042. From Minergie to Minergie-P (rejoice!)

Figure 33. The new calculated energy balance for the house. The numbers are in kWh/(m²). Compare to figure 23.

Back in April I had started to write a little about the calculations of the projected energy usage that had been done based on the components making up the house. Since then much has changed and the structure of the house is now going to be made entirely of wood[1]. The windows have been upgraded to have better energy performance. We also have a new energy planner who is a better fit for our project. He changed the heat pump to a better-tested model, added 4 m² of solar collector (for hot water) and an Erdregister (earth tube heat-exchanger). The result of all these changes is that we now have an optimized system with lower numbers. Numbers that put us firmly in Minergie-P territory!! Read on for more information...

First, the quantities listed in the diagram above are:

- Q_iP: heat generated by the residents.
- Q_iE: heat generated by the electrical equipment.
- The total internally generated energy is Q_i = Q_iP + Q_iE.

- Q_S: heat delivered by the sun.
- The total gain Q_g = Q_i + Q_S.

It is assumed that only 60% of Q_g can be used by the house:
- Q_g,u = 0.60 Q_g
- Q_V is the energy lost through the ventilation system.
- Q_t is the energy lost via transmission through the shell.

Also back in April, in post 021, I had written about the requirements of Minergie. Here again is the relevant equation (refer to the old post for details):

With the new construction plan, the various energy demands are computed to be:
Q_H,eff = 13.9 kWh/(m²·a)
Q_WW = 6.7 kWh/(m²·a) and the rest is met via solar energy
Q_V = 2.51 kWh/(m²·a)

This gives a weighted energy demand of 20.7 kWh/(m²·a). The limit for Minergie-P is 30 kWh/(m²·a), so we are comfortably within this requirement. This is great news!

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[1] In the earlier plan, the lower level was going to be made of concrete and bricks, and the upper level out of wood.

039. Back to the heat pump

There was a misunderstanding somewhere and it appears that the overall cost for the district heating system would exceed that of the version with a heat pump. I still don't understand this clearly so I obviously cannot explain it. So, it's back to the air-source heat pump, though perhaps not the one by Heliotherm.

I'm looking at some of the results of performance tests of these devices done by independent labs[1] to see if I find a favorite. One new issue that I've become aware of is that our technical room[2] is a little on the small side. The heat pump and its associated plumbing will fit, but we really could use the space if we can save it. Also, this type of installation requires large openings from the room to the outside and because this room is partly underground, construction elements called Licht- or Kellerschächte[3] have to be used. I have a great aversion to these things, partly from the maintenance point of view. To circumvent both these problems, I think a unit that is installed outside the house is called for (we had been considering only internal units so far). I have yet to find any indications that they're less effective than the ones installed inside.

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[1] One such facility is in Switzerland → Wärmepumpen-Testzentrum WPZ.

[2] Here's a link to an old sketch → Figure 7g. The lower level.

[3] I'm not sure these things even exist in the same form in the Anglo-world. You can do an image search with the keyword "Kellerschacht" or "Lichtschacht" to see some examples.

038. District heating instead of a heat pump

Back in March (link to the post here), I had written that we would be using a heat pump made by a company called Heliotherm to heat the house and warm water. That model had been suggested to WA by the company that did the first round of energy calculations for us. Since then, there have been some significant changes in the construction elements, and a new energy calculation is being done by a different firm. Also, a new opportunity became available in May, that of district heat[1]. Earlier in the year REFUNA, the company that runs this network, was in the process of adding capacity and they couldn't promise that they could serve us. By May they were able to give us a positive answer.

The advantages of this type of heating system is that they're generally more energy efficient and cleaner than local systems and boilers. They take up less room for installation and are very low maintenance. Wikipedia had a good write up on it, see the link in the footnotes below. In our case, the costs associated with the installation and hook-up should be below that of what it would be for the heat pump system because a lot of the necessary earth removal work will have to be done for the other connections anyway.

