Energy-Efficient Refrigeration Systems
The demand for HVAC industry for various needs like preservation of food, medicines and especially, human comfort has always been there and is now at its peak. In the process, to meet this demand man has been depleting resources and polluting the environment for years. The use of natural refrigerants is the best solution to check environmental destruction…
- Bijan Kumar Mandal,
Madhu Sruthi Emani,
Refrigeration plays a significant role in our daily life which allows keeping the temperature below the environmental temperature for human comfort. The energy from fossil fuels is used to run the conventional HVAC systems. But, the fossil fuels are being rapidly depleted. Also, the increased demand for refrigeration and air-conditioning has contributed to two major environmental issues namely, global warming and ozone layer depletion caused due to the refrigerants that are being used. Therefore, finding novel ways towards green technology without compromising comfort and indoor air quality remains a challenge for research and development. There are different techniques, which can be implemented on HVAC systems to improve their energy efficiency and also new refrigerants can be used to reduce the negative environmental impact. The natural refrigerants like ammonia, hydrocarbons and CO2 are considered as environmental friendly and future refrigerants. The control and optimization strategies have been used recently to improve the energy consumption rates of these systems. However, implementing these strategies is either expensive or complicated, and requires constant monitoring. The other way to achieve energy efficiency is to combine different HVAC components to create an energy-efficient configuration. The modifications can result in substantial savings in the long term by reducing maintenance costs associated with control and optimization strategies.
Environmental Impact of Refrigerants
The refrigerants play a very crucial role in HVAC industry. The substances chosen as refrigerants decide many factors of a refrigeration system such as compressor work, COP and the extent to which global warming and ozone layer depletion are affected. The natural refrigerants such as water, methyl chloride, sulphur dioxide, carbon dioxide and ammonia were used in the beginning of the invention of mechanical refrigeration. But unfortunately, most of those refrigerants proved to be toxic and flammable. Thus, chlorofluorocarbon, (CFC) refrigerants came into picture and served as excellent and efficient refrigerants for years till the 1970s. In 1973 Prof James Lovelock discovered Freon to possess high ozone layer depletion potential (ODP). The Montreal protocol stopped the production and consumption of such ozone layer depleting CFC refrigerants. Then, the hydro fluorocarbon (HFC) refrigerants were used as alternatives to CFCs. But, HFCs have also high global warming potential (GWP). More than 190 countries gathered in Kigali, in 2016 and adopted an amendment to the 1989 Montreal Protocol to eliminate HFC gases. Now, the search for new alternative refrigerants which can replace the conventional CFC and HFC refrigerants, without compensating the performance of the system has become a challenge to the scientists and researchers working in this area.
The natural refrigerants like CO2, NH3 and hydrocarbons have zero ODP and GWP and are considered to be the long term replacements to CFCs and HFCs. The natural refrigerants failed back in those days, due to the problems of toxicity and flammability. The present day technology can easily handle such problems. Thus, the use of natural refrigerants could be the best possible solution to stop the environmental destruction caused by the conventional CFC and HFC refrigerants.
Properties of Refrigerants
The properties of refrigerants play a vital role in economic and environmental friendly application. The thermo-physical properties of different refrigerants along with their ODP and GWP have been shown in table 1. Some of the desirable properties of refrigerant are as follows:
• The refrigerant should have low boiling point and low freezing point.
• It must have high critical pressure and temperature to avoid large power requirements.
• It must have low specific heat and high latent heat. Because high specific heat decreases the refrigerating effect per kg of refrigerant and high latent heat at low temperature increases the refrigerating effect per kg of refrigerant.
• It should have low specific volume to reduce the size of the compressor.
• It must have high thermal conductivity to reduce the area of heat transfer in evaporator and condenser.
• It should be non-flammable, non-explosive, non-toxic and non-corrosive.
• It should give high COP in the working temperature range. This is necessary to reduce the running cost of the system.
• It should have zero ODP and very low GWP.
