IAQ, ENERGY, AND COST IMPLICATIONS OF UNDERFLOOR AIR DISTRIBUTION SYSTEMS

Transcription

1 IAQ, ENERGY, AND COST IMPLICATIONS OF UNDERFLOOR AIR DISTRIBUTION SYSTEMS R Brahme 1,V Loftness 1, M Mondazzi 1, E Vineyard 2, and M MacDonald 2 1 Center for Building Performance and Diagnostics, Carnegie Mellon University 2 Oak Ridge National Laboratory ABSTRACT A recent comprehensive review of underfloor air distribution (UAD) systems literature and interviews with engineers (who have a significant number of completed projects using these systems) has shown that the use of UAD system results in measurable improvements in thermal comfort, indoor air quality, and user satisfaction. These systems support changing space layouts with continued levels of HVAC delivery, and provide high level of user control. Frequently, the UAD systems show first-cost savings over more conventional ceiling systems, churn-cost savings ranging from $10 - $50 per square meter per move, and savings of 5-35% in energy consumption. Finally, facility management costs are often reduced. In this paper, we discuss the findings from the above-mentioned review in the context of different UAD system approaches, comparing their performance to traditional ceiling-based systems. Specifically, we discuss the reasons for better air quality, reduction in peak and operational energy use, and lower first and life cycle costs of these systems. INDEX TERMS IAQ, Energy, Cost, Underfloor Air Distribution Systems, Displacement Ventilation INTRODUCTION The blanket conditioning offered in present day office buildings cannot accommodate organizational and technological changes. Both new technologies and space planning concepts are often introduced into buildings without any modification of the buildings base systems - cooling, ventilation, lighting, or networking - with serious failures occurring in the areas of air quality, thermal comfort, energy efficiency, and cost of change. To address these failures, there have been several innovations in engineering practice towards delivering flexible, user-based services for cooling, ventilation, and network access, using raised floors to move beyond embedded technologies in buildings to end-user technologies. As part of the project titled Energy Savings Potential of flexible and adaptive HVAC distribution systems for office buildings, (Loftness et. al. 2002) funded by the Airconditioning and Refrigeration Technology Institute (ARTI), the authors sought journal and conference articles, manufacturers materials, case studies, and interviews with engineers, to document the state of the knowledge, engineering diversity, and performance of recent developments in flexible and adaptive distribution systems in office buildings. In this paper, we will present the key findings from the above-mentioned project, related to the effect of the different under floor air distribution (UAD) systems on indoor air-quality, energy efficiency, and cost. Contact author s rohini@cmu.edu 254

2 UAD SYSTEM CONFIGURATION AND TYPES The UAD system configuration differs from the traditional HVAC systems mainly in the way conditioned air is distributed, with a raised floor plenum for distribution of air, instead of the traditional ceiling based distribution. The main drivers for the variations include combined vs. separate ventilation and thermal conditioning, ducted vs. unducted, passive terminal units vs. fan-powered terminal units, and floor vs. desk diffusers. These four drivers have resulted in at least 11 significant system variations around the world and can be grouped into the following three distinguishing factors: 1. Pressurized or Push systems: These can be unducted/partially ducted UAD pressurized plenums (e.g. Owens Corning HQ, Toledo, OH; Alcoa World HQ, Pittsburgh, PA) or fully ducted UAD; 2. Distributed fans or Pull systems: These systems can have distributed floor fans and UAD plenums (e.g. York Mills Center, Toronto, Canada), distributed desk fans and UAD plenums (e.g. West Bend Mutual HQ, West Bend, WI), distributed under floor fans in UAD plenums, ducted to desk (e.g. Lloyds of London, London, England), and distributed desk fans with fully ducted UAD (e.g. Intelligent Workplace south end, Pittsburgh, PA); 3. Underfloor ventilation and separate thermal conditioning systems: Some of the variations in this category include displacement ventilation (DV) with separate thermal conditioning (e.g. Intelligent Workplace north end, Pittsburgh, PA; Gartner HQ, Gundelfingen, Germany), underfloor pressurized ventilation with underfloor thermal conditioning (e.g. Nixdorf Building, Koln, Germany), underfloor pressurized ventilation with thermal conditioning above the floor (e.g. Adaptable Workplace Laboratory, Washington, D.C.), underfloor pressurized ventilation with ceiling-based Variable Air Volume (VAV) cooling (e.g. PNC Firstside Bank, Pittsburgh, PA). Here, it is important to mention the distinction between DV system mentioned above and the other UAD systems. DV system relies on low supply-air velocities of m/s combined with low temperature differentials 1-3 o C (Int-Hout 2000), and is used mainly for ventilation. The low volumes (as a result of low velocities) are suitable for loads from W/m 2 and may require separate air- or water-based thermal conditioning systems. For the typical UAD system, the supply-air velocity is in the range of m/s. These systems are used for cooling, heating, and ventilation purposes. COMPARATIVE ANALYSIS FA systems have a significant effect on indoor air quality (IAQ), energy consumption, and cost. In the following sections we describe the key findings (advantages/concerns) for each of these parameters and their impact on building design and operation. It is also important to note that not all references, mentioned in brackets after each key finding, address all the issues/implications listed below that finding. The aim here is to give the reader an overall idea of the relevant literature. IAQ: In general, the literature shows that there is a substantial improvement in the IAQ with the use of UAD systems. Three variables are often cited to quantify how these systems ensure greater indoor air quality increased ventilation effectiveness, removal of pollutants out of the breathing zone, and the ability to effectively maintain the floor plenums. 1. Ventilation effectiveness (Fisk et. al. 1991, Hedge et al. 1990, Heinemeier et al. 1990, Loudermilk 1999, Mahdavi et. al. 2000, Milam 1992, Seppänen et al. 1989, Shute 1992a, Yuan et al. 1999) is 1-2 due to the proximity of the diffusers to the individual s breathing 255

