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Abstract

With the increase in electricity consumption and price, energy efficiency in industrial refrigeration systems becomes an even more important issue. The performance of the industrial refrigeration system can be reported by creating energy labeling according to the energy efficiency index value. In this study, the industrial refrigeration system was evaluated in terms of energy performance with proposed a fan control technique and performed an environmentally friendly design using propane (R290) refrigerant. In addition, a new control process was developed for the management of evaporator and condenser fans, and the effect of this process on the cooled package temperatures and energy consumption was investigated. With this new technique, a 25.2% improvement was achieved in the total energy consumption of the industrial refrigeration system. It was observed that the cooled package temperatures were between −1 and +5°C throughout the experiment. The EEI value of the industrial refrigeration system has increased from class “E” to class “D” using a new control technique.

Keywords

refrigerated display cabinet energy labeling fan control EEI value

Introduction

Refrigerated display cabinets (RDCs) can be defined into two main groups as display cabinets and storage cabinets. Industrial-type display cabinets have many features such as horizontal, vertical, combined, open, closed, positive, and negative set temperatures.

In cases where the surface temperatures of the evaporators working as air RDCs in the vapor compression cooling cycle in RDC fall below 0°C, problems of moisture accumulation in the air as snow and ice on the surface occur. Due to these problems, there is a decrease in the amount of heat transfer in the system. Defrosting is done in order to destroy or melt the ice and snow that forms and accumulates on the cold surfaces of the evaporators. There are pieces of fabric in the condensate pan where the melted ice is collected. The fabric pieces should be heated and dried by the heater at specific intervals. As the heaters are activated, the electricity consumption of the system increases.¹

Various studies have been conducted on energy efficiency, coefficient of performance (COP) values, energy efficiency index (EEI) and fans in RDCs and systems. Erten et al.² conducted tests on the RDC using the R290 refrigerant and air defrosting method. They observed the positive effects of defrosting on cooling performance. During the experiments in the RDC, eight defrosting events occurred. According to the values measured before these eight conditions occurred, the COP values were, respectively, 2.13, 2.31, 2.30, 2.29, 2.6, 3.36, 3.29, 3.30 and 3.49. Afshari et al.³ compared the COP values of heat pumps and chillers with similar working principles and used different refrigerants in heating and cooling conditions. They used R134a, R407c, R22 and R404a refrigerants as refrigerants. In the study, the suction and discharge lines of the compressor were closely examined considering the temperature, pressure, enthalpy, entropy and compression ratio. It was determined that the optimum charge amount of the freezer was 15–25% lower than the system when operating in heat pump mode. Finally, the effect of condenser flow rate on performance was examined to compare another difference between the heat pump and chiller devices. As a result of the experiments, it was found that in some cases the COP of the heat pump is 100 times higher than the COP of the chiller.

Zhang et al.⁴ observed the effect of the temperature and humidity of the air entering the evaporator on COP and electricity consumption. Calculation showed that COP of this system was 3.1, and the daily electricity consumption of the compressor was 10.5 kWh when the temperature (TRA) and relative humidity (RHRA) of the return air entering the evaporator are 12°C and 0.6, respectively. Furthermore, it was found that TRA and RHRA significantly affect COP. COP increased by about 115% when TRA varies from 8 to 14°C and increased by 35% when RHRA varied from 0.5 to 0.8. Suamir et al.⁵ researched the effect of airflow rates on COP and EEI values in display cabinets. 1200 m³/h airflow was used in the first test condition and 1800 m³/h airflow was used in the second test condition. According to the results of the first test, the COP value was 3.17 and the EEI value was 8.25. According to the results of the second test, the COP value was 3.03 and the EEI value was 8.95.

According to the study by Geilinger et al.,⁶ the annual energy consumption for glass door vertical cabinets was 2168 kWh if beverage coolers, 1606 kWh for ice cream cabinets, 1348 kWh for storage refrigerators, and 3690 kWh for storage freezers. Energy efficiency classes were determined based on the energy efficiency index value calculated from these consumption values. EEI values differed according to classes. In the best energy efficiency classes, the EEI value was <10 in class A and 10–20 in class B. In the middle energy efficiency classes, the EEI value was 20–35 in class C and 35–50 in class D. In the worst energy efficiency classes, it was 50–65 in the E class, 65–80 in the F class, and >80 in the G class.⁷ Waide et al.⁸ examined the classification of the energy efficiency index (EEI) of RDCs according to five types of energy efficiency classes according to China. EEI ≤ 55% in first grade, 55% – <65% in second grade, 65% – <80% in third grade, 80% – <90% in fourth grade, and 90%–100% in fifth grade.

