Ice storage offers many opportunities
to use other tcchnologies, such as heat
recovery and cold-air distribution. In
fact, by using them, the designer can
improve the efficiency and lower the
construction cost of an ice system.
This paper presents a case study of a
large (550,000 square feet) office building
which combines these ideas into one HVAC
system. By integrating these technologies
to take advantage of the unique
characteristics of ice storage, the
engineers created a system that provided a
1-1/2 year simple payback. The system
reduces operating costs by over $0.16 per
square foot each year, yet it increased the
net HVAC budget by only $0.22 per square
foot

Tackett, R. K.

Texas A&amp;M Repository

OFFICE BUILDING USES TCE STORAGE, HEAT RECOVERY, AND COLD-AIR DISTRIBUTION ROBERT K. TACKETT Marketing Engineer TI1 Electric Dallas, INTRODUCTION Ice storage offers many opportunities to use other tcchnologies, such as heat recovery and cold-air distribution. In fact, by using them, the designer can improve the efficiency 4 lower the construction cost of an ice system. This paper presents a case study of a large (550,000 square feet) office building which combines these ideas into one HVAC system. By integrating these technologies to take advantage of the unique characteristics of ice storage, the engineers created a system that provided a 1-1/2 year simple payback. The system reduces operating costs by over $0.16 per square foot each year, yet it increased the net HVAC budget by only $0.22 per square foot. SYSTEM OVERVIEW AND ECONOMICS The 550,000 square foot office building is located in Addison, Texas, north of Dallas. It is owned by Sunbelt Savings -- one of the "few" Texas savings and lcan institutions that was still afloat at the time of writing this paper. The owner asked the engineers to meet two objectives: provide a state-of-the-art system, but do it at a reasonable cost. After conceptual designs, an HVAC system was developed to include the fol1owin.g features: 1. Partial Ice Storage - Two centrifugal, qlycol chillers and 90 ice storage tanks are located in the lower parking levels (see Figure 1). The chillers operate at night to make ice, which is then used the next afternoon to supplement chiller cooling. Ice storage reduces 800 kW from the peak electric demand, saves about $55,000 each year in demand charges, and initially earned a cash incentive of more than $200,000 from the local utility. (See Table 1, which provides a summary of the system's incremental costs and savings.) 2. Heat Recovery One of the glycol chillers has a double-bundle condenser to recover heat in the winter. During the winter days, heat is removed from the building's interior and stored in the ice tanks. Then at night., the heat recovery chiller removes this heat from the tanks and delivers it to the building's Texas perimeter office spaces. The extra cost for thr? heat recovery system was about $60,000, but it will. supply about two-thirds cf the buildings annual heating needs and s a w  an estimated $30,000 each year in electric heating costs. 3. Cold-Air Distribution Ice storage delivers 34 F water to the cooling coils of four large, built-up air handling units, which supply air at 44 F. Compnred to a standard supply air temperature of 55 F, the colder supply air temperature has four benefits: * First, it reduced the air quantities, duct sizes, and air-handling units by about one-third, and squeezed $90,000 from the air system cost. * Second, the reduced air quantities also lowered the total fan power by 140 brake-horsepower, which will save an estimated $12,500 in annual fan energy costs. * Third, the smaller air-handling units gave the owner an extra 3,500 square feet of usable lease space. * Fourth, it improved comfort conditions by lowering the space relative humidity to about 40%. 