Harvesting Surplus Energy, Off-Grid

Intermediate

Inside this Article

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.

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.

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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