Dynamic Thermal Simulation of A PCM Lined Building with Energy

Plus

MURAT OZDENEFE, JONATHAN DEWSBURY

School of Mechanical Aerospace and Civil Engineering

The University of Manchester

Manchester M13 9PL

UNITED KINGDOM

murat.ozdenefe@postgrad.manchester.ac.uk

Abstract: - Thermal transmittance of the building fabric plays a significant role in building heat transfer. As the

thermal transmittance value decreases, building heat losses and gains through building fabric reduces. In

addition to the thermal transmittance, heat storage in the building fabric either in sensible or latent form has an

important effect on indoor temperature. The objective of this work is to investigate the effect of temperature

reduction capacity of a PCM lining which is applied to a simple building under the weather conditions of

Cyprus. Dynamic thermal simulations are carried for the simple building and it is found that by application of

15 mm PCM lining temperature reductions achived up to 1.2 oC without supplying any means of mechanical

ventilation or cooling. It is also found that for the indoor temperatures in the band of 26 oC- 30

oC PCM lined

building has always less hours exceeding a temperature value than a building having no PCM lining.

Key-Words: - Building simulation, thermal, phase change materials, Energy Plus, enthalpy method, heat storage

1 Introduction Heat can be stored in building fabric either in

sensible or latent form. Heavy materials are usually

employed for sensible heat storage. On the other

hand, phase changing materials (PCMs) can be

incorporated to the building fabric for latent heat

storage. The isothermal behaviour of the fabric

(thermal stability) when incorporated with PCMs is

the advantage of storing heat in the latent form.

The main feature of the PCMs is to keep built

environment thermally stable by storing more heat

per unit volume than the other conventional

materials, which is achieved by latent heat storage.

When the temperature increases, bonds between

molecules breaks and material melts and turns from

solid to liquid. Phase change from solid to liquid

absorbs heat. Conversely when the temperature

drops below the melting point of material it freezes

and turns from liquid state to solid state. During this

phase heat is released. This process stabilizes the

indoor temperature of the building without changing

thermal transmittance of the building fabric but by

stabilizing the surface temperature by storing heat.

This cycle reduces peak cooling load and decreases

heating load as well [1], [2].

In this work, PCM lined building which

composed of single room is simulated with Energy

Plus program under the weather conditions of

Cyprus. The objective is to investigate the effect of

PCM lining placed on the inner surface of building,

on the indoor temperatures for the cooling season.

Indoor temperatures of the building with and

without PCM lining is investigated and compared to

reveal the thermal stabilizing effect as well as the

peak temperature reducing capability of the PCM

lining.

2 Energy Plus Program and Enthalpy

Method for modelling PCMs Energy Plus is an energy analysis and thermal load

simulation program, which calculates the heating

and cooling loads of a building that is necessary to

maintain specified thermal conditions. It also

evaluates many other parameters related with the

thermal performance of the building [3]. Energy Plus follows the “ASHRAE heat balance

method” [4] principles for heating and cooling load

calculations. Heat balance method is based on

balancing all energy flows into a thermal zone. This

involves solving a set of equations, which are

energy balance equations for outside surface, inside

surface and indoor air for each element (wall, roof

etc.) of a building.

The advantage of the heat balance method is that

it reduces the transient heat transfer equations into

simple linear equations with constant coefficients,

which could be solved easily for inside and outside

face temperatures. The coefficients involved in

these linear equations are evaluated only one time

for each type of construction. This simplicity makes

this method to simulate buildings involving

materials, which have constant properties. Therefore

more advanced materials such as PCMs, which have

variable heat storing capacity depending on

temperature can not be modelled by this method. To

overcome this obstacle a finite difference algorithm

is included in Energy Plus enabling to model

materials, which have variable thermal properties.

This algorithm employs the enthalpy method, which

makes it possible to account all the enthalpy of the

PCMs during the simulation process [5].

Solution of phase change problems are difficult

because two phases (solid and liquid) present

simultaneously and phase change interface is

moving and its position is unknown as latent heat is

absorbed or released at the interface. The energy

equation is needed to be written separately for both

of the phases and their temperatures should be

coupled in the phase interface. This requires finding

out the location of phase interface, which is difficult

if the problem is to be solved by finite difference

methods.

The enthalpy method is an approach that

overcomes these difficulties by applying enthalpy

form of the energy equation, which could be used

for both of the phases. Therefore, there is no need to

have separate equations for solid and liquid phases

making finite difference solution methods

applicable. The enthalpy form of the energy

equation is given in the following expression [6],

[7].

