Feasibility Study on Variable-Speed Air Conditioner Under Hot Climate Based on Real-Scale Experiment and Energy Simulation

energies

Article Feasibility Study on Variable-Speed Air Conditioner under Hot Climate based on Real-Scale Experiment and Energy Simulation

Jaehun Lim 1,2, Myung Sup Yoon 2, Turki Al-Qahtani 3 and Yujin Nam 1,*

1 Department of Architectural Engineering, Pusan National University, 2 Busandaehak-ro 63, Geomjeong-gu, Busan 46241, Korea; [email protected] 2 Korea Testing Laboratory, 87, Digital-ro, Guro-gu, Seoul 08389, Korea; [email protected] 3 Saudi Standards Metrology and Quality Organization, Al Imam Saud Ibn Abdul Aziz Road, Al Mohammadiyah, Riyadh 11471, Saudi Arabia; [email protected] * Correspondence: [email protected]; Tel.: +82-51-510-7652; Fax: +82-51-514-2230

Received: 19 March 2019; Accepted: 18 April 2019; Published: 19 April 2019

Abstract: It is well known that inverter-driven variable-speed compressor (or inverter) air conditioners are more efficient than constant-speed compressor air conditioners. Therefore, most countries have employed part-load assessment test standards such as ISO 16358, EN 14825 and ASHRAE 116 in addition to the conventional ISO 5151 full-load test standard to appropriately evaluate the part-load efficiencies of variable-speed air conditioners. However, many countries in the Middle East and South America still use the ISO 5151 standard owing to policy or high- temperature environmental considerations. In this study, we experimentally verify the energy saving effect of the inverter air conditioner with respect to the constant-speed air conditioner under the Korean climate with distinct temperature changes of four seasons and under the hot climate of Saudi Arabia throughout the year. ISO 5151 defines test conditions for a single temperature, whereas ISO 16358, EN 14825 and ASHRAE 116 simulate seasonal efficiencies using interpolation of several climate test results. Herein, we directly employ the environmental changes during a day or season in a qualified test room with specific dimension. Using extensive regional and seasonal climate data for Saudi Arabia and South Korea, the changes in temperature conditions are applied directly to the outdoor side and appropriate building cooling load conditions are applied to the indoor side of the air-enthalpy-type test room. The energy savings of the inverter air conditioner were analyzed experimentally according to the spatial and temporal temperature changes. The energy reduction effects of the inverter air conditioner largely depended on the temperature and cooling load changes for a day or season. Furthermore, a feasibility study based on an energy simulation showed that the variable-speed air conditioner could be economical even in hot climates.

Keywords: energy saving; inverter air conditioner; energy eﬃciency; Saudi Arabia; Korea

1. Introduction Air conditioners are essential not only in “hot” countries such as Saudi Arabia but also in Korea. In Korea, air conditioners were luxury items until the 1980s, but gradually became popular since the 1990s. Nowadays, they are used in most homes owing to the increased temperatures in summer by climate changes. The energy labelling program of air conditioners started in 1992 in Korea, as energy resources were scarce and the demand for cooling energy in summer was a large portion of the total electricity demand. In particular, the peak power demand management in summer was a big concern every year.

Energies 2019, 12, 1489; doi:10.3390/en12081489 www.mdpi.com/journal/energies Energies 2019, 12, 1489 2 of 14

Saudi Arabia is the world’s largest oil producer having approximately 20% of the world’s oil reserves. Despite the supply of electricity by the abundant oil resources, the Saudi government has been interested in energy conservation owing to the rapid industrialization and population increase in the 2000s. Saudi Arabia’s crude oil production is not expected to rapidly increase. In addition, the crude oil exports are expected to decrease with the increases in domestic demand. In this regard, Saudi Arabia has mandated the minimum energy efficiency performance standard for room air conditioners since 2013, as the autonomous Saudi Standards, Metrology, and Quality Organization (SASO) 2663/2007 standard has been amended to an enhanced compulsory SASO 2663/2012 standard [1]. The energy efficiency regulation of air-conditioners is applied in most countries, not only in these two countries. The definition of energy efficiency of an air conditioner is largely managed by two approaches based on the simple energy efficiency ratio (EER) by International Organization for Standardization (ISO) 5151 and seasonal EER (SEER) by ISO 16358, European (EN) 14825 and ASHRAE 116 [2–5]. Manufacturers have carried out extensive technological studies to improve the performances of air conditioners. One of the most notable approaches is the compressor technology. A compressor control technology has been developed, which could vary the speed of the compressor in response to a cooling load. It provides a high efficiency of the compressor such as a high efficiency of the compressor motor section, reduction in the mechanical friction of the rotating section, and reduction in the pressure loss of the refrigerant entering and exiting sections. The variable-speed compressor (or inverter compressor) can reduce the operation cost, compared to the constant-speed compressor. In this regard, we need to assess the efficiency of the air conditioner during a part-load operation. Some countries in the Middle East and South America still prefer to use full-load evaluation methods as the ISO 5151 full-load test is simple and the hot climate in most the seasons requires full operation of the air conditioner. However, the winter period in Saudi Arabia does not require air conditioning; in addition, some months before and after the summer peak require a part-load operation. Several studies compared performance differences between constant-speed and variable-speed air conditioners. Chen et al. [6] carried out a test in a specific-size real room with an operation start at a pre-heated temperature and evaluated the effects of the product temperature setting and space heat load. Yurtseven et al. [7] studied real office usage circumstances in a relatively long-term power input measurement. Al-tamimi et al. [8] analyzed the thermal performance and energy consumptions of air conditioner devices for two identical houses located in the Dhahran area of Saudi Arabia for one year. Khatri et al. [9] carried out a long-term field test to compare the energy savings of two types of air conditioners in two specific rooms in India. However, these studies did not analyze the performance differences under controlled temperature variations and building cooling load variations in accurately controlled test rooms. In addition, the performances and economic efficiencies of the air conditioners have been investigated. Wu et al. [10] contributed to the variable air conditioner efficiency standard by monitoring the outdoor temperature distribution and operation time in China. Oropeza-Perez [11] analyzed the economic efficiencies based on several variables such as the set temperature of the air-conditioner in Mexico. Shah et al. [12] carried out a simple payback period analysis according to the efficiency and type of the air conditioner in India. In this study, we analyze the energy saving effects of inverter and constant-speed air conditioners during the cooling periods in Saudi Arabia and South Korea. People usually think that no difference in energy efficiency between constant-speed and inverter-type air conditioners exists in countries with hot climates such as Saudi Arabia. However, few experimental studies based on dynamic energy simulations of variable air conditioners were carried out. In this study, dynamic energy simulation results are applied to a real-scale environmental test chamber. Using the test results, the energy consumptions are compared and the suitability of the constant and variable-speed air conditioners are analyzed. The energy usage of each air-conditioner under the laboratory environmental conditions is evaluated by simulating the climate of South Korea where the four seasons are evenly distributed and hot climate of Saudi Arabia throughout the year. EnergiesEnergies 2019,2019 12 FOR, 12, PEER 1489 REVIEW 3 of 14 Energies 2019, 12 FOR PEER REVIEW 3 2. Materials and Methods 2. Materials Materials and and Methods 2.1. Summary of the Experimental Approach 2.1. Summary Summary of of the the Experimental Experimental Approach In general, the condenser of the air-conditioner is installed at the outdoor side, while the In general, the condenser of the air-conditioner is installed at the outdoor side, while the evaporatorIn isgeneral, installed the at condenserthe indoor ofside the of air-conditionethe building, asr is shown installed in Figure at the 1. outdoor As the outdoor side, while and the evaporator is installed at the indoor side of the building, as shown in Figure1. As the outdoor and indoorevaporator temperatures is installed are not at fixed,the ind i.e.,oor they side vary of the with building, time, the as performanceshown in Figure of the 1. Asair theconditioner, outdoor and indoor temperatures are not fixed, i.e., they vary with time, the performance of the air conditioner, sensitiveindoor to temperatures the temperature, are not changes fixed, i.e.,accordingly. they vary Inwith this time, regard, the performancea performance of thetest air of conditioner, the air sensitive to the temperature, changes accordingly. In this regard, a performance test of the air conditionersensitive is to carried the temperature, out in a calorimeter-type changes accordingly. or air-enthalpy-type In this regard, test rooma performance (Figure 2) testwhere of thethe air conditioner is carried out in a calorimeter-type or air-enthalpy-type test room (Figure2) where the temperatureconditioner environment is carried outis exactly in a calorimeter-type controlled according or air-enthalpy-type to the ISO 5151 standard.test room (Figure 2) where the temperature environment is exactly controlledcontrolled accordingaccording toto thethe ISOISO 51515151 standard.standard.

