Blog of Passive House in Darmstadt - Kranichstein

Before December 5th: no heating

Data logging of the temperatures in the days before starting operation: up to 26th November the temperatures measured in all zones of the building were always between 21 and 23 °C (completely without heating). Whenever direct solar radiation was incident on the southern side (e.g. as was the case on 26th and 27th November), the building heated up again perceptibly as a result 1) . However, since 28th November the sun had no longer appeared from behind the clouds – and with outdoor temperatures between 3 and 6°C 2) the heat loss was around 9 kWh each day. With the heat capacity of the interior walls and ceiling, this corresponds to an average daily temperature decline of about 0.3 degree. For a couple of days one is able to bear this, but at some point it will become uncomfortably cold; before that happens, we will start operation of the split unit air conditioner in order to compensate for any further heat losses.

December 5th, 2022: the mini split unit is put into operation on the ground floor

In the morning a “room” temperature of 19.4 °C prevailed in the dining room (where the indoor unit of the split unit was installed). In a Passive House building this is still a comfortable temperature when wearing a sweater, warm pants and slippers with thick soles. It is comfortable mainly because there is hardly any difference between the temperatures in the room: the chairs, table surfaces, interior walls, and even the exterior walls and the flooring on the ground floor have temperatures that differ by a maximum of only a few tenths of a degree, so there is no radiation temperature asymmetry and no thermal buoyancy leading to draughts. A further reduction in the temperature would still be quite bearable here, but this would require further adjustment to the clothing.

Because the general specification for offices in the public sector in Germany at present is 19°C, we now see a meaningful target in this area for our new self-experiment with temperature set points: at this time of the year we want to keep the rooms within the range between 19 and just above 20°C, and observe what it takes in terms of energy input. The split unit had last been in operation on 31.8.2022 for cooling, and was put into operation at 9:29 CET on 5th December in the mode:
“heating, automatic volume flow, internal thermostatic control at 21°C”

In the picture 1: Temperature progression on 2022-Dec-5: the air conditioning unit quickly raises the temperatures in the dining room and kitchen, the study room in the attic follows just a little later (Temp_DG_SE). An equilibrium state has not yet been reached; the heat capacities of the concrete ceilings in particular need several days to adjust to this level. However, the desired level of comfort is already ensured after 1 hour - with extremely low electrical power (see text).

This is done quite simply by turning on the mains power via a switch. The settings are then selected using the original remote control, which is intuitive and not much different from the operation of a thermostatic valve3) . The unit started up immediately and drew between 424 and 1140 watts electrical power in the first 30 minutes (also to heat up the device parts). At 10:00 a.m., the temperature in the installation room was already 19.6 °C. As we already knew from previous years, all other rooms are “taken along” step by step through the open staircase here over the following days. This system is very inert - that's why we didn't wait until the temperatures were below 19° everywhere, instead we already started early on in order to counteract the temperature drop a little. The temperatures in the rooms occupied by us were then a few tenths of a degree higher than the average, because each occupant also contributes appreciably 4) towards keeping these warm - such outputs already play a role in well-insulated buildings, and even the few watts which enter as passive solar irradiation 5) through the windows additionally during the daytime help provide 2 to 3 tenths of a degree 'more' than in the early morning.

The power consumption on the first day (up to 9:29 on 6th December) was 10.7 kWh, equating to an average 24h output of about 350 Wel . From this, the heat pump in the split unit generates about 800 Wtherm of heating energy (“thermal”) in the present configuration. This is also roughly the same power that this building needs on average just to compensate 6) for the net losses towards the outside 7) .

Evaluation: the average power consumption for all other applications (such as the refrigerator, lighting, dishwasher, workstation computers, etc.) in this home is about 350 Wel in winter 8). The current consumption of the air conditioner heating is quite a realistic value for December: it roughly doubles the power taken from the grid compared to the annual average 9); although the annual sum for heating energy consumption in the Passive House10) is only about a quarter of the total final energy consumption: heating is only a “winter issue”, this is actually well-known, but unfortunately very often forgotten. In less well-insulated buildings, the required power is (very) much higher: as opposed to the 350 W, this can easily be 2000 to 4000 W; such an average power consumptions would be higher by a factor 6 to 10 than previously in a typical household.

