Available online at www.sciencedirect.com
Solar Energy 86 (2012) 2504–2514
www.elsevier.com/locate/solener
Field thermal performance of naturally ventilated solar roof
with PCM heat sink
Jan Kośny a,⇑, Kaushik Biswas b, William Miller b, Scott Kriner c
b
a
Fraunhofer Center for Sustainable Energy, Cambridge, MA, USA
Oak Ridge National Laboratory, 1 Bethel Valley Road, Oak Ridge, TN 37831, USA
c
Metal Construction Association, Glenview, IL, USA
Received 16 February 2012; received in revised form 13 May 2012; accepted 19 May 2012
Available online 20 June 2012
Communicated by: Associate Editor Ruzhu Wang
Abstract
For decades, residential and commercial roofs have been considered a prime location for installation of building integrated solar systems. In climatic conditions of East Tennessee, USA, an experimental solar roof was tested during 2009/2010, by a research team representing Metal Construction Association (MCA), and a consortium of building insulation companies, photovoltaic (PV) manufacturers,
and energy research centers. The main objective was to thermally evaluate a new roofing technology utilizing amorphous silicon PV laminates integrated with the metal roof panels. In order to mitigate thermal bridging and reduce roof-generated thermal loads, this novel
roof/attic assembly contained a phase change material (PCM) heat sink, a ventilated air cavity over the roof deck, and thermal insulation
with an integrated reflective surface. During winter, the experimental roof was expected to work as a passive solar collector storing solar
heat absorbed during the day, and increasing overall attic air temperature during the night. During summer, the PCM was expected to
act as a heat sink, reducing the heat gained by the attic and consequently, lowering the building cooling-loads.
In this paper, field thermal performance data of the experimental PV-PCM roof/attic system are presented and discussed. Performance of the PV-PCM roof/attic is evaluated by comparing it to a control asphalt shingle roof. The test results showed about 30% heating and 50% cooling load reductions are possible with the experimental roof configuration.
Ó 2012 Elsevier Ltd. All rights reserved.
Keywords: Building energy; Phase change material; Thermal insulation; Building-integrated PV; Metal roofs
1. Introduction
This article describes a new solar roof technology utilizing metal roof panels with integrated amorphous silicon
photovoltaic (PV) laminates, a ventilated air cavity, dense
fiberglass over-the-deck insulation, and phase change
material (PCM) heat sink. During the last decade, computer modeling and a number of lab-scale and field experiments led to a new paradigm for designing thermally
active building envelopes (Tyagi et al., 2011; Zhou et al.,
⇑ Corresponding author.
E-mail address: jkosny@fraunhofer.org (J. Kośny).
0038-092X/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.solener.2012.05.020
2012). Since roofs and attics are subjected to greater temperature extremes than any other component of the building, a large number of studies have been conducted with a
focus on novel material configurations containing coolroof surfaces, PCM heat storage, reflective insulations,
and attic/roof ventilation (Levins and Karnitz, 1987;
Desjarlais and Yarbrough, 1991; Parker and Sherwin,
1998; Petrie et al., 1998; Medina, 2000; Miller and Kosny,
2007; Kosny et al., 2007, 2011).
Venting of attic spaces and its effect on heat transmission, moisture, and condensation were studied often in
conjunction with an application of radiant barriers and
reflective surfaces. Roof and attic radiant barriers (RBs)
J. Kośny et al. / Solar Energy 86 (2012) 2504–2514
present a different way of increasing the thermal performance of existing or to-be-installed insulation within the
space between roofs and ceilings of buildings (e.g., attic
spaces in residential buildings or the space between roofs
and suspended ceilings in commercial buildings).
Roof and attic RBs are proven technologies that significantly reduce the radiation exchange across roof cavities
and attic spaces (Yarbrough, 1983, 1990; Wilkes, 1991;
Medina, 2010). This reduction in radiation heat transfer
lowers heat flows across the ceilings of buildings that translate into lower space cooling and heating thermal loads.
Radiant barriers and reflective insulation facings are mainly
available as aluminum foil laminates or aluminized synthetic films sheets. These foils are most-often laminated to
paper, synthetic films, oriented strand board (OSB), or plywood. They are characterized by having at least one low
emittance surface with emissivity of 0.1 or less (ASTM C
1313, 2010).
