The 14th International Symposium on District Heating and Cooling,
September 7th to September 9th, 2014, Stockholm, Sweden
EVALUATION OF VACUUM INSULATION PANELS USED IN HYBRID INSULATION
DISTRICT HEATING PIPES
1
1
A. Berge & B. Adl-Zarrabi
1
Chalmers University of Technology, Department of Civil and Environmental Engineering, Division of Building
Physics
ABSTRACT
It is of interest to lower the energy losses from district
heating pipes both for economic and environmental
reasons. This paper evaluates a hybrid insulation
solution where Vacuum Insulation Panels (VIP) are put
around the supply pipe in a district heating pipe and the
rest of the casing pipe is filled with polyurethane foam
(PUR).
work focus on a hybrid pipe concept where vacuum
insulation panels are used together with polyurethane
foam, as shown in Fig. 1. The concept is based on a
conventional pipe type where polyurethane foam fills
the cavity between a steel service pipe and a
polyethylene casing pipe.
The apparatus for the “guarded hot pipe” method was
used to estimate the thermal properties of single pipes,
which have been used as input in finite element models
for simulation of twin pipes in field. The simulations
indicate a total reduction in the energy loss between
18% and 32% compared to pipes of the same size with
pure PUR insulation. Furthermore, the losses from the
supply pipe decrease by up to 56%.
To achieve the low energy losses, the vacuum in the
panels has to be preserved over the life span of the
VIP. In field measurements, a hybrid pipe prototype
was connected to the district heating grid in Varberg
(southwest Sweden). After almost two years, the pipe
is still working without any detectable deterioration of
the insulation performance. The panels have also
been tested at high temperatures in laboratory with
promising results.
INTRODUCTION/PURPOSE
In 2012, 57% of the energy consumption in Swedish
buildings was distributed by district heating [1].Some of
the energy, fed into a district heating network is lost
due to heat transfer from the heated water to the
surrounding. For a sparser district heating grid or a grid
with lower energy outtake, the energy losses in the
distribution will be a proportionally larger part of the
input energy.
As a part of the research program “Värmegles”,
environmental impact of district heating in sparse
Swedish neighbourhoods were evaluated [2]. One of
the conclusions was that it is very important for the
environmental performance of sparse district heating
networks with a district heating system with small heat
losses.
The purpose of this work is to evaluate the possibilities
to reduce the thermal losses from district heating pipes
by the use of vacuum insulation panels. The presented
Fig. 1 Description of the hybrid insulation pipe concept. S
stands for supply pipe and R stands for return pipel.
STATE OF THE ART
The work with hybrid insulated district heating pipes
started investigating a variation of concepts for high
thermal performance district heating pipes [3]–[5]. The
project finished with the hybrid insulation concept
where a vacuum insulation panel was used as high
performance insulation close to the hot pipe. A
prototype was created and installed in field for
measurements and an initial estimation of the payback
time was 12 years [4].
The work continued in a new project focusing on hybrid
insulation concept with vacuum insulation panels
showing the progression of laboratory measurements
and field measurements [6]. This paper reports this
continuation and an up to date presentation of the
latest results for the laboratory measurements, the
numerical simulations and the field measurements.
The fundamental idea of the concept, shown in Fig. 1,
is that, in a cylindrical geometry, the effect of the
insulation is closely related to how close it is to the
centre of the cylinder. This means that a high
performance insulation close to the centre can have a
large impact on the heat flow out from the pipe. The
thermal conductivity of polyurethane foam, presented
by district heating pipe producers range from 23
mW/(m∙K) to 27 mW/(m∙K) dependent on the
The 14th International Symposium on District Heating and Cooling,
September 7th to September 9th, 2014, Stockholm, Sweden
production method [7]–[9]. These values can be
compared to measurements on vacuum insulation
panels where the thermal conductivity in the centre of
the panel range in between 2.5 mW/(m∙K) for glass
fibre and 7 mW/(m∙K) for polyurethane foam for
pressures below 0.1 mbar[10]. Commonly for long life
span applications, nano-porous materials like fumed
silica is used since the thermal conductivity of the
material is reduced already at higher pressures.
