A study on a ventilation stack integrated with a light pipe

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 ATE4152_proof ■ 5 May 2012 ■ 1/9 Applied Thermal Engineering xxx (2012) 1e9 Contents lists available at SciVerse ScienceDirect Applied Thermal Engineering journal homepage: www.elsevier.com/locate/apthermeng A study on a ventilation stack integrated with a light pipe Q1 Thanyalak Taengchum m a, Surapong Chirarattananon n a, b, *, Robert H.B. Exelll a, b, Kuskana Kubaha a c, a, b Pipat Chaiwiwatworakull a Joint Graduate School of Energy and Environment, King Mongkut’s University of Technology Thonburi, Bangkok, Thailand Science and Technology Postgraduate Education and Research Development Office, Ministry of Education, Thailand c School of Energy Environment and Materials, King Mongkut’s University of Technology Thonburi, Bangkok, Thailand b h i g h l i g h t s < An experimental and theoretical study of solar heated stack-light pipe is reported. < Solar heated water exchanges heat with air to create a stack flow. < Wind, stack, and heat transfer effects are simultaneously coupled. < The volume of air flow is sufficient for ventilation and passive cooling. < The light pipe transmits sufficient natural daylight for general illumination. a r t i c l e i n f o a b s t r a c t Article history: Received 11 November 2011 Accepted 19 April 2012 Available online xxx A theoretical and experimental study has been conducted on the performance of a vertical light pipe that also functions as an air flow stack for night ventilation. The rectangular light pipe of height 3 m and cross-section area 0.0625 m2 surrounded by an air duct of total cross-section area 0.23 m2 is situated above a room of height 3.8 m and floor area 9 m2. Heat transfer from the hot water in the wraparound hot water jacket to the air in the duct is assisted by stainless steel fins. The ventilation of the room, due partly to the buoyancy of the air in the duct and partly to the wind effect, amounted to nearly 10 air changes per hour which is sufficient for passive cooling during cooler night periods. The light pipe has specular reflecting walls. It was found that the transmission of daylight through the light pipe in the middle of a partly cloudy day was sufficient for illuminating the room to general illumination level. Ó 2012 Published by Elsevier Ltd. Keywords: Natural ventilation Daylighting Thermal buoyancy Light pipe Counter flow heat exchanger Solar water heater 1. Introduction The climate of Thailand is hot and humid. Air-conditioning to provide cooling for thermal comfort has been increasingly used in consonant with the increase in disposable income. Air-conditioning load accounts for over 70% of total electric load in a small household. However, night time air is cool and it is possible to use natural ventilation to achieve thermal comfort in detached houses in suburban areas for most nights of a year. A report of the National Statistical Office of Thailand on the appliance ownership in Thailand, [1], shows that 14.3% and 12.8% of all households own air-conditioners and electric hot water heaters * Corresponding author. Joint Graduate School of Energy and Environment, King Mongkut’s University of Technology Thonburi, Bangkok, Thailand. Tel.: þ66 (2) 4708309. E-mail addresses: [email protected] (T. Taengchum), surapong@jgsee. kmutt.ac.th (S. Chirarattananon), [email protected] (R.H.B. Exell), kuskana. [email protected] (K. Kubaha), [email protected] (P. Chaiwiwatworakul). (used for personal hygiene-bathing) in 2008. Among all households, more than 40% of middle and high income households own both air-conditioners and electric hot water heaters. It seems that ownerships of air-conditioners and electric hot water heaters correlate strongly and both are increasing steeply. The authors perceive that warm water (at 40e50  C) produced by heat pump air-conditioner while running as room air-conditioner, or by solar hot water heater, could be used to produce and store warm water to drive stack air flow for night ventilation. In the latter case, cooling and ventilation are completely passive. The device in this paper integrates a light pipe and a ventilation stack in the same unit that will reduce construction cost. The whole unit is expected to be situated in a large space at the middle of a house. Daylight transmitted through the light pipe will provide general lighting for the space during daytime while stack air flow y will provide night ventilation. In this paper, raytracing is used for calculation of both daylight from sun and that from sky. A wellknown sky luminance distribution model is used with measured 1359-4311/$ e see front matter Ó 2012 Published by Elsevier Ltd. doi:10.1016/j.applthermaleng.2012.04.045 Please cite this article in press as: T. Taengchum, et al., A study on a ventilation stack integrated with a light pipe, Applied Thermal Engineering (2012), doi:10.1016/j.applthermaleng.2012.04.045 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119 120 121 122 123 124 125 126 127 128 129 130 131 132 133 134 135 136 137 138 139 140 141 142 143 144 145 146 147 148 149 150 151 152 153 154 155 156 157 158 159 160 161 162 163 164 165 166 167 168 169 170 171 172 173 174 175 ATE4152_proof ■ 5 May 2012 ■ 2/9 