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The findings show that the supply structure that needs to be built and its efficiency depend greatly on energy-consuming applications' load curves; it turns out that short fluctuations of a few days are less important1), since medium-term grid storage (pumped storage, etc.) can generally balance them out. The situation is different when an application's demand has significant seasonal fluctuations, such as when heating drops to zero for several months at a time. Energy generated from primary power sources (wind turbines, etc.) then has nowhere to go – unless this excess power is sent to the P2G system and turned into methane. The requirement for complete supply is:
PEprim = Edir + EMS / ηMS + ESS / ηSS + EPL
whereby Edir is the electricity from renewable primary power generators that can be directly and immediately used by the building, EMS is electricity from short/mid-term storage, ESS is reconverted electricity from seasonal storage, EPL is power losses, and ηMS and ηSS are the respective overall seasonal efficiency rates. The PER factor can then be determined with
PER = (Edir + EMS / ηMS + ESS / ηSS + EPL ) / (Edir + EMS + ESS )
The results show that while PER factors determined with this equation are relatively stable compared to, say, changes in the primary power mix or the technology mix, they also depend greatly on the individual energy application and its demand profile.
In this paper, domestic electricity includes all electricity used in a dwelling except for power used for hot water, heating, and air conditioning; in other words, electricity for:
Figure 6 shows how the PER factor for domestic power supply depends on the primary power mix used: with 90 percent wind, for example, it comes out to 1.75 kWhPER/kWhel; with 90 percent PV, 1.5 kWhPER/kWhel. An PER factor of 1.39 kWhPER/kWhel is achieved for the “ideal mix” (here) of 54 percent PV, 36 percent wind, and 10 percent hydropower. We suggest a somewhat conservative factor of PERDEel = 1.4 kWhPER/kWhDPel for the analysis. This PER factor is relatively low, so domestic power can easily be provided by renewable primary power generators; almost all buffering results from an improvement of grid simultaneities and from short-term grid storage capacities, which are in some need of expansion. Costs are still quite low for storage cycles of less than 106 hours. Seasonal storage capacity (with a little more than two cycles but low efficiency) accounts for only 11 percent of domestic electricity demand; the storage capacity required is low at less than 62 Nm³ (cubic meters at normal conditions), and the conversion infrastructure required can also be relatively small. The infrastructure could easily be entirely located at sites for regional combined-cycle plants used for reconversion; the methane network can serve as the connection to the underground methane storage (the current natural gas network can continue to be used here).
Primary renewable energy (primary electricity) required
for total domestic power demand from PV, wind, and hydropower (high efficiency).
Figure 7 depicts the equivalent area for renewable generators that would be needed to cover total domestic electricity demand. A (very flat) optimum of only about 35 m² can be reached with a solar share of 35 to 75 percent. The actual area for PV (with a share of 55 percent) would come out to 19 m². Such a small area could almost always be located very close to the building in question, often directly on the roof.
Size of the equivalent PV array (primary electricity) needed to cover total domestic power
demand with PV, wind, and hydropower (with buffer storage in the grid and
seasonal P2G methane storage) with high efficiency.
This study assumes a tap water schedule for a four-person household and a hot water heat pump with a seasonal performance factor SPF of 2.5, resulting in annual power consumption for hot water of 1,123.5 kWh/a [AkkP 49]. The heat pump has a (heating) water storage with a usable volume of 260 liters. The amount of time that the heat pump is in operation can be adjusted to a large extent based on the availability of direct renewable primary power – that is, it can serve as a kind of “storage.” This ability to serve as storage is optimally (for the entire grid!) taken advantage of in this study's basic model with the assumption that there will be an ideally functioning smart grid in the future that consumers will actually use.
Figure 8 shows the PER factors from the simulation relative to the share of PV in the production mix for hot water supply. A quite flat optimum can be seen with a PV share of 55 percent. Because of the locally available storage, this subsystem requires very little power from seasonal storage (only 10 Nm³ of methane). With a small safety margin, the PERDHW-hp factor result is 1.23 kWhPER/kWhel, suggesting that renewable power is well suited to supplying domestic hot water systems. An equivalent PV area of about 8 m² is enough to cover all domestic hot water demand including conversion losses.
|Figure 8: Renewable primary energy (primary electricity) needed to cover power demand
for hot water with solar, wind, and hydropower (grid buffer storage / seasonal P2G methane
storage); hot water heat pump with seasonal performance factor SPF of 2.5.
For our Passive House example, we have assumed a heat pump with a SPF of 2.53, resulting in annual power consumption for heating of 1,080 kWh/a.
