An Analysis Model for Domestic Hot Water Distribution Systems

Abstract

A thermal model was developed to estimate the energy losses from prototypical domestic hot water (DHW) distribution systems for homes. The developed model, using the TRNSYS simulation software, allows researchers and designers to better evaluate the performance of hot water distribution systems in homes. Modeling results were compared with past experimental study results and showed good agreement. The model was also compared with existing domestic hot water distribution system modeling software HWSIM for verification. The developed model has several capabilities that are not available in HWSIM, including the ability to integrate any new or existing types of water heater systems, the ability to handle several simultaneous draws to different end uses, and the ability to handle unique annual draw profiles instead of weekly draw profiles. It also allows for draw profiles and ambient conditions to be considered using any time resolution. To demonstrate the abilities of this new model, a series of sensitivity analyses were performed using a benchmark domestic hot water distribution system. The effects of adding insulation to the domestic hot water distribution system of homes with a gas water heater and a solar water heater were also examined.

Circulo

“…We calculate the time delay between the fixture event's time stamp and the most recent circulation event, including either pump-initiated recirculation or user-initiated circulation due to water fixture usage. Once the time delay is known for each fixture usage event, we calculate ∆T based on the fact that the water in the pipe would drop in temperature by approximately 1 • C approximately every 4 minutes [10].…”

Validation of a Hot Water Distribution Model Using Laboratory and Field Data

“…The ongoing development of advanced simulation models, such as the HWSIM distribution system model (Springer et al 2008) and recent TRNSYS modeling work completed by the University of Colorado at Boulder and NREL (Maguire et al 2011), represent valuable steps in the process of developing the tools to improve quantification of hot water system performance, ultimately leading to the identification of cost-effective improvement options that will contribute to Building America's goal of 30%-50% energy savings.…”

“…Given that TRNSYS is widely recognized as a mainstream building system simulation tool, the decision was made to focus the validation effort on using TRNSYS with the newly developed Type 604 "bi-directional node pipe model." This pipe model, described in full detail in Appendix A, is an enhanced version of the Type 709 model used in prior work (Maguire et al 2011). The Type 604 dynamically models pipe exterior surface convective and radiant heat transfer effects based on fluid, pipe, and environmental properties, as opposed to Type 709's use of a fixed pipe outside heat transfer coefficient.…”

“…Previous analysis of this model suggests that a 6-s time step captures the realistic minimum duration of hot water draw events while saving some processing time, when compared to 1-s time step (Maguire et al 2011). The discrete draw events are then processed to make a TRNSYS input file that lists draw volumes and fixture type indicators (see coding in Table 2) for every time step.…”

Hot Water Distribution System Model Enhancements

“…In addition, copper surface emissivities, which will vary over time due to oxidation, are difficult to determine. Overall, based on the validation results, we feel confident that the enhanced HWSIM model, and the recently developed TRNSYS model (Maguire et al 2011), are ready for the next validation step of using detailed site monitoring data (NREL's Solar Row dataset) to drive the model. Model outputs in the form of end use point hot water outlet temperatures can then be compared to model-predicted outlet temperatures.…”