Night Ventilation and Air Conditioned Offices
Countries in the European Union have agreed the ‘burden share’ by each EU country required to meet the 8% cut in greenhouse-gas emissions between 2008 and 2012 as outlined by the Kyoto Protocol. These figures include a 12.5% reduction for the UK. The combined efforts of all countries equate to a total reduction of 8% of the European Community’s emissions. It is expected that much of the burden for reducing greenhouse-gas emission will fall on the EU’s energy, manufacturing, transport and farming industries.
Buildings consume approximately 40 to 50% of primary energy in the UK and other EU countries. According to a recent survey, energy for cooling in commercial office buildings amounts to 278000 tonnes-of-oil-equivalent delivered energy. This represents slightly over 10% of the total energy consumed by commercial office buildings. In addition, according to the same survey, the percentage of full air-conditioned floor-space is also increasing; 7.5% of floor area was air-conditioned in 1970, 12% in 1980, 19% in 1990, and 27% in 1994.
Therefore, there is potential for significant savings in CO2 emissions if a passive method could be used to reduce the energy requirements of air-conditioning plants. In addition, reduction of the peak cooling-energy demand would be of great interest to the power generating industries. It would also be of interest to building investors, as it would imply a possible reduction in the required installed plant capacity.
Cooling energy requirements can be reduced by using low-energy technologies. Of the available technologies, night ventilation is particularly suited to office buildings because these are usually not occupied during the night. Night ventilation works by using natural or mechanical ventilation to cool the surfaces of the building fabric at night and is more effective where a building includes a reasonably high thermal mass, so that heat can be absorbed during the day. Night ventilation can affect internal conditions during the day in four ways:
• reducing peak air-temperatures;
• reducing air temperatures throughout the day, and in particular during the morning hours;
• reducing slab temperatures; and
• creating a time lag between the occurrence of external and internal maximum temperatures.
A typical effect of night ventilation in an office is shown in Fig. 1, which is reproduced from Ref. [3].
There has been extensive recent research in Europe that addresses, in detail, night ventilation combined with thermal mass as one of the most appropriate techniques for hot, cold and temperate climates in reducing the need for air conditioning and improving the internal thermal conditions. It should also be noted that night ventilation and thermal mass is one of the techniques shown to be used in many of low-energy buildings in the UK.
It can be claimed that thermal mass and night ventilation has started to be implemented as one of the default design options for ‘green’ office buildings. However, the potential for using the technology in air-conditioned offices has not been investigated in detail and there are no practical applications reported as yet for typical air-conditioned offices.
This paper investigates the potential for applying night ventilation to typical air-conditioned offices and gives estimates of the potential energy-savings and reductions in the required plant capacity. This is done by using a thermal and air flow simulation program to calculate this potential in typically-constructed air-conditioned offices in England.
The calculations are based on the BRE’s high-speed thermal simulation program 3TC. In this method, the thermal response of each room has three time-constants and each room is modelled as a network of thermal conductances and capacitances. The ventilation calculations are based on the equations included within CIBSE’s AM10. These models have been implemented in an easy-to-use pre-design tool, which has been developed especially for UK office buildings and climate, with the aim of facilitating comparisons with energy-consumption and comfort benchmarks. They provide an opportunity to explore, quickly, variations in internal heat-gains, ventilation rates, occupancy patterns and external temperatures.
The whole cooling season (May to September) was chosen as the design assessment period in order to obtain results representing a high percentage of the cooling requirements. A maximum external-air temperature of 23.5°C was chosen with a corresponding minimum of 14.5°C. These represent a risk factor of 5% for typical weather data in south-east England.
A risk factor of 5% implies that the external temperature will exceed 23.5°C for about 7?8 days over the 150 days of the summer cooling-season of 5 months. Lower and higher risk factors (of 50, 25, 5 and 2%) were also taken into account while performing the calculations. These correspond to maximum temperatures of 24.5°C (2%), 22°C (10%), 19.5°C (25%) and 18°C (50%). The last value of 50% could be taken as an average value over the cooling season, i.e. indicating that 50% of the maximum temperatures are above this value and 50% below. In addition, a maximum temperature of 29.5°C was considered to examine the scenario of a hot spell during the summer.
The building model is based on a typical cellular office of 10 m width, 6 m depth and 3 m floor-to-ceiling height. It is positioned in the middle of a row of offices on the middle floor of a 3-storey office-block. The building weight is defined by the construction of the ceiling. The constructions of the various building elements are (the outermost layer being given first):
• Walls: 3 mm steel-cladding, air gap, 100 mm insulation, 10 mm plaster.
• Windows: double glazed, U-value=3.0 W/m2 K.
• Partition construction: 5 mm plaster, 13 mm plasterboard, 100 mm air gap, 13 mm plasterboard, 5 mm plaster.
• Ceiling
• Light weight: 12 mm Wilton carpet, 150 mm light-cast concrete, 400 mm air-gap, 20 mm glass-fibre quilt, 2 mm steel-cladding.
• Medium weight: 12 mm Wilton carpet, 20 mm plywood, 100 mm air-gap, 100 mm heavy-cast concrete, 400 mm air-gap, 20 mm glass-fibre quilt, 2 mm steel-cladding.
• Heavy weight: 12 mm Wilton carpet, 5 mm plywood, 200 mm air-gap, 300 mm case concrete.
Occupancy is assumed to occur from Monday to Friday inclusive: 8:00 am to 18:00 pm and Saturday 8:00 am to 12:30 pm.
The air-conditioning system selected for day cooling is a 100% fresh-air fan-coil system with a COP of 3.0, with a supply fan of 1 W/ls specific power while the extract fan has a 0.75 W/ls specific power; both values were selected to represent ‘best practice’.
Several types of night-cooling system have been investigated. Three natural-ventilation configurations (single-sided, cross and stack ventilation) and balanced mechanical ventilation using the day-cooling system.
Tags: night ventilation
