Hot water debate

70sbudgie

Reed_Richards said:

Water has about 5 times the specific heat capacity of sand.  So to store the same amount of energy in a 300 l tank of sand you would need to heat the sand to 220 C instead of water to 60 C (I'm assuming "background" temperature is 20 C.  So "domestic scale" sand batteries are never going to happen, if by "domestic scale" you mean one per house.  A sand battery might just work as an add-on to a district heating scheme (as per your link) but we're not big on district heating in the UK.    

I think the article said the sand was heated to 500 C. So with heater exchangers, (discharge) could be used to heat water over a long period of time. I think the premise is that high temperature storage is more efficient and therefore is more suitable for longer term storage. The science behind it is outside my comprehension (maybe why I can help but be interested).

It is the opposite of the cryogenic "batteries" they are building.

When you say water has 5 times the specific heat capacity of sand, do you mean that it needs 5 times the energy to raise the temp 1 degree? 

I will admit that my "wish" for domestic scale isn't much more than pipe dreaming / speculating where the technology might go. I agree, it is currently looking more suitable for district heating.

70sbudgie

Magnitio said:

Pile_o_stone said:

We have our PV attached to two immersion heaters on our tank. The iBoost diverts excess solar to the top immersion heater  first, where I  set the thermostat to 80C. Once the top of the tank reaches 80C, it  diverts to the immersion at the bottom of the tank. This immersion maxes out at 60C. On a good day we get a 350L tank full of water at 60C. We have a mixer valve that prevents scalding water getting to the taps.

Not only are we saving money because we are not using the gas boiler to supply hot water for showers, etc., we have had weeks where the boiler has not been used at all, which I  assume will prolong the life of our boiler.

What rate do you pay for your gas and what rate do you get paid for exporting electricity? The large difference between these (i.e. electricity export being greater than gas) and a comparison of environmental impact was the main reason for the debate.

My export is 7.5p/kWhr and my gas is 7.34p/kWhr. If I was able to generate enough to get rid of my gas, then the biggest savings would be from the standing charge.

70sbudgie

I've just thought, that if you have Octopus Go, with 4 hrs at 5p/kWhr, surely that would be the time to run an immersion heater for a hot water tank. That is then cheaper than gas. 

michaels

QrizB said:

michaels said:

WE are going to do our morning hot water from 4.5p per hour overnight leccy rather than 14p per hour (October rate) gas.  What is the hottest we can safely set the immersion to max storage?

Your immersion heater will have a thermostat. The max setting is likely to be 60-65C or so. Mine is set to the max and it's not causing any problems. (If you have very young, very old or other vulnerable people in the house, 65C might be too hot for safety and you might need a blend valve as mentioned previously.)

Unfortunately it is only a 180l tank so will only do the morning showers not the evening ones too. 

Depending on exactly where in the tank the immersion heater is fitted, you might have far less. My tank is around 150 litres but the immersion heater is mounted in the crown of the tank, not the side, and only heats about half of it.

WE have a 24kwh leaf to also store cheap rate leccy but it may only store about 12-14kwh which may not be enough to provide water heating as well as all our other leccy needs during the expensive period.

Per this calculator, to heat 180kg (litres) of water from 10C to 60C will take about 10.5kWh.

WE are lucky, two horizontal 3kw immersions, one at the bottom, one in the middle.  iBoost is on the bottom one.

Not sure how the set up that iBoosts does the top then bottom one works?  All our baths/showers have thermo-mixers but sink/basin taps are standard mixers so I guess it might be worth getting a blend valve on the cylinder out for safety - do they restrict flow when the tank is cool though?

Thinking of setting the gas cylinder temp to 45 which should be just enough for shower and then the iBoost/night rate thermostat to 70 or perhaps even 75.

Also have my second user heat pump which is going into the heating circuit, will obviously use it during the 4.5p night period but beyond that I guess it depends if the relative gas and electricity price vs the COP make sense so will use gas or heat pump depending on the outside temp and thus required flow temp.  Hmm - price ratio is 3.5 so need a cop of at least 3.2 for it to be worth using the hat pump with standard rate electricity although obliviously any that can come from the car storage makes sense.  I need a bigger battery car!