2 T. Taengchum et al. / Applied Thermal Engineering xxx (2012) 1e9 Nomenclature1 DPstack g ra hs hNL Ts Ta V_ sin V_ sout As CDi DPsin DPsout ns Pwi Cpi vi stack pressure at the stack inlet or outlet constant of gravity, 9.81 density of air at inlet stack height neutral height temperature of air at stack outlet temperature of air at stack inlet rate of volumetric flow of air at stack inlet rate of volumetric flow of air at stack outlet cross-sectional area of stack discharge coefficient of air flow through an opening stack pressure inlet stack pressure outlet exponent of stack pressure wind pressure at wall i local pressure coefficient at wall i speed of wind into wall i daylight illuminance data obtained from a nearby station on the same campus. A heat exchange model is integrated with a stack air flow model to predict the rate of air flow in the stack. However, the actual air flow is disturbed by wind and the model is modified to account for the effect of wind. The integrated model is used to calculate air flow under the situation of the experiment. Zhai and Previtali, [2], in a review of vernacular architecture, identify local climate and cultural heritage as the main influences on ancient architecture of a location. The authors use vernacular houses in two locations in Indonesia where the houses are constructed on raised floor with open windows and doors for natural ventilation for illustration. Natural ventilation and daylighting are two methods that can help reduce energy needs in modern buildings. Normally, they are separate and independent operations. However, in a project supported by the European Union and participated by five European institutions in five countries called ‘TripleSave’, [3], it was demonstrated that a light pipe can be integrated with a ventilation stack and used with passive or active heating or cooling system to serve three functions of daylighting, ventilation, and heating or cooling of buildings. A number of publications by participating institutions illustrate theoretically and experimentally the effectiveness of light transmission of light pipes fitted with different light entry ports and surface material, [4e6], and the effectiveness of air flow through different ventilation terminals that are subjected to wind from different directions, [7,8]. Air flow by natural means is due to pressure differences caused by wind and thermal buoyancy. In the case of buoyancy air flow in a solar chimney, a number of studies have been reported, such as those in Refs. [9e11], where heat transfer from solar radiation to air creates buoyancy force to drive air flow through stack, [12,13]. Studies of wind effect on air flow into buildings have also been conducted, [14e16]. The methods used in Refs. [9e16] have not been directly applied to the case of simultaneous heat transfer to air in a stack when the air flow is subjected to the influence of wind at the stack outlet. Superposition method with simplified model has been employed for the mixed wind and stack driven flow in some studies, [17,18]. However, computational fluid dynamics (CFD) instead has been reportedly used to study stack air flow that is simultaneously subjected to the force of wind, [19], and also to solar heating, [20]. 1 Listed in order of appearance. V_ wi Awi nw dp 3 N Ma Mw R U UAa DT rs Pf f Dh Cps rate of volumetric flow of air due to wind at wall i area of wall i exponent of wind pressure internal counter balancing pressure against wind pressure effectiveness of (stack) heat exchanger number of transfer units heat capacity flow rate of air heat capacity flow rate of water ratio of heat capacity flow rate overall heat transfer coefficient air-side heat transfer parameter log mean temperature difference (LMTD) density of air at stack outlet pressure loss due to friction friction factor hydraulic diameter wind pressure coefficient at stack outlet Light pipes are hollow light guidance systems with highly reflective interior surfaces that are used to transfer natural daylight from both the sun and the sky from the exterior of a building into its interior spaces. Light pipes comprising circular cross sections (or cylindrical shape) are more commonly used than those comprising y rectangular cross sections. Raytracing and flux transfer have been applied successfully to the study of façade-mounted rectangular pipes by Hien and Chirarattananon, [21]. Dutton and Shao, [22], use long thin rectangular sections to form approximate circular shaped pipes and simulate light pipe transmission by the use of Photopia, a computer program. Swift et al., [23], develop theoretical model of transmission of rectangular light pipe for collimated rays and report that results from the model agree well with experimental results. 