In order to reduce the investment and operation cost of distributed PV energy system, ice storage technology was introduced to substitute batteries for solar energy storage. Firstly, the ice storage air conditioning system (ISACS) driven by distributed photovoltaic energy system (DPES) was proposed and the feasibility studies have been investigated in this paper. And then, the theoretical model has been established and experimental work has been done to analyze the energy coupling and transferring characteristics in light-electricity-cold conversion process. In addition, the structure optimization analysis was investigated. Results revealed that energy losses were high in ice making process of ice slide maker with only 17.38% energy utilization efficiency and the energy efficiency and exergy efficiency of ISACS driven by DPES were 5.44% and 67.30%, respectively. So the immersed evaporator and cointegrated exchanger were adopted for higher energy utilization efficiency and better financial rewards in structure optimization. The COP and exergy efficiency of ice maker can be increased to 1.48 and 81.24%, respectively, after optimization and the energy utilization efficiency of ISACS driven by DPES could be improved 2.88 times. Moreover, ISACS has the out-of-the-box function of ordinary air conditioning system. In conclusion, ISACS driven by DPES will have good application prospects in tropical regions without power grid.
With the dramatic climate changes, the cooling demand has been increased and led to a rapid growth of energy consumption, which causes traditional fossil fuel energy shortage and great damage to climate and environment with the emissions of CO2 and harmful particles by extensive use of traditional fossil energy. Furthermore, a large number of the uses of the electric air conditioning can increase the tense situation between power supply of grid and demands of people. Therefore, refrigeration driven by solar energy becomes one of the promising approaches to reduce or partially replace the conventional refrigeration systems. Solar thermal refrigeration and solar photovoltaic refrigeration are two main working modes [
There are two refrigeration models: thermoelectric refrigeration and vapor compression refrigeration can be driven by PV. As early as 2003, Dai et al. [
Firstly, Aktacir [
According to comprehensive analysis, PV refrigeration system research is currently mainly concentrated on ice maker driven by PV. Batteries are essential component to store energy and to solve the intermittency of solar energy in PV refrigeration system. However, the use of batteries can increase the investing and running costs. So the PV refrigeration without batteries or batteries replacement technology was investigated. Axaopoulos and Theodoridis [
The performances of ice storage were analyzed. Pu et al. [
According to the above analysis, nowadays, the research and utilization of PV refrigeration and ice storage are relatively independent. In order to integrate the advantages of the two technologies together, the ice storage air conditioning system (ISACS) driven by distributed photovoltaic energy system (DPES) was established based on our previous research results [
Ice storage air conditioning system (ISACS) driven by distributed photovoltaic energy system (DPES) was mainly configured by DPES, ice maker, storage system, and air conditioning system. The working diagram of ISACS driven by DPES is shown in Figure
Work diagram of ISACS driven by DPES with batteries.
PV modules convert solar energy into electric energy which can be regulated by controller with maximum power point tracking to drive ice maker, ice storage system, and air conditioning system. In daytime, DPES receives solar energy and turns it into direct-current (DC) electric power which can be converted to alternating current (AC) electric power by inverter to drive AC compressor, water pump, ethylene glycol pump, and fan coil. To maintain the stability of electric energy supply, batteries were adopted and connected with controller to maintain the energy conversion and supply in the most optimized way. Ice maker and storage system were made up with AC compressor, condenser, expansion valve, disc evaporator, and ice storage tank. Circulating water can be frozen in a disc evaporator and the ice can be drooped into the ice storage tank when the hot refrigerant flowed into the evaporator which was controlled by the solenoid valve. Thereby, the ice maker worked as vapor compression refrigeration. In AC compressor, cryogenic R134a was compressed to high temperature and high pressure gas to be filtered in economizer and to release heat in condenser. Refrigerant was condensed to mild temperature and high pressure gas. When the gas inflows into throttle valve, it can be throttled to low temperature and low pressure liquid and then feeds into plate evaporator. And then, refrigerant flows into the other gas-liquid separator to be sucked into the compressor. Thereby, the refrigeration cycle will be completed. Air conditioning system was mainly made up of coil heat exchanger which was fixed in ice storage tank, ethylene glycol pump, solenoid valve, proportional control valve, and fan coil. Ethylene glycol is an adopted cold exchanging medium.
