Vertically Integrated Systems in Stand-Alone Multistory Buildings

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1 2005, American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc. ( org). Rerinted by ermission from ASHRAE Journal, (Vol. 47, No. 6, June 2005). This article may not be coied nor distributed in either aer or digital form without ASHRAE s ermission. Vertically Integrated Systems in Stand-Alone Multistory Buildings By Robert Bean, Associate Member ASHRAE, Tim Doran, Member ASHRAE, Bjarne Olesen, Ph.D., Fellow ASHRAE, and Peter Simmonds, Ph.D., Member ASHRAE T he term district energy tyically is considered horizontal munici- Tal heating/cooling and domestic iing systems linking thermal roduction to consumtion (Figure 1). An alternative aroach is to take the system and rotate it 90 to service stand-alone multistory buildings (Figure 2). This involves having roduction units distribute to the consuming units through a single vertical and central distribution network. Each floor is served by an indirect connected substation (Figure 3). Enhancing comfort 1 and reducing energy demand 2 is ossible when floorto-floor (rather than building-to-building) consumtion is moderate (cooling) or low (heating) temerature radiant-based surface conditioning systems are used (ossibly with dedicated outdoor air systems). This is articularly true when the design considers activating the building floor mass as a thermal caacitor. In a vertically integrated system, the heating and cooling lant acknowledges the low-temerature requirements associated with radiant-based systems and is designed to take advantage of condensing or renewable technologies or use standard boiler and chiller equiment. As noted by Olesen, Hydronic concrete slab cooling and heating systems can use relative high water temeratures for cooling and relative low water temerature for heating. This increases the ossibility of using renewable energy sources such as ground heat exchangers, solar energy for heating and cooling and free night cooling. It also increases the efficiency of boilers, refrigeration machines and heat ums (Steimle, 1999). On to of that the active concrete system may About the Authors Robert Bean is a registered engineering technologist with Tim Doran is the technical suort manager for Uonor Wirsbo. Bjarne Olesen, Ph.D., is director, rofessor, International Centre for Indoor Environment and Energy, Technical University of Denmark. Peter Simmonds, Ph.D., is an associate with IBE Consulting Engineers, Sherman Oaks, Calif. 4 8 A S H R A E J o u r n a l a s h r a e. o r g J u n e

2 Dedicated Outdoor Air System Indoor Air Quality & Heat Recovery Deals with eak (sulemental) sensible loads, latent loads, and ventilation requirements. Signifi cant reduction in horseower requirements and building height rofi les. Economizers can assist in core conditioning in some geograhy. DOAS Ventilation to ASHRAE Standard 62 Indirect Substations Radiant Heating and Cooling Systems Indoor Comfort Quality Has a direct and immediate infl uence over thermal environmental conditions for human occuancy. Provides motive for couling to activated core and district energy systems for reduced thermal and ower consumtion. Thermal Comfort to ASHRAE Standard 55 Radiant Designed For a Nominal 10 F to 20 F T Activated Core Systems Addresses base loading by taking advantage of the caacitance caabilities of the building mass. Has direct and immediate imact on reducing ower consumtion by using lower evening ower rates and by eliminating the sensible load comonent from the air system. District Energy Systems Takes advantage of the temerature differences between roduction and consumtion to reduce fl ows and caital and oerating costs. Mechanical system is adatable to any high or low temerature roduction technologies and becomes district energy ready. Distribution Designed For a Nominal 40 F to 80 F T T Based On Loads And Equiment Summary and benefits of vertically integrated systems (simlified illustration). use cheaer night rate electricity. 2 As stated in the 2004 ASHRAE Handbook HVAC Systems and Equiment, Central lants generally have efficient baseload units and less costly eaking equiment for use in extreme loads or emergencies. 