UNIVERSIDADE FEDERAL DE SANTA CATARINA Centro Tecnológico Departamento de Engenharia Mecânica Coordenadoria de Estágio do Curso de Engenharia Mecânica CEP 88040 970 Florianópolis SC Brasil www.emc.ufsc.br/estagiomecanica INTERNSHIP REPORT 1/3 CREATIVE THERMAL SOLUTIONS Intern: Beatriz Mibach Supervisor: Roberto Pereira Advisor: Claudio Melo Urbana, October 2010.
THE COMPANY Creative Thermal Solutions (CTS) is a research and consulting company located in Urbana, central Illinois. The main activities developed by the company include experimental investigation and evaluation of refrigeration systems and components, simulation of components and entire systems, improvement and optimization as well as fundamental research. Besides that, the company provides validation and analysis services as well as fabrication of components and complete facilities for measurements. In addition, education and training courses are provided for practicing engineers. The company was founded in 2003 by Predrag S. Hrnjak, a Professor of refrigeration, air conditioning and environmental technology of the Air Conditioning and Refrigeration Center (ACRC) at the University of Illinois at Urbana Champaign (UIUC), in partnership with Professor Will Stoecker. In the beginning, only five employees worked at CTS, and the company building was an old house. At that time, four air chambers and two closed loop wind tunnels were available. In 2006 the building was extended. A new building was added to the company, housing eleven new environmental chambers, workshops for metal and wood processing, an electrical engineering lab and a conference room. Already in 2008, CTS expanded again. An office and small conference room were added. Soon after that, the original house was torn down and connected another hall in order to create space for new growth rooms, a laboratory for calibration of measuring instruments and components of a refrigeration cycle. CTS now has an area of approximately 1900m 2. By the end of 2010, the area is going to increase in three other buildings, now under construction. New climate chambers and laboratories will create more room to supply the incoming orders. The range of expertise with full refrigeration systems spans from breadboard constructions to complete units. Certain projects have even covered the full spectrum from the laboratory to the application (frequently vehicle), where system optimization made on a mobile AC breadboard system was implemented and compared to the performance of the real vehicle. Residential split systems, military unitary systems, and commercial cassette systems have also been explored in a wide variety of configurations to evaluate the application of laboratory test data to the design process. Individual system components have also been examined for the purpose of analysis, design, and optimization. Distribution in heat exchangers, their headers, efficiency and design of various flow separators, destructive and nondestructive testing, acoustic issues in heat exchangers and expansion devices, as well as visualization of flow through novel expansion devices and compressor discharge are some of the subjects of investigations that have been performed. Studies of numerous alternative Page 2 of 21
refrigerants have also been made, both natural and man made refrigerants, as well as precise measurement of leakage rates of various refrigerants. The engineering staff is qualified by research, design, and close cooperation with industrial needs. The engineers hold advanced degree levels in a variety of engineering disciplines, predominately mechanical, so their experience provides a very good environment for learning despite working in the same field as previously during graduation. All the internship work will be developed in the Research and Development division, which comprises the core of the company's business. Page 3 of 21
SCHEDULED ACTIVITIES The internship program is divided into 5 phases, each one with a different objective. Phase 1: The first activities of the training plan were conducted in order to become familiar with the specifics of transcritical CO 2 heat pump water heater systems. The main objective is to develop the ability to realize the differences in comparison to the conventional systems already studied during education at the Federal University of Santa Catarina (UFSC) in Florianópolis, Brazil. Phase 2: The purely mechanical system built at first shall be instrumented and a data acquisition system added. The testing apparatus should be complete by the end of this phase, giving the chance to assess the quality of the work. A functioning test apparatus at the end of the phase will be a rewarding experience for the trainee. Phase 3: The next phase goes beyond building and operation of the transcritical CO 2 heat pump water heater. The main purpose is to analytically evaluate the experimental results obtained with the test stand earlier designed, built, instrumented, debugged, and operated. Phase 4: The next phase is intended to increase the ability of conducting computational investigations in the system combined with the analysis of the components designed in the previous phases of the work. Phase 5: By the end of the internship program, a report will be written for the company in order to evaluate the ability regarding technical documentation and communication. Page 4 of 21
