Help please

[johnkilfoil]

Hi Everyone,
My first post and I am going to jump in the deep end...
I live in Cape Town, South Africa and am planning to install Solar Hot water to reduce my electricity bill.
I have spoken to a number of companies locally and it seems the general consensus is that I will need 2 banks of 18 tubes each to adequately run my 300 liters.
I have some issues, though, that stand out from what I have been told and what I noticed from browsing this forum and I was wondering if the more knowledgeable among the members would be so kind as to set me straight with the issues.....

1. My cylinders are 2 x 150 liter horizontals that are able to run both electric and solar and will be about 6 feet apart. They have a removable manifold that allows the cylinder to connect to the solar array separately from the main feed but the water in the cylinder is not heated by a closed loop. In other words, the water in the cylinder is circulated through the panels.
Do I need to connect them in parallel with one bank of tubes per cylinder or can I run the cylinders in series through both banks of tubes to achieve maximum heating??
2. I am told that the solar powered 12volt pumps are not reliable and should go for a 220 volt pump.
True or False?
3. I am told that all I need to have is a relief valve to compensate for boiling. In fact, when I mentioned a cooling loop I was met with a blank look and asked what it was....

I am currently running one 150 liter cylinder and it is not sufficient at bath times, so the second will follow to coincide with the solar fit.
The whole solar thing is still in it's infancy here, so in short, opinions are like arseholes---everyone has one, and they are mostly different to each other and often rubbish the contrary views from possible competition, hence my need for an objective opinion from some professionals....

I am not posting the makes of equipment, but will in answer to a specific query to avoid the wrath of the mods..

Thanks in advance for your time, I really need all the help I can get.
John

[dhaslam]

Do you have frost in your area.  If so you need  to use antifreeze which rules out  direct heating of water.   In any case  it isn't  a good idea to heat the water  directly and especially if there is a heat dump radiator in the circuit.      It is better to use a heat exchanger, the usual one to use is a plate heat exchanger.      If possible  arrange  that one cylinder is heated first  and used first  but 300 litres will still heat fairly quickly  if connected in parallell.    In Northern Europe it is recommended to use one of the smaller 47mm tubes per five litres.  Since the sun in your area is close to being overhead in summer  it is a much different situation, approximately double  heat gain,  it sounds about right to use  one tube for  8.33 litres. 

http://re.jrc.ec.europa.eu/pvgis/countries/afr/4-gs13.png

[langstroth3]

Do you have frost in your area.  If so you need  to use antifreeze which rules out  direct heating of water.

Not necessarily. You can use a direct system, and rely on a control mechanism to pump water if temp < 4C to prevent freezing. Not as reliable I admit due to risk of power failure, but if costs are limited...

In any case  it isn't  a good idea to heat the water  directly and especially if there is a heat dump radiator in the circuit.

If there's a radiator as a heat dump I agree, but if not then direct heating does work providing the water is not 'hard' which would cause rapid furring of narrow copper pipes. Thought has to be given to pipe runs to ensure that any air that comes out of solution (and it will) can be easily pushed around and to the main collector tank where it can vent.

> 3. I am told that all I need to have is a relief valve to compensate for boiling
With a direct system boiling is handled by virtue of the steam / hot water being pushed into the main collector tank.

[EccentricAnomaly]

I have spoken to a number of companies locally and it seems the general consensus is that I will need 2 banks of 18 tubes each to adequately run my 300 liters.

Sounds about right. They'll probably want to be quite big tubes for 300 litres if you want to run the system through your winter. The usual rule of thumb is 1 47 mm tube for each 5 litres or 1 58 mm tube for each 7 litres. You'll probably get a bit more sun than most northern Europeans even in the winter - may be as cloudy but the days are longer at 34° from the equator (only day I spent in Cape Town was very cloudy (August 1968 or thereabouts) - couldn't see anything from the top of Table Mountain).

