The Hybrid Solution: Part 2 of Mike’s Guide to Hybrid Generators

If you are following this series in order, you just finished this overview of how generators work and how these operating limitations play out on the jobsite. If you are just jumping in now, or your eyes glazed over when you got to alternators and rpm targets in the previous article, here are the key takeaways.

1. Large generators run best at a fixed load, normally 60-80% of rated capacity 

2. It costs extra time and money to run them at a less than optimal load

3. Variable demands on a jobsite ensure that running at an optimal load is nearly impossible 

How do you like them apples? So now that that’s established, we are left with two options; 

1. Accept all this as the cost of doing business, chase economies of scale, design a business model that is highly focused on capitalizing on the high service and fuel needs of these systems, invest in emissions reduction technology to reduce the smog output from running engines 24/7

2. Or, look for innovative solutions that solve the problems without also introducing crippling second-order effects. 

Hybridization is the answer we are putting forward to the second option. In this article I will cover the what’s, how’s, and why’s of hybridization. I would like to make an argument for why I think this is the solution, while giving a fair shake to the limitations and some exciting new workarounds in the follow-up article.

The What’s 

Hybridization is the combining of two systems to result in a complete system that is more than the sum of its parts. It’s the old “you got your chocolate in my peanut butter” trick that results in something better by playing on the synergies between the combined elements. To bring this into context we are talking about combining a regular old internal-combustion engine powered generator with a set of batteries and inverters. 

By themselves, these two elements serve a purpose and are limited by their normal use case. We have covered this on the generator side in-depth. For the batteries and inverters, you typically see them in an “off-grid” system where the overall system efficiency is limited by the inconsistent solar power source they are usually connected to. This inconsistency of power production that comes from solar is exactly WHY these systems need batteries, the sun doesn't shine at night and so we need to store that power…in batteries. 

This is the key to what batteries provide, they serve as the suspension in a power system, absorbing the highs and lows to provide consistent, available power. To be technical, they bridge the delta between production and consumption. In a solar off-grid system, they allow inconsistent production to provide power to more consistent loads. But what if we turned this around and used them to make inconsistent loads efficient for a consistent power source? 

The How’s

Let's go back to the hypothetical jobsite from the last article, the one with the big 40kVa diesel generator that is sized to handle the startup load of a compressor but outputs 1,200W the other 22 hours a day. How would we go about hybridising this system? 

For starters, let's hypothetically break down this load profile over the course of a day. 

1. Absolute Peak: 40kW 

2. Continuous Peak: 20kW x 2 hours

3. Background Load: 1.2kW x 22 hours

4. 24-Hour Consumption: ((20kW x 2)+(1.2kW x 22)) = 66.4kWh

Now that we have our load profile lined out in terms of absolute peak, peak continuous load, and 24-hour consumption, let's translate this into the battery and inverter system.

1. Inverters: 4 x 10kW inverters with a 140A DC charger

2. Batteries: 40kW of batteries with a 1C discharge rating

First we take these batteries and inverters, build them out into a complete system with controls and remote monitoring, and connect them to the output of the 40kVa generator. The output of the inverter system is then connected to the jobsite’s main distribution panel. The batteries are sitting at a 60% state of charge. Now what happens? 

The 24-hour duty cycle will kick off with the new power system using a 2-wire start to remotely power up the generator to charge the batteries which are currently sitting at 60% SOC. The generator will then run at a 31.2kW output for a half an hour to provide the remaining ~16kWh of power needed to top off the battery bank. A 40kw battery bank at 60% SOC has 16kw of capacity before it gets to 100% SOC, assuming this charge happens off peak loading than the generator will also be supplying the minimal 1.2kW standby load.

Next, the system will turn off the generator when the batteries hit 100% SOC and continue powering the load from the now full battery bank. When the compressor kicks on and runs for a few minutes, the inverters and batteries will smoothly handle the peak demand with perfectly clean power from the inverters. This will continue on for 11 HOURS… until the battery bank hits the pre-programed discharge floor, in this case 20% SOC. Then the generator will start again, charge the batteries for an hour and a few minutes to recharge the 32kWh consumed, and the cycle starts again.

The Why’s

I think this is a good place to start exploring why we hybridize. In the thought experiment we just completed, the jobsite started with a 40kVa generator that ran 24 hours a day, with 22 of those 24 hours at an average 3% load. With the addition of some inverters and batteries, those 24 hours of runtime just turned into 2.07 hours of runtime at a completely stable 75% load. 

What does that mean for this jobsite and the crew that works it?

1. Daily runtime has gone from 24-hours to 2-hours, a 92.4% reduction. 

2. Time between service intervals has increased 13x

3. Mean time between failures has also been increased by up to 13x (power systems don't have many moving parts to break)

4. If the generator’s life expectancy is 30,000 runtime hours, we can now expect it to run for ~40 years rather than 3. 

5. Fuel consumption has been significantly reduced, most generators don't even list gallons/kWh at loads that low… for a reason

6. Emissions and noise, along with their associated state or municipal fines/penalties, are reduced by 92.4% 

7. The power is now perfectly clean, no voltage/frequency drops and associated equipment wear when that big inductive load kicks on

8. Generator run times can be scheduled outside of the desired quiet hours 

These are the what’s, how’s, and why’s of hybrid generators through the lens of an established jobsite with a clear issue, but that's not all there is to cover. On one hand, it's never a good idea to totally trust the word of a salesman that paints a rosy picture without any acknowledgements of the compromises and setbacks that have to exist. On the other hand, now that we have shattered the restraints imposed by tying production to consumption, there is new territory to explore outside that box. 

All of this will be explored in the next chapter of this series. There, we will take a stab at why hybridized systems aren't commonplace everywhere you look, what a dedicated system looks like once we move past hybridizing existing generators, and how some very innovative technology from Victron Energy disrupts all of this and opens the door to new possibilities.