These actual values illustrate how energy efficiency, switching to electrical systems with higher efficiency (heat pumps) and the use of renewable energy can successfully work together to make us independent of fossil energy in the end.

December 7th : the sun sometimes peeks out behind the clouds

First, the result for 6th December: the sky was no longer quite “so dark” as it had been the day before, our PV system was now also providing almost 4W 11) ; this is still little compared to typical power consumption in winter (around 350 W, as already mentioned)12) . The average measured electrical power consumption of our air-conditioning split unit, used as the only heating source, was around 312 Wel.

Despite low outside temperatures, somewhat less power was needed than for the previous day: this is because the building is now somewhat closer to the new dynamic equilibrium. In particular, the concrete ceiling above the ground-floor room with the air-conditioning unit has now already warmed up - so the unit's heating requirement is lower. However, it will take a few more days before equilibrium is reached throughout the house - which may be disrupted again right away, because the weather services are forecasting an influx of cold air coming from Siberia for the coming week. The suspense continues.

Fig. 2
Fig.3 :Hourly values for the electrical power consumption of the split unit: a declining profile, because a steady state is gradually established. On average, this is 'only' about 320 watts

Fig. 2, a few rays of sunshine appeared even in December: it became bright in the south-facing rooms, providing heat which already more or less made up for the losses through the glazing - and some electricity is generated, at present actually 350 W; if the weather in December were always so sunny, operation would then be almost self-sufficient (in a Passive House building that is). But unfortunately, this only occurs during a fourth of the time of the day at the most, on average on less than 10 days of December.

Fig. 3 shows the time course of the hourly measured average power. That is about 104 watts for each person - for operating the heat pump for heating, which is twice the heat output in calories given off by this number of people due to their basal metabolic rate. In a Passive House, this is typical for the winter period: the power outputs are in the same order of magnitude as the original output capacity of the persons on a human scale. Of course, there is the basic electricity consumption in addition (of the same level for computers, lights, refrigerator, etc.), and the consumption for mobility – however, this is extremely low in this specific case, because for the past two years we have been going everywhere on foot, on bicycles and occasionally by tram. Food intake is then approximately of a similar level again.

December 8th : Will things remain the same? A justified critical question from the media

Today, on 8th December, the weather is a little colder and sunshine is very scarce. Indoor temperatures have settled into the 20 °C range (plus or minus a few tenths of a degree). We already know from previous years that it will stay like this - because even in case of a cold spell the air conditioner will be able to continuously deliver about twice the heat output; however, the present control adjustment for this is not optimal - but it works. More information about this can be found in the publication [Feist 2022].

Fig. 4: Heat emission in the winter of 2019/20: from the split unit at the top in red, added to this is the body heat of occupants (in orange), waste heat from electrical appliances such as our home office computers (green), and from some other sources

Here you can also find detailed measurement data on heat generation, electricity consumption values and the efficiency of the split unit used. The amount of heat generated by the split unit and the internal heat sources such as people, hot water pipes and electricity consumers in winter 2019/20 are shown in Fig.4. Typically, the required heat output of the split unit is highest in December (in the year shown: around 600 watts on average), although this is not the 'coldest month'. As already mentioned, for buildings with this insulation standard and sufficient windows for good daylight, solar radiation is just as important at least. From mid-March at the latest therefore, “heating stops” in the Passive House in Kranichstein. The electrical power consumption of the split unit for generating this heat is lower by the factor of the “COP” (coefficient of performance, i.e. the ratio of the heat provided to the electricity used). For our device, this power consumption is then just below 300 Wel on average. This matches the values we had measured in the last days of this December and documented here.

Point to note: even in a highly efficient building like the Passive House, energy consumption for heating still predominates from about December to February; this leads to very uneven annual demand patterns. However, the value (around 600 watts on average in December) is within the manageable range in the Passive House. With a heat pump, it takes about 300 watts on average additionally, which can always be handled by the grid, even “if everyone did the same”. However, what is important still is that the corresponding renewable power stations (i.e. wind generators!) are also built.

