The amount of wastewater created by the system can directly be retrieved from the amount of DHW required and then be used as the low temperature source for a heat pump.
Water-water HP 1
Water-water HP 2
Heat 70-80°C forward
For correctly modelling the wastewater, three energy carriers must be defined (names are up to the users):
Hot water (as the output of the technologies)
Hot water forward (as the demand)
Hot water return (the wastewater)
A virtual technology will then create one unit of hot water forward and hot water return for each unit of hot water produced.
It is necessary to add an export of wastewater, to assure that the energy balance can be solved, when DHW is to be produced but there is not heating demand that requires wastewater to be used.
In a similar reflexion, there need to be a technology allowing to create heating, which is not based on wastewater (for cases where no wastewater is available). In this case, the heat pump is considered to have a second mode, working on an alternative anergy source.