Hexayurt project/Cooling: Difference between revisions

(This is theoretical. Can someone with more practical experience of using the Hexayurt please review). When the Hexayurt is used in a hot climate it will get hot inside. The heat comes from three sources:

• Solar gains due to sunlight coming in through the fabric
• Fabric heat gains due to outside air being hotter than inside air
• Ventilation heat gains due to incoming air being hotter than the inside air.
• Heat from people and equipment in the Hexayurt.

The Hexayurt has good insulation. It is highly reflective (when new and clean) so it reflects most of the solar energy. However it has very low thermal mass so that, with no air conditioning, it will heat up when the sun is out.

Increasing the Thermal mass

Increasing the thermal mass will slow down the speed with which the Hexayurt heats up during the day, reducing the temperature inside the yurt during morning and early afternoon. The simplest way to increase the thermal mass of the Hexayurt is to use the thermal mass of the ground under it. During the night maximise the exposed area of ground. Roll up any floor coverings, open all the vents. Sleep on camp beds raised off the ground so the night time cold air can circulate below the beds (or sleep outside). Wrap the floor coverings round you so they keep you warm but not the floor. Get the ground as cold as you can. As soon as the outside air temperature rises above the ground temperature (probably soon after dawn) close the vents and put back the floor coverings. In some cases the simplest way of doing this may be to move the entire Hexayurt at dawn and put it down on a nice cold bit of ground.

Controlling Ventilation heat gains

During the day limit the amount of hot outside air which comes into the yurt. This will reduce the heat gains due to incoming air.

Bring the air in at low level so the cold ground can cool this incoming air so it doesn't heat the yurt. This doesn't reduce the ventilation heat gains but it does improve conditions in the Hexayurt.

The hottest air will accumulate at the highest point in the yurt so your exhaust vent should be higher up, on the downwind side of the yurt. As we have limited the ventilation to a minimum this will make sure the exhaust air takes away as much heat as possible.

Alternatively you can cover the ground with an insulating layer (sleeping bags, carpet etc.) to keep the ground cold during the day. You now have some nice cold ground to sit on when the Hexayurt seems hot; just pull back the floor covering and sit down. Sitting on the floor also means you are out of the hot air at the highest point in the yurt. In this way the coolth stored in the ground is controlled and used for personal cooling rather than cooling the entire yurt.

Reduce the solar heat gains

Any shading which reduces the amount of sunlight hitting the Hexayurt will reduce the solar heat gains. Shading in the morning will keep out heat which would be in the Hexayurt all day. Putting the Hexayurt under trees or next to a hill can give this effect. A large banner if properly sited can cast a shadow which reduces the solar gains.

Keeping the Hexayurt shiny will mean more sunlight is reflected away and less is absorbed by the roof.

Reduce Fabric gains

Even if sunlight impinging on the Hexayurt is reduced the Hexayurt will still heat up till the outside is close to the temperature as the outside air and if this is hotter than the inside air then heat will leak through into the Hexayurt and heat the inside air adjacent to the walls and roof. Lining the walls and roof with net drapes will keep this hot air from getting into the rest of the Hexayurt.

Reduce equipment heat gains

As the Hexayurt is so well insulated any heat in the yurt will stay inside so be wary of operating any machinery in the Hexayurt during the day. Any heat given off will serve to heat the unit. Any computers should be laptops, not towers. Limit the amount of sunlight you let into the yurt - a lumen of light from an LED or a fluorescent lamp gives off fewer watts of heat than a lumen of sunlight. Any fridge or cooling unit should be set up so it's heat rejection (the pipe coil on the back) is to outside the yurt. If you do not do this then a fridge or cooling unit will just heat up the yurt.

SleepBreeze personal cooler

We are currently working closely with Andy Buxton and his SleepBreeze personal cooler. Unfortunately we can't tell you very much about their personal cooling system at this point because it has not launched yet, but if you check that page you can get more of the picture. We can reveal a few key features:

• Aimed at cooling the person, not the entire Hexayurt.
• Low energy consumption, battery powered.
• Simple enough that, in volume production, the price would fall into our general price range.

It's important to understand that the Hexayurt project is a mix of proprietary and open systems. Specific functions, like solar power generation, require technologies with extremely expensive development costs, of a kind which open source projects can't fund. But the fruits of those developments are what we call COTS (common, off the shelf) technologies. The SleepBreeze system is a very good technology in a COTS area: you could substitute a fan, but it would not work as well.

Andy is also consulting with us on more general cooling and thermal issues, like where to position vents and how many we would need, and what kinds of interior temperature ranges we could expect to see in various conditions.