4. After-Hours Cooling The ice system also eliminated the cost of a condenser water loop for night-time cooling. Normally, night-time cooling loads are too small to practically operate a centrifugal chiller. The owner's options included installing a condenser water loop or a 200 ton reciprocating compressor. Both would have cost an extra $50,000. With ice storage, though, the chillers al.ready operate at night, and with slightly modified piping and controls the chillers can simultaneously satisfy after-hours cooling and make ice. ICE STORAGE Ice Storage Tanks The heart of the HVAC system is the 90 ice tanks located on the lower parking level. These tanks are conr-ected in parallel with the centrifugal chillers so that the ESL-HH-89-10-13Proceedings of the Sixth Symposium on Improving Building Systems in Hot and Humid Climates, Dallas, TX, October 3-4, 1989 TABLE 1 Comparison of Alternate HVAC Schemes and Costs -- .................... -.- . . . . . . . . . . . . . . . . . . . . . . . . . .  -.- .---. --- Incremental Storage Costs & Savings Standard D e s i c  Modified Storage Added Added Increnental System Costs: --. Quantity Cost Quantity Cost .. 1) Ice Storage.. (ton-hours required) 0 Base 7500 $4-5- 2) Central plant OPE) 1400 Rase 1150 ($31,250) .............. 3) Utility incentive (kW) 0 Base 850 ($215,000) 4) Air systems ...................( cfm) 540,000 Base 360,000 ($90,000) ................ 5) Hezt recovery (tons) 0 Rase 550 $60,000 ....... 6) Condenser water loop.. (tons) 200 Base 0 ($50,000) -- Net incremental cost..........($) Base $124,250 - Annual Savings: ............. 1) Demand savings ($/year) 2) Chiller ~enalty ............($/ year) 3) Fan savings .................... ear) .............. 4) Heat recovery ($/year) Base Base Base Base --........... Total savings ($/year) Base - $92,750 Simple payback.. ......... (years) Base 1.34 building's cooling load can be met by using either the ice tanks, the chillers, or both (look at Figure 2). Each tank is about 6 feet in diameter, 6 feet high, and contains thousands of feet of spiral wound plastic tubing surrounded by water. Each night a 24 F glycol/water solution (25% ethylene glycol) circulates through the tubing in the tanks to produce up to 600,000 pounds of ice, which can provide about 7,500 ton-hours of cooling the next day. Glycol Chillers The centrifugal chillers are three stage compressors that produce low temperature glycol at relatively high efficiencies. For example, each chiller produces a 34 F solution at an efficiency of 0.76 kW/ton to provide 568 tons of cooling capacity. Likewise, each chiller produces a 24 F solution at an efficiency of 0.85 kW/ton to provide 425 tons of ice-making capacity. Note that in the ice-making mode, each chiller provides only 75% of its standard capacity. However, the compressors still operate at relatively high efficiencies and compare favorably to positive displacement machines that require about 1.0 kW/ton. The chiller capacity is slightly smaller than the building's peak load (1150 tons of capacity to meet a 1400 ton cooling load) because ice storage is available to supplement the load. The smaller chillers reduced cost6 by another $30,000. These chillers, however, incur a slight energy penalty of about $5,000 each year, because they cannot make ice as efficiently as they make chilled water. System o p e r a t .  ~ ~ b r i d  Partial Storage Some engineers support the use of partial storage. This strategy uses less storage and smaller chillers \rhich operate 24 hours each day. It is cost-competitive with a standard HVAC system, but it may provide smaller savings and constrained operating flexibility. Others support the use of full storage. This strategy uses enough storage to meet all cooling needs during the utility's on-peak hours. It provides the greatest demand savings and operating flexibility, but it may not be cost-competitive without substantial cash incentives. The author submits that the best design (which was used in "his project) may be a hybrAd ?f the two. With this strategy, the storige and chiller sizes fall somewhere between the partial and full storage strategies. One-half of the chillers operate 24 hours each storage), while the ot during the on-peak hou storage). The idea ie chillers and storage t costs and savings. Th this project operates 1. During the on-peak completely off; the se> is demand limited to m load of about 500 tons meet the remaining 1021 diversified part; this demand reduct-ion and s investment in storage. 