ENERGY EFFICIENCY OF CO2 HEAT PUMP WATER HEATING SYSTEM IN BUILDING

Sunmonu Gbenga Adewale, Zhongjie Huan Department of Mechanical Engineering, Tshwane University of Technology, Private bag x680, Staatsartillerie Road,

Pretoria West, 0001

ABSTRACTThis paper would highlight the implications of various gas coolers on the CO2 heat pump water heating system and how it can be applied in building for energy efficiency purpose. These highlights will open more lights on different gas coolers that have been used and how effective they have been. However, the gas cooler is made possible due to the fact that the refrigerant is working above the condensation state. Carbon dioxide has been the only refrigerant that can replace the about to be-phased-out refrigerants like CFCs, HCFCs, and HFCs. In this paper, a brief highlight of the advantages of different gas coolers such as; shell and tube; tube in tube, plate and fin, microchannel etc. gas coolers will be highlighted.

Key words: energy management, building, gas cooler, heat exchanger, CO2, heat pump, water heating

1. INTRODUCTIONThe first era of refrigerant started around 1830 and ended up to about 1930, where any available substance that produced refrigeration was used. This era was qualified as “whatever refrigerant in use is to be used” period, since the main selection criterion is based only based on practical system use regardless of its toxicity, compatibility with the environment. Although most of them were toxic and/or flammable, solvents and volatile fluids were then used. With the development of comfort air conditioning and refrigeration to a very large scale and to the public at large, new and safer refrigerants became essential.A second era, dated around 1931 to 1994, manifest by the development and endorsement of the CFCs (Chlorofluoro-Carbon refrigerants) and HCF C (Hydro-ChloroFluoro-Carbon refrigerants) as aerosol propellants, blowing agents and refrigerants. These refrigerants were called phenomenon substances because they met all the criteria required to be used at the time: stability, safety, efficiency, etc. However, these substances (mostly chlorinated compounds proved to be too steady and their long-term effects were totally ignored until their buildup in the stratosphere resulted in the partial destruction of the ozone layer.In order to refurbish the ozone layer, a programmed worldwide phase-out of the CFCs and HCFCs was agreed upon by the Montreal Protocol and its subsequent amendments. As an example, HCFC refrigerant was

taken out to no longer be used in new equipments in Europe since 2004.A third era, dated around 1994 during which HFCs (Hydro-Fluoro-Carbon refrigerants) have been massively introduced in the AC&R industry. Because of their nil contribution to ozone layer depletion, these substances were endorsed as the new marvel solution to the global environmental problems.Nevertheless, the HFCs have been listed amongst the substances contributing to global warming (Greenhouse gases – GHG) in the Kyotol Protocol in 1999, and the prevalent use of these refrigerants is not expected to last.Moreover, the first international action was aimed at banning the consumption of R-134a, one of the “new” refrigerants developed in response to the Montreal Protocol in 1987, which phased out chlorine-containing refrigerants to protect the Earth’s ozone layer.This brief look at history shows that as refrigerants evolved, no global approach has been adopted to simultaneous address the various issues, mainly due to the lack of knowledge.The effect of chlorine in the ozone layer was only discovered in 1975, but also because scientific evidence of the effects of refrigerants was not accepted. So far, mainly synthetic, man-made substances have been used as refrigerants, and their environmental effects could only be identified on the long term [1].Therefore, CO2 has been surviving refrigerants that most researchers have found effective to replace the halocarbon refrigerants that are raising environmental issues today throughout the world. Due to the effectiveness of CO2 that falls mostly on is characteristics such as; is non-toxicity, non-flammability, is abundant availability in the atmosphere and also is technical characteristic such as having the low critical temperature of about 31.10C and the pressure that falls in the range of 73.8 bar, this significant characteristics have made it possible for the CO2 to have an edge over other halocarbon refrigerants and also some percentage of chances in working perfectly in water heating area over is traditional refrigerants due to is non toxicity and non flammability.

