1、392 2010 ASHRAEABSTRACTHVAC with wheel desiccant dehumidification (WDD)with a low ambient impact is more efficient system compare tothe traditional systems. Hybrid desiccant cooling systems(HDCS) can be used to control indoor air quality (temperatureand humidity) in commercial and industrial buildin
2、gs. Hybriddesiccant cooling systems (HDCS) can be used as a technologyfor controlling indoor air quality (temperature and humidity)in many commercial and industrial buildings. Results increas-ingly show stringent guidelines for outdoor ventilation rate.Application of these systems depends on site-sp
3、ecific condi-tions and the loading rate, while; a high percentage of a build-ing load can be latent. Desiccant cooling systems removemoisture from the air by using a wheel desiccant similar tocondensation on coils in conventional systems. Climate condi-tions and buildings requirements should be anal
4、yzed and eval-uated before using this system for any application. There area few important variables, which drive performance and cost-benefit analysis. When more number of the just mentioned vari-ables favor a hybrid desiccant system will be over a conven-tional system. In such conditions a detaile
5、d economic andperformance analysis of a hybrid system is required. This studypresents important variables analysis for different areas inMalaysia. The correlation shows that a high potentiality existsfor using hybrid desiccant systems in all states of Malaysia.Moreover the corresponding electricity
6、saving would beconsiderable.INTRODUCTIONIn the last decade, all-desiccant and hybrid desiccantsystems have emerged as an alternative to conventional vaporcompression systems for cooling and conditioning air in sometype of buildings such as presented in Refs. 1-6. A typicalhybrid system combine a des
7、iccant cooling system with aconventional vapor compression cooling system was presentedin Ref. 2. The main goal of application of hybrid desiccantcooling systems is using the high capability of desiccants inmeeting the latent load of buildings. When combining with thehigh capability of vapor compres
8、sion system in meeting thesensible load of buildings, the systems efficiency could beenhanced. Conventional vapor compression systems use a verylow evaporator temperature in order to meet the high latentload of the air and reheat is often needed to bring the temper-ature to required temperature of t
9、he supply air. This problemcauses the system to have poor energy efficiency. Differentstudies have been conducted to compare hybrid desiccantcooling systems with conventional vapor compression systemsuch as presented in Ref. 7. This study developed and demon-strated a different and important variabl
10、e of climate conditionsin several states in Malaysia for energy saves and cost-benefitsto air conditioning applications. Figure 1 shows a typical twowheel desiccant dehumidification system uses a heat-regener-ated desiccant wheel to remove the latent load, as shown in thisstudy when using hybrid coo
11、ling system will be saved a highportion of electricity in Malaysia.METHODOLOGY To evaluate hybrid desiccant cooling system in Malaysiathe climate conditions and performance-benefits must beconsidered and quantified. There is few important variabledrive performance and benefits. For this reason a stu
12、dy ofthese key variables was performed for different cities inMalaysia then it has been modelled and its total loads calcu-lated with conventional engineering software for a buildingFeasibility Study of Hybrid Wheel Desiccant Dehumidification Cooling Systemsin Malaysia Salman Khosravi T.M.I. Mahlia,
13、 PhD Yat H. Yau, PhD, PEMember ASHRAESalman Khosravi is a doctoral student, T.M.I. Mahlia is an associate professor of energy conversion and policy, and Yat H. Yau is an associateprofessor and M gas turbine exhaust andhot water from solar collectors can be used. Most places inMalaysia recorded solar
14、 radiation ranging from 12 to 16 MJm-2per day 8. The highest solar radiation of 18 MJm-2per daywas recorded 8. However, these free and clean heats makethis system most attractive over compression system.High-Demand Electrical EnergyMalaysia with a high demand exists for electricity tariff (/Kwh), an
15、d with onset of oil price increasing more and moreefforts have been turned to the techniques that can efficientlyemploy low-grade thermal energy. Desiccant dehumidifica-tion is prospective, as it is entirely a low-grade thermal energydriven device. The price of 3412.128 Btu (1kWh) electricalenergy i
