wanted to find out how many kwh I'm producing

[tik33]

hi there, I'm running 40 tubes @ 47mm diameter with a TDC (I think 3?) controller & a 250L tank. 

My question is - how do I find out how many kwh I'm using? I noticed that in the current manual of TDC2 & 3 (not at home so can't say which I have for certain) - it has current kwh on the front of the screen - but mine doesn't.

I've looked through the functions but can't find anything - am I being really dumb?

I've currently got the boiler on for the morning and the solar seems to warm up enough water for the evening shower & baths and I suppose water during the day for washing up etc.

I've never seen the panel temp go above 60 odd degrees - does that seem strange on 40 panels with the summer that we've had?

that's really the reason I wanted to work out kwh - because it is probably a better indicator of how much energy the panels are producing than reading the temperature of the water.

thanks, Gus

[KLD]

Gus,

The panel temperature tracks the tank temperature. If you normally extract most of the heat from the tank, i.e. it is relatively cool in the morning, say below 40°C, then you'd hardly ever see it go beyond 60°C in the evening. On most days our 40x 47mm ET system produces 3 to 4 kWh, and peaks at 8kWh. The heat capacity of water is 4200J/°C kg, or 0.00117 kWh/°C L. Thus, a 20°C temperature increase in a 250L tank is therefore achieved by an energy input of 5.8kWh.

The TDC3 controller can only produce an estimate of the solar heat harvested. For the calculation it needs to know the temperature of the solar loop where it enters the tank, and the temperature in the return pipe to the panel. Together with the flow rate (and the heat capacity of the solar fluid) you can then calculate the heat flow, in kilowatt. By multiplying with the time span that the pump was running, you  get the energy in kWh. None of the necessary values are actually measured. The TDC only knows the panel temp (this is taken for the flow temperature) and the store temp (assumed to equal return temp). The flow rate is your own guess :-) The specific heat of the fluid depends on the concentration and type of antifreeze. In short: all highly scientific, but the values you get can be out by a large factor. 

If you have more than one sensor in the store, you can record the average temperature increase of the whole volume. As long as you don't draw any HW off during that day, you can calculate the heat input much more reliably. Let me know if you help with setting up such a measurement.

Klaus

[Baz]

Really with solar you should try to avoid using the boilier until the evening so that the sun has the opportunity to do as much as possible first.
The navitron 20 tube panel is 2.5m2 and suggested tank is 150L, so you have 5m2 and a better tube to tank ratio. In southern UK according to calculators in this summer you should be getting >3kWh average per m2 so that's 15KWh on some days which is about a 60C rise.
If you are using 250L of hot water a day you must have one heck of a water bill.
Something not right.

[tik33]

thank  you both for your replies.

Klaus - I have taken some readings during the day - but with a wife a lodger and two young children generally at home it is proving quite hard to get some accurate 'no draw down' readings.

I have an s2 & an S3 in the tank (one sensor at the bottom of the tank between the solar flow & return & the other half way up between boiler flow and return).  would be interested to know how to calculate the heat input - what kind of minimum 'no draw down' would I need? all day or just a few hours???

Baz, I would love not to turn on the boiler in the morning - but I don't seem to be getting enough heat out of the tank to allow me to do that - I can cope with cold showers, but not sure my wife is willing...

my gut tells me something isn't right - but since the system is new and put in by my builder's plumber, I don't know exactly how to proove it one way or the other - hence my original post.  If I can get an idea of how many kwh input I'm getting that seems to be an easier reality check than pure temperatures - and then I can work out if the solar isn't performing properly or whether the tank is not keeping water hot enough for long enough.  i.e. if I'm getting less than 10kwh on a sunny day then something probably isn't right.  However, my entire water usage is around 700L per day I think - so 250L on hot water? that's probably possible isn't it?  1 bath @ say 50L, 3-4 showers @ 50L - adds up quite quickly...

thanks, Gus

[desperate]

tik33

700l/day wow, even 250 hot/day??  has your cylinder got a bottom on it?? must be shome mishtake shurly?? I have a 220 l cylinder and three of us make it last 2 days no sweat, thats 5 showers, washing up and bits n bobs.

Desp

[KLD]

Baz,

Don't know where you get your figures from. My own measurements say, a 40x 47mm system has a max power around 1.1kW, which is in accord with the figures published by the SPF. See also the plot I posted earlier about how much energy is harvested during one day.

Gus, if you want to go down the route of measuring the heat input into the store, you really have to make sure there is no draw-off (calculating the amount extracted would most likely introduce too much extra error.) You want to collect data for longer then the typical time constant of your system (why did you ask a physicist?), so half a day is good, 10 min is not.  Before I installed my 1-wire monitoring system, I collected data by hand. I have four sensors, at bottom, 1/3, 2/3, and top of the tank.  If you have only 2, then ideally the upper one should be at the top. Now, on a good sunny day, send your family to go and play outside  , lock yourself in  , and take temperature readings every 20 to 60 minutes or so. See the attached spreadsheet for an example how to calculate the energy input (kWh) and the heating rate (kW).

Good luck!

Klaus


Edit: the dip in the graph around midday is due to clouds moving over, later in the afternoon the sky was clear again.

[tik33]

Baz, I may be wrong on the 700L per day - but I remember working out that we were approximately on target for 120L per day per person - however, I have proper readings at home - so if I can dig them out I will post.  That of course includes wc use, garden watering, washing machine, dishwasher etc.

