DC Distribution System for Energy Efficient Buildings R. K. Chauhan School of Computing and Electrical Engineering Indian Institute of Technology Mandi Mandi, India
[email protected] Abstract— In the present scenario, electronic load is continuously increasing in the buildings which needs Direct Current (DC) input. These loads require conversion from AC to DC power. On the other hand the Renewable Energy Sources (RES) such as Solar Photovoltaic (PV) produce DC power which has to be converted into AC to tie into electric power systems and again converted into DC for DC compatible loads. These AC-DC and DC-AC-DC conversion stages introduce the energy losses. The DCDS eliminates the conversion stages. It can also decrease the power losses to an acceptable extent. In this paper two case studies have been described 1) the building is based on the AC Distribution System (ACDS) and supplied by public utility including PV and battery bank, 2) The building is based on DC Distribution System (DCDS) equipped by PV and battery bank including public utility. Results have been simulated in the LABVIEW environment. Outcomes show that the DC distribution system with DC internal technology appliances provide the largest energy saving and reduce the building load. Keywords— Solar photovoltaic, DC distribution system, Power loss optimization, DER size optimization, DC microgrid, DC Appliances.
I.
INTRODUCTION
The Renewable Energy Sources (RESs) like wind, hydro and solar power plants play an important role to produce electricity without the harmful emission of greenhouse gasses. The RES can also help to come over the peak demand and shortage of electricity, especially in India as well as in the world [1]. According to Central Electricity Authority of India (CEA), India is running shortage of around 11% peak demand and more than 24% transmission and distribution losses [2]. The energy consumption in commercial and residential building has been increased 20% and 40% in developed countries [3]. The electrical distribution infrastructure is also not very well extended in rural areas. But the technical and economical development during the last decades has established opportunity to create a new competitive distribution system based on modern power electronic technology. From a technological point of view, the DC distribution system is a new concept in electrical power system and it generates new opportunities for power electronic device manufacturers. Photovoltaic (PV) system can be the good solution for getting a good electrification in rural areas with the implementation of DC distribution system. The PV panels can
978-1-4799-5141-3/14/$31.00 © 2014 IEEE
B. S. Rajpurohit School of Computing and Electrical Engineering Indian Institute of Technology Mandi Mandi, India
[email protected]
be directly utilized at the value of DC microgrid without converting it into AC [4]. In general, output of PV panel is DC power and this DC power is injected into an electric grid by using suitable power converters by converting DC into AC [5]. In the present scenario most of the loads in the residential/commercial buildings are based on DC loads or AC loads which can be easily converted into DC loads [6]. Hence, the DC output of the RES can be directly used within the buildings without converting into AC. It can be made possible by implementing a DC distribution system (DCDS) [7]. The DCDS also helps to reduce the shortage of electricity, to improve the reliability of the power systems and to reduce the transmission and distribution losses [8]. The estimate lifecycle energy savings, carbon emission reduction, and costeffectiveness of energy efficiency measures in new commercial buildings have been discussed in [9]. The DC appliances are energy more efficient than the AC appliances [10]. On the other hand the DC system is free from inductance, capacitance, and skin effects. Due to no inductance effect, the DC system has low voltage drop than AC system for same load and sending end voltages and hence a better voltage regulation can be attained. As there is no capacitance in the DC systems, the power losses which occur in charging and discharging of capacitance will be removed. So there are less power losses in DC system in comparison to AC systems [11]. The conductor resistance is inversely proportional to the cross section area of the conductor. Due to the skin affect the entire cross section area of the line conductor is used in DC system. Hence the DC system has less line resistance than the AC system. In this paper, a DC distribution system with PV for energy efficient buildings has been proposed. The concept of using DC distribution is extended by comparing system efficiency and power losses for two following cases: AC Distribution System (ACDS) equipped by PV and battery bank including with public utility, DCDS supplied by PV and battery bank including with public utilities. Previously, a number of research papers [12-15] have been written on DCDS but no one describes the system which is completely based on DC distribution. It is found that DC compatible load with DC supply is more energy efficient. This paper is arranged as follows. Section I having the related literature, introduction and summary of sections. In section II, the changes in total daily load, current, voltage and power losses with AC and DC distribution system have been described. Section III, proposed DC distribution system for energy efficient buildings and
shows its controlling and monitoring with LabVIEW simulator. Two case studies have been described in section IV and a conclusion has been given in section V by stating the main results. The proposed DC distribution system can easily tie with the distributed generation such as PV and can be directly fed to the consumers. II.