Figure 1. Basic model for the analysis. Figure 1. Basic model for the analysis.

FigureFigure 2. Air-enthalpy 2. Air-enthalpy type typetest room test room used usedfor the for experiment. the experiment. Figure 2. Air-enthalpy type test room used for the experiment. Unlike the (Tout/T ) environmental setting of the ISO test methodology, we used the temperature Unlike the (Tout/Tin) inenvironmental setting of the ISO test methodology, we used the in theUnlike outdoor the side (T andout/Tin the) environmental building cooling setting load in of the the indoor ISO side test ( Toutmethodology,/q ). In other we words,used the we temperature in the outdoor side and the building cooling load in the indoor sidebuild (Tout/qbuild). In other words,temperatureset thewe set temperature the in temperature the outdoor of the outdoor of side the and outdoor side the for building side a simulation for coolinga simulation of load the heat inof thethe reservoir indoorheat reservoir outsideside (Tout outside and/qbuild applied). Inand other the appliedwords,cooling the we loadcooling set condition the load temperature condition in the indoor of in the the sideoutdoor indoor for a side simulationside for for a simulationa simulation of the heat of inﬁltrationtheof the he atheat reservoir orinfiltration generation. outside or Tandout appliedand q thewere cooling provided load time-dependently.condition in the indoor In the side test setup,for a simulationTout(t) was of realized the heat by infiltration a cooling coil or generation.build Tout and qbuild were provided time-dependently. In the test setup, Tout(t) was realized by a generation.and heater atTout the and outdoor qbuild were side, provided while q time-dependently.(t) was implemented In the test by asetup, heater Tout at(t) the was indoor realized side, by as a cooling coil and heater at the outdoor side, whilebuild qbuild(t) was implemented by a heater at the indoor side,coolingshown as shown incoil Figure in and Figure 2heater. The 2. The largestat the largest outdoor di ﬀ erencedifference side, was while was that qthat thebuild(t) coolingthe was cooling implemented coil coil at the at indoorthe by indoor a sideheater side did at notdid the operatenot indoor operateside, and as shown the refrigeration in Figure 2. function The largest was difference operated was only that by thethe cooling indoor coiltest atunit the (air-conditionerindoor side did not operate and the refrigeration function was operated only by the indoor test unit (air-conditioner

Energies 2019, 12 FOR PEER REVIEW 4 indoor unit). Therefore, the temperature of the indoor room was decreased by the operation of the air conditioner while the target temperature was set. The wall of the test room minimized heat penetration and air leakage from the outside with the use of a sufficient thermal insulation material; the relevant test room specification is described in Section 2.2. In order to measure the energy consumption in each case (constant and variable-speed air conditioners), the outdoor temperature changes (Tout(t)) and indoor cooling load changes (qbuild(t)) were applied in the air-enthalpy test room shown in Figure 2. The outdoor temperature data were applied for the capital cities of Saudi Arabia and South Korea during the cooling season, while the cooling load data were obtained by TRNSYS modeling. As the latent heat in the cooling load cannot be easily experimentally controlled according to the time, the latent heat was included in the sensible heat. The cooling load was simulated by a heater, which could be easily controlled and had a quick response.

2.2. Air-Enthalpy-Type Test Room Specifications The Korea/International Laboratory Accreditation Cooperation (ILAC) accredited facility Energiesfollowing2019 ,the12, 1489ISO 5151 standard was used as the air-enthalpy test room (Figure 3). The maximum4 of 14 cooling capacity that could be measured was 18,600 W. The inner dimensions (W × D × H) of the outdoor and indoor rooms were 4.1 m × 4.8 m × 3.6 m and 5.7 m × 4.8 m × 3.6 m, respectively. H and the refrigeration function was operated only by the indoor test unit (air-conditioner indoor unit). included an upper airflow passage above the punched ceiling. The walls of the test room were 75 Therefore, the temperature of the indoor room was decreased by the operation of the air conditioner mm poly-urethane panels. The main instruments were shown in Table 1. while the target temperature was set. The wall of the test room minimized heat penetration and air leakage from the outside with the use of a sufficient thermal insulation material; the relevant test room Table 1. Measurement error of measuring equipment. specification is described in Section 2.2. In order toItem measure the energyModel consumption Manufacturer in each case (constantMeasurement and Uncertainty variable-speed air Temperature (RTD) Pt100 CHINO ±0.1 °C conditioners), the outdoor temperature changes (Tout(t)) and indoor cooling load changes (qbuild(t)) ±0.6 V WT230 Yokogawa were appliedPower in consumption the air-enthalpy test room shown in Figure2. The outdoor±0.04 temperature A data were applied for the capital cities of SaudiKDY-AC Arabia and South Light star Korea during the cooling ±1.1 W season, while the cooling load dataData logger were obtained by DA100 TRNSYS modeling. Yokogawa 0.05% of reading, 2 digits As the latent heat in the cooling load cannot be easily experimentally controlled according to the 2.3.time, Heat the Balance latent heatEvaluation was included of the Air-Enth in thealpy-Type sensible heat. Test Room The cooling under the load Steady was State simulated by a heater, which could be easily controlled and had a quick response. In order to experimentally implement qbuild(t), it was necessary to evaluate the heating components affecting the both inside and outside of the test room. As shown in Figure 2, the heat 2.2. Air-Enthalpy-Type Test Room Specifications generation element in the test room was not only the heater, but also the turbo, air handling unit (AHU)The and Korea air/ Internationalsampler fans. Laboratory In addition, Accreditation heat penetration Cooperation from (ILAC)the outside accredited of the facility test room following wall existed.the ISO The 5151 heat standard balance was of usedthe test as room the air-enthalpy was compared test roombetween (Figure the heating3). The components maximum cooling in each configurationcapacity that could and cooling be measured capacity was of 18,600 the air W. conditioner. The inner dimensions (W D H) of the outdoor and × × indoorBased rooms on werethe measurement 4.1 m 4.8 m for3.6 each m and component, 5.7 m 4.8 the m total3.6 heat m, respectively. generation Hwasincluded compared an upper with × × × × theairflow tested passage cooling above capacity the punchedof the air ceiling.conditioner, The wallsas shown of the in testTable room 2, under were the 75 mmsteady poly-urethane state before panels.the time-dependent The main instruments simulation were tests. shown in Table1.