Measurements have shown a centre of panel
conductivity below 5 mW/(m∙K) already at pressures
below 10 mbar and still below 8 at pressures around
100 mbar [10].
To obtain the vacuum in the vacuum insulation panels,
the core material is enveloped by a diffusion barrier.
The diffusion barrier is commonly a metalized polymer
laminate. Thin layers of aluminium are alternating
layers of some organic polymer. This leads to a high
thermal conductance along the surface and through the
edges of the panels. Consequently, this creates an
optimization conflict since more aluminium in the
diffusion barrier prolong the life span of the panel but at
the same time it increases the thermal transport in the
edges of the panel [10].
The use of vacuum insulation for high temperature
applications has also been suggested for the use in
heat storage where the ratio between the insulation
volume and the thermal storage volume can be of
importance, especially for small storage tanks where
the surface to volume ratio is large [11]. No tests on the
long time performance was presented.
METHODS/METHODOLOGY
The work can be separated into three main parts;
laboratory measurements of thermal performance,
numerical simulations of heat transfer and field
measurements on prototype pipes connected to an
active district heating network.
Laboratory measurements
In the laboratory, an apparent thermal conductivity of
the hybrid pipes has been measured with the
methodology from EN 253 using a “guarded hot pipe”
apparatus [12]. The standard method gives the
conductivity for a homogenous insulation material in
the pipes. To obtain the apparent thermal conductivity
of the vacuum panel, the insulating effect of the
polyurethane have been back calculated from the
performance of the whole pipe according to Eq (1)
based on the equation for a 1 dimensional axisymmetrical heat flow [13].
(1)
where Q is the power input to the pipe in the “guarded
hot pipe” apparatus (W/m). λappVIP and λPUR are the
apparent thermal conductivity of the vacuum insulation
panels and the thermal conductivity of polyurethane
foam obtained from measurements on a reference pipe
produced with the same foam at the same time as the
test pipe (W/(m∙K)). The terms r1, r2 and r3 are the outer
radius of the steel pipe, the outer radius of the vacuum
insulation panel and the outer radius of the
polyurethane foam insulation (m). The terms T1 and T3
are the temperatures at the inner and outer surfaces of
the test pipe (°C). The terms are also described in the
pipe section in Fig. 2.
Fig. 2 Description of input for calculation of apparent
thermal conductivity.
The resulting total conductivity for the hybrid pipe, from
the “guarded hot pipe” measurements, can also be
seen as indication of the energy loss. The results can
be compared to polyurethane but the thermal
conductivity is only representative for hybrid pipes with
the same dimensions and material proportions.
The high temperature performance of the vacuum
panels has been tested by heating some panels in an
oven and continuously measuring their internal
pressure with a measurement device supplied by the
panel producer. The panels have been held in an oven
at a constant temperature of 70°C for almost a year.
The results from “guarded hot pipe” can be seen in Fig.
8
and the results from the high temperature
performance measurements can be seen in Fig. 16 in
the results chapter.
Numerical simulations in Comsol
Numerical simulations on thermal performance were
made in the finite element software Comsol 4.3b [14].
The pipes were modelled in 2 dimensions assuming
the flows along the pipe to be small because of
symmetry. Two cases were modelled; a model of a
single hybrid pipe in a laboratory setting comparable to
“guarded hot pipe” measurements and a model of a
twin pipe in field.
The results from the simulations of the single pipe in
the laboratory were used to estimate the properties of
the vacuum insulation panels in more detail. The
vacuum insulation panels introduce an extra complexity
to the pipe section geometry through the high thermal
conduction in the diffusion tight envelope of the panels,
shown as a dashed line in Fig. 3. To separate the heat
conduction through the envelope from the heat
conduction in the core of the vacuum insulation panels,
a simulation model was created with the boundary
conditions shown to the right image of Fig. 3 described
in Table 1.