According to the working principle diagram, 0.2 kW ISACS driven by DPES was established as shown in Figure
Main component parameters of ISACS driven by DPES.
Component | Model | Parameters | |
---|---|---|---|
DPES | PV module | JN-245 | |
Controller | PL60 | 12–48 V 60 A charge, 30 A load | |
Inverter | Solar 48 V | | |
Batteries | SP12-65 | Battery capacity: 12 V 65 Ah, four batteries in series | |
ISACS | Refrigerant | R134a | Molecular formula: CH2FCF3, boiling point: −26.1°C, critical temperature: 101.1°C |
Ice maker | IM50 | Ice production: 2.12 kg/h, | |
Ice storage tank | / | Capacity: 20 cm | |
Cold exchanging medium | Ethylene glycol | Melting point: −12.6°C, viscosity: 25.66 mPa·s | |
Pump | RS15-6 | Power: 46–93 W, life: 6 m, maximum flow rate: 3.4 m3/h | |
Fan coil | / | Fan type: YS 56-2, power: 180 W, voltage: 380 V, current: 0.53 A, speed: 2800 r/min, number of fins: 95, size: 23 cm |
Pictures of 0.2 kW ISACS driven by DPES.
DPES were made up of two 245
Measuring instrument parameters and uncertainty.
Instrument | Model | Range | Accuracy | Application scope | Maximum relative error | Maximum absolute error | Uncertainty |
---|---|---|---|---|---|---|---|
Pyranometer | Kipp & Zonen CMP-6 | 0–2000 (W/m2) | ±5% | 0–1000 (W/m2) | ±10% | ±100 W/m2 | 57.7348 W/m2 |
Thermocouples | T | −200 to 350 (°C) | ±0.4% | 0–150 (°C) | ±0.93% | ±1.4°C | 0.8083°C |
Wind speed transducer | EC-9S | 0–70 (m/s) | ±0.4% | 0–10 (m/s) | ±2.8% | ±0.28 m/s | 0.1617 m/s |
Electromagnetic flow meter | KROHNE OPTIFLUX 5000 | DN 5; 0–12 (m/s) | ±0.15% | 0–5 (m/s) | ±0.36% | ±0.018 m/s | 0.0104 m/s |
Pressure transducer | YOKOGAWA EJA430E | 0.14–16 (MPa) | ±0.055% | 0–2 (MPa) | ±0.44% | ±0.0088 MPa | 0.0051 MPa |
Electronic balance | AHW-3 | 0–3 kg | ±0.05 g | 0–3 kg | ±0.05% | ±0.0015 kg | 0.0009 MPa |
Wattmeter | DELIXI DDS607 | 0–10000 kW·h | ±0.01 kW·h | 0–100 kW·h | ±1% | ±1 kW·h | 0.5774 kW·h |
Digital multimeter | FLUKE F-179 | Voltage: 0–1000 V | ±0.9% | 0–380 V | ±2.37% | ±9.006 V | 5.1996 V |
Current: 0–10 A | ±1% | 0–10 A | ±1% | ±0.1 A | 0.0577 A |
Theoretical model was established to analyze the energy conversion and transfer characteristics of ISACS driven by DPES as follows.
Energy balance equation of PV modules is expressed as
Solar energy absorbed by PV modules can be estimated as
Electric power is expressed by
Exergy and exergy losses of PV modules are given in [
Energy balance equation of controller can be estimated as follows:
Exergy and exergy losses of controller are expressed as in [
Energy balance equations of batteries are given by [
Here,
Exergy balance and exergy loss equations of batteries are written as [
Energy balance empirical equations of inverter are given in [
Exergy balance and exergy loss equations of inverter are expressed in [
ISACS are driven by stable electric energy outputted by DPES. The thermodynamic cycle refrigeration process of ice maker, refrigerant, R134a thermodynamic properties, and
R134a thermodynamic curve.
Energy and exergy analyses are shown as follows. The compressor energy balance equation is given by Compressor exergy model is given as The condenser energy balance equation is shown as Condenser exergy model is expressed as It is isenthalpic throttling process to refrigerant in throttle valve, so Throttle valve exergy model is given by Evaporator energy balance equation is shown as
Evaporator exergy model is given by
Energy balance equations in cold exchanging and supplying process are written as
And exergy calculation equation is shown:
The energy conversion and transmission efficiency
The related parameters are shown in Table
Related parameters of ISACS driven by DPES.
