This article is based on a paper published in the 56th volume of the Research Group - Energy efficiency and renewable energy: Conflict of goals or synergy?. The original article by Wolfgang Feist is availabe in German in the proceedings, which can be accessed here.

Fig. 1: Renewable energy, on the right: visible to all, on everyone's lips. Energy efficiency, on the left in the picture: effective, inconspicuous, hardly noticed.

In the scientific context, it is clear that the world must free itself from dependence on fossil energy within a few decades. Very small amounts (a few single-digit percent of today's fossil fuel consumption) can still be borne by the atmosphere for some time - but in about 50 years [IPCC 2018] we will have to achieve a good 90% reduction in CO2. Key decision-makers are already questioning the feasibility of such a target.

For some time, built and scientifically monitored projects have been demonstrating that these goals are achievable. In the course of the contributions to this working group, it will become clear that the combination of renewable energy and energy efficiency even offers comprehensive advantages and significant economic, social and ecological opportunities.

This will be illustrated using the Passive House in Darmstadt Kranichstein, which has been in use for 30 years by four families (Fig. 2). Energy can be supplied entirely on the basis of electrical power – in this way it will be possible to build up a completely renewable energy supply system (see Section 10 in this article). image: Efficiency and renewables: measurement in a PH Plus Passive House Plus in Kranichstein Measured consumption values: Heating energy consumption: 10.2 kWh/(m²a) Useful heat consumption for hot water: 10.2 kWh/(m²a) Annual consumption domestic electricity: 11.3 kWh/(m²a) Heating: mini-split unit heat pump 2 kW Hot water: Split-heat pump 0.5 kW 260 litres PV system: on roof and façade (26.6 m² 4 kWp). The seasonal distribution of the energy consumption in the building is decisive: the relevant results are reproduced in Fig. 3 [Feist 2019].

Fig. 2: Brief characterisation of the world's first Passive House building; inhabited by 4 families since 1991, all terraced houses are certified Passive Houses, the components are systematically described e.g. in [Feist 1997] and [Feist 2019]; the heating demand is around 10 kWh/(m²a). The end-of-terrace house has been heated using a split unit heat pump for the past 4 years; the building is now a “Passive House Plus” due to a PV system with a total of 4 kWp.

Efficiency: measurement in PH Kranichstein Jahresverbrauch Heiz-Split WP…=Annual consumption for heating with split unit heat pump: Strom Heizung=Electricity for heating, WP-Strom für Warmwaser= HP electricity for hot water: Jahresverbrauch WW-WP…=Annual consumption for hot water with heat pump Jahresverbrauch=annual consumption, Haushaltsstrom (Licht…)=domestic electricity (lighting, refrigerator, computer etc.) Strom (Erzeugung bzw. Verbrauch)= Electricity (generation and consumption)

Fig. 3: Monthly profile for electricity consumption in the world's first Passive House (heating/domestic electricity measured values); average values for the last three years of operation in the western end-of-terrace house which is heated electrically with a heat pump. The total consumption for the whole year is also given (domestic electricity: 11.4 kWh/(m²a); DHW heat pump electricity 4.4 kWh/(m²a); electricity for heating with split unit 4.5 kWh/(m²a)).

Three main insights emerge here:

• The respective consumption values are extremely low; e.g. 86 % below current common values for heating in new constructions in Germany.
• With the high level of efficiency of the building envelope, the consumption of domestic electricity predominates here. This electricity demand is relatively evenly spread out over the year with only a slight additional demand in winter (particularly due to lighting).
• Although the electricity demand of the heat pump for heating is small (4.5 kWh/(m²a)), it is distributed very unfavourably throughout the year: consumption is primarily concentrated on the months of December and January. This is not surprising, because that is when the lowest passive solar heat gains are available and the coldest outside temperatures prevail at the same time. We refer to this as the “winter peak” in the energy demand.

In conventional buildings, the winter peak is even more pronounced, especially since heating accounts for more than 70 % of the total consumption - and in the peak months heating consumes 7 to 10 times as much as was measured here in the Passive House.

