Hypothetical: Augmenting a diesel generator with solar

stephendv

In a hypothetical off-grid applications where batteries would be impractical or too costly, and where currently only a diesel generator is in use, would it be possible to augment the output of the gen with solar WITHOUT installing a battery based system?

I've been thinking specifically about the SunnyBoys and their frequency based backoff system (Basically they regulate their output based on the grid frequency, 50Hz grid frequency = full ouput, 52Hz = no ouput, and everything in between).

So, if you could control the frequency of the generator based on the loads and the output of the SB's you could potentially have a safe system where the output of gen + SB exactly meets the loads(?).
This increase in frequency could even happen naturally on the generator, no load = 52 Hz means the SB output nothing, as the loads increase the gen works harder, frequency goes down and the SB's start outputing.

Do you think this could work? or are there better ways of augmenting a gen with solar?

BB.

Re: Hypothetical: Augmenting a diesel generator with solar

It certainly is a possibility--but to a degree adding solar "generator tied" inverters is probably a looser overall.

Assuming that it all could be made to work (certainly not that difficult engineering wise)--we need to look at the power characteristics of the genset.

Generally, you want to load the generator at around 50% or more load for optimum fuel costs and life. Running below 25% (SWAG) is expensive in both fuel and possible maintenance costs (fuel flow does not reduce much below 50% rated electrical output and engine starts to coke up).

So--for a "cost effective solar" setup, you would need to operate the genset in the 40-100% load range when you have solar GenTie connected and working.

So--at best, near 100% load, you could offset around 50% of it with solar GenTie--and that would be OK.

However, for a random load type setup (say a home, small business, etc.), you would have to run fairly steady and large loads on your genset all of the time, and your genset would have to be able to manage 100% of the loads when the sun is not shining (night, cloudy weather, etc.).

Perhaps if you had a programmable/variable load (such as water pumping)--you could mix the two together nicely. Basically, vary the pumping so that the genset load remains around 50% rated power.

In the end, in my humble opinion, most non-industrial loads are random and have quite a broad range (10:1 or larger) of Max/Min power requirements.

A genset+solar is just not going to efficiently manage those loads dynamically (variable nature of solar power).

Personally, I still believe the "Hybrid" Generator is the best solution at this time. Basically, a battery bank and inverter to handle loads 18 hours a day, and a genset to charge and handle peak loads the other 6 hours a day.

These have been found to be a real fuel saver in Africa (run the genset in the evening when loads are heavy such as cooking, pumping, lights, etc.; and run the inverters the rest of the day to support smaller loads such as schools, fans, computers, etc.).

Even so, that is trading off the ~$1 or so per kWhr cost of a fuel driven generator vs the $0.45 per kWhr costs of a battery backed inverter system (using US pricing for fuel and batteries which are both variable in time and location).

The nice thing about adding the battery in there--is you can now any any Renewable Energy source that is cost effective in the region--solar, hydro, wind, etc. as simply another charging source for the battery bank without having to work on the diesel "GenTie" inverter interfacing.

-Bill

Cariboocoot

Re: Hypothetical: Augmenting a diesel generator with solar

I don't know how prices range where you are, but if it were here it would be cheaper to buy an additional generator rather than the equivalent Watts in solar panels plus a GT inverter. Less convenient to use, perhaps, but cheaper.

stephendv

Re: Hypothetical: Augmenting a diesel generator with solar

This question came up in another forum, the user works for a mining company in Chile, they have 4 x 100kW pumps that need to be driven for 7 hours each day and they're currently using more than 1 diesel gen.

I guess assuming all the gens are synchronized to the same output - then this would solve the loading issues. They could just spin down gens as the panels output more.

What would happen if the GT's backfed too much power into the gen?

BB.

Re: Hypothetical: Augmenting a diesel generator with solar

Here is what you need... A salt water rheostat was used in decades past as variable loads for diesel gensets. Made out of scrap drums and cheap to operate.

Regarding a diesel hybrid solution... I was thinking setting it up differently. Use an Off Grid inverter as the main energy source. And setup up a Diesel Genset with a Generator Synchronization panel so the generator assists the inverter (the GT genset).

Now, if you could control the power output of the generator based on the battery current and state of charge--you would now be able to use the battery bank as your variable load (keep genset at 50-70% load) and once the battery is charged, shut down the diesel until time to recharge again.

