Model Selection for Predicting the Return Time from Night Setback

Night setback is a common strategy used to reduce energy use in buildings. It involves increasing the cooling setpoint and decreasing the heating setpoint in a zone during unoccupied periods. To ensure occupant comfort and maximize energy savings, the zone temperature must be returned to the range defined by the occupied cooling and heating setpoints at occupancy, but not before. The time required to cool down or warm up a zone from a night setback condition is referred to as the return time and algorithms for predicting return time are commonly referred to as optimal start algorithms. Optimal start algorithms generally employ a model for predicting return time. This study describes the selection of separate return time models for cooling (i.e., a model for predicting the return time when cooling is required) and heating from 57 candidates. The following model forms were considered: τ = f (Tf - Ti), τ = f ((Tf - Ti), u), τ = f ((Tf - Ti), Tout), and τ = f ((Tf - Ti), u, Tout) where τ is the return time, Tf is the zone temperature at the end of the optimal start period, Ti is the zone temperature at the beginning of the optimal start period, u is exponentially weighted moving average (EWMA) of the zone cooling or heating demand at the beginning of the optimal start period, and Tout is the outdoor air temperature at the beginning of the optimal start period. Computer simulations were used to generate year-long data sets relating return time to the model inputs. The simulations considered the influence of climate, building mass, controller tuning, zone orientation, and the unoccupied control strategy on the return time. In all, 140 cooling data sets and 104 heating data sets were generated. For each data set, least squares regression was performed to determine the parameters for each of the 57 models considered. The performance of each model was quantified using the average root mean square prediction error across all simulations. The study revealed that the best models for predicting return time use the zone temperature change and the EWMA of the zone cooling or heating demand as inputs. The EWMA of the zone cooling or heating demand provides an indication of the recent history of the cooling or heating load on a zone and can account for intermittent cooling or heating that is required to keep the zone temperature within the bounds of the unoccupied setpoints. Notably, outdoor air temperature, a common input in optimal start algorithms, is not used. To the best of the authors\u27 knowledge, zone cooling and heating demand have not been previously used as an input in an optimal start algorithm. The full paper will provide a detailed description of the simulations and model comparison undertaken in this study

Which strategies work best to prevent obesity in adults?

Regular physical activity decreases long-term weight gain (strength of recommendation [SOR]: B, 2 high-quality, randomized controlled trials [RCTs]). Decreasing fat intake (SOR: B, 1 high-quality systematic review) and increasing fruit and vegetable consumption (SOR: B, 1 high-quality RCT) also may decrease weight gain. Combined dietary and physical activity interventions prevent weight gain (SOR: B, 1 high-quality systematic review)

On-board demux/demod

To make satellite channels cost competitive with optical cables, the use of small, inexpensive earth stations with reduced antenna size and high powered amplifier (HPA) power will be needed. This will necessitate the use of high e.i.r.p. and gain-to-noise temperature ratio (G/T) multibeam satellites. For a multibeam satellite, onboard switching is required in order to maintain the needed connectivity between beams. This switching function can be realized by either an receive frequency (RF) or a baseband unit. The baseband switching approach has the additional advantage of decoupling the up-link and down-link, thus enabling rate and format conversion as well as improving the link performance. A baseband switching satellite requires the demultiplexing and demodulation of the up-link carriers before they can be switched to their assigned down-link beams. Principles of operation, design and implementation issues of such an onboard demultiplexer/demodulator (bulk demodulator) that was recently built at COMSAT Labs. are discussed

Evolution: Lending a Helping Hand in Sperm Competition?

