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 byJürgen Schnieders, is availabe in German in the proceedings, which can be accessed here.

The Passive House standard primarily stands for energy efficiency. However, Passive House principles were combined with the use of renewable energies very early on. This may go to such an extent that the building no longer needs to be connected to the gas or electricity grid.

In this article, the current climate protection targets (as of May 2020) in the German state of Hesse and the EU are summarised as a basis. Against this background, some concepts will be explained in which Passive House buildings were equipped with renewable energy sources - some already in the last century. Other recent examples can be found in other contributions in this Protocol Volume. In this context, an initial look at the use of solar thermal energy and especially seasonal heat storage should not be omitted.

The Federal Republic of Germany and the EU have set themselves the goal of drastically reducing their greenhouse gas emissions in the coming decades. Targets for 2020, 2030 and 2050 are clearly defined, figures can be found in Table 1. These goals are generally defined in relation to the status in 1990. For some time now, Germany has been aiming for a reduction of 80 %₁₉₉₀ by 2050. After the Paris Agreement where the urgency of climate protection became clearer to countries, a tougher goal of -95 %1990 was agreed. The difference is quite significant, as is also made clear in the article on coupling of sectors in this Protocol Volume, because the still permissible emissions differ by a factor of 4.

Similar reductions are also being sought at the EU level in the long term. At present the goal of “climate neutrality” has already been proposed for 2050 in the context of the “European Green Deal”, and the intermediate goals would accordingly also become more stringent.

Table 1: Climate protection goals in Germany and the EU (table taken from [BMU 2019])

With [KSG 2019], the Federal government has specified permissible annual emissions for each year up to 2030 and has specified permissible annual emissions for the different sectors which will allow any necessary adjustments at short notice, so that the goals for 2030 are actually also achieved (Table 2).

Table 2: Permissible annual emission volumes based on [KSG 2019], Appendix 2

It can thus be seen that considerable savings are already being strived for in the next few years. In Table 2 the building sector seems to be of relatively small importance, but this is misleading, since a large share of emissions from the energy sector is also caused by buildings.

Defined intermediate steps have been set for the climate protection goals in the German state of Hesse as well. The Integrated Climate Protection Plan 2025 with a total of 140 measures has already been in place since 2017. It aims to reduce greenhouse gas emissions by 40 %₁₉₉₀ by 2025, corresponding to minus 1.03 %₁₉₉₀ p.a. For the following years minus 2 %₁₉₉₀ p.a. has been planned, so the greater efforts have been postponed into the future. All in all, the possibilities for the federal states to exert influence in this area are limited.

The model project “Development Area as a Passive House Settlement” ([Hessen 2019]) was initiated as a possible option for taking action. Taking into account the fact that Passive House buildings today are state-of-the-art, the state of Hesse supports municipalities in urban land-use planning and public relations work when areas are to be designated as Passive House construction areas. This relates, for example, to urban development contracts, land-use plans and advice for property developers.

The first Passive House in Darmstadt-Kranichstein which was completed in 1991 already used renewable energy successfully in the form of a solar collector heating system for hot water in summer. The project was designed to achieve a very high level of efficiency at an acceptable cost; for this reason solar power generation was not included due to costs which were still very high at the time.

It is worth mentioning that it was possible to operate one of the residential units completely without heating for some years through improved efficiency alone, with the installation of individually produced insulation shutters for all of the windows. The technical feasibility of a zero heating energy house without any additional seasonal storage was thus proved. This measure proved to be at least as effective as concepts with seasonal heat storage, but was much more cost-effective. Nevertheless, it was disproportionately expensive in relation to the energy savings that could still be achieved. Completely dispensing with space heating also isn't necessary for a sustainable building.

As a basis for the subsequent contributions in this Protocol Volume, we will discuss in this section how different possibilities for using renewable energies in Passive House buildings as well as in relation to the Passive House concept should be classified, based on some examples.

The energy self-sufficient (energy autarchic??) Solarhaus in Freiburg was realised in 1992 by the Fraunhofer Institute for Solar Energy Systems ([Schirmer 1998]). The aim behind this research building, which is a detached house with a living area of 145 m², was to meet its entire energy consumption through the solar radiation falling on the building itself. This was achieved due to an unshaded location, a highly insulated building envelope, a geothermal heat exchanger and heat recovery ventilation as well as a combination of transparent thermal insulation, PV, solar thermal power, an electroly¬ser and a fuel cell. A hydrogen-powered gas cooker was also part of the equipment. The self-sufficient energy supply functioned even when room temperatures dropped below 20 °C at times in winter.

