- The purple line shows the use of electricity (excluding gas replacement) by month. It’s relatively stable through the year, although it does rise in the autumn as my daughter started school and so my electric car mileage increased.
- The green line shows the output from the solar panels. From April to August (5 months) solar panel output exceeded usage giving the potential not to buy electricity with sufficient smart capability – be that electricity storage or alignment of consumption with availability.
- The red line shows import of electricity from the mains. It tends to be the reverse of the solar panel output. It’s never zero indicating potential (at cost) to improve smart usage. Solar power is a more significant energy source than imported power from March to September (7 months).
- The blue line shows diversion of surplus electricity to water heating as gas replacement.
- The turquoise line shows export of electricity to the grid. This occurs when there is insufficient energy smart resource available to store or self-use the surplus power. Export amounts to about 12% of total solar panel output. While this potentially free energy, the economics of storage or smart controls make using this remainder increasingly costly from an investment perspective. In my case this surplus occurs on particularly sunny summer days when the electric car is not at home or is already fully charged – which might be vacation periods when the house is unoccupied for example.
Here we have the system in action earlier today. With 1542 Watts coming from the solar panels, the house (including the PowerVault storage battery) is running at a maximum of 1097 Watts, with the balance of the available power controlled by the ImmerSUN- 410 Watts to water heating and at the point of this snapshot 35 Watts into the grid. At this moment that’s 97% of generation used as such self-consumption and 100% of energy being consumed coming from the solar panels.
if the ImmerSUN had priority then it would have taken all the available power leaving nothing for the PowerVault storage battery.
In a prior post I described the use of current clamps to prioritise smart loads that are enabled by surplus solar power to maximise self-use of this ‘free’ electricity. That’s free in the sense that a deemed export tariff doesn’t pay any more for an extra kWh exported, or pay any less for an extra kWh used, and so the marginal cost of using that (and every other) kWh is zero. In that post three current clamps were visible in the picture – though I described only the function of the right-most.
In fact my home currently has 6 current clamps which is probably more current measurement than the substation that supplies my area. The six clamps are as follows:
|Solar panel output||Report (1)||-||Report (4)|
|Immersion heater||Control||Measure sum (3)||-|
|Import / Export||Measure (2)||Report (5)|
|Battery In / Out||-||Control||Report (6)|
I’ve tried to distinguish between their functions as follows:
- Control – is an output current actively controlled by a device so there’s generally no need to measure it with a clamp.
- Measure – a clamp whose output is analysed automatically to create a control action such as divert more or less power to some device.
- Report – it’s just reporting something for the purposes of understanding, but it’s not used to directly control anything.
- The optional ImmerSUN monitoring package adds a clamp to measure the output of the solar panels which then enables the charts and self-use calculations that I’ve used before to illustrate system behaviour.
- The ImmerSUN fundamentally operates by measuring export via this clamp and then responding to minimise that export.
- The operation of the PowerVault uses this clamp. In most installations that’s simply around the live of the incoming supply, but for me it’s also around the live feed to the immersion heater to set priorities.
- I also have 3 data loggers at my home to provide a year’s data for UKPN around self-usage so that they can assess the impact on the grid from large scale battery adoption. Each logger measures one of the fundamentals for battery behaviour: output from the solar panels, ..
- .. import to / export from my home, ..
- .. and current to / from the PowerVault. From those three you can infer what is being used by my home in its entirety, but not how power is divided between (for example) car charging and water heating.
This week I was discussing how to maximise the benefit of self-consumption with an installer. The issue here is, where one has multiple independent systems (such as a battery and a water heater) each looking to use any surplus self-generated electricity, how does one set the priorities of the devices or is it just a lottery which gets the surplus power first? In my own case, for example, I recognise that the surplus invested in my battery storage is better value than investing that surplus in hot water as electricity is considerably more expensive than gas.
I have previously written on this subject Prioritising the battery, but the installer was unaware of my solution and recommended an alternative which I consider flawed. The solution recommended by the installer is to enter the maximum charging power of the battery as the export threshold in the ImmerSUN controller. This does indeed create headroom for the battery to charge, but I believe does not use the ImmerSUN to best advantage. Let me use a table to contrast what might happen at 3kW generation with both the installer’s and my own solutions:
|Installer's solution||My solution|
|Total load / total generation||3.0 kW||3.0 kW|
|Baseload of house||0.2 kW||0.2 kW|
|ImmerSUN Export threshold||0.8 kW||-|
|Battery charging||0.8 kW||0.8 kW|
|Balance for water heating||1.2 kW||2.0 kW|
The installer’s solution will potentially always waste an amount of power equal to the maximum battery charge power / ImmerSUN export threshold. My solution doesn’t do this, although there is an aspect of immersion heater operation that may or may not concern you.
In my scheme I modify use of the current clamp for the priority device – the battery for me. Such current clamps have the property of summing the current in all the cables which they surround and, as you’ll see from the pictures, my control clamp for the battery surrounds both the incoming power cable from the grid and the outgoing cable to the immersion heater.
