24

CHAPTER 2

LITERATURE REVIEW

The present research is focused on the free cooling of buildings

with the PCM-based thermal energy storage heat exchanger. Hence, a detailed

literature survey has been made on the various studies carried out by the

researchers on the different passive cooling techniques, free cooling potential

and night ventilation, free cooling with direct ambient air circulation, free

cooling with PCM for cold storage applications, studies on PCM and

modelling of the phase change storage system.

2.1 PASSIVE COOLING TECHNIQUES

2.1.1 Solar and Heat Protection Techniques (Reduce Heat Gains)

The presence of water, plants and trees contributes to microclimate

cooling, and is an important source of moisture within the mostly arid urban

environment (Robitu et al (2006)). Indoor simulations still tend to be isolated

from an important element affecting urban microclimate, such as urban trees.

The main advantage of urban trees, as a bioclimatic responsive design

element is to produce shade, whereas its main disadvantage is blocking the

wind (Yoshida et al (2006)). In addition, the effects of specific urban tree

types - for example, the different leaf area densities and evapotranspiration

rates of urban trees influence solar access and heat exchanges, if planted

around buildings (Radhi (2009)). Eumorfopoulou and Kontoleon (2009) and

Kontoleon and Eumorfopoulou (2010) analysed thoroughly the influence of

the orientation and proportion (covering percentage) of plant-covered wall

25

sections on the thermal behaviour of typical buildings in Greece during the

summer. Limor et al (2010) have analysed the thermal effect on an urban

street due to three levels of building densities. The study indicated the

importance of urban trees in alleviating the heat island effect in a hot and

humid summer. In all the studied cases, the thermal effect of the tree was

found to depend mainly on its canopy coverage level, and planting density in

the urban street, and a little on other species characteristics.

Gijón-Rivera et al (2011) made an assessment of the thermal

performance of an office on the top of a building with four different

configurations of window glass, and their influence on the indoor conditions.

NohPat et al (2011) observed that the use of solar control film in their

numerical analysis observed that the double glassing unit (DGU) is highly

recommended due to the energy gain reduction by 55%, compared to the

traditional DGU without the solar control film. Baetens et al (2010a) made a

survey on the prototype and the currently commercial dynamic tintable smart

windows, and concluded that the commercial electro-chromic windows seem

most promising to reduce cooling loads, heating loads and lighting energy in

buildings, where they have been found most reliable, and able to modulate the

transmittance of up to 68% of the total solar spectrum.

Belusko et al (2011) investigated the thermal resistance for the heat

flow through a typical timber framed pitched roofing system, measured under

outdoor conditions for the heat flow up. However, with higher thermal

resistance systems containing bulk insulation within the timber frame, the

measured result for a typical installation was as low as 50% of the thermal

resistance determined, considering two dimensional thermal bridging using

the parallel path method. This result was attributed to the three-dimensional

heat flow and insulation installation defects, resulting from the design and

construction method used. Translating these results to a typical house with a

26

200 m2 floor area, the overall thermal resistance of the roof was at least 23%

lower than the overall calculated thermal resistance, including the two

dimensional thermal bridging. Ong (2011) reported that the heat transmission

through the roof could be reduced by providing insulation in the attic under

the roof or above the ceiling. A roof solar collector could provide both

ventilation and cooling in the attic. Several laboratory-sized units of passive

roof designs were constructed and tested side-by-side under outdoor

conditions to obtain the temperature data of the roof, attic and ceiling, in order

to compare their performances.

2.1.2 Heat Modulation or Amortization Techniques (Modify Heat

Gains)

The PCM wallboard is considered to be an effective and less costly

replacement of the standard thermal mass, to store solar heat in buildings, in

which the PCM is embedded into a gypsum board, plaster or other building

structures. Neeper (2000) impregnated fatty acid and paraffin waxes into the

gypsum wallboard and examined the thermal dynamics under the diurnal

variation of the room temperature (the radiation absorbed was not considered)

with the PCM on the interior and exterior portions respectively. Their

investigation indicated that when the PCM’s melting temperature was close to

the average room temperature, the maximum diurnal energy storage occurred

and diurnal energy storage decreased if the phase change transition occurred

over a range of temperature.

In order to evaluate the capacity of the PCM to stabilise the internal

environment when there were external temperature changes and solar

radiations, Kuznik et al (2008) designed an experimental test room

MINIBAT, using a battery of 12 spotlights to simulate an artificial sunning,

and they got the results that the PCM wallboard can reduce the air

temperature fluctuations in the room and enhance the natural convection

27

mixing of the air, avoiding uncomfortable thermal stratifications. Kuznik and

Virgone (2009) also tested two identical test cells under two kinds of external

temperature evolutions, heating and cooling steps with various slopes and

sinusoidal temperature evolutions, in a 24 hours period. They found there was

a time lag between the indoor and outdoor temperature evolutions and the

external temperature amplitude in the cell was reduced.

