Conduction Cooling Loads

What You Need to Know Describe the components that make up a cooling load Understand the fundamental differences between heating and cooling loads

What You Need to be Able To Do Calculate simple conduction cooling loads Evaluate systems to identify energy savings

Terms Cooling load Total Equivalent Temperature Differential (TETD) Storage effect Time Lag Thermal mass

Cooling Load “The amount of energy that must be removed from a space in order to maintain the space within the comfort zone.” I. Cooling Load A. “The amount of energy that must be removed from a space in order to maintain the space within the comfort zone.” B. Same essential definition as heating load, only in the opposite direction C. Notice we are still building on our definition of comfort.

Good News! Same ‘R’ values Same ‘U’ values Same conduction heat transfer Same convection heat transfer Same radiation heat transfer I. Good News! A. Same ‘R’ values B. Same ‘U’ values C. Same conduction heat transfer D. Same convection heat transfer E. Same radiation heat transfer II. Fundamentally, it is not any different from our study in heating loads

Cooling Load Components roof lights partition wall people infiltration glass solar equipment glass conduction exterior wall floor

Sensible and Latent Gains load latent load cooling load components conduction through roof, walls, windows, and skylights solar radiation through windows, skylights conduction through ceiling, interior partition walls, and floor people These load components contribute sensible and/or latent heat to the space. Conduction through the roof, exterior walls, windows, skylights, ceiling, interior walls, and floor, as well as the solar radiation through the windows and skylights, all contribute only sensible heat to the space. The people inside the space contribute both sensible and latent heat. Lighting contributes only sensible heat to the space, while equipment in the space may contribute only sensible heat (as is the case for a computer) or both sensible and latent heat (as is the case for a coffee maker). Infiltration generally contributes both sensible and latent heat to the space. The cooling coil has to handle the additional components of ventilation and system heat gains. Ventilation contributes both sensible and latent heat to the coil load. Other heat gains that occur in the HVAC system (from the fan, for example) generally contribute only sensible heat. lights equipment/appliances infiltration ventilation system heat gains

Major Differences from Heating Loads Peak conditions Heat storage effect Consideration of both latent and sensible gains More unique sources of heat gain I. Major Differences from Heating Loads A. Peak conditions are usually different 1. Depends on the amount of glass and the direction of glass, but usually from noon to 5:00 P.M. 2. This means that ΔT is variable! B. Heat storage effects 1. The total heat is not immediately transferred into the space. In other words, there is a time lag between when the heat is generated and when the heat actually reaches the room. Sometimes, the heat doesn’t reach the room until after the peak occurs! C. Consideration of both latent and sensible heat gains D. More unique sources of heat gains

Time of Peak Cooling Load east-facing window roof heat gain One of the more difficult aspects of estimating the maximum cooling load for a space is determining the time at which this maximum load will occur. This is because the individual components that make up the space cooling load often peak at different times of the day, or even different months of the year. For example, the heat gain through the roof will be highest in the late afternoon, when it is warm outside and the sun has been shining on it all day. Conversely, the heat gain due to the sun shining through an east-facing window will be highest in the early morning when the sun is rising in the east and shining directly into the window. Determining the time that the maximum total space cooling load occurs will be discussed later in this clinic. 12 6 12 6 12 mid a.m. noon p.m. mid

Storage Effect (thermal lag)

Thermal Mass Dilemma

Time Lag solar effect 12 6 12 6 12 time lag B A mid a.m. noon p.m. mid The walls and roof that make up a building’s envelope have the capacity to store heat energy. This property delays the heat transfer from outdoors to the space. The time required for heat to be transferred through a structure into the space is called the time lag. For example, the heat that is transferred through a sunlit wall into a space is the result of sunlight that fell on the outer surface of the wall earlier in the day. Curve A shows the magnitude of the solar effect on the exterior wall. Curve B shows the resulting heat that is transferred through the wall into the space. This delay in the heat gain to the space is the time lag. The magnitude of this time lag depends on the materials used to construct the particular wall or roof, and on their capacity to store heat. 12 6 12 6 12 mid a.m. noon p.m. mid

Conduction – Sunlit Surfaces Total Equivalent Temperature Difference (TETD) is used to account for the added heat transfer due to the sun shining on exterior walls, roofs, and windows, and the capacity of the wall and roof to store heat. The TETD is substituted for T in the equation for conduction. Q = U  A  T TETD

Conduction Gains (Walls and Roofs and doors) Q = U x A x TETD where TETD is the Total Equivalent Temperature Differential, which accounts for Temperature difference Mass Color Solar Gain I. Conduction Gains (Walls and Roofs) A. The formula for this is: Q = U x A x CLTD where CLTD is the Cooling Load Temperature Differential B. The cooling load temperature differential accounts for 1. The storage effect 2. Unstable and highly variable temperatures

Step #1 – Select Wall Type

Step #2 Select Sun time Select color of wall Select wall orientation D = dark L = light Select wall orientation

Step #3 – Read Value of TETD

Same Steps for Roofs

For Windows Btuh = (U x A x TD) + (A x SC x SHGF) A= Area TD = outdoor design – indoor design temp. SC = shading coefficient SHGF = solar heat gain factors

EQ Credit 8.1 - Daylighting ASHRAE Standard 90.1 10% lighting load credit for harvesting 10% lighting load credit for occupancy sensors

Heat Gain from People A function of activity Always contains both sensible and latent components

Equipment - Office Best obtained from manufacturers Can be reduced by using Capture Hood Usually is sensible, but may have a latent component

Equipment Loads (ASHRAE Fundamentals)

Ventilation Load Must consider both sensible and latent loads QS = 1.1 x CFM x (T2 – T1) QL = .68 x CFM x (W2 – W1) QT = QS + QL