2. Thermal environment and vernacular architecture Traditional practice of housing construction is influenced by and reflects the climate of the location and cultural heritage, [2]. Prior to the advent of air-conditioning, buildings in Thailand were designed for natural ventilation. Dry-bulb temperature of the central region of Thailand varies from 25 to 35  C over a daily range of about 10  C for every day except those in the cooler period of November to e For most January. Daily range of relative humidity is 40e80%. nights, air temperature in suburban and less densely populated area surrounding a detached house is sufficiently cool for night ventilation. Table 1 shows the percentage number of hours in a year between 10 PM and 7 AM that ambient temperature falls below the given value. Among the middle income people who own air-conditioners, the present practice is to turn the air-conditioners in the bedrooms on to sleep at night. The configuration of the stack proposed in this paper will enable utilization of cool natural air for ventilation for the same purpose. Table 1 Number of hours between 10 PM and 7 AM in a year that the ambient temperature falls within the given value. Temperature,  C 30 29 28 27 26 25 Number of hours % of number of hours between 10 PM and 7 AM 3286 100 3256 99 3095 94 2664 81 1925 59 1099 33 Please cite this article in press as: T. Taengchum, et al., A study on a ventilation stack integrated with a light pipe, Applied Thermal Engineering (2012), doi:10.1016/j.applthermaleng.2012.04.045 176 177 178 179 180 181 182 183 184 185 186 187 188 189 190 191 192 193 194 195 196 197 198 199 200 201 202 203 204 205 206 207 208 209 210 211 212 213 214 215 216 217 218 219 220 221 222 223 224 225 226 227 228 229 230 231 232 233 234 235 236 237 238 239 240 ATE4152_proof ■ 5 May 2012 ■ 3/9 3 T. Taengchum et al. / Applied Thermal Engineering xxx (2012) 1e9 241 242 243 244 245 246 247 248 249 250 251 252 253 254 255 256 257 258 259 260 261 262 263 264 265 266 267 268 269 270 271 272 273 274 275 276 277 278 279 280 281 282 283 284 285 286 287 288 289 290 291 292 293 294 295 296 297 298 299 300 301 302 303 304 305 Fig. 1 shows monthly average hourly temperatures. Hourly temperatures of the four months of November to February exhibitt distinct pattern of the cooler season where the values are lower than those of all other months during the night to the early morning period. If the acceptable temperature is 27  C, the air is sufficiently cool for over 80% of the night time period in a year, as seen from Table 1. 3. Experimental set up The configuration of the light pipe and solar-heated ventilation stack is shown in Fig. 2, and a cross-section of the stack interior, which has five concentric layers, is shown in Fig. 3. The fins, which transfer heat from the water to the air in the duct, are of stainless steel 1.2 mm thick. The volume of hot water to be contained by the jacket is 0.286 m3. The insulation layers are of fiberglass 12.5 mm thick. Fig. 4 shows a vertical side view of the stack, which is 3 m high and is situated above the ceiling of the experimental room. Thermally insulated half-inch copper pipes were used between the solar water heater tank and the stack water jacket. Hot water from the solar collector tank was pumped downwards through the water jacket causing the air inside the stack to rise in counter flow. The stack water jacket and the air duct act as a counter flow heat exchanger. Typical values of thermal conductivities of copper and stainless steel are used for calculation of heat transfer parameters. The water side heat transfer area is 6.6 m2 while that for the air-side is 22.5 m2. The estimated value of the overall heat transfer parameter (UA value) is 56 W K1. The light pipe transmits daylight to the interior space, which has four doors, as shown in Fig. 5. A chamber of 1.1 m height at the top of the stack has downward-pointing black-painted slats to allow air to escape and keep out rain. The glass cover at the top of the pipe has transmittance 88%. The walls of the light pipe are covered with specularly reflecting plastic film having reflectance 99%. The bottom of the light pipe is covered with a translucent plastic plate used to diffuse the light. The size of the light pipe was chosen so that there was sufficient light flux to illuminate the room. The size of air duct was chosen to allow sufficient air flow for ventilation. The height of the stack of 3.0 m was determined to create sufficient stack pressure and flow. A daylight measurement station has been set up on top of a nearby 7-storey building on the seaside campus of the university. Beam normal illuminance and irradiance are measured directly by a suntracker. The tracker also holds two shading balls to shade out sunlight and sun irradiance from an illuminance sensor and an Fig. 1. Monthly average hourly temperature. Fig. 2. Light pipe and solar heated ventilation stack. irradiance sensor to give values of diffuse sky illuminance and diffuse irradiance. Global illuminance and global irradiance as well as total illuminance and total irradiance in each cardinal direction are measured by individual sensors. Measured data of sun and sky illuminance from the station were used in the calculation described in the followings. 4. Mathematical models The stack with hot water that flows through its jacket performs as a water to air heat exchanger. The heated air flows up through the stack by buoyancy force. The room where the stack is situated has four openings (door and windows) that are subjected to the force of wind. The mechanisms of heat transfer and air flows in the three connected components can be described by three coupled mathematical models as follows. 