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Abstract

The present study reports air-based novel energy saving compact bank type earth air heat exchanger system (EAHE) (having only 25m² area) fabricated from Polyvinylchloride pipes of nominal diameter 0.203 mm (8 inches) for inlet and outlet headers and main pipes and 0.150 mm (6 inches nominal diameter) for branched pipes, which is installed in Shaheed Bhagat Singh State University, Ferozepur in Punjab state, India (North West border area of India) and studied for the duration from April to August 2021 using the full factorial design with four inlet variables at multilevel to find a model equation between inlet and outlet variables, i.e., heat exchanger effectiveness and achieved temperature difference. An induced airflow mode has been used to provide near-uniform flow through all corresponding pipes of symmetrical-shaped earth air heat exchangers. The values of Dry bulb temperature (DBT) and wet bulb temperature (WBT) have been measured for air inlet, for air outlet, as well as for ambient air using resistance temperature detectors (RTD) installed at the requisite locations. Earth's undisturbed temperature (EUT) was also noted by installing RTD at a depth of 2 m, and the average measured value of EUT is found to be 28.5°C. The current system has imparted the temperature difference variation from 0.4 ^oC to 9.4 ^oC, and the effectiveness varied from 0.16 to 0.82 during the whole season. This system could give a cooling potential of 0.0523 kJ/s to 1.587 kJ/s using a mass flow rate of 0.163 kg/s.Experiments have been designed methodically to apply the full factorial technique. Most favorable parameters have been found for hot and dry and hot and humid weather. The current study is novel in terms of significantly improving the effectiveness of EAHE and addressing the central issue of space limitation in urban areas.

Keywords

Earth air heat exchanger full factorial design factor interactions effectiveness temperature difference dry bulb temperature wet bulb temperature earth's undisturbed temperature

Introduction

An earth air heat exchanger helps regulate the temperature of ambient air flowing through it before being fed to the conditioned space. These are placed 1–3 m below the ground surface, and the air may pick up or lose heat to the surrounding soil depending on weather conditions. The concept behind this system is that the temperature below the earth's surface beyond a certain depth becomes independent of ambient air temperature, which is called Earth's Undisturbed Temperature (EUT). The EUT is higher and lower than the ambient temperature in the winter and summer seasons, respectively. When ambient air is passed through the tubes buried underground at a considerable depth, the air becomes warmer in winter and cooler in summer than ambient air. So it is considered to be a promising technique for energy savings. Although it is a very energy-efficient option, it has not been used on a large scale as it requires considerable space. EAHE is a very inert cooling technique with no environmental problems and consumes minimal electrical energy. India's Energy Conservation Building Code (ECBC 2017) has considered the EAHEs, the best air conditioning technique for the buildings sector. Almost 40% of the world's high-grade energy is consumed in the heating and cooling of buildings. Combating global warming through reduced energy consumption in buildings is one of the European Union strategies. The use of renewable energy sources is being encouraged across the globe;¹ thus, EAHEs ideally provide comfortable temperatures for preheating and pre-cooling in the winter and summer seasons, respectively.

Climatic conditions affect the performance of EAHE. Studies have been carried out on EAHEs across the globe under different climatic conditions.^2–5 These studies have reported that EAHEs work best in semi-arid and temperate climates but worst in humid climates.^5,6 The places with alternate heating and cooling demands have been shown to utilize this technology optimally.³ Soil composition also affects the performance; hence the effect of soil composition has been studied,^7–11 and it was concluded that sandy soil gives the best performance as far as heat transfer is concerned.⁸ The soil with a higher humidity level performs better than dry soil.¹² It has been reported that the addition of Bentonite to soil can improve the performance of EAHE; thus, the length of the tube can be reduced, and the increase in energy gain achieved is 13–14%.¹¹ The thermal conductivity of dense soil is high and thus gives better performance.¹³ The material of pipes does not have much impact on the performance of EAHE.¹⁴

