Difference between revisions of "Detailed DC system design"

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! style="width: 20%"|Total Ah required
 
! style="width: 20%"|Total Ah required
 
! style="text-align:left;"| = Average daily Watt-hours required ÷ System voltage parameter × Battery temperature correction factor (step 3) × Days of autonomy parameter (Step 2) ÷ Depth of discharge parameter (Step 1)
 
! style="text-align:left;"| = Average daily Watt-hours required ÷ System voltage parameter × Battery temperature correction factor (step 3) × Days of autonomy parameter (Step 2) ÷ Depth of discharge parameter (Step 1)
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| style="text-align:left;"| = 140 Wh ÷ 12 V × 1.08 × 2 days ÷ .4 = 63 Ah
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! style="width: 20%"|Total Ah required
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! style="text-align:left;"| = 140 Wh ÷ 12 V × 1.08 × 2 days ÷ .4 = 63 Ah
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! style="width: 20%"|Batteries in series
 
! style="width: 20%"|Batteries in series
 
! style="text-align:left;"| = System voltage parameter ÷ Chosen battery voltage
 
! style="text-align:left;"| = System voltage parameter ÷ Chosen battery voltage
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{| class="wikitable" border=1 style="width: 80%;"
 
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! style="width: 20%"|Batteries in series
 
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! style="text-align:left;"| = 12 V ÷ 12 V
 
 
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! style="text-align:left;"| = 1 × 12 V battery is sufficient
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| style="text-align:left;"| = 12 V ÷ 12 V
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| style="text-align:left;"| = 1 × 12 V battery is sufficient
 
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! style="width: 20%"|Batteries in parallel
 
! style="width: 20%"|Batteries in parallel
 
! style="text-align:left;"| = Total Ah required (step 4) ÷ Chosen battery Ah rating
 
! style="text-align:left;"| = Total Ah required (step 4) ÷ Chosen battery Ah rating
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|}
 
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{| class="wikitable" border=1 style="width: 80%;"
 
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! style="width: 20%"|Batteries in parallel
 
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! style="text-align:left;"| = 63 Ah ÷ 75 Ah = .84
 
 
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!
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! style="text-align:left;"| = Round up to 1 × 75 Ah battery.
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| style="text-align:left;"| = 63 Ah ÷ 75 Ah = .84
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| style="text-align:left;"| = Round up to 1 × 75 Ah battery.
 
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! style="width: 20%"|Final Ah capacity
 
! style="width: 20%"|Final Ah capacity
 
! style="text-align:left;"| = Number of batteries in parallel (Step 7) × Chosen battery Ah rating
 
! style="text-align:left;"| = Number of batteries in parallel (Step 7) × Chosen battery Ah rating
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|}
 
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{| class="wikitable" border=1 style="width: 80%;"
 
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! style="width: 20%"|Final Ah capacity
 
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! style="text-align:left;"| = 1 battery in parallel × 75 Ah
 
 
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!
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! style="text-align:left;"| = 75Ah
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| style="text-align:left;"| = 1 battery in parallel × 75 Ah
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| style="text-align:left;"| = 75Ah
 
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Revision as of 09:50, 8 December 2020

Location: Pampachiri, Apurimac, Peru
GPS coordinates: 14°11'37.65"S 73°32'31.73"W
Altitude: 3378m
Description: A two story adobe home in the Peruvian Andes with only DC power needs.

  • Photo of the home.

  • Location of the home.

DC load evaluation

# Load Quantity Watts Total watts Duty cycle Hours per day Days per week Average daily DC watt-hours
1 Cree 5 W LED 6 5 W 30 W 1 3 hours 7 days 90 Wh
2 Retekess radio 6 W 1 6 W 1 5 hours 7 days 30 Wh
3 Samsung cell phone 2 10 W 20 W 1 1 hour 7 days 20 Wh
Total average daily DC watt-hours 140 Wh
  • Load: The make and model or type of load.
  • Quantity: The number of of that particular load.
  • Watts: The power rating in watts of the load.
  • Total watts = Quantity × Watts
  • Duty cycle = Rated or estimated duty cycle for the load. If the load has no duty cycle a value of 1 should be used. A load with a duty cycle of 20% would be inputted as .2
  • Hours per day: The maximum number of hours the load(s) will be operated per day. If the load has a duty cycle 24 hours should be used.
  • Days per week: The maximum number of days the load(s) will be operated per week.
  • Average daily DC watt-hours = Total watts × Duty cycle × Hours per day × Days per week ÷ 7 days
  • Total average daily DC watt-hours = sum of Average daily DC watt-hours for all loads

Average daily watt-hours required

Average daily watt-hours required = Total average daily DC watt-hours + Total average daily AC watt-hours
Average daily watt-hours required = 140 Wh

Weather and solar resource evaluation

Maximum ambient temperature = 23°C
Minimum ambient temperature = 2°C
Minimum indoor temperature = 10°C

  • Retrieving PVGIS monthly insolation data.

