Solar PV Hot Water system Project

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argentum66

18 posts

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#230423 23-Feb-2018 10:55

The promise of Solar PV Power is not really worth the effort due to such low payback rates here in NZ - the best is a measly 6c a kw. So our proposal is this, to use a 36v 450w / 900w DC water heating element which also has a 2.1kw Dual AC element (trademe, $70 ).

We are a household of 6, and our main drain on the energy we use is Electric How water with a solar tube array. Our bills and usage (night time rates for water heating) average about $220 a month, using 1100kW our current provider is Ecotricity (previously Electric Kiwi - but they just bumped up their rates by 15%).

The plan A - use solar pv to offset what little electricity is used during the day ( about 0.4kw an hour 8am to 6pm ) with 2 x 270w panels. This will save the 0.27c a kW cost at this time , with 2x solar pv panels @ 270w costing $550 for both, and small inverter $300.

The potential saving : $350 a year - with -0.06c factored in.

The plan B- we are going to test a single 270w panel with the DC element and see how long it takes under real conditions to heat up a 42 Litre Rheem tank fitted with this element. Total outlay $330.

Should this succeed then our potential savings for this tank alone are (based on night rates and 1 hour heat at 3kw ) : $150 a year. If it works we will add / replace an element on our main tank.

Will keep you posted with pictures.

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8171 posts

Uber Geek

#1962803 23-Feb-2018 11:11

You local lines company and retailer will likely slug you the cost of installing a new meter to account for all those lover 6c units being returned to the grid, so you might want to factor that into you figures,

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kotuku4

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#1962870 23-Feb-2018 11:50

When I installed 4.9 kw solar PV a few years ago I had to fill in a form with details of the system and Inverter. And pay a $220 ish fee to local lines company for approval to connect equipment to their network.

Yes there was a fee to change the meter to an import export meter, and since been changed to a smart import export meter with another fee.

5 of us in a modest 3 bed, single bathroom house. Upgraded insulation, and retro fit doubled glazed, single 6kw heat pump. Just upgraded 20 year old 180 L low pressure hot water cylinder to 270 L low pressure. Annually we export more kw than we use. Always been on low user plans, before solar install.

Summer power bills are about zero, December $11 credit, January $16 to pay, but a power trust rebate has the account currently $220 in credit. Supplier is Ecotricity, plan residential low solar.

Buy back rates started at 24 cents/kW and now down 7.02 cents/kW. There is a small export fee charged by the local lines company 0.5 cents/kW, daily lines charges, metering charges, electicity authority levy.

Mattnzl

281 posts

Ultimate Geek

#1962921 23-Feb-2018 12:03

I believe the OP isn't looking to connect to the grid though - so no fees/meter needed......?

Just a direct connection to the cylinder - shouldn't need the inverter either if you get a lower voltage DC element?

wellygary

8171 posts

Uber Geek

#1962928 23-Feb-2018 12:19

Mattnzl:

I believe the OP isn't looking to connect to the grid though - so no fees/meter needed......?

Just a direct connection to the cylinder - shouldn't need the inverter either if you get a lower voltage DC element?

For option B yes it not grid connected, , but in option A the OP was talking about offsetting current grid power usage and that would certainly need to be grid tied...

argentum66

18 posts

Geek

#1962950 23-Feb-2018 13:18

If the PV water heating option works we will increase panels and run either a DC element at 1kw, (4x 270w panels) on all day ( so kw equivalent approx 8kw heating working in conjunction with the solar.

The saving for this would be 0.14 (night rate equivalent) a kw, or $420 a year.

Running 2 elements of course doubles the saving. This is why we are running a real world test.

Aredwood

3885 posts

Uber Geek

#1963213 23-Feb-2018 22:38

Since you already have a solar hot water system, Far cheaper to just add more tubes. Very little extra cost apart from the cost of buying the extra solar hot water panels. How many tubes do you currently have? And how many Litres is your cylinder?