(a) (b)

Figure 3. Air-enthalpy-type test room: ( (aa)) Korea/International Korea/International Laboratory Accreditation Cooperation (ILAC) accredited chamber; ((b)) testtest sample.sample.

Table 1. Measurement error of measuring equipment.

Item Model Manufacturer Measurement Uncertainty Temperature (RTD) Pt100 CHINO 0.1 C ± ◦ 0.6 V WT230 Yokogawa ± Power consumption 0.04 A ± KDY-AC Light star 1.1 W ± Data logger DA100 Yokogawa 0.05% of reading, 2 digits

2.3. Heat Balance Evaluation of the Air-Enthalpy-Type Test Room under the Steady State

In order to experimentally implement qbuild(t), it was necessary to evaluate the heating components aﬀecting the both inside and outside of the test room. As shown in Figure2, the heat generation element in the test room was not only the heater, but also the turbo, air handling unit (AHU) and air sampler fans. In addition, heat penetration from the outside of the test room wall existed. The heat balance of the test room was compared between the heating components in each conﬁguration and cooling capacity of the air conditioner. Energies 2019, 12, 1489 5 of 14

Based on the measurement for each component, the total heat generation was compared with the tested cooling capacity of the air conditioner, as shown in Table2, under the steady state before the time-dependent simulation tests.

Table 2. Measurement results of the test room components.

Categories Description Measured (W) Total (W) Cooling capacity 5680 5 5680 5 Test unit (Sensible) ± ± Cooling capacity 231 10 231 10 (latent) ± ± Heater (E ) 4175 100 H ± AHU’s fan (E ) 1150 20 Indoor room heating AF ± Turbo fan (E ) 250 10 5760 145 components TF ± ± Heat penetration 15 5 ± Air sampler fan 170 10 ± Condensate - 213 10 213 10 ± ±

During the simulation test described below, the variable components such as the heater, AHU fan and turbo fan were measured in real time with a power meter. By adjusting the power consumption of each component aﬀecting the cooling load in real time, the time-variant building cooling load could be provided in the indoor side of the test room.

2.4. Climates in Korea and Saudi Arabia Korea is located in the mid-latitude temperate climatic zone; the four seasons, spring, summer, fall, and winter are conspicuous. In winter (December–February), it is cold and dry under the influence of the continental air from northern China and Siberia. In summer (June–August), hot and humid weather is observed, attributed to the Pacific atmospheric pressure. In spring (March–May) and autumn (September–November), many sunny days are observed owing to the influence of the migratory anticyclone. The climate of Saudi Arabia is characterized by high temperatures during the day and low temperatures during the night. Most of the country follows the pattern of the desert climate, except the southwest region having a semi-arid climate [13]. Table3 shows the distributions of the monthly temperatures in Seoul and Riyadh, the capital cities of South Korea and Saudi Arabia, respectively [14,15]. In Seoul and Riyadh, the cooling seasons are June–September and March–November, respectively. In both cities, continuous air conditioning is not required for all the cooling periods. In Seoul, the air conditioners operate intermittently throughout the cooling period. In Riyadh, the air conditioners operate intermittently in March, April, September, October and November.

Table 3. Temperatures in Seoul and Riyadh (average high, daily mean, average low). *

City Temp. Jan. Feb. Mar. Apr. May. Jun. Jul. Aug. Sep. Oct. Nov. Dec.

