Issues Related to air conditioning units

Environmental issues

Air-conditioning plants consume a lot of energy. This means air or/and water and/or sound pollution at the source of the electricity. A rage number of these plants uses chemicals such as CFCs, HCFCs, and HFCs as the colling agents.

When such agents are released to the environment either accidentally or at the end of life, they have negative effects on the ozone layer and thus leads to global warming over time.

The recent times, many air-conditioners are being manufactured from plastics. If disposed to the environment, plastics can be detrimental as they are non-biodegradable. However, a large percentage of plastics can be recycled.

Energy Issues

Air-conditioning plants consume a lot of energy for them to function as expected. This deprives other energy needs as well as being very expensive to operate especially during extreme weather conditions (cold or hot) (Davis and Gertler, 2015).

Health Issues

A large percentage of the air-conditioning plants use ducts in their operations. Over time, these ducts accumulate dust and bacterial such as Legionella bacteria which causes Legionnaires' disease (Orkis et al., 2018). 

Whenever the air-conditioning plant is turned on, the dust and other contaminants are released to the surroundings and are inhaled by the users and thus affecting the health of people. 

 While there are ductless air-conditioners in the market, they are expensive to acquire. On the other hand, air-conditioning can save lives, especially in extreme weather conditions. 

Air conditions can help people with asthma as they are designed to condition are and also to filter it before provided it to the users. Filter removes dust and also harm full gases and this help people with breathing problems Orkis et al., 2018).

Policies

Montreal Protocol – This is an international treaty that was designed for ozone layer protection. it was signed on 26th August 1987. 

The treaty aims to phase out the production of various products that depletes the ozone layer such as those used in air-conditioning plants. The treaty has 46 signatories and ratified by 20 states.

The new model guidelines for room air conditioners and refrigerators – The guidelines require energy-efficient appliances as well as the use of refrigerants with a lower global warming potential.

United for Efficiency (U4E) guidelines - This guide focuses on room air conditioners. 

 It is intended to provide policymakers with information and best-practice case studies on how to promote energy-efficient and climate-friendly room air conditioners in their respective national markets.

Effect of Nano-fluids on Heat Transfer Enhancement in Automotive Cooling Circuits

Unceasing technological development has compelled the automobile industry to manufacture engines with higher efficiencies. To meet the weight requirements of a vehicle, a radiator with optimum size and design is required. Fins are generally used in radiators to increase the rate of heat transfer. Moreover, conventional approach of improving the heat transfer by using micro-channels and fins have some limitations. Additionally, Ethylene Glycol and water have very low thermal conductivities, therefore there is a dire need of introducing new fluids to increase the heat transfer in automobile cooling circuits. Recent researches have shown that Nano-fluids perform better than conventional fluids in heat transfer applications. Leong et al. [33] studied the heat transfer performance of Copper and Ethylene Glycol Nano-fluid and found a better rate of heat transfer than the base fluid alone. The Reynolds number for air and coolant was kept 5000 and 6000 respectively and a 3.8% increase in the rate of heat transfer was narrated for 2% volume fraction. The study also suggested a decrease in the dimensions of frontal surface. Hussein et al. [34] carried out a study on the heat transfer performance of SiO2 and TiO2 Nano-fluid with water. Volume concentration, volume flow rate and inlet temperature of Nano-fluid was kept 1-2%, 2-8 LPM, 60°-80°C respectively and flow was kept laminar. The results indicated that Nusselt number increased slightly with the volume concentration and inlet temperature of the Nano-particles and showed a significant rise with the increase in volume flow rate. 

