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Lecture Objectives:
• Learn about automatic control
• Use life-cycle cost analysis integrated in eQUEST
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
Basic Control loopExample: Heat exchanger control
– Modulating (Analog) control
air
water
Cooling coil
(set point temperature)
x
Cooling coil control valve
Position (x)
fluid
Electric (pneumatic) motor
Vfluid = f(x) - linear or exponential function
Volume flow rate
The PID control algorithm
For our example of heating coil:
Proportional Integral Differential
time
Position (x)
constants
e(t) – difference between set point and measured value
d
TTdTKdTT
T
KTTKx d
i
)()()( measuredpointset
measuredpointset measuredpointset
Proportional(how much)
Integral(for how long)
Differential(how fast)
Position of the valve
The control in HVAC system – only PI
dTTT
KTTKx
i
)()( measuredpointset measuredpointset
Proportional Integral
Proportionalaffect the slope
Integralaffect the shape after the first “bump”
Set point
Set point
value
Detail control system simulationMatLAB - Simulink
Control system simulation - take into account HVAC component behavior but focus more on control devices and stability of control scheme
Models integrated in HVAC System simulation Example:
Economizer (fresh air volume flow rate control)
mixing
damper
fresh air
T & RH sensors
recirc. air
Controlled device is damper
- Damper for the air - Valve for the liquids
HVAC Control
Economizer (fresh air volume flow rate control)
mixing
damper
fresh air
T & RH sensors
recirc. air
Controlled device is damper
- Damper for the air - Valve for the liquids
% fresh air
Minimum for ventilation
100%
Economizer – cooling regime
How to control the fresh air volume flow rate?
% fresh air
Minimum for ventilation
100%
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
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
Sequence of calculation in energy simulation modeling is different than sequence of operation !
We often assume perfect aromatic control
Example of Sequence of calculation in energy simulation models
HVAC solver calculates Q using
plant_real Matrix solver results
Matrix solver solves newT and T
for Qrad_surf air_real
plant_real
T < Trad_sur f max
Matrix solver solves Q for
T or Tplant_corec
max min
for heating:
or for cooling:T > Trad_surf_corec min
yes
yes
no
no
controlled Trad_surf controlled Tsupply
Matrix S.
for
air_setpoint
solves required T and Q
Trad_surf plant
HVAC solver calculates Q using
plant_real
Matrix solver results
Matrix solver solves new T and T
for Q supply air_real
plant_real
T < T < Tmin supply max
Matrix solver solves Q for
T or Tplant_corect
max min
yes
yes
no
no
controlled msupply
Matrix S.
for
air_setpoint
solves required T and Q
Tsupply plant
Q < Qplant_real plant Q < Qplant_real plant
HVAC solver calculates Q using
plant_real
Matrix solver results
Matrix solver solves new m and T
for Qsupply air_real
plant_real
Matrix solver solves Q for
m or m and Tplant_corect
max min supply
yes
yes
no
no
Matrix S.
for
air_setpoint
solves required m and Q
Tsupply plant
Q < Qplant_real plant
Matrix Solver
for
air_setpoint
solves Q
Tplant_recqured
Matrix solver
for Q
solves T
= 0air
plant_recqured
Matrix solver
for air_setpoint
solves Qplant_recqured
T
HVAC solver calculates Q using
plant_real
Matrix solver results
Matrix solver so lves T
for Qair_real
plant_real
yes
no Q < Qplant_real plant_required
contro lled pure convective source
m < m < mmin supply max
If
corect
zone reheater and m > m
T and check
T < T < T
min supply
supply
min supply max
load calculation HVAC is OFF
HVAC control is ON
II III IV V
VII
Life Cycle Cost Analysis
• Engineering economics
Life Cycle Cost Analysis
• Engineering economics
• Compound-amount factor (f/p)• Present worth factor value (p/f) • Future worth of a uniform series of amount (f/a)• Present worth of a uniform series of amount (p/a)• Gradient present worth factor (GPWF)
Parameters in life cycle cost analysis
Beside energy benefits expressed in $,you should consider:
• First cost• Maintenance• Operation life• Change of the energy cost • Interest (inflation)• Taxes, Discounts, Rebates, other Government
measures
Example
• Using eQUEST analyze the benefits (energy saving and pay back period)
of installing
- low-e double glazed window
- variable frequency drive
Floor heating system
Solar radiation
Floor heating tank
Perforated tube
Floor heating system
P2
T3
T4
Example project
Solar collector system
)]([ OAiLossRSCSC TTUSFAQ
)( iopSC TTmcQ
)()( OAiTANKheatingSCp TTUAQQd
dTmc
TANKUA)(
LossU
Solar collector
Water flow
Water tank
Area
Property of solar collector
Total solar radiation
coefficient which define lost of energy from solar collector surfaces to surrounding
define lost of energy from water tank to surrounding
Used energy