The effect of this arrangement leaves the ImmerSUN acting normally – it sees any export and diverts it to hot water.
The current clamp for battery however sees the sum of the export current and the current diverted to the immersion heater, so it doesn’t matter how much the ImmerSUN diverts (and thus reduces the export) – the battery ‘sees’ what would have been exported without the ImmerSUN and responds according.
The ImmerSUN clamp then ‘sees’ the export current drop as the battery charge current increases and thus reduces its own current proportionately.
To gain access to both the incoming live mains supply cable and the outgoing live feed to the immersion heater, my current clamp is located within the consumer unit. It’s the blue clamp on the right around the larger incoming cable and the smaller brown cable for the immersion heater. You need to be a competent person to work inside a consumer unit. (There are two further blue clamps to the left but their function isn’t relevant to this post.)
The aspect of this that may concern you (but doesn’t bother me) occurs if you want to run the immersion heater from stored electricity rather than just using surplus from the solar panels immediately. If you turn on the immersion heater using the Immersun (known as ‘boosting’ in ImmerSUN vocabulary) then the current from the consumer unit to the immersion heater is equal and opposite to the current from the grid to the consumer unit, which is summed by the battery’s current clamp to zero and thus the battery neither ‘sees’ that current nor discharges to meet that load. To me this isn’t an issue as I consider transferring energy stored in one device (a battery) to another storage device (a hot water cylinder) is poor practice; and in any case the battery doesn’t have enough power capability (maximum 1.2 kW) to run the immersion heater at full load (3 kW) so you’d always be importing at least 1.8 kW to run the immersion heater.
As a final note – it is critical to orientate the two cables in the battery’s current clamp correctly for the system to work as intended. Current from the consumer unit to the grid, and from the consumer unit to the immersion heater, must flow in the same direction through the battery’s clamp.
Around 25 years ago I purchased my first washing machine. Smart capability was optional – it came in the form of an electro-mechanical timer which allowed the operation of the washing machine to be shifted into the Economy 7 hours with absolute confidence.
Recently I replaced said washing machine. The new washing machine (my second) isn’t compatible with external timers, instead it offers a pushbutton which delays the start time by one hour every time that it is pressed. However that doesn’t give absolute confidence of the cheapest energy costs because these days it’s a lottery between will the day be sunny (and thus free solar electricity) or will Economy 7 be cheaper?
Where are the smart controls? Where are the washing machines that link to smart meters to automatically operate on the cheapest energy? It will potentially be another 25 years before my washing machine is smart – if I live long enough to buy my third.
(And in case you’re thinking that you can use some sort of smart socket to control the washing machine, turning on the power externally will just get you to the point that you can configure the washing machine program and options, not start a washing cycle.)
In combining both battery storage and water heating from surplus PV output I was keen to ensure that the two solutions worked together well. I consider that there are potentially two issues with such an arrangement:
- Firstly, the system may go unstable. With two independent systems each trying to absorb the same PV output, they may overreact both claiming the excess and putting the house into import, then both back-out in response to the input restoring the system to export, and then repeat the cycle endlessly etc. In control system terms you might consider that it had too much gain.
- Secondly, even if stable, the resulting split of diversion into two systems (battery and immersion heater) may not be optimal. In my case a kWh of stored electricity is worth more than four times a kWh of gas saved – so I’d clearly want to prioritise battery charging.
I considered various relatively complex solutions and then thought of one of great simplicity which required no additional parts.
My solution is simply to install the current clamp for the high priority device around both the incoming power cable and the cable to the lower priority device. The nature of a current clamp is that the current it reports is the sum of all the cables that it encircles. Thus, by placing the battery clamp around both the incoming supply and immersion heater feed, it doesn’t matter how much of the potential export has been diverted to the immerSUN, the battery clamp still sees what would have been exported were the immerSUN not active and gradually takes that power. The immerSUN in turn sees the reducing export / increasing import on the meter tail and backs off to restore the balance.
My installer had previously combined immerSUN and battery without such an arrangement and advised that the slow response of the battery compared to the immerSUN remained stable, but as the immerSUN responded most quickly it was prioritised.
However the above graph from the immerSUN shows my battery successfully tracking the PV output, so my arrangement is working successfully.
The only downside, which for me isn’t an issue, is that the immersion heater becomes invisible to the battery. Even if the immersion heater is operated via an immerSUN boost when no PV output is available, the incoming mains current is entirely matched by current in the immersion heater feeder in the opposite direction, and the battery’s current clamp sees nothing. Thus the battery cannot support the immersion heater because it doesn’t know that the immersion heater is drawing any current. I however don’t see this an issue since we use gas to heat the water when the ImmerSUN’s diversion is insufficient. Even if we did use electricity the battery is not able to deliver sufficient power to run the immersion heater at 3kW.
In my installation the battery current clamp is actually inside the consumer unit to minimise the additional wiring to allow the clamp to encircle both the incoming mains and the immersion heater feed.