PV Divert to immersion difficulties

[S2D2]

8 minutes ago, Radian said:

someone decides to bake a cake

This is optimal PV divert benefit

[Radian]

2 hours ago, dpmiller said:

I wouldn't overthink it tbh, they just... work.

That would be a refreshing change!

 

2 hours ago, S2D2 said:

This is optimal PV divert benefit

I'm doing my best to encourage such 'optimal usage' and am quite pleased with the response so far. Fresh hot cakes cooked by the Sun are a delight and reasonably guilt-free. But if I baked them it would still add to the carbon problem.

[ProDave]

We have had such a good run of sun this week my immersion maxed out mid afternoon.  I would (for once) have loved someone to take a long shower.

[dpmiller]

Same here, yesterday. Whole tank above 90c...

[S2D2]

£17.04 in export payments for the previous three days, £3.06 in gas hot water usage. Important to remember it's an unusual few days and not get tempted into investing in a divert system based on the "worst" case, it's the argument solar installers use to sell more unnecessary batteries too.

[Radian]

Terrific. Woke up to the RCD tripping off the house power just before 7AM. A quick look at my power plots confirmed my suspicion that the my Solar PV diverter had just started to dump the excess into the immersion. A megger test reveals that the insulation resistance of the immersion heater element was less that 1M. I can't seem to get a break with this project.

[Radian]

The faulty immersion has been replaced under warranty and, for the time being, extra expense on the plumbing has been vetoed while other more pressing issues are being addressed. This means I'm trying to make the most of what I've got: A 0.6m long immersion in the top half of a 1.2m tall (160l) cylinder. I've put in a permanent temperature sensor right at the top in addition to the one 1/3 the way up that was governing the boiler demand so I can plot the goings-on in the cylinder.

 

Now the problem remains being how to stop the boiler firing-up when the partial draw-off of lovely hot Solar PV diverted water is replaced by unmixed cold when there's every prospect of there being sufficient 'free energy' up at the top of the tank to make this unnecessary. Take today so far for example:

                      A nasty case of stratification: HW LO falls after 7AM while HW HI goes up.

 

Ignoring the shenanigans until the Sun comes up at 7AM, the immersion gradually builds up the water temperature in the top half of the tank (red: HW HI), even while the lower half loses heat through ambient losses and occasional draw-off (orange: HW LO).

Gas boiler demand is currently governed by the average of these two temperature readings (orange dashed: HW Average) so that at 12:30PM a 10 minute shower (🙄) displaces a significant amount of water causing the bottom sensor to fall to 20^oC thus pulling the average below the HW Setpoint.

The immersion was going great guns at this time and the HI sensor in the top of the tank shows plenty of HW was available - but still the boiler fired up.

 

I guess I could weight the two sensors differently - 50/50 seemed like a reasonable starting point. But taking extreme cases to aid understanding, if the sensing was biased to 100% of HW HI, there might only be a tiny bit of HW just 'floating' at the top and nearly 160l of room temperature water below it.

I could move the LO sensor up a bit but the current position has worked well before I built the PV diverter and in the winter its height reflects how long it takes to reheat the cylinder during sustained HW use. Any suggestions?

[ProDave]

Simplest way would be on a day you know it is going to be sunny all day, manually turn the boiler off.

 

Or have a summer boiler program, that turns the boiler off during the daytime.  This is what I am trialling at the moment,  we still need some heating (average about 5 degrees outside today!!!!) so I only have the heating timed to come on for a few hours around mid day when it is likely there might be some good PV generation to power most of the ASHP demand.

[S2D2]

56 minutes ago, Radian said:

The faulty immersion has been replaced under warranty and, for the time being, extra expense on the plumbing has been vetoed while other more pressing issues are being addressed. This means I'm trying to make the most of what I've got: A 0.6m long immersion in the top half of a 1.2m tall (160l) cylinder. I've put in a permanent temperature sensor right at the top in addition to the one 1/3 the way up that was governing the boiler demand so I can plot the goings-on in the cylinder.

 

Now the problem remains being how to stop the boiler firing-up when the partial draw-off of lovely hot Solar PV diverted water is replaced by unmixed cold when there's every prospect of there being sufficient 'free energy' up at the top of the tank to make this unnecessary. Take today so far for example:

                      A nasty case of stratification: HW LO falls after 7AM while HW HI goes up.

 

Ignoring the shenanigans until the Sun comes up at 7AM, the immersion gradually builds up the water temperature in the top half of the tank (red: HW HI), even while the lower half loses heat through ambient losses and occasional draw-off (orange: HW LO).

Gas boiler demand is currently governed by the average of these two temperature readings (orange dashed: HW Average) so that at 12:30PM a 10 minute shower (🙄) displaces a significant amount of water causing the bottom sensor to fall to 20^oC thus pulling the average below the HW Setpoint.

The immersion was going great guns at this time and the HI sensor in the top of the tank shows plenty of HW was available - but still the boiler fired up.

 

I guess I could weight the two sensors differently - 50/50 seemed like a reasonable starting point. But taking extreme cases to aid understanding, if the sensing was biased to 100% of HW HI, there might only be a tiny bit of HW just 'floating' at the top and nearly 160l of room temperature water below it.

I could move the LO sensor up a bit but the current position has worked well before I built the PV diverter and in the winter its height reflects how long it takes to reheat the cylinder during sustained HW use. Any suggestions?

Use either the current immersion power or the forecasted solar generation for the rest of the day as your blending coefficient. If you want to get really fancy model the current energy in the tank and the current loss rate.

[Radian]

3 hours ago, ProDave said:

Simplest way would be on a day you know it is going to be sunny all day, manually turn the boiler off.

