Variable chilled water supply temperature regulation strategy guided by thermal comfort studies

Since chiller plants consume a large portion of the energy use in the buildings with HVAC systems, growing attention has been paid to chiller plant optimization. A typical chiller plant includes devices such as chillers, cooling towers, primary pumps and condenser pumps. Operation settings of a device are zeros when the device is off; otherwise, they are within positive ranges. With nonlinearity in heat exchange, chiller plant optimization is a mixed-integer nonlinear problem. Establishing a problem formulation with a good balance between accuracy and simplicity is usually difficult. For example, water temperature is critical in improving chiller efficiency while chiller power consumption is a highly nonlinear function of the temperature. In this paper, for simplicity, devices of the same class are assumed identical. To save energy and simplify the problem, a formulation with chilled water supply temperature as an additional decision variable and condenser water supply temperature as a parameter is established. To obtain good solutions and save computational effort, surrogate Lagrangian relaxation (SLR) combined with sequential quadratic programming (SQP) with good initialization is used. Numerical testing shows that power consumption is significantly reduced with maximum savings around 18% in a partial load condition by using our method as compared with a baseline using current strategies.

Users' cooling needs also change as the outdoor climate continues to change. In order to ensure the cooling demand and timely grasp the cooling capacity transmission situation and the dynamic characteristics of the heating system of the cooling pipeline network, a simulation model was built to analyze the time delay of the cooling capacity transmission and the influence of the chilled water supply flow rate and temperature on the cooling system. The results show that the chilled water supply temperature has almost nothing to do with the time delay of cold transport, and the chilled water supply flow is the main factor affecting the user's time delay, and the lower the system load, the smaller the chilled water supply flow and the longer the cold transport time. The system load decreases by 10% step, and the cooling capacity of the pipe network, cooling capacity of a chiller, power consumption of the chiller and return water temperature of the cold station gradually decrease with time. The step of chilled water supply temperature increases by 10%, and the step of chilled water supply flow decreases by 10%. The cooling capacity of the system, the cooling capacity of the cooler, the power consumption of the cooler and the return water temperature of the cold station gradually increase with time. Moreover, the system transition time is longer when the chilled water temperature and water supply flow are disturbed simultaneously.

A Model Predictive Control algorithm was developed for the UC Merced campus chilled water plant. Model predictive control (MPC) is an advanced control technology that has proven successful in the chemical process industry and other industries. The main goal of the research was to demonstrate the practical and commercial viability of MPC for optimization of building energy systems. The control algorithms were developed and implemented in MATLAB, allowing for rapid development, performance, and robustness assessment. The UC Merced chilled water plant includes three water-cooled chillers and a two million gallon chilled water storage tank. The tank is charged during the night to minimize on-peak electricity consumption and take advantage of the lower ambient wet bulb temperature. The control algorithms determined the optimal chilled water plant operation including chilled water supply (CHWS) temperature set-point, condenser water supply (CWS) temperature set-point and the charging start and stop times to minimize a cost function that includes energy consumption and peak electrical demand over a 3-day prediction horizon. A detailed model of the chilled water plant and simplified models of the buildings served by the plant were developed using the equation-based modeling language Modelica. Steady state models of the chillers, cooling towers and pumps were developed, based on manufacturers performance data, and calibrated using measured data collected and archived by the control system. A detailed dynamic model of the chilled water storage tank was also developed and calibrated. Simple, semi-empirical models were developed to predict the temperature and flow rate of the chilled water returning to the plant from the buildings. These models were then combined and simplified for use in a model predictive control algorithm that determines the optimal chiller start and stop times and set-points for the condenser water temperature and the chilled water supply temperature. The report describes the development and testing of the algorithm and evaluates the resulting performance, concluding with a discussion of next steps in further research. The experimental results show a small improvement in COP over the baseline policy but it is difficult to draw any strong conclusions about the energy savings potential for MPC with this system only four days of suitable experimental data were obtained once correct operation of the MPC system had been achieved. These data show an improvement in COP of 3.1% {+-} 2.2% relative to a baseline established immediately prior to the period when the MPC was run in its final form. This baseline includes control policy improvements that the plant operators learned by observing the earlier implementations of MPC, including increasing the temperature of the water supplied to the chiller condensers from the cooling towers. The process of data collection and model development, necessary for any MPC project, resulted in the team uncovering various problems with the chilled water system. Although it is difficult to quantify the energy savings resulting from these problems being remedied, they were likely on the same order as the energy savings from the MPC itself. Although the types of problems uncovered and the level of energy savings may differ significantly from other projects, some of the benefits of detecting and diagnosing problems are expected from the use of MPC for any chilled water plant. The degree of chiller loading was found to be a key factor for efficiency. It is more efficient to operate the chillers at or near full load. In order to maximize the chiller load, one would maximize the temperature difference across chillers and the chilled water flow rate through the chillers. Thus, the CHWS set-point and the chilled water flow-rate can be used to limit the chiller loading to prevent chiller surging. Since the flow rate has an upper bound and the CHWS set point has a lower bound, the chiller loading is constrained and often determined by the chilled water return temperature (CHWR). The CHWR temperature is primarily comprised of warm water from the top of the TES tank. The CHWR temperature falls substantially as the thermocline approaches the top of the tank, which reduces the chiller loading. As a result, it has been determined that overcharging the TES tank can be detrimental to the chilled water plant efficiency. The resulting MPC policy differs from the current practice of fully charging the TES tank. A heuristic rule could possible avoid this problem without using predictive control. Similarly, the COP improvements from the change in CWS set-point were largely captured by a static set-point change by the operators. Further research is required to determine how much of the MPC savings could be garnered through simplified rules (based on the MPC study), with and without prediction.

