DC Home Appliances for DC Distribution System

This paper strengthens the idea of DC distribution system for DC microgrid consisting of a building of 50 apartments. Since the war of currents AC system has been dominant because of the paucity of research in the protection of the DC system. Now with the advance research in power electronics material and components, generation of electricity is inherently DC as by solar PV, fuel cell and thermoelectric generator that eliminates the rectification process. Transformers are replaced by the power electronics buck-boost converters. DC circuit breakers have solved the protection problems for both DC transmission and distribution system. In this paper 308V DC microgrid is proposed and home appliances (DC internal) are modified to operate on 48V DC from DC distribution line. Instead of using universal and induction motors in rotary appliances, BLDC (Brushless DC) motors are proposed that are highly efficient with minimum electro-mechanical and no commutation losses. Proposed DC system reduces the power conversion stages, hence diminishes the associated power losses and standby losses that boost the overall system efficiency. So in view of all this a conventional AC system can be replaced by a DC system that has many advantages by cost as well as by performance.


INTRODUCTION
I n late the 1880s, the war between two rivals in favor of AC and DC power was won by the AC because of the lack of technologies in DC system. Carbon emissions coexist with the generation of AC by traditional dominant fossil fuels that seriously not only balancing out the earth environment but may also be a cause of the extinction of earth species. In DC distribution system with DC sources and DC appliances, 47% energy saving is observed by Gholase and Fernandes [1] and 22% system efficiency is improved by Anand and Fernandes [2]. In this paper system efficiency is improved by removing conversion stages and replacing induction motors with efficient BLDC motors. Keeping an eye over the present rate of research on DC, future is pro to DC technology. A typical DC distribution system is shown in Fig. 1. Such stand-alone DC distribution systems are flexible enough to integrate with other distributed generations [3].
Most appliances work internally on DC power; they take AC as input and after conversion process bring the required DC voltage level. Garbesi et. al. [4] 33% energy saving is estimated by replacing standard technology with the DC internal efficient appliances and a further 14% by operating DC internals on direct DC. These DC internal appliances take AC as input followed by AC/DC converter. This conversion process creates heat losses both in working mode and standby mode. If these appliances are fed with solar PV power, transformer and rectifier would be terminated from the circuit [5] that saves a considerable amount of energy [6].
The absence of skin effect in DC system [7] reduces the material cost of the wiring as the whole cross section carries the current. The overall cost of the system is greatly reduced as the equipment for the power factor improvement, AC/DC and DC/AC conversion and reactive power compensation is not required in DC distribution system [8]. Enhanced energy savings and improved reliability by DC distribution system is recommended by Vagelis et. al. [9]. Soo and Jung [10] have studied the efficiency of the DC distribution system by using multiple DGs (Distributed Generations) while Kamran et. al. [11] has studied MPPT based DC-DC buck-boost converter for multiple DGs.
With the advancement in semiconductor materials and devices DC can directly be generated by using solar PV cell. Solar PV cell generates clean and green renewable energy. Historically, this energy is either stored in batteries or delivered to the AC grid through inverters that brings major conversion losses.

EFFICIENCY IMPROVEMENT OF DC SYSTEM
The efficiency of a DC distribution system depends upon the losses in the cables and conversion losses (power electronics + electro-mechanical conversion stage) as given by Equation (1). P loses = P cables + P converter + P mechanical (1) P cables are the losses occurring in the distribution cables. They depend upon the voltage level of the distribution line. In this paper 308 V DC, the peak value of 220V AC, is selected as a distribution line voltage since the voltage level below this increases the losses in cables as given in Table 1 calculated by Equation (2) [12]. Where ρ is the electrical resistivity (2x10 -8 Ω.m), A is the X-sectional area of cable (60mm 2 ), is the total length of the cable (200m), P is the total power that load absorbs (100 W assumption for each apartment), η is the converter efficiency (85% assumed), V DC is the DC distribution line voltage level, N is the number of apartments in the building that is assumed 50 in this paper.
P converter are the losses that occur in the converter. They are in inverse relation with the efficiency of the converter as given in Equation (3) [12].
Efficiency (η) of the DC-DC converter is higher than DC-AC-DC because of the double conversion in the latter converter [13]. This is one of the reasons that DC home appliances operated on DC from DC distribution line are proposed here.