  • Retrieving PVGIS monthly insolation data.

  • Retrieving PVGIS weather data.

  • Retrieving PVGIS weather data.

Load and solar resource comparison

Month Average daily insolation Average daily Watt-hours required Ratio
January 193.85 kWh/m² / 30 = 6.46 kWh/m² 140 Wh 21.67
February 162.2 kWh/m² / 30 = 5.41 kWh/m² 140 Wh 25.90
March (179.81 kWh/m² / 30 = 6.00 kWh/m² 140 Wh 23.36
April 174.98 kWh/m² / 30 = 5.83 kWh/m² 140 Wh 24.00
May 214.31 kWh/m² / 30 = 7.14 kWh/m² 140 Wh 19.60
June 200.05 kWh/m² / 30 = 6.67 kWh/m² 140 Wh 20.10
July 210.35 kWh/m² / 30 = 7.01 kWh/m² 140 Wh 19.97
August 229.96 kWh/m² / 30 = 7.67 kWh/m² 140 Wh 18.26
September 126.87 kWh/m² / 30 = 4.23 kWh/m² 140 Wh 33.10
October 214.82 kWh/m² / 30 = 7.16 kWh/m² 140 Wh 19.55
November 212.91 kWh/m² / 30 = 7.10 kWh/m² 140 Wh 19.73
December 176.98 kWh/m² / 30 = 5.90 kWh/m² 140 Wh 23.73
  • Month: The month of the year.
  • Average daily insolation: Solar resource data from PVGIS.
  • Average daily Watt-hours required from load evaluation.
  • Ratio = Average daily Watt-hours required ÷ Average daily insolation

Design parameters

System voltage parameter = 12 V

  • The system, based upon the load evaluation, will be relatively small. A 12 V system makes the most sense.

Irradiance safety parameter = 1.25

  • The irradiance safety parameter is always the same.

Continuous duty safety parameter = 1.25

  • The continuous duty safety parameter is always the same.

Low voltage disconnect parameter = 11.5 V

  • A simple charge controller with a pre-programmed low voltage disconnect will be used.

Energy storage sizing and selection

A flowchart depicting the primary inputs and outputs of the energy storage sizing and selection process.

The energy storage system is sized based upon the average daily energy requirements for the system and the design parameters. The first 5 steps of this process output a suggest Ah size for the energy storage system, but then it is necessary to determine a series and parallel configuration based upon the available battery voltages and sizes.

Step 1: Determine depth of discharge parameter For this project a depth of discharge of .4 (40%) is a good compromise.

  • Depth of discharge = .4

Step 2: Determine days of autonomy parameter The home is used daily and providing lighting is very important, but at the same time the budget for the project is limited. The users are willing to adjust their consumption during periods of poor weather according to the state of charge of the energy storage system.

  • Days of autonomy = 2

Step 3: Determine battery temperature correction factor The minimum indoor temperature was determined to be 10°C. An AGM battery will be used to avoid regular maintenance.

  • Battery temperature correction factor = 1.08

Correction factors for various battery types:[1]

Temperature FLA AGM Gel
25°C 1.00 1.00 1.00
10°C 1.19 1.08 1.11
0°C 1.39 1.20 1.25
-10°C 1.70 1.35 1.42

Step 4: Calculate total Ah required

Total Ah required = Average daily Watt-hours required ÷ System voltage parameter × Battery temperature correction factor (step 3) × Days of autonomy parameter (Step 2) ÷ Depth of discharge parameter (Step 1)
= 140 Wh ÷ 12 V × 1.08 × 2 days ÷ .4 = 63 Ah
= 63 Ah

Step 5: Calculate number of batteries in series
A 12 V battery is ideal for a system of this size.

Batteries in series = System voltage parameter ÷ Chosen battery voltage
= 12 V ÷ 12 V
= 1 × 12 V battery is sufficient

Step 6: Calculate number of batteries in parallel
In Peru 12 V AGM batteries are widely available in 40 Ah, 55 Ah and 75 Ah sizes. 55 Ah is too small, so a 75 Ah battery will have to be used.

Batteries in parallel = Total Ah required (step 4) ÷ Chosen battery Ah rating
= 63 Ah ÷ 75 Ah = .84
= Round up to 1 × 75 Ah battery.

Step 7: Calculate final Ah capacity

Final Ah capacity = Number of batteries in parallel (Step 7) × Chosen battery Ah rating
= 1 battery in parallel × 75 Ah
= 75Ah


Notes/references

  1. ↑ Trojan Battery Company - Battery Sizing Guidelines https://www.trojanbattery.com/pdf/TRJN0168_BattSizeGuideFL.pdf
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