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The #Water-from-Air Resource Chart Explained

5/3/2014

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The Water-from-Air Resource Chart for Praia, Cape Verde
The Water-fom-Air Resource Chart for Praia, Cape Verde (click to enlarge).
Why use a Water-from-Air Resource Chart? Well, this colourful output from a computer model is a marvelous tool for understanding how well water-from-air machines (atmospheric water generators; AWGs) would perform at your location. "Knowledge is power"--there is value to being well-prepared before talking to equipment suppliers, consultants, or project colleagues.

Let me guide you through this information-packed chart.

  • Local water vapour resource compared to standard conditions: At standard conditions of 26.7° C dry bulb temperature and 60% relative humidity and 1 atmosphere air pressure (sea level) there is 15.3 grams of water vapour in every cubic metre of moist air. The local water vapour density (the resource) may be less, equal, or greater. By using a standard it is possible to compare one location to another, compare one month to another, compare one hour to another, and so on.
  • Elevation is an important input to the model because water vapour density normally decreases with altitude.
  • Latitude and longitude are for reference—usually water vapour density decreases with increasing latitude north or south (moving from the equator to the poles).
  • Monthly average temperatures for the location are obtained from various reliable sources.
  • Monthly average relative humidity values for the location are calculated by the model once it knows the monthly average dew-points.
  • Monthly average air pressures are calculated from the elevation information relative to the standard atmosphere at sea level which is 1.013 bar.
  • Humidity ratio, specific volume, and dry air density are intermediate values the model needs to calculate the average water vapour density in kilograms of moisture per cubic metre of air.
  • The highlighted-in-blue values of water vapour density have the units [g/cubic metre] to make the values easier to read and comprehend. A gram of water is represented by the 1 mL division on graduated cylinder laboratory glassware.
  • Proportion of water vapour density (WVD) at standard temperature (T) and relative humidity (RH) = (monthly average water vapour density) / (15.3 g/cubic m); This proportion is also called the index value.
  • Maximum mechanical dehumidification efficiency = [(average water vapour density - 6.8 g/cubic m)/(average water vapour density)] × 100%; Where 6.8 g/cubic m is the water vapour density of air at 5°C and 100% relative humidity at 1 atmosphere pressure—this models air leaving a wetted, unfrozen dehumidifier coil as being warmer than the freezing point of water and containing all the water vapour it can hold at the leaving temperature; The efficiency value is constrained to be less than 100% because in practice the coil temperature is regulated to 5­°C or above to prevent freezing of water onto the coil which causes overload and damage to the machine's compressor(s). An exploration of maximum efficiency for mechanical dehumidifiers is found in an earlier blog post.
  • Index value colours are applied to the index and efficiency values so the chart user can grasp at a glance the seasonal pattern of the water-from-air resource. The colour key and grade wording (excellent, good, fair, poor) for the index values is shown at the right-hand side of the page.
  • The annual index value is the average of the monthly index values. 
  • Chart 1 is a month-by-month graphical display of average values of temperature (dry bulb), dew-point, relative humidity, and efficiency.
  • Chart 2 is a month-by-month graphical display of average values of water vapour density (WVD; left blue columns) relative to the constant (right pink/red columns) standard condition WVD = 15.3 g/cubic metre.
  • An analysis section for the location enables each chart to be a stand-alone document with a concise explanations about standard conditions, the dehumidification process, and efficiency. This section also identifies the location's climate zone and expresses in words how effective a water-from-air machine would be at the site throughout the year.

Charts for many different locations are available from Atmoswater Research. You are welcome to ask me to produce charts for places of interest that are not listed yet.
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#Water-from-Air Resource charts backstory

30/9/2013

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Picture: Measuring water productionMeasuring water production from an AWG in Belize
While helping a client commercialize water-from-air systems, I got frequent questions about how well atmospheric water generators would perform at various locations around the world. A common misconception was that relative humidity values told the whole story. Higher humidity means more water in the air and better water production. Right? Not really!