Little has been studied with regard to the venting and
flow patterns observed in inclined channel created by tile
roofs. Rose (1995) and Romero and Brenner (1998) give
an overview of the evolution of attic venting. Parker
et al. (1995) studied heat transfer through direct-nailed
and counter-batten tile roofs compared to direct-nailed
asphalt shingles. The counter-batten roof in general
reduced heat transmission by almost 50%.
For centuries conventional thermal mass played an
important role in building energy conservation. Today,
advanced latent heat storage materials are being integrated
into building envelope materials and components. Due to
high temperature fluctuations generated by the solar loads
on the roof surface, a great variety of novel roof and attic
components are used today, which can store heat during
times of peak power operation and release the same during
reduced power operation. Phase change materials and radiant barriers are most commonly used for this purpose
(Miller et al., 2006; Kosny et al., 2007; Alawadhi and
Alqallaf, 2011; Ravikumar and Srinivasan, 2012). Some
of these new roofing configurations also contained solar
thermal systems (Saman and Halawa, 2005; Eicker and
Dalibard, 2011). One of the first experimental metal roofs
containing a building-integrated photovoltaic system and
a PCM heat sink was tested in Oak Ridge, Tennessee,
USA during 2009–2011 (Kosny et al., 2011). Standing
seam, cool-painted metal roofing and amorphous silicon
PV laminates were used in this test assembly.
This paper summarizes field test data for the experimental solar roof/attic containing PCM heat sink. A traditional
shingle roof/attic was used as a control assembly to compare and evaluate the experimental roofing technology.
Both test attics incorporated ridge and soffit vents. It was
expected that increasing the thermal capacitance of the roof
or attic, combined with the application of a thin layer of
thermal insulation on the roof deck, would reduce diurnal
temperature swings, and, in turn, reduce both the total
energy use and peak demand in moderate and cooling-dominant climates. Further details of the above attic assemblies
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are provided by Miller et al. (2006), Kosny et al. (2007)
and Kosny et al. (2011). The testing was conducted in
Oak Ridge, Tennessee, which falls under climate zone 4
(mixed-humid) according to the International Energy Conservation Code (IECC, 2009).
2. Novel designs of dynamic roof assemblies
One of the first experimental metal roofs containing a
PCM heat sink was tested during 2006/2007. Standing
seam, cool-painted metal roofing was used in this test
assembly. In comparison to a conventional shingle roof,
it eliminated about 90% of attic heat gains crossing the roof
deck during peak solar irradiance (Miller and Kosny,
2007). This roof design used 10 cm air channels to exhaust
excess heat during peak irradiance (sub-venting). Two
reflective membranes were placed above the roof sheathing
with the low-emittance surfaces facing each other across
the 10 cm air gap. The PCM heat storage was placed above
the roof deck but below the reflective foil. A multilayer
configuration of PCM-enhanced polyurethane foams and
PCM-impregnated fabrics, combined with highly reflective
aluminum foil was used in this project. Organic PCM loading was about 0.39 kg/m2 of the surface area. Two types of
PCMs were utilized. Their melting temperatures were
around 26 °C and 32 °C. The total storage capacity of
the PCM was about 54 kJ/m2 of the roof area.
During July 2007, a new test roof was constructed. This
roof contained an increased amount of inorganic PCM
with a storage capacity (650 kJ/m2) 12 times higher than
the previous test roof and improved configuration of the
above-sheathing ventilation channels. A multilayer configuration of reflective insulations was used to create an additional air space of 5 cm adjacent to the roof deck, which
had two radiant barriers facing each other to negate almost
all the radiation heat transfer crossing the air space. The
PCM (calcium chloride hydrate) was contained in sealed
aluminum pouches. Similar to the earlier design, this roof
also eliminated over 90% of the attic peak hour cooling
loads (Kosny et al., 2010). However, just a few weeks after
the start of this experiment, aluminum pouches began to
leak PCM and the experiment had to be terminated.
Finally, a third “dynamic” roof was installed during
September–October, 2009 by a research team with members from the Metal Construction Association (MCA),
CertainTeed Corp., Uni-Solar, Phase Change Energy Solutions and Oak Ridge National Laboratory (ORNL). MCA
is a North American trade association of metal building
manufacturers, builders, and material suppliers; CertainTeed is a manufacturer of thermal insulation and building
envelope materials; Uni-Solar produced the amorphous silicon photovoltaic (PV) laminates used in the test roof; and
Phase Change Energy Solutions manufactured the PCM.