Extensive testing has been done on EAHEs in both physical and simulative modes. A few studies have reported actual fabrication and installation of EAHE in full scale. Many studies have been conducted which have used simulation techniques^15–19 using software, i.e., CFD, ANSYS, TRANSYS. Numerical modeling^20–23 and analytical modeling at different parametric values using different configurations have been done to a large extent. Most of the experiments have been performed with pipes laid in a straight line^24–35 and serpentine style^36–39 in both open mode⁴⁰ and closed mode⁴¹ to study their usefulness. Large space requirement is a significant constraint for that. So pipes were laid in meandering style by other researchers²⁷ to overcome this problem. U, S, and Z configurations have also been used.^{22,39,42–44} Temperature difference ranging from 5°C to 20°C has been reported. In one of the studies, a higher temperature difference of 21.3 has been achieved with a water jacket on the outlet pipe.⁴⁵ The longer the EAHE, the more the effectiveness as the outlet temperature gets closer to EUT. Vertical borehole heat exchangers have also been studied with groundwater flow. A new convective ground resistance was added for a peclet number greater than 0.01.⁴⁶

Some studies are based on combined systems where EAHE is coupled with evaporative coolers, desiccant dehumidification systems (experimentally^47,48 as well as numerically^,49–52⁾ and vapor compression systems^42,53,54 to improve its competence in the air conditioning industry. Energy savings of 71%,⁴⁹ 74%,⁵⁵ and 75%⁵⁶ and CO₂ emissions savings of 76 to 91%⁵² have been reported in coupled systems. Indoor air quality can be improved by coupling EAHE with a solar-assisted desiccant cooling system also.⁵⁷

The ground heat source pump was examined for cooling and heating purpose and the co-efficient of performance was found to be 4.29 for cooling and 3.97 for heating purpose in Korea.⁵⁸

Bank type EAHE can be easily fitted in while laying the foundation of buildings as compact and does not meddle with the civil foundation restraints. EAHEs with long straight pipes can be practicable in countryside areas, but metropolitan cities are unsuitable due to space constraints. The cost of land, as well as quarrying, is high in urban areas. So for the present study, a compact bank type EAHE(requiring only 25m² area) was conducted in Shaheed Bhagat Singh State University, Ferozepur in Punjab state, India, from April to August 2021.

Materials and methods

Bank shaped earth air heat exchanger consisting of two headers (inlet and outlet) with four parallel distribution lines has been decided. EAHE was fabricated with Polyvinylchloride pipes of nominal diameter 0.203 mm (8 inches) for inlet and outlet headers and main pipes and 0.150 mm (6 inches nominal diameter)for branched pipes. Leak-proof joints were made between the lines. The symmetrically shapedEAHE was installed in a trench, as shown in Supplementary Figure 1. The diameter of the main pipes is 1.33 times the diameter of connecting lines, as the literature confirms that the primary pipe diameter should be 1.2 to 1.8 times the parallel branched pipes.⁴³ The labeled top and front views of the header connected to the main pipe, with all dimensions, are shown in Supplementary Figure 2.

Two similar setups for experimentation were created and installed at the depths of 1 m and 2 m below the earth's surface. Induced draught of airflow is supplied using a circular duct fan. The values of Dry bulb temperature (DBT) and wet bulb temperature (WBT) have been measured at (a) air inlet (b) air outlet (c) ambient air, with the help of resistance temperature detectors (RTD) installed at the requisite locations. Earth's undisturbed temperature (EUT) was also noted by installing RTD at a depth of 2 m, away from the effect of EAHE. Instruments used of the required specifications are given in Table 1, and their pictures are given in supplementary Figure 3.

Table 1.

Instrumentation and component used.

Use	Instrument	Specifications
For measuring airflow velocity through the pipes	Digital vane type Anemometer (AVM-03) of Metrix make	Range of the instrument is 0.4 m/s to 30 m/s; resolution of 0.1 and accuracy of + 2% for an operational temperature range of 0 °C to 50 °C.
For induced airflow through the pipes	Circular duct type fan “ Asteberg Ventilation” make, model AEE-250	0.8 A of current: 175 W; 2420 rpm; a flow of 949 cubic meters per hour; the pressure of 694 Pa; single-phase fan that works at a voltage of 220 to 240V

For measuring the temperature of inlet and outlet air	Resistance temperature detectors	3-wire RTD with four inches length, diameter 5 mm, 5 m long wire
For taking readings of temperature	6-channel Temperature indicator	Digital type with the range of -10°C to 50°C

