2
Freezing Fridge
Research to the influence of phase change materials in a fridge.
Thijs Jansen
Commissioned by Flexines
Course: International Power Generation and Distribution
Institute of Engineering
Hanze Hogeschool Groningen
Groningen, February ’16
3
Foreword
For my graduation, I had a choice of assignments at different companies.
When I heard about the project Flexines and the assignment with the fridge, I was immediately
enthusiastic. I find it very interesting and fun to be working on new innovations in the field of energy.
I have enjoyed working on this project. Because it was not only electrical but also physics.
It was also nice to work with the colleagues within the project Flexines who occasionally gave good
advice. For that I thank them.
Groningen, June 2010
Thijs Jansen
4
Summary
Electrical energy is increasingly produced in a sustainable way.
The disadvantage is that this kind of generation is highly depending on the weather conditions and
therefore not always present at times when we need it and the other way around.
Purpose is to identify ways to save energy at times when it is there but we do not need it.
This research examined the extent to which energy can be stored using a thermal buffer in a
refrigerator and thus the time of switching on and off. With this study, two kinds of Phase change
materials (PCM) are used as a thermal buffer. These materials are water with a phase change
temperature of 0°C and an organic solution with a phase change temperature of 4°C.
With an ambient temperature of 20°C, these two materials showed that the temperature difference
between the PCM and the compartment of the fridge is 10°C when the temperature in the fridge
reaches a constant level. The problem with these two materials is that their phase change
temperature is to high. With water the temperature in the fridge stays constant at 10°C and for the
organic solution this is at 14°C to 15°C. Because the temperature in the fridge has to be 5°C to 7°C a
PCM with a phase change temperature of -4°C is needed.
The range of extending the time between turning on and off the refrigerator is depending on the
amount of material that is used. The time can be extended from 1 hour without PCM till up to eight
hours with 1,5 liters of PCM.
The use of PCM’s will not only help to extend the time of on and off but it also makes the fridge more
efficient. In this study the efficiency of the fridge became 3.3% better.
To prevent the temperature in the fridge getting too cold, the PCM needs to cover the whole surface
of the cooling element. Otherwise, the cooling element has more influence on the temperature in
the refrigerator than the PCM. Therefore the temperature will drop beneath 0°C before the whole
phase change is completed.
Due to time constraints in this study practical research on the results of a PCM with a phase change
temperature of -4°C has not been performed.
However, a simulation model of the fridge is made which gives a very good presentation of reality.
This model also shows that a material of -4°C is best to use.
5
1 Table of content
2
3
4
Introduction ..................................................................................................................................... 8
2.1
Background.............................................................................................................................. 8
2.2
Phase Change Materials (PCM’s)............................................................................................. 8
2.2.1
Organic solutions ............................................................................................................. 9
2.2.2
Eutectics ........................................................................................................................ 10
2.2.3
Salt solutions ................................................................................................................. 10
2.3
Research assignment ............................................................................................................. 11
2.4
Plan of approach.................................................................................................................... 11
The fridge....................................................................................................................................... 12
3.1
What is a fridge ..................................................................................................................... 12
3.2
Values .................................................................................................................................... 12
3.2.1
Average heat flow.......................................................................................................... 13
3.2.2
Resistance fridge ........................................................................................................... 14
3.2.3
Capacity fridge ............................................................................................................... 16
3.2.4
Full fridge ....................................................................................................................... 18
Measurements .............................................................................................................................. 19
4.1
5
6
Temperature sensors ............................................................................................................ 19
4.1.1
Application..................................................................................................................... 19
4.1.2
Type ............................................................................................................................... 19
4.1.3
Calibration ..................................................................................................................... 19
4.2
Phidgetboard ......................................................................................................................... 20
4.3
Amplifier ................................................................................................................................ 20
4.4
Energy consumption.............................................................................................................. 20
4.5
Measurements fridge ............................................................................................................ 21
4.5.1
Average temperature .................................................................................................... 21
4.5.2
Influence products......................................................................................................... 22
4.5.3
Power............................................................................................................................. 23
4.5.4
PCM ............................................................................................................................... 24
4.5.5
Energy consumption of the fridge ................................................................................. 26