The 14th International Symposium on District Heating and Cooling,
September 7th to September 9th, 2014, Stockholm, Sweden
The thermal conduction through the envelope was
simulated by using the results from the “guarded hot
pipe” measurements of the hybrid pipes. The thermal
conductivity of the core of the vacuum insulation panel
was obtained from the panel producer and the thermal
conductivity of the polyurethane foam was taken from
measurements on reference pipes. The properties of
the envelope were adjusted until the heat losses in the
simulations corresponded to the heat losses from the
“guarded hot pipe” measurements.
Fig. 4 Description of the boundary conditions material
properties in the finite element model of twin pipes in the
ground.
Fig. 3 The image describes the material input and
boundary conditions for the finite element model of single
pipes in the laboratory. The thick dashed line show the
thermal bridge in the diffusion tight envelope surrounding
the vacuum insulation panels.
Table 1 Description of materials and boundary conditions
for simulations of single pipes in laboratory.
Description
Set value
λi
Thermal conductivity of
polyurethane.
from
reference
λi+
Thermal conductivity in VIP core.
4.5 W/(m∙K)
Ti
Temperature on the inner surface
of the steel pipe.
~ 80°C
Temperature on the outer surface
of the casing pipe.
21°C
To
1
Table 2 Description of materials and boundary conditions
for simulations of twin pipes in ground.
Description
Set value
λi
Thermal conductivity of
polyurethane
26 mW/(m∙K)
λi+
Thermal conductivity of VIP
varies
λa
Thermal conductivity in ground
1.5 W/(m∙K)
Ts
Supply water temperature
85°C
Tr
Return water temperature
55°C
Ta
Ambient temperature
5°C
D
Domain size
16 m
H
Burying depth
0.8 m
1
From the producer of the vacuum insulation panel.
The simulation of twin pipes in ground was used to
estimate the reduction in heat losses from replacing
polyurethane foam with vacuum insulation panels for a
twin pipe in field. The input data and boundary
conditions are explained in Fig. 4 and Table 2.
In the twin pipe, the thermal bridge along the vacuum
insulation panel disturbs the symmetry. One point of
interest for implementation of the hybrid pipes is the
effect of an overlap of the panel to reduce the heat flow
through the thermal bridge by increasing its length.
This was modelled in the twin pipe model as described
in Fig. 5 which also show the definition of the thermal
bridge length.
Fig. 5 The definition of overlap length used in some of the
simulations. The thermal bridge length is defined as the
sum of the distances S1 and S2.
The results of the simulations are shown in Fig. 9 to
Fig. 12 in the results chapter.
Field measurements
In field, two hybrid insulation pipes have been
connected to the district heating network in Varberg, a
city on the southwest cost of Sweden. The district
heating network is a low temperature network with
maximum temperature in the supply pipe below 90°C.
In the pipes, thermocouples have been embedded into
the polyurethane foam and measure the temperature at
various positions in the pipe section. The temperatures
The 14th International Symposium on District Heating and Cooling,
September 7th to September 9th, 2014, Stockholm, Sweden
The first pipe of the two pipes has the dimensions
DN 2*80/250 and the placements of the thermocouples
are shown in Fig. 6. The second pipe has the
dimensions DN 2*25/140 and the placement of the
thermocouples are shown in Fig. 7.
The data from the field measurements have been
analysed to get an estimate of the long time
performance of the hybrid pipes.
Fig. 6 Thermocouple placement for the DN 2*80/250 field
measurement pipe. The uppercase S and R represent
supply and return. The lowercase u, s and d represent the
orientations up, side and down. VIP, PUR and PE
represent the outside of the vacuum insulation panel,
corresponding position in the Polyurethane foam and
measurements on the polyethylene casing pipe. The term
tb represents the thermal bridge along the panel.
Fig. 7 Thermocouple placement for the DN 2*25/140
field measurement pipe. The uppercase S and R
represent supply and return. The lowercase u, s and d
represents the orientation up, side and down. VIP
outside of the vacuum insulation panel and PUR
represent a reference part of the pipe with only
polyurethane foam. The terms tb-A and tb-E represents
the thermal bridges along the panel and at the panel
edge.
The results from the field measurements are shown in
Fig. 13 to Fig. 15 in the results chapter.
RESULTS
The results can be divided into two main parts; first an
analysis of the thermal performance of the hybrid pipes
and how they compare to regular polyurethane foam
pipes and secondly an analysis of the durability of
vacuum panels under district heating temperatures.