INTRODUCTION Over the last decades the Refrigeration, Air Conditioning and Heat Pump industry has been forced through major changes due to the restriction on refrigerant fluids. The change to ozone friendly substances was the first big revolution on the industry, and the upcoming HFC were expected to be the permanent solution. However, nowadays these fluids are on the list of regulated substances due to their high GWP. As a concern to these environmental issues, two agreements have been signed, the Montreal protocol in 1987 for banning production and consumption of ozone depleting compounds and the Kyoto Protocol in 1997 for reducing consumption of global warming substances. These events open an interesting research field on eco friendly refrigerants, most of them desired to be natural. Among the available options, carbon dioxide (CO 2, R 744) has excellent properties to be used as a refrigerant, such as: non toxic, non flammable, low priced, excellent thermodynamic properties and the opportunity to design compact system components due to its high density. Carbon dioxide is not a new refrigerant; rather, it was rediscovered in the early 90's. The primary application was in marine refrigeration, a field where CO 2 dominated as a refrigerant until the mid 50 s. In Europe, CO 2 machines were often the only choice due to legal restrictions on the use of toxic or flammable refrigerants like NH 3 and SO 2. However, as the CFC fluids were introduced in the 30 s, the old working fluids were replaced in most applications. Although the major argument for the replacement was improved safety when compared to fluids like ammonia and sulfur dioxide, CO 2 was also displaced by this transition to CFC. The factors that contributed to the disuse of CO 2 as a working fluid were the problems found in high pressure containment, capacity and efficiency loss at high temperature, aggressive marketing of CFC products, low cost tube assembly in competing systems, and a failure of CO 2 system manufacturers to improve and modernize the design of systems and machinery. With the CFC problem becoming a pressing issue in the late 80 s, the whole industry was searching for viable refrigerant alternatives. In Norway, Professor Gustav Lorentzen believed that the old refrigerant CO 2 could have a renaissance. In 1989, he filed an international patent application, regarding the transcritical CO 2 cycle system, where the high side pressure was controlled by a throttling valve. One of the intended applications for this system was automobile air conditioning, a sector that dominated the global CFC refrigerant emissions, and also an application where a non toxic and nonflammable refrigerant was needed. Page 5 of 21
Based on results published since the beginning of the 90's, the interest in CO 2 as a refrigerant increased considerably throughout the nineties, as shown in Figure 1. Figure 1: Number of papers on CO2 as a primary refrigerant presented in Gustav Lorentzen Conferences along the years. Page 6 of 21
R744 ON HEAT PUMPS The CO 2 properties are well known, although they are quite different from most other refrigerants. The primary distinction between CO 2 and other refrigerants is the fact that it has a low critical temperature of 31 C and a very high critical pressure of 73.8 bar. This characteristic leads to different considerations when designing vapor compression systems using CO 2 as its working fluid, since most of the time the system will operate close to its critical region when rejecting heat to the ambient. During summer or in tropical countries, the outdoor air temperature will be close to the critical temperature of CO 2 most of the time, leading to transcritical operation. Owing to the higher average temperature of heat rejection, and the larger throttling loss, the theoretical cycle work for CO 2 increases under the same ambient conditions, if compared to most HFCs as can be seen in Figure 2. Figure 2: T s diagram for a transcritical CO 2 cycle and its losses. The most promising result of the application of transcritical cycle has been for hot water heat pumps, due to its unique characteristics. In this situation, the transcritical cycle is often more efficient cycle compared to subcritical cycle. The energy released during the gas cooling process that is used for water heating, makes it possible to obtain high temperature water, which is difficult to be achieved in a subcritical cycle. Hot water up to 90 C can be achieved without major operating problems, but a small loss in efficiency. Heat pumps using CO 2 as a refrigerant are well suited for heating sanitary water, which requires temperatures Page 7 of 21