3 zone, compared to for ceiling based systems (Figure 1). This allows the designer to reduce required outdoor air quantities while sizing systems. Ventilation effectiveness is also better in cases where the user has the ability to relocate/add diffusers to match use patterns. Split ventilation and thermal conditioning systems have superior air quality since ventilation can be increased proportionally to need without awaiting thermal demand. There are some concerns regarding delivery of ventilation air in UAD systems, in the event that diffusers are closed by occupant based on thermal comfort. This can be avoided by establishing minimum settings for diffusers and specifying diffusers for shared areas. Ventilation effectiveness Loudermilk 1999 Milam 1992 Mahdavi et al Yuan et al Matsunawa et al UDA Ai Ceiling Fisk et al Overview study Case study Lab study Figure 1: Ventilation effectiveness values mentioned in literature Preferable range 2. Pollutant removal (Burnley 1993, Kim and Homma 1992, Loftness et. al. 2002) is effective in UAD systems since air from floor sweeps pollutants up and away from breathing zone and reduces cross-contamination. This is specially effective if contaminant sources are associated with heat sources, specially in DV systems. 3. Floor plenum maintenance (Bauman and Arens 1996, Houghton 1995, Int-Hout 2000, Kim et. al. 2001) in UAD systems is better due to ease of access. But there are some concerns of condensation in UAD systems since warmer chilled water reduces the coil's capacity to dehumidify resulting in condensation where water-based systems are used for cooling. This can be avoided by using separate and/or well-designed dehumidification systems and by operating the water-based cooling system above dew-point conditions. One should also design a reliable condensate management. There are also some concerns of dust/debris in unducted/partially ducted plenums due to construction materials/debris and introduction of dirt and pollutants from the occupied space. This can be reduced by sealing the concrete slab properly and avoiding diffusers in locations with possibility of liquid spillage. Energy: The literature identifies between 5-35% savings in energy consumption, which are attributed to a combination of reasons - ventilation effectiveness, stratification savings, higher supply temperatures, and in some cases, split task/ambient conditioning. These savings can be seen in both the peak loads as well as operational loads. 1. Reduction in peak loads for UAD systems occurs due to (Akimoto et. al. 1999, Bauman et al. 1999, Hu et. al. 1999, Int-Hout 2000, Loudermilk 1999, Mass 1998, Shute 1992, Sodec and Craig 1990, Spoormaker 1990, Webster et. al. 2000) a) reduction in required outdoor air quantity; b) less volume flows due to stratification, so that the space can be considered 256

