Designing an Off-Grid PV System For a Residential Consumer in Mashhad-Iran Morteza Khatami,Hashem Mortazavi
Mostafa Rajabi Mashhadi,Mahdi Oloomi
Khorasan regional electric Company, KREC Mashhad, Iran
[email protected],
[email protected]
Khorasan regional electric Company, KREC Mashhad, Iran
[email protected],
[email protected]
Abstract— this article suggests an off-grid solar power system for a typical home at Mashhad, IRAN. In order to computing the off-grid solar system components. The design was done based on the shortest day of the year. The solar data is obtained from NASA web site and has been used from RETSCREEN software. The solar system set must capable to supply load current for 4 days; therefore we need to install Batteries and battery charger. In this computation the shortest day of the year has been assumed. Although the total design is acceptable from engineering point of view, from economical view the total Cost of system is higher than the case that has been designed for summer days that the electricity cost is higher in IRAN country. Keywords— Solar Power System; off-Grid PV system; Inverter; Charge Controller: PGC
I. INTRODUCTION
The world-wide demand for solar electric power systems has grown steadily over the last 20 years. The need for reliable and low cost electric power in isolated areas of the world is the primary force driving the world-wide photovoltaic (PV) industry today. For a large number of applications, PV technology is simply the least-cost option. Typical applications of PV in use today include stand-alone power systems for cottages and remote residences, navigational aides for the Coast Guard, remote telecommunication sites for utilities and the military, water pumping for farmers, and emergency call boxes for highways and college campuses, to name just a few. Significant growth in demand for PV systems is expected to occur in developing countries to help meet the basic electrical needs of the 2 billion people without access to conventional electricity grids. In addition to this demand for cost effective off-grid power systems, environmental and longer-term fuel supply concerns by governments and electric utilities are beginning to help accelerate the market for demonstration programs for PV systems connected to central electric grids in industrialized countries. PV modules are integrated into systems designed for specific applications. The components added to the module constitute the “balance of system” or BOS. Balance of system components can be classified into three categories:
Batteries - store electricity to provide energy on demand at night or on overcast days; • Inverters - required to convert the DC power produced by the PV module into AC power; • Controllers - manage the energy storage to the battery and deliver power to the load Not all systems will require all these components. For example in systems where no AC loads s present an inverter is not required. For on-grid systems, the utility grid acts as the storage medium and batteries are not required. Batteries are typically not required for PV water pumping systems, where a water reservoir “buffers” short-term demand and supply differences. Some systems also require other components which are not strictly related to photovoltaic. Some stand-alone systems, for example, include a fossil fuel generator that provides electricity when the batteries become depleted; and water pumping systems require a DC or AC pump. This article suggests an off-grid •
solar power system, for a typical home at Mashhad, IRAN. In order to computing the off-grid solar system components, as it was requested the design was done for the shortest day of the year. The solar data is obtained from NASA web site and Retscreen software. The total solar system must capable to supply load current for 4 days; therefore we need to install Batteries and battery charger. II. A TYPICALLY HOME PLAN SPECIFICATIONS A. Solar Information A Typical home is a 120 square meter apartment in a 2 apartment’s house located in Mashhad- IRAN. Mashhad location: latitude:36.3N, longtitude:59.6E. In Fig.1 the solar information of the home Based on the NASA data from the Retscreen database represented. As it can be seen daily solar radiation in Mashhad varies between 2.17 to 7.81 KWh/M2/day during the year.
C. Technical Step for The Solar Home Power For the Generation of Solar power, the following components should be to install. • solar PV modules • solar charge controllers • solar inverters • battery systems These above components are united in one circuit as seen in Fig.2.