I'm actually very pleased about this development as I was not so happy about the air-source heat pump. At very low ambient temperatures, they basically become electric heaters.

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[1] District heat is also known as waste heat, or utility-supplied heat. The term in German is Fernwärme, literally "distance" heat or warmth. REFUNA gets its heat from the waste heat of the nuclear reactor in Beznau near where we'll be. The Wikipedia article mentions this → District heating

036. Design consideration: window shades

I've mentioned a few times already that it is crucial that a good plan be made for shading the (transparent elements of the) house from the sun in the summer to avoid overheating. Generally speaking, this topic is especially relevant in these times of rising global temperatures and there is a strong effort underway to find the best (low-energy) solutions. Organizations such as the European Solar Shading Organization (ES-SO) have some information in English on their websites[1]. Minergie in Switzerland provides guidelines for good practice. See Figure 22 in Post 29 to get an idea of the size of the problem for our house, as calculated for a particular set of parameters.

The German Institute for Standardization (DIN = Deutsches Institut für Normung) provides a useful listing of the rough efficiency factors for different types of shading systems as given in Table 4 below (visit the ES-SO link to see illustrations of these systems). Of course, the actual numbers will vary with the details of the type of materials used. In general, the best systems are those that are installed outside the glazing. For residential applications, roller shutters[2] seem to be the usual choice over here. Aesthetically they're not my favorite as I find them chunky and obstructive. A better alternative are external venetian blinds, with adjustable louvers that allow a view of the outside. They're rather high-maintenance from a cleaning point of view but WA assure us that in our locality we won't have to clean them more than once every couple of years.

Table 4. Solar gain reduction factors for different window shading systems. Smaller is better. Source: DIN 4108-2

	Reduction factor
Type of shade	Fc
No shading	1
Internal installation or between the glass panes
White or reflective surface with low transparency	0.75
Light colors or low (less than 15%) transparency	0.8
Dark colors or high transparency	0.9
External installation
Rear-ventilated adjustable louvers	0.25
Blinds and materials with low transparency, rear-ventilated	0.25
Blinds, general	0.4
Shutters, roller shutters	0.3
Overhangs	0.5
Awnings, ventilated	0.4
Awnings, general	0.5

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[1] A nice list of the different types of shading systems can also be found at their site → European Solar Shading Organization

[2] Actually, I'm not sure this is the right term for the objects I'm thinking of which are called Rollläden in German

031. The energy balance

Figure 23. Illustration of the calculated yearly energy balance for our house. The numbers are in kWh/(m²).

In posts 027, 028 and 029 we looked at the heat flow into, and heat flow out of, the house on a monthly basis. The information is then put together for the year to calculate the yearly energy demand of the house. A depiction of this can be seen in figure 23 which is an illustration based on one generated by a software package called NOVA[1] which is what our energy planner used. Note that the diagram is not to scale!

The quantities listed in the diagram are:

- Q_iP is the heat generated by the residents.
- Q_iE is the heat generated by the electrical equipment.
- The total internally generated energy is Q_i = Q_iP + Q_iE.

- Q_S is the heat delivered by the sun.
- The total gain Q_g = Q_i + Q_S.

Only 69%[2] of Q_g can be used by the house:
- Q_g,u = 0.69 Q_g

- Q_V is the energy lost through the ventilation system[3].
- Q_t is the energy lost via transmission through the shell.

The box labelled WRG (Wärmeruckgewinn) represents the heat recovery aspect of the ventilation system. I think the term E_hww represents the electrical energy required to run the heat pump and related equipment that we have planned. Q_r and Q_L must give an indication of the amount of energy that is extracted from the environment (the air in the case of our air source heat pump).

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[1] NOVA is made by → Plancal AG

[2] I don't know how exactly this number is computed.

[3] This number is calculated using an Aussenluftvolumenstrom of 0.37 . I will try to explore this on the blog at some later point.

029. The energy balance of the house: losses and gains

Figure 22. Heat lost and gained by the house.

