropwise condensation is characterized with higher heat transfer coefficients and is therefore preferabe over fim condensation, it is difficut to mode and strongy affected by temperature, pipe materia and surface finish. The performance considering fimwise condensation is therefore a conservative mode, which is ikey to be surpassed in an actua system. The temperature difference across the pipe wa is, from Fourier's aw: T wo ( z) T wi ( z) = q wi d d wi n d 2k sp wo wi (7) Forced convection boiing heat transfer within the pipe is evauated using a method suggested in [12] and wi not be discussed here. One notabe detai is that the heat transfer coefficient is dependent on steam quaity, meaning that heat fux varies aong the pipe, affecting both the inner and outer wa temperatures. From [12], the oca heat fux coud be represented as a function of the inner wa temperature, water saturation temperature and steam quaity: q = q q ( T (, T (, x( z)) (8) s wi = s wi This, in combination with equations (6),(7) aows soving for oca wa temperatures and heat fux given water and HTF saturation temperatures Overa baance and power bock mode Since the HTF is considered to be at saturation temperature, the entire therma power discharged from the PCM contributes soey to HTF evaporation. Simiary, subcooing of HTF that condenses over the steam pipes is negected. This approximation can be evauated by estimating the ratio between the sensibe and atent heat extracted from the HTF using the Jakob number: sat C( Tv T Ja = h m wo ) (9) The temperature difference that was used in this estimate is between the HTF vapour and the externa face of the steam pipes. In a the case studies examined in this work Ja , and therefore negecting of sensibe heat is acceptabe. The tota therma power discharged from the PCM capsues then equas the tota power transferred to the water in the pipes. The ratio of the surface area of the PCM capsues to the area of the steam pipes A p /A sp is defined as a major design input, and this ratio determines the heat fuxes and temperature differences on the two sides of the process. Given materia properties and the geometry of the system, four parameters are eft as variabes: the HTF

6 1066 R. Yogev and A. Kribus / Energy Procedia 49 ( 2014 ) and steam saturation temperatures, and the heat fux vaues at the PCM capsue wa and into the steam T v (, T sat (, q p (t,y) and q s (t,z) respectivey. Setting the vaue for one of these parameters defines the other three via the reations described in equations (1)-(8). With T sat ( and q s (t,z) known, it is possibe to address the performance of the thermodynamic cyce. Steam generated by the PCM storage is superheated by a separate sensibe heat storage unit, which is not addressed in this work, and then fed into the turbine inet. The performance of the turbine is affected by the steam pressure that corresponds to T sat and by the steam mass fow rate, defined by q s. The detaied thermodynamic anaysis of the power cyce has been presented in a previous work [3] and assumes a fixed turbine isentropic efficiency Numerica soution The set of equations presented above was soved numericay using Matab computer software. The inputs of the numerica mode incude materia properties of the PCM, HTF and water, as we as the geometry of the system and the desired heat fux from the PCM wa. The mode was soved as a quasi-steady-state probem over short time increments. For every time increment, heat fux on the face of the PCM sabs and the corresponding HTF evaporation rate were cacuated from equations (4,(5). By mutipying the heat fux with the PCM surface area, we get the instantaneous discharge power. Appying the assumption of negigibe therma osses, this equas the therma power that is transferred to the steam. Soving equations (6-(8) determines the steam evaporation temperature, the steam pressure and the steam evaporation rate. 3. Resuts and discussion 3.1. Case study description It is most desirabe to examine system feasibiity without requiring the deveopment of new materias. For this reason, the mode has been soved using the properties of the common PCM sodium nitrate (NaNO 3 ) as storage materia [13] and of the two-phase therma fuid Thermino VP-1 [14] as HTF, both satisfying the requirements presented in Section 2.1. Key parameters, incuding materia properties and eement sizing are reported intabe 1. Tabe 1 Case study parameters Property Symbo Vaue PCM Density ρ kg m -3 PCM Heat of Fusion h m kj kg -1 PCM Meting Temperature T m PCM Therma Conductivity k p (mk) -1 Nomina Discharge Time τ 2 hours Nomina Discharge Fux q 0 W m -2 PCM Sab Thickness - Tota PCM sabs area m 2 HTF fim thickness - Number of steam pipes - Steam pipe ength L Steam pipe interna diameter - Steam pipe externa diameter - Steam and HTF properties vary with temperature and pressure and are determined from [15] and [16] respectivey. The dimensions of the steam pipe are those of a schedue 40 seamess stainess stee pipe with a nomina diameter of [17].

