L2 Compliance: Project Darwin

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L2 Compliance: Project Darwin

SBEM vs Apache Sim


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 CO 2 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.


HVAC System Specification – in SBEM

HVAC system specification is limitedsystem type must be specified from pre-defined listheating 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 algorithmdefault options for any additional system adjustments,

choices made here effect the calculation of SCoP, SSEER & auxiliary energyfinal 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 selectionwhich is calculated using a default algorithm within SBEM


HVAC System Specification – in Apache Sim

fuel type selected from drop down

generator seasonal efficiency entered manually

SCoP entered manually

i.e. not calculated via SBEM algorithm

SCoP linked to generator efficiency via “heating delivery efficiency”

note: this approach differs from SBEM

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

generator seasonal

Energy Efficiency Ratio (EER)

entered manuallySystem 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

“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


DHW System Data

DHW specification in SBEM mode allows the system type to be selected from a pre-defined list

empty

DHW specification in Apache Sim mode is

handled differently. A dedicated system must be

created within the main HVAC view


Lighting Data

in SBEM a dedicated lighting dialogue allows for a range of lighting control options to be specified at a basic level.

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.

LIGHTING POWER DENSITY

LIGHTING CONTROLS

OCCUPANCY SENSING

SENSOR SETTINGS FOR DETAILED DYAMIC SIMULATION

OF DAYLIGHT CONTROLS USING APACHE SIM MODE

LIGHTING POWER DENSITY

LIGHTING CONTROLS & OCCUPANCY SENSING CONTROLLED VIA DYNAMIC

“DIMMING PROFILES”


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.

LOCAL MECHANICAL EXHAUST

& DESTRATIFICATION FAN POWER SETTINGS

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.

AUXILIARY ENERGY ENTERED MANUALLY IN 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.

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.

CHP SETTINGS WHICH ARE AVAILABLE WHEN USING APACHE SIM MODE


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.

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.

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.

L2 Compliance: Project Darwin

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