A REVIEW OF THERMAL ENERGY STORAGE SYSTEMS WITH
SALT HYDRATE PHASE CHANGE MATERIALS FOR COMFORT
COOLING
Justin Ning-Wei Chiu*, Dr. Viktoria Martin, and Prof. Fredrik Setterwall
*KTH – Department of Energy Technology, Brinellvägen 68, SE-100 44 Stockholm, SWEDEN
Tel: +46 (0)87907476
[email protected]
Abstract
This paper presents a review of cold thermal energy storage technologies. Latent heat
thermal energy storage (LHTES) with phase change materials (PCMs) deserves attention as
they provide high energy density and small temperature change interval upon
melting/solidifying. Salt hydrates are especially interesting since they demonstrate high latent
heat of fusion, high thermal conductivity, low flammability, and facilitate the use in buildings
as compared to organic PCMs. A review of system performance obtained from experimental
work, theoretical analyses and real case studies has however shown some material
shortcomings. To reach cost effectiveness, future work in the field of LHTES with salt
hydrates lies in finding suitable methods for limiting incongruent melting and subcooling
without compromising the storage density. Also, system integration of LHTES in cold
applications can be further developed in terms of innovative design for high power and
storage capacity, load optimized sizing, controls, and elimination of PCM encapsulation.
1 Introduction
Many reviews have been conducted in the light of providing the state of the art of latent
heat thermal storage in buildings (e.g., [1, 2, 3, 34, 77, 95]). Here, the focus is on reviewing
cold thermal energy storage (CTES) systems using salt hydrates for comfort cooling.
CTES systems can be effectively implemented in cooling systems where peak cooling
demands are high and the capacity is limited. Advantages of running thermal energy storage
(TES) systems have for example been high-lighted as cost effectiveness, and energy
reduction [4]. However, when implemented in a district cooling network, peak cooling load
may be reduced by offsetting the load to off peak period, therefore allowing district cooling
plants to provide additional cooling power without the need of expanding existing
infrastructure and hence reduction in capital cost. When installed in an electrical chiller based
cooling system, peak grid electricity load may be alleviated by producing cold during offpeak hours and releasing cold during peak hours. Not only does peak shaving lower the
marginal electricity cost by cutting down premium fuel use [5], but power plants operating at
night also demonstrate higher electricity production efficiency. In addition, lower ambient
temperature at night increases chiller performance if ambient air is used as heat sink, which
further reduces the energy consumption.
TES provides new means of thermal energy management and electric power distribution
over the existing network. However, the payback period of a potential cold storage
installation is not always as attractive as other energy saving technologies [4]. Research focus
is now being put in improving the energy storage density, increasing the power
extraction/storage rate and minimizing investment cost. This paper will first delve into water
and ice based TES, followed with an introductory overview on material properties of salt
hydrates in comparison with organic and eutectic mixtures, then a state of the art on salt
hydrate based TES system performance will be illustrated with emphasis on cold storage
applications.
2 Thermal Energy Storage Materials
Water, as one of the most abundant natural resource, has been largely used in TES
systems. Stratified chilled water storage (SCW) is technologically mature and is utilized in
thermal storage applications. However, the small temperature interval in comfort cooling
limits the stored capacity. Typically, a 10°C temperature difference of water storage
corresponds to 12 kWh/m³ energy storage density. Therefore, the volume required for SCW
for cooling storage may be enormous. The cost of SCW has been evaluated by Hasnain [6],
who reported that such system is only economical in system where at least 7,000kWh of
cooling is needed, and where space is available.
Phase change material (PCM) utilizes the latent heat of the melting/solidification process
to store energy. They are characterized by high volumetric storage density and a small
temperature interval for thermal energy extraction and regeneration [ 7 ]. The feed in
temperature for cooling may thus be maintained at constant level facilitating controls.
Ice, as one of the first PCMs used, has been implemented in various cooling systems.
Commercial ice storage systems have for example been installed in capacities ranging from
1,000kWh to 120,000kWh [8 9]. The advantages of using ice are the low cost and the high
storage energy density, eight times the storage capacity of a SCW with a temperature
difference of 10°C. However, ice storage systems require chillers capable of energy
production below freezing point of water and shows large volume expansion upon
solidification [10]. Thus, ice storage is not suitable for integration with district cooling, and
limits the amount of natural cooling (free cooling) that can be used in a system. Using other
PCM materials with higher melting temperatures overcomes the low energy storage density
experienced with SCW, and facilitates direct integration to an existing cooling system [2].