December 9th : very little heat losses despite the cold

On December 9, it was already about 2 °C colder outside – due to which the heat losses are increased, of course. These are actually approximately proportional to the difference between the indoor and outdoor temperature(s) 13)) . The fact $ H_e $ ,with which the heat loss rate is obtained from the temperature difference by multiplication is called the “specific heat loss” of the building. In norms 14) it is described how this can be calculated from the heat loss coefficients, the areas of the exterior building components, the glazed areas, and the air change rate. For example, for our building here this can be calculated using the PHPP; in the present case it results in about $ H_e $= 82 W/K 15) By way of example, let us calculate the total heat losses $ \dot{Q} $ in the last days, with around 3°C outside and 20.4°C inside: $ \dot{Q}=H_e \cdot ( \theta_i - \theta_e ) = $ 82 W/K $ \cdot $ (20,4 - 3 ) K = 1427 W

This heat loss rate is significantly higher than the heat output of around 720 watts provided by the split unit. The rest of the losses are compensated through heat dissipation from people and appliances and water pipes 16) and by the passive solar contribution through the windows, even though this is small at present.

In our Passive House building, the heating system (in this case the split unit air conditioner) clearly only has to compensate for less than half of the heat losses - all the rest is compensated for through the occupants themselves, the waste heat from e.g. lighting and computers, and the solar heat radiating through the windows. Because today, 9th December 2022, was quite a sunny day, tomorrow we will talk about solar energy - active and passive.

December 10th: Passive solar gains: important in buildings with a good standard of thermal insulation

Today (10th December) – in the middle of December in Germany – we will take a small excursion into the topic of energy. In Central Europe, this is typically the month with the lowest solar radiation – but even then solar radiation is not zero, it still provides (some) energy.

In order to get an idea of the amounts of energy concerned, let us take a look at the recorded PV yields: electricity was produced every day; Fig. 5 shows the profile for the generated electrical output. This was very meagre from 3rd to 5th December – typical cloudy weather in Darmstadt. On 10th December however, PV electricity generation temporarily reached 550 W. We will take a closer look at this not entirely cloud-free, but relatively sunny day in December. The logged measurement data shows available solar radiation of about 1.2 kWh/(m²d) on the southern façade on this day 17) . That is almost 24 kWh solar incident on the approximately 20 m² glazed area on the southern side. However, not all these solar gains actually “enter” through the window; there is shading by neighbouring buildings, by the balcony parapet, there is dirt on the panes, radiation is partly reflected, partly absorbed 18) - in fact only about 4.8 kWh actually reach the inside19) . However, for the entire apartment, this is definitely about 200 watts of continuous power averaged over 24 hours. A measurable contribution to the energy balance which, with a total heat loss of only 1427 watts in this well-insulated home 20) , also contributes significantly, namely about one seventh 21) .

Fig. 5: Electrical output of the PV system
Fig. 6: Temperature increase in the south-eastern room on the top floor on a sunny day in December 10.12.2022)
Fig. 7: Monthly balance with the PHPP. This clearly illustrates why we must heat additionally in the winter, and also the fact that this does not have to be a drastic issue in a well-insulated building and that the passive solar gains still contribute a little in December. The calculation shown here was performed based on the average climate in Darmstadt and an indoor temperature of 20 °C; currently, in 2022, it was much warmer in November, so that no heating was needed in this month.

This solar contribution can actually be seen in the temperature readings. This is documented in Fig. 6 for the south-eastern room; a steep rise in the temperature from 10.30 onwards is recognisable here, with a peak temperature of up to 1.4 °C. It is now really pleasantly warm here due to free energy supplied by the sun. And because the losses in this building are so low, this even lasts until 3:00 in the following night! That's how long the temperature remains above the selected set point. 'Passive solar' works, even in December! However, it is necessary to assume that the users also ensure this, meaning that they do not close the external blinds every time the sun shines in winter. Of course, this presupposes that there is an alternative system installed on the inside for glare protection22) . In such a building, this saves about 17% of the heating energy in the main winter months; in poorly insulated buildings, the absolute contribution (2 to 5 kWh/(m²a)) is equally high, but relatively insignificant compared to the high losses.

