A review of variable refrigerant flow HVAC system components for residential application

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A Review of Variable Refrigerant Flow HVAC System Components for Residential Application Andrew C. Hernandez III , Dr. Nelson Fumo PII: DOI: Reference:

S0140-7007(19)30470-0 https://doi.org/10.1016/j.ijrefrig.2019.11.005 JIJR 4572

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International Journal of Refrigeration

Received date: Revised date: Accepted date:

12 July 2019 21 October 2019 5 November 2019

Please cite this article as: Andrew C. Hernandez III , Dr. Nelson Fumo , A Review of Variable Refrigerant Flow HVAC System Components for Residential Application, International Journal of Refrigeration (2019), doi: https://doi.org/10.1016/j.ijrefrig.2019.11.005

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A Review of Variable Refrigerant Flow HVAC System Components for Residential Application Andrew C. Hernandez IIIa and Dr. Nelson Fumoa a Mechanical Engineering Department, The University of Texas at Tyler Student: [email protected] Mentor/Corresponding Author: [email protected]; Tel: +1-903-565-5588, Fax +1-903-5655907 Abstract A variable refrigerant flow (VRF) classification is a multi-split Heating, Ventilation and Air Conditioning (HVAC) system that controls refrigerant flow to control separable zones to residential consumer’s needs. VRF specific components regulate refrigerant flow control for system performance and reliability. The objective of this review is to recognize the VRF system components that affect various aspects of operation and performance. The investigate of preceding experimentation will better assist in the direction of future progress as development of expansive residential VRF systems are still in primary phases. Findings show that specific testing through different compressor arrangements, electronic expansion valve (EEV) positioning, and air flow operations affect performance and thermal comfort. The system responsiveness and sensitivity are related to the quantity of indoor evaporators that are connected to the system. Keywords VRF; Multi-evaporator; Inverter compressor; Thermal comfort 1. Introduction In recent decades strides for efficiency and performance in the heating, ventilation, and air conditioning (HVAC) industry have been momentous. Where this drive comes from is the introduction of Variable Refrigerant Flow (VRF) systems that are beginning to replace traditional HVAC systems in both residential and commercial applications. VRF systems are direct expansion systems that use the vapor compression cycle to produce cooling and heating by means of one compressor-condenser (outdoor unit) with variable compressor speed and possible multiple evaporators (indoor units) regulated by controllable expansion valves allowing a variable refrigerant flow in response to zone thermal measurements. In these systems, as dualmode heat pumps, the cycle can be reversed to provide heating and therefore only one system can provide cooling or heating. So, for residential applications, the traditional HVAC system that can be compared to a VRF system is a dual-mode heat pump (HP). The main differences between these two systems is that the VRF has possibility of more than a single evaporator and does not require ducts for air distribution. Table 1 shows these and other differences between a HP and a VRF for residential buildings, while Figure 1 shows a schematic of a VRF.

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Table 1. Comparison between a HP and a VRF. Characteristic Dual Mode HP (Ducted System)

VRF (Ductless System)

Compressor variable speed Evaporators Temperature sensor Attic Space Humidity Control

Preferable/not required One (air handling unit) One (thermostat) Required High

Required Several (wall mounted) One per wall unit/evaporator Not required Low

Airflow Appearances Maintenance

Balanced and pressurized Only registers are shown Single unit

Specifically targeted per zone Ductless unit mounted on wall Multiple individual units

Fig. 1 Schematic of a VRF system.

The HVAC industry recognize that Daikin Industries [1] [2] invented a variable refrigerant volume system (VRV) in 1982 consisting of singular outdoor unit with multiple indoor evaporator units. VRV is a trademarked term where it is only seen within Daikin Industries since other iterations of the technology follow under VRF systems. Daikin Industries created this space saving and efficient HVAC option where it thrived in Japan where the dense population and living spaces greatly benefited from VRV/VRF technology. Studies, such as the one done at the Pacific Northwest National laboratory by Thornton and Wagner [3], suggest that although VRF systems have a higher initial cost and maintenance repair, a lower life-cycle cost analysis is obtained because of energy cost savings. In HVAC systems, the efficiency and performance have a number of subcategories such as compressor and superheat control alongside temperature control distribution that need to be 2

evaluated in order to deem system developments successful. The importance of VRF systems is divided among a number of factors including improved energy efficiency, zone thermal comfort, and flexibility of design capabilities. The interest in VRF systems stems from speculation of future VRF market demand and technology advances. Trends in research specifically target performance analysis and aspects of temperature distribution independently. Aynur [4] reviewed papers including all aspects of VRF technology that fall into heat recovery where heating and cooling is required by the system. Heat recovery is not considered in this scope as the extensiveness of VRF components can quickly increase even in only heat pump operation. However, what remains similar is the type of field testing and how many various interpretations of the system can come to be. Findings of studies in this review are deduced and synthesized in attempt to generalize conclusions to provide a comprehensive source of various component and configuration research. With a trailing domestic recognition for VRF, new and future studies are accommodating development in this technology. Relatable work involves the university cooperation with a leading industry company to develop, test, and calibrate an innovative VRF installation within a research friendly residential home. Understanding the operation and contribution of each component that were not specifically described in detail in various textbooks outside of ideal conditions aided in keeping pace with industry professionals. As system knowledge advanced, issues and failures were diagnosed by investigating variation in refrigerant line pressure and temperature with EEV and fan coil operation. Variable zone testing was conducted and reported for expert review with feedback to further make testing alterations and outlook for expected operation. This disclosed project and research is in efforts to drive the domestic VRF progression. Notions and themes throughout this paper include studies of VRF that have multiple components with different configurations along with their field-testing conclusions. Investigations on compressors moving refrigerant throughout various pipe design choices with respect to refrigerant type and system control are all considered. 2. Overview Different from the principles introduced from a standard vapor compression refrigeration system, the refrigerant flow throughout a VRF system is controlled instead of operating at the speed of a one- or two-staged compressor setting. The set-positioned expansion valve is replaced with an electronic expansion valve (EEV) that is controlled in conjunction with the rest of the system. An EEV is calibrated for different orifice sizes to allow any amount of refrigerant flow through to the evaporator. The positioning of the EEV orifice size is correlated to how much load is required to control zone temperature. Generally, the compressor works at a variable speed to match load required by the overall system. Energy efficiency increases since the compressor is capable at operating from a range of speeds depending on the load required to maintain temperature throughout single zone or multiple zone arrangements. Instead of a system turning on at full capacity for a short time to cool a zone, a VRF system can turn on for the same amount

