Not a problem

Traditionally, this technology featured a spray coil or a cooling tower. The performance and hygiene problems associated with these devices are well known.

Spraying water into the supply air stream results in an unacceptable rise in the room moisture content, so there is no overall comfort benefit. Spray coils used in conjunction with regenerative heat recovery can only deliver a modest cooling effect (2-3ºC). Although spray cooling is indirect, the supply air moisture content rises because of leakage and the regenerative process itself. Hygiene problems are a significant risk in the thermal wheel matrix.

To achieve a significant cooling effect without an increase in supply air moisture content it is necessary to depart from conventional air handling unit design and construction. The energy of the outside air must be harnessed in addition to the exhaust air.

On the MHS Homes project, Fulcrum decided to approach Menerga, a company that developed alternative techniques to increase temperature efficiency in multistage and asymmetric recuperators during the 1990s.

Computer modelling combined with the use of polymers now helps to achieve temperature efficiencies in excess of 80%, along with separation of the exhaust and outside air streams.

Maguire questioned the ability of the return air to absorb additional moisture but, in practice, the rise in moisture content of the return air from Termodeck buildings such as MHS homes is relatively small, partly as a result of the air volumes involved but also because of the absorption effect of the internal environment. This allows the return air to achieve a significant reduction in dry bulb temperature as a result of the adiabatic cooling effect.

Modelling of the MHS homes project showed that under summer design conditions (outside air 29ºC db; 20ºC wb and 24ºC return air) the supply air temperature achieved would be approximately 20.1ºC. Broadside has not been in operation long enough to prove these predictions, but we have observed dry bulb reductions of at least 6ºC in the supply air.

Useful data have been observed on many similar systems operating throughout Europe; data for the summer performance for a Menerga Adsolair unit at Daresbury Laboratory on a day when outside conditions were far above accepted design values, shows that the indirect adiabatic cooling effect achieved by the Menerga unit was 13.9ºC.

Hygiene problems are related to high operating water temperatures, stagnation and accumulated material in the system. These are avoided by careful design of the air handling unit so that the water temperature does not rise to a level where bacteria can become a problem.

Peter Mark, director, Fulcrum Consulting