As said earlier, water heating has been the strong area for CO2. Different experimental research that has been carried out will be drawn attention to in this paper. For instance, the measurements of the coefficient of performance of the CO2 taking into account its temperature at different

conditions and also characterized the optimum high side pressure of the CO2 heat pump water heating system. The results gotten from this research have been promising by exhibiting COPs at the range between 3-5 according to Neksa. [2] The volumetric heat capacity is said to be five to eight times higher than that of conventional refrigerants illustrated by (Groll and Kim), creating room for more

compact equipments and systems. In supercritical condition, instead of having a constant temperature condensation process, the refrigerant will be cooled from a vapor state to a liquid state in the component known as gas cooler. This temperature condition profile is demonstrated according to Groll and Garimella [3] in a T-h plot shown in Fig. 1.

Fig. 1, Transcritical cycle T-h diagram.

A review provided by (Groll and kim) closed to four years ago identified many advanced technologies that can be included into the cycle increase system efficiency in various applications that include; residential air conditioning and heat pump systems, but water heating has been the most promising area in the application of CO2

transcritical cycle [4].

Special design considerations are required for the gas coolers due to the high operating temperature and the temperature glide due during supercritical cooling of CO2. In order to achieve maximum coefficient of performance the system, the gas cooler must be designed in a kind of way as to minimize the approach temperature between the heat sink and refrigerant. As CO2 gas cooler design steps from a conventional tube and fin geometry to more compact microchannel designs, which allow high operating pressures (in excess of 120 bar) and improved heat transfer coefficients, experimental investigations and models for heat transfer and pressure drop of supercritical CO2 in these heat exchangers geometry are required as described by Brian and Garimella [5].

Many experimental and analytical studies have been conducted and performed on the performance of transcritical CO2 cycles for water heating and space conditioning. Neksa used tube in tube gas cooler design to demonstrate COPs up to 4.3 when heating water from (9 to 60)0C [2]. Richter also developed a prototype split air-to-air CO2 system intended for residential heat pump applications. The system was designed to fit in the footprint of a commercial R410a system and used an aluminum microchannel heat exchanger for the gas cooler

component. They now compared the experimental system to a commercial R410a system and noted that the increased capacity of the CO2 system at low ambient temperatures resulted in higher annual heating efficiency [6]. A simulation model for air source CO2 heat pump water heating by Laipradit, the evaluation of the system performance was carried out under varying temperature of the gas coolers, speed of the compressor and the ambient temperature of the evaporator. Tube in tube gas cooler was designed for the system [7]. Similar to previous studies, Sarkar and company also developed and validated a simulation model for simultaneous water heating and cooling, the water coupled gas cooler was a tube in tube design, with an inner tube diameter of 6.35mm and thickness of 0.8mm, outer tube diameter of 12mm and thickness of 1mm. The refrigerant heat transfer co efficient was modeled using the correlation designed by Pitla which finally showed a maximum deviation of 15% from experimental results [8].

Experimental study of an air-coupled microchannel gas cooler was also conducted by Zhao and ohadi, in which the gas cooler considered used microchannel tubes with a diameter of 1.0 mm. the gas cooler was composed of several microchannel slabs with which each of them has a refrigerant heat transfer of 0.46m2. Also in this system, two parallel rows of five slabs are connected serially. Tests were also conducted at refrigerant mass flow rates in the range of 15 to 40 g s-1, refrigerant inlet pressure between 69 to 125 bar and refrigerant inlet temperature from 79 to 120 0C. The air inlet temperature was set at 210C with a mass flow rate at 520 g s-1. The experimental heat duties ranged from 4 to 8 kW given the air and refrigerant balances more or less in 3% [9]. A relative performance evaluation of an air-coupled gas cooler was also conducted by Hwang similar to the one conducted by Zhao and Ohadi, instead of a microchannel heat exchanger, a more conventional tube and fin heat exchanger with the ID of 7.9 mm was tested. The heat exchanger had 3 rows and 18 tubes in cross flow with the incoming air. Air inlet temperatures were set at 29.4 and 35 0C with frontal velocities of 1.0, 2.0 and 3.0 m s-1. Refrigerant mass flow was set at 38 and 76 g s-1 with gas cooler inlet pressures of 90, 100 and 110 bar. The refrigerant inlet temperature to the gas cooler was not fixed and was allowed to vary with high-side pressure and other system operating conditions. The heating capacity of the gas cooler ranged from 6 to 14 kW. For every refrigerant mass flow and pressure, the

