Evaporative Air-Conditioning phần 2 pps

Evaporative air-conditioning: lowering of dry-bulb temperature as air passes over water.. Evaporative air-conditioning EAC technologies are being used increasingly in residential and com

Although evaporative coolers cannot be used in all countries and at all times, they are generally very much underutilized in places where they can be used successfully This is unfortunate, both for the potential user, the country, and the global environment Benefits include lower cooling equipment costs and a much reduced electricity bill for the user, reduced electrical energy and power demand at peak-times for the country, and lower greenhouse gas and CFC/HFC emissions for us all

This handbook is designed for those who do not know evaporative coolers, but might be convinced

to try using or promoting them It provides the advantages and disadvantages of using evaporative coolers while comparing them to the commonly used, energy guzzling, and expensive vapor compres-sion air conditioners Existing markets where evaporative coolers are currently used, local manufactur-ing possibilities, operational aspects are discussed along economic and global aspects A world-wide list

of manufacturers and suppliers is included in the Annex

James Bond Director

Energy, Mining and

Telecommunications Department

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As the harmful environmental effects of chloro-fluorocarbons (CFCs) and greenhouse gases have be-come better known, interest has grown in environmentally friendly cooling technologies Evaporative air-conditioning (EAC) is such a technology Whereas conventional vapor compression air-conditioning (VAC) uses CFCs as cooling liquids, EAC uses water EAC technology is simple, functional, and has both residential and commercial applications in industrialized and developing countries EAC can provide superior cooling and ventilation while consuming less energy (and hence contributing less to green-house gas emissions) than VAC EAC works best in hot, dry climates, but it can be used in more humid climates as well

This paper elucidates some of the technical characteristics and fields of application for EAC and out-lines the climatic conditions under which EAC can be most effectively employed The document begins with a general outline of the applications and limitations of EAC and explains the differences between

"direct" and "indirect" EAC Chapter 2 discusses the applicability of EAC in different climates and ex-plains the use of wet-bulb temperature as a useful tool for predicting the applicability of EAC Chapters 3 and 4 discuss the economics of EAC versus VAC in terms of energy consumption, required investments, and life-cycle costs Production costs, the paper points out, are low enough so that EACs can be manufac-tured relatively easily in the developing world, as is now being done in South Asia and the Middle East Chapters 5 and 6 review the market for EACs and try to show how EAC can increase individuals'

"feeling of comfort."

Chapter 7 explains the basic technology of EAC The difference between direct and indirect coolers is elaborated on through the use of a psychrometric chart The hardware components of the EAC are ex-plained: pads, motor, pump, and fan Chapter 8 lists the equipment available on the market It also points out that the capacity of the cooler and the size of the room to be cooled are key elements in selection of an EAC A simple example is given to aid in sizing Like any sort of mechanical equipment, EACs need to be maintained regularly to perform well and last longer Maintenance requirements for each component are discussed in chapter 9 EACs require little energy, and because the presence of strong sunshine coincides with the need for cooling, a link with solar energy appears to be attractive

In chapter 10 the usefulness of solar EAC and the present market situation are outlined EAC is an attractive cooling solution, for industrial as well as for less developed countries too The requirements for the introduction of a relatively new technology like EAC are discussed in Chapter 11 In Chapter 12 the usefulness of EAC for commercial applications is outlined Commercial kitchens, laundry and dry clean-ing and industrial applications are three areas where EAC could be useful

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Abbreviations, Symbols, and Glossary

Design temperatures: outdoor temperatures at a fixed percentage more temperate than worst-case fig-ures, which are a standard air-conditioning system design parameter

Enthalpy: total heat content of air-water vapor atmospheric gas Not altered by adiabatic cooling

Evaporative air-conditioning: lowering of dry-bulb temperature as air passes over water Two methods using evaporating water to cool air: (1) direct, which is adiabatic and humidifies the air; and (2) indirect, which is nonadiabatic and cools the air being treated

Indirect evaporative air-conditioner a heat and mass transfer device used to sensibly cool a primary airstream, without addition of moisture, by means of an evaporatively cooled secondary airstream Since the secondary air provides wet-bulb depression, it represents a heat sink to the primary air

Latent heat load: heat carried by water vapor in air; varies with humidity Wet-bulb temperature is an index to latent heat

Saturation (cooling) effectiveness: the primary air dry-bulb temperature reduction divided by the pri-mary air entering dry-bulb temperature less the entering secondary wet-bulb temperature

Temperature, dry-bulb: the air temperature measured by a dry temperature sensor

Temperature, wet-bulb: the temperature measured by a temperature sensor covered by a water-moist-ened wick and exposed to air in motion When properly measured, it is a close approximation of the temperature of adiabatic saturation

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Evaporative air-conditioning (EAC) technologies are being used increasingly in residential and com-mercial applications worldwide EAC technologies-which rely on water as a coolant rather than on chemical refrigerants-are economical to produce and use and have important environmental ben-efits This paper introduces the technical aspects of EAC, reviews EAC's scope of application, and surveys the specific climatic conditions under which EAC can be used most effectively in industrial-ized and developing countries