above 80 C, increasing the temperature in a single passage. A conventional heat pump, for example a heat pump using R 134a as a refrigerant, requires a high condensing temperature, once it releases the heat at a constant temperature, reducing the COP of the system. Despite the reduced COP, the overall efficiency of heating may then be increased in multi energy conversion systems, due to reduced supplementary heat. In this respect, the system energy efficiency is calculated as heating system COP, i.e. the ratio of the system s heat output plus supplementary heat, to heat pump energy use plus supplementary heat input. By reducing the need for supplementary heat the system COP of a CO 2 heat pump may often be higher than the baseline. The reason is that the baseline system does not maintain its heating capacity at lower heat source temperature, and more supplementary heat is needed. The high process efficiency is partly due to good adaptation of the process to the application, but also due to the good heat transfer characteristics of CO 2. Heat rejecting process in transcritical cycle takes place in supercritical region where temperature and pressure are independent properties. As the gas cooling process is performed around the critical region, the thermophysical properties of CO 2 vary greatly. The temperature characteristics of the transcritical cycle match the temperature profiles of both the heat source and the heat sink, resulting in small heat transfer losses and high efficiency, as shown in Figure 3. Figure 3: T s Diagram of a transcritical CO 2 cycle used for water heating. Page 8 of 21
Heat rejection that occurs in single phase region is an ideal condition for water heating process with large temperature lift. In water heating applications, the inlet temperature is often quite low, and the CO 2 temperature glide during heat rejection in a triangular process with low inlet temperature is ideal for heating of service water to high temperatures. Gas cooling processes occurring in supercritical region will follow isobar lines with decreasing temperature monotonously. Pressure drops in heat exchangers become higher as the pressure level increases, and this gives a possibility of improving heat transfer through higher flow velocities in high pressure systems. This is of particular importance for single phase heat transfer in the gas cooler of CO 2 systems. High pressure and proximity to the critical point gives increased specific heat, again leading to improved convective heat transfer. Heat pump water heaters are today widely accepted in both Japan and Europe where energy costs are high and government provides incentives for their use. In the other hand, the acceptance of such products in the US has been slow due to a few issues related mostly to performance, reliability and operating costs. Several Japanese manufacturers put CO 2 heat pump water heaters on the market in 2001 and 2002. The typical rated heating capacity and COP are 4.5 kw and over 3.0, respectively, based on the JRA Standard 4050 which provides test standards and terminologies of small CO 2 heat pump water heaters. These water heater systems produce hot water using cheap late night electric power, and stores hot water in a tank for daytime use. These CO 2 heat pump water heaters used several different types of system and components design in order to provide the necessary cost effective compact systems. Page 9 of 21
CO2 SYSTEM DESIGN Heat pumps using CO 2 as working fluid may achieve high cycle efficiency due to the favorable reasons previously mentioned. However, this requires that the individual components, the heat pump unit as well as the secondary systems on the cold and hot sides are designed to utilize the particular properties of the fluid. Compressor As the compressor is one of major components of air conditioning and refrigeration systems and has an important effect on the system performance, compressor technology for the CO 2 transcritical systems has reached an advanced level after years of development. The vapor pressure of CO 2 is higher than conventional refrigerants and the transcritical cycle operates at much higher pressures than the conventional vapor compression systems. Therefore, compressors in CO 2 systems will operate at high mean effective pressure and with large pressure differentials, but the pressure ratios will be quite low due to operation close to the critical point. In a theoretical approach, the displacement of the R 134a machine is about 6 times larger, and the pressure ratio is 5 as compared to 3 with CO 2. Re expansion losses are much smaller in CO 2 process. Owing to the higher pressure level and the different shape of the p V diagram, as seen in Figure 4, the negative effect of valve pressure drops tends to be small in CO 2 compressors, thus giving higher efficiency. Figure 4: p V compression diagram for both R 744 and R 134a. The compressor for the transcritical CO 2 cycle requires thicker walls to contain the high operating pressure, but since the volumetric capacity of the fluid is large, the compressor itself will actually be smaller than refrigeration compressors for the same capacity using conventional condensing refrigerants. For that reason, the compressor displacement is often reduced by 80 85% for a given Page 10 of 21
