That's true, and depends on what the design objectives are. Water has a specific heat capacity about 4 times that of air per given mass. So, a kilogram of water (approximately one litre volume) would require about 4 kg of air to have the same heat capacity, but on a volumetric basis, that means 3400 litres or 120 cubic feet.
From an energy standpoint, assuming ideal heat exchangers, a certain amount of heat transferred from the charge air will increase the temperature of the same volume of ambient air ~4X as many degrees Celsius as water, although the quantity of heat transferred is the same.
My point is -- and this must not be lost -- is that the water is just a heat transfer medium and a heat sink with a large thermal inertia. The environment is ultimately still ambient air, and heat must be ultimately rejected into the environment from a metal heat exchanger to said air. Heat transfer works in both directions -- of course, only from a higher temperature to a lower temperature -- and what is superficially an advantage for water (a high heat capacity) is also an Achille's heel: water takes longer to heat up X degrees, but it also takes more time for it to cool back down the same X degrees. In transients, an A2A cooler would then have a higher temperature difference to the environment than water. But the problem is more acute -- heat transfer relies on a temperature delta, meaning that a water HX requires a larger area to transfer the same quantity of heat Q through a smaller temperature difference:
Q=h*A(T
medium-T
environment)
The take home message is three-fold:
1) The water in an A2W system MUST circulate and MUST have a second, ambient heat exchanger (sounds trivial but more installations I've seen don't follow this principle than do);
2) A2W have particular advantages to A2A where packaging constraints are prohibitive (e.g. front-mounted HXs in a mid/rear-engine application, or an A2A system would require undesired long tract lengths/volume;
3) The ambient HX should be as large as practical. Using water as a transfer medium does not excuse this requirement and in fact increases the need for a larger HX, as previously explained. The HX that between the charge air and water may be substantially more compact, however; this is the packaging advantage of A2W systems.