A finer and higher spatial scale measurements of atmospheric variables, Air Temperature (AT), Relative Humidity (RH), Wind Speed (WS), and Mean Radiant Temperature (MRT), are essential for human-scale thermal comfort and Urban Heat Island (UHI) studies to help architects, urban designers and landscape architects to make thermally comfortable and climate-responsive design decisions. Fixed wireless sensor units with real-time data monitoring facilities are impractical and not cost-effective as they require more sensor units in a short spatial distance to capture the finer spatial scale measurements. On the other hand, the commercially available sensor units are expensive. Also, they do not facilitate the Globe Temperature (GT) measurements, a cost-effective way to calculate MRT, according to the ISO 7726 (1988) standards. Hence, we developed an affordable and portable weather station platform, with an edge device (containing Raspberry Pi micro-controller and sensors to measure the required atmospheric variables) and a centralised server. This GPS-enabled device is capable of measuring AT, GT, RH and WS, and can also perform some data manipulation, and pre-processing, store the data, and communicate the collected data to a centralised server via cellular for further processing and storage. To assess its performance, especially for human-scale thermal comfort and UHI studies, we carried out three tests under two scenarios ‘On the Move’ and ‘Stop-and-Go’: Test 1 to find the best housing and shielding design for the device, Test 2 to assess the GPS accuracy and performance, and Test 3 to understand data consistency, latency, sampling rate and stabilisation. We found that the ‘3-D printed semi-circular louvre’ performed well in protecting the AT sensor. Overall, the portable weather station worked well under the ‘Stop-and-Go’ scenario compared to ‘On the Move’ due to the finer and higher spatial inaccuracies of the GPS sensor.

Warship variable frequency constant-pressure water supply instrument is a new kind of water supply device applying alternating current frequency conversion technology and programmable logic control technology to warship water supply. The instrument can keep the water pressure at a constant value according to the rate of water flow by controlling the frequency of the pump. Its characteristics of nonlinearity, hysteresis and variability bring difficulties to system modeling. By interpolating and fitting of input and output data obtained from experiment making use of data processing tools in MATLAB software, the model of variable frequency constant-pressure water supply instrument for warship was established in the thesis based on least square system identification. Also, the error check and simulation research were conducted. The results indicated that the model can fully reflect working characteristics of variable frequency constant-pressure water supply instrument with high veracity and reliability.