Standing seam metal roofing was used for this test assembly as well. The amorphous silicon PV laminates were
integrated with the roof panels. In general, material configuration of this roof was very similar to the previous
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J. Kośny et al. / Solar Energy 86 (2012) 2504–2514
designs. Organic PCM with a melting temperature of
around 32 °C was utilized in this experiment. The nominal
heat storage capacity of the PCM packed in the plastic film
pouches was about 560 kJ/m2 of the roof area.
anells
M l roo
Metal
of pa
cavitty
A c
Air
s
glass
Fib
berg
S
Shingles
s
3. Description of the experimental roofs
A “dynamic” roof containing PV laminates and PCM
heat storage was constructed during September–October,
2009. As stated above, it was a continuation of earlier analytical and experimental efforts focused on the development
of a new generation of dynamic attic and roofing systems
using metal roof panels. Fig. 1 shows the experimental
roofs/attics. On the PV-PCM roof, three metal panels with
pre-installed amorphous silicon PV laminates can be seen.
Thermal performance of the experimental solar roof/attic
containing PCM heat sink was compared against the traditional shingle roof/attic that was used as a control.
Both experimental roofs were identical except the roofdeck area of the PV-PCM roof, where the PCM heat sink
and fiberglass insulation with reflective facing were
installed. Fig. 2 shows the cross section of the control attic
using conventional shingles and a material configuration of
the PV-PCM roof-deck area. The placement of sensors was
similar in both test attics. The PV-PCM attic had additional temperature sensors within the roof deck assembly.
The PV-PCM roof consisted of a layer of macro-packaged bio-based PCM placed on an oriented strand board
(OSB) roof deck, followed by a 2.5 cm thick layer of dense
fiberglass insulation with a reflective surface facing the
metal panels above, and finally, metal panels with preinstalled PV laminates on top. The metal panels were
placed on top of metal sub-purlins that provided an air
gap of about 5.1 cm over the fiberglass insulation; the air
gap is vented both at the ridge and the eave – Figs. 1 and 2.
In order to compare energy performance of test assemblies, both experimental roof decks had 5.1 cm-square by
0.5 cm deep routed slots with a heat flux transducer
(HFT) inserted to measure the heat flow crossing the deck.
Each HFT was placed in a guard made of the same OSB
material used in construction. HFTs were calibrated using
a Heat Flow Meter Apparatus to correct for shunting
effects (i.e. distortion due to three-dimensional heat flow).
As depicted in Fig. 3, attic cavities are also instrumented
to measure temperatures and the heat flow through the ceiling into the conditioned space. The local solar and sky
PV
V-PC
CM R
Roo
of
Co
hing
gle Rooff
onv
ventiiona
al Sh
CM
PC
Heat Flux Transducer
OSB
1.6--cm
Tar pap
per
1.3
3-cm
m
2.5
5-cm
m
Fig. 2. Construction details of tested roofs.
Fig. 3. Location of the PCM heat sink directly on top of the roofing deck
material with air channels above and below the fiberglass insulation.
radiation are measured using spectral pyranometers and
radiometers.
Two configurations of the attic floor insulation were
tested during the time period considered for this paper:
1. Winter 2009/2010 testing was performed with a 3.8-cm
thick layer of wood fiber board – as depicted in Fig. 2.
Thermal conductivity of this wood fiber product was
about 0.05 W/m2 K, which yields a thermal resistance
(R-value) of 0.73 m2 K/W.
2. During the summer of 2010, experimental attics were
tested with fiberglass batt insulation with R-value of
RSI – 6.7 m2 K/W installed on the attic floor, giving a
total attic floor (ceiling) resistance of RSI – 7.2 m2 K/W.
The PV laminates were PVL-144 models from UniSolar. In building integrated applications, the relatively
Fig. 1. Shingle roof and photo-voltaic (PV)-phase change material (PCM) roof – on the left; open air cavity in the PV-PCM roof – on the right side.
J. Kośny et al. / Solar Energy 86 (2012) 2504–2514
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high solar absorption of amorphous silicon laminates can
be utilized during winter for solar heating purposes with
PCM providing the necessary heat storage capacity. Conversely, PV laminates can generate increased cooling loads
during the summer. In the tested PV-PCM roof, the PCM
heat sink was expected to offset the thermal effects of the
PV laminates and reduce summer cooling loads.
