Immersun Perfomance Estimation

[The Solar King]

Have been looking at the possibility of providing customers with an idea of the likely savings they can expect from using an immersun in conjunction with their PV system. This can then be added to the PVSol economic model as additional savings.

The following is a first stab at this. Unfortunately as far as I know it is not possible to upload an xls spreadsheet, so if anyone wants a copy, please pm me. The performance data (consumption requirement, energy from inverter & direct use of PV energy) is cut and pasted out of PVSol for a 4kW system where I live. This has been extrapolated to average daily values. The hot water heating requirement is extracted from draft SAP2012. The figure of 2400kWhrs is typical for a four person household.

The interesting bit is, what is the likely utilisation of available power especially during the summer months? The chart shows average daily outputs for a system. On how many days of the month due to poor weather will the PV system fail to produce sufficient energy to heat the water? The values put in are either logic (Nov-Feb) or guess-ology (rest of the year) Does anyone have more empirical evidence from monitoring equipment to give a more accurate figure?

From what is already there it is probably safe to say a 50% contribution is highly likely, but it could well be higher.

There are some other observations that can be made. The hot water cylinder needs to be of sufficient size to hold 24hours hot water use. As with solar thermal, auxiliary back up heating should only be added in the evening after maximum solar gain has occurred. If a new tank is fitted, there will be further energy saving due to reduced standing losses. If possible (ie if there is room), always fit a new-build Part L compliant tank as they come with 50mm of insulation rather than 35mm. Also ensure there is a 3kW immersion element fitted. In the summer months there may be times when the PV system is capable of delivering more energy than the capacity of a 2kW item.

Savings in kWhrs or money need to be divided by the Seasonal Performance Factor of the heating device from which energy is displaced by the use of the Immersun. eg for a modern condensing gas boiler this is 0.768 and for oil condensing 0.754. This increases the savings.

Interested to know what others think.

 

[EWP]

The immersun has a 130 watt export threshold have you tried the SOLiC 200 with a zero threashold?

SOLiC 200 stops Electricity Meter Disk dead. - YouTube

And there's a the proof a old style electric meter stopped and held dead in its track. !

 

[Earthstore]

How many times do you need to post this?
This is post 4....

 

[EWP]

I guess to as many threads as is relivent.

The figures are all wrong in the first post as they don't take into account the 130kw threshold the immersun has.

 

[Earthstore]

Ok, Just one question, where are you in Oxfordshire?

 

[EWP]

Ok, Just one question, where are you in Oxfordshire?

Yes I am based in Didcot.

 

[Gavin A]

The immersun has a 130 watt export threshold have you tried the SOLiC 200 with a zero threashold?

SOLiC 200 stops Electricity Meter Disk dead. - YouTube

And there's a the proof a old style electric meter stopped and held dead in its track. !

didn't look stopped to me, looked like it was still exporting slowly but you moved the camera away quickly and pressed a button before it was too obvious.

Not that I'm bothered about the units exporting a bit, just pointing out that your statement isn't really accurate.

 

[The Solar King]

The immersun has a 130 watt export threshold have you tried the SOLiC 200 with a zero threashold?

The figures are all wrong in the first post as they don't take into account the 130kw threshold the immersun has.

Having checked the literature from Immersun I can find no mention of this. Could you substantiate this with empirical evidence and also clarify whether this is 130W or 130kW as you seem to contradict yourself. If you have evidence that this restricts performance of the Immersun, please provide this, and using the data I supplied suggest what limitation it places on it's potential contribution.

Simply saying the figures are all wrong is not particularly helpful. What I am endevoring to do is establish a realistic model that allows me to evaluate the suitability of a property for the use of such a device.

As you appear to be an expert in this field, your positive contribution would be appreciated.

 

[The Solar King]

Have finally found the reference to 130w start up threshold on the Immersun. This is quite low. To an extent the number of times this affects when the Immersun is not contributing will be affected by the configuration of the PV system and its start up voltages. Just checked a 2kw system here in Edinburgh and it is already producing 0.28kW. Taking in to account self use, this weedy little system should now be over the threshold to allow the Immersun to work. a 4kW system would be well on its way.

So just how 'all wrong' are the figures I have used?

 

[EWP]

didn't look stopped to me, looked like it was still exporting slowly but you moved the camera away quickly and pressed a button before it was too obvious.

Not that I'm bothered about the units exporting a bit, just pointing out that your statement isn't really accurate.

The amount of load in the house is changing all the time as is the amount being generated, so the disk will move slightly in each direction, as it samples the electricity 1000s of times a second and corrects.

If you keep the your eye on it for longer you will see it stops moves a little stops as it corrects.

A film maker i am not! :LOL:

 

[EWP]

Have finally found the reference to 130w start up threshold on the Immersun. So just how 'all wrong' are the figures I have used?

Ours is effectively Zero. Now that is low.

As for the figures being all wrong, i only remarked on the 130 watts threshold. With that corrected, i would need to look at how you calculated it in detail. It is very hard to get a 100% accurate figure as there are so many variables in each home. For instance an immersion heater often only heats the top of a tank.

I have tried calculations with the SOLiC 200, but you have make a few assumptions and not everyone will agree those are the ones they would use.

We have accurately measured the power put though to the immersion circuit, in our original test installation over a 12 month period and came out with a saving of over £260 a year vs oil, using the SOLiC 200.

 

[Earthstore]

How did you quantify the cost of oil to heat your DHW?

Sorry that should be calculate.

 

[EWP]

By knowing how much energy was put into the tank, you can compare how much oil you would need to do put the same amount of energy into the tank then multiply that by the average cost of oil for the period. Including a 95% average boiler efficiency for the boiler from memory.

 

[Gavin A]

95%?

