Dynamic behavior of mobile air-conditioning systems.Passenger cars and light trucks consume 80% of the total oil imported by the United States United States, officially United States of America, republic (2005 est. pop. 295,734,000), 3,539,227 sq mi (9,166,598 sq km), North America. The United States is the world's third largest country in population and the fourth largest country in area. . Mobile air-conditioning systems (MACs) increase vehicle fuel consumption and exhaust gas Exhaust gas is flue gas which occurs as a result of the combustion of fuels such as natural gas, gasoline/petrol, diesel, fuel oil or coal. It is discharged into the atmosphere through an exhaust pipe or flue gas stack. emissions. They operate most of the time in a transient state The exact point at which a device changes modes, for example, from transmit to receive or from 0 to 1. . To investigate the dynamic behavior of a typical R-134a MAC, the "dynamic simulator (1) Software that enables the execution of an application written for a different computer environment. Same as emulator. (2) Software that models the interactions of hypothetical or real-world objects or business processes. ," which is a laboratory transient A malfunction that occurs at random intervals and lasts for a short duration such as a spike or surge in a power line or a memory cell that intermittently fails. See spike and power surge. transient - 1. test facility for MACs, was utilized. The facility depends on simulation software Simulation software is based on the process of imitating a real phenomenon with a set of mathematical formulas. It is, essentially, a program that allows the user to observe an operation through simulation without actually running the program. that measures the conditions of the air supplied by the MAC and subsequently adjusts the conditions of the air returning to the MAC, depending on the results of a thermal numerical model of the car cabin. The transient tests conducted include pull-down, drive-cycle tests and drive-cycle tests with a thermostat thermostat, automatic device that regulates temperature in an enclosed area by controlling heating or refrigerating systems. It is commonly connected to one of these systems, turning it on or off in order to maintain a predetermined temperature. . The results show that the most energy-efficient method to pull down the air temperature inside a hot-soaked cabin is to start with fresh air, as long as the temperature in the cabin exceeds that of the ambient temperature Outside temperature at any given altitude, preferably expressed in degrees centigrade. , and then switch to recirculated air. The effect of the thermostat action on the relative humidity relative humidity n. The ratio of the amount of water vapor in the air at a specific temperature to the maximum amount that the air could hold at that temperature, expressed as a percentage. (RH) inside the car cabin shows that the RH inside the cabin increases due to re-evaporation of the condensate condensate, matter in the form of a gas of atoms, molecules, or elementary particles that have been so chilled that their motion is virtually halted and as a consequence they lose their separate identities and merge into a single entity. on the evaporator evaporator Industrial apparatus for converting liquid into gas or vapour. The single-effect evaporator consists of a container or surface and a heating unit; the multiple-effect evaporator uses the vapour produced in one unit to heat a succeeding unit. . This re-evaporation of the condensate during the compressor-off period adds cooling load to the MAC when it starts again. This study provides some insights on the dynamic behavior of the MAC and proposes a new transient test method in the laboratory and a new transient performance index. INTRODUCTION As recent climate changes result in warmer summers and extended cooling-demand hours, residential and mobile air conditioners Conditioners used on leather take many shapes and forms. They are used mostly to keep leather from drying out and deteriorating. A very old and widely used conditioner is dubbin. face increasing demands. A mobile air-conditioning system (MAC) requires a vehicle to consume more fuel not only for its operation but also for its additional weight to transport. Since the extra fuel consumption by the MAC means more greenhouse gas greenhouse gas n. Any of the atmospheric gases that contribute to the greenhouse effect. greenhouse gas emissions, enhancement of the MAC's efficiency and evaluation of its performance are both important. Substantial efforts are required to evaluate the performance of the MAC, since it is subject to highly varying conditions: initial conditions determine air temperatures entering the evaporator and condenser condenser Device for reducing a gas or vapour to a liquid. Condensers are used in power plants to condense exhaust steam from turbines and in refrigeration plants to condense refrigerant vapours, such as ammonia and Freons. ; user choices determine the evaporator airflow rate and ventilation mode; driving patterns determine the compressor compressor, machine that decreases the volume of air or other gas by the application of pressure. Compressor types range from the simple hand pump and the piston-equipped compressor used to inflate tires to machines that use a rotating, bladed element to achieve revolutions per minute (rpm) and the condenser airflow rate; and cabin material, occupancy, and weather determine internal and external thermal loads. When the compressor is turned off after operation, the refrigerant re·frig·er·ant adj. 1. Cooling or freezing; refrigerating. 2. Reducing fever. n. 1. A substance, such as air, ammonia, water, or carbon dioxide, used to provide cooling either as the working substance of starts to migrate from the condenser at high pressure to the evaporator at low pressure, passing through the expansion device. Migration continues until both pressures are equalized. Migration is also driven by temperature differences. The refrigerant also carries some energy to the evaporator during its migration. The amount of energy that is carried with the migrating refrigerant to the evaporator air was estimated by Rubas and Bullard (1995) for a household refrigerator to be 4% of the steady-state capacity in the case of liquid migration and 7% of the steady-state capacity in the case of vapor migration. This means that during any transient period, the MAC is subjected to additional loads that result from redistribution re·dis·tri·bu·tion n. 1. The act or process of redistributing. 