4.1. Air flow through stack The pressure difference that drives airflow through a stack due to buoyancy force can be determined by the following relationship, [12,13], g ra ðhs  hNL ÞðTs  Ta Þ ; Ts DPstack ¼  (1) where DPstack is the pressure difference at the stack inlet (when hs ¼ 0) or the stack outlet (when hs ¼ stack height), g is the gravity constant, ra is the density of air at the inlet, hNL is the neutral height, Fig. 3. Horizontal cross-section of stack. Air duct is divided into 12 square channels by stainless steel fins. Layers of thermal insulation are shaded. Please cite this article in press as: T. Taengchum, et al., A study on a ventilation stack integrated with a light pipe, Applied Thermal Engineering (2012), doi:10.1016/j.applthermaleng.2012.04.045 306 307 308 309 310 311 312 313 314 315 316 317 318 319 320 321 322 323 324 325 326 327 328 329 330 331 332 333 334 335 336 337 338 339 340 341 342 343 344 345 346 347 348 349 350 351 352 353 354 355 356 357 358 359 360 361 362 363 364 365 366 367 368 369 370 ATE4152_proof ■ 5 May 2012 ■ 4/9 4 371 372 373 374 375 376 377 378 379 380 381 382 383 384 385 386 387 388 389 390 391 392 393 394 395 396 397 398 399 400 401 402 403 404 405 406 407 408 409 410 411 412 413 414 415 416 417 418 419 420 421 422 423 424 425 426 427 428 429 430 431 432 433 434 435 T. Taengchum et al. / Applied Thermal Engineering xxx (2012) 1e9 Fig. 4. The three system components. Ts is the temperature of the air at the stack outlet, and Ta is the temperature of the air at the stack inlet. The neutral height hNL is an unknown. For a simple stack where the stack dimension and air temperatures are known, equation of balance of air flows can be set up from which the neutral height can be obtained. The volumetric rates of air flow V_ sin into and V_ sout out of the stack are determined by V_ sin ¼ As CDi DPsin jDPsin j 1ns and V_ sout ¼ As CDi DPsout jDPsout j1ns (2) where As is the cross-sectional area of the stack, CDi is a discharge coefficient, and ns is an exponent. In this research the inlet area equals the outlet area. When wind is present, the wind pressure on each external facade of a room superimposes a pressure to the air at a given location. The pressure difference Pwi between the wind pressure on the wall i and the (static) local outdoor air pressure at the same location is given by Ref. [16] as 1 C r v2 2 pi a i (3) where Cpi is a local pressure coefficient the value of which is dependent on the direction of the wind on the facade, and vi is the wind speed. References [14,15] give average value of local wind pressure coefficient on a wall in a given direction. The volume flow rate V_ wi of air into the room through an opening of area Awi in the wall is given by the formula, [16], A CDi Pwi nw ¼ wi 1nw V_ wi ¼ Awi CDi Pwi jPwi j Pwi ¼ 1 C r v2  dp 2 pi a i (5) 4.3. Heat transfer in the stack 4.2. Air flow due to wind Pwi ¼ where CDi is a discharge coefficient, and nw is an exponent between 0.4 for large openings and 1.0 for small openings. The second equality in Equation (4) is used to preserve the sign of the volumetric flow V_ wi which can be negative. In a contiguous room with openings, the sum of air flows into the room must balance to zero for all wind velocity and for any room configuration. This implies that there is a counter-balancing pressure from the interior of the room to enable the flows of air into and out of the room to balance out. Let this counter balancing pressure in the room in Fig. 5 be dp. Then for opening i, the net wind pressure on the opening is given as (4) The temperature of the air entering the stack at the bottom is assumed to be the ambient air temperature Ta in the room. Let Twi be the temperature of the water flowing into the top of the water jacket around the stack from the solar water tank. Then the temperature Ts, which is the temperature that air emerges from the stack is raised to, is given by Ref. [24] as Ts ¼ Ta þ 3 ðTwi  Ta Þ (6) where 3 is the effectiveness of the heat exchange process in the stack. This value is obtained from 3 ¼ 1  exp½Nð1  RÞ 1  R exp½Nð1  RÞ (7) where N is the number of heat transfer units, and R is ratio of the heat capacity flow rates of air Ma and water Mw. The quantity N is related to heat capacity flow rates and the heat exchanger Please cite this article in press as: T. Taengchum, et al., A study on a ventilation stack integrated with a light pipe, Applied Thermal Engineering (2012), doi:10.1016/j.applthermaleng.2012.04.045 436 437 438 439 440 441 442 443 444 445 446 447 448 449 450 451 452 453 454 455 456 457 458 459 460 461 462 463 464 465 466 467 468 469 470 471 472 473 474 475 476 477 478 479 480 481 482 483 484 485 486 487 488 489 490 491 492 493 494 495 496 497 498 499 500 ATE4152_proof ■ 5 May 2012 ■ 5/9 5 T. Taengchum et al. / Applied Thermal Engineering xxx (2012) 1e9 501 502 503 504 505 506 507 508 509 510 511 512 513 514 515 516 517 518 519 520 521 522 523 524 525 526 527 528 529 530 531 532 533 534 535 536 537 538 539 540 541 542 543 544 545 546 547 548 549 550 551 552 553 554 555 556 557 558 559 560 561 562 563 564 565 Fig. 5. The experimental room and stack (sloping roof not shown). parameter UAa, where U is the overall heat transfer coefficient and Aa is the heat transfer area (air-side) of the heat exchanger. The