amp-hour is variable, depending on the battery bank size. For batteries that are used in residential solar energy systems, the cost is $1.6 /Ah. For a large scale storage system, the cost is $60/Ah (or $0.52/Watt- hour). Some major cost adders for the large scale storage systems are the wiring and electrolyte mixing. But the worst feature of large scale battery storage is that, without sophisticated charging, their lifetime is very limited. They may only last for 2 or 3 years, if they are deep cycled (to more than 50% depth of discharge) every day. The cost of either replacing or installing the fancy charging is prohibitive. Batteries are very practical for residential or other small solar energy systems. They are only about 5% of the system cost, will last 10 years or more under light duty of a house load and take up a very small amount of floor space (8 to 16 sq. fl.). While in the USA, we tend to plan for industrial loads when building new generating systems, most of the developing world does not need such large amounts of power. In fact, much of the need is made up of thousands of homes, villages and towns which could prosper with very small solar energy systems (10 kW or less) and in such cases, batteries are fine. Water Pumping Energy can be stored by pumping water to a high reservoir. The potential energy stored in the reservoir is the mass of the water time the height times the acceleration of gravity (mgh). When power is needed, the water is allowed to fall through a turbine, which turns a generator (just like in a hydro-electric power plant). This is one way to accomplish large scale power storage and such a system can be kept operating indefinitely with proper maintenance. The cost of suclra system involve the cost of creating a reservoir, cost of land for reservoir, pumps, pipes and labor to install it. Such a system can’t be installed just anywhere. It requires land with certain topography for putting a reservoir above the generator. A 0.6 acre pond 10 m deep, raised 150 m above the generator stores 10 MWh (gravitational potential, mgh). Such a system would require an acre or two of land, a large pumping system, labor for excavation and engineering design. Also, conversion to electricity is probably around 50% efficient. The cost to install such a storage system is about $1M (or $0.1/Watt-hour). This is about 5 times cheaper than batteries and has the advantage of being able to store megawatt-hours. Operating costs of such a system are not trivial, since the system would have to be monitored and controlled by at least one person per shift and routine maintenance would have to be performed several times per year. Pumps do wear out also. They will last about 10 years before needing repair or replacement, though better pumps may last 20 years. This modest system would have to be expanded to a very large civil project to store power for a large city. Scaling up to such a large system would bring in land-use issues and make it difficult to find suitable (convenient location) land. These types of systems are already in use for storing hundreds of megawatts (for example at a power plant in western NY near the Kinzue area). It’s safe to assume then the at the cost per kWh over the system’s life is probably a cent or two, since power in that area is about 10 cents/kWh.
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