You should be going into boiler manufacture if you can get that efficiency from an oil boiler

 

[The Solar King]

These calculations with regard to oil all seem a little odd. If oil is 60p/litre and you get 10kWh per litre, thats 6p/kWh. Divided by the seasonal performance factor of a high efficiency condensing oil boiler gives you approx. 7.95p/kWh. A £260.00 annual saving would mean 3270kWh of fuel saved. As this is only going to be a partial contribution to the total hot water heating requirement, was this a hot water tank or a swimming pool? Was it even insulated?

As I mentioned in my original post, the figure given as a hot water requirement comes from draft SAP2012, 2400kWh being not untypical for an average household, based on a daily requirement of around 160 litres at a usage temperature 45deg C. Even getting a 60% contribution from a proportional controller would mean a saving of 1440kWh worth £114.48.

On the same basis a £260.00 saving would mean a hot water heating requirement of 5450kWh and a daily hot water consumption of 320 litres, enough for 8 people. I should add that the 40 litres a day figure is higher than that in SAP2005 (25litres a person plus 36litres overhead) or SAP2009/2012 which uses a totally bonkers formulation based on floor area.

Hot water consumption and hot water cylinder sizing can start to become a black art, on which there is little agreement. There are even calculation differences in different MCS standards depending on technology. This is why objectivity is so important, and the real purpose of me starting this thread. I do get a little hacked off when somebody tries to hijack it for the purpose of promoting their own product.

 

[EWP]

sorry, not meanting to hack you off.


It takes 9,5Kw to heat 180l of water by 45deg multiple that by 365day and you cab see you need about 3500kwh to heat hot water in many houses. Not sure how the government got to a 2400 watt figure from as it is not my experience. We personally run our boiler at 55 deg for instance.

 

[methley]

Yes but you don't raise the temperature of the whole 180L by 45degC every day! This is where the lower figure comes from.

 

[The Solar King]

sorry, not meanting to hack you off.


It takes 9,5Kw to heat 180l of water by 45deg multiple that by 365day and you cab see you need about 3500kwh to heat hot water in many houses. Not sure how the government got to a 2400 watt figure from as it is not my experience. We personally run our boiler at 55 deg for instance.

The assumptions you are making mis-understand those used in SAP and other calculations of hot water usage.

SAP is based on a usage temperature of 47degC which is on the high side. Most hot water is used for showering. Get in a shower at 47 deg and you will get out again bloody quick! Most people shower between 38 and 42 deg. Anything above this is pretty uncomfortable. Humans are very sensitive to contact heat above body temperature. There is also an inherent assumption that water is heated to 60deg for Legionella disinfection.

Calculation of daily hot water consumption is still open to debate. The calculation in SAP2005 was based on 25litres a person plus a 36litre overhead. Your 180litres is enough by this calculation for 5.76 people. SAP2009 and draft 2012 both use a calculation based on floor area. This came about from a survey that showed the average occupancy of a domestic dwelling is 3.1 people. An algorithm was then built to reflect this. Whilst this may be OK for the general purposes of SAP, it is completely bonkers for use in planning/designing domestic heating systems or renewable heating systems. The Draft of the revision of MIS3001 'Requirements For Contractors Undertaking The Supply, Design, Installation, Set To Work, Commissioning and Handover of Solar Heating Microgeneration Systems' goes some way to address this by allowing use of the SAP2005 formula if average occupancy is known. Practical experience suggests that this still understates usage. I personally work on 40 litres per person with no overhead.

These parameters can then be input into the SAP hot water energy requirement. This is seasonalised to take into account variance in water inlet temperature and usage. Using the parameters you state gives a figure of 3450kWh. However, using the SAP2005 usage formula and the SAP standard temperature gives 2145kWh. Using what I consider more realistic parameters (4 x 40ltrs @ 45deg) gives 2387kWh or to more closely align with SAP (4 x 40ltrs @ 47deg) gives 2523kWh. As such you will see that the given figure of 2400kWh is entirely justified.

This is based on usage and not storage volume. This can make tank sizing a black art when taking into consideration heating to 60deg (required in solar thermal standard), standing loss from tank, and pipe losses. As alluded to the simple act of replacing a tank leads to increased savings due to reduced standing loss.

Blithely selling or fitting an Immersun or other proportional controller without taking in to account the factors outlined in my original post is doing no one any favours. The purpose of this thread was to generate constructive comments on how to better what I had already put together to allow evaluation.

 

[Gavin A]

IME usage is very dependent upon whether the system is mains pressure / pumped for showers, or open vented just pressured from the header tank.

If it's from an open vented system then the shower flow rate is probably in the 2-4 l/minute range, whereas mains pressure will be at around 8-12l/minute, so a mains pressure shower will probably use around 3 times as much water as a vented system, and showering is the main hot water use in most houses.


These figures are from memory, but IMO you really ought to factor this in to any calcs on hot water consumption levels as with either solar water heating or immersun you ought to know what system you're working with.

This is partly why I generally advise customers for solar water heating to stick with open vented if they have it already, from an energy saving perspective there's not much will increase your energy consumption faster than switching to main pressure showers... other than actual electric showers.

 

[The Solar King]

Wise words Gavin.

It underlines the need to take holistic approach when altering hot water systems. I tend to favour unvented systems due to their many advantages. However, we measure shower flow rates post installation and advise on upgrading to modern mixers and shower heads designed for use with unvented systems to overcome the potential problems you highlight.

If a property already has an vented system, keeping it the same does make installation more straight forward.

As with so many aspects of renewables, the devil is in the detail. To serve customers well, knowledge and experience are paramount. We all see too many examples where at best an installer or sales person has only had their eye on the profit, not the interests of the customer.