Fullscreen HTML5Med Res (??MB)HTML4

Hot Water System Solar Diverter Part 1 by Ray Berkelmans & John Clarke Solar-optimised hot water system (HWS) heating using power purely from excess solar generation Solar export data is obtained from the inverter and updated every five seconds Shows operational parameters on a 2.4-inch OLED screen WiFi logging of operational parameters to a ThingSpeak database every five minutes Automatic override if the HWS temperature is still cold by the end of the solar day Night-time power-down Active heatsink cooling Email alert (one per day) if communication with the inverter is lost Over-the-air program updates via WiFi Manual override switch You can save a lot of money with this device! It lets you use excess solar power generation to power your electric water heater. It’s a lot less expensive to put together than an equivalent commercial unit. siliconchip.com.au Australia's electronics magazine Background Image: unsplash.com/photos/sunset-view-5YWf-5hyZcw June 2025  35 S olar hot water diverters enable you to use surplus electricity generated by your solar photovoltaic system to heat water. Commercial versions have been around for decades, although they are pretty rare to find. The main reason is price; brand-name diverters often cost upwards of $900, with some (eg, Fronius) close to double that! At that price, it is hard to justify the up-front cost in terms of the savings. Thankfully, this Solar Diverter can be built for a fraction of that price! In this era of ever-diminishing solar feed-in tariffs, it makes sense to maximise consumption of your own solar power. The reality is that often, when you have plenty of solar energy to export, the grid does not need it! As of February 2023, all new solar installations in Queensland above 10kVA require a Generation Signalling Device (GSD) to be fitted so that electricity distributors can remotely curtail your solar feed-in when required. Even without this, residential supply voltages can often exceed 250V AC on sunny days, causing most inverters to shut down. Other states are considering similar so-called ‘backstop’ mechanisms. A simple timer to divert power to a load during peak solar times is a good start to optimising the usage of the available solar power. However, excess solar power is a highly variable thing with passing, or persistent, clouds decreasing solar output by an order of magnitude or more. If you have a conventional hot water system with an electric element, this project will help you make the most of your solar generation on those challenging solar days (see Fig.1). If you have splashed out on a fancy whole-of-home battery system, this project will be especially useful, because it will prevent your HWS from sucking your battery dry during the night and during poor solar conditions! How it works Our solar diverter consists of an ESP8266 WiFi-enabled microprocessor that connects to your solar inverter and reads the solar export data directly from it. If more than 0.5kW of power is being exported, the microprocessor produces a pulse-width-modulated (PWM) signal with the duty cycle being a percentage of the available export power to the maximum power demand of the HWS element. We have an Aquamax 250L HWS system fitted with a 3.6kW electric heating element. So, for example, if 1kW of solar export power is available, the duty cycle is set to 14% ([1kW – 0.5kW] ÷ 3.6kW). The duty cycle increments and decrements in steps of 2% from 0 to 100% with each program cycle, providing hysteresis during highly variable conditions. The PWM signal passes to a zero-crossing detection (ZCD) opto-coupler. This converts the PWM signal to a timing suitable for switching a Triac with the AC mains waveform. A Triac typically needs to be switched in synchrony with the sinusoidal(ish) mains waveform, switching it on near the zero voltage point (zero crossing), either from positive to negative or vice versa. To do otherwise would cause unacceptably high peak currents and excessive electromagnetic interference (EMI). Before zero-crossing detection opto-couplers came along, the timing of power switching is something that had to be handled in software, with voltage monitoring and interrupts or with much more involved hardware setups. With the ZCD opto-coupler, we are spared from such complexity! Obtaining solar export data The solar export data is extracted from your solar inverter by reading the relevant register over WiFi using the Modbus communication protocol. Before setting off to build this project, you will need to establish if your inverter supports Modbus and, if so, which register(s) contain(s) the solar export data needed. The finished and wired Hot Water System (HWS) Solar Diverter. Note the two visible acrylic covers (green and clear acrylic), which are placed to prevent contact with high-voltage components. 