Energy Modelling and Indoor Air Quality Analysis of Cooling Systems for Buildings in Hot Climates

fluids

Article Energy Modelling and Indoor Air Quality Analysis of Cooling Systems for Buildings in Hot Climates

Zohaib Shaikh and Hassam Nasarullah Chaudhry *

Department of Architectural Engineering, School of Energy, Geoscience, Infrastructure and Society, Heriot-Watt University, P. O. Box 294 345, Dubai, UAE; [email protected] * Correspondence: [email protected]; Tel.: +971-(04)-435-8775

Received: 8 August 2018; Accepted: 18 October 2018; Published: 22 October 2018

Abstract: Energy consumption due to cooling and ventilation of buildings has grown significantly within the last two decades, and therefore advancement in cooling technologies has become imperative to maximise energy savings. This work numerically investigates the performance of vapour-compression unitary and centralised cooling systems for high rise buildings using an office case-study in the United Arab Emirates (UAE). Energy modelling, thermal comfort and indoor air quality analyses have been carried out using the Integrated Environmental Simulation Virtual Environment (IES-VE). Using the benchmark system based on fan-coil units, the findings have indicated that attaching a Variable Speed Drive (VSD) fan can reduce the overall energy consumption of the building by 8%, with 20% reduction in the cooling loads. The unitary cooling system operating on variable refrigerant flow principle achieved an energy reduction of approximately 30%; however, this system is not recommended in high-rise buildings as the CO2 concentration obtained is in excess of 3000 ppm, which is considerably higher than ASHRAE standards. It is essential for buildings running in hot climates to incorporate hybrid cooling techniques to relieve the load on conventional active cooling systems.

Keywords: buildings; hybrid; indoor air quality (IAQ); cooling; thermal modelling

1. Introduction

1.1. Background Active cooling systems are responsible for consuming 80–85% of the building’s total electricity because of the extremely hot climatic conditions where outdoor temperatures reach up to 50 ◦C with relative humidity up to 60% during peak summer months [1]. Intensive use of air-conditioning system emits greenhouse gases like carbon dioxide, methane, nitrous oxide, chlorofluoro carbon into the atmosphere causing ozone depletion. In areas of extreme climates, users spend most of the time indoors, which makes it compulsory to maintain an optimum level of thermal comfort and indoor air quality. However, at the same time, these cooling systems have to be resilient against the effects of climate change. There is a growing interest among the local community to adapt towards more energy efficient cooling technology and sustainability is also the main theme for the United Arab Emirates (UAE) Expo 2020 [2]. The ducted split and packaged air conditioning systems mostly dominate the local market by having 30% of the local market share in UAE. The centralised air conditioning systems also account for 25% of the local market share. While district cooling and split units/window AC have similar market share of about 18%. The variable refrigerant flow systems only accounts for 2% of the market share since they are not very common in the local market [3]. Office buildings usually have higher energy consumption because the cooling systems have to compensate for the continuous internal heat gains produced by office occupancy and equipment.

Fluids 2018, 3, 77; doi:10.3390/ﬂuids3040077 www.mdpi.com/journal/ﬂuids Fluids 2018, 3, 77 2 of 20

In this research, energy modelling of a high-rise office case-study building was performed using Integrated Environmental Simulation Virtual Environment software (IES-VE). The impact of different cooling systems based upon the energy consumption, carbon emission, thermal comfort and indoor air quality was investigated. Fan Coil Unit (FCU) with a Variable Speed Drive (VSD) fan, Variable Air Volume (VAV), Variable Refrigerant Flow (VRF) and Cool Phase Hybrid cooling system were some of the cooling systems evaluated on the baseline office model. The aim of this study is to investigate a range of hybrid and active building cooling systems for buildings in hot climates in order to recommend the most appropriate cooling technique for reducing the energy consumption, carbon emission and improve the thermal comfort and indoor air quality. The analyses will include dynamic modelling of the VAV, VRF and hybrid cooling systems using a case-study office building located in hot climates such as the UAE.

1.2. Previous Related Works Numerous research works have highlighted the significance of adopting active retrofits in order to enhance the building performance. The impact of various cooling technologies had been studied individually to find the most optimum solution to improvise the energy efficiency, thermal comfort and indoor air quality. AlAwadhi et al. [4] examined the energy saving potential by retrofitting federal public housing villas by using Integrated Environmental Simulation Virtual Environment (IES-VE). The retrofits were based upon the Estidama 1 Pearl standard being limited to insulation and glazing. While Estidama 2 Pearl standard was only based upon window wall ratio (WWR). An overall energy reduction of 30% was attained using Estidama 1 and additional 1% reduction was achieved by applying the Estidama 2 standard. This research proved the capability of IES-VE software to analyse the retrofits performance and also highlighted the UAE government’s ambition to control the buildings energy consumption by bringing it under certain limit by implementing Estidama Pearl Rating to go according to 2021 vision. Alkhateeb and Hijleh (2016) [5] had performed the energy modelling of federal government building with an aim to reduce the energy demand by adopting different retrofit techniques. Integrated Environmental Simulation Virtual Environment (IES-VE) was used for assessing the effectiveness of passive and active retrofit strategies to reduce the energy-consumption for Ministry of Infrastructure development building (MOID) in Ras Al Khaimah (RAK). The study revealed that the active retrofits such as improving the Coefficient of Performance (COP) and changing set point were more ideal as it provided 64% of electricity saving. While passive retrofits like shading, insulation and glazing provided 14.5% of electricity saving proving that the energy saving potential is far greater when adopting active retrofits such as energy efficient cooling systems or variable speed drive fans (VSD) as they are directly associated with building services. Hartungi and Jiang [6] conducted a research to find the most appropriate energy conservation retrofitting solution in order to make the office building comply with the UK Part L regulation. Integrated Environmental Simulation Virtual Environment (IES-VE) was used for modelling to determine the carbon emission rates. A combination of retrofits such as insulation, heat recovery system, automatic blind system and energy efficient artificial lighting components like lamps, ballast, luminaires and control gear were adopted. The Target Emission Rates (TER) had reduced by 28% making it compliant with Part L building regulation. This research proved that how IES-VE can be used as an energy modelling tool to make buildings compliant with latest government regulations. Taleb [7] studied the impact of various passive retrofit techniques such as sunshade, external wall, glazing and green roof for a hospital case-study building to enhance the building skin performance. Integrated Environmental Simulation Virtual Environment (IES-VE) and Computational Fluid Dynamics (CFD) were utilized to evaluate the performance of the hospital building before and after retrofitting. The results indicated heat gain reduction of 1.5% from sunshade and 24.7% from green roof, proving that the green roof was one of the most suitable solutions in reducing the heat gains and energy consumption. This research proved that green-roof is a viable option for roofing because it Fluids 2018, 3, 77 3 of 20 will help to improve the building fabric performance and creates more habitable space to support the health and well-being of building users. The influence of occupancy pattern on energy consumption was studied by Pan et al. [8] using IES-VE to model the building and set the occupancy profiles replicating an actual operation throughout the year in different seasons. The aim of this study was to identify occupant’s behavior, which leads to energy-wastage. Room-occupancy, window operation, indoor and outdoor air-temperature were some of the parameters that were monitored. Switching off the Heating, Ventilation and Air-Conditioning (HVAC) system when the office was unoccupied proved to provide the most-savings. The findings showed that the way in which the occupants use the building had a major impact on its performance; educating occupants about energy conservation is a great solution and is more beneficial in terms of cost, implementation and knowledge. This research demonstrated capability of IES-VE simulation to assume the occupancy behavior pattern for predicting the energy saving potential. A feasibility study was led by Stansbury and Mittelsdorf [9] to analyse the economic and environmental benefits by adopting energy efficient lighting. This study mainly wanted to prove how retrofitting of lighting system can make a drastic impact in reducing the overall emissions of SO2, NO2, CO2 and CO from power plants by reducing the energy-consumption. Audit was carried out to know the exact lighting requirements, occupancy patterns, room dimension before retrofitting the office and the annual energy data for the office was obtained from general service administration. The results showed an annual energy saving of 53% by implementation of an energy efficient lighting system. Hammad and Abu-Hijleh [10] examined the impact of integrating dynamic louvers on building’s external façade to reduce the energy consumption in office building by using an Integrated Environmental Simulation Virtual Environment (IES-VE) to predict the energy consumption. The results showed energy saving potential of 34% at south, 28.5% in east and 30.31% from the west direction of the building. This research study gave an imprint about how IES-VE could be utilized to analyse the performance of the dynamic facades to reduce the energy consumption and improve the productivity of employees within the work environment as these facades will also help to control the glare. A research study was conducted by Khatami and Hashemi [11] to find the most appropriate retrofitting strategy to improve the energy consumption, thermal comfort and indoor air quality (IAQ) of the office environment by performing dynamic modelling in integrated Environmental Simulation Virtual Environment (IES-VE). Energy efficient lighting system, automatic thermostat and automated window controls were some of the adopted retrofitting strategies. The results revealed that the IAQ improved by 90% and reduced the energy consumption by 2.1%. This research gave an impression about how thermal comfort and IAQ could be analyzed using IES simulation. A comparative research was led by Yu et al. [12] to investigate the energy consumption between Variable Refrigerant Flow (VRF) and Variable Air Volume (VAV) in typical office buildings. Site surveys, electricity bills and design simulation toolkit (DeST) were used to evaluate the performance of the cooling systems. Simulations revealed that VRF systems had 48% less cooling load in contrast to a typical VAV system, which was mainly due to the flexibility of the VRF system to have a part-time part-space operation mode, which allowed the user to turn on as per occupancy and had individual controls for occupants in different spaces. This research gave a general overview about the benefits of Variable Refrigerant Flow (VRF) with regards to the potential savings and design flexibility for retrofitting VRF systems in existing office buildings. Aynur et al. [13] performed an experimental study between Variable Air Volume (VAV) and Variable Refrigerant Flow (VRF) air conditioning with energy recovery ventilator (ERV) module using energy plus to determine the energy consumption and meet the ventilation criteria set by ASHRAE. The results from the simulation concluded that the VRF system was 27.1–57.9% more energy efficient than the VAV system. The findings also proved that there was a trade-off between temperature control and energy consumption in the VAV system because the VAV box had to be utilised in order to maintain the temperature precisely and it was also energy demanding because it increased the energy Fluids 2018, 3, 77 4 of 20 consumption by 65.8%. This research highlighted major drawbacks of VRF and VAV systems with solutions to overcome the challenges and also emphasized the fact that having a greater number of components on the air side and water side will lead to greater energy demand. There is a lot of literature that has successfully performed energy modelling, but failed to realise the significance of carbon emission because none of it extensively provides carbon data comparison before and after modelling. It is also worth mentioning that adoption of an energy efficient cooling technique in the existing building stock shall play a significant role in reducing the energy consumption of the built environment as cooling accounts for more than 70% of the buildings electrical load in UAE. This research will therefore investigate the most optimum solution to meet the heavy cooling demands in the most energy efficient manner with comprehensive analysis of energy consumption, carbon emission, thermal comfort and indoor air quality. Designing, comparing and recommending mechanical cooling systems assisting with the hot climatic condition will be of great interest to the building community as cooling is requisite for most of the year.

