Barra system
Encyclopedia
The Barra system is a passive solar building technology developed by Horazio Barra in Italy
. It uses a collector wall to capture solar radiation in the form of heat. It also uses the thermosiphon
effect to distribute the warmed air through channels incorporated into the reinforced concrete
floors, warming the floors and hence the building. Alternatively, in hot weather, cool nighttime air can be drawn through the floors to chill them in a form of air conditioning
.
Barra's are said to have more uniform north-south temperature distributions than other passive solar houses. Many successful systems were built in Europe
, but Barra seems fairly unknown elsewhere.
, but with the advent of cheap, robust Polycarbonate
glazing most designs use twin- or triple-wall polycarbonate greenhouse sheeting. Typically the glazing is designed to pass visible light, but block IR
to reduce losses, and block UV
to protect building materials.
The next layer is an absorption space. This absorbs most of the light entering the collector. It usually consists of an air gap of around 10cm thickness with one or more absorption meshs suspended vertically in the space. Often window fly screen mesh is used, or horticultural shade cloth. The mesh itself can hold very little heat and warms up rapidly in light. The heat is absorbed by air passing around and through the mesh, and so the mesh is suspended with an air gap on both the front and back sides.
Finally a layer of insulation sits between the absorption space and the house. Usually this is normal house insulation, using materials such as polyisocyanurate
foam, rock wool, foil and polystyrene
.
This collector is very agile - in the sun it heats up rapidly and the air inside starts to convect. If the collector were to be directly connected to the building using a hole near the floor and a hole near the ceiling an indirect solar gain system would be created. One problem with this that, like Trombe wall
s, the heat would radiate back out at night, and a convection current would chill the room during the night. Instead, the air movement can be stopped using automatic dampers, similar to those used for ventilating foundation spaces in cold climates, or plastic film dampers, which work by blocking air flow in one direction with a very lightweight flap of plastic. The addition of the damper makes the design an efficient isolated solar gain system.
Whether water or concrete is used, the heat is transferred from the air in the collector into the storage material during the day, and released on demand using a ceiling fan into the room at night.
Where "spancrete" slabs are used, the ceiling also heats the house by radiation. Some houses are fitted with louvers
(similar to those used on satellites) to adjust the radiation transfer. Warm air travels through the slab tunnels from south to north, where it exits and travels back north through the bulk of the room to the air heater inlet near the floor.
Italy
Italy , officially the Italian Republic languages]] under the European Charter for Regional or Minority Languages. In each of these, Italy's official name is as follows:;;;;;;;;), is a unitary parliamentary republic in South-Central Europe. To the north it borders France, Switzerland, Austria and...
. It uses a collector wall to capture solar radiation in the form of heat. It also uses the thermosiphon
Thermosiphon
Thermosiphon refers to a method of passive heat exchange based on natural convection which circulates liquid without the necessity of a mechanical pump...
effect to distribute the warmed air through channels incorporated into the 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...
floors, warming the floors and hence the building. Alternatively, in hot weather, cool nighttime air can be drawn through the floors to chill them in a form of air conditioning
Air 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...
.
Barra's are said to have more uniform north-south temperature distributions than other passive solar houses. Many successful systems were built in Europe
Europe
Europe is, by convention, one of the world's seven continents. Comprising the westernmost peninsula of Eurasia, Europe is generally 'divided' from Asia to its east by the watershed divides of the Ural and Caucasus Mountains, the Ural River, the Caspian and Black Seas, and the waterways connecting...
, but Barra seems fairly unknown elsewhere.
Passive solar collector
To convert the sun's light into heat indirectly, a separate insulated space is constructed on the sunny side of the house walls. Looking at the outside, and moving through a cross section there is an outside clear layer. This was traditionally built using glassGlass
Glass is an amorphous solid material. Glasses are typically brittle and optically transparent.The most familiar type of glass, used for centuries in windows and drinking vessels, is soda-lime glass, composed of about 75% silica plus Na2O, CaO, and several minor additives...
