OPINION
Your letters
This month: The performance gap – 18 years on; insulation versus district heating; and the importance of the admittance method
Performance gulf In 1994, I came into facilities management (FM) from aircraft maintenance. Soon after, I joined a quality improvement team dedicated to producing a ‘cradle to grave’ project process. The need arose because many new facilities were uncomfortable for various reasons, including where ‘heating systems were fighting cooling systems’. When, 18 years later, I read those same words in the article ‘Work in Progress’ in the March 2012 CIBSE Journal (page 44, first column), I felt a profound sense of déjà vu. Why is it that in this sector, there
is such a gap between predicted energy performance and actual performance? When an aircraft flies the Atlantic, the fuel needed for the weather conditions and payload has to be calculated very accurately. When Werner Von Braun did his sums for the Apollo programme, slide rules were commonplace and mainframe computers were about 64 kB, yet he got those right. Some of it is poor client specification; for example putting more people into a facility than the designer had been told. While some of it is poor design and some of it poor commissioning. Last minute ‘value engineering’ does not help. But does it go further than that? Is there something wrong in the way the sums are done? Do we have people that don’t
always know what they are doing? Do we give facilities managers systems that are too complex to run,
16 CIBSE Journal August 2012
or don’t we train them well enough or give them enough system design information? Is too much of the ‘design’ left to the contractor? When we worked on ‘cradle
to grave’, someone said what we produced was probably better than anything CIBSE had. Certainly, it did result in much better projects. With building design, construction and handover taking up to three years, say, it is still only 5% of the 60-year life typically expected. Yet, mistakes made cast a shadow over the remaining 95%. As always, it is easier to pose questions than answer them, but what do others think? Rob Farman
Admittance method provides accurate load estimates It was with some delight that I read the article by Tom de Saulles entitled ‘Learning Tool’ in the March Journal. It is some years since I was involved in the development of the admittance method at BRE, but I always believed that the method, if applied properly, could – and should – provide accurate estimates of the heating/ cooling loads of building. This belief was based on the fact
that the method had been developed over many years by the application of the basic laws of physics to the transfer of energy within and around buildings, and the development of the appropriate equations to simulate these transfers.
BUILDING DESIGN MODELLING
A new early stage design tool aims to provide educational and practical benefits to professionals
Why is there a gap between predicted energy
performance and actual performance in building engineering? When an aircraft flies the Atlantic, fuel needed has to be calculated accurately
It is therefore somewhat reassuring to hear that the ‘laws of physics’ still apply and that the admittance method is still a valuable tool. I look forward to reading a copy of the paper by White et al in the near future. John Harrington-Lynn
Focus on insulation not district heating I am writing in response to the ‘Power debate’ article in CIBSE Journal’s May 2012 edition, and the ‘Right solution’ opinion piece from the April 2012 edition. This debate is obviously polarised.
LEARNING TOOL
A new free tool that combines the CIBSE admittance method with a user-friendly front-end aims to make building physics more accessible to non-specialists and students in the construction sector. Tom De Saulles explains
T
The PDA tool allows the user to create a room and experiment with its design to see how the internal temperature and loads change in response to ‘what if?’ questions
he current consultation on Part L 2013 will undoubtedly produce plenty of feedback. But whatever the outcome, the direction of travel
will continue to be underpinned by the ‘fabric first’ approach, which is central to government policy. Ensuring the fabric is made to work hard
does of course require a good understanding of building physics, which is typically limited to one or two members of the design team. But, with an increasing emphasis on fabric energy efficiency, a basic working knowledge of the subject would perhaps be beneficial to all involved in designing and procuring buildings. With this in mind, and to make building
physics more accessible to professionals and students alike, a new modelling tool called the Passive Design Assistant (PDA) has been produced, aimed specifically at educational and early stage design needs.
