Friday, August 4, 2017

Reduce heat wave harm with solar air heaters

Humid Heat Waves Will Top Limits of Human Survivability 
Heat danger in coming years. But warm humid climates have one advantage - it might easily be possible to create convectional rain, merely by heating the air. Usually the solar energy goes into heating ground and air, but with solar air heaters the ground is prevented from heating because solar air heaters shade the ground. Most of the energy would go into heating the air, so we could have massive convection and rain. The clouds formed would reflect and radiate solar energy back to space. This is especially so with low clouds in low latitudes. See
on how to build a solar air heater. We could have huge solar air heaters on every roof.
Using Espy's equation shows that with high relative humidity, low clouds form.
H = 125 (T-Tdew), where H is the height of the base of the clouds, where T is the temperature (deg C) of the parcel at near ground level and Tdew is the dew point temperature at near ground level. With high relative humidity Tdew is close to T. The formula gives the altitude of the cloud base in metres.The heated air parcel that we are applying the above formula to is the air heated by the solar air heaters.
Example: The temperature of the air parcel heated by all the solar air heaters is 45 deg C. The dew point is 35.9 deg C.
Then H=125(45-35.9) = 125x9.1=1138 m.
The whole process of causing the rain takes heat away from near the ground and moves it higher up. It also removes water vapour from the air, dehumidifying the air (the water vapour has become water). In the tropical forests the temperature usually stays below 35 deg C or so because evaporation reduces temperatures near the ground.

The present design has a greenhouse at the bottom providing hot air. My concern is that air does not come into intimate contact with hot surfaces with a greenhouse and if the hot air is not transferred quickly, there will be heat losses through the glass of a greenhouse and so on. Air is not heated much by radiation, but it is heated efficiently by direct contact with hot surfaces. I therefore propose that solar air heaters be used for the base of the solar updraft towers, rather than greenhouses. With greater efficiency one would not have to have such a large area (the greenhouse needs a huge area). Also, with solar air heaters, the heaters can be mounted vertically saving huge space. See
Example on air pollution: Kathmandu has an air pollution problem and solar air heaters could be used to cause convection and dilute the air pollution. Kathmandu, in winter, has about 4.2 kWh of solar energy falling on every square metre in a day. Because of a fairly high altitude, the air pressure is about 87 kPa (instead of 101.325 kPa). With a temperature of 25 deg C, 4.2 kWh could heat 2954 cubic metres of this low pressure air by 5 deg C. If a one square metre solar air heater was 50% efficient, it could heat 1477 cubic metres of air by 5 deg C every day. In summer Kathmandu has about 7.6 kWh of solar energy falling on every square metre every day.
VOLUMETRIC HEAT CAPACITY OF AIR: I had a hard time finding figures for the volumetric heat capacity of air. They will be useful for calculating how many cubic metres of air at a certain temperature and pressure of 101.325 can be heated by a solar air heater, for example. I calculated the figures using Specific heat of mixture of gases=sum of (mass fraction x specific heat of each gas). I then calculated the mass of a cubic metre of air with RH=50% for various temperatures and multiplied specific heat by mass of a cubic metre of air with RH=50 and P=101.325 kPa at various temperatures. The RH makes very little difference to the volumetric heat capacity (although it does make a bigger difference to the specific heat). For instance the volumetric heat capacity of dry air at 35 deg C at P=101.325 kPa=1.154 kJ/degC.m^3 and the volumetric heat capacity for an RH=50% parcel at the same temperature and pressure is 1.159 kJ/degC.m^3.

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