Lv et al (2006) built an ordinary room as well as a room using PCM

gypsum wallboard in the northeast of China, and they found that the PCM

wallboards can attenuate indoor air fluctuation, reduce the heat transfer to the

outdoor air and have the function to keep warm. Recently, Kuznik et al (2011)

used Dupont de Nemours PCM wallboards for the renovation of a tertiary

building, and found that they were really efficient if the outside temperature

was varying in melting temperature by monitoring the building for a whole

year.

Another method of applying PCMs into building structures is to

incorporate them into the concrete matrix or open cell cement. This composite

is called thermocrete. Cabeza et al (2007) studied a new innovative concrete

with PCM on its thermal aspects, in order to develop a product which would

not affect the mechanical strength of the concrete wall. They set up two real

size concrete cubicles to demonstrate the possibility of using

microencapsulated PCM in concrete. They found that the concrete reached a

compressive strength, over 25 MPa and a tensile splitting strength, over

6 MPa and no difference occurred in the effects of the PCM after 6 months of

operation. Baetens et al (2010b) reported that enhancing the thermal mass of

concrete buildings seemed better than the use of PCM wallboards; however,

the high cost of PCMs was the biggest concern.

Koschenz and Lehmann (2004) put forward a new concept of a

thermally actived ceiling panel for refurbished buildings. In this system, the

28

mixture of microencapsulated PCM and gypsum was poured into a sheet steel

tray which was used as a support for maintaining the mechanical stability of

the panels. A capillary water tube system was applied to control the thermal

mass. They tested the thermal performance of this system, and indicated that

only a 5 cm layer of microencapsulated PCM and gypsum was enough for a

standard office to keep within comfortable temperatures.

Pasupathy et al (2008a) presented a detailed review on the PCMs’

incorporation in buildings, and the various methods used to contain them for

thermal management in residential and commercial establishments. Among

all the PCM applications for high-performance buildings, the PCM integration

in wallboards, roof & ceiling, and windows is most commonly studied, due to

its relatively more effective heat exchange area and more convenient

implementation. Pasupathy et al (2008b) constructed an experimental setup

consisting of two identical test rooms, to study the effect of having a PCM

panel in the roof for the thermal management of a residential building. One

room is constructed without the PCM on the roof, to compare the thermal

performance of an inorganic eutectic PCM which has a melting temperature

in the range of 26-28°C. A numerical model was also developed by them, and

the results of the model were validated with the experimental results, and

several simulation runs were conducted for the average ambient conditions for

all the months in a year, and for various other parameters of interest.

Pasupathy and Velraj (2008) recommended a double-layer PCM concept in

the roof to achieve year round thermal management in a passive manner.

2.1.3 Heat Dissipation Techniques (Remove Internal Heat)

Bassindowa et al (2007) and Farmahini et al (2009) investigated the

experimental and theoretical applications of long-wave radiance, nocturnal

radiative cooling, its potential in different climate conditions, and the effects

of various parameters on this passive method. Prapapong and Surapong

29

(2007) showed that a cooling tower could be employed to provide cooling

water for radiant cooling and for pre-cooling of the ventilation air to achieve

thermal comfort. No active cooling is required. If a more exacting condition is

required, then pre-cooling the ventilation air with cooling water generated

from active cooling can help achieve thermal comfort, superior to that of

conventional air-conditioning, while substantial energy saving can still be

achieved.

Farahani et al (2010) studied the results of an investigation on a

two-stage cooling system; it consists of a nocturnal radiative unit, a cooling

coil and an indirect evaporative cooler; this investigation has been carried out

in the weather conditions in the city of Teheran, and the results demonstrated

that the first stage of the system increases the effectiveness of the indirect

evaporative cooler. Also, the regenerative model provides the best comfort

conditions. Heidarinejad et al (2010a) studied a hybrid system of nocturnal

radiative cooling, and direct evaporative cooling in Teheran. This system

complements direct evaporative cooling, as it consumes low energy to

provide cold water, and is able to fulfil the comfort conditions, whereas direct

evaporative alone is not able to provide summer comfort conditions, and the

results showed that the overall effectiveness of the hybrid system is more than

100%.

Heidarinejad et al (2010b) analysed a ground-assisted hybrid

evaporative cooling system in Teheran. A ground coupled circuit (GCC)

provides the necessary pre-cooling effects, enabling a Direct Evaporative

Cooler (DEC) that cools the air even below its wet-bulb temperature. The

simulation results showed that a combination of the GCC and DEC systems

could provide comfort conditions, whereas the DEC alone could not. Based

on the simulation results the cooling effectiveness of a hybrid system is found

30

to be more than 100%. Thus, this novel hybrid system could decrease the air

temperature below the ambient wet-bulb temperature.

Maerefat and Haghighi (2010) studied a low-energy consuming

passive cooling technique (solar chimney together with earth-to-air heat

exchanger) to remove undesirable interior heat from a building in the hot

seasons. He found that it is possible to use the solar chimney to power the

underground cooling system during the daytime, without any need for

electricity. Moreover, this system with a proper design may also provide a

thermally comfortable indoor environment for a large number of hours during

the scorching summer days.

