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Analyses of phase change materials’ efficiency in
warm-summer humid continental climate
conditions
To cite this article: J Ratnieks et al 2017 IOP Conf. Ser.: Mater. Sci. Eng. 251 012119
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IMST 2017 IOP Publishing
IOP Conf. Series: Materials Science and Engineering 251 (2017) 012119 doi:10.1088/1757-899X/251/1/012119
Analyses of phase change materials’ efficiency in
warm-summer humid continental climate conditions
J Ratnieks1, S Gendelis1, A Jakovics1and D Bajare2
1University of Latvia, Laboratory for mathematical modeling of environmental and
technological processes, Zellu str. 23, Riga, Latvia
2Riga Technical University, Faculty of Civil Engineering, Azenes str. 16, Riga, Latvia
E-mail: janis.ratnieks@fizmati.lv
Abstract. The usage of phase change materials (PCMs) is a way to store excess energy pro-
duced during the hot time of the day and release it during the night thereby reducing the
overheating problem. While, in Latvian climate conditions overheating is not a big issue in
traditional buildings since it happens only a couple of weeks per year air conditioners must still
be installed to maintain thermal comfort. The need for cooling in recently built office buildings
with large window area can increase significantly. It is therefore of great interest if the ther-
mal comfort conditions can be maintained by PCMs alone or with reduced maximum power of
installed cooling systems. Our initial studies show that if the test building is well-insulated (nec-
essary to reduce heat loss in winter), phase change material is not able to solidify fast enough
during the relatively short night time. To further investigate the problem various experimental
setups with two different phase change materials were installed in test buildings. Experimental
results are compared with numerical modelling made in software COMSOL Multiphysics. The
effectiveness of PCM using different situations is widely analysed.
1. Introduction
In order to reduce cooling energy consumption, PCMs are being used as a passive system for
temperature condition stabilization in the room. This kind of system saves energy that would
be used for cooling and more important, in Latvian climate conditions, could even save the
costs of installing air cooling devices. A lot of papers have dealt with phase change materials
being installed in a testing environment and have achieved results where the effect of PCM is
shown compared with the similar environment without PCM [1] and [2]. Other papers deal with
thermal comfort condition improvement in the living environment [3]. The modelling is also
done to some extent, like in [4]. An issue that occurs for PCMs is that passive systems do not
solidify fast enough during the night when the temperature is lower. High air exchange rates
are used in [2] and [4] to ensure that solidification occurs during the night.
The aim of the study is to evaluate how many cycles can PCM effectively use in hotter periods
as a passive system in warm-summer humid continental climate conditions. To achieve the goal,
two different PCMs are installed in different test buildings that are located in Riga, Latvia.
The testing ground consists of five buildings, with the ones without PCM used for comparison.
Data from summer seasons of years 2015 and 2016 are taken and analyzed. Typical overheating
cycles are selected and by employing numerical model the phase change phenomena are studied
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IMST 2017 IOP Publishing
IOP Conf. Series: Materials Science and Engineering 251 (2017) 012119 doi:10.1088/1757-899X/251/1/012119
in greater detail. The testing ground, sensors used and measurement periods are described in
section 2.
The experimental data are analyzed in section 3 where some already known as well as expected
patterns are observed. This leads to the next task of evaluating the time a PCM is working.
As there is no good way of telling whether the inside of the material is in molten state, a
mathematical model is used that is described in section 4.
2. Experimental setup
2.1. Testing environment
The test buildings are localized in the urban environment, under natural conditions in Riga,
Latvia, characterized by warm-summer humid continental climate. The average heating period
is 203 days and average annual temperature is 6.2 C. Therefore buildings with good thermal
insulation are necessary. While the summer season is short, the overheating issues are still
present as shown later [5] [6] [7] .
Each experimental stand is a free-standing building, placed in equal relation to the sun and the
surrounding shading objects. They have 9 m2floor area and 3 mceiling height with a window
on the south facade and a front door on the north facade see figure 1. Each building is placed on
pillars and has no direct contact with the ground. The U-values of wall assemblies are calculated
approximately as 0.15 [ W
m2·K]. The basic materials used for the ventilated facade exterior wall
Figure 1. Testing ground, located in Riga, Latvia with sensor positions on the left.
construction are:
1) perforated ceramic blocks (440 mm) with flexible stone wool insulation outside (type CER);
2) aerated concrete blocks (375 mm) with flexible stone wool layer outside (type AER);
3) modular plywood panels with flexible stone wool filling (200 mm) and fibrolite (70 mm)
inside (type PLY);
4) laminated beams (200 mm) with flexible stone wool insulation layer and wood paneling inside
(type LOG). [6], [7]
The thermal masses of each building’s wall envelope are calculated as [ MJ
K·m2] and are given
in table 1. The calculated heat capacity of PCM latent heat is taken from the manufacturers’
data. Heating/cooling systems, windows, doors, as well as roof and floor constructions are made
equal with the same spatial direction so the wall assemblies could be compared.