(1)

Where,

∇ = Del operator

k = Thermal conductivity

∇T = Temperature gradient (vector)

ρ = Density

(∂H (T)) /∂t = Enthalpy change with respect to time

In building simulation, heat transfer through

building elements (walls, roofs, etc.) is usually

treated as one-dimensional. Let us consider a

solidification of a liquid, which is initially at a

uniform temperature of T0 that is higher than

melting temperature having boundaries 0≤x≤B. If

the boundary temperature at x=0 for times t>0 are

kept at f which is lower than the melting

temperature and if it is assumed there is not any

temperature gradient at the boundary x=B then (1)

and the related boundary conditions become as (2)

[6]. The physical illustration of this phenomenon is

given in Fig. 1.

0

T=T0>Tmelt

(liquid)

B

t = 0

x0

ρ.δH/δT=

k.δ2T/δx2

B

t > 0

x

T=f<Tmelt δT/δx=0T=T0>Tmelt T=T0>Tmelt

Fig. 1: Physical illustration of enthalpy form of the

energy equation and boundary conditions.

(2)

In Energy Plus, (2) is approximated with implicit

finite difference method as in (3). Then, (3) is

solved within the algorithm by using user entered

enthalpy temperature values to obtain enthalpy-

temperature function. Therefore, it is necessary that

the user inputs the temperatures and corresponding

enthalpies of the substance that is going to be

modelled. (3) and enthalpy-temperature function is

generated for the each node of the PCM material.

The node temperatures are updated after each

iteration so the node enthalpies are updated as well

and are used to develop a variable Cp for phase

change material. This is done by the Cp equation,

which is given in (4). By this algorithm the correct

enthalpy is used for each time step thus the correct

Cp is used [5].

(3)

(4)

Where,

Cp = Specific heat

∆t = Time step

∆x = B/M where M is the number of parts that the

region 0≤x≤B is devided

i = spatial discretization

n = time discretization

3 Properties of Applied PCM Lining

and Incoorporation to Energy Plus In this work to be realistic, it is thought to employ a

real PCM end product, which exists in the market.

Due to passive applications of PCMs to the the

buildings is relatively new technology the variety of

the products in the market are limited. Few

companies produce these materials for building

applications. One of them is a chemical company

called BASF, which they named their product as

Micronal PCM. Micronal PCM is a product that

encapsulates the paraffin wax that is phase change

material in polymer capsules having size of

approximately 5 µm. This product has three types

having three different phase change temperatures;

21, 23 and 26 oC [8].

The Micronal PCM, which is the raw material,

can be incorporated in the construction materials to

produce end products. Such products exist and they

are already available in the market. One of the end

products produced from Micronal PCM is wall

boards which is called Micronal PCM Smartboard

(will be referred to as Smartboard from this point

forward). This product has thickness of 15 mm and

contains 26% mass fraction of microcapsulated

PCM [8]. Its thermal conductivity and density are

0.196 W/m.K and 770 kg/m3 respectively

[9]. It has

three types, which each of them has different phase

change temperatures; 21, 23 and 26 oC. Because the

main objective of this work is to control the

excessive temperatures the one having phase change

temperature of 26 oC is used as lining in the

simulations. Properties of the Micronal PCM which

is the raw material for the Smartboard 26 is given in

Table 1. Temperature vs. enthalpy curve for the

Smartboard 26 is given in Fig. 2. The data points

which are entered to the Energy Plus program are

also seen in Fig.2 [9].

Table 1: Properties of Micronal PCM which is the

raw material of Smartboard 26 [8].

Product type Dispersion Powder

Melting point 26 oC 26

oC

Operational range 10-30 oC 10-30

oC

Overall storage

capacity 59 kJ/kg 145 kJ/kg

Latent heat capacity 45 kJ/kg 110 kJ/kg

Density 0.98 kg/m3 ---

Apparent density --- 250-350

kg/m3

Fig. 2: Temperature vs. enthalpy curve of the

Smartboard 26 [9].

4 Simulated Building The model generated is one room, one storey

building having single thermal zone. Plan of the

model and its 3-D drawing are given in Fig. 3 and in

Fig. 4 respectively. Illustration of the fabrics of the

model is given in Fig. 5. The building fabric of the

model especially the walls and roof are kept simple

to be able to observe the effects of PCM. The

materials employed in the model are widely used in

Cyprus. No ventilation or cooling system assigned

to the model. Two simulations are carried out; one

for the model, which does not employ PCM lining,

and one, which has PCM lining on the inner surface

of the walls and roof. The model having PCM has

the same configuration as given in Fig. 5 and the

fabric called lining in the figure is the PCM lining.