Recently, Geetha and Velraj (2012) reviewed the various possible

methods of passive cooling for buildings and discussed the representative

applications of each method. They broadly classified the passive cooling

methods under three categories: (i) Heat prevention techniques (ii) Thermal

moderation techniques and (iii) Heat dissipation techniques.

2.2 FREE COOLING POTENTIAL

Various studies carried out to assess the free cooling potential in

different regimes of the world are summarised. Lam et al (2005) measured the

long-term hourly and daily weather data for five cities in China, namely

Harbin, Beijing, Shanghai, Kuming and Hong Kong and analysed them.

These cities were selected to represent the five main architectural climates -

severe cold, cold, hot summer and cold winter, mild and hot summer, and

warm winter. Statistical techniques and graphical methods were used to study

the long-term weather characteristics of these five climatic zones. Three

common climatic variables, namely temperature (dry-bulb and wet-bulb),

solar radiation (global, direct and diffuse) and wind conditions (wind speed

and wind direction), were investigated.

31

The potential of free cooling represents a measure of the capability

of ventilation to ensure indoor comfort without using the mechanical cooling

system (Ghiaus and Allard (2006)). The potential for passive cooling of

buildings by nighttime ventilation was evaluated by analysing the climatic

data, without considering any building-specific parameters (Artmann et al

(2007)). An approach for calculating the degree–hours based on a variable

building temperature – within a standardised range of thermal comfort – is

presented and applied to the climatic data of 259 stations all over Europe. The

results show a high potential for night-time ventilative cooling over the whole

of Northern Europe and still significant potential in Central, Eastern and even

in some regions of Southern Europe.

Medved and Arkar (2008) studied the free-cooling potential for

different climatic locations in Europe. The size of the LHTES was optimised

on the basis of the calculated cooling degree-hours. Six representative cities

were selected in Europe that covers a wide range of different climatic

conditions. Numerical investigations of the free-cooling potential were made

for a time period of 3 summer months. Bulut and Aktacir (2011) studied the

free cooling potential of Istanbul, Turkey by using hourly dry-bulb

temperature measurements during a period of 16 years. It is found that the

free cooling potential varies with the supply air temperature and months. It is

determined that although there are substantial energy savings during a

significant portion of the year, especially in the transition months (April, May,

September and October), the high outdoor air temperatures from June to

August, made the system not beneficial for free cooling except at high supply

air temperature. The HVAC systems which have a free cooling option should

be preferred, if the climate is favourable.

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2.3 NIGHT VENTILATION

Various literatures pertaining to free cooling by night ventilation

are reviewed and presented. Pfafferott et al (2003) have done a study on the

design of passive cooling by night ventilation. They identified the

characteristic building parameters and determined the night ventilation effect

with these parameters. Geros et al (2005) have investigated the impact of the

urban environment on night ventilation, and reported that the increase in air

temperature and decrease in wind velocity decreases the efficiency of night

ventilation.

Becker et al (2007) determined the internal air quality improvement

for school buildings by night ventilation techniques. Kubota et al (2009)

investigated the effectiveness of the night ventilation technique for residential

buildings in the hot humid climate of Malaysia. The results from the field

experiment showed that night ventilation would provide better thermal

comfort for occupants in Malaysian terraced houses, compared with the other

ventilation strategies in terms of operative temperature.

2.4 FREE COOLING WITH DIRECT AMBIENT AIR

CIRCULATION

Olsen et al (2003) showed that low energy cooling systems that

maximise free cooling from the outside air have the best energy performance

under mild UK climatic conditions. Emmerich et al (2011) described an

adaptive thermal comfort option as a tool for implementing the climatic

suitability methodology for natural ventilation in the U.S. climate. The

adaptive thermal comfort option has the potential to substantially increase the

effectiveness of natural ventilation cooling for many U.S. cities. Parys et al

(2012) studied the feasibility of passive cooling of buildings, solely by the

diurnal manual window operation in the temperate climate of Belgium.

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2.5 FREE COOLING TECHNIQUES WITH PCM STORAGE

Free cooling is a concept developed for air conditioning

applications, in which coolness is collected from the ambient air during the

night and is released into the room during the hottest hours of the day. The

effectiveness of free cooling applications is highly dependent on the local

climate. Interior and desert regions are the most suitable because they have

large diurnal temperature variations (15°C is desired). The properties of the

PCM are very important, the melting temperature in particular. It should be in

the midrange of the diurnal temperature variation. Great importance for the

successful operation has an air flow. If it is large, then the heat transfer will be

sufficient, but at the expense of energy needed for the fans.

Free cooling differs from natural ventilation in which air exchange

takes place through mechanical fans. Otherwise, an additional power is

needed for the operation of fans but there is an improvement of the cooling

potential of the AHU. The significance of improvement depends on several

factors and one of them is the diurnal temperature range, where 15K between

the day and night temperature is desired (Zalba et al (2004)). Another is the

mode of air circulation. Some authors have proposed systems where during

the day indoor air circulation is used (Zalba et al (2004), Turnpenny et al

(2000a)), which is favourable in terms of the cooling load, but it is less

favourable when it comes to indoor air quality.