At the simplest level considering conservation of energy, once we have the house at a temperature we're happy with, we want to balance the heat gain and the heat loss so as to maintain a steady state on the inside. In figure 22[1] the blue line represents the total heat energy that is lost from the house through ventilation and transmission (details at → post 028) and the red line depicts the total heat energy that is added to the house, mainly through solar gain (details at → post 027).

It is clear from the graph that in the winter months more energy flows out of the house than flows in. In order to maintain a constant temperature[2] we must add heat and the blue shaded regions represent roughly the amount of heat that must be added[3]. In the summer, the situation is reversed unless some action is taken to suppress the gain. For example, by shading the windows as they're responsible for the largest amount of gain in our case[4]. Another way to cool the house is to bring in cooler air from the outside during the nighttime. Both of these can be automated to reduce "user error", e.g. a situation where we forget to close the window shutters when we leave the house one summer morning.

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[1] The numbers here are raw numbers in that I have not included the effect of some weighting factors and such. They make a small difference.

[2] It is not true that we maintain the same temperature throughout the year. In the winter the temperature is taken to be 20°C (68°F). Exactly what the maximum summer temperature is taken to be, I do not know yet, however 22°C to 24°C (71°F to 75°F) is probably not unreasonable.

[3] The gain is pretty much maximized in this case. However there is room to play in the loss side of the equation, i.e. more insulation and reduction of heat bridges. As with most things, it's a matter of optimizing the system within the parameters of affordability.

[4] If we happened to run a computer farm at home, we'd have to implement some additional cooling methods.

028. The energy balance of the house: losses

Figure 21. Heat lost from the house.

In the previous post I showed the calculated heat gain for the house. Here we have the calculated heat loss, based on walls and roof (opaque elements, as they're called) with a heat transfer coefficient, U, of 0.13 W/(m²·K) and windows (transparent elements) with a heat transfer coefficient of 1.3 W/(m²·K). One item under discussion at the moment is window upgrades. About 43% of the heat that is lost, is lost through the windows. Reducing the heat transfer coefficient of the windows could have a significant effect on the total heat loss.

027. The energy balance of the house: passive gain

I've been slowing working my way through the results of the energy calculations that the energy planner did for the house based on the provisional plans. The document is about 60 pages long. Parts of the input information and intermediate steps in the calculations are not in it so I first have to try to reconstruct it. Then I can put the information together in a format I want for the purposes of posting here. Let's start to look at the calculation of the energy balance of the house in parts. I won't go into a discussion of the details right now there are already sites[1] where that's done.

Figure 20. Heat gained by the house passively.

Figure 20 shows the passive heat gain predicted for the house, broken down by month. I believe this is before the sun-shading system is taken into account for the summer months. It is clear that without the proper system to suppress the heat gain in the summer, the house would become unbearably hot. Note the drop in June. This must be related to the input data (see figure 16[2] in post 024) and my first reaction was to think this can't be real. However, I've found a similar thing in a plot on another site[3] so it's still an open question for me. On the other hand, it's not really very important.

As I understand, these detailed calculations are usually only done for passive houses, i.e. houses with energy demands that are about half —15 kWh/(m²·a) instead of 38 kWh/(m²·a)— of what ours is going to have. These houses have such low heating needs that the heat generated by people[4] and appliances have a substantial influence. In the graph the lowest two lines represent these internal heat gains. The heat output of a person is taken to be 70 W and a daily 12 hour presence is assumed. The appliances are estimated to contribute 15 kWh/(m²·a). The heavy grey line at the top is then the sum of the solar heat gain and the internal gain. In my next post, I'll talk about the other side of the energy balance (the losses) and the need for additional heating.

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[1] One good example is here (in English) → Energy Balances Passive House

[2] Direct link to figure 16 → Solar data from Buchs

[3] PHPP → Passive House Design Package

[4] One passive house joke is that if you feel your house is too cold then you can invite a few of your friends over for dinner to warm it up. We'd have to invite a dozen or so.