ANALYSIS OF AC VS. DC SYSTEM
In this section mathematical descriptions have been derived to find out voltage, current and power losses during AC as well as DC distribution system supply. A. Total Daily Load Total Daily Load (TDL) has been calculated with respect to the different DC voltage level supply as well as the different AC voltage level and inverter efficiency. When the DC Load is applied to system then the direct current load in amperes is given as:
γ= (2)
losses in DC to AC conversion are lesser and the system TDL will be reduced. As the system voltage is high the system TDL will also decrease, i.e. system losses may decrease. On the other hand if the inverter efficiency or DC supply voltage is lower than TDL will be higher. It means battery bank will discharge at fastest rate which decreases battery life and efficiency.
k
(1)
υ
And Total Daily Load (TDL) in ampere hours is as in eq.
(2) Γ =γ ×χ Where γ is the direct current load in Amperes; κ = Load rating in kilowatt; υ = DC system voltage; χ =Number of operating hours per day. If there is variable DC load connected to the system, the calculation of total daily load is as follows Γ=
Ω
γ δ χδ ∑ δ
(3)
=1
where γ 1 , γ 2 ,...γ Ω and χ1 , χ 2 ,...χΩ are DC loads in amperes and DC loads run time in percentage of the day respectively. If Alternating Current (AC) load is connected to the system, then the AC voltage must be converted to DC voltage and the inverter efficiency is also considered. The calculation of total daily load is as follows: k μ= (4) ν ac
μ× γ=
ν ac υ
ηinv
(5)
Where μ is AC load in amperes at rated voltage, ν ac =AC load at rated voltage and ηinv = Inverter efficiency As shown in table I, if the DC load 2.4 kW is supplied at 24 Volt and 48 Volt, the Total Daily Load (TDL) is 2400 Ah, 1200 Ah respectively. The 2.4 kW load at 120 VAC, 220 VAC supplied by the same efficiency 92% inverter then TDL 2608.7 Ah remains same but higher than the 2.4 kW load at 24 VDC. If the 2.4 kW load is supplied at 230 VAC by the inverter of efficiency 95% the TDL 2526.32 Ah. A table I shows, system TDL depends upon the DC system voltage and inverter efficiency. If the inverter efficiency is higher, the
TABLE I.
TOTAL DAILY LOAD ON DC SYSTEM
Load Rating
Inverter Efficiency
Total Daily Load (Ah)
24V
2.4kW
-
2400
-
48V
2.4kW
-
1200
3.
120 V
-
2.4kW
92%
2608.7
4.
220 V
-
2.4kW
92%
2608.7
5
230 V
-
2.4kW
95%
2526.32
S.N. AC
DC
1.
-
2.
B. Voltage, Current and Power Losses at DC Supply In India the AC distribution system for residential buildings is single phase 230 volt (rms). The equivalent DC voltage (325 volt) applied to the same load can reduce the current ratings. For 230 volt and 110 volt AC supply the current rating can be reduced to 30% and 70% respectively [13]. DC equivalent voltage can be calculated as follows: (6) For single phase system: Vdc = 2×Vac Volt
For three phase system: Vdc =
3 3
π
V
p
Volt
(7)
where Vdc is the DC system voltage and V p is the peak value of the phase (line to neutral) input voltages. Ρ=VI Watt (8) P (9) I= Ampere V where P = power transfer, V and I is the system voltage and current respectively. As shown in Eq. (6) and (7), if DC voltage is applied instead of AC voltage then system voltage rating can be increased to 41.3%. Eq. (9) gives the current is inversely proportional to the voltage. TABLE II.