1. My cylinders are 2 x 150 liter horizontals that are able to run both electric and solar and will be about 6 feet apart. They have a removable manifold that allows the cylinder to connect to the solar array separately from the main feed but the water in the cylinder is not heated by a closed loop. In other words, the water in the cylinder is circulated through the panels.

Should be OK but, as dhaslam mentions, freeze protection is a concern. With very rare freezing and only getting to around 0°C anyway it's not much of a concern, though. I wonder if manually cycling a few litres of water though the panels before you go to bed on any night where freezing is a possibility might be sufficient.

Do I need to connect them in parallel with one bank of tubes per cylinder or can I run the cylinders in series through both banks of tubes to achieve maximum heating??

My thought would be to connect the cylinders in series for flow to the house then connect the panels in series to flow in the opposite direction (counter-flow) so that the cylinder on the output of the panels (i.e., the warmer one) will be closer to the house, plumbing wise.

2. I am told that the solar powered 12volt pumps are not reliable and should go for a 220 volt pump.
True or False?

I don't think there's any intrinsic reason for this to be true. However, maybe the ones available in your market are a problem.

3. I am told that all I need to have is a relief valve to compensate for boiling. In fact, when I mentioned a cooling loop I was met with a blank look and asked what it was....

That's a bit surprising considering the difference in weather between summer and winter. If you want a system to operate reasonably well in winter with high usage then surely it'll get a bit hot in summer low usage conditions.

Are the cylinders at mains pressure, then? Otherwise, wouldn't any expansion just flow back into the cold water tank?

The whole solar thing is still in it's infancy here, ...

Pity - much better place for it than northern Europe. Also, doesn't SA still produce a lot of electricity from coal?

[johnkilfoil]

Thanks for the responses, guys!!
OK, in no particular order....
1. We don't have frost where my house is. Minimum recorded ever between 1961-1990 was -1 and average minimum is +7,this winter we went down to 5.
Summer is the main concern for me, where days can get to 40.. but 30's are common..
2. The tubes are 58mm.
3. Water quality is good, kettles don't fur up and when my last cylinder failed, the element was not furred beyond a thin coating after more than 8 years installed.
4. The system is going to run at 400kpa, reduced with a reducing valve from mains which is around 700kpa average.
5. The roof where it's going to go is North facing and has sun on it from around 10am to sunset, at a 45 degree pitch.
6. My cylinders are from the local manufacturer and are designed to be full electric or solar/electric with the addition of a "flange assembly" that has the necessary fittings to allow the solar panels to be plumbed in. Here is a link to their .pdf and the flange is on pg 3.
http://www.climatetemp.info/south-africa/cape-town.html
Mods, my apologies if it infringes the rules, please delete.
7. I agree on the pumps, probably ours are suspect. I will run a 220v for now.
8. Connecting the cylinders to feed the house in one direction and the solar panels to feed the other way will not be a problem with the way the cylinders are designed.
9. Is there a control system with a valve that switches to bypass at a certain preset temperature available to route to a heat dump? I will probably need to import that due to the absence of any idea what a heat dump is here.....
10. Can the system in 9 also be programmed to circulate if temps go below a preset minimum for freezing (if it ever happens)?
11. If I use a heat exchanger I assume I will require 2 pumps? One to circulate through the panels and the other to circulate through the cylinders. Yes/No?
The house is a double story with the upper floor under the full roof (like an attic with a reduced footprint), and the cylinders will be on that level. Height difference will be +- 5m to the top of the panels and 4m laterally.
12. If I use a heat exchanger, would it benefit me to use a wetting agent/antifreeze in the closed system or will water be OK?
13. Plumbing fittings are 3/4" (20mm) as standard, is that OK or should I go to 1/2" (15mm)?
14. ALL hot lines will be lagged to prevent loss, of course. I will investigate the specifications for max melting temps.
15. We do produce electricity from coal, but also have one nuclear station and another 2 on the cards. Problem is, the thieving barstewards in the national grid have allowed the network to decay to the point where it now needs a huge injection of money and have secured permission from government to hit us with huge tarriff hikes over a few years to pay for their incompetence.... http://www.engineeringnews.co.za/article/nersa-approves-248-eskom-tariff-hike-for-201011-2010-02-24
Not to mention that you can buy SA generated electricity as far as Botswana for less than I pay at home a few miles from the nuclear station....
The next mission will be addressing my electric usage and reducing that exposure, but one thing at a time......