3 Regardless of the lant selection, the otential oerating temerature differential between roduction and consumtion could reduce distribution flows significantly. For heating, floors are limited to less than 84 F (29 C) surface temeratures and for cooling, above 66 F (19 C) (ANSI/ASHRAE Standard , Thermal Environmental Conditions for Human Occuancy). Therefore, lants oerating at 180 F (82 C) or chillers at 42 F (6 C) have significant motive force for heat transfer. Examle (Figure 4 ): Given a 1.5 MBtu/h (440 kwh) directconnected heating load, oerating with traditional baseboard or fan/coil units, 180 F (82 C) entering fluid temeratures, and a design T T of 20 F (11 C) equates to flow using: Q w = q w /(60 min/h w C T ) where Q w = flow, gm (L/s) q w = heat transfer, Btu/h (kwh) w = fluid density, lb/gal, (kg/m 3 ) (e.g., I-P units 8.2 at 150 F, 8.1 at 180 F) C = secific heat, Btu/lb F, (kj/kg K) Q w = 1.5 MBtu/h/(60 min/h 8.1 lb/gal 1 Btu/lb F 20 F) Q w = 155 gm (9.46 L/s) To stay within friction losses of 1 ft (0.31 m) to 4 ft (1.22 m) er 100 ft (31 m) of ie and flow velocities of 2 fs (0.61 m/s) to 5 fs (1.5 m/s), the rimary distribution ie would be a 4 in. (100 mm) line. However, (indeendent of discussions on energy roduction efficiencies), lants running at maximum design with weathercomensated target suly temeratures of 180 F (82 C), indirectly connected to a radiant system oerating with a sec- J u n e A S H R A E J o u r n a l 4 9

3 ondary design return temerature of 100 F (38 C) rovide for an 80 F (44 C) T T in the vertical distribution network serving each substation (Figure 5 ). Substituted into the formula, the rimary vertical distribution flow becomes: /( Q w = 1.5 MBtu/h/(60 min/h 8.2 lb/gal 1 Btu/lb F 80 F Q w = 38 gm (2.39 L/s) = 75% distribution flow reduction Using the criteria for velocity and friction loss, the line size is reduced from 4 in. to 2 in. (100 mm to 50 mm) offering considerable savings on caital costs and oerating transmission losses. An analysis of uming horseower could accet a 2.5 in. (64 mm) ie, which reduces the head loss from 2.45 ft/100 ft (0.75 m/31 m) to 0.85 ft/100 ft (0.26 m/30 m) while still oerating within accetable velocities. A single and central distribution line reduces the number of vertical risers, fire-stos, hangers, suorts, insulation, and labor, creating further savings on materials tyically associated with traditional designs. Reductions in any quantity of assemblies, translates to a corresonding reduction in otential failure oints. With indirect connected substations, the quantity of fluid oerating at a higher ressure is contained on the roduction side of the heat exchangers, resulting in significant volume reductions. A reduction in volume renders significantly smaller lant exansion tanks, and savings in fluid conditioning chemicals. Production If the roduction side is required to oerate with glycol for makeu air or reheat/recool units, the reduced volume means less caital and maintenance cost associated with antifreeze. Since the ressure on the consumtion side of the heat exchangers is always low ressure, valves, circulators, exansion tanks, etc., can be standard off-the-shelf comonents tyically found in low-rise residential systems. The low-ressure requirement and consistency in load distribution creates standardization on each floor leading to further simlification in stocking relacement or reair arts. Regarding consumtion side circulators, Ottmer and Rishel found that circulators sized for just the ressure loss imosed at that articular location can reduce total chilled water um ower by 20% to 25% in very large systems, 3 which leads to further ositive economic imact on caital costs and ongoing maintenance cycles. Vertical integrated district energy systems kee all serviceable equiment outside of the suites. Therefore, maintenance ) Distribution Consumtion Substations Figure 1: Traditional horizontal district energy system. has little imact on occuants. The indirect systems also contain any mechanical failure on the consumtion side to a secific site rather than imact the entire lant or other floors. They also ermit fine-tuning of consumtion side temerature with weather resonsive controls er substation. Energy meters can be used for er floor allocation of energy costs. Indirect systems also eliminate many difficulties associated with balancing and commissioning since the hydraulics of the roduction side are searated from the multile zones on the consumtion side. When the lant is ied in a rimary/secondary configuration, constant flow can be emloyed using three-way diverting valves at the substations, making balancing the single distribution riser a Consumtion Substations Distribution Production simle task. With all energy consumtion equiment mounted within the substation, multizone balancing is carried out from one location, again simlifying the entire balancing and commission rocess. Finally, the coordination during cold weather building construction with this tye of system often ermits the use of the floor system to condition the sace rather than having to bring in temorary construction heaters with their associated health and safety issues. Exlored in the next discussion is the concet of shifting district fluid thermal storage concets to the building mass. Figure 2: Vertically integrated district Activated Core energy system. As stated in the ASHRAE Final Reort for Research Project 985-RP, Imlementation of Thermal Storage in Building Mass, There is inherent energy storage within a building, which can be used to change the time distribution of the equiment cooling loads and significantly reduce the cooling costs for a building. 