capacity. Compressor and heat exchanger size and weight reductions seem possible due to the reduced refrigerant side volumes and cross sections. Internal leakage losses in valves and as piston blow by were initially expected to be a problem with the large pressure differentials in CO 2 compressors, but across the years it has been shown that these losses account for less than 1% difference in cylinder charge compared to an ideal zero leakage model, leading to the conclusion that the influence of leakage can be neglected in properly designed lubricated reciprocating compressors. Heat Exchangers Gas Coolers Compared to the conventional vapor compression refrigeration cycle, the function of the gas cooler in the transcritical CO 2 cycle is similar to the condenser. But in the condenser, the phase change heat transfer is occurring, while in the gas cooler the single phase forced convective heat transfer is taking place. So the heat transfer mechanism for the two processes and their heat transfer performances are different. In the transcritical CO 2 cycle, system performance is very sensitive to gas cooler design. A small change in refrigerant exit temperature can produce a large change in gas cooler exit enthalpy because of the big changes in specific heat close to the critical point. This indicates that a CO 2 transcritical cycle is so sensitive to the refrigerant exit condition that a counterflow configuration is important for the gas cooler to exploit the large refrigerant side temperature glide. Gliding temperature can be useful in heat pumps for heating water or air. With proper heat exchanger design the refrigerant can be cooled to a few degrees above the entering coolant (air, water) temperature, and this contributes to high COP of the system. Due to the higher pressures, optimum compact heat exchanger designs for CO 2 generally tend to use small diameter flow channels, in many cases based on extruded multiport tubing with parallel flow of refrigerant in several tubes and flow channels. In some cases it may be more economical to use conventional flat fin/round tube heat exchangers with small diameter tubes. Also in this case, the internal diameter will most likely be smaller than for conventional refrigerants. Compared to conventional flat fin/round tube designs, microchannel heat exchangers increase refrigerant side area by about a factor of three, and have far less air side pressure drop due to the streamlined profile presented by the tubes. The flat tubes enable higher face velocities that increase the air side heat transfer coefficient. Page 11 of 21
Differences in fan and pump power requirements should also be considered when comparing CO 2 and other condensing fluids, particularly since air side pressure drops and air flow rates are likely to be different, thus giving differences in fan power. In a comparison with heat pumping systems using secondary fluid circuits, the pumping power should not be overlooked, particularly if a directevaporation CO 2 system may offer the same environmental and personal safety. Another issue in compact gas cooler design is internal conduction due to large temperature differences across small lengths. Internal conduction in fins, tubes and manifolds may lead to performance reduction. Solutions to avoid these problems include splitting of fins, use of several heat exchanger sections, and careful design of manifold geometries. Evaporators Microchannel evaporators are currently the subject of research because of the potential performance improvements obtainable from further increases in refrigerant side area and higher face velocities. Brazed plate designs are also used, commonly using aluminum plates patterned with chevrons or dimples to facilitate refrigerant distribution between the upwind and downwind sides, thus usually less effective. The enhanced performance is attributable to the fact that microchannel tubes are thinner than the brazed plates, allowing the same air volume to pass with greater face velocity through a deeper heat exchanger, without a pressure drop penalty. Evaporator pressure drop leads to reduced temperature differences due to the corresponding drop in saturation temperature. Nucleate boiling heat transfer is also affected to a large extent by pressure, since the wall superheat needed to initiate boiling becomes lower as