capacity increased as the frontal air velocity increased. However, at the 38 g s-1 refrigerant flow condition, the air side then showed signs of temperature pinch as the velocity increased from 2.0 to 3.0 m s-1. With the fixed gas cooler size, higher average temperature differences were seen at the higher refrigerant mass flow rates. Due to the higher refrigerant outlet temperature, specific enthalpy differences across the gas cooler for the 76 g s-1 cases were seen to be 57-81% of those at 38 g s-1. It was also found out that by doubling the mass flow rate, the heating capacity increased by 14-62% [10]. A development of a segmented model of an air coupled, parallel-serpentine gas cooler consisting of three passes of 13, 11 and 10 microchannel tubes with louvered fins was designed by Yin [11], where there were 1 circular ports per tube with ID = 0.79 mm. the refrigerant heat transfer area in each segment was calculated from the Gnielinski [12] the correlation and pressure drop taken from Churchill [13]. Conduction between the tubes was neglected, and the incoming air was assumed to have uniform velocity and temperature. Local heat duty in each segment was calculated based on the log mean temperature difference (LMTD) method. The simulation model was validated with 358 data points corresponding to 48 different indoor/outdoor operating conditions. They showed agreement in predicted heat duty of more or less about 2%, while refrigerating pressure drop was systematically under predicted by approximately a factor of 3, which was attributed to potential manufacturing defects in the microchannel tubes. A model for a near-counter flow for air-coupled CO2 gas cooler using serpentine refrigerant tubes and louvered fins on the air side was designed by Garimella. The geometry required the tracking of refrigerant and air temperature along both the length and width of each refrigerant tube. The simulated gas cooler had 36 tubes with six 1.905 mm diameter circular ports per tube. The effectiveness-NTU method was employed to obtain the local heat duty in each segment [14]. A correlation was used to predict the local heat transfer coefficient. With a volumetric air flow rate of 0.334 m3 s-1 and a refrigerant flow rate of 31 g s-1, the model predicted a heat duty of 6.97 kW and an approach temperature difference of 5.3k this prediction was made by Krasnoshchekov [15]. This analysis was extended by Garimella to include axial conduction losses due to heat transfer between adjacent tubes through the louvered fins, and demonstrated a reduction in heat duty of 13% at an unlouvered fin fraction of 30% [16]. A segmented model of an air-coupled microchannel gas cooler into a split-system simulation model was developed by Ortiz, which the gas cooler design is made in a cross flow, extruded aluminum design. The system heat duty is solved in iterative way, where an initial duty tube wall temperature is estimated, after which the heat duty is determined from the effectiveness-NTU method and from evaluating the local [17] Refrigerant heat transfer efficiency is calculated using the modified Gnielinski correlation which

falls at high mass flux of pettersen result of (G > 350 kgm -

2s-1) [18] and correlation at lower mass fluxes gotten from Petrov and Popov [19]. And refrigerant pressure was then calculated in each segment applying the friction factor correlation of Kuraeva and Protopopov [20]. Furthermore, entrance and compressibility effects were considered. The individual model was validated with Zhao experimental data, which featured microchannel tubes with ten 1.0 mm diameter ports per tube [21]. The model predicted heat duty within 3% of CO2 side measurements and 5% of air-side measurements, with a poorer agreement at low refrigerant mass fluxes attribute to the potential inapplicability of the modified Gnielinski correlation.

