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Anyone who has built or lives in a Passive House building already has this part of the energy transition taken care of. After all, the low energy demand in a Passive House can sustainably come from regional energy sources. The supply structure is transitioning from fossil sources to renewables at an encouragingly rapid pace. The old assessment systems for energy demand in buildings are based on the old supply system and do not work in the new one. The Passive House Institute therefore developed a new evaluation system based on renewable primary energy (PER, Primary Energy Renewable). It also takes proper account of the energy that a building generates. This new evaluation system consists of three Passive House classes:
This paper illustrates these classes based on specific reference projects and shows how you can take your project to the next level.
Most people probably think of a single number when they hear the term Passive House: 15 kWh/(m²a). It describes the maximum demand for annual heating energy for compliance with the Passive House Standard. This figure is still included in all of the classes in the new evaluation system because it provides a starting point by limiting the amount of useful energy made available for heating purposes indoors. Useful energy demand for cooling, airtightness, and criteria for comfort and hygiene also remain the same.
But heating energy demand does not tell the whole story; after all, heating energy demand is roughly equal to hot water demand in a Passive House. Demand for household electricity is usually much higher. A building’s total energy demand – including the energy needed to provide the building with final energy – therefore also needs to be taken into account. This is where the new Passive House classes come in. They divide buildings into categories based on renewable primary energy demand and their own renewable primary power production (Figure 1).
Power from a solar roof, say, is considered primary electricity with a PER of 1.0. It is exported to the grid and not calculated against the building’s energy demand. The PER model is used to calculate demand. For example, solar power generated in the summer should not be treated as though it directly offsets heating energy in the winter because energy from the summer would need to be stored seasonally for the winter, a process that entails additional losses. If this factor is not taken into consideration during planning, buildings are not properly optimized. By taking account of renewable primary energy, the new system allows the building to be made future proof.
Often, energy demand and generation are stated with reference to a building’s treated floor area. If a building has a photovoltaic array, it can produce a certain amount of energy, but the amount per square meter of floor area decreases as the number of stories (and hence floor area) increases. Single-story bungalows thus seem to perform better than row houses and duplexes/complexes, although bungalows actually consume much more area and resources per resident.
Stating renewable energy production in terms of floor area can thus also lead to improper optimizations. In the new concept, energy generation is instead stated relative to the building’s ground area, defined as the vertical projection of the thermal envelope towards ground (for details, please see PHPP 9 manual). Whether a bungalow or a complex is built, the assessment is therefore the same in terms of energy generation. This approach is better because the space a building takes up is then no longer available for other types of usage. If this area is used to generate electricity, there are additional benefits, and these benefits are then assessed in terms of this area. After all, the sun shines on the roof, not on the treated floor area on every story.
Both within Germany and worldwide, biomass is only available in limited amounts. There is a clear usage hierarchy for biomass: 1) food production, 2) materials, and 3) energy [Krick 2012]. Because biomass can be stored and has a high energy density, it will mainly be needed in mobile applications (transport). Only a small amount will be left over for consumption in buildings. The new PHPP 9 sets the amount of renewable primary energy left over at 20 kWh/(m²a), and the PER factor is set at 1.10 for biomass in general. Because biomass can be used to generate electricity and produce liquids or gases, it can be used in any supply system, so it is credited to all supply variants. And because biomass can be stored, it is perfect for use in the winter. The budget is then prioritized as follows: heating, hot water in the winter, and household electricity.
For instance, if a building has a condensation boiler (PER of renewable gas: 1.75), the first 20 kWh/(m²a) of PER demand is calculated with the PER factor of 1.10 for biomass. The PER factor of 1.75 for renewable synthetic gas is then applied for subsequent applications. If the PER demand for heating is lower than 20 kWh/(m²a), the rest of the budget is applied to hot water supply, followed by household power demand. If biomass is used to cover this demand, it is only available within this budget. Furthermore, the PER factor of electricity is used for heating purposes because the additional consumption of biomass comes at the expense of other users.
