Energy Calculations: Why, What and When

Tim Shinnick, PE, QCxP, CEMGrumman/Butkus Associates

AIA Quality Assurance

The Building Commissioning Association is a Registered Provider with The American Institute of Architects Continuing Education Systems (AIA/CES). Credit(s) earned on completion of this program will be reported to AIA/CES for AIA members. Certificates of the Completion for both AIA members and non-AIA members are available upon request.

This program is registered with AIA/CES for continuing professional education. As such, it does not include content that may be deemed or construed to be an approval or endorsement by the AIA of any material of construction or any method or manner of handling, using, distributing, or dealing in any material or product.

Questions related to specific materials, methods, and services will be addressed at the conclusion of this presentation.

BCxA Conference – Nashville, TN – October 2018 2

Learning Objectives

1. Cite the energy and HVAC engineering calculations necessary for energy savings.

2. Identify data required for typical facility improvement measure savings calculations.

3. Apply calculations to airflow setback measure.

4. Apply calculations to heat recovery measure.

BCxA Conference – Nashville, TN – October 2018 3

Outline• Energy Calculation Overview• Review Specific Energy Savings Measures

o Measure Descriptiono Data Required to Estimate Energy Savingso Formulas Necessary for Calculationo Review the Calculationo Review Verification Processo Discuss Potential Mistakes

• Discuss Multiple Measure Interaction• Discuss Back-Checking Savings

Previous Webinar: 11.29.17 Diving into Energy Savings Calculations available on BCxA website, Recorded Webinars

Introduction

• Why do we do energy calcs?• Justify energy project• Prioritize different energy measures• Prove value of doing Cx• Apply for utility incentives

Components of a Calculation• Operating Parameters

• Field measurements• Trend data• Equipment design information• Equipment nameplate information• Assumptions

• Weather data used to normalize calculation for annual estimates • Engineering formulas to calculate energy savings• Utility rates to estimate cost savings

Energy Calculations

• Understand the level of effort needed for different calculations. • Do a first pass, quick calculation• Minimal energy savings: Tweak the calc if necessary but

don’t spend too much time.• Significant energy savings: Start a new calc and collect

additional data to improve assumptions• Review M&V requirements for project

Engineering Formulas

• Air Total Heat Transfer Formula: Q (btu/hr) = 4.5 x cfm x Δh • 4.5 = 60 minutes/hour x 0.075 lb/ft3

• Air Sensible Heat Formula: QS (btu/hr)= 1.08 x cfm x ΔT • 1.08 = 60 minutes/hour x 0.075 lb/ft3 x 0.24 btu/lb/˚F

• Water Energy Formula: Q (btu/hr)= 500 x gpm x ΔT • 500 = 60 minutes/hour x 8.33 lb/gal x 1 btu/lb/˚F

• Pump Brake Horsepower: BHP = GPM x Head x Specific Gravity (3960 x Pump Efficiency)

• Fan Brake Horsepower: BHP = CFM x Total Pressure (6356 x Fan Efficiency)

Engineering Formulas

• Desired outside air ratio: • MAT = OA% x OAT + RA% x RAT• %OA = (MAT – RAT) / (OAT – RAT)

• Affinity Laws• Reduce exponent to account for real world

efficiency losses

Typical Weather Data

• Apply observed operation to typical weather data in order to normalize energy savings estimate on an annual basis.• Allows you to estimate annual energy savings for typical

weather conditions instead of recent actual weather conditions.

• Typical Meteorological Year 3 (TMY3) Weather Data• http://rredc.nrel.gov/solar/old_data/nsrdb/1991-2005/tmy3/by_state_and_city.html

• Future Weather Scenarios• https://energyplus.net/weather/simulation

Typical Weather Data• From 2016 ACEEE Summer Study on Energy Efficiency in Buildings

Energy Calculations

• Resources for Assumptions• State or Utility Energy Efficiency Programs

• Illinois Technical Reference Manual

• NEMA minimum motor efficiency requirements• ASHRAE 90.1 – Energy Standard for Buildings

• Appendix G provide standard assumptions for building occupancy based on building type, lighting density, equipment efficiencies

• Start with standard material, modify as necessary based on discussions with building operators or site observations.