, but with the advent of cheap, robust Polycarbonate
Polycarbonate
PolycarbonatePhysical PropertiesDensity 1.20–1.22 g/cm3Abbe number 34.0Refractive index 1.584–1.586FlammabilityV0-V2Limiting oxygen index25–27%Water absorption – Equilibrium0.16–0.35%Water absorption – over 24 hours0.1%...
glazing most designs use twin- or triple-wall polycarbonate greenhouse sheeting. Typically the glazing is designed to pass visible light, but block IR
Infrared
Infrared light is electromagnetic radiation with a wavelength longer than that of visible light, measured from the nominal edge of visible red light at 0.74 micrometres , and extending conventionally to 300 µm...
to reduce losses, and block UV
Ultraviolet
Ultraviolet light is electromagnetic radiation with a wavelength shorter than that of visible light, but longer than X-rays, in the range 10 nm to 400 nm, and energies from 3 eV to 124 eV...
to protect building materials.
The next layer is an absorption space. This absorbs most of the light entering the collector. It usually consists of an air gap of around 10cm thickness with one or more absorption meshs suspended vertically in the space. Often window fly screen mesh is used, or horticultural shade cloth. The mesh itself can hold very little heat and warms up rapidly in light. The heat is absorbed by air passing around and through the mesh, and so the mesh is suspended with an air gap on both the front and back sides.
Finally a layer of insulation sits between the absorption space and the house. Usually this is normal house insulation, using materials such as polyisocyanurate
Polyisocyanurate
Polyisocyanurate, also referred to as PIR, polyiso, or ISO, is essentially an improvement on polyurethane . The proportion of methylene diphenyl diisocyanate is higher than for PUR and instead of a polyether polyol, a polyester derived polyol is used in the reaction...
foam, rock wool, foil and polystyrene
Polystyrene
Polystyrene ) also known as Thermocole, abbreviated following ISO Standard PS, is an aromatic polymer made from the monomer styrene, a liquid hydrocarbon that is manufactured from petroleum by the chemical industry...
.
This collector is very agile - in the sun it heats up rapidly and the air inside starts to convect. If the collector were to be directly connected to the building using a hole near the floor and a hole near the ceiling an indirect solar gain system would be created. One problem with this that, like Trombe wall
Trombe wall
A Trombe wall is a sun-facing wall separated from the outdoors by glass and an air space, which absorbs solar energy and releases it selectively towards the interior at night. The essential idea was first explored by Edward S. Morse and patented by him in 1881...
s, the heat would radiate back out at night, and a convection current would chill the room during the night. Instead, the air movement can be stopped using automatic dampers, similar to those used for ventilating foundation spaces in cold climates, or plastic film dampers, which work by blocking air flow in one direction with a very lightweight flap of plastic. The addition of the damper makes the design an efficient isolated solar gain system.
Thermal store
To store the thermal energy from the collector, the Barra system suspends a "spancrete" slab of concrete as a ceiling to store heat. This is fairly expensive and requires strong support. An alternative is to use water, which can store 5 times as much heat for a given weight. A simple, cheap and effective way is to store the water in sealed 100 mm diameter PVC storm pipe with end caps.Whether water or concrete is used, the heat is transferred from the air in the collector into the storage material during the day, and released on demand using a ceiling fan into the room at night.
Where "spancrete" slabs are used, the ceiling also heats the house by radiation. Some houses are fitted with louvers
Louver
A louver or louvre , from the French l'ouvert; "the open one") is a window, blind or shutter with horizontal slats that are angled to admit light and air, but to keep out rain, direct sunshine, and noise...
(similar to those used on satellites) to adjust the radiation transfer. Warm air travels through the slab tunnels from south to north, where it exits and travels back north through the bulk of the room to the air heater inlet near the floor.