4 as two zones allowing the top zone to be conditioned to less stringent comfort standard (this is offset to some extent by increased volume requirements due to higher supply-air temperatures). However, some engineers do not agree that the system sizes can be reduced; and c) reduced central fan pressure requirements when using fan terminal units and/or unducted/partially ducted plenum. 2. Savings in energy consumption during operation of UAD systems (Bauman et al. 1997, Drake et. al. 1991, Heinemeier et al. 1990, Houghton 1995, Hu et. al. 1999, Int-Hout 2000, Loudermilk 1999, Sodec and Craig 1990.) occur due to use of higher supply air temperatures. This allows for use of alternative cooling methods (e.g. geothermal energy). If chiller is used, one can expect higher chiller efficiencies, although this is not possible if it is used for latent load also. The higher supply temperatures also allow the use of economizer cycle for longer time, especially in mild climates. Systems that have split taskambient conditioning, savings can occur if task-conditioning operation is flexible and have occupancy sensors. Cost: The professional community is divided on whether first costs for FA systems are slightly lower (Int-Hout 2000, Hu et al. 1999, Wilson 1998, Milam 1992, Thomas 1995, Sodec and Craig 1990), slightly higher (Loftness et. al. 2002, Loudermilk 1999, Bauman et al. 1999, Ellison and Ramsey 1989, Mass 1998) or neutral (York 1994). Whenever networking and churn costs are considered, UAD systems are typically competitive with ceiling systems. Value must also be placed on the ability to redesign for tenant requirements, the ability to pursue just-in-time purchasing of terminal units, and productivity gains (Kroner et. al. 1992). Key findings related to first costs and operation costs (includes churn and facility management costs) are mentioned below. The effect on cost due to the reduction in the system component sizes and reduced energy consumption is not included here. 1. First cost savings (Loftness et. al. 2002, Milam 1992, Seppänen et. al. 1989, Shute 1992) in UAD systems are due to a) ability to increase/decrease diffusers and outlet boxes as needed. This allows the building owner/tenant to purchase a percentage of the infrastructure as needed over time, removing the expenditures from the first cost package. The developer can shift these nodes of service into the tenant fit-out budget; b) decrease/no change in floor-height. The floor to ceiling heights can be higher than regular buildings in some cases; and c) reduction in labor costs since most work is conducted at floor-level. Significant cost savings also result due to reduction in ductwork in unducted/partially ducted systems. These savings can be invested in high performance enclosures. 2. Churn cost savings (Loftness et. al. 1999, Int-Hout 2000, Ellison and Ramsey 1989, Shute 1992) are high since UAD systems are specifically designed to make office moves and changes inexpensive in labor, materials, and lost professional work time. The ease of assembly/disassembly of the raised floor is critical. Floor-tiles with screws reduce flexibility. In heavier tiles, the ability to lift the tile must be tested. Carpet modules and connections to the floor tile must allow ease of reconfiguring HVAC interfaces. Same dimensions for carpet tile and floor tiles improves flexibility. The distributed fans must be carefully selected for ease of relocatability. 3. Facility management costs (Mass 1998, Thomas 1995, Hedge et al. 1990, Houghton 1995) can be reduced since there is reduction of calls to facility management about overheating and drafts due to use of UAD systems. Their also reduction in maintenance costs, especially when using passive diffusers. Also, since the diffusers operate at nearly the same pressure, their are reductions in commissioning requirements. These benefits allow for reduction in facility staff requirements and give the facility staff the opportunity to complete preventive maintenance tasks. 257