For instance, higher coefficient of performance (COP) of cooling units is achieved with
higher melting temperature PCMs as compared to ice production [11]. Thus, PCM LHTES
may provide more economic and cost effective storage solutions than ice storage systems.
PCMs are here divided into two main families: organic and inorganic [12]. Organic
materials can be further classified into paraffin and non paraffins such as esters, fatty acids,
alcohols and glycols. Inorganic materials are subdivided into salt hydrates and metallics.
Mixtures of chemical compounds constitute practical PCM and this leads to a
melting/solidification temperature range as compared to using pure chemicals. Eutectic
mixtures of organic and/or inorganic materials are desirable in that they melt/solidify at a
constant temperature like a pure chemical. A large variety of PCMs are currently available,
however they must exhibit correct operating temperature range, desirable thermodynamic,
kinetic and chemical properties as well as reasonable costs to be effectively integrated in TES
systems [12, 13].
Table 1 Salt Hydrate and Wax Paraffin Comparison
Organic
Inorganic
Eutectic
Low Cost (120Euro/kWh) [14]
Moderate cost (130 Euro/kWh)
Sharp melting point
Self nucleating
[ 15 16]
Low volumetric storage density
Chemically inert and stable
High volumetric storage density
No phase segregation
(180-300 MJ/m³)
Recyclable
Higher thermal conductivity
Available in large temperature
(0.6W/m°C)
range
Non flammable
Low volume change
Flammable
Subcooling
Limited available material property
Low thermal conductivity
Phase segregation
data
(0.2W/m°C)
Corrosion of containment material
Low volumetric storage density
(90-200 MJ/m³)
Some key characteristics of PCMs are summarized in Table 1. Organic compounds exhibit
favorable characteristics such as congruent melting, low volume change, and non
corrosiveness. However, they also show low thermal conductivity, high flammability, varying
level of toxicity and non compatibility with certain plastic containers [17, 18, 19]. Salt
hydrates have high storage density and higher thermal conductivity as compared to paraffins.
For certain building codes, non toxicity and non flammability are also assets of salt hydrates.
Nonetheless, corrosion, incongruent melting and subcooling are drawbacks that need to be
mastered to provide technically and economically sound storage solutions.
Several compilations of material properties exist [1, 11, 12, 20, 21, 22, 23]. From these
sources, a summary of phase change enthalpy of commercialized salt hydrate mixtures has
been derived (Figure 1, left). As shown, in the temperature range of comfort cooling,
commercialized products have latent heat between 180kJ/L and 300kJ/L as compared to
90kJ/L and 200kJ/L of organics. Available salt hydrates are however limited in numbers.
Potential analytical grade salt hydrate with phase change temperature located between 0°C
and 40°C are shown in Figure 1 to the right. Many of the analytical salt hydrates fit in the
temperature above 25°C, however from 0°C to 25°C, promising materials are limited. It is of
great necessity to perform advanced material research to broaden the prospect of available
salt hydrate materials covering wider range of comfort cooling temperatures at high storage
density.
Figure 1 Commercialized Salt Hydrate Products (left) Analytical Grade Salt Hydrates (right).
3 Salt Hydrate Based Thermal Energy Storage System Performance
PCM-based CTES needs further development in order to cost-effectively serve as an
alternative to SCW storage or direct cooling. To form the basis for this work, a state-of-theart assessment on PCM-based cold storage, with special interest in salt hydrates, is presented
in terms of cold storage findings from the literatures. First, the optimal containment of PCM
will be addressed, followed by power extraction of the TES. Then, findings on minimizing
subcooling and incongruent melting are discussed along with control strategies.
