HKIE TRANSACTIONS, 2017VOL. 24, NO. 2, 63–69https://doi.org/10.1080/1023697X.2017.1313135

Changing cooling in Hong Kong through dehumidifying in addition to chilling

Joseph Y Huia,b and Mark H C Chan c

aMonarch Power Corp, Scottsdale, AZ, the USA; bDepartment of Electrical, Computer and Energy Engineering, Arizona State University,Tempe, AZ, the USA; cDepartment of Civil and Environmental Engineering, The Hong Kong Polytechnic University, Hong Kong, People’sRepublic of China

ABSTRACTMuch of the residential housing in Hong Kong is cooled by compressive air conditioning wallunits using hydrofluorocarbons (HFCs) as refrigerants. The recent Kigali Agreement in Rwandahas proposed the rapid phasing out of HFCs, a global warming gas that is 1400 times moreheat trapping than carbon dioxide. Such air conditioning is ineffective in hot and humid HongKong. Condensinghumidity also causes irritatingdrippingontopedestrians. This paper proposescompressing hot and humid air directly using a recently invented Hui turbine, instead of thedisplacement pressurisation of the refrigerant. Compressed air is cooled in a condenser withhumidity condensing out, releasing a substantial amount of heat of condensation. The cooledpressurised air is expanded in the Hui turbine, producing chill and recovering work. This innova-tive air conditioningmethodwhich does not use a refrigerant is called Firefly Air. This paper looksat the thermodynamic heat pump cycle of this cooling and dehumidifying process, comparingit with the usual Rankine cycle heat pump process which employs a refrigerant. Chill productionis more than doubled and the resulting chilled air has greatly reduced humidity, while irritatingdripping is avoided.

ARTICLE HISTORYReceived 7 November 2016Accepted 26 March 2017

KEYWORDSAir conditioning;thermodynamic heat pump;cooling and dehumidifyingprocess; Hui turbine; FireflyAir

Introduction: the Kigali Agreement to banglobal-warming HFCs for air conditioning

The recent Kigali Agreement in Rwanda [1] proposesthe rapid phasing out of hydrofluorocarbons (HFCs),a global warming gas that is 1400 times more heattrapping than carbon dioxide [2]. The Kigali Agree-ment is an amendment to the Montreal Agreement,which banned the chlorofluorocarbons (CFCs) thatwere depleting the Earth’s ozone layer. At the time,HFCs were heralded as the saviour of the ozone layer;their global warming effect was of a lesser concern.China and India objected to the early abandoning theuse of HFCs, because HFC replacements are morecostly and flammable than HFCs [3]. Africa whole-heartedly endorsed the Kigali Agreement, as the con-tinent would rather suffer from scorching heat withoutair conditioning than suffer the fate of drowning due toa rising sea level.

Current efforts are focused on finding a replacementfor HFCs. It is believed that refrigerants are unnec-essary if the fluid dynamics and thermodynamics ofheat pumps are fully understood. The Hui turbine [4]was invented with a complete analytical solution thataccounts for the velocity, temperature, and pressure ofthe gas that flows around an exponential, tapering spi-ral, providing a solution for the efficient compressionand expansion of air.

Asking developing countries to avoid the use of basiccomforts such as air conditioning is a form of climateinjustice. In developed countries, these daily comfortsare often taken for granted. It is hypocritical to ask poorpeople from developing countries to refrain frommak-ing use of these comforts when it is the developedworldthat is the primary cause of global warming, inflict-ing devastating climate change with the rampant useof fossil fuels. The only way to turn the tide againstglobal warming and climate injustice is to use tech-nological innovation to provide affordable and cleanhuman comforts such as air conditioning – not justfor the fortunate, but also for the poorest of the world.Abundant energy sources such as solar energy must befostered, while in solar-deprived locations or at night,fungible energy sources such as bottled petroleum gascould be lifesaving. Currently, almost all petroleum gasinAfrica is flaredwithout being bottled. This gas, if bot-tled, could fulfil half of the energy needs of more than1 billion Africans who are living in dire energy poverty.The technology is already available, but the political willand economic incentives to meet the energy needs ofthe world’s poor is sadly lacking.

