Harvesting Surplus Energy, Off-Grid: Page 3 of 3


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With wind and hydro systems that can’t be allowed to operate at open-circuit, dump loads, like these resistance air heaters, are necessary—but they usually don’t do useful work.
ProStar MPPT charge controllers can switch diversion loads directly (based on a fixed battery setpoint), using LVD mode with a rather high setpoint.
Morningstar TriStar can be configured for stand-alone PWM diversion using simple DIP switches that also select the battery type.
Blue Sky Energy’s charge controllers (left) mostly contain a 20 A load relay that can be used for diversion. Also, the DUO Option software upgrade converts the Solar Boost 3024 controller’s auxiliary output into a 20 A diversion-type PWM charge controller.
OutBack Power’s FLEXmax series can divert PWM current to loads via SSR based on prevailing battery-charging voltage setpoint.
The Schneider Electric C-series of PWM charge controllers can be configured for stand-alone PWM diversion using jumpers. Two potentiometers and a multimeter are used for adjusting the battery-charging setpoints.
MidNite Solar’s Classic controller has two aux outputs, one of which can divert PWM current to loads via SSR. The KID controller offers PWM diversion directly (without SSR) from its load terminals. In both cases, control can be based on prevailing battery-charging voltage setpoint.
MidNite Solar’s Classic controller has two aux outputs, one of which can divert PWM current to loads via SSR. The KID controller offers PWM diversion directly (without SSR) from its load terminals. In both cases, control can be based on prevailing battery-charging voltage setpoint.
Morningstar products can operate relays based on a variety of parameters via the optional Relay Driver module. Their MSview software has many advanced tools for customizing relay drivers and charge controllers.
Solar converters offer low-cost stand-alone circuit boards that you can build into your own control cabinet. The LDR is a PWM load controller, whereas the VCS is a mechanical relay that switches on and off based on battery setpoints configured with a screwdriver.
Mechanical relays are simple, inexpensive, and efficient, but they switch more slowly than solid-state relays. They have a limited service life, so it’s wise to buy a relay base that you can plug them into for easy replacement (as shown in the lower photo).
Mechanical relays are simple, inexpensive, and efficient, but they switch more slowly than solid-state relays. They have a limited service life, so it’s wise to buy a relay base that you can plug them into for easy replacement (as shown in the lower photo).
Solid-state relays are ideal for PWM or rapid, frequent switching, as they will not wear out, but they do generate some waste heat and need a heat-sink to prevent damage.
Specialty water heating elements often come with several subelements. Use them in series for higher voltage or for lower power. Use in parallel for operation at nominal voltage, switching as many as needed, using one or more relays.

Water-Heating Elements

The most popular opportunity diversion load for off-grid PV systems is an electric heating element in a large hot water tank. Heaters do not care whether they get AC or DC, but they are sensitive to voltage. For example, a 1,600-watt 110 V heater will only give 400 W at 55 V as a diversion for a 48 V battery system. Half the voltage means half the current, and thus only one-quarter of the wattage.

When choosing the heating element, there is no need to aim for high wattage. Diverting low power steadily works better than a very powerful heater. The big load will switch on and off frequently when there is only a small excess, cycling the battery and creating power quality issues, such as flickering lights. Higher power loads also need heavier wiring. If the controller’s aux mode works at a fixed voltage setpoint, then it is preferable to use a lower-wattage diversion so that the battery can still reach full absorption voltage later in the day, even with the heater active.

If you cannot find a standard AC water-heating element that works at your battery’s voltage, then you can buy DC elements online. Often, these have multiple subelements that can be configured in series or parallel to match your system’s battery voltage and optimum power. For safety’s sake, put a notice next to the drain valve to remind you where to turn your heater off before you drain the tank!

Safety & Thermostats

Even a small heater operating over a long period can produce dangerously hot water. The conventional solution is to use a thermostat to turn the heater off. But switching high DC current may damage a standard thermostat.

One solution is to use an AC water heater that draws power through the inverter. Another is to use a very large tank which, due to the greater volume of water, will be less likely to reach scalding temperatures. A third option is to wire the relay-control signal through the thermostat, so that when it opens, the relay turns off the heater or diverts the current to another load.

If you use a PWM diversion controller to run your water heater, then various strategies are possible. The TriStar has a battery-voltage-sensing circuit that can be wired via the thermostat. When the contacts open, the sensing is diverted through a diode string. A couple of diodes step the voltage down by a volt or so, making the controller think the voltage has fallen, and it turns off the heater. As the actual battery voltage rises further, the MPPT controller starts to limit the charging rate, so the TriStar is defeated. Be aware that a voltage difference exceeding 5 V will produce an error in the controller.