2. Numerical Methodology

2.1. Governing Equations The dynamic building simulation was carried out using IES-VE’s module ApacheSim, where the conduction of each building element was presumed to be unidirectional. A different set of equations was utilised to calculate the boundaries, heat differentiation and heat transfer by air movement. Furthermore, the thermo-physical properties λ, ρ, and c of each layer of element were assumed to be uniform within the layer: ∂2T ρc = ∂T/∂t, (1) ∂x2 λ where λ was the conductivity of solid element (W/(m2·K)), ρ was the density of solid in kg/m3 and c was the specific heat capacity of the solid in J/(kg·K). IES adopts a finite difference approach to the solution of the heat differentiation equation, which involves replacing the element with a finite number of discrete nodes at which the temperature is calculated: T − 2T + T ∂T n−1 n n+1 = −( ) 2 ρc/λ , (2) δn ∂t ◦ where Tn is the temperature ( C) at node n and δn is the local node spacing in m. Nodes are distributed within the layers in such a way as to ensure accurate modelling of the heat transfer and storage characteristics for a chosen time-step. This choice is based on constraint imposed on the Fourier number: λ F = ∆/δ2 , (3) ρc n where ∆ is the simulation timestep in seconds:

. j j+1 j Tn = (Tn − Tn)/∆, (4)

. j ◦ j where Tn is the temperature ( C) at node n and time-step j, Tn is the time derivative of temperature (K/s) at node n and time-step j. The rate of heat transfer associated with a stream of air entering calculated was using:

Q = mCp(Ti − Ta), (5) where m is the air mass ﬂow rate in kg/s, Cp is the speciﬁc heat capacity of the air at constant pressure in J/kg/K, Ti is the air supply temperature and Ta room mean air temperature:

m = ρre f aV/3600, (6) Fluids 2018, 3, 77 5 of 20

Fluids 2018, 3, x FOR PEER REVIEW3 3 5 of 20 where ρre f is 1.2 kg/m as the reference air density, V is the room volume in m and a is the air change rate in ach: w== m((gi − g)).. (7) (7)

𝑖𝑖 CarbonCarbon dioxide dioxide gain gain is is treated treated in in a asimilar similar𝑤𝑤 way,𝑚𝑚 way,𝑔𝑔 where− where𝑔𝑔 c cis is the the carbon carbon dioxide dioxide gain gain in in kg/s, kg/s, k iisis thethe car carbonbon dioxide dioxide concentration concentration of of the the supply supply air air in in kg/kg kg/kg and and k kisis the the carbon carbon dioxide dioxide concentration concentration 𝑘𝑘𝑖𝑖 ofof the the room room air air in in kg/ kg/kgkg [14] [14. ].

22.2..2. Physical Physical Domain Domain IESIES-VE-VE (Integrated (Integrated Environmental Environmental Simulation Simulation Virtual Virtual Environment) Environment) was was used used for for performing performing thethe Dynamic Dynamic Thermal Thermal Simulation Simulation in in this this study. study. The The mixed mixed-use-use building building occupied occupied an an area area of of 2731 2731 m m2,2 , thethe height height ofof thethe tower tower was was 235 235 meters meters and consistedand consisted of 59 floorsof 59 includingfloors inc commercialluding commercial and residential and residentialspaces. The spaces. site hasThe asite level has terrain a level withterrain scarce with vegetationscarce vegetation as most as ofmost the of nearby the nearby sites sites have have been beenconstructed constructed with with high-rise high towers.-rise towers. The UAE The UAE is well is knownwell known for dry for arid dry climatic arid climatic conditions condition as its lies as onit liesthe on tropic the of tropic cancer of with cancer extremely with extremely hot temperatures hot temperatures during summer during months summer and months low temperatures and low temperatureswith less rainfall with in less winter rainfall months. in winter Summer months. seasons Summer last seasons for almost last nine for almost months nine being months followed being by followedwinter for by three winter months. for three June, months. July and June, August July are and the August hottest are months the hottest in the yearmonths when in thethe temperatureyear when thecan temperature reach up to can 50 ◦reachC with up 90% to 50 humidity °C with 90% and humidity 11.5 h of and average 11.5 maximumh of average sunshine. maximum The sunshine. office is Theserved office by is a served Fan Coil by Unit a Fan (FCU) Coil Unit air conditioning (FCU) air conditioning system that system was created that was in thecreated Apache in the HVAC Apache tool HVACfor a detailed tool for analysisa detailed of analysis systems andof systems energy and consumption. energy consumption. TheThe modelled modelled office office space space had had an an area area of of 687.18 687.18 m m22. .The The office office had had an an open open floor floor plan plan that that was was replicatedreplicated in in the the model model by by replacing replacing the the walls walls with with holes holes for for accurate accurate simulation simulation purposes. purposes. Some Some of of thethe inaccessible inaccessible spaces underunder thethe control control of of building building management management were were considered considered as voids as voids and haveand havebeen been highlighted highlighted in red in inred Figure in Figure1. 1.

Figure 1. Office floor plan (m). Figure 1. Ofﬁce ﬂoor plan (m).

2.3. Indoor Design Criteria The indoor design criteria (Table 1) were set to replicate the actual functioning of the building in accordance with the local standards and regulation for office environment. The indoor temperature was set to 24 °C and for server room it was kept at 18 °C. Fluids 2018, 3, 77 6 of 20

2.3. Indoor Design Criteria The indoor design criteria (Table1) were set to replicate the actual functioning of the building in accordance with the local standards and regulation for ofﬁce environment. The indoor temperature was set to 24 ◦C and for server room it was kept at 18 ◦C.

Table 1. Indoor design criteria.

Indoor Summer Indoor Winter Illuminance Zones Occupants Inﬁltration Temperature Temperature Level Name ◦C ◦C Lux People ACH Circulation 24 24 500 5 0.167 Conference Rooms 24 24 500 18 0.167 Ofﬁces 24 24 500 56 0.167 Pantry 24 24 500 0 0.167 Server Room 18 18 500 0 0.167 Main Entrance Lift Lobby 20 20 500 0 0.167 Adjacent Floors 24 24 500 0 0.167

High performance construction materials have been used for modelling the ofﬁce. A green patterned curtain wall has been replicated in IES-VE by adding 100% glazing. The buildings core structure has been assigned with external walls (U value = 0.56 W/(m2·K)) and glazing (U value = 1 W/(m2·K)) taken in line with Dubai Municipality green building standards. Zone templates were created to imitate the 12-h operation with a one-hour break (Table2).

Table 2. Occupancy proﬁles for main spaces.

Occupancy Proﬁles Corridors Conference Rooms Ofﬁces Timing % % % 00:00–07:30 0 0 0 07:30–08:00 50 10 100 08:00–09:00 50 25 100 09:00–12:30 50 50 100 12:30–14:00 100 10 50 14:00–18:00 50 50 100 18:00–20:00 50 20 100 20:00–22:00 0 0 10

Table3 shows the operational profile for the office and pantry equipment. Computers were some of the most commonly used equipment’s in the office. Most of the pantry equipment’s were expected to have peak usage at break time during 12:30 p.m. to 2:00 p.m. Lighting operational profile has been presented at Table4.

Table 3. Equipment operational proﬁles.

