Energy Plus & Open Studio Class

• Today, 5:45 PM

• Computer lab ECJ 3.402

• Instructor: Wesley Cole

ASHRAE Student Chapter Meeting

• Monday Nov 26th at 6pm

• ECJ 5.410

Lecture Objectives:

• Finish with common HVAC system configurations

• Discuss control systems

• Discuss the life cycle cost analysis

• Learn about empirical modeling

HVAC systems 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 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

What are the reasons for energy simulations?

• System Development (research)

• Building design (evaluate different design solutions)

• Economic benefits

• Budget planning

Life Cycle Cost Analysis

• Engineering economics

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

in the school building in NYC

What are the reasons for energy simulations?

• System Development (research)

• Building design (evaluate different design solutions)

• Economic benefits

• Budget planning

For budget planning

5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 900

50

100

150

200

250

300

350

400

450

500

Q [t

on]

t [F]

Load vs. dry bulb temperature Measured for a building in Syracuse, NY

5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 900

50

100

150

200

250

300

350

400

450

500

Q=-11.33+1.2126*t

Q=-673.66+12.889*t

Q [t

on]

t [F]

8760

1i ii

ii

57 tif t889.1266.673

57 tif t126.133.11(Q

Model

Empirical model

8760

1i ii

ii

57 tif t889.1266.673

57 tif t126.133.11(Q

For average year use TMY2

=835890ton hour = 10.031 106 Btu