Energy Savings Calculations

• Energy measures to review• VAV Airflow Setback• Heat Recovery Operation• Simultaneous Heating and Cooling

VAV Airflow Setback

• Existing OperationLarge variable air volume system serving spaces that are occupied during the day and several spaces that are unoccupied at night, but the unit airflow remains high during unoccupied periods.

• Proposed OperationReduce the airflow based on time of day for the terminal units serving unoccupied periods.

VAV Airflow Setback

• Identify Measure / Document Baselineo Discuss with building operatorso Review of trend data showed minimal supply fan speed

reduction during unoccupied hours.

14

15

16

17

18

19

20

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23

Cont

rol S

igna

l (4-

20 m

Amps

)

Hour of the day (0-23)

Baseline Average Supply Fan Signal

SF-1

SF-2

SF-3

SF-4

VAV Airflow Setback

• Proposed Changeso Review building terminal unit schedule or BAS to determine

potential airflow reductiono Discuss building occupancy schedule with building manager

VAV Airflow Setback

• Data Requiredo Fan speed or airflow based on time of dayo Discharge air temperatureo Return air temperatureo Mixed air temperature vs OATo Measured fan power

• Estimated Typical reheat performanceo Utilize MBCx software to compile terminal unit datao Otherwise review individual terminal unit DAT datao Determine typical DAT during unoccupied hours

VAV Airflow Setback

• Baseline Calculationo Average fan speed/airflow based on time of dayo Use fan law to estimate fan power at estimated speedo Use measured fan power as a reference for the fan law

o Assumingwemeasured35kWat97%speed

o 78%79%

:.<x 35 kW = Fan Power at 91% speed = 29.8 kW

VAV Airflow SetbackProposed Calculationo Assume airflow reduction based on selected terminal units

o 3,200 cfm during unoccupied hourso Assume fan speed reduces proportional to airflow

compared to design airflowo AHU design is 52,000 cfm. o 3,200/52,000 = 6.1%

o Calculate annual baseline and proposed energy usage using TMY3 data

o Include a safety factor to account for underperformance

*Gray values are estimated reduction

VAV Airflow Setback

• Verification Calculation

14

15

16

17

18

19

20

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23

Cont

rol S

igna

l (4-

20 m

Amps

)

Hour of the day (0-23)

Verified Average Supply Fan Signal

SF-1

SF-2

SF-3

SF-4

VAV Airflow Setback

Common Mistakeso Assume the fan motor power at 100% speed equals the

motor nameplateo Use measured power or assume a load factor of around

70%o No safety factor

Heat Recovery Operation• Existing Operation

System has run around heat recovery system with hydronic coils installed in outdoor and exhaust air streams. The hydronic loop has a constant volume pump and a three-way valve to modulate the amount of heat recovery. The system was not operational due to a fitting leak.

• Proposed OperationRepair the piping leak, refill the system and operate the system as designed.

Heat Recovery Operation• Identification

o Pump disconnect was found OFFo Building automation system showed no temperature rise

across the outdoor air heat recovery coil in winter mode

Heat Recovery Operation• Collect Data

o Review trend data to document baseline performanceo Review design drawings to determine outdoor and exhaust

airflowo Review trend data to observe performance of other similar

units.

10

20

30

40

50

60

70

80

90

4/2 4/3 4/4 4/5 4/6 4/7 4/8 4/9 4/10

Tem

pera

ture

(F)

Heat Recovery Coil Operation

OAT HR DA Temp RH COIL TEMP BLDG 665 MAU-C1 TEF HR COIL DA TEMP

Heat Recovery Operation• Heat Recovery Calculation

o Heat recovery effectiveness = ε = "̇$% &%$'(&$%)"̇*% &%$'(&*%'

Entering Outside Air-10 F

Leaving Outside Air49 F

Entering Return Air73 F

OA cfm = RA cfm(-10 – 49) = -49 = 59%(-10 – 73) -83

Heat Recovery Operation• Proposed Calculation

*ASHRAE HVAC Systems and Equipment Handbook

o Assume 55% average for Run-Around sensible effectivenesso Use formula to calculate MAT based on OAT and RAT