For renewable energy supply, the installation of a PV system is obviously ideal precisely because Passive House buildings require very little electricity for heating due to their high level of efficiency. Such a system was installed in autumn 2015 on a total area of approx. 27 m² (4 kWp) of the end-of-terrace house on the western side; Fig. 2 shows the distribution of the panel surfaces on the façade (1/3) and roof (2/3). While the façade collectors have an optimal winter orientation, the monopitch roof is slightly tilted towards the north and therefore is not really optimal. However, the situation here is not uncharacteristic for retrofitting with PV systems in existing buildings developments; the system here is at least largely free of shading.

Fig. 4 shows the measured PV electricity generation (mean value of years 2016 to 2019). The measurement results are not surprising; they indicate the following:

• PV powergeneration functions reliably and largely delivers the yields that were expected from the system. The total annual yield of 19.9 kWh/(m²a) is almost as high as the total annual electricity consumption of 20.3 kWh/(m²a) (the reference area here in each case is the 'treated floor area' according to the [PHPP 2015]; this area corresponds quite accurately with the heated living area according to the German living space ordinance). Apart from a negligible difference, the Passive House in Kranichstein is already a “net zero-energy house” simply with this system alone.
• As expected, the PV generation has a strong yearly cycle: while the electricity yield is high from April to September (at least 400 kWh/month corresponding to a continuous output of more than 500 watts), it drops sharply in the transitional periods and especially in winter (in December/January below 25 kWh/month, corresponding to less than 33 watts).
• This yearly variation in generation does not at all match the yearly variation in the demand, which exhibits a significant peak especially in winter.

If we optimistically (and for the future; also realistically) assume that due to advancements in storage technology (especially increasingly cheaper battery storage; see also Section 7), the balancing of the generation profile and the load profile within a few days (around 50 cycles per year) would be reasonable in economic and logistical terms, then according to Fig. 4 the entire electricity demand of the building from March to October can be fully met with the PV system installed on the house. Significant surpluses are even observable from April till September. In contrast, solar power generation is not even sufficient for meeting the domestic electricity demand from mid-October to February, contribution of the PV system towards heating is practically non-existent.

Efficiency and renewable energy: measurement → net zero Strom Heizung=Electricity for heating WP-Strom für Warmwaser= HP electricity for hot water, PV strom= PV electricity, PV stromerzeugung= PV electricity generation Summe Stromverbrauch =Total electricity consumption Strom (Erzeugung bzw. Verbrauch)= Electricity (generation and consumption)

Fig. 4: Monthly profile of electricity generation by the PV system (average values of the last four years of operation in the western end-of-terrace house). The total generation is 19.9 kWh/(m²a). This compares to an annual requirement of 20.3 kWh/(m²a) - “net zero” is almost exactly already achieved with the PV system; the residents also used wind power plants on a public participation basis (see next figure).

• The energy demand for heating and generation available for this through PV is practically disjunctive. PV and energy efficiency of the building envelope are therefore not competitors at all, but rather they complement each other entirely. The lower the heating demand, the higher the proportionate coverage of the total energy demand of buildings via solar energy can be.

Although this installation makes the property a “net-zero” energy building, about half of the electricity still has to be made available from other sources (Fig. 5)). As long as significant parts of the electricity generation still come from fossil energy, especially in winter, the “net zero” designation is misleading for users: neither is the consumer self-sufficient, nor is the supply at this level particularly environmentally friendly. It should be noted that as yet the European power grid does not have seasonal energy reserves for renewable energy that are even remotely adequate.

Fig. 5: Lack of electricity in winter (the so-called winter gap). Just under 50% of the electricity actually comes from the building's own system, the rest still has to come from the grid, especially in winter (grey area). Stromlücke=electricity shortfall

Of course, this can and must change in the future - one prerequisite for this is that the property remains connected to the public power grid for the time being. This has several advantages:

Surplus electricity in summer can be fed into the grid and thus made available to other users; industrial consumers with daily peaks and accumulators for electrically powered vehicles are useful here.

• In addition, the creation and use of future storage facilities will be more cost-effective and better regulated.
• The supply of electricity in the winter will ensure that residents do not have to bear the cold and can also obtain all other energy services. The peak power drawn from the grid with this concept has never exceeded 1000 watts on an hourly average. If necessary, this could even be reduced to less than about 750 watts through deflection of demand, especially for heating; in terms of power requirements, this would not overburden the grids, even if everyone did that.

