Towards battery storage

For quite a long time, I have been thinking about adding battery storage to my set up; the recent eye-watering rise in the cost of grid-purchased electricity here in the UK is finally driving me from thinking about it to acting on it.

It is not a simple matter however; choices about battery size, whether to include grid outage back-up capability, where to physically locate the battery and charger/inverter, - all these and much more have to be worked through, and inevitably compromises identified and accepted.

I have persuaded myself that the case for battery storage is unassailable. My reasoning goes like this: my Powerspout is good at delivering energy over long periods at low power levels; batteries are good at delivering energy over short periods at high power levels; by combining the two, advantage can be taken of their respective strengths.

In effect, for a grid connected Powerspout like mine, a battery can be used to address the limited capability of the Powerspout to deliver sufficient power when domestic appliances like a kettle or cooker are turned on; the battery opens the door to saving on the energy which would, without a battery, be drawn from the grid.

In the complicated journey of working out how to configure a battery system, a major 'road block' has been how to integrate the existing diversion device I have, a Solarcache; it harvests surplus energy and diverts it to 3 different heating loads in the house, always maintaining a bare minimum of power to trickle back to the grid; it has been a very valuable component of our system which we have come to rely on as our sole source of domestic hot water in the summer months; I am keen not to have to jettison it with the introduction of battery storage.

The 'road block' was because my Solarcache was an early model which lacked the later firmware which enabled Solarcache to be adjusted so the battery management system (BMS) got first priority of surplus power; 

Happily, yesterday that block was removed; I discovered I had a spare Solarcache with the required firmware update; below is the screen from it which shows how the Solarcache's settings can be adjusted to give the BMS priority.

The settings allow for a threshold level of Watts flowing back to the grid, and a delay time during which that power needs to flow, before Solarcache will start to divert power to the loads connected to it. Discrimination between the settings of the BMS and Solarcache can thus be achieved to ensure the BMS gets first priority.

I was really excited at discovering I could keep Solarcache alongside the battery system; having removed that particular 'block', I now feel free to progress the battery scheme further.

I'll post more in due course.

How I use the electricity my Powerspout generates - in winter months

In January and February my Powerspout produces more electricity than at any other time of year; its power output is usually a constant 924 Watts giving an energy yield of just over 22 kWh per day.

The question I want to answer in this blog post is: how does this electricity get used ?

Below are two plots which show how the power and energy was used on one, unexceptional day last year: the 20th February 2020; the data was captured by the diversion device I use (a Solarcache) which has the ability to record information every 6 seconds to an SD card.

Plot 1

Plot 2

Explanation of plot 1.

the vertical axis is power in watts; the horizontal axis is the time of day.

the green line represents power being generated; during the hours of darkness, fairly obviously this will only be from the Powerspout, but between 08:30 and 16:00 a variable contribution from solar panels is added giving the spiky appearance mid-way through the day.

power in excess of what is being consumed in the house as 'base load' is diverted by Solarcache to two 'non-essential loads'.

these two non-essential loads are prioritised so that one, a 3 kW immersion heater giving the main house its domestic hot water (DHW), receives power first until the water is up to the temperature of its thermostat;  when power is being fed to this load the plot represents it by the red bars.

when the thermostat opens, Solarcache senses there is no load to supply and starts sending power to the second priority load, represented by blue bars; 

this second load is comprised of another 3 kW immersion heater supplying hot water to our 'granny' annexe plus, in parallel with it, a 3 kW storage heater for space heating in the annexe; the storage heater is set to allow air to convect through it at all times so it never gets hot enough for its thermostat to open; in this way the priority 2 load never switches off and is always available to receive excess power.

'base load' consumption in the house at any moment can be determined from the plot fairly exactly by the number of watts by which generation exceeds the power being sent to one or other diversion load; I say fairly exactly because Solarcache always allows a trickle of power to flow back to the grid, of about 40 to 80 W; this amount will contribute to the difference between generated and diverted power and makes it difficult to be precise about base load consumption.

base load is made up mostly of lighting (LED or compact fluorescent) but includes also a Grundfoss central heating pump (48 W), a fridge and a freezer (111 W each) and innumerable switch-mode battery charging devices; we have no TV.

when a power hungry household appliance is switched on, requiring more power than is being generated by the sum of hydro and solar generation, the diversion loads automatically switch off and the extra power needed to supply the appliance is drawn from the outside grid. 

the household appliances which make up the power hungry category are: kettle, hob, oven, microwave, vacuum cleaner, washing machine and dishwasher, - the latter two only being power hungry when they are heating water which they have to do as each is only fitted with a cold water supply.

the discerning reader will detect the times when power hungry appliances were in use on 20th Feb 2020 by the white gaps within the red and blue bars.

the provision of DHW in the main house does not only rely on its immersion heater; the tank also has a heating coil which receives hot water from the back boiler of a log burner in our lounge and in winter this is burning 24/7; thus the water quite quickly gets hot enough to open the immersion heater thermostat and for this reason the cumulative energy taken by the priority 1 load is less than that of the second priority load in the annexe.