4. Construction details and thermal characteristics
As shown in Figs. 2 and 3, a 1.9 cm thick layer of macropackaged bio-based PCM was placed on an the oriented
strand board (OSB) roof deck. An organic PCM was
packed in arrays of plastic pouches/cells 4.4 cm 4.4 cm 1.3 cm with 1.3 cm spacing (air pockets in plastic
cells represented about 20% of the total volume). It was followed by a 3.4 cm thick layer of dense fiberglass insulation
with a reflective surface facing the metal panels above, and
finally, metal panels with pre-installed PV laminates were
attached on top (Kosny et al., 2011). Bio-based, organic
PCM was utilized in this experiment. The nominal heat
storage capacity of the PCM was about 560 kJ/m2 of the
roof area. The PCM had nominal melting and freezing
temperatures of about 30 °C and 26 °C, respectively. Melting phase transition enthalpy of the PCM was about 190 J/
g. As shown in Fig. 3, PCM was macro-packaged between
two layers of heavy-duty plastic foil forming arrays of
PCM cells. The fiberglass insulation had a nominal thermal
resistance of 0.76 m2 K/W. Air cavities between PCM cells
and air gap above the fiberglass insulation provided abovesheathing ventilation to help reduce attic-generated cooling
loads during summer. Due to the strong three-dimensional
character of the heat transfer within the PCM heat sink, a
conventional DSC testing had to be performed together
with transient simulations during the thermal analysis. A
numerical–experimental methodology was developed during an earlier project for a similar thermal storage system
with a melting temperature of 30 °C (Kosny et al., 2009,
2010).
During 2009, a series of differential scanning calorimeter
(DSC) tests were performed on PCM samples used in the
field experiment (Kosny et al., 2011). A Seiko DSC220
was programmed to heat from approximately 0 °C to
40 °C and then cool from 40 °C to 0 °C. Two different heating/cooling rates of 0.3 and 1.0 °C per min were used for
the DSC tests.
As presented in Fig. 4, results of the DSC test with a
heating rate of 0.3 °C per min. indicated that the threshold
temperatures to initiate melting and freezing were 25.7 °C
and 27.2 °C, respectively. Temperature phase transition
range, including both melting and freezing processes, for
this PCM was from 24.0 °C to 31.8 °C.
Table 1 summarizes the results of DSC testing for both
heating rates. It can be observed that for the lower heating
rate, the phase transition range is narrower with melting
and freezing points that are closer to each other. The
relationship between measured melting and freezing
Fig. 4. Thermal characteristics of PCM measured during the DSC testing
with a heating rate of 0.3 °C per min.
Table 1
Results of DSC testing for PCM used in field experiments – detailed DSC
results for 0.3 °C heating rate are depicted in Fig. 4.
Property
Heating/cooling rate
0.3 °C per min
Heating/cooling rate
1.0 °C per min
Melt point (°C)
Melt energy (kJ/kg)
Freeze point (°C)
Freeze energy (kJ/kg)
30
185
26
181
31
190
24
186
temperatures for different heating rates is well-known
(Günther et al., 2009; Mehling and Cabeza, 2008; Castellón
et al., 2008). Generally speaking, it can be assumed that
lower the heating/cooling rate during the DSC testing,
the narrower the measured temperature range of the phase
transition. It is important to notice that in most reported
DSC measurements, the temperature change rates were significantly faster than natural processes taking place in
building envelope applications (Günther et al., 2009; Mehling and Cabeza, 2008; Castellón et al., 2008). In addition,
the DSC sample sizes are too small to correlate well with
the conditions encountered in the practice. That is why it
can be assumed that in case of the PCM application analyzed in this paper, the threshold temperature to initiate
melting will most-likely be lower than 25.7 °C – the temperature recorded during the DSC test with heating/cooling
rates of 0.3 °C per min. Similarly, in real conditions, the
freezing threshold temperature may be even higher than
27.2 °C.
