Choice of intelligent switches ?

sly_dog_jonah

Hobbo2006 wrote: »

wow.... was easier for me to calculate how many units we were using.... I just slapped an energy monitor round the cable!

wouldn't your cold water be a lot colder than 19C though?

I measured the cold water temperature yesterday and it was 19C, but in winter it will be a good few degrees less according to this source. It also states that boilers are much less efficient at hot water heating alone, which might account for the relatively high gas kWh's needed to heat our water.

Cardew

S-D-J

This report indicates the average household consumption of Hot water is 122 litres.

http://www.bsria.co.uk/news/est-water/

There are so many unknown factors that it is very dificult to agree or disagree with your assumptions.

Much will depend on your lifestyle pattern. For instance on a day when you run out of hot water, you will put on the CH and there will inevitably be warm water in the tank next day.

My main reservation would be the assumption that the HW tank has an almost infinite capacity to 'absorb' generated electricity.

Subjectively I would have thought 1,000kWh pa would be a ball park figure - but you may be correct this it will be a lot higher - time will tell!

Hobbo2006

sly_dog_jonah wrote: »

OK you hooked me Cardew, I couldn't resist taking a look through my figures again to come up with a more considered figure.

PVGIS generation estimate for our system is 3060 kWh/year. In fact we should surpass that figure tomorrow with over 7 weeks left until our anniversary. That's an average of 8.4kWh/day generation according to PVGIS, but actually we're running at 9.5kWh/day since installation (+13%).

Assuming our electric consumption hasn't changed since installation, then the reduction in our import is solely due to our generation that has NOT been exported. I don't have any Q4 2010 import figures to compare with as we only moved into the house late 2010, but I've done a comparison of the figures for period Jan-Aug for this year and last year (the only complete months I have full comparable figures for):

Avg Import pre-solar: 10.77kWh/day
Avg Import post-solar: 7.13kWh/day (34% down)
Avg Generation: 11.01kWh/day
Avg Export: 7.37kWh (assuming consumption is unchanged. If consumption has dropped then the export figure would be higher)
Avg Export: 67%

The export average peaked in May at 12kWh/day but in January was <1kWh/day (and was probably worse in Dec). As I've omitted Sept-Dec, when Export will be less than average I've estimated that the annual export average would be about 6.5kWh.

So that's what is theoretically available to dump into the tank each day, maybe slightly more as we've probably cut our usage down since getting the solar installed. Also this is probably an underestimate of our export as in 2011 year my wife and baby were at home whereas in 2012 they are at work/nursery. So electricity consumption was almost certainly higher in 2011 which perturbs the above assumption about export somewhat.

Our hot water is currently heated by gas twice a day to 45C during 2x 1hr programs in the morning and evening. The thermostat is positioned in the lower 1/4 of the tank, so most of the water will be hotter than that (because it is hottest at the top), probably 48C is the average in the tank when hot.

Assuming the cold water is coming in at 19C our gas consumption of ~14kWh (when central heating is off) for a 90% efficient boiler would heat around 375 litres of hot water to 48C. That ignores the losses between the boiler & tank (although the distance between is quite short) and any losses from the tank itself (rated ~1C loss per day). Perhaps the 90% figure is based on heating hot water and the central heating at the same time?

I'm sure our actual usage is much less than that though, and here's why:
Our total water usage per day (hot & cold) is only 360litres (metered). The only use of hot water is in baths/showers and from sinks, as all the applicances use cold water. I find it hard to believe then that we would use more than 200litres of hot water per day, let alone 375litres. I can only attribute the above figures to losses between our boiler and tank.

So, lets go back to the export electricity. Assuming the immersion is 95% efficient (which is conservative), then heating 200litres of water per day to 48C would require 7.1kWh. That's less than our average export, so we shouldn't have a problem fulfilling that on the average day. On very good days of course there will still be some export once the tank is up to temperature.

Of course some days the spare power will be only be enough to part-heat the tank and we'll need to top up with gas heating, even if only a little. The question is how many such days will there be, and how much gas would that require? Only time will tell, but I think the Immersun will probably save us 20% of our gas hot water usage in Nov-Jan, 50% in Oct & Feb and 80% for the rest of the year (even that is conservative I think), or an average of 60% per month. That would be equate to 3060kWh of gas each year, which will save us £113 a year (based on EDF Blue Price promise prices inc VAT).