Parameters | Value |
---|---|
Outside | |
| 5778 |
| 286.36–298 |
| 0.87–1.55 |
| 22.17 |
PV modules | |
| 2.76 |
| 2.88 |
| 3.24 |
| 89.35 |
| 8.18 |
| 50.90–54.20 |
| 0.50–8.50 |
| 34.50 |
| 7.10 |
| 17.50 |
| 0.92 |
| 0.90 |
| 23 |
| 1179.06 |
| 287.04–325.15 |
Controller | |
| 303.15 |
Storage batteries | |
| 48 V 65 Ah |
| 313.15 |
| |
| 0.80 |
VF | 56.53 |
| 47.60–52.53 |
| 8.00 |
| 0.060 |
| 0.041 |
| 95.234 |
| 51.856 |
| |
| 0.70 |
VF | 54.88 |
| 54.88–44.60 |
| 5.50–7.40 |
| 0.052 |
| −0.012 |
| 4.113 |
| −100.653 |
Inverter | |
| 318.15 |
| 10.045 |
| 1.1885 |
Compressor | |
| 0.0127 |
| 328.31–340.25 |
| 268.15 |
| 313.15 |
| 243.71 |
| 770.21 |
| 395.01 |
| 425.00 |
| 1.7276 |
| 1.7500 |
Condenser | |
| 313.15 |
| 303.15 |
| 700.00 |
| 700.00 |
| 425.00 |
| 241.80 |
| 1.7500 |
| 1.1437 |
| 0.21 |
| 1000 |
| 286.36–296 |
| 291.36–301 |
Throttle valve | |
| 303.15 |
| 263.15 |
| 700.00 |
| 200.00 |
| 241.80 |
| 241.80 |
| 1.1437 |
| 1.1500 |
Evaporator | |
| 263.15 |
| 268.15 |
| 200.00 |
| 243.71 |
| 241.80 |
| 395.01 |
| 1.1500 |
| 1.7276 |
Cold exchanging and supplying | |
| 272.20–278.15 |
| 276.62–285.15 |
| 293.45–289.95 |
| 293.45–287.65 |
| 16.98 |
| 335 |
| 0.12 |
| 14400 |
08:00–17:00 is the working time from Monday to Friday. During working time, the cold demand in the house is very low, so the refrigerator can be driven by PV modules and then the cold can be stored in the ice storage tank. After work, the cold demands of the house increased and the cold stored in the daytime can be charged to service family. So the ISACS driven by DPES has great application prospects in domestic cooling field.
The ISACS driven by DPES operated from 08:00 to 23:00. In daytime, solar energy was converted into electrical energy by PV modules. Before 11:00 AM, the irradiation was too low and the electricity generated by PV modules cannot drive ice maker. At the same time, batteries discharged to make up for the lack of electricity produced by PV modules and the ice maker could operate stably and reliably driven by photovoltaic battery hybrid energy supply system. And then, the irradiation gradually increased and the electricity increased along with irradiance. In hybrid energy supply system, the system output power was equal to the power rating of ice maker and was always the same but PV modules output power increased and batteries output power decreased accordingly. About 11 AM, the output power of PV modules was enough to drive ice maker so the output power of batteries is zero. After 11 AM, with the continuous increase of irradiance, not only was the electricity of PV modules used to drive ice maker but also the surplus electricity was stored in batteries. So the batteries were in charge. After noon, the irradiation from sun decreased step by step and so the electricity generated by PV modules reduced gradually. About 14:00, the electricity reducing step by step was only enough to drive ice maker. The charge state of batteries was end. Then, ice maker was driven by PV modules and batteries once again until the ice was enough at 16:00. All of the ice was stored in ice storage tank and the cold can be exchanged by the coil with refrigerating medium ethylene glycol flowing in it at 19:00. At the other end of the coil the cold of refrigerating medium can be blown into the air in the house by fan. So the temperature of house dropped step by step until it tended to balance. About 23:00, the cold of ice in the tank was released completely. And then, the fan and glycol pump would be shut down and the cold supply process was over. Through all of the process, energy is transformed from light to electricity by PV modules and it is stored with ice through phase change latent heat of water. In conversion and transmission process, the energy and exergy of system changed with the change of external environmental conditions. In order to describe system performance clearly and intuitively, the overall performance of the system was evaluated. So the energy efficiency and exergy efficiency of each component of the system in energy conversion and transmission process were calculated and the results are shown in Table
Calculated results of total energy and total exergy of each component of the system in energy conversion and transmission process.