4 The base-loading concet of using the building mass as thermal storage is not new. Braun [rincial investigator] used dynamic otimization, alied to simulations, to determine otimal building control strategies and determined that energy costs could be reduced u to 50%, with greater savings being achieved with time of day rates. Similarly, he demonstrated that demand charges could be reduced by about 30%, with greater savings in buildings not occuied for the whole day. Braun roosed a combination of mechanical recooling and free recooling to utilize the thermal mass otential of the building and stated that by recooling the building to five degrees below the comfort zone, both the demand and energy charges could be reduced effectively. 4 Simmonds showed it was ossible to use night ventilation urge to cool the building mass and reduce the refrigeration 5 0 A S H R A E J o u r n a l a s h r a e. o r g J u n e

4 system size and still maintain comfortable occuant saces (this would be used only when the nighttime dew oint is less than or equal to the desired indoor humidity level). 1 Also from Olesen, et al., A new trend, which started in the early nineties in Switzerland (Meierhans, 1993, 1996), is to use the thermal storage caacity of the concrete slabs between each storey in multi-storey buildings. Pies carrying water for heating and cooling are embedded in the centre of the concrete slab. 2 Thermal storage is not new to district energy. According to the 2004 ASHRAE Handbook, Both hot and chilled water thermal storage can be imlemented for district systems. Deending on the lant design and loading, thermal storage can reduce chiller equiment requirements and lower oerating costs. By shifting a art of the chilling load, chillers can be sized closer to the average load than the eak load. Shifting the entire refrigeration load to off eak requires the same (or slightly larger) chiller machine caacity, but removes the entire electric load from the eak eriod. Since many utilities offer lower rates during off eak eriods, oerating costs for electric-driven chillers can be substantially reduced. 3 Outdoor Tem. Sensor District Heating Suly Radiant Circuit District Heating Return Radiant Heat Exchanger The receding text establishes the foundation for defining the benefits of using the building mass as the storage medium for both heating and cooling, and the design rincile is based on the following statement from Olesen, et al., As the heat transfer between the heated or cooled surfaces, the sace and eole in the sace is mainly by radiation, it is imortant to use the oerative temerature for secifying comfort conditions and for load calculations. With concrete slab systems, where the dynamic effects and thermal storage caacity of the slabs are used, the oerative temerature should during the day ram inside the comfort range. Studies by Knudsen (1989) show that as long as the temerature change is less than 5 K er hour the temerature range based on steady state conditions (ISO 7730) is still valid. 2 (See Standard , which establishes oerative range and allowable temerature drifts and rams.) So the rincile becomes, For a well designed building with a low heating and low cooling load, a concrete slab system may be controlled at a constant core (water) temerature year-round. If, for examle, the core is ket at 72 F (22 C), the system will heat at room temeratures below 72 F (22 C) and cool when the room temerature increases above 72 F (22 C). 3 The heating and cooling flux limitation of an activated core is established by the building/mechanical thermal inertia characteristics for ceiling systems to be stable (avoiding uncontrolled heating and cooling) at a nominal 10 Btu/h ft 2 (30 W/m 2 ) based Control and Regulating Units Domestic Hot Water Heat Exchanger Tem. Sensor DHW Main Heater DHW Circulation Preheater Cold Water Figure 3: Building indirect connection for both heating and domestic hot water (substation). on ideal fluid temeratures between 64 F (18 C) and 77 F (25 C). It is also imortant to distinguish the core conditioning caacity from those of a radiant floor system whose flux erformance in the heating mode eaks around 32 Btu/ft 2 (100 W/m 2 ) and 12 Btu/ft 2 (38 W/m 2 ) for a cooling. Olesen, Simmonds, et al., have noted, A secial case