Detail Artikel

The Application of Variable Speed Controls to Air-Conditioning System

INTRODUCTION

Most air conditioning systems countries located in the tropics operate at constant compressor speed as these countries experience a quite moderate diurnal temperature variation on the order of five to ten degrees throughout the year. The temperature inside the building is maintained constant using a simple on/off system to the air-handling unit. In many cases, no proper control system is used to conserve energy. The selection of these systems for most application is mainly based on capital cost of the equipment and the use of control system to conserve the electrical energy is not of prime importance.

In cases where the accurate control of temperature of an environment is needed, for example in the manufacturing of electronic components, cooling and dehumidifying of air is accomplished through heating and cooling of air to the required conditions in the air handling unit. Currently, there is a wide concern about the optimum use of energy in building, as the price of fuel has doubled in the last five years. Energy conservation and thermal comfort in buildings are topics of general interest. One of the methods that has been suggested and investigated to maintain a thermal comfort of an environment room and to reduce the energy consumption from an air-conditioning unit is through the use of well-tuned controller for the Air Handling Unit (AHU) and Variable Speed Compressor (VSC). This involves the development of various types of controller either for AHU or the compressor system. Among many control methods for Heating and Ventilation of Air-conditioning application, the PID (proportional-integral-derivative) algorithm is very common.
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For example Nesler (1984) reported the behavior of the proportional and integral constants in combination to provide responsive, yet stable, control in the HVAC system. Three–way bypass valve were used in this study and the results are valid for valve controller application. Ho (1993) developed and evaluated a software package for self-tuning of three-Term DDC Controllers using a searching technique for optimization. A simulation model for a practical air-handling system was studied. The behavior under a conventional system of PID controllers was investigated. A new controller based on system identification model was developed and tested where input and actuating variables were incorporated into system identification model. This model could predict the new system status based on past records and suggest the optimum control actions. Computer simulation had proved that such a system identifications based controller was superior to the conventional PID controller in at least three major aspects: adaptation to system change, response rate and energy conservation. The result of the study has not been tested for variable speed compressor and may be valid only for AHU controller mechanisms.

Lloyd (1982) presented an analysis the use of inverter on Variable Speed Motors of a compressor. He compared the two types of waveforms used on the motor- the six-step and Pulse Width Modulated (PWM) and a pure Sinusoidal forms and indicated that the efficiency of the compressor can be maintained high while the output power varied over a wide range for a sinusoidal waveform. His study suggests that varying the power supply frequency to the motor is an excellent way of modulating its output.

Riggers (1988) demonstrated a potential of a VSC for providing enhanced load matching capability in both cooling and heating modes. He mentioned that there is a good opportunities for enhancing system performance in both the air conditioning and heating modes. The largest opportunities lie at speeds below 60 Hz for air conditioning, where a significant increase in efficiency can be achieved. For heating mode, operation above 60 Hz can significantly increase the capacity of the system.
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Krakow (1995) investigated the use of Proportional-integral-differential (PID) controller on an AHU and a compressor of an air-conditioning unit. He showed that such methods are suitable for attaining compressor and evaporator fan speeds such that sensible and the latent components of the refrigeration system capacity equals the sensible and latent component of the system loads. The investigation also indicated the space temperature and humidity were not successfully controlled simultaneously by the variation of evaporator fan speed and compressor speed, respectively. However, the study did not include the energy and the performance analysis of the air-conditioning unit.

Koury (2001) conducted a numerical study to simulate the transient and steady state behavior of a vapor compression refrigeration system. The result of the simulation indicates that the control refrigeration capacity by varying the compressor rotational speed leads to increasing the degree of superheat and hence impaired the COP of the system.

This paper reports result of a research work aimed at quantifying the performance an air conditioning system operating on an inverter and a controller installed to vary the speed of the compressor for load matching and thermal comfort. The emphasis is on the energy consumption using PID controllers and the effect of PID control gain factor in the determination of optimum energy. The investigation consisted of three parts. The first part to was to determine the temperature and energy for the room at various speed of the compressor. The second phase was to determine the PID control gain factor, Kp, Ki and Kd. The third phase is to compare the use energy under on/off condition and PID controlled conditions.

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METHODOLOGY

Experimental tests were performed with following objectives : to determine the energy consumption and temperature distribution at various compressor speeds, to determine PID control gain for optimum energy consumption, and to compare the energy consumption for on/ff and variable speed conditions.

RESULTS

Energy Consumption and Temperature Distribution at Constant Speed

The rate of decrease in temperature to the steady state condition is different for each speed, indicating that the cooling rate and the time to reach a steady state temperature are different. For this system, the steady state temperature varies from 18 C to 26 C. The energy consumption is proportional to frequency of the input current to the motor.

PID Control for Optimum Energy Consumption

The determination of optimum value of Kp, Ki and Kd was done by a trial and error. The optimum energy consumption for the system studied was found to be at Kp = 3.3, Ki = 0.180 and Kd = 0.040 both for the room with thermal load and without load. The detailed experimental determination of optimum values of all control gain (Kp, Ki and Kd) is given in (Henry 2005). From the response curves shown for both under load and unload conditions, it was found that the control systems are stable, no offset from the set point is observed and the settling times are acceptable. The system response is almost the same for both no load and with load conditions.

On/Off and Variable Speed Condition

Under on/off condition and at a temperature setting of 22 C, the compressor stopped seven times in two hours, indicating that the compressor is over sized. However, when the room was under the conditions of thermal load, the compressor stopped eight times. The compressor rotates at 1420 rpm. The speed variation for controlled system was found to be between 200 and 600 rpm. At temperature setting 22 C, the energy saving for system is obtained to reach : P = 13.44 to 21.49 %, PD = 8.74 to 16.03 %, and PID = 4.98 to 18.74 %.
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CONCLUSION

The application of variable speed controls to air-conditioning offers the potential for substantial energy savings or energy efficiency.

The investigation indicates the space temperature may be controlled by inverter driven motor to achieve energy saving. Under no load condition, the frequency setting of the inverter can be predetermined. Under various load conditions, to obtain a required temperature, further work is required to obtain accurate correlation between frequency and load imposed to the room.

The employment of well tuned PID controller may produce satisfactory solution to energy saving for the room. Under on/off condition, the motor speeds fluctuate from zero to maximum speed and stops several times. This may affect the lifetime of the motor.


Henry Nasution

PhD Student
Mechanical Engineering Faculty
Universiti Teknologi Malaysia