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of time but only at required compressor speeds that result in same zone temperature control but at lower cost due to less energy consumption. The vapor compression cycle has vital requirements to ensure equipment longevity and proper operation. Refrigerant entering the compressor must be superheated vapor as any liquid entered to the compressor has potential to harm the compressor internals. This is achieved by balancing the amount of warm return air introduced through the evaporator coils to evaporate the refrigerant mixture. The refrigerant is not capable of undergoing a complete phase change from liquid to superheated vapor without proper airflow properties through the evaporator coils. Having superheated vapor ensures that the refrigerant has no liquid in the lines during this phase, so the EEV is in place to regulate refrigerant flow and allow less refrigerant to flow through and properly acquire superheated vapor. A superheat variable is considered during the reviewed investigations to monitor how effective the current refrigerant flow is. Alternatively, the airflow across the evaporator coils can be increased to manipulate the amount of superheat at the evaporator exit. Efficiency is optimized by developing a balance of operation between each component within the system. Multiple inputs can be altered to achieve a single output. This is where controlling of the system plays a vital role in efficiency and reliability. Control is looked at overall in this paper, however specific details nearing coding and logic are discussed minimally. VRF systems offer small indoor evaporating units that require minimal space compared to ducted systems. VRF system customization is where the advantageous small form factor plays a large role. The modularity and versatility of VRF systems comes with being able to add additional indoor evaporating units with adjustments to refrigerant flow to each unit. Multi-split systems include a number of indoor evaporators that include the EEV and airflow fan in a compact unit all connected in parallel to the same refrigerant lines leading to and from an outdoor unit consisting of the compressor and condenser. As different zones require varying loads to maintain their desired air temperature the EEV will be told to allow more refrigerant to increase capacity to remove heat. Each of the indoor units’ EEV position is communicated to the Outdoor Unit (ODU), in order that the compressor operates to match the capacity provided by the flow rate. To better present the research work that has been done in VRF systems, the information has been grouped in six sections covering the major components or characteristics of a VRF (sections 3 to 8). As a summary of the research works discussed in the following sections 3 through 8, Table 2 shows the references that focus the research on a specific component or characteristic of the VRF system investigated. For example, reference [2] in Table 2 focus the study on performance of a VRF system with triple evaporator and with consideration on the effect of the piping system. In Table 2, each section corresponds to a major column with selfexplanation with the label of each column, but for clarification on the last column, COP and EER refers to coefficient of performance and energy efficiency ratio, respectively. This column and the one on Thermal Comfort are not parts of a VRF system but were treated as independent groups to highlight that those studies focused on that performance characteristic of the VRF 4

systems. The reader will notice that since information is plentiful from certain studies, there will be overlap from section to section where research is referred to multiple times throughout. Therefore, Table 2 acts as a path for this review to take place in each of their respective categories.

X

COP & EER

Thermal Comfort

Compressor Digital Scroll Inverter Scroll Single Speed

X X

R717

X

R600

R410A

R290

R125

R134A

R32

R22

4+

Triple

Refrigerants

R407C

[5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29]

Double

Reference

MultiEvaporator

Pipe Dimensions

Table 2. VRF Component Reference Chart.

X X X

X

X

X

X

X X

X

X X X

X

X X

X

X X

X X X

X X X

X X

X X X

X X

X X

X X

X

X

X X X X

X

X X X

X

X X X

X

X X X

X

X

X X

X X X

3. Multi-Evaporator Air Conditioners It is observed that multi-evaporator configurations generally consist of two to five for residential and more than five for commercial uses. Since VRF systems are expansive the first segment of review investigates just that as subsequent components will be resulting conditions of evaporator unit expansion. Modeling and simulations are included within this review as calculations for more than two units prove to be cumbersome for continually experimenting physically. Computer aided calculations effectively provide a preliminary position to start at for field investigations. 5

3.1. Dual Evaporator Where a traditional HVAC split-system or single evaporator VRF system has one single input and single output, a dual evaporator introduces additional inputs from the newly controlled zone with an additional output. Usually, this input refers to target set point for that zones’ temperature. The output occurs after many controlling parameters are met in order to provide a specific airflow with an appropriate temperature given that the remaining components are maintained at a level of acceptable reliability. In order to test conditions of a dual evaporator configuration Shah et al. [9] investigated situations of reducing airflow in one of the evaporators in order to investigate the superheat response. Since both evaporators were running identically, when the first evaporator held a reduced airflow the superheat decreased as well as zone temperature. As a response to the first evaporator airflow reduction, the superheat measurements from the second evaporator slightly increased and a small decline in zone temperature took place. This response indicates how this type of system is fully attached to itself and how experimentation is vital in the full development of future VRF systems as the response may be different in a scenario with different sized zones. Alternative variations of this scenario will be investigated later when compressors are in discussion since the compressor was in steady state during this time. As the development of VRF systems is still underway, test conditions have yet to be normalized and are logically in placed for pertinent resulting information. Choi and Kim [13] tested a dual evaporator VRF system where there are minor differences between the two indoor units within the design of the finned-tube heat exchanger ultimately allowing a difference of 1.6 CFM between the indoor units. The first evaporator had a moving range of load conditions while the second evaporator maintained a specific EEV position and load condition, all the while the compressor was held at a constant speed. The EEV used by Choi and Kim [13] was from Saginomiya and included a 4-stepping motor for a total of 120 potential testing positions. Total load capacity is monitored and compared between tests alongside superheat and mass flow rate. Similarly, to the results of Shah et al. [9] there are adverse effects when altering one of the evaporator units that show changes in control need to be handled appropriately to accommodate for the variable mass flow rate and capacity that is being called in the first evaporator. Superheat is monitored while compressor speed is incrementally increased from 30Hz to 63Hz and it shows that when the air temperature at each evaporator is equal that the superheat difference between the evaporators fluctuates between 0.25°C and 2°C. Alternatively, when the air temperature at the evaporator units has a 5°C difference the superheat is less volatile and responds similarly across both units. Developing for general VRF system is slow in comparison to a specific set up as response between various configurations has yet to be generalized. Yan et al. [22] modeled a dual evaporator system that was focused primarily also on pipeline length however the end results loosely play into response results from Choi and Kim [13] and Shah et al. [9] as it was found best to locate the outdoor unit equidistance between each evaporator. The dual evaporator