5 CONCLUSION AND IMPLICATIONS This paper shows that the substantial advantage of using FA systems for conditioning of office spaces is due to the cost-effective delivery of air quality and energy efficiency in the face of spatial and technological dynamics. However, depending on the type of configuration, the degree of advantage is different. Also, there are very few field studies that have started logging these data. Some of the areas where field data is needed to understand the effect of FA systems in actual office environments are as follows: Air quality effect of different types of diffusers under different thermal loads; Energy effect of different types of diffusers and thermal loads on stratification; Facility management and organizational churn cost/benefit. The results of these studies would dramatically improve the affordability, performance, reliability, and professional commitment to floor-based air-conditioning systems. ACKNOWLEDGEMENTS The authors would like to acknowledge ARTI for its support of the project Energy savings potential of flexible and adaptive HVAC distribution systems for office buildings. REFERENCES Akimoto T, Nobe T, Tanebe S, and Kimura K Floor-Supply Displacement Air- Conditioning: Laboratory Experiments. ASHRAE Transactions. Vol. 105(2). pp Bauman F, Pecora P, and Webster T How Low Can You Go? Air Flow Performance of Low-Height Underfloor Plenums. Center for the Built Environment. October Bauman F, Baughman A, Carter G, and Arens E A Field Study of PEM Performance in Bank of America s San Francisco Office Building. Center for Environmental Design Research, University of California at Berkeley, April Bauman, Fred S., and Edward A. Arens Task/Ambient Conditioning Systems: Engineering and Application Guidelines. Final Report, October Burnley H Design and Application of an Upward Plug Flow Ventilation System. Building Design, Technology and Occupant Well-Being in Temperate Climates, Brussels, February Drake P, Mill P, and Demeter M. Implications of User-Based Environmental Control Systems: Three Case Studies. Healthy Buildings, IAQ 91, Washington, D.C., Sept Ellison, Jim and Bill Ramsey Access flooring: Comfort and convenience can be costjustified. Building Design and Construction. April Fisk W, Faulkner D, Pih D, McNeel P, and Sullivan D Indoor Air Flow and Pollutant Removal in a Room With Task Ventilation. Annual Report to California Institute for Energy Efficiency. June Hedge A, Michael A, and Parmelee S Improving Thermal Comfort in Offices: The Impact of Underfloor Task Ventilation. Proceedings of the Human Factors Society 34th Annual Meeting. Heinemeier KE,. Schiller GE, and. Benton CC Task Conditioning for the Workplace: Issues and Challenges. ASHRAE Transactions. Vol.96(2). pp Houghton, PE David Turning Air Conditioning on Its Head: Underfloor Air Distribution Offers Flexibility, Comfort, and Efficiency. Tech Update TU-95-8, E Source, Inc., Boulder, CO, August Hu S, Chen Q, and Glicksman L Comparison of Energy Consumption between Displacement and Mixing Ventilation Systems for Different U.S. Buildings and Climates. ASHRAE Transactions. Vol. 105(2), p (RP-949). Int-Hout, D Pressurized Plenum Access Floors. Internal Report. May 11, Kim Y, Lee K, and Cho H Experimental Study of Flow Characteristics of a Diffuser 258

6 for Under Floor Air-Conditioning System. ASHRAE Transactions. Vol. 107 (1). Kim I, and Homma H Possibility for Increasing Ventilation Efficiency with Upward Ventilation. ASHRAE Transactions 1992; Vol. 98(1), pp Loftness V, Brahme R, Mondazzi M, Vineyard E, and MacDonald M Energy Savings Potential of flexible and adaptive HVAC distribution systems for office buildings. Air- Conditioning and Refrigeration Technology Institute, Loftness V, Mathew P, Gardner G, Mondor C, Paul T, Yates R, and Dellana M Sustainable Development Alternatives for Speculative Office Buildings: A Case Study of the Soffer Tech Office Building. Final Report. Center for Building Performance and Diagnostics, May 26, Loudermilk, Kenneth J Underfloor Air Distribution Solutions for Open Office Applications. ASHRAE Transactions. Vol. 105(1), pp Mahdavi A, Ries R, and Cho D Infiltration, Natural Ventilation, HVAC Performance in the Intelligent Workplace. ASHRAE Transactions Vol. 106(1) pp Mass D Underfloor Air Still an Underused Tool. Facilities Design & Management. June Milam, J A Underfloor Air Distribution HVAC System Analysis. Prepared for USG Interiors, Inc. and published by Environmental Design International, September 16, Seppänen O, Fisk W, Eto J, and Grimsrud DT Comparison of Conventional Mixing and Displacement Air-Conditioning and Ventilating Systems in U.S. Commercial Buildings. ASHRAE Transactions. Vol. 95(2). pp Shute R Integrated Access Floor HVAC. ASHRAE Transactions. Vol.98(1) pp Sodec F, and Craig R The Underfloor Air Supply System The European Experience. ASHRAE Transactions. Vol.96(2). pp Spoormaker H.J Low-Pressure Underfloor HVAC System. ASHRAE Transactions. Vol.96(2). pp Thomas G Floor-Based Air Conditioning Offers Major Long-Term Benefits. Facilities. Volume 13, Number 2, February, 1995, pp Webster T, Bauman F, and Ring E Supply Fan Energy Use in Pressurized Underfloor Air Distribution Systems. Center for the Built Environment. University of California at Berkeley. October Wilson A Access Floors: A Step Up For Commercial Buildings. Environmental Building News, Volume 7, Number 1, January, Yuan X, Chen Q, and Glicksman LR. 1999b. Models for Prediction of Temperature Difference and Ventilation Effectiveness with Displacement Ventilation. ASHRAE Transactions. Vol. 105 (1), pp York T Conquering the Last Ergonomic Barrier: Temperature Comfort. Facility Management Journal, November/ December