BU-413a: How to Store Renewable Energy in a Battery

Solar Energy: Economists predict rapid growth in micro-grid technology using batteries. Environmentally conscious Germany, Japan, Australia and other countries have made use of solar panels for many years to reduce energy costs. In parts of Africa where the AC grid is not sufficiently developed to support all household activities, solar panels with battery backup are mandatory.

Personal energy production is moving to the US Sunbelt; cheaper solar panels and longer-lasting batteries make this attractive. Batteries store energy during peak production when output is in over-supply to bridge the gap when free energy goes to rest at night or when the wind stops. Batteries will moderate peak consumption when the AC grid is stressed to the breaking point.

Renewable energy makes economic sense, but it is expensive. Most of the Western World is served with cheap and reliable electricity from the AC grid with a per kilowatt-hour cost as low as US $0.06 in parts of Canada, to $0.15 in many cities and up to $0.40 in some European countries. Electricity produced by a solar panel comes in at about $0.20 per kWh. When including peripheral expenses, solar power in most parts of the world is more expensive than buying electricity from the utilities, and as a rough guideline, stored energy doubles the price.

In spite of the apparent higher cost, putting solar panels on houses is becoming fashionable. Hardware prices are falling and so is the installation. The most common photovoltaic (PV) solar cells are the crystalline silicon type with efficiency of about 20 percent. Flexible panels for portable use, in comparison, have an efficiency of only about 10 percent. The hardware costs to generate 1 watt of electricity with solid panels is $2.00–2.50, with cost trending lower.

In solar-rich countries where electricity is expensive, energy from solar panels is being fed back to the AC grid. This causes the electrical meter to spin backwards, offsetting previously consumed energy, but it can also induce a problem. The amount of power generated cannot exceed consumption. Dumping more energy into the grid than consumed makes the system unstable, resulting in voltage fluctuations that can overload the circuit and lead to brownouts.

Renewable energy has friends and foes in high places. On one side, governments hand out subsidies to install renewable energy systems, while on the other side utilities try desperately to stem the move of home electricity generation by reducing incentives and adding fees. The utilities argue that spurious energy production by homeowners complicates control and cuts into the revenue stream. They see it as creating glut and famine by means of excess supply during times of plenty and famine when demand is high but renewable contributions are not available.

The conflict is understandable because utility companies are responsible for providing stable energy at all times while independent producers are unable to reduce the concern of pending failure caused by an aging grid that moans during peak demand. Right or wrong, producing clean energy from a renewable resource should never be curtailed, especially if the resource can be stored, and solar companies are fighting back through regulators, lawmakers and the courts.

Pumped hydro: Storing electrical energy is not new. One of the most effective storage media for large hydroelectric power stations is to pump water back up to the reservoir during low electrical demand and make it available during peak times. With an efficiency factor of 70–85 percent, pumped hydro is easier to manage than adjusting the generators to satisfy fluctuating power needs. Flywheels also serve as energy storage. Large electric motors spin one-ton flywheels when excess energy is available to fill brief energy deficiencies and stabilize the grid. Pumping compressed air into large underground cavities is another way to store energy but for small to medium installations, batteries work best. (See BU-1001: Batteries in Industries)

Flywheels serve as energy storage in lieu of batteries. Large electric motors spin one-ton flywheels when excess energy is available to fill brief energy deficiencies and stabilize the grid. High-speed flywheels whirl at 30,000–50,000 rpm often on magnetic bearings in a vacuum chamber. Electric motors/generators with permanent magnets charge and discharge kinetic energy on demand.

Flywheels deliver high power density rather than excessive energy. Whereas a modern Li-ion battery has a specific energy of 250Wh/kg, the equivalent in a flywheel is about 5Wh/kg, but the specific power is up to 2500W/kg, a level that is 10-times higher than a Li-ion battery.

In terms of delivering power, the flywheel has comparable qualities to the supercapacitor. See: BU-209: How does a Supercapacitor Work? A flywheel maker says: “For applications <20–30kW you'd rather use supercaps. Above this power requirement, the flywheel offers advantages.” In terms of longevity, the flywheel delivers a few million full cycles whereas the longevity of the supercap is around 500,000 cycles. The drawback of the flywheel is mechanical wear-and-tear with mechanical bearings.

Compressed Air: Pumping compressed air into large underground cavities is another way to store energy. Such systems provide energy for large installations in lieu of batteries. (See BU-1001: Batteries in Industries) Compressed air is not in common used. It needs access to a large pool of water and the cavity in the form of a balloon can be punctured.

Compressed air in underground caverns is an interesting effort that didn’t work out as planned. Inefficiencies by turbulent losses, leakage of underground caverns and cost to build the system work against this innovative effort. Money and talent cannot solve every problem.

Other mechanical energy storage systems are elevating weight in a tower and building a railroad that pulls a loaded car up the tracks, and the releasing as energy storage. Tidal turbines are also used with various rates of successes.

Battery Chemistries: Storage batteries have mostly been lead acid and users complain about their short life span. This is in part caused by excessive cycling as the battery charges during the day and discharges at night. Lead acid has a limited cycle count and suffers from sulfation when not periodically fully charged. A fully saturated charge takes 16 hours, and no solar system can deliver energy for this long. In addition, electrical consumption tends to increase with time while the solar panels reduce their output due to dirt buildup and aging. This often leaves lead acid with insufficient charge.

The switch to Li-ion solves this in part. Li-ion is more resistant to cycling than lead acid and does not need to be fully charged; in fact a partial charge is better as it relieves stress. But Li-ion is still double or three times the cost of lead acid in terms of system purchase.

The Tesla Powerwall offers a 7kWh and a 10kWh battery, enough energy to keep a home lit for several hours. Both packs have the same number of cells; the 7kWh battery uses the robust NMC that is used in many industrial applications while the 10kWh makes use of the NCA that powers the Tesla S-models. NCA offers high energy density and short charge time, while the NMC delivers a high cycle count at a lower capacity. (See BU-205: Types of Lithium-ion)

Both the NCA and NMC are Energy Cells that dislike heavy loads. The power of the Powerwall is limited to 2kW. This is sufficient to run a fridge, brown toast and perhaps iron a shirt, but the wattage is too low to cook a meal on an electric stove, run an electric dryer or keep the air conditioner going; high-energy appliances consume more than 2kW. To fill the gap, the AC grid kicks in seamlessly during peak household activity. A 10kWh battery with 2kW peak power cannot disconnect a household from the grid, but it reduces the electrical bill by one third to one half.

To fully charge a 10kWh battery during 5 hours of optimal sunshine requires a solar system that delivers 5–12kW. At an estimated cost of $2 per watt, the 10kW solar hardware comes in at $20,000. Installation and the DC-AC inverter to convert the solar DC to compatible AC power and synchronize it with the grid might double the cost. The battery will be extra also.

Another hidden cost that is often overlooked is end-of-life. Solar panels have a life span of 25 years and batteries are commonly guaranteed for 10 years. At a cost-of-money of 5 percent and a 20 year amortization, a $25,000 system could cost the owner $2,500 per year. The energy savings should be greater than this or else the exercise may be misconstrued. Even larger energy savings can be made by reducing personal transportation or scaling down on the size and power of such a carriage.

References

^[1] Source: Elliot Green Power
^[2] Source: Nant de Drance