Within the joint project , a single-family house is to be developed, constructed and measured, which can inde-pendently cover its demand for electricity, heating and domestic hot water at any time of the year from locally used renewable energies. The starting point is a commercially available system for the electrical self-sufficiency of homes, which includes a closed hydrogen cycle consisting of fuel cell, electrolyser and hydrogen storage. In addition a model-based quantification and optimization of the potential of the building to provide grid services for the distribution grid is carried out. For this purpose, new control approaches such as model predictive control will also be investigated. Finally, the life cycle analysis method will also be used to classify the CO2 foot-print of the building.
Within the first project phase, several component variations for the extension of self-sufficiency to the thermal range will be simulatively investigated. In the second project phase, the most promising energy supply concept will be implemented and measured. Furthermore, the development focus will be on the integrated control of the entire supply structure with the aid of various control strategies including grid signals.
The current state of the art in the field of energy-efficient residential buildings is the Efficiency House Plus (also called PlusEnergy House) . In the annual balance, this building feeds more electrical energy from renewable energies (mostly photovoltaics) into the grid than it needs to supply its own electricity and heating requirements. PlusEnergy buildings thus have the characteristic of generating an excess supply in summer and a strong demand for electricity in winter. The general challenge in dealing with renewable energies, i.e. the adjustment of generation and demand in a seasonal context, is not solved in this way, but merely outsourced to the electricity grid or even exacerbated in some cases.
In Germany, solar electricity is mainly fed into the distribution grid in a decentralized man-ner. In the case of a locally high density of PV systems, or high installed capacities of individual systems, as in the case of the Efficiency House Plus, this can lead to the electricity feed-in exceeding the electricity demand in the grid section. In this case, measures must be taken to stabilize the distribution grid. For example, transformers are used to transfer excess electricity to the medium-voltage grid. If this is no longer possible due to transformators’ power limits, the distribution system operator is also allowed to reduce the nominal power of individual PV systems.
Thus, it becomes clear that especially in connection with an increasing share of renewable energies in the power grid, the temporal supply profile – also of residential buildings – is becoming more and more important in addition to the pure power consumption or balance surplus . The electricity grid does not gain in flexibility due to the rigid producer/consumer structure of PlusEnergy Houses (even with battery storage) .
The project therefore aims to realize and investigate a further development of PlusEnergy buildings that is as grid-friendly as possible. These buildings should be able to cover their ther-mal and electrical energy demand not only in the annual balance, but also in a seasonally balanced manner from locally generated renewable energies, while continuously being able to provide flexibility to the electricity grid.
A typical single-family house in the sense of a pure “consumer”, i.e. without local use of renewable energies but according to the EnEV standard, generates a burden of about 38 kg CO2 equivalents per square meter and year  (see Figure 1). A PlusEnergy house, on the other hand, as a “prosumer”, is able to provide more renewable electrical energy over the year than it needs itself. One could assume that the CO2balance is therefore also negative.
However, PlusEnergy houses usually feed unregulated electricity into the grid at times when there is already a surplus of renewable electricity and possibly even grid bottlenecks or negative prices on the electricity market. The additional CO2avoidance through the feed-in is therefore rather low. However, if there is an undersupply of renewable energy in the electricity grid, as is the case during a winter dark period, there is usually a significant demand for electricity even in PlusEnergy houses, and this at a time when grid electricity is comparatively CO2-intensive.
The goal of is to solve this phenomenon and to enable a full supply with local renewable energies around the year and not to “dispose” of unregulated surpluses into the electricity grid. To this end, solar surpluses are stored in the home’s own seasonal hydrogen storage system in summer using electrolysis. In winter, the house can then be supplied with electricity and heat from this storage by means of a fuel cell.
This results in two major advantages: on the one hand, a complete relief of the power grid with regard to the inflexible power feed-in and demand, and on the other hand, a flexibilisation potential for the power grid with the help of the existing system and storage technology. As a result, the building can become a flexibly acting part of the electricity grid, a socially acting power plant of the future, so to speak. This flexibilisation potential will be demonstrated, optimized on a model basis and quantified within the project.