Heat Recovery Operation• Proposed Calculation

o Assume 55% average for Run-Around sensible effectiveness o Calculation for each 8760 hour

o Run around is sensible only, so only concerned about temperature not enthalpy

o Knowns = OAT, RAT, effectiveness, airflowso Example condition OAT= -10F and RAT = 72F

• ε = "̇$% &%$'(&$%)"̇*% &%$'(&*%'

17,600 x (-10 – MAT) = 55%12,970 x (-10 – 72)

(-10 – MAT) = (12,970) 55% = 40%(-10 – 72) (17,600) (-10 – MAT) = 40% x (-82)

-MAT = 40% x (-82) +10 = -23F

Heat Recovery Operation

1/1 0 10 1 65.0 100% 12,970 17,600 35.1 0 5051/1 1 11 1 65.0 100% 12,970 17,600 35.7 0 4941/1 2 12 1 65.0 100% 12,970 17,600 36.3 0 4831/1 3 12 1 65.0 100% 12,970 17,600 36.3 0 4831/1 4 13 1 65.0 100% 12,970 17,600 36.9 0 4711/1 5 13 1 65.0 100% 12,970 17,600 36.9 0 4711/1 6 13 1 65.0 100% 12,970 17,600 36.9 0 4711/1 7 14 1 65.0 100% 12,970 17,600 37.5 0 4601/1 8 16 1 62.6 100% 12,970 17,600 38.7 0 3921/1 9 20 1 56.5 100% 12,970 17,600 41.1 0 231

1/1 10 23 1 55.8 100% 12,970 17,600 42.9 0 1831/1 11 26 1 55.6 100% 12,970 17,600 44.6 0 1461/1 12 28 1 56.6 100% 12,970 17,600 45.8 0 1421/1 13 30 1 57.0 100% 12,970 17,600 47.0 0 1261/1 14 30 1 57.4 100% 12,970 17,600 47.0 0 1351/1 15 30 1 57.9 100% 12,970 17,600 47.0 0 1451/1 16 30 1 58.2 100% 12,970 17,600 47.0 0 1511/1 17 29 1 63.4 100% 12,970 17,600 46.4 0 2611/1 18 28 1 65.0 100% 12,970 17,600 45.8 0 3021/1 19 30 1 65.0 100% 12,970 17,600 47.0 0 279

OA(CFM)

Actual MAT(F)

Heating(MBH)

Cooling(MBH)

SAT(F)

Flow(CFM)

Flow(%)

On?Date and

TimeDB(F)

Heat Recovery Operation• Verified Calculation

• After implementation there is a temperature rise across the heat recovery coil

10

20

30

40

50

60

70

80

90

4/11 4/12 4/13 4/14 4/15 4/16 4/17

Tem

pera

ture

(F)

Heat Recovery Coil Operation

OAT HR DA Temp RH COIL TEMP BLDG 665 MAU-C1 TEF HR COIL DA TEMP

Heat Recovery

Heat Recovery Operation• Verified Calculation

• Use this data to estimate the actual heat recovery effectiveness• Replace the assumed value in the calculation• Predicted 40%, in reality it was 32-41%

0

0.1

0.2

0.3

0.4

0.5

0.6

20 25 30 35 40 45 50 55 60

Effe

ctiv

enes

s

Outside Air Temperature (F)

Heat Recovery Effectiveness

Heat Recovery OperationCommon Mistakeso Not including the pump power as a penaltyo This measure had existing coils. If the measure was adding

heat recovery you also need to account for the additional air pressure drop energy

Simultaneous Heating and Cooling• Existing Operation

Variable volume air handling unit has steam heating coil and chilled water cooling coil that operate to maintain a discharge temperature setpoint. The steam valve is failed open at all times, but the chilled water valve is able to operate and maintain temperature setpoint.

• Proposed OperationRepair the steam valve so the it closes properly during cooling mode.

