Ice storage air conditioning is the process of using ice for

thermal energy storage Thermal energy storage (TES) is achieved with widely different technologies. Depending on the specific technology, it allows excess thermal energy to be stored and used hours, days, months later, at scales ranging from the individual process, ...

. The process can reduce energy used for cooling during times of peak electrical demand. Alternative power sources such as solar can also use the technology to store energy for later use. This is practical because of water's large

heat of fusion In thermodynamics, the enthalpy of fusion of a substance, also known as (latent) heat of fusion, is the change in its enthalpy resulting from providing energy, typically heat, to a specific quantity of the substance to change its state from a s ...

: one

metric ton The tonne ( or ; symbol: t) is a unit of mass equal to 1000 kilograms. It is a non-SI unit accepted for use with SI. It is also referred to as a metric ton to distinguish it from the non-metric units of the short ton (United States c ...

of water (one cubic metre) can store 334 megajoules (MJ) (317,000

BTU The British thermal unit (BTU or Btu) is a unit of heat; it is defined as the amount of heat required to raise the temperature of one pound of water by one degree Fahrenheit. It is also part of the United States customary units. The modern SI u ...

) of energy, equivalent to 93 kWh (26.4 ton-hours). The original definition of a " ton of cooling capacity" (heat flow) was the heat needed to melt one ton of ice in a 24-hour period. This heat flow is what one would expect in a house in Boston in the summer. This definition has since been replaced by less-archaic units: one ton of

HVAC Heating, ventilation, and air conditioning (HVAC) is the use of various technologies to control the temperature, humidity, and purity of the air in an enclosed space. Its goal is to provide thermal comfort and acceptable indoor air quality. HV ...

or refrigeration capacity is approximately equivalent to 3520

Watts Watts is plural for ''watt'', the unit of power. Watts may also refer to: People *Watts (surname), list of people with the surname Watts Fictional characters *Watts, main character in the film '' Some Kind of Wonderful'' *Watts family, six chara ...

. A small storage facility can hold enough ice to cool a large building from one day to one week, whether that ice is produced by

anhydrous ammonia Ammonia is an inorganic compound of nitrogen and hydrogen with the formula . A stable binary hydride, and the simplest pnictogen hydride, ammonia is a colourless gas with a distinct pungent smell. Biologically, it is a common nitrogenous wast ...

chillers or hauled in by horse-drawn carts.

Ground freezing Ground freezing is a construction technique used in circumstances where soil needs to be stabilized so it will not collapse next to excavations, or to prevent contaminants spilled into soil from being leached away. Ground freezing has been used f ...

can also be utilized; this may be done in ice form where the ground is saturated. Systems will also work with pure rock. Wherever ice forms, the ice formation's heat of fusion is not used, as the ice remains solid throughout the process. The method based on ground freezing is widely used for mining and tunneling to solidify unstable ground during excavations. The ground is frozen using bore holes with concentric pipes that carry brine from a chiller at the surface. Cold is extracted in a similar way using brine and used in the same way as for conventional ice storage, normally with a brine-to-liquid heat exchanger, to bring the working temperatures up to usable levels at higher volumes. The frozen ground can stay cold for months or longer, allowing cold storage for extended periods at negligible structure cost. Replacing existing air conditioning systems with ice storage offers a cost-effective energy storage method, enabling surplus wind energy and other such intermittent energy sources to be stored for use in chilling at a later time, possibly months later.

Early ice storage, shipment, and production

Before the advent of mechanical refrigeration, ice was cut from frozen lakes or rivers and transported to cities for use as a coolant. Ice was widely shipped and stored year-round in icehouses. If there was no readily-accessible source of ice, then shallow, shaded pools were often built to nearby, and the ice removed from them during the freezing season.