36 Silicon Chip Australia's electronics magazine siliconchip.com.au Fortunately, most modern inverter manufacturers have subscribed to the SunSpec Alliance, which sets open information standards for the Distributed Energy Resource industry. You can see if your inverter manufacturer is part of the alliance by checking the membership at https://sunspec.org/ members/ Even if you don’t see your inverter manufacturer listed, not all hope is lost. For example, my manufacturer, SolarEdge, is not listed, yet they still comply to the SunSpec Modbus standard and provide a very detailed Application Note (siliconchip.au/link/ ac4z) to guide users through the hundreds of registers. You will need to do some research to find out which register addresses are used by your inverter. Be aware that some models don’t have Modbus enabled by default. Check your inverter instructions and/or with your solar installer. To test whether your inverter is compliant, and to explore its data registers, you can download a Modbus simulation tool such as Modbus Poll (www.modbustools.com), which has a free 30-day trial. Under the “Connection” tab, you simply enter the LAN IP address for your inverter (check your network client list in your router), together with your server port. Most inverters use a default port of 502, but SolarEdge uses 1502. Under “Setup → Read/Write Definitions”, select the slave ID (the default is 1) and enter a start register address like 40001. After hitting OK, Modbus Poll will then display the content of the next ten registers. Until recently, my inverter was a three-phase Solar Edge SE-10K model. Since our house and HWS are connected to phase C, the most relevant register is 40209: “Phase C AC Real Power”. Readings are displayed in watts (int16) with solar export shown as positive values and grid import as negative values. During development of this project, we upgraded our solar system to a Sigenergy 5-in-1 battery with an integrated 25kW, 3-phase inverter. It stores its export data as 32-bit integer (int32) values, which are spread across two registers. For us, they are 30056 & 30057. Its export values are the opposite of the SolarEdge’s: negative for export and positive for import. The SigEnergy siliconchip.com.au Fig.1: this shows why having a HWS Solar Diverter may be required to make the most of your solar power during highly variable solar conditions. Modbus protocol can be found at siliconchip.au/link/ac50 WiFi It is worth ensuring you have an adequate WiFi signal at the point where you intend to mount the Solar Diverter. There are many free smartphone apps that will show your WiFi signal strength. You will need at least -70dB for a reliable WiFi connection. Otherwise, you may need to invest in a WiFi range extender. HWS element The other check you should make before launching into construction is to ensure that your HWS has a resistive heating element and is not a heatpump or another type of HWS. If it has an element, its specification will be written on the compliance plate, possibly near the base of the HWS. A rating of 3600W or less will confirm that this design is suitable for your HWS. Circuit details The full circuit is shown in Fig.2. MOD2 is the microprocessor module that communicates with the inverter and generates the PWM signal from its GPIO14 pin. That is fed to OPTO1, a MOC3083 zero-crossing opto-coupler Australia's electronics magazine Triac driver. This features a low 5mA trigger current (IFT) and high isolation, with a rated peak blocking voltage of 800V between the line and control circuitry. Current flowing through OPTO1’s internal LED generates an infrared signal that triggers the monolithic silicon detector, then its internal Triac and finally the external Triac to switch the HWS load. The 360W resistor on pin 1 of OPTO1 was chosen to supply the necessary