Subhedar [35] carried out an experimental study to observe the heat transfer performance of water monoethylene glycol (MEG) and Nano-particles of Al2O3 with a volumetric ratio of 50-50%. MEG is usually used as a coolant in radiators. The flow was kept laminar and an automobile cooling system was replicated in the experimental system. The Nano-fluid preparation consisted of two-step method. Ultrasound technique was used to disperse the Nano-particles (20nm diameter) of Al2O3 in a mixture of MEG. The experimental study showed an increase in the heat transfer relative to the base fluid. With the increase in volume concentration from 0-8%, an increase in the overall heat transfer coefficient was noticed. The increase of the inlet temperature from 65 °C to 85 °C. Lastly, the study also established that when the volume concentration of Nano-particles of Al2O3 is 2%, it permits a reduction of 36.9% in the heat exchanger surface. Bozorgan et al. [36] performed numerical analysis on the radiator of a diesel engine. He circulated the Nano-fluid of CuO-water through the flat tubes. The study concluded that an increase of 23.8% and 10% compared to base fluid was attained in pumping power and overall heat transfer coefficient respectively when the speed of the vehicle was 70km/h and the volume concentration of Nano-fluid was 2%. 

Hwa Ming Nieh [7] carried out an experimental study in which he used Nano-coolant (NC), Titania (TiO2) and Alumina Al2O3 to increase the heat transfer of an air-cooled radiator. The study concluded that Nano-fluids perform better than Ethylene Glycol in the terms of heat dissipation and efficiency. Moreover, experimental data revealed that Nano-fluid of TiO2 is efficient than the Nano-fluid of Al2O3. Gulhane and Chincholkar [37] performed experimental analysis on the radiator of a car to determine the heat transfer rate. The Nano-fluid of Al2O3 with water was taken in lower concentration in this study. The result revealed that with the increase in inlet temperature of the coolant, coolant flow rate and particle concentration, heat transfer coefficient of the coolant also rises. They noticed a maximum increase of 45.87% in the heat transfer coefficient. Ray and Das [39] used three different types of Nano-particles i.e. silicon dioxide, copper oxide and aluminum oxide in a car radiator. The base fluid for all three Nano-particles was a mixture of water and Ethylene Glycol in a ratio of 40:60. It was observed that a mixture with lower concentrations i.e. 1% performed better than the mixture that had higher concentration of Nano-particles. A reduction of 35.3% in the pumping power and 7.3% in the heat transfer surface was achieved for 1% volume concentration of Al2O3 Nano-fluid. 

In the case of CuO Nano-fluid, pumping power reduction was comparatively lesser i.e. 33.1% and heat transfer surface reduction i.e.7.2% was almost similar to Al2O3 Nano-fluid. SiO Nano-fluid performed lowest but still a reduction of 26.2% and 5.2% was attained in the pumping power and heat transfer surface respectively. Ali et al. [40] studied the heat transfer performance of ZnO Nano-particles at different volume concentrations i.e. 1%, 2%, 3%, and 8%. The flow rate and Reynolds number was kept in the range of 7-11 LPM and 17,500-27,600 respectively. All the concentrations of Nano-fluid showed a better heat transfer performance compared to the base fluid. The maximum increase of 46% was achieved for 2% volume concentration of the Nano-particles. When the concentration was increased up to 3%, a decrease in the heat transfer was noticed. In addition to that change in inlet temperature from 45°C to 55°C, an increase of 4% was observed in heat transfer velocity. Moghaieb et al. [41] performed the experimental analysis to study the convective heat transfer of a car engine for water and Al2O3 Nano-fluid. He used different diameters [21-37 nm] of Nano-particles in his study. He found that with the increase in flow velocity, the heat transfer coefficient increase whereas it decreases with the increase in the temperature. For a volumetric concentration of 1%, 78.76 % increase in the heat transfer was attained as compared to the traditional fluid. 