Relevance for the energy balance

With the Passive House Planning Package, the balance calculation of heat losses and gains can be determined on a month by month basis. Fig. 7 demonstrates this for the current average climate in Central Europe 23) . It can be seen clearly how the solar gains24) decrease drastically particularly in winter - and how strongly the heat losses increase in turn25) . It is precisely in December that solar gains are lowest while heat losses are high; in our case, they are not very high, because the building is very well insulated. In an uninsulated existing building these losses can be 5 to 10 times as high. The availability of solar radiation already increases again from February onwards – and then covers a large share of the losses.


1.Passive solar works, even in winter; however, for the solar contribution to actually be relevant in Europe, the building should be well-insulated.
2.A prerequisite is that users should also allow the sunlight to enter and should not block it, for example.
3.“Passive solar” in our case means a heating energy contribution of almost 200 W on a sunny day in December averaged over 24 hours. Compared to this, the 4 kW peak PV yield on the same day supplied an average output of around 50 Wel throughout the entire day. This is not a lot, even for us, but at least it may be enough for e.g. operation of the ventilation system26).

December 11th: Siberian cold forecast

Fig. 8: Some hoarfrost in the adjacent meadow.

But we haven't noticed much of that yet: the meadows in the area and the grass on the roof were covered with frost last night; these areas with very low heat capacities quickly radiate their heat content into the cold night sky and thus cool down even below the air temperature27). Incidentally, this is why the outer surface temperatures of many building components are often 28) colder than the outside air in winter - we will describe this in more detail in another article later. Despite this, the building components are warm on the interior surface - because hardly any heat escapes due to the good level of thermal insulation, and all surfaces towards the interior have temperatures that differ only slightly from the room air temperature (max. 3 K in the case of windows, less than 1 K in the case of external walls and roofs). These are very good prerequisites for a good level of thermal comfort: as long as the temperature level matches the clothing and activity.

Speaking of the temperature level: this now remains in the range between 19.7 and 21.3 °C; except for the room that we are currently purposely connecting as little as possible29), where temperatures can be down to 19.2 °C at times (upper floor NE). The electrical power consumption of the split unit, our only heating at the moment, is still below 400 watts on a 24-hour average.

December 12th: Quite comfortable today: the sun is shining!

Although the temperatures outside are below freezing - but the outdoor unit of the air conditioner30) can cope with this without any problems. Of course, the COPs 31) will then decrease - remaining at values significantly higher than 1, and the output of the unit is still more than enough to keep the whole house warm. The solar radiation on the southern side helps in addition, as has already been described in the blog for December 10th.

Especially those who understand a bit more about heating technology will certainly be wondering how it even works that a 156 m² end-of-terrace house can be comfortably heated entirely using only a mini-split unit with a maximum heat output of 3.6 kW which is mounted on the wall of a room in the ground floor. And scepticism is justified here, because such approaches with “heating using only one heat generator with heat emission on the ground floor”, which had already failed in the past, namely the “masonry heater” 32) that were often installed in the middle of the 19th century. The final verdict: certainly it was warm and cosy in the main room where it was installed (usually the living room), but on the other floors there was often not enough heat, even on the 2nd floor. And if there was, it caused strong draughts. Water-conducting central heating systems with radiators in every room then took over in Germany and 'the usual' active air handler systems in the US33) – these systems were reliable, comfortable and convenient 34).

Why should this be possible all of a sudden now? And that also with a mini heat pump (maximum 3.6 kW) instead of a stove with over 10 kW? The answer here lies in the considerable reduction of heat losses! In this building these are only about one eighth (12.5%) as high as in a typical existing building in the days when tile stoves were used. For this reason, much less heating capacity is needed - less than one tenth of the previous values, because the freely available heat is now a little higher due to the larger windows and higher electricity consumption nowadays. The available output of the mini-split heat pump unit is even twice as high as the maximum heating load that arises in the dwelling we discuss here.

The question that still remains is: if all the output from one source is released in just one room on the ground floor - how can the rest of the dwelling become reasonably warm?

Abb. 9: Fig. 9: Section through the Passive House building: the good level of insulation ensures that thermal coupling between the rooms is much greater than localised losses of heat towards the outside. This forces at least 'similar' temperatures to prevail throughout the house, at least on average in the longer term.

