Managing Battery Charging Using Diversion Loads

 

PVcharge regulation can be relatively simple: When the batteries are full, the controller disconnects the PV array. Adding wind or microhydro to the system makes charge regulation more complicated, since turbines may overspeed if unloaded.

Often, diversion control is the solution. Off-grid “hybrid” combinations of solar, wind, or microhydro have been around for decades. These renewable electricity systems can power buildings and other sites far from the electrical grid. But hybrid systems can present unique design challenges, since multiple sources of generation—for example, a PV array, wind turbine, and a backup generator—increase the complexity of how to control battery charging so your batteries don’t get overcharged, and damaged.

In a PV system, the charge controller is placed between the energy source and the battery. Its job is to regulate the voltage and current coming from the energy source to charge the battery and protect the battery from overcharge (and damage). Modern charge controllers have a three-stage charge cycle. During the “bulk” phase, the voltage rises to the bulk level while the batteries draw maximum current. Once this level is reached, the absorption phase begins: the voltage is maintained at the bulk phase level for a specified time, while the current tapers off as the battery reaches a full charge. Once the battery is fully charged, the voltage drops to the float level and the battery only draws a small current until the next cycle.

Today’s charge controllers mostly use pulse width modulation (PWM) to control current into the battery. Pulses of current in rapid succession are allowed to pass from the energy source to the battery (or from the battery to the diversion load—more on this to follow). The controller cannot limit the size of this current, but instead controls the duration so that it can achieve the correct average current in the circuit. By modulating the width of the pulses, the controller regulates the battery charge rate. The target is to maintain the correct battery voltage for the prevailing stage of the charging cycle.

Some charge controllers also offer maximum power point tracking (MPPT), which no longer chains the input voltage to the output voltage. By doing so, it potentially allows more energy to be harvested from the generating source.

With a PV array, charge control regulation is fairly straightforward. When the batteries are full, the controller throttles the voltage/current accordingly to prevent overcharge, turning off the array in a sophisticated way. Chargers and inverter/chargers connected to generator power control the charge rate in the same way, limiting the load on the generator to regulate the charge rate. But charge control with wind and microhydro turbines is more difficult, since some turbines can overspeed when unloaded (i.e., disconnected from the battery). In an unloaded condition, a turbine will “freewheel,” increasing rpm and voltage. Excessive freewheeling can damage bearings and rotor components, and harm the electronics with the high voltage that’s produced. Turbine output must remain connected to a load at all times, yet not be allowed to overcharge the batteries. And this is most effectively accomplished with a diversion controller.

Diversion-Load Control Options

Diversion controllers are an effective method for managing wind or hydro turbine output and preventing battery overcharge. The turbine output is connected directly to the battery, in parallel with a diversion load controller (usually separate from any PV controller). This device shunts excess energy from the battery to a diversion (aka “dump”) load, usually a large resistor in air or a water-heating element, either of which is sized to enable constant turbine operation at its full output.

Some PV charge controllers can be reconfigured as diversion controllers for managing turbine output. Commonly available models range in size from 35 to 60 A for various nominal (12 V, 24 V, 48 V) battery voltages. These controllers typically allow field-selection of battery voltage; mode of operation (charge, load, or diversion control); and voltage setpoints for battery charging or load management. Often, a combination of dip switches, jumpers, and potentiometers are used to adjust these settings.

Programming the controller diversion setpoints is the same as setting the charging setpoints for a PV system. The goal is to set the bulk- and float-charge parameters to ensure full battery charging—without overcharge. However, if two separate controllers are being used, it may take a little time to adjust these parameters to keep the two units working together properly. It may be helpful to program the diversion controller to slightly higher or lower setpoints than the solar controller so as to encourage or discourage diversion of power to the dump load as desired. For example, a 48 V system with flooded batteries that has the PV bulk charge set to 58.4 V could have the diversion controller set to divert power during the bulk-charging cycle at 58.6 V so as to minimize diversion of PV power. Or vice-versa—for example, if we want to maximize the energy capture into the dump load for water heating. Float- and equalize-charge setpoints would be fine-tuned in a similar manner, but bear in mind that the two units may not agree on the timing of these stages of charge. The default strategy is to set the two devices to the same setpoints that best suit the battery charge regime.