Performance of simple Vapour Compression Refrigeration (VCR) system
The performance of a simple vapour compression refrigeration system using different refrigerants has been broadly studied by researches. The main objective of testing different refrigerants in VCR system is to identify proper replacements to conventional refrigerants which have high ODP and GWP values and to develop new environmental friendly refrigerants. Thus, as shown in table 1, R600a has been found to be a replacement for R12 and R134a. The R1234ze and R1234yf are suitable replacements for R134a. Also, the refrigerants R32 and R410a are noted to replace R22. The variations of compressor input power and COP with evaporator temperature at a constant condenser temperature of 50°C for a simple vapour compression refrigeration system have been shown in figures 1 and 2 respectively (Mohanraj et al., 2008). The trend in variation of COP and compressor input power has been noticed for refrigerant R134a and its replacements R152a, R600, R600a, R1270 and R290. The compressor input power for R1270 is the highest and for R600 is the lowest. The compressor input power of R290 and R1270 are about 42% and 54% higher than that of R134a, respectively. R600a, R600, and R152a are about 46%, 65%, and 4% lower than that with R134a. It is shown from figure that R600 and R600a yield highest COP and R1270 and R290 yield the lowest. The COP of R152a is higher than that of R134a by about 7–9% across the considered range of operating temperatures. The COP of R290 and R1270 are lower than that of R134a by about 2.5% and 2%, respectively. The COP of R600a and R600 are higher than that of R134a by about 2.4% and 6.9% respectively.
Figure 1: Effect of evaporator temperature on compressor input power
Figure 2: Effect of evaporator temperature on COP
Limitations of Vapour Compression Refrigeration Cycle
It is well known that a simple vapour compression refrigeration system is the most commonly used system for the purpose of refrigeration and air-conditioning. But, when the atmospheric temperature is high and a very low temperature is to be maintained, this simple vapour compression refrigeration system becomes inefficient. In case of high temperature difference between evaporator and condenser (Temperature Lift), compressor power requirement increases and the specific refrigeration effect decreases. This leads to the operation of system for a long period of time to meet the desired load which sometimes results in failure of the compressor. So, the application of simple vapour compression refrigeration system for high temperature lift is not advisable. The major drawbacks in operating a simple vapour compression refrigeration system with high temperature lift are:
• Throttling loss increases
• Superheat loss increases
• Compressor discharge temperature increases
• Quality of the vapour at the inlet to the evaporator increases
• Specific volume at the inlet to the compressor increases
• Refrigeration effect decreases
• Volumetric efficiency decreases
• Compressor work increases
• Energy requirement increases
Thus, as the temperature lift increases the single stage systems become inefficient and impractical.
Figure 3: Two-stage vapour-compression plant with flash intercooler.
Figure 4: Temperature versus entropy diagram for two stage refrigeration cycle
All the above mentioned drawbacks resulting from high temperature lift, lead to increased consumption of energy and decreased efficiency of the system. These issues can be overcome by modifying and using different configurations of vapour compression refrigeration system . Some of such configurations have been discussed below.
1. Multi Stage Vapour Compression Refrigeration System with Flash Intercooling
The working of two stage vapour compression refrigeration system with flash intercooling along with its representation on T-s plane has been shown in the figures 3 and 4 respectively. The configuration consists of an evaporator, low pressure compressor, high pressure compressor, condenser, high pressure throttle valve, low pressure throttle valve and a flash chamber. The refrigerant in the evaporator absorbs heat from the cooling space that has to be cooled and gets converted into vapour. The refrigerant vapour is then compressed in a low pressure compressor. The compressed refrigerant vapour from the low pressure compressor and the condensed liquid refrigerant expanded in the high pressure throttle valve are sent into the flash chamber. In the flash chamber the liquid and the vapour refrigerant are separated and the refrigerant vapour is sent to the high pressure compressor while the liquid refrigerant goes to the low pressure expansion valve, where it expands to the evaporator pressure. In this configuration, the amount of vapour entering the evaporator which is also known as flash gas is reduced due to presence of flash chamber. The amount of flash gas entering the evaporator should be as minimum as possible as the flash gas is already in vapour state and does not contribute to the refrigerating effect and increases the pressure drop in the evaporator.