The results from the “guarded hot pipe” measurements
are shown in Fig. 8. For measurement number 1 in Fig.
8, two vacuum panels with a length of 0,5 m were used
in opposite to 1 m long panels which were used in all
the other measurements.
For measurements number 2-5 in Fig. 8, the panels
were the same as those used in the field
measurements. It is important to point out high
apparent conductivities of measurement number 2 and
number 5. For these two measurements, the vacuum
insulation panels have probably collapsed and have
become air filled. This illustrates a measurement
problem where there is a difficulty ensure the correct
location of the panels in the measurement pipes,
creating a risk of perforating the panels during sample
preparation. It is although important to see that the
apparent thermal conductivity of the vacuum insulation
panels are in the same order of magnitude as the
polyurethane foam, even if they are air filled.
Thermal conductivity [mW/m/K] or Thickness [mm]
have been measured every second hour since
instalment in January 2012.
40
35
30
VIP thickn. [mm]
λ.tot [mW/m/K]
λ.app.VIP* [mW/m/K]
*Calculated from measurements on corresponding reference pipes
25
20
15
10
5
0
¹Two 0.5 m long panels instead of one 1 m long.
²Two layers of 10 mm thick panels.
Fig. 8 Results from “guarded hot pipe” measurements of
hybrid pipes. The figure shows the vacuum insulation
panel thickness, the total thermal conductivity calculated
according to EN 253:2009 and the effective conductivity of
the vacuum insulation panels.
The 14th International Symposium on District Heating and Cooling,
September 7th to September 9th, 2014, Stockholm, Sweden
The last measurement, number 6 in Fig. 8, is made for
a pipe with a new improved type of vacuum insulation
panel. That is why the measurements on pipe 6 have
been used for the further analysis of the hybrid pipes.
The measured data from “guarded hot pipe”
measurement number 6 in Fig. 8 was used as input
data in the single pipe in laboratory model. The
simulations gave an estimated thermal conductivity of
14.3 W/(m∙K) for the envelope assuming an envelope
thickness of 0.1 mm. This data was used in the twin
pipe model to evaluate the effect of the overlap.
Pipes with a number of different dimensions between
DN25 and DN 150 and a 8 mm thick vacuum insulation
panel around the supply pipe were simulated in the
twin pipe in field model. For each dimension three
lengths on the overlap was modelled; 2 cm, 4 cm and 6
cm. A reference value for the pipes was also modelled,
without the vacuum insulation and polyurethane foam
in its place.
The results from the simulations are shown in Fig. 9 as
the total energy loss from the pipe and in Fig. 10 as the
reduction in energy loss compared a conventional pipe
with polyurethane foam insulation. The results show a
reduction of between 15% and 30% in the heat loss
due to the addition of vacuum insulation panels.
30
2 cm
4 cm
6 cm
2 cm
10
5
0
6 cm
30%
20%
10%
Fig. 10 The reduction in total energy losses when
simulated results for hybrid pipes are compared to the
result for a reference pipe with only polyurethane foam.
The pipes are twin pipes of different dimensions with a 8
mm vacuum insulation panel mounted around the supply
pipe.
In Fig. 10 it can be seen that for the small dimensions,
the total energy loss increase for an overlap of 6 cm
compared to a 4 cm overlap. This is due to a lower
temperature where the thermal bridge reaches the
polyurethane in the 4 cm case. While a longer overlap
always decrease the loss from the supply pipe, the
return pipe losses increases which makes the effect on
the total loss complicated to predict.
60%
20
15
4 cm
0%
total
Reduction in energy loss [%]
Total energy loss [W/m]
Ref.
25
40%
Reduction in energy loss [%]
For measurement number 3 in Fig. 8, two layers of 10
mm thick vacuum insulation panels were used. The
high apparent thermal conductivity indicates that one of
the panels might be damaged.
50%
supply
40%
30%
20%
10%
0%
Fig. 9 Simulated total energy loss from twin pipes of
different dimensions with a 8 mm vacuum insulation panel
mounted around the supply pipe. The different colours
represent different overlap in the vacuum insulation panel.