3.1 Optimum Encapsulation Geometry
Three types of PCM containment are identified: macro encapsulation, micro encapsulation
and bulk storage. Encapsulation serves as heat transfer surface, prevents PCM from reacting
with outside environment, and adds mechanical strength to the structure. Macro
encapsulation has been produced in pouches, tubes, and ball type storage geometry [8, 24,
25, 26, 27]. They provide self supporting structure and are easily maneuverable [5]. It has
been found by Wei [28] that the thermal transfer performance is the highest with spherical
capsules, followed by cylindrical, plate type and tubular encapsulations. Microencapsulation
overcomes the shortcoming of phase segregation [13, 29] and demonstrates notable heat
transfer increase where Barba [30] showed the solidification time of the PCM being reduced
as inversely proportional to the square of capsule radius. However, microencapsulation
exhibits the highest production cost among the three containments [5] which limits the
economical viability of such technology. The third type of storage, bulk storage of PCM,
requires extensive heat exchanger design for sufficient thermal energy extraction power. One
of the main concerns with bulk storage may be the air tightness of storage tank, without
which salt hydrate would lose water content resulting in incomplete crystallization heat
release. On the other hand, bulk storage can significantly reduce the PCM TES cost with
elimination of the packaging process. Furthermore, the storage density can be made higher
since bulk storage increases the ice packing factor (IPF). This reduces the cost by reducing
the storage size and space needed for a certain amount of stored cooling. Thus it appears that
enhancement of heat transfer through low conductive PCMs in bulk storage could be a highly
desirable area for research and development.
3.2 Heat Transfer Enhancement
The rate of heat transfer in gelled salt hydrate depends on three major heat transfer
processes: convection from the heat transfer fluid (HTF) to the heat exchanger wall,
conduction within the heat exchanger, and heat transfer from heat exchanger wall to the core
PCM through conduction and/or convection. While salt hydrates exhibit thermal conductivity
two to three folds that of organic materials, property acquired from databases shows that it
rarely surpasses 0.6W/m.K [11, 20, 21, 22]. Moreover, in the solidification process, the solid
layer formed on heat exchanger surface reduces the heat transfer rate. A large body of
literatures commented on observing low TES power rate in freezing/crystallization [6, 31].
However, the issue of a slower solidification as compared to melting is, in CTES, presumably
not a problem since charging of cold (freezing) may be extended over a longer period. The
critical part in heat transfer is thus to have a high power at discharging (melting). Therefore,
heat transfer enhancement should prioritize the discharging process.
Experimental studies of heat transfer of CaCl2⋅6H2O in vertical cylindrical tubes led to a
conclusion that the convective process was the major heat transfer process with non gelled
PCM [32, 33]. Wei [28], Zalba [34] and Ismail [35, 36] have concluded from experimental
and numerical parametric studies that the solidification rate of a LHTES can be improved
with increasing HTF flow rate and decreasing HTF entry temperature. It was further
mentioned that the duration of solidification time shortens with decreasing capsulation size
and that increasing heat exchanger wall thickness lead to faster solidification rate. Non
thickened organic PCMs have a higher heat transfer rate during melting process due to
convective process [37], but such configuration may lead to phase separation if applied on
salt hydrates. Numerical models for fixed bed of encapsulated PCM have been examined by
Ismail [38]. In the same study, the HTF flow rate was shown to have less influence on storage
performance as compared to the inlet temperature and void fraction of the bed. In addition,
larger temperature difference between HTF and PCM was found to give higher
charging/discharging rate.
Investigation has been done to improve heat transfer coefficient by adding external fins
[39, 40, 41, 42], internal fins [43] or with other types of heat transfer enhancing methods such
as lessing rings or water steam [44]. Choi [45] studied the efficiency of thick finned tube
showing heat transfer rates between 90W/m²K and 250W/m²K. Ismail [46] examined finned
tube configurations and concluded that, for optimal performance, the fins should be placed in
the same direction as the tubes with four to five fins of twice the tube diameter around the
tube circumference. Also, the study showed that fin thickness has small influence on the
solidification time of PCM.
Other means of enhancing the heat transfer have been examined: dispersion of aluminum
[47], and graphite [48 49] into the PCM; and absorption of PCM into expanded graphite [50].
Dispersing a highly conductive substance into high storage capacity PCM for thermal
conductivity improvement has been tested since the 80’s by Knowles [51]. Recently, the
scientific field has shown renewed interest in the concept; investigations are however more
concentrated around organic compounds. Introduction of 50wt% of aluminum powder to
paraffin wax increased the thermal conductivity by a factor of 20 [52]. Promising results on
improving the conductivity of organic PCM have been reported: the introduction of 10% by
weight of expanded graphite (EG) to a Capric–myristic acid/expanded perlite composite, the
thermal conductivity is increased by 58% [53]; by introducing 2% of EG into Capric–
myristic acid/vermiculite composite, 85% thermal conductivity improvement was observed
[54] ;adding 10% into stearic acid increased its thermal conductivity by 266% [55]. Study of
heat transfer enhancement by impregnating PCM into a graphite matrix has also been carried
out [56, 57, 58]. Here, thermal conductivity improvements between 20 and up to a 1000 times
were observed depending on the specific blends and methods. Among the salt hydrates, EG
added to sodium acetate trihydrate has been studied within the IEA Solar Heating and
Cooling program - Task 32 [59]. However, extensive studies on salt hydrate thermal property
enhancement in the range of comfort cooling are lacking and thus still need to be carried out.