for floor cooling is when there is direct sun radiation on the floor. In this case the cooling caacity of the floor may exceed 100 W/m 2 (Borresen, 1994). This is also why floor cooling is increasingly used in saces with large glass surfaces like airorts, 5 atriums and entrance halls. The heat exchange coefficients (convection + radiation) are listed in Table 1. A summary of the benefits of activated core cooling (also alies to heating) rinciles from the ASHRAE Research Project 985-RP follows: thermal mass recooling hels reduce demand eaks and on-eak energy usage because the cooled mass has a greater availability to absorb heat gains to the conditioned sace. Oerating the cooling lant at night may result in enhanced cooling equiment erformance. The efficiency of the cooling lant is directly deendent on the oerating temerature of the condenser and hence, oerating the equiment at night, i.e., at lower oerating temeratures, translates to imroved cooling equiment erformance. Enhanced equiment efficiencies may be obtained by oerating the cooling equiment at art loads. Shifting the loads causes the lant to oerate at a lower art load ratio during the daytime eriods, thus giving higher lant efficiencies for lants with favorable art load characteristics. Sensible cooling using embedded ies can be assisted by Nighttime ventilation cooling (which) uses the air handler to feed in cold nighttime air (when the dew oint is low enough) into the building to reduce the cooling loads for the subsequent day. In cases where the outdoor air temerature dros below the daytime setoint, this may serve as a means for sensible cooling of the zone and reduce total cooling lant requirements. Conditioning the building mass is best achieved by direct conduction of heating and cooling ies embedded in the slab, and, by its nature, roduces a by-roduct of an indoor radiant comfort conditioning system. Radiant Heating and Cooling Systems Practitioners and students of HVAC design are encouraged to review the details of Standard and consider that the human body transfers between 40% and 50% of its energy via J u n e A S H R A E J o u r n a l 5 1

5 radiation. Many ASHRAE and other studies state comfort levels are highly influenced by floor/ceiling surface temeratures, temerature stratification, radiant asymmetry, and air velocities (draft). It is not logical to ignore the scientific benefits of radiant heating and cooling not only from a hysiological ersective but also from a controllability oint of view. Consider this: relative small temerature differences between the heated or cooled surface and the sace are tyical for surface heating and cooling systems. This results in a significant degree of self control, because a small change in this temerature difference will influence the heat transfer between the cooled or heated surface and the sace significantly. 2 This control stability is suitable for all regions and ideal for geograhic regions, which exerience extreme temerature and humidity swings since it s ractical to use radiant for sensible base loading (core conditioning) and the ventilation system for eak temerature, and latent/ humidity control. Olesen rovides this guidance, The imortant factors for the heating and cooling caacity of hydronic systems embedded in concrete slabs are the heat exchange coefficient between the slab surface and the sace, the accetable minimum and maximum slab surface temeratures based on comfort and consideration of the dew oint in the sace and heat transfer between the ies and the surface. 1.5 MBtu/h 180 F EWT 160 F LWT Production Distribution Consumtion Figure 4: Traditional design rincile. Simlified flow and temerature rofile for direct connected high-temerature baseboard, radiator or fan/coil system. 1.5 MBtu/h 180 F EWT It is imortant to distinguish between the caacities obtained from core conditioning system and those of a radiant system. 5 2 A S H R A E J o u r n a l a s h r a e. o r g J u n e gm 120 F LWT s 38 gm 155 gm 100 F LWT 100 F EWT s Production Distribution Consumtion Figure 5: District energy rincile. Simlified flow and temerature rofile for indirect connected substation serving low-temerature radiant floor heating systems. Mode Heating Cooling Surface Btu/h ft 2 F W/m 2 K Btu/h ft 2 F W/m 2 K Floor Wall Ceiling Table 1: Total heat exchange coefficient (convection + radiation) between surface and sace for heating and cooling. 