REHVA Journal – December 2011

 

Bjarne W. Olesen
Professor Ph. D.
International Centre for Indoor Environment and Energy
Technical University of Denmark
bwo@byg.dtu.dk

 

Introduction

A trend, which started in the early nineties in Switzerland (6, 7), is to use the thermal storage capacity of the concrete slabs between each storey in multi storey buildings to heat or cool buildings. Pipes carrying water for heating and cooling are embedded in the centre of the concrete slab.

By activating the building mass there will not only be a direct heating-cooling effect, but due to the thermal mass also the peak load will be reduced and transferring some of the load is transferred to outside the period of occupancy will be possible. Because these systems for cooling operate at water temperatures close to room temperature, they increase the efficiency of heat pumps, ground heat exchangers and other systems using renewable energy sources.

Relatively small temperature differences between the heated or cooled surface and the space are typical for surface heating and cooling systems. This results in a significant degree of self control, because a small change in the temperature difference will influence the heat transfer between the cooled or heated surface and the space significantly.

In an earlier study Olesen et. al. (8) studied for the summer season different control parameters (time of system operation, intermittent operation of circulation pump and supply water temperature control) by dynamic computer simulation. It was found that operation of the system during the night was sufficient, intermittent operation of the pump was possible and that the water temperature should be controlled over the season based on outside temperature.

The present paper presents the results of additional dynamic computer simulations of such a system. In the present study two different climatic zones (Würzburg, Germany and Venice, Italy) are studied both for summer and winter season. Further algorithms for water temperature control and the effect of a room temperature dead-band are investigated.

Method

The study was performed with the aid of the dynamic simulation program (9).The multidimensional heat transfer processes in the slab were modelled via a special module developed by Fort (4). The following describes the test space and other boundary conditions, which were very similar to the conditions reported by Olesen et. al. (8) and Hauser et. al. (5).

Description of system and test space

The system considered is shown in Figures 1 and 2. The ceiling/floor consists of an 18 cm thick concrete slab with 20 mm plastic pipes embedded in the middle with 150 mm spacing. The slab is finished with 20 mm of acoustical insulation and 45mm screed. Heat is supplied or removed by the heated or cooled water flowing in the embedded pipes. The mass flow rate of the system is constant at 350 kg/h.

The effect of heating and cooling the ceiling is described using a central room module in an office building with offices on either side (west and east) of the corridor. This characterises the thermal behaviour of all rooms that are at least two rooms away from the roof, corner and ground floor rooms. The geometrical dimensions of the room module are shown in Figure 1.

 

Figure 1. Central room module used for the computer simulation of a building with concrete slab cooling. All dimensions are in meter.

 

Figure 2. Position of the plastic pipes in the concrete slab between two stories.

The floor (Figure 2) consists of 45 mm screed (l = 1.4 W/m²K, c = 1 kJ/kgK, r = 2000 kg/m³), 20 mm insulation (l = 0.04 W/m²K, c = 1.5 kJ/kgK, r = 50 kg/m³) and 180 mm concrete (l = 2.1 W/m²K, c = 1 kJ/kgK, r = 2400 kg/m³). The outside pipe diameter is 20 mm and the spacing is 150 mm. The window has a U-value = 1.4 W/m²K.

The room volume is 55.44 m³ with a thermal capacity of 700 kJ/K.

Table 1. Design day outdoor temperatures for Würzburg, Germany and Venice, Italy.

City

Lat.
[°]

Long.
[°]

Elev.
[m]

Heating Dry Bulb
[°C]

Cooling Dry Bulb
[°C]

99.6%

99%

0.4%

2%

Venice

45.30 N

12.20 E

6

–4.9

–3.1

30.8

28.2

Würzburg-Frankfurt

50.05 N

8.60 E

113

–11

–8.2

30.3

26.7

Boundary conditions

The meteorological ambient boundary conditions correspond to those of Würzburg/Germany and Venice/Italy. The external temperature data for winter and summer design days are shown in Table 1. Summer was the period May 1 to September 30, winter was the period October 1 to April 30.

Time of occupancy was Monday to Friday from 8.00 to 17.00, 12.00 to 13.00 lunch break.

System was only in operation outside the occupancy from 18:00 to 06:00.

Internal Heat Sources:

During occupied periods 550 W corresponding to 27.8 W/m².

 

This corresponds to two occupants, two computers, a printer and light. During the lunch break 350 W corresponding to 17.7 W/m², 50% convective, 50% radiant.

Moisture Production:

During occupation, 100 g/h.

Ventilation (ach):

Outside time of occupation 0.3 h-1 (infiltration). During occupation 1.5 h-1 (~11 l/s per person).

Sun Protection:

During occupation by direct exposure of sunlight and operative temperature above 23°C, reduction factor z = 0.5.