Fig. 11 A comparison between total losses and supply flow
losses for twin pipes with a 8 mm thick vacuum insulation
panel around the supply pipe.
The 14th International Symposium on District Heating and Cooling,
September 7th to September 9th, 2014, Stockholm, Sweden
The difference between the losses from the supply and
the return pipes are shown in Fig. 11 for pipes with a
2 cm overlap. The results show that the reduction in
heat losses from the supply pipe is almost twice the
reduction of the total heat loss from both the supply
and return pipes.
As the insulation effect is a combination of material
properties and material thickness, the simulation
results were compared to the results from simulations
of conventional polyurethane pipes where the diameter
was changed until it achieved the same thermal
performance. The resulting diameters are shown in Fig.
12, which shows that a large reduction in size can be
achieved, especially for small pipe dimensions. This
would mean that less ground have to be removed for
the pipe trenches and it would also make it easier to
install the pipes in narrow areas as in central parts of a
city.
Pipe diameter [mm]
400
100%
2 cm
4 cm
6 cm
2 cm %
4 cm %
6 cm %
300
200
50%
PUR and s-PUR) which shows that the heat loss are
smaller for this part of the pipe although the losses
cannot be quantified from the temperatures.
A similar result can be seen in Fig. 14 where the
temperature on the side of the vacuum insulation panel
(s-VIP) is shown together with the temperature
measured on the thermal bridge along the vacuum
insulation panel (tba) and the reference measurement
(s-PUR). The temperature in the thermal bridge is
higher than the temperature on the middle of the panel,
which is expected, but the temperature is lower than
the reference temperature. This indicates that the
vacuum insulation panel insulate better than the
polyurethane foam even in its weakest spot, the
thermal bridge.
For both pipe dimensions, DN 2*80/250 shown in Fig.
13 and DN 2*25/140 shown in Fig. 15, the
temperatures on the vacuum panels (u-VIP s-VIP and
d-VIP) seem to follow the variation in the temperatures
of the supply and return pipes (S and R).There is no
unique jump in temperature which would indicate
damage in a panel. Over the time frame of the
measurements, there is so far no visible slow
increment in the temperatures from the diffusion of air
through the envelope of the panels. This is a positive
result for the vacuum insulation panels as the life span
have been one of the main questions.
80
100
70
0%
DN 2*80/315
DN 2*80/250
DN 2*40/200
DN 2*40/160
DN 2*20/180
DN 2*20/140
Fig. 12 The diameter for the polyurethane pipe, required to
achieve the same thermal performance as the hybrid
pipes. The results are shown both as absolute value and
as relative values.
60
Temperature [°C]
0
50
40
30
20
10
The temperature measurements from the field, are
shown in Fig. 13 and Fig. 14 for the dimension DN
2*80/250, and in Fig. 15 for the dimension 2*25/140. All
shown temperatures are the weekly mean temperature
averaged from two or three measurement points at
each position.
In Fig. 13, the temperature on the outside of the
vacuum panels are shown together with corresponding
positions in the reference part of the pipe, with only
polyurethane foam, and the supply and return
temperatures of the heat carrier. The temperature on
the vacuum insulation panel (u-VIP and s-VIP) is
significantly lower than the reference temperatures (u-
u-PUR
R
u-VIP
s-PUR
s-VIP
0
2012-04
2013-04
2014-04
Fig. 13 Temperature measurements from the field
measurements on the pipe with dimensions DN 2*80/250.
The figure shows the temperature on the outside of the
vacuum insulation panels together with the temperatures
on corresponding positions in the polyurethane foam and
the return and supply water temperatures.
The 14th International Symposium on District Heating and Cooling,
September 7th to September 9th, 2014, Stockholm, Sweden
6
5
50
40
Pressure [mbar]
30
20
10
0
s-PUR
tba
s-VIP
2012-04
2013-04
2014-04
Fig. 14 Temperature measurements on the field
measurements pipe with dimensions DN 2*80/250. The
figure shows the temperature on the side of the vacuum
insulation panels together with corresponding position in
the polyurethane foam and the temperature at the position
of the thermal bridge along the vacuum insulation panel.