2. An economic theory or policy that advocates reducing inequalities in the distribution of wealth. of the refrigerant among the different components of the system; readjusting the operating parameters, such as the temperature of the different parts of the system and the temperatures and pressures of the refrigerant, to their new operating values; and reconditioning the thermal mass Thermal mass, in the most general sense, is any mass that absorbs and holds heat. In the architectural sense, it is any mass that absorbs and stores heat during sunny periods when the heat is not desirable in the living space of a building, and then releases the heat during of the conditioned space to the steady-state air temperature. These loads are, by nature, time dependent. They are at their peak in the beginning of the transient period and decrease with time until they diminish as steady state is reached. Therefore, the case during the transient period is similar to the case of an undersized undersized see dwarfism, runt. system working at lower than design efficiency. The performance and behavior of the system during the transient period is mainly dependant on Adj. 1. dependant on - determined by conditions or circumstances that follow; "arms sales contingent on the approval of congress" contingent on, contingent upon, dependant upon, dependent on, dependent upon, depending on, contingent the performance of the compressor and expansion device and the existence of auxiliary auxiliary In grammar, a verb that is subordinate to the main lexical verb in a clause. Auxiliaries can convey distinctions of tense, aspect, mood, person, and number. components such as accumulator A hardware register used to hold the results or partial results of arithmetic and logical operations. (processor) accumulator - In a central processing unit, a register in which intermediate results are stored. , receiver, suction suction /suc·tion/ (suk´shun) aspiration of gas or fluid by mechanical means. post-tussive suction a sucking sound heard over a lung cavity just after a cough. line heat exchanger heat exchanger Any of several devices that transfer heat from a hot to a cold fluid. In many engineering applications, one fluid needs to be heated and another cooled, a requirement economically accomplished by a heat exchanger. , and oil separator, since these components affect the duration and consumption of power during the transient period (Mulroy and Didion 1983). The coefficient of performance The coefficient of performance, or COP (sometimes CP), of a heat pump is the ratio of the output heat to the supplied work or (COP) of the system during the transient period suffers from a loss whose magnitude depends on the deviation of the real operating conditions from the design conditions. This loss is called the transient loss. When the transient loss is estimated for the deactivation de·ac·ti·vate tr.v. de·ac·ti·vat·ed, de·ac·ti·vat·ing, de·ac·ti·vates 1. To render inactive or ineffective. 2. To inhibit, block, or disrupt the action of (an enzyme or other biological agent). 3. of the MAC for five minutes after every fifteen minutes of operation, it is 33% of the delivered capacity in an hour of operation. The magnitude of the cyclic cyclic /cyc·lic/ (sik´lik) pertaining to or occurring in a cycle or cycles; applied to chemical compounds containing a ring of atoms in the nucleus. cy·clic or cy·cli·cal adj. 1. loss draws even more attention to the automotive application due largely to the widely varying rotational speed Rotational speed (sometimes called speed of revolution) indicates, for example, how fast a motor is running. Rotational speed is equivalent to angular speed, but with different units. Rotational speed tells how many complete rotations (i.e. of the compressor. The MAC needs to deliver acceptable capacity at a low compressor rpm, thus, the systems usually have charge management devices, such as a suction accumulator or a receiver (Althouse et al. 2000). The goal of this research is to provide insight into the dynamic behavior of MACs by conducting transient tests using a dynamic simulator as a laboratory transient test facility. By conducting laboratory experiments on a MAC, important performance indicators, such as the sensible and latent Hidden; concealed; that which does not appear upon the face of an item. For example, a latent defect in the title to a parcel of real property is one that is not discoverable by an inspection of the title made with ordinary care. capacities, which would be difficult or impossible to measure in field tests, are measured under transient conditions. TEST FACILITY AND TEST SYSTEM The dynamic simulator, a transient test facility for the MAC, was developed by modifying controllers of a typical steady-state environmental test facility, integrating computer controls and software that substitute for the car and adjust the conditions inside the test chambers to the air conditions that occur inside a real vehicle under transient operation (Gado et al. 2004). In this way, the air-handling unit (AHU A´hu n. 1. (Zool.) The Asiatic gazelle. ) of the environmental chamber simulates the cabin loads, and the conditions of the return air to the evaporator coil are controlled to reflect the return air conditions from an actual vehicle cabin. Thus, the air-conditioning system operates as if placed in an actual vehicle. Gado et al. (2004) provide more details on the dynamic simulator. Test Facility Details of the specifications for the indoor and outdoor simulators are listed in Table 1. The air-flow rates through the wind tunnels wind tunnel, apparatus for studying the interaction between a solid body and an airstream. A wind tunnel simulates the conditions of an aircraft in flight by causing a high-speed stream of air to flow past a model of the aircraft (or part of an aircraft) being tested. of both heat exchangers were set to the target values by controlling the speed of fan motors, which were operated by the variable-frequency inverter (1) A logic gate that converts the input to the opposite state for output. If the input is true, the output is false, and vice versa. An inverter performs the Boolean logic NOT operation. (2) A circuit that converts DC current into AC current. Contrast with rectifier. drivers.
Table 1. Specifications of Test Facility
Simulator Specification
Temperature: -40[degrees]C to 60[degrees]C
RH: 20% to 85%
Indoor Simulator Typical airflow rate: 0.16 [m.sup.3]/s
Maximum airflow rate: 0.57 [m.sup.3]/s
Wind tunnel size: 0.356 (width) X 0.305 (height) m
Temperature: -20[degrees]C to 60[degrees]C.