However, in such a case, the additional electricity supply in winter will still mainly be provided through fossil energy. Some improvement is possible if existing wind power plants in the power grid are preferably used for winter coverage (see Section 6).

An idea that is particularly popular with many non-experts is to bridge the winter shortfall by means of energy storage facilities. In the last few decades this approach has been pursued several times using various technologies:

• Seasonal heat storage ([Kriesi 1990], [Fisch 2005], [Colclough 2015]): it is easy to calculate how large such a storage facility would have to be to keep a conventional building warm throughout the winter: it would have to be larger than the house itself. For this reason, a prerequisite for such a thermal storage system to even come close to providing an appreciable amount of coverage is that the building's heating demand should already be within the considerably better efficiency range of a Passive House building. We carefully implemented this in a pilot project in Dörpe, and then tested and published the findings [Hinz 1994]. Jürgen Schnieders will discuss such seasonal heat storage systems in more detail in his contribution to this Conference Proceedings.
• Anergy storage (anergy is the part of an energy flow that complements exergy; according to the laws of thermodynamics, exergy is the maximum amount of work that can reversibly be recovered from provided energy [Exergy 2020], [Buchholz 2009]): as ice storage or ground storage under the floor slab, not directly for heating, but as a heat source for a heat pump - the coefficient of performance of which is noticeably increased in this way to reduce the electricity demand even further. In the case of a new building, this can be done for example by laying some pipe loops (PE tubing) under the insulation of the floor slab of the building. In the following summer, the extracted heat is replaced again by waste heat (from a solar thermal system or from the condensation heat of a heat pump used for cooling). Mr. Linnig will analyse this type of storage in more detail in his contribution to the Conference Proceedings. The advantage of this is that it does not cost much, since the ground under the house is already “present” in any case. According to earlier publications which documented failures in this respect, most were due to a much too high heat demand of the buildings being supplied [Schwarz 1981], which simply could not be gained from the limited heat capacity of the ground - the winter temperatures of the pipe loops dropped to well below 0 °C, which could even have endangered the statics of the building. However, the heat demand of a Passive House is about 3 to 4 times lower than was the case in these early experiments. Today, such approaches are successfully used in Passive Houses.
• Electrical energy storage: this is where the exergy is stored so that the heat pump for heating can still be operated in the main winter months. Often there is the concept of making this possible through decentralised accumulators (e.g. Li-ion batteries). In Fig. 6 we illustrate this approach with a wall-mounted electricity storage element with a capacity of 13.5 kWh that is commercially available today (2020). Since 1,573 kWh are needed to cover the gap still existing here, 117 such wall-mounted storage units would have to be installed for this building alone. It is clear that this is out of the question, even with an extreme drop in the price of accumulators. In order to make progress in this direction, the demand must first be reduced further (which would be possible through even better insulation and more efficient heat pumps, see previous point) and coverage in the winter must be improved through directly generated renewable energy (which is also possible, see next section). However, even then it still seems wiser to allocate such exergy storage to the electricity grid as a whole and not to individual buildings.

Fig. 6: Determination of the electricity storage capacity for seasonal coverage: because the capacity is needed just a little more than once a year (referred to as seasonal storage), it must be able to fill the entire gap of 1,573 kWh. More than 100 large Li-ion wall accumulators/powerwalls would be needed for this purpose – apart from the space demand, this is not economically feasible

• Biomass storage: a stack of wood with over two cubic metres would be able to cover this winter gap. That is so little that it could be worth considering this for Passive House buildings – however, in view of the overall availability of wood and biomass, it is doubtful whether its use for heating can and may play a role at all in the future – if wood is used for energy then this must increasingly occur to an extremely high exergy level, e.g. in heat and power plants with which significant amounts of electricity become available in the winter. B. Krick and O. Kah will discuss biomass utilisation in their contributions to this volume of the Conference Proceedings.

