Application Note



November 9, 2008

Application Note

Recommended Design Practices of Basic Off-Grid Solar Photovoltaic (PV) Systems

Jakub Mazur – B.S. EE, Michigan State University

Abstract:

This document will outline design practices that should be taken into consideration while designing a basic “off-grid” system, particularly for deployment into rural areas. This includes the following topics: determining load power consumption, solar insulation, solar panel array size, battery array size and voltage. This document will use examples of calculations used by Michigan State University’s Team 2 senior design project for deployment near Arusha, TZ.

Keywords: Solar System Setup, Photovoltaic power

Table of Contents

Introduction 3

Block Diagram 4

Section 1: Determining Photovoltaic Array Size 5

• Load Power Consumption 5

• Determining Solar Insolation Levels 6

Section 2: Sizing Battery Array 8

Section 3: Wire Sizing and Connections 10

Section 4: DC-AC Inverter 12

Section 5: Charge Controller 12

Conclusion 14

References 15

Appendix 1: Solar Panel Selection 16

Appendix 2: Battery Selection 17

Appendix 3: Inverter Selection 18

Introduction:

There are many types of solar systems but most can be categorized into a variation of the following: A “grid-tie” system where there are no batteries and the power grid provides back-up power. A hybrid “grid-tie” system where the power grid provides back-up for the solar panels and batteries act as a backup for the grid. In cases where there is no access to grid power an “off-grid” system is used, in which the battery bank stores and provides all the energy for the system without a backup. Since this is generally the case in under-developed areas this will be the system discussed here. There are also systems with generators as backups, they are comparable to “grid-tie” systems and will also be omitted from discussion here.

Block Diagram of Team 2 PV System

[pic]

Figure 1

Section 1 - Determining Size of Photovoltaic Panel Array:

There are several steps involved in sizing the PV array, determining load power consumption, accounting for losses and dividing by solar insolation levels for deployment region.

Determining Load Power Consumption:

The first step is to determine how much power the total system load will draw. Power is measured in Watts:

P = V x I (Joule’s Law)

However, the power rating is more useful when looked at in terms of time, this is indeed how electric companies charge consumers. For example a 200Watt light bulb running for 24 hours uses 4.8 KWh.

200Watts x 24hrs = 4800 Watt-Hours or 4.8 KWh

A list of all devices connected to the system should be made with their appropriate power draw available from specifications sheets or better yet, actual measurements.

Power Measurements (11/11/2008)

|Component |Power (Watts) |

|Lenovo S10 (Idle) |91 |

|Lenovo S10 (Full Processor + Hard Drive) |116.76 |

|Lenovo S10 (30% Duty Cycle) |98.73 |

|Satellite Router (Idle) |53.8 |

|Satellite Router (Busty) |72.5 |

|Est. Typ. Satellite (30% Duty Cycle) |59.41 |

|17” LCD Screen x 4 |(20*4) = 80 |

|Total |238.14 Watts |

Figure 2

* Headsets, Keyboards & Mice are currently not included in calculations because the team is not in possession of them and their power consumption should be minimal.

Since these devices are designed to plug into AC power, a DC-AC power inverter is needed. The power inverter ideally operates at 90% efficiency. Therefore the maximum inverter draw from batteries is:

238 Watts / 0.90 = 264.60 Watts

This system power draw is then multiplied by the amount of hours per day that it will operate.

264.60 Watts * 8hrs/day = 2116.80 Watt Hrs/day

To compensate for system losses during battery charge/discharge cycles the total system power consumption is multiplied by a 20% compensation factor (Sunwize).

2115.52 Watt Hrs/day * 1.2 = 2540.16 Watt Hrs/day

Determining Solar Insolation Levels:

In order to determine a good approximation of how much power the PV panels will output, solar insolation levels need to be considered. Solar insolation is the amount of incoming solar radiation incident on a surface, for PV applications the surface of interest is the earth’s surface. The values of solar insolation are commonly expressed in kWh/m2/day, which is the amount of solar energy that strikes a square meter of the earth's surface in a single day. This is commonly referred to as a “Sun-Hour-Day”. The amount of insolation received at the surface of the Earth is controlled by the angle of the sun, the state of the atmosphere, altitude, and geographic location.

