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THERMAL ENERGY STORAGE

SECTION S

WHY IS THERE INTEREST INTHERMAL ENERGY STORAGE?

Reduced peak demand costspSome utilities offer rebates and rate

incentivesReduced equipment size and cost (new)May be improved reliability due to

production and storageproduction and storageSmaller fans and pumps (colder water with

ice storage)

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ECONOMIC PAYBACK TIME

Typical simple payback of 5 to 7 years (maybe 3 to 5 in some cases) for existing (maybe 3 to 5 in some cases) for existing buildings and chillers.

Recent examples from ASHRAE and others are showing the payback may be immediate to 1 – 2 years for good design in new constructionconstruction.

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CONVENTIONAL AIRCONDITIONING OPERATION

CAC system peaks at peak cooling timeCAC system is sized to meet peak cooling

loadCAC system may have its lowest efficiency

at the time it is needed the most

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CAC OPERATION

Building Cooling Load ProfileConventional Air Conditioning (CAC)

1000

Building Load Chiller Load

250

500

750

1000

Load

(kW

)

Peak chiller load 1000 kW

0

Midnigh

t 1 2 3 4 5 6 7 8 9 10 11Noo

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

Time of Day

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OFF-PEAK AIRCONDITIONING OPERATION

CAC together with storage is used to meet peak cooling loadspeak cooling loads

Chilled water or ice is used for storage medium

Daytime peak load is reduced or eliminated

OPAC system operates at night when efficiencies are usually higher due to lower outside temperatures

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OFF-PEAK AIR CONDITIONING OPERATION

The Total Daily Cooling Load (plus system losses) must be met

The Instantaneous Cooling Loads must also be met when they occur, just not directly from the chillers.

We are simply decoupling the Load (demand) from the Chiller (supply)

If we take advantage of optimal chiller If we take advantage of optimal chiller loading (sweet spot) and cooler condenser temperatures, we may gain significant efficiencies.

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Building Cooling Load Profile

750

1000

Building Load

Load x hours = 14,000 kWh

250

500

750

Lo

ad (

kW)

250 kW x 8hrs = 2000 kWh

750kW x 4hrs = 4000 3000 2000

0

Midn

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

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

Time of Day

250 kW x 8hrs = 2000 kWh3000 kWh

4000 kWh

3000 kWh

2000 kWh

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OPAC OPERATING STRATEGIES

• Load leveling– Partial shifting of AC load to off-peak hours

Chill l d l d – Chiller runs at constant load or near constant load for 24 hours per day

– Very cost effective for new construction– Less costly to purchase– Less space needed– But ~ less savings

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LOAD LEVELING CHILLER LOAD

CALCULATIONS

Where would we need to operate the chiller(s) in order to satisfy the building load? Peak period between 12:00 p.m. to 8 p.m.

Total kWh / Hours available to operate chillers

For the Load Leveling Strategy, the chiller will operate 24 hours per day at a load of:will operate 24 hours per day, at a load of: 14,000 kWh / 24 hours = 583.3 kW

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Building Cooling Load ProfileLoad Levelling TES

1000

Building Load Load Levelling TES

Peak Chiller Load= 583.3 kWWith TES

250

500

750

Lo

ad (

kW)

Peak SavingsWith TES

P k P i d0

Midn

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

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

Time of Day

Peak Period

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Load shifting Complete shifting of peak hour AC load to off-peak hours OPAC system must be sized to meet peak cooling load in

kWh Usually more cost effective for retrofit situations because

of large existing chiller load that can be moved mostly off peak

More costly to purchase and install Requires more space for storage tanks But ~ more savings

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OPAC LOAD CALCULATIONS

Total kWh Hours / Hours available to operate chillers A peak period from noon to 8 p.m. would leave 16 hours to

generate cooling capacitygenerate cooling capacity.

For the Load Shifting Strategy, the chiller will operate at a load of:

14,000 kWh / 16 hours = 875 kW

Section S - 13

Building Cooling Load Profile

1000

Building Load Load Shifting TES

250

500

750

Lo

ad (

kW)

PeakSavings

0

Midn

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

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

Time of Day

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TES STORAGE MEDIA

Chilled water storage Simple ~ but large tanks needed; lots of space.

Requires 4 to 5 times the space of ice storageq p g Typical water temperatures of 4C Practical considerations for water storage tanks

Need to minimize mixing of warm returnwater with the cold water in storage

May need two tanks ~ if full capacity ofa tank is needed. If temperature stratification of tank is used the tank stratification of tank is used, the tank may need to be up to 20% bigger

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Ice Storage More complex tanks and auxiliary equipment needed;

more complex to maintain Ice/water requires around 20 to 30% of the space needed

for chilled water tanksfor chilled water tanks Solid ice requires around 10% of the space needed for

chilled water tanks Very low temperature water can be used ~ around 1C Can use ice harvester, ice on coil, or ice/water (slush)

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PROPERTIES OF STORAGE MEDIA

Chilled water systems are typically operated in a manner to use only sensible p yheat storage and thus stores 4.2 kJ per kg of water for each C of temperature difference between the stored water and the returned water.

