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 collectors
, 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 soil 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 heating. Such systems can also be seen as an extension to the building design (normally passive solar building design), as the design involves some simple but significant differences when compared to 'traditional' buildings.
- 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 HVAC and water heating systems. Water is normally the storage medium, stored in tanks at temperatures that can approach boiling point. Phase change materials (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 / 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.
Low-temperature seasonal heat stores
One of the original motivations of early man's movement into caves
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:
- 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 radiation) 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.
- 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: www.greenershelter.org
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 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 tarmac 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 heating. A high Coefficient of Performance is obtained because the heat pump starts with a warm temperature from the thermal store.
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 heating
Although the use of high-temperature seasonal thermal stores within individual buildings dates back to at least 1939 (MIT Solar House #1
), the United States
have all been notable pioneers in this field.
Perhaps the best known international example of this active approach is the experimental “Jenni-Haus” built in 1989 in Oberburg, Switzerland. This has 3 tanks storing a total of 118m³ (4,100 cubic feet) 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 (195 °F) inside a 20m³ (700 cubic feet) tank in the basement , and is now one of a growing number of similar properties.
At the neighbourhood level, the Wiggenhausen-Süd solar development at Friedrichshafen
has received international attention. This features a 12,000 m³ (424,000 cubic feet) reinforced concrete
thermal store linked to 4,300m² (46,000 square feet) 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. Here the store is created from the ground itself, with solar heated water pumped into a Borehole Thermal Energy Storage (BTES) system. This consists of 144 boreholes, each 37m (121 ft) deep, which heat the ground to a maximum of around 90 °C (195 °F)
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 aquifer, 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.