Vissim model ................................................................................................................................. 27
5.1
Turning the fridge on and off ................................................................................................ 27
5.2
Heat flow of the cooling element .......................................................................................... 27
5.3
Heat flow PCM ....................................................................................................................... 28
6
5.4
Heat flow compartment fridge and products. ...................................................................... 29
5.5
Evaluation .............................................................................................................................. 29
5.6
PCM of -4ºC............................................................................................................................ 31
Conclusion ..................................................................................................................................... 32
References ............................................................................................................................................. 33
Attachment 1. Temperature range organic solutions ........................................................................... 34
Attachment 2. Temperature range eutectics ........................................................................................ 35
Attachment 3. Measurement protocol. ................................................................................................ 36
Attachment 4. Datasheet LM35. ........................................................................................................... 37
Attachment 5. Calibration Amplifier. .................................................................................................... 38
Attachment 6. Vissim model ................................................................................................................. 39
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2 Introduction
2.1 Background
This project “Freezing Fridge” is part of a bigger project called Flexines. Flexines is a collaborative
project of ECN, RuG, Kema, TNO and HG. The goal of this project is to balance the supply and demand
of energy. The World is running out of fossil fuels so we will make use of sustainable energy more
and more. The amount of sustainable energy is highly depending on the weather conditions which is
very unpredictable. In the future the energy supply will be irregular while the demand of energy will
not automatically change its behavior to match the supply.
The demand of energy is controllable between certain limits. Under special circumstances some
devices could have the ability to regulate their energy consumption by using a thermal buffer system
to store energy. Thereby their energy usage could be shifted in time. In this case we speak of time
shifting of energy demand. The project “Freezing Fridge” is a research project to see what is possible
with a fridge regarding this time shifting by making use of phase change materials.
2.2 Phase Change Materials (PCM’s)
One of the biggest energy users in our houses is the fridge. The fridge uses the energy in a short
period of time. Thereby it is a device which is not easy to shift in time. PCM’s are a valuable tool for
thermal buffers with an extremely low increase of volume. This means that the time shift can be
increased. PCM is a discipline which is quite new. It appears that application of these materials for
smart grids (i.e. matching supply and demand of energy) is an extremely important value.
We all know the phenomenon of water. We find it very natural that water turns into ice at a
temperature of 0°C. Phase change materials are materials that in many respects are similar to water.
Phase change materials have the same property as water, they change phase at a certain
temperature. The only difference between water and phase change materials is that these materials
are available in different configurations. This means the temperature at which the phase change
takes place can be selected.
For the phase change from liquid to solid a lot of energy is needed in the form of cold while the
temperature of the material remains the same until the phase transition from liquid to solid is fully
completed. After that the temperature of the material will go further down.
The other way around it takes the same amount of energy to go from solid to liquid again. And again
the temperature will be the same during the phase change. The only difference is that the “cold”
energy is now not entering the material but it is extracted from it by the environment. The amount
of energy to be added or removed from the material for a complete phase change is called latent
heat and is given in kJ/kg or MJ/m3.
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The different types of compounds that are currently used to make phase change materials can be
divided into three groups, namely:
- Organic solutions
- Salt Solutions
- Eutectic
These three groups are all made of different materials and therefore each has its own specific
properties. These different properties each have their advantages and disadvantages.
One of the different properties between the materials is that they each have a different range of
temperatures for which they are available as phase change material.
Table 2.4.1 shows the different ranges at which the materials are available.
Organic solutions
Salt solutions
Eutectic
Minimum temperature Maximum temperature
2°C
164°C
7°C
117°C
-173°C
-2°C
Table 2.4.1. Temperature range different phase change materials.
2.2.1
Organic solutions
Organic solutions are mostly made of paraffin. Paraffin has some advantages and disadvantages.
For instance it freezes with almost no change of super cooling. Super cooling means that a liquid can
be cooled down below its freezing point without changing in to its solid state.
It has a very sharp phase change temperature so there is no big temperature change during the
phase change.
Organics solutions have no segregation, even after a long period of time. So the material will be able
to freeze and melt for many times without losing its capacity.
One of the disadvantages of the material is that in the solid state the material has a low thermal
conductivity. Also the latent heat capacity per volume is not very high.
Attachment 1 shows the complete range of available temperatures for organics with corresponding
heat properties.
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2.2.2
Eutectics
Eutectics consist of a mixture of two or more substances. These substances are mixed in such a way
to provide the desired melting/freezing point. Each substance has its own phase change
temperature. Combining these materials gives a lower phase change temperature than the
substances had on their own.
Eutectics have an even sharper phase change temperature point then organics. Eutectics melt and
freeze completely at the designed temperature. The latent heat capacity of eutectics is a little higher
than with organics. Attachment 2 shows the complete range of available temperatures for eutectics