To solve the problem of insufficient energy supplywhilst also preserving the environment, the first authorand others have filed patents on spiral turbines, alter-nating currect (AC) motors, heat engines, heat pumps,

CONTACT Joseph Y Hui jhui@asu.edu

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and solar desalination [5]. The present paper focuseson how Firefly Air can help the citizens of Hong Kongenjoy cool and dry living conditions in their homes andplaces of work, with a view to providing similar low-cost air conditioning solutions to the people of China,India, and Africa without contributing to the problemof global warming through the use of HFCs.

Heat index comparison of Phoenix and HongKong

The first author of this paper resides in Phoenix, Ari-zona in the United States (US) and the second authorresides in Hong Kong. As is known, Phoenix is locatedin a desert region – so, do the hot days in Phoenixreally feel hotter than those in Hong Kong? This ques-tion can be answered using the “perceived temperature”(i.e. the temperature that is actually felt by the resi-dents of an area), which can be measured by a heatindex [6]. The heat indices of these two cities on twohot days are compared in Figure 1 [7,8]. Heat indextables taking into account the contribution of humid-ity in order to indicate the heightened temperature thatis felt by the people who live in a given region. Theheat index factors in the enthalpy of the evaporationof water that is contained in humid air, as humid airinhibits the sweat evaporation which allows the human

body to cool down. The temperature indicated by athermometer does not correctly reflect the subjectiveexperience of heat when one’s body temperature is notbeing regulated through the evaporation of sweat.

In the comparison of two hot days in Phoenix andHong Kong in June 2016, Phoenix had a tempera-ture of 49◦C and 10% humidity, while Hong Kong hada temperature of 34◦C and 81% humidity. The heatindex table indicates that Hong Kong felt much hot-ter with a perceived temperature of 52◦C, comparedto the dry heat of Phoenix, which felt like 45◦C. HongKong is arguably hotter than Phoenix, as Hong Konghas a latitude of 22◦ while Phoenix is more northerlyat 33◦. Figures 2 and 3 show the annual weather pat-terns of Phoenix andHongKong, respectively. The highhumidity and precipitation of Hong Kong indeed makeit much hotter than Phoenix, and both cities employthe lion’s share of the electricity they generate for airconditioning.

The first author visits Hong Kong frequently andsuffers in the oppressively hot and humid Hong Kongweather more than in the dry heat of Phoenix. Theseexperiences brought about a determination to inventa personal air conditioner to improve his comfort thattakes not only the heat out of the air but also the humid-ity that prevents the profuse amounts of sweat that aregenerated by the body’s attempts to cool itself down

Figure 1. Heat index table for comparisons of the perceived temperatures in Phoenix and Hong Kong [6].

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Figure 2. Climate pattern of Phoenix [7].

Figure 3. Climate pattern of Hong Kong [8].

from evaporating. The resulting invention employs theHui spiral turbine to compress hot and humid air, whichcools with condensing humidity in a small condenserholding cooled compressed air. This air is cooled fur-ther as it expands in the second Hui spiral turbine thatis coaxial and conjoint with the first Hui spiral turbine.More work is required for air compression than thework produced by air expansion. This work deficit is

provided by a further motor invention, which is a mod-ification of Michael Faraday’s homopolar generatorinvented in 1831. The Hui compressor, Hui expander,and Hui motor combine to form a single 3D-printedunit that is rotor-driven by 12VAC from a solenoid sta-tor with no iron core and has a dry weight of about 1 kg.Called Firefly Air, it consumes up to 300W of powerand can remove more than 1KW of heat from the air

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and another 1KW of heat through the condensationof humidity. It can also use the heat generated by thecompression component of the process to heat water.

