Low Cost energy Storage for Rural Renewable Energy Micro-grids

Small to medium farm/ranch development and electricity demand are a closely related phenomenon, and when combined with responsible environmental protection, renewable energy projects become an excellent means for sustainable economic growth. Renewable energy projects benefit rural communities in multiple areas by creating new employment opportunities, stimulating economic activity, reducing environmental impacts (no greenhouse gas production) and by meeting the electricity demand created by rural economic growth without additional utility transmission/generation infrastructure. Renewable energy projects also provide further economic diversification, expanding from traditional agriculture into off-farm activities and farm-related industries, which are needed if agricultural economies are to remain vital well into the 21st century and beyond.Rural electric generation and transmission (G&T) co-ops and larger farms/ranches have the credit and resources to be the primaryimplementers of renewable energy systems. But, renewable energy systems have a distinct drawback; variable or intermittent power, since the amount of energy generated can vary quickly with wind speed and sunlight intensity. This quickly varying energy availability can be mitigated through fast response energy storage systems which store energy at peak production times and then release energy back to the micro-grid when the renewable production wanes due to natural weather and solar cycles. Renewable energy projects have a wide selection of commercial energy storage systems that are portable and can react quickly to the rapidly changing energy generation. These include electro-chemical (e.g., deep cycle lead acid and Li-ion batteries), electro-static (e.g., ultra-capacitors) and kinetic (e.g., flywheels).Each of these commercial fast reaction energy storage systems have limitations. Chemical batteries have the most technical maturity, lowest initial cos ($400 - $1,200 per kWh)and a range of capabilities. but the largest limitation for chemical batteries is that the medium wears out (low cycle life) and the batteries must be replaced after 2000-3000 expected power cycles, incurring increased lifecycle costs. Another key limitationis that to maintain the expected 2000-3000 power cycles, chemical batteries cannot be discharged completely, typically with a 50-70% depth-of-discharge (DOD),reducing their effective efficiency. Also, current chemical batteries degrade over operational use, with a 80% end-of-life (EOL)capabilityconsidered"used up" when operated atthe expected power cycle rate. Finally, chemical batteries have inherent internal efficiencies that dissipate energy (usually as heat) during charging and discharging round trip, with average Lead Acid battery round trip efficiency (RT-Eff) at 83% and Li-Ion at 95%. Current ultra-capacitors have high efficiencies and are great forapplications requiring fast charge/discharge and nearly unlimited power cycles. The main limitation to ultra-capacitors comes fromtheir low energy density (they take up a lot of space and are heavy) and high cost ($3,000-$4,000 per kWh), whichhampers its ability to compete effectively. Traditional high-mass flywheel systems are commercially available and fall between chemical and electrostatic with regard to capability. Traditional flywheels have a long operational life (100,000+ power cycles) andhighefficiencies (99% DOD, 100% EOL, and 88% RT-Eff), but with low energy densities and higher initial purchase costs ($2,000-$3,000 per kWh). For small to medium farm/ranches with limited capital investment assets to install a renewable energy micro-grid, the energy storage design must account for the initial investment cost, energy storage efficiency and lifecycle cost to make the decision profitable compared with using local utility power or on-site generator power.Low initial cost - drives the initial investment. Directly related to a battery's cost per kWh of storage capacity.H