One of the roles of my Home Energy Management System (HEMS) is to switch the control state of my Powervault storage system. Many users would probably leave the system in Normal where it always either charges or discharges proportionally to solar surplus or deficit respectively. Other users on time of use tariffs might also program charging on cheap night time electricity in a fixed time window. However my HEMS integration allows for automatically charging in varying cheap windows only when solar production won’t be enough to fully charge the battery.
Powervault G200 storage system
The Powervault has six Control States covering different combinations of charging and discharging behaviour. Only three of these are used in my implementation. The other states could be more relevant if I was paid to export to the grid, and the value of that export started to exceed bought electricity costs; but in my case I’m not paid for export and thus any export is an error state.
Control state
Charge
Discharge
When used
Force discharge
No
Full power
unused
Only discharge
No
Proportionate
unused
Normal
Proportionate
Proportionate
High electricity cost
Only charge
Proportionate
No
Medium electricity cost
Force charge
Full power
No
Low electricity cost
Disabled
No
No
unused
Powervault Control States
While I could use the HEMS to load a full daily schedule into the Powervault, instead I store all my schedules in the HEMS itself and switch the state of the Powervault (and indeed my other devices) every half hour as dictated by the schedule. This approach also gives me some flexibility to nuance Control State as a function of state of charge which isn’t available via the Powervault’s own scheduler.
The HEMS thus has three roles:
Creating the daily schedule from future electricity prices and the solar forecast.
Switching the states of the devices every 30 minutes following the schedule.
Real time data display and data upload to Solcast the solar forecasting service.
A recent move to full fibre broadband prompted a reconfiguration of my home network as the incoming cable moved to the opposite side of the house from where the incoming telephone line had been. That’s not a normal situation but it turns out that my home had previously had two telephone lines in different places, but the cable that got upgraded to fibre was not the cable that had been in use in recent time. I’ve also had further additions which prompted the idea to create a diagram of how all the devices are connected.
Home networks
I was somewhat surprised to count as many as nine protocols/networks in use between different devices.
Like many homes we have a WiFi router where the fibre or telephone line enters the home. However over time I’ve been moving some devices from WiFi to Powerline adapters which carry the internet over the house wiring. This helps reduce issues where devices on different WiFi networks can’t communicate with each other and provides more bandwidth so more data can be carried more quickly.
I have four Powerline adapters which communicate with each other over the house wiring and then have different internet-enabled devices plugged into them. One such Powerline adapter is in the lounge with the WiFi router plugged into it.
Smart metering and external networks to home
The second external connection to the home is to the smart meters. There are two networks associated with smart meters: the Wide Area Network (WAN) and the Home Area Network (HAN). The HAN (which uses a protocol called ZigBee) interconnects the meters (both electric and gas) and the in-home display IHD. The WAN communicates outside the home to send consumption data to one’s supplier via a national database called the DCC. The WAN can send data as frequently as half-hourly.
I also have an additional device known as a Consumer Access Device (CAD). The CAD connects the HAN side of the smart meter to an independent database. This offers me two two things: (i) a third party app to review my electricity and gas use and (ii) data on a minute-by-minute basis rather than at most half-hourly. The CAD acts as a bridge between the ZigBee and WiFi.
Much of my home automation, particularly home heating, uses Apple HomeKit. HomeKit runs on home hubs, rather than an external cloud server. The hubs commonly connect to the individual smart devices (such as radiators valves, smart plugs, door and window sensors etc) by Bluetooth Low Energy (BLE). The low power demand of BLE allows battery powered devices to operate for months between battery replacements. My original hub was an Apple TV.
Apple HomeKit smart devices
Subsequently I’ve added an Apple smart speaker which can also act as a hub. HomeKit decides which of the available hubs to put in charge, but may relay signals to and from the devices via other hubs. The smart speaker also supports a protocol called thread which allows communications to and from smart devices like door sensors via other smart devices on their way to and from a hub. Thread thus helps to extend the area that can be covered by a hub by allowing signals to and from distant sensors and actuators to travel via nearer devices rather than directly to or from hubs.