2. GAS COOLERS OF CO2 HEAT PUMP WATER HEATING SYSTEM

Due to the rate of energy that is consumed in buildings, different methods have been taken in solving the energy usage in building. Therefore, this area will be looking at another alternative ways of saving or managing the energy consumption in building using different gas coolers. Although parts of this gas coolers have been technically described earlier in the beginning of this papers where different researchers have used it for a lot of application, there is need to focus in is application area, specifically in building.

2.1 SHELL AND TUBE GAS COOLER

This gas cooler is built generally of round tubes mounted in a cylindrical shell with the tube axis parallel to that of the shell. Fluid flows inside one tube and the other flows across or along the tubes. The main components of these heat exchangers are tubes, shell, front head, rear-end head, baffles, and tube sheets. Various internal constructions are used in shell-and-tube exchangers, depending on the desired heat transfer and pressure drop performance with the method to reduce the performance of thermal stress, preventing leakages, easing cleaning, to contain operating pressures and temperatures, controlling corrosion, to accommodate highly asymmetric flows etc. the shell and tube gas coolers are organized and constructed in accordance with the widely used TEMA (Tubular Exchanger Manufacturers Association) standards (TEMA, 1999). In Fig 2, the diagram below shows typical image and parts description of shell and tube gas cooler [22].

Fig 2, a shell and tube gas cooler with is functioning parts

With the rate of energy consumed on building today, there is need to manage the energy demand and supply in a building. Energy is basically consumed by building occupants on water heating for; washing; bathing; cooking etc. these three areas which energy are consumed need to be considered thoroughly.

However, with the use of shell and tube gas cooler, energy can be saved if different parameters and consideration that will be highlighted can be considered, such parameters as; the inlet and outlet temperature of the gas coolers, water mass flow rate in the gas coolers, the thermal conductivity etc. if these areas are focused on some percentage of energy can be saved by the gas cooler which will at the end contribute to the energy efficiency or energy management in building.

2.2 MICROCHANNEL GAS COOLER

This gas cooler is commonly used for heating homes, the heat exchanger furnace has been used in many countries in part of the world today and is been known for an effective household heating device in the Europe and America today. Parts of these gas cooler advantages are that it can work with high temperature and pressure. The image of the gas cooler in displayed in fig 3 below [23].

Fig 3, a typical microchannel gas cooler

2.3 VACUUM BRACED PLATE GAS COOLER

This gas cooler is a new development in heat exchangers technology because it results in parts with extremely strong joints and with no residual corrosive flux. It is a process in which two base metals, such as aluminum plates are joined together using a filler metal that has a melting point below that of the base metal. The filler metal, also known as a braze alloy, is drawn into the closely mated parallel surfaces of the aluminum plates by capillary action. The features of this gas cooler includes, uniform heating, tight temperature control, no post cleaning processes, and process repeatability. This gas cooler consists of;

aluminum cold plates, plate-fin heat exchangers, flat tube heat exchangers. Fig 4 shows the typical image of a vacuum brazed gas cooler [24].

.

Fig 4, a descriptive brazed plate gas cooler image

The technical vacuum advantages are is high strength, due to is design approach it does not give room to any leakage, it also proof the pressure of up to 800 psi and burst pressure up to 1300 psi, and does have a temperature resistance of about 1760C

2.4 PLATE AND FIN GAS COOLER

This type of gas cooler uses slot in passages containing fins to increase the effectiveness of the heat exchanger’s unit. The design includes various fin configurations such as straight fins, offset fins and wavy fins.

Plate and fin gas cooler are usually made of aluminum alloys which provide higher heat transfer efficiency. The materials enable the system to operate at a lower temperature and reduce the weight of the equipment. Fig 5 below shows the typical image of a plate and fin gas cooler

Fig 5, described a typical image of plate and fin gas cooler that is applicable for water heating in building

The benefits of the plate and fin heat gas cooler are such that it transfers heat in high quantity most especially in gas treatment, it also includes is large heat transfer area and also light in wait [25].

