Influence of mixing valve dynamics and recirculation loop connection to solar tank on large hot water system performances

Abstract

A centralized solar domestic hot water system for a multi-family building is analyzed numerically. The solar preheated water in the solar tank is flowing into an auxiliary tank, connected to a gas boiler. At its output, a mixing device is used to ensure hot water at steady temperature before going through the recirculation loop. This reference system is sized as a typical installation under mild climate in France. The load profile is generated statistically with a one-minute time step. Small time scale phenomena have consequences on the global system behavior: recirculation flow rate variations due to pressure drop change is modeled and validated experimentally. This work's goal is to analyze these effects on two sub-parts of the plant: mixing valve and recirculation loop connection to solar storage. In each case, yearly simulations with 6 seconds time step provide quantitative performance indicators for different sizing and control options. Results show that mixing valve slow response, due to ageing or bad design, leads to a loss of 3.5% energy savings compared to using a fast component. Furthermore, health risks increase as the minimal temperature in the recirculation loop is often under 50°C, which promotes legionella development due to cold water plug flows generated in the loop by the mixing valve dynamic response. The recirculation loop, linked to a solar tank, leads to performance improvement with 6% to 15% increase in energy savings following solar oversizing and reduced overheating risks.