High (◦C) 2 5 10 18 23 27 29 30 26 20 12 4 Seoul Mean ( C) 2 0 6 13 18 22 25 26 21 15 7 0 ◦ − Low ( C) 6 3 2 8 13 18 22 22 17 10 3 3 ◦ − − − High (◦C) 19 23 27 32 38 41 43 42 40 34 27 22 Riyadh Mean (◦C) 14 17 21 27 32 34 36 36 33 28 22 17 Low (◦C) 11 15 21 26 28 29 27 26 20 16 11 9 * Climate data from Wikipedia (“Climate of Seoul”, “Climate of Saudi Arabia”); originally from the Weather base. Energies 2019, 12 FOR PEER REVIEW 6 building properties such as geometric dimensions and wall configuration data and windows. In this module, the heat transfer by conduction, convection and radiation, heat gains were calculated considering the presence of occupants and equipment, and the storage of heat in the room air and building mass. Furthermore, it is possible for the temperature, solar radiation, bearing etc. to multi-zone module by applying a Meteonorm climate (for 20 years average temperature). Based on the transfer function method, a dynamic thermal energy analysis was performed in consideration of various types of complex heat gain occurring in space and associated radiation, convection, heat storage and heat dissipation of equipment [16,17]. The TRNSYS energy rate control method was used to analyze the seasonal building cooling load. Modules of type 15-2, type 56, TRNSYS Energies 2019, 12, 1489 6 of 14 three-dimensional (3D) plugin and Google SketchUp were utilized. We set a single zone of the same size in Seoul and Riyadh; however, the size of the window was set according to each country (Figure 4).2.5. The Summary architectural of Energy style Simulations changes according for the Building to the Coolong life and Load environment aspect, and various studies are being conducted on this [18,19]. However, the comparison in the form of the same building is The TRNSYS program (version 17) was used to obtain the building cooling load for the cooling likely to be suitable for comparing the energy consumption by the type of air conditioner of this periods. We applied the meteorological data of Seoul and Riyadh for several years embedded in the study. The Saudi households typically use small windows to block direct solar radiation. The TRNSYS program. TRNSYS (TRaNsient SYSTem Simulation), which is widely used as a building load properties of glass and wall materials were applied by referring to the Korea’s architectural calculation program, was developed by SEL (Solar Energy Lab) of University of Wisconsin, USA. The regulation notice and Saudi’s paper [20,21]. The room temperature in the TRNSYS simulation was calculation algorithm of building load in TRNSYS is based on ISO 13790 and ISO 7730. Simulation for set to 26 °C, which was the same as the air-conditioner temperature setting during the experiment. estimating the heating and cooling load on the building was performed through type 56. A multi-zone The recommended room temperature is different for each country. In the case of Korea, 26 °C is building analysis module of the TRNSYS program. Multi-zone module includes building properties recommended according to the "Ministry of Land, Infrastructure and Transport", and 23–25 °C are such as geometric dimensions and wall conﬁguration data and windows. In this module, the heat recommended according to the government guidelines in Saudi Arabia. For an equal comparison, transfer by conduction, convection and radiation, heat gains were calculated considering the presence we set the temperature as 26 °C in this study [20,22]. of occupants and equipment, and the storage of heat in the room air and building mass. Furthermore, According to ISO 7730, heat from the human body under light work circumstance is 65 W for it is possible for the temperature, solar radiation, bearing etc. to multi-zone module by applying a sensible heat and 55 W for a latent per person [23]. Meteonorm climate (for 20 years average temperature). Based on the transfer function method, a The building components for the TRNSYS simulation in Seoul and Riyadh are summarized in dynamic thermal energy analysis was performed in consideration of various types of complex heat Table 4. gain occurring in space and associated radiation, convection, heat storage and heat dissipation of equipment [16,17]. TheTable TRNSYS 4. TRNSYS energy simulation rate control conditions method in was Seoul used and to Riyadh. analyze the seasonal building cooling load. Modules of type 15-2, type 56, TRNSYS three-dimensional (3D) plugin and Google SketchUpInput were variables utilized. We set a single zone Seoul of the same size in Seoul and Riyadh; Riyadh however, the Space (single zone) 10 m × 8 m × 2.5 m 10 m × 8 m × 2.5 m size of theWindow window was set according 60% of the to southern each country wall area—15 (Figure m2 4). 30% The of architectural the southern wall style area—7.5 changes m2 according to the life and environmentLocated aspect, between and various floors studies are beingLocated conducted between on floors this [18,19]. Floor of space However, the comparison in the form(Up of and the down same insulations) building is likely to be(Up suitable and down for insulations) comparing the 2 2 energy consumptionGlass by the type of air conditioner1.20 W/m K of this study. The Saudi1.720 households W/m K typically Wall 0.21 W/m2K 0.345 W/m2K useRoom small temperature windows & humidity to block direct solar 26 radiation. °C & 60 %R.H. The properties of glass 26and °C wall& 60%R.H. materials were appliedHeating by referring& Light element to the Korea’s architectural 25 W/m regulation2 notice and Saudi’s paper25 W/m [202, 21]. The room temperature in the TRNSYS simulation was4 persons set to 26 ◦C, which was the same4 as persons the air-conditioner temperaturePersons setting during the experiment.(Sensible: The 65 recommendedW / person room temperature(Sensible: 65 is W di ﬀ/ personerent for each Latent: 55 W/person) Latent: 55 W/person) country. InInfiltration the case of Korea, 26 ◦C is recommended 0.4 (1/h) according to the "Ministry of 0.4 Land, (1/h) Infrastructure and Transport",Ventilation and 23–25 ◦C are recommended 0.6 (1/h) according to the government 0.6 guidelines (1/h) in Saudi Arabia. For an equal comparison, we set the temperature as 26 ◦C in this study [20,22].

(a) (b)

Figure 4. Models of households in ( a) Seoul and ( b) Riyadh.

2.6. SpecificationsAccording to of ISOthe Compared 7730, heat Air from Conditioners the human body under light work circumstance is 65 W for sensible heat and 55 W for a latent per person [23]. The building components for the TRNSYS simulation in Seoul and Riyadh are summarized in Table4. Energies 2019, 12, 1489 7 of 14

Table 4. TRNSYS simulation conditions in Seoul and Riyadh.

Input variables Seoul Riyadh Space (single zone) 10 m 8 m 2.5 m 10 m 8 m 2.5 m × × × × Window 60% of the southern wall area—15 m2 30% of the southern wall area—7.5 m2 Located between floors Located between floors Floor of space (Up and down insulations) (Up and down insulations) Glass 1.20 W/m2K 1.720 W/m2K Wall 0.21 W/m2K 0.345 W/m2K Room temperature & humidity 26 ◦C & 60%R.H. 26 ◦C & 60%R.H. Heating & Light element 25 W/m2 25 W/m2 4 persons 4 persons Persons (Sensible: 65 W/person Latent: (Sensible: 65 W/person Latent: 55 W/person) 55 W/person) Infiltration 0.4 (1/h) 0.4 (1/h) Ventilation 0.6 (1/h) 0.6 (1/h)

2.6. Specifications of the Compared Air Conditioners For the comparison test of constant and variable-speed air conditioners in the test room, products with similar specifications were selected (Table5). The two products had similar capacities, power inputs and EERs with relative differences within 5%. The purpose of this study was to evaluate the energy saving effects of the variable-speed air conditioner in real climate conditions even though the specifications were similar to those of the constant-speed air conditioner. To operate under the Korean and hot Saudi Arabian climates, hot-climate (T3) air conditioners were employed.

Table 5. Speciﬁcations of the constant and variable-speed air conditioners.

Contents Constant Speed Variable Speed Electricity Single phase, 230 V, 60 Hz Single phase, 230 V, 60 Hz Rated cooling capacity (T1 climate) 5130 W 5275 W Rated power input (T1 climate) 1520 W 1,490 W Rated current (T1 climate) 6.8 A 6.7 A EER (T1 climate) 3.375 W/W 3.540 W/W Cooling capacity (T3 climate) 4420 W 4484 W Power input (T3 climate) 1820 W 1754 W Current (T3 climate) 8.14 A 7.8 A EER (T3 climate) 2.429 W/W 2.556 W/W Maximum power input 2610 W 2900 W Maximum current 11.68 A 14.0 A Refrigerant R410a, 0.85 kg R410a, 0.95 kg Product type Wall mount split type Wall mount split type Annual energy consumption by 4104 kWh/year 4023 kWh/year SASO energy label

3. Results

3.1. Simulation Results Before the experiments with the constant and variable-speed (inverter) air conditioners, TRNSYS simulations were carried out for building cooling loads under Seoul and Riyadh temperature conditions (Figures5b and6b). As the TRNSYS program contained long-term data (more than 10 years), it seemed that the recent temperature increase owing to global warming is buried in Figures5a and6a. Therefore, it seemed that 1–3 ◦C were underestimated compared with the climate felt nowadays. The building cooling loads of Seoul and Riyadh have maximum values of approximately 4000 W in August. The two cities have similar maximum building loads as the humidity in summer is very low in Riyadh and very high in Seoul. Energies 2019, 12 FOR PEER REVIEW 7

For the comparison test of constant and variable-speed air conditioners in the test room, products with similar specifications were selected (Table 5). The two products had similar capacities, power inputs and EERs with relative differences within 5%. The purpose of this study was to evaluate the energy saving effects of the variable-speed air conditioner in real climate conditions even though the specifications were similar to those of the constant-speed air conditioner. To operate under the Korean and hot Saudi Arabian climates, hot-climate (T3) air conditioners were employed.

Table 5. Specifications of the constant and variable-speed air conditioners.