4.1. Impact of material configuration on the performance of
the PV-PCM attic
Current practice bases roof thermal design on steadystate resistance (R-value). However, the day-to-day change
in weather causes varying component loads that, through
proper design, can be exploited to enhance the thermal performance of the envelope. Conventional attic design with
soffit and ridge ventilation is an example of a working
dynamic thermal system, where the ventilation air redirects
some of the heat emanating from the roof deck away from
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the insulation on the attic floor. The attic floor insulation
works against the internal attic air temperature instead of
the dynamic temperatures observed on the roof surface.
In general, benefits of the advanced attic thermal systems
can be listed as follows:
1. Effective reduction of roof solar loads,
2. heat conduction break between the attic floor and the
roof deck,
3. reduction of nocturnal cooling effects,
4. stratification of the attic air and addition of thermal
resistance to attic insulation,
5. shifting of attic-generated peak-hour thermal loads.
The PV-PCM roof contains, in addition to the PCM
heat storage, a layer of thermal insulation and an air cavity
with a reflective surface, located just over the roof deck. All
of these significantly change overall roof/attic thermal performance, reducing peak-time thermal excitations and
delaying the time when thermal peaks occur within the attic
space.
5. Experimental results and discussion
In the following sections, experimental data collected
from the test roofs/attics during the winter of 2009, and
spring, summer and early fall of 2010 are presented. Particularly, thermal performance of the roof-deck area and
overall attic thermal performance are presented and discussed. The data sampling occurred every 15 s and they
were averaged over 15 min before being recorded by the
data acquisition system. The data collection period for this
paper extends from November, 2009 to October, 2010.
5.1. Roof top temperature profiles
The PV laminates and shingle roof analyzed in this
paper had infrared emissivities of 0.67 and 0.91, respectively, which influenced the night-time radiation heat
losses. Fig. 5 shows 24-h variations of roof surface temperatures and outside ambient temperatures over typical winter and summer days, January 9, 2010 and July 15, 2010.
No significant temperature differences were seen between
the two roofs.
A detailed numerical and experimental analysis of a similar assembly containing dense fiberglass and an array of
the PCM pouches was performed with the use of a finite
difference computer code and dynamic heat flow meter
apparatus (DHFMAs) testing (Kosny et al., 2010). This
research indicated that due to the relatively-low thermal
conductivity of the frozen PCM, the temperature difference
between the top and the bottom of the heat sink can be
noticeable. As shown in Fig. 6, this fact was confirmed
by field test data recorded during winter. During summer,
however, when PCM was melted, its higher thermal conductivity yielded a relatively low temperature difference
Fig. 5. Daily roof surface temperature variations.
across the PCM pouches – see Fig. 7. This can be considered as an indicator that PCM is fully melted.
In addition, the data presented in Figs. 6 and 7 show
that during both heating and cooling seasons, a layer of
the dense fiberglass, located on the top of the PCM, significantly reduced thermal excitations generated by the roof
surface. This fact makes it possible for the PCM to work
in more optimum conditions without being overwhelmed
by high energy fluctuations, which take place closer to
the roof surface.
5.2. Melting and freezing processes within the PCM heat
sink
Since roofs and attics experience large temperature fluctuations, they are attractive candidates for applications of
the PCM systems. The main goal is to reduce those fluctuations and, hence, the attic/roof-generated thermal loads to
the conditioned space.
To evaluate the influence of the PCM on the attic behavior under field conditions, local heat fluxes and temperatures above and below the PCM layer were recorded and
analyzed. This analysis showed that the following three
forms of PCM phase transition can occur during the cooling season within the PV-PCM roof deck area:
1. Top of the PCM sink is melting and freezing, the bottom
part is frozen,
2. whole volume of PCM is going through the phase transition, and
J. Kośny et al. / Solar Energy 86 (2012) 2504–2514
2509
Fig. 6. Temperature distribution across the PCM heat sink measured during winter.
Fig. 7. Temperature distribution across the heat sink with melted PCM measured during summer.
3. the bottom part of PCM is melted, while the upper part
of the PCM heat sink is going through phase transition.
Aforementioned phase change processes within the heat
sink are easily traceable. Each of these thermal progressions takes place during a different time period and each
of them leaves a different footprint on the recorded heat
flux profiles and temperature distributions. Figs. 8 and 9
present an example of the phase change process when the
PCM is initially fully melted. It can be observed that
PCM subcooling took place at just over 27 °C. In Fig. 8
the nucleation temperature is detected by sharp rises in
the “bottom of PCM” and “top of PCM” sensors’ signals,
This means that in field conditions (when thermal processes
during the night are very slow) the PCM used in this experiment could still freeze in temperatures over 27 °C. Intense
melting of the PCM occurred over 30 °C, which confirms
the DSC data depicted in Fig. 4.