Accepted energy/W | Accepted exergy/W | Useful power/W | Useful exergy/W | | | ||
---|---|---|---|---|---|---|---|
PV module | 100007.5 | 112560.5 | 15889.79 | 95910.39 | 15.89 | 85.21 | |
Controller | 15889.79 | 95910.39 | 15254.2 | 95894.55 | 96.00 | 99.98 | |
Batteries | 15254.2 | 95894.55 | 15162.49 | 95887.94 | 99.40 | 99.99 | |
Inverter | 19772.17 | 95887.94 | 16222.1 | 95621.2 | 82.05 | 99.72 | |
Distributed photovoltaic energy system | | | |||||
Compressor | 156936.5 | 95621.2 | 151397.8 | 87286.14 | 96.47 | 91.28 | |
Condenser | 151397.8 | 87286.14 | Air | 51450 | 86445.17 | 90.88 | 99.04 |
Refrigeration | 86136.41 | ||||||
Throttle valve | 86136.41 | 86445.17 | 86136.41 | 85784.72 | 100 | 99.24 | |
Evaporator | 86136.41 | 85784.72 | Output | 140714.4 | 79810.74 | 336 | 93.04 |
Absorbing | 54578 | ||||||
Ice | 54578 | 79810.74 | 9484.22 | 76271.06 | 17.38 | 95.57 | |
Ice maker system | | | |||||
Air conditioning system | 94842.22 | 76271.06 | 81009.64 | 75747.45 | | | |
ISACS driven by DPES | | |
Figure
Energy efficiency and exergy efficiency of ice maker driven by DPES.
Energy efficiency and exergy efficiency of fan coil driven by DPES.
In daytime, ice maker system was driven by DPES batteries and the system energy efficiency and exergy efficiency were 5.44% and 67.30%, respectively. It was found that the total energy efficiency and total exergy efficiency of DPES were 12.44% and 84.95%, respectively, and the total energy efficiency and total exergy efficiency of ice maker system were 51.20% and 79.77%, respectively. After 16:00, the total ice production was 16.98 kg. All of the ice was stored in the tank and air conditioner was open at 19:00 and sustained for about 4 hours with 85.42% total energy efficiency and 99.31% exergy efficiency.
It was observed that in ice maker thermodynamic cycle refrigeration efficiency was only 17.38% and the wastage in cooling process was 45093.78 W. In the ice making process, the water can be pumped by a recycle pump and flowed through evaporator to absorb heat. A part of water can be frozen on the evaporator and the rest of water flowed back to the water storage. Thereby, in the water circulating process, the circulating water consumed much of the energy, as shown in Figure
Ice slide maker (internal picture).
In ice maker, compressor was the maximum exergy loss component with 8.78% exergy loss efficiency and it is clear that the evaporator is the most important component of ice maker; the total exergy of evaporator was 79810.74 W and the exergy efficiency was 93.04%. At the same time, the exergy flow from water to the evaporator was 76271.06 W with 95.57% exergy efficiency in ice making process.
The output performance of PV modules was greatly affected by the external environment such as solar irradiance, wind speed, ambient temperature, and PV modules temperature. Therefore, the external environment should be tested through the experiment to calculate and analyze PV output performance. ISACS driven by DPES tested on October 22 in Kunming. Results showed that the irradiation quantity
Variations of global solar radiation and ambient temperature on 22nd Nov., 2015, in Kunming.
Variations of wind speed and PV modules temperature on 22nd Nov., 2015, in Kunming.
The voltage and current variations of PV modules were also tested on October 22 in Kunming. The variation curves are shown in Figure
Voltage and current variations of PV modules.
Voltage and current variations of batteries.
It was observed that the maximum tested current was 8.6 A at 12:35. The output voltage changes were 50.8 V–54.2 V and the maximum voltage appears at 12:36. In batteries discharging, ice maker can be driven for 10 hours by batteries only which were in full power state and disconnected with PV modules until the batteries electricity decreased and cannot drive the ice maker. The voltage declined from 54.88 V to 44.6 V step by step and the output current of PV modules was approximately 7 A with small fluctuations between 5.4 A and 7.4 A. In charging process for batteries, batteries disconnect with ice maker and connected with PV modules. Batteries current changed with solar irradiance and voltage increased gradually until batteries were in floating state.
