Cooling floor, radiant floor cooling, cooling capacity
Andrzej ODYJAS*
THE RELATION BETWEEN TYPE OF COOLING LOADS
AND CAPACITY OF COOLING FLOOR SYSTEM
Floor cooling system performance largely depends on the type of cooling loads occurring in the
room. In the case of a convective cooling load capacity of the system is small, however, receive in the
event of radiation directly on the floor of the system significantly increases productivity. The article
describes the results of numerical simulations, whose aim was to determine system performance, depending on the type of cooling load for the fixed assumptions of utility. It also sets out the maximum
system performance while maintaining a minimum temperature of the floor 20 0C.
1. INTRODUCTION
Floor cooling systems do not belong to the most popular air conditioning systems,
they are quite new systems and the conditions of performance are not well known.
They can be successfully used for cooling in conjunction with an air system, which
aims to ensure the quality of indoor air and humidity. It was commonly thought that
their cooling capacity is low and that they should be more as a supporting and complementary systems compared to traditional air conditioning systems. However,
in certain conditions, their performance can be greatly increased. This is true in the
case of thermal radiation directly receive in the floor. In commercial buildings usually
occur two types of radiant flux. Short wave radiation which comes from direct solar
radiation and long wave radiation that comes from internal sources of heat, land radiation, and spaces inside the room. In practice, the largest flux of radiation on the floor
is the direct solar radiation from the sun.
__________
* Institute of Environmental Engineering, Poznan University of Technology.
2. SIMULATION MODEL
In order to determine the maximum possible efficiency of the cooling floor system
in the rooms numerical model which allows to simulate thermal conditions was developed. The numerical model take into account the short-wave radiation (sunlight) falling directly on the floor, as well as long wave radiation emitted by internal heat
sources and radiation flux exchanged between surfaces of the room. In the model,
short-wave radiation emission coefficient for the floor was adopted equal to 0.8, while
for the long-wave radiation emission coefficient of all the surfaces assumed equal 0.9.
Also it was taken into account convective heat streams put into the indoor air by internal heat sources and possible heat flows delivered to the room with the air vent. Simplified diagram of heat exchange in the room shown in Figure 1.
Ventilation
Heat
sources
Water
Short wave radiation
Long wave radiation
Convection
Fig. 1. Simplified schematic of heat transfer in a room
Room dimensions used in the simulation were a x b x h 4.68 x 3.00 x 4.00 m with
a window in the outer wall with an area of 3.4 m2. The floor and pipes system was
divided into 100 rectangles, and the inside air with adjoining walls on three separate
areas with a height: lower 0.3m, middle 2.4m, upper 0.3m. The floor was assumed
two-dimensional, while the other divisions one dimensional heat flow. Each of the 100
rectangles and each surface separately exchange the heat with the rest of the surfaces
in the room. Coefficients of heat transfer by convection was conditional on the temperature difference between air and the each surface. A more detailed description of
the model is in a separate articles on this topic [2,3].
3. ASSUMPTIONS FOR THE SIMULATION
It is known that the main limitation of floor cooling performance is its temperature,
which should not fall below 190C - 200C, both in terms of comfort as well as to the
possible condensation of moisture on the floor [1,4,5]. In addition to the type of cooling loads occurring in the room the system performance also depends on other factors.
The basic ones are: the layout and thermal resistance of flooring materials, in particular the external cladding, the emission coefficient of them, short wave radiation long
all surfaces in the room, spacing and arrangement of the pipes in the floor, the temperature and flow rate of water in the floor. The simulations were performed assuming no
contribution from mechanical ventilation. Structural solutions of the room and other
parameters were adopted in the most typical way for the system. The most important
of these are presented in Figure 2.
