Absorption cooling is essentially an air conditioner driven not by electricity, but by a heat source such as natural gas, propane, solar-heated water, or geothermal-heated water. Because natural gas is the most common heat source for absorption cooling, it is also referred to as gas-fired cooling. Although mainly used in industrial or commercial settings, absorption coolers are now commercially available for large residential homes.
Absorption cooling usually only makes sense in homes without an electricity source, but may also be employed to make use of renewable energy. Absorption cooling is essentially a heat pump technology; absorption coolers are absorption heat pumps that are not set up to allow their use as a heating device.
Absorption Heat Pumps
Absorption heat pumps are essentially air-source heat pumps driven not by electricity, but by a heat source such as natural gas, propane, solar-heated water, or geothermal-heated water. Because natural gas is the most common heat source for absorption heat pumps, they are also referred to as gas-fired heat pumps. There are also absorption coolers available that work on the same principal, but are not reversible and cannot serve as a heat source. These are also called gas-fired coolers.
Residential absorption heat pumps use an ammonia-water absorption cycle to provide heating and cooling. As in a standard heat pump, the refrigerant (in this case, ammonia) is condensed in one coil to release its heat; its pressure is then reduced and the refrigerant is evaporated to absorb heat. If the system absorbs heat from the interior of your home, it provides cooling; if it releases heat to the interior of your home, it provides heating.
The difference in absorption heat pumps is that the evaporated ammonia is not pumped up in pressure in a compressor, but is instead absorbed into water. A relatively low-power pump can then pump the solution up to a higher pressure. The problem then is removing the ammonia from the water, and that's where the heat source comes in. The heat essentially boils the ammonia out of the water, starting the cycle again.
A key component in the units now on the market is generator absorber heat exchanger technology, or GAX, which boosts the efficiency of the unit by recovering the heat that is released when the ammonia is absorbed into the water. Other innovations include high-efficiency vapor separation, variable ammonia flow rates, and low-emissions, variable-capacity combustion of the natural gas.
Although mainly used in industrial or commercial settings, absorption coolers are now commercially available for large residential homes, and absorption heat pumps are under development. The 5-ton residential cooler systems currently available are only appropriate for homes on the scale of 4,000 square feet or more.
Absorption coolers and heat pumps usually only make sense in homes without an electricity source, but they have an added advantage in that they can make use of any heat source. Because of this, they can make use of solar energy, geothermal hot water, or other heat sources. They are also amenable to zoned systems, in which different parts of the house are kept at different temperatures.
The efficiency of air-source absorption coolers and heat pumps is indicated by their coefficient of performance (COP). COP is the ratio of either heat removed (for cooling) or heat provided (for heating) in Btu per Btu of energy input. Look for a heating efficiency of 1.2 COP or greater and a cooling efficiency of 0.7 COP or greater.
Radiant cooling cools a floor or ceiling by absorbing the heat radiated from the rest of the room. When the floor is cooled, it is often referred to as radiant floor cooling; cooling the ceiling is usually done in homes with radiant panels. Although potentially suitable for arid climates, radiant cooling is problematic for homes in more humid climates.
Most radiant cooling home applications in North America have been based on aluminum panels suspended from the ceiling, through which chilled water is circulated. To be effective, the panels must be maintained at a temperature very near the dew point within the house, and the house must be kept dehumidified. In humid climates, simply opening a door could allow enough humidity into the home to allow condensation to occur.
The panels cover most of the ceiling, leading to high capital expense for the systems. In all but the most arid locations, an auxiliary air-conditioning system will be required to keep the home's humidity low, adding further to the capital cost. Some manufacturers do not recommend their use in homes, while the Radiant Panel Association suggests they could yield added comfort and energy savings when used in combination with a central air conditioning system.
In addition, the limited U.S. experience with radiant cooling creates concerns about the quality and availability of professionals for installation, maintenance, and repair of a residential system.
Despite these caveats, there may be cases where radiant cooling is appropriate for homes, particularly in the arid Southwest. Radiant cooling systems have been embedded in the ceilings of adobe homes, taking advantage of the thermal mass to provide a steady cooling effect.
