Annualised Geosolar-Sustainable Buildings
Annualised Geo Solar (AGS) permits passive solar heating to operate in sustainable buildings in cold areas which experience little sun for a large part of the year. The system allows the ground under the sustainable building project to act as a thermal mass to heat and cool the structure. Depending on the season heat is either returned to or taken from the building. In hot climates for example the collector when exposed to the cold night air in the Winter, can have the effect of cool the building six months hence or if it’s a hot day the next day there and then. In this respect Annualised Geosolar is the sustainable building dream.
A six month thermal lag is put in place in the form of three metres of soil under the building with a six metre wide skirt of insulation buried around it. The skirt construction keeps melted snow and rain out of the soil. The contribution to sustainable building objectives arises from the soil providing cooling and heating through the walls and floor depending on the relative temperatures at any time. A thermal syphon (see below) transfers the heat between the soil and the solar collector, which might be a compartment in the roof made of sheet metal or alternatively, a flat box on the side of a hill or a building. These syphons are typically made of plastic pipe and carry air. The advantage of air is that it prevents water leaks and corrosion and although plastic is not a particularly environmentally friendly material for short term use it is nevertheless popular with environmentalists involved in sustainable building projects owing to its longevity. It’s long life span makes a ‘greener’ option in the long run than metals which eventually corrode in the ground
AGS sustainable building installations typically take the form of:
- A heavily insulated, energy efficient, environmentally sustainable building;
- A heat capture system in an attic space, sub roof, greenhouse or sunspace used for heat accumulation in the summer. Alternatively, a ground-based flat-plate may be in use. The system also comprises a thermosyphon collector, or some alternative other solar-heat collection device;
- The heat transferred from the collection source is stored in the earth mass under the sustainable building itself, with the storage surrounded by perimeter “umbrella” or ‘cape’ located below surface which provides insulation so avoiding heat loss back into the environment and moisture transfer the other way
- A dense floor of the type popular in all sustainable buildings designs for the purpose of radiating heat into the living area, to match the time lag built into the sub floor design
- An (often Photovoltaic powered) control system which controls dampers and fans, and which operates when the sustainable building’s sensors determine that the air is colder in the storage mass than in the collection location, or alternatively which allows passive convection to move the heat into the storage zone. This is sometimes achieved by means of thermally activated dampers and a solar chimney
It usually tales several years for the earth mass storage volume to fully heat up to the necessary temperature and this period can vary widely depending on locality and orientation. The optimum ‘Fall level’ which allows sustainable building practitioners to achieve 100% heating requirement in the Winter can therefore can take several years to reach. This fact makes it an unsatifisying prospect for some sustainable building projects but the technology continues to evolve. For climate reasons the system is almost exclusively confined to Northern Europe where nearly all methods of sustainable building techniques appear to being explored for practicality but to date one at least has been built in Drake Landing in the USA
Thermosyphons are a commnon feature of sustainable building projects and are usually incorporated into AVG sustainable building design. Basically a thermosyphon is a simple solar water heater operating by means of a process known to physicists which causes a substance whether it be liquid, air or gas to circulate without needing the assistance of a mechanical pump. Thermosyphoning has application in sustainable building projects owing to its capacity to circulate volatile gases intended for heating and cooling various locations at various times. For example heat pumps furnaces, boilers and water heaters. The circulation can either be in the form of an open loop where the content of a holding tank passes in one direction to a distribution point along a heated transfer tube positioned at the bottom of the tank, or alternatively by a vertical closed loop which returns it to the original container. The purpose of the system in a sustainable building heating system is to transfer the liquid or gas without the costs and complexity of a conventional pump so making it ideal for sustainable building projects which include Annualised Geosolar installations. Convective movement of the liquid commences when the content of the loop becomes heated. The contents expand and become less dense making it more buoyant than the cooler water lower down in the loop. Convection transports the heated liquid vertically in the system and it is replaced by cooler liquid which comes back down by the force of gravity. The best thermosyphons have low hydraulic resistance and allow the liquid to move easily. A variation on the theme used in some sustainable buildings is a ‘heat pipe’ where the loop is not completely full of liquid. In this instance the system cannot move the heat by convection but it can still be transferred by evaporation and subsequent condensation of the vapour. Sometimes the system might contain other fluids such as air where the heat flux density will be less than in a heat pump proper containing only one fluid.
Sometimes confusion can arise in the use of these different terms with thermosyphons being wrongly described as ‘gravity return heat pipes’. But proper heat pipes require a wick to return the condensate to the evaporator whereas a true thermosyphon needs no wick because gravity alone moves the liquid. A thermosyphon proper is therefore simpler than a heat pipe. Single phase thermosyphons only transfer heat upwards and away from the vector of acceleration so orientation is much more important than for heat pipes and sometimes thermosyphons fail owing to bubbles in the loop. So sustainable building promoters need to ensure for a circulating loop of pipes. If the piping in a thermosyphon is resistant to flow, or too much heat is applied, the content might even boil because the convective pressure of the gas is greater owing to gas being more buoyant than the liquid. Thermosyphons are typically used to heat the liquid in solar heating systems in constructions built to sustainable building principles. The liquid is heated passively using solar energy which has been transferred to a solar collector. The heat from the collector is transferred to the liquid either directly by circulating it through the collector, or indirectly where a solution of anti-freeze carries it heat via a heat exchanger to the water tank from the collector. Convection allows for the movement of the heated liquid which is convected away from the solar collector and is subsequently replaced by colder liquid
Thermosyphons in sustainable buildings have to be mounted so that the vapour rises up and liquid can flow down to the boiler with no bends in the route so avoiding pooling. So the fan that cools the liquid needs cool air to operate. Thermosyphons are therefore not suited to all sustainable building projects but are used in engines and computers. Modern computer processors in particular are prone to become very hot