Mixed Mode Cooling Systems for efficient air conditioning

The new Part L2 of the Building Regulations demands cuts in energy use in buildings, as well as compliance with temperature guidelines. The mixed mode cooling system which combines natural ventilation and comfort cooling can fulfil these requirements with lower energy use and predictable, controllable indoor temperatures.

In order to design an efficient mixed mode system, it is vital that the natural ventilation strategy operates to maximum effect.

There are two main traditional methods of natural ventilation: cross flow and passive stack. Buildings which are entirely natural ventilation use less energy than those with mechanical cooling methods and therefore produce lower CO₂ emissions. However, natural ventilation has its limitations. The heat loads a natural ventilation system can deal with is limited, often only half the level experienced in a typical modern office building. Also, occupants want a predictable indoor environment, which is difficult to achieve with a natural ventilation system.

A mixed mode system offers an excellent solution, as it can bring all the low energy advantages of natural ventilation, with the benefits of controlled air conditioning, to ensure predictability and reliable temperature control.

As ventilation and cooling account for more than 50% of energy use in a well insulated office, a well controlled and energy efficient system is essential for a low energy building. There are two main types of mixed mode strategy; 'Changeover' and 'Concurrent'.

Changeover occurs when a building is usually cooled and heated mechanically, but in mild conditions it can revert to natural ventilation only. With Concurrent systems, the comfort cooling and natural ventilation systems work in parallel and move from one form of cooling to the other throughout the day.

For a mixed mode system to operate to its maximum potential, a key element is control. The system needs to reflect the external as well as the internal conditions and requires software to track external and internal conditions to select the optimum system settings. Ideally, a mixed mode system will be included at the early planning stages of a new building so the building is designed for the system to work at maximum energy efficiency, however, it is possible to introduce mixed mode systems into the refurbishment of existing buildings.

How the Mixed Mode system works

1. 1st Floor in Winter Air conditioning heats the pace with natural ventilation providing the fresh air.

2. Ground Floor in Winter High heat loads inside the building require cooling. This is provided free of carbon emissions and at no cost by the natural ventilation.

3. 1st Floor in Summer Air conditioning cools the space and fresh air is supplied by the natural ventilation.

4. Ground Floor in Summer The ambient temperature is low enough to provide free cooling using natural ventilation and there is no need for the air conditioning to run.