The Great 'Get Paid To Generate Energy' Hunt

zeupater

green_as_grass wrote: »

I will have 60 evacuated tubes feeding into a 300 litre thermal store. The store has two solar coils, with heat feeding into the top coil initially to provide rapid usable heat, then will divert to the bottom coil to heat the remainder of the tank. This second coil will act as a primary heat dump, but also provide some benefit in tiding us over periods of cloudy days when little input is provided by the EV tubes.

Hi green as grass

I can see the logic in what you're attempting to do, but have a question. Logically the morning irradiance increases as the sun's angle above the horizon increases and the azimuth becomes more benificial to the installation, so waiting on the panel to raise to a temperature which would be higher than the top of the tank before the system delivers any heat is surely a lost opportunity when considering efficiency (same applies in the late afternoon).

I would have thought that the following should apply in your case (subject to set temperature differentials, of course) .

(a) - Lower Coil > Panel < Top Coil = Off
(b) - Lower Coil < Panel < Top Coil = Deliver to Lower Coil
(c) - Lower Coil < Panel > Top Coil = Priority deliver to Top Coil until max temp reached then deliver to bottom coil until max temp reached then divert to heat dump.

Is this what you meant to describe, or is the logic (c) only ? (in which case the efficiency and total delivered heat will be relatively poor).

Regards
Z

green_as_grass

Hi Z

Your little arrows > and < mean 'greater than' and 'less than' and they refer to whether the panel is hotter or cooler than the temperature around the Top Solar Coil or around the Lower Solar Coil.

I just thought I'd better explain that for other readers who may not be familiar with symbols used in this way.

Your suggestion is to make use of 'lower grade' sunshine in the early morning and late afternoon when the temperature of the panel is moderately warm - lower than the temperature of the top coil, but greater than the temperature at the bottom coil.

My reasoning is that I want the top of the tank at a high temperature quickly to get usable heat early in the day. To use the 'low grade' heat in the panel through the bottom coil first would delay the panel getting hot enough to circulate through the top coil.

Explaining for other readers not in the know...
I have a thermal store rather than a standard hot water tank. No water flows into or out of the thermal store, input of heat is via coils linked to the solar panel plumbing circuit. Output of hot water to taps, shower etc. is via a massive coil sited in the top 2 thirds of the store. One advantage of this set up is that the heat in the tank remains stratified - hotter at the top, cooler at the bottom. A flow of water through the tank would destroy the stratification.

Now, I have designed the system so that the thermal store is larger than it needs to be for our daily hot water needs. The bottom coil's main purpose is as a primary 'heat dump' for when the top of tank has reached max. safe temperature but the sun is still shining on the panels - the bottom of the tank then gets heated. Most heat dumps just dissipate heat where it goes to waste. My thinking is that this extra heat in the bottom of the tank will help keep the tank warm through periods of minimal sunshine (not a rare occurrence in the UK!). This should allow us to minimise times we have to light our fire for hot water.

In case you're wondering, I do have a secondary heat dump system for if the WHOLE tank starts to get too hot - a pump switches on to circulate through the back-boiler of our woodburner

Z, I can see your suggestion being more useful for late afternoon if the top of tank is much hotter than the bottom. However this would mean being around to change settings on the controller at that time - not always possible if out at work.

Another time when it may be useful is beginning and end of season (spring & autumn) when there is very little high grade heat to be had and to waste the low grade heat would not be sensible, even if it is only useful for pre-warming the store before lighting the fire. May help by not needing to burn so much wood in this case.

I'd value your comments.

zeupater

green_as_grass wrote: »

My reasoning is that I want the top of the tank at a high temperature quickly to get usable heat early in the day. To use the 'low grade' heat in the panel through the bottom coil first would delay the panel getting hot enough to circulate through the top coil. .........
........ Z, I can see your suggestion being more useful for late afternoon if the top of tank is much hotter than the bottom. However this would mean being around to change settings on the controller at that time - not always possible if out at work.

I'd value your comments.

Hi

Regarding the above, there is a very strong arguement that, on what turns out to be an overcast day, the system could be dormant for long periods awaiting the high temperatures the control system requires to switch the circulation on, therefore reducing potential overall system efficiency. ..... I'm no expert on these systems but would have logically thought that a stratified system would be attempting to reach a state of thermal equilibrium over a period of time, therefore you may just have a system which waits for the top of the store to cool to the panel supply temperature before requesting heat .... Do you have any data on the timescales over which stratification maintains a thermal gradiant within the store ?

Regards
Z

albyota

green as grass, are you not overlooking the fact that the top section of the thermal store would/should normally be hot...or at least quite warm as you would be requiring a good amount of instant hot water for showers and baths etc...?
The way you describe your proposed set up, the panel temp would have to be at least 10C above...say, 50C in the store and as Zeupater suggests, the system could be missing out on earlier low grade heat into the bottom coil.

green_as_grass

Zeupater and Albyota

I don't have any hard data regarding thermal store stratification and cooling but I'm given to understand that, yes, there is a slow tendency towards thermal equilibrium, mainly via conduction down the sides of the store, but that stratification is largely maintained in practice during cooling periods.

You are both making valid points regarding missing out on low grade heat through the bottom coil on overcast days. But remember that heat at the bottom of the store is not as useful (it's not where the DHW heat exchanger coil is sited).

I envisage my set up as similar to having two tanks with a solar coil in each. The bottom half of the the store is like a second tank used as a heat dump for excess heat. But it also has some benefit is slowing down cooling of the store.