Electricity conversed from light by distributed photovoltaic energy system was transferred among batteries, ice maker, and air conditioning system. It was very important to optimize and match the energy among PV modules, storage batteries, and power consumption machine, which can provide an important reference for the future work. Therefore, the experimental study on ISACS driven by DPES was carried out in order to analyze the light-electricity-cold conversion process and the energy transfer characteristics. The results are shown in Figure
The conversion and flow characteristics of power generated by PV modules among batteries, ice maker, and air conditioning system.
There were ice making and ice melting abscission in ice maker operation process. In ice making process, the system current was 1.8 A, the sum of compressor working current and condenser fan current. However, in ice melting abscission process, the system current was 1.4 A, the sum of condenser fan current and solenoid valve operating current. On the other hand, the voltage of AC compressor was 220 V and it was stable all the time. Voltage and current of pump and fan were 220 V and 0.4 A and 380 V and 0.5 A, respectively. In DPES, photovoltaic modules and batteries were untied and used to drive ice maker. Before noon the PV modules output current increases along with solar irradiance and batteries output current gradually decreases. When PV modules output current increased to 6.5 A, batteries output current reduced to 0 A at 10:50 and ice maker can be completely driven by PV modules. Since then, ice maker was driven by PV module and the rest electricity can be stored in batteries. Batteries charging current increased step by step and reached the maximum of 2.0 A until the solar irradiance reached the peak values at 12:20. After noon, batteries charging current gradually reduces along with solar irradiance decreases. It was found that at 15:37 the charging current and PV modules output current were 0 A and 6.5 A. Therefore, ice maker was only driven by PV modules once again. Subsequently, the generated electricity by PV modules was not enough to drive the ice maker and batteries discharged to supplement the shortage. Batteries discharging current increases step by step along with the solar irradiance decreases. Batteries voltage reduced in discharging process and increased in charging process. Batteries separately derived pump and fan coil to supply cold from 19:00 to 23:00. Output current remained constant with 5 A and output voltage slightly decreased gradually. At 23:00, ice melts completely and then all machines were shut down. Until 07:30 of the next day, a new ice making and cold supplying cycle starts. Through the 27-hour experiment, batteries capacity decreased from 65 Ah to 9.76 Ah with a 55.24 Ah decrease. In the first day, batteries discharged 13.7 Ah in the morning and then were charged 7.39 Ah by PV modules. Batteries discharged 12.17 Ah from 15:37 to 19:00. And then ice maker stopped and cold exchanger ran, driven by batteries. The electricity consumption was 20.22 A from 19:00 to 23:00. In the next day, ice maker starts to work at 07:00 and electricity consumption in the morning discharging process and afternoon discharging process was 12.55 Ah and 11.61 Ah, respectively. Batteries were charged 7.62 Ah by PV modules from 10:11 to 15:36.
The ice production is shown in Figure
Ice production of ice maker driven by DPES from 8:00 to 16:00 on 22nd Nov., 2015, in Kunming.
It was found that the total amount of ice was 16.98 kg. Ice maker operation cycle time was 10 min and the ice production was about 0.35~0.36 kg every cycle. Ice making efficiency was about 2.12 kg/h. Another experimental test has been conducted at various temperatures of ice maker with every component in thermodynamic cycle of ice making process, as shown in Figure
Temperature variations of ice maker components during two refrigeration thermodynamic cycle processes.
In ice making process, ice was pasted well together with five solid walls of grid plate evaporator and cannot be separated off by ice gravity. Thus, compressor must be shut down and a special solenoid valve as shown in Figure
Electromagnetic valve photo adopted in ice maker system.
When the electromagnetic was opened, the evaporator plays the role of condenser to release heat to ice through solid walls and ice interfaces begin melting. Ice was divorced from evaporator and fell into tank. Meanwhile, evaporator inlet temperature and outlet temperature increased sharply until the solenoid valve was closed and compressor was turned on and a new ice making process begins. The refrigerant flowed out of evaporator into throttle valve and became low temperature and low pressure liquid. And then, the refrigerant flowed into condenser to absorb heat from outside and condenser inlet and outlet temperature declined sharply when low temperature refrigerant flowed into it. Condenser outlet temperature was higher than condenser inlet because refrigerant absorbed heat from outside through condenser, as shown in Figure Ice maker operation period extended for 200 s; furthermore, the ice melting abscission time was one-third time of an ice making cycle. Service life of compressor will be shortened by frequent start and stop compressor, which has a great impact on ice making process and was extremely unfavorable for energy supply process of DPES. Electricity consumption of solenoid valve was 3.0 × 10−4 kW h in ice melting abscission process and the total electricity consumption was 0.024 kW h from 7:30 to 19:00.