024. Solar radiation intensity over Switzerland and our region

Figure 15. Annual solar radiation over Switzerland.

Solar radiation is a major source of energy for our house. Here is an overview of how much energy is actually available to us from the sun. The map in figure 15 (pvgis-solar-optimum) shows the yearly sum of irradiation available to optimally inclined collectors in different parts of Switzerland. The high intensity regions in brown in the south are the Alps. Our house will be in the north near Zürich, in one of the least sunny areas of the country. But, there is still quite a bit of energy to be gained as can be seen in the graph in figure 16.

Figure 16. The monthly solar radiation measured at our reference weather station in Buchs, broken down by the cardinal directions. That drop in the intensity in the south in June is strange. Must look into that.
Figure 17. The average monthly temperature.

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Data and calculators covering most of Europe is available here → Photovoltaic Geographical Information System

023. Where our electricity comes from

Figure 14. Breakdown of the electricity sources in our locality.

As our plans currently stand we will be using electricity to cover the parts of our energy demand that are not met by either solar gain or the energy extracted from the environment via the heat pump.

In the locality where we're building the major share of the electricity supply is from nuclear power plants. About three-quarters of that is generated domestically while the remainder is purchased from abroad (most likely France). In the hydroelectric and renewable group, hydro actually accounts for most of it. Only about 0.01% of the total supply comes from sources such as wind, biomass and photovoltaic arrays.

About 2.7% of the supply comes from waste. There are facilities that incinerate garbage (Kehricht-verbrennungs-anlage = KVA, aka Müllverbrennungsanlage in Germany) and use the energy from the process to produce both electricity and also provide heat for direct use (Fernwärme which can be translated as district heat). One such facility in the region is in Turgi near Brugg and they serve a total of about 200,000 people. They generate 7.7 MW of electricity and 18 MW of thermal energy for the Fernwärme system. District heat could have been a possibility for our house[1]. The problem is that the connection costs are rather high.

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[1] Using district heat would lower our weighted energy demand to 33.2 kWh/(m²·a) from the 35.7 kWh/(m²·a) calculated → here

021. Calculation of our energy demand (Minergie-Nachweis)

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 II

We 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) Q_H,eff is the amount of heat required to maintain a comfortable indoor temperature (usually taken to be 20°C) (ii) Q_WW is the amount of heat used to prepare hot water, and (iii) Q_V 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: Q_H,eff = 31.4 kWh/(m²·a) Q_WW = 13.9 kWh/(m²·a) Q_V = 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 (η_H = 3.88) and 80% of the hot water needs (η_WW = 3.04). The remaining 20% of the hot water needs is covered by an electrical heater (η_WW = 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):

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[1] Wikipedia entry on thermal bridges → Thermal bridge

020. Canton Aargau doubles subsidies for solar collectors and more this year

If you're thinking of building or renovating in canton Aargau, then this might be of interest. This year the subsidies for solar collectors and photovoltaic cells for homes have been doubled. In addition, there's now a subsidy to replace electrical heating systems with underfloor heating systems which use water as the heat transfer medium. Furthermore, for renovations of existing homes, the canton is doubling the subsidies given by the Klimarappen Foundation! The following is an overview of the items relevant to new construction. For more information, visit the links at the very bottom of the post.

Solar thermal collectors

Flat plate systems (Flachkollektoren)
CHF 3000.- for 4 m² to 8 m²
CHF 1250.- plus CHF 220.- per m² for installations between 8 m² and 15 m²

Evacuated tube systems (Röhrenkollektoren)
CHF 3000.- for 3 m² to 6 m²
CHF 1250.- plus CHF 280.- per m² for installations between 6 m² and 12 m²

Solar cells (Photovoltaic)

CHF 3500.- per kW_P for an integrated system
CHF 2900.- per kW_P for an add-on system
CHF 2500.- per kW_P for a free-standing system

Heat pumps (Wärmepumpen)

CHF 3000.- for either a ground-source or groundwater-source unit up to 20 kW

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Aargau Departement Bau, Vehrkehr und Umwelt → Fachstelle Energie (Aargau)

Climate Cent Foundation → Stiftung Klimarappen

The weighted energy demand, part II

Yesterday's post was about the calculation of the energy demand of a house and the weighting factors assigned to different energy sources. Another factor that must be taken into account is the efficiency of the particular device, e.g. a furnace, used to produce the heat energy. A note on terminology: In the case of heat pumps, the word efficiency is not accurate and the phrase coefficient of performance[1] is used instead.