3. Results

3.1. Simulation Results Before the experiments with the constant and variable-speed (inverter) air conditioners, TRNSYS simulations were carried out for building cooling loads under Seoul and Riyadh temperature conditions (Figures 5b and 6b). As the TRNSYS program contained long-term data (more than 10 years), it seemed that the recent temperature increase owing to global warming is buried in Figures 5a and 6a. Therefore, it seemed that 1–3 °C were underestimated compared with the climate felt nowadays. The building cooling loads of Seoul and Riyadh have maximum values of Energiesapproximately2019, 12, 1489 4000 W in August. The two cities have similar maximum building loads as8 ofthe 14 humidity in summer is very low in Riyadh and very high in Seoul.

o ( C) (W)

40 4,000

30 3,000

20 2,000 Temperature Jun Jul Load Cooling Aug Sep Jun Jul 10 1,000 Aug Sep

0 0 0 4 8 12162024 0 4 8 12162024 Daily hour(h) Daily hour(h) (a) (b)

EnergiesFigure 2019, 5.12 5. (FORa)( aSeoul) PEER Seoul outdoor REVIEW outdoor temperatures; temperatures; (b) Seoul (b) Seoulbuilding building cooling cooling loads obtained loads obtained by the TRNSYS by the 8 TRNSYSsimulation. simulation.

o ( C) (W)

40 4,000

30 3,000

20 2,000 Temperature Cooling Load Mar Jun Sep 10 Mar Jun Sep 1,000 Apr Jul Oct Apr Jul Oct May Aug Nov May Aug Nov

0 0 04812162024 0 4 8 12162024 Daily hour(h) Daily hour(h)

(a) (b)

FigureFigure 6.6. ((a)) Riyadh outdoor temperatures; ( b) Riyadh building cooling loadsloads obtainedobtained byby thethe TRNSYSTRNSYS simulation.simulation.

3.2.3.2. Experimental Results FiguresFigures7 7–9–9 show show experimental experimental resultsresults for for comparison comparison of of the the constant-speed constant-speed and and inverter inverter air air conditioners.conditioners. TheThe indoor indoor building building loads loads and and outdoor outdoor temperatures temperatures were consistentwere consistent with the with TRNSYS the simulationTRNSYS simulation results (circle results points (circle and points rectangular and rectangular in the ﬁgures). in the figures). The indoor The indoor building building loads loads were experimentallywere experimentally implemented implemented through through the heater the atheat theer upperat the end upper of theend AHU, of the as AHU, shown as in shown Figure 2in, whereFigure the2, where powers the of powers other heatingof other elementsheating elemen (e.g., AHUts (e.g., and AHU turbo and fans) turbo in fans) the indoorin the indoor side were side simultaneouslywere simultaneously measured measured and the and test the was test controlled was controlled by the by AHU the heater.AHU heater. The dotted The dotted lines with lines small with oscillations represented the indoor room temperature which varied in the range of 26.0 1.5 C as small oscillations represented the indoor room temperature which varied in the range of 26.0± ± 1.5◦ °C theas testthe producttest product temperature temperature was set was at 26set◦C. at Both 26 the°C. constant-speedBoth the constant-speed and inverter and air conditionersinverter air wereconditioners automatically were automatically operated during operated the experiments during the andexperiments only the indoorand only building the indoor load building and outdoor load temperatureand outdoor weretemperature applied were according applied to theaccording TRNSYS to simulationthe TRNSYS results. simulation results. The largest difference in the graph is the variation in the power input. The constant-speed air conditioner exhibited simple on/off control whereas the inverter (variable speed) air conditioner exhibited changes corresponding to the building load or outdoor temperature. In the case of the inverter air conditioner in Riyadh, the power input in May was highly activated during the daytime (12–19 h), whereas it was activated at any time in August. In the hottest season (August) in Riyadh and Seoul, the building loads were similar under the considered simulation conditions (Section 3.1).

o o (W) ( C) (W) ( C) Outdoor temperature(TRNSYS result from Fig.6(a)) Outdoor temperature(TRNSYS result from Fig.6(a)) 5,000 Indoor cooling load(TRNSYS result from Fig.6(b)) 60 5,000 Indoor cooling load(TRNSYS result from Fig.6(b)) 60 Outdoor temperature(T ) Outdoor temperature(T ) out out Indoor cooling load(q ) Indoor cooling load(q ) build 50 build 50 Indoor temperature(T ) Indoor temperature(T ) 4,000 in 4,000 in Power Input Power Input 40 40 3,000 3,000 30 30 2,000 2,000 20 20

1,000 1,000 10 10

0 0 0 0 0 4 8 12162024 0 4 8 12162024 Daily hour(h) Daily hour(h)

(a) (b)

Figure 7. Experimental results for Riyadh in May: (a) Constant speed; (b) variable speed.

Energies 2019, 12 FOR PEER REVIEW 8

o ( C) (W)

40 4,000

30 3,000

20 2,000 Temperature Cooling Load Mar Jun Sep 10 Mar Jun Sep 1,000 Apr Jul Oct Apr Jul Oct May Aug Nov May Aug Nov

0 0 04812162024 0 4 8 12162024 Daily hour(h) Daily hour(h)

(a) (b)

Figure 6. (a) Riyadh outdoor temperatures; (b) Riyadh building cooling loads obtained by the TRNSYS simulation.

3.2. Experimental Results Figures 7–9 show experimental results for comparison of the constant-speed and inverter air conditioners. The indoor building loads and outdoor temperatures were consistent with the TRNSYS simulation results (circle points and rectangular in the figures). The indoor building loads were experimentally implemented through the heater at the upper end of the AHU, as shown in Figure 2, where the powers of other heating elements (e.g., AHU and turbo fans) in the indoor side were simultaneously measured and the test was controlled by the AHU heater. The dotted lines with small oscillations represented the indoor room temperature which varied in the range of 26.0 ± 1.5 °C as the test product temperature was set at 26 °C. Both the constant-speed and inverter air conditioners were automatically operated during the experiments and only the indoor building load and outdoor temperature were applied according to the TRNSYS simulation results. The largest difference in the graph is the variation in the power input. The constant-speed air conditioner exhibited simple on/off control whereas the inverter (variable speed) air conditioner exhibited changes corresponding to the building load or outdoor temperature. In the case of the inverter air conditioner in Riyadh, the power input in May was highly activated during the daytime Energies(12–192019 h),, 12 whereas, 1489 it was activated at any time in August. In the hottest season (August) in Riyadh9 of 14 and Seoul, the building loads were similar under the considered simulation conditions (Section 3.1).

