Solar boilers

[Eddie Dane]

A question:

Me and the wife are implementing some things that will reduce our humongous energy bill.

One of the things we're looking at are solar boilers. Here's the model we're considering. Apparently the newest model (looks like the pods from Alien). Sorry, the site is in Dutch.
This model is the boiler and the water reservoir in one.


We have a three floor home with a flat roof
I'm located in the Netherlands.
The gas-fired boiler is in the basement.

Would passive solar energy work in this climate (moderate)?
Does anybody have any real life experience with this technology?

 

[Pixel42]

Do you mean a solar water heater? I have one on my (south facing) roof which provides virtually all the hot water I need between about May and September. I sometimes need to "top it up" with the immersion heater if there are several overcast days in a row. I'm in the southern UK, so at about the same latitude as you.

 

[Eddie Dane]

Yes, a water heater.

Boiler is probably a English word that sneaked into the Dutch language, while in actual English a different term is used.
Germans call a mobile phone a "Handy", then when they speak English nobody knows what the hell they mean.

According to the advertising blurb a solar water heater could reduce our heating bill by 40%. But I thought I'd check for actual user experiences before going for it.

 

[Pixel42]

According to the advertising blurb a solar water heater could reduce our heating bill by 40%.

I'd estimate that it's reduced my bill by about 25%.

I decided to go for it based on my calculation that it would give a better return on investment than my savings account, i.e. it would save me more per year in fuel costs than the interest the capital cost of it would earn per year if I left it in the bank. It was just about doing that initially (four years ago), and is certainly doing so now that fuel prices have gone up so much. It also adds to the value and saleability of the house, so I should get at least some of the capital back if and when I sell.

 

[neutrino_cannon]

Prior to her death my grandmother used one, and to good effect. Granted, she was fairly near the equator.

Is there any chance you could do some back of the envelope math on this? Figure out how much hot water you use, approximately how much hotter than ambient it is, take the specific heat of water and get a figure for how much energy you're using to heat water. Then figure the sun collection area on this gadget. You'll get one kilowatt per square meter perpendicular to the sun.

 

[GreyICE]

A question:

Me and the wife are implementing some things that will reduce our humongous energy bill.

One of the things we're looking at are solar boilers. Here's the model we're considering. Apparently the newest model (looks like the pods from Alien). Sorry, the site is in Dutch.
This model is the boiler and the water reservoir in one.


We have a three floor home with a flat roof
I'm located in the Netherlands.
The gas-fired boiler is in the basement.

Would passive solar energy work in this climate (moderate)?
Does anybody have any real life experience with this technology?

Pulling up PV Watts, you have a reasonably low 3kWh/day average heating capacity, peak 4.5, low around 1, so you have a very variable climate with a low, low average.

Assuming an efficiency of 90%, you have 985 kWh/m^2/year. The heat of combustion of 1 cubic meter of gas is about 10.8 kWh. Assuming you have a typical air-fired boiler (you didn't say anything about it) you probably get 40-50% efficiency. If you have a good boiler, 80-90.

Therefore, with a bad boiler, you will use 230 cubic meters of Gas to supply this heat. A good boiler will take around 100.

It typically costs $1,000-$2,000 per meter, possibly higher.

You haven't provided me with pricing information, but I can only assume its not insanely expensive, so the payback on this improvement is roughly speaking, 10-50 years. Not economically efficient.

If numbers on prices are substantially different provide and we can adjust accordingly.

 

[I Ratant]

I had all the dirt around my mobile home concreted in when I moved in.
I fingered the savings on lawn care would pay for it in the long run.
My water bill is so low, the DWP is continually replacing the water meter, as it shows so little usage, month to month.

 

[Eddie Dane]

Thanks for all the answers so far.

I'm a complete idiot with math and physics. So keep your answers simple, use words with few syllables and type really slowly.

We have an electric boiler/ water heater with an 80-90% efficiency (three years old)
Nominal yield: 40 KW (translating technical terms now, hope you understand what it is, I certainly don't)
As far as I can see we use 2316 cubic meters of gas per year.
And 91 cubic meters of water.
The price of gas: € 0.20559

The solar water heater is supposed to get a yearly yield of 3.1 GJ (according to the brochure)
The heater costs roughly € 2000,- excluding installing the thing.
we should get a € 500 subsidy.

I truly have no idea how that thing performs under our (grey and lukewarm) weather conditions. Pixel42's experiences are encouraging though.

 

[GreyICE]

Thanks for all the answers so far.

I'm a complete idiot with math and physics. So keep your answers simple, use words with few syllables and type really slowly.

We have an electric boiler/ water heater with an 80-90% efficiency (three years old)
Nominal yield: 40 KW (translating technical terms now, hope you understand what it is, I certainly don't)

*shiver*

In simplest terms - an electric boiler/hot water heater is the worst thing you could possibly have. Ever. Their efficiency rating is a total joke, don't pay any attention to it. Your thermal efficiency is MAYBE 15%. Tops.

Maximum load is 40 kW, but that unfortunately doesn't tell usage over the year.

As far as I can see we use 2316 cubic meters of gas per year.
And 91 cubic meters of water.
The price of gas: € 0.20559

Hmm, we can do some back of the envelopes just with the gas. If the price is 0.2 per cubic meter, that's a bill of $476 (and yes I'm using $, my keyboard doesn't have your awesome euro symbol).

The solar water heater is supposed to get a yearly yield of 3.1 GJ (according to the brochure)
The heater costs roughly € 2000,- excluding installing the thing.
we should get a € 500 subsidy.

Okay, so 3 year payback, IF it can replace gas. Can it?

