Cost-Benefit Calculation - Pay back time on Solar Tubes

[quentingargan]

According to most energy authorities, (SEAI included) the average household uses a lot more energy to heat domestic water than you do.

Fair play to you, and clearly solar isn't for you, but the average is 2,000 Kw Hrs per year for hot water.

 

[jcash]

am i right in this ,
with solar system ,you have a bigger water cylinder, hence in winter your oil boiler has more work to do in heating this cylinder before it puts any heat into the rads ???

 

[quentingargan]

am i right in this ,
with solar system ,you have a bigger water cylinder, hence in winter your oil boiler has more work to do in heating this cylinder before it puts any heat into the rads ???

I can see where you would draw this conclusion, but there are two coils in the cylinder, one near the top is supplied by your boiler and this will only heat the top 120L or so of hot water that you would always have needed with a standard system. It won't heat water below its level.

The lower coil is supplied by the solar panel and heats the whole cylinder. And even in winter, your solar panel will often enable this coil to bring the contents of the bottom of the cylinder to 40 degrees. So effectlvely your boiler actually has less work to do.

Finally, most solar cylinders have about 50mm of insulation, and are better at storing the heat, so there is less heat loss in the winter. This is often a forgotten benefit of installing solar water heating. In a good cylinder, if it has been brought up to 60 degrees the night before, in the morning the water temperature will only have fallen very slightly, so in fact your boiler will have much less work to do on its daily rounds to the radiators. :~)

 

[Barney Magoo]

We looked at this last year as part of an energy efficiency upgrade to our house, and talked to several potential suppliers and 'experts'. Most were more interested in the sale than doing a personalised cost-benefit analysis for our situation and all appeared ill-informed about their technology.
We found the linked article very helpful and unbiased. Decided not to fit water-heating panels after all as it did not make economic sense to us.

[broken link removed]

 

[quentingargan]

We found the linked article very helpful and unbiased. Decided not to fit water-heating panels after all as it did not make economic sense to us.

I beg to differ. To my mind, that is a bit of a rant, suggesting that it might cost €5K to €8K to install a system that would meet as little as 15% to 20% of your domestic hot water needs. The article also totally misunderstands zero loss efficiency in tests conducted by SPF on vacuum tubes (by simply ignoring IAM). The author favours DIY panels over commercial products, and DIY installation over professional installation. It isn't exactly unbiased, but then neither am I.

 

[Leo]

The author favours DIY panels over commercial products, and DIY installation over professional installation. It isn't exactly unbiased, but then neither am I.

The article favours 'an indigenous, localised solar thermal industry producing low cost easy to install solar collectors for mass deployment in a manner similar to Austria.' Not exactly DIY panels.

DIY installation makes sense if you're trying to reduce the overall system cost to reduce the payback period.
Leo

 

[onq]

Hi chipclub,

I see no reason to doubt your figures and what I see is more of the same empirical data coming back - not great returns - and you will have to replace the system well before three decades + 2 years expires.

ONQ.

 

[onq]

Finally, most solar cylinders have about 50mm of insulation, and are better at storing the heat, so there is less heat loss in the winter. This is often a forgotten benefit of installing solar water heating. In a good cylinder, if it has been brought up to 60 degrees the night before, in the morning the water temperature will only have fallen very slightly, so in fact your boiler will have much less work to do on its daily rounds to the radiators. :~)

Agreed but this latter point is a sore one with De Wife.

She uses the hot press to part dry the clothes in really inclement weather when you cannot get them fully dry outside and she say she could never do that if the cylinder was fully insulated - go figure.

ONQ.

 

[quentingargan]

The article favours 'an indigenous, localised solar thermal industry producing low cost easy to install solar collectors for mass deployment in a manner similar to Austria.' Not exactly DIY panels.

DIY installation makes sense if you're trying to reduce the overall system cost to reduce the payback period.
Leo

Ah, I was talking about the author rather than the article. The author is an advocate of homemade systems. I don't mean to slag that off by the way - I made my own first solar water heater, and as long as you know what you are doing, DIY is a valid way of cutting costs.

A DIY system using old radiators, or clip fins from the Centre for Alternative Technology ([broken link removed]) for example, will be extremely cheap to make, but its performance would not compare with modern collectors using the latest absorber materials.

There is a market for this sort of DIY system, but the vast majority of installations will be certified product manufactured to standards. Whether that can be done as part of a low cost indigenous industry remains to be seen, but producing quality solar panels anywhere requires heavy investment and a sizeable market.

 

[Leo]

Ah, I was talking about the author rather than the article. The author is an advocate of homemade systems. I don't mean to slag that off by the way - I made my own first solar water heater, and as long as you know what you are doing, DIY is a valid way of cutting costs.

Ah OK, thanks for clarifying. I was unfamiliar with his work/advocacy outside of that article. I'd agree that DIY panels will never yield great efficiencies. Even the article above states that the low-complexity Solar Twin option yields poor results, so I was wondering how the use of DIY panels would fit with that. It's unlikely anyone going down the DIY panel route will use intelligent control mechanisms.
Leo