The results of a trial using multiple e-stacks to naturally ventilate large buildings
The e-stack natural ventilation system was developed and patented by the University of Cambridge as an energy-saving alternative to displacement ventilation systems. It is supplied and marketed by a spin-off company, E-Stack.
To date, e-stack has been installed in a number of schools, theatres and halls, but the firm’s commercial and development team were interested to learn how multiple e-stacks would perform in a larger space. So, in a project supported by the Carbon Trust, e-stack was put to the test in the laboratory and, over eight winter weeks, in a 250-person hall by Saunders Boston Architects at Unity College, a school in Northampton.
There are two types of natural ventilation - ‘displacement’ and ‘mixing’. Displacement systems draw cold air into a building at low level. The air is warmed by occupants and other heat sources, such as lighting and computer equipment, and rises by convection before evacuating through vents in the roof. The drawbacks are that, in the colder winter months, air drawn in from the outside has to be preheated to be comfortable and in the summer the air will only circulate if the inside temperature is sufficiently warmer than outside. If not, interiors can become uncomfortable and extra mechanical cooling is called for.
Mixing ventilation systems, such as e-stack, on the other hand, draw in cool air through high-level vents. It descends in a plume, becoming more dilute as it approaches the floor. Usually, such installations have to be high enough to ensure occupants do not experience localised pockets of cold air, but the e-stack mixing system incorporates a series of dampers and low-wattage fans, which ensure that adequate mixing and an appropriate ventilation rate is achieved at lower installation heights. Once the flow pattern has become stable, the fans can be turned off.
E-Stack’s experimental work established that, in winter mode, energy savings achieved through mixing ventilation using a multiple e-stack, compared to displacement ventilation, was 10-50MW/hr of pre-heat for a 250-person hall, equivalent to carbon dioxide emissions of between two and 10 tonnes a year. Compared with a mechanically air-conditioned system, the saving is a further 40MW/hr in a year (depending on occupancy patterns), equivalent to CO2 emissions of eight tonnes.
Shaun Fitzgerald, managing director of E-Stack, says: ‘When one considers what will happen as this technology becomes more widely deployed, the potential reduction in carbon emissions is enormous. This is one of the reasons why the Carbon Trust was keen to support the development. The multiple e-stack is now proven and there are a number of installations across the UK.’
Data collected from the Unity College trial has helped to determine the appropriate control algorithm for a multiple e-stack system in winter mode. The four e-stacks installed at Unity College are controlled by a central panel in the plant room. Three control strategies were tested over eight weeks and the room temperature, air quality (CO2 measured in parts per million or ppm) and operation
of the units compared. The first strategy used localised control, whereby each e-stack operated independently. The second used global control, all units operating in unison. A third strategy separated the stacks into two groups, so that two operated as usual but two operated as outflow stacks only (rather than combining inflow and outflow).
The results show that the cheapest option in terms of control complexity - all e-stacks operating in unison - was equally effective as the other two in keeping CO2 levels below 1,000ppm (when the e-stacks were not operational, the reading was nearly 2,000ppm). Although some of the e-stack unit inlet temperatures came close to the exterior temperature at some points, this could be mitigated by regulation of the damper settings.