Off grid systems, solar DC/battery ratio

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In this article we continue on from part 1. To recap, the customer has received a preliminary costing and the design process is seriously underway.

It's all about the relationships between solar DC/battery storage ratio, genset contribution and load profile in part 2 of off system questions for CEC installer, designers and customers interested in renewable energy, solar PV, off grid systems and sustainability.

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Design Process

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Now we need to get into the nitty gritty of the design by looking  carefully at all factors. This is when we get the customer to fill out a load audit table in detail.

The load audit shows exactly all of the electrical loads: 

• Their kW rating
• How long are they on for hours per day?
• What loads will be on simultaneously?

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Day night ratio

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Let’s assume that the customer's actual daily load will be 12 kWh compared to the stated 10 kWh and from the analysis it is highly likely that most of the daily loads will be outside of solar production hours.

In fact the split is 60 night:40 day!       

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So this means:

• 12 kWh x 0.6 = 7.2 kWh outside solar production hours
• This must come directly from battery and possibly the generator
• The remaining 4.8 kWh comes from the solar component if the sun is out!

But the sun doesn’t shine every day, can have extended periods of cloudy weather and sometimes we can have a week of cloudy weather, so what then?

This puts extra load on the battery system, which needs to be sized accordingly, and or the generator.

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Design questions 

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Questions must be asked from a design perspective such as do we design for a bigger battery system or more generator run time or both? Pros and cons?

Maybe the customer has said “I only want the generator to run during winter months”!

If this is the case, we must take this into account with the battery storage capacity design. But remember, batteries do not produce energy, they only convert energy that has been inputted!

So with limited generator contribution, we need the right amount of panels.

As mentioned it is a balancing act between all components and if you alter one component this affects the others.

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But what costs more?

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Altering the main components has cost ramifications and batteries and generators are still quite expensive compared to solar which is fairly cheap and there are still financial incentives available but not for batteries and generators.

But in this case the roof size physically limits the amount of solar we can factor in and in this case, the roof can comfortably hold 40 x 350 watt panels = 14 kW.

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AC or DC coupled?

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After looking at all the factors we will design an AC coupled system and we will select our inverter charger based on the load audit results. In regards to the energy storage component we will select LiFePO4 ( lithium solution).

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Lithium solution

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Lithium can have C1 discharge rate and still maintain stated cycle life while Lead based solutions usually designed with max DoD of 50% @ C10 rate and take up more room, weigh more and tend not to last as long.

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Inverter charger

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In our preliminary costing we offered a 5kW Inverter charger but after further load audit analysis we will go with a 7 kW model.

This was due to reverse cycle air conditioning being used in winter*in addition to potential simultaneous surges from inductive loads.

* Originally a wood heater was going to be used as the primary heat source but this decision was changed at the last minute.

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So we have selected a 7 kW continuous inverter charger.

Next is the AC grid connect inverters

Looking at 8 kW of solar, capable on average of producing 28 kWh/day, more in summer, less in winter so have selected an 8kW single phase grid connect inverter*.

These off grid certified grid connect inverters are tricked into thinking that the output of the inverter charger is the grid.

Batteries

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With the batteries looking at 2 days of autonomy so that requires 24 kWh available 9 based on the 12 kWh/day average daily load.

As we are using lithium solution we can discharge 80% of entire capacity of battery at adequate C rating so the total battery capacity required is 24 kWh /0.80 = approximately 30 kWh

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Battery voltage is 48 volts, so individual cells are 625 AH but the next cell size up from 600 is 700 Ah @ 48 volts.

But the max size of some other batteries we have looked at are 350 AH @ 12 Volt and this means we have to series and parallel the batteries.

Will need 4 x batteries in series, gives 350 AH @ 48 Volt and the parallel another bank, 8 x cells in total, 700 AH @ 48 Volt.

Lead based gel solution?

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We could have selected lead acid gel solution but lead acid gel batteries are less energy dense, depth of discharge not as deep, C discharge rate not as good as lithium.

Let’s say we contemplated a lead based gel solution!

Regardless of the battery type we need the same amount of available capacity which is 24 kWh but with lead based we don’t want to discharge the batteries deeper than 50 % at C10 rate.

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Will need: 

• 24 kWh @ 48 volts = 500 AH
• But can only take max 50% out of battery
• This means will need 1000 AH @ 48 volts, 500/0.5 = 1000

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• Lead based this large come in 2 volt configurations 
• So a 48 volt system will need 24 cells.
• Each cell may weigh in excess of 100 kg
• Have to take this into consideration when installing
• OHS
• Structure to hold the batteries
• Battery freight
• Enclosure 

DOD and C rate

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In this case we have a C10 rating and that means we can discharge 100 Amps for 10 hours, 100 Amps @ 48 volts = 4.8 kW.

The cycle life of 1000 AH battery @ 50% DoD and C10 rate is approximately 2000 cycles max.

Now with the lithium: 

• Have a 700 AH @ 48 volts
• Can take out 80%
• Still get 3000 cycles

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And, if required discharge at C1, which means we can service large short term loads if required without reducing the lifespan of the battery.

At the end of the day you have to weigh the pros and cons of what energy storage chemistry system you select: tried and true lead based technologies versus relatively new lithium solutions.

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Generators

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In most cases when designing an off grid system a generator is required due to:

• Large loads for sustained periods may be better off serviced by generator
• During cloudy conditions , batteries may be put under undue strain hence generator 
• Limited solar due to space constraints so generator shares the workload

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Conclusion

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As an off grid system generally involves a substantial outlay from the customer the ability to quickly separate the “wheat from the chaff” is very important and this is achieved by a yes no question answer process culminating in a preliminary costing.

If accepted the installer designer moves to putting together a formal proposal based on a load audit. When that happens the real work begins as comparisons are made between the different strategies and components available. Look out for part 3.

If you’d like to see more of what Greenwood Solutions get up to in the real world of renewable energy, solar, battery storage and grid protection check out the following pages:

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 https://www.greenwoodsolutions.com.au/industry 

https://www.greenwoodsolutions.com.au/commercial

https://www.greenwoodsolutions.com.au/commercial/customer-stories

https://www.greenwoodsolutions.com.au/news