Category Archives: Energy Smart

Cooling it down (or not)

As regular readers will know, I shift my major loads (including battery, car and water heating) around to exploit the cheapest periods on my tariff. Yesterday I had a conversation with someone who was very keen to do the same with a fridge. I’ve never considered doing this a fridge on two grounds:

  • Is it safe to mess with a fridge or freezer if that risks compromising the temperature and thus the safe storage of food?
  • Is there enough saving to be worth even considering this?

Although I don’t explicitly measure the power consumption of the fridge, we can make some assumptions from data that I do have.

According to the US Department of Health, a fridge can be safely left for 4 hours during a power cut, but you should avoid opening the door. That period at least corresponds to the duration of the early peak (and thus potentially the highest cost power), but I still don’t think that this would be worth the effort.

Overnight electricity consumption #1

The above illustration shows my home drawing 168 Watts overnight. That load is all the standby loads in the house (TV, DVD, oven, microwave etc), assorted phone and iPad chargers, cordless phone power, WiFi router, alarm clock, central heating controls, smart plugs and the fridge-freezer.

Overnight electricity consumption #2

For a couple of periods overnight the consumption dips to around 114 Watts, a difference of 54 Watts. I can think of nothing on my list of loads that might cycle on/off except the fridge-freezer so potentially that’s the near-continuous 54 Watt load. It’s conceivable of course that the fridge-freezer load cycling between two levels, but the lower level cannot be more than 114 Watts. 114 Watts is 4 p/hour at my highest electricity cost of 35 p/kWh or 1 p/kWh at my average rate of 9 p/kWh. This doesn’t seem a worthwhile saving to me.

Different perspectives

The above images show four different perspectives on the same day of data (April 24th) from different sources within the home.

Smart Meter HAN

Firstly, the Smart Meter HAN image shows bought electricity to the home. Each smart meter sits on a Home Area Network (HAN) which is how the In-home display provided with the meter gets its data. The in-home display is an example of a Consumer Access Device (CAD). In my case I also have a Hildebrand Glow Stick as a CAD. The Glow Stick, which looks something like an oversized USB stick, also connects as a CAD to the smart meter allowing the meter to be read. An associated app, Hildebrand’s Bright, allows the Glow Stick to be read via the cloud. In principle the Bright app can display either energy in kWh or cost, but in my case can only display energy in kWh as Octopus don’t push the price data into the smart meter so energy cost always reports as zero. The data is presented by the minute.

Smart Meter WAN

Secondly, the Smart Meter WAN image shows the same data but from the perspective of the Wide Area Network (WAN) whch connects the smart meter to the energy retailer (Octopus for me). This half-hourly data is reported via the Octo Watchdog app. The data reported is cost per kWh (the blue line) and energy consumer / kWh (the red columns). The energy data in the red columns follows that of the red line in the prior illustration but in lower resolution (half-hourly versus minute-by-minute). You can clearly see most energy being bought when the price is lowest.

Powervault

Thirdly, the Powervault image shows grid in/out and battery in/out. The green grid-in line mimics the red data from the above images. The battery in/out data is solely visible in this image. The resolution is good enough to see shorter events like kettle boil cycles.

Immersun

The final image, from the Immerun, is probably the most useful although it lacks energy price and hides battery in/out within the House data (hence ‘House’ being zero at times). The immersun alone reports output data from the solar panels and diversion to the immersion heater. It also lumps the car charger energy within ‘House’, indeed none of these views can directly report the car charger behaviour although its the dominant energy consumer here.

I’m planning to construct my own view showing all the different prices of data together in one place. I already have access to:

  1. The Immersun data via the same API called by their app. I came across a blog post that described how to do this.
  2. The Powervault data API (I only have a control API at the moment) which should give me battery in/out (at least I’m on a promise of the API at the moment).
  3. The Hilbebrand data which duplicates the Powervault Import/Export at the moment, but has the potential to provide independent monitoring of my car charger.

In principle then that would leave me able to report 3 x energy sources (grid, panels and battery; of which grid and battery would be bi-directional) and report 3 x energy consumers (car, water heating and home).

Home Energy Management System (HEMS) hardware

My current energy management arrangements are designed to maximise use of the output of my solar panels for lowest energy cost by diverting any excess to PowerVault storage system, car charger or immersion heater.  I can also manually configure the PowerVault and ImmerSUN to minimise costs of bought energy from the grid (I get 7 hours of cheaper night time electricity) by setting time periods for charging.

