Air quality up, energy cost down
A key factor in current building design sees the coming together of two issues of ever-increasing importance: indoor air quality and energy efficiency.
While research continues internationally into the precise correlation between indoor air quality and health, nobody questions that performance and productivity are affected by the working environment. Noise, lighting and temperature all play a part, but air quality is accepted as being a crucial factor. It is recognised that even relatively minor falls in air quality, deriving from inadequate ventilation, can exacerbate such Sick Building Syndrome symptoms as headaches and inability to concentrate.
Typically, the need for buildings to be properly ventilated has been considered alongside the cost of the solutions. The drive for energy conservation, however, means that it is increasingly important to look at the whole life-cycle cost of providing indoor air.
Ventilation is no longer the necessary evil of building services. With aesthetic considerations traditionally having been regarded as paramount during building design, ventilation has seemed to represent an area offering initial cost savings and compromise. This is becoming more readily understood to be wrong. To put it into context, while an individual may consume 1kg of food and 2kg of liquid each day, they will breathe somewhere in the region of 20kg of air.
A host of regulations affect how ventilation and air conditioning systems are designed into buildings, including the most recent updates to the Building Regulations. While the latest Part F deals with ventilation, it has to be considered alongside the updated Part L, regarding energy use in buildings. Among other matters, one of the most significant Part F changes is the 25% increase in the mandatory minimum fresh air quantity per person in buildings, from 8 litres per person to 10 litres per person. One of the major objectives for this increase is to reduce carbon-dioxide build-up within occupied spaces. The CO2 levels that we breathe in, however, can be reduced merely by changing the method of introducing the fresh air into the space.
One answer is displacement ventilation which, although in use in the UK for some years, offers benefits which are still not widely understood. A key benefit is that it addresses both the indoor air quality issue and the energy issue. Displacement ventilation supplies clean fresh air at the correct temperature, directly into the occupied area at low level. It displaces the area’s warm contaminated air upwards, where it is effectively extracted through the exhaust air system.
The simple principle is based on the fact that warm air rises, carrying any contaminants with it at the same time. Clean air moves to replace spent air in the places where ventilation is needed. The system is self-regulating, provided that the supply air flow is adequate. Displacement ventilation is seen to offer a ventilation efficiency around four times more effective than that of traditional mixing systems when introducing fresh air into the target zone. This remains consistent regardless of whether the space is occupied.
In a comfort cooling context, the system functions at relatively low flows and is capable of meeting the CO2 requirements while providing a suitable room temperature. Generally, displacement systems operate at both very low pressure and low velocity. In consequence, they typically offer low noise with minimal draught within the occupied zone, providing a high air quality.
Displacement ventilation naturally promotes a temperature gradient, meaning that the temperature at ceiling level is much higher than that in the occupied zone. This means that, firstly the supply temperature does not need to be as low as with a traditional mixing system. Secondly, the higher return temperature makes it ideal for use with any energy recovery device within an air handing unit. The amount of heating energy required to meet room conditions after the energy recovery device is reduced dramatically and, in some cases, can be removed altogether, particularly when a thermal wheel is the energy recovery device. Finally, because the supply temperature is higher than is required in a traditional mixing system, free cooling is delivered for much of the year.
If there is a thermal wheel in the air handling unit, utilising an evaporative humidifier in the extract before the thermal wheel enables a reduction in the cooling load required by any chiller or condensing unit of 50% or more. Although the use of a thermal wheel and indirect evaporative cooling can allow a reduction in peak loads on both cooling and heating to around 50%, the actual running energy cost is reduced by significantly more.
A key factor in maximising the energy efficiency of such a system is its energy recovery capabilities. At the leading edge in this area is Econet, the Flakt Woods packaged liquid-coupled energy recovery system. This features a unique optimising control system, incorporating highly efficient use of energy as well as space and installation flexibility. It comprises supply and extract coil, invertor driven pump set, dedicated optimising controls set and all the valves and sensors required for the correct use of the system. The coils are installed within the air handling unit, but the rest comes as a pre-piped, preconfigured, preprogrammed and pre-commissioned pump pack suitable for direct connection to the supply coil.
Large coils are used to maximise the amount of energy recovered from the extract air, with water used as the most efficient medium to improve energy recovery. There are up to 12 rows of coils, with up to 76 waterways inside the coil. The result is the achievement of energy recovery efficiencies in the region of 70-75% at equal supply and extract airflow. Having such large coils enables them to be used for all heating and cooling as and when required, by supplementing the system either by injecting low pressure hot water or chilled water through water heat exchangers. It means that no subsequent coils are required unless there is an additional need for dehumidification, in which case a small re-heater is required.
The coil size also allows low grade cold or hot water to be used, making it ideal for use with waste water from chillers for heating or bore hole water for cooling. If normal temperatures (6°C and 82°C) apply, then the lower water flow rates are required (allowing for the higher temperature difference on waterside), thereby reducing pipe sizes, valves and fittings.
The system’s own optimising controls take input water temperatures on both supply and extract. This calculates the optimum method for providing the required air temperature, whether through recovery, waste heat or any other scenario that may occur. In addition, because of the optimiser and invertor driven pump, efficiency is maintained throughout the unit’s operation: if the unit runs at 73%, it will maintain 73% as the airflow reduces or other changes occur in the system.
Finally, Econet is not just about energy saving; it also delivers high quality indoor air. With both airstreams kept entirely separate, it is eminently suitable even for hygiene-critical applications. Both thermal wheels and plate heat exchangers are less appropriate in such cases, due to the potential crossover of contaminated air from exhaust to supply.
Whatever system is installed to provide ventilation, it is no longer sufficient simply to provide a cooling airflow at a low installation cost. The indoor air quality must be high, for the sake of the building occupants, but it is more important than ever that it is provided in an energy-conscious way.
As a general rule, the running cost of an air handing unit over 20 years can be approximately 10 times the initial capital cost at current plant and energy prices, so any saving in energy use provides significant savings in whole life-cycle cost. It means that a displacement ventilation system addresses every component of indoor quality; temperature, noise, contamination, velocity and humidity conditions. More importantly, it does so while providing low pressure, free cooling, high return temperatures for winter energy recovery and, with the use of an evaporative humidifier in the extract airstream, indirect evaporative cooling.