Seasonal thermal store
Encyclopedia
A seasonal thermal store (also known as a seasonal heat store or inter-seasonal thermal store) is a store designed to retain heat deposited during the hot summer months for use during colder winter weather. The heat is typically captured using solar collector
s, although other energy sources are sometime used separately or in parallel.
In all cases, very effective above-ground insulation
or superinsulation
of the building structure is required to minimize heat-loss from the building, and hence the amount of heat that needs to be stored and used for space heating.
Despite the differences in design that they involve, low-temperature systems tend to offer simple and relatively inexpensive implementations which are less vulnerable to equipment failure. They do, however, require the site of the building to be clear of the water table
, bedrock
and existing buildings, and are limited to temperate (or warmer) climate zones and to space heating only. High-temperature systems share the same vulnerabilities as conventional space and water heating systems due to their 'active' mechanical and electrical components, as well as their advantage of enabling greater control. They can also be employed in colder climates.
s was probably the ability of the earth to naturally even out variations in temperature. At depths of about 20 feet (6m) temperature is naturally “annualised” at a stable year-round temperature.
With the development of modern passive solar building design
, during the 1970s and 1980s a number of techniques were developed in the US that enabled thermally and moisture-protected soil to be used as an effective seasonal storage medium for space heating
, with direct conduction
as the heat return method.
Two basic techniques can be employed:
These concepts are compared in greater detail at the Greener Shelter site.
. A high Coefficient of Performance is obtained because the heat pump starts with a warm temperature of 25 °C (77 °F) from the thermal store, instead of a cold temperature of 10 °C (50 °F) from the ground.
.
, Switzerland
and Germany
have all been notable pioneers in this field.
One example of this active approach is the experimental “Jenni-Haus” built in 1989 in Oberburg
, Switzerland
. This has three tanks storing a total of 118 cubic metres (4,167.1 cu ft) providing far more heat than is required to heat the building.
The more recent “Zero Heating Energy House”, completed in 1997 in Berlin
as part of the IEA Task 13 low energy housing demonstration project, stores water at temperatures up to 90 °C (194 °F) inside a 20 cubic metres (706.3 cu ft) tank in the basement
, and is now one of a growing number of similar properties.
Another similar example was set up in Ireland
in 2009. The solar seasonal store consists of a 23 m3 tank, filled with water, which was installed in the ground, heavily insulated all around, to store heat from evacuated solar tubes during the year. The system was installed as an experiment to heat the world's first standardised pre-fabricated passive house in Galway, Ireland
. The aim was to find out if this heat would be sufficient to eliminate the need for any electricity in the already highly efficient home during the winter months. The system is monitored and documented by a research team from The University of Ulster and the results will be included in part of a PhD
thesis.
has received international attention. This features a 12000 m³ (423,776 cu ft) reinforced concrete
thermal store linked to 4300 m² (46,284.8 sq ft) of solar collectors, which will supply the 570 houses with around 50% of their heating and hot water.
A different approach is illustrated by the Drake Landing Solar Community development in Okotoks, Alberta
. This community consists of 52 houses built to the stringent R-2000 building code. Here the store is created from the ground itself, with solar heated water pumped into a Borehole Thermal Energy Storage (BTES) system. It consists of 144 borehole
s, each 37 m (121.4 ft) deep, which heat the ground to a maximum of around 90 °C (194 °F). During the winter, the hot water flows from the BTES field to the houses through a distribution network. Once inside the house, it flows through coil units, over which air is blown. The hot air then heats the house. Each house also has an independent solar thermal system installed on its sloped roof to provide domestic hot water. This system has a 90% solar fraction, meaning 90% of the energy required to heat the air and water within the community is provided by the sun. This results in a reduction of over five tonnes of CO2 equivalent, per house.