with corresponding heat properties.
2.2.3
Salt solutions
Salt solutions are mostly bases on Glauber’s salts or sodium sulphate decahydrates.
These salts are available on a very large scale which makes them very cheap compared with the
other two. The major problem with salt solutions is that every cycle of freezing and melting, the salts
have the tends to crystallize in such a way that they get separated from the saturated solution they.
After melting these crystals sink to the bottom of the substance and with the next frozen they will
not be able to recombine with the rest of the substance. This will result in a continual loss of capacity
performance.
Salt solutions also have tendency to super cooling. And there change of volume between liquid en
solid is big compared with organics and eutectics.
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2.3 Research assignment
The goal of this project is to do research to the influence of PCM’s when they are installed in a fridge.
Important research questions are:
• How do PCM's behave in this application.
• What is the extra room for negotiation, i.e. how we can improve supply and demand matching with
these materials?
• What effects do the PCM’s have on the energy consumption of the fridge?
2.4 Plan of approach
First the behavior of the fridge without PCM’s is determined. This is done to see what the differences
are between the fridge with and without PCM’s. To be able do to this, temperature sensors are build.
Together with a program to read and store the measurements from the sensors.
During these measurements, information about the PCM’s is collected. What kind of materials are
there and what properties does each one have? With these information and the measurements a
good decision can be made for which one is best to use.
As soon as the PCM’s are ordered, time is used to research what would be the best place in the fridge
to attach the PCM’s. In this phase of the project a simulation program of the fridge is made to
simulate the fridge. This simulation will help to make a prediction of the effects of the PCM’s.
After the PCM’s are delivered, they will be placed in the fridge and new measurements can start.
After these measurements the results have to be evaluated to see what the effects are and if this is
in line with the simulation program.
In this phase the advantages and disadvantages of this system will be evaluated. The energy
consumption of the fridge is part of this.
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3 The fridge
3.1 What is a fridge
A fridge can be seen as a device that transports heat.
The compressor of the fridge takes out the heat inside the fridge by a cooling element.
When the fridge is off, the environment heats up the fridge again. The speed of cooling down and
heating up is depending on the heat capacity of the different elements in the fridge and the
resistance that the heat flow undertakes. This process can be compared with an electrical circuit.
In an empty fridge, the heat undertakes three resistances. These are:
R1
The resistance from the environment to the compartment of the fridge.
R2
The resistance from the compartment of the fridge to the cooling element of the fridge.
R3
The resistance from the environment to the cooling element of the fridge.
An empty fridge has two capacities:
C1
The compartment of the fridge.
C2
The cooling element of the fridge.
For the electrical circuit there is a voltage over the resistors and a current goes through.
In this case, the voltage is the temperature difference and the current is the heat flow.
Underneath is a diagram of an electrical circuit which represents an empty fridge.
R3
I1
1
R1
V1
2
R2
3
C1
compartment
environment
compressor
C2
element
4
Figure 3.1.1. Electrical diagram of an empty fridge.
3.2 Values
To make a good model of the fridge we have to determine the values of each component.
Once the fridge is running, it turns on and off at the same temperature every time.
If we measure the energy that is used to cool the fridge, we know that at the time the fridge will turn
on again, the energy (heat) that was extracted from the fridge has now returned back into the fridge
from the environment. So the capacities are unloaded and loaded again. These capacities won’t
consume energy, they only store it for a while. So for a hole period of cooling down and heating up
again we can say all the energy that went into the fridge also came out. All this energy went through
the resistors during the whole period.
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3.2.1
Average heat flow
With the first measurement, the temperature is measured on three places.
- The temperature of the environment (point 1 in the diagram)
- The temperature of the compartment on several places (point 2 in the diagram)
- The temperature of the element (point 3 in the diagram)
With the total energy that entered the fridge and the time that it took, the average heat flow can be
calculated. A measurement of ten cool down and heat up cycles is used. At first the electrical
consumption of the fridge is determined. With this energy consumption, the average power during
cool down is calculated.
Electrical consumption of the fridge:
Pulses  384
t 1  12173sec
t tot  42965sec
Etotal  Pulses  0.5W hr  192 W hr
Etotal
Pcompressor 
 56.781 W
t 1
This is the electrical power of the fridge. To get the power in terms of heat this has to be multiplied
with the COP of the fridge. With the amount of energy which is subtracted from the fridge during
cool down, the average heat flow of the fridge warming up again can be calculated by dividing this
over the total time period.
Amount of energy in heat to fridge:
COPcompressor  1.30
Pheat  Pcompressor  COPcompressor  73.816W
5
Eheat  Pheat t 1  8.986  10 J
Eheat
Pout 
 20.914 W
t tot
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3.2.2
Resistance fridge
With the average heat flow and the average ΔTemperature between the fridge and the ambient the
total the total resistance of the fridge can be calculated.
Paverage  20.914W
2
Surfacecompartment  1.425m
2
Surfaceelement  0.3m
Taverage.compartment  3.54K
Taverage.element  6.72K
Taverage.ambient  19.94K
T average  Taverage.ambient  Taverage.element  26.66K
Rt 
T average
Paverage
 1.275
K
W
This is the total resistance of the fridge.
To divide this resistance in to three separated resistances, the difference in temperature between
the ambient, the compartment of the fridge and the element of the fridge are used. To be able to
solve this question I assumed the material at the back of the fridge is the same as the material used
at the other surface of the fridge. Otherwise this problem can’t be solved.
T compartment.ambient  Taverage.ambient  Taverage.compartment  16.4K
T element.compartment  Taverage.compartment  Taverage.element  10.26K
Tcompartment  T compartment.ambient  16.4K
Telement  T element.compartment  10.26K
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Given
R2
Telement
Tcompartment
 R1
Tcompartment  Telement
Rt
Tcompartment
Tcompartment  Telement
Tcompartment
R3
Surfacecompartment
Surfaceelement
 Surfacecompartment
 R1 