Much of my energy smart equipment is divided between my study and the the airing cupboard. Each has a Powerline adaptor allowing wired internet communication. The Home Energy Management System (HEMS) in the airing cupboard gets electricity pricing and solar predictions from the cloud and then directs the Powervault battery and the ImmerSUN diverter (partially) via the cloud. The Powervault is connected to the internet via an internet switch in my study and thence via the Powerline adapter. The ImmerSUN in the airing cupboard connects to the internet via a proprietary protocol to a bridge in my study and then onwards via the same internet switch.
The nine forms of communication are:
Bluetooth – low power and low range device-to-hub communications for battery-powered smart home devices on Apple HomeKit.
Ethernet – wired mains-powered internet devices
Fibre broadband – high bandwidth connection from home to external internet
ImmerSUN proprietary – wireless communications between mains-powered ImmerSUN devices: power diverter, bridge to internet, and remote current sensors
Radio 868 MHz – radio signal to control car wallbox in garage from home
Powerline communications – carries the internet over mains house wiring using mains-powered adapters
Thread – an extension of Bluetooth allowing signals to travel from distant devices via nearer devices to a hub thus extending the area that a hub can cover
WAN – external connection to a smart meter
ZigBee HAN – internal connection between smart meters. ZigBee, like Bluetooth, is a low power low range protocol suitable for battery-powered devices (like a gas meter). It can be used for smart devices like Bluetooth, but in the form used in smart meters is secured and not accessible directly to consumers.
Last December I wrote of reviving my Powervault home storage battery with new internal batteries. However subsequently I decided that my old batteries were worth more to me for their remaining storage capacity than they were to me as scrap, and instead I would try to use both my old and new batteries.
I knew that there wasn’t space inside the case for two sets of batteries so I planned to add the additional batteries in a separate rack outside the standard case.
Powervault G200 storage
Electronics side of G200
I decided to explore the right hand side of the Powervault where the controls reside. The picture shows a large blue box which is the inverter/charger and handles the conversion of AC to DC to charge the battery, and DC to AC to discharge the battery. The open green circuit board to the right controls the inverter/charger and connects it to the internet for monitoring and control. Down the middle is an umbilical that connects the batteries (on the reverse side of the panel) to the inverter/charger. Each pair of batteries on a shelf is connected separately to the inverter/charger.
I fairly rapidly determined that there wasn’t space to add additional battery cables to the inverter/charger so instead I added two terminal blocks (see right) in the free space to the right, moved the existing battery cables from the inverter/charger to the blocks, and then added new cables from the blocks to the inverter. At this point the system was checked to ensure that all was working correctly.
Terminal block with applications
DC fuse
I then added two DC cables from the blocks, through a suitable fuse, and through new cable glands to the external batteries.
All the additional parts including batteries, blocks, cables, fuse and glands are intended for similar battery storage applications in boats or caravans.
The results of this are that the battery continues to function correctly supporting the needs of the house. The above graphs, from Powervault and smart meter respectively, show:
00:00 to 00:30 – home still running on stored energy from yesterday having already run through yesterday evening on stored energy.
00:30 to 01:00 – battery charges for half an hour on cheap rate power (7.5 p/kWh) as HEMS predicts that today may not be sunny enough to fully charge the battery.
01:00 to 04:30 – battery idle while other household loads charge from cheap power.
04:30 to 07:00 – house runs from stored energy until sun comes up.
07:00 to 18:00 – house running from solar with battery charging, car charging and water heating at times from solar. Some export from 15:00 as all storage devices (battery, car and water cylinder are fully charged).
18:00 to 00:00 – home runs from battery.
Through the day the battery supported the house to the tune of 5.18 kWh and then the next day for 30 minutes until the cheap rate started. The Powervault had notionally 4 kWh usable capacity as supplied, although theoretical capacity was higher (3 x 120 Ah x 24 Volts = 8.64 kWh). Theoretical capacity is higher now, but I suspect that the control board continues to limit the usable capacity. A low ratio of usable capacity to theoretical capacity should be good for battery longevity as depth of discharge is limited. The older batteries have now been in use for over six years, although they have only been supported by the newer batteries for a couple of months.