The performance of air conditioning system was tested at 19:00. At this time, the temperature of ice stored in ice storage tank and ice storage tank temperature were −3°C and −1°C, respectively. The indoor temperature was 20.5°C. The changes of temperature for exchanging cold and supplying cold processes are shown in Figure
Temperatures change of exchanging cold and supplying cold process.
Ice storage temperature remained constant at −1°C in ice absorbing and melting phase transition processes. However, after 22:20, all ice melted and the tank filled with water at low temperature. Subsequently, the temperature of water gradually increased when the water was employed to cool. Subsequently, it increased from 0°C to nearly 5°C within 40 min. At the end of the experiment, ice storage tank outlet temperature increased to 11.5°C from initial temperature of 3.4°C. In ice phase transition process, ice storage tank inlet and outlet temperatures should be constant values in theory; however, the temperature difference between inlet and outlet increased 7.15°C mainly as a result of the working temperature of pipeline pump. It was found that the fan outlet temperature decreased from 20.5°C to 15.1°C with a 5.4°C decline and the indoor temperature decreased from 20.5°C to 18°C with a 2.5°C decline within 4 hours. Cold power blown out by fan coil was 0.298 W and the cold exchanging and supplying efficiency of the air conditioning system was 90.4%. Furthermore, the ice storage tank has a good thermal insulation performance at 0.10 m thick polyurethane foam. The ice cold loss in ice storage tank was 218.59 kJ as 3.89% of input cold from 19:00 to 23:00 and the cold loss of fan coil was 313.92 kJ as 5.82% of input cold.
The system energy efficiency was 5.44% calculated as shown in Table
Analog computation and experimental tests results showed that there were some deficiencies of current system as follows: Usually the cold demand during the daytime is high, but the system can supply cold through ice melting made by ice maker driven by electricity generated by PV modules in daytime. Therefore, there is no matching between cold supply and demand in the same time. Energy loss is huge and ice making efficiency is low in ice making process of ice slide maker.
Therefore, in order to improve the performance of ISACS driven by DPES and promote the project commercial promotion, some optimization and improvement measurements were proposed as follows: Evaporator immersion static refrigeration mode was adopted to replace ice harvester refrigeration mode. The optimized coil evaporator was immersed into water to absorb heat and make ice and all the energy was utilized, as shown in Figure The coil cold exchanger was cointegrated with coil evaporator. In refrigeration process, coil cold exchanger has the priority to get cold transferred from the coil evaporator next to the cold exchanger to supply cold for user. Surplus cold is used to make ice to store cold. Consequently, ISACS not only has the out-of-the-box function of ordinary air conditioning, but also effectively improves the appearing phenomenon of overcooling and remedies the disadvantage of cold supply after ice making process in traditional submerged ice making system. The top view of cointegration evaporators and cold exchanger immersed in the ice storage tank is shown in Figure
Profile of evaporators and cold exchanger immersed in the ice storage tank.
Top view of cointegration evaporators and cold exchanger immersed in the ice storage tank.
Another immersion static ice maker with evaporator cointegrated with coil cold exchanger was constructed according to the optimization design shown in Figure
The photo of evaporator immersion static ice maker.
The performance of optimized ISACS driven by DPES can be tested and calculated as mentioned above. The system operated on May 12, 2016. The tested and calculated results were shown in Figure
The performance of two kinds of ISACS driven by DPES.
DPES | Ice maker system | Air conditioning system | System | |||||||||||||||
---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
Compressor | Condenser | Throttle valve | Evaporator | Ice | Total | |||||||||||||
| | | | | | | | | | | | | | | | | | |
1 | 12.44 | 84.95 | 96.47 | 91.28 | 90.88 | 99.04 | 100 | 99.24 | 336 | 93.04 | | | | | 85.42 | 99.31 | | |
2 | 12.44 | 84.95 | 96.47 | 91.28 | 90.88 | 99.04 | 100 | 99.24 | 336 | 93.04 | | | | | 85.42 | 99.31 | | |
Note: 1 stands for the ISACS driven by DPES mentioned above; 2 stands for the optimized ISACS driven by DPES.
