My smart(er) home

Distributed Energy Resources – which are a form of IoT – and their effects on the electricity distribution network fascinate me. I am a geek, I know! But having the opportunity to install a roof-top solar system, a battery, and an automation system is; a) a good way to get my hands dirty and understand DER first-hand, and b) presents an excellent opportunity to trial a couple of ‘smart’ things that will help my family and I reduce our carbon footprint. As an added bonus, it will help us save money on our monthly power bill.

However, as someone who knows first-hand just how the S, R, and L in IoT stand for security, reliability, and longevity, I get anxious at the thought of anything reliant on a connection to “The Cloud” to function. Whilst there are plenty of innovative DER/IoT products and services that I could install in my home, I am mindful that it is not just me that needs to be able to operate and maintain them – it’s also my family (if I get hit by a bus) and any family who may own this home 5 years from now.

All of this is to say that there are a couple of principles that I try to follow for anything that I install in my home:

• If the “thing” provides an essential service around my home (the definition of which is specific to me), then it must be able to operate fine without an Internet or wireless connection. i.e. Local first and hardwired networks are desirable. I want this as any new homeowner may have no idea that these systems may exist inside the walls of my home, so they must be as resilient as I can possibly make them.
• If the “thing” relies on a cloud connection, then it needs to be trivial to use and troubleshoot (i.e. it needs to pass the ‘wife test’). Similarly, depending on the function that the “thing” provides, it needs to provide an alternative way to achieve the same outcome should its connection to the cloud fail.
• Lastly, the system as a whole should fade into the background. My days are busy, and what spare time I do have I want to spend with my family. I’d rather not spend it patching security flaws, updating firmware, troubleshooting wireless connections, changing batteries, etc.

This is a multi-part series. Be sure to follow the “smart home” tag for more posts on this topic.

What have I built?

The short story

I have installed both a 5kW solar inverter and a 7kWh battery energy storage system in my home. On top of this, I have used an Eastron power meter, a Raspberry PI home automation system, and a digital temperature controller to; a) direct any excess solar generation into my hot water cylinder (a.k.a. a secondary thermal battery), and b) capitalise on the “free” power that my electricity retailer provides to “top up” my thermal battery.

The long story

Everything in my home is driven by electricity (e.g. the oven, cooktop range, hot water system, and HVAC system). I could have installed a natural gas connection when we built our home, but I decided against this idea as; a) I cannot generate gas myself (well, not in any significant quantities 😉), and b) I have always dreamed of owning a sustainable home that was able to source all of its energy needs locally (so far as reasonably practicable).

In the lead-up to purchasing a rooftop solar system, I sat down and looked at both my home energy consumption and (typical) peak loads. I was able to use my electricity retailer’s online portal to review 12 months of daily energy usage in my home, and I calculated peak loads by summing up the kW rating of those “big” appliances that typically run at the same time. Big in this respect means anything that makes either a lot of heat (i.e. ovens, HVAC, and microwaves) or a lot of noise (i.e. a vacuum cleaner), as it is these things that are usually consuming between 2-4kW when they run. Over the year, daily energy consumption did not fluctuate wildly in our home and was usually less than 20kWh per day. This is primarily because water heating represents our biggest consumer of energy and this does not change much throughout the year.

With this information in hand, I was ready to start looking at different types of solar inverter and battery energy storage systems.

The equipment

Based on the typical energy usage in my home, and having now spoken to a handful of NZ solar resellers, I purchased:

• 1x Goodwe GW5000-EH 5kW single phase inverter.
• 12x Trina Vertex 495W PV solar panels.
• 1x 7kWh Pylontech Force H2 battery system.

In this case, the Goodwe inverter is designed to work with all sorts of battery energy storage systems, and thankfully the Pylontech Force H2 battery is one of those systems.

The installation

Solar panels - My preference would have been to install all twelve solar panels facing north, as this orientation would have captured the most sunlight hours during a typical New Zealand day. However, the orientation of my roof doesn’t allow for it.