Most females mate with many males. This can be costly, but the benefits to females are often unclear. A new study raises the possibility that females could benefit through an unconventional genetic pathway, while also showing that males can inadvertently increase rival males' fitness in surprising ways

Model-free Control and Automatic Staging of Variable Refrigerant Flow System with Multiple Outdoor Units

For efficient operation of a variable refrigerant flow (VRF) air conditioning system with multiple outdoor units (ODUs), we propose a model-free control strategy based on extremum seeking control along with automatic staging control logic. The proposed strategy is evaluated with a representative VRF system consisting of 12 indoor units (IDUs) and three ODUs. The IDU zone temperature is regulated by EEV opening, and the compressor pressure is regulated by compressor speed. To optimize load sharing among multiple ODUs in operation, a set of bypass valves (BPVs) are added to the suction side of the compressors to manipulate refrigerant flow distribution among different compressors as needed. A penalty-function based multivariable extremum seeking control (ESC) method is used for real-time optimization of system operation. The performance index as the ESC feedback is the total power of the compressors, the ODU fans and the IDU fans, augmented with penalties for securing minimum superheat at the suction side of compressors. The manipulated inputs include the compressor suction pressure setpoint, the openings of BPVs at the suction side of the compressors, and a uniform setpoint of fan speed for all ODUs. As for the ESC feedback, the compressor power is normalized by its capacity. A set of control strategies for staging on/off particular ODUs is developed based on the compressor speed of the operating ODUs. Under increasing load, if the operating compressor(s) speed exceeds the higher limit of operation speed range (80% of rated speed), an additional ODU turned on to meet the load demand. Under decreasing load, it is desirable to turn off the least efficient ODU in a model-free fashion. In this study, an ESC based ODU staging-off strategy is proposed, for which the compressor shaft power normalized by the rated capacity is adopted as the ESC input. In addition to the compressor pressure setpoints and ODU fan speeds, the manipulated inputs of ESC also include the openings of suction-side BPVs in order to optimize load sharing among the multiple ODUs. With online optimization of ODU load sharing based on the normalized compressor power, the ESC can drive less efficient compressor(s) to operate at lower speed/capacity. If the compressor speed of an ODU falls below the preset lower limit of operational speed range (e.g. 20% of the rated speed) for long enough time, this ODU will be turned off. A dynamic simulation model of the multi-ODU VRF system is developed with Dymola and TIL Library. Simulation studies have been performed to evaluate the proposed ESC strategy for energy efficient operation during constant load patterns and the control logic for staging on and off ODU during load increase and decrease. The total power searched by the ESC is shown to be close to that obtained by a genetic algorithm based global optimization procedure in Dymola. Also, ESC is shown to be able to turn off least efficient ODU during load decrease without model knowledge. The load-sharing BPV at the compressor suction-side demonstrates bearable pressure loss except for the scenarios of large split ratio

Experimental Evaluation for an Extremum Seeking Control Strategy based on Input-output Correlation with a Mini-split Air Conditioning System

Extremum Seeking Control (ESC) has emerged as a model-free real-time optimization framework, typically based on dither-demodulation driven gradient estimation. However, such conventional ESC suffers from slow convergence. Salsbury et al. have recently proposed an input-output correlation based ESC (IOC-ESC) strategy anchored on a statistical analysis. The IOC-ESC algorithm is less sensitive to changes in its internal parameters because of the use of a normalized correlation coefficient in the feedback loop. The design goal of the algorithm is to have only two tunable parameters: (1) a time scale parameter that relates to the time open loop time constant of the system; and (2) the amplitude of the dither signal. A suitable set of generic internal parameters is still in the process of being identified as more test data become available from different system types. For the work reported here, the feedback gain (referred to as the tuning factor) with the IOC-ESC was also tuned for optimal performance. This study aims to conduct an experimental evaluation for the IOC-ESC strategy with a ductless mini-split air conditioning system, compared with conventional ESC (CON-ESC). The system features variable-capacity compressor operation and variable-speed operation for the evaporator and condenser fans. In this study, both single-input and two-input ESC scenarios are tested. The manipulated inputs include the evaporator and condenser fan speeds, while the total power consumption is used as feedback for all cases. The experimental setup is developed with a 9000 BTU variable-speed mini-split AC system serving a 4’x8’x6’ insulated chamber, and an electrical fan heater is used to provide an artificial heat load. The data acquisition and control algorithms are implemented on a National Instruments CompactRIO platform. Both IOC-ESC and CON-ESC are tested with the same setup. For single-input scenario, the manipulated input is the condenser fan speed. The testing results of five trials of IOC-ESC are used to evaluate the impact of the two tuning parameters, i.e. dither frequency and tuning factor, on the ESC performance. IOC-ESC#1, IOC-ESC#4 and IOC-ESC#5 have the same dither frequency but different tuning factors, while IOC-ESC#1, IOC-ESC#2 and IOC-ESC#3 have the same tuning factor but different dither frequencies. The testing results of two trials of CON-ESC are then compared with the IOC-ESC results. Both CON-ESC and IOC-ESC can effectively reduce the power consumption of the mini-split system without sacrificing zone temperature regulation. Moreover, the settling time of IOC-ESC ranges from 300 to 600 seconds, while the settling time of CON-ESC ranges from 900 to 1200 seconds. Overall, the IOC-ESC converges faster than the CON-ESC. For two-input scenario, the manipulated inputs are condenser fan speed and evaporator fan speed. The testing results of the two-input IOC-ESC are compared with the result of a two-input CON-ESC trial by Yan et al. with the same system. The settling times for CON-ESC and IOC-ESC are about 1800 and 1200 seconds, respectively. In summary, both CON-ESC and IOC-ESC can optimize the condenser fan speed and evaporator fan speed for energy efficient operation, while the IOC-ESC converges faster and has fewer tuning parameters

