1. Lecture Objectives
Review what we learned about Eclectic Energy Production
Learn about Thermal Comfort
Introduce Psychrometric Chart

2. Basic energy principles
Primary
Energy
Site vs. Primary Energy
Site (End-use) energy is the energy directly consumed by end users
Primary energy is site energy plus the energy consumed in the production and delivery of energy products
Light
Thermal
Fresh
air
HVAC System
Site energy (End use)
HVAC – Heating, Ventilation and Air-Conditioning
Site
Energy
Primary
Energy
Distribution
Storage
Generation

3. Example: Energy Consumption Monthly Profile for 100,000 sf Commercial Building in Austin
~12%
~96 MWh

4. Comparison of Energy for Heating and Cooling
How to compare heating energy from gas and electric energy for cooling ?
1) Convert all to primary energy
2) Convert end use energy from gas to electric energy you would get from this gas
You will need:
- Conversion factors: 1000 BTU = KWh, 1,000,000J=0.278 KWh
- Average efficiency of electric generation systems: ≈33%

5. Thermal Comfort
Definition

6. Thermal Comfort
We need to be in thermal balance

7. Important to understand (basic thermodynamics)
What is thermal balance?
How it effects everyday life?
Earth
Humans
…..

8. Thermal balance Energy loss (work and heat) = energy production
Heat loss by: convection, radiation, conduction, evaporation

9. Attempts to quantify thermal balance

10. Fanger’s model

11. Factors Influencing Thermal Comfort
Occupant related
Clothing
Activity level

12. Impact of Relative Humidity
How relative humidity affect thermal comfort?

13. Other factors Air velocity Surrounding surface temperature
Impacts convective heat transfer on human surfaces
Surrounding surface temperature
Impacts radiative heat transfer (heat exchange with surrounding surfaces)

14. Environmental apartments that affect thermal comfort
Temperature
Relative humidity
Air velocity
Surrounding surface temperature
Measured as mean radiant temperature
HVAC systems control all these
- directly or indirectly

15. Curtail to understand What is relative humidity ?
How do we measure it ?
How it affects us?

16. Psychrometric Chart

17. Thermodynamics of Moist air or Psychrometrics
Variables
Temperature
Relative Humidity
Absolute Humidity
Enthalpy (total energy)
Dew Point Temperature
Wet Bulb Temperature ….

18. Temperatures Absolute Temperature (T) (K, R)
Dry-bulb temperature (t) [°F, °C]
Wet-bulb temperature (t*)
Dew-point temperature (td)

20. Which temperature do you expect to be higher?
Wet-bulb
Dry-bulb

21. Wet-bulb temperature (t*)
Temperature measured by a psychrometer
Lower than dry-bulb temperature
Evaporating moisture removes heat from thermometer bulb
The higher the humidity
Smaller difference between wet-bulb and dry-bulb temperature

23. Dew-Point Temperature, td
Define temperature at which condensation happen
td is defined as temperature of that air at saturation
i.e. RH = 100%
Surfaces below the dew point temperature will have condensation
Measured with a chilled-mirror apparatus

25. Absolute Humidity or Humidity Ratio
Humidity ratio (W) [lb/lb, g/kg, grains]
[grains/lb = 1/7000 lb/lb]

26. Humidity Ratio, W Mass of water vapor/divided by mass of dry air
Orthogonal to temperature
Not a function of temperature
Most convenient form for calculations involving airflow
Very hard to measure directly

28. Relative Humidity, RH or 
Ratio of partial pressure of water vapor to partial pressure of water vapor at same T and P at saturation
Strong function of temperature
For constant humidity ratio
Higher temperature, lower relative humidity
Saturation

30. Enthalpy or Total Energy
Enthalpy h or i [J/kg] or [Btu/lb]
Defines amount of energy contend in moist air

31. Enthalpy

32. Psychrometric Chart

33. Reading Assignment Chapter 1 and
Chapter 2 (Section 2.1 & 2.2) Tao and Janis