3kW Grid Tied Solar PV: Watts and Warts.

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1268 posts

Uber Geek

#1014924 28-Mar-2014 23:52

If you have an electric hot water cylinder and PV it makes sense to have some kind of accommodation for the element to draw more of its power from the panels as it will take up 30 to 40% of electricity usage. This thread talks about timers
http://forums.whirlpool.net.au/forum-replies.cfm?t=2235707

Replacement elements are cheap at less than $20 for a 2kW. A large system may work well enough for a 3kW element on a timer to draw from panels.

There are dedicated intelligent controllers to have a cylinder make use of PV production where possible instead of mains power
http://www.diykyoto.com/au/aboutus/optimmersion
But this is expensive product compared with a cheap timer. I don't know if there is anybody in NZ importing and installing this kind of technology.

If you install a timer you need to make sure your cylinder is reaching 60'C daily to prevent bacteria growth. A 2kW element heats 1 litre of water by 1'C in 2.1 seconds. So with a 2kW element in a 250 litre cylinder would take 8 hours to heat 5'C water to 60'C, or heat by over 40'C during the six peak production hours that Porboynz's 3kWp PV system produces over 2kW. Setting the thermostat higher such as to 85'C in cylinders that support high temperatures would help keep ahead in some situations.

I think a timer setup is only going to work if you're not a heavy hot water user and have a large tank or at least one that lets you optimise the thermostat above the default 60/65'C. An insulation wrap would help if the intention is to store solar produced heat.

How long will it take for 5kWp systems to go down to $8000?

The overcharging going on for solar thermal systems is severe. One retailer is selling a solar thermal equipment kitset for $6660 more than an online seller is selling a better kit for. The technology makes sense but not the prices being charged for it here.

Now the pump shuts off at 85 degrees and the safety valve on the panels opens to vent super hot water which is quickly replaced by cooler water and the valve shuts off by itself and the cycle repeats.

If that happens several times daily won't it wear the valve out?

Porboynz

110 posts

Master Geek

#1018699 4-Apr-2014 18:52

A controller that diverts export generation to a HWC element makes a lot of sense, I have been considering a similar setup to charge 4 x 12V batteries to power an inverter that runs my base load after the sun goes down. Actually it would run my base load 24 x 7 as its simpler and provides a fairly steady load. (Fridge, freezer, ventilation system, modems, switches and AP's plus the entertainment centre Tivo and My Sky boxes) Simpler to buy the controller but more fun to build one, I could not easily find the price of the readymade controller. Not that I looked very hard.

The solar water panels rarely get a chance to vent via the roof safety valve, normal house HW use keeps the temp down, the highest I saw the tank reach this summer was about 80C. It really only activates if we are away and its very sunny. If we are away a few weeks I cover the panels with a cheap tarpaulin, takes about 10mins to fit with bungy cords. I manually activate the safety valve every time I up on the roof or at least every 6 months to ensure its functioning, the same with the cylinder mounted safety valve. I'll bet there are thousands of these safety valves on cylinders across the country that are never activated with the test lever. They are very simple mechanically but could still corrode shut if not exercised occasionally.

The solar panels are rocking, I'm up to 2650kWh generated and 1280kWh exported since install, during which time we imported 2321kWh. So just roughly I am using half and exporting half of what is generated. Now for winter.

Porboynz

110 posts

Master Geek

#1041796 11-May-2014 18:03

Autumn Update.

A few days ago saw 3000kWh achieved since the PV install last October. My power bills are still around $80 - $90 but will soon increase as the heat pump gets more use and the HWC needs more electricity boost than the solar water panels can provide as winter settles in.

If you look back at earlier posts my 3kW of PV generated about 15kWh per day on average and we used about 50% and exported 50%. Now that its late Autumn take a look below, the panels are barely reaching 10kWh per day and the export ratio has dropped to about 35%.

mattwnz

20147 posts

Uber Geek

#1041829 11-May-2014 19:53

Anyone got one of those ones being advertised on TV for about 7 grand. Although I see in the fine print that it appears to be 1.5kW, so appear to be half the generation capacity.

