1. BEM CLASS 5 Building Thermodynamics – 2 Air-conditioning Load Calculation – latent heat, solar and internal gains

2. Problem from Class 4 Calculate Building Heat Loss
A 50’ x 150’ x 10 story free-standing building has an overall R-value of 3 (taking into account all walls, windows, roof). Each story is 10’ tall. Ventilation, as calculated at 15 cfm per occupant at design occupancy, provides .85 air-change per hour. Ignore basement/foundation losses. Calculate the design heat load at 10 dF outside temperature and 70 dF indoor temperature
[(50 x 2) + (150 x 2)] x 10 x 10 = 40,000 sf surface area
40,000 x 1/3 x (70-10) = 800,000 BTUH conduction
50 x 150 x 10 x 10 = 750,000 cf volume
750,000 x .85 x .018 x (70-10) = 688, 500 BTUH ventilation
Answer = 800, ,500 = 1,488,500 BTUH

3. Next Step: Convert to Fuel Use
[(50 x 2) + (150 x 2)] x 10 x 10 = 40,000 sf surface area
40,000 x 1/3 x (70-10) = 800,000 BTUH conduction
50 x 150 x 10 x 10 = 750,000 cf volume
750,000 x .85 x .018 x (70-10) = 688, 500 BTUH ventilation
Answer = 800, ,500 = 1,488,500 BTUH
(1) Account for plant efficiency loss
if plant is 75% efficient,
1,488,500 / .75 = 1,984,667 BTUH
(2) Convert BTU to Fuel
Natural gas, 100,000 BTU = 1 therm
1,984,667 / 100,000 = BTUH
If heating is Electric, what is next question?
Next exercise, how much energy would you expect this building to use on an annual basis? How would you calculate?

4. AC Load Calculation
Cooling Load, Q = conduction + infil/ventil + SG + IG
For cooling design calculation, infil/ventil has two components: (1) Sensible Heat and (2) Latent Heat
SG = solar gain
IG = internal gains (people, equipment/electricity)

5. Solar gain Solar Constant: 433 btuh/sf Relation to lighting
Actual gain on a surface varies by orientation, season, time-of- day
Desirable in winter but can be excessive
Major instantaneous load in summer – through fenestration; lagging through walls.
Relation to lighting
Day-lighting
Glare

6. Fenestration treatments for solar control
Architectural features
Adjusting glazed areas, adjusting floor plates, atriums
Overhangs, light shelves, external shading
Active facades
Curtain and shade systems
Electro-chromic

7. Fenestration treatments for solar control
Reflective films and tinting, replaced by spectrally selective coatings
solar heat gain coefficient (SHGC) and visible transmittance
“low-e”
Optimize for heating, cooling

8. Internal Gains: People
300 btuh per person at normal office work
Design occupancy. Density by usage
Variable loads in places of assembly
Scheduling and modeling of where people are
big savings in controlling to occupancy
What people DO in their spaces. Relation to ZONES.
Lights
shades
Windows
Thermostats
Diffusers

9. Internal Gains: Electricity
All electric use converted to heat
3414 btu / kwh
Lighting, Motors, "plug-loads"
typically watts per sf in typical office space
Computers
operate at a fraction of rated power
data centers w/sf (data processing + cooling)

10. How does day-light harvesting work?
Lighting
Comfort & productvity
illuminance levels - IEEE stds by task
lighting quality
Lighting Design, Lighting Modeling - RADIANCE
Lighting Power Density – watts per sf
Basis of code
1 w/sf and less
CALCULATE
Usage hours
Lighting and lighting retrofit SCHEDULES
How does day-light harvesting work?

11. LATENT HEAT LOAD
Humidity in hot air. Enthalpy.
Psychrometric chart.

12. LATENT HEAT LOAD
From Tao & Janis Mechanical and Electrical Systems in Buildings

13. Exercise
It is a 90 dF, 70% RH day outside. You want to deliver air at 65 dF, 50% RH. On the psychrometric chart describe the work that has to be done at the air-handling unit and coils, showing lines for sensible cooling, latent heat removal and re-heat.
sensible
latent
reheat

15. CONTROL OF OUTSIDE AIR Fans off at night? OA dampers closed?
"Minimum Outside Air" - fix to code based on full occupancy
Economizer mode - use max OA when conditions are suitable
Dynamic Ventilation Control - CO2 - match OA to occupancy