2. Both chillers opera ESL-HH-89-10-13Proceedings of the Sixth Symposium on Improving Building Systems in Hot and Humid Climates, Dallas, TX, October 3-4, 1989 cooling needs before the on-peak houre. This conserves the storage so that it can be used during the on-peak period. Alternatively, the chillers of a partial storege system would he too small to meet the cooling load before the on-peak hours. A? a result, storage would be wasted to provide cooling when savings could not be realized. IlEAT RECOVERY Since the ice storage tanks had already been bought and paid for, it only made sense to find some way to reuse them for heat storage. This would reduce electric costs and improve the system's overall payback. Sequence of Operation - The project's heat recovery system does not provide heating around the clock; it provides heating only at night when the building is unoccupied. GeneraI.ly, it costs much more to inhtcll a hest recovery system that has the flexibility to provide heating and cooling anytime of the day. Fortunately, about 80% of the heating needs occur at night. The idea, then, is to satisfy the gre?.test portion of heating with the least first cost. (Note: Some people have trouble believing that ice storage can be used to heat a building. For those people, the following paragraph explains how it works. If the explanation is not clear, the author asks you to trust him on this point.) In operation, the heat recovery system works in two steps. First, ice is melted during the day to cool the interior spaces. Now as long as you believe that cooling is really the removal of heat, then you can visualize this process as the removal of heat from the interior. The tanks absorb this heat as the ice melts. (Obviously, it takes heat to melt ice.) Then at night, the heat recovery chiller removes this heat to refreeze the melted ice. The heat removed from the tanks is rejected to a double-bundle condenser, where a 95 F water solution is produced and d-elivered to the air-handling units. Keeping Construction Costs Down "Now," you might ask, "what does the air-handling units do with the heated water? Weren't these units designed as cold-air distribution systems?" The answer, of course, is yes and no. Most heat recovery systems cost more because of the extra. hot water piping, heating coils, and controls. To eliminate these extras, the contractor developed a scheme that reused the existing air-handlers and water risers. It works like this: 1. Description. Two of the four air-handlers serve the south side of the building, while the other two serve the north. A north unit is connected to a south unit through a series of crossover ducts that run between the main supply air ri~ers. (Look at Figures 1 and 2.) The water coils in the south units receive chilled water only. The north units, however, have the option to receive either chilled or heated water. 2. Operation. If cooling and heating are required at night, chilled water is delivered to the south units, while heated water is delivered to the north units. To provide heating on an unoccupied floor, the floor damper at the south supply riser ie closed, the floor damper at the north supply riser is opened, and heated air is delivered from the north unit, through the crossover ducts, and to the south side of the building. Likewise, to provide cooling on a floor, the north floor damper is closed, the south floor damper is opened, and cold air from the south unit is sent through the crossover duct to the north side of the building. Heat. Recovery vs. Economizer Cycles Some engineers contend that heat recovery only wastes energy. Why? "Because," they say, "the chillers must operate to provide cooling when an outside air economizer cycle could have been used for free cooling.'' The notion, of course, is absolutely absurd. A heat recovery chiller can absorb about 3 kwh of heat for every 1 kwh expended. (The I kwh of compressor heat can also be reclaimed.) Thus every kwh used for cooling provides about 4 kwh of heating. An economizer cycle, on the other hand, saves only 1 kWh of cooling energy for every 3 kwh of heating energy that is rejected. The choice is obvious. COLD AIR DESIGN Little needs to be said about the virtues of