United Kingdom

a r t i c l

Building applicationsPhysical parameters

area, this paper presents and discusses physical and performance parameters of heat recovery unit and theeters on operation and efciency of the system. In addition, areas that have notntion and that warrant future analysis of this technology are also highlighted.

& 2013 Elsevier Ltd. All rights reserved.

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. . . . . . . . . . . . 181. . . . . . . . . . . . 182. . . . . . . . . . . . 183. . . . . . . . . . . . 183. . . . . . . . . . . . 183. . . . . . . . . . . . 184. . . . . . . . . . . . 185. . . . . . . . . . . . 186. . . . . . . . . . . . 187

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 187

Contents lists available at ScienceDirect

Renewable and Sustainable Energy Reviews

Renewable and Sustainable Energy Reviews 28 (2013) 174190E-mail addresses: mardianaidayu@usm.my, drmia707@gmail.com (A. Mardiana).1364-0321/$ - see front matter & 2013 Elsevier Ltd. All rights reserved.http://dx.doi.org/10.1016/j.rser.2013.07.016

n Corresponding author. Tel.: +604 653 2214; fax: +604 657 3678.2.3.5. Wick and n structures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2.3.6. Corrugated structure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

2.4. Congurations/ow arrangements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3. Performance parameters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

3.1. Heat and mass transfer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3.2. The effectivenessNTU method. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3.3. Effects of airow . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3.4. Effects of temperature, moisture and pressure drop . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

4. Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2.3.3. Fibre . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1802.3.4. Membrane-based . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 180

. . . . . . . . . . . . 181Efciency

Contents

1. Introduction . . . . . . . . . . . . . . . . .2. Physical parameters. . . . . . . . . . .

2.1. Size and heat transfer are2.2. Fans and ducting . . . . . . .2.3. Materials and structures .

2.3.1. Metal type . . . .2.3.2. Polymer-based .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 174

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 176. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 177. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 177. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 178. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 179. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 179Performance parameterKeywords:Heat or energy recovery

signicances of these paramreceived much research atteArticle history:Received 28 January 2013Received in revised form5 July 2013Accepted 14 July 2013

Owing to global energy crisis, various technical strategies are adopted for energy conservation in buildingsthrough energy-efcient technologies. One of the signicant ways for this purpose is by installation or usage ofheat or energy recovery device which is known as one of main energy-efcient systems that will decrease thepower demands of building heating, cooling, air conditioning and ventilation loads. In order to have an insightinto existing knowledge leading to understanding of previous works and researches carried out concerning theArchitecture and Built Environment, Faculty of Engineering, University of Nottingham, NG7 2RD University Park, Nottingham,

e i n f o a b s t r a c tReview on physical and performance parameters of heat recoverysystems for building applications

A. Mardiana a,n, S.B. Riffat b

a School of Industrial Technology, Universiti Sains Malaysia, 11800 USM, Pulau Pinang, Malaysiab Department ofjournal homepage: www.elsevier.com/locate/rser

1. Introduction standard, policies, regulations, recognition and new technologies.For instance, in the EU, the Energy Performance of Building

Nomenclature

CFD Computational Fluid DynamicsHi enthalpy of intake air, kg/kgHs enthalpy of supply air, kg/kgHr enthalpy of return air, kg/kgHe enthalpy of exhaust, kg/kgME mass ow rate of exhaust air stream, kg/sMS mass ow rate of supply air stream, kg/sMmin mass ow rate (minimum), kg/sNTU number of transfer unitsTi temperature of intake air in, 1CTr temperature of return air to heat or energy

recovery, 1C

Te temperature of exhaust air out, 1CTs temperature of supply air to room, 1C

Symbols

S sensible efciency/temperature efciency, %L latent efciency/moisture efciency, %H enthalpy efciency, %HR heat/energy recovery efciency, %NTU the effectivenessNTU methodi moisture/humidity ratio of intake air, kg/kgs moisture/humidity ratio of supply air, kg/kgr moisture/humidity ratio of supply return air, kg/kge moisture/humidity ratio of exhaust air, kg/kg

A. Mardiana, S.B. Riffat / Renewable and Sustainable Energy Reviews 28 (2013) 174190 175Over the past 30 years, the world has experienced largeincreases in energy consumption as a result of economic andpopulation growth. In the context of built environment, literaturehas proven that buildings are responsible for about 40% of nationalenergy demand in EU [1], 23% in Spain [2], 35.4% in Greece [3], 30%in China [4,5], 41% in US [6], 39% in UK [7,8], 2040% in developedcountries [2] and predicted to increase by 34% in the next 20 years[9]. In Singapore, the use of electricity in buildings constitutesaround 16% of national energy demand [10]. Energy demand isalso estimated to increase at the rate of 6.3 annually in Malaysia inorder to sustain the nation's economic growth [11]. This increasingtrend of energy consumption in buildings is contributed in largeproportion by building space heating, cooling and ventilation. TheInternational Energy Agency (IEA) estimated that by the year 2030,the consumption of energy sources will increase by 53%, whereby70% will be derived from developing countries [12]. In relation tothis, a study of World Energy Council (WEC) found that withoutany changes in our current practice, the world energy demand in2020 would be 5080% higher than 1990 levels [13].

In order to overcome energy consumption and at the same timeto promote energy conservation in buildings, most countries in theworld have shown their commitments by setting up new buildingFig. 1. Link between building and energy andDirective (EPBD) was adopted in December 2002 whilst in theUK, Building Regulations, a standard for limiting heat gains andlosses through elements and other parts of the building services,was developed in October 2010. In Japan, Basic Energy Plan (BEP)was adopted in June 2010 which represents the signicant state-ment of Japanese energy policy and energy crisis [14]. NationalEnergy Policy was also formulated in Malaysia in 1979 with theprincipal energy objectives to ensure efcient, secure and envir-onmentally sustainable supplies of energy, including electricity.Apart from that, in 2009 Malaysian Green Building Index (GBI) wasintroduced to promote green building design and sustainability inbuilt environment. These policies, standards and rating toolswould encourage the building design to adopt energy-efcientbuilding materials, technologies and at the same time ensure thatadequate means of ventilation are provided. Thus, in order tomaintain good and healthy indoor environment, provide a habi-table and comfortable place for human occupation as well asconserve the energy, energy-efcient technologies should beadopted in building services. Therefore, the challenge nowadaysis to develop an energy-efcient system which can make a bigcontribution to CO2 emission reduction, to fulll the building coderequirements and energy conservation either as an alternative tothe needs for heat or energy recovery.

the conventional systems or as an incorporating system to theexisting technologies. In this approach, one of the signicant waysis by installation or usage of heat or energy recovery device whichis known as one of the main energy-efcient systems that willdecrease the power demands of building heating, cooling, airconditioning and ventilation loads as illustrated in Fig. 1.

As stated by Shurcliff [15], heat or energy recovery is dened as adevice that removes in terms of extracts, recovers or salvages heat ormass or energy from one airstream and transfers it to anotherairstream and is classied into two major categories which aresensible heat recovery [16,17] and latent heat or enthalpy recovery[1820]. This means that the energy that would otherwise be lost isused to heat the incoming air, helping to maintain a comfortabletemperature in a building. In other words, this system introducesmechanisms to recover the coldness and/or hotness from theexhaust stale air via heat transfer surfaces with the induced fresh airduring the ventilation processes. There are many different types ofheat or energy recovery systems that are available for transferringenergy from the exhaust air to the supply air or vice versa for buildingapplications [2129] and lots of researches have been conducted inthis area to date [3034]. It is therefore vitally important to know andhave an in-depth understanding of this system and its mechanismtowards development of energy-efcient technologies. This paperaims to review the system in terms of its physical and performance

parameters and the signicances of these parameters on the opera-tion and reliability of the system in recovering heat or energy.

