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Computational Fluid Dynamics

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					Heating and Air Conditioning I
  Principles of Heating, Ventilating and Air
                 Conditioning
  R.H. Howell, H.J. Sauer, and W.J. Coad
               ASHRAE, 2005

        basic textbook/reference material
                   For ME 421
                  John P. Renie
        Adjunct Professor – Spring 2009
  Chapter 6 – Energy Estimating Methods
• General Considerations.
    • Energy Resources and Sustainability
        • Because energy used in buildings and facilities comprises a significant
          amount of the total energy used for all purposes – affecting energy
          resources
        • ASHRAE recognizes the “effect of its technology on the environment and
          natural resources to protect the welfare of posterity”
        • Regulation of energy conservation through building permits
        • Energy sources – on-site energy in the form that it arrives at or occurs in
          a site (electricity, gas, oil, coal).
        • Energy resource is the raw energy that (1) is extracted, (2) is used to
          generate the energy sources delivered to the building (coal used to
          generate electricity), or (3) occurs naturally and is available at a site
          (solar, wind, geothermal)
        • This chapter takes an introductory look at the methods for estimating
          energy use.
        • Primary objective is economic – which option has the lowest total
          (lifetime) cost.
        • Compliance with energy performance codes
  Chapter 8 – Energy Estimating Methods
• General Considerations.
    • Energy Estimating Techniques
        • Share three elements – based on calculation of (1) space load, (2)
          secondary equipment load, and (3) primary equipment energy
          requirements.
        • Primary – central plant equipment that converts fuel or electric energy
          for heating and cooling
        • Secondary – equipment used to distribute the heating, cooling, or
          ventilating medium to the conditioned space.
        • Space load – amount of energy that must be added or extracted from
          a space to maintain thermal comfort – simple method would function
          of outdoor dry-bulb temperature only – more complex involves solar
          effects, internal gains, heat storage, etc. – most sophisticated involves
          hour-by-hour analysis.
        • Translation into secondary equipment load
        • The translation into the fuel and electricity required by the primary
          equipment considering efficiencies and part-load characteristics
        • Economic analysis – cost effectiveness of energy conservation,
          capital equipment, time of energy use, maximum demand, etc.
  Chapter 8 – Energy Estimating Methods
• General Considerations.
    • Energy Estimating Techniques
        • Sophistication of calculation procedures – function of number of
          ambient variable and/or time increments used.
        • Simpliest method – one measure such as degree-days – single-
          method measures
        • Bin methods – using more information such as the number of hours
          under an anticipated condition – simplified multiple-measure method.
        • Detailed simulation methods – require hourly weather data, as well as
          hourly estimates of internal loads such as lighting or occupants.
        • Calculations are nonlinear, dynamic, and very complex – need for
          computer modeling. See US DOE for list of software.
  Chapter 8 – Energy Estimating Methods
• Component Modeling and Loads.
    • Loads
       • After determining peak load, select equipment to offset load. Since
         most of time will be at partial loading, this aspect of sizing is also
         important
       • Calculating instantaneous space load is key to simulation
              •   Heat balance method
              •   Weighting factor method
              •   Both use conduction transfer functions to calculate heat gain or loss –
                  differences arise in the subsequent internal heat transfers to the room
    • Secondary System Components
       • Everything between the overall building energy system between a
         central heating and cooling plant and the building zones
              •   Air handler equipment, air fans, ductwork, dampers, humidifying
                  equipment, etc.
              •   Divided into distribution components and heat and mass transfer
                  components.
              •   All methods approximate the effect of the interactions with part-load
                  performance curves – shape of curve depends on the effect of flow
                  control on the pressure and fan efficiency – detailed analysis
  Chapter 8 – Energy Estimating Methods
• Component Modeling and Loads.
    • Primary System Components
       • Consumes energy and deliver heating and cooling to a building –
         includes chillers, boilers, cooling towers, cogeneration equipment,
         and plant-level thermal storage equipment – the major energy-
         consuming equipment – important to accurately model
       • Energy consumption based on design, load conditions,
         environmental conditions, and equipment control strategies
       • Usually the energy consumption characteristics of primary
         equipment is modeled using regression analysis on manufacturer’s
         published design data – based on full-load with correction for partial
         load.
       • Many forms of data curves … sometimes the use of data
         interpolation from tables in employed.
  Chapter 8 – Energy Estimating Methods
• Overall Modeling Strategies.
    • In developing a simulation model – two basic issues must be
      considered
        • Modeling of the components or subsystems (equations)
        • The overall modeling strategy (sequence and procedures to solve the
          equations)
    • Building energy programs – load models are executed for each
      space for every hour.