Under the right conditions and applicafions, EAC can provide excellent cooling and ventilation with minimal energy consumption using water as the working fluid and avoiding the use of ozone-destroying chlorofluorocarbons (CFCs) Policymakers in particular should become better informed about EAC be-cause of the opportunities it affords to reduce the use and emission of CFCs and hydrofluorocarbons (HFCs), the reduction in CO2emissions that come from the energy efficiency of the technology, and the potential for mitigating problems of peak electricity demand during the hot season in many countries The viability of using EAC will depend on the particular application and on the local climatic conditions For example, for comfort cooling, EAC is most suited to dry regions, although technical improvements such as indirect/direct and desiccant-assisted systems widen the zone of applicability On the other hand, some commercial applications of EAC are suitable even in humid climates

In general, several sectors have significant reasons for considering employing EAC technologies:

* Utilities Dissemination of EAC appliances can serve as a significant demand-side management

(DSM) tool for utilities Power savings of EAC technology versus VAC are on the order of 70 percent for direct EAC and 50 percent for indirect EAC This differential presents substantial peak-saving opportunities for utilities that can promote the use of EAC within their service areas

* Governments For goverrnent agencies and planners, cost savings from reduced electrical con-sumption can be realized directly by incorporating EAC technology into buildings and other

in-2 Evaporative Air-Conditioning: Applications for Environmentally Friendly Cooling

* Consumers Consumers who use EAC at home can save money on cooling costs The typical capi-tal, installation, and operation costs are significantly lower for EAC technologies than for VAC technologies Moreover, EAC technology is simple enough so that most homeowners can main-tain their own units

* Private enterprise The manufacture and sale of EAC appliances presents significant opportunities for both small and large enterprises It is particularly suited to manufacture even in relatively poor developing countries because-unlike the comparatively complex technical requirements for production of chemical air-conditioners-EAC production requires only the basic infrastruc-ture and skills mix related to sheet metal, motor, pump, and fan fabrication Hence, marketers of EACs can underbid VAC prices while maintaining comparatively high profit margins In the right climates, EACs can gain far more than a "niche" market: in some of the larger cities in the southwestern United States and northern Mexico, for example, 95 percent of the residential air-conditioning market is taken by EAC units, most of them manufactured locally

Benefits of Evaporative Cooling

The following benefits of EAC can be cited:

* Significant local fabrication and employment

* Substantial energy and cost savings

* No chlorofluorocarbon (CFC) usage

* Reduced peak demand

* Reduced CO2 and power plant emissions

* Improved indoor air quality

* Life-cycle cost effectiveness

* Easily integrated into built-up systems

* Wide variety of packages available

* Provide humidification when needed

* Easy to use with direct digital control (DDC)

* Greater regional energy independence

Opportunities and Limitations

EAC works best for comfort cooling where it is hot and dry EACs are widely used in the Middle East, Australia, the Indian subcontinent, Eastern African, northern Mexico, and the southwestern United States

Residential EACs are known in India as desert coolers, and in such desert or dry-steppe climates EACs do

give "significant relief" during the hot months "Significant relief" is considered to be provided when the final supply-air temperature leaving the EAC is about 20' to 250C (680 to 77°F) Even in a tropical savanna climates such as in the northeast of Brazil, the Sahel region of Africa, the southwest Dominican Republic, EAC can be useful in some comfort cooling applications and also for many commercial applications such

as greenhouses and poultry houses

A limiting factor for the application of EAC is the definition of comfort A residential cooler bringing

down the temperature from 450 to 30°C (1130 to 860T) may still be appreciated even if it does not provide

"significant" relief

Environmental Benefits

EAC technologies represent significant enviromnental benefits related to reducing CFC/HCFC use and for obviating C02 and other emissions, as well as for reducing peak electrical demand For example, the

Introduction 3

4 million EAC units in operation in the United States provide an estimated annual energy savings equiva-lent to 12 million barrels of oil and an annual reduction of 5.4 billion pounds of CO2emissions They also avoid the need for 24 million pounds of refrigerant traditionally used in residential VAC systems Similar energy savings and environmental benefits are also made by commercial applications of evaporative cooling technologies in the United States and elsewhere Through increasing use of EAC technologies, countries can save energy, reduce power plant emissions, obviate CFC usage, and improve indoor air quality Basic air conditioning with water is a relatively simple process

Direct Evaporative Air-Conditioning

Direct EAC is the simplest, the oldest, and the most widespread form of air-conditioning This system typically uses a fan to draw hot outside air into a dwelling through a porous wetting medium Heat is absorbed by the water as it evaporates from the porous wetting medium, and the air thus leaves the EAC

at a lower temperature The amount of cooling provided is determined by efficiency of the wetting me-dium, the fan, and the overall design and construction of the unit

A critical component in EAC is the use of water This may vary from a few liters per day in small residential coolers to perhaps a hundred liters or more in pad-and-fan EAC systems in greenhouses and complicated duct-systems in laundries and hotel kitchens

