Choice of intelligent switches ?

sly_dog_jonah

zeupater wrote: »

If you think about it, the temperature at the top of the tank cannot be higher than the temperature at the heating element if the contents of the tank are allowed to reach saturation ... and that's what they will do in a solar environment because it's not restricted with a timer ....

Heating elements do not raise the temperature of the convective cell significantly once stratification layers have been destroyed and true convection has started, it simply adds a little to the temperature of the water which is passing over the heated surface and, as you correctly mention, rises due to a lower density. What needs to be remembered is that the denser, cooler displaced water from the tank then falls to the bottom (where both your & my elements/coils are) and then is reheated .... if your control thermostat is at the bottom of the tank it will stop the heating process for a while, the relative densities will move cooler water to the bottom and then the thermostat will switch the power on again ... this is what will equalise the temperature at the top and bottom of the tank ... as it is your time limited heating period of ~1Hr is preventing you from seeing this .....

(snipped your quote a bit so it's not a huge quote).

When the tank reaches 'saturation' (or in our case, when the thermostat setting is reached) the tank is 'up to temperature' and the heating source (either the boiler coil, or immersion) switches off. But that does not mean that temperature is constant throughout. Liquids that have been heated will always have differences notable differences in temperature through the body of liquid unless it has been sufficiently agitated (eg through stirring).

I think by saturation you mean 'my solar thermal can't raise the temperature any more'? That's quite different to the situation where a tank's thermostat turns off the heat source. An immersion can probably keep heating up to 95C or more, but that's probably inadvisable!

As our thermostat is not at the very bottom of the tank, there will always be some cooler water below it, not cold but cooler. Even when the thermostat has tripped. Similarly there will always be warmer water above it, as the hottest 'pockets' of the water will have risen to the top of the tank. I think you underestimate the convection current during heating as well, the immersion will not toggle on and off frequently since the convection current will keep a flow of water going past the element, until such time as the passing flow reaches the thermostat temperature.

This is why the hot water outlet on a tank is always at the top, its the hottest water that can be delivered at a given time.

zeupater

sly_dog_jonah wrote: »

(snipped your quote a bit so it's not a huge quote).

When the tank reaches 'saturation' (or in our case, when the thermostat setting is reached) the tank is 'up to temperature' and the heating source (either the boiler coil, or immersion) switches off. But that does not mean that temperature is constant throughout. Liquids that have been heated will always have differences notable differences in temperature through the body of liquid unless it has been sufficiently agitated (eg through stirring).

I think by saturation you mean 'my solar thermal can't raise the temperature any more'? That's quite different to the situation where a tank's thermostat turns off the heat source. An immersion can probably keep heating up to 95C or more, but that's probably inadvisable!

As our thermostat is not at the very bottom of the tank, there will always be some cooler water below it, not cold but cooler. Even when the thermostat has tripped. Similarly there will always be warmer water above it, as the hottest 'pockets' of the water will have risen to the top of the tank. I think you underestimate the convection current during heating as well, the immersion will not toggle on and off frequently since the convection current will keep a flow of water going past the element, until such time as the passing flow reaches the thermostat temperature.

This is why the hot water outlet on a tank is always at the top, its the hottest water that can be delivered at a given time.

Hi

... and the cold water feed is at the bottom and stratificaton will separate the two and provide hot water between heating periods ..... but you'll find that there will be a noticable difference when you are heating from an inconsistent source over an extended period ....

We have thermostats at the top and bottom of the tank, so know the temperatures now, not the theory ... stagnation simply happens when the sensor at the bottom of the tank is calling for no-more heat ... at this point the temperature will be the same as the top of the tank ... this is not theoretical, it's what happens .... however, the radiation collected by the panels doesn't tend to synchronise with the heating demand too well, so with no demand to cool the circuit the panels simply heat up to a point where there is equalibrium between thermal loss and solar gain - it doesn't happen very often as we're pretty fragrant in this household, hot water being plentiful and pretty-near free :) .... the only reason we don't run the cylinder at higher temperatures is the stress it would place on the mechanical elements (pump etc) ... I'm sure that the system we have is well capable of making a cup of tea or two if the settings were pushed up a little as stagnation temperatures on the roof are usually around 60C-80C above boiling point ... :D

HTH
Z

Hobbo2006

Just wanted to post a very early update on the Immersun product.

I had it installed last Thursday afternoon so we've had 3 full days of use. First impressions are that it is every bit as professional as I'd hoped. The box and instructions are great. The fitting was very neat and tidy and nothing needed to be changed on the hot water tank.

So far it's provided

3.5kWh on Friday
5kWh on Saturday (no immersion boost needed)
2kWh today.

So 11kWh's so far over 3 days.

Using only electricity to heat our hot water, as we have warm air central heating means that I spend more on hot water than most people and I would say that the results seen so far are slightly higher than expected, although it's early days.

I think that this could save me £100 - £160 per year which puts the payback at around 3 years. (Paid £420 for the installation).

So very happy so far but looking forward to seeing the figures for winter.

sly_dog_jonah

Thanks Hobbo, what was your generation each day just out of interest? Our immersion replacement is being fitted Thursday so will be able to start monitoring the performance of the Immersun then.

Hobbo2006

Friday was 11.9kWh
Saturday 14.9kWh
Sunday 7.8kWh

We had high usage too with dishwasher, washing machine and tumble drier going daily

dan_c_73

Quick question - could you install one of these http://www.sailwider-smartpower.com/category/home-energy-monitoring-and-control-system-solar-version.htm and then control your immersion heater ?

sly_dog_jonah

dan_c_73 wrote: »

Quick question - could you install one of these http://www.sailwider-smartpower.com/category/home-energy-monitoring-and-control-system-solar-version.htm and then control your immersion heater ?

Yes, but from a quick look that setup would only turn on the immersion (load 3kW) when the solar generation exceeded 3kW.

The advantage of the Immersun and similar devices is that it regulates the power delivered to an immersion heater (or other resistive load, such as a towel rail) to ensure that only 'spare' electricity is used (it minimises export). This is better than the above system because an 'export' of 500W (or less) can be diverted to the load, rather than needing 3kW of excess generation.

sly_dog_jonah

Had our immersion element replaced under warranty this morning, so the ImmerSUN is now fired up and has already diverted 0.5kWh into the tank (which is cold as it was drained). Pretty decent day down here in cider country so hoping the water will be heated sufficiently by the immersion and the boiler doesn't have to fire up at 6pm.

sly_dog_jonah

Our ImmerSUN diverted 5kWh out of 7.8kWh generation into our tank via the immersion yesterday, but didn't get it fully up to temperature from the cold starting point (tank had to be fully drained). So during that period we would have otherwise exported 64% of our generation, and would have had to use at least 5kWh of gas to heat the water.

I have had some other thoughts about additional the boiler heating inefficiencies compared to immersion heating. Firstly the heat exchanger in the boiler has to be heated in order to warm the water looping between the boiler and primary coil in the tank. Secondly the looping water itself will also need to raised in temperature.

sly_dog_jonah

Generation was a meagre 6.1kWh today, yet the ImmerSun still managed to dump 1.9kWh of spare electricity into the tank. Even then the dishwasher was put on after lunch when the sun made a brief appearance, but we didn't see blue skies at all.

Days like these are where it will considerably out-perform other devices that rely on a threshold of generation to be reached before turning on a fixed load.