the critical pressure is approached. In evaporators the biggest challenge is to distribute the two phase flow uniformly through the multiple parallel circuits. The current strategy for dealing with this problem is to find ways of eliminating it, such as flash gas bypass.tube fin evaporators usually require flow redistribution in order to achieve compatible flow velocities and improve the heat transfer. Another peculiarity of the CO 2 cycle is the smaller influence on heating capacity and COP by varying evaporating temperature, which enables the system to maintain a high heating capacity at low ambient temperature. Expansion Devices High side pressure in a transcritical system is mostly determined by refrigerant charge and not by saturation pressure. The system design thus has to consider the need for controlling high side pressure to ensure sufficient COP and capacity. Page 12 of 21
COP of the transcritical carbon dioxide system is significantly influenced by the gas cooler pressure, and interestingly non monotonically. In these systems, for fixed gas cooler exit temperature (dependent on external fluid inlet temperature and gas cooler design), as the pressure increases the COP increases initially and then the added capacity no longer compensates for the additional work of compression and COP decreases. Hence, there exists an optimum pressure where the system yields the best COP and the knowledge of the optimum operating conditions corresponding to maximum COP is a very important factor in the design of a transcritical carbon dioxide cycle. The optimum discharge pressure and corresponding performance are dependent on evaporator temperature, gas cooler exit temperature, degree of superheat and compressor design. However, the studies showed that the effects of first two parameters are more pronounce on optimum discharge pressure than the others. For these reasons, it is common to use variable expansion devices in transcritical cycles, allowing the system to adjust the operational point close to the maximum achievable COP for each condition. Accumulator In transcritical R 744 systems, an expansion valve is often used to control the high side pressure rather than to control a constant amount of superheat at the evaporator exit like in thermal expansion valve systems. The liquid surplus may be an advantage when the high side pressure is raised, to avoid drying up the evaporator. As a result, the evaporator of R 744 systems is typically operated in flooded mode, meaning that the refrigerant flow at the evaporator exit still contains a certain amount of liquid refrigerant which then enters the accumulator. The accumulator is installed to store excessive refrigerant, since the mass stored in the system components strongly depends on the operating conditions. High side pressure is controlled by adjusting the expansion valve, temporarily changing the balance between compressor mass flow rate and valve flow rate. By reducing the valve opening, a temporary reduction in the valve mass flow rate gives refrigerant accumulation in the high side, and the pressure rises until a new balance point between valve flow rate and compressor flow rate is found. The vapor fraction at the evaporator outlet may temporarily rise while pressure is rising, and the additional high side charge is transferred from the low side buffer. By installing an internal heat exchanger in the suction line, the liquid is evaporated before the compressor inlet, and the COP is improved at high heat rejection temperature. Page 13 of 21
System Assembly The design of heat pump systems is strongly dependent on the components placing and distribution, therefore it becomes very important to consider that when designing such systems. Low side refrigerant line diameters are typically reduced by 60 to 70% compared to HFC systems, due to the higher vapor density and flow velocity. High side piping dimensions may also be reduced. Assuming a wall thickness that is more or less the same as in HFC piping of equal capacity, the pressure capability will be sufficient for CO2 due to the reduced inner diameters. The high working pressure and favorable heat transfer properties of CO2 enable reduced tube diameters and small refrigerant side surface areas. Since these reductions may give room for more airside surface per unit core volume, the compactness can be increased. Depending on the heat exchanger type, the size and mass of high pressure manifolds or headers may offset some of these advantages. Innovative solutions that benefit from reduced refrigerant side volume are therefore needed. Heat pump storage systems can be assembled with external or internal gas coolers. When using external gas cooler, it is connected to a single shell storage tank by means of a closed water loop. A pump circulates the water from the bottom of the tank through the gas cooler and back to the top of the tank. An integrated gas cooler can