dnb

Working out a scheme for DHW

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3 minutes ago, gravelld said:

You have to replace the inverter of course.

 

Depends.  A PV system with microinverters may well last well over 25 years, as they don't have the potential issue with electrolytic capacitors ageing.  A conventional inverter mounted in a cool location may well last 20 years or more, as electrolytic capacitor life is very temperature dependent.  A 50,000 hour life capacitor at 50°C may well have four times that life at 25°C, for example.

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40 minutes ago, JSHarris said:

Modelling is great fun (I spent months doing it...), but ultimately there are lots of parameters that are not easy to fully model. 

 

Isn't that the truth! 

 

I have spent the last 15 years doing mathematical modelling work (amongst other things) in the defence industry. Different subject matter but always the same question of finding the best thing.

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26 minutes ago, dnb said:

 

Isn't that the truth! 

 

I have spent the last 15 years doing mathematical modelling work (amongst other things) in the defence industry. Different subject matter but always the same question of finding the best thing.

 

 

Early in my career I spent several years modelling the underwater plunge behaviour of air launched torpedoes, to try and predict the 3D underwater trajectory for the short time between water entry and motor start up, so the range of attitudes, depth and velocities in all axes could be predicted (the aim was to inform the guidance control people of the sort of start up conditions they would have to deal with).  Back then we were coding in Fortran 77, using an ICL located 200 miles away, that compiled our code (transmitted via a 300 baud teleprinter link) overnight.  Each day started by going through the errors flagged in the printouts that had piled up overnight, before trying to fix the code and move on to the next section.

 

The best bit was validating the model, as that meant designing and building a pretty robust sensor and data logger (using 1/2" magnetic tape and missile data recorders) that we fitted inside dummy torpedoes that were released over as wide a range of drop conditions as we could manage.  It did mean I got to fly a lot, though, which led to a shift to a career in flight test.  To get the highest release speeds, we had to chuck the things out of a Canberra, doing around 400kts at maybe 150ft ASL.  Lots of fun...

 

74088634_WT309-lasttrialssortie.thumb.jpg.21c0f7116b44d200040542a6cec52d96.jpg

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I suspected you were involved in the industry somehow. My experience is mostly radars. I have been fortunate to sail a lot of miles with the RN and witnessed many missile firing trials to test the systems I have helped to build. It is as you say lots of fun.

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6 minutes ago, dnb said:

I suspected you were involved in the industry somehow. My experience is mostly radars. I have been fortunate to sail a lot of miles with the RN and witnessed many missile firing trials to test the systems I have helped to build. It is as you say lots of fun.

 

 

I guessed you may have been a radar person, being where you are.  I was OiC of the ranges at DERA Funtington and DERA Fraser during the late 90's, when they were still a part of DERA Above Water Systems at Portsdown.

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Have you factored in that DHW is pretty useless below 30°C and has an upper limit of 65°C (though that can go higher when grown ups are not looking).

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On ‎11‎/‎06‎/‎2019 at 08:30, ProDave said:

Instead use an unvented cylinder.  the water temperature in the tank is how hot it comes out of the tank.  You only need to store water and say 47 degrees, I actually have mine at 48 degrees and that is hotter than I can hold my hand in for more than a few seconds, so why would you want it hotter?

 

I thought the well-understood answer to that is 'to minimise the risk of legionella'? I see a few on this thread alone are storing at below the recommended 60 degrees to kill of legionella bacteria so clearly not everyone abides by this?

Edited by MJNewton

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23 minutes ago, SteamyTea said:

Have you factored in that DHW is pretty useless below 30°C and has an upper limit of 65°C (though that can go higher when grown ups are not looking).

I had factored in the 30 deg usefulness but had used 85 as an upper limit because there aren't any grown ups around and I assumed a thermal store...  I probably should do a run with an upper limit of 65 to properly simulate the UVC case.

 

I was puzzled by the apparent lack of legionella protection too. It is one of the advantages of a thermal store

Edited by dnb

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8 minutes ago, MJNewton said:

 

I thought the well-understood answer to that is 'to minimise the risk of legionella'? I see a few on this thread alone are storing at below the recommended 60 degrees to kill of legionella bacteria so clearly not everyone abides by this?

 

It's a question of evaluating the real, versus imaginary, risk. 

 

If the DHW system is closed, so that disinfected mains pressure water comes in and hot water comes out, with no means for anything to enter the system, then the legionella risk is miniscule, as there's no path for the bacteria to make their way into the system.  If the DHW is fed from an open cold water tank, or a water source that has not been disinfected, then there's clearly a significant risk that bacteria may be able to make their way into the DHW system and then multiply.

 

Similarly, if the DHW only has a small volume, such as a thermal store or Sunamp, where the only volume of hot water is in the heat exchanger and pipes, then the risk is also small, as the water will be flushed through often enough to reduce the risk of any bacteria multiplying.

