Guru
Joined: 02/02/2017
Location: AustraliaPosts: 1432
| Posted: 05:33pm 02 Jul 2017 |
Part 10: sizing the inverter for my particular requirements and efficiency concerns
Given:
- the cost of power is only going to increase, and faster than CPI
- zero political desire or capability to reduce these cost increases
- gas is also only going to go up in price, faster than CPI for the same reasons.
- I live in Melbourne, this means lots of cloudy skies
- (this may be very controversial) we are headed for the mother of all economic depressions when the house market collapses, taking banks, personal savings, jobs, etc with it
- I have some savings in one of the big 4 banks
I thought
- make a solar power system.
- go for the easy 75% of value return, not 100% which would get costly.
- accept I will never supply 100% in Melbourne. About 30kW array needed plus a huge battery.
- get a ROI at 10-15 years.
- slowly transition to a house using less high power things. We use electric cooktop, microwave, clothes iron, kettle. This can be reduced as items need replacement.
- get some money out of the bank so I don't loose it when we all “bail-in” the banks.
So I have a 3kW solar array, feeding a 400Ah 48V battery.
All data is logged to non-volatile storage every 2 minutes.
I have 1 1/2 years of data showing street power and inverter supplied power consumed by my house. Looking at it gives me clear guidance in inverter sizing.
First, see the total energy consumed by the house from Jan 2016 to date, there are two lines here. Note my solar system can supply nearly 1/2 house needs. But when Winter comes, forget about it. Not even close.
But with 4.5MwHr self supplied, I can say this has returned me 4,500 x $0.29/kWhr (my rate) or $1,300 for this hybrid system.
Let’s have a look at the average power consumption level at any time in the 24 hr day.
It shows me that I need 1500W on average peak power, around 19:00 or 7pm which is when we cook. That is why the peak is at that time. We have a tropical fish tank too. Hence the high night time power levels.
The house spends a lot of time less than 1000W. I like to imagine how it would look if we installed a gas cooktop and oven and gave the fish away.
This is about 18kWhr/day average house consumption.
Now, here is the total time (in 2 minute slices, added together) the inverter has spent at various power levels. The inverter is a Victron 3000VA inverter/charger.
It is clear to me that I need to build a replacement inverter that is optimised for power levels up to 700W because it spends more than 90% of it’s time running at these levels.
Finally here is the cumulative energy total, as you totalise the different inverter power outputs. The small time spent at 1000W and more needs to be carefully designed so as to not kill the total efficiency equation.
If the about 500kWhr made at 1500W levels is at an efficiency of 85% only, then
that is 75kWh loss. The 3000kWhr made at 93% (which I am demonstrating right now on my bench) means I lose 210 kWh while operating in that regime.
So it’s 1/4 lost at high levels and 3/4 lost at low levels of inverter output.
Consequentially I am designing a replacement for the Victron that is 93% at 700W but can handle peaks in excess of 3000VA.
I'm currently obsessed by mosfet switching and cross conduction. This is where I can find further efficiency gains. A post on this subject will come when it is ready.
wronger than a phone book full of wrong phone numbers