be designed as desired and mounted inside the tank, at the tank surface or in a thermosyphon unit. Advantages can be seen in any of the options, depending on the operating conditions of the systems Improvement in storage may be achieved by thermal stratification; that is, water of a high temperature than the overall mixing temperature can be extracted at the top of the container and water of a lower temperature than the mixing temperature can be drawn off from the bottom to run the gas cooler with high efficiency. In practice, perfect stratification is not possible since the water entering the tank will cause a certain amount of agitation and mixing. Moreover, there would be a certain amount of diffusion from the entering water (to the stored water) before it reaches the appropriate density level. Having obtained good thermal stratification by eliminating mixing, it is equally important to maintain the temperature layers. Due to the heat losses from the surface of the storage tank, the temperature of water near the vertical walls is lower, leading to natural convection currents that destroy the temperature layers. In order to maintain stratification over long time intervals, the tank should be provided with extremely good thermal insulation or with special installations. In the case of thermal stratification in storage, an improvement in both storage and collector performance is achieved. There are three main advantages: a) in a thermally stratified hot liquid tank, liquid at a higher temperature than the overall mixed mean temperature can be extracted at the top of Page 14 of 21
the tank, thereby improving the satisfaction of the load; b) the efficiency in the heat exchanger is improved since the collector inlet fluid temperature is lower than mixed mean storage temperature; c) the stratified storage can be at a lower mixed mean temperature for any given temperature requirement from the load, thereby reducing heat losses from the storage tank. The mean collector fluid temperature will, therefore, not necessarily be lower for a low flow stratified system than for the fully mixed system. This implies that the collector efficiency is not necessarily enhanced through operating with storage stratification. Page 15 of 21
TESTING AND EVALUATION The project consists of the conversion of an R 134a heat pump water heater into a CO 2 heat pump water heater. The changes in the original package aim to reach both high efficiency and great levels of compactness. The heat pump extracts the available heat from the ambient air and produces heat suitable for heating water in a storage tank. The objective is to provide higher energy efficiencies than condensing fluid water heaters and electric water heaters on the market today. By utilizing CO 2 as the refrigerant and significantly improving cycle efficiency, performance of the system can be improved. CO 2 is an excellent HPWH refrigerant due to the reasons pointed in the review. The expected heating capacity for the unit will be approximately 40 kw. The performance of the CO 2 Heat Pump will be compared against a state of the art R 134a heat pump system. Therefore, a whole new system was designed. The compressor was selected specifically to operate on the transcritical CO 2 cycle. It is a semi hermetic compressor, which has the capability to deliver the capacity required by the system, which is similar to the R 134a compressor used on the original unit. No major problems were found to fit the compressor in the original package. A brazed plate heat exchanger is being used as a gas cooler. The component has the nominal capacity of 40 kw and working pressure of up to 140 bar on the CO 2 side. Tests conducted in the company ensure that the component can operate at a pressure up to 200 bar. The water side of the heat exchanger is rated to a working pressure that can reach up to 30 bar. It should also be emphasized that the gas cooler provides nearly 50% of reduction in heat exchanger volume from the condenser installed in the baseline. The evaporator designed has close to half of the size of the original R 134a evaporator, but the width was kept the same to make it fit into the existing enclosure. An accumulator is being used at the evaporator outlet. This component consists of a container, a splash plate, a coolant inlet, outlet and an oil return line. Two phase refrigerant passes through the inlet into the container. While the liquid portion of the fluid settles down, the vapor is sucked by the compressor. Due to gravity effects, lubricant accumulates at the bottom together with the liquid phase and has to be returned to the compressor. The suction pipe is further connected with the oil return. A valve is placed connecting the oil and suction line, where the two streams can be mixed, pumping the oil back to the compressor. The splash plate prevents the droplets from being dragged by the suction of the compressor. It is important to notice that the accumulator also operates as a buffer during load changes, allowing the system to run with less sensitivity to charge effects. Page 16 of 21