SYSTEM PARAMETER FOR 325 VOLT DC Reduction in
Reduction
Current Rating
Power losses
230
29.24%
50.07%
110
66.15%
88.54%
DC Voltage
AC Voltage
325 325
As shown in table II, the 230 volt (rms) AC system voltage is equal to the 325 volt DC system voltage. It means if a system is design for 230 volt AC than it can operate at 325 volt DC [7]. From table II have been proved, the distribution system current rating and power losses are less at the 325 volt DC than 230 volt AC. Table III shows the energy savings switching from AC technologies to the most energy efficient
DC-internal technologies. It is seen from the table III, that the total energy saving with most efficient DC internal technologies at most of the residential loads is 12% to 71%. Where the rectifiers losses have exist in some cases like refrigerator, dishwasher, cloth-washer and fans etc TABLE III.
As shown table IV shows, the power consumption by appliances and there the corresponding AC-DC conversion efficiencies. It is clear from this table that higher the consumption high is the AC-DC conversion efficiency.
ENERGY SAVING POSSIBLE FROM MOST EFFICIENT DCINTERNAL TECHNOLOGY Total Energy Saving
Appliances
Efficient DC Compatible Replacement Technology
Fan
Run by brushless DC motor in place of single-phase AC induction motors.
47%
Room Air Conditioners
Variable-speed compressor
35%
LightingIncandescent
14LPW incandescent goes to CFL (electronic ballast) at 52LPW
73%
LightingReflector
15LPW goes to CFL (electronic ballast) at 52LPW
71%
LightingTouchier
Assuming 80% incandescent at14 LPW goes to CFL at 52 LPW and 20% CFL stays the same
69%
Electric Water Heaters
Heat pump
50%
Refrigerators
Assuming 85% standard-size at 587 kWh AEU with savings of 51% and 15% compact at 331 kWh AEU with savings of 75%
Fig. 1 AC distribution system for buildings 53%
Clothes Washers
BDCPM variable speed
30%
Ceiling Fans
BDCPM variable speed
30%
It means as system voltage and current will increase and decrease simultaneously. On the other hand system copper losses will also decrease. (10) p=ι 2 R Watt where p = power losses in the system, ι= system current rating and r is the resistance of feeder cable. TABLE IV.
APPLIANCES, TYPICAL POWER CONSUMPTION AND CORRESPONDING AC-DC CONVERSION EFFICIENCIES
Appliances
Typical Power(W)
AC-DC Efficiency
Lighting
11/16/20/30
TV
3/7(standby), 45+ (Full)
0.79/0.81/0.82/0.84
DVDs/VCRs, home Audio, Computer
standby considered
Rechargeable Electronics
10-20
0.83
Security System
20-30
0.83
Others
100-2000+
0.85 0.69,0.79,0.8
0.87-0.89
The quantity of hourly energy consumption has been calculated by subtracting the value of past hour point from the value of current hour point by the author in [16] as follows: E n −1 = Vn − Vn −1 (11) III.
PROPOSED DC DISTRIBUTION SYSTEM FOR ENERGY EFFICIENT BUILDING Fig. 2 shows the DC distribution system equipped with PV as well as battery bank including, public utility and energy efficient DC internal technology appliances used for case studies in this paper. In the layout of this building L1, L2, L3, L4 represents DC lamps and F1, F2 represent DC fans (Brushless DC (BLDC) and Brushless DC Permanent Magnate (BDCPM) motor). The Water pump is designed by the BDCPM motor. Some heavy loads like washing machine, DC cooler are also DC compatible. The key difference between AC and DC air conditioner and refrigerator is the type of compressor used. The DC operated audio/video devices give good results as compared to AC supply. Laptop and computer having very sensitive memory devices like RAM and hard drive. So the DC supply makes move safe its memory. The DC bus is considered to be lossless and tied to PV and battery bank. This system is also equipped with public utility by AC-DC converter. It is assumed that the bus is supplied by a battery bank via bidirectional DC-DC converter. When the PV output power becomes higher than the load, the battery can be utilized when the load is higher than the output power of PV or the PV outage. The cable conductor size is selected as the explanation and standard given in literature [18] and
according to NEC code 2008. Based on this article the standard cross sectional area of cable should be 42.41mm2 with a permitted capacity of 60 A at 90°C. The wiring for this system is based on 3 wires (one positive pole, one negative pole and one neutral). This wiring is able to feed the loads of two different voltage rating without the DC-DC converter at the appliance end. This reduces converter stages as well as converter losses, the line current and power losses in the system.