7 R. Yogev and A. Kribus / Energy Procedia 49 ( 2014 ) As the PCM storage is discharged, a graduay increasing ayer of soid PCM is formed on the boundary of the sab. This transates to an increase in the therma resistance of the system that eads to a reduction in therma power, an increase in the temperature difference, or a combination of the two. Previous work [3] discussed different operation strategies of the thermodynamic cyce, and their impact on the variation in time of the therma power extracted from storage and the eectrica power produced. Here we appy two operation scenarios: first with therma power ineary changing with time (incuding the case of zero change rate, i.e., constant therma power); and second with therma power varying so as to produce a constant eectrica power Lineary decreasing therma power In this scenario, the saturation temperatures of the HTF and water are controed to sustain a inear reduction in therma power. The rate of reduction is defined by the dimensioness parameter, which represents the fraction of power reduction at the end of the nomina discharge time τ : Qth ( Q th,0 = 1 β t τ (10) The specia case of represents discharge at a constant therma power. Fig. 2 shows the reative reduction with dimensioness time in eectrica power for three vaues of. Achieving smaer vaues of requires a arger reduction rate in HTF and water saturation temperatures to compensate for the increasing therma resistance of the PCM. This eads to a higher decrease in the thermodynamic cyce efficiency. Therefore, discharge at a constant therma power cannot provide constant eectrica power, as aso shown in [14] for conventiona storage configurations. Fig. 2 Operation at ineary decreasing heat fux The tota amount of eectrica energy produced throughout discharge depends on the operation strategy. This coud be quantified as the discharge efficiency (DE): the tota eectricity produced as a fraction of the eectricity that woud have been produced if the soid PCM had infinite therma conductivity. In the idea case, the eectrica power production woud remain constant at nomina vaue for the entire nomina storage time. ttot Qe ( dt 0 DE = τ Q e,0 (11)

8 1068 R. Yogev and A. Kribus / Energy Procedia 49 ( 2014 ) The corresponding DE vaues for the three cases from Fig. 2 are given in Tabe 2. In a cases the eectricity produced from storage is ess than the potentia of an idea PCM. Higher vaues of produce higher DE, but at the cost of higher variabiity over a onger discharge time. In constant therma power operation about 10% of the potentia to produce eectricity is ost due to the reduction in cyce efficiency whie operating at ower steam pressure Constant eectrica power Tabe 2 Discharge efficiency (DE) vaues Case DE = = = As demonstrated in the previous section, maintaining a constant therma discharge power does not provide constant eectrica power due to the reduced thermodynamic efficiency of the cyce. This effect can be compensated by a further increase in the temperature difference, which causes the therma power to increase in time. Fig. 3 presents the required increase in therma power and in steam mass fow rate, and the corresponding decrease in the cyce s thermodynamic efficiency, when operating at constant eectrica power. The first two are normaized by their nomina vaues and the atter is presented at its absoute vaue and potted against the secondary vertica axis. Fig. 3 Steam mass fow rate, therma power and thermodynamic efficiency at constant eectrica power operation The required increase in therma power is significant, due to the increasing therma resistance faced by the soid PCM as we as the higher heat of evaporation at the reduced water pressure. The constant eectrica power generation is achieved with a penaty of increased oss, with discharge efficiency of DE = , which can aso be observed as shorter discharge time of about 11% ess than the nomina vaue. It is interesting to note that in spite of the significant increase in therma power and the significanty shorter discharge period of this case compared to the case of constant therma power, the DE is amost the same (the difference is ony 1.4%). Therefore, operation at constant eectrica power does not require a significant further penaty compared to operation at constant therma power Comparison to standard storage geometry We compare the performance of the active heat pipe storage system to the simper reference configuration composed of steam tubes surrounded by PCM annui, with one-dimensiona conduction heat transfer in the radia direction [3]. For both systems, the same steam pipes were used, and the same amount of PCM per unit steam pipe ength. Hence, the reference geometry of the PCM is an annuus with interna diameter 2.13 cm and an externa