value of UAa (The same UA value as that in Section 3) is related to the heat transfer rate Q by the equation, (8) Q ¼ UAa DT; where DT is the log mean temperature difference (LMTD) across the heat exchanger, see Ref. [21]. In Equation (6), Ts (and Two as well) is dependent on 3 , which is in turn dependent on the rate of heat capacity flow of air, Ma or the volumetric flow rate of air through the stack, V_ sin or V_ sout in Equation (2). where rout is the density of air at the outlet of the stack. The counter balancing pressure dp for this experimental room also exerts on the inlet (bottom) of the stack, thus modifying the pressure at the inlet. The wind pressure also exerts at the top of the stack, thus modifying the pressure at the stack outlet. The roughness of the interior surface of the stack can act to reduce the flow of air through the stack. The pressure loss due to this friction force is given by the y equation, [21], as DarcyeWeisbach Pf ¼ f h v2in r Dh a 2 where f is the friction factor and Dh is the hydraulic diameter. The resultant mass balance equation at the stack then becomes 4.4. Combined equations When wind exerts pressure onto the experimental room with the stack-light pipe in Fig. 5, air can flow through all four openings and through the outlet on top of the stack. The mass balance equation for air flowing through the room is written as " X i ¼ 1e4 # ra V_ wi þ rs V_ sout ¼ 0; or 3 1 Cpi ra v2i  dp 7 6 X ra Awi CDi  2 4 1nw 5 þ rs V_ sout ¼ 0   i ¼ 1e4 1Cpi r v2  dp  a i 2  2 (9) ra V_ sin þ rs V_ sout ¼ 0; or where, ra DPsin jDPsin j1ns þ rs DPsout jDPsout j1ns ¼ 0 DPsin ¼ ðgra ðhNL ÞðTs  Ta Þ=Ts Þ þ dp  Pf DPsout ¼ ðgra ðhs  hNL ÞðTs  Ta Þ=Ts Þ þ 1=2Cps ra v2i . (10) and Here, Cps is the wind pressure coefficient at the stack outlet. It is clear that dp and hNL are the two unknowns in Equations (9) and (10). Section 4.3 also identifies Ma as another unknown variable. Equations (6), (9) and (10) form 3 simultaneous nonlinear equations that can be used to solve for the problem of air flow through the heated stack that is subjected to the force of wind. The approach taken differs from those in Refs. [17,18]. Here, all phenomena are dealt with explicitly without any simplifying assumption and the equations are obtained from application of heat and mass balances. Please cite this article in press as: T. Taengchum, et al., A study on a ventilation stack integrated with a light pipe, Applied Thermal Engineering (2012), doi:10.1016/j.applthermaleng.2012.04.045 566 567 568 569 570 571 572 573 574 575 576 577 578 579 580 581 582 583 584 585 586 587 588 589 590 591 592 593 594 595 596 597 598 599 600 601 602 603 604 605 606 607 608 609 610 611 612 613 614 615 616 617 618 619 620 621 622 623 624 625 626 627 628 629 630 ATE4152_proof ■ 5 May 2012 ■ 6/9 6 631 632 633 634 635 636 637 638 639 640 641 642 643 644 645 646 647 648 649 650 651 652 653 654 655 656 657 658 659 660 661 662 663 664 665 666 667 668 669 670 671 672 673 674 675 676 677 678 679 680 681 682 683 684 685 686 687 688 689 690 691 692 693 694 695 T. Taengchum et al. / Applied Thermal Engineering xxx (2012) 1e9 4.5. Transmission of daylight through light pipe Hien and Chirarattananon n [21], use raytracing principle in calculating transmission of light rays through rectangular light pipes. The configuration of each flat section that together with other sections forms a rectangular light pipe is defined using plane geometry and each of its corner point is referenced to a Cartesian coordinate. When light rays traveling along a given direction incident on an internal surface, the rays are specularly reflected onto another surface. Daylight from the sun is considered to comprise collimated rays and raytracing method is used to trace its travel. However, for diffuse light from the sky, the method of flux transfer is used to calculate its transmission through a pipe. The authors in Ref. [21] encode the raytracing and flux transfer methods in a computer program called BESim. In this paper, the ASRC-CIE sky luminance model is used to model diffuse daylight, [25], and use raytracing y to trace light rays from each of the standard 145 sky zones. The ASRC-CIE model uses four CIE sky models, clear, intermediate, overcast, and a high turbidity clear sky model. Perez’s clearness index and brightness index in Ref. [25], calculated from values of measured beam irradiance and diffuse irradiance, are used to identify sky condition and to weigh contribution from each of the four CIE sky models. The model references zenith luminance. 