block represent the proposed energy efficient building while AC load block represents ordinary building. Building loads are varying with respect to time (some appliances operate ‘ON’ for particular hours of the day while ‘OFF’ on the other time) of a typical day as shown in table V. Two case studies have been done in this paper to study power consumption for both DCDS and ACDS case.
Fig. 3 LabVIEW simulator of proposed system
IV.
Fig. 2 Proposed DC distribution system for energy efficient buildings
The LabVIEW simulator of building load, controlling and monitoring of the proposed distribution system is shown in the Fig.3. The control block contains 18 virtual switches associated with the particular load. The load can be switched ‘ON’ or ‘OFF’ through these switches. Load block is again containing DC load block and AC load bock for the building shown in Fig.2. The building total AC compatible load is 2160 watt while this load has been reduced to 1209 watt with more efficient DC internal technology, i.e. 54.6% of the AC technology based load. The inverter losses are 4.5 times higher than the rectifier losses. This reduces the power rating of the PV panel, battery bank and the consumed energy from public utility. The PV panel of 900 watt has been installed with the battery bank of 500 Ah by the public utility. So the ordinary building of the AC distribution system including AC technology compatible load is connected directly to the public utility including the PV and battery via an inverter. It is assumed the energy efficient DC internal technology appliances rating is equivalent to the AC technology based appliances. Fig.3 shows the LabVIEW simulator of ACDS and DCDS of the buildings. Load block of LabVIEW simulator consists of AC load block and DC load block. A DC load
RESULTS AND DISCUSSION
A. AC Distribution System (ACDS) In this case building is based on the AC Distribution System (ACDS) as shown in Fig.1. The building is supplied by PV and public utility including with battery bank storage. In this case, primarily the PV DC power is converted into AC power to connect with the building AC distribution system. The AC load is directly connected to the AC bus. This AC bus excessive power has to be converted into DC power to charge the battery bank. The DC compatible loads are connected to the AC bus by AC-DC converters. It is assumed that public utility is available for few hours during night only. When public utility is available then load is supplied by public utility. The battery bank has been recharged by the excessive PV power in AC bus. When the public utility is an outage and PV system is not able to supply load, then the PV and battery bank supply the building load and balances the power flow in AC bus.
There are many AC-DC and DC-AC conversion stages introduced in ACDS buildings. These conversion stages increase the energy consumption as well as increase the system cost and also affect the system reliability. As shown in table V, each appliance is not running for the typical day i.e. building load vary with respect to time. For ACDS system at 2 am, the maximum power drawn from the battery reaches at 560 watt. As discussed earlier, the load is varying with respect to time and supplied by the different source at different times. At 3 am the building load is 120 watt, which is supplied by battery bank then the converter losses are 9.6 watt. These converter losses are due to the addition of battery inverter and appliances internal converter losses. The total converter losses at 10 am are 99.55 watt while all the appliances are in off condition and only battery is connected to the AC bus. In this condition the total converter losses are the combination of the losses in PV converter and the battery converter.
B. DC Distribution System (DCDS) In this case the building is based on the DC Distribution System (DCDS) including energy efficient DC internal technology appliances as shown in Fig.2. It is also assumed that the building DC bus is supplied by public utility through AC-DC converter, including PV and battery bank. As shown in Fig. 2 this topology is free from the PV, battery bank converter including the appliances internal converters. Hence the numbers of energy conversion stages in DCDS are less than ACDS in conventional buildings. It means DCDS reduces the conversion loss as well as system cost. In Fig. 4, a comparison has been done between building load in the ACDS and DCDS system during a typical day. It can be analyzed from the graph that the ACDS system has much more building load in comparison to DCDS system. The simulation results also show that 300 watt PV panel with 500 Ah battery bank is sufficient for uninterrupted power supply. The converter losses only exist between 18-21, 4-6 and 1 due to the building is supplied by public utility.