Many observations have been recorded daily from April to August 2021 at two different depths and three airflow velocity values. The values of DBT and WBT have been measured with the help of RTDs with an uncertainty level of ± 0.1 °C, and velocity of air has been measured with Vane type anemometer with the measurement uncertainty of ± 0.1 m s − 1. The airflow rate has been calculated using airflow velocity, and the uncertainty falls within ± 2% of the measured value. The specific humidity of air was calculated from the DBT and WBT of inlet and outlet air, respectively. The uncertainty in the calculated values from the observed values is found to be ± 2.67 × 10–4 kg kg − 1 of dry air. Output response variables are the effectiveness of the heat exchanger and temperature difference. They have been calculated from the expressions given below.

\begin{aligned} ε = (T_{d i} - T_{d o)} / (T_{d i} – E U T) \\ d T = T_{d i} – T_{d o} \end{aligned}

dT is directly proportional to the cooling potential which is given by the formula

Cooling Potential = m C p (T_{d i} – T_{d o})

Where m is the mass flow rate of air through the pipes and

Cp is the specific heat at constant pressure.

The detail of inlet parameters and response variables, along with their minimum and maximum values, is given in Table 2.

Table 2.

Comparison of results with previous studies.

Reference	Place	Specifications						Inlet Temp	Performance parameter
Reference	Place	Length (m)	Depth (m)	Diameter (m)	Material	Velocity m/s	Shape	(^oC)	dT (^oC)	$E$	Citation
Sakhri, Menni, Chamkha, et al., 2020	ALGERIA	60	1.5	0.11	PVC		Open loop	36.2	6.2	0.75	¹⁷
Rangarajan et al., 2019	PUNE	As per plot	2, 2.75,4				Serpentine	36	7	0.58	³²
Belloufi et al., 2017	ALGERIA	53,16	3	0.11	PVC	3.5	Serpentine	43.5	13.5	0.79	³⁸
Nayak et al., 2016	I.N.D.I.A.	39	1	0.06	PVC		Serpentine	48	8	0.38	⁴⁰
Menhoudj et al., 2018	ALGERIA	20	2	0.12	PVC	2	Straight	38	6 TO 6.5		⁴
Lekhal et al., 2021	ALGERIA	20	2	0.118	PVC	0.45	Straight	40	17		⁹
Serageldin et al., 2020	ALGERIA	60	1, 1.5		PVC		Straight	40	8.4	0.67	¹¹
LEKHAL et al., 2020	ALGERIA	20	2	0.118	PVC		Straight	25	5.5		¹²
Wei et al., 2020	CHINA	40–45	1, 2, 3	0.11, 0.16, 0.11, 0.075	----		Straight	41.2	22.13		¹⁴
Pakari et al., 2019	QATAR	21.5	0.5	0.15	AL	9.4	Straight	40.6	6.5	0.49 to 0.52	³³
Benhamza et al., 2017	ALGERIA	50	3	0.11	PVC	3.5, 4.5	Straight	49	20	0.8	³⁹
Current work	FEROZEPUR	35	1,2	0.15	PVC	2.4, 3.2, 4	Bank type	31–43	0.4 to 9.4	0.16 to 0.82

Results and discussion

Bank type EAHE has not been explored much. Previous studies have also not explained the effect of specific humidity as well as the effects of 2-factor or 3-factor interactions. The current study is very much novel in terms of the shape of EAHE and its compact design as it occupied an area of only 25m² and the values of effectiveness ( $E$ ) and temperature difference ( dT) achieved are pretty similar to those obtained in previous studies where very long pipes have been used for the fabrication of EAHE. Hence its compact design with no compromise on output parameters makes it sustainable for modern urban architecture where space is a constraint. The variation of response variables for input variables as two-factor interactions is explained with the help of plots as follows:

Effect of factors interactions on dT

The dT between T_di and T_do as response variable has been plotted against individual inlet parameters. Their two-factor and three-factor interactions and the patterns of graphs are explained in this section. It is directly proportional to the amount of heat removed from the air, thus the cooling potential of the EAHE.