Even at that time it was known that a renewable supply system for buildings could only be achieved on the basis of high energy efficiency. The building envelope itself is roughly of the Passive House quality.

At the outset, this was not intended to serve as a model for buildings on a broad scale; it was more of an experimental building for testing different technologies. The construction costs alone, equating to 1.3 million euros, i.e. almost 10,000 € per square metre of living space (approximately 15,000 €/m² adjusted for current price levels using the construction price index) simply would not be justifiable.

If one concentrates on the essentials with the technology available today, in general energy self-sufficient houses can be implemented by private builders. One example is the energy self-sufficient Passive House in Berg near Lake Starnberg (Fig. 1).

Picture 1: Energy self-sufficient Passive House in Berg near Starnberg Lake, Vallentin + Reichmann Architekten ([Vallentin 2018])

The building does not need any electricity since the Passive House basis is combined with maximum efficiency of all components and a 50 m² PV system with battery storage. A 200 m² area with ground absorbers in the garden serves as a heat source for direct-evaporation and direct-condensing heat pump. It initially supplies hot water by means of hot gas heat dissipation, and in winter the refrigerant then flows through the underfloor heating at a lower temperature level. Space heat can be stored in the solid building components, which absorb about 50 kWh/K of heat. The heat pump is dimensioned large enough so that in just a few sunny hours the heat demand of the building can be supplied for one day.

Experience in practice has shown that the occupants have to restrict their energy use for about 2 months of the year by being particularly economical and accepting moderate room temperatures, which however are always above 19°C. Highly efficient components, from the FTP server to the inverter of the battery storage, are a prerequisite for functioning of the building in the winter. On the other hand, in summer there is a surplus of energy.

This example shows in practice what Wolfgang Feist has already discussed in his introductory article: energy self-sufficient buildings with Passive House technology are feasible, but are ultimately they are not a prototype for the building of the future. The winter peak in energy consumption on the one hand and surplus energy in the summer that cannot be utilised meaningfully on the other hand indicate that connection to the electricity grid is necessary. Expensive individual storage facilities will also be just as unnecessary as foregoing of thermal comfort in winter, which would certainly not be acceptable to all users.

With this in mind, the climate-neutral Passive House development in Hanover-Kronsberg shown in Fig. 2 was already planned in 1998 as one of the first Passive House settlements. The buildings themselves are classic Passive House terrace houses, with large south-facing windows, thermal insulation with thicknesses of 30 cm to 40 cm, insulated window frames, a good level of airtightness and heat recovery ventilation.

The additional costs for the Passive House buildings were 12 % of the total construction costs. However, a similar amount was saved elsewhere, so that the Passive House buildings were ultimately no more expensive than the other buildings in the development.

Picture 2: Climate-neutral Passive House development in Hanover-Kronsberg

Picture. 3: Specific heating energy consumption for three different heating periods. Illustration taken from [Peper 2002]

As Picture 3 shows, the heating energy consumption of the settlement measured over three years was less than 15 kWh/(m²a). Efficiency was also improved for domestic buyers received free advice and a subsidy for efficient household appliances. This resulted in a measured saving of 38 % compared to typical reference households.

The heat supply for heating and hot water is primarily provided via connection to the district heating network with a high share of CHP; the heat is distributed via a supply air heating system plus bathroom radiators. In addition, each house has a solar thermal system, with which it was possible to reduce the hot water consumption to 1.3 kWh/(m²a) in the five summer months. Nevertheless, the absolute contribution of the solar thermal systems is small, as can be seen in Fig. 4.

In addition, buyers paid the equivalent of 1300 € for a share in a wind power plant installed on Kronsberg hill. Its annual yield is so high that with the primary energy factors at the time (European electricity mix: 2.5, district heating from CHP: 0.7), more primary energy was generated in the annual balance than the total consumed in the buildings for all energy applications. This was only possible at such a low cost due to the high overall efficiency of the Passive House buildings. Further details on this project can be found in [Kron 2020].

Picture 4: Final and primary energy balance. The settlement consumes less primary energy than is generated through renewable energy sources – this is thus a net-zero district. As with other net-zero projects, it goes without saying that there is neither self-sufficiency nor simultaneity of consumption and generation here. Illustration from [Peper 2001]

Solar hot water heating has been a tried and tested, almost classic application of renewable energy sources in buildings for decades. Here too, it is important to give attention to a very good level of efficiency so that the highest possible solar fractions can be achieved. One example of this is the renovation of the Tevesstrasse apartment building in Frankfurt am Main.