The Goodwe solar inverter has two MPPT PV inputs, allowing two independent strings of solar panels to be connected. In light of not being able to install a northern-facing solar array, the two MPPT inputs would have allowed me to split my twelve solar panels into two sets of six panels, and install both a western and an eastern-facing set of panels – one to catch the afternoon sun and the other to catch the morning sun.

However, for ease of installation, I installed all twelve panels facing west. In my location, a western-facing system allows me to capture the late morning, early afternoon, and late afternoon sun, which works out well as my family and I typically use the most energy in the afternoon / early evening for cooking dinner, bathing, heating/cooling, etc.

I will add the comment that installing solar panels onto a tin tile roof comes with its own set of challenges. Be sure to check out the section on lessons learnt if you’re thinking about something similar.

Inverter and battery - I installed the Goodwe inverter and Force H2 battery system in my garage where there is plenty of space and good airflow to dissipate heat. One thing you may not be aware of - I certainly wasn’t - but PV inverters generate a lot of heat. On a sunny day, it is not uncommon for the case of the Goodwe inverter to reach temperatures of up to 70 degrees Celcius!

With such high temperatures and a battery nearby, fire is a very real concern. The GIB (plaster) board around the inverter system is fire retardant and rated for higher-than-normal temperatures. Equally, I like the design of the Force H2 battery – I am no battery expert but the batteries design is very robust (i.e. it is built like a rock that is likely to contain any battery failure long enough for the smoke/heat detector above to trigger and for use to evacuate).

The Goodwe inverter is wired directly into my single-phase main distribution board via an electrical isolator and circuit breaker, all of which are handily located right next door to the inverter itself. With respect to the main distribution board, I also:

• Installed an additional (smaller) distribution board below the main board to house the Goodwe power meter, which is essential for the inverter to make use of any battery system, and to house an Eastron power quality meter that I’ll use as part of my home automation system.
• Split my distribution bus into two independent single-phase buses – an “essential” and a “non-essential” bus. With a battery connected, the Goodwe inverter can act like an uninterruptable power supply (UPS) and can keep the essential bus live during a network outage on the grid – cool!

Distribution board and inverter connection diagram

Let’s get smart and automate!

By itself, the Goodwe inverter will largely do its own thing and make use of the battery depending on one of the four modes that you select during installation. The mode I chose to run is Goodwe’s “general” mode which means:

• Any power generated by the system will first supply the appliances (loads) within my home.
• Any excess generation (over and above the load in my home) will be stored within the battery.
• Once the battery is charged, any excess generation is exported onto the grid.
• Any home load over and above my solar generation will draw energy from the battery, or when there is zero generation, then the battery will supply all essential and non-essential loads until it is depleted (in reality, it’ll do this until the battery state of charge reaches a lower threshold I set during installation).
• The Goodwe inverter will also attempt to consume or inject kVArs depending on the voltage at my point on the electricity distribution network (i.e. Volt/VAr control). Equally, if the voltage goes too high at my point on the distribution network, the inverter will reduce the amount of kW’s being exported back onto the grid (i.e. Volt/kW control). These are all settings I program within the inverter.

In my case, for every kWh I export back into the grid, my retailer will credit my energy bill by 9 cents (i.e. I get paid 9c per kWh of exported generation). Conversely, I need to pay 33c per kWh for the energy I consume (import) from the grid. Together, these values mean it makes more sense for me to ‘bank’ (store) as much of my energy generation as I can so that I can avoid importing energy from the grid and paying that high(er) rate.

Smarter hot water

And banking on my excess generation is just what I did! Heating water is the biggest consumer of energy in my home – it represents nearly 70% of our monthly energy consumption. Because of this, I wanted to heat my water more smartly.