Using the Aux Output for Turbine Control

Commonly used in PV systems, maximum power point tracking (MPPT) charge controllers may also offer options for turbine control. MPPT means that the input voltage of the controller is adjusted to maximize the PV array’s productivity. This function is independent of the actual charge control process and offers the advantages of higher-voltage transmission as well as enhanced energy production. On the charge control end of things, most PV charge controllers have at least one auxiliary output feature for diversion control of battery charge rate. Blue Sky Energy, MidNite Solar, OutBack Power, and Schneider Electric all have MPPT models with auxiliary output and programming to support diversion-load applications. This allows for the connection of turbines and/or for maximizing energy usage by diverting excess solar energy to useful heating. Blue Sky Energy offers its Duo Option Diversion Control upgrade component, which converts the auxiliary output for its Solar Boost 3024 controller into a 20 A diversion control. This unique conversion allows simultaneous MPPT operation for the PV array while also diverting up to 20 A through the converted auxiliary output. (Note this controller model is limited to 12 V and 24 V battery systems.) This can be a good solution for a smaller system, since it minimizes the space required for control equipment. Another upgrade option can increase the diversion capability to 40 A.

Dedicated MPPT Controllers for Turbines

MPPT voltage conversion can also be used to maximize turbine output in some cases. Some MPPT controllers can track microhydro turbine output by adjusting a few setpoints. In this application, the turbine’s DC output would be connected directly to the charge controller’s DC input, much like a PV array. MidNite Solar and Morningstar also offer wind turbine MPPT functions in some of their controllers. However, wind turbine output cannot be tracked to find maximum power. The installer must enter values or a “power curve” into the controller’s memory to suit the particular wind turbine.

MidNite Solar’s integrated solution couples its Classic charge controller with the “Clipper” add-on. This unique solution can be used for both wind and microhydro applications. The Clipper provides protected DC input into the charge controller, which in turn performs MPPT-like management of the turbine for maximum power output. Advantages include built-in components, such as diversion load, solid-state relay, and a run/stop breaker. The Clipper is similar to the integrated diversion controls offered with some wind turbines. Units are available in DC and AC options. AC models rectify the turbine’s wild AC output into controlled DC input for the charge controller to process. The resistor bank can be configured to match the turbine’s output, and multiple units can be paralleled for larger turbines.

Morningstar has a variety of controllers designed for MPPT for PV and wind systems, including the MPPT 600 V TriStar controller. This controller has a battery output rating of 60 A and also can be paralleled (up to four units) for larger wind units. However, these controllers do not have auxiliary outputs for diversion control, nor does Morningstar offer DC voltage protection add-ons. OutBack Power’s FLEXmax series (60 and 80 A models) can be used for hydro but not wind. They do have diversion features but no protection add-ons, so it is important to ensure that the turbine output cannot exceed the Voc of these controllers.

Diversion Load Control Approaches

The wind and/or microhydro turbine and the type of charge controller used for the PV array are two main factors in determining an optimal diversion-control strategy. For a PV array and a turbine with DC output, a common scenario is to use an MPPT charge controller for the array with an auxiliary output to control diversion of turbine energy. Used with a relay and an air-heating diversion load, this is one of the most economical, and easiest, approaches. Depending on the voltage and wattage ratings, an air-heating dump load with a wattage range between 1,000 W and 2,100 W can cost between $150 and $250; a suitable DC-rated relay may cost about $50. Mounting a prefabricated air-heating diversion load can often be much easier than retrofitting a DC circuit to a water heater tank.

For larger systems, multiple dump loads may be necessary to handle full output diversion from the turbine. The number of relays and the number of dump loads will depend on their power ratings. Loads can be paralleled to achieve a sufficient capacity. In smaller hybrid systems, or ones that receive only seasonal use, a separate PWM controller for managing the turbine and preventing battery overcharge is common, since these systems may not be able to fully realize the advantages of MPPT charge controllers. This system type could be designed with two separate PWM controllers—one unit manages the PV array, and another manages output from the turbine. Costs depend on the size and model. For a 24 V system with turbine output of 25 A or less, a basic PWM controller rated for 40 A with a 25 A to 40 A diversion load would cost about $300. Adding another PWM controller for the PV array brings the cost to $450—still less than most MPPT controllers, which typically start at $550.