Fig. 6. Battery power utilization in ACDS and DCDS system
Fig. 7. Power consumption from PU in ACDS and DCDS system
Fig. 4 Building load in ACDS and DCDS system
Fig. 5. SPV power consumption in ACDS and DCDS system
In Fig. 5, a comparison has been made between SPV power consumed by the ACDS and DCDS system during a typical day. It can be analyzed from the graph that the ACDS system consumes much more power in comparison to DCDS system. So resulting in DCDS system requires less number of solar panels with less wattage. In Fig. 6, a comparison has been done between battery power utilization of the ACDS and DCDS system during a typical day. It can be analyzed from the graph that the ACDS system requires to use more battery power however in case of DCDS system the use of battery power is very less. So the less number of battery banks of less ampere hours (Ah) are required in this case.
A comparison between the ACDS and DCDS system has been made based on public utility utilization in Fig. 7 during a typical day. It can be analyzed from the graph that the ACDS system consumes much more power from public utility in comparison to DCDS system. It means less electricity bill appear.
Fig. 8. Converter losses in ACDS and DCDS system
In Fig. 8, a comparison has been made between the ACDS and DCDS system based on the converter losses during a typical day. It can be analyzed from the graph that the ACDS system having the more converter losses than DCDS system consumes much more power in comparison to DCDS system. In DCDC system the converter losses appear only when the building is connected to the public utility as shown in Fig. 8. In table V the energy saving has been shown with DC technology when the particular appliance is operated in ‘ON’ condition for a particular time duration mentioned in table V. For example there is a 2968 watt-hour saving in case of 400 watt rating refrigerator which is operating in ‘ON’ condition for 14hrs on a typical day.
TABLE V.
ENERGY SAVING IN APPLIANCE AFTER ELIMINATING AC-DC CONVERTER
Name of Appliance
Appliance Appliances Power 'ON' time Rating per day (Watt) (hour ) Air conditioner 270 11 Washing Machine 100 1 Refrigerator 400 14 Water Pump 150 2 Water Heater 750 3 Lamp1 30 8 Lamp2 15 12 Lamp3 30 10 Lamp4 15 8 Fan 1 60 7 Fan 2 60 6 Laptop/PC 100 8 Audio/Video 80 7 Cooler 100 8 Total Saving in a typical day (kW hour)
Energy Saving with DC Technology (Watt-hour) 12365.9 30 2968 90 750 175.2 131.4 213 82.5 1776.6 108 160 117.6 376 19.35
V. CONCLUSION The Alternating Current Distribution System (ACDS) requires DC-AC converter to interconnect the PV and battery bank. Moreover, the AC-DC converters are used to interconnect the DC compatible appliance to the ACDS. The ACDS system requires large number of conversion stage which increases the conversion losses in the ACDS. In ACDS the energy consumption includes the energy consumed by AC compatible appliances, DC compatible appliances including AC-DC conversion losses in DC compatible appliance. These energy consumptions also depend on converters efficiency. In proposed DCDS for energy efficient buildings, the PV panel and battery bank are directly interconnected to building DCDS. It means the load and RES ends conversion stages such as AC-DC, DC-AC, and DC-DC have been eliminated by the proposed DCDS system. The conversion losses have been decreased to a significant level in DCDS than ACDS as shown in Fig. 8 which helps to decrease the power losses in distribution system as compared to the ACDS. This leads to optimization of the size of PV panel and battery bank. Simulated results show the energy saving by using DCDS in comparison to ACDS system. Results also show that proposed DCDS decrease the PV panel size to 67.67 % in comparison to the ACDS for the same building with DC compatible load. The cost of overall system decreases with the reduction in the conversion stages in DCDS. DCDS system also proves that the system current rating reduces up to 29.24% with equivalent ACDS voltage, while system power losses decrease up to 50.07%.