3.3 Phase Separation Minimization
Incongruent melting, or phase separation, can cause a loss in enthalpy of solidification as
reported by Cantor [60]. Such phase segregation can be reduced with the addition of extra
water, use of microcapsules, rotary storage tank and thickening agents [63]. The addition of
3% super absorbent copolymer (SAP) has been found to completely prevent phase separation
of Glauber’s Salt [61]. Thixotropic (attapulgite clay) and alginate have been tested as well
[62]. Ryu [63] reported the effectiveness of SAP made from carboxymethyl cellulose (CMC),
and polyvinyl alcohol (PVA) with addition of 3wt% to 5wt% for high hydrate inorganic salts
and 2 wt% to 4 wt% for low hydrate inorganic salts. However, in another study it was shown
the addition of bentonite, starch and cellulose to sodium acetate trihydrate still resulted in
20% to 35% of latent enthalpy drop [64]. Polymeric polycarboxylic acid, silica gel and
diatomaceous earth are all potential gelling agents [63].
Although thickening agents have been added to prevent phase separation, thermal cycling
may still lead to thermal capacity drop. For example a 48% drop in stored capacity for
Glauber’s salt was noted after 200 cycles [65]. As explanation, it has been found that the
cause of capacity drop is due to PCM crystal boundary behavior which leads to parting of
thickeners and nucleating agents upon solidification. With the addition of 1%
acrylamide/acrylic acid copolymer (AACP) and 0.1% of sodium hexametaphosphate (SHMP,
(NaPO3)6) as crystal habit modifier, the storage capacity was maintained at 82% of ideal
value after 1600 cycles [66]. Calcium chloride hexahydrate has shown good stability after
1000 cycle test, and may thus be a potential promising PCM for TES [67]. In order to
minimize phase separation, rolling cylinder heat storage system that mixes the salt hydrate
under constant rotation has been proposed by Herrick [68], after 200 cycles no performance
degradation was found. However, a cost analysis for economical validation of the concept is
required, as is the assessment of the parasitic electricity load due to the rolling.
In order for salt hydrates to be fully competitive as PCM materials, technical design, phase
separation and cyclic stability must be mastered, along with the subcooling minimization.
3.4 Subcooling Minimization
The use of nucleating agents, cold finger and porous heat exchange surface has been utilized
to prevent subcooling [69]. Carbon nanofibers, copper, titanium oxide as well as potassium
sulfate and borax have also been demonstrated as good nucleating agents [63, 70, 71].
An extensive study of nucleating agents added to CaCl2·6H2O, MgCl2·6H2O, and Mg(NO3)
·6H2O was investigated by Lane [71]. The optimum addition in weight percentage of borax in
Glauber’s salt was studied by various researchers [71 72], 0.95% to 1.9% of pulverized borax
was found to be the most effective in subcooling minimization. The addition of
Na2P2O7·10H20, K2SO4, TiO2, Na2SO4, SrSO4, K2SO4, SrCl2, BaI2, BaCl2, Ba(OH)2, BaCO3,
CaC2O4, Sr(OH)2, SrCO3, CaO, MgSO4, and carbon power have also been tested by various
investigators and are shown to be promising nucleating agents. Regarding subcooling,
although research has given light to nucleating agents that limit subcooling from 20°C to 2°C,
in comfort cooling where usable temperature difference is limited, 2°C subcooling may still
lead to high efficiency loss from energetic approach. Incorporation of PCM LHTES in an
application will have to address this challenging design issue.