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configuration is more common in early development phases but the push for larger configurations is growing. 3.2. Triple Evaporator Before discussing fully expansive systems the next iteration is for a triple evaporator set up. To reiterate, a single compressor is still traditionally used with each evaporator having a respective EEV to modulate refrigeration flow rate. While Lin et al. [5] focused aspects of the paper on specifically controlling algorithms, there were some realizations and responses to testing the triple evaporator set up that pertain to this review. The control parameters of previous configurations start with single input and single output for single speed systems and grow but not in a linear fashion. The expansion and full control of a system increases the number of outputs to dictate input parameters. Outputs consist of pressures and temperature at the end of the phase change region in the evaporator. These must work in collaboration to adjust the compressor speed and each EEV however it is not a simple increase or decrease in orifice openings; the EEV adjustments must be analytically arrived at in near real time for respective system response time. Lin et al. [5] held the evaporating temperatures (located at the exit of the evaporator) as a primary output to monitor to keep appropriate levels of superheat with correct EEV openings. The testing consisted of a triple evaporator setup with variable options in the model for direct expansion of an additional evaporator and EEV. As seen in dual evaporator configurations superheat remains volatile with multiple units handled by a single compressor with a shared refrigeration line. At the time study from Lin et al.[26], the balance of control and reliability is a fine line. The controlling range of the triple evaporator system is widened however it increases the unreliability as superheat is harder to control for each evaporator. The indoor unit fans were held at a constant speed while EEV positions were altered. When the EEV positions of individual units were adjusted the superheat collectively had increased fluctuations which is troublesome as higher superheat values mean that the system is not operating efficiently and lower superheat values mean there is possibility that the phase change in the evaporators is not always exiting the evaporators at 100% vapor. Superheat fluctuation were reduced when each EEV was maintained equally to provide a more stable range of superheat that is guaranteed a vapor and a respectable level of system efficiency. Particular studies from Meng et al. [18] are carried out dealing with triple evaporator systems however their focus is in the heat exchanger fins and alterations to the inner components. The microchannel heat exchangers contain a level of effectiveness that is correlated to compressor performance that will be conferred in subsequent component segments. Since the progress of VRF systems is still in development, the driving parameters vary from study to study. Yan and Deng [23] investigated controlling options for a triple evaporator system that concentrates on the response of indoor relative humidity. As the experiment developed from a single to a triple evaporator setup the controlling algorithm became more rudimentary from a full variable speed system to essentially a two-stage system with high and low settings in order that humidity response was relatively acceptable. The criteria for high or low settings are if the 7

target zone temperatures were not yet met then the system would operate under higher speed conditions and on the lower speed settings if zone temperature had been satisfied. Tests transitioned from variable speed to the high-low settings which resulted in a tighter controlled range of relative humidity for each respective indoor unit zone. While the targets were met there was room for improvement to lengthen how fast the temperature and RH falls and rises. The changes to the experiment were carried out by changing from transient to steady-state operating conditions ranging from varying indoor target temperatures, sensible and latent loads. Yan et al. [29] experimented with a three-evaporator air conditioning system to show how the system responded as individual components operated to meet their respective zone testing parameters. Airflow was maintained throughout each test and the parameters surrounding the refrigeration and performance were collected. The EEV position was not let free to adjust, it was set at a value for each segment of testing ranging from 20% to 60% only. The compressor was maintained at a constant speed of 3960 rpm with a constant supply air temperature. There is meaning behind keeping a number of variables constant, and it strives to find how each indoor unit responds to changes of other units since they are so complexly related to one another. In the first test only one indoor unit was modified to change EEV position and air temperature in the space separately to view how indoor units 2 and 3 would respond with their constant conditions. In the second test, indoor units 1 and 2 were changed and indoor unit 3 was held constant. There was a correlation between the two parts of the first test. Introducing higher air temperature in indoor zone 1 steadily increased the cooling capacity for the evaporator and opening the EEV from 20% to 60% in increments also increased the cooling capacity. This seemed to be expected as more refrigerant mass flow rate increases the total capacity for that evaporator. When indoor unit 1 was altered, indoor units 2 and 3 slowly lost cooling capacity as the indoor unit was increasing from air temperature or EEV opening position. Degrees of superheat were also gathered, and as the EEV position opened there was loss in control of degrees of superheat, not by much but it was a rather low value that would begin to raise flags. In test 2 where indoor space 1 and 2 were modified, results are showing similar trends however the sharing of refrigerant with a constant compressor speed is a limiting factor. The test starts both indoor units at 40% and indoor unit increases to 60% while indoor unit 1 decreases to 20%. As expected, the cooling capacity rises for indoor unit 2 and decreases for indoor unit 1. Alternatively, when the temperature rises for space 2 and lowers for space 1, the cooling capacity increases and decreases for each respective zone. During each of the testing periods, the unaltered indoor unit maintains initial conditions within reason to not affect the remainder of the system to create instability. This indicates how crucial the system controller is to stable operation for every component to maintain reliability. 3.3. Four+ Evaporators VRF systems in development for more than four evaporators usually pertain to specific project and geolocation. Since physical experimentation can be costly, most studies conclude by means of simulations and mathematical models for results. Studies were conducted on a nine8

