Keeping the Heat Out: Cooling Load Avoidance Strategies
This article takes a detailed look at cooling load avoidance in residential and commercial buildings.
Cooling our homes and commercial buildings is becoming a more and more significant environmental concern. Both the total amount of energy we expend for cooling and the fraction of peak electricity use for cooling are on the rise. At the same time, our ability to reduce cooling loads in buildings is improving. We have new materials and technologies available to us, and we are improving our understanding of how buildings provide—or don’t provide—comfort.
For both economic and environmental reasons, it makes a great deal of sense to reduce our energy use for cooling. There are three ways to do this: keeping heat out of buildings (reducing cooling loads); expanding our comfort range (modifying work attire and providing air circulation, for example); and getting rid of heat once it gets in (heat rejection)—which can include both passive and active means. This article takes a detailed look at the first option: cooling load avoidance in residential and commercial buildings.
Energy Use for Cooling
Cooling accounts for about 15% of total U.S. electricity use. Approximately 64% of houses in the U.S. have mechanical air conditioning systems, according to the U.S. Department of Energy, and 37% have central air conditioning (1990). In hot climates, those percentages are far higher. These air conditioning systems consume 16% of residential electricity use at a cost of $11.3 billion dollars per year. Among new houses, some 77% are now built with central air conditioning. Mechanical cooling is required in virtually all commercial buildings and accounts for roughly 30% of electricity use.
Total electricity use for cooling tells only part of the story, though. Even more significant is the effect of cooling on peak utility electric demand. Buildings account for 43% of summer peak loads in the U.S., according to E Source, an energy and environmental research organization affiliated with the Rocky Mountain Institute. The impact of peak cooling loads can be dramatic. A 1°F increase in temperature on a hot summer afternoon in Los Angeles, for example, translates into a 300-megawatt increase in peak electric load.
The potential for reducing cooling loads—both peak and annual—is tremendous. In some northern parts of the country, cooling can be totally eliminated through careful attention to building design, with almost no sacrifice in comfort. Nationwide, E Source estimates that energy use for cooling can be reduced approximately 50% through cooling load reduction strategies.
Along with reducing operating energy use, cooling load reduction strategies can significantly cut up-front costs for equipment. The cost of air conditioning equipment is roughly proportional to size. Each additional ton of air conditioning capacity (12,000 Btu/hr. of cooling) costs about $1,000. (This is not the case at all with heating equipment: a 200,000 Btu/hr. furnace costs little more than a 100,000 Btu/hr. model.) Also, because air flow requirements for a cooling system are directly proportional to tonnage, downsizing the air conditioning equipment by half means the cross-sectional area of the ductwork can be cut in half, providing substantial savings in material and cost. The savings in mechanical system costs can often pay for the extra design and construction costs for measures that reduce cooling loads.
Finally, many of the strategies for reducing cooling loads have synergistic benefits, including savings in other energy end-uses, enhanced occupant comfort, and increased resale value of buildings.
Comfort and Cooling Processes
The ultimate goal of both heating and cooling buildings is to keep occupants comfortable. We produce heat internally through metabolic processes, and we gain and lose heat through our skin via conduction, convection, radiation, and evaporation. The rate of heat gain and heat loss from our bodies is influenced by activity level, clothing, and environmental conditions in the building: temperature, humidity, and air flow.
The relationship between temperature and humidity is particularly important in understanding comfort and strategies for reducing heat gain in buildings. A given volume of air contains both sensible heat and latent heat.
Sensible heat is the energy associated with temperature. It takes about 0.018 Btus to raise the temperature of dry air one degree Fahrenheit.
Latent heat is the energy locked up in the water vapor in a volume of air. Evaporating a pound of water takes about 1,050 Btus, and condensing that pound of water vapor releases 1,050 Btus. This is called the
latent heat of vaporization.
During the summer, high humidity levels are uncomfortable because they impede evaporation of heat from our skin. If a room is 78°F with a relative humidity of 50% we are likely to be comfortable, but if the relative humidity is 80%, we may feel too hot because our bodies can’t get rid of heat as easily. When outside air is introduced into a building, it contains sensible heat and latent heat, both of which contribute to the cooling load.