 

Or have a summer boiler program, that turns the boiler off during the daytime.  This is what I am trialling at the moment,  we still need some heating (average about 5 degrees outside today!!!!) so I only have the heating timed to come on for a few hours around mid day when it is likely there might be some good PV generation to power most of the ASHP demand.

 

I can easily change the HW setpoint - I just bark at Alexa: "set hot water to zero" (or any other desired temperature). This has been one of the more useful hacks I've done with my heating system as anyone in the house can give it a boost when planning to use HW without opening an app or using a wall mounted control. The programmed values are then restored at the given times.

 

So yes, I can effectively override the boiler and even change the schedule but being lazy, and being an engineer, means I'm looking for the maximum amount of automation.

 

3 hours ago, S2D2 said:

Use either the current immersion power or the forecasted solar generation for the rest of the day as your blending coefficient.

 

This is a good suggestion. To begin with I will try it out by shifting the weighting towards HW HI as more PV divert Joules are accumulated.

 

4 hours ago, S2D2 said:

If you want to get really fancy model the current energy in the tank and the current loss rate.

 

I'll have a ponder on that.

 

[nigeldodd]

The observation that hw tanks exhibit such stratification in temperature is very interesting. I am not sure if this thread is still live, but I will give my two Eurocents worth.

 

My situation is slightly different. I have a Powermax gas boiler with an integrated 100 litre tank and the immersion heater is at the bottom. I have a homemade diverter with the temperature sensor also at the bottom. Recently this temperature sensor was reading 40 deg C and, alarmingly, the pressure/temperature relief valve at the top of the tank opened and hot steaming water started gushing through the tundish. When opened a hot tap, the temperature of the water was 90 deg C. 

 

The stratification effect is real and in my case a barrier to storing the maximum amount of energy from the solar pv panels in the form of hot water. 

 

I will put another temperature sensor at the top of the tank and use this to turn off the diverter.

 

My strategy now is to dump power into the hot water at the beginning and end of the day while in the middle of the day, when the sun is at its strongest, to dump the energy into the car. 

[JohnMo]

22 minutes ago, nigeldodd said:

Recently this temperature sensor was reading 40 deg C

Have you checked the calibration? Your immersion should also have an overheat trip in addition to any thermostat to prevent this from occurring.  Something is not correct in your set up.

[nigeldodd]

The sensor is correctly calibrated. The temperature stratification is as I have described, 40 deg C at the bottom and 90 deg C at the top. It roughly agrees with the graph that Radian has posted above.

[SteamyTea]

1 hour ago, nigeldodd said:

integrated 100 litre tank and the immersion heater is at the bottom.

What power rating is the immersion heater?

 

Stratification is a bad term to use when describing fluids.

It implies a very distinct change in temperature at a fixed position.

Think of it as geological stratification.

In reality, there will be a temperature gradient from bottom (colder) to top (hotter) once there is no fluid mixing happening (given time to settle down).

As electrical immersion heaters are generally fitted low down in the vessel, and when powered, are very hot (maybe 650K), it is not unusual for water vapour to be created on the surface of the element. This vapour will bubble away upwards, causing a chaotic turbulence in the fluid (water in this case).  Some of this gas will reach the top of the vessel as steam (steam is the term used for water at phase change temperature between liquid and gas).

Steam is a dangerous material. While only at 373K (100°C) as it cools, the entropy of vaporisation is 2257 kJ.kg^-1, so 550 times more energy is realised compared to liquid water dropping 1K in temperature (vaporisation and fusion have not delta T).

So if you are getting visible water vapour coming out of the tap, you have a very dangerous situation (think pulling the cling film off the top of some over microwaved soup, the steam really hurts).

You may find that fitting a smaller heating element will reduce, or even eliminate, the overheating.

It is possible to design a system that is intrinsically safe by balancing the energy input to the totally storage system losses, governing the maximum water temperature, though they can take a long time to heat up.

 

Thank goodness for PRV, which should really be called ERV (energy release valves).

[JohnMo]

2 hours ago, nigeldodd said:

The sensor is correctly calibrated. The temperature stratification is as I have described, 40 deg C at the bottom and 90 deg C at the top. It roughly agrees with the graph that Radian has posted above.

If the temp sensor is below the immersion then you will get what you are seeing. Immersion heats water, this lowers it's density, it floats to the top of cylinder. Water below the immersion really doesn't get heated.

 

Immersion should have its own thermostat not an add on one, the inbuilt immersion will have an auto cut overheat protection thermostat also. But if you have circumvented the inbuilt thermostat not really that surprised you are having issues.

[marshian]

Useful thread

 

I've added the bottom of the tank immersion to my new HW tank criteria (we've only ever had top mount immersions as it's only ever used in the event of HW/CH circuit drain down but with a plan to install PV at some stage in the future it makes sense to avoid the issue of not fully utilising the excess electric.

[JohnMo]

49 minutes ago, marshian said:

bottom of the tank immersion to my new HW tank

If you get a heat pump cylinder that's generally how they come. Top immersion being an option you can sometimes specify 

[marshian]

23 minutes ago, JohnMo said:

If you get a heat pump cylinder that's generally how they come. Top immersion being an option you can sometimes specify 

Also on my spec list

[Nickfromwales]

4 hours ago, marshian said:

Useful thread

 

I've added the bottom of the tank immersion to my new HW tank criteria (we've only ever had top mount immersions as it's only ever used in the event of HW/CH circuit drain down but with a plan to install PV at some stage in the future it makes sense to avoid the issue of not fully utilising the excess electric.

You know to approach Trevor @ cylinders2go for any cylinders, ya?

[marshian]

2 minutes ago, Nickfromwales said:

You know to approach Trevor @ cylinders2go for any cylinders, ya?

I have that down as my first port of call from previous mentions you've made

 

I don't want to thread jack this thread