Lecture Objectives:

• Discuss Project 1

• Learn about thermal storage systems

For Project 1 you will needWet bulb temperature

Empirical equation based on T DB [°C] and RH [0-100]

Exact calculation require solving 3 equations

For this modeling the empirical equation is fine.

Merging Two Models

Finally: Find P() or

The only fixed variable is TCWS = 5C (38F) and Pnominal and Qnominal and m cooling tower (Rnominal) for a chiller (defined in nominal operation condition): Based on Q() and WBT you can find P() and COP().

Temperature difference:

R= TCTR -TCTS

22444

2444

2444 ][][ RWBTiWBThgRWBTfWBTedWBTcWBTbaTCTS

Model:

Link between the chiller and tower models is the Q released on the condenser: Q condenser = Qcooling + Pcompressor - First law of Thermodynamics

Q condenser = (mcp)water form tower (TCTR-TCTS) m cooling tower - property of a tower

TCTR= TCTS - Q condenser / (mcp)water

)(

)()(

P

QCOP

Example of CHP + Cooling

We need a thermal storage somewhere in this system !

Thermal storage

• Store heat• Many issues to consider (∆T, pressure, losses,…. )

• Store cooling energy• Chilled water

• For cooling condenser

• For use in AHU (cooling coils)

• Ice storage • Compact but…

• Other materials (PCMs) that change phase the temperature we need in cooling coils

• Many advantages, but disadvantages too!

On-Peak and Off-Peak Periods

This profile depends on the type of building(s) !

Chilled water tank

Use of stored cooling energy

Store Use

Which one is better ?

Depends on what you want to achieve:-Peak electric power reduction-Capacity reduction-…..

Downsizing the Chiller

• Lower utility costsLower on-peak electrical consumption(kWh)Lower on-peak electrical demand (kW)

• Smaller equipment sizeSmaller chillerSmaller electrical service (A)

• Reduced installed costMay qualify for utility rebates or other incentives

Sizing storage system (use Annual Cooling-Load Profile)

How often you need to use it?What are the cost-benefit curves ? What is the optimum size ?

Stratified chilled water tanks

Temperature and dynamics Temperature at outlet during the changing cycle of the tanks

Stratified chilled water tanksdiffuser geometry

Challenge:“Pull” large amount of energy withoutdisturbing stratification

Ice Storage Tank

Many issues !….As freezing progresses progress the ice becomes thicker and significantly impedes heat transfer

Open Ice Storage Tank

Also issues !…..

Fluid Flow Rate and Freeze ΔT

Impact on Chiller Efficiency

PCMs

A. Sharma et al. / Renewable and Sustainable Energy Reviews 13 (2009) http://www.docunator.com/bigdata/1/1367142535_c566e7d75e/reviewexample2.pdf

Other latent heat thermal storage systems (research is ongoing)

Construction and benefits Active

Passive (wall)

Modeling(stratified tank)

Write energy and mass balancer equation for each section

Installation of thermal storage system

DownstreamUpstream

• Increases chiller efficiency • Increases chiller capacity • Overall system efficiency ??? • Decreases storage capacity• Simplifies system layout• …..

• Decreases chiller efficiency• Decreases chiller capacityOverall system efficiency ???• Increases storage capacity•……• Does not allow chiller shut down!