Higher temp in water cylinder, as energy store. With mixer to give sensible hot water temp @ taps?

[Smallholder]

 

We've been doing endless calculations as part of our design for an off-grid home we are hoping to build.

Energy storage is key to smoothing out the erratic energy produced from the large PV array, and it seems wise to try and use a large water cylinder as a secondary energy storage system as well batteries.

 

Now this may well have been done before, but I had the idea of increasing the max allowable temp of the water in the hot water cylinder as a means of storing more energy when it's available.

I have seen these clever little mixer valve things for public hot water taps. They have a hot water and cold water input, and they automatically mix the two to ensure that the output temp is never over 40 degrees C. Anti-scald valves I think they are called. 

Now we usually limit the temp of the water in the cylinder to about 60 degrees C, presumably to stop us burning the skin off our hands, and because we never need anything hotter than this coming out of the taps?

If we allowed the water in the tank to be heated to, say 80 degrees... Could we then add a mixer valve on the hot water output of the cylinder, and mix to give the desired hot water output temp, say 60 degrees?

 

This would allow a greater storage of energy in a given volume of water cylinder. Or maybe I am missing something, and it would be a foolish thing to do.

 

Feedback appreciated. 

[ToughButterCup]

There are more qualified people than I to answer your plumbing question. 

I wish merely to pass on a conclusion drawn by one member ( well regarded, technically ueber-confident, not so active on this board these days) who remarked that he wish he'd kept it simple(r) ... in terms of costs and diminishing returns from time and effort invested.

[Fly100]

I run my tank to max immersion temp ( 67c near bottom) anything above that it trips out. I dont have a mixer on the top of the tank as the house is new and all the taps are mixer taps and we know where to set them, muscle memory if you will.

That said we dont have children either so that could be a concern.

 

The tank drops 5C over night at the bottom, but remeber there is some water usege through the evening which is replaced with the cold mains.Our boiler doesnt run from March to late Septemember other than the odd run when we have had a few grey days and it needs a boost to get back ahead of the curse so to speak.

[Smallholder]

That sounds like a good setup. I wonder if it's possible to push the idea further, without running into problems.

 

One of our red lines with this house build, is to be 100% off grid. Relying almost entirely on solar has it's limitations, and I'm wondering how clever we can be with it. I say almost entirely, as we've got a diesel generator, but I really only want to keep that for occasional emergency situations.  

[SteamyTea]

Because you get a temperature gradient over the height of the storage cylinder (mine is about 25⁰C once settled), you may find circulating the water within the cylinder stores a usable amount of energy.

Note I say usable.

DHW and space heating water is of little use once below 30⁰C, but 500 Lt at 60⁰C is very useful.

 

[SimonD]

Yes, you can get a mixer valve. I have one included in my thermal store and you just set the flow temp to whatever you want. The store also has an immersion heater that heats to approx 80 deg C. Once budget permits this will be used with PV. The TS is designed for 80 degrees.

[ma9mwah]

On 11/05/2022 at 11:37, SimonD said:

Yes, you can get a mixer valve. I have one included in my thermal store and you just set the flow temp to whatever you want. The store also has an immersion heater that heats to approx 80 deg C. Once budget permits this will be used with PV. The TS is designed for 80 degrees.

Do you remember what model mixer you have?

[Nickfromwales]

Reference above to a conclusion drawn……was irrelevant as that chap was NOT off grid.  

 

Anyhoo. 
 

[joth]

On 11/05/2022 at 11:25, Fly100 said:

I dont have a mixer on the top of the tank as the house is new and all the taps are mixer taps and we know where to set them, muscle memory if you will.

I went for the same plan, however note you need thermostatic mixer taps for this to work. the cheap tap we ended up choosing for the utility room turned out to be a basic mixer tap so does scold on a sunny day when the PV has been busy. Also the bath filler doesn't have a thermostat so the amount of manual mixing needed varies by season. We'd have to fix both these if we ever decided to let the property

 

[joth]

Overall, while smart way of maximizing self use of PV  this probably isn't a very helpful for achieving fully off grid.