2. Physical parameters

Heat or energy recovery system has particular physical compo-nents, which makes it unique in its operation and behaviour.The physical conditions and characteristics of the elements thatcongure the heat recovery determine how it behaves in terms ofperformance. As discussed in [26], the whole typical heat recoverysystem is a system comprising of ductworks for inlets and outlets,fans and a core which is a heat exchanger. A basic physical modelof heat or energy recovery system as explained by Min and Su [35]is shown in Fig. 2.

Overall, we can differentiate a heat recovery system accordingto its type, size and conguration/ow arrangement as discussedin [28,3644]. Specically, each component of the heat recoverysystem has its own physical parameters that need to be consideredin designing the system. Recently, researchers have produceddevelopments of high interest in heat recovery technologies forbuilding applications. Lots of studies have been conducted toanalyse the application and design of the physical model of heatrecovery systems. This section highlights the physical parameters

OUTDOORAIR

INDOORAIR

A. Mardiana, S.B. Riffat / Renewable and Sustainable Energy Reviews 28 (2013) 174190176xf

EXHAUSTAIR

Membrane

Supply in

Computational domain

Exhaust in

Fig. 2. Basic physical model of heat or energy recovery.

Source: [35].xd

zyf y

o

SUPPLYAIR

Exhaust out

Supply out

A. Mardiana, S.B. Riffat / Renewable and Sustainable Energy Reviews 28 (2013) 174190 177of heat recovery technologies and their signicances on theoperation and efciency of the systems.

2.1. Size and heat transfer area

The heart of heat recovery system is heat exchanger which isthe core or a matrix containing the heat transfer area. This heattransfer area is an area of the exchanger core that is in directcontact with uids and through which heat or energy is trans-ferred. Thus, physical sizes in terms of length, width and height aswell as surface area of heat exchanger used in heat recoverysystem are crucial to the overall efciency and cost of the system.Generally, by increasing the heat transfer area or size of heatexchanger, the efciency would also increase but this will add tobulk and cost of the equipment. In order to avoid those weak-nesses, optimisation of heat exchanger size for heat recoveryapplication is extremely important. In the literature, several workshave been conducted on heat recovery core or heat exchanger[4548] including efforts on the optimisation of heat exchangersfor various heat recovery types [49,50]. For instance, Soylemeyz[51] conducted an optimisation analysis of heat transfer area forthree different unmixed type heat exchangers, i.e. counter-ow,parallel-ow and single uid. In the study, he developed a thermo-economic optimisation analysis for estimating optimum heattransfer area for heat or energy recovery applications. The resultsillustrated that heat transfer area or size of the heat exchangeraffected the effectiveness of heat recovery, which is an importantperformance indicator of heat or energy recovery system. Scien-tically, in the study it was proven that the effectiveness of heatrecovery increased as the area increased for three differentunmixed type heat exchangers. This was also agreed by Manzand Huber [52] in their study that the area in terms of length ofheat exchanger had an impact on the effectiveness (temperatureefciency), whereas in the context of heat pipe recovery unit, sizeis termed by the number of rows and it has been proven that asthe number of row increases, the effectiveness would also increase[5356]. The logical explanation for this phenomenon is that, withincreasing number of rows, the overall heat transfer area isincreased, thus increasing the heat transfer between the airowand heat pipes as stated in Ref. [54]. Meanwhile, in rotary heatrecovery, the concern is addressed on the optimum rotary speed tomaximise the heat transfer rate per unit of area. Effects of channelwall thickness on heat and moisture transfer were discussed inRef. [57]. It was proved that there was a strong inuence of wallthickness on the system performance and the optimum rotaryspeed where the higher the thickness, the lower the optimumspeed. Numerical optimisation of rotary heat exchanger was thenpresented in a later study by Dalkilic et al. [58] with the objectiveto maximise the heat transfer rate of frontal surface area of rotaryheat exchanger. Results proved that the ow with the highest heatcapacity rate occupied a smaller frontal area. With the capability totransfer both heat and moisture and used for enthalpy recovery[40] nowadays rotary heat exchanger has become one of thesuccessful alternatives to the conventional mechanical dehumidi-cation system for building applications [59].

Besides system efciency, economy and optimum energy sav-ing are vitally important in considering heat recovery system forbuilding applications [60]. An investigation by Soylemeyz [40]highlighted that the initial and operational cost and net saving ofthe heat recovery system mainly depend on the size including thedimension of the heat exchanger itself and this indirectly reectsthe economics of the heat recovery system. Adamski [61,62]estimated the nancial effect due to the use of the heat recoverysystem instead of a simple ventilation system. On the other hand,Nasif et al. [53] discussed the effect of varying the heat exchanger

face area on energy consumption where Kuala Lumpur, Malaysia,weather data were used as a benchmark. The results indicated thatas the heat transfer area of exchanger increased, the amount ofsaved energy increased. Dalkilic et al. [58] approached a new wayfor determining the area and type of the most appropriate heatrecovery core for maximum net gain which was based on a newmodel. By using the model, they found that the best heatexchanger type and its area can be determined by comparing netgains or effectiveness of heat exchangers at maximum NTU.

Recently, to increase heat transfer area, appendages known asns have created much attention among researchers in this eldwhich can be intimately connected to the primary surface toprovide extended surface of a heat exchanger [63]. The additionof ns reduces thermal resistance on uid side and therebyincreases net heat transfer from the surface. In relation to this,Pongsoi et al. [49], in their work, conducted several experimentson the optimised n-pitch having a two-row conguration with asize range of 2.46.5 mm. From the results, it can be observed thatthe convective heat transfer coefcient for a n-pitch of 2.4 mmwas relatively low compared with that of other n-pitches withthe same air frontal velocity. This shows that n-pitch with biggersize gives higher convective heat transfer coefcient whichstrongly inuences the efciency of heat recovery system ingeneral. On the other hand, Tang et al. [64] investigated the air-side heat transfer of ve kinds of ns through both experimentaland numerical investigations. Their results indicated that differentns gave different pressure drops and provided different air-sideheat transfer performances. Both of these optimised ndings arevaluable and useful when designing physical model of heat orenergy recovery system for building applications.

From the literature, it can be seen that various researches havebeen conducted in the eld of heat recovery since the 1980s [24]in relation to the application, construction, design and efciency ofthe systems. Previous researchers have extensively observed theeffects of heat transfer area on the effectiveness of various types ofheat recovery systems. More recently, major research and devel-opment directed at optimisation of heat transfer area have beeninitiated in order to obtain optimum heat exchanger area for heator energy recovery applications. Although a great deal of optimi-sation work has been performed, a gap still exists betweenresearch results and practical application. Thus, more future worksshould be established in this area by taking into account variousoperating parameters for different climate zones and economicanalysis.

2.2. Fans and ducting

Besides the heat exchanger or the core, when designing a heatrecovery system, it is also important to take into consideration thesize of fan with high energy efciency as well as the ductingcharacteristics. In relation to this, an earlier study by Routlet et al.[65] investigated global fan efciency of heat recovery unit as afunction of measured power as shown in Fig. 3 and found thatthere were signicant differences between fans within the samepower class used for heat recovery system. These results were alsosupported by a later research work conducted by Min and Su [35]which found that larger fan power leads to a larger total heattransfer rate, with the maximum total heat transfer rate occurringat a smaller channel height.