I am wondering if it isn't better to centrally generate all power, rather than each house having some generation capacity. There is potentially better economies of scale in this, and all maintained is centrally done. I think the current system fails because there are too many companies clipping the ticket along the way providing that power. I mean NZ did used to have the cheapest power in the world. There will also be a growth in electricity needs as more car users switch to electricity.

SumnerBoy

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#1041889 11-May-2014 22:19

I am very interested in your findings Porboynz. I have a solar outfit coming to look at my place and give me a quote this week. I am ChCh based and they have just installed a system at my old mans EQ rebuild. They use the same Enasolar inverter you have I believe. I have a pretty comprehensive home automation system so I am hoping to use that to try and shift as much load as I can when there is plenty of sunlight and good generation happening. Not quite sure how it will all work but the general idea is to 'schedule' high load tasks in the morning and the HA software will monitor the PV panels and when they hit a certain level start kicking off tasks.

I also have underfloor hydronic heating - powered by a 9kW air to water heatpump - but I think I am right in saying there is no way I could run this using PV (I will likely have a 3kW system). However if I ran the heatpump during the day and was generating 3kW presumably I would then only be importing 6kW? I guess this is the benefit of the grid tied inverter.

Still, I am on day/night rates. So roughly 30c/kWh day and 11c/kWh night. Currently I only run the underfloor heat pump from 9pm - 7am so I would need to do some calculations in order to work out if running the heatpump during the day when PV is cranking, is actually worth it, if I am still having to pay for 6kW. Could be another job for the HA software...

Porboynz

110 posts

Master Geek

#1041981 12-May-2014 09:34

SumnerBoy: I am very interested in your findings Porboynz. I have a solar outfit coming to look at my place and give me a quote this week. I am ChCh based and they have just installed a system at my old mans EQ rebuild. They use the same Enasolar inverter you have I believe. I have a pretty comprehensive home automation system so I am hoping to use that to try and shift as much load as I can when there is plenty of sunlight and good generation happening. Not quite sure how it will all work but the general idea is to 'schedule' high load tasks in the morning and the HA software will monitor the PV panels and when they hit a certain level start kicking off tasks.

I also have underfloor hydronic heating - powered by a 9kW air to water heatpump - but I think I am right in saying there is no way I could run this using PV (I will likely have a 3kW system). However if I ran the heatpump during the day and was generating 3kW presumably I would then only be importing 6kW? I guess this is the benefit of the grid tied inverter.

Still, I am on day/night rates. So roughly 30c/kWh day and 11c/kWh night. Currently I only run the underfloor heat pump from 9pm - 7am so I would need to do some calculations in order to work out if running the heatpump during the day when PV is cranking, is actually worth it, if I am still having to pay for 6kW. Could be another job for the HA software...

I recommend the Enasolar Inverter. They are made in NZ (Christchurch), they are feature rich and have world class construction and finish.   Why would you not support NZ made when the local product is this good? I will send you a link so you can login to the Inverter and have a play with the reports.

Your HA sounds great, mine is fairly mantronic. I have the a central RF timeclock that moves as much baseload to daylight hours as possible.   There are simple things that can be done like boiling the kettle first then turning on the toaster while you make a cup of tea so the load is spread out while the panels are exporting. The dishwasher and washing machine are also operated during daylight wherever possible.

Yes you are correct, when you are using say 6kW and the panels are making 2kW then you only pay for the net import of 4kW. I think with your heatpump the night rate power is roughly equivalent to the $0.10 you get for exporting more than 5kWh per day and much less that the $0.25 per kWh you get for the first 5kWh per day exported. Assuming you have enough HWC storage to heat all day then I think your current operation 100% on night rates makes the most sense.    Note also that Meridian do not measure your actual daily export but average it over the full month at $0.25 per kWh for 5kWh per day. So if I export 10kWh per day for 15 days and none for 15 days they still pay out 150kWh at $0.25 for the 30 days. This means you benefit slightly by averaging the good and bad generation days.

SumnerBoy

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#1041986 12-May-2014 09:40

It would be great to see those graphs - many thanks for the offer. Definitely keen to support a local business so will more than likely go with an Ensolar inverter. I am starting to understand how to best utilise this stuff, thanks to your advice and tips. Obviously want to minimise large spikes in usage when generating, i.e. during the day. Good to know about the averaging by Meridian. Do Contact do the same or are they more accurately monitoring usage?

