Task 40
Task 40
SHC Task 40

Net Zero Energy Solar Buildings

Net ZEB Evaluation Tool - FAQ

What are the main differences between the selected definitions?

The four Net ZEB definitions differences are summarized in the table you get in the results’ sheet by pushing the button “click here to compare definitions”. The main differences regard the balance metric, balance boundaries and generation systems localization.

In particular the first two balances (Net ZEB limited and Net ZEB primary) are in terms of primary energy and differ only in balance boundaries: plug loads are not considered as negative items for the Net ZEB limited balance (as EU directive 2010/31 suggests), whereas they are included in the Net ZEB primary balance.

The Net ZEB carbon definition calculates the balance in terms of equivalent carbon emissions and includes all balance items and on-/off-site generation systems.

The Net ZEB strategic definition is meant to be used to test every kind of combination not included in the other three definitions as all supply and demand items are allowed. In this way strategic factors can be asymmetric primary energy factors or carbon factors. Furthermore, strategic factors can be used to promote or discourage the adoption of certain technologies and energy carriers as well as include the conversion of primary sources into energy carriers. More can be seen in Sartori I., Napolitano A., Karsten V., Net Zero Energy Buildings: A Consistent Definition Framework. In: Energy and Buildings (48), S. 220–232, 2012.

Can I calculate the balance for off grid buildings?

One of the assumptions made is that Net ZEB are buildings connected to any energy infrastructure with which they exchange energy. Autonomous buildings are not addressed in the task, as they are considered suitable only under special boundary conditions (e.g. remote locations, missing energy infrastructure…).

Which is the difference between load/generation balance and import/export balance?

In the load/generation balance interactions between generation systems and loads are overlooked. It is equivalent to assume that, per each energy carrier, the load is entirely satisfied by delivered energy while the generation is entirely exported to the grid.

Load/generation balance is easier to use with design data, as most simulation or calculation tools do not offer the chance of monthly of higher disintegrated data comparison (as building codes do not require). Therefore design calculations with estimated self-consumption which end up in a balance of energy import and export with the grid infrastructure are not possible. Import/export balance is more suitable to monitored data as only few meters at the physical boundary of the building are needed to check the balance.

Which is the difference between energy generation and exported energy?

The exported energy is the energy produced by renewable energy sources (RES) flowing from buildings to the grids. The generation is the overall building energy generation from RES. The generation may not coincide with exported energy due to self-consumption of energy generated on-site (see above).

Which is the difference between energy load and delivered energy?

The delivered energy is the energy flowing from the grids to buildings. This is the energy imported by the building and supplied to the technical building systems through the system boundary, to satisfy the uses taken into account (heating, cooling, ventilation, domestic hot water, lighting, appliances etc.) or to produce electricity. Delivered energy can be calculated for defined energy uses or it can be measured. The load is the building energy demand to satisfy the uses taken into account by the definition.  The load may not coincide with delivered energy due to self-consumption of energy generated on-site (see above).

Which is the difference between estimated and monitored building-grid interaction data?

They both refer to the import/export data calculation. Estimated building grid interaction data refer to dynamic simulations results, accounting for the simultaneousness of the energy supply from renewable sources and the loads. Monitored building grid interaction data refer to measured data about delivered and exported energy during a defined time period of building operation. Although the two sections require the same type of data, they have been separated to allow comparison between estimated and monitored import/export balance.

What is the difference between static and quasi-static weighting factors?

A static weighting system does not vary over time (the same weighting factors are used over the whole year). A quasi-static weighting factors uses weighting factors evaluated on monthly bases. Dynamic weighting factors may vary over shorter time steps, but they are not included in this tool.

What is the difference between symmetric and asymmetric weighting factors?

A symmetric weighting system uses the same weighting factors for both energy demand and energy supply. An asymmetric weighting system uses different weighting factors for energy demand and energy supply.

What is the difference between “Incentives for generation” and “Specific costs for exported energy”?

Incentives are meant as subsidized feed-in tariff for energy generated by renewable sources. Specific costs for exported energy are meant as the price of energy sold to the energy infrastructure.

All values should be specified in kWh/m². Which area do I have to refer to?

Users should refer to their relevant national methodologies and regulations for the area to consider. Please be sure that the area value is the same for all the data entered in the spreadsheet. Otherwise result comparison would not be consistent.

How can I deal with solar collectors generation?

The generation of heat form solar thermal collectors can be accounted in two cases:

  1. If the building exchanges heat to the district heating grid, it is recommended to write in the table the entire amount of heat generated by solar collectors and o write the load as it would be in absence of any generation and self-consumption in the table "Energy load of building systems".
  2. Else, if the building does not exchange heat with the district heating grid, two options are available:               
  • consider the solar collectors’ heat generation as totally self-consumed, thereby no value in this table should be entered, but in the table "Energy load of building systems" load should take into account the effect of generation and self-consumption; 
  • consider it as a generation, thereby the overall heat load should be entered in the table "Energy load of building systems" without accounting for the effect of generation and self-consumption.             

Do I have to enter total loads?

No, total loads have to be entered only if final uses itemization is not known. If the total energy load is entered, the balance calculation relies on that value and a message warns that the balances relevant to the Net ZEB limited and the Net ZEB primary definitions will output the same result, as the tool does not recognize the inclusion or exclusion of plug loads and built-in lighting.

Why do I get the same balance results for Net ZEB limited and Net ZEB primary definition?

As the two definitions differ only in balance boundaries, a same balance result is due to the lack of data about estimated delivered energy or load for plug loads and built-in lighting (in non-residential buildings only).

Are simulation data time step or the monitored data time step needed to calculate the balance?

No, they aren’t. Nevertheless they can be a useful information about the accuracy of the data.

Which values are required for the calculation of the thermal load match index?

Thermal load match index evaluates the fraction of thermal energy load covered by on-site energy generation. Data required are monthly energy load and generation for thermal energy exchanged with the district heating.

Solar fraction of solar thermal collectors could be calculated too entering solar collectors generation of thermal energy and thermal energy loads. However, these values are not consistent for the calculation of the balance by quasi-static weighting factors if the building is not connected to any district heating.

Where can I find weighting factors for my country?

Conversion factors for primary energy and carbon emission depend on several considerations, e.g. the mix of energy sources within certain geographical boundaries (international, national, regional or local), average or marginal production, present or expected future values and so on. By selecting the country from the location dropdown list in the sheet “building data”, default values for static weighting factors, provided by Task40/Annex52 participants, are automatically entered in the weighting factors sheet. Otherwise users should refer to their national regulation and codes.