It has been noted from the graph (Figure 1) that the value of dT is increased as the T_di is increased for both lower and higher values of v . The dT is lower for a higher value of the velocity at all points as the air does not get enough time to interact with soil for heat transfer. The amount of decrease is almost constant for all inlet values of T_di . The difference is slight, indicating that T_di has a more pronounced effect on the T_do than v . It is noted from the graph that a higher value of dT is obtained at higher depth, i.e., 9.4°C, whereas the maximum dT obtained at a depth of 1 m is 7.6°C (Figure 3(a) and 3(b)). It has been concluded from the graph (Figure 2) that the value of dT is increased with the increase in T_di at both lower and higher values of h . At higher h , the temperature of soil decreases and becomes independent of ambient weather conditions.

Figure 1.

Variation of dT with T_di for three different velocities at a specific humidity of 0.015 kg/kg of dry air (a) at h = 1m (b) at h = 2 m.

Figure 2.

Variation of dT with T_di for two different depths at specific humidity of 0.015 kg/kg of dry air and velocity of 3.2 m/s.

Figure 3.

Variation of dT with T_di for three different specific humidities at a velocity of 3.2 m/s (a) at h = 1m (b) at h = 2 m.

The similar trends as shown in Figures 1 and 2 were obtained in earlier literature studies. In one such study, temeprature difference of 6.2 ^oC was obtained at a depth of 1.5 m with a length of 60 m⁴⁰ and, in another study, temeprature difference of 8.4 ^oC was obtained at a depth of 1.5 m with a length of 60 m³⁰ both conducted in Algeria. One more study has reported a temperature difference of 8 ^oC at a depth of 1 m with a length of 39 m³⁸ in India. However, the current study has reported temeprature difference of 9.4 ^oC at a depth of 2 m at Ferozepur, India. Moreover, the detailed comparison of present study with previous studies is shown in Table 2. However the results reported in current study are better in terms of temperature difference as well as size and shape.

The dT is decreased as the w is increased at both the highest and lowest values of d and all values of T_di (Figure 3(a) and 3(b)). The dT is decreased at lower values of depth and higher velocity values. The decrease in the value of dT is considerable as the depth decreases but nominal as the w is increased. So it is concluded that change in specific humidity has lesser impact on the performance of EAHE, whereas the difference in depth has a good effect. The value of dT increases with an increase in T_di and d but decreases with an increase in w . An increase in v reduces the dT, but the effect is negligible. With increased T_di , the sensible heat transfer is increased. In contrast, with an increase in w, sensible heat transfer is decreased marginally due to increased specific heat of air given by the addition of sensible heat and latent heat. As the d increases, the soil temperature decreases and becomes independent of ambient weather conditions; thus dT increases till the depth where EUT is achieved. It has been concluded from the interaction graph (Figure 3) between T_di and w that the dT increases as T_di is increased at both higher and lower values of w_, and if the humidy level is considerably increased from 0.0106 kg/kgof dry air to 0.205 kg/kg of dry air, the temperature difference value decreases by 2 ^oC. A considerable variation could have been achieved if there had been condensation of moisture in the air. However, since the size of EAHE is relatively compact, the air does not get enough time for the moisture to condense.

To the best of knowledge, no study has reported the values of specific humidity in literature for EAHE system, which makes the current study novel.

It is noted that the lowest value of 0.32°C of dT is at the interaction of lowest values of T_di and d, and the largest value of w . The highest value of 9.4 ^oC of dT is obtained where the T_di and d are highest, and w is lowest The value of cooling potential would range from 0.0523 kJ/s to 1.587 kJ/s at a mass flow rate of 0.163 kg/s which seems significant for cooling purpose.

Effect of factors interactions on effectiveness ( $E$ )

Figure 4 shows the variation of $E$ with all inlet parameters. The value of $E$ increases with an increase in T_di up to the middle value of 37°C and then decreases. Though the temperature difference increases as the value of T_di increases, the difference between Tdi and EUT is widened, thus leading to the decreased value of $E$ . The value of $E$ decreases with an increase in v and w and increases with an increase in h. With an increase in w,_, the specific heat of the humid air is increased; thus, lower dT is achieved, leading to lower $E$ . As the v increases, the retention time to transfer heat from it to the soil gets reduced, thus leading to the lower $E$ . With the increase in d , the soil temperature decreases up to EUT at a certain depth, and therefore lower T_do is achieved due to increased heat transfer rate, which leads to an increase in the $E$ .

Figure 4.