The limitations for off-grid will be in the depths of winter, and you won't have enough excess solar to achieve these higher temps on those days.

If you're only using the cylinder for DHW that's a very small part of the challenge anyway, but if it doubles as a thermal store for the heating system then this could make a dent.

 

I'd say this maybe one case where a wood burning stove with a back burner  has its place. Even if you can't grow all your own wood fuel on site, it can be grown a lot more locally than diesel which feels closer to the ethos of self-sufficiency

[Nickfromwales]

On 11/05/2022 at 11:25, Fly100 said:

I dont have a mixer on the top of the tank as the house is new and all the taps are mixer taps and we know where to set them, muscle memory if you will.

That said we dont have children either so that could be a concern.

Anti-scald mitigation is a core safety remit, and BRegs requirement, so how do you tick that box with <46oC at the bath when turned to max hot temp? 

[Nickfromwales]

On 11/05/2022 at 10:56, Smallholder said:

 

We've been doing endless calculations as part of our design for an off-grid home we are hoping to build.

Energy storage is key to smoothing out the erratic energy produced from the large PV array, and it seems wise to try and use a large water cylinder as a secondary energy storage system as well batteries.

 

Now this may well have been done before, but I had the idea of increasing the max allowable temp of the water in the hot water cylinder as a means of storing more energy when it's available.

I have seen these clever little mixer valve things for public hot water taps. They have a hot water and cold water input, and they automatically mix the two to ensure that the output temp is never over 40 degrees C. Anti-scald valves I think they are called. 

Now we usually limit the temp of the water in the cylinder to about 60 degrees C, presumably to stop us burning the skin off our hands, and because we never need anything hotter than this coming out of the taps?

If we allowed the water in the tank to be heated to, say 80 degrees... Could we then add a mixer valve on the hot water output of the cylinder, and mix to give the desired hot water output temp, say 60 degrees?

 

This would allow a greater storage of energy in a given volume of water cylinder. Or maybe I am missing something, and it would be a foolish thing to do.

 

Feedback appreciated. 

Hi.

If you want an optimum design for off-griddyness then I would suggest a dual cylinder arrangement. A 500L TS ( thermal store ) with 2x 3kW immersions, and a 300L UVC ( unvented hot water cylinder ) also with 2 immersions, with the TS preheating the cold feed to the UVC. This will massively increase your potential to store energy as heat, plus the TS can be depleted ( for heating ) without sapping all of your DHW for bathing. 

 

All excess electrical energy into the UVC first, then cascading to the TS. Batteries and PV can dump into either on demand, whichever needs a nudge.

 

Another advantage is, if you have a diesel genny which is water-cooled you can divert the heated water from that into a low mounted coil in the TS to absorb every ounce of energy you create, wherever it gets created. When you have the least solar revenue it will be the winter, and you'll be needing heating, plus the batteries will need topping up via the genny, so, one problem gets solved and the bi-product ( wasted heat from the genny running ) helps to solve another by using even low grade heat at start up to the high grade heat after it's been running for a while. Zero is wasted.

 

What are you doing in terms of heat emitter? Concrete slab with UFH to be used as a further energy store? 

Edited May 15, 2022 by Nickfromwales

[SimonD]

2 hours ago, ma9mwah said:

Do you remember what model mixer you have?

 

Reliance Heatguard Tempering Valve

[Nickfromwales]

11 hours ago, SimonD said:

 

Reliance Heatguard Tempering Valve

Reliance are very good bits of kit. 

[Wil]

16 hours ago, Nickfromwales said:

Hi.

If you want an optimum design for off-griddyness then I would suggest a dual cylinder arrangement. A 500L TS ( thermal store ) with 2x 3kW immersions, and a 300L UVC ( unvented hot water cylinder ) also with 2 immersions, with the TS preheating the cold feed to the UVC. This will massively increase your potential to store energy as heat, plus the TS can be depleted ( for heating ) without sapping all of your DHW for bathing. 

