December 11, 2024 Aaron Hanson

The Generator Rut & Modern Hybrid Power Systems

In an off-grid or hybrid power system, the generator is often the most problematic piece of equipment. Generators usually don't communicate with the rest of the system and seem to fail at the most inconvenient times.

To be fair, this occurs mainly because these units sit unused for long periods and suffer from poor maintenance. Another major factor is the flood of low-cost, low-quality generators from discount retailers into the market. As expected, you get precisely what you pay for—a shorter generator life expectancy and a throwaway unit at the end.

Generators also take up quite a bit of space since they're typically oversized for their applications—whether being used in a home, RV, emergency vehicle, security trailer, or other mobile power system.

In this article, we'll discuss current limitations with generators, the technological advances we're experiencing now, and our recommendations for engineers and manufacturers integrating generators into hybrid energy systems for off-grid and mobile markets.

Generators are not an obsolete technology

During overcast skies, snowfall, or when the power grid is unavailable, generators become your only viable option. While a power system without a generator can meet about 80% of needs in many climates—and some sites with large battery banks might rarely need one—the vast majority of off-grid power users require a generator. People use the term "backup generator," but if it's required for the job, it's not a backup. The backup is the extra one you have ready to go when your primary fails.

So, the key question becomes: How do we minimize our generator use, fuel consumption, and maintenance issues? You may have specific priorities your system around, for example, noise reduction or CO2 emissions. If your efficacy calculation is based on the time between refueling or servicing, we can combine these metrics into one that best addresses your specific needs.

As a refresher, a standard generator consists of an engine connected to an electrical end, the generator. The engine runs at a relatively fixed RPM because there's a direct relationship between its speed and the output power frequency. This is called a fixed-speed generator, with its speed determined by the number of poles in the generating unit and the RPM. If the engine runs too fast, the frequency will spike above normal—and if it runs too slow, you'll get a nice low frequency.

Oversized = Inefficient

Traditional generators are sized for maximum expected peak power, as shown by their fuel consumption versus output efficiency charts. These generators typically reach peak efficiency at around 70% of their total potential output. The challenge is that as power demands fluctuate, there's only one peak efficiency point—and this ideal operating point rarely matches where the generator typically runs. How bad can this mismatch get?

Consider the worst-case scenario: a generator idling without purpose. It's like leaving your car running outside while you go to work, only to return to an empty gas tank. What did that wasted fuel get you? In diesel engines, idling is known as 'cold stacking.' Without a load, diesels create excess carbon on their valves, and in the end, you can seriously damage these engines by 'not working them enough.'

You might say, "I would never let my generator run without a load—that would be foolish." However, see the graph below, which shows the charging curve for a lead-acid battery.

As you can see, the float stage operates at a painfully low current, yet that's necessary to maintain your batteries' full charge. This low charge range effectively idles your generator. Lithium batteries have revolutionized this process, especially with communication capabilities that allow charging at full rate until completion. Their ability to communicate means we can precisely determine when charging is complete for our purposes. This might be 100%, but it is also often 85% to give the solar plenty of work to do.

The point here is that advances in one technology, like lithium batteries, often lead to improvements in generator capabilities—especially when different parts of the system can communicate with each other.

Variable speed generators

These are typically smaller gas generators designed for portable applications. They run quietly, partly because they can idle at lower speeds but mostly because they are sound-damped to the max. The RPM can vary because they have an inverter that converts the generator output to 60 Hz, 120V power. These are very popular and really changed the market when they were released in the early 2000s.

While options are limited for larger sizes, these generators significantly improve portable use. Some users have attempted to add auto-start capabilities, which we've supported when requested. Though some models have remote control buttons for activation, the remote start options are still limited. Honda and Yamaha pioneered this market, but now, 10 to 20 other brands compete for your portable inverter-generator business. You can install stepper motors to run the choke and hotwire the push button. We have seen companies do this to take advantage of the functionality of an inverter-generator even with the added cost and complication of these custom modifications.

Diesel generators

Why do semi trucks use diesel engines instead of gas? The answer is simple: better fuel efficiency and longer service life. Like it or not, the energy density of a gallon of diesel fuel is phenomenal. In the US, however, we've had the luxury of ignoring fuel efficiency when making decisions. That mindset is changing, but the impact on diesel generators has been clear.

In a nutshell, diesel generators became overly complicated while manufacturers worked around the clock to meet new emissions regulations, which negatively affected reliability. They became excessively expensive, and the emissions regulations reduced fuel efficiency. It was a dark time for what had been the premium generator option.

What’s next?

What features should we look for in a generator when developing modern, self-managed, fully communicating hybrid systems for the U.S. market?

We need a generator we can communicate with and monitor remotely.

We need great fuel efficiency—which means properly sized generators with variable speed.

We need diesel for its superior lifespan and fuel economy.

We need something reasonably priced and easy to maintain.

As for the footprint, we need something compact and lightweight. We'd also like to be able to customize its enclosure to suit our needs.

In short, an old technology has been updated, resulting in the best generator charging combination we have ever seen. In the next article, we'll focus entirely on the Hatz fiPMG generator's communications and fuel efficiency.

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