I realise of course that the gas kWh savings is much greater than the electricity we currently export, but I think that is down to the inefficiencies of the boiler rather than a flaw in my calculation. I welcome any pulling apart of the above logic though!

PS you'd need to get the water over 60C to be safe from legionella bacteria.

sly_dog_jonah

Hobbo2006 wrote: »

PS you'd need to get the water over 60C to be safe from legionella bacteria.

That's why we are going to have a blending valve fitted, so we can ramp up the immersion thermostat but not the temperature delivered to taps. At the moment Mrs SDJ insists the thermostat shouldn't be higher than 45C because of our 2 year old, and I'm inclined to agree.

Cardew, thanks for the links and your thoughts. We never have to 'boost' the hot water outside of the normal 2hr/day heating program.

Once we have the blending valve fitted, the plan is to keep the gas thermostat quite low, but the immersion thermostat much higher. Therefore if the immersion has been on during the day, the boiler will only fire up in the evening if the temperature hasn't reached the lower thermostat's setting. Our morning water requirements pre-empt sufficient solar radiation in the day for the majority of the year, so the idea is to ensure there is sufficient hot water each night.

zeupater

sly_dog_jonah wrote: »

.... Assuming our electric consumption hasn't changed since installation, then the reduction in our import is solely due to our generation that has NOT been exported ....

Hi sdj

Depending on how energy aware you were before the installation of the pv that could be quite a large assumption ....

We put considerable effort into energy reduction in our property for a considerable time (years!!) before installing our pv resulting in a saving of well over 1000kWh/year, a good proportion of which was simple awareness (standby etc) .... if this had been done post pv it would be very easy to believe that it was a pv saving, especially the awareness .... this is why I'm so sceptical about many of the claims of high self-generated energy usage based on year-on-year metered imports as I'm sure that many haven't accounted for this ...

sly_dog_jonah wrote: »

Our hot water is currently heated by gas twice a day to 45C during 2x 1hr programs in the morning and evening. The thermostat is positioned in the lower 1/4 of the tank, so most of the water will be hotter than that (because it is hottest at the top), probably 48C is the average in the tank when hot.

If you heat the water at the bottom of the tank you will be heating the stratified cold water at the bottom (cold water entry) and the heat will build at the bottom until it's as warm/warmer than the layers above, then convection will take the warmer water to the top and the (slightly) cooler water at the top will sink within the convection cell .... at the end of heating the water will be the same at the top of the tank as the bottom (we have multiple sensors on our ST tank). When you draw hot water off then the bottom will cool rapidly and reestablish the stratified layers ...

sly_dog_jonah wrote: »

.... and any losses from the tank itself (rated ~1C loss per day).

I'd certainly look at this ... tanks are usually rated to lose heat at somewhere around 2kWh/day at a standard temperature. The warmer you store the water the higher the heatloss ... our overnight heatloss is far higher in terms of degreesC after a really good day's solar input ...


HTH
Z

zeupater

Cardew wrote: »

... My main reservation would be the assumption that the HW tank has an almost infinite capacity to 'absorb' generated electricity ....

Hi

I agree .... don't overestimate the amount of energy which can be dumped into the tank ..... our (SW) thermal array was only supplying heat for the early part of the afternoon and then shut down .... this would be exactly the same if the pv was supplying the power .... a couple of days ago we only got ~2kWh.t all day ... this is why you generally have larger tanks as well as heating to higher temperatures on solar thermal systems - just to smooth demand to availability ....

If s-d-j is currently heating the cylinder twice per day then unless the temperature differential (cold in to hot out) is doubled then the tank will still need to be heated twice/day, but I would have concerns that the timing of hot water draw wouldn't have a good fit to the generation pattern so the current heating method would likely be needed so satisfy one heating cycle ....

HTH
Z

sly_dog_jonah

Thanks for the points Z.

The tank is a tribune HE indirect 210Litre with standing heat loss of 1.85kWh/day. This is apparently measured at 65C, and corresponds to a loss of 7.55C for 210litres if left standing for 24hrs. So that's approximately 4C between solar radiation periods on average. Not sure where my 1C figure came from, but that was clearly quite wrong!

http://www.range-cylinders.co.uk/pdfs/sales/tribune.pdf

Although the gas heating is currently timed for 2x 1hr periods per day, it usually doesn't take long for the thermostat to open and the boiler to switch off.