Fig. 9 shows a characteristic heat discharge process,
which took place during the night and early morning. It
started with subcooling of the PCM, which can be observed
on the top and bottom layers of the PCM on Figs. 8 and 9
(98th hour mark). The PCM discharging processes and
Fig. 8. Temperature distribution across the heat sink during the PCM
phase change process – as measured during two mid-summer days.
characteristic foot prints of the subcooling are the most
easily identifiable evidences of the phase change process.
Test data presented in Fig. 9 demonstrates that, for midsummer Tennessee weather conditions, it is possible to
reduce initial peak-hour cooling loads generated by the
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J. Kośny et al. / Solar Energy 86 (2012) 2504–2514
Fig. 9. Heat fluxes generated on the level of the roof deck – as measured in
two test roofs during two mid-summer days.
roof, by about 90%. This is a confirmation of results from
earlier studies using similar configuration of the PCM heat
sink (Kosny et al., 2007; Miller et al., 2007; Kosny et al.,
2010). Additional findings from this part of the analysis
finds an approximate 19 h lag time in the transmission of
the peak-hour load by the PCM heat sink – as shown in
Fig. 9. It can be also observed that during the mid-summer
(when all PCM was going through the phase transition),
the tested PCM roof system had two heat flux peaks. The
first one can be observed about 2–4 h after the heat flux
peak on the conventional roof and the second final one
at about 104th hour, when PCM started melting.
Fig. 10 shows the variation of the weekly maximum and
minimum PCM layer temperatures. The DSC-generated
threshold temperatures (Fig. 4) at which phase changes –
melting and freezing – are initiated, are also shown for comparison. The weekly maximum temperatures indicate that,
for several weeks during the winter (November 20, 2009
to March 19th 2010), the PCM did not melt. These periods
are indicated by “No PCM Cycling” in the figure. Conversely, the weekly minima indicate that during the peak
summer days (July 2010), the PCM was not always able
Fig. 10. Weekly maximum and minimum PCM surface temperatures.
to freeze. A detailed count of days when the phase change
transition took place is presented in Fig. 11. As stated
above, (i) sharp morning-time rises of either the “bottom
of PCM” or “top of PCM” temperatures, (ii) significant
temperature differences between these two locations, (iii)
heat flux peaks observed about 12–20 h after the heat flux
peaks on the conventional roof, and (iv) characteristic foot
prints of the subcooling, are the most noticeable evidences
of the phase change processes. If carefully analyzed, they
also enabled identification of which PCM layer was going
through the phase transition and which one was not. For
example, a single sharp rise, recorded for the “top of PCM”
temperature in the middle of the night, indicates subcooling
process and that only a top layer of the PCM is starting to
freeze. Similarly, a relatively rapid rise of the “top of PCM”
temperature, observed in the middle of the day, indicates
that top part of the PCM just melted and will follow top
of the roof temperature changes.
Based on the above analysis, it can be observed that
between April and October of 2010, phase change process
took place during about 2/3rd of the total number of days
– see Fig. 11. There was not a single week without at least
one day when PCM was cycling. However, during the midsummer days, the freezing ability of the PCM was compromised by high temperatures during the night. For better
thermal performance, it might be beneficial to use PCM
of a slightly higher freezing threshold temperature. This
would reduce the number of days with phase transition
during the shoulder seasons, but will improve conditions
for the full phase change processes during the mid-summer
days. Detailed thermal modeling is necessary to evaluate
this option.
5.3. Attic thermal performance during winter
The performance evaluation period was divided into the
winter–spring and spring–summer–fall periods. During
winter, PV-PCM roof behavior was expected to be different
than during summer, due to the lack of thermal mass effects
associated with the phase transition. Fig. 12 illustrates the
weekly average attic center temperatures during the period
defined as winter–spring, between November 13, 2009 and
March 11, 2010.
