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.
DISCLAIMER This report is provided under a confidentiality agreement. Therefore, all the information here provided shall not be used for any purposes, besides the intern evaluation. Page 2 of 19
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 Urbana Champaign (UIUC), in partnership with professor Will Stoecker. At the beginning, only five employees worked at CTS, and the company building was an old house. At that time, were available four air chambers and two closed loop wind tunnels. 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 has 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 measures an area of approximately 1900 m 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, 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 refrigerants have also been made, both Page 3 of 19
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 degrees and advanced levels in a variety of engineering disciplines, predominately mechanical, their experience provides a very good environment for learning. All the internship work will be developed in the Research and Development division, which comprises the core of the company's business. Page 4 of 19
SCHEDULED ACTIVITIES The internship program has been 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 5 of 19
R 744 HEAT PUMPS 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 ecologically friendly and safe 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 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. The CO 2 properties are well known, thus they are quite different from most other refrigerant. The primary distinction between CO 2 and other refrigerants is the fact that it has a low critical temperature of 31.0 C and a very high critical pressure of 73.8 bar. This characteristic leads to different consideration 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. The most promising result of the application of transcritical cycle has been for hot water heat pump, due to its unique characteristic. In this situation, transcritical cycle is more efficient cycle compared to subcritical cycle. 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 matches the temperature profiles of both the heat source and the heat sink, resulting in small heat transfer losses and high efficiency, as shown in Fig. 1. Heat rejection that occurs in single phase region is an ideal condition for water heating process with large temperature lift. Gas cooling process occurring in supercritical region will follow isobar line with decreasing temperature monotonously. Page 6 of 19
Fig. 1. T s Diagram of a transcritical CO 2 cycle used for water heating. In addition, if the energy released during this gas cooling process is used for water heating, it is 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. Due to its excellent performance in hot water heat pump system while rather inferior in airconditioning system compared to subcritical cycle, transcritical cycle would be an interesting option to be implemented in areas where there is a need for cooling and heating simultaneously. The chance to combine water heating with an air conditioning system may offer both saving energy consumption for producing hot water and also improving performance of the air conditioning side. Page 7 of 19
THE PROJECT The project consists of the development of a state of the art heat pump water heater reaching great efficiency and levels of compactness. Objective of this first project is to develop two high efficiency transcritical CO 2 (R 744) heat pump water heaters (HPWH), with different capacities, offering advantages over existing options. The heat pump extracts the available heat from the ambient air and produces heat suitable for heating water in a storage tank, providing higher energy efficiencies than the best condensing gas water heaters and the best electric water heaters on the market today. HPWHs 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. By utilizing CO 2 as the refrigerant and significantly improving cycle efficiency, performance of the system can be addressed. CO 2 is an excellent HPWH refrigerant due to the reasons pointed in the review. The performance of both CO 2 Heat Pumps will be compared against state of the art R134a heat pump systems. Page 8 of 19
THE HIGH CAPACITY SYSTEM R 134a UNIT The original system to be modified is a commercial potable water heater that claims to achieve a maximum 140 C final tank temperature at common indoor temperatures, using non ozone depleting R 134a and achieving a COP up to 4.2. The unit cabinet is constructed of formed, painted aluminum panels. The panels are insulated with matte finish, fiberglass insulation for thermal barrier and noise reduction. The cooling coil drain pan is constructed of stainless steel, and on the back side of the unit there are two inch pleated air filters. A unit mounted circulating pump is used to pump water between the storage tank and heat pump. The blower assembly is consisted of a double inlet forward curved blower, an open drip proof motor, drive belt and adjustable sheaves. The blower assembly is all mounted on vibration pads to reduce vibration noise through the unit cabinet. Fig. 2: Overview of the R 134a HPWH The refrigeration circuit consists of a scroll type compressor, suction line accumulator, direct expansion valve, liquid line filter dryer, sight glass, cooling coil (copper tube and aluminum fin) and a brazed plate refrigerant to water heat exchanger stainless steel, double wall). The refrigerant tubing is Page 9 of 19
