Ice storage air conditioning
Encyclopedia
Thermal energy storage
using ice is practical because of the large heat of fusion of water. One metric ton of water, one cubic metre, can store 334 million joule
s (MJ) or 317,000 BTUs (93kWh or 26.4 ton-hours). In fact, ice was originally transported from mountains to cities for use as a coolant, and the original definition of a "ton" of cooling capacity (heat flow) was the heat to melt one ton of ice every 24 hours. This is the heat flow one would expect in a 3000 square feet (278.7 m²) house in Boston in the summer. This definition has since been replaced by less archaic units: one ton HVAC
capacity = 12,000 BTU/hour. Either way, an agreeably small storage facility can hold enough ice to cool a large building for a day or a week, whether that ice is produced by anhydrous ammonia chillers or hauled in by horse-drawn carts.
Another method of storage of cold energy is ground freezing. Although this may be in the form of ice where the ground is saturated, the system will work as well with pure rock. Indeed where ice is formed the heat of fusion of ice formation is not used as the ice stays solid throughout the process. The method is based on ground freezing as widely used for mining and tunnelling to solidify unstable ground during excavations. The ground is frozen using bore holes with concentric pipes of 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.
Partly originated due to the benefits of using off peak power, clearly the replacement of existing air conditioning systems with ice storage offers a cost effective method of energy storage enable surplus wind energy and other such Intermittent energy source to be stored for use in chilling at a later, possibly months later, date
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 that 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 turbine
s 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.
A new twist on this technology uses ice as a condensing medium for the refrigerant
. In this case, regular refrigerant is pumped to coils where it is used. Instead of needing a compressor
to convert it back in to a liquid however, the low temperature of the ice is used to chill the refrigerant back in to 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 chill water technology. In addition, unlike water-cooled chill 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 daytime temperatures and off peak temperatures, this type of unit is typically more energy efficient than the equipment it is replacing.
air inlet cooling. Instead of shifting electrical demand to the night, this technique shifts generation capacity to the day. To generate the ice at night, the turbine is often mechanically connected to the compressor of a large chiller. During peak daytime loads, water is circulated between the ice pile and a heat exchanger
in front of the turbine air intake, cooling the intake air to near freezing temperatures. Because 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
system.
Thermal energy storage
Thermal energy storage comprises a number of technologies that store thermal energy in energy storage reservoirs for later use. They can be employed to balance energy demand between day time and night time. The thermal reservoir may be maintained at a temperature above or below that of the...
using ice is practical because of the large heat of fusion of water. One metric ton of water, one cubic metre, can store 334 million joule
Joule
The joule ; symbol J) is a derived unit of energy or work in the International System of Units. It is equal to the energy expended in applying a force of one newton through a distance of one metre , or in passing an electric current of one ampere through a resistance of one ohm for one second...
s (MJ) or 317,000 BTUs (93kWh or 26.4 ton-hours). In fact, ice was originally transported from mountains to cities for use as a coolant, and the original definition of a "ton" of cooling capacity (heat flow) was the heat to melt one ton of ice every 24 hours. This is the heat flow one would expect in a 3000 square feet (278.7 m²) house in Boston in the summer. This definition has since been replaced by less archaic units: one ton HVAC
HVAC
HVAC refers to technology of indoor or automotive environmental comfort. HVAC system design is a major subdiscipline of mechanical engineering, based on the principles of thermodynamics, fluid mechanics, and heat transfer...
capacity = 12,000 BTU/hour. Either way, an agreeably small storage facility can hold enough ice to cool a large building for a day or a week, whether that ice is produced by anhydrous ammonia chillers or hauled in by horse-drawn carts.
Another method of storage of cold energy is ground freezing. Although this may be in the form of ice where the ground is saturated, the system will work as well with pure rock. Indeed where ice is formed the heat of fusion of ice formation is not used as the ice stays solid throughout the process. The method is based on ground freezing as widely used for mining and tunnelling to solidify unstable ground during excavations. The ground is frozen using bore holes with concentric pipes of 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.
Partly originated due to the benefits of using off peak power, clearly the replacement of existing air conditioning systems with ice storage offers a cost effective method of energy storage enable surplus wind energy and other such Intermittent energy source to be stored for use in chilling at a later, possibly months later, date
Air conditioning
The most widely used form of this technology is in large building or campus-wide air conditioningAir conditioning
An air conditioner is a home appliance, system, or mechanism designed to dehumidify and extract heat from an area. The cooling is done using a simple refrigeration cycle...
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
Fuel
Fuel is any material that stores energy that can later be extracted to perform mechanical work in a controlled manner. Most fuels used by humans undergo combustion, a redox reaction in which a combustible substance releases energy after it ignites and reacts with the oxygen in the air...
used at night to produce electricity is a domestic resource in most countries, so that 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 turbine
Gas turbine
A gas turbine, also called a combustion turbine, is a type of internal combustion engine. It has an upstream rotating compressor coupled to a downstream turbine, and a combustion chamber in-between....
s that provide peaking power during the day. And because the load factor
Load factor
Load factor may refer to:* Load factor , the ratio of the lift of an aircraft to its weight* Load factor , the ratio of the number of records to the number of addresses within a data structure...
on the plants is higher, fewer plants are needed to service the load.
A new twist on this technology uses ice as a condensing medium for the refrigerant
Refrigerant
A refrigerant is a substance used in a heat cycle usually including, for enhanced efficiency, a reversible phase change from a liquid to a gas. Traditionally, fluorocarbons, especially chlorofluorocarbons, were used as refrigerants, but they are being phased out because of their ozone depletion...
. In this case, regular refrigerant is pumped to coils where it is used. Instead of needing a compressor
Gas compressor
A gas compressor is a mechanical device that increases the pressure of a gas by reducing its volume.Compressors are similar to pumps: both increase the pressure on a fluid and both can transport the fluid through a pipe. As gases are compressible, the compressor also reduces the volume of a gas...
to convert it back in to a liquid however, the low temperature of the ice is used to chill the refrigerant back in to 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 chill water technology. In addition, unlike water-cooled chill 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 daytime temperatures and off peak temperatures, this type of unit is typically more energy efficient than the equipment it is replacing.
Combustion gas turbine air inlet cooling
Thermal energy storage is also used for combustion gas turbineGas turbine
A gas turbine, also called a combustion turbine, is a type of internal combustion engine. It has an upstream rotating compressor coupled to a downstream turbine, and a combustion chamber in-between....
air inlet cooling. Instead of shifting electrical demand to the night, this technique shifts generation capacity to the day. To generate the ice at night, the turbine is often mechanically connected to the compressor of a large chiller. During peak daytime loads, water is circulated between the ice pile and a heat exchanger
Heat exchanger
A heat exchanger is a piece of equipment built for efficient heat transfer from one medium to another. The media may be separated by a solid wall, so that they never mix, or they may be in direct contact...
in front of the turbine air intake, cooling the intake air to near freezing temperatures. Because 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
Compressed Air Energy Storage is a way to store energy generated at one time for use at another time. At utility scale, energy generated during periods of low energy demand can be released to meet higher demand periods....
system.