1. L2 Compliance: Project Darwin
SBEM vs Apache Sim

2. Introduction
Neil,
This document is just to give you a feel for the major differences which exist between the SBEM & Apache Sim L2 Compliance Frameworks.
SBEM is by definition “simplified”. It is a form of steady state calculation which runs a series of iterations to obtain the annual overall energy results. Apache Sim however uses Dynamic Thermal Simulation (DTM).
DTM breaks the analysis down into 6 minute time steps - or less if required. By monitoring the building at this very detailed level over the full year and using real site historical weather data recorded at hourly intervals a very detailed energy analysis can be simulated.
SBEM uses simple calculations whereas Apache Sim models the complex building physics which are at work. By modelling these building physics at 6 minute intervals a multitude of variables can be monitored which are simply not possible using the SBEM framework – these include solar shading, interzonal bulk airflow, thermal storage of concrete slabs etc
What this really means is that an SBEM calculation can not really be compared with an Apache Simulation. The main purpose of SBEM is to provide a free tool endorsed by the UK government which can be used to assess all new buildings for compliance under the newly implemented National Calculation Methodology (NCM).
The NCM is the UK’s official framework which has been created in order to allow the energy performance and CO2 emissions of all new buildings to be assessed and compared against target benchmarks in order that the UK can achieve the targets specified by the European Performance of Building Directive (EPBD).
Although SBEM is the UK’s official tool for assessing the performance of a building under the NCM it is not exclusive. Other tools are available and can be used to assess whether or not a building complies - providing the tool is approved by the UK Department of Communities & Local Government (DCLG).
The IES Apache Sim method has been approved by the DCLG as an official tool which can be used to test a building under the NCM – but this does not mean that the results obtained from an Apache Simulation will be the same as results obtained for an identical building modelled using the SBEM interface.
There a number of major differences which exist between both methods – these are summarised on the following pages.

3. HVAC System Specification – in SBEM
final tab allows for any additional control corrections to be defined,
choices made here again effect the calculation of SCoP, SSEER & auxiliary energy
auxiliary energy is non-editable & is a function of system type selection
which is calculated using a default algorithm within SBEM
default options for any additional system adjustments,
choices made here effect the calculation of SCoP, SSEER & auxiliary energy
heating system options are restricted,
heating SCoP is calculated using a default algorithm
cooling system options are also restricted,
cooling SSEER is calculated using a default algorithm
system type must be specified from pre-defined list
HVAC system specification is limited

4. HVAC System Specification – in Apache Sim
SCoP linked to generator efficiency via “heating delivery efficiency”
note: this approach differs from SBEM
generator seasonal efficiency entered manually
SCoP entered manually
i.e. not calculated via SBEM algorithm
fuel type selected from drop down
“Auxiliary Energy” is fully editable – any W/m² value can be entered in order to make an allowance for the HVAC systems combined annual energy usage.
note: auxiliary energy is non-editable in SBEM and is dependent on the system type selection.
a default algorithm is then used to calculate the final auxiliary energy value
generator seasonal
Energy Efficiency Ratio (EER)
entered manually
cooling mechanism chosen manually
[nat vent, mech vent
or air-con]
Note: this is not an option in SBEM, instead
cooling mechanism is automatically determined from system type selection
System Seasonal
Energy Efficiency Ratio (SSEER)
entered manually
i.e. not calculated via SBEM algorithm
SSEER linked to EER via “cooling delivery efficiency”
and “heat rejection pump & fan power”
note: this approach differs from SBEM & is required due to the differences between steady state & dynamic simulations

5. DHW System Data
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DHW specification in Apache Sim mode is handled differently. A dedicated system must be created within the main HVAC view
DHW specification in SBEM mode allows the system type to be selected from a pre-defined list

6. Lighting Data
Apache Sim requires that lighting is handled differently due to the nature of dynamic thermal simulation. Sensors can be placed in zones and lighting controlled via “dimming profiles” which allow complex controls to be modelled at each time step of the simulation using the real site solar intensity weather data.
in SBEM a dedicated lighting dialogue allows for a range of lighting control options to be specified at a basic level.
LIGHTING POWER DENSITY
LIGHTING CONTROLS
LIGHTING POWER DENSITY
SENSOR SETTINGS FOR DETAILED DYAMIC SIMULATION
OF DAYLIGHT CONTROLS USING APACHE SIM MODE
OCCUPANCY SENSING
LIGHTING CONTROLS & OCCUPANCY SENSING CONTROLLED VIA DYNAMIC “DIMMING PROFILES”

7. LOCAL MECHANICAL EXHAUST & DESTRATIFICATION FAN POWER SETTINGS
Additional Extract & Destratification Fan Energy
SBEM provides check-boxes for specifying “mechanical extract” & “destratification fans” at local zone level.
These options exist because auxiliary energy cannot be edited directly in SBEM, so by specifying these options at zone level correction factors are then made to the final auxiliary energy algorithms used in the overall energy calculation.
Apache Sim mode allows an exact value of auxiliary energy to be entered manually – so in this way the energy requirement for extract & destratification fans can be allowed for at system level.
LOCAL MECHANICAL EXHAUST
& DESTRATIFICATION FAN POWER SETTINGS
AUXILIARY ENERGY ENTERED MANUALLY IN APACHE SIM MODE

8. CHP SETTINGS WHICH ARE AVAILABLE WHEN USING APACHE SIM MODE
CHP & Link to Absorption Chiller
Apache Sim allows a CHP link to be modelled and monitored at every time step of the dynamic simulation.
Project Darwin utilises CHP which serves both space & water heating requirements in addition to cooling requirements via an absorption chiller.
Fuel Type, Heat Output and Thermal & Power Efficiencies at both full & part load can all be allowed for using Apache Sim mode.
CHP SETTINGS WHICH ARE AVAILABLE WHEN USING APACHE SIM MODE
SBEM does not support CHP and it is therefore not possible to model Project Darwin directly. Some form of assumption would need to be made to allow for an approximate figure on CO2 reduction due to CHP and tri-generation.

9. SBEM does not support solar shading calculations.
Apache Sim uses a geometric model to represent the building. This then allows the Suncast link to be used to simulate solar shading in detail which monitors the exact percentage of surfaces which are in/out of shade for each time step of the simulation.
SBEM does not support solar shading calculations.
This means that the benefits of self shading on the building cannot be modelled which could potentially lead to large differences in results when compared against Apache Sim mode.
When linked into the Apache Simulation engine these geomteric shading calculations translate directly into reductions in solar gain based on the real site weather data.