Solar water heating

Forums › Home Workshop › Solar water heating

Clawhammer

13 posts

Geek
+1 received by user: 2

Topic # 240494 10-Sep-2018 21:28

I'm a newby so please excuse if this posting should be in an existing Forum that I haven't discovered yet.

Approx. 30% of electricity used in this country is used in the home and approx. 30% of that is used for water heating, something like 10% of our total electricity usage.

What if every new house was required to have a small percentage ( 2-3%? )of it's roof area covered with solar water heating panels to be built into the conventional electric hot water system. I reason that most hot water is used in the daytime, coincidentally when the sun shines so reduce total electricity demand and also help with the midday and maybe evening peak loads.

As part of a new build it should keep the costs to a sensible level, will provide long term savings to the owner and won't cost the government one cent.

Any thoughts?

1 | 2

3013 posts

Uber Geek
+1 received by user: 1155

Subscriber

Reply # 2087181 10-Sep-2018 22:00

One person supports this post

As a plumber, who has solar hot water on his own house - no. And that is despite such a rule, that if implemented would give me extra income.

Biggest problem, the average house uses 60% more energy on water heating during winter than summer. Yet there is way more sun during summer than winter. So during summer, you have problems with managing excessive system temperatures. And in winter, the system is unlikely to provide even half of your hot water needs.

I don't agree with most hot water being used during the daytime either. If you are at work during the day, you are therefore not at home. So you won't be using any hot water at home during the day. Or you will only be at home in the early morning, or late afternoon/ evening. Even if is still daylight then, there is not much energy in the sun's rays, and it will be at the wrong angles for your solar panels to use.

And that assumes that your house is even a suitable design, and is in a suitable area to be feasible for solar hot water to be installed.

Far better, on a cost Vs environmental point of view. Is large electric hot water cylinders, which are connected to ripple controlled night rate power. The power used is cheap, late night renewable generation. The ripple control helps with management of the national grid, and further reduces fossil fuel generation. As less fossil fuel generation is needed to provide spinning reserves.

Solar hot water along with solar PV, are far better suited to countries like Australia. Which get more sun than NZ. And which have power grids that are supplied mostly by fossil fuel generation.

In NZ, replacing a fossil fuel burning car, with an EV. Helps the enviroment far more than a solar system ever will.

edited to add

As for cost, it would cost the government a lot extra. As you have just pushed up the cost of all of the kiwibuild homes and new housing NZ homes. And you have made private sector housing more expensive. Which in turn will flow through to higher rents. All of which will then add extra costs onto the government via the social welfare budget.

SATTV

712 posts

Ultimate Geek
+1 received by user: 195

Reply # 2087186 10-Sep-2018 22:08

I agree with what you say, I believe some new subdivisions this is a requirement for water heating.

I believe there should be a requirement for at least a 10000l water tank to catch rain water, this can be used for watering the garden and flushing the toilet etc, even putting a restricter so it all does not go stormwater all at once and overflow the stormwater drains, this is a big issue in out area with lots of infill houseing and the council not making the stormwater drain any bigger.

John

I know enough to be dangerous

Geektastic

11831 posts

Uber Geek
+1 received by user: 3836

Trusted

Lifetime subscriber

Reply # 2087187 10-Sep-2018 22:11

How would a heat pump hot water cylinder compare in terms of cost?

nickb800

1997 posts

Uber Geek
+1 received by user: 331

Trusted

Reply # 2087251 11-Sep-2018 07:27

I agree with aredwood, large hot water cylinders on controlled rate would the way to go - for the household, society, the environment and the govt.

If this was a mandated change and became widespread, it would probably encourage investment in geothermal and hydro (particularly run of river) power stations, as it would nudge wholesale prices to be more stable (less peaky and more demand overnight).

Most households could easily replace their 180 litre cylinder with a 300 litre one, as they both have the same diameter (300l is just taller). If you run the cylinder with a tempering valve (required for new installations) then you could heat the cylinder to (say) 75 degrees, to store more energy.

For most households, 8-10 hours of night rate power would be sufficient to heat this, except for the highest usage households, which could go on an Night Boost/Plus (or similar) tariff, which gives an extra four hours in the afternoon.

Solar hot water is great when it's great, but it has complications that reduce it's usefulness and simplicity e.g. frost protection, electric boost to prevent legionella. Solar PV (electricity) has a very high embodied energy in manufacture, but we usually ignore that because the coal gets burnt in China, not here.

tdgeek

13243 posts

Uber Geek
+1 received by user: 2376

Trusted

Reply # 2087261 11-Sep-2018 08:04

I agree with the aboves. I have Solar HW and for us I love it, never would not have it.

