How to design a solar water heating controller using MPPT

introduce

Solar heating of domestic hot water has traditionally been achieved using "solar thermal" technology (flat panel or evacuated tube collectors). While solar thermal is more space efficient than photovoltaics (PV) in terms of energy captured per unit area, it generally has higher installation costs, is less efficient in cold weather or cloudy days, has a shorter lifespan, and can present a number of ongoing maintenance challenges and costs (mechanical pumps, frost and corrosion protection chemicals, leaks, roof space positioning and plumbing wiring, etc.). With larger solar collection areas available (and few limitations in most installations), the use of PV panels now offers a more cost-effective, near zero maintenance and better performing "solid state" solar water heating alternative. The rapid and continuing decline in the cost of PV panels has helped make PV water heating an attractive and economically viable approach. Designed to sit between the PV panel array and a conventional electric immersion heater, the Loadmaster is designed to adjust the load impedance of the heating element to maximize the efficiency of the PV electrical energy collected regardless of sky conditions.

Photovoltaic systems are also more versatile than solar thermal systems and any excess electricity can easily be used for a variety of other purposes. The LoadMaster includes an optional second output that allows excess solar energy to be diverted to electric baseboard heaters (cheap) and used for space heating once the water has reached maximum temperature.

If the property has a full "grid-tied" solar electric installation, there are a number of commercial products available to divert excess electricity to the water immersion heaters rather than feeding it into the grid. The complexity and cost of a grid-tied system extends the payback time and is beyond the scope of a DIY project for many people who are connected to the grid. Any solar system will be much more expensive if you pay someone else to install it!

The LoadMaster PV hot water method shown here offers a very simple and inexpensive solution, perfect for the capable DIY person. It is a "fit and forget" project with virtually zero maintenance, with occasional cleaning of the panels being the only maintenance requirement.

The LoadMaster project discussed here has two additional "add-on" project pages, one project that adds remote internet connectivity using a DT-06 (or ESP8266-01S) Wifi-Serial terminal, and another project that details a method for remotely displaying the Loadmaster's operating status on a remote, wirelessly connected Nextion display:

Photovoltaic water heating, MPPT basics!

To heat the water, the electricity generated by the photovoltaic panels must be delivered to the electric heating element in a standard hot water cylinder. Sounds simple? Well, unfortunately, for efficient and safe operation, it's not as simple as just connecting the heating element (and thermostat) to the DC output of the solar array.

Connecting the high voltage DC output of a PV array to a standard AC rated mechanical thermostat can result in arcing and welding of the contacts, potentially seriously compromising the safety of the thermos. Furthermore, under real world variations and non-perfect sky conditions (i.e. most of the time!), there will often be a severe mismatch between the load resistance of the heater and the ideal "matched" impedance required by the PV array to deliver maximum power. This results in a loss of potential heating power, very low efficiency, and is a ridiculous waste of the investment in the PV panels.

Let's use my 3kW array as an example to illustrate the importance of MPPT (2 strings in parallel each consisting of 5x REC Solar 300WTP2 panels in series): - In full sunlight the array will have a similar voltage to current relationship as shown by the blue curve in the graph below (for a single panel the VI response graph is usually shown in the datasheet). The ratio of voltage to current at any point in the graph implies a load resistance of V/I ohms connected, as shown by the green trace.

Multiplying the voltage and current at all points on the V–I curve will show the array’s output power curve (the red trace). We can clearly see that there is a peak in the array output power (at the vertical line) corresponding to a particular voltage and current. This is called the maximum power point. In this example, to operate at the maximum power point and therefore deliver 3kW, we would need a load of 162V/18.48A=8.8Ω. So, now let’s look at the array characteristics when the sun is about half as strong (i.e. it’s cloudy, not midday, etc.):

This time we can see that the peak power is being delivered at a slightly lower voltage, but the current is now about half of what it was when the sun was full. In this reduced insolation condition, in order for the array to operate at its peak power point and deliver 1.47kW, we now need a matched load impedance of 160V/9.24A = 17.3Ω. If we still had the original 8.8Ω load from this curve then connected we can see that an 8.8Ω load will pull the array voltage down to just 85V, only producing about 834W of output power to the 8.8Ω load, rather than the 1.47kW we would have had if a 17.3Ω matched load was connected! By having the wrong load impedance to accommodate the reduced solar conditions, we have just lost 636W - the equivalent of two of my 300W PV panels just lost because of the wrong load impedance! . The effective power loss due to load mismatch becomes extreme when solar conditions are not at maximum (i.e., on a sunny day when it is not midday, - which is most of the time!!). Circuits such as LoadMaster are designed to provide Maximum Power Point Tracking (MPPT), they continuously adjust the effective load impedance presented to the solar panel so that the maximum available power is always extracted under any solar conditions and at any time of day. LoadMaster has and will continue to provide us with free hot water for a total outlay of << £1,800 (3kW new PV panels, 150L cylinder, roof rails etc, plus some bargain hunting). The PV panels have a 25 year life guarantee and there is no reason why the system should not last more than 25 years.