Homes built on concrete slabs are prime candidates for radiant heating systems and radiant floor cooling takes advantage of the same principle using chilled water. This is particularly economic in homes with existing radiant floor systems. Again, condensation is a concern, particularly if the floor is covered with heavy carpeting, and the effect is intensified by the tendency of cool air to collect near the floor in stratified layers. This limits the temperature to which the floor can be lowered.
Despite that limitation, a study performed by the Oak Ridge National Laboratory found that some early morning cooling of a home' s concrete slab, combined with nighttime ventilation, could shift most of the cooling loads for a house to off-peak hours, reducing the electrical demand on electric utilities.
Earth Cooling Tubes
In the 1970s and early 1980s, earth cooling tubes received a great deal of attention from architects, builders, and homeowners as an alternative or aid to conventional air conditioning. While the concept of routing air through underground tubes or chambers to achieve a cooling effect seems like a good idea, in practice it is not very effective, both technically and economically. Perhaps a few hundred systems were constructed, but information on the practical application of the concept is limited. There are few functioning installations, and limited quantitative performance data exists.
How They Are Supposed To Work
Cooling tubes are long, underground metal or plastic pipes through which air is drawn. The idea is that as the air travels through the pipes, it gives up some of its heat to the surrounding soil, entering the house as cooler air. This will occur only if the earth is at least several degrees cooler than the incoming air.
A cooling tube system uses either an open- or closed-loop configuration. In an open-loop system, outdoor air is drawn into the tubes and delivered directly to the inside of the home. This system provides ventilation while hopefully cooling the home's interior. In a closed-loop system interior air circulates through the earth cooling tubes. An alternative is to direct the cooled air from either type of system into a mechanical air conditioning system to reduce the air conditioner's cooling load.
A closed loop does not exchange air with the outside; instead the system recirculates the home's air through the earth cooling tubes. This makes the closed loop system more efficient than an open loop design, since it does not require as high a degree of dehumidification as an open loop system.
The main considerations in selecting tube material are cost, strength, corrosion resistance, and durability. Tubes made of aluminum, plastic, and other materials have been used. The choice of material has little influence on thermal performance. PVC or polypropylene tubes perform almost as well as metal tubes; they are easier to install, and are more corrosion resistant.
Optimum tube diameter varies widely with tube length, tube costs, flow velocity, and flow volumes. Diameters between 6 and 18 inches (15.2 and 45.7 centimeters) appear to be most appropriate.
Earth temperatures and, consequently, cooling tube performance vary significantly from sunny to shady locations. Where possible, the inlets in open loop systems and the cooling tubes themselves should be placed in shady areas.
Tubes should be buried at least 6 feet (1.8 meters) below grade. Only rarely is burying them more than 12 feet (3.7 meters) justifiable. When digging trenches at these depths, cave-ins are an extreme hazard, and appropriate precautions should be taken.
The temperature of the earth at depths of 20–100 feet (6.1–30.5 meters) remains about two to three degrees higher than the mean annual air temperature. At depths less than 10–12 feet (3.1–3.7 meters), earth temperatures may be strongly influenced by air temperatures and may vary during the year, depending on the locale. Near the surface, earth temperatures closely correspond to air temperatures.
There is no simple formula for determining the proper tube length in relation to the amount of cooling desired. Local soil conditions, soil moisture, tube depth, and other site-specific factors should be considered to determine the proper length.
The amount of heat conducted and how widely it is diffused varies from one soil type to another. The moisture content of the soil is a major influence on conductivity and diffusivity, and accounts for large variations on how heat moves through the earth.
Earth cooling tubes are likely to perform poorly in hot, humid areas, because the ground does not remain sufficiently cool at a reasonable depth during the summer months. Moreover, dehumidification, another equally important aspect of cooling, is difficult to achieve with earth cooling. Mechanical dehumidifiers will most likely be necessary.
The dark and humid atmosphere of the cooling tubes may be a breeding ground for odor-producing molds and fungi. Furthermore, condensation or ground water seepage may accumulate in the tubes and encourage the growth of bacteria. Good construction and drainage could eliminate some of these problems.
Insects and rodents may enter the tubes of an open-loop system. You should install a sturdy grille and insect screen at the tube inlet to deter potential intruders.
Earth cooling tube systems can be very expensive. Considering current electric power rates and the cost of materials and labor, it is unlikely that an earth cooling tube installation can be justified on economic grounds alone.