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Published byLindsey Jacobs Modified over 4 years ago

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Lecture Objectives: Learn about automatic control Use life-cycle cost analysis integrated in eQUEST

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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

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

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Cooling coil control valve Position ( x ) fluid Electric (pneumatic) motor V fluid = f(x) - linear or exponential function Volume flow rate

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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 Proportional (how much) Integral (for how long) Differential (how fast) Position of the valve

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

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Detail control system simulation MatLAB - Simulink Control system simulation - take into account HVAC component behavior but focus more on control devices and stability of control scheme

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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

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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%

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Economizer – cooling regime How to control the fresh air volume flow rate? % fresh air Minimum for ventilation 100% If T OA < T set-point → Supply more fresh air than the minimum required The question is how much? Open the damper for the fresh air and compare the T room with the T set-point. Open till you get the T room = T set-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

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Economizer – cooling regime Example of SEQUENCE OF OERATIONS: If T OA < T set-point open the fresh air damper the maximum position Then, if T indoor air < T set-point start closing the cooling coil valve If cooling coil valve is closed and T indoor air < T set-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

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Example of Sequence of calculation in energy simulation models

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Life Cycle Cost Analysis Engineering economics

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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)

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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

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Example Using eQUEST analyze the benefits (energy saving and pay back period) of installing - low-e double glazed window - variable frequency drive

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Floor heating system Solar radiation Floor heating tank Perforated tube Floor heating system P2 T3T4 Example project

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Solar collector system 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

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