Simultaneous Heating and Cooling• Identification

• Measured temperature after each device in the AHU, found the heating coil active when valves were commanded closed.

Simultaneous Heating and Cooling• Baseline Calculation

• Equation that will be used is 1.08 x cfm x (Tout – Tin )• Necessary Inputs:

• Airflow• Entering Temperature (Mixed Air Temp)• Leaving Temperature (Heating Coil Leaving Temp)

• Assumptions• Airflow is directly proportional to supply fan speed

• 100% fan speed = 100% design airflow• Entering Temperature assumes economizer operation

works to maintain MAT of 50F• Leaving air temp is based on system operation

Simultaneous Heating and Cooling• Baseline Calculation

• Initial calculation to determine order of magnitude of savings• Site measurements determine heating DT = 40F• Steam plant is active year round• Chilled water plant is active year round• Fan was observed operating at 70% speed

• 1.08 x (29,000 cfm x 0.70) x 40 F x 8760 = 11,045 Mbtu/year• Cost savings around $70,000/year• Need more in depth analysis

Simultaneous Heating and Cooling• Collect Data

• Data log heating coil leaving air temperature• Utilize BAS trending for mixed air temperature and discharge

air temperature

3040

50

60

70

80

90100

110

4/10

/13

4/11

/13

4/12

/13

4/13

/13

4/14

/13

4/15

/13

4/16

/13

4/17

/13

4/18

/13

4/19

/13

4/20

/13

4/21

/13

4/22

/13

4/23

/13

4/24

/13

4/25

/13

4/26

/13

4/27

/13

4/28

/13

4/29

/13

4/30

/13

5/1/

13

Tem

pera

ture

(F)

LB56 Air Temperatures

LB53.OAT LB56DT LB56MT LB56RT 56 HCT

Simultaneous Heating and Cooling• Collect Data

• Compare heat transfer to MAT• As it gets warmer outside the heat transfer reduces

0%

20%

40%

60%

80%

100%

51015202530354045

45 50 55 60 65 70 75 80 85

Air D

T ov

er co

il (F

)

Mixed Air Temperature (F)

Heating Coil Air DT for Calculations

53 DT AVG 54 DT AVG 55 DT AVG 56 DT AVG Average %

Simultaneous Heating and Cooling• Verified Calculation

• Confirm temperature rise across the heating coil is eliminated when the steam valve were closed.

• Remove safety factor if all valves were corrected

Simultaneous Heating and CoolingCommon Mistakeso Assume steam heat loss is constant at all timeso Assume wasted cooling energy is equal to wasted heating

energy.

Measure Interaction

• The order that measures are implemented/calculated matters• If incremental savings are desired, each measure should be

added to the calculation incrementally. • The following slides discuss an example of economizer

optimization and unoccupied scheduling measures implemented on the same AHU.

Measure Interaction

• Calculate both measures assuming both are implemented. RCM1 (economizer) calculation assumes the reduced schedule from RCM2 (scheduling), RCM2 assumes the reduced load from RCM1.

0

2

4

6

8

10

12

14

16

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24

Ener

gy U

nit (

not t

o sc

ale)

Hour of Day

Savings from RCM 1 Unaccounted savings Savings from RCM2 Baseline

Measure Interaction

• Calculate the savings from RCM1. Use the proposed final load after RCM1 to calculate the savings for RCM2.

0

2

4

6

8

10

12

14

16

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24

Ener

gy U

nit (

not t

o sc

ale)

Hour of Day

Savings from RCM 1 Savings from RCM2 Baseline

Energy Calculation Completion

• Back-Check Savings Total• Compare savings to entire building usage (utility data)• Compare savings to expected HVAC portion of annual

energy usage• Use Commercial Building Energy Consumption Survey

(CBECS) • https://www.eia.gov/consumption/commercial/

• Compare savings to annual equipment usage ( % of annual fan energy usage)• Example: Nameplate Motor kW x Annual Operating Hours vs

Calculated Savings

Tim Shinnick, PE, QCxP, CEMGrumman/Butkus Associatestshinnick@grummanbutkus.com

BCxA Conference – Nashville, TN – October 2018 44