ACR rated and brazed with a 15% silver alloy. The suction line and suction line accumulator are totally insulated. The electronic controller is configures so the water heating is initiated when the tank temperature drops below the heating set point. After a factory set time delay, the compressor is energized to heat water until the tank set point is reached. The compressor counts with protection against high and low pressure switches as well as thermal overload. The initial testing on the baseline R 134a unit was developed in the company, previous to the CO 2 unit design and construction. The data is available for comparison with the designed system. Some results can be seen in the pressure enthalpy diagram for the case with an inlet water temperature of 8 C and an ambient temperature of 10 C. In this case, the cooling capacity was determined to be 20.5 kw and the heating capacity was 23.7 kw. This results in a COP for cooling of 4.3 and 5.0 for heating. P [kpa abs] 2500 0 2375 10 2250 2125 2000 1875 1750 1625 1500 1375 1250 1125 1000 875 750 625 500 375 250 125 20 30 40 50 60 CTS R134a P-h diagram Baseline x=.2 x=.4 x=.6 x=.8 x=.1 x=.3 x=.5 x=.7 x=.9 40 60 80 100 120 140 160 180 200 220 240 260 280 300 320 340 360 h [kj/kg] 380 Fig. 3: P h diagram for baseline Heat Pump system with water inlet temperature of 8 C and ambient temperature of 10 C 10 20 30 40 50 60 70 80 90 100 110 120 150 140 130 Page 10 of 19
CO2 UNIT detailed below. In order to ensure the conversion of the heat pump, several changes were conducted as COMPRESSOR The vapor pressure of CO 2 is higher than conventional refrigerants and the transcritical CO 2 cycle operates at much higher pressures than the conventional vapor compression systems. Higher pressure gives special requirements regarding the design of suitable components, especially compressors for the CO 2 systems. Therefore, a new compressor was selected to operate on the new CO 2 cycle. It consists of a semi hermetic compressor, with 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 current package. HEAT EXCHANGERS The high working pressure and favorable heat transfer properties of CO 2 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. With these considerations in mind, the evaporator designed has close to half of the size of the original R134a evaporator, but the width was kept the same to make it fit into existing enclosure. 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 (and as a consequence, the evaporator inlet enthalpy) because specific heat becomes infinite at the critical point. So the design of the gas cooler has to be carefully considered. Therefore, 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. The pressure drop on both sides of the component is already predicted, within the range of 8.5 kpa for the CO 2 side and 48 kpa for the water side. It should also be pointed that the gas cooler is nearly a 50% reduction in heat exchanger volume from the condenser in the baseline. Page 11 of 19
Fig. 4: Brazed plate Gas Cooler. EXPANSION DEVICE The expansion device used in the heat pump is comprised by two electronic expansion valves to supply the flow demand. Both valves are installed with a single control board and respond to a 4 20mA or 0 10V signal. ACCUMULATOR In transcritical R 744 systems the expansion valve is 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. 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. 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 Page 12 of 19
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 less sensible to charge effects. STORAGE TANK The storage tank used for the HPWH system is originally part of another system, composed by several elements. As there is no interest in the other components of the system, only the tank was used and no significant modifications were performed. However, instrumentation was added inside the tank in order to evaluate the stratification of the water stored. Improvement in storage is actually 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 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 13 of 19
UNIT ASSEMBLY Several modifications to the second unit for use with CO 2 were performed. The changes from the original R 134a unit are worth noting, specially regarded to the package reduction and components redistribution. One minor change is the placement of the distributor to the evaporator in comparison to the compressor chamber of the unit. In the baseline unit, the distributor was on the far side of the evaporator from the compressor. This required a long liquid line, which increases charge. In the proposed design for the CO 2 unit the distributor is placed on the near side of the evaporator to both reduce charge and the amount of piping in the system. In addition, all of the new components were placed in a manner that will highlight the reduced package size for CO 2 systems in comparison to other conventional refrigerants. With the reduction in the evaporator, the motor for the evaporator blower was moved from the top of the blower frame to the side, which underscores the reduction in package height. The modifications can be seen in Fig 5. Fig. 5: Full system full assembly. Page 14 of 19
SYSTEM TESTING AND EVALUATION 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. A simplified schematic of the system and the instrumentation used for measuring performance is shown in Fig. 6. 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 are previously installed in the chambers, and can be used by all the facilities in test. The Coriolis mass flow meters used are manufactured by MicroMotion 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. Fig. 6: Simplified system diagram. The system is installed in a calorimetric 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 Fig. 7. Page 15 of 19