Explanation of plot 2.

the vertical axis is in watt hours (Wh); dividing by 1000 gives the more common unit for energy: the kilowatt hour, - kWh, - aka a unit of electricity for metering purposes.

the blue and red plot lines represent respectively the cumulative energy diverted to the house load and the annexe load;

as explained above the priority 2 (blue line) receives the greater share because DHW in the main house receives additional energy from a log burner and can switch off, whilst the priority 2 load can never switch off and is always available to soak up power.

Conclusion.

this is how the system works in winter when winter conditions prevail: hydro power is plentiful, the lounge fire is alight and solar power is minimal.

the situation in summer is quite different and will be the subject of a later blog post;

the 'brains' of the system is the Solarcache and it is a huge pity this device is no longer available; mine has been bomb proof since installation 5 years ago and I am not aware of anything being marketed now which has quite the same functionality.

the SD card (8 GB size) which stores all the data has been quietly recording everything since August 2015 and last week covid lockdown stimulated yet another job to be tackled that had long been put off, - I ventured to download the accumulated data; there was 2.3 GB of it with each day accounting for 1.3 MB; considerable patience was needed because it took ages to get the data off the card to my desktop, and then MS Excel went into extra slow mode trying to crunch the figures.

such a huge amount of data is probably excessive but its usefulness is in being able to see how much surplus energy my combined hydro and solar installation yields at different times of the year; such information can help decide whether storing the energy in a battery, rather than use it for diversion load heating, might be a worthwhile road to take. 

For the moment, I don't think that is the way I want to go, but if battery prices come down and grid electricity prices go up, at least I'll have the hard data to help me make the decision.

A fuller description of Solarcache is in this blog post.

Making use of what you generate

In the course of the last ten days, as the flow from the spring has picked up, output has doubled from 225 W to 518 W .

At 518 W, more than enough power is being generated to meet the 'base load' of our house, by which I mean the total load of all those electrical gadgets which are taking power in the background most of the time.  For us, 'base load' is made up of fridge and freezers, central heating pumps, lights, computers and a good number of small devices connected through mains rectified power units.  The total load comes to about 350 W, but it fluctuates, particularly depending on whether fridge and freezers happen to coincide their 'on' times.

So there is a surplus of generated power much of the time, and as water flow picks up further, this surplus will get greater.  What to do with it ?

Back in August this year, I had some solar PV panels put in, 3.25 kWp in total, and at the same time installed a Solar Cache, which is a device to divert surplus home generated power to useful loads 'in-house'.  The greater benefit of Solar Cache comes from diverting the huge amounts of surplus power arising from a sunny day, but it works just as well in diverting the smaller amounts of surplus hydro power.  By so wiring it that it registers the total of home generated power, it works seamlessly to divert power whether it is solar, hydro or a combination of both.

Here is a picture of the Solar Cache screen taken last night:

Taken as it was at 22.50, there was no solar generation and the 518 W indicated to be solar all comes from the Powerspout.  House 'base load' at the time is recorded as 197 W so there was a surplus available of 321 W.  The cleverness of Solar Cache is that it diverts as much of this surplus as it can to a useful load, whilst just maintaining a trickle of export to the grid.  From the screen, it is trickling 95 W to grid and sending 230 W to the immersion heater in our domestic hot water tank.

Under the feed-in tariff scheme implemented in the UK, payment for 'exported' energy is not calculated from a metered export reading but from a 'deemed' amount, which for hydro is 75% of energy generated.  Thus the way Solar Cache works which is to keep exported energy to a minimum (the makers claim around 50 W) does not mean that the revenue one receives for exported energy is reduced.

This, however, may be about to change.  In the UK, the government are embarked on a public consultation regarding changes to the feed in tariff scheme, and one of the questions in the consultation is:
"Given our intention to move to fully metered exports for all generators, do you agree with the proposal that new and existing generators should be obliged to accept the offer of a smart meter when it is made by their supplier?"

The effect of this change, were it to be implemented, would certainly reduce the income coming from a Powerspout, and this would be the case whether one had a Solar Cache or not.  A Powerspout installation, by the smallness of its output, will never generate sufficient for 75% to be exported, so any change from the present calculation method based on '75% deemed' will reduce income and prolong the 'payback time'.

But hey ! - it's not about the finances but the fun of having such an elegant source of home power.