How traditional air conditioning usestemperature reduction to condense water outof humid air

The enthalpy of air has twomajor components. The firstis the internal energy of a gas, which is αnRT in whichn is the number of moles of gas, R is the ideal gas con-stant, T is the temperature of the gas in Kelvin, and α

is the degree of freedom (five for diatomic gas oxygenand nitrogen molecules) divided by two. The second isthe heat of the evaporation of water, which is fairly highat 2257 J/g at boiling temperature. Thus, the presenceof humidity substantially increases the enthalpy of air,which is a major reason why humid places do not reachthe same temperature as high as dry and hot places suchas Phoenix.

Humidity can be defined as the percentage by weightof water in the air. Relative humidity at a given mea-sured air temperature is defined as the percentage ofwater content comparedwith thewater content of 100%humid air [9]. This saturation of water in the air causeswater condensation to form as dew. Take for examplean air temperature of 14◦C. Air with 100% humiditycontains 1 g of water per 100 g of air. If air containsonly 0.5 g of water, then it has a relative humidity of50%. Take for another example an air temperature of25◦C. Saturated 100% humid air contains 2 g of waterper 100 g of air. If that air contains only 1 g ofwater, thenit also has a relative humidity of 50%.Humidity levels of30%–40% are considered comfortable to humans overa broad temperature range.

If the air temperature is cooled to 25◦C and the airhas 50% relative humidity (i.e. it contains 1 g of waterper 100 g of air) then the relative humidity of the airwould increase. Cooling the air to 14◦Cwould raise therelative humidity to 100%,meaning that further coolingwould cause dewing of air when the air humidity con-denses out of the air as dew. Thus the humidity of theair can also be measured by its dew point – a temper-ature at which the water in the air begins to condenseout.

Indeed, lowering the temperature is an effective wayto get rid of water in air. Relative humidity remainshigh at 100% when the air temperature reaches the dewpoint. Removing water from humid air is an additionalheat load on air conditioning, as the heat of the con-densation also has to be removed, and this heat is anextra load which otherwise could be used to lower theair temperature. Thus, the cool air output of an air con-ditioner in Phoenix has amuch lower temperature thanthe same air conditioner consuming the same amountof electric power in Hong Kong. The heat load of the airin Hong Kong is therefore higher than that in Phoenix

due to the higher latent heat of the evaporation of itswater content.

Using pressure can also drive out water, as shownin the next section, without the need to use a refriger-ant. Firefly Air has the extra advantage of being able toextract water out of the air without increasing the heatload, even though water condensation is an exothermicprocess with heat that requires dispersal.

Using an isothermal compressor as a heatpump for air conditioning

Air conditioning on aeroplanes is achieved withoutusing HFCs through bleeding pressurised air from theearly turbine stages of the compressor of a jet engine(Figure 4). Heat is then removed from the bled pres-surised air. An expansion valve allows the pressurisedair to release its pressure, causing the air to cool down.The chilled air is then fed into the cabin for the comfortof the people on board.

Key to compressing air efficiently is the use of theHui turbine, which is small and simple compared withlarge and expensive jet engines. Jet engine technologycan be brought to themasses for the purpose of air con-ditioning if the cost of the turbines is reduced throughthe creation of a simple design that can be printed using3D additive manufacturing.

Compression of a gas imparts work energy into thegas in order to raise its internal heat energy. Compres-sion can be isothermal at a fixed temperature. Com-pression can also be adiabatic, meaning that there isno heat exchange between the gas being compressedand the environment. Consider the work that goes intocompressing a gas in an isothermal manner. The com-pression work Wc performed on the gas is convertedinto compression heat Qc that is completely exchangedwith the environment. Isothermal compression can beexpressed as:

Wc = Qc = nRTH lnpHpL

, (1)

where the compression ratio pH/pL is the pressureincrease of the high pressure pH over the low pressurepL, and the amount of work required is proportionalto the temperature TH of the isothermal gas, which ishigher than the ambient air temperature. Subsequently,isothermal expansion of the gas at the low temperatureTL generates work of expansion. Isothermal expansionproduces work of expansion:

We = Qe = nRTL lnpHpL

. (2)

The net amount of work required for isothermal heatpumping is:

W = Wc − We = nR(TH − TL) lnpHpL

. (3)

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Figure 4. Air conditioning of jet aeroplanes by bleeding air out of the jet engine compressors.