5. Results and discussion 5.1. Ventilation 5.1.1. Design calculations for the ventilation stack For ventilating the experimental room, the preliminary target was to have an air flow of 0.1 m3 s1, or about 10 air changes per hour in this experimental room. Assuming ventilation for 8 h per night and a rise in temperature of the air passing through the stack of 5  C, the amount of heat required on the air-side was about 4.0 kW h per day. Average solar insolation per day at the location of experiment is 18.2 MJ m2 or 5 kW h m2. Assuming an average efficiency of a solar collector of 50%, the required collector area is 1.6 m2. The collector available in the market, chosen and used in the experiments had an area of 2.16 m2 and a water storage tank with a capacity 160 l. The capacity of a small water pump available and used was 280 l per hour, or 0.078 l s1. The temperature drop of the hot water through the stack jacket was expected to be 1.5  C. 5.1.2. Experiments on ventilation Experiments were conducted mainly during daytime to determine the amount of ventilation produced by the stack with solar heated water pumped through the water jacket. Measurements were made of the water flow rate, the temperature Twi of the water entering the top of the stack, the temperature Two of the water from the bottom of the stack, the temperature Ts of the air emerging from the top of the stack and the air speed vin into the bottom of the stack. Type K thermocouples were used for the temperature measurements, and hot wire anemometers were used for measuring the air speed into the stack. Measurements were also made of the wind speed and direction, the ambient air temperature Ta, and the ambient relative humidity outside the building. The measured temperatures Ta, Twi, Two, and Ts for an experiment conducted on a day in June 2010, when solar insolation was high and wind speed was low, are shown in Fig. 6. Fig. 7 shows the measured volumetric airflow rate V_ sout through the stack in the same experiment together with the calculated total airflow rate V_ sin . The distinctive patterns of the temperatures of the inlet Twi Fig. 6. Measured temperatures in the stack. and that of the outlet Two hot water between 10.07 and d 10.59 in Fig. 6 require a detailed examination. As is described subsequently, there are 3 distinct periods of the temperature patterns: 10.07 to 10.59, 10.59 to 12.00, and beyond 12.00. Period 1: 10.07 to 10.59. At the start of the experiment, the water in the stack jacket was all at 32.5  C. The solar collector had accumulated hot water in its own tank during the previous day to stratify the water in the tank to different temperature levels. Fig. 8 together with Fig. 6 help illustrate the situation. Part of the water from the lowest stratification at the bottom of the tank, with temperature 45  C, had flowed by natural thermosyphon through the solar collector in the morning that resulted in raising the temperature of water in the top stratum to 62  C when the pump operated to supply hot water to the jacket. Fig. 8a illustrates the situation at the start of the experiment at 10.07 h. Fig. 9 shows graphs of global solar irradiance, and of solar illuminance for the day, the two graphs at the top. The temperature of the water that flowed subsequently from the remainder of the solar Fig. 7. Comparison between experimental and calculated volumetric airflow rates. Please cite this article in press as: T. Taengchum, et al., A study on a ventilation stack integrated with a light pipe, Applied Thermal Engineering (2012), doi:10.1016/j.applthermaleng.2012.04.045 696 697 698 699 700 701 702 703 704 705 706 707 708 709 710 711 712 713 714 715 716 717 718 719 720 721 722 723 724 725 726 727 728 729 730 731 732 733 734 735 736 737 738 739 740 741 742 743 744 745 746 747 748 749 750 751 752 753 754 755 756 757 758 759 760 761 762 763 764 765 766 767 768 769 770 771 772 773 774 775 776 777 778 779 780 781 782 783 784 785 786 787 788 789 790 791 792 793 794 795 796 797 798 799 800 801 802 803 804 805 806 807 808 809 810 811 812 813 814 815 816 817 818 819 820 821 822 823 824 825 ATE4152_proof ■ 5 May 2012 ■ 7/9 7 T. Taengchum et al. / Applied Thermal Engineering xxx (2012) 1e9 Fig. 8. Illustration of the water temperatures during the two periods. Fig. 9. Irradiance and illuminance on the day of experiment. hot water tank into the stack jacket was lower while the temperature of the leaving water from the jacket, Two, remained at 32  C. By 10.59 all water from the stack jacket had emptied into the solar collector tank and the mixture of water from the head of the jacket at 10.07, with temperature 48  C, had reached the bottom of the jacket. During this period, heat from only the upper part of the jacket was transferred to air to drive the stack flow since the temperature of water at the lower part remained the same as that of the air in the stack. Effective height of the part of the stack that drove air flow started from 0 m at the beginning to reach 3 m at the end of the period. Stack driven air flow was lowest during this period. Period 2: 10.59 to 12.00. Fig. 8b illustrates the situation at the start of this period. The water in stack jacket had totally replaced the water in the solar water tank and this water had been heated by the solar collector to 45  C. The temperature of the water in the jacket had reached 45  C. Stack driven air flow had increased from the first period. Period 3: from 12.00 onward. Solar radiation from a relatively clear sky had increased its intensity and the temperature at the top of the collector tank increased beyond 45  C at the beginning of this period. The temperature of water flowing into the stack exceeded 48  C near the end of the period. Stack driven air flow became highest in this period. Summary of results. Table 2 shows average temperatures, wind speed, air flow, heat transfers and other quantities from the experiment for the 3 periods. The average inlet water temperature in Period 