The week starting on March 12 is deemed an appropriate partition date for the two periods because of the addition of attic floor insulation on March 9. On average, the
PV-PCM attic remained warmer than the shingle attic by
4.6 °C, as seen in the bar chart on the right. As shown earlier in Fig. 10, the PCM apparently did not undergo any
phase changes during the heating season. Therefore, the
reduction in daily temperature fluctuations and increased
attic air temperature are combined effects of the fiberglass
insulation with reflective surface, two air cavities, and frozen PCM – acting as an additional insulation layer. During
this time, the weekly minimum PV-PCM attic air temperatures were higher on average by 8.9 °C when compared to
the shingle attic.
2511
7
6
5
4
3
2
top melting/bottom frozen
all layers cycling
01-Oct-10
08-Oct-10
24-Sep-10
17-Sep-10
03-Sep-10
10-Sep-10
27-Aug-10
20-Aug-10
13-Aug-10
30-JulJ l 10
06-Aug-10
23
J l 10
23-Jul-10
09 Jul 10
09-Jul-
16-Jul16
J l 10
02-Jul-10
02-Jul-
25
Jun
25-Jun-10
11-Jun-1
J
0
18 Jun
18-Jun-10
04-Jun-10
21-May-10
28-May-10
y
14-May-10
30-Apr-10
07-May-10
16-Apr-10
23-Apr-10
0
02-Apr-10
1
09-Apr-10
Number of days with phase
transition per week
J. Kośny et al. / Solar Energy 86 (2012) 2504–2514
top freezing/bottom melted
Fig. 11. Weekly count of days when phase transition took place in the PCM heat sink.
Fig. 12. Average weekly attic center temperatures during winter–spring period with no attic floor insulation.
Fig. 13 shows the average weekly heat flow into the attic
from the conditioned space below, between November 13,
2009 and March 11, 2010. The heat flow from the conditioned space to the attic represents the attic-generated heating load. The PV-PCM attic performed significantly better
than the shingle attic by reducing the heating load by about
30%. The PV-PCM and shingle attics generated average
heating loads of 9.55 W/m2 and 13.54 W/m2, respectively.
It needs to be noted that, during the winter months, the
shingle attic added some heat to the conditioned space, presumably during the sunny days, while the PV-PCM attic
essentially eliminated any heat addition.
Fig. 13. Average weekly attic-generated heating loads during winter–spring period (with no attic floor insulation).
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J. Kośny et al. / Solar Energy 86 (2012) 2504–2514
5.4. Attic thermal performance during summer and the
shoulder months
During the cooling season, significant differences in
maximum PV-PCM and shingle attic temperatures were
observed. As seen in Fig. 14, the temperature band between
the weekly attic center maximum and minimum temperatures in the PV-PCM attic was notably narrower than the
asphalt shingle attic. Combined air cavity, thermal insulation and PCM in the roof deck area mitigated daily temperature fluctuations in the PV-PCM attic. As a result, the
maximum weekly shingle attic center temperatures were
about 10.5 °C higher than the PV-PCM attic during this
period (bar chart in Fig. 14). During the shoulder months
and early- and late-summer, the attic center temperatures
for the PV-PCM roof were almost identical to the ambient
air temperatures. It should again be noted that the PCM
underwent a significant number of phase change cycles during almost each week during this period – see Fig. 11. During the mid-summer weeks, however, the attic center
temperature, in the case of the PV-PCM roof, was slightly
higher than the ambient air. This is a consequence of the
combined impacts of the following three factors: (i) too
low thermal resistance of the insulation material located
directly on top of the PCM, (ii) insufficient heat storage
capacity of the PCM heat sink, or (iii) too low freezing
threshold temperature of the PCM. The corresponding
minimum weekly attic center temperatures for the shingle
roof were, on average, about 6.8 °C lower than the PVPCM roof.
Fig. 15 shows the daily heat flux variation between the
attics and the conditioned space below. The sign convention for the heat transfer direction is such that the heat flow
from the attics to the conditioned space is positive. Thus,
positive values represent attic-generated cooling loads (heat
gain by the conditioned space). The effect of reduced attic
temperature fluctuations and peak load shifts due to the
PV-PCM roof is seen in the recorded heat flux data as well.
Fig. 15. Daily heat fluxes between attic and conditioned space.
On average, when phase transition of PCM was not occurring, there occurred an approximate 2–4 h peak load shift
in the PV-PCM attic.