, which is the cold source. This heats the water, which is then stored by the system in a warm source. In winter, the warm water is pumped up to supply heat. The now cooled water is returned to the cold source. The combination of cold and heat storage with heat pumps has an additional benefit for greenhouses, as it may be combined with humidification. In the (closed circuit) system, the hot water is stored in one aquifer, while the cold water is stored in another. The water is used to heat or cool the air, which is moved by fans. Such a system can be completely automated.
Solar collector
-See also:*Solar thermal collector*Solar water heating*Solar air heating*Photovoltaic module*Renewable heat*Concentrating solar power...
s, although other energy sources are sometime used separately or in parallel.
Types of seasonal thermal storage system
Seasonal (or "annualized") thermal storage can be divided into three broad categories:- Low-temperature systems use the soilSoilSoil is a natural body consisting of layers of mineral constituents of variable thicknesses, which differ from the parent materials in their morphological, physical, chemical, and mineralogical characteristics...
adjoining the building as a low-temperature seasonal heat store (reaching temperatures similar to average annual air temperature), drawing upon the stored heat for space heatingSpace heatingA space heater is a self-contained device for heating an enclosed area. Space heating is generally employed to warm a small space, and is usually held in contrast with central heating, which warms many connected spaces at once...
. Such systems can also be seen as an extension to the building design (normally passive solar building designPassive solar building designIn passive solar building design, windows, walls, and floors are made to collect, store, and distribute solar energy in the form of heat in the winter and reject solar heat in the summer...
), as the design involves some simple but significant differences when compared to 'traditional' buildings. Rock caverns filled with water have also been proposed.
- Warm-temperature interseasonal heat stores also use soil to store heat, but employ active mechanisms of solar collection in summer to heat thermal banks in advance of the heating season.
- High-temperature seasonal heat stores are essentially an extension of the building's HVACHVACHVAC 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...
and water heatingWater heatingWater heating is a thermodynamic process using an energy source to heat water above its initial temperature. Typical domestic uses of hot water are for cooking, cleaning, bathing, and space heating...
systems. WaterWaterWater is a chemical substance with the chemical formula H2O. A water molecule contains one oxygen and two hydrogen atoms connected by covalent bonds. Water is a liquid at ambient conditions, but it often co-exists on Earth with its solid state, ice, and gaseous state . Water also exists in a...
is normally the storage medium, stored in tanks at temperatures that can approach boiling point. Phase change materialPhase Change MaterialA phase-change material is a substance with a high heat of fusion which, melting and solidifying at a certain temperature, is capable of storing and releasing large amounts of energy...
s (which are expensive but which require much smaller tanks) and high-tech soil heating systems (remote from the building) are occasionally used instead. For systems installed in individual buildings, additional space is required to accommodate the size of the storage tanks.
In all cases, very effective above-ground insulation
Thermal insulation
Thermal insulation is the reduction of the effects of the various processes of heat transfer between objects in thermal contact or in range of radiative influence. Heat transfer is the transfer of thermal energy between objects of differing temperature...
or superinsulation
Superinsulation
Superinsulation is an approach to building design, construction, and retrofitting that dramatically reduces heat loss by using much higher levels of insulation and airtightness than normal...
of the building structure is required to minimize heat-loss from the building, and hence the amount of heat that needs to be stored and used for space heating.
Despite the differences in design that they involve, low-temperature systems tend to offer simple and relatively inexpensive implementations which are less vulnerable to equipment failure. They do, however, require the site of the building to be clear of the water table
Water table
The water table is the level at which the submarine pressure is far from atmospheric pressure. It may be conveniently visualized as the 'surface' of the subsurface materials that are saturated with groundwater in a given vicinity. However, saturated conditions may extend above the water table as...
, bedrock
Bedrock
In stratigraphy, bedrock is the native consolidated rock underlying the surface of a terrestrial planet, usually the Earth. Above the bedrock is usually an area of broken and weathered unconsolidated rock in the basal subsoil...
and existing buildings, and are limited to temperate (or warmer) climate zones and to space heating only. High-temperature systems share the same vulnerabilities as conventional space and water heating systems due to their 'active' mechanical and electrical components, as well as their advantage of enabling greater control. They can also be employed in colder climates.
