Evaporative Air-Conditioning phần 4 pptx

Typical Investment Costsfor Evaporative Air-Conditioning in the United States 2,500 -, E 1,000 , Room size in sq m for EAC and AC ES Installation cost Cost cooler Source: R.. Technolog

indoor units are available for US$40 and up; however, the largest and most expensive units sell for more than US$1,200

The investment cost for a direct-indirect system is roughly double that for a direct EAC unit (and in fact approaches the level as VAC) However, the direct-indirect EAC's power consumption is only about

25 percent higher than direct EAC on an annual basis, and the total cost of electricity and maintenance for indirect-direct EAC systems amounts to only about 50 percent of that of conventional VACs of compa-rable performance

Investment Costs

Figure 3.1 compares typical total investment costs of EAC and VAC systems for different room sizes (20,

60 and lOOm2) for the United States In all cases EAC is the cheaper option

Figure 3.1 Typical Investment Costsfor Evaporative Air-Conditioning in the United States

2,500

-,

E 1,000 ,

Room size in sq m for EAC and AC

ES Installation cost Cost cooler

Source: R Foster

It is striking that although the cost of EAC coolers in the United States is low, the cost of installation

is relatively high, because of the labor involved in placing the cooler, connecting it to water and electric power sources, and providing a drain for the flush water

The same has been done for India in Figure 3.2 Here the difference between EAC and VAC is much more pronounced because EAC units are made by small wayside industries at very low cost, whereas VAC units are either imported or made by large, inefficient industries at much higher cost

The cost of installation in India is low because labor is cheap These typical investment costs for India and the United States illustrate that the relative economic merits of EAC are more pronounced in devel-oping countries than in the industrialized world

Life-Cycle Costs

The life-cycle and operational costs have also been analyzed for these two countries, as depicted in Fig-ures 3.3 and Figure 3.4.

Economics 17

Figure 3.2 Typical Investment Costs for Evaporative Air-Conditioning in India

1,000-Cei- 800

600

-i 400

-

200-0

Room size in sq m for EAC and AC Installation cost Cost cooler

Source: R Foster

Figure 3.3 Typical Life-Cycle Costs: Evaporative Air-Conditioning versus Air-Conditioningfor the United States

6,000

-,5,000

m) 4,000

8 3,000

2,000

1,000

0

Room size in sq m for EAC and AC

* Depreciation 2 Energy n Water

Source: R Foster.

Figure 3.4 Typical Life-Cycle Costs: Evaporative Air-Conditioning versus Air-Conditioningfor India

rA

5,000-4,000

8 3,000

2,000

-1,000

Room size in sq m for EAC and AC

* Depreciation E Energy n Water

Source: R Foster

For the calculation of the operational costs it was assumed in all cases that the maintenance is done by

a hired professional, which explains the rather high annual maintenance cost for EAC in the United States

In reality, however, many EAC owners do their own maintenance because it is easy and saves money In developing countries where labor is cheap, maintenance is generally done by professionals In India, for example, it is common for owners of EAC units to have a maintenance contract with an EAC dealer

Market Situation

At least 20 million residential EAC units are in operation worldwide Of these, some 8 to 10 million are in India, and more than 4 million are in the United States Other significant markets also exist in Australia, South Africa, Pakistan, and Saudi Arabia EAC also has significant market potential in many other areas

of the world (e.g., in the Sahel); yet in most of these areas, EAC technology is unknown

A significant reason why EAC units are not in operation in more areas around the world is that half

or more of the world's population lives in coastal regions, or within 100 kilometers of coasts, and these areas are typically humid and hence generally not the most favorable sites for EAC units In contrast, the most favorable climatic conditions for using EAC are in dry and hot desert regions, and these are com-paratively sparsely populated

Population differences notwithstanding, sufficient populations live in dry and hot regions to consti-tute meaningful markets for EACs In the United States, for example the current sales of direct EACs are more than US$150 million per year Moreover, the recent growth of the U.S EAC market has been signifi-cant, with annual increases of 10 percent reported by manufacturers

California, which traditionally has used VAC, represents one of the world's fastest-growing EAC markets The California Energy Commission (CEC), noting the 50 to 80 percent energy savings pos-sible with EAC (as opposed to VAC) technologies statewide, adopted energy credits for EAC as part of the Title 24 code compliance program in January 1993 Inclusion of EAC in the Title 24 program facili-tates significant prospective growth of the industry in California The CEC is also promoting an EAC