 R1 
Surfaceelement

 R1
Surfacecompartment
Surfaceelement
 R1

 R1
 1.0525309160824062037 K  s 3 


2


kg  m


 0.65847360969545656401 K  s 3 
Find  R1 R2 R3  

2

kg  m


3 
 4.9995218513914294675 K  s 


2
kg  m


These resistances are the total resistances of the different surfaces.
To calculate the specific thermal resistance of the materials, these numbers have to be multiplied by
the surface.
Specific thermal resistance:
Rthermical
Rcircuit surface
2 K
Rthermical.1  R1 Surfacecompartment  1.5 m 
W
2 K
Rthermical.2  R2 Surfaceelement  0.198 m 
W
2 K
Rthermical.3  R3 Surfaceelement  1.5 m 
W
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3.2.3
Capacity fridge
Now that the resistances are known, the capacities of the fridge can be calculated.
To do this one period of the measurement which was used to calculate the average heat flow is used.
For this period the heat flow between the different parts of the fridge is calculated for every
moment. These heat flows be expressed in a formula.
The graphic below shows the heat flow for the element with the formula. On the next page you will
find the heat flow of the compartment of the fridge.
Figure 3.2.3.1. Total heat flow of the element of the fridge
From the thermodynamics we know that:
Q
Qtotal.element
C T
 element
t
Rewriting these formulas we are able to calculate the capacity of the fridge:

 13 4
 ( t)  7.917302 10
T  2.98K  5.12K  8.1 K
C 
1s 

T 
1410
 ( t) dt
0
3 J
C  1.875  10 
16
K
 10 3
 t  2.263173 10
62
t  5.974066 10
2
t  1.223658 10

 t  15.90731 W
Figure 3.2.3.2. Total heat flow of the compartment of the fridge.
For the compartment of the fridge:

 15 5
 ( t)  3.156939 10
 11 4
 t  2.182989 10
83
 t  5.935284 10
52
t  8.342730 10