Of late, Laverge and Janssen [66] stated that the electric load forfan operation in the heat recovery system used for buildingventilation is highly dependent on fan and ducting characteristics.They had conducted a study on primary energy and exergyframework of mechanical ventilation and heat recovery systemfor different climates in Europe. From their results, it was foundthat only low specic fan power would make the heat recovery

system advantageous if operating energy is concerned. In support

A. Mardiana, S.B. Riffat / Renewable and Sustainable Energy Reviews 28 (2013) 174190178of this hypothesis, El-Fouih et al. [33], on the other hand, hadcarried out a sensitivity analysis by simulation in their study toinvestigate the inuence of exchanger efciency and specic fanpower on the global energy performance of heat recovery systemfor different types of low energy buildings and different Frenchclimate zones. They had analysed the exchanger efciency andspecic fan power by means of a parametric study to test theperformance of heat recovery system in function of design options.From the results of the simulation, it was concluded that the fanpower of heat recovery system was inuenced by three factors:(i) the overall efciency of the fan; (ii) the resistances in thesystem (pressure drop); and (iii) the air ow velocity through theunit and ductwork. Aside from various studies for climatic condi-tions in Europe, there are also several papers that discuss aboutthe performance of heat recovery system and its physical char-acteristics with regard to primary energy implications in otherdifferent climatic conditions [32,67]. In relation to this, Kang et al.[68] carried out an investigation on heat recovery systems with avariable rotational speed fresh air fan to introduce fresh air intothe buildings during winter in four cities in different climate zonesof China. Their results showed that a variable rotational speed fanwith COP value of 2.5 was able to still operate heat recoverysystem at optimum efciency and maximum energy saving inmost of the selected cities. From these studies, it can be seen thatthe value of fan COP, which is dened by the ratio of recoveredheat and used electrical power indirectly represents the perfor-mance of a heat recovery system. In relation to this, Martinez et al.[69] performed an investigation on the COP of a mixed energyrecovery by considering fan energy consumption in order to dene

Fig. 3. Global fan efciency of heat recovery unit.Source: [65].the relationship between recovered heat ow and energy con-sumption of energy or heat recovery system. Their results showedthat using the COP characteristic as an integrating factor, theoptimum results for the mixed recovery system were obtained forthe condition of maximum outdoor air temperature with COPvalue of 9.83.

Furthermore, depending on the size of fan, the size of duct interms of diameter can be determined for this system for the casewhere installation of the fan is in the ductwork. As suggested byShurcliff [15], ducts should be short and free of sharp bendsas feasible and of ample diameter in order that the pneumaticresistance will be low and fans or blowers of small, low power,quiet type will sufce. Usually, the air owing in the ducts isturbulent and the duct length ranges from 3 to 18 m and ductdiameters of 0.10.2 m are satisfactory, producing pressure dropsat a low ow rate [15]. Besides, duct structure and material willalso inuence ow distribution and are substantial in order tominimise heat loss from the duct system. Otherwise, the heat losscan reduce total efciency of the system signicantly. In addition,duct cross-sections are the predominant factor inuencing pres-sure drop and heat transfer coefcient [70].

In the literature, numerous duct cross-section investigationsrelated to physical characteristics of heat exchanger and heattransfer can be found either experimentally or numerically [7174].A study of a prototype unit of duct/heat exchanger for buildingventilation was conducted by Manz and Huber [75] by measuringairow rates, temperatures, air humidity and pressure differences.In the study, the duct/heat exchanger unit for building ventilationwas designed to unite two functions in one device: uid transportand heat recovery. Based on the results, it was observed that usingthis concept it was possible to realise a heat recovery unit with high-efciency heat exchange, with temperature efciency of 0.7 at a ductlength of 6 m. Besides, uid-to-uid heat transfer in the prototypeunit has to be improved by additional ns in the ducting system.Nonetheless, the success of the concept would not only dependon thermodynamic aspects of the duct/heat exchanger unit, but verymuch on other aspects such as cost-effective production, easyintegration into a building and user aspects. In another study,Fernndez-Seara et al. [76], quite recently, conducted experimentalanalyses of an air-to-air heat recovery unit equipped with a sensiblepolymer plate heat exchanger which was arranged in a paralleltriangular duct for balanced ventilation systems in residential build-ings to investigate the inuence of changing the operating conditionson the heat exchanger performance. The experimental resultsindicated that the heat transfer rate in the heat exchanger in paralleltriangular duct decreased almost linearly as the inlet fresh airtemperature increased.

These days, obtaining enhanced heat transfer and reducingequipment cost are two major concerns that need to be satisedwhen designing a heat or energy recovery system. To fulll thisdemand, Wu et al. [77] presented exergo-economic performancecomparison between enhanced heat transfer duct and referencesmooth duct subjected to constant wall temperature under variousdesign constraints. From their results, it can be seen that theenhanced duct posed the greatest possibility to be superior to thereference smooth duct under the equivalent pressure drop ratherthan the other two constraints. More recently, an experimentalinvestigation to determine optimum values of the design para-meters of heat exchanger and ducting was described by Taguchimethods in [78].

Since the fan power will typically increase with higher heatrecovery efciency, in order to obtain a system operating at theoptimum energy saving, a more complete feasibility evaluation fora specic study should be conducted based on system character-istics and climate data. In addition, the choice between thedifferent physical systems (fans, ducting, lters etc.) includingoperating costs, building specic characteristics, maintenancecosts and component replacement should be considered. Thusfor future works, in designing a heat recovery system for buildingapplications, fans and ducting characteristics should be balancedwith operating costs and building energy saving.

2.3. Materials and structures

Materials and structures of heat or energy recovery core haveevolved along with heat transfer technologies over the pastdecades. From the engineering approach, materials and structuresare selected by how they perform because this will bring somesignicant effects to their surroundings or how they physicallyresist the inuence of the surroundings. For instance, capillaryforces or porosity and pore diameter of the materials playsignicant roles in moisture transfer. Higher porosity materialcan hold more transfer and bigger pore diameter has lower massow resistance [79]. Besides, thickness of the heat or mass transfer

surface heavily affects the heat and mass transfer resistance. On

amount of research is still required into material properties andlife-time behaviour. With regard to this, a recent experimentalstudy was conducted by Gendebien et al. [82] on a polystyrene air-to-air heat exchanger with the purpose of gathering performancepoints. From their work, it was stated that the main disadvantageof polystyrene heat exchangers is related to their low thermalconductivity. This weakness conversely can be counter-balancedby the high enlargement factor which could be attained withpolystyrene heat exchangers compared to traditional plate heatexchangers made of metal. Since this new approach is quite new inthe eld of heat or energy recovery application, further studiesshould be conducted in this area to investigate design parameters,

Se pa

rator

Fig. 5. Horizontal cross-section of the exchanger.Source: [84].

A. Mardiana, S.B. Riffat / Renewable and Sustainable Energy Reviews 28 (2013) 174190 179the other hand, durability of material is another important part toevaluate the optimisation and economic performance of the heatrecovery. Up to now, we can observe that lots of research anddevelopment efforts on material and structure of heat exchangeror heat recovery core have been performed widely. This sectionpresents several types of material and structure which are com-monly used in heat exchanger or recovery core design.

2.3.1. Metal typeIn the earlier studies, heat exchanger and recovery applications

rely heavily on n-and-tube or plate-n heat exchanger designs,often constructed using metal type such as copper, aluminium orsteel. Using these materials, only sensible heat can be transferredfrom one stream to other stream neglecting the latent heat. Inaddition, these materials also have less porosity to retain thecondensed moisture from the hot and humid air in heat pipeexchangers. To compete with current market needs, enhancementof the existing metal type should be executed. Thus, to increasethe transfer surface, porous structure is considered to replace thesmooth surface of the sheet or tube [80] such as wicked metal,foams or wools or ns. From this point of view, Manz and Huber[52] had carried out experimental and numerical investigations onheat recovery (duct/heat exchanger) system by adding aluminiumns on the heat transfer surfaces as shown in Fig. 4. The resultsshowed that by adding the aluminium ns, heat recovery up to70% at a duct/heat exchanger length of 6 m could be achieved forbuilding ventilation. In another study, the heat pipe recoveryconsists of 25 copper tubes with the evaporator and condensersections were nned with 50 square aluminium sheets of 0.5 mmthickness were used in research work by El-Baky and Mohamed [22].These data denote that aluminium ns are effective transporters ofheat from the extract airow to the supply airow of heat recoverysystem. From the literature, it can be concluded that the operating

Fig. 4. Segment of an extruded aluminium prole used as a combined duct andheat exchanger.Source: [52].limitations of metal type materials in some applications have createdthe need to develop appropriate structures to increase heat and masstransfer as well as efciency of exchanger for heat recovery purpose.