16、s $0.07 in Malaysia19 but about $0.924 in theUnited States 10, that is about 13 times lower for Malaysia.Also, the price for 3.5 ft3(0.1 m3) natural gas with the sameenergy is $0.0525 in Malaysia 9 compare to $0.035 in theUnited States 11. Although prices are lower in Malaysia,natural gas prices are
17、 much less than electrical energy price.Latent Load Ratio Desiccant-based systems are most effective in reducinglatent loads than are conventional systems; therefore, they aremore attractive when the latent loads as a fraction of total loadsare high. For this reason must be consider this key variabl
18、ebefore applying these systems. The design-cooling load is theamount of heat energy to be removed from building by theHVAC equipment to maintain building at indoor designtemperature when worst case outdoor design temperature isbeing experienced temperatures. For appreciable quantity ofratio latent t
19、o total loads shown typical outdoor heat designconditions for a number of cities of Malaysia shown Figure 28. To determine the outdoor air load these expression shownbelow could be applied 12.(1)(2)(3)In above equations minimum latent heat load for air loadhas been calculated based on air load. As t
20、he density of peopleand appliances increase the fraction of latent load will beincreased. The regions that this ratio of further than 25% thisareas will satisfy the important variable and hybrid desiccantcooling systems over a conventional system in performanceand cost-benefits.1.$1.00 = 3.4 RM (Rin
21、git Malaysia)Figure 1 Hybrid desiccant system including heating and cooling options.HS1.944qdT=HL4.84qd=HLHt- 25% 2010, American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc. (www.ashrae.org). Published in ASHRAE Transactions 2010, Vol. 116, Part 1. For personal use only. Ad
22、ditional reproduction, distribution, or transmission in either print or digital form is not permitted without ASHRAEs prior written permission. 394 ASHRAE TransactionsTo have the latent load fraction of above ( 25% or 50%) we have:(4)(5)Choosing a point with conditions in the psychrometricchart as a
23、n example, 26C for indoor temperature and humid-ity ratio of 10.57 g/kg air2and drawing two lines with theabove slopes on the right hand side of the point upper zone ofthe line would be the zone we will consider:As discussed in the last part in order to find the properzones, the line of 50% latent l
24、oad over total loads must bedepicted and in the same way as previously explained.We can draw the line of 25% latent load ratio. Then thepsychrometric chart has been divided into two different zonesby these lines. By illustrating these lines on Figure 2, Figure 3 will becreated, which demonstrate tha
25、t all regions of Malaysia coun-try located in zone 2 and satisfy the key variable of high latentload ratio.(More than 50%), as shown in Figure 3.Site-Specification Condition StudiedMalaysia is an equatorial country and as a result has auniform temperature throughout the year. The variation intempera
26、ture annually is less than 12C except for hilly areasMalaysia 8. However the average temperature in a dayranges from 36 degree Celsius during the day to 24 degreeCelsius during the colder in the hilly regions of the country atsome days 8. Malaysia boasts of two district weatherpatterns, warm and hum
27、id. In the month of October climatesare hot and humid with average day temperature at 35 degreeCelsius and night temperature at 25 degree Celsius 8. Thehumidity in Malaysia is very high. The mean monthly relativehumidity falls within 70 to 90%, varying from place to place.The monthly relative humidi
28、ty varies from a minimum ofabout 3% to a maximum 15%in Malaysia 8. For example thecity of Kuantan located in state of Pahang has a high and drydesign point of 33.3 C and 31.9 g/kg air humidity ratio. Thisrepresents about 22% more humidity ratio than the peak dew-point design for the Mediterranean is
29、land of Mallorca, Spain. Fresh-Air Intakes and QualityA fraction of fresh air to total supply air basically dependon building types. According to ASHRAE for Ventilation toacceptable indoor air quality has number of requirements andrecommendations with which the HVAC industry is struggling2.g/kg air
30、= 6.99 grain/lb airFigure 2 Climate design conditions of Malaysian cities.HLHLHS+()HLHt- 25%:HLHS-13-ddT- 0.134HLHt- 50%:HLHS-13-ddT- 0.401 2010, American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc. (www.ashrae.org). Published in ASHRAE Transactions 2010, Vol. 116, Part 1.