A new energy-saving method of variable frequency and constant pressure water supply system based on fuzzy control theory and software control technique is presented in this paper. Firstly, collecting the pipe network pressure periodically through the pressure sensor, the differential pressure signal will be sent into fuzzy controller after comparing the pipe network pressure with the set pressure. Then send a precise control quantity into the PID regulator to control output frequency of the frequency converter after a series of fuzzy arithmetic and solution of fuzzy arithmetic, so as to achieve the purpose of pump speed and consumed power controlling. The operating principle of variable frequency and constant pressure water supply system and the design procedures of software programming are elaborated. Practical application shows that the variable frequency and constant pressure water supply system based on fuzzy control has good effect on energy saving and obvious economic benefits and is worth popularizing and applying.

Increasing chilled water supply temperature is recommended to save the energy as a control tool in buildings using BEMS. Though indoor humidity affects occupants' thermal sensation, health, and business efficiency work efficiency in office buildings, office building operate cooling system focusing on indoor temperature. In this study, the goal of the research is to set supply air temperature using new effective temperature comfort index which is considering both temperature and humidity and to compute chilled water supply temperature using the set supply air temperature. Therefore, to prevent indoor uncomfort of misoperation and to reach energy saving, unskilled building operator can easily set chilled water supply temperature up by suggested guide line that can set chilled water supply temperature based on mixed wet bulb temperature, chilled water flow and supply air temperature. We researched comfort distribution of buildings, and readjusted comfort range of new effective temperature limit based on report of complaint. Building of subject of study exceeded comfort range <TEX>$26ET^*$</TEX> suggested by ASHRAE, but we arranged comfort range limit to <TEX>$26.6ET^*$</TEX> since no complaint was reported. We proposed setting guide line to have highest chilled water supply temperature based on analysis of mixed wet bulb temperature that can consider both outdoor air and indoor air at the same time, a set supply air temperature by operator, and data of cool water flow rate passing through cooling coil to find the most appropriate chilled water supply temperature for set supply air temperature.

An office building located at Jinan equipped with ground-source heat pump (GSHP) system was selected as the research object. The GSHP system model was established using TRNSYS software. With the total energy consumption of the system as the objective function, several control strategies were proposed for the optimization work of water supply temperature at the load side of the heat pump unit. Firstly, a variable water temperature control strategy was adjusted according to the load ratio of the unit. In addition, the TRNSYS-GENOPT (TRNOPT) optimization module in TRNSYS was used to find the optimal water supply temperatures for different load ratios. After simulating and comparing the system’s energy consumption under the three control strategies, we found that the total annual energy consumption under the variable water supply temperature scheme is less than that under the constant water supply temperature scheme by 10,531.41 kWh. The energy saving ratio is about 5.7%. The simulation found that the total annual energy consumption under the optimized water supply temperature based on TRNOPT is lower than that under the variable water supply temperature scheme by 1072.04 kWh, and it is lower than that under the constant water supply temperature scheme by 11,603.45 kWh. The annual energy saving ratio of the system is about 6.3%. It is concluded that the optimized water supply temperature scheme based on TRNOPT has a better energy saving effect than the first two water supply temperature schemes.