[pic]

World Insolation Map, Figure 3

This map divides the world into six solar performance regions based on winter peak sun hours.

It is important to keep this in mind when designing the system because as seen below in Figure 4, during the winter you have a much smaller ‘Solar Window’. Worst case scenarios should be calculated as it is better to have extra energy in the summer than not enough in the winter. Therefore the “Sun-Hour-Day” values for December (since December days are shortest) are generally used.

[pic]Figure 4

Solar Insolation Levels for Arusha, the prototype deployment area are seen below in Figure 5.

Figure 5 (Hankins)

The compensated total power consumption per day value calculated above is then divided by the solar insolation values for given deployment region to determine minimum PV panel array power output requirement.

2540.16 Watt Hrs/day / 5.5 = 461.84 Watts

Section 2 - Sizing Battery Array

Nearly all large rechargeable batteries in common use are Lead-Acid type, although there are three variations, flooded, gelled electrolyte (“Gell Cells”) and absorbed glass matt (“AGM”). Flooded is the oldest and cheapest technology used but can be dangerous, in case of a malfunction acid can spill. Gell Cells contain acid that has been "gelled" by the addition of Silica Gel, turning the acid into a solid mass, therefore even if the battery where cracked open, no acid would spill. Gell batteries need to be charged at a slower rate (capacity / 20) but this is not a concern in the PV setup as the panels will not be outputting nearly this much current. AGM batteries are the newest technology and have all the advantages of Gell Cells without the charging limitations.

All deep cycle batteries are rated in amp-hours. An amp-hour (Amps x Hours) is one amp for one hour, or 10 amps for 1/10 of an hour and so forth. The accepted AH rating time period for batteries used in solar electric and backup power systems is the "20 hour rate". This means that it is discharged down to 10.5 volts over a 20 hour period while the total amp-hours it supplies is measured (Windsun).

The compensated total power consumption per day value is used again to calculate minimum battery array size.

2540.16 Watt Hrs/12 Volts = 211.68 AmpHrs/day

Number of days of autonomy to support: 1 (8hrs)

211.68 * 1 = 211.68 AmpHrs

“Battery life [deep cycle] is directly related to how deep the battery is cycled each time. If a battery is discharged to 50% every day, it will last about twice as long as if it is cycled to 80% DOD [depth of discharge]. If cycled only 10% DOD, it will last about 5 times as long as one cycled to 50%. Obviously, there are some practical limitations on this - you don't usually want to have a 5 ton pile of batteries sitting there just to reduce the DOD. The most practical number to use is 50% DOD on a regular basis (Windsun).”

Depth of discharge for battery: 0.5

211.68 / 0.5 = 423.6 AmpHrs

This means that after 8 hrs of use without sun the battery will be discharged to 50%

8 Hrs of autonomy and battery depth of discharge at 0.80 (Half the life-span of 0.50):

264.60 Amp Hrs

6 Hrs of autonomy and battery depth of discharge at 0.50:

264.6 Watts * 6 Hrs = 1587.6 Watt Hrs / day * 1.2 = 1905.12 Watt Hrs / day

1905.12 Watt Hrs/12 Volts = 158.76 AmpHrs

142.8 AmpHrs / 0.5 = 317.52 Amp Hrs

4 Hrs of autonomy and depth of discharge at 0.50:

238 Watts * 4 Hrs = 1058.4 Watt Hrs / day * 1.2 = 1270.08 Watt Hrs / day

1270.08 Watt Hrs / 12 Volts = 105.84 Amp Hrs

105.84 Amp Hrs / 0.5 = 211.68 Amp Hrs

Section 3 - Wire Sizing and Connections:

Another important consideration for the system is the electrical wiring. All wiring needs to safely accommodate the amount of current draw of the system with an acceptable amount of losses. In a DC system losses quickly become an issue. This is especially a concern PV systems as they can only handle a small voltage drop as there must be enough potential to charge the battery array, and of course it is good practice to keep energy loss sourced from the sun to a minimum. Generally a 3% drop between PV array and batteries is acceptable. Also, “any type of connection bigger than AWG 10 should have a proper compression connector, with appropriate joint compound and preparation. This does require special tools and dies. Otherwise you are running the risk of burning up your connections if you get any kind of heavy current flowing. (SolarForum)”

Losses associated with transmission of DC power:

CM = (22.2 x A x D)/VD

CM = Circular Mills In Copper

A = current in amps

D = one-way cable distance in feet

VD = Voltage Drop

22.2 = Constant for Copper

For wiring from the PV panels to charge controller the maximum PV short circuit current specification (from PV data sheet) is used.

