Going off-grid is a big step. It requires careful consideration, good design and engaged users.
Recently, sustainability trailblazer Michael Mobbs talked of his own experience, and the unexpected challenges he has faced.
Having supplied equipment to Mobbs in 2015, Alpha-ESS was particularly interested in drawing out lessons for others looking to ‘cut the wire’.
Living Off-Grid is a Mindset
When you are effectively the power customer and the power provider, energy comes into sharp focus.
The first step we always advise our customers is to become an ‘energy bloodhound’. Cut energy use any way you can.
Lesion 1: Reduce your load, and plan to use energy when it is available. Also design essential circuits that still operate when power is scarce.
Same Solar Output?
Mobbs is right to point out that in an off-grid situation, when your battery is full and there is not much power being used in the home, the excess solar energy has no-where to go, so is curtailed by the inverter.
Lesson 2: Depending on the design, an off-grid solution may yield less solar energy than a grid connected system (that exports excess solar energy to the grid regardless of local loads).
System design is critical. Getting the right sizing of both solar and storage will reduce this problem.
A System Bug?
Mobbs also says there is a bug causing the system to trip.
In this case our measurements show that large concurrent loads are shutting the 5kW inverter down.
The size of the inverter puts a ceiling on the amount of energy that can be drawn at any one time.
In Mobbs’ case the inverter has an absolute capacity of 5kW. This in turn means running power hungry devices concurrently may trip the system. Increasing the size of the inverter may help.
Lesson 3: Bump up your inverter even if you have cut your load, and avoid using big loads such as water pumps, irons, washing machines and microwaves all at the same time.
Expect the Unexpected – Back up is Smart
Australia’s weather is changing as extreme and unusual weather events become more commonplace. A system designed on assumptions of the ‘average pattern’ is fine – until it isn’t.
Battery storage is finite, so you either have to design for the ‘outlier events’ (and get a battery that is oversized for the rest of the time), or have back up for exceptional circumstances.
Most off-grid customers will design in a small emergency generator for that backup. Mobbs did not want a generator, and that is fine, but that will lead to beefing up your storage capacity.
Lesson 2: Think about your options in managing outlier weather events and have a plan.
Our Solution
Going off-grid takes careful planning and requires expert advice. Alpha-ESS are experts in this field.
Given all of the variables, our advice to others in a similar situation is:
Monitor and model the variables, we can do this for you, to come up with an ideal design.
Get a backup generator (only for extreme events). It will be much cheaper than getting a larger battery bank to increase the days of autonomy.
Potentially look at upgrading to a larger inverter, and adding some more solar PV. We have other inverters in the range that might suit the purpose.
Make the most of sunny days and target times when your battery is full to run appliances.
Alpha-ESS stands by our customers. We are very happy to measure all of Michael Mobbs’ unique energy flows, and provide further design advice at no charge.
Dong Lin is managing director of Alpha ESS Australia
This post was published on April 6, 2017 2:47 pm
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View Comments
Dong, I agree with what you have said here, but have a look at the PV yield. His system is only producing less than half of what it should and that needs to be sorted as well !
Dong, one of the best explanations in simple terms of the challenges and pitfalls of going off-grid. Well done. I think I'll quote you in my training sessions.
A generator instead of a bigger battery?
If you go deeper into discharge on your battery, the battery will have a shorter life span. So I would opt for a bigger battery. Even try to stay in the top 10% if possible.....
Am I wrong? I've seen depth of discharge charts.
https://energymag.files.wordpress.com/2014/02/nickel_iron_battery_-_depth_of_discharge_life-wikipedia.jpg
http://www.mpoweruk.com/images/dod.gif
https://www.valence.com/wp-content/uploads/2014/05/DOD.png
http://www.exact-co.com/images/stories/gel-pictures/image001.jpg
I'm no expert.
But that is how it looks to me.
5,000 cycles is 13 years...
IMO if you are using a generator, you are not off grid.
There are technologies coming to increase the number of cycles....
Great article Dong, a further nuance is the nature of the loads and their surge rating. First I have a small backup system with a 500VA inverter with close to 1500W surge rating and it can run the house but I have encountered issues with the fridge. I have a fridge/freezer with standard compressor, a couple of issues arise;
- the defrost 450W is random and unable to be set to a convenient time which means it can be activated for the 20 minute cycle at any time, then of course not much room for anything else. Ideally I would like to set this for 4am or some time when no other loads are present.
- the operating power is about 115W but the surge measured is 12A (>2500W reduces with voltage sag) for seconds which gives the inverter a shutter each time it starts, thankfully it does not exceed the circuit breaker limits so it some how manages to start the motor each time and keep working.
- I also looked at the power factor and added in an extra capacitor to adjust the power factor to 1.
The ultimate solution would be to change the fridge with a super efficient one as possible and one which has a much reduced surge rating, has variable speed compressor, programmable defrost and unity power factor.
With the washing machine, a front loader and a single inlet cold water hose, as it has a heating element. We normally do a cold wash but in winter this can be an issue with how detergents work (i.e. not so well in winter). The solution I've used is to use a water tempering valve adjusted to around 30C on the single inlet hose, this then uses my 'free' solar thermal hot water for each wash to keep operation of the detergent effective. Problem solved no 2,400W heating element used.
Re the solar hot water. My preference is for the close coupled units, simple, no pumps, no controllers and frankly just work without any hassles, giving a supper high level of resilience. To get the most out of the systems I've designed the size of the system to have a year round Solar Fraction >0.95, which in my location in SE Queensland is easily achieved. The temptation by the industry is to under size the systems to keep the quotes as competitive as possible which then tends to lower the Solar Fraction and require more frequent extra boosting.
So by using a range of solutions and paying attention to the extra technical details (which can take considerable expert effort) of all the appliance operating characteristics lowers the need for more expense inverters, which are then likely to operate the majority of the time at less than ideal efficiency.