2
t  6.586337 10
 t  16.77832 W
T  6K  1.58K  4.42 K
1s 
C 

T 
2310
 ( t) dt
0
3 J
C  2.14  10 
K
Now all the components of the fridge are determined. These components are used to make the
simulation model in Vissim.
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3.2.4
Full fridge
When you fill the fridge with products or PCM’s, figure 3.1.1. will change and look like figure 3.2.4.1.
For each product its own capacity and resistance has to be determined. This also count’s for the
PCM.
I1
R3
1
5
2
V1
C3
product A
8
R1
R2
R4
R5
R7
R6
6
4
C4
product B
environment
compressor
C1
compartment
C5
PCM
C2
element
3
Figure 3.4.2.1. electrical diagram of a loaded fridge.
As you can see R2 is still in the same position but due to the fact that the PCM is attached to the back
of the fridge, the resistance from the element directly to the compartment of the fridge will be
bigger because the surface of the element directly to the compartment will decrease with the surface
of the PCM covering the element.
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4 Measurements
For this project I made use of a fridge of ZANUSSI, type ZRG 616 CW, model TT 160 C.
To determine the behavior of the fridge I had decided to collect seven measurements, which are:
- Temperature in the fridge on four different places.
- Temperature of the PCM.
- Ambient temperature of the fridge.
- The energy consumption of the fridge.
All the data is collected per time unit.
To determine the amount of power the fridge uses, these measurements are done in seconds.
Once this is done the measurements will take place in minutes.
Because a fridge responds different when it’s empty than when it is full, different scenarios are
tested. This is done by the measurement protocol (attachment 3)
4.1 Temperature sensors
4.1.1
Application
The fridge has three sections separated by glass, therefore each Floor gets its own sensor. Also the
temperature of the cooling element is measured. This way there is a good representation of the
temperatures in the fridge. Because the temperature change in the fridge is strongly depending on
the ambient temperature, this is also measured.
4.1.2
Type
Two different temperature sensors are used. The LM35 and the LM335.
The LM35 is a precision temperature sensor which is able to work fine on a 5V dc coming from the
pc. The temperature range for this sensor is set on -5°C till +40°C (50mV-500mV) because the
temperature in the fridge and the ambient temperature will not exceed these limits. To do so, the
circuit in figure 8 of the datasheet (attachment 4) is used. Because the temperature of the cooling
element and the PCM will get very low, the LM335 is used to measure these temperatures.
4.1.3
Calibration
All sensors are calibrated. This is done by determining the temperatures on two different known
values. The first value is created by a bucket of melting ice. The temperature of the ice is first
measured with a calibrated sensor. After that the other sensors where placed in the bucket.
The second value was room temperature. The deviations of the sensors are corrected in the read and
storage program on the pc.
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4.2 Phidgetboard
For the connection between the sensors and the pc, a phidget board is used. This board converts the
analog signals from the sensors to digital signals which can be read by the program.
Testing the board, showed that the low voltage change of the sensors were too small to be detected
by the board for every step. This resulted in a resolution of 0,5 °C. To get a better resolution from the
sensors (LM35) in the fridge, an amplifier is build.
4.3 Amplifier
Op-amps are used to amplify the sensor signals. De LM324N to be exact. This type Works well for
these low voltages and it doesn’t need a Vcc- supply to work properly.
The temperature in the fridge will never exceed the ambient temperature. In this case it would never
exceed 30°C (400mV output voltage from the temperature sensor). With the available 5Vdc from the
computer, the signal can be amplified eleven times without the risk of the signal being smoothed by
the Op-amp. To do so, a non-inverting amplifier as shown in figure 4.3.1 is build.
Figure 4.3.1. Amplifier circuit amplification factor 11 for every input signal.
This amplifier is also calibrated to determine the exact amplification.
The calibration report can be found in attachment 5.
4.4 Energy consumption
To measure the energy consumption of the fridge a kWh-meter is used. This meter gives a pulse for
every 0,5 watt hour that is used. These pulses are monitored by the computer.
These signals are used to determine the power of the fridge on a certain moment.
20
4.5 Measurements fridge
With this project there is been a lot of measuring to determine the behavior of the fridge.
To see what the influence is of products in a fridge and what the possibilities are for PCM’s.
Also to determine what PCM is most likely to use for storing energy in it and what the possibilities
are for PCM’s. In this chapter the most important measurements are shown and for each
measurement the results are evaluated.
4.5.1
Average temperature
To determine the average temperature in the fridge several measurements are done. One of the
