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ENERGY EFFICIENCY OF CO2 HEAT PUMP WATER HEATING SYSTEM IN BUILDINGSunmonu Gbenga Adewale, Zhongjie HuanDepartment of Mechanical Engineering, Tshwane University of Technology, Private bag x680, Staatsartillerie Road, Pretoria West, 0001 ABSTRACT This 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. INTRODUCTION The 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 (ChlorofluoroCarbon refrigerants) and (Hydro-ChloroFluoroCarbon 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 and was agreed upon by the Montreal Protocol and its subsequent amendments. As an example, refrigerant was taken out to no longer be used in new equipments in Europe since 2004. A third era, dated around 1994 during which (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 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 Earths 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. 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

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-toair 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 f


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