What can the PDA do? The PDA allows the user to create a room and experiment with its design, to see how the internal temperature and loads change in response to ‘what if?’ questions relating to orientation, glazing, thermal mass, ventilation, and so on. The consortium behind the PDA comprises Arup (project
60 CIBSE Journal March 2012
leader), the Concrete Centre and AHMM architects, with a financial contribution from the Technology Strategy Board (TSB). In addition to being free to download,
a key feature that sets the PDA apart is its front-end, which has been specifically configured to allow the user to simultaneously view input and output data, helping highlight connections between cause and effect. This is achieved with a permanently visible graph of the hourly operative temperature or heating/cooling load across the chosen design day. The graph is updated almost instantaneously in response to user inputs – made possible by use of the CIBSE admittance method, which is capable of performing a very rapid thermal analysis of the design. The near real-time feedback helps enhance the overall user experience and the educational value of the tool. The interface also includes an interactive
3D depiction of the room to help visualise its form, orientation and fenestration. Other features of the PDA include the ability to: l Directly compare the performance of up to four designs;
l Use sliders and spinners for much of the data input;
l Choose from a large range of UK locations or input user defined climate
www.cibsejournal.com data for any location in the world;
l Choose between normal or warm summer conditions;
l Calculate and display U-values and other thermal properties information; and
l Provide help and guidance within the tool.
The admittance method The admittance method was used in the PDA as it provides a good balance between speed and accuracy, appropriate to the sketch design stage when ideas are being explored. It was originally created in the 1960s by Danter and Loudon1,2
and,
although developed as a manual calculation procedure, it remains a useful and widely used technique that takes account of the dynamic effects of heat storage in the building fabric. A more detailed explanation of the admittance method can be found in CIBSE Guide A3
, but a brief overview is
offered here. The underlying principal of the admittance method is that the behaviour of space can be characterised as being in a steady cyclic state, in which daily variations in heat gain and loss about the mean value are assumed to be equal across 24 hours. This allows the dynamic nature of the space to be calculated in response to a sine
www.cibsejournal.com
wave within a 24 hour period. It also enables the calculation to be split into a constant (mean) response and the variation from the mean at any point during the day (swing). The mean response of the building fabric
is expressed by the U-value, whilst its cyclic response is described by the decrement factor and admittance value (referred to as the Y-value). Finally, a solar gain factor deals with the absorption and release of solar gain from the fabric, along with the transmission and absorption properties of the glazing. Use of the admittance method in the PDA
follows five basic steps: l The mean heat gains/losses are calculated for the chosen design day;
l The mean operative temperature is then determined using the result from step 1 and mean weather data for the design day;
l The swing (mean-to-peak) heat gain/loss is calculated for each hour of the day;
l The swing in operative temperature is calculated for each hour of the day and added to the mean value to give the actual operative temperatures across the day; and
l The absolute heating/cooling load can now be determined for each hour of the day.
March 2012 CIBSE Journal 61
The admittance procedure is capable of reproducing the results of more complex calculation methods when the same assumptions are made
One side of the debate, as set out by Jones and Hamnett, believes district heating and combined heat and power (CHP) can save carbon emissions and is the best investment in our future. The other side – comprising housing associations using the technology, a few professional engineers such as myself and, indeed, the Department of Energy and Climate Change chief scientific adviser David Mackay – look at the available figures and are not convinced. As I understand it, the theoretical benefits of the technology are marginal and could be lost in practice. The discussion would be clearer if we had data on the actual running of CHP and district heating systems – how much fuel was consumed, how much electricity was sold and how much useful heat was sold to the consumer at what cost. In my experience, the industry is reluctant to provide this. I’ve asked the CHPA and been
witness to DECC asking the industry – I don’t know if they have the data yet. I presume the energy services companies (ESCOs) have this data, because their business model is based on it. A householder can pay 4p/kWh
for gas burnt in an efficient boiler (and there are already concerns about
www.cibsejournal.com
StockLite/
Shutterstock.com
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48 |
Page 49 |
Page 50 |
Page 51 |
Page 52 |
Page 53 |
Page 54 |
Page 55 |
Page 56 |
Page 57 |
Page 58 |
Page 59 |
Page 60