2.3.2. Polymer-basedMuch of the initial attention in the growth of polymer-based

material of heat exchanger or recovery core was motivated by theircapability to deal with various uids, their resistance to foulingand corrosion, and their potential use in integrated heat or energyrecovery with humidication and/or dehumidication systems.A thorough review on polymer heat exchangers for heating,ventilation, air-conditioning and recovery has been discussed inT'joen et al. [81]. From the review it can be said that polymermatrix composites do hold promise for use in the design andconstruction of heat exchangers in heating, ventilation, air-conditioning and recovery applications; however a substantialSection 1

Insulation

Section 2

Zoom

Inlet airExtracted airInlet air

Polycarbonate plate

5mm

5m

m4

mmthermal performance, effectiveness, heat transfer mechanism andeffects of operating parameters.

In addition, another famous polymer-based material used inthe development of heat exchanger is polycarbonate which is aparticular group of thermoplastic polymers that can be easilyworked, moulded and thermoformed. Looking at its properties,polycarbonate has good chemical resistance to acids but poorresistance to alkalis and solvents and is able to prevent corrosion.It normally has service temperature ranges from 4 to 135 1C.Because of its low thermal conductivity around 0.190.22 W/mK, itnds many applications and it does hold promise for use in theconstruction of heat exchangers in HVAC and heat recoveryapplications [81]. For instance, one earlier study of polycarbonateplates in heat exchanger was performed by Kho et al. [83]. Then,after a decade from that study, Kragh et al. [84] conductedperformance investigations of a polycarbonate xed-plate heatexchanger with 0.5 wall thicknesses as shown in Fig. 5 for heat

A. Mardiana, S.B. Riffat / Renewable and Sustainable Energy Reviews 28 (2013) 174190180recovery of mechanical ventilation in arctic climates. Even thoughpolycarbonate plates do have potential in heat or energy recoveryapplication, only few studies can be found in this eld in theliterature. Thus, more fundamental investigations should be estab-lished in this area including numerical analyses, simulation studiesas well as experimental works. As a whole, it can be seen thatthrough careful design modication, polymer-based materialshave a bright future in heat recovery applications. Thus, futureworks should look into the emerging technologies of nanoscalepolymer-based materials for heat and/or mass transfer.

2.3.3. FibreFibre is one of the common hydrophilic materials that are able

to transfer both heat and moisture effectively. Besides its hydro-philic characteristics, it also has lower thermal conductivity. Fromprevious studies, it was found that bres have much lowerthermal conductivity than metals, ranging from 0.01 to 0.3 W/mK [85,86]. In addition, the pore size of various bres is thinenough to carry the heat or mass transfer with less resistance andhas strong absorption ability [87]. However, in terms of hardness,most bre materials are not strong enough to be used asexchanger plates and the lifespan of the bre exchanger is shortas it is easy to be deformed or damaged when being soaked withwater [88].

With regard to the above factors, Liu et al. [89] performed acomparative study of hydrophilic materials including bres,metals, ceramics, zeolite and carbons for air-to-air heat and massexchanger. From the study, it was indicated that bres and carbonswere suitable materials for making heat and mass transfer surfacesdue to their higher performances. In another study, Dallaire et al.[90] presented a conceptual optimisation of bre porous core ofrotary wheel recovery and they found that the performance ofrotary wheel recovery could be drastically improved by properlyselecting its thickness and the porosity of the internal thermalmass. On the other hand, an analytical solution for heat masstransfer of heat or energy recovery was studied by Zhang [91]using a hollow bre with a diameter of 13 mm and contact areabetween the two air streams of 1000 m2/m3. Results of the studyshowed that using this bre, the heat and moisture exchangeeffectiveness has the potential to attract commercial interest. Sincebres have the potentials as heat and mass transfer surfaces forheat or energy recovery application, the requirements for eco-nomic design, optimisation of operating conditions and enhance-ment of heat transfer performance investigations are essential forfurther development of this material. Numerical and experimentalinvestigations as well as CFD simulation involving relevant vari-ables that affect the performance should be established in thefuture works.

2.3.4. Membrane-basedNowadays, membrane-based heat exchangers constitute a

competitive technology in air-to-air recovery systems mainly dueto their high capability as an enthalpy recovery that can simulta-neously transfer both sensible and latent heat when compared toother materials [9295]. Heat exchanger made of vapour perme-able membrane such as treated paper or new micro-porouspolymeric membrane has the capabilities to transfer both heatand moisture from one airstream to the other, thus providing totalenergy exchange [35]. Due to these advantages, membrane-basedmaterials have attracted much attention in recent years in thedevelopment of heat exchanger for various types of heat or energyrecovery [96]. The membrane-based materials can economicallyachieve high sensible heat recovery and high total energy effec-tiveness because they have only a primary heat transfer surface

area separating the air streams and are therefore not inhibited bythe additional secondary resistance inherent in other exchangertypes. There are many studies that have been presented in thiseld over the last decade. Zhang [96] presented a thorough reviewwhich covered fundamental and engineering approach on theprogress of heat and moisture recovery with membranes. Niu andZhang [97] performed a theoretical model to evaluate the perfor-mance of a membrane-based energy recovery ventilator andinvestigated the effects of the entrance states of the two air-streams. Zhang and Niu [93] then further developed performancecorrelations for quick estimation of enthalpy effectiveness of amembrane-based energy recovery ventilator. Min and Su [35]carried out a numerical investigation to study the effects ofmembrane spacing and membrane thickness. The results indicatedthat it was necessary to use a thin membrane to obtain a goodperformance of heat or energy recovery ventilator. Most recently,Woods and Kozubal et al. [94] investigated various support spacersfor airow through membrane-bound channels in energy recoveryventilators to enhance heat and mass transfer.

Due to low latent effectiveness of current enthalpy recoverycore [92,98] novel material, the composite supported liquidmembrane (CSLM) which used a liquid membrane to selectivelytransfer moisture was proposed by Zhang [99] in his study. Insolving the weaknesses of traditional hygroscopic paper as plateand n materials, CSLM was used as the plate material. Compara-tive study was also made between paper-n and paper-plate, andpaper-n and membrane-plate. Results indicated that the latenteffectiveness of the membrane-plate core is 60% higher than thetraditional paper-n and paper-plate core, due to high moisturediffusivity in the CLSM. On the other hand, Zhang and Jiang [100]conducted a study to compare the performance of different corematerials and ow arrangements of heat or energy recoveryventilator. Sensible, paper and membrane core with cross andcounter-ow were compared and results indicated that thesensible core has the lowest enthalpy effectiveness, whilemembrane-based core with counter ow arrangement has thebest performance. Moreover, a study on steady-state performanceof a run-around membrane energy exchanger (RAMEE) for a widerange of outdoor air conditions was presented in Ref. [101]. It wasobserved that the effectiveness values were signicantly depen-dent on outdoor conditions which results in some effectivenessvalues exceeding 100% or being less than 0% for several of theoutdoor air conditions investigated. Taking into account the resultsin Ref. [101] and as an enhancement of it, Ge et al. [102] thendeveloped an analytical model for optimisation studies for exchan-ger size and solution ow rate RAMEE. Results showed thatoptimisation control of the solution ow rate could enhanceannual energy recovery rate up to 7%. Up to date, new approachesfor better-designed studies of membrane-based technologies aretaking place worldwide. For instance, poly (vinyl chloride)/sodiummontmorillonite (Na+-MMT) hybrid membranes with varied Na+-MMT content were introduced by Liu et al. 2013 using castingprocesses for potential use in total heat recovery ventilationsystems. Even though this hybrid membrane has high potentialapplications in heat or energy recovery system, this material still hassome defects such as lower water vapour permeability, moisture andenthalpy exchange effectiveness compared to polymeric ionic orcellulose membranes. Therefore further works and deep research inimprovement of correlative properties of this material need to becarried out for different types of heat or energy recovery system.From the existing literature, it can be concluded that lots of funda-mental works have been carried out. Thus to balance this situation, inthe future more cost-effective membranes for energy recovery shouldbe developed with the direction for engineering and commercialapplications. Further works should also include studies on theinuences of membrane conguration and material properties on