The building which was refurbished with Passive House components has a rather small sized solar thermal system to support water generation. As Fig. 5 shows, this provides an annual total of 7.6 kWh/(m²a), just under half of the domestic hot water demand. From a slightly different perspective, the contribution of the solar thermal system is as large as the hot water distribution losses (1.4 kWh/(m²a) for the branch lines, 5.7 kWh/(m²a) for the circulation). It should be noted that these distribution losses are an extremely good result for a system with central hot water generation; it is not uncommon for the distribution losses to be twice as high. If the hot water distribution had been planned and executed less efficiently, the solar fraction would have been even smaller.

Picture 5: Annual balance for the solar thermal system in Frankfurt Tevesstrasse. Illustration and picture [Peper 2009]

The Passive House office building belonging to Wagner & Co in Cölbe was completed in 1998. The Passive House building envelope with heat recovery ventilation and geothermal heat exchanger achieved a heating energy demand of less than 15 kWh/(m²a). The hot water requirement is very small, as is typical for office buildings. The net floor space is 1950 m².

A 65 m² flat-plate collector system and a stainless steel tank (formerly a wine tank) with a volume of 87 m³ for seasonal thermal storage were integrated into the building. The remaining heat demand is met by a gas-powered CHP unit (Fig. 6).

Picture 6: Passive House office building for Wagner & Co in Cölbe near Marburg. A 85 m³ seasonal storage tank was integrated into the building design. Illustration from [Wagner 2001]

The storage tank was insulated with five layers of mineral wool to a total thickness of 50 cm. Fig. 7 shows the temperature curve in autumn 1999 at different levels of the storage tank. At the end of the summer, the storage tank reached a temperature of 90 °C, which incidentally was quite compatible with a high level of summer comfort, achieved using slat blinds, a geothermal heat exchanger and automated night-time ventilation across several floors. In the following weeks, until around 1 November, the storage tank cools down very slowly, the heat losses benefit the building and delay the start of the heating season. The storage tank is then actively discharged by the heating system until it is empty around 1 December. The solar thermal system also keeps contributing towards space heating throughout the winter.

Fig. 7: Temperature course for the seasonal storage tank and shares of the energy balance assessed in terms of primary energy. Illustration taken from [Wagner 2001]

In the overall balance (Picture 7, right) the electricity demand predominates, mainly for work tools and lighting. The contribution of the solar heating system (yellow) is relatively small.

This is a typical example in this respect. The term “seasonal storage” only inadequately reflects the actual behaviour. Although the system as a whole functions as planned, the storage tank is already empty at the beginning of the main winter period. At the same time, the technical effort is relatively great, and the system is also very apparent in terms of design (which was certainly the intention here). Nevertheless, the contribution of seasonal heat storage to the energy supply remains moderate overall. The contributions of the solar heating system are no longer negligible only in the context of the very low heat demand of a Passive House building.

In principle, such storage strategies can also be implemented on a smaller scale down to single-family homes. An example from Ireland shows the possibilities and limitations ([Colclough 2012], [Colclough 2015]). Based on TRNSYS simulations, the authors conclude that mild, maritime climates such as Ireland offer the greatest potential for solar heating support, which is a quite plausible outcome. The practical test took place in a detached Passive House with a living area of 215 m². Vacuum tube collectors with an area of 10 m² and a 23 m³ seasonal storage tank were installed here. The storage tank is made of concrete with EPS insulation and is located in the ground outside of the thermal envelope. The heat losses measured during the cooling down test were just under 10 W/K, which was significantly higher than the calculated value of 4.2 W/K.

The building was used as an office and demonstration building and stood vacant for some time. Due to this, the room temperatures dropped below 15 °C in December and January, outside this period they were above 17 °C. Under these conditions, the measured heating energy consumption was 7.4 kWh/(m²a), the hot water consumption was just 3.3 kWh/(m²a).

46 % of the heating energy consumption was met directly by the solar collectors, 26 % by the seasonal storage tank, and 28 % was met via direct electric post-heating. The seasonal storage tank made relevant contributions until the end of December; in January and February, electric heating was additionally required in addition to direct solar heating (Fig. 8). According to the TRNSYS simulations, even with a collector area of 100 m² and a storage tank size of 130 m³ the total solar coverage of 72 % could only have been increased to 77 %.

Picture 8: Monthly balance for the solar heating system. Direct utilisation of solar energy for space heating was only possible from 26 November 2010 onwards. Data based on [Colclough 2012]

Since heat from solar thermal systems is available almost exclusively in the summer months, and that in excess, attempts were made early on to store the heat in seasonal storage tanks in the summer and use this for heating in the winter directly without a heat pump. In addition to excellent insulation, such storage tanks must have the greatest possible volume, because this improves the ratio of heat-transferring enclosing area to volume (corresponding to the amount of heat that can be stored).

