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    The most widely used form of thermal ice storage systems is in large building or campus-wide air conditioning or chilled water systems. Air conditioning systems, especially in commercial buildings, are the most significant contributors to the peak electrical loads seen on hot summer days. In this application, a relatively standard chiller is run at night to produce a pile of ice. Water is circulated through the pile during the day to produce chilled water that would normally be the daytime output of the chillers.

    A partial storage system minimizes capital investment by running the chillers 24 hours a day. At night they produce ice for storage, and during the day they chill water for the air conditioning system; their production augmented by water circulating through the melting ice. Such a system usually runs in ice-making mode for 16 to 18 hours a day, and in ice-melting mode for 6 hours a day. Capital expenditures are minimized because the chillers can be just 40 to 50% of the size needed for a conventional design. Ice storage sufficient for storing half a day’s rejected heat will do.

    A full storage system minimizes the cost of energy to run the system by shutting off the chillers entirely during peak load hours. Such a system requires chillers somewhat larger than a partial storage system and a larger ice storage system, so that the capital cost is higher. Ice storage systems are inexpensive enough that full storage systems are often competitive with conventional air conditioning designs.

    The efficiency of air conditioning chillers is measured by their coefficient of performance (COP). In theory, thermal storage systems could make chillers more efficient because heat is discharged into colder nighttime air rather than warmer daytime air. In practice, this advantage is overcome by the heat losses while making and melting the ice.

    There are some advantages to society from air conditioning thermal storage. The fuel used at night to produce electricity is a domestic resource in most countries, so less imported fuel is used. This process also has been shown in studies to significantly reduce the emissions associated with producing the power for air conditioners since inefficient “peaker” plants are replaced by low emission base load facilities in the evening. The plants that produce this power are often more efficient than the gas turbines that provide peaking power during the day. And because the load factor on the plants is higher, fewer plants are needed to service the load.

    Article by Jerry Rollen, Director of Preconstruction at Energy Air

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