31、 For personal use only. Additional reproduction, distribution, or transmission in either print or digital form is not permitted without ASHRAEs prior written permission. ASHRAE Transactions 395to comply 13. The primary prescription outline is to increasethe amount of fresh air intakes to occupied sp
32、aces. Table 1shows this outline for some spaces. Refrigeration-based cool-ing has a difficult time efficiently handling the High latentloads in fresh air over a large part of the country, especiallywith packaged equipment. That latent load, if not adequatelytreated, creates conditions where meld and
33、 mildew flourish.Major requirement and recommendation of ASHRAE newstandard affecting ventilation and humidity can be summa-rized as follows:1. Acceptable air quality to be achieved by providing venti-lation air of the specified quality and quantity to thespace. The specified quantity averages nearl
34、y 0.566 m3/min for person313. Compared with an average of 0.14m3/min/person in the previous standard.2. Humidity can support the growth of allergenic organismssuch as fungi, mycotoxins, and dust mites. Relativehumidity in habitable spaces should be the ranges 30% to60%. Humidity above 70% in spaces,
35、 ducts, or plenumscan allow fungal contamination to accrue 13, 14. The base desiccant system meets up these requirementswith lower costs to compete conventional system becausethere is typically little moisture on a post-desiccant coolingcoil, in the drain pan and the air distribution ducts. As a res
36、ult,special buildings that need a specific level of indoor humidity,3.m3/min = 35.3 ft3/minFigure 3 Selected zone cities for hybrid cooling.Table 1. Comparative Ventilation Standards ASHRAE 62-2005 and 62-1981 13ApplicationASHRAE 62-2005 m3/min/personASHRAE 62-1981m3/min/personHealth careDining room
37、CafeteriaKitchenConference roomHotel (bedroom)Theaters (audio)Theaters (lobby)0.7070.5660.5660.4240.5660.8490.4240.5660.1980.1980.1980.2830.1980.4240.1980.198 2010, American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc. (www.ashrae.org). Published in ASHRAE Transactions 2010
38、, Vol. 116, Part 1. For personal use only. Additional reproduction, distribution, or transmission in either print or digital form is not permitted without ASHRAEs prior written permission. 396 ASHRAE Transactionssuch as hotels, mosques and hospitals, satisfy the importantvariable of indoor air quali
39、ty.Availability of Return Air for Post CoolingBased on according ASHRAE 62-2005(new version ofASHRAE 62-1989) the amount of fresh air intakes increased,so the return air can be used in post-cooling coil to cool thesupply air (Figure 1) and so recover a part of sensible coolingenergy requirement. In
40、the previously mentioned buildingswhere fresh air requirements are high (greater than 20%)desiccant-based systems using a combination of fresh andexhaust air can be more cost-effective than others. Although insome buildings such as those with large smoking areas, usingexhaust air may not be easy due
41、 to the high contaminantcontent of the air, and so the exhaust air cannot be used forsensible heat recovery. In fact, this situation occurs for someof the buildings that have high fresh air ratios.RESULTSA study of site-specifications about application of hybriddesiccant system was conducted using k
42、ey variables, and thenusing with the climate conditions for a building consistentwith the typical architecture standard in Malaysia cities. Byanalyzing all of them, we concluded the following.Selected Regions for Using Base-Desiccant SystemIn the cities of highly humid areas, in zone 2 shown inFigur
43、e 3, nearly any building that requires air conditioningsystem is an excellent candidate for a desiccant-assistedsystem. In particular crowded buildings such as hospitals,mosques, hotels, etc that has a high potential for benefits fromthis system. System Performance and Energy SavingTo discuss the pe
44、rformance of the system we limit theanalysis to the most significant parameters, viz., temperature,and humidity ratio of air entering the evaporator. For simplic-ity of presentation, only results conducted to highly humidcities (zone 2). To check this correlation complete calculation,were made for s
45、ome building type in different cities as anexample, a typical layout is prepared with consider 30 m3/minflow rate for create comfort condition indoor dry bulb temper-ature 26C and specific humidity 10.57 g/kg air calculatedwith conventional coil design software for restaurant buildingswith the typic
46、al architectural standards has been modeled 15-17. The results show consistency of the selected zone withthe more detailed consideration and calculated latent loadfractions. These conclusions for some the cities in Figure 3 areshown in Table 2. The thermal loads of the building consist ofpeople, lig
47、hting and kitchen appliance having about 2.18 Kwof latent load and 7.52 Kw of total load. The selected buildinghas about 29% fraction of latent to total load, which by takinginto outdoor air conditions and taken from another heat source.Load calculation results of Table 2. Show that the buildingssat
48、isfied the important variable for cities in zone 2, whichconfirms the mentioned results.CONCLUSIONThis investigation considers the possibility of usinghybrid desiccant cooling system instead of conventionalcompression systems through some important variables.These variables can determine if a specia
49、l building in an areaneeds a more detailed consideration. Finally based on the information presented here, itappears that a high potential exists for using hybrid desiccantsystems in Malaysia. As below:a. All states of Malaysia have latent fraction of total loadmore than 50%. These building will satisfy the previousimportant variables.b. Low price of natural gas usage in hybrid system com-pare to High demand of electrical energy in compressionsystem.c. Availability of cheapest and clean t
