Detailed DC system design

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.

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  Photo of the home.

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  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

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  Retrieving PVGIS monthly insolation data.

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  Retrieving PVGIS monthly insolation data.

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  Retrieving PVGIS weather data.

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  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:

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

Minimum PV source size

The size of the PV source, which is determined based upon the load evaluation and weather and solar resource evaluation will determine the necessary size of the charge controller. The charge controller must be selected at the same time as the PV source as the charge controller type - PWM or MPPT - will also determine the possible configurations of PV modules.

In this phase of the design process, more than in any other phase, it is necessary to explore different designs using PV module, series and parallel wiring configurations, and charge controllers in order to achieve the highest performance at the lowest cost possible. This phase may have to be performed several times.

An off-grid PV system that depends upon the PV as its single charging source requires an array that is sufficiently sized to be able to generate sufficient energy to both meet the energy needs of the users and to recharge the energy storage system under less than ideal conditions. Any sizing decisions should therefore lean towards an oversized PV source.

Step 1: Deteremine PV source loss parameters

The PV module(s) will be mounted on a pole mount system.

• Module degradation parameter = .944
• Shading loss parameter = .95
• Soiling loss parameter = .97
• Wiring loss parameter = .96
• Module mismatch parameter = 1
• PV source temperature loss parameter = -.48%/°C

The mounting system will also affect the ability of the PV source to cool itself. A mounting system temperature adder should be added to the maximum temperature that is used to calculate losses due to temperature:

• 20°C for pole mount
• 25°C for ground mount
• 30°C for roof mount

PV source temperature loss parameter	= 1 + (Maximum ambient temperature + Mounting system temperature adder - 25°C) x Temperature coefficient of max power %/°C ÷ 100
	= 1 + (23°C + 20°C - 25°C) x -.48%/°C ÷ 100
	=

Total PV source loss parameter	= Module degradation parameter × Shading loss parameter × Soiling loss parameter × Wiring loss parameter × Module mismatch parameter × PV source temperature loss parameter

Step 2: Determine charge controller type

There are two different charge controller types - pulse width modulation (PWM) and maximum power point tracking (MPPT) - each of which has advantages and disadvantages that are are detailed in Charge controller types. The charge controller will be sized further on, but a charge controller type must be selected at this point to proceed with the design.

PWM:

The PV source must be configured to operate at the charging voltage of the energy storage system and below the maximum PV source current rating of the charge controller.

• Nominal system voltage: 12V, 24V, 48V.
• Maximum PV source current: 6A-60A

MPPT:

The PV source must be configured to operate below the maximum PV source voltage rating of the charge controller, above the minimum PV source voltage based upon the maximum charging , and below the maximum PV source current rating of the charge controller.

• Nominal system voltage: 12V, 24V, 48V
• Maximum PV source voltage: varies up to 600V
• Minimum PV source voltage: depends upon nominal voltage and charge controller type
• Maximum PV source current: up to 100A+

Step 3: Charge controller efficiency parameter

All charge controllers lose a certain percentage of all energy that is produced as heat as it is transferred to the energy storage system and loads. PWM charge controllers are more efficient than an MPPT charge controller, nonetheless the design of a MPPT charge controller will lead to it outproducing a PWM charge controller under most conditions - see Charge controller for more info. Specifications sheets for MPPT charge controllers will often give a maximum efficiency rating that is typically an overestimate - it is better to use a more conservative value. Two seperate designs may be performed with each type of charge controller to determine the best systerm design. The specification sheet for a particular MPPT charge controller can be consulted for a maximum effiency number, but this number will typically be an over-estimation of efficiency.

Charge controller efficiency parameter values for different charge controller types:

• Pulse width modulation(PWM) charge controller: .99 (99% efficient)
• Maximum Power Point Tracking (MPPT) charge controller: .95 (95% efficient)

Step 4: Energy storage efficiency parameter

All energy storage systems lose a percentage of all energy produced to heat as it is stored and released to power loads.

Energy storage efficiency parameter values for lead acid battery types:

• Flooded lead acid (FLA) batteries : .75 (75% efficient)
• Valve-regulated lead acid (VRLA) batteries: .85 (85% efficient)

Step 5: Deteremine minimum size of the PV source

Finding the minimum size of the PV source will inform the rest of the design.

Minimum PV source size	= Average daily Watt-hours required ÷ Design insolation ÷ Total PV source loss parameter (Step 1) ÷ Charge controller efficiency parameter (Step 3) ÷ Energy storage efficiency parameter (Step 4)