Equipment Computers + Open Pantry Equipment Conference Room Microwave Refrigerator Operational Proﬁles Photocopier (Coffee Machine + Water Filter) Equipment Timing % % % % % 00:00–07:30 0 15 0 100 0 07:30–08:00 100 15 20 100 10 08:00–09:00 100 15 25 100 25 09:00–12:30 100 15 25 100 50 12:30–14:00 50 100 60 100 10 14:00–18:00 100 15 25 100 50 18:00–20:00 100 15 25 100 20 20:00–22:00 10 0 0 100 0 FluidsFluids 20182018, 3, ,3 x, 77FOR PEER REVIEW 7 of7 20 of 20

Table 4. Lighting operational profiles. Table 4. Lighting operational profiles. Lighting Operational Profiles Office Pantry Circulation Server Room Lighting Operational Profiles Office Pantry Circulation Server Room Timing % % % % 00:00–Timing07:30 0 % 0 % 0 % %0 07:3000:00–07:30–08:00 100 0 25 0 50 0 00 08:0007:30–08:00–09:00 100 100 50 25 100 50 050 08:00–09:00 100 50 100 50 09:00–12:30 100 50 100 0 09:00–12:30 100 50 100 0 12:3012:30–14:00–14:00 50 50 100 100 100 100 00 14:0014:00–18:00–18:00 100 100 50 50 100 100 5050 18:0018:00–20:00–20:00 100 100 25 25 100 100 00 20:0020:00–22:00–22:00 10 10 0 010 10 00

TheThe office office systems systems were were modelled modelled to to indicate indicate the the heat heat gains gains due due to toexternal external equipment equipment used used in in thethe building. building. Table Table 55 indicates indicates the the internal internal heat heat gains gains from from the the equipment equipment,, while while Table Table 6 6provides provides a a summarysummary of of the the office office systems systems’’ internal heat gain.gain. FiveFive individualindividual coolingcooling systems,systems, namelynamely the the fan fan coil coilsystem, system variable, variable air volume,air volume, variable variable refrigerant refrigerant flow, flow, monodraught monodraught cool-phase cool-phase and districtand district cooling coolingwere modelled were modelled to study to itsstudy effect its oneffect energy, on energy, IAQ and IAQ thermal and thermal comfort. comfort. Full details Full details of the of systems the systemsmodelled modelled are presented are presented in Tables in7 Tables–12. 7–12.

TableTable 5. 5.EquipmentEquipment’s’s internal internal heat heat gains gains (W) (W)..

EquipmentEquipment Heat HeatGains Gains (W) (W)Number Number of Equipment’s of Equipment’s Computer 50 58 Computer 50 58 PeoplePeople 140 14080 80 PhotocopierPhotocopier 540 5403 3 MicrowaveMicrowave 11411141 1 1 DishwasherDishwasher 264 2641 1 RefrigeratorRefrigerator 310 3101 1 Coffee Machine 352 2 CoffeeWater Machine Filter 352 3522 2 Water Filter 352 2

Table 6. Ofﬁce system internal heat gains (W/m2). Table 6. Office system internal heat gains (W/m2). Systems Heat Gains (W/m2) Systems Heat Gains (W/m2) LightingLighting 12 12 Computer Rack 200 Computer Rack 200 Conference Equipment 5 Conference Equipment 5

Table 7. Cooling systems modelled. Table 7. Cooling systems modelled.

SchematicSchematic Description Fan Coil Unit with Variable Speed Drive Fan: The air side Fan Coil Unit with Variable Speed Drive Fan: The air side consisted of the cooling coil, heating coil, supply fan and consisted of the cooling coil, heating coil, supply fan and extract extract fan. The fan coil cooler controller temperature was set fan. The fan coil cooler controller temperature was set to 24 °C. to 24 ◦C. Similar air ﬂow rates had been inputted in the Similar air flow rates had been inputted in the simulation. The simulation. The water side had district cooling performance water side had district cooling performance curve. Apache curve. Apache Heating, Ventilation and Air-Conditioning Heating, Ventilation and Air-Conditioning (HVAC) did not (HVAC) did not allow the user to manually modify the allow the user to manually modify the imported HVAC systems imported HVAC systems components and henceforth they components and henceforth they could not be removed, and the couldonly option not be was removed, to switch and off thethe heating only option coil. was to switch off the heating coil. Fluids 2018, 3, 77 8 of 20

Table 7. Cont. Fluids 2018, 3, x FOR PEER REVIEW 8 of 20 FluidsFluids 2018 2018, 3, 3, ,x x FOR FOR PEER PEER REVIEW REVIEW 88 of of 20 20 Fluids 2018, 3, x FOR PEERSchematic REVIEW Description 8 of 20

Variable Air Volume: This system had been modelled in Variable Air Volume: This system had been modelled in Apache ApacheVariableVariable HVAC Air Air Volume: Volume: by importing This This system system a prototype had had been been modelled Variable modelled in Air in Apache Apache Volume HVAC by importing a prototype Variable Air Volume (VAV) (VAV)HVACHVAC system by by importing importing from the a a prototype library.prototype The Variable Variable air side Air Air network Volume Volume consisted(VAV) (VAV) system from the library. The air side network consisted of the ofsystemsystem the cooling from from the coil,the library. library. supply The The and air air extractside side network network fan, reheating consisted consisted coil of of the inthe cooling coil, supply and extract fan, reheating coil in VAV box. VAVcoolingcooling box. coil, coil, Different supply supply spacesand and extract extract had fan, beenfan, reheating reheating zoned separatelycoil coil in in VAV VAV to box. box. help Different spaces had been zoned separately to help maintain the maintainDifferentDifferent thespaces spaces spatial had had bee functionbeenn zoned zoned using separately separately multiplexing. to to help help maintain maintain For the the the spatial function using multiplexing. For the waterside the system spatialspatial function function using using multiplexing. multiplexing. For For the the waterside waterside the the system system watersidehad been connected the system from had district been cooling connected to supply from chilled district water. cooling hadhad been been connected connected from from district district cooling cooling to to supply supply chilled chilled water. water. tohad supply been connected chilled water. from district cooling to supply chilled water.

VariableVariable RefrigerantRefrigerant Flow: Flow: ThisThis system system had had been been modelled modelled in in VariableVariable Refrigerant Refrigerant Flow: Flow: This This system system had had been been modelled modelled in in thethe ApacheApache HVAC HVAC by importingby importing a packaged a packaged terminal terminal thethe ApacheApache HVACHVAC byby importingimporting aa packagedpackaged terminalterminal air-conditioningair-conditioning system (PTAC)(PTAC) fromfrom thethe library.library. Different Different airair-conditioning-conditioning systemsystem (PTAC)(PTAC) fromfrom thethe library.library. DifferentDifferent spacesspaces hadhad been specifiedbeen specified using multiplexing. using multiplexing. spacesspaces hadhad beenbeen specifiedspecified usingusing multiplexing.multiplexing. Part-load-chiller-curvePart-load-chiller-curve in in chiller componentcomponent was chosenchosen toto PartPart-load-load-chiller-chiller-curve-curve inin chillerchiller componentcomponent waswas chosenchosen toto replicatereplicate the d directirect expansion expansion cooling cooling model model that thatonly onlyconsists consists of replicatereplicate the the d directirect expansion expansion cooling cooling model model that that only only consists consists of of ofcompressor, compressor, condenser condenser and and evaporator. evaporator. The The air airside side network network compressor,compressor, condensercondenser andand evaporator.evaporator. TheThe airair sideside networknetwork comprisedcomprised of PTAC fan, damper, damper, cooling cooling coil, coil, heating heating coil, coil, time time comprisedcomprised of of PTAC PTAC fan, fan, damper, damper, cooling cooling coil, coil, heating heating coil, coil, time time switch,switch, airair flowflow rate rate controller. controller. Other Other parameter parameterss such such as chiller as switch,switch, air air flow flow rate rate controller. controller. Other Other parameter parameters ssuch such as as chiller chiller water pumps, condenser water pump and cooling tower had chillerwaterwater waterpupumps,mps, pumps, condenser condenser condenser water water pumppump water and and pump cooling cooling and towercoolingtower had had been set to zero. Then, this system was finally linked in Apache towerbeenbeen set hadset to to beenzero. zero. setThen Then to, , zero.this this system system Then, was thiswas finally systemfinally linked linked was in finallyin Apache Apache linkedHVAC into Apachesimulate HVACthe results. to simulate the results. HVACHVAC to to simulate simulate the the results. results. Monodraught Cool-Phase: Monodraught Cool-Phase was MonodraughtMonodraughtMonodraught Cool-Phase:CoolCool-Phase:-Phase: MonodraughtMonodraughtMonodraught Cool-PhaseCoolCool-Phase-Phase waswaswas modelled using a navigator in IES-VE. Specific requirements for modelledmodelledmodelled using using a aa n navigatornavigatoravigator in inin IES IES IES-VE.-VE.-VE. S Specificpecific Specific requirements requirements requirements for for assigning zones, setting operation templates were further set in forassigningassigning assigning zones, zones, zones, setting setting setting operation operation operation templates templates templates were were further further were furtherset set in in navigator. The cool-phase system was assigned to a zone setnavigatornavigator in navigator.. . TheThe cool Thecool-phase cool-phase-phase systemsystem system waswas wasassignedassigned assigned toto a toa zone azone zone followed by setting the operational parameters and profiles. followedfollowedfollowed by byby setting settingsetting the thethe operational operationaloperational parameters parametersparameters and andand profiles. profiles.profiles. Then this system had to be further applied to Apache HVAC for ThenThenThen thisthis this system system had hadhad to to be be be further further further applied applied applied to to Apa toApa Apachecheche HVAC HVAC HVAC for for saving the set up. Room and system load calculations had been forsavingsaving saving the the theset set up. setup. up.Room Room Room and and system andsystem system load load calculations loadcalculations calculations had had been been had performed to size the system and finally the system was linked in beenperformedperformed performed to to size size tothe the size system system the and system and finally finally and the the finally system system the was was system linked linked in was in Apache Simulation to simulate the results. linkedApacheApache in Simulation Simulation Apache Simulation to to simulate simulate the to the simulateresults. results. the results.