Since individual storage facilities for buildings prove to be (unrealistic) /non-perspective (except for the anergy type), everything points to connection of the building to the existing power grid. This has several advantages at the same time:

• Enough electricity will always be available in the winter; the 1573 kWh that is missing so far will never go beyond an output requirement of approximately 1 kW – the network will always provide this output (for everyone), also generation capacities of this scale are not a problem (this is approximately 15 GW in total). However: currently this output in winter is largely provided from fossil and nuclear energy. This would have to change in the future – and this aspect will also be addressed in this Research Group session.
• Electricity is the easiest way to bring renewable energy to the consumer. It is an already existing network, the outputs in the grid are sufficient (see first point: at least if the connected buildings have the EnerPHit standard as a minimum). Some surplus wind power will even be available in the winter - but only because electricity so far only has a small share in the heating market ([Reinwald 2018]). If we want to change this in the future, then the winter peak which presently affects mainly natural gas will increasingly extend to the electricity supply. Significantly higher wind power capacities will then be required.
• If the building is connected to the grid, then the excess electricity generated by the PV system in the summer can be fed into the grid for other consumers - preferably for industry and as electricity for charging electric vehicles. By offering this to all consumers, PV power generation can be operated far more cheaply. Synergy with other renewable energy generators will be drastically improved – as well as the uniformity of solar power supply due to the wider dispersion of generators regionally.

The integration of storage facilities is also much more efficient at the grid level; initially, it is possible to indicate the direct use of the electricity fed into the grid, which saves storage capacity and storage losses. The most favourable storage facilities respectively can then be charged or the required electricity can be generated from the most favourable storage facilities.

In this specific case, we now add wind power from a plant operating in a windy area: Fig. 7 shows that the wind power can now largely cover the winter gap for the household and even cover the domestic hot water demand. However, even now the heating demand stands out neatly from the otherwise matching profile. Less than a quarter of the electricity is not generated in a timely way from renewables in this situation. This could be further optimised by cleverly integrating hydropower and biomass power plants to a certain extent, as well as through load shifting. The winter gap would ultimately be less than approx. 14 % - how this gap could be filled through renewable energy is the subject of a contribution by Jessica Grove-Smith in this volume of the Conference Proceedings. The keyword here is PER - 'Primary Energy Renewable', a sophisticated system for complete supply on the basis of renewable energy globally (see also [Grove-Smith 2015]). Only the absolutely necessary part is obtained from seasonal storage, because obviously this is the most expensive of all alternatives; but it would be even more expensive if high non-usable excess capacities in wind energy and PV remained idle for longer periods of the year, which inevitably happens if there is no seasonal storage at all.

We therefore need to briefly look at the prospects of seasonal storage - based on elementary physics.

Fig. 7: Grid connection and inclusion of the wind power percentages; ideally, building owners can acquire a corresponding share in a wind turbine. The winter shortfall (outlined in bold) is reduced to less than half as a result of connection to the grid, mainly because now wind energy can also be easily integrated. Even more can be achieved through smart optimisation - simulations show that a shortfall and thus a power demand for electricity made available from long-term storage can ultimately be reduced to less than 1/7th of the electricity workload. Of course, the output of the backup systems must still be high, which accounts for part of the high cost.

“Energy storage” is a technical term - actually a double designation. The physical quantity “energy” in itself describes the work capacity “stored” in a system.

A system can store this work capacity based on different mechanisms; each of these mechanisms is based on at least one of the four basic physical interactions: gravity (including inertia), electromagnetism, weak and strong nuclear forces. Mechanical storage uses the laws of inertia (e.g. flywheel masses) and gravity (pumped storage plant), while electromagnetic storage uses attraction and repulsion of charges, which usually become effective at the chemical level. Direct electromagnetic storage, such as superconductive storage rings, is actually under discussion for ultra-short term storage with huge capacities - but it cannot be taken into consideration for long-term storage. For the foreseeable future, nuclear energy storage is beyond the technical feasibility of human civilisation (Fig. 8).