Variation of ɛ with T_di at two different depths (a) at w = 0.0105 kg/kg of dry air and v = 2.4 m/s (b) w = 0.015 kg/kg of dry air and v = 3.2 m/s ( c) w = 0.0206 kg/kg of dry air and v = 4.0 m/s.

Similar trends were obtained in literature and one such study has reported effectiveness of 0.79 at a velocity of 3.5 m/s and inlet temeprature of 43.5 ^oC³⁷ in Algeria and in another study, effectiness of 0.8 at a velocity of 3.5 m/s and inlet temeprature of 49 ^oC is reported²⁷ in Algeria. The current study has reported efectivess of 0.82 at a velocity of 2.4 m/s with inlet temeprature of 43 ^oC and specific humidity of 0.105 kg/kg of dry air.

Statistical modeling

Full Factorial design with 04 inlet parameters (Table 3) at multilevel has been used to analyze 02 response variables (Table 2). All input, as well as outlet parameters, are numeric and continuous. Depth of buried pipes as inlet parameter has been taken at two levels, whereas other three parameters, i.e., DBT, specific humidity, and airflow velocity, have been taken at three levels. So a set of 54 experiments has been taken from the whole set of readings. Seven extra design points have been taken to improve the model's accuracy.

Table 3.

Detail of parameters along with their minimum and maximum values.

Name	Units	Minimum	Maximum
Inlet Variables
DBT( T_di )	^oC	31.00	43.00
Specific humidity ( w )	kg/kgof dry air	0.0106	0.0205
Velocity of air( v)	m/s	2.40	4.00
Depth of EAHE( h )	m	1.0000	2.00
Output variables
Temperarure Difference( dT )	Celsius	0.4	9.4
Effectiveness( $E$ )		0.16	0.82

The data analysis has been done with the help of software - Design Expert 13.0.5.0. Analysis of data and prediction of response variables was tried with the help of linear, 2FI, quadratic, cubic, quartic, fifth, and sixth-order models, and it was seen that the reduced 2FI model fitted well for the analysis of effectiveness as well as temperature difference as response variables. Though the higher-order models gave better fitting polynomial, the highly complex equation became cumbersome to use. These models have an acceptable range of residual errors. Significant terms were identified after running AICc forward regression on reduced 2FI models. Some nominal terms were added to maintain the hierarchy of variables to make the analysis of variance viable. Factor coding was coded, and the Sum of squares was Type III – Partial. The Model F-value proves the significance of the model. Only those model terms are significant that have P-values less than 0.0500. The difference between Predicted R²and Adjusted R² is less than 0.2. Adeq Precision measured the signal-to-noise ratio. A ratio greater than 4 is desirable. All these values for both models have been tabulated in Table 4. The values conform to the requirements for valid models. So these models can be used to navigate the design space.

Table 4.

Models for response variables .

	Model for response variable 1: dT	Model for response variable 1: $E$
Type of model	Reduced 2FI model	Reduced 2FI model
F value at 95% confidence level	529.7	31.34
Significant terms added after forward regression	T_di, w, v,h, T_dih	T_di, w, v, h, T_div, T_dih
Predicted R²	0.9780	0.7745
Adjusted R²	0.9803	0.7478
Adeq Precision	67.86	21.71

The normal plots of residuals versus various terms were plotted, and it was noted that maximum points lie on the straight line, and some points were scattered. No specific S pattern could be observed, proving that a normal distribution pattern was followed and no further transformation was required. The final equations for predicting temperature difference and effectiveness are given below.

Final equation in terms of actual factors:

\begin{aligned} d T = - 11.89534 + 0.464815 T_{d i} - 102.37622 w \\ - 0.246528 v - 2.07778 h + 0.088889 T_{d i} h \\ ε = + 0.139328 + 0.012959 T_{d i} - 13.84184 w \\ - 0.232144 v + 0.566755 h + 0.005076 T_{d i} v \\ - 0.010652 T_{d i} h \end{aligned}

The graphs indicate that the model equations were appropriate for predicting the values of $E$ and dT at non-experimental parameters. The predicted and actual values of $E$ and dT at modeled parameters are shown in Figure 5. Minimal residuals can be seen as the points lying near the straight line.

Figure 5.

Predicted vs. actual values (a) for response 1: DT (b) for response 2: $E$ .

