Controls to minimise PV export + supercapacitor storage in NZ

Solar Questions and Comments
1

I am considering a 5kW PV system with 10kWh battery or supercapacitor storage.
As an aside, are there any good reviews available comparing Li-ion batteries vs supercaps?

Including line charges and tax I pay about NZD0.25 / kWh importing and would receive about NZD0.07 / kWh exporting to the grid. Minimising exporting electicity to the grid is therefore of interest!
Are there automatic switching devices available that prioritise feeding a heirachy of house circuits, to minimise exporting PV electricity to the grid
e.g.

  1. Lighting (on dedicated / fused circuits) + fridge / freezer (would need to ensure it is on a dedicated circuit?) has 1st priority
  2. General wall socket circuits have 2nd piority, when the lighting & fridge load has been satisfied
  3. Hot water cylinder (on a dedicated circuit / fuse) receives power after loads 1 & 2 have been satisfied
  4. “Night store” heat storeage heater (on a dedicated circuit / fuse) receives power after loads 1- 3 loads have been satisfied.
    Any informative suggestions / comments would be gratefully received.
2

Hi Annie & Richard

I’m not sure I understand the need for a hierarchy system, perhaps I’m getting out of my technical depth. As I understand it, you won’t export solar to the grid if you can use it in your home. You will only export excess power, or it would first charge your battery, until the battery is full.

7c is a pretty low feed in tariff, however, I’m still not sure the financials would be in favour of a battery if you crunched the numbers. What % of electricity to do you consume in daylight vs night time hours approximately? And what is your average daily kWh usage?

Battery vs supercapacitor? I think your main obstacle in getting a supercapacitor is the dearth of manufacturers offering it in the small scale space. It’s early days for the technology, and when you have the likes of Tesla and Samsung competing to drive down prices for lithium batteries (a decades old technology), I’m not sure you’ll find a competitive option. However, @Svarky will know more.

Hope that helps.

Marty

3

Sounds like you’re over complicating it. You don’t need any switching controls as many of the hybrid inverters like Redback, Selectronic and Victron already have relay controls built in which can be programmed for time of use.

As for supercapacitors, they seem promising but I have heard some strange stories about issues with voltage drop etc. I personally prefer to go with proven lithium battery technology.

4

Hi Marty,

The following may help the discussion.

For my house in southern New Zealand:

Unregulated electricity average usage: 7,500 kWhr/year (at 26.98c /kWhr incl line & GST)

Winter average use (June–Sept) 1100 kWhr/month with about 34% of this used during daylight hours. Winter max 1500 kWhr/month (7 kW heat-pump on during the evenings)

Summer average use (Dec – March) 250 kWhr/month with about 45% of this used during daylight hours

Ripple controlled (hot water) av. usage: 3000 kWhr/year. (at 18.08c /kWhr incl line & GST)

Use range: 200 – 400 kWh / month with about 60% of this used during daylight hours

Total average usage 10,500 kWhr/year

PV generation potential: Note about a 3x difference between best summer & worst winter month generation figures at my house-site in southern NZ.

Values below are average solar irradiance (POA) kWh/m2

January 203 Feb 174 March 158

April 115 May 86 June 70

July 87 Aug 107 Sept 142

October 166 Nov 185 Dec 208

I suspect about a ¼ of my June consumption could be provided by a 5 kW PV system and in December I would generate about 2.7x more PV than I could use!

Battery storage: The attraction is resilience for fridge, lighting and basic electricity consumption after an expected “AF8” earthquake event and no grid power for months.

The other attraction is whether a PV / battery system would give a better rate of return than a 10 year bank term deposit!

Batteries vs super capacitors: KilowattLabs (www.kilowattlabs.com) make a “Sirius” 3.55 kWh capacitor at a cost of NZD5119. It has a 10 year swap out warranty and claimed to have no degradation of cycle life. Size 600 x 534 x 200 / fits 19” telco rack

This capacitor compared with a 5kW Sungrow/Samsung Li-ion battery at NZD5796 with a 10 year / 6000 cycle warranty

Are there any reviews available that compare commercially available Li-ion batteries with super capacitors?

Relay controls in hybrid inverters & comments about 4 quotes

Your further comments would be appreciated about the following systems reputation & smart features e.g. incorporation of relay controls in the hybrid inverter.