Electricity for heating could be considered the “problem child” of the renewable power supply system, since it is not needed at all for more than half the year (summer!) and then is disproportionately needed in the winter. Worse, in conventional buildings (including Low Energy Houses), power consumption for heating is much higher than for all other applications. Although wind turbines on the coast do deliver (slightly) more electricity in the winter, the seasonal simultaneity of primary power production remains unbalanced relative to the load curve for heating energy demand. Direct primary power can only contribute a small amount to heating (despite the high time constant of Passive House buildings). The grid and mid-term storage systems are not required for short-term storage here, since Passive House allows a flexible choice when to run the heating system – based on how much electricity is available – within each period of about three days. In this case, the building's heating system must have enough load capacity. In a Passive House building with a regular maximum heat load of about 1.5 kW, this is not at all a problem; in buildings with a higher heat load (about three times as much in a Low Energy House), it would require such high outputs that this kind of control strategy is not a good idea (not to mention that the heat cycle's temperature amplitude would increase tremendously). Despite all of these options, even with the Passive House Standard there is generally too little primary power available for heating.
Expanding wind power is a particularly ideal solution for power supply for heating (see Figure 9), although a PV share of 10 to 50 percent is still acceptable. The rounded PER factors then come out to
PERheat= 2.2 kWhPER/kWhel,
Heating thus requires much higher numbers when it comes to producing, converting, and storing energy. If PV provides 90 percent of renewable primary energy, the PER factor for heating increases to more than 2.6 kWhPER/kWhel. Clearly, PV systems are not particularly ideal for heating. This situation reinforces the importance of bringing together multiple power generators and consumers in the grid. In this case, for example, homeowners could be encouraged to invest in a share of a wind turbine – indeed, this is exactly the concept behind the first climate-neutral residential district in Hannover-Kronsberg, built in 1998, and the experience has been positive [Peper/Feist 2001].
The amount of renewable power generation that needs to be installed to cover the Passive House heat pump amounts to about 22 m² of equivalent PV area, although this is mostly covered by a corresponding amount of wind power. Using P2G methane produced and stored in the summer to fill the winter gap requires at least 72 Nm³ of storage volume under normal conditions. The required storage is already available today (even if all buildings use heat pumps for heating), and the technology for electrolysis and methane synthesis could easily be installed on the grounds of combined-cycle plants used for reconversion. The main reason this would all be so simple is the very low heating demand required in the Passive House Standard.
The picture is very different if the building is instead built as a Low Energy House with a heating demand of 56 kWh/(m²a). Electricity demand for running the heat pump then increases to 3,631 kWh, while the PERheat factor stays about the same and the equivalent renewable primary electricity generator area required jumps to 67 m² for heating alone.
Renewable primary energy (primary electricity) needed for heating power demand
in a Passive House building, wind power and PV (grid buffer storage / seasonal P2G methane);
heat pump with an SPF of 2.53 (German test reference year 12).
In terms of time, potential cooling demand and direct primary power production seem to be well correlated at first glance – in the summer.
The remaining renewable electricity produced in seasons that do not require cooling can be completely used directly for other applications (for example heat pumps that urgently require power), thereby reducing the amount of power that needs to be drawn from storage. The factor for cooling as an isolated application is:
PERcool,is = 2.5 kWhPER/kWhel
At the same time, the marginal value relative to total power consumption is only about 0.5 kWhPER/kWhel (more power is directly generated for heating, leading to lower storage losses).
A resulting insight significantly changes priorities related to building design as we have previously known them. In the future, it will be much more environmentally sound and cost-effective to use renewable energy to cover energy demand for any cooling that may be needed than for heating. In addition, since the same heat pump system can generally be used for both cooling and heating in Central Europe (and even the same distribution system in a Passive House building), the technical complexity required for cooling can also be drastically reduced. This situation has four consequences:
1. With climate change resulting in increasingly hot summers everywhere, including Central Europe, we will be able to provide people in Passive House buildings with a comfortable indoor temperature without environmental guilt or high costs, even in the summer.
2. Of course, cooling demand should not be unnecessarily increased – all technical recommendations (such as those in AkkP 22, 31, and 41) for limiting summertime heat input should still be followed. But, now it is better to undertake optimisation measures to minimise cooling demand based using the PHPP's “Cooling” sheet rather than focus on passive optimisation using the “Summer” sheet (it’s not difficult to realise, that the results are different).
3. Especially expensive solutions for passive summertime optimisation (more complicated duct systems, larger duct cross-sections, special cooling towers, etc.) may make less and less sense in the future, since PV-operated cooling systems are becoming more affordable, both economically and ecologically.
4. When solutions for conserving heating or cooling energy are competing, reductions in cooling have tended to be more important in central Europe. This priority will be turned on its head, since cooling energy can be provided with overall less ecological concern and lower costs (with an annual heat pump SPF of 2.5, a PERheat factor of 2.2 results in a primary heating energy expenditure factor of 0.88 kWhPER/kWhuseful; with a cooling SPF of 2.5 and a marginal PER factor for PV cooling power of 0.5, the primary cooling energy expenditure factor is only 0.20 kWhPER/kWhuseful).
These findings are revolutionary, considering that environmental associations and policymakers have frequently championed the consistent avoidance of air-conditioning systems, but these must now be taken into account when designing a sustainable supply structure.
