The Question

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Most off grid solar systems incorporate a fossil fuel component being a genset and in many cases diesel is the fuel of choice. The cost of running a generator on diesel is substantial and not very sustainable but what if veggie oil was used with diesel?

This presentation looks at how much diesel is used with an off grid solar system with assumed average daily load, the method of introducing a veggie oil component and what are the cost savings after implementing this strategy.

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Diesel usage with off grid systems

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The generator is there to:

• Charge the batteries when there is insufficient solar renewable energy resource
• Spare the batteries large loads that maybe better handled by a generator
• Service loads outside of solar production hours
• All of the above

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Assumptions made concerning the generator

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In regards to the genset will assume the following:

• Genset is 12 kVA
• The Power factor is 0.8
• The generator will never operate continuously at more than 85% of full load
• So max continuous output is 12 x 0.8 x 0.85 = 8.16 kW

Assumptions continued

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A generator this size will consume the following in regards to diesel fuel:

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But we are limiting to 85% of the PF corrected figure so looking at approximately:

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How hours per day/week/year will the genset run?

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To answer this question we have to make some assumptions concerning the solar battery system :

• Let’s assume that customer’s actual daily load will be 12 kWh
• And from the load 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
• This means that 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! 

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And there is more:

• But the sun doesn’t shine every day, can have extended periods of cloudy weather 
• Sometimes there can be 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
• Also there are some large loads that the customer wants the genset to deal with

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How hours per day/week/year will the genset run?

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What the customer wants must come into play:

• Maybe customer has said “I only want the generator to run during winter months”
• If this is the case must have to 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 as you can see there are many factors to consider but in this scenario we have to make some decisions:

• Generator to only run through winter months, so the solar battery ratio has been designed well
• The total hours run by the generator over the three month period equates to 90 hours
• This averages to about 3 hours per day

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How hours per day/week/year will the genset run?

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Issue is the customer also has some really big loads that exceed 85% capacity of the inverter charger and the decision has been made to service these infrequent loads with the genset.

These loads occur all year round.

When designing off grid system from the perspective of the genset ideally needs to run at almost full load:

• Basically it’s not an efficient use of a genset if underloaded
• This is because with diesel gensets, glazing of the cylinders can be an issue with under-loading
• Also, we want to minimise overall run time

So, we have to fully utilise the output of the genset as much as possible!

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Now we have to establish how many hours per year outside of the winter only normal running, of the genset is required to service large intermittent loads:

• The customer has a workshop with welders and other heavy equipment
• Weekends are when these additional heavy loads may occur
• Approximately 2 - 4 hours every weekend

So, total of say 50 x 3 = 150 hours/year at full load for the workshop and:

• 90 hours at full load for the winter period
• A total of 240 hours at full load per year
• Will also assume another 50 hours/year at 3/4 load and
• 20 hours/year at 1/2 load

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How hours per day/week/year will the genset run?

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We can see all the information the table below:

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How much fuel in total per year?

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We can see all the information the table below:

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How much will this cost for the year?

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Have a total of 710.04 litres per year and will assume $2/litre for the diesel so total is $1420.08.

Now, what about after 10 years with the assumption that diesel fuel costs increase 4, 5, 6, 7% per year?

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Is there another alternative to diesel?

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It can be seen that the annual cost for diesel is considerable and after 10 years assuming different % increases in fuel costs can spend up to $19,619 @ 7% increase cost increase per year after the first year.

What if we used another fuel that is acceptable to a diesel genset?

We are talking spent/used vegetable oil!

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The idea

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This particular approach uses both vegetable oil and diesel:

• Collect the spent vegetable oil from a local source preferably
• Secure a ‘fuel tank’, can use old oil heater storage tanks
• Filter the oil through an appropriate mesh
• Connect veggie oil tank to genset

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The issues

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All fluids have a viscosity, the more viscous the more sticky and vegetable oils tend to be more viscous than diesel so there are some potential issues with fuel lines clogging due to cold temperatures. 

To overcome this I am suggesting the following approach:

• The genset when it first comes on, runs off diesel
• At a certain point the tank flicks over to the vegetable oil
• Before the genset finishes its load servicing cycle, flicks back to diesel

In addition a small heater or heat exchange system could be used to ensure that veggie oil didn’t solidify in the colder months.

This is definitely not going to be an issue in the tropics and in fact, there are all manner of machinery running on straight veggie oil, coconut to be precise all year round.

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How can this be achieved?

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As there is 2 x fuels involved the system must take in to consideration a few things:

• When running off diesel no back feed to the veggie oil tank and vice versa
• This can be achieved with solenoids in the respective fuel lines or similar
• Will need some kind of pre heater or heat exchange to deal with veggie oil’s viscosity

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Anything else to consider?

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With an off grid system using say a Selectronic SP PRO or similar the generator can be programmed to go through a warm up stage before loads are serviced by it.

The same thing can be achieved after the pre-programmed generator run time has expired, and can program a ‘warm down period’.

I am suggesting that in the warm up and warm down period the generator is running on diesel.

This means that there must be a relay that signals the solenoids to open and close on the respective diesel and veggie oil tanks.

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How much can I really save using veggie oil?

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Will have to make some assumptions concerning the following parameters:

• We know that the genset will run 310 house per year in total
• Full load for 240 hours
• 3/4 load for 50 hours
• 1/2 load 20 hours

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The question we have to determine is the number of times the genset starts and stops on diesel, in other words the number of events and the actual duration of each event.

Will have to make some more assumptions that include:

• Each generator event occurs for 15 minutes
• Out of each event 2 minutes warm up and 2 minutes warm down
• So generator running on veggie oil 9 minutes per session

So now we have to work out:

• Number of events per load status, i.e full load, 3/4 load and 1/2 load
• Then calculate the amount of fuel used for both diesel and veggie oil
• Then calculate the cost of diesel with this approach

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Calculating exact hours for each load status

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Calculating exact costs for each load status

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Cost comparison between the two

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Can be seen that there is considerable savings if the genset is run on veggie oil for the majority of the run time:

• Diesel only spending about $1,420 for the first year
• With veggie oil option only spending $379
• Saving of $1041

Over 10 years assuming the same increase in fuel costs the savings would be considerable!

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Questions to ask

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Obviously there is a lot more to the whole veggie oil scenario. 

Things to consider include:

• What dollar value do we attach to the collection and filtration of the veggie oil?
• Veggie oil is less energy dense so compared to diesel more fuel will be used
• Finding an appropriate storage tank and converting it will cost x amount
• There is the cost of the solenoids and additional fuel pump for the veggie oil
• Also need to consider how to keep the veggie oil from completely solidifying in colder climates

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