Note that it is more efficient to generate electricity with biomass first and then use a heat pump for heat supply second. If some of the biomass is combusted in a household stove, around 80 percent of the primary energy can be converted into useful heat. If biomass is consumed in a cogeneration unit, around 50 percent of the energy is used to produce electricity and 30 percent to produce useful heat, with only 20 percent losses. A heat pump allows three units of heat to be generated from a single unit of electricity. In this case, 50 percent electricity becomes 150 percent heat in addition to the 30 percent useful heat from the cogeneration unit. As a result, biomass produces 180 percent useful heat in combination with a heat pump instead of 80 percent useful heat from direct combustion. Nonetheless, Passive House buildings can continue to have biomass heating systems; the overall PER demand will simply be relatively high in such cases.
This section uses reference projects to shed light on the new classes. General questions are answered with reference to the specific reference cases.
In the basic variant, a boiler fired with wood pellets is used to heat this single-family home, which also includes an office room. The roof has a 74 m² photovoltaic (PV) array on it (variant 1). At the outset, this building is already very energy-efficient, with an annual heating demand of 11 kWh/(m²TFA*a). With an airtightness of 0.14 h-1, it is also very well built. Its PER demand of 60 kWh/(m²TFA*a) means that it just barely fulfills the criteria for Passive House Classic, whereas the 53 kWh/(m²ground*a) of PER generation puts it below the threshold for Passive House Plus (60 kWh/(m²ground*a)).
If a small solar thermal array for hot water supply with six square meters of collector area is added, PER demand drops to 47 kWh/(m²TFA*a), while energy generation increases to 65 kWh/(m²ground*a) (1a). In terms of energy generation, the level of Passive House Plus is reached, and we are not far away in terms of demand either. In this variant, the building already generates slightly more final energy than it consumes. If the collector area triples to 18 square meters, however, PER demand drops even further to 43 kWh/(m²TFA*a), which fulfills Passive House Plus. However, the effects are minor (especially in the light of the high cost of three times more collector area) because the additional energy produced in the summer cannot be completely used; the hot water tank (now 2,000 liters) will be fully loaded (1b).
Variant 1c with six square meters of collectors and additional heat recovery from shower water is a good option in combination with an improved hot water distribution system. This variant has a PER demand of 43 kWh/(m²TFA*a) and generates 61 kWh/(m²ground*a) and is therefore in compliance with Passive House Plus. A comparison with the original variant 1a with a solar thermal array shows that less energy is generated even though the array has the same size. The difference results from far lower demand for hot tap water. Because less energy is needed, the solar thermal array has less energy to offset. In this variant, the balance of renewable primary energy is even. Because of its high efficiency, the Passive House building in Gerstetten does not need much energy beyond the biomass budget, so the switch from a pellet stove to a gas condensation boiler does not make a big difference (variant 2).
If a heat pump is used, however, the evaluation changes considerably. Variant 3 has an air-air heat pump that reduces PER demand to 40 kWh/(m²TFA*a). The level of Passive House Plus is then reached for demand, and the PER balance is already even. A switch to a brine-water heat pump (3a) reduces demand further to 34 kWh/(m²a), and 85 square meters of PV puts the Passive House Plus building clearly in the black for PER.
We still have a gap to close before reaching Passive House Premium. Simply optimizing building services will not get us there; we have to make changes to the building envelope. Instead of the original window frames of efficiency class phB, Passive House windows of class phA can be used to reduce annual demand for heating energy down to only 8 kWh/(m²TFA*a). In combination with heat recovery from shower water and optimized hot water distribution as in variant 1c, the roof can be completely covered with 123 square meters of photovoltaics as shown in variant 3b. Then, demand comes in at 30 kWh/(m²TFA*a) to fulfill Passive House Premium. However, generation is still too low at 88 kWh/(m²ground*a), even though twice as much energy is produced as is needed. One option is to add PV to the garage roof or southern façade of the building; alternatively, residents could invest in a community wind turbine. If a mere three-kilowatt stake is purchased in a local community wind farm (equivalent to 1/500 of a modern onshore wind turbine), the Passive House Premium level is reached, and nearly three times the amount of energy consumed is produced.
In the basic variant (variant 1), the day care center (Figure 3) has a gas condensation boiler for both space heating and hot tap water. The facility does not generate any renewable energy. The building has an annual heating energy demand of 15 kWh/(m²a). Renewable primary energy demand comes in at 84 kWh/(m²a); in other words, the upper limit for renewable primary energy of 60 kWh/(m²a) is exceeded.
