influence of floor plenum on energy performance of

1

Influence of floor plenum on energy performance of buildings with 1

UFAD systems 2

Yan Xue1 and Qingyan Chen2,1* 3

4 1School of Mechanical Engineering, Purdue University, West Lafayette, IN 47907, USA 5

2School of Environmental Science and Engineering, Tianjin University, Tianjin 300072, China 6 *Tel. (765)496-7562, Fax (765)494-0539, Email: [email protected]

8

Abstract 9

10

The heat transfer through the floor slab in buildings with Under-Floor Air Distribution (UFAD) 11

systems may have a negative impact on the energy performance of these buildings, although very 12

few studies have been reported in the literature. By using an energy simulation program, 13

EnergyPlus, this investigation compared the energy use in a Philadelphia office building with a 14

UFAD system to that with a well-mixed ventilation system. When the heat transfer through the 15

floor slab was taken into consideration, the thermal load of the building with the UFAD system 16

was higher than with the well-mixed system. On the other hand, the higher supply air 17

temperature of the UFAD system enables the use of more free-cooling. The annual energy 18

consumption by the chillers in the building with the UFAD system was 16%-27% lower than 19

with the well-mixed system, but energy consumption by the boiler was 12%-30% higher, and the 20

energy consumption by the fan was 22-50% higher, depending on the manner in which the heat 21

was supplied to the floor plenum. When the UFAD system was used with an un-ducted floor 22

plenum and without heating coils under the diffusers, it consumed slightly more energy than the 23

well-mixed system. 24

25

1. Introduction 26

27

Unlike conventional overhead well-mixed systems, UFAD systems provide directly conditioned 28

air to the occupied zone through diffusers in a raised floor, as shown in Figure 1. Thermal 29

buoyancy causes temperature stratification in the occupied zone, and the air temperature in the 30

lower part of the zone is lower than that in the upper part. Therefore, UFAD systems are believed 31

to use less energy for cooling [1,2]. In addition, the thermal stratification in a room with a UFAD 32

system can create better indoor air quality than in a room with a well-mixed air distribution 33

system [3,4]. A UFAD system also allows individual control of airflow rate in order to meet the 34

thermal comfort requirements of different occupants [5,6] and can thus provide a more 35

comfortable environment than overhead well-mixed systems [7]. However, if the air supply and 36

return in the plenums are un-ducted or un-insulated, the air temperature in the floor plenum is 37

very low during the cooling mode, while the ceiling plenum is very warm. The temperature 38

Xue, Y. and Chen, Q. 2014. “Influence of floor plenum on energy performance of buildings

with UFAD systems,” Energy and Buildings, 79, 74-83.

2

difference between the floor plenum and downstairs ceiling plenum leads to heat transfer across 39

the floor slab, as shown in Figure 1,which may have an impact on energy use. 40

41

Studying the influence of the floor plenum on cooling load and energy use requires that the 42

energy flow between the occupied zone and the floor plenum also be considered, so do the 43

thermal stratification in the occupied zone [8]. Many previous researchers have analyzed the 44

thermal stratification, and various models have been developed to predict the air temperature 45

stratification in rooms with UFAD systems [9-15]. Air flow and heat transfer in the floor plenum 46

contribute further complexity to the simulations. Linden [16] determined that there was a 47

significant air temperature differential and air velocity variation in the floor plenum. These non-48

uniform air temperature and flow distributions could affect heat transfer in the floor plenum. 49

50

In regard to the impact of the floor plenum on the energy performance of a UFAD system, 51

Bauman et al. [17] found that the heat transfer in the floor plenum can be as high as 30%-40% of 52

the room cooling load. However, this conclusion was made on the basis of a simplified first-law 53

model that may not be accurate. Schiavon et al. [8] indicated that the presence of a raised floor 54

changed the cooling load profile greatly, and the peak cooling load could vary in the range of −7 55

to + 40% as compared to the load without the raised floor. However, their study did not address 56

heating. Lee et al. [18] simulated a three-floor office building with a UFAD system on two 57

design days: a summer day and a winter day. They found that thermal decay in the floor plenum 58

could result in a higher supply airflow rate and greater use of energy by the fans and chiller. 59

However, the non-uniform flow in the floor plenum was not taken into account in their study. 60