current to trigger the LED. This is calculated as (3.3V – 1.5V) ÷ 5mA, where 3.3V is the supply voltage, 1.5V is the LED forward voltage and 5mA is the trigger current. When a sufficient LED current (IFT) is supplied and the AC line voltage approaches the zero point, the Triac driver latches on. This introduces a gate current in the main Triac, triggering it from the blocking state into full conduction. The main Triac here is a BTA41800B, capable of handling up to 40A RMS; more than ample for our ~15A RMS heating element load. We recommend fitting the 360W resistor at pin 6 of OPTO1, as it prevents the Triac driver from being damaged in applications where the load is inductive. It helps to limit the gate trigger current (Igt) if there is a transient June 2025  37 Fig.2: the HWS Solar Diverter circuit is based around MOD2, an ESP8266 microcontroller with WiFi. IC1-IC3 provide a way to monitor the current drawn by the HWS while OPTO1 and TRIAC1 provide PWM control of the HWS element, so its power draw can be modulated. in the mains waveform while the Triac driver was off. The 330W (1W) Triac gate resistor provides better noise and thermal immunity when the internal gate impedance of the Triac is high, which is the case for sensitive-gate Triacs. These resistors are 1W types mainly for their voltage rating. An externally mounted 20A 250V override switch (S2) allows you to bypass all electronic control circuitry, if required, and force the HWS element on. 38 Silicon Chip Current monitoring While current sensing isn’t an essential part of the circuit, it provides a helpful insight into how well the circuit is working. The main current-sensing element is an ACS770LCB-050B 50A bidirectional current sensor (IC1). A TLC2272 dual operational amplifier (IC2) buffers the output of the current sensor, feeding an ADS1115 16-bit analog-­todigital converter (ADC), IC3. So IC1 converts the load current into a voltage which is buffered by Australia's electronics magazine IC2, then converted to a digital value by IC3 and passed to the microcontroller. The ADS1115 is the fastest available ADC that communicates using an I2C serial bus, with a sampling rate of up to 860 samples per second. This is about the minimum acceptable for accurately sampling the AC current waveform. Since each complete AC sinewave lasts 20ms, this provides us with a bit over 16 samples per full wave. When measured over 100 cycles (two seconds), that gives us a fair estimate siliconchip.com.au Fig.3: the PCB is populated with a mixture of SMD and through-hole components. Note the three acrylic covers over OPTO1, IC1 and the mains terminals that prevent accidental contact with high-voltage parts. During assembly, be careful to fit OPTO1, IC2 and IC3 with the correct orientations; other parts are polarised, but their correct orientations should be obvious. between IC3 and MOD2. This is performed by general-­ purpose N-channel Mosfets Q2 & Q3, plus a few 10kW pull-up resistors. They reduce the voltage levels of the SCL and SDA lines from 5V at IC2 to 3.3V at MOD2 & MOD3, while still allowing bi-directional communication. Two temperature sensors (DS18B20s) connect via CON5 and CON6 for monitoring the HWS and heatsink temperatures. CON2 provides a connection for a light-dependent resistor (LDR), which allows our circuit to go into sleep mode when the sun goes down. Momentary switch S1 and associated 47kW resistors and a 100nF capacitor form the reset circuitry of the ESP8266. The CON10 header and jumper JP1 provide a means for programming the microcontroller on the ESP8266 module. Power supply of the current flow. ADCs that communicate using an SPI serial bus can sample faster, but require a few extra pins on the microprocessor, which we don’t have. Since we already have another I2C device in our circuit, the 2.4-inch, 128 × 64 pixel monochrome OLED siliconchip.com.au screen, communication with both devices required just two of MOD2’s pins (GPIO4 & GPIO5). The current sensing components operate at 5V to give sufficient resolution, while the microcontroller is strictly a 3.3V-tolerant device, so we need some digital level shifting Australia's electronics magazine DC power for the circuit is derived from a PCB-mounted 230V AC to 5V DC power supply (MOD1), which has a 250mA fuse (F1) in case of a fault. The 5V rail powers the heatsink fan (FAN1, connected via CON4) and the components involved in sensing the current drawn by the HWS element. CON3 provides a handy way of powering the circuit with a 5V DC power supply, or a 3.7V (nominal) Li-ion/ LiPo battery, so programming and testing can be done without having to connect AC mains power. June 2025  39 The 3.3V rail is derived from the 5V DC bus via low-dropout linear regulator REG1 (AP7365). Other LDO regulators in the same SOT-23-5 package would be equally suitable (eg, ME6211, MCP1802T or TPS7A2033), provided they can supply at least 250mA and have a compatible pinout. Construction The first step is to create the PCB assembly, which can then be mounted in a plastic box and wired up. The Solar Diverter is built on a double-­ sided, plated-through PCB with a red solder mask that’s coded 18110241 and measures 134 × 207.5mm. It is installed within an enclosure measuring 222 × 146 × 55mm. The locations of components on the PCB are shown in Fig.3. Many (but not all) of the components used are surface-mount types that can be soldered by hand using a fine-tipped soldering iron. Starting from the smallest component and working up to the largest, solder one end first (for capacitors and resistors) or one lead first (for the ICs and MOD2). Make sure the component is lined up with the other pad or pads; if necessary, remelt the initial solder joint and gently realign the part before soldering the remaining pins. If any solder bridges form between IC leads, they can be cleared using solder wick. Adding a small amount of flux paste from a syringe will make both soldering and clearing bridges easier. For MOD2, apply solder over the outside edges of the pads on this module to join them to the PCB pads, treating it like the large surface-mounting part that it is. For the through-hole parts, such as the 1W resistors, switch S1, OPTO1 and IC1, insert the leads through the associated PCB holes and solder them on the underside of the PCB. All polarised parts, including OPTO1, IC2 and IC3, must be orientated as shown in Fig.3 for the circuit to work. IC1 has large, high-current leads that must be soldered on both the top and bottom sides of the PCB to ensure low-resistance connections. The three 45A two-way barrier terminal connectors (CON7, CON8 & CON9) require sufficient solder and heat for the solder to flow over the full underside pad and to the connector terminals to provide low-resistance connections. 40 Silicon Chip Parts List – Hot Water System Solar Diverter 1 double-sided plated-through PCB coded 18110241, 134 × 207.5mm 1 ABS enclosure, 222 × 146 × 55mm [Jaycar HB6130 (ABS) or HB6220 (Polycarbonate)] 1 100 × 110 × 33mm heatsink cut to 100 × 70 × 33mm [Altronics H0563 with half the fins cut off (see text)] 1 40mm 5V fan (FAN1) [Altronics F1110 or DigiKey 102-4361-ND] 1 M205 fuse holder with cover (F1) [Altronics S5985] 1 M205 250mA fast-blow fuse (F1) 1 Light-dependent resistor (LDR1) [Jaycar RD3480, Altronics Z1619] 1 SPST pushbutton two-pin switch (S1) [Jaycar SP0611, Altronics S1127] 1 20A 240V AC IP66 weatherproof switch (S2) [Bunnings I/N 4430626] 1 3-6.5mm cable gland, for LDR and TS2 wiring 1 OLED display bezel (see text and Fig.5 next month) Connectors 1 4-pin JST XH header with 2.54mm spacing plus matching plug (CON1) * 3 2-pin JST PH headers with 2mm spacing plus matching plugs (CON2-CON4) * 2 3-pin JST XH headers with 2.54mm spacing plus matching plugs (CON5, CON6) * 3 45A 600V 2-pin barrier connector strips, 0.5-inch/12.7mm pitch (CON7-9) [DigiKey YK7010223000G-ND] 1 3-way polarised pin header with 2.54mm pin spacing (CON10) 1 2-way pin header with 2.54mm pin spacing (JP1) 1 jumper shunt (JP1) * all available in the Jaycar PT4457 JST Connectors Kit Hardware 1 3mm-thick sheet of clear acrylic, 340 × 307mm (for weather shield) 1 acrylic or fibreglass piece, 106 × 79.5 × 3mm (see Fig.5 next month) 1 acrylic or fibreglass piece, 32.5 × 15 × 1.5mm (see Fig.5 next month) 1 acrylic or fibreglass piece, 26 × 33 × 1.5mm (see Fig.5 next month) 1 transistor clamp to secure TS1 to the heatsink [Jaycar HH8610] 2 5.3mm inner diameter crimp eyelets suitable for 2.5mm2 wire 1 M4 × 15mm panhead machine screw 1 M4 × 10-12mm panhead machine screw 4 M3 × 12mm tapped spacers 8 M3 × 6.3mm tapped spacers 2 M3 × 20mm panhead machine screws 2 M3 × 15mm panhead machine screws 6 M3 × 12mm