Turnpenny et al (2000b) studied a novel ventilation cooling system

for reducing air-conditioning in buildings (Figure 2.1). A latent heat storage

unit incorporating heat pipes embedded in the PCM was developed and tested

for a novel application in low energy cooling of buildings.

34

Figure 2.1 Schematic of proposed design by Turnpenny et al

A theoretical model of heat transfer between the air and the PCM

via a heat pipe was designed and tested. The developed model gives heat

storage of about 270 Watt-hour over an eight hour office day. Assuming heat

gains of 30 W/m2 in an office floor area of 15 m2, over eight hours, 3600

Watt-hour of storage is required, i.e. 13 units. In most cases, the model over-

predicted the heat transfer rate by about 100%, but predicted the heat pipe

surface temperature within 2°C. The measurements of the model showed that

a large temperature difference between the air and the PCM (15°C or more)

was needed to melt and freeze the material in practical timescales (7 ± 10

hour). At more reasonable temperature differences and flow rates (e.g. 5°C,

0.18 m3/s), the heat transfer falls below 40 W, and the melt time increases

significantly.

A Night Ventilation system with PCM packed bed storage (NVP)

was proposed by Yanbing et al (2003), for increasing a building's energy

efficiency (Figure 2.2). During the night, the cold air from outside is blown

through the LHTES system, which charges it with the cold. In the daytime,

35

the cold stored in the PCM is released to the air, which circulates between the

LHTES system and the room.

Figure 2.2 Schematic of the NVP system proposed by Yanbing et al

In order to analyse the thermal behaviour of the NVP system, a

mathematical model was derived, which considered the energy storage

(LHTES) system and the ventilation scheme. Besides that, an experimental

installation was also made, where 20 shelves with three layers (2.4 m × 3 m ×

0.12 m) were attached to the ceiling of an experimental room with 10 m2. All

together, about 2000 capsules containing some kind of fatty acid developed

by the authors (190 kJ/kg, temperature of melting between 22°C and 26°C)

were used and the total mass of the PCM was 150 kg. The experimental

results proved that with the NVP system the room temperature decreases and

the thermal comfort level increases.

Zalba et al (2004) outlined the development of an installation for

free cooling (Figure 2.3.a) that allows testing the performance of the PCMs.

Part of the experimental setup was a flat plate heat exchanger (Figure 2.3.b)

36

with encapsulated PCM (RT25, Rubitherm GmbH). Experiments were

performed using 3 kg of PCM, which means a storage density of 28 kWh/m3.

(a) (b)

Figure 2.3 Schematic of proposed design by Zalba et al (a) General

sketch of the installation (b) Configuration of the TES

device

The main focus of the study was on: the ratio of energy/volume in

the encapsulates, the load / unload rate of storage, and the cost of the

installation. It was found that the effects with significant influence in the

solidification and melting process are the thickness of the encapsulation, the

inlet temperature of the air, the air flow, and the interaction between the

thickness and the temperature. The authors later proposed an improved

system, using a graphite matrix in a flat plate encapsulate (Marin et al (2005))

to overcome the low heat transfer rates. Comparing both systems it was

concluded that with the same thickness for the plates, the response time was

much lower (50% in time) and could be reached with a very low reduction of

the energy stored.

A paper by Takeda et al (2004) discusses the development of a

ventilation system, using direct heat exchange between the PCM granules and

the air. A PCM packed bed was installed vertically in a supply air duct, and

37

air was blown through the packed bed under certain conditions, releasing heat

or cold to it (Figure 2.4). The total weight of the PCM bed was 4.59 kg, and it

consisted of 65% ceramic materials and 35% paraffinic hydrocarbon by

weight, having a latent heat of 38 kJ/kg. First, they periodically varied the

inlet air temperature between 21.5°C and 28.0°C, and measured the outlet air

temperature in order to simulate changes of the outdoor ambient air

temperature. It was shown that the outlet air temperature was stabilised and

remained within the phase change temperature range. Then, computer

simulations were performed, analysing the potential of such a system in

reducing the ventilation load during summer for eight Japanese cities. It was

found that the maximum benefit could be obtained in Kyoto with a reduction

of ventilation load by 62.8%. Besides that they concluded that the range of

daily temperature variation has a greater influence on the benefit, than the

average temperature.

Figure 2.4 Elevation view of the experimental apparatus by Takeda et al

38

Nagano et al (2004) embedded the PCM directly on the floor

boards in the form of granules, several millimeters in diameter. This PCM

packed bed is permeable to air, and so is suitable for use in floor supply air-

conditioning systems. During the night, the circulation of cool air through the

under floor space allows cool energy to be charged to the concrete slab, floor

board and the PCM packed bed. During the daytime, the cool energy can be

used to remove the cooling load in the room. This method shown in

Figure 2.5 is superior compared to a sensible storage system, because the

building’s thermal mass storage capacity is limited.