Fig.2. Schematic of all components for solar power generation
III. PANEL SPECIFICATIONS WITH DATA SHEET Fig 1. The home solar information based on Retscreen software Data
A. PV selection B. The Home Load A typical home loads are described in Table.1. Note that the nominal voltage of Iranian low voltage distribution system is 220 V and the frequency is 50Hz. Table1. The Home Load-All Loads are AC Watt
h/d
∑A
Ah/d
Wh/d
140
10
0.636
6.36
1400
1
1190
0.2
5
1.08
238
1
200
10
0.901
9.01
2000
1
150
5
0.68
3.4
750
6
100
3
2.724
8.172
1800
Microwave
1
1800
0.2
8.18
1.636
360
Vacuum
1
1000
0.2
4.55
0.91
200
22.996
30.568
6748
Appliance
N
1 TV- 29 Inch Washing machine Refrigerator Desktop PC Lighting
First of all, for finding the actual PV size, i t s h o u l d b e calculated the corrected load based on the: depth of battery discharge, wiring efficiency and the other factors. With the following assumptions: • Depth of discharge allowed in the batteries: 50 %==> we use it for battery Sizing • Wiring Efficiency: 98% • Charge/discharge efficiency of batteries: 90% The Home load current for 4 days is about: IL=122.4 Ah Corrected load current (without the inverter efficiency) = 30.6/ (0.98*0.90) =34.7 Ah Considering Inverter efficiency: If it has been assumed that the inverter efficiency is 96% (Sunnyboy3600TL) then the final corrected load current is: IL-final= 34.7/ 0.96=36.12 Ah WL-Final=Vn*ILfinal=220*36.12=7950.7wh
Cleaner Total
If it has been considered the solar system supplying energy for 4 days energy: For 4 days total energy consumption is: 6748*4= 26992 Wh for 4days For 4 days total load current based on Ah is: 30.6*4= 122.4 Ah for 4days
B. Panel choosing Based on the Retscreen software data sheet for a home city (Mashhad-IRAN) the minimum daily solar radiation2 Horizontal is 2.17 KWh/m /d to a maximum 7.81 2 KWh/M /day and as if it has been considered the shortest day of the year, for a city like Mashhad it will be 2.17 daily sun radiation. Minimum daily solar radiation- Horizontal for 2 Mashhad- Iran= 2.17 KWh/m /d. for 4 days, the output a of 2 1m panel is Ppanel=4*2.17=8.68 KWh, Based on the
project assumption no Dust and no clouds so Dust effect= 1 and PGF for Mashhad is 4.3 hour. So, for finding the number of panel: WL-Final/PGF= 7950.7/4.3=1849W If it has been chosen solar panel SW-255-mono-ds from Sunmodule company:No of panels= 1849/ 255=7.25 ==>7 panels So it will be chosen the 7, SW-255 W panels from Sunmodulecompany. By choosing 7 panels the actual output of this system is Pmax= 7*255= 1785 W. Hence 7 Panels 2 on average 4 KWh/M /day for C o n s i d e r e d location will produce the following amount of energy throughout the year: Total P o w e r p r o d u c t i o n i n t h r o u g h t h e y e a r : ( 1785/4)*365=162881W/year= 0 . 1 6 3 MW/year C. Panel installation Based on the Home location, it has to been chosen the panel Declination. As in IRAN country the electricity in summer is more expensive but as the design is for the shortest day of the year δ is equal to δ= - 23.4. Mashhad location: latitude: 36.3N, longitude: 59.6E. So: α=16.87 then Өz= 73.12 and Ψ=102.143 in degree. As Mashhad located in northern hemisphere, so the panel will install at 73.12 (Degree) from horizontal face West at 102.143 due to South. D. PV Weight As has been chosen solar panel SW-255-mono-ds FromSunmodule Company, each SW- 255-mono-ds panel weights near 21.2 Kg. so the total net- weight of panels is 15*21.2=318 Kg, if a simple structure has been chosen the maximum weight of the structure will be less Than 200 kg. So: • Total net- weight of panels is 7*21.2=148 Kg • Simple structure weight=200 kg • the total load on roof top= 200+148= 348 Kg • the total needed area is: 12 square meter • the average load per square meter: 348/12= 29 2 Kg/m So in each square meter there are near 29 kg additional loads on the building rooftop; Which roof structural framing can easily handle the load.
Fig.3. Xantrex C35 Charge Controller selected
In ideal condition, the VA rating of the inverter would have been same as the power need. But in practical conditions, the power factor of Inverter is less than 1 hence power supplied by 1785 VA Inverter won't be 1785watts but lesser than that. So the inverter of SMA-Sunnyboy 2100TL, that has a very good characteristic of reactive power c o n t r o l h a s b e e n s e l e c t e d . so i t w i l l b e a s s u r e d a l l u s e r s that the inverter is capable not only to produce needed reactive power in the home , but also; regulate the nominal voltage of home. TheSunnyboy 2100TL [ 1 ] is the selected inverter from a leading company of inverter with name of SMA.It provides 220V/50Hz 1950Watts, 11A AC. the efficiency of inverter is >96%. It produces pure sine wave appropriate for motor, fan, pump and inductive loads.