Elbadawy [42] performed a numerical analysis to study the flow characteristics and thermal performance of water based Nano-fluid of Al2O3 and CuO. He observed that Nano-fluids can increase the rate of heat transfer and obtained a major volume reduction in the radiator. He found that pumping power is depends upon the concentration of Nano-particles and Reynolds number of the flow. Moreover, 38% and 45% increase in the rate of heat transfer was obtained for CuO and Al2O3 Nano-fluid respectively. Optimum value of volume concentration of both Nano-fluid was found to be 4.5%. At this value, a reasonable increase and decrease in the heat transfer and pumping power was achieved respectively. Harsh et al. [43] carried out an experimental analysis on the heat transfer performance of Nano-fluids in an engine. A solution of Ethylene Glycol (EG) and water (80:20) was used as a base fluid for the dispersion of Al2O3 Nano-particle. Three different values of flow rate were chosen to be 4, 6 and 7 LPM whereas the speed of air flow inside the duct was 4.9 m/sec. The study concluded that with the use of Nano-particle, 24.5% and 13.9% increase in the overall heat transfer coefficient and rate of heat transfer can be obtained. In this research, the diameter of Nano-particles of Al2O3 was 45 nm and its volumetric concentrations was kept between 2% and 6%. The analysis was carried out on three different operating ranges of car (High, Medium and Low) and study was focused on determining the rate of heat transfer, pumping power and outlet temperature of radiator. The obtained results were then compared with the results obtained using base fluid. 

Factor affecting the nanofluid performance in heat transfer

A Nano fluid can of the size of 100 nm particles of non-metallic and metallic substance or it can also be a Nano fiber particles. When these particles are mixed in any fluid then they are called Nano fluids. Adding Nano fluids in any regular fluid like water will enhance the thermal conductivity of that fluid as they provide better flow of mixing. Masuda work on Al2O3 Nano fluid and mixed about 4.3 percent by volume of these Nano fluids in water. This increases the thermal conductivity of water mixed Nano fluid by 30 percent as compared to pure water. Similarly Eastman performs an experiment with Nano crystalline copper oxide and water. Nano crystalline copper oxide particle with 5 percent by volume were mixed in water which result an increase in thermal conductivity of water by almost 60 percent as compare to simple water.

An experimental work was done by Tzeng to investigate the thermal performance of Nano fluids on a real vehicle cooling system. Al2O3, antifoam and CuO Nano particles were used this this practical work. These Nano particles were added in the transmission oil of the vehicle engine. Four different engine speed (400, 800 1200 and 1600 revolution per minute) were analyzed in this test. Test result concluded that CuO perform the best among the Nano particles and it provides the best heat transfer from engine to fluid Test result concluded that CuO perform the best among the Nano particles and it provides the best heat transfer from engine to fluid Test result concluded that CuO perform the best among the Nano particles and it provides the best heat transfer from engine to fluid as it has the best effect in terms of heat transfer and heat transfer distribution for all four different speeds.

Vajjha performed numerical study using CFD and ansys fluent to check and analyze the thermal performance of the CuO and Al2O3 Nano fluid. A flat tube radiator was used for this numerical study and result show that at the Reynolds number of 2000 and 10 percent by volume of Al2O3 improve the heat transfer coefficient up to 94 % as compare to simple fluid. For CuO case a 6 percent by volume increase the heat transfer coefficient of heat exchanger to about 82 percent as compared to pure water.

Factor affecting the nanofluid performance

Coolant efficiency and effective tube geometry of a radiator are the two most important factors that affect the pressure drop and heat transfer taking place in the cooling system of a vehicle engine. Flat tubes and Nano-fluids are used in lieu of circular tubes and conventional fluids (e.g. EG, water) respectively to enhance the cooling system efficiency.  Factors that affect the performance of the cooling system are highlighted in Fig. 3.