2. Cascade Refrigeration System
The cascade refrigeration system is a freezing system that uses two kinds of refrigerants having different boiling points, which run through their own independent freezing cycle and are joined by a heat exchanger. In a cascade refrigeration system, two simple vapour compression refrigeration cycles (Low temperature cycle (LTC) and high temperature cycle (HTC)) are connected with each other in series with a cascade heat exchanger. Figure 5 shows a two stage vapour compression cascade refrigeration system, which consists of low and high side refrigeration systems indicated as A and B respectively. The refrigeration systems A and B are coupled to each other by means of a heat exchanger in which the total heat from refrigeration system A is rejected to refrigeration system B. The refrigerants flowing in both systems are usually different from each other although there are some cases where the same refrigerant can be used in both systems. The refrigerant circulating through system B has a higher boiling temperature than the refrigerant in system A. The process has been represented on T-s plane as shown in figure 6. As the system allows use of refrigerants that have suitable temperature characteristics for each of the higher-temperature side and the lower-temperature side the energy is saved.
Figure 5: Schematic diagram of the vapour cascade refrigeration system
Figure 6: Vapour cascade refrigeration system in T-s plot
3. Vapour compression refrigeration system with dedicated subcooler
The complete system is constructed by coupling two cycles in series. The schematic diagram of the complete system has been shown in the figure 7 and the corresponding p-h plot has been presented in figure 8. The main cycle is connected to the dedicated subcooler system through a heat exchanger. This heat exchanger serves two purposes. It acts as a subcooler for the main cycle and evaporator for the dedicated subcooler cycle. Components of the main cycle are bigger in size than those of the dedicated subcooler cycle. In the main cycle refrigerant leaves the evaporator at state 1 and enters the main cycle compressor. Refrigerant leaves the compressor at state 2 and enters the condenser. Main cycle refrigerant rejects heat to the environment in the condenser and leaves it as a saturated liquid at state 3. Then this saturated liquid enters the subcooler heat exchanger to get cooled below its saturation temperature (state 4). It then enters into the expansion valve in the main cycle and leaves it as low quality vapour and enters the evaporator. On the other hand, subcooler cycle refrigerant gets evaporated by taking the heat from the main cycle refrigerant in the subcooler and enters in to the dedicated cycle compressor at state 6. Compressed high pressure and high temperature refrigerant is then entered into the condenser in the dedicated cycle at 7 and leaves the condenser at state point 8 as saturated liquid and enters in the expansion valve.
Figure 7: Schematic diagram of the vapour compression refrigeration system with dedicated subcooler
Figure 8: Vapour compression refrigeration system with dedicated subcooler in p-h plot.
