Lecture Objectives: Answer questions related to HW 4 Solar Systems Continue with HVAC system

Building HVAC Systems (Primary and Secondary Building HVAC Systems) AHU – Air Handling Unit Distribution systems Fresh air for ventilation AHU Primary systems Air transport Electricity Secondary systems Cooling (chiller) Heating (boilers) Building envelope HVAC systems affect the energy efficiency of the building as much as the building envelope. In many situation even more! (or Gas) Gas

Examples of HVAC System Multizone Dual Duct System Multi zone VAV with Re-heaters 55°F 90°F 55°F P C P C Perimeter (P) Core (C)

Dual Duct vs. VAV with Re-heaters for Different Weather Conditions What happens if outdoor air is A, B, C A B C

Example of a Plant System (Chilled Water System) Air cooled chiller Chiller with a cooling tower COP ~ 3 COP ~ 5 COP = Cooling Energy / Electric Energy ( same units)

Two Basic Approaches for Modeling of HVAC and Building Envelope Load System Plant model Building Qbuiolding Heating/Cooling System Q including Ventilation and Dehumidification Plant Integrated models Building Heating/Cooling System Plant

Example of a HVAC Model Schematic of simple air handling unit (AHU) Mixing box m - mass flow rate [kg/s], T – temperature [C], w [kgmoist/kgdry air], r - recirculation rate [-], Q energy/time [W]

Example of a Plant Models (Chiller) P electric () = COP () x Q cooling coil () TOA What is COP for this air cooled chiller ? T Condensation = TOA+ ΔT Evaporation at 1oC TCWS=5oC TCWR=11oC water Building users (cooling coil in AHU) COP is changing with the change of TOA

Plant model Refrigeration Cycle Released energy (condenser) T outdoor air T cooled water - What is COP? - How the outdoor air temperature affects chiller performance? Cooling energy (evaporator)

Chiller model: COP= f(TOA , Qcooling , chiller properties) Chiller data: QNOMINAL nominal cooling power, PNOMINAL electric consumption for QNOMINAL The consumed electric power [KW] under any condition Available capacity as function of evaporator and condenser temperature Cooling water supply Outdoor air Full load efficiency as function of condenser and evaporator temperature Efficiency as function of percentage of load Percentage of load: The coefficient of performance under any condition:

Models integrated in HVAC System simulation Example: Economizer (fresh air volume flow rate control) Controlled device is damper - Damper for the air - Valve for the liquids fresh air damper mixing recirc. air T & RH sensors

HVAC Control Economizer (fresh air volume flow rate control) Controlled device is damper - Damper for the air - Valve for the liquids fresh air damper mixing recirc. air % fresh air T & RH sensors 100% Minimum for ventilation

Economizer – cooling regime How to control the fresh air volume flow rate? 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 % fresh air 100% Minimum for ventilation