cold-air. It saves money; it Eaves energy; it saves space. However, it does not save the engineer the trouble of paying attention to some design details. The most important are noted below. Coolinq Coil Selection This project uses 10 row cooling coils, which must cool mixed air from an 85 F to a 42 F temperature. The coil's water side operates on a 24 F rise between a 34 F supply and a 58 F return temperature. Because of the high temperature rises, the water velocities within the coils are generally outside the range at which cooling coils are rated. It is very important that the meaufacturer double-check his selection. Terminal Devices Usually, it is difficult to dump 44 F air ESL-HH-89-10-13Proceedings of the Sixth Symposium on Improving Building Systems in Hot and Humid Climates, Dallas, TX, October 3-4, 1989 d i r e c t l y  i n t o  t h e  space .  For t h i s  p r o j e c t ,  t h e  c o n t r a c t o r  des igned  s p e c i a l  b l e n d i n g  boxes t h a t  h a n d l e  about  6,000 CFH each.  To reduce  t h e  c o s t  o f  t h e  secondary  ductwork,  t h e  c o n t r a c t o r  c o n t r o l l e d  t h e  boxes t o  p r o v i d e  50 F r a t h e r  t h a n  55 F  s u p p l y  a i r .  The r e s u l t  i s  t h a t  t h e  secondary  ductwork i s  s i z e d  on a  25 F  r a t h e r  t h a n  a  s t a n d a r d  20 F  t e m p e r a t u r e  r i s e .  L a t e n t  v s .  T o t a l  Cool ing Loads- Some e n g i n e e r s  worry t h a t  c o l d - a i r  d e s i g n s  i n c r e a s e  t h e  d e s i g n  c o o l i n g  l o a d  because  o f  t h e  i n c r e a s e d  l a t e n t  v e n t i l a t i o n  l o a d .  I t  is  t r u e  t h a t  more m o i s t u r e  i s  removed from t h e  o u t s i d e  a i r  a s  it i s  c o o l e d  t o  4 2  P. However, t h e  s m a l l e r  f a n s  and c o r r e s p o n d i n g  r e d u c t i o n  i n  f a n  h e a t  g a i n s  o f f s e t  t h i s  c o c l i n g  load .  CONCLUSIONS By i t s e l f ,  i c e  s t o r a g e  would have  prov ided  a b o u t  a  t h r e e  y e a r  s i m p l e  payback. However, by i n c o r p o r a t i n g  h e a t  r e c o v e r y  and c o l d - a i r  d i s t r i b u t i o n ,  t h e  payback was reduced t o  less t h a n  1-1/2 y e a r s .  Cold-a i r  p rov ided  t h e  most s i g n i f i c a n t  r e d u c t i o n s .  O v e r a l l ,  t h e  a i r  s i d e  c o s t  was reduced by $75 p e r  t o n ,  and t h e  s m a l l e r  a i r - h a n d l e r s  gave t h e  owner an a d d i t i o n a l  3,500 s q u a r e  f e e t  of u s a b l e  l e a s e  space .  The r e n t a l  income from t h i s  c o u l d  be a l m o s t  a s  s u b s t a n t i a l  a s  t h e  s y s t e m ' s  t o t a l  s a v i n g s .  Heat r e c o v e r y  improved t h e  o v e r a l l  payback. By r e u s i n g  t h e  ice t a n k s  t o  a b s o r b  h e a t ,  enough h e a t  can be removed from t h e  b u i l d i n g ' s  i n t e r i o r  t o  m e e t  most o f  t h e  b u i l d i n g ' s  h e a t i n g  needs on t h e  p e r i m e t e r .  T h i s  p r o j e c t  emphasizes  t h e  need f o r  t h e  d e s i g n e r  t o  examine s e v e r a l  o p t i o n s .  I n  t h i s  c a s e ,  t h e  o p t i m a l  s o l u t i o n  was n e i t h e r  a  f u l l  o r  p a r t i a l  s t o r a g e  sys tem,  b u t  a  combinat ion o f  t h e  two. ESL-HH-89-10-13Proceedings of the Sixth Symposium on Improving Building Systems in Hot and Humid Climates, Dallas, TX, October 3-4, 1989 ESL-HH-89-10-13Proceedings of the Sixth Symposium on Improving Building Systems in Hot and Humid Climates, Dallas, TX, October 3-4, 1989 ESL-HH-89-10-13Proceedings of the Sixth Symposium on Improving Building Systems in Hot and Humid Climates, Dallas, TX, October 3-4, 1989 

Office Building Uses Ice Storage, Heat Recovery, and Cold-Air Distribution

http://oaktrust.library.tamu.edu/bitstream/handle/1969.1/6574/ESL-HH-89-10-13.pdf?sequence=3&isAllowed=y

Office Building Uses Ice Storage, Heat Recovery, and Cold-Air Distribution

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Office Building Uses Ice Storage, Heat Recovery, and Cold-Air Distribution

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