MEEB Chapter 10: Passive Cooling

What is the problem with earth tubes?

Winter heat loss will likely exceed summer loss if solar heat can be admitted to counterbalance the increased winter loss, uninsulated walls sized for the desired summer loss become more attractive

Is it true that passive cooling can introduce "coolness" without the use of mechanical assistance?

Yes

What is important with the passive cooling guidelines?

You must first check the match between the climate and the cooling strategy before applying the guidelines to a building design

Before doing calculations for Night Ventilation of Thermal Mass, Fan- Assisted Evaporative Cooling, and Roof Pond, what should you check?

1. Check your summer climate against the passive cooling strategies 2. Your building and climate have been checked for approximate performance based on the design guidelines

What are the two variations of natural ventilation?

1. Cross Ventilation 2. Stack Ventilation

How do you calculate for Fan-assisted Evaporative cooling?

1. Determine the outdoor air conditions. enter the psychrometric chart at the Summer design DB and mean coincident WB temperatures for your climate 2. Determine supply air temperature 3. Calculate the heat removed and airflow rate indoors.

What is a designers checklist of considerations for heat gain:

1. Characteristics of the building envelope: materials, sizes, external surface colors, and shapes 2. Building location and orientation, as well as the extent of external shading of the building by trees or adjacent structures 3. Outdoor design conditions 4. Indoor design conditions: DB, WB, and ventilation rate 5. The schedule of lighting, occupancy, equipment, and any other processes that contribute to internal heat gain 6. Thermal zoning requirements

How do you calculate the Roof Pond Cooling?

1. Find the maximum DB temperature, mean daily range, minimum DB temperature, design DB temperature, average maximum RH for July, and July average temperature 2. Calculate the peak hourly heat gain but exclude internal gains 3. Approximate the daily total heat to be stored in the roof pond 4. Determine the rate of the internal gain in Btu/ h while the building is occupied 5. Approximate the daily total internal heat to be stored in the roof pond 6. Calculate the daily heat gain directly to the roof pond through its insulated covers in Btu 7. Consider whether fans will be used to stir the room air below the roof pond 8. Determine the highest comfortable internal air temperature 9. Calculate the maximum allowable internal air temperature 10. Determine the allowable temperature swing of the roof pond 11. Determine the required pond depth

What are the type of "gains"?

1. Gains through roof and walls 2. Gains through glass 3. Gains through outdoor air 4. Gains from people 5. Gains from lighting 6. Gains from equipment 7. Gains from latent heat gains

How do you calculate the Night Ventilation of Thermal Mass?

1. List the hourly outdoor temperatures for the design condition (Do not list temperatures above 80 degrees because outdoor air will not be used for cooling above that temperature) 2. Calculate the 24 hour heat gain for the building in Btu 3. Find the total area of the thermal mass surface that is exposed both to the space to be cooled and to moving night air during ventilation cycle 4. Find the mass heat capacity for the entire space to be cooled: mass volume x density x specific heat 5. For supplementary cooling due to surfaces other than those of the principal thermal mass, list the space's floor area 6. Complete column 3 hour by hour after determining the mass temperature 7. Calculate volume 4 which is Btu/ h (column 3) 8. Add all of the hourly cooling Btu/ h values to obtain the total mass cooling 9. The final mass temperature will be at least 5 degrees above the lowest air temperature of the night

What are some of the design strategies that are considered for cooling?

1. Location 2. Orientation 3. Wind 4. Land Massing 5. Vegetation 6. Microclimate

What data do you need to know for Roof Pond Cooling?

1. Minimum WB temperature 2. Average July operating hours for residential air conditioning

What are four basic approaches to control the heat gains in a building?

1. Natural Ventilation Cooling 2. High- mass cooling 3. Night ventilation thermal mass 4. Evaporative cooling

What are two forms of high mass cooling?

1. Roof ponds 2. Earth contact

What does the heat removed from the buildings by airflow depend on?

1. The DB temperature difference between the supply air and the exhaust air 2. The airflow rate usually expressed in cubic feet per minute

What are the three detailed methods for calculating heat gains and equipment cooling loads?

1. The transfer function method 2. The total equivalent temperature differential, time- averaging method 3. A one- step method, cooling load temperature difference/ cooling load factor

What are the three ways for calculating heat gains and equipment cooling loads?

1. Transfer Function Method 2. The total equivalent temperature differential, Time- averaging method 3. One- step method, cooling load temperature difference/ cooling load factor

How big are the earth tubes?

8 to 20 inch in diameter and buried at a depth of 5 to 10 feet and are up to 200 feet long

What temperature do you set the roof pond at?

80 degrees because a higher temperature would fail to produce cooling for the building interior

What is the standard assumption for gains from equipment?

1200 to 1600 Btu/h of sensible heat gain is produced by appliances

What are roof ponds?