Tai et al.⁹ EEI value of 80 in vertical RDCs, EEI value of 50 in horizontal freezer cabinets, EEI value of 70 in beverage coolers and EEI value of 75 in vending machine cabinets were considered according to the minimum energy performance standard. In addition, the COP value of the cooling system in accordance with ISO 23953 and ARI standards and the COP values of the CO₂ and R404a refrigerant cooling systems made by Girotto et al.¹⁰ COP values were taken as 2 according to the ISO 23953 standard, 3.25 according to the ARI standard, 3 according to the CO₂ cooling cycle made by Girotto and 3.75 according to the R404a fluid cooling cycle. Özkan and Ünal¹¹ aimed to reduce the energy consumption in RDCs and increase the efficiency of the defrosting process. They developed a theoretical correlation as a function of frost thickness, heat flux, and frost density to estimate the defrosting time of the evaporator fin surface. The melting time of the frost on the surface was calculated with a mathematical model and compared with the experimental results. The results differed from the actual melting time by 4.7%.

Liu et al.¹² stated that the defrosting of the RDC by storing the heat discharged from the condenser in the phase-change material will create 71% less energy consumption than the electric defrosting application. Mitsopoulos et al.¹³ studied refrigerant replacement and different refrigeration systems to reduce electricity consumption in the refrigeration system. Choosing CO₂ as the refrigerant, they compared ten different refrigeration systems with the conventional refrigeration system using R404A refrigerant. As CO₂ system types, a simple booster system, parallel compression system, medium temperature parallel compression and supercharged evaporator system, both medium and low temperature parallel compression and supercharged evaporator system, adding an intercooler after low-temperature compressors and after gas cooler. They designed systems such as the addition of a mechanical subcooling cycle. According to the comparison results, the most promising configuration was the system with parallel compression, medium and low-temperature supercharged evaporators, intercooler and mechanical subcooling, which saved 8.53% electricity per year compared to the traditional R404A system. They found that it was less efficient than the traditional R404A system, with 15.23 higher electricity consumption.

Yu and Chan¹⁴ emphasized that air-cooled chillers should perform optimum condenser fan control in order to improve the COP. This control included determining the optimum set point of the condensing temperature with optimized power relations of the compressors and condenser fans and improving the airflow and heat transfer area of the condensers. An exemplary application of this control for an air-cooled centrifugal chiller showed that COP could increase by 11.4%–237.2% depending on operating conditions. Such an increase in COP results in a reduction of up to 27.3% in the annual refrigeration energy of the chiller. Smith and King¹⁵ showed in their study that a 10% reduction in total power consumption of a 35 kW reciprocating chiller was achieved by operating the condenser fan at a higher speed when the outdoor temperature drops below 25°C.

Simulating an industrial refrigeration system, Manske et al.¹⁶ found that all fans of an evaporative condenser must be continuously cascaded to ensure that the condensing pressure remains at its lowest level, regardless of the cooling load. Yu and Chan¹⁷ discussed how the stage of condenser fans could be optimized to increase the efficiency of air-cooled chillers at part load. An experiment on an air-cooled reciprocating chiller confirmed that when all condenser fans are arranged to increase heat removal airflow, the heat transfer coefficient of the condenser can be optimized, allowing the condensing temperature and chiller power to be significantly reduced with less fluctuation. Chiller efficiency, on the other hand, could be improved by 4.4%–40.1% depending on load conditions and outdoor temperatures.

Wang et al.¹⁸ considered the inhomogeneous air curtain of the dual-temperature open display cabinet and performed a numerical study by modeling the air between the dual-temperature open display cabinet, the air curtain, and the cabinet. It was observed that air distribution affected the structure of the curtain by changing the size or position of the holes in the back panel. In addition, it was found that the fan flow rate had little effect on the structure of the air curtain. In another study, Wang et al.¹⁹ improved a dual-temperature open display cabinet that reduces power consumption by recovering condensing heat. At 25°C ambient temperature, a saving of 5.63 kWh/24h in total system power consumption was achieved, which was equivalent to a reduction of 188.57 kg of CO₂ emissions per year.

