Large inverters create big heat dissipation problems that are clearly
beyond the capabilities of conventional enclosure air conditioning
systems. An effective alternative is the air-water heat exchanger, but
some system builders can take a bit of convincing
The emergence of the air-water heat exchanger as a viable alternative to
conventional cooling for high power inverters, is seen by some systems
integrators as being a potentially hazardous development, yet by others
as being long overdue. For the first group, whether convinced or
otherwise of the performance benefits of air-water heat exchangers, there
is that dyed-in-the-wool instinct that liquids and electricity should
never be mixed. And, of course, there's all that additional plumbing to
worry about, isn't there?
Equipment safety is, and will always be, an important part of system
building, but in the development of more reliable components and
monitoring and control devices, air-water heat exchangers can be
considered every bit as safe as alternative cooling methods. A chilled
water supply is required, of course, but should a local supply not be
available, cooling system suppliers should be able to provide chillers as
part of the package. The associated pipework, too, should not present too
great a problem. It is when the performance of the air-water heat
exchanger is weighed against that of its counterparts, that these
prerequisites become proportionately less significant.
The advantages
The advantages of using water as a cooling medium is based upon its
superior specific heat capacity (4,200J/kg/°C compared with air's
1,200J/kg/°C). Because of this, the power to weight/volume ratios tend to
be high, so water-based cooling units can be small while at the same time
offering a high power output. Besides this, the air-water heat exchanger
has fewer components, as it does not rely on a refrigeration cycle. The
Rittal unit, pictured on this page, has a single fan (the air conditioner
type cooler requires two of these), the heat exchanger matrix itself and
an electromagnetic valve for opening/closing the water circuit.
Significantly, there is no requirement for an external air circuit.
Rittal has developed a complete air-water heat exchanger solution for
large inverter cooling, including the enclosure, heat exchanger unit and
(where required) a closed loop supply of chilled water from its range or
'Recool' chillers with capacities from 4 to 25kW. As an example, the
cooling of a 315kW inverter would require two 5kW air-water heat
exchangers plus a 10kW Recool chiller (should a chilled water supply not
be available). And if sufficient chilling capacity is built in at the
outset, future expansions can be accommodated by simply installing heat
exchanger units on the additional enclosures, and connecting them, in
turn, to the chilled water supply.