A form of high mass cooling for one and two story buildings. Because they require only the roof to be massive, they allow for considerable design freedom in walls and fenestration. Where cooling is the only objective, this approach uses water that is stored between the metal ceiling and the roof insulation: by night, the water is pumped (and/ sprayed) over the exposed roof and allowed to trickle back through the insulation to the storage pond.

What are the DETD values based on?

An average indoor temperature of 75 degrees

What should be remembered with stack ventilation?

An equal (or greater) area of stack outlet openings as well as at least an equal cross sectional area through the vertical stack is also required. Also consider internal obstructions: partitions must have a total of openings at least equal to this required inlet area.

What must be remembered with cross ventilation?

An equal or greater area of outlet openings must be provided

What is earth used for in earth tubes?

As a heat sink

Facts about roof ponds?

Average depth is 3 and 6 in Maximum temperature is 80 degrees because if there is a higher pond temperature that would fail to produce cooling for the building interior

What strategy is most successful in locations with warm, dry summers where the extremes of hot days can be tempered by the still- cool thermal mass of a building?

High- mass cooling

Are earth tubes used for the whole building?

Because earth tubes need to be well underground as well as rather long in order to cool outdoor air, it is rarely economical to install enough earth tubes to completely meet a building's need for cooling

Why is passive cooing not the most common evaporative cooler?

Because it depends on a fan force large quantities of outdoor air through a wet filter, thereby lowering the air temperature and raising the relative humidity before delivering the air to the space to be cooled

Why does Evaporative cooling impose constraints on designers?

Because the equipment resembles conventional HVAC systems

What principle drives stack ventilation?

Hot air rises

What happens to the floors in Night Ventilation thermal mass?

Both sides of the floor slab is available for thermal storage

What kind of buildings work well with natural ventilation?

Buildings should be very open to breezes simultaneously closed to direct sun. The materials of these buildings may be thermally lightweight as well, because the temperature of the night air is not cool enough to remove much stored daytime heat.

How is the ventilation rate determined?

By the "best hour" of cooling during the night

How is the ventilation rate in Night Ventilation thermal mass determined?

By the "best hour" of cooling during the night which would be when the hour between inside mass and outdoor air is greatest

What is important to note with passive cooling guidelines?

Check the climate with the cooling strategy

What is the typical climate for Evaporation Cooling?

Conditions are more uncomfortably dry than uncomfortably hot, lower air temperature in exchange for higher humidity

What should you consider before making any detailed calculations for passive cooling performance?

Consider a simplified heat gain procedure that has been developed for residential buildings with some risk and some judgment and can be used for quick approximation of the conditions in commercial buildings.

What have we most been successful with?

Controlling environments that require heat and maintaining warmth during cold seasons

What is the first step in passive cooling?

Controlling the heat that a building gains from its environment and estimating the extent of the heat gains in the building

What happens when a climate surpasses the boundaries of all four strategy zones?

Conventional air conditioning is almost certainly desirable

What type of strategy is related to natural ventilation cooling?

Cross and stack ventilation

What strategy provides fresh air but maintains a building temperature that is slightly above the outdoors?

Cross- ventilation

What is stack ventilation?

Depends upon very low openings to admit outdoor air and very high openings to exhaust air

What does stack ventilation do?

Provides plentiful fresh air but maintains a building at temperatures slightly above outdoor temperatures.

What is cross ventilation?

Driven by wind and is accomplished with windows

What is the first step to designing a passive cooling approach?

Estimating the extent of the heat gains in a building

What strategy relies on the principle that when moisture is added to air, relative humidity increases while dry- bulb temperature decreases?

Evaporation Cooling

What strategy requires water?

Evaporation Cooling

What did shading begin with?

Flat roofs

What are some of the ways in which the Greeks and Chinese practiced solar design planning for proper solar orientation and evaporative cooling?

Fountains, pools, water streams, and vegetations

What is a cool tower?

Has wetted pads on all four faces at the top. Hot air is cooled as it passes through the pads, dropping to the base of the tower and then into the house

How do "closed" buildings react with heat gain from infiltration?

Heat gain from infiltration or ventilation must be added, because these structures maintain internal temperatures lower than outside temperatures

What strategy is highly compatible with passive solar heating strategies that rely on large areas of thermal mass such as direct gain and large high buildings?

High Mass Cooling with Night Ventilation

What strategy is most successful in hot- dry summer with sub comfortable nighttime temperatures to flush away heat stored during the daytime?

High Mass Cooling with Night Ventilation

Where is cross ventilation most desired?

Humid climates

What is needed for Evaporative Cooling?

Large quantities of both water and outdoor air are needed; fan- driven evaporative coolers are the most common way to provide this kind of cooling.

What is High Mass Cooling with Night Ventilation?

Maintains a building at temperatures lower than those outside by day and flushes the building with plentiful air by night

What is Night Ventilation of Thermal Mass?

Maintains a building at temperatures lower than those outside by day and flushes the building with plentiful fresh air by night

Why is the "passive approach" becoming abandoned?