Economics 19

commercialization program that seeks to accelerate adoption of EAC to maximize its energy saving, environmental, and economic development potentials

Several California utilities are promoting EAC for commercial and residential applications as well Pacific Gas and Electric (PG&E) offers rebates for commercial use of evaporative cooling equipment Under the utility's customized program, hybrid and two-stage EACs can receive a US$200/kW reduc-tion as replacements for VAC technologies PG&E also offers a line-item rebate for the installareduc-tion of commercial evaporative cooling equipment at US$80 per ton displaced of VAC for new construction as part of a "Retrofit Express" program

Locally in California, the Sacramento Municipal Utility District (SMUD) has a new construction rebate program that provides rebates to EAC in the commercial sector based on calculated energy savings com-pared with conventional cooling In late 1992 Southern California Edison began offering US$100 rebates for installation of residential EAC (direct and indirect-direct) in their service territory The company has promoted these rebates actively in desert locations, offering an incentive of US$125 for replacement of residential VAC units with EAC equipment Southern California Edison also provides and maintains EACs

at no cost to qualifying low-income residents in their service area On the commercial front, the company

is interested in energy conservation in the retrofit market and offers rebates at US$75 per ton for direct EAC and US$100 per ton for indirect-direct EAC for displaced tonnage of VAC (they use 1,250 cfm = 1 ton cooling) About 30 to 50 commercial installations are taking advantage of this program each year

The State of New Mexico is requiring the use of EAC (mainly indirect-direct systems) instead of VAC systems in new public schools and additions New Mexico places about 100 new EAC applica-tions per year in schools

The Stratospheric Ozone Protection Division of the U.S Environmental Protection Agency (EPA) has included EAC as an acceptable technology in the EPA's Significant New Alternatives Policy (SNAP) rulings on alternative refrigerants and technologies This should further encourage the adoption of EAC technologies in the United States

Greenpeace and other environmental organizations are advocating EAC as an environmentally re-sponsible technology worldwide This type of interest from environmental organizations should also further global market development

The greatest market development problem facing the EAC industry currently is the lack of a normal-ized test standard for performance ratings Saudi Arabia and Australia have some limited general test standards However, the American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) standards committees on EAC have submitted a proposed test standard for testing indirect evaporative air conditioning equipment adopted by ASHRAE in 1996 Similarly, a proposed ASHRAE test standard for direct EAC units should be adopted in 1998 When these standards are adopted, the industry worldwide will benefit from a proposed certification program for rating EACs based on the ASHRAE test standards by the Evaporative Cooling Institute This certification program will provide design engineers worldwide with an independent performance-based test standard for rating EAC units The EAC market should continue to grow worldwide as interest from utilities and countries in-creases in applying the technology as an energy conservation tool Given advances with indirect and hybrid systems that widen the climatic range of application, the potential market penetration of this technology is large Indeed, when coupled with desiccant technologies, EAC could displace VAC tech-nologies in many applications in the coming century

Technology

Direct Evaporative Air-Conditioning

A residential evaporative air-conditioner consists of a cubical box with large, vertical filter-like "pads," a sump at the bottom, an electric-motor-driven fan, a water pump, and a water distribution system (see Figure 4.1) The fan draws in warm outside air through the wet pads, cooling the air The water pump lifts the water from the sump through the distribution system on top of the pads from where it trickles down by gravity back to the sump The cooled air is then delivered either directly through a grill into a single room or into a duct system to cool more than one room

This is a "direct" evaporative air-conditioner in which the cooled and humidified outside air flows to the room and removes the heat An efficient wetted pad can reduce the air temperature by as much as 95 percent of the wet-bulb depression (ambient dry-bulb temperature less wet-bulb temperature), while an inefficient and poorly designed pad may only reduce this by 50 percent, or worse A simplified process diagram for direct evaporative air-conditioning is shown below There is actually very little change in energy state of the air (i.e there is no sensible cooling) other than energy inputs from the fan and

make-up water Direct EAC is simple and cheap but it has the disadvantage that if the ambient wet-bulb tem-perature is higher than 21°C (69.8°F), the cooling effect is not sufficient for indoor comfort cooling The saturation effectiveness of a direct evaporative air-conditioner best describes the performance of the unit Saturation effectiveness is defined as the difference between the entering and exit dry-bulb (DB) temperatures over the wet-bulb (WB) depression and can be defined as follows:

Saturation effectiveness = DBI - DB2

DB, -WB1 where

DB1= Entering (typically ambient) dry-bulb temperature

DB2= Exiting dry-bulb temperature

WB1= Entering (typically ambient) wet-bulb temperature

21

Figure 4.1 Simplified Evaporative Air-Conditioning Process

Latent energy

to evaporate water Direct

You feel 35°C Sensible and You feel 21°C

latent heat energy

Sensible heat in the air is used to evaporate water

(transfered to latent energy in the moist air)

Source: Authors.