6 months a year the power is off, apart from the odd boost when weather and usage collide. (I do have a free hour with EK heating the top element 9pm to 10pm) Winter, I have it on 4pm to 5pm. I'm unsure of cost savings in winter, based on I only have it on max 2 hours a day, compared to 24/7 thermostat control

For those that don't have Solar PV here is a rundown how mine works

40 Tubes I think, 5 BR house suitable. 300L cylinder, two elements. Bottom element is mains powered, so flick a switch if needed. Top element is panel powered so its always off unless I set one or more of the three timers. I can also turn it on manually via the panel. In the off season, set the timers as needed, and back them off as sun improves. Its Sept now, ChCh, I have turned off the late afternoon timer, I will need to boost that occasionally, from late October, manual intervention will be minimal.

If everyone showers morning or night, that changes the benefit, as you will need the entire cylinder hot. In our case its morning and evening, so if the bottom of the cylinder is too cool, it rarely matters as we use the top.

When the Collector is 8C higher than the bottom cylinder sensor, it moves the water. When the collector heats up again to 8C higher, it moves the water. Right now, on a good day in Sept, ChCh, it gets the entire cylinder to 60, just. A trick is to heat the bottom half for an hour, the collector then heats up quite quick as no transfer for a while as the variance is well over 8C , that gives me more value than the 4pm to 5pm topup of the top of the cylinder (more of the cylinder is hot)

Its great, works for us.

Clawhammer

13 posts

Geek
+1 received by user: 2

Reply # 2087277 11-Sep-2018 08:49

Aredwood: As a plumber, who has solar hot water on his own house - no. And that is despite such a rule, that if implemented would give me extra income.

Biggest problem, the average house uses 60% more energy on water heating during winter than summer. Yet there is way more sun during summer than winter. So during summer, you have problems with managing excessive system temperatures. And in winter, the system is unlikely to provide even half of your hot water needs.

I don't agree with most hot water being used during the daytime either. If you are at work during the day, you are therefore not at home. So you won't be using any hot water at home during the day. Or you will only be at home in the early morning, or late afternoon/ evening. Even if is still daylight then, there is not much energy in the sun's rays, and it will be at the wrong angles for your solar panels to use.

And that assumes that your house is even a suitable design, and is in a suitable area to be feasible for solar hot water to be installed.

Far better, on a cost Vs environmental point of view. Is large electric hot water cylinders, which are connected to ripple controlled night rate power. The power used is cheap, late night renewable generation. The ripple control helps with management of the national grid, and further reduces fossil fuel generation. As less fossil fuel generation is needed to provide spinning reserves.

Solar hot water along with solar PV, are far better suited to countries like Australia. Which get more sun than NZ. And which have power grids that are supplied mostly by fossil fuel generation.

In NZ, replacing a fossil fuel burning car, with an EV. Helps the enviroment far more than a solar system ever will.

edited to add

As for cost, it would cost the government a lot extra. As you have just pushed up the cost of all of the kiwibuild homes and new housing NZ homes. And you have made private sector housing more expensive. Which in turn will flow through to higher rents. All of which will then add extra costs onto the government via the social welfare budget.

So are you saying that most people installing these vacuum concentrator tubes on their rooves are wasting their money?

tdgeek

13243 posts

Uber Geek
+1 received by user: 2376

Trusted

Reply # 2087281 11-Sep-2018 08:59

Clawhammer:

So are you saying that most people installing these vacuum concentrator tubes on their rooves are wasting their money?

He is saying its cheaper to use off peak low rate power.

Our one was 8k on a new build. For us, its worth it, zero grid power for 6 months, and the other 6 months max of 2 hours per day, not needing to keep all the the 300L cylinder at 65C all of the time. And I can manage it easily by way of timers, manual if needed.

wellygary

3315 posts

Uber Geek
+1 received by user: 889

Reply # 2087287 11-Sep-2018 09:25

tdgeek:

40 Tubes I think, 5 BR house suitable. 300L cylinder, two elements. Bottom element is mains powered, so flick a switch if needed. Top element is panel powered so its always off unless I set one or more of the three timers. I can also turn it on manually via the panel. In the off season, set the timers as needed, and back them off as sun improves. Its Sept now, ChCh, I have turned off the late afternoon timer, I will need to boost that occasionally, from late October, manual intervention will be minimal.