3.5 Control System Analysis
While control strategy optimization has mainly been focused on heat storage [73], it may
be extended to CTES as well. Henze [74] investigated the potential of combination of active
and passive cooling through effective control strategies. Commonly, storage control strategies
are classified as full storage or partial storage. Full storage performs complete load shift from
peak to off-peak power and is most cost effective with high peak hour energy fee. Partial
storage is further divided to load leveling and demand limiting schemes, where refrigeration
units run at either constant or variable power with storage that complements the load
requirement. Peak electric demand for cooling on peak hour with use of proper control
strategy may in the best case be reduced by 40% to 90% [6]. Integrated thermal storage
system optimized by means of proactive control with use of local weather forecasts [75] may
be extended for use in TES model enhancement. The correct control strategy and optimized
load distribution management have a significant impact on the energy saving and deserve
high consideration in system design.
3.6 Experimental and Numerical System Studies
Many application studies on PCM storage are available, though, the major part concerns
heating applications. Much information can however be deduced by examining heating
applications using PCM technology. One case comparison of TES systems with chiller use
scenario led to concluding that TES has lower operating cost and hence is preferred over
chiller units [76]. Hasnain [77] performed an in depth study of CTES integration for peak
electricity load leveling in Saudi Arabia, showing that the peak electrical load may be
reduced by 15% to 23% and peak time cooling rate may be lowered by 35% to 40% upon
adoption of CTES technology. Air conditioning application was studied numerically
assuming CaCl2·6H2O and KF·4H2O as PCMs [78].
Among the numerous issues, one to be mentioned is the lack of accurate PCM thermal
property data, especially for commercial grade products where divergence in heat capacity
and phase change temperature may lead to discrepancies between design model and actual
TES system [79]. He [80] conducted various phase equilibrium study of PCMs and concluded
the results may further be altered with use of differential scanning calorimetry (DSC).
Development of T-history method has been investigated and is shown as one important step
towards standardization of property measurement [ 81 , 82 ]. Analytical and experimental
investigation on heat removal of PCM [83] as well as numerical model for TES simulation
[84] were developed to reproduce solidifying and melting phenomena. Blir [85] suggested a
list of correlations expressing inward solidification time as a function of Stefan number, Biot
number and superheat parameter for cylindrical container and spherical container. Numerical
models of melting within capsules [86] and around cylindrical tube [87] were investigated;
space thickness, HTF entry temperature, tube wall conductivity and solid phase PCM
conductivity were shown to have significant influence on the overall solidification process.
The focus of using phase change materials in comfort cooling applications has mainly
been put in passive thermal control. Passive walls impregnated with PCM were shown to
have better performance than masonry wall of five times the thickness [88] and may lead to
effective energy saving in building [89]. Other passive designs that were widely studied are
sandwich panels [90], floor heating systems [91], ceiling board thermal storage systems [92],
wall boards [93] and window shutters [94]. Building materials impregnated with PCMs, such
as gypsum wallboards, concrete ceilings and floors are cost efficient method of shifting
thermal loads. Hydrated salts with phase change close to comfort temperature range are
shown to be possible candidates as presented in Khudhair’s review [95]. Effect of heat leak
through containment wall of cold LHTES was studied by Seeniraj [96] and limiting values of
heat leak ratio were proposed.
Finally, with the goal of achieving higher efficiency in charging and discharge of LHTES,
Farid [97, 98] proposed the use of different melting temperature materials. Watanabe [99]
further conducted the experiment using three PCMs with water as heat transfer medium and
optimization of such system with exergetic approach was analyzed [100].
4 Concluding Remarks
Thermal energy storage comes in various forms and salt hydrate based latent heat thermal
storage seems to be one of the most promising technologies in terms of energy conservation,
grid load alleviation, and integration in a built environment. From this review it can be
concluded that, significant findings are available in the literature for advancing the practical
system design into functional storage units. However, system performance modeling, design
optimization and experimental work in the area of cold storage are still required. More
techno-economical feasible PCM systems with suitable material properties (e.g. lower cost,
higher conductivity, and larger storage density) will be needed for comfort cooling
applications. Here, adequate power and capacity sizing through advanced control scheme and
novel design of compact TES by elimination of encapsulation are yet to be performed.
Proposed work of PCM based cold storage should be focused in lowering the investment cost
with elimination of expensive containment and provision of higher power extraction rate
through enhancement of heat transfer design, while maintaining stable charge capacity with
use of improved PCMs.
Acknowledgement
The authors would like to acknowledge Swedish Energy Agency for their financial
support. We would also like to acknowledge Capital Cooling AB, Climator AB, Ecostorage
Sweden AB, Fortum Värme, and Vesam AB for providing useful suggestions during the
study.
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