Ice systems are typically operated in a Ice systems are typically operated in a manner to use only latent heat associated with freezing and melting, and one kg of ice at 0C absorbs 335 kJ to become 0C water.

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SIZING CHILLED WATER STORAGE TANKS

Assume that chilled water is stored at 4C and is returned at the standard temperature f 12Cof 12C.

• This is an 8 ∆T for the AC system• Thus, one kg of water stores 33.6 kJ• One kWh of AC is 3600 kJ

So, to store 1 kWh you need:k f t • kg of water; or,

• litres of water; or,• cubic metres of water.

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EFFICIENCY AND CAPACITY CONSIDERATION

OPAC system is less efficient at lower water temperatures ~ but night time operation is temperatures ~ but night time operation is more efficient ~ these might cancel each other

We might suffer a 10 to 15% capacity reduction because the OPAC system has less capacity at lower water temperatures

We might lose 5 to 10% of capacity because of lstorage losses

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HOW BIG WOULD A CHILLED WATER TANK

BE?(THEORETICALLY)

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HOW BIG WOULD ANICE TANK BE?

ABOUT 10% AS BIG

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CONDITIONS THAT FAVOR TES

High peak demand charges

Low cost of energy used at nightLow cost of energy used at night

High on-peak loads

Low AC loads at night

Need for increased cooling system capacity

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CEM REVIEW PROBLEMS1. TES systems yield large energy savings.

A) True B) False

2. Use of full storage allows use of smaller chillers than partial storage systems.

A) True B) False3. Why are utilities encouraging TES?

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4. Temperature stratification can occur in(A) Chilled water storage(B) Hot water storage(C) A & B(D) None of the above

5. TES for heating uses some of the following storages: 1) building mass; 2) hot water; 3) ground couple; 4) compressed air tanks; 5) rocks; and 6) propane containers. Select the right combination:right combination:

(A) 1,2,3,4 (B) 3,4,5,6(C) 1,2,3,5 (D) 2,4,5,6

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6. With a load leveling TES strategy, a building manager will

(A) Not operate the chiller during peak hours(B) Essentially base load the chiller (i.e., operate

at high load most of the time)at high load most of the time)(C) Operate only during the peak times(D) Operate in the “off” season

7. A large commercial building will be retrofitted with a closed loop water to air heat pump system. Individual department meters will meter costs to

h d D d billi i ll f each department. Demand billing is a small part of the total electrical cost. Would you recommend a TES?

(A) Yes (B) No

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TES APPENDIX

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TES CHILLED WATER TANK DFW

DFW TES DATA The DFW Airport TES tank:

• Tank fabricated and installed by CBI

• Physical dimensions: 56 ft. in height with a diameter of 138 ft.

• Storage volume: 6,000,000 gallons

• Storage capacity: 90,000 ton-hours

• Shifts over 15 MW off-peak

• Simple payback on the incremental investment was 4 years

One other interesting fact, the DFW CHW system was originally designed for a 24 degree F delta T with a leaving CHW temperature of 36 degrees F. The buoyancy of water inverts at 38 degrees F so 36 degree entering water would compromise the stratification in the TES. So, a buoyancy depressant called So Cool is used to depress the minimum buoyancy below 36 F. It has worked perfectly.

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ANOTHER STORAGE MEDIUM

There is one more storage medium that is available but it is almost never used It is available, but it is almost never used. It is Eutectic Salt.

Eutectic salt was used some in the 1970s and early 1980s for storage of heat, but its use for air conditioning is not common gtoday. But, it could be used.

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Eutectic Salt Storage Expensive, high tech solution Allows use of existing 6 C chillers Typical melt range is 5 to 8 C Requires only 30 to 50% of the space needed for chilled

water tanks Requires secondary heat exchanger May be considered hazardous The salt has a useful life of about five years, and must

then be sent back and replaced

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STORAGE CAPABILITY OF EUTECTIC

SALTS

Eutectic salts use latent heat associated with freezing and melting but one kg of with freezing and melting, but one kg of solid eutectic salt absorbs only about 115 kJ to become liquid.

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SUMMARY OF STORAGE TANK SIZINGTHESE ARE REAL WORLD, PRACTICAL NUMBERS – NOT FOR USE ON

CEM TEST

Chilled water.12 to .15 cubic metres per kWh

Eutectic salt.03 to .05 cubic metres per kWh

Ice.025 to .035 cubic metres per kWh.025 to .035 cubic metres per kWh

Section S - 33

END OF SECTION S

Section S - 34