Point prediction and model validation:

The comparison has been made between predicted and observed output parameters (Table 5) at given inlet parameters that were not used for modeling.

Table 5.

Predicted Vs. Observed Values.

				Outlet parameters
Inlet Parameters				dT			$E$
T_di	w	v	d	Observed	Predicted	% Error	Observed	Predicted	% Error
41.5	0.0192	4.0	2	7.5	7.67	2.27	0.5769	0.57	−1.2
37.6	0.012	3.2	2	5.8	6.09	5.0	0.64	0.66	3.13
31.8	0.015	3.2	2	1.9	2.05	7.89	0.6	0.57	−5.0
43	0.015	4	1	7.3	7.25	−0.68	0.50	0.53	6.0
35.2	0.019	2.4	2	3.9	4.03	3.33	0.58	0.59	1.72
42.6	0.010	2.4	2	9.5	9.7	2.11	0.68	0.71	4.41
Mean deviation						3.32			1.51

The predicted values are almost consonant with the observed values, confirming the models' validity. Hence the present study has explored an energy-saving natural compact air conditioning system that can be commercialized for urban buildings.

Response Surface Model has been used in literature for Parametric optimization of vertical ground heat exchanger⁵⁹ and parametric optimization of horizontal earth to air heat exchanger.^27,60 No study has been found to the best of knowledge for the optimization of EAHE based on Full Factorial design which makes the current study novel.

Conclusion

The compact-shaped bank type EAHE is very effective in pre-cooling the air that can be directly fed to the conditioned space or the air handling unit of the conventional air conditioning system, depending on weather conditions. The effectiveness was found to be 0.82 in the present study, which is comparatively better than many studies giving a significant improvement in effectiveness. As the value of temperature difference decreased with the increase in moisture content of the air, it was found that the system was more effective in hot and dry weather. The temperature difference varied from 0.4°C to 9.4°C, and the effectiveness varied from 0.16 to 0.82 during the whole season. The system can provide a cooling potential of 0.0523 kJ/s to 1.587 kJ/s at a mass flow rate of 0.163 kg/s. The airflow velocity had a minimal effect, and lower velocity increased the temperature difference. Increased depth of EAHE increased the temperature difference by 2–3 ^oC. A polynomial has been fitted between the inlet parameters and response variables using full factorial design, taking 63 experimental readings fitted into the design table. The model equations have given −5% to 5% error between the experimental and predicted values. So the model is considered to be appropriate. Hence, the bank type compact-sized EAHE is seen as a promising and suitable energy-saving alternative or to support conventional air conditioning systems, especially in urban areas where space availability is a constraint.

Supplemental Material

sj-docx-1-pie-10.1177_09544089221096052 - Supplemental material for Experimental studies on natural bank type heating/cooling energy saving Air conditioning system

Supplemental material, sj-docx-1-pie-10.1177_09544089221096052 for Experimental studies on natural bank type heating/cooling energy saving Air conditioning system by Vaishali Goyal, Arun Kumar Asati, Rajeev Kumar Garg and Amit Arora in Proceedings of the Institution of Mechanical Engineers, Part E: Journal of Process Mechanical Engineering

Footnotes

Acknowledgements

The authors are highly grateful to the university's civil and electrical maintenance staff for helping with the experimental setup's fabrication and installation.

Disclosure statement

There is no conflict of interest with anyone.

Declaration of conflicting interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) received no financial support for the research, authorship, and/or publication of this article.

ORCID iD

Vaishali Goyal

Supplemental material

Supplemental material for this article is available online.

Appendix

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Supplementary Material

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Experimental studies on natural bank type heating/cooling energy saving Air conditioning system

Abstract

Keywords

Introduction

Materials and methods

Results and discussion

Effect of factors interactions on dT

Effect of factors interactions on effectiveness ( E )

Statistical modeling

Final equation in terms of actual factors:

Point prediction and model validation:

Conclusion

Supplemental Material

sj-docx-1-pie-10.1177_09544089221096052 - Supplemental material for Experimental studies on natural bank type heating/cooling energy saving Air conditioning system

Footnotes

Acknowledgements

Disclosure statement

Declaration of conflicting interests

Funding

ORCID iD

Supplemental material

Appendix

References

Supplementary Material

Effect of factors interactions on effectiveness ( $E$ )