The Generator Paradox

A minor challenge arises when a generator is connected to an inverter-charger, as the inverter-charger attempts to push the battery voltage up to its own charging setpoint. Often, this is coordinated with the MPPT controller’s setpoints in a control system common to both. If the diversion controller has a lower setpoint, it will divert generator power as if this were another opportunity to harvest excess PV energy—but it is not. You can defeat the diversion load using a relay that opens its (normally closed) contacts when its coil is energized by the generator’s AC voltage. The relay may simply interrupt the heater circuit, or again be used to distort the battery sensing of a TriStar controller (as before).

Wiring Examples

Choose wire with suitable ampacity and temperature rating, and also check that the voltage drop is acceptable. You need appropriate wire terminals, circuit breakers, and a heat sink or relay socket. Conform to all local codes, and hire a professional electrician if necessary. Read the manuals and plan how to program your controller(s) to optimally charge your particular battery type. In some cases, you may find useful videos on YouTube.

The diagrams show some of the possible wiring configurations for using relays to drive diversion loads based on signals from aux ports in MPPT controllers. Before you start, shut down any turbines and turn off all circuit breakers, starting with your PV array. Take note of the polarity of the wiring between the aux port and the SSR input, connecting positive to positive. Connect the positive of the SSR output to the battery positive busbar through a suitable breaker that is rated above the heater current and below the wiring ampacity. Double-check that everything is correct before powering up the system.

Looking Beyond

There are other techniques for harvesting surplus PV energy. For example, Morningstar’s Relay Driver can be networked with charge controllers and programmed with a computer to operate diversion loads according to a wide range of criteria. Another method uses the diversion controller’s DC load output as a signal to trigger a special type of SSR that modulates the inverter’s current to an AC heater using phase-controlled switching. This combines the convenience of a conventional AC heating element with the smoothness of PWM control—but it’s a component-level project beyond the scope of this article.

This is a fascinating arena for creative homebrew, but “turnkey” products are rare. It’s sad to think of all the solar energy that is wasted because manufacturers and installers consider it such a low priority.

Comments (6)

Mark S's picture

Is there a reason that the "View article as a single page" does not function and that these articles are not available as downloadable PDFs?

Michael Welch's picture
Hi Mark. The single page view does work, providing the entire text of the article. It is strange and potentially confusing, though, that over to the right it still leaves the option to choose which page you are on.

As a digital subscriber, you can download that entire issue, but we don't have a setup for downloading single articles.
dave@atomicsolar.biz's picture

I like using a 120V (or 240V depending on inverter) water heater element(1000-2500 watts), placed in the top element location in a conventional electric water heater, to dump excess. I use a relay and aux 2 output from a Midnite Classic charge controller. It is a bit easier than dumping DC, as you can use smaller wire and cheaper AC elements. This applies to places where NEM is not available, or self consumption is a priority.

solarKings's picture

Enphase's AC Battery is a good residential storage solution which allows you to build out a solar battery farm in your home; each storage battery is independent of the others, uses safe Lithium-Iron-Phosphate, has a long longevity, is scalable up to 14 units on a single DP 20A circuit breaker, and can be installed into any existing or new solar PV system. Because it it is an integrated AC solution combining inverter with charge-controller and battery, it can be installed in under and hour and connect right to a home's utility lug panel. It is an intelligent battery that can be easily monitored from your computer, cell phone, tablet, etc. Cost is about $1,000/kWh installed.

fowlerrudi's picture

Has anyone ever tried a geothermal "invertor series" heat pump that would directly modulate with available surplus power to store hot water in a storage tank? I am considering trying this using by pulling data from Schneider Electric's Combox (or a solar insolation sensor combined with relay from Combox) to adjust load on a variable speed compressor geothermal heat pump off grid. Storing hot water in 1000 gallons of insulated storage for low-temp infloor heating system. Thoughts?

Secundius's picture

@ fowlerrudi.

I don't know if this will answer your question or not?/! But consider placing "SharkBite" PEX/XLPE (Polyethylene) Tubing under the Flooring or Foundation of you're Homestead. Pressure Rated at ~160-lbs/in. and Temperature Rated at ~200F. Comes in 1/2-inch by 500-foot lengths. Some Universities use it as a "Poor Mans" method of Shoveling Snow, to keep the Walkways clear of Heavy Snow and/or Ice. Try "FarmTek.com" for better Information or their Zendesk (Customer Services), their Seasonal so some Info Digging might be required...

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