evaporator system in a hot and humid setting. Saab et al. [16] [17] further investigated the system response with a focus on system exergy since harsh environments provide a higher challenge. With exergy analysis refrigerants begin to take precedence among VRF systems that follow a differing set of guidelines outside of the United States. For testing each of evaporators, a model was created and a different load was applied to each of the units between 1.3kW and 6.7kW. The coefficient of performance (COP) for this nine-evaporator system was dependent on how the balance of evaporator pressures and compressor speed. COP increased as evaporator pressure did however the COP decreased with higher compressor speeds and a decline in evaporator pressures. A wide range of results values for each unit is appropriate with this large expansion of a system. The expansive system includes a larger outdoor unit than the common residentially sized unit of 1.5- to 3-ton systems. With this comes a wider range of operation modes for the system compressor. Hu et al. [25] investigated partial loads and midrange of compressor ranges to determine more than one type of compressor can be used which will be compared among other compressors. Zhang et al.[27] modeled four indoor evaporators that posed challenges from incorporating the equation models for each component when they are all working simultaneously to achieve various parameters. A finite volume method was used for a fin and tube heat exchanger with a spatial distribution model for simpler portions of the paper. Equations presented in ARI standards were used in predicting the compressor performance alongside the flow coefficients in the ASHRAE handbook for electronic expansion valve performance. With having a working model of their chosen system, a validation was carried out in a test facility to monitor superheat between each of the evaporators and testing load conditions and further explore compressor mode compatibilities. 3.4. Multi-Evaporator Highlights:  



Adding evaporators increases quantity of inputs and outputs nonlinearly for proper system integration, stability, and reliability. Testing includes, but not limited to: o Airflow reduction in single zone for zone response and stability o Variety of load conditions per single zone for stability and response in other zones and constant load o Different constant loads per zone for system response through EEV controller and variable compressor speed o EEV position changes for response in superheat measurements with a fixed compressor speed o Limiting ranges of airflow and compressor speed for part load condition experimentation Double- and triple-evaporator HVAC systems are recommended to be targeted to investigate for future residential usage as control variables are less excessive and system is more responsive than more than four evaporators. 9

4. Refrigerants Choice of refrigerants is not solely based on performance. Environmentally friendly and nonhazardous properties drive the development in rising refrigerants. Amarnath and Blatt [15] observed that years leading into residential VRF systems there was an array of refrigerants starting with R-22 and R-407C that eventually led to the more common R-410A. Shah et al. [9] used R-22 in the dual-evaporator system validation where pressure and superheat response were focus but refrigerant was not compared as preceding review work for the experiment was based on R-22 since it is the predecessor to R-410A. R-32 is only used in R-407C along with R-125 and R-134a in a mixture. In a study by Sencan et al. [6], R-134a was resulted to have the highest efficiency in a MATLAB modeled system for superheat and subcooling properties. The system tested on was not specifically and variable system as compressor efficiencies were applied to the model for calculations. COP was recorded from first and second law of thermodynamics analysis with R-134a, R-407C, and R-410A. R-410A was shown that although a more recent and safer refrigerant, it held a lower efficiency rate compared to the other refrigerants. In another R-407C mixture experiment, Atas et al. [8] investigated a VRF system with three evaporators resulted in a COP minimum value of 2.24 where the study from Sencan had a range of COP from 2 to 6.5 depending on the compressor efficiency and operating temperatures. The lower COP range is an additional indication of VRF efficiency over a single speed system. Temperature control of each unit remained similar as the EEV control was maintaining proper pressure and temperature at the exit of the evaporators. Saab et al. [16] [17] experimented with nine-evaporators and five refrigerants; R-134a, R290, R-410A, R-600, and R-717. The highest COP over a range of compressor pressures comes from R717 which is 24% higher than R410A with the other refrigerants falling behind 12% to 37% lower. Using R717 (ammonia) is toxic so R410A is chosen as there must be some position where one refrigerant is better on average with various features as ozone depleting potential, global warming potential, lifetime that all factor into choosing the correct refrigerant. Aside from being environmentally harmful, R717, if used, would need to be replaced yearly increasing the costs of maintenance even more. Between the compressor and evaporators there is a tradeoff between COP values as for the evaporator as pressure increases the COP does as well however for the compressor the COP decreases as the pressure increases. Similar responses were shown for each refrigerant over the range of pressure where R717 has the highest COP potential but incurs environmental and health threats. For comparative purpose the COP for R-410A and R134a were 3.9 and 3.4 respectively. The remainder of refrigerants did generally maintain a lower COP they are not used due to safety and maintenance reasons. Tu et al. [7] charged a VRF system with 16 kg of R-410A in order to record performance data. A large amount of charge was set into this system as portions can be stored in a sub-cooler that affected to this experiment. In other systems with multiple evaporators there are multiple input variables to dictate the compressor speed, fans, EEV position but this controller checks if the unit is on then looks for change in pressures and alters compressor speed if it needs to change 10

or completely stop. A positive component to system developed controllers is how this experiment used a pressure compensation to keep the capacity dynamic to reflect the changes in the system. The balance of compressor speed and EEV operation remain to be the controllers of performance results. Cheung et al. [19] investigated heating mode with different types of expansion valves for VRF systems using R-410A through a thermostatic expansion valve (TXV) or a fixed orifice with an accumulator (FXO) primarily used in single speed systems. The single speed systems were compared directly to the VRF systems. COP and efficiencies of heat production, heat transfer, degradations and second law efficiency are measured and reported for the systems each kept at 2.2kg of R410A of charge. When ambient temperatures were below target set points where the system was kept at near full capacity the COP between the VRF system and the TXV and FXO systems were within a respectable range of each other. However, when temperatures within range of partial load capacity and the VRF system began to modulate compressor speed the COP exceeded that of the TXV and FXO system by up to 2.5. As a general finding, R-410A remains a common goal of uniform residential refrigerant while new refrigerants are always being researched. 4.1. Refrigerant Highlights: 

 



Numerous factors affect choice of refrigerant aside from performance o Ozone depleting potential o Global warming potential o Refrigerant lifetime o Overall hazardousness R-717 was shown to be best performing however is known to be most hazardous and highest required maintenance Coefficient of performance and environmental factors place R-134a and R-410A close in contention but R-410A pressure and temperature properties appropriate for a residential a HVAC system R-410A is current common refrigerant over pervious R-22 due to ozone harmfulness and heat transfer potential