Figure 2.5 System proposed by Nagano et al

An investigation into the efficiency of free cooling in a

heavyweight and lightweight low energy building was performed by Arkar et

al (2007). Two units of latent heat thermal energy storage (LHTES), one for

cooling the fresh supply air and the other for cooling the recirculated indoor

air, were integrated into a mechanical ventilation system (Figure 2.6). Both

LHTESs were filled with the PCM (RT 20, Rubitherm GmbH) encapsulated

into spheres. With the help of the developed and experimentally verified

39

numerical model, the temperature response functions, the air flow rates and

the PCM’s thermal properties were implemented into the TRNSYS building

thermal response model. The results showed that the total mass of the PCM in

LHTES-1 and LHTES-2 is 6.75 kg/m2 of floor area in the heavyweight

building and 13.5 kg/m2 in the lightweight building. Moreover, the free

cooling enables a reduction in the size of the mechanical ventilation system

and provides more favourable temperatures. Arkar and Medved (2007)

proposed another system but with only one LHTES. For their case study, they

analysed the optimum PCM peak temperature, optimum size of the LHTES

and the efficiency of the free cooling. It was found that the peak temperature

should be between 20°C and 22°C (continental climate), spheres should have

a diameter of 25 mm, and the optimal size should be 6.4 kg/m2 of floor area.

Figure 2.6 Different mechanical ventilation modes proposed by Arkar

et al (a) Mechanical ventilation, (b) free cooling - daytime

operation, (c) free cooling - nighttime operation

Based on the outcome of the experiments of Zalba et al, two

different real scale prototypes of the air-to-PCM heat exchangers (Figure 2.7)

were designed and tested by Lazaro et al (2009) following the

ANSI/ASHRAE standard 94.1-2002 (Method of testing the active latent heat

storage devices based on thermal performance). In this method, in order to

obtain accuracy in the measurement of the air flow, and the temperature

40

difference between the inlet and the outlet, precision thermopiles were used

for the measurement of the inlet and outlet temperatures.

Figure 2.7 Prototype 1 configuration proposed by Lazaro et al

A latent heat storage unit was proposed by Stritih and Butala (2009,

2010) for free cooling (Figure 2.8). An experiment was conducted using

3.6 kg of paraffin with a melting point of 22°C (RT 20, Rubitherm GmbH)

and heat storage capacity of 172 kJ/kg. A metal box with external and internal

fins was filled with the PCM, and the air was blown through it. The authors

have presented outlet air temperatures, heat fluxes and heat as a function of

time for different air velocities and inlet temperatures. A numerical model

was developed and there was good agreement with the experimental results. It

was shown that the cold storage can cool the air to a temperature below 24 °C

for more than 2.5 hours when the air velocity is 1 m/s and the inlet air

temperature 26°C.

41

Figure 2.8 Testing line scheme for measuring daily cold storage

efficiency proposed by Stritih and Butala

A detailed survey on the free cooling of building using phase

change materials was carried out by Antony Aroul Raj and Velraj (2010). In

addition to various researches on free cooling, the heat transfer problems and

design considerations associated with free cooling were also discussed by

them. In their view, the type of cooling system is to be designed, based on the

diurnal temperature of the place that depends on the geographical and micro

climatic conditions of the location.

An experimental study carried out by Waqas and Kumar (2011)

was conducted to investigate the thermal performance of the latent heat

storage for the free cooling of buildings in a dry and hot climate. The PCM

storage unit was an open air circuit type (Figure 2.9) which was fabricated

and installed in a controlled environmental chamber (to simulate the ambient

conditions of hot and dry climate). 13 kg of PCM (SP 29, Rubitherm GmbH)

was used, encapsulated in the containers of galvanised steel (0.5m × 0.5m ×

42

0.01m). The authors focused on the influence of the air flow rate and the inlet

air temperature on cold accumulation, and it was found that the solidification

of the PCM was more sensitive to the charging air temperature compared to

the air flow rate. When the charging air temperature was reduced from 22°C

to 20°C, 33% less time was needed to completely solidify the PCM. In the

case when the air flow rate changed from 4 m3/h to 5 m3/h, the solidification

time reduced up to 16%. Experimental observations also showed that lower

ambient temperatures and higher air flow rates are advantageous to complete

the solidification.

Figure 2.9 The experimental PCM storage unit proposed by Waqas

and Kumar

Antony Aroul Raj and Velraj (2011) developed a shell and tube

type heat exchanger (Figure 2.10.a) with the PCM in the shell portion of the

module, and a passage for the flow of air through the tubes. Its arrangement

(modules of the heat exchanger are stacked one over another with air spacers

in between) allows using it in free cooling applications where the diurnal

temperature variation is low.

43

(a) (b)

Figure 2.10 Schematic of the proposed system by Antony Aroul Raj and

Velraj (a) Operation of the free cooling system

(b) Photographic view of one module

Recently, Zhou et al (2012) reviewed the thermal storage with

phase change materials in building applications. This paper summarises

previous works on latent thermal energy storage in building applications,

covering PCMs, the impregnation methods, current building applications and

their thermal performance analyses, as well as the numerical simulation of

buildings with PCMs. Osterman et al (2012) studied the PCM based cooling

technologies for buildings. All studies have shown that the use of PCMs

helped to improve the energy performance of buildings, and they also

discussed the problems encountered in heat transfer, and the amount of PCM

needed for storage.