1,000 1,000 10 10

0 0 0 0 0 4 8 12162024 0 4 8 12162024 Daily hour(h) Daily hour(h)

Energies 2019, 12 FOR PEER REVIEW(a) (b) 9 Energies 2019, 12 FOR PEER REVIEW 9 FigureFigure 7. 7.Experimental Experimentalresults results forfor Riyadh in May: (a (a) )Constant Constant speed; speed; (b ()b variable) variable speed. speed. o o (W) ( C) (W) Outdoor temperature(TRNSYS result from Fig.6(a)) ( C) Outdoor temperature(TRNSYS result from Fig.6(a)) o o (W) ( C) (W) Indoor cooling load(TRNSYS result from Fig.6(b)) ( C) 5,000 OutdoorIndoor cooling temperature(TRNSYS load(TRNSYS result result from from Fig.6(b)) Fig.6(a)) 60 5,000 Outdoor temperature(TRNSYS result from Fig.6(a)) 60 Outdoor temperature(T ) Outdoor temperature(T ) Indoor cooling load(TRNSout YS result from Fig.6(b)) 5,000 Indoor cooling load(TRNSYSout result from Fig.6(b)) 60 5,000 60 Indoor cooling load(q ) OutdoorIndoor cooling temperature(T load(q )) Outdoor temperature(Tbuild ) buildout out 50 Indoor temperature(T ) 50 4,000 IndoorIndoor coolingtemperature(T load(q )) 4,000 Indoor cooling load(q in ) buildin build 50 IndoorPower temperature(TInput ) 50 IndoorPower temperature(TInput ) in 4,000 in 4,000 Power Input 40 Power Input 40 3,000 40 3,000 40 3,000 30 3,000 30 30 2,000 2,000 30 2,000 20 2,000 20 20 20 1,000 1,000 10 10 1,000 1,000 10 10 0 0 0 0 0 0 4 8 121620240 0 0 4 8 121620240 0 4 8Daily 12162024hour(h) 0 4 8Daily 12162024hour(h) Daily hour(h) Daily hour(h) (a ) (b ) (a) (b) Figure 8. Experimental results for Riyadh in August: (a) Constant speed; (b) variable speed. FigureFigure 8. 8.Experimental Experimental results results forfor RiyadhRiyadh in August: ( (aa) )Constant Constant speed; speed; (b ()b variable) variable speed. speed.

o o (W) ( C) (W) ( C) Outdoor temperature(TRNSYS result from Fig.5(a)) o Outdoor temperature(TRNSYS result from Fig.5(a)) o (W) ( C) (W) ( C) 5,000 OutdoorIndoor cooling temperature(TRNSYS load(TRNSYS result result from from Fig.5(b)) Fig.5(a)) 60 5,000 OutdoorIndoor cooling temperature(TRNSYS load(TRNSYS result result from from Fig.5(b)) Fig.5(a)) 60 Outdoor temperature(T ) Outdoor temperature(T ) 5,000 Indoor cooling load(TRNSYSout result from Fig.5(b)) 60 5,000 Indoor cooling load(TRNSYSout result from Fig.5(b)) 60 OutdoorIndoor cooling temperature(T load(q )) OutdoorIndoor cooling temperature(T load(q )) buildout 50 buildout 50 4,000 IndoorIndoor coolingtemperature(T load(q ) ) 4,000 IndoorIndoor coolingtemperature(T load(q ) ) buildin 50 buildin 50 IndoorPower temperature(TInput ) IndoorPower temperature(TInput ) 4,000 in 4,000 in Power Input 40 Power Input 40 3,000 40 3,000 40 3,000 30 3,000 30 30 2,000 30 2,000 2,000 20 2,000 20 20 20 1,000 1,000 10 10 1,000 1,000 10 10 0 0 0 0 0 048121620240 0 0 4 8 121620240 04812162024Daily hour(h) 0 4 8Daily 12162024hour(h) Daily hour(h) Daily hour(h) (a ) (b ) (a) (b) FigureFigure 9. 9.Experimental Experimental results results for for SeoulSeoul inin August:August: ( (aa)) Constant Constant speed; speed; (b (b) variable) variable speed. speed. Figure 9. Experimental results for Seoul in August: (a) Constant speed; (b) variable speed. TheHowever largest the diﬀ erencepower variations in the graph were is different the variation owing in to thethe poweroutdoor input. temperature The constant-speed differences. However the power variations were different owing to the outdoor temperature differences. airAlthough conditioner the exhibitedbuilding cooling simple loads on/oﬀ werecontrol similar whereas in this the study, inverter the (variableoutdoor temperature speed) air conditioner in Saudi Although the building cooling loads were similar in this study, the outdoor temperature in Saudi exhibitedArabia was changes significantly corresponding higher than to the that building in Korea. load Usually, or outdoor the outdoor temperature. unit of the In air the conditioner case of the Arabia was significantly higher than that in Korea. Usually, the outdoor unit of the air conditioner inverterwas affected air conditioner by the dry-bulb in Riyadh, temperature the power rather input than in May by the was wet-bulb highly activatedtemperature during in the the outdoor daytime wastest roomaffected during by the the dry-bulb cooling temperature test in which rather no condensation than by the wet-bulb on the outdoor temperature unit heat in the exchanger outdoor testwas room observed. during the cooling test in which no condensation on the outdoor unit heat exchanger was observed.The power inputs in the above graph could be averaged to obtain the daily average power consumption.The power Table inputs 6 shows in the the above monthly graph power could consum be averagedption, obtained to obtain by multiplyingthe daily average 30 (days) power with consumption.the daily average Table power 6 shows consumption. the monthly During power the consum coolingption, seasons obtained of Seoul by multiplying(four months) 30 and(days) Riyadh with the(nine daily months), average powerthe monthly consumption. power During consumptions the cooling of seasons the constant- of Seoul (four and months) variable-speed and Riyadh air (nineconditioners months), were the compared monthly (Figures power 10 consumptions and 11). Considering of the theconstant- air conditioners’ and variable-speed specifications air in conditionersTable 5, the werevariable-speed compared air(Figures conditioner 10 and had 11). aConsidering higher EER the than air thatconditioners’ of the constant-speed specifications airin Tableconditioner 5, the byvariable-speed approximately air 5%. conditioner Therefore, had the arela highertive difference EER than was that subt of theracted constant-speed by –5% in Table air conditioner6. In Riyadh, by theapproximately variable-speed 5%. airTherefore, conditioner the rela hastive a smallerdifference energy was subtconsumptionracted by –5%by 18.3%in Table in 6.August In Riyadh, and 47.1% the variable-speed in November. airThis conditioner implies that has the a smallervariable-speed energy airconsumption conditioner by provides 18.3% in a Augusthigher energy and 47.1% saving in than November. that of the This constant-spe implies thated airthe conditioner variable-speed in the air part-load conditioner condition. provides Seoul a higherin August energy is less saving hot than Riyadhthat of the (Figures constant-spe 5a anded 6a), air which conditioner leads to in athe larger part-load energy condition. saving (36.3%) Seoul inof Augustthe variable-speed is less hot than air conditioner.Riyadh (Figures In Saudi 5a and Arabia 6a), which and Korea,leads to total a larger inverter energy energy saving savings (36.3%) of of the variable-speed air conditioner. In Saudi Arabia and Korea, total inverter energy savings of