On the other hand, No PCM model also has the

same configuration as in Fig. 5 but the fabric called

lining in the figure is a hypothetical material which

has the same thermal and physical properties

(thermal conductivity, specific heat (for solid state),

density) as PCM lining except phase change feature.

Thus, the PCM and NO PCM models are identical

except for the phase change feature. Simulations are

carried out for the Cyprus (city of Larnaca)

simulation weather data, which exist in the Energy

Plus database. 5.0

3.01

.2

2.2

1.02.0

2.0

5.0

N

Volume (m3)

Floor area (m2)

External wall area (m2)

External opening area (m2)

75

25

60

5.8

All dimensions in m

1.0

1.3

Fig. 3: Plan of the modelled building and its

physical features.

Fig. 4: 3-D drawing of the model.

dX (mm)

Plaster

U value (W/m².K)

Perforated

clay brick

Lining

25 200 15

Reinforced

concrete

Lining

150 15

Marble Screed

30 20

Sand

50

Concrete

100

Hardcore

150

Outdoor Indoor

Outdoor

Indoor

Indoor

dX (mm)

dX (mm)

Soil

External wall

Roof

U value (W/m².K)

Floor

U value (W/m².K)

1.682

6.759

2.719

Fig. 5: Thermal properties of the fabric. (windows

and door U values; 3.61 W/m2K and 4.25 W/m

2K

respectively)

5 Results of Simulations Simulations are carried out for whole year.

However, only the months May to October

(inclusive) are considered for the results of

simulations due to reducing indoor temperatures for

cooling season is the main objective. The results are

presented in the form of comparison of building

having PCM lining with the one having no PCM

lining.

Initially the comparison of hourly indoor air

temperatures of No PCM and PCM building are

presented in the form of charts for particular days

for each month considered (May-October). The

effect of PCM lining can be observed for each

individual hour from this charts enabling micro

scale investigation. Peak temperature shaving could

be seen in these charts. Then the CDF (cumulative

distribution function) curves of these occurring

temperatures are generated and presented in order to

observe macro effect of PCM lining.

Hourly indoor air temperatures of the PCM and

No PCM building as well as outdoor air

temperatures for May-October period for three days

(14th, 15

th and 16

th) are given in Fig.6.

Fig. 6: Hourly indoor air temperatures for No PCM

and PCM building as well as outdoor for May-

October period.

It is seen in above figures that the effect of PCM

lining occurs as peak shave and reduction in

temperature fluctuations. The effect in June, July

and September is very clear whereas in May,

August and October is not very well apparent.

Due to indoor air temperature does not rise well

above or drops well below the phase change

temperature (26 oC) in May, August and October

phase change process is not activated, hence the No

PCM and PCM indoor air temperatures becomes

almost same. For instance in May and October for

three days (14th, 15

th and 16

th) indoor air temperature

never rises above phase change temperature keeping

PCM lining inactive. Whereas during three day

period in August indoor air temperature is above

phase change temperature, almost all hours keeping

the PCM lining always melted.

On the other hand, in June, July and September

indoor air temperature fluctuates mainly about 26 oC, which is phase change temperature thus

continuously activating phase change process

(continuous cyclic melting and freezing).

Maximum reduction in hourly indoor air

temperature by application of PCM lining during

May-October period reaches up to 1.2 oC, whereas

monthly average reduction can reach up to 0.6 oC.

Maximum and average temperature reductions by

application of PCM lining for May-October period

are shown in Table 2 for each month.

Maximum temperature reduction in a month due

to application of PCM lining occurs when the indoor

air temperature difference between No PCM

building and PCM building i.e. TNo PCM-TPCM reaches

maximum value. Whereas, monthly average

reduction in indoor air temperature is evaluated by

calculating the hourly temperature differentials

when TNo PCM-TPCM is positive and averaging them

over the month. This process is illustrated in Fig. 7

for one day as an example.

Table 2: Maximum and average temperature

reduction by application of PCM lining during

daytime.

Months: Max.T

reduction (oC)

Avg.T

reduction (oC)

May 1.2 0.3

June 1.1 0.5

July 1.1 0.3

August 0.8 0.2

September 1.2 0.6

October 1.1 0.4

Time

Me

an

in

do

or

air te

mp

era

ture

1' 3'

3

2

2'

1

(T1-T1')+(T2-T2')+(T3-T3')

3

A particular day in a particular

month within May-October period

No PCM

PCM

4 5 6

4'

5'

6'

Daily average T reduction

by PCM application =

Fig. 7: Illustration of average temperature reduction

by the application of PCM lining.