2.6 AIR CONDITIONING SYSTEM BASED ON PCM

Weather conditions, and industrial, commercial and residential

activities change during the day, and an air-conditioning system has to adapt

to these changes. This means that the electrical demand varies significantly,

reaching its peak load during the day. By using the cold storage with the

44

PCM, the load can be reduced and even shifted to the nighttime when the

electricity tariff is low. This entails steadier operation; moreover, the size of

the air-conditioning (AC) system can be adjusted (smaller power).

A detailed study of the phase change materials based cool thermal

energy storage (CTES) system integrated with a large building air-

conditioning system as shown in Figure 2.11, was presented by Velraj et al

(2006). In this system, the storage tank is kept separate away from the

building.

Figure 2.11 Layout of air conditioning system using thermal energy

storage

45

The major focus of this study was to provide technical information

about the encapsulated phase change material (PCM) based storage system for

air-conditioning applications, and the importance of a careful design load

calculation. The economic benefits of load shift operations are highlighted. It

was found that the PCM based storage system reduces the monthly demand

charge of INR 1.2 million. It was also estimated that a cost saving of INR

2.26 million per annum could be achieved through energy management. The

author suggested that the CTES system can be introduced economically for

air conditioning in residential / commercial establishments.

Kondo and Ibamoto (2006) examined the effects of a peak shaving

control of air-conditioning systems using PCM for ceiling boards in an office

building (Figure 2.12). A rock wool PCM ceiling board was enhanced by

adding a micro capsulate PCM with a melting point of about 25°C, and the

floor area was approximately 16 m2. The experiment was designed so that

during the night (from 4 AM to 8 AM) cold air from the air-handling unit

(AHU) flows into the ceiling chamber space and chills the PCM ceiling

board. The cold is then stored for peak hours (from 1 PM to 8 PM) when the

air from the room flows through a ceiling chamber to the AHU. With such an

arrangement, the air is pre-cooled and the result was that the maximum

thermal load using the PCM ceiling board was 85.2% of that, using the rock

wool ceiling board. The results also revealed that the transition rate of the

thermal load to the night was 25.1%, thus lowering the running cost to 91.6%,

compared to the rock wool ceiling board (for Japan).

46

Figure 2.12 Outline of the proposed system by Kondo and Ibamoto

2.7 PHASE CHANGE MATERIALS FOR BUILDING

APPLICATIONS

Thermal energy can be stored in PCMs as the heat of vaporization

(liquid–vapour transition) or heat of fusion (solid–liquid transition), where the

latter is mainly used nowadays (Farid et al (2004), and Tyagi et al (2011)).

PCMs allow large amounts of energy to be stored in a relatively small

volume, resulting in some of the lowest storage media costs of any storage

concepts. Most of the comparative studies for phase change heat energy

storage and sensible heat storage have shown that a significant reduction in

storage volume can be achieved, using the PCM compared to sensible heat

storage (Morrison et al (1978)).

47

Phase change materials are categorized as organic, inorganic and

eutectic materials. Organic materials are further divided into fatty acids,

paraffins and non-paraffins. Inorganic materials are divided into salt hydrates

and metallics. However, a eutectic is a minimum-melting composition of two

or more organic or inorganic PCMs like organic-organic, organic-inorganic

and inorganic-inorganic (Sharma et al (2009)). The materials to be used for

phase change thermal energy storage must have a large latent heat and high

thermal conductivity. They should have melting / freezing temperatures in the

practical range of application (Agyenim et al (2010)). The hydrated salts,

paraffin waxes, fatty acids and eutectics of organic and non-organic

compounds have been widely used as phase change materials for the last 30

years. In building applications, the PCMs with a phase change temperature

(18–30°C) are preferred to meet the need of thermal comfort. Some potential

PCMs are listed here, including organic PCMs, salt hydrates and eutectics, as

well as commercial PCMs, seen in Tables 2.1 and 2.2.

2.7.1 PCM Encapsulation

The technology of PCMs encapsulated in a container, for example,

tubes, spheres or panels, is called macro encapsulation. Several studies have

been made of various configurations like flat plate, cylindrical, and spherical

encapsulation as shown in Figure 2.13. With flat plates, it is possible to

achieve more surface area per unit volume of storage material, and low PCM

thickness for reducing the solidification time. According to Zalba et al (2004),

the flat plate configuration with a channel width of 15 mm, resulted in a

charging time of 4 hours and discharging time of 6 hours, which is reasonably

accepted for free cooling. PCM cylindrical pipes have lesser fabrication

difficulty, comparable heat transfer characteristics and a lower heat loss rate.