A single 270W panel won't be correctly matched to that element that you want to use.

argentum66

18 posts

Geek

#1963300 24-Feb-2018 09:16

We have a 30 Tube system at present which preheats our water, however we have 4 teenagers who drain this quite regularly, and have limited roof space on the side that faces North. The element is rated at 36v, so will work quite well with the 270w panel, just not as hot. The advantage of the Solar PV is there are no moving parts and the system can be installed on the detached garage roof and face North, and its input will be more significant in winter months when the evacuated tube system is less efficient. Hot water in tank today at 67 C !

argentum66

18 posts

Geek

#1963301 24-Feb-2018 09:17

Cylinder is 310 Litres

tdgeek

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#1963323 24-Feb-2018 10:10

argentum66:

We have a 30 Tube system at present which preheats our water, however we have 4 teenagers who drain this quite regularly, and have limited roof space on the side that faces North. The element is rated at 36v, so will work quite well with the 270w panel, just not as hot. The advantage of the Solar PV is there are no moving parts and the system can be installed on the detached garage roof and face North, and its input will be more significant in winter months when the evacuated tube system is less efficient. Hot water in tank today at 67 C !

Interesting. We have solar HW, Apricus, 30 rings a bell. In any case its the middle size for a 5 bedroom, multi adult home, which we have, although only 3 adults now.

How do you manage it?

I have the bottom element permanently off. There are 3 timers, I have one timer set to 9pm to 10pm, as that tops up the top element if needed. If its been cool, the top 1/3 of the cylinder will be 62, the bottom 2/3 a bit less. This time of year it rarely tops up as on a sunny day the cylinder top and bottom will be well over 70. When needed I can set another timer for 1 -2 hours early morning to take care of morning showers. If needed in winter, a one hour popup at say 5pm.

The other thing is if the HW is less than 60, it doesnt matter as it will have been 60+. Ive had a shower at 47 it was fine, a minor tweak on the mixer. Along as the cylinder gets topped up once daily I dont see an issue wth Legionaires. If you lived with less than 60 because it was fine, for days on ed, you cant take that risk.

Im with EK, so that one hour topup is my hour of free power. ChCh If I had a one off need, its easy to turn the top element on vis the panel, and to turn the bottom element on at the wall connection.

argentum66

18 posts

Geek

#1963575 24-Feb-2018 17:53

We were with electric Kiwi, but since they raised their price structure Ecotricity seemed a better option for the night rates - 11c vs 27c. The main reason for this was that we still needed to heat the water well beyod the hour of power. We had originally upgraded the x 3kw elements in the tank by replacing 1 3kw with a 4kw in the upper tank. If you have the same system as us, the timed heating operates both elements simultaneously so when the correct temperature for one is reached they both cut off ( the top can be 60c the bottom still 30c.

We'd like to use both solar pv for water heating as well as direct solar, and feed back the small constant drain of electricity from fridges, sewerage etc - 2 x 270w panel would do this and save 4kw a day.

Our experience with EK was the initial pricing structure was fair at 24c a kw (now its 27c) but one hour of free power wasnt enough to run the DW or WM, so we run these after 11pm on timers now. We did utilise a lot of plug in electric heating for an hour on EK during the coldest nights - probably some over utilisation just for the sake of it being free - fine unless you fall asleep and reawaken at 1am with that dreadful omg we've left the heaters on !

Regarding how we manage the hotwater, used timed heating between 1am and 5am to top up the solar shortfall. Most showers are taken in the morning, and we have as mentioned in the post a backup tank of 42L for kitchen use which will have the dc element fitted to one or two solar panels @ 270w each.

tdgeek

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#1963770 25-Feb-2018 10:48

argentum66:

We were with electric Kiwi, but since they raised their price structure Ecotricity seemed a better option for the night rates - 11c vs 27c. The main reason for this was that we still needed to heat the water well beyod the hour of power. We had originally upgraded the x 3kw elements in the tank by replacing 1 3kw with a 4kw in the upper tank. If you have the same system as us, the timed heating operates both elements simultaneously so when the correct temperature for one is reached they both cut off ( the top can be 60c the bottom still 30c.

We'd like to use both solar pv for water heating as well as direct solar, and feed back the small constant drain of electricity from fridges, sewerage etc - 2 x 270w panel would do this and save 4kw a day.

Our experience with EK was the initial pricing structure was fair at 24c a kw (now its 27c) but one hour of free power wasnt enough to run the DW or WM, so we run these after 11pm on timers now. We did utilise a lot of plug in electric heating for an hour on EK during the coldest nights - probably some over utilisation just for the sake of it being free - fine unless you fall asleep and reawaken at 1am with that dreadful omg we've left the heaters on !