Control parameters studied

Two control parameters were studied:

·         Control of water temperature

·         Dead-band for room temperature.

Control of water temperature

The goal for the system used in the present study is to operate water temperatures as close to the room temperature as possible. If very high or very low water temperatures are introduced into the system it may result in over heating or under cooling.

In the present study supply water temperature was limited not to be lower than the dew point in the space. For this purpose a humidity balance (latent loads from people, outside humidity gain from ventilation) were also included in the simulation. It was then possible to calculate the dew point in the room for each time step in the simulation.

Instead of controlling the supply water temperature it may be better to control the average water temperature. The return water temperatures are influenced by the room conditions. By constant supply water temperature an increase in internal loads from sun or internal heat sources will increase the return temperature. The average water temperature will then increase and the cooling potential will decrease. If instead the average water temperature (½ (treturn – tsupply)) is controlled an increase in return temperature will automatically be compensated by a decrease in supply water temperature.

In well designed buildings with low heating and cooling loads it may be possible to operate the system at a constant water temperature. The following concepts for water temperature control were studied:

Supply water temperature is a function of outside temperature according to the equation:

tsupply = 0.52 (20 – textenal) + 20 – 1.6 (to – 22)    °C        (case 801)

Average water temperature is a function of outside temperature according to:

taverage = 0.52 (20 – textenal) + 20 – 1.6 (to – 22)    °C        (case 901)

Average water temperature is constant and equal to: 22°C in summer and 25°C in winter.

                        (case 1201)

Supply water temperature is a function of outside temperature according to the equation:

tsupply = 0.35 (18 – textenal) + 18   °C        Summer           (case 1401)

tsupply = 0.45 (18 – textenal) + 18   °C        Winter  (case 1401)

Dead-band of room temperature

To avoid a too frequent change between cooling and heating it is recommended to stop the circulation pump during a certain room temperature range, dead-band. In the study by Olesen et. al. (8) a dead-band of 22°C to 23°C was used. This means when the room operative temperature increases above 23°C the system will start in the cooling mode. If the room operative temperature is less than 22°C the system will start in the heating mode. In between the circulation pump is stopped.

In the present study following dead-bands were tested:

    22–23°C      (case 0901-1)

    21–23°C      (case 0901-8)

    21–24°C      (case 0901-9)

Results and discussion

The simulations were done for both an East and a West facing room. Only results for a West facing room are presented in this paper. In a pre-test it was found that the highest exposures occurred in the room facing West.

Results from the summer period May 1st to September 30th and the winter period October 1st to April 30th are presented.

The total number of hours in each period is ~ 3690, number of working days ~ 109 and number of working hours ~ 981. The results will be evaluated based on comfort (operative temperature ranges, daily operative temperature drift during occupancy) and energy (running hours for circulation pump, energy removed or supplied by the circulated water)

The calculated operative temperatures may be compared to the comfort range 23 to 26°C recommended for summer (cooling period) and 20 to 24°C recommended for winter (heating period) (1, 2, 3). This is based on a fixed level of clothing insulation for summer (0.5 clo) and winter (1.0 clo), which may not be relevant for the whole period.

Study of water temperature control

The results of the simulation are shown in Table 2 for summer conditions and in Table 3 for winter conditions.

The operative temperature of the cases 0801, 0901 and 1401 (Table 2) is for most of the time (>85%) in a comfortable range (22–26°C). In Würzburg 27°C is never exceeded and 26°C is exceeded less than 5% of the time. In Venice only 5% of the temperatures are above 27°C. The difference between controlling the supply water temperature (case 0801) or the average water temperature (case 0901) is very small. In the case 1401 the control do not take into account the internal operative temperature, but the results are almost identical to case 0801 and 0901. With a constant average water temperature (22°C) the cooling effect is too low and the operative temperature is often too high (60% of the time above 27°C in Venice and 27% in Würzburg).

The energy use is the same for the cases 0801, 0901 and 1401 in Venice. For Würzburg case 1401 is the energy use however about 10% lower than case 801 and 901. Energy use the case 1201 with constant water temperature is relatively high.

The pump running time for case 1401 is equal or lower than the other cases.

In the summer case 1401 is overall better than the others. Due to the warmer climate in Venice (Table 1) the room temperatures are higher, energy use and pump running time are also higher compared to Würzburg.

Table 2. Operative temperatures, temperature drift, pump running time and energy transfer for different water temperature control strategies. Summer conditions. Dead-band 22–23°C. Ventilation rate: 0.3 ach from17:00 to 8:00, 1.5 ach from 8:00 to 17:00.