A psychrometric chart, which shows moist air properties, more clearly demonstrates the evaporative cooling process The initial dry-bulb and wet-bulb temperatures are shown at the start of the process, and the endpoint of the evaporative cooling process is found to the left at the end of the arrow along the line

of constant wet-bulb temperature For example, taking 1 percent design conditions for Ciudad Juarez, Mexico, of 37.7°C (99.9°F) dry-bulb temperature at a mean coincident wet-bulb temperature of 17.7°C (63.9°F), and using evaporative media that has a saturation effectiveness of 85 percent, we find that the evaporative media will change the state of the airstream to a dry-bulb temperature (supply air) of 20.7TC (69.3°F) This process is shown in Figure 4.2 for Ciudad Juarez

Figure 4.2 Psychrometric Processfor Direct Evaporative Cooling, Mexico

20.70CDB for S.E.=85%

a / \ ~~~Direct Evaporative oa

/4/ / \ti~~Coling Process +;o0

Cd Jukrez, Mexico i

37.7 DBJ17.7°C WB 2

Dry-Bulb Tempearture °C

Source: ECI.

Technology 23

Direct evaporative coolers do not recirculate air in applications Instead, air is passed only once through the system and then exhausted This leads to superior indoor air quality Evaporative cooling media also act as a wetted filter that scrubs out many contaminants (see also Figure 1.1)

Pads

The pad-or medium, as it is often called-serves to bring the water and air into contact so that the air can absorb moisture and lower the dry-bulb temperature (cooling effect) An ideal pad should have the following characteristics:

* Minimum resistance to airflow

* Maximum air-water contact for vaporization

* Equal distribution of airflow resistance, air-water contact, and water flow

* Resistance to chemical or biological degradation

* Ability to self-clean airborne matter

* Durability and consistent performance over life-cycle

* Low cost

In reality, all pads fall short of this ideal and thus require some trade-offs among advantages There are at present three major types of pads: aspen (or other similar type) wood, rigid pads, and synthetic pads Each has its own advantages and disadvantages

Aspen Wood Pads These pads are composed of thin shredded wood slivers, packed loosely to a

thickness of 3 to 5 cm This material is spread equally over the pad-holder surface and held in place by a flexible steel or plastic grid The thin wood strands absorb water and ensure good diffusion of the water over the surface of the pad, which gives it sufficient cooling characteristics This good cooling, combined with the very low cost (US075 per replacement pad) has made aspen wood the most popularly used pad material worldwide Aspen pads have some serious deficiencies in performance and durability, however First, because wood is an organic material, it degrades fairly quickly in humid conditions In application, this means that the strands decrease in strength and sag under the weight of the water they have absorbed T'he sagging means that some areas of the pad become more compact, blocking the airflow, while other areas become more open, increasing airflow at the cost of reduced saturation efficiency This combination leads to reduced cooling Moreover, dust, pollen, and other airborne organic or inorganic matter are trapped between the strands of the pad, increasing resistance to airflow and imparting unpleasant odors to the cooling air if the pad is not properly dried during daily use Similarly, when the EAC is turned off and the remaining water in the pad evaporates, it leaves behind a deposit of minerals, called scale This scale is not completely dissolved when the unit is restarted and it impairs the airflow and blocks the pad

Depending on the intensity of usage, the level at which mineral concentrations are controlled (ad-equate bleed-off), and the outside air quality (quantity of dust in the air) aspen pads may be replaced once a cooling season or sometimes after two cooling seasons Even so, optimum performance of the EAC may only be achieved in the first weeks after installation of pads A properly packed pad may start with 70 percent saturation efficiency but may decline to 50 percent efficiency after only a few weeks, operating at that level or less until it is replaced

Another problem with aspen wood pads is their sensitivity to installation technique That is, the pads must be installed so as to ensure that the woody material is spread in equal density across the pad's total area If this is not done, the saturation efficiency will be reduced from the start Because replacement of pads is needed regularly and appears to be a relatively simple task, many EAC owners will do it-with varying results in terms of efficiency-themselves

Rigid Pads Rigid pads became available in the early 1980's They are made of a specially

impreg-nated type of paper or glass fiber and typically use a honeycomb type structure They are made of strips

of corrugated paper alternative with upward and downward slopes, cemented together where the corru-gations touch (Figures 4.3 and 4.4) This arrangement eliminates most of the problems associated with aspen wood because rigid pads have the following advantages:

Figure 4.3 Commonly Available Rigid Cellulose Pads Provide Superior Saturation and Cooling Compared

with Ordinary Aspen Pads

Source: Munters Corporation.