Methods and Innovations
Based on the home energy supply system Picea, which is currently being developed and tested by the company HPS Home Power Solutions, the necessary innovations and develop-ment goals, which are to be worked out within the project, will be outlined. These goals relate to the home energy supply solution itself, but also to the building envelope and intelligent energy management, which is necessary to make the flexibility potential available on the grid side.
1.1 The hydrogen system
HPS Home Power Solutions GmbH (HPS) from Berlin – Adlershof develops home power supply solutions for single and multi-family homes. The goal is to enable a complete supply of electrical energy from locally generated photovoltaics not only in the summer months, but around the year. For this purpose, a photovoltaic system is supported by a battery storage system in order to adapt the energy supply to the household demand within one day or a few days. In addition, a seasonal energy balance is necessary, especially between winter and summer. For this purpose, a storage volume of 700 to 1500 kWh, depending on household demand, is to be provided. This seasonal storage is realized via a closed hydrogen path. In concrete terms, this means that in the summer months excess solar power is used to generate and store hydrogen using an alkali membrane electrolyser. The resulting heat due to conversion losses in the electrolyser is used to heat domestic hot water. In the winter months, when the average daily solar yield is below the household electrical demand, the hydrogen is converted back into electrical and also thermal energy via a fuel cell and made available to the house (see Figure 2).
This system combines power-to-gas with cogeneration at household level. This makes it possible to use about 90% of the locally generated photovoltaic energy electrically or thermally within the household. The components currently available in Picea and the services provided with them are summarised in Figure 2 within the grey area.
While on the electrical side a winter supply through fuel cells and photovoltaics is already successful, there is currently still an undersupply with regard to space heating in single-family houses according to the energy efficiency standard KfW-40. In the case of space heating by means of a heat pump, this inevitably leads to a heavy load on the electricity grid in the winter months, given the relatively low heat capacity of modern single-family houses. In Figure 3 the space heating contribution that Picea is typically able to provide over the year is shown in red. In addition Figure 3 compares this heat supply (red) with the characteristic demand of a single-family house (blue) with 145 m² of usable floor space in accordance with the KfW 40 standard at the Berlin location. While Picea in this example provides a usa-ble space heating of 2120 kWh over the year, a deficit of 4006 kWh remains, which would be necessary to cover the total heat demand of 6126 kWh and would currently have to be provided by a separate heating system.
1.2 Development goals and innovations
In , the following two main objectives are to be pursued and achieved::
- Coverage of the electricity, domestic hot water and space heating requirements of a single-family house completely from on site photovoltaic system (cf. Figure 2 dashed area) using low-cost, mass-produced components (no special system construction) and without the need to achieve passive house standards.
- Model-based optimization and quantification of the potential of such a home energy system to provide flexibility to the higher-level electrical supply system with regard to supporting the system integration of renewable energies with fluctuating power output such as wind power and photovoltaics.
The objectives described above are to be achieved without any noticeable reduction in living comfort. To achieve this, it is necessary, on the one hand, to further develop the building system technology beyond the standard currently available within Picea in terms of process technology and to expand it with heat pumps and/or electric surface heating systems as well as innovative air distribution systems. On the other hand, it is necessary to further develop the building concept beyond the currently available KfW-40 standard in the most cost-efficient way possible. Finally, the operation of fuel cell, heat source and storage system must also be optimally matched to each other. These three research and development challenges will now be described in more detail.
1.2.1 System technology
The aim of system technology development is to create the technical boundary conditions to enable the extended storage of energy for space heating and domestic hot water demand. For this purpose, detailed analyses must be carried out to find out what storage capacity is required for the respective heat demand and how high the additional demand for electrical energy is for the respective heating system.
In order to answer this question, a detailed thermal house model is created, with the help of which various electricity-based heating systems are integrated into the system technology model of HPS and compared on the basis of annual simulations.
For some of the system configurations to be investigated, a hot water storage tank will be necessary in order to obtain a sufficiently large displacement potential for the heat supply. An important aspect of the storage tank will be the comparison of the different possibilities of introducing space heat into the building. Here, the direct air-based heat input is compared to an indirect input via a water buffer storage as an intermediate step.