District Cooling: District Cooling had been integrated to DistrictDistrictDistrict Cooling: Cooling:Cooling: District DistrictDistrict Cooling CoolingCooling had hadhad been beenbeen integrated integratedintegrated to toto replicate the actual scenario with the chilled water served from a replicatereplicate the the actual actual scenario scenario with with the the chilled chilled water water served served from from a a replicatedistrict cooling the actual plant scenario located 4 with KM theaway. chilled Thus, watera virtual served district from districtdistrict cooling cooling plant plant located located 4 4 KM KM away. away. Thus Thus, ,a a virtual virtual district district acooling district had cooling been plantadded located in the 4Apache KM away. HVAC Thus, water a virtualside by coolingcooling had had been been added added in in the the Apache Apache HVAC HVAC w waterater side side by by districtadding coolingand modifying had been the added default in electric the Apache water-cooled HVAC chiller water addingadding and and modifying modifying the the default default electric electric water water-cooled-cooled chiller chiller sidewith by part adding load andcurve modifying in chiller theset defaultand setting electric zero water-cooled watts for withwith part part load load curvecurve in in chiller chiller set set and and setting setting zerozero watts watts for for chillerprimary with pump part power, load condenser curve in chiller pump setand andcooling setting tower zero fan wattson primaryprimary pump pump power, power, condenser condenser pump pump and and cooling cooling tower tower fan fan on on forthe primary cooling side pump. power, condenser pump and cooling tower thethe cooling cooling side side. . fanthe oncooling the cooling side. side.

Table 8. Design variables for Baseline Fan Coil Unit (FCU). TableTableTable 8 8. 8..D DesignDesignesign variables variables variables for for for Baseline Baseline Baseline Fan Fan Fan Coil Coil Coil Unit Unit Unit ( FCU(FCU (FCU).).) . BaseBase Model Model FCU FCU OpenOpen Offices Offices ConferenceConference Room Room ServerServer Room Room CorridorsCorridors BaseBase Model Model FCU FCU Open Open Offices Offices Conference Conference Room Room Server Server Room Room Corridors Corridors Central Fan Airflow (L/s) 800 CentralCentral Fan Fan Airflow Airflow ( L/(L/s)s) 800800 LocalCentral Fan Airflow Fan Airflow (L/s) (L/s) 300 800 LocalLocal Fan Fan Airflow Airflow ( L(L/s)/s) 300300 FanLocal Energy Fan Airflow Source (L/s)Office 300Meter FanFan FanEnergy Energy Energy Source Source Source OfficeOffice Office Meter Meter Meter Cooling Capacity (kW) 30.976 30.976 3.88 2.99 CoolingCoolingCooling Capacity Capacity Capacity (kW) (kW) (kW) 30.97630.976 30.976 30.97630.976 30.976 3.883.88 3.88 2.992.99 Controls Time Switch ControlsControlsControls TimeTime Time Switch Switch Switch

Table 9. Design variables for FCU with Variable Speed Drive (VSD). TableTable 9 9. .Design Design variables variables for for FCU FCU with with Variable Variable Speed Speed Drive Drive ( VSD(VSD).) . VSD FCU Open Offices Conference Room Server Room Corridors VSDVSD FCU FCU OpenOpen Offices Offices ConferenceConference Room Room ServerServer Room Room CorridorsCorridors Central Fan Airflow (L/s) 800 CentralCentral Fan Fan Airflow Airflow ( L(L/s)/s) 800800 Local Fan Airflow (L/s) 300 LocalLocal Fan Fan Airflow Airflow ( L(L/s)/s) 300300 Fan Energy Source Office Meter FanFan Energy Energy Source Source OfficeOffice Meter Meter Fan Characteristic VSD FanFan Characteristic Characteristic VSDVSD Cooling Capacity (kW) 30.976 30.976 3.88 2.99 CoolingCooling Capacity Capacity (kW) (kW) 30.97630.976 30.97630.976 3.883.88 2.992.99 Controls Time Switch ControlsControls TimeTime Switch Switch Fluids 2018, 3, 77 9 of 20

Table 9. Design variables for FCU with Variable Speed Drive (VSD).

VSD FCU Open Offices Conference Room Server Room Corridors Central Fan Airflow (L/s) 800 Local Fan Airflow (L/s) 300 Fan Energy Source Office Meter Fan Characteristic VSD Cooling Capacity (kW) 30.976 30.976 3.88 2.99 Controls Time Switch

Table 10. Design variables for the Variable Air Volume (VAV) system.

Conference Room + VAV System Open Offices Conference Rooms Corridors Server Room Central Fan Airflow (L/s) 4719.47 Air Handling Unit Fan Energy Source Office Meter Cooling Capacity (kW) 100 Controls Time Switch

Table 11. Design variables for the Variable Refrigerant Flow (VRF) system.

VRF System Open Offices Conference Room Server Room Corridors Packaged Terminal Fan 471.95 Airflow (L/s) Fan Energy Source Office Meter Cooling Capacity (kW) 30 30 30 14 Controls Time Switch, Independent controller with sensor

Table 12. Design variables for the Monodraught Cool Phase system.

Monodraught Cool Phase System Open Offices Conference Room Server Room Corridors Cool-Phase Fan Airflow (L/s) 294 Fan Energy Source Office Meter Fan Characteristic Variable Volume Cooling Capacity (kW) 4.1 Controls Time Switch, controller with CO2 Sensors, differential controllers

Most of the design variables for different HVAC systems are very similar because the office had an open plan layout as most of the spatial footprint had been kept very repetitive and were also being served by the same Air Handling Unit (AHU) (central fan). The system was designed as per to the overall cooling capacity replicating the actual FCU layout of the office. All of the modelled HVAC systems have totally different operating mechanism as the other systems are completely mechanical based, while the monodraught (Table 12) is a hybrid system. Hence, there is difference in power capacity and these figures are directly taken from the IES-VE database in response to the actual cooling load of the space. For thermal comfort modelling input variables (Table 13), the default values from IES in comfort parameters were retained with a clothing level of 0.69, activity level 90 and nominal design air speed of 0.15 m/s.

Table 13. Thermal comfort modelling input variables. Activity Variable Clothing Level (clo) 0.69 Activity Level (W) 90 Nominal Air Speed (m/s) 0.15 Fluids 2018, 3, 77 10 of 20

3. Results and Discussion

3.1. Energy and Carbon The annual energy consumption (Table 14) for the baseline office model was 227.78 MWh with a breakdown of 59.7 MWh for district cooling and 168 MWh from office utility. Energy intensity unit Fluids(Figure 20181, )3, of x FOR 331.5 PEER kWh/m REVIEW2 was competitive with the industry benchmarks as it was 0.45%10 lower of 20 than the local Emirates Green Building Council (EGBC) guidelines. When compared with the Energy baselineConsumption office model Guide 19was (ECON19) 118,219 KgCO [15], it2, waswhich 7.40% translated lower thanto a thecarbon typical intensity prestige (Table air-conditioned 14) of 172 2 KgCOoffice2 and/m .29.4% higher than a good practice benchmark. While the annual carbon emission for baseline The comparison of carbon intensities with ECON19 showed a variation of 7.63% against the2 office model was 118,219 KgCO2, which translated to a carbon intensity (Table 14) of 172 KgCO2/m . typicalThe office comparison benchmark. of carbonHowever, intensities at the same with ECON19time, the showedbaseline aoffice variation model of was 7.63% 29.4% against higher the thantypical the office good benchmark.practice carbon However, benchmark at the from same ECON19 time, the, baselinewhich could office only model be wasbecause 29.4% the higher ECON19 than isthe a British good practice standard carbon which benchmark takes into fromaccount ECON19, the climatic which conditions could only of be UK because region the over ECON19 a longer is a periodBritish of standard time. The which energy takes intoand accountcarbon theresults climatic proved conditions that the of baseline UK region office over model a longer was period very of competitivetime. The energy as it was and able carbon to retain results its proved performance that the even baseline after officeeight years model of was operation. very competitive as it wasThe able implementation to retain its performance of Variable even Speed after eight Drive years (VSD) of operation. fan in a baseline FCU system was a practicalThe retrofit implementation option with of Variablea reduction Speed of 8% Drive in annual (VSD) fan energy in a baselineconsumption FCU systemand carbon was aemission practical byretrofit targeting option cooling with aand reduction fan energy of 8% with in annual a significant energy reduction consumption of 20% and and carbon 9.90 emission% (Figure by 2) targeting, which cancooling be further and fan seen energy from with Table a significant5. Ease of installation reduction of in 20% office and environment 9.90% (Figure causing2), which less can downtime be further at moderateseen from cost Table can5. be Ease regarded of installation as some in of office the advantages. environment On causing the other less hand, downtime the ad atoption moderate of VAV cost coolingcan be regardedsystem was as some not a of practical the advantages. solution On as theit increased other hand, the the energy adoption consumption of VAV cooling and carbon system emissionwas not by a practical 42% because solution of the as greater it increased number the of energy air side consumption and water side and components. carbon emission The air by side 42% consistedbecause ofof thethe greaterfilter, cooling number coil, of air supply side andfan, waterexhaust side fan components. and VAV box. The However, air side consisted for the water of the side,filter, virtual cooling district coil, supply cooling fan, performance exhaust fan curve and VAVset, and box. pumps However, had for been the utilised water side, for modelling virtual district the VAVcooling system. performance curve set, and pumps had been utilised for modelling the VAV system.

