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Example: Overall power provision for a Passive House

A Passive House single-family home was used to simulate a renewable supply structure with the method described in the previous section. Figure 1 shows primary power gene­ration for a mix of 60 percent photovoltaics (PV) and 40 percent wind energy (wind) in the TRY12 (Oberrheingraben) region from January 17 to February 4. Production is clearly irregular, with PV providing almost 1,600 watts around noon on February 26 (a clear day) and absolutely nothing at night; from January 28 to 30, the sky is cloudy and primary production from PV drops to a maximum of 150 watts around noon. On those cloudy days, winds are also not high enough for the turbines in the region. On days like that, power must mainly come from mid-term grid storage or even from P2G methane reconversion. On January 17, on the other hand, wind speeds allow the turbines to produce the maximum amount of electricity, which is more than is immediately needed for consumption. Wind and solar tend to have an anticyclical relationship. It is therefore much more cost-effective to use a mix of different energy sources rather than rely on only one primary power generator. A much higher share of power produced can then be directly used and there is less need for investing in storage technology, which itself leads to additional losses and costs, anyway.

Figure 2 depicts power demand in this four-person Passive House home over the same time period (top curve). As usual, energy is taken from the grid for normal household appliances (fully equipped), cooking, and lighting. However, all devices in question are highly efficient, with the current market's top devices used as a standard (domestic power including lighting; all electronics and cooking appliances; and ventilation system operation and any pumps for heating, hot water, and auxiliary power for all building services equipment: 2,884 kWh/a). Throughout the year, water is heated using an electrical hot water heat pump with storage; the overall hot water system's COP (as designed for this study) is about SPFHW = 2.6 (system performance factor – hot water), while annual power consumption for the system comes to about 1,123 kWh/a. A standard tap plan and a useful storage volume of 260 liters are assumed. Heating is also provided by a heat pump, which could be the same heat pump that heats domestic water or a compact unit; we purposely have not focused on a certain technology or configuration here. The simulation of the moderately efficient system results in an SPFheat (system performance factor – heating including all auxiliary energy) of 2.3. The power curve for heating demand for the Passive House building under investigation was determined using a dynamic thermal simulation for the TRY12 site (average).

Figure 1:
Primary power production, mix of 40 % wind and 60 % PV – late January / early February.

Figure 2:
Electricity demand for total power and share of primary power (circles),
from short-term grid storage (for example pumped storage, thin solid line)
and seasonal storage using P2G methane (dashed line).
Excess electricity (thick solid line) is sent to the P2G converter.

Annual heat demand amounts to 2,398 kWh/a. In terms of consumption, the peaks from cooking activities are particularly noticeable in Figure 2 – but those peaks become less noticeable when overall grid load is considered. Since not all households cook at the same time, the power used for such tasks is actually spread out over a longer period of time – one of the major advantages of the grid, since this balancing function is particularly important for an energy supply based on renewable primary energy sources (this is acknowledged in the model used here using convolution with a distribution function). PV power also makes a significant contribution to covering those daily “cooking peaks.” Nevertheless, for this winter period, primary power generation over time and the load curve are absolutely not in harmony (thin solid line with squares: directly useable primary power). The simultaneity of various consumers (the grid effect described above) and available short-term storage, however, mean that total power demand could still be covered with direct generation and short-term storage (solid thin line without symbols) from January 17 to 19, for example. This situation did not hold true for the next three cloudy days, when PV generation was minimal, winds were low, and short and mid-term grid storage had been used up. The grid then had to turn to reconversion from synthesized P2G energy (from the previous year, for example). In Figure 2, the dashed line represents the share of power coming from reconversion plants (combined-cycle systems, in some cases with cogeneration). July 14 (see Figure 4, day 196), for example, is a completely different story, with primary power almost completely able to cover power demand; while there is significant excess energy that initially fills mid-term storage “to the brim,” it is then used for the P2G methane synthesis process (thick solid line). Energy can thus be saved for times of lower production (especially in the winter), although with significant losses (efficiency for the entire chain is about 30 percent). Power provided in those time periods will therefore be noticeably more expensive in the future, especially for extremely anticyclical applications (relative to renewable power production). The amount of primary energy needed for an application in the future will therefore depend greatly on when it requires power.

Figure 5 depicts the load curve for the energy stored to supply the house under investigation, measured in kWhCH4. The capacity in this case comes to 1,270 kWhCH4; a maximum of about 127 Nm³ (normal cubic meters) of natural gas could be stored for this building under normal conditions1). In order to load the storage system, about 1.75 times as much primary power needs to be generated to compensate for losses from electrolysis and methanation. Primary power demand for seasonal storage therefore amounts to about 1,688 kWh.

Figure 3:
Primary power production, mix of 40 % wind and 60 % PV, July 14 to 28 in Frankfurt am Main, Germany.

Figure 4:
Electricity demand for total power and share of primary power (circles),
from mid-term grid storage (for example pumped storage, thin solid line)
and seasonal storage using P2G methane (not required in this time period, July 14 to 27).
Excess electricity (thick solid line) is sent to the P2G converter.

Since, however, a significant share of power demand can be covered by primary power generators and a similar share can come from mid-term grid storage (with an efficiency of more than 70 percent, much higher than the P2G system), the average renewable primary power expenditure over all applications PERall comes out to about 1.46 kWhPER/kWhel in this case. This figure is acceptable for the sum of all applications in a residential building. Equivalent PV area (including the share of wind power) is 68 m² for this example, about the same as the building's floor space (or 45 percent of its residential space). Complete supply from primary power generated on site is therefore absolutely realistic for a home built to the Passive House Standard, even one with two to three stories located in central Europe. For a Low Energy House with an annual heating demand of 60 kWh/(m²a), the equivalent solar area required almost doubles to 124 m², about as much as the building's entire residential space. This example alone shows how well suited Passive House energy efficiency is to a completely renewable energy supply [Feist 2013a][AkkP 46].

Figure 5:
Load curve for seasonal storage using P2G methane

Showing the heating value of the stored P2G methane at each ANm³ of methane;
From this, about 672 kWh of electricity can be produced in combined-cycle systems (or fuel cells),
covering about 13 percent of total power demand.

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In existing houses, an average about 3,600 m³ of natural gas equivalent (fossil energy) is “consumed” in a building of 150 m²; the remaining methane demand for this Passive House building is therefore only about 3.5 percent of today's value. This amount could even be generated from biogas in a sustainable way. Even if such P2G energy were much more expensive than natural gas is today, the small amount needed would still be quite affordable. Good energy efficiency – especially for heating – guarantees an economically and socially acceptable supply, even in the future.
basics/passive_house_-_assuring_a_sustainable_energy_supply/example_-_overall_power_provision_for_a_passive_house.txt · Last modified: 2014/09/18 18:19 (external edit)