Automatic Mode Switching for A Multi-functional Variable Refrigerant Flow System

Multi-functional variable refrigerant flow system (MFVRF) is designed to realize simultaneous heating and cooling for individual zones. It is desirable to use existing measurements to determine switching between different modes under changes of ambient and load conditions, i.e. reversing the mode of indoor unit (IDU) and/or outdoor unit (ODU) heat exchangers (HX), as well as the bumpless transfer for controller switching. In this study, a set of mode switching logic is proposed for a four-zone MFVRF system, which involves both IDU and ODU mode switching actions. For the ODU-HX mode switching, thermodynamic analysis under different load changes reveals the qualitative trend for the air-side and refrigerant-side characteristics as the operation approaches to marginal scenarios. The ODU mode switching is thus based on the air-side temperature difference. Mode switching involving IDU action only is studied with a 1H3C (one heating three cooling) mode, in which IDU-1 is in heating mode and IDU-2, IDU-3 and IDU-4 are in cooling mode. For a given zone load conditions, when the zone temperature of IDU is higher than upper limit of a preset cooling mode hysteresis band, IDU enters the cooling mode by simultaneously opening all related cooling mode valves and closing heating-mode valves within time duration. On the other hand, the cooling mode is turned off by closing all related cooling valves when the zone temperature is lower than the lower limit of the cooling mode temperature band. Similarly, when the zone temperature is lower than lower limit of heating mode temperature band, IDU enters its heating mode. When the zone temperature for IDU is higher than the upper limit of heating mode temperature band, the heating mode is turned off. For ODU Mode Switching, it is proposed in this paper to use the temperature difference between the inlet and outlet air of the ODU HX. To justify the use of ODU air-side temperature differential as the indicator variable for ODU mode switching, several cases of 2H2C (two-heating two-cooling) mode are first simulated, in which the IDU-1 and IDU-2 are operated in heating mode and IDU-3 and IDU-4 are operated in cooling mode. A negative ramp of load change applied to IDU-3 within 1000 seconds. For the ODU-HX, the air inlet temperature is fixed at the ambient 20oC, while the air outlet temperature approaches closer and closer to 20oC under reducing cooling load in IDU-3. Simulation results have revealed the decreasing trend of COP. The T-s diagram for the refrigerant cycle of 2H2C mode is evaluated under several scenarios of reduction in IDU-3 cooling load. It reveals that a decreasing temperature difference at the air side or refrigerant side can be candidate probing variables for mode switching of ODU HX. Also, similar study is conducted when the ODU HX works as evaporator, with the MFVRF system operated in 3H1C (three heating one cooling) mode. Simulations for ODU HX mode switch case have been performed, and the results validate the effectiveness of the proposed scheme of mode switching