Magnitio

70sbudgie said:

I've just thought, that if you have Octopus Go, with 4 hrs at 5p/kWhr, surely that would be the time to run an immersion heater for a hot water tank. That is then cheaper than gas. 

Unfortunately, you can't be on both Octopus Go and Agile Outgoing.

Reed_Richards

70sbudgie said:

When you say water has 5 times the specific heat capacity of sand, do you mean that it needs 5 times the energy to raise the temp 1 degree? 

Yes, that's exactly what I mean.  The energy required to heat water to 60 C (from 20 C) would heat the same weight of sand to about 220 C.  Sand is not an efficient medium with which to store heat, it's cheap and it doesn't melt if heated to 600 C but you need a huge volume of it.

The Finnish "sand battery" has 100 Tonnes of sand, that's 100,000 kg.  The specific heat of sand is 830 J/kg.C.  If it's heated to 600 C and used to heat water to 50 C then the useful stored energy is (100,000*550*830)/ (3,600*1000) = 12680 kWh.  When I lived in a house heated by gas my annual gas usage was about 18,500 kWh per year so the 100 Tonne sand battery would not store enough energy to heat one UK house for a year.

That's so very little I wonder if I have made a mistake in my calculation; if so I cannot spot it.    

Heedtheadvice

Nothing wrong with your physics or maths!

Some points to consider though:

I have a fairly well insulated but detached property. My annual gas use is approx the energy figure you quote that is for heating and gas portion of hot water.

I presume Finish properties have much better insulation? Could that amount of heat warm two or even three properties there? Many finish properties have a land area that could utilise 100 tons of sand storage. Long term cost of sand is cheap, maybe £50 per tonne bulk installed and does not degrade. £5,000 over 50 years looks attractive!

Uk is however more problamatic property has lower land area for storage (non at all in many cases), poor insulation values (virtually sod all in some). So similar problems to geothermal for many on an individual basis.!

Brie

I know the OP has said that financial considerations are the most important thing but has anyone mentioned the actual cost of replacing a boiler?  

And what about the environmental costs?  Building a boiler of any sort is energy neutral, add the simple things like the actual energy use of having some gas/electric person come to remove, replace and dispose of stuff.  What do you do with the unwanted boiler?  Dump it? (hopefully not if it's that new) Recycle it?  (is there a market for them?)

Would love to have solar panels or some other more environmentally friendly power source but haven't quite figured out the hows for our place yet.

Pile_o_stone

Magnitio said:

Pile_o_stone said:

We have our PV attached to two immersion heaters on our tank. The iBoost diverts excess solar to the top immersion heater  first, where I  set the thermostat to 80C. Once the top of the tank reaches 80C, it  diverts to the immersion at the bottom of the tank. This immersion maxes out at 60C. On a good day we get a 350L tank full of water at 60C. We have a mixer valve that prevents scalding water getting to the taps.

Not only are we saving money because we are not using the gas boiler to supply hot water for showers, etc., we have had weeks where the boiler has not been used at all, which I  assume will prolong the life of our boiler.

What rate do you pay for your gas and what rate do you get paid for exporting electricity? The large difference between these (i.e. electricity export being greater than gas) and a comparison of environmental impact was the main reason for the debate.

For our 4kw array, we are on FITs, so we get paid a fixed amount for generation not how much  we export. For the additional 1.8kw array, we don't get paid for anything we export and don't receive anything for generation.

From the calculations of the OP, the difference between heating his water and earning money from exporting the electricity  sounded marginal and so in that case I'd use  the energy myself and have lower gas bills. 

Pile_o_stone

Reed_Richards said:

Pile_o_stone said:
....we have had weeks where the boiler has not been used at all, which I  assume will prolong the life of our boiler.

Or shorten its life, it's hard to know.  Some things benefit from regular use.

I'm not sure if 'use it or lose it' would apply to boilers, they don't have any gaskets that could perish if they dry out (like a car or other vehicle that needs to be kept running every so often), fuel deteriorating in a tank and clogging anything, nor will it suffer from rust as it's inside the house. The only issues I have ever had with gas boilers is the PCB needing to be replaced, and that's caused by the heat from the boiler damaging components or flexing the PCB and causing the cracks in the traces. If there is no heat in the boiler for weeks, then no expansion/contraction to cause damage.