Energy Storage Remains A Roadblock for Solar and Wind

Feb 03, 2014

A few years back, I built several UPS systems to run MTSOs (mobile telephone switching office) for the cellular industry. They provided power at -48 volts DC and were made up of eight (8) racks of 24 batteries per rack, (shown below). They power a MTSO for up to eight hours in the event of a power outage, assuming the 750 kW diesel generator failed to start or ran out of fuel.

Lead acid batteries were used in my UPS systems because they’re relatively inexpensive at roughly $2,000 USD each. Notice the term “relatively.” Other battery technologies were available but cost prohibitive. The entire UPS system used 192 batteries at an operational install cost of around $600,000 USD.

Each individual battery weighs a little over 400 pounds. All batteries combined weighed in at 38 tons, not including the racks and cables. They took up the space of a small house.

The batteries for a MTSO UPS system. Notice the batteries along the back wall too.

To put capacity into perspective, a MTSO UPS system could power seven average sized homes for a day. It might power a textile mill for several minutes or a single electric arc furnace steel mill for less than a minute.

Why all this talk about batteries and capacity? I want to give you some context for one of the greatest technical challenges for solar and wind power generation. The sun only shines during half the day and not at all on cloudy days. The wind doesn’t always blow. Practically speaking, solar and wind generators need to store enough excess energy when they generate power to feed the electrical grid when they’re not generating. Otherwise, they wreak havoc with load management and voltage regulation in the grid.

What about using batteries like I used for switching centers? Using present battery technology, you would need 24 of these beastly sized batteries for every house supplied by solar and wind to provide any resemblance of constant energy. It would take a whopping 67,200 tons of batteries to keep a small city of 14,000 homes in constant power. Even then, power outages will occur when weather conspires to block the sun and still the air for more than a day. In this scenario, the cost of maintaining our sun, wind, and battery power plant after installation adds an additional $9,600 USD per year to each home’s electrical bill based on today’s electricity and battery prices.

What about other forms of energy storage? Some fairly practical designs exist for heat storage systems that store heat from solar thermal power plants to run steam powered turbine generators after the sun sets. However, they’re impractical for photovoltaic (commonly referred to as PV) solar panel or wind power generation due to inefficiencies in converting electricity into thermal energy for storage and back to electricity to feed the grid.

So, lets just use solar thermal power generation to take advantage of thermal storage. Whoa, says a number of environmental groups and communities. Solar thermal power plants require a lot of water to operate, making them a conflicted choice for installation in desert areas where solar insolation is best for these plants but limited water resources put the kibosh on building them. A water war broke out in Amargosa Valley, Nevada over a proposal to build a wet cooled solar thermal plant when it was discovered it would require 20% of all available water resources in the area to operate.

Nevada One Solar
Steam rising from the wet cooling towers at Nevada One Solar

There is considerable research being done on building solar thermal dry systems. The advantage is the design is estimated to use 90% less water. The disadvantages are the design is a complicated air cooled closed system that is considerably more expensive to build and operate, occupies significantly more land, and 10% to 15% less efficient in power generation. Until costs and inefficiencies are overcome, dry thermal solar plants will see little interest.

For the time being heat storage systems don’t offer a practical solution because they’re of very limited use.

Flywheel energy storage systems appeared in the 90’s. They claim an 80% smaller footprint as compared to battery storage but they’re plagued with costly maintenance problems and inability to deliver power for long enough periods to be practical in grid use. They also require wasted power to keep them spinning. Flywheel storage plants are seeing their fair share of financial problems.

Beacon Energy Company
Bankrupt Beacon Flywheel Energy Company

Some solar and wind advocates have championed pumped storage hydropower systems using water reservoirs, and combination pump/generators to store power from solar and wind generators. The greatest challenge to this concept is the large reservoirs need to be located in mostly wilderness areas and have a reliable high volume water source that can be diverted. While a few electric companies like the Tennessee Valley Authority’s (TVA) Raccoon Mountain plant do use this concept, it isn’t practical or environmentally friendly on the scale needed to store energy for solar and wind power generation.

Pumped-Storage Plant
Artist’s drawing of the TVA’s Raccoon Mountain power plant.

Raccoon Mountain Reservoir
Photograph of the Raccoon Mountain reservoir

The TVA Raccoon Mountain reservoir was built in the 1970’s. Today, such a project would receive fierce opposition from environmental and other interest groups.

A recent proposal is vehicle-to-grid (VTG) energy storage. The concept is built around the idea that at some point in time, enough electric vehicles (EV) will be plugged into the grid, storing power when power generation exceeds demand and giving it back when demand exceeds power generation.

PHEV Vehicle
Ford PHEV Plug-in Vehicle featuring VTG concept

There’s a number of immediately apparent downsides to this concept. One is related to battery technology. The additional charging and discharging of the batteries or “cycling” by the grid will significantly reduce vehicle battery life. Each vehicle will need to have reserve battery capacity to store and give back to the grid without affecting driving range. The extra cost of reserve battery capacity will add cost to the ownership of an EV. Alternatively, owners will need to opt for a vehicle without reserve capacity and accept lower driving distance per charge. Irrespective, I see the VTG as a pie in the sky concept that is predicated upon too many technical and social challenges.

There’s the much overhyped notion championed by various myopic individuals that all we need to do is redesign the electrical grid infrastructure to work with solar and wind power generation. That begs the question, redesign it how?

Let’s scale down the problem so we can understand the implications. If your house ran off only solar and wind power and didn’t have storage batteries (remember, your house now represents the grid where energy storage isn’t practical), how would you rewire your house to keep the lights on when the sun isn’t shining and the wind isn’t blowing?

One solution is to cheat and buy a gasoline or diesel powered generator and implement a complex set of sensors, switches, voltage regulators, and phase synchronizers to cleanly switch between solar, wind, and generator power. But remember, the generator needs to run all the time as a “spinning reserve” else your house will loose power for 15 to 20 seconds at a time waiting for the generator to start. It’s going to happen frequently throughout the day. You’ll be replacing air-conditioner compressors and appliances constantly because they can’t tolerate such dirty power.

Say, isn’t that what we’re doing now - using traditional spinning reserve power generation to prop up solar and wind power? Ahem, yes.

Until a major breakthrough occurs in energy storage, solar and wind power generation will remain relegated to being relatively minor and problematic sources of energy. We are at least decades away from a viable and cost effective large-scale energy storage technology that can truly leverage solar and wind generated power as a significant contributor to our growing energy needs. Fossil fueled, nuclear and hydroelectric powered energy will remain our primary energy sources for the foreseeable future. We need not be shutting them down irresponsibly thinking solar and wind will keep the home lights lit.

This article is an updated reprint of my original work of the same title published at Suyts Space, an on-line science and politics venue.

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