Evaporative cooling is a powerful way of passively (no energy input) reducing the effective temperature below the ambient drybulb temperature. It takes advantage of the latent heat of evaporation to remove heat from the system as water is transformed from liquid to vapour. Common methods for buildings are roof ponds and roof sprays [1], however the first requires a strong roof and the second requires pipes etc and energy for pumping.
This idea is for a passive indirect evaporative cooling device based on wicking in a fabric layer. The principle is the same as placing a wet flannel on the forehead to cool a person. A layer of fabric is placed on the roof of a building with the lower edge in a reservoir of water along the edge (like a gutter). The wicking draws water up through the material, and as it evaporates heat is removed.
This would be ideal for buildings in developing countries with metal roofs, which are good conductors so removal of heat from the outer surface will quickly cool the inside of the dwelling. It would ideally be combined with a shading system, since water evaporated off by solar gain is not removing heat from the dwelling. The shade could be as a second layer (with a gap for airflow), or as an overarching canopy.
The system is very cheap and easy to install, with no moving parts necessary (if the reservoir is refilled manually). This is obviously not suitable for areas with very low water resources, but would be fine for areas with sufficient non-potable water, e.g. beside rivers or lakes (I don't think sea water would work).
Also remember that this doesn't have to be drinkable water so wastewater could be used.
[1] - Shade cover with evaporative cooling
www.google.com.au/patents/U...
[2] - Analysis of the Wicking and Thin-Film Evaporation Characteristics of Microstructures
docs.lib.purdue.edu/cgi/vie...
My initial thoughts were in terms of a very low-tech device for very poor areas (slums, off-grid rural villages). It would cost a few dollars, use no power, be refilled manually. This could improve comfort in places with zero possibility of air conditioning (the majority of the world). It wouldn't reduce carbon emissions, but would improve the lives of many people. Not suitable for the middle of a desert, but many habitations are next to water sources (rivers etc).
A device aiming to reduce AC electricity use could be suitable for any low-rise development (obviously high-rise doesn't work). The simplest implementation (no internal loft space) could work for garages etc (as well as factories, retail malls...). For normal houses with loft spaces (typical suburban sprawl), the arrangement would be similar to that pictured, with a narrow airspace directly under the roof from which cooled air would drop down into the room at the sides. Warm air could return through a central 'chimney' (if recirculating, i.e. with supplementary AC), or fresh air could be drawn in through a vent at the ridge. Either way the circulation would be entirely natural (no fan power needed).
For everything other than the lowest-tech solution, the reservoir could refill using a floating control valve (like a toilet cistern). This could use mains water (already under pressure), rain water stored in a roof tank (fed by gravity), or rain water stored at ground level (via a pump, perhaps solar powered).
Juan, I think wicking from ground level would be difficult: would need to be sealed to avoid evaporation on the way up; would need a small 'pipe' to avoid covering the whole side of the building. Trees do this using very complex microstructure to allow the 'evaporative tension'... I think it would complicate things hugely to try and develop an equivalent.
In terms of materials, the shade could be sheet metal (needs to be strong to resist wind but also lightweight), or maybe wooden slats (some ventilation between). PV could be installed on top of the shade (cooling the underside would also aid panel efficiency). For the very low-tech option, perhaps stretched fabric (like a tent). For the wicking material itself, anything with the right fiber-density to give good pore sizes would work.
As a general comment, in terms of low carbon materials, local is not always better. It's better to transport sheet aluminium and fabric a long way by container ship (very low embodied carbon since they're lightweight) rather than use local timber and plant material if the latter option causes local problems like deforestation etc.
[1] www.coolmax.com.au/evaporat...
Can you clarify why you think there will be problems with solar gain? Are you saying that the energy absorbed by the water would otherwise be reflected from the roof and so there is no net cooling effect wihout the reflective shade?
My main concerns would be that this may be a good way to prevent the roof becomming hotter than the surrounding air, but it may be difficult to get the roof much colder than the surrounding air (in order to take noticeable amounts of heat out of the room). If this is the case, then the occupants could sit outside under a shadely tree and feel the same benefit. Another concern for this competition is how it will save carbon if it does not replace existing air conditioning - as voiced by other commentators.
But I like the lateral thinking evident in this approach and think it would be well worth trying to do some rough and ready heat flux modelling to try and quantify the cooling effect. Good luck with it!
Regarding solar gain: water evaporated directly by the sun does not extract any energy from the building, so has no active cooling benefit but requires lots of water. (It is true that normally the solar gain would heat the roof surface, but for a well-insulated roof this would have little effect on internal temperatures and thus cooling loads). It is better to shade the roof, and the double-skin concept is commonly used, sometimes called a tropical roof.
The big benefit of evaporative cooling is that it allows cooling to well below the ambient air temperature [1]. The limit of cooling is governed by the wet bulb (saturation) temperature, e.g. for an air temperature of 32C, at 15% humidity you can cool to 16C, or at 50% humidity to 24C. (Note that those values are for an evaporative cooler in an HVAC system so are only roughly comparable, but the principle is the same). I will work on a more detailed analysis.