the effectiveness, optimisation of heat or energy recovery

result in a larger surface area and would increase the effectiveness

force the ow in the plate channels to experience a continuous

A. Mardiana, S.B. Riffat / Renewable and Sustainable Energy Reviews 28 (2013) 174190 181as stated by Yau [54]. Fig. 6 shows the wick structure with heatand uid ow paths in the evaporation surface studied in Ref.[105] where this design was used to increase the performance ofperformances and full-scale experimental measurements from thereal membrane-based heat or energy recovery system.

2.3.5. Wick and n structuresIn heat exchanger design for heat or energy recovery applica-

tions, heat transfer enhancement is one of the major considera-tions that attracts much attention these days. For this purpose,wick is one of the structures that could increase the heat or masstransfer area and at the same time increase the capillary force ofheat exchanger [103]. Wick porosities are enough to contain thecondensed moisture in heat pipe exchanger [104]. From this pointof view, numerous investigations have been performed with theinnovating wick structure in heat pipe exchanger technology inthe past. The present wick structure in heat pipe exchanger would

Fig. 6. Schematic of the wick geometry in the evaporator of heat pipe.Source: [105].heat pipes. Furthermore, a numerical model for transport in atheat pipe exchanger considering wick microstructure effects wasstudied by Ranjan et al. [106] using the three-dimensional model.Results showed that the inuence of the wick microstructure onevaporation and condensation mass uxes at the liquidvapourinterface of heat pipe exchanger was accounted for by integratinga microstructure-level evaporation model (micromodel) with thedevice-level model (macromodel). On the other hand, an experi-mental investigation on capillary pumped loop with the mesheswick for heat recovery applications in the eld of air-conditioningwas reported in Ref. [104]. From the results, it was found that theoptimal charging rate could be obtained in the range of 7076% forthe given experimental conditions of the proposed capillarypumped loop with the meshes wick and that it was able toperform the heat recovery applications.

Similar to wick, n structure is attached to the primary surfaceon one or both uid sides of heat exchanger to increase the surfacearea and consequently to increase the total heat transfer rate[94,17,107,108]. As a result of this, recently, lots of studies can befound related to n structure of heat exchanger in the literature[109]. For instance, a study by Zhang [63] proved that theenhanced heat transfer with ns was useful to overcome majorresistance of heat transfer in plate. Recently, Mehrizi et al. [110]highlighted the heat transfer enhancement in a ventilated porouschange in direction and ow area [83]. The benets of this geometryare that they have efcient heat exchange capabilities and strongmechanical strength, evenwith very thin material wall thickness. Thestructure also gives better heat mass transfer [113]. There are a lot ofstudies in literature that have been conducted to investigate theperformance of corrugated structures of heat exchanger's channelsuch as [114120]. Islamoglu and Parmaksizoglu [121] suggested thatin order to achieve enhanced heat transfer, corrugated structureshould be applied in the construction of heat exchanger's channels asshown in Fig.7. Besides, in corrugated structure of heat exchangersand to achieve enhanced heat transfer, various factors such as platethickness, geometry, plate spacing and plate height that affected thethermal performance need to be considered in the elementary heatexchanger design theory. From this point of view, much attention hasbeen paid concerning this recently. For instance, Doo et al. [123] intheir study had used CFD to perform a quantitative assessment of thethermal performance of a cross-corrugated heat exchanger includingthe longitudinal heat conduction effect for various design optionsincluding different plate thicknesses and corrugation geometries fora typical operating condition. Qi et al. [124] proved that thermalperformance of the heat exchanger with corrugated louvered nswas affected signicantly by ow depth, ratio of n pitch and nthickness and the number of the louvers for an optimum design of aheat exchanger. Zhang et al. [125] conducted experimental andtheoretical investigations of uid-side fouling which have beenperformed inside four corrugated plate heat exchangers with differ-ent geometric parameters, such as plate height, plate spacing, andplate angle.

Recently, Woods and Kozubal [122] presented a study on thetheoretical pressure drop and heat transfer for an open channelmedia plate heat exchanger in their works. In addition, effects ofn positions on streamlines, temperature contours, average Nus-selt number and outlet mean temperature were also investigated.It was signicantly found that the n position showed a mean-ingful effect on Nusselt number.

Fins can be of a variety of geometries such as plate-n and tube-n and amongst all types of plate-ns, a cross-ow plate-nstructure is the most popular arrangement for the exchanger coredue to its compactness and high mechanical strength evenwith verysmall channel wall thickness. Since cross-ow plate n is widelyused in the development of heat exchanger, it was optimallydesigned using multi-objective optimisation technique by Sanayeand Hajabdollahi [111] in their study which also included asensitivity analysis. The design parameters and n characteristicswere selected such as n pitch, n height, n offset length, coldstream ow length, no-ow length and hot stream ow length. Theresults revealed that n pitch, n height, n offset length, hot streamow length and cold stream length (in small effectiveness values)were found to be important design parameters in heat exchangers.

Based on the reviewed literature, it can be concluded thatgeometries play an important role in heat transfer for heat or energyrecovery purpose. Fluid-to-uid heat transfer in the heat exchangerhas improved by additional wicks or ns; however variable wick orn thicknesses do not substantially improve heat transfer. Besides,to fulll the better-designed studies of wick and n structures forenergy recovery applications, CFD simulation studies that have beenproven as one of the effective approaches for detailed design shouldbe established which could offer a very detailed solution containinglocal values of all relevant variables that are difcult to measure.

2.3.6. Corrugated structureCorrugated structures are basic channels construction in xed-

plate exchangers of the heat recovery system [112]. The corrugationsand for simple triangular corrugation (or plan-n) spacers made of

the superior effectiveness of counter-ow over the co-current

A. Mardiana, S.B. Riffat / Renewable and Sustainable Energy Reviews 28 (2013) 174190182polypropylene, which is common in heat exchangers and heatrecovery systems. In contrast, the effects of channel spacing andphase shift variations on heat transfer and pressure drop incorrugated channels were discussed in Ref. [126] and results ofcorrugated channels ow showed a signicant heat transferenhancement accompanied by increased pressure drop penalty.Afterwards, Gendebien et al. [82] investigated an air-to-air heatrecovery in cross-counterow arrangement made of several cor-rugated plates in synthetic material in which the study employed

Fig. 7. A schematic representation of corrugated structure channel.Source: [121].the same ratio of maximum to minimum air ow rate as the onepresented by Fernandez-Seara et al. in Ref. [76].

Many theoretical and experimental studies are available inthe literature on the corrugated structures. In addition, there arefew works reporting on simple geometries of corrugated struc-tures and heat transfer characteristics under different conditions.Further works on more complex corrugation structures should becarried out to better evaluate the convenience of assigning thisstructure to the heat transfer surfaces. Such studies are useful inunderstanding and modelling the performance of heat exchangersfor heat or energy recovery applications.