5. Pipe Dimensions The length of refrigerant pipe lines has been found to play vital roles in system response and sensitivity. Lin et al. [5] investigated a triple evaporator VRF system where pipeline length for each indoor unit measured 11m, 17m, and 28m. As superheat was a focal point for controller purposes it was seen that the levels of superheat increase between each unit due to length of refrigerant pipe length. The control aspect of the study neutralized large differences from pipe length differences with error feedback to the controller in order to keep a uniform level of superheat. The air temperature responses remained similar throughout their optimization process. Pan et al. [12] reiterated that the longer refrigerant pipes impose pressure drop that can affect 11

system performance. Initially the single evaporator configuration is considered and evaluated for total power requirement while maintained constant refrigerant line lengths. However, the diameter of said pipe is considered and delivers optimal amount for performance versus evaporator size. This is changed when introducing further evaporator expansion passed two. Saab et al. [17] investigated a number of refrigerants across nine evaporators with a 20-meter pipe length as this study was not directed toward residential use as far as the number of evaporators. There was no altering of lengths, but the pressure increases across the compressor reduced COP for the system and each refrigerant. Min et al. [21] focused on a vapor-injection to the VRF system in colder seasons that was found to be more beneficial with longer pipe lengths to fully activate with additional components. Yan et al. [22] primarily focus was for building use but the study is carried for dualevaporator system that can be generalized for a residential setting. The variable that was affected the most between testing scenarios was the evaporating pressure at the indoor units at longer lengths began to increase thus creating a larger drop between suction and evaporating pressure. At this point in modifications to a system, changes made elsewhere such as compressor can negate some differences found in pipe length. Refrigeration control management with EEVs can also alleviate issues found in pipe dimension differences however, this is counterproductive in developing a system that can be applied generally. Tuning and slight modifications are due upon installation but not on a software control level. It is noted that as evaporator are added to the system, pressure drop is to be maintained between each branch of refrigeration line to be relatively close to each other. Alternatively, the introduction of larger diameter refrigeration pipes can help reduce pressure drop. This can only take place if the capacity of the entire system deems it necessary otherwise COP and efficiency will suffer throughout. 5.1. Pipe Dimensions Highlights:     

Underlying concern with pipe length is maintaining vapor from the evaporator to the compressor inlet Systems with longer refrigerant pipes may require a higher control point of superheat to compensate and maintain vapor upon exit Pressure drop naturally occurs for longer pipe lengths thus alters temperature control for superheat and performance System balance favors equal pipe lengths for multiple evaporator systems Pipe diameter remains dependent upon indoor unit capacity and refrigerant choice however a tradeoff between length and diameter should be investigated for various evaporator distances

6. Compressors 6.1. Digital Scroll Compressor

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These types of compressors operate at full speed which would appear counter intuitive of the types of investigations in discussion. The scroll that is orbiting another scroll is what is operating at full speed, but the compressor controller lifts the scrolls away from each other when capacity needs to be adjusted. When the scrolls are separated the work done on the refrigerant is zero thus minimal energy consumption and when capacity is required the scroll is set back in starting to increase pressure and temperature of the refrigerant to the desired discharge pressure and temperature. The variation of when the scrolls are together(closed) or separated(open) is averaged by time of operation to calculate an output ratio which was used to gather the output capacity. The ductless system that was compared to a TXV and FXO system is the proof of levels of increased efficiency when using a digital scroll compressor over a simple single stage from Cheung and Braun [19]. The testing methodology was structured to allow testing in partial load range for the single system to further showcase where VRF based systems excel. Superheat targets were maintained at the same temperature with slight variation in target compressor speed settings due to variation in expansion valve operation. While there are segments of ambient temperature where COP values are relatively close the subject is of longevity of efficiency for energy consumption which over a longer period of time the VRF system ventures away from the single stage efficiency base line. Hu et al. [25] further proves the efficacy and potential reliability of digital scroll compressors in a multi-evaporator air-conditioning system. Integration between EEV, compressor speed and capacity output were crucial in its success. The further levels of controllability allowed for zoning options of up to five indoor units on the same scroll compressor and part-load scenarios resulting in energy consumption reduction as the range of operation in this type of system ranges from 17% to 100%. Calibration was required for the usage of single EEV usage up to four components to develop a linear regression when adding or removing controlled zones and remain under the same compressor. The response parameters in check for the test were sub-cool and superheat temperatures compared with the outdoor fan speed. Compared to the traditional variable frequency-controlled compressor this was a promising increase in existing operation range of 48% to 104%. Multiple compressor systems hold validation in evaluating part load conditions for VRF systems however the efforts are not often seen in consumer residential production. Tu et al.[28] experimentation focuses on just that with a digital scroll compressor and single speed compressor for five indoor units. Initial results indicated that the system remained stable when the digital scroll compressor had an output ratio of 100% thus closing the valve and the scrolling produced maximum capacity. This is where the secondary compressor engages. Although the digital scroll compressor is running stable, the target pressure has yet to been reached thus enabling the standard compressor to alleviate output ratio in order that the pressure be reached while attempting to remain stable. While both systems were on, the target pressure still had issues of being reached until the output capacity was reduced. When the digital scroll compressor had a low output ratio of 20% is when the standard compressor reverts to on-off status. Results 13