2.5 HYDRONIC GAS COOLER

This gas cooler has been around over the years, and is said to have been effective for is heat transfer ability, and can work with a high temperature and high pressure. This can also be used to reduce the rate of energy consumed in a building. In fig 6, the hydronic gas cooler image is displayed [26].

Fig 6 shows and describes the functional parts of the hydronic gas cooler.

3. IMPLICATIONS OF GAS COOLERS IN BUILDING

The gas coolers described above in references to the way each of them is being applied, and if all parameters like the

inlet and outlet temperature of the gas cooler, the water mass flow rate, the water inlet and outlet temperatures, refrigerant inlet temperature are considered the energy consumption in building can be reduced to a level of percentage. For instance, if the inlet temperature of the gas cooler changes the condition of all other components in the CO2 heat pump water heating will also change with respect to the rate of heat transfer. Therefore, since energy is being consumed massively in building basically on water heating, there is need to save or manage the amount of energy supply per building. With the aid of the above points, we can now say that with the effective application of gas cooler in CO2 heat pump water heating, some amount of energy can be saved in other to also assure or increase the rate of energy supply to other locations.

Furthermore, with an adoption of these gas cooler in buildings, new development can start taking place, by considering the gas coolers referred to and that have been described in this paper, with the help of research. South Africa can choose to adopt the better solution in reducing the rate of energy consumption in building by considering various gas coolers that have been researched on.

The suggested areas which the gas coolers can have an effect or be applied to are the most noticeable parts of the energy consumption in building such as; cooking, bathing, and washing. These areas are basically connected to water heating application and would be briefly described in the following paragraphs.

3.1 ENERGY MANAGEMENT IN ‘COOKING’

The rate of energy consumed in cooking is clearly high, if simple survey is made in a local restaurants in South Africa today, we would see that energy is majorly

consumed on any meal that are prepared in a local restaurants. Therefore, there is a particular rate of energy that is consumed only on cooking with the numbers of restaurants we have in the typical streets in South Africa, this shows that the rate of energy consumed on building could be high.

3.2 ENERGY MANAGEMENT IN ‘WASHING’

There is need for energy management in washing areas as well, this in effects to the local laundries shops in South Africa or when we are able to account of the washing machines in different homes today, we will see that the rate of energy consumed on those washing machines is also high but might be low compared to the rate of energy consumed in cooking.

3.3 ENERGY MANAGEMENT IN ‘BATHING’

This area of energy consumption area is another common one that is known by many of us today. If the rate of energy consumed in bathing is measured three times a day, for instance if it is measured in the morning, afternoon or night, we would see that the rate of energy that will be consumed in the morning will be higher due to the numbers of people that are using their geysers in our various homes. If one of the gas cooler can be applied to the homes that use geysers, some percentage of energy supply would be saved for other usage.

In conclusion, the rate of energy consumed on building can be saved if these three areas which energy can be consumed in building are considered. With the application of the gas coolers technologies that have been introduced and suggested in this paper, the rate of energy consumption in building would be reduced and if also some of the parameters that are referred to in this paper are considered which some technologists considered too before designing the gas cooler technology for water heating.

4. CONCLUSION

With all the points explained in this paper, there are things that are discussed that need to be taken into considerations, and the following points are:

(i) For the consumption of energy to be reduced there is a need to consider the type of technology to adopt in solving the heat transfer rate in the gas coolers.

(ii) With the aid of different models that are developed, results have shown that gas coolers are effective for domestic water heating.

(iii) Evaluating the performance of each gas coolers have been recommended with the help of references that are made available.

(iv) Suggestions have been made on how to reduce the rate of energy consumption in South Africa.

REFERENCES

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AUTHOR1

PRESENTER: This paper will be presented by Sunmonu Gbenga Adewale

Sunmonu Gbenga Adewale holds a degree in Mechanical Engineering at Lagos State University and presently pursuing his Masters degree in Mechanical Engineering at Tshwane University of Technology. He has also worked in Nigerian Brewery as a technician in the company. He is also member of the Nigeria institute of mechanical engineers (Lagos branch).