Thanks for the replies, and I hope I have covered the points raised so far. I am off to get more info on the available control systems to see what they can do.

[guydewdney]

15. We do produce electricity from coal, but also have one nuclear station and another 2 on the cards. Problem is, the thieving barstewards in the national grid have allowed the network to decay to the point where it now needs a huge injection of money and have secured permission from government to hit us with huge tarriff hikes over a few years to pay for their incompetence.... http://www.engineeringnews.co.za/article/nersa-approves-248-eskom-tariff-hike-for-201011-2010-02-24
Not to mention that you can buy SA generated electricity as far as Botswana for less than I pay at home a few miles from the nuclear station....
The next mission will be addressing my electric usage and reducing that exposure, but one thing at a time......

from the link:-
Nevertheless, the average electricity price would rise from around 33c/kWh to 41,5c/kWh this year, and then to 52c/kWh in 2011/12 and to over 65c/kwh in 2012/13. This, however, was, in Eskom's view, below a cost-reflective level, which the utility reported to be northwards of 70c/kWh.


33c/ unit? thats 2.5p per unit - we are in the realms of 15p / unit or R1.91 per unit. But I suspect your wages are a bit lower...

[dhaslam]

Pipes should be as  small as possible unless the pipe run is very short.   I have 10mm pipe  for 60 47mm tubes.  The reason for the small pipes is to minimize  the amount of cold water in the  pipe when the pump starts, this is important in dull weather to  pick up the maximum heat.  Controllers normally have  twp ways to prevent overheating.  They can stop circulation that  makes the tubes stagnate but obviously that isn't  such a good idea.  The other control is a relay output that operates a pump or valve.    The usual  way is to open a valve to a radiator placed somewhere  that  you don't mind heating in hot weather, in your location that is probably outside the house.  I haven't used a heat exchanger but  it probably does need  two pumps, the one on the cylinder circuit could use the lowest setting with  larger pipe than the other circuit. 

[johnkilfoil]

33c/ unit? thats 2.5p per unit - we are in the realms of 15p / unit or R1.91 per unit. But I suspect your wages are a bit lower...

Not quite the full picture given there. That is the price prior to retail.
We are charged on a system called "Incline block tarriffs" where you are penalised for your usage....
Block 1 is 50kWh or less is 65.72c/kWh
Block 2 51-350 kWh is 75.42c/kWh
Block 3 351-600 kWh is 109.5c/kWh
Block 4 600 or more kWh is 120.1c/kWh
I currently am in Block 4 and thus my desire to reduce my bill somewhat..... Ironically, the producer is also my supplier, so there is actually no middle man as such AND I am on a pre-paid meter which means noone has to be hired to come and read the meter, but I am paying more per unit than the Cape Town Monkeypality users who buy from a middle man.......
As for wages, I am a Licensed Aircraft Maintenance Engineer with 21 years in the trade and if I worked at home, I would be lucky to take home a tad over R20 - 25k a month after tax. Currently the rate is ZAR12.55/1GBP.....

[johnkilfoil]

I managed to find a plumber who has done a solar installation course and he is au-fait with heat dumps, controllers and all the associated gubbins, so it looks like my project is a step closer to fruition early in the new year.
All I need now is to do my sums and give the piggy bank a good thump on the head.
I will be back to solicit advice as and when I have questions, though.