Figure 4.4 Close-up of Rigid Cellulose Pad Made of Corrugated Paper

Source: Munters Corporation.

* Long and fairly constant service life between three and seven years, depending on maintenance

* Largely self-cleaning (i.e., dust washes off)

* No biological deterioration of the pad material

* More consistent saturation efficiency of about 75 to 90 percent

* Low pressure drop across the pad

The disadvantage is that rigid media are more costly (about US$100 more on an EAC that would cost US$300 if using aspen wood pads) They are also bulkier, which makes them difficult to use in smaller units At present, about 25 percent of the EACs sold in the United States are fitted with rigid pads, a share

Technology 25

that is growing In fact, some U.S manufacturers expect that eventually most EACs will be fitted with rigid pads because of their performance advantages over aspen pads

Other Pad Materials In a bid to improve on aspen wood, some manufacturers are supplying pads

made of woven plastic The plastic pads avoid many of the disadvantages of aspen wood but have the disadvantage of poor cooling efficiency because of the poor wetting characteristics (low saturation effec-tiveness) of the plastic material Other substances have been tried as pad materials such as woven ex-panded paper, fabrics, wood wool made of pine, fir, cottonwood, cedar, redwood, spruce, plain and etched glass fibers, copper, bronze and galvanized screening, but none of these are extensively used

Country-Specific Pad Materials In each country where evaporative air-conditioners are used or are

intended to be used it may be advisable to look for an inexpensive and easily available indigenous pad material-such as Khus-khus grass in India-or a long-lasting alternative such as a rigid pad The objec-tive, of course, is to avoid the need for continuous large-scale shipment of pad materials such as aspen wood from the United States or Australia or if corrugated paper from Europe

Cabinet

The cabinet of the air-conditioner is usually made of hot dip galvanized steel, coated with baked on high quality paints (see Figure 4.5) Corrosion can be a problem with drip air-conditioners because most parts come into contact with highly oxygenated water and concentrated solutions of waterbome or airborne chemicals To eliminate corrosion problems some manufacturers supply stainless steel air-conditioners and some others air-conditioners made entirely of polypropylene, polyurethane, or glass fiber In Austra-lia at least one manufacturer brings an aluminum air-conditioner on the market Stainless steel air-condi-tioners are expensive and very sensitive to electrolytic corrosion (one screw of the wrong material may cause corrosion of the whole air-conditioner) and glass fiber or plastic models are subject to deterioration due to ultraviolet radiation If galvanized steel cabinets are cleaned and repainted inside after every sea-son, they should last 10 years or more

Fan and Motor

Small air-conditioners (up to 55m3/min of washed air), serving only one or two rooms are often fitted with an axial propeller type fan These fans, with 2 to 4 blades, operating at 900 to 1,400 rpm are noisier than centrifugal types but are about twice as efficient For higher airflow resistance, as is usually the case for larger air-conditioners delivering air to a duct system, centrifugal fans are more suitable They are very quiet in operation but the efficiency is only half of that of an axial fan

Axial fans are usually fitted directly on the motor shaft but centrifugal fans are belt driven and geared down to roughly 1/3 of the motor speed In general it can be said that the larger the fan and the lower the speed the more quiet it is

The motors for most residential air-conditioners are two-speed, single-phase, shaded-pole and four-pole types in the range of 200 to 1000W They should have a drip proof construction and a 50°C allowable temperate rise, certified by some recognized authority More advanced designs are beginning to incorpo-rate variable speed motors

Recirculation Pump

The most popular pump is a small submerged centrifugal pump driven through a vertical shaft from an air-cooled motor mounted dry above the waterlevel in the sump These pumps are inexpensively made (US$15 retail price) and may last no more than three to five seasons They require no maintenance but can

be vulnerable to dry running The capacity is generally not more than 20 1/min against a head of about

lm In many cases there is a small outlet besides the pump discharge for the purpose of continuously