The heat pump generates an amount of heat of com-pression Qc. The coefficient of performance of heatingis:

COPh = Qc

W= TH

TH − TL. (4)

The heat pump also takes out an amount of the heatgenerated by expansion Qe. The coefficient of perfor-mance of cooling is:

COPc = Qe

W= TL

TH − TL. (5)

In an enhanced Firefly Air unit called Firefly Air+,the heat generated by compression is used to heat waterfor bathing. As an example, consider TH = 330 K(57°C) and TL = 280 K (7°C). The coefficients perfor-mance for heating and cooling areCOPh = (330/50) =6.6 and COPc = (280/50) = 5.6, respectively. If thepower is provided at 300W, then heat is produced at1980W and cooling at 1680W. More than half of thatproduction is anticipated in practice.

Using pressure to condense water out of air

The heat produced by evaporation is a major compo-nent of enthalpy in humid air. Air conditioning thatuses a refrigerant such as HFCs does not reduce theair humidity of the output air, which is cold but alsowet. Sweat fails to evaporate in an indoor environmentthat is cold and wet as a result of being in a hot andhumid climate such as that of Hong Kong, resulting inthe occupants of buildings feeling uncomfortable andsometimes becoming sick.

Instead of using chill to condense water out of air, itis preferable to use pressure. The heat resulting from theprocess of condensation is dissipated at the same timeas the heat resulting from the process of compression.To explain how pressure condenses water, vapour pres-sure due to the gas components of water, oxygen, andnitrogen is considered. The partial vapour pressure is

proportional to the molar fraction of the gas compo-nents by the Ideal Gas Law, pV = nRT, broken downinto partial pressures andmolar components as follows:

(pO2 + pN2 + pH2O)V = (nO2 + nN2 + nH2O)RT,(6)

where the water vapour pressure pH2O = pvp (kPa) ispurely a function of temperature according toAntoine’sequation,[10] log pvp = A − B/(T + C) , where A =16.54, B = 3985, and C = −40, and T is in Kelvin.

Figure 5 plots the saturation pvp versus temperature.The water vapour pressure above the plot causes watervapour to condense as water, while the water vapourpressure below the plot causes water to boil. Considerincreasing the pressure of air with 80% relative humid-ity while keeping the temperature constant. Withoutcondensing the water, the water vapour pressure wouldbecome double. Increasing the pressure causes the rel-ative humidity to reach 100%, at that point water con-denses out to restore the saturation vapour pressure atthat temperature.

Figure 5. Saturation vapour pressure separating phases ofwater and steam.Source: Engineeringtoolbox.com.

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Consider how much water is compressed out of airthat is 30◦C and 80% humidity. Dry air has a weightof 1.225 kg/m3, containing 42.24 moles of molecules.For 80% relative humidity, the water content is 42.24moles × (4.25 kPa/101.3 kPa) × (18 g/mole) × 80% =25.53 g/m3. The heat of evaporation of that water con-tent is 25.53 g × 2257 J/g = 57.6 kJ. For chillers usingHFCs to cool air from 30◦C to 10◦C, the heat load forthe water condensation is 57.6 kJ × (4.25 kPa × 80% −1.23 kPa/(4.25 kPa × 80%)) = 36.8 kJ. This heat mustbe removed first before the air is cooled from 30◦Cto 10◦C. The specific heat of gas per mole under con-stant pressure is Cp = 3.5R. The heat that needs to beremoved in order to drop the temperature of a cubicmetre of air down from 30◦C to 10◦C is 42.24 moles ×3.5R × 20 K = 24.8 kJ.