Fig. 9 provides the answer: the excellent level of thermal insulation of this building is decisive. Very little heat escapes towards the outside, as we have already discussed in one of the previous blog posts 35) . Almost half of this heat is replaced decentrally due to occupants and appliances and some via light entering through the windows - the remaining “residual loss” must of course also be met, otherwise it will become colder than desired at some point in spite of everything. In terms of the total output needed, the mini-split unit can easily manage this - and because it is installed on the ground floor, this energy can spread throughout the entire house in a completely natural way: via a phenomenon known as “warm air loop” that also occurs with space heating systems with radiators. The only difference is that here, warm air travels through the entire house from the ground floor into the top floor, as shown in the illustration. This is assisted by the fact that this building has an open floor plan: the central stairwell is completely open towards the single space ground floor consisting of the dining and living area with a centrally integrated kitchen. This works if the doors of all the rooms (which we want to heat) that lead to the stairwell are left open!

That this actually works can be followed in this publication: [Feist 2022] Heating using a split unit air conditioner. From experiences documented here in the blog for this year, it can also be seen that the temperature on the ground floor is about 1 °C higher than on the top floor for example. This is barely perceptible 36) ; depending on the priority it is then “slightly warmer” 37) on the ground floor or “a little cooler”38) on the top floor. We can choose the level at which this temperature difference 'rests'. In the past years, this was a comfortable 21 to 23 °C in winter. In this natural gas crisis winter we are now testing operation between 19 and 21 °C.

The “air movement” throughout the house should not be understood to be a kind of “warm air typhoon” rushing through the entire stairwell. Air velocities everywhere are below 0.15 m/s (!!) and are not perceptible . However, I can actually perceive that it gradually gets warmer as I come down the stairs.

What is crucial for an assessment is that in such a building with three levels, with just one unit this can naturally only work with an excellent level of thermal insulation to the Passive House standard, and even then only with an open floor plan. In a renovated existing building, however, solutions with two (or even three) such split unit air conditioners are conceivable. This will still be a fairly cost-effective solution. It may be surprising that such split units can still undertake quite a large share of the space heating in addition to an existing heating system, even in an older building. This is a serious approach to use in the gas crisis - because these devices are very inexpensive, installation is much easier to organise than a renewal of the entire heating system39) - and further devices may possibly be added later. Or, if the thermal protection is improved, then just one appliance will be sufficient for an increasingly larger share until the old heating system becomes obsolete40).

December 13th: Freezing cold

Fig. 10: Frost patterns – does anyone still encounter these

The cold has arrived, but only outside – on the inside it remains warm; and it would remain warm even if we couldn't heat. However, we can heat because the split unit heat pump runs without any problems, even when it's below zero outside. Of course, the coefficient of performance is lower then, but it is still well above 1. And electrical power consumption is still only around 400 watts; this is neither a problem for the regional grid nor for electricity generation, even if everyone did this.

First, let us go back to the outdoor climate: frost patterns are forming on the inside of our glazed porch on the north side of the house! On the inside surface of the single-glazing. Where is the humidity coming from? From the air in the glazed porch. Why is water present there? At the moment it is coming from the materials stored in the porch, which even now have still a bit higher temperatures (slightly above 0°C) – the moisture contained in the pores of these materials is evaporating. And also moisture from us, whenever we come through the porch, naturally we breathe out moisture. However, the interior surface of the single glazing has temperatures below 0°C. The air from this space cools down on this surface… and then can no longer hold so much moisture. In this case the surplus water remains on the surface as ice of course. This results in slowly growing crystal structures (Fig. 10). If you want to understand in more detail how this happens, you can find basic information here: Physics of humid air, particularly, what actually is the relative humidity of air?41)

The sun is also shining here today, and that is not a coincidence in Central Europe: the cold coming here is mainly due to the (Siberian) continental climate (high-pressure area!) encroaching on Europe. However, this air is quite dry - and for this reason the sky is clear, apart from ground fog. This makes it even colder42) for one thing, and for another it allows the sun through even if it is above the horizon for only a few hours. This then often fills a well-insulated building with lots of available passive solar energy. In such cold weather, no more heating than would be required on a cloudy day in November43) is therefore needed in most Passive House buildings.