Looking at the literature survey, it is observed that energy consumption in refrigeration systems has great importance. In this study, a new fan control technique has been suggested to reduce energy consumption in the available industrial refrigeration system. In the current system (System 1), the condenser fans run in parallel with the compressor and the evaporator fans operate at 1400 rpm when the cooling curtain is open and closed. In the proposed system (System 2), the condenser fans operate for 24 h and the evaporator fans are operated at 1400 rpm when the curtain is open, and at 900 rpm when the curtain is closed. In other words, the recommended fan control technique means reducing the fan motor speed when the cooling curtain is closed. The main objectives of this research are the following:

To obtain RDCs with a higher energy class by reducing the energy consumption of industrial refrigeration systems.

To increase the usage of waste condenser heat for energy efficiency.

To preserve the shelf life of products displayed in RDCs.

According to these purposes, a new design has been made that increases performance in industrial refrigeration systems and its effects on the system. Moreover, the originality of this study is to show the effect of the proposed system on the energy consumption—EEI value of the RDC by running the fan continuously using waste heat.

Material and Method

In this study, an industrial refrigeration system was designed, using two different controlled fans in order to analyze the effects of airflow on the refrigeration system and on the control techniques of the fans used in industrial refrigeration systems.

In System 1, the condenser fans operate in conjunction with the compressor and stop when the compressor is deactivated. In addition, the evaporator fans operate at 1400 rpm for 24 h. Since there is not enough heat and mass transfer in System 1, the electrical heaters in the condensate pan are activated, therefore it is in the high levels of the energy labeling class. As a result of the analyses made in System 2, the condenser fans were operated for 24 h. Evaporator fans, on the other hand, were reduced to 900 rpm after the night curtain was closed (last 12 h) as the cooling load decreased. Thus, in System 2, it was seen that the electric heaters in the condensate pan work less.

The technical representation of the tested refrigeration system is given in Figure 1. In terms of testing the effects of fan control and airflow, two separate RDC with different fan control techniques and different features were tested for 24 h in accordance with ISO 23953–2:2015 standards and their average energy consumption was calculated.

Figure 1.

Technical demonstration of the refrigeration system.

Moreover, it is aimed to reduce energy consumption by ensuring that the heater in the condensate pan is kept on for less time by means of effective drying on the side of the condenser fans by making speed adjustments under the control of the evaporator fans. Environmental-friendly R290 (Propane) is used as a refrigerant in both systems. The ambient temperature and relative humidity values (Class 3) where the experiment was carried out were set at 25°C and 60%. During the experiment, the temperature values of the cooling package, the temperature–pressure (evaporator, condenser, and compressor) values at certain points in the refrigeration system equipment, the condenser and evaporator air inlet–outlet temperature measurement values and the energy consumption values of the systems were measured, and the test data were recorded. In System 1 and System 2, axial, ECM type, 70% efficient fans are used in the condenser and evaporator. The characteristics of the fans in the system are given in Table 1.

Table 1.

Technical specifications of the fans used in System 1 and System 2.

	System 1evaporator	System 1condenser	System 2evaporator	System 2condenser
Fan type	Axial	Axial	Axial	Axial
Electric motor type	ECM	ECM	ECM	ECM
Number of fans	5	4	5	4
Curtain open fan motor speed (r/min)	1400	1400	1400	1400
Curtain closed fan motor speed (r/min)	1400	1400	900	1400
Input power of fans (W)	13	13	13	13
Working period of fans (h/24h)	24	15.77	24	24

Frost or icing occurs on the evaporator due to the moist air coming into contact with the cold surface of the evaporator surface. After this frost or ice melts, the water that emerges is collected in a structure called a condensate pan. As the cooling cycle continues, the water flow to the condensate pan continues. This container has a certain capacity, and when it is overloaded, overflows may occur. The overflow problem is undesirable. Therefore, the water in the condensate pan must be removed. There are many different methods for this. The system structure used in the study is given in Figure 2. Compressor discharge pipes were passed through the condensate pan, and some of the defrost water was heated there and evaporated. Afterward, the remaining water is absorbed by antibacterial cloths and dried by the hot airflow from the condenser fan. If the water does not evaporate completely, the remaining defrost water is removed by means of an electric heater. If the electric heater works for a long time, the energy consumption increases, and it also affects the energy class of the RDC.

Figure 2.

Schematic representation of the removal of water from the condensate pan.

The airflow in System 2 is continuous, while the airflow in System 1 is intermittent. In this way, the heat and mass transfer rate in the condensate pan was increased, and antibacterial fabrics were dried in a shorter time by using the waste heat in the condenser. In this way, the electrical heater in condensate pan operated for less time in System 2 than in System1. At the same time, the problem of overflowing the water in the condensate pan is prevented.