48

Table 2.1 Thermal properties of potential PCMs

PCMs TypeMelting

temperature(°C)

Heat of fusion

(kJ/kg)

Paraffin C16–C18 Organic 20–22 152

Paraffin C13–C24 Organic 22–24 189

Paraffin C18 Organic 28 244

Butyl stearate Organic 19 140

1-Dodecano 1 Organic 26 200

n-Octadecane Organic 28 200

Vinyl stearate Organic 27–29 122

Dimethyl sabacate Organic 21 120–135

Polyglycol E600 Organic 22 127.2

45/55 capric + lauric acidOrganiceutectic

21 143

Propyl palmitate Organic 19 186

Octadecyl 3-mencaptopropylate Organic 21 143

KF.4H2OHydrate

salts18.5 231

Mn(NO3).6H2OHydrate

salts25.8 125.9

Cacl2.6H2OHydrate

salts29.7 171

CaCl2.6H2O + Nucleat +MgCl2.6H2O (2:1)

Inorganiceutectics

25 127

48% CaCl2 + 4.3% NaCl +0.4% KCl + 47.3% H2O

Inorganiceutectics

26.8 188

Source: Zhou et al (2011)

49

Table 2.2 Thermal properties of commercial PCMs

PCMs

Melting

temperature

(°C)

Heat of

fusion

(kJ/kg)

Specific heat

(kJ/kgK)

Thermal

conductivity

(W/mK)

Source

RT 20 22 172 - -Rubitherm

GmbH

RT 25 25 1472.9(s)

2.1(l)

1.02(s)

0.56(l)

Rubitherm

GmbH

RT 27 26–28 1791.8 (s)

2.4 (l)0.2

Rubitherm

GmbH

STL 27 27 213 - -Mitsubishi

Chemicals

Climsel C23 23 148 - - Climator

Climsel C24 24 216 - - Climator

S 27 27 1901.5 (s)

2.22 (l)

0.79 (s)

0.48 (l)Cristopia

TH 29 29 188 - - TEAP

SP 22 A 17 22 150 - 0.6Rubitherm

GmbH

SP 25 A 8 25 180 2.5 0.6Rubitherm

GmbH

SP 29 29 157 - 0.6Rubitherm

GmbH

50

Figure 2.13 Various types of PCM encapsulations

PCM balls have a larger surface area per unit volume compared to

the cylindrical geometry of the length equal to the diameter. Heat transfer and

pressure drop can be controlled by selecting the size of the balls. Arkar and

Medved (2005) used 25 mm balls. PCM pouches and panels were tested by

Lazaro et al (2009), and the panel was found to be superior to the pouch.

2.7.2 Thermal Stability of PCMs

The long-term stability of the PCMs is required by the practical

applications of latent heat storage, and therefore, there should not be major

changes in the thermal properties of PCMs after undergoing a great number of

thermal cycles. Thermal cycling tests to check the stability of PCMs in latent

51

heat storage systems were carried out for organics, salt hydrates and salt

hydrate mixtures by many researchers (Ting et al (1987), Fernanda et al

(1988), Sharma et al (1999), Sharma et al (2002), and Kimura et al (1988)).

Some potential PCMs were identified to have good stability and thermo-

physical properties. Shukla et al (2008) carried out the thermal cycling tests

for some organic and inorganic PCMs selected on the basis of thermal,

chemical and kinetic criteria, and their results showed that organic PCMs tend

to have better thermal stabilities than inorganic PCMs. Tyagi and Buddi

(2008) conducted the thermal cycling test for calcium chloride hexahydrate,

and found minor changes in the melting temperature and heat of fusion; only

about 1–1.5°C and 4% average variation respectively was recorded during

1000 thermal cycles. They recommend calcium chloride hexahydrate as a

promising PCM for applications.

2.8 MODELLING OF THE PHASE CHANGE STORAGE

SYSTEM

The numerical approach of solving phase change problems is

categorized as Temperature based models or variable domain models, and

Enthalpy based models or fixed domain models. In the temperature based

models, also called as variable domain models the volume of each region

changes with respect to time, and the temperature is the sole dependent

variable. The energy conservation equations are written separately for each

region, and the solutions of these equations are coupled through the energy

balance at the interface. The major disadvantage in the temperature based

model applied in PCMs is the continuous tracking of the solid-liquid

interface, by solving simultaneously all the three energy equations.

The most common method used is the enthalpy method. The

enthalpy model introduced in the 1940s is widely employed in modelling

52

phase change problems. The main advantage of this model is its ability to

accommodate PCMs with a wide phase change temperature range. In

addition, the phase change problems can be reduced to a single equation in

terms of enthalpy. There is no boundary condition to be satisfied at the

interface, and the total domain volume does not change with respect to time.

But from the calculation point of view, this can be determined from the

enthalpy function H (T) of the PCM, which is equal to the sum of the sensible

and the latent heat required for the phase change.