PS (at 14:00): a clear winter day all throughout! And that brings so much solar energy into the inside that I have measured over 22°C in the study for hours now. Despite a set point temperature of only around 20°. Of course, this also increases heat losses (a little), but it's really very, very pleasant - “refuelling with heat” once in a while, and that in decidedly cold weather. The house also refuels with heat, which we notice in the following days when, despite the cold, not much electricity is required for the heat pump.

December 14th: The first snow

Fig. 11: Low sun hitting the southern facade in December
Fig. 13: This is snow – oh well!

First an addendum to yesterday's “sunbathing”. Fig. 11 shows what it looked like from the outside. This also reflects the mood well: the sun stands low even in the middle of the day44) . The radiation enters through the glazing because it is transparent for the visible part of the spectrum 45) . And with the light comes energy into the house. Some of this radiant energy is reflected by the surfaces it falls on inside 46) . Fig. 12 shows the “spot of light” entering the house through the glazing deep beyond the middle of the floor plan - 'passive solar' is by no means a new idea, it was even attributed to Socrates in classical antiquity (“Socrates' solar house”).

Does it bring energy into the house? Certainly; how much that is can be estimated somewhat from Fig. 13. At its peak (11:00 to 12:00), global radiation on the south façade was definitely over 800 W/m². The daytime duration is very short - on such a sunny day 47) this is 2 kWh/(m²d) in total. It would be nice if we could use all of this energy; however, even the most advanced Passive House technology is not fully able to do that; rather, only a quarter of that 48) passes through almost 20 m² of a south-facing glazing surface. 10 kWh per day is a deliberately rough estimate. We don't have to heat for all of the losses - although, the losses were still higher overall because of the cold weather. However, the split unit still didn't have to do much: an average of 425 Wel yesterday; and yes, this is more than twice the domestic electricity that is otherwise needed. So this will be a significant total load on the electricity network if heating is to be predominantly provided via heat pumps in the future, even if all buildings are energy-efficient. If they are not, then this load will be 5 to 10 times as high - and that will not be available so quickly for everyone. But efficiency and renewable energy go perfectly together: with the low loads we have here, this is feasible.

But now let us come to the “first snow”, if you want to call it that. OK, it was enough for a thin layer of powder on the PV panels on the roof. It will disappear again quickly in case of solar radiation, but there is very little today: it looks like a rather “dark day”, but the outdoor temperatures are still below freezing. A little wind is coming up, that will increase renewable electricity generation, so our heat pump will run with less initial CO2 from the electricity grid from now on49). The inside of the house, walls and ceilings have become warmed by about a third of a degree due to the free solar heat yesterday. We could see that in the morning - the measured values for the room temperatures on the south side were even a good half degree higher than those of the previous day. Over time, the heat is distributed throughout the house, but it is currently not enough to compensate for all the heat losses of the entire house - not even in this Passive House building. It then helps that these losses are much lower than in poorly insulated buildings - the 1000 watts that remain as a net loss can easily be covered sustainably in many different ways; one of these is the use of a heat pump, but it will only become truly sustainable when electricity supply from renewable energy is really expanded.

December 15th: A completely different topic: breakthrough in nuclear fusion

We should always keep the big picture in sight: the importance of the energy supply for our prosperity today should have become clear to everyone in recent months with reference to the latest energy crisis, this time relating to fossil gas. I appreciate a well-heated room - especially when it's really cold outside. How this can be done without releasing the last fossil carbon atoms back into the atmosphere in the form of CO2 is the core subject of these pages on Passipedia.

The fusion of atomic nuclei (such as hydrogen, deuterium or tritium) into heavier ones is a physical way of generating energy without carbon combustion 50). That nuclear fusion can do this no longer needs to be proven: stars do it all the time; indeed, it is by far the predominant source of energy in the entire cosmos today. We have also succeeded in doing that on Earth: Edward Teller was the one who advanced this - unfortunately it has just been limited to military “applications” so far.