When the condenser fans were operated continuously, the condenser surface temperature was lower than System 1 and when the cooling cycle started again, the condenser continued at a low temperature. Since the cooling load of the system is reduced when the night curtain is closed, the evaporator fans in System 2 are reduced to 900 rpm after the night curtain is closed.

According to the ISO 23953-2:2015 standard,²⁰ while the average energy consumption for 24 h is calculated in open type RDCs, they were tested with curtains and lighting open for 12 h and curtains and lighting closed for 12 h.

In System 1, the evaporator fans operated at 1400 rpm when the cooling curtain was open and closed. In System 2, energy saving was achieved by reducing 1400 rpm when the curtain was open to 900 rpm when the curtain was closed.

On the side of the condenser fans, while the condenser fans were running for 15.77 h/24 h in System 1, fan control was set to run for 24 h in System 2. The reason for this was that the water puddles condensed from the evaporator during defrosting are collected in the condensate pan through the drainage. In order to remove the collected water from the system, antibacterial fabrics with high absorbency are used in the pan and these fabrics were dried by means of condenser airflow. If sufficient airflow cannot be provided on these antibacterial fabrics and the fabrics do not dry, water flows to the overflow pan with a heater in the second condensate pan position, and the heater is activated and the energy consumption increases tremendously. Detailed information about the measuring instruments used in the measurements made within the scope of the ISO 23953-2:2015 standard in the test rooms is given in Table 2.

Table 2.

Information on measuring instruments used in tests.

Measuring device	Measuring range	Accuracy
Thermocouple	−40/ + 150°C	±0.1
High-pressure gauge	0–30 bar	±0.1
Low-pressure gauge	0.5–8 bar	±0.01
Thermohygrometer	0–100%RH 0/ + 50°C	±%1.5 ±0.03
Anemometer	0–2 m/s	±0.01
Digital manifold	−50/ + 150°C −1/ + 60 bar	±0.1 ±0.01
Energy analyser	95–240 V AC 0.001–7.4 A	±%0.1 ±%0.2
Flow Meter	0–1000 kg/h	±%0.1

Electrical heater in condensate pan equipment that converts the electrical energy passed over it into heat energy. In the operation, the electric heater positioned inside the condensate pan is activated when the water cannot be removed and heats the water and evaporates it. In System 2, the evaporation performance of antibacterial fabrics increased due to the increase in the operating time of the condenser fans and effective drying was achieved. Since the defrost water is dried on the fabrics, the electric heater remains active for a shorter time, and the energy consumption of the RDC is reduced.

Theoretical analysis

The mechanical vapor compression refrigeration cycle evaporator capacity, compressor capacity and coefficient of performance (COPs) are calculated using the equations given below.^21,22 The evaporator and compressor capacity are found by calculating the mass flow rate and enthalpy. The coefficient of performance of cooling is found by the ratio of the cooling capacity of the evaporator to the power of the compressor:

{\dot{Q}}_{e} = \dot{m} * (h_{1} - h_{4})

(1)

{\dot{W}}_{c} = \dot{m} * (h_{2} - h_{1})

(2)

COPs = \frac{{\dot{Q}}_{e}}{{\dot{W}}_{c}} = \frac{(h_{1} - h_{4})}{(h_{2} - h_{1})}

(3)

Theoretical power of the fan (

N_{t h}

Δ P

the total pressure difference (Pa) between the inlet and outlet of the fan, Q the volumetric flow of the air (m³/s), ρ the density of the air (kg/m³), g the gravitational acceleration (m/s²) and the discharge head H (mmSS) units are used. The power and efficiency expressions for the fans in the system are obtained with the following equations and the theoretical power of a fan can be calculated by the equation given below²³:

N_{t h} = Δ P * Q

(4)

Δ P = ρ * g * H

(5)

Some of the discharge air by the impeller in the fans passes back to the inlet channel of the fan between the impeller and the body. A small part of it escapes from the place where the fan shaft cuts the body. For this reason, the airflow at the fan outlet is less than the amount discharged by the impeller. If the difference is indicated by ΔQ, the volumetric efficiency of the fan is defined as follows:

η_{v} = \frac{Q}{Q + Δ Q}

(6)

The friction efficiency of the fan is found by the ratio of the power consumed by the shaft friction from (