1 is higher than those of the other two periods. In period 2, the average outlet water temperature is higher than that at the inlet, thus straight forward calculation of heat transfer from hot water would give misleading results for Period 1 and 2. The values of log-mean temperature difference LMTD and heat transfer to air Q_ a in Period 3 are used in Equation (6) to obtain the air-side heat transfer parameter UAa a value of 47.9 W K1. This value is used in the calculation of air flow through the stack to be described next. 5.1.3. Calculation of stack air flow In applying Equations (1)e(10) for calculation of stack air flow, first the coefficients, exponents, and areas where air flows through of Equations (2)e(4) are set to values appropriate for the problem. Table 3 shows the given values, where the discharge coefficients, wind pressure coefficients and exponents assume values typically recommended d [13e16]. Note that the wind was from the southwest direction. Next, the values of heat transfer parameters in Equations (7) and (8) are given as Mw ¼ 325.7 W K1 (for the pump flow of 280 l per minute) and UAa ¼ 47.9 W K1. In the experiment, the physical dimensions of the room and the stack are already known. From the experiment, the temperatures of entering water and of air into the stack are also known. In the absence of wind, there is no wind pressure at the openings into the room, and the air flows through the stack due to buoyancy force of the difference of air temperatures at the entry and exit of stack. The air temperature difference is due to heat transfer by water to air. In this case, the temperature of air at stack exit and the rate of air flow are two unknown variables in the heat transfer equation (6) or (7) and the stack flow equation (10). When wind exerts pressure at the openings it causes a counter balancing pressure dp. This same pressure adds to the buoyancy pressure at the bottom of the stack. The number of unknowns becomes 3. The problem of calculation of stack air flow now is reduced to the following problem statement: Given a set of values of condition of air at stack inlet (Ta and relative humidity), inlet water temperature, and wind velocity, find a set of values for va, stack air velocity, dp, counter pressure to wind, hNL, neutral height of air flow in the stack, that satisfy the flow balance and heat transfer equations (6), Table 2 Experimental average temperature, air flow, and heat transfer values. Period Wind speed, m s1 Twi  C Two  C Ta  C Ts  C V_ sin m3 s1 Q_ a W Q_ w W LMTD UAa W K1 1 2 3 0.71 0.92 1.05 51.9 44.6 47.9 32.6 46.3 45.7 32.8 33.1 33.8 39.8 40.7 39.0 0.025 0.055 0.089 152 428 496 NA NA 703 NA 7.67 10.34 NA NA 47.9 Please cite this article in press as: T. Taengchum, et al., A study on a ventilation stack integrated with a light pipe, Applied Thermal Engineering (2012), doi:10.1016/j.applthermaleng.2012.04.045 826 827 828 829 830 831 832 833 834 835 836 837 838 839 840 841 842 843 844 845 846 847 848 849 850 851 852 853 854 855 856 857 858 859 860 861 862 863 864 865 866 867 868 869 870 871 872 873 874 875 876 877 878 879 880 881 882 883 884 885 886 887 888 889 890 891 892 893 894 895 896 897 898 899 900 901 902 903 904 905 906 907 908 909 910 911 912 913 914 915 916 917 918 919 920 921 922 923 924 925 926 927 928 929 930 931 932 933 934 935 936 937 938 939 940 941 942 943 944 945 946 947 948 949 950 951 952 953 954 955 ATE4152_proof ■ 5 May 2012 ■ 8/9 8 T. Taengchum et al. / Applied Thermal Engineering xxx (2012) 1e9 Table 3 Values of coefficients and exponents for Equations (2)e(4). Air flow opening North East South West Stack Area (m2) Wind pressure coefficient Discharge coefficient Exponent in (2) and (4) 0.1 0.231 0.65 0.65 0.1 0.231 0.65 0.65 0.1 0.213 0.65 0.65 0.1 0.213 0.65 0.65 0.237 0.297 0.5 0.65 (9) and (10). The solution to the three simultaneous nonlinear equations was obtained by the use of multiple-variable Newton Raphson numerical method. The first step in the calculation was to assume a value each for va, dp, and hNL. With given value of va, the heat capacity flow rate Ma and consequently the temperature Ta were calculated. These values were then used to calculate the left hand side functions in Equations (9) and (10). Numerical Jacobian (matrix of difference approximation to partial derivatives of functions) were then obtained and the calculation step continued. In the experiment, all data were recorded on 1-min n interval, but the measured input values were averaged as 5-min values for use in the calculation. The calculated volumetric stack air flow obtained is shown in the graph of Fig. 7, together with graphs of experimental values, and of wind speed. For the first period, the effective stack height used in the calculation was varied linearly from 0 m at the beginning of the period to 3 m at the end of the period. The effective stack height for the other two periods was set at 3 m. The resulting graph of volumetric air flow appears to match reasonably well with that from experiment. However, since 5-min average values of inlet water temperature and wind speed were used in the calculation, the resulting graph appears smoother than that from experiment. The calculated volumetric flow rates of air through the stack for the 3 