design cooling load part 2 -...

1

Ball State Architecture | ENVIRONMENTAL SYSTEMS 1 | Grondzik 1

DESIGN COOLING LOAD – part 2


Quick Review/Preview• External cooling loads (previously discussed)

– Result from envelope/climate interactions

– Include both sensible and latent loads

– Architectural design decisions regarding the building enclosure have a huge effect on these loads

• Internal cooling loads (to be discussed now)

– Result from heat sources (people, lighting, equipment/appliances) within a building

– Include both sensible and latent loads

– User (as well as design) decisions have a major effect

– In low-energy buildings, these become critical

2


People (Occupant) Loads

• Sensible gains to a building– A result of radiation, convection, and

conduction heat losses from occupantsq = (# people)(BtuS/h per person)(CLF)

– CLF is cooling load factor (it adjusts for heat flow that is initially stored in building mass)

• Latent gains to a building– A result of evaporation heat loss from occupants

q = (# people)(BtuL/h per person)– The latent load (and percent latent) increases

with higher occupant activity– No CLF, as water vapor storage is assumed nil


People (Occupant) Loads

ww

w.e

nerg

y-m

odel

s.co

m

ww

w.a

scd.

org

heat gain peroccupant isfound from

design tables

number ofoccupants isknown todesigner

3


Equipment/Appliance Loads• Sensible gains to a building

q = (design wattage)(usage factor)(CLF)(3.41)– wattage is the design plug load or fixed-equipment load

(toaster, TV, photocopier, computer, elevator motor, deep fryer, etc.)

– usage factor accounts for non-coincident loads and part-time loads

– CLF adjusts for heat stored in building mass (as with people and lighting loads, only the radiated portion of heat flow is substantially stored)

– 3.41 is a watt-to-Btu/h conversion

• Latent gains to a buildingq = (Btu/hL)(usage factor)

no CLF (assumes no storage of moisture)


Equipment Loads

ww

w.s

lides

hare

.net

ww

w.a

scd.

org

heat gain per equipment item is found

from design tables

number of equipment items is known or estimated by designer

ths.

gard

enw

eb.c

om

4


People/Equipment Loads — CLF

ASHRAE Handbook --1997 Fundamentals

reminder: q = (# people)(BtuS/h per person)(CLF)


Electric Lighting Loads

• Sensible gains to a buildingq = (design lighting watts)(usage factor)(CLF)(3.41)

– Wattage includes ballasts (more on this in Arch 373)

– Usage factor accounts for non-coincident loads (for example, if there are two lighting systems in a room but they would never be used at the same time)

– CLF adjusts for heat from lights that is stored in the building mass—values for CLF are obtained from tabulated data (as with occupant loads)

– 3.41 is a watt-to-Btu/h conversion factor

• Latent gains – none (there is no water vapor involved with electric lighting or daylighting)

5


Electric Lighting Loads — CLF

zone types describe interior construction (thermal mass); D is heavier


Daylighting Loads

Cooling loads attributable to daylighting are addressed during analysis of external cooling loads. The heat that accompanies even well-designed daylight admittance is part of the analysis conduced for solar heat gains—

Q = (A) (SCL) (SHGF)

6


Total Design Cooling Load Equals:

• External loads (resulting from the climate interacting with the various spaces through the building envelope)– Sensible– Latent

plus

• Internal loads (resulting from functional use of the various building spaces)– Sensible– Latent


Typical Building Cooling Loads

energy end-use in a typical U.S. office building www.mge.com/

external

internalpartially external

andpartially internal

7


Cooling Terminology• Design heat gain

– The term heat gain is used to describe the results of an analysis that includes all heat flows into and within a building at a defined time of interest—whether or not these flows immediately affect building air temperature

• Design cooling load– The term cooling load is used to describe the results

of a more discriminating analysis that includes onlythose heat flows that will change building/room air temperature at the time in question

– A cooling load analysis excludes radiation heat flow that is stored in internal building mass and does not directly affect room air temperature (making this distinction is the purpose of the various “CLF” values)


Heat Gain vs. Cooling Load

AS

HR

AE

Han

dboo

k --

1997

Fun

dam

enta

ls

difference in “answers”heat gain

cooling load

The effect of thermal mass on radiation loads from lighting systems—time lag.