Diversion Load Sizing

Properly sizing the diversion load is fairly straightforward, but important—if too small, it will not divert all of the turbine power, subjecting the battery to overcharge. If the diversion load is too big, it could overload the controller and cause it to disconnect, leading to unregulated battery overcharging. Add up all of your uncontrolled charging sources (i.e., wind turbine and/or microhydro turbine), then include a safety factor—the 2011 National Electrical Code (NEC) suggests adding a safety factor of 150%. (This factor accounts for potential spikes in output and provides longevity of the load component since, under normal turbine operation, the load would only be operating at two-thirds capacity.) Then, choose a diversion controller with a rating equal to or higher than this (see an example in the “Diversion Load Calcs” sidebar). Choose a diversion load equal or higher to this, but not any higher than the controller’s capacity.

Using Your Diverted Energy

Stand-alone renewable energy systems suffer a bit of a disadvantage in relation to grid-tied ones, which is that much of the energy they can produce will be wasted when the battery is full. With grid-tied systems, the excess is always exported; with stand-alone systems, you have to use it, or lose it. True conservationists will tailor their usage habits relative to the battery voltage, effectively acting as human diversion controllers, with a mission to maximize the effective use of precious energy: washing clothes, sawing firewood, running the vacuum cleaner, when the wind is blowing or the sun is blazing.

But not everyone wants to have their lives dictated by battery voltage. How can we automate usage and prevent wasting energy? The obvious answer is to use the surplus as a useful source of heat, which is the secondary function of diversion loads, which create heat energy from surplus electricity. Often, these devices are simple wire-wound resistors, installed in the power shed, that “dispose” of the energy surplus safely and economically. We can harvest some (or all) of this heat and avoid burning fossil fuel to heat water or to keep warm.

Two main types of diversion loads are air-cooled resistors and water heating elements. The advantage of air-cooled resistors is that they are always available (nobody will turn them off). But PWM-driven wire-wound resistors tend to be noisy, and their whining is usually unwelcome in a living space, so this heat is usually just “dumped” into a power shed.

Water heating elements in a water heater tank are another type of diversion load. They can provide water heating and perhaps space heating. If your renewable energy system is adequately sized to maintain the battery at a healthy state of charge, quite a bit of surplus heat energy will be available to divert. The disadvantage is that you cannot readily put a thermostat on these heaters because they must always be available to control battery charge, and they will be working on DC, which may damage the thermostat.

To use the output of the diversion charge controller directly, you will need to obtain special elements designed to work at lower voltage, or put up with much lower power output from heaters designed to work at grid voltage—for example, 120 VAC heaters will only yield one-quarter of the heating capacity if operated at 60 VDC.

The solutions to this problem are many and various. One option is to obtain specialized low-voltage heating elements, or use inverter power to operate grid-voltage elements via a relay. In the low-voltage case, you can use a changeover relay to shift the power to an air-heating element when the water heater thermostat opens, or you can design a water system that copes with the excess heat without the need for a thermostat. In the inverter-powered case, you can prioritize diversion to hot water via the inverter, but program a backup diversion at battery voltage that takes over after the water reaches a certain temperature.

Diversion control of battery charge is more than just a way to deal with renewable energy from turbines. It’s an opportunity to increase the efficiency of your energy system by automatically using any energy surplus. The amount of effort you put into this needs to be commensurate with the gains, but in the case of hydro turbines, for example, there can be a very large amount of surplus and it is well worth doing.

Manufacturers

Apollo Solar • apollosolar.com

APM Hydro • apmhydro.com

Blue Sky Energy • blueskyenergyinc.com

MidNite Solar • midnitesolar.com

Morningstar • morningstarcorp.com

OutBack Power • outbackpower.com

Schneider Electric • solar.schneider-electric.com

Solar Converters • solarconverters.com

 

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