2.4. Congurations/ow arrangements

From the literature, it is believed and proved that ow arrange-ments have direct impact on heat exchanger effectiveness. In addi-tion, heat transfer efciency is maximised if the two airstreams haveopposite directions in airow arrangement [15]. There are numerouspossibilities that exist for airow arrangement in heat exchanger forheat recovery system. It is well known that effectiveness of a cross-ow heat exchanger is 10% less than that of a counter-ow heatexchanger and the maximum effectiveness is conned to 0.8% [26].Several research works can be found in the literature from overtwo decades ago regarding ow arrangement or conguration ofheat exchanger or energy recovery core for building applications[41,26,74,84,127,128].

In the earlier study, Kandlikar and Shah [129] analysed severalusual congurations and presented guidelines for selecting appro-priate ow arrangement from among those considered. It wasveried that in most cases symmetric congurations with counter-ow yielded the highest effectiveness. From that point of view,recently Yaici et al. [130] presented a detailed numerical analysisof heat and membrane-based energy recovery ventilators usingCFD to investigate the thermal performance of co-current andcounter-ow designs under typical summer/winter Canadianconditions. It was found that the numerical results conrmed

Fig. 8. Schematic of a quasi-counter ow parallel-plates total heat exchanger.Source: [91].ow. These days, major researches directed at optimisation andadvanced ow arrangements as these factors are signicant instructural design and performance of heat exchanger. For instance,optimisation of the xed-plate heat exchanger ow arrangementto yield a minimum annual operating cost was studied byJarzebski and Wardas-Koziel [131]. Pinto and Gut [132] presentedan optimisation method for determining the best ow congura-tion. In relation with advanced ow arrangement, Zhang [91] haddeveloped a mathematical model and experiments of a owarrangement called quasi-counter ow arrangement (Fig. 8). Inthe study, he investigated the performance of the system andfound that the effectiveness of the arrangement lay between thosefor cross-ow and counter-ow arrangements. In addition, thecomparisons of ow arrangements in case of effectiveness werealso studied and it was generally shown that sensible and latenteffectiveness values were improved by 5% in quasi-counter owarrangement. On the other hand, Nasif et al. [53] conducted aninvestigation of advanced Z-ow conguration heat exchanger whichprovides counter-ow arrangement over most of the transfer surfaceto maximise heat and moisture transfer. Afterwards, Al-Waked et al.[133] developed a CFD model which supports conjugate heat andmass transfer problem across the membrane of air-to-air energyrecovery heat exchangers that use cross-counter-ow (hybrid-ow)conguration.

From all of these studies, it was proved that the effectiveness ofexchanger depends heavily on airow congurations, conditionsand patterns of the supply and exhaust air streams. In addition, thesearch for new heat exchangers congurations (hybrid or quasiairow conguration) for heat or energy recovery application

order to have an in-depth understanding about the system, this

spacers. For future work, the experimental results from this studycan be used to validate a CFD model, which can then be used toinvestigate spacer-lled channel designs.

3.2. The effectivenessNTU method

A second denition of efciency was originally made by Nusselt[137], called number of transfer units (NTU). Method of NTU isbased on the fact that the inlet or exit temperature change(difference) of a heat exchanger or heat recovery core is a functionof NTU and capacity rate. The NTU is a non-dimensional expres-sion which depends on the heat transfer area and the overall

A. Mardiana, S.B. Riffat / Renewable and Sustainable Energy Reviews 28 (2013) 174190 183section presents previous and current works conducted in thisarea which is related to the performance parameters of heatrecovery, their impacts on the system and comparisons to andbetween previous research and development in this area to date,seems to be a very important aspect and future works should becarried out including theoretical, experimental, optimisation, andpractical applications as well as optimisation studies.

3. Performance parameters

This section comprises proven techniques of mentoring theperformance of heat or energy recovery system in relation to itsefciency or effectiveness from existing established data andprevious works in the literature. Efciency or effectiveness of heatrecovery system is usually dened with respect to balancedairows and the performance of this unit can be studied by asimple mass and energy balance. According to ASHRAE Standard[134], sensible efciency is expressed by the temperature ratio ortemperature efciency when mass ow rates of supply andexhaust air are equal as shown in

S TsTiTrTi

TrTeTrTi S;S0 S;E0 S 1

Sensible heat recovery is used where heat transfer withoutmoisture is desired. Enthalpy recovery such as rotary wheel,xed-plate with membrane based is used to transfer both moist-ure and heat between air streams [134]. Moisture/latent efciencyis dened as

L MS0 siMminri

ME0 reMminri

2

When mass ow rates of supply and exhaust air are equal,moisture efciency is calculated based on the amount of moisturetransferred and is presented by

L siri

3

Enthalpy efciency or energy efciency can be dened as

H MS0 HsHiMminHrHi

ME0 HrHeMminHrHi

4

Another way to calculate efciency was introduced in Ref. [65]by conducting measurements of the heat recovery systemas shown in Fig. 9. In the study, they suggested a global efcie-ncy of heat recovery which depends signicantly on the airin-ltration and ex-ltration. Based on their study, it was foundthat by taking the in-ltration and ex-ltration rate, the globalheat recovery efciency was between 60% and 70% for units having80% nominal heat recovery efciency. However, the results of theirstudy were only limited to conditions in Switzerland and Germanyand thus, more measurements should be conducted in othercountries or different climatic conditions in order to get similarinformation. Afterwards, Jokisalo et al. [135] carried out a study toinvestigate relation between air tightness of a building envelope,inltration, and energy use of a typical modern Finnish detachedhouse in the cold climate of Finland using a simple adapted IDA-ICE simulation model and it was found that the average inltrationrate and heat energy use increase almost linearly with the buildingleakage rate n50 in the typical Finnish detached house. From thesestudies, it can be seen that in-ltration and ex-ltration havesignicant impact on global efciency of heat or energy recoverysystem for building applications. In addition, more studies shouldbe expanded in this area and taking into account the leakagedistribution as it has a signicant effect on the inltration rate. Inwhich includes heat and mass transfer, the effectivenessNTUmethod, effects of airow, effects of temperature, moisture andpressure drop.

3.1. Heat and mass transfer

There have been numerous investigations of heat and/or masstransfer in heat exchanger for heat or energy recovery application[74,112,100,99,136]. Quite recently, Zhang [91] conducted heat andmass transfer studies of a membrane-based total heat exchangerfor heat recovery in air conditioning system. On the other hand,a detailed heat and mass transfer investigation was studiedby Zhang and Jiang [100] of a novel porous hydrophilic polymermembrane for two different ow arrangements. The resultsshowed that for the cross-ow arrangement, the membrane wasnot effectively used either for sensible heat or for moisturetransfer as compared to counter-ow arrangement. On the otherhand, heat and mass transfer mechanisms in a cross-ow parallelplate membrane-based enthalpy exchanger for heat and moisturetransfer recovery from exhaust air streams were numericallyinvestigated by Zhang [74]. It was concluded that the results ofthe study would give some insight for future applications of heator energy recovery design. Zhang [63] conducted an experiment tomeasure the steady heat and mass transfer through the exchangercores, by the measurements of inlet and outlet temperature,humidity and airow rates. Results found that the mass transferresistance for plate was larger than heat transfer resistance, so thehumidity changes more slowly than humidity does. In anotherstudy, Zhang [96] proposed some novel concepts for heat andmoisture transfer analysis in his review on the progress of heatand moisture recovery with membranes. The performance corre-lations for the effectiveness of heat and moisture transfer pro-cesses in an enthalpy exchanger with membrane cores werepresented in Ref. [95]. In the study, the total enthalpy effectivenesscan be calculated from sensible effectiveness, latent effectivenessand the ratio of latent to sensible energy differences across theunit. Most recently, Woods and Kozubal [122] performed a studyon heat and mass transfer enhancement by introducing newcorrugated mesh spacers with one spacer in three orientationsand comparing it with triangular corrugation spacers. It was foundthat this new corrugated design could improve heat transfer withlittle pressure drop penalty compared to the triangular corrugation