 

Figure 1 factor effecting nano fluid working

Tube geometry used for nanofluids heat exchanger 

Flat tubes usually pronounced as flattening of tube profile is an important parameter. Rate of heat transfer is largely dependent on the flattening of tube profile. When the tubes are flattened, the upper and lower walls come more close to each other which escalates the boundary layer phenomenon and an increase in the heat transfer is attained. It can also be said that when the tubes are flattened the hydraulic diameter decreases which increases the heat transfer coefficient. Safikhani et al. [21] performed CFD analysis to analyze the heat transfer coefficient of five flat tubes of different cross-section. Al2O3 Nano-fluid was used in this analysis and flow was kept laminar. With the help of Multi-objective or vector optimization techniques, they noticed an improvement in HTC with the flattening of tubes. Abbassi and Safikhani [22] studied the thermal-hydraulic performance of Al2O3 Nano-fluid with relation to the tube flattening. In this case, they modelled two-phase mixture using FORTRAN programming language and the results were found similar to the previous study. The experimental study on tubes of different internal heights i.e. 8.6 mm, 10.6mm, 11.7mm and 13.4 mm was performed. It concluded that hydraulic diameter reduces with the tube flattening and hence an increase in HTC is noticed at the same flow rate. In the experimental studies performed by Razi et al. [23], he used four different flat tube geometries and one circular tube to study the pressure drop and heat transfer characteristics of flat tubes and found that when the tube is flattened pressure drop becomes high. To find a trade-off between the minimum value of pressure drop and maximum value heat transfer, they calculated a performance index. A tube with internal height of 7.5 mm gave the maximum value of factor as compared to the tube with the internal height of 6.3mm.

Nanofluids (Nanoparticle) concentration 

Weight concentration or volume fraction of Nano-particles in a Nano-fluid is a very important parameter that affects the performance of any thermal system or heat exchanger devices like radiator. When the concentration of Nano-particles is increased, the Brownian motion increases and hence it escalates the micro-convection. The phenomenon of micro-convection and Brownian motion delay the formation of thermal boundary layer. This delay causes the HTC at the entrance region to increase which helps in the rise of overall value of HTC. Weight concentration or volume fraction of nano-particles in the base fluid largely affects its the viscosity and thermal conductivity [24]. Increase in the concentration of Nano-particles increases the thermal conductivity of Nano-fluid but it has adverse effects on its viscosity. Heat transfer is inversely proportional to the viscosity and directly proportional to the thermal conductivity of the Nano-Fluid. Thermal conductivity is an important parameter for effective heat transfer at lower concentrations but at higher concentrations viscosity also becomes very significant. So the HTC increase with the increase in particles’ concentration up to a certain limit and beyond that it goes on decreasing due to the adverse effects of viscosity. Ali et al. [25] studied the thermal behavior of Al2O3 and water Nano-fluid and narrated an increase of 14.97% for 1 vol. %. He found that with increase in the vol. % to 1.5 % and 2 %, HTC started to decrease. Vajjha studied the Nano-fluids of CuO and Al2O3 to predict their thermal behavior by changing their concentration. He noticed that, at lower concentration HTC remained unchanged but its value increases when the vol. % was increased beyond 1 %. Ali et al. [25] used the Nano-fluid of ZnO and water at different volume concentrations i.e. 0.3%, 0.2%, 0.08% and 0.01%. He noticed an increase of 46% in the value of HTC at 0.2 vol. % and the value of HTC started to decrease beyond the vol. % of 0.3. Author used MWCNT/water Nano-fluid and narrated a decrease in the value of HTC with the increase in the concentration of Nano-particles beyond 0.16 wt. %. Ahmed et al. [26] also narrated the similar trend for TiO2 Nano-fluid. He noticed a rise in the value of HTC at 0.2vol. % and it started to decrease beyond the vol. % of 0.3.

Flow rate/Reynolds number of coolant (nanofluid) 