Temperature changes of ice maker components after optimization.
The water tank was full of 13.4 kg water. The system operated at 08:35 and all of the water changed into ice at about 10:40. And then the ice started subcooling and the ice surface became hard at 12:04. At the moment, the surface of evaporator temperature (the temperature of ice core) was −12.11°C and the water temperature was −8.09°C with a 4.20°C temperature difference between ice surface and ice core. At 12:04, the temperatures of evaporator inlet and outlet were, respectively, −20.49°C and −16.51°C and the compressor inlet temperature was −14.28°C. The energy efficiency and exergy efficiency of system components of optimized ISACS driven by DPES were calculated through above formulas and the performances of the system mentioned above and the optimized system were compared in Table
By structure optimization, the ice making efficiency of evaporator immersion static ice maker was improved to 6.00 kg/h which was 2.88 times of ice slide maker with the same input power. Through theoretical calculation, it was observed that the ice making efficiency and exergy efficiency increased from 17.38% to 50.12% and from 95.57% to 97.32%, respectively. So the ice maker system energy efficiency was improved 2.88 times and exergy efficiency was improved from 76.85% to 78.52%. After the structure optimization, the energy efficiency of ISACS driven by DPES can be improved from 5.44% to 15.69% and the exergy efficiency can be improved from 67.30% to 68.54%. When evaporator cointegrated with coil cold exchanger, ISACS achieved out-of-box functionality as the ordinary vapor compression air conditioning system, which can effectively solve the problem of cold supplying process must lagging behind ice making process in traditional ice storage air conditioning system.
The energy utilization ratio and exergy efficiency of ISACS driven by DPES were 5.44% and 67.30%, respectively. DPES conversion efficiency was 12.44% with 84.95% exergy efficiency and ice maker system energy efficiency was 51.20% with 79.77% exergy efficiency. In order to improve the refrigeration performance of ISACS, evaporator immersion static refrigeration mode was adopted to replace ice harvester refrigeration mode to achieve high efficiency refrigeration. The coil cold exchanger was cointegrated with coil evaporator. Consequently, ISACS not only has the out-of-the-box function of ordinary air conditioning system, but also effectively improves the appearance of overcooling phenomenon. By simulation, the system energy utilization efficiency could be improved from 5.44% to 15.69% with exergy efficiency increasing from 67.30% to 68.54%.
Batteries capacity (Ah)
Empirical constant
Empirical constant
Specific heat capacity (J·kg−1·K−1)
Exergy loss (W)
Exergy (W)
Solar irradiance (W·m−2)
Enthalpy (J·kg−1)
Current (A)
Mass (kg)
Mass flow (kg·s−1)
Power per unit time (W)
Empirical constants
Entropy (J·kg−1·K−1)
Area (m2)
Runtime (s)
Temperature (K)
Voltage (V)
Full charge rest voltage (V)
Speed (m·s−1)
Compressor operating power (W).
Solar cell absorption coefficient
PV module cover glass transmittance
Symbolic coefficient
Energy efficiency
Exergy efficiency.
Refrigerant in state 1
Refrigerant in state 2
Refrigerant in state 3
Refrigerant in state 4
Refrigerant in state 5
Ambient
Indoor air
Evaporator absorption
Batteries
Cell
Convective
Controller
Compressor
Condenser
Electricity
Emission
Evaporator
Ice maker
Fan coil
Input
Inverter
Ice
Loss
Maximum power point
Open circuit
Output
Parallel
PV modules
Refrigeration
Sky
Short circuit
Sun
Storage
Throttle valve.
The authors declare that they have no competing interests.
The authors gratefully acknowledge the financial support provided by National Natural Science Foundation of China (51666018) and National International Scientific and Technological Cooperation Program (2011DFA62380). The authors are also grateful to Renewable Energy Research and Innovation Development Center in Southwest China (05300205020516009), Research and Innovation Team of Renewable Energy in Yunnan Province and Yunnan Provincial Renewable Energy Engineer Key Laboratory (2015KF05), and Yunnan Provincial Department of Education Science Research Fund Project (2015J035).