showed where the fluctuation of supply air temperatures were from on-off cycling of the standard compressor than attempt to aid the digital scroll compressor. In efforts to improve the experiment, average suction pressures were taken during a period of on and off stages of the digital compressor to control the output capacity differently in order the target pressure has been met. The goal was met to have the pressure maintain an oscillation that was within a defined range due to an averaged suction pressure across cycle periods. A testing methodology has been documented for VRF systems with a multi-compressor configuration as previous by Xia et al. [11]. While this methodology is for a heat recovery system, the compressor configuration response is relatable. The performance for part load conditions was exceptional as this is where both the variable speed and single speed compressor are engaged. This band of higher efficiency is where the system begins to engage the single speed compressor as where the previous investigation sought out to find the optimal frequency at which to engage the secondary compressor. Part load conditions is the beginning of increase efficiency, so it is no surprise there are numerous investigations regarding a dual-compressor configuration. Similar to previous multi-compressor studies, Zhang et al. [20] also experiments with a VRF system with a single speed compressor complementary to a digital scroll compressor to evaluate part load conditions for a four-zone system. This compressor configuration is controlled by pulse width modulation (PWM) so it was determined using COP and energy efficiency ratio (EER) is not optimal as PWM varies with time. A simulation model was presented by Min et al. [21] using an injected scroll compressor and sub-cooler. Parameters measured and developed were degrees of superheat and sub-cool, alongside efficiency and the amount of injection ratios. Carried out for the cold seasons, an increase in injection ratio also increased efficiency of the compressor found by differences of pressures. Varying portions of mass flow rate made the same periods of injection windows longer thus making ratios higher and increasing mass flow rate from the compressor. The digital scroll compressor in different climates proves to be versatile for further development such as vapor injection. 6.2. Inverter Driven Compressor These types of compressors similar to the digital counterpart also have scrolls orbiting however there is no moving of the scroll to disengage the compressing of refrigerant. The variable part of this compressor is a variable motor that adjusts the speed of the scroll motor thus adjusting the compressor capacity. Atas et al. [8] evaluates a system ran by an inverter driven compressor. The compressor was initially operated at a static 50Hz with a set temperature for the system targets where the pressure between each of those scenarios were nearly identical to each other indicating good reliability throughout a cooling mode range of set points. The PLC (programmable logic controller) introduces variable speed to the compressor that is calculated from input temperature throughout the system response. Remainder of results were comparison between static 50Hz compressor speed and variable compressor speed. Overall the PLC method maintained a lower energy 14

consumption rate. The notable point when comparing the COP to the energy consumption is that when the compressor speed is held at a static 50Hz, the COP is lowest at 2.24. This does not automatically dictate that single speed compressors are superior; it just remains that for this speed and configuration for testing shows that single speed systems do have their place. The PLC method locates the balance of energy consumption with wanting to keep the COP higher. The actual temperature control is similar between both methods. Lin et al. [26] experiment contains a 0 to 100Hz inverter driven compressor alongside a 480step EEV. Feedback control loops are discussed for control but not heavily considered for this review paper. Alongside the control loops, EEV control was also discussed as theories are presented in order to control the flow throughout the evaporator and EEV positioning effects on temperature parameters. Lin et al. [26] experiments and models the system as all evaporators are active each requiring EEV control for capacity. Typical situations where various zones may require different capacities to reach their respective temperature settings rather than all constant values throughout. It was considered that each zone is on and demanding respective refrigerant flow. Variation of targeted evaporator temperatures proved that different evaporators can be controlled by this logic of flow distribution. In development of multiple (more than two) evaporators the control development can be extrapolated to conform to as many evaporator units as needed. The effectiveness of this particular control is currently limited to the whole system being completely on and simplifying EEV modulation of refrigerant flow distribution. Zhang et al.[27] studied finding an alternative or competitive system to variable speed compressor systems posed many challenges as variations needed to be validated. Where inverter driven compressors generally include a variable frequency drive to control the motor speed from frequency and voltage, a rapid cycling compressor is relied upon cycling period variations. One cycle period in a rapid cycling compressor consists of a load and idle portion. The amount of time spend under load is affected by the system determined duty ratio. Going into this study, concerns were held regarding the idling portion of the cycle period where there is no compression during idle phase and a designated amount of power consumption for the idling motor function to fill the duty ratio and complete the cycling period. Introducing partial loads to compressors truly tests the reliability and efficiency of a compressor as some may indicated higher efficiency but only at higher loads. Parameters of cycle periods indicated that change in cycle period time will not affect the overall performance. The coefficient of performance only shows slight differentiation when the cycle period is at its lengthiest time frame. Benefit of a rapid cycling compressor over an inverter driven compressor for a VRF system has yet to be seen. Choi and Kim [13] tested variations between the two indoor units was a 21°C (70°F) and 27°C (81°F) for indoor unit 1 and 2 respectively. At a testing condition of constant compressor speed the primary source of cooling capacity is focused into indoor unit 1 at lower temperature conditions of 21°C (70°F). Variations of keeping EEVs separately constant and or controlled is tested alongside changes in compressor speeds that ultimately grant a maximum cooling capacity overall when the superheat is maintained at 4°C. Superheat without control continued to rise as 15

there was a segment of the experimentation where the EEV was maintained at constant opening where the alternate indoor unit was controlled and the superheat was between 4°C – 10°C. Subcooling was kept also between 4°C – 7°C. Meng et al. [18] experiments with a three evaporator system with an inverter driven compressor and additional solenoid valves to increase system pressure during start up periods. There is an oil management phase where if the compressor runs below a designated speed (35Hz) for longer than 60 minutes then a valve opens to introduce more oil to the compressor to improve longevity. Without safety measures, failure would be imminent for a system that can run for extended hours. The remainder of the investigation included focus on a developmental type of condenser under part load conditions ranging from 20% to 120%. The trends found between condenser modifications were minimally different with an increase in cooling capacity from increase in compressor speed between each design change in heat exchanger microchannels. Tu et al. [10] continued to investigate a multi-compressor arrangement consisting of an inverter driven compressor and a single stage compressor. The goal of this was to create a larger band of efficiency while using both compressors to achieve a higher demand while maintaining acceptable noise levels. The odd occurrences throughout this testing experiment were found to be due to pressure variances during early attempts at stabilizing phases. At a certain compressor speed the frequency of the driver will engage the single stage compressor to aid the inverter driven compressor. Determining the point at which the secondary compressor engages is crucial to ensuring stability and system performance. Field experimentation showed that in the midrange efficiency performance was high while the outer bands of operation showed poor levels of efficiency with an increase in noise levels at the higher band. The multi-compressor has potential but not specifically a direct need within VRF development. 6.3. Single Speed For a number of studies regarding improved efficiencies and performance, results are often compared to that of a single unit split system that has a single speed compressor. However, from the variable speed compressors discussion it is not uncommon for a test to operate a variable speed compressor at a constant speed in order to test other response and variables throughout the system. The study by Cheung and Braun [19] included the TXV and FXO based expansion system containing the data from testing two different types of single speed compressor configurations against an EEV based variable speed compressor system. This data shows where variable speed compressor excels and where the single speed compressors are similar in COP. The inclusion of single speed compressors to variable speed compressors to make a multicompressor system is worth the time spent on the research however the cost effectiveness is less than a single variable speed compressor for the VRF system. 6.4. Compressor Highlights:

