District Energy - 2012 1st Quarter

Table 1. A Comparison of Latent Heat and Sensible Heat TES Technologies.

Latent Heat TES Technologies

Ice TES

How It Works

Water is frozen during off-peak periods (typically at night), stored and melted to meet cooling loads during peak periods (typically the next day).

Benefits

•;Relatively;compact;storage;volume
compared to other TES technology options
•;Capability;of;some;designs;for;low;supply
temperatures during TES discharge (typically
34 F to 44 F), which minimizes network
piping size and cost – depending on
characteristics of selected equipment, rate
of TES discharge or ice melting, and amount
of remaining ice inventory in the TES
•;Availability;of;standard;modular;units;in;small
to moderate capacities, allowing for phased
capacity growth

Drawbacks

•;Low;chiller;operating;temperatures;required
for charging ice TES, which increase
chiller energy consumption and operating
energy cost
•;Relatively;little;economy;of;scale,;due;to
modular nature of ice storage equipment

District Cooling Applications

•;Generally;used;where;available;space;for TES equipment is limited (e.g., some urban systems) and/or potentially low supply temperature is desired (e.g., to minimize network piping size and cost).

•;Ice;TES;has;traditionally;dominated;in smaller individual building TES applications where its small modular equipment provides a better fit.

Source: The Cool Solutions Company.

Sensible Heat TES

Chilled-water TES or low-temperature fluid (LTF) TES Water or LTF is chilled during off-peak periods (typically at night), stored in a thermally stratified tank (with cooler, denser supply fluid beneath the warmer, less dense return fluid) and used to meet cooling loads during peak periods (typically the next day).

•;Simplicity;and;high;energy;efficiency;due;to;relatively;constant,;warm chiller operating temperatures (in which TES discharge temperatures are essentially equal to TES charging temperatures)

•;Dramatic;economy;of;scale;inherent;to;tank;construction;costs,;thus
producing lower capital costs per ton-hour or per ton, for larger
applications such as district cooling
•;Large;storage;volume;for;chilled-water;TES;(which;can;be;reduced;by
33 percent to 50 percent using LTF TES, though still requiring larger
volume than ice TES)

•;Generally;used;where;space;is;available;for;the;large;tank
•;Note;that;the;tank;site;can;be;remote;from;the;chiller;plant;(generally
not practical when using ice TES) and perform as if it were a satellite
chiller plant when discharging TES during peak load periods.

•;Tank;can;also;be;quite;tall;(commonly;40-80;ft;or;even;150;ft), minimizing the required footprint to a value that can be equal to or less than that needed for an equivalent ice TES or non-TES chiller plant capacity. LTF TES is chosen where further volume reduction or reduced operating supply temperature is desired.

Table 2. Typical Characteristics of Cool TES Technologies.

Unit Volume Per Ton-hour Unit Footprint Per Ton-hour Modularity Economy of Scale Energy Efficiency Low Temperature Capability Ease of Retrofit to Existing District Cooling System Rapid Charge/Discharge Capability Simplicity and Reliability Ability to be Sited Remotely From Chillers Dual Use as Fire Protection