Lecture Objectives: Discuss Projects 1 and 2 Learn about thermal storage systems Learn about control systems

Project 1: Modeling of Water Cooled Chiller (COP=Qcooling/Pelectric) Chiller model: COP= f(TCWS , TCTS , Qcooling , chiller properties)

Modeling of Water Cooled Chiller Chiller model: Chiller data: QNOMINAL nominal cooling power, PNOMINAL electric consumption for QNOMINAL Available capacity as function of evaporator and condenser temperature Cooling water supply Cooling tower supply Full load efficiency as function of condenser and evaporator temperature Efficiency as function of percentage of load Part load: The consumed electric power [KW] under any condition of load The coefiecnt of performance under any condition Reading: http://apps1.eere.energy.gov/buildings/energyplus/pdfs/engineeringreference.pdf page 597.

Combining Chiller and Cooling Tower Models Function of TCTS 3 equations from previous slide Add your equation for TCTS → 4 equation with 4 unknowns (you will need to calculate R based on water flow in the cooling tower loop)

Merging Two Models Temperature difference: R= TCTR -TCTS 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 is given - property of a tower TCTR= TCTS - Q condenser / (mcp)water Finally: Find P() or The only fixed variable is TCWS = 5C (38F) and Pnominal and Qnominal for a chiller (defined in nominal operation condition: TCST and TCSW); Based on Q() and WBT you can find P() and COP().

Low Order Building Modeling Measured data or Detailed modeling Find Q() = f (DBT)

For HW3a (variable sped pump efficiency) you will need Q() Yearly based analysis: You will need Q() for 365 days x 24 hours Use simple molded below and the Syracuse, NY TMY weather file posted in the course handout section 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 4 8 12 16   Q=-0.45 +0.0448*t Q=--27.48+0.5152*t Q [ton] t [F] TMY 3 for Syracuse, NY http://rredc.nrel.gov/solar/old_data/nsrdb/1991-2005/tmy3/by_state_and_city.html

For Austin’s Office Building Model: (Area = 125,000sf) Hours in a year kW Used for component capacity analysis Model =0 when building is off Reading assignment: http://www.taylor-engineering.com/downloads/cooltools/EDR_DesignGuidelines_CoolToolsChilledWater.pdf Chapter: 2 Number of hours

Thermal storage for adjustment production to consumption We need a thermal storage somewhere in this system !

Stratified chilled water tanks

Installation of thermal storage system Upstream Downstream • Increases chiller efficiency • Increases chiller capacity • Overall system efficiency ??? • Decreases storage capacity • Simplifies system layout • ….. • Decreases chiller efficiency • Decreases chiller capacity Overall system efficiency ??? • Increases storage capacity •…… • Does not allow chiller shut down!

Modeling of chilled water tank (stratified vs. mixing) From building To chiller Stratification To building From chiller Mixing happens if the supply temperature vary Mixing model: mcpDT/D = Qin –  Qout

Stratification Dr. Jing Song’s PhD results Flow time at 20 minutes CFD domain Flow time at 1 minute

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

Stratified chilled water tanks diffuser geometry Challenge: “Pull” large amount of energy without disturbing stratification

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 costs Lower on-peak electrical consumption(kWh) Lower on-peak electrical demand (kW) Smaller equipment size Smaller chiller Smaller electrical service (A) Reduced installed cost May qualify for utility rebates or other incentives

HVAC Control Example 1: Economizer (fresh air volume flow rate control) Controlled device is damper - Damper for the air - Valve for the liquids fresh air damper mixing recirc. air T & RH sensors

Economizer Fresh air volume flow rate control % fresh air TOA (hOA) enthalpy 100% Fresh (outdoor) air TOA (hOA) Minimum for ventilation damper mixing Recirc. air T & RH sensors

Economizer – cooling regime How to control the fresh air volume flow rate? If TOA < Tset-point → Supply more fresh air than the minimum required The question is how much? Open the damper for the fresh air and compare the Troom with the Tset-point . Open till you get the Troom = Tset-point If you have 100% fresh air and your still need cooling use cooling coil. What are the priorities: - Control the dampers and then the cooling coils or - Control the valves of cooling coil and then the dampers ? Defend by SEQUENCE OF OERATION the set of operation which HVAC designer provides to the automatic control engineer % fresh air 100% Minimum for ventilation

Economizer – cooling regime Example of SEQUENCE OF OERATIONS: If TOA < Tset-point open the fresh air damper the maximum position Then, if Tindoor air < Tset-point start closing the cooling coil valve If cooling coil valve is closed and T indoor air < Tset-point start closing the damper till you get T indoor air = T set-point Other variations are possible

HVAC Control Example 2: Dew point control (Relative Humidity control) fresh air damper filter cooling coil heating coil filter fan mixing T & RH sensors Heat gains Humidity generation We should supply air with lower humidity ratio (w) and lower temperature We either measure Dew Point directly or T & RH sensors substitute dew point sensor

Relative humidity control by cooling coil Mixture Room Supply TDP Heating coil

Relative humidity control by cooling coil (CC) Cooling coil is controlled by TDP set-point if TDP measured > TDP set-point → send the signal to open more the CC valve if TDP measured < TDP set-point → send the signal to close more the CC valve Heating coil is controlled by Tair set-point if Tair < Tair set-point → send the signal to open more the heating coil valve if Tair > Tair set-point → send the signal to close more the heating coil valve Control valves Fresh air mixing cooling coil heating coil Tair & TDP sensors

Sequence of operation (ECJ research facility) Set Point (SP) Mixture 2 Mixture 3 Mixture 1 DBTSP DPTSP Control logic: Mixture in zone 1: IF (( TM<TSP) & (DPTM<DPTSP) ) heating and humidifying Heater control: IF (TSP>TSA) increase heating or IF (TSP<TSA) decrease heating Humidifier: IF (DPTSP>DPTSA) increase humidifying or IF (DPTSP<DPTSA) decrease humid. Mixture in zone 2: IF ((TM>TSP) & (DPTM<DPTSP) ) cooling and humidifying Cool. coil cont.: IF (TSP<TSA) increase cooling or IF (TSP>TSA) decrease cooling Humidifier: IF (DPTSP>DPTSA) increase humidifying or IF (DPTSP<DPTSA) decrease hum. Mixture in zone 3: IF ((DPTM>DPTSP) ) cooling/dehumidifying and reheatin Cool. coil cont.: IF (DPTSP>DPTSA) increase cooling or IF (DPTSP<DPTSA) decrease cooling