Of the physically possible storage systems, some technically feasible ones have been in use for many decades in practice. Established systems for storage on a seconds, hours and also daily basis are successfully used in the energy sector and in industry - flywheels for brief power peaks, pumped storage power plants for daytime balancing in the electricity sector; this technology is still noticeably cheaper today than the cheapest batteries currently available. The expansion of pumped-storage plants in Europe can still proceed strongly in Europe, especially in Austria and Norway. Against this background, concerns about bridging a couple of hours or even days are certainly out of place; concepts with large flywheel masses (inertia train on magnetic levitation circuit in an evacuated tunnel, see [Powell 2010]) are also under development.

Reversible electrochemical storage systems (accumulators) are power sources which have been adopted on a wide scale, especially for mobile applications. They are used with storage cycles in the range of a few hours to a few days.

Thermal storage facilities are also widely used - storage cycles of about one day are common (e.g. for domestic hot water). Storage systems based on phase transitions are generally more expensive than simple water storage systems - a construction principle that relies on very inexpensive materials and decades of experience.

Fig. 8: Physics enables many different energy storage systems based on the four elementary interactions, inertia and thermodynamics. However, miracles should not be expected here: the storage types that are possible in principle are all known and have been researched intensively for decades.

All these technologies can be used technically and economically for a storage period of perhaps a few weeks – provided that 30 to 50 or more charging and discharging cycles (called storage cycles) are possible per year. If the number of cycles is less, then economic efficiency will be dramatically reduced: investment costs will remain almost the same, but the respective storage system can only supply energy less frequently and thus generates considerably lower yields.

This is the main reason why most of the in principle available and otherwise tried and tested storage facilities are not suitable for seasonal storage: flywheel and pumped storage energy would be in the price range of several euros per kWh if the respective storage facility were only loaded and unloaded once a year (Fig. 9). An exact analysis provides only two realistic alternatives for expensive but possibly still affordable seasonal storage: hydrogen or, more likely, methane generated from it.

The recharging of old natural gas fields with methane produced from renewable sources and using the combustible gas stored here when it is needed is technically not a problem at all. We already make use of this system today, for example by storing Russian natural gas all year round (even in summer, when demand is actually low) as a reserve for the following winter. These storage facilities are large enough to allow future loading with biogas and synthesis gas generated from surplus electricity (PtG: Power to Gas) to an adequate extent. With additional infrastructure costs of around 10 to 12 cents/kWh and at least 60 % conversion losses, this is not a particularly economically attractive approach - but this system remains financially viable if the total amount of energy to be stored in this way remains within reasonable limits: additional costs of around 18 cents/kWh are then quite acceptable if they account for less than about one sixth of the generated energy.

Fig. 9: The two realistic technological approaches conceivable for seasonal storage: Fuels derived from renewable sources, in the simplest case hydrogen or methane synthesised from it; the storage facilities for this are already available and are already being used: these are old depleted natural gas fields where methane is stored in the summer and retrieved in the winter; the storage process itself is virtually without losses. Seasonal heat storage in the ground is also possible in principle, but is more expensive and with higher losses.

Fig. 10: Here's how it works in principle: approx. 50 % of the renewable energy can be used directly by the consumer. Short-term storage, such as pumped-storage power plants and comparable systems, can adjust demand to generation over the course of an hour or even a day. These systems have hardly any losses. Austria, Norway and Switzerland have enormous potentials in this respect - which in a properly functioning EU can be integrated for everyone's benefit. In the European network, more than 80 % of the increased electricity demand (for electric vehicles and a major part of heating via heat pumps) can be covered in this way. The rest must come from biomass, reconverted RE gas or/and a small remaining share from natural gas. The RE gas is produced from surplus solar or wind power by a process called “Power-To-Gas” (electrolysis of water to obtain hydrogen and oxygen). With the Sabatier process [Sabatier 1913], the hydrogen is converted into methane by binding it to CO2 because it is easier to store in already existing caverns.