 

 

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As above, I store at 48’ with an UVC so have concluded the risk is very minimal. On one of these threads I am sure someone wrote that there has never been a domestic case of legionella causing a problem.

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2 hours ago, JSHarris said:

There's also the fact that solar thermal is a one trick pony; all your investment can do is heat hot water.  A PV system can heat hot water, supply power to the house and export power to the grid.

 

 how much have you spent on multiple  sunamp units and how long you  have you been trying to get them to work correctly,

so PV is not trouble free as you imply

 my post earlier was trying  giving a balanced approach on using both technologies  , both have their place if used correctly and together 

you could even use solar thermal to heat your sunamps!!

 seems to me that a lot of energy usage in a house goes to heating water ,in space heating and DHW- one way and another -so why not use the best one when possible and save your PV for other things  or sell it back -

I think your comment on costings was also a bit skewed as you fitted all your PV yourself and hunted for cheapest 

I reasonably  sure you did not do the same for the cursory glance at solar thermal .

when i did my system years ago +prices have dropped since then, my 8kw system of solar(4sqmish) was £2k--

I would not argue that PV prices have dropped or that installation is simpler for retro fit .

,when i get round to new system I,m sure i will find prices have dropped if self fit   with solar thermal as well and much better controllers are available now --

and if as you say it is very expensive ,then i will  not be doing it

I am not so in favour of solar thermal ,that i would use it if the numbers do not stack up,same as the numbers on PV -still really are not right yet .

the   LPG my solar thermal saved  paid for system in 5 years 

was easy to see by how much gas bills dropped--did,nt need any complicated spread sheets and computer modelling 

 

 

 

 

 

Edited by scottishjohn

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2 hours ago, dnb said:

I modelled the efficiency of these as 80% when the thermal store (or UVC - it makes no difference to the model) temperature is low, rolling off as an exponential decay as the store temperature increased according to [efficiency = nominal * exp(-tank_temperature/constant)]. I set up the constant to give an efficiency of approximately 25% at 60 deg C tank temperature.

 

The usual model is to assume a particular efficiency (η₀) at 0 °C temperature difference (between the water being heated and outside ambient) then a heat loss proportional to that temperature difference and the square of that temperature difference.

 

ΔT = water temperature - outside temperature (°C or K).

G = insolation in W/m².

Q = power output in watts = Gη₀ - α₁ΔT - α₂ΔT²

 

It's not just the curve that will be different with this model, the losses are also proportionally larger as the insolation drops. This makes sense, in the extreme case (like at night) the panels can actually lose heat.

 

A somewhat old blog post on the subject: https://edavies.me.uk/2012/01/pv-et-flat/

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3 minutes ago, scottishjohn said:

 

 how much have you spent on multiple  sunamp units and how long you  have you been trying to get them to work correctly,

so PV is not trouble free as you imply

 my post earlier was trying  giving a balanced approach on using both technologies  , both have their place if used correctly and together 

you could even use solar thermal to heat your sunamps!!

 seems to me that a lot of energy usage in a house goes to heating water ,in space heating and DHW- one way and another -so why not use the best one when possible and save your PV for other things  or sell it back -

I think your comment on costings was also a bit skewed as you fitted all your PV yourself and hunted for cheapest 

I reasonably  sure you did not do the same for the cursory glance at solar thermal .

when i did my system years ago +prices have dropped since then, my 8kw system of solar was £2k--

I would not argue that PV prices have dropped or that installation is simpler for retro fit .

,when i get round to new system I,m sure i will find prices have dropped  with solar thermal as well --if self fit  

and if as you say it is very expensive ,then i will  not be doing it

 

 

 

 

 

 

I've only bought one Sunamp, I've spent nothing on getting it working, and Sunamp have now sorted out the controller problems, it seems.

 

Our PV system has been faultless, unlike my neighbours solar PV system that I've now repaired twice, and which had been repaired three times prior to that.  That's in an eight year period.  My neighbours PV system (same age as the solar thermal system) has been as faultless as ours.  A read through some of the renewable energy forums quickly shows that solar thermal systems have a lot more maintenance issues than PV systems.

 

Not much use in having an 8 kWp solar thermal system that can only heat hot water, and then sits there doing sod all for several hours a day.  At least a PV system can heat the hot water as well as offset the cost of other loads in the house during all the hours of daylight.  Even at £2k (which seems very cheap compared to the cost of a Navitron evacuated tube system I costed up a few years ago) that would have bought well over 13,000 kWh of electricity to heat hot water directly with an immersion  13,000 kWh would be around 6 years worth of hot water at peak rate electricity prices, or around 10 years at off peak electricity prices.  When I checked solar thermal prices about a year ago they seemed no cheaper than they were when I looked at them around 2012.  I've just had a quick look, and a ~2 kWp solar thermal system (30 off, 58mm tubes) system costs about £1.67k, plus delivery, plus installation.