&amp;lt;p&amp;gt;Water uptake under variable soil water supply is highly critical for the functioning of trees and the services provided by forests. Current climate projections predict an increasing variability of precipitation and thus a higher frequency of droughts alternating with extreme precipitation events. Reduced water availability is the most critical driver for tree mortality and impairment of trees&amp;amp;#8217; functions. Under variable water supply, both the ability of a plant species to utilize remaining water under drought and to immediately capitalize on soil rewetting from subsequent rainfall events will be crucial for its survival and competitiveness. High uncertainty still exists regarding the ecohydrological belowground interactions at the soil&amp;amp;#8211;root interface on short to seasonal time scales.&amp;lt;/p&amp;gt;&amp;lt;p&amp;gt;To overcome previous limitations, we carried out high-resolution &amp;lt;em&amp;gt;in situ&amp;lt;/em&amp;gt; observations of &amp;amp;#948;&amp;lt;sup&amp;gt;18&amp;lt;/sup&amp;gt;O&amp;amp;#160;in soil and xylem water to track the water uptake of beech trees based on the approaches of Volkmann et al. (2016a &amp;amp; b) in the hot dry summer 2018. We set up a laser isotope system to continuously probe the &amp;amp;#948;&amp;lt;sup&amp;gt;18&amp;lt;/sup&amp;gt;O signature in the water vapor in equilibrium with the soil water at different soil depths and with the xylem of beech trees in a forest in Switzerland and applied a Bayesian isotope mixing model (BIMM) to resolve the origin of the water taken up. Moreover, we installed xylem flow sensors, dendrometers and soil moisture sensors in the trees.&amp;lt;/p&amp;gt;&amp;lt;p&amp;gt;Mid of June the drought period started with extended phases of high temperature and only infrequent precipitation. At the same time, soil water content sharply decreased, especially in the upper soil layers and transpiration as well as radial growth started to decline, and this pattern became more pronounced until the end of August. In the soil water, strong &amp;lt;sup&amp;gt;18&amp;lt;/sup&amp;gt;O enrichment in the upper 5 cm and slighter enrichment in 15 cm developed during this period. The BIMM results indicated that tree xylem water was made up by &amp;gt; 80% of shallow soil water (0-15 cm) at the onset of the drought and that this contribution continuously dropped to &amp;lt; 20% by the end of August, when deeper soil water and groundwater became more important. End of August, intensive rainfall events along with decreasing temperatures terminated the drought period when shallow soil water pools became partially replenished, and transpiration increased again. Within days, the contribution of shallow soil water to tree xylem water increased and reached a share of &amp;gt; 70% a couple of weeks after the end of the drought. &amp;amp;#160;With the&amp;lt;em&amp;gt; in situ&amp;lt;/em&amp;gt; method applied here, real-time information of the plasticity of soil water use becomes available and we can l trace the effect of drought and drought release on root activity of trees in different soil depths.&amp;lt;/p&amp;gt;&amp;lt;p&amp;gt;&amp;amp;#160;&amp;lt;/p&amp;gt;&amp;lt;p&amp;gt;&amp;lt;strong&amp;gt;Volkmann THM, Haberer K, Gessler A, Weiler M. 2016a.&amp;lt;/strong&amp;gt;High-resolution isotope measurements resolve rapid ecohydrological dynamics at the soil&amp;amp;#8211;plant interface. The New phytologist&amp;lt;strong&amp;gt;210&amp;lt;/strong&amp;gt;: 839-849.&amp;lt;/p&amp;gt;&amp;lt;p&amp;gt;&amp;lt;strong&amp;gt;Volkmann THM, K&amp;amp;#252;hnhammer K, Herbstritt B, Gessler A, Weiler M. 2016b.&amp;lt;/strong&amp;gt;A method for in situ monitoring of the isotope composition of tree xylem water using laser spectroscopy. Plant, Cell and Environment&amp;lt;strong&amp;gt;9&amp;lt;/strong&amp;gt;: 2055&amp;amp;#8211;2063.&amp;lt;/p&amp;gt;

Energy Use and Performance of a New Variable Primary-Flow Chilled Water System

A high temperature differential between supply and return water is most cost effective in district cooling systems. The chilled water supply temperature relies on central plant operation while the chilled water return temperature mainly depends on cooling coil performance of air handling units (AHU) at consumers. Many factors affect the AHU cooling coil performance at the consumers, such as cooling coil size, chilled water supply temperature, AHU supply air temperature, space cooling load, outside airflow and conditions, cooling coil fouling condition and cooling coil control valves types. The AHU chilled water return temperature may deviate from its design valve under off-design conditions. The common method to maintain the required consumer chilled water return temperature is to install a bypass bridge between the chilled water supply and return at consumer connections. This paper studies the impact of these factors on the AHU chilled return water temperature and finally develops the improved chilled water return temperature control method.

In this paper, we study the mathematical model about variable frequency water supply system in the process of applying glue to particleboard. Based on the original linear ordinary differential model, the effects of time delay and the nonlinear factor are considered. Then, we obtain the delayed nonlinear differential equation associated with variable frequency water supply system. Further, we consider the existence and stability of the equilibria and the existence of several types of bifurcations in this functional differential equation. Next, we derive the normal forms of Hopf bifurcation and Bogdanov–Takens bifurcation by using the multiple time scales method and the center manifold reduction method, respectively, and analyze the classifications of local dynamics. Finally, by using Matlab software, we obtain numerical simulations with estimated values of parameters and show the existence of stable equilibrium, stable periodic-1, periodic-2, and periodic-4 solutions, and a complex chaotic attractor from a sequence of period-doubling bifurcations.