Annual energy Intensity unit 400

350 358 300 331.5 333

250

200 234

150

100 Annual Energy kWh/m2 Annual Energy 50

0 IES Benchmark Model EGBC Retrofit Guide ECON19 (Good Practice) ECON19 (Typical)

Annual Energy kWhr/m2

Figure 2. Annual energy intensity unit comparison. Figure 2. Annual energy intensity unit comparison. The VRF system was one of the most energy efficient cooling systems, as it was able to cut down The VRF system was one of the most energy efficient cooling systems, as it was able to cut the annual energy consumption and carbon emission by 30% with 79.2% reduction directly from down the annual energy consumption and carbon emission by 30% with 79.2% reduction directly cooling because of the use of direct expansion cooling coil on the air side and scroll compressor on the from cooling because of the use of direct expansion cooling coil on the air side and scroll compressor water side to regulate the flow of refrigerant as per the cooling demand. It can also be observed that on the water side to regulate the flow of refrigerant as per the cooling demand. It can also be the number of components on the air side and water side were significantly less because the system observed that the number of components on the air side and water side were significantly less was completely based upon the split system. because the system was completely based upon the split system. Monodraught cool-phase was the second most energy efficient cooling system which was able to reduce the annual energy consumption and carbon emission by 22% with specific cooling energy and central fan energy saving up to 18.5% and 70.6%. This cooling system took advantage of the intelligent AHU system and thermal batteries to absorb heat [16]. It also did not have any compressor or chiller on water side. While the air side only had sensors, actuators, VSD fan, controls and humidity monitoring systems.

Fluids 2018, 3, 77 11 of 20

Monodraught cool-phase was the second most energy efficient cooling system which was able to reduce the annual energy consumption and carbon emission by 22% with specific cooling energy Fluids 2018, 3, x FOR PEER REVIEW 11 of 20 and central fan energy saving up to 18.5% and 70.6%. This cooling system took advantage of the intelligentTable AHU 14 system. Energy andand thermalcarbon variation batteries due to to absorb implementation heat [16]. Itof alsodifferent did notcooling have systems any compressor. or chiller on water side. While the air side only had sensors, actuators, VSD fan, controls and humidity Annual Energy Annual Carbon monitoring systems.Annual Annual Carbon Cooling System Energy Reduction % Reduction % Energy Carbon Emission Type (kWh/m2) Compared to Compared to (MWh) (kgCO2) (KgCO2/m2) Table 14. Energy and carbon variationBaseline due toimplementation of different cooling systems.Baseline Fan Coil Unit 227.78 Annual331.5 Annual- Energy 118,219Annual Carbon172 Annual- Carbon Energy (BaselineCooling Office) System Type Energy Reduction % Carbon Emission Reduction % (kWh/m2) Fan Coil Unit with (MWh) Compared to Baseline (kgCO ) (KgCO /m2) Compared to Baseline 209 304.1 8% 108,494 2 158 2 8% Fan CoilVSD Unit (Baseline Office) 227.78 331.5 - 118,219 172 - FanVAV Coil Unit with VSD322.82 209469.8 304.1−42% 8%167,547 108,494 244 158 −42% 8% VRF VAV158.67 322.82230.9 469.8 30% −42%82,351 167,547 120 244 −30%42% MonodraughtVRF 158.67 230.9 30% 82,351 120 30% Monodraught Cool Phase178.01 178.01259.0 259.022% 22%92,391 92,391 134 134 22% Cool Phase

AsAs observedobserved fromfrom Figure Figure3, July3, July had thehad highest the highest electricity electricity consumption consumption because ofbecause the extremely of the extremelyhigh temperatures high temperatures during peak during summer peak months summer and months February and had February the lowest had energy the lowest consumption energy consumptionbecause of lower because temperatures of lower duringtemperatures winter monthsduring winter showing months a strong show correlationing a strong between correlation the site betweenmicroclimate the site and microclimate energy consumption. and energy The consumption energy trend.had The started energy to trend gradually had started increase to from gradually March increaseonwards from and March came back onwards to normal and came during back mid-October. to normal during While themid energy-October trend. While declined the energy signiﬁcantly trend dbetweeneclined significantly the months of between November the months and February. of November and February.

40.00

35.00

30.00

25.00

20.00

15.00 Energy (MWh) 10.00

5.00

0.00 Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Months

Baseline FCU VSD Fan in Baseline Model VAV VRF Monodraught Cool Phase

FigureFigure 3. 3. TTrendrend in in monthly monthly energy energy consumption consumption for for different different cooling cooling systems systems.. During the peak month of July, the VAV system had demonstrated 53% higher energy consumption During the peak month of July, the VAV system had demonstrated 53% higher energy in contrast to baseline ofﬁce FCU system because of the greater number of components on the air side consumption in contrast to baseline office FCU system because of the greater number of components and water side. The energy trend showed a sharp variation throughout the different months of the on the air side and water side. The energy trend showed a sharp variation throughout the different year because of the external conditions. The VAV system is commonly known to occupy larger spatial months of the year because of the external conditions. The VAV system is commonly known to footprint in any building because of the large sized components such as AHU, chillers, pumps and occupy larger spatial footprint in any building because of the large sized components such as AHU, cooling towers, which are also very energy demanding. chillers, pumps and cooling towers, which are also very energy demanding. Other cooling systems such as the VRF system had demonstrated an energy saving potential of Other cooling systems such as the VRF system had demonstrated an energy saving potential of 41% by eliminating AHU on the air side and chiller, pumps, cooling towers on water-side during peak 41% by eliminating AHU on the air side and chiller, pumps, cooling towers on water-side during month. The annual energy trend for VRF system was very steady throughout the months. The adoption peak month. The annual energy trend for VRF system was very steady throughout the months. The adoption of DX cooling coil in the split system was also very beneficial as it allowed the system to have indoor and outdoor units. The evaporator and fan were integrated in indoor units and the scroll compressor, condenser, expansion valve was located in an outdoor unit of the system. The implementation of VSD in the FCU system had reduced the energy by 13.24% in comparison with the baseline office FCU as the variable speed drive fan matches the air supply to Fluids 2018, 3, 77 12 of 20

of DX cooling coil in the split system was also very beneficial as it allowed the system to have indoor and outdoor units. The evaporator and fan were integrated in indoor units and the scroll compressor, Fluidscondenser, 2018, 3, x expansionFOR PEER REVIEW valve was located in an outdoor unit of the system. 12 of 20 The implementation of VSD in the FCU system had reduced the energy by 13.24% in comparison coolingwith the requirement baseline office of space FCU which as the helps variable to conserve speed energy drive and fan provided matches themore air tighter supply control to cooling. The annualrequirement energy of trend space was which quite helps similar to conserve to the baseline energy FCU and system provided with more an tighteraverage control. variation The of annual 8%. energyThe trendenergy was trend quite for similar the toMonodraught the baseline FCUcool- systemphase was with a an bit average different variation as the of peak 8%. energy consumptionThe energy was trendobtained for during the Monodraught August with cool-phase 27% energy was reduction a bit different against asthe the baseline peak energyoffice FCUconsumption as this system was took obtained advantage during of Augustintelligent with AHU 27% with energy CO2 reductionsensors to match against the the cold baseline-air supply office withFCU cooling as this systemdemand took, which advantage represented of intelligent August AHU as withthe peak CO2 sensorsmonth and to match February the cold-air as thesupply base month.with cooling The annual demand, energy which trend represented for the cool August-phase as theshowed peak a month slight and increase February throughout as the base different month. months.The annual energy trend for the cool-phase showed a slight increase throughout different months. FurtherFurther detailed detailed analysis waswas performedperformed by by interpreting interpreting the the space space cooling cooling energy energy and fanand energy fan energyof different of different proposed proposed cooling cooling systems. systems. The annual The space annual cooling space energy cooling (Figure energy4) was(Figure determined 4) was determinedby integrating by integrating all the proposed all the cooling proposed systems cooling to thesystems IES baseline to the IES office baseline model. office The model. VAV system The VAVpossessed system the possessed highest space the highest cooling space energy, cooling which energy even surpassed, which even the baselinesurpassed FCU. the Thebaseline VRF systemFCU. Thehad VRF outperformed system had all outper the otherformed cooling all the systems otherby cooling showing systems a reduction by showing of 79% a against reduction the baselineof 79% againstFCU, which the baseline proves FCU it to, be which of the proves most energy it to be efficient of the most cooling energy systems efficient in the cooling industry. systems However, in the the industry.space cooling However, energy the from space FCU cooling with energy VSD and from the FCU monodraught with VSD cooland phasethe monodraught was very close cool by ph 1.43%ase wasbecause very bothclose of by the 1.43% systems because utilised both VSD of the fans. systems Energy utilised saving VSD of 19.62% fans. wasEnergy achieved saving by of implementing 19.62% was achievedVSD fans by in implementing FCU, which proves VSD fans it to in be FCU a very, which feasible proves option it to considering be a very feasible the aspect option of effort, considering time and thecost, aspect as it of is aeffort, plug time and playand cost application., as it is a plug and play application.