Distributed Extremum Seeking Control for a Variable Refrigerant Flow System

The variable refrigerant flow (VRF) technology has facilitated the development of multi-split ductless air conditioning systems, in which multiple indoor units (IDU) are used to regulate the refrigerant flow to achieve individualized zoning control. Model based control for VRF system demands for more modeling efforts in part due to diverse configuration, as well as changes in load and ambient conditions. As a model-free control strategy, Extremum Seeking Control (ESC) has been investigated for VRF systems. Dong et al. (2015) applied the standard centralized ESC scheme to a VRF system that consists of one outdoor unit (ODU) and four IDU’s. Simulation results have indicated the effectiveness of such strategy. As the number of IDU’s increases, the complexity of centralized controllers will increase accordingly. Therefore distributed ESC becomes a natural consideration for VRF systems with large number of IDU’s. In this paper, the Shashahani gradient based distributed ESC scheme proposed by Poveda and Quijano (2013, 2015), is applied to the four-zone VRF system simulated by Dong et al. (2015). In particular, this scheme is enhanced by appending a band-pass filter array at the output to achieve a better “isolation†among individual input channels. A single-input ESC is applied to the ODU, while the distributed ESC is applied to the four IDU’s with each acting as an agent. For each agent, the respective power consumption is used as feedback. The objective is to minimize the total power consumption of all agents. For the ODU ESC, the compressor suction pressure (PCS) set-point is employed as the manipulative input. For the IDU DESC, the evaporator superheat (SH) set-point is used as the manipulative input for each IDU agent. The distributed ESC scheme assumes full information communication among all IDU’s. Simulation study is performed to evaluate the proposed strategy with the Modelica based dynamic simulation model developed by Dong et al. (2015). The ESC is designed under the ambient condition of 35oC and 40 %RH, respectively. The initial temperature of all four IDUs zone is 29oC, and the zone temperature set-point is 26oC. The heat loads for IDU1 through IDU4 are 3000W, 2600W, 2400W and 2000W, respectively. It takes the average total power about 10000 seconds to converge to about 3200W in steady state, with PCS around 13bar, and the SH values of IDU1 through IDU4 at 4.5oC, 4.5oC, 6oC, and 5.5oC, respectively. The total power consumption was decreased from 4500 W to 3200 W, i.e. by 29%. In comparison with the centralized ESC Dong et al. (2015), the steady state error of total power is less than 50w. Work is under way to improve transient and steady-state performance, as well as simulation of other operation modes.  Â

Modelling the impact of local reactive school closures on critical care provision during an influenza pandemic

Despite the fact that the 2009 H1N1 pandemic influenza strain was less severe than had been feared, both seasonal epidemics of influenza-like-illness and future influenza pandemics have the potential to place a serious burden on health services. The closure of schools has been postulated as a means of reducing transmission between children and hence reducing the number of cases at the peak of an epidemic; this is supported by the marked reduction in cases during school holidays observed across the world during the 2009 pandemic. However, a national policy of long-duration school closures could have severe economic costs. Reactive short-duration closure of schools in regions where health services are close to capacity offers a potential compromise, but it is unclear over what spatial scale and time frame closures would need to be made to be effective. Here, using detailed geographical information for England, we assess how localized school closures could alleviate the burden on hospital intensive care units (ICUs) that are reaching capacity. We show that, for a range of epidemiologically plausible assumptions, considerable local coordination of school closures is needed to achieve a substantial reduction in the number of hospitals where capacity is exceeded at the peak of the epidemic. The heterogeneity in demand per hospital ICU bed means that even widespread school closures are unlikely to have an impact on whether demand will exceed capacity for many hospitals. These results support the UK decision not to use localized school closures as a control mechanism, but have far wider international public-health implications. The spatial heterogeneities in both population density and hospital capacity that give rise to our results exist in many developed countries, while our model assumptions are sufficiently general to cover a wide range of pathogens. This leads us to believe that when a pandemic has severe implications for ICU capacity, only widespread school closures (with their associated costs and organizational challenges) are sufficient to mitigate the burden on the worst-affected hospitals