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Thoughts on Solar HWS and Boosting

(9 posts) (6 voices)

Started 3 years ago by me3neuralfibrecom
Latest reply from Bobthe...

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me3neuralfibrecom
Member

I have done my own solar hot water
http://neuralfibre.com/paul/tree-hugging/diy-solar-hot-water
And have been playing with boosting a bit.

After I gave up on the idiocy of the controller I bought, and built my own, I moved the booster over to running on a timeclock.

Here's the theory.
The booster is contolled by the timeclock AND the thermostat (set to 55c).
The termostat senses the bottom of the tank, and the tank tends to remain stratified from use. Solar heating circulation is all at the bottom - so that de-stratifies it during solar days. The booster is at the bottom, so that de-stratifies is during boosting.
At 4pm - the booster kicks in. If the thermostat (bottom of tank) is not at 55c, the booster will run until either 5:15PM, or it hits 55c.

I chose these times as at about 5:30, we throw the kids in the bath, and this pulls cold water into the bottom of the tank, possibly tripping the thermostat again.

So this way, we ensure that at the end of each day, we have a full tank of hot water, enough for our next 24hrs.

BUT - I have been thinking.

Say I set the thermostat to 85C and ran the booster 24/7 without the timeclock.
Solar would take that up further. Now assuming that it doens't open the releif valve and waste water / heat (it didn't when I tried this last week) - I have a tank of very hot water.
I would only need to use 1/2 the hot water i was before, and mix it down with more cold in the shower (my safety valve would do that automatically).
SO I would be using the same number of JOULES as before, it's jsut the tank has a higher standing temperature.

SO therefore.
My use of X Joules of heat / day is constant.
The input of Y solar Joules / day is out of my control
I have to add electricity to ger from Y to X - regardless of tank temperature or the time it is added.

SO really, timing the boosting time, makes no difference the number of Joules needed / day.

BUT
THe higher the tank standing temperature, the greater the parasitic heat loss.
AND
Every time you use hot water you pull about 3-5L into the pipes, that stays there when you turn off the tap, and then cools and the heat is lost. The higher standing temp will increase that loss (4L*50C vs 4L*90C)

So - after that huge explanation
Would this parasitic and pipe loss matter? Are we talking 1% of total HWS energy, or 50%.

It's worth thinking on.
I'm googling for info atm - but anyone with thoughts - I'm interested to hear them.

Thanx
Paul

Posted Monday 1 Sep 2008 @ 12:36:58 am from IP #
sun2steam
Member

Paul,
You can run just a simple test. (But you cannot use any hot water from morning until sun set!)

This is for a day when the weather forecast promises a cloudy day.
Bring you system to about 55C +-5C the day before.

Then switch the booster off.
In the morning (no sun!) destroy stratification in the tank by switching on the solar circulation pump for some time. Read the temperature of the tank. (T1)
Read the time (t1)

Keep the solar system switched off.

Read the air temperature every hour.

In the evening (no sun!) destroy stratification in the tank by switching on the solar circulation pump for some time. Read the temperature of the tank. (T2)
Read the time (t2)

This is the end of the experiment. you can enjoy your hot water again.

Calculate the average of all air temperature readings. (T air)
Read the volume of the tank. (V)

The equation to calculate the change in thermal energy (Q) is:
Q = m * cp * (T1-T2)

m is the mass. If you use V in litres and call them kg your are close enough.
cp is the heat capacity of water (4.2 kJ/kg/K)

let me give some data for an example:
T1 = 55C
T2 = 50C
T air =20C
t2-t1 = 10 h
V =250L

Q= 250 * 4.2 * 5
= 5250 kJ

The difference of the average temperature of the tank (52.5C) and T air is 52.5-20 =32.5C =32.5K

So for just a 1K difference between the average tank temperature and and 1 hour time the heat loss would be: 5250/(32.5 *10) =16 kJ/(K h)

You can now use this number to calculate any heat loss for any temperature difference between tank and air and any time.

So for instance for 24 h and an average difference between air and tank of 40K :

Heat loss = 16 * 40 * 24
= 15360 kJ

To make it a bit more practical you can convert the energy units from kJ to kW h using the conversion factor of 1 kW h = 3600 kJ or 1 kJ= 0.000278 kWh:

Heat loss = 15360 * 0.000278 = 4.25 kW h

The result in the example is only indicative and should only tell you how to run the calculation. But it shows you that heat losses can be substantial and may cost you money. And of course with all other things being equal: higher tank temperatures will lead to higher heat losses.

The result will also depend on the precision of the temperature measurements and on the length of time you run the experiment. The resulting data are not lab quality but they should give you an indication.

Posted Sunday 28 Jun 2009 @ 2:16:17 pm from IP #
petanque
Member

Isn

Posted Monday 29 Jun 2009 @ 12:32:24 am from IP #
me3neuralfibrecom
Member

FYI:
With a 30 tube system and a 250L tank in Brisbane, it will not sustain household use in winter unboosted, struggling to stay over 45C without boosting, and falling to lukewarm after 1-2 overcast days.

In summer, with boosting set to 50C, it never boiled over. Peak temp seen was 95C when I set the booster to 70C as a test.

