October 30 2007

Ice Storage Systems

This post is a primer on Ice Storage Systems that will be continued by interviews of designers and operators that have used these systems. We will also tour a local facility using such a system and see how these things really work.

Ice Storage Systems refer to building conditioning systems that use ice as a storage media for thermal energy. These systems are technically simple, cost effective, and can increase the efficiency of a building HVAC system. Ice storage technology is currently going through a resurgence as building designers look for ways to reduce the energy consumption and impact of buildings on the environment. Given the statistics published by the EIA (US Energy Information Administration) on energy consumption by the built environment, ice storage is another valuable tool in the kit for designers attempting to reduce the footprint of their building.

Ice storage has been around for ages. Centuries ago people would climb up mountains and return with ice to cool buildings or preserve food in cities below. The principle in today's technology remains the same. It is worth mentioning that the term "ton" of cooling capacity has its origins in the amount of energy it took to melt a one ton block of ice over 24 hours.

Water/Ice is a great storage medium because of its high specific heat coefficient and large heat of fusion. What this effectively means is that water can hold large amounts of thermal energy in a relatively small volume. For instance, one cubic meter of water can store 334 MJ / 93kWh / 317 k BTU of energy. Furthermore that energy, if the ice is properly insulated, can be stored for extended periods of time and still retain most of its cooling capacity. In fact, we will later show how relevant this attribute is to the use of ice in a building conditioning system.

An ice storage system typically operates by running the building chillers 24 hours a day. During the day these chillers cool the building, and at night, during off-peak hours, the chillers are used to create ice. That ice in turn helps offset the cooling requirements for the chillers the following day. On average it takes 16 to 18 hours cycle of the chillers running in ice-making mode to produce enough ice for a typical 6 hour melt mode cycle.

The beauty of the system is that chiller equipment can be drastically downsized because the load on each chiller is reduced at peak times due to the aid of the coolth stored in the ice storage system. Some figures put the capital expenditure on chillers as being 40-50% reduced when coupled with an ice storage system. The building operator is also saving money because they are paying off-peak utility rates for the ice production done at night. Their bill can be further reduced if their overall peak wattage load does not cross predefined thresholds defined by utilities that, once reached, increase the price paid for all electricity used in a given period.

The second benefit to ice storage, besides lower capital and operational costs, is the reduced environmental impact of using off-peak power. Peak power is generated from a number of different sources depending on the energy mix in your particular region, but what is true in all areas is that peak power production facilities, "peaker" plants, are not held to the same environmental standards as off-peak plants. That means that in order to generate peak power, utilities are turning on dirty, older, polluting plants to meet demand. If peak load is lower, then additional "peaker" plants are not needed!

There are several variants to ice storage systems in use today, and they all use relatively simple and similar technologies. We hope this introduction to ice storage systems helps lay the groundwork for our more in depth analysis these systems coming soon.

The following slideshow shows a few ice storage systems...

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October 30th, 2007 at 4:17 PM. Posted in Other. Share this post.