Objective: To analyze the pollution status and influencing factors of Legionella pneumophila in a secondary water supply facility in a city. Methods: From June to August 2020, a survey on the level of Legionella pneumophila in secondary water supply unit was carried out in a city in northern China, and 304 sets of secondary water supply facilities were included in the study. A total of 760 water samples were collected from the inlet and outlet water of the secondary water supply facilities and some water samples in the water tank were collected for the detection of Legionella pneumophila, standard plate-count bacteria and related physical and chemical indicators. Through questionnaire survey, the basic information of secondary water supply facilities and daily management of water quality were collected. Multivariate logistic regression model was used to analyze the influencing factors of Legionella pneumophila contamination. Results: Among 304 sets of secondary water supply facilities, most of them were located in residential buildings [57.24% (174/304)]. High and low water tank water supply, low water tank variable frequency conversion water supply and non-negative pressure water supply accounted for 26.6% (81/304), 36.8% (112/304) and 36.5% (111/304), respectively. About 25.7% of facilities (78/304) were positive for Legionella pneumophila. Among them, the positive rates of Legionella pneumophila in high and low water tank water supply, low water tank variable frequency conversion water supply and non-negative pressure water supply facilities were 38.3% (31/81), 29.5% (33/112) and 12.6% (14/111), respectively. The results of multivariate logistic regression model analysis showed that the disinfectant residue could reduce the risk of Legionella pneumophila contamination in water samples, and the OR (95%CI) value was 0.083 (0.022-0.317). The increase of the standard plate-count bacteria and conductivity might increase the risk of Legionella pneumophila contamination in water samples. The OR (95%CI) values were 3.160 (1.667-5.99) and 1.004 (1.001-1.006), respectively. Compared with the non-negative pressure water supply, the risk of Legionella pneumophila contamination of secondary water supply facilities was increased by water supply from high and low water tanks and variable frequency conversion water supply from low water tanks, with OR (95%CI) values of 4.296 (2.096-8.803) and 2.894 (1.449-5.782), respectively. Conclusion: The positive rate of Legionella pneumophila in secondary water supply in the study city is high. Disinfectant residue, conductivity and method of water supply are associated with the positive rate of Legionella pneumophila.

In this paper, a kind of variable frequency and multi-pressure water supply control system based on STM32 processor is designed. It can adjust the water pressure dynamically, according to the changes of water flow. Therefore, it can economize the energy consumption significantly. In order to improve real-time performance, the system adopts STs STM32F407 series processor which is based on ARM® CortexTM-M4 32-bit RISC core. It is used to collect the water flow, pressure and other data in real time, and automatically adjust the output frequency to control the motor speed, according to the set pressure value, and ultimately control the flow and pressure of the water supply system. According to the test, this variable frequency and multi-pressure water supply control system based on STM32 processor can meet the design requirements.

With the increase of population density and the improvement of human economic level, the emergence of high-rise buildings has put forward higher requirements on infrastructure such as building hydropower, traditional water supply methods have disadvantages such as secondary pollution, unstable water pressure, and high energy consumption. Therefore, variable frequency and constant pressure water supply have gradually become the main method of water supply for high-rise buildings. In order to meet the higher water supply requirements of urban residential quarters, a PLC-based variable frequency speed regulation and constant pressure water supply monitoring system is designed. The main control objective is to stabilize the water supply pressure. The inverter is driven by a three-step variable frequency control method. The pressure sensor of the water supply pipeline detects the pressure value in real time, uses the PID control algorithm, and outputs the frequency setting value to the inverter through the PLC, and then realizes the control of the water pump outlet pressure and achieves the purpose of constant pressure water supply. On the basis of considering the program capacity and expansion requirements, this system selects S7-1200 compact PLC as the main controller, uses modular programming as the basic concept to control the PID control program, logic switching program, and alarm breaking the program has been programmed and debugged. The test result shows that the designed control algorithm is effective, the logic function is complete, and it has a certain application and reference value. The frequency conversion constant pressure water supply system can realize the automatic switching of the operation status and the number of pumps in operation under different water supply requirements, and further improve the automation degree of the system on the basis of meeting the control requirements.