160

140

120

100

80 141.67 60

40 59.72 20 48 48.69 12.48

Annual Space Cooling Energy (MWh) Energy Cooling Space Annual 0 Baseline FCU FCU with VSD VAV VRF Cool Phase Monodraught Cooling System Types

Figure 4. Annual space cooling energy for different cooling systems. Figure 4. Annual space cooling energy for different cooling systems. The annual fan energy consumption (Figure5) was obtained from IES to determine the performanceThe annual of differentfan energy cooling consumption systems. (Figure It can be 5) observed was obtained that both from of IES the fanto determine coil unit (FCU) the performancesystems had of central different and cooling local fans systems. to provide It can a sufficientbe observed amount that both of airflow of the for fan cooling, coil unit while (FCU) the systemsVAV, VRF had and central Monodraught and local onlyfans reliedto provide on central a sufficient fans. The amount baseline of airflow FCU had for an cooling annual, w fanhile energy the VAV,consumption VRF and of Monodraught 73.69 MWh as theonly highest. relied Theon central source offans. energy The forbaseline central FCU and localhad fansan annual is the officefan energymeter-1. consumption Since different of 73.69 offices MWh within as thethe buildinghighest. The have source their own of energy submeter. for central The central and local fan energy fans is is thehigher office because meter- it1. is Since the primary different fan offices inside within the AHU. the Thebuilding fan for have the monodraughttheir own submeter. cool-phase The proved central to fanbe theenergy most is energy higher efficient because as it it is had the dropped primary the fan fan inside energy the consumption AHU. The fan by 74%for sincethe monodraught it had utilised coola single-phas VSDe proved fan as to a be central the most fan. energy The VRF efficient system as and it had FCU dropped with VSD the had fan demonstrated energy consumption a fan energy by 74%saving since of it 30% had and utilised 9.62%. a Thesingle central VSD fanfan inas VAVa central system fan. had The the VRF highest system energy and consumptionFCU with VSD because had demonstrated a fan energy saving of 30% and 9.62%. The central fan in VAV system had the highest energy consumption because it heavily relied on traditional fan technology. This overall proved that the VRF and Monodraught cool phase are some of the most energy efficient cooling technologies. The central fan energy use was much higher than the joint FCU fan energy use because it was the assortment of the complete energy use of individual FCU units. Fluids 2018, 3, 77 13 of 20 it heavily relied on traditional fan technology. This overall proved that the VRF and Monodraught cool phase are some of the most energy efficient cooling technologies. The central fan energy use was much higher than the joint FCU fan energy use because it was the assortment of the complete energy Fluidsuse of2018 individual, 3, x FOR PEER FCU REVIEW units. 13 of 20

70 8.72 6.33 60 50 40 69.56 30 64.97 60.34 51.49 20

10 19.1 0 Annual Energy Consumption (MWh) Consumption Annual Energy Baseline FCU FCU with VSD VAV VRF Cool Phase Monodraught Types of HVAC

Central Fan MWh Local Fan MWh

Figure 5. Annual energy consumption for the compared cooling systems. Figure 5. Annual energy consumption for the compared cooling systems. 3.2. Indoor Air Quality (IAQ) 3.2. Indoor Air Quality (IAQ) CO2 concentration was used as a parameter to examine the indoor air quality after the implementationCO2 concentration of various was cooling used systemsas a parameter in office model to examine because the the healthindoor andair well-beingquality after of office the implementationemployees plays of a crucialvarious role cooling towards systems utmost in productivityoffice model ofbecause any organization. the health and Numerous well-being studies of officehave indicatedemployees the plays significance a crucial of role evaluating towards the utmost indoor productivity air quality becauseof any organization people spend. Numerous more than studies90% of theirhave workingindicated hours the significance in the office environment.of evaluating Inadequatethe indoor ventilationair quality bybecause HVAC people systems spend could morebe identified than 90% as of some their of working the major hours causes in forthe poor office indoor environment. air quality. Inadequate ventilation by HVAC systems could be identified as some of the major causes for poor indoor air quality. The average CO2 concentration (Table 15) for the baseline office model was 624.2 PPM being fairlyThe adequate average as CO it2 was concentration within the acceptable(Table 15) for range the ofbaseline 600 PPM office and model 800 PPM was [ 17624.2]. The PPM adoption being fairly adequate as it was within the acceptable range of 600 PPM and 800 PPM [17]. The adoption of of the VSD fan in the baseline fan coil unit (FCU) did not impact the indoor air quality as the CO2 theconcentration VSD fan in was the fairlybaseline consistent fan coilbecause unit (FCU) it was did a not part impact of medium the indoor retrofitting, air quality which as was thebeing CO2 concentrationimplemented was by replacing fairly consistent the fan. because The VAV it systemwas a part was of able medium to enhance retrofitting the indoor, which air was quality being by implemented by replacing the fan. The VAV system was able to enhance the indoor air quality by showing a reduction of 10.36% in CO2 concentration only because the system heavily relied on treating showingoutdoor fresha reduction air in AHU of 10.36% and the in proportion CO2 concentration of fresh air isonly always because maintained the system at 30%. heavily The VRF relied system on treating outdoor fresh air in AHU and the proportion of fresh air is always maintained at 30%. The had shown an average CO2 concentration of 3597 PPM, which meant that the indoor air quality was VRFundesirable system had because shown of the an lackaverage of availability CO2 concentration to supply of fresh 3597 air PPM as the, which cooling meant system that was the completelyindoor air qualitybased upon was undesirable unitary vapor because compression of the withlack of direct availability expansion to technologysupply fresh [18 air]. Onas the the cooling other hand, system the was completely based upon unitary vapor compression with direct expansion technology [18]. On Monodraught Cool Phase system was also able to maintain an appropriate level of CO2 concentration thebeing other 6.1% hand, lower the than Monodra baselineught FCU Cool system Phase because system it was utilised also outdoorable to maintain fresh air an to supplyappropriate the treated level ofair CO into2 concentration the space. being 6.1% lower than baseline FCU system because it utilised outdoor fresh air to supply the treated air into the space.

Table 15. Average CO2 Concentration level for different cooling systems. Table 15. Average CO2 Concentration level for different cooling systems. Fan Coil Unit Monodraught Model BaselineBaseline FCU Fan Coil Unit VAV VRF Model with VSD VAV VRF MonodraughtCool Cool Phase Phase FCU with VSD CO Concentration Levels CO2 Concentration2 Levels (PPM) for 624.2624.2 624.2 624.2 559.5 559.5 3597.0 3597.0586.0 586.0 (PPM)Office for Ofﬁce Model Model

Figure6 shows the CO 2 concentration level maintained throughout 17th July by various cooling systems. The CO2 concentration trend started increasing once the occupants started entering the office from 8:30 a.m. onwards and persisted until 8:30 p.m. The peak CO2 concentration for baseline office model was at 5:30 p.m. due to greater occupancy. The VRF system had drastically failed to Fluids 2018, 3, x FOR PEER REVIEW 14 of 20 maintain an optimum level of CO2 concentration as it had surpassed the safety threshold causing sick building syndrome [17]. While the VAV system and Monodraught cool-phase were able to maintain maintain an optimum level of CO2 concentration proving that the quality of indoor air was suitable an optimum level of CO concentration proving that the quality of indoor air was suitable for human for human occupancy in 2office environment. The cooling system’s ability to supply fresh air into the occupancy in office environment. The cooling system’s ability to supply fresh air into the indoor indoor spaces could be considered as the paramount to maintain a suitable level of CO2 spaces could be considered as the paramount to maintain a suitable level of CO concentration for concentration for creating a pleasant work environment. The trend for the VRF in Figure2 6 somehow creating a pleasant work environment. The trend for the VRF in Figure6 somehow supports the supports the average CO2 concentration level from Table 15 and justifies the incapability of VRF average CO concentration level from Table 15 and justifies the incapability of VRF cooling systems cooling systems2 to provide sufficient indoor air quality to meet the industry standards due to the to provide sufficient indoor air quality to meet the industry standards due to the lack of additional lack of additional ventilation systems. ventilation systems.

Co2 Concentration

4500 4000 3500 3000 2500 2000 1500

Co2 (PPM) Co2 Concentration 1000 500 0

Baseline Office Fan Coil Unit with VSD VAV VRF Monodraught Cool Phase

Figure 6. Trend in CO2 Concentration for different cooling systems on 17th July (reference day). Figure 6. Trend in CO2 Concentration for different cooling systems on 17th July (reference day). 3.3. Thermal Comfort 3.3. Thermal Comfort Predicted Mean Vote (PMV) was used to perform the thermal comfort analysis using different coolingPredicted systems Mean in the Vote office (PMV) model was and used the values to perform had been the thermal obtained comfort from IES-VE analysis Vista-Pro. using different The PMV coolingis based systems upon the in Fangersthe office scale, model which and isthe also values referred had tobeen as ASHRAEobtained from thermal IES- sensationVE Vista- scalePro. The and PMVmostly is based ranges upon between the F−angers3 (Cold) scale, to +3 whic (Hot).h is also referred to as ASHRAE thermal sensation scale and mostlyThe average ranges PMVbetween for − the3 (Cold) baseline to +3 office (Hot). model was obtained for different cooling systems to analyseThe theaverage performance PMV for of the cooling baseline system office to model achieve was the obtained desirable for thermal different comfort cooling satisfaction systems forto analyseoccupants. the performance According to of ASHRAE cooling system standard to achieve 55, the PMVthe d valuesesirable varying thermal between comfort −satisfaction0.5 to +0.5 for are occupants.within the acceptableAccording humanto ASHRAE comfort standard range. As55, perthe TablePMV 16 values above, varying the baseline between office −0.5 with to FCU+0.5 are had withinan average the acceptable PMV of 0.53human being comfort close range. to +0.5, As which per Table means 16 thatabove it, wasthe baseline compliant office with with ASHRAE FCU had 55. anWhile average the adoption PMV of 0.53 of VSD being fans close in FCU to +0.5 system, which had means slightly that increased it was thecompliant PMV by with 16.5%, ASHRAE which was55. Whilestill within the adoption the acceptable of VSD range.fans in However, FCU system the had VAV slightly system increased had outperformed the PMV otherby 16.5% cooling, which systems was stillas the within PMV the value acceptable was higher range. than theHowever, baseline the FCU VAV by 22.64%. system had outperformed other cooling systems as the PMV value was higher than the baseline FCU by 22.64%.