The current investigation systematically studied the influence of heat transfer through a floor 61

slab on the energy performance of an office building with a UFAD system. The objective was to 62

accurately simulate the thermal load and energy use as influenced by the floor plenum and to 63

assess the impact of non-uniform flow in the floor plenum on energy modeling. 64

65

2. Method 66

67

2.1. Load and energy simulations for an office building 68

69

To accurately study the impact of the floor plenum on energy performance, this investigation 70

used EnergyPlus [19] as the main tool with the implementation of a room air stratification model. 71

To simplify the study, this investigation simulated a typical middle floor, Floor N, in a multi-72

floor building, as shown in Figure 2, rather than an entire building. Accurate simulations must 73

take into account (1) the vertical air temperature stratification in the room air and (2) the non-74

uniform air distribution in the floor plenum. 75

76

3

2.2. Vertical air temperature stratification in the simulated room 77

78

Because the EnergyPlus program did not calculate the room air distribution, the air temperature 79

model from Lin et al. [12], implemented into the program by Liu et al. [20], was applied to 80

calculate the vertical air temperature profiles in the room. The model predicts the vertical 81

temperature profiles by simulating the buoyancy plumes from internal heat sources and the jets 82

from air supply diffusers. The buoyancy plumes generate the air stratification and the jets lead to 83

air mixing. 84

85

2.3. Non-uniform air distribution in the floor plenum 86

87

Along with the air temperature stratification in the room, non-uniform air velocity and air 88

temperature in the floor plenum also contribute additional complexity to the simulation[16]. To 89

simulate the non-uniform flow in the floor plenum, Computational Fluid Dynamics (CFD) can be 90

used in combination with the energy calculation [21,22], but doing so is too computationally 91

demanding. Instead, this investigation used a multizone model [23] to simulate the non-uniform 92

air distribution in the floor plenum for coupling with EnergyPlus. The floor plenum was divided 93

into several subzones according to the temperature distribution determined by CFD simulation. 94

The air velocity and temperature differences among these subzones represented the non-uniform 95

air temperature in the floor plenum. The simulation results of the multizone model were then 96

compared to a single zone model in which the floor plenum had uniform air distribution. The 97

difference between them could indicate the impact of the non-uniform air distribution on the 98

energy modeling. 99

100

2.4. Validation of the computer models 101

102

The computer models described above use a number of approximations, and coupling them with 103

EnergyPlus can lead to additional errors. The models needed to be validated before they could be 104

used to simulate the cooling load and energy use in the building. This investigation used an 105

environmental chamber as shown in Figure 3 to measure the air and surface temperatures in the 106

floor plenum and the surface temperatures in the room. 107

108

This chamber had dimensions of 4.80 m in length, 4.20 m in width, and 2.73 m in height, 109

including a floor plenum with a height of 0.30 m. The room contained several pieces of furniture, 110

lighting fixtures, and heated boxes that were used to simulate internal loads such as electrical 111

appliances, occupants, etc. The supply air duct was connected to the floor plenum, and two linear 112

grille diffusers were installed in the floor. The walls and ceiling were well insulated with a 113

thermal resistance of 5.45 m2-K/W. The raised floor panels were made of 0.1 m thick lightweight 114

concrete with a thermal resistance of 0.16 m2-K/W. A double-glazed window with dimensions of 115

4

4.65 m in width and 1.55 m in height and a thermal resistance of 0.25 m2-K/W was installed in 116

the east wall of the chamber. Table 1 shows the power levels of the internal heat sources. 117

118

Eight anemometers were placed in the middle of the floor plenum (0.15 m from the slab) for 119

measuring air temperature and velocity in the plenum. Temperatures of the different surfaces of 120

the floor plenum and room were measured by a number of T-type thermocouples. The 121

experiment was conducted under steady-state conditions in summer. The inlet temperature was 122

17.3°C, and the air change rate was 6 ACH. 123

124

3. Validation of the computer models and case setup 125

126

3.1. Validation of the computer models 127

128

The CFD simulations for this investigation were performed for the floor plenum using a 129

commercial software program FLUENT [24]. The CFD results provided greater understanding of 130

the temperature distribution in the floor plenum, which was the basis of the multizone model. 131

Figures 4 and 5 show the air velocity and temperature profiles, respectively, in two sections of 132

the floor plenum. The air supply was located on the bottom wall under Position 7, as shown in 133