My priority is to have useable heat at the top of the store. My reasoning is that if I were to allow low-grade heat in the morning to go the bottom of the store this would delay the panel reaching a high enough temperature to go to the top of the store (the bottom coil would effectively be keeping the panel cool).

I can forsee there will be occasions, on overcast days when the top of the tank is still warm, when there will be a long delay before the panel reaches a high enough temperature to circulate through the top coil (if ever). But do I need to worry about this? - There is still useable heat at the top of the store and the benefit of adding heat to the bottom of the store is minimal. And how often is this scenario likely to happen?

I'm not recommending this set up for everyone, but in my case, with panels and store both oversize for our daily DHW needs I think it could work.

But I'm still open to arguments!

Dave_Fowler

Green as Grass,

I'd be interested to see some calculations on the advantages / disadvantages of the two tank system you describe over the system I envisaged at post #152. Your system has the advantage of maintaining stratification in the heat dump/storage tank but 'suffers' from the disadvantages of 1) not using all the possible heat generated by the panels on a cloudy day. 2) feeding cold water from the header tank directly into the hot water domestic supply tank. 3) requiring an extra pump to circulate the water through the top coils of the storage tank to heat the domestic hot water tank.

I take your point that in your system, using low-grade heat to flow through the lower coil in your heat storage tank would not give much if any advantage. The two tank system I described used the panels to heat directly the domestic hot water tank if the panel temperature was higher than the domestic tank (and the water needed heating) but heated the feed tank with the low-grade heat on cloudy days so that the water fed to the domestic tank was pre-heated above the normal header tank temperature whenever any hot water was drawn off through the domestic water supply.

albyota

Dave, how I read it, g-a-g is only using one tank (thermal store)....but he describes it as similar to using two.

Dave_Fowler

albyota wrote: »

Dave, how I read it, g-a-g is only using one tank (thermal store)....but he describes it as similar to using two.

Hi albyota,

Having re-read g-a-g's post, I do agree with what you say. Ok I need to re-word part 2) of my post to say 'cold water from the header tank is fed to the 'Massive heating coil'.

green_as_grass

Dave
As you've discovered I just have the thermal store.

There is no feed-in from a header tank, in fact no feed in of water at all. The water in the thermal store just stays put. The large DHW coil is supplied directly by mains water (and so supply to taps, shower etc. are all at mains pressure) rather than a header tank.

So yes, the input to the DHW is cold. Although in the long term I do intend to pre-warm this by circulating around a thermophilic compost heap but that's another story altogether!!

Disadvantage 3. doesn't apply - no pump needed.

I originally considered a two-tank system to try to make use of excess heat, rather than just dissipate it wastefully. But for simplicity, for minimising pipe runs and for space reasons I went for the single, large thermal store. I couldn't say with any certainty which would work out most efficient. Taking more advantage of low-grade heat could well tip things in favour of your 2-tank system. If you had a single large thermal store set up to take advantage of low-grade heat I've no idea which would work out best. Ability to minimise pipe runs and maximise insulation could be a deciding factor. If you find out the answer to that question I'd be interested to hear.

zeupater

green_as_grass wrote: »

..... I originally considered a two-tank system to try to make use of excess heat, rather than just dissipate it wastefully. But for simplicity, for minimising pipe runs and for space reasons I went for the single, large thermal store. I couldn't say with any certainty which would work out most efficient. Taking more advantage of low-grade heat could well tip things in favour of your 2-tank system. If you had a single large thermal store set up to take advantage of low-grade heat I've no idea which would work out best. Ability to minimise pipe runs and maximise insulation could be a deciding factor. If you find out the answer to that question I'd be interested to hear.

Hi

This would just be space & cost dependant .... the two tank option should always be more efficient due to thermal conductivity. What must be taken into consideration with the large thermal store is that there will be a migration on heat throughout the tank, this will be via both convection and conductivity. The thermal store should be constructed with baffles to reduce the effect of convection, but this will not be totally eliminated. Even if the convection is restricted to a minimum you will be left with conductivity.

Water conducts heat at a given rate, see link (http://en.wikipedia.org/wiki/Thermal_conductivity), which equates to approximately 0.6W/m2 (of cross sectional area of the tank) heat transfer per m of separation between the top and bottom sections of the tank, per degree C difference, per hour.

The construction of the tank will likewise transfer heat at the given rate for the material (copper/stainless steel) ... the CSA of the tank walls (assuming a circular tank) can be calculated using (pi r 2 (od))-(pi r 2 (id)).

If you compare the conductive transfer of heat within the single tank model with a twin tank model, you'll get an idea of the difference. However, my guess is that the primary concern for most installations would be the space requirements for two tanks, therefore the selection of a single unit.

When I looked at installing, after much deliberation, I was considering a twin tank hot water supply model using the following logic (Tank B= Preheat/Tank A=Supply) ....

Separate switchable logic for Summer/Winter seasons (PLC controller) ...

Summer - Use high grade solar to heat tank A / Low grade solar tank B / Tank B is primary heat dump to capture excess high grade.

Winter - Use all (any ) energy to heat B, (and logically use A as the heat dump :rotfl:) to reduce primary heat source load. Primary heat source (boiler) is therefore only heating tank A.

Why didn't I do it, well quite simply, however I looked at it the figures didn't stack up, especially after replacing the old floorstanding boiler with a more efficient condensing unit. Recent developments regarding renewables may change this view, but I've not bothered checking yet.

Hope this helps

Regards
Z