The achievement of a positive local energy balance by LLNL is definitely an important breakthrough. And I want to be quite honest about this: this is a pleasing future prospect because I am sure that this technology will gradually develop further and open up completely new possibilities for people: interstellar space travel, for example.

Unfortunately, it will be quite a long time before this happens. The scientists at LLNL themselves have emphasised this. I don't have to explain this in detail, because Anton Petrov has already done that very well in his Youtube video: here is the honest scientific background to the latest breakthrough in nuclear fusion: Anton Petrov makes it really understandable.

One more thing: if fusion energy becomes available on a large scale in e.g. 30 years, it will be used to generate electricity. So it will be the same even then: heating buildings, fueling our vehicles… we will have to switch to electricity for everything - this is something we will have to do anyway, regardless of the primary energy source we use in the future. We are therefore well advised to switch to heat pumps, in the same way as has been described in detail here with the split unit, for example. Everything needed to make this work appropriately is also required - whether that is powered at the end by nuclear fusion or, for example, wind power. For the grid to accomplish this, significantly improved efficiency is pivotal, as we have described here and reported in our experiences.

The really good thing about it
This efficiency technology is already available for mass application - it is tried and tested and extremely cost-effective, even compared to fossil energy sources that are still sold cheaply. It can be used everywhere and anyone can get started themselves: structural measures for energy efficiency.

And what is the situation in the house right now?
For the loyal and curious followers of our blog, here is the data from the previous day: outdoor temperatures ranged between -2 and -8°C yesterday - it was continuously overcast, with not even 50 W/m² at the most on the outside of the southern façade. It was colder than average in December, but still “typical December weather”. Temperatures between 19.2 and 21 °C were maintained inside the house, and the split unit heat pump was in operation. It drew 461 Wel (from the electricity grid, for heating and comfort for a total of 3 people), while solar electricity generation was only 8.5 W on average. For comparison, here is the report for the average power consumption in Germany: about 880 W/person was drawn from the electricity grid, and unfortunately this was mostly fossil-fuel generated electrical energy 51). The gas consumption values have also been at peak levels for days, so high that the Federal Network Agency repeatedly had to call for more economic use: it was over 4 TWh a day 52). If we subtract the temperature-independent consumption (from August) of 1 TWh/d, the 24-hour average is around 1500 W per person in Germany 53). If approximately half of the heating is covered by natural gas, the overall heating load per person is no less than 3 kW in such a cold period in December. This adds up to a huge output, which makes the extent of the task transparent: if we want to have a chance at meeting the demand with sustainable energy sources, these heating losses must be reduced considerably. In a typical Passive House building like the one documented here, this is only a twentieth after all, and for this wind and sun as well as backup from biomass power plants are sufficient even in the winter, even “if everyone did this”54).

Goto December 16-31 Key words: climate gases; frost; performance in extreme cold period; indoor air qulity; detail: wall insulation performance.