IV. CHARGECONTROLLER AND INVERTER SELECTION Charge controllers, protect battery from over charging and/or excessive discharge, are the essential components of Solar PV systems. Whereas 7 panels has been calculated and the capacity of batteries are 24 V, The Xantrex C35 charge controller should be chosen.A l s o ,Due to the actual power of C35 is about 840 W, T h e 2 charge controllers each feed from three solar panels should be selected. Figure 3 shows the technical specification of Xantrex C35. Actual output of panels is Pmax= 7*255= 1785 W.
V. CALCULATING BATTERY CAPACITY The number of batteries in battery bank depends on many factors such as: • The number of appliances and the amount of • Power they take. • The number of days the batteries goes without charging, due to bad weather or other factors. • The temperature of the area where the batteries are stored. • The size of charging system.
•
The size of budget.
A. Days of Autonomy the solar system supplying energy for 4 days energy should be considered • For 4 days total energy consumption is: 6748*4= 26992 Wh for 4days • For 4 days total load current based on Ah is: 30.6*4= 122.4 Ah for 4days • Corrected load current (without the inverter efficiency) = 30.6/(0.98*0.90)=34.7 Ah • Considering Inverter efficiency: the inverter efficiency is 96% (Sunnyboy 2100TL ). Then the final corrected load current is: IL-final= 34.7/ 0.96=36.12 Ah. B. Depth of Charge The depth of discharge is the limit of energy withdrawal to which user will subject the battery (or battery bank). DoD is expressed as a percent of total capacity. It’s recommended that user never discharge a deep-cycle battery below 50% of its capacity; so for this calculation DOD=50% has been considered.
C. Temprature Effect Based on the project assumption, designing of system has been calculated for the shortest day of the year, and based on Fig.2 the average temperature of Dec. and Jan. in The home town is less than the 25C (degree), so the temperature has no any adverse effect on my PV output. D. Battery Selection Hence, the corrected value of the Ampere hour capacity of the batteries can be given by: Battery Bank Capacity= final corrected load current/ DoD= 36.12/0.5=72 Ah, So Battery Bank Capacity= 72 Ah. So I will choose 2 4 V o l t - 100Ah Solar Series Battery-12xA602300 from Sonnenschein company [5]. Sonnenschein A600 Solar Series batteries are developed for medium to large solar powered applications. VI. FINANCIAL ASPECT In Appendix it can be seen the result of Retscreen software for the solar project. Table2 presents the current price of main items of the project such as: Inverter, Batteries and solar cells. As it can be clearly seen from the Figure 3 ; with considering inflation rate of 1%, 20 years project life and 90% debt ratio without interest rate this project after 8 years will have income ( which is not acceptable from financial view).
The main reason is that the calculation has been done for whole system in the worst condition of solar power (shortest day of the year). Definitely for any other cases such as Fall, Spring and specially for summer this system can sell power to grid and be more economical. Fig 3. Retscreen Results
Table2. The current price of main Components Price ($) Component
PV Mono 255 Watt Panel
SMA Inverter
VARA Battery
1.22/watt
1410
1-200
REFERENCES [1] [2] [3]
[4]
[5]
[6]
http://www.sma.de/en/solutions/medium-power-solutions/sma-smarthome.html. http://www.solarguru.com.au/PDFs/NG12-200.pdf. http://www.altestore.com/store/Charge-Controllers/Solar-ChargeControllers/PWM-Type-Solar-Charge-Controllers/Xantrex-SolarCharge-Controllers-PWM/Xantrex-C35-Charge-Controller-35A-12-or24V-Solar-Charge-Controller/p2069/. http://www.tlcdirect.co.uk/Technical/Charts/VoltageDrop.html?cable=S WA_4_CoreXLPE&application=clipped_direct&max_perct_volt_drop= 3&ambient_temp=30&no_circuits=3&circuit_layout=bunched&power= 3&power_units=1000&voltage=220&length=6&submit=Calc ulate+Min+Cable+Size. http://www.energymatters.com.au/sonnenschein-24volt-300ah-solarseries-battery12xa602300p951.html?zenid=lpo1vlvv4nvmhjb4j9o2bb90v4. www.retscreen.net.