It is important to note that these are the "standard" values accepted in the Minergie system. There are heat pumps which beat these numbers and our architects have recommended one of the best performing models on the market. The COPs for the air-source heat pumps made by this company[2] are claimed to range from 3.56 to 4.2, depending on the particular model. I plan to write a post about these systems before too long. Update 23 August 2009: Instead of a heat pump we will probably use district heating. Please see post 038.

Table 3. Efficiencies or coefficient of performance (COP) for a selection of different heat generating systems. Larger is better.

	Heating	Warm (hot) water
Source	ηH	ηWW
Oil or gas furnace	0.85	0.85
Oil, condensing furnace	0.91	0.88
Wood-fired furnace	0.75	0.75
Wood pellet furnace	0.85	0.85
Waste (district) heat	1.0	1.0
Electricity	1.0	0.9
Heat pump, outside air	2.3	2.3
Heat pump, ground source	3.1	2.7
Heat pump, ground water	3.2	2.9
Photovoltaic	0	0
Solar collector	0	0

The effect of this weighting system can be seen in figure 8 below. The standard values for the weighting factors for the different energy sources have been used here to compare the different results for the same house. I've left off the numbers on the graph because I just want to give an idea of the relative differences. A process of optimization leads to the the best solution for a particular house.

Figure 8. The weighted energy demand calculated using different energy sources.

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[1] Read about COP at this Wikipedia page → Coefficient of performance

[2] The company is Heliotherm → Heliotherm air-source heat pump

The weighted energy demand

All this talk of insulation, but I haven't yet gone over the fundamental point of Minergie which is to reduce the energy demand of buildings, in this case our house. I've mentioned several times that the limiting value for houses is 38 kWh/(m²·a). Then the question is how this energy demand is defined. Different systems of certification (e.g. the German KfW-40) have different ways of calculating this and in the Minergie system under consideration here it is a weighted sum of the energy required to
(i) maintain a comfortable indoor temperature (usually taken to be 20°C), call this Q_H,eff
(ii) to heat water Q_WW and
(iii) to run the ventilation system[1], Q_V.

We have the following relationship (click on the equation to see a larger version):

In Equation 1 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. You can see from the inequality that small g's and large η's are good.

More about the η's in the next post, here I'll just say a few words about the weighting factors g. This is where the differences in the different certification systems become apparent[2]. They are basically an attempt to compare the losses associated with the conversion of the energy from different sources to heat (see Table 2 below for the list). Burning fossil fuels to generate heat is taken to have a g of unity. Using the sun directly, as in absorbing the radiation and storing it as heat 'costs' nothing so it is given a weighting of 0. Using electricity, say to run a heat pump or (horror!) a resistance heater is considered least desirable (I imagine because the electricity itself is generated from other sources and there are losses in that chain of production and in the transmission). However, in the case of heat pumps this is mitigated by the ability of these devices to extract energy from the surroundings and this should be clear in tomorrow's post. The energy source to run the ventilation system is almost certainly electricity. A typical value for the Q_V is between 3 kWh/(m²·a) and 4 kWh/(m²·a).

Table 2. Weighting factors for different energy sources. Smaller is better.