Appliances with Motors
Most of the appliances having rotatory part use universal dc motors or 1-θ induction motors which have low efficiency because of carbon brush and mechanical commutation bearing a major maintenance cost and mechanical losses. All such motors can be replaced with BLDC which has a high efficiency, low noise [14], low power consumption and a wide range of speed.
In BLDC motor permanent magnet acts as a rotor while 3-θ DC supply is given to the static stator coil. There is low stator and rotor air gap which increases the efficiency by increasing the magnetic torque on the rotor by the stator. So no need of carbon brushes to supply the current which makes it maintenance free. But because of the presence of magnet, electronics and sensor, BLDC motor is costly than induction and universal motor [15]. With the adoption of DC grid and BLDC motor in industrial and residential appliances, the cost will definitely be reduced in future.  [16,17] which can be operated on the proposed DC distribution system with the incorporation of a DC-DC converter.

Air Conditioner
The proposed BLDC motor based DC operated air conditioner is depicted in Fig. 3.

Microwave Oven
Microwave oven consists of two sections with separate independent circuits and power supplies. HVS (High Voltage Section) and LVS (Low Voltage Section). HVS contains magnetron whose power supply consists of a high voltage transformer followed by a voltage doubler.
While LVS contains turntable motor, magnetron fan motor (1-θ induction motor) [18] and switching device. In AC operated microwave oven 220V from input AC source is supplied to the turntable and magnetron fan motor as shown in Fig. 4. In some modified ones, the 220V is stepped down to operate a 21V turntable AC motor that reduces manufacturing cost and power consumption [19].
In DC microwave oven input AC is rectified into DC using AC induction motor [23]. These fans would be highly efficient with minimum conversion losses.

Cloth Washer
Most of the cloth washers available in markets incorporate 1-θ induction motors. They use transformer and rectification process to obtain a DC lowlevel voltage to operate the control circuit. Fisher and Paykel have some models that use BLDC motors but incorporate a rectifier.
In lieu of using AC power followed by transformer and rectifier, DC input from a DC distributed line is proposed to operate BLDC motor and control circuit of the washing machine and dryer.

Water Pump
Current home water pumps use a 1-θ induction motor operated on 220V AC. In solar DC water pumps for flood irrigation an inverter is used to run 1-θ induction motor.

Air Cooler
In air cooler 3 motors are used. 1-θ induction pump motor, 1-θ induction fan motor and a swing motor. In proposed air cooler all these motors are replaced with BLDC motor.
In solar air cooler DC power is inverted to AC which is eliminated in the proposed system.

Electronics Appliances
Laptop computer, TVs, security cameras and data centers are the major electronics load which can be made efficient or even energy star just by replacing input AC/DC converter with the buck-boost converter.

Laptop
In current laptop adapter, 220V AC is taken from the distribution line. After stepping it down and rectification process it is decided by the switching technique whether to charge the battery or run the laptop or both. Using buck converter the output 19V DC is reduced to 5, 3.3, 2 and 1.5V for various components to operate. The power architecture is shown in Fig. 7.
In proposed scheme input is directly taken from 48V DC distribution line followed by a buck converter with the 19V output. Hence this proposed DC adapter is much efficient than current AC adapter as major power conversion losses are eliminated in proposed architecture as shown in Fig. 8. to 10% of the incandescent bulbs [25]. Table 2 shows that LED consumes less power as compared to old technologies and average life is much larger.

LED television
In AC fed LED TVs, input AC is passed through EMI

DC Home Appliances for DC Distribution System
The power consumption of different television technologies of different screen sizes is given in Table 3.
From Table 3 it is obvious that LED TV is the most efficient one which can further be improved by excluding rectification stage proposed in Fig. 10.

Reduction in Power Consumption
In order to estimate the energy savings by adopting a DC distribution system, power ratings of different home internally DC appliances taking AC as input are collected from [4]. AC-DC power conversion losses are calculated by using Equation (3). Then a load profile for a family of 3 members for a month is calculated using calculator [27] and it is observed that 65.4 kWh/month/home energy can be saved if the internally DC appliances are operated directly on DC as shown in Table 4.
The other advantage of removing AC-DC converter is the savings of standby electricity which is consumed even the device is switched off but still plugged in standby power of different appliances is given in  [1,21,22,28] and it is estimated that an overall 24% efficiency of the system is improved by replacing AC motors with BLDC motors shown in Table 6.

CONCLUSION
This paper investigates the energy savings by adopting DC distribution system instead of conventionally dominant AC distribution system. A DC microgrid of 308V is chosen as a distribution line voltage. Home appliances