Off-peak cooling systems use electricity to freeze water in special insulated tanks that contain refrigerant-filled coils. These coils circulate ethylene or propylene glycol and a water mixture that's super chilled to well below freezing. Once the water freezes, at night when energy demand is low, it's ready to be used to cool the air the next day. This is called charging, and a charged off-peak cooling system takes very little energy to keep cold in stand-by mode until it's ready to be used to cool the air in an office building or home.
As the building starts to warm up during the day, the air-conditioning kicks on, and the chilled refrigerant from the off-peak cooling system keeps the building's air cool. The glycol cycles through the ice filled tanks periodically to cool back down after being exposed to the hot air, and eventually, this exchange of hot for cold melts the ice. In the evening, the system charges again, freezing the melted ice with a chiller, and preparing the system for the next hot day.
This is an example of a simple off-peak cooling arrangement. More complex systems can use other forms of refrigerant for cooling or use different design strategies for storing ice. Some systems only use off-peak storage for part of their overall demand, supplying some of their cooling requirements from chillers running during peak energy hours. Although these partial systems have higher operating costs, the capital investment is lower because the chillers can be sized smaller, making the equipment itself less expensive.
Off-peak cooling, or thermal energy storage (TES), has typically been used in large buildings, like office complexes, hospitals or schools. Older TES systems needed large tanks and were hard and expensive to retrofit for existing construction or residential applications. Nowadays, smaller, more cost-effective systems are being developed for residential use, and large installations can place tanks almost anywhere, from buried underground to inside the building itself.
A lion's share of the savings in off-peak cooling systems comes from taking advantage of lower energy costs available at off-peak times of the day. If energy rate structures change and off-peak savings become less attractive, the monetary advantages of off-peak cooling could almost disappear. In this instance, the value of off-peak cooling is in being able to take advantage of the way power companies sell energy today, not in the design or monetary value of the equipment itself.
In the next section, we'll explore some of the components of off-peak cooling systems and how they relate to residential applications.