Maximum Solar Power Output:

24 Volt Systems:

|Configuration |Max Current Out (Amps) |

|6 x PW080 |3 x (5.14A-ISC) = 15.42 |

|3 x ST-165 |20.63 |

|4 x KY125 |20.83 |

Figure 6

12 Volt Systems:

|Configuration |Max Current Out (Amps) |

|6 x 80 Watt |6*(5.14A-ISC) = 30.84 |

|3 x PW165 |41.25 |

Figure 7

Using the loss equation above the following result was obtained for the selected system:

Distance: 50ft

Voltage Drop: 0.72

Current: 15.42 Amps

Circular Mills: 23772.5

AWG: 6

Inverter to Battery Wiring

For current level estimates from the battery to inverter maximum power draw levels are used although this distance is generally short and maximum available wire gauge is recommended. This is also due to the fact that the system will encounter surge currents as various components are ‘turned on’. Since the system used as an example here is not continuously running and is to be turned off every night and back on in the morning this was a serious issue that needed to be tested. (Refer to figure X).

Maximum Power Draw:

|Component |Power (Watts) |

|Lenovo S10 (Full Processor + Hard Drive) |116.76 |

|Satellite Router (Busty) |72.5 |

|17” LCD Screen x 4 |(20*4) = 80 |

|Total |270 Watts |

Figure 8

Assuming the inverter that will be sourced in deployment area is operating at 90% efficiency:

270 Watts = 300 Watts x 90%

Maximum current draw in 12 Volt system = 300 Watts / 12 Volts = 25 Amps

Maximum current draw in 24 Volt system = 300 Watts / 24 Volts = 12.5 Amps

[pic]Figure 9

Figure 9 shows DC current draw as measured during power-up of Lenovo S10 Workstation (custom configuration) and L193p Monitor. Although the system is only drawing 5 amps while running the surge current spikes are clearly visible. This is indeed one of the reasons why proper electrical connections are crucial.

Section 4 - DC-AC Inverter:

Since the computer and monitors are designed to plug into AC power and accessory plugs for phone charging are a project specification an inverter is necessary. There are two types of inverters, pure sine wave and modified sine. Most devices will work from modified sine, this is what common uninterrupted power supplies provide and what was selected for this system. It is important to make sure that the inverter is rated to provide enough power for everything running off of it.

Section 5 - Charge Controller:

The charge controller chosen for this system is the Outback Power FlexMax 60. This decision was based on versatility, efficiency, robustness, and availability in deployment area. The Outback can accept a wide range of voltage inputs as well as various battery arrays, this was important for this specific system as ultimately whatever solar panels are in stock at the time of deployment in the region will be used.

Note, the efficiency curves (Figure X and X) are for the Flexmax80, they are identical to the Flexmax60 other than the fact that the FX60 does not accept 85 and 100V.

Figure 10, (Outback)

The highlighted area on the graph represents the highest efficiency while charging a 12V battery array. The charge controller is operating at about 95.5% efficiency with an input Voltage between 17-34V. Typically a 12V PV panel's Voltage at Peak Power is around 17 Volts.

Figure 11 (Outback)

The highlighted area in this graph represents the optimum efficiency if the system where charging a 24V battery array. The charge controller is operating at about 98% efficiency with an input Voltage around 34V. Two 12V panels in series will typically have 34 Volt equivalent Voltage at peak power.

In an ideal setup the FlexMax 60 would operate at 98.1% efficiency with an input of 68V while charging a battery array at 48V. This would be the case with the optimum PV panel chosen in section 1, the Kaneka G-EA060 as the VPM is 67Volts.