The PCMs installed on the inner walls of test buildings can be seen in figure 2 and their
properties in table 2. The placement of PCMs have been researched previously and it is shown
in [8] and [9] that the inner surface placement is not the best option, however due to the fact that
testing environment already existed there was no optin to change the placement. The work of
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IMST 2017 IOP Publishing
IOP Conf. Series: Materials Science and Engineering 251 (2017) 012119 doi:10.1088/1757-899X/251/1/012119
Table 1. Thermal mass of each construction.
wall assembly LOG CER PLY AER
thermal mass (w/o pcm), MJ
K·m20.19 0.37 0.11 0.18
PCM latent heat, M J
m21.45 0.52 0 0
PCM present + + - -
Figure 2. Phase change materials installed: LOG builing on the left and CER building on the
right
[9] is purely experimental and does not take into account that there is a direct thermal radiation
through the windows that can change the actual performance.
Table 2. Phase change materials’ properties.
LOG CER
Latent heat 200 121 kJ
kg
Thermal conductivity 0.2 0.14..0.18 W
m·K
Density 860 810 kg
m3
Melting temperature 25 21.6 C
2.2. Data acquisition system and sensors
To monitor ambient temperature data a meteorological station located on the top of AER
building collects temperature, humidity, precipitation and other information. In each test
building more than 20 sensors collect data on temperature, humidity, solar radiation etc. The
indoor temperature is calculated as average of 5 sensors that are placed in the middle of
horizontal plane at various heights (0.1, 0.6, 1.1, 1.7 and 1.9 mabove the floor). Data are
collected every minute and once per day sent to the server. The data experimentally acquired
are averaged over an hour and used as an input for the numerical model described in later
chapters. More on monitoring system can be read in [10] and [11].
3. Data analysis
Two different time periods were experimented with during summer 2016 . The first was from 1
May to 1 July where no cooling was provided and the mechanical ventilation was switched off.
And the second period started 1 July where cooling and mechanical ventilation was switched
on. The measured temperatures are shown in figure 3. The outside weather conditions are given
as MET.
It can be seen that during period one CER building has lower temperature values for hot sub-
periods and higher temperature values for cooler sub-periods. This can be attributed to thermal
mass that is almost twice as big as LOG and AER buildings have and more than three times
the PLY building. It is, therefore, difficult to correctly evaluate the effect of PCM. In figure
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IMST 2017 IOP Publishing
IOP Conf. Series: Materials Science and Engineering 251 (2017) 012119 doi:10.1088/1757-899X/251/1/012119
Figure 3. Inner temperatures in buildings and outside temperature during the summer 2016.
4, it is shown that at the phase change temperature of CER building’s PCM the behaviour of
daily cycle slightly changes. This can be seen in more detail in the subsequent numerical model
section. PLY, LOG and AER buildings the behaviour is quite similar except for PLY that is
cooling down faster than LOG and AER. During the brief moment from June 3rd until June
10th the AER building shows faster decrease than LOG building and this period lies in the phase
change temperature of LOG PCM’s phase change temperature. For the second period cooling
temperature was set to 21 Cand the air exchange of n= 0.45[h−1] [12]. The set temperature for
cooling was below the phase change temperature of any of two PCMs installed (see tables 1 and
2) and therefore no heat is saved due to PCMs. Temperature control gives a clue that there is a
temperature level difference between test buildings. This is believed to be due to temperature
sensor offset between cooling units. The total energy consumption is given for all the buildings.
Also, the average temperatures during the cooling period are shown in table 3.
Table 3. Average temperatures, inner heat gains and cooling energy for test buildings during
second (cooling) period form June 1st until August 31st.
LOG PLY CER AER
Qcooling,kW h 31.3 33.9 35.5 35.2
Qinner, kW h 20.6 22.8 25.5 46.5
Tavg ,◦C21.2 20.6 20.1 21.0
The inner sources are due to sensors, data loggers and other consumers found in buildings.
The AER test building has higher energy gains from inner sources because the meteorological
station is placed there that consumes extra power. The energy required is lowest for LOG
building but this is compensated by the fact that it has the highest average inside temperature
during the second period. To fully interpret data and see which construction consumes the
lowest amount of energy an energy balance calculation must be made. However it’s obvious that
the difference will be marginal as it is now.
A 2015 summer season was also considered in this study. The only difference was that the
outside weather was colder on average and the mechanical ventilation was turned on. These
data are later used in section 4 where they are compared with those of the year 2016.
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IMST 2017 IOP Publishing
IOP Conf. Series: Materials Science and Engineering 251 (2017) 012119 doi:10.1088/1757-899X/251/1/012119
4. Numerical model
A 1D numerical model is set up in COMSOL Multiphysics with a single material wall of the
same thermal mass and effective thermal conductivity as the CER building has. The thermal
mass in this context is to be understood as the amount of heat in Ja square meter of wall
assembly can store per one Ktemperature difference. A heat transfer equation is solved that
is well known and therefore not given in this paper. There are third type boundary conditions
towards inner and outer environment with heat transfer coefficients being 7.7[ W
m2·K] for inner
environment and 25[ W
m2·K] for outside environment. The time dependent temperature is taken
from experimental data and shown in figure 4 as red and green lines for outside and blue lines
for inner temperature. The end of summer 2015 is cut off in figure 4 as these results are not
presented here.