My system is configured to transfer water to <40°C by preheating a compatible LPGCombi boiler. Even in November in the UK, a 3kW array provides a large proportion of our hot water usage (2 person household). Of course, on cloudy, rainy days the solar output will be much lower.

Hot water typically accounts for 25-30% of a household's total annual energy budget. This is a straightforward and cost-effective project with PVPhotwater. It has no grid connection issues and is within the capabilities of anyone with qualified DIY and electronics skills. LoadMaster is a small step towards protecting our fragile and densely populated planet. Our planet's climate is on fire. What are you doing to help?

A spreadsheet can (finally) be downloaded to estimate possible hot water performance for a given PV array size and your location.

Loadmaster Specifications

The LoadMaster project is based on ArduinoNano, and the basic circuit architecture is as follows:

C1 is a high voltage film type capacitor effectively connected across the solar array. The small value shunt resistor and voltage divider enable the Arduino to monitor the current and voltage (and therefore the power) produced by the PV array.

As we all know, the impedance of a capacitor depends on its state of charge. When an uncharged capacitor is initially connected, the voltage across it will be low and the charging current will be high, so it will present a low impedance. Once fully charged, the capacitor will present a very high impedance. Ultimately, for MPPT operation, we want to keep C1 in a charged state that presents an impedance that perfectly matches the prevailing maximum power point condition of the PV array. To achieve this, the Arduino quickly switches the load on and off (using PWM at 5kHz) changing the ON:OFF ratio to effectively "shed the load" when sky conditions are not optimal. In technical terms, the ON:OFF switching times of the heater load are adjusted so that just the right amount of charge (Ixt) is removed from C1 to keep the array at its maximum power point. The LoadMaster monitors the PV output voltage and current (VxI = Power) and uses the "PerterbanObserve" (P&O) method to adjust the ON:OFF ratio 10 times per second to track the maximum power under any sky condition.

To minimize losses, the MOSFET is turned "hard" on and off (using a strong gate drive). Such fast switching edges are notorious for radiating RF noise. You certainly can't connect such high power, sharp switching edges to a length of cable (i.e. an antenna) without risking disrupting your wifi, your local radio station, your neighbors, or the radio regulator.

The inductor and C2 provide filtering to limit dI/dt and dV/dt on the external cables. D1 is used to capture the back EMF spike and energy released by the inductor when the MOSFET turns off. While this arrangement may look like a low-side switching buck, it is not! This arrangement is sometimes called a linear current booster - it maximizes output current without overloading the input. With some adjustments, this circuit can easily be used to maximize the current (and therefore torque) delivered to a DC solar irrigation pump.

Using high voltage DC with a specified AC mechanical thermostat could result in arcing and welding of the contacts, potentially seriously compromising the safety of the thermos. The Loadmaster modulates its electrical output waveform by forcing brief periods of zero load current (extinguishing any arcing) at 30Hz intervals. No evidence of arcing was observed using this technique (at 3kW and Vmp155V). It should also be noted that during normal operation the immersion thermostats are not used to control the cylinder temperature, they are simply adjusted to open to a higher temperature as a safety back-up. Temperature control during normal operation is achieved using a sensor (DS18B20) and software.

Loadmaster Specifications

Power supply: -12VDC, 500mA. 2.1mm power connector center +Ve

PV input: - Maximum current < 20A, Maximum Vmp < 200V, recommended array PMax = 3.8kW

Relay output x1 (for Combi preheating/diverter valve control, water below a specified temperature can be diverted to the Combi inlet)

Optically isolated logic inputs - for future connections or spare I/O

RGB Status LED - provides an "at-a-glance" indication of current operating status

Single button control - short press = display toggle. long press = on/off.

20x4 I2C Graphic LCD

Connectivity: -Serial interface available at connector. Tested with HM-10 (BLE), DT-06 (Wifi), and (ESP8266-01S). Serial menu-driven terminal interface provides access to all monitoring and control functions. Wifi module enables LAN or Internet remote access (Telnet). KaiMorich's Android serial terminal app is highly recommended.