Lacroix (1993) developed a theoretical model to predict the

transient behaviour of a shell-and-tube storage unit with the PCM on the shell

side, and the heat transfer fluid (HTF) circulated inside the tube. Rady and

Mohanty (1996) had applied an enthalpy-porosity fixed grid method to the

melting and solidification of pure metals in a rectangular cavity. Kurklu et al

(1996) developed a numerical model for the prediction of the thermal

performance of a PCM, the polypropylene tube, utilising air as the heat

transfer fluid. Velraj et al (1997) carried out an experimental analysis and

numerical modelling of the inward solidification in a finned vertical tube for a

latent heat storage unit. Patrick and Lacroix (1998) numerically estimated the

thermal behaviour of a multi-layer heat storage unit. The model is based on

the conservation equation of energy for the PCM and the fluid heat transfer.

Zivkovic and Fujii (2001) simulated the transient behaviour of a phase change

material for both cylindrical and rectangular geometries, and their rectangular

geometry showed a good agreement with the experiment. Elgafy et al (2004)

developed a computational model to investigate and predict the thermal

performance of a high melting point phase change material, during its melting

and solidification processes within a cylindrical enclosure. Trp et al (2005)

carried out a theoretical and experimental heat transfer analysis of a shell and

tube heat exchanger, with the cold fluid inside and solidification outside.

53

An alternative formulation called the apparent heat capacity

method, is employed to solve the melting and solidification problem, by

including the effect of the PCM storage through the apparent heat capacity

model in the energy equation. Various shapes of the apparent heat capacity,

like rectangular and triangular profiles were studied by Beasley et al (1989)

and Lamberg et al (2004). Arkar and Medved (2005) simulated the heat

transfer in the cylindrical packed bed by using the apparent heat capacity

method, utilising the DSC results as a model of the heat capacity value.

Hed and Bellander (2006) developed a mathematical model of the

PCM air heat exchanger, which was verified with the measurement on a

prototype heat exchanger. Their model can fit into indoor climate and energy

simulation software, based on the finite difference method where the thermal

properties of the material are considered. A fictive heat transfer coefficient

was established, which includes aspects of the geometry and the airflow as

well as the PCM properties. For an air velocity of 4 m/s it ranged between 16

and 30 W/m2K depending on the surface of the unit. If the surface was rough,

then the heat transfer coefficient increased significantly, but at the same time

a higher input of energy for the fans is needed. Considerations were also taken

of different shapes of the cp(T) curve. It was concluded that special attention

should be paid to the time-dependent behaviour of the equipment, since there

is a great difference between the ideal and commercially available material.

Dubovsky et al (2011) analysed a shell and tube type tubular heat

exchanger in which the PCM melts inside the tubes while air flows across the

tube banks. Considering the sensible heat capacity of the liquid PCM, and the

tubes as small in comparison with the latent heat of melting, a system of

partial differential equations, which describes the heat transfer and melting of

the PCM inside the tubes and heat transfer in the air, has been derived and

solved. An analytical solution of the system of equations has been compared

54

with the results of a numerical solution, demonstrating a very good

agreement.

A one-dimensional liquid-based model of a flat slab phase change

thermal storage unit was developed by Liu et al (2011). The melting and

freezing processes are analysed, based on the temperatures of the heat transfer

fluid nodes, the wall nodes and the PCM nodes during melting and freezing.

The mathematical model developed was validated using two sets of

experimental data. The numerical results show a good agreement with the

experimental ones.

Antony Aroul Raj and Velraj (2011) carried out a transient and

steady state CFD analysis, for a single module (Figure 2.10.b) and two air

spacers. With the steady state CFD analysis, the authors determined the

pressure drop across the module, the flow and temperature variations in order

to select the right geometrical and flow parameters. With the transient

analysis, they determined the PCM solidification characteristics and verified

the suitability of the selected geometrical dimensions.

2.9 SUMMARY OF THE LITERATURE SURVEY

Much research has been done on cool energy storage, using solid-

liquid phase change materials focusing on applications for the cooling of

buildings. The studies were based on experimental work, and numerical

simulations primarily focused on the variation of indoor air temperature due

to the installation of PCMs. A lot of emphasis was also placed on the

possibility of reducing the electricity consumption for cooling. To determine

this possibility, it was necessary to know the characteristics of the system,

such as the charging / discharging rate, mass of the PCM, air flow rate, inlet

and outlet air temperatures, etc. Though plenty of research has been done on

free cooling potential, the free cooling concept was not implemented in any

55

buildings, due to the need for demonstration setups to create confidence

among the building owners. Further, this free cooling technology is site

specific. Hence the design also has to be carried out with respect to the

particular site of the building.

2.10 SPECIFIC OBJECTIVES OF THE PRESENT RESEARCH

A study carried out to assess the free cooling potential for

Bangalore city in India, revealed that this concept is highly suitable for this

city, and it is possible to eliminate totally a mechanical air conditioner.

Considering the potential and also the need for the demonstration setup, the

major objectives of the present work are formulated as below.

To develop a PCM based storage type heat exchanger suitable

for free cooling applications of buildings in Bangalore city.

To investigate the charging characteristics of the PCM under

various ambient conditions that prevails during the early-

morning hours in Bangalore city.

To determine the temperature reduction possible with the

present heat exchanger under various room heat load

conditions.

To carry out the CFD analysis for the configuration similar to

the experimental setup, to make it useful for designing such a

system under various operating conditions.