How to Model Bivalent HVAC Systems in IESVE
VE version 6.1 and earlier
A work-around we could employ would be to use the CHP function to represent the lead heating system. With this approach we can enter the thermal efficiency properties of the GSHP to the CHP generator and set the electrical efficiency to 0.
Consider the following example system:
- Primary Heating Source = ground source heat pump (80kW base load )
- Secondary Heating Source = Natural gas boilers (size @ 350kW)
We can set the maximum output to the system to 80 kW so that when the system is exhausted it will be topped up with the gas boilers defined within ApacheSystems.
Also, we may want to reduce the minimum fraction from the default so that the GSHP operates at loads lower than 40kW.
The main issue here is that electricity is not an available fuel selection for the CHP system. What we can do however is select a different fuel type not being used (i.e. Anthracite) but on the Fuel factors table change the emissions factor of anthracite to match that of electricity.
On the ApacheSystems dialog define the thermal properties for the gas boilers and check the CHP link. The base load will be provided by the CHP (in this case GSHP) up to it's maximum capacity. Above this the Gas boilers will top up the load.
VE version 6.2 and later
The only permissible way will be to use the Bivalent Systems option which essentially calculates a weighted Carbon emission factor and a weighted System efficiency based upon the annual contribution each system makes. We could use this same approach in 6.1.1 however the weighted Carbon Emission Factor and system efficiency will need to be manually calculated. With this approach we would need to determine what proportion of the load is supplied by each of the systems.