All vaccuum tubes are cool to touch except one (it must have lost vacuum). Pump cycling is regualar, and close to 100% in summer, and 30% in winter (at a guess from watching it).

Boosting is needed for me.
We use 3000L/week with 2 ppl and 3 kids uner 5yo.
My house runs off my 10,000L rainwater tank in BNE 80% of the time. No rain for 3wks+ puts me back on mains.
Whilst I am don't waste water (efficient fittigns etc) - we have long hot showers and baths and utilise the bathroom greywater. I would guess we use 150L of hot water / day.

Paul

Posted Monday 29 Jun 2009 @ 2:39:53 am from IP #
munter
Member

Interesting. People's different consumption patterns seem to lead to different levels of satisfaction with solar hot water. Our total household water consumption is 170 L/day (for three people) so our hot water use much be much lower (perhaps 50-70L/day). I don't bring this up to preach, just to note that there are big differences in consumption and that what works for some will not work for all.

Paul - is you collector panel north facing and at a suitable angle?

Posted Monday 6 Jul 2009 @ 3:07:31 am from IP #
petanque
Member

Is it set up correctly?

Unless you use a lot of water you should be fine in the hotter months.

Posted Monday 6 Jul 2009 @ 5:36:06 am from IP #
me3neuralfibrecom
Member

Collector faces 5deg off north to the west, and is at the roof angle of 30deg. I am in Brisbane, so that's pretty close to perfect.

My water is free, so why restrict myself? (5 star washing machine etc, so not wastful)

Hotter months are fine, unless you get 3 days of rain.
250L tank isn't a huge reserve.

Brisbane gets a lot of sunny days with scattered cloud. This makes a large impact to the insolation levels. You can see it on the PV inverter the difference a bit of lunchtime cloud shadow makes.

Paul

Paul

Posted Tuesday 7 Jul 2009 @ 2:24:05 am from IP #
Sunshine
Member

I agree with Munter, the amount of hot water consumption obviously affects the amount of boosting a solar hot water system requires.

We use around 170-200l/day for a family of 3, so hot water use would be around what Munter says (50-70l per day), so a 315l tank I have found has enough capacity to see us through 3-4 days of solid rain.
Keep in mind, the solar storage tank potentially stores water at a higher temperature than gas/electric storage units, as they are thermostatically controlled around 60 degrees, whilst solar storage tanks go up to 95 degrees, before they cut out.
So potentially a 315l solar storage tank will have greater yield than a 315l gas/electric system.

Posted Saturday 11 Jul 2009 @ 9:46:53 am from IP #
Bobthe...
Member

I'm a newbie here, but have been dabbling in low budget water/energy efficiency for... well I guess its 30 years now.
I'm located (have been since 1981)between Tamworth and Gunnedah NSW in a rural area, close to a river in a 128 yo brick ex schoolroom/teachers house.
Thermally a very inefficient building, no insulation (there is some now), high thermal mass connected to the outside, gaps etc. We love it! We notice and enjoy daily/seasonal change.
There is a fireplace in almost every room, and in winter we use a lot of wood (which we grow on our 150 acres, so its a carbon neutral cycle, and I guess, ultimately its solar heating) We also have a big black cast iron wood burning stove in the kitchen, which is in use through most of April to October.
We use roof collected rainwater for everything in the house except the toilet flush.
For that and the garden we use river water pumped (electric) up the hill to a 30,000L tank, which gives good gravity pressure in the garden taps.

About 18 years ago I installed a Solarhart black chrome HWS with glycol in the panels. I did 2 things when I installed it.... no booster at all, and put the ugly tank inside the ridge of the roof with the panels down the slope a bit so the thermosyphon works normally.
Its been wonderful. We very rarely get 3 consecutive cloudy days, so for a few years the no boost was tolerable. After a few too many complaints from the family (4 kids wife & self) I finally put a heat exchange manifold in the back (not in the fire) of the kitchen stove connected to a separate 60L HWS in the roof above the stove. I put a change over tap in the system so we can select "sun" or "fire" HWS, for kitchen and bathroom. The laundry stays on solar.
It works very well..... and has needed almost no maintenance for 15 years.

Now....... my elderly mum (in the hunter valley, frost and a bit more cloud) had a 10 panel PV installed last year, and now wants to install a solar HWS. She asked my advice....
I feel a bit behind the times, 'cos now its all tubes and pumps....
I don't mind the tubes, but the complication of a pump and control system goes against my kiss principles.........
She has a 45 degree roof (150 yo) with good space for me to put a horizontal tank in the roof ridge, with the panels down the slope a bit, just like mine. But she will want boost........ and frost tolerance.

Q. Where you have electric boost in the solar HWS (that's just about everyone but me), In order to get the most efficiency from boost energy shouldn't the boost thermostat and element be located well up in the tank, say only 60L down from the top, so that you only boost that days needs ?
That way the solar will input significantly as soon as the sun shines.

Q. Do tube systems lend themselves to thermosyphon, and what about frost tolerance ?

Bob >>

Posted Friday 6 Nov 2009 @ 10:41:05 pm from IP #

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