Low-temperature seasonal heat stores
One of the original motivations of early man's movement into caveCave
A cave or cavern is a natural underground space large enough for a human to enter. The term applies to natural cavities some part of which is in total darkness. The word cave also includes smaller spaces like rock shelters, sea caves, and grottos.Speleology is the science of exploration and study...
s was probably the ability of the earth to naturally even out variations in temperature. At depths of about 20 feet (6m) temperature is naturally “annualised” at a stable year-round temperature.
With the development of modern passive solar building design
Passive solar building design
In passive solar building design, windows, walls, and floors are made to collect, store, and distribute solar energy in the form of heat in the winter and reject solar heat in the summer...
, during the 1970s and 1980s a number of techniques were developed in the US that enabled thermally and moisture-protected soil to be used as an effective seasonal storage medium for space heating
Space heating
A space heater is a self-contained device for heating an enclosed area. Space heating is generally employed to warm a small space, and is usually held in contrast with central heating, which warms many connected spaces at once...
, with direct conduction
Heat conduction
In heat transfer, conduction is a mode of transfer of energy within and between bodies of matter, due to a temperature gradient. Conduction means collisional and diffusive transfer of kinetic energy of particles of ponderable matter . Conduction takes place in all forms of ponderable matter, viz....
as the heat return method.
Two basic techniques can be employed:
- In the Passive Annual Heat Storage (PAHS) and similar direct solar gain systems, solar heat is directly captured by the structure's spaces (through windows and other surfaces) in summer and then passively transferred (by conduction) through its floors, walls (and, sometimes, roof) into adjoining thermally-buffered soil. It is then passively returned (by conduction and radiationRadiant energyRadiant energy is the energy of electromagnetic waves. The quantity of radiant energy may be calculated by integrating radiant flux with respect to time and, like all forms of energy, its SI unit is the joule. The term is used particularly when radiation is emitted by a source into the...
) as those spaces cool in winter. These techniques were advocated in Daniel Geery's 1982 book Solar Greenhouses: Underground and John Hait's 1983 Passive Annual Heat Storage — Improving the Design of Earth Shelters.
- The Annualized Geothermal Solar (AGS) concept involves the capture of heat by isolated solar gain devices (rather than the building structure). From here it is deposited in the earth (or other storage masses or mediums) adjoining the building using active or passive technology. The depth at which the heat is deposited is calculated (according to soil type) to provide a controlled 6-month heat-return time-lag to the building through conduction as the building cools. This alternative was posed by Don Stephens.
These concepts are compared in greater detail at the Greener Shelter site.
Warm-temperature seasonal heat stores
Warm-temperature heat stores are a development of low-temperature stores in that solar collectors are used to capture surplus heat in summer and actively raise the temperature of large thermal banks of soil so that heat can be extracted more easily (and more cheaply) in winter. Interseasonal Heat Transfer uses water circulating in pipes embedded in asphalt solar collectors to transfer heat to Thermal Banks beneath the insulated foundation of buildings. A ground source heat pump is used in winter to extract the warmth from the Thermal Bank to provide space heating via underfloor heatingUnderfloor heating
Underfloor heating and cooling is a form of central heating and cooling which achieves indoor climate control for thermal comfort using conduction, radiation and convection...
. A high Coefficient of Performance is obtained because the heat pump starts with a warm temperature of 25 °C (77 °F) from the thermal store, instead of a cold temperature of 10 °C (50 °F) from the ground.
High-temperature seasonal thermal stores
High-temperature seasonal thermal stores are found on a variety of scales, from those installed in individual houses to those serving neighbourhoods via district heatingDistrict heating
District heating is a system for distributing heat generated in a centralized location for residential and commercial heating requirements such as space heating and water heating...
.