Figure 6, and therefore Positions 3 and 7 were located in the jet region. As a result, the air 134

velocities measured at these positions were high, and the air temperatures were low. The 135

agreement between the CFD results and experimental data for air velocity and temperature is 136

quite good. Figure 6 shows a non-uniform air temperature distribution in the floor plenum. 137

138

Our EnergyPlus simulation divided the floor plenum into three subzones, as shown in Figure 6, 139

for studying the impact of the non-uniform air temperature distribution on the load. Figure 5 140

depicts the air temperature profiles calculated with the multizone model in the two sections of 141

the floor plenum, and again they are in good agreement with the measured data. The results 142

shown in Figures 5 and 6 have validated the computer models for calculating air temperature in 143

the floor plenum. 144

145

Table 2 further compares the air temperature from the diffusers, air temperature at the exhaust, 146

and EnergyPlus-simulated surface temperatures with the measured data. The columns of "Error" 147

show the percentage difference between the experimental measurements and simulation results. 148

Once again, agreement between the simulations and measurements is quite good. Therefore, the 149

EnergyPlus program with the multizone air models can be used to predict thermal load and 150

energy use in buildings. In addition, Table 2 shows the results obtained by EnergyPlus with the 151

assumption of uniform air temperature in the floor plenum (one subzone). The air temperature 152

with one subzone differed by only 0.5 K from that with three subzones, which was insignificant, 153

although the air temperature between zones can differ by 2 K, as shown in Figure 6. Thus, the 154

5

impact of the non-uniform air distribution in the floor plenum on the energy simulation was very 155

limited and our subsequent simulations used only one zone for the floor plenum. 156

157

3.2. Case description for a building with UFAD systems in Philadelphia 158

159

Using the validated EnergyPlus model, this investigation conducted energy simulations for a 160

mid-level floor of a multi-floor office building in Philadelphia. An office area with dimensions 161

of 30 m 40 m 3.7 m, as shown Figure 7, was divided into five thermal zones: a central zone 162

and four perimeter zones, because solar radiation would have a significant impact on the 163

perimeter zones. The perimeter zones had a width of 5.0 m [25]. The construction and material 164

information for the building envelope were taken from the default data for Chicago in 165

EnergyPlus Version 7. Figure 8 shows the internal load profiles for weekdays and Saturdays. 166

This study further assumed that the building was completely closed on Sundays, without an 167

internal heat load. 168

169

This investigation simulated three different scenarios for the office building, as shown in Figure 170

9: a well-mixed ventilation system, a UFAD system without heating coils in the diffusers, and a 171

UFAD system with heating coils in the diffusers [26]. For each of the two UFAD scenarios, three 172

cases were simulated with different floor plenum configurations: completely ducted floor 173

plenums; partially ducted floor plenums (ducted floor plenums in the perimeter zones and an un-174

ducted core zone); and completely un-ducted floor plenums. The arrangement led to a total of 175

seven simulation cases, as shown in Table 3. All of the cases used the same HVAC system, 176

which incorporated an electric chiller and a gas boiler. The system used variable-air-volume 177

control and an economizer. The thermostat in the occupied zone was set at 21°C during the 178

winter and 24°C during the summer. The minimum fresh air rate was 0.3 L/(s·m2), in accordance 179

with ASHRAE Standard 62.1-2010 for indoor air quality [27]. For mixed ventilation, the supply 180

air temperature was 13°C for cooling and 32°C for heating [28]. In the UFAD system without 181

heating coils in the diffusers, the supply air temperature was 17°C [1,29] for cooling and 32°C 182

for heating. In the UFAD system with heating coils, the air supply temperature from the HVAC 183

system was 17oC all year round. When heating was called for, the air was heated to 32°C by the 184

heating coils. The HVAC system operated from 6:00 to 22:00. 185

186

4. Results 187

188

In order to explain the effects of the heat transfer through slabs on the energy performance of 189

buildings, this section will show the heat flux profiles across slabs and analyze their influence on 190

the thermal loads. Then the year-round energy consumption results will be reported. 191

192

6

4.1. Thermal load simulations for the building 193

194

Figure 10 shows the heat flux at the two surfaces of the floor slab in the office building on July 195