[2]
[3] [4]
[5]
[6]
[7] [8]
[9]
[10]
[11]
[12]
[13]
[14]
[15]
[16]
[17]
REFERENCES [1]
D. Nilsson and A. Sannino, “Efficiency analysis of low and medium voltage dc distribution systems,” in proc. IEEE Power Engineering Society General Meeting, vol. 2, pp. 2315-2321, June 2004.
[18]
B. S. Rajpurohit, S. N. Singh and L. Wang, “Technical and economical practices for alternative energy in India,” in proc. IEEE Power Engineering Society General Meeting, pp. 1-8, July 2012. L.P. Lombard, and elt, “A review on buildings energy consumption information,” Energy and Buildings, vol. 40, no. 3, (2008) 394-398. K. Clement, I. Pardon, and J. Driesen, “Standby power consumption in Belgium,” in proc. 9th International Conference on Electrical Power Quality and Utilisation, pp. 1-4, Oct. 2007. J. Mitra, N. Cai, M.Y. Chow, S. Kamalasadan, W. Liu, W. Qiao, S. N. Singh, A. K. Srivastava, S. K. Srivastava, G. K. Venayagamoorthy, and Z. Zhang, “Intelligent methods for smart microgrids,” in proc. IEEE Power and Energy Society General Meeting 2011, pp. 1-8. R. K. Chauhan, B. S. Rajpurohit, and etlt, “DC grid interconnection for conversion losses and cost optimization,” Renewable Energy Integration: Challenges and Solutions. Springer Singapore, 2014, pp. 327-345. J.P. Benner, and L. Kazmerski, “Photovoltaics gaining greater visibility,” IEEE Spectrum, vol.36, no.9, pp. 34-42, Sept. 1999. P. A.D.V. Raj, M. Sudhakaran and P. P.D.A. Raj, “Estimation of standby power consumption for typical appliances,” Journal of Engineering Science and Technology, Review, vol.2, no.1, 71-75, July 2009. J. Kneifel, “Life-cycle carbon and cost analysis of energy efficiency measures in new commercial buildings,” Energy and Buildings, vol. 42, no. 3, pp. 333-340, March 2010. K. Garbesi, V. Vossos and H. Shen, “Catalog of DC appliances and Power Systems,” Lawrence Berkeley National Laboratory, pp. 1-76, Oct. 2011. F. Dastgeer, and A. Kalam, “Efficiency comparison of DC and AC distribution systems for distributed generation,” in proc. IEEE Power Engineering Conference, pp. 1-5, Sept. 2009. P. Salonen, T. Kaipia, P. Nuutinen, P. Peltoniemi and J. Partanen, “A LVDC distribution system concept,” in proc. Nordic Workshop on Power and Industrial Electronics, June 2008. K. W. E. Cheng, “Overview of the DC power conversion and distribution,” Asian Power Electronics Journal, vol. 2, no.2, pp. 75-82, Oct. 2008. M. Amin, Y. Arafat, S. Lundberg, and S. Mangold, “Low voltage DC distribution system compared with 230 V AC,” in proc. IEEE Electrical Power and Energy Conference, pp. 340-345, Oct. 2011. A. Lana, T. Kaipia and J. Partanen, “Investigating into harmonics of LVDC power distribution network using EMTDC/PSCAD Software,” in proc. International Conference on Renewable Energies and Power Quality, pp. 13-15, April 2010. L. Zhao, J. L. Zhang, and R. Liang, “Development of an energy monitoring system for large public buildings,” Energy and Buildings , vol. 66, pp. 41–48, Nov. 2013. D. Salomonsson, and A. Sannino, “Low-Voltage DC Distribution System for Commercial Power Systems with Sensitive Electronic Loads,” IEEE Trans. power Delivery, vol. 22, no. 3 pp. 1620-1627, July 2007. Z. Gershony, and etl, Optimal DC Cable Selection in PV Design, article with link:http://solarprofessional.com/articles/design-installation/optimal -dc -cable-selection-in-pv-designs.
