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

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

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

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 = 0.293 KWh, 1,000,000J=0.278 KWh - Average efficiency of electric generation systems: ≈33%

Thermal Comfort Definition

Thermal Comfort We need to be in thermal balance

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

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

Attempts to quantify thermal balance

Fanger’s model

Factors Influencing Thermal Comfort Occupant related Clothing Activity level

Impact of Relative Humidity How relative humidity affect thermal comfort?

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)

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

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

Psychrometric Chart

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

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

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

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

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

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

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

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

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

Enthalpy

Psychrometric Chart

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