21, the summer design day in EnergyPlus. The sum of the heat transfer at the two surfaces is 196

equal to the thermal storage in the slab. For cooling, the heating coils in the diffusers were not 197

activated, and thus the two UFAD scenarios were identical. For the well-mixed system, from 198

8:00 to16:00, heat was transferred from the occupied zone to the slab, as shown in Case 1 of 199

Figure 10(a, b). In the perimeter zones as shown in Figure 10(a), this heat transfer was caused by 200

the high level of radiation from the direct sunlight on the floor slab and the internal heat sources, 201

With the UFAD systems, however, the radiation from the sun and the internal load had no direct 202

impact on the slab. Thus, the heat transfer profiles in these cases were very different from those 203

with the well-mixed system. In the thermal zones with ducted floor plenums, there was a small 204

amount of heat transfer across the floor slabs. As for the un-ducted UFAD systems, cool air was 205

supplied to the floor plenum, and the downstairs ceiling plenum was warm. The significant 206

temperature difference between the two sides of the floor slab led to a high heat transfer rate 207

from the downstairs ceiling plenum to the floor slab and further to the floor plenum. In the well-208

mixed system, from 6:00 to 8:00 and 18:00 to 22:00, when the internal load and solar radiation 209

were small, heat was transferred from the floor slab to the room air as a result of the high floor 210

slab temperature. In the UFAD system, however, the total air supply was lower during these two 211

periods than during the occupied hours, and thus the heat transfer from the floor plenum to the 212

slab was smaller. 213

214

Despite the drastic difference in heat transfer profiles between the well-mixed ventilation and 215

UFAD, the cooling loads of these systems had similar shapes on the summer design day, as 216

shown in Figure 11. 217

218

For the winter heating, the heat transfer between the well-mixed system and UFAD system could 219

be different but the heating load is also very similar as shown in the cooling scenario. Due to 220

limited space in this paper, the detailed results are not presented here. 221

222

4.2. Annual energy consumptions of the building 223

224

Figures 12 and 13 illustrate the monthly energy consumption by the chiller and boiler, 225

respectively, in the HVAC system in year-round simulations. With the well-mixed ventilation 226

system, more energy was used by the chiller, especially during the shoulder seasons when the 227

outdoor air temperature was suitable for free-cooling. In the two UFAD scenarios, the levels of 228

electricity consumption by the chillers were almost identical. They were lower than that in the 229

well-mixed system by 27% for the ducted cases (Cases 2 and 5), 23% for the partially ducted 230

cases (Cases 3 and 6), and 16% for the un-ducted cases (Cases 4 and 7). The percentage numbers 231

are based on the UFAD systems. However, the natural gas consumption levels by the boiler with 232

7

the UFAD systems were higher than that with the well-mixed system. Without heating coils 233

under the diffusers, the boiler energy consumption in the UFAD system was higher than that in 234

the well-mixed system by 18% in the completely ducted case (Case 2), 21% in the partially 235

ducted case (Case 3), and 30% in the un-ducted case (Case 4). When there were heating coils 236

under the diffusers, the boiler energy consumption in the UFAD system was higher than that in 237

the well-mixed system by 12% in the completely ducted case (Case 5) and by 18% in the 238

partially ducted (Case 6) and the un-ducted (Case 7) cases. 239

240

The levels of electricity consumption by the fans in the building’s HVAC system were 22-50% 241

higher with the UFAD systems than with the well-mixed system by using the energy use of the 242

UFAD system as the reference, as shown in Figure 14. This difference occurred because the 243

supply air temperature in the UFAD systems was higher than that in the well-mixed ventilation 244

system, and a higher airflow rate is required in order to remove the same amount of heat. 245

246

A comparison of the different cases indicates that the ducting of floor plenums can reduce energy 247

consumption by the chiller and the boiler, and that this effect is more significant in perimeter 248

zones. Furthermore, during the heating mode, supplying warm air directly to the floor plenum 249

without ducts is not recommended because heat flow in the plenum would cause significant 250

energy loss. In addition to ducting of the floor plenums, the use of heating coils under the 251

diffusers is recommended in order to reduce energy use by the boiler. 252

253

Using the energy consumption data, this investigation conducted a cost analysis for these cases 254

with EnergyPlus. Table 4 shows the monthly price of natural gas [30]. The cost of electricity [30] 255

was calculated using the block method shown in Table 5. The tax rate was 8%, and the monthly 256

service fee for electricity was $8.81. Table 6 shows the annual electricity costs for the chiller and 257

fans and the gas costs for the boiler. For the same building, the use of electricity by lights and 258

electrical equipment was exactly the same with either the well-mixed ventilation or UFAD. 259