Consider now using compression instead of coolingto force out moisture. The air pressure is raised usingthe Hui turbine compressor and then the air is cooledback down to room temperature. If the air pressure of100% humid air is doubled, then half of the water willbe forced out. If the air pressure of 80% humid air isdoubled, then (2 × 0.8 − 1)/1.6 or 3/8 of the waterwill be condensed out. If the air pressure is quadrupled,then (4 × 0.8 − 1)/3.2 or 11/16 of the water will becondensed out, generating heat from the condensationprocess of 57.6 kJ × 11/16 = 39.6 kJ, which can alsobe used to heat water. The condensing water can alsobe captured without dripping down on unsuspectingpedestrians.

Conclusion

In essence, the solar evaporation of water is the mothernature’s form of refrigeration, using water as refrig-erant. Water evaporation reduces the temperature asmeasured by a thermometer. Humidity however pre-vents the natural way in which humans are cooleddown by means of sweat evaporation. Getting rid ofhumidity is an additional benefit of compressing wetair directly instead of compressing a refrigerant. Somework is needed to compress water vapour, and usinganother refrigerant to replace global-warming HFCsseems unnecessary and convoluted. Firefly Air attainsa higher efficiency than compressive air conditioningusing HFC due to simplicity and good thermodynam-ics. Much of this advance is due to a new understandingof the fluid dynamics of gas flow in spiral channels.

It is expected that Firefly Air will substantially lowerthe equipment, installation, and operating costs of air-conditioning systems. Firefly is portable and providesintimate and personal cooling. It is indispensable forlabourers and public servants who spend hours undera scorching sun or in oppressive humidity.

More importantly, it is believed that this is a majorparadigm shift in how society cools itself: through

dehumidification. Firefly Air can benefit not just thosein Hong Kong but also poor people in many countriesaround the world, who can avoid contributing to theproblems of global warming and the rising sea levelwithout being deprived of the comfort of cooling andlifesaving refrigeration due to the ban on HFCs.

Acknowledgements

Mr Ng Cheuk Hei Michael prepared the figures based on theinformation gathered from the Internet.

Notes on contributors

Prof Joseph Y Hui obtained his B.Sc.,M.Sc. and Ph.D. degrees in Electri-cal Engineering from the MassachusettsInstitute of Technology in 1981, 1981and 1983, respectively. He was the Inter-national Switching Symposium (ISS)Chair of the Department of Electri-cal, Computer, and Energy Engineer-ing at the Arizona State University, and

became a Professor Emeritus in 2014 in order to take on therole of the President and CEO of the Monarch Power, anR&D company devoted to solving the 3Es of human needs:energy, environment, and education. Prof Hui holds morethan 40 patents in communication, networking, cloud com-puting, and renewable energy. He has received many awards,such as the Institute of Electrical and Electronics Engineers(IEEE) Fellow, the IEEEWilliam Bennett Prize Paper Award,and the NSF Presidential Young Investigator Award. He isthe author of many papers and books, including the seminaltextbooks Integrated Switching and Traffic Theory for Inte-grated Broadband Networks (Kluwer Academic) and What’sthe Matter with Energy! (Solar Man Press).

IrDrMarkHCChan obtained his B.Sc.and M.Sc. degrees in Civil Engineering,and Sc.D. degree inGeotechnology fromthe Massachusetts Institute of Technol-ogy in 1979, 1981 and 1986, respectively.He has about 30 years of academic andindustrial experience in the US, the UK,Canada and Hong Kong in numericalanalysis, geotechnical design and con-

struction. He is a Research Fellow of The Hong Kong Poly-technic University and has worked on various R&D projectsand geotechnical control at the Geotechnical EngineeringOffice (GEO) of the Civil Engineering and DevelopmentDepartment (CEDD) of the HKSAR Government. He hasbeen a member of a working group of the Geotechnical Divi-sion Committee of The Hong Kong Institution of Engineers(HKIE) since 2006 and is the leader of theGeotechnical Com-puter Program Users Group. Ir Dr Chan has organised andspoken at geotechnical seminars of the HKIE, the Ameri-can Society of Civil Engineers (ASCE), the Association ofGeotechnical and Geoenvironmental Specialists (AGS) andthe GEO.

ORCID

Mark H C Chan http://orcid.org/0000-0002-9669-1584

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