Individual structures
Although the use of high-temperature seasonal thermal stores within individual buildings dates back to at least 1939 (MIT Solar House #1), the United StatesUnited States
The United States of America is a federal constitutional republic comprising fifty states and a federal district...
, Switzerland
Switzerland
Switzerland name of one of the Swiss cantons. ; ; ; or ), in its full name the Swiss Confederation , is a federal republic consisting of 26 cantons, with Bern as the seat of the federal authorities. The country is situated in Western Europe,Or Central Europe depending on the definition....
and Germany
Germany
Germany , officially the Federal Republic of Germany , is a federal parliamentary republic in Europe. The country consists of 16 states while the capital and largest city is Berlin. Germany covers an area of 357,021 km2 and has a largely temperate seasonal climate...
have all been notable pioneers in this field.
One example of this active approach is the experimental “Jenni-Haus” built in 1989 in Oberburg
Oberburg
Oberburg is a municipality in the administrative district of Emmental in the canton of Bern in Switzerland.-Geography:Oberburg has an area, , of . Of this area, or 55.8% is used for agricultural purposes, while or 33.3% is forested...
, Switzerland
Switzerland
Switzerland name of one of the Swiss cantons. ; ; ; or ), in its full name the Swiss Confederation , is a federal republic consisting of 26 cantons, with Bern as the seat of the federal authorities. The country is situated in Western Europe,Or Central Europe depending on the definition....
. This has three tanks storing a total of 118 cubic metres (4,167.1 cu ft) providing far more heat than is required to heat the building.
The more recent “Zero Heating Energy House”, completed in 1997 in Berlin
Berlin
Berlin is the capital city of Germany and is one of the 16 states of Germany. With a population of 3.45 million people, Berlin is Germany's largest city. It is the second most populous city proper and the seventh most populous urban area in the European Union...
as part of the IEA Task 13 low energy housing demonstration project, stores water at temperatures up to 90 °C (194 °F) inside a 20 cubic metres (706.3 cu ft) tank in the basement
Basement
__FORCETOC__A basement is one or more floors of a building that are either completely or partially below the ground floor. Basements are typically used as a utility space for a building where such items as the furnace, water heater, breaker panel or fuse box, car park, and air-conditioning system...
, and is now one of a growing number of similar properties.
Another similar example was set up in Ireland
Ireland
Ireland is an island to the northwest of continental Europe. It is the third-largest island in Europe and the twentieth-largest island on Earth...
in 2009. The solar seasonal store consists of a 23 m3 tank, filled with water, which was installed in the ground, heavily insulated all around, to store heat from evacuated solar tubes during the year. The system was installed as an experiment to heat the world's first standardised pre-fabricated passive house in Galway, Ireland
Galway
Galway or City of Galway is a city in County Galway, Republic of Ireland. It is the sixth largest and the fastest-growing city in Ireland. It is also the third largest city within the Republic and the only city in the Province of Connacht. Located on the west coast of Ireland, it sits on the...
. The aim was to find out if this heat would be sufficient to eliminate the need for any electricity in the already highly efficient home during the winter months. The system is monitored and documented by a research team from The University of Ulster and the results will be included in part of a PhD
PHD
PHD may refer to:*Ph.D., a doctorate of philosophy*Ph.D. , a 1980s British group*PHD finger, a protein sequence*PHD Mountain Software, an outdoor clothing and equipment company*PhD Docbook renderer, an XML renderer...
thesis.
Neighbourhoods
At the neighbourhood level, the Wiggenhausen-Süd solar development at FriedrichshafenFriedrichshafen
This article is about a German town. For the Danish town, see Frederikshavn, and for the Finnish town, see Fredrikshamn .Friedrichshafen is a university city on the northern side of Lake Constance in Southern Germany, near the borders with Switzerland and Austria.It is the district capital of the...
has received international attention. This features a 12000 m³ (423,776 cu ft) reinforced concrete
Reinforced concrete
Reinforced concrete is concrete in which reinforcement bars , reinforcement grids, plates or fibers have been incorporated to strengthen the concrete in tension. It was invented by French gardener Joseph Monier in 1849 and patented in 1867. The term Ferro Concrete refers only to concrete that is...
thermal store linked to 4300 m² (46,284.8 sq ft) of solar collectors, which will supply the 570 houses with around 50% of their heating and hot water.