Flow rate or the Reynolds number of the Nano-fluid is also an important parameter that decides the amount of heat transfer taking place. At higher values of Reynolds number, the flow becomes turbulent which increases the molecular collisions and better mixing of the Nano-particles takes place. So high Reynolds number increases the value of HTC. It is evident from the literature that Reynolds number of the flow and concentration of Nano-particles affect the heat transfer. It was noticed an increase of 1.4 % in the rate of heat transfer with the increase in Reynolds number from 4000 to 7000. Abdolbaqi et al. [26], believed that increase of flow rate enhances the rate of heat transfer. They mentioned that increasing the Reynolds number increases the pressure drop and decreases the friction factor.  Nano-fluid temperature also plays an important role in deciding the thermal performance of radiator devices. When the temperature of the fluid is high, kinetic energy of the particles also becomes high and their collisions increases. A rise in heat exchange is noticed with the increase in number of collisions. Initial temperature of Nano-fluids also affects their viscosity and thermal conductivity. With the rise in temperature, viscosity decreases while thermal conductivity increases. A slight increase in the thermal performance of a radiator with the increase in the inlet temperature of Nano-fluid was observed. Selvam et al. [27] increased the inlet temperature of Nano-fluid up to 35°C and noticed an increase of 10% in the HTC. The HTC kept rising with the increase in inlet temperature and at 45°C, it showed an increase of 21 %. According to literature, the temperature of Nano-fluid has a very nominal effect on the thermal performance of a radiator.  The changed the temperature of Nano-fluid from 56°C to 64° and narrated an increase of only 6.7% in the value of HTC. In the same study, only 4% rise in the value of HTC was observed when the temperature was varied form 45 ◦C to 55 ◦C. A study related to reduction in the friction factor when the temperature of Nano-fluid was increased.  Because at high temperature, the viscosity of the fluid decreases and hence shear stress are also less. With the change in temperature from 35°C to 45°C, a significant decrease in the friction factor was noticed. It was also noticed that a decrease in the value of pressure with the increase in temperature from 35 ◦C to 45 ◦C.

What is the effects of shear force on Newtonian and non-Newtonian fluids

Newtonian fluids are those which flow newton law of viscosity and Non-Newtonian fluid are those that does not follow that rules. The Newtonian fluid viscosity is independent of the shear force applied on the system and its does not change with the change in the shear force for a given temperature. On the other hand the Non Newtonian fluids are those whose viscosity changes with change in shear force. They have four different conditions in this case; Dilatant, Pseudoplastic, Rheopectic, and Thixotropic 

In the case of Newtonian fluid temperature effect the lubrication of steam turbine as increase in temperature decreases the viscosity of the lubricant and thus compromising the lubrication process. For Non Newtonian fluids the temperature and pressure both affects the lubrication process with high pressure increases the viscosity of the fluid.

Organic Rankine Cycle and Vehicle Energy Recovery System

Increase in human population and rapid industrialization has increased the burden on already limited natural resources of energy production like fossil fuel. Industrial process wastes a lot of energy which can be extracted and used to meet human need. Organic Rankine Cycle ORC is one of the processes which can be operated even on low waste energy. Organic Rankine Cycle ORC are also being installed in vehicles to extract waste heat from radiator and on diesel engine exhaust. Fluids for Organic Rankine Cycle ORC either refrigerant or any organic fluid which is selected on basic of ISO 817:2014/AMD 1:2017, ISO 17584:2005 and ISO 11650:1999 standards.

Introduction

Today’s world has a never ending demand of energy due to continuous increase in the world population, rapid industrialization and highly dependence of humans of electrical and thermal devices to meet day to day needs. This never ending demand of the energy has forced the maximum utilization of the limited natural resource for the energy production purpose. The quantity of the fossil fuel is limited but the conventional energy generating systems are not effectively converting and utilizing the energy produce by burning the fossil fuel due to design and process limitation. According to the world energy consumption report about the use of energy by the geographical regions and the use of natural resources to meet the need of energy, the requirement of energy will only increase in future but the availability of the natural resources will decrease. So there is a need of setup which can extract the maximum energy from the existing energy generation or processing systems to meet the world energy need.

 

Figure 1 global energy consumption [(energy matter, 2017).

 

Figure 2 energy consumption by resources (energy.gov, 2017).

One of the methods to extract the maximum energy out of the system is to utilize the energy being wasted into the environment. The wasted energy can be utilized to do useful work which either part of the same process like ventilation cooling system that utilize the low temperature air to cool down the fresh air or the completely separate system like waste energy recovery organic Rankine cycle power plant that utilize the waste heat rejected by a system to heat an organic fluid which then is used to power turbine for energy production. 