Klaus, thanks for that.  I have been investigating the use of some data buttons - which can record temperature at whatever intervals I need - then I can have one at the top of the tank (i.e. on the hot water out) and at various other (accessible) places.  Problem of course is that while I have some control (and I say some!) over my wife - less over my tenant - but if they go away for the weekend...

going back to your max power of 1.1kw - does that mean that to raise 250L by 20 deg - which you calculated to be 5.8kwh - would need 5+ hours of max power (which sounds like my system)?   but 1.1kw seems quite low given that my boiler is 28kw?

best, Gus

[KLD]

Gus,

Very true, 1.1kW looks like nothing when compared to the 28kW combi boiler. That said, the combi has to have enough umph to heat the water while it's flowing through it. Say, your mains cold is at 15°C, HW required at 45°C, so an increase in 30°C. It's easy to calculate the power necessary to heat a given flow rate:
power = spec. heat * dT * flow rate.  A flow rate of 13L/min and dT = 30°C translate to 27.3kW. There you are, with a combi at it's limit. But you only need it while the water is flowing, say 5min while you're under the shower. Energy used is 27.3kW * (5/60)h = 2.28kWh.

The solar panel, though, puts heat into the store all day long. To power your 5min 13l/min shower, you'd have to have 2 hours of good sunshine on your panels.

1.1kW is plenty for us, but we are only 2 adults one child in the household. Your requirements are way higher then ours (according to the figures you posted earlier.) There are many ways how to reduce the HW usage, but that another story altogether. Alternatively, add one or two panels. Standing losses from the tank shouldn't be too significant, since you use the heat pretty quickly. So, your exiting tank volume should be fine, just heat it to higher temperatures.

Klaus

[tik33]

thanks Klaus,

this is where correct 'sizing' comes into it I suppose? I was thinking of adding another panel - but I don't currently have a heat dump so didn't want to oversize...

re: the maths - I'm assuming that the panels are 550w each - where does one find that info?

also, in your equation to calculate power - what is spec.heat in the equation? power = 13 * 30 * ? = 27.3kw my physics is rusty to say the least... but the maths is beginning to make sense - especially when I consider that 5 mins is pretty short for my wife... I'm going to get a shower timer and a bucket like they do in Australia.  Actually if I did get a bucket I could then measure flow rates from my shower too?

Best, Gus

[KLD]

Gus,

Putting a bucket under the shower and time how long it takes to fill it. That's a good start to find where all that water goes ;-) I also found it informative to time how long a shower really takes.

Check out the solar collector data on www.spf.ch

The physics: sorry for the abbrev, spec. heat is the specific heat. It is a measure for the amount of energy that you need to put into a certain amount of material, in order to raise its temperature by a set difference. Phew. For example, if you want to raise the temperature of 1 gram of water by 1°C, you need 1 calorie of heat. But the SI unit of energy is Joule (J), not calorie (cal). There are 4.2J in one calorie. So, you need 4200 J to raise the temperature of 1kg of water by 1°C. One Joule is a pretty small unit. More useful is the kWh. Now, 1 Joule equals 1 Wattsecond, that is 1W times 1 second. 1000W in 1 kW, and 3600s per hour, mix it all together, and you find the specific heat of water is 4200J/kg °C = 0.0017kWh/kg °C.
And of course, 1 kg of water fills 1 litre.

E voila!

Klaus

[Baz]

http://www.apricus.com/html/solar_collector_size.htm  Appologies for linking another companies website but it just came up when googling solar tube efficiency.
This site has a calculator you can play with. My understanding is that effective insolation in summer for southern UK is about 3kwh/m2/day. This site doesn't have a value for UK but gives 2.4 for Helsinki. You can allow various factors for efficiency from 80% down to 50%, explained elswhere on the site above.

[brackwell]

The temptation is to compare tank temps with one another but this is largely a waste of time due to the many variable differences eg, type of set up,tank size v panel size,draw off patterns,additional heat patterns, heat losses and so on. 

Your panels are almost certainly working as they should as the technology does get any more basic.  Peoples expectations however of continual hot water or whatever are unrealistic.    Even in winter my panels are collecting energy and saving my gas even though the water temp might only have been raised to say 20C at the top from a input temp of say 6C. 

The consensus of opinion on this forum is that 5L per tube is about right ie 200L in your case so you will tend to have a larger volume of cooler water than some others but the embedded energy will be the same.

However there are things which you can do to improve matters the biggest of which is to cut that massive water usage. Instead of 13L/Min showers try 10L/min and then perhaps 8L/min with one of these aerating showers(providing you have mains pressure) and then encourage folks to reduce the time in order to save the planet ( the kids are not interested in your money until it dries up of course).

The addition of heat at the beginning of the day is unfortunate as the panels are most efficient the cooler the input water but also introducing heat tends to ruin stratification in the storage tank.

My personal choice is to allow the tank to do what it wants and use a inline heater to raise the temp if necessary to that required 40C for shower/bath for just the amount being used! All other things in the house can use the "hot" water at whatever temp the tank is.  I believe that storing tanks of hot water just in case of a undefined requirement is flawed but when gas is so cheap many people cannot be bothered with efficiency.

I switched off my boiler in April and just use our 30X47m tubes for the 2 of us ( showers and hot fill washing Mc)

Ken

[tik33]

thanks again all.

Klaus good link - are these guys the be all and end all of data testing for the navitron tubes? also, thanks for the physics reminder! -so power nec. to heat water in my shower: 4200 * 13/60 * 30 = 27300 w   - got it!

Baz, your link seemed to suggest my system is oversized - so who knows?

Ken - you're right reduce water - already have aerated showers - I'm going to test the flow rate on them and start timing my wife!

Best, Gus