Energies 2019, 12, 1489 10 of 14

(12–19 h), whereas it was activated at any time in August. In the hottest season (August) in Riyadh and Seoul, the building loads were similar under the considered simulation conditions (Section 3.1). However the power variations were different owing to the outdoor temperature differences. Although the building cooling loads were similar in this study, the outdoor temperature in Saudi Arabia was significantly higher than that in Korea. Usually, the outdoor unit of the air conditioner was affected by the dry-bulb temperature rather than by the wet-bulb temperature in the outdoor test room during the cooling test in which no condensation on the outdoor unit heat exchanger was observed. The power inputs in the above graph could be averaged to obtain the daily average power consumption. Table6 shows the monthly power consumption, obtained by multiplying 30 (days) with the daily average power consumption. During the cooling seasons of Seoul (four months) and Riyadh (nine months), the monthly power consumptions of the constant- and variable-speed air conditioners were compared (Figures 10 and 11). Considering the air conditioners’ specifications in Table5, the variable-speed air conditioner had a higher EER than that of the constant-speed air conditioner by approximately 5%. Therefore, the relative difference was subtracted by –5% in Table6. In Riyadh, the variable-speed air conditioner has a smaller energy consumption by 18.3% in August and 47.1% in November. This implies that the variable-speed air conditioner provides a higher energy saving than that of the constant-speed air conditioner in the part-load condition. Seoul in August is less hot than Riyadh (Figures5a and6a), which leads to a larger energy saving (36.3%) of the variable-speed air conditioner. In Saudi Arabia and Korea, total inverter energy savings of 28.9% and 44.5% were Energies 2019, 12 FOR PEER REVIEW 10 achieved during the cooling season, respectively. These results were very similar to those in the 28.9%study forand Istanbul 44.5% were and Saudiachieved Arabia, during where the energy coolin savingsg season, of 11–38%respectively. and 22–50%, These results respectively, were very were similarobtained to forthose an in inverter the study air conditionerfor Istanbul inand a real Saudi offi Arabia,ce usage where [6,8]. energy savings of 11–38% and 22– 50%, respectively, were obtained for an inverter air conditioner in a real office usage [6,8]. Table 6. Monthly power consumption (kWh) during the cooling season.

City Air ConditionerTable 6. Monthly Type power Mar. consumption Apr. May. (kWh Jun.) during Jul. the Aug. cooling Sep. season. Oct. Nov. Tot. City AirConstant Conditioner speed Type Mar. - Apr. - May. - Jun. 341 Jul. 490 Aug. 542 Sep. 319 Oct. - Nov. - 1692 Tot. ConstantVariable speed speed ------341 150 490 249 542 318 319 138 - - - - 8551692 Seoul (Con.-Var.)Variable/Con. speed 100% ------15056.0 249 49.2 318 41.3 138 56.7 - - - - 49.5 855 Seoul × (Con.-Var.)/Con.5% compensation × 100% ------56.0 51.0 49.2 44.2 41.3 36.3 56.7 51.7 - - - - 44.5 49.5 −−5% compensation - - - 51.0 44.2 36.3 51.7 - - 44.5 Constant speed 305 421 603 672 745 747 595 470 330 4888 Constant speed 305 421 603 672 745 747 595 470 330 4888 Variable speed 149 237 395 459 529 573 435 298 158 3233 Riyadh Variable speed 149 237 395 459 529 573 435 298 158 3233 Riyadh (Con.-Var.)/Con. 100% 51.1 43.7 34.5 31.7 28.9 23.3 26.9 36.6 52.1 33.9 (Con.-Var.)/Con. ×× 100% 51.1 43.7 34.5 31.7 28.9 23.3 26.9 36.6 52.1 33.9 5% compensation 46.1 38.7 29.5 26.7 23.9 18.3 21.9 31.6 47.1 28.9 −−5% compensation 46.1 38.7 29.5 26.7 23.9 18.3 21.9 31.6 47.1 28.9

800 Constant speed 700 Variable speed 600 500 400 300 200

Power consumption[kWh] 100 0 Jun. Jul. Aug. Sep. Month FigureFigure 10. 10. MonthlyMonthly power power consumption consumption during during the the cooling cooling season season in in Seoul. Seoul.

Constant speed 800 Variable speed 700 600 500 400 300 200

Power consumption[kWh] Power 100 0 Mar. Apr. May. Jun. Jul. Aug. Sep. Oct. Nov.

Month Figure 11. Monthly power consumption during the cooling season in Riyadh.

3.3. Economic Analysis In Korea, most of the household air conditioners in the market are inverters and the actual operation time is supposed to be remarkably different from the test condition. As the electricity rates in Korea are high, the conditioners do not operate during the whole day, but only when it is very hot. However, we carried out an economic analysis in both countries. The results of the economic analysis were based on the experimental results. Based on a simple payback period analysis, the initial investment and running costs were calculated to compare the economic benefits of the different types of air conditioners. The initial investment cost was the average price of an 18,000 British-thermal-unit (BTU)/h air conditioner sold in Korea and in Saudi Arabia; the annual operation cost was calculated based on

Energies 2019, 12 FOR PEER REVIEW 10

28.9% and 44.5% were achieved during the cooling season, respectively. These results were very similar to those in the study for Istanbul and Saudi Arabia, where energy savings of 11–38% and 22– 50%, respectively, were obtained for an inverter air conditioner in a real office usage [6,8].

Table 6. Monthly power consumption (kWh) during the cooling season.

City Air Conditioner Type Mar. Apr. May. Jun. Jul. Aug. Sep. Oct. Nov. Tot. Constant speed - - - 341 490 542 319 - - 1692 Variable speed - - - 150 249 318 138 - - 855 Seoul (Con.-Var.)/Con. × 100% - - - 56.0 49.2 41.3 56.7 - - 49.5 −5% compensation - - - 51.0 44.2 36.3 51.7 - - 44.5 Constant speed 305 421 603 672 745 747 595 470 330 4888 Variable speed 149 237 395 459 529 573 435 298 158 3233 Riyadh (Con.-Var.)/Con. × 100% 51.1 43.7 34.5 31.7 28.9 23.3 26.9 36.6 52.1 33.9 −5% compensation 46.1 38.7 29.5 26.7 23.9 18.3 21.9 31.6 47.1 28.9

800 Constant speed 700 Variable speed 600 500 400 300 200

Power consumption[kWh] 100 0 Jun. Jul. Aug. Sep. Month Energies 2019, 12, 1489 11 of 14 Figure 10. Monthly power consumption during the cooling season in Seoul.

Constant speed 800 Variable speed 700 600 500 400 300 200

Power consumption[kWh] Power 100 0 Mar. Apr. May. Jun. Jul. Aug. Sep. Oct. Nov.

Month Figure 11. MonthlyMonthly power consumption during the cooling season in Riyadh.

