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Energy Storage For Island Microgrids And Diesel Hybrids

Many of our islanded clients are converting from diesel-only electricity generation to either a diesel-hybrid or a pure renewable-powered microgrid. This client requires both energy and power storage. Energy storage to be able to dispatch the electricity when it is needed. Power storage to provide stability to the island’s electricity system, or “grid”. The more that solar-PV is relied on as the prime generator, usually the more energy storage is required. Wind generation can reduce the energy storage demands depending on the consistency of the wind resource. But in both cases, storage is a big part of the picture. And storage is probably the least understood and fastest growing piece of the islanded energy puzzle.

There is a difference between power and energy. Simplistically, power is like the speedometer of a car and energy is like the odometer. Sometimes you need power and sometimes you need energy. The most common need is energy, so we’ll stick to that and keep it simple(r).

There are several factors to consider when buying an energy storage system. These include safety, life-cycle cost, first cost, land/space requirements, reliability, efficiency, environmental impact, and permitting difficulty, to name a few. This is a look at just three of these.

  • First cost (what is the cost to install the system initially)
  • Life-cycle cost (what is the total cost including maintenance, repairs, replacement, and other costs over the service life of the system)
  • Environmental impacts

There are dozens of alternatives for energy storage, and these options continue to increase as energy prices rise and entrepreneurs develop new ideas for the market. Batteries probably are the most practical for typical applications, since our clients like to innovate and be on the leading edge, but not the bleeding edge, so to speak. We also work with pumped-hydro storage and UPS as commercially viable systems, but I’m sticking to batteries for this post.

Looking only at chemical storage batteries, there are options there also: various lead-acid technologies, various lithium-ion chemistries, other hybrid-ion products, and flow batteries. I’m sure there are others and that some manufacturers will let me know that. But these are the common players in the market.

Again, for simplicity, let’s consider only one application: an island microgrid powered by solar-PV that is going to have a deep discharge every day. And what are the merits of battery types for that application based on the criteria above.

My experience is heavily weighted in lead-acid batteries, and I’ve developed probably 30-40 flooded lead-acid battery plants and about the same number using valve-regulated lead-acid batteries. These range from 6 small batteries on a rack in the back of an electric-powered truck to almost a thousand in a large battery room. Add to this a few handfuls of lithium-ion installations and other stuff. But I have not yet engineered or installed a flow-battery plant.

Lots of islanded systems seem to go with lead-acid batteries. They’re proven, they’re ubiquitous, they’re inexpensive. And I am a fan of lead-acid batteries – for the right application.

Consider what a lead-acid battery manufacturer says about their products, and this is from a major battery manufacturer’s industry seminar in March 2015: the best case for flooded batteries (the big plastic “jars” filled with liquid sulfuric acid) in a deep-cycle application like this island example is less than 3 years. The same manufacturer’s valve-regulated batteries (“VRLA”, “maintenance free, sealed” type) are good for 18 months.

Compare that to a lithium-ion or hybrid chemistry that warrants deep-cycle life of 4-10 years. Again, this is for deep-cycle, daily discharge applications like an islanded microgrid system.

These life-cycles mean the batteries will have to be removed and disposed (in some legal way) and new ones installed multiple times over the life of the PV system. If the PV system performs for 25 years, the lead-acid plant will be replaced at least 8 times, the VRLA batteries maybe 15 times (yes, that is hard to believe), and the lithium-ion or hybrid probably 3-6 times. So when modeling the finances of the plant, make sure to include these life-cycle replacements plus labor and disposal or recycling costs.

Depth of discharge, which means how much of the rated capacity you can use in a cycle. All chemistries have allowable depth of discharge. For example, if a battery has a rated energy capacity of 100 kWh and an allowable depth-of-discharge of 30%, that means only 30 kWh are “useable” in a cycle. Lead-acid batteries usually should not be discharged more than 30% to 50%, and lithium or hybrid chemistries usually range from 60% to 90%. That’s the max that they’re designed for, according to manufacturers. Usually the deeper the discharge, the fewer cycles are covered by the warranty. Translating that to cost, it means if you need 100 kWh of energy every day, you would need about 200-333 kWh of lead-acid batteries and 110-125 kWh of the lithium or hybrid product.

Then some additional aspects to keep in mind are environmental controls (cooling and ventilation for hydrogen off-gassing), shipping costs, and disposal of the spent batteries. Lithium chemistry batteries are fairly new on the market and do not have established recycling channels. Lead-acid, on the other hand, has been around for a very long time and the lead can be readily recovered and recycled. But the drawback to the lead recycling is that the battery has to be shipped to a smelting plant. There aren’t many of those – maybe a dozen or more around the world, and it costs money to ship old batteries there. As a result, unfortunately, lead-acid batteries often get disposed in the bush, off the back of a boat, or in the landfill. Personally I have seen old lead-acid batteries on the sea floor miles from the nearest land.

There are some very innovative products on the market that promise to change the paradigm of battery storage. Without naming names, one electric vehicle manufacturer has recently gone to market with a lithium-ion battery that is roughly half the cost of comparable lithium-ion products. My guess is that is due to the amazing economies of scale in their manufacturing process. “Half-priced” lithium-ion batteries will be a game changer. The other that comes to mind is an aqueous-hybrid-ion battery that is similarly priced to the previous example, and is much more benign and recyclable than most other batteries.

Engineering, building, and operating an islanded energy storage system doesn’t need to be complicated or unreliable. The more time invested up front doing the homework usually pays off many times over with lower costs and higher performance.