	Weighting factor
Source	g
Solar, geothermal, ambient	0
Biomass (Wood, biogas)	0.5
Waste heat	0.6
Fossil fuels	1.0
Electricity	2.0

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[1] A little aside about the ventilation system: The aspect of the building envelope that I've talked about so far, namely the insulation, deals with the loss of heat via conduction through the shell. Another very important mechanism of heat loss is the movement of warm air from (i.e. leakage) and cold air into (i.e. infiltration) the house through gaps in the shell. In a well-insulated house, this can account for upto 50% of the total heat loss. It turns out that it is possible to neutralize this effect and still have good air quality by building a very tight shell and by relying on a high-efficiency mechanical ventilation system with a heat exchanger to capture back more than 80% of the heat of the exhaust air. A topic for other posts.

[2] An entry on the German language Wikipedia compares three systems: Primärenergiebedarf

Limits on the overall heat transfer coefficient through the building envelope

I've mentioned before that the first step in reducing energy consumption is to invest in a well-insulated building shell. Just like choosing a thick and warm winter jacket. I've also stated before that in the end what is important for the Minergie specification is that the weighted (I'll talk about this weighting very soon) energy consumption per unit area be less than 0.38 kWh/m² per year. It is possible to achieve this by adjusting the different factors that play into this equation and to simplify the matter the Minergie group has come up with a set of "standard solutions". The limits given in the table below are those for the building elements. It is possible to build a Minergie house with components that exceed these values but make up the difference in other ways. In fact, not only it is possible, it seems to be often the case. This clarifies some of the confusion I had in earlier posts.

Table 1. Limits on the total heat transfer coefficient for building components.

	Separation from exterior	Separation from unheated interior	Separation from ground
	W/(m2·K)	W/(m2·K)	W/(m2·K)
Roof	0.15	0.20	-
Wall	0.15	0.20	0.20
Floor	0.15	0.20	0.20
Window	1.00	1.60	-
Door	1.20	2.00	-

Limits on the overall heat transfer coefficient through the building envelope

Figure 3. A simplified representation of the heat transfer coefficient limits for a few specific building components.

In general, the first step towards reducing energy consumption in a house is to use a lot of insulation. The lower the heat transfer through the building envelope, the better. One limitation to this is that the thicker you make your shell, the smaller you make your interior area. In the case of our house, we're probably looking at a minimum of 35 cm (≡ 14") thick exterior walls. This is for timber construction[1], construction in masonry or concrete usually ends up being thicker for the same insulating properties.

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[1] Actually, it is more appropriate to say "mixed" construction. About a quarter to a third of the shell will probably be of concrete because our building land is on a slope.

Two things the label does not include

A couple of points that I think are important to note. - The label certification is based on the projected energy consumption of buildings. There is no oversight of the actual construction and subsequent usage. The good news is that a study[1] in 2004 (see graph below, remember that 1 MJ ≡ 0.28 kWh) carried out by the University of St. Gallen of about 500 certified buildings showed that single family homes (SFH) and renovation projects typically were better than the standard prescribed. Newly constructed larger residential units (MFH) fell short, though not by much. There have been a lot of developments and many lessons learnt since the time the study was conducted, and I think things are much improved. Figure 2. Results of a survey of Minergie-certified residential units conducted by the University of St. Gallen in 2004.

- A second important point for me is that the requirements say nothing about the consumption per person. They just set a limit on the amount consumed per heated square meter of floor space. To make an extreme comparison, person A can have 300 m² to himself or herself while family B with 4 members can live in a 180 m² house and both houses can be Minergie certified. For a vision of a per person consumption limit see, for example, the 2000 Watt society project.

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[1] A copy of a presentation of the results (in German) can be downloaded here: Minergie Praxistest 2004.

Simple representation of the basic points

Figure 1. A simplified representation illustrating the main points of a Minergie house.

The current maximum for the weighted energy demand of a single family house is set at 38 kWh/m² per year in the Minergie category. The weighting involves the type of energy source that is chosen to meet the heating demands. Direct solar energy is highly preferred while electricity from the grid[1] is not. More about this later.

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[1] I make this distinction because electricity generated via a photovoltaic system is viewed favorably.