Hillybilly heating of storage tank?

[garethg]

I have a hot water storage tank, with what I believe is a short immersion element in it (about top third gets warm at most. The main heating is through a gas system which does appear to heat the whole tank up.
However, I have a solar system which diverts energy into one or two loads. This is set to go to the immersion. Unfortunately the electricians who put in the solar attempted to remove the immersion and have caused the tank to start bending, meaning that attempting to remove the immersion (for example to replace with a 3' one) will possibly result in the tank needing to be replaced.
So I am wondering, why can't a device similar to an electric plate heater be used to add heat to the bottom of the tank when there is excess energy? (The bottom of my tank is uninsulated but concave.) Searching google there appears to have been a product by willis renewables, but they have disappeared. (further looking today shows it wasn't a non-plumbing solution).

Surely a ceramic 'plate' heated by electricity of 1KW, with an internal thermostat at say 45 degrees would raise 200 litres (of a ~300l tank?) by 10 degrees in around 2 hours. Looking at the temperatures at the outside of my tank more energy could be sunk thank this and every little helps.
(So obviously the 'correct' solution would be a dual immersion tank, however, as the tank currently is usable, the question is)
Why does such a product not exist?

 

[halfway]

http://www.plumbingproducts.ie/cylinder-accessories-ireland/3285-willis-immersion-heater.html

Would it work for you?

 

[JohnD]

If your HW is normally heated by gas, I doubt that spending money to use the Solar PV will be a paying proposition.

In my own house, the cost of gas during summer (HW) is about £7 per month, excluding the standing charge.

In the unlikely event that I halved this usage during the six summer months, that would give me a saving of £21 per year. It would take me more than ten years to regain the cost of an Immersun or similar, never mind installation costs, opportunity costs, or changing the cylinder or element.

If and when your cylinder needs to be replaced, you may as well get a large one, with an upper and lower element, and heat the lower one by solar. I don't think it is worthwhile spending more money until you need to. Use your solar for your electrical appliances and heaters, if any. With energy from electricity costing four times as much as energy from gas, it is only worthwhile to substitute for electrical use.

 

[bernardgreen]

a ceramic 'plate'

uninsulated but concave

How would you thermally connect a flat plate heater to the concave base of the cylinder ?

To produce a standard heater with a convex surface that would efficiently connect thermally to a wide range of cylinder bases would be difficult. A pad of thermally conductive gel could be used but it would still be necessary to drain and remove the cylinder to fit the heater and the gel pad underneath it.

Removing the cylinder and replacing with a dual immersion cylinder would be more cost effective than buying a heater and gel pad and fitting it under the existing cylinder.

There are wrap around heaters, but these require that a strip of the insulation has to be removed from the cylinder and then new insulation put in place over the heating strip.

 

[garethg]

http://www.plumbingproducts.ie/cylinder-accessories-ireland/3285-willis-immersion-heater.html

Would it work for you?

It might ultimately, but doesn't really meet the no-plumbing I was thinking of. If I am going down the plumbing route, then in some ways I might as well replace the tank.
Another alternative, as this just sits by the side of the tank, would be to put in a small bypass with a slow pump, taking the hot water from the top and introducing it at the bottom (a de-stratification pump). It is the same effort ultimately.

 

[garethg]

If your HW is normally heated by gas, I doubt that spending money to use the Solar PV will be a paying proposition.

In my own house, the cost of gas during summer (HW) is about £7 per month, excluding the standing charge.
.....
With energy from electricity costing four times as much as energy from gas, it is only worthwhile to substitute for electrical use.

So I realise that a bit better now. We originally purchased the panels, and the diverter with them. It works by detecting when we generate more than we use, and the puts the extra energy somewhere else. In the last 28 days, it has put ~39 units into the third of the water tank. This equates to £4.98 at 12p per unit. As you point out, gas is cheaper (although I would need to check the joules conversion/equiv. [edit: I always thought Gas was done by the m3, however SSE list it as 4.8p per KWHr]). This is however electricity we are paid for and not using. So it is a case of paying back the current outlay now.

How would you thermally connect a flat plate heater to the concave base of the cylinder ?

To produce a standard heater with a convex surface that would efficiently connect thermally to a wide range of cylinder bases would be difficult.

So this was kind-of the type of thinking I was after. The only blocker to me seems to be one of efficiency/mechanics rather than safety related.
So long as the unit was earthed and sealed it wouldn't be excessively more dangerous than a current immersion. It shouldn't generate any weaknesses into the system. Assuming that the thermostat was forcibly set low. The advantage of an immersed element is that it will almost 100% cause an electrical trip before causing a hazard should the thermostat fail, this may not be the case for an external element.

However, transferring the heat from one device to the other is a right PITA. I did look to begin with at a heat pipe convection loop. However, for an 8mm pipe the transfer rate is rated at 100W per pipe. Sprung pipes with flattened ends might be a close enough approximation, but then 10 would be needed to get a transfer rate of 1kw.

 

[garethg]

As a for-interest

The graph shows three temperature lines (in grey, scale far left), one from the top of the boiler, one from the middle where the HW thermostat is, and one from the bottom. You can see that the stratification is working quite well. The HW cut in around 7am this morning and you see the middle temperature spike up, which is about 2/3rds of the tank being heated up. Then about 8:15 two baths are taken which reduces the volume of warm water, and you can see the effects of the slug of replacement cold water entering the tank at the bottom.
The yellow line, verses the red line (measured in Watts), once the yellow which is the incoming sun power goes above the amount being used (red filled) by a 'decent amount' (~200W) then the immersion starts off (the green line, which is included as part of the red line). You can see the top temperature increase slightly as the green line puts energy into the top of the tank. This peaks at about 9:10 when the immersion thermostat kicks in, the green (and red) line drops. However due to the stratification in the tank about 2/3rds of it is now 'cool'

 

[JohnD]

As you point out, gas is cheaper (although I would need to check the joules conversion/equiv. [edit: I always thought Gas was done by the m3, however SSE list it as 4.8p per KWHr]).

Some mistake, surely?
My gas costs me 2.656p per kWh (plus 5% VAT) this year
1m³ is roughly 11.2kWh and they will calculate the conversion for your bill

The standing charge is extra and does not vary with usage, so is irrelevant to savings from reducing use.

Edit
I am told that my price for gas is unusually good.

 

[JohnD]

The graph shows three temperature lines (in grey, scale far left), one from the top of the boiler, one from the middle where the HW thermostat is, and one from the bottom.

When you use the word "boiler" I think you mean "hot-water cylinder"