The level of energy efficiency necessary for a successful energy transition is considerably higher than is usual today; many sectors can contribute with efficiency technologies, which are mentioned here only briefly:

• Transportation: electric traction has higher efficiencies compared to internal combustion engines with the basic approach already. Since electric motors can also be used reversibly as generators, braking energy can also be recovered through reversible braking. Furthermore, electric drives with modern control technology for electronically commutated DC motors (also: “brushless DC motors”, English: ECM) can be adapted considerably better to partial loads [ECM 2020]. The efficiency potential alone for the conversion to electric traction is therefore very large (> 50 %). The possibility of using renewable energy is always the main motive for the changeover - this is also quite correct, provided that power generation from fossil energy is reduced expeditiously.
• Other drive systems: many existing drives still operate with older asynchronous motors with poorer efficiencies and above all very unfavourable partial load behaviour. In the course of renewal of facilities, they can be replaced by modern ECM motors. Here, too, there is considerable potential for savings.
• The utilisation of heat in industry uses many processes in which materials are brought to high temperatures - in the process, they absorb heat which later has to be cooled out of the end product again. There is an efficiency potential here, e.g. in the form of counterflow firing kilns. Alternative process control options, reduction of co-heated auxiliary materials and waste and packaging materials can also help to improve efficiency.
• Artificial lighting accounts for an increasing share of energy services. Due to now much improved lamps (LED light can now be generated with around 150 lm/W [lumens/watt]; incandescent lamps are as low as 12 lm/W; Fig. 11), the amount of electricity required for lighting has dropped significantly in recent years. Better access to daylight, a functional architectural task, brings further contributions.
• There is much discussion about the still growing demand for electricity for IT applications. Even modern digital systems are often still shockingly inefficient: a good notebook, including display, requires less than 25 W for normal operation. A desktop PC consumes three times that amount – then there is the monitor which consumes another 30 watts. The latest electronic ink technology makes it possible to reduce the power consumption of flat screens to around less than 1 W. MOS technology would also have to become mandatory for desktop PCs and server electronics. Power management software can decrease clocking frequencies at low capacity utilisation, saving electricity significantly and extending the life of the systems.

Fig. 11: The energy efficiency of artificial lighting has steadily improved over the centuries: but it is only with the latest technology (LED lamps) that we are approaching the physical limits of optimal efficiency. 3.5 watts per person are thus enough to provide an optimal amount of light for reading comfortably (at night time) - this is so little that it can easily be supplied by a renewable supply system (less than 300 MW for all of Germany; source: lecture “Energy Systems” W. Feist, University of Innsbruck, 2019).

Kunstlicht= artificial lighting, Quelle Kerze…= candle, pressurised mantle lamp, incandescent bulb, CFL, LED lamp, best LED on the market in 2019, optimum LED in terms of physics 2025?),Energiedienstleistungen mit immer niedrigerem Energiebedarf =energy services with increasingly lower energy demand, Einsparung= saving (construction)

The overview given here is far from complete. Improvement in energy efficiency offers huge potentials - but a systematic implementation strategy for applying this is still lacking. Noticeable progress has been made only in respect of the energy efficiency of buildings, thanks to the commitment of research, development, architectural design and further training of skilled professionals on a broad scale. The Passive House concept plays a key role here.

The Passive House concept is a documented and detailed planning and quality assurance concept that shows how really comprehensively improved energy efficiency of buildings can actually be achieved [PHI 2016]. It is based on a reliable forecast of the heating demand of the planned (or to be converted) building using the project planning and energy balance tool PHPP ([PHPP 2015], [Dermentzis 2016]). The building must be optimised in such a way that the heating demand does not exceed the limit value of 15 kWh/(m²a). This is therefore a purely functional requirement that is geared to the actual performance of the building. The remaining demand is so low that a fully sustainable supply system is possible in the long run (see the results from 30 years of experience in the first Passive House presented in Section 2).

In order to provide architects and planners information about target-oriented measures, especially for their first projects, the most important measures that allow the standard to be achieved are summarised here in brief (Fig. 12):

• Significantly improve thermal protection of the building envelope (U-values around 0.15 W/(m²K) for opaque areas in Central Europe [Feist 2007])
• Passive House windows (in Central Europe these are insulated window frames with triple glazing, separate edge seal and a certificate from an independent body [AkkP 14])
• Thermal bridge free design (this is a planning principle which can be applied by architects and construction engineers even without calculation if need be; achieved most easily by selecting certified components [AkkP 16], [AkkP 35])
• Airtight design (“red line planning principle” [Elmroth 1983], [Peper 2005])
• Balanced home ventilation with highly efficient heat recovery [AkkP 17]

These principles have been dealt with several times in the Research Group and extensively documented. A constantly updated description can be found on e.g. [passipedia 2014].