measurements is shown below. The other measurements with the bottles of water will follow later
on.
Figure 4.5.1. Temperature process of an empty fridge.
With these measurements I have been able to determine the average temperature in the
compartment of the fridge under different circumstances. This shows that the average temperature
in the fridge is 4ºC. This fact indicates that the best PCM to use is a PCM with a phase change
temperature of 4ºC.
21
4.5.2
Influence products
The first series of measurements was to determine what the difference is between an empty fridge
and a fridge which contains some products as it normally does.
Figure 4.5.2. Temperature process in a fridge with water in the fridge on different levels.
As you can see in figure 4.5.2., adding eight bottles of water do have an effect on the timeline of the
fridge. The time that it takes to warm up increases from 36 minutes till 50 minutes.
This is only a difference of eleven minutes while the capacity of the fridge has become much higher.
The capacity of the water is:
vwater  12L
kg
water  1000
3
m
mwater  vwater  water  12kg
cv.water  4182
J
kg K
4 J
Cwater  mwater  cv.water  5.018  10 
K
So the total capacity of the compartment of the fridge would now be
50180
22
J
K
 2140
J
K
4 J
 5.232  10
K
This is 24 times more which means that the total amount of heat which has to leave the fridge is also
24 times more. Because: Q  C T
If the resistance of the fridge stays the same you would expect the time of warming up would also be
around twenty-four times longer.
To figure out why this is not 24 times longer one of the sensors is placed in a bottle with water. The
result of this can be seen in figure 4.5.3.
Figure 4.5.3. Temperature process of a fridge with a bottle of water with a sensor in the bottle.
Expected was that the temperature in the bottle would change its temperature the same way as the
temperature in the fridge. But as you can see the bottle of water is not changing temperature very
rapidly. So the energy that is given back to the compartment of the fridge is very little and thereby
the temperature in the fridge will not stay at a low temperature for a very longer period of time.
The reason for this is that there is a resistance between the bottle of water and the fridge.
Also the surface of the water is not so big compared with the amount of water.
So there has to be a very big temperature difference between the water and the fridge before a heat
current between the fridge and the bottle will occur.
4.5.3
Power
Another thing that was measured is the energy that is used by the fridge. Because the measurements
are by second, this energy is convertible to power.
The first thing that was noticed was that the amount of power the fridge uses is not the same for the
whole period that it is cooling. It appears that there is a strong relation between the energy
consumption of the fridge and the temperature difference between the element of the fridge and
the ambient. In the graphic on the next page you can see the result.
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Figure 4.5.4. Power use of the fridge for several cool down periods.
4.5.4
PCM
Because during the project I got to the discovery that the temperature of the cooling element of the
fridge becomes much lower than the compartment of the fridge and the PCM’s were still not
delivered, I had decided to use water on the back of the fridge. Because water is also a PCM with a
phase change temperature of 0°C. This measurement is shown below.
Figure 4.5.5. Measurement of a fridge with water as a PCM mounted on the back plate of the fridge.
This measurement is done to find out how the temperature in the fridge would behave when a PCM
is added. A total of 1 liter of water is used at the backside of the fridge. What you see is that the
water makes its phase change at 0°C every period. You can also see that the first phase change takes
much longer than the others. This means that after the first cool down the ice is not fully changing
back to water again during warm up. The temperature difference between the ice and the
compartment of the fridge is not big enough to get a heat flow from the ice at 0°C to the
compartment which is big enough to keep the compartment under the right temperature.
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To determine the temperature difference that is needed between the ice and the compartment of
the fridge to keep the temperature at a constant level I turned off the fridge at a certain moment to
measure this. The result of this measurement is shown below.
Figure 4.5.6. Warm-up curve of a fridge with water as a PCM mounted at the back of the fridge.
In this graphic you can see that the temperature in the fridge stays around 10ºC for a very long time.
Six hours to be precise.
This shows us that it is possible to keep the temperature in the fridge at a stable temperature for a
very long time with the use of PCM’s. The only problem with water is that the temperature at which
the phase change takes place is too high for the desired temperature at which food has to be stored