RH: 15% to 80%
Outdoor Simulator Typical airflow rate: 0.57 [m.sup.3]/s
Maximum airflow rate: 1.13 [m.sup.3]/s
Wind tunnel size: 1.016 (width) X 0.610 (height) m
Test System A typical R-134a MAC designed for a midsize passenger car was installed inside the chambers of the dynamic simulator. It had a fixed area orifice orifice /or·i·fice/ (or´i-fis) 1. the entrance or outlet of any body cavity. 2. any opening or meatus.orific´ial aortic orifice and a suction accumulator. Table 2 and Figure 1 show details of the test system. The open-drive compressor was driven by a 7.5 kW electric motor, which was operated by a variable-frequency inverter. [FIGURE 1 OMITTED]
Table 2. Specifications of System Components
Component Specification
Type: reciprocating piston
Compressor Swept volume: 155 c[m.sup.3]
Oil type: Plyalkylene glycol
Type: fin-and-tube
Dimension: 44.5 (height) X 57.8 (width) X 2.24 (depth) cm
Condenser Fin pitch: 0.8 mm
Fin thickness: 0.1 mm
Total outer surface area: 9.2 [m.sup.2]
Material: all aluminum
Type: serpentine
Dimension: 27 (height) X 25.5 (width) X 7.8 (depth) cm
Evaporator Fin pitch: 2.1 mm
Fin thickness: 0.16 mm
Total outer surface area: 3.7 [m.sup.2]
Material: all aluminum
Instrumentation Thermocouples and pressure transducers Pressure transducer An instrument component which detects a fluid pressure and produces an electrical, mechanical, or pneumatic signal related to the pressure. were installed along the circuit as indicated by the "T" and "P" symbols in Figure 1. A torque meter was mounted between the compressor and the electric motor. Also, the rotational speed of the compressor was measured by means of an rpm sensor. A Coriolis mass flowmeter See flow meter. was placed in the liquid line to measure the mass flow rate (MFR MFR, n See myofascial release. ) of the refrigerant. During transient operation, the refrigerant temperature cannot be measured from the outer surface of the pipes because of the thermal storage of the pipe metal. All the thermocouples used were in-stream. To ensure the quickest possible measurements, thermocouples were selected to have exposed junctions and to be of 1.6 mm in diameter, which is the thinnest diameter commercially available. Grids of nine equally spaced thermocouples were used to measure the average air dry-bulb temperatures The dry-bulb temperature is the temperature of air measured by a thermometer freely exposed to the air but shielded from radiation and moisture. In construction, it is an important consideration when designing a building for a certain climate. upstream and downstream of the coils. To calculate the latent capacity of the system, dew-point sensors were placed before and after the evaporator. These dew-point sensors had the smallest time constant among commercial dew-point sensors but the largest time constant among instruments used. The flow rates of evaporator air and of condenser air were measured by nozzles. In steady-state operation, the capacity as calculated from the air side should be the same as calculated from the refrigerant side. This presents a useful criterion to check the accuracy of the instrumentation and data acquisition. During steady-state operation of the system, the latent capacity calculated using the dew-point values measured by the dew-point meter, agreed within a 2% margin of error with the latent capacity calculated by collecting the evaporator condensate. During steady-state operation of the system, the error in energy balance between the refrigerant-side and air-side capacities was less than 4%. This energy balance check was repeated several times throughout the battery of tests, and the error was less than 3%. Dynamic Simulator The dynamic simulator used the cabin model to simulate simulate - simulation real car-cabin conditions and interaction with the MAC being tested. The cabin model used the following variables, which were measured and fed to the model as inputs: supply air temperature ([T.sub.s]), supply air humidity humidity, moisture content of the atmosphere, a primary element of climate. Humidity measurements include absolute humidity, the mass of water vapor per unit volume of natural air; relative humidity (usually meant when the term humidity ratio ([W.sub.s]), and evaporator airflow rate ([m.sub.e]). In the transient tests, the parameters listed in Table 3 were provided as user inputs for the cabin model. The cabin model consists of five main governing equations (Equations 1-5) to solve the mass and energy balances for the cabin.
Table 3. Input Variables for Cabin Model
No. Parameter Value Source
1 Solar load 950 W Huang, 1998
2 Surface area of cabin 30 [m.sup.2] Huang, 1998
3 Number of passengers 1 Typical value
4 Internal volume of cabin 8 [m.sup.3] Huang, 1998
5 Interior mass of cabin 200 kg Huang, 1998
6 Specific heat of interior 400 J/kg - K Huang, 1998
mass
7 Surface area of interior 3 [m.sup.2] Typical value
mass
8 Overall heat transfer 4 W/[m.sup.2] - K Meyer, 2002
coefficient of cabin
wall
9 Convective heat transfer 100 W/[m.sup.2] - K Estimated
coefficient between
interior mass and cabin
air
The sensible part of the cabin model is given by Equations 1-3, which represent the energy balance of cabin air sensible heat Sensible heat is potential energy in the form of thermal energy or heat. The thermal body must have a temperature higher than its surroundings, (also see: latent heat). The thermal energy can be transported via conduction, convection, radiation or by a combination thereof. , energy balance of interior mass, and adiabatic ad·i·a·bat·ic adj. Of, relating to, or being a reversible thermodynamic process that occurs without gain or loss of heat and without a change in entropy. mixing of dry air, respectively. Similarly, two equations are solved for humidity: namely, cabin air latent energy balance and moisture mass balance at the mixing point before the coil. These are provided as Equations 4-5, respectively, and represent the latent part of the cabin