ChW coil basline sizing and modeling update

joesinger12 commented 8 years ago

Background: Currently, we ask users to specify the gross capacity and design fluid flow rate of their ChW coils. We also default the fluid flow rate based on the ChW loop design dT and the total capacity, so really, all users need to do is define the gross capacity. This was done to simplify the inputs, and also because when we developed the initial SDD data model and translator, we did not review the impact of use E+ defaults for characterizing the performance of ChW coils in simulations.

The Issue: There are two: 1) Proposed model: The inputs for defining ChW coils are simple, but they do not allow explicit distinction between total and sensible capacity. The current default assumptions we use (from E+) to calculate the total/sensible capacity are probably not that not far off for most cooling applications (design SAT of 55F, leaving RH ~90%), but for design conditions that differ from this, I believe our current approach may lead to unexpected results. 2) Baseline model: The design conditions assumed in reporting the sizing run capacities, and the conditions we specify the capacities for the annual baseline run, are not "apples to apples". What I mean is: Currently, we are doing a E+ sizing run using design days. We retrieve capacity values from the E+ sizing run output. These capacities are reported at a specific design condition, which is not entirely clear unless you dig into the E+ engineering reference. In the rules, we manipulate the capacities to include a 1.15 sizing factor, and and also make an adjustment for fan heat. We then specify the final total capacity for the sizing run. However, we have confirmed that the default design conditions used by E+ in the annual simulation differ from design conditions used in the reporting sizing run capacities. Similar to the proposed model, we also have the issue of not clearly defining the ratio of total to sensible capacity.

Impact on Results: We've done some testing, using the 040012-OffLg-Std model, to evaluate the impact of this potential change on the standard design results. That is, we created a model that represents (what I believe is) the correct interpretation of E+ sizing run results representation of capacities in the annual standard design simulation. The impact is small, in our test only changed the compliance margin by 0.3%. The difference is not large, however, my concern is larger differences for atypical design conditions or coil selections, such as may be the case for DOAS and other potential low-energy designs.

Proposed Changes to Address the Issue: 1) 360: Updates to CBECC UI and data model: Basically, these amount to asking users to specify sensible capacity and their design inlet and outlet air conditions (dry bulb and wet bulb) for the coil. In my experience, total and sensible capacities are readily available on drawings, and most good engineers also include the inlet/outlet air conditions. However, we can minimize the user's inputs by defaulting the air conditions using total/sensible capacities and the design leaving air drybulb. This additional information would be used to explicitly specify the conditions in the annual E+ simulations, thereby making sure we represent the coils as they should be rather than utilizing E+ default conditions. 2) 360: Updates to rules to deal with these new inputs properly. 3) Scott: Adding the capability to do psychometric calculations in CBECC. Basically, a new function that allows specifying two properties of moist air, such as dry bulb and wet bulb temps, and from this calculating the other properties, such as density, enthalpy, etc. Rather than developing from scratch, I'd proposed we adapt existing E+ software code for psychometric calcs, or from some other open-source project, such as CoolProp. 4) Kyle: Some straightforward SDD to OS translator updates to utilize the new properties we add to the data model.

Reported by: joesinger12

Original Ticket: cbecc-com/tickets/1532

joesinger12 commented 8 years ago

Milestone: Com2016 --> Com2016B