(7ºC). This graphic shows that a temperature difference of 10ºC is needed between the PCM and the
compartment to get the same heat flow from the PCM to the compartment as the heat flow from the
ambient to the compartment, with an ambient temperature of 20ºC.
Because we want a temperature around 5ºC or 6ºC, a PCM with a phase change temperature of -4ºC
or -5ºC is needed. However if we look at how people use their fridge at home you will see that most
households have their fridge on a level 4 of 6. This means the temperature in the fridge as we use it
in our houses will be higher as shown in figure 6.4.3.
Figure 4.5.7. Temperature in a fridge at level 4.
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Because I had to order the PCM in a much earlier state of the project I didn’t knew the cooling
element of the fridge was much colder then the compartment of the fridge at that point.
Therefore I have ordered the wrong PCM. Instead of -4ºC I had chosen +4ºC because this was the
average temperature in the fridge.
But with this PCM I am able to check if the above theory is correct.
If I do the same measurement with the PCM of +4ºC as I did with the ice (warming up the fridge). You
should see the same appearance only at a temperature that is about 4ºC higher than it was with the
ice. Figure 4.5.8. shows this.
Figure 4.5.8. Warm-up curve of a fridge with a PCM with a phase change temperature of 4°C
4.5.5
Energy consumption of the fridge
To determine the effect of PCM on the energy consumption of the fridge, A measurement is done
where the temperature of the fridge is set between two levels. This measurement showed that for
one cycle with PCM the fridge used 142,5Wh to cool it down. The total time between cooling down
and heating up was 11,33 hours.
The empty fridge needed 22,5Wh to get the temperature down. The total time for this cycle was
1,732 hours. This means that to keep the temperature between these levels for 11,33 hours the
energy consumption of the empty fridge would be.
11.33hr
1.732hr
 22.5W  hr  147.185 W  hr
This means that due to the PCM, the efficiency of the fridge became better with:
147.2W  hr  142.5W  hr
142.5W  hr
 3.298 %
This is because the temperature of the cooling element of the fridge is higher when a PCM is
attached. And thereby the COP (Coefficient Of Performance) of the fridge will increase because:
COP
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Tcold
Theat  Tcold
5 Vissim model
Because it’s easier and cheaper to see how the temperature in the fridge will behave when a PCM is
attached, a Vissim model is made to simulate the fridge.
With the measurements of the empty fridge al the values of the different components are calculated.
In this model mathematical solutions have to be determined for each component.
The simulation model can be divided into five separated parts.
These parts are:
- Turning on and off the fridge with a certain power.
- The heat flow of the cooling element.
- The heat flow of the PCM.
- The heat flow of the compartment.
- The heat flow of the products in the compartment.
If these parts are connected to each other they will make a complete simulation of the whole fridge.
5.1 Turning the fridge on and off
To simulate turning on and off of the fridge, the power measurement, mentioned earlier is used.
As seen in this measurement, the power used by the fridge is depending on the temperature
difference between the ambient temperature and the temperature of the cooling element of the
fridge. There is also a difference between the first cool down (when the fridge is at room
temperature) and the following sequences. Therefore I a formula is determined for both situations.
The amount of power is multiplied by the COP of the fridge to determine the power to the fridge in
terms of heat. The fridge is turned off when the temperature of the cooling element is beneath
-19,5°C and is turned back on when the temperature of the element gets above 3ºC.
5.2 Heat flow of the cooling element
The element of the fridge has a few heat flows [J/s]. The sum of all these heat-flows results in a
certain amount of heat which makes the temperature of the element changing.
The total heat flow of the cooling element of the fridge is depending on:
- The heat that is produced by the compressor. (entering the element)
- The losses from the element to the environment. (leaving the element)
- The heat flow from the element to the PCM. (leaving the element)
- The heat flow from the element to the compartment of the fridge. (leaving the element)
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Each of these heat flows have to be calculated separately. The amount of power from the
compressor is already mentioned. For the other heat flows the next formula is used:
 element.environment
T element.environment Surfaceelement
Relement.environment
This is the formula for the losses from the element to the environment but this same formula can be
used to calculate the other heat flows only with their own specific numbers.
The temperature change of the element at every moment depends on the amount of energy in heat,
entering or leaving the element at every moment [J] and the heat capacity of the element in [J/K]
which is calculated in chapter 5.
T element

