History

Liquid Desiccant-Based Air Conditioning Systems have been recurrently proposed, and investigated, since the early 1930's. In the past 80 years, many developments, following many different ideas — and dead ends — have been undertaken, as a short selection of references below demonstrates^1..9.

The question why this Air Conditioning technology has not imposed itself, despite its many advantages, is thus legitimate. A short review of the history of Air Conditioning can uncover some of the main reasons.

Concise Time-Line of Air Conditioning Technologies

Averaged over the Globe and the Seasons, the Earth Climate offers wonderful conditions to Life! Local variations, both with Latitude and Season of the year, may prove quite uncomfortable, however.

Shelters provided by nature, were certainly the only known and possible alleviations to extremes for a long time. However, as soon as toiling to survive gave place to other kinds of occupation, means were sought to create man made shelters, or other means, to alleviate hardships born out by Location and Season.

Nature provided shadows and breezes have, possibly, inspired simple solutions to the early Creative Man. The observation of natural phenomena brought ever more ingenious solutions that imitated nature's work, e.g. the combination of windcatchers with evaporative cooling¹⁰, or the use of Atria, combined with evaporative cooling, to actively influence the indoor climate.

In desertic regions, with clear night skies, heavy masonry would also improve living conditions. In any case, most of these solutions sought to influence the temperature, while leaving the humidity content of the air to follow whatever resulted from the actual situation.

The history of modern Air Conditioning also shows that the control of temperature has been the main objective in the early air conditioning devices of modern times. Probably, the first modern attempt at simultaneous control of temperature and humidity is that reported by John J. Harris in 1903¹¹. Harris' task was to air-condition an auditorium for at least 1400 people at the Scranton Highschool (PA), USA. He set out to use ice for cooling, as was already common practice at the time. To reduce the humidity content of the supply air, he had a variable number of trays containing calcium chloride (CaCl₂) placed between the ice racks and the ventilation openings to the auditorium. In Harris' words, "The quantity of CaCl₂, as well as the number of trays, should be determined by the volume of air and the required humidity. A whirling psychromer was used in this case for that purpose. The trays containing the CaCl₂ were placed in the duct, one at a time, until the desired result was obtained."

Ice used for cooling was at first natural ice, which enjoyed a lively commercial trade up to the 1930s. However, since the mid XIX Century, the development of cooling machinery for ice production was pursued intensively, and the first ice producing air cycle machine, for air conditioning purposes, was patented by John Gorrie in 1851¹². Another machine of this kind was also patented by Leicester Allen in 1882¹³.

Strong commercial interests — trade in perishable goods such as meat, and the beer production, —  supported this endeavour, the thermodynamic knowledge basis having been established already. The technology was based on the same principles we continue using today: Absorption systems (essentially ammonia-water basedd^{14, 15}), including Combined Heat & power¹⁵; Vapour compression systems (Ammonia, Carbon Dioxide¹⁶, Methyl Chloride), and air cycle devices. The term Air Conditioning however, was first coined by Stuart W. Cramer¹⁷ in his patent of 1907 for an evaporative cooling device.

Evaporative Cooling (EC) is, mostly, a passive step when applied to air conditioning processes. This because power is only required to drive a small water circulator pump. EC can be produced by two methods: Direct contact of air and water flows, as in an air washer, or indirect contact (IEC), when the air to be cooled is contacted to a surface of a separator, itself cooled directly by water. In conditioning of air for comfort, the last method avoids a number of hygienic issues that affect direct contact EC, e.g. Legionela Pneumophila infections.

Evaporative Cooling is also combined with solid desiccants (e.g. Silica Gel, Zeolite Molecular Sieves, Hydrates of Alkali Halide Salts, etc.) in air conditioning systems, so-called DEC Systems^{18, 19} operating on the Pennington Cycle²⁰. Although these systems are, in general, good dehydrators of air, or other gases for that matter, they require regeneration temperatures higher than those required for liquid desiccant-based systems. Furthermore, as porous solids, cooling during the dehydration phase is not practical, if not impossible.

Summarizing this tourd'horizont, it would be fair to state that air conditioning has been, since the beginning of industrialization, a stepchild of the refrigeration industry. In many cases, even today, being reduced to mechanical cooling devices.

From the point of view of the Thermodynamic Efficiency of the whole process, this has been an unfortunate path.

References & Notes