Variations in energy consumption arose from the HVAC systems. 260

261

Because only the perimeter zones of the building required heating and natural gas was much 262

cheaper than electricity, the gas costs for the boiler were much lower than the electricity costs for 263

the chiller. As discussed above, energy consumption by the chiller was greater with the well-264

mixed system than with the UFAD systems. Therefore, as shown in Table 6, chiller operation 265

was more expensive in the well-mixed case (Case 1) than in the UFAD cases (Cases 2-6). 266

Among the UFAD cases, those with un-ducted floor plenums (Cases 4 and 7) had higher 267

electricity costs than the other cases. However, the addition of heating coils under the diffusers 268

did not contribute significantly to the electricity costs. 269

270

Although chiller operation was cheaper in the UFAD cases, both the gas costs for the boiler and 271

the electricity costs for the fan were higher with the UFAD systems than with the well-mixed 272

8

system. These higher costs are consistent with the energy consumption levels shown in Figures 273

13 and 14. In the UFAD cases, the addition of heating coils under the diffusers (Cases 5-7) 274

reduced the gas cost for the boiler, especially in the cases with un-ducted floor plenums. The 275

total energy cost for a building with a UFAD system could be lower than that with a well-mixed 276

system if the floor plenum was ducted. Unfortunately, ducting of the floor plenum is not a 277

common practice at present. 278

279

5. Conclusions 280

281

Using validated computer models, this investigation assessed the energy performance of a 282

building in Philadelphia with several different UFAD systems and with a well-mixed ventilation 283

system. The study led to the following major findings: 284

The airflow and air temperature distribution in the floor plenum can be highly non-285

uniform. The non-uniform air temperature distribution can be calculated with the use of a 286

simple subzone model. By comparing the supply air temperature, exhaust air temperature, 287

and enclosure surface temperatures between the multizone model and the single zone 288

model, this study found that the impact of the non-uniform air temperature distribution on 289

the energy modeling is small. 290

In cooling situations, the temperature difference between the cold air in the floor plenum 291

and the warm air in the downstairs ceiling plenum can result in significant heat transfer 292

through the floor slab in a building with a UFAD system. This heat transfer leads to a 293

slightly higher cooling load than with a well-mixed ventilation system. 294

In heating situations, when a UFAD system is used in the building without heating coils 295

in the air supply diffusers, the presence of warm air in the floor plenum can lead to a 296

higher heating load than that with the well-mixed ventilation system. This increased heat 297

load is again attributed to heat transfer from the plenum air to the floor slab. 298

By conducting an annual energy analysis for the building, this investigation found that 299

the chiller used 16%-27% less energy with the UFAD systems than with the well-mixed 300

system because of the use of free cooling during the shoulder seasons. However, the 301

boiler consumed 18%-30% more energy with the UFAD systems because of heat loss in 302

the floor plenum. The use of heating coils in the air supply diffusers and/or the utilization 303

of ducts for supplying air to the floor plenum can reduce energy use by the boiler. Finally, 304

because the UFAD systems had a higher supply air temperature for cooling, the total 305

supply airflow rate was higher than that with the well-mixed ventilation system. As a 306

result, energy consumption by the fans in the UFAD systems was 22-50% higher. 307

The total energy cost for a UFAD system with un-ducted floor plenums and without 308

heating coils under the diffusers could be slightly higher than that for a well-mixed 309

system. Either ducting the floor plenums or using the heating coils under the diffusers 310

would reduce the energy cost. However, ducting the floor plenums would increase the 311

9

initial investment and maintenance costs. Therefore, although the UFAD system with 312

completely ducted floor plenums and with heating coils (Case 5) had the lowest energy 313

costs, the partially ducted UFAD system with heating coils (Case 6) would be the most 314

favorable one. 315

316

Acknowledgement 317

318

This research was funded partially by the Energy Efficient Building Hub led by the Pennsylvania 319