Fig. 7: 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. The test matrix for the system baseline was defined to achieve the most common operating conditions when in field. The lowest ambient temperature was defined to reach the crossover point for each fluid (R 134a and R 744). The water inlet in the gas cooler (or condenser) is the most influent temperature in the results on the hot side of the system. The complete water heating system will be tested with the tank and a secondary water pumping unit. THE SMALL CAPACITY SYSTEM The main internship work was developed in the commercial system, though the first activities were related to the conversion of a small capacity HPWH into a CO 2 operated system. CO2 UNIT CONVERSION In this project, the performance of two heat pumps will be determined. Originally two identical units were supplied. The first operates with R 134a and is commercially available, while the second unit was converted to operate with CO 2. Due to the higher pressures seen in a CO 2 system than in the original R 134a system, several new components needed to be sourced. Page 16 of 19
The main focuses of interest in performance are the heat exchangers. While the R 134a system operated only with a single gas cooler, which was wrapped around the tank, the CO 2 system was designed to use two separated gas coolers. The internal gas cooler is very similar to the original coil wrapped around the tank, differing by the pipe wall thickness and by the wrap starting point. While in the R 134a gas cooler the inlet is close to the middle of the tank, in the CO 2 unit the inlet is placed exactly at the lowest point of the tank. These changes were made in order to provide the hottest stratified water available. The external gas cooler can be used to separate the effects of the storage tank in the system, the same as in the commercial system. In order to maintain the package size, an evaporator with the same external geometry as the R134a unit has been sourced, however suitable for the CO 2 characteristics. The expansion device in use consists of an electronic expansion valve and controller with the proper size. The original R 134a compressor was replaced by a proper CO 2 compressor, which supplies the demanded capacity. Temperature and pressure measurements will be made at appropriate points within the refrigerant cycle to ensure that all of the relevant state points can be determined. A Coriolis mass flow meter will be used to measure the refrigerant mass flow rate allowing for a determination of both the heating and cooling capacity of the system. The power consumption of the water heater unit will be measured in both compressor and fan, by using a power meter to provide the cooling and heating COP. A wind tunnel has been constructed and attached to the exit of the evaporator, in order to measure cooling capacity on the air side. To overcome the additional pressure drop of the air flow nozzle, an additional blower has been installed on the wind tunnel. In addition to the instrumentation on the refrigerant and air sides, thermocouples were be placed inside the water heater tank to allow for examination of the stratification of the temperature layers as the tank heats up. UNIT ASSEMBLY Since the residential unit will be tested by a customer, the system has to be contained into a single cart which can be easily transported. The operation has also to be simple and straightforward. Page 17 of 19
CONCLUSIONS The fundamentals of a CO2 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 commercial system is fully assembled and ready to be tested. The residential unit will be shipped to the customer, who will be responsible for the testing and evaluation. Page 18 of 19
REFERENCES AGRAWAL, N. 2008. Optimized transcritical CO2 heat pumps: Performance comparison of capillary tubes against expansion valves. International Journal of Refrigeration 31 (2008) 388 395. ANSI/ASHRAE Standard 118.1:2008 Method of Testing for Rating Commercial Gas, Electric and Oil Service Water Heating Equipment. ASHRAE, 2001 ASHRAE Fundamentals Hanbook (SI), 2001. BOWMAN, N. et al 1981. Stratified Solar Storage For Use in Domestic Scale Systems. Sun at Work in Britain 12/13:39 42. DANFOSS, Transcritical Refrigeration Systems with Carbon Dioxide (CO2). Instructions article. FURBO, S. 1989. Solar water heating systems using low flow rates. Experimental investigations. Internal Report No. 89 9, Thermal Insulation Lab. Technical University of Denmark. KIM, M. et al 2004. Fundamental process and system design issues in CO2 vapor compression systems. Progress in Energy and Combustion Science 30 (2004) 119 174. LAIPRADIT, P. et al 1998. Simulation Analysis of CO2 Heat Pump Water Heaters: Comparative with other natural working fluids. Proceeding of the conference of natural working fluids 98, Norway, pp.203 211. ORTIZ, T. et al 2003. Evaluation of the Performance Potential of CO2 as a Refrigerant in air to air Air Conditioners and Heat Pumps: System Modeling and Analysis. Final Report prepared for the Air Conditioning and Refrigeration Technology Institute. SARKAR, J. 2010. Review on Cycle Modifications of Transcritical CO2 Refrigeration. Journal of Advanced Research in Mechanical Engineering 1 (2010) pp. 22 29. SKAUGEN,G. 2002. Investigation of Transcritical CO2 Vapour 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 CO2 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 19 of 19