1.2.2 Building envelope and comfort
The objective with regard to the thermal energy demand of the grid-connected house is to determine the extent to which better external insulation than that of the kfW40 standard is required so that the heating demand can be covered solely by local renewable energies while at the same time maximizing the flexibility potential. This objective involves a highly integrated problem with influencing factors from the areas of orientation and design of the building envelope, the environment and user behaviour. While many of these influencing factors are of minor importance in conventional heat supply concepts, their share in low-energy concepts increases to a decisive size, so that their consideration becomes obligatory for a successful design of the heating energy and house concept.
The calculation initially focuses on the building envelope. The aim is to find a compromise that meets today’s architectural requirements and at the same time takes into account the technically necessary criteria.
Here, attention must be paid to a low ratio of surfaces to volume of the house in order to minimize heat-transferring surfaces with a constant floor area. The avoidance of porches such as bay windows, projecting balconies and loggias also helps to reduce the heat transfer to the surroundings through the design of the building envelope.
A trade-off arises with regard to the high share of glass surfaces of residential buildings that are preferred today. From an energy point of view, these allow high solar gains during winter, which reduce the necessary heating demand. In summer, on the other hand, there is a danger of the building overheating, which should be countered by suitable measures.
In this context, various shading concepts and the orientation of the building should be mentioned as degrees of freedom. The orientation and location of the building are not always freely selectable. Thus, within the project, design guidelines are to be developed which, within certain limits, allow the basic house design to be replicated in almost any orientation and position without having to rely on costly, temporally and spatially high-resolution simulation.
In terms of shading concepts, passive or active shading as well as concepts with the help of planting are conceivable, which provide sufficient shade in summer while not hindering the use of solar gains in winter. Active shading also requires intelligent integration into the building automation system, including a connection to the Picea energy management system.
In addition to the factors influencing the building envelope, the individual needs and behaviour of the building’s occupants are also an increasingly important factor that must be taken into account.
The detailed house model should contain a room-accurate resolution of the comfort para-meters of air and radiation temperature as well as relative humidity. The consideration of the relative humidity becomes particularly relevant for the air heating system, since these systems often only achieve lower relative humidities without additional measures. However, since water is produced during the regeneration of hydrogen during the heating period, the extent to which this can be used for humidification of the supply air is to be investigated.
1.2.3 Energy management and grid flexibility
The energy management of the described output system of the HPS home energy supply system regulates the flow of electricity from the single energy source photovoltaics to the energy consumers, energy converters and storage devices. The focus here is on reliably meeting the demand for electricity. Likewise, the reconversion of the hydrogen into elec-tricity, for example, is oriented exclusively to the electricity demand.
However, future energy management will have to treat heat demand with the same priority as electricity demand. This circumstance will complicate the heuristic decision structures since additional component- and time-dependent restrictions will have to be taken into account.
It will not be sufficient to cover plant operation based only on current storage levels and known conversion efficiencies. Since photovoltaics is still the only relevant energy source, an intelligent control system based on predictive data is required, which incorporates forecasts for weather data (outdoor temperature and expected global radiation) into the control decisions for covering the final energy types and is also able to respond to flexibility require-ments on the part of the power grid.
The influence of other relevant parameters, such as the expected energy demand, flexibility requirements on the part of the electricity grid and minimum storage levels, must then also be included in the decision-making process for the optimal heat supply for the entire system.
A further focus of research lies in the identification and quantification of the flexibility potential available on the system side, which can be made usable for grid-serving operation, taking into account the aforementioned component and system restrictions. Since components for controllable electrical energy generation, consumption and storage are available, such a building would be predestined for grid-serving operation. In addition, the price of electricity is an important input variable in the deployment planning for flexible operation that serves the higher-level energy system. It enables a monetary evaluation of the flexibility options. Furthermore, forecasts of the electricity price trend can be incorporated into the model predictive control, e.g. in order to keep power reserves available for times of particularly high or low electricity prices.
To complete the comprehensive analysis of the building, a life cycle assessment will be carried out to be able to quantify the burden of CO2 connected to the construction, use and disposal of the building. Hereby the system technology used will be focused in order to draw a comparison with alternative house energy supply concepts.
is a publicly funded collaborative research project, which is supported with funds from the BMWI within the 7th Energy Research Programme.
There is an exchange of results with the accompanying research and with the ENER-GIEWENDEBAUEN network. The FlexEhome project is funded in the period 12/2020-11/2024.
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