Table 16. Average Predicted Mean Vote (PMV) for different cooling systems.

Baseline Fan Coil Unit Monodraught Cool Model VAV VRF FCU with VSD Phase Average Predicted Mean Vote (PMV) 0.53 0.62 0.65 1.06 2.22 for the Office Model

The degree of discomfort amongst occupants was greater with the use of VRF and Monodraught because the VRF system did not have any provision for fresh air supply, which somehow deteriorated the thermal comfort satisfaction for occupants, while in Monodraught the Fluids 2018, 3, 77 15 of 20

Table 16. Average Predicted Mean Vote (PMV) for different cooling systems.

Fan Coil Unit Monodraught Model Baseline FCU VAV VRF with VSD Cool Phase Fluids 2018, 3, x FOR PEER REVIEW 15 of 20 Average Predicted Mean Vote 0.53 0.62 0.65 1.06 2.22 thermal(PMV) battery for the (PCM) Ofﬁce Model was not able to absorb the surrounding heat enough to meet the heavy cooling demand of the Middle East. The average PMV for the VRF system was 1.06, which was manageableThe degree as the of values discomfort mostly amongst ranged occupants between 0.7 was and greater 1.41. with the use of VRF and Monodraught because the VRF system did not have any provision for fresh air supply, which somehow deteriorated 3.3.1the. thermal Annual comfort PMV Prediction satisfaction for occupants, while in Monodraught the thermal battery (PCM) was not ableThe annual to absorb PMV the trend surrounding indicates heat long enough-term tothermal meet the comfort heavy satisfaction cooling demand for the of occupants the Middle after East. adoptionThe average of different PMV for cooling the VRF techniques. system was Fro 1.06,m Figure which 7, was it can manageable be observed as thethat values the level mostly of thermal ranged discomfortbetween 0.7 was and mostly 1.41. high between July and August because of the extremely high summer temperatures. A cumulative trend was observed as PMV started increasing from May onwards 3.3.1. Annual PMV Prediction because of the summer outset and remained high until December when PMV started to get back to normalThe at 0.5 annual because PMV of the trend winter. indicates long-term thermal comfort satisfaction for the occupants afterThe adoption PMV values of different for the cooling baseline techniques. office were From mostly Figure within7, it can the beacceptable observed thermal that the comfort level of rangethermal of − discomfort0.5 to +0.5 (ASHRAE was mostly 55) high because between the Julyvalues and wer Auguste within because the range of the and extremely only became high summerhigher bytemperatures. 43% during Amid cumulative-July because trend of was excessive observed heat. as The PMV adoption started increasing of VSD in frombaseline May Fan onwards coil unit because had increasedof the summer the PMV outset range and, which remained had high also untilled to December an increase when in the PMV thermal started discomfort to get back level to normal by 80% at during0.5 because the peak of the interval. winter.

FigureFigure 7. 7. TTrendrend in in annual annual PMV PMV for for different different cooling cooling systems. systems.

TheThe VAV PMV cooling values forsystem the baseline had increased ofﬁce were the mostlyPMV range within with the values acceptable mostly thermal between comfort +1.5 rangeand −of0.05−,0.5 indicating to +0.5 (ASHRAEthat the level 55) of because thermal the discomfort values were had within drastically the range increased and only during became peak highersummer by months.43% during The PMV mid-July range because in VRF of system excessive was heat.a bit constrained The adoption as ofthe VSD upper in baselinelimit was Fan reduced coil unit by 7% had andincreased the level the of PMV thermal range, dissatisfaction which had alsowas ledalso to very an increase similar into thebaseline thermal office discomfort Fan Coil levelUnit by(FCU) 80% becauseduring the peakVRF system interval. used sophisticated controls maintain an optimum temperature for the office occupant.The VAVThe monodraught cooling system cool had-phase increased had drastically the PMV deteriorated range with valuesthe thermal mostly comfort between as the +1.5 PMV and range−0.05, was indicating significantly that the in levelcreased of thermalbecause discomfort the system had used drastically a hybrid increased coolingduring technology peak summerwith a thermal battery, which was not able to absorb the surrounding heat because of the extreme hot climatic conditions of the Middle East.

3.3.2. Daily PMV Prediction Figure 8 displays the PMV trend of the office model on 17th July with the adoption of different cooling techniques. The PMV trend was at peak between 3:30 p.m. and 5:30 p.m. because of the Fluids 2018, 3, 77 16 of 20 months. The PMV range in VRF system was a bit constrained as the upper limit was reduced by 7% and the level of thermal dissatisfaction was also very similar to baseline office Fan Coil Unit Fluids 2018, 3, x FOR PEER REVIEW 16 of 20 (FCU) because the VRF system used sophisticated controls maintain an optimum temperature for the officeintense occupant. solar radiation The monodraught during the peak cool-phase summer had month drastically of July. deteriorated The PMV trend the thermal for the comfort baseline as office the PMVmodel range showed was an significantly acceptable increasedlevel of thermal because comfort the system as most used of athe hybrid values cooling ranged technology between 0.44 with and a thermal0.79, which battery, were which somehow was not between able to 0 absorb (Neutral) the surroundingand 1 (Slightly heat Warm). because The of adoption the extreme of hotthe climaticVariable conditionsSpeed Drive of the(VSD) Middle Fan East. had slightly increased the thermal discomfort level by an average 17% because it gave the fan coil unit the ability to adjust the fan speed according to set point temperature. 3.3.2. Daily PMV Prediction The PMV values mostly varied between 0.45 and 1.14 showing a rise in degree of discomfort in upperFigure range8 displays by 44.30%. the PMV trend of the office model on 17th July with the adoption of different coolingThe techniques. VAV system The was PMV able trend to wasmaintain at peak an betweenoptimum 3:30 level p.m. of andthermal 5:30 p.m.comfort because as the of PMV the intense values solarmostly radiation fluctuated during between the peak 0.28 summer and 1.5 month indicating of July. that The the PMV level trend of discomfort for the baseline had officeincreased model by showed24.5% when an acceptable compared level against of thermal baseline comfort FCU. The as mostPMV oftrend the for values VAV ranged system between had started 0.44 gradually and 0.79, whichincreasing were from somehow 8:30 a.m. between onwards 0 (Neutral) and continued and 1 (Slightly increasing Warm). until The 5:30 adoption a.m. The of VRF the Variablesystem showed Speed Drivea steady (VSD) rise Fan in had PMV slightly from increased 8:30 a.m. the and thermal continued discomfort increasing level byuntil an average6:30 p.m 17%. However, because it in gave the theMonodraught fan coil unit Cool the ability Phase to System, adjust the the fan PMV speed trend according had significant to set point variation temperature. as the The PMV PMV gradually values mostlyincreased varied from between 9:30 a.m. 0.45 and and only 1.14 declined showing from a rise 7:30 in degreep.m. of discomfort in upper range by 44.30%.

2.5

1.5 PMV 1

0.5

0 0:30 1:30 2:30 3:30 4:30 5:30 6:30 7:30 8:30 9:30 10:30 11:30 12:30 13:30 14:30 15:30 16:30 17:30 18:30 19:30 20:30 21:30 22:30 23:30 Time of the day Baseline Office Fan Coil Unit with VSD VAV VRF Monodraught Cool Phase

FigureFigure 8.8. TrendTrend inin PMVPMV forfor differentdifferent cooling cooling systems systems on on 17th 17th July. July.

TheThe VAV overall system thermal was comfort able to maintainanalysis proves an optimum that the level VRF of and thermal monodraught comfort ascool the-phase PMV systems values mostlywere not fluctuated able to comply between with 0.28 andthe ASHRAE 1.5 indicating 55 because that the of level the ofdesign discomfort failure had and increased climatic variation. by 24.5% whenThe VRF compared cooling against systems baseline suffered FCU. with The the PMV problem trend forof poor VAV indoor system ai hadr quality started, which gradually somehow increasing also fromaffected 8:30 its a.m. thermal onwards comfort and continued performance increasing, while until the Monodraught 5:30 a.m. The VRFcool system-phase was showed a hybrid a steady cooling rise insystem PMV fromthat heavily 8:30 a.m. relied and continuedon the thermal increasing batteries until for 6:30 space p.m. cooling. However, However, in the Monodraughtthe thermal battery Cool Phasecould System,not meet the the PMV sufficient trend hadcooling significant demands variation because as of the the PMV heavy gradually cooling increaseddemands fromas the 9:30 thermal a.m. andbatteries only declinedrequire some from time 7:30 p.m.to get recharged. The overall thermal comfort analysis proves that the VRF and monodraught cool-phase systems were4. Model not able Comparison to comply with with Literature the ASHRAE 55 because of the design failure and climatic variation. The VRF cooling systems suffered with the problem of poor indoor air quality, which somehow also affectedActual its thermal office characteristics comfort performance, and FCU while system the Monodraughthad been modelled cool-phase in the was IES a- hybridVE software cooling to systemreplicate that the heavily baseline relied operation on the of thermal the office batteries building. for The space validation cooling. of However, the model the was thermal performed battery by couldcomparing not meet against the sufficient industry cooling benchmark demands standards because and of theliterature. heavy coolingIES-VE demandssimulations as theare thermal heavily batteriesreliant on require the weather some time data to get of recharged.a particular location as this can have a direct impact on the performance of the building. There are two types of weather files test reference year (TRY) and design summer year (DSY) which include data sets of dry bulb and wet bulb temperature, solar radiation, wind speed, atmospheric pressure, and cloud cover [19]. Dynamic simulation results of the baseline office with the FCU system determined the annual energy consumption to be 227.78 MWh that translated to an energy intensity unit of 331.4 kWh/m2, which was validated against the local building standard as it was 0.45% lower than typical office buildings’ benchmarks from Emirates Green Building Council guidelines [20]. The average CO2 Fluids 2018, 3, 77 17 of 20