The gas thermostat is positioned at the same height as the indirect coil, and this is also where the immersion and it's thermostat is positioned. I'm familiar with the principles behind stratification, but have to correct you on the comment that when the thermostat opens, the temperature at the thermostat height will be the same as the very top of the tank. The hottest water will always be found at the top of a tank as it is less dense than any colder water, so rises to the top. The temperature difference may not be much, but it will not be neglible.

What is the maximum temperature drop you've seen on your solar store overnight?

Regarding energy awareness, we have indeed been very conscious both before and after moving to this house. I've no doubt that the solar has focused our thoughts even more, but this has more so been to shift usage to daytime periods where possible. We've not replaced appliances, as they are already very efficient (and new). As I said before though, if our consumption has reduced then that means that my estimation of export is too low, ie there will be greater potential to satisfy our water heating requirements via the Immersun than I've calculated.

The immersion thermostat is going to be set at 65C or higher, so has the potential to approximate double the energy stored compared to the current 45C gas thermostat setting.

zeupater

Hi

Just read the post on my phone so fired up the laptop when we got home as it's easier ....

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 .....

Overnight the water which is in direct contact with the skin of the cylinder (even if highly insulated) will cool and migrate to the bottom, creating a small temperature gradient between the core of the tank and the skin which will effectively maintain stratification in the core ... however the cooled water will migrate to the bottom which will look like a higher temperature loss at the bottom of the tank - to this, due to the higher water storage temperature, you will experience heat migration into the cold water feed and into the central heating coil which will further cool the bottom of the tank .... you'll therefore find that the temperature difference between the bottom and top of the tank will be due to heatloss ....

With a variable load immersion system you should see very little difference to our system as we usually pump somewhere between 1kWh.t & 3kWh.t into our cylinder, depending on irradiation and can be quite a bit higher if the cylinder is cool, so the only difference should be a reactive time as our thermal system needs more time to react to intensity changes, but with this being both positive and negative it's probably almost negligable ....

When our system cut out today the temperature at the top and bottom of out tank was equal at 77C (summer setting temperature) and had been held at that for a while before the panels finally entered the stagnation stage (a pretty rare event this year) ... the temperature at the top of the tank has fallen by 5C and the bottom by 13C with very little (/hardly any) hot water drawn off over this period (approx 12 Hrs) ... I would estimate that the average core temperature over the whole tank would be pretty close to the current 72C at the top ....

HTH
Z

zeupater

sly_dog_jonah wrote: »

... What is the maximum temperature drop you've seen on your solar store overnight? ...

Hi

Current temperatures (10:45) are 68C top and 59C bottom, so that's over approx 21Hrs. We're heating at about 1.4kW.t at the moment but meter showing less than 1kWh.t so far this morning (due to a clear sky and a SW aspect) .... this should give you enough to calculate the likely real-world heatloss on your tank at higher temperatures ....

Time to cool the bottom of the tank now with a couple of showers .....

HTH
Z

hp4020

We installed http://solarimmersionheaterswitch.co...FcfKtAodCGkAgg today.

Father in law (Part P certified Electrician) and myself, it took around 20 minutes and does what it says on the tin.

Immersion was set to switch on at 1500w and thats what happened, took the kettle into the garage, each time I switched it on power was instantly turned off to the immersion.

The switch has two leds, a 3mm and a 5 mm led.

If both leds are red you are exporting and sending to the immersion.

If the small led is red and the large 1 green, then you are exporting but not generating enough to power the immersion.

If both leds are green you are importing and not sending to the immersion.

I did notice that both leds were red indicating that I was exporting and sending power to my 1kw immersion, however my conumption meter told me the house was only using 400w, then the penny dropped, the tank was up to heat and so although the switch was still diverting power to the immersion the thermostat had kicked in and the element was not heating ( It is set to 65 degrees )

If I raised the thermostat threshold, then the house consumption rose by 1kw.

Had generation ranging fromm 400w to 3800w as clouds moved in and out, pleased there wasnt constant relay chattering and system responds very quickly almost instantly to dips in generation and spikes in house useage, would recommend.