model. [M.sub.r][C.sub.pr][[dT.sub.r]/dt] + [M.sub.c][c.sub.c][[dT.sub.c]/dt] = -[m.sub.e][C.sub.pe]([T.sub.m] - [T.sub.s]) + [Q.sub.sol] + [Q.sub.ps] + [U.sub.o][A.sub.o]([T.sub.amb] - [T.sub.r]) + [m.sub.iv][C.sub.p, amb]([T.sub.amb] - [T.sub.r] (1) [M.sub.c][C.sub.c][[dT.sub.c]/dt] = -[h.sub.c][A.sub.c]([T.sub.c] - [T.sub.r]) (2) [m.sub.iv][C.sub.p, amb][T.sub.amb] + ([m.sub.e] - [m.sub.iv])[C.sub.pr][T.sub.r] = [m.sub.e][C.sub.pm][T.sub.m] (3) [M.sub.r][h.sub.fg][[dW.sub.r]/dt] = - [m.sub.e][h.sub.fg]([W.sub.m] - [W.sub.s]) + [m.sub.iv][h.sub.fg]([W.sub.amb] - [W.sub.r]) + [Q.sub.pl] (4) [m.sub.e] - [m.sub.iv])[W.sub.r] + [m.sub.iv][W.sub.amb] = [m.sub.e][W.sub.m] (5) Equations 1-3 were numerically solved for the cabin air temperature ([T.sub.r]), interior mass temperature ([T.sub.c]), and temperature of return air to evaporator coil ([T.sub.m]). Equations 4-5 were solved for the cabin air humidity ratio ([W.sub.r]) and the humidity ratio of return air to the evaporator coil ([W.sub.m]). During the calculation, air properties at each time step were calculated at the corresponding values of temperature and humidity ratio, which were used for the control of the AHU. To construct the dynamic simulator, some modifications were made to the controllers of the typical steady-state test facility. Both the indoor and outdoor simulators control the temperature and relative humidity (RH) of supply air to test units by using AHUs with proportional integral derivative (PID (1) (Process IDentifier) A temporary number assigned by the operating system to a process or service. (2) (Proportional-Integral-Derivative) The most common control methodology in process control. ) controllers. The temperature controller actuates the hot-gas bypass valve, liquid-line valve, and electric heater. The temperature and humidity PID controllers See PID. and frequency inverter drivers all accept remote setpoints in the form of an analog signal An analog or analogue signal is any time continuous signal where some time varying feature of the signal is a representation of some other time varying quantity. It differs from a digital signal in that small fluctuations in the signal are meaningful. that the controller uses to scale the value of temperature, RH, or frequency between specified upper and lower limits. The cabin model program sends remote setpoints to the temperature and humidity controllers to control the indoor loop temperature and RH. At each time step, the software measures the value of the controlled parameters and compares it to the desired values based on the simulation. The difference between the two values is the error, e, which the software uses to calculate the adjusted setpoint that is sent to the corresponding controller. In this case, the next setpoint is already known in each time step; therefore, the dynamic control software takes advantage of this feature while sending the setpoints to the controllers by further adjusting the setpoints. Verification of Dynamic Simulator Two types of tests were conducted to verify the interaction between the dynamic simulator and the MAC being tested. In the first category, the unloaded test, which means operating only AHUs of the test facility but not the test system, parameters such as temperature, RH, airflow rate, and compressor rpm were controlled according to according to prep. 1. As stated or indicated by; on the authority of: according to historians. 2. In keeping with: according to instructions. 3. a pre-set profile to check the accuracy of dynamic simulator control. The absolute error in controlling temperature was within [+ or -]0.5[degrees]C, and the absolute error in controlling RH was within [+ or -]2% RH. In the second category, the New European Drive Cycle (NEDC NEDC (formerly) National Economic Development Council Also (informal): (Neddy) NEDC n abbr (BRIT) (= National Economic Development Council) → Consejo Económico y Social ) (Wertenbach 2003) was imposed on the system according to the test conditions shown in Table 4 and Figure 2. The MAC was started in the beginning of the test. Test results showed that the difference in the cabin air temperature profiles as calculated by the cabin model and as measured inside the indoor simulator loop was less than 0.7[degrees]C. The difference between the calculated and measured air RHs was less than 4% [FIGURE 2 OMITTED]
Table 4. Test Conditions of Drive-Cycle Tests
Amb. Temp., Amb. Cabin Air Comp., Evap. Cond. Ventilation
[degrees]C RH, Temp., rpm Air Air Mode
% [degrees]C Speed, Speed,
m/s m/s
30 50 35.6 NEDC 2.3 2.5 Recirculated
profile air
EXPERIMENTAL RESULTS Three types of transient tests, which take place during a typical driving pattern, were conducted to measure the transient behavior of the MAC: pull-down tests, drive-cycle tests, and drive-cycle tests with a thermostat. Details of each test result are presented as follows. Pull-Down Tests The first category of tests focuses on the time period and the energy consumption needed for the MAC to lower the cabin air temperature to 24[degrees]C. The test matrix shown in Table 5 was designed for this purpose. All the tests started at the same cabin air temperature of 41[degrees]C and at two different ambient temperatures, 41[degrees]C and 30[degrees]C, to simulate cases for both the ambient temperature of 41[degrees]C without hot soak and for the ambient temperature of 30[degrees]C with 11[degrees]C hot soak. The 32% RH used for the hot-soak case is a result of sensible heating from 30[degrees]C and 50% RH to 41[degrees]C. To investigate the effects of various ventilation modes, three modes were chosen: recirculated air, fresh air, and actively controlled air. Test number 9 represents the actively controlled mode, which starts with fresh-air mode until the cabin air temperature reaches ambient temperature and then switches to recirculated-air mode. Test number 10 was added to investigate the effect of the elevated condenser air temperature due to radiation and recirculation Noun 1. recirculation - circulation again circulation - the spread or transmission of something (as news or money) to a wider group or area near the ground, as reported by Inui and Tomatsu (2004) and Sumantran et al. (1999). All the tests ended either when the cabin air temperature (the temperature of the air returning to the evaporator) decreased below 24[degrees]C or when the pull-down period reached 40 minutes.