panhead machine screws 2 M3 × 10mm panhead machine screws 13 M3 × 5mm panhead machine screws 2 M4 star washers 6 M3 flat washers 3 M4 hex nuts 10 M3 hex nuts This is what you will typically see displayed on the OLED screen. It is mounted to the lid of the case as shown in the photo opposite. We have glued it onto the case for a flush fit, but you might prefer to use the standoffs to screw it in. Australia's electronics magazine siliconchip.com.au 2 20mm or 25mm corrugated conduit glands [Bunnings I/N 4330875 or 4330876] 1 small tube of thermal paste Cable & conduit 3 lengths of 4-core shielded cable for the DS18B20 temperature sensors and LDR, length to suit installation [Jaycar WB1510, Altronics W3040] 3 lengths of 2.5mm2 round cable or 2.5mm2 flat twin and Earth for S2, mains input and mains output wiring [Bunnings I/N 4430139 or 4430080] lengths of 20mm or 25mm conduit, to suit installation Modules 1 Meanwell IRM-03–5 5V/3W AC-to-DC converter (MOD1) [DigiKey 1866-3020-ND] 1 ESP8266 – ESP-12F programming and development board (MOD2) [AliExpress, eBay] 1 2.42-inch 128×64 I2C OLED display module (MOD3) [AliExpress 1005006267098554 or 1005006267098554] Semiconductors 1 ACS770LCB-050B-PFF-T bidirectional current sensor (IC1) [DigiKey 620-1541-5-ND] 1 TLC2272CD dual op amp (IC2) [DigiKey 296-1305-2-ND] 1 ADS1115DGSx 16-bit ADC (IC3) [DigiKey 296-24934-2-ND] 1 MOC3083M opto-isolated Triac driver (OPTO1) [DigiKey MOC3083M-ND] 2 DS18B20 temperature sensors (TS1, TS2) [DigiKey 4518-DS18B20-ND, Altronics Z7280] 1 AP7365-33WG-7 3.3V linear regulator (REG1) [DigiKey AP7365-33WG-7] 3 BSS138 N-channel Mosfets, SOT-23 (Q1-Q3) [DigiKey 4530-BSS138TR-ND] 1 BTA41-800BQ 800V 40A Triac, TO-3P (TRIAC1) [DigiKey BTA41-800BQ-ND] Capacitors (all SMD M2012/0805-size X7R ceramic) 1 22μF 6.3V 2 10μF 16V 1 1μF 50V 5 100nF 50V Resistors (all SMD M2012/0805-size ⅛W unless noted) 2 47kW 5 10kW 1 6.8kW 1 4.7kW 1 2.2kW 1 360W 1 360W axial 1W [DigiKey 738-RSMF1JT360RCT-ND] 1 330W axial 1W 1 120W MOD1 (the 230V AC to 5V DC converter) and fuse holder F1 can be installed now. With the holder soldered to the board, insert the M205 250mA fuse, then the transparent cover can be clipped over the top. Connectors There are several different types of connectors used on the PCB. These include a 4-pin XH JST plug and socket with 2.54mm spacing for CON1; 2-pin PH JST connectors with 2mm spacing for CON2, CON3 & CON4; and 3-pin XH JST connectors with 2.54mm pin spacings for CON5 & CON6. These are available in the Jaycar JST Connectors Kit (PT4457) or separately from online suppliers. The 3-way pin header with 2.54mm pin spacing for CON10 and the 2-way pin header with 2.54mm pin spacing for JP1 are standard headers available in strips. Heatsink & Triac mounting The Altronics H0563 heatsink is supplied with cooling fins on either side of a central flat area for mounting power transistors in TO-3 packages. For our design, one side of the heatsink with fins will need to be removed so the TO-3P packaged Triac can mount on the central flat area. Cut it off using a hacksaw, leaving a 30mm wide flat section next to the fins (see Fig.3). Use the PCB as a template to mark the six holes required, then remove the heatsink and drill them. Place the heatsink on the PCB and check they all line up. Bend the Triac leads at right angles so it can be mounted tabdown onto the heatsink with the leads inserted into the PCB pads. The Triac tab is electrically isolated from the A1 and A2 leads, so an insulating washer is not required. Apply a thin layer of thermal paste (heatsink compound) between the Triac tab and the heatsink to improve heat transfer. Secure the Triac with an M4 machine screw and nut, then solder the Triac leads to the mounting pads on the PCB. Next month Warning: Mains Voltage This Solar Diverter operates directly from the 230V AC mains supply; contact with any live component is potentially lethal. Do not build it unless you are experienced working with mains voltages. A licenced electrician is also required to install the project. At this stage, we are ready to prepare the case to install the PCB, wire it up and start testing. We’ll have the details on how to do that in the final article next month, with detailed testing instructions, as well as information on the final installation, setup, calibration and use. SC June 2025  41