ASHRAE Journal - July 2021 - 34

ENGINEER'S NOTEBOOK
COLUMN
Kelley Cramm
Modeled Energy vs. Actual Performance
Does the Lab Building
Follow the Model?
BY KELLEY CRAMM, P.E., MEMBER ASHRAE
Clients often ask me to produce an energy model for a new laboratory building project
or renovation. Typically, they ask for it to help predict the magnitude of utility
bills. But,can an energy model accurately predict building performance? This column
highlights the inherent difficulty of forecasting building utility consumption, particularly
for laboratory buildings where load profiles are driven by several factors, including
weather, plug loads and occupancy schedules.
Weather Data
Because laboratories are typically fully exhausted with
no recirculation, meaning the supply is 100% outdoor
air, weather has a large influence on HVAC energy consumption.
Weather databases used in energy modeling
are multiyear averages and do not reflect actual conditions
in any given year. So, differences between weather
data used in a model and actual weather experienced
for any given year are an important factor in explaining
the differences between modeled and actual energy
performance for a laboratory. Refer to Figures 1 and 2 for
normal, observed and record temperatures in Kansas
City, Mo., for 2016 and 2020.1
A deviation of 5°F (2.8°C) in the actual high or low
temperature from the long-term average would result
in a 39% difference in cooling energy and a 23% difference
in heating energy.* The example is intended to
*Calculated using a modified bin analysis using dry-bulb temperature,
wet-bulb temperature and enthalpy. Total cooling energy
equal to 1.5 kW/ton (0.43 kW/kW). Total heating system efficiency
equal to 80%. $0.10/kWh and $0.0095/ft3 ($0.335 m3)
natural gas.
34
ASHRAE JOURNAL ashrae.org J U LY 2021
illustrate how weather in any given year will affect the
comparison between the theoretical energy consumption
and actual energy consumption.
Plug Loads/Laboratory Equipment
Predicting the actual energy use of laboratory equipment
and plug loads in an energy model is difficult. One
reason is the equipment installed on moving day almost
never matches the equipment in the design equipment
list. Researchers continuously chase new grants that
come with new equipment. In addition, lab equipment
is constantly being upgraded to newer models.
Photo 1 shows two mass spectrometers that were added
to a lab when construction was nearly complete and
the users were moving in. The design should be flexible
enough to allow for these additions, but because unexpected
equipment may get added at the end, there is no
accurate way to predict energy performance in advance
if the equipment is not in the design equipment list.
Kelley Cramm, P.E., is an associate and mechanical technical leader at Henderson
Engineers in Lenexa, Kan.
http://www.ashrae.org

ASHRAE Journal - July 2021

Table of Contents for the Digital Edition of ASHRAE Journal - July 2021

Contents
ASHRAE Journal - July 2021 - Intro
ASHRAE Journal - July 2021 - Cover1
ASHRAE Journal - July 2021 - Cover2
ASHRAE Journal - July 2021 - 1
ASHRAE Journal - July 2021 - Contents
ASHRAE Journal - July 2021 - 3
ASHRAE Journal - July 2021 - 4
ASHRAE Journal - July 2021 - 5
ASHRAE Journal - July 2021 - 6
ASHRAE Journal - July 2021 - 7
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ASHRAE Journal - July 2021 - Cover3
ASHRAE Journal - July 2021 - Cover4
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