N_{s}

) the effective power (

N_{i}

) of the electric motor running the fan and the ratio of the effective power of the electric motor running the fan:

η_{s} = \frac{N_{i} - N_{s}}{N_{i}}

(7)

The total pressure (

Δ P

) difference between the inlet and outlet of the fan is calculated by adding up and proportioning the head losses (

\sum ξ Δ z

) caused by the friction of the fluid as the air passes through the fan:

η_{h} = \frac{Δ P}{Δ P + \sum ξ Δ z}

(8)

The internal efficiency of the fan (

η_{i}

); It is calculated by multiplying the volumetric (

η_{v}

), friction (

η_{s}

) and hydraulic (

η_{h}

) efficiencies:

η_{i} = η_{v} * η_{s} * η_{h}

(9)

The total efficiency (

η_{t}

) is calculated by multiplying the internal efficiency of the fan (

η_{i}

) and the efficiency of the electric motor to be used to drive the fan (

η_{e}

η_{t} = η_{i} * η_{e}

(10)

The energy efficiency index (EEI) value can be calculated with equation (11). The AE value in the equation represents the annual energy consumption (kWh/year), and the SAE value represent the reference value of the annual energy consumption amount⁷:

E E I = \frac{A E}{S A E}

(11)

AE expressed in kWh/year and rounded to two decimal places is calculated as follows:

A E = 365 * E_{d a i l y}

(12)

E_{d a i l y}

is the average energy consumption of the refrigeration appliance with the direct selling function over 24 h, expressed in kWh/24h and rounded to three decimal places. SAE for all compartments with the same temperature class, rounded to two decimal places (M and N values—Table 3, C value—Table 4 and P value—Table 9; Commission Delegated Regulation (EU) 2019/2018)⁷:

S A E = 365 * P * (M + N * Y) * C

(13)

expressed by the equation. For multi-compartment refrigeration appliances with different temperature classes, excluding refrigerated vending machines, the SAE is calculated as follows:

S A E = 365 * P * \sum_{c = 1}^{n} (M + N * Y_{c}) * C_{c}

(14)

Equation of daily (24 h) energy consumption of industrial refrigeration system:

T E C = F E C + L E C + P E C + C E C

(15)

Here, TEC, FEC, LEC, PEC and CEC refer to total daily energy consumption, the daily energy consumption of fans, the daily energy consumption of lighting, the daily energy consumption of electrical heaters and daily energy consumption of the compressor, respectively.²⁰

In order to calculate the total mass transfer amount, Reynolds, Schmidt and Sherwood numbers were calculated using equations (16)–(18). The Reynolds number is found with the help of equation (3). Reynolds number calculated, v is the air velocity (m/s), L is the size of the antibacterial fabric (m), and $ν$ is kinematic viscosity (m²/s)²⁴:

R e = \frac{v x L}{ν}

(16)

The Schmidt number is calculated with the help of equation (17). It is calculated with the help of Schmidt number (Sch), kinematic viscosity (m²/s) and thermal diffusion coefficient (m²/s):

S c h = \frac{ν}{D_{A B}}

(17)

The Sherwood number is found with the help of equation (18). Sherwood numbers are found with the help of the previously calculated Reynolds and Schmidt numbers:

S h = 0, 037 * (R e^{0, 8}) * S c h^{1 / 3}

(18)

The mass transfer coefficient is calculated with the help of equation (19). Mass transfer coefficient (h_m); calculated by Sherwood number (Sh), thermal diffusion coefficient (D_AB) (m²/s) and size (m) of antibacterial fabrics:

h_{m} = \frac{S h x D_{A B}}{L}

(19)

The heat energy required to remove the water accumulated in the condensation pan is found with the help of equation (20). Heat energy (

\dot{Q}

) (W); It is calculated with the help of amount of mass evaporated per unit time

{\dot{m}}_{b}

(kg/s), enthalpy (saturated vapor) (

h_{f g}

) (kJ/kg)²³:

\dot{Q} = {\dot{m}}_{b} x h_{f g}

(20)

The parameters used in the analyses are given in Table 3.

Table 3.

Parameters used in thermodynamic analysis of the system.