If we assume 40 cubic meters goes down the toilet (typical usage pattern) and you mostly use hot water for sinks, etc., you're probably talking about heating 40 cubic meters or so a year. A typical delta t is 30 centigrade.

Easy calculation there, it's a megajoule to raise the temperature of a cubic meter one degree.

That gives us 330 kWh, with a nominal calculated capacity of 985 kWh/sq. meter/year.

So, final question: What is the surface area of the system?

If we can get an answer, we're good.

I truly have no idea how that thing performs under our (grey and lukewarm) weather conditions. Pixel42's experiences are encouraging though.

Nuts to the nominal yield. PV Watts isn't inaccurate, and I was using Amsterdam.

 

[neutrino_cannon]

Pulling up PV Watts

Why are we screwing around with PV? Mirrors and fresnel lenses will waste far less of the incoming energy and are cheaper to boot.

 

[GreyICE]

Why are we screwing around with PV? Mirrors and fresnel lenses will waste far less of the incoming energy and are cheaper to boot.

Err... you have no idea what PV Watts actually is, do you?

Not that I particularly mind that you don't know obscure software packages, but you could just ask. This is just insanely irritating.

 

[soylent]

(and yes I'm using $, my keyboard doesn't have your awesome euro symbol).

To produce € on a US layout, press and hold right alt, type 0 1 2 8 on the number pad and let go of alt and a € should appear.

If you're not using the US layout, try alt graphics + e(I believe that's standard for all the EU countries).

 

[neutrino_cannon]

Err... you have no idea what PV Watts actually is, do you?

Not that I particularly mind that you don't know obscure software packages, but you could just ask. This is just insanely irritating.

Aha! I have learned something. I thought you were just doing manual calculations of a solar panel array's performance.

Does this program do calculations for system that are not photovoltaic in nature? My point still stands, that would be stupidly wasteful and expensive, and all the solar water heaters I have seen are optical for exactly that reason.

 

[Eddie Dane]

I think "electric boiler" does not translate in English very well. i got te term from the manual.
Our boiler burns gas and probably uses electricity to ignite.

The surface of the house (is that what you mean?) is 220 meters.
But this is an old house from the 1920's and the ceilings are high. 800 cubic in all.
Got these numbers of the internet. There is a an identical house for sale down the street.

Why are yu guys arguing about Photovaltic? Isn't this just simple passive solar (black surface, copper piping)? There are no actual PH cells in this thing are there?

 

[GreyICE]

Aha! I have learned something. I thought you were just doing manual calculations of a solar panel array's performance.

Does this program do calculations for system that are not photovoltaic in nature? My point still stands, that would be stupidly wasteful and expensive, and all the solar water heaters I have seen are optical for exactly that reason.

No. This program contains a database of the solar radiation averages recorded for many locations from around the globe.

I was working with the net radiant energy, in kWh, output by the sun for each month, and the yearly average kWh/m^2/day output.

If you don't personalize the solar output to location, you will get bad results - latitude is NOT sufficient. I then assumed 90% efficiency on the solar collector in collecting that energy, which I assure you is a very acceptable assumption for a back of the envelope like this.

 

[GreyICE]

I think "electric boiler" does not translate in English very well. i got te term from the manual.
Our boiler burns gas and probably uses electricity to ignite.

The surface of the house (is that what you mean?) is 220 meters.
But this is an old house from the 1920's and the ceilings are high. 800 cubic in all.
Got these numbers of the internet. There is a an identical house for sale down the street.

Why are yu guys arguing about Photovaltic? Isn't this just simple passive solar (black surface, copper piping)? There are no actual PH cells in this thing are there?

There's no photovoltaics. Don't worry about us, we're having an arcane little debate.

The area I need is actually the surface area of the collector. I could get it from the website... if I spoke the language

I fail at Dutch. Sorry.

Hopefully the area information is right on the site (it should tell you size, dimensions, the like)

 

[balrog666]

It's very, very simple technology - you tilt it up to face the sun most of the time (easily determined by your latitude) and it will pay for itself in just a few years. After that it's just gravy - easily maintained, augmented, or replaced entire.

 

[neutrino_cannon]

No. This program contains a database of the solar radiation averages recorded for many locations from around the globe.

I was working with the net radiant energy, in kWh, output by the sun for each month, and the yearly average kWh/m^2/day output.

If you don't personalize the solar output to location, you will get bad results - latitude is NOT sufficient. I then assumed 90% efficiency on the solar collector in collecting that energy, which I assure you is a very acceptable assumption for a back of the envelope like this.

Between the reflectivity and absorbency of realistic materials for a solar energy collector those numbers sound about as close as matters. I saw PV and, you know, assumed that something inefficient was going on, my bad.

What's the difference between the "good" and "bad" boilers? That's a fairly enormous discrepancy in efficiency you quote.

 

[GreyICE]

Between the reflectivity and absorbency of realistic materials for a solar energy collector those numbers sound about as close as matters. I saw PV and, you know, assumed that something inefficient was going on, my bad.

What's the difference between the "good" and "bad" boilers? That's a fairly enormous discrepancy in efficiency you quote.

There's a rather pernicious species of furnaces and boilers that were very common 20-40 years ago. Usually stove pipe coming out the top. They're cheap, they're easy to make, and they were rather ubiquitous for a long time. All the heat goes right up the chimney unfortunately. Mostly you can't really buy anything that bad anymore, but since people leave these horrible things in until they break... gah. The old boilers are by far and away the worst offenders - they started with an out of the box efficiency of like 55% and age doesn't improve that.

A good modern boiler is in the 80-90% range.

 

[Eddie Dane]

In closing.

I've found two neighbours who use solar boilers and are happy with the result.
looks like we'll go for it.

Thanks everybody for the input.