 

May to September

 

Time of operation 18:00–06:00

 

 

Venice

 

 

 

Würzburg

 

 

 

Water temperature control

 

Supply= F (outside)
0801

Average= F (outside)
0901

Average= 22°C
1201

Average= F (outside)
1401

Supply= F (outside)
0801

Average= F (outside)
0901

Average= 22°C
1201

Average= F (outside)
1401

Operative temperature interval

°C

%

%

%

%

%

%

%

%

<20

0

0

0

0

0

0

0

0

20–22

0

0

0

0

3

3

1

5

22–25

56

58

8

56

75

78

30

77

25–26

26

25

13

25

18

16

21

14

26–27

13

12

19

14

5

4

22

4

>27

5

5

60

5

0

0

27

0

Temperature drift [days]

<1

0

0

0

0

3

2

6

4

1–2

9

9

14

10

26

27

26

24

2–3

56

54

65

49

33

33

46

35

3–4

35

37

21

41

38

38

22

37

4–5

0

0

0

0

1

1

0

1

5–6

0

0

0

0

0

0

0

0

>6

0

0

0

0

0

0

0

0

Pump running

hours

1254

1190

1417

1214

1091

971

1327

953

 

% of time

34

32

39

33

30

26

36

26

Energy

Cooling

1104

1109

1297

1106

763

785

978

749

kWh

Heating

1

2

0

0

29

41

2

2

 

Table 3. Operative temperatures, temperature drift, pump running time and energy transfer for different water temperature control strategies. Winter conditions. Dead-band 22–23°C. Ventilation rate: 0.3 ach from17:00 to 8:00, 1.5 ach from 8:00 to 17:00.

 

October to April

 

Time of operation 18:00–06:00

 

 

Venice

 

 

 

Würzburg

 

 

 

Water temperature control

 

Supply= F (outside)
0801

Average= F (outside)
0901

Average= 25°C
1201

Average= F(outside)
1401

Supply= F (outside)
0801

Average= F (outside)
0901

Average= 25°C
1201

Average= F (outside) 1401

Operative temperature interval

°C

%

%

%

%

%

%

%

%

<20

0

0

0

1

0

0

4

4

20–21

1

1

6

14

9

7

19

24

21–23

72

75

50

63

77

80

50

63

23–24

14

15

5

14

8

7

7

7

24–26

12

10

23

8

6

5

15

2

>26

0

0

16

0

0

0

5

0

Temperature drift [days]

<1

33

34

30

32

57

57

58

57

1–2

44

43

49

41

29

29

28

29

2–3

21

21

20

23

12

12

13

13

3–4

2

2

0

4

2

2

1

2

4–5

0

0

0

0

0

0

0

0

5–6

0

0

0

0

0

0

0

0

>6

0

0

0

0

0

0

0

0

Pump running

hours

837

642

1487

1166

813

664

1533

1322

% of time

16

13

29

23

16

13

30

26

Energy

Cooling

144

144

143

143

57

64

63

45

kWh

Heating

551

554

407

421

816

834

684

717

Also for the winter period (Table 3) the cases 801, 901 and 1401 results in the most comfortable conditions. In Venice the room temperatures exceed the interval 20–24°C less than 12% of time. In case 1401 the room temperature is, however, below 20°C for 4% of the time.

On the energy side case 1401 is again about 10% better than case 801 and 901, but the pump running time is significant higher.

In winter the energy use in Würzburg is as expected higher than in Venice.

It is clear that with the proper control the activated slab system is not only capable of reducing the indoor temperatures to a comfortable range, but also capable of heating up the space to the comfort range as the only heating system.

Table 4. Operative temperatures, temperature drift, pump running time and energy transfer by different room temperature dead-bands. Control of water supply temperature according to outside and internal temperature (case 0901). Summer conditions. Ventilation rate 0.8 ach.

 

May to September

 

Time of operation 18:00–06:00

 

 

Venice

 

 

Würzburg

 

 

Room temperature dead-band

 

22–23°C
0901-1

 21–23°C
0901-8

 21–24°C
0901-9

22–23°C
0901-1

21–23°C
0901-8

21–24°C
0901-9

Operative temperature interval

°C

%

%

%

%

%

%

<20

0

0

0

0

0

0

20–22

0

0

0

2

6

5

22–25

58

58

38

81

78

69

25–26

25

25

33

14

14

20

26–27

12

12

22

2

2

6

>27

5

5

7

0

0

0

Temperature drift [days]

<1

0

0

0

0

0

0

1–2

5

5

5

19

19

19

2–3

44

44

44

31

31

32

3–4

51

51

51

49

49

48

4–5

0

0

0

1

1

1

5–6

0

0

0

0

0

0

>6

0

0

0

0

0

0

Pump running

hours

1094

1094

878

709

657

378

 

% of time

30

30

24

19

18

10

Energy

Cooling

1035

1035

983

669

657

606

kWh

Heating

0

0

0

50

25

15

Study on room temperature dead-band

To minimise the risk for both heating and cooling within the same day and also decrease pump running time it is recommended to let the building float within a certain room temperature interval, i.e. dead-band. In the study by Olesen et. al. (8) it was always 22–23°C. In this study too additional dead-bands, 21–23°C and 21–24°C was tested. The results for the summer period are shown in Table 4 and for the winter period in Table 5. In all cases the supply water temperature was controlled according to case 901 and with a constant ventilation rate of 0.8 ach the whole day.