The system you have that requires manual switching and timers, rather then thermostats sounds like a risk for Legionnaires’ disease

HWCs should remain above 60C

http://www.arphs.govt.nz/Portals/0/Health%20Information/Communicable%20Disease/Water%20borne%20illness/Legionellosis/legionellosis.pdf

MikeAqua

4952 posts

Uber Geek
+1 received by user: 2011

Reply # 2087288 11-Sep-2018 09:26

We put evac tubes on our previous house in Nelson. This allowed us to leave the HWC switched off for at least 7 months per year (except for a weekly heat for safety purposes in the shoulder season). The decrease in our power bill more than offset the repayments on the system.

However ... that included replacing an old cylinder with a modern one, which would have accounted for big chunk of the savings.

Mike

tdgeek

13243 posts

Uber Geek
+1 received by user: 2376

Trusted

Reply # 2087292 11-Sep-2018 09:39

wellygary:

tdgeek:

40 Tubes I think, 5 BR house suitable. 300L cylinder, two elements. Bottom element is mains powered, so flick a switch if needed. Top element is panel powered so its always off unless I set one or more of the three timers. I can also turn it on manually via the panel. In the off season, set the timers as needed, and back them off as sun improves. Its Sept now, ChCh, I have turned off the late afternoon timer, I will need to boost that occasionally, from late October, manual intervention will be minimal.

The system you have that requires manual switching and timers, rather then thermostats sounds like a risk for Legionnaires’ disease

HWCs should remain above 60C

http://www.arphs.govt.nz/Portals/0/Health%20Information/Communicable%20Disease/Water%20borne%20illness/Legionellosis/legionellosis.pdf

Its thermostatically controlled on both elements, if power is on. It doesn't "require" manual switching or timers, timers are there to allow the system to be a solar solution with boost. A grid system with solar boost is a waste of money. The top cylinder will be 65 at 10pm and when I was boosting 4pm to 5pm, it will be 65 at 5pm. As seasons change, the boosting can be altered to suit. Right now, the collector is 18, bottom cylinder is 15, top is 51 due to two morning showers (Bottom is 2/3, top is 1/3) The temps are slightly understated as the sensor I think is at the bottom and 2/3 up. I always manage this with 60+ in mind

Varkk

178 posts

Master Geek
+1 received by user: 88

Reply # 2087356 11-Sep-2018 10:46

Another option I have seen start to be used was a PV panel connected directly to a heating element in the cylinder. It certainly simplifies the plumbing of the system as you don't need to circulate water through the roof panel. It also gives more flexibility on cylinder placement as it is easier to shift electricity with less energy loss from the roof to the cylinder.

tdgeek

13243 posts

Uber Geek
+1 received by user: 2376

Trusted

Reply # 2087372 11-Sep-2018 10:54

Varkk:

Another option I have seen start to be used was a PV panel connected directly to a heating element in the cylinder. It certainly simplifies the plumbing of the system as you don't need to circulate water through the roof panel. It also gives more flexibility on cylinder placement as it is easier to shift electricity with less energy loss from the roof to the cylinder.

Isn't that standard Solar PV? If the PV is generating more energy than the HW needs, or of the HW is now fully heated, does the excess go to the house?

Probably be easier just to use Solar PV and timeshift

Hammerer

1657 posts

Uber Geek
+1 received by user: 392

Lifetime subscriber

Reply # 2087401 11-Sep-2018 11:09

Varkk:

Another option I have seen start to be used was a PV panel connected directly to a heating element in the cylinder.

That option has been there for decades and I'm glad that I didn't choose it. It would be economically worthwhile for me individually but not when I consider the total costs of such systems as has already been brought up in this topic and almost every other PV topic

nickb800:

Solar PV (electricity) has a very high embodied energy in manufacture, but we usually ignore that because the coal gets burnt in China, not here.

Varkk

178 posts

Master Geek
+1 received by user: 88

Reply # 2087402 11-Sep-2018 11:09

The ones I saw don't convert the DC from the panel to 240AC. Instead it is just straight in to an appropriate DC heating element. This saves some of the losses associated with that change. Also since it is low voltage DC it doesn't require the same level of design etc as a mains voltage system.

wratterus

353 posts

Ultimate Geek
+1 received by user: 106

Reply # 2087431 11-Sep-2018 11:29

Good info here. Sorry for the incoming hijack....

We'll be making some changes to our hot water system soon - currently it is low pressure, header tank on roof. Want to get rid of header tank and get a higher pressure system (would probably be re-plumbing the whole house too as it's currently all old copper and dodgy as hell).

Have been considering wetback & possibly solar too. I've not looked into this too much, what is the general consensus on wetback these days? I assume this can be run with a higher pressure system? Thanks.

1 | 2