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Lecture Objectives:
• Clarify issues related to eQUEST – for midterm project
• Learn more about various HVAC - economizer
- heat recovery
• Discuss about the final project assignment
Integration of HVAC and building physics models
BuildingHeating/Cooling
SystemPlant
Load System Plant model
Qbuiolding Q
including
Ventilation
and
Dehumidification
Primary and Secondary Building Systems
AHU
ECJ buildingCooling(chiller)
(or Gas)
Electricity
Gas
Heating(boilers)
Fresh air For ventilation
Distribution systems
Air transport
Secondary(HVAC) systems
Primary (Plant)systems
AHU – Air Handling Unit
Processes in AHU
filterfan
cooling coil
heating coil
filter steamhumidifier
Building
mixing
damper fresh air
Processes in AHU presented in psychrometric chart - cooling
OA Case forSummer in Austin
IA
MA
SA
Processes in AHU presented in psychrometric chart - heating
OA
Case forWinter in Austin
IA
MA
SA
Simple modeling of HVAC system
Models for heat transfer in building elements
-External boundaries (weather data)- convection - conduction - radiation - infiltration
QLoad – for every hour
Does not include ventilationby HVAC system
)(/
)()(/
)()(
sup
sup
sup
airpLoadindoorply
plyindoorairpLoad
plyindoorairpLoad
cVQTT
or
TTcQV
or
TTcVQ
Disadvantages of this model:- Does not include the characteristics of HVAC components- Assume that whole building is a single zone- Assume always all air system
Supply volume flow rate for every hour
Psychrometric Chart
- External boundaries (weather data)- Points in chart - Heating, cooling, mixing, humidification, dehumidification- Find enthalpies and V
QHVAC – for every hour
VAV
Supply temperature for every hour
CAV
Final project topics:Software based• Energy analysis of building form Integrated design course, • Envelope analysis of glass buildings• ….
Detail Modeling (your model)• Heat recovery systems, • Economizers, • Water cooled chiller,• Geothermal heat pump, • Solar hot water systems,• Mass transfer (moisture, ozone, VOCs,…) • ….Your ideas
Requirement for Thursday Email me:• Group members • Project title• One paragraph (200-300 words) including
- Project objective - Methodology- Expected results
Any schematic drawing is welcome
Thursday afternoon long office hours to discuss
your project
Modeling of AHU systemEconomizers & Heat recovery systems
IA
OAsummer
OAwinter
OA
What to do if OAIs here ?
Economizer
mixing
damper
fresh air
T & RH sensors
recirc. air
Controlled devices are damper
Economizer Fresh air volume flow rate control
mixing
damper
Fresh(outdoor) air
T & RH sensors
Recirc. air
% fresh air
Minimum for ventilation
100%
TOA (hOA)
enthalpy
Economizer – cooling regime with CAV
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
Heat Recovery SystemsAir to Air Heat Exchanger
Plate heat exchangers
Enthalpy wheel Effectiveness ~60%
Effectiveness ~75%
Effectiveness ~50%
Ventilation Heat Recovery Systemsfor cooling and heating
Commercial buildings Residential buildings
Saving % - depends on how much ventilation we have in the building
Water Cooled Chiller
Outside air 95°F
Inside 75°FWater 42°F
BuildingWater 52°FWater 120°F
Water 100°F
Cooling tower
Chiller
Task: analyze COP for the whole yearand different locations
Solar hot water system
Geothermal Energy Systems(in combination with a cooling machine / heat pump)
Summer Winter
In the summer, the earth acts as a cooling tower. The Cooling Machine loads the loop with heat, sending warmed water to be cooled by the earth
In the winter, the earth acts as the boiler. The Heat Pump extracts heat from the loop, sending cooled water to be warmed by the earth.