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Digital scroll compressors run at full speed uninterruptedly with a mechanical part that separates top and bottom of the scroll compressor to reduce work to zero and create no pressure difference Inverter driven compressor, while similar to the scrolling compressor principle, contains a motor that adjusts the speed of the scrolling action to appropriate temperature and pressure control for partial load conditions; no top and bottom separation Multiple compressor systems contain a variable speed and a single speed compressor that achieved higher COP values o While max load conditions are met with both compressors, the variable speed compressor begins to decrease with respect to declining ambient temperatures in later portion of a day o COP values are averaged over a shorter time span with two-compressors show increase over single compressor systems in early phases of VRF development Inverter driven compressor variability allows for testing in stages of a fixed speed to evaluate the remainder of system components then adjust to a higher or lower speed and analyze Testing between the studied systems show variation in results and system response as each control is particular to that system reliability Future investigations should continue with partial load conditions utilizing only one compressor with at least two evaporators in efforts to generalize a controlling application and guidelines for a residential system

7. Thermal Comfort Proper thermal comfort does not rely solely on target temperatures but relative humidity as well. System testing for controlling sensitivity is necessary in order that zone temperature is steadily reached opposed to immediate cooling that leads to consumer discomfort or noise issues from having too fast of air flow speed. Yan and Deng [23] developed a controlling method for a triple evaporator system based on previous research regarding a single and double evaporator system. The leading information that drove this study held a temperature-based control for one system and a humidity-based control for another system. Yan and Deng [23] integrated these systems to simulate overall response in both temperature and relative humidity means. The initial experiment consisted of each of the three indoor units’ target temperature set to 26°C(79°F) with an outdoor ambient temperature of 35°C(95°F). The sensible load varied between the indoor units from 1150 W to 1350 W and the latent load varied from 450 W to 650 W. The results showed that the variation of sensible and latent loads affected only the patterns of response of temperature and relative humidity while maintaining good control of the system within range of the target temperatures. The first indoor unit had the lowest sensible and latent loads at 1150 W and 450 W, respectively. This showed where the lower temperature control threshold was met before the other two indoor units. Since this order of operation took place, the controller reduced airflow and refrigerant across and through the evaporator coils. This is how the controller was 17

programmed to respond. The lowest band of relative humidity, 51%, was attained between the three zones by this indirect dehumidification process. The other two zones maintained a relative humidity up to 58%. Yan and Deng [23] carried out a second test that put each unit’s target temperature within 1°C (approximately 1.9°F) of each other with indoor unit 1 at 27°C(80.7°F), indoor unit 2 at 26°C(78.8°F), and indoor unit 3 at 25°C(77°F). Sensible and latent loads are held constant throughout each component at 1300 W and 500 W. The test variation happens at 40 minutes where the indoor unit 1 is altered from 1300 W to 1400 W sensible load and 500 W to 700 W latent load. The results showed that the change in load increased the relative humidity by 3% while maintaining target temperature at 27°C(80.7°F) in a looser pattern of modulation. The response time for zone 1 was approximately 30 minutes for the zone temperature and zone RH to reach a steady level. The other two zones were hardly affected and showed no noticeable response. Park et al. [24] experimented with EnergyPlus simulations to compare to configurations of a VRF system with and without outdoor ventilation. This study stretches the limit of this review that investigates into ventilation contribution however needs to be addressed in efforts to convey the importance of possible humidity control for future usage. Using ASHRAE thermal comfort standards Park et al. found that for this non-residential building with a VRF system without any ventilation grants less than 20% acceptable comfortable conditions for winter and summer seasons. The addition of any ventilation, whether it be an energy recovery ventilation (ERV) or a dedicated outdoor air system (DOAS), moves ASHRAE acceptable comfortable conditions in the winter and summer season in the range of 65% to 94%. This goes to justify the system response from Yan and Deng [23] for the triple evaporator configuration with adjusted target temperatures and reduced relative humidity response from load changes in order to maintain a level of thermal comfort that is acceptable. Alahmer and Alsaqoor [14] simulated a multi-level building with multiple split systems throughout each level but investigated thermal comfort for a room during summertime that further responded to Yan and Deng [23] that efforts are to be made that as temperature rises in the zone, the relative humidity should be reduced in order to remain within acceptable comfort ranges. 7.1. Thermal Comfort Highlights:  

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After general system response with stability and reliability is achieved, controlling methods are investigated with thermal comfort in consideration Relative humidity (RH) held a focal point for a specific triple evaporator configuration that showed moderate speed and steady RH fluctuation with sensible and latent load adjustments o Measurement of thermal comfort comes with several qualifications and standards to follow that will need to be followed more closely after multi evaporator systems become more common More common systems will introduce ventilation variables that were shown to drastically increase thermal comfort based upon ASHRAE standard 18

