I'm about to install an Apricus 30 tube 315L electric boosted SHWS. I'm just trying to decide how to boost it. I don't want to connect to off peak because if the kids have a bath in the evenings the water will be cool and the off peak will heat it before the sun gets a chance in the morning. The installer recommends a timed booster set for about 5pm for 1 hr to come on if the water is below 65 degrees. But still it will boost even if I have no intention of using it before morning. I'd thought of a manual timed boost that I can push a button when necessary then have it turn itself off, but I don't know how long I'd have to wait for the hot water. Can anyone help?
Solar hot water boosting
(35 posts) (15 voices)-
rsigmund
MemberI think some of these issues where covered in an article in the most recent renew mag....
Heres my attempt at a calculation using waters specific heat and knowing that a Watt is a Joule per second....
4.1813kJ kg?1 K?1
4.1813kJ * 315 * C-1 = 0
1C = 1317.1095kJ
1Watt = 1 Joule/Second
2000W = 2kJ/second
1C = 1317.1095kJ * second / 2kJ
1C = 658.56 Seconds
So 1 Degree C increase would take ~ 11 Minutes with a 2000W element.
Posted Wednesday 11 Feb 2009 @ 6:03:50 am from IP # -
Sunshine
MemberBurtons40,
I have a Hills 30 tube 315l electric boosted system, which is basically the same system as yours will be. Just a different brand collector, exactly the same storage unit. It has been in for around 4 months or so, and the electric boost switch is permanently off in the switchboard. We have had alot of overcast days, in that time, and even with baths, dishwasher etc, we haven't looked like running out.
I am amazed at how efficient this system is, I have observed it pumping hot water at 9:45 in the morning, when it is installed on a western facing roof, on a gloomy rainy morning!
I was worried about the electric boost cutting in as well, by using hot water at night, the system will try to use electric boost to recover the lost heat. Easy solution is to leave electric boost off, and only manually switch it on, when you require it. However I believe this will be a rare event, going from my experience with the system.Posted Wednesday 11 Feb 2009 @ 6:50:23 am from IP # -
burtons40
MemberThanks rsigmund
That seems like it could take quite a while to heat. I thought I'd seen an article on this and couldn't find it. I'll try to find the latest edition, but I think someones borrowed it.
Thanks sunshine
It's good to hear from someone who actually has one installed. I didn't think it would need boosting too often and that's why I'm leaning towards a manual boost. I'm off to have a look at the boosting switches today.
Is your system at your latitude, greater or less? We are at 36.6 degrees south but our roof pitch is only 26. I know in theory that the collectors should be at latitude + 10 degrees for extra winter solar gain. However, that is all worked out with traditional flat plate collectors and not tubes.
Posted Monday 16 Feb 2009 @ 10:09:08 pm from IP # -
Sunshine
Memberburtons40,
You may be able to ask the installer to add the booster switch in a convenient location in the house somewhere, but I just use the switch in the meter box, as the electric boost has it's own switch in the meter box. The meter box is conveniently located, so it's not a problem for me, but a friend of mine has a booster switch located in the house (he has a flat plate SHWS).
I am located in Brisbane, and mine is facing west on a relatively steep roof, so not very optimaly located, but as I have 3 kW of solar PV, they had first choice of the premium roof space that I had available.
From what I read on the internet the evacuated tube SHWS track the sun very well due to there inherant design, so eventhough facing west on a steep roof, seems to work fine still. Neverless, I ahve observed it pumping hot water off the roof before 9am in the morning, so it still works well.Posted Tuesday 17 Feb 2009 @ 7:02:59 am from IP # -
johnny
MemberWe have a conventioal Solarhart Black Chrome system facing true North witha 315 Lt storage and NEVER need to "off-peak" boost from End Oct until Mid May (Manual switch) and even during the winter only have to use the "off-peak" infrquently We hade 9 people stay for a week-end last ski season and had to go to "day-rate" for 2 hours. Our system has been in use for 3 years and before that in previous dwelling 15 years (that's why we went Solarhart again)
Posted Monday 23 Feb 2009 @ 10:07:55 am from IP # -
vk6tbv
MemberBoosting the storage tank seems very very inefficient. Firstly if the water is to cold to use, the whole tank needs to be heated and then heat is lost though the insulation.
A better idea if you are using electric assist is to use a instantaneous electric ,gas might be better but more expensive to install. That is only turning it on when needed. Or you could use a bit of electronics to turn on the instantaneous when the temperature is below a set value.
A three phase electric instantaneous unit running on two phases would give just the right amount of heating.
This way only the water being used is heated.
Posted Tuesday 24 Feb 2009 @ 7:53:12 am from IP # -
Solgen
MemberI have had a solar tube system for about 3 months now and am very impressed with the performance. I have only had to turn on the booster element once after about 7 days of rain and heavy cloud. However, I have been giving this some thought and am concerned that unless the user is an active user/enthusiast then they may not get the best benefit from a SHWS. If the system was capable of continuous boosting from either electricity or gas then depending on the time of usage of hot water the boost system may well be providing most of the heating of the water.