 

All excess electrical energy into the UVC first, then cascading to the TS. Batteries and PV can dump into either on demand, whichever needs a nudge.

 

Another advantage is, if you have a diesel genny which is water-cooled you can divert the heated water from that into a low mounted coil in the TS to absorb every ounce of energy you create, wherever it gets created. When you have the least solar revenue it will be the winter, and you'll be needing heating, plus the batteries will need topping up via the genny, so, one problem gets solved and the bi-product ( wasted heat from the genny running ) helps to solve another by using even low grade heat at start up to the high grade heat after it's been running for a while. Zero is wasted.

 

What are you doing in terms of heat emitter? Concrete slab with UFH to be used as a further energy store? 

 

This is exactly what I’d like to do, except swapping the diesel genny for grid (as it’s still cheaper per kWh) and put a WBS through the TS coil to be that secondary source of heat when the PV isn’t performing. 

 

For me it’s that I’d like to minimise grid dependence, I don’t need to go as far as the genny and fuel storage to remove it, however tempting to stick fingers up at the DNO. True grid independence can only be achieved as Nick says above.

[SimonD]

4 hours ago, Nickfromwales said:

Reliance are very good bits of kit. 

Good to know, ta 🙂

[Iceverge]

 

We have one of these. It works fine. 

 

We heat our 300l to about 70deg and blend everything to "hottish" at the taps. 

 

I have a mate who does pv solar installs. If you exclude DHW and space heating a very modest solar array and battery can suffice year round. Must ask him again but I seem to remember 2kWp of panels and 2kWh of battery being mentioned. 

 

I agree with @jothre the boiler stove.  

[SteamyTea]

6 minutes ago, Iceverge said:

have a mate who does pv solar installs. If you exclude DHW and space heating a very modest solar array and battery can suffice year round. Must ask him again but I seem to remember 2kWp of panels and 2kWh of battery being mentioned

Have to be low electrical energy users. That is about what I use in the winter excluding space heating and DHW.

Currently using about 5 kWh a day.

[Iceverge]

I think one of the keys is to find a hot fill dishwasher + washing machine and cook on a fire of some description. 

[ProDave]

12 minutes ago, Iceverge said:

I think one of the keys is to find a hot fill dishwasher + washing machine

I have often wondered why those no longer exist.  But I suspect they use so little water that unless they were inteligent enough to dump the slug of cold water in the pipe and only actually really start filling when they detected hot water arriving, the saving might be minimal.

[SteamyTea]

11 minutes ago, Iceverge said:

I think one of the keys is to find a hot fill dishwasher + washing machine and cook on a fire of some description. 

I would think that most washing machines and dish washer could take water at 35°C. That would not be an unusually high temperature for Central Europe's taps.

[joth]

2 hours ago, Iceverge said:

 

I have a mate who does pv solar installs. If you exclude DHW and space heating a very modest solar array and battery can suffice year round. Must ask him again but I seem to remember 2kWp of panels and 2kWh of battery being mentioned. 

 

Hmmmmm. Our evening cooking generally starts after the sun is down. Either of our 3kW oven or the 5kW on the hob is going to empty that battery PDQ and then there's all the evening's lighting and entertainment. And in winter the battery may not get recharged for days if it's cloudy.

 

I think paramount to going off-grid is completely changing to an off-grid compatible lifestyle. Taking that to the extreme, why bother with electricity at all?

 

 

Edited May 16, 2022 by joth

[SteamyTea]

54 minutes ago, joth said:

why bother with electricity at all?

Because is is the only practical form of energy that is storable, transportable, relatively cheap and can be generated in a very low carbon dioxide (and other pollutants) manner.

[Temp]

23 hours ago, Nickfromwales said:

Reliance are very good bits of kit. 

 

+1 we have two or three of their ufh mixers in our system. Been very reliable for 15 years.