Air conditioning

The most widely used form of this technology can be found in campus-wide air conditioning or chilled water systems of large buildings. Air conditioning systems, especially in commercial buildings, are the biggest contributors to peak electrical loads seen on hot summer days in various countries. In this application, a standard chiller runs at night to produce an ice pile. Water then circulates through the pile during the day to produce chilled water that would normally be the chiller's daytime output. A partial storage system minimizes capital investment by running the chillers nearly 24 hours a day. At night, they produce ice for storage and during the day they chill water for the air conditioning system. Water circulating through the melting ice augments their production. Such a system usually runs in ice-making mode for 16 to 18 hours a day and in ice-melting mode for six hours a day. Capital expenditures are minimized because the chillers can be just 40 - 50% of the size needed for a conventional design. Ice storage sufficient to store half a day's rejected heat is usually adequate. A full storage system minimizes the cost of energy to run that system by entirely shutting off the chillers during peak load hours. The capital cost is higher, as such a system requires somewhat larger chillers than those from a partial storage system, and a larger ice storage system. Ice storage systems are inexpensive enough that full storage systems are often competitive with conventional air conditioning designs. The air conditioning chillers' efficiency is measured by their

coefficient of performance The coefficient of performance or COP (sometimes CP or CoP) of a heat pump, refrigerator or air conditioning system is a ratio of useful heating or cooling provided to work (energy) required. Higher COPs equate to higher efficiency, lower energy ( ...

(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, heat loss overpowers this advantage, since it melts the ice. Air conditioning thermal storage has been shown to be somewhat beneficial in society. Off-peak electricity is cheaper, as demand is lower. It also reduces the demand at peak times, which is often provided by expensive and unenvironmental sources. A new twist on this technology uses ice as a condensing medium for the

refrigerant A refrigerant is a working fluid used in the heat pump and refrigeration cycle, refrigeration cycle of air conditioning systems and heat pumps where in most cases they undergo a repeated phase transition from a liquid to a gas and back again. Ref ...

. In this case, regular refrigerant is pumped to coils where it is used. Rather than needing a

compressor A compressor is a mechanical device that increases the pressure of a gas by reducing its volume. An air compressor is a specific type of gas compressor. Compressors are similar to pumps: both increase the pressure on a fluid and both can transp ...

to convert it back into a liquid, however, the low temperature of ice is used to chill the refrigerant back into a liquid. This type of system allows existing refrigerant-based HVAC equipment to be converted to Thermal Energy Storage systems, something that could not previously be easily done with chilled water technology. In addition, unlike water-cooled chilled water systems that do not experience a tremendous difference in efficiency from day to night, this new class of equipment typically displaces daytime operation of air-cooled condensing units. In areas where there is a significant difference between peak day time temperatures and off-peak temperatures, this type of unit is typically more energy efficient than the equipment that it replaces.

Combustion gas turbine air inlet cooling

Thermal energy storage is also used for combustion

gas turbine A gas turbine, also called a combustion turbine, is a type of continuous flow internal combustion engine. The main parts common to all gas turbine engines form the power-producing part (known as the gas generator or core) and are, in the directi ...

air inlet cooling. Instead of shifting electrical demand to the night, this technique shifts generation capacity to the day. To generate ice at night, the turbine is often mechanically connected to a large chiller's compressor. During peak daytime loads, water is circulated between the ice pile and a

heat exchanger A heat exchanger is a system used to transfer heat between a source and a working fluid. Heat exchangers are used in both cooling and heating processes. The fluids may be separated by a solid wall to prevent mixing or they may be in direct contac ...

in front of the turbine air intake, cooling the intake air to near freezing temperatures. Since the air is colder, the turbine can compress more air with a given amount of compressor power. Typically, both the generated electrical power and turbine efficiency rise when the inlet cooling system is activated. This system is similar to the

compressed air energy storage Compressed-air energy storage (CAES) is a way to store energy for later use using compressed air. At a utility scale, energy generated during periods of low demand can be released during peak load periods.Wild, Matthew, LWind Drives Growing Use ...

system.

References

{{HVAC, state=collapsed Energy storage Heating, ventilation, and air conditioning

Early ice storage, shipment, and production

Air conditioning

Combustion gas turbine air inlet cooling

See also

References