My thoughts on this were crystallised when I was recently at Geelong where my sister had recently installed a gas boosted SHWS installed. The system was amazing - a remote tank solar system plumbed into a conventional 130l gas storage system, very complicated plumbing. Effectively 2 hot water systems in series - the whole system was provided by their local gas supplier for around $2000, I presume after rebates,etc. I don't believe the company could have made much profit from the installation but obtain an income from the gas consumption which would be constantly topping up the solar hot water contribution.
Consequently, I have come to the same conclusion as vk6tbv that an instantaneous system at the outlet would be the best option, particularly the gas Rinnai and similar systems where you can set the water temp. It can be done with electricity but you need a lot of power, some of us remember the old 3 phase instantaneous systems of the 1940/50 which required 20A a phase.Posted Tuesday 24 Feb 2009 @ 10:48:08 am from IP # -
Solgen
MemberThe formula for finding out how much energy is required to heat water is:
Energy=specific heat of water *temp diff*litres
=Joules
to convert to kWh divide the answer by 360000.
Specific heat of water is 4186A useful approx is 1 kWh will raise 16 litres of water 50 degC
Posted Tuesday 24 Feb 2009 @ 11:05:08 am from IP # -
Sunshine
MemberIt will be cheaper if you live in the southern states or have a larger family to install a 40 tube collector with the 315 litre tank, that will provide a 25% increase in heating ability, and would further reduce the need for electric/gas boost.
The installer told me it costs around $1 to heat the storage tank, if it gets cool after heavy usage.Posted Tuesday 24 Feb 2009 @ 12:20:47 pm from IP # -
rsigmund
MemberThanks Solgen, I just re-ran my calculation to confirm if I (may) have got it right
2000W 30min 16L
4.1813kJ * 16 = 1C
1C = 66.9008Kj
2000W = 2kJ/second
1C = 66.9008Kj * second / 2kJ
1C = 33.4504 seconds
In 30 Min
30Min * 60 Sec/Min = 1800Seconnds
53.81C = 1800 / 33.4504
Cheers...
Posted Wednesday 25 Feb 2009 @ 12:52:44 am from IP # -
gully
Membervk6tbv: instant electric hot water... not sure it exists. It takes like 1 minute for a hot water kettle to boil 4 litres of water - be hard to do this and keep the water pressurised. If anything it would draw a monster current - very eco-unfriendly to the grid's peak load capacity. Just get the smallest tank you need, lots of hot water tubes/panels and boost it in the early evening.
Posted Wednesday 17 Mar 2010 @ 1:28:26 am from IP # -
tintac
MemberI'm also looking into the installation of one of these and I was originally going to get a gas boosted one. I don't want to run out. But I'm also going to install a solar PV system.
The question I'm now thinking through is although gas is a more environmentally sustainable fuel than coal based electricity it seems to me that if I'm getting a big enough PV system (say 2 to 3 kW) then I can use this to offset the power used by the water boosting.
What does everyone think? Am I being too simplistic?
Posted Wednesday 17 Mar 2010 @ 8:59:55 am from IP # -
gully
Membertintac: No your idea is fine in principle. I spoke to a family recently who wanted a 3kw PV system installed at their house. Their thinking is, if we've got the panels, why not get electrical everything (floor heating, hot water, oven) etc.
It's a personal choice but i'd be arguing that gas is more environmentally friendly
than coal (power plants). Even if you have green power you are just paying for RECS, which is a falicy. Plus if you're on a Net Tariff you won't get money back from the PV array unless the power is pumped back into the grid.Obviously if you don't have gas your options are limited
Posted Wednesday 17 Mar 2010 @ 10:39:54 pm from IP # -
Tacker
MemberGot to go the gas option where available, the CO2 emissions per unit of energy for gas is far less than for brown coal at 1.3tonnes of CO2 per MWhr.
Should be trying to reduce electricity use as much as possible and pumping as much as you can back into the grid. This will provide the greatest environmental benefit, which is the sole reason for a household to install PV panels (I guess they do pay back eventualy).
Posted Wednesday 17 Mar 2010 @ 11:01:49 pm from IP # -
dymonite69
MemberDon't forget that a 3-4 person Solar HWS produces as much energy as a 1.5kW solar PV array (but is one third the cost to install). If adequate hot water is all you want for winter it would be far more cost-effective to increase the size of the HWS array then using PV electricity to boost it.
Of course you will have excess thermal energy (hot water) during summer that has only limited applications. Load balancing electricity is easier than load balancing heat.
Posted Thursday 18 Mar 2010 @ 12:32:16 am from IP # -
dymonite69
MemberCoal source electricity is a big emitter because of the huge inefficiency of coal plants. You need to burn 3 units of coal energy to produce 1 unit of electrical energy. Electricity generated from other sources tends to be more efficient.
