Clarifications contributed by CIBSE members on last month's CPD In January's CPD, Simple thermal analysis for buildings, the electrical analogy used for admittance was somewhat confused. A more appropriate description would be '
....in electrical terms the U value is analogous to the reciprocal of the total resistance (conductance), and Y additionally includes the susceptance, that accounts for the storage effects of the structure'
Figure 5: Summary of annual heating energy supplied by ventilation systems and additional fan energy required to overcome heat recovery device resistance
(relating to each band) is then calculated,
but this time only requiring heating from θB to the supply temperature, θS
. The additional fan power (Wh) required
to overcome the resistance of the heat exchanger and the additional return air filter may be calculated from: (Q·∆P/ηfan
) x hours of operation.
For the cross-flow heat exchanger, a pressure drop of 100Pa (for each of the flow and return paths) is typical, and a panel filter would add 30Pa. The total fan efficiency is related to the fan type and the motor/drive mechanism – in this example, a value of 70% has been used although direct drive EC motors can approach 90% as may be needed to meet regulatory Specific Fan Power requirements. So additional annual energy use =
(1.02 x (100 + 100 + 30)/0.7) x 8766/1000 = 2938 kWh. This will be electrical power, whereas the savings in heating energy could be savings in gas or electricity (or other fuels), depending on the method used for heating on site. The combined counter-flow/cross-flow
heat exchanger can be examined in exactly the same way as the cross-flow but with the efficiency amended to 0.85 (85%) and the pressure drop increased to 150Pa. The results from that analysis, together with the others, are summarised in Figure 5. The annual energy consumption may be
used to provide a simple approximate cost saving (in terms of heating energy less the additional fan energy), as shown in Figure 6. A similar exercise could be undertaken to show notional CO2
saving. It is important to note that this simple
model is based on a 24-hour-a-day application with low casual gains. In other applications of 10-hour working and higher occupancies and equipment loads, the savings will not be so dramatic. The application of a compliance tool such as
64 CIBSE Journal February 2013
SBEM or, preferably, dynamic modelling software, together with independently certified performance data, will provide a more complete picture of the whole life impact of a heat recovery device. Appropriately applying this simple technology can increase flexibility in building design, while still meeting rigorous carbon targets. The size of associated heating systems may also be reduced, saving on both capital and installation costs. © Tim Dwyer, 2013.
Further Reading Air Conditioning Engineering, Jones WP, Butterworth 2001, Section 6.6. ASHRAE HVAC Handbook 2008, Chapter 25. CIBSE Guide B, Section 2.5.6, 2005.
References
1 Fisher, J., Reducing your Total Building Emissions using High Efficiency Energy Recovery, FläktWoods, 2012
2 CIBSE Guide A, Section 5.6.2, 2006 3 CIBSE Guide F, Section 5.2.5.5, 2012.
Figure 2 also lost some details – principally the omission of decimal points in the ordinate values. We thank David Findler for his input on the electrical analogy and were particularly pleased to be contacted by the original author of the work on which much of the article was based, John Harrington-Lynn.
John pointed out that, although the mathematics behind the admittance concept assumes that the weather data and usage patterns are repeated over several days, the majority of buildings are subject to regular 24-hour usage patterns and so the method can be applied to assess the performance of a building both in winter as well as summer, with appropriate data. The method will cope with a variable infiltration/fresh air rate. To cope with more complex systems, it would be necessary to use the calculated cooling/heating load and the designers knowledge of the proposed air handling system to calculate the air handing volumes. As part of his clarification of the figure in the article, he explained that Admittance or U values should meet on the Y axis at a value of approximately 5.5 Wm-2
K-1 +Rse ,
since the solution of the matrix equation for an infinitely thin membrane produces a Y-value and U-value equal to 1/(Rsi a vertical surface, Rsi
Y or U value of 5.55 Wm-2
= 0.12, Rse K-1
.)
). (Typically for = 0.06 giving a
Figure 6: Approximate cost savings applying heat recovery for a system that uses gas heating and one that uses electric heating
www.cibsejournal.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 |
Page 61 |
Page 62 |
Page 63 |
Page 64 |
Page 65 |
Page 66 |
Page 67 |
Page 68 |
Page 69 |
Page 70 |
Page 71 |
Page 72 |
Page 73 |
Page 74 |
Page 75 |
Page 76