2016 ASHRAE Annual Conference

Both the ASHRAE Standard 90.1 and California Title 24 Building Energy Efficiency Standards require AHU supply duct pressure setpoints on variable-air-volume (VAV) systems with direct digital controls (DDC) be reset at the zone level. While many different implementation methods have been proposed, the Trim and Respond (TR) method is one of the more popular strategies. Although the TR methods are popular many are difficult to successfully implement due to issues maintaining stable control, complexity of tuning parameters, or sacrificed zone level comfort. A newer “Tiered Trim and Respond” (TTR) strategy has improved control stability, increased response time and eased implementation in the field compared to the traditional TR method, while achieving similar fan energy savings. The TTR method recognizes that the instability of the Trim and Respond (TR) methods is often caused by targeting the system’s maximum damper position at nearly 100% open. At these extreme positions the system is most sensitive to disturbances or load changes and therefore stable control is difficult. The TTR method compares the maximum VAV damper command or position value to three different tiers of high/low thresholds, and responds by varying the trim and respond rates to adjust the static pressure set points. The targeted average maximum VAV damper value is lowered from the traditionally recommended 95% or 98% open position to a lessor range of 80% to 90% open. The TTR method pushes the setpoints slightly off the “ideal static pressure curve”, but provides more stable system control while maintaining a quick response to load changes. The TTR method is being implemented in a year-long demonstration at five different building sites in various building types and usage on four different building automation systems. Preliminary demonstration results show that among seven AHUs fan energy savings vary from 6% ~ 47% with the TTR compared their normal or existing fixed static pressure control setup.  In four of the RTUs the fan energy savings are 33% to 36%. Additionally, two of the demonstration AHUs utilize two implementations of Trim and Respond strategies. The TTR method is more responsive to load changes and maintains better indoor air quality while using a similar amount of fan energy. This paper describes the TTR methodology, the five demonstration sites and their direct digital systems, the preliminary energy savings compared to fixed static pressure control, and control performance characteristics compared to two different TR methods.

A rapidly growing amount of office buildings in the Netherlands is using an Aquifer Thermal Energy Storage (ATES) system. An ATES system uses a well pump to extract cold groundwater for cooling. The returned warm water is injected and stored in a second well. During winter this warm water is used for heating and the returned cold water is injected again in the first well. An optimal functioning ATES system can significantly reduce energy use and CO₂emissions of an office building. An essential condition for optimal ATES operation is the thermal balance of the system. Office buildings typically store much more heat than cold, causing the entire underground slowly to heat up and causing cooling capacity problems on the long term. This is compensated by using cold outdoor air to store additional cold during the winter, called regeneration. In this research a methods were evaluated to keep the thermal storage in balance Model Predictive Control (MPC) is used to control the amount of regenerated cold to maintain the ATES balance. The key element in the method is the reference model to calculate the expected stored amount and use as model for MPC. A reference model was constructed based on a case study building and it contains three main blocks: ATES, Heating/Ventilation/Air-conditioning (HVAC) and load simulation. For the ATES system a lightweight finite element simulation method is developed, based on an axisymmetric grid. An additional method was developed to reconstruct the injected water temperatures and volumes, because these were not measured in the case study installation. The HVAC and load simulation models are based on logged building management system (BMS) data. The use of BMS data has the large advantage that models are easily configured and can automatically adjust to changes in the building. Using MPC it was possible to keep the ATES in balance over a simulated 20 years period. By using a slight cold surplus as target, the effect of exceptionally warm winters is minimal and extraction temperatures are very constant. For the case study building it can be concluded that MPC, using the developed reference model, is capable of automatically maintaining the ATES balance. Because the case study building type and size is comparable to the majority of the new Dutch office buildings, it is expected that large parts of the method are universally applicable.

District cooling system (DCS) consists of chillers, cooling towers and pumps, and is widely used owing to high energy efficiency. However, due to higher energy costs and greenhouse gas emission issues, more energy-efficient total plant operation, not only the chiller system but also the cooling water system, is required to reduce the energy consumption and CO2 emission. In this report, we will present real-time online optimization control, to minimize the energy consumption, in district cooling plants.  As well as the theoretical background for real-time online optimization control methodology, the report contains examples of successful application including the reduction in primary energy consumption rates. Herein, the objective function of the optimization is to minimize the primary energy consumption rate whilst satisfying chilled water demand, and the optimization controls are constructed and realized by:  Optimum Chiller control, Optimum cooling tower control, Optimum cooling water pump control, Optimum primary pump control and Optimum secondary pump control. The effectiveness of the real-time online optimization control methodology and the actual reduction of the primary energy consumption by applying the optimization are shown based on the result of operation data.

Single zone (SZ) air handling units (AHU) are widely applied in a large conditioned spaces. A SZ AHU typically consists of a chilled water cooling coil, a hot water heating coil and a supply fan. For a constant volume (CV) SZ AHU or variable air volume (VAV) SZ AHU operating at a minimum airflow, the control valve of either the cooling coil or heating coil is modulated to vary the supply air temperature and consequently control space air temperature. Traditionally, a single control loop is applied to modulate the control valve directly based on the space air temperature. The traditional control is simplistic in nature however, suffers significant drawbacks. Due to the thermal capacity of both the water in the coils and the air in the conditioned space, the system often becomes unstable if the controller is not well tuned. On the other hand, cascade control makes the control system more adaptive and robust. A cascade control can be applied to SZ AHU in order to stabilize the system. The primary controller reads the room air temperature and determines the required supply air temperature for secondary controller which then controls the heating/cooling coil valve. The purpose of this paper is to demonstrate the stability of the cascade control method in SZ AHU system, theoretically and experimentally. First a theoretical model of the single zone AHU system with transfer functions is developed and root-locus analysis is performed with MATLAB. Then the experiment is conducted on a SZ AHU to evaluate the system performance using the traditional and cascade control respectively. The simulation and experiment results shows the cascade control stabilizes the system operation.