More affordable air- conditioning systems, hotter cities, and people's changing expectations about comfort has caused people to have dependence on mechanical equipment

What are toldos?

Movable shading devices

What does cross ventilation rely upon?

Narrow plans with large ventilation openings on either side - naturally compatible with daylighting

What is the most obvious strategy that is suggested by the comfort charts?

Natural Ventilation Cooling

What is the only passive strategy available in humid, hot climates in which temperatures are only slightly lower by night than by day?

Natural Ventilation Cooling

How big are roof ponds that are sized for cooling?

Nearly equal the area of the floors of the buildings they cool

What most be the case with temperature in High Mass Cooling with Night Ventilation?

Night- time temperatures must be cooler than the comfort zone temperature if this strategy is to be effective.

How are the various approaches to passive cooling in temperate climates best assessed?

On a building bioclimatic chart

What is an underlying assumption to all passive design and energy performance calculations?

Optimization of the building envelope to minimize heat gains

What is the assumed depth for Roof Pond Cooling?

Optimum pond depth of 4 in. but allows for other depths as well

What is High- mass cooling with night ventilation most compatible with?

Passive solar heating strategies that rely on large areas of thermal mass such as direct gain

What is High Mass Cooling with Night Ventilation most compatible with?

Passive solar heating strategies that rely on large areas of thermal mass such as direct gain. It is suitable for large, high buildings, particularly those that have concrete structures.

What does cross- ventilation do?

Provides plentiful fresh air but maintains a building at temperatures slightly above those outdoors. The cross ventilation inlet (window) areas is expressed as a percentage of total floor area, are related to wind speed, and resulting in heat removal

What type of strategy is related to high- mass cooling?

Roof ponds and earth contact

What is one of the most influential variables in earth tube performance?

Soil conductivity

In what season does the building bioclimatic chart focus on?

Summer conditions

What are the roof pond design guidelines based off of?

Summer night DB temperature

Why are summer calculations complicated?

Summer worst hour heat gain is assumed to occur during the daytime under active occupancy conditions, so solar heat gains and internal gains from lights, people, and equipment must be included. Also the hourly change in summer load can be very great.

What should one remember with cross ventilation?

That an equal or greater area of outlet openings must be provided

What happens to a building using High- Mass cooling with Night Ventilation?

The building switches from a thermally closed condition by day (to exclude sun and hot outdoor air) to an open condition by day (to allow ventilation to cool the mass).

What does a building bioclimatic chart do?

The chart allows the designer to visualize the suitability of four approaches to passive cooling under summer conditions. The climate data is plotted on the chart and if they do not exceed the "boundaries" of a strategy zone, then a match is indicated between the climate and that strategy.

What happens when high mass cooling depends upon earth contact?

The earth acts as a heat sink, keeping walls and floors (even roofs when earth- covered) cool. However, if the earth is allowed to continue to act as a heat sink in the winter, heating needs could be greatly increased. Thus, a strategy for summer contact and winter isolation might be appropriate.

Why does Evaporative Cooling impose few constraints on designers?

The equipment used for this strategy resembles conventional HVAC systems

Why is the long term potential of earth as a heat sink lessened?

The fact that soils are slow heat conductors

What element in the High-mass cooling design strategy needs to be protected from exposure to the sun in the day?

The roof

What role does the roof play in high- mass cooling?

The roof has the advantage of radiating to the cold night sky but it should be protected from exposure to sun by day.

How does stack ventilation work?

The stack effect needs several conditions: warmer air indoors that can enter the bottom of the stack, cooler air outdoors, and low inlets to admit that cooler outdoor air to the building. This cooler outdoor air picks up heat from the building and enters the bottom of the stack. Within the stack, this now- warm air rises, because it is less dense- therefore lighter- than the cooler outdoor air that surrounds the top of the stack.

What is high- mass cooling?

The thermal mass can be located in the floors, walls, or roofs but will need a sink to which it can reject its heat by night

Why don't "open" buildings that are naturally ventilated not have heat gains from infiltration?

They are assumed to maintain internal temperatures that are slightly above exterior temperatures. They will experience heat gains through windows, walls, and roofs due to solar impact

What do the stack inlet areas of stack ventilation do?

They are expressed as percentage of total floor area and are related to stack height resulting in heat removal

What happens with cooling loads?

They are sometimes more closely related to individual building characteristics than to climate

What are Earth tubes?

They provide a way to cool outdoor air before it enters a building with a fan that forces sufficient quantities of air through long tubes that are underground

What is the lowest temperature in which air can be cooled in the earth tubes?

Tubes will approach ground temperature

How does a roof pond work?

Uses water that is stored between the metal ceiling and the roof insulation. At night the water is pumped over the roof surface and allowed to trickle back through the insulation to the storage pond.

When will cross ventilation and stack ventilation work?

When the outside is cooler than the inside