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Solar panels

Straight answers to factual questions
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csearle
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Re: Solar panels

#419206

Postby csearle » June 13th, 2021, 11:20 am

Mike4 wrote:Or... can a modern storage heater compete with a water-filled radiator size-for-size nowadays?
They do seem to be getting slimmer, but I'm not convinced. My limited understanding is that they can only really be made smaller if the material used to store the heat can store more (and frankly even the bricks they typically use never seem to quite match up to expectations). Is there a material in existence that can store even more heat? Could they be designed so that they reach even greater temperatures internally and are able to retain enough to supply heat 17 hours later? Could this be achieved in something thinner and more elegant? I somehow doubt it. Chris

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Re: Solar panels

#419216

Postby Mike4 » June 13th, 2021, 11:44 am

csearle wrote:
Mike4 wrote:Or... can a modern storage heater compete with a water-filled radiator size-for-size nowadays?
They do seem to be getting slimmer, but I'm not convinced. My limited understanding is that they can only really be made smaller if the material used to store the heat can store more (and frankly even the bricks they typically use never seem to quite match up to expectations). Is there a material in existence that can store even more heat? Could they be designed so that they reach even greater temperatures internally and are able to retain enough to supply heat 17 hours later? Could this be achieved in something thinner and more elegant? I somehow doubt it. Chris


Broadly speaking, the measure of a material to look at is the "specific heat" if I think back to early physics lessons. Ceramic blocks have a 'high' specific heat which is why they are used in storage heaters but I think any improvements will be just fiddling at the margins. The specific heat of a material is often closely proportional to its density so I wonder if a steel or lead (or even gold!) core might give a much smaller storage radiator than the old low-tech ceramic blocks.

Going back to the Caldera heat battery, they are dead coy about the core material they use. I'm not sure if this is because it is just basic ceramic and they don't want to say so, or if they have developed something really good which would work well in storage heaters too, thereby undermining their USP. I also note the price of £12k on their website does not look so outrageous given it included delivery to site and installation, including plumbing it in, installing the electrical supply and obtaining the 'continuous load' permission from the leccy supplier.

The FAQs on their website are quite candid too. Unusually there are no punched pulled to questions like "how much heat does it store?" A straight answer of 100kWhrs instead of a load of marketing waffle is very refreshing!

UncleEbenezer
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Re: Solar panels

#419313

Postby UncleEbenezer » June 13th, 2021, 6:08 pm

Mike4 wrote:Broadly speaking, the measure of a material to look at is the "specific heat" if I think back to early physics lessons. Ceramic blocks have a 'high' specific heat which is why they are used in storage heaters but I think any improvements will be just fiddling at the margins. The specific heat of a material is often closely proportional to its density so I wonder if a steel or lead (or even gold!) core might give a much smaller storage radiator than the old low-tech ceramic blocks.

That's capacity, but not retention (some materials will lose retained heat faster than others). Metals have very low specific heats: steel is less than half of air, and either lead or gold far less again. Which is also why in everyday life they're cold to the touch but warm up very quickly!

Water has probably the highest specific heat among everyday substances. Your kettle takes much longer to boil than its element takes to heat, and massive (albeit brief) power consumption!

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Re: Solar panels

#419314

Postby 9873210 » June 13th, 2021, 6:10 pm

csearle wrote:
Mike4 wrote:Or... can a modern storage heater compete with a water-filled radiator size-for-size nowadays?
They do seem to be getting slimmer, but I'm not convinced. My limited understanding is that they can only really be made smaller if the material used to store the heat can store more (and frankly even the bricks they typically use never seem to quite match up to expectations). Is there a material in existence that can store even more heat? Could they be designed so that they reach even greater temperatures internally and are able to retain enough to supply heat 17 hours later? Could this be achieved in something thinner and more elegant? I somehow doubt it. Chris


The problem with "thinner" is you want to minimize the surface area/volume ratio to minimize heat loss and the cost of insulation (since thin high R-value insulation is expensive). In practice that means you want the storage element to be approximately a cube. The insulation is also going to add to the thickness. So flat panels, which could cover an entire wall to get enough volume, are not going to work well.

This will give an advantage to a single central system. A single large, roughly cubic, storage element will have a better volume to surface area ratio than distributed units. You don't have to worry about aesthetics if it's in a utility closet.

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Re: Solar panels

#419324

Postby 9873210 » June 13th, 2021, 7:14 pm

Mike4 wrote:Broadly speaking, the measure of a material to look at is the "specific heat" if I think back to early physics lessons.


Specific heat capacity is usually given "per kg". A storage heater will be more interested in the "per litre" and "per pound sterling" values.

If you compare bricks to iron ceramic is perhaps twice the specific capacity on a per kg basis, but half on a per litre basis. Ceramics will win on a per cost basis. Water is better than ceramics on all three measures, but using a liquid would add other costs.

There is also a question of working temperature. Ceramics store heat at around 250C. Using pressurized water at 250C would draw significant attention from the health and safety brigade.

Engineering uses physics, but is not physics.

(Fun fact: The best specific heat capacity on a per kg basis is probably hydrogen gas. There are several good reasons not to use hydrogen as a thermal storage medium.)

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Re: Solar panels

#419332

Postby JohnB » June 13th, 2021, 8:15 pm

Storage radiators don't run out of steam late afternoon in a well insulated house. Indeed with the correct insulation you can just use the fabric of the house for thermal mass, and PassivHaus standard buildings hardly need extra heating beyond their occupants daily activity, but its just VERY hard to insulate existing housing stock that well.

