We received a copy of our Minergie-Nachweis (der Nachweis = certificate), computed by an energy planning firm, several weeks ago and since then I've been busy with the different aspects of it. What this is is the calculation of the projected energy needs of the house and how they are to be met. It's a detailed document with inputs ranging from the location and orientation of the house (to calculate the solar gain and also heating needs based on the average monthly temperatures) to the construction details of the connection between the walls and the foundation (to calculate the amount of heat lost through thermal-bridges[1]). Not all the details are finalized yet; however no subsequent change should increase the heat demand so let's take a look at the end results of the calculations. I'll explore the individual parts of the work-up later as time permits.
I'll start off by referring to these two old posts where I had talked about the Minergie limit on the weighted energy demand: The weighted energy demand The weighted energy demand, part IIWe had the following relationship (click on the equation to see a larger version) which states that the weighted energy demand of a standard Minergie house may not exceed 38 kWh/(m2·a). So per year,
where
(i) QH,eff is the amount of heat required to maintain a comfortable indoor temperature (usually taken to be 20°C)
(ii) QWW is the amount of heat used to prepare hot water, and
(iii) QV is the amount of energy required to run the ventilation system.
The g terms are the weighting factors for the particular type of energy source chosen and the η's (eta) are (or are analogous to) the efficiencies of the devices used. Small g's and large η's are good, for lists of some of the commonly used ones see the old posts.
For our house, the energy demands are computed to be: QH,eff = 31.4 kWh/(m2·a) QWW = 13.9 kWh/(m2·a) QV = 3.02 kWh/(m2·a)
You will note that this adds up to (31.4+13.9+3.02) kWh/(m2·a) = 48.3 kWh/(m2·a). This exceeds the limit of Minergie, but this is not yet weighted. Let's do that now. Some additional information specific to the heat pump chosen for the house is necessary for this step. The heating device is an air-source heat pump which runs on electricity and covers 100% of the room heating needs (ηH = 3.88) and 80% of the hot water needs (ηWW = 3.04). The remaining 20% of the hot water needs is covered by an electrical heater (ηWW = 0.9). Some of you might recall that the weighting factor g for electricity is 2.0. The weighted energy demand is then calculated to be 35.7 kWh/(m2·a) and thus satisfies the Minergie requirement. Here it is written out in equation 2 (click on the image to enlarge it):
[1] Wikipedia entry on thermal bridges → Thermal bridge
0 comments:
Post a Comment
Thank you for your comment. It should appear here within 24 hours of posting.