Fig. 16 shows the weekly average heat flows into the
conditioned space from the attic between April 23 and September 17, 2010. The average cooling loads during this period were 0.80 W/m2 and 1.76 W/m2 for the PV-PCM and
shingle attics, a 55% reduction for the PV-PCM roof compared to the shingle roof.
Based on above discussions, during the cooling season
of 2010, for about 2/3rd of the total number of days, the
PCM heat sink behaved in the following three ways: (i)
the top of the PCM sink was cycling, while the bottom part
was frozen, (ii) the whole volume of PCM was going
through phase transition, or (iii) the bottom part of the
PCM was melted, while the upper part of the PCM sink
was going through the phase transitions.
It is important to remember that thermal performance
of the tested PV-PCM roof represents the combined effects
of all components of the roof deck area. That is why, in
order to evaluate the real impact of the thermal insulation
with reflective surface and air cavities on the cooling load
Fig. 14. Maximum and minimum weekly attic center temperatures recorded during the spring–summer–fall period with RSI – 6.7 attic floor insulation.
J. Kośny et al. / Solar Energy 86 (2012) 2504–2514
2513
Fig. 16. Average weekly attic-generated cooling loads measured during the spring–summer–fall time period (with RSI – 6.7 attic floor insulation).
reductions generated by the PV-PCM roof, cooling load
reductions were computed for two weeks starting July 23
and July 30, 2010. As shown in Fig. 11, during these two
weeks the PCM stayed melted for most of the time and
only limited cycling of the top layer took place during a single day each week. So, it can be assumed that almost no
dynamic thermal contribution from the PCM took place.
This happened due to relatively high night temperatures
preventing the PCM from freezing (see Fig. 10). Based on
these calculations, cooling loads generated by the PVPCM roof were 25% and 27% lower for the weeks of July
23rd and 30th, respectively – in comparison to the conventional shingle roof. These numbers are consistent with the
30% heating load reductions recorded during the winter
months (as shown in Fig. 13), when the PCM was predominantly frozen.
About 55% overall cooling load reductions (as observed
in Fig. 16), combined with a more than 90% peak hour roof
heat flux decrease and noticeable peak load shifting (see
Fig. 9), are confirmations of previously recorded performance data from the similarly-constructed roof assemblies
containing PCM heat sinks (Kosny et al., 2007; Miller and
Kosny, 2007; Kosny et al., 2011).
Current results indicated that it is possible to further
improve the performance of the PV-PCM roof with
enhancement of thermal properties of the PCM and the
insulation placed on top of the PCM. Further, the choice
of the PCM was not ideal for the weather conditions
encountered. Greater load reductions and, hence, energy
savings can be expected by using PCMs of slightly higher
freezing threshold temperatures. Another option for future
consideration is a potential increase of the thickness of the
array of PCM containers.
6. Summary and conclusions
A full-scale experimental attic using building integrated
PV laminates and dynamic roof deck technologies was
tested in the mixed climatic conditions of Oak Ridge,
Tennessee, USA. The thermal performance of the experimental roof was compared to a control attic with a conventional asphalt shingle roof. Field-test data collected
between November, 2009 and October, 2010 are presented
in this paper. The experimental roof consisted of roof-integrated PV laminates, air cavities, and dense fiberglass insulation with reflective foil facing, and PCM heat sink.
The test data demonstrated that, during winter, without
the phase change contribution, the PV-PCM attic had a
30% reduction in roof-generated heating loads compared
to a conventional shingle attic. Conversely, during the
cooling season, the attic generated cooling loads from the
PV-PCM attic were about 55% lower than the shingle attic.
In addition, about 90% reductions in peak daytime roof
heat fluxes were observed with the PV-PCM roof.
It was found that for almost 2/3rd of the total number
of days during the cooling season of 2010, the PCM went
through the phase transition process.
Recorded test data indicate that it may still be possible
to improve the thermal performance of the PCM heat sink
by further enhancement of the phase change temperature
profile of the PCM and modification of thermal properties
of insulation placed on top of the PCM.
Acknowledgments
The authors would like to thank Joe Harter (ATAS),
Mike Walters and Eric Akkashian (UniSolar), Sam Yuan
(CertainTeed) and Peter Horwath (Phase Change Energy
Solutions) for their contributions. The technical assistance
of Phillip Childs and Ed Reed (ORNL) in setting up the
data acquisition systems is gratefully acknowledged.
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