State University and funded by the U.S. Department of Energy and other government agencies, 320

where Purdue University was a subcontractor to the grant. 321

322

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324

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National Conference, Boulder, CO. 363

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(UFAD) Systems, ASHRAE Transaction 112 (2006) Part 2. 365

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Applied Energy 91 (2012) 197-207. 368

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Lawrence Berkeley National Laboratory, 2012. 370

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393

394

11

Table 1.Power inputs of the lights and heated boxes. 395

Number Power [W]

Lighting

1 43.0 2 44.0 3 26.5 4 43.5

Heated boxes for manikins 1 93.3 2 84.0

Heated boxes for equipment 1 65.0 2 65.0 3 65.0

396

Table 2.Comparison of simulated and measured temperatures for the environmental chamber. 397

Locations

Experimental measurements

E+ simulation with1-zone floor plenum

(uniform air temperature)

E+ simulation with 3-zone floor plenum (non-uniform air

temperature)

℃ ℃ Error (%) ℃ Error (%) Air at the diffusers 20.2 19.9 1.5 20.4 1.0 Air at the exhaust 24.2 24.4 0.8 24.2 0.0 Slab surface (facing floor plenum)

23.7 23.6 0.4 23.6 0.4

Ceiling surface 24.3 24.7 1.6 25.4 4.5 North wall of floor plenum 23.8 22.7 4.6 23.7 0.4 South wall of floor plenum 22.7 22.7 0.0 23.0 1.3 West wall of floor plenum 23.5 22.7 3.4 23.0 2.0 East wall of floor plenum 22.7 22.7 0.0 23.0 1.3 North wall of room 25.1 25.2 0.4 25.5 1.6 South wall of room 24.1 25.2 4.6 25.5 5.8 West wall of room 24.7 25.2 2.0 26.4 6.9 East wall of room 25.4 25.2 0.8 25.5 0.4

398

Table 3.Simulated cases. 399

Case Ventilation system Floor plenum configuration

Heating coils under diffusers

1 Well-mixed N/A N/A 2 UFAD Completely ducted No

3 UFAD Ducted perimeter zones

Un-ducted core zone No

4 UFAD Completely un-ducted No 5 UFAD Completely ducted Yes

6 UFAD Ducted perimeter zones

Un-ducted core zone Yes

7 UFAD Completely un-ducted Yes 400

12

Table 4. Monthly natural gas price (unit: $/kWh). 401

Jan Feb Mar Apr May Jun

0.344 0.343 0.343 0.346 0.371 0.400

Jul Aug Sep Oct Nov Dec

0.391 0.386 0.376 0.375 0.322 0.331

402

Table 5.Electricity price (unit: $/kWh). 403

Summer (Jun – Sep)and Winter (Oct - May) Charge Energy [$/kWh] Transition [$/kWh] Distribution [$/kWh]

< 80 hrs. 0.1088 0.0669 0.0344 80-160 hrs. Summer: 0.0592

Winter: 0.0428 Summer:0.0319 Winter: 0.0205

Summer:0.0162 Winter: 0.0103

160-400 hrs. 0.0428 0.0205 0.0103 > 400 hrs. 0.0275 0.0095 0.0046

404

Table 6. Annual energy cost for the whole building in different cases. 405

Case Electricity cost for the chiller

($)

Electricity cost for the fans

($)

Gas cost for the boiler

($)

Total cost as a percentage of Case 1

(%) 1 4275 369 244 100.0 2 3425 481 300 86.1 3 3553 660 320 92.7 4 3797 756 372 100.7 5 3424 481 274 85.7 6 3553 659 290 92.4 7 3814 744 297 99.6

406

13

407

408

409

410

411

412

413

414

415

416

417

Figure 1.Schematics of well-mixed (left) and UFAD (right) systems. 418

419

420

421

422

423

424

425

426

427

428

429

430

431

432

433

434

435

436

Figure 2. Floor N in a multi-floor building with a UFAD system. 437

438

439

Floor plenum

Heat transfer

Troom= Tplenum

Ceiling plenum

Occupied zone

Ceiling plenum

Floor slab

Floor plenum

Room

Return plenum

Floor plenum

Room

Floor plenum

Return plenum

14

1 2 3 4

5 6 7 8

0.00

0.40

0.80

1.20

1.60

0 1 2 3 4 5

U[m/s]

x[m]