Parameter	System 1-1	System 1-2	System 2-1	System 2-2
Ambient temperature (T_ambient) (°C)	25	25	25	25
The amount of water (kg)	25.77	27.17	21.29	23.99
Condenser air outlet temperature (°C)	31	29.7	30	26
Relative humidity of the air (%)	0.45	0.47	0.46	0.55
Volumetric flow of the air out of the condenser (m³/h)	1400	0	1400	1400
Diffusion coefficient (D_AB) (m²/s)	0.00002608	0.0000258	0.0000259	0.0000248
Prandtl number (Pr)	0.72792	0.7281	0.7282	0.72986
Density of air (ρ) (kg/m³)	1.16	1.165	1.164	1.188
Kinematic viscosity (γ)(m²/s)	0.00001617	0.00001605	0.00001608	0.00001552
Saturation pressure of water (kPa)	7.3851	7.3851	7.3851	7.3851
Air velocity (m/s)	2	0,01	2	2
h_fg (kJ/kg)	2427.42	2430.514	2429.28	2444.08
Width of object (m)	0.15	0.15	0.15	0.15
Water vapor gas constant (R_b) (kJ/kgK)	0.46152	0.46152	0.46152	0.46152

Results and Discussion

In this study, while the energy consumption in System 1 was 27.38 kWh/24h, the energy consumption in System 2 was reduced to 20.48 kWh/24h with the improvements made. The reasons for this decrease are:

While the airflow in System 1 is dependent on the operation of the compressor, the airflow in System 2 is continuous as the condenser fans operate for 24 h. In this way, the heat and mass transfer rate in the condensate pan was increased, and the waste heat in the condenser was recovered. Thanks to the waste heat used, the electrical heater in condensate pan remained in operation for less time in System 2 than in System1. As a result of the experiments, the amount of energy consumed by the condensate pan heater in System 2 compared to System 1 was reduced by 74% and the daily total energy consumption of the RDC by 25.2%. Although the condenser fans operate 24 h/day, the daily energy consumption of the RDC is less in System 2. Because the energy consumed by the electric heater is much higher than the fans. At the same time, when the condenser fans were operated for 24 h, the condenser surface temperature was reduced and it was observed that the condenser started at a low temperature every time the compressor started. This has increased the cooling efficiency. In Open type RDC, the night curtain is closed for 12 h. Thus, the cooling load of the system is reduced. Therefore, in System 2, the evaporator fans were reduced to 900 rpm after the night curtain was closed. Although the fan speed decreased, the package temperatures were kept within the M1 temperature class range according to ISO 23953-2:2015 standard.²⁰

As a result of the experiments, the compressor operating time decreased by 3.7% and the electrical heater in condensate pan operating time by 73.8% in System 2 compared to System 1. The reason why the electrical heater in condensate pan operating time was reduced so much was that the condenser fans were operated for 24 h and most of the water in the condensate pan was evaporated faster thanks to the airflow provided by the fans continuously.

Figure 3 demonstrates the comparison chart of EEI values and 24 h energy consumption values. With the fan technique control, while the EEI value was 59.5 in System 1, it was reduced to 44.5 in System 2. Since the EEI value calculated according to the experimental data depends on energy consumption, the EEI value of System 2 is lower than that of System 1. As a result of the calculations, System 1 is in the “E” class on the energy label of the RDC, while System 2 is in the “D” class, resulting in an improvement. Percentages of energy consumption, System 1 energy consumption is 27.38 kWh/24 h, with the improvements made, energy consumption in the new system, namely System 2, has been reduced to 20.48 kWh/24 h.

Figure 3.

EEI values – 24 h energy consumption (kWh) values comparison chart.

In Figure 4, the energy consumption graph of the equipment of the refrigeration system is given. In these graphs, it is seen that the compressor consumes 53% of the total energy consumption in System 1 and 69% in System 2. This situation is followed by electrical heater in condensate pan and fans. Since the condenser fan speeds increase in System 2, the energy consumption of the fan increases, but the energy consumption of the heater was greatly reduced compared to System 1. As a result, the total energy consumption of System 2 decreased. It was observed that condenser fans consume 3% of the total energy consumption in System 1 and 6% in System 2. In addition, it was concluded that 34% of the total energy consumption in System 1 and 12% in System 2 was consumed by the electrical heater in condensate pan.

Figure 4.

Energy consumption graph of refrigeration system equipment.