The CO 2 Heat Pump Water Heater has been fully instrumented to measure the properties of the fluid on the inlets and outlets of all the components. The thermocouples used for the system are all immersion probes positioned in the center cross section of the pipes. The absolute and differential pressure transducers were previously installed in the chambers, and can be used by all the facilities in test. The Coriolis mass flow meters used are manufactured by Micro Motion and the ranges vary depending on where they are placed in the system. All instruments were calibrated before installation in order to provide the best measurement accuracy. The system is installed in an environmental control chamber for testing. A wind tunnel will be used to evaluate the performance in the air side of the system, which comprises the evaporator secondary fluid. The wind tunnel was designed according the ASHRAE Standard 41.2:1987, which provides information about Standard Methods for Laboratory Airflow Measurement. In order to provide a second and third method of determining the heating capacity of the heat pump unit a small glycol water facility was constructed. A picture of this facility is shown in Figure 5. Figure 5: Glycol cart. All the tests will be conducted following ASHRAE Standard 118.1:2008, which describes a Method of Testing for Rating Commercial Gas, Electric and Oil Service Water Heating Equipment. In total 5 energy balances are possible: air side cooling balance, water side heating balance, glycol side heating balance and the refrigerant side in both cooling and heating sides, ensuring good precision to the results obtained. A picture of the facility installed in the environmental chamber can be seen in Figure 6. Page 17 of 21
Figure 6: Facility and chamber. Page 18 of 21
CONCLUSIONS Carbon dioxide can be considered as an alternative refrigerant for most applications due to its non flammability, non toxicity and since emissions to the environment is not harmful. Important is also the low price of CO 2 and the high availability worldwide. In various applications it shows superior energy efficiency compared to other alternatives. Nowadays, the most favorable application of CO 2 systems on the market is heat pump water heaters, where the thermodynamic properties are very favorable, especially because the temperature characteristics of the transcritical cycle matches the temperature profiles of the heat source and heat sink, giving small heat transfer losses and high efficiency. A pre condition for high efficiency is a low water inlet temperature, giving a low refrigerant inlet temperature to the throttling device, which may be achieved by stratification in the hot water storage tank. With systems designed and operated on the premises of the refrigerant and by taking advantage of the thermodynamic properties of CO 2, it has been shown superior energy efficiency. CO 2 gives in general higher average working pressures than many of the alternatives, however, has the benefit of typically 5 10 times higher volumetric capacity, resulting in much smaller flow areas in the system. Thus, smaller diameter tubing and components can be used, which can lead to systems with less weight, less volume and gives also in many applications reduced system costs. Differences in heating capacity characteristics between CO 2 and conventional refrigerants have to be taken into account in the comparison of CO 2 to baseline systems, since differences in supplementary heating requirements may significantly affect the system energy efficiency. The use of CO 2 as refrigerant has been commercialized in some applications and demonstration systems exists for many others. All the ongoing projects world wide and the results achieved so far should indicate a good prospective for the use of CO 2 as refrigerant in the future. The fundamentals of a CO 2 Heat Pump Water Heater were introduced. Bibliographic reviews as well as a literature research were conducted in order to provide the fundamental knowledge to the project development. A brief summary of the work developed in the company was described, as well as the equipments and materials used in the process. So far the system is fully assembled and ready to be tested. Page 19 of 21
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SKAUGEN,G. 2002. Investigation of Transcritical CO 2 Vapor Compression Systems by Simulation and Laboratory Experiments. Doctoral Thesis at the Norwegian University of Science and Technology (NTNU), Dept. of Energy and Process Engineering, 2002 141. STENE, J. 2004. Residential CO 2 heat pump system for combined space heating and hot water heating. International Journal of Refrigeration, Volume 28, Issue 8, December 2005, Pages 1259 1265. Page 21 of 21