However as I move to a smart meter and smart tariff then I’m looking to start automating the selection of when to draw power from the grid based on costs that change half-hour-to-half-hour and day-to-day.  The hardware to achieve this is illustrated here.  To the right is a Raspberry Pi – a small computer with a wide range of connectivity – and to the left is a module that sets on top and has four relays able to switch mains loads, although at the moment I only anticipate needing 2 of them.

One of the relays will switch the boost input to the ImmerSUN to enable water heating, potentially when electricity is cheaper than gas, and a second relay will operate the transmitter that turns the car charger on alongside the ImmerSUN’s relay output during the cheapest available energy times.

The image to the right shows the timers that can be used to enable the ImmerSUN outputs to draw power from the grid.  I never use this for water heating as currently gas is always cheaper than bought electricity, but do use it to more or less effect seasonally to charge the car from cheap night rate power when there isn’t enough daytime solar.  For the new HEMS I plan a table of 7 days specifying the number of hours required for each output and let the HEMS find the cheapest half hours to deliver the total hours required and enable the charger or water heating as required.

 

 

Smart meter and tariffs

Earlier in the week I received notification from my electricity and gas supplier that my 12 month contract was coming to an end.  I did my usual search for the best value Economy 7 tariff but drew a blank – everything including renewal with my existing supplier was rather more expensive than I’m paying now – so I decided to be rather more adventurous.

My decision was a significant change – not just a move from Economy 7 to a smart meter, but also the addition of a smart tariff (one that changes rate multiple times per day), and indeed my chosen tariff is dynamic so it potentially changes every half hour and day-to-day.  As I don’t yet have a smart meter then I’ll continue on Economy 7 until the meter is replaced, but then adopt the dynamic tariff.  With flexible loads like electric car charging and my storage battery then I should be well equipped to make the most of such a tariff.

On the dynamic tariff rates are published each day at 4:00 PM for the next day.  Some times (not very often!) prices even go negative so one is being paid to consume.  At other times electricity is relatively expensive (early evening’s principally) but the battery should help me minimise purchases during such times.

I’ve already checked the battery storage and it has the ability to be programmed very flexibly around different electricity prices at different times of day so that it doesn’t just absorb surplus solar but charges at lower cost times to discharge at higher cost times.

I also want to explore opportunities to automate the response to tariff changes – potentially linking storage battery, car charging, and water heating to tariff as well as self-consumption.

Mid-year thoughts

Summer is definitely upon us now as we enjoy the glorious summer weather.  Disappointing weather earlier in 2018 has given way to two record months in May and June which yielded the highest monthly outputs for their respective months since the system was installed back in 2015.  Some days we buy no measurable electricity or gas  (given that the electricity meter has a resolution of 1 kWh) depending on what the need to charge the car.  If the car is at home then I can fully charge it from the solar panels, whereas if the car is at away from home during the day then I may need to give it some charge overnight.  When charging overnight I have been tending to charge for the minimum number of hours up to 7:30 AM when I typically leave home on a weekday – that pattern provides for mostly Economy 7 Energy from the grid less whatever comes off the solar panels from the rising sun less whatever might be left in the PowerVault from the previous day as illustrated below:
The green ‘hill’ from around 3:30 to 7:30 AM is created by car charging.  Normally this would be seen as a rectangular block as the car charger effectively runs at a constant 10 Amps (2.3 kW) through the operating period.  However in the illustration the charging event (at least in terms of power drawn from the grid) seems rounded at both the beginning and the end.  At the beginning of the car charging period the mustard ‘Device Power Out’ curve shows the last remaining stored energy from yesterday being drawn from the PowerVault, while at the end of the charging period the ramp down is a result of increasing output from the solar panels reducing the need for power from the grid.  Hence at the moment the car charging ends there’s a sudden switch to charging the PowerVault at full power (the blue line) and some surplus power not used by the PowerVault (the purple line) – suggesting that something around 1 kW is suddenly available.  Although the purple line is described as ‘Grid Power Out’ that’s not strictly true here as much of that surplus is being diverted to make hot water (although this is invisible to the PowerVault).

After that digression, my actual purpose in making this post was to reflect upon relative energy costs and the best use of my solar power to reduce energy cost.