A different approach is illustrated by the Drake Landing Solar Community development in Okotoks, Alberta
Alberta
Alberta is a province of Canada. It had an estimated population of 3.7 million in 2010 making it the most populous of Canada's three prairie provinces...
. This community consists of 52 houses built to the stringent R-2000 building code. Here the store is created from the ground itself, with solar heated water pumped into a Borehole Thermal Energy Storage (BTES) system. It consists of 144 borehole
Borehole
A borehole is the generalized term for any narrow shaft bored in the ground, either vertically or horizontally. A borehole may be constructed for many different purposes, including the extraction of water or other liquid or gases , as part of a geotechnical investigation, environmental site...
s, each 37 m (121.4 ft) deep, which heat the ground to a maximum of around 90 °C (194 °F). During the winter, the hot water flows from the BTES field to the houses through a distribution network. Once inside the house, it flows through coil units, over which air is blown. The hot air then heats the house. Each house also has an independent solar thermal system installed on its sloped roof to provide domestic hot water. This system has a 90% solar fraction, meaning 90% of the energy required to heat the air and water within the community is provided by the sun. This results in a reduction of over five tonnes of CO2 equivalent, per house.
Greenhouses
Thermal storage (sometimes referred to as heat and cold storage) is also used extensively for applications as the heating of greenhouses. In summer, the greenhouse is cooled with ground water, pumped from an aquiferAquifer
An aquifer is a wet underground layer of water-bearing permeable rock or unconsolidated materials from which groundwater can be usefully extracted using a water well. The study of water flow in aquifers and the characterization of aquifers is called hydrogeology...
, which is the cold source. This heats the water, which is then stored by the system in a warm source. In winter, the warm water is pumped up to supply heat. The now cooled water is returned to the cold source. The combination of cold and heat storage with heat pumps has an additional benefit for greenhouses, as it may be combined with humidification. In the (closed circuit) system, the hot water is stored in one aquifer, while the cold water is stored in another. The water is used to heat or cool the air, which is moved by fans. Such a system can be completely automated.
See also
- Central solar heatingCentral solar heatingCentral solar heating is the provision of central heating and hot water from solar energy by a system in which the water is heated centrally by arrays of solar thermal collectors and distributed through district heating pipe networks .For block systems, the solar collectors are...
- District heatingDistrict heatingDistrict heating is a system for distributing heat generated in a centralized location for residential and commercial heating requirements such as space heating and water heating...
- Geosolar
- Ice pondIce pondAn ice pond is a large volume of ice or snow produced and stored in the winter to be used for cooling/air conditioning in the summer. The best known experiment is the 'Princeton ice pond' by Ted Taylor in 1981. He then convinced the Prudential Insurance Company to use a bigger pond to provide air...
- Solar thermal collectorSolar thermal collectorA solar thermal collector is a solar collector designed to collect heat by absorbing sunlight. The term is applied to solar hot water panels, but may also be used to denote more complex installations such as solar parabolic, solar trough and solar towers or simpler installations such as solar air...
- Solar pondSolar pondA solar pond is a pool of saltwater which acts as a large-scale solar thermal energy collector with integral heat storage for supplying thermal energy...
- Thermal energy storageThermal energy storageThermal 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...
- Zero energy buildingZero energy buildingA zero-energy building, also known as a zero net energy building, Net-Zero Energy Building , or Net Zero Building, is a popular term to describe a building with zero net energy consumption and zero carbon emissions annually. Zero energy buildings can be independent from the energy grid supply...