periods are obtained respectively as: 0.052, 0.071, and 0.093 m3 s1. These values are slightly larger than the corresponding values in Table 2. The corresponding RMSD (root of mean square difference between measured and calculated) values for each of the three periods and for the whole experiment are 0.034, 0.021, 0.056, and 0.027 m3 s1. The fact that the method employed enable calculated results to match reasonable well with experimental results illustrates that this method of directly couple the three equations of wind pressure flow, stack pressure flow, and heat exchange through mass and heat balances is applicable. However, it is noted here that values of discharge coefficients are also highly relevant to the outcomes. The resultant volumetric air flow through the stack obtained corresponds to the value of discharge coefficient of 0.5 used. 5.2. Daylighting through light pipe 5.2.1. Experiment on the light pipe During daytime the stack functions as a light pipe to transmit daylight into the building interior, as shown in Fig. 3. Measurements were made of the beam, diffuse and global illuminance using sensors in the daylight measurement station and the results are shown in Fig. 9. Measurements were also made of the total daylight illuminance at the top of the light pipe under the glass cover and the illuminance under the diffuser plate at the exit of the light pipe and the results are shown in Fig. 10. The very low illuminances before 10:20 and after 14:10 are caused by shading of the entrance to the light pipe by the slatted cover (see Fig. 4). At noon the illuminance at the entrance was 59 klux and the illuminance at the exit was 33 klux, giving a transmission of 56%, but at 12:40 the transmission was 84%. The sky of the day was relatively clear and beam illuminance featured prominently. The smaller angle between the beam rays and the pipe during noon led to the observed results. The Fig. 10. Comparison between experimental and calculated daylight illuminances. total flux of transmitted diffuse daylight was 2.5 kilo lumens (40 klux from Fig. 10 multiplied by 0.0635 m2 of aperture area), which is about the same as the volume of light flux from a common long fluorescent lamp. This is sufficient for general illumination in the room. 5.2.2. Calculation of daylight transmission through the light pipe The calculation of the daylight transmission through the light pipe was aided by the use of a modified version of BESim, where y raytracing is now used for both daylight from sun and that from sky. There is fairly good agreement between the experimental and calculated values. The graphs in Fig. 10 include calculated daylight illuminance at light pipe entry to show that BESim could also handle shading from obstruction. The calculated results correspond to a light pipe transmission of 75%, similar to that from experiment. 6. Conclusion The results from the experiment described in this paper demonstrate that a solar heated stack surrounding a light pipe can performs both function of providing cool natural ventilation air during night time and bringing daylight to the center of a large space in a house for general illumination during daytime. The specific configuration in this research differs from that described in Ref. [3] in that here there is a separate channel for ventilation air to flow, the light pipe itself is not used for air flow, and that hot water is used to transfer heat to air directly in the stack to create buoyancy force. The results presented in this paper demonstrate that the simple method of calculation used is applicable. The significance of this result is that it is possible to utilize the method to design a system to achieve a rate of ventilation desired, to transmit given volume of light flux, and to evaluate the cost effectiveness of a configuration. The specific experimental results illustrate that designed ventilation rate of 0.1 m3 s1 or 100 l s1 was achieved. However, it seems that the capacity of hot water tank used was not sufficient. Acknowledgements This work was jointly funded by the National Science and Technology Development Agency of the Ministry of Science and Technology and the National Research University Programme of the Commission for Higher Education of the Ministry of Education of Thailand. Please cite this article in press as: T. Taengchum, et al., A study on a ventilation stack integrated with a light pipe, Applied Thermal Engineering (2012), doi:10.1016/j.applthermaleng.2012.04.045 956 957 958 959 960 961 962 963 964 965 966 967 968 969 970 971 972 973 974 975 976 977 978 979 980 981 982 983 984 985 986 987 988 989 990 991 992 993 994 995 996 997 998 999 1000 1001 1002 1003 1004 1005 1006 1007 1008 1009 1010 1011 1012 1013 1014 1015 1016 1017 1018 1019 1020 1021 1022 1023 1024 1025 1026 1027 1028 1029 1030 1031 1032 1033 1034 1035 1036 1037 1038 1039 1040 1041 1042 1043 1044 1045 1046 1047 ATE4152_proof ■ 5 May 2012 ■ 9/9 T. Taengchum et al. / Applied Thermal Engineering xxx (2012) 1e9 References [1] Economic and Social Statistic Bureau, National Statistical Office of Thailand, Statistics on household appliance ownerships. Available at: <http://www.nso. go.th/keystat/keystatDB>. [2] Z. Zhai, J.M. 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