Ventilated

space

eAHU

o s

x

Rxs

exf

Re

inf

rs

HR

i

re

Rie

a

Fig. 9. The tested system of global efciency study.Source: [65].coefcient of heat transfer from uid to uid and has been the

Quite recently, Mathew and Hegab [141] conducted a study onapplication of the effectivenessNTU relationship to parallel owmicrochannel heat exchangers subjected to external heat transfer.In the study he had theoretically analysed thermal performance ofparallel ow microchannel heat exchangers and an equation fordetermining the heat transfer between the uids was formulated.It was observed that, irrespective of the heat capacity ratio, for aspecic NTU, as external heating decreased, the effectiveness ofthe hot and cold uids increased. In addition, at a given NTU,reduction in heat capacity ratio improved the effectiveness of theuids. Thus, it can be concluded that the model developed inRef. [141] can be used to predict the axial temperature as well asthe effectiveness of the uids in parallel ow microchannel heatexchangers, operating in the laminar ow regime, subjected toexternal heat ux. However, the model is limited to incompres-sible and single phase working uids of microchannel owapplications.

3.3. Effects of airow

The airow rate has a signicant effect on the efciency of heator energy recovery [28]. Shao et al. [56] conducted a study on heatpipe recovery with low pressure loss for natural ventilation and airvelocity in the systemwas measured around 0.51 m/s. The resultsindicated that the heat recovery efciency decreased with increas-ing air velocity as shown in Fig. 11. Nasif et al. [53] conducted theexperiments of thermal performance of enthalpy or membraneheat exchanger for heat recovery. The measurements were per-formed for air face velocity ranging from 0.3 to 2.89 m/s. Theresults discovered that as the air velocities increase, the effective-

A. Mardiana, S.B. Riffat / Renewable and Sustainable Energy Reviews 28 (2013) 174190184most convenient methodology to predict performance [63]. Whenthe NTU is small the efciency of heat recovery unit is low, andwhen the NTU is large the effectiveness approaches asymptoticallythe limit dened by ow arrangement and thermodynamic con-siderations. The effectiveness correlations for different types ofheat exchangers are discussed in Ref. [72]. Niu and Zhang [97]explained the inuence of NTU number on the effectiveness and itwas found that the effectiveness increased with increasing NTUnumber as shown in Fig. 10. It can be seen that the effectivenessincreased with increasing NTU number.

In addition, NTU is changed numerically by changing theconvective heat transfer coefcients, the transfer area and themass ow rate of dry air [131]. Thus, the efciency of heat recoveryusing the effectivenessNTU method can be dened with thefollowing equation when the ow arrangement, NTU and the heattransfer ratio are known (Kays and London [138]). For the cross-ow heat recovery, when both air streams are unmixed, C0, theeffectiveness is dened as

NTU 1expfNTU0:22expNTU0:781g 5

For parallel ow heat recovery, the effectiveness is

NTU 1expNTU1 C

1 C 6

For counter-ow heat recovery, the effectiveness is as follows:

NTU 1expNTU1C1C expNTU1C 7

and when C1, the effectiveness is presented as

NTU

Fig. 10. Effects of total number of heat transfer units on the effectiveness.Source: [97].NTU 1 NTU 8

There are several works conducted in the literature related to theeffectivenessNTU method of heat exchangers. For instance, astudy carried out by Zhang and Niu [93] showed that the sensibleeffectiveness is a function of NTU, the number of transfer unitsfor heat, while the latent effectiveness is a function of NTUL,the number of transfer units for moisture. Nellis and Pfotenhauer[139] performed an analytical study of the effectivenessNTUrelationship for a counter-ow heat exchanger which was sub-jected to an external heat transfer. On the other hand, Wetter [140]conducted a simple simulation model of air-to-air plate heatexchanger to be used for yearly energy calculations where theeffectivenessNTU relations were used to parameterise the con-vective heat transfer for a cross-ow with both streams unmixed.Navarro and Gomez [48] conducted a study on the effectivenessNTU computation with a mathematical model for cross-ow heatexchangers.Fig. 11. Heat recovery efciency of plain n unit.Source: [56].

Fig. 12. Experimental sensible, latent and total effectiveness for 60gsm paper heatexchanger.ness declines as for this range of air velocities the larger theresident time, the higher the heat transfer and effectiveness asshown in Fig. 12.Source: [53].

A. Mardiana, S.B. Riffat / Renewable and Sustainable Energy Reviews 28 (2013) 174190 185Fig. 13. Comparison of measured heat transfer with the literature.Source: [142].A most recent study by Al-Waked et al. [133] also proved thatthe effectiveness of the heat exchanger decreased as the airowrate increased. From their study, it was found that maximumvariations of 1.8% and 3.7% in sensible and latent effectiveness,respectively, were noticed when results from their CFD model andresults of Zhang [91] are compared. On the other hand, Yaici et al.[130] conducted thermal performance investigation by presentinga detailed numerical analysis of heat and membrane-based energyrecovery ventilators using CFD under typical summer and winterCanadian conditions. It was also found that effectiveness of heatrecovery decreased with increasing supply/exhaust air velocity.

Contrary to this, heat transfer in terms of heat recoveredincreased with increasing airow rates. Hviid and Svendsen [142]compared the heat transfer against airow rate in their study withseveral literature sources as shown in Fig. 13. A study conducted inRef. [28] found that the temperature efciency and the effectivenessNTU method decreased as the velocity increased as shown in Fig. 14and it can be seen in the gure that temperature efciency isreasonable consistent with the effectivenessNTU method results.On the other hand, airows also have signicant effects on thepressure loss of heat recovery system. As airow increases, thepressure loss increases as shown in Fig. 15 [56]. In addition, it hasbeen proven that the temperature of air after the heat recovery unit(supply air to room) varies with air velocity [37].

In a different case, Manz et al. [143] conducted a study toobserve the impact of unintentional airows on the ventilationunit with heat recovery system on the energy requirement forheating. The system studied is shown in Fig. 16. It was found thatintentional airows can considerably reduce the performance ofventilation units in combination with unintentional heat ows

Fig. 14. Efciency of the heat recovery system.Source: [28].Fig. 15. Pressure loss against velocity.Source: [56].through the casing. On the other hand, Woods and Kozubal [122]investigated various support spacers for airow throughmembrane-bound channels in heat or energy recovery ventilatorsto enhance heat and mass transfer. Results showed that unsteadyow occurs in the mesh spacers once a certain ow rate wasreached. Lots of numerical, simulation and experimental workscan be found in the literature related to effects of airow on theperformance and efciency of heat or energy recovery system forbuilding applications. However, a gap still exists between theseresearch results and practical application, which mainly lies in thefull-scale experimental measurements on real buildings. Furtherworks should be established concerning this area to betterevaluate the reliability of this technology in building servicesectors.

3.4. Effects of temperature, moisture and pressure drop

In order to determine the sensible and total efciency, tem-perature in terms of dry bulb and wet bulb must be known. From atheoretical point of view, the performance of heat recovery systemshould not be affected by inlet temperatures [34]. For bothsensible and total efciency, the temperature of the inlet airappears to have minor inuence [38] in heat recovery systemused in natural ventilation system. On the other hand, the effect ofheat-pipe inlet temperature was studied by Shao and Riffat [144]for six cases in naturally ventilated buildings and results indicatedthat the temperature of heat pipe had a small effect on ow lossperformance and pressure loss. Min and Su [35] studied thevariations of latent-to-sensible heat ratio with outdoor air relative

Fig. 16. Main and unintentional air ows in a system consisting of a ventilationunit, room and outdoor space.Source: [143].