Fig. 12: The five principles for successful energy efficient construction in Central Europe: thermal insulation, highly efficient windows, thermal bridge free design, airtightness, and hygienic heat recovery ventilation.

Improvement measures for pursuing these principles do not require a new construction method or fundamentally altered building components; instead, all methods of construction (including, for example, timber constructions, steel constructions, solid constructions) are still possible, including all conceivable architectural designs. The improvements relate to details of the components that are necessary in any case - which are easy to manufacture in an improved form and hardly involve any additional effort. The essential requirement is careful planning of the decisive details right from the start.

As a result, Passive House buildings can be realised with very little additional investment, provided that the components are available and planners have the necessary knowledge; this also applies to the “additional initial expense for ecological reasons” (e.g. more glass due to triple glazing), which can even be avoided completely if the materials are chosen carefully.

Fig. 13 illustrates this based on the example of costs of the opaque envelope components. These are an additional €0.75 to €2.50 per square meter of component area and cm of insulation thickness, which translates into approx. 4 to 6 cents per kWh of heat saved. This is already significantly more cost-effective than heating with fuel oil or natural gas. The use of the insulation levels shown here is therefore strongly recommended for every new building for economic reasons alone.

Fig. 13: Economic efficiency of thermal insulation of opaque envelope areas (according to [AkkP 55]).

Economic efficiency of insulation of the roof, wall and basement Dämmung= insulation, gespart kWh =saved kWh, Dach=roof, Kellerdecke=basement ceiling Wärmedämmung verbessern…=improving thermal insulation to the NZEB standard: already economically effective today by 4 to 6 cents/kWh: cheaper than gas and oil

The costs for transparent components are summarised in Fig. 14; these amount to a total of around 50 €/m² of window area, including triple glazing, thermally separated edge seal, improved window frame, and thermal bridge free and airtight connection details ([Krick 2015]). This is the equivalent of less than 5 cents per kWh of heat saved. These improvements are therefore recommended to the full extent in view of the current energy prices.

The purely planning-related measures such as “thermal bridge-free design” ([AkkP 16], [AkkP 35], Fig. 15) and planning of “airtight building envelopes” ([Peper 2005]) are even more attractive in terms of economic efficiency. An up-to-date handbook with numerous tested and documented details is shown in Fig. 16 ([Hazucha 2016]).

For those designers who do not want to deal with the technical details, there are also pre-tested details made available by this sector. These are subject to careful quality assurance for the “certified Passive House suitable component” certificate. More than 1000 products are now available from a wide range of manufacturers all over the world. Passive House components are available on the market - and they already come with all the crucial connection details and application guidelines (Fig. 17).

Fig. 14: Passive House window – in view of the offers on the market nowadays and the market prices for heating energy, this better quality is recommended in general.

NZEB suitable window, triple-paned, warm edge, insulated frame, installed in a thermal bridge free way, in Deutschland= in Germany

Fig. 15: Instructions and tools for thermal bridge free design [AkkP 16], [AkkP 35]

Construction details thermal bridges and thermal bridge free design Research Group for Cost-effective Passive Houses 35 Thermal bridges and planning of load-bearing structures – The limits of thermal bridge free design (and other Protocol Volumes)

Fig. 16: Construction details in practice – in particular airtightness and absence of thermal bridges [Hazucha 2016]

Construction details All construction methods are possible: concrete, masonry, aerated brick, solid timber, timber frame, formwork elements, straw)

Fig. 17: Passive House components: the key to ensuring the building quality in relation to energy efficiency; website: database.passivehouse.com/de/components/

The approaches presented here are not new - on the contrary, since the late seventies of the last century, scientists have already been pointing out the importance of renewable energy and improved energy efficiency ([Bossel 1979], [Lovins 1978], [Shurcliff 1981]). Certainly, these analyses were followed up in some industrial developments and also by some governments. However, they were never systematically developed. Even so, energy efficiency now accounts for the largest single supply proportion of all “energy carriers” (in the sense of [Meyer-Abich 1979]: “energy efficiency as an energy carrier”) in Germany and renewable installations already provide almost half of the generated electricity.

