By: Hugh Piggott
Published In:Home Power Magazine
Issue #179, May / June 2017
Many off-grid users of renewable energy abhor wasting energy. We obsess about load efficiency, switching off lights, and putting phantom loads on plug strips. But few people realize how much energy is wasted by charge controllers. This article can help you use most of your system’s available energy.
PV system generates electricity during the sunny hours (as do wind turbines in windy hours), but much of this energy is needed at other times, such as evenings or periods of calm. The solution to this mismatch is to store energy in batteries.
RE sources will produce much more energy on one day than on another, depending on the weather and the season. Surpluses occur when the battery and the loads cannot absorb all the available energy. Also, the rate at which a battery can absorb current tapers off as the battery approaches its fully charged state. For a battery to remain healthy, this situation ought to be commonplace, but it often results in unused energy.
The principle of charge control is to regulate the battery voltage to an optimum level for the specific stage of the charging process. The installer must program the controller with the correct voltage “setpoints” for each stage—absorption, float, and equalize. At first, the battery will need a high charging current, but this will taper off over time, even though the voltage is kept at its setpoint. A quality charge controller uses information from a temperature sensor to further adjust the charging, and it runs a timer to determine when the absorption stage is complete and the battery is “charged.” After this, it will limit the current to a very low trickle that maintains the float voltage.
A PV charge controller limits the current going into the battery bank based on the setpoints. This prevents the battery from charging too fast, which can result in damage. The downside is that it also reduces the system’s efficiency by using less energy. Wind and hydro sources are not as easy to control. If their generated output is not used, turbines can be damaged by overspeed. For these sources, we must use a diversion controller that shunts unwanted energy into a load. This “protective diversion load” or “dump load” protects the battery from overcharging and the turbine from overspinning, but it can waste energy. The key to improved efficiency is using “opportunity diversion loads” instead of, or as well as, protective diversion loads.
Lifestyle Adjustments
Most off-gridders try to get the laundry done and the floor vacuumed when the sun is shining (or wind blowing) and the batteries are full. Just as we switch off loads as the battery voltage falls (due to reduced RE), we try to use electricity when the voltage is high. We’re taking advantage of energy that would otherwise go to waste.
In my home, for example, we have a single-burner induction cooktop that does most of our cooking when there’s ample electrical energy available. It’s a great feeling to use free, clean energy and to avoid the cost and pollution of using propane that is also likely derived from fracking.
While there are aspects of this that are satisfying, constantly having to monitor system energy can be irksome. Most of us have other priorities in our lives, and that’s where “opportunity diversion” comes in. A diversion relay can often do our job better than we can because it has no other purpose in life than switching things on and off automatically. Don’t ask a relay to make your breakfast, but it can heat your water tank, pump your irrigation, or switch on air conditioning.
Diversion Controllers & Relays
One way to set up an opportunity load is with a separate controller. Use a second pulse-width modulation (PWM) controller, such as Morningstar’s TriStar or the Xantrex C-40, configured for diversion mode, to do the job of controlling the battery voltage. Keep your solar controller for the sake of its maximum power point tracking to maximize energy capture. Set the MPPT controller’s charging setpoints slightly higher than the PWM unit’s setpoints, so the PWM is activated first by rising voltage. If your MPPT controller has no means of driving a relay, then adding a second controller is a good way to set up opportunity DC water heating.
Often, your MPPT controller will “know” when there is excess power. MidNite Solar’s KID charge controller can be set to “PWM Divert” and run a DC load (like a heating element) directly on its load output. Many of Blue Sky Energy’s SolarBoost controllers can also switch loads using an internal 20 A relay, while its DUO upgrade option contains a PWM diversion control function. Most other makes of MPPT controllers offer an auxiliary output or “aux port” that can produce a 12 V signal (or close some switch contacts) when a battery voltage setpoint is reached. Connect this to the coil or input of a relay and it will switch on a load to make good use of the surplus energy.
Aux ports need to be configured for a particular “mode” that determines the criteria for switching. MidNite’s Classic and OutBack’s Flexmax charge controllers offer modes that energize their aux ports when the battery voltage setpoint is reached for the prevailing stage of charging—absorption, float, or equalization—just like a dedicated diversion controller. If you plan to use diversion whenever possible, then you should use these modes. In other MPPT controllers (and inverters), the aux port modes offer only fixed voltage setpoints. A fixed voltage setpoint for diversion will either be too low to allow proper absorption or too high to be activated during the float stage of charging. It may work well for relatively low power loads or for heating a small water tank with a thermostat that opens after an hour or so, but otherwise it will prevent your RE system from properly charging the battery.
Some controllers offer Aux port modes that signal when charging has reached the float stage. Several offer modes for a certain percentage state of charge (SOC). These modes may be worth considering for operating motorized opportunity loads, such as irrigation pumps that can only work at full power. But they will miss out on the gradually rising surplus of power that occurs during the absorption stage.
High-array-voltage triggers are another possible mode to use for diversion. If the controller is rejecting surplus PV power, then the array voltage will rise beyond the maximum power point. You can choose an Aux port mode to trigger diversion as the array rises a little above its normal, observed MPP voltage. This should not interfere with the battery’s ability to achieve absorption voltage setpoint, but your trigger point may be a moving target. This mode is ideal for hydro turbines, but less so for PV arrays, as the energy capture drops fast as the array voltages rises above MPP voltage and the MPP voltage will vary with array temperature.
Relays
A relay is a switch for high current that is operated by a tiny current. Older mechanical relays use an electromechanical solenoid to close physical contacts. They can open and close many thousands of times but will eventually wear out. Choose these relays for modes that cycle on/off over periods of minutes—for example, driving motorized loads. Use a “plug-in” relay that is easy to replace every couple of years.
For more rapid cycling (many times per second) conditions, choose a solid-state relay (SSR) that uses semiconductor technology. SSRs are more costly and they need a heat sink, but they can be driven fast enough to implement pulse width modulated (PWM) switching that ramps the average diverted current up and down smoothly.
Use a solid-state relay (SSR) when you need frequent, rapid switching. For example, an OutBack or MidNite controller’s Aux port in PWM mode linked directly to the input terminals of a Crydom D1D40 SSR can reliably modulate a DC load up to about 25 A at 60 V (2.4 ohms, 1,500 W). Mount it on a heat sink rated for less than 2°C temperature rise per watt. You can also use an AC SSR to switch AC loads via the inverter, but a heavy load may cause your lights to flicker as it pulses.
When choosing a relay, make sure the is rated well above the working voltage and current. As with most products, they have a higher failure rate when pushed to their limits, so good safety margin, such as a factor of two, is wise. Some relays are designed for AC, and some for DC—which require heavy-duty contacts due to arcing potential. Aux ports typically provide 12 V, which will work for most SSRs. If you only have aux contacts switching 48 V battery power, it would need to somehow be stepped down below 30 V to be usable for SSR input. Mechanical relays can be found with many different coil voltages, including 48 V DC.
You can set your relay to operate a DC heater that draws current directly from the battery. But if your inverter is large enough, you may prefer to divert to an AC load, which has the same effect. The Aux ports in OutBack inverters have an “AC diversion” mode that prevents overloading the inverter. Irrigation pumps and air conditioning are among the possible loads, along with AC water heating elements. (Note that 120 V or 240 V heaters are easier to find than battery-voltage ones, and most thermostats are not designed to switch high DC currents.)
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.
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