A. Mardiana, S.B. Riffat / Renewable and Sustainable Energy Reviews 28 (2013) 174190186Fig. 17. Variations of latent-to-sensible heat ratio.Source: [35].humidity at three outdoor air temperatures of 32, 35 and 38 1C asshown in Fig. 17. From the study, it was found that for xed outdoortemperature, as the outdoor humidity increased, the latent-to-sensibleheat ratio increased because the latent heat increases. Niu and Zhang[97] investigated the variations of sensible, latent and enthalpyeffectiveness against different inlet air temperatures as shown inFig. 18. From their study, it was found that the sensible effectivenessdoes not change much with supply air inlet temperature, while thelatent effectiveness is very sensitive to temperature and decreasedwith rising temperatures. In addition, a most recent study by Yaiciet al. [130] also indicated that the outdoor temperature and humidityhad only minor effects on heat or energy recovery performance.

However, the efciency of the heat recovery system in realsituation varies with the temperature of supply and return air(temperature difference) [145] and because of air leakage, motorheat generation and so on [146]. In the heat recovery system usedto recover energy in air conditioning, El-Baky and Mohamed [22]proved that the inlet fresh air temperature is the dominantparameter to enhance the heat transfer rate in the evaporatorside of the heat pipe recovery and the effectiveness is increasedwith increasing inlet fresh air temperature. Mardiana and Riffat[28] investigated the impact of temperature difference on theenergy recovered of a heat or energy recovery system. From theirstudy, it was found that maximum sensible recovered energy of134 W was calculated at 4.3 1C temperature difference across theheat recovery unit at 3.0 m/s air velocity, whilst, the highest latentrecovered energy of 32.6 W was achieved at the same air velocityand temperature difference across the heat recovery unit.

Effect of supply air humidity was studied in Ref. [97] and it wasfound that the supply air humidity has slight effects on thesensible efciency but it has a major inuence on the latent

Fig. 18. Effectiveness against inlet temperature.Source: [97].Fig. 19. Effects of relative humidity of supply air on effectiveness with supply intemperature (35 1C).Source: [97].effectiveness as shown in Fig. 19. In addition, moisture diffusivityin membrane core of heat or energy recovery system is a keyparameter inuencing system performance [147,98]. An investiga-tion in Ref. [35] also claimed that with increasing outdoorhumidity, the latent-to-sensible heat ratio increases rapidly, sothe enthalpy effectiveness changes from near the sensible effec-tiveness toward the latent effectiveness.

Experimental measurements of pressure losses across heat piperecovery unit were carried out in Ref. [56]. The results show that,the pressure loss coefcient reduced as velocity increased. Inanother study, pressure drop across the membrane-based heatexchanger for energy recovery was measured in Ref. [53]. Theresults indicated that pressure drop was proportional to the airvelocity as shown in Fig. 20. Manz and Huber [52] conducted aninvestigation of pressure drop in the heat exchanger of heatrecovery system by calculating the friction factor as a function ofReynolds number as shown in Fig. 21. From their study, it wasfound that pressure drop increased with increasing airow rates.From the existing works, it can be seen that a considerabledatabase already exists for various operating parameters in termsof temperature, humidity and pressure drop and their effect on theeffectiveness of heat or energy recovery in which most of thesedata are based on laboratory conditions and CFD simulationstudies. Thus, further works are needed on practical applicationsin different climatic conditions especially in hothumid regions.

4. Conclusion

Throughout the literature, it can be seen that there are severalparameters that inuence the efciency of effectiveness of heat or

Fig. 20. Pressure drop measurement.Source: [53].

Source: [52].

A. Mardiana, S.B. Riffat / Renewable and Sustainable Energy Reviews 28 (2013) 174190 187energy recovery systems for building applications. These parameterscan be categorised into physical and performance parameters. It wasclearly discussed that in order to study the performance in terms ofefciency of heat or energy recovery system, either sensible orenthalpy, the main parameters that should be emphasised are(i) size, structure and material of heat recovery core (heat exchan-ger); (ii) size of ducts and fans; (iii) conguration or ow arrange-ment; (iv) heat and mass transfer; (v) ow and pressure drop in theducts; (vi) temperature and humidity and (vii) airow rates. It wasproven that the efciency or effectiveness of heat recovery increasesas the area increases. Flow arrangements have direct impact on heatexchanger effectiveness and heat transfer efciency is maximised iftwo airstreams have opposite directions in airow arrangement.Besides, the selection of fan and size of duct should be considered indesigning heat or energy recovery system for building applications asthese components are signicant to the performance of the system.In addition, duct structure and material will inuence the owdistribution and are also substantial in order to minimise heat lossfrom the duct system. Otherwise, the heat loss can reduce the totalefciency of the system signicantly. Furthermore, material andstructure of heat exchanger are the predominant factors of heat andmass transfer. Recently, membrane-based materials constitute a com-petitive technology in heat or energy recovery systems due to theirhigh capability to transfer both heat and moisture simultaneously.

Heat recovery efciency can be determined by application ofFig. 2thre

(i)

(ii)

(iii)

Inincreeffectranefcinuheatof sutemppera1. Friction factor against Reynolds number of a heat recovery system.e suggested methods as follows:

The ASHRAE standard method which can be dened in termsof sensible/temperature efciency, latent efciency andenthalpy (total) efciency.The effectivenessNTU method which can be dened by thecalculation of NTU and heat capacity ratio, C, when owarrangement is known.Global efciency which takes into account the ex-ltrationand in-ltration rate in the ventilation system.

any heat recovery system, reducing airow rates alwaysases efciency. On the other hand, airows have signicantts on all types of heat recovery efciency, pressure loss andsferred (recovered) heat or mass. For both sensible and totaliency, the temperature of the inlet air appears to have minorence in the heat recovery system. However, the efciency of therecovery system in real situation varies with the temperaturepply and return air (temperature difference). In addition, theerature change increases with increasing inlet fresh air tem-ture. Thus, as a conclusion, performance and efciency of heat or

regenerators using a generalized effectiveness NTU method. International

Journal of Heat and Mass Transfer 2009;52:226572.

[20] Sphaier LA, Worek WM. Analysis of heat and mass transfer in poroussorbents used in rotary regenerators. International Journal of Heat and MassTransfer 2004;47:341530.

[21] Li M, Simonson CJ, Besant RW, Mahmood G. Run-around energy recoverysystem for air-to-air applications using cross-ow exchangers coupled with aporous solid desiccant part I: model development and verication. HVAC &Research 2009;15:53759.

[22] El-Baky MA, Mohamed MM. Heat pipe heat exchanger for heat recovery inair conditioning. Applied Thermal Engineering 2007;27:795801.

[23] Juodis E. Extracted ventilation air heat recovery efciency as a function of abuilding's thermal properties. Energy and Buildings 2006;38:56873.

[24] Fehrm M, Reiners W, Ungemach M. Exhaust air heat recovery in buildings.energy recovery system are related to the ambient climatic condi-tions and operating parameters in terms of temperature and relativehumidity, effects of airows, pressure drop and fan power. Therefore,to improve the performance of heat or energy recovery system, moreefcient heat transfer materials, structures and more efcient fansmust be explored. In addition, more future works and optimisationstudy should be established concerning the physical and perfor-mance parameters of heat or energy recovery to better evaluate thereliability of this technology for building applications.

Acknowledgements

We wish to express our acknowledgements to FRGS Grant (203/PTEKIND/6711274), Ministry of Higher Education of Malaysia(MOHE), USM Short Term Grant (304/PTEKIND/6312055) and Indus-try Funding United Kingdom for the nancial supports associatedwith this study.

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