According to the ISO 23953-2:2015 standard,²⁰ the night curtain is tested with the night curtain open for the first 12 h and closed for the next 12 h in the RDC (ISO 23953-2:2015). In Figure 5, the power consumed in the first 12 h (night curtain open) and the last 12 h (night curtain closed) is given. According to this graph, when the night curtain is open, the power consumption of System 2 is 29% less than System 1. The reason for this is that the condenser fans are operated for a longer time, and the water in the condensate pan is evaporated by the fans without the electrical heater in condensate pan being activated. With the night curtain closed, the power consumption of System 2 is 10.3% less than System 1. Because the cooling load decreased due to the closing of the night curtain, the evaporator fans were reduced from 1400 to 900 rpm. In this way, System 2 consumed less power than System 1. In both tests, the power consumption is much higher when the curtain is open than when the curtain is closed. In open display cabinets, when the night curtain is open, the cooling load increases, and the compressor stays on longer. At the same time, since the evaporator surface is more frosted, the water mass collected in the condensate pan is more. In order to remove this water, the condenser fans are insufficient and the electrical heater in the condensate pan is activated. Both of the mentioned situations are among the factors that increase power consumption. After the night curtain is closed, the cooling load decreases and less moist air passes over the surface of the evaporator and less ice is formed. The two mentioned factors reduce the power consumption of the chiller by reducing the compressor and heater operating time.

Figure 5.

First and last 12 h energy consumption comparison graph.

Package temperatures comparison graph is shown in Figure 6. Although the evaporator fan revolutions were reduced after the night curtain was closed in System 2 compared to System 1, the test package temperatures (M package) were kept in the temperature range of −1, +5°C in accordance with the ISO 23953-2:2015 standard.²⁰

Figure 6.

Package temperatures comparison graph.

In the current RDC, the fans stop when the compressor is off and start running when the compressor is activated. In the newly developed fan control management, the fans continue to operate even if the compressor is turned off. Thus, the defrost water in the condensation pan evaporated faster with the help of fans.

In the graph given in Figure 7, the average condenser air outlet temperatures were compared. System 1 was examined as System 1-1 (the compressor and condenser fans are on mode) and System 1-2 (the compressor and condenser fans are off mode) according to the working condition of the condenser fans. In addition, the evaporator fans operated at 1400 rpm with the cooling curtain open and closed. System 2 was studied as System 2-1 (the compressor and condenser fans are on mode) and System 2-2 (the compressor is off and condenser fans are on mode). However, the evaporator fans were operated at 1400 rpm when the curtain was open, and at 900 rpm when the curtain was closed. It was observed that System 2-2 had the lowest average condenser air outlet temperature. In addition, when the condenser outlet temperatures were compared, it was determined that there was a 5°C difference between System 2-2 and System 1-1.

Figure 7.

A 24-h average condenser air outlet temperature values (°C).

When the heat transfer coefficients of the systems were compared, it was calculated that the hm value of System 2-2 was 98.5% higher than the hm value of System 1-2. The reason for this is that the air velocity in System 2-2 is 2 m/s and the air velocity in System 1-2 is 0.01 m/s. As the air velocity increases, the Reynolds number increases and the heat and mass transfer by convection increases.

According to the graph given in Figure 8, 0.01 kW of heat is lost in system 1-2 where the compressor and condenser fans are off, while 0.83 kW of heat is lost in system 2-2 where the compressor is off but the condenser fans are on. In this way, the heat dissipation when the cooling system is off has allowed the condensation pressure to decrease when the compressor is on. The analysis results of the parameters used in the calculation are given in Table 4.

Figure 8.

The heat energy required to remove the defrost water.

Table 4.

Analytical results.

Parameter	System 1-1	System 1-2	System 2-1	System 2-2
Reynolds number (Re)	18,552.88	93.46	18,656.72	19,329.90
Schmidt number (Sc)	0.6200	0.6221	0.6208	0.6258
Sherwood number (Sh)	81.9845	1.1912	82.3883	84.9830
Mass transfer coefficient (h_m) (m/s)	0.0143	0.0002	0.0142	0.0141
Amount of mass passed through unit surface in unit time (kg/h) (m_b)	1.4856	0.0207	1.4605	1.2182
The heat energy required to remove the defrost water ( $\dot{Q}$ ) (kW)	1.0017	0.0140	0.9855	0.8270

Table 3 illustrates a comparison chart in terms of the performance with related studies. As it is clearly presented in Table 5, the obtained COP, energy consumption and EEI values in the present research are seen in agreement when compared to the related studies shown in table.

Table 5.

Comparison with the studies in the literature.

