Future Energy Sources 3.3 Solar Powered HVAC

Does this evaporative tower actually work? I have NOT done the math... Since water vapor is significantly lighter (MW18) than dry air (MW≈29), I'm not sure if the contraction due to ∆T overcomes the effect of decreased molecular weight. And of course the increased humidity will reduce the cooling perceived by the human body.

Perhaps some form of heat exchanger could transfer almost as much heat from the downdraft without adding humidity, with the humidified air being carried away outside.

Classic case of bolting the stable door after the horse has gone!

HVAC (solar or otherwise) shouldn't be an add on.

The building should be designed for it in the first place... Blimey in the middle East and plenty of other hot countries they have managed to design buildings to keep cool for hunderds of years with no waste of electricity (Or water as this system uses).

Avoid the problem in the first place rather than trying to patch it up afterwards.

I think Dkwarner has confused the molecular weights of water and air (average) with the density difference between air and water-saturated air. These two are much closer than his numbers suggest. Therefore, his stated concern disappears.

I looked at the two links, and believe the installed cost per ton of cooling is quite disproportional to the available funds of residential users. Actual values would depend on a host of factors, such as rebates, tax credits, utility development costs, utility rates, climate, etc.

Del the Cat, has a somewhat pithy post, but is spot-on. The centuries-old use of towers in many areas of the world is an excellent example of the use of thermal mass. The relatively cool tower, during the day, is soaking up solar heat on its outside during the day while it is still cooling the rising hot air from the room(s) below thus causing a downward convection of this cooler air back into the room(s). In the night, the solar heating has raised the interior surface of the tower's mass to be above the ambient temperature, so the convection becomes a chimney to draw the air out of the house and replace it with cooler outside air. By morning, the tower will once again be at a lower temperature than the surrounding air and the cycle will repeat daily with no need for intervention by people or connection to a grid.

Another approach would be to combine a solar chimney (low mass) and an earth-coupled supply air source, to use natural convection currents to pull air through tubes which temper it via conduction to the surrounding earth. I believe this is being done in a number of locations. Again, there is no need for human intervention or grid connection. It would have the advantage of being able to work even when the night-time temperatures are not low enough, and in locations where a very high humidity is a problem.

Engineers designing low-energy buildings in Davis CA found that combining many individual changes to reduce the heating and cooling loads resulted in a design which eliminated the central heating/cooling unit entirely. This lump-sum savings then justified and paid for all the other steps' costs. Del the Cat alluded to this also, by saying that a solar HVAC system couldn't be just an add-on to an otherwise unwise building.

How about radiative cooling to the sky at night. I believe that buildings have been built with blackened thermal mass on the roof, and a movable insulation cover. The moving of the insulation is done (manually or automatically) twice a day, so some human intervention is needed and a small amount of power (in some form) is used. However, the thermal mass can be cooled a number of degrees below night-time ambient temperatures because it is radiating energy through the warmish atmosphere to the much colder space above (doesn't work too well when the sky is cloud-covered). You may have experienced this in one form, when you climb into an automobile in the early morning and find its inside colder than the air outside. This approach has been used to freeze a shallow layer water overnight when the air temperature is even 10 deg.C above freezing.

These are just some of the approaches or combinations of approaches that could be used, Masu. We need to be knocked away from our use of the same approaches that worked yesterday or last year or whenever. The factors we need to include in design would be the micro-climate surrounding the structure, what mediating changes we could and should make to the surroundings (such as plantings), how to make the structure responsive to the needs of the users instead of imposing an energy-consuming system of HVAC equipment and controls, and a host of others including the ones that have been mentioned in this thread.

Thanks--John M.

I said in my first post that I hadn't done the math! I do know that the density of any gas/vapor mixture is directly dependent on the effective molecular weight of the mixture, at a given pressure. So adding water vapor to unsaturated air MUST lower its density as it lowers the effective molecular weight of the mixture.

Without too much difficulty I was able to find the following: Dry air (no H20 vapor) at 40°C and 760mmHg has a density of 1.127 g/l or kg/m3.
If we cool that air to 20°C without adding water, the density increases to 1.205 g/l.

If we add enough water vapor to bring the dew point of the 20°C air up to 10°C, the density goes back down to 1.199 g/l

I still haven't done the math to find out how much water vapor we have to add so the evaporation of that water cools the mixture from 40° down to 20°, but based on these numbers, it does appear that the increase in density due to cooling IS more significant than the decrease in density due to lowered effective molecular weight from added water vapor, so the tower should indeed work. I wasn't sure - that's why I asked!

This tower makes me frightened for deseases like legionella.

The water is kept at normal evaporating temperature, ideal for bacteria to grow and spread through the ventilating system.

What will happen when the air is very humid? This thing will not work, and it is just in those areas that air conditioning is mostly used.

• This tower makes me frightened for deseases like legionella.
• The water is kept at normal evaporating temperature, ideal for bacteria to grow and spread through the ventilating system.

Problems with bacteria and other hazardous things growing in the water can easily be overcome with the addition of antifungal and antibacterial agents. You need to be careful that the agent is not harmful to humans but there are such things readily available

• What will happen when the air is very humid? This thing will not work, and it is just in those areas that air conditioning is mostly used.

While air-conditioning is definitely desirable in tropical climates where high humidity and temperature go hand in hand there are plenty of places that have high temperatures with low or even extremely low humidity. Anywhere that has what is referred to as a Mediterranean climate and desert areas are perfect for evaporative cooling systems and they have been used very effectively in such climates for a very long time. Actually in some of the really dry climates it is beneficial to humidify the air as constant exposure to very dry air can cause respiratory tract problems that range from a sore throat to life threatening inflammation.

In response to Dick's question:

• Does this evaporative tower actually work? I have NOT done the math... Since water vapor is significantly lighter (MW18) than dry air (MW≈29), I'm not sure if the contraction due to ∆T overcomes the effect of decreased molecular weight. And of course the increased humidity will reduce the cooling perceived by the human body.

While I have not built such systems myself I have seen it use documented and I believe one of the larger applications is in Texas where they use it to cool larger covered outdoor areas. However the latent heat of vaporization for water is 2.272 kJ g^-1 while the specific heat of air is around 29 J mol^-1 K^-1, so you don't need to evaporate too much water to reduce the temperature. If I get the time over the next few days I will do the numbers properly, that is unless somebody else would care to do them first.