Organic Rankine Cycle for Waste Energy Recovery

Organic Rankine cycle power plant are just like a standard Rankine cycle power plant with the only difference is that an organic fluid is used as a working fluid instead of water. Benefit of utilizing the organic fluid instead of water is that organic fluid require very less heat to get vaporize as compared to water which enable this power plant to operate at the low waste heat of different process for energy recovery process. Recently a lot of work is being done to develop small scale organic Rankine cycle power plant which can extract waste energy from low heat waste process. Yinji (2018) worked on the development of the organic Rankine cycle power plant which can run of the waste heat of the vehicle exhaust and engine waste heat of radiator. According to work 30% of the heat developed inside engine is wasted by the radiator which can be utilized by the specially designed organic Rankine cycle to produce and store electrical energy. The electrical energy produce can be utilized to power the vehicle after engine is turn off.

 

Figure 3 ORC integrated in Honda vehicle

Other than the vehicle the internal combustion engine which use diesel as a fuel are used for the electricity production purpose in many industries. These engines waste about one third of the heat produce in engine cylinder through the exhaust gases. Cipollone (2015) worked to develop an organic Rankine cycle power plant which can operate on the exhaust gases of the internal combustion engine to produce electricity using its own turbine and generator system. Regular work conditions (regular load) and off work conditions (maximum load) were tested in this setup and result show the overall efficiency of 4% which includes 4.8% for the regular working conditions and 4.4 percent for off work conditions. As organic Rankine cycle combines two different engineering fields which include refrigerants and power plant setup so two different ISO standards are used to develop, evaluate and improve these power plants. A special purpose turbine will be used for ORC so ISO 10437:2003 revised in 2017 will be used for development, evaluation and improvement of ORC along with the ISO 10494:2018 for noise pollution. As per energy related product directive (ERP) of European commission the minimum efficiency of the machined used for energy recovery system should 75 % by the year 2015 and should reach 80 to 85 % by the year 2017.

Organic Fluids of ORC

Organic fluid used in organic Rankine cycle is one of the most important factors in defining the efficiency of the plant. Kamyar (2015) worked on nine different fluids of organic Rankine cycle which includes refrigerants (R 134a, R123, R227ea, R245fa, R600) and organic fluid (Iso butane, Iso pentane, n pentane and toluene) in order to find the best working fluid. Result show that for the given condition refrigerant R134a and organic fluid n-butane provide best result in terms of overall efficiency of 21.3 % and 21.9 % respectively. Working fluids for organic Rankine cycle are selected based on criteria which includes requirements like environment sustainability, ozone depletion potential, global warming potential, safety (non-toxic, non-corrosive and non-flammable) and properties like critical vapor pressure, critical temperature and thermal stability (Kamyar, 2015). Hung (2019) worked on the development of a small scale organic Rankine cycle for waste heat recovery and investigated the effect of pure refrigerant and mixture of refrigerant on the performance of the organic Rankine cycle power plant. Two pure liquids R 245 fa and R 123 and two mixture 67 % R 245 fa with 33 % R 123 and 33% R 245 fa with 67 % R 123 were test and result show that the performance of pure fluid or mixture depends on the working parameters of the organic Rankine cycle. Working parameters includes working pressure, temperature, pressure ratio and the mass flow rate of the fluid. For refrigerants ISO 817:2014/AMD 1:2017 designate and classify the safety of refrigerant, ISO 17584:2005 defines the properties and ISO 11650:1999 define the refrigerants recycling and recovery. All these refrigerants are used to select the proper refrigerant for ORC. Other than this general ISO rules which includes, ISO 9001:2015 for standard quality, ISO 14001:2015 for environmental protection and ISO 45001:2018 for safety and health standard are used for the advancement and future development of energy recovery organic Rankine cycle.