Experiment Simulation

0.00

0.40

0.80

1.20

1.60

0 1 2 3 4 5

U[m/s]

x[m]

Experiment Simulation

440

Figure 3.Schematic of the environmental chamber. 441

442

443

444

445

446

447

448

449

450

451

452

453

454

Figure 4. Comparison of CFD results with experimental data for air velocity in two sections of 455

the floor plenum (horizontal cross-section of the plenum at 0.15m above the floor). 456

457

ExhaustLights

Heated boxes

for manikin and

equipment

Diffusers

Conditioned air inlet

Floor plenum

Desk

Room

North

1 2

3

4

5 6

7

8 x

y

15

458

Figure 5. Comparison of measured air temperature with that computed by CFD and by the 459

multizone model in two sections of the floor plenum 460

461

462 Figure 6.Air temperature distribution at 0.15m above the floor slab as simulated by CFD, and 463

division of the plenum into three subzones. 464

465

17

18

19

20

21

22

23

24

25

0 1 2 3 4 5

T[ ]

x[m]

CFD Experiment E+ (three subzones)

17

18

19

20

21

22

23

24

25

0 1 2 3 4 5

T[ ]

x[m]

Experiment CFD E+ (three subzones)

(K)

16

466

467

468

469

470

471

(a). Top view of thermal zone layout. 472

473

474

475

476

477

(b). Side view of thermal zone layout. 478

Figure 7.Medium-sized office building in Philadelphia. 479

480

481

482

Figure 8.Internal heat load of the office building on weekdays (left) and Saturdays (right). 483

484

0

2

4

6

8

10

12

0:00

2:00

4:00

6:00

8:00

10:0

0

12:0

0

14:0

0

16:0

0

18:0

0

20:0

0

22:0

0

W/m2

Time/h

Internal heat on weekdaysElectric Lights Occupancy

0

1

2

3

4

5

6

0:00

2:00

4:00

6:00

8:00

10:0

0

12:0

0

14:0

0

16:0

0

18:0

0

20:0

0

22:0

0

W/m2

Time/h

Internal heat on SaturdaysElectric Lights Occupancy

Ceiling Plenum

Floor Plenum

Room

0.4m

3.0m

0.3m

N

W

S

E

40.0m

30.0m

5.0m

Central Zone

5.0m

5.0m

5.0m

Ceiling Plenum

Room

0.7m

3.0m

17

485

486

487

488

489

490

491

492

493

494

495

(a) (b) (c) 496

497

Figure 9. Simulated scenarios: (1) well-mixed ventilation system, (2) UFAD system without 498

heating coils in the diffusers, and (3) UFAD system with heating coils in the diffusers. 499

500

501

(a) (b) 502

-15

-10

-5

0

5

10

15

0:00

2:00

4:00

6:00

8:00

10:0

0

12:0

0

14:0

0

16:0

0

18:0

0

20:0

0

22:0

0

Hea

t flu

x [W

/m2 ]

Time[h]

Perimeter zonesHeat flux from room/floor plenum to slab

1.Well-mixed2/5.UFAD (completed ducted)3/6.UFAD (ducted perimeter zones)4/7.UFAD (completed un-ducted)

-15

-10

-5

0

5

10

15

0:00

2:00

4:00

6:00

8:00

10:0

0

12:0

0

14:0

0

16:0

0

18:0

0

20:0

0

22:0

0

Hea

t flu

x [W

/m2 ]

Time[h]

Core zoneHeat flux from room/floor plenum to slab

1.Well-mixed2/5.UFAD (completed ducted)3/6.UFAD (un-ducted core zone)4/7.UFAD (completed un-ducted)

Diffuser Exhaust

Supply air

Return air

Diffuser

Exhaust

Supply air

Return air

Diffuser

Exhaust

Return air

Supply air Heating

coil

18

503 (c) (d) 504

Figure 10. Heat transfer across the floor slab of the office building on the summer design day - a 505

weekday. Refer to Table 3 for case descriptions. (a) Heat flux from the room/floor plenum to the 506

floor slab in perimeter zones, (b) Heat flux from the room/floor plenum to the floor slab in the 507

core zone, (c) Heat flux from the downstairs ceiling plenum to the floor slab in perimeter zones, 508

and (d) Heat flux from the downstairs ceiling plenum to the floor slab in core zones (right) 509