Table 5. Test Matrix of Pull-Down Tests
Test Amb. Temp., Amb. RH, % Cabin Air Temp.,
No. [degrees]C [degrees]C
1 30 50 41
2 41 32 41
3 30 50 41
4 41 32 41
5 30 50 41
6 41 32 41
7 30 50 41
8 41 32 41
9 30 50 41
10 35 50 41
Test Comp., rpm Evap./Cond. Air Ventilation Mode
No. Speed, m/s
1 700 (idling) 2.3/2.5 Recirculated air
2
3 Fresh air
4
5 2300 (driving) Recirculated air
6
7 Fresh air
8
9 2300 (driving) Actively
controlled air
10 700(idling) Recirculated air
The results of the first category of tests are shown in Figure 3a, which shows the pull-down period of the tests with hot soak (tests number 1, 3, 5, 7, 9, and 10 in the test matrix) and Figure 3b, which shows the pull-down period of the tests without hot soak (tests number 2, 4, 6, and 8 in the test matrix). The cabin air temperature measured during the tests is plotted against time in those figures. Both figures show that the cabin air temperature did not drop below 24[degrees]C in the case of fresh air mode, which indicates that the MAC was undersized for the cabin size used in the tests; however, the final temperature in the case of driving condition was lower than for idling condition. The final temperature was also lower in the case where the ambient temperature was 30[degrees]C than in the case where the ambient temperature was 41[degrees]C. In the four tests with recirculated air (idling and driving with recirculated air in Figure 3), the cabin temperature dropped below 24[degrees]C. As expected, the pull-down speed of the driving case was faster than that of the idling case due to the higher refrigerant MFR. The cabin air temperature reached 24[degrees]C after approximately 29 minutes in the idling case and after 17 minutes in the driving case, regardless of the ambient temperature in either case. When the air temperature entering the condenser was raised 5K higher than the ambient temperature, the pull-down period to 24[degrees]C was approximately 3.5 minutes longer than the case without the temperature rise. At driving conditions with recirculated air, the pull-down period was much shorter than for other cases. When two ventilation modes, recirculated air and fresh air, were compared for the hot-soak condition, the cooling speed of the cabin air was faster in the fresh-air mode than in the recirculated-air mode until the cabin air temperature reached ambient temperature, since the initial cabin air temperature was higher than the ambient temperature. However, when the cabin temperature decreased below the ambient temperature, the cooling speed of the cabin air was reversed. Therefore, it is beneficial to start the system in the fresh-air mode until the cabin air temperature reaches ambient temperature. However, when the cabin temperature decreases and reaches the ambient temperature, the mode is better switched to recirculated air. This scenario is referred to as "actively controlled air mode" and is demonstrated by the thin solid line in Figure 3a. The first part of the thin line representing this scenario and the line representing the pull-down with fresh air scenario overlapped, which indicates sound repeatability of test results. When the actively controlled air mode was used, the pull-down period was about two minutes faster than that of the recirculated air mode. Figure 3c shows the humidity ratio changes for three different ventilation modes. As the figure shows, the cabin humidity ratio was maintained above a 10 g/kg level under the fresh-air mode; it decreased to 7 g/kg after 20 minutes under the recirculated-air mode. However, it decreased for the first two minutes, maintained a constant level of 9.5 g/kg for two and eight minutes, started to decrease when the mode was changed to the recirculated-air mode, and then decreased to a 4.5 g/kg level after 20 minutes under the actively controlled air mode. This result means that the fresh-air mode enhances the moisture removal from the cabin only for the first two minutes and adds moisture to the cabin between two and eight minutes. The moisture removal was enhanced again by providing cold air to the evaporator when the ventilation was switched to the recirculated-air mode. [FIGURE 3 OMITTED] Figure 4 shows the energy consumption and the overall COP of the MAC during the pull-down tests at two ambient Surrounding. For example, ambient temperature and humidity are atmospheric conditions that exist at the moment. See ambient lighting. conditions. Here, the overall COP, which is defined as the ratio of the cooling capacity provided to the cabin air by the MAC to the energy consumed by the compressor, which are both integrated over the entire test period, is proposed as a metric to compare the transient performance of a MAC, as shown in Equation 6: [FIGURE 4 OMITTED] [MATHEMATICAL EXPRESSION A group of characters or symbols representing a quantity or an operation. See arithmetic expression. NOT REPRODUCIBLE IN ASCII ASCII or American Standard Code for Information Interchange, a set of codes used to represent letters, numbers, a few symbols, and control characters. Originally designed for teletype operations, it has found wide application in computers. ] (6) where Q = cooling capacity of the MAC, kW [W.sub.comp comp See comparison. ] = compressor power input, kW [t.sub.i] = initial time of the transient period, s [t.sub.f] = final time of the transient period, s Figure 4a also shows the energy consumptions for the two cases of idling and the two cases of driving at 30[degrees]C and 50% RH ambient condition with hot soak. The energy consumption until the cabin air temperature reaches 24[degrees]C is given in kJ. When the condenser air was 5[degrees]C hotter than the ambient air, the energy consumption was 30.7% higher and the overall COP was 18.1% lower than for the idling case. The actively controlled air mode had 12.3% less energy consumption and 21.6% higher overall COP than just recirculated air for the driving case. If the driving and the idling cases are compared for the same recirculated mode, the driving case consumed 64% more energy and had 52.5% less overall COP than the idling case. Figure 4b shows the energy consumption and the overall COP for the tests without hot soak. When the pull-down tests with and without hot soak were compared, the energy consumption was slightly higher, and the overall COP was slightly lower in the case without hot soak than for the case with hot soak. This can be attributed both to the lower condensing con·dense v. con·densed, con·dens·ing, con·dens·es v.tr. 1. To reduce the volume or compass of. 2. To make more concise; abridge or shorten. 