Energy storage requirements

Battery selection

While any energy obtained from wave action can be expected to be reasonably constant (depending on location), energy from wind and solar sourced will necessarily fluctuate, so we'll need some way to store energy.

What will be the energy storage requirements? How much and for how long? The bio-diesel powered generator will be used for reserve power, but we'd probably want to restrict its operation to say half an hour per day, so batteries will have to store energy for 24 hours in the event of no power generation from solar, wind or wave generators.

The traditional and the proposed solutions:

see http://en.wikipedia.org/wiki/Lead-acid_battery and http://en.wikipedia.org/wiki/Vanadium_redox_battery

A lead-acid battery has an energy density of 20-40 Wh/Kg, compared to 10-20 (increasing as new research is done) for a VRB.

While a lead-acid battery might be expected to last for 500-800 cycles, a VRB will last for more than 10000, does not self-discharge and is environmentally better.

(From http://www.pdenergy.com/):

The VRB Energy Storage System (VRB-ESS) is an electrical energy storage system based on the patented vanadium-based redox regenerative fuel cell that converts chemical energy into electrical energy. Energy is stored chemically in different ionic forms of vanadium in a dilute sulphuric acid electrolyte. The electrolyte is pumped from separate plastic storage tanks into flow cells across a proton exchange membrane (PEM) where one form of electrolyte is electrochemically oxidized and the other is electrochemically reduced. This creates a current that is collected by electrodes and made available to an external circuit. The reaction is reversible allowing the battery to be charged, discharged and recharged.

The principle of the VRB is shown in more detail in Figure 1 - it consists of two electrolyte tanks, containing active vanadium species in different oxidation states (positive: V(IV)/V(V) redox couple, negative: V(II)/(III) redox couple). These energy-bearing liquids are circulated through the cell stack by pumps. The stack consists of many cells, each of which contains two half-cells that are separated by a membrane. In the half-cells the electrochemical reactions take place on inert carbon felt polymer composite electrodes from which current may be used to charge or discharge the battery.

The VRB-ESS employs vanadium ions in both half-cell electrolytes. Therefore, cross-contamination of ions through the membrane separator has no permanent effect on the battery capacity, as is the case in redox flow batteries employing different metal species in the positive and negative half-cells. The vanadium half-cell solutions can even be remixed bringing the system back to its original state.

The open circuit cell voltage at a concentration of 2 mole per liter for each vanadium species is 1.6 V when fully charged. The relatively fast kinetics of the vanadium redox couples allows high Coulombic and voltage efficiencies to be achieved without costly catalysts. The same current is passed through all of the cells as they are arranged in series. Such systems have many admirable properties including high efficiency, long cycle life, ease of scalability and negligible environmental impact.

Battery Type	Cost $ per Wh	Wh/kg	Joules/kg	Wh/liter
Lead-acid	$0.17	41	146,000	100
Alkaline long-life	$0.19	110	400,000	320
Carbon-zinc	$0.31	36	130,000	92
NiMH	$0.99	95	340,000	300
NiCad	$1.50	39	140,000	140
VRB	?	25	?	?
Lithium-ion	$4.27	128	460,000	230

From http://www.batteryuniversity.com:

Below is a summary of the strength and limitations of today's popular battery systems. Although energy density is paramount, other important attributes are service life, load characteristics, maintenance requirements, self-discharge costs and safety. Nickel-cadmium is the first rechargeable battery in small format and forms a standard against which other chemistries are commonly compared. The trend is towards lithium-based systems.

Nickel-cadmium - mature but has moderate energy density. Nickel-cadmium is used where long life, high discharge rate and extended temperature range is important. Main applications are two-way radios, biomedical equipment and power tools. Nickel-cadmium contains toxic metals.

Nickel-metal-hydride - has a higher energy density compared to nickel-cadmium at the expense of reduced cycle life. There are no toxic metals. Applications include mobile phones and laptop computers. NiMH is viewed as steppingstone to lithium-based systems.

Lead-acid - most economical for larger power applications where weight is of little concern. Lead-acid is the preferred choice for hospital equipment, wheelchairs, emergency lighting and UPS systems. Lead acid is inexpensive and rugged. It serves a unique niche that would be hard to replace with other systems.

Lithium-ion - fastest growing battery system; offers high-energy density and low weight. Protection circuit are needed to limit voltage and current for safety reasons. Applications include notebook computers and cell phones. High current versions are available for power tools and medical devices.

Table 1 summarizes the characteristics of the common batteries. The figures are based on average ratings at time of publication. Lithium-ion is divided into three versions: The traditional cobalt that is commonly used in cell phones, cameras and laptops; the manganese (spinel) that power high-end power tools and the new phosphate that competes head-on with spinel. Lithium-ion polymer is not listed as a separate system. Its unique construction performs in a same way to cobalt-based lithium-ion.