8. Coefficient of Performance and Energy Efficiency Ratio Coefficient of performance (COP) and energy efficiency ratio (EER) are common means of evaluating a HVAC system. The drive for VRF systems falls within the increase in COP and EER over that of single speed systems. Part load conditions is where the majority of improvements of VRF over single speed systems resides. On lower and higher bands of operation, COP and EER are often closer together but while the single speed system can only be on or off, the partial load requirement favors the variable speed of VRF thus averaging higher performance values. Also, seasonal energy efficiency ratio (SEER) is considered from some studies. Although the study by Tu et al. [7] researched adding in a sub-cooler into the system for charge management and performance variation with variable speed compressor, the results show the importance of refrigeration control via means of compressor speed and EEV opening. A 25% increase in EER occurred while there was a pressure compensation addition to the input variables of control that determined the compressor speed. The compensation and compressor speed were the changes to that of without while during both tests the EEV position of the respective subcooler was held at same positions throughout. This 25% increase puts more priority toward the compressor operation over the EEV position control. Atas et al. [8],mentioned previously a triple-evaporator set up that was experimented with variable speed and a 50 Hz speed compressor setting. As expected from relevant information, the variable speed system performed better with a COP of 2.2 over 1.9 from the single speed 50 Hz system. The direct correlation between COP and compressor speed is that at the highest set compressor speed the COP is lowest but the range of variable speed is to be averaged throughout a test thus resulting in an overall higher COP. Xia et al. [11] had a multiple evaporator experiment with two compressors; one single speed and one variable speed. This system showed a steady range of COP values throughout the dedicated band of compressor speeds in conjunction with differing ambient temperatures. From 15°C(59°F) to 35°C(95°F) outdoor temperatures the range of COP was between 2 and 2.4 for cooling mode in range of 30% to 60% compressor operation. Directly correlated to compressor and COP consistency follows a pressure trend that follows similarly. While superheat and subcooling temperature control was main topic for Choi and Kim [13] COP was visited while the compressor speed was held constant while and EEV remained constant in order to observe response when only adjusting inlet air temperature of a single indoor unit. From 20°C(68°F) to 32°(90°F) under specified conditions COP went from 2.6 up to 2.66 as the cooling capacity exceeded the total power consumption. Saab et al. [16] [17], as discussed, tested five types of refrigerants and monitored their COP in response to pressure created from the compressor ranging from 2500 kPa (362 psi) to 3600 kPa (522 psi) that resulted in the two most common HVAC refrigerants, of the tested, together under the more hazardous option (R717). Each of the refrigerants responded similarly throughout the compressor pressure range, so trends can be generalized for a VRF system with multiple evaporators involved. In discussion of refrigerant properties, pipeline length has some effect on performance. Yan et al. [22] found that there is a decrease in COP with increase in refrigerant 19

line length. The maximum length changed within the simulation was approximately 20 meters (65 feet). An 8% decrease in COP can change of course with even longer lengths but at a much faster rate per foot added. With a variation in COP of 0.05, the highest performance value was when the evaporators were equal distance from the outdoor unit. The COP was impacted when the outdoor unit was exceptionally close to an indoor unit. This goes hand in hand with having similar distances between units. Cheung and Braun [19] compared ductless VRF systems with single speed systems with different types of expansion valves. In this study, the residential building load maxes out near 8000 W and produces a COP for the ductless heat pump and single speed system both at approximately 3.1. However, prior to the max building load during medium load, the ductless VRF system averages a COP of 3.8 where the single speed system falls to 2.8 and below. In further comparison while experimenting with a range of ambient temperatures, -15°C(5°F) to 15°C(59°F), there is a range of COP percentage increase from 5% up to 50% as indoor temperatures approach their target. As the target temperature is approached the part load conditions of the variable speed compressor begin to activate and further push the variable speed system over the single speed since the single speed system is still at 100% speed when that capacity is not truly required. Zhang et al. [20] found the hourly energy efficiency ratio (HEER) is more suitable over EER or even seasonal EER as PWM can be checked over smaller periods of time for evaluation. It was found that the HEER responds to the ratio of actual capacity to maximum capacity as most other compressor configurations do where the part load conditions hold a higher HEER value where both compressors are operating providing cooling capacity efficiently. The response of total power consumption per hour mirrored throughout the part load value conditions. This reflects the response correlation from ambient temperatures for performance better at lower ambient temperatures. 8.1. COP and EER Highlights: 

   

Coefficient of performance and energy efficiency ratios are a direct reply to how well the system is controlled with various integration techniques and compensation factors o Pressure compensation can grant up to a 25% increase in EER Coefficient of performance is improved for variable speed compressors over single speed largely in the part load condition range Measuring performance is relative to measurable time as experiments used multiple compressors that averaged COP over their hourly operation that netted in a higher value Systems with pulse width modulated compressors can be analyzed effectively in smaller time windows of hourly energy efficiency and potentially seasonal energy efficiency ratios In development, COP and EER come with fine tuning an already stable system

9. Conclusion 20

Progress of VRF systems is reliant on integration of each component input and output variables. This review encapsulates the vast array of individual and grouped categories of VRF technology that consisted of experimental testing environments for operation in heat pump configurations. Increasing the number of evaporators proliferates the range of control variables which is how zone control is handled by VRF HVAC systems. The selection between digital scroll and inverter driven compressors is a sole contributor to increase performance in partial load conditions. Existing literature showcases various paths to investigate as it has been in developmental phases. These developmental phases have potential to be as creative as possible. An intriguing proposal is a scenario where a traditional split system is hybridized with a number of wall units all for larger and individual zone control. This path leads potential testing with pipe diameters and length, and EEV control for the larger capacity airflow air handler and smaller sized wall units for total efficient control with improved operation and increased reliability. Although VRF systems are being well accepted as technology for saving energy, the literature reviewed shows that more development is needed to make the technology more robust and economically feasible. EEV control is a key factor to ensure the system to operate as desired and compressors technology must be further studied to decide on what type of compressor is the most appropriated for VRF systems. Future work includes research in varying topics leading to optimal synergized operation of VRF technology used in even most unique applications. Variable speed compressor operation and variation in fixed speed is the foundation in controlling a VRF system efficiently, thus leading to effectiveness of control with pipe layout and EEV operation. The unique applications and final layer to perfect will pertain mostly to thermal comfort. Thermal comfort has potential to remain most volatile output since nearly all applications will be different. The uniqueness comes in custom configurations based on the residence in question. Open floor plans that consist of two or more fan coils propose issues with air mixture and zone domination throughout daily heat load orientation. Simulations would fall short of the many variables to attempt to generalize exceptional design layouts where as physical experimental will provide a more dynamic heat load in a residential home for potential orientation generalizations to apply in specific installations. Therefore, research on thermal comfort provided by a VRF system in typical residential layouts has the potential for complementing the proven benefits of this technology regarding energy savings.

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Declaration of interests

☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:

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