By default, most electric hot water cylinders have a heating element installed, and power to this element is controlled by a thermostat (i.e. an adjustable temperature switch) that monitors the temperature of the water within the cylinder. The operation of this thermostat is simple – when the water temperature within the bottom of the cylinder is below a certain threshold, the temperature switch closes and the heating element turns on. When the water’s hot, the switch opens and the heating element turns off.

To more smartly control the process of heating the water within my water cylinder, I have supplemented the inbuilt thermostat with a digital temperature controller. The idea here is that I can dynamically change the temperature setpoint to control when, and for how long I run the heating element. With a digital temperature controller installed, I’ve reconfigured the inbuilt thermostat to act only as an over-temperature safety switch and the digital temperature controller (via a high current relay), now modulates temperature by controlling when the cylinder’s heating element is turned on and off.

How do I modulate the hot water temperature setpoint?

With a Raspberry PI of course! The digital temperature controller incorporates a Modbus RTU RS-485 serial interface, so using the Raspberry PI and USB to RS485 adapter I can send commands to raise and lower the temperature setpoint as needed.

To know when to raise and lower the temperate setpoint, I monitor the total electrical energy flowing through the main switch in my distribution board. I do this using an Eastron power meter, which is a meter that also happens to support Modbus RTU over RS-485 serial.

When excess energy is being exported to the grid (indicating the sun is shining and the battery is full) I raise the setpoint on the temperature controller to begin using the hot water cylinder as a secondary battery. I have set up a ‘dumb’ algorithm to constantly monitor power flows, adjust the temperature, and ensure I don’t thermally cycle my hot water cylinder too much (potentially burning out the element).

I will cover off more details about how my Raspberry PI works in a future post.

Free power from my retailer

The same automation system that I described above is also used to capitalise on a window of free power that my electricity retailer offers. Contact Energy offers 3 hours of free power between 9pm and midnight. So guess what I do… regardless of whether the water in my hot water cylinder is already hot, the setpoint on the temperature controller is raised to its (safe) maximum. If the water needs to be heated, then it’s heated to make use of that free power.

In parallel, via a set of hardwired connections to the Goodwe inverter DRED interface, the Raspberry PI also tells the inverter to halt the use of the battery during this same window of time. I would love to be able to charge the battery (from the grid) during this period but the inverter’s firmware does not support that just yet.

There is a story to be told about hot water ripple control systems and unintentionally paying for your own excess solar generation. I will explain this in a future post - keep an eye out for it if you are installing a solar system and have an electric hot water cylinder of your own.

Lessons learnt

Installing a solar inverter, PV panels, and battery energy storage system wasn’t as difficult as I thought it was going to be. But….. I’ve learnt that tin tile roofs are the worst to mount panels too (my opinion only). They’re the worst for two reasons:

One. Before you install your solar panels, you will first need to mount the horizontal rails that they – the panels – will attach to. These horizontal rails attach to your roof using 90-degree angle brackets, and to reduce the likelihood of a roof leak, you’ll want to attach them on the ‘peaks’ on each roof tile, as attaching them in the ‘trough’ means they’ll see more rainwater flow past them.

However, the trouble here is there is very little strength in the peaks of each tile, and as you tighten the screws which hold each bracket to your roof, you will likely end up crushing/deforming the tile’s peak. In the end, I attached each bracket in the ‘trough’ of the tile, such that when the screw is tightened it pulls up hard against the wood directly underneath the tile. Rubber gromets and plenty of Bostik sealant in between the bracket’s foot, and the tile, help here.

Two. The second thing to be aware of is that the pitch of your roof does not align with the pitch of the trough in each individual tile – and hence the pitch of your rails will not match the pitch of panels when they are affixed to the rails. The picture to the right speaks a thousand words.

If you are mounting your rails on a tin tile roof, it pays to adjust each of the 90-degree angle brackets to account for this discrepancy between the ‘tile pitch’ and the ‘roof pitch’, otherwise your PV panels will be under strain when they are affixed to the rails.

In the future, I think I’d look for a house with long-run roofing iron – it would save a lot of hassle!