Heating up water requires a lot of energy. In a day care center, however, little hot water is used relative to the long distribution and circulation lines. As a result, heat losses are high. The useful energy demand for hot water of 8.4 kWh/(m²a) is actually lower than the losses of 12.1 kWh/(m²a). In such cases, it’s a good idea to use a different system, such as electronic instantaneous heaters. This option improves the efficiency of hot water supply because the PER factor of electricity for hot water is less than that of renewable methane. In addition, an instantaneous heater reduces the risk of Legionella and usually lowers investment costs. Finally, less auxiliary energy is needed for circulation and storage pumps. Overall, this option reduces PER demand to 59.5 kWh/(m²a), making the building compliant with Passive House Classic (variant 1a).
Using a heat pump (variant 2) for heat supply reduces final energy demand for space heating to 5 kWh/(m²a). As a result, PER demand drops to 47 kWh/(m²a), nearly the level of Passive House Plus. The high line losses make the use of a heat pump for hot water supply less attractive than distributed instantaneous heaters. But unlike residential buildings, day care centers need a lot of energy for lighting. More efficient lamps can offset an additional 3 kWh/(m²a) of PER demand to comply with Passive House Plus (variant 2a). A 79 square meter PV array covering 31 percent of the roof also makes the building compliant with Passive House Plus in terms of generation (variant 2a).
To generate the 120 kWh/(m²a) of renewable primary energy needed for Passive House Premium, 157 square meters of PV would need to be installed, thereby covering 63 percent of the roof. It is harder to reduce energy demand to 30 kWh/(m²a) of renewable primary energy. In the heating system chosen, a kilowatt-hour of offset heating energy would only reduce the heat pump’s COP. Therefore, it’s better to take another look at lighting. LED lights of utmost efficiency can be installed throughout the building, and electrical efficiency increased in general. Because there is no shower, heat recovery from shower water would not increase efficiency – but the use of energy-saving taps would save about one kilowatt-hour of PER. And when windows of efficiency class phA are used, the Passive House Premium level is reached.
A cogeneration unit next to the building produces electricity and heat with gas from the water purification process (Figure 4). The heat is exported to a district heat network for use in space heating and hot water. The cogeneration share of heat is estimated at 94 percent. The lines are very short, so losses are low. The biogas from purification has an PER factor of 1.1, but only within the biomass budget of 20 kWh/(m²a) PER. Within that budget, the PER factor is 0.53, which results in an efficiency factor of the grid of 85% and a cogeneration share of 94% (with 46% electricity and 44% heat). Then, the biogas is considered to have a PER of 1.75, and the factor worsens to 0.93. Although the hot water distribution system is relatively inefficient, the initial PER demand is 44.3 kWh/(m²a). With the 247 square meters of PV covering 35% of the roof, the Passive House Plus level is reached.
If 495 square meters of PV is installed covering 70 percent of the roof, the energy generated fulfills Passive House Premium. For demand, the item with the highest PER factor is optimized first. In this case, we are talking about electricity for lighting, office equipment, and auxiliary equipment. Once again, LED lamps of utmost efficiency are used and office applications are also highly efficient, reducing the PER demand to 33.4 kWh/(m²a). When hot water circulation is adjusted according to demand and energy-saving taps are used, the remaining 3 kWh/(m²a) can be offset so that Passive House Premium is reached for demand. The amount of renewable primary energy generated is even more than twice as great as demand in absolute terms.
[Feist 2014] Feist, Wolfgang: Passive House – the next decade. In: Feist, Wolfgang (Hrsg.): Tagungsband zur 18. Internationalen Passivhaustagung 2014 in Aachen. PHI Darmstadt 2014
[Krick 2012] Krick, Benjamin: Zukünftige Bewertung des Energiebedarfes von Passivhäusern. In: Feist (Hrsg.): Protokollband des Arbeitskreises kostengünstige Passivhäuser Nr. 46: Nachhaltige Energieversorgung mit Passivhäusern. PHI Darmstadt 2012
[Ochs 2013] Ochs, Dermentzis, Feist: Energetic and Economic Optimization of the Renewable Energy Yield of Multi-Storey PHs. In Feist, Wolfgang (Hrsg.): Tagungsband zur 17. Internationalen Passivhaustagung 2013 in Frankfurt/Main. PHI Darmstadt 2013