P2G methane can be created with a conversion efficiency of 57 percent out of primary electricity with
PERCH4 = 1.75 kWhPER/kWhmethane
Here, we are purposely limiting the definition of P2G methane to methane synthesised as primary power from wind and/or PV systems (for energy budget reasons, the biogas share is considered separately – see next section). This renewable primary energy factor is almost as high as that of electricity for heating – a paradox only at first glance, since if P2G gas is used for, say, heating, it first needs to be completely synthesised from primary power, resulting in losses. If, however, energy is taken from the renewable power grid – even for heating – it is still mostly primary electricity; only the (for heating, not insignificant) “rest” needs to take the detour to P2G storage and then be reconverted with additional losses.
Methane's very high PER factor means that, for example, boilers should not be used to provide heat for domestic hot water. Even providing hot water using direct electricity is more efficient within a renewable structure – and a hot water heat pump can lead to primary power expenditures three or four times lower. For heating, too, any heat pump now has significant advantages compared to a fuel-based system. The latter can still be tolerated during a certain transition period but should be replaced with heat pump systems whenever the opportunity presents itself. P2G methane is mostly used as an easily stored fuel for mobile applications (simple conversion into methanol), where it can provide greater yields, as well as in any reconversion to electricity that may be needed, particularly in cogeneration systems with combined-cycle power generators.
In this scenario, biogas is also cleaned to its methane component and sent to the same gas network as the P2G gas discussed in the previous section. An average PER factor could therefore also be calculated for this type of energy when mixed in with gas from solar and wind, but doing so would cover up an important consideration: as a renewable primary gas, biogas is directly recovered from biomass. This is a very efficient process with a favorable PER factor of
PERbio = 1.1 kWhPER/kWhmethane
At first glance, biogas therefore seems like it should be given priority for applications that rely on storable energy (traffic, heating, etc.). The problem, however, is that biogas is only available to a very limited extent [Krick 2012]. In terms of sustainable agriculture and forestry, there are limitations related to
Clearly, there are significant limits on the use of bioenergy as a resource. It makes absolutely no sense to use it for hot water and domestic power in houses and other buildings, especially since direct electricity results in better PER factors; if there is still biomass available beyond traffic needs, it should therefore be sent to combined-cycle plants (perhaps with waste heat recovery). Overall, only a very small budget of 20 kWh/(m²a) (relative to a building's residential/useful area) remains for direct domestic use in any kind of system (reconverted biogas, biogas in boilers for heating, wood or pellets for biomass boilers, etc.). Biomass fuel within this budget has an PER factor of 1.1; any methane used beyond that must be given the solar/wind gas factor of 1.75.
Analyses of combined systems always have to deal with a certain amount of uncertainty regarding how, exactly, the benefits are distributed to the various components. For the system in question here, we suggest a consistent electricity credit method, for the following reasons:
It is important to note that this method is appropriate here largely because the substituted energy source – P2G gas – is the same. This method would not work if the primary energy sources are different, especially when there is a mix of renewable and nonrenewable sources (see [Vallentin 2011]).
Another important consideration is, that the heat distribution can be quite expensive and also leads to additional heat losses [Kaufmann 2012]. The energy chain is considered with all distribution losses; the key figure is heat available to the end user. It is thus clear that a pure heat supply system without cogeneration via district heating systems is not recommended as part of a renewable energy system; electric heat pumps would be much more efficient. District heat plants also have potential if they use a heat pump as an additional, alternative heat generator. If enough renewable primary electricity is available, the heat pump can generate heat at the most affordable prices.
In a renewable energy systems, cogeneration plants must have high conversion rates and conversion efficiency. The share of distributed heat from cogeneration (or a heat pump) must be higher than 75 percent (and a maximum of just 25 percent of the heat can come from peak boilers). Only combined-cycle systems and fuel cells are really interesting; the electricity generation efficiency should be high (here: 54 percent compared to 60 percent for a combined-cycle system without cogeneration in the future). The technology in such systems will allow them to generate on low power as well, as can already be seen in the latest fuel cell advances.
With a combined-cycle conversion efficiency rate of 54 percent in cogeneration, a peak boiler efficiency of 92 percent, and district heat distribution losses of 15 percent, the primary power demand PERdistrict for district heat generation, including electricity credit, is shown in the following table for different cogeneration rates. It is easy to see that the share of cogeneration heat is absolutely decisive for determining a system's suitability. In light of this, moderate heat storage seems like a good idea so that the systems can actually run based on how much electrical power is needed at the time. This approach is less complicated than it may seem, since P2G energy mostly needs to be reconverted to provide electricity for indoor heating applications. This, of course, works well with district heat systems, which mostly require heat for indoor heating.
|Cogen share of heat production||0%||60%||65%||70%||80%||85%||90%||95%|
|Combined-cycle district heat from P2G||2.52||1.88||1.75||1.61||1.29||1.12||0.93||0.73|
Table 1: PER factors for district heat from cogeneration
For comparison's sake, the PER factor of renewable electricity for heating is 2.2 (wintertime gaps covered with P2G reconversion);
of heating with a condensation boiler using P2G methane, 1.75.
Passive House – the next decade - Focus, Consequences and outlook and References