3. Physics a. temperature in the case with hot soak and the elapsed time e·lapsed time n. The measured duration of an event. Noun 1. elapsed time - the time that elapses while some event is occurring to cool. Here, the time elapsed e·lapse intr.v. e·lapsed, e·laps·ing, e·laps·es To slip by; pass: Weeks elapsed before we could start renovating. n. during the transient period is defined at the time consumed from the initial time until the cabin air temperature reaches 24[degrees]C. Drive-Cycle Tests The second category of tests focuses on the pull-down performance while the compressor rpm varies under the NEDC (Wertenbach 2003). The system was started from the beginning of the test. Figure 5a shows the change in temperature of the cabin supply air with time, as well as the change in the temperature of the internal mass. The figure also shows the air temperature inside the cabin as calculated by the cabin model and measured inside the indoor simulator loop. The difference between the calculated and measured cabin air temperatures, which was less than 0.7[degrees]C, is an indicator for how accurate the temperature control of the dynamic simulator was during the test. Figure 5b shows the supply air and cabin air RHs as well as air MFR. It is clear that the supply air RH fluctuated as a result of the fluctuations in compressor rpm. The compressor rotational speed fluctuations affected the evaporation evaporation, change of a liquid into vapor at any temperature below its boiling point. For example, water, when placed in a shallow open container exposed to air, gradually disappears, evaporating at a rate that depends on the amount of surface exposed, the humidity temperature, which in turn affected the latent capacity of the evaporator and, therefore, the supply-air RH fluctuated. The difference between the calculated and measured cabin air RHs was less than 4%. The air MFR increased from 176 to 185 g/s due to the increase in air density, which was caused by the drop in cabin air temperature. Figures 5c-5h illustrate changes in cycle parameters and performances, which were all affected by the changes in the compressor speed. To help understand the trends of these changes, the refrigerant MFR, which is an indicator of compressor rpm, is plotted in Figure 5c on the second y-axis. Figure 5c shows that the refrigerant MFR fluctuated from 15 to 28 g/s. It also shows the inlet inlet /in·let/ (-let) a means or route of entrance. pelvic inlet the upper limit of the pelvic cavity. thoracic inlet the elliptical opening at the summit of the thorax. and outlet pressures of both the evaporator and condenser. Whenever the compressor rpm increased at the given refrigerant charge, the compressor circulated more refrigerant. As a result, the condensing pressure increased, the evaporating pressure decreased, the pressure drops of both heat exchangers increased, and the pressure ratio increased. Figure 5d shows refrigerant temperatures. As the condenser heat transfer was limited by the air-side heat transfer, the condenser outlet temperature remained almost the same regardless of the refrigerant MFR change. This resulted in increased vapor quality Steam Engines use water vapor to drive pistons which effects work through movement. The quality of steam can be quantitatively described. Vapor quality is a quantitative description of the usefulness of a vapor to do work. as the evaporating pressure decreased. To transfer more heat at the lower evaporating pressure and at the increased refrigerant MFR, the outlet temperature of the evaporator became higher, since the saturated vapor enthalpy enthalpy (ĕn`thălpē), measure of the heat content of a chemical or physical system; it is a quantity derived from the heat and work relations studied in thermodynamics. decreased as the evaporating pressure decreased. This resulted in a higher degree of superheating
In physics, superheating (sometimes referred to as boiling retardation, or boiling delay , as illustrated in Figure 5e. When the refrigerant MFR increased, the compressor suction entropy entropy (ĕn`trəpē), quantity specifying the amount of disorder or randomness in a system bearing energy or information. Originally defined in thermodynamics in terms of heat and temperature, entropy indicates the degree to which a given and the condensing pressure increased. This resulted in an increase in condenser inlet temperature. As the condenser outlet temperature remained almost unchanged while the condensing pressure increased, the degree of subcooling increased. The refrigerant-side capacity and the air-side sensible, latent, and total capacities are shown in Figure 5f. [FIGURE 5 OMITTED] It can be seen that whenever the rpm increased, the capacities increased due to the increased refrigerant MFR. The locations in which the refrigerant-side capacity curve is discontinuous discontinuous /dis·con·tin·u·ous/ (dis?kon-tin´u-us) 1. interrupted; intermittent; marked by breaks. 2. discrete; separate. 3. lacking logical order or coherence. are where the superheat su·per·heat tr.v. su·per·heat·ed, su·per·heat·ing, su·per·heats 1. To heat excessively; overheat. 2. at the evaporator outlet was lost and, therefore, the refrigerant-side capacity could not be calculated. The refrigerant-side capacity was obviously more than the air-side capacity, as expected in a transient case, but the difference between them was decreasing. Whenever the rpm increased, the latent capacity increased because of the decrease in evaporating temperature. The increase in pressure ratio and refrigerant MFR caused an increase of compressor power, as shown in Figure 5g. It can be seen in the figure that the compressor power fluctuated between 0.5 and 2.0 kW. It is interesting to note the phase shift between the refrigerant MFR and the air-side capacity. In the figure, it is clear that the air-side capacity lagged the refrigerant MFR, while the compressor power was in phase with the refrigerant MFR. This is because the heat transfer from the air to the evaporator coil and then to the refrigerant was a relatively slower process than the redistribution of the refrigerant inside the system. As the com-pressor rpm increased, the compressor power increased more than the capacity did, and, therefore, the instantaneous in·stan·ta·ne·ous adj. 1. Occurring or completed without perceptible delay: Relief was instantaneous. 2. COP decreased with the increase of rpm. As illustrated in Figure 5h, the instantaneous COP fluctuated with time as the compressor rpm varied over time. For this reason, the air-side capacity and the compressor power consumption were both integrated over time for the duration of the test to provide a more meaningful metric in such a transient situation. As the instantaneous COP varied, the slope of their cumulative values changed slightly over time. The overall COP until any elapsed time can be obtained from the cumulative cooling capacity and power consumption value at that time. In addition, in Figure 5h, the slopes of the cumulative energy consumption and cooling capacity are different. This means that the overall COP until any elapsed time is not equal to 1. In fact, the overall COP value during 19 minutes was 1.72, as noted in Figure 5h. The overall COP can be used as a transient performance index when different MAC are compared under the same transient operating conditions, such as a standardized standardized pertaining to data that have been submitted to standardization procedures. standardized morbidity rate see morbidity rate. standardized mortality rate see mortality rate. drive cycle. Drive-Cycle Tests with a Thermostat In the third category, the thermostat function was imposed on the system under the previous drive-cycle test conditions. The thermostat was set to 24[degrees]C [+ or -]1 [degrees]C so that when the cabin air temperature decreased to 23[degrees]C, the compressor clutch was disengaged dis·en·gage v. dis·en·gaged, dis·en·gag·ing, dis·en·gag·es v.tr. 1. To release from something that holds fast, connects, or entangles. See Synonyms at extricate. 