Fig. 18 shows the success of the “Energiewende” (energy transition) in Germany; we will discuss the progress made between 2000 and 2016, since the ten years 1990-1999 were shaped by the economic peculiarities of the reunification of Germany:

• Between 2000 and 2016 in Germany, the energy service increased by 22 % (more living space, higher room temperatures, more cars, more kilometres travelled, more products consumed; top curve).
• Nevertheless, during the same time period the non-renewable primary energy demand decreased by more than 19 %.

Fig. 18: The success of the energy transition in Germany: synergy of energy efficiency and renewable energy

Fossile=fossil fuels, plus nuclear, completely inc. renewables, energy service Wachstum der Energiedienstleistung (BIP)= growth in energy service (GDP) Effizienz Verbesserung=improved efficiency, Primärenergie Deutschland=primary energy in Germany -28% with consumption of non-renewable energy, Between 2003 and 2018 (15 years) this equates to -1.9%/a This would take around 50 years, the following is necessary: Continued commitment, more is better, but above all: Recognition of success and positive communication of the same

Thus a 19 % reduction in consumption brings a 22 % higher performance. The 33% improvement is due mainly to increased efficiency (light green area) - vehicles have become more energy efficient, new buildings are better insulated, some (too few) existing buildings have even been retrofitted, old industrial plants have been renewed, more efficient power plants have been put into operation, old household appliances have been replaced by significantly more efficient new ones, lighting has been replaced by CFL and LED, etc. Material efficiency has also been improved, the recycling rate has increased. A certain structural change also plays a role: electronic dissemination of information requires fewer non-sustainable raw materials such as tons of newly printed paper every day (although considerable savings could still be achieved here); garden gazebos, electric bicycles and cell phones are less energy-intensive than fighter jets or heavy industry production facilities. However, efficiency contributes enormously: hardly perceived by the general public, its share is now higher than that of crude oil or natural gas in the supply of energy services.

Renewable energy sources also account for an increasingly larger share of the reduction in the primary energy demand (dark green area). The non-sustainable part of the primary energy supply has been reduced by more than 33% during this period (fewer non-renewable energy resources of all kinds: hard coal, lignite, oil, natural gas, uranium). That's about 2% less non-renewable resources mined for each of 16 years. A major success!

If Germany can continue this trend, maybe even put a little more emphasis on it - then the country could do without non-sustainable energy sources within 34 years.

It is striking that currently this development is communicated in an almost uniformly negative way:

• One group is declaring the energy transition a failure because this wasn't their intention in the first place, and is calling for a return to increased expansion of unsustainable energy sources. The supposed problems are widely communicated to the public as “failed (efforts)”. Publicly and with the support of some media outlets, calls are made for the expansion of gas terminals, new opencast lignite mines and additional fossil and nuclear power plants due to electricity that is allegedly too expensive in Germany (for which the expansion of renewable energies is blamed).
• For others, the pace (“only minus 2% per year”) is not fast enough. They, too, are quick with the analysis that the “energy turnaround has failed”

This unholy alliance of bad-mouthers is grist to the mill of the opponents of a sustainable energy transition. Attempts are being made to turn this clearly visible success into the opposite by stirring up public opinion.

The consequences of such irresponsible actions are fatal: the German economy is right in that the quantitative significance of CO2 emissions in Germany is very small in view of the antics of China and India with regard to fossil fuels, but how can anyone present a convincing case to China and India if their interlocutors rightly point out that we have to go down the sustainable path ourselves before we can convince others to do so? And there is also the fact that the German press itself constantly reports more about supposed problems with the energy transition than about its obvious success. Spreading word about the positive experiences would be the most important impulse we could offer the world: a successful path that leads away from unsustainable energy sources, and constructive communication about this.

In this regard this can work with the combination which has already proven thoroughly successful in the past 20 years despite all the reluctances and resistances: the combination of renewable energy and significantly improved energy efficiency: a dream team.

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Building design for the renewable energy future: Primary Energy Renewable