Specific heat capacities needn't be just in the solid or water phase, if you can take advantage of the latent heat of condensation you can have heat storage that falls towards the melting point and them hovers there for hours. Molten salt is used for grid scale energy storage, perhaps something equivalent could be used in home systems.

PhaseThree

Re: Solar panels

#419342

Postby PhaseThree » June 13th, 2021, 9:32 pm

JohnB wrote:Specific heat capacities needn't be just in the solid or water phase, if you can take advantage of the latent heat of condensation you can have heat storage that falls towards the melting point and them hovers there for hours. Molten salt is used for grid scale energy storage, perhaps something equivalent could be used in home systems.


I have hot water heating installed that uses this method. Energy is stored using a "Phase Change Material" (a non-toxic salt) with a melting point of 58C. Energy is stored by melting the salt using a simple immersion heater and released by passing cold water through the system to re-freeze the salt and release energy. The energy appears as mains pressure hot water.

https://sunamp.com/residential/

csearle
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Re: Solar panels

#419348

Postby csearle » June 13th, 2021, 9:44 pm

PhaseThree wrote:Energy is stored using a "Phase Change Material" (a non-toxic salt) with a melting point of 58C. Energy is stored by melting the salt using a simple immersion heater and released by passing cold water through the system to re-freeze the salt and release energy. The energy appears as mains pressure hot water.
Absolutely fascinating. Thank you. C.

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Re: Solar panels

#419354

Postby JohnB » June 13th, 2021, 10:42 pm

https://www.youtube.com/watch?v=N8aGV3Z8dOA talks about Mixergy hot water tanks, that only heat the top of the tank, and pump water around to move the thermocline, and can be programmed for standard usage patterns and overridden by app. Interesting that merely monitoring the grid frequency gives them an idea when oversupply is occuring and prices are likely to be cheap. A nice alternative to a hot water on demand boiler system. It can connect to electric and thermal solar panels.

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Re: Solar panels

#419387

Postby Nocton » June 14th, 2021, 9:05 am

I have hot water heating installed that uses this method. Energy is stored using a "Phase Change Material" (a non-toxic salt) with a melting point of 58C. Energy is stored by melting the salt using a simple immersion heater and released by passing cold water through the system to re-freeze the salt and release energy. The energy appears as mains pressure hot water.
https://sunamp.com/residential/

Very interesting, but the web site is short on facts. Nowhere that I can see does it mention costs or the storage capacity of their heat stores, so it is not clear if they can really replace an oil boiler for CH rather than just hot water.
In contrast, Caldera mentions costs and heat capacity of 100kwh with discharge rate of up to 30kw, so I can easily see that the capacity can match our oil boiler.
Our solar thermal panels can heat all of a 225 litre tank to 75-80C in a few hours in the summer and to 50-60C in the winter, when sunny. Enough for 24 hours use without any extra complexity of heat stores. With our oil boiler in the winter, HW heating is cheaper than electricity.

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Re: Solar panels

#419415

Postby swill453 » June 14th, 2021, 10:30 am

ReallyVeryFoolish wrote:(As an aside, they run washing machine and dishwasher during the night too on economy 7 electricity).

... against the advice of the fire service, on safety grounds.

Scott.

PhaseThree

Re: Solar panels

#419480

Postby PhaseThree » June 14th, 2021, 3:02 pm

ReallyVeryFoolish wrote:
PhaseThree wrote:
JohnB wrote:Specific heat capacities needn't be just in the solid or water phase, if you can take advantage of the latent heat of condensation you can have heat storage that falls towards the melting point and them hovers there for hours. Molten salt is used for grid scale energy storage, perhaps something equivalent could be used in home systems.


I have hot water heating installed that uses this method. Energy is stored using a "Phase Change Material" (a non-toxic salt) with a melting point of 58C. Energy is stored by melting the salt using a simple immersion heater and released by passing cold water through the system to re-freeze the salt and release energy. The energy appears as mains pressure hot water.

https://sunamp.com/residential/

Sledgehammer to crack a walnut? In the flats that my grown kids live in, they have economy 7 water heating that once heated overnight lasts the entire next day with perhaps a 20 to 30 minute top up at tea time. Why bother with the complexity of introducing molten salt energy storage when simply heating the water in a very well insulated cylinder does the exact same thing? What am I missing?

(As an aside, they run washing machine and dishwasher during the night too on economy 7 electricity).

RVF


The volumetric energy storage efficiency is a lot better. I have the equivalent of 200Ltrs of hot water stored in a couple of boxes each the size of a computer tower. Because the energy store is so much more compact it allows the use of very high quality insulation (Vacuum panel). This means that the standing energy losses are tiny around (0.5Kwh/24hr), the losses from a megaflow type stored water tank are 3-4 times greater.

Why is this an issue ? - I live in a very well insulated house (Passiv House levels). Uncontrolled heat input can be a big problem at certain times of year. I know of people who have binned perfectly working and well insulated cylinders for this very issue.

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Re: Solar panels

#419541

Postby 9873210 » June 14th, 2021, 6:28 pm

ReallyVeryFoolish wrote:Sledgehammer to crack a walnut? In the flats that my grown kids live in, they have economy 7 water heating that once heated overnight lasts the entire next day with perhaps a 20 to 30 minute top up at tea time. ... What am I missing?

Large enough tanks. A properly sized system will not need "topping up".

(An alternative may be to increase the storage temperature. Possibly with a mixing valve to control the temperature at the taps.)


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