510

511

512 (a) Perimeter zones (b) Core zone 513

514

Figure 11. Cooling loads on the summer design day for the office building with different 515

ventilation systems. The numbers are for different cases as described in Table 3. 516

-15

-10

-5

0

5

10

15

0:00

2:00

4:00

6:00

8:00

10:0

0

12:0

0

14:0

0

16:0

0

18:0

0

20:0

0

22:0

0

Hea

t flu

x [W

/m2 ]

Time[h]

Perimeter zonesHeat flux from ceiling plenum to slab

1.Well-mixed2/5.UFAD (completed ducted)3/6.UFAD (ducted perimeter zones)4/7.UFAD (completed un-ducted)

-15

-10

-5

0

5

10

15

0:00

2:00

4:00

6:00

8:00

10:0

0

12:0

0

14:0

0

16:0

0

18:0

0

20:0

0

22:0

0

Hea

t flu

x [W

/m2 ]

Time[h]

Core zoneHeat flux from ceiling plenum to slab

1.Well-mixed2/5.UFAD (completed ducted)3/6.UFAD (un-ducted core zone)4/7.UFAD (completed un-ducted)

0

10

20

30

40

50

60

70

0:00

2:00

4:00

6:00

8:00

10:0

0

12:0

0

14:0

0

16:0

0

18:0

0

20:0

0

22:0

0

Coo

ling

load

[W

/m2 ]

Time[h]

Perimeter zones_cooling load1.Well-mixed2/5.UFAD (completed ducted)3/6.UFAD (ducted perimeter zones)4/7.UFAD (completed un-ducted)

0

10

20

30

40

50

60

70

0:00

2:00

4:00

6:00

8:00

10:0

0

12:0

0

14:0

0

16:0

0

18:0

0

20:0

0

22:0

0

Coo

ling

load

[W

/m2 ]

Time[h]

Core zone_cooling load1.Well-mixed2/5.UFAD (completed ducted)3/6.UFAD (un-ducted core zone)4/7.UFAD (completed un-ducted)

19

517

518

Figure 12.Monthly energy consumption by the chiller in the building’s HVAC system.The 519

numbers are for different cases as described in Table 3. 520

521

522 Figure 13.Monthly energy consumption by the boiler in the building’s HVAC system. The 523

numbers are for different cases as described in Table 3. 524

525

526

0

1000

2000

3000

4000

5000

6000

7000

8000

1 2 3 4 5 6 7 8 9 10 11 12

Ele

ctri

c C

onsu

mpt

ion

[kW

h]

Month

Chiller energy consumptionNo heating coils in diffusers

1.Well-mixed2.UFAD (completed ducted)3.UFAD (partially ducted)4.UFAD (completed un-ducted)

0

1000

2000

3000

4000

5000

6000

7000

8000

1 2 3 4 5 6 7 8 9 10 11 12

Ele

ctri

c C

onsu

mpt

ion

[kW

h]Month

Chiller energy consumptionWith heating coils in diffusers


0

500

1000

1500

2000

2500

3000

3500

4000

1 2 3 4 5 6 7 8 9 10 11 12

Gas

Con

sum

ptio

n [k

Wh]

Month

Boiler energy consumptionNo heating coils in diffusers


0

500

1000

1500

2000

2500

3000

3500

4000

1 2 3 4 5 6 7 8 9 10 11 12

Gas

Con

sum

ptio

n [k

Wh]

Month

Boiler energy consumptionWith heating coils in diffusers


20

527 Figure 14.Monthly energy consumption by the fans in the building’s HVAC system.The numbers 528

are for different cases as described in Table 3. 529

530

0

100

200

300

400

500

600

700

800

1 2 3 4 5 6 7 8 9 10 11 12

Ele

ctri

c C

onsu

mpt

ion

[kW

h]

Month

Fan energy consumptionNo heating coils in diffusers


0

100

200

300

400

500

600

700

800

1 2 3 4 5 6 7 8 9 10 11 12

Ele

ctri

c C

onsu

mpt

ion

[kW

h]

Month

Fan energy consumptionWith heating coils in diffusers


influence of floor plenum on energy performance of

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