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11 minutes ago, JSHarris said:

 Our PV system has been faultless, unlike my neighbours solar PV system that I've now repaired twice

 

You mean "unlike my neighbour's solar thermal system" I'm guessing?

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33 minutes ago, Onoff said:

 

You mean "unlike my neighbour's solar thermal system" I'm guessing?

 

 

Yes, my error.  It's their solar thermal system that seems to need maintenance.  Last time I fixed it was when the pump controller packed up, before that it was air in the system (might have been the pump controller starting to fail and letting it occasionally overheat, looking back).

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When I first looked into this I was very keen to build my own solar thermal system but the more i looked into it the more there appeared to be issues with solar thermal that PV did not have. The only thing I thought of regarding the using of the excess heat from a solar thermal system was dumping heat into a hot tub in the summer 😎.

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2 minutes ago, joe90 said:

dumping heat into a hot tub in the summer

 

Heating a compost heap for biogas? Greenhouse?

 

I've seen a homebrew solar thermal project in Canada where the chap is growing tropical plants/fruit in his greenhouse and there's snow on the ground.

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1 minute ago, Onoff said:

 

Heating a compost heap for biogas? Greenhouse?

 

I've seen a homebrew solar thermal project in Canada where the chap is growing tropical plants/fruit in his greenhouse and there's snow on the ground.

 

 

Bit like the pineapple house at the Lost Gardens of Heligan: https://www.heligan.com/explore/news/the-story-of-the-10000-pineapple

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37 minutes ago, Onoff said:

Heating a compost heap for biogas

 

Hmmm. Home made petrol for the classic car collection... now there is an idea.

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1 hour ago, Onoff said:

I've seen a homebrew solar thermal project in Canada where the chap is growing tropical plants/fruit in his greenhouse and there's snow on the ground.

My Aunt lies in Canada, 10° South of me, in Cornwall.  So a lot more sun power as well as a lot more snow.

We have palm trees down here, just like they do in the Isle of Skye.  But they are not real palm trees, they are Cordylines.

There is a Banana tree in Penlee Park, but never seen a banana on it.

But back to ST, shame that there is not a simple, cheap drainback system anymore.

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Re solar PV Vs Solar thermal.

 

Solar thermal is a one trick pony.  Once your tank is up to temperature (hopefully a bit less than boiling) any more energy it could produce is wasted.

 

Solar PV on the other hand can power anything electric,  So the normal system of an automatic diverter to send excess power to the immersion heater, favours the generated power first being used by anything in the house, and only heats water to prevent it being exported to the grid.

 

I think it was my HW tank we did a legionairs analysis on.  Our drinking (mains) water comes from mountain loch so starts at a very low temperature, far too low for legionella to survive.  and having been through the treatment plant, including microfilters, no bacteria could pass though.  So being a sealed system with a "clean" source there really is no risk.

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I think there are still not any confirmed cases of legionairs disease, as opposed to the presences of the bacteria, caused by DHW systems.  I may be wrong as it is a few years since a heated debate about this happened over at the other place.

When I worked for the council, we had all the taps and tanks tested, part of the system got closed down and sterilised.  Up until then, no one had reported any symptoms.  After the system was isolated, a few people got ill and blamed it on the water.

Go figure.

Edited by SteamyTea
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7 hours ago, ProDave said:

Solar thermal is a one trick pony

 

But hot water is a fairly a useful trick!  I take the point though - in the summer, the 9m^2 of ETs are not pulling their weight for much of the day, when their equivilent area of PV could be doing something useful like powering the A/C from my other thread. But of course only if the DHW is charged or it's more cost effective to heat the DHW over night with economy 7 (or whatever other base load incentive I might find)

 

I have come to the conclusion that I can make whatever system I install work and meet my DHW requirements, and all I am going to achieve is to get the last few percent of efficiency out of the system. It should be worth it though. I updated the model with Ed Davies suggested ET efficiency model and used the parameters from the Navitron system I was considering. It makes surprisingly little difference to the conclusions.

 

What I really need is a near infinite capacity thermal store that has next to no loss. Then I can average the energy across the seasons... So, a 10m^3 concrete cube buried deeply  ;)in the garden ought to do it. ;)

 

I got the first draft of the SAP calculations this afternoon. It will change a bit, I am sure, because some design details have been entered as guesses. It indicates the house will be annoyingly close to carbon negative, but won't quite achieve it. Still, it's a good A rating so nothing too bad for a first go at house building. 

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If you really want to see DIY solar thermal working for people who think outside the box head across the pond to here:

 

https://simplysolar.supporttopics.com/

 

Yes, they get more Sun. They also seem to go for size to compensate for lower efficiency of the designs.

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