8


Heat Gain vs. Cooling Load• The numerical difference between heat gain and cooling

load can be substantial (on the order of 15-25%)• The typical active climate control (HVAC) system

operates to directly maintain building air temperature under the control of a thermostat that can only sense air temperature

• Thus, the typical HVAC system does not detect and does not impact stored radiant heat gain

• The typical HVAC system responds to cooling load• Not considering this distinction (through the use of

simplified load calculations) results in oversized systems (which are more expensive and less efficient … thus, not in the client’s best interests)


Cooling Load Calculations• Steps to reduce design cooling load can be intuitively

deduced from the equations that define load relationships and associated correlations — this is certainly more complex, however, than for design heat loss

• Manual calculation methods are becoming increasingly complex

• In practice, computer software is commonly used for load calculations (doing 8760 analyses, for each hour of the year, and taking the guess-work out of finding the “worst” load)

• But … such software is typically used in a way that provides little feedback to improve architectural decision making (it is not intuitive); and the software often is run after all key design decisions have been made (reactively versus proactively)

• What does this imply for full implementation of BIM (building information modeling)?

9


Cooling Load Calculations

• Accurate calculations are important, because:– Under-sizing of equipment and systems will result in

failure to meet the OPR (users will inhabit uncomfortable spaces)

– Over-sizing of systems will require more investment for unneeded equipment; greater energy use due to system operating characteristics (systems often operate less efficiently at part load than full load); and the potential for discomfort (high humidity often results from oversized cooling equipment)

– Right-sizing is the place to be for the owner and for the environment

• But, informed design is equally important … since being very accurate while doing the wrong thing is not a virtue


Follow-up heating degree days (HDD)

talking about the weathercode requirements based upon HDD

construction has improved greatly since the HDD concept was first established (and linked to 65 deg F), yet HDD65 still plays a useful role here and there

10


Follow-up

parallel versus series heat flow paths

heat flowing through the blue pathencounters insulation (with a high R)and “sees” a U that includes that R

heat flowing through the green pathbypasses insulation (going through a stud

with a lower R) and “sees” a U thatincludes that lower R

the wall has two distinct (parallel) flow paths with different U-factors; thecomposite U is an area-weighted average of the two U factors

series 1

series 2


Envelope Design Details radiant barriers

a radiant barrier is a reflective material placed facing an air space (where radiantheat flow may predominate) and installed so as to reflect radiation

back toward the “source”

radiant barriers are most effective in reducing cooling loads; they mustface an air space to be effective

ww

w.c

dc.g

ov/

ww

w.b

tubu

ster

s.co

m/

11


Envelope Design Details

low-e glazing

e stands for “emissivity” and refers to theability of a material to radiate energy (radiate,

versus absorb or transmit)

each pane of glass in an assembly couldin theory be coated with a low-e substance

to reduce its ability to emit (or pass on) radiation;the surface that is actually coated is indentified by

a numbering scheme (see adjacent)

a low-e coating affects U-factor, and is generallyindependent of (although intertwined with)shading performance; longer-wavelengthIR radiation is the target of low-e coatings

low-e coatings will improve the U-factor of a glazing assembly


Low-e Glazing Window Technologies: Low-E Coatings

Low-emittance (Low-E) coatings are microscopically thin, virtually invisible, metal or metallic oxide layers deposited on a window or skylight glazing surface primarily to reduce the U-factor by suppressing radiative heat flow. The principal mechanism of heat transfer in multilayer glazing is thermal radiation from a

warm pane of glass to a cooler pane. Coating a glass surface with a low-emittance material and facing that coating into the gap between the glass layers blocks a significant amount of this radiant heat transfer, thus lowering the total heat flow

through the window. Low-E coatings are transparent to visible (sic) light. Different types of Low-E coatings have been

designed to allow for high solar gain, moderate solar gain, or low solar gain.

www.efficientwindows.org/lowe.cfm

12


www.bchydro.com

www.treehugger.com

Good Design Decisions Reduce Building Energy Use


www.architecture2030.org/

U.S.Energy

Use

U.S.Electricity

Use

Building Energy Useis Very Important to the

Environment and to the Economy

design cooling load part 2 -...

Documents