 
element.total ( t) d t


capacity element
This is the temperature change at every moment. To get the temperature of the element itself, this
temperature change has to be subtracted from the begin temperature.
5.3 Heat flow PCM
For the PCM the same calculations can be used. With their own specific values.
With the heat flows from the element to the PCM and the heat flow from the PCM to the
compartment of the fridge.
The difference with the PCM is that at the phase change temperature the heat capacity of the
material is changing. This is the latent heat of the material.
I have done this by taking the phase change temperature and between +0,5°C en -0,5°C from the
phase change temperature, the capacity of the material changes to the value of latent heat. When
these limits are exceeded the capacity changes back to the normal value.
The capacity of the material can be calculated by the volume of the material and the specific heat of
the material.
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5.4 Heat flow compartment fridge and products.
Here are also the same calculations used to calculate the temperature.
For the compartment of the fridge there are four heat flows.
- From the element to the compartment of the fridge. (if the PCM isn’t covering the whole
backside of the fridge)
- From the PCM to the compartment of the fridge.
- From the compartment of the fridge to the products.
- From the compartment of the fridge to the environment.
For the products in the fridge there is actually only one heat flow, from the compartment of the
fridge to the product and back
Connecting all these heat flows together results in the Vissim model which can be found in
attachment 6.
5.5 Evaluation
To see how close the simulation corresponds to reality, some measurements are compared with the
simulation.
At first the period of warming up and cooling down of an empty fridge is compared.
If the fridge is empty, measurements show us that the time of warming up takes 36 minutes.
When we take out one period of the simulation you will see that the warm up time in the simulation
takes
17850-15750 = 2100sec which is 35 minutes. This gives us an accuracy of 97,22%
For cooling down, we measured a total time of 21,6 minutes. This model says 21,5 minutes which
gives an accuracy of more than 99%.
The pictures below show the measurement and de simulation for one period of the fridge.
Figure 5.5.1 Real measurement of an empty fridge and the simulation.
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The next picture is a simulation of the measurement with the sensor in the bottle of water.
The simulation is made over the same period of time as the measurement.
If you look back at the previous chapter you can compare the pictures yourself.
My conclusion is that they match perfectly.
Figure 5.5.2. Simulation of a fridge with a sensor in a bottle of water.
Last but not least a simulation of the measurement with the water as a PCM is made.
For this simulation also counts that it is done over de same period of time as the measurement in
chapter 4.
As you can see in the picture, the simulated temperature in the fridge behaves the same as with the
measurement. With the first cool down it get’s lower than with the cycles after that.
For the PCM you see that the first time the temperature drops less under 0ºC than the others.
Compared with the measurement this is correct. The only difference between this simulation and the
measurement is the temperature change of the PCM after its phase change. Unfortunately I haven’t
been able to find out what causes this. I even have thought about the change of heat capacity
between water and ice but even with that I get this result.
Beside this I can conclude that this model gives a good representation of the reality.
Figure 5.5.3. Fridge with PCM controlled by temperature cooling element.
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5.6 PCM of -4ºC
Because this simulation represents the fridge in a good way, we can use this simulation to see what
will happen in the fridge when we will add 1 liter of PCM with a phase change temperature of -4°C on
the backside of it. The first graphic shows what the fridge will do when the PCM is added to the
fridge and the fridge is controlled just the same way as it is normally.
Figure 5.6.1. Simulation of a fridge with a PCM of -4ºC, controlled by element temperature.
What we see in this picture is that the PCM is not changing its phase completely during cool down.
The temperature of the cooling element drops below -20ºC before the phase change is completed.
To overcome this problem the power of the fridge has to be regulated in a different way.
In the next simulation the power not regulated by the temperature of the cooling element but by the
temperature of the PCM. This way it is sure that the phase change has completed every time.
Figure 5.6.2. Simulation of a fridge with a PCM of -4°C, controlled by PCM temperature.
In this graphic we see the complete phase change. By controlling the temperature in the fridge at a
different way it is possible to keep the temperature in the fridge at a good level of 7°C for 6 hours if
we use 1 liter of PCM.
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6 Conclusion
Phase change materials can be used in a fridge to store energy in the form of heat/cold.
Depending on the amount of materials you use, you are able to scale up the process time for a very
long time. This project shows that even with 1.5 liters of phase change materials you are able to scale
this up to eight hours.
This project has only been performed with two phase change materials, water with a phase change
temperature of 0ºC and an organic solution with a phase change temperature of 4ºC. Water is able to
keep the temperature in the fridge at 9ºC to 10ºC for a long time. For the regulations regarding food
storage this temperature is too high. Although if we look at the practical side of this we will see that
households have their fridge often set on level 4 instead of level 6. Measurements to our fridge tells
us that the temperature in the fridge on this level is also moving at this temperature.
To meet the regulations on food storage it is advisable to do another measurement with the fridge
and make use of a phase change material with a phase change temperature of -4ºC or -5°.
This means an eutectic solution is the only compound to use for this application.
This material can be used the whole life time of the fridge without losing its capacity.
Due to the eight hours, it is possible to let a fridge cool down during the night time. In daytime the
fridge is able to keep the temperature at a good level without adding more energy.
If every household would have this principle, this can help with reducing the energy consumption
during daytime and thereby reduce the peak of energy.
Using PCM’s in a fridge will not only make it possible to shift in time, it also makes the fridge more
efficient.
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References
[1]
PCM products 2010
Homepage (http://www.pcmproducts.net) 15 February 2010.
[2]
Thermodynamics 2010 COP calculations
(http://en.wikipedia.org/wiki/Coefficient_of_performance) 05 May 2010.
[3]
Phase change 2010
Fase(http://wapedia.mobi/nl/Fase_(stof)) 21 April 2010.
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Attachment 1. Temperature range organic solutions
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Attachment 2. Temperature range eutectics
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Attachment 3. Measurement protocol.
To determine the effects of phase change materials (PCM), we need to determine in advance how a
refrigerator behaves in standard operating conditions. Within a household a refrigerator is filled with
products one day and might be empty the next day. The heat content (or cold content) thereby is a
variable and not constant. This is done to get a good reference for when the PCM’s are used.
The next measurements are chosen:
1. The fridge is completely empty.
2. Only the glass plates are in the fridge.
3. The bottom plate is filled with eight bottles of water each filled with 1,5 liters of water.
4. The middle plate is filled with eight bottles of water each filled with 1,5 liters of water.
5. The top plate is filled with eight bottles of water each filled with 1,5 liters of water.
6. The bottom and middle plate are both filled with four bottles of water each filled with 1,5
liters of water.
7. The middle and top plate are filled with four bottles of water each filled with 1,5 liters of
water.
8. The bottom and top plate are filled with four bottles of water each filled with 1,5 liters of
water.
These measurements are done without and later on with the PCM.
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Attachment 4. Datasheet LM35.
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Attachment 5. Calibration Amplifier.
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Attachment 6. Vissim model
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