4. Vapour compression refrigeration system with integrated subcooler
The major components of an integrated mechanical subcooling vapor-compression refrigeration system includes two reciprocating compressors, two expansion valves, condenser, evaporator, receiver and a subcooler. The system consists of two simple cycles coupled to each other via a subcooler as shown in figure 9, while its pressure enthalpy diagram is shown in figure 10. The bigger cycle is known as the main cycle and the smaller cycle is known as the subcooler cycle. The two cycles have a common condenser, and the components of the two cycles are connected in a closed loop through a piping system that has heat transfer with the surroundings. The figure shows that the main-cycle refrigerant leaves the main-cycle evaporator at state 1 as a low pressure, low temperature, saturated vapor and enters the main cycle compressor at state 2. The refrigerant, from state 1 to 2 takes heat from the surroundings in the suction line. At state 3, it leaves the compressor as a high temperature, high pressure, superheated vapor. The refrigerant, from state 3 to 4 rejects heat to the surroundings in the discharge line. At state 4, it mixes with the subcooler cycle refrigerant coming from the subcooler cycle compressor and attains state 13, and the mixture enters the condenser. The mixture after leaving the condenser is collected in the receiver. Some of this liquid refrigerant mixture is extracted from the receiver and is expanded in the expansion valve of the subcooler cycle and is then passed through the subcooler. The remaining liquid refrigerant in the receiver enters the subcooler, where it is cooled below the saturated liquid state at a constant pressure to state 6 by the subcooler cycle refrigerant. It enters the main cycle expansion valve and at state 7 it leaves the expansion valve as a low quality vapor and enters the evaporator. In the evaporator, it is evaporated at a constant pressure to the saturated vapor state. The subcooler-cycle refrigerant after cooling the main-cycle refrigerant in the subcooler, leaves as a low-pressure, low-temperature, saturated vapor at state 9 and enters the subcooler cycle-compressor at state 10. The refrigerant from state 9 to 10 takes heat from the surroundings. At state 11, it leaves the compressor as a superheated vapor where it is mixed with the main cycle refrigerant coming from the main-cycle compressor, and attains state 13.
Apart from multistage vapour compression refrigeration there are many other methods which can be used for energy saving in HVAC industry. The evaporative cooling technology has been widely used for years. It uses water as the working fluid and hence it does not cause any negative environmental effects. Another important technology is the ground-coupled technology which relies on the fact that, at depth, the earth has a relatively constant temperature that is colder than the air temperature in summer and warmer than the air temperature in winter. The thermal storage systems are quite important as they shift the energy usage of the HVAC systems from on-peak to off-peak periods to avoid peak demand charges. In this system, energy for cooling is stored at low temperatures normally below 20°C for cooling, while energy for heating is stored at temperatures usually above 20°C. The heat recovery techniques can be used to recover energy that might otherwise be wasted. The objective of heat recovery is to reduce the cost of operating an HVAC system by transferring heat between two fluids such as exhaust air and fresh air. Also, many studies have been carried out on energy savings for HVAC systems by using materials with enhanced capacity to absorb, store and release the mass or heat.
Figure 9: Schematic diagram of the vapour compression refrigeration system with integrated subcooler.
Figure 10: Vapour compression refrigeration system with integrated subcooler in p-h plot.
Case study with dedicated sub cooler
The performance of a vapour compression refrigeration system can be improved by subcooling of condensed saturated liquid which permits the low quality refrigerant to absorb more heat from the refrigerated space in the evaporator. A performance enhancement model for a vapour compression refrigeration system by adding a small dedicated mechanical subcooler system to the main system has been developed to operate with comparatively large temperature difference between the evaporator and the condenser. Subcooler cycle provides subcooling to the main cycle refrigerant by absorbing heat from the condensed liquid of the main cycle refrigerant. R134a is chosen as main cycle refrigerant, whereas, R152a and R134a are separately considered as subcooler cycle refrigerants. Engineering Equation Solver (EES) has been used to analyze the performance of the system and compared with two different refrigerants in the subcooler cycle. The basic assumptions for the simulations have been tabulated in table 2. It is found that the overall performance of the system is improved for using both R152a and R134a as subcooler cycle refrigerants. Results also depict that R152a shows slightly better performance compared to the R134a as the subcooler cycle refrigerant. Predicted results show that an optimal subcooler temperature exists for fixed condenser and evaporator temperature.
The improvement in the performance characteristics of the refrigeration cycle with dedicated subcooler over vapour compression refrigeration cycle without subcooler the can be expressed by plotting the predicted results in the normalized curve. Normalized compressor work for a particular condenser and evaporator temperature can be expressed as:
where WTotal is the total required work by the refrigeration system with dedicated subcooler and WWS is the total compressor power requirement by the refrigeration system without subcooler. The normalized COP can be expressed as:
and similarly here, COPWS and WWS are the COP of the system and compressor power without subcooler respectively.