References	Key descriptions	Refrigerant	COP	Energy consumption or change	EEI Value
Modi et al.²⁵	A conventional household refrigerator was chosen and redesigned by adding a battery bank, inverter and transformer and powered by solar photovoltaic (SPV) panels. Various performance tests have been carried out to examine the performance of the system	R134a	2.10	—	—
He et al.²⁶	They conducted some researches on natural refrigerant propane (R290) and its mixtures substituting for R134a used in a large-capacity chest freeze	R290 R134a	1.62 1.65	1.73 kWh/24 h 2.36 kWh/24 h	—
Redo et al.²⁷	An evaluation tool is constructed using the seasonal energy consumption of ORDCs commonly used in Japan and driven by a transcritical CO₂-compression chiller within Japanese climatic conditions.	CO₂			—
	Hokkaido		3.95	25.867 kWh
	Tokyo		2.98	37.045 kWh
	Okinawa		2.50	51.598 kWh
Kasera et al.²⁸	An experimental investigation on the performance of vapor-compression based milk refrigerator using R290 as refrigerant is carried out. Milk refrigerator is driven by solar photovoltaic. (25°C)	R290	—	158.58 W	—
Suamir et al.⁵	Airflow in the cabinet loaded with M-packages as test products was studied in order to analyse the effects of airflow rate (1200 and 1800 m³/h) in the cabinet to its temperature and energy performances.	R404a
					—
	1200 m³/h		3.17	2.69 kW
	1800 m³/h		3.03	2.85 kW
Evans J. A.²⁹	The energy consumption of RDC and freezers using different door types was evaluated according to the door type used, by performing door opening-closing tests at 30°C and 55% relative humidity conditions according to EN 441 (EN 23953) standard. As a result of these tests, power consumption and EEI values were compared.		—	-Solid Door (Chilled) System 15.24 kWh/48 h/m³-Chest Freezer System 16.26 kWh/48 h/m³-Glass Door (Chilled) System 22.40 kWh/48 h/m³-Multi-deck (Chilled) System 24.31 kWh/48 h/m³-Glass Door (Frozen) System 38.34 kWh/48 h/m³-Solid Door (Frozen) System 39.28 kWh/48 h/m³-Well (Frozen) System 43.15 kWh/48 h/m³	—
Bahar et al.³⁰	For RDC, resistance glass was used in the 1st system and anti-fog glass in the 2nd system was used and tested. The energy label class and EEI value of the designed systems were determined and compared. In these compared systems, since the EEI value of the 2nd system is lower, the energy label class is at a higher level than the 1st system.	R290
	System 1		1.69	48.34 kWh/24 h	60.53
	System 2		1.68	39.34 kWh/24 h	49.25
This study	With a new fan control technique, energy consumption was reduced and the energy class achieved next level.	R290
	System 1		2.24	27.38 kWh/24 h	59.5
	System 2		2.24	20.48 kWh/24 h	44.5

In today's world where energy demand is increasing and available resources are limited, energy consumption plays a major role in industrial cooling systems. Especially, the increment in energy costs has been effective in accelerating energy efficiency studies. In this study, a new fan control technic was recommended, which was simple but significantly affects energy consumption. By reducing the fan motor speed by 500 rpm while the cooling curtain in the current system was closed, an improvement of approximately 6.9 kWh/24 h was achieved in energy consumption. This situation led to a decrease in the EEI value and consequently an enhancement in the energy class from E to D.

Conclusion

In this paper, a new system that increased performance for RDC was proposed. The performance of the system was determined by calculating the energy index value. The specific results can be listed below:

While the total energy consumption of the industrial refrigeration system was 27.38 kWh/24 h in System 1, it was measured as 20.48 kWh/24 h in System 2. With the applied new control System 2, 25.2% energy efficiency was achieved using waste condenser heat.

The EEI value was reduced from 59.5 to 44.5 with System 2. As a result, the energy class was increased from “E” to “D.”

It was observed that the energy consumption of System 2 is 29% less than System 1 when the night curtain is open.

Package temperatures were successfully kept in the temperature range of −1 to +5°C (M1) in accordance with the ISO 23953-2:2015 standard.

RDC designers have to take into account the proposed system in terms of energy consumption. In addition, they should select the refrigerant considering the damages and effects it may cause to the environment.

Footnotes

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 iDs

Meltem Koşan

Elif Merve Bahar

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Investigation of the effects of fan control technique on energy consumption in industrial refrigerated display cabinet: An experimental study

Abstract

Keywords

Introduction

Material and Method

Theoretical analysis

Results and Discussion

Conclusion

Footnotes

Declaration of conflicting interests

Funding

ORCID iDs

Appendix

References