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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.

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Heating and air conditioning i l.jpg

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


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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


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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.


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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.


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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


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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.


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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???


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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.


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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.


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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.


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Chapter 8 – Energy Estimating Methods

  • Integration of System Models.

    • VAV algorithm


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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.


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Chapter 8 – Energy Estimating Methods

  • Integration of System Models.

    • Forward modeling


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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.


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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


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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)


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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


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Chapter 8 – Energy Estimating Methods

  • Degree-Day Methods.


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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:


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Chapter 8 – Energy Estimating Methods

  • Degree-Day Methods.

    • Heating Degree-Day Method

      • General form of the degree-day equation for fuel consumption


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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


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Chapter 8 – Energy Estimating Methods

  • Degree-Day Methods.

    • Heating Degree-Day Method – Table 8-2


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Chapter 8 – Energy Estimating Methods

  • Degree-Day Methods.

    • Heating Degree-Day Method – Example 8-1


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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.


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Chapter 8 – Energy Estimating Methods

  • Degree-Day Methods.

    • Cooling Degree-Day Method


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Chapter 8 – Energy Estimating Methods

  • Degree-Day Methods.

    • Example 8-2


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Chapter 8 – Energy Estimating Methods

  • Degree-Day Methods.

    • Example 8-3


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Chapter 8 – Energy Estimating Methods

  • Degree-Day Methods.

    • Example 8-3


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Chapter 8 – Energy Estimating Methods

  • Degree-Day Methods.

    • Example 8-3


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Chapter 8 – Energy Estimating Methods

  • Degree-Day Methods.

    • Example 8-3


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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


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Chapter 8 – Energy Estimating Methods

  • Bin Method (Heating and Cooling).

    • Table 8-3 Hourly Temperature Occurrences


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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


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Chapter 8 – Energy Estimating Methods

  • Bin Method (Heating and Cooling).

    • Degree-Day from Bin Data

      • First determine the balance point temperature


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Chapter 8 – Energy Estimating Methods

  • Bin Method (Heating and Cooling).

    • Bin-method – Data form (Table 8-4)


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Chapter 8 – Energy Estimating Methods

  • Bin Method (Heating and Cooling).

    • Bin-method – Example 8-4


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Chapter 8 – Energy Estimating Methods

  • Bin Method (Heating and Cooling).

    • Bin-method – Example 8-4


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Chapter 8 – Energy Estimating Methods

  • Bin Method (Heating and Cooling).

    • Heat pump capacity and building load


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Chapter 8 – Energy Estimating Methods

  • Bin Method (Heating and Cooling).

    • Bin-method – Example 8-4


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