Conclusion

Designing an off grid photovoltaic system involves many steps and although the math is simple all calculations should be double checked. If the calculations for one component are off chances are the whole system will not work, every stage relies on the previous one. Designing the system for worst case scenarios is good practice, it is better to have extra energy than not enough. All safety precautions should be followed especially on electrical connections that have a possibility of carrying a lot of current. Breaker boxes before and after battery connections for easy power disconnect should be implemented. These breakers should be rated for DC voltages.

References:

Hankins, Mark, and Francis Njeru. Solar Electric Systems for Africa : A Guide for Planning and Installing Solar Electric Systems in Rural Africa. Ed. Timothy Simalenga. Beverly: Commonwealth Secretariat, 1995. 7-8.



World Insolation Map:

Sun path chart:

Outback Power Flexmax User Manual:

Wiring Safety Concerns:

Also,

** Source: Sunwize PV installation guide as well as other installer personal experiences



Appendix 1: Solar Panel Selection

Optimum Panel available in North America:

Kaneka G-EA060

Power: 60 Watts

IPM: 0.90 Amps

VPM: 67.00 Volts

Price: $227.00 USD (Retail)

Value ($/Watt): $3.783

Locally Available in prototype deployment area Arusha, TZ:

24 Volt Setups:

|PV Panel |Quantity |Total Wattage |Price |Value ($/Watt) |

|Photowatt PW080 |6 |480 |2706 |5.64 |

|Suntech ST-165 (24V) |3 (//) |495 |3150 |6.36 |

|Kyocera KY125 |4 |500 |2768 |5.54 |

|Suntech ST-080 |6 |480 |3180 |6.63 |

12 Volt Setups:

|PV Panel |Quantity |Total Wattage |Price |Value ($/Watt) |

|Photowatt PW080 |5 |400 |2255 |5.64 |

|Photowatt PW165 |3 |495 |2778 |5.61 |

|Suntech ST-060 |7 |420 |2870 |6.83 |

A 24 Volt setup is recommended, one reason being that current levels are reduced. This allows for easier current measurement and smaller wire gauge which results in more manageable wiring and lower costs.

Appendix 2 – Battery Selection

Batteries locally available in deployment area

|Description |Voltage |AH at c/20 |Type |Retail Price |Value ($/AH) |

|Surrette 27-HT-90 |12 |90 |Flooded |$190.00 |2.11 |

|Surrette T12-250 |12 |200 |Flooded |$500.00 |2.5 |

|Deka 8G4D |12 |210 |Gel |$575.00 |2.74 |

The Deka 8G4D batteries is chosen because it is the only non-liquid acid battery available. Two are suggested as they will allow for a 24V setup and provide enough capacity to run the system for approximately 8 hours with a discharge depth of 50%. With this discharge depth the Deka batteries are rated for 1000 cycles (at 77*F). Batteries self-discharge faster at higher temperatures. Lifespan will also reduce at higher temperatures, the Deka datasheet does not include this information and their engineers claim that temperature will not affect the lifespan. Most other manufacturers state a 50% loss in life for every 15 degrees F over a 77 degree cell temperature. Temperature and depth of discharge data will be collected to determine the actual temperature effects.

Appendix 3 – Inverter Selection

Inverter used in first 12V lab prototype:

Xantrex Xpower, 1750 Watts, Peak efficiency 90%, Retail Price: $210.00

10-15VDC Input, 115VAC 60 Hz (+/- 4Hz) Out

Inverter suggested for 24V setup:

Samlex PSE-24125A, 1250 Watts, Peak efficiency 90%, Retail price: $250.00

20-33 VDC Input, 120V (+5% / -10%) 60 Hz (+/- 5%) Out

Inverters available locally in Arusha, TZ:

24 Volt:

Xantrex DR1524E, 1500VA, Peak efficiency 94%, Retail price in Arusha: $1050

24VDC Input, 230VAC 50Hz Out

12 Volt:

Xantrex CR1012E, 1000 Watt, Peak Efficiency 86%, Retail price in Arusha: $554

10.5 - 14.5VDC Input, 230VAC 50Hz Out

Charge Controller Prices:

Outback Flexmax60 sourced in MI for: $563.44

Available in Arusha, TZ for: $744

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