at this time of year, we as users find temperatures occasionally over 24°C a decidedly pleasant reminder of summer; of course, I then take off my jacket or roll up my sleeves, for example, but I never find it uncomfortable.
Quite typical for December weather in Darmstadt
just more convenient
with a round 100W
even on such a cloudy day like today
Of course, initial operation is also a special “transient” situation. Since the temperature distribution in the steady state will be different in a few days from what it is now, some heat capacities will be discharged (partition walls, for example), others will be charged (e.g. the ceiling in the installation room of the split unit). Therefore measurement on a single day cannot be considered as completely typical - but the order of magnitude is already correct. We will provide more information later on
Heat losses through the windows, exterior walls, roof etc.
about 3000 kWh/a or just below 20 kWh/(m²a) with 3 persons basically “working from home”
while the output fed in by the PV system during this time is naturally rather small, on 5 December this was still 1.25 Wel on average over 24 hours
only in such a building
watts, converted to a 24-h daily average
Here it becomes clear that for renewable energy in the winter, a massive expansion of wind power is absolutely necessary; even 10 times as much PV would not help much in December. However, PV complements wind power quite well during the months February till October, so that its expansion is important.
Strictly speaking, the temperature above the roof is still slightly lower than that around the south-facing façade, and even inside the house temperatures differ a little from place to place. However, the differences are not very large compared to the difference between the inside and the outside. For this reason, for simplification it is quite possible to use the outside air temperature (around 3°C) measured using the radiation-protected outside thermometer halfway up the north façade with active air flow around it, and a living area-averaged inside temperature (now around 20.4°C
In essence correct in terms of physics, set out in ISO 13789. We have explained this in detail here: Building energy balance
This is an extremely low specific heat loss for a building with a living space of 156 m²; converted to the envelope area, this is about 0.2 W/(m²K) including ventilation heat losses. For typical new buildings, 3 to 5 times higher values are still achieved today, and an average existing building can achieve ten times that amount.
Hot water minus cold water
ultimately described by the solar heat gain coefficient sghc
Solar enthusiasts are usually very disappointed when they have to realise this for the first time: the main contribution to “bad” energy transfer are banal things like the angle of incidence, self-shading (e.g. by the window frame), dirt on the panes, back reflection (also from the room) etc. By the way: 'single glazing' doesn't really do it any better (maybe 30%), but then increases the losses by a factor of more than 7; the balance then becomes much worse, despite the higher g-value.
See blog for 9 December
However, in existing buildings with even poorer thermal insulation which often exhibit ten times higher heat losses, such an amount is of very little importance
E.g. for 'home office' work at the computer
In Fig. 7, these are the calculated values for the mean outdoor temperatures in the past decade, i.e. a “theoretical pre-calculation” (a priori). After the winter we will recalculate this with the measured boundary conditions (ex post) and then compare these with the measured consumption values
yellow bars
turquoise line with square symbols
this is probably the most important electrical device in this building.
this is the exact reason why there is frost on windscreens of vehicles, also
especially at night; not if the sun is shining directly on the surface, of course. In winter, the radiative cooling effect outweighs by far
interior door kept permanently closed
which now functions as a so-called 'evaporator'
now increasingly called COP 'Coefficient Of Performance' also in Germany
usually covered in tiles
that was, becaues thes can also wih the same system provide cooling durcing summer
… well, the consequence was a huge increase in energy consumption
and even assigned a value: around 1500 watts
Unfortunately this is also difficult to measure, and simulation (computational fluid dynamics) still has a hard time with such things (thermally induced free flow). We will go into this elsewhere.
take off the sweater
but only if the sun didn't come out at all: put on the sweater again!
Especially when there is a lack of skilled workers
Oh yes: these devices can also provide cooling of course. That's what they were originally developed for.
everybody 'seems' to Know this, because it's in the news a lot - but, there is a surprise: quite often people get this wrong
high long wave radiation emission into space
even though this is not that cold at all
The hazelnut tree had already bloomed 'by mistake', but has now lost almost all its leaves. Did someone just say we could have cleaned the windows at least? They're quite right…
Sure, that's what they're there for, we want to look outside. The rows of houses here are just far enough apart so that some sunlight can still enter the first floor in December - that's what the city planners had in mind at the time. Interestingly, this still results in a high construction density, precisely because of the rows of terraced houses, each with a large floor plan depth.
And thus distributed throughout the space, where it is ultimately absorbed by other surfaces, sometimes after being reflected several times. Except for a small part, which simply goes out again through the glazing. That is why, for example, the lemon on our little tree can also be seen from the outside.
d=day, kWh per square metre and day
at least that!
I'll explain what this balance finally looks like in a future article here.
One of many ways: just like wind energy, solar collectors of all kinds and also fission energy.
Only 26% renewables - a winter wind power lull that has been going on for a few days now; but more on that some other time. For now, it is obvious how important it is to reduce the average load of the heating systems.
TWh = Terawatt-hour; T=1012=1 000 000 000 000
continuous load in winter for heating, here just for natural gas alone
better: only if everyone did that with at least an EnerPHit certified retrofit – see the link given above. “Insulate instead of freezing” is the cue here, and indeed, everyone can contribute here according to their capabilities
examples/residential_buildings/multi-family_buildings/central_europe/the_world_s_first_passive_house_darmstadt-kranichstein_germany/kranichsstein_blog/december_1_2022.txt · Last modified: 2023/10/10 17:38 by wfeist