INTERNAL
WALLS
3
2300kg/m ; 840 J/kgK; 1,5 W/mK
6 cm
4 cm
2200kg/m3; 1000 J/kgK; 1,6 W/mK
30kg/m ; 1450 J/kgK; 0,04 W/mK
STYROFOAM
11 cm
2500kg/m3; 1000 J/kgK; 1,8 W/mK
1cm
900kg/m3; 1000 J/kgK; 0,25 W/mK
CONCRETE
PLASTER
3
CERAMIC TILES
JOINTLESS CONCRETE
ADIABATE
600kg/m3; 8400 J/kgK; 0,3 W/mK
900kg/m3; 1000 J/kgK; 0,25 W/mK
3
900kg/m ; 1000 J/kgK; 0,25 W/mK
COOLING FLOOR
1 cm
6 cm
PLASTER
CELLULAR CONCRETE
1cm
1cm
PLASTER
600kg/m ; 8400 J/kgK; 0,3 W/mK
CELLULAR CONCRETE
3
24 cm
STYROFOAM
10 cm
30kg/m3; 1450 J/kgK; 0,04 W/mK
EXTERNAL
WALL
CEILING
Fig. 2. The design parameters of the room
Cooling loads occurring in the room was divided into 5 types:
A - 100% convection to the air space into the central air zone
B - Mixed: convection 50% + 50% long wave radiation brought proportionately on all surfaces in the room
C - 100% long-wave radiation brought on all surfaces in the room in
proportion to their share in total room area
D - 100% short-wave radiation (solar) brought directly on the entire
floor space
E - Mixed: convection 25% + 25% of long wave radiation brought
proportionately on all surfaces in the room + 50% short-wave radiation equally brought to the entire floor space
The closest to the real cooling load type is the type E, which corresponds to 50%
share of the burden from the cold internal sources and 50% receive in the solar radiation directly to the floor. The simulation algorithm was varied levels of cooling loads
in order to achieve the set temperature in the central zone of air space equal to 24 0C
and 260C alternatively.
4. RESULTS AND DISCUSSION
The first series of simulations were performed assuming a fixed set of the power
system: the input water temperature equal to 160C, water flow rate 20dm3/hm2 of
floor, pipes spacing in the floor 10cm of a double meander in the system, stood outside the room temperature equals 240C and 260C alternatively. The simulation results
are shown graphically in Figure 3. The figure also describes the minimum floor surface temperature, which occurs in the places where pipes enter into the floor.
The assessment of the results shows that in the case of convective cooling load type
(type A), system performance is small, the order of 22 to 28 W/m2, depending on the
maintenance of the internal air temperature 240C and 260C. In the case of mixed cooling loads (type E), reaching values of 39-50 W/m2, whereas in the case of direct radiation receive in the floor (type D) are much larger in a row from 68 to 86 W/m2. Be
noted, however, that the minimum floor surface temperatures are exceeded in cases A,
B, and part C and E, but well above 200C in the case of D.
Because of to low temperatures of the floor surfaces an additional series of simulations were done. In these simulations was assumed a variable water inlet temperature
in order to reach the floor surface temperature equal to 200C. In addition to D-type
load flow were increased to 100 dm3/hm2 and two-collector pipe system were used so that the water velocity in the pipes was real. The results are shown in Figure 4.
The figure also indicated the inlet water temperature in order to reach the min. floor
temperature equal 200C.
The unit floor cooling capacity [W/m2]
The floor average cooling capacity, depending on the type of cooling loads
(inside air temperature 240C and 260C)
200,0
180,0
160,0
140,0
120,0
100,0
80,0
60,0
A
Internal heat
sources
convection
100%
B
Internal heat
sources:
convection 50% +
long wave
radiation 50%
tair=240C
tair=260C
twater=160C
V =20dm3/h m2
40,0
22,4
28,3
C
Internal heat
sources:
long wave
radiation 100%
D
Short wave
radiation on the
floor 100%
85,8
E
Internal heat
sources 50%
(c.25% + r.25%)
+ short wave
radiation on the
floor 50%
68,5
49,5
47,6
27,3
34,5
39,2
38,2
20,0
0,0
18,0 18,6
18,5 19,1
19,5 20,3
22,0 23,5
19,5 20,4
The minimum floor surface temperature [0C]
Fig. 3. The average cooling floor capacity for the fixed parameters of power, depending on the type of
cooling loads.