After writing that post last night I realised that my heatpump actually only draws about 2.6-2.8kW when running. Therefore in theory on a sunny day most of it could be provided by solar (assuming I had a 3kW array). I guess it will just be something to play around with. Something else to consider is that I don't want to be cycling the heat pump on and off many times a day. So there will need to be significant hystersis in any rules governing when it can turn on/off.

Sounds like a lot of fun trying to program that up!

Really enjoying reading about your experiences (as are obviously many others). Keep up the great work!

Porboynz

110 posts

Master Geek

#1041991 12-May-2014 09:51

mattwnz: Anyone got one of those ones being advertised on TV for about 7 grand. Although I see in the fine print that it appears to be 1.5kW, so appear to be half the generation capacity.

I am wondering if it isn't better to centrally generate all power, rather than each house having some generation capacity. There is potentially better economies of scale in this, and all maintained is centrally done. I think the current system fails because there are too many companies clipping the ticket along the way providing that power. I mean NZ did used to have the cheapest power in the world. There will also be a growth in electricity needs as more car users switch to electricity.

The NZ hydro schemes are a fantastic natural asset but distributed power systems are very much the way to go to avoid transmission losses. I think if houses were able to generate perhaps 1.5kW it would certainly be beneficial over 10 to 20 years, but like double glazing and insulation, not everyone can justify the initial investment. If every house was generating 1.5 to 3kW and was able to divert excess generation into onsite storage you avoid messing with a power distribution sytem that is not designed for large scale distributed generation. Batteries are expensive, but a device that takes any excess and heats your HWC makes some sense. There are a bunch of gotchas of course, some HWC are small, some are gas, some cannot go above 60C. Electric cars will be a natural onsite storage but they are probably out being driven during the daytime and need to be charged after hours on night rates. Its never simple.

Porboynz

110 posts

Master Geek

#1042242 12-May-2014 13:18

SumnerBoy: It would be great to see those graphs - many thanks for the offer. Definitely keen to support a local business so will more than likely go with an Ensolar inverter. I am starting to understand how to best utilise this stuff, thanks to your advice and tips. Obviously want to minimise large spikes in usage when generating, i.e. during the day. Good to know about the averaging by Meridian. Do Contact do the same or are they more accurately monitoring usage?

After writing that post last night I realised that my heatpump actually only draws about 2.6-2.8kW when running. Therefore in theory on a sunny day most of it could be provided by solar (assuming I had a 3kW array). I guess it will just be something to play around with. Something else to consider is that I don't want to be cycling the heat pump on and off many times a day. So there will need to be significant hystersis in any rules governing when it can turn on/off.

Sounds like a lot of fun trying to program that up!

Really enjoying reading about your experiences (as are obviously many others). Keep up the great work!

I was wondering about a heat pump that uses 6kW, it would have a massive hot water ouput at 300% efficiency! If you can get $0.25 per kWh for exported power then you are better to export it and acrue the credits if you can run your heat pump on cheap night rate power at $0.11 per kWh. The hysteresis needed for running the heat pump when the suns shining might make it impractical given that every time a cloud reduced the output you would be using imported power at $0.25+ approx per kWh. Thats why a gadget that can monitor output and feed any ecess power directly to a resistive element in a HWC would be nice, it can react instantly to changing generation. I envisage something like a speed controller in reverse that can vary its operating current input to match the excess power available and deliver a steady 240V with a varying AC current to the HWC element. For optimal return it would need to predict the daily export based on a usage algorithm so it diverts any exports beyong the 5kW per day Meridian pay a good return for. Gives me a headache just thinking about it, seasonably adjusted, predictive load, predictive output modified by time of day and generation up to that moment, oh my goodness.

About Contact, I gave up on them when I was told I had to invoice them monthly. (they were my incumbent provider as well) At that time they paid a linear rate of $0.199 per kWh inc GST for however much you can export. They may well be worth a second look when I have more data to analyse after winter. I pay $0.29/kWh on the low user rate with Meridian.