Figure 4. Temperature dependence on time for mathematical model.
The first calculation was done from 17 May 2016 to 17 June 2016 and the second from 19
May 2015 to 01 July 2015. The PCM was considered in a solid state initially. Calculations were
started a week before the first melting in order to get realistic temperature distribution as the
initial conditions depicted constant temperatures.
5. Results
The results are best viewed as a phase diagram where time is shown on X axes and phase - either
solid or liquid - is shown on Y axes where unity represents liquid state and zero represents solid
state, see figures 5 and 7. Temperature inside, on the surface, and between the PCM and wall is
shown in figures 6 and 8. The experimental data for second period - cooling below phase change
temperature - shows that there is a temperature measurement offset from one air conditioning
unit to another. This must be considered in future experiments when they include cooling above
the phase change temperature.
Qualitatively looking at the experimental data it is hard to notice the difference of temperature
regimes for LOG and AER buildings despite the fact that they have similar constructions’
thermal mass but one has PCM while the other doesn’t. This gives a rise to doubt if the PCMs
are of any use. The cooling consumption is shown in table 3. It has been measured previously
that mechanical ventilation alone needs approximately 40 W that adds up for 59.2 kW h just for
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IMST 2017 IOP Publishing
IOP Conf. Series: Materials Science and Engineering 251 (2017) 012119 doi:10.1088/1757-899X/251/1/012119
Figure 5. PCM phase dependence on time from mathematical model for overheating cycle in
2016; inner surface being PCM and inner air boundary.
Figure 6. Temperature dependence on time from mathematical model for overheating cycle in
2016.
ventilation in the given period. The cooling energy required therefore is comparable to energy
needed for ventilation system to work and negligible compared to heating energy consumption
during the winter. In figure 4, it is clearly seen that average temperature is higher in summer
2016 and the peak values are also higher during that period. The hot period (when outside
temperature is higher than phase change temperature) starts on day 11 and mathematical model
shows that until end of day 13 all the PCM have molten down, see the phase indicator diagram
figure 5. After this melting for remaining of hot period, although outside temperature goes
below phase change temperature the solidification does not happen and overheating occur.
For year 2015 the ventilation is n= 0.45[h−1] and the outside temperature on average is lower.
In this case the PCM melts and solidifies from day 13 until day 18 when the inside temperature
gets too high for solidification to occur again, see figure 7. For a given period the effect of the
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IMST 2017 IOP Publishing
IOP Conf. Series: Materials Science and Engineering 251 (2017) 012119 doi:10.1088/1757-899X/251/1/012119
Figure 7. Temperature dependance on time for mathematical model for overheating cycle in
2015; inner surface being PCM and inner air boundary.
Figure 8. Temperature dependance on time for mathematical model for overheating cycle in
2015.
PCM can be clearly seen by looking at the figure 8 where inside temperature is not rising above
23 Ctill day 18 and then after the PCM have molten down, it rise rapidly. In warm summer
humid continental climate conditions the buildings must be built with good thermal insulation -
U=0.15 W
m2·K. This effect can be seen in figures 5 and 7 at days 11 and 15, respectively. In case
where the temperature rises, melting happens more on the inner surface where more than half
of the material undergoes phase change (it is assumed that phase change occur over interval of
0.5 C) while at the wall/PCM interface only 20% has undergone phase change. The reverse is
true for solidifying where wall/PCM boundary can’t solidify while the PCM closer to inside of
room haven’t solidified.
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IMST 2017 IOP Publishing
IOP Conf. Series: Materials Science and Engineering 251 (2017) 012119 doi:10.1088/1757-899X/251/1/012119
6. Discussion
The cooling energy demand in Latvian climate conditions is low and almost negligible compared
with other devices. The PCM usage therefore is not justified in sense of cooling energy saving.
However taking into account short summer seasons with typical temperature peak periods
the PCM could be used as a passive system that can completely eliminate the need for air
conditioning device. In this sense the use of PCMs could be justified and further studies on the
subject are recommended.
Despite CER building having lower overall PCM’s latent heat, the effect is qualitatively seen,
compared to other buildings. In the case of LOG building, the temperature level is the same
as in similar buildings without PCMs, the only difference being at the end of hot period when
temperatures are decreasing.
7. Conclusion
Qualitatively it can be seen for the CER case that PCMs reduce peak temperatures during
hotter periods. The effect, if any, is barely seen in LOG building despite higher latent heat
being released. Numerical model shows that the peak temperature reduction only happens
during first few days of longer high temperature periods and for periods long enough or hot
enough the overheating is not eliminated. A larger amount of PCM could be used to overcome
those periods. Additional studies are suggested regarding: validation of mathematical model.
Additional temperature sensors will be installed in cooling season 2017; viability of PCM usage
in Latvian climate conditions are proposed; an increase of mechanical air exchange during the
night is proposed to achieve solidification during the night.
Acknowledgments
This publication is part of a project that has received funding from the European Unions Horizon
2020 research and innovation program under grant agreement No. 657466 (INPATH-TES).
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