Off-grid and Renewable Energy Power Conversion

An off-grid system is a chain: something generates, something stores, and something converts stored energy into the form the loads need. Each link is sized against the others, and the whole system is limited by whichever was sized optimistically.

What distinguishes these installations from grid-connected work is that the DC bus is not a stable rail. A battery bank moves substantially between fully charged and deeply discharged, and it sags further under load. Equipment specified against the nominal bus voltage will misbehave at both ends of that range, and the site where it happens is usually the one that takes half a day to reach.

Modified sine wave or true sine wave?

A true sine wave inverter produces an output resembling the utility waveform. A modified sine wave inverter produces a stepped approximation that is cheaper to generate and adequate for some loads.

Resistive loads — heaters, incandescent lamps, most simple tools — do not care. Beyond that the list of loads that object is longer than it first appears. Induction motors run hotter and less efficiently on a stepped waveform because the harmonic content produces losses without torque. Many switch-mode supplies behave unpredictably. Audio equipment picks up audible buzz. Some medical devices, and a good deal of modern electronics with power factor correction on the input, will not operate at all. Mains-dimmable lighting and anything using the zero crossing for timing is unreliable.

The modified sine wave category is genuinely the economical answer for a known, simple, resistive load set. The true sine wave category — the NTS, NTU, NTN and TN families across a broad power range — is what a general-purpose installation needs, and the difference in cost is smaller than the cost of discovering at handover that the site's one critical load is incompatible.

Size for surge, not for running watts

Motors, compressors and pumps draw several times their running current for the first moments of a start. A refrigeration compressor rated at a few hundred watts can demand well over a kilowatt while its motor comes up to speed, and an inverter sized on the running figure will shut down on overload every time the fridge cycles.

Inverters are therefore specified with two numbers: continuous power, and a surge rating for a stated short interval. Both matter, and the surge figure is meaningless without the duration it applies to — a unit holding twice its rating for a few seconds is a different product from one holding it for a fraction of one.

Working through it properly means listing the loads, identifying which ones have a starting surge, and then deciding whether they can start simultaneously. Two compressors that can start together need a surge rating covering both. Where the loads can be sequenced — staggering a pump and a compressor with a simple timer — a smaller inverter often does the job, which is worth considering before buying capacity that exists only for a few seconds a day.

Solar pumping and driving DC loads directly

Every conversion stage costs efficiency, and in an off-grid system the losses come out of a generation budget that is fixed by the array. Where a load can be driven from the DC bus directly, skipping the inverter is worth real energy.

Water pumping is the clearest case. The solar inverter category holds the PV-SMI and ISI families for this work, and the DC-input drive modules in the VFD category accept 48 V DC directly and produce the variable-frequency output an AC pump motor needs. Feeding one from the battery bus avoids inverting to AC only to rectify it again inside the drive.

Variable speed suits solar pumping for a second reason. A pump that can run slowly when the sun is weak keeps delivering water instead of cycling on and off at the threshold where a fixed-speed pump can just barely start, and cycling is what wears out pumps and contactors.

Designing around a bus that moves

A nominal 48 V battery bank is not 48 V. Fully charged and under charge it sits well above that; deeply discharged and under heavy load it sits well below. The span between those two states is the input range every piece of connected equipment has to cover.

Specify against the extremes rather than the nominal figure. An inverter that shuts down at a threshold the bank reaches during a normal evening's discharge will trip on an ordinary night, not an exceptional one. Equipment with a wide input range — the 2:1 and 4:1 input ranges common in DC/DC converters — is what gives margin, and the margin is more valuable here than in almost any other application because the alternative is a site visit.

Cable resistance compounds it. Currents on a low-voltage DC bus are large, and the drop between the battery terminals and the inverter input is subtracted from an already reduced voltage at exactly the moment the bank is at its lowest. Conductor sizing on the DC side of an off-grid system is part of the electrical design rather than an installation detail.

Charging from an intermittent source

Charging equipment in an off-grid system has a harder job than a mains charger, because its input is not dependable. Generation varies through the day and disappears at night, and a charger that expects a steady input has to cope with one that is not.

Where the site has a generator or an intermittent grid connection, chargers from the charger category handle the AC side. The battery charging page covers the profile and chemistry questions, all of which apply here unchanged; what off-grid adds is that the charger must handle being interrupted mid-cycle repeatedly without mismanaging the pack, and must tolerate the voltage and frequency a small generator actually produces rather than the clean supply a laboratory does.

Temperature is the other off-grid specific. Equipment in an unventilated enclosure in direct sun runs far above ambient, and derating curves have to be read against that internal temperature. A cabinet that is comfortable in the spring commissioning visit is a different environment in August.