The water vapour content of the air, sometimes referred to as "absolute humidity", has the proper scientific name "water vapour density" with units of [grams of water vapour per cubic metre of air]. The water vapour density depends on three measures all at the same time: the air's dry bulb temperature, relative humidity, and atmospheric pressure.

Monthly average climate data for air temperature (dry bulb) and relative humidity is fairly easy to obtain for many places. Average air pressure at a location can be estimated knowing the site's altitude above sea level. Using well-known formulas used in the heating, ventilation, air-conditioning field (HVAC) the average monthly water vapour density can be calculated.

Once water vapour density was known, I could use information about the airflow (in units of cfm or cubic feet per minute) through my client's machines to estimate water production of various designs. The focus on water production was fine for a manufacturer and its customers.

Later, as an independent consultant, potentially dealing with end-users of atmospheric water generators having all sorts of different specifications for airflow I decided it was better to focus on the actual water vapour resource to make charts of wider usefulness.To make it easier to compare how good a site is for atmospheric water generators I had the idea of indexing the water vapour density by dividing the density values by the water vapour density 15.3 g/cubic metre which is the density at the standard measurement conditions of 26.7°C air temperature; 60 % relative humidity. When the water-from-air resource monthly index = 1.00, the expected drinking water production rate from an atmospheric water generator (AWG) at the site should be the same as the machine's specified water-from-air production rate. I gave the Water-from-Air Resource (WFAR) annual index grades (Excellent: Index is greater than or equal to 1.00; Good: Index range 0.76 to 0.99; Fair: Index range 0.51 to 0.75; and Poor: Index range is less than or equal to 0.5) and assigned colours to make it easier to interpret the charts.

A lot of information about a site, all relevant to using water-from-air systems is packaged onto the 8.5 inch x 11 inch landscape format of the charts!
  • Geographic coordinates and elevation;
  • Monthly average temperature, humidity, pressure, and dew-point;
  • Monthly average water vapour density;
  • Monthly average maximum mechanical dehumidification efficiency;
  • Graph to visualize how temperature, humidity, dew-point, and efficiency vary together by month;
  • Graph to visualize and compare monthly average water vapour density with water vapour density at standard conditions;
  • Summary of the analysis, identifying the climate zone and expected atmospheric water generator performance for various periods during a year; and
  • Annual index to compare the water-from-air resource between locations
After a bit of experience using the charts my clients discover some of their business stress evaporates! Here's why:
  • They don't have to spend their own time and resources responding to ongoing questions about how good or bad a place is for operating water-from-air systems. This gives just a bit extra time for concentrating on growing their business and profits;
  • Risk reduction by understanding the characteristics of the water-from-air resource at a site. Why alienate their customer by selling them a machine that might only work properly a couple of months of the year at a site? Better to forgo that sale and focus on sites where the machine return on investment is more favourable;
  • The charts make their business more valuable—they become more of an expert at what they do;
  • They find it easier to achieve sales targets because quantitative information, rather than a guesstimate is available; and
  • Surprises are removed with having solid quantitative knowledge about the water-from-air resource at sites.

Picture: Example of a Water-from-Air Resource Chart
Water-from-Air Resource Chart for Praia, Cape Verde (click to enlarge).
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#Cape #Verde #Water from Air Resource

19/2/2013

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Picture: Map of Cape Verde
Location of Praia, Cape Verde in Atlantic Ocean west of Senegal and Mauritania (Source: Google Maps)
A Water-from-Air Resource Chart is available for Praia, Cape Verde. This arid | desert | hot arid climate site is a good site for atmospheric water generator operation on an annual basis. Some months are only fair, however, but some months are excellent. See the chart for details.
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    Roland Wahlgren

    I have been researching and developing drinking-water-from-air technologies since 1984. As a physical geographer, I strive to contribute an accurate, scientific point-of-view to the field.

    Discover previous interesting and informative scientific/technical posts by clicking "<<Previous" at the bottom of each page!

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