The unit floor cooling capacity [W/m2]
The floor average cooling capacity, depending on the type of cooling loads
(the floor min. temperature 200C)
500,0
450,0
400,0
350,0
300,0
250,0
A
Internal heat
sources
convection
100%
B
Internal heat
sources:
convection 50% +
long wave
radiation 50%
C
Internal heat
sources:
long wave
radiation 100%
D
Short wave
radiation on the
floor 100%
226,1
200,0
150,0
tair=240C
tair=260C
E
Internal heat
sources 50%
(c.25% + r.25%)
+ short wave
radiation on the
floor 50%
150,4
100,0
50,0
29,8
33,6
53,5
50,2
14,4 22,5
19,3
18,7 18,0
18,3 17,3
17,0 15,4
Inlet water temperature [0C]
34,7
0,0
9,7
4,5
16,9 15,2
Fig. 4. The average cooling floor capacity for a minimum floor surface temperature equal to 20 0C, depending on the type of cooling load.
The results obtained further reduce the efficiency of the floor in the conditions of
convective heat gain to the level of 14-22 W/m2 floor and increase system performance significantly in the case of direct radiation on the floor, up to 150 - 226 W/m2
floor. However, due to very low supply water temperature (9,70C and 4,50C), to obtain
such performance is quite nonrealistic in practice, but shows a "hidden" capabilities of
the system under specific conditions (radiation flux directly on the floor surface). But,
this is only possible if the water which flow in the floor is able to receive such a large
heat.
5. CONCLUSION
Before drawing final conclusions from the simulations, one has to take into account, that the results of simulation were obtained by the system after reaching
a steady thermal state. Achieve similar performance in terms of normal day conditions
will not always be possible because both the large thermal resistant of the system and
other surfaces of the room as well as variation of cooling loads in the rooms during the
day. The simulations show that floor cooling system achieves the greatest efficiency in
the case of direct thermal radiation receive in the floor. This situation may occur when
solar radiation receive the floor through the windows, glass facades or roof illuminates.
In further stages of research, it is planned to study performance of the cooling floor
system under conditions similar to the real, alternating a day of sunlight falling on the
floor in a manner dependent on the world direction of the outer wall and glazing and
the current position of the sun on the horizon.
REFERENCES
[1] CAUSONEA F., BALDINB F., OLESEN B.W., CORGNATIA S., Floor heating and cooling combined with displacement ventilation: Possibilities and limitations, Energy and
Buildings 42 (2010) 2338–2352.
[2] GÓRKA A., ODYJAS A., Numeryczny model działania systemu posadzki chłodzącej, XIII
International Conference : Air & Heat - Water & Energy 2011, Wrocław-Kudowa, 2011.
[3] ODYJAS A., GÓRKA A., Określanie obciążeń chłodniczych w radiacyjnych systemach
chłodzenia pomieszczeń, XII International Conference : Air conditioning protection and
district heating 2008, Wrocław-Szklarska Poręba, 2008, PZITS, 327-332.
[4] OLESEN B.W., Possibilities and limitation of radiant floor cooling. ASHRAE Transaction: Reserch 103, 1997, 42-48.
[5] OLESEN B.W., Thermal comfort requirement for floors occupied by bare feet. ASHRAE
Transactions 83 (2), 1977.
WPŁYW TYPU OBCIĄŻEŃ CHLODNICZYCH NA WYDAJNOŚĆ SYSTEMU POSADZKI
CHŁODZĄCEJ
Wydajność systemu posadzki chłodzącej w znacznym stopniu uzależniona jest od typu obciążeń
chłodniczych występujących w pomieszczeniu. W przypadku obciążeń chłodniczych typu
konwekcyjnego wydajność systemu jest niewielka, jednak w przypadku promieniowania trafiającego
bezpośrednio na posadzkę wydajność system znacząco wzrasta. W artykule opisano wyniki symulacji
numerycznych, których celem było określenie wydajności systemu w zależności od typu obciążeń
chłodniczych przy ustalonych założeniach użytkowych. Określono również maksymalną wydajność
systemu przy zachowaniu minimalnej temperatury powierzchni posadzki równej 20 0C.