Application: This document describes the use of a battery storage system in conjunction with a PV system to avoi d or reduce the purchase of more costly on-peak power. However, energy storage systems can also play a flexible, multifunction role in an electric supply network to manage resources effectively. Battery energy storage systems are use d for a variety of applications, such as: power quality assurance, transmission and distribution (T&D) facility deferral, voltage regulation, spinning reserve, load leveling, peak shaving, and integration with renewable energy generatio n plants [6]. Battery systems appear to offer the most benefits for utilities when providing power management suppor t (i. e., voltage regulation, spinning reserve, customer peak shaving, integration with renewables, and T&D facilit y deferral) and when responding to instant voltage spikes or sags and outages.
Benefits: Specific studies at electric utilities considering battery energy storage systems revealed a number o f
generation, T&D, and customer-based benefits that are generally site-specific [5,6]. A number of factors determin e the benefits of installing energy storage systems, such as storage size, location, system load profiles, and load profiles at individual substations and T&D lines.
A few battery energy storage systems are currently being demonstrated, some with U. S. DOE Energy Storage Systems (ESS) Program funding. Crescent Electric Membership Cooperative (CEMC) has been using a 500 kW lead-aci d battery energy storage system for peak shaving purposes since 1987. CEMC has been able to significantly reduce the demand charges paid to its generation and transmission cooperative, North Carolina Electric Membership Cooperative
[7].
Niagara Mohawk funded an investigation into peak load reduction with PV and buffer battery storage. The utility and the Empire State Electric Energy Research Corporation installed a 13 kW (AC) PV system on an energy-efficient office building in Alba ny, NY in 1990. The PV system operated as designed, but because afternoon clouds were reducin g the PV system’s effect on peak demand somewhat, Niagara Mohawk added a 21 kW/1-hour battery storage system in July 1993 [8]. The PV/battery prototype had the two systems operate in parallel, with off-peak grid power used to recharge the battery. It acted as a “ quasi-dispatchable” unit, protecting against local load excesses and, thus, guaranteeing T&D benefits [9].
The manufacturer has since improved on this PV/battery system, by creating a compact system that can be installe d on rooftops. Delmarva Power & Light is testing these units to determine whether, after PV generation cuts back a t 4 P. M., the battery can provide three more hours of output to help shave peak loads in the summer. The prototypes were installed July 1996-April 1997 [10]. The unit can be operated locally or remotely; the batteries are charged from the grid overnight. Delmarva has successfully obtained peak shaving benefits from their operation. This quantity o f storage is being evaluated to determine if the benefits of multiple hours of storage capacity justify the additional costs.
EPRI, Sandia National Laboratories, and the Salt River Project electric utility installed a 2.4 kW PV array an d
25.2 kWh battery in an experimental residence owned by the utility. The system was designed to discharge the P V generated electricity stored in the batteries to match specific three-hour peak loads. The PV/battery system has operated continually and reliably since its installation in August 1995. No repairs or homeowner involvement has been needed. The only maintenance performed was periodic watering of the battery cells and manually changing the dispatc h schedule each season [11].
There are many examples of battery energy storage integrated with PV facilities at national parks and militar y installations. For example, Dangling Rope Marina on Lake Powell in Utah is the largest PV system ever installed a t a national park. The Dangling Rope PV system replaced an existing diesel generator and consists of a 115 kW P V array, a 250 kW power conditioning unit and a 2.4 MWh battery bank. The Yuma Proving Ground in Arizona ha s a grid-tied 441 kW PV system with 5.6 MWh of lead-acid batteries. During the summer peak season, the system ca n deliver 825 kW to the grid to help reduce peak demand. The system can also operate stand-alone in the event of an extended outage.
A number of studies have examined the contribution of storage coupled with renewable generation [9-15]. A recen t study examined the benefits and costs of installing an integrated MW-scale windfarm with battery storage to defer the upgrade of a 25 kV circuit to 69 kV for Orcas Power and Light Company. Although sufficient wind potential wa s identified, the high winds did not generally occur coincidentally with peak loads on the distribution line. A transportable 500 kW/2-hour battery was considered for use during low wind periods to defer the upgrade in the distribution line until the year 2000 [15]. The study concluded that extremely high winds and high utility costs appear to economically justify the addition of MW-scale windfarms and battery storage.
Impacts: There are no emissions, solid wastes, or effluent produced during the operation of PV/battery energy storage systems. Flooded lead-acid batteries are closed, and VRLA and advanced batteries are essentially sealed. Electrolyt e leakage from batteries is a rare occurrence because each lead-acid cell is surrounded by a double container. In the rare event of a leak, the fuid is captured by a containment system, neutralized and cleaned up as a chemical spill. Th e volume of leakage is typically small as each cell contains little liquid and there is very low likelihood that a larg e number of cells would break open simultaneously.
When the battery subsystems are replaced, essentially all battery materials (e. g., lead, acid, plastic casing) are captured and recycled. According to the Battery Council International, 95% of all lead available in scrapped batteries wa s recycled on average during 1990-1995. Batteries used in stationary applications represent less than 4% of the total tonnage of lead available for recycling during that period [16].
3.0 Technology Assumptions and Issues
Currently, there are a variety of PV array materials and battery energy storage technologies in use and unde r development. This document assumes off-the-shelf silicon-based PV panels are used, although the specific choice i s not an issue. PV technology descriptions are provided elsewhere.