This would use the (more or less) undocumented ability of a TSW Off Grid inverter to charge the battery bank if back driven by a GT connected source.

Down side would be the need to find a small generator synchronizer (I found one that may go as small as 50kW, but generally in the 100's kW to 1+ MW range). As well as the software/monitoring regarding battery charging and generator output. Still does not get rid of the battery bank though.

What would happen if you back drive an Alternator--In theory, it would attempt to drive the diesel motor faster than 60 Hz (1,800 RPM or whatever base speed is). I assume the actual issues may be more severe (overheating, damaging control circuits/switch gear, over-voltage/over frequency cutout by the FT inverter). If was an inverter/generator (alternator)--A lot of the behaviors would depend on the exact hardware involved (for example, a simple regulation method is to run part of the load current through the exciter coils--more current, more excitation, more voltage--a good way to adjust for voltage droop under load.

In the end, you are probably going to want to setup some operational models in Excel and play with your "realistic" options.

Cost of equipment + fuel + maintenance / (number of actual kWhrs used over xx years of equipment life) = $$$/kWhr costs

Get a small 5-6kW used RV genset (propane, gas, or diesel) for "low power operation". Vs get the solar panels now. Vs just loading up the current genset with loads to prolong life (at higher fuel costs)....

You might find out that all of the solutions will end up with similar costs over the life of the various solutions...

-Bill

stephendv

Re: Hypothetical: Augmenting a diesel generator with solar

BB. wrote: »

Regarding a diesel hybrid solution... I was thinking setting it up differently. Use an Off Grid inverter as the main energy source. And setup up a Diesel Genset with a Generator Synchronization panel so the generator assists the inverter (the GT genset).

Now, if you could control the power output of the generator based on the battery current and state of charge--you would now be able to use the battery bank as your variable load (keep genset at 50-70% load) and once the battery is charged, shut down the diesel until time to recharge again.

Thanks Bill, as you said it's going to come down to number crunching the costs of the various solutions. I think the battery based systems will have a problem scaling up for larger systems, for example the largest battery based (of the well known) inverter systems I can find is 72kW provided by 9 x Studer Xtender in 3 phase config. Although a local company seems to build bigger systems to 120kW, but their's is a very new system in the battery based field:
http://www.ingeteam.com/ingles/productos_servicios/comun/ficha_producto.asp?id_producto=68

BB.

Re: Hypothetical: Augmenting a diesel generator with solar

Stephen,

If you are talking about that large of system--then I would look at Flywheel based storage. Not terribly efficient but you could store enough power to give you 5-15 minutes or so of standby power to spin up the next generator to allow for the "bumpyness" in solar RE (wind, solar PV) output (both abosrb excess energy and give up energy for a few minutes at a time before cranking up or shutting down a genset).

Some larger computer facilities use flywheel standby UPS systems of various mix and match setups. Those are apparently designed for 15 second ride through / generator startup:

For the Culpeper site, Terremark bought 11 900 kVA UPS flywheel systems, which are made by Active Power Inc.. The company bought them in a package from Caterpillar that includes backup diesel generators.
Terremark's flywheel-only approach stands in contrast to other companies that employ flywheel UPSs to complement battery-powered systems. Flywheel vendors such as Active Power and Pentadyne Power Corp. say that flywheels can prolong the life of batteries by handling power blips that barely register on a radar but nonetheless drain battery juice. Robert McFarlane, president of the Interport Financial Division of New York-based Shen Milsom & Wilke Inc. and a data center power expert, says companies use both means to increase their backup power and reduce the number of batteries. Meriter Hospital, a nonprofit hospital in Madison, Wis., uses a mix.
...
One is data center space savings. "One nice thing about the flywheel is it uses a lot less space than long strings of batteries," said Ben Stewart, Terremark's senior VP of facility engineering. Case in point: Terremark's 200,000-square-foot Miami data center. Aside from IT equipment, the facility holds a 6 MW generator and 6 MW worth of flywheel UPS. If the company were running only battery UPSs, the batteries wouldn't fit in 2,000 square feet.
And while Terremark anticipates cost savings over time, flywheel technology is more expensive up front. Stewart said a 900 kVA flywheel unit sells for a little more than $300,000 but "compares very closely to the static UPS equivalent" -- that is, batteries. But because batteries need to be replaced every three to five years, a cost Stewart estimated at about $100,000 for each replacement, "the total cost [of flywheels] is a lot lower. It

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Another option to think about.

-Bill