To understand how to reduce cooling loads in buildings, we need to understand where it comes from. There are four ways heat gets into buildings:
1. Solar heat gain.In energy-efficient houses, solar heat gain is usually the largest component of cooling loads, particularly in more northern climates, where it can account for up to 75% of the cooling load. Solar heat gain occurs through windows, glass doors, and skylights. It is also an important factor in the conductive heat gains discussed below. Solar gains involve sensible heat only. Solar gain is dependent on the incident angle at which sunlight strikes the glass. Thus it varies according to the orientation of the glass and the season. Figure 2 shows the maximum solar intensity during July on vertical glass at various orientations and on horizontal glass. Solar gain through east and west windows is much greater than that through south windows. An understanding of this principle is crucial to controlling cooling loads during the summer.
2. Conduction through the building envelope. Heat gain via conduction is proportional to the temperature difference across the building envelope (∆T). Because the outside air temperature in the summer isn’t that much greater than the indoor temperature in most of North America (∆T usually less than 25°F), conductive heat gain is usually relatively minor, particularly in energy-efficient buildings with good insulation levels. But in some situations, these gains can be significant. Sunlight shining on walls and roofs can boost outer surface temperatures as high as 140°F—which provides plenty of driving force for significant conductive heat gains. Conductive gains do not involve humidity; they involve sensible heat only.
3. Air infiltration and ventilation. Heat gain via air infiltration is dependent on the temperature difference across the envelope, outside wind conditions (and other factors that influence pressure differences across the building envelope), and the nature of intentional and unintentional openings in the envelope. Because outside air often has a high relative humidity, infiltration and ventilation introduce both latent and sensible heat gain. Air infiltration heat gain is much more significant in the south than in the north. In some situations, air with high moisture content (and latent heat) can be transferred from one part of a house, such as an attached greenhouse or basement, to another where the humidity causes discomfort.
4. Internal gains. Internal gains are often the largest component of cooling loads in commercial buildings; in energy-efficient houses they may account for about 25% of the cooling load. Internal gains include heat given off by building occupants, electrical use within a building, hot water flowing through pipes, water vapor from cooking and bathing. Virtually all electricity use in a building is ultimately converted into heat at a rate of 3,413 Btu/kWh. Internal gains can include both sensible and latent components. In homes, latent heat gains can be significant. Each pound of water vapor introduced contributes a latent heat gain of 1050 Btu.
Reducing Cooling Loads
Through careful attention to building design, material selection, landscaping, and building management, it is usually possible to reduce cooling loads by 50% compared to conventional buildings. In some parts of North America, cooling loads can be eliminated altogether. Recommended strategies for reducing cooling loads are summarized below and described in detail in the checklists at the end of the article.
In residential buildings, the greatest attention should usually be focused on reducing solar gains. Careful building orientation, providing vegetative shade, minimizing window area on the east and west, careful selection of glazings, and shading windows with architectural features or window treatments to reduce gain are the most important strategies. In commercial buildings, use of low-solar-gain glazings is the most important strategy for reducing solar gain.
Internal gains are by far the most significant cooling load in commercial buildings, and they are usually second most important in houses. Strategies for reducing these gains include installation of energy-efficient electrical equipment, insulation of cooling system ducts, insulation of hot water tank and pipes in the occupied space, spot ventilation of concentrated heat sources, and elimination or reduction of water vapor sources.
Conductive and infiltration heat gains should be fairly minor in energy-efficient buildings that are managed carefully. Conductive heat gain can be controlled through insulation, radiant barriers, reflective roof and wall surfaces, and good attic or roof ventilation. Infiltration gains can be minimized through airtight construction, careful management of windows and whole-house ventilation by building occupants, and isolation of humidity sources from occupied spaces.
Simple strategies to reduce summer heat gain are among the most significant environmental building practices. Many cooling load avoidance strategies are inexpensive, though they do require an investment in design. Extra design costs can often be recovered through downsizing of mechanical cooling and air distribution systems. Savings in operating costs are often synergistic: more efficient lighting, for example, costs less to operate and also reduces air conditioning loads.
While cooling load reduction is extremely important, it is only part of the solution in most parts of the country. Remaining cooling loads must be satisfied through a combination of expanding the comfort range of building occupants and implementing cooling strategies (both natural and mechanical). Here, too, there is room for great improvement from conventional practice.