Question-and-Answer Resource for the Building Energy Modeling Community
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Hello Konstantinos,
Please confirm that you want to model zone level terminal units and not central ASHPs and central GSHPs.
Assuming that you want to model zone level equipment and vertical, closed loop ground heat exchanger, here is my advice:
In DesignBuilder, there are two ways to model ASHPs: (1) using PTHP. This simulates "mini-split" systems, where a single fan coil unit is connected to an outdoor, air source condensing unit. This object conditions a single zone. You can add this object from the HVAC zone level of detailed HVAC. (2) using air-source VRF. This object allows you to connect multiple indoor VRF units to a single outdoor VRF unit. You add the indoor unit the same way you would the PTHP. From the HVAC zone level, go to the "forced air unit" icon and select the indoor VRF. For the outdoor unit, you add this from the "add loop" icon. The outdoor VRF object dialog has the open to select the condenser type: water-cooled or air-cooled.
For the GSHP:
(1) zone indoor unit: from the HVAC zone level, click the "add forced air unit" icon and select zone water to air heat pump. This object can be connect to the ground heat exchanger plant loop.
(2) Ground heat exchanger plant loop: to add a ground heat exchanger (GHX), you will need to add a condenser loop and swap out the cooling tower for a GHX. I size GHX using a different software, called GroundLoopDesign (GLD). This will provide a much more accurate representation of the intended borefield thermal response. This program calculates and exports a G-Function for import back into DesignBuilder. However, for just getting started, DesignBuilder does offer some default GHXs in the component dialog.
(3) Be sure to use the DesignBuilder help website for setting this up.
For ventilation: what are you doing for ventilation air? We typically specify central ERVs with heat recovery (energy recovery) devices. you can add outdoor air direct to the zone level units, but that is typically not done and would require many exterior penetrations.
DHW: you will need to add the DHW loop from the HVAC system level. Use the Design Builder help. If you want to use an ASHP for DHW, you can find that from the "load HVAC template" dialog, or you can add the "heat pump water heater" from the DHW loop supply side level. The ASHP DHW can be added from the HVAC zone level, meaning that the unit is inside a zone and sourcing heat from that zone.
Things to keep in mind: for the ASHP: remember that capacity is degraded as temperatures get cold and there are low temperature cut offs. However, in Athens, I doubt this will be much of an issue. Also remember, that you will need multiple VRF outdoor systems to condition all 10,000 m2 of space. We usually, don't specify outdoor units much larger than 20 tons due to refrigerant distribution issues. The outdoor units offer "heat recovery" option that allows for simultaneous heating and cooling. I would recommend specifying and modeling this feature. You will need to understand a little bit about the refrigerant distribution so you can address the piping loss factors.
for the GSHP, remember to modeled the pump energy as accurately as you can. Typical pump head for vertical boreholes are 30-60ft, depending on the depth of the bore hole. We use 500 ft depth. This has a large impact an annual energy consumption. I have found that it is best to expend pump energy for cooler water temperatures to the heat pumps. The improvement in heat pump COP is worth driving the pumps harder.
Bottom line: The water (ground) source heat pumps is the way to go. The energy sharing in the water loop is less complicated than in a refrigerant system and the water temperatures are going to send the heat pump COPs much higher than the air source alternative. Maintenance is far easier for the water source system. Water is much safer to have distributed all over the building compared to refrigerant. A refrigerant leak is a big deal, but water, not so much.