Temperature Sensors:-Default DS18B20 sensors are used for radiator, water tank top and bottom temperatures. (Alternatively analog sensors such as MCP9701 can also be used for cables > 12m)

Second Load Output - A second MOSFET channel is included in the hardware and can be used to dump excess power into a second load, such as space heating (see assembly files)

Photovoltaic load capacitance:

This should be a film type capacitor with suitable voltage and ripple current ratings (typically >350VDC and >15ARMS). Suitable capacitors are often used for motor operation or "DC link applications". (Consider capacitors such as the EPCOSB32363 series which offer M10 stud terminals, overvoltage disconnect and are particularly suitable for high pulse applications. These are often used in large DC power supplies or solar installations - look for bargains on eBay!). Electrolytic caps are not suitable.

The capacitor value affects the ripple amplitude of the PV current and voltage around the peak power point. The higher the value, the less ripple. The recommended value is 200 to 400uF.

The capacitors provide high current pulses to the load and are connected to the Loadmasters PV+ and C- terminals. IMPORTANT: - To ensure low cable inductance (which can cause large voltage spikes and possible circuit damage when switching large currents), it is imperative that short (ideally <10cm) and large gauge (6mm^2 or larger) cables are used to connect the capacitors to the Loadmaster. If possible, it is best to connect the positive terminal of the PV array directly to the positive terminal of the capacitor as shown below:

System Design, How Much Hot Water Will I Get?

The average energy (KWhrs) delivered to the tank per day will obviously depend on your location, the size of your PV array, the time of year and other efficiency losses such as shading, panel angle and orientation etc. During the winter some days will be significantly above or below average, this is called weather. In the Downloads section (at the end) the LoadMaster Design Spreadsheet includes an approximate performance calculator, plus there is a SystemSizing document. The following examples show typical performance for the UK and US regions:

An efficient 150L hot water cylinder might have a standing loss of <= 50W. Overnight it might cool by 3-4°C. The final temperature values ​​shown above assume the cylinder starts up each morning at 15°C. In reality, even in mid-winter, if usage is lower for a day or two, then the stored energy will build up to reach the set temperature limit (60°C in my case). A larger cylinder can be used in good weather to efficiently accumulate more energy. The heated water rises by stratification and fills the top of the cylinder. While a larger cylinder can store more energy and hold more hot water, its standing losses affect the net solar gain in poor solar conditions. You need to consider your daily hot water consumption, the average daily energy required to heat the water, and the size and location of your PV array. A balancing act needs to be struck.

Unfortunately, given that everyone has different geographic locations and solar conditions, different PV panel specifications, different array configurations, different hot water consumption needs, different commercial heater specifications, etc., then it is simply impossible to provide advice on every single person’s setup!

A spreadsheet with design calculations and background information is provided in the download to help you understand and make system design choices. Calculating PV energy and water heating performance is not magic, it is just physics, equations, and basic math to get a good idea of ​​possible performance.

Obviously, a person living frugally on a hillside in Oregon who only needs a small amount of hot water for a more comfortable life is very different from a family of four in Brisbane. However! I still get asked "what is the best configuration". Please check out the System Tuning document.

Maybe consider the system I present here, which is a 5Sx2P300W panel array using a 150-180L cylinder driving 2 3kw/240V heaters in parallel.

If more hot water and energy is needed, I would recommend that you not drive 6kw again or 2x3kW/240Vheaters in parallel and use a 200-250L cylinder. (Higher voltage configurations are not needed, higher voltages are more dangerous (Youtube's "DC arc"), switching higher voltages puts higher dV/dt stress on components, plus a 4.5 or 6S configuration of a 60Cell panel (4 or 5S if 72Cell) is usually a good match for many common 240V heater resistors). You are responsible for system configuration, safety and performance!

efficiency

LoadMaster losses are low. MOSFET power losses (RdsON = 17mΩ when cool) depend mainly on the heater load current, I^2xRdsWatts (plus some small switching losses) will result in MOSFET losses IRO2 to 10W. Capacitor losses are negligible. The CR snubber resistor loses about 2.5W (an unavoidable result of dissipating the unwanted "ringing" energy caused by the switching and stray inductance). The total power loss to deliver 2.985kW from a 3kW PV input is 15W, equivalent to >99% efficiency.

PV hot water systems have been shown to outperform solar thermal in winter. Photovoltaic panels are more efficient in cold conditions, producing about 10% more power at -12°C than at 25°C.

The cooling system becomes less effective in cold ambient conditions due to increased heat losses. Smaller temperature differences mean fewer activations of the circulation pump and therefore less energy is delivered to the water.