For this competition aiming for carbon savings, the focus will be on reducing the need for AC, e.g. fitting to buildings with AC installed allowing it to be used less often. I would also argue that avoiding emissions increases should count. As temperatures rise (especially in urban areas), more people will consider installing AC (and there is concern that we are approaching a tipping point for this). If they install my device instead, these emissions are avoided.
For the low-tech options where no AC-offsetting opportunity exists, this is a good chance for some corporate social responsibility. Primarily, the technologies developed here could be transferred to a non-profit organisation making the basic version. Maybe even a charitable subsidy: for each device sold, a basic model is sent to a developing country.
[1] en.wikipedia.org/wiki/Evapo...
- Swamp Cooler link upload.wikimedia.org/wikipe... Illustration from "Passive Cooling Systems in Iranian Architecture" Scientific American, February 1978
- Solar Chimney link upload.wikimedia.org/wikipe...
I certainly think the wind tower principle could be incorporated: I mentioned a vent at the ridge to draw in air, which could be rotatable / openable on either side depending on wind direction. There are several other ways this could be combined with ways of assisting natural ventilation, so I'm confident it won't need any fans to move the air around.
The solar chimney alone is another means of driving an airflow for natural ventilation, similar to the wind tower. In your image it's combined with an earth tube that provides the cooling. The issue with the earth tube / labyrinth / qanat is that they require lots of stuff underground. This is doable (though can still be very expensive) for new-build, but impractical for existing buildings. The solar chimney also has to be very tall to work well, which is not practical in many places.
I am fan of evaporative cooling system. As you well say, if you have a free air current, you can cool it, just using water and no added energy.
You will see soon in A+HOUSE entry, that I am using this concept but in a different way.
Regards.
Your idea could dovetail nicely with my idea for a 'solar skin' for both heating and cooling: http://marblar.com/challenge/earthhack-sustainable-homes/idea/1259
Nice idea,
A few thoughts about the fabric used.
If the water used is dirty, which is pretty likely in developing countries, then the fabric would also get quite filthy. The fabric would have to have some antibacterial properties to prevent bacterial growth and also be able to be easily rinsed to remove build up of contaminants. Without these features it would quickly start to smell and then would be removed by the user no matter how cool it may keep them.
As a side note. If this fabric exists and can wick well then why not allow the water to wick all the way up the fabric and drip off the other end? Could this water then be safe enough to drink? Cooling solution and water purification all in one.
Hi Ralph. A good idea - many thanks - and lots of good comments and contributions already. One rather trivial question in the scheme of things, but any thoughts on what you could do to prevent insects being attracted to the roof? In some emerging markets this could be a problem but there should be a clever way to overcome this I presume.
Potential carbon savings
There is an accelerating expansion of the air conditioning capacity in developing countries: energy consumption for AC in these regions is predicted to increase from 200TWh (in 2013) to 750TWh (in 2030) http://escholarship.org/uc/item/64f9r6wr. This is equivalent to 525 million tonnes of CO2 per year using a carbon factor of 7kg CO2 per kWh http://www.epa.gov/cleanenergy/energy-resources/calculator.html (a very conservative estimate for developing countries, which will most likely have much less green electricity generation). Therefore it is possible to hit the target of 1 million tonnes of CO2 per year by reducing this by 0.2%.
As noted in the above paper, energy efficiency improvements that lead to demand reductions, of which this is an example, could reduce consumption by 23%, or 118 million tonnes of CO2 per year.
Expansion to include pre-cooling of incoming air
The initial idea is best suited to single storey buildings, but to benefit a greater number of dwellings the concept can be extended to exterior spaces from which air is drawn. For example, a porch or awning could be used as the "roof" element to cool air which in drawn into the building though the lobby, thus reducing the air conditioning demand by pre-cooling the incoming air. The improved comfort in external spaces is an additional benefit.
This is similar to the 'pot in pot' system being used to store foodstuffs in hot countries (that uses wet sand packed between the walls of two earthenware pots stacked one inside the other)
This is similar to a "swamp cooler" for air conditioning homes in arrid environments is it not? Although it is a different process which seems alot better, the concept is the same, taking advantage of the cooling properties of evaporating water (desert survival 101) This requires a dry climate as it would not work in the humid tropical environments
Here is a report detailing the final version of this idea, termed CoolSkin: https://docs.google.com/file/d/0B9ozpo36sm4ldG0tV3R6UFNBdEU/edit?usp=sharing
The most significant addition is an alternative configuration that uses a convective cell rather than wind-driven air to drive the evaporation. This should be mounted vertically (so works for multi-storey dwellings), and incorporates insulation to counteract external temperatures.
Material specifications are provided, and an alternative flow mechanism based on a fluidyne pump is suggested.
It is estimated that 370,000 such devices would save 1MtCO2/a, and that between now and 2030 47 million homes in suitable climatic regions will purchase AC units for which this would be a cheaper substitute.
- Which regions is this likely to be most relevant in? Where would you have the combination of high temperatures and water availability? Ideally (thinking in terms of carbon savings), these would be areas that have at least some ongoing usage of electric ACs.
- What materials would you make the shade and wick out of? Should be something easily accessible in the relevant regions and with a low carbon footprint itself (such as locally grown plant materials).
- How far could you minimise the need for actively topping up the reservoir? That should help with adoption.
Looking forward to your thoughts!