Blowing Hot and Cold
Meeting the Carbon Reduction Commitment is high on the agenda for many FMs. Nick Barnard explores how adapting comfort temperatures can significantly reduce energy consumption.
By Nick Barnard
So how to meet the Carbon Reduction Commitment without spending millions? The best approach is most likely by a combination of measures. One simple measure is to use adaptive comfort temperatures to optimise comfort and reduce energy consumption.
Adaptive comfort temperatures
Recent design standards1 and guidance2,3 advocate the use of adaptive comfort temperatures. These are based on occupants adapting their dress, behaviour etc to outside temperature.
To the average person a building at 24C will feel cool in summer but hot during the winter.
Taking the example of dress, people wear warmer clothes in winter. This means that optimum comfort temperatures will vary with outside temperature, being lower in winter, higher in summer. This is in keeping with most people’s experience – a building at 24C will feel cool in summer but feel hot during cooler periods of the year. Adaptive comfort temperatures are now being adopted for design purposes – why not also use them for operation of existing buildings? Throughout the year we adapt to the climate, so our buildings should adapt to us.
Carbon savings
In addition to optimising comfort, adaptive comfort temperatures will also reduce carbon consumption. As comfort temperatures vary, so heating and cooling set-points should be adjusted in harmony to maintain optimum comfort. Lower set-points in winter mean less heating would be needed to achieve meet the required temperature saving carbon. In summer the situation is reversed. Higher set-points in summer mean less cooling would be needed providing further carbon savings.
Adaptive comfort temperature variation with location
The variation in adaptive comfort temperature summer to winter is typically 6C for a moderate climate giving heating4 and cooling5 energy savings of up to 20 per cent.
Adaptive comfort temperatures are most appropriate to buildings where the occupant has control over themselves and their environment — ie, they have adaptive opportunity6. These buildings are often referred to as “free running”. Where this is not the case, for example in air conditioned “close control” buildings which are sealed and operate a strict dress code, a smaller variation is appropriate.
Equations (based on EN15251 and Cibse)
Adaptive comfort temperatures are based on outside temperature during the preceding few days. The weighting or influence given to the outside temperature is largest for the previous day, reducing for the preceding day, reducing again for the day preceding that and so on as people “forget” with time.
A weighted running mean of outdoor temperatures Trm is calculated as follows:
Trm= (1 – arm) [Te(d-1) + arm Te(d-2) + arm2 Te(d-3) …]
where arm is a constant between 0 and 1 which defines the speed at which the running mean responds to outdoor temperature, Te(d-1) is the daily mean outdoor temperature (C) for the previous day, Te(d-2) is the daily mean outdoor temperature (C) for the day before that, and so on.
The adaptive comfort temperature is then calculated from the running mean:
Tacf = 0.33Trm + 18.8 where Tacf is the adaptive comfort temperature for free running buildings.
For close control buildings this is modified to:
Tacc = 0.09Trm + 22.6
where Tacc is the adaptive comfort temperature for close control buildings.
Free data
Adaptive comfort temperatures may be calculated from weather readings and forecasts. Temperatures will vary with location as well as season. Current and predicted adaptive comfort temperatures for locations in the UK and throughout the world are available free at www.lowcarboncomfort.com. The website also provides guidance on further reducing cooling by using predictive pre cooling.
Nick Barnard is director at lowcarboncomfort.
Notes
- “EN 15251:2007 Indoor environmental input parameters for design and assessment of energy performance of buildings addressing indoor air quality, thermal environment, lighting and acoustics”.
- “Cibse Guide A: Environmental Design: 2006”.
- “ASHRAE Standard 55-2004: Thermal Environmental Conditions for Human Occupancy”.
- “Carbon Trust website: Cut carbon and reduce costs: Heating: Housekeeping”.
- “BCO 24C Study: Comfort, Productivity and Energy Consumption”.
- “Revival Technical Monograph 2: Adaptive thermal comfort and controls for building refurbishment”.