For the summer period a dead-band of 22–23°C and 21–23°C give the same results regarding operative temperature distribution, energy use and pump running time. The dead-band 21–24°C results in somewhat higher room temperatures especially in Venice. The pump running time decreases significantly, but the energy use is about the same as for the two other dead-bands.

In winter the biggest effect is lowering the dead-band from 22 to 21°C. This reduces the energy for heating with 20% and a more time with operative temperatures in the range 20–21°C, but always higher than 20°C. Conclusions of the dead band analysis

With an optimisation of the dead band the energy use for heating-cooling and running the pump can be reduced without sacrificing the comfort. The dead band should not be larger than 2 K.

Table 5. Operative temperatures, temperature drift, pump running time and energy transfer by different room temperature dead-bands. Control of water supply temperature according to outside and internal temperature (case 0901). Winter conditions. Ventilation rate 0.8 ach.

 

October to April

 

Time of operation 18:00–06:00

 

 

Venice

 

 

Würzburg

 

 

Room temperature dead-band

 

22–23°C
0901-1

21–23°C
0901-8

21–24°C
0901-9

22–23°C
0901-1

21–23°C
0901-8

21–24°C
0901-9

Operative temperature interval

°C

%

%

%

%

%

%

<20

0

0

0

0

0

0

20–21

1

8

8

2

13

13

21–23

62

71

68

78

77

77

23–24

27

13

12

16

7

7

24–26

10

7

11

4

2

2

>26

0

0

0

0

0

0

Temperature drift [days]

<1

0

0

0

5

1

1

1–2

49

47

47

64

69

69

2–3

43

44

44

22

22

22

3–4

8

9

9

9

9

9

4–5

0

0

0

0

0

0

5–6

0

0

0

0

0

0

>6

0

0

0

0

0

0

Pump running

hours

761

526

443

841

634

594

% of time

15

10

9

17

12

12

Energy

Cooling

101

83

61

35

19

11

kWh

Heating

842

713

695

1194

1138

1113

Conclusion

The results of a dynamic computer simulation of different control concepts for a water based radiant cooling and heating system with pipes embedded in the concrete slabs have been presented. The system was studied for both the summer period May to September and the winter period October to April in two geographical locations, Venice, Italy and Würzburg, Germany.

The best performance regarding comfort and energy is obtained by controlling the water temperature (supply or average) as a function of outside temperature. There is no need to take into account the room temperature.

The energy performance (energy use for heating and cooling, pump running time) can further be reduced by introducing a 2 K room temperature interval (dead band), where the circulation pump is stopped.

The system was able to keep the room temperatures within a comfortable range, both summer (cooling) and winter (heating) and in both climatic zones.

References

1.   ASHRAE 55. 1992. Thermal Environmental Conditions for Human Occupancy, ASHRAE.

2.   CEN CR 1752. 1998. Ventilation for Buildings: Design Criteria for the Indoor environment, CEN, Brussels.

3.   ENISO 7730. 1993. Moderate thermal environments – determination of the PMV and PPD indices and specification of the conditions for thermal comfort.

4.   Fort, Karel. 1996. Type 160: Floor Heating and Hypocaust.

5.   Hauser, G., Kempkes, Ch., Olesen, B. W. 2000. Computer Simulation of the Performance of a Hydronic Heating and Cooling System with Pipes Embedded into the Concrete Slab between Each Floor. ASHRAE Trans. V. 106, pt.1

6.   Meierhans, R.A. 1993. Slab cooling and earth coupling, ASHRAE Trans. V. 99, Pt.2.

7.   Meierhans, R. A. 1996. Room air conditioning by means of overnight cooling of the concrete ceiling. ASHRAE Trans.V. 102, Pt.2.

8.   Olesen, B. W., Sommer, K. and Düchting, B. 2000. “Control of slab heating and cooling systems studied by dynamic computer simulations., ASHRAE Trans.V.108, Pt.2.

9.   TRNSYS. 1998. TRNSYS 14.2 - User’s Manual.

Bjarne W. OlesenPage

Stay Informed

Follow us on social media accounts to stay up to date with REHVA actualities

0

0 product in cart.products in cart.