9 R. Yogev and A. Kribus / Energy Procedia 49 ( 2014 ) diameter of 7.4 cm. The discharge of the reference system is determined using a heat baance between the soidifying PCM and evaporating steam; detais are given in [3]. Fig. 4 compares the normaized eectrica power output of the two storage configurations, both operated in constant therma power mode (β=0). The heat pipe system offers a major improvement with respect to the radia conduction configuration, where the eectrica power output drops by ony 18% at the end of discharge, compared to 35% for the radia system. Fig. 4 Normaized eectrica power output for the two storage configurations, both operated in constant therma power mode Another aspect for comparison is the required heat transfer area per unit voume of storage materia. In the case study presented here, this ratio is 46.5 m 2 /m 3. For comparison, in the conduction-based finned tube storage system presented in [18], a much higher surface area was used, about 200 m 2 /m Discussion The active heat pipe configuration of PCM therma storage has a potentia to mitigate the effect of ow therma conductivity of the PCM. The physica decouping of the PCM heat transfer area from the geometry of the steam pipes aows an additiona degree of freedom for shaping PCM capsues and reducing the impact of therma conduction. The therma simuation shows that a storage system with the active heat pipe can achieve significanty higher performance compared to the more traditiona conduction-based configurations. In constant therma power discharge, the active heat pipe system maintains ower temperature differences, higher steam pressure, and higher thermodynamic efficiency and eectrica power output. These advantages impy higher storage efficiency regardess of the discharge regime. For exampe, if constant eectrica power is desired, the active heat-pipe system shoud aow a onger discharge period at the nomina power compared to an equivaent conduction-based storage system discharged in a simiar manner. The anaysis of the constant eectrica power mode is currenty underway. The mass of the PCM in the case study above is 2.7 times the mass of the pipes, a ratio that may be increased in future optimization for cost reduction. A higher PCM mass fraction in the radia conduction configuration wi reduce the cost of the stee pipes per unit stored energy, but wi aso reduce the system performance due to increased therma resistance in the thicker PCM ayer. In the active heat pipe system, on the other hand, it is possibe to increase the PCM mass fraction by adding PCM capsues without a significant change in the heat transfer performance. Note that the heat pipe system anaysis used ony natura PCM properties; combining conductivity enhancements in the PCM capsues with the heat-pipe storage may further reduce the amount of structura materias required whie maintaining satisfactory performance. The active heat pipe requires additiona eements in the storage system, incuding: a pump, HTF distribution piping, and a contro system. However, these eements are impemented standard technoogy that is ubiquitous in any power pant and do not represent a significant addition of cost or risk. Therefore the added compexity of an active vs. passive storage system may be a reasonabe investment to achieve the significant potentia gain in power generation.

10 1070 R. Yogev and A. Kribus / Energy Procedia 49 ( 2014 ) The mode presented here is simpified and is intended ony as a preiminary evauation of the thermodynamic feasibiity of the active heat pipe approach. A more detaied engineering and cost mode shoud be used for a more detaied evauation, and obviousy an experimenta vaidation is needed for the operation of the proposed heat pipe geometry. Acknowedgment This work was supported by a grant from the Ministry of Science and Technoogy, Israe, and by the Ministry of Foreign Affairs and the Ministry of Nationa Education and Research, France. References [1] A. Gi, M. Medrano, I. Martore, A. Lázaro, P. Doado, B. Zaba, et a., State of the art on high temperature therma energy storage for power generation. Part 1 Concepts, materias and modeization, Renewabe and Sustainabe Energy Reviews. 14 (2010) [2] D. Laing, C. Bah, T. Bauer, D. Lehmann, W.-D. Steinmann, Therma energy storage for direct steam generation, Soar Energy. 85 (2011) [3] R. Yogev, A. Kribus, Operation strategies and performance of soar therma power pants operating from PCM storage, Soar Energy. 95 (2013) [4] L. Fan, J.M. Khodadadi, Therma conductivity enhancement of phase change materias for therma energy storage: A review, Renewabe and Sustainabe Energy Reviews. 15 (2011) [5] M. Liu, W. Saman, F. Bruno, Review on storage materias and therma performance enhancement techniques for high temperature phase change therma storage systems, Renewabe and Sustainabe Energy Reviews. 16 (2012) [6] R. Adinberg, D. Zvegisky, M. Epstein, Heat transfer efficient therma energy storage for steam generation, Energy Conversion and Management. 51 (2010) [7] J.B. Kessei, D.E. Lacy, The Cavity Heat Pipe Stiring Reciever for Space Soar Dynamics, in: IECEC-89, Washinton, DC, 1989: pp [8] D. Laing, M. Påsson, Hybrid Dish/Stiring Systems: Combustor and Heat Pipe Receiver Deveopment, Journa of Soar Energy Engineering. 124 (2002) 176. [9] A. Faghri, Y. Zhang, Transport phenomena in mutiphase systems, Academic press, Amsterdam, [10] R. Yogev, Latent Heat Storage in Soar Therma Power Pants, Te Aviv University, [11] J.P. Homan, Heat Transfer, 10th ed., McGrew-Hi, [12] R.W. Bjorge, G.R. Ha, W.M. Rohsenow, CORRELATION OF FORCED CONVECTION BOILING HEAT TRANSFER DATA, Internationa Journa of Heat and Mass Transfer. 25 (1982) [13] R. Tamme, T. Bauer, J. Busche, D. Laing, W. Steinmann, Latent heat storage above 120C for appications in the industria process heat sector and soar power generation, (2008) [14] R. Yogev, A. Kribus, Performance of Soar Therma Power Pants Operating from PCM Storage, in: SoarPACES, Marrakech, Morocco, [15] M. Homgren, X Steam, thermodynamic properties of water and steam, (2007). [16] Thermino VP-1 datasheet, downoaded from: (n.d.). [17] ASME Code for pressure piping, B31, an american nationa standard, 2004, (n.d.). [18] R. Bayón, E. Rojas, L. Vaenzuea, E. Zarza, J. León, Anaysis of the experimenta behaviour of a 100 kwth atent heat storage system for direct steam generation in soar therma power pants, Appied Therma Engineering. 30 (2010)