    • This is followed by running models for every secondary system,
      one at a time, for every hour of the simulation
    • Finally the plant simulation model is executed again for the entire
      period. Each sequential execution processes the fixed output of
      the preceding step – (load, systems, plant interation can cause
      unmet conditions only reported, not corrected)
    • Alternative approach is to solve all calculation simultaneously –
      superior but costly computationally. More accurate???
  Chapter 8 – Energy Estimating Methods
• Overall Modeling Strategies
    • See Figure 8-1 Overall Modeling Strategy flowchart
    • Figure 8-1 represents an economic model to calculate energy
      costs based on the estimated required input energy – maintaining
      running sums yields monthly or yearly energy usage or costs
    • These methods only compare design alternatives – uncontrolled
      number of variables usually rule out these methods for accurate
      prediction of utility bills.
    • Most energy analysis programs include a set of preprogrammed
      models that represent various systems – equations are arranged
      so that they can be solved sequentially. Or a smaller number of
      equations are solved simultaneously
    • Inflexibility in this approach
    • Solution – series of components may be organized in a component
      library and individually selected by the program – resolution of the
      specifications between components is simultaneously solved.
  Chapter 8 – Energy Estimating Methods
• Integration of System Models.
    • Energy calculations for secondary systems involve construction of
      the complete system from the set of HVAC component.
        • Example of a VAV system which is a single path system that controls
          zone temperature by modulating the airflow while maintaining a
          constant supply air temperature.
  Chapter 8 – Energy Estimating Methods
• Integration of System Models.
        • VAV system simulation consists of a central air-handling unit and a
          VAV terminal unit with reheat coil located at each zone.
        • Central air-handling provides the air at a controlled setpoint
        • VAV unit at each zone varies the airflow to meet the cooling load
        • As the zone cooling load decreases, the VAV terminal decreases the
          zone airflow until the unit reaches it minimum position – then reheat
          coil is used to meet the zone load.
        • Variable-speed fan/drive is used to control the supply fan
        • Algorithm for performing the calculations is given in Figure 8-3
        • The algorithm directs sequential calculations of system performance.
          Calculations proceed from the zones forward along the return air path
          to the cooling coil inlet and back through the supply air path to the
          cooling coil discharge.
        • Subsequent modifications to the basic algorithm – heat balance and
          weighted factor approaches – zone temperature variation and
          readjustment, limits, enhancements, etc.
  Chapter 8 – Energy Estimating Methods
• Integration of System Models.
    • VAV algorithm
  Chapter 8 – Energy Estimating Methods
• Integration of System Models.
    • Forward modeling
        • Description of building system or component of interest and defines
          the building being modeled according to its physical description.
        • Based on sound engineering principles and widespread acceptance
        • Order of analysis is presented in Figure 8-4 that is typically performed
          by a building energy simulation program
    • Inverse modeling
        • Based on empirical behavior of the building as it relates to one or
          more driving forces. This approach is referred to as system
          identification, parameter identification, or inverse modeling.
        • A structure or system is assumed first and then important parameters
          are identified by a statistical analysis.
  Chapter 8 – Energy Estimating Methods
• Integration of System Models.
    • Forward modeling
  Chapter 8 – Energy Estimating Methods
• Degree-Day Methods.
    • Introduction
        • Simplest methods for energy analysis and are appropriate if the
          building use and the efficiency of the HVAC equipment are constant
        • Where efficiency or conditions of use vary with outdoor temperature,
          the energy consumption can be calculated for different values and
          multiplied by the corresponding number of hours – bin methods
        • When indoor temperature is allowed to vary as well as interior gains,
          these simple models can’t be used.
        • Cooling methods less established than heating method – smaller
          temperature differences and more dependent on solar and interior
          gains.
        • However, similar cooling degree-day methods have been established
          and used.
        • Accurate for seasonal calculations (long term versus short term) when
          indoor temperatures and internal gains are constant
        • Valid for envelope dominated heating and cooling and loads based on
          temperature difference only.
  Chapter 8 – Energy Estimating Methods
• Degree-Day Methods.
    • Balance-Point Temperature and Degree-Days
        • Balance-point temperature is the average outdoor temperature at
          which the building requires neither heating or cooling for the HVAC
          system.
        • Degree-day procedures recognize that heating equipment need to
          meet only the heating not covered by internal sources and solar gain
        • Energy requirements of the space is proportional to the difference
          between the balance-point temperature and the outside temperature
  Chapter 8 – Energy Estimating Methods
• Degree-Day Methods.
    • Balance-Point Temperature and Degree-Days
        • Balance temperature, tbal, found when setting qH = 0 and solving for to
        • Heating only required when temperature drops below tbal.
        • Determination of heating-degree day – summed over month, season,
          or entire year