With PV hot water, even at dawn, dusk or in inclement weather you will still see the LoadMaster pushing all available Watts of energy into the water, even if this is 80W the water is still heating whereas the Solar Thermal system would be completely inactive.

Matching heater resistors to photovoltaic arrays

A key consideration in this design is to select the heating element (power rating and resistance) to match the peak output specifications of the connected PV array. The Loadmaster cannot be connected to any old array and any old heater resistance without considering matching. This is a simple task but must be considered early before purchasing panels or heaters.

It can be seen that at maximum solar array output, the PWM will be 100% and the MOSFET will be continuously on, so the load resistance presented to the PV array will be that of the heating element. Under lower solar conditions, the MPPT will adjust the PWM's ON:OFF ratio so that the LoadMaster presents a higher impedance, matching the reduced maximum power point of the PV array.

Domestic immersion elements are generally 3kW, 240V (R=V^2/P=19.2Ω). An ideal array suitable for such elements would obviously produce Pmp=3kW@240Vmp, which is impractical and too high a voltage for the Loadmaster, and different applications may require different PV powers. In all cases, the PV array should ideally have a Pmp and Vmp relationship such that Vmp^2/Pmp=Rheater.

Ideally, we would aim to have the heater resistance ideally the same as or slightly lower (or at least within +/- 10%) than the maximum Vmp/Imp of the array.

The LoadMaster design spreadsheet includes a worksheet (“Load Matching Configuration”) to help you review possible PV specifications, array configurations, and heater selections.

If a single 3kW 240V heating element (19.2Ω) is used, then for a small hot water consumption system a good array option might be a 5Sx1P array using 275W 60Cell panels (i.e. Vmp = around 32.5V per PV panel). The resulting array would be 1,375W @ 162.5V Vmp = 19.2Ω.

For larger hot water needs, a 5Sx2P array configuration of 60Cell270 to 300W panels used with a heater having a resistance of about 9 ohms (e.g. two 240V 3kW heaters in parallel) may be preferred.

If using a 72 cell panel with a higher Vmd (about 40V), then aim from a 4Sx1P or 2P array configuration.

Avoid exceeding the voltage and power limits of the Loadmasters as this will cause excessive voltage transients and stress on the MOSFETs.

My system uses 2x240V 3kW immersion heaters in parallel (ie 9.6Ω) and has a 5Sx2P array consisting of REC300W (60 element) panels with Vmp=32.5V, producing 3kW@Vmp=162.5V, representing a matched load of (V^2)/P=8.8Ω. This is very close to the 9.6Ω heater load. In hindsight! A 6Sx2P array configuration (ie 3.6kW@195Vmp would equate to a 10.56Ω Mppt load, which would be better matched by the lower 9.6Ω resistance of my paralleled components.

My Direct150L cylinder has two immersion heater elements in parallel. Each 3kw heating element only draws 1.5kw max and should last many years. My excess PV in the summer will be used for battery charging.

Some may wish to use the LoadMaster in smaller, lower power and lower voltage applications, for this purpose a range of DC heaters are available with various resistances and power ratings. In the USA there are a number of AC rated heater elements available:-

3500Wx480V=65.8Ω，1500Wx277V=51.15Ω，1250Wx240V=46.08Ω，1500Wx240V=38.4Ω，2000Wx240V，20V=20Wx20V，20V=20V，20Wx240V，20V=20V240V=19.2Ω，2500WX208V=17.3Ω，4500WX277V=17.05Ω，3500WX240V=16.46Ω，3800WX240V=15.16Ω，4500WX240V=12.8Ω，3500WX208V=12.36Ω，5500WX240V=10.47Ω，6000Wx240V=9.6Ω，5000Wx208V=8.65Ω，5500Wx208V=7.87Ω，6000Wx208V=7.21Ω。

Safety!

Hopefully by now you've discovered that the project includes:

High voltage, high current, high power (risk of electric shock and fire)

Hot water (scalding risks and understanding the safety issues of pressurized hot water storage systems)

Capacitors with high charge energy (never short this or any high voltage - burns, sparks, eye injuries, etc.)

Install solar panels at height, etc.

As with any commercial high power solar inverter or charger, the electronics must be housed within a sealed metal grounded enclosure to provide a degree of protection against fire. This project is as safe as any other mains powered project if you apply the relevant safety precautions and common sense. I take no responsibility for your project and if you doubt your knowledge, skill or appreciation of the safety aspects involved then I really recommend that you consider a different project!

The power supply side of the circuit was modeled using TiTina.