SumnerBoy

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#1042243 12-May-2014 13:22

Yep - good point. The other thing I was considering though was the benefit of running the heatpump during the day. It would not be having to work as hard extracting heat out of the warmer daytime air, rather than the very cool mid-winter nighttime air. But I think you are right, that I will still be better off in the long term exporting as much power as possible (up to 5kW per day) when I can and using this credit to pay for the night time charges.

Does anyone know if anyone offers a day/night rate along with buy back?

Seems I have plenty more research to do!

SumnerBoy

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#1042262 12-May-2014 13:33

Looks like Meridian have a day/night tariff - 28.26/11.76c/kWh excl. GST. So with the 25c/kWh buy back (assuming I can have the day/night tariff) I will be much better off continuing to run the heating at night.

SumnerBoy

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#1045505 15-May-2014 12:43

Not sure if you guys have come across the following but I found them very interesting reading;

Open Energy Monitor
mk2pvrouter

The first is an open source energy monitor which looks to have a pretty decent following, especially in the UK. It is quite modular and they have a lot of guides and WIKI pages dedicated to helping setup/install/use the devices.

The second is a guy who has developed a system for diverting excess power to a secondary load, instead of exporting to the grid. It is based on the Open Energy Monitor technology I believe. He seems to have done a lot of work and has some good YouTube clips showing it in action. Probably doesn't apply quite as much to NZ since our export tariffs are structured differently, but it would be a good way to ensure you are using all of your excess power rather than relying on power company buy backs (at lower rates).

The final thing I have come across is micro inverters - i.e. an inverter under each panel converting DC to AC and then daisy chained together. These have advantages in terms of install costs as there is no dangerous high voltage DC cabling running across your roof that has to be secured and isolated etc. It also means your array can continue operating if a panel is damaged or partly shaded, without affecting any others. According to www.alosun.co.nz the prices look comparative.

Just wondering if anyone has come across these micro inverter systems and has a view on them. Some online forums claim they have a lower efficiency but then some of the manufacturer sites claim they are better than standard inverters. Just interested in your thoughts on this technology.

Cheers,
Ben

Porboynz

110 posts

Master Geek

#1045676 15-May-2014 17:05

I do like the look of the PV Router for diverting excess energy from the PV panels into a HWC element. My bottom element is 3.5kW so I would probably need to downgrade to a 2kW element so I did not overwhelm the triac. It does seem that the Arduino calcs the power available in every half cycle so it should closely follow the PV power available. More detailed investigation needed, thanks for the links.

K

dwl

371 posts

Ultimate Geek

#1046707 16-May-2014 22:05

SumnerBoy:
...snip...
Just wondering if anyone has come across these micro inverter systems and has a view on them. Some online forums claim they have a lower efficiency but then some of the manufacturer sites claim they are better than standard inverters. Just interested in your thoughts on this technology.

Cheers,
Ben

At the recent home show in Wellington I asked two suppliers of PV systems their views and both sell the micro inverters as an option. I was told by one that the gains with getting optimal MPPT per panel amount to around 20% improvement in efficiency compared to a single inverter and I have heard this claim from other sources. The other supplier was more circumspect saying the benefits kick in when there is either shading of a few panels or for other reasons single panel outputs drop then the whole string output can drop with a single inverter.

When splitting the panels across different parts of the house roof in groups then micros could also make more sense, especially if the orientation differs slightly. I note that EnaSolar larger models have dual MPPT which suits this problem if you don't want micros.

The costs are slightly higher using micros with one supplier saying around about $300 for moderate sized system. The other supplier uses 500W micros and two panels per micro. I was told if the array got even sun there wasn't so much benefit so I guess it gets down to individual circumstances.

For the geeks, some of the micros can each report their output so you are getting down to 1 or 2 panel resolution and can more quickly see where the problem lies if there is a drop off in total output. With a single inverter I'm not sure how you can tell whether a single panel isn't quite right - just on total power output?

The suppliers should hopefully provide guidance and price comparisons. Just another variable to add to the mix.

Porboynz

110 posts

Master Geek

#1062781 10-Jun-2014 17:59

No sun today, just wind and rain and no solar gain. Yesterday was bad but today is terrible, 0.5kWh was all we made all day.

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