We are also going to run of natural gas as well but not for a little while yet.
Posted Thursday 18 Mar 2010 @ 12:36:22 am from IP # -
Terry Leach
MemberThis might be worthy of a new thread, but it is on topic. I'm near Adelaide and have a large gravity HWS with solar collector, wet back on my combustion stove and off peak boost. There are only two of us and we're careful with hot water usage (totally reliant on rain water for the house). We use only solar for 6 months. In summer the water is scaldingly hot such that you need only a tiny bit of hot water in a shower, while in winter the wet back struggles unless I keep the fire roaring.
I've just been in the roof and worked out why. The supply and return for the solar collectors are at the top of the tank, same level as the hot water outlet for the house. Accordingly, the solar heats the hottest water at the top, and then relies on convection and conductance to heat the rest of the water. My understanding is that it would do this inefficiently, as you get stratification of temperature levels in tanks. The thermosiphoning effect sets up only horizontal convective currents in the tank. However, when the solar heat gain is marginal, then theoretically I should get some hot water. In practice if it is cool and cloudy I get very little hot water.
However, with the wet back, the supply is from the very bottom of the tank, and the return is half way up. So in winter I'm heating a large column of water, and the thermosiphoning would set up vertical convection currents, such that I'm having to heat at least half of the tank. No wonder my wet back struggles.
What I'm thinking of doing is swapping the supply to the solar collectors and the return of the wet back. In this way I would not get the water so scaldingly hot in summer, a taller column of hot water via the solar, and the wet back would deliver the hot water to the top of the tank for immediate use, rather than mixing from halfway up.
Does this sound like a reasonable solution? Would I likely get any reverse thermosiphoning of hot water through the solar, or are there non-return valves to prevent this?
Might it be better to put a T on the solar return and return the wet back at the same point? Or should I just leave well enough alone and trust the plumber to have connected it all up optimally?
Advice from people with experience or expertise would be much appreciated.
Posted Thursday 18 Mar 2010 @ 5:06:17 am from IP # -
questionman
MemberI just had a 30 tube, 315 liter, electric-boosted Apricus system installed on a west-facing roof. I noticed that the water is not hot or warm enough during night time, no matter how hot it is during the day. Tonight, the water in the tank is only 28 degrees centigrade. What can or should I do to have hot water at night? As much as possible I dont want to use the electric booster. Can a rubber insulation be wrapped around to cover the tank so that whatever hot water is collected and heated during the daytime, will stay hot throughout the night?
Otherwise , I will probably buy an instantaneous gas heater at the point of use , like in the kitchen and bathroom or maybe turn my electric-boosted hot water system into gas-boosted much later. Anybody have any useful advice, solutions or suggestions about this?
Posted Friday 19 Mar 2010 @ 3:12:41 pm from IP # -
questionman
MemberI am in Perth, and 18 degrees is the angle the unit is mounted at ( I presume that this is also the pitch of my roof). My roof, and therefore, the unit is facing west; I dont have a north-facing roof. If you draw a line running along the west, the tubes are placed perpendicular to that line.
I noticed that without using the electric booster, the 'hot water' temperature only ranges between 40 and the 50's; very rare does it reach 60.
Any useful advice/suggestions/input will be appreciated.
Posted Monday 22 Mar 2010 @ 2:34:33 am from IP # -
questionman
MemberI have 30 tubes, 315 liter tank ; 2-4 people in the household.
March 20 - at 9 am, tank temperature was 38; collector temperature was 44
at 11 am, tank was 41; collector was 49
at 7:30 pm, tank was 49; collector was 39Today, March 22 - at 10:15 am , tank was 44; collector was 48
now at 1:56 pm, tank is 57; collector is 62.
I will measure temperatures again at around 5-6 pm.What is the normal temperature of unheated tap water?
Will there be a danger of legionnaires organism growing in the tank if the temperature are always in the 50's only?Posted Monday 22 Mar 2010 @ 6:01:36 am from IP # -
dymonite69
MemberYour figures sound about right.
Intake water temp is approximately ground temperature ~ 15 degrees but can vary a lot.
Aperture area of 30 tubes is about 2.9 m2
Insolation is about 21.4 MJ/m2 (or 5.94 kWh/m2) - March Perth - see BOM
It takes 16.4 kWhr to bring 315 L from 15 to 60 degrees (1.16 Wh/L/degree = 4186 J/L/K)
Assume best efficiency of SHWS at 80%
Muliply this by 80% for non ideal orientation.
Gives you tank temp of 47 degrees for 315L tank.
Posted Monday 22 Mar 2010 @ 8:13:49 am from IP # -
questionman
MemberWhich one should I ask about the panel sensor's proper installation and the possible misreading, causing insufficient pump run time, - the plumber and/or the electrician hired by the solar hot water company, who directly installed this unit or the solar hot water company's solar consultant?
Posted Monday 22 Mar 2010 @ 8:15:11 am from IP #
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