Regarding how we manage the hotwater, used timed heating between 1am and 5am to top up the solar shortfall. Most showers are taken in the morning, and we have as mentioned in the post a backup tank of 42L for kitchen use which will have the dc element fitted to one or two solar panels @ 270w each.

Tks for the detail.

My 300L cylinder is mains powered at the bottom and timer powered at the top. EK did put their price up, I rang, they offered 24c incl GST on a 12 month contract so I took that.

The bottom element will take longer as the water will be cooler, but you use the top first, hence why I turned the bottom off permanently. If you have night rates it makes sense to heat then but I would only heat enough to last the morning showers. Solar will then start to take care of the bottom mid morning. My mantra is to only use electrical HW heating when I have to. Unattended, Solar HW is useless. HW is hot in the morning and paid for, showers happen it heats up again then when the sun is out there is no need, it just heats water you wont be using and to hotter than needed.

Are you sure the bottom heats up on the timer? If it did. top and bottom would heat at a similar rate, after allowing that the top os 1/3 the bottom is 2/3. If top was 60, Id expect 60/50

EK hour of power is worth 11% here during summer, which is low use. power bill is currently $35 a fortnight (we have gas cooktop). it peaked at 30%. At 9pm we run DW and WM if needed. I start them at 8-45 so by 9pm they are starting to use more power once WM filled and DW rinsed. Turn both heatpumps to 25 if we are using them, then instead of going back tp 21, we turn them off, so the free hour built up heat to cover 10pm to bed, so an added saving over the free hour.

I'd love solar PV, but its a hard sell. To cover standby items isnt much, or lunch, isnt a lot either. I work from home so I would get more of a benefit with the Citrix box and two screens, and heat pump maybe. Winter is less generation when I want it, summer is more when I dont want it. Gimme a 30kW battery and Id be in, but thats an arm and half a leg price.

Having said all that, it worth keeping up to date with solar install costs and efficiency. A mate got a bunch of PV panels that generate 50% more then his older ones

Aredwood

3885 posts

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#1964135 25-Feb-2018 22:43

argentum66:
We have a 30 Tube system at present which preheats our water, however we have 4 teenagers who drain this quite regularly, and have limited roof space on the side that faces North. The element is rated at 36v, so will work quite well with the 270w panel, just not as hot. The advantage of the Solar PV is there are no moving parts and the system can be installed on the detached garage roof and face North, and its input will be more significant in winter months when the evacuated tube system is less efficient. Hot water in tank today at 67 C !

You will actually get better output wiring the 2 450W elements in series, to operate from a single 270W panel. Reason - the panel will only output around 8A at max power. But 450W at 36V = 12.5A meaning that the panel will be operating way outside of its max power point.

argentum66

18 posts

Geek

#2126775 14-Nov-2018 19:33

The promise of Solar PV Power is not really worth the effort due to such low payback rates here in NZ - the best is a measly 6c a kw. So our proposal is this, to use a 2kw heating element which also has a 2.1kw heating element connected to grid tie inverter, connected to an array of 7 x 300w panels wired straight to the tank, no grid tie.

Thus our water is heated during the day by the solar array, wiring is standard 2.5mm TPS as its carrying AC current

We are a household of 6, and our main drain on the energy we use is Electric Hot water with a solar tube array. Our bills and usage (night time rates for water heating) average about $220 a month, using 1100kW our current provider is Ecotricity (previously Electric Kiwi - but they just bumped up their rates by 15%).

irongarment

280 posts

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#2126830 14-Nov-2018 21:17

If you use a solar PV panel to run a heating element in your hot-water tank then your payback is 23 cents a unit (or whatever it is), not 6 cents. Because you are getting the benefit of not buying that unit from the electric company. Your benefit drops to 6 cents when you make it but can't use it. And zero cents when you make it and can't sell it.

A hot-water tank is probably better and safer than a battery bank, plus you already have it.

argentum66

18 posts

Geek

#2126833 14-Nov-2018 21:30

Absolutely - solar panels are now $1 a Watt - I was wondering if I could use the grid tie inverter I have (never connected to grid ) which is rated to 2.5kw straight to the element

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