Residential Coolers

A residential EAC typically consists of a cubical box of sheet metal or plastic containing large vertical filter "pads," an electric-motor-driven fan, a water pump, a water distribution system, and a water sump

at the bottom As Figure 1.1 and Figure 1.2 show, the fan draws in warm outside air through the wetted media, cooling the air The water pump lifts the water from the sump through the distribution system to the top of the pads from where it trickles down by gravity back to the water sump The cooled air is then delivered either directly through a grille into a single room or into a duct distribution system

This is a "direct" EAC in which the cooled and saturated outside air flows into the room, displacing the hot air It is simple and cheap but is not sufficient for indoor comfort cooling once ambient wet-bulb temperatures reach 21°C (69.8°F)

Figure 1.1 Typical Direct Evaporative Air-Conditioner

Distribution

Manifold

Recirculation Pump

4 Evaporative Air-Conditioning: Applicationsfor Environmentally Friendly Cooling

Figure 1.2 Roof-Mounted Downdraft Evaporative Air-Conditioning Unit, El Paso, Texas

7."~ ~ ~ ~ ~~~~.

Source: R Foster.

Indirect Evaporative Air-Conditioning

Indirect-direct EAC is a method established only over the past 15 years It is not as widely used as direct EAC, but it is gaining in popularity because it cools air more than direct EAC, and cools the air down

from higher wet-bulb temperatures hindirect EAC accomplishes these effects by building an additional step into the cooling process That is , the incoming air is cooled first with a normal air-to air heat ex-changer This is the "indirect" stage because it does not add moisture to the supply air Instead, only one side of the heat exchanger is cooled with evaporating water as the supply air passes through the other

side, dropping in temperature as it does Only then, as it passes through the direct EAC stage, is the

supply air moisturized The final air leaving an indirect-direct EAC unit is generally 3.5C (6.30F) cooler than what could be achieved with a direct EAC unit alone

Because it cools the air first without moisturizing it, the indirect-direct process also allows the EAC unit to provide more comfort in slightly more humid areas Commonly these units achieve 65 percent indirect stage efficiency (performnance factor), which allows an ambient wet-bulb temperature of up to

250C to provide acceptable room temperatures for real comfort

Two-stage air-conditioners combinLing indirect and direct EAC are becoming popular in the United States and Australia, particularly in locations where slightly higher wet-bulb temperatures (i.e., conditions

of higher ambient humidity) do not permit sufficiently comfortable supply-air temperatures via direct EAC

On the downside, however, the two-stage units have higher construction and maintenance costs

Desiccant-Assisted Evaporative Air-Conditioning

The use of dehumidifying chemicals (e.g., desiccants such as silica gel) further widens the scope for EAC Desiccant technologies can widen the scope for comfort cooling to even the most humid regions In such systems, the desiccant is used first to dehumidify the ventilation air to a desired state; then, EAC (either

direct or indirect or a combination thereof) is used to cool the air to the desired supply-air temperature.

Introduction 5

Commercial Evaporative Air-Conditioners

Commercial EAC applications are of several types Commercial comfort cooling applications are used for offices, retail establishments, and so on, as determined by local climates and comfort preferences

In other commercial applications, EAC may be used to moderate the effects of an additional internal heat source that does not depend (only) on the climate or the time of the year For example, temperatures may rise inside warehouses or buildings because of the operation of ovens, machines, or the presence of livestock These heat sources sometimes exacerbate already high ambient temperatures Although the cooling requirements differ as a matter of degree, so to say, cooling of buildings affected by both internal and external sources of heat does require a somewhat different approach from residential cooling to moderate high outside ambient temperatures For one thing, such commercial EAC systems may well need to be designed for operation the year round rather than just in a "hot season." A commercial kitchen

or bakery, for example, might need cooling year-round Moreover, the internal cooling requirements may

be quite localized within the building (e.g., spot-cooling in a manufacturing plant)

Another difference between commercial and comfort cooling with EAC is that EAC in some com-mercial applications is the only practical alternative; that is, where VAC technologies cannot function or compete effectively because of high operating costs The most salient example here is the cooling towers

in a power plant, but on a smaller scale, EAC is the only real alternative in agricultural applications such

as greenhouses, where VAC is both inappropriate and far too costly

Common commercial applications for EAC include the following:

* Commercial kitchens

• Hotels and restaurants

- Hospitals

• Other institutions

* Laundry and dry cleaning

* Industrial applications

- Agricultural applications

- Poultry sheds

- Greenhouses

* Schools and offices

* Transit buses (Figure 1.3)

* Industrial applications

- Warehouses

- Spot cooling

- Factories

Comparing Vapor-Compression and Evaporative Air-Conditioning

Table 1.1 compares the basic characteristics of VAC with those of EAC

Outlook

Worldwide, the potential for EAC is much greater than is currently realized Investment, operation, and replacement costs can be lowered significantly by foregoing or replacing VAC technologies and using EAC The potential applications are manifold: from buildings and homes to buses and kitchens In some developing regions of the world where air-conditioning has scarcely arrived, EAC could bring comfort,