3.3. Economic Economic Analysis Analysis In Korea, most of the household air conditionersconditioners in the market are inverters and the actual operation timetime isis supposedsupposed to to be be remarkably remarkably di ffdifferenterent from from the the test test condition. condition. As the As electricity the electricity rates ratesin Korea in Korea are high, are thehigh, conditioners the conditioners do not do operate not operate during during the whole the day, whole but day, only but when only it iswhen very it hot. is veryHowever, hot. weHowever, carried outwe ancarried economic out analysisan economic in both analysis countries. in Theboth results countries. of the The economic results analysis of the wereeconomic based analysis on the experimentalwere based on results. the experimental results. Based onon a a simple simple payback payback period period analysis, analysis, the initial the investmentinitial investment and running and costsrunning were costs calculated were calculatedto compare to the compare economic the benefitseconomic of benefits the different of the types different of air types conditioners. of air conditioners. The initial investment cost was the average price of an 18,000 British-thermal-unit (BTU)/h (BTU)/h air conditioner soldsold inin KoreaKorea and and in in Saudi Saudi Arabia; Arabia; the the annual annual operation operation cost cost was was calculated calculated based based on theon monthly power consumption (Tables7 and8). To calculate the annual operation cost, electricity rates proposed by the power authorities of both countries were employed [24,25].

Table 7. Calculation results of the simple payback period (Seoul).

Running Cost Categories Initial Cost (USD) Payback Period (Years) (USD/year) Constant speed 727 291 Basis Variable speed 1182 104 2.5

Table 8. Calculation results of the simple payback period (Riyadh).

Running Cost Categories Initial Cost (USD) Payback Period (Years) (USD/year) Constant speed 432 238 Basis Variable speed 635 157 2.5

The results of the economic analysis for both types (constant, variable) of air conditioners are shown in Figure 12. Energies 2019, 12 FOR PEER REVIEW 11

the monthly power consumption (Tables 7 and 8). To calculate the annual operation cost, electricity rates proposed by the power authorities of both countries were employed [24,25].

Table 7. Calculation results of the simple payback period (Seoul).

Initial Cost Running Cost Payback Period Categories (USD) (USD/year) (Years) Constant speed 727 291 Basis Variable speed 1182 104 2.5

Table 8. Calculation results of the simple payback period (Riyadh).

Initial Cost Running Cost Payback Period Categories (USD) (USD/year) (Years) Constant speed 432 238 Basis Variable speed 635 157 2.5

The results of the economic analysis for both types (constant, variable) of air conditioners are shown in Figure 12. With respect to the constant-speed air conditioner, the payback period of the inverter type was 2.5 years in both countries. For the whole lifetime (10 years, assumed using the usual warranty period for the compressor), in Korea and Saudi Arabia, the inverter air conditioner is 1400 USD and 600 USD more economical than the constant-speed air conditioner, respectively. A total of 21 million of split and window air conditioners are used in Saudi Arabia, while the inverter type is rarely used owing to its high initial cost. If all of the constant-type air conditioners Energieswere replaced2019, 12, 1489 by inverter-type air conditioners, the power consumption could be reduced by12 of34.8 14 TWh per year (the average electric consumption can be obtained by the employed models) [26].

4000 4000

3500 3500

3000 3000

2500 2500

2000 2000

Cost(USD) 1500

Cost(USD) 1500 Costant speed Costant speed 1000 2.5 years 1000 Variable speed 2.5 years Variable speed 500 500

0 0 012345678910 012345678910 Year Year (a) (b)

Figure 12. Periods of recovery of the initial investment: (a) Korea; (b) Saudi Arabia.

4. ConclusionsWith respect to the constant-speed air conditioner, the payback period of the inverter type was 2.5 years in both countries. For the whole lifetime (10 years, assumed using the usual warranty period In this study, the energy saving effect of the inverter (variable-speed) air conditioner with for the compressor), in Korea and Saudi Arabia, the inverter air conditioner is 1400 USD and 600 USD respect to the constant-speed air conditioner was analyzed. The ambient temperature and building more economical than the constant-speed air conditioner, respectively. cooling load were obtained using TRNSYS climate data and simulation. The results were obtained in A total of 21 million of split and window air conditioners are used in Saudi Arabia, while the the precisely controlled air-enthalpy-type test room (outdoor side: Temperature-controlled, indoor inverter type is rarely used owing to its high initial cost. If all of the constant-type air conditioners side: Cooling-load-controlled) with specific dimensions, which contained the air conditioner set at were replaced by inverter-type air conditioners, the power consumption could be reduced by 34.8 TWh the temperature of 26 °C throughout the day. For the Korean and Saudi Arabian cooling periods of per year (the average electric consumption can be obtained by the employed models) [26]. four and nine months, respectively, TRNSYS simulations and experiments in the air-enthalpy test 4.room Conclusions were carried out. In standards such as ISO, EN and ASHRAE, methods are introduced to determine the part-load efficiency of an air conditioner. However, in this study, we directly measuredIn this the study, actual the air energy conditioner saving e ffenergyect of theconsum inverterption (variable-speed) using a real climate air conditioner environment with respect rather tothan the calculation constant-speed of the air SEER conditioner based on was some analyzed. test points. The ambient temperature and building cooling loadThe were conclusions obtained using of this TRNSYS study can climate be summarized data and simulation.as follows: The results were obtained in the precisely controlled air-enthalpy-type test room (outdoor side: Temperature-controlled, indoor side: Cooling-load-controlled) with specific dimensions, which contained the air conditioner set at the temperature of 26 ◦C throughout the day. For the Korean and Saudi Arabian cooling periods of four and nine months, respectively, TRNSYS simulations and experiments in the air-enthalpy test room were carried out. In standards such as ISO, EN and ASHRAE, methods are introduced to determine the part-load efficiency of an air conditioner. However, in this study, we directly measured the actual air conditioner energy consumption using a real climate environment rather than calculation of the SEER based on some test points. The conclusions of this study can be summarized as follows:

1. The variable-speed (inverter) air conditioner was more eﬀective than the constant-speed air conditioner in the months and location of part-load operation; in all of the preset test cases, it led to a smaller energy usage. 2. Inverter energy savings of 18.3–47.1% and 36.3–51.7% were observed during the Riyadh’s (March–November) and Seoul’s (June–September) cooling months, respectively. 3. The energy savings depended on the month; larger energy savings were observed before and after the summer peak. Larger energy savings were observed in the months and location of part-load operation. 4. With respect to the constant-speed air conditioner, the payback period of the inverter type was 2.5 years in both countries. For whole lifetime (10 years, assumed using the usual warranty period for the compressor), in Korea and Saudi Arabia, the inverter-type air conditioner is 1400 USD and 600 USD more economical than the constant-speed air conditioner, respectively.

It is widely known that no differences in energy efficiency between constant-speed and inverter-type air conditioners exist in countries with hot climates such as Saudi Arabia. However, Energies 2019, 12, 1489 13 of 14 according to the results of this study, even in hot countries, the building load and outside temperature change with time. Consequently, energy saving was obtained by part-load operation of the inverter air conditioner in the morning and night periods. The actual energy consumption analysis through the proposed approach can provide good empirical evidence to establish air conditioner policies in different countries.

Author Contributions: J.L., M.S.Y. and Y.N. defined the performance analysis models, developed the methodology, and wrote the full manuscript. The author T.A.Q. checked the contents regarding Saudi Arabia. Funding: This research was funded by Basic Research Project (No.CBS18126) of the Korea Testing Laboratory (KTL). Conflicts of Interest: The authors declare no conflict of interest.

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