2. , and when the cabin air temperature increased to 25[degrees]C, the compressor clutch was engaged. Since it took approximately 17 minutes for the cabin air temperature to reach 24[degrees]C, the NEDC cycle was repeated two times. Figure 6a shows the different cabin temperatures. The supply air temperature is shown by the dashed line, and the internal mass temperature is shown by the solid line with cross symbol; the internal mass temperature decreases slower than the supply air temperature. Two cabin air temperatures, one for the calculated values and another for the measured values, are plotted together. The cabin air temperature decreases to 23[degrees]C and then starts to cycle around 24[degrees]C. The difference between the calculated and measured cabin air temperatures is shown on the second y-axis of Figure 6. The difference is bounded by [+ or -]0.9[degrees]C, which indicates the accuracy of the cabin temperature control of the current test facility. Figure 6b shows the supply air and cabin air RHs. As the system started, the RH downstream of the evaporator increased. When the system stopped, the moisture captured on the evaporator re-evaporated. Therefore, the RH of the supply air increased and, subsequently, the RH inside the cabin increased as well. Due to the thermostat action, the cabin-air RH fluctuated between 15% and 35%. The difference between the calculated and measured cabin RH is shown on the second y-axis of Figure 6. The difference is bounded by -2% RH and 10% RH, which indicates the accuracy of the cabin RH control of the current test facility. [FIGURE 6 OMITTED] Figure 6c shows the capacities and COPs during the test. The line with cross symbol indicates the latent capacity. It shows a negative latent capacity during the compressor-off period due to the re-evaporation as explained. This re-evaporation of the condensate during the compressor-off period adds a cooling load to the MAC when it starts again. Furthermore, for cycle times in the order of a few minutes, it can eliminate any net latent cooling capacity of the MAC. The line with circle symbol indicates the sensible capacity. It shows that during the off period, it did not drop to zero instantaneously in·stan·ta·ne·ous adj. 1. Occurring or completed without perceptible delay: Relief was instantaneous. 2. due to the thermal storage effect of the evaporator. In total, the air-side capacity had a small positive value but was lower than the sensible capacity during the compressor-off period. Therefore, from an energy point of view, the re-evaporation of the condensate during the compressor-off period adds a cooling load to the MAC when it starts again. Figure 6c also shows the refrigerant-side capacity and the instantaneous COP calculated using the air-side capacity. It shows that the COP was slightly lower when the compressor restarted after the thermostat action than when the compressor rpm varied without being interrupted in·ter·rupt v. in·ter·rupt·ed, in·ter·rupt·ing, in·ter·rupts v.tr. 1. To break the continuity or uniformity of: Rain interrupted our baseball game. 2. by the thermostat. CONCLUSIONS MACs are subjected to time-dependent loads under typical driving patterns. These include redistribution of refrigerant and oil among the different components of the system; readjustment re·ad·just tr.v. re·ad·just·ed, re·ad·just·ing, re·ad·justs To adjust or arrange again. re of operating parameters, such as the temperature of the different parts of the system and the temperatures and pressures of the refrigerant to their new operating values; and reconditioning the thermal mass of the conditioned space to the steady-state air temperature. During this period, the COP suffers losses. To test the dynamic behavior of a MAC, we used the dynamic simulator, which is able to run weather cycles and drive cycles, simulate the thermal storage load of the conditioned space, simulate changes in space loads and allows for different user settings, including fan speed and percentage of fresh air. The accuracies of the cabin temperature and RH controls by the dynamic simulator used in the current study were [+ or -]0.9[degrees]C and -2 RH to +10 RH from their respective target values. Three types of transient tests, pull-down tests, drive-cycle tests, and drive-cycle tests with a thermostat, were conducted to provide detailed transient behaviors of the MAC. From the test results, the following was observed: * The effect of the thermal storage in the system mass was obvious in the difference between the refrigerant-side capacity and the air-side capacity and also in the refrigerant temperature at the accumulator outlet. * Different time constants of the system were observed due to the different rates at which the power and the capacity built up. The power built up faster than the capacity and, hence, the COP decreased. * An optimum control strategy to pull down the cabin temperature was developed. In the case of a hot-soaked car cabin under the specific test conditions used, starting the pull down with fresh air an then switching to recirculated air when the cabin temperature reached the outdoor ambient temperature saved two minutes in the time required to reach 24[degrees]C, reduced energy consumption by 12.3%, and enhanced the COP by 21.6% for the pull-down period until the cabin air temperature reached 24[degrees]C. * In the case where the condenser air was 5[degrees]C hotter than the ambient air (for example, due to air-recirculation) under the specific test conditions used in this study, the energy consumption was 30.7% higher and the COP was 18.1% lower than the case without the hot soak. * The effect of the thermostat action on the RH inside the car cabin was observed. During the compressor- off period, the RH inside the cabin increased by the re-evaporation of the condensate on the evaporator. This re-evaporation of the condensate during the compressor-off period adds a cooling load to the MAC when it starts again. Furthermore, for cycle times in the order of a few minutes, it can eliminate any net latent cooling capacity of the MAC. * The effect of rpm variations on the latent capacity and the sensible capacity was noted at the specified test conditions. The latent capacity was more affected than the sensible capacity, as the evaporating temperature responded quickly to rpm changes. The dynamic simulator, a means to run transient tests for MACs in the controlled environment of a laboratory, can be utilized to enhance the MAC's efficiency and to set new test standards for rating the dynamic performance of MACs, taking into consideration interactions between the test system and the controlled space. As a new transient performance indicator, an overall COP is proposed by integrating the air-side capacity and the compressor power consumption over the entire test period, such as a standardized drive cycle. The overall COP can be used as a transient performance index when different MACs are compared under the same transient operating conditions. ACKNOWLEDGMENTS The support of this research through the Alternative Cooling Technologies and Applications Consortium of the Center for Environmental Energy Engineering at the University of Maryland University of Maryland can refer to:
NOMENCLATURE nomenclature /no·men·cla·ture/ (no´men-kla?cher) a classified system of names, as of anatomical structures, organisms, etc. binomial nomenclature AHU = air-handling unit C = specific heat, kJ/kg*K [C.sub.p] = specific heat at constant pressure, kJ/kg*K h = enthalpy, kJ/kg M = mass, kg P = pressure, kPa Q = cooling capacity of the MAC, kW t = time, s T = temperature, [degrees]C W = humidity ratio, kg/kg, or power input, kW Subscripts amb = ambient air c = core, or interior mass comp = compressor cond = condenser e = evaporator f = final [f.sub.g] = latent heat latent heat, heat change associated with a change of state or phase (see states of matter). Latent heat, also called heat of transformation, is the heat given up or absorbed by a unit mass of a substance as it changes from a solid to a liquid, from a liquid to a gas, i = initial iv = infiltration infiltration /in·fil·tra·tion/ (in?fil-tra´shun) 1. the pathological diffusion or accumulation in a tissue or cells of substances not normal to it or in amounts in excess of the normal. 2. infiltrate (2). and/or ventilation air m = mixture, or return air o = cabin, overall p = pipe [p.sub.s] = passengers, sensible [p.sub.l] = passengers, latent r = room, or inside of cabin s = supply air sol = solar radiation solar radiation, n the emission and diffusion of actinic rays from the sun. Overexposure may result in sunburn, keratosis, skin cancer, or lesions associated with photosensitivity. REFERENCES Althouse, A.D., C.H. Turnquist, and A.F. Bracciano. 2000. Modern Refrigeration refrigeration, process for drawing heat from substances to lower their temperature, often for purposes of preservation. Refrigeration in its modern, portable form also depends on insulating materials that are thin yet effective. and Air Conditioning air conditioning, mechanical process for controlling the humidity, temperature, cleanliness, and circulation of air in buildings and rooms. Indoor air is conditioned and regulated to maintain the temperature-humidity ratio that is most comfortable and healthful. , 2d ed. Tinley Park Tinley Park, village (1990 pop. 37,121), Cook and Will counties, NE Ill., a residential suburb of Chicago; inc. 1892. Its population grew significantly in the late 20th cent. , IL: Goodheart-Willcox Co., Inc. Gado, A., Y. Hwang, and R. Radermacher. 2004. Measurements of the dynamic performance and behavior of air conditioning systems using a dynamic test facility. Proceedings of the 10th International Refrigeration and Air Conditioning Conference at Purdue, West Lafayette West Lafayette, city (1990 pop. 25,907), Tippecanoe co., W Ind., a suburb of Lafayette, on the Wabash River; inc. 1924. A primarily residential city, it is the seat of Purdue Univ. , IN. Huang, C.D. 1998. A dynamic computer simulation model for automobile passenger compartment compartment a part of the body as a whole and divided from the rest by a physical partition. fluid compartment that liquid part of the body excluded by cell membranes. Includes intravascular and intercellular compartments. climate control and evaluation. PhD thesis, Michigan Technological University Michigan Technological University (abbr. Michigan Tech or MTU) is an American public university with a range of degree offerings. Michigan Tech's main campus is in Houghton, Michigan, in the Upper Peninsula. , Houghton, MI. Inui, K., and Y. Tomatsu. 2004. Concerns and solutions for CO2 A/C systems for compact vehicles. SAE sae abbr (BRIT) (= stamped addressed envelope) → sobre con las propias señas de uno y con sello Mobile Air Conditioning Summit, April 14-15, Washington D.C. Meyer, J. 2002. HMC HMC Harvey Mudd College (Claremont, CA) HMC Harborview Medical Center (Seattle, Washington) HMC Hosted Messaging and Collaboration HMC Hoffman Modulation Contrast sonata: Thermal energy thermal energy Internal energy of a system in thermodynamic equilibrium (see thermodynamics) by virtue of its temperature. A hot body has more thermal energy than a similar cold body, but a large tub of cold water may have more thermal energy than a cup of boiling efficient vehicle. SAE Automotive Alternate Refrigerant Systems Symposium, July 9-11, Scottsdale, AZ. Mulroy, W., and D. Didion. 1983. A laboratory investigation of refrigerant migration in a split unit air conditioner conditioner, n 1. an additive substance used to increase the effectiveness of another substance. 2. a substance added to enamel that improves a sealant's ability to adhere. . Report #NBSIR 83-2756, National Bureau of Standards National Bureau of Standards: see National Institute of Standards and Technology. National Bureau of Standards - National Institute of Standards and Technology , Gaithersburg, MD. Rubas, P.J., and C.W. Bullard. 1995. Factors contributing to refrigerator cycling losses. International Journal of Refrigeration 18(3):168-76. Sumantran, V., K. Saka, and S. Fischer. 1999. An assessment of alternative refrigerants Chemical refrigerants are assigned an R number(sometimes the label replaces it with the word Freon) which is determined systematically according to molecular structure. The following is a list of refrigerants with their R numbers, IUPAC chemical name, molecular formula, and CAS number. for automotive applications based on environmental impact. Proceedings of SAE Alternate Refrigerant Systems Symposium, Scottsdale, AZ. Wertenbach, J. 2003. Energy analysis of refrigerant cycles. Proceedings of the Automotive Alternate Refrigerant Systems Symposium, Scottsdale, AZ. Amr Gado, PhD Associate ASHRAE ASHRAE American Society of Heating, Refrigerating & Air Conditioning Engineers Yunho Hwang, PhD Member ASHRAE Reinhard Radermacher, PhD Fellow ASHRAE Amr Gado is a researcher, Yunho Hwang is a research associate professor, and Reinhard Radermacher is professor and director of the Center for Environmental Energy Engineering, Department of Mechanical Engineering, University of Maryland, College Park The University of Maryland, College Park (also known as UM, UMD, or UMCP) is a public university located in the city of College Park, in Prince George's County, Maryland, just outside Washington, D.C., in the United States. , MD. |
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