        • Cooling degree-days (note balance point could be different)
  Chapter 8 – Energy Estimating Methods
• Degree-Day Methods.
    • Seasonal Efficiency, h
        • Depends on factors such as steady-state efficiency, sizing, cycling
          effects, and energy conservation devices installed.
        • Can be lower or nearly equal to steady-state efficiency
        • Neglecting ducting loss (from NIST)




        • CFpl is a trait of the part-load efficiency of the heating equipment
  Chapter 8 – Energy Estimating Methods
• Degree-Day Methods.
  Chapter 8 – Energy Estimating Methods
• Degree-Day Methods.
    • Heating Degree-Day Method
        • Assumption is that in the long term, solar and internal gains offset
          heat loss when the mean daily outdoor temperature is equal to the
          balance-point temperature.
        • Assumption that fuel consumption is proportional to the difference
          between the daily mean and the balance-point temperature
        • Heat loss per degree difference being constant
        • Theoretical heating requirement is given by:
  Chapter 8 – Energy Estimating Methods
• Degree-Day Methods.
    • Heating Degree-Day Method
        • General form of the degree-day equation for fuel consumption
  Chapter 8 – Energy Estimating Methods
• Degree-Day Methods.
    • Heating Degree-Day Method
        • Typical heating values – h found in Table 19-5
         • Heating degrees-days for balance point of 65 F have been widely
           tabulated in past
         • Today, it may overestimate due to improved building construction –
           error adjust due to CD factor in equation
         • Recommend using variable-base degree-day approach
  Chapter 8 – Energy Estimating Methods
• Degree-Day Methods.
    • Heating Degree-Day Method – Table 8-2
  Chapter 8 – Energy Estimating Methods
• Degree-Day Methods.
    • Heating Degree-Day Method – Example 8-1
  Chapter 8 – Energy Estimating Methods
• Degree-Day Methods.
    • Variable-Base Heating Degree-Day (VBDD)
        • Variable-base degree-day method count degree-days based on actual
          balance-point temperature rather than 65 F.
        • Can give good results for the annual heating energy of single-zone
          buildings dominated by gains through the walls and roof and/or
          ventilation
        • Table 8-2 provides multiple base values for cooling and heating
          degree-days.
  Chapter 8 – Energy Estimating Methods
• Degree-Day Methods.
    • Cooling Degree-Day Method
  Chapter 8 – Energy Estimating Methods
• Degree-Day Methods.
    • Example 8-2
  Chapter 8 – Energy Estimating Methods
• Degree-Day Methods.
    • Example 8-3
  Chapter 8 – Energy Estimating Methods
• Degree-Day Methods.
    • Example 8-3
  Chapter 8 – Energy Estimating Methods
• Degree-Day Methods.
    • Example 8-3
  Chapter 8 – Energy Estimating Methods
• Degree-Day Methods.
    • Example 8-3
  Chapter 8 – Energy Estimating Methods
• Bin Method (Heating and Cooling).
    • Introduction
        • Sometimes the degree-day methods shouldn’t be employed because
          the heat loss coefficient K, the equipment efficiency, and the balance-
          point temperature may not be constant.
        • Annual consumption can be determined if different temperature
          intervals and time periods are evaluated separately
        • Energy consumption bin, Ebin, determined at several outdoor
          temperatures and multiplied by umber of hours, Nbin
  Chapter 8 – Energy Estimating Methods
• Bin Method (Heating and Cooling).
    • Table 8-3 Hourly Temperature Occurrences
  Chapter 8 – Energy Estimating Methods
• Bin Method (Heating and Cooling).
    • Modified Bin Method
        • Refinements – such as seasonal variation on solar gains
        • Use of a diversified (part-load) rather than a peak-load value to
          establish the load as a function of outdoor temperature
        • Effect of primary and secondary equipment included
        • Effect of reheat and recovery included
        • Characterization of time-dependent diversified loads
        • Transient effects of building mass
    • Degree-Day from Bin Data
  Chapter 8 – Energy Estimating Methods
• Bin Method (Heating and Cooling).
    • Degree-Day from Bin Data
        • First determine the balance point temperature
  Chapter 8 – Energy Estimating Methods
• Bin Method (Heating and Cooling).
    • Bin-method – Data form (Table 8-4)
  Chapter 8 – Energy Estimating Methods
• Bin Method (Heating and Cooling).
    • Bin-method – Example 8-4
  Chapter 8 – Energy Estimating Methods
• Bin Method (Heating and Cooling).
    • Bin-method – Example 8-4
  Chapter 8 – Energy Estimating Methods
• Bin Method (Heating and Cooling).
    • Heat pump capacity and building load
  Chapter 8 – Energy Estimating Methods
• Bin Method (Heating and Cooling).
    • Bin-method – Example 8-4

				
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