2,9 Dedicated Outdoor Air Systems By directly conditioning the building core temerature and, thus, addressing sensible base loads with radiant, the only remaining design conditions to consider are sulemental eak heating loads, latent cooling loads and ventilation requirements. All three can be handled with a dedicated outdoor air system (DOAS). Dieckmann, Roth, Brodrick, rovide a concise benefit exlanation of using a combination of a sensible base systems and DOAS by saving energy in at least four ways. 7 First, a DOAS reduces ventilation energy consumtion by reducing the total ventilation airflow needed to meet Standard 62 ventilation requirements. This is due to the inherent recision of the DOAS in delivering required ventilation flows in the aggregate and in the individual zones in the building. Second, reductions in the total ventilation airflow decrease the energy exended to condition the ventilation air during cooling and heating seasons. Simle analyses erformed by TIAX suggest that a DOAS decreases total sace heating energy consumtion by aroximately 10%. Third, because the ventilation makeu air is searately conditioned from the internal loads, with the entire building humidity load handled in the rocess. This enables the use of higher chilled water temeratures for the internal (sensible) loads (aroximately 55 F [13 C] evaorating temerature vs. 40 F to 45 F [4 C to 7 C], tyically), increasing the COP of the air conditioner comressor. Fourth, by decouling temerature and humidity control, it creates an ideal situation for VAV, where the volume of conditioned airflow rate varies in roortion to the net cooling or heating load. This significantly reduces blower ower during the large roortion of the year when full heating or cooling caacity is not required (or where activated core is used). Note that this alies to both chilled water based systems and to DX systems. 7 By effectively managing indoor humidity levels, a DOAS also enables the alication of energy-efficient radiant ceiling systems for sensible cooling, where water is used to transort cooling instead of air. The DOAS architecture readily incororates energy recovery heat exchange between the makeu air and exhaust, reducing eak and seasonal cooling and heating loads needed to condition makeu air. 7 This aroach allows each comonent of the HVAC system to do what it does best. 6 As stated by Mumma, The integrated anel-cooling/doas aroach rovides suerior indoor air quality and thermal comfort, and that alone should be sufficient incentive for the industry to use the concet. This is esecially true since the Lawrence Berkeley National Laboratory estimates that U.S. comanies could save as much as $58 billion annually by reventing sickbuilding illnesses and could benefit from u to $200 billion in roductivity increases each year. 8

6 Conclusions Two dominant challenges influencing modern building sace conditioning system design are energy costs and human comfort. One is driven by economic conditions and the other by the hysical condition of an aging oulation. A review of ASHRAE research and documentation suggest the integration of the four common and roven systems identified here is a otential concet that could easily and simly solve both challenges while romoting the natural rogression of green building habitation. References 1. Simmonds, P Designing comfortable office climates. Proceedings for the International Conference on Building Design, Technology and Occuant Well-Being in Temerate Climates, eds., Sterling, E.M., C. Bieva, C.W. Collett. 2. Olesen, B.W Hydronic Radiant Heating and Cooling of Buildings Using Pies Embedded in the Building Structure. 41 AICARR Conference ASHRAE Handbook HVAC Systems and Equiment, Chater Analytical Tools for Dynamic Building Control Sonsored by ASHRAE s Final Reort for Research Project 985-RP, Imlementation of Thermal Storage in Building Mass. 5. Simmonds, P., et al Using radiant cooled floors to condition large saces and maintain comfort conditions. ASHRAE Transactions 106(1): Morris, W The ABCs of DOAS: dedicated outdoor air systems. ASHRAE Journal 45(5): Dieckmann, J., K. Roth, J. Brodrick Dedicated outdoor air systems. ASHRAE Journal 45(3): Mumma, S.A Ceiling anel cooling systems. ASHRAE Journal 43(11): Olesen, B.W., Possibilities and limitations of radiant floor cooling. ASHRAE Transactions 103(1): Bibliograhy Bean, R. and R. Kilkis. Fundamentals of Panel Heating and Cooling. ASHRAE Learning Institute Short Course. Borresen, B Floor cooling in atrium. Velta Congress, St. Christohe, Austria (in German). Knudsen, H.N., et al Thermal Comfort in Passive Solar Buildings. Technical University of Denmark. Meierhans, R.A Slab cooling and earth couling. ASHRAE Transactions 99(2). Meierhans, R.A Room air conditioning by means of overnight cooling of the concrete ceiling. ASHRAE Transactions. 102(2). Steimle Entwicklung der Wärmeumentechnik der Fußboden als Heiz- und Kühlfläche. 21 Internationaler Velta-Kongreß, St. Christoh/Tirol. Advertisement formerly in this sace. J u n e A S H R A E J o u r n a l 5 3