80
Temperature [°C]
103 -A
349 -B
434 -C
330 -D
1
0
Days
The results shown here are made for vacuum
insulation panels adapted to room temperature
applications.
If the demand of high temperature
applications would increase, the product development
would follow. This could give an improved high
temperature performance of the diffusion barrier.
60
50
40
For “guarded hot pipe” measurements there have been
some problems with preparing the samples without
damages to the vacuum insulation panels. Although,
the panels mounted for field are shown to be
undamaged.
30
F
s-VIP
d-VIP
R
u-VIP
0
2012-10
2
DISCUSSION
70
10
3
Fig. 16 Pressure measurements on vacuum insulation
panels put in an oven at 70°C.
90
20
4
13
15
19
21
23
33
75
119
125
127
129
198
289
313
Temperature [°C]
60
2013-07
2014-04
Fig. 15 Temperature measurements from the field
measurements on the pipe with dimensions DN 2*25/140.
The figure shows the temperature on the outside of the
vacuum insulation panels together with the return and
supply water temperatures.
The durability was also tested with the panels put in an
oven at 70°C. The results are shown in Fig. 16. Under
almost a year, the pressure have increased in average
around 1,5 mbar. Panel D collapsed after 119 days.
This is although contradicted by the field
measurements where all panels have survived with no
visible deterioration in the panels. This is an indication
that the protected position in the polyurethane foam
increases the vacuum insulation panel’s life time. Also,
in a pipe, the high temperature load will only affect one
side of the panel. In the oven both sides were affected.
The numerical simulations have been made in two
dimensions with the effect on the edge of the panel
baked into the panels overall performance. This is a
simplification but it should have a small influence on
the total performance since the edge thermal bridge
follow the geometry of the panel.
OUTLOOK
The evaluation of the hybrid insulation concept will
continue. More variations of the set-up of the vacuum
insulation panel will be tested.
The thickness of the vacuum insulation layer and the
orientation of the thermal bridges will be further studied
for optimization.
A method for relative measurements of the thermal
performance for twin pipes is under development. This
will be used to validate the indicated thermal
improvement seen in the simulations.
The durability of the vacuum insulation will also be
investigated further.
The 14th International Symposium on District Heating and Cooling,
September 7th to September 9th, 2014, Stockholm, Sweden
CONCLUSIONS
The numerical simulations show a reduction in between
15% to 30% on the total losses from a twin pipe when
an 8 mm vacuum insulation panel was added around
the supply pipe. The losses from the supply pipe were
reduced by more than 50%.
The field measurements show that the energy loss is
reduced by the application of vacuum insulation. More
important, there are no sudden large increases in the
temperatures measured on the surface of the vacuum
insulation panels, indicating a damaged panel, and so
far there are no sign of the slow increment of the
thermal conductivity of the vacuum insulation panels
due to diffusion of gas into the panel. The experiments
in an oven show a clear increment in the pressure but
the influence on the temperature loggings in field can
still not be seen.
These two conclusions show that the vacuum
insulation panels can be used to decrease the energy
losses from district heating distribution.
For single pipes with dimensions between DN 50 and
DN 100 we saw a possible decrease in the calculated
thermal conductivity from 26 mW/(m∙K) for pure
polyurethane to below 20 mW/(m∙K) for a hybrid
solution with around 10 mm vacuum insulation panel.
For the best case, a DN 50/140 pipe with 8 mm
vacuum insulation the calculated thermal conductivity
was as low as 17.5 mW/(m∙K). The calculated thermal
conductivity directly correlates to the energy losses
from the pipe, the improvement will be a little less in
field since the thermal resistance in the ground will be
the same for both cases, but the main part of the
thermal resistance is in the pipe insulation.
The simulations show that the effect of the overlap of
the vacuum insulation panel on the total energy losses
is complex. The effect has to be examined more in
detail to give good recommendations for optimization of
the production.
ACKNOWLEDGEMENT
This project have been made possible by funds from
svensk fjärrvärme AB through the research programme
fjärrsyn.
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