As well as Table 2, Figure 6 and Figure 7, CDF

curves for indoor air temperatures for No PCM and

PCM lined buildings are generated in order to have

a broader aspect. This curves and their difference

are given in Fig. 8. Hours exceeding a particular

temperature (between 26 oC and 30

oC) for No PCM

and PCM building are given in Fig. 9 in the form of

bar chart as well in order to have a better

visualization.

It is seen in Fig. 8 that until certain temperature,

(around 21 oC) CDF curves of the two buildings are

almost same. This shows that both buildings

experiences almost same amount of hours equal and

exceeding a certain value until 21 oC. This is due to

the inactivity of PCM lining caused by insufficient

temperature rise in PCM building. Between 21 oC

and 25 oC PCM building has more hours than NO

PCM building at and exceeding a certain

temperature value. During this occurring

temperature range PCM lining releases stored heat

yielding a temperature rise in the PCM building

while No PCM building experiences no temperature

rise because its lining has not phase change feature.

Throughout this stage No PCM building has

maximum of 164 hours less than PCM building for

the temperatures at and exceeding 24 oC. Around the

phase change temperature (26 oC), the two curves

intersect. Between 26 oC and 29

oC PCM lining

absorbs heat keeping the indoor air temperature in

the PCM building stable and causes it to have less

hours than No PCM building exceeding a certain

temperature value. In this range maximum of 220

less hours occurs for PCM building than No PCM

building for the temperatures at and exceeding 26.9 oC. When the Indoor air temperatures are above 29

oC two curves are almost the same due to the PCM

lining is almost always melted enabling no heat

storage.

Fig. 9 is the simplified form of the Fig. 8

showing only the particular temperature range from

26 oC- 30

oC. It is seen in Fig. 9 that No PCM

building has always more hours exceeding a

temperature value for this range; 129 hours for 26 oC, 197 hours for 27

oC, 135 hours for 28

oC, 55

hours for 29 oC and 9 hours for 30

oC.

Fig. 8: CDF curves of occurring indoor air

temperatures for NO PCM and PCM building as

well as their difference.

Fig 9: Hours exceeding a particular temperature for

No PCM and PCM building (between 26 oC and 30

oC).

6 Conclusions The results show that application of PCM lining

reduces the indoor temperatures up to 1.2 oC without

application of any other ventilation and cooling

system either in mechanical or natural way for the

May- October Period. It is also seen that for the

temperature range 26 oC- 30

oC PCM lined building

has always less hours exceeding a certain

temperature value than the No PCM building.

It is also revealed that during the months which

the indoor air temperatures are not enough (less than

26 oC) to activate phase change process or

excessively high (above 30 oC) the effectiveness of

the PCM lining reduces.

It should be considered that in this work no

ventilation and cooling system is employed. If the

released heat by the PCM lining during the night

were dissipated by a ventilation or air conditioning

system the effect of the PCM lining for reducing the

indoor air temperatures could be increased.

References:

[1] V. Tyagi and D. Buddhi, PCM thermal storage

in buildings: A state of art, Renewable and

Sustainable Energy Reviews, Vol.11, No.6, 2007,

pp. 1146-1166.

[2] R. Baetens, B. P. Jelle, and A. Gustavsen,

Phase change materials for building applications: A

state-of-the-art review, Energy and Buildings,

Vol.42, No.9, 2010, pp. 1361-1368.

[3] Energy Plus, Energy Plus Documentation. US

Department of Energy, 2010.

[4] ASHRAE, ASHRAE HANDBOOK OF

FUNDAMENTALS, Atlanta: American Society of

Heating, Refrigerating and Air-Conditioning

Engineers, 2009.

[5] C. O. Pedersen, Advanced Zone Simulation in

EnergyPlus: Incorporation of Variable Properties

and Phase Change Material (PCM) Capability,

Building Simulation, 2007, pp. 1341-1345.

[6] M. N. Ozisik, HEAT CONDUCTION Second

Edition, New York: John Wiley & Sons, INC., 1993.

[7] V. Voller and M. Cross, Accurate Solutions of

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Method, International Journal of Heat and Mass

Transfer, 1981, Vol.24, pp.545- 556.

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Management for Buildings, BASF The Chemical

Company, 2009.

[9] Valentin EnergieSoftware, PCM Express, 2008

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