Energy usagePetrolDaytime ElectricityNighttime ElectricityGasSolar Electricity
Battery Storagen/an/a - no economic case to charge battery from grid during dayOptional - need to consider value of saved energy versus cost of 1 cycle of battery cycle-lifen/aSelf-use Priority #1 via PowerVault (daytime electricity -> solar)
Car ChargingManual 3rd backup (typically only used for long journeys when charging en-route becomes impractical) Manual 2nd back-upAutomated 1st backupn/aSelf-use Priority #2 via ImmerSUN (nighttime electricity -> solar)
Water Heatingn/aManual 3rd backup (never used in 3 years)Manual 2nd backup (never used in 3 years)Automated 1st backup for dull daysSelf-use Priority #3 via ImmerSUN (gas to solar)
Space Heatingn/aManual backupManual backupDefaultn/a - a summer solar surplus is a poor match to winter heating demand but could be Priority #4

 
The table above shows columns of energy sources ordered by reverse energy cost versus the major energy consumers in the house: battery storage, car charging, space heating and water heating. Energy consumers are ranked according to the value of displacing the the alternative energy course if not solar:

  1. Battery storage – I currently only charge the battery storage from solar, although there would be a seasonal economic argument to charge from cheap rate electricity if the differential between day and night rates was higher.
  2. Car charging – I generally charge on cheap night-rate electricity when I don’t have enough solar. In summer I program my car charger via the ImmerSUN’s 7-day timer to deliver sufficient charge for the day ahead, but sufficient headroom to make use of any available solar.
  3. Water heating – water heating is my 3rd priority for solar self-use and is automatically based up by the gas boiler which runs for an hour making hot water in the early evening if the tank isn’t already hot from diverted solar power during the day. The gas thermostat is also set slightly colder than the immersion heater – still very usable for a bath or shower from gas but giving some ability to delay water heating from a dull day to a following sunny day.
  4. Space heating – my space heating is generally gas. It would be possible to run a heater (or heaters) such as storage radiators via the ImmerSUN’s third output, but I consider that the cost of the heater(s) and installation is unlikely to be recouped given the major mismatch between surplus solar generally being in summer and heat demand being in winter.

Clamp orientation for ImmerSUN and PowerVault

In the last few days I’ve been assisting a reader of this blog who also has an ImmerSUN immersion heater controller plus PowerVault storage battery combination.  Like me, he had the immerSUN first and later added a PowerVault, but had immediately disabled the ImmerSUN to get the PowerVault to work.  Left to their own devices, the ImmerSUN will normally take the surplus power first before the PowerVault has chance to respond since it has a more dynamic control system, however economically it makes more sense to prioritise the PowerVault.

I previously posted on this topic in Prioritising smart loads for self-consumption but wanted to provide more clarity on the orientation of the clamps.

Both ImmerSUN and PowerVault rely on current clamps to get their control signal. Such clamps fit around an electrical cable and measure the flow of electricity through that cable.  Normally clamps for both these devices alone would be around the live in the incoming mains cable, but do that with both and the ImmerSUN will always take the available power first which is undesireable.

The illustration shows my solution, as per the prior post, where the PowerVault clamp surrounds both the incoming live and the live output to the immersion heater.  If the clamps are correctly orientated, this allows the PowerVault storage battery to be prioritised over the ImmerSUN immersion heater controller.  When the PowerVault clamp surrounds the two cables, it is important that outgoing power to the grid and outgoing power to the immersion heater from the consumer unit pass through the clamp in the same direction.  This solution should work for clamp-driven solutions too.

The PowerVault clamp is directional – it has an arrow which should point towards the consumer unit.  That means, if you pair the two cables as I described, so outgoing power flows in the same direction in both cables through the clamp, then the arrow should point in the opposite direction i.e. towards the consumer unit.

Fundamentally it doesn’t matter which way the ImmerSUN clamp faces, as during commissioning the ImmerSUN will work it out by cycling the power several times, but you shouldn’t change it after commissioning.

Electricity use 2017

The image above shows my solar electricity generation and usage in 2017 by calendar month.

  • 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.

Energy Smart in action

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.

An attack of the clamps

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:

 ImmerSUN PowerVaultData Logger
Solar panel outputReport (1)-Report (4)
Immersion heaterControlMeasure sum (3)-
Import / ExportMeasure (2)Report (5)
Battery In / Out-ControlReport (6)

I’ve tried to distinguish between their functions as follows:

  1. Control – is an output current actively controlled by a device so there’s generally no need to measure it with a clamp.
  2. Measure – a clamp whose output is analysed automatically to create a control action such as divert more or less power to some device.
  3. Report – it’s just reporting something for the purposes of understanding, but it’s not used to directly control anything.

So the 6 clamps are:

  1. 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.
  2. The ImmerSUN fundamentally operates by measuring export via this clamp and then responding to minimise that export.
  3. 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.
  4. 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, ..
  5. .. import to / export from my home, ..
  6. .. 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.

 

 

Prioritising smart loads for self-consumption

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 solutionMy solution
Total load / total generation3.0 kW3.0 kW
Baseload of house0.2 kW0.2 kW
ImmerSUN Export threshold0.8 kW-
Battery charging0.8 kW0.8 kW
Balance for water heating1.2 kW2.0 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.