4. Model Comparison with Literature Actual office characteristics and FCU system had been modelled in the IES-VE software to replicate the baseline operation of the office building. The validation of the model was performed by comparing against industry benchmark standards and literature. IES-VE simulations are heavily reliant on the weather data of a particular location as this can have a direct impact on the performance of the building. There are two types of weather files test reference year (TRY) and design summer year (DSY) which include data sets of dry bulb and wet bulb temperature, solar radiation, wind speed, atmospheric pressure, and cloud cover [19]. Dynamic simulation results of the baseline office with the FCU system determined the annual energy consumption to be 227.78 MWh that translated to an energy intensity unit of 331.4 kWh/m2, which was validated against the local building standard as it was 0.45% lower than typical office buildings’ benchmarks from Emirates Green Building Council guidelines [20]. The average CO2 concentration was 624.2 PPM, being fairly appropriate as it could be confirmed from an experiment conducted by Ibrahim [21], showing that an average CO2 concentration of 538.5 PPM for office building is tolerable. The American Society of Heating Refrigeration and Air-Conditioning Engineers (ASHRAE) states that any CO2 concentration level below 1000 PPM is fairly acceptable. On the other hand, the baseline office model had an average PMV of 0.53 indicating a moderate satisfaction level (−0.5 < PMV < +0.5) of thermal comfort for occupants in office environments based on the thermal acceptability criteria of ISO 7730 [22]. The addition of VSD in a baseline FCU system had reduced the overall annual energy consumption by 8.24% by targeting space cooling with a significant reduction of 20%. The CO2 concentration was not affected as it remained constant similar to the baseline FCU system, while the PMV value for thermal comfort had slightly increased due to the adoption of a VSD fan that gave the FCU system the ability to control the air-flow with respect to thermostat setting, causing the thermal discomfort to increase because a constant set point temperature was maintained throughout the office [23,24]. The adoption of a VAV cooling system drastically increased the annual energy consumption by 42% due to the greater number of components, which can also be confirmed by a simulation study conducted by Kim et al. [25], showing that VAV consumes 15–42% greater energy than any other HVAC system. Meanwhile, the average CO2 concentration was 559.5 PPM, meaning that it was compliant with the ASHRAE and the average thermal comfort PMV was 0.65, indicating that it was meeting the least satisfaction level (−0.7 < PMV < +0.7) of thermal comfort criteria as per the ISO 7730 [22]. The CO2 concentration within the space can also be used as an indicator to recognize if the ventilation provided by the HVAC system is adequate. A VRF system was proposed as a cooling system to reduce the overall energy consumption and improve the thermal comfort for occupants. The cooling system had successfully demonstrated an annual energy saving potential of 30%, which could also be validated with the help of research undertaken by Aynur et al. [13], showing that the overall energy savings potential mostly ranges between 27.1–57.9% in contrast to other conventional HVAC systems. The performance of the VRF cooling system in terms of indoor air quality and thermal comfort was unsatisfactory as the average CO2 concentration was 3957 PPM and PMV was 1.06, being fairly unacceptable, and this could also be validated with the research led by Bhatia [26] highlighting that one of the main challenges associated with the integrating of VRF systems is the failure to comply with the ASHRAE 62.1 to achieve a satisfactory level of indoor air quality. Another piece of literature by Goetzler [18] pointed out the necessity to integrate additional ventilation systems in conjunction with VRF systems to maintain an appropriate level of level of indoor air quality. Reiterating from the literature, an experimental study conducted by Aynur et al. [13], with the help of integrating an energy recovery ventilator, reflected that the VRF system had shown a vast improvement in the thermal comfort and indoor air quality. The monodraught cool-phase was proposed as a natural cooling technique that helped to cut down the annual energy consumption of the office by 22%. This system was able to maintain an average CO2 concentration of 586 PPM, which is quite acceptable for the office environment in accordance Fluids 2018, 3, 77 18 of 20 with ASHRAE and could also be validated by a study conducted in the University of West England to integrate the monodraught cool-phase system, showing that an average CO2 concentration of 675 PPM is also adequate [16]. The PMV value of 2.22 was unacceptable as the level of thermal discomfort was significantly higher, which could mostly be due to the inability of the thermal battery to get recharged to absorb surrounding heat because of the extreme climatic conditions of UAE.

5. Conclusions This research numerically investigated the most energy efficient cooling technique in order to reduce the energy consumption and carbon emission for office buildings by making sure that the thermal comfort and indoor air quality was appropriate for the office environment. The findings and conclusion of the study have been highlighted as follows. Energy modelling of the office with the FCU system proved that the energy consumption and carbon emission was 331.47 kWh/m2 2 and 172 KgCO2/m being fairly competitive with the typical industry benchmarks considering its age and the fan coil unit (FCU) system also played a crucial role in maintaining a higher level of energy efficiency. However, the CO2 concentration and thermal comfort were also in accordance with the ASHRAE standard. Adoption of variable speed drive (VSD) fans in FCU systems proved to be expedient in reducing the energy consumption and carbon emission by 8%. Its performance in terms of CO2 concentration and thermal comfort was acceptable to meet the occupant comfort satisfaction in office environment. Implementation of VAV systems in the baseline office drastically increased the energy consumption and carbon emission by 42%. However, it had outperformed all the other HVAC systems in terms of thermal comfort satisfaction by having an average PMV of 0.66 and CO2 concentration of 559.5 PPM in accordance with the ASHRAE standards [27]. The VRF cooling technique was one of the most energy efficient systems as it had significantly dropped the energy consumption and carbon emission by 30%. Its overall performance with regards to CO2 concentration and thermal comfort was quite unsatisfactory due to the lack of ability to provide the sufficient ventilation within the office space. This means that having a supplementary ventilation system with the VRF system would be advisable, as it would help to maintain the required ventilation rates to achieve the thermal comfort standards of ASHRAE 55 [27]. On the other hand, the monodraught cool phase system was the second most energy efficient cooling technique as it had proved to reduce the energy consumption and carbon emission by 22%. Its average CO2 concentration was 586 PPM being acceptable with regards to ASHRAE and the PMV was 2.22 being extremely high for occupancy thermal comfort satisfaction because the thermal battery could absorb heat greater than its capacity and thus having an outdoor condenser unit would have been a good solution as it would provide chilled water for cooling coil to provide more effective cooling. Overall, the findings from this study have strongly verified that the cooling systems have a drastic effect on the energy consumption, thermal comfort and indoor air quality of the buildings. The limitation endured due to an excessively high CO2 concentration could be overcome by adding a supplementary ventilation with a VRF system (unitary cooling system) because it would help to provide fresh air supply. In conclusion, finding the most appropriate cooling technique based upon energy efficiency and thermal comfort is challenging considering the local climatic conditions with excessive cooling requirements. The adoption of variable speed drive fans in the FCU system and VAV system can be regarded as one of the most suitable cooling techniques in consideration with the site microclimate. For future work, it would be beneficial to model the VRF system with an additional ventilation system to investigate its impact on thermal comfort and indoor air quality using carrier HAP software. Furthermore, using demand control ventilation in FCU systems will also be beneficial to reduce the energy consumption.

Author Contributions: Conceptualization, H.N.C.; Methodology, Software, Validation and Formal Analysis, Z.S.; Investigation, Z.S. and H.N.C.; Writing—review and editing, Z.S. and H.N.C. Fluids 2018, 3, 77 19 of 20

Funding: This research received no external funding. Acknowledgments: The authors would like to thank Heriot-Watt University for the use of computational facilities in conducting this research. Conﬂicts of Interest: The authors declare no conﬂict of interest.

Nomenclature

Variable Description λ Conductivity of solid element (W/(m2·K)) ρ Density of solid element (kg/m3) C Specific heat capacity of the solid (J/kgK) ◦ Tn Temperature ( C) at node n δn Local node spacing (m) j ◦ Tn Temperature ( C) at node n and time-step j . j Tn Time derivative of temperature (K/s) at node n and time-step j m Air mass flow rate (kg/s) Cp Specific heat capacity of the air at constant pressure (J/kg/K) ◦ Ti Air supply temperature ( C) ◦ Ta Room mean air temperature ( C) V Room volume (m3) a Air change rate (ach) w Water vapour gain (kg/s) gi Humidity ratio of supply air (kg/kg) g Humidity ratio of the room air (kg/kg) ki Carbon dioxide concentration of the supply air (kg/kg) k Carbon dioxide concentration of the room air (kg/kg) Abbreviation Description IES-VE Integrated Environmental Solution Virtual Environment VAV Variable Air Volume System AHU Air Handling Unit VRF Variable Refrigerant Flow System HVAC Heating Ventilation and Air Conditioning FCU Fan Coil Unit VSD Variable Speed Drive

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