Objectives Discuss final project deliverables Control Terminology Types

Objectives • Discuss final project deliverables • Control – Terminology – Types of controllers • Differences – Controls in the real world • Problems • Response time vs. stability

FINAL PROJECT DELIVERABLES AND GRADING DELIVERABLES 1) PROJECT REPORT: - Project statement (introduction) 2 pages Explain what are you designing/analyzing and why is that important On the second page clearly identify (bullet list) project outcomes - Building description (geometry) 1 -3 pages Schematics that focus on your system(s) Identify all assumptions and simplifications you introduced - Methodology 1 -3 pages Describe methodology (equations, schematics, …) Provide a list of assumptions used in your methodology - Results 3 -5 pages Formatted results with comments Tables, Charts, Diagrams, … Analysis and Results discussion - Conclusion 0. 5 -1 page Summary of most important results - 2) PRESENTATION: - 5 minutes (exactly) Power point presentation (4 -6 slides) GRADING CRITERIA: 1) Analysis approach: - Methodology - Accuracy analysis - Result analysis 2) Deliverables: - Final report - Presentations 60% 20% 20% 40% 30% 10%

Sequence of operation for the control system design OA CC Adiabatic humidifier HC SA mixing RA Define the sequence of operation for: WINTER operation and: - case when humidity is not controlled - case when humidity is precisely controlled Solution on the whiteboard

Economizer % fresh air Fresh air volume flow rate control enthalpy damper Fresh (outdoor) air TOA (h. OA) mixing Recirc. air T & RH sensors 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

Basic purpose of HVAC control Daily, weekly, and seasonal swings make HVAC control challenging Highly unsteady-state environment Provide balance of reasonable comfort at minimum cost and energy Two distinct actions: 1) Switching/Enabling: Manage availability of plant according to schedule using timers. 2) Regulation: Match plant capacity to demand

Terminology • Sensor – Measures quantity of interest • Controller – Interprets sensor data • Controlled device – Changes based on controller output Figure 2 -13

outdoor Direct Indirect Closed Loop or Feedback Open Loop or Feedforward

• Set Point – Desired sensor value • Control Point – Current sensor value • Error or Offset – Difference between control point and set point

Two-Position Control Systems • Used in small, relatively simple systems • Controlled device is on or off – It is a switch, not a valve • Good for devices that change slowly

• Anticipator can be used to shorten response time • Control differential is also called deadband

Residential system - thermostat • ~50 years old DDC thermostat - Daily and weekly programming

Modulating Control Systems Example: Heat exchanger control – Modulating (Analog) control Cooling coil air x water (set point temperature)

Modulating Control Systems • Used in larger systems • Output can be anywhere in operating range • Three main types – Proportional – PID Electric (pneumatic) motor Position (x) fluid Volume flow rate Vfluid = f(x) - linear or exponential function

The PIDconstants control algorithm time e(t) – difference between set point and measured value Position (x) Proportional Integral Differential For our example of heating coil: Proportional (how much) Position of the valve Integral (for how long) Differential (how fast)

Proportional Controllers x is controller output A is controller output with no error (often A=0) Kis proportional gain constant e= is error (offset)

Unstable system Stable system

Issues with P Controllers • Always have an offset • But, require less tuning than other controllers • Very appropriate for things that change slowly – i. e. building internal temperature

Proportional + Integral (PI) K/Ti is integral gain If controller is tuned properly, offset is reduced to zero Figure 2 -18 a

Issues with PI Controllers • Scheduling issues • Require more tuning than for P • But, no offset

Proportional + Integral + Derivative (PID) • Improvement over PI because of faster response and less deviation from offset – Increases rate of error correction as errors get larger • But – HVAC controlled devices are too slow responding – Requires setting three different gains

Ref: Kreider and Rabl. Figure 12. 5

The control in HVAC system – only PI Proportional Integral value Set point Proportional affect the slope Integral affect the shape after the first “bump”

The Real World • 50% of US buildings have control problems – 90% tuning and optimization – 10% faults • 25% energy savings from correcting control problems • Commissioning is critically important