At first, simulation has been carried out taking R134a in both the cycles as refrigerant keeping the evaporator temperature constant at -30°C and condenser temperature is varied from 40°C to 55°C. Effect of subcooler temperature on the normalized COP of the system for different main cycle condenser temperatures has been shown in figure 11. Also, variation of normalized COP with subcooler temperature for different evaporator temperatures in the main cycle has been shown in figure 12. It can be seen from the figure 9 that normalized COP initially increases, reaches its maximum value and then decreases with the increase in subcooler temperature. This is because, as subcooler temperature increases beyond a certain value, the subcooler cycle COP increases but the COP of the main cycle decreases. It can also be noted from the figure that maximum normalized COP is achieved when condenser temperature is maximum. It is clear that COP reaches its peak at the subcooler temperature midway between the evaporator and condenser temperature. The results show that at optimal condition of the subcooler the system with subcooler cycle shows almost 28% higher COP compared to that of the without subcooler cycle at condenser temperature of 55°C.
Figure 11: Effect of subcooler temp. on normalized COP for different condenser temp. for fixed evaporator temp. of -30°C.
In figure 12, evaporator temperature is varied from -15°C to -30°C while condenser temperature is kept constant at 40°C and refrigerant R134a is considered in both the cycle for the analysis. The figure depicts that with the decrease in evaporator temperature in the main cycle, normalized COP increases. For a particular evaporator temperature, COP first increases and then decreases with the increase in subcooler saturation temperature. Therefore, an optimum point exists where system shows its highest performance. It can also be observed from the figure that the optimal condition is obtained at the subcooler temperature which is at the mean temperature between the corresponding condenser and evaporator temperature. Predicted results showed about 18% enhancement in COP over the simple VCR cycle when the subcooler cycle is attached to the main cycle at evaporator temperature of -30°C.
Figure 12: Effect of subcooler temp. on normalized COP for different evaporator temp. for fixed condenser temp. of 40°C
The variations of normalized compressor power with subcooler temperature for fixed evaporator and condenser temperatures of -30°C and 40°C respectively have been presented in figure 13. In the figure, red line and black line represents the normalized compressor power consumed by the system using R152a and R134a as subcooler cycle refrigerants respectively. It is clearly seen from the figure that compressor power requirement initially decreases and reaches the minimum value and then again increases with the increase in subcooler temperature. As the subcooler temperature increases, the amount of subcooling in the main cycle decreases. This leads to increase in refrigerant mass flow rate in the main cycle which results in increase in compressor power in the main cycle. It is evident from the figure that compressor power requirement is slightly less for the system with R152a in the subcooler. Differences in compressor power for both R152a and R134a subcooler configurations have been calculated and it is found to be 15.5% and 15.1% less than that of the without subcooler system. Also, the comparison of system with and without subcooler has been shown in table 3. The percentage improvement in COP for system with subcooler over the system without subcooler varied from 17.74 to 20.57 in the given range of evaporator and condenser temperatures.
Figure 13: Effect of subcooler temperature on normalized compressor power for specified evaporator and condenser temperature
In a scenario where fossil fuels are fast depleting and the demand for HVAC is growing rapidly, it is now high time to look for alternatives which can save energy usage in HVAC industry. Energy-efficient HVAC system can be designed into new configurations of conventional systems that make better use of existing parts. The alternative configurations of vapour compression refrigeration system have proved to be effective and energy efficient especially in case of high temperature lift. The other methods like evaporative cooling, thermal storage systems and heat recovery systems should be implemented in HVAC to increase the energy-efficiency of the systems. The demand for HVAC industry for various needs like preservation of food, medicines and especially human comfort has always been there and is now at its peak. In the process, to meet this demand man has been depleting resources and polluting the environment for years. The use of natural refrigerants is the best solution to check environmental destruction. Now, the world has to come together to stop the damage by carrying research and developing technologies which result in minimum energy resource depletion and destruction of our environment.
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