I am hoping that someone might know the answer to a funny effect I’m seeing in a recent model - I’ve put together a quick sample .gh file that illustrates the effect and attached it here.
I’m modeling a simple ‘Ideal Air Loads’ system and trying to look at the effect of Heat Recovery on the ventilation air. I’ve set up the example here to isolate just the mech vent (I’ve zero’d out all the windows, economizers, infiltration, other loads, etc… ) and I’m using just a simple ‘Office’ program with default schedules.
My question is why it might the case that even with ‘perfect’ (or near perfect) 'sensible and latent heat recovery I still see a fairly substantial amount of ‘Mechanical Ventilation Energy’ in the ‘readEPResult’ component?
I have tested it at several different efficiency % values and compiled just a few in a excel I’m attaching here as well. Just looking at 3 steps (0%, 50%, 100% HR) I am getting results like this:
As you can see in the output, even at ‘perfect’ HR it still registers quite a bit of energy loss over the year. Does anyone know what might be going on here and why I’d still be getting Mech Energy output even with ‘perfect’ HR applied? I double checked the csv output and I do believe I don’t have any other source of air into the zone other than the ‘mechanical’ on this zone.
Does anyone have an idea what might be causing this to occur?
No I never did figure out what was going on there. If you ‘0’ the Ventilation Loads you can eliminate any ‘Mechanical Ventilation’ losses from the results, but as soon as you include any ventilation airflow, even with a 99.99% effective HRV you still see ventilation losses in the final results. Granted, its a small amount compared to the transmission losses - so maybe its just that last 0.01%?
If I’m understanding your question correctly, I think the ventilation losses you are getting are correct as from a thermodynamic perspective, you can only recover a fraction of the heat from exhaust air.
For example, if you have a system where your exhaust air is recirculated into a mixing chamber with fresh outdoor air, and assuming the box is perfectly insulated, you will have the following relationship between the input and output air:
So the resulting enthalpy is the weighted average of the exhaust and outdoor air, by their air mass flow rate (same thing also applies to temperature assuming the same specific heat capacity, and humidity ratio). Enthalpy is the total heat content of our air, so this equation proves that you can’t transfer all your heat from one air mass to another. Assuming equal exhaust and outdoor air rates, the best you can recover is half the heat content.
In your bar chart, you can also see that going from 0 to 99.99% heat recovery is reducing the mechanical ventilation more then 50% which is something I can’t explain. I’m not an expert on HVAC systems, so there may be something I’m missing on how mechanical heat recovery systems work, but this is how I would think about it.
Thats interesting - I imagine you might be right. But I guess I though the Heat Recovery Ventilation in the E+ modeling was something like a dedicated 100% fresh air flow though, without any recirculation or ‘mixing’ with exhaust air. Certainly you could model that as a system, but for the ‘ideal air loads’ I’d have thought they were separate?
If you have an exhaust air stream (1), and a supply air steam (2), and they are fully separate inside the HRV core (no mixing), I would have thought with 100% (99.99999%) sensible and latent recovery efficiency that means you’d pass 100% (99.99999%) of the thermal energy from air stream 1 into air stream 2, no?
But maybe you are right and this component isn’t modeling this 100% fresh air scenario?
I think this might have something to do with the “Ideal air load”, and also the Mechanical Ventilation value is not a direct output from the simulation (HB post-processes from zone heating and cooling energy), this might not be a good indicator for testing heat recovery efficiency in ideal air load.
But it is possible to test this recovery efficiency in a real DOAS system with heat recovery, and generate outputs for energy consumption of downstream cooling and heating coil, for mech vent cooling and heating.
@edpmay, yeah I see what you’re saying. Your reasoning makes sense to me for a theoretical system (adiabatic duct boundaries so no ambient waste heat loss, discounting mechanical work done by fans to move air through ducts - which I believe is discounted in ideal air loads anyway), not sure why it wouldn’t work. HB calculates mechanical ventilation by subtracting the energy from the supply coil for that zone, from the zone energy .
I agree with @MingboPeng that there may be some intricacies in the HB ideal air loads implementation that might be creating unknown side-effects to mitigate the heat recovery.
I may be able to shed a little light here by at least pointing out that you have not changed the temperature of the ideal system supply air to be close to that of a DOAS, which at least pokes a hole in one of your assumptions. True to its name, the ideal air system works by trying to produce ideal air at a specific heating/cooling supply temperatures and humidities, which are usually different than the thermostat setpoints of the zone. The default supply temperatures for the ideal air system are meant to mimic what is typically seen in most air-based HVAC systems (I think we set them at 40 C for heating and 13 C for cooling). If you set them to be closer to what you’d expect to see coming out of a DOAS (like 18 C for cooling and 22 C for heating), this would result in huge (unrealistic) flow rates of supply air for a zone that has typcial heating/cooling demands. However, for your adiabatic, 0-loads space that you are describing here, narrowing this supply air setpoint band might allow you to see the effect you are hoping to see.
So I think you should be able to get your ventilation load to be closer to 0 by narrowing the band that the ideal air supply temperatures are allowed to operate in by using the corresponding inputs on the “Honeybee_HVAC Air Details” component. If you narrowed this band all of the way to 0 and set both supply air temperatures to be the exact same as both the heating and cooling setpoints of the zone, you might be able to get the “0 ventilation energy” effect that you are hoping to see. But ultimately, this effect really is one based on a stead-state assumption and it doesn’t lend itself well to the transient calculation that EnergyPlus is running. So no promises here.