Around the turn of the millennium, several such systems were realised and intensively scientifically monitored, but ultimately these were not successful. Several installations can be found described in detail e.g. in [Bodmann 2005].

By way of example, the results of the “Solar Local Heating Friedrichshafen” project are summarised here. Here a storage tank with a diameter of 32 m and a volume of 12,000 m³ was built and connected to a new housing estate nearby via a local heating network. Depending on the year of operation, a solar coverage of 20 to 30 % could be achieved. Fig. 9 shows an example annual balance sheet.

Picture 9: Energy balance of the “Solar Local Heating Friedrichshafen” project. Illustration: [Bodmann 2005]

In this context, it should be noted that the heat demand of the connected buildings for space heating and hot water was approx. 90 kWh/(m²a). A reduction of at least 50 % would have been possible easily and economically with a Passive House construction method; the other way around, this would certainly have increased the solar coverage, although it probably wouldn't have doubled it.

The grid losses amounted to 5 to 8 %, a quite acceptable value. However, when evaluating the system in relation to decentralised gas boilers, this share must be deducted from the solar coverage rate, because without the solar supply, there wouldn't have been any local heating network. 50 % more efficient buildings would double the relative grid losses (which are constant) to 10 to 15 %.

This weak result is also due to the fact that the system did not reach the planned level of performance by far. The heat losses of the storage tank were 50 % higher than planned, probably due to unexpectedly high moisture in the soil (the lower third of the storage tank is not insulated). The return temperatures were around 50 °C, so they were 10 K higher than planned, which impaired the efficiency of the solar collectors and the effective heat capacity of the storage tank. A few leaky bypass valves in the fresh water or heat transfer units are enough for such malfunctioning. From today's perspective, it is also questionable that the collectors were primarily used for return flow boosting, so an additional high-temperature (fossil fuel based) heat source is always needed.

As can be seen from Fig. 10, the storage tank was fully charged in August. The heat output increased from September onwards, and heating was obviously already taking place. At the end of November, the storage tank is already empty in the sense that it can no longer release any relevant amounts of heat at the given return flow temperature, so one cannot really speak of seasonal storage.

Picture 10: Monthly balance for the solar heating system. Illustration: [Bodmann 2005]

The situation described in this example is not unusual, unfortunately. The other systems with large seasonal storage tanks described in [Bodmann 2005] also look similar structurally: the solar coverage is about 30 %, there are higher storage losses than planned (up to a factor of 4), the return flow temperatures are too high (also because the return flow temperature is raised as an immediate measure if the heating capacity is not sufficient), and the collector fields are in part not in operation. The solar heating costs in these projects are up to 40 cents/kWh, and those involved consider about 10 cents/kWh to be achievable in the long term.

It should be mentioned that large solar thermal systems in combination with heat exchangers are being used more frequently in heating grids, e.g. in less sunny Denmark. The grids here are operated at lower temperatures and the hydraulic problems can be solved as a result, thus it cannot be ruled out that similar systems can make a useful contribution to supporting district heating networks in the future.

Germany and the EU have set themselves ambitious climate protection goals: CO2 emissions are to be reduced to almost zero by 2050. This is unquestionably necessary both from a scientific point of view and in view of the Paris Agreement. The combination of high energy efficiency and the use of renewable energy sources is a good way to achieve this in the building sector.

Since the late 1990s there have already been examples of buildings that combine the Passive House standard as an ideal basis with the use of renewable energies and thus implement cost-effective and sustainable solutions. Energy self-sufficient buildings without grid connection are also technically feasible and, if implemented intelligently, are at least economically feasible. However, this solution doesn't make sense on a broad scale.

In the case of newer projects, the trend is towards PV instead of solar thermal systems, which were often used in the past. This is partly due to the drop in the price of PV, but also because electricity can be used more flexibly and because it can be fed into the grid. It has also been shown that the complexity of solar thermal systems, especially large-scale systems, is difficult to control in practice. In tests on seasonal storage of solar heat, it usually hasn't been possible to provide heat at an immediately utilisable temperature